Electropolymerization: Concepts, Materials and Applications

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Edited by Serge Cosnier and Arkady Karyakin

Electropolymerization Concepts, Materials and Applications

Edited by Serge Cosnier and Arkady Karyakin Electropolymerization

Further Reading Leclerc, M., Morin, J.-F. (eds.)

Hadziioannou, G., Malliaras, G. G. (eds.)

Design and Synthesis of Conjugated Polymers

Semiconducting Polymers

2010

2007

Hardcover

Hardcover

ISBN: 978-3-527-32474-3

ISBN: 978-3-527-31271-9

Kumar, C. S. S. R. (ed.)

Staikov, G. T. (ed.)

Nanostructured Thin Films and Surfaces

Electrocrystallization in Nanotechnology

2010

2007

Hardcover

Hardcover

ISBN: 978-3-527-32155-1

ISBN: 978-3-527-31515-4

Eftekhari, A. (ed.)

Geckeler, K. E., Nishide., H. (eds.)

Nanostructured Materials in Electrochemistry

Advanced Nanomaterials

2008

2010

Hardcover

Hardcover

ISBN: 978-3-527-31876-6

ISBN: 978-3-527-31794-3

Chemistry, Physics and Engineering

2 Volumes

Edited by Serge Cosnier and Arkady Karyakin

Electropolymerization Concepts, Materials and Applications

The Editors Dr. Serge Cosnier Universit´e Joseph Fourier D´epartment de Chimie Mol´eculaire UMR-5250 Boite postale 53 38041 Grenoble Cedex 9 France Prof. Arkady Karyakin Moscow State University Analytical Chemistry Department Lenin Hills, GSP - 3 11991 Moscow Russia

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliograﬁe; detailed bibliographic data are available on the Internet at .  2010 WILEY-VCH Verlag & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microﬁlm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not speciﬁcally marked as such, are not to be considered unprotected by law. Cover Design Adam Design, Weinheim Typesetting Laserwords Private Ltd., Chennai, India Printing and Bookbinding Strauss GmbH, M¨orlenbach Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-32414-9

V

Contents

Preface XI List of Contributors 1

1.1 1.2 1.3 1.3.1 1.3.2 1.3.3 1.4

2

2.1 2.2 2.3 2.4

3

3.1

XIII

Electropolymerized Films of π-Conjugated Polymers. A Tool for Surface Functionalization: a Brief Historical Evolution and Recent Trends 1 G´erard Bidan Introduction 1 Electropolymerization: Epistemological Analysis within the ICP Saga 2 Electropolymerization: from Pristine Heterocyclic to Sophisticated Functional and Conjugated Architectures 4 Electropolymerization of Pristine Aromatic Heterocycles 5 Electropolymerization of Substituted Heterocycles 7 Electropolymerization as a Tool to Elaborate Functional Conjugated Architectures 10 Conclusion 12 References 13 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects 27 Mikhail A. Vorotyntsev, Veronika A. Zinovyeva, and Dmitry V. Konev Electropolymerization: General Aspects 27 Redox Activity of Polymer Films 32 Effect of Polymerization Parameters on Properties of Deposited Polymer Films 38 Conclusions 47 References 47 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization 51 Gy¨orgy Inzelt and Gy˝oz˝o G. L´ang Introduction 51

Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

VI

Contents

3.2 3.3 3.3.1 3.3.2 3.3.3 3.4 3.5 3.5.1 3.5.2 3.5.3 3.5.4 3.6

4

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.5

5 5.1 5.2 5.2.1 5.2.2 5.3 5.3.1

Experimental Arrangements 53 Impedance Spectra of Polymer Films 55 Effect of the Film Thickness and Thickness Distribution of Polymer Films 56 Characteristic Quantities for Modiﬁed Electrodes 57 Impedance Associated with Polymer Films in Contact with Media Allowing both Ionic and Electronic Interfacial Exchange 60 Analysis of the Impedance Spectra 61 Models of Polymeric Layers 63 ‘‘Homogeneous’’ or ‘‘Uniform’’ Models 63 ‘‘Heterogeneous’’ or ‘‘Porous Layer’’ Model 64 Theories Dealing with Two or Three Charge Carriers 65 Brush Model 66 Summary 70 Acknowledgment 70 References 70 Recent Trends in Polypyrrole Electrochemistry, Nanostructuration, and Applications 77 Pierre Audebert Introduction 77 Advances in Synthetic Procedures – New Polymers 78 New Monomers and Polymers 78 Fundamental Research 78 New Polymerization Methods 79 Nanostructuration of Polypyrrole 80 Nanostructuration of Polypyrrole 80 Polypyrrole Nanocomposites 80 Applications 83 Batteries and Supercapacitors 84 Actuators 85 Anticorrosion 85 Miscellaneous 86 Conclusion 87 References 87 Electropolymerized Azines: a New Group of Electroactive Polymers 93 Arkady A. Karyakin Introduction 93 Electropolymerized Azines as a New Group of Electroactive Polymers 93 Electropolymerization of Azines 94 Hypothesis of Polyazine Structure 96 Polyazines in Electroanalysis 98 Electrocatalysis by Polyazines 98

Contents

5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.3.2.5 5.4 5.4.1 5.4.1.1 5.4.2 5.5

6 6.1 6.2 6.2.1 6.2.2 6.2.3 6.3 6.3.1 6.3.2 6.3.3 6.4

7 7.1 7.1.1 7.2 7.3 7.4 7.5 7.6

Electropolymerized Azines as Advanced Electrocatalysts for NAD+ |NADH Regeneration 99 Dehydrogenase Enzymes and Electrocatalysis of NAD+ |NADH Regeneration 99 Mimetics of Enzyme Catalysis 100 Electropolymerized Azines as NADH Transducers 101 Electroreduction of NAD+ to Enzymatically Active NADH at Poly(Neutral Red)-Modiﬁed Electrodes 102 Observation of the Equilibrium NAD+ |NADH Potential at Poly(Neutral Red) Electrodes 103 Electropolymerized Azines as Promoters for Bioelectrocatalysis 105 Attempts to Involve Glucose Oxidase in Mediator Free Bioelectrocatalysis 105 Bioelectrocatalysis by Hydrogenase and Peroxidase 106 Bioelectrocatalysis by Cellobiose Dehydrogenase on Polyazines 106 Conclusion 108 References 108 Electropolymerization of Phthalocyanines 111 Ninel M. Alpatova and Elena V. Ovsyannikova Introduction 111 Immobilization of Transition-Metal Phthalocyanines on Conducting and Nonconducting Substrates 111 Phthalocyanines in Electron-Conducting Polymers 111 Phthalocyanines in Matrices of Artiﬁcial Lipids 113 Composites of Ultrathin Layers of Oppositely Charged Ions 115 Electropolymerization of Phthalocyanines 117 Electropolymerization of Phthalocyanines with Ligands Bonded to Radicals of Electron-Conducting Polymer Precursors 118 Electropolymerization of Tetra-Amino-Substituted Phthalocyanines 119 Electrochemical Modiﬁcation of Electrodes with Nickel Tetra-Sulfonated Phthalocyanine 125 Conclusion 128 References 130 Imprinted Polymers 133 Michael J. Whitcombe and Dhana Lakshmi Introduction 133 What is Molecular Imprinting? 133 Molecular Imprinting in Conjugated Polymers 135 Solgel Imprinted Films Prepared by Electropolymerization 138 Integration of MIPs with the Surface of Transducers 139 Nanostructured Materials 140 Other MIP-Based Sensors 143

VII

VIII

Contents

7.6.1 7.6.2 7.6.3 7.6.4 7.7

Piezoelectric Sensors 144 Capacitive Sensors 144 Amperometric and Voltammetric/Potentiometric Sensors 145 Miscellaneous Sensing Systems 146 Conclusion 147 References 148

8

Gas Sensing with Conducting Polymers 153 Karin Potje-Kamloth Introduction 153 Electronic Properties of Conducting Polymers 153 Preparation of Polymer Gas-Sensing Layers 155 Solvent Casting 155 In situ Electrochemical Deposition 155 Tuning of Electronic Properties of Conducting Polymers 156 Effect of Primary Doping on Work Function 156 Electrochemical Work Function Tuning 156 Mechanism of Gas/Polymer Interactions 157 Secondary Doping by Donor/Acceptor Interactions 157 Work Function Modulation – Modulation of Carrier Density 157 Bulk Resistance Changes 158 Contact Resistance Changes (Schottky Barrier) 158 Types of Conducting Polymer-Based Gas Sensors 159 Potentiometric (Zero-Current) Sensors 159 Kelvin Probe 159 CHEMFET 159 Examples of Kelvin Probe and CHEMFET Gas Sensors 160 Conductometric (Nonzero-Current) Sensors 163 Chemiresistors – Bulk Resistance Modulation 163 Schottky Barrier Diodes – Contact Resistance Modulation 165 OFETs – Field-Modulated Chemiresistors 168 Examples of Conductometric Gas Sensors 168 Examples of Polymer Schottky Diode Gas Sensors 168 Conclusion 169 References 169

8.1 8.2 8.3 8.3.1 8.3.2 8.3.3 8.3.3.1 8.3.3.2 8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.5 8.5.1 8.5.1.1 8.5.1.2 8.5.1.3 8.5.2 8.5.2.1 8.5.2.2 8.5.2.3 8.5.2.4 8.5.2.5 8.6

9 9.1 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.3

Chemical Sensors Based on Conducting Polymers 173 Johan Bobacka and Ari Ivaska Introduction 173 Electrochemical Signal Transduction 174 Potentiometric Sensors 175 Amperometric and Voltammetric Sensors 179 Conductimetric Sensors 181 Chemically Sensitive Transistors 182 Optical Signal Transduction 184

Contents

9.4

Conclusions 184 Acknowledgments 185 References 185

10

Biosensors Based on Electropolymerized Films 189 Serge Cosnier and Michael Holzinger Introduction 189 Chronological Evolution of the Concept of Biosensors Based on Electropolymerized Films: Principal Stages 190 Formation of Polymer Films by Direct Electropolymerization of the Biomolecule 191 Adsorption on Electrogenerated Polymers 194 Mechanical Entrapment within Electropolymerized Films 195 Covalent Binding at the Surface of Electropolymerized Films 200 Noncovalent Binding by Afﬁnity Interactions with the Electropolymerized Films 203 Outlook 205 References 206

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8

11

11.1 11.1.1 11.1.2 11.1.3 11.2 11.2.1 11.2.2 11.2.3 11.3 11.3.1 11.3.2 11.4 11.4.1 11.4.2 11.4.3 11.5 11.5.1 11.5.2 11.6

Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage 215 Gordon G. Wallace, George Tsekouras, and Caiyun Wang Introduction 215 Electrochemical Techniques 216 Substrates 217 The Electrolyte 217 Energy Conversion 218 Polythiophenes via Electropolymerization of Simple Precursors 219 Polythiophenes via Electropolymerization of Precursors Functionalized with Electron Accepting/Withdrawing Moieties 222 Polythiophenes via Electropolymerization of Precursors Functionalized with Light-Harvesting Moieties 225 Energy Storage 227 Application of Inherently Conducting Polymers in Rechargeable Batteries 228 Application of Conducting Polymers in Supercapacitors 229 Electropolymerization to Form Electrodes for Energy Storage Applications 230 PPy 230 PANi 231 PTh and Derivatives 232 Nanostructured Conducting Polymers 233 Template-Assisted Electropolymerization 233 Direct Electropolymerization 233 Conducting Polymer Composites 234

IX

X

Contents

11.7

Conclusions 236 References 236

12

Electrochemomechanical Devices: Artiﬁcial Muscles 241 Toribio F. Otero and Joaqu´ın Arias-Pardilla Introduction 241 Conducting Polymers as Reactive Materials: Electrochemical Reactions 242 Oxidation 242 Prevailing Anion Interchange 243 Prevailing Cation Interchange 243 Reduction of Neutral Chains 244 Complex Actual Ionic Interchanges and Polymeric Structure 244 Giant Nonstoichiometry 245 Electrochemical Properties: Multifunctionality and Biomimetism 246 Electrochemomechanical Properties and Artiﬁcial Muscles 246 Basic Molecular Motor 247 Macroscopic Dimensional Changes and Mechanical Properties 248 Anisotropy Obtained from Isotropic Changes: Macroscopic Devices 248 Electrochemical Transducer 249 Efﬁciency 251 Bending Structures 252 Asymmetrical Monolayers 252 Bilayers 253 Triple Layers 253 Structures Giving Lineal Movements 254 Fibers and Films 254 Tubes and Films with Metal Support 254 Combination of Bending Structures 255 Microdevices and Microtools 255 Electrochemical Characterization 257 Sensing Capabilities of Artiﬁcial Muscles 258 Tactile Sensitivity 259 Intelligent Devices 263 Muscles Working in Air 264 Advantages, Limitations, and Challenges 264 Artiﬁcial Muscles as Products 265 References 266

12.1 12.2 12.2.1 12.2.1.1 12.2.1.2 12.2.2 12.2.3 12.2.4 12.3 12.3.1 12.3.2 12.4 12.5 12.5.1 12.5.2 12.5.3 12.5.3.1 12.5.3.2 12.5.3.3 12.5.4 12.5.4.1 12.5.4.2 12.5.5 12.5.6 12.6 12.7 12.8 12.9 12.10 12.11 12.12

Index 273

XI

Preface Well before the activity explosion in the nanosciences and their huge success, electropolymerization, from a fundamental point of view, had already become one of the key methods making the creation of molecular wires or nets possible. The process of electropolymerization leads to a simple and reproducible formation of organic or organometallic ﬁlms with precise spatial resolution over surfaces, whatever their size and geometry. This formation of polymer ﬁlms led to a broad material diversity of applications which extended from automobile industry to biotechnologies using, for example, the DNA chip. Thanks to the versatility of the monomer design and the polymer processability, a fascinating range of conductive, electromagnetic, mechanical, electrochromic, light-emitting, and optical properties was ascribed to these ﬁlms. The functionalization of repeating units opens a broad ﬁeld of application of polymers in the ﬁelds of corrosion protection, biochemistry, analytics, photovoltaics, renewable energy and the environment, as well as actuators. Electropolymerization presumes polymer formation upon oxidation or reduction of certain organic compounds. Upon electropolymerization, new covalent bonds are formed between these monomer precursors distinguishing this process from simple electrodeposition. Electropolymerization is thus a powerful tool for development of modiﬁed electrodes. First, electropolymerization provides simplicity of targeting in the selective modiﬁcation of multielectrode structures. Indeed, even by exposing the whole structure to the precursor solution, one can target polymer formation on the electrode of interest, applying to it the required current or potential. Second, the electropolymerized ﬁlms are usually much more stable on electrode surfaces compared to both adsorbed and covalently linked modiﬁers such as low-molecular-weight organic compounds or chemically synthesized polymers. Third, the electropolymerized materials usually possess some unique properties that are not those of the corresponding monomers. These properties mainly concern electroactive polymers, which demonstrate new sets of peaks in cyclic voltammograms due to the appearance of new conjugative chains or modiﬁcation of the existing ones in their structure. The formation of electroactive polymers upon reduction or oxidation of different organic compounds became one of the major topics in modern electrochemistry Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

XII

Preface

following the discovery of conducting polymers. Electrochemical synthesis of the latter undoubtedly dominates over chemical synthesis. We invited world-recognized scientists who are specialists in different ﬁelds to contribute speciﬁc chapters to this book. This was in order to cover major aspects of electropolymerization from the fundamentals to various applications. We believe the book will be accepted by the wide scientiﬁc audience as well as by graduate students.

February 2010

Serge Cosnier (Grenoble) Arkady Karyakin (Moscow)

XIII

List of Contributors Ninel M. Alpatova A.N. Frumkin Institute of Physical Chemistry and Electrochemistry Russian Academy of Sciences Leninskii Prospect 31 119991 Moscow Russia Pierre Audebert Ecole Normale Sup´erieure de Cachan Laboratory of Supramolecular and Macromolecular Photophysic and Photochemistry Bˆatiment d’Alembert 61, avenue du Pr´esident Wilson 94235 Cachan C´edex France Joaqu´ın Arias-Pardilla Technical University of Cartagena Center for Electrochemistry and Intelligent Devices School of Industrial Engineers C/Carlos III 30203 Cartagena Spain

G´erard Bidan CEA/Institut Nanosciences et Cryog´enie 17 rue des Martyrs 38054 Grenoble cedex 09 France Johan Bobacka ˚ Abo Akademi University Process Chemistry Centre Laboratory of Analytical Chemistry Biskopsgatan 8 ˚ FI-20500 Abo-Turku Finland Serge Cosnier Universit´e Joseph Fourier D´epartement de Chimie Mol´eculaire UMR CNRS 5250 BP-53 38041 Grenoble Cedex 9 France Michael Holzinger Universit´e Joseph Fourier D´epartement de Chimie Mol´eculaire UMR CNRS 5250 BP-53 38041 Grenoble Cedex 9 France

Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

XIV

List of Contributors

Gy¨ orgy Inzelt E¨otv¨os Lor´and University Institute of Chemistry Laboratory of Electrochemistry and Electroanalytical Chemistry P´azm´any P´eter S´et´any 1A 1117 Budapest Hungary

Gy˝oz˝o G. L´ang E¨otv¨os Lor´and University Institute of Chemistry Laboratory of Electrochemistry and Electroanalytical Chemistry P´azm´any P´eter S´et´any 1A 1117 Budapest Hungary

Ari Ivaska ˚ Abo Akademi University Process Chemistry Centre Laboratory of Analytical Chemistry Biskopsgatan 8 ˚ FI-20500 Abo-Turku Finland

Toribio F. Otero Technical University of Cartagena Center for Electrochemistry and Intelligent Devices School of Industrial Engineers C/Carlos III 30203 Cartagena Spain

Arkady A. Karyakin M.V. Lomonosov Moscow State University Department of Chemistry 119991 Moscow Russia

Elena V. Ovsyannikova A.N. Frumkin Institute of Physical Chemistry and Electrochemistry Russian Academy of Sciences Leninskii Prospect 31 119991 Moscow Russia

Dmitry V. Konev University of Bourgogne ICMUB - UMR 5260 CNRS Bat. Mirande 9 avenue A. Savary BP 47 870 21078 Dijon Cedex France Dhana Lakshmi Cranﬁeld University Cranﬁeld Health Vincent Building Cranﬁeld Bedfordshire MK43 0AL UK

Karin Potje-Kamloth Institut f¨ur Mikrotechnik Mainz Carl-Zeiss-Strasse 18–20 55129 Mainz Germany George Tsekouras University of St Andrews School of Chemistry North Haugh St. Andrews Fife KY16 8DA Scotland UK

List of Contributors

Mikhail A. Vorotyntsev University of Bourgogne ICMUB - UMR 5260 CNRS Bat. Mirande 9 avenue A. Savary BP 47 870 21078 Dijon Cedex France

Michael J. Whitcombe Cranﬁeld University Cranﬁeld Health Vincent Building Cranﬁeld Bedfordshire MK43 0AL UK

Gordon G. Wallace University of Wollongong Intelligent Polymer Research Institute ARC Centre of Excellence for Electromaterials Science Wollongong NSW 2522 Australia

Veronika A. Zinovyeva University of Bourgogne ICMUB - UMR 5260 CNRS Bat. Mirande 9 avenue A. Savary BP 47 870 21078 Dijon Cedex France

Caiyun Wang University of Wollongong Intelligent Polymer Research Institute ARC Centre of Excellence for Electromaterials Science Wollongong NSW 2522 Australia

XV

1

1 Electropolymerized Films of π-Conjugated Polymers. A Tool for Surface Functionalization: a Brief Historical Evolution and Recent Trends G´erard Bidan

1.1 Introduction

Electrodeposition of conducting polymer ﬁlms at the surface of an electrode has opened a ﬁeld at the convergence between two rich domains: electrochemistry of modiﬁed electrode [1–3] and conjugated systems [4]. Consequently, applications of modiﬁed electrodes in electrocatalysis, electrochromism, energy storage, electroanalysis, and sensors have been enriched by the speciﬁc properties of intrinsically conducting polymers (ICPs), for example, electrochemically tunable doping and dedoping (equivalent to adjustable redox states), polymeric matrix affording electrical wiring, use as immobilized redox mediators, and the capacity to induce new functionalities by the use of speciﬁc gratings. Reciprocally, electrochemistry has opened up the route to easy-to-handle polymer ﬁlms in a manner similar to the way that polyacetylene, (CH)x , prepared as a ﬁlm by a modiﬁcation of the Natta reaction [5], resulted in the discovery, in 1977, of the doping effect as presented in the seminal paper of Shirakawa and coworkers [6]. In addition, this cross-fertilization enlarged the panel of new ICP-based materials, such as electrogenerated composites [7], and strengthened or brought in new applications such as energy conversion and storage (Chapter 11); electrotriggered drug delivery [8]; soft actuators (Chapter 11); chemical, bio-, and gas sensors (Chapters 8–10); biocompatible ﬁlms [9]; and artiﬁcial muscles (Chapter 12). Considering the intense and widespread research activities in these ﬁelds, the aim of this historical survey is not to cover the entire ﬁeld of the various electropolymerization facets detailed in the following chapters, but to give an overview of the successive contributions to and acquisition of knowledge. The electropolymerization reported here is restricted to oxidative condensation; as a matter of fact, it should be mentioned that as early as in 1983, Fauvarque [10] reported the synthesis of poly(p-phenylene) ﬁlm by electroreduction-assisted catalysis by Ni(0) complex. In the ﬁrst part, electropolymerization is described in the context of π-conjugated polymers. Four generations have been distinguished in this saga: the ‘‘era of physicists,’’ the ‘‘era of electrochemists,’’ the ‘‘era of polymerists,’’ and the ‘‘era of molecular electronics.’’ This division appears a little artiﬁcial, since Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

2

1 Historical Evolution and Recent Trends in Surface Functionalization

the progress in each of these eras resulted from mutual enrichment between these scientiﬁc communities; however, this book provides an enlightening presentation of each determining step of the evolution. The ‘‘era of electrochemists’’ starts with the early use of electropolymerization in the 1980s. The second part presents the major milestones reached by the process of electropolymerization in the light of the functionalization of surfaces for the electrodeposition of increasingly sophisticated conjugated architectures endowed with speciﬁc functionalities from sensors to active photovoltaic layers. Recent trends in the use of the electropolymerization concerning the elaboration of nanowires or nanotubes of ICPs for sensors or molecular electronics, nanostructured materials (interpenetrated networks with ICPs, carbon nanotubes/ICPs combination, etc.) are not presented here. It is emphasized that the compilation of bibliographic data has been a very difﬁcult task, since it is pointless to duplicate all the references that are given in the other chapters; so the selection here is a mix of citations of pioneering teams with key contributions made in the 1980s–1990s, easily accessible reviews, and recent representative publications on the new trends in the ﬁeld.

1.2 Electropolymerization: Epistemological Analysis within the ICP Saga

It is possible to distinguish four generations in the still active saga of ICPs. The ﬁrst generation, the ‘‘era of physicists,’’ corresponds to their historical identiﬁcation as synthetic or organic metals, and parallel to the development of mixed valence crystals in the family of TTF-TCNQ [11, 12], the domain of electroconductive polymers appeared driven by the interest of physicists in the semiconducting/conducting or even supraconducting conductivity transition. Polysulfurnitride, (SN)x , was investigated in the early 1960s and the metallic properties were studied in the 1970s [13], with a superconducting transition below 0.3 K evidenced in 1975 [14], and the ‘‘doping effect’’ of halogens reported in 1977 [15]. Similarly, (CH)x , the ﬁrst chemically unsophisticated representative of the π-conjugated structure, that is, with alternative C–C single and double bonds, was extensively investigated after the discovery of halogen doping in 1977 [6]. It must be emphasized at this point that the preparation of (CH)x as an easy-to-handle ﬁlm [16, 17] instead of a pressed pellet of powder considerably boosted the ﬁeld and allowed to carry out electrochemical doping on (CH)x ﬁlms as electrodes [18]. The chemical modiﬁcation of poly(p-phenylene) [19] by AsF5 or alkali metals was reported soon after in 1979. The second generation of ICPs, the ‘‘era of electrochemists,’’ began with the electropolymerization of aromatic heterocycles and derivatives. In addition to the ‘‘easy-to-handle effect’’ previously illustrated by (CH)x , electropolymerization is based on a new concept of oxidative condensation through the generation of radical cations (Chapter 2). Early work in 1916 [20] and 1937 [21] on chemically prepared ‘‘neri di pirrolo’’ had not been aware of the electronic properties of these powders of poly(oxipyrrole). Chemical oxidation of aniline, reported by Buvet and coworkers

1.2 Electropolymerization: Epistemological Analysis within the ICP Saga

` Christoﬁni in 1973 [24], in 1968 [22, 23], and of pyrrole, reported by Hautiere was recognized to provide electrically conductive materials. Electropolymerization allowed handling polypyrrole [25, 26], polyaniline [27–29], and poly(p-phenylene) [30] ﬁlms, resulting in completely new polymers ﬁlms, some of the better known being the polythiophene [31, 32], polyﬂuorene [33], and polycarbazole [34] classes. Consequently, in early 1980s, the electrochemist community was drawn to use electrochemistry not only as a tool to prepare ICP ﬁlms [35] but also as a methodology (Chapter 3) to investigate the doping/dedoping process electrochemically tuned with the associated movements of ionic dopants and the concomitant evolution of the electronic structure using electron spin resonance (ESR) spectroscopy [36], UV–vis [37] and surface IR [38] spectrophotometries, ellipsometry [39–41], quartz crystal microbalance (QCM) [42], and mirage effect [43], coupled with voltamperometric methods. In addition to the use of ICPs as substitutes for metals, new ICP applications, traditionally falling into the ﬁeld of electrochemistry such as electrocatalysis [44, 45], sensors [46–51], biosensors (Chapters 8–10), energy storage (e.g., batteries [52–56] and supercapacitors [57, 58]) (Chapter 11), anticorrosion deposits onto metals [59–61] and semiconductors [62–65], and electrochromism [66–69], were rapidly developed. However, the concept of functionalization was the key breakthrough [45, 70–73]. It is possible to deposit a polymer ﬁlm including functional moieties into the polymeric backbone in just one step. The tremendous progress in research on sensors and biosensors (Chapters 8–10) originates with the study of sensitive layers based on (bio)functionalized ICP ﬁlms. The third generation, the ‘‘era of polymerists,’’ emerged from the inputs of chemists, particularly the macromolecularists, to the ICP domains. The intrinsic advantage of electropolymerization – a straightforward deposition of a redox and an electroconducting ﬁlm of an electrocontrollable thickness, with tunable interfacial properties for numerous electrochemical applications – is counterbalanced by the complete insolubility of the deposit. Thus, the determination of classical characterization parameters for polymers such as the average chain length, dispersion, crystallinity, and the handling by spin or dip coating for large-scale applications are both impeded. Chemists have played an important role in the development of new routes in chemical synthesis, providing structurally well-deﬁned conducting polymers. In the large family of ICPs, polythiophenes have been by far the more studied, and as early as in 1980, Yamamoto [74] reported the Ni-catalyzed condensation of 2,5-dibromothiophene. Three main properties have been tuned via structural and chemical modiﬁcations: the gap, the solubility, and the conductivity. The existence of low-gap thiophene-based ICPs [75, 76] such as poly(isothianathene) was reported in 1984 by Wudl [77], polyfused heterocycles such as poly(thienothiophene) was reported by Taliani in 1986 [78, 79], and poly(dithienylethylene) and related systems by Roncali [80] in 1997. Poly(ethylenedioxithiophene) (PEDOT), reported by Heinze et al. in 1994 [81] and Reynolds et al. in 1996 [82] and considered as one of the most stable ICPs, is now commercially available and is used in numerous applications. Soluble poly(3-alkylthiophenes) (P3-ATs) were ﬁrst reported by Elsenbaumer in 1986 [83]. Regioregularity with the so-called McCullough method

3

4

1 Historical Evolution and Recent Trends in Surface Functionalization

[84] reported in 1993 in the P3-ATs family has been the cornerstone for the development of applications in organic electronics (vide infra). While classical polymerizations by oxidative coupling using Fe(III) salts provides polymers with 3,4- and 2,5-linkage defects, low molecular weights, and weak conductivities (in the range of 0.1–1 S cm−1 ), the metal-catalyzed C–C coupling of heterocycles (e.g., Suzuki-, Sonogashira-, and Stille-type reactions) allows to improve their conductivities by more than 2 orders of magnitude [85]. In addition to processable ICP-based materials [86], the above-mentioned chemical methods were also used for the step-by-step synthesis of well-deﬁned length oligothiophene [87]. Considerable progress has been made from the simple sexithiophene reported in 1989 by Garnier et al. [88] to the sophisticated oligothiophene-based nanoarchitectures reported in the recent remarkable review by B¨auerle et al. [89]. The fourth generation covers the wide domain of organic electronics in its extended acceptation and can be considered as a ‘‘renaissance’’ of the ICP domain of applications by the fruitful cross-fertilization between synthetic chemistry and electronics. It is contemporary to the third generation, and mainly concerns organic light-emitting diodes (OLEDs), ICP-based photovoltaic devices, organic thin ﬁlm transistors (OTFTs), and molecular electronics. After the ﬁrst report by Garnier on OTFTs based on sexithiophene [88] in 1989, a signiﬁcant step in 1990 was the description by Friend and coworkers of the electroluminescent device based on poly(p-phenylene vinylene) (PPV), placed between an indium tin oxide (ITO) and an Al electrode [90]. Polymer light-emitting diodes were extended to different classes [91] of conjugated polymers such as poly(carbazole)s, poly(ﬂuorene)s, PPVs, and poly(thiophene)s. The reverse phenomenon of photovoltaic cells based on ICPs [92] was soon reported, with the next decisive step resulting in the ultrafast photoinduced electron transfer from ICPs to the C60 fullerene, developed independently by Sariciftci et al. [93] and Yoshino et al. [94] in 1992. These ﬁelds are well detailed in the second volume of the third edition of the Handbook of Conducting Polymers, edited by Skotheim T A, Reynolds in 2007. Interestingly, we will see in the second part of the following that, in spite of the leading processes of dip or spin coatings to implement ICPs in electronic devices, electropolymerization is still being developed as an alternative method for the ﬁne control of thickness for numerous applications [95–97].

1.3 Electropolymerization: from Pristine Heterocyclic to Sophisticated Functional and Conjugated Architectures

Having, very brieﬂy, traced almost 30 years of scientiﬁc venture on ICPs, which have replaced electropolymerization as one of the most important inputs, this section examines more speciﬁcally the evolution of this methodology (Figure 1.1) in the context of functionalization of surfaces.

1.3 Electropolymerization: Toward Conjugated Architectures

1980

1990

2000

5

2010

Pristine and fused heterocycles Templated microstructures Polymer composites

Nanostructured ICPs: nanowires and nanotubes, IPN*, inverse opals, etc.

Doping and pending group functionalizations Metallated conjugated architectures, hybrid and alternated polycycles («in chain» functionalization) Figure 1.1 Some key milestones in the evolution of electropolymerized heterocycles in the saga of ICPs. These successive main steps are not independent but mutually enriched. * IPN, Interpenetrated network.

1.3.1 Electropolymerization of Pristine Aromatic Heterocycles

The ﬁrst electropolymerization experiment reported in 1968 by Dall’oxlio [25] on pyrrole in water medium was not the trigger event; as a matter of fact, except for aniline in acidic media, pristine pyrrole, and, as reported later, for thiophene and derivatives in micellar medium [98–100], few heterocycles are soluble and able to electropolymerize in aqueous media. The ﬁrst determining event was the electropolymerization in an organic medium of pyrrole (0.06 M) in acetonitrile (1% aqueous) containing 0.1 M Et4 NBF4 , reported in the seminal article of Diaz et al. in 1979 [101] from works carried out at IBM San Jose [102]. The route was then opened for the screening of electrochemical synthesis of conducting polymers by anodic oxidative condensation of aromatic heterocycles. Over the next 10 years, electropolymerization of polythiophene [103, 104], poly(p-phenylene), polynaphthalene, polyanthracene, polypyrene, polyindole, polyazulene [103], polyﬂuorene, and polycarbazole [34, 105–107] was reported and reviewed in detail by Simonet and Rault-Berthelot [108]. Many more sophisticated units are still being reported [109]. A special mention must be made of the electropolymerization of thiophene (from among thiophene, azulene, and furan) by Tourillon and Garnier in 1982 [31], since this family, including thiophene-fused cycles [54, 78, 79], has been the most ﬂourishing of the ICP domain. PEDOT, pioneered by Heinze [80] and then by Reynolds [82], still appears as the leading material, including alkyl derivatives [110]. Polyaniline, which was ﬁrst chemically synthesized and considered as early as in 1968 by Buvet and coworkers [22, 23] as a semiconductor whose redox properties can be tuned by the pH, was electrosynthesized by Diaz et al. in 1980 [27], and then ‘‘rediscovered’’ around 1985 in the light of the new concepts of conjugated structures; among leading teams we may cite MacDiarmid [111], Geni´es [28, 112], and Bard [29]. After a brilliant start, owing to the attractive potential applications

6

1 Historical Evolution and Recent Trends in Surface Functionalization

in energy storage with the pioneering paper by MacDiarmid and coworkers in 1984 [113], polyaniline (Pani) appeared to ﬁnd greater use in solution-processable applications [114], since as soon as the phenyl cycle is substituted, the electropolymerization is strongly disturbed [115, 116]. A renewing of Pani applications, such as supercapacitors, beneﬁts from the recent use of ionic liquids in addition to the contribution of nanoscience concepts to produce nanostructures. An important step toward the functionalization of surfaces was the inclusion of a functional dopant during electrodeposition. Applications of these pristine polyheterocycles were limited to the exploitation of their redox properties (batteries and supercapacitors) and related modulations in conductivity (electrochemical transistors), color (electrochromism), and volume changes (actuators). Functionalization by the inclusion of speciﬁc dopants (Figure 1.2) soon appeared as a straightforward route pioneered by Skotheim in 1985 [124] and Shimidzu in 1987 [117], since no speciﬁc chemistry on the heterocycle is required – only the choice of a functional anionic (Figure 1.2a) or cationic (Figure 1.2c) (for self-doped ICPs) species. The inclusion of the functional dopant present in the electrolyte during electropolymerization was preferred to the exchange of ‘‘classical’’ nonfunctional dopants (perchlorate, chloride, tetraﬂuoroborate, etc.) after the electrodeposition just by dipping in a solution containing the functional dopant. In fact, the reverse reaction, that is, the retrodiffusion of the functional dopant in a renewed solution cannot be avoided, even though it had been exploited in a certain − + − n (2 + d) e − 2n H

n

+ nd A−

N H

(a)

+ nde

(c)

N H

−

n

+ nde d+

n

, d A−

d+ N H

(b)

, d A−

d+ N H

N H

+ nd A−

n

−

, d A−

+ nd C+

n

Figure 1.2 Functionalization by doping. (a) The function is introduced during the electrodeposition using a speciﬁc dopant A− entrapped into the polymeric backbone. (b) When the dopant is small, the anionic dopants A− are mobile and mainly expelled during the dedoping by electroreduction. (c) Provided that the anionic dopants (bulky ˚ entangled ionomeric groups, i.e., ≥ 10 A, chains, grafted dopants) are immobilized

, d A− , d C+ N H

n

(the wavy line symbolizes this immobilization), the dedoping results mainly in the entrance of a cationic ‘‘pseudodopant’’ C + and vice versa during doping. The arrows of electronic transfers and ionic movements (from left to right) correspond to the dedoping and must be inverted for the reverse doping reaction. These processes have been exploited for the electrocontrolled delivering of charged drugs.

1.3 Electropolymerization: Toward Conjugated Architectures

manner for the electrocontrolled release of species (Figure 1.2b) [118–120]. A fruitful method to increase the ratio of dopants versus monomeric units consisted in the grafting of an additional positive charge on the pyrrole unit via an alkylammonium substituent [121]. This was exploited for the incorporation of enzyme via a solid-state electropolymerization [122]. The inclusion of a speciﬁc dopant was demonstrated as an efﬁcient method to incorporate electrochromes [71, 117] and photosensitizers [71, 117]. Sulfonated metalloporphyrins or metallophtalocyanines [123–126], polyoxometallates [127, 128], enzymes [129–131], and single-stranded DNA [132, 133] were inserted in an ICP matrix mainly to induce electrocatalytic and biorecognition properties. On the other hand, use of a bulky dopant (i.e., with a ˚ during electrodeposition allows an almost irreversible diameter more than 5–6 A) trapping in the polymeric matrix due to the entanglement of the polymeric chains around the dopant. The shape of the dopant is also a determining factor and for linear dopants, such as alkylsulfonates, the retention starts from chain length of C10 [134]. In this case, other holding factors such as hydrophobicity of the dopant chain must be considered. When the anionic dopant is immobilized, the dedoping results in the insertion of a cation to insure the electroneutrality [135] (Figure 1.2c). This cationic pseudodoping is also performed when the anionic dopant, usually sulfonate, is directly grafted on the conjugated polymer skeleton (self-doped polymers [136–142]) or is a pending group of an ionomeric polymer such as Naﬁon [136, 143, 144] or polystyrene sulfonate [145, 146]. Such interpenetrated polymeric networks [70] were investigated for applications in batteries [71, 117], water deionization [71, 117], and electrochemical delivery of drugs [118–120]. This last example was pioneered by Miller for the electrodelivery of dopamine [147]. Bidan and Kaneto introduced doping-speciﬁc cavities such as sulfonated cyclodextrines [148] or calixarenes [149] in polypyrrole for the selective trapping of drugs and ions. 1.3.2 Electropolymerization of Substituted Heterocycles

The concept of functionalization by covalent grafting seems to be very familiar by now. In the one-step approach (Figure 1.3a), the monomeric unit is presubstituted by a functional group. Provided some rules are followed, the electropolymerized ﬁlms exhibit new functionalities while keeping the attributes of ICPs. Instead of prefunctionalization of the starting monomer, more generic methods have been developed based on a two-step approach (Figure 1.3b). Classically, an active group (e.g., amino [150], carboxylic [151], or afﬁnity groups [152, 153]) is grafted by prefunctionalization; then, after electrodeposition via a coupling reaction (e.g., a peptide link), a bond is formed with a specie in solution (e.g., amino–protein). This postfunctionalization of the ﬁlm mainly affects its surface; this is a limitation in ionic complexation for applications of the modiﬁed electrode in electroanalysis and electrocatalysis. On the other hand, this approach is effective and efﬁcient with bulky groups (enzymes, DNA [154, 155]); the active layer acts as a functional interface for biorecognition in biosensors (Chapters 8–10).

7

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1 Historical Evolution and Recent Trends in Surface Functionalization

F

F −

− n (2 + d) e − 2nH+

n

+ n dA−

X

d+

, dA−

X

n

X = NH; S

(a)

Step 1

R1

R1 −

− n (2 + d) e − 2nH+

n

+ n dA−

X

d+

, dA−

X

n

X = NH; S F

Step 2 R1

R1 R2 F

d+ X (b)

R2

, dA−

n

Figure 1.3 Functionalization by covalent grafting via a pending function. (a) One-step functionalization of ICPs by covalent grafting of the function, via a spacing arm, at the 3-position to the precursor monomer, then submitted to electropolymerization. For pyrrole, the N-substitution has also been fully exploited. (b) Two-step functionalization: Step 1, a generic coupling group R1 is end-grafted as performed

d+ X

, dA−

n

in the one-step strategy (a) and Step 2, after electrodeposition the modiﬁed electrode is functionalized by a heterogenic coupling reaction via a reactive group R2 classically the R1 –R2 coupling reaction is a peptide condensation, an afﬁnity recognition (biotine–avidine, single-stranded DNA hybridization, antibody–antigen) or, more recently, click chemistry.

The two-step approach was recently renewed by using click chemistry. Li et al. opened the route to a general method for a modiﬁed electrode based on ‘‘clickable’’ polypyrrole [156]: two types of N-substituted pyrrole with azide and terminal alkyne were synthesized and functionalized by complementary redox or bioactive elements. One-step functionalization by grafting of conducting polymers (Figure 1.3a) has been widely reviewed [45, 70–73]. The history of this process is brieﬂy described here: in 1982, Diaz et al. reported on poly(N-alkylpyrrole) [157] and

1.3 Electropolymerization: Toward Conjugated Architectures

on p-phenyl-substituted poly(N-phenylpyrrole) [158]. Then, in 1985, Skotheim and coworkers reported the ﬁrst redox-active pending function illustrated by the N-(p-nitrophenyl) group [159]. An important improvement was made with the grafting of functional molecules using an aliphatic spacing arm. In fact, the use of an alkyl chain spacer arm leads to a more tunable product than that by a phenyl spacer arm. The length of this alkyl arm can be tailored in a manner so as to control the steric hindrance. In the N-substituted polypyrrole series, a C6 spacing arm appears to be a good compromise between a too short C3 that decreases the electropolymerizability by steric hindrance interaction and a C11 length that moves away the conjugated chains and reduces electronic accessibility [160, 161]. However, the steric hindrance induced by short spacer can be reduced by linking two pyrrole units [160]. In 1984, Bidan and coworkers reported the grafting of the viologen (4,4 -bipyridinium) system [162, 163] and poly(pyridinyl) complexes of ruthenium [164]. This class of polypyrrole–Ru complexes was investigated in detail for their electrocatalytic properties by Deronzier et al. [165]. Various substituents, such as the ferrocene group [166], nitroxide functions [167] and anthraquinone groups [168], the camphor chiral unit [169], porphyrin [170], phenothiazine [171], enzyme [172], calixarene [173] and ﬂuorene [174], biotine [152, 153, 175], and single-stranded DNA [176, 177] were soon grafted at the nitrogen – there is no limitation to the substituents that can be used. The basic chemistry to perform N-substitution is counterbalanced by a loss in conductivity by 5–7 orders of magnitude. This is not really a limitation in electronic transfer for electrodes modiﬁed by a thin ﬁlm of micrometer thickness; however, for a better transduction of recognition event, grafting in 3-position appeared more attractive in spite of the need to implement a more elaborated chemistry. The 3-substitution was applied on pyrrole by Audebert et al. [178] and on thiophene derivatives by Garnier and Lemaire [179] simultaneously in 1989. This, particularly in the thiophene class, opened a route to another ﬁeld of research. The structural effect of the functionalization in 3-position was rationalized, and a ‘‘functionalization space’’ was deﬁned in terms of three parameters: the length of the spacer, the intrachain distortion, and the interchain distance [180–182]. A wide range of functionalizations of pyrrole and thiophene at the 3-position was developed mainly for sensor applications. Among these functions, the following can also be cited: alkyl chains, ﬂuoroalkyl chains, oligo(oxyethylene) chains [179], chiral and redox groups, and anionic species (self-doped polymers) crown ether [183–185] reviewed by Roncali [186] and by Swager [187]; boronic ester [188]; and peptides [189] (Chapters 8–11).Unlike polypyrrole or polythiophene derivatives, the poor functionalization of Pani has been pointed out. This poor electropolymerizability of aniline derivatives could be got around by an inverse approach; functional molecules bearing a phenyl or naphthalene group were amino-substituted to promote the electrocoupling. In this manner, amino derivatives of porphyrines [190, 191], anthraquinone [192], and naphthol [193] were polymerized. In some cases, substitution by a hydroxyl group also promotes the electropolymerization of aromatic molecules and allows to reach conjugated structures such as those from juglone monomer [194].

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1 Historical Evolution and Recent Trends in Surface Functionalization

1.3.3 Electropolymerization as a Tool to Elaborate Functional Conjugated Architectures

An important step forward was achieved by moving from ‘‘simple’’ functionalization with active pending group to preorganized conjugated architectures. Electropolymerization of ICPs can be exploited to form or freeze certain molecular shapes. Sauvage, Bidan and coworkers have taken advantage of the relative rigidity of the polymeric matrix to incorporate preformed complexing cavities into polypyrrole by grafting at the N- [161] or 3-position [195]. In the ﬁrst approach, the spacer between the metallic complex and the pyrrole units was still a single alkyl chain. However, the presence of four pyrrolic monomers allowed to maintain the phenanthroline ligands interlocked in a pseudocatenane assembly even after demetallation. More sophisticated molecular units such as rotaxanes were electrodeposited, and it was possible to electrochemically induce gliding motion inside the polypyrrole matrix [196]. Recently, Ikeda et al. have prepared polythiophene polyrotaxane by electrochemical polymerization [197], including the Stoddart viologene ring; however, there is no evidence of gliding of the ring during cyclic voltammetry. It should be noted that in these cited works the metal complexes and polypyrrole matrix behave almost independently in terms of electronic interactions. The goal of the direct transduction of a chemical event in an electrical signal for sensing was achieved by grafting or doping a receptor into the ICP matrix. Provided that the recognition event results in the trapping of a charged or bulky analyte (the guest) such as alkali ions, single-stranded DNA, proteins, and so on, the electrostatic or steric hindrance interaction with the ICP chain conformation modiﬁes the electroactivity of the ICP-electrodeposited ﬁlm. This ICP-receptor-couple acts as an electrochemical transducer for sensing. This opened a route to a large number of sensor applications (Chapters 10 and 11). As illustrating examples, in the vast literature, the coupling with crown ether, DNA, or enzyme was investigated and reviewed the most; see, for instance, the works of Bryce [198], Swager [199], Bidan [176, 200], Cosnier [201], Wallace [202], Mirsky [203], Bobacka [204], Josowicz [205], and Singh [206]. A step toward chemtronic materials, that is, the direct interaction, via molecular orbital overlap, of a function with the conjugated chain was achieved by moving from the concept of a pending group to the one of an intercalated group. Now, the concept of end-terminated polymerizing groups (Figure 1.4), based on the use of thiophene [207] and EDOT [208, 209] units as the external heterocyclic rings, is currently used to promote the one-step electrodeposition of molecules unable to electropolymerize alone [210]. Among them, a new class of conjugated materials with novel electronic and electrochemical characteristics was obtained by the hybridization of ICPs with transition metal complexes [211–213]. The cooperative behavior between a large number of photonic and electronic properties of metallic centers and the conjugated matrix can be achieved by forcing the π-conjugation to go through a metallic center or by an electronic coupling via the metal–ligand orbitals. In

1.3 Electropolymerization: Toward Conjugated Architectures Electropolymerization

P

P

F

P

F

P

n

O

O

P

F

=

S

S S

S

S

= Vinyl, phenyl, diphenyl, thiophene and derivatives; furans; pyrrole(N-substituted by bulky group); ligands such as pyridine, bipyridine, phenanthroline and their metal complexes; salen types metal complexes; dithiolene metal complexes; fluorenes and carbazoles derivatives; tetrathiafulvalenes; fluorenone; tetrazine; silole; carbine; etc...

Figure 1.4 Functionalization by the strategy of the end-terminated polymerizing groups. The function is inserted between two electropolymerization-promoting heterocycles. In this manner, it is possible to electropolymerize central blocks with speciﬁc properties,

whereas, on their own, they exhibit poor electropolymerizability. This ‘‘in chain’’ approach is preferred to the pending-group one for applications exploiting the electronic delocalization of the tricyclic units, such as OPVs and molecular electronics.

the ﬁrst approach, the conjugated chain is interrupted by the metal [214–216] and is not considered here as representative of hybrid compounds. The ﬁrst hybridization was pioneered by Yamamoto from polypyridine [217] complexed by ruthenium. However, these chemically prepared materials were almost insoluble and the postpolymerization metallation is difﬁcult. A step forward – mutual metal-conjugated chain interaction – was achieved by Wrighton et al. in 1994 using electropolymerization of 5,5 -(2-thienyl)-2,2 -bithiazole, followed by metallation of the bithiazolyl units by Re(CO)3 (CH3 CN). Oxidation of the conjugated backbone to the conducting state causes a decrease in electron density at the Re metallic center evidenced par IR spectroscopy [218]. Zotti et al. [219] tried to compare the efﬁciency of electron transfer between metal sites by electron hopping or via conjugated linkage using a polythiophene backbone with pending ferrocene unit bound either by an alkyl or by a vinyl spacer. However, their results are ambiguous. To avoid the difﬁculty of postmetallation of the polymer, the direct electropolymerization of a Ru complex of 5,5 -bis(2 -bithiophene)-2,2 -bipyridine was reported by Swager et al.; however, interconnection between the Ru centers and the conjugated poly(bipyridine–bithiophene) chain was not clearly evidenced [220]. A key step had been achieved with the anodic polymerization of metal-directed preassembly of rotaxane unit end terminated by two EDOT or thiophene units reported by Swager et al. in 1997 [221] and then by Sauvage et al. in 1998 [222]. In situ conductivity measurements on interdigitated electrodes coupled with cyclic voltammetry had showed that the redox conductivity of the copper-metallated polymer is strongly ampliﬁed compared to the demetallated one [223]. These

11

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1 Historical Evolution and Recent Trends in Surface Functionalization

metallo-pseudopolyrotaxanes can exhibit remarkably high conductivities when the redox potential matches the oxidation potential of the π-system, such as in copper complexes associated with polythiophene and derivatives. This is well illustrated by the variation in conductivity reported by Swager in the sophisticated structure of the three-strand conducting ladder polymers using a two-step electropolymerization process of metallorotaxanes [224]. During the same period of time, similar approaches to enhance the communication between metal centers and conjugated backbones were developed on other structures of transition-metal-containing conjugated polymers prepared by electropolymerization. Reynolds et al. reported the tuning of the electrochromic properties of polythiophene link to salen-type metal complexes [225]. Audebert et al. had electropolymerized anil- and salen-type complexes, which exhibit enhanced charge transport when a conjugated backbone is electroassembled [226]. Wolf et al. [227] studied polythiophene cross-linked via different Pd complexes and the groups of Pickup et al. [228] and Shabara et al. [229] presented electropolymerized thiophene derivatives containing dithiolene Ni, Pd, or Au complexes.

1.4 Conclusion

The elaboration of increasingly sophisticated conjugated architectures such as insulated molecular wires reviewed in 2007 by Anderson et al. [230] involves an important part of chemical coupling [87, 231] compared to anodic coupling. Nevertheless, it should be noted that the building up of conjugated polymer networks cross-linked by organometallic bridges reviewed in 2005 by Weder [232] accounted for a large panel of electrogenerated structures. Recently, chiral polysalen–thiophene chromium complex was electrosynthesized and used to promote asymmetric reactions [233]; electropolymerization of carbine ended by bithiophene units was reported by Cowley et al. [234]. The determining role of conducting polymers as active materials in organic photovoltaics (OPVs) was boosted by the chemical coupling synthesis of new polymers [235] or oligomers [236]. However, here too, the use of electrochemistry for the electrodeposition of active layer in OPV cells is still widely exploited. The beneﬁt of this strategy is the well-controlled thickness at a nanometer scale of the electrodeposited ﬁlm. Double-cable polymers of poly(thiophenes) containing pendant C60 –fullerene groups was electrogenerated [237–240]. Polycarbazole derivatives appear among the more promising polymers for OPVs. Their direct electropolymerization is difﬁcult; however, electrocopolymerization [241] or derivatization-enhanced solubilization [242] allows their electrodeposition. Recently, the application of the end-terminated polymerizing heterocycle strategy has involved the synthesis of a central C60 -linked carbazole ended by EDOT units [243]. Although the mechanism of anodic electropolymerization of conducting polymers [244, 245] (Chapter 2) such as polypyrrole [246–248] has been well investigated, there are still gray areas with regard to the relationship between

References

the morphologies (dense, open structure, ﬁbrillar) and the conditions of electropolymerization, for example, solvent (aqueous, organic) and potential (value, potentiostatic, cyclic potential sweep). The recent use of ionic liquids [249, 250] for the electrosynthesis of ICPs has boosted the renewal of electrochemical applications, which suffered from a lack of stability toward electrochemical cycling such as batteries, supercapacitors [251], artiﬁcial muscles [252], or sensors [253, 254]. In addition, the one-step elaboration (without template) of ICP-based nanostructures [255] such as nanowires [256–258] is also exploited in similar applications [259]. Therefore, the electropolymerization of conjugated structures will deﬁnitely continue to be an important tool of functionalization and elaboration of nanoobjects as well as an active ﬁeld of researches for many years.

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Composites of intrinsically conducting polymers as sensing nanomaterials. Chem. Rev., 108, 746–769. Zhou, Q.-X., Miller, L.L., and Valentine, J.R. (1989) Electrochemically controlled binding and release of protonated dimethyldopamine and other cations from poly(N-methylpyrrole)/polyanion composite redox polymers. J. Electroanal. Chem., 261, 147–164. Ateh, D.D., Navsaria, H.A., and Vadgama, P. (2006) Polypyrrole-based conducting polymers and interactions with biological tissues. J. R. Soc. Interface, 3, 741–752. Fauvarque, J.-P., Petit, M., Pﬂuger, F., Jutand, A., and Chevrot, C. (1983) Preparation of poly(1,4-phenylene) by nickel(0) complex catalyzed electropolymerization. Makromol. Chem. Rapid Commun., 4, 455–457. Torrance, J. (1979) The difference between metallic and insulating salts of tetracyanoquinodimethane (TCNQ): how to design an organic metal. Acc. Chem. Res., 12, 79–86. J´erome, D. (2004) Organic conductors: from charge density wave TTF-TCNQ to superconducting (TMTSF)2 PF6 . Chem. Rev., 104, 5565–5591. Walatka, V.V., Labes, M.M., and Perlstein, J.H. (1973) Polysulfurnitride, a one-dimensional chain with a

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Electrochemical polymerization of pyrrole in BMIMPF6 ionic liquid and its electrochemical response to dopamine in the presence of ascorbic acid. Synth. Met., 159, 1542–1545. Yin, Z.-H., Long, Y.-Z., Gu, C.-Z., Wang, M.-X., and Duvail, J.-L. (2009) current–voltage characteristics in individual polypyrrole nanotube, poly(3,4-ethylenedioxythiophene) nanowire, polyaniline nanotube, and CdS nanorope. Nanoscale Res. Lett., 4, 63–69. Kemp, N., Cochrane, J., and Newbury, R. (2009) Characteristics of the nucleation and growth of template-free polyaniline nanowires and ﬁbrils. Synth. Met., 159, 435–444. Catherine Debiemme-Chouvy, C. (2009) Template-free one-step electrochemical formation of polypyrrole nanowire array. Electrochem. Commun., 11, 298–301. Mo, X., Wang, J., Wang, Z., and Wang, S. (2004) The application of polypyrrole ﬁbrils in hydrogen evolution reaction. Synth. Met., 142, 217–221. Ghenaatian, H., Mousavi, M., Kazemi, S., and Shamsipur, M. (2009) Electrochemical investigations of self-doped polyaniline nanoﬁbers as a new electroactive material for high performance redox supercapacitor. Synth. Met., 159, 1717–1722.

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2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects Mikhail A. Vorotyntsev, Veronika A. Zinovyeva, and Dmitry V. Konev

2.1 Electropolymerization: General Aspects

The discovery that electrochemical methods represent a perfect means both for the synthesis of conducting polymers and for manipulations with them was made by Diaz et al. [1, 2] in 1979, very shortly after the ﬁrst publications of Heeger, MacDiarmid, and Shirakawa [3]. These studies demonstrated the electrochemical method to produce electrodes covered by polypyrrole (PPy) (or its N-substituted derivatives) [1, 2, 4–7] or polyaniline (PAni) [6] ﬁlms. These polymers (with their various derivatives) represent two of the three principal families (together with polythiophene, PTh) which are the subject of research of numerous electrochemical teams. More than a hundred other families of conjugated polymers (e.g., polyparaphenylene, polyalkylcarbazole, etc.) have also been obtained electrochemically; see, for example, an extensive list in review [8]. The overviews of numerous publications for the principal families of conjugated polymers and their applications in various ﬁelds are available in books [9–12]. A classiﬁcation of various types of conjugated polymers and copolymers has been proposed in [13]. Electrochemical polymerization of such polymers is seemingly a very simple process: it is enough to impose a sufﬁciently positive potential, Epolym (for potentiostatic regime), or to cycle with a sufﬁciently high anodic limit (generally, it should exceed Epolym by 100–200 mV), or to pass an anodic current through the solution of a monomer, and the ﬁlm of the corresponding polymer progressively grows at the electrode surface. For most monomers representing conjugated molecules, their electrooxidation results in linear polymer chains, with chemical bonds between neighboring monomer units formed in the way that the chain also corresponds to a conjugated structure, for example, in positions 2 and 5 for Py and Th rings, or in para position for benzene and aniline. The value of the deposition potential, Epolym (or the anodic limit of potential variation for the cyclic voltammetry (CV) regime) varies largely for different polymers, mostly parallel to the oxidation potential of the corresponding monomer (being relatively low, e.g., for Py and EDOT, much higher for thiophene and Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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benzene). It may be changed by chemical modiﬁcation of the basic monomer, in particular, by substitution in positions not used for polymerization, for example, 3 and 4 for Th and Py (also at nitrogen for Py). Electron donating groups (e.g., alkyl and especially oxo-alkyl) facilitate the monomer oxidation, shifting Epolym to lower values, while electron accepting ones give an opposite effect. The polymerization potential may also be lowered by the use of an oligomer as the building block; for example, bithiophene as the starting component allows one to avoid a high value of Epolym necessary for oxidation of Th. For each monomer the potential, Epolym , and the deposition rate depend on the concentration of the monomer: the higher the concentration, the faster the ﬁlm growth (at ﬁxed Epolym ), or Epolym may be lowered (for the same growth rate). The process is inﬂuenced strongly by the choice of the solvent, for example, oxidation of Py in aqueous acetonitrile (AN) solutions results in PPy ﬁlms with markedly different properties. The process may also be affected by the type of the ‘‘background’’ electrolyte (the term background means the absence of its redox transformation) since its components are trapped by the ﬁlm, thus affecting its properties. The polymerization rate and properties of the ﬁlm may also depend on temperature, pH (e.g., for the PAni family of polymers), convective motion of solution (with or without stirring), and other factors. Various illustrative examples of polymerization processes and the redox activity of deposited ﬁlms are given in Section 2.3. The polymer may be deposited usually on any electrode surface stable to its oxidation up to Epolym : Pt, glassy carbon, highly oriented pyrolytic graphite (HOPG), indium tin oxide (ITO), or SnO2 coated glass, Au (for monomers with relatively low oxidation potentials), stainless steel, and so on. It is technically much easier to obtain a uniform and well-adhered ﬁlm at the electrode with a small surface area (below 0.1 cm2 or so). For the deposition of the ﬁlm with similarly advantageous properties at the electrode with a size of about 1 cm or greater, it is required to deal carefully with signiﬁcant ohmic potential losses (preferably, by making the potential drop inside the solution identical for all points at the electrode surface, by the proper geometrical conﬁguration of electrodes), with variation of convective-diffusion transport conditions along the surface (which affects the distribution of the limiting diffusion current), with speciﬁc transport conditions near the solution/air surface (such as meniscus, capillary waves, etc.). In general, a slower deposition rate results in a more uniform ﬁlm formation. Optical microscopy represents a perfect tool for primary control of the quality of the coating. Electrochemical route to synthesize conjugated polymers possesses both merits and shortcomings, compared to alternative polymerization routes: by reaction of the monomer with an oxidative agent (frequently, in the presence of extra components, e.g., emulsion stabilizer or solute polyelectrolyte molecules, for their incorporation inside the polymer matrix) or via organometallic catalysis [13–15]. The electrochemical method, besides its simplicity to realize, allows one to avoid extra chemical agents (from the oxidant, extra agents, or catalyst) inside the polymer. It gives the polymer in the form of a ﬁlm at the electrode surface, or a free-standing ﬁlm (e.g., for membrane applications) while the chemical routes result in a powder.

2.1 Electropolymerization: General Aspects

The powder form of the polymer is less advantageous for most applications, for example, in catalysis because of the necessity for the postreaction separation of the powder from products. The drawback of electrochemically deposited polymers is their insolubility in solvents (a rare example of such a polymer soluble in organic solvents [16] is related to bulky substituents) and their decomposition before melting (expectedly, due to the chemical bond formation between polymer chains), which does not permit their redeposition at another (in particular, insulating) substrate. The possibility to vary the type of the oxidant and its concentration (together with extra added components of solution) in the chemical method resulted in the synthesis of polymers soluble in organic solvents (e.g., nonsubstituted PPy [17]) or/and possessing a regioregular structure of their pendant groups (e.g., polyalkylthiophenes [14, 18]). These polymer solutions provide the information on the distribution of chain lengths [14, 16] (see also [13] for discussion). They are used for casting polymer layers at any substrate. However, the absence of the chemical bonds between polymer chains inside such materials results in much poorer mechanical properties of such layers, compared to ﬁlms of the same polymer deposited electrochemically, an important drawback for many applications. The amount of the deposited polymer can be regulated by the passed anodic charge, Q polym (or by the difference between the anodic and cathodic charges for the CV regime). If the polymerization potential/current is reasonably low, one can observe a proportionality between the number of deposited monomer units, Nmon (characterized by ‘‘the redox charge’’ of the ﬁlm, Q redox ) and the polymerization charge, Q polym , except maybe for the initial period of the process. For potentiostatic conditions, Nmon = Q polym /(2 + αpolym )F

(2.1)

where the term ‘‘2’’ is related to two electrons withdrawn from each monomer unit, to form σ -bonds with two neighboring units inside the chain, while the extra charge (dependent on Epolym ), αpolym e, is spent in the course of potentiostatic deposition to withdraw electrons from the neutral polymer chain, to get its oxidized/doped state corresponding to Epolym . The charge, Q polym , is smaller than the overall charge, Q tot , spent during the deposition: Q polym = fpolym Q tot

(2.2)

where the ‘‘polymerization efﬁciency factor,’’ fpolym < 1, describes all electrical losses related to side reactions, generation of oligomers in the bulk solution, deposition of oligomers and polymer outside the electrode surface, formation of intermolecular bonds between polymer chains, and so on. The amount of the deposited material (Nmon ) in the course of CV procedure may be determined for most conjugated systems from the shape of the anodic and cathodic branches of these curves in the potential interval below the monomer oxidation range. This amount can also be estimated with the use of various in situ techniques in the course of the polymerization process (both for the CV and

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potentio/galvanostatic regimes), such as electrochemical quartz crystal microbalance (EQCM) (tracing the mass of the deposit), ellipsometry, and UV–visible spectroscopy (ﬁlm thickness and its optical constants), and so on. Such studies have shown that for carefully chosen conditions (e.g., for sufﬁciently low monomer concentration and slow polymerization rate) the deposition in the course of the CV procedure takes place in a stabilized manner, that is, the mass and the thickness of the ﬁlm increase proportionally to the number of potential cycles [19, 20]. It implies a constancy of the factor, fpolym , in the course of the whole process. Frequently, this conclusion is also valid for potentiostatic conditions of the process if the deposition current becomes stabilized after the initial ‘‘relaxation period’’ (the ﬁrst seconds or tens of seconds after the potential step). On the other hand, even for the same monomer and the solvent, this factor depends generally on the deposition parameters: cell conﬁguration (electrode shapes and sizes, inter-electrode distances), Epolym , monomer concentration, hydrodynamic conditions. The very ﬁrst cycle for the bare electrode surface in contact with the monomer solution generally gives a markedly different CV response (compared to those for the subsequent cycles), with a very rapid (‘‘super-Nernstian’’) increase of the anodic current, with the crossing of the cathodic and anodic branches, and so on. Anomalous features are also observed in the i–t curves for the potentiostatic conditions where the current passes through a minimum (the depth and position of which depend on Epolym ), with a subsequent maximum for higher values of Epolym , before it becomes stabilized or slowly varying. This behavior is usually attributed (on the basis of the analogy with metal deposition) to the nucleation limitations and the subsequent growth of nuclei [21–23]. Recently, it has been demonstrated [24] that similar behavior of the current also takes place for the oxidation of conjugated molecules which cannot give a solid deposit. Their interpretation was proposed on the basis of a catalytic mechanism with participation of multicharged oligomers as redox intermediates [24]; see also review paper [25]. The polymerization process is initiated by the monomer oxidation at the electrode surface (maybe, passing via its preceding adsorption): •

HMH − e− → HMH+

(2.3)

where hydrogen atoms (H) in the positions of the further bond formation (e.g., those in 2 and 5 positions for Py or Th) are explicitly indicated. Sometimes, the further reaction of the cation radical with a neutral monomer species is postulated. However, it has been proven experimentally (at least, for basic monomers and their alkyl derivatives) that the subsequent chemical steps include the radical dimerization, with formation of the σ bond between two sp3 carbon atoms. The further loss of two protons restores the aromaticity of the dimer (giving, e.g., bypyrrol from pyrrol): •

•

HMH+ + HMH+ → HMHHMH++ → HMMH + 2 H+

(2.4)

Frequently, it is the proton release which is the rate-determining step [5, 25–27].

2.1 Electropolymerization: General Aspects

Since the oxidation potential diminishes with the increase of the chain length, the dimer is oxidized as soon as it approaches the electrode surface. The further transformation of the cation radical of the dimer (as well as of longer chain oligomers) is actually under discussion regarding whether it reacts mostly with the cation radical of the monomer or of a similar oligomer (e.g., dimer); see a detailed discussion of this point in [25, 27, 28]. For oligomers of a sufﬁcient length, the imposed electrode potential is high enough for the further oxidation steps, giving dications, trications, etc., which are able to participate in the disproportionation reaction with the starting monomer, thus accelerating its oxidation, compared to this reaction at the electrode surface [24, 25]. The solubility of oligomers and their cationic derivatives diminishes drastically with the chain length so that the oligomers are deposited at the electrode after a certain threshold is reached. It is estimated as about 6–10 for basic conjugated monomers but it should vary essentially depending on substituent groups, solvent, electrode material, polymerization potential, monomer concentration, temperature, and so on. After the formation of the deposit, the reactions between charged oligomers may lead to the further extension of the chain and/or to the formation of chemical bonds between neighboring chains. These processes result in practical insolubility of the ﬁlm, even in solvents with a much higher solvation ability for oligomers. Morphology of the deposited ﬁlm depends largely on its thickness and the reaction conditions. Rapid growth results frequently in a ‘‘cauliﬂower’’ shape of the external surface. Slower deposition (especially, in organic solvents) may provide layers with a very ﬂat surface for ﬁlms with the thickness within a submicrometer range. Figure 2.1 presents an example of the atomic force microscopy (AFM) image of a ﬁlm [29], the surface of which is composed of round elements with diameters of 25–30 nm and is extremely ﬂat: the maximum variation of the height is only about 10 nm within the 2.5 × 2.5 µm2 area while the mean roughness is below 1 nm. An important feature of the internal structure of the ﬁlm is the existence of a ‘‘compact layer’’ (thickness of this layer is estimated as 10–30 nm) near the

Figure 2.1 AFM image of the ﬁlm with PEDOT matrix with attached titanocene dichloride complexes [A, #27]. Horizontal scale: 2.5 µm. Relative height: 10 nm. Mean roughness over the area (512 × 512 points): 0.7 nm. Oscillating contact mode.

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electrode surface which prevents the reaction of solute species at the electrode surface [30]. Outside of this layer, the ﬁlm represents a porous system, with pore diameters being determined by the size of the basic elements of the polymer phase (e.g., it should be within the nanometer range for the sample in Figure 2.1).

2.2 Redox Activity of Polymer Films

For numerous conjugated molecules (e.g., for Py or Th or their derivatives), the ﬁrst potential scan in the positive direction leads (for a bare electrode in contact with monomer solution) to an oxidation wave of the monomer, Equation 2.3, accompanied by the further electrochemical and chemical steps. Repetition of the cycle many times does not signiﬁcantly change the monomer wave while the intensity of the anodic and cathodic responses is growing progressively within a broad potential range; see illustrations in Section 2.3 below. This additional current is related to the variation of the ‘‘redox/electronic charge’’ of the ﬁlm as a function of the electrode potential, that is, to the oxidation of the polymer matrix and its reduction in the cathodic scan. For most conjugated systems, the interval of this redox response of the polymer starts at a much more negative potential than that of the monomer oxidation but this interval is extended over a very broad potential range including the monomer oxidation one (so that the latter response is superimposed over the response of the polymer). In advantageous cases, the increase of each current compared to that for the previous cycle remains about the same over numerous cycles. Then, within the potential interval below the monomer oxidation range, only the intensities of these currents grow progressively (being roughly proportional to the number of cycles) while the shape of the CV response remains unchanged. It implies the addition of a polymer layer with identical properties within each cycle, without a change of redox properties of previously deposited layers. In general, this simple behavior breaks down after a sufﬁciently great number of cycles, that is, for sufﬁciently thick ﬁlms. A frequent scenario is a progressive shift of the oxidation wave of the polymer to higher potentials which may be due to various reasons: because of the change of polymer properties in the course of cycling, or (most frequently) because of the ohmic potential drop across the ﬁlm. Since the charge for the polymer oxidation has to be supplied from the electrode to all redox elements of the ﬁlm, the ohmic potential drop increases as the square of the number of cycles because both the redox current, and the ﬁlm resistance (related to its thickness) are proportional to the number of cycles. This ohmic drop remains negligible even for thick ﬁlms (well above micrometer range) for polymers with a high electronic conductivity, as nonsubstituted PPy, PAni, or PTh. However, the substitution results frequently in a strong diminution of the conductivity, for example, by 4 or 5 orders of magnitude for poly(N-alkylpyrroles) compared to PPy [7]. For such systems, the polymer oxidation in the course of the

2.2 Redox Activity of Polymer Films

CV deposition starts at increasingly higher potentials, ﬁnally reaching the range of the monomer oxidation, which makes a further ﬁlm growth impossible. After the termination of the deposition, one can measure the CV response of the ﬁlm-coated electrode in contact with the same monomer solution but with a lower anodic limit of the scan, to avoid the additional monomer oxidation. Another useful procedure is to transfer the ﬁlm-coated electrode (after rinsing it, to remove absorbed monomer moieties) into a monomer-free solution, preferably with the same background electrolyte. For polymer ﬁlm with a stable redox response, these two CV curves should be close to the CV response in the course of the last deposition cycle while a marked difference between these plots means the instability of the redox response; see examples in Section 2.3. This redox response of the polymer matrix is frequently called ‘‘p-doping’’ since the mobile carriers are charged positively. This signiﬁcant current (proportional to the amount of the deposited polymer material) is observed within a certain potential range. An important feature of this redox response of the polymer matrix is its marked difference for the anodic and cathodic potential sweeps (‘‘redox hysteresis’’). The lower limit of this electroactivity interval for the anodic sweep is called oxidation onset. Below the p-doping electroactivity interval, the intensity of the CV response becomes very low (comparable to the level of the EDL response of the bare electrode). The ﬁlm within this interval is called electroinactive. For polymers with a relatively high oxidation onset (e.g., PTh or polyparaphenylene and some of their derivatives), the potential sweep from this ‘‘inactivity range’’ in the negative direction allows one to reach another electroactivity range (‘‘n-doping’’ of the polymer) where the polymer matrix becomes negatively charged. For other polymers, irreversible changes take place even before reaching the n-doping range. If the anodic limit of the potential sweep into the p-doping interval is not very high (several hundred millivolts lower than that for the ﬁlm deposition), the redox response of the ﬁlm in the course of multicycle experiment is generally very stable. The only exception is the response during ‘‘the ﬁrst anodic sweep’’ if the electrode with the ﬁlm was kept at the potential inside the inactivity interval, the effect being strengthened by a longer holding period or/and a more negative holding potential. Then, the oxidation of the polymer starts at a markedly higher potential (compared to that in the multicycle CV experiment) after which the current shows a high peak, strongly shifted in the positive direction with respect to the (much lower) maximum in the stabilized response. The origin of this phenomenon which is generally called ﬁrst cycle or memory effects is still the subject of intensive discussions [25, 31–36]. Even for polymers with the well-expressed n-doping range, the redox response is much less stable in this potential interval and its observation imposes stronger requirements on the purity of the solvent and electrolyte. The manifestation of the redox activity in the p- and n-doping ranges (the ability of the polymer matrix to be charged positively or negatively) is immediately related to the conductivity [5]. In particular, the polymer in its neutral state within the inactivity range represents a semiconductor or even insulator, with a low electronic conductivity. On the contrary, in situ measurements of the electronic conductance

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2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects

of the ﬁlm with the use of microband electrodes demonstrate its drastic increase at the onset of the polymer oxidation or reduction. This behavior in the p-doping range is the key condition for the deposition of the ﬁlm by the monomer electrooxidation since the electronic charge has to be transported across the ﬁlm between the electrode and the ﬁlm/solution interface. This difference in the ﬁlm resistance in the electroactivity and inactivity potential intervals plays a crucial role in the kinetics of redox reactions of solute components of the solution in contact with the ﬁlm. Such reactions at ﬁlm-coated electrode take place mostly near their standard potentials if this potential corresponds to the electroactivity interval, owing to the electronic charge transport across the ﬁlm. On the contrary, if the standard potential belongs to the inactivity interval the high resistance of the ﬁlm prevents the charge transfer near this potential, and the wave related to this reaction is shifted (in the positive direction for the anodic process and in the negative direction for the cathodic one) to the potential range of the ﬁlm electroactivity [30, 37, 38]. Except for extremely thin polymer layers, the ﬁlm represents an additional three-dimensional phase between the electrode and the solution so that it has to be electroneutral (except for charges in its interfacial regions). Therefore, in the electronically charged state of the polymer matrix this charge has to be compensated by a countercharge inside the ﬁlm. In most cases, the latter is provided by the counterions (anions or cations) which have a charge opposite to that of the matrix. For a p-doped polymer without pendant-ionizable groups, it means incorporation of anions in the course of the polymer oxidation and their removal from the ﬁlm in the course of its reduction. Charged substituents (e.g., COO− or SO3 − ) reverse the situation, making the polymer ﬁlm a cation exchanger in the course of the potential variation. This ‘‘countercharge rule’’ breaks down in certain cases, for example, if the counterion is either trapped inside the ﬁlm or cannot enter into the ﬁlm, due to its large size or its complexation with the matrix. It results in peculiar features in the redox response in such systems [39–41]. The compensation of the electronic charge by the ionic one should be achieved not only for the ﬁlm as a whole but also for all its elements (the size of which signiﬁcantly exceeds the Debye screening length of this medium). It means the necessity for the corresponding ion not only to cross the solution/ﬁlm interface but also to propagate through this medium, partially along pores of the ﬁlm and partially by the diffusion inside its morphological elements. In this context, the ﬁlm has to represent a mixed (electron–ion) conductor where the transport properties are essentially different compared to those in, for example, electrolyte solutions [42–44]. The processes of the ion exchange between the ﬁlm and the solution and the ion transport across the ﬁlm may be monitored with the use of the EQCM technique [45, 46] or its further development, ‘‘electrogravimetric impedance’’ (or ‘‘ac-electrogravimetry’’) [47, 48]. The stability of the CV response in the multicycle experiment depends on the limits of the potential sweep. For sufﬁciently high positive potentials (their values vary largely depending on the type of the polymer), one observes a strong current

2.2 Redox Activity of Polymer Films

in the course of the ﬁrst anodic sweep related to a change in the chemical structure of the polymer, accompanied by the strong diminution of the redox response of the ﬁlm in the ‘‘usual electroactivity range.’’ It is interesting to note that this irreversible transition in the course of the CV experiment in the monomer-free solution takes place already in the potential range which is optimum for the ﬁlm deposition (in particular, performed under the same CV regime within the same potential range), that is, this degradation does not occur (or it is much slower) at the same potentials for the monomer solution. This modiﬁcation of the ﬁlm is called overoxidation. Being mostly harmful, it is performed intentionally in certain cases to transform the ﬁlm into a passive and nonconducting support, for example, for incorporated centers or nanoparticles. The potential interval in which the redox response of the ﬁlm is stable may be strongly inﬂuenced by the solution composition. In particular, the effect of the same species may depend crucially on the solvent. For example, the chloride anion which is widely used in numerous conducting polymer-systems as a chemically inert background anion (‘‘dopant’’) behaves in quite a different manner in dry organic solvents (like AN or THF) resulting in a rapid loss of the electroactivity of the PPy derivatives in these media, or preventing the ﬁlm deposition [49–51]. The stability of the response of conjugated polymers also diminishes due to the presence of dissolved oxygen, in the monomer-containing or/and monomer-free solution. If the variation of the potential in the CV experiment is sufﬁciently slow, the state of the ﬁlm is close at each moment to its ‘‘quasi-equilibrium’’ state corresponding to the instantaneous value of the electrode potential, E. In particular, the electronic charge of the polymer matrix, Q redox , varies in time only due to the change of the potential, E(t). Then, the CV response in the course of the anodic or cathodic sweep should be directly related to the ‘‘redox capacitance of the ﬁlm,’’ C redox (a,c) , that is, to the derivative of the redox charge: ia,c ∼ = ±vCredox (a,c) , C redox (a,c) = dQ redox (a,c) /dE

(2.5)

The condition that the system in the course of the CV study may be considered as ‘‘quasi-equilibrium’’ is usually veriﬁed by plotting the current in a characteristic point, for example, for the anodic or/and cathodic maximum (called peak even if the shape of the maximum and its width are far away from those for faradaic reactions of solute species), iap or ipc , correspondingly, versus scan rate, v. This dependence is to be ‘‘proportional,’’ which means linear with zero intercept. A shift of the ‘‘peak potential’’ dependent on the sweep rate is usually ignored while this effect shows an evident deviation from Equation 2.5. A more informative analysis is based on the plots of ia,c /v versus E for the set of scan rates (see an illustration in Section 2.3). If these plots are close to each other for sufﬁciently slow scan rate, one may determine the redox charge, Q redox (a,c) (E), by integrating the CV current from the inactivity range (Equation 2.5). A common feature of these curves for conjugated polymers is the hysteresis in the redox response, that is, a difference in the anodic and cathodic branches of both C redox (a,c) and Q redox (a,c) , even for slow scan rates. On the other hand, for ﬁlms

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with a stable redox response the overall anodic and cathodic charges for the whole interval of the potential sweep should be (and frequently it is) well-balanced, which means that the whole electronic charge at the polymer matrix incorporated up to the anodic limit of the sweep is withdrawn from the ﬁlm in the course of the cathodic scan. The ‘‘maximum redox charge,’’ which is equal to the redox charge at the highest potential where the ﬁlm is still sufﬁciently stable with respect to the overoxidation process, Q redox max , represents a very useful parameter of the ﬁlm since it is proportional to the overall number (in moles) of redox-active monomer units, Nmon(redox) : Q redox max = α redox FNmon(redox)

(2.6)

where F is the Faraday’s constant and the ‘‘charging/oxidation degree,’’ α redox , is equal to the electronic charge per monomer unit inside the polymer at this potential. Combination of Equations 2.1, 2.2, and 2.6 give a relation between two experimentally measurable quantities, maximum redox charge and total deposition charge: Q redox max = ηQ tot ; η = α redox fmon(redox) (2 + αpolym )−1 fpolym

(2.7)

where fmon(redox) = Nmon(redox) /Nmon is the fraction of redox-active monomer units inside the ﬁlm. Frequently, deposition of a series of ﬁlms with various charges, Q tot (keeping other parameters of the process unchanged), results in the proportional variation of the redox charge, Q redox max , which implies a constancy of the other parameters in Equation 2.7. In the course of interpretation of this result, the formation of inactive monomer units is mostly disregarded while the oxidation degree at the polymerization potential is identiﬁed to be α redox so that Equation 2.7 is simpliﬁed: Q redox max = ηQ tot ; η ∼ = α redox (2 + α redox )−1 fpolym

(2.8)

Under carefully chosen polymerization conditions (in particular, for a slow deposition rate), the factor fmon(redox) is really close to 1 while another factor, fpolym , is probably below 0.9 even in the best cases [52]. The value of the (maximum) oxidation degree, α redox , depends on numerous factors, ﬁrst of all on the monomer and the solvent. It is often estimated as about 0.2–0.3 for PPy and PTh and their derivatives, while it is about 0.5 for PAni or 0.6 for PEDOT [5, 25, 27, 53, 54]. In rare cases, a much higher charging degree was achieved, close to one electronic charge per monomer unit [55, 56], which represents probably a ‘‘natural limit’’ for the oxidation level to be reached by electrochemical means. The ratio of these charges, η = Q redox max /Q tot , is sometimes called polymerization efﬁciency. According to Equation 2.7, its maximum value is ηmax = α redox (2 + αpolym )−1 , which is much smaller than 1, for example, it is equal to 0.11 for α redox = αpolym = 0.25. In this context it seems to be more logical [52] to use the term polymerization efﬁciency, y, for the ratio of the experimentally found value of

2.2 Redox Activity of Polymer Films

η to its ‘‘theoretical maximum’’ for this polymer family the deviation of which from 1 characterizes the losses of the electroactive monomer units in the course of deposition: y ≡ η/ηmax = fmon(redox) fpolym

(2.9)

The use of the above values for the charging degree, α redox or/and αpolym , for the particular polymer family gives the value of ηmax . Its combination with the value of η found from the ratio of the charges in Equation 2.7 provides an estimate for the ‘‘polymerization efﬁciency,’’ y, that is, the overall loss, fmon(redox) fpolym (Equation 2.9). Then, an assumption on one of these parameters, fmon(redox) or fpolym , allows one to crudely estimate the number of monomer units inside the ﬁlm, Nmon , from Equation 2.1 or Equation 2.6. Another useful parameter is the molar volume of the monomer units inside the ﬁlm, vmon = Vpolym /Nmon , Vpolym being the total volume of the polymer phase which is equal to the product of the overall volume of the ﬁlm, Vﬁlm , and the ﬁlm porosity factor, fpor ( fpor < 1). It gives the relation Vﬁlm = vmon Nmon /fpor

(2.10)

The molar volume, vmon , may be crudely estimated from the data for the pure monomer, for example, X-ray diffraction (XRD) study gives its value for the monomer crystal while for the liquid monomers it is equal to the ratio of the molar mass of the monomer and the density: vmon = Mmon /d. This treatment disregards the diminution of the monomer–monomer distance inside the polymer chain because of the bond formation, this effect being counterbalanced by a better packing in the monomer crystal or liquid. The porosity factor depends not only on the synthesis conditions but also on the physical state of the ﬁlm, either in contact with solution or after its rinsing and drying. For slowly deposited polymer ﬁlms with a compact structure, this factor, fpor , is probably not especially small (above 0.5) even for the swollen state of the ﬁlm, and it is even higher (closer to 1) for the dry state. Knowledge of all three quantities on the right-hand side of Equation 2.10 provides an estimate for the ﬁlm thickness, Lﬁlm = Vﬁlm /A, A being the (visible) electrode surface area. Despite its very approximate nature, it is often not less reliable than the values of this important parameter given by ‘‘direct methods’’ of its measurement. In particular, various proﬁlometric measurements of the thickness have to overcome the problem of a very broad transition region between the uncovered electrode surface and the area with the ‘‘stabilized ﬁlm thickness.’’ Its large width originates from various effects, such as very nonuniform deposition conditions near the three-phase boundary (electrode/solution/air), rapid propagation of the polymer deposit across the surface boundary between the electrode and the insulator (in the direction of the insulator area), and so on.

37

38

2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects

2.3 Effect of Polymerization Parameters on Properties of Deposited Polymer Films

The polymerization efﬁciency, y, in Equation 2.9, and properties of the deposited ﬁlm depend on numerous parameters of the process: composition of the monomer solution (ﬁrst of all, solvent, monomer concentration and its purity, but the choice of the background electrolyte and even its concentration may affect ﬁlm properties), electrochemical regime (potentiodynamic, potentio- or galvanostatic, periodical series of potential steps [57]), parameters of the regime (cathodic and especially anodic potential limits for CV regime, intensity of the current or the deposition potential for static regimes etc.), presence of extra components of solution (electron donors or acceptors, coordinating agents, oxygen), temperature, hydrodynamic conditions (e.g., stirring or its absence), arrangement of the cell (material, size, and shape of the working electrode, pretreatment of its surface, conﬁguration of the cell, etc.), and others. Examples given below for polymerization of Py and its derivatives illustrate effects of some of these factors. One should keep in mind that both the deposition process and redox properties of the ﬁlm may be inﬂuenced strongly by the purity of the starting monomer, solvent, and other solution components, especially for such liquid and easily oxidizable (by oxygen, under light illumination etc.) monomers as aniline, Py, and N-methylpyrrole. The most reliable way to ensure reproducible results with such species is to use them freshly distilled and transferred without contacts with atmosphere into the hermetic cell under nitrogen or Ar. Their storage (especially, under illumination and/or action of molecular oxygen) results in rapid formation of various derivatives, in particular oligomers, the presence of which enormously affects the results. To minimize these effects in the course of storage, the monomers are kept in a dark vessel and at low temperature. Figure 2.2a demonstrates a series of chronoamperometric curves at different potential steps from the same initial potential (without a faradaic process) to one of the ﬁnal potentials, Epolym . A characteristic feature of these curves is the minimum of the current, with its subsequent increase and the further stabilization (or a slower change) at the long-time scale. Increase of Epolym results in shorter periods of the current diminution and stabilization, as well as in a higher stabilized current. For even lower values of Epolym , the current diminishes monotonously. In this case, and also for the potentials with a very extended relaxation period, the electroactive ﬁlm is not formed, or the polymerization efﬁciency, Equation 2.9, becomes low. Slightly higher values of Epolym (curves 1 and 2 in Figure 2.2a) diminish the relaxation period up to the scale of tens of seconds (but the deposition of a sufﬁciently thick ﬁlm still requires a very long period) while a further increase of Epolym (e.g., curves 3 and 4 in Figure 2.2a) leads to a more rapid ﬁlm deposition. To compare redox responses for a series of such ﬁlms deposited at various potentials, Epolym , and measured in contact with the monomer-containing solution, each CV curve was divided by overall charge, Q tot , to exclude the role of this parameter owing to the linear dependence (Equation 2.7). Such normalized plots

2.3 Effect of Polymerization Parameters on Properties of Deposited Polymer Films 4

50.0µ

Curve 1 Curve 2 Curve 3 Curve 4

10.0m

I /Q polym (s−1)

I (A)

40.0µ 3

30.0µ 20.0µ

2

39

5.0m 0.0 −5.0m

10.0µ 1

−10.0m −0.6 −0.4 −0.2

0.0 0 (a)

50

100

150

t (s)

200

80.0µ

0.2

1

60.0µ 2

40.0µ

I (A)

0.0

E (V)

(b)

20.0µ 0.0 −20.0µ −40.0µ −60.0µ −1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4

(c)

Figure 2.2 (a) Potentiostatic deposition of PPy ﬁlms from aqueous 50 mM Py + 0.1 M LiClO4 solution at different polymerization potentials: 0.57 V (curve 1), 0.61 V (2), 0.65 V (3), 0.69 V (4). All potentials in this chapter are given versus aqueous SCE. Glassy carbon electrode (A = 0.1 cm2 ). (b) Redox responses of PPy ﬁlms after the

E (V)

deposition procedure (corresponding to the curves in Figure 2.2a) measured in the same monomer solution. (c) Redox responses of the same PPy ﬁlm (deposited at 0.65 V) in the monomer solution (1) and after its transfer into the background (0.1 M LiClO4 + H2 O) solution (2).

demonstrate a practical identity of redox properties of all these ﬁlms, despite the variation of the polymerization potential within these limits (Figure 2.2b). It also means identical values of the ‘‘maximum redox charge,’’ Q redox max , and of the polymerization efﬁciency, Equation 2.9, for all these ﬁlms. Another useful characterization of the ﬁlm is to measure its redox response within the same potential interval (ensuring a stable response in the multicycle treatment), both in the monomer-containing solution (used for the ﬁlm deposition) and in the background-electrolyte one. Figure 2.2c shows similar CV curves for these conditions, with a slightly stronger and more reversible response for the background solution, expectedly due to ‘‘a progressive activation’’ of the ﬁlm. The use of higher potentials leads to a rapid ﬁlm deposition but the onset of its redox CV response is shifted to higher potentials (thus, diminishing the deposition efﬁciency). The further increase of Epolym initially results (after passing the minimum) in a very high oxidation current but then after a maximum the current diminishes

0.4

0.6

2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects

gradually, because of the ‘‘overoxidation’’ which takes place during the deposition itself. Similar tendency is observed for the galvanostatic or CV polymerization regimes. Once again, the reasonably slow deposition results in higher efﬁciencies and a broader electroactivity range while an excessively high polymerization potential/ current diminishes the efﬁciency. An important advantage of the CV regime is the possibility to trace the evolution of the redox response of the polymer during the whole deposition process, for example, visible in the range below 0.4 V in Figure 2.3a. A regular increase of the current intensity in this interval (with about the same distances between the CV 400.0 µ 300.0 µ

I (A)

200.0 µ 100.0 µ 0.0 −100.0 µ −0.6

−0.3

0.0

(a)

0.3

0.6

0.9

E (V) 150.0 µ 100.0 µ

1 3

50.0µ

I (A)

40

0.0 2

−50.0 µ −100.0 µ −1.2 (b)

−0.8

−0.4

0.0

0.4

0.8

E (V)

Figure 2.3 (a) Potentiodynamic deposition of a PPy ﬁlm from aqueous 50 mM Py + 0.1 M LiClO4 solution. Anodic sweep limit: 0.86 V. Scan rate 100 mV s−1 . Glassy carbon electrode (A = 0.1 cm2 ). (b) Redox responses of the PPy ﬁlm after the deposition

procedure (Figure 2.3a) in the monomer solution (2) and after its transfer into the background (0.1 M LiClO4 + H2 O) solution (3). The CV curve corresponding to the last deposition cycle in Figure 2.3a (1) is given for comparison. Scan rate 100 mV s−1 .

2.3 Effect of Polymerization Parameters on Properties of Deposited Polymer Films

curves for neighboring cycles) testiﬁes in favor of the layer-by-layer deposition of the electroactive polymer ﬁlm. Another advantage is the possibility to terminate the process at the moment when the redox response of the polymer reaches a planned intensity. The next question is whether the CV curve in the course of the last deposition cycle is identical to the redox response of the ﬁlm (after the end of the deposition) measured either in the same monomer solution or after the ﬁlm transfer in contact with a background solution. The afﬁrmative answer to the former question (the anodic branches of the CV curves for the last deposition cycle and after the deposition termination but still in the same monomer solution) is shown in Figure 2.3b, curves 1 and 2. Similar to the observation for ﬁlms deposited potentiostatically (Figure 2.2c), the transfer of the electrode with potentiodynamically deposited ﬁlm into the monomer-free solution retains the shape of the redox response of the polymer but slightly increases its intensity (curve 3 in Figure 2.3b). A greater variety of shapes of the CV response has been discovered for PPy ﬁlms deposited from AN solution (or AN with water additions) [58–60]. Another important speciﬁcity of the AN medium is the possibility to perform the deposition of electroactive polymers not only from monomer solutions with 50–100 mM concentrations but also from very dilute ones, for example, 1 mM Py (with the shift of the deposition potential to higher values), while the ﬁlm formation from the aqueous solutions of the same concentration is very slow or does not occur at all. Potentiodynamic polymerization from AN + TBAPF6 + 1 mM Py solutions at a medium potential, Epolym , proceeds similarly to that in AN solutions with a higher monomer concentration [58–60] or in aqueous solutions; compare Figures 2.3a and 2.4a. One can also observe the same effect: a small increase of the intensity and reversibility of the redox response of the ﬁlm after the termination of the deposition process, both in the same monomer solution and especially in the background one (lines 1–3 in Figure 2.4b). Also in analogy with results for higher monomer concentrations [58–60], the increase of the deposition potential makes the shape of CV curves much more complex, especially during the ﬁrst anodic sweep (Figure 2.5a). This process leads (besides a much more rapid ﬁlm growth) probably to the formation of a rigid structure since its redox response does not change signiﬁcantly after the termination of the deposition, either in the same monomer solution or after the ﬁlm transfer into the monomer-free solutions (Figure 2.5b). Also similar to the Py polymerization in AN from a more concentrated solution [58–60], the deposition from 1 mM Py solution in AN + TBAPF6 manifests itself as CV curves of a complex shape (including peaks near the onset of the polymer oxidation) in conditions of a very slow polymerization (Figure 2.6a). The shape of the redox response of such ﬁlms is very unstable: even cycling in the monomer solution (but with a lower anodic limit than that used for the ﬁlm deposition) results in a progressive change of the CV curve (Figure 2.6b, lines 2 and 3 compared to line 1). The transfer to the background electrolyte leads to a further strong change of the response: both anodic and cathodic peaks disappear, with a decrease of the current intensity (Figure 2.6b, lines 4 and 5). At the same time, the potential interval of

41

2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects

6.0µ

I (A)

4.0µ

2.0µ

0.0

−2.0µ −1.0

−0.5

0.0 E (V)

(a)

0.5

1.0

2.0µ 2 1.0µ 1

3

I (A)

42

0.0

−1.0µ

−2.0µ −1.0 (b)

−0.5

0.0 E (V)

Figure 2.4 (a) Potentiodynamic deposition of a PPy ﬁlm from 1 mM Py + 0.1 M TBAPF6 + AN solution. Anodic sweep limit: 1.20 V. All experiments in AN solutions were performed with Ag/0.01 M AgNO3 +AN reference electrode separated from working AN solution with double frit. These data are presented in this ﬁgure and in Figures 2.5–2.7 for the potential scale versus aqueous SCE (the potential shift

0.5

1.0

between these scales is 320 mV [52,54]). Scan rate 100 mV s−1 . Pt electrode (A = 0.01 cm2 ). (b) Redox responses of the PPy ﬁlm after the deposition procedure (Figure 2.4a) in the monomer solution (2) and after its transfer into the background (0.1 M TBAPF6 + AN) solution (3). The CV curve corresponding to the last deposition cycle in a) is given for comparison. Scan rate 100 mV s−1 .

the ﬁlm electroactivity is extended strongly in the negative direction (Figure 2.6b, line 5). It is interesting to note that the overall redox charge, Equations 2.6 and 2.7, does not change signiﬁcantly in the course of these transformations of the ﬁlm (it even increases slightly in the ﬁnal state), that is, the electronic charge carriers

2.3 Effect of Polymerization Parameters on Properties of Deposited Polymer Films 20.0 µ 15.0 µ

I (A)

10.0 µ 5.0 µ 0.0 −5.0 µ −1.0

−0.5

(a)

0.0

1.0

0.5 E (V)

1.5

2.0

2

5.00 µ

3 1

I (A)

2.50 µ

1

0.00 3 −2.50 µ

−5.00 µ −1.0 (b)

2

−0.5

0.0

0.5

1.0

E (V)

Figure 2.5 (a) Potentiodynamic deposition of a PPy ﬁlm from 1 mM Py + 0.1 M TBAPF6 + AN solution. Higher anodic sweep limit: 1.82 V. Scan rate 100 mV s−1 . Pt electrode (A = 0.01 cm2 ). (b) Redox responses of the PPy ﬁlm after the deposition

procedure (Figure 2.5a) in the monomer solution (2) and after its transfer into the background (0.1 M TBAPF6 + AN) solution (3). The CV curve (1) corresponding to the last deposition cycle in a) is given for comparison. Scan rate 100 mV s−1 .

are redistributed inside the ﬁlm. One may expect that the corresponding electronic states become more delocalized along the chain. In conformity with this hypothesis, the potential step experiment has demonstrated that the ﬁlm increases its electronic conductivity within an extended range of its electroactivity. Figure 2.7 presents comparison of stabilized redox responses (in the monomer-free solution) for ﬁlms deposited from 1 mM Py + AN solution potentiodynamically with various anodic limits. For the sake of visibility, the responses of all ﬁlms were ‘‘normalized’’ by division by the corresponding maximum current (to take into account different deposition charges for these ﬁlms). One can see a progressive shift of the onset of the polymer oxidation toward

43

2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects

2.0µ

I (A)

1.0µ

0.0

−1.0µ −1.0

−0.5

2.0µ

0.5

0.0 E (V)

(a) Curve 1 Curve 2 Curve 3 Curve 4 Curve 5

1

3

2

1.0

1.0µ

I (A)

44

4 0.0 5 −1.0µ −1.5

(b)

−1.0

−0.5

0.0

0.5

1.0

E (V)

Figure 2.6 (a) Potentiodynamic deposition of a PPy ﬁlm from 1 mM Py + 0.1 M TBAPF6 . + AN solution. Low anodic sweep limit: 1.07 V. CV curves for deposition cycles: 1, 3, 4, 5, 10, 15, 20, 25, 30, 35, 38. Scan rate 100 mV s−1 . Pt electrode (A = 0.01 cm2 ). (b) Redox responses of the PPy ﬁlm after the deposition procedure (Figure 2.6a) in the monomer solution

(curve 2: ﬁrst cycle, curve 3: stabilized response) and after its transfer into the background (0.1 M TBAPF6 + AN) solution (curve 4: ﬁrst cycle after the transfer, curve 5: stabilized response in a broad potential interval). The CV curve (1) corresponding to the last deposition cycle in a) is given for comparison. Scan rate 100 mV s−1 .

lower potentials in parallel with the decrease of the anodic limit of the potential sweep during the deposition. The extension of the electroactivity interval for the ﬁlm deposited in the low-potential range (line 3 in Figure 2.7), compared to those at higher potentials, is especially striking: the difference reaches 1.5 V or even 2 V. Qualitatively analogous results are valid for the potentiostatic deposition from the same 1 mM Py + TBAPF6 + AN solution. The shape of chronoamperometric

2.3 Effect of Polymerization Parameters on Properties of Deposited Polymer Films

1

1.0 3

I /I max

0.5 2 0.0

−0.5

−1.0 −2.0

−1.5

−1.0

−0.5 E (V)

0.0

0.5

1.0

Figure 2.7 Comparison of the stabilized redox responses of the PPy ﬁlms deposited with different anodic sweep limits (Figures 2.4a–2.6a). Background solution: 0.1 M TBAPF6 + AN. Experimental CV curves (Figures 2.4b–2.6b) are ‘‘normalized’’ by division to the corresponding anodic peak current.

curves and their dependence on the deposition potential, Epolym (Figure 2.8a) resemble those for aqueous solutions (where a much higher Py concentration is required). In particular, the polymerization at intermediate potentials (lines 3–6 in Figure 2.8a) gives ﬁlms with a redox response (after the deposition termination) over rather a broad potential range (Figure 2.8b). It does not change signiﬁcantly after the electrode transfer into the background solution. Moreover, the redox response demonstrates an almost ‘‘equilibrium’’ dependence on the sweep rate (proportionality of the CV response at each potential to the scan rate, v), Equation 2.5. Correspondingly, i/v versus E plots are almost coincident, except for scan-rate-dependent anodic waves of the monomer oxidation at the anodic sweep limit, a slight shift of the oxidation onset to lower values for slower scans, and the approach to a more symmetrical (with respect to the anodic branch) cathodic response near the current maximum (Figure 2.8b). Another interesting result is a very high value of the ratio of the redox and deposition charges, Equation 2.7, η = Q redox max /Q tot = 0.16–0.17. Since its maximum value is given by the formula: ηmax = α redox (2 + αpolym )−1 and αpolym ≥ α redox one can derive a lower estimate for the charging/oxidation degree, α redox : α redox ≥ 2η(1 − η)−1

(2.11)

Equation 2.11 with the use of the above experimental value of η results in a much higher charging degree for these PPy ﬁlms, α redox ∼ = 0.4 or even higher, than the values about 0.25 reported earlier for AN or aqueous media. In analogy with the potentiodynamic polymerization, a slow potentiostatic ﬁlm deposition results in the redox response with strong and narrow peaks if cycling

45

46

2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects

1.4µ 1.2 µ

20.0 µ 5

800.0n 600.0n

I /v (F)

I (A)

1.0 µ

4

1

10.0 µ

4

0.0 −10.0µ

400.0n 3 2

200.0n 0.0 0

50

100

150

−20.0µ

1

−0.8−0.6−0.4−0.2 0.0 0.2 0.4 0.6 0.8 1.0

200

t (s)

E (V)

(b) 4.0µ 1

3.0µ 2.0µ

I (A)

(a)

Curve 1 Curve 2 Curve 3 Curve 4

30.0 µ

6

1.0µ 0.0

3 4

2

−1.0µ −2.0µ −3.0µ

−1.6 −1.2 −0.8 −0.4

(c)

Figure 2.8 (a) Potentiostatic deposition of PPy ﬁlms from 1 mM Py + 0.1 M TBAPF6 + AN solution at different polymerization potentials: 0.87 V (curve 1), 0.89 V (2), 0.91 V (3), 0.93 V (4), 0.95 V (5), and 0.97 V (6). Pt electrode (A = 0.01 cm2 ). (b) Stabilized redox responses of the PPy ﬁlm (deposited at 0.97 V, curve 6 in Figure 2.8a) measured in the same monomer solution for a set of scan rates: 20 mV s−1 (curve 1), 50 mV s−1

0.0

0.4

0.8

E (V)

(2), 100 mV s−1 (3), 200 mV s−1 (4). (c) Stabilized redox responses of the same PPy ﬁlm (deposited at 0.96 V): in the monomer solution after the polymerization termination (curve 1), after its transfer into the background (0.1 M TBAPF6 + AN) solution (curves 2 and 3 in different potential intervals), and after its back transfer into the monomer solution (curve 4).

is realized in the same monomer solution (line 1 in Figure 2.8c). The electrode transfer in contact with the monomer-free solution leads to a strong diminution of the response in the same potential range (anodic and cathodic peaks disappear, line 2 in Figure 2.8c) while the electroactivity range is extended enormously in the negative direction (line 3 in Figure 2.8c), with about the same overall redox charge, Q redox max . This transformation of the ﬁlm is irreversible, that is, the redox response of the ﬁlm after electrode transfer into the monomer solution remains practically unchanged (line 4 in Figure 2.8c), compared to that in the background solution (line 3 in Figure 2.8c). The ability of a ﬁlm (with the thickness well above 1 nm) to change its oxidation level in a uniform manner under variation of the potential is directly related to its sufﬁciently high electronic conductivity. Therefore, the observation that the redox

References

activity of the PPy ﬁlms deposited at low potentials (line 3 in Figure 2.7 and lines 3, 4 in Figure 2.8c) is extended up to very high negative potentials implies its electron-conducting state within this potential range. This extension of the electroactivity range toward very high negative potentials looks as prospective for its various applications, in particular, in electrocatalysis or for synthesis of hybrid materials.

2.4 Conclusions

The information in this chapter was selected to demonstrate that conjugated polymers possess broadly varying properties, in particular they have shown surprisingly different redox responses, depending on deposition conditions, even for systems corresponding formally to the same chemical composition and molecular structure. This ﬂexibility represents a powerful tool for variation of their properties in a controllable way, to adjust them to the needs of the corresponding application.

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(1984) The electrodeposition of polypyrrole ﬁlms from aqueous solutions. J. Electroanal. Chem., 177, 229–244. Asavapiryanont, S., Chandler, G.K., Gunawardena, G.A., and Pletcher, D. (1984) The electrodeposition of poly-N-methylpyrrole ﬁlms from aqueous solutions. J. Electroanal. Chem., 177, 245–251. Randriamahazaka, H., Sini, G., and Tran Van, F. (2007) Electrodeposition mechanisms and electrochemical behavior of poly(3,4-ethylenedithiathiophene). J. Phys. Chem. C, 111, 4553–4560. Heinze, J., Rasche, A., Pagels, M., and Geschke, B. (2007) On the origin of the so-called nucleation loop during electropolymerization of conducting polymers. J. Phys. Chem. B, 111, 989–997. Heinze, J., Frontana-Uribe, B.A., and Ludwigs, S. The electrochemistry of conducting polymers – persistent models, new concepts. Chem. Rev., submitted. Genies, E.M., Bidan, G., and Diaz, A.F. (1983) Spectroelectrochemical study of polypyrrole ﬁlms. J. Electroanal. Chem., 149, 101–113. Heinze, J. (2004) Conducting polymers, in Encyclopedia of Electrochemistry, vol. 8, Chapter 16 (eds A.J. Bard and M. Stratmann), Wiley-VCH Verlag GmbH, Weinheim, p. 607. Smie, A., Synowczyk, A., Heinze, J., Alle, R., Tschuncky, P., G¨otz, G., and B¨auerle, P. (1998) Disubstituted oligothiophenes, a new oligomeric approach towards the synthesis of conducting polymers. J. Electroanal. Chem., 452, 87–95. Skompska, M., Vorotyntsev, M.A., Refczynska, M., Goux, J., Lesniewska, E., Boni, G., and Moise, C. (2006) Electrosynthesis and properties of poly(3,4-ethylenedioxythiophene) ﬁlms functionalized with titanocene dichloride complex. Electrochim. Acta, 51, 2108–2119. Kazarinov, V.E., Levi, M.D., Skundin, A.M., and Vorotyntsev, M.A. (1989) Phenomenological description of dark redox reactions at electrodes covered with conducting polymer ﬁlms. Part 1. Mechanism and kinetics off Ferrocene

References

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oxidation at polythiophene. J. Electroanal. Chem., 271, 193–211. Inzelt, G. (1989) Role of polymeric properties in the electrochemical behavior of redox polymer-modiﬁed electrodes. Electrochim. Acta, 34, 83–91. Kalaji, M., Peter, L.M., Abrantes, L.M., and Mesquita, J.C. (1989) Microelectrode studies of fast switching in polyaniline ﬁlms. J. Electroanal. Chem., 274, 289–295. Aoki, K. (1991) Nernst equation complicated by electric random percolation at conducting polymer-coated electrodes. J. Electroanal. Chem., 310, 1–12. Odin, C. and Nechtschein, M. (1991) Slow relaxation in conducting polymers. Phys. Rev. Lett., 67, 1114–1117. Otero, T.F. (1997) Artiﬁcial muscles, electrodissolution, and redox processes in conducting polymers in Handbook of Organic Conducting Molecules and Polymers, vol. 4 (ed. H.S. Nalwa), John Wiley & Sons, Inc., New York, p. 517. Vorotyntsev, M.A., Aoki, K., and Heinze, J. (2003) Charging process in percolating systems. Russ. J. Electrochem., 39, 182–191 (English edition). Levi, M.D., Vorotyntsev, M.A., Skundin, A.M., and Kazarinov, V.E. (1991) Phenomenological description of dark redox reactions at electrodes coated with conducting polymer ﬁlms. Part II. Mechanism of tetracyanoquinodimethane and iodine electroreduction at polythiophene. J. Electroanal. Chem., 319, 243–261. Levi, M.D., Alpatova, N.M., Ovsyannikova, E.V., and Vorotyntsev, M.A. (1993) Mechanism of cathodic reactions at the electron-conducting polymer ﬁlms: electroreduction of chloranyl and tetracyanoquinodimethane at the poly-3-methylthiophene coated glassy carbon electrode. J. Electroanal. Chem., 351, 271–284. Vorotyntsev, M.A., Vieil, E., and Heinze, J. (1996) Ion Exchange between the polypyrrole ﬁlm and the lithium perchlorate PC solution. Electrochim. Acta, 41, 1913–1920. Levi, M.D., Lopez, C., Vieil, E., and Vorotyntsev, M.A. (1997) Inﬂuence of ionic size on the mechanism of electrochemical doping of polypyrrole

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ﬁlms studied by cyclic voltammetry. Electrochim. Acta, 42, 757–769. Vorotyntsev, M.A., Vieil, E., and Heinze, J. (1998) Charging process in polypyrrole ﬁlms: effect of ion association. J. Electroanal. Chem., 450, 121–141. Vorotyntsev, M.A., Daikhin, L.I., and Levi, M.D. (1994) Modelling the impedance properties of electrodes coated with electroactive polymer ﬁlms. J. Electroanal. Chem., 364, 37–49. Vorotyntsev, M.A., Badiali, J.P., and Vieil, E. (1996) Multi-Component diffusion approach to the transport across the electroactive-polymer ﬁlms with two mobile charge carriers. Electrochim. Acta, 41, 1375–1381. Vorotyntsev, M.A., Badiali, J.P., and Inzelt, G. (1999) Electrochemical impedance spectroscopy of thin ﬁlms with two mobile charge carriers: effect of the interfacial charging. J. Electroanal. Chem., 472, 7–19. Buttry, D.A. (1991) Quartz crystal microbalances in electrochemistry in Electroanalytical Chemistry, vol. 17 (ed. A.J. Bard), Dekker, New York, p. 1. Inzelt, G. (2000) Simultaneous chronoamperometric and quartz crystal microbalance studies of redox transformations of polyaniline ﬁlms. Electrochem. Acta, 45, 3865–3876. Bourkane, S., Gabrielli, C., and Keddam, M. (1993) Investigation of gold oxidation in sulfuric medium. II. Electrogravimetric transfer function technique. Electrochim. Acta, 38, 1827–1835. Gabrielli, C., Keddam, M., Nadi, N., and Perrot, H. (2000) Ions and solvent transport across conducting polymers investigated by ac electrogravimetry. Application to polyaniline. J. Electroanal. Chem., 485, 101–113. Qi, Z., Rees, N.G., and Pickup, P.G. (1996) Electrochemically induced substitution of polythiophenes and polypyrrole. Chem. Mater., 8, 701–707. Soudan, P., Lucas, P., Breau, L., and B´elanger, D. (2000) Electrochemical modiﬁcation of poly(3-(4-ﬂuorophenyl)thiophene). Langmuir, 16, 4362–4366.

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2 Mechanisms of Electropolymerization and Redox Activity: Fundamental Aspects 51. Skompska, M. and Vorotyntsev, M.A.

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(2004) Inﬂuence of chloride anion on electrodeposition and electroactivity of the polymer matrix in polypyrrole, poly(N-methylpyrrole) and polypyrrole derivative functionalized by titanocene centers, in dry non-aqueous solutions. J. Solid State Electrochem., 8, 360–368. Vorotyntsev, M.A., Skompska, M., Pousson, E., Goux, J., and Moise, C. (2003) Memory effects in functionalized conducting polymer ﬁlms: titanocene derivatized polypyrrole in contact with THF solutions. J. Electroanal. Chem., 552C, 307–317. Zotti, G., Schiavon, G., Zecchin, S., and Groenendaal, L. (1999) Conductive and magnetic properties of poly(3,6-bis(2-(3,4-ethylenedioxy)thienyl)N-dodecylcarbazole). A polyconjugated polymer with a high spin density polaron state. Chem. Mater., 11, 3624–3628. Vorotyntsev, M.A., Casalta, M., Pousson, E., Roullier, L., Boni, G., and Moise, C. (2001) Redox properties of titanocene-pyrrole derivative and its electropolymerization. Electrochim. Acta, 46, 4017–4033. Dietrich, M. and Heinze, J. (1991) Poly(4,4 -dimethoxybithiophene) – A new conducting polymer with extraordinary redox and

56.

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58.

59.

60.

optical properties. Synth. Met., 41, 503–506. Levi, M.D., Lankri, E., Gofer, Y., Aurbach, A., and Otero, T. (2002) The behavior of polypyrrole-coated electrodes in propylene carbonate solutions I. Characterization of PPy ﬁlms by a combination of electroanalytical tools and XPS. J. Electrochem. Soc., 149, E204–E214. Schuhmann, W., Kranz, C., Wohlschl¨ager, H., and Strohmeier, J. (1997) Pulse technique for the electrochemical deposition of polymer ﬁlms on electrode surfaces. Biosens. Bioelectron., 12, 1157–1167. Zhou, M. and Heinze, J. (1999) Electropolymerization of pyrrole and electrochemical study of polypyrrole. 3. Nature of ‘‘water effect’’ in acetonitrile. J. Phys. Chem. B, 103, 8451–8457. Zhou, M. and Heinze, J. (1999) Electropolymerization of pyrrole and electrochemical study of polypyrrole: 1. Evidence for structural diversity of polypyrrole. Electrochim. Acta, 44, 1733–1748. Zhou, M., Pagels, M., Geschke, B., and Heinze, J. (2002) Electropolymerization of pyrrole and electrochemical study of polypyrrole. 5. Controlled electrochemical synthesis and solid-state transition of well-deﬁned polypyrrole variants. J. Phys. Chem. B, 106, 10065–10073.

51

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization Gy¨orgy Inzelt and Gy˝oz˝o G. L´ang

3.1 Introduction

Electrochemical impedance spectroscopy (EIS) represents a powerful tool for investigation of the rate of charge transfer and charge transport processes occurring in electrochemical systems [1–4]. Therefore, it is widely used also for the characterization of conducting polymer ﬁlms and membranes [5–191]. Owing to the marginal perturbation from equilibrium (steady state) by low-amplitude (<5 mV) sinusoidal voltage, its advantage over other techniques involving large perturbations (e.g., chronoamperometry) is evident. For instance, even the potential dependence of the charge transport diffusion coefﬁcient can be determined that may reveal the nature of charge carriers and interactions within the ﬁlm. Although there are several variations, usually an alternating voltage U(t) = Um sin(ωt)

(3.1)

is applied to an electrode and the resulting current response I(t) = Im sin(ωt + ϑ)

(3.2)

is measured, where ω (ω = 2πf , where f is the frequency) is the angular frequency of the sinusoidal potential perturbation, ϑ is the phase difference (phase angle, phase shift) between the potential and the current, and Um and Im are the amplitudes of the sinusoidal voltage and current, respectively. The impedance (Z) is deﬁned as U(t) = |Z| exp(iϑ) = Z + iZ (3.3) Z= I(t) where Z and Z are the real and imaginary part of Z, respectively, and i = (−1)1/2 . (For the real and imaginary parts, the symbols ZR and ZI , respectively, are also used.) Impedance and admittance (Y) are related as follows: Y=

1 = Y + iY Z

(3.4)

Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

52

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

For an RC circuit with components Rs , Cs (series) and Rp , Cp (parallel), the following relations are valid: Z = Rs , Z = −1/ωCs , Y = 1/Rp , and Y = ωCp . Usually the impedance is measured as a function of frequency, and its variation is characteristic to the electrical circuit consisting of passive and active circuit elements. Under appropriate conditions, that is, at well-selected cell geometry, working and auxiliary electrodes, and so forth, the impedance response will be related to the properties of the working electrode and the (ohmic) resistance between the working electrode and the reference. The tools used to interpret impedance data fall into two categories: (i) analogs and (ii) physical models. Analogs, which almost always take the form of electrical equivalent circuits (EECs), do not pretend to describe the electrochemical properties of the system, but simply reproduce the impedance response. In addition, physical signiﬁcance of the parameters may not always be obvious when data analysis is performed in this manner. On the other hand, physical models should not only reproduce the phenomenon of interest (e.g., the impedance spectrum) but should also account for the mechanism of the processes occurring at the interface in terms of physicochemically (electrochemically) valid concepts. EIS data are often interpreted in terms of EECs. However, EECs are analogs, not models, and hence the information that they can deliver on the physicoelectrochemical processes involved is very limited. As has been pointed out [4], EEC analysis, while providing a termination point for a paper, is not the end of the road; the amount of information that is to be gleaned from a full mechanistic analysis far outweighs that which can be obtained by simply ﬁtting EECs. For instance, for the description of a simple electrode reaction, the Randles equivalent circuit is often used, in which the solution resistance (R ) is in series with the charge transfer resistance (Rct ) and the Warburg impedance (Zw ) expressing the diffusion of the electroactive species, and the double-layer capacitance (Cdl ) is in parallel with Rct and Zw . (ZF = Rct + Zw is called Faraday impedance.) Because an explicit expression exists for the Warburg impedance, it is possible, in this case, to relate the electrical elements to chemical parameters. However, it is also important to note that the Warburg impedance has no simple electrical analog, although a constant phase element (CPE) emulating Warburg impedance (phase angle = π/4) can be devised as an electrical transmission line. In the Randles model, semi-inﬁnite linear diffusion is considered, and the capacitive current is separated from the faradaic current, which is justiﬁed only when different ions take part in the double-layer charging and the charge transfer processes, that is, a supporting electrolyte is present in high concentration. Finite diffusion conditions should be considered at well-stirred solution when the diffusion takes place only within the diffusion layer, and also in the case of surface ﬁlms which have a ﬁnite thickness. However, the two cases are different since in the previous case one has a practically ‘‘inﬁnite’’ source of electroactive species (transmissive boundary condition), while in the case of surface ﬁlms, both transmissive boundary conditions and reﬂective boundary conditions may prevail. The latter means that at the interfaces the complete blocking of the diffusion occurs. This is the case when a polymer-modiﬁed electrode is investigated, and no

3.2 Experimental Arrangements

electrochemically active species are dissolved in the contacting electrolyte or there is no charge leakage, that is, a reaction between the conducting polymer and one of the components of the solution can take place. It means that redox sites remain in the surface layer, the charge propagates through the layer by electron hopping or electric conduction as well as by the diffusion and/or migration of freely moving ions (usually counterions), and electron can cross the metal|polymer, while ions can cross the polymer|electrolyte solution interfaces, respectively. It is evident that also in the case of polymer ﬁlms, a theoretical approach, which assumes the model structure a priori, would be preferable; however, due to the high complexity of the polymer ﬁlm electrodes none of the current theories can be regarded as satisfactory in all respects. Consequently, the derivation of an appropriate and adequate impedance function is rather problematic. Therefore, the so-called structural approach is also employed. The structural approach means that the model structure is derived from experimental data and procedures for parametrical identiﬁcation are then applied.

3.2 Experimental Arrangements

For the detailed characterization of conducting polymers, it is very important to investigate their electrochemical properties in different cell and electrode conﬁgurations. Experimental arrangements usually fall into four main categories: 1) The polymer ﬁlm is supported on a metal or other electronic conductor (often a rotating disk electrode) and dipped in an electrolyte solution containing only electrochemically ‘‘inert’’ species (Figure 3.1a), that is, the solution only contains ions that do not possess a redox activity (‘‘background electrolyte’’). In this electrode arrangement, two different interfaces exist: a metal/ﬁlm interface where only electrons may be exchanged and a ﬁlm/electrolyte solution interface, which is permeable only for counterions that are able to cross the ﬁlm/solution boundary to retain the bulk ﬁlm electroneutrality, and for neutral (e.g., solvent) molecules (‘‘blocking’’ interface). 2) The polymer ﬁlm is supported on a metal or other electronic conductor and dipped in an electrolyte solution containing a redox couple (Figure 3.1b). In this case also two interfaces exist: a metal/ﬁlm interface where only electrons may be exchanged and a ﬁlm/electrolyte solution interface where both electrons and ions as well as solvent molecules can be exchanged [38, 39]. Of course, the situation represented in Figure 3.1b is only a limiting case for the electron exchange between the ﬁlm and the redox couple. More generally, this electron transfer may occur within the polymer, after penetration of the redox couple. Even if one may assume that no penetration of redox species occurs, the data analysis must take into account the existence of two interfaces of different nature, each of which may be the site of an impedance.

53

54

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization Metal

Film

Metal

Solution

Film

R

O− e

e−

e−

e−

e− AzA−

AzA−

Cz C +

R

O

O− e

R

e−

AzA−

Red

e− AzA−

AzA−

AzA− Cz C +

Cz C +

Ox

e−

e− AzA−

Cz C +

Cz C +

Solution

Cz C +

AzA− Cz C + Cz C +

(b)

(a)

Solution Ox

Red

e−

Film R

O

O− e

R

e− AzA−

Solution

e− AzA−

AzA−

AzA− Cz C +

Cz C +

Ox

e−

e− AzA−

Cz C +

Red

Cz C +

AzA− Cz C + Cz C +

(c)

Figure 3.1 Schematic representation of the main charge transfer and charge transport processes occurring on (a) a polymer ﬁlm supported on a metal or other electronic conductor and dipped in an electrolyte solution containing only electrochemically ‘‘inert’’ species (‘‘modiﬁed electrode’’); (b) a polymer

ﬁlm supported on a metal or other electronic conductor and dipped in an electrolyte solution containing a redox couple (‘‘modiﬁed electrode’’); and (c) a freestanding membrane in contact with electrolyte solutions containing a redox couple. Only the limiting case of surface reaction is considered.

These two arrangements are often called modiﬁed electrodes and are schematically represented in Figure 3.1a,b. 3) The ﬁlm is located between two electrolyte solutions that may be identical or not. In this case, either the solutions only contain ions which do not possess a redox activity or redox active species are present inside the solutions [41, 43, 44]. Figure 3.1c schematically shows a symmetrical arrangement for the study of redox processes occurring at a polymer ﬁlm/electrolyte solution interface (again as a limiting case). In this case, the polymer ﬁlm is a freestanding membrane, symmetrically bathed at both faces by the same electrolyte solution. Each of

3.3 Impedance Spectra of Polymer Films

the two identical faces allows the transfer of both electrons and ions. Carriers of both types are also transported across the ﬁlm. Thus, in the steady state, the overall current may be assumed as due to two additive components, one resulting from convective diffusion of the redox species in solution, electron transfer at the polymer/electrolyte interface, and electron transport within the ﬁlm, and the other due to migration of nonelectroactive ions in solution, ion transfer at the polymer/electrolyte interface, and ion transport within the ﬁlm. As a ﬁrst approximation, in the non-steady state, the respective contributions will be inversely proportional to the impedance of each path. Actually, the overall process is more complex since (i) electroactive species (if charged) may contribute to the ionic transport in both the solution and the ﬁlm and (ii) ionic and electronic transport are coupled in the ﬁlm [45]. General treatments can be found, for example, in [174, 175]. 4) The ﬁlm is located between two electronic conductors (metals) [62].

3.3 Impedance Spectra of Polymer Films

The theory of the impedance method for an electrode with diffusion restricted to a thin layer is well established [6, 12, 26, 29, 40, 52, 58, 70, 88, 107, 110, 130, 158, 160, 174, 177]; however, the ‘‘ideal’’ response, that is, a separate Randles circuit behavior at high frequencies, a Warburg section at intermediate frequencies, and a purely capacitive behavior due to the redox capacitance at low frequencies (see Figure 3.2), seldom appears in real system. The deviations of the impedance responses [24, 54, 60, 72, 74, 76, 82, 89, 101, 102, 106, 110, 119, 128, 139, 140, 161, 172, 178] from those predicted by the theories have been explained by taking into account different effects such as interactions between redox sites [85, 119], ionic relaxation processes [167], distribution of diffusion coefﬁcients [161], migration [26, 37, 107, 130, 131, 178], ﬁlm swelling [108, 168, 190], slow reaction with solution species [38, 108], nonuniform ﬁlm thickness [106], inhomogeneous oxidation/reduction processes [76], experimental artifacts [82], and so on. The CPE has been used to describe both the double-layer capacitance and the low-frequency pseudocapacitance as well as the diffusion impedance [19, 20, 22, 23, 60, 83, 88, 108, 119, 120, 172]: ZCPE = A (iω)−α

(3.5)

where 0 < α < 1 is the CPE exponent, which is a dimensionless parameter, and A is the CPE coefﬁcient. It follows that the exponent is less than 1, which is expected for an ideal capacitor (ϑ < 90◦ ), and it differs from 0.5, as is expected for the ideal diffusion impedance. The dispersion of the high-frequency capacitance has been attributed to the microscopic roughness of the electrode surface [22, 88, 168, 180] and an adsorption pseudocapacitance connected with the charging/discharging process within the

55

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

35 1 30 1

2 3 4

5

25 −Z ′′ (Ω)

56

6 1

20

1

100 15 10

10

10 5

1000

10 10

100 0 0

5

10

15 20 Z ′ (Ω)

25

Figure 3.2 The complex-plane impedance plot representation (also called Argand diagram or Nyquist diagram) of the ‘‘ideal’’ impedance spectra in the case of reﬂective boundary conditions. Effect of the ratio of the ﬁlm thickness (L) and the diffusion

30

35

coefﬁcient (D). L/D1/2 : (1) 0.005; (2) 0.1; (3) 0.2; (4) 0.3; (5) 0.5; and (6) 1 s1/2 . R = 2 , Rct = 5 , σ = 50 s−1/2 , Cdl = 20 µF. The smaller numbers refer to frequency in hertz. All values are related to unit area.

ﬁrst layer of the ﬁlm at the metal interface [108]. The frequency-dependent nature of the low-frequency capacitance has been explained by considering the irregular geometry of the surface of the polymer network and the counterions’ binding to the sites of different energies [19, 108, 181], by the roughness of the blocking metal electrode [23], by a distributed charge–transfer resistance in the internal polymer/solution interface [172]. Consequently, the measured impedance spectra can be quite complicated, and by merely inspecting the experimental data, it is not possible to ascertain whether or not the data are valid or have been distorted by some experimental artifact. For instance, the selection of the amplitude of the potential perturbation is of particular interest. It has been shown in [4] that, in principle, the amplitude of the input potential perturbation used for impedance measurements should be adjusted for each experimental system. The validation of the impedance spectra can be executed, for example, by using Kramers–Kronig (K–K) transformations as described in [3, 135, 136, 192]. Of course, the K–K transforms are purely mathematical results and do not reﬂect any assumptions concerning the physical properties of the system. 3.3.1 Effect of the Film Thickness and Thickness Distribution of Polymer Films

The ﬁlm thickness is very often nonuniform. The effect of thickness distribution is shown in Figure 3.3. If the surface is very rough, that is, the ﬁlm consists of very

3.3 Impedance Spectra of Polymer Films

14

9

12

8

5

7

−Z ′′ (Ω)

10

6

4

8

R

6

3

4

2

2

0.98 2

1 10

0 0

2

4 6 Z ′ (Ω)

8

10

Figure 3.3 Impedance spectra demonstrating the effect of ﬁlm thickness and thickness distribution at constant ohmic resistance (R = 2.35 ), charge transfer resistance (Rct = 0.9 ), double-layer capacitance (Cdl = 24 µF), and diffusion coefﬁcient (D = 9.04 × 10−9 cm2 s−1 ). Thicknesses are (1) 0.006; (2) 0.06; (3) 0.6; (4)

1.6; (5) 2.6; (6) 3.6; (7) 4.6; (8) 5.6, and (9) 6.6 × 10−5 cm. The resulting curve (R) was constructed using thicknesses of (3–9) with the following frequency factors: (3) and (9): 1; (4) and (8): 3; (5) and (7):6; and (6): 10. The smaller numbers refer to values of frequency in hertz. The average thickness is L = 3.6 × 10−5 cm. (Adapted from [87].)

thin and thick regions, no Warburg section appears. It should be mentioned that a similar problem arises when two parallel diffusion paths exist in the ﬁlm as has been assumed, for example, for the Ru(bpy)3 3+/2+ /Naﬁon system [161]. It is evident that the shape of the impedance spectra varies with the potential since the values of the charge transfer resistance (Rct ), low-frequency (redox) capacitance (CL ), and the Warburg coefﬁcient change with the potential; more exactly, they depend on the redox state of the polymer. In many cases, D is also potential dependent. The double-layer capacitance (Cdl ) usually shows only slight changes with potential. The ohmic resistance (R ) is a sum of the solution resistance and the ﬁlm resistance, and the latter may also be a function of potential due to the potential-dependent electron conductivity, sorption of ions, and the swelling of the ﬁlm. In Figure 3.4, three spectra are displayed, which are constructed by using the data obtained for a poly(tetracyanodimethane) (PTCNQ) electrode at three different potentials close to its equilibrium potential [106]. 3.3.2 Characteristic Quantities for Modiﬁed Electrodes

In the case of the ‘‘ideal’’ reﬂective spectra (surface response), the following relationships are valid, and can be used to derive the quantities characterizing the

57

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

30 1

2

3

25 0.98

0.22

20

0.98

−Z ′′ (Ω)

58

15 2

10

2

5

20

0.98 2000 2

5.2 5.2

20 215

0 0

5

10

15 −Z ′ (Ω)

20

25

30

Cdl = 25.8 µF; CL = 8.88 mF; D = 8.72 × 10−9 cm2 s−1 ; (3) E = −0.1 V; R = 2.3 ; Rct = 1.78 ; σ = 50.35 s−1/2 ; Cdl = 34 µF; CL = 9 mF; D = 3.2 × 10−9 cm2 s−1 . The smaller numbers refer to frequency in hertz. (Adapted from [106].)

Figure 3.4 The effect of potential on the impedance spectra. The data used for the simulation of the spectra: (1) E = 0 V; R = 2.35 ; Rct = 0.9 ; σ = 7.65 s−1/2 ; Cdl = 24 µF; CL = 35 mF; D = 9.04 × 10−9 cm2 s−1 ; (2) E = 0.1 V; R = 2.6 ; Rct = 3.85 ; σ = 30.7 s−1/2 ;

electrode and the electrode processes. Zf = Rct + (1 − i) σ ω−1/2 coth (sL)

(3.6)

where σ is the Warburg coefﬁcient, L is the ﬁlm thickness, and s=

iω D

1/2 =

(1 + i) ω1/2 (2D)1/2

(3.7)

The Warburg coefﬁcient depends on the diffusion coefﬁcient (D), the concentrations (c) of redox sites, and the temperature (T) σ = √

RT

2n2 F 2

1 1/2

cO D O

+

1 1/2

cR D R

(3.8)

or, when cO = cR and DO = D (indexes O and R for the oxidized and reduced forms, respectively). σ =

√ 2 2RT n2 F 2 D1/2 c

(3.9)

3.3 Impedance Spectra of Polymer Films

ω−1/2 Rct ω1/2 + σ F1 Z = R + 2 2 Rct ω1/2 + σ F1 1 + σ Cdl ω1/2 F2 + ωCdl −i where F1 =

F2 =

2 Cdl Rct ω1/2 + σ F1 + σ F2 ω−1/2 + F2 Cdl σ 2 2 Rct ω1/2 + σ F1 1 + σ Cdl ω1/2 F2 + ωCdl

coth K 1 + cot2 K + cot K 1 − coth2 K coth2 K + cot2 K coth K 1 + cot2 K − cot K 1 − coth2 K coth2 K + cot2 K

K = 2−1/2 L (ω/D)1/2 From Equations 3.10–3.13, it follows that Rct + L2 /3DCdl lim Z = R + ω→0 (1 + Cdl /CL )2

(3.10)

(3.11)

(3.12)

(3.13)

(3.14)

where CL is the redox capacitance (low-frequency capacitance) that, in principle, can be obtained from the Z versus ω−1 plot since lim Z = (CL + Cdl )−1 ω−1

ω→0

(3.15)

The diffusion coefﬁcient can be derived either from the Z versus ω−1/2 plots or from the low-frequency impedance or resistance, RL =

L2 3DCL

(3.16)

However, if the Warburg section is small or nonexistent, which is the case when the ﬁlm is thin, this procedure is not applicable. The low-frequency capacity is equal to CL =

n2 F 2 L c O c R RT cO + cR

(3.17)

n2 F 2 Lc 4RT

(3.18)

or at Ecθ CL =

It follows that while CL (E) function has its maximum at Ecθ , σ has its lowest value at this potential, that is, when cO = cR . (CL can also be estimated from the cyclic voltammograms: CL = I/ν, where ν is the sweep rate.) While CL is independent of the ﬁlm swelling, because as L increases c decreases, σ and D usually vary with the swelling.

59

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

The temperature dependence of σ is determined by the exponential temperature dependence of D. R decreases with increasing concentration of the supporting electrolyte and with increasing temperature. Rct decreases with temperature, and has a minimum value at Ecθ . It follows that carrying out measurements for a range of potentials, temperatures, and electrolyte concentrations helps to achieve an adequate analysis of the EIS results by resolving some ambiguities. 3.3.3 Impedance Associated with Polymer Films in Contact with Media Allowing both Ionic and Electronic Interfacial Exchange

In this experimental setup, the polymer ﬁlm is a freestanding membrane bathed at both faces by the same electrolyte [28, 44, 45, 53]. Each of the two identical faces allows the transfer of both electrons and counterions. In some cases, in order to quantitatively control mass transport of the redox system in the solution, a well-deﬁned forced convection has been imposed, for example, in a cell with two coaxial jets impinging in opposite directions on the two faces of the bipolar electrode [44, 45]. It has been shown, that steady-state current-potential curves for both Pt foil and polypyrrole (PPy) membrane, in contact with 0.1 M Na2 SO4 solution containing K4 [Fe(CN)6 ] and K3 [Fe(CN)6 ], are very similar. Figure 3.5 shows two complex-plane plots of the impedance measured at the same ﬂow rate. A Pt sheet was used in the ﬁrst experiment and a PPy freestanding membrane in the second one. In the ﬁrst case, the impedances on both faces are identical and can be represented for such a system by a Randles equivalent circuit in which the Faradaic impedance is the series combination of a charge transfer resistance Rct and a diffusion impedance Zd = Rd · f (ω) accounting for the concentration relaxation of the redox species in solution. Rd is called the diffusion resistance, and f (ω) is a complex function of angular frequency such that f (ω) → 0 when ω → ∞, and f (ω) → 1 when ω → 0. 1 Hz

50

1 Hz

−Z ′′ (Ω)

60

0 100

150

200

Z ′ (Ω) Figure 3.5 Comparison of the impedance diagrams obtained on a Pt sheet (•) and on a PPy freestanding membrane (◦) in 0.1 mol · dm−3 Na2 SO4 solution containing 0.01 mol · dm−3 K4 [Fe(CN)6 ] and 0.01 mol · dm−3 K3 [Fe(CN)6 ]. (Adapted from [44].)

3.4 Analysis of the Impedance Spectra

The second plot shows the behavior for the PPy ﬁlm. A marked difference can be observed in the Rct values measured on Pt and on PPy. The plot displays a small, depressed semicircle at high frequencies, which is not completely resolved from the (low frequency) diffusion loop. It means that a large decrease in the charge transfer resistance is evident when Pt is replaced by PPy. It has been previously mentioned that the electronic path involving redox reaction and the ionic path play additive roles in the impedance response. As an intuitive way of assessing its frequency dependence, a suggested equivalent circuit is proposed in [44, 45] where the interfacial ionic impedance (Zi ) and the interfacial electronic impedance (Rct + Zd ) are placed in parallel. In the absence of redox reactions and by neglecting the double-layer capacitance, the overall impedance is 2Zi + Zp where Zp is the impedance of the bulk membrane. It may be concluded that PPy is a better electrode material than Pt for the [Fe (CN)6 ]4− / [Fe (CN)6 ]3− couple [44].

3.4 Analysis of the Impedance Spectra

Complex nonlinear least squares (CNLS) ﬁtting of the data to a theoretical model and/or equivalent electrical circuit is the best method of quantitative analysis. Such ﬁtting provides estimates of the parameters and their standard deviations. Unfortunately, in the majority of papers no standard deviations of the parameters are given, and the goodness of ﬁt is merely illustrated in the ﬁgures. Usually, the Argand diagram is used for this purpose; however, the deviation between the measured and calculated data is more striking in the transformed plots, for example, in the Bode diagrams (log |Z| vs log f and ϑ vs log f plots) or in the changes of pseudocapacitance as a function of frequency (log Y ω−1 vs log f plots). It should also be checked whether the derived parameters depend on the number of elements or not, or on the method of weighting. While linear estimation theory is an important branch of mathematical statistics, practical considerations are equally important in nonlinear parameter estimation. As emphasized by Bard [193], in spite of its statistical basis, nonlinear estimation is mainly a variety of computational algorithms that perform well on a class of problems but may fail on some others. In addition, most statistical tests and estimates of variability are formulated for linear models, and in the nonlinear case, most often, the best we can do is to apply these linear results as approximations. Regarding the calculation of the conﬁdence interval, it is well known that the ‘‘classical’’ method of the determination is rigorous only for linear regressions. In nonlinear models, we can use an approximation as follows [194]: in the case of a weighted least squares estimation of parameters in multivariable nonlinear models, analogous to the linear case, the goodness of ﬁt is measured in terms of the residual sum of squares Q pˆ and the residual variance (or sigma square) s2 , deﬁned by Q pˆ 2 s = (3.19) nm × ny − np

61

62

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

with the degrees (nm × ny − np) of freedom in the denominator. (In the above equation, pˆ is the vector of least squares estimate, nm is the number of sample points, ny is the number of dependent variables, and np is the number of parameters.) Interpretation of estimates is based on the observation that each iteration of the Gauss–Newton algorithm is equivalent to solving a linear regression problem. The covariance matrix of estimates is approximated by −1 Cp = s2 JT pˆ W J pˆ

(3.20)

where J pˆ is the Jacobian, corresponding to the linear approximation of the response function in a neighborhood of pˆ , and W is the matrix of weights. On the basis of the same linear approximation, the conﬁdence region is described by

2 p − pˆ Cp−1 p − pˆ χp,nx

(3.21)

Where nx is the number of independent variables. This is clearly an approximate relationship, and, in some cases, it may differ considerably from the exact conﬁdence region given by Q p − Q pˆ ≤ χ 2 , where χ 2 depends on the probability level. The exact conﬁdence region has little practical value for np > 2, since it is very difﬁcult to compute (e.g., by Monte Carlo methods), whereas the linear approximation is very useful for the characterization of the goodness of parameters. It is, however, well known that only a statistical analysis of the ﬁtting procedure is not always enough to decide whether the parameters obtained by CNLS ﬁtting are acceptable or not. The same is true for the graphical representation of the measured and ﬁtted points and curves. As has been already pointed out in [88, 102, 112, 195], the investigation of the transformed curves (Bode plots, admittance plots, etc.) supplies more valuable information concerning the goodness of a ﬁt than that of the complex-plane plot. It is always expedient to follow the four-step procedure proposed in [88, 102, 112] for checking the goodness of ﬁt: 1) the statistical analysis of the results of the ﬁtting procedure (sum of squares, variance, correlation matrix, conﬁdence intervals, etc.), that is, information about the ‘‘mathematical’’ goodness of the ﬁt; 2) comparison of impedances calculated by using the estimated parameters, as well as the transformed curves with the measured series of data. A ‘‘visual’’ inspection on both the Argand diagrams and the transformed curves (complex-plane admittance plots, Bode diagrams, etc.); 3) analysis of the distribution of the deviations between the measured and ﬁtted curves; 4) analysis of the physical signiﬁcance of the estimated parameters (e.g., dependence of the parameters on electrode potential, electrolyte concentration, temperature, comparison of the estimated values with independent experimental or theoretical data, etc.), evidently the most important step.

3.5 Models of Polymeric Layers

63

3.5 Models of Polymeric Layers

One of the crucial points of the modelling is in connection with the structure and morphology of the surface of the polymer layer. Essentially, there are two different approaches, which are called ‘‘homogeneous’’ or ‘‘uniform’’ [12, 29, 40, 62, 69, 70, 83, 88, 107, 112, 130, 177, 178] and ‘‘porous medium’’, ‘‘heterogeneous’’, or ‘‘distributed’’ models [5, 6, 21, 52, 58, 83, 93, 132, 146, 149, 153, 154, 158, 160], respectively, based on two different perceptions regarding the structure of the surface polymer layers or membranes (Figure 3.6). 3.5.1 ‘‘Homogeneous’’ or ‘‘Uniform’’ Models

The ‘‘homogeneous’’ models assume three phases, that is, metal, polymer ﬁlm, and an electrolyte solution. Electronic, mixed electronic (electron or polaron), and ionic charge transport processes are considered in the metal, within the polymer ﬁlm, and in the solution, respectively. The polymer phase itself consists of a polymer matrix with incorporated ions and solvent molecules. A one-dimensional model is used, that is, the spatial changes of all quantities (concentrations, potential) within the ﬁlm are described as a function of a single coordinate x, which is a good approach when an electrode of usual size is used. The metal/polymer and the polymer/solution interfacial boundaries are taken as planes. The interfacial potential differences at the two interfaces and a potential drop inside the ﬁlm when current ﬂows are considered. The thicknesses of the electric double layers

(a)

Ru

Electrolyte solution

Polymer film

Zif,s/f

Zb

Zif,f /s

Metal substrate

Figure 3.6 Schematic pictures of the two models for polymer modiﬁed electrodes. (a) ‘‘Homogeneous model’’: Zif ,s/f , Zif ,f /s : interfacial impedances (s|f: solution|ﬁlm; f|s: ﬁlm|substrate); Zb : impedance of the bulk phase; Ru : solution resistance. (b) ‘‘porous (heterogeneous) model’’: Z1 : the impedance

Counter electrode

Counter electrode

Electrolyte solution

(b)

Ru

Polymer film

Z2 Z2

Z2 Z2

Z3 Z3

Z3 Z3 Z3

Z1

Z1

Z1 Z1

Metal substrate

per unit length of the transport channel in the polymer phase; Z2 : the impedance per unit length of the transport channel in the pores; Z3 : the speciﬁc impedance at the inner interface, which corresponds to charge transfer and charging processes; Ru : solution resistance. (Adapted from [112].)

64

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

at the interfaces are small in comparison with the ﬁlm thickness, and therefore are neglected. In an asymmetrical (polymer ﬁlm) arrangement, electron transfer at the metal/ﬁlm interface is combined with a charge transport process in the ﬁlm and the ion transfer at the ﬁlm/solution interface. The ﬁrst theoretically well-established models of uniform ﬁlms considered a pure diffusional transport of a single charge carrier across the ﬁlm under ﬁnite diffusion conditions [12, 38, 63, 70]. It is also assumed that the electrolyte concentration is high enough, and as a consequence, the diffusion of ions in the bathing electrolyte is not rate-determining, and the migration contribution to the ﬂux can be neglected. It follows that the branches of the double-layer capacitance and the Faraday impedance can also be separated. The advanced homogeneous models [29, 62, 107, 130, 176, 178] consider diffusion–migration transport of electrons and ions as mobile charge carriers in a uniform medium, coupled with a possible nonequilibrium charge transfer across the corresponding interfaces at the boundaries of the ﬁlm. The contributions of the capacitive charging of the metal/polymer and polymer/electrolyte interfaces have been taken into account a posteriori by inserting one or two capacitive elements in parallel with the charge transfer resistance in the equivalent circuit. The uniform ﬁlm model has also been elaborated by introduction of an adsorption pseudocapacitance and a resistance connected with the charging/discharging processes within the ﬁrst layer of the ﬁlm at the metal interface, as well as a CPE in order to describe the capacitor at the ﬁlm/electrolyte interface, considering the irregular geometry of the surface of the polymer network and the counterions binding sites of different energies [24, 106]. This may be considered as an inhomogeneous homogeneous model inasmuch as the properties of the ﬁrst layer differ from those of the bulk ﬁlm. The CPE elements have been used to describe both the double-layer capacitance and the low-frequency pseudocapacitance, their frequency-dependent nature being attributed to the nonuniformity of the electric ﬁeld at rough electrode surfaces [22, 88, 168, 180]. 3.5.2 ‘‘Heterogeneous’’ or ‘‘Porous Layer’’ Model

Within the alternative approach, the ﬁlm is considered as a porous medium [6, 52, 57, 58, 83, 138, 146, 147, 149, 158, 160]. Physically, it represents a porous membrane that includes a matrix formed by the conducting polymer and pores ﬁlled with an electrolyte. Mathematically, in this approach, the ﬁlm is modeled as a macroscopically homogeneous, two-phase system consisting of an electronically conducting solid phase and an ionically conducting electrolyte phase. Considering a planar geometry, each layer perpendicular to the electrode surface contains these two phases, and therefore it may be described at any point by two potentials depending on the time and the spatial coordinates. Each of the phases has a speciﬁc electric resistivity, and the resistivities of the two phases, are interconnected continuously by the double-layer capacitance

3.5 Models of Polymeric Layers

(or a more complicated element) at the surface between the solid phase and the pores. A further interconnection results from the charge transfer at the surface of pores. There is also an electron exchange between the regions in the polymer having different degrees of oxidation [52, 138, 149, 158]. Charge transfer within the material is determined by diffusion process. In the advanced porous membrane model, inhomogeneous resistivities are considered. By the use of this model, the low-frequency CPE can be interpreted [138], as well as two sublayers having different resistivities are assumed. While considerable efforts having been spent on the elaboration of the model of faradaic impedance, is an adequate description of the double-layer charging effects has mostly been neglected. The essential problem is that the Randles–Ershler approach, that is, when the interfacial charging is described by a double-layer capacitance in parallel to the faradaic branch, is justiﬁed in the presence of an excess of a supporting electrolyte, which strongly diminishes the electric ﬁeld inside the system so that the transport of each electroactive component corresponds to pure diffusion, and the interfacial charging is realized mostly by the supporting electrolyte owing to its higher concentration. As a result, current passage across the transport zone (which includes the diffusion and interfacial layers) takes place as the sum of two noninteracting partial currents – those of the electroactive species and those of the background electrolyte. Therefore, the impedance of this region (which is equal to the overall impedance without ohmic resistances) can be represented by a parallel combination of the impedances of these two branches. Evidently, this reasoning does not hold any more for more complicated systems without a background electrolyte, in particular, for those containing two mobile charge carriers. 3.5.3 Theories Dealing with Two or Three Charge Carriers

If the same charged species take part in both the electrode reaction and the double-layer charging, the interfacial processes are coupled to the same ﬂux of the electroactive component. Moreover, since the distributions of the charged species inside the ﬁlm are interrelated owing to the electroneutrality condition and the self-consistent electric ﬁeld, their transport cannot be considered as pure diffusion. This is the case where at least one of the ions of a binary electrolyte participates in the charge transfer process or crosses the interface; a similar situation also arises when charging of an electrochemically active polymer via electron transport is accompanied by the movement of the charge compensating ions, that is, when mixed electronic–ionic conductivity prevails. A detailed analysis of the effects of the interfacial charging of surface ﬁlms with two mobile charge carriers on the impedance spectra has been discussed by Vorotyntsev et al. in detail by using the homogeneous model and taking into account the corresponding interfacial thermodynamics [175, 177]. This problem has also been analyzed within the framework of the porous membrane model [52].

65

66

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

It should be mentioned that a theoretical model involving diffusion and migration charge transport mechanism with three charge carriers has also been developed by L´ang and Inzelt [107]. The essential feature of this model is the assumption of a coupling of the oscillation amplitude of the concentration for the charge carriers. The derivation of the impedance function was possible, and a good ﬁt has been achieved over a wide potential range by using the general functional form of the impedance containing 12 parameters and the data obtained for poly(o-phenylenediamine) electrode used as a test system. However, it became clear that only the uncompensated ohmic resistance and the L/D2 ratio could be determined unambiguously. It was found that several parameters were strongly correlated. The simpliﬁcation of the general formula resulted in similar equations that have been derived by Vorotyntsev et al. [178] and Mathias and Haas [130, 131] when two mobile charge carriers and diffusion–migration transport have been considered. The case of three charge carriers is rather general in conducting polymer systems because, beside the transport of the electrons and counterions, very often, hydrogen ions also participate in the charge transport and charge transfer processes in the course of the redox transformation of the polymers. 3.5.4 Brush Model

On the basis of the observation that in many cases electrochemically active constituents of the electrolyte can react at the metal surface, for example, oxide formation and reduction at Au and Pt, and also hydrogen adsorption at Pt, which can take place, it was concluded that the polymer chains are attached to the metal by only a few points or at islands with a small area, like a brush. Experimental evidence is presented in Figure 3.7, which shows the cyclic voltammogram obtained for a thick (L = 2900 nm) poly(o-phenylenediamine) (PPD) ﬁlm deposited on gold. Cyclic voltammetric waves typical for gold oxide formation and reduction, respectively, appear at high positive potentials beside the PPD redox transformations that occur between −0.2 and +0.2 V. It should be mentioned that no decomposition of the polymer ﬁlm was observed [110]. It follows that the metal surface is not fully covered by the polymer, as assumed in the majority of the models, and that the solvent molecules ﬁlling the micropores and nanopores are in contact with the substrate surface. (It is assumed that in the case of such a thick ﬁlm, macropores do not reach the metal surface.) According to the ‘‘brush model’’ developed by L´ang et al. [110–112], the polymer chains are linked to bundles containing nanopores and micropores. Between the bundles, there are macropores of considerably greater cross-sections than that of the micropores. A distribution of short and long chains is also considered. The ratio of short and long chains may depend on the surface roughness of the substrate. The schematic picture of the structure of a polymer ﬁlm grown on smooth and rough surfaces, respectively, is shown in Figure 3.8.

3.5 Models of Polymeric Layers

1.0 0.5

I (mA)

0 −0.5 −1.0 −1.5 −2.0 −0.4

0

0.4 0.8 E (V) vs SSCE

1.2

1.6

Figure 3.7 Cyclic voltammogram obtained for an Au|PPD electrode in contact with 1 mol dm−3 HClO4 . Film thickness: 2900 nm; roughness factor: 1.71; scan rate: 50 mV s−1 . (Adapted from [110].)

Electrolyte solution Pores "Long" chains (bundles)

Ru

(a)

Zl

Zs

Micropores (c) "Short" chains (bundles)

(d)

Substrate (b)

Figure 3.8 Schematic picture of the structure of a polymer ﬁlm grown on smooth (a) and rough surfaces (b), respectively, a section of a bundle with micropores (c), and the equivalent circuit proposed (d). Ru : the uncompensated ohmic resistance;

Zl : the impedance which is attributed to the conductivity path along the long chains and long micropores; Zs : the impedance of the short chains with short micropores connected to the long pores. (Adapted from [110].)

67

68

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

On the basis of these ideas, the following theoretical models have been derived and applied in the analysis of impedance spectra obtained for Au/PPD electrodes [110]. The Z impedance corresponding to the double-channel transmission line model can be given by the expression 2f z1 z2 z2 + z22 +f 1 L+ coth L/f (3.22) Z= z1 + z2 z1 + z2 sinh L/f where the element z1 is the impedance per unit length of the transport channel in the polymer phase, z2 is the impedance per unit length of the transport channel in the pores, L is the thickness of the ﬁlm, and f = [z3 /(z1 + z2 )]1/2 where z3 represents the speciﬁc impedance at the inner interface, which corresponds to charge transfer and charging processes [142]. Equation (3.22) can be transformed in the following form: L L z1 z2 (z1 − z2 )2 (z1 + z2 )2 +f (3.23) coth tanh +f Z=L z1 + z2 2 (z1 + z2 ) 2f 2 (z1 + z2 ) 2f The impedance corresponding to the homogeneous model [107, 178] can be given as PC PB L L coth + tanh (3.24) Z = RA + s 2s s 2s where, in the model of Vorotyntsev et al. [178], RA , PB , andPC are frequencyindependent elements, while in the model of L´ang and Inzelt [107], they may be frequency dependent. In the simplest cases s can be expressed as

iω (3.25) s= D∗ with a frequency-independent D∗ , representing the effective diffusion coefﬁcient of the moving species. It can be seen that Equations 3.23 and 3.24 have similar mathematical structures, if it is assumed that z1 and z2 are resistances per unit length, and z3 is a pure capacitance. For this special case, R1T R2T 1 (R1T + R2T )2 (iω)−1/2 1/2 Z= + coth + R C (R ) (iω) 1T 2T 3T 1/2 R1T + R2T 2 2 (R1T + R2T )3/2 C3T −1/2 2 1 (R1T − R2T ) (iω) 1/2 + (3.26) tanh + R C (R ) (iω) 1T 2T 3T 1/2 2 2 (R1T + R2T )3/2 C3T where R1T and R2T are the total resistances distributed in the polymer channel and in the ionic channel, respectively, and C3T is the total capacitance of the pore walls. In order to simplify the notation, Equation (3.26) can be rewritten in the following form:

P2∗ P1∗ (3.27) coth F ∗ (iω)1/2 + tanh F ∗ (iω)1/2 Z = R0 + 1/2 (iω) (iω)1/2

3.5 Models of Polymeric Layers

69

800 0.02407 Hz

700

4

500

3 log(ZΩ)

−Z ′′ (Ω)

600

400 300

0.2639 Hz

200

2

1

100 0

0 0

100 200 300 400 500 600 700 800

Z ′ (Ω)

(a)

−2

−1

0

1

2

3

4

log (f (Hz))

(b)

0 −2 log((Y ′′ (Ω−1))/(w (Hz))

−10 −20 J (°)

−30 −40 −50 −60 −70 −80 −90 (c)

−3 −4 −5 −6

−2

−1

0

1

2

3

log (f (Hz))

Figure 3.9 Impedance spectra obtained for an Au|PPD electrode in contact with 1 mol dm−3 HClO4 at different potentials: −0.075 V (∗ or and ); 0.025 V ( and ); 0.05 V (• and ◦); 0.05 V ( and ); and 0.1 V ( and ). The roughness factor of Au

−2

4 (d)

−1

0

1

2

log (f (Hz))

is fr = 2.41. Simulated curves are indicated by continuous lines and open symbols. (a) Complex plane; (b) log |Z| versus log f; (c) ϑ versus log f ; and (d) log Y /ω−1 versus. log f plots. (Adapted from [110].)

Equation 3.27 can be modiﬁed heuristically by introducing an exponent β in order to describe the anomalous behavior:

P2 P1 (3.28) coth F (iω)β + tanh F (iω)β Z = R0 + (iω)β (iω)β where parameters R0 , P1 , P2 , β, and F are frequency independent and real. The introduction of the CPE element is justiﬁed since the distributed polymer/solution interface does not respond as an ideal capacitor. The impedance of the electrode can be represented by an equivalent circuit with parallel combinations of two impedances. The two impedances belong to the individual branches of long chains and long micropores, as well as short chains with short micropores connected to long pores, that is, Equation (3.28) can be used for both impedances completing the impedance expression with an ohmic resistance that corresponds to the solution resistance but may also involve the ohmic resistance of the long pores. At a given potential, the total impedance can be described by the following

3

4

70

3 Electrochemical Impedance Spectroscopy (EIS) for Polymer Characterization

function: ZT (ω) = Ru +

1 1 (ω) + (ω) Z Zs

(3.29)

The complete expression of the impedance contains 11 parameters. On the basis of the mathematical structure of Equation (3.29), the parameters are expected to be strongly correlated. It was indeed found, therefore, that the number of parameters has been decreased on the basis of reasonable assumptions. However, it was executed in such a way that the contributions of the individual branches to the total capacity of the ﬁlm could be determined. Figure 3.9 illustrates the goodness of ﬁt. It has been concluded that the low-frequency distortion effect (CPE behavior) is most likely connected with the ﬁlm nonuniformity; however, the surface roughness of the underlying metal substrate inﬂuences the ratio of the long and short polymer chains. At low frequencies, the characteristics of the impedance spectra are mainly determined by the long polymer chains. With the help of these models, reasonable values for different parameters characterizing the polymer ﬁlm electrodes can be derived.

3.6 Summary

EIS has become the most powerful technique to obtain kinetic parameters such as rate of charge transfer, diffusion coefﬁcients (and their dependence on potential), double-layer capacity, pseudocapacitance of the polymer ﬁlm, and resistance of the ﬁlms. The essential elements of this technique are discussed here, in order to help the researchers working on electrochemically active polymer ﬁlms, and to encourage them to use EIS as a basic tool for characterization of polymer ﬁlms and membranes.

Acknowledgment

This work was supported by the grants OTKA K71771 (GI) and OTKA67994/OMFB-01078/2007 (CGL) from the National Scientiﬁc Research Fund, Hungary, and GVOP-3.2.1-2004-040099 National Research and Technology Ofﬁce NKTH-OM-00123/2008 (GI).

References 1. Bard, A.J. and Faulkner, L.R. (2001)

Electrochemical Methods, 2nd edn, John Wiley & Sons, Inc., New York, pp. 368–415. 2. Retter, U. and Lohse, H. (2002) Electrochemical Impedance Spectroscopy in

Electroanalytical Methods (ed. F. Scholz), Springer, Berlin, pp. 149–166. 3. Lasia, A. (1999) Electrochemical Impedance Spectroscopy and its applications in Modern Aspects of Electrochemistry, vol. 32, Chapter 2

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77

4 Recent Trends in Polypyrrole Electrochemistry, Nanostructuration, and Applications Pierre Audebert

4.1 Introduction

This chapter covers recent original research on polypyrrole, especially that pertaining to the last four or ﬁve years. It focuses on the synthesis and electrochemical properties of polypyrrole and polypyrrole derivatives, at the same time describing some interesting chemical syntheses and related composites. Physicochemical properties and related applications are mainly discussed, with a brief mention of the ﬁeld of related biosensors and bioactive polymers. In fact, a more detailed discussion on these topics would make this article too long; moreover, there are several chapters in this book that are dedicated to this area. Since its original discovery by Diaz et al. [1a], polypyrrole progressively tends to become an old polymer. Meanwhile, the trend of the whole polypyrrole related research ﬁeld follows its way, including many more technical progresses and focusing more on precise applications. However, it is possible to distinguish major and minor ﬁelds within this area. While research on new monomers has slowed down, investigations of new polymerization methods, both chemically and electrochemically, are still being carried out. The ﬁeld of electrochemistry, for example, has seen a notable breakthrough in the use of ionic liquids for electrochemical synthesis, and there is a clear increase in research in the direction of syntheses of all kinds of prestructured media such as templates, emulsions, gels, nanoparticles arrays, membranes, and so on. After the initial excitement on research in the ﬁeld of accumulators, enthusiasm in this area of application has dwindled; however, related applications such as supercapacitors continue to stimulate a steady amount of research work. Furthermore, the main focus of scientists is now on applications, especially on the design of polypyrrole derivatives ﬁtted to speciﬁc purposes. Interesting reviews covering this ﬁeld have appeared in recent years [1b,c]. These will be referred to in the chapter. This chapter is divided into three parts. The ﬁrst part discusses the advances in polypyrrole synthesis, including new original monomers that appeared in the last ﬁve years, new polymerization procedures, and composites; a few recent fundamental studies on redox processes in polypyrrole are also mentioned. The Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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4 Recent Trends in Polypyrrole Electrochemistry, Nanostructuration, and Applications

second part is devoted to nanostructuration of polypyrrole. Finally, the last part discusses the applications, starting from improvements in traditional applications (supercapacitors, electrocatalysis, actuators, etc.) and going on to more recent developments such as corrosion protection, trying not to exclude marginal but noteworthy recent applications.

4.2 Advances in Synthetic Procedures – New Polymers 4.2.1 New Monomers and Polymers

After the remarkable variety of new monomers and subsequent polymers prepared in the late 1980s and 1990s by several groups [2] and maybe because of the limitation of the synthetic possibilities, activity in this area of research has slowed down. One should, however, acknowledge some recent works of Zotti [3], Reynolds [4], and Audebert [5], who steadily continued to provide new functional pyrrole monomers. Most of their work was related to production and applications of low band gap materials, and Reynolds’ alkyldioxopyrroles [4] achieved notable success in this direction. However, new functional polymers were also prepared, with optically active groups, for example [3b]. More classical monomers such as α,ω-bis-(3-pyrrolyl)alkanes [2e, 6], have been prepared, as also polypyrrole-bearing silane chains [7] and original, attached counterions [8]. However, the conductivity and the properties of the polymers are not very different from what had been obtained earlier, with the exception of poly(ProDOP), which exhibits particularly interesting electrochromic properties [4]. A very interesting and, to some extent, rarely investigated topic is the preparation of ﬂuorinated polypyrrole. Indeed, the preparation of poly(diﬂuoropyrrole) has been reported in the literature [9]. However, although the polymer displays very interesting characteristics such as a very high redox potential and a high doping level, its low stability, especially in residual water, precludes its use in devices. However, perﬂuoromethyl derivatives could be an attractive alternative, as reported in a recent theoretical analysis [10a] and in the related experimental work describing several preﬂuorinated conducting polymers, including N-perﬂuoroalkylated polypyrrole [10b]. 4.2.2 Fundamental Research

Some fundamental ab initio calculations have been performed on the electropolymerization of polypyrrole [11]. The aim of these studies, as in the earlier ones, was to attempt to demonstrate the best possible methods for electropolymerization of polypyrrole and also to improve its conductivity. The mobility of polarons and bipolarons continues to attract the attention of researchers [12]. Similar claims of a polypyrrole composite with a very high conductivity (4000 S cm−1 ) have been

4.2 Advances in Synthetic Procedures – New Polymers

reported [13], but then these have been contested by another team [14]. However, in the 1990s, a conductivity of up to 1000 S cm−1 had already been reported for a polypyrrole/Naﬁon composite directly grown in a Naﬁon gel [15], which, if one considers that the actual polypyrrole content of the composite was about 10–15%, makes the extrapolated value even higher for the conducting polymer itself. Meanwhile, fundamental studies on the redox mechanisms of polypyrrole [12] are now attracting much less attention though scattered papers still continue to appear in this area [16]. It should be emphasized that some years ago, several interesting papers reported by Heinze and coworkers [17] gave an almost complete picture of the polymerization mechanism of pyrrole and its derivatives in the most possible classical conditions. 4.2.3 New Polymerization Methods

The last few years have seen a tremendous improvement in the polymerization techniques for pyrrole and its derivatives. While nonoxidative polymerization of pyrrole is quite rare and difﬁcult, there has been a tremendous increase in the number of oxidative methods, both chemical and electrochemical. The main development during the last 10 years was the use of ionic liquids for the electrochemical synthesis of polypyrrole [18] (and other various conducting polymers), which led to a signiﬁcant increase in some properties, in particular, the cyclability, along with an increase in the doping level. Among recent attempts, an increase in the number of interfacial polymerization methods can be noted. Instead of simply mixing the oxidant and pyrrole, successive impregnation can lead to interesting composites. Electrochemical polymerization of pyrrole at biphasic interface, as well as from the gas phase, has also been described. Beautiful crystalline nanoneedles (Figure 4.1) can be obtained from interfacial polymerization of pyrrole and other monomers at a water/organic solvent interface [19]. Finally, it is worth mentioning two very original recent polymerization processes – pulse radiolysis polymerization [20] and two-photon polymerization [21] with the help of a two-photon sensitized photooxidant. Since the polypyrrole formed is black, this could be an interesting opening for enhancing the performance of two-photon absorbents. (a)

(b)

100 nm

Figure 4.1 Nanoneedles of polypyrrole produced by chemical polymerization at the chloroform/water interface. (Adapted from [19].) The insert shows the crystalline character of a needle at higher magniﬁcation.

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4 Recent Trends in Polypyrrole Electrochemistry, Nanostructuration, and Applications

4.3 Nanostructuration of Polypyrrole

Following the new trend to fabricate original nanostructures, many researchers have prepared nanosized polypyrrole by using several techniques. The work can be divided into two main parts: the ﬁrst deals with pure nanosized polypyrrole, in the shape of nanoparticles, nanowires, nanocontainers, and so on, and, the second focuses on the preparation of polypyrrole-based nanocomposites; the size of the polymer is most often controlled by the preexisting nanostructure of the precursor, silica being predominantly used as precursor. 4.3.1 Nanostructuration of Polypyrrole

The historical way to obtain nanosized polypyrrole is templated polymerization. This has been achieved primarily by Martin [22], who used several types of membranes to electrochemically grow polypyrrole. This approach has been continued by varying the substrates, such as various kind of oxides [23] or porous polymeric membranes [24], and has been reviewed by De Paoli [24a]. The nanostructuration is obtained in four steps: (i) generation of the nanostructured membrane, (ii) soaking with the monomer solution, (iii) oxidative polymerization, and (iv) dissolution of the membrane by an appropriate reagent, which is able to dissolve the template while keeping the polypyrrole intact. Since pyrrole is by far the easiest monomer to polymerize (when compared, for instance, to aniline, thiophene, etc.), this approach has been more often adopted and successfully realized with polypyrrole than with any other conducting polymer. Another advantage of pyrrole is that it has a high vapor tension and is therefore readily evaporated. This makes possible the formation of very uniform coatings on substrates that had been previously exposed to an oxidizing agent solution [25]. However, besides nanostructuration, there are several other methods used to prepare nanoparticles of polypyrrole. For example, one report describes the use of a biphasic system for electrodeposition of polypyrrole nanowires [26] or a surfactant mixture [27]. This allows the preparation of luminescent polypyrrole nanoparticles [27b], polypyrrole nanoobjects of various sizes and shapes [27c], or nanostructured ﬁlms (Figure 4.2) [27d]. Some works also aim to seed the polymerization; nanotubes [28] have been efﬁciently used in this direction, since they are both conducting and nanosized. 4.3.2 Polypyrrole Nanocomposites

Of late, polypyrrole has been intimately mixed with an extremely wide range of precursors. As already mentioned, silica is the main precursor used to amalgamate polypyrrole [29], since the preparation of silica nanospheres is a well-known and well-documented process. In most preparations, silica nanospheres are formed by

4.3 Nanostructuration of Polypyrrole

(a)

(b)

500 nm

Figure 4.2

500 nm

Picture of two nanoporous polypyrrole ﬁlm. (Adapted from [27d].)

the classical [30] or a modiﬁed [31] St¨ober process and the resulting silica nanoparticles are impregnated with pyrrole and submitted to oxidative polymerization. Since the particles are highly porous, polymerization can proceed efﬁciently and a conductive composite can be formed. It should be noted that silica is not the unique support for the fabrication of polypyrrole composites. Analogous methods have also been used with various oxides such as ZnO [32], TIO2 [33], Al2 O3 [34] as a nonlimitative list of examples; it should be noted that electrochemical polymerization is systematically preferred [34] in the case of alumina. The advantage of silica is that the conducting polymer can be inserted inside or outside the particle, according to the process of synthesis. It is also possible to coat composite particles with an additional silica layer so that they become conducting inside but surrounded by a nonconducting shell. Such interesting structures have found applications, for example, in dielectrophoretic devices [29f ] (see Section 4.4). Precious metals such as gold [35] and silver [36] have also been widely used, because, in this case, the synthesis of the particles by a standard method such as that of Turkevitch is easy as well as convenient. However, anchoring the polymer is more difﬁcult and often requires the metal particle to be covered with a covalently bound layer like a functionalized thiol layer. Seeding the polypyrrole formation with nanotubes [28] allows to postfunctionalize the composite with metals or oxides, by simply exchanging metal salts in the polypyrrole and subsequently reducing it. This has been performed with ruthenium and platinum [37]. An interesting paper describes the preparation of a glucose-sensing electrode by simultaneous formation of polypyrrole with platinum nanoparticles, while embedding the glucose oxidase within the same step [38]. The range of metals used for making the nanocomposites is now larger; recently, for example, the growth of polypyrrole around silicon nanoparticles has been described [39]. Figure 4.3 shows the nice structure of the core-shell particles obtained, which may ﬁnd application in hybrid ﬁeld-effect transistor (FET)-type devices. More complicated structures can also be prepared, such as oxide–polystyrene– polypyrrole nanoparticles. Often the polystyrene is adsorbed onto oxide particles, and then the classical polymerization of pyrrole by an impregnation/oxidation sequence is effected [40]. The advantage of such particles is the high ﬂexibility of the possible structures that can be formed by selective dissolution of one

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4 Recent Trends in Polypyrrole Electrochemistry, Nanostructuration, and Applications Figure 4.3 Silicon nanopowder PPy composites. (Adapted from [39].)

PPy

Si 100 nm

component of the system. For example, dissolving the polystyrene polymer leads to nonbonded (ﬂoating) oxide cores in a hollow polypyrrole nanocontainer [41]. Inverse opals have been prepared chemically or electrochemically by templating polypyrrole with polymer nanoparticles [42]. An especially interesting paper [42a] describes the preparation of an inverse opal of polypyrrole taking advantage of the extremely narrow polydispersity of polystyrene latex particles, which can therefore form a direct opal onto a substrate. When this direct opal is soaked ﬁrst in an oxidant and then exposed to pyrrole vapor, the polypyrrole is formed in the interstices of the opal. After dissolution of the polymer opal template is achieved, a perfectly regular inverse opal of conducting polypyrrole is produced (Figure 4.4). Sometimes, electrochemistry can prove very advantageous in preparing polypyrrole nanostructures. A speciﬁc case concerns the use of carbon nanotubes. Since they are conducting, a carbon nanotube carpet can be used as an electrode, and performing the electropolymerization reaction through ultrashort pulses allows to coat the nanotubes with an ultrathin layer of polypyrrole [43]. This has the advantage of drastically increase the conductivity of any composite material loaded with the modiﬁed nanotubes, because the coating enhances both the dispersion of the nanotubes and the intertube electrical connection. Following the same trend, (b)

(a)

235.6 nm

Figure 4.4

236.3 nm

(a) Direct templating and (b) polypyrrole inverse opal. (Adapted from [42a].)

4.4 Applications Cycle 5 Cycle 2 5

Cycle 1

Current (µA)

0 −5 −10 −15

400 nm 200 0 0 0 1000

2000 2000

−20

nm

3000 4000

4000

nm

5000

−25 −1.2 −1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 E / V vs SCE

Figure 4.5 Electrodeposited polypyrrole nanoparticles, along with their cyclic voltammetry response featuring no capacitive tailing. (Adapted from [44].)

ultrashort pulses were utilized to polymerize pyrrole on a gold electrode polished to the nanometer scale [44]. This produces scattered nanoparticles on the gold surface, which are highly conducting. The latter display cyclic voltammograms that do not exhibit the classical capacitive tailing of standard polypyrrole, although the oxidation and the reduction processes show a marked hysteresis (Figure 4.5). Finally, some recent work has shown that while the use of weak acid counteranions leads to a passivation of the polypyrrole ﬁlm [45a], the addition of small quantities of unreactive (e.g., perchlorate) counterions to the same medium results in a drastic modiﬁcation of the morphology of electrodeposited polypyrrole. Elegant nanotubes of various lengths and morphology (Figure 4.6) were thus obtained without any template [45b], but the reason for this occurrence is still subject to conjectures.

4.4 Applications

Nowadays, in addition to the previously mentioned nanodevices, applications are the main driving force for preparing new polypyrroles and new polypyrrole composites or formulations including polypyrroles. As in the case of many other conducting polymers, accumulators have been the ﬁrst application of polypyrroles. While there was considerable research in this area during the 1980s and even in the 1990s, it has more or less come to a standstill now. The reason is probably the success of new electrode materials for energy storage such as nanodivided metal

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4 Recent Trends in Polypyrrole Electrochemistry, Nanostructuration, and Applications

200 nm

Figure 4.6 Templateless electrodeposited polypyrrole nanotubes, in the presence of both a weak acid and a noninteracting counterion. (Adapted from [45b].)

oxides or chalcogenides, which now outrank most conducting polymers. Similarly, the relatively ancient development of polypyrrole-based modiﬁed electrodes has decreased with the exception of applications in the area of biosensors, which are not being discussed here (some review papers already exist [46]). Electrochromism with polypyrrole-derived polymers still continues to be published, although PEDOT-derived materials happen to be generally much more effective. Interesting ﬂuorescent polypyrrole nanoparticles have been prepared recently [47]. Some other applications of polypyrrole have emerged in the 1990s and continue to be the subject of increasing research effort, while new and original applications also appear regularly from time to time. Among the conventional applications that appeared more than a decade ago, one especially has gone through considerable development – the search for efﬁcient anticorrosion coatings. Parallel to these applications is the use of polypyrrole derivatives for high-energy capacitors, which always constitute an attractive research topic. 4.4.1 Batteries and Supercapacitors

Although, as previously mentioned, there is no ongoing research on conducting polymers for standard batteries, research for better polymers to be used in the preparation of supercapacitors is still going on. For example, pulse polymerization has been shown to lead to a performing polymer in this direction [48]. The search for better contacts in many types of accumulators has triggered technical studies. It seems that polypyrrole remains one of the best conducting ﬁllings between the active matter of a device and the metal electrode, in particular with divided metal supports. Likewise, many composites such as polypyrrole/polyoxometallates [49] have been used for such purposes [49b, 50]. It seems that the large size of the doping

4.4 Applications

ions improves the porosity of the composite and, therefore, its performance as a supercondensator. The use of polypyrrole in fuel cells has also been attempted recently [51]. Although some results are encouraging, it seems unlikely that this polymer could efﬁciently resist the harsh conditions of a fuel cell over its lifetime. 4.4.2 Actuators

Since the pioneering work of Otero [52] and Inganas [53], and further developments by Smela [54], the ﬁeld of polypyrrole-based actuators has narrowed down, but despite this, publications dedicated to the improvement of the strength of the actuator continue to appear [55]. However, it seems that the performances of the devices have reached a limit now. In addition, some successful work has helped improve the kinetics of motion [56] of the actuators, which now encompass the 10-Hz frequency range. 4.4.3 Anticorrosion

Application of conducting polymers for anticorrosion protection dates back to the end of the 1990s especially to the work of Lacaze et al. [57]. The fact that the polymer is redox active allows to deﬁne the electrochemical potential at the metal/polymer interface and therefore, importantly, bend the Fermi level at this interface; this had been recognized even earlier. This prevents attack by aggressive oxidants like oxygen (humid corrosion). This process is totally different from the protection provided by classical agents that behave either as sacriﬁcial electrodes or simply like physical barriers. Initially, however, the coatings were too coarse and porous, allowing the penetration of corroding agents, which often exfoliated the metal/polymer interface, leading to a rapid loss of the protection. The relative frailty of polypyrrole, which is also easily submitted to chemical degradation or dedoping from atmospheric agents, was also a drawback for this kind of application. However, researchers quickly learn to protect the polypyrrole with a strong mechanical barrier. Solgel coatings and related polymers proved particularly efﬁcient in this regard, as noted by Audebert and Pagetti [58]. Along a parallel path, many works aimed and succeeded at polymerizing pyrrole onto several types of oxidizable metals. Now, electropolymerization techniques have been developed for not only almost every corrodible metal, including course steel [59], but also for more reactive metals such as copper [60], nickel [61], zinc [57a, 62], lead [63], and even aluminum [64]. In some cases, composite paints of polymers have been successfully elaborated [65], and polypyrrole has been doped with metal pigments such as zinc phosphate [66] with a notable improvement in the anticorrosion protection. Despite the fact that the area might, at ﬁrst sight, appear slightly technical from the number of works in the ﬁeld (besides the biosensing applications not reviewed here), the anticorrosion ﬁeld has by far been the most attractive and

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4 Recent Trends in Polypyrrole Electrochemistry, Nanostructuration, and Applications

extensively investigated topic on polypyrrole derivatives in recent years. If one compares these with other more robust conducting polymers featuring analogous redox properties like polyaniline, the relative success of polypyrrole is probably a consequence of the ease of the electropolymerization of this polymer in various useful conditions, and maybe also the ease with which it forms processible composites. The adherence of the polymer layer onto the metal has been considerably improved, thus increasing the duration of corrosion protection. In this area also, many types of composites speciﬁcally aimed at increasing the overall robustness of the polymer in view of anticorrosion applications have been investigated with varied levels of success. 4.4.4 Miscellaneous

Electrorheology is among the most notable of the emerging applications of polypyrrole. In fact, an electrorheologic ﬂuid is made of (nano)particles dispersed into an low-dielectric-index viscous liquid (typically an oil); application of an electric ﬁeld results in the organization of the particles into stacked phases, which, in turn, induces a marked increase in the macroscopic viscosity of the ﬂuid. The optimization of the dielectrophoretic forces requires a high dielectric index contrast between the ﬂuid solvent and the particles responsible for the electrorheologic behavior. Since most of the high-index materials are conductive, the preparation of particles requires the existence of a coating of nonconductive layer insulating the nanoparticle core, to avoid short circuits when the system operates. For all these reasons, conductive polymers, especially polypyrrole, are target materials for this purpose. Current means to prepare the particle involve in situ polymerization into a nonconducting particle (e.g., made from mesoporous zirconia that serve as a matrix [67]) or preliminary polypyrrole particles preparation, followed by the deposition of a nonconducting coating like silica [29f ]. Several oxide particles have been used in this manner. Dielectric ﬂuids have been prepared and demonstrated to be efﬁcient to promote viscosity increase or to induce particle motion for low energy displays [29f ]. Electrochemical monitoring of an active substance through the redox state of a polypyrrole membrane is also a recent application. However, ﬁne-tuning of the delivery is usually not possible. Several types of chemical sensors have recently been prepared using different forms of polypyrrole prepared through different approaches, for example, polypyrrole packed into a syringe [68] or used for red wine control. Various types of polypyrrole ﬁlms have also been deposited on a chip ﬁtted with electronic control [69], and operated into fully integrated systems. Polypyrrole is used for ammonia sensing by depositing it on titanium dioxide [70]; one work [70c] shows that a single PPy nanowire can be efﬁciently used as the sensing element. A polypyrrole composite has also been tested for folic acid oxidation [71]. Other unexpected applications continue to emerge, such as thermal transduction of NIR radiation [72].

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4.5 Conclusion

As can be seen, active research on polypyrrole continues to take place. However, one can note that while interest in the fundamental properties of the polymer and its mechanisms has considerably decreased, there is continuous work going on in the ﬁelds of synthesis of composites and applications of polypyrrole and its derivatives. Actually these two ﬁelds are closely related, since polypyrrole composites are frequently designed with a precise application in view or at least with the aim of the improving a given set of properties of the composites. While improvement in conventional properties, such as conductivity, is not considered important, the ability of polypyrrole and derived products to possess electrochemical properties, especially corrosion protection, continues to be at the root of many more applications. One characteristic of the most recent work on polypyrrole is obviously that of a greater focus on technical aspects. This trend is however questionable, since the later steps of the process of polymerization and the preparation–conductivity relationship have not yet been completely understood, while apparently the efforts of the researchers in this area have turned toward better performing polymers such as polythiophenes and PEDOT. However, many questions on polypyrrole and its composites have not yet been fully explored and these will continue to be the subjects of future work for some years to come.

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5 Electropolymerized Azines: a New Group of Electroactive Polymers Arkady A. Karyakin

5.1 Introduction

Electroactive polymers are an independent new class of organic materials. Being exposed to certain electrolyte solutions, these materials exhibit both electronic and ionic conductivity. It is accepted that the existence of electronic conductivity was ﬁrst proved in the case of polyacetylene [1]. However, the intensive study of electroactive polymers started only after the discovery of electropolymerization of pyrrole [2] and aniline [3]. Electropolymerization is a powerful tool for the development of modiﬁed electrodes. One of the advantages of the technique of electropolymerization is that it provides a simple method of targeting for selective modiﬁcation of multielectrode arrays. In addition, electropolymerized materials usually possess some unique properties that are not characteristic of the corresponding monomers.

5.2 Electropolymerized Azines as a New Group of Electroactive Polymers

The formation of electroactive polymers upon reduction or oxidation of different organic compounds became one of the major subjects of modern electrochemistry after the discovery of conducting polymers. Among these compounds, the azines represent a group that has already found wide use as redox indicators and mediators in various branches of biochemistry and bioelectrochemistry. This group can be deﬁned by the general formula: H N X

Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

94

5 Electropolymerized Azines: a New Group of Electroactive Polymers

where X = N, S, O for phenazines, phenothiazines, and phenoxazines, respectively. 5.2.1 Electropolymerization of Azines

It is generally accepted that acidic media are preferable for the deposition of electroactive polymers. This is conﬁrmed by electropolymerization of pyrrole, aniline, thiophene, and so on. However, among azines, only thionine (Figure 5.1) was successfully electropolymerized from acidic solutions [4–6]. On the other hand, in the case of methylene blue (Figure 5.1) in such conditions, only a minor amount of electroactive polymer was obtained [7, 8]. Moreover, the growth rate of poly(methylene blue) increased as the pH is increased, indicating basic solutions to be the optimal media for polymerization [7]. These unique properties of methylene blue electropolymerization and the structure of the monomer molecule allowed poly(methylene blue) to be considered as representative of a new group of electroactive polymers [9]. Electropolymerization of azines from neutral and basic aqueous solutions occurs due to irreversible oxidation of the monomer. The latter process is seen in cyclic voltammograms as an asymmetrical peak current at high anodic potentials (Figure 5.2). The electropolymerization was noticed from the increase in peak current attributed to the monomer-type redox activity, as well as from the appearance and growth of the new set(s) of peaks at more positive potentials (Figure 5.2a). Further electrochemical and spectroelectrochemical [10] investigations have conﬁrmed that the new redox activity appearing in the course of the deposition can be attributed to the resulting polymer. Both monomer-type and polymer-type sets of peaks remained in cyclic voltammograms of corresponding modiﬁed electrodes recorded in monomer-free supporting electrolyte solutions. The subsequent electrochemical quartz crystal microbalance (EQCM) study has conﬁrmed formation of the polymer in the course of methylene blue irreversible oxidation in neutral and basic media [11]. Similar to methylene blue [7, 12], other azines, such as methylene green [10, 13], azure [10, 14], toluidine blue [10, 15, 16], brilliant cresyl blue [10], phenosafranine [17], and neutral red [10, 18, 19], have been successfully electropolymerized. The general properties of the electropolymerization processes are (i) the growth rate increases as the solution pH is increased making basic media preferable for electropolymerization and (ii) polyazines are characterized by an appearance of a new, polymer-type electroactivity in cyclic voltammograms. A special case is the electropolymerization of neutral red. First, it is polymerized from neutral solutions (pH 6). Second, the only monomer-type redox activity is seen in its cyclic voltammograms (Figure 5.2b). This is unusual for azines, and such observations can be easily explained by considering the properties of neutral red. A neutral solution was chosen for polymerization because at pH 6.8, neutral red monomer deprotonates and becomes insoluble in water [20]. So pH 6.0 is the most basic allowable solution. As was found from the pH dependences of

5.2 Electropolymerized Azines as a New Group of Electroactive Polymers

N +

H3 C N

N

S

CH3

Methylene blue

CH3

H 3C N +

H3 C N H3C

N

S

CH3

Methylene green

CH3

NO2 N +

H2N

S

H3C

N

H2N

S

CH3

N

Azure A

CH3

+

CH3

N

Toluidine blue

CH3 H 3C

N

H2N

O

+ N CH2 CH3

Brilliant cresyl blue

CH2 CH3 H3C

N

H2N

N

+

CH3

N

Neutral red

CH3

H N

H2N Figure 5.1

+

S

NH2

Thionine

Chemical structures of azines.

poly(neutral red) electroactivity at pH 6.0, the monomer-type and the polymer-type redox activities overlap and display single peaks in cyclic voltammograms. In more acidic solutions, these redox activities can be observed separately [9]. An appearance of the two types of redox activity (one similar to that of monomers, and the other at more positive potentials) in electropolymerized azines was conﬁrmed from cyclic voltammograms of the corresponding modiﬁed electrodes recorded in monomer-free solutions [7, 9]. Charge compensation upon charging of these electroactive polymers was initially attributed to entrapment of only the anions in polymer ﬁlms [11]. However, more recent studies on electropolymerized neutral red have also shown participation of inorganic cations and solvated protons in charge compensation [21–23].

95

96

5 Electropolymerized Azines: a New Group of Electroactive Polymers

0.1 mA cm−2

−0.4

0.4

0.8

E (V)

(a)

0.1 mA cm−2

−0.5 (b)

0.0 E (V)

0.5

Figure 5.2 Cyclic voltammograms of toluidine blue (a) and neutral red (b) electropolymerization, recorded at 50 mV s−1 in 0.01 M borate buffer pH 9.1 + 0.1 M NaNO3 ; 0.4 mM azine solutions.

Hence, electropolymerized azines, including thionine, which was also later polymerized from neutral aqueous solutions [24], can be considered as a new group of electroactive polymers. The reasons except for their unique chemical structure are (i) the increase of growth rate with the increase in pH, (ii) an appearance of a new polymer-type redox activity, and (iii) remaining monomer-type redox activity in the resulting polymer. 5.2.2 Hypothesis of Polyazine Structure

A hypothesis on the structure of polyazines has been made on the basis of their electrochemical and spectroelectrochemical properties, and, in addition, by considering some organic reactions that are able to simulate the polymerization process [9]. Cyclic voltammograms and visible and infrared spectra of the polyazines

5.2 Electropolymerized Azines as a New Group of Electroactive Polymers

indicate that the polymers contain conjugating chains and aromatic rings similar to those of the corresponding monomers. The only difference between them is found in the substituents of the aromatic rings. Hence, azine electropolymerization occurs without destruction of monomer rings. An appearance of polymer-type redox activity in polyazines can be explained as follows. Such redox activity occurring at more positive potentials compared to that of the monomer requires existence of the polymer in oxidized state. Visible spectra of the oxidized polymer indicate the presence of additional electron acceptors as ring substituents. At more negative potentials, when the polymer is reduced by more than 50%, both spectroelectrochemical and electrochemical properties of the polyazines are similar to those of the corresponding monomers. Assuming that the corresponding monomer units in the polymer are bound to each other, the former, in an oxidized state, can serve as electron acceptor substituents, causing the appearance of polymer-type redox activity. In order to build a model of polyazine structure, it is essential to analyze what kind of bonding between monomer units would occur in the polymer chain. Since we have not found the direct indication of that from spectroelectrochemical investigation of polymers, the only way to discuss monomer-to-monomer bounds in polyazines is to consider similar organic reactions. In the case of azines containing primary amino groups, we can assume that azine electropolymerization could be similar to polymerization of aniline. Polyaniline synthesis occurs in acidic media via ‘‘head-to-tail’’ bonding: the amino group binds to the aromatic ring. Indeed, the electropolymerization of thionine, which contain only primary amino groups, occurs in acidic media similar to aniline polymerization. However, the azines that contain one primary and one tertiary amino group are cannot be polymerized from strongly acidic media. As we have already pointed out, their polymerization rate increased as the solution pH was increased. This behavior is completely different from aniline polymerization. Moreover, methylene blue and methylene green do not contain any primary amino groups. Thus, polymerization of these azines cannot be explained in a manner similar to that of aniline. It is known that upon oxidation of aniline even in strong acidic media some amount of benzidine is formed [25, 26]. This indicates the possibility of ‘‘ring-to-ring’’ coupling upon aniline oxidation. Another evidence of this process is oxidation of N,N-dimethylaniline resulting in the formation of N,N,N ,N -tetramethylbenzidine [27]: CH3

CH3

H3C

N

N

CH3

H3C

N

CH3

The possibility of ‘‘ring-to-ring’’ coupling of aromatic amines allows one to expect that in polymer azines (for example, poly(methylene blue), Figure 5.3) a similar type of bonding between monomer units occurs. Taking into account that the destruction of methylene blue monomer causes the demethylation of amino group

97

98

5 Electropolymerized Azines: a New Group of Electroactive Polymers

H 3C N

S

H

N

+

CH3

N

CH3

N H3C N

N CH3

S

CH3

N H3 C N H 3C Figure 5.3

+

S

N

CH3 CH3

A hypothesis on poly(methylene blue) structure.

[20], we suppose the existence of another type of bonding in poly(methylene blue). We believe that monomer units could also be bound via amino bridges (Figure 5.3). The hypothesis ﬁrst formulated in [9] is that electropolymerization results in the polymer chain formed by azine molecules and the latter are bound to each other either via direct ring-to-ring coupling or through nitrogen bridges (Figure 5.3). This hypothesis was recently conﬁrmed by recognition of the polymerization intermediates by means of electrospray mass spectrometry [28]. Indeed, methylene blue dimers and trimers were observed at m/z values consistent with either ‘‘nitrogen-to-ring’’ coupling of the monomers or with ‘‘ring-to-ring’’ coupling of demethylated monomers during electropolymerization [28].

5.3 Polyazines in Electroanalysis

Conductive and electroactive polymers have found a wide application in electroanalysis. Electropolymerized azines are not an exception. Immediately after their synthesis, polyazines were discovered as electrocatalysts of various organic reactions, which has led to elaboration of the corresponding sensors. More recently, poly(methylene green) has been employed as a molecularly imprinted polymer (MIP) matrix for electrochemical sensing [29]. A potentiometric poly(neutral red) sensor showing high selectivity to the citrate anion was employed for its determination in soft drinks [30]. 5.3.1 Electrocatalysis by Polyazines

Among azines, the electropolymerized methylene blue and neutral red have attracted particular attention. Poly(methylene blue) was used for detection of hemoglobin in whole blood by its oxidation [31]. Conversion of vitamin B6

5.3 Polyazines in Electroanalysis

(pyridoxine hydrochloride) at poly(methylene blue)-modiﬁed electrode was found to be quasi-reversible, being diffusion-controlled at low scan rates and adsorption-controlled at high scan rates [32]. The detection of nitric oxide in the course of its oxidation to NO2 − or reduction to N2 O [33] was made possible by electropolymerized neutral red. Owing to its low redox potential, poly(neutral red) was used as an electrocatalyst for the reduction of BrO3 − and IO3 − [34]. Among catalytic applications of other azines, the poly(toluidine blue) ﬁlm electrode and its ability to detect nitric oxide released in liver homogenate [35] as well as the poly(Nile blue) sensing ﬁlm and its application in food analysis as a possible alternative for electrochemical detection of nitrite [36] can be mentioned. The determination of easily oxidizable compounds like ascorbic acid, dopamine, and serotonin at poly(phenosafranine) [37] is rather obvious. Recently, electropolymerization of methylene blue on carbon ionic liquid electrode and its electrocatalytic activity toward 3,4-dihydroxybenzoic acid [38] has been reported. However, the widest applications of electropolymerized azines as electrocatalysts of NAD+ |NADH conversion are described in the following sections. 5.3.2 Electropolymerized Azines as Advanced Electrocatalysts for NAD+ |NADH Regeneration 5.3.2.1 Dehydrogenase Enzymes and Electrocatalysis of NAD+ |NADH Regeneration Nicotinamide adenine dinucleotide (phosphate) (NAD(P)+ /NAD(P)H) dependent enzymes, which are commonly called dehydrogenases (CE 1.6), constitute the largest group of the enzymes known. The number of characterized dehydrogenases is more than 500. As in the case of a coupled redox reaction, these enzymes use the turnover of the same mediator – nicotinamide adenine dinucleotide. O

O C NH2 N+ R

C

2e− H+

NH2 N R

Obviously, the electrochemical regeneration of this cofactor is extremely important from both the fundamental and practical points of view. The most important biotechnological application is the development of biosensors for a great number of analytes, which covers the majority of key metabolites. In addition, this reaction can be used for the development of biofuel cells with different cheap fuels and would provide stereospeciﬁc electrosynthesis of organic compounds. However, (NAD+ /NADH) is the mediator with the lowest potential found in aerobic organisms (−0.56 V vs SCE, pH 7) [39]. It is not surprising that nature discovered how to prevent oxidation of its reduced form in aerobic media. As a result, regeneration of (NAD+ /NADH) on bare electrodes requires extremely high overvoltages. One-electron NADH oxidation on glassy-carbon electrodes occurs irreversibly with half-wave potentials from +0.62 to +0.72 V (SHE), which

99

100

5 Electropolymerized Azines: a New Group of Electroactive Polymers

corresponds to an overvoltage of approximately 1 V. NADH oxidation at platinum electrodes occurs at even higher potentials (E1/2 ≈ 0.92–0.98 V (SHE)) [40]. The polarographic reduction of NAD+ in alkaline buffers exhibits two waves at −1.1 and −1.7 V (SCE) [41]. The ﬁrst wave corresponds to the formation of NAD• radicals, which is accomplished by their fast dimerization [42, 43]. The dimers are not further reduced in the potential range down to −1.8 V and can be only oxidized back to NAD+ at a potential of −0.1 V (SCE) [43]. In the continued reduction of the NAD• radicals in the second polarographic wave, both 1,4-NADH and 1,6-NADH are produced, the latter being enzymatically inactive [42]. Considering the number of known dehydrogenases, one can conclude that a search for efﬁcient electrocatalysts for NAD+ |NADH regeneration is extremely important for bioanalysis, because it allows elaboration of biosensors for the majority of the key metabolites. Mediators for (NAD+ |NADH) regeneration have been proposed, following continuous efforts of bioelectrochemists during the last three decades. Quinones [44, 45] and phenoxazine derivatives containing quinoid structures [46, 47] have been used successfully as two-electron catalysts for NADH oxidation. Several phenazines have also been found to be suitable mediators of pyridine nucleotide electrooxidation [48]. More recently, dihydroxybenzaldehyde [49], caffeic acid [50], and trinitroﬂuorenes [51, 52] were discovered to be effective mediators. The highest rate constants in NADH oxidation are reported for the latter [51, 53], as well as for phenothiazine [54] and dihydroxyphenyl-dimethylbutane [55]. In contrast to a number of electrocatalysts proposed for NADH oxidation, the only mediator system known for NAD+ electroreduction, up to recent times, is the rhodium(III)-bipyridyl complexes [56]. 5.3.2.2 Mimetics of Enzyme Catalysis As stated above, nature protected its most reductive mediator (NAD+ /NADH) from oxidation by oxygen and other conventional redox reactions. However, the dehydrogenases provide highly efﬁcient regeneration of NAD+ /NADH close to equilibrium conditions. Nearly all dehydrogenase catalyzed reactions are completely reversible. In nature, there are special enzymes (NADH-dehydrogenases) that catalyze the reduction of NAD+ and the oxidation of NADH by various redox active compounds. Mimicking their active sites, it is possible to synthesize an electrocatalyst for NAD+ /NADH regeneration. Active sites of these enzymes have been compared and it has been found that lipoamidehydrogenase (CE 1.6.4.3), glutathionedehydrogenase (CE 1.6.4.2), and NADH dehydrogenase (CE 1.6.99.3) [57] use the same prosthetic group (ﬂavin) for NAD+ /NADH regeneration. To mimic biological catalysis, it is possible to use electropolymerization. This is a powerful tool, allowing highly controlled synthesis of polymers from solutions of the corresponding monomers by their electrochemical oxidation or reduction. According to our hypothesis, the electropolymerization of azines occurs via binding of azine monomers without their destruction [9]. When the structure of the monomer is similar to the chemical structure of the biological system of interest,

5.3 Polyazines in Electroanalysis

the desired afﬁnity or catalytic center can appear in the resulting polymer due to various substitutions upon electropolymerization. 5.3.2.3 Electropolymerized Azines as NADH Transducers As methylene blue is known to act as a highly active electron-transfer mediator for the electrocatalytic oxidation of NADH, it was expected that poly(methylene blue)-covered glassy-carbon surfaces may similarly be used as electrodes for NADH oxidation. In Figure 5.4, the cyclic voltammogram of a poly(methylene blue)-modiﬁed electrode in the absence and presence of NADH (5 mM) deﬁnitely proves the electrocatalytic properties of these polymeric ﬁlms in the oxidation of NADH. Obviously, two redox waves with signiﬁcantly different potentials are present in the poly(methylene blue) cyclic voltammogram, one with the lower potential (about −300 mV) resembling the monomer-analog redox couple, and the second (about −80 mV) due to a polymer-related redox reaction. According to the hypothesis on poly(methylene blue) structure (see above), polymer-type redox activity with the more oxidative potential appears to be due to oxidized neighbors being electron acceptor substituents. Hence, it could be expected that a polymer-type redox reaction serves as a more efﬁcient electrocatalyst for NADH oxidation. Indeed, this new set of peaks exhibits a dramatically improved catalytic activity for the oxidation of NADH, possibly due to the higher redox potential and thus the higher driving force for the electron-transfer reaction [58]. This result is additionally proved by measuring the steady-state current on addition of known concentrations of NADH at ﬁxed electrode potential in neutral solution. A sufﬁciently high electrocatalytic current is obtained at electrode potentials higher than −100 mV, corresponding well with the redox wave of the cyclic voltammogram. The diffusion limit of the current is obtained at potentials higher than 100 mV [58].

I (µA)

0.5

0.25

−250

0

E (mV) −0.25

Figure 5.4 Cyclic voltammograms of a poly(methylene blue)-modiﬁed electrode in the absence (lower curve) and the presence of 5 mM NADH (upper curve); sweep rate 1 mV s−1 ; 0.1 M tris buffer, pH 8.5 + 0.1 M KCl.

250

101

102

5 Electropolymerized Azines: a New Group of Electroactive Polymers

Hence, the advantage of poly(methylene blue) compared to the corresponding monomer is improved catalytic activity in NADH oxidation due to a polymer-type redox reaction. Another advantage of using electropolymerized methylene blue as a catalyst for NADH oxidation is its improved long-term stability. The half-inactivation time of the corresponding modiﬁed electrode was approximately one week, which was at least 10 times higher than that of electrodes based on adsorbed or covalently linked mediators [58]. The electrocatalytic properties of different polyazines in NADH oxidation were investigated. Poly(toluidine blue), poly(brilliant cresyl blue), poly(Azure A), and poly(methylene blue) exhibited redox activity in a similar potential region. In relation to the catalytic activity of different electropolymerized azines, one can make the following conclusions: (i) an additional substitution of the benzene ring by an alkyl group reduces catalytic efﬁciency, (ii) in general, phenoxazines are better catalysts than phenothiazines, and (iii) ring substitution with only tertiary nitrogen atoms as ligands provides higher catalytic activity [59]. The redox potential of the polyphenothiazine affected the kinetic parameters of NADH oxidation, as expected. Higher potentials promoted higher catalytic efﬁciency. Poly(methylene green) was found to be the best electrocatalyst among polyazines under this study. One can conclude that electropolymerization provides great advantages for known mediators with respect to their catalytic efﬁciency in NADH oxidation: (i) a considerably improved catalytic activity caused by polymer-type redox reaction and (ii) operational stability prolonged by an order of magnitude. Polyazine-modiﬁed electrodes can be optimally used for the electrocatalytic oxidation of NADH at potentials between 0 and 100 mV versus SCE, which is simultaneously the optimal region for the operation of amperometric enzyme electrodes with respect to possible interferences by cooxidizable or coreducible interfering compounds. These advantages provide the use of polyazines in NADH sensors [6, 12, 13, 15], and for construction of corresponding dehydrogenase-based biosensors [12, 16, 58, 60]. 5.3.2.4 Electroreduction of NAD+ to Enzymatically Active NADH at Poly(Neutral Red)-Modiﬁed Electrodes To realize the electroreduction of NAD+ to enzymatically active NADH, it is highly attractive to use the biomimetic approach described above. Unfortunately, the redox potentials of the accessible ﬂavins are too high to catalyze the reduction of NAD+ . That is why the azine neutral red, similar in structure to ﬂavins, but with much lower redox potential was chosen. Neutral red had already found wide use as an artiﬁcial enzyme substrate [61] and a redox indicator for electrochemical investigations of biological redox systems [62]. Its formal potential at pH 7.0 is −0.325 V (NHE) [20]. The possibility of NAD+ reduction at poly(neutral red) electrodes was tested in both steady-state and potentiodynamic conditions. At negative potentials, a current of NAD+ reduction was observed on poly(neutral red)-modiﬁed electrodes. The cathodic current density was dependent on NAD+ concentration.

5.3 Polyazines in Electroanalysis

To identify the product of the NAD+ reduction, the alcohol dehydrogenase enzyme containing a Naﬁon membrane was put onto a poly(neutral red)-modiﬁed electrode surface. In the presence of acetaldehyde, the enzyme was able to oxidize NADH back to NAD+ . Hence, if the product of NAD+ reduction at poly(neutral red) electrodes is enzymatically active, one should expect a response of the enzyme electrode to acetaldehyde due to mediator recycling. Indeed, acetaldehyde generated a cathodic current response at the enzyme electrode in the presence of a small concentration of NAD+ . Cathodic current density was dependent on NAD+ concentration, and was more than 10 times higher than that of the acetaldehyde background reduction on a bare poly(neutral red)-modiﬁed electrode. Thus, the main product of NAD+ electroreduction at poly(neutral red)-modiﬁed electrode was enzymatically active NADH [19]. 5.3.2.5 Observation of the Equilibrium NAD+ |NADH Potential at Poly(Neutral Red) Electrodes As mentioned above, as a result of continuous efforts of bioelectrochemists over the last three decades, mediators for (NAD+ |NADH) regeneration have been proposed. However, neither the various electrode materials, nor the proposed electrocatalytic systems provide the NAD+ |NADH turnover close to its equilibrium potential. The only exception was the poly(neutral red)-based electrocatalyst able to reduce NAD+ to enzymatically active NADH (see above). Thus, an ability to observe electrochemically the NAD+ |NADH equilibrium potential, which obviously was impossible using all previous catalytic and mediator systems, was highly attractive. Electrocatalysis for NAD+ |NADH regeneration was investigated by analyzing the steady-state response of freshly prepared poly(neutral red)-modiﬁed electrodes to nicotinamide adenine dinucleotide. Steady-state current–potential curves are most useful because the inﬂuence of capacitive currents is excluded. As seen in Figure 5.5a, the addition of pure NADH provides anodic (oxidation) currents on a poly(neutral red)-modiﬁed electrode, whereas the addition of pure NAD+ causes a cathodic (reduction) current. Thus, poly(neutral red) is active in both NAD+ reduction and NADH oxidation. Indeed, in the presence of an equimolar mixture of NAD+ and NADH, the current–potential curve intersects the abscissa axis (Figure 5.5b). The solid lines in Figure 5.5 are the corresponding polarographic waves ﬁtted to typical exponential equations. The observed zero-current potential of −0.59 V (SCE) (Figure 5.5b) was rather far from both the determined poly(neutral red) redox potential (−0.53 V) and the reported [39] standard potential for NAD+ |NADH (−0.53 V). It was proved that the reported (rather long ago) standard potential for NAD+ |NADH [39] had not been precisely determined, in particular, at pH 6.0; its real value is equal to the zero-current potential (−0.59 V) observed in Figure 5.5b [63]. How important is the difference of 60 mV for biochemistry? According to the Nernst equation, the deviation of 59 mV in potential corresponds to a change of the substrate-to-product ratio by 1 order of magnitude. On the other hand, the average

103

5 Electropolymerized Azines: a New Group of Electroactive Polymers

j (µA cm−2)

1

0

−0.60

−0.55

E (V)

−1

(a)

1.0

0.5

j (µA cm−2)

104

0.0

−0.60

−0.55 E (V)

−0.50

−0.5 (b) Figure 5.5 Current responses of poly(neutral red)-modiﬁed electrodes to nicotinamide adenine dinucleotide at different potentials: (a) 0.1 mM NADH (o) or 0.1 mM NAD+ (); (b) mixture of 0.1 mM NADH and 0.1 mM NAD+ ; 0.02 M phosphate pH 6.0 with 0.1 M KNO3 .

values of the potentials across the living biological membranes range from 50 to 70 mV [64]. Finally, it is important to note that by using poly(neutral red)-modiﬁed electrodes, a reversible polarographic wave for nicotinamide adenine dinucleotide reduction–oxidation and an equilibrium (NAD+ |NADH) potential were observed. This was not possible using all other known catalytic and mediator systems. The unique poly(neutral red)-based electrocatalyst allowed us to determine the standard (NAD+ |NADH) potential more precisely (E ∼ = 0.59 V SCE, pH 6.0).

5.4 Electropolymerized Azines as Promoters for Bioelectrocatalysis

5.4 Electropolymerized Azines as Promoters for Bioelectrocatalysis

Bioelectrocatalysis is an acceleration of the electrochemical reactions by biological catalysis. The credit for the discovery of direct (mediator-free) bioelectrocatalysis by the enzymes goes to the Russian enzymology and electrochemistry schools. The ﬁrst oxygen enzyme electrode made by immobilization of the enzyme laccase on carbon black electrode had been reported in the late 1970s [65]. The main problem with direct bioelectrocatalysis is how to achieve an efﬁcient electron exchange between the enzyme active site (its electron transport chain) and the electrode. This depends on two main factors. First, the enzymes being macromolecules have to be properly oriented on the electrode surface bringing their terminal redox group to the distance suitable for electron transfer. Random enzyme orientation obviously decreases the efﬁciency of bioelectrocatalysis. Second, electrochemical reaction of the terminal redox group has to be rather fast and occur at low overvoltages. This is not always the case in biological redox mediators. As mentioned, electrochemical conversions of NAD+ |NADH couple on platinum, gold, and carbon electrodes requires more than 1.0 V of overvoltage [40]. 5.4.1 Attempts to Involve Glucose Oxidase in Mediator Free Bioelectrocatalysis

Immediately after their synthesis, electropolymerized azines were investigated as promoters for bioelectrocatalysis. The enzyme glucose oxidase was chosen. The active site of this enzyme was immersed deep inside the protein globule (20 A˚ from the surface [66]), and there was no electron transport chain from the active center to protein surface. As a result, despite particular attention to this enzyme due to the importance of glucose tests, its bioelectrocatalysis without diffusion-free mediators [67] or linked to the long spacer [68] mediators has not been practically achieved. It is therefore not surprising that even soluble glucose oxidase has shown only a minor electrocatalytic effect on electropolymerized methylene blue. However, recently, there have been a number of reports on the use of polyazines as promoters of bioelectrocatalysis by glucose oxidase [69–71]. The enzyme was immobilized on the top surface of the electropolymerized azine by cross-linking with glutaraldehyde. The claimed anodic (oxidative) response, however, was always increased rather than descreased as expected, when the potential was shifted towards cathodic region. Moreover, and the optimal response was often apart from the redox activity of polyazine, which makes doubtful the catalytic role of the latter. An attempt to deaerate the solution caused a decrease in the response [71], whereas avoiding the competition of oxygen should have resulted in its increase. Finally, a possibility for hydrogen peroxide (the product of glucose oxidation with oxygen catalyzed by glucose oxidase) electroreduction on polyazines was considered [70, 71]. Hence, if avoiding a confusion with the sign of the observed anodic responses (which are sometimes truly referred to as cathodic ones [71]), polyazines in [69–71],

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5 Electropolymerized Azines: a New Group of Electroactive Polymers

most probably, serve as rather poor electrocatalysts of H2 O2 reduction rather than as mediators for bioelectrocatalysis. 5.4.1.1 Bioelectrocatalysis by Hydrogenase and Peroxidase Peroxidase and hydrogenase are the enzymes known for already 25–30 years to be involved in direct bioelectrocatalysis even being immobilized directly on carbon supports [72, 73]. The most popular horse radish peroxidase (HRP) can be involved in bioelectrocatalysis almost independent of the material of electrode support. Hence, it is not surprising that HRP, being embedded in electropolymerized methylene green [74] and thionine [24], showed an appropriate electrocatalysis and served as a sensor for hydrogen peroxide. Electropolymerized neutral red, in which redox potential is closer to hydrogen equilibrium potential compared to other polyazines, was used as promoter for bioelectrocatalysis by hydrogenases. Indeed, as seen in Figure 5.6, the enzyme electrode with hydrogenase immobilized over poly(neutral red) shows 3–4 times improved current–potential characteristics as compared to the hydrogenase electrode without intermediate polyazine layer. 5.4.2 Bioelectrocatalysis by Cellobiose Dehydrogenase on Polyazines

Cellobiose dehydrogenase (CDH) bioelectrocatalysis was investigated using screen-printed carbon supports. As seen from the current–potential dependence (Figure 5.7), when the enzyme is immobilized directly on carbon material, the response of the resulting electrode around the potential of Ag|AgCl reference 1000 800

i (µA cm−2)

106

600 400 200

0

0

50

100 Er (MB)

150

200

Figure 5.6 Current–potential dependencies of a hydrogen enzyme electrode in H2 atmosphere for hydrogenases from Thiocapsa roseopersicina, immobilized on carbon (•) and over electropolymerized neutral red ().

5.4 Electropolymerized Azines as Promoters for Bioelectrocatalysis

2.0

I (nA)

1.5

1.0

0.5

0.0 −100

0

100 200 E (mV)

300

400

Figure 5.7 FIA response of CDH electrode to 1 mM lactose: CDH on carbon (); CDH over poly(methylene green) (30 cycles) (); and CDH over poly(methylene blue) (15 cycles) (•).

is rather low. Similar to CDH behavior on graphite supports [75], the response becomes sufﬁcient for analytical applications only at +0.3–+0.4 V. However, at potentials higher than +0.1 V electrochemical sensors are known to decrease their selectivity due to the presence of a large number of reductants able to produce a parasitic signal. Azines are known to be substrates for a number of dehydrogenases including CDH. It was expected that electropolymerized azines containing monomer structures can serve as anchors for proper orientation of CDH on the electrode surface. Indeed, as seen in Figure 5.7, CDH immobilized over electropolymerized methylene green and, in particular, methylene blue, results in enzyme electrode with higher electrocatalytic activity. In contrast to the enzyme adsorbed directly onto the carbon surface, the enzyme electrodes with an intermediate polyazine layer display their maximum activity around 0.0–+0.2 V. The potential of the highest electroactivity almost coincides with the redox potential of the corresponding electroactive polymer, conﬁrming that the electropolymerized azines act as immobilized mediators [76]. It is important to note that electropolymerized azines really improve CDH electrocatalysis rather than simply increase the amount of immobilized enzyme. It was independently shown that surface concentration of CDH immobilized over polyazine layers is similar to that of the enzyme adsorbed directly on carbon [76]. In conclusion, an intermediate layer of the electropolymerized methylene blue drastically improves performance characteristics of the resulting cellobiose dehydrogenase electrodes. The operating potential of the corresponding biosensor is shifted toward the negative region and reaches the optimal range. Electroactivity of the improved biosensor at operating potential is an order of magnitude higher

107

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5 Electropolymerized Azines: a New Group of Electroactive Polymers

compared to the enzyme electrode made by CDH immobilization directly on carbon.

5.5 Conclusion

In this chapter, an introduction to electropolymerized azines has been presented. It has been proven that these materials comprise a novel group of electroactive polymers. A hypothesis on polyazine structure has also been presented. Among the main applications of polyazines is their use both as electrocatalysts for various reactions and as promoters of enzyme bioelectrocatalysis. Biomimetic approach based on electropolymerization of azines is highly successful in the case of dehydrogenase enzymes. The most efﬁcient electrocatalysts for NAD+ |NADH regeneration have been synthesized allowing one to observe the equilibrium NAD+ |NADH potential electrochemically. Improvement of bioelectrocatalytic efﬁciency, most probably resulting in enzyme orientation on the electrode surface and accelerating the enzyme–electrode electron exchange, is shown for different biological catalysts – hydrogenase and dehydrogenase. Despite the fact that electropolymerized azines were discovered rather recently, there is already considerable interest in these unique materials, which have found a number of applications that hold promise for further development of this particular ﬁeld of electroactive polymers and electropolymerization.

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Varfolomeyev, S.D. (1994) Electroanalysis, 6, 821–829. Karyakin, A.A., Karyakina, E.E., Schuhmann, W., and Schmidt, H.L. (1999) Electroanalysis, 11, 553–557. Curulli, A., Carelli, I., Trischitta, O., and Palleschi, G. (1997) Talanta, 44, 1659–1669. Thauer, R.K., Jungermann, K., and Decker, K. (1977) Bacteriol. Rev., 41, 100. Fultz, M.L. and Durst, R.A. (1982) Anal. Chim. Acta, 140, 1–18. Karyakin, A.A., Ivanova, Y.N., and Karyakina, E.E. (2003) Electrochem. Commun., 5, 677–680. Metzler, D.E. (1977) Biochemistry. The Chemical Reactions of Living Cells, Academic Press, New York. Berezin, I.V., Bogdanovskaya, V.A., Varfolomeev, S.D., Tarasevich, M.R., and Yaropolov, A.I. (1978) Dokl. Akad. Nauk SSSR, 240, 615–618. Wilson, R. and Turner, A.P.F. (1992) Biosens. Bioelectron., 7, 165–185. Cass, A.E.G., Davis, G., Francis, G.D., Hill, H.A.O., Aston, W.G., Higgins, I.J., Plotkin, E.V., Scott, L.D.L., and

68.

69. 70.

71. 72.

73.

74. 75.

76.

Turner, A.P.F. (1984) Anal. Chem., 56, 667–671. Ohara, T.J., Rajagopalan, R., and Heller, A. (1993) Anal. Chem., 65, 3512–3517. Ghica, M.E. and Brett, C.M.A. (2006) Electroanalysis, 18, 748. Pauliukaite, R., Ghica, M.E., Barsan, M., and Brett, C.M.A. (2007) J. Solid State Electrochem., 11, 899. Ghica, M.E. and Brett, C.M.A. (2009) J. Electroanal. Chem., 629, 35. Yaropolov, A.I., Malovik, V., Varfolomeev, S.D., and Berezin, I.V. (1979) Dokl. Akad. Nauk SSSR, 249, 1399–1401. Yaropolov, A.I., Karyakin, A.A., Varfolomeyev, S.D., and Berezin, I.V. (1984) Bioelectrochem. Bioenerg., 12, 267–277. Yang, R., Ruan, C., and Deng, J. (1998) J. Appl. Electrochem., 28, 1269. Stoica, L., Ludwig, R., Haltrich, D., and Gorton, L. (2006) Anal. Chem., 78, 393. Karyakin, A.A., Vinogradova, D.V., Morozov, S.V., and Karyakina, E.E. (2009) Electrochim. Acta, in press.

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6 Electropolymerization of Phthalocyanines Ninel M. Alpatova and Elena V. Ovsyannikova

6.1 Introduction

A book devoted to electropolymerization, which considers the formation of electroactive conducting layers, logically involves a chapter on the electropolymerization of phthalocyanines (Pcs). Pcs of transition metals (Figure 6.1) are N4 -complexes of cations of these metals. In most cases, Pc ligands of complexes consist of plane two-charge anions containing condensed pyrrole and benzene fragments. Chemical structure of Pc rings (combined double and ordinary bonds) leads to a high π-conjugation of the entire ligand structure. Electron structure of Pc rings allows both their oxidation (loss of electrons) and reduction (gain of electrons). Along with the redox conversions of Pc rings themselves, variation in oxidation state of the central ion occurs. An example of this process is Co in CoPc: its bivalent cation can be oxidized to Co(III) and reduced to Co(I). Pc systems have been attracting both scientiﬁc and practical interest for their unique electrochemical, electrochromic, and electrocatalytic properties, photoactivity, and so on [1–4]. In this chapter we describe the properties of electropolymerized Pcs in comparison with the properties of Pcs that were immobilized on the electrode surface by alternative methods. Major attention is paid to the immobilized systems, which are of interest for electrochemists. We also focus on Pcs intercalated into various matrices, which makes it possible to estimate the effect of Pc–Pc and Pc–matrix noncovalent interactions. 6.2 Immobilization of Transition-Metal Phthalocyanines on Conducting and Nonconducting Substrates 6.2.1 Phthalocyanines in Electron-Conducting Polymers

Among electron-conducting polymers (ECPs), polypyrrole and polyaniline (PANI) are commonly used as matrices for Pc immobilization. The latter polymer is easily Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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6 Electropolymerization of Phthalocyanines

MTRPc

R (1, MTSPc)

R

−

SO3 Na+

=

O N

N

N

M

N

(2, MTNhtPc) R

N N

N

R

=

R SO3−Na+

N (3, AB)

R

=

N

CH2

+

Cl− R

Figure 6.1

(4)

R

=

(5)

R

=

O

(CH2)2

(CH2)2

N

O

N

S

(6)

R

=

O

(7, MTAPc)

R

=

NH2

Tetra-substituted MPc complexes.

synthesized either electrochemically or chemically in acidic aqueous solutions. This polymer can be anodically doped in aqueous solutions of inorganic acids. Doping process is chemically reversible. The conditions (solutions and potential range) under which the polymer is doped, that is, presents in conducting state, can be used to carry out several electrocatalytic reactions. Among them, the cathodic reduction of oxygen is one of the most important processes. Anions of inorganic and organic acids used for anodic synthesis of PANI are incorporated into the polymer matrix during electropolymerization. Potentials of anodic synthesis correspond to the doped state of PANI. However, when the polymer is undoped, the behavior of the anion depends on its dimension. Small anions are released from PANI ﬁlm to solution bulk, whereas large anions remain immobilized in the polymer matrix due to steric restrictions. This causes incorporation of cations into polymer matrix to maintain electroneutrality. Pcs containing anionic substituents are among the anions, which are strongly immobilized in the conducting polymer. The Pc ring is 1.2 nm in diameter [5, 6], which makes immobilization of its anionic derivatives possible. Four-substituted Pc derivatives containing sulfoand naphthoxysulfo-groups (Figure 6.1, compounds 1 and 2, respectively) are used for the synthesis of PANI [7–10]. Both substituents provide high solubility of Pc in

6.2 Immobilization of Transition-Metal Phthalocyanines on Conducting and Nonconducting Substrates

water within wide pH range, including acidic media required for the synthesis of PANI. PANI modiﬁed with anionic Pcs was synthesized by anodic polymerization of monomer from the 0.1–1 M sulfuric acid solutions in the presence of anionic Pc (10−3 M) [7–10]. Despite the large excess of sulfuric acid, Pc anions were incorporated in the polymer. This points to a strong interaction between the positively charged polymeric chains and these four-charged anions of Pcs [7, 9, 10]. As a result of such interaction, anionic Pc catalyzes electrochemical synthesis of PANI. For instance, in the presence of CoTNhtPc, the rate of potentiodynamic synthesis of PANI is twice higher [10]. Metallic tetra sulphophthalocyanines (MTSPcs) of iron group metals (Fe, Co, Ni) [7–11] and copper [12] were investigated as electroactive compounds immobilized in PANI by various physicochemical methods including electron absorbance spectroscopy [10], elemental analysis [11], quartz microbalance [11, 12], and so on. The results of these investigations reliably proved the fact of entrapment of MTSPcs in the PANI matrix. It was also shown that, on average, four aniline units in PANI are doped with one sulfo-group [11]. The immobilization of MTSPcs within PANI suppresses the ionic transport and renders the proton exchange to be predominant [11, 12]. Moreover, the results obtained indicate that PANI contains anionic Pcs as nonaggregated ions. The latter virtually do not interact with each other and strongly interact with the PANI matrix in doped and undoped states. Interaction between the Pc anions and polymer matrix is of electrostatic type and donor–acceptor type. This interaction prevents the dimerization (aggregation) of Pcs, which is typical of their aqueous solutions. 6.2.2 Phthalocyanines in Matrices of Artiﬁcial Lipids

Solid salts of quaternary ammonium cations with long alkyl radicals possess some properties that allow us to consider them as artiﬁcial lipids. Lipid-like behavior is especially typical for the asymmetric quaternary ammonium salt (QAS), which contains one or two long and three or two short (methyl) radicals. The speciﬁc feature of lipids is the presence of two parts in their molecules: the nonpolar hydrophobic part and polar hydrophilic part. In the case of QASs, alkyl radicals represent the hydrophobic part and N+ −A− (commonly, Br− ) dipoles represent the hydrophilic part. It is a large polarizable bromine anion that readily forms such dipoles. Drying solutions of these salts in nonpolar organic solvents, the polybilayer coatings can be formed on solid substrates (Figure 6.2). Matrices of QAS or artiﬁcial lipids are widely used to form composites with fullerene C60 and its derivatives [13–15]. Bromides of quaternary ammonium with long alkyl substituents are also suitable for immobilizing Pcs of transition metals. The composites are known to involve anions of CuTSPc [16–18] and CoTSPc [19]. There are three ways to immobilize MTSPc within the QAS.

113

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6 Electropolymerization of Phthalocyanines

−

− N+

N+ Br− Br− N+

− N+

N+ Br−

−

Figure 6.2 Simpliﬁed model of polybilayer structure based on asymmetrical tetra-alkylammonium salts.

The ﬁrst one is the immobilization of MTSPc in the form of salts MTSPc (DDDMA)4 and MTSPc(DMTA)4 , where MTSPc is the four-charged anion and DDDMA and DMTA are the didodecyldimethylammonium and dodecyltrimethylammonium cations, respectively. Casting of salt solutions in chloroform on solid substrates followed by their drying results in salt ﬁlms [17–19]. The second immobilization technique involves a preliminary application of bromides of DDDMA or dioctadecyldimethylammonium on a solid substrate. The obtained ﬁlm is then transferred to an aqueous solution with MTSPc, where bromine anions in the ﬁlm are replaced by the MTSPc anions [16, 17, 20]. The third immobilization technique involves application of composites prepared by water dispersion of DDDMA bromide vesicles and water-soluble MTSPc sodium salt [18]. The ﬁrst technique results in conventional salt ﬁlms [17–19] and the surfactants cannot form bilayers. The absorption spectra of salt coatings show a sharp decrease in dimerization of Pc anions as compared to their dimerization in aqueous solutions [18, 19]. This led the authors to assume that the MTSPc anions strongly interact with the QAS cations and weakly bind to each other. The ﬁlms prepared by the second and third techniques are of quite different nature. They can be characterized by the presence of polybilayers formed by the surfactant component [17, 18, 20]. It is interesting that the absorption spectrum of the polybilayer coating demonstrates an increase in the fraction of associates (Pc dimers) as compared to the salt ﬁlm [18]. It was proposed in [20] that this is the result of dimerization (aggregation) of Pcs that provides their ﬁrm binding to the cationic bilayer. Investigations of the electrochemical behavior of MTSPc–artiﬁcial lipid composites revealed the redox transitions of the Pc and its participation in the electrocatalytic processes. The polybilayer composites transfer electric charge more effectively than the salt ﬁlms and, hence, can play the role of catalysts, for example, in the reaction of electrochemical dehalogenation [18].

6.2 Immobilization of Transition-Metal Phthalocyanines on Conducting and Nonconducting Substrates

Nevertheless, the salt ﬁlms, for example, with CoTSPc, can be used in electroanalysis. The salt ﬁlms based on cobalt compounds form complexes with nitrogen monoxide, thus catalyzing its electroreduction [19]. The above polybilayer systems contain the Pc anions. However, an alternative method of Pc immobilization in the artiﬁcial lipid can exist. As follows from [21, 22], lutecium diphthalocyanine (LuPc2 ) with four or eight long-chain thioalkyl substituents – SR, where R = (CH2 )11 CH3 [21] or R = (CH2 )5 CH3 , respectively [21, 22], forms the electroactive composite with the matrix of tetra-n-octylphosphonium bromide or tetra-n-octylammonium bromide. These matrices with symmetrical alkyl radicals (C > 6), as well as asymmetrical QAS, can form polybilayer structures [13, 15]. The presence of hydrophobic alkyl substituents in the Pc ring allows the lutecium compound to intercalate into the hydrophobic part of polybilayer coating. This leads to the dispersion of LuPc2 within the matrix and to the appearance of inert microenvironment around the molecules of LuPc2 . Because of such environment, LuPc2 can participate in the cathodic processes in the aqueous solutions. Furthermore, these processes proceed under the effect of electrostatic interaction of formed anions with cations of matrix that shifts the formal potentials of the cathodic redox transitions of LuPc2 in the positive direction. Therefore, the matrices of artiﬁcial lipids are able to form Pc composites of different nature. 6.2.3 Composites of Ultrathin Layers of Oppositely Charged Ions

The electrostatic application of alternating, oppositely charged, ultrathin ionic layers is known as the layer-by-layer (LbL) method. The latter is one of the simplest methods of modifying solid substrates. A prototype of this method has been developed by Iler [23] who showed the possibility of obtaining assemblies with oppositely charged colloid particles. More recently this method was developed by Decher and coworkers [24–27]. The process is realized by dipping a substrate into the solution of the ﬁrst electrolyte which is chemisorbed on the substrate as the ﬁrst ionic layer. After washing, the substrate is transferred into the solution of the second electrolyte wherein, under the electrostatic attraction, a layer of oppositely charged ions is deposited. The LbL method can be used both for conducting substrates (dipping into solutions at an open-circuit potential) and for insulators. The main types of LbL assemblies based on Pcs are given in Table 6.1 (numbers 1–9) [6, 28–36]. In addition, the table presents two composites containing two thick layers, which were prepared by drop casting of electroactive component solutions on the electrodes followed by drying of each layer (numbers 10, 11) [37–39]. As a result, the insoluble salt layer is formed.

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6 Electropolymerization of Phthalocyanines

116

LbL composites based on phthalocyanines.

Table 6.1

No.

Components

1 2 3 4 5 6 7 8 9 10 11

Substrate

Reference

Anionic

Cationic

CoTSPc NiTSPc FeTsPc CuTSPc CuTSPc-capped TiO2 nanoparticles CuPc(COONa)8 CuTSPc CuTSPc P2 W18 pure or mixed with PSSb CoTSPc, CuTSPc FeTSPc

PDDA PDDA PAH PDDA PDDA

Au ITO (ITO/TiO2 ) ITO ITO Quartz, ITO, mica

PEI AB ABa AB

Quartz Silicon wafer, glass slide HOPG Quartz, ITO

FeT4MPyPc FeT4MPyP

GC GC

[6] [28] [29, 30] [31] [32] [33] [34] [35] [36] [37, 38] [39]

a Alcian

Blue (AB) (compound 3, Figure 6.1). (PSS). c (FeT4MPyP) – iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin. b Poly(styrenesulfonate)

Several polyelectrolytes, such as poly(allylamine) hydrochloride (PAH), poly(diallyldimethylammonium) chloride (PDDA), and poly(ethyleneimine) (PEI) are used most frequently as the cationic components. (

(

)

)

n

n

CH2 +

NH3 Cl−

(PAH)

N + Cl−

(PDDA)

N H

x

N

y

(PEI)

CH2CH2NH2

Being surfactants, these compounds are readily and ﬁrmly adsorbed on different solid substrates. For composites without polyelectrolytes, the problem of application of the ﬁrst layer arises. The problem is commonly solved by a combination of one of the polyelectrolytes and poly(sodium 4-styrenesulfonate) (PSS), which are applied in two or even three bilayers. Some physical and physicochemical methods are used for investigating the LbL ﬁlms with polyelectrolytes. Among these methods, UV–vis absorbance spectroscopy should be mentioned ﬁrst. Such measurements are most convenient for monitoring the formation of electrostatic assemblies based on colored Pcs (numbers 1, 3–7, 9). Figure 6.3 gives an example of such step-by-step control. The UV–vis spectroscopy enables one to follow the degree of dimerization (aggregation) of Pcs in the LbL ﬁlms. Earlier, for anionic Pcs, it was shown that the presence of monomeric and dimeric forms of Pcs was typical for their assembling with polyelectrolytes [6, 29–31, 40, 41].

6.3 Electropolymerization of Phthalocyanines 0.5

Absorbance (a.u.)

0.4

0.3

0.2

0.1

0.0 200

400 600 Wavelength (nm)

800

Figure 6.3 PSS–AB ﬁlms (13 bilayers, quartz) assembled from aqueous solutions of PSS (4 mg per ml H2 O) and AB (2.5 × 10−4 M). The AB peak shifts to the long-wave range (conjugation increases) and the dimer-to-monomer ratio increases (aggregation enhances).

The results of Kelvin probe (number 2) and ellipsometry methods (numbers 4–7) conﬁrm a regular layered structure of LbL composites. Atomic force microscopy (AFM) (numbers 4–7) showed that the outer surfaces of LbL coatings have a closely packed nanodimensional granular structure. As a rule, the systems prepared on conducting substrates were characterized by cyclic voltammetry (CV) and voltammetry on a rotating disk electrode (numbers 1, 3–5, 9, 11). The redox transitions of negatively charged MTSPcs (M = Co, Fe, Cu) which were electrostatically assembled through the cationic electrolyte layers were investigated in [6, 29–31]. The data presented in these works are fragmentary and contradictory. However, in some cases, the nature of the limiting stage is established reliably. For instance, the redox transitions in the PDDA–CoTSPc [6] and PAH–FeTSPc [29, 30] assemblies were diffusion controlled. In some cases (numbers 10, 11), electrochemical measurements were the basis for developing amperometric sensors (nitrite oxidation, number 10; oxygen reduction, number 11). Therefore, the assembling of LbL composites based on Pcs is a new and rapidly progressing technology, which requires the development of general theory for these systems.

6.3 Electropolymerization of Phthalocyanines

To prevent Pc leakage from the matrix into solution and, hence, to avoid the loss of catalytic activity of electrodes, the conducting polymers based solely on Pcs were

117

118

6 Electropolymerization of Phthalocyanines

created. These polymers consist of Pc units that form a uniﬁed π-polyconjugated system. According to W¨ohrle [42], Pcs are polymerized by several methods which can be divided into three groups. 1) Chemical methods which enable to produce direct cross-linking of benzene fragments of Pc ligand or bridging of these fragments. In the former case, the polymer forms at the stage of synthesis of Pc ligands; in the latter case, low-molecular-weight Pcs undergo polymerization due to suitable substituents in aromatic rings. 2) Linking of Pcs to a foreign polymer matrix. In this case, Pc and polymer can be linked by covalent bonds or via the donor–acceptor or electrostatic interactions. In addition, ﬁnely dispersed particles of low-molecular-weight Pcs can be included mechanically in the foreign polymer matrix. 3) Polymerization of Pc through the formation of stacks, that is, the parallel structures consisting of plane rings of Pc ligands. In stacked MPcs, the metal ion can acquire an octahedral environment owing to the presence of additional ligands. The interest in the plane-parallel stacks composed of macrocycles is due to their noticeable electron conductivity even in undoped state and an increased conductivity in doped state. As a rule, extra ligands tighten macrocycles in the axial direction, thus ensuring that their π-systems overlap and providing overall electron conduction of the system. Additional bidentate ligands can be bound to the metal ion of Pc via a covalent or a donor–acceptor binding. In addition to these polymerization methods, the electrochemical fabrication of Pc ﬁlm was also attempted through the introduction of an electropolymerizable group into the benzene fragment of the Pc ligand. Commonly, the amino group is most frequently used (see Section 6.3.2), although other substituents such as pyrrole can be used as well. The electrochemical polymerization is one of the most convenient techniques for immobilizing MPcs on conducting substrates. Film thickness can be easily controlled by varying the quantity of charge passed. 6.3.1 Electropolymerization of Phthalocyanines with Ligands Bonded to Radicals of Electron-Conducting Polymer Precursors

Examples of successful realization of this process can be found in [43, 44]. For compounds 4 and 5 (Figure 6.1) in aprotic media, the authors observed irreversible anodic oxidation of pyrrole fragment of the substituent leading to the formation of conducting electroactive ﬁlm. It can be supposed that this process is similar to the electropolymerization of pyrrole and, hence, involves formation of cation radicals and their coupling followed by removal of hydrogen ions from β positions of the heterocycle.

6.3 Electropolymerization of Phthalocyanines

The structure of these polymers is unknown. One can suppose formation of the two-dimensional polymer backbone in which Pc ligands are bounded through bipyrrole oxy-bridges. An absence of the typical polypyrrole responses in cyclic voltammograms (CVs) is a good although indirect evidence for this hypothesis. However, even in the absence of the common conjugated system of pyrrole fragments, the ﬁlm can exhibit conductivity due to the formation of stack structures of Pc rings. Electropolymerization of Pcs with benzene fragments containing substituents, capable of forming radical precursors of ECP, occurs under conditions required for regular grafting of the heterocyclic substituent to the Pc ligand. The electropolymerization of compounds 4 and 5 [43, 44] is possible because positions 2 and 5 of pyrrole ring in the substituent are free and hence can be involved in ﬁlm formation. The attempts to electropolymerize tetrathiophene-substituted Pcs of cobalt, manganese, and zinc (compound 6, Figure 6.1) failed because position 2 in the substituent was occupied [45, 46]. Investigation of electrocatalytic activity of compounds 4 and 6 showed that the adsorbed compounds and polymerized compound 6 constitute efﬁcient catalytic sites for the oxidation of 2-mercaptoethanol and 1-cysteine, and reduction of glutathione [47]. 6.3.2 Electropolymerization of Tetra-Amino-Substituted Phthalocyanines

As mentioned, for electropolymerization of Pcs most commonly the tetra-amino-substituted Pcs of transition metals (compound 7, Figure 6.1) are taken as precursors [48–51]. Till now, the electrochemical polymerization is known for a wide range of amino-substituted Pcs containing Cu, Zn, Fe, Al, Mn, Co, Ni, Pd, and so on [48–60]. The popularity of amino-substituted precursors is caused by the relative ease of oxidation of aromatic amino group leading to polymerization (analogously to oxidation of aniline and formation of PANI) [51]. In particular, classic investigations in this ﬁeld were performed by Gwarr and coworkers [48]. Most commonly, MTAPc are electropolymerized from dimethylsulfoxide (DMSO) and DMF, wherein tetra-amino-substituted precursors are highly soluble. The peculiarities of MTAPc electropolymerization can be considered by taking CuTAPc as an example. Figure 6.4 shows the ﬁrst cycle in the CV for the saturated solution of CuTAPc in DMSO. The single-electron reversible oxidation of the Pc ligand in CuTAPc and the four-electron irreversible oxidation of the four amino groups in this compound are observed. As follows from Figure 6.5, in initial cycles, the reversible redox activity increases and, then, the current in CVs decreases. The current of irreversible oxidation (electropolymerization current) increases during cycling at all the stages of synthesis. This fact suggests that synthesis is in progress; however, DMSO is an inappropriate solvent for monitoring the growth of ﬁlms with time. When the ﬁlm growth is controlled by cathodic redox activity

119

6 Electropolymerization of Phthalocyanines

20

I × 103 (mA)

15

I2

10 5

I1

0 −5 0.0

0.2

0.4

0.6 E (V)

0.8

1.0

Figure 6.4 First cycle of CuTAPc electropolymerization in DMSO containing 0.1 M Bu4 NBF4 ; v = 0.025 V s−1 . 30

I × 103 (mA)

20 10 0 −10 −20 −0.2

0.0

0.2

0.4 E (V)

0.6

0.8

0.0

0.2

0.4 E (V)

0.6

0.8

(a)

60

40

I × 103 (mA)

120

20

0 −20 −0.2 (b)

Figure 6.5 CuTAPc electropolymerization: (a) 1–10 and (b) 21–40 cycles; arrows indicate the direction of current change.

6.3 Electropolymerization of Phthalocyanines

40

I × 103 (mA)

20 0 −20

I1

−40 −2.0

−1.6

−1.2

−0.8 E (V)

−0.4

0.0

Figure 6.6 Cyclic voltammogram of electropolymerized CuTAPc (10 cycles) in 0.1 M Bu4 NBF4 in PC; v = 0.1 V s−1 . 300

I1 × 103 (mA)

2 200

1

100

0

0

10

20 30 Number of cycles

40

Figure 6.7 Dependence of polymer redox activity I1 on the number of cycles of CuTAPc polymerization in (1) DMSO and (2) DMF; v = 0.1 V s−1 .

I1 in propylene carbonate (PC) (Figure 6.6), it is seen that the ﬁlm thickness increases whereas the rate of polymerization decreases gradually upon cycling. The polymerization of CuTAPc in DMF (Figure 6.7) enables to enhance the rate of ﬁlm formation and to avoid its decrease with time. Acceleration of polymerization process in DMF as compared to the polymerization in DMSO can be explained by a weaker donor–acceptor interaction of DMF molecules with radical cations of Pcs and polymer chains which are formed in course of synthesis. Blocking of radicals and chains by the donor solvent hampers further conversions of radical cations and polymer doping, thus reducing the rate of polymerization. Figure 6.8 gives the normalized absorption spectra for CuTAPc dissolved in DMF and for the CuTAPc ﬁlm polymerized on conducting glass.

121

6 Electropolymerization of Phthalocyanines

1.2 Absorbance (a.u.)

122

0.8

2

0.4

1 0.0

400

600 800 Wavelength (nm)

1000

Figure 6.8 Normalized absorbance spectra for (1) 1 × 10−4 M CuTAPc solution in DMF and (2) polymerized CuTAPc ﬁlm (20 cycles).

Both spectra contain two typical bands: the Q-band in the visible range and the B-(Soret) band in the near-UV range [61]. The polymer spectrum shows signiﬁcant broadening of the Q-band and a slight hypsochromic shift of the maximum in the absorption spectrum, λmax (from 724 nm in the solution to 713 nm in the polymer). The shortwave shoulder corresponding to the associates of CuTAPc in solution vanishes in this band. Furthermore, the Soret band intensity relatively increases. The observed spectrum evolution highlights the intermolecular interaction between the polymer fragments (π-coupling and the formation of stack structures) [61]. Similar spectrum evolution occurs for polymerized AlTAPc [62]. Films of polymerized CuTAPc can be doped both anodically and cathodically, and the doping process depends on the nature of the solvent and supporting electrolyte salt. For instance, anodic doping of polymer is practically impossible in DMSO and DMF; however, it can be realized in other aprotic solvents with low donor ability, namely, in PC with Bu4 NClO4 as supporting electrolyte [49]. The loss of the anodic doping ability of polymerized CuTAPc in solvents with high donor ability (DMSO and DMF) may be more likely ascribed to the formation of donor–acceptor complexes between the positively charged, oxidized fragments of the polymer system and the solvent molecules of electron-donor type. This complex formation blocks the interaction of dopant anions with oxidized polymer’s fragments, which is necessary for anodic doping of polymer ﬁlm. Anodic behavior of poly(CuTAPc) in acidic aqueous solutions looks like that of PANI. Two redox transitions are observed in the corresponding CV (Figure 6.9, curve 1). Similar behaviors were observed for poly(AlTAPc) [62] and poly(CoTAPc) [63]. This is consistent with the assumption of the formation of a PANI-like conjugated system. In neutral solutions CVs of the corresponding polymers (Figure 6.9, curve 2) are transformed into the form of one set of peaks. The potentials required for cathodic doping of poly(CuTAPc) can be attained only in aprotic media. Table 6.2 lists the formal potentials of the two redox transitions

6.3 Electropolymerization of Phthalocyanines

60 2

I × 103 (mA)

40 1

20 0 −20 −40 0.0

0.2

0.4

0.6

E (V) Figure 6.9 Cyclic voltammograms for CuTAPc (10 cycles) in (1) 0.1 M HClO4 and (2) 0.1 M NaClO4 aqueous solutions; v = 0.025 V s−1 .

in these systems. Redox behavior of polymer ﬁlm is close to that of the polymer precursor in solution and corresponds to the successive two-stage reduction of the ligand. The attempts to carry out the reduction of poly(CuTAPc) in PC with LiClO4 as a supporting electrolyte have failed. This fact and the data presented in Table 6.2 point to the signiﬁcant role of the counterions as active participants in cathodic doping. Large cations (for example, Bu4 N+ ) or the alkali metal cations, which are strongly solvated in solvents with high donor ability, can preserve their solvate shell upon entrapment in polymer and thus can act as the counterions. The large size of the dopant provides its interaction with the fragments of negatively charged polymer matrix via the formation of bridging structures. The data on the redox behavior of the electropolymerized MTAPcs allow us to assign them to electroactive conducting polymers. Electroactive polymers can be divided into two large, well-studied classes of compounds, the ECP and redox Table 6.2

Potentials of cathodic redox conversions of electropolymerized CuTAPc.

No.

Solution

Electrode

EfI (V)

EfII (V)

∆(V)

1 2 3 4 5 6 7 8

DMSOa + Bu4 NClO4 DMSO + LiClO4 DMSO + LiClO4 DMF + Bu4 NBF4 DMF + LiClO4 DMF + NaClO4 PC + Bu4 NBF4 PC + Bu4 NClO4

GC GC Pt (micro) GC GC GC GC GC

–1.03 –1.24 –1.22 –1.03 –1.08 –1.07 –0.99 –0.99

–1.45 – – –1.46 –1.48 –1.48 –1.33 –1.34

0.42 – – 0.43 0.40 0.41 0.34 0.35

a The

solvents are arranged in the order of decreasing donor ability.

123

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polymers. In ECP, the current is transferred by delocalized electrons appearing in long polyconjugated π-electron systems. Redox polymers consist of stationary redox centers embedded into low-conducting polymer matrix. Their conductivity results from an electron hopping process between redox centers. The conductivity of redox polymers is commonly several orders of magnitude lower than that of ECP. Poly(MTAPc) ﬁlms behave similarly to such ECP as PANI. However, the peculiarities of the anodic doping and cathodic behavior of poly(MTAPc) ﬁlm as well as its precursor in solution are similar to those of the redox polymer. This allows us to refer electropolymerized MTAPcs to the hybrid class of electroactive conducting polymers which combine the properties of electron-conducting and redox polymers. However, polymerized Pcs differ from the conventional ECPs due to the existence of two electron subsystems providing the conductivity through delocalized charge carriers. The two anodic redox transitions in an acid medium seem to correspond to two oxidation states conventionally observed for PANI. This corroborates the validity of one of the models of electropolymerization of MTAPcs, which assumes the formation of a PANI-like conjugated system. However, solid MPcs are capable of self-assembling into stacks without any additional substituents. We can suppose that two polyconjugated systems exist in polymerized MTAPc. The systems form a common electron system. One system mimics the PANI structure (binding through amino groups), while the other has a stack structure (Pc ligands assembled in stacks). If this model is valid, then, not only the amino groups but also mesoatoms of nitrogen can undergo protonation in an acid environment. The spectroscopic analysis of these subsystems indicates that the width of the Q-band for polymerized ﬁlm only slightly exceeds the width of the corresponding band for the cast ﬁlm, which is free of the PANI-like structures. Therefore, the major contribution in the ﬁlm conductivity is more likely from the stacking electron subsystem. The interest in immobilizing substituted CoPcs [64–66], which catalyze the cysteine oxidation, gaves rise to attempts to electropolymerize not only traditional CoTAPc but also its analogs. Tetra-substituted CoTRPcs, where R = NO2 , SO3 H [64, 65], and SO3 Na [64], were anodically electropolymerized in DMF + Bu4 NPF6 as a supporting electrolyte. However, the mechanisms of these processes are unknown. Since the benzene fragments in Pc ligands are substituted by amino groups, Pcs can be immobilized on electrode surfaces thanks to the anodic oxidation of aromatic NH2 groups. In this case, the electropolymerized MTAPcs exhibit redox activity in a wide potential range. This opens a possibility for their application in the chemical power sources and supercapacitors. The electropolymerized MTAPcs retain the electrocatalytic features of their precursors, which makes them promising candidates for the development of electrochemical sensors. Some of these polymerized Pcs are already applied as sensors to detect tert-butylhydroxyanisol [52], hydrazine [54, 55], 2-mercaptoethanol [47, 56], nitrite [58], glycine [59], and nitric oxide [60]. Applications of such Pc polymers in pH [53] and luminescence sensors [57] are also described.

6.3 Electropolymerization of Phthalocyanines

6.3.3 Electrochemical Modiﬁcation of Electrodes with Nickel Tetra-Sulfonated Phthalocyanine

There are several publications [67–74], as well as our own investigations, where voluntarily the ﬁeld is referred to as Pc electropolymerization. However, the authors working in this ﬁeld use the term electropolymerization [67–74], and hence its usage in the present chapter. NiTSPc is the most popular precursor for a very peculiar anodic modiﬁcation of electrode surface. Interest in this process is caused by a high electrocatalytic activity of the resulting ﬁlm in oxidation of several compounds. This enables one to detect NO in biological systems [67–70] and some other substances, for example, 2-chlorophenol [58]. The investigations were started in 1997 by Bedioui et al. [67]. This group of researchers made a great contribution to the concept of electrochemical NO sensors based on nickel ﬁlms [67–70]. In contrast to the electropolymerization of MTAPcs, which takes place in aprotic solvents, the anodic modiﬁcation of the electrodes with NiTSPc can be carried out in aqueous solutions: 0.1 M NaOH or carbonate–bicarbonate buffer (pH 11) containing 1–2 × 10−3 M of a precursor. Carbon materials (GC, microﬁbers), Pt (including Pt microelectrodes, Au electrodes), and conducting glasses are used as the working electrode supports. Figure 6.10 shows CVs recorded in buffer solution in a potential range of 0.0–1.25 V (SCE) on indium tin oxide (ITO) glass. The origin of a current at high 0.9

I (mA)

0.3

I (mA)

0.2

0.6

0.0

−0.2

0.4

0.6

0.8

E (V)

0.0

−0.3 0.0

0.4

0.8

1.2

E (V)

Figure 6.10 Synthesis of electroactive ﬁlm of nickel compounds (40 cycles) on ITO (0.83 cm2 ) in aqueous carbonate buffer solution (0.1 M NaCO3 + 0.1 M NaHCO3 , pH 11) containing 1 × 10−3 M NiTSPc; v = 0.025 V s−1 . Inset: a cyclic voltammogram of the ﬁlm of nickel compounds (40 cycles) in the carbonate buffer solution; v = 0.02 V s−1 .

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0.28 0.24 0.20

I (mA)

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0.16 0.12 0.08 0.04 0.00

0

5

10

15

20

25

30

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Number of cycles Figure 6.11 Dependence of anodic peak current I of the ﬁlm of nickel compounds on the number of cycles.

potentials was ascribed to the anodic oxygen evolution. In parallel to this process, the electroactive nickel compounds insoluble in the aqueous solutions are formed. As a result, the set of peaks at lower anodic potentials appears and the current peaks continuously increase with the number of scans. The anodic peak currents I of the ﬁlm were used to evaluate its redox activity. Figure 6.10 also shows that the current of oxygen evolution increases during cycling. This allows us to make an assumption that the formation of electroactive ‘‘nickel’’ ﬁlms catalyzes oxygen evolution. At the initial stage (1–8 cycles), the ‘‘nickel’’ ﬁlm grows very slowly (Figure 6.11). This can be related to the difﬁculties in nucleation of a new phase formed by nickel compounds. The CV, which was recorded after transferring the electrode to the buffer solution free of precursor (the inset in Figure 6.10), exhibits the conventional Ni(II)/Ni(III) chemically reversible redox transition [72]. The rather small peak width and a signiﬁcant potential difference allow us to assume that the redox transitions are accompanied by the transformation in the solid-phase ﬁlm of nickel compounds. Restructuring of the ﬁlm can be caused by entrapment of anions involved in the redox transitions of ‘‘nickel’’ (nickel compounds). Since the peak current varies linearly with the square root of the sweep rate, the polymer electroactivity is controlled by diffusion. A similar behavior is observed for the synthesis of nickel compounds at the platinum electrode. In contrast to ITO glass, the conducting oxide layer of which is stable in the buffer solution but destroyed in the alkaline solution, Pt can be used in both types of solutions. Cyclic voltammograms of Pt electrodes modiﬁed with the ﬁlm of nickel compounds are shown in Figure 6.12 (curves 1, 2). As seen, the formal potentials of the redox transitions depend on the pH of the solution. This indicated that OH− anions are involved in the redox transitions of electroactive ﬁlm of nickel compounds.

6.3 Electropolymerization of Phthalocyanines

0.02 2

1

I (mA)

0.01 4

3 0.00

−0.01 0.0

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0.4

0.6 E (V)

0.8

1.0

Figure 6.12 The redox transitions of electroactive ﬁlms of nickel compounds on (1, 2) Pt (30 cycles) and (3, 4) GC (50 cycles) electrodes in (1, 3) alkaline and (2, 4) carbonate buffer solutions; v = 0.025 mV s−1 (the electrode surface area: 0.03 cm2 ). Films of nickel compounds were synthesized in 0.1 M NaOH + 1.3 × 10−3 M NiPcTS solutions.

The hypothesis is also corroborated by the fact that the ‘‘nickel’’ ﬁlm coated with the OH− -impermeable Naﬁon ﬁlm loses its redox activity [67]. Dependence of the formal potentials of the ‘‘nickel’’ ﬁlm on pH is similar for the ﬁlms deposited onto glassy carbon (GC) substrates (Figure 6.12, curves 3, 4). Nevertheless, a sharp decrease in the rate of ﬁlm formation appears in the case of GC compared to Pt. This conﬁrms the earlier assumption [69] that Pt oxides formed at the potentials of oxygen evolution have a catalytic effect on the ‘‘nickel’’ ﬁlm synthesis. A combination of electrochemical methods (CV and impedance), XPS and electron absorption spectra, showed that the ﬁlm of nickel compounds has a complex structure. The intensive oxygen evolution gives rise to the destruction of initial nickel compound causing formation of nickel–oxygen bonds. However, the very low intensity of absorption bands corresponding to the Pc rings (visible spectrum range) indicates that the major fraction of nickel occurs in its oxygen compounds. Moreover, as a result of precursor destruction, the Pc rings partially lose nickel cation. It is interesting that the ‘‘true’’ NiPc polymer, which is synthesized from NiTAPc compound in aprotic solvents, converts into (Ni–O–)-containing coating upon long-term cycling in the aqueous alkaline solutions [71, 72]. The available XPS data do not allow us to unambiguously discriminate between two types of bindings, namely, the clusters of Ni(OH)2 and the compounds containing Ni–O–Ni bridges. Nevertheless, we believe that the formation of Ni(OH)2 is the more likely way. The coatings described in this section are typical of nickel–oxygen compounds. This element forms oxides and hydroxides capable of undergoing reversible conversions, which is the basis for their application in alkaline chemical sources [75]. The

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hypothesis of the presence of Ni(OH)2 in the electroactive ‘‘nickel’’ ﬁlm was also reinforced by electrochemical investigations. The nickel ﬁlm undergoes the redox transition at a formal potential, which is close to the redox potential recorded for Ni(OH)2 + OH− ↔ NiOOH + H2 O + e− reversible reaction. The latter occurred in the oxygen-containing layer on the nickel electrode in alkaline solutions as previously demonstrated by Kudryavtseva et al. via the ellipsometric method [76]. Meso-tetra(4-sulfonatephenyl) porphyrinate manganese presents an electrochemical behavior similar to that of NiTSPc in aqueous solutions [77]. In similar conditions to those described for producing electroactive ﬁlms of NiTSPc [67–76], Mn porphyrin also leads to a ﬁlm containing manganese oxides. However, to initiate their formation, metal silver nuclei are required. For this purpose, a small additive of silver salt is added to the initial solution. As the salt is reduced, the clusters of metal silver are formed. It is interesting that both manganese oxides and nickel oxides are applied in the chemical power sources. The coatings of manganese compounds [77] can also be used as catalysts for hydrazine electrooxidation.

6.4 Conclusion

The content of Section 6.3.2 shows that the electropolymerization of Pcs is one of most convenient and reliable ways of immobilizing the transition-metal Pcs on conducting substrates. In addition, the measurement of the charge consumed for the electrochemical polymerization constitutes an elegant tool for quantitatively controlling the thickness of the electroactive coating. Commonly, in a reasonable potential range, unsubstituted Pcs do not undergo the irreversible anodic oxidation leading to the destruction of some covalent bonds and the formation of other covalent bonds for the cross-linking of Pc molecules in the polymer. To facilitate the electropolymerization of Pcs, the readily oxidizable substituents (amino groups or pyrrolic radicals) are introduced into the Pc benzene fragments. The necessity of an additional synthesis stage imposes certain limitations on the application of Pc electropolymerization. Moreover, the electrochemical procedure cannot be used for insulating substrates necessary for some particular investigations and engineering purposes (Table 6.1). An attractive alternative may be the chemical polymerization of these derivatives, for instance, pyrrole groups can be oxidized and polymerized by FeCl3 . A comparison between the properties of Pcs immobilized on solid substrates in various matrices reveals the determining role of Pc–Pc and Pc–matrix noncovalent interactions in the synthesis of the Pc-based composites and their operation. The following classiﬁcation can be proposed for the basic types of these interactions. 1) The intermolecular interaction between π-conjugated systems of the Pc ligands leading to the formation of stacking structures of the Pc complexes. The electropolymerized Pcs (Section 6.3.2) and phase Pc ﬁlms, which are not considered in this chapter, can serve as the examples of the systems, in which this effect is most pronounced.

6.4 Conclusion

2) The electrostatic interaction between the oppositely charged Pc fragments and the matrix (or the second component) leading to the formation of composites. The composites consist of anionic Pcs in the ECP whose chains in the doped state bear a positive charge (Section 6.2.1) and the LbL assemblies composed of the ionic Pcs and the oppositely charged polyelectrolytes or the Pcs. The extreme event of the electrostatic interaction is the formation of phase salt ﬁlms insoluble in both water and organic solvents. These salts are formed by the interaction of the anionic and cationic Pcs (Section 6.2.1, Table 6.1, numbers 10, 11). It is expected that ionic Pcs and polyelectrolytes can form similar salts. 3) The hydrophobic interaction between the nonpolar Pc fragments and the matrix. The most demonstrative example of such interactions is the hydrophobic interaction between quaternary ammonium salts with long alkyl radicals and the long alkyl substituents of the Pc ring (Section 6.2.2). The Pcs immobilized by the different techniques have one common feature: they are capable of extra-coordinating a transition-metal cation. In the electrochemical investigations, the supporting electrolyte anions, the molecules of electron–donor solvents, and other compounds such as O2 , NH3 , NO, and so on can act as supplementary ligands. This extra-coordination is more likely at the origin of the high electrocatalytic activity of immobilized Pcs. Two examples of this Pc application are given in Table 6.1 (numbers 10, 11) [37, 39]. These are the composite salt ﬁlms containing FeT4MPyP, which is the common cationic component in both works, and the anionic electroactive component, Co(II)TSPc [37] or Fe(II)TSPc [39]. Both composites were employed for the development of amperometric sensors for the detection of NO2 − [37] and O2 [38] via, respectively, their oxidation and reduction of these targets. An excellent catalytic activity of the composite ﬁlms was demonstrated in both cases. Among possible applications of immobilized Pcs, their use as the working elements in the sensor devices shows considerable promise. The extra-coordination of Pc can be useful not only in the analytical studies but in other ﬁelds, for example, in medicine. The photodynamic therapy (PDT) of oncological diseases also constitutes a promising ﬁeld of application for Pcs. If Pcs are used as the light-sensitizing agent, under illumination with light with wavelengths of the Pc bands (600–700 nm), an active form of oxygen, namely, singlet oxygen, which can destroy cancer cells, appears. It is the extra-coordination of MPcs that gives rise both to the adsorption of Pcs on the cell membranes and to the oxygen bonding. The content of this chapter shows that Pcs can be immobilized by several methods. A number of matrices can be used for Pc immobilization. The conducting materials and insulators can be used as the substrates. Thus the obtained composite systems have a great variety of remarkable properties. A wide range of Pc unique features result in their actual and potential applications.

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7 Imprinted Polymers Michael J. Whitcombe and Dhana Lakshmi

7.1 Introduction

In this chapter, we present a brief introduction to the concept of the imprinting of templates in polymers and how it is achieved in practice, followed by a discussion of how electropolymerization has been used to prepare molecularly imprinted polymers (MIPs). The major part of the chapter deals with the applications of electropolymerized MIPs. By the nature of the formation of these materials, this will mainly revolve around electrochemical sensing of the template species and their analogs. 7.1.1 What is Molecular Imprinting?

Molecular imprinting can be described as a synthetic approach to mimicking the molecular recognition properties of natural systems (such as receptor proteins, antibodies, and enzymes) by inducing substrate-selective recognition in an artiﬁcial matrix by the use of a template [1–3]. The template interacts with monomers and cross-linkers present in the precursor mixture which are subsequently ‘‘locked-in’’ by cross-linking or polymerization of the matrix-forming material. The combination of steric exclusion and cooperative interactions between matrix functionality and the template both contribute to the formation of the imprinted site. This complementary space and arrangement of groups is preserved after removal of the template and retains a speciﬁc afﬁnity or ‘‘memory’’ for the templating species. Potential applications of MIPs have been discussed in a number of review papers [4–6] and a comprehensive survey of scientiﬁc papers in the area of molecular imprinting (up to 2003) has also been published and lists around 1500 references to prior work [7]. The latter review deﬁnes imprinting as The construction of ligand selective recognition sites in synthetic polymers where a template (atom, ion, molecule, complex or a molecular, ionic or macromolecular assembly, including micro-organisms) is employed in Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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order to facilitate recognition site formation during the covalent assembly of the bulk phase by a polymerization or polycondensation process, with subsequent removal of some or all of the template being necessary for recognition to occur in the spaces vacated by the templating species [7]. The formation of imprinted materials is often represented schematically and one such scheme is given in Figure 7.1. This deﬁnition encompasses a broad range of cross-linking strategies, polymerization chemistries, polymerization conditions, cross-linkers, and functional monomers as well as diverse formats such as bulk materials, membranes, beads, ﬁlms, ﬁbers, grafted coatings, and nanoparticles (NPs). The majority of MIPs reported in the literature are based on cross-linked acrylic polymers, with the combination of methacrylic acid as functional monomer and ethylene glycol dimethacrylate as cross-linker, as introduced by Mosbach and coworkers [8, 9], being the classic example. In polymers of this type, template–monomer interactions are noncovalent in nature and result from relatively weak interactions such as hydrogen bonding (noncovalent imprinting). Stronger interactions have also been utilized, such as boronate ester bonds used in the imprinting of monosaccharide derivatives (covalent imprinting), as pioneered by Wulff et al. [10–12], and hybrid approaches, such as that of Whitcombe and coworkers, where covalent bonds Step 1

Step 2

x

x

x

Non covalent assembly

c

x

a

d

x

x

b

C

B

D

Covalent modification L

e Template

x

M

x

Polymerize with cross-linker

A M

x

x

E

x x

M

Ligand exchange

Disrupt polymer-template interactions Remove template

Association L

Dissociation

M

M

Step 3

Step 4

Figure 7.1 Schematic representation of the imprinting process. Step 1: prearrangement of functional monomers around the template via covalent, noncovalent, semicovalent, electrostatic, and ligand-exchange interactions, ‘‘X’’ in the monomer structure

indicates polymer-precursor functionality; step 2: formation of cross-linked polymer, freezing-in the template–monomer interactions; step 3: template removal; and step 4: rebinding. (Adapted from [7].)

7.2 Molecular Imprinting in Conjugated Polymers

are employed in the polymerization step, but the rebinding event occurs through noncovalent interactions (semicovalent imprinting) [13, 14]. Despite the dominance of acrylic-based materials in the ﬁeld of MIPs, many other polymeric materials are commonly used, including inorganic polymers. In fact, the ﬁrst materials that were shown to display template-derived binding properties were silica-based. These were prepared by Polyakov in the 1930s [15–17] and similar work was also undertaken by Dickey around a decade later [18]. Electropolymerization has also been exploited as a means to prepare molecularly imprinted materials, the ﬁrst examples being the formation of templated metal ion binding sites in an electrosynthesized polypyrrole (PPy) matrix reported by Bidan et al. [19] and the creation by Hutchins and Bachas of a nitrate-selective electrode by imprinting doping of PPy electropolymerized on a glassy carbon electrode [20].

7.2 Molecular Imprinting in Conjugated Polymers

Among the materials that can be readily prepared by electropolymerization, π-conjugated polymers prepared from heterocycles such as pyrrole, thiophene, aniline, and their derivatives are attractive, as they can combine conducting properties with the formation of molecular imprints. The generally poor solubility and structural rigidity of conjugated polymers also contribute to their suitability as imprinting matrices, as these features will help to preserve the integrity of the imprinted sites after template removal. Among the various types of conjugated polymers studied so far, PPy has many attractive features as a molecular recognition material, since it can be used at neutral pH and its ﬁlms are stable and compatible with various substrates. The observation by Shinohara and coworkers that PPy retained permeability for chloride ions, used to dope the polymer ﬁlm, but excluded larger ions, such as ethanesulfonate, was a clue that this material could retain a memory for the anion of the electrolyte used in its preparation [21]. This ‘‘ion-sieving’’ property was demonstrated during doping and dedoping cycles with electrolytes with different-sized anions and the recorded current in cyclic voltammetry (CV) experiments could be correlated with the size of the anions. PPy undergoes overoxidation at positive potentials and this process can lead to an irreversible loss of conductivity and dedoping (Figure 7.2). While this may be undesirable for some applications, it has been reported that overoxidized polypyrrole (oPPy) also has ion-selective properties, in this case favoring cation transport [22]. These properties were capitalized upon by Deore et al. who proposed H N

H N +

X Figure 7.2

−

N H

−

−nX

n

O

H N N H

H N n

Overoxidation of polypyrrole, leading to dedoping and loss of conjugation.

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oPPy as a new matrix for molecular imprinting in 1999 [23]. This was demonstrated using l-glutamic acid (l-Glu) as the template, which can be positively charged, neutral, or negatively charged depending on pH. Stereoselective binding of l-Glu over d-Glu was shown in electrochemical quartz crystal microbalance (EQCM) measurements on an oPPy ﬁlm, electrochemically grown in the presence of l-Glu. A similarly prepared d-Glu-imprinted polymer showed the opposite selectivity. This was not the ﬁrst use of oPPy as a matrix for molecular templating, since it had been used before by Spurlock et al. to imprint a number of neutral molecules [24]; however, their materials did not show such high template selectivity as those of Deore. In a later paper, Deore and coworkers describe the interaction of l-Glu with an oPPy-based sensor prepared by the controlled overoxidation of PPy with the template incorporated as a doping anion [25]. The complementary cavity permits recognition of the anion, which is captured and released by charging and discharging of the polymer ﬁlm. The preparation process of a molecularly imprinted oPPy ﬁlm has been studied using a quartz crystal microbalance (QCM) [26]. The ﬁlm was derived from a 1-naphthalenesulfonate (1-NS)-doped PPy through overoxidation. The extraction of 1-NS (dedoping) created a shape-complementary cavity at the surface. The resulting oPPy ﬁlm imprinted with 1-NS exhibited an excellent selectivity to 1-NS with respect to 2-NS. The electrodeposition of an oPPY ﬁlm imprinted with taurocholate for use in a bile acid sensor was also reported by the same group [27]. oPPy has also been used by Syritski et al. on the EQCM to make an enantioselective sensor for l-aspartic acid [28]. Chen and coworkers described the enantioselective uptake of l-amino acids by l-lactate-imprinted oPPy colloids [29]. Colloidal oPPy was prepared by chemical oxidation in the presence of template and a stabilizing polymer (poly(vinylpyrrolidone)) followed by electrochemical oxidation of the polymer particles. Shiigi and coworkers also described the preparation, characterization, and performance of imprinted oPPy colloids that can discriminate between amino acid enantiomers and structural isomers of naphthalene sulfonate, according to the template [30]. A more convenient one-step preparation of oPPy colloidal MIPs, imprinted with l-lactate, was also reported by the same authors [31]. This involved the use of an excess of chemical oxidant (ammonium persulfate) in order to achieve overoxidation of the templated-colloid particles in a one-pot process. PPy has also been employed as a material for MIP formation in its native state, ¨ without overoxidation. A (PPy)-based imprinted ﬁlm was fabricated by Ozcan and coworkers [32] as a sensor for the determination of ascorbic acid (AA). The ﬁlm was constructed by incorporation of AA during the electropolymerization of pyrrole onto a pencil graphite electrode (PGE) using CV. A schematic representation of imprinting and removal of the template from an AA-imprinted PPy-modiﬁed PGE is shown in Figure 7.3. The template was conveniently removed using 0.05 M phosphate buffer solution. A similar synthetic strategy was followed by the same authors to prepare a PPy-based sensor for the determination of paracetamol (acetaminophen) [33]. The PPy ﬁlm was prepared by cyclic voltammetric deposition of pyrrole in the presence of a supporting electrolyte (LiClO4 ) in the presence

n

7.2 Molecular Imprinting in Conjugated Polymers

H N

OH HO HN

O

n

Polypyrrole

HO

OH

H

H N

N N H

N H

n

Pencil graphite electrode

n

(a)

H N HN

n

Polypyrrole

N H

H

H

N

N N H

n

Pencil graphite electrode (b)

Figure 7.3 Schematic representation of (a) molecular imprinting of ascorbic acid, dashed lines indicate potential hydrogen-bonding interactions and (b) ascorbic acid-imprinted polypyrrole-modiﬁed pencil graphite electrode following template removal. (Adapted from [32].)

or absence of template (paracetamol) onto a PGE to prepare imprinted and control sensors, respectively. The detection limit for paracetamol was 7.9 × 10−7 M. Molecularly imprinted polypyrrole (MIPPy)-modiﬁed PGEs showed stable and reproducible responses without any inﬂuence of interferents commonly found in pharmaceutical samples. The feasibility of integrating electrosynthesized PPy, imprinted with a neutral template (caffeine), with a piezoelectric quartz crystal (PEQC) transducer was explored by Ebarvia and coworkers [34, 35]. A similar detection system was employed by Suedee et al. for PPy-based sensors imprinted with trichloroacetic acid [36].

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An imprinted PPy-based PEQC sensor for sodium dodecylsulfate (SDS) determination has also been reported [37, 38]. Galvanostatic polymerization was used to deposit PPy membranes onto one side of the quartz crystal electrodes in the presence of the template salt. Various studies have provided proof-of-concept of the fabrication of microbially imprinted ﬁlms using conducting polymers. In this way, Bacillus subtilis endospore-imprinted conducting polymer ﬁlms and subsequent electrochemical detection of spore binding were reported by Namvar and coworkers [39]. In their work, imprinted ﬁlms were prepared by absorbing spores on the surface of glassy carbon electrodes upon which a PPy layer, followed by a poly(3-methylthiophene) layer, was electrochemically deposited. Binding of endospores to imprinted ﬁlms was detected via impedance spectroscopy by monitoring changes in Y (susceptance) using Mn(II)Cl2 (0.5 M, pH 3) as the supporting electrolyte and the change in Y could be correlated to spore densities between 104 and 107 cfu ml−1 . However, it was not possible to regenerate the imprinted ﬁlms without a loss of electrode response. PPy-based MIPs for label-free detection of bovine leukemia virus glycoprotein gp51 have been reported [40]. Reusability was also an issue with the glycoprotein-imprinted PPy ﬁlms as only a few redoping/dedoping cycles were shown to be effective. Other examples of electropolymerized MIPs that may be of interest include the following examples: Boyle and coworkers reported the creation of a sensor by electropolymerization of pyrrole in the presence of adenosine tri-phosphate (ATP) on a platinum electrode where the PPy-ATP ﬁlm imparted reversible electrochemical behavior, which further depends on the concentration of the ATP molecule [41]. That electropolymerization of phenol in the presence of phenylalanine imparted memory for amino acid and also demonstrated high sensitivity, which was reported by Panasyuk et al. [42].

7.3 Solgel Imprinted Films Prepared by Electropolymerization

The use of self-assembled solgel materials has recently emerged as a new technique for the preparation of biosensors [43] and solid-state electrochemical devices due to their high stability and sensitivity [44, 45]. The formation of a solgel-derived hydrophobic ﬁlm using an electrodeposition method was reported by Shacham and coworkers [46] but the ﬁlm was not sufﬁciently mechanically stable, especially in the piezoelectric quartz crystal (PEQC) sensor. Therefore, it was necessary to improve the solgel ﬁlm adhesion to the surface. In order to achieve a stable layer, Zhang and Zao [47] reported a novel stabilized solgel imprinted ﬁlm deposited on the surface of the PEQC Au-electrode by using the self-assembly and electrodeposition technique. This was achieved using 1,6-hexanedithiol and Au NPs during the preparation step. A signiﬁcantly negative potential was applied to increase the pH near the gold surface, which resulted in the deposition of a homogeneous thin cytidine-imprinted solgel ﬁlm on the Au-electrode surface.

7.4 Integration of MIPs with the Surface of Transducers

3-(Aminopropyl)trimethoxysilane (APTMS) was used as the silane precursor. The results show that the solgel electrodeposition method is feasible for the preparation of imprinted sensing ﬁlms. The piezoelectric response was able to sense over a broad concentration range (5.0 × 10−9 to 5.0 × 10−5 M) while electrochemical impedance was proven to be a convenient and sensitive method for studying the imprinted solgel ﬁlm and its swelling behavior upon rebinding the template molecule.

7.4 Integration of MIPs with the Surface of Transducers

For the reason of their recognition properties, robustness, reproducibility, low cost, and ease of synthesis, MIPs are considered to be promising candidates for replacing biomolecules as the recognition element in a wide range of chemical sensors. The most important part of a sensor is the recognition element, which is in close contact with a transducer. The recognition element speciﬁcally recognizes the target analyte and the transducer translates the chemical signal generated upon target binding into an output signal. Molecularly imprinted receptor systems are capable of binding target molecules with speciﬁcities on par with natural receptors. Their integration with standard industrial fabrication processes is comparatively easier than natural receptors and they are also relatively cheap to produce [48]. Indeed, MIPs have been used as the recognition element in combination with optical, acoustic, and electrochemical transducers. The response of the sensors strongly depends on how intimately the MIP and the transduction element are integrated and how efﬁcient is the communication between them. Using electropolymerization it is possible to deposit an MIP ﬁlm at a precise spot on the detector surface and to coat an electrode having a complex geometry with a homogeneous ﬁlm. Thus MIP ﬁlms have been used in combination with electrochemical [49], acoustic [34, 50], and optical transducers [51]. Other techniques for integration of transducer and polymer include the use of self-assembled monolayers [52] and surface grafting using chemical [53], UV [54], or plasma initiation. Electropolymerization of MIP ﬁlms at the surfaces of electrochemical transducers [47, 55–57] or piezoelectric transducers [58–62] is possible. In this way, uniform layers of MIPs can be deposited with good surface adhesion [58] and control over the ﬁlm thickness by varying the amount of circulated charge. However, in both types of approaches it is not possible to separately select the best condition for imprinting and binding, so the simultaneous optimization of both processes is required. Some of the problems associated with fouling of the electrode and surface renewal can be mitigated by preparation of the MIP as a membrane or thin ﬁlm on the electrode surface. There are several ways to attach conjugated MIPs to transducer surfaces. The most common and direct way is electropolymerization. In many cases, conducting monomers such as aniline and pyrrole are used, which can be oxidized at a certain potential, giving rise to the formation of free radical intermediates which condense

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to form polymers. Polymer growth can be affected by the electrochemical properties of the monomers, the applied potential, and the reaction time. Substrate-selective polyaniline electrodes were prepared by Vinokurov [63]. Although the technique is versatile, the choice of monomers through which interaction with templates occurs is rather limited compared with the range of monomers used in free radical polymerization of (meth)acrylates, which has proved successful for the imprinting of templates such as amino acids, phenols, nucleic acid, sugars, peptides, and proteins. It would therefore be beneﬁcial to utilize conventional MIPs within conducting polymer layers and a number of groups have attempted to do just that. For example, poly(3,4-ethylenedioxythiophene) (PEDOT) [56] was utilized to immobilize MIP particles onto indium tin oxide (ITO) glass as an MIP/PEDOT-modiﬁed electrode. More recently, Mazzotta et al. [64] published their work on the preparation of an electrochemical sensor for ephedrine based on the immobilization of chemically synthesized MIP particles in an electrosynthesized and oPPy matrix. This approach is quite novel and decouples the synthesis of MIP from its integration with the transducer surfaces and, therefore, the two steps can be separately optimized. The lowest detectable concentration was found to be 0.5 mM. A more elegant approach to this problem was presented by Lakshmi et al. who recently reported a new difunctional monomer, N-phenylethylenediamine methacrylamide (NPEDMA), containing both an aniline and a methacrylamide functionality, which can help to bridge the gap between electrosynthesis and traditional acrylic MIPs [65]. This monomer is capable of electrochemical polymerization onto the surface of a suitable electrode, giving a conductive polyaniline surface with a high density of double bonds, capable of being utilized as reactive sites for further grafting of acrylate-based MIPs. Thus a sensor was constructed by grafting a methacrylate-based MIP, which acts as a redox catalyst for the oxidation of catechol, onto the substituted polyaniline ﬁlm after activation with a photochemical initiator [66]. The result was closer integration between the catalytic sites of the MIP and the electrode, thought to be due to the presence of ‘‘molecular wires’’ facilitating conduction between the catalytic centers of the MIP and the gold electrode surface (Figure 7.4). The resulting sensor showed selectivity for catechol and dopamine over a range of analogs and the limit of detection for catechol was found to be 228 nM.

7.5 Nanostructured Materials

Some of the main problems associated with MIPs are low selectivity and slow kinetic responses. A good solution to these problems can be surface grafting, where thin polymer ﬁlms are attached to the support and all the binding sites are near the surface and readily accessible to the target molecules. This approach can however give rise to a large reduction in the binding capacity within the same device area, thereby reducing the sensitivity when integrated with a transducer.

7.5 Nanostructured Materials

141

O NH

Gold on glass electrode (5 nm Cr + 45 nm Au, evaporated coating)

H N

HN

N

n O

Poly[NPEDMA]

NPEDMA

Electro polymerization UV

S

Immobilization of photo-iniferter

S N

MIP layer

UV, photochemical grafting of MIP

Iniferter-modified double bonds on surface of poly[NPEDME] layer

} ∼100 nm

Catechol (template), CuCl2, urocanic acid ethyl ester, ethyleneglycol dimethacrylate, DMF

Template removal

Catalytically active imprinted sites

1 2

OH

O

OH

O

O2 +

+ H2O Oxidation of catechol to o -quinone

NH N

N

NH

Imprinted polymer layer

Cu(II)

Cu(II) N NH

N

NH

2e − 2e −

Poly[NPEDMA] Gold

Figure 7.4 Schematic representation of the construction of a catalytic MIP-based sensor for catechol using a novel electropolymerized monomer to create ‘‘molecular wires’’ between the catalytic MIP sites and the underlying gold electrode. (Adapted from [66].)

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This problem can be mitigated by the use of nanostructured materials for molecular imprinting, to enhance the surface area for the same amount of polymer. Recently, Wei and coworkers [67], have proved this concept by using both carbon nanotubes (CNTs) and semiconductor quantum dots (encapsulated into a PPy ﬁlm) to increase the surface area of their microsolid-phase preconcentration device for ochratoxin A (OTA) in red wine. Since the discovery of CNTs in 1991, they represent a new kind of carbon material that has been widely recognized as the quintessential nanomaterial [68]. CNTs possess a high electrocatalytic effect and a fast electron-transfer rate [69] and they are promising materials for design of electrochemical sensors and biosensors. In addition to the potential electrochemical applications of CNTs, their high conductance, tensile strength, chemical stability, ultrasmall size, and poor solubility make them potential candidates for the development of novel analytical devices. Integration of CNT and CPs in the form of composites presents new opportunities to produce superior materials for various applications. Yu and coworkers [67, 70–72] prepared MIPs by electropolymerization of pyrrole (MIPPy) onto a stainless steel frit using OTA as the template and CNTs as nanostructured ﬁbers to make a microsolid-phase preconcentration device, where the limit of detection for OTA was 12 ng ml−1 (ppt). The use of MIPPy/CNT-modiﬁed stainless steel frits in the rapid extraction of small amounts of OTA was compared with traditional microsolid-phase extraction cartridges and MIPPy-modiﬁed stainless steel frits [73] (prepared without CNTs), where the limit of detection was only 0.05 mg ml−1 (ppb). Recently, Choong and coworkers proposed a new sensing platform prepared by surface imprinting electropolymerization of a PPy thin ﬁlm on an array of vertically aligned CNTs which gave rise to high surface-to-volume, 3-D scaffold for the deposition of MIPPy. The performance of CNT-MIP architecture was assessed for caffeine imprinting (dense and sparse-type CNT arrays). Owing to the small tube-to-tube spacing in the dense CNT array, electrode fouling was observed during the detection of saturated caffeine. These types of sensors outperformed the conventional thin ﬁlms by 3.6 times (sparse) and could be suitable for the attachment of almost any desired chemical species [74]. NPs are another aspect of nanotechnology and they can also play a part in electropolymerized MIP sensors, as the following examples show. A thin ﬁlm of an electropolymerized MIP with sensitive and selective binding sites for dimethoate is described in the work of Du and coworkers [75]. Thus a ﬁlm of poly(o-phenylenediamine) was prepared on a glassy carbon electrode by electrochemical polymerization with the template (dimethoate) from solution with further deposition of Ag NPs (Figure 7.5). Under the optimal experimental conditions, the peak currents were proportional to the concentrations of dimethoate in two ranges, from 1.0 to 1000 ng ml−1 and from 1.0 to 50 µg ml−1 , with a detection limit of 0.5 ng ml−1 . Owing to the high afﬁnity, selectivity, and stability, the imprinted sensor provides a simple detection platform for organophosphate compounds. A novel approach for the design of functionalized Au-NP electrodes for the sensitive electrochemical analysis of trinitrotoluene (TNT) was recently described

7.6 Other MIP-Based Sensors

Electropolymerization Glassy carbon electrode

20 cycles Deposition

Extraction

Selectivity

AgNPs Monocrotophos

Dimethoate Carbaryl

Figure 7.5 Schematic diagram of the construction of a dimethoate-imprinted sensor for recognition of dimethoate. (Adapted from [75].)

[76]. Thioaniline-capped Au NPs were electropolymerized on the electrode. The oligoaniline units that bridge the Au NPs act as π-donor sites that concentrate TNT at the electrode surface, and Au NPs provide three-dimensional conductivity for the electrochemical detection of TNT. Imprinted PPy nanowires with an average diameter of 100 nm were successfully prepared by electrochemical polymerization for chiral amino acid recognition in which l- or d-camphorsulfonic acid (l-or d-CSA) molecules acted as both the dopant and molecular imprinting pseudotemplate [77]. PPy nanowires have greater surface areas, giving rise to higher sensitivity and faster response times than bulk thin ﬁlms of the same material. This method provides a potential route for the fabrication of PPy nanostructures with enantioselective recognition properties that could be employed in sensing or in chiral separations.

7.6 Other MIP-Based Sensors

On the basis of the type of binding event occurring on the transducer, MIP-based sensors can be categorized as electrochemical, optical, or piezoelectric [32, 78–81]. Further subdivision is possible where afﬁnity sensors can be classed either as immunosensors (or receptor-type sensors) or as catalytic sensors. Immunosensors represent the most common type of MIP-based sensor.

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7.6.1 Piezoelectric Sensors

During recent years, there have been many examples of the use of acoustic wave transducers for the design of MIP-based sensors. The QCM [82] is a bulk acoustic wave sensor platform that is relatively low cost, highly robust, and easy to use. A piezoelectric quartz crystal sensor incorporating a PPy-based MIP for sodium dodecylsulfate was described by Albano and Sevilla [37] where the PPy membrane was deposited onto the side of the electrode by galvanostatic polymerization. Similarly, a piezoelectric biomimetic sensor for the determination of caffeine, based on PPy, was described by Cabanilla and coworkers [35]. This sensor exhibited a linear response over the range of 0.1–10 mg ml−1 . One of the major problems associated with this type of transducer system when used in the liquid phase is that it is sensitive to changes in viscoelasticity close to the surface. Therefore, reference sensors with nonselective polymers have been used to mitigate these problems. This approach was used by Malitesta et al. who used QCM to detect glucose using imprinted poly(o-phenylenediamine) electropolymerized directly onto the surface of electrode in the presence of 20 mM glucose. In this way, one can achieve very thin polymeric layer which could rebind glucose with selectivity over other interfering compounds [58]. Various electrochemical sensors and piezoelectric enantioselective sensors and biosensors are described in a review written by Trojanowicz and Wcislo [83]. This report covers the development of sensors for potentiometric, amperometric, and mass-detecting devices involving QCM. Despite the increasing number of applications of biosensors in various ﬁelds, the construction of stable biosensors still remains very challenging. Feng and coworkers [60] fabricated biosensors based on an electropolymerized MIP for the detection of sorbitol. Imprinted ﬁlms were prepared using o-phenylenediamine and QCM was employed as a method of detection. The sensitivity of piezoelectric sensors is generally lower than those of optical and electrochemical sensors. One way to improve the sensitivity of acoustic devices is by increasing the resonance frequency of the sensor [84]. 7.6.2 Capacitive Sensors

Capacitive sensors incorporating electropolymerized phenol-based MIPs were prepared for phenylalanine [42]. Using electrochemical impedance spectroscopy, the insulating properties of the polymer layer were studied. Electrical leakage through polymeric layers was reduced by the use of self-assembled monolayers of mercaptophenol before polymerization and of alkanethiols after polymerization, followed by template removal. The same authors also reported a capacitive sensor for creatinine based on a photografted MIP [85]. Capacitive sensors based on electropolymerization of an o-phenylenediamine ﬁlm doped with an ion-pair complex as selective element for the detection of pentoxyverine were described by Yin in

7.6 Other MIP-Based Sensors

2004 [86]. Cheng also employed electropolymerized, o-phenylenediamine-based polymers to make a glucose biosensor [87]. CV and capacitive measurements were used to monitor the process of electropolymerization. Minimal interferences were observed with AA and fructose. A poly(p-aminobenzene sulfonic acid) ﬁlm for the detection of pazuﬂoxacin mesilate was reported by Zhou and coworkers [88]. Similarly, a novel electrosynthesized poly(dopamine)-imprinted ﬁlm for the capacitive sensing of nicotine was described by Liu and coworkers [89]. On the other hand, a capacitive immunosensor on a glassy carbon electrode was fabricated successfully by using an insulating poly(o-phenylenediamine) ﬁlm where covalent coupling and copolymerization techniques were used to immobilize antitransferrin onto the electrode for sensitive detection of transferrin [90]. Compared to self-assembled monolayers this method is simple and provides a new approach to prepare capacitive immunosensor on carbon substrates. Recently, using electrochemical copolymerization of o-phenylenediamine (o-PD) and dopamine, an MIP was synthesized for the recognition of Glu [91]. The resulting MIP capacitive sensors showed good reproducibility, stability, and high precision. Liao and coworkers [61] reported a capacitive MIP sensor for tegafur constructed by electropolymerization of m-aminophenol onto the surface of gold and gold-coated quartz crystal electrodes. QCM and electrochemical impedance were used to characterize the electrode performance. Unlike earlier work done on capacitive sensors, they did not use alkanethiols after electropolymerization but the results were satisfactory. 7.6.3 Amperometric and Voltammetric/Potentiometric Sensors

Piletsky and coworkers developed the ﬁrst MIP-based amperometric sensors for the detection of aniline and phenol [92]. Other electrochemical MIP sensors based on conductometric transducers have been constructed. These measure change in conductivity of a selective layer, which is in close proximity to a pair of electrodes, when it interacts with the chosen analyte. Conductometric sensors are often based on ﬁeld-effect devices. The review published in 2004 by Blanco-L´opez and coworkers [93] examines the literature on noncovalent imprinted polymer-based electrochemical sensors over the last 10 years. The same group [94] also published an interesting and useful work which summarizes the different recognition elements and integration strategies for voltammetric sensors which are (i) membrane-electropolymerized at electrode surfaces; (ii) drop-coating and casting of polymeric membranes; (iii) composite membrane preparation and acrylic-type MIPs, and (iv) in situ polymerization of a thin layer of acrylic monomers, prepared by spin coating. Preparation of potentiometric MIP sensors based on electropolymerization was ﬁrst described by Vinokurov [63], where the monomers themselves acted as templates to form speciﬁc polymers. In this way, sensors for pyrrole, amines, and substituted phenols were prepared using PPy, polyaniline, and an aniline-p-aminophenol copolymer, respectively. Merkoc¸i and coworkers [95] described various aspects

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7 Imprinted Polymers

of electrochemical sensing, presenting an overview of the use of MIPs for the design of electrochemical sensors based on different signal-transduction schemes. Poly(2-mercaptobenzimidazole) (PMBI) ﬁlms were prepared on the surface of gold electrodes by electropolymerization using imprinting technology for the detection of cholesterol [96]. A cholesterol-selective sensor based on PMBI ﬁlms was used with differential pulse voltammetry for detection, using ferricyanide as the mediator. The limit of detection was found to be 0.7 µM. On the other hand, a novel voltammetric sensor has been described, with electropolymerization of o-phenylenediamine and resorcinol on a glassy carbon electrode in the presence of the template molecule 2,4-dichlorophenoxyacetic acid (2,4-D), to form the receptor layer [81]. 7.6.4 Miscellaneous Sensing Systems

Electrochemical sensors, fabricated on nickel electrodes using d- and l-tyrosine as templates, to form complementary cavities in PPy for subsequent template recognition were reported by Liang [49]. Using coulometry, the performance of the imprinted ﬁlms was evaluated using an applied positive potential to induce adsorption of the target compound and to impart high enantioselectivity on each of the imprinted ﬁlms for the respective template. Several other conducting polymers have also been employed to construct electrochemical sensors using MIP technology. For example, saccharide-imprinting of poly(aminophenylboronic acid) in the presence of ﬂuoride was described by Deore and coworkers [97]. An attempt was made to combine imprinting and electropolymerization of self-assembled o-aminothiophenol (o-AT) to prepare imprinted ﬁlms using nitrobenzene as the target analyte [98]. Stable responses could be achieved in just 2 min and the imprinted polymer ﬁlm also showed good stability, even under harsh conditions. The use of a competitive electrochemical assay, employing ferrocene-modiﬁed monosaccharides as the redox labels and imprinted polymer ﬁlms for the selective detection of d-glucose and d-mannose, was reported [99]. To prepare the MIPs, phenol was co-electropolymerized with 3-hydroxyphenylboronic acid–monosaccharide complexes of either d-glucose or d-mannose on a gold support. Voltammetric microsensors for paracetamol, based on electro-copolymerized molecularly imprinted ﬁlm-modiﬁed carbon ﬁber microelectrodes, were reported by G´omez-Caballero and coworkers [100]. The polymeric ﬁlm was obtained by electro-copolymerization of o-phenylenediamine and aniline in the presence of the template molecule using CV. The response of the imprinted microsensor to paracetamol was linearly proportional to its concentration over the range 6.5 × 10−6 to 2.0 × 10−3 mol l−1 , with good stability and reproducibility (RSD < 5.6%). The detection limit was 1.5 µM. Under the experimental conditions, the MIP sensor was able to differentiate between paracetamol and other closely structurally related interferents present in biological ﬂuids, such as certain catecholamines.

7.7 Conclusion

Riskin and coworkers [101] have demonstrated that π-donor–acceptor interactions between a polymer and an imprint molecule can be used in the electrochemically induced imprinting of a polyphenol ﬁlm on Au electrodes. The modiﬁed electrode could be used for selective measurement of the herbicide, N,N -dimethyl-4,4 -bipyridinium, (methylviologen, MV2+ ). In their method the electropolymerized dialkoxybenzene units exhibit π-donor properties, and hence generate π-donor–acceptor complexes with MV2+ during the synthesis of the ﬁlm. The formation of π-donor–acceptor complexes leads to a buildup in the concentration of MV2+ near the electrode surface, and the wrapping of the polymer around the MV2+ substrate molecules. The subsequent rinsing of the polymer and exclusion of the substrate from the polymer yielded the MIP for MV2+ . A highly cross-linked polyviologen ﬁlm [102], prepared by reductive electropolymerization of a trifunctional monomer with three 4-cyanopyridimium moieties, was described by Kamata et al. [103]. The polymeric ﬁlms showed clear electrochromic behavior, characteristic of a viologen redox process. A microﬂuidic system for the detection of morphine using an MIP ﬁlm and amperometric sensor has been described [104]. 3,4-Ethylenedioxythiophene (EDOT) was used as monomer which was mixed with morphine prior to the electropolymerization process on a sensing electrode. The sensitivity for morphine was 171.5 µA cm−2 mM−1 . These types of multifunctional electrochemical detection systems are feasible with microﬂuidic/MIP-based electrochemical sensing technologies. Recently, metal-complexing polymer-coated electrodes, synthesized by oxidative electropolymerization of ethylenediamine tetra-N-(3-pyrrole-1-yl)propylacetamide (monomer L), have been reported [105]. On the same EDTA skeleton, four polymerizable PPy fragments were used to confer enhanced rigidity and controlled dimensionality to the resulting complexing materials which were used for the electrochemical detection of Hg(II), Cu(II), Pb(II), and Cd(II) using a preconcentration-anodic stripping technique. It was observed that using imprinted polymer-coated electrodes prepared by electropolymerization of L in the presence of metal cations signiﬁcantly improved the detection limits: down to 5 × 10−10 mol l−1 for Hg(II) and Cu(II) species.

7.7 Conclusion

The general principle of molecular imprinting is based on a process where functional and cross-linking monomers are copolymerized in the presence of a target analyte (the imprint molecule), which acts as a molecular template. Electropolymerization can allow for the generation of thin ﬁlms of MIP, synthesized in situ at an electrode surface. This technique has some attractive features including the ready adherence of the polymeric ﬁlms to the surfaces of conducting electrodes of any shape and size and the ability to control thickness of the ﬁlms under different deposition conditions.

147

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There is no doubt that the domain of electropolymerized MIPs will eventually reach well beyond the pioneering applications of sensing, but for now a better understanding of the fundamentals associated with electropolymerized MIPs, MIP-based sensors and the challenges associated with them are still ripe areas for study. We have set out, therefore, in this brief chapter to give a ﬂavor of the role that electropolymerization methods have to play in molecular imprinting and how they can impinge on quantitative analytical chemistry in particular. The work also demonstrates that there is signiﬁcant room for improving the integration of MIPs with transducer surfaces and points to some methods by which this may be achieved. References 1. Sellergren, B. (2001) Molecularly

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8 Gas Sensing with Conducting Polymers Karin Potje-Kamloth

8.1 Introduction

Conducting polymers belong to the group of organic semiconductors. They have received considerable attention recently owing to their numerous potential applications in electronic components such as Schottky diodes [1], ﬁeld-effect transistors (FETs) [2], and electroluminescent diodes [3, 4]. Because of the inherent ﬂexibility of organics and their relative ease of processing, as well as the easy tunability of electric properties by means of chemical substitutions, these materials provide new avenues in the development of electronic devices. Furthermore, these materials exhibit a particularity concerning their interaction with gases and vapors [5]. Interacting electrically neutral gas molecules can donate/accept fractional electronic charge, thereby changing the electronic properties of the matrix. The modulation of electrical properties of polymers upon interaction with gas depends on the type of the conducting polymer. The following sections give an overview of the basic properties of conducting polymers that are required to understand the operation of conducting polymer-based gas sensors. The mechanism of interactions between conducting polymers and gas or vapor species that is used as a transduction principle in gas sensors is discussed. Examples of the gas-sensing properties of conducting polymer gas sensors are given. They show how experimental and structural parameters of conducting polymers can inﬂuence the sensor characteristics.

8.2 Electronic Properties of Conducting Polymers

Polymers are typically associated with ﬂexible, processible materials having electrically insulating properties. Although this is true of most polymers, a special class of these materials called conjugated polymers has the electrical and optical properties traditionally associated with metals and semiconductors, yet they retain the mechanical properties and the processibility of plastics [6]. Conjugated polymers Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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8 Gas Sensing with Conducting Polymers

*

*

n

S

*

n

*

trans - Polyacetylene

[PA]

Eg = 1.4 eV

Polythiophene

[PT]

Eg = 2.1 eV

Polyaniline

[Pani]

Eg = 2.2 eV

Poly(p -phenylene vinylene)

[PPV]

Eg = 2.5 eV

Poly(p -phenylene)

[PPP]

Eg = 3.0 eV

Polypyrrole

[PPy]

Eg = 3.2 eV

H N

*

n

*

* n

*

*

N

n

*

n

*

*

Figure 8.1 Schematic representation of chemical structures of the most commonly studied conducting polymers, as well as their common nomenclature and energy band gap Eg .

are characterized by repeated units, in which the π-electrons are delocalized over large segments of the polymer chain. They form the so-called π-conjugated system, which is responsible for the electronic properties of the conducting polymers. The chemical structures of the most thoroughly studied conducting polymers are shown in Figure 8.1. Typical conjugated polymers include straight chain units, ﬁve or six-membered rings, and their combinations. The addition of heteroatoms (atoms other than carbon and hydrogen) and side chains allows for an even larger variety in this class. Their electronic properties can be tailored by the synthesis, and their electrical conductivity can be varied from about 10−12 S cm−1 to values in excess of 105 S cm−1 . Charge injection (holes or electrons) into the conjugated backbone of the polymer is called doping. It leads to the formation of a self-localized electronic state within the previously forbidden semiconductor bandgap and causes a chain deformation around the charge. Thus, the added charge on the polymer is not a free electron or hole, but is a localized particle connected with a chain deformation, which is known as polaron or bipolaron state depending on the degree of doping. To match conventions between the physicist and chemist, a polymeric cation-rich material is called p-doped, and a polymeric anion-rich material is called n-doped.

8.3 Preparation of Polymer Gas-Sensing Layers

Besides this charge injection, doping in polymers also implies the insertion or the repulsion of counter-ions, that is, dopant anions, in order to maintain charge neutrality. Because every monomer is a potential redox site, conjugated polymers can be doped to a high density of charge carriers; the doping level is up to 5 orders of magnitude greater than that in common inorganic semiconductors. Doping in polymers can be accomplished in a number of ways: chemically, electrochemically, and photochemically, as well as by charge injection at the metal–insulator–semiconductor interface. In the case of electrochemical and/or chemical doping, the neutral polymer chain is oxidized (p-doping) or reduced (n-doping) to form polaron or bipoloaron state. The induced electrical conductivity is permanent, until the charge carriers are purposely removed by undoping, that is, by reversing the (electro)chemical reaction at the redox sites of the polymer chain. The creation of polaron or bipolaron electronic defects allows the charge transport within a single chain. Even though there is a high density of charge carriers, they may be spatially delocalized so that they cannot participate in electronic transport except through hopping. The prime source of localization of charge carriers in conducting polymers is structural disorder, which decreases the conductivity by lowering the charge carrier mobility. The dopant compounds play a very important role in hopping transport.

8.3 Preparation of Polymer Gas-Sensing Layers 8.3.1 Solvent Casting

Solvent casting processes are usually applied to sensor devices, which demand the deposition of sensitive conducting polymers on nonconductive or over partially nonconductive areas, for example, on top of interdigitated electrodes of conductometric sensors or on the gate dielectric of chemical-sensitive ﬁeld-effect transistors (CHEMFETs). Solvent casting methods such as spin coating, spray coating, and drop casting lack in pattern precision and in uniformity in the layer thickness. Moreover, most conducting polymers are insoluble in the doped form, which can be overcome by doping after solvent processing by, for example, electrochemical or photochemical doping (Section 8.3.3) or vapor impregnation. Other approaches to overcome this drawback use either tensides as dopant anions [7, 8] or conducting polymers substituted with pendant solubilizing groups [9]. 8.3.2 In situ Electrochemical Deposition

The electrochemical deposition of conducting polymers is a reasonably controllable process because ﬁlms can be electrodeposited onto metallized areas and the ﬁlm thickness can be varied by monitoring the total charge passed during the deposition

155

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8 Gas Sensing with Conducting Polymers

process. Polymers can be deposited using a number of different electrochemical waveforms, for example, potentiostatically, galvanostatically, cycled potential, or pulsed potentiometry. The polymerization is usually carried out in a three-electrode conﬁguration, in which the working electrode is the sensor substrate. A positive oxidation potential that is sufﬁcient to oxidize the monomer is often applied to the working electrode, following which the monomer undergoes oligomerization and the oligomers electroprecipitate onto the electrode surface. Measurement of the total charge passed gives an approximation of the ﬁlm thickness, and the ﬁnal potential applied to the material will control the extent of doping. Several strategies can be employed to obtain conducting polymers used as sensing element, each of them having a different response toward a number of analytes. They include (i) electrochemical polymerization of different single-ring heterocycles and multiring fused/unfused heterocycles, such as pyrrole, thiophene, aniline, indole, and carbazole; (ii) application of different polymerization conditions (i.e., by changing the oxidation potential, oxidant, temperature, solvent, electrolyte concentration, monomer concentration, etc.) to the same monomer to obtain polymers with different properties such as morphology, molecular weight (or conjugation length), connectivity of monomers, conductivity, band gap, and so on [10, 11]; and (iii) the use of different counterions (i.e., dopants) to compensate the positive charge (for a p-doped polymer) generated upon oxidation of the conducting polymer to produce the electrically conductive state of the polymer [12, 13]. 8.3.3 Tuning of Electronic Properties of Conducting Polymers 8.3.3.1 Effect of Primary Doping on Work Function Besides the preparation condition, discussed above, the nature of dopant anion is one of the main factors that inﬂuences the sensing properties of the conducting polymers [10, 12, 14–18]. The dopant anion is incorporated during the in situ oxidation of the polymer to the conducting state, which is also called the primary doping process. This opens the possibility of tuning the conducting polymer with different dopant anions to obtain a series of materials exhibiting different values of electron afﬁnity of the polymer matrix, expressed as the initial work function, which is shown to be an important property of conducting polymers used in gas-sensing devices [16]. 8.3.3.2 Electrochemical Work Function Tuning Electrochemical tuning of the doping level and, hence, of the work function is preferably carried out in solution. Doping by electrochemical procedure offers the inherent advantage of precise control over the doping process via the charge injected and the amount of counterion incorporated during electrochemical oxidation/reduction of the conducting polymer. When compared to chemical doping by strong oxidants or reductants, no interference from reduced/oxidized species remaining in the polymer matrix as by-products after chemical doping is obtained. For p-doped conducting polymers, the oxidation state can be controlled

8.4 Mechanism of Gas/Polymer Interactions

by applying different oxidation potentials, whereas the doping level was adjusted by regulating the amount of charge injected.

8.4 Mechanism of Gas/Polymer Interactions 8.4.1 Secondary Doping by Donor/Acceptor Interactions

In 1980, Van Ewyk et al. proposed the formation of a charge-transfer complex associated with the transfer of a partial amount of electron density due to an electron donor/acceptor interaction during the adsorption of strong electron donating or accepting gases on a matrix, which is considered to be an inorganic p-type semiconductor [19]. While the classiﬁcation of donor and acceptor gases is useful, these terms are relative when conducting polymers are concerned, since they are redox-active materials and can be either an electron donor or electron acceptor depending on the work function of these matrices. The gas exposure introduces changes, in analogy to inorganic semiconductors, in the occupancy level at the valence band edge and the conduction band edge, respectively, and leads to a variation of the doping level of the conducting polymer. It was assumed that this type of reaction is also responsible for the chemical modulation of the work function and conductivity of the polymer layer due to gas absorption [16, 20]. This type of interaction process is often described as a secondary doping of the sensitive polymer layer [21]. 8.4.2 Work Function Modulation – Modulation of Carrier Density

Janata proposed a model that describes the potential concentration relationship based on the formation of a charge-transfer complex between the secondary dopant (neutral gas or vapor molecule) dissolved in the conducting polymer matrix and the activated sites of the matrix (i.e., sensitive phase). It is based on a redox process, which is combined with the transfer of a fractional charge δe per molecule [21]. Assuming that the conducting polymer has a discrete energy band structure and that the gas is an electron donor, the charge density is transferred to the conduction band (i.e., δ < 0), whereas for an electron-accepting gas, a charge-transfer complex is formed between the gas and the valence band (i.e., δ > 0). In either case, the direction of the charge ﬂow is governed by the difference in the electron afﬁnity of the conducting polymer matrix and the gas molecule. This interaction can be viewed as a normal doping process, which inﬂuences the position of the Fermi level EF of the conducting polymer. The dependence of the Fermi level of the sensitive phase on the partial pressure of the dopant gas or vapor is given by EF = E0∗ +

kT ln pgas + const 2δ

(8.1)

157

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8 Gas Sensing with Conducting Polymers

where pgas is the partial pressure of the gas or vapor, k is the Boltzman constant, and T is the absolute temperature. In E0∗ the donor/acceptor level and the equilibrium coefﬁcients of all relevant reactions are combined. A change of the Fermi level CP is reﬂected in the change of the work function φWF of the conducting polymer. Equation 8.1 has the familiar form of the Nernst equation for ion and electron transfer across the interface of two condensed phases, except that they account for the fractional charge transfer δe. Whether the vapor acts as an electron donor or electron acceptor and what fraction δ of charge is transferred depend on the position of the Fermi level of the conducting polymer relative to the Mulliken electronegativity χgas [22] of the guest gaseous species (8.2) δ = ξ EF − χgas where the coefﬁcient ξ is an unspeciﬁed coupling factor between the gas molecule and the electron afﬁnity of the interacting, sensitive polymer matrix [23]. 8.4.3 Bulk Resistance Changes

In chemical sensing, the most interesting electronic property of conducting polymers, besides work function and optical absorption, is the conductivity σ , or its reciprocal, the resistivity. The bulk conductivity is a function of both the concentration and the mobility of the charge carriers of the conducting polymer: σ = nn µn + np µp

(8.3)

The subscripts n and p denote electrons and holes, respectively. The environment affects both properties. The carrier concentrations can be changed by the interactions of the conducting polymer with electron donor/acceptor gases, that is, by the formation of a charge-transfer complex. A nonlinear response of conductivity (or resistance) change of a chemiresistor to various analyte concentrations is difﬁcult to interpret. It is most likely due to the modulation of the contact resistance change of the electrode metal–polymer junction of the chemiresistive sensor rather than a modulation of the bulk resistance of the conducting polymer. 8.4.4 Contact Resistance Changes (Schottky Barrier)

A Schottky barrier is created by the intimate contact of a metal of work function S S m m and a semiconductor of work function φWF , if φWF is larger than φWF for φWF an n-type semiconductor and vice versa for a p-type semiconductor. The band of an organic semiconductor in contact with a metal is bent in order to achieve the alignment of the Fermi level of the metal with the Fermi level of the organic layer ﬁxed somewhere in the bandgap (i.e., Mott–Schottky limit), simply by analogy to an interface between a doped inorganic semiconductor and a metal [24]. The zero-bias band bending or built-in voltage Vbi can be exploited in gas sensors. It is determined by the difference in the Fermi levels of the metal and the semiconductor.

8.5 Types of Conducting Polymer-Based Gas Sensors

The secondary doping process of the gas or vapor is expected to cause a change in the characteristic band bending at organic semiconductor/metal interfaces and, hence, a change in the current–voltage characteristics of the Schottky barrier junction sensor (Section 8.5.2.2). It should be noted that semiconductor junction gas sensors are strongly related to the nature of the interfacial electronic structure of the Schottky barrier [25]. An extensive review on the properties of conducting polymer Schottky barrier junction and their sensor applications is given in [26].

8.5 Types of Conducting Polymer-Based Gas Sensors

The broadest classiﬁcation of gas sensors is based on whether the current passes through the organic semiconductor or not. There are several sensor conﬁgurations in which the transduction principle of the work function modulation of conducting polymers can be employed [27]. It is utilized in equilibrium potentiometric sensors, such as the macroscopic Kelvin Probe and its solid-state miniature counterpart, the CHEMFET. Both are based on zero-current potentiometry. The functional relationship between sensor response and the partial pressure of the gas can be described by a generalized form of the Nernst equation (Equation 8.1). The well-known chemical modulation of work function also enters into the operation of a second type of sensor based on organic ﬁeld-effect transistors (OFETs), which is discussed in Section 8.5.2.3. Other types of sensors based on work function modulation are Schottky barrier junctions (Section 8.5.2.2) and various forms of chemiresistors (Section 8.5.2.1). In these sensors, the response is obtained while current passes through the material. 8.5.1 Potentiometric (Zero-Current) Sensors 8.5.1.1 Kelvin Probe Measurements of the work function with the vibrating capacitor (Kelvin probe) are based on the fact that there is an electric ﬁeld in the capacitor formed from two materials of different work function. Both bulk and surface contributions are included in the overall result [28]. The resultant potential difference φWF between the capacitor plates is equal to the contact potential difference. For thermodynamic reasons, it is not possible to measure the absolute work function of one plate of a capacitor. For gas-sensing measurements, one plate acts as the working plate covered with the gas-sensitive layer, for example, conducting polymer, and the second one is the reference plate, which preferably should not interact with gases or vapor species. 8.5.1.2 CHEMFET The interaction between conducting polymer and electrically neutral gas species has been used as a transduction principle in insulated gate ﬁeld-effect transistors

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8 Gas Sensing with Conducting Polymers

Sensing layer Insulator

Gas

VG

Au

VG

Au

n-type

Organic semiconductor

Au

n-type

p-type silicon (a)

Gas

Au

n-type

VDS

n-type

VDS

Insulator

p-type silicon (b)

IGFET/CHEMFET

OFET

VG

Figure 8.2 Schematic diagram of (a) a CHEMFET and (b) an OFET based on IGFET structure. The sensing layer is a conducting polymer. (Adapted from [30].)

(IGFETs) sensors since the late 1980s [29], but has been mostly neglected in other nonsensing applications of the same device structures. The structure of the IGFET is shown in Figure 8.2a. The metal-insulator-semiconductor (MIS) structure (sensing layer/gate dielectric/Si) represents the gate itself. The gate conductor material is chosen such that its properties can be modulated by an external chemical stimulant, for example, gas or vapor species able to interact electronically with the gate conductor material. The conductivity of the channel region between the drain and the source electrodes is modulated by the electric ﬁeld in the gate capacitor. The threshold voltage VT is directly related to the ﬂat band voltage VFB , which is the difference between the Si CP work function of silicon φWF and the gate electronic conductor φWF , in this case, the conducting polymer. The behavior is described by well-known equations for the IGFET operation, which can be found in [31]. For sensing applications, the saturation regime is preferable, because it yields the explicit relationship between the change of the Fermi energy level (i.e., work function) and the operating gate voltage, VG . If the transistor is operated in the feedback mode (i.e., constant current ID ), the change of output voltage VG can be expressed as a function of change of partial pressure of the analyte gas pg as   kT  pgas o ln + 1 (8.4) VG − VG = VG = 2eδ Ki p i

VG0

where is the gate voltage at zero analyte pressure, and Ki is the selectivity coefﬁcient with respect to background gases [23]. Changes in concentration of the CP analyte cause a change in the work function of the gate electronic conductor φWF , that is, the conducting polymer, leading to a corresponding change of the output gate voltage VG [28]. Thus, the sensitivity of the CHEMFET is a logarithmic function of the vapor concentration. 8.5.1.3 Examples of Kelvin Probe and CHEMFET Gas Sensors Several studies deal with the inﬂuence of the nature of the dopants on the chemical-sensing behavior of conducting polymers to selected vapors and gases

8.5 Types of Conducting Polymer-Based Gas Sensors

200 Acceptor

100

Isopropanol

Chloroform

−100 Methylene chloride −200

Donor

∆fWF (meV)

0

Methanol −300 −400 0 200 400 600 −600 −400 −200 fWF, initial − cgas / meV vs Au

800

Figure 8.3 Work function modulation φWF of conducting polymers exhibiting different initial work functions φWF,initial upon exposure to different vapors. The designations ‘‘acceptor’’ and ‘‘donor’’ refer to the vapor. (Adapted from [16].)

[13, 16, 32, 33]. The initial value of the work function φWF,initial of conducting polymers based on PPy and p-polyphenylene doped with different anions was adjusted by the nature of the dopant anions [13, 16] and the afﬁnity of doped conducting polymer to different gases was investigated [16]. The results shown in Figure 8.3 indicate that a given gas can behave with respect to the matrix either as a donor or as an acceptor of electrons depending on the initial value of the φWF,initial of the polymer and the Mulliken electronegativity χgas of the vapor (Equation 8.2). The importance of this semiquantitative result is that by adjusting the initial value of the work function, it is possible to tune the afﬁnity of the conducting polymer to different gases according to Equation 8.1 Since the gas type and the doping concentration remained constant throughout those experiments, the only variable parameter was the initial value of the work function of the polymer. The points at which these lines intercept the ‘‘zero’’ work function modulation line correspond to the condition when no charge transfer takes place, that is, δe = 0 for a given gas molecule/polymer combination. The tuning of the work function can be accomplished by different means as discussed in Section 8.3.3. Several studies deal with the inﬂuence of the nature of the dopants on the morphology and the chemical-sensing behavior of PPy to selected vapors and gases [13, 32, 33]. Anions commonly used for the electropolymerization of PPy, such as ClO4 , BF4 , and tosylate (TOS), were chosen for comparison with PPy layers doped with metallophthalocyanines (MPcTS, M = Cu, Pb, Al, Fe, and Ni). Pure MPcTS layers show good sensitivity toward nitrogen oxide gas (NOx ) and organophosphorous vapors. The sensitivities (i.e., work function changes) of

161

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8 Gas Sensing with Conducting Polymers

NOx 18 ppm

AlPcTS Inactivation not sensitive Fe(II)PcTS PbPcTS NiPcTS CuPcTS ClO4 BF4 TOS

NiPcTS CuPcTS ClO4 BF4

PER 92 ppm

TOS

AlPcTS Fe(II)PcTS Not sensitive PbPcTS NiPcTS CuPcTS Not sensitive ClO4 BF4

DMMP 44 ppm

TOS

−200

−100

0

100

200

Work function ∆fWF vs Au (meV) Figure 8.4 Comparison of work function change φWF of PPy layers doped with various anions in the presence of nitrogen oxide (NOx ), perchloroethylene (PER), and dimethylmethylphosphonate (DMMP).

PPy ﬁlms doped with various anions to dimethylmethylphosphonate (DMMP), perchloroethylene (PER), and NOx are compared in Figure 8.4. The interaction of these gases or vapors with PPy (i.e., secondary doping) leads to both a positive and negative change in the work function, depending on the nature of the dopant anions incorporated into PPy. The measured signals conﬁrm the results shown in Figure 8.3. It indicates that the intrinsic electronic state of doped PPy, which can be described by the initial work function, is strongly dependent on the nature of the dopant anion. Moreover, the difference and the opposite direction of φWF for PPy/CuPcTS and the other PPy/MPcTS layers after exposure to NOx indicate that the central metal ion of the phthalocyanine anions plays an important role in the sensing characteristics. The results discussed above show that the same secondary dopant molecule can exhibit either electron donor or electron acceptor properties depending on the type of the primary dopant [13, 16]. In light of this ﬂexibility, it is not surprising to ﬁnd

8.5 Types of Conducting Polymer-Based Gas Sensors

that different vapors affect the conducting polymer differently. The most important outcome of this derivation is that a fractional value of the slope in Equation 8.1 is possible and can be determined. It simply depends on the ability of the entering molecule to exchange a charge density with the matrix. The selectivity and the response time of PPy layers doped with MPcTS-anions are not sufﬁcient for practical applications. It is assumed that, because of the highly ordered layered structure and the relatively high packing density of MPcTS-doped PPy, the vapor diffusion in and out of the layer is very slow, resulting in a long response time. Several attempts were made to address this problem. One approach is in situ electrochemical grafting or copolymerization of PPy with chemical inert, nonconducting polymer, for example, polyoxyphenylene, forming a polymer blend or polymer composite, which increases the porosity of the conducting polymer and, hence, leads to faster diffusion of the vapor molecules into and out of the bulk of the polymer ﬁlm [32]. Another approach is based on the so-called competitive doping process [34], which consists of the additional incorporation of a small spherical dopant, such as BF4 or ClO4 , besides the anion determining the sensitivity of PPy. Examples for PPy doped with CuPcTS + ClO4 and NiPcTS + BF4 have been given [13]. Besides the improvement in the time response of PPy ﬁlms, competitive doping strongly improves selectivity of the doped PPy ﬁlms [13]. Transients in the work function response of PPy/CuPcTS + ClO4 layer to NOx can be seen in Figure 8.5a. In Figure 8.5b, the work function–log cNOx dependence is shown. The sensitivity of PPy/CuPcTS + ClO4 obtained by the slope of a straight line in the signal versus log (concentration) presentation is 69 mV (ppm decade)−1 , which gives δ = 0.49, indicating the formation of a charge-transfer complex according to Equation 8.1. A large number of gas sensors based on ﬁeld-effect devices have been developed in the last few years. In CHEMFETs exhibiting an IGFET structure, as shown in A, the chemically sensitive conductive polymer layer, which is deposited on top of the gate dielectric, acts as the gate electrode. The deposition can be carried out by solvent cast or by photolithographic patterning. CHEMFETs employing a conductive polymer gate, based on PANI [35] and PPy [15, 21], have been fabricated with sensitivity to organic compounds, such as ethanol, isopropanol [35], NOx [15], ammonia [35], tributyl phosphate [36], and hydrogen cyanide [37]. 8.5.2 Conductometric (Nonzero-Current) Sensors 8.5.2.1 Chemiresistors – Bulk Resistance Modulation Chemiresistors are bipolar devices. The schematic of a chemiresistor and its equivalent circuit is shown in Figure 8.6a,b, respectively. In its simplest form, it consists of a pair of electrodes forming contacts with the conducting polymer, deposited on an insulating substrate. Both electrical contacts in a chemiresistor m > S φWF ) for a p-type semiconductor and are supposed to show ohmic behavior (φWF vice versa for an n-type semiconductor. When a constant current is applied, the resulting potential difference at the electrodes becomes the response signal. The

163

8 Gas Sensing with Conducting Polymers

Work function fWF vs Au (meV)

0.02

1.9 ppm

3.8 ppm

0.00 1.9 ppm

−0.02

0.95 ppm

−0.04

0.95 ppm

0.95 ppm

0.5 ppm

−0.06 −0.08 PPy / CuPcTS + ClO4 (1 : 1)

−0.10 0 ppm

−0.12 0

240 360 Elapsed time (min)

120

(a)

480

600

100 Work function fWF vs Au (meV)

164

PPy/CuPcTS + ClO4− (1 : 1) 50

0 Linear fit R = 0.9962 −50 0.2

(b)

1

10

60

Concentration NOx (ppm)

Figure 8.5 (a) Transient response of PPy/CuPcTS + ClO4 (1 : 1) layer to various concentrations of NOx measured with Kelvin Probe and (b) dependence of work function on the concentration of NOx (data taken from Fig. 8.5a).

primary interaction between the sample and the sensor involves the selective layer and the sensory information originating from the surface, the contact and the bulk of the conducting polymer is obtained by the selective modulation of at least one of the equivalent circuit elements shown in Figure 8.6b [27]. According to Equation 8.3, a linear current–concentration dependence is obtained, provided that the overall signal source is the change in bulk resistivity. If the signal arises from the chemical modulation of the contact resistance, the device operates in the kinetic regime and the response is a nonlinear function of concentration.

8.5 Types of Conducting Polymer-Based Gas Sensors

Gas Contact I

Surface Surface

Contact II Electrode II

Electrode I

Contact I Substrate (a)

Bulk

Contact II

Conducting polymer Interface

(b)

Interface

(c) Figure 8.6 (a) Cross-sectional view, (b) equivalent circuit (according to [27]), and (c) interdigital electrode array of a typical chemiresistor used for chemical-sensing application.

8.5.2.2 Schottky Barrier Diodes – Contact Resistance Modulation In Figure 8.7a–c, a cross section of a typical chemical sensor based on an ideal Schottky junction, its schematic band diagram, and the equivalent circuit are shown, respectively. Each Schottky barrier diode comprises two contacts or junction areas and one bulk area, by deﬁnition: (i) the junction area between metal I and the semiconductor forming the Schottky barrier junction, which is the origin of sensor signal mostly by variation of the junction resistance RJ , which is equivalent to the charge carrier resistance in electrochemical experiment and a measure of the Schottky barrier φb . (ii) The neutral bulk area of bulk resistance RB . The latter dominates the overall resistance of the Schottky barrier diode and, hence, the sensor response, if the Schottky barrier height is only in the range of a few millivolts. In this case, the sensor device functions as a simple chemiresistor. (iii) The junction area between metal II and the semiconductor forming the ohmic contact, which has to be inactive and, hence, should not contribute to the sensor response. It should be noted that the area of the ohmic contact of the sandwich structure shown in Figure 8.7a is much larger than that of the Schottky junction in analogy to auxiliary and working electrodes in electrochemical experiments, when the smaller electrode always yields the response. The Schottky barrier junction shows a rectifying behavior indicated by the ratios of the forward to reverse current of the nonlinear current density versus bias voltage curve (J–V curve). The current density J in the low forward bias voltage region varies exponentially with the applied voltage Vappl , which can be described by the standard diode equation (Richardson’s equation) for charge transport governed by thermionic emission [31]. The rectifying behavior of the polymer Schottky diodes relates directly to barrier height φb and, hence, to the work function of the polymer

165

166

8 Gas Sensing with Conducting Polymers

Gas/Vapor Metal I Semiconductor

Contacts to external circuit

Metal II

Substrate

(a) Metal I Depletion region EF fB

Semiconductor Neutral bulk region − + −− + +

− − +− + +

CJ

WB CB

Barrier region (c)

Ohmic contact EF

eVbi

WJ

(b)

Metal II

Bulk region

RJ

RB

Figure 8.7 (a) Cross-sectional view, (b) schematic band diagram, and (c) equivalent circuit of a typical geometrical arrangement of a Schottky barrier diode used for chemical-sensing application.

layer, which can be changed by exposure of the polymers to certain gaseous species exhibiting electron donor or acceptor behavior. A gas-induced voltage shift V of the J–V characteristic can be observed, which can be exploited for gas-sensing measurements. It is deﬁned as [17] V = Vgas − Vair

at J = constant

(8.5)

where Vgas and Vair are the measured voltages at an applied constant current density in gas and air, respectively. In Figure 8.8, a typical change in the J–V characteristic of a Schottky barrier diode due to interaction with a gas or vapor of interest is shown. The extraction of the sensing information from Schottky barrier diodes can be done by operation in constant current mode. Figure 8.9 shows a typical example of an experimentally obtained transient response of the output voltage of such a Schottky barrier diode operated in the constant current mode. It is important to realize that in DC current measurements, the resistance at the Schottky junction area consists of two resistances in series: the junction resistance

Current density (A cm−2)

8.5 Types of Conducting Polymer-Based Gas Sensors

102 ∆VS 1

10

At constant current density ∆VJ

100 10−1

In air Exposure to gas

−0.4

−0.2

0.2

0.0

0.4

0.6

0.8

Bias voltage (V) Figure 8.8 Typical change in J–V characteristics of a Schottky barrier diode due to interaction with a gas or vapor of interest. VJ and VB indicate the gas- or vapor-induced voltage shift at the junction and in the bulk, respectively, measured at applied constant current density.

0.40 In air after exposure to NOx

Output voltage (V)

0.35 0.30 0.25 0.20 In air

0.15 0.10

Exposed to 11 ppm NOx

0

20

40

60

80

100

120

140

Time (s) Figure 8.9 Response characteristic of an Au/PPy Schottky barrier diode toward NOx by applying a constant current density.

RJ and polarization resistance RP , which is caused by the depletion of the carriers in the vicinity of the forward-biased junction upon passage of current. In many cases, particularly in chemiresistors, the operating voltage greatly exceeds the Schottky barrier height. In that case, the junction resistance becomes much smaller than the polarization resistance, which then dominates the overall resistance of the junction contact. This situation is common to both chemiresistors and ﬁeld-modulated chemiresistors (aka OFETs) [38].

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8 Gas Sensing with Conducting Polymers

8.5.2.3 OFETs – Field-Modulated Chemiresistors It has been known that operation of an OFET is affected by the chemical ambient, namely, by the presence of donor–acceptor gases and by humidity [39]. However, the OFET operates in a fundamentally different way as the CHEMFET considering the current path (Figure 8.2a,b). In the OFET structure, silicon serves as the OFET ‘‘gate’’ electrode and the current passes through the conducting polymer (Figure 8.2b), which encounters at least three resistances: two at the contacts and one in the conducting polymer, which is very similar to the Schottky diode arrangement. The chemical modulation of work function and the perpendicular electric ﬁeld for |VG | > 0 affect all of these resistances in a highly nonlinear manner [25, 40], which indeed shows a ‘‘voltage-dependent resistor’’ behavior. It is obvious that, because of the low mobility of the charge carriers in conducting polymers, the polarization space charge region (expressed as polarization resistance RP ) is created by the depletion of the charge carriers at the vicinity of the drain electrode, analogous to the formation of a diffusion depletion layer in the electrochemical experiment. It has been shown that the effect of gate voltage on the modulation of the contact resistance of the drain electrode greatly exceeds the modulation of the interfacial conductivity of the organic semiconductor itself [25, 38, 41], and the device functions as a ﬁeld-modulated junction. 8.5.2.4 Examples of Conductometric Gas Sensors To build a conductometric gas sensor, the conducting polymers are generally applied over a substrate with interdigitated electrodes (Figure 8.6c) by evaporation (sublimation) or by electrochemical or solvent cast techniques. Different conducting polymers, such as PPy, PANI, and poly(alkylthiophenes) exhibiting different counterions and different resistivities depending on the preparation method, have been used [42]. Sasaki et al. investigated electrochemically treated cast PANI-HgCl2 ﬁlms for the detection of HCN in the range of 1–20 ppm [37]. They used a sensing platform that can be used as CHEMFET and as chemiresistor. Conducting polymers have a numbers of advantages, which make them applicable in chemiresistor arrays [42]. 8.5.2.5 Examples of Polymer Schottky Diode Gas Sensors Gas-sensitive characteristics toward water vapor, chloroform, and ethanol were observed in poly(3-dodecylthiophene) Schottky diodes [43]. The two types of gases have opposite effects on the diode characteristics, which were explained by the acceptor and the donor behavior, respectively. Chemical sensitivities of PANI toward gases and vapors, such as ammonia, hydrogen, methane, and NOx have been shown [14, 17, 44]. Nguyen et al. studied the inﬂuence of different metal phthalocyanines incorporated in PPy on the NOx sensing properties of the Au/PPy diodes [17]. They show that the sensitivity, that is, the change in junction voltage VJ , and the diode parameter are strongly inﬂuenced by the nature of the dopant. In Figure 8.9, the transient response of the output voltage of an Au/PPy-CuPcTS Schottky barrier diode to exposure to NOx , measured at an applied constant current density, is shown. The exposure of a Schottky diode with PPy to NOx causes an

References

increase in the doping level, leading to a positive shift of the Fermi level and, therefore, increases the output voltage of the Au/PPy Schottky barrier diode. Impedance measurements show that the secondary doping of PPy upon gas–polymer interaction with NOx leads to a change in the junction resistance RJ in the low bias region as well as in the bulk resistance RB in the high bias region (Figure 8.7a–c). In this case, the sensor device works as both a junction controlled device in the low bias region and a bulk controlled device in the high bias region with a diode current linearly related to the change in conductivity according to Equation 8.3 The latter represents a simple chemiresistor device (Section 8.5.2.1). Both effects have a compensating inﬂuence on the overall sensor response.

8.6 Conclusion

Conducting polymers are widely acknowledged as useful sensing materials for selective detection of neutral species based on charge-transfer complex formation, which causes a change in the electronic properties of the polymers. These changes are reversible, which is a key requirement for sensors. The tunability of the sensing properties, that is, sensitivity, selectivity, as well as response time by the nature of the dopants as well as by the preparation procedures is an important beneﬁt. In sensing arrays, individual modiﬁcation of each sensor is possible in only one processing step. The mechanical ﬂexibility, environmental stability, and solution processability offer an enormous potential for applications within the ﬁeld of microsensors. The transduction principle of conducting polymer can be exploited in different solid-state chemical devices, such as CHEMFETs and Schottky barrier diodes, leading to controlled electrical effects on the device structure. The transduction mechanism of chemiresistors and OFETs is less clear. Their functional characteristics are dominated by poorly deﬁned processes that depend mostly on the conditions at the metal–conducting polymer interface of the contacts. The high degree of ﬂexibility of conducting polymers allows for the inexpensive fabrication of multisensing arrays, an aspect that makes sensors based on conducting polymers suitable for commercialization.

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Klavetter, F., Colaneri, N., and Heeger, A.J. (1992) Flexible light-emitting diodes made from soluble conducting polymers. Nature, 357, 477–479. 2. Tsuma, A., Koezuka, H., and Ando, T. (1986) Macromolecular electronic device: Field-effect transistor with a

polythiophene thin ﬁlm. Appl. Phys. Lett., 49, 1210–1212. 3. Bradley, D.D.C. (1992) Electroluminescence: a bright future for conjugated polymers? Adv. Mater., 4, 756–758. 4. Burroughes, J.H., Bradley, D.D.C., Brown, A.R., Marks, R.N., MacKay, K., Friend, R.H., Burn, P.L., and Holmes,

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5.

6.

7.

8.

9.

10.

11.

12.

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A.B. (1990) Light-emitting diodes based on conjugated polymers. Nature, 347, 539–541. Miasik, J.J., Hooper, A., and Toﬁeld, B.C. (1986) Conducting polymer gas sensors. J. Chem. Soc., Faraday Trans., 82, 1117–1126. Skotheim, T.A., Elsenbaumer, R.L., and Reynolds, J.R. (eds) (1998) Handbook of Conducting Polymers, 2nd edn, vol. 1, Marcel Dekker, New York. Ko, J.-M., Kim, S., Kim, K.M., and Chung, I.J. (1994) Electrochemical properties of dodecylsulfate-doped polypyrrole ﬁlms in aqueous solution containing NH4 Cl and ZnCl2 . Synth. Met., 64, 9–15. Wernet, W., Monkenbush, M., and Wegner, G. (1984) A new series of conducting polymers with layered structure: polypyrrole n-alkylsulfates and n-alkylsulfonates. Makromol. Chem. Rapid. Commun., 5, 157–164. Ken, K.Y., Miller, G.G., and Elsenbaumer, R.L. (1986) Highly conducting, soluble, and environmentally-stable poly(3-alkylthiophenes). J. Chem. Soc., Chem. Commun., 1346–1347. Feller, J.F., Langevin, D., and Marais, S. (2004) Inﬂuence of processing conditions on sensitivity of conductive polymer composites to organic solvent vapours. Synth. Met., 144, 81–88. Satoh, M., Kaneto, K., and Yoshino, K. (1986) Dependences of electrical and mechanical properties of conducting polypyrrole ﬁlms on conditions of electrochemical polymerization in an aqueous medium. Synth. Met., 14, 289–296. Vork, F.T., Schuermans, C.A.M., and Barendrecht, E. (1990) Inﬂuence of inserted anions on the properties of polypyrrole. Electrochim. Acta, 35, 567–575. Cabala, R., Meister, V., and Potje-Kamloth, K. (1997) Effect of competitive doping on sensing properties of polypyrrole. J. Chem. Soc., Faraday Trans., 93, 131–137. Van Nguyen, C. and Potje-Kamloth, K. (2000) The inﬂuence of thickness and preparation temperature of doped

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9 Chemical Sensors Based on Conducting Polymers Johan Bobacka and Ari Ivaska

9.1 Introduction

Chemical sensing is an important and fascinating phenomenon that allows us to obtain information about the chemical composition of matter in space and time. However, the development of efﬁcient chemical sensors is challenging and a lot of research work is done to explore new materials with improved sensor characteristics. Conducting and electroactive polymers, such as polypyrrole, polythiophene, polyaniline and their derivatives are multifunctional materials that are particularly useful for the development of chemical sensors. Chemical information carried by an analyte in solution or in the gas phase can be converted into a measurable signal by using conducting polymers for signal transduction and/or analyte recognition. The choice of monomer unit(s) in the conducting polymer backbone, the degree of doping, and inclusion of different doping ions and substituents allow tailoring of several properties for conducting polymers, including electroactivity, electronicand ionic conductivity, conformation, optical properties (band gap), lipophilicity, and processability (Figure 9.1). Interactions between conducting polymers and ions/molecules in the surrounding medium can simultaneously inﬂuence several properties of the conducting polymer, which leads to many possibilities in the design of chemical sensors. Conducting polymers have been applied in chemical sensors since the mid-1980s. Polypyrrole was initially explored in electrochemical sensors for the detection of lower aliphatic alcohols [1], immunoglobulin G [2], anions [3–5], glucose [6, 7], metal ions [8], and gases such as NH3 , NO2 , and H2 S [9]. The signal transduction mechanisms employed in these early studies include changes in work function [1], potentiometry [2–4], amperometry [5–7], voltammetry [8], and conductimetry [9]. Chemically sensitive microelectrochemical devices were developed based on polyaniline [10] and poly(3-methylthiophene) [11]. The great potential of functionalized conducting polymers for the construction of so-called intelligent materials, including chemical sensors, had already been realized by the end of the 1980s [12]. In the 1990s, conducting polymer-based Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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Molecular recognition

Charge

Optical properties

Electronic conductivity

− S

S

Electro activity

S

+

S

S

n

S

Band gap

Ionic conductivity

Processability Conformation

Lipophilicity

Figure 9.1 Schematic illustration of the multifunctionality of conducting polymers (using p-doped functionalized polythiophene as an example) that opens many possibilities for chemical sensing.

chemical sensors and biosensors were mostly based on electrochemical signal transduction [13–31]. Today, electrochemical sensors continue to dominate [32–60], although optical signal transduction has gained more interest in recent years [32, 36, 40, 42, 51, 55, 61, 62]. More than 50 reviews or book chapters have been devoted to different aspects of chemical sensors based on conducting polymers [13–66]. Some reviews cover a variety of conducting polymer-based chemical sensors [13–16, 19, 23, 24, 27–30, 32, 36, 38, 40, 42, 51, 55, 62, 63, 66] while others focus on speciﬁc types, such as biosensors [17, 18, 22, 25, 31, 33, 41, 47, 49, 60, 61, 64, 65], potentiometric ion sensors [21, 35, 37, 44, 45, 48, 50, 54, 56, 58], micro- and nanostructured sensors [20, 26, 46, 59], transistor-based sensors [34, 43, 53, 57], electronic noses [39], and gas sensors [52]. Some interesting possibilities offered by conducting electroactive polymers in the ﬁeld of chemical sensors are discussed below. The main purpose of this chapter is to serve as an introduction to the ﬁeld of chemical sensors based on conducting polymers. Because of the large number of articles published in this ﬁeld, we decided to refer mainly to review articles and book chapters that are excellent starting points for researchers who want to dig further into the original publications on a speciﬁc topic [13–66]. Since biosensors and gas sensors are treated elsewhere in this book, this chapter is focused mainly on ion sensors.

9.2 Electrochemical Signal Transduction

Electropolymerization of various monomers onto high-work function electronic conductors such as platinum, gold, and carbon offers a unique route toward

9.2 Electrochemical Signal Transduction

electrochemical sensors. Since conducting polymers are electroactive materials with mixed ionic and electronic conductivity, they can transduce an ionic signal into an electronic signal in the solid state, which can be utilized in potentiometric sensors. Conducting polymers also function as electron-transfer mediators, which is useful in amperometric and voltammetric sensors. Furthermore, analytes that are electron acceptors or donors can inﬂuence the concentration of charge carriers (doping level) of the conducting polymer and this phenomenon can be used in conductimetric (chemiresistors) and transistor-based chemical sensors. However, to achieve high selectivity to a given analyte, the molecular recognition properties of the conducting polymer have to be tailored by using functionalized monomers, speciﬁc counterions, or by deposition of another membrane on top of the conducting polymer. Different possibilities to immobilize receptor units (molecular recognition sites) in a p-doped conducting polymer are shown schematically in Figure 9.2. If the receptor unit has a negative charge, it can simultaneously work as counterion (Figure 9.2a). This functionalization can be done in a single electropolymerization step. Alternatively, the receptor can be covalently bound to the conjugated polymer chain (Figure 9.2b) or entrapped in the polymer ﬁlm via, for example, steric effects or hydrophobic interactions (Figure 9.2c). In the two latter cases, the positive charge (polarons/bipolarons) of the p-doped polymer is compensated by other counterions (Figure 9.2b,c) or, for example, by an anionic polyelectrolyte. In practice, the receptor unit may be ﬁrst covalently bound to the monomer, which is then electropolymerized or copolymerized with a nonfunctionalized monomer. Alternatively, the receptor may be immobilized after polymerization, but, in this case, steric effects may lead to functionalization of the polymer surface only, depending on the size of the receptor and the morphology of the ﬁlm. The receptor units may also be covalently bound to an anionic polyelectrolyte that is immobilized in the conjugated polymer and simultaneously works as counterions that in the ideal (unlikely) case may exactly compensate the positive charge of the conjugated polymer (Figure 9.2d). However, if the negative charge of the polyelectrolyte exceeds the positive charge of the conjugated polymer, then charge-compensating cations are required in the polymer (Figure 9.2e). Finally, if the receptor is bound to an uncharged (nonconjugated) polymer, the positive charge (polarons/bipolarons) of the p-doped polymer must again be compensated by other anions (Figure 9.2f) or, for example, by an anionic polyelectrolyte. These examples indicate some of the possibilities available to achieve functionalized conducting polymers for chemical sensor applications. Similar strategies can be used to obtain biosensors, where the receptor unit is an enzyme, antibody, oligonucleotide, or some other biomolecule. 9.2.1 Potentiometric Sensors

Potentiometric sensors are based on the measurement of the electrical potential of the sensor versus a reference electrode that has a constant potential. By

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(a)

(b)

(c)

(d)

(e)

(f)

Receptor unit

Uncharged monomer unit

Positively charged unit/cation

Figure 9.2 Some ways to immobilize the receptor units in a p-doped conducting polymer. (a) Receptor immobilized as doping anion, (b) Receptor covalently bound to the conducting polymer, (c) Receptor entrapped via non-electrostatic (e.g. steric or hydrophobic) interactions, (d) Receptor covalently

Negatively charged unit/anion

bound to a polyanion having the same charge density as the p-doped conducting polymer, (e) Receptor covalently bound to a polyanion having a higher charge density than the p-doped conducting polymer, (f) Receptor covalently bound to an uncharged polymer.

9.2 Electrochemical Signal Transduction

using a potentiometer with high input impedance, only a negligible electrical current ﬂows in the measuring circuit and therefore the potential is measured when the electrode reaction is at equilibrium. Potentiometry is an attractive measurement technique that makes it possible to use small-size, portable, and low-cost instrumentation. Conducting polymers offer many possibilities in the development of potentiometric ion sensors [21, 35, 37, 44, 45, 48, 54, 56, 58]. Oxidized (p-doped) conducting polymers have a polycationic backbone and can, therefore, function as anion exchangers, resulting in an anionic potentiometric response, as indicated in Figure 9.3a. The anionic response is often rather nonselective, which means that different small anions, including Cl− , Br− , NO3 − , ClO4 − , BF4 − , and SCN− , may contribute to the measured potential. However, the doping anion included in the polymer ﬁlm during electrosynthesis may signiﬁcantly inﬂuence the structure of the conducting polymer by a process similar to molecular imprinting. Immobilization of doping anions in order to obtain an excess negative charge versus the positive charge of the oxidized polymer gives a conducting polymer with cation-exchange behavior, resulting in a cationic potentiometric response,

Conducting polymer

Solution

Conducting polymer

Solution

(a)

(b) Solution

Conducting polymer

e− Red/Ox

(c)

Red/Ox Uncharged monomer unit

Positively charged unit/anion

Negatively charged unit /anion

Electroactive (redox) species

Figure 9.3 (a) Anionic, (b) cationic, and (c) redox response of a p-doped conducting polymer (see text).

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as indicated in Figure 9.3b. As a result of their electroactivity and electronic conductivity, conducting polymers also show a redox response, as indicated in Figure 9.3c. Such an electron-transfer reaction between a redox couple in solution and the conducting polymer ﬁlm usually also induces ion transfer between the conducting polymer and solution. The special case in which the redox species simultaneously oxidizes/reduces the conjugated polymer and enters the polymer ﬁlm as counterion is called chemical doping. The anionic, cationic, and redox sensitivity of conducting polymers are three limiting cases and often a mixed response can be observed, as shown both theoretically [67] and experimentally [68]. In addition to ionic and redox responses, several conducting polymers show pH sensitivity depending on the acid–base properties of the conjugated polymers. To enhance the selectivity to the target analyte, it is possible to modify the chemical structure of the conducting polymer by covalent binding of suitable molecular recognition sites or by immobilization of functional dopants that are selective to the target analyte [35], following the same principles as shown in Figure 9.2. In this way, conducting polymers can be functionalized by ion-complexing ligands or highly selective supramolecular receptors (ionophores) forming host–guest complexes with the target analyte. Covalent binding of ion receptors to conjugated polymers offers possibilities to construct potentiometric ion sensors where both molecular recognition and signal transduction take place in a single macromolecule, which is of particular interest for the development of nanosized sensors. Another possibility is to include the ion receptors in a separate membrane (ion-selective membrane) that is in contact with the conducting polymer ﬁlm, resulting in a solid-contact ion-selective electrode (ISE) [35, 37, 44, 45, 54, 56, 58]. In this conﬁguration, the conducting polymer acts as a solid-state ion-to-electron transducer and the outer membrane determines the selectivity of the sensor. A third possibility is to integrate the conducting polymer transducer into the ion-selective membrane in the form a polymer blend, resulting in the so-called single-piece ISE [35, 54]. Also in this case, the role of the conducting polymer is to convert an ionic signal into an electronic signal. The three different types of conducting polymer-based potentiometric ion sensors are schematically shown in Figure 9.4a–c, together with the conventional (liquid-ﬁlled) ISE (Figure 9.4d) and the so-called coated-wire electrode (Figure 9.4e). The conducting polymer gives a well-deﬁned ion-to-electron transduction pathway that stabilizes the electrode potential of these solid-contact electrodes (Figure 9.4a–c); this is their major advantage in comparison to the coated-wire electrode, which had a poorly deﬁned capacitive transduction mechanism (Figure 9.4e). Replacement of the internal ﬁlling solution of conventional ISEs (Figure 9.4d) by a solid contact results in more durable ion sensors that are easier to miniaturize and fabricate. The solid-contact approach shown in Figure 9.4a, where the conducting polymer functions only as transducer and the selectivity is given by the ion-selective membrane, has received a lot of attention, and found to be particularly useful for practical applications [35, 37, 44, 45, 54, 56, 58]. In this approach, conducting polymers based on derivatives of polypyrrole, polythiophene, and polyaniline

9.2 Electrochemical Signal Transduction

Electronic conductor

Internal reference electrode (a)

(b)

(c)

Internal filling solution Ion-selctive membrane (d)

(e)

Electronic conductor Conducting polymer

Ion-selective membrane containing conducting polymer

Ion-selctive membrane

Conducting polymer with ionrecognition sites (ionophore)

Figure 9.4 Different types of ion-selective electrodes (ISEs): (a) ISE with a conducting polymer as solid contact that is coated with a conventional ion-selective membrane, (b) ISE with a conducting polymer

dissolved in the ion-selective membrane, (c) ISE based on a conducting polymer containing ion-recognition sites (ionophores), (d) conventional liquid-ﬁlled ISE, and (e) coated-wire ISE.

are frequently used as ion-to-electron transducers in combination with classical ionophore-based ion-selective plasticized PVC membranes. In recent years, the analytical performance of the solid-contact ISEs based on conducting polymers as ion-to-electron transducers has been dramatically improved, which includes the achievement of sufﬁcient potential stability and lowering of the detection limit to 1 × 10−9 M or even lower [45, 54]. Not only solid-contact ion sensors but also solid-state reference electrodes are currently of great interest. Future combinations of solid-contact ion sensors with solid-state reference electrodes may give rise to mass production of miniaturized ion-sensor systems with powerful analytical capabilities. 9.2.2 Amperometric and Voltammetric Sensors

Amperometric and voltammetric sensors are based on the measurement of the electrical current ﬂowing through the sensor as a result of oxidation or reduction of the analyte (or a reaction product from the analyte) at a constant applied potential (amperometry) or during a potential scan (voltammetry). Normally a three-electrode

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system is used in such a way that the potential is applied between the sensor and the reference electrode, and the current is measured between the sensor and the counter electrode by using a potentiostat. The oxidation (p-doping) or the reduction (n-doping) of conducting polymers is associated with ion transfer in order to preserve electroneutrality in the conducting polymer. Electrochemical oxidation or reduction of the conducting polymer takes place only if charge-compensating ions are available in the solution in which the conducting polymer is immersed. This unique property can be used to detect even electroinactive ions amperometrically or voltammetrically in a ﬂow-through cell [13]. By using a carrier solution that does not contain any anions that are able to enter the polymer ﬁlm, a positive potential can be applied without causing any oxidation (p-doping) of the conducting polymer and the current remains small. When the analyte is injected into the carrier stream and the analyte anions pass the detector, oxidation of the conducting polymer takes place resulting in a current peak that can be used for analytical purposes. Electroactive ions can be oxidized or reduced directly at the conducting polymer surface, following the same principle as shown in Figure 9.3c. Conducting polymers can, therefore, be used as voltammetric sensors for a variety of electroactive ions in solution [13–16, 19]. Furthermore, electroactive ions may enter the conducting polymer ﬁlm, depending on the size, charge, and chemical structure of the ion (guest) as well as the conducting polymer (host). This provides a means to accumulate and preconcentrate metal ions before electrochemical oxidation or reduction, similar to voltammetric stripping analysis. Functionalization of conducting polymers by covalent binding of receptors such as crown ethers can be used to inﬂuence the host–guest interactions in order to enhance the response to the target ion [28, 42, 55, 63, 66]. Modiﬁcation of electrodes by conducting polymer ﬁlms is a way to enhance sensitivity and selectivity of the electrode to the analyte and to suppress interfering reactions. Electroactive organic compounds may be oxidized or reduced at the conducting polymer/solution interface or the electroactive compound may diffuse into the conducting polymer ﬁlm before electron transfer takes place. In these cases, electron transfer occurs between the analyte and the conducting polymer. Electroactive species may even diffuse through the whole polymer ﬁlm so that electron transfer takes place between the analyte and the underlying electrode (onto which the conducting polymer was deposited). In this case, the conducting polymer acts only as a (selective) diffusion barrier and does not participate in the electron-transfer reaction. Furthermore, interaction between organic compounds and conducting polymers may inﬂuence the redox (doping) process of the conducting polymer, resulting in a useful current response, for example, a change in the cyclic voltammogram of the conducting polymer-modiﬁed electrode, even if no electrons are transferred to or from the organic compound (analyte). Such an indirect amperometric or voltammetric detection of organic compounds is similar to amperometric detection of electroinactive ions, as discussed above. In both cases, the analyte inﬂuences the doping–undoping reaction of the conducting polymer, which is observed as a change in current versus time (amperometry) or

9.2 Electrochemical Signal Transduction

current versus potential (voltammetry). Since the inherent redox process of the conducting polymer involves both electron and ion transfer and transport, the detection mechanism may often be very complex. Conducting polymer-coated electrodes are useful in voltammetric determination of biologically important organic compounds, including neurotransmitters such as dopamine. Since the detection of these organic compounds does not involve any biological reagents (enzymes, antibodies, etc.) immobilized in the conducting polymer ﬁlm, these sensors are different from biosensors, which are discussed in another chapter of this book. Oxidation of organic compounds at bare electrodes, such as glassy carbon, gold, and platinum, often results in fouling of the electrode by reactants or reaction products that adsorb on the electrode surface. By using conducting polymer-modiﬁed electrodes, the electrode passivation can be signiﬁcantly reduced or completely eliminated and the electron-transfer rate can be signiﬁcantly increased. For example, irreversibly oxidized (overoxidized) polypyrrole shows increased sensitivity to dopamine, while the response from ascorbic acid is suppressed, which is useful in voltammetric determination of dopamine in vivo. Polypyrrole itself has no signiﬁcant catalytic activity for the oxidation of β-dihydronicotinamide adenine dinucleotide (NADH), which is an important coenzyme. However, copolymers of pyrrole and N-substituted pyrrole derivatives of chloranil or 2,3-dichloro-1,4-naphthoquinone or poly(thionine)-modiﬁed electrodes show excellent electrocatalytic properties in oxidation of NADH. These examples show the importance of the chemical structure of the conducting polymers used as voltammetric or amperometric sensors for organic compounds. 9.2.3 Conductimetric Sensors

The electrical conductivity of conducting polymers results from charge transport along individual polymer chains, charge transfer between polymer chains, and possibly charge transfer between polymer aggregates or particles. This means that the conductivity is sensitive to the chemical structure (π-electron conjugation, doping) as well as to the morphology of the conducting polymer, whose properties in turn are sensitive to the chemical compounds in contact with the conducting polymer ﬁlm. This makes it possible to develop conductimetric sensors based on conducting polymers. This is widely explored in the ﬁeld of gas sensors [52] and electronic noses [39]; however, other chemical sensors can also be based on conductimetric signal transduction [55, 63]. Conductimetric sensors are also called chemiresistors [34]. Conductimetric sensors do not require any reference electrode, which simpliﬁes the experimental setup compared to potentiometric and amperometric/ voltammetric sensors. Normally, a thin conducting polymer ﬁlm is deposited electrochemically or by solution casting (spin coating) on a pair of electrodes (two-point probe) or on four electrodes (four-point probe), for example, in the form of interdigitated electrodes on an insulating substrate. The conducting polymer covers the electrodes and bridges the gap between them. The electrical resistance and the

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corresponding conductivity of the conducting polymer ﬁlm are then obtained from DC (direct current) or AC (alternating current) measurements of current versus potential or vice versa. By depositing the conducting polymer over two closely spaced microelectrodes, it is possible to construct microelectrochemical devices to be used as conductimetric sensors [10, 11]. In the conductimetric sensors described here, the conducting polymer acts as the resistor the value of which varies when it comes in contact with different chemical compounds (analyte). Conducting polymers functionalized with ion receptors are potentially useful materials for conductimetric ion sensors [63]. Interactions between the ion and the receptor can inﬂuence the conductivity along the conjugated polymer chain via electrostatic effects or via changes in conformation (planarity) of adjacent monomer units in the polymer chain. It is interesting to note that conductimetric signal transduction of the ion-recognition process can be much more sensitive than voltammetric transduction. This can be related to an inherent signal ampliﬁcation of the ion recognition resulting from a collective system response [63]. In other words, a single ion-recognition event causing a change in conductivity in a single point along a conducting polymer chain acting as a molecular wire inﬂuences all electrons or holes transported through that point. Therefore, a small number of ion-recognition events may cause a large change in the electronic conductivity of the material. The electrical conductivity of conducting polymers such as polypyrrole and polyaniline is sensitive to pH. This property can be utilized to produce disposable conductimetric sensors for measurement of pH. By using functionalized conducting polymers, other analytes can also be sensed using conductimetry [63]. By combining an array of conductimetric sensors with mathematical signal treatment, each individual sensor in the array does not have to be selective to any particular analyte but each sensor should give different selectivity patterns to different ions. This is an application area where even relatively nonselective conducting polymers are particularly useful. 9.2.4 Chemically Sensitive Transistors

Different types of chemically sensitive transistors based on conducting polymers can be used to detect gases, vapors, biomolecules, and ions [34, 43, 53, 57, 69]. Such transistors can be fabricated by using conducting polymers as the semiconductor and/or chemically sensitive layer in the form of a thin-ﬁlm transistor (TFT) or as an insulated-gate ﬁeld-effect transistor (IGFET) [34]. Both TFT and IGFET are three-terminal devices [34]. However, in the development of chemically sensitive transistor devices, it is important to note the differences in operation principle between the IGFET and TFT [34, 57]. The TFT using conducting polymers as the semiconductor is sometimes called an organic thin-ﬁlm transistor (OTFT ), organic ﬁeld-effect transistor (OFET ), or ﬁeld-modulated chemiresistor [43, 57]. When used as a chemical sensor, the TFT responds to changes in both work

9.2 Electrochemical Signal Transduction

function and conductivity of the conducting polymer that acts simultaneously as the semiconductor between the source and the drain and as the molecular recognition layer [34, 57]. On the contrary, the IGFET is a chemical-sensitive ﬁeld-effect transistor (CHEMFET) that responds only to changes in work function of the conducting polymer, because the current between the source and the drain ﬂows through the inorganic semiconductor, usually based on silicon (Si–SiO2 –Si3 N4 ), which is not sensitive to changes in the chemical environment [34, 57]. Conducting polymers can also be used to develop an ion-selective organic electrochemical transistor (IS-OECT), also referred to as an ion-selective organic electrochemical junction transistor (IS-OEJT) [53, 69]. These devices are called electrochemical transistors because the operation principle is not based on the ﬁeld effect, but on electrochemical doping/undoping of the conducting polymer ﬁlm. An example of a cation-selective OEJT is shown schematically in Figure 9.5. In this case, the selectivity is obtained by depositing a cation-selective membrane on the conducting polymer layer based on poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) that would otherwise show a nonselective

B PEDOT-PSS

VB Solution

Ion-selective membrane

e−

(PSS) C

(PEDOT)

E Electrical insulator

IC

VC Cation for which the ion-selective membrane is selective

Figure 9.5 Schematic structure of an IS-OEJT, where B, base; C, collector; E, emitter; VB , base potential, VC , collector/emitter potential, IC , collector/emitter current [69].

183

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9 Chemical Sensors Based on Conducting Polymers

cationic response following the same principle as shown in Figure 9.3b. The IS-OEJT device is still at an early stage of development, but it may be an interesting option for mass production of chemical sensors by using printing technology [69].

9.3 Optical Signal Transduction

Chemical sensors in which molecular recognition is transduced into an optical signal are called optical sensors. Optical sensors based on conducting polymers are gaining more interest alongside electrochemical sensors. Conducting polymers are potentially useful materials in optical sensors for several reasons. In their undoped state, most conjugated polymers absorb electromagnetic radiation of wavelengths between 300 and 800 nm, that is, in the UV–visible range, corresponding to band gaps between 4 and 1.5 eV. The doping process results in new energy levels in the band gap, allowing electronic excitations at lower energies, which results in absorption of electromagnetic radiation at longer wavelengths. Therefore, not only the electrical conductivity but also the optical properties of conjugated polymers depend strongly on the degree of doping. Additionally, the optical properties depend on the chemical structure of the conjugated polymer backbone as well as on the conformation of the polymer chain. Consequently, chemical species (analytes) that inﬂuence the charge carrier concentration (doping level) or the planarity of the adjacent monomer units in the conjugated polymer chain will inﬂuence the optical properties of the polymer that thus works as an optical transducer. As in the case of electrochemical sensors, the selectivity and sensitivity of optical sensors based on conducting polymers can be inﬂuenced by functionalization of the conjugated polymers by different receptor groups, such as crown ethers, calixarenes, and pyridyl-based ligands [32, 61–63, 66]. For example, polythiophenes containing crown ether groups give an optical response to alkali metal ions resulting from changes in the conjugation length along the polymer backbone induced by ion complexation. Active research in this ﬁeld has resulted in novel optical biosensors based on conjugated polyelectrolytes [61] and novel chemical sensors based on amplifying ﬂuorescent conjugated polymers [62].

9.4 Conclusions

Conducting polymers show a unique combination of properties that cannot be found in any other material known at present. The electrical, electrochemical, and optical properties of conjugated polymers are utilized to convert chemical information (concentration, activity, partial pressure) into electrical or optical signals in the solid state. Conjugated polymers are useful both as transducers and sensitive layers in chemical sensors. Immobilization of synthetic receptors

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in conducting polymer ﬁlms allows the construction of novel chemical sensors based on various electrochemical and optical transduction schemes. Analogously, immobilization of biological recognition systems results in various types of biosensors. The research on chemical sensors based on conducting polymers has increased rapidly since the mid-1980s. Referring to sensor devices based on electrodeposited polymers, Wallace et al. stated in 1991 that ‘‘there is little doubt that only the tip of the iceberg is visible in terms of applications for which these devices are suitable’’ [14]. Now we can deﬁnitely see more than the ‘‘tip of the iceberg’’ but a lot still remains to be explored.

Acknowledgments

˚ This work is part of the activities at the Abo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Program (2000–2005 and 2006–2011) by ˚ the Academy of Finland. Support from the Research Institute of the Abo Akademi University Foundation is also acknowledged.

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10 Biosensors Based on Electropolymerized Films Serge Cosnier and Michael Holzinger

10.1 Introduction

Over the last four decades, the development of biointerfaces has been the subject of increasing research effort and it now constitutes a major challenge for research in the ﬁelds of health, environmental monitoring, and energy. These biointerfaces, whose applications include nanodevices, bioreactors, biosensors, and biofuel cells, require, by deﬁnition, the immobilization of a biological entity (vitamin, coenzyme, protein, DNA, polypeptide, cell, microorganism). The immobilization procedure should ideally not alter the biological properties of the immobilized macromolecules to ensure a good accessibility to the recognition of catalytic sites and provide a spatial orientation of the biomolecule for facilitating its interaction with its target. In addition, this deposition should lead to a high density of biomolecules on the transducer surface and also establish an electrical communication with these biomolecules. As a consequence, the stable and reproducible immobilization of biological macromolecules on a surface with complete retention of their biological activity remains a crucial problem, in particular, for the commercial development of biosensors. Deposition of biological macromolecules has been achieved in many different ways such as physical adsorption, cross-linking, covalent binding, and entrapment in organic or inorganic gels, membranes, sol–gels, or microcapsules. In addition to these conventional methods, Langmuir–Blodgett deposition, electropolymerization, and self-assembled biomembranes were also developed. Taking into account that most of the substrates used for biointerface construction are constituted of conductive surfaces, the electropolymerization via electrogeneration of polymeric ﬁlms covers most of these various approaches. This approach confers at the stage of immobilization the advantages of an easy control over the properties of the polymeric coating such as morphology and thickness and of the spatial addressing of polymer whatever the size and form of the conducting surface. In addition, the quality of the electrogenerated polymer ﬁlms (absence of manufacturing defects, storage stability, and chemical stability in water and organic solvents) constitutes an attractive advantage for the regularity at the molecular Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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level of additional functionalization of the polymer ﬁlm surface. Moreover, the considerable ﬂexibility in the monomer structures through the tunable synthetic modiﬁcation of the monomer can provide polymer ﬁlms with a wide diversity of properties. These ﬁlms thus constitute a powerful platform for the development of biomaterials. In particular, this kind of electrode material aroused widespread attention in the design of biomimetic sensors, enzyme sensors, biochips, DNAand protein sensors, and biofuel cells [1–6]. This chapter brieﬂy reviews the methods and mechanisms for the immobilization of biomolecules in or on electrogenerated polymers and the properties of the resulting biomaterials in sensing applications. In addition, current perspectives in one-dimensional (1D) conducting polymer nanowires for the fabrication of nanobiosensors and the three-dimensional (3D) bioarchitecture fabrication combining polymer and nanomaterials are highlighted.

10.2 Chronological Evolution of the Concept of Biosensors Based on Electropolymerized Films: Principal Stages

Conducting and insulating polymers can be electrogenerated using monomers like thiophenes, phenols, anilines, and pyrroles. The latter are particularly appropriate for the fabrication of biosensors since pyrrole derivatives can be polymerized in both, water at neutral pH under mild conditions and in organic solvents. The physical and chemical properties of the resulting ﬁlms can be modulated by the electropolymerization conditions and the chemical modiﬁcation of the pyrrole monomer by speciﬁc functionalities. As a consequence, the ﬁrst example of a biosensor was based on polypyrrole ﬁlms. Thus, in 1986, Foulds and Lowe [7] and Umana and Waller [8] simultaneously reported the entrapment of enzymes in polymer ﬁlms during their electrochemical growth on electrode surfaces. In 1988, this concept was extended to the oxidative wiring of an immobilized enzyme by a redox polymer, an N-substituted ferrocene polypyrrole [9]. Then, in 1990, Schuhmann et al. described the ﬁrst biosensor design based on chemically grafted biomolecules on polymers [10]. In 1994, the electropolymerization of oligonucleotides and peptides functionalized by a pyrrole group led, for the ﬁrst time, to the simple fabrication of DNA and protein electrodes [11, 12]. In the same year, the simultaneous immobilization and reductive wiring of an immobilized enzyme (nitrate reductase) by redox polypyrrole ﬁlms was realized [13]. Besides entrapment and covalent binding, a new method of biomolecule immobilization based on the anchoring by afﬁnity interactions with the underlying ﬁlm was reported in 1998 [14–16]. Biotinylated polypyrrole and polyphenol ﬁlms illustrated this concept with the immobilization of biotinylated proteins via avidin bridges. Five years later, the ﬁrst nanostructured polymer-biocomposite was designed: a polypyrrole-carbon nanotube (CNT)-DNA sensor that opened the way for the electrogenerated nanostructured bioarchitectures [17]. In addition, an innovative photoelectrochemical procedure was developed providing the direct covalent binding of protein by irradiation of polypyrrole ﬁlms

10.3 Formation of Polymer Films by Direct Electropolymerization of the Biomolecule

bearing photoactive benzophenone entities [18]. In 2005, a second afﬁnity-binding system using an electropolymerized ﬁlm bearing nitrilotriacetic acid (NTA) was proposed [19]. The immobilization strategy consists in the chelation of metal ions such as Cu2+ or Ni2+ by the polymerized NTA groups followed by the binding of histidine-tagged proteins via axial coordination on the chelated metal. In the same year, a novel pyrrole–alginate was synthesized providing after Ca2+ cross-linking and electropolymerization a composite polypyrrole–gel matrix [20]. This composite exhibited a greater stability and enzyme retention properties than conventional alginate gels. In 2007, the use of polymers containing enzyme and redox mediators was successfully applied for the construction of biofuel cells [21]. A new generation of afﬁnity-binding polymer without a bridging block between the ﬁlm and the biomolecule was described in 2009. A poly(pyrrole–adamantane) ﬁlm allowed the immobilization of enzymes and gold nanoparticles modiﬁed by β-cyclodextrin thanks to supramolecular host–guest interactions [22]. The evolution of biosensor development based on electropolymerized ﬁlms is summarized in a graphical timescale in Figure 10.1.

10.3 Formation of Polymer Films by Direct Electropolymerization of the Biomolecule

The easiest way for biomolecule immobilization is the direct electropolymerization of the biomolecule itself after its modiﬁcation with electropolymerizable entities. The advantages of this approach are the simplicity and rapidity of a one-step procedure, unhindered accessibility of the biomolecules, and the high biomolecule density of the resulting ﬁlm. Some examples of developed models of such direct electropolymerizations are shown in Figure 10.2. This approach was widely exploited for the development of biomimetic electrodes based on electropolymerized catalytic or redox compounds that mimic the structure and/or the activity of prosthetic sites of enzymes. These immobilized biological models are more stable than enzymes in aqueous or organic media, particularly at extreme pH values or temperatures. Furthermore, their immobilization on electrodes is an attractive way for understanding the mechanism of enzyme-catalyzed transformations. Among the conventional enzyme analogs, electropolymerized metalloporphyrinoids play a main role owing to their functions as redox mediators, catalysts, or selective complex centers via axial coordination [23–28]. In addition to the biomimetic sensors, the electropolymerization of biomolecules was mainly focused on the fabrication of polypeptide ﬁlms and poly(pyrrole–oligonucleotide) ﬁlms, initially designed for the construction of biochips. For instance, the copolymerization of pyrrole and pyrrole–oligonucleotides allowed the electrochemical construction of a DNA matrix on microelectrodes (50 µm × 50 µm). This matrix was then applied to the genotyping of hepatitis C virus in blood samples [11, 29]. More recently, DNA array chips were fabricated by electropolymerization of oligonucleotides that allowed the label-free detection of point mutations using surface plasmon resonance

191

Figure 10.1

1985

1990

N H

S

S

N

N

N H

N H

N

N

HN H N

1995

NH

H N

H N

N

N

N

O

H N

O

O O

N H

O

N

n

CH2 10

N H

S

HN

O

S

HN

H N

O

S

N

S N H

H N O

hn HN N

O

O

HHN R

n

2009: Anchoring of b-cyclodextrin modified GOX on poly(pyrroleadamantane)

O

N H

O

2000

2005

1999: Reversible anchoring of histidine-tagged protiens on a poly(pyrrole)-Ni(II) film

N H

O

H

1998: Biotinylated films for the protein immobilization via biotin–avidin bridges

O

2003: Photografting of enzymes on a photoactivable polypyrrole film

O

N

n

CH2 10

O

N

O

2010

n

NH

R

NH

Graphical timetable of the milestones achieved in the development of biosensors based on electrogenerated ﬁlms (1986–2009).

1986: Entrapment of glucose oxidase in conductive polypyrrole film

O

H N

N H

H N

1992: Direct electropolymerization of a pyrrole modified enzyme

O

1990: Covalent binding of enzymes

OH

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10 Biosensors Based on Electropolymerized Films

NH2

N

N

N H

N

N

N

N

N

(f)

N+

M

N

CH3

F

F

O

O

H N

OH

N

N

N

OH

O

O

NaOOC

HO

OH

HO AcHN

HO

OH OH O

N H

N H

HN

OH

O

OH

O HO O OH

N GOX-pyrrole

(g)

N

HO O

Metalloporphyrine-pyrrole

F

F

Atrazine-pyrrole

N H

HO

OH S

OH

N

O

OH

H N

H N

N

H N

S

O

N

H N

N

N

N

O

O

N

O

HO

O

P O

H3C

O−

N

N

NH(CH2)6NHOC(CH2)8CONH

ODN-pyrrole

Peptide-pyrrole

Oligonucleotide

O

RYNRNAVPNLRGDLQVLAQK

(e)

HN

(d)

N

3′-sialyllactosyl-pyrrole

O

NH

H N

Lactosyl-pyrrole

O

Figure 10.2 Schematic presentation of the direct electropolymerization of (a) synthetic redox centers like porphyrin derivatives; (b) biomimetic models; or (c and f ) saccharides. More sophisticated entities like (d) oligopeptides; (e) oligonucleotides; and even (g) pyrrole-functionalized glucose oxidase could also be successfully electropolymerized.

(a)

(b)

Cl

(c)

10.3 Formation of Polymer Films by Direct Electropolymerization of the Biomolecule 193

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(SPR) imaging [30]. These simple measurements of reﬂection angle changes as a function of time gave access to real-time kinetics simultaneously with analytical measurements. Following the pioneering work of Cosnier et al. [31], several recent reports have focused on the binding of oligosaccharides [32–36]. In particular, the speciﬁc binding of lectins such as Arachis hypogaea and Maackia amurensis to polypyrrole–lactosyl and polypyrrole–3 -sialyllactosyl ﬁlms was nicely demonstrated by electrochemical SPR providing speciﬁc detection limits at the nanomolar level [34]. In this vein, the electropolymerization of natural oligosaccharides (molecular weight ranging from 3 to 45 kDa) was carried out for elaborating an SPR chip array. This allowed measurements of kinetic data reﬂecting the interactions in real time with different proteins of biological interest such as interferon-γ [35]. Recently, the successive coelectropolymerization of phenol and phenol derivatives modiﬁed with d-glucose or d-mannose and the release of the bound saccharides by acidic hydrolysis led to a new kind of electrogenerated imprinted polymers [36]. Such templates showed enantioselective recognition of monosaccharides. Localized electrochemistry is a very promising method for direct addressing and patterning of conductive surfaces by biomolecule electropolymerization. For instance, Fortin et al. demonstrated the advantages of scanning electrochemical microscopy (SECM) for local electrocopolymerization of pyrrole and DNA-pyrrole [37]. Localized electropolymerization was also realized mechanically by the use of mobile electrochemical microcells. By following this route, Descamps et al. developed an array of 10 cantilevers each containing an individual microﬂuidic electrochemical cell. This setup enabled the simultaneous fabrication of 10 localized micrometer-sized poly(pyrrole-DNA) spots [38]. Such immobilization concepts are better applicable for small biomolecules such as peptides, short oligonucleotides, vitamins, or coenzymes [39]. In spite of this, some attempts of immobilized bulky proteins were reported. In their pioneering work in 1992, Lowe et al. have described the electropolymerization of a glucose oxidase (GOX) chemically modiﬁed with 30 pyrrole groups [40]. However, this concept of polypyrrole formation, also described by Schuhmann et al. in 1993, required, in fact, a copolymerization process with regular pyrrole [41, 42]. As a consequence, under these conditions, it was not possible to distinguish between direct electropolymerization and a conventional entrapment process.

10.4 Adsorption on Electrogenerated Polymers

A comfortable way of biomolecule immobilization is their adsorption on electropolymerized ﬁlms. Owing to the instability of adsorbed enzymes, this approach had to be coupled with an electrochemical process of polymer doping. Initially, the electrochemical doping of polyaniline was used for the incorporation of negatively or positively charged enzymes such as GOX, galactose oxidase, and peroxidase during the oxidation or reduction process of the ﬁlm [43]. Polyaniline, indeed,

10.5 Mechanical Entrapment within Electropolymerized Films

presents a chemical and structural ﬂexibility that facilitates the adsorption and the incorporation of biomolecules. As an extension of this work, the direct adsorption of DNA probes, antibodies, and antigens onto such polymers was used for the development of afﬁnity sensors. For instance, DNA probes were incorporated in electrodeposited polyaniline or polydiaminobenzene ﬁlms leading to DNA sensors with AC impedance or ellipsometry detection [44]. In particular, recombinant antiatrazine antibodies, antihuman-α fetoprotein IgGs were attached to polyaniline ﬁlms by cyclic voltammetry while scanning the oxidative range. Then, the immunoreactions were detected with impedance spectroscopy or competitive immunoassays [45]. Moreover, potentiometric immunosensors were elaborated by direct adsorption of speciﬁc monoclonal antibodies on the polypyrrole surfaces. The determination of hepatitis B surface antigen, troponin, and digoxin was realized via formation of enzyme-labeled immunocomplexes, providing very sensitive detection limits, for instance 50 fM for the hepatitis B antigen [46]. The phenomena of DNA adsorption and desorption on the conducting polypyrrole surface were nicely utilized by Wang and coworkers for the amperometric detection of DNA and RNA in ﬂuid samples [47]. In 2005, Dong et al. demonstrated that conducting polypyrrole ﬁlms displays an attractive support for the deposition of bilayer lipid membranes [48]. Polypyrrole nanoﬁbers (30–90 nm in diameter) were also electrosynthesized and modiﬁed by adsorption of dsDNA [49]. The resulting physisorbed dsDNA was used as recognition element to detect small molecules such as spermidine via the decrease in the guanine oxidation current. In a parallel development, the optimization and characterization of polyaniline ﬁlms generated by electrochemical oxidation were described and applied to the electrostatic adsorption of peroxidases and antibodies [50]. Recently, another way reﬂecting a combination between adsorption and entrapment was described for the GOX immobilization. This route consists in the initial electrogeneration of a Prussian blue polymer followed by the electrodeposition of a mixture of GOX and silica as used for sol–gel coatings [51].

10.5 Mechanical Entrapment within Electropolymerized Films

In 1986, Foulds et al. [7] and Umana et al. [8], followed by Bartlett et al. in 1987 [52], published a simple but ingenious strategy for biomolecule immobilization based on the entrapment of biomolecules in polymer ﬁlms during their electrogeneration on electrode surface. This method consists in the application of an appropriate potential to the electrode soaked in aqueous solution containing both biomolecule and monomer molecules. The biological macromolecules at the immediate vicinity of the electrode surface are thus physically incorporated in the growing polymer. This simple one-step entrapment procedure occurs without chemical reaction between the formed polymers and the proteins, thus preserving their biological activity. The entrapment in electropolymerized ﬁlms remains today the most

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popular electrochemical immobilization procedure of coenzymes, proteins and oligonucleotides, and even phages and cells [53, 54]. Thanks to the electroactive skeleton of conducting polymers, the electrogeneration of polymer coatings is not limited to the thickness of the polymer ﬁlm and hence the amount of entrapped biomolecule, can easily be controlled. In addition, the successive electrogeneration of different polymer–biomolecule layers is possible. This principle was applied to the elaboration of multienzyme conﬁgurations with heterogeneous enzyme location. As an example, Dale et al. reported the sequential electrochemical immobilization of three enzymes (xanthine oxidase, purine nucleoside phosphorylase, and adenosine deaminase) in polypyrrole ﬁlms for the development of an amperometric biosensor for the detection of purines [32]. This construction of multienzyme ﬁlms was also used to amplify the biosensor sensitivity by enzymatic recycling of the substrate or to change the electrochemical detection to more appropriate conditions. This concept was illustrated by salicylate biosensor based on the entrapment of salicylate hydroxylase in polypyrrole ﬁlms. The enzyme catalyzes the formation of catechol, NAD+ , and carbon dioxide from salicylate in the presence of NADH and dioxygen. This setup led to the amperometric detection of salicylate via oxidation of the enzymically generated catechol at +0.4 V versus Ag/AgCl. By electrochemical entrapment of salicylate hydroxylase and polyphenol oxidase in one polypyrrole ﬁlm, a bienzyme electrode amplifying the detection of salicylates could be obtained. In contrast to a monoenzyme electrode,

HO

NADH/O2 +

O OH OH OH

−CO2

Salicylate hydroxylase

+ NAD+

+2e− (−0.2 V)

O

Polyphenol oxidase O

Electrode Figure 10.3 Reaction scheme of an example of a bienzyme electrode biosensor. Here, salicylate hydroxylase and polyphenol oxidase were entrapped in one polypyrrole ﬁlm for amplifying the detection of salicylates. Salicylate hydroxylase catalyzes the formation

of catechol, NAD+ , and carbon dioxide from salicylate in the presence of NADH and dioxygen. Polyphenol oxidase regenerates the substrate o-quinone by oxidizing the enzymatically generated catechol.

10.5 Mechanical Entrapment within Electropolymerized Films

the bienzyme electrode is based on the amperometric reduction of o-quinone at −0.2 V as shown in Figure 10.3 [55]. The generated catechol is enzymatically re-oxidized into o-quinone by polyphenol oxidase inducing thus a regeneration cycle. Owing to the conductivity and electrochemical addressing of such polymers, this approach was used for the functionalization of microelectrochemical systems such as biochips or interdigitated ultramicroelectrodes. The electropolymerization of poly(3,4-ethylenedioxythiophene) was thus used for the entrapment of GOX onto microelectrodes combined with a microchannel [56]. The resulting microﬂuidic system with integrated prereactor allowed the interference-free detection of glucose. Moreover, conducting polymers were elegantly applied to the achievement of direct electron transfers between the electrode surface and entrapped biomolecules (in particular oxidases and peroxidases). The use of the conductivity of the host polymer to establish direct electrical communication avoids the use of redox mediators and is more compatible with in vivo conditions. This concept of direct electrical wiring of a biomolecule was implemented for the ﬁrst time with an enzyme (GOX) immobilized in a polypyrrole ﬁlm [57]. An amperometric biosensor for glucose based on a direct electron transfer via the polypyrrole chains at 0.35 V versus Ag/AgCl was thus successfully constructed. Van Os et al. reported that a polypyrrole-wired GOX biosensor exhibited glucose sensitivity in anaerobic conditions, which represented almost 50% of the sensitivity obtained via the amperometric detection of H2 O2 in aerobic conditions [58]. Nevertheless, the mechanism of the proposed direct electron transfer remains questionable and may involve an electrical wiring of the active site of the enzyme by polypyrrole oligomers. Indeed, small and mobile redox mediators usually ensure the electrical wiring of enzyme since most of the oxidoreductases have their prosthetic group deeply buried within the protein shell. As a consequence, this kind of electrical wiring was mainly applied to heme proteins such as peroxidase that have their prosthetic site exposed at the protein surface [59, 60]. For instance, Schuhmann et al. reported a direct electron-transfer process between a conducting polypyrrole and the therein entrapped quino-hemoprotein alcohol dehydrogenase [61]. This phenomenon was ascribed to the presence of a redox intermediate located at the periphery of the enzyme, thus allowing communication with the conductive polymer chains. Kauffmann and coworkers investigated the mechanism involved in the electrical wiring of peroxidase by polypyrrole [62]. They demonstrated that the direct electron transfer occurred via the conductive polymer chains. The electron transfer by oligomers is negligible. The electrodeposition of insulating polymers like polyphenols, poly (dichlorophenolindophenol), polyphenylenediamines, or overoxidized polypyrrole, led to very thin ﬁlms due to their self-limiting growth. Such ﬁlms present a restricted permeability conferring an improved selectivity as well as antifouling properties to reactions occurring at the electrode surface. These insulating ﬁlms were thus successfully exploited to provide interference-free biosensors based on immobilized oxidases [63–66]. Besides the conventional polyphenol ﬁlms electrogenerated from simple phenolic compounds such as phenol, 4-aminophenol,

197

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10 Biosensors Based on Electropolymerized Films

and 3-nitrophenol, the formation of passivating poly(2-naphthol) was also applied to the GOX immobilization [67]. The resulting modiﬁed electrode efﬁciently eliminated interferences from uric acid, acetaminophen, ascorbic acid, and cysteine at 0.65 V versus Ag/AgCl. However, the biosensor sensitivity remained extremely low (176 µAM−1 cm−2 ). In contrast to the insulating ﬁlm discussed earlier, the poly (ethacridine)-GOX electrode provided sensing capabilities for glucose (5 mAM−1 cm−2 ) similar to those observed for glucose biosensors based on polypyrrole ﬁlms [68]. In addition, this polymeric ﬁlm diminished the electrochemical interference from ascorbic acid, uric acid, and acetaminophen to 4–7% of the H2 O2 response. Nevertheless, it should be noted that conducting polypyrroles were also exploited to generate sensitive biosensors based on ultrathin polymer ﬁlms. For instance, the entrapment of sulﬁte oxidase in a 54-nm-thick polypyrrole ﬁlm led to determination of sulﬁte in beer and wine samples with better performance than those recorded with thicker polypyrrole-sulﬁte oxidase electrodes [69]. In particular, the ultrathin polypyrrole-sulﬁte oxidase biosensor enabled a 50-fold improvement in detection limit (0.9 µM). The advantage of the biomolecule immobilization by entrapment lies in the easy and fast elaboration of biopolymer coatings and their stability. Its weak points concern mainly the poor accessibility to the entrapped biomolecules or oligonucleotides that is in conﬂict with the development of immunoreactions or hybridization events. In addition, this approach was obviously restricted to the use of monomers soluble in aqueous solutions. Consequently, the concept of enzyme wiring is limited by the lack of solubility of monomers functionalized by redox mediators. Moreover, this procedure requires, usually, high concentrations of monomer (0.05–0.5 M) and biomolecule (0.2–3.5 mg ml−1 ) for the electropolymerization process. These conditions may limit the applicability of such an approach to biosensor fabrication due to the cost of commercially available biomolecules. Two alternatives were developed to overcome the last drawback. First, small-volume electrochemical cells (0.1 ml) were designed to save signiﬁcantly the amount of the used biomolecule. Another approach for the electrochemical entrapment of biomolecules consists ﬁrst in the immobilization of monomer and biomolecule together, by adsorption, on the electrode surface before electropolymerization [70]. Taking advantage of the adsorption and electropolymerization properties of pyrrolic surfactants in their adsorbed state, an original two-step procedure of biosensor construction has been developed by the Cosnier’s group [71]. Brieﬂy, the electropolymerization of the adsorbed monomers induced the physical entrapment of the adsorbed biomolecule in the in situ generated polypyrrole ﬁlms. In contrast to the preceding method, this strategy allows to determine the amount of immobilized biomolecule if the biomolecule preserves its catalytic activity. In addition, this concept was exploited for the electrical wiring of immobilized enzymes by the use of amphiphilic redox monomers. Thus, the resulting redox polymer encapsulated the entrapped enzymes establishing an electrical communication between the prosthetic site of an enzyme and the electrode surface. For example, nitrite and nitrate reductases were efﬁciently immobilized and wired by polypyrrolic ﬁlms N-substituted by viologen groups (Figure 10.4a) [13, 72, 73].

10.5 Mechanical Entrapment within Electropolymerized Films

199

(c) 1-(3-D-Gluconamido)-pyrrole HO O OH O

N

O

O O

HO

O O

O

HO

OH OH

O

NH

OH

(b)

(d)

O

N H

OH OH

N

Diethyleneglycole dipyrrole

Lactobionamide-pyrrole

O N

N

(a) Viologene-pyrrole

N

N S

(e) Triethyl ammonium-pyrrole

O

BF4−

+ N

(f)

O

Ethylenedioxythiophene

N+ BF4−

BF4− N +

Figure 10.4 Examples of developed electropolymerizable monomers capable to entrap biomolecules during electropolymerization without affecting their properties. Such strong hydrophilic groups are (a) violog`ene; (b) (poly) ethylene glycol; (c) glucose; (d) lactobionamines; (e) triethyl ammonium groups; or (f ) cyclic ethyl ethers.

Another disadvantage of the entrapment concept lies in the hydrophobic character of the organic host polymers. This hydrophobicity may alter the 3D structure of the entrapped biomolecules and hence diminish their catalytic activity or their recognition properties. In order to improve the biocompatibility of polymer ﬁlms, the modiﬁcation of the organic character of the polymers was attempted by the introduction of hydrophilic additives within these ﬁlms by incorporation during polymerization or by chemical synthesis of the monomers. Thus, pyrrole, carbazole, and thiophene were functionalized with linear chains bearing ammonium, ethoxy (Figure 10.4b), hydroxy groups, or oligosaccharides such as glucosyl or lactobionamide groups (Figure 10.4c–e) [31, 32, 74–76]. These entities should preserve the hydration layer of biomolecules into the polymer backbone itself. In this context, Tran-Minh et al. reported the use of a poly 3,4-ethylenedioxythiophene (Figure 10.4f) as conducting polymer for the entrapment of GOX and polyethylene glycol leading to a glucose biosensor exhibiting a sensitivity of 15.2 mAM−1 cm−2 [75]. In this vein, a poly(pyrrole–lactobionamide) was employed as an additional polymer ﬁlm during the GOX entrapment in polypyrrole matrix [31]. The resulting amperometric biosensor exhibited an improved glucose sensitivity (40 mAM−1 cm−2 ) compared to the preceding glucose biosensors, highlighting the beneﬁcial effect of the polymerized lactobionamide groups. Moreover, a marked improvement in biosensor

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10 Biosensors Based on Electropolymerized Films

stability (more than four months) was described for the entrapment of GOX into a poly(gluconyl-pyrrole) [74]. An easier way of counterbalancing the hydrophobic nature of organic polymers consisted in the incorporation of hydrophilic additives. The co-entrapment of enzymes and highly hydrophilic laponite nanoparticles into polypyrrole ﬁlms thus led to a strong enhancement of biosensor performance for the amperometric detection of cholesterol [77] and lactate [78]. Recently, the incorporation of a polysaccharide, chondroitin-4-sulfate A, as polyanions during the polypyrrole growth, was also described to promote cell adhesion and tissue formation onto the polypyrrole biomaterial [79].

10.6 Covalent Binding at the Surface of Electropolymerized Films

As an alternative to biomolecule entrapment, the formation of covalent bonds between the biomolecule and the underlying polymer constitutes the strongest immobilization procedure. The main advantage of this sequential procedure, electropolymerization followed by covalent attachment, is the possibility to use optimal conditions for each step. In particular, the initial formation of polymer ﬁlms can be carried out in organic solvents at all appropriate potentials for optimal polymerization. Also, strong acidic or basic environment can be chosen, which is usually deleterious for biomolecules. Moreover, the covalent coupling of biomolecules to the functionalized polymer can be performed in aqueous buffer solutions at adequate pH values. Additives and stabilizers can be added to preserve the catalytic activity and/or the recognition properties of the biomolecules. This two-step strategy consists, in a ﬁrst step, in the electrogeneration of polymer ﬁlms bearing amine or carboxyl groups followed, in a second step, by their chemical activation in order to bind the biomolecule to the polymer–solution interface via the formation of a peptide bond. Schuhmann et al. initiated this procedure in 1990 by chemical attachment of GOX on an electrode by postfunctionalization of nitro or amino groups modiﬁed polypyrrole ﬁlms in the presence of the water-soluble 1-ethyl-3-(3-dimethylaminopropyl) carbodiimine (EDC) hydrochloride [10]. This procedure is still used for the development of enzyme electrodes, immunosensors, or DNA sensors based mainly on polypyrrole, polyaniline, and polythiophene ﬁlms substituted by amino or carboxylic groups and currently remains among the most popular electrochemical procedures of biomolecule immobilization [80–96] (Figure 10.5a,b). In the area of enzyme electrodes, the group of Gooding, for instance, reported on the covalent attachment of pyruvate oxidase on electrogenerated polytyramine ﬁlm presenting permselective properties [84]. More recently, the covalent binding by carbodiimide coupling of cytochrome c3 , cytochrome c, and xanthine oxidase on conducting poly(terthiophene-3-carboxylic acid) provided biosensors for the measurement of superoxide radicals, NO, and xanthine, respectively [92–94]. The resulting three amperometric biosensors displayed

10.6 Covalent Binding at the Surface of Electropolymerized Films

(c)

O N O F

(b)

(a)

F

F

O

N

O

O

O OH

O

F

F

O

(d)

(e)

CH2 n

N

N

O

O

O

CH2 n

O O

Fe

NH2

O

HN

CH2 n N

N

S

S S

n

n

Figure 10.5 Examples for functionalized polymer ﬁlms bearing reactive groups for covalent anchoring of biomolecules (a) carboxyls (polythiophenes); (b) amines (polyazulenes); (c) N-hydroxysuccinimide esters (polypyrroles)

n

201

n

n

n

and pentaﬂuorophenyl esters (polypyrroles); (d) N-hydroxyphthalimide esters (here, poly 1,3-pyrrole-(3-amidopropyl)-1-butyroxy-Noxyphthalimidoferrocene); and (e) photosensitive groups (poly(pyrrole-benzophenone)).

an elegant direct electron transfer between the grafted proteins and the electrode via the polymer skeleton. DNA sensors were also developed via the covalent binding of oligonucleotides on polymer ﬁlms. For instance, an electrochemical DNA sensor was elaborated from the copolymer ﬁlm poly(5-hydroxy-1,4-naphthoquinone-co-5 hydroxy-3-thioacetic acid-1,4-naphthoquinone) (Figure 10.3), the carboxyl and quinone entities served for the chemical grafting and the transduction of the hybridization event, respectively [85, 89]. The chemical grafting of 21-mer peptide nucleic acid on a polypyrrole–polyvinylsulfonate ﬁlm provided an extremely sensitive sensor for rRNA from Mycobacterium tuberculosis [95]. The electrochemical detection of the RNA target was carried out directly via guanine oxidation or indirectly with methylene blue and ruthenium complex as redox indicators. A detection limit of 0.1 amol was thus obtained by square wave voltammetry using methylene blue. Concerning the immunosensor ﬁeld, recently, Dong et al. elaborated an immunosensor for mouse IgG by electropolymerization of pyrrole propylic acid (conducted by cyclic voltammetry and controlled by SPR) and chemical binding of sheep antimouse IgG. In an original approach, they compared the electrochemical and optical (SPR) signals for the protein detection, demonstrating a similar sensitivity for both transductions [96]. The postfunctionalization of polymer ﬁlms bearing amino or carboxyl groups, however, requires the presence of additional chemical reagents such as the coupling reagent EDC and N-hydroxysulfosuccinimide in the aqueous solution with long reaction times. These conditions may partly denature the biomolecule and/or lead to incomplete derivatization of the polymer surface. To circumvent these drawbacks, an alternative consists in the electrochemical polymerization of monomers functionalized with activated esters such as N-hydroxysuccinimide, N-hydroxyphthalimide, or pentaﬂuorophenyl esters (Figure 10.5c). The resulting

O

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10 Biosensors Based on Electropolymerized Films

polymers can form, under very mild conditions (reagentless approach), amide bonds with the amino groups of biomolecules. This strategy, initiated by Hiller et al., was used for the development of enzyme electrodes [97]. For instance, the oxidative electropolymerization of chiral electropolymerizable dicarbazole derivatives functionalized with activated esters provided chiral polymers that allowed the covalent binding of GOX, tyrosinase, and alcohol dehydrogenase with stereoselective recognition [98]. With regard to the direct chemical grafting of receptor reagents, in their pioneering work, Korri-Youssouﬁ et al. have chemically grafted DNA probes onto a ﬁlm substituted in β-position of the polymerized pyrrole units by activated esters and exploited the conductivity of polypyrrole ﬁlms to detect the duplex formation [99]. As an extension of this earlier work, polypyrrole ﬁlms functionalized by a redox mediator (ferrocene) bearing an N-hydroxyphthalimide group were thus applied to the immobilization of amino-terminated DNAs (Figure 10.5d). The resulting DNA sensors exhibited an improvement in the detection limit (2 pM) by a factor of 10 compared to preceding sensors [100]. In addition to the conventional approach of DNA sensor fabrication, a new interesting concept of sensor emerged: the detection of DNA by ‘‘ﬁshing’’ a dsDNA directly from an aqueous solution. For this purpose, the covalent attachment of an intercalator (9-chloro-6-[(3-propylaminopropyl)amino]-4H-pyrido[4,3,2-kl]acridin-4-one) onto poly (pyrrole N-hydroxysuccinimide) provided an electrode capable of extracting and detecting a DNA duplex by simple insertion of the grafted intercalator into the present dsDNA [101]. A new and interesting research direction in biomolecule deposition lies in the direct covalent binding of proteins by irradiation of electrogenerated photoactive polymer ﬁlms. The strengths and weaknesses are the same than those of the covalent binding via activated ester groups except that the procedure is faster and spatially addressable by light. This photografting procedure was initiated by Cosnier et al. who reported the ﬁrst example of an electropolymerized ﬁlm (poly(pyrrole–benzophenone)) allowing, upon irradiation at 345 nm, the reagentless covalent grafting of proteins [18] (Figure 10.5e). This approach was successfully applied to the elaboration of electrochemical and optical immunosensors. In particular, the electrogeneration of poly (pyrrole–benzophenone) ﬁlms at the end face of an optical ﬁber allowed the grafting of antigens by internal light irradiation. For example, the hepatitis C virus as antigen was photochemically bound to the ﬁber and applied to the detection of the corresponding antibody in real blood samples [102]. This methodology was now extended to the biofunctionalization of ITO-coated glass chips modiﬁed by a poly(pyrrole–benzophenone) ﬁlm. The latter served as a substrate platform for the pattern photoimmobilization of bioreceptor reagents used for the simultaneous analyte detection of anticholera toxin B, antihepatitis B virus surface, and core protein antibodies [103]. This approach was also exploited for the elaboration of polypyrrole–benzophenone SiO2 nanocomposites. This photoactive nanocomposite served for the photografting of cholera toxin B and was used as a reagent in a visual immunologic-agglutination test [104].

10.7 Noncovalent Binding by Afﬁnity Interactions with the Electropolymerized Films

10.7 Noncovalent Binding by Afﬁnity Interactions with the Electropolymerized Films

The anchoring of biomolecules on electropolymerized ﬁlms can be achieved by afﬁnity interactions between the biomolecules and the polymer surface, summarized in Figure 10.6. Among the different afﬁnity-binding systems involving electropolymerized ﬁlms, the main approach was based on the strong associations (association constant Ka = 1015 ) [105] between biotin (a vitamin) and avidin (a protein, displaying four binding sites for biotin). The electrogeneration of biotinylated polymer ﬁlms thus enabled the successive attachment of avidin and biotinylated probes (Figure 10.6a). The advantage of the anchoring by afﬁnity interactions is the absence of chemical reactants that could deactivate the biomolecule. In addition, this immobilization approach is based on the formation of a single attachment point preserving thus the activity and accessibility to the immobilized biomolecule. The ﬁrst examples of biotinylated ﬁlms were reported for the polymerization of biotin derivatives functionalized with phenols or pyrroles [14–16]. Although different electropolymerizable groups such as phenol or carbazole were H N H

S

N O

O

N

HN

H

O S N N H

HN

H O

O

S

N

N

N

H

O

N O

O H

O N H

N

Ni

N

O O

N

N

O O

S O

(a)

(b)

O

(c) NH

O CH2 10

H2C 10 O

n

CH2 10 N

N

N

O

n

Figure 10.6 Commonly used afﬁnity systems for biomolecule immobilization utilizing speciﬁc supramolecular interactions(a) Biotin/avidin/biotin; (b) NTA/Cu2+ /histidine; and (c) adamantane/β-cyclodextrin.

n

203

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10 Biosensors Based on Electropolymerized Films

investigated [106], an overwhelming majority of biotinylated ﬁlms are based on polypyrrole skeleton. Such biotin–polypyrrole was mainly used for the design of DNA chips, immunosensors, and protein sensors [2]. It should be noted that the chemical modiﬁcation of enzymes with biotin introduces several biotin groups on the protein shell. As a consequence, the avidin–biotin interactions can be used for the reproducible successive deposition of enzyme layers intercalated by avidin. In contrast to conventional covalent binding, this approach enabled to build multienzyme layers on polymer surfaces and even bienzyme multilayers combining complementary activities [107]. In addition, such afﬁnity binding is surprisingly stable in organic solvents and was hence applied to the fabrication of organic-phase enzyme electrodes. With the aim to confer speciﬁc properties to the biotinylated polymers (hydrophilicity, chirality, redox conductivity, and photosensitivity), various biotinylated monomers or building blocks, such as a hydrophilic pyrrole–biotin, a tris(bipyridyl)iron(II) complex bearing six preoriented biotin groups, a chiral dicarbazole–biotin, or a biotinylated tris bipyridine–pyrrole ruthenium (II) complex [106, 108–110], were synthesized. In addition to the electropolymerization of biotin derivatives, another strategy consists in the covalent binding of one of the afﬁnity partners onto polyaniline ﬁlms. For instance, the anchoring of a biotinylated 20-mer oligonucleotide speciﬁc to Escherichia coli on an avidin-grafted polyaniline ﬁlm led to an efﬁcient electrochemical genosensor for E. coli cells without using PCR ampliﬁcation [111]. The hybridization of E. coli cell lysate was detected by differential pulse voltammetry in the presence of methylene blue. In this vein, the group of Higson developed immunosensors via the covalent binding of biotin–NHS onto polyaniline ﬁlms. Different biotinylated antibodies were immobilized on this platform via avidin–biotin interactions. By using impedance measurements, the resulting sensors led to label-less detection of prostate-speciﬁc antigen, ﬂuoroquinolone antibiotics, and neuron-speciﬁc enolase, a marker of cerebral injury [112–114]. However, the main weak point of the immobilization strategy based on the avidin–biotin interactions is the necessity of an intermediate avidin layer as building block that has a detrimental effect on the sensitivity of the transduction step. As an alternative to this approach, two new afﬁnity systems based on electropolymerized ﬁlms involving a metal afﬁnity immobilization concept (Figure 10.6b) or the formation of a supramolecular interaction between adamantane and β-cyclodextrin (Figure 10.6c) recently overcame this drawback. An further alternative afﬁnity-binding system is based on the interactions between histidine-tagged biomolecules and metal ions such as Ni2+ , Zn2+ , or Cu2+ previously complexed on functionalized polymers. Cooper et al. described the ﬁrst attempt by using such an afﬁnity system by immobilizing histidine-tagged enzymes onto electrogenerated polypyrrole ﬁlms N-substituted by Ni (II) doped carboxylate or imidazole groups [115]. More recently, a chelating group, NTA, was efﬁciently incorporated into an electropolymerized polyaniline–poly(vinyl sulfonate) ﬁlm without alteration of its conductivity properties and enabled the ﬁrm coordination of Ni (II) [116]. Nevertheless, the most convenient avenue consisted in the electropolymerization of a pyrrole–NTA derivative enabling the successive coordination of Cu2+ and

10.8 Outlook

histidine-tagged GOX to obtain a close-packed enzyme monolayer [19]. Recently, the anchoring of histidine-tagged antiatrazine antibody on such ﬁlm provided an extremely sensitive impedimetric immunosensor for atrazine (10 pg ml−1 ). The second afﬁnity system uses the host–guest interactions between adamantane and β-cyclodextrin. The electropolymerization of an adamantane–pyrrole derivative provided thus a new afﬁnity-binding polymer [22]. This principle was successfully applied to the direct attachment of β-cyclodextrin modiﬁed GOX or the indirect attachment of adamantane tagged GOX via anchoring of β-cyclodextrin modiﬁed gold nanoparticles.

10.8 Outlook

Among numerous procedures for biomolecule immobilization, conventional electrochemical methods based on electropolymerized ﬁlms combined with nanoparticles or templates continue to be the source of an extremely large diversity of electrochemical biosensors, biodevices, and biofuel cells. With the aim to improve the performance of immobilized biological coatings, 3D structures were designed. In particular, many research efforts have been focused on the combination of CNTs with electrogenerated polymers for designing biosensors [117–120]. Owing to the geometry of the CNTs, this material has an impressive high speciﬁc surface of more than 1000 m2 g−1 [121] that makes CNTs promising candidates for the construction of highly porous 3D nanostructured frameworks. Biosensors based on CNT–polymer composites were commonly prepared by electropolymerization of a monomer in the presence of biomolecules on electrodes already modiﬁed by adsorbed CNT coatings. More recently, the electropolymerization of afﬁnity-binding polymers based on avidin–biotin or adamantane–β-cyclodextrin interactions has opened attractive routes for the design of nanostructured biological architectures [22]. In this vein, the design of composite biosensors based on nanoparticles, electrogenerated polypyrrole or polyaniline ﬁlms, and biomolecules has demonstrated the validity of such approach for enhancing the transduction properties. For instance, a label-less impedimetric immunosensor exhibited an excellent detection limit for interleukin (10 fg ml−1 ), thanks to the combined conductivity of polypyrrole and Au nanoparticles [122]. Owing to their chemical tunable electrical conductivity, and facile controlled processing down to nanoscale dimensions by electrochemical polymerization, conducting polymers also emerged as promising materials for the development of nanobiosensors. As nicely illustrated by Martin et al. [123], there is currently a considerable interest in the idea of using template-based electropolymerization methods for the electrogeneration of nanowires. For instance, poly(pyrrole)-NTA nanotubes were synthesized by electrochemical polymerization using nanoporous alumina templates. Such tubes were employed to develop a chemiresistive sensor for detection and quantiﬁcation of heavy metal ions and histidine-tagged

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proteins [124]. Anodic aluminum oxide membrane was also used as template for the fabrication of polyaniline nanotubes that enabled direct electron transfer with GOX immobilized into the inner wall of these polymer nanotubes [125]. Other original templates consisted in self-assembled polystyrene particles or onion-type multilamellar vesicles [126]. In particular, the electrochemical growth of polyaniline through the interstitial spaces of the self-assembled template polystyrene spheres followed by their dissolution led to a cauliﬂower-like nanostructure of polyaniline. Such nanostructured ﬁlms were successfully used for enzyme immobilization [127].

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11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage Gordon G. Wallace, George Tsekouras, and Caiyun Wang

11.1 Introduction

Inherently conducting polymers (ICPs) have found widespread application as electrode materials for energy conversion and storage. The use of electropolymerization to produce such materials and structures containing them has a number of advantages including the fact that localized polymerization onto electrode structures ensures • • • • • • •

the ability to incorporate a wide range of dopants intimate electrical connection spatial control ease of fabrication of microstructures careful control over the polymer oxidation state the ability to form copolymers sequential deposition to produce layered structures.

In this chapter, we examine the use of electropolymerization to produce ICPs for use in solar cells (an example of energy conversion) and for polymer electrodes used in batteries and supercapacitors (energy storage). ICPs prepared via electrodeposition have also found use in catalytic electrodes for fuel cells; however, this application is not covered here. It has been widely accepted that the physicochemical and electrochemical properties of ICPs depend strongly on many factors encountered during electrodeposition [1]. These include electrochemical technique, working electrode substrate, and electrolyte [2–5]. The following considers each of these variables in turn, before the discussion is shifted to consider the application of ICPs in energy conversion and storage.

Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

11.1.1 Electrochemical Techniques

The electropolymerization of ICPs is generally achieved using galvanostatic (constant current), galvanodynamic (pulsed current), potentiostatic (constant potential), or potentiodynamic (cyclic voltammetry (CV) or pulsed potential) methods. Polymerization at constant current is most convenient for quantitatively controlling the thickness of the deposited polymer. The charge density may be calculated by the current density applied and the time elapsed according to the equation Q = I × t. Pulsed current methods have been employed to synthesize shorter chain length and/or obtain a higher degree of conjugation [6, 7]. When the current pulse is on, polymer chains are formed and nucleate over the substrate. When the current is off, no further nucleation occurs. A long off-time helps the already grown chains to oxidize completely before the next current pulse is applied and another polymer chain forms [6, 7]. Constant potential or potentiodynamic methods are preferred for the electrodeposition of ICPs whose overoxidation potentials are close to the monomer oxidation potential, for example, as in the case of polyaniline (PANi) [2]. In this way the time spent at potentials potentially damaging to the ICP is minimized, while this time period is sufﬁcient to oxidize the monomer and thereby allow electropolymerization to occur. Moreover, constant potential and potentiodynamic methods have been utilized to investigate ICP growth mechanism. Electrodeposition of polypyrrole [8], polythiophene (PTh) [9], and PANi [10] all involve a nucleation and growth mechanism. Chronoamperograms recorded during formation of polypyrrole include an initial current drop, which is typical of conducting polymer formation. It can be ascribed to the oxidative electroadsorption of monomer and substrate passivation [11]. Following the initial current drop, the current rises slowly to a maximum value before gradually falling again. The nucleation and growth mechanism are studied by comparing and ﬁtting the current density maximum obtained from the chronoamperometric curves with the theoretical curves developed by Harrison and Thirsk [12]. The polymerization potential can be determined either by the onset potential from the cyclic voltammograms or the lowest potential where a progressive increase in the current is observed over 200–300 s by applying potential increment at small amplitude [13]. CV is most commonly used to investigate the polymerization of a new monomer and obtain qualitative information about the redox processes involved in the early stage of the polymerization. Polymerization and ﬁlm deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potential below the onset of monomer oxidation. A ‘‘nucleation loop’’ in which the magnitude of the charge passed on the cathodic scan is higher than that on the anodic scan is commonly observed during the ﬁrst cycle.

11.1 Introduction Table 11.1

Oxidation potentials for common heterocyclic and aromatic ICP precursors [21].

Precursor

Oxidation potential (V vs SCE)

Pyrrole Bipyrrole Terpyrrole Aniline

1.20 0.55 0.26 0.71

Precursor

Thiophene Bithiophene Terthiophene –

Oxidation potential (V vs SCE) 2.07 1.31 1.05 –

11.1.2 Substrates

Since ICPs are deposited on a working electrode substrate by an oxidative process, it is necessary that the substrate should not be oxidized concurrently with the aromatic monomer. The oxidation potential of the substrate should therefore be higher than that of the monomer. Table 11.1 presents the oxidation potentials for a range of common ICP precursors. For electrochemical studies, the most commonly used substrates include platinum, gold, glassy carbon, or indium tin oxide (ITO)-coated glass. Stainless steel substrates have also been used due to the low cost compared with noble metals. Stainless steel has been used as a substrate for the electrodeposition of polypyrrole [14], PTh or its derivatives [15, 16], and PANi [17]. Another commonly used substrate for energy storage is carbonaceous materials such as carbon mat, carbon ﬁber, and carbon paper [18–20]. Some oxidizable substrates have also been used for electropolymerization. These include chromium, titanium, iron, mild steel, zinc, and aluminum [22–25]. Normally a passive coating is formed on these metal surfaces prior to ICP deposition. In addition, copper has also been used as a substrate for ICP electrodeposition [26]. 11.1.3 The Electrolyte

In an electrochemical polymerization, the monomer, dissolved in an appropriate solvent containing the desired anionic doping salt, is oxidized at the surface of an electrode by application of a positive potential and an insoluble, conducting polymeric material is deposited at the electrode [1, 27]. Both the solvent and electrolyte salt should be stable at the potential required to produce the polymer and they should provide an ionically conducting medium. The solvent should be capable of dissolving the monomer and electrolyte salt at appropriate concentrations. Aqueous solvents are preferred to organic solvents in terms of cost, ease of handling, safety, and the range of the counterions that can be used. The electrolyte inﬂuences the conductivity of the solution, the polymer properties, and the rate of polymerization. For efﬁcient polymerization, the anion

217

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11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

from the electrolyte should be readily incorporated into the conducting polymer, and it should also be stable both chemically and electrochemically during the electropolymerization process. Ionic liquids, a class of organic molten salts liquid at ambient temperature, have attracted attention for use as electrolytes in the conducting polymers formation [28, 29].

11.2 Energy Conversion

The utilization of electropolymerization for energy conversion (i.e., conversion of sunlight to electricity) may offer some advantages compared to more common processing techniques (e.g., spin-coating). Advantages include the ability to use simple precursors without solubilizing groups and subsequent deposition of otherwise nonprocessable materials, and the deposition of pinhole-free thin ﬁlms. As such, a considerable number of investigations concerning electropolymerization for energy conversion have been reported. The vast majority of such studies utilize electropolymerization to yield ICPs, in particular PThs. Dedoped, semiconducting PThs have been utilized as photoactive layers, while PThs have also been utilized in junctions with n-type inorganic materials (e.g., CdS, TiO2 ). The following, therefore, largely considers the application of electrodeposited PThs for energy conversion. Direct comparison of the photovoltaic (PV) performance of electrodeposited PThs reported in the literature is difﬁcult due to the great variation in the systems considered and measurements presented. The systems considered vary between half-cells and complete devices including solid-state and liquid electrolyte devices, while measurements presented vary from fundamental light-induced phenomena to those obtained using monochromatic and white-light illumination. A major difﬁculty in the comparison of literature reports is the general absence of overall device power-conversion-efﬁciency (η) values obtained under white-light illumination. Owing to such difﬁculties, quantitative comparison of the PV performance of electrodeposited PThs reported in the literature is not attempted here. Rather, the following will aim to capture the directions pursued by researchers in their attempts to improve the PV performance of electrodeposited PThs. In order to understand the directions pursued by researchers to improve the PV performance of PThs, it is necessary ﬁrst to brieﬂy consider the PV effect in ICPs, which includes light absorption and dissociation of resultant photoexcitations, or ‘‘excitons’’ (Figure 11.1). The PV effect is completed with the migration of separated charges to opposite electrodes for collection and passage of electrons through an outer loop where useful work is done. In Figure 11.1a, light of energy hν is absorbed, resulting in the photoexcitation of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), leaving empty HOMO levels, or ‘‘holes,’’ with charge +1. Resultant excitons, or electron–hole pairs, remain bound until they either recombine (radiatively (photoluminescence)

11.2 Energy Conversion

LUMO

Acceptor

Acceptor (i)

E

hn

Acceptor (ii)

Band gap Donor

Donor = Electron

HOMO (a)

219

Exciton

Donor = Hole

(b)

Figure 11.1 PV effect in ICPs: (a) light absorption and (b) exciton dissociation including (i) diffusion to interface and (ii) dissociation.

or nonradiatively) or are dissociated at an interface between two materials of different electron afﬁnity, as shown in Figure 11.1b. In order to maximize the PV effect, the ‘‘bandgap’’ of the photoactive layer should match as closely as possible the peak energy of sunlight (1.8 eV), while excitons should be dissociated into separate charge carriers in order to avoid recombination. The importance of light absorption and exciton dissociation is well reﬂected in the directions pursued by researchers to improve the PV performance of electrodeposited PThs. 11.2.1 Polythiophenes via Electropolymerization of Simple Precursors

Some of the earliest investigations on the utilization of electropolymerization for energy conversion were concerned with the simplest possible alkyl thiophene, 3-methylthiophene (3MT). The PV effect in electrodeposited P(3MT) thin ﬁlms was demonstrated in half-cells employing an aqueous electrolyte containing the redox couple methyl viologen (MV2+ /MV+ ) [30], or in nondegassed organic electrolyte with dissolved oxygen acting as the redox couple (O2 /O− 2 ) [31, 32]. Complete solid-state P(3MT)-based photoelectrochemical cells (PECs) employing poly(ethylene oxide) (PEO)-based electrolytes ﬁlled with the I3 − /I− redox couple have also been prepared and tested [33–35]. Very low photocurrents of ∼1 µA cm−2 under white-light illumination were obtained, largely due to poor diffusion of I3 − and I− species within the solid electrolyte. The utilization of thiophene dimer (2,2 -bithiophene (BTh) and thiophene trimer (2,2 :5 2 -terthiophene (TTh), Figure 11.2a) as precursors to PThs has the advantages of lowered oxidation potential compared to monomeric precursors (refer Table 11.1), allowing milder electropolymerization conditions to be employed, and a certain degree of predetermined and more favorable α − α linkages. A very limited number of studies considering (in part) the use of P(BTh) for energy conversion have been carried out [35, 36]. The relatively poor PV properties of P(BTh) compared to other electrosynthesized PThs has meant that further studies have

220

11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

generally not been undertaken. However, BTh was utilized in a study considering electropolymerization initiated at a self-assembled-monolayer (SAM)-modiﬁed ITO-coated glass substrate [37]. In this study, 3,4-ethylenedioxythiophene (EDOT) and BTh were sequentially electropolymerized onto a ter(EDOT) SAM to yield an ITO/SAM/PEDOT/P(BTh) photoactive electrode. When tested in a half-cell using aqueous MV2+ /MV+ as the redox couple, ITO/SAM/PEDOT/P(BTh) demonstrated more than double the photocurrent of ITO/PEDOT/P(BTh) due to improved bulk layer molecular organization of the former. Compared to the relatively poor PV properties of P(BTh), P(TTh) has demonstrated more promising results as described below. P(TTh) has been the benchmark material in the investigations of Wallace and coworkers regarding the application of electropolymerization to energy conversion [35, 38, 51–56]. In all of these investigations, the PV performance of P(TTh)-based PECs has been used as a reference to which the effect of the incorporation of electron-withdrawing and light-harvesting moieties has been measured (discussed later). One of the best η values reported by Wallace and coworkers has been for P(TTh)-based PECs, which demonstrated η = 0.101% under 50 mW cm−2 white-light illumination [54]. This result was obtained under optimized electropolymerization conditions with the solvent (electrolyte), growth technique, and temperature used – previously critical – inﬂuencing among other things the polymer morphology. The importance of P(TTh) morphology to the PV performance of PECs was further explored in the works of Dastoor and coworkers, who found that P(TTh) was electrodeposited in the form of an initial compact two-dimensional ﬁlm of ∼650-nm thickness, followed by a bulk three-dimensional overlayer [57, 58]. It was also determined that the initial compact ﬁlm constituted the photoactive layer, and that the bulk overlayer acted to limit device performance. An interesting variation to the utilization of P(TTh) for energy conversion was also reported by Wallace and coworkers, where an acetyl-linked bis-TTh (Figure 11.2b) was coelectropolymerized with TTh in an optimized ratio of 2 : 1, respectively, to yield a PEC demonstrating η = 0.069% under 50 mW cm−2 white-light illumination [38]. For this particular study, the PV performance of copolymers was an improvement over the P(TTh) homopolymer. The utilization of electrodeposited PThs with simple alkyl and alkoxy functionalities, within solid-state, Schottky-type devices of structure ITO/(photoactive PTh)/Al, has been extensively investigated by Casalbore-Miceli and coworkers [39–41, 43–50]. A summary of the precursors investigated is presented in Figure 11.2. According to the authors of these numerous reports, alkyl groups were employed to aid supramolecular organization, and, by variation of the alkyl chain length, manipulate interchain distance, and therefore PV properties. In addition, alkoxy groups were used to lower precursor oxidation potential, while it would also seem that the positioning of alkoxy groups in the β-position was done in order to promote α-α linkages in the resultant PTh backbone. Earlier reports by Casalbore-Miceli and coworkers [39–41] focused on the PV properties of 4,4 -dipentoxy-2,2 -bithiophene (Figure 11.2c) electropolymerized via CV onto n-doped Si and ITO substrates. The PV properties of the same material were

11.2 Energy Conversion

221

S C5H11O

S S

S

S

S

S

S S

(b)

S

H

H

H

C16H33

OC5H11

(c)

S

(a)

C16H33

R

C16H33

S

S S S

S

(d)

S

S

(e)

Figure 11.2 Structures of simple precursors utilized for energy conversion following electropolymerization: (a) 2,2 :5 2 -terthiophene (TTh), (b) 1,2-bis(3 -(2,2 :5 ,2 -terthiophenyl))ethene [38], (c) 4,4 -dipentoxy-2,2 -bithiophene [34, 39–42], (d) 4H-cyclopenta[2,1-b:3,4-b ]

S

S

(f)

C5H11O

(g)

dithiophene [43–45], (e) hexadecyl-substituted derivative of (d) [44, 45], (f) dihexadecyl-substituted derivative of (d) [44, 45], and (g) 4,4 -dipentoxy-3 -alkyl-2,2 :5 ,2 terthiophenes, R = −C6 H13 , −C8 H17 , or −C12 H25 [46–50].

also subsequently investigated by De-Paoli and coworkers in solid-state PECs employing the I3 − /I− redox couple; however, the reported short-circuit-current (Isc ) values were quite low, between 1 and 4 µA cm−2 under 100 mW cm−2 white-light illumination [34, 42]. The attention of Casalbore-Miceli and coworkers then shifted to the effect of hexadecyl substitution on the PV properties of 4H-cyclopenta[2,1-b:3,4-b ] dithiophene precursors [43–45] (Figure 11.2d–f ) electropolymerized via square wave and CV techniques, whereby the primary structure (Figure 11.2d) was chosen for improved structural order and carrier mobility. Hexadecyl substitution of the primary structure resulted in more regular polymers as evidenced by narrower CV and UV–vis peaks, and increased interchain distance according to X-ray diffraction (XRD) analysis [44, 45]. The inﬂuence of hexadecyl substitution on PV properties was higher open-circuit voltage (Voc ), lower Isc values, and enhanced device stability under ambient conditions due to raised hydrophobicity [44]. The most recent investigations reported by Casalbore-Miceli and coworkers have been concerned with the PV properties of electropolymerized 4,4 -dipentoxy-3 -alkyl-2,2 :5 ,2 -terthiophenes [46–50] (Figure 11.2g), with particular focus on the inﬂuence of alkyl chain length [48–50]. Increasing the alkyl chain length from hexyl to octyl to dodecyl led to increased interchain distance according to the results of XRD analysis, resulting in a drop in Isc values for Schottky-type devices from a high of ∼12 µA cm−2 under 20 mW cm−2 white-light illumination

OC5H11

222

11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

for the hexyl-substituted derivative [50]. Fluorescence measurements revealed that the precise reason for the drop in Isc values with increasing alkyl chain length and increasing interchain distance was an increase in the ﬂuorescence quantum yield (φpl ), due to the localization of excitons on polymer chains and subsequent recombination [49]. 11.2.2 Polythiophenes via Electropolymerization of Precursors Functionalized with Electron Accepting/Withdrawing Moieties

One of the most signiﬁcant breakthroughs in conducting polymer solar cell research came in 1995 when Heeger and coworkers reported a Schottky-type device based on the blend between the donor conducting polymer poly(2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) and the acceptor C60 derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) [59]. A two-orders-of-magnitude improvement in η values was obtained for MEH-PPV/PCBM blend devices compared to devices based on MEH-PPV alone. This breakthrough was based on the ﬁndings of an earlier report by Sariciftci and coworkers in 1992, which demonstrated photoinduced electron transfer from MEH-PPV to C60 using light-induced electron spin resonance (LESR) spectroscopy [60]. An alternative to the blending of ICPs and C60 derivatives for the formation of donor–acceptor heterojunctions is the electropolymerization of PTh precursors bearing pendant C60 groups. This approach avoids the problems associated with the simple blending of donors and acceptors, including phase separation and immiscibility of materials. Several examples of the electropolymerization of PTh precursors with covalently attached C60 functionalities have been reported (Figure 11.3) [61–71]. The ﬁrst such investigation was reported by Sannicolo and coworkers [61] for a cyclopentadithiophene-fullerene precursor (Figure 11.3a), which was electropolymerized by CV. Postgrowth CV of the electrodeposited polymer in precursor-free electrolyte demonstrated retention of the characteristic p-type and n-type electrochemistry of the PTh backbone and pendant C60 , respectively, indicating that no ground-state electronic transitions took place. Unfortunately, no photoelectrochemical measurements were performed in order to probe PV properties in this study. Several investigations following a similar pattern of electrochemical characterization have also been reported concerning the electropolymerization of PTh precursors with pendant C60 groups, namely, by the groups of Ferraris [62, 63], Cravino [64–67], Komatsu [68, 69], and Wallace [70, 71]. Beyond the similar pattern of electrochemical measurements established in the ﬁrst report by Sannicolo and coworkers, a limited number of more insightful investigations have been reported. Cravino et al. [64] conﬁrmed photoinduced electron transfer from the C60 group to the PTh backbone of an electrodeposited polymer based on a bithiophene-fulleropyrrolidine (Figure 11.3c) using LESR spectroscopy. Yamazaki et al. [69] measured Isc values of ∼1 µA cm−2 under 50 mW cm−2 white-light illumination in a half-cell employing a photoanode, based on the

11.2 Energy Conversion

223

N

S

(b)

S S

(a)

S S S

S S N

S

R

(OCH2CH2)3O

(d)

(c)

O

O

O

O S

S

S

S

S

S

(f) H3C

(e)

Figure 11.3 Structures of C60 -functionalized PTh precursors utilized following electropolymerization as possible photoactive materials for energy conversion: (a) a cyclopentadithiophene-fullerene [61], (b) a 3 -alkyl-2,2 -bithienyl-fullerene [62, 63],

N

(c) a bithiophene-fulleropyrrolidine [64–67], (d) a terthiophene-fullerene dyad, R = −H or −CH3 [68], (e) an ethylenedioxy-substituted terthiophene-fullerene [69], and (f) N-methyl2-(2-[2,2 ;5 ,2 -terthiophene-3 -yl]ethenyl) fullero[3,4]pyrrolidine [70, 71].

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11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

electrodeposited polymer of an ethylenedioxy-substituted terthiophene-fullerene (Figure 11.3e) and MV2+ /MV+ redox couple in aqueous electrolyte. Finally, the only example of a complete photoelectrochemical device was that based on the electrodeposited polymer of a C60 -functionalized TTh (Figure 11.3f ) [70]. In this study, a Voc of 0.25 V was demonstrated under 50 mW cm−2 white-light illumination, which was signiﬁcantly higher compared to the Voc of 0.15 V observed for PECs based on P(TTh) without any pendant C60 groups. This was possibly indicative of photoinduced electron transfer from the P(TTh) backbone to the pendant C60 , although no additional measurements were carried out to conﬁrm this. Only a few reports have considered electrodeposited PThs bearing pendant electron accepting or withdrawing groups besides C60 for energy conversion applications. PTh precursors bearing pendant nitrostyryl and cyanostyryl electron-withdrawing groups were electropolymerized and the polymers used in liquid PECs employing the I3 − /I− redox couple [36, 51]. Slightly improved Voc values under white-light illumination were observed for the polymer formed via the electropolymerization of a nitrostyryl-substituted TTh (Figure 11.4a) compared to P(TTh); however, the overall η value of the former was signiﬁcantly lower due to lower Isc values [51]. Unfortunately, no improvement in PV properties was observed for copolymers formed via the coelectropolymerization of nitrostyryl or cyanostyryl-substituted thiophenes (Figure 11.4b) with BTh, compared to P(BTh) [36]. In contrast, Semenikhin and coworkers have demonstrated ∼5× improvement in the PV performance under monochromatic illumination of photoanodes based on copolymers formed via the coelectropolymerization of a thienyl-silole (Figure 11.4c) and BTh, compared to P(BTh) [72]. In this study, the silole functionality acted as an electron acceptor due to its low-lying LUMO, while photoanodes were characterized as half-cells in an air-saturated organic electrolyte with O2 /O2 − acting as the redox couple. The authors cited the stark improvement in PV properties as evidence that a donor–acceptor material was formed. NO2

R

S

S S

S

S Si

S

S

(a)

(b) Figure 11.4 Structures of PTh precursors bearing electron accepting or withdrawing groups other than C60 utilized for energy conversion: (a) (E)-3 -(p-nitrostyryl)terthiophene [51], (b) styryl-substituted thiophenes, R = −NO2 or –CN [36], and (c) 2,5-bis([2,20-bithiophene]-5-yl)-1, 1-dimethyl-3,4-diphenyl-silole [72].

S CH3

CH3

(c)

11.2 Energy Conversion

11.2.3 Polythiophenes via Electropolymerization of Precursors Functionalized with Light-Harvesting Moieties

A current limitation to the PV performance of conducting polymer solar cells is the lack of broad light absorption across the visible light spectrum. In particular, the ICPs commonly utilized for solar cell applications (e.g., poly(3-hexylthiophene), P(3HT)) typically demonstrate absorption maxima around 2.5 eV, whereas natural sunlight has a peak intensity at 1.8 eV [73]. One possible solution to this problem has been to covalently attach light-harvesting moieties to PTh precursors and to electropolymerize these to yield photoactive materials with broadened light absorption. Copolymers of 3MT, BTh, or TTh with porphyrin-substituted Th (Figure 11.5a) or TTh (Figure 11.5b) were formed via coelectropolymerization [35]. Copolymers were grown owing to the difﬁculties associated with the electrodeposition of homopolymers of the porphyrin-substituted precursors, due to the steric bulk of the porphyrin moiety. Unfortunately, almost no beneﬁt to device PV performance was observed for PECs based on porphyrin-substituted PThs compared to P(TTh). Later efforts aimed at combining the known beneﬁt to PV properties afforded by the formation of cross-linked PThs demonstrated previously [38], and the projected beneﬁt of covalent attachment of porphyrin light-harvesters to the PTh backbone. In this way, copolymers between TTh and a porphyrin-linked bis-TTh (Figure 11.5c) were coelectropolymerized and utilized in liquid PECs employing the I3 − /I− redox couple [52, 53]. Copolymers were considered since homopolymers of the porphyrin-linked bis-TTh were difﬁcult to electrodeposit due to the steric bulk of the porphyrin moiety. An overall η value of 0.12% under 50 mW cm−2 white-light illumination was achieved both for devices utilizing a Zn-coordinated porphyrin moiety with a CH3 CN-based electrolyte [52], and a Cu-coordinated porphyrin moiety with an ionic liquid-based electrolyte [53]. Signiﬁcantly, the PV properties of electrodeposited PThs with a covalently attached porphyrin moiety were improved compared to those of optimized P(TTh) without any added light-harvesting functionality reported soon after [54]. An alternative and perhaps more direct approach to that described above for the incorporation of porphyrin light-harvesting moieties has been reported involving the electropolymerization of an aminophenyl-substituted porphyrin (Figure 11.5d) to yield conducting polymer ﬁlms that were utilized in energy conversion devices [74]. A very modest Isc value of 3 µA cm−2 was obtained under 100 mW cm−2 white-light illumination for a liquid electrolyte PEC utilizing I3 − /I− as the redox couple, although the authors failed to employ a catalyst (e.g., Pt) on the counter-electrode surface for I− oxidation, which would have severely limited PV performance. A much higher Isc value of 180 µA cm−2 was obtained under the same illumination conditions for a bilayer Schottky-type device employing PCBM as the acceptor material. The ability to dope ICPs with a wide variety of charged species led Wallace and coworkers to investigate the incorporation of commercially available, light-harvesting dyes into electrodeposited P(TTh)s [55, 56]. This approach

225

226

11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

N N NH NH

HN

HN N N

S

S S

S (a)

(b)

H2N

NH2

S S N

NH

N S

NH

HN

S N

HN

N S

S NH2

H2N (c) Figure 11.5 Structures of PTh precursors with covalently linked light-harvesting moieties utilized for energy conversion following electropolymerization: (a) trans1-(2 -(5 ,10 ,15 ,20 -tetraphenylporphyrinyl))2-(3 -thienyl)ethene [35], (b) trans-1-(2 (5 ,10 ,15 ,20 -tetraphenylporphyrinyl))-2(3 -terthienyl)ethene [35], (c) trans-5,

(d) 15-bis([2 ,2 :5 ,2 -terthiophene]-3 -yl)-2,8, 12,18-tetra-n-butyl-3,7,13,17-tetramethylporphyrin (52, 53), and (d) structure of polyporphyrin precursor tetrakis-5,10, 15,20-(4-aminophenyl)porphyrin utilized for energy conversion following electropolymerization [74].

11.3 Energy Storage

represents an elegant solution to the incorporation of light-harvesting moieties into PThs for energy conversion applications, since it avoids the need for covalent functionalization of PTh precursors. An anionic dye (A− ) may be incorporated during electropolymerization of, for example, TTh (Equation 11.1). For P(TTh) to be useful as a photoactive material, it must be electrochemically reduced to the semiconducting form. A relatively bulky, immobile anionic dye incorporated during electropolymerization will generally be retained upon electrochemical reduction. In this scenario, electrical neutrality may be achieved by carrying out the electrochemical reduction of P(TTh)+ /A− in the presence of a cationic dye (C+ ), to yield P(TTh) doped with two light-harvesters (Equation 11.2). nTTh + A− − e− → P(TTh)+ A− +

−

+

−

(11.1) −

+

P(TTh) A + C + e → P(TTh) A C 0

(11.2)

The approach described above has been utilized to prepare a photoactive material based on P(TTh) doped with both the anionic dye Erioglaucine (Erio) and the cationic dye brilliant green (BG) [55]. A signiﬁcant improvement in PV performance was observed for PECs based on P(TTh)0 /Erio− /BG+ compared to PECs based on P(TTh)0 under 50 mW cm−2 white-light illumination. This was largely due to improvement in Voc values (260 mV for P(TTh)0 /Erio− /BG+ compared to 134 mV for P(TTh)0 ), while photocurrent action spectra revealed the contribution of both Erio− and BG+ to the output photocurrent. The success of this approach was measured in terms of the η value of 0.11% achieved for PECs based on electrodeposited P(TTh)0 /Erio− /BG+ , which was comparable to the η value of 0.12% reported for PECs based on more complicated, electrodeposited copolymers of a porphyrin-linked bis-TTh and TTh [52, 53]. Finally, the approach of Wallace and coworkers was also utilized to incorporate the anionic dye sulforhodamine-B (S-B) during the electropolymerization of TTh [56]. UV–Vis spectroscopy revealed that S-B− was retained upon electrochemical reduction of P(TTh) to yield P(TTh)0 /S-B− . PV testing of PECs based on P(TTh)0 /S-B− under 50 mW cm−2 white-light illumination demonstrated a more than 2 × improvement in Voc to 318 mV compared to the 152 mV achieved earlier for optimized P(TTh)-based PECs. Unfortunately, the lack of clear evidence for the direct contribution of S-B− to device photocurrent suggested that the motivation for incorporation of this light-harvesting dye was not realized. However, according to the electrochemical characterization results presented, it is possible that the presence of S-B− within the P(TTh) matrix may have resulted in the establishment of a donor (P(TTh))–acceptor (S-B− ) material, which may also have explained the large improvement in device Voc .

11.3 Energy Storage

ICPs can be reversibly switched between metallic and insulating states (from p- or n-doped states to neutral state; see Scheme 11.1). During the doping/dedoping process, counterions are expelled from or integrated into the polymer backbone to

227

228

11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

(P0)n + ny A− (P0)n + ny M+

p-doping undoping n-doping undoping

Insulating state

(P)y+ (A−)y

n

(P)y− (M+)y

n

−

+ ny e−

A− = PF6 , --- ---

− ny e−

M+ = (C2H5)4N+, --- ---

Metallic state

Scheme 11.1 Doping/undoping processes of conducting polymers. (P: conducting polymers; y: doping level; A: dopant anion; M: dopant cation).

maintain electroneutrality. The oxidation and reduction processes in ICPs make it possible to use these polymeric materials for charge-storage devices, such as secondary batteries and supercapacitors [75, 76]. 11.3.1 Application of Inherently Conducting Polymers in Rechargeable Batteries

The theoretical values of speciﬁc charge of ICPs are comparable or higher than those of metal oxides (see Table 11.2), and their application in rechargeable batteries has been intensively reviewed by Nov´ak et al. [77]. Most research into rechargeable Table 11.2

Selected values of speciﬁc charge for ICPs and other materials.

Polymer or dimera

Pac PANi (aqueous) PANi (organic) PPy PT PMT PPP Paz PCz TETD DMS EEDS X1 X7 a PAc:

yb

Equivalent weight (g/MU; g/dimer)

Speciﬁc charge (without counterions) (Ah kg –1 )

0.07 0.5 1.0 0.33 0.25 0.25 0.4 0.25 0.25 – – – – –

13 91 65 65 82 96 76 126 165 296 94 210 148 142

144 147 295 136 82 70 141 53 41 181 570 255 362 377

polyacetylene; PANi: polyaniline; PPy: polypyrrole; PT: polythiophene; PMT: poly(3-methylthiophene); PPP: poly(p-phenylene); PAz: polyazulene; PCz: polycarbazole; TETD: tetraethylthiuram disulﬁde; DMS: dimethyl disulﬁde; EEDS: bis(ethoxyethyl) disulﬁde; X1: bis(2-mercaptoethyl) ether polymer; X7: 2,4-dimercaptopyrimidine polymer. b y: doping level. Reprinted from Nov´ak et al. [77].

11.3 Energy Storage

batteries has focused on the use of p-type ICPs as cathodes together with metal or metal alloy anodes, such as Li, Li–Al, or Zn [78, 79]. n-Type ICPs usually exhibit low charge capacity and poor chemical stability [80, 81]. Consequently, only a few studies investigating ICPs as both the cathode material and the anode material in rechargeable batteries have appeared. These all-ICP battery systems have been polypyrrole-based [82, 83], PTh-based [18, 84], or PANi-based [85, 86]. However, the drawback of self-discharge has deferred their development and commercialization. Some promising results have been obtained using poly(3,4-ethylenedioxythiophene) (PEDOT) which has exhibited a high average speciﬁc capacity of 425 mAh g−1 in 44 charge/discharge cycles in LiN(CF3 SO2 )2 /ethylene glycol dimethyl ether/1,3-dioxolane solution with a Li foil counter electrode [87]. The development of ﬂexible batteries, which could ﬁnd application in modern gadgets, such as roll-up displays, wearable devices, radiofrequency identiﬁcation tags, and integrated circuit smart cards [88–90], is an area of current interest. ICPs are promising candidates for use as electrodes in ﬂexible energy storage systems. 11.3.2 Application of Conducting Polymers in Supercapacitors

Supercapacitors are charge-storage devices, which possess much higher power densities (>500 W kg−1 ), excellent reversibility, and longer cycle life (>105 cycles) compared with rechargeable batteries. The differences between capacitors and batteries are clearly illustrated by the discharge curves shown schematically in Figure 11.6, where the voltage on the capacitors declines linearly (for potential-independent double-layer capacitance) with the extent of charge, while that for an ideal battery remains constant as long as two phases remain in equilibrium [91]. Capacitors can Recharge Discharge

Potential

Recharge

Ideal battery

Ideal capacitor I/C Discharge State of charge indication Charge/discharge

Figure 11.6 Difference of charge and recharge relations for a capacitor and a battery: potential as a function of state of charge. (Reprinted from [91].)

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11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

deliver energy at very high rates (very high power), but are not capable of holding large quantities of energy. Supercapacitors are intermediate in power and energy density. The application of supercapacitors in combination with batteries in hybrid electric vehicles (HEVs) is an important area of application. The energy storage system discharges at a high rate during acceleration or hill climbing, and recharges when traveling at constant speed and during deceleration, either from the battery or by regenerative braking. For ICP-based supercapacitors, in addition to the double-layer capacitance, pseudocapacitance involved in Faradaic reactions contributes to capacitance. Charge can also be stored throughout the polymeric material, rather than only at the solid/electrolyte interface as for conventional, high-surface-area carbonaceous material. Compared with metal oxide, most ICPs are inexpensive and environment friendly.

11.4 Electropolymerization to Form Electrodes for Energy Storage Applications

For energy storage applications, it is necessary to synthesize polymers that have a high doping level and undergo reversible electrochemical reactions. ICPs may be prepared either chemically or electrochemically. The chemical methods involve the use of various oxidants. For electrochemical studies of chemically prepared ICPs, particularly for application in batteries or supercapacitors, the polymers are either pressed [92] or pasted onto the electrode surface with the addition of binder polyvinylidene ﬂuoride (PVDF) or polytetraﬂuoroethylene (PTFE>) and conductive additives (for example, carbon black, graphite). In comparison, the use of electropolymerization to form ICPs for energy storage provides better control of ﬁlm thickness and morphology. In addition, many of the polypyrroles, PThs, and PANi’s to be used for energy storage are not readily processable and so electropolymerization provides a means of preparing the material in situ in the electrochemical device. The most intensively investigated ICPs for the application in energy storage are polypyrrole, PANi, PTh, and PTh derivatives owing to their stability, good electronic conductivity, and tunable E ◦ value for the electrochemical processes of interest. 11.4.1 PPy

Generally, p-type polypyrroles are used as cathode materials with a nonaqueous electrolyte, particularly in lithium batteries, although they have also been used in aqueous-based rechargeable batteries [93]. Generally, high conductivity, a high doping level, and a porous structure are beneﬁcial to the application in energy storage, particularly for supercapacitors. The size of the anion incorporated during electropolymerization determines the microstructure and porosity of the polymer, and hence dictates the diffusion characteristics of ions into and out of the polymer during charge/discharge.

11.4 Electropolymerization to Form Electrodes for Energy Storage Applications

The incorporation of electroactive dopants increases the quantity of charge that can be stored. Using the electroactive dopant anthraquinone-1-sulfonate, PPy ﬁlms exhibited a high energy density of 150 Ah l−1 , which was more than twice that observed for PPy ﬁlms containing electrochemically inactive anions such as naphthalene-1-sulfonate or ClO4 − [94]. Naﬁon is a molecular dopant capable of providing ionic conductivity. PPy/naﬁon ﬁlms exhibit promising redox properties when used as a battery or supercapacitor material. A battery comprising Li/PEO-LiClO4/PPy/Naﬁon showed a coulombic efﬁciency exceeding 95% with up to 0.1 mA cm−2 discharge current density being used. In contrast, only 43% coulombic efﬁciency was observed for PPy/ClO4 [95]. The diffusion coefﬁcients measured for the dopant ion were 2.78 × 10−11 (PPy/Naﬁon) and 1.63 × 10−14 (PPy/ClO4 ). PPy/Naﬁon is also a highly stable supercapacitor electrode material, which retains 98% of the initial capacitance over 3000 cycles, whereas PPy/ClO4 retained only 70% over this period [96]. The electropolymerization temperature has a substantial inﬂuence on the kinetics of polymerization as well as on the conductivity, redox properties, and mechanical characteristics of the ﬁlm prepared. In general, higher conductivities are observed in materials when lower synthesis temperatures are employed. PPy ﬁlms prepared at 4 ◦ C with a growth charge ≤ 1.0 C cm−2 possess ideal capacitive response and have a high speciﬁc capacitance of 268 F g−1 [97]. 11.4.2 PANi

PANi has properties that make it an attractive electrode material for energy storage [98], and it has been used as an electrode material for aqueous batteries [99], lithium batteries [100], and electrochemical supercapacitors [101]. PANi can be produced using either an aqueous or nonaqueous electrolyte. The use of a constant potential or potentiodynamic methods are preferred for the electrodeposition of PANi since the overoxidation potential for the polymer is close to the monomer oxidation potential [1]. In aqueous solution, generally an acidic medium (e.g., H2 SO4 , HCl, HClO4 ) with pH ≤ 2 is used as the electrolyte. This low pH is required to solubilize the monomer and to generate the emeraldine salt (conducting) form of PANi [1]. PANi ﬁlm prepared from a solution containing 0.5 M H2 SO4 and 0.5 M aniline was deposited onto a stainless steel substrate. When used as a capacitor electrode this material exhibited a capacitance of 815 F g−1 . PANi’s obtained from organic-solvent-based electrolytes have been used as electrodes in Li ion batteries or supercapacitors [102, 103]. PANis electropolymerized in these two systems exhibit similar performance with application in energy storage, although different surface morphologies are obtained. Agar gel [104] or polyacrylonitrile aerogel [105] has been used as a template for PANi formation. Agar gel is an interconnected cellular material with relatively large pores, and it can be easily removed using water at temperatures above 90 ◦ C. The electrical conductivity and the bulk density of PANi prepared using

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11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage

an agar-template were 1.5 S cm−1 and 1.53 g cm−3 , respectively, which is three orders and four times larger than those prepared without the template. With the polyacrylonitrile template, the aerogel is ﬁrst deposited on carbon paper, and then used as substrate for PANi electrodeposition. A speciﬁc capacitance of 230 F g−1 was obtained using this material, and it also shows an excellent cycle life (over 3000 cycles). 11.4.3 PTh and Derivatives

PThs can exhibit reversible p-doping and n-doping properties, which make them promising materials for the application in all-polymer batteries and supercapacitors [106]. Polypyrrole and PANi cannot be n-doped since the n-doping processes are expected to occur at much lower potentials than those allowed by the electrochemical windows of the most common polar aprotic electrolytes. PTh with relatively high oxidation potential can be n-doped, although the maximum possible charge is usually considerably lower than that for the corresponding p-doping processes. Efforts to develop π-conjugated polymers with high n-doping ability and stability when reduced are summarized by Levi and Aurbach [107], which include the introduction of an electron-withdrawing group in the 3-position of the thiophene and the introduction of electron-withdrawing moieties (subunits) together with π-conjugated bi- or quarto-thiophene subunits in the polymer backbone. The nature of the solvent used during electropolymerization also affects the intensity of the charge trapping phenomena upon the π-conjugated polymers. Sulfolane-based electrolytes facilitate stable and highly reversible doping of π-conjugated polymers, in particular in the n-doping domains. The most investigated electrodeposited PTh for application in energy storage is PEDOT, which has mainly been used as an electrode in supercapacitors. The electropolymerization of EDOT (and other thiophene derivatives) generally requires anhydrous organic media [108, 109]. Electropolymerization of EDOT in aqueous media is limited by the low solubility of the monomer. In addition, the thienyl cation radicals produced react with water, and the oxidation potential is close to or higher than that for water oxidation. However, it is reported that the addition of surfactants such as sodium dodecylsulfate (SDS) [110] or hydroxylpropyl-β-cyclodextrin (HP-β-CD) [111] improves the solubility of EDOT. PEDOT electrodes prepared in aqueous electrolyte containing 0.1 M H2 SO4 and the surfactant SDS are found to yield higher speciﬁc capacitance than those prepared from neutral aqueous electrolyte. Speciﬁc capacitances of 250 F g−1 were obtained during the initial stages of cycling [15]. A speciﬁc capacitance of 130 F g−1 and an enhanced cycle life (up to 70 000 cycles) were obtained for a PEDOT ﬁlm prepared in ionic liquid electrolyte [112]. 1-Butyl-3-methylimidazolium tetraﬂuoroborate was used as both the growth medium and the supporting electrolyte.

11.5 Nanostructured Conducting Polymers

11.5 Nanostructured Conducting Polymers

Superior electrochemical properties can also be expected for the nanostructured ICPs, particularly for the ordered structures, which should be ideal for high electrochemical efﬁciency and fast kinetics. Two methods are employed to electrochemically synthesize nanostructured ICPs: template-assisted and nontemplate-assisted electropoymerization. 11.5.1 Template-Assisted Electropolymerization

Anodic aluminum oxide (AAO) is a commonly used template for electropolymerization. Electrodeposition within the pores is accomplished by coating one face of the template with a metal ﬁlm, for example, Pt or Au (usually either ion sputtering or thermal evaporation) and using this metal ﬁlm as the electrode substrate [113]. Cao et al. have produced PANi nanowire arrays using the AAO template [114]. They demonstrated that an increase in sulfuric acid concentration induces a transition from solid nanowires with tubular ends to open nanowires. The speciﬁc capacitance of the array structures was found to be 710 F g−1 at a charge/discharge rate of 5 A g−1 , about 20% more than that of conventional bulk ﬁlm. PEDOT has also been electrosynthesized using an AAO template [115, 116]. At potentials higher than 1.4 V (vs Ag/AgCl), hollow nanotubes can be grown instead of solid nanowires. However, at a lower oxidation potential (1.2 V vs Ag/AgCl) nanotubes are produced regardless of the monomer concentration. PPy or P3MT nanotubes were grown using a similar approach. PEDOT-nanotube-based supercapacitors can achieve a high power density of 25 kW kg−1 while maintaining 80% of their energy density (5.6 Wh kg−1 ). This high power density is attributed to the fast charge/discharge of nanotubular structures: hollow nanotubes allow counterions to readily penetrate into the polymer while the thin wall provides a short diffusion distance to facilitate the ion transport [117]. 11.5.2 Direct Electropolymerization

For the template-assisted electropolymerization of ICPs, the polymer was grown and ﬁlled in the nanopores of the membrane, so the dimensions and the morphology of the polymer structures are deﬁned (or limited) by the porous template support. Liu et al. have developed a three-step electrochemical method to directly produce large arrays of oriented ICP nanowires on a variety of substrates (Pt, Si, Au, Carbon, SiO2 ) without the use of a template [118, 119]. A large number of nuclei are ﬁrst created on the substrate when a large current density is applied (0.08 MA cm−2 for 0.5 h). After nucleation, the current was reduced stepwise ﬁrst to 0.04 mA cm−2 (for 3 h) and then to 0.02 mA cm−2 (also for 3 h). Stepwise growth produces uniform, oriented nanowires on a variety of ﬂat and rough surfaces. Large arrays of oriented

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11 Inherently Conducting Polymers via Electropolymerization for Energy Conversion and Storage Table 11.3

Capacitance for different types of PANi ﬁlms.

Types

Synthesis method

PANi/Pt

Three-step One-step Three-step One-step Three-step One-step

PANi/Au PANi/Carbon

CLF (F g –1 )a

Cs (F g –1 )a

Cp (F g –1 )a

344 119 314 122 330 176

415 157 419 155 437 181

596 189 541 129 589 234

a C , low-frequency capacitance estimated from AC impedance; C and C , speciﬁc capacitance LF s p estimated from cyclic voltammograms and galvanostatic charge/discharge. Results from [120].

PANi nanowires have been produced using this three-step method. The structures produced exhibited capacitance values several times higher than those obtained for structures produced using the bulk polymer one-step electropolymerization method (see Table 11.3) [120].

11.6 Conducting Polymer Composites

ICPs can be formed electrochemically in the presence of metal oxide or carbon materials to form composite electrodes. In these composite electrodes, the polymers form a conducting matrix, which provides a conducting binder for the other material. This has a direct impact on improving coulombic efﬁciency, rate capability, and the cycle life of the electrode materials. When the electrochemical properties of the conducting polymer overlap with the operative redox couple of the other material used in the composite, then the polymer will also contribute to the charge/discharge properties. Electropolymerization and simultaneous chemical polymerization methods have been used to prepare LiFePO4 /polymer composite electrodes [121]. The results clearly demonstrated that higher rate capacity can be obtained in the composite electrode, and the electrodeposited composite exhibits superior performance to those produced by chemical polymerization (Figure 11.7). MnO2 /PEDOT composite nanowires have been formed within AAO templates [122]. In the application of these composites in supercapacitors, the core MnO2 provides high energy storage capacity, while the highly conductive, porous, and ﬂexible PEDOT shell facilitates the electron transport and ion diffusion as well as providing mechanical reinforcement. High speciﬁc capacitances can be provided by these coaxial nanowires particularly at high current densities (Figure 11.8). PPy/graphite composites have been used as anode materials in lithium ion batteries [123]. The irreversible capacities of the composites during galvanostatic cycling are signiﬁcantly decreased and the reversible capacities are increased compared with bare graphite.

11.6 Conducting Polymer Composites

Charge capacity(mAh g−1)

200 160 120 C-LFP LFP/16% PPy-ED LFP/7% PPy-Chem LFP/7% PANi-Chem

80 40 0

0.1

1

10

C-rate

Specific capacitance (F g−1)

Figure 11.7 Comparison of discharge rate capacity for the C-LFP-based composite cathodes with PPy and PANi. The data were obtained by discharging at various rates after charging at 0.1 C. (Reprinted from [121].)

210 180 150 120 90 60 30 5

10

15

20

25

Current density (mA cm−2) Figure 11.8 Speciﬁc capacitance of MnO2 nanowires (closed square), PEDOT nanowires (open dots), MnO2 thin ﬁlm (open square) and MnO2 /PEDOT coaxial nanowires (closed dots) at difference charge/discharge current densities. (Reprinted from [122].)

A synergistic effect has been reported between the two components of the PPy/carbon nanotubes (CNTs) composites as supercapacitor electrode materials [124]. The composite was obtained by electrodeposition of PPy on nanotubes using a solution of monomer in sulfuric acid. The total capacitance of the composites combines a pure electrostatic attraction of ions and the electrochemical redox reactions of the π-conjugated system of PPy. A very high capacitance value of 165 F g−1 was obtained for the composites.

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11.7 Conclusions

Electropolymerization has been utilized for energy conversion, where investigations have been generally concerned with various photoactive PThs. Efforts have gone beyond investigations concerning the utilization of simple PTh precursors and those with simple added functionalities to address the critical processes of light absorption and exciton dissociation that give rise to the PV effect in ICPs. However, the potential advantages of electropolymerization for energy conversion have not been realized to date. An expanded research effort will be required to improve fundamental understanding necessary to raise the PV performance of solar cells based on electrodeposited materials. Electropolymerization has proven to be an effective method to produce a variety of energy storage electrodes, such as ICPs, nanostructured ICPs, and ICPs composites. Their electrochemical properties can be tuned and improved by controlling the electrodeposition conditions. Nanostructured ICPs and ICPs composites are promising electrodes in this application. References 1. Wallace, G.G., Spinks, G.M.,

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241

12 Electrochemomechanical Devices: Artiﬁcial Muscles Toribio F. Otero and Joaqu´ın Arias-Pardilla

12.1 Introduction

Science is one of the most robust conceptual constructs developed by human beings. Theoretical physical models have been developed involving the smallest and the largest systems over the full scale of the universe. At either extreme, the models, which provide for constant interaction forces between their components, describe experimental observations and are predictive. Life evolved from systems of intermediate size in relation to the extremes of universal scale. Life, biological organs, and cells develop functions only under chemical driving conditions. Natural organs can be considered as biological devices that are very efﬁcient in transforming chemical energy at constant temperature into functions, unlike servitude of machines to the Carnot cycle. Inside any living cell, thousands of simultaneous reactions occur. Every reaction promotes changes from reactants to products, with subsequent changes to hundreds of intramolecular and intermolecular interactions. Moreover, most of those reactions link conformational changes of biopolymers with ionic and electronic movement driving water ﬂow. Hence, many simultaneous chemical reactions, intermolecular, and intramolecular interactions involving conformational movements are outside the possibilities of current theoretical models. Theoretical descriptions of any living cell and predictions of its behavior when unhealthy are unavailable within our scientiﬁc models. This constitutes the proximity paradox [1] because we have been able to develop good and predictive theoretical models for subatomic or galactic systems, far removed from our everyday surroundings. Actuation of natural organs such as muscles involves, moreover, the chemical reaction of ATP hydrolysis simultaneous with sensing processes that provide living creatures with a perfect consciousness of both the characteristics of their mechanical movements and their interactions with their environment: they are intelligent machines. Most of our technological developments were inspired by biological functions and organs. One of our aims is to overcome the efﬁciency of the biological Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

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12 Electrochemomechanical Devices: Artiﬁcial Muscles

organs. Technological products, by contrast with biological organs, are constituted by dry materials that keep a constant composition under actuation. Toward the end of the 1970s, new artiﬁcial organic materials, conducting polymers (CPs), which are discussed in the next section, were discovered [2–4]. These materials have some characteristic physical properties similar to those of commodity polymers. At the same time, like metals and inorganic semiconductors, they can be oxidized and reduced. These reactions result in a complex material, polymer–ion–solvent. The composition, properties, and functions of this complex material, as we try to show here, are much more similar to those of biological material than to those of any other synthetic material. They can be considered as the basic model for biological reactions involving reactive biopolymers (Table 12.1).

12.2 Conducting Polymers as Reactive Materials: Electrochemical Reactions

All the families of CPs described below are suitable for oxidation (or reduction) processes, transforming the neutral material into oxidized (or reduced) materials. These basic electrochemical reactions originate changes in properties and are useful in applications such as artiﬁcial muscles. 12.2.1 Oxidation

During electrochemical oxidation (or reduction), positive (or negative) charges are stored along the polymeric chains and balancing counterions are forced to penetrate from the solution [5–9]. The transformation from a neutral material to an oxidized material can occur through two different processes.

Table 12.1

Electrochemical properties related with physical properties and biological organs.

Electrochemical property

Physical property

Mimicked organ

Electrochemomechanical (shrinking) Electrochromic (color A) Electroactivity (discharge) Electroporosity (compacted)

Volume Color Stored charge Porosity

Artiﬁcial muscles (swelling) Skin mimicking (color B) Electric organ (charged) Smart membrane (porous)

Transducter Dosage

Nervous interface (free ion) Glande

Transduction Electron/ion (free electron) Electron/chemical

12.2 Conducting Polymers as Reactive Materials: Electrochemical Reactions

12.2.1.1 Prevailing Anion Interchange During oxidation, stimulated conformational movements generate free volume and anions penetrate from the solution to balance the positive charges along the chains [10–12]. The high concentration of charges inside the oxidized material induces the entrance of water, which is required to maintain the osmotic pressure, from the solution. The volume of the material increases during oxidation. Reverse processes and volume decrease are observed during reduction:

pPy0 s + n A− aq + m (H2 O) ←→ pPyn+ s A− n (H2 O)m gel + n e− metal

Neutral chains

Oxidized chains

(12.1)

where the subscripts s and aq stand for solid and aqueous solution, respectively; pPy represents the polypyrrole (or any other CP) chains; and A− represents the anions required for charge balancing. Changes in volume and a transition, promoted by the reaction, from a packed solid state to a gel are illustrated in Figure 12.1. 12.2.1.2 Prevailing Cation Interchange The macroanions remain inside the material, whatever the oxidation state, because of their dimensions relative to the interchain distances in the network or by strong intermolecular Van der Waals interactions [14–19]. Cations penetrate during the reduction process to compensate the macroions. The material shrinks during oxidation and swells during reduction (Figure 12.2).

pPy0

MA−

n

C+

n s

Red/Oxid

←→

Neutral chains

pPyn+

MA−

n s

+ n C+ aq + n e− metal

Oxidized chains

(12.2)

where MA− represents any macroscopic anion (organic, polymeric, or inorganic) trapped inside the CP during polymerization, pPy represents the polypyrrole (or any other CP) chains, and C+ represents a cation; other subscripts are as previously deﬁned. Reactions 12.1 and 12.2 produce positive charges on the polymeric chains, and the process, from a physical point of view, is named p doping. +

Polymeric chains

+

−

Oxidation n− +

Reduction Solvent molecules

−

+ +

− +

−

−

− +

Anions

+

− +

−

−

− +

Figure 12.1 Schematic representation of the reversible volume change associated with the electrochemical reactions of polypyrrole in electrolytes including a small anion. (Modiﬁed from [13] with permission.)

+

−

+

+ ne−

− +

Polarons and bipolarons

243

12 Electrochemomechanical Devices: Artiﬁcial Muscles

+

−

−

+

+

+

−

+

−

+

+

−

−

+

−

Full reduction under diffusional control

+

−

+

+

−

−

−

+

−

244

+

−

Partial oxidation under diffusional control

− + −

Full oxidation + compaction under conformational relaxation control

Partial reduction under conformational relaxation control

+ +

+

Solvated cation

−

−

Solvated anion

+

−

Immobile macroanion or polyelectrolite

+

Positive charges fixed on polymeric chains

+ +

−

−

−

−

+ −

−

+

+

Solvent molecules Polymeric chains

Figure 12.2 Schematic representation of the reversible variation of volume of a polymeric blend constituted by a conductive polymer and a polyelectrolyte, or an organic macroanion. Changes in free volume are mainly due to exchange of cations and solvent molecules. (Reproduced from [19] with permission.)

12.2.2 Reduction of Neutral Chains

Moreover, in those oxidation/reduction processes, some CPs have electronic afﬁnity high enough to allow transitions from the neutral state to a reduced state [20, 21], storing negative charges (n doping from a physical point of view) on the chains at high cathodic potential (very stable solvents and salts are required as electrolytes for this reaction):

pPy0 s + n C+ aq + n e− metal ←→ pPyn− s C+ n (H2 O)m gel Neutral chains

Reduced chains

(12.3)

12.2.3 Complex Actual Ionic Interchanges and Polymeric Structure

The above-described reactions are simpliﬁed expressions and an initial approach to real processes. Any ﬁlm of a CP acts as a polymeric membrane. As for any other membrane (biological or one that is formed by commodity polymers) inside a liquid electrolyte, a physical equilibrium is stated between the ﬁlm and the electrolyte: solvent, anions, and cations getting distributed between the polymer and the electrolyte. So, during oxidation or reduction process, there are two possibilities to

12.2 Conducting Polymers as Reactive Materials: Electrochemical Reactions

keep the charge balance: ingress of anions from the solution or expulsion of cations from the polymer. A mixed interchange of anions, cations, and solvent is present during most of the electrochemical reactions [22–24]. Nevertheless, as indicated above, for most polymers, one of the two ionic interchanges prevails, carrying over 90% of the balancing charge [25, 26]. Because of this, most of the literature assume that only the biggest ionic interchange takes place; meanwhile, experimental methodologies such as photothermal deﬂection spectroscopy (PDS) [27–29], probe beam deﬂection (PBD) [30–33], and quartz crystal microbalance (QCM) [34–38] can be improved upon, or a new one discovered, to quantify unambiguously both absolute and relative amounts of ions and solvent interchanged during electrochemical processes. 12.2.4 Giant Nonstoichiometry

Electrochemical oxidation or reduction of an ideal chain of a CP occurs through n consecutive steps. Considering a ideal linear chain of CP constituted by m monomeric units, the reaction goes on through n consecutive oxidation (energetic conformational states, with increasing conjugated planar segments) steps (m > n): 1.CP + (A− )solv ←→ (CP+ )A− + e− − 2.(CP+ )A− + (A− )solv ←→ (CP2+ )A− 2 +e 3+ − − − 3.(CP2+ )A− 2 + (A )solv ←→ (CP )A3 + e − 4+ − − 4.(CP3+ )A− 3 + (A )solv ←→ (CP )A4 + e − 5+ − − 5.(CP4+ )A− 4 + (A )solv ←→ (CP )A5 + e

−−−−−−−−−−−−−−−−−−−−− − n+ − − n.(CP(n−1)+ )A− n−1 + (A )solv ←→ (CP )An + e

(12.4)

The oxidized chain passes through n consecutive ‘‘stoichiometric’’ states. In accordance with the H¨uckel model, the longer the chain, the lower is the ﬁrst ionization potential [39]. This means that under the same oxidation conditions, potential chains having a different length will have a different number of positive charges per monomeric unit and so a different stoichiometry. As a result, at a constant oxidation potential, the ﬁlm is constituted by chains of different lengths having different stoichiometries. Under these conditions, it can be stated that the oxidized polymer is, on average, a nonstoichiometric material. The average counterion composition, understood as the polymer/counterion ratio, inside the ﬁlm increases continuously during oxidation by ﬂow of a constant anodic current or by anodic potential sweep, giving, as average, a nonstoichiometric material [40, 41]. This effect has been widely observed [42–62] and was found to be dependent on the counterion molecular weight [63–73].

245

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12 Electrochemomechanical Devices: Artiﬁcial Muscles

12.3 Electrochemical Properties: Multifunctionality and Biomimetism

Owing to their unique nonstoichiometric nature, any of those properties of CPs that are a function of their composition, for example, volume, color, stored charge, stored chemicals (counterions), porosity, and conductivity, will also be tuned, under electrochemical control, along a wide range of the concomitant property [5–7, 24, 74]. During the electrochemical reaction, the material composition includes polymer, ions, and water. This composition mimics that of the living cells. The properties involved are also related to those that change in living functional organs: muscles, mimetic skins, electric organs, glands, or membranes. These are biomimetic functions [1, 75–77]. Properties and devices from reactive and wet materials include the following: electrochemical kinetics, polymer science, mechanics, optics, and so on. Theoretical models integrating all these parts of science are still not available, constituting a great challenge for future research. The central point of this challenge will be to arrive at the theoretical description of shifting physical properties of the polymer during electrochemical reactions. The reverse variation of the magnitudes of those variables under control of the reverse electrochemical reaction envisages the development of new biomimetic (electrochemical) devices and products such as artiﬁcial muscles, electrochromic windows [78, 79], polymeric batteries [80, 81] and/or supercapacitors [82–84], smart drug delivery devices [85–91] and nervous interfaces [92–96], or smart membranes [57, 97–104]. In this chapter, we focus on artiﬁcial muscles understood as electrochemomechanical actuators. 12.3.1 Electrochemomechanical Properties and Artiﬁcial Muscles

As stated above, the ﬂow of an anodic current starts breaking with reorganization of the double bonds along the chain (Figure 12.1) stimulating conformational movements with the formation of radical cations and dications, generation of free volume, entrance of balancing counterions, and water from the solution. As a result, a macroscopic swelling process is observed. During reduction, reverse processes and macroscopic shrinking are observed. These changes in volume that are linked to the electrochemical reaction, which means that we expect a faradic behavior, were named electrochemomechanical properties [74]. This chemical property is different in origin from the asymmetric dimensional variations originated by electric ﬁelds on nonconducting polymers: electromechanical properties (piezoelectric, electrostrictive, electrostatic, electroosmotic, ferroelectric, etc.) [105, 106]. The resulting devices are also named artiﬁcial muscles in literature, and the nonconducting polymers and elastomers are named electroactive polymers (EAPs) [107–109]. These are physical devices without the presence of any chemical reaction. The above-described consecutive events that occur during the oxidation of a CP are similar to those occurring during the actuation of a muscle: electric pulse (ionic)

12.3 Electrochemical Properties: Multifunctionality and Biomimetism

arriving from the brain, ionic interchange (liberation of calcium ions inside the sarcomere), water interchange, chemical reaction (ATP hydrolysis), and stimulated conformational movements on the actin (biopolymer) heads. On the basis of these similarities, CPs were envisaged as suitable materials to develop devices able to produce mechanical energy that can be named, by analogy, artiﬁcial muscles [110–113]. 12.3.2 Basic Molecular Motor

The n consecutive conformational states of a chain (Reaction 12.4) involving the evolution of covalent transformations and noncovalent interactions are useless as molecular motors when the chain is in a solvent and is submitted to thermal ﬂuctuations (free rotation and molecular collisions) (Figure 12.3a). If we imagine one of the ends of the chain to be attached to a metallic electrode, both electrochemical and thermal energy from the surroundings can be transferred to an electrical molecular machine, resulting in one-dimensional movement toward the solution bulk (Figure 12.3b). This physical attachment could be considered equivalent to a Maxwell demon for a molecular motor [114–119]. The strong intramolecular interactions in the neutral state originate a coil compact structure of the chain [7]. On oxidation, consecutive electrons (from 1 to n) are extracted from the chain, positive charges are generated, and the above-described processes now progressively result in the transformation of the coil-like structure to a rodlike structure. (a) +

(b) Metal

+

− +

Solution

−

−

+ −

+

−

+

Oxidation

Reduction

5e− −

− −

5e−

5e−

− −

Figure 12.3 Oxidation–reduction-induced conformational movements on a conducting polymer chain; (a) in solution with free rotation and (b) with one of the chain ends attached to a metal electrode. (Reproduced from [7] with permission.)

ox red

5e−

247

248

12 Electrochemomechanical Devices: Artiﬁcial Muscles

This is the basic molecular motor for a CP, which works under electrochemical stimulation of the conformational movements added to the electrochemical control of the intramolecular interactions. This basic molecular motor includes electric current, chemical reactions, ions and water interchanges between the polymer and the solution, chemical reactions, stimulation of the conformational movements along a polymeric chain, and changes in the inter- and intramolecular interactions. These processes occurring in soft and wet materials mimic, at molecular level, the consecutive events involved in the actuation of a natural anisotropic muscle [120–123].

12.4 Macroscopic Dimensional Changes and Mechanical Properties

The longitudinal electrochemomechanical properties of free standing ﬁlms obtained by different methods are studied by using different kinds of test frame machines. These machines include special electrochemical cells that allow any in situ characterization of the mechanical response of the ﬁlms under current ﬂow inside the electrolyte [124–126]. The test can be performed by means of any electrochemical methodology (voltammetric [127–129], chronopotentiometric, chronocoulometric [130]). Length variation under a constant stress can be followed under cyclic oxidation/reduction processes, as well as stress variations under a constant length. In this way, characterization of the mechanical energy produced under different chemical, electrical, or physical conditions can be performed. In situ atomic force microscopy (AFM) [131–133], ellipsometry [134], optical microscopy [135, 136], conductivity [113], and in situ mass [137, 138] measurements also allow to follow the evolution of a ﬁlm thickness during oxidation/reduction reactions. At present, the claimed evolution of the material length during the ﬁrst voltammetric cycle has been improved from a few percent to more than 30%, that is, this surpasses the natural muscles by almost one order of magnitude. Nevertheless, a faster decrease is observed on the initial consecutive cycles up to 1–3% of length variation, this value being the most usual and stable. The Young’s modulus of these reactive and wet CPs also overcomes that of the natural muscles. The present state of the attained mechanical characteristics and efﬁciencies has been reviewed by different authors [139–141]. The movement obtained by macroscopic devices has also been characterized from an electrochemical point of view, as discussed below, in order to determine the electrochemical origin of the movement [41].

12.5 Anisotropy Obtained from Isotropic Changes: Macroscopic Devices

To generate macroscopic machines from the isotropic conformational movements originated inside the ﬁlm, we must induce anisotropy, for instance, by selecting conformational parallel movements to the ﬁlm length to produce a macroscopic

12.5 Anisotropy Obtained from Isotropic Changes: Macroscopic Devices

work. Such transducers were successfully built by 1992 with the construction of bilayers constituted by a tape adhered to the ﬁlm of a CP [110–113]. The CP ﬁlm was electrogenerated on a metallic electrode (Figure 12.4a). Once rinsed and dried, a tape of a nonconducting polymer was attached to the CP ﬁlm. The bilayer was peeled off from the electrode and then the all-organic bilayer was used as a new electrode by connecting the ﬁlm of CP to the working electrode (WE) of a potentiostat, in an electrolyte. To allow the current ﬂow through the ﬁlm of CP, a metallic counterelectrode (CE) immersed in the electrolyte is required. Bilayers are transducers from the isotropic conformational movements of the chains to anisotropic stress gradients across the interface (Figure 12.4b), giving macroscopic angular movements greater than 360◦ (Figure 12.4c). Similar results were obtained by different interfaces: CP–tape [111, 142–144], CP–metal [145–147], CP–solid-state electrolyte [148], or CP–CP [149, 150]. The described arrangements allow the electrochemical stimulation of the conformational energetic states from every individual chain inside the ﬁlm, fulﬁlling the fat ﬁnger paradox established by Prof. Smalley [152]. At the same time, a macroscopic response from the total volume of the entangled chains in the ﬁlm is obtained: the sticky ﬁnger paradox. 12.5.1 Electrochemical Transducer

The above-described transducer from isotropic molecular motors to anisotropic stress variations and macroscopic devices performing mechanical work are based on an electrochemical reaction. The rate of this reaction must follow the Buttler–Volmer expression linking the reaction rate (the current density, i) and the electric potential, overpotential (η), the CP is submitted to α·F·η (12.5) i = i0 · exp R·T where α is the transfer coefﬁcient (0 ≤ α ≤ 1) and i0 is the exchange current n density when η = 0 i = i0 = kr · cr · A− · exp(α · F · E0 /R · T) and includes the reduction kinetic constant (kr ), the concentration of active centers in reduced polymer ﬁlm (cr ), and the concentration of counterions, A− , in solution, and η is the applied overpotential (η = E − E0 , where E is the applied potential and E0 is the equilibrium potential) required to inject the electrons in the polymeric chains. The overpotential can be used as a unique tool for the CP-based actuation: a shift of the overpotential for a few millivolts is expected to promote a massive extraction of electrons from the material (anodic shift) or a massive injection of electrons (cathodic shift) with transformation of double bonds, conformational movements, interchange of ions and water molecules, and so on. As a consequence of the electrochemical nature of the device, the ﬂow of a constant current (i) produces an overpotential (η), which is expected to depend, through Reaction 12.1, on electrolyte concentrations [A–], working temperatures (α · F · E0 /R · T), and the polymeric nature (cr ). These predicted dependencies of

249

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12 Electrochemomechanical Devices: Artiﬁcial Muscles

(1)

(2)

(3)

(a) Ppy

Flexible polymer

Solution (H2O)n (ClO−4)

Stress gradient through the bilayer

(H2O)n (ClO−4)

Contraction Active Passive

Expansion Active Passive Electric contact

Air

Air

Solution

(b)

(1)

(2)

(c)

(4)

(3)

(5)

12.5 Anisotropy Obtained from Isotropic Changes: Macroscopic Devices

the potential with physical and chemical variables denote sensing properties of the materials and unique sensing properties of the electrochemomechanical actuators. The use of a reference electrode (RE) allows a perfect control of the CP potential (potentiostatic experiment). The potentiostat provides the ﬂow of the required charge between the WE and the CE to produce the electrochemical reaction (Equation 12.4) of suitable extension and direction to attain the ordered potential according to the Nernst expression:

pPyn+ s A− n (H2 O)m gel R·T

ln (12.6) E = E0 + n·F pPy [A− ]n Equation 12.6 indicates that different oxidation degrees of CPs will have different [pPyn+ ]/[pPy] relationships, responding with a different potential to the ambient. Moreover, the same polymer balanced with different counterions will also respond in different ways to the same physical or chemical variables. This is a fast method of obtaining a large array of sensors from the same basic polymer: by different oxidation with the same counterion or by oxidation (doping) with different counterions. 12.5.2 Efﬁciency

At this point, we must distinguish between the efﬁciency of the device and the overall efﬁciency of the system. The transduction from isotropic three-dimensional variations to anisotropic one-dimensional stress gradient supposes that at least two-thirds of the involved electrical energy is useless for producing mechanical energy. Until we learn how to produce artiﬁcial isotropic devices, efﬁciencies greater than 33% on the transformation of electrical to mechanical energy using bilayer devices will remain a difﬁcult task. Nevertheless, a good fraction of that stored energy (two-thirds of the ﬂowing charge) could be recovered during movement in the opposite direction. In any case, despite the explosion of new organic molecular motors [153, 154] developed during the last 10 years, this is the best macroscopic assembling available today. Up to now, the attained efﬁciencies for the direct transformation of electrical energy to mechanical energy range between 0.01 and 10% [74, 155–158]. However, the system efﬁciency is lower owing to the electrochemical reactions produced at metallic CE/electrolyte interface. Using an aqueous electrolyte, the

Figure 12.4 (a) (1) Stainless steel electrode partially coated with an electrogenerated polypyrrole ﬁlm, (2) adhesion of the nonconducting polymeric ﬁlm on one side of the electrode, (3) the CP/tape bilayer is removed from the surface. (b) Polypyrrole/tape bilayer-induced stress gradients by electrochemical reactions (c) Angular movement described by the free end of a bilayer muscle (CP/tape) immersed in a 0.1 M aqueous electrolyte under a current ﬂow of 15 mA (1, 2, and 3), or of −15 mA (4 and 5). (Reproduced from [9, 74, 151] with permission.)

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12 Electrochemomechanical Devices: Artiﬁcial Muscles

most common of these reactions at a neutral pH are 2(H2 O) + 2e− ←→ H2 + 2 OH− (When the CE acts as a cathode) 2(H2 O) ←→ O2 + 4H+ + 4e−

(When the CE acts as a anode)

(12.7) (12.8)

These reactions require a higher overpotential, which means that most of the applied electrical energy is consumed by reactions at the CE, although, these reactions produce changes of the solution pH (see OH− generation, Reaction 12.7, or H+ generation, Reaction 12.8) inducing the chemical degradation of the CP. This crucial problem was solved by producing triple layers, CP/tape/CP, with each ﬁlm of CP connected to one of the potentiostat/galvanostat outputs: the WE or the CE (Figure 12.5) [112]. Now, the same current ﬂows through the two polymeric ﬁlms: the anode swells (preferential anionic interchange) and pushes the device; the cathode shrinks and trails the device. Two electrochemomechanical transducers (anode and cathode) are included to work cooperatively in the same device. Improved efﬁciencies, related to these devices are expected with the use of a metallic CE. Moreover, both polymeric reactions occur at lower overpotentials than the respective oxygen or hydrogen release, contributing again to a higher efﬁciency. 12.5.3 Bending Structures

The generation and control of transversal stress gradients producing macroscopic bending movements can be attained from different structures: asymmetrical monolayers, bilayers, and triple layers. 12.5.3.1 Asymmetrical Monolayers We can design monolayers of the same CP, but having an internal asymmetry that are able to produce asymmetric swelling or shrinking processes across the ﬁlm under the same electrochemical process (oxidation or reduction) [159–165]: one-half of the ﬁlm has a prevalent anionic interchange, while the second half experiences a prevalent cationic interchange [149, 150, 166]. Some other possibilities are being explored to produce asymmetric monolayers: by physical means by growing the CP

WE

Figure 12.5 Sequence followed for the construction of a triple-layer muscle. (Reproduced from [9] with permission.)

CE/RE

12.5 Anisotropy Obtained from Isotropic Changes: Macroscopic Devices

on adsorbed and porous materials [167], or by electrochemical means by generating a ﬁlm of CP with a counterion concentration [159, 164, 168, 169], conductivity [163, 170], or morphology [171–173] gradients, by cross-linking network, or by generating a bilayer of the CP with a macroanion (which shrinks by oxidation) and then of the same CP with a small anion (which swells by oxidation) [149, 150], or even by placing a metal sheet between both ﬁlms [166]. These asymmetric ﬁlms require the presence of a metallic CE to allow the current ﬂow, when there is subsequent efﬁciency drop on degradation processes.

12.5.3.2 Bilayers A ﬁlm of a CP electrogenerated on a metallic electrode is stuck to a polymeric tape. (Figure 12.4a) The bilayer CP/tape is removed from the electrode. The new bilayer is used as a new WE in an electrolyte. (Figure 12.4c) The mechanical stress gradient generated across the bilayer interface (Figure 12.4b) by swelling/shrinking processes induced by the electrochemical Reactions 12.1, 12.2, or 12.3, depending on the family of the studied CP ﬁlm, originates the bending movement of the free end of the bilayer [110–113]. Films of CP having a prevailing interchange of anions swell by oxidation, pushing the bilayer free end and staying at the convex side of the bended device. CPs having cation-prevailing interchange suffer shrinking processes during oxidation, trailing the device and staying at the concave side of the bended device. To allow the current ﬂow, a metallic CE is required. At present, the construction of bilayers, for example, plastic/CP [149, 174], paper/CP, or a thin ﬁlm of any ﬂexible metal-coated material (i.e., by sputtering)/CP [175] is very attractive for most of the research groups involved in this ﬁeld.

12.5.3.3 Triple Layers By using a two-side polymeric tape and sticking one ﬁlm of CP on either side, short circuits are avoided and a triple layer is produced. One of the CP ﬁlms is connected to the WE output from a potentiostat. The second CP ﬁlm is connected to the CE output and short-circuited with the RE. By immersion of the triple layer in an electrolyte we complete the structure of the electrochemical cell allowing the current to ﬂow through them. Now the same current ﬂows through the two polymeric ﬁlms [40, 176–178]. (Figure 12.6) The evolution of the potential difference between the ﬁlms is the muscle potential. The three-layer muscles can work outside the liquid electrolytic media using an ionic conducting tape membrane. This membrane can be obtained using a solution containing the solvent, a polymer, and the salt, and by adding a UV cross-linker product, the membrane is then obtained by casting, solvent evaporation, and UV irradiation [179–181] or by the formation of interpenetrated networks [182–185] by chemical polymerization of the CP on the external part of the membrane and then cutting the edges.

253

254

12 Electrochemomechanical Devices: Artiﬁcial Muscles Reduction Contraction

Oxidation

Oxidation

(Counterions) Expansion

Expansion

Reduction

(Counterions)

(Counterions)

(Counterions)

Electric contact

Electric contact

Contraction

Air

Conducting polymer Solid electrolyte

Figure 12.6 Scheme of ionic interchange, induced stress gradients, and generated angular movements during current ﬂow of a triple-layer device working on air. (Reproduced from [178] with permission.)

12.5.4 Structures Giving Lineal Movements 12.5.4.1 Fibers and Films Different methodologies are being used to obtain ﬁbers of CPs [186, 187]. Bundles of ﬁlms or ﬁbers are checked to produce vertical displacements of weights [188]. (Figure 12.7) Different models have been proposed to describe mechanical behavior [124, 189, 190] or electrochemomechanical deformations [125, 191, 192] of these devices. Individual ﬁbers or bundles are used as WEs, therefore a metal CE is required to allow the current ﬂow generating similar problems to those described above for bending asymmetric ﬁlms. Origami structural form ﬁlms also provide good linear movements [193]. 12.5.4.2 Tubes and Films with Metal Support Another approach consists of the electropolymerization of a CP on springs and helical metallic wires until a tube is generated, or a CP on zigzag metal wires to generate ﬁlms [126, 194–197]. These efforts are based on the named conducting/ nonconducting transition during the reduction of very thin ﬁlms of CPs. The

12.5 Anisotropy Obtained from Isotropic Changes: Macroscopic Devices

1 piece

10 pieces

(a)

(b)

Figure 12.7 (a) Schematic diagrams of some bundles of pPy–metal coil composites. (b) Photographs of a pPy–W coil composite actuator and a bundle of its 10-piece actuator. (Reproduced from [194] with permission.)

supporting metal wire should guarantee a uniform potential and current distribution, greater strain, and length variation rates under electrochemical reactions. However, most of the described experimental conditions involve devices working under partial oxidation of the material, outside deep reduction states, having a high enough electronic conductivity as the electrochemically stimulated conformational relaxation (ESCR) model predicts [57, 198]. This gives a uniform actuation, not requiring embedded metallic wires to guarantee a uniform conductivity along the device and a uniform bending along the device, as proved by digital image analysis [199, 200]. These structures also need a metallic CE to allow the current ﬂow. 12.5.5 Combination of Bending Structures

The present state of the technology provides an unsolved problem related to structural assembling of elements: how to design a basic element constituted by bilayers, triple layers, and so on, that repeated n times can originate three-dimensional structures having any shape (lineal, cylindrical, conical, thick sheet, etc.) and any volume. The expected mechanical properties and work production should be n times that of the basic element (Figure 12.8). Different combinations of triple layers [201, 202] and bilayers [203] are being constructed and checked: the key point is to keep simultaneous sensing and actuating capabilities despite the complexity of the device. 12.5.6 Microdevices and Microtools

Any material, such as metals, that can be electrodeposited or electrogenerated can be used by means of microtechnological processes to produce micrometrical

255

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12 Electrochemomechanical Devices: Artiﬁcial Muscles

CE WE

(1) Oxidation of WE Reduction of CE (3) Reduction of WE (4) (5) (6)

(a)

Oxidation of CE

(2)

(b) Figure 12.8 (a) Basic element constituted by four bilayers including WE, CE, and RE suitable for following the device potential at any instant. (b) Basic element constituted by two triple layers moving by opposition.

(c) (c) Basic three-dimensional electrochemomechanical unit constituted by 36 basic elements. (Reproduced from [9, 203] with permission.)

structures. The electrochemical synthesis of ﬁlms of CPs, and the electrochemical actuation, are suitable for the construction and manipulation of elegant and imaginative microdevices [204–207] and microtools constituted by bilayers [145–147, 205, 206, 208–211] or triple layers [212].

12.6 Electrochemical Characterization

257

12.6 Electrochemical Characterization

All the artiﬁcial muscle structures described above are considered to be electrochemical devices working under faradic conditions. The volume changes between reduced and oxidized polymers are under control of the consumed charge. This charge controls the number of counterions interchanged with the solution. Different experimental currents ﬂowing through the device produce proportional movement rates. Figure 12.9a shows the expected linear relationship between current and rate of angular movement for a triple layer; furthermore, the angle described by a triple layer (Figure 12.9b) or by any other artiﬁcial muscle [5, 6, 213, 214] can be controlled. This means that, whatever the testing current, a constant charge is consumed to describe the same angle from the same muscle, as shown in Figure 12.9c. We can obtain the charge required to describe an angular movement of 1◦ (a in mC/degree) from the slope of Figure 12.9c. From this value, we can calculate how much charge is required to attain any new position: Q (mC) = a mC/degree · angle degree (12.9)

0.2 0.15 0.1 0.05 0

(a)

7

14

21

28

Current (mA) 8

35

Electric charge (mC)

Movement rate (rad s−1)

0.25

Electric charge (mC)

The result is an electrochemo-positioning device, requiring a constant charge to describe a deﬁned angle, that is, 90◦ , the time consumed for the movement must be under control of the charge ﬂowing through the device per unit of time: the current [74, 213, 214]. The faradic nature of artiﬁcial muscles based on CPs is underlined by Figure 12.10. There different artiﬁcial muscles, having different surface area, or constructed with pPy ﬁlms having different thicknesses, produce the same angular movement rate under ﬂow of the same current per unit of CP weight. The faradic nature of the movement allows a perfect control of both the movement rate (by the current) and the angular position (by the charge). This means that we have a machine capable of transforming electrical energy into mechanical energy using molecular motors as mechanical components.

360 300 240 180 120 7

(b) 10

Figure 12.9 (a) Linear relationship between the applied current and the angular rate determined from the time required to describe an angular movement of 90◦ in 1 M LiClO4 aqueous solution under seven

14 21 28 35 Current (mA) 15

20

600 500 400 300 200 100 0

0

50

(c) 25

100

150

Angle (°) 30

different currents. Electric charge consumed by (b) the triple-layer muscle to move through 90◦ and (c) through different angles (30, 45, 60, 90, 120, 135, and 180◦ ). (Reproduced from [214] with permission.)

35

200

12 Electrochemomechanical Devices: Artiﬁcial Muscles

Current (mA mg−1)

258

8 7 6 5 4 3 2 1 0

0

0.1

0.2

0.3

0.4

0.5

0.6

Angular rate (rad s−1) Figure 12.10 Angular rate measured through a movement of 90◦ using triple-layer muscle of different weights of polypyrrole ﬁlm (8.3, 7.8, 7.4, 6, 5.5, 5.1, 5, 4, 3.7, 3.5, 3, 2.3, and 2 mg) and checked in 1 M LiClO4 aqueous solution under different currents (10, 15, 20, 25, and 30 mA). (Reproduced from [214] with permission.)

12.7 Sensing Capabilities of Artiﬁcial Muscles

By stopping consecutive reactions (Equation 12.4) at any intermediate point, a chemical equilibrium is stated and any physical or chemical variable acting on the chemical equilibrium will change the Nerst electrodic potential. The stopped muscle should act as any traditional potentiometric sensors do. Any physical or chemical property inﬂuencing the conformational energetic states of the polymer chains will change any macroscopic property of the polymer: electrodic potential, ﬁlm color, ﬁlm resistance, and so on. In conclusion, these materials should be suitable for the development of different kinds of sensors: gas [215–217], pH [218, 219], ion-selective [220], humidity [221], solvent vapors [222–224], or biosensor [225–227] devices, working as amperometric [221] or potentiometric [228] sensors. Very interesting applications can be electronic tongues [229–232] or artiﬁcial noses [216, 231]. These polymeric sensors are available for both, CPs and commodity polymers. As conjugated CPs are expected to react very fast to the change of any external physical or chemical variable and the fast equilibrium stated between conjugated electronic levels in the polymer and electronic levels in the connecting metal, the change of the muscle potential is expected to be detected by the potentiostat simultaneously with the change of the variable. These fast events should allow developing actuators capable of sensing the surroundings while working, during actuation, and outside chemical equilibrium. Sensing muscles, batteries, windows, smart membranes, intelligent drugs, and chemical delivery devices, and so on, can be envisaged. Thus, the ﬂow of a constant anodic current through a bilayer muscle under constant ambient conditions gives a progressive increase of the electrodic potential

12.8 Tactile Sensitivity

(Figure 12.11). This is the potential of the CP from the bilayer related to the RE (the muscle potential) [151, 233–237]. Figure 12.12a–d show similar evolutions of the muscle potential (CP of the WE versus CP of the CE) from a triple layer for different values of the surrounding variables: electrolyte concentration, temperature, different weights attached to the bottom of the muscle, or different current ﬂowing through the device. As expected from the inﬂuence of those variables on the equilibrium states of reactions (12.4), rising electrolyte concentrations and working temperatures promote a shift toward lower muscle potentials: the inﬂuence of a variable is similar under chemical equilibrium and under electronic equilibrium. Rising weights adhered to the muscle require higher energies to keep a constant movement rate (under constant current) giving increasing muscle potential. This is equivalent to saying that increasing resistances for the volumetric changes are present to trail rising weights. The ﬂow of rising currents, assuming constant resistances (electrolyte, anode reaction, and cathode reaction), produces higher potentials and requires lower times of current ﬂow because the same charge is consumed. Figure 12.13a–d shows the linear evolution of the electrical energy consumed by the artiﬁcial muscle as a function of the studied experimental variables. Rising driving currents, or increasing trailed weights, produce higher overpotentials consuming more electrical energy. Rising electrolyte concentrations, or temperatures, require lower overpotentials consuming less electrical energy during the movement. Those results underline the simultaneous sensing capabilities of the device during actuation, under reactive conditions and outside the chemical equilibrium. Both signals, the actuating current and the muscle potential response, are included in the same two connecting wires (Figure 12.11) opening a new paradigm for robotic devices.

12.8 Tactile Sensitivity

The ability of this type of device to sense mechanical variables opens the way for the exploration of one of the most exciting possibilities that scientists are trying to reproduce – the tactile sense. Considering that we obtain increasing muscle potentials by trailing increasing weights, if a muscle moves freely and meets an obstacle with a mechanical resistance lower than the mechanical energy produced by the device, the device must touch, push, and shift the obstacle (Figure 12.14). When the muscle touches the obstacle, it feels a mechanical resistance and produces extra energy by raising the muscle potential [41, 238, 239]. A potential step is observed on the muscle potential evolution (Figure 12.15) as expected from the hypothesis of a fast equilibrium stated between electronic levels in the CP and in the metal. Moreover, this ﬁgure indicates that when we repeat the experiment by using obstacles of increasing weights, increasing potential steps, proportional to the obstacle weight, are produced at the contact time: this is a tactile sensor.

259

12 Electrochemomechanical Devices: Artiﬁcial Muscles

WE CE RE

WE

CE RE

Reference electrode Ag/AgCl Cathode

Anode

Anode

Non conducting film

Cathode

Non conducting film

pPy films

pPy films

+

+

Li ClO4−

Li ClO4−

(a)

(b) 2.0

1.5

E (V)

260

1.0

0.5

0.0 −5 (c)

0

5

10

15 20 t (s)

Figure 12.11 (a) Electrochemical equipment, electrical contacts, and scheme of the triple-layer muscle (CP/two-sided tape–CP). (b) Distribution required for potentiostatic control, or during galvanostatic control, of

25

30

35

the muscle in order to follow the potential of the CP acting as WE. (c) Muscle potential evolution during the experiment. (Reproduced from [9] with permission.)

(a)

E (V)

2000 1750 1500 1250 1000 750 500 250 0

0

0.5

1

1.5

−5

0

0

4

5

10 t (s)

15

10 mA

25 mA 20 mA 15 mA

t (s)

20

5 mA

25

12 16 20 24 28

30 mA

8

0.25 M 0.5 M 1M 3M

0.1 M

0.05 M

(b)

(d)

0

0.5

1

1.5

2

2.5

1600 1400 1200 1000 800 600 400 200 0

0

−1 0

5

10

20 mA

1

2

15

4

20 t (s)

t (s)

3

25

5

30

6

35

180 times 170 times 150 times 120 times 50 times 0 time

7

5 °C 15 °C 25 °C 35 °C 45 °C

Figure 12.12 Chronopotentiograms obtained when a triple layer (2 × 1.5 cm2 , 12 mg of pPy) describes 90◦ (a) in aqueous solutions of LiClO4 (3, 1, 0.5, 0.25, 0.1, and 0.05 M) under a constant current of 10 mA, (b) in 0.1 M LiClO4 at different temperatures (5, 15, 25, 35, and 45 ◦ C), (c) under ﬂow of different currents (5, 10, 15, 20, 25, and 30 mA), and (d) trailing different attached steel weights (0.6, 1.4, 1.8, 2.04, and 2.16 g). (Reproduced from [237] with permission.)

(c)

E (mV)

E (mV) E (V)

2

8

12.8 Tactile Sensitivity 261

1.6

15

1.4

14

Ee (kJ kg−1)

log(Ee) (kJ kg−1)

12 Electrochemomechanical Devices: Artiﬁcial Muscles

1.2 1 0.8 −0.5

0

11

0

0.5

Ee (mJ mg−1)

Y = 59 + 6.5 X r = 0.99

250 200 150 5

10

15

20

25

10

30

35

40

80 70 60 50 40 30 20 10

0

(d)

i (mA)

20

30

40

50

60

T (°C)

(b)

log [LiClO4] (M) 300

(c)

12

9 −1

(a)

100

13

10

0.6 −1.5

Ee (mJ)

262

500

1000

1500

2000

2500

Load (mg)

Figure 12.13 Electrical energy consumed by the device in Figure 12.12 as a function of the different studied variables: (a) electrolyte concentration, (b) temperature, (c) current, and (d) trailed weight. (Reproduced from [9] with permission.)

(a)

(d)

(b)

(e)

(c)

(f)

Figure 12.14 (a) The triple-layer muscle initiates its movement under a constant current of 5 mA, in 1 M LiClO4 aqueous solution; 10 s later, (b) the muscle meets the obstacle weighing 6000 mg, pushing

and sliding it (c) and (d). (e) The angular movement allows the muscle to overcome the border of the obstacle. (f ) The free movement goes on until the current stops. (Reproduced from [238] with permission.)

12.9 Intelligent Devices

2.5 Potential (V)

e:4800 mg d:3600 mg c:2400 mg b:1200 mg a:nonobstacle

j:14400 mg i:9600 mg h:8400 mg g:7200 mg f:6000 mg

3

2 1.5 1 0.5 0 −0.5

0

10

20

30

40

50

60

70

Time (s) Figure 12.15 Chronopotentiograms obtained from a triple-layer macroscopic muscle containing two polypyrrole ﬁlms (2 cm × 1.5 cm × 13 µm), each weighing 6 mg, under ﬂow of 5 mA in 1 M LiClO4 . The muscle

moves freely, contacting an obstacle of different weight. The muscle was unable to push and slide an obstacle weighing 14 400 mg. (Reproduced from [238] with permission.)

When the mechanical resistance of the obstacle exceeds the mechanical energy produced by the device, the muscle is unable to shift the obstacle and the muscle potential steps to very high values at the moment of the contact. The system indicates when it touches an obstacle and how much mechanical resistance the obstacle opposes.

12.9 Intelligent Devices

The development of sensing and tactile devices allow envisaging the construction of intelligent devices: a current source connected to the electrochemomechanical artiﬁcial muscle, a computer with very simple software able to follow the potential evolution, detecting the potential jump, and quantifying the potential step. These results are immediately transformed to useful information: the touching moment, the weight of the obstacle, whether the machine can shift the obstacle or not and allowing, in the last case, taking decisions such as to switch off the current. Moreover, the sensing abilities of the device can provide simultaneous information about the ambient conditions: working temperature or electrolyte concentration. All this information can be obtained through the only two connecting wires carrying actuating and sensing signals. The described ensemble: sensing muscle, generator, and software constitutes a quite primitive conscious device.

263

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12 Electrochemomechanical Devices: Artiﬁcial Muscles

12.10 Muscles Working in Air

Despite the unprecedented simultaneous actuating and sensing properties of soft, wet, and nonstoichiometric materials and devices, human technology is nowadays based on dry materials and devices. Different attempts are being made in this direction in order to develop all-solid-state CPs actuators with polyaniline [240, 241], polypyrrole [242–248], or PEDOT [249]. The most common structure is CP/ionic conductive membrane/CP. Also encapsulation [243] should be convenient for devices of this kind. Some manufacturing technologies have been described in part 12.5.3.3.

12.11 Advantages, Limitations, and Challenges

The main advantage of reactive materials is the possibility to construct devices mimicking biological organs that are capable of sensing the physical and chemical conditions of the surroundings. Considering that basic electrochemical and polymeric principles can be applied to any new batteries, smart windows, smart membranes, nervous interfaces, or smart drug delivery devices made of CP, sensing and giving information about the surroundings and about conditions of work of the device are expected. For the assembling of artiﬁcial muscles, microscopic or macroscopic, three-dimensional tools or robots need to be constructed. On the other hand, any approach to the construction of molecular sarcomeric units will allow the muscle efﬁciency to step up by one order of magnitude. The present state of this emerging area presents some difﬁculties that need to be overcome. Most of the devices developed to produce linear movements require a metallic CE. The result is that at least one-half of the consumed electrical energy is wasted in the generation of chemical products that contribute to the CP degradation. When we try to assemble a number (n) of the muscles elements in order to obtain n times the mechanical energy, irregular electric ﬁeld distributions promote nonuniform movement. Failure on the electrical connections between the copper wires or any of the ﬁlms of CPs produces irregular movements. Mechanical rigidity is caused by combination of several basic elements, resulting in decrease in amplitude of the linear movement and a loss of some of the simultaneous sensing properties, the tactile sense being the subtlest, and the most evanescent. Actuation of this type of devices takes a few seconds. In order to get faster muscles, it is necessary to identify the rate-limiting step for the actuation: the electronic conductivity of the CP, the ionic conductivity in the electrolyte, the ionic diffusion inside the polymer ﬁlm, the ionic migration inside the ﬁlm, the conformational movements of the polymeric chains, or the chemical reaction. The low electronic conductivity (10−3 < σ < 104 S cm−1 ) along a thick ﬁlm (> 3 µm)

12.12 Artiﬁcial Muscles as Products

of CP could be overcome with intermediate metal wires or metal deposition. This results in extra stiffness and possible peeling and the rate is not increased; see Section 12.5.4.2. To overcome a possible low ionic conductivity of the electrolyte, ionic liquids [188, 246, 249–257] are being proposed as electrolytes. The results point to extended lifetimes probably related to a dry ambient condition, but without an increase in rates. The kinetics and mechanism for the simultaneous electropolymerization, degradation, and chemical polymerization of CPs has not been well clariﬁed. A good challenge for electrochemists should be a parallel kinetic study by Tafel slopes on metals, Tafel slopes on polymer coated electrodes, in situ microgravimetric study, microgravimetric study of the dry electrogenerated ﬁlms, following the productivity of the electropolymerization, following the charge stored per weight unit of the electrogenerated ﬁlm, and so on. The clariﬁcation of the polymerization mechanism should allow the production of tailored materials. A clariﬁcation of the oxidation–reduction mechanism for the CP ﬁlms should provide good predictive models for actuating and sensing properties. The packing conformational energy obtained from the oxidation of CPs includes structural information in kinetic magnitudes. Nowadays, models from chemical kinetics do not include any structural information about the conformational changes brought about by enzymes, nucleic acids, or reactive proteins. These structural aspects of the reaction kinetics appear as an important scientiﬁc challenge for the twenty-ﬁrst century, involving responses to questions related to life, health, and illness. Owing to the magnitude of the challenge, these responses can only be obtained by cooperative work involving chemists, physicist, engineers, biologists, and clinicians.

12.12 Artiﬁcial Muscles as Products

Despite difﬁculties, the present state of the technique is allowing the start up of new companies. Following 17 years of evolution, products and applications are being put into the market by companies around the world : EAMEX in Japan, Quantum Technology Pty. Ltd in Australia (development of Braille screens), Micromuscles AB in Sweden, Molecular Mechanisms in Massachusetts, Artiﬁcial Muscles Inc (AMI) in California, Danfos Polypower A/S in Denmark, EMPA in Switzerland, Environment Robots Incorporated in Virginia, and NanoSonic Inc. in Vancouver are some of the pioneers and new companies developing different medical tools or devices for technological applications. This considerable technological interest is stimulated by the fact that developing new polymeric actuators is a priority area from the defense advanced research projects (DARPA) (USA) and the european space agency (ESA) (EU). This implies a strong competition to obtain funds for developing new devices and to get new products at the expense, in most of the cases, of basic development in this emerging area of the electrochemomechanical devices. In any case, a new world of soft and reactive, sensing and tactile actuating machines are being developed and new ones,

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already envisaged, are expected to challenge the intellectual energy and engineering ability of scientists.

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273

Index

a acetonitrile (AN) solutions 27–47 actuators 1–12, 85 3-(Aminopropyl)trimethoxysilane (APTMS) 139 amperometric sensors 145–146, 179–181 analogs, in EIS 52 anodic aluminum oxide (AAO) 233 anodic sweep 33 anticorrosion 85–86 artiﬁcial lipid matrices, phthalocyanines in 113–115 – immobilization of MTSPc within QAS 113 – – composites application prepared by water dispersion of DDDMA bromide 114 – – in the form of MTSPc (DDDMA)4 and MTSPc(DMTA)4 salts 114 – – preliminary application of bromides of DDDMA 114 – – preliminary application of dioctadecyldimethylammonium 114 artiﬁcial muscles 241–266 – advantages 264–265 – anisotropy obtained from isotropic changes 248–256 – – macroscopic devices, 248–256, see also individual entry – basic molecular motor 247–248 – challenges 264–265 – conducting polymers as reactive materials 242–245 – – complex actual ionic interchanges and polymeric structure 244–245 – – electrochemical reactions 242–245 – – giant nonstoichiometry 245 – – neutral chains, reduction of 244

– – – –

– oxidation 242–243 – prevailing anion interchange 243 – prevailing cation interchange 243–244 electrochemical characterization 257–258 – electrochemical properties 246–248 – – biomimetism 246–248 – – multifunctionality 246–248 – intelligent devices 263 – limitations 264–265 – macroscopic dimensional changes and mechanical properties 248 – muscles working in air 264–266 – as products 265–266 – sensing capabilities of 258–259 – tactile sensitivity 259–263 ascorbic acid, MIP of 136–137 asymmetrical monolayers 252–253 azines, electropolymerized 77–87, 93–108, see also polyazines in electroanalysis – azur a 95 – brilliant cresyl blue 95 – electrochemical quartz crystal microbalance (EQCM) study 94 – electropolymerization 94–96 – electropolymerized azines – – as advanced electrocatalysts for NAD+ |NADH regeneration 99–100 – – as bioelectrocatalysis promotors 105–108 – – as NADH transducers 101–102 – from neutral and basic aqueous solutions 94 – methylene blue 95 – methylene green 95 – neutral red 94–95 – as new group of electroactive polymers 93–98

Electropolymerization: Concepts, Materials and Applications. Edited by Serge Cosnier and Arkady Karyakin Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32414-9

274

Index azines, electropolymerized 77–87, 93–108, see also polyazines in electroanalysis (contd.) – polyazine structure, hypothesis of 96–98 – – ‘head-to-tail’ bonding 97 – – ‘nitrogen-to-ring’ coupling 98 – – ‘ring-to-ring’ coupling 97 – thionine 95 – toluidine blue 95 azur A 95

b background electrolyte 28 batteries 84–85 bending structures, macroscopic devices 252–254 – asymmetrical monolayers 252–253 – bilayers 253 – triple layers 253 bienzyme electrode biosensor 196 bioelectrocatalysis 77–87 – by cellobiose dehydrogenase (CDH) bioelectrocatalysis 106 – electropolymerized azines in 105–108 – – glucose oxidase in mediator free bioelectrocatalysis 105–108 – – by hydrogenases 106 – – by peroxidase 106 biomimetic models 193 biomimetism, artiﬁcial muscles 246–248 biosensors based on electropolymerized ﬁlms 189–206 – chronological evolution 190–191 – – 1986–2009 192 – – principal stages 190–191 – covalent binding at electropolymerized ﬁlms surface 200–202 – by direct electropolymerization 191–194 – electrogenerated polymers, adsorption on 194–195 – electropolymerized ﬁlms, mechanical entrapment within 195–200 – nitrilotriacetic acid (NTA) 191 – noncovalent binding by afﬁnity interactions with electropolymerized ﬁlms 203–205 – polypyrrole-carbon nanotube (CNT)-DNA sensor 190 – polypyrrole–gel matrix 191 brilliant cresyl blue 95 brush model 66–70 bulk resistance modulation 163–165 – in gas/polymer interactions 158

c capacitive sensors 144–145 – o-phenylenediamine-based polymers 145 – poly(p-aminobenzene sulfonic acid) ﬁlm 145 carbon nanotubes (CNTs) for molecular imprinting 142 carrier density modulation in conducting polymers 157–158 cellobiose dehydrogenase (CDH) bioelectrocatalysis 106–108 chemical doping 178 chemical methods, of phthalocyanines electropolymerization 118 chemical polymerization 77–87 chemical-sensitive ﬁeld-effect transistors (CHEMFETs) 155, 159–160, 183 chemical sensors based on conducting polymers 173–185, see also electrochemical signal transduction – amperometric 179–181 – chemically sensitive transistors 182–184 – – CHEMFET 183 – – insulated-gate ﬁeld-effect transistor (IGFET) 182 – – ion-selective organic electrochemical junction transistor (IS-OEJT) 183 – – ion-selective organic electrochemical transistor (IS-OECT) 183 – – optical signal transduction 184 – – organic ﬁeld-effect transistor (OFET) 182 – – organic thin-ﬁlm transistor (OTFT) 182 – – thin-ﬁlm transistor (TFT) 182 – conductimetric 181–182 – electroactive ions 180 – electroinactive ions 180 – potentiometric 175–179 – voltammetric 179–181 chemiresistors 181 – bulk resistance modulation 163–165 coated-wire electrode 178 competitive doping process 163 complex actual ionic interchanges 244–245 complex nonlinear least squares (CNLS) 61 conductimetric sensors 181–182 conducting polymers 1–12, see also gas sensing with conducting polymers – composites 234–235 – electronic properties of 153–155 – electronic properties tuning in 156–157 – – electrochemical work function tuning 156–157

Index – – primary doping effect on work function 156 – gas sensors based on, see gas sensing with conducting polymers – as reactive materials, 242–245, see also under artiﬁcial muscles conductometric (nonzero-current) sensors 163–169 – chemiresistors, bulk resistance modulation 163–165 – OFETs, ﬁeld-modulated chemiresistors 168 – Schottky barrier diodes, contact resistance modulation 165–167 – Schottky diode gas sensor polymers 168–169 conjugated polymers 153 – molecular imprinting in 135–138 – – overoxidized polypyrrole (oPPy) 135–136 π -conjugated polymers, electropolymerized ﬁlms of 1–12, 154, see also intrinsically conducting polymers (ICPs) – from pristine heterocyclic to sophisticated functional and conjugated architectures 4–12 – surface functionalization 1–12 contact resistance modulation 165–167 – in conducting polymers 158–159 ‘countercharge rule’ 34 counter-ions 155 covalent binding at electropolymerized ﬁlms surface 200–202 – amines (polyazulenes) 201 – carboxyls (polythiophenes) 201 – N-hydroxyphthalimide esters 201 – N-hydroxysuccinimide esters (polypyrroles) 201 – poly(pyrrole–benzophenone) ﬁlms 202

d dehydrogenases 99–100 deposited polymer ﬁlms properties 38–47 – polymerization parameters effect on 38–47 – – potentiodynamic polymerization 41 – – starting monomer purity 38 diffusion resistance 60 dimethoate-imprinted sensor 143 dimethylmethylphosphonate (DMMP) 162 dimethylsulfoxide (DMSO) 119 direct electropolymerization 191–194 – biomimetic models 193 – oligonucleotides 193

– oligopeptides 193 – synthetic redox centers like porphyrin derivative 193 direct electropolymerization 233–234 dopant anions 155 doping effect 2 doping in conducting polymers 154 – n-doped 154 – p-doped 154

e easy-to-handle effect 2 electroactive ions 180 electrocatalysis 77–87 – by polyazines 98–99 electrochemical impedance spectroscopy (EIS) for polymer characterization 51–70 – experimental arrangements 53–55 – – background electrolyte 53 – – blocking interface 53 – – modiﬁed electrodes 54 – – polymer ﬁlm placement 53 – ﬁlm thickness distribution effect 56–57 – ﬁlm thickness effect 56–57 – interpretation tools 52 – – analogs 52 – – physical models 52 – ionic and electronic interfacial exchange media 60–61 – modiﬁed electrodes 54 – – characteristic quantities for 57–60 – polymeric layers models 63–70 – – brush model 66–70 – – heterogeneous or porous layer model 64–65 – – homogeneous or uniform 63 – – two or three charge carriers, theories dealing with 65–66 – spectra, analysis 61–62 – – ‘classical’ method 61 – – comparison using estimated parameters 62 – – complex nonlinear least squares (CNLS) 61 – – distribution of deviation, analysis 62 – – four-step procedure 62 – – Gauss–Newton algorithm 62 – – linear estimation theory 61 – – physical signiﬁcance analysis 62 – – statistical analysis 62 electrochemical quartz crystal microbalance (EQCM) study 30, 94

275

276

Index electrochemical signal transduction 74–184 – receptor units immobilization in p-doped conducting polymer 176 electrochemical techniques, in ICPs preparation 216–217 electrochemical transducer 249–251 electrochemical work function tuning in conducting polymers 156–157 electrochemically stimulated conformational relaxation (ESCR) 255 electrochemomechanical devices 241–266, see also artiﬁcial muscles electrogenerated polymers, adsorption on 194–195 electrogravimetric impedance 34 electroinactive ions 180 electroluminescent diodes 153 electron-conducting polymers (ECPs), phthalocyanines in 111–113 – ECP precursors, in phthalocyanines electropolymerization 118–119 – metallic tetra sulphophthalocyanines (MTSPcs) 113 – polyaniline (PANi) 111–113 – polypyrrole 111–113 electropolymerization, general aspects 27–32 electropolymerized azines, see azines, electropolymerized electropolymerized ﬁlms, mechanical entrapment within 195–200 – advantage 198 – bienzyme electrode biosensor 196 – drawbacks 198–199 – insulating polymers, electrodeposition 197 electrorheology 86 end-terminated polymerizing groups 10 energy conversion, in ICPs electropolymerization 218–227 – photovoltaic (PV) effect in 218–219 – polythiophenes 219–224 energy storage, in ICPs electropolymerization 227–230 – to form electrodes for 230–232 – – Naﬁon 231 – – PANi 231–232 – – Ppy 230–231 – in rechargeable batteries 228–229 – in supercapacitors 229–230 enzyme catalysis, mimetics of 100–101 3,4-Ethylenedioxythiophene (EDOT) 147

f Faraday impedance 52 Fermi level 77–87 ﬁeld-effect transistor (FET)-type devices 81, 153 ﬁeld-modulated chemiresistor 182 ﬁrst cycle or memory effects 33 functional conjugated architectures elaboration 10–12 – end-terminated polymerizing groups 10 functionalization 1–12 functionalization by covalent grafting 7–8 functionalization by doping, in Pristine aromatic heterocycles 6

g gas sensing with conducting polymers 153–169 – conductometric (nonzero-current) sensors 163–169, see also individual entry – doping 154 – polaron or bipolaron 154 – poly(p-phenylene vinylene) 154 – poly(p-phenylene) 154 – polyaniline 154 – polypyrrole 154 – polythiophene 154 – potentiometric (zero-current) sensors, 159–163, see also individual entry – preparation 155–157 – – solvent casting 155 – – in situ electrochemical deposition 155–156 – trans-polyacetylene 154 – types 159–169 gas/polymer interactions mechanism 157–159 – bulk resistance changes 158 – contact resistance changes (Schottky barrier) 158–159 – secondary doping by donor/acceptor interactions 157 – work function modulation – modulation of carrier density 157–158 Gauss–Newton algorithm 62 Giant nonstoichiometry 245

h ‘head-to-tail’ bonding 97 heterogeneous polymeric layer model 64–65 homogeneous polymeric layer model 63 hopping 155

Index

m

horse radish peroxidase (HRP) 106 hydrogenase, bioelectrocatalysis by 106

i imprinted polymers, 133–148, see also molecularly imprinted polymers (MIPs) – deﬁnition 133–134 – process of imprinting 134–135 – solgel imprinted ﬁlms by electropolymerization 138–139 in situ electrochemical deposition 155–156 indium tin oxide (ITO)-coated glass 217 inherently conducting polymers (ICPs) 215–236 – conducting polymer composites 234–235 – via electropolymerization for energy conversion and storage, 215–236, see also under energy conversion; energy storage – – electrochemical techniques 216–217 – – electrolyte 217–218 – – indium tin oxide (ITO)-coated glass in 217 – – substrates 217 insulated-gate ﬁeld-effect transistor (IGFET) 182 insulating polymers, electrodeposition 197 intelligent devices 263 interfacial polymerization methods 79 intrinsically conducting polymers (ICPs) 1–4 – epistemological analysis of electropolymerization within 2–4 – ‘era of electrochemists’ 2 – ‘era of molecular electronics’ 4 – ‘era of physicists’ 2 – ‘era of polymerists’ 3 ion-selective electrodes (ISE) 178–179 ion-selective organic electrochemical junction transistor (IS-OEJT) 183 ion-selective organic electrochemical transistor (IS-OECT) 183 ‘ion-sieving’ property 135

k Kelvin probe 159 Kramers–Kronig (K–K) transformations

56

macroscopic devices 248–256 – bending structures combination 255 – bending structures 252–254 – – asymmetrical monolayers 252–253 – – bilayers 253 – – triple layers 253 – efﬁciency 251–252 – electrochemical transducer 249–251 – structures giving lineal movements 254–255 – – ﬁbers and ﬁlms 254 – – tubes and ﬁlms with metal support 254–255 mediator free bioelectrocatalysis, glucose oxidase in 105–108 metal afﬁnity immobilization concept 204 metallated structures 1–12 metallic tetra sulphophthalocyanines (MTSPcs) 113 methylene blue 95, 77–87 methylene green 95 microdevices 255–256 microtools 255–256 modiﬁed electrodes, in EIS 54, 57–60 – characteristic quantities for 57–60 molecularly imprinted polymers (MIPs) 98, 133–148 – amperometric sensors 145–146 – of ascorbic acid 136–137 – capacitive sensors 144–145 – in conjugated polymers 135–138 – integration with transducers surface 139–140 – molecular wires in catalytic MIP-based sensors for catechol construction 141 – nanostructured materials for 140–143 – – carbon nanotubes (CNTs) 142 – – dimethoate-imprinted sensor for dimethoate recognition 143 – – NPs 142 – – ochratoxin A (OTA) 142 – o-aminothiophenol (o-AT) 146 – piezoelectric sensors 144 – voltammetric/potentiometric sensors 145–146 Mott–Schottky limit 158 multifunctionality, artiﬁcial muscles 246–248

l last deposition cycle 33 layer-by-layer (LbL) method 115 linear estimation theory, EIS 61 lutecium diphthalocyanine (LuPc2 )

n 115

nanoneedles 77–87 nanostructuration of polypyrrole – direct templating 82

80–83

277

278

Index nanostructuration of polypyrrole (contd.) – electrodeposited 83 – oxide–polystyrene–polypyrrole nanoparticles 81 – polypyrrole inverse opal 82 – polypyrrole nanocomposites 80–83 – – analogous methods 81 nanostructured conducting polymers 233–234 – direct electropolymerization 233–234 – template-assisted electropolymerization 233 nanostructured materials for molecular imprinting 140–143 – carbon nanotubes (CNTs) 142 – dimethoate-imprinted sensor for dimethoate recognition 143 – NPs 142 – ochratoxin A (OTA) 142 neutral red 77–87, 94–95 nickel tetra-sulfonated phthalocyanine, electrochemical modiﬁcation of electrodes with 125–128 nitrilotriacetic acid (NTA) 191 nitrogen oxide (NOx ) 162 ‘nitrogen-to-ring’ coupling 98 noncovalent binding – by afﬁnity interactions with electropolymerized ﬁlms 203–205 – – adamantane/β-cyclodextrin 203–204 – – biotin/avidin/biotin 203–204 – – NTA/Cu2+ /histidine 203–204 nonoxidative polymerization 79 N-phenylethylenediamine methacrylamide (NPEDMA) 140 ‘nucleation loop’ 216

o o-aminothiophenol (o-AT) 146 ochratoxin A (OTA) in molecular imprinting 142 oligonucleotides 193 oligopeptides 193 one-step functionalization by grafting 8 o-phenylenediamine-based polymers 145 optical signal transduction 184 organic ﬁeld-effect transistor (OFET) 182 organic thin-ﬁlm transistor (OTFT) 182 overoxidation 35 overoxidized polypyrrole (oPPy) 135–136 overoxidized polypyrrole 197 oxidation onset 33 oxidation potential 31

oxide–polystyrene–polypyrrole nanoparticles 81

p p-doped conducting polymer 33 – receptor units immobilization in 176 – – anionic 177 – – cationic 177 – – redox response of 177 pencil graphite electrode (PGE) 136–137 perchloroethylene (PER) 162 peroxidase, bioelectrocatalysis by 106 [6,6]-phenyl C61-butyric acid methyl ester (PCBM) 222 photothermal deﬂection spectroscopy (PDS) 245 phthalocyanines – in electron-conducting polymers (ECPs) 111–113 – electropolymerization of, 117–129, see also transition-metal phthalocyanines immobilization – – chemical methods 118 – – with ligands bonded to radicals of electron-conducting polymer precursors 118–119 – – nickel tetra-sulfonated phthalocyanine 125–128 – – Pcs linking to foreign polymer matrix 118 – – tetra-amino-substituted phthalocyanines, 119–124, see also individual entry physical models, in EIS 52 piezoelectric quartz crystal (PEQC) 137–138 piezoelectric sensors 144 polaron or bipolaron 154 poly (dichlorophenolindophenol) 197 poly (pyrrole–benzophenone) ﬁlms 202 Poly(p-phenylene vinylene) 154 Poly(p-phenylene) 154 Poly(2-mercaptobenzimidazole) (PMBI) ﬁlms 146 poly(2-methoxy-5-(2 -ethylhexyloxy)p-phenylene vinylene) (MEH-PPV) 222 poly(3,4-ethylenedioxythiophene) (PEDOT) 140 poly(allylamine) hydrochloride (PAH) 116 poly-(diallyldimethylammonium) chloride (PDDA) 116 poly(ethyleneimine) (PEI) 116 poly(neutral red)-modiﬁed electrodes 102–103 poly(o-phenylenediamine) (PPD) ﬁlm 65

Index poly(p-aminobenzene sulfonic acid) ﬁlm 145 polyaniline (PANi) 111–113, 154 ‘polymerization efﬁciency factor’ 29 polyazines in electroanalysis 77–87, 98–104 – electrocatalysis by polyazines 98–99 – enzyme catalysis, mimetics of 100–101 – equilibrium NAD+ |NADH potential at poly(neutral red) electrodes 103–104 – NAD+ electroreduction to NADH at poly(neutral red)-modiﬁed electrodes 102–103 – in NAD+ |NADH regeneration 99–100 – – dehydrogenase enzymes 99–100 polyoxometallates 77–87 polyphenylenediamines 197 polypyrroles (Ppy) 27–47, 111–113, 154, 230–231 – applications 83–86 – – actuators 85 – – anticorrosion 85–86 – – batteries 84–85 – – in fuel cells 85 – – supercapacitors 84–85 – electrochemistry, recent trends in 77–87, see also nanostructuration of polypyrrole – in MIP formation 136 – polypyrrole-carbon nanotube (CNT)-DNA sensor 190 – polypyrrole–gel matrix 191 – synthetic procedures, advances in 78–79 – – fundamental research 78–79 – – interfacial polymerization methods 79 – – new monomers and polymers 78 – – new polymerization methods 79 – – nonoxidative polymerization 79 – – pulse radiolysis polymerization 79 – – two-photon polymerization 79 polythiophenes (PTh) 154, 219–224, 232 – via electropolymerization of precursors functionalized – – with electron accepting/withdrawing moieties 222–224 – – with light-harvesting moieties 225–227 – via simple precursors electropolymerization 219–222 porous polymeric layer model 64–65 potentiodynamic polymerization 41 potentiometric (zero-current) sensors 159–163 – CHEMFET 159–160 – dimethylmethylphosphonate (DMMP) 162 – Kelvin probe 159

– nitrogen oxide (NOx ) 162 – perchloroethylene (PER) 162 potentiostatic deposition 44 primary doping effect on conducting polymers 156 Pristine aromatic heterocycles, electropolymerization of 5–7 – functional dopant inclusion 6 probe beam deﬂection (PBD) 245 pulse radiolysis polymerization 79

q quartz crystal microbalance (QCM) 245 ‘quasi-equilibrium’ 35 quaternary ammonium salt (QAS) 113–115

r Randles model 52 Randles–Ershler approach 65 rechargeable batteries, ICPs in 228–229 redox activity and electropolymerization 27–47 – anodic charge inﬂuence 29 – deposition and 38–47, see also under deposited polymer ﬁlms properties – monomer oxidation at electrode surface 30 – redox activity of polymer ﬁlms 32–37 – – additional three-dimensional phase 34 – – ‘countercharge rule’ 34 – – electrogravimetric impedance 34 – – electroinactive ﬁlm 33 – – ﬁrst cycle or memory effects 33 – – overoxidation 35 – – oxidation onset 33 – – p-doping 33 – – polymerization efﬁciency 36 – – ‘quasi-equilibrium’ 35 ‘relaxation period’ 30 ‘ring-to-ring’ coupling 97

s Schottky barrier – contact resistance modulation 165–167 – in conducting polymers 158–159 Schottky diode gas sensor polymers 168–169 secondary doping by donor/acceptor interactions 157 sensing capabilities of artiﬁcial muscles 258–259 solgel imprinted ﬁlms prepared by electropolymerization 138–139 solvent casting 155

279

280

Index St¨ober process 81 substituted heterocycles, electropolymerization of 7–9 – functionalization by covalent grafting 7–8 – one-step functionalization by grafting 8 supercapacitors 84–85 – ICPs in 229–230 surface functionalization 1–12

t tactile sensitivity 259–263 template-assisted electropolymerization 233 tetra-amino-substituted phthalocyanines electropolymerization 119–124 – CuTAPc electropolymerization 120–121 – – anodic redox transitions in 122–124 – – B-(Soret) band in near-UV range 122 – – poly(CuTAPc) anodic behavior 122 – – Q-band in the visible range 122 – – spectra of 122 – from dimethylsulfoxide (DMSO) 119 thin-ﬁlm transistor (TFT) 182 thionine 95 toluidine blue 95–96 transducers surface, MIPs integration with 139–140

transition-metal phthalocyanines immobilization 111–117 – on conducting and nonconducting substrates 111–117 – – in artiﬁcial lipids matrices 113–115 – – phthalocyanines in electron-conducting polymers 111–113 – ultrathin layers composites of oppositely charged ions 115–117 – – layer-by-layer (LbL) method 115 trans-polyacetylene 154 two-photon polymerization 79

u ultrathin layers composites of oppositely charged ions 115–117 undoping 155 uniform polymeric layer model 63

v voltammetric sensors 179–181 voltammetric/potentiometric sensors 145–146

w work function modulation in conducting polymers, carrier density modulation 157–158