Electroactive polymer redox conduction

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

The above mechanistic aspect of electron transport in electroactive polymer films has been an active and chemically rich research topic (13-18) in polymer coated electrodes. We have called (19) the process "redox conduction", since it is a non-ohmic form of electrical conductivity that is intrinsically different from that in metals or semiconductors. Some of the special characteristics of redox conductivity are non-linear current-voltage relations and a narrow band of conductivity centered around electrode potentials that yield the necessary mixture of oxidized and reduced states of the redox sites in the polymer (mixed valent form). Electron hopping in redox conductivity is obviously also peculiar to polymers whose sites comprise spatially localized electronic states. 

An enormous number of polymers have been used to prepare chemically modified electrodes. Some examples are given in Table 13.2 Albery and Hillman provide a more extensive list [8]. As indicated in Table 13.2, these polymers can be divided into three general categories—redox polymers, ion-exchange and coordination polymers, and electronically conductive polymers. Redox polymers are polymers that contain electroactive functionalities either within the main polymer chain or in side groups pendant to this chain. The quintessential example is poly(vinylferrocene) (Table 13.2). The ferrocene groups attached to the polymer chain are the electroactive functionality. If fer-... 

The third class of polymers used to prepare chemically modified electrodes is the electronically conductive polymers [25]. The polymer chains in this family of materials are themselves electroactive. For example, the polymer redox reaction for polypyrrole (Table 13.2) can be written as follows ... 

Two nitrogen-containing electroactive polymers, polypyrrole (PPY) [21] and polyaniline (PAN) [22], have been of particular interest because of their environmental stability, high electrical conductivity and interesting redox properties associated with the chain heteroatoms. More importantly, PAN has been found to exhibit solution processability [23, 24] and partial crystallinity [25,26]. 

The modification of electrode surfaces with electroactive polymer films provides a means to control interfacial characteristics. With such a capability, one can envisage numerous possible applications, in areas as diverse as electronic devices, sensors, electrocatalysis, energy conversion and storage, electronic displays, and reference electrode systems [1, 2]. With these applications in view, a wide variety of electroactive polymeric materials have been investigated. These include both redox polymers (by which we imply polymers with discrete redox entities distributed along the polymer spine) and conducting polymers (by which we imply polymers with delocalised charge centres on the polymer spine). 

These are presented by two subclasses of electroactive polymer (i) -conjugated polymers of both organic and inorganic nature [5-15] and (ii) conventional redox polymers [26], and by inorganic ion-insertion (intercalation) compounds [27, 28[ (see the top of Scheme 11.1b). Despite the different nature of their chemical bonds, all of these compounds are mixed, electronic-ionic conductors [29], and hence, their electronic and/or ionic conductivity is expected to change with the applied potential in a predictable, characteristic manner (see Section cl 1.4). 

In contrast to mediators, also redox polymers can be used. These polymers are mainly characterized by the presence of specific electrochemically active sites. There are different possibilities to facilitate redox transfer by shuttling electrons via redoxactive groups in non-conductive or conductive polymers. In general, a redox polymer consists of a system where a redoxactive molecule is covalently bound to a polymer backbone which may or may not be electroactive. Erequently, electroactive polymers are formed by the electropolymerization of suitable monomer complexes. A few representative examples of electron shuttle molecules are shown in Eig. 1. 

Direct registration of DNA hybridization advanced a lot with the use of electroactive films of conductive polymers as electrodes. Conductive polymers, which consist of conjugated backbones that are easily oxidized or reduced (doped) with a concomitant increase or decrease in conductivity, with each polymer having its own redox characteristics. 

J.M. Pemaut and J.R. Reynolds, Use of conducting electroactive polymers for drug dehvery and sensing of hioactive molecules A redox chemistry approach, J. Phys. Chem. B, 104(17), 4080-4090 (2000). 

Electroactive polymers can be divided into three broad classes electronically conducting polymers, such as poly(pyrrole), in which conduction is associated with motion of charge carriers along the chains redox polymers, such as poly(vinylfer-rocene), in which conduction is associated with cross-exchange reactions between discrete redox sites and polymer electrolytes, such as Nafion, in which conduction is associated with ion motions within the film. Examples of polymers from all three classes have been used in bioelectrochemical applications. 

To date studies using electroactive polymers with redox proteins have been much less numerous despite the fact that it should be possible to extend the general principles elucidated by Hill and colleagues using adsorbed monolayers at electrode surfaces to design suitable electroactive polymers for this application. For example studies with poly(5-carboxyindole), a conducting polymer formed by electropolymerization of 5-carboxyindole have shown that it can be used for the direct... 

In contrast to the area of redox protein electrochemistry, redox enzyme electrochemistry has received much greater attention, driven in many cases by the desire to construct practical, self-contained enzyme electrodes for commercial applications. Redox enzyme electrochemistry is also easier to study in many ways because the substrate or product is often detected electrochemically rather than the enzyme itself. Various types of electroactive polymers have been used with redox enzymes, including redox polymers, redox-active hydrogels, and electropolymer-ized films of conducting and nonconducting, polymers. We discuss each type of polymer in turn, starting with electropolymerized films. 

The oxidation and reduction (redox) processes in electroactive polymers (EAPs) make it possible to use these polymer materials as charge storage devices, either as battery electrodes or as supercapacitors. The potential for reduced cost, weight, and enviromnental impact of EAP electrodes relative to the metals and metal oxides that are traditionally used in such devices makes these polymers attractive alternatives. While inorganic options are limited, EAPs can be tailored to provide specific properties, such as conductivity, voltage window, storage capacity, porosity, reversibility, and chemical and environmental stability. 

FIGURE 12.9 Schematic illustration of chemical and biological sensor configurations involving the use of conducting electroactive polymers in redox switches. (Reprinted from Guiseppi-Elie, A., Wallace, G.G., and Matsue, T., Handbook of Conducting Polymers Marcel Dekker, New York, 1998. With permission.)... 

Redox active polymer films are ideally suited to tackling these issues. The properties of many of these electroactive polymers, e.g., their conductivity, charge distribution, shape, etc., can be changed in a controlled and reproducible way in response to environmental stimuli, e.g., a change in the nature of the contacting solution, an applied voltage, light intensity, or mechanical stress. 

The redox properties of the conducting polymer film are the primary interest of the present chapter, because most of the important applications are associated with switching the electroactive polymer films from the neutral (reduced) state of the doped (oxidized) state. The voltage range in which representative polymers show electroactivity is shown in Figure 2.2, compared with inorganic materials. Li metal is chosen as the reference because of the interest in using the intercalation materials in lithium battery systems. 

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