Conducting polymer films

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Fig. 8. Entrapment of mediator-modified enzymes within a conductive polymer film where ( ) represents the mediator ferrocene and (B) the active site...

Electrogenerated conducting polymer films incorporate ions from the electrolyte medium for charge compensation (182). Electrochemical cycling in an electrolyte solution results in sequential doping and undoping of the polymer film. In the case of a -doped polymer, oxidation of the film results in the... 

Equations (37) and (38), along with Eqs. (29) and (30), define the electrochemical oxidation process of a conducting polymer film controlled by conformational relaxation and diffusion processes in the polymeric structure. It must be remarked that if the initial potential is more anodic than Es, then the term depending on the cathodic overpotential vanishes and the oxidation process becomes only diffusion controlled. So the most usual oxidation processes studied in conducting polymers, which are controlled by diffusion of counter-ions in the polymer, can be considered as a particular case of a more general model of oxidation under conformational relaxation control. The addition of relaxation and diffusion components provides a complete description of the shapes of chronocoulograms and chronoamperograms in any experimental condition ... 

It is now 20 years since the first report on the electrochemistry of an electrode coated with a conducting polymer film.1 The thousands of subsequent papers have revealed a complex mosaic of behaviors arising from the multiple redox potentials and the large changes in conductivity and ion-exchange properties that accompany their electrochemistry. 

The electrochemistry of conducting polymers has been the subject of several reviews2-8 and has been included in articles on chemically modified electrodes.9-14 The primary purpose of this chapter is to review fundamental aspects of the electrochemistry of conducting polymer films. Applications, the diversity of materials available, and synthetic methods are not covered in any detail. No attempt has been made at a comprehensive coverage of the relevant literature and the materials that have been studied. Specific examples have been selected to illustrate general principles, and so it can often be assumed that other materials will behave similarly. 

The electrochemistry of conducting polymer films involves ion expulsion or insertion to maintain electroneutrality. As illustrated in Eqs. (1)... 

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can... 

The reproducibility of the electrodeposition of conducting polymer films has been a very difficult issue. It has long been realized that each laboratory produces a different material and that results from different laboratories are not directly comparable.82 We have experienced reproducibility problems with almost all of the electrochemically polymerized materials used in our work. 

A number of approaches are available to improve the morphology and homogeneity of electrochemically deposited conducting polymer films. Priming of the electrode surface with a monolayer of adsorbed or covalently bonded monomer leads to more compact deposits of polyaniline,87,88 poly thiophene,80 and polypyrrole.89,90 Electrode rotation has been shown to inhibit the deposition of powdery overlayers during poly(3-methylthiophene) deposition.81... 

The electrochemistry of a polymer-modified electrode is determined by a combination of thermodynamics and the kinetics of charge-transfer and transport processes. Thermodynamic aspects are highlighted by cyclic voltammetry, while kinetic aspects are best studied by other methods. These methods will be introduced here, with the emphasis on how they are used to measure the rates of electron and ion transport in conducting polymer films. Charge transport in electroactive films in general has recently been reviewed elsewhere.9,11... 

Impedance spectroscopy is best suited for the measurement of electronic conductivities in the range 10 -7to 10 2S cm 1.145 In principle, it is perhaps the best method for this range, but it is often difficult to interpret impedance data for conducting polymer films. The charge-transfer resistance can make measurements of bulk film resistances inaccurate,145 and it is often difficult to distinguish between the film s ionic and electronic resistances.144 This is even more of a problem with chronoamperometry146 and chronopotentiometry,147 so that these methods are best avoided. 

It was reabzed early on that because of their high electron transport rates, the charging rates of conducting polymer films would be controlled predominantly by the rate at which charge-compensating ions [Eq.(l)] could be extracted from, or ejected into, the bathing electrolyte solution.160,161 However, these and some other studies employing chronoam-... 

The results from EQCM studies on conducting polymer films can be ambiguous because the measured mass change results from a combination of independent ion transport, coupled ion transport (i.e., salt transport), and solvent transport. In addition, changes in the viscoelasticity of the films can cause apparent mass changes. The latter problem can be minimized by checking the frequency response of the EQCM,174 while the various mass transport components can be separated by careful data analysis.175,176... 

Although cyclic voltammetry in a variety of electrolyte systems, and with a variety of doped polymers, has shown strong effects due to ion transport, it has provided little understanding. In fact, one of the important uses of ion transport data from the techniques discussed in the preceding subsections is that they help to provide an understanding of the cyclic voltammetry behavior of conducting polymer films. Their importance will... 

As illustrated in the previous sections, the electrochemical properties of conducting polymer films are strongly influenced by polymer-ion interactions. These interactions are in turn influenced by the nature of the solvent and the solvent content of the film. Consequently, the electrochemical behavior of conducting polymer films can be highly solvent dependent. Films can even become electrochemically inactive because of lack of solvation.114,197... 

The kinetics of charge transfer between metallic electrodes and conducting polymer films have proved to be difficult to investigate, and little reliable data exist. Charge-transfer limitations have been claimed in cyclic voltammetry, and Butler-Volmer kinetics have been used in a number of... 

The unusual cyclic voltammograms and responses to large-amplitude potential steps of a variety of conducting polymer films have prompted a number of groups to develop nucleation models for their oxidation. The key features that they have sought to explain are the peaks observed in anodic chronoamperometry (see Fig. 14), and the dependence of the anodic peak position on scan rate207 and the time spent in the undoped state.20 ... 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

The electrochemical characteristics of an electroactive ion immobilized in a conducting polymer film depend on whether the film is conductive at the formal potential of the ion.243... 

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