Conducting Polymer-Based Reference Electrodes

The deposition conditions, postdeposition treatment, ionic and redox sensitivity, and selected applications of the conducting polymer-based reference electrodes are summarized in Table 12.2. 

Conducting polymer-based immobilization or wired enzymes is a global enzyme immobilization method that differs in many respects from those just described. In one example, a redox polymer is formed on the surface by the oxidation of pyrrole molecules to pyrrole radical cations, which then polymerize on the surface to form conductive polypyrrole [60,68]. Other conducting polymers include polyvinylpyridine, polythiophene, polyaniline, and polyindole. If enzymes are present in the solution as polymerization takes place, they are entrapped within the polymer. When these polymers are cross-linked with redox mediators such as [Os(bpy)2Cl]+ 2 the resulting amperometric (or potentiomet-ric) biosensors are referred to as wired enzyme electrodes [5-7]. The distance between the redox centers of the polymer and the FADH2 centers of the reduced enzyme is reduced sufficiently for electrons to be transferred and, therefore, for the mediated electro-oxidation of glucose on conventional electrodes. These electrodes do not require diffusing redox mediators or membranes to contain the enzyme and the redox polymer. 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

Although all solid state reference electrodes based on conducting polymers have been tested, they are still in the testing phase. The most promising systems appear to be bilayers with different ion-exchanger properties such as glassy carbon/polypyrrole in conjunction with a polypyrrole(polystyrene sulfonate) layer 64,65). 

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