ELECTROCHEMISTRY OF ORGANIC CONDUCTING SALT ELECTRODES - A UNIFIED MECHANISTIC DESCRIPTION

A detailed, unified description of electrocatalytic mechanisms at organic conducting slat (OCS) paste electrodes (TTF-TCNQ, tetrdthiafulvalene-p-tetracyanoquinodimethane; HMTTeF-TCNQ, hexamethylenetetratellurofulvalene-p-tetracyanoquinodimethane) has been presented. A wide spectrum of behavior is observed depending on the electrochemical and kinetic characteristics of the analyte of interest, the nature of the OCS, the applied potential and the electrolyte composition. These electrodes behave like metal electrodes in the presence of an analyte (i.e. Fe(CN)63-/4-) whose E0' is greater than the E0' of either the donor or the acceptor molecule or if competing indirect, mediated pathways are not kinetically viable. Current-potential and current-concentration studies are consistent with ascorbate (pH 8.3) and NADH oxidation occurring via a homogeneous mediation mechanism. Rotating disk electrode behavior is consistent with a homogeneous mechanism by involving the concept of replacing an electrochemical mechanism involving homogeneous kinetics with a "heterogeneous equivalent". It is concluded that electrooxidation of a reduced enzyme (glucose oxidase) occurs via a homogeneous mechanism on both electrode materials. A detailed examination of background currents of OCS electrodes is shown to be very useful in understanding their intrinsic redox activity and their solubilities. Evidence for Faradaic chemistry and potential-induced solubility of components of the OCS electrodes well within the stable limits of the material shows that surface redox phenomena differ considerably from bulk or lattice redox phenomena. This "background" generation of soluble mediators is shown to be the source of the homogeneous mediators present in bioelectrocatalysis experiments.