Studies of the Kinetics of Mediated Electrochemical Oxidation at Polymer-Coated Electrodes

Abstract

During the past few years interest in the electrochemical behaviour of biological molecules has been further stimulated by the quest for practical biosensors¹. These devices combine the inherent selectivity of a biochemical process, e.g., an enzyme reaction, with the advantages of measuring the analyte concentration in terms of a potential or current. Both potentiometric (equilibrium) and amperometric (dynamic) modes of operation, however, require facile electron transfer between the biomolecule and the surface of an electrode. For most large, electroactive biomolecules such direct heterogeneous electron transfer is extremely slow and even for the measurement of equilibrium potentials, which requires only very small currents, a suitable mediator must be added to the solution. Clearly, provision for charge transfer mediation is essential for the successful construction of sensors of both types. The selection of an appropriate mediator is usually based on the fulfillment of certain criteria. Its heterogeneous and homogeneous redox reactions must take place rapidly at a well-defined potential in the medium of interest and involve the transfer of a definite number of electrons to and from stable redox species. In addition the mediator should be adequately soluble, usually in aqueous media at pH ~ 7, and should not inhibit the reaction of the biomolecule with its substrate. Very few substances meet all these demands.