ELECTROCHEMICALLY PREPARED GLUCOSE BIOSENSORS - KINETIC AND FARADAIC PROCESSES INVOLVING ASCORBIC-ACID AND ROLE OF THE ELECTROPOLYMERIZED FILM IN PREVENTING ELECTRODE-FOULING

The influence is discussed of ascorbic acid (AA) on the response of a glucose biosensor based on glucose oxidase immobilized in electropolymerized poly(o-phenylenediamine) (PPD) or overoxidized poly(pyrrole) (oxPPy) films on a Pt electrode. The kinetics of the homogeneous reaction between AA and H2O2 has been investigated by two independent methods and found to be too slow to influence the response of typical glucose biosensors. Therefore, the decrease in the sensor response, observable when working in batch under typical experimental conditions, can in no way be ascribed to a depletion of H2O2, which is produced in the biocatalytic cycle via the homogeneous reaction with AA. While the purely additive Faradaic interference can practically be nullified by both entrapping membranes, electrode fouling by electro-oxidation products of AA (responsible for the observed decrease in glucose sensitivity) might still represent a problem when working with PPD based biosensors. In this respect the permeability characteristics of the film markedly influence the magnitude of the observed phenomena. The oxPPy film achieves the goal of completely eliminating ascorbate without blocking the access of glucose to the immobilized enzyme.