ELECTROCHEMICAL CHARACTERIZATION OF POLYPYRROLE BIENZYME ELECTRODES WITH GLUCOSE-OXIDASE AND PEROXIDASE

Polypyrrole bienzyme electrodes with glucose oxidase and horseradish peroxidase (GOD/HRP/PPy electrodes) were fabricated by electropolymerization of pyrrole on an SnO2 electrode in the presence of the two enzymes. The bienzyme electrode thus obtained functions as a glucose sensor on the basis of H2O2 reduction at +150 mV vs. Ag/AgCl, retaining the electrochemical activity and conductivity of PPy. A linear relation between the steady-state cathodic current and the glucose concentration was obtained from 10(-5) to 10(-3) M. At an electrode potential of +700 mV vs. Ag/AgCl where HRP oxidized by H2O2 cannot be reduced back and PPy loses its electrochemical activity and conductivity because of overoxidation, the bienzyme electrode responds to glucose on the basis of H2O2 oxidation on the SnO2 surface and exhibits much lower sensitivity than at +150 mV. This was explained in terms of the efficiency of the electrochemical signal transfer; signal transfer from GOD to SnO2 via H2O2, HRP, and PPy is much more efficient than that via H2O2 alone. The electrochemical activity and conductivity of PPy and the activity of HRP are responsible for this difference. Dependencies of the sensitivity and the signal transfer efficiency of these electrodes on the film thickness were also examined. Further, a bilayer electrode carrying a GOD/PPy film as an inner layer and an HRP/PPy film as an outer layer was fabricated and was found to exhibit signal transfer efficiency higher than that of the GOD/HRP/PPy homogeneous film-coated electrodes, while the former showed lower sensitivity owing to the outer layer inhibiting the glucose supply to the inner layer.