Effect of proton donors on direct electron transfer in the system gold electrode-horseradish peroxidase

The effect of proton donors (PD) on the direct electron transfer (ET) reaction between polycrystalline Au electrodes and horseradish peroxidase (HRP) was investigated. HRP was immobilised directly on the bare Au surface. The pH of the contacting solution was varied at a constant ionic strength and the following different PDs were used as additives: H3O+, NH4+, [La(H2O)](3+), [Y(H2O)](3+), [Lu(H2O)](3+). The kinetics of the bioelectrocatalytic reduction of H2O2 catalysed by HRP was studied with linear sweep voltammetry (LSV) in the potential range from 700 to -100 mV vs. SCE as well as amperometrically at -50 mV vs. Ag\AgCl with the HRP-modified An electrodes placed in a wall-jet flow through electrochemical cell. An increase of [H3O+] results in an enhancement of the current of the bioelectroreduction of H2O2 due to a more facilitated direct ET between Au and the enzyme over the potential range involved. It is shown that at high overvoltages (E<0.4 V) the PDs do not affect the rate of the enzymatic reduction of H2O2 but rather increase significantly the rate of direct ET between Au and HRP and the efficiency of acting as a PD is strongly correlated with their PD properties. The dependence of the apparent heterogeneous rate constant of direct ET, k(s), on [H3O+] makes it possible to suggest that the reaction mechanism involves the participation of a proton in the elementary step of the charge transfer. (C) 2001 Elsevier Science B.V. All rights reserved.