Role of Conductive Nanoparticles in the Direct Unmediated Bioelectrocatalysis of Immobilized Sulfite Oxidase

We report efficient bioelectrocatalytic sulfite oxidation by human sulfite oxidase ( h SO) immobilized on a gold nanoparticle (AuNP) modified gold electrode. The AuNP were synthesized in aqueous phase by using branched polyethyleneimine (PEI) as reducing as well as stabilizing agent. Gold electrodes were modified by a self assembled monolayer of dithio‐bis(N‐hydroxysuccinimidyl propionate) (DTSP) onto which the NP and h SO were immobilized. Cyclic voltammetry of the h SO modified electrode in the absence of substrate revealed a quasi‐reversible direct electrochemical reaction of the heme domain of h SO with fast electron transfer rate. The electron transfer rate constant of k s =32 s −1 and the formal potential E 0 ′=−0.155 V vs. Ag/AgCl/1 M KCl were estimated. Comparative studies with nanoparticles of BaSO 4 indicate the importance of the NP conductivity for charge transfer and enhancement of direct electron transfer communication. Sulfite addition resulted in a largely enhanced oxidation current at low overpotential. Use of longer thiols for assembly of the AuNP resulted in a decrease of electron transfer rate and increase of overpotential for sulfite bioelectrocatalysis. The electrode modified with DTSP and AuNP exhibited a very high steady state catalytic current density of 1 µA/cm 2 . The assembly can be applied for the amperometric biosensing of sulfite at a potential of 0 mV vs. Ag/AgCl/1 M KCl.