Topological and electron-transfer properties of glucose oxidase adsorbed on highly oriented pyrolytic graphite electrodes

The topological and electron-transfer properties of the redox enzyme glucose oxidase (GOD) adsorbed on highly oriented pyrolytic graphite (HOPG) were studied at a single molecule level with in situ electrochemical scanning tunneling microscopy (EC-STM). In situ STM and cyclic voltammetry were performed to understand the potential-dependent tunneling process in the presence of adsorbed GOD. Potential-dependent apparent changes in the GOD morphology on HOPG were observed in STM images and are discussed. For example, an issue in the STM scanning of adsorbed GOD was the influence of EC currents on the measured tip current. These Faradaic currents might be due to involved redox species that react at the enzyme-modified electrode surface and subsequently diffuse to the STM tip. Such effects of local enzyme activity on the contrast in STM images have been studied and are evaluated within a proposed model.