REDOX POLYMER-FILMS CONTAINING ENZYMES .2. GLUCOSE-OXIDASE CONTAINING ENZYME ELECTRODES

Glucose oxidase is covalently bound in a film of cross-linked, redox-conducting epoxy cement on the surface of electrodes. The binding simultaneously immobilizes the enzyme and connects it electrically with the electrode. The effects of cross-linker concentration, film thickness, enzyme concentration, temperature, and oxygen concentration on the steady-state electrocatalytic oxidation of glucose at the redox-epoxy enzyme electrodes are described. The catalytic "reaction layer" extends through the entire film, even for films as thick as ca. 5-mu-m. The limiting catalytic current density is a function of the enzyme concentration, reaching a maximum near 35 wt % enzyme for films about 1-mu-m thick. In such films, the activation energy for the electrocatalytic reaction at high glucose concentration is 63 kJ/mol, and the apparent Michaelis constant monotonically decreases with increasing enzyme concentration and increases with increasing oxygen concentration. These results are explained by postulating that in such approximately 1-mu-m thick redox-epoxy enzyme films the rate-limiting kinetic step at high substrate concentration is related to electron transfer away from the enzyme active site, a process involving flexing of the cross-linked redox chain segments. This bottleneck may be attributed to the high activity of the enzyme and the small contact area between the redox polymer and the enzyme-active site that is recessed inside the insulating protein shell of the enzyme.