ENZYME CATALYSIS AT HYDROGEL-MODIFIED ELECTRODES WITH SOLUBLE REDOX MEDIATOR

The kinetics of amperometric enzyme electrodes are reported for glucose oxidase immobilized in three different bovine serum albumin (BSA) hydrogel modified electrodes using as soluble electron mediator either ferrocene sulfonate or a pentammine(pyrazine)ruthenium complex. The effect of hydrogel layer thickness, enzyme loading, substrate and redox mediator concentrations on the amperometric response are reported. The kinetic data are analysed in terms of a physico-chemical model that takes into account diffusion of substrate, redox mediator and non-linear kinetics in the hydrogel layer and external mass transport in the electrolyte outside the enzyme layer. Analytical solutions for selected conditions are presented and tested with experimental results. Simulated concentration profiles give an insight into the detailed diffusion-kinetic process and allow the conditions under which the analytical equations are valid to be tested. No difference in the enzyme kinetic pattern (ping-pong mechanism for glucose oxidase) is found for soluble enzyme and for immobilized glucose oxidase (GOx). Rate coefficients are reported for the two redox mediators.