Modelling competition of the enzyme-catalysed glucose oxidation and redox reactions in scanning electrochemical microscopy

Kinetics of the glucose oxidation catalysed with immobilized enzyme (glucose oxidase) and redox reactions that compete for dissolved oxygen in an electrochemical cell is studied numerically by employing a mean-field model of a scanning electrochemical microscopy. The model accounts for: the bulk diffusion of glucose and dissolved oxygen towards the ultramicroelectrode (UME) and catalyst (enzyme-modified surface) and the products (gluconolactone and hydrogen peroxide) bulk one from the catalyst surface into the same cell, adsorption on and desorption from the catalyst surface of particles of both reactants. We have modified the known similar model and included into it two important processes: the surface diffusion of the adsorbed particles and intermediate reaction products, and a possibility to simulate the oxygen flux on the UME surface. The full mathematical model is described by a coupled system of nonlinear partial differential equations. It is approximated by using the finite volume method in space and the alternating direction implicit finite difference technique for integration in time. The influence of the immobilized enzyme concentration, distance between the catalyst surface and the UME, surface diffusivity of all intermediate reaction products, reaction rate constants, and bulk diffusivity of reactants on the evolution of the oxygen reduction current into water at the UME is studied. In particular it is found that the surface diffusion of the intermediate reaction products decreases the oxygen reduction current while the reverse reaction between the oxidized enzyme and hydrogen peroxide increases it. The mechanism of these effects is investigated.