Diffusion-kinetic theory of stationary behaviour of amperometric bienzyme electrodes

Abstract

Steady-state kinetics of amperometric bienzyme electrodes have been analysed at relatively low concentrations of the assayed substrate. The electrodes are classified into eight types which differ in the character of their interrelationships between enzymatic conversions and in the role of the detected depolarizer. The influence of the activities of the immobilized enzymes which are distributed in the layer in uniform or asymmetric manner, the permeabilities of the components of three-layer coating and the rate of the indicator reaction on the substrate sensitivity of the electrodes are discussed. The structural similarity of the steady-state equations for the electrodes involving consecutive conversions of the substrate and parallel conversions of the cosubstrate has been demonstrated. The criteria of the functioning of such electrodes in the diffusion-controlled regime are formulated. The conditions of kinetic equivalence of the enzymes uniformly distributed in the electrode coating have been determined. It is shown that the electrodes involving cyclic conversions of the substrate are characterized by the higher sensitivity to substrate: (1) in the case of uniform distribution of the enzymes in comparison with the asymmetric distribution of the enzymes; (2) in the case of detection of one of the products or cosubstrates of the process of substrate consumption in comparison with detection of the corresponding substance for the process of regeneration of the substrate; and (3) when the permeability of the outer membrane decreases.