Kinetic analysis of wired enzyme electrodes.: Application to horseradish peroxidase entrapped in a redox polymer matrix

A new approach to the description of the steady-state voltammetric behavior of wired enzyme electrodes in the presence of substrate mass transport polarization is presented. Starting from the exact analytical solutions corresponding to two-dimensional mediator-enzyme structures, experimental conditions are identified where the same equations can be applied to the analysis of the more often encountered three-dimensional catalytic films. These conditions are shown to involve a uniform redox conversion throughout the film. Case diagrams have been developed to assess the validity of this approach and to ascertain the influence of mass transport polarization and electron hopping on the voltammetric response. The relevance of the catalytic half-wave potential, as a direct measure of the ratio of the rates of redox mediation and enzyme turnover, is stressed. The kinetic analysis is applied to the electrocatalytic behavior of taurine-modified horseradish peroxidase, entrapped within a polyvinyl pyridine polymer containing osmium redox centers. This integrated electrochemical system is shown to be characterized by an efficient electronic connection between the catalytic and mediator centers, easy permeation of the substrate through the film, and a low value of the enzyme-substrate Michaelis constant. A sensitivity 20% higher than the maximum value previously reported in the literature for polymer-based peroxidase electrodes is obtained, and it appears to be related to a stronger electrostatic interaction between the negatively charged taurine modified HRP and the positively charged redox polymer. A comparison with kinetic parameters obtained in homogeneous solution (J. Am. Chem. Soc. 2002, 124, 240) suggests that further improvement of this electrode configuration would require a higher fraction of the immobilized enzymes to be effectively connected to the redox network.