Modelling the steady-state current at the inlaid disc microelectrode for homogeneous mediated enzyme catalysed reactions
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The steady-state currents at an inlaid microdisc electrode have been modelled for a redox mediated enzyme catalysed reaction (such as the glucose/glucose oxidase/ferrocene system) in which all the components are present in homogeneous solution and the reaction of the redox mediator at the electrode is assumed to be either reversible or mass transport limited. The numerical solution for the non-linear system in the axisymmetrical geometry of the inlaid disc is achieved by using the finite element method in an iterative scheme. The resulting concentration and reaction profiles provide useful insight into the process, and show for some parameter values the formation of an almost spherical, sharp reaction layer or shell, whose position can be predicted approximately. Conditions under which the scheme reverts to the pseudo-first-order EC′ mechanism and expressions to interpret the corresponding current are discussed. A simple approximate expression is worked out under the assumption of constant substrate concentration on the electrode surface. This approximate expression is useful in determining a combination of the parameters from the slope of measurements at low substrate concentration. The effect of some parameters is discussed in detail, leading to two suggestions which can improve the analytical performance: (i) use of the lowest mediator concentration compatible with the background current and (ii) use of the largest microelectrode which reaches steady state in a reasonable time. Experimental measurements using ferrocene monocarboxylic acid as the mediator for the glucose/glucose oxidase system agree with the model and confirm the order of magnitude of the previously reported parameter values.