Amperometric enzyme electrodes

Abstract A complete theoretical treatment is presented for an enzyme electrode where electron transfer from the enzyme to the electrode is achieved by a mediator reacting in homogeneous solution. The second-order differential equations describing the transport and kinetics of the enzyme and of the mediator in the diffusion layer of the electrode are solved for a number of different cases. Seven different cases are found and 11 equations describing all the boundaries between the cases are derived; concentration profiles are presented for each of the cases. The behaviour of the system is a function of three parameters and the self-consistency of the treatment is demonstrated by the fact that each equation that describes the interface between two cases can be simplified into two results for the two cases on either side of the interface. The relation of the three parameters to three convenient experimental variables, the concentrations of enzyme and substrate and the rotation speed of a rotating disc electrode, is discussed. Case diagrams in three alternative parameters, each of which is a linear function of one of the three experimental variables, are presented. Six out of the seven cases each have a unique set of orders with respect to the three experimental variables. Furthermore, the pattern of variation from case to case as the three-dimensional space of the case block is explored will aid diagnosis and testing of the model.