Model of an immobilized enzyme conductimetric urea biosensor

A model for predicting the response of a conductimetric urea biosensor was developed and validated experimentally. The biosensor under consideration is formed by immobilizing the enzyme urease onto the surface of a planar interdigitated electrode array. The enzymatic hydrolysis of urea produces ionic products, such as ammonium and bicarbonate ions, which increase the electrical conductivity of the solution proximal to the electrode array. The model combines an analysis of the diffusive transport and enzymatic hydrolysis of urea in the vicinity of the biosensor surface with an electric fields model for calculating interelectrode impedance. To validate the model, urea biosensors were constructed by immobilizing urease to the interdigit space of microfabricated interdigitated electrodes. The responses of these sensors were investigated in urea solutions prepared in deionized water, at concentrations ranging from 10 μM to 5 mM. Using reasonable estimates for the parameters, the predictions of the model were in good agreement with the experimental data over the entire range of concentrations.