NOVEL, REAGENTLESS, AMPEROMETRIC BIOSENSOR FOR URIC-ACID BASED ON A CHEMICALLY-MODIFIED SCREEN-PRINTED CARBON ELECTRODE COATED WITH CELLULOSE-ACETATE AND URICASE

Amperometry in stirred solution has been used for the systematic evaluation of modified screen-printed carbon electrodes (SPCEs) with a view to developing a reagentless biosensor for uric acid. The developed system consists of a base cobalt phthalocyanine (CoPC) electrode tailored to the electrocatalytic oxidation of H2O2 by means of a cellulose acetate (CA)-uricase bilayer. Uricase was immobilized by drop-coating the enzyme onto the CA membrane covering the CoPC-SPCE. The de,ice exploits the near-universal H2O2-generating propensity of oxidases, the permselectivity of the CA film towards H2O2 and the electrocatalytic oxidation of this product at the CoPC-SPCE. The electrochemical oxidation of the resulting Co+ species was used as the analytical signal, facilitating the application of a greatly reduced operating potential when compared with that required for direct oxidation of H2O2 at unmodified electrodes. The time required to achieve 95% of the steady-state current (t(95)i(ss) was 41 s [relative standard deviation = 7.5% (n = 10)]. Amperometric calibrations were linear over the range from 13 X 10(-6) to 1 X 10(-3) mol dm(-3), with the former representing the limit of detection. The CA membrane extended the linear range of the biosensor by over two orders of magnitude, when apparent Michaelis-Menten constants (K-m') of immobilized and free enzymes are compared. This suggests that the process is diffusion-controlled and not governed by the kinetics of the enzyme. The precision of electrode fabrication was determined by cyclic voltammetry to be 4.9% (n = 6). Successive amperometric calibrations (n = 7) over 7 d using a single sensor revealed only a 14.0% diminution in sensitivity from the original response. Sensor stability and the dependence of the steady-state current on the pH, ionic strength and temperature of the supporting electrolyte were studied and the results are presented. The functioning of the biosensor is indifferent to a wide range of potential interferences studied in a synthetic sample and results correlate favourably with those of a standard hospital method.