An optical biosensor is presented for continuous determination of uric acid. The scheme is based on the measurement of the consumption of oxygen during the oxidation of uric acid that is catalyzed by the enzyme uricase. The enzyme is immobilized in a polyurethane hydrogel next to a metal-organic probe whose fluorescence is quenched by oxygen. The consumption of oxygen was followed by measurement of changes of luminescence intensity of two kind of probes and can be related to the concentration of uric acid. Analytical ranges (0-2 mM), the response times (80-100 s), reproducibility, and long-term stability were investigated. The biosensors; are stable for at least 1 month and are not interfered by common interferents. One kind of biosensor was applied to the determination of uric acid in human blood serum. The results agree with those of a commercial colorimetric detection kit. (C) 2008 Elsevier B.V. All rights reserved.