ELSEVIER Sensors and Actuators B 21 (1994) 205-208 Urea CHEMICAL biosensor based on ammonia gas-sensitive Pt/GaAs Schottky diode L.M. Lechuga, G. Mer, A. Calle, D. Golmayo, F. Brlones Centro National de MmveIectr6nrca (CSIC), Ci Semmo, 144, 28006 Madnd, Spun Recewed 9 May 1993, tn remed form 15 March 1994, accepted 25 March 1994 Abstract Pt/n-GaAs Schottky diodes with dlscontmuous catalytx metal are sensltlve detectors of ammoma gas m a wide temperature range mcludmg room temperature The combmatlon of such devices wth lmmobxhzed urease allows sensitive lxosensors to be developed, whxh determine the urea concentration by means of the ammoma release m the enzymesatalysed reactson In this paper we report urea determmatlons m the concentration range 0 4-10 mM Kqwords Ammonra sensor, Capaatance, Polyethemmde, Urease, Urea 1. Introduction Blosensors are small analytical devices that combme a transducer wth a blologlcally active substance There IS an mcreasmg demand for the development of mm- latunzed and low-cost blosensors that enable rapld and sensltlve detection of a wide range of environmental pollutants, and can be used on line to assess the impact of an increasing number of chemicals on the quality of atmospheric, aquatic and sod environments [l-4] In a blotechnologxal process, high yield can be achieved by control mvolvmg on-line determmatlon of a multitude of parameters In a fermentation process, e g, fabn- cation of antlblotlcs, a defined hme course of the concentrations of nutrients such as carbohydrates, ammo acids, phosphates and ammonmm has to be stnctly followed A knowledge of the product concentration permits a direct evaluation of the state of the bloprocess Also m the medical field there 1s currently great interest m the development of blochemlcally selective sensors that can be used to monitor the concentrations of several blologxal constituents m body-flmd samples P-71 The rapid development of solid-state transducers over the past two decades has been brought about m part by the apphcatlon of integrated-arcult technologies to fabricate preclslon nucrostructures and devices [8] These systems are being used m a number of apphcatlon areas, mcludmg transportation, robotics, mdustrlal man- ufacturing, consumer electronics and health care The apphcatlon of integrated-clrcmt techniques allows hlgh- 0925-4005/94/$07 00 Q 1994 Elsewer Saence S A All nghts reserved SSDI 0925-4005(94)01244-C yield fabrication of these structures with small size, precise dnnennons and reproducible physical charac- tenstics The prmclple of the enzyme sensor 1s simple An enzyme 1s chosen that acts, either specdically or highly selectively, v&h the compound to be assayed [9] Many blosubstances are very good catalysts and both the selectlvlty and sensltlvlty of blosensors are frequently substantially better than for other sensor systems The major disadvantage of blosensors 1s often the limited stab&y of the blosubstances m an environment different from their natural one Enzymes are blocatalysts m- volved m the performance of metabohc reactions In blosensors, enzymes specifically recogmze their sub- strates and produce secondary chermcal signals that can be recognized by suitable transducers On the basis of the stomhlometry, either a product or a reactant of the catalysed reaction can be determmed The determination of urea in blood 1s an important test m clmlcal analysis Urea can be degraded through the enzymatic reaction W,N),CO UreaSe ) 2 NH,+ +HCO,- I 2 NH, + H,O CO, + H,O Ammonia gas produced by the blochemlcal reaction can be detected by an NH, gas sensor In 1984 Wmqulst