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The direct electron transfer between covalently immobilized glucose oxidase (EC 1.1.3.4) and a cyanuric chloride modified graphite electrode Is observed by use of differential pulse voltammetry. A well-defined peak at -0.51 V vs. Ag/AgCI, due to the reduction of flavin adenine dinucleotide (FAD), Is obtained which Is 100 mV more positive than the free enzyme. Comparisons to adsorbed FAD and glucose oxidase Indicate that the peak In covalently attached enzyme Is due to the reduction of the prosthetic group as part of the enzyme molecule, rather than FAD adsorbed on the electrode surface. Suggestions for the analytical utility of direct electron transfer at modified electrodes are discussed.

Chemically modified graphite electrodes containing covalently Immobilized xanthine oxidase (E.C. 1.2.3.2) have been employed for the potentlometric and amperometrlc detection of xanthine. Potassium hexacyanoferrate( III) Is used as the

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A novel thermistor enzyme probe (TEP) has been developed for the determination of urea in solution. Response time is 1 min or less. Linear response in urea concentration has been obtained in the 5-30 mM range. Stability and factors influencing the response of the probe are examined. Finally, a design for a modified TEP, useful for small sample volumes, is described.