KINETIC-STUDIES ON MEMBRANELESS AMPEROMETRIC BIOSENSORS PREPARED FROMXANTHINE-OXIDASE, ORGANIC CONDUCTING SALT, AND SILICONE OIL

Membrane-free amperometric biosensors have been prepared from xanthine oxidase powder (XOD) (E.C. 1.2-3.2), the organic conducting salt TTF- TCNQ, and silicone oil. The sensor response to hypoxanthine can be des cribed by the enzyme-kinetic Michaelis-Menten formalism with good corr elation over a wide range of potential (-100 to +300 mV vs. Ag/AgCl) a nd pH (6.10 to 8.80) values. Measurements with rotating disk electrode s reveal that XOD is oxidized by at least two mediator species in a ho mogeneous mechanism. Electron transfer to oxygen is apparently slower than transfer to the mediators so that the sensor can be operated in a ir-saturated solutions. Since open circuit conditions alter the kineti c parameters of the sensor. operation in flow analysis systems is reco mmended. Analytical features including response time (t95 almost-equal -to 10 seconds), detection limit (ca. 10 nM hypoxanthine), selectivity over interferents (e.g., ascorbate), and simplicity of preparation as well as operation and storage stability compare very favorably with o ther XOD biosensors.