THE POLYCRYSTALLINE FLUORIDE ION-SELECTIVE ELECTRODE AND HORSERADISH-PEROXIDASE - AN ALTERNATIVE ELECTROCHEMICAL BIOSENSOR

Optimization of the enzymatic cleavage of fluoride ion from 4-fluorophenol by horseradish peroxidase, in the presence of hydrogen peroxide, has demonstrated a maximum release of 7.7-mu-mol F-1 l-1 s-1, with a Michaelis-Menten constant (K(m)) of 1.1 mmol l-1 and a turnover number of 1.2 x 10(5). The role of bovine serum albumin (1 g l-1) in the reaction was variable and related to substrate concentrations. At 80 mmol l-1 4-fluorophenol and 14 mmol l-1 H2O2 a 46% decrease in fluoride ion release was observed (7.50 to 4.09-mu-mol l-1 s-1); a reduction of the H2O2 concentration to 9 mmol l-1 demonstrated an identical 46% fall (6.50 to 3.50-mu-mol l-1 s-1). Effects on the shape of the reaction curves were also noted. Increasing the 4-fluorophenol concentration to 100 mmol l-1 with the 1 mmol l-1 H2O2 showed no effect on the rate of fluoride ion released; at 200 mmol l-1 4-fluorophenol and 5 mmol l-1 H2O2 a 37% increase in fluoride ion released was found (3.45 to 4.72-mu-mol l-1 s-1) together with an apparent removal of inhibition of the enzymatic reaction. The mechanism involved may be related to free radical scavenging, and complementary experiments involving glutathione and other plasma proteins support this hypothesis. The development of polycrystalline fluoride disposable sensors has resulted in the production of reusable sintered LaF3/CaF2 sensors.