CARBON-PASTE ELECTRODES WITH INCORPORATED LACTATE OXIDASE AND MEDIATORS

Amperometric bioelectrodes based on carbon paste modified with L-lactate oxidase and mediators are described which are sensitive to L-lactate. Several ferrocenes and phenoxazine derivatives as mediators are evaluated, with Meldola blue exhibiting the best results. With ferrocene derivatives the bioelectrodes were operated at potential of 0.2-0.4 V (vs SCE). The apparent Michaelis constants for the calibration curves of the bioelectrodes are different depending on the chosen mediator and varied in the range from 0.9 to 3.6 mM L-lactate. The pH optimum in the case of the 1,1'-dimethylferrocene bioelectrode was 9.3 and the limiting sensitivity was determined to be 31-mu-A/mM cm-2. Covered by a Nafion membrane, the electrode sensitivity was in the range of 3.2-4.4-mu-A/mM cm-2 at pH 7.0 and the calibration range was linear up to 2-2.5 mM L-lactate. The bioelectrode based on Meldola blue mediated electron transfer at potentials between 0.05-0.25 V. The K(m)(app) was 17.0 and 18.2 mM at 0.1 and 0.05 V, respectively. The pH optimum of the bioelectrodes was 7.9 and limiting sensitivity - 164-mu-A/mM cm-2. At normal physiological level of ascorbic acid (50-mu-M) in blood the response of the Meldola blue based sensors was 5.3 % at 0.1 V, whereas that of the 1,1'-dimethylferrocene bioelectrode was 83 % at 0.25 V in comparison to the response observed with physiological levels of L-lactate (2 mM).