SIMULTANEOUS USE OF DEHYDROGENASES AND HEXACYANOFERRATE(III) ION IN ELECTROCHEMICAL BIOSENSORS FOR L-LACTATE, D-LACTATE AND L-GLUTAMATE IONS

The L-lactate, D-lactate and L-glutamate selective amperometric electr ochemical biosensors presented were designed so that the last electron -transfer step is hexacyanoferrate(II) oxidation on a platinum electro de. A single enzyme sensor is described for L-lactate assay, where a l actate dehydrogenase extracted from yeast, immobilized on a membrane, will accept potassium hexacyanoferrate(III) as an electronic relay. It is possible to determine L- and D-lactate using bienzymatic sensors w ith NAD+-dependent dehydrogenases immobilized or in solution. In such a case, a second enzymatic reaction [a diaphorase-catalysed NADH oxida tion by hexacyanoferrate(III)] enabled the detection limit to be lower ed. For the L-glutamate-specific sensor, the two preceding enzymes wer e associated with a third one that catalyses a substrate product trans formation, making it possible to exploit the enzyme amplification phen omenon. In each instance, the required presence of hexacyanoferrate(II I) in the samples to be assayed makes it possible to suggest a simple apparatus with two slightly polarized electrodes. The advantages of en zyme fixation in increasing sensor stability and lowering the detectio n limit are also highlighted.