A BIENZYME-POLY-(O-PHENYLENEDIAMINE)-MODIFIED CARBON-PASTE ELECTRODE FOR THE AMPEROMETRIC DETECTION OF L-LACTATE

A reagentless lactate amperometric biosensor, constructed by immobilis ing the enzymes glutamic pyruvic transaminase (GPT) and L-lactate dehy drogenase (LDH) together with its cofactor nicotinamide adenine dinucl eotide (NAD(+)) in carbon paste using an electropolymerised poly(o-phe nylenediamine) (PPD) film, is described. The response of the electrode is based on the electrocatalytic oxidation, at low applied potentials , of the enzymatically produced NADH by the PPD conducting film. This sensor construction was optimised, the final biosensor contained 80 un its of LDH, 5 units of GPT and 0.1 mg of NAD(+) mg(-1) of carbon paste . Interference by direct electrochemical oxidation of easily oxidizabl e substances such as uric acid, L-ascorbic acid, L-cysteine, glutathio ne and paracetamol, are drastically reduced by covering the PPD-modifi ed electrode with a second electrochemically synthesized, non-conducti ng poly(o-aminophenol) film. The bienzyme-double polymeric layer-modif ied electrode at an applied potential of 0 V (vs. Ag/AgCl), gives a li near response for lactate in the range 6x10(-7) M8.5x10-5 M and a resp onse time of 80 s. This sensor was used for the quantification of lact ic acid in cider. Results compared favourably with a standard spectrop hotometric method.