Immobilized enzyme electrode for the determination of L-lactate in food samples

A sensitive immobilized enzyme electrode for L-lactate is described. Toluidine blue O (TBO) and lactate dehydrogenase (LDH) were co-immobilized onto the surface of a graphite electrode by means of a dialysis membrane. L-lactate is oxidized to pyruvate by LDH in the presence of nicotinamide adenine dinucleotide NAD(+) which is then reduced to NADH. The NADH formed is electrocatalytically oxidized by TBO and the reduced mediator is detected at O V vs. a Ag/AgCl reference electrode. Different dialysis membranes were tested in order to improve stability. The most stable enzyme electrode was fabricated with a benzoylated membrane, showing a half-life of eight days. This amperometric electrode responds fast and linearly to L-lactate in a concentration range 10(-6)-6x10(-5) M. The limit of detection is 4x10(-7) M. The enzyme electrode was applied to the determination of L-lactate in food samples. Results were compared to those obtained using a spectrophotometric method, showing a good agreement.