Direct electrochemistry of lactate dehydrogenase in aqueous solution system containing L(+)-lactic acid, β-nicotinamide adenine dinucleotide, and its reduced form

Using lactate dehydrogenase (LDH) for lactate detection in biosensors is fascinating but still challenging because in-situ bioelectrochemical sensors are still at the research stage due to the lack of reliable information about the direct electrochemistry of systems not involving redox mediators. To provide reliable information, the direct electrochemistry of aqueous solution systems containing L(+)-lactic acid (Lac), β-nicotinamide adenine dinucleotide (NAD⁺) and its reduced form (NADH) is investigated using (i) free-standing LDH and (ii) LDH immobilized on carbon nanotubes (CNT). Results show that there is one peak for NADH oxidation and one peak for the NAD⁺ reduction while the conversion of lactate is observed in the aqueous solution. In addition, the peak for NADH oxidation may disappear or be shifted irreversibly. As a result, the detection of lactate relies on the type of solution. The conventional mechanisms explaining these phenomena are too superficial and hard to interpret, thus, in this work, we suggest a new interpretation method explaining the empirical behavior of lactate detection. In this prospect, the role of chemical equilibrium related to the conversion of lactate into pyruvate that is catalyzed by LDH is important and this is sequentially elucidated, while the rate-determining step for lactate detection is suggested.