Amperometric l-lactate-sensing electrode based on a polyion complex layer containing lactate oxidase. Application to serum and milk samples

An amperometric enzyme electrode for l-lactic acid was prepared by immobilizing lactate oxidase (LOD) in a polyion complex membrane. An aqueous solution containing poly-l-lysine and LOD was placed on a glassy carbon electrode, and an aqueous solution of poly(4-styrenesulfonate) was added to the polycation/LOD mixture and dried. The anodic current (at 1 V vs. AgAgCl) of this enzyme electrode increased immediately after the addition of L-lactic acid and the response time was < 5 s. A linear response to l-lactic acid was observed up to 0.3 mM with a detection limit of 0.1 μM. The enzyme electrode was applied to the determination of l-lactic acid in sera and sour milk: the polyion complex membrane showed permselectivity based on the solute size with the molecular weight cut-off of ca. 100, which was very effective in suppressing the electrochemical interference by l-ascorbic acid, uric acid and acetaminophen. The electrode could be used for at least two months.