An amperometric bi-enzyme sensor for determination of formate using cofactor regeneration

A biosensor for detection of formate at submicromolar concentrations has been developed by co-immobilizing formate dehydrogenase (FDH, E.C. 1.2.1.2), salicylate hydroxylase (SHL, E.C. 1.14.13.1) and NAD+ linked to polyethylene glycol (PEG-NAD+) in a poly(vinyl alcohol) (PVA) matrix in front of a Clark-electrode. The principle of the bi-enzyme scheme is as follows: formate dehydrogenase converts formate into carbon dioxide using PEG-NAD+. Corresponding PEG-NADH produced is then oxidized to PEG-NAD+ by salicylate hydroxylase using sodium salicylate and oxygen. The oxygen consumption is monitored with the Clark-electrode. The advantages of this biosensor approach are the effective re-oxidation of PEG-NADH, and the entrapment of PEG-NAD+ resulting in avoiding the addition of expensive cofactor to the working medium for each measurement. This bi-enzyme sensor has achieved a linear range of 1–300 μM and a detection limit of 1.98×10−7 M for formate (S/N=3), with the response time of 4 min. The working stability is limited to 7 days due to the inactivation of the enzymes. Only sodium salicylate was needed in milli-molar amounts.