Surface-modified electrodes for NADH oxidation in oxidoreductase-catalysed synthesis

Three surface-modified electrodes were investigated: 3,4-dihydroxybenzaldehyde electropolymerized on glassy carbon, hexacyanoferrate modified nickel electrodes, Meldola Blue adsorbed on graphite, for application to NADH oxidation in oxidoreductase-catalysed synthesis. The high overpotential, which is generally required for NADH oxidation on clean electrodes, was perfectly overcome with the three electrodes and especially with the Meldola blue modified graphite electrode. This electrode allowed fast NADH oxidation at potentials as low as 0.00 V vs SCE. It also ensured good storage stability. It is stressed that the key problem in applying this electrode to synthesis is the decrease in its electrocatalytic properties under operating synthesis conditions. An operating procedure was established that allowed ageing of the electrode to be directly linked to the current measurement. Following this procedure, several methods were investigated for improving the half-life of the electrode: pulse electrolysis, replacing NADH by NAD(+) in the initial solution, lowering the NAD(+) concentration and using dextran-bound NAD(+). In the best conditions, the half-life reached 152 min instead of 34 min with the initial conditions, proving that fouling by anodically produced NAD(+) was first of all identified as a very important cause of electrode ageing. Modification of intrinsic surface properties of the electrode also led to the decrease of the interface efficiency.