Effect of pH on the catalytic electrooxidation of NADH using different two-electron mediators immobilised on zirconium phosphate

A detailed study is reported on the electrocatalytic oxidation of β-nicotinamide adenine dinucleotide at three different carbon paste electrodes modified with redox mediators commonly used in bioelectrochemistry, viz. Meldola blue, methylene green and riboflavin, adsorbed on zirconium phosphate. Cyclic voltammetric investigations of these chemically modified electrodes performed in aqueous tris buffer solutions at different pHs allowed us to conclude that the immobilised redox species presented very stable redox properties, with midpoint potentials (E_m, ?125, ?40, and ?140 mV versus SCE for Meldola blue, methylene green and riboflavin, respectively) virtually independent of the pH of the contacting solution. The second-order rate constant for electrocatalytic oxidation of NADH as well as its pH dependence (from 6 to 8) were evaluated from rotating disk electrode (RDE) experiments, using the Koutecky–Levich approach.