A paradoxical method for NAD+/NADH accumulation on an electrode surface using a hydrophobic ionic liquid

In this communication, we describe a novel and facile method for the immobilization of NAD+/NADH on an electrode surface using a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]). By taking advantage of the insolubility of NAD+/NADH in hydrophobic ionic liquids, it is expected that NAD+/NADH can be retained on the electrode's surface. Alcohol dehydrogenase (ADH) and NAD+/NADH were immobilized with a gelatin hydrogel on an electrode that was modified with an electropolymerized ruthenium complex containing 5-amino-1,10-phenanthroline (pAPRu) as a mediator for NADH oxidation. The (ADH, NAD+)/pAPRu-immobilized electrode exhibited the electrocatalytic oxidation of ethanol in [C4mim][Tf2N]. The obtained catalytic current in [C4mim][Tf2N] was comparable to that in buffer solution containing NAD+. It was confirmed by UV–vis spectroscopy that NAD+ did not dissolve in the [C4mim][Tf2N] and was retained on the electrode's surface. Furthermore, we succeeded in constructing an ethanol/O2 biofuel cell comprised of an (ADH, NAD+)/pAPRu anode and a bilirubin oxidase cathode using [C4mim][Tf2N] as an electrolyte.

► A method for the immobilization of NADH using an ionic liquid is proposed.
► We construct a novel bioanode for an ethanol/O2 biofuel cell.
► A hydrophobic ionic liquid can be used as an electrolyte for a biofuel cell.