Factors affecting the electrochemical regeneration of NADH by (2,2′-bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes: Impact on their immobilization onto electrode surfaces

Complexes of the (2,2′-bipyridyl) (pentamethylcyclopentadienyl)-rhodium family ([Cp*Rh(bpy)Cl]+, which is actually hydrolyzed in the form of [Cp*Rh(bpy)H2O]2+ in aqueous medium) are suitable solution-phase mediators likely to regenerate nicotinamide cofactors associated to dehydrogenases involved in many biocatalytic applications. Their practical application as bioelectrocatalysts, e.g., in fine chemicals synthesis or biosensors, remains however restricted to their durable immobilization in an active form onto solid electrode surfaces. This paper reports some new observations on the electrocatalytic properties of this mediator towards NAD+ reduction, notably the critical effect of pH and cofactor-to-mediator concentration ratio, and investigates the behavior of a series of ([Cp*Rh(bpy)Cl]+) derivatives bearing various substituents on the bipyridine ligand in view of their subsequent integration in electrochemical bioreactors. It will be shown that such compounds containing S- or N- moieties (i.e., often used as precursors to functionalize electrode surfaces) lead to inactivation of the electrocatalyst because their interaction with the Rh center prevents the formation of the active rhodium hydride complex. It was thus necessary to find another strategy of immobilization, and we found that adsorption of [Cp*Rh(bpy)Cl]+ by π-stacking on single-walled carbon nanotubes is an effective mean to reach this goal, leading to efficient and stable catalytic responses for NAD+ reduction. Preliminary electroenzymatic experiments in the presence of d-sorbitol dehydrogenase further point out the interest of this approach for bioelectrocatalysis purposes and provide the proof-of-concept for this immobilization strategy.

Research highlights
► [Cp*Rh(bpy)Cl]+) are suitable solution-phase mediators for NADH regeneration but their activity is function of many experimental parameters.
► We report here the effect of pH, cofactor-to-mediator concentration ratio, and substituents on the mediator.
► It is shown that compounds containing S- or N- moieties are inactive electrocatalyst, preventing their immobilization on electrodes.
► We propose an alternative: [Cp*Rh(bpy)Cl]+ adsorption by π-stacking on carbon nanotubes.
► We conclude this is a good way to get efficient and stable catalytic responses for NAD+ reduction.