Optimization of a photoregeneration system for NADH using pristine TiO₂ as a catalyst

Coenzymes are essential participants in the oxidoreductase-catalyzed reactions, but the stoichiometric consumption and high cost limit their applications. In this study, the reduced nicotinamide adenine dinucleotide (NADH) was regenerated with high efficiency via UV photocatalysis using original TiO₂ nanoparticles as the catalyst, disodium salt of ethylenediamine tetraacetic acid (EDTA) as the electron donor and [Cp*Rh(bpy)(H₂O)]²⁺ as the mediator. When the loading amount of TiO₂ was increased, the initial reaction rate first increased and subsequently remained almost unchanged. In the presence of 1.5 mg/mL TiO₂, the NADH yield reached approximately 90% after 30 min of irradiation. Several electron donors were investigated. Disodium and tetrasodium salt of EDTA were demonstrated to be superior to triethanolamine, CH₃OH, trisodium nitrilotriacetate monohydrate and H₂O under appropriate pH conditions. The optimal concentration for disodium EDTA and rhodium mediator was 0.15 and 0.25 M, respectively, and the preferred molar ratio of NAD⁺ to TiO₂ and [Cp*Rh(bpy)(H₂O)]²⁺ was 1:0.1:0.25. Regenerative NADH was successfully applied in the enzymatic reduction of formaldehyde to methanol with a high conversion percentage (up to 95% after several minutes), which was notably close to fresh NADH. This work provides an alternative strategy to catalyst modification for the high-efficiency photoregeneration of coenzyme by optimizing the regeneration system.