Bioinspired Mineralization and Assembly Route to Integrate TiO2 and Carbon Nitride Nanostructures: Designing Microstructures for Photoregeneration of NADH

A polyamine-mediated assembly route to mineralization and assembly of nanoparticles to generate titania (TiO2)-based hollow spheres (HS) is demonstrated. In this bioinspired strategy, poly(allylamine) hydrochloride (PAH) while mineralizing TiO2 nanoparticles via hydrolysis of titanium(IV) bis(ammonium lactato)dihydroxide (TIBALDH) in an aqueous medium further aids in the assembly of TiO2 nanoparticles along with silica (SiO2) or graphitic carbon nitride (CN) on calcium carbonate (CaCO3) microspheres (acting as a sacrificial template). The assembly method results in mineralized TiO2 with photocatalytic efficacy as verified in the SiO2-TiO2-HS system that instigates it to be further utilized in mimicking the photosystem I type of catalyst. Accordingly, CN-TiO2-HS is designed to be used for the reduction of NAD+ (nicotinamide adenine dinucleotide) to NADH under visible-light irradiation. The designed photocatalytic system circumvents the use of toxic rare metal (ruthenium and iridium) chelated ligands as electron mediators for the regeneration of NADH. It not only shows efficient activity but also facilitates its coupling with an enzymatic reaction involving a horseradish peroxidase-mediated H2O2 reduction, making the whole reaction process cyclic with respect to NADH regeneration. The light-harvesting properties of the hollow structures are illustrated with a 66.3% conversion of NAD+ to NADH, in contrast to a 40.0% conversion using solid structures. The biggest challenge in the NADH regeneration from NAD+ is the selective formation of enzymatically active 1,4-NADH unaccompanied by enzymatically inactive products (e.g., 1,6-NADH, 1,2-NADH, NAD2, and decay products). The 1H nuclear magnetic resonance (NMR) spectroscopic data indicate the selective formation of 1,4-NADH during the photocatalytic regeneration reaction using CN-TiO2-HS as a photocatalyst. This easily adaptable assembly strategy is therefore believed to allow the fabrication of photocatalytic systems with potential applications ranging from environmental remediation to enzymatic reactions and mimicking natural photosystems.