Early instability of MIL-125-NH2 in aqueous solution and mediation of the visible photogeneration of an NADH cofactor

Abstract

Metal organic frameworks (MOFs) based on titanium and carboxylic acid derivatives have been recently proposed as mediators of multiple photoinduced processes in substitution of traditional semiconductor materials. In the case of MIL-125-NH₂, the introduction of the amino group extends its photoactivity to the visible region with respect to MIL-125, but it is also responsible for reducing its hydrostability, which strongly limits its application in aqueous media. Here, it is shown that the destabilization of the frame by hydrolysis and dissolution processes can be controlled and minimized if the medium is regulated to pH 7 or lower. By taking into account these considerations, we explored the use of MIL-125-NH₂ as a mediator in the visible photogeneration of reduced nicotinamide adenine dinucleotide (NADH) by using triethanolamine (TEOA) as an electronic donor in environments where the framework structure is not compromised. It was found that the concentration of NADH produced depends on the pH of the medium, since the degree of protonation of the donor and the surface charge of the particles are markedly affected in the working range. The maximum yield was obtained in neutral conditions (pH 7) provided by a buffer of cationic nature. Thus, MIL-125-NH₂ is envisaged as a potential suitable support for photochemical activation of NAD-dependent enzymes operating in aqueous neutral media.