Two-electron transfer photoreduction of methyl viologen and perfluorooctanoic acid mediated by flavin mononucleotide at colloidal titanium dioxide interfaces

Abstract

Flavin mononucleotide (FMN) in methanol was photochemically reduced to FMNH₂, which undergoes electron transfer with Methyl viologen (MV²⁺) in ethanol. This charge transfer interaction results in the reduction of MV²⁺ into MV⁺˙, as well as the storage of electrons. The spectroscopic experiments allow for the elucidation of quantitative electron transfer into MV²⁺. To study the redox potential for FMN species, the HOMO and LUMO energies were calculated using density functional theory (DFT) method on the B3LYP level with a basis set of 6-311G(d,p). The HOMO and LUMO energy calculations show that the redox state of FMNH₂ is energetically more favorable than FMNH˙ for MV²⁺ reduction. These studies revealed good agreement between the experimental results and computational predictions. Moreover, molecular electrostatic potential (MEP) was performed on the FMNH₂-MV²⁺ complex to determine the site of electron transfer, and the results were compared to the experimental results. FMN functionalized to the surface of colloidal titanium dioxide nanoparticles (TiO₂ NPs) showed enhanced reactivity toward the reduction of perfluorooctanoic acid (PFOA). The two-electron transfer reduction process is generated by irradiating FMN attached to TiO₂, during which FMN reduces to FMNH₂. The FMNH₂ then reacts with PFOA to produce shorter chain structures with fewer fluorine atoms, such as C₅F₁₁COOH. The results indicate that the rational organization of FMNH₂/TiO₂ to mediate two-electron transfer is effective for the reductive degradation and remediation of PFOA.