Sensitization of an Endogenous Photosensitizer: ElectronicSpectroscopy of Riboflavin in the Proximity of Semiconductor, Insulator,and Metal Nanoparticles.

Riboflavin (Rf) is a class of importantvitamins (Vitamin B2) and a well-known antioxidant. Herewe have synthesized nanohybridsof Rf with a number of inorganic nanoparticles (NPs); namely zincoxide (ZnO), titanium oxide (TiO2), aluminum oxide (Al2O3) and gold NPs of similar sizes. While high resolutiontransmission electron microscopy (HRTEM) confirms integrity and sizesof the NPs, intactness of the molecular structure of the drug Rf isrevealed from absorption and steady-state emission spectra of thedrug in the nanohybrid. Raman spectroscopy on the nanohybrids showsthe nature of molecular complexation of the drug with the inorganicNPs. For the semiconductor and insulator NPs, the complexation isfound to be noncovalent, however, a covalent attachment of the drugwith the dangling bonds of metal atoms at the surface is observed.In order to investigate antioxidant activity of the nanohybrids, wehave performed 2, 2-diphenyl-1-picrylhydrazyl (DPPH) assay of thenanohybrids in dark as well as under blue light irradiation. Whereaschange of the antioxidant activity of the nanohybrids with respectto free riboflavin in the absence of light is observed to be insignificant,a drastic change in the activity in the case of TiO2andZnO in the presence of light is evident. No change in the case ofAl2O3and a significant decrease in the antioxidantactivity for gold nanohybrids are also remarkable. Picosecond-resolvedfluorescence studies on the nanohybrids reveal a molecular pictureof the differential antioxidant activities. An ultrafast photoinducedelectron transfer from Rf to ZnO and TiO2are clearly evidentfrom the corresponding fluorescence transients. We have compared thepicosecond-resolved transients with that of Rf in the presence ofa well-known electron acceptor benzoquinone (BQ) and found similartime scales. No temporal change in the fluorescence transient of riboflavinin Al2O3nanohybrids compared to that of freeRf is observed indicating uneventful excited state relaxation of thenanohybrids. Nanosurface energy transfer (NSET) over Försterresonance energy transfer (FRET) is found to be the prevailing de-excitationmechanism in the case of gold nanohybrids, because of the strong spectraloverlap between Rf emission and surface plasmon absorption of thegold NPs. Different excited state mechanisms as revealed from ourstudies are expected to be useful for the design of NP-sensitizeddrugs, which are reported sparsely in the literature. [ABSTRACT FROM AUTHOR]