Generation of superoxide anion and most likely singlet oxygen in irradiated TiO2 nanoparticles modified by carotenoids

Photosensitization of the TiO2 nanoparticles with carotenoids leads to the formation of superoxide anion and singlet oxygen on red light irradiation. It has been shown that carotenoids facilitate generation of superoxide radical anion O2− in irradiated TiO2 colloids. At low carotenoid concentrations (<3×10−5 M) the rate constant of O2− production exceeds that obtained in the absence of carotenoid. periments with 2,2,6,6-tetramethyl-4-piperidone and spin trapping with α-phenyl-N-tert-butyl nitrone and 5,5-dimethyl-pyrroline-N-oxide showed that both superoxide anion and singlet oxygen can be produced in irradiated TiO2 suspensions in toluene and CH2Cl2. The PBN–O2− and DMPO–O2− spin adducts were detected by the 9 GHz EPR/ENDOR spin trapping technique. The dismutation of superoxide radical anion was proposed to generate singlet oxygen through intermediate formation of HO2 radicals. The PBN–HO2 spin adduct was identified by the 9 GHz EPR/ENDOR and 95 GHz EPR spin trapping technique.