In recent times, significant achievements in the use of zinc oxide (ZnO) nanoparticles (NPs) as delivery vehicles of cancer drugs have been made. The present study is an attempt to explore the key photoinduced dynamics in ZnO NPs upon complexation with a model cancer drug protoporphyrin IX (PP). The nanohybrid has been characterized by FTIR, Raman scattering and UV-Vis absorption spectroscopy. Picosecond-resolved Förster resonance energy transfer (FRET) from the defect mediated emission of ZnO NPs to PP has been used to study the formation of the nanohybrid at the molecular level. Picosecond-resolved fluorescence studies of PP-ZnO nanohybrids reveal efficient electron migration from photoexcited PP to ZnO, eventually enhancing the ROS activity. The dichlorofluorescin (DCFH) oxidation and no oxidation of luminol in PP/PP-ZnO nanohybrids upon green light illumination unravel that the nature of ROS is essentially singlet oxygen rather than superoxide anions. Surface mediated photocatalysis of methylene blue (MB) in an aqueous solution of the nanohybrid has also been investigated. Direct evidence of the role of electron transfer as a key player in enhanced ROS generation from the nanohybrid is also clear from the photocurrent measurement studies. We have also used the nanohybrid in a model photodynamic therapy application in a light sensitized bacteriological culture experiment.
Vitamin B2 has been studied as a conventional antioxidant (in the dark) since its discovery in 1926. The effect of visible light on vitamin B2-containing food has a long history of scientific investigation. Although photodegradation of the vitamin producing several photoproducts is evident in certain experimental conditions, phototoxicity revealing an additional oxidative stress in the medium is also clear from some reports. Here we report the photosensitized antioxidant effect of the vitamin, which is found to be greater than 2 orders of magnitude more efficient than that in the dark condition. The photoinduced antioxidant property is apparently paradoxical compared to the reported phototoxic effect of the vitamin. Our present study unravels a unified picture underlying the difference in character of vitamin B2 under visible light irradiation. UV-vis absorption and fluorescence studies in a number of physiologically relevant nanoscopic environments (micelles and reverse micelles) reveal the antioxidant activity to a well-known oxidative stress marker 2,2-diphenyl-1-picrylhydrazyl (DPPH) as well as a phototoxicity effect resulting in self-degradation of the vitamin. Picosecond-resolved Förster resonance energy transfer (FRET) from the vitamin to the marker DPPH in the biomimetic environments clearly reveals the role of proximity of an oxidizing agent in the photoinduced effect of the vitamin. Our systematic and detailed studies unravel a simple picture of the mechanistic pathway of the photosensitized vitamin in the physiologically important environments leading to the antioxidant/phototoxicity effect of the vitamin. The excited vitamin transfers its electron to the oxidizing agent in proximity for the antioxidant effect, but otherwise it employs oxygen to generate reactive oxygen species (ROS), resulting in phototoxicity/self-degradation.
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