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Abstract
Publisher Summary Titanium dioxide (TiO 2 ) and zinc oxide (ZnO) are photo-semiconducting catalysts used in the oxidation of various organic compounds. Spin-trapping studies using electron paramagnetic resonance (EPR) spectroscopy have demonstrated the formation of active oxygen species on ultraviolet (UV) irradiation: hydroxyl radicals from TiO 2 and superoxide anion radicals or protonated perhydroxyl radicals from TiO 2 and ZnO. The photochemical generation of hydrogen peroxide and singlet oxygen by irradiation of TiO 2 and ZnO has also been reported. The cytotoxicity of irradiated TiO 2 to prokaryotic and eukaryotic cells TM has been ascribed to the production of these active oxygen species, especially to that of highly reactive hydroxyl radicals. Despite their known photoreactivity, TiO 2 and ZnO have been widely used in sunscreening and cosmetic products without complication. Given that studies have shown that singlet oxygen causes the oxidation of nitrone spin-trapping reagents to yield apparent hydroxyl radical adducts, this chapter reexamines the photochemical production of active oxygen species from irradiated TiO 2 and ZnO. It discusses the formation of singlet oxygen by examination of the oxidation products of methyl oleate and 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) and compared the rate of oxidation of uric acid and 2,6-di- tert -butyl-4-methylphenol (butylated hydroxytoluene, BHT).
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