Reactive oxygen species produced upon photoexcitation of sunscreens containing titanium dioxide (an EPR study)

Commercial sunscreen products containing titanium dioxide were irradiated with λ > 300 nm and the formation of oxygen- (OH, O2-/OOH) and carbon-centered radicals was monitored by EPR spectroscopy and spin trapping technique using 5,5-dimethyl-1-pyrroline N-oxide, α-phenyl-N-tert-butylnitrone (PBN), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone as spin traps, and free nitroxide radical 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl. The photoinduced production of singlet oxygen was shown by 4-hydroxy-2,2,6,6-piperidine. The generation of reactive oxygen radical species upon irradiation of sunscreens significantly depends on their composition, as the additives present (antioxidants, radical-scavengers, solvents) can transform the reactive radicals formed to less harmful products. The continuous in situ irradiation of titanium dioxide powder, recommended for cosmetic application, investigated in different solvents (water, dimethyl sulfoxide, isopropyl myristate) resulted in the generation of oxygen-centered reactive radical species (superoxide anion radical, hydroxyl and alkoxyl radicals).