DNA Damaging Potential of Photoactivated P25 Titanium Dioxide Nanoparticles

Titanium dioxide nanoparticles (TiO₂ NPs) are found in numerous commercial and personal care products. Thus, it is necessary to understand and characterize their potential environmental health and safety risks. It is well-known that photoactivated TiO₂ NPs in aerated aqueous solutions can generate highly reactive hydroxyl radicals (^•OH), which can damage DNA. Surprisingly, recent <i>in vitro</i> studies utilizing the comet assay have shown that nonphotoactivated TiO₂ NPs kept in the dark can also induce DNA damage. In this work, we utilize stable isotope-dilution gas chroma­tography/tandem mass spectrometry to quantitatively characterize the levels and types of oxidatively generated base lesions in genomic DNA exposed to NIST Standard Reference Material TiO₂ NPs (Degussa P25) under precisely controlled illumination conditions. We show that DNA samples incubated in the dark for 24 h with TiO₂ NPs (0.5–50 μg/mL) do not lead to the formation of base lesions. However, when the same DNA is exposed to either visible light from 400 to 800 nm (energy dose of ∼14.5 kJ/m²) for 24 h or UVA light at 370 nm for 30 min (energy dose of ∼10 kJ/m²), there is a significant formation of lesions at the 50 μg/mL dose for the visible light exposure and a significant formation of lesions at the 5 and 50 μg/mL doses for the UVA light exposure. These findings suggest that commercial P25 TiO₂ NPs do not have an inherent capacity to oxidatively damage DNA bases in the absence of sufficient photoactivation; however, TiO₂ NPs exposed to electromagnetic radiation within the visible portion of the light spectrum can induce the formation of DNA lesions. On the basis of these findings, comet assay processing of cells exposed to TiO₂ should be performed in the dark to minimize potential artifacts from laboratory light