Photo-induced toxicity of titanium dioxide nanoparticles to Daphnia magna under natural sunlight

Titanium dioxide nanoparticles (TiO₂ NP) are one of the most abundantly utilized nanoparticles in the world. Studies have demonstrated the ability of the anatase crystal of TiO₂ NP to produce reactive oxygen species (ROS) in the presence of ultraviolet radiation (UVR), a co-exposure likely to occur in aquatic ecosystems. The goal of this study was to examine the photo-induced toxicity of anatase TiO₂ NP under natural sunlight to Daphnia magna. D. magna were exposed to a range of UVR intensities and anatase TiO₂ concentrations in an outdoor exposure system using the sun as the source of UVR. Different UVR intensities were achieved using UVR opaque and transparent plastics. AnataseTiO₂-NP demonstrated the reciprocal relationship seen in other phototoxic compounds such as polycyclic aromatic hydrocarbons (PAHs) at higher UVR treatments. The calculated 8h LC₅₀ of anatase TiO₂ NP was 139ppb under full intensity ambient natural sunlight, 778ppb under 50% natural sunlight, and >500ppm under 10% natural sunlight. Mortality was also compared between animals allowed to accumulate a body burden of anatase TiO₂ for 1h and organisms whose first exposure to anatase TiO₂ aqueous suspensions occurred under UVR. A significantly greater toxic effect was observed in aqueous, low body burden suspensions than that of TiO₂ 1h body burdens, which is dissimilar from the model presented in PAHs. Anatase TiO₂ presents a unique photo-induced toxic model that is different than that of established phototoxic compounds.