Dependence of the irradiation conditions and crystalline phases of TiO₂ nanoparticles on their toxicity to Daphnia magna

The widespread proliferation of titanium dioxide (TiO₂) and its inevitable release into aquatic environments have led to growing concerns about its hazards to the environment. However, knowledge regarding the influence of irradiation and crystalline phases of TiO₂ nanoparticles (NPs) on their mechanisms of toxicity is lacking. This study systematically evaluated the toxicity of five types of TiO₂ NPs with varying percentages of their crystalline forms under three different irradiation conditions. The 50% effective concentrations (EC₅₀) of all the anatase/rutile mixtures illuminated with ultraviolet (UV) light were lower than those of their counterparts under visible light or in the dark. Among the mixtures, the 4 : 1 anatase/rutile mixture yielded the lowest EC₅₀ of 3.58 mg L⁻¹ under UV illumination, likely due to the higher level of bioaccumulation and oxidative stress, as quantified by reactive oxygen species (ROS) and superoxide dismutase enzyme (SOD). TiO₂ NPs under UV illumination showed higher photocatalytic activity towards ROS production due to the stronger absorption in the corresponding wavelengths, leading to higher oxidative stress. The anatase/rutile mixture in a ratio of 4 : 1 displayed an appropriate band gap to generate excitons related to oxidative stress, which induced high toxicity. Our study suggested that the oxidative stress-related toxicity of TiO₂ NPs to daphnids may be significantly related to the band gap of exciton generation.