Effects of surface-functionalized TiO₂ nanoparticles on copper ion toxicity under simultaneous and sequential exposure conditions

Most research on the ecotoxicity of titanium dioxide nanoparticles (nano-TiO₂) has focused on the effects on organisms of co-exposure to nano-TiO₂ and other environmental contaminants. However, different harmful pollutants often do not act on organisms at the same time, and organisms are more likely to be exposed to multiple pollutants sequentially. We investigated the toxic effects of three types of surface-functionalized nano-TiO₂ and copper ions (Cu²⁺) on zebrafish embryos under different sequential exposure conditions. The results showed that Cu²⁺ and nano-TiO₂ exposure exerted toxic effects in three ways: oxidative stress, oxidative defense, and oxidative damage. Compared with the co-exposure scenario, the Cu²⁺ pre-exposure scenario resulted in a significantly higher mortality and reactive oxygen species production. In contrast, the nano-TiO₂ pre-exposure scenario decreased the mortality and oxidative damage as well as invalidated the antioxidant defense system. Moreover, the three types of surface-functionalized nano-TiO₂ exerted significantly different effects on Cu bioaccumulation and toxicity. These results indicate that research on the toxicity of nano-TiO₂ should consider both the nature of nano-TiO₂ and the exposure scenarios.