Aging effects of titanium dioxide on Cu toxicity to Daphnia magna: Exploring molecular docking and significance of surface properties

Cosmetics and personal care products containing titanium dioxide nanoparticles (TiO₂ NPs) may enter aquatic environments, where the surface coatings of TiO₂ NPs may change with aging due to environmental factors such as light, and potentially affect their bioaccumulation and toxicity. This study examined how aging impacted the physicochemical properties of three commercially available TiO₂ NPs and subsequent influence on the bioaccumulation and toxicity of copper (Cu) in Daphnia magna (D. magna). We demonstrated that aging significantly affected the hydrophobicity of TiO₂ NPs, which affected their binding to water molecules and adsorption of Cu. Changes of bioaccumulation of TiO₂ NPs and Cu in D. magna ultimately affected the activities of intracellular antioxidant enzymes such as SOD, CAT, GSH-Px, and the transmembrane protein Na⁺/K⁺–ATPase. Molecular docking calculations demonstrated that changes of activities of these biological enzymes were due to the interaction between TiO₂ NPs, Cu, and amino acid residues near the sites with the lowest binding energy and active center of the enzyme. Such effect was closely related to the hydrophobicity of TiO₂ NPs. Our study demonstrated the close relationship between surface properties of TiO₂ NPs and their biological effects, providing important evidence for understanding the behavior of nanomaterials in aquatic environments.