Combined toxicity of nano-CuO/nano-TiO₂ and CuSO₄/nano-TiO₂ on Escherichia coli in aquatic environments under dark and light conditions

Copper(II) oxide nanoparticles (n-CuO) and copper(II) sulfate (CuSO₄), commonly used in industrial and agricultural applications, are released in substantial amounts to the environment where they may then interact in unexpected ways with other materials such as engineered nanomaterials (ENM). In this study, we assessed the combined effects of their interaction with titanium(IV) dioxide nanoparticles (n-TiO₂), a widely used ENM, on the ATP levels and cell membrane integrity of Escherichia coli in a natural aqueous medium (Lake Michigan water). Under dark conditions, we observed that n-TiO₂ addition had negligible effects on copper toxicity, except in the case of higher doses of CuSO₄ where n-TiO₂ attenuated bacterial stress effects presumably by the adsorption of copper ions at the n-TiO₂ surface. However, an opposite effect was observed under exposure to simulated solar irradiation (SSI) with synergistic stress occurring in mixtures of n-TiO₂ with either CuSO₄ or n-CuO. Enhanced photoactivity (as measured by methylene blue decay) in both CuSO₄/n-TiO₂ and n-CuO/n-TiO₂ mixtures compared to single-component solutions explains the amplified stress effects of the mixtures under SSI, but we were unable to directly measure an increase in the production of reactive oxygen species (ROS) in the mixtures. Under environmental conditions that include light exposure, then, Cu/n-TiO₂ mixtures exert amplified cellular stress exceeding that expected based on individual component effects.