Nano-TiO₂ enhances the toxicity of copper in natural water to Daphnia magna

The acute toxicity of engineered nanoparticles (NPs) in aquatic environments at high concentrations has been well-established. This study demonstrates that, at a concentration generally considered to be safe in the environment, nano-TiO₂ remarkably enhanced the toxicity of copper to Daphnia magna by increasing the copper bioaccumulation. Specifically, at 2 mg L^-1 nano-TiO₂, the (LC₅₀) of Cu²⁺ concentration observed to kill half the population, decreased from 111 μg L^-1 to 42 μg L^-1. Correspondingly, the level of metallothionein decreased from 135 μg g^-1 wet weight to 99 μg g^-1 wet weight at a Cu²⁺ level of 100 μg L ^-1. The copper was found to be adsorbed onto the nano-TiO ₂, and ingested and accumulated in the animals, thereby causing toxic injury. The nano-TiO₂ may compete for free copper ions with sulfhydryl groups, causing the inhibition of the detoxification by metallothioneins.