Time-Dependent Toxicity Responses in Daphnia magna Exposed to CuO and ZnO Nanoparticles.

Aggregation and dissolution of CuO and ZnO nanoparticles (NPs) increased with increasing exposure time (24, 48, and 72 h). Acute toxicity of CuO NPs to Daphnia magna also increased significantly with increasing exposure time (p < 0.05), whereas exposure time did not significantly affect acute toxicity of ZnO NPs. The dissolved Cu concentration of CuO NPs was much lower than the median effective concentration (EC) value (44 μg L at 72 h), implying that the increase in acute toxicity was caused by particles rather than by dissolved ions. However, the dissolved Zn concentration of ZnO NPs was higher than the EC value (600 μg L at 72 h), suggesting this acute toxicity may be caused by dissolved ions. Moreover, CuO NPs induced greater lipid peroxidation than Cu ions did at an exposure time of 72 h, whereas converse results were observed for ZnO NPs.