Acute toxicity of Ag and CuO nanoparticle suspensions against Daphnia magna: The importance of their dissolved fraction varying with preparation methods

A variety of methods to prepare nanoparticle suspensions have been employed for aquatic toxicity tests, although they can influence the dispersion property and subsequent toxicity of nanoparticles. Thus, in this study, we prepared stock suspensions of silver (Ag) and copper oxide (CuO) nanoparticles using different methods and compared their acute toxicity against Daphnia magna. The results showed that the dispersion method, filtration and initial concentration largely affected their toxicity, when the toxicity was expressed as the total concentrations of Ag and Cu. In case of Ag nanoparticles, the toxicity was also influenced by their different particle size. However, negligible differences in 24h-median effect concentration (EC ₅₀) values, which were calculated in terms of their dissolved concentrations, were observed. When expressing toxicity on the basis of dissolved concentrations, 24h-EC ₅₀ values of the Ag and CuO nanoparticles were also found to be similar to those of the counterpart ionic species, i.e., Ag (as AgNO ₃) and Cu (as CuCl ₂·2H ₂O). These findings indicate that the dissolved fraction of nanoparticles largely contributes to their acute toxicity. Our results may help in establishing a useful guideline for preparing nanoparticle suspensions with reproducible toxicity.