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The aqueous media could affect the physicochemical properties (e.g., surface charge, morphology, and aggregation) of ZnO nanoparticles (nZnO), leading to their different environmental impacts. In this study, the toxicity of nZnO toward the green fluorescent protein-expressing Escherichia coli cells under UV-365 light irradiaiton in various media was assessed, including deionized (DI) water, 0.85% NaCl, phosphate-buffered saline (PBS), minimal Davis medium (MD), and Luria-Bertani medium (LB). The toxicity of nZnO was assessed by the conventional plate count method and the fluorescence intensity method, which consistently demonstrated that the nZnO toxicity was dependent on the medium components that varied the potency of reactive oxygen species (ROS) generation. In DI, NaCl, PBS, and MD medium, nZnO generated three types of ROS (O2(•-), •OH, and (1)O2), whereas in LB medium, nZnO generated O2(•-) and (1)O2. The total concentrations of ROS generated by nZnO in DI, NaCl, PBS, MD, and LB were 265.5 ± 15.9, 153.6 ± 8.6, 144.3 ± 6.9, 123.0 ± 6.0, and 115.6 ± 4.5 μM, respectively. Furthermore, a linear correlation was established between the total concentrations of three types of ROS generated by nZnO and their bacterial mortality rate (R(2) = 0.92) in various media. Since the released Zn(2+) from nZnO under UV irradiation only accounted for less than 10% of the total Zn in all media, the ionic forms of zinc did not significantly contribute to the overall toxicity. This work aims at providing further insight into the medium type influences on the ROS production and the toxicity of nZnO toward the E. coli cells.

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The impact of perfluorocarboxylic acids (PFCAs) with carbon chain length C2 to C8 on the dissolution, aggregation, reactive oxygen species (ROS) generation, and toxicity of citrate-coated AgNPs was investigated under UV irradiation. The presence of PFCAs decreased dissolution, aggregation, ROS generation, and toxicity of AgNPs because the negatively charged PFCAs sorbed on AgNP surface enhanced their stability. Both dissolution and aggregation rate of AgNPs decreased with chain length of PFCAs under UV irradiation, primarily because PFCAs with longer chain length sorbed on AgNP surface could form thicker coatings. The dissolution of AgNPs followed pseudo-first-order kinetics, and the rate constant decreased from 0.58 h(-1) with C2 to 0.30 h(-1) with C8. The hydrodynamic diameters of AgNPs linearly increased under UV irradiation with aggregation rates ranged from 72.1 to 143.5 nm/h. O2(•-) generation was observed in AgNP suspension with quantum yield of 0.12%, but was completely suppressed by PFCAs because they inhibited the interaction between photoelectrons and O2. A linear correlation was established between dissolved Ag(+) and bacterial survival rates of AgNPs with and without PFCAs under UV irradiation. This study highlights the necessity of considering coexisting organic contaminants when investigating the environmental behaviors of AgNPs.