Effects of Chloride Ions on Dissolution, ROS Generation, and Toxicity of Silver Nanoparticles under UV Irradiation

This work investigates the effect of chloride ion (Cl–) on dissolution, reactive oxygen species (ROS) generation, and toxicity of citrate-coated silver nanoparticles (AgNPs) under UV irradiation. The dissolution rate was decreased by 0.01 M Cl– due to AgCl passivation on the AgNP surface. By contrast, high concentrations of Cl– (0.1 or 0.5 M) promoted dissolution due to the formation of soluble Ag–Cl complexes (AgClx1–x). The generation of O2•– in the AgNPs/Cl–/UV system was promoted by 0.01 M Cl–, whereas it was retarded by 0.1 or 0.5 M Cl–, which was probably because the aggregation of AgNPs at high ionic strength reduced the nanoparticles' surface areas for radical formation. Additionally, Cl– contributed to •OH generation in the AgNPs/Cl–/UV system, in which the produced •OH concentrations increased with increasing Cl– concentrations. The reduction reaction between silver ions and O2•– resulted in lower dissolution rates of AgNPs/Cl– mixtures under UV irradiation than those in the dark. The phototoxicity of AgNPs toward E. coli with different concentrations of Cl– followed the order of 0.5 M > 0 M > 0.1 M > 0.01 M. Both ROS and dissolved Ag played significant role in the phototoxicity of AgNPs. This work demonstrates the potential importance of anions in the fate and biological impact of AgNPs.