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 (AgCl x1- 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.