Photochemical interactions between n-Ag2S and n-TiO2 amplify their bacterial stress response

Abstract

Under the reducing conditions of wetlands and anaerobic wastewater treatment processes, nanosilver (n-Ag) released from consumer products reacts with sulfide to form Ag₂S, which is believed to be stable and practically benign. However, in the natural environment, these nanosized silver sulfide particles (n-Ag₂S) may interact with light and other nanomaterials to create oxidizing conditions that may destabilize n-Ag₂S, releasing Ag⁺ and causing stress to microorganisms. We investigate this scenario by monitoring the ATP levels of E. coli exposed to mixtures of n-Ag₂S and n-TiO₂, another nanomaterial with high likelihood of environmental release. Under dark conditions, we find that sulfidation increases the threshold concentration for microbial stress relative to n-Ag. Under simulated solar irradiation (SSI), exposure to n-Ag₂S alone does not depress bacterial ATP levels in the concentration range tested (≤1 mg L⁻¹ as Ag). However in the presence of 1 mg L⁻¹ n-TiO₂ and under SSI, n-Ag₂S causes synergistic toxic stress and enhances the production of reactive oxygen species (ROS). Based on the photochemistry of these nanomaterials and our measurements of dissolved Ag, we propose that under environmental conditions the ROS produced by n-TiO₂ dissolves n-Ag₂S, releasing Ag⁺ that can then be photoreduced on the surface of n-TiO₂ to form a highly photoactive and phototoxic nanocomposite. This work reveals that under oxidizing conditions n-Ag₂S is prone to transformations that cause microbial stress. Overall, we provide further evidence that chemical interactions between multiple nanomaterials under irradiation can dramatically change their toxic effects.