Synergistic Bacterial Stress Results from Exposure to Nano-Ag and Nano-TiO₂ Mixtures under Light in Environmental Media

Due to their widespread use and subsequent release, engineered nanomaterials (ENMs) will create complex mixtures and emergent systems in the natural environment where their chemical interactions may cause toxic stress to microorganisms. We previously showed that under dark conditions n-TiO₂ attenuated bacterial stress caused by low concentrations of n-Ag (<20 μg L^-1) due to Ag⁺ adsorption, yet, since both n-Ag and n-TiO₂ are photoactive, their photochemistries may play a key role in their interactions. In this work, we study the chemical interactions of n-Ag and n-TiO₂ mixtures in a natural aqueous medium under simulated solar irradiation to investigate photoinduced stress. Using ATP levels and cell membrane integrity as probes, we observe that n-Ag and n-TiO₂ together exert synergistic toxic stress in Escherichia coli. We find increased production of hydrogen peroxide by the n-Ag/n-TiO₂ mixture, revealing that the enhanced photocatalytic activity and production of ROS likely contribute to the stress response observed. Based on STEM-EDS evidence, we propose that a new composite Ag/TiO₂ nanomaterial forms under these conditions and explains the synergistic effects of the ENM mixture. Overall, this work reveals that environmental transformations of ENM mixtures under irradiation can enhance biological stress beyond that of individual components.