Oxidative and Toxicological Evolution of Engineered Nanoparticles with Atmospherically Relevant Coatings

The health impacts associated with engineered nanoparticles (ENPs) released into the atmosphere have not been adequately assessed. Such impacts could potentially arise from the toxicity associated with condensable atmospheric secondary organic material (SOM), or changes in the SOM composition induced by ENPs. Here, these possibilities are evaluated by investigating the oxidative and toxicological evolution of TiO₂ and SiO₂ nanoparticles which have been coated with SOM from the O₃ or OH initiated oxidation of α-pinene. It was found that pristine SiO₂ particles were significantly more cytotoxic compared to pristine TiO₂ particles. TiO₂ in the dark or under UV irradiation catalytically reacted with the SOM, increasing its O/C by up to 55% over photochemically inert SiO₂ while having negligible effects on the overall cytotoxicity. Conversely, the cytotoxicity associated with SiO₂ coated with SOM was markedly suppressed (by a factor of 9, at the highest exposure dose) with both increased SOM coating thickness and increased photochemical aging. These suppressing effects (organic coating and photo-oxidation of organics) were attributed to a physical hindrance of SiO₂-cell interactions by the SOM and enhanced SOM viscosity and hydrophilicity with continued photo-oxidation, respectively. These findings highlight the importance of atmospheric processes in altering the cytotoxicity of ENPs.