Anti-microbial activities of aerosolized transition metal oxide nanoparticles

This study used the electrospray method to create airborne droplets of metal oxide nanoparticles (NPs) and examined their anti-microbial activities, employing Escherichia coli as a model microbial species. We tested the anti-microbial activities of six metal oxide NPs (NiO, ZnO, Fe₂O₃, Co₃O₄, CuO, and TiO₂) in both an aqueous culture medium and an aerosol exposure mode (spraying the particles directly onto the cell surface). In the aqueous medium, the both NPs and stressed E. coli cells severely aggregated. Only NiO NPs (>20mgL^-1) showed significant growth inhibition of E. coli (∼30%). In contrast to aqueous exposure, where the direct interactions between NPs and bacteria were limited, aerosol exposure of three metal oxide NPs to E. coli enhanced NP toxicity to cells and dramatically reduced cellular viability. Electrospraying NiO, CuO, or ZnO NPs (20nm, 20μg, in 10min) reduced the total number of living E. coli by more than 88%, 77% and 71%, respectively (compared to the control experiments). However, TiO₂, Co₃O₄, and Fe₂O₃ NPs showed no significant antibacterial activities in either the aqueous exposure mode or the aerosol exposure mode. The above observations suggest the potential application of electrosprayed metal oxide NPs to disinfect airborne pathogens.