Copper, silver, and titania nanoparticles do not release ions under anoxic conditions and release only minute ion levels under oxic conditions in water: Evidence for the low toxicity of nanoparticles

It is widely assumed that the discharge of nanoparticles into the environment may cause adverse effects on organisms. Nonetheless, so far most nanoparticles have demonstrated little to no observed hazards in multiple biological test systems. It is not until nanoparticles undergo transformations, e.g., release of metal ions, that most environmental toxicities are induced, yet the ionization of nanoparticles in natural oxic and anoxic conditions is poorly known. We hypothesized that in anaerobic conditions, where oxidation is absent or limited, metal nanoparticles should not release metal ions. We investigated the transformation of three commercially produced materials, i.e., copper nanoparticles, silver nanoparticles, and titania nanocrystals with an average particle size of 50 nm. The nanoparticles were subjected to different environmental conditions including oxic/anoxic suspension, incubation with natural organic matter, and pH gradient, then subsequently analyzed for zeta potential, hydrodynamic diameter, concentration of released metal ions, and generation of reactive oxygen species. Transmission electron microscopy was used to assess morphological changes. Under oxic conditions, results show that only 9.4 μg/mL of copper ions and 6.9 μg/mL of silver ions were released from nanoparticles, when continuously stirred over 48 h. These levels are low compared to levels found in natural media. Moreover, under anoxic conditions, an insignificant amount of copper ions of 0.9 μg/mL and silver ions, of 0.2 μg/mL, were released. For titania nanoparticle, less than 0.05 μg/mL ions were released under either oxic or anoxic conditions. Overall, our findings reveal the absence of ion release under anoxic conditions, and the very low levels of ion released under oxic conditions. As a consequence, the toxicity of Cu, Ag, and Ti nanoparticles should be very low in natural media.