Scientists have found that the antibacterial properties of silver nanoparticles are caused by the release of silver ions and not by the nanoparticles themselves, a finding which has significant implications for the risk assessment of nanomaterials.
The researchers from Rice University in Texas, US, initially began testing nanoparticles of different sizes to investigate whether there was a correlation between size and toxicity. However, they were unable to get consistent results in these experiments and therefore decided to test the toxicity of particles when sealed inside a chamber with no exposure to oxygen, which would control the release of silver ions from the nanoparticles. This showed that nanoparticles inside the chamber were a lot less toxic to microbes than expected.
The researchers then synthesized silver nanoparticles inside the chamber, to eliminate entirely the process of oxidation which produces silver ions. "We found the particles, even up to a concentration of 195 parts per million, were still not toxic to bacteria," said Zongming Xiu, a Rice postdoctoral researcher and lead author of the paper. "But for the ionic silver, a concentration of about 15 parts per billion would kill all the bacteria present. That told us the particle is 7,665 times less toxic than the silver ions, indicating a negligible toxicity."
These results indicate that experiments to determine the toxicity of nanoparticles may be confounded by the release of metal ions from the material which are not always accounted for. The authors suggest that an anaerobic chamber could be used in toxicity testing to determine the effects of nanoparticles independent of any ion release that occurs due to oxidation.
Their findings also suggest that the harmful effects of nanoparticles may be reduced, and the antibacterial effects maximised, by controlling the release of ions, for example using specifically designed polymer coatings.
The research was funded by the US Environmental Protection Agency (EPA) and the UK Natural Environment Research Council (NERC), and is published in the journal Nano Letters.
Fonte: Chemical Watch