ACS Nano http://dx.doi.org/10.1021/nn2031319 (2011)

Silver nanoparticles are used as an antimicrobial agent in various consumer goods but it is unclear whether these nanoparticles are released and if they cause hazards specific to their nanoscale form. This is partly because detecting nanomaterials in the environment and monitoring their transformations due to weathering is difficult. Researchers at the University of Oregon and Dune Sciences have now shown that under ambient conditions new metal nanoparticles can spontaneously form close to parent nanoparticles.

James Hutchison and colleagues immobilized silver nanoparticles with diameters of 75 nm on positively charged transmission electron microscopy (TEM) grids and exposed the samples to light and different levels of humidity. Samples that were exposed to relative humidities greater than 50% showed that new silver nanoparticles formed around the parent nanoparticles. More new nanoparticles were observed when samples were stored for longer times at 100% relative humidity, suggesting that humidity can influence their formation. Potential effects of the electron beam from the TEM and the effects of the charged surface were ruled out by control experiments. The researchers propose that the new nanoparticles are formed in three stages: oxidation of silver nanoparticles into silver ions, diffusion of ions in the absorbed water layer, and finally reduction of the ions into new silver nanoparticles.

Hutchison and colleagues also found that new nanoparticles could be formed from macroscopic objects including a sterling silver earring and a silver cutlery set, as well as from copper objects. The results suggest that metal nanoparticles may have been present in our environment for many years.