New evidence for natural synthesis of silver nanoparticles

Silver nanoparticles are being discovered more and more in the environment - as well as in environmental science laboratories. In light of the fact that they contain a variety of useful properties, especially as antibacterial and antifungal agents, their use is becoming more and more common in a wide variety of consumer and industrial products. This development, in turn, gave rise to concerns about the release of these nanoparticles into the environment. Now another piece of evidence has been added in this field - it is possible that nature itself produces silver nanoparticles.

A team of American researchers from the Florida Institute of Technology (FIT), the State University of New York (SUNY) and the National Institute of Standards and Technology (NIST) reports that in the presence of a source of silver ions, natural humic acids (a combination of a variety of organic substances found during process of decomposition into a different organic state) produce stable silver nanoparticles.

"One of our colleagues, Virender Sharma, came across an article that described how wine was used to produce nanoparticles. He believed that based on similar chemistry, we could produce silver nanoparticles using humic acids," explains chemist Mary Sohn. "At first we made them using normal methods and then we tried to do it using one of the humic acids taken from the river sediments near us. We were very excited when we saw the characteristic yellow color of the nanoparticles."

"humic acid" (Wikipedia term) is a complex mixture of many organic acids formed during the decomposition of organic matter. Despite the fact that the exact composition varies from place to place and from season to season, humic acid is everywhere in the environment. Metal nanoparticles, the researchers explain, have characteristic colors that are born as a direct function of their size. Silver nanoparticles appear yellowish-brown in color.

The team of researchers mixed silver ions with humic acid that came from a variety of sources at varying temperatures and concentrations and found that acids originating from rivers or sediments produced silver nanoparticles in measurable quantities at room temperature within two to four days. Moreover, the humic acid appears to stabilize the nanoparticles by wrapping them while preventing them from clumping together into a larger pool of silver. "We believe that this is actually a process similar to the one by which we synthesize nanoparticles in the laboratory," the researcher points out, except for the fact that in the laboratory we usually use citric acid at high temperatures.

"These findings surprised us since most of our research focused on examining how silver nanoparticles might dissolve when they are released into the environment and emit silver ions," explains the researcher. Many biologists believe that the toxicity of silver nanoparticles, the reason for their use as antibacterial and antifungal agents, originates from their high surface area which makes them an effective source of silver ions, adds the researcher, however "this belief gives rise to the idea that there may be different types of cycles natural ones that turn the silver ions back into silver nanoparticles." It could also help explain the discovery, which has emerged over the past few years, of the presence of silver nanoparticles in sites such as old mining areas that are unlikely to have been exposed to synthetic, man-made nanoparticles and still contain high concentrations of silver ions.   The news about the study