AFM (Atomic force microscopy) image of silver nanoparticles formed from silver ions in solution with humic acid. Color tone in this image indicates height (0 to 10 nanometers) above the base plane, so brighter spots are taller, larger nanoparticles. Image is roughly 1,700 nm on a side. Image: MacCuspie, NIST |
Nanoparticles of silver are being found increasingly in the
environment—and in environmental science laboratories. Because they have a
variety of useful properties, especially as antibacterial and antifungal
agents, silver nanoparticles increasingly are being used in a wide variety of
industrial and consumer products. This, in turn, has raised concerns about what
happens to them once released into the environment. Now a new research paper*
adds an additional wrinkle: Nature may be making silver nanoparticles on its
own.
A team of researchers from the Florida Institute of Technology
(FIT); the State Univ. of New York (SUNY), Buffalo; and the National Institute
of Standards and Technology (NIST) reports that, given a source of silver ions,
naturally occurring humic acid will synthesize stable silver nanoparticles.
“Our colleague, Virender Sharma, had read an article in
which they were using wine to form nanoparticles. He thought that, based on the
similar chemistry, we should be able to produce silver nanoparticles with humic
acids,” explains FIT chemist Mary Sohn. “First we formed them by traditional
methods and then we tried one of our river sediment humic acids. We were really
excited that we could see the characteristic yellow color of the
nanoparticles.” Samples were sent to Sarbajit Banerjee at SUNY Buffalo and
Robert MacCuspie at NIST for detailed analyses to confirm the presence of
silver nanoparticles.
“Humic acid” is a complex mixture of many organic acids that
are formed during the decay of dead organic matter. Although the exact
composition varies from place to place and season to season, humic acid is
ubiquitous in the environment. Metallic nanoparticles, MacCuspie explains, have
characteristic colors that are a direct consequence of their size**. Silver nanoparticles
appear a yellowish brown.
Transmission electron microscopy (TEM) image of silver nanoparticles formed from silver ions in solution with humic acid. The acid tends to coat the nano particles (visible here as a pale cloud), keeping them in a colloidal suspension instead of clumping together. (Color added for clarity.) Image: SUNY, Buffalo |
The team mixed silver ions with humic acid from a variety of
sources at different temperatures and concentrations and found that acids from
river water or sediments would form detectable silver nanoparticles at room temperature
in as little as two to four days. Moreover, MacCuspie says, the humic acid
appears to stabilize the nanoparticles by coating them and preventing the
nanoparticles from clumping together into a larger mass of silver. “We believe
it’s actually a similar process to how nanoparticles are synthesized in the
laboratory,” he says, except that the lab process typically uses citric acid at
elevated temperatures.
“This caught us by surprise because a lot of our work is
focused on how silver nanoparticles may dissolve when they’re released into the
environment and release silver ions,” MacCuspie says. Many biologists believe
the toxicity of silver nanoparticles, the reason for their use as an
antibacterial or antifungal agent, is due to their high surface area that makes
them an efficient source of silver ions, he says, but “this creates the idea
that there may be some sort of natural cycle returning some of the ions to
nanoparticles.” It also helps explain the discovery, over the past few years,
of silver nanoparticles in locations like old mining regions that are not
likely to have been exposed to man-made nanoparticles, but would have
significant concentrations of silver ions.
* N. Akaighe,
R.I. MacCuspie, D.A. Navarro,
D.S. Aga, S. Banerjee, M. Sohn and V.K. Sharma. Humic acid-induced silver
nanoparticle formation under environmentally relevant conditions. Environmental
Science & Technology, Published online Apr. 1, 2011.
dx.doi.org/10.1021/es103946g.
** The effect is called “surface plasmon resonance” and is
caused by surface electrons across the nanoparticle oscillating in concert.
