Silver nanoparticles are highly
indicated for a range of biologically interesting applications. Here we get the good news that nature has
been there ahead of us. Apparently the particle
is formed through humic acid chemistry and they are simultaneously protected by
the acid to prevent immediate clumping or dissolution. They do in fact show up naturally.
The next question is to determine
what other element(s) pulls the same trick.
Nanoparticle process chemistry is slowly emerging as an important new
tool and it is already an important method of medical delivery.
Recently we had a successful test
done with gold nanoparticles at MIT that established a certain size would
preferentially invade cancer cells allowing the cells to be heated and
killed. Silver may be even easier to
work with in the same way.
It also explains the likely
reason for a number of erratic therapeutic effects of silver.
Silver cycle: New evidence for natural synthesis of silver nanoparticles
by Staff Writers
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. Credit: 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 University 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
"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.
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.
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