University researchers have settled a long-standing controversy over
the mechanism by which silver nanoparticles, the most widely used
nanomaterial in the world, kill bacteria.
Their work comes with a Nietzsche-esque warning: Use enough. If you don’t kill them, you make them stronger.
have long known that silver ions, which flow from nanoparticles when
oxidized, are deadly to bacteria. Silver nanoparticles are used just
about everywhere, including in cosmetics, socks, food containers,
detergents, sprays and a wide range of other products to stop the spread
scientists have also suspected silver nanoparticles themselves may be
toxic to bacteria, particularly the smallest of them at about 3 nm. Not
so, according to the Rice team that reported its results this month in
the American Chemical Society journal Nano Letters.
fact, when the possibility of ionization is taken away from silver, the
nanoparticles are practically benign in the presence of microbes, said
Pedro Alvarez, George R. Brown Professor and chair of Rice’s Civil and
Environmental Engineering Department.
would be surprised how often people market things without a full
mechanistic understanding of their function,” said Alvarez, who studies
the fate of nanoparticles in the environment and their potential
toxicity, particularly to humans. “The prefix ‘nano’ can be a
double-edged sword. It can help you sell a product, and in other cases
it might elicit concerns about potential unintended consequences.”
said the straightforward answer to the decade-old question is that the
insoluble silver nanoparticles do not kill cells by direct contact. But
soluble ions, when activated via oxidation in the vicinity of bacteria,
do the job nicely.
figure that out, the researchers had to strip the particles of their
powers. “Our original expectation was that the smaller a particle is,
the greater the toxicity,” said Zongming Xiu, a Rice postdoctoral
researcher and lead author of the paper. Xiu set out to test
nanoparticles, both commercially available and custom-synthesized from 3
to 11 nm, to see whether there was a correlation between size and
“We could not get consistent results,” he said. “It was very frustrating and really weird.”
decided to test nanoparticle toxicity in an anaerobic environment—that
is, sealed inside a chamber with no exposure to oxygen—to control the
silver ions’ release. He found that the filtered particles were a lot
less toxic to microbes than silver ions.
with the lab of Rice chemist Vicki Colvin, the team then synthesized
silver nanoparticles inside the anaerobic chamber to eliminate any
chance of oxidation. “We found the particles, even up to a concentration
of 195 parts per million, were still not toxic to bacteria,” Xiu said.
“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
point of that experiment,” Alvarez said, “was to show that a lot of
people were obtaining data that was confounded by a release of ions,
which was occurring during exposure they perhaps weren’t aware of.”
suggested the team’s anaerobic method may be used to test many other
kinds of metallic nanoparticles for toxicity and could help fine-tune
the antibacterial qualities of silver particles. In their tests, the
Rice researchers also found evidence of hormesis; E. coli became stimulated by silver ions when they encountered doses too small to kill them.
we want to control the rate of (ion) release to obtain the desired
concentrations that just do the job,” Alvarez said. “You don’t want to
overshoot and overload the environment with toxic ions while depleting
silver, which is a noble metal, a valuable resource—and a somewhat
expensive disinfectant. But you don’t want to undershoot, either.”
said the finding should shift the debate over the size, shape and
coating of silver nanoparticles. “Of course they matter,” Alvarez said,
“but only indirectly, as far as these variables affect the dissolution
rate of the ions. The key determinant of toxicity is the silver ions. So
the focus should be on mass-transfer processes and controlled-release
findings suggest that the antibacterial application of silver
nanoparticles could be enhanced and environmental impacts could be
mitigated by modulating the ion release rate, for example, through
responsive polymer coatings,” Xiu said.
of the paper are postdoctoral researcher Qingbo Zhang and graduate
student Hema Puppala, both in the lab of Colvin, Rice’s Kenneth S.
Pitzer-Schlumberger Professor of Chemistry, a professor of chemical and
biomolecular engineering and vice provost for research.
work was supported by a joint U.S.-U.K. research program administered
by the Environmental Protection Agency and the U.K.’s Natural
Environment Research Council.
Source: Rice University