How Nanosilver Zaps Germs | July 23, 2012 Issue - Vol. 90 Issue 30 | Chemical & Engineering News
Volume 90 Issue 30 | p. 8 | News of The Week
Issue Date: July 23, 2012 | Web Date: July 19, 2012

How Nanosilver Zaps Germs

Antimicrobials: Ions, not nanoparticles, are the killers
Department: Science & Technology
News Channels: Nano SCENE, Biological SCENE, Materials SCENE
Keywords: silver nanoparticles, nanosilver, toxicity, nanotechnology, silver, ions
Transmission electron micrograph of the Rice team’s silver nanoparticles synthesized under anaerobic conditions.
Credit: Nano Lett.
Hundreds of evenly spaced dark grey dots on a light grey background; in two spots, the dots are smooshed together. A transmission electron microscopy image of silver nanoparticles.
Transmission electron micrograph of the Rice team’s silver nanoparticles synthesized under anaerobic conditions.
Credit: Nano Lett.

Curious germophobes can rejoice: Researchers have figured out how silver nanoparticles, found in many consumer products, kill bacteria. The culprits are the silver ions the nanoparticles emit, not nanoparticle-specific biological effects (Nano Lett., DOI: 10.1021/nl301934w).

Manufacturers add silver nanoparticles to cosmetics and clothing to kill germs. Yet debate rages about how they work. When exposed to air in an aqueous solution, silver nanoparticles release silver ions, which have known antibacterial properties. Researchers have been unable to rule out a role for the nanoparticles themselves.

Now, postdoc Zongming Xiu, professors Vicki L. Colvin, and Pedro J. J. Alvarez, and colleagues at Rice University have synthesized and tested silver nanoparticles’ antimicrobial properties under anaerobic conditions, which prevent release of silver ions. In their tests, ionic silver killed Escherichia coli even at concentrations as low as 15 parts per billion. The silver nanoparticles themselves weren’t toxic to the bacteria, even at concentrations thousands of times higher. The team found that it was important to add enough silver ions; sublethal concentrations boosted bacterial survival rates compared with controls.

Sizes, shapes, and coatings for silver nanoparticles do matter, but only because they tune the rate of silver ion release, Alvarez says. “You want the release rate to be fast enough to kill bacteria, but it must be slow enough to avoid excessive depletion of silver,” which would drive up product costs, he explains.

Because silver can wash away from products and go into the water supply, scientists are concerned about these products' environmental footprint. And the Environmental Protection Agency is rolling out a new plan to review nanosilver pesticides (C&EN, July 16, page 36). It might be possible, Alvarez says, to diminish silver nanoparticles' environmental impact by controlling silver ion release with a responsive polymer coating.

The Rice team's findings underscore the need for continued development of analytical measurements and reference standards for nanomaterials, says Robert I. MacCuspie, who characterizes nanoparticle surfaces at the National Institute of Standards & Technology. He notes that NIST is currently developing silver nanoparticle reference materials so researchers will be better able to understand the particles' chemical behavior and environmental fate.

Bernd Nowack of Empa, the Swiss Federal Laboratories for Materials Science & Technology, praises the team for its carefully designed experiments. “I doubt that this work will actually settle the debate because the debate is not based on science but more on feelings that nano is dangerous,” he says. “But it will definitely be helpful to move away from condemning nanosilver and focus on silver as whole, or even better, on biocides as a whole.”

Chemical & Engineering News
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Steve Barwick (July 20, 2012 2:18 AM)
This is what I've been saying all along: Metallic silver (also called particulate silver or nanoparticle silver) is only effective to the extent it sheds silver ions. I'm surprised this is being touted as "new" news, however. I just finished reading Lansdown's great book from 2010, "Silver in Healthcare: Its Antimicrobial Efficacy and Safety in Use (Issues in Toxicology)" and he repeatedly emphasizes the point that metallic silver is inert except to the extent that it sheds silver ions. Most every expert on antimicrobial silver since 1954 has said the same thing. You can read some of their quotes here:
Kevin (August 2, 2012 10:41 AM)
Just want to correct a notion here:
Particulate silver is not equal to nano silver.
Everybody knows particulate silver is not toxic to bacteria, since it’s relatively big for bacteria. But nano silver, which could be as small as 3 nm, is extremely small compared to bacteria, which is around 2-5nm. This is why EPA spent around 50million $ to set up two research center recently (lead by Duke and UCLA) to investigate nano silver. You can read the report at the following link:
At Duke’s Center for the Environmental Implications of NanoTechnology, a multidisciplinary team of scientists, chemists, biologists and engineers have stepped up to take on the challenge of studying these extremely small particles and their effects on the environment.
“This is a revolution of science and tech,” said CEINT Director Mark Wiesner. “We want to get this one right. We don’t want the next DDT.”

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