How Nanosilver Zaps Germs

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, Vicki L. Colvin, Pedro J. J. Alvarez and colleagues of Rice University have synthesized and tested silver nanoparticles' antimicrobial properties under anaerobic conditions, which prevents 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” to save money, 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 U.S. Environmental Protection Agency is rolling out a new plan to review nanosilver pesticides. 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 their 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."