Featured Research

from universities, journals, and other organizations

Ions, not particles, make silver toxic to bacteria: Too small a dose may enhance microbes' immunity

Date:
July 11, 2012
Source:
Rice University
Summary:
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.

Silver ions delivered by nanoparticles to bacteria promote lysis, the process by which cells break down and ultimately die, which makes silver nanoparticles a superior and widely used antibacterial agent. New research by Rice University found that silver ions, not the particles themselves, are toxic to bacteria. They also found that ligands in the vicinity of a bacteria can bind silver ions and prevent them from reaching their target.
Credit: Zongming Xiu/Rice University

Rice 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.

Scientists 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 of germs.

But scientists have also suspected silver nanoparticles themselves may be toxic to bacteria, particularly the smallest of them at about 3 nanometers. Not so, according to the Rice team that reported its results this month in the American Chemical Society journal Nano Letters.

In 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.

"You 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."

He 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.

To 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 nanometers, to see whether there was a correlation between size and toxicity.

"We could not get consistent results," he said. "It was very frustrating and really weird."

Xiu 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.

Working 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 toxicity."

"The 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."

Alvarez 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.

"Ultimately, 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."

He 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 mechanisms."

"These 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.

Co-authors 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.

The 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.


Story Source:

The above story is based on materials provided by Rice University. The original article was written by Mike Williams. Note: Materials may be edited for content and length.


Journal Reference:

  1. Zong-ming Xiu, Qing-bo Zhang, Hema L. Puppala, Vicki L. Colvin, Pedro J. J. Alvarez. Negligible Particle-Specific Antibacterial Activity of Silver Nanoparticles. Nano Letters, 2012; 120709145707008 DOI: 10.1021/nl301934w

Cite This Page:

Rice University. "Ions, not particles, make silver toxic to bacteria: Too small a dose may enhance microbes' immunity." ScienceDaily. ScienceDaily, 11 July 2012. <www.sciencedaily.com/releases/2012/07/120711123016.htm>.
Rice University. (2012, July 11). Ions, not particles, make silver toxic to bacteria: Too small a dose may enhance microbes' immunity. ScienceDaily. Retrieved July 22, 2014 from www.sciencedaily.com/releases/2012/07/120711123016.htm
Rice University. "Ions, not particles, make silver toxic to bacteria: Too small a dose may enhance microbes' immunity." ScienceDaily. www.sciencedaily.com/releases/2012/07/120711123016.htm (accessed July 22, 2014).

Share This




More Matter & Energy News

Tuesday, July 22, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Government Approves East Coast Oil Exploration

Government Approves East Coast Oil Exploration

AP (July 18, 2014) The Obama administration approved the use of sonic cannons to discover deposits under the ocean floor by shooting sound waves 100 times louder than a jet engine through waters shared by endangered whales and turtles. (July 18) Video provided by AP
Powered by NewsLook.com
Sunken German U-Boat Clearly Visible For First Time

Sunken German U-Boat Clearly Visible For First Time

Newsy (July 18, 2014) The wreckage of the German submarine U-166 has become clearly visible for the first time since it was discovered in 2001. Video provided by Newsy
Powered by NewsLook.com
Obama: U.S. Must Have "smartest Airports, Best Power Grid"

Obama: U.S. Must Have "smartest Airports, Best Power Grid"

Reuters - US Online Video (July 17, 2014) President Barak Obama stopped by at a lunch counter in Delaware before making remarks about boosting the nation's infrastructure. Mana Rabiee reports. Video provided by Reuters
Powered by NewsLook.com
Crude Oil Prices Bounce Back After Falling Below $100 a Barrel

Crude Oil Prices Bounce Back After Falling Below $100 a Barrel

TheStreet (July 16, 2014) Oil Futures are bouncing back after tumbling below $100 a barrel for the first time since May yesterday. Jeff Grossman is the president of BRG Brokerage and trades at the NYMEX. Grossman tells TheStreet the Middle East is always a concern for oil traders. Oil prices were pushed down in recent weeks on Libya increasing its production. Supply disruptions in Iraq fading also contributed to prices falling. News from China's economic front showing a growth for the second quarter also calmed fears on its slowdown. Jeff Grossman talks to TheStreet's Susannah Lee on this and more on the Energy Department's Energy Information Administration (EIA) report. Video provided by TheStreet
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins