Plain cotton cloth dipped in a high-tech broth that has silver nanowires and carbon nanotubes makes an effective filter that purifies water 80,000 times faster,researchers at Stanford have discovered.
Instead of physically trapping bacteria as most existing filters do, the new filter lets them flow on through with the water.
But by the time the pathogens have passed through, they have also passed on, because the device kills them with an electrical field that runs through the highly conductive "nano-coated" cotton.
In lab tests, over 98 percent of Escherichia coli bacteria that were exposed to 20 volts of electricity in the filter for several seconds were killed.
Multiple layers of fabric were used to make the filter 2.5 inches thick.
"This really provides a new water treatment method to kill pathogens. It can easily be used in remote areas where people don't have access to chemical treatments such as chlorine," said Yi Cui, an associate professor of materials science and engineering.
Cui said the new filter could be used in water purification systems from cities to small villages.
Filters that physically trap bacteria must have pore spaces small enough to keep the pathogens from slipping through, but that restricts the filters' flow rate.
Since the new filter doesn't trap bacteria, it can have much larger pores, allowing water to speed through at a more rapid rate.
"Our filter is about 80,000 times faster than filters that trap bacteria," said Cui.
The larger pore spaces in Cui's filter also keep it from getting clogged, which is a problem with filters that physically pull bacteria out of the water.
The researchers already knew that carbon nanotubes were good electrical conductors, so the researchers reasoned the two materials in concert would be effective against bacteria.
"This approach really takes silver out of the folk remedy realm and into a high-tech setting, where it is much more effective," said Heilshorn.
The amount of silver used for the nanowires was so small the cost was negligible, said Cui.
Still, they needed a foundation material that was "cheap, widely available and chemically and mechanically robust." So they went with ordinary woven cotton fabric.
"We got it at Wal-mart," said Cui.
"With one filter, we can kill 98 percent of the bacteria. For drinking water, you don't want any live bacteria in the water, so we will have to use multiple filter stages," said Cui.
The study will be published in an upcoming issue of Nano Letters.