Illness-inducing bacteria, meet nano-engineered cotton--and a quick death. Researchers have created a new "filter" that zaps bacteria with electric fields to clean drinking water. They say their system may find use in developing countries since it requires only a small amount of voltage (a couple of car batteries, a stationary bike, or a solar panel could do the job) and cleans water an estimated 80,000 times faster than traditional devices. Instead of trapping bacteria in small pores like many slow-going traditional filters, the cotton and silver nanowire combo uses small electric currents running through the nanowires to kill the bacteria outright. In a paper to appear in the journal Nano Letters researchers say that 20 volts and 2.5 inches worth of the material killed 98 percent of Escherichia coli in the water they tested in their lab setup. The authors argue that the filter's silver nanowires and carbon nanotubes are cheap; the small amount of silver required makes its expense "negligible," coauthor Yi Cui says in a press release, and the group chose to use cotton because of its abundance.
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," Cui said. [Stanford University]
They made the potent combination by dipping the cotton first in a "broth" containing carbon nanotubes and then the silver nanowires, allowing the structures to coat the cotton fibers. The scanning electron microscope image above shows the silver nanowires compared to the large cotton fibers (the red line is 10 microns long). The current running though the material, a few milliamperes, may be fatal for the bacteria but it would barely makes a human tingle.
[B]ecause the voltage is so low, it doesn't require serious electricity generation. A person could generate the power from a stationary bike or a hand-cranked device. No pumping is required either. The force of gravity is enough to allow the water and its nasties to pass through the cotton and get zapped! [Discovery News]
Next, the group hopes to test the device on other microorganisms--perhaps those responsible for other waterborne illness such as cholera, typhoid and hepatitis. The researchers will also continue testing the filter to make sure only clean water comes out and not any nano-structures.
"So far, our evidence suggests that they don't come off," Cui told New Scientist. "It is an interesting academic study," says nanoengineer Eric Hoek at the University of California, Los Angeles. He says proving that the potentially harmful CNTs [carbon nanotubes] do not leach into the water will be a key step in finding out if it is useful on a practical level. [New Scientist]
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