Although carbon-nanotube membranes are promising candidates for filtering out small impurities and organic materials like DNA and proteins from water, the arrays are hydrophobic, that is, they strongly repel water. Now, Nikhil Koratkar and colleagues at Rensselaer Polytechnic Institute have discovered that low applied voltages can be used to manipulate the flow of water through the membranes, turning the essentially hydrophobic surfaces into hydrophilic ones. This is the first time that applied voltage has been shown to control the way that water interacts with the surfaces of these nanomaterials.

Koratkar and colleagues found that when a small positive voltage of 1.7 V was applied to the nanotube membrane, and the water had a negative potential, the nanotubes quickly switched from repelling water to pumping water. When the charge on the water was increased, the water flowed at an exponentially faster rate. In contrast, when the nanotube was made negatively charged, it took a much higher voltage of 90 V to move the water through the tube.

The team also found that they could start and stop the flow of water through the tube by simply reversing the polarity of the nanotubes. When a small positive charge was applied, the water moved through the tube and when the charge was reversed, the flow of water stopped.

Koratkar and co-workers found that the walls of the nanotubes had been electrochemically oxidized because of water electrolysis, meaning that oxygen atoms had coated the surface of the tubes. This allows water to flow through them. Once the charge was reversed, the oxidization stopped and water could no longer flow through the unoxidized portion.

Finally, the researchers observed that they could control the rate of water flow through nanotubes that were directly next to each other, allowing one tube to pump quickly while the one beside it did not pump water at all. This discovery could be important for time-released drug coatings, for instance, and lab-on-a-chip devices.

"In this century one of the big challenges is how to get clean drinking water," said Koratkar. "If you can remove salt from water you can solve this problem. Nature does this all the time. The first step to getting to this process is to control the flow of water through nanochannels, which we have now successfully demonstrated. The next step would be to capture specific proteins, DNA, or impurities within the water with specifically designed nanotubes."

The researchers reported their work in Nano Letters.