CAMBRIDGE, MA -- Like a pea going through a straw, tiny molecules can pass through microscopic cylinders known as nanotubes. This could potentially be used to select molecules according to size for example, to purify water by allowing water molecules to pass through while blocking salt or other substances.
Now, researchers at MIT, Seoul University in Korea and Ursinus College in Pennsylvania have found that such tubes are more selective than had been thought: Molecules of a precise size can zip through five times faster than those that are a bit smaller or larger. The new findings are published in the journal Nature Communications by MIT professor Michael Strano, graduate students Wonjoon Choi and Zachary Ulissi, and three others.
This size-dependence in nanotube transport was completely unexpected, says Strano, a professor of chemical engineering at MIT. "This work illustrates how transport in pores of this type remains exotic and relatively unexplored," he says.
The team "looked at ion transport through the smallest single nanopores that have been studied," Strano says. The carbon nanotubes they studied had widths ranging from 0.9 to 2 nanometers about the diameter of a DNA helix and were about 1 millimeter long.
"What we found was not predicted by theory," he says: Up to a certain diameter, the flow of ions through a nanotube increased steadily but then beyond that diameter, the flow decreased. "The dependence is a volcano-shaped plot," Strano says.
The peak flow, at the center of that plot, allows transport that is five times greater than transport at smaller or larger diameters. "The experimental results are counterintuitive," Strano says, "that there appears to be an optimal diameter."
This size-dependence of transport could turn out to be useful in a variety of technologies, he suggests, including proton-exchange membrane (PEM) fuel cells, where molecules of oxygen or hydrogen must pass thr
|Contact: Sarah McDonnell|
Massachusetts Institute of Technology