Carbon nanotube acts as size sorter

Tiny molecules passing through nanotubes can be propelled or slowed down depending on their size say researchers who found that nanotubes are more selective than previously thought.

A team from MIT, Seoul University and Ursinus College were studying ion transport through small single nanopores – carbon nanotubes measuring between 0.9 and 2 nanometres in width and 1mm in length – when they found that molecules of a precise size could zip though the nanotube five times faster than molecules slightly too small or slightly too large.

“What we found was not predicted by theory,” said MIT professor of chemical engineering, Michael Strano. “Up to a certain diameter, the flow of ions through a nanotube increased steadily – but then beyond that diameter the flow decreased.”

“The experimental data results are counterintuitive,” he said. “That there appears to be an optimal diameter.”

He suggests the size-dependence is the result of an attractive force in which ions’ electrical charge causes them to be pulled by an electric field through the pore. Since everything is submerged in water, some water gets pulled along too. Up to a certain diameter the water layer or layers are simply pulled through, but as the opening gets bigger water begins to behave as a bulk material and slows the ion down.

The work – published in Nature Communications – could help design better membranes for desalination of water, or new sensors capable of detecting specific contaminants, like arsenic, in water. It may also find a use in proton-exchange membrane (PEM) fuel cells where oxygen or hydrogen must pass through tiny pores in a membrane to produce electricity, or in DNA sequencing devices where transit speed could be tuned to slow DNA sequences down for analysis.

Diameter-dependent ion transport through the interior of isolated single-walled carbon nanotubes http://www.nature.com/ncomms/2013/130912/ncomms3397/full/ncomms3397.html