"We are the first to measure transport of gas and water through sub-2 nm carbon nanotubes," Olgica Bakajin of Lawrence Livermore National Laboratory told nanotechweb.org. "The fast transport was predicted by molecular dynamics simulations."

Bakajin and colleagues fashioned the membrane from a vertically-aligned array of double-walled carbon nanotubes made by catalytic chemical vapour deposition. They surrounded the nanotubes with a matrix of silicon nitride, before ion milling to remove excess silicon nitride, and reactive ion etching to open up the ends of the nanotubes.

"We wanted to use something that can be uniformly deposited from the vapour phase," said Bakajin. "We tried silicon nitride because it is often used for fabrication of large but thin membranes in MEMS applications."

Transport of gas and water through the nanotubes was much higher than predicted by classical diffusion or hydrodynamics models. The researchers believe the fast transport occurs because of the smoothness of the nanotube interiors or because of molecular ordering phenomena. They estimated the pore density at 2.5 x 1011 per sq. cm.

The nanotube membranes could offer a combination of high selectivity and high flow rate. "Though our membranes have an order of magnitude smaller pore size, the enhanced flow rate per pore and the high pore density makes them superior in both air and water permeability compared to conventional polycarbonate membranes," said Bakajin.

Now the researchers hope to understand why the transport through nanotubes is so fast, as well as to work on practical applications.

"We are at the point where we can make structures that are similar to aquaporins, the 'water channels' that exist in cells," said Bakajin. "We are hoping to understand transport in carbon nanotubes, which should give us an insight into the physical principles that govern transport of water and ions though cellular channels."

The researchers reported their work in Science.