Biological nanopores can rapidly transport selected ions and molecules through membranes. Aquaporins, for example, transport water at extremely high rates and can also desalinate. These proteins contain channels that are just 0.3 nm wide, which forces the water molecules to pass through in a single file.

A team of researchers led by Aleksandr Noy has now made the first artificial nanopore system that is not only better than aquaporin at filtering water but which can also filter out specific ions as well as biological ion channels can. The new membrane is made of carbon nanotubes (CNTs) that are 10 nm long and 0.8 nm wide embedded in a lipid bilayer. Again, the nanotubes are narrow enough to squeeze water molecules so that they pass through in 1D single file.

Testing out different CNT widths

The researchers, reporting their work in Science, obtained their results by testing out nanotubes of different widths to find out which were the best at filtering water. “We did this by embedding the tubes in lipid vesicle walls, applying osmotic pressure to the vesicles and then measuring how fast they shrunk as water flowed out of them through the ‘nanotube porins’,” explains team member and co-first author of the study Rama Tunuguntla.

The researchers found that nanotubes with a diameter of 0.8 nm are six times more efficient at filtering water than aquaporins. The flux through these tubes is also 10 times greater than those through 1.5 nm-sized ones.

Atomically smooth CNT walls help speed up water transport

The higher water transport rate comes thanks to the absence of interactions between the water molecules and the atomically smooth CNT walls, they say. In comparison, the amino acids inside aquaporins can donate or accept hydrogen bonds to water and the kinetics of this bond formation or breakage slows down water transport.

The researchers also found that the membranes are able to remove anions from saltwater as well as aquaporins and only fail at very high salt concentrations. “They are also much more robust than their biological cousins and are cheaper to make too,” says team member and co-first author Robert Henley.

CNT pores can be made into switchable ionic diodes

And that's not all: by modifying the potential at the mouth of a nanotube, the researchers discovered that they could change the membrane’s ion selectivity. The device thus acts as a switchable ionic diode that transports ions in just one direction.

According to Zuzanna Siwy of the University of California at Irvine and Francesco Fornasiero of the LLNL, who discuss Noy and colleagues’ research in a related Science Perspective, this means that such narrow CNTs embedded in a lipid bilayer could form the basis of artificial cells in the future. They might even make for key components in advanced, lipid-bilayer-mediated interfaces between living organisms and artificial devices.

Towards improved desalination membranes

“We hope that CNT porins, and more generally small diameter CNTs, will eventually result in improved desalination membranes that could deliver high flux and better fouling resistance to help alleviate the global water threat,” say Tunuguntla and Henley. “Of course, the economics of desalination is much more complicated than just making high flux membranes, but we hope that our work will renew interest in this field,” adds Noy.

The researchers say that they are now studying the fundamental transport mechanisms in CNT pores. They are also trying to make more robust and scalable architectures with their nanotube porins.