Self-assembling Nanotubes Can Desalinate Seawater

Self-assembling hollow tubes with diameters of only a few billionths of a metre could be used to delivering cancer-fighting drugs inside cells or to desalinate seawater.

Constructing nanostructures has proved to be a difficult and complex process, especially when attempting to create a large quantity with the same traits, such as millions of nanotubes with identical diameters.

But a team from the US Department of Energy's Berkeley Lab has discovered a family of nature-inspired polymers that spontaneously assemble into hollow crystalline nanotubes when placed in water.

The nanotubes can also be tuned to all have the same diameter of between five and ten nanometres, depending on the length of the polymer chain.

The polymers have two chemically distinct blocks that are the same size and shape. The team found that the blocks act like molecular tiles capable of forming rings which stack together to form nanotubes up to 100 nanometres long, all with the same diameter.

"This points to a new way we can use synthetic polymers to create complex nanostructures in a very precise way," said Ron Zuckermann, who directed the team.

"Creating uniform structures in high yield is a goal in nanotechnology," he added. "For example, if you can control the diameter of nanotubes, and the chemical groups exposed in their interior, then you can control what goes through - which could lead to new filtration and desalination technologies, to name a few examples."

The research is the latest in the effort to build nanostructures that approach the complexity and function of naturally occuring proteins but are made of durable materials.

The Berkeley team studied a polymer that is a member of the peptoid family, rugged synthetic polymers that mimic peptides, which are used to form proteins. They can be tuned at the atomic scale to carry out specific functions.

They looked specifically at ‘diblock copolypeptoid’ because it binds with lithium ions and could be used as a battery electrolyte.

Although they are still unclear about how the nanotubes are actually formed, the research sheds light on their structure, and hints at a new design principle that could be used to build nanotubes and other complex nanostructures.

Cryo-electron microscopy imaging of 50 of the nanotubes showed the diameter of each tube is highly uniform along its length, as well as from tube to tube.

They also assemble themselves without the usual nano-construction aids, such as electrostatic interactions or hydrogen bond networks.

"You wouldn't expect something as intricate as this could be created without these crutches," Zuckermann said. "But it turns out the chemical interactions that hold the nanotubes together are very simple.

“What's special here is that the two peptoid blocks are chemically distinct, yet almost exactly the same size, which allows the chains to pack together in a very regular way. These insights could help us design useful nanotubes and other structures that are rugged and tunable--and which have uniform structures."

An Australian team from Melbourne University recently demonstrated that nanostructures interwoven into clothes can degrade organic matter when exposed to light, eliminating the need for washing.

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Taxonomy

• Nano Materials
• Desalination
• Nano Technology
• Nanotextiles