Adaptable nanotubes make way for custom-built structures

Tiny self-assembled tubes may now be used as a scaffold to custom-build molecular wires and other components for use in nanometer-sized electronic devices.

Purdue University researcher Hicham Fenniri has developed a method to create self-assembling nanotubes that can be easily manipulated with specific dimensions or chemical properties.

The nanotubes can be used as a frame on which various objects – in this case chemicals, molecules or even metals – can be added to give the structure a specific property or direct it toward a selected target, Fenniri said. Tailoring structures in such ways will reportedly allow scientists to develop high performance materials or new tools to diagnose and treat disease.

To develop the structures, Fenniri and his group created a series of molecules that are ‘programmed’ to link into groups of six to form tiny rosette-shaped rings. The rings are maintained by hydrogen bonds.

The molecules that make up the rings are bipolar, with one end of the molecule working to attract water and the other end repelling it. As the molecules join to form a ring, the water-attracting ends connect on the outside of the ring, burying the hydrophobic ends on the inside.

He said the molecule’s attempts to make order out of these contradictory conditions help spark a self-assembly process (developed by Fenniri and his group last year), allowing the molecules to form tubes without intervention.

‘The inside surface of the ring is trying to avoid water, but the outside surface of the ring is attracting water,’ he said. ‘In response to this situation, the assembly links to another ring to protect the inside molecules.’

This self-assembly process takes place in water and, although driven by hydrophobic interactions, it is in fact orchestrated by hydrogen bonds, Fenniri said.

As the rings stack to form a tube, electrical charges on the outside of the tube create an electrostatic ‘belt’ that wraps around the structure. Fenniri said the electrostatic belt serves to hold the nanotube together and keep it stable, and provides an anchor to which chemicals or other molecules can be added to the structure.

The structures developed using Fenniri’s self-assembling system may prove to be especially useful in industrial applications because they remain stable under high temperature conditions. In fact, the tiny structures, fuelled by hydrophobic attractions between the molecules, actually increase in size under high temperatures, Fenniri said.

‘This opposes common wisdom, because generally when you heat something it falls apart,’ he said. ‘Our demonstrations show that these structures become more stable under the influence of temperature and attain a new level of self-organisation.’