By R. Colin Johnson
WEST LAFAYETTE, Ind. -- Using a more complex system of atoms than carbon nanotubes, scientists at Purdue University have devised a tunable approach to nanotube creation that allows them to build application-specific varieties. Called "rosette nanotubes" and built from a combination of carbon, nitrogen, hydrogen and oxygen, the new structures offer unique physical, chemical and electrical properties, the researchers said.
"The physical and chemical properties of our rosette nanotubes can now be modified almost at will through a novel dial-in approach," said Purdue professor Hicham Fenniri. The scheme "gives them the properties and biomimetic behaviors needed for specific applications," said Fenniri, who was assisted by fellow Purdue researchers Bo-Liang Deng and Alexander Ribbe.
This patented class of self-assembling organic structures is shaped with a hollow central interior channel that runs the length of the nanotube. Unlike carbon nanotubes, it has tunable inner and outer diameters. On the outside, smaller hollow channels are custom-tailored to harbor specific molecules useful for a given application.
Fenniri begins with a nanotube "seed" molecule, which self-assembles in water into tiny rings. Then the rings snap together into tubes that can be grown to any desired length. Self-assembly is accomplished by coding the edges of the rings so they can form bonds only with other rings in the correct orientation. Since the rosettes are hydrophobic (repelled by water) on the inside and hydrophilic (attracted to water) on the outside, they self-assemble by stacking into tubes in an attempt to protect their insides from water. The result is self-assembly operations that, by design, can occur in one, and only one, manner.
Fenniri previously demonstrated two seed molecules for electronics: one that grows conventional wires, for electricity, and one for growing photonic nanotubes that process light.
"We are optimistic that we can make many specialized nanotubes that are useful for a new generation of computer memory systems, high-definition displays, biosensors and drug delivery systems," said Fenniri.
The results confirm that not only is it possible to produce nanotubes of specific sizes, but also that a specific shape and composition can literally be made to order by including application-specific properties, such as for strength and electrical conductivity. To add strength, for instance, a nylon molecule could be hung on the outside hooks, in effect adding a tough insulator around nanoscale wires.
Since Fenniri's nanotubes catalyze their own formation, the addition of application-specific raw materials on their outsides can be done in a test tube. Then, by adjusting the temperature, pressure and other environmental factors, the nanotubes self-organize into the desired configuration.
One of the most interesting new capabilities harnessed by Fenniri's approach is the so-called "chiroptical" properties of nanotubes. Like DNA molecules, the rosette nanotubes can be grown in spiral configurations that match up with others t are similar. DNA spirals intertwine because they both twist in the same direction. However, unlike nature, Fenniri has found a way to make nanotubes spiral in either a left- or right-handed manner, permitting two different kinds of otherwise identical nanotubes.
"We tune the chiroptical properties of our nanotubes by controlling their supramolecular helicity," said Fenniri. "That is, we can make them turn left or right to varying degrees using external chemical inducers called promoters."
According to Fenniri, a promoter induces both dominant and recessive behaviors, just as in real DNA. The nanotube "prefers" to bind with a dominant promoter, but will express its recessive behavior if its promoter is in low concentration. Fenniri demonstrated two routes to building the same nanotube, the slow recessive route or the fast-track dominant route, which is called a "supramolecular chain reaction." Supramolecular chain reactions speed up the self-assembly process with an external trigger promoter.
Since their discovery in 1991, nanotubes have been demonstrated as a channel transistor at IBM Corp., as the electron emitter for microscopic vacuum tubes by Agere Systems Inc., and in fuel cell batteries that can power a laptop computer for days instead of hours by NEC Corp.
In addition to electronic circuitry, Fenniri claims nanotubes will find myriad new applications in fields ranging from disease treatment to plastics manufacturing to optical information storage.