March 11, 2002
Adaptable nanotubes make way for custom-built structures, wires
WEST LAFAYETTE, Ind. Tiny self-assembled tubes, about 1/1,000th the width of a grain of sand, may now be used as a scaffold to custom-build molecular wires and other components for use in nanometer-sized electronic devices, including some that could be inserted into the body.
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.
Like molecular-sized Tinkertoys, 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 says. Tailoring structures in such ways will allow scientists to develop high performance materials or new tools to diagnose and treat disease.
"By using different chemicals on the outside of the structure, you can modify its function or make it bind to a specific target, such as an amino acid," he says. "It's like we have a skeleton, and we just have to put a dress on it. And we can decorate the tube with all sorts of dresses."
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, fueled by hydrophobic attractions between the molecules, actually increase in size under high temperatures, Fenniri says.
"This opposes common wisdom, because generally when you heat something it falls apart," he says. "Our demonstrations show that these structures become more stable under the influence of temperature and attain a new level of self-organization."
The findings currently on the Proceedings of the National Academy of Sciences' Web site and soon to be published in a print issue of the journal may pave the way for designing new materials, electronic devices and drug delivery systems for use in the atom-size realm of nanotechnology. Purdue has applied for a patent on the process.
The idea of using very small components, or nanotechnology, to make computers and electrical devices including biomedical devices that could be inserted into the body has been the subject of much scientific interest and research. Nanotechnology refers to components only a few nanometers in size. A nanometer is one-billionth of a meter.
To develop the structures, Fenniri and his group expanded on a system they developed last year to produce self-assembling nanotubes. Self-assembly is a principle familiar in biology, where the right mix of biological molecules will interact on their own to form distinctive structures, such as cells, tissues and organs.
"The advantage of using a self-assembly process is that it dramatically simplifies the development process," Fenniri says. "The tubes form naturally and spontaneously. The process also is self-correcting, so the resulting structures are predictable and error-free."
Fenniri and his group created a series of molecules that are "programmed" to link in 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-loving ends connect on the outside of the ring, burying the hydrophobic ends those with an aversion to water on the inside.
Fenniri says the molecule's attempts to make order out of these contradictory conditions help spark the self-assembly process, 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 says. "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 says.
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 says 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.
"This belt, produced from electrostatic bonds, creates a new level of organization that can be manipulated to change the chemical properties of the molecule," Fenniri says.
For example, by attaching a photoactive substance one that can absorb solar energy and transfer it to another chemical scientists can create a tube that is capable of absorbing energy from one end and delivering it at the other.
Because the tiny structures flourish in the heat, the custom-built nanotubes may be especially useful in developing applications such as molecular electronics and photonics wires, or biomedical devices that could be inserted into the body, Fenniri says.
The research at Purdue was funded by the National Science Foundation, the American Cancer Society, the American Chemical Society, the Showalter Foundation and Purdue University.
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Entropically-driven self-assembly of
Hicham Fenniri, Bo-Liang Deng, Alexander E. Ribbe,
Rosette nanotubes are a new class of organic nanotubes obtained through the hierarchical self-assembly of low molecular weight synthetic modules in water (Fenniri, H. et al. 2001, J. Am Chem. Soc. 123, 3854-3855). Here we demonstrate that these materials can serve as scaffolds for the supramolecular synthesis of multi-channel nanotubular architectures, and report on the discovery of their entropy-driven self-assembly process.