WEST LAFAYETTE, Ind. — Inspired by nature's own building blocks, Purdue University researchers are using the same principle that makes DNA strands link together to create tiny structures that may someday be used to manufacture molecular wires and other components for use in nanometer-sized electronic devices.
Purdue chemist Hicham Fenniri has created molecules designed to automatically find each other and link to form elaborate but tiny tubes. The new technique allows scientists, for the first time, to use self-assembly techniques to develop nanoscale structures with specific dimensions and chemical properties.
The findings, which will be detailed in the April 25 issue of the Journal of the American Chemical Society, may help pave the way for designing new materials, electronic devices and drug delivery systems for use in the atom-size realm of nanotechnology.
"The beauty of this system is that, by designing the molecules that make up the system, we have perfect control over every part of the system," says Fenniri, an assistant professor of chemistry who directed the effort. "We not only dictate how the molecule behaves, but we also can control the dimensions and chemical properties of the resulting nanotube."
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.
The nanotubes developed at Purdue are the first to use a self-assembly process capable of producing the tiny structures in large quantities, making the process more feasible for use. 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 synthetic effort," Fenniri says. "Along with its high yields — generally close to 100 percent — the process also is self-correcting, so the resulting structures are predictable and error-free."
To develop the structures, Fenniri and his group borrowed chemistry from deoxyribonucleic acid, or DNA, to create a series of molecules that are "programmed" to link in groups of six to form tiny rosette-shaped rings. Numerous rosettes then combine to form tiny, rod-like structures, or nanotubes.
"We took the chemical bond that binds the base pairs guanine and cytosine in DNA and put it in a synthetic system to develop a new molecule," Fenniri says. "The result is a single molecule that contains the traits of both guanine and cytosine."
Endowed with the characteristics of its two parent molecules, the new molecule is somewhat "confused," Fenniri says. Created from elements that normally bind together, the molecule recognizes and wants to mate with itself. In addition, one end of the molecule serves to attract water while the other end repels it.
Fenniri says the molecule's attempts to make order out of its confusion help spark the self-assembly process, allowing the molecules to form rings and tubes without intervention.
"The way the molecule is designed, the only way it can recognize itself is by forming a ring," Fenniri says. "In this case, it forms a six-membered ring, or rosette. The ring is maintained by hydrogen bonds, exactly as DNA is maintained."
As the molecules form a ring, the water-loving ends join on the outside of the ring, burying the ends with an aversion for water on the inside.
"The inside surface of the ring is trying to avoid water, but the outside surface of the ring is attracting water," Fenniri says. "In response, the new assembly looks for something that looks like itself — another ring — to protect the inside molecules."
The rings then begin forming stacks, naturally and spontaneously. "It's totally self-assembled" Fenniri says. "You don't have to input any energy."
In addition, the water-loving ends on the outside of the tube have both a negative and a positive charge. As the stacks form, the ends line up with a positively charged particle on one level binding to a negatively charged particle on the next level, creating an electrostatic "belt" that wraps around the tube. This electrostatic belt serves to hold the structure together and keep it stable, Fenniri says.
"This belt, produced from electrostatic bonds, creates a new level of organization and can be manipulated to change the chemical properties of the molecule," he says. For example, by attaching a photo active substance — one that can absorb 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.
"In this way, we can tailor structures to perform specific tasks," Fenniri says.
Nanotubes made in this manner may be used in applications ranging from new structural materials that are stronger and lighter weight to electronic components for new supercomputers to drug delivery systems, Fenniri says. Purdue has applied for a patent on the process.
The new nanotubes also may offer some advantages over other types of structures, such as carbon nanotubes, Fenniri says.
"One major advantage is that we can produce these nanotubes in very large quantities in the laboratory," he says. "Carbon nanotubes, with all their promise, also have some limitations. For example, the process used to produce them is very tedious and results in only small quantities. Also, researchers, at this time, have no way to control the dimensions of carbon nanotubes. They get only a random mixture of pieces and then have to find a way to purify it."
Fenniri and his research group are now studying the thermodynamics of the system to learn more about how the structures self-assemble and how each component contributes to the process.
"If we can dissect and understand all the components leading to self-assembly and self-organization processes we would be in position to predict and tailor new functional materials with unparalleled performances," Fenniri says. "Once this exploratory phase is completed, we will have more control over matter, and that's the ultimate goal of nanotechnology".
The research at Purdue was funded by the National Science Foundation (Career Award), the American Cancer Society, the American Chemical Society, the Showalter Foundation and Purdue University.
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