Brighter Nanotubes Unveiled

In a discovery that could have significant applications in medical imaging, homeland security, biological sensors and other areas, chemists have found a way to increase the luminescence efficiency of single-walled carbon nanotubes.

According to University of Connecticut chemists, increasing the luminescence efficiency of carbon nanotubes could make it possible for doctors to inject patients with microscopic nanotubes to detect tumors, arterial blockages and other internal problems. Rather than relying on potentially harmful x-rays or the use of radioactive dyes, physicians could simply scan patients with an infrared light that would capture a very sharp resolution of nanotubes’ luminescence in problem areas.

The research was performed in the Nanomaterials Optoelectronics Laboratory at the Institute of Materials Science at UConn. A patent for the process is pending.

UConn chemist Fotios Papadimitrakopoulos describes the finding as a major breakthrough and one of the most significant discoveries in his 10 years of working with single-walled carbon nanotubes. Assisting Papadimitrakopoulos with the research were Polymer Program graduate student Sang-Yong Ju (now a researcher at Cornell University) and William P. Kopcha, a former chemistry undergraduate assistant in the College of Liberal Arts and Sciences, who now is a first-year graduate student at UConn.

Although carbon is used in many diverse applications, scientists have long been stymied by the element’s limited ability to emit light. The best scientists have been able to do with solution-suspended carbon nanotubes is to raise their luminescence efficiency to about one-half of 1 percent, which is extremely low compared with that of other materials, such as quantum dots and quantum rods.

By tightly wrapping a chemical “sleeve” around a single-walled carbon nanotube,
Papadimitrakopoulos and his research team reduced exterior defects caused by chemically absorbed oxygen molecules.

This process can best be explained by imagining sliding a small tube into a slightly larger diameter tube, Papadimitrakopoulos says. For this to happen, all deposits or protrusions on the smaller tube have to be removed before the tube is allowed to slip into the larger tube. What is most fascinating with carbon nanotubes, however, Papadimitrakopoulos says, is the fact that, in this case, the larger tube is not as rigid as the first tube (i.e. carbon nanotube) but rather is formed by a chemical “sleeve” comprising a synthetic derivative of flavin (an analog of vitamin B2) that adsorbs and self-organizes onto a conformal tube.

Papadimitrakopoulos claims that this process of self-assembly is unique in that it not only forms a new structure but also actively “cleans” the surface of the underlying nanotube. It is the active cleaning of the nanotube surface that allows the nanotube to achieve luminescence efficiency to as high as 20 percent.

“The nanotube is the smallest tube on Earth, and we have found a sleeve to put over it,” Papadimitrakopoulos says. “This is the first time that a nanotube was found to emit with as much as 20 percent luminescence efficiency.”

Papadimitrakopoulos has been working closely with the UConn Center for Science and Technology Commercialization (CSTC) in transferring his advances in research into the realm of patents, licenses and corporate partnerships. The CSTC was created several years ago as a way to help expand Connecticut’s innovation-based economy and to help create new businesses and jobs.

This is the second major nanotube discovery at UConn by Papadimitrakopoulos in the past two years. Last year, Papadimitrakopoulos and Sang-Yong Ju, along with other UConn researchers, patented a way to isolate certain carbon nanotubes from others by seamlessly wrapping a form of vitamin B2 around the nanotubes. It was out of that research that Papadimitrakopoulos and Sang-Yong Ju began wrapping nanotubes with helical assemblies and probing their luminescence properties.

The more luminescent the nanotube, the brighter it appears under infrared irradiation or by electrical excitation (such as that provided by an LED). A number of important applications may be possible as a result of this research, Papadimitrakopoulos says. Not only are carbon nanotube emissions extremely sharp, but they also appear in a spectral region where minimal absorption or scattering takes place by soft tissue.

Moreover, carbon nanotubes display superb photobleaching stability and are ideally suited for near-infrared emitters, making them appropriate for applications in medicine and homeland security as bioreporting agents and nanosize beacons. Carbon nanotube luminescence also has important applications in nanoscaled LEDs and photodetectors, which can readily integrate with silicon-based technology. This provides an enormous repertoire for nanotube use in advanced fiber optics components, infrared light modulators and biological sensors, where multiple applications are possible due to the nanotube’s flavin-based (vitamin B2) helical wrapping.

For more information, visit: www.uconn.edu