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Posted: April 1, 2009

Surface smoothing increases carbon nanotube luminescence 40-fold

(Nanowerk News) Imagine you are looking through a very high-powered microscope at the smallest tube in the world – a single-walled carbon nanotube so tiny that a million can fit on the head of a pin.
Imagine too that the exterior of the tube is covered in small irregular bumps caused by oxygen molecules that cling to the outside like barnacles on a pier. Now imagine trying to slide something – a slightly larger tube perhaps – over the bumpy tube to smooth out the surface.
In this molecular, microscopic world, it isn’t easy; a near impossibility, in fact, that has proven a barrier to scientists for years.
But now, chemistry professor Fotios Papadimitrakopoulos and a team of researchers in the Nanomaterials Optoelectronics Laboratory at the Institute of Materials Science have found a way to smooth the surface of nanotubes, in what Papadimitrakopoulos describes as a major nanotechnology breakthrough that could have significant applications in medical imaging and other areas.
By developing a process in which a chemical ‘sleeve’ tightly wraps itself around the nanotube, Papadimitrakopoulos managed to not only create a smooth new surface on the nanotube but also to ‘clean’ its underlying exterior of defects in a way that has never been accomplished before.
Carbon nanotubes have traditionally been very poor emitters of light because of their bumpy exterior defects and have therefore been limited in some of their technological and medical applications.
As a result of the newly discovered wrapping process, Papadimitrakopoulos managed to increase the luminescence efficiency – the light emitting capability – of the nanotube 40-fold.
That increased luminescence, he says, opens the way for broad new advancements in science.
“The nanotube is the smallest tube on earth and we have found a sleeve to put over it,” says Papadimitrakopoulos, whose discovery is featured in the March 6 issue of Science magazine ("Brightly Fluorescent Single-Walled Carbon Nanotubes via an Oxygen-Excluding Surfactant Organization").
“This is the first time that a nanotube was found to emit with as much as 20 percent luminescence efficiency.”
The more luminescent the nanotube, the brighter it appears under infrared irradiation or by electrical excitation (such as that provided by a light-emitting diode or LED).
Carbon nanotube emissions are not only extremely sharp, but they also appear in a spectral region where minimal absorption or scattering by soft tissue takes place, making them ideal for medical imaging, Papadimitrakopoulos says.
Increasing the luminescence efficiency of carbon nanotubes may someday 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 the luminescence of the nanotubes in problem areas in very sharp resolution.
Carbon nanotubes also have properties that make them ideally suited for near-infrared emitters, Papadimitrakopoulos says, making them appropriate for applications in homeland security as bio-reporting agents in the case of terrorist attacks and as nano-sized beacons.
Their luminescence also allows them to 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.
The key to the discovery was a flavin-based (Vitamin B2) helical wrapping that formed an especially tight and seamless barrier around the nanotube.
Working closely with Papadimitrakopoulos in discovering the wrapping process were Sang-Yong Ju, a graduate student in the Polymer Program (now a researcher at Cornell University), and William Kopcha, a former undergraduate in chemistry in the College of Liberal Arts and Sciences who is now a first-year graduate student here.
The Center for Science and Technology Commercialization (CSTC) is assisting Papadimitrakopoulos in obtaining a patent for the process.
In addition, Xiao-Ming Xu, a graduate student in the Department of Pharmaceutics under the supervision of Professor Diane Burgess, created a computerized animation of the wrapping process that has allowed Papadimitrakopoulos to receive international media attention for the discovery.
The animation can be found at: http://www.ims.uconn.edu/~papadim/research.htm
This is the second major nanotube discovery at UConn by Papadimitrakopoulos in the past two years. Last year, Papadimitrakopoulos, Sang-Yong Ju, and other UConn researchers patented a way to isolate certain carbon nanotubes from others by using a similar method of 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.
Source: University of Connecticut