Ozone makes nanotubes shine brighter

A breakthrough in carbon nanotubes could expand their biological and material applications, allowing them to be tracked in single cells or used in novel lasers and more.

By adding minute amounts of ozone to batches of single-walled carbon nanotubes and then exposing them to light, a team at Rice University succeeded in covering all the nanotubes with oxygen atoms, which systematically changed their near-IR fluorescence. Although chemical reactions on the surfaces of nanotubes tend to kill their already limited natural fluorescence, the researchers said the new process actually enhances the intensity and even shifts the wavelength.

Previous studies had looked at the interaction between ozone and nanotubes, but the team said those studies had used too much ozone exposure, killing the fluorescence. Instead, the new approach involves adding one oxygen atom for every 2000 to 3000 carbon atoms.

Single-walled carbon nanotubes treated with ozone incorporate oxygen atoms that shift and intensify the nanotubes’ near-infrared fluorescence emission. The discovery could lead to new uses for nanotubes in biomedicine and materials science. Courtesy of Bruce Weisman, Rice University.

After doping, most sections of the nanotubes continue to absorb IR light as normal, forming excitons that jump back and forth along the tube until they come into contact with oxygen. When an exciton encounters a doping site, it tends to get trapped and emit red-shifted light. So the nanotube becomes an antenna that absorbs light and funnels the energy to the doping site, according to the researchers.

The fluorescent properties that result from the doping process are stable for months at a time.

Using IR instead of visible light overcomes the challenge of background emissions from cells and tissues. The team tested the visibility of the doped nanotubes in a tissue culture and found that images of the cells that were excited in the near-IR wavelength showed single nanotubes shining brightly. When the same sample was excited with visible light, a background haze appeared and rendered the tubes much less visible.

The results were reported online in the journal Science on Nov. 25, 2010.