A puff of ozone can make carbon nanotubes fluoresce more intensely, according to researchers in the US. The discovery might pave the way to devices for tracking single cells or a new class of near-infrared laser.

Rice University scientists in the lab of Bruce Weisman, a nanotube spectroscopy pioneer, realized that the natural fluorescence of carbon nanotubes is quenched by reactions on their surfaces. However, by adding a tiny amount of dissolved ozone to batches of single-walled carbon nanotubes in aqueous suspension and then exposing them to light, it is possible to pepper the nanotubes with oxygen atoms. This systematically shifts their fluorescence to longer near-infrared wavelengths while avoiding the quenching processes [Weisman et al., Science, (2010) doi: 10.1126/science.1196382]. Weisman, a physical chemist, points out that the preparation is simple enough even for a physical chemist to do!

"We're not the first people to study the effects of ozone reacting with nanotubes," Weisman concedes. "That's been done for a number of years. But all the prior researchers used a heavy hand, with a lot of ozone exposure. When you do that, you destroy the favorable optical characteristics of the nanotube. It basically turns off the fluorescence. In our work we only add about one oxygen atom for 2000-3000 carbon atoms, a very tiny fraction."

The researchers explain that the sparse nature of the oxygen distribution allows the doped nanotubes to absorb infrared light normally across most of their surface, forming excitons that can hop back and forth along the tube until they encounter an oxygen atom, at which point they stop because this is an energetically stable environment. Emission at a longer, red-shifted, wavelength can then take place. For biological sensing the use of longer wavelengths for fluorescence is important as it avoids noisy background emission.

"Essentially, most of the nanotube is turning into an antenna that absorbs light energy and funnels it to the doping site. We can make nanotubes in which 80 to 90 percent of the emission comes from doped sites," Weisman says. His team tested the doped nanotubes and found their fluorescent properties to persist after several months. Experiments with human uterine adenocarcinoma cells clearly revealed bright individual nanotubes under infrared excitation, whereas under visible excitation the nanotube emission competed with a background haze.

The researchers are now refining the process of doping the carbon nanotubes and suggest that this is currently the best approach to observing single nanotubes inside cells and tissues for biodistribution studies.

David Bradley