Ozone makes nanotubes shine brighter
HOUSTON – 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.
- The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
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