Chemists discover lighter side of carbon nanotubes

Chemists at Rice University, Texas, have added fluorescence to the list of unique physical properties exhibited by the fullerene family of carbon molecules.

A report in the current issue of Science magazine details how a team of Rice University chemists, led by fullerene discoverer and Nobel laureate Richard Smalley, first observed fluorescence in carbon nanotubes.

Fluorescence occurs when a substance absorbs one wavelength of light and emits a different wavelength in response. The Rice experiments, conducted by Smalley’s group and the photophysics research team of chemist R. Bruce Weisman, found that nanotubes absorbed and gave off light in the near-infrared spectrum, which could prove useful in biomedical and nanoelectronics applications.

‘Some of the most sophisticated biomedical tests today – such as MRI exams – cannot be performed in a doctor’s office because the equipment too large and too expensive to operate,’ said Smalley, University Professor at Rice. ‘Because nothing in the human body fluoresces in the near-infrared spectrum, and human tissue is fairly transparent at that spectrum, one can envision a test apparatus based on this technology that would be as inexpensive and simple to use as ultrasound.’

Optical biosensors based on nanotubes could also be targeted to seek out specific targets within the body, such as tumour cells or inflamed tissues. Targeting would be achieved by wrapping the tubes with a protein that would bind only to the target cells.

Since nanotubes fluoresce with a single wavelength of light, and different diameter nanotubes give off different wavelengths, it may be possible to tailor different sizes of tubes to seek specific targets, and thus diagnose multiple maladies in a single test using a cocktail of nanotubes.

Carbon nanotubes are a member of the fullerene family of carbon molecules, a third molecular form of carbon that is distinct from diamond and graphite.

Like all fullerenes, carbon nanotubes are very stable and almost impervious to radiation and chemical destruction. They’re small enough to migrate through the walls of cells, conduct electricity as well as copper, conduct heat as well as diamond and are 100 times stronger than steel at one-sixth the weight.

Much of Smalley’s current research involves bridging the gap between ‘wet’ nanotechnology – the molecular, biochemical machinery of life – and ‘dry,’ insoluble nanomaterials like fullerenes.

Toward that end, Smalley’s lab has churned out dozens of varieties of soluble fullerenes by wrapping nanotubes in various polymers, including proteins, starches and DNA.

In the fluorescence experiments, Smalley and Weisman’s teams observed the effect only in nanotubes that were untangled and isolated from neighbouring tubes.

Researchers bombarded clumps of nanotubes with high-frequency sound waves to separate them, and they encased each individual tube in a molecule of sodium dodecylsulfate in order to isolate it from its neighbours. Fluorescence was observed in both plain and polymer-wrapped nanotubes.

In addition to biomedical applications, the fluorescence research could prove useful in the field of nanoelectronics because it confirms that nanotubes are direct band-gap semiconductors, which means they emit light in a way that could be useful for engineers in the fibre optics industry.