Carbon nanotubes found to fluoresce

Optical properties could prove useful in biomedical, nanoelectronic applications

Add fluorescence to the growing list of unique physical properties associated with carbon nanotubes — the ultrasmall, ultrastrong wunderkind of the fullerene family of carbon molecules.

In research detailed in the current issue of Science magazine, a team of Rice University chemists led by fullerene discoverer and Nobel laureate Richard Smalley describes the first observations of 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 tumor 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. The discovery of fullerenes in 1985 earned Smalley a share of the Nobel Prize.

Like all fullerenes, carbon nanotubes are extraordinarily 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 neighboring 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 neighbors. 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 fiber optics industry.

The Rice research team included Michael O’Connell, Sergei Bachilo, Chad Huffman, Valerie Moore, Michael Strano, Erik Haroz, Kristy Rialon, Peter Boul, William Noon, Carter Kittrell, Jianpeng Ma and Robert H. Hauge. The research was funded by the National Science Foundation and the Robert A. Welch Foundation.