You Light Up My Nanolife

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 July 26 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 radiation at one wavelength then reradiates the energy, usually at a different wavelength. 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.

In the fluorescence experiments, the researchers observed the effect only in nanotubes that were untangled and isolated from other tubes. They bombarded clumps of nanotubes with high-frequency sound waves to separate them, then they encased each individual tube in a molecule of sodium dodecyl sulfate in order to isolate it from its neighbors. Fluorescence was observed in both plain and polymer-wrapped nanotubes.

“Some of the most sophisticated biomedical tests today—such as MRI exams—cannot be performed in a doctor’s office because the equipment is too large and too expensive to operate,” says Smalley. “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 be used 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 specified 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 different specific targets and thus diagnose multiple maladies in a single test using a cocktail of nanotubes.

The fluorescence research also could find application 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.

Like all fullerenes, carbon nanotubes are extraordinarily stable and almost impervious to radiation and chemical destruction. They are small enough to migrate through the walls of cells, they conduct electricity as easily as copper and heat as easily as diamond, and they 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.

The Rice research teams 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.

—Jade Boyd