Fluorescent carbon nanotubes successfully harvested

Michael A. Greenwood

Researchers have separated highly fluorescent carbon nanotubes from their nonlight-producing brethren, a technique that could allow the tiny structures to be employed in a variety of medical imaging applications, and even in cancer treatment.

Nanotubes are manufactured in a variety of forms, including metallic and semiconducting as well as single- and multiwalled designs. Of these, only the single-walled semiconducting variety produces fluorescence. Previously, it was difficult to separate the various types in meaningful quantities, and no method exists to produce only the highly fluorescent variety.

Soapy separation

Researchers from Vanderbilt University in Nashville, Tenn., and from Brigham Young University in Provo, Utah, led by Tobias Hertel of Vanderbilt have developed a method that effectively isolates the fluorescent nanotubes from those that are useless for imaging purposes. The technique involves mixing the nanotubes with a special soap, accompanied with a blast of ultrasound, which breaks the clumps apart and dissolves them. The resulting liquid then undergoes density gradient ultracentrifugation to separate impurities.

Hertel’s team removed the most buoyant layer, let it set for a while and then placed it back in the ultracentrifuge for another 12 hours to further separate the layers according to their buoyant density. A uniform population of the single-walled nanotubes was contained in one of the layers. The density gradient technique has been around for decades but only recently has been applied to nanotubes.

This three-dimensional model shows a carbon nanotube encapsulated in a special soap that helps break apart clumps of nanotubes. The soap is part of a technique to separate fluorescent nanotubes from their nonlight-producing cousins.

Although much work remains to be done before nanotubes can be used in medicine, Hertel said that the structures have several potential advantages compared with other imaging agents, including quantum dots.

The harvested nanotubes exhibit a surprisingly high quantum efficiency and photostability. The fluorescence generated by the nanotubes can last for months. In contrast, the longevity of other imaging approaches is measured in hours or days. The nanotubes also emit at about 1000 nm, a wavelength that is ideal for use in the life sciences because skin and other tissue are transparent in the near-infrared, and the nanotube’s light is not absorbed as strongly.

Nanotubes eventually could be used as chemical probes and as contrast agents and could be employed in cancer treatment through laser-induced hyperthermia, Hertel said. In the latter technique, the nanotubes would be attached to cancerous tumors and exposed to a wavelength of light that would cause them to heat up to a point at which they would destroy the afflicted cells.

But those applications are likely at least several years away, as researchers continue to study the safety of nanotubes in humans. The nanotubes are nontoxic, and there is no evidence that they damage cells. But more testing will be needed to ensure that there are no unintended side effects, Hertel said.

Journal of the American Chemical Society, July 4, 2007, pp. 8058-8059.