Brightening prospects of using fluorescent nanotubes in medical applications

The researchers had expected this approach to boost the quantum efficiency by five to ten times. The fact that the improvement was considerably larger – 20 to 100 times – came as a pleasant surprise.

“Quantum efficiency is critical, but there are several other factors that make nanotubes particularly well suited for use in living systems,” says Hertel. These factors include:

• Nanotubes emit light in a very narrow range of wavelengths, or colors. This makes it easier to pick out their signal against background noise. Furthermore, they produce light in a part of the spectrum – the near infrared where skin and other tissue is transparent – that allows the nanotube light to stand out.

• Nanotubes are made entirely from graphitic carbon, which is non-toxic and, at least so far, experiments that have been done indicate that they do not damage living cells. By comparison, quantum dots, which are a popular alternative fluorescent tagging technology, contain cadmium, which is highly toxic, and so may not be appropriate for “in vivo” applications.

• Nanotube fluorescence is extremely stable and can last for months. Fluorescent proteins – widely used for imaging living systems – begin fading within a few hours. Quantum dots last several days before degrading.

Hertel’s team is currently working on the next step necessary for many biomedical uses: finding a way to attach molecules to the surface of the nanotubes that will allow them to bind to specific biological targets. The trick is to do so without dimming or extinguishing the nanotubes’ delicate fluorescence.

An example of the possible medical applications of nanotube fluorescence is a collaboration that Hertel and Associate Professor of Biomedical Research Duco Jansen are planning. Jansen has been pursuing research that uses gold nanoclusters to burn away cancer cells. He has developed a selective method for attaching the gold clusters to the surface of tumors and then exposing them to wavelengths of light that cause them to grow hot enough to destroy nearby cells. The approach has one drawback: He doesn’t have an easy way to identify the locations where the clusters attach. Nanotubes should work as well as gold clusters as microscopic blow torches while their fluorescence should make them easy to locate. At least that is the hypothesis the researchers hope to test.

Hertel’s co-authors on the paper are Vanderbilt graduate student Jared Crochet and Michael Clemens, a graduate student from Brigham Young University. The research was funded by grants from the American Chemical Society and the National Science Foundation.