Nanotubes shine brightly in batches

05 Jul 2007

Large batches of fluorescent nanotubes have been produced which shine brightly enough for nanoscale medical applications.

A centrifugal processing method is helping researchers in the US to produce large batches of highly fluorescent carbon nanotubes (CNTs). What's more, the CNTs emit light with a quantum efficiency that is claimed to be significantly higher than previously reported. (Journal of the American Chemical Society 129 8058)

"We achieved a quantum efficiency of slightly above 1 %. This figure has never been achieved for large batches," Tobias Hertel, a professor at Vanderbilt University, told optics.org. "So far, high quantum yields have only been observed in a few single nanotubes by microscopic techniques."

Separating nanotubes

Now that the researchers know how to separate out the brightest CNTs, they hope that the emission properties can be exploited in clinical applications. "We believe that CNTs find uses in the life sciences arena as chemical probes, contrast agents or as nanoscopic energy converters for laser-induced thermal cancer treatment," commented Hertel.

CNTs have a high surface-to-volume ratio making them particularly susceptible to their environment. "This means that they may find uses as probes of biochemical changes and processes in living organisms," said Hertel.

The team is now optimizing the separation process. "We hope to improve the purification protocols to prepare samples with tubes of a desired bandgap that emit at designated wavelengths," said Hertel. "We will also replace the surfactants used in the initial purification with functional molecules, allowing us to target specific cancer cells."

The method that researchers use to produce nanotubes creates a soot containing different types of nanotubes: metallic, semiconducting and single-walled for example. Hertel's team used density gradient ultracentrifugation (DGU) - a technique that sorts objects by their buoyant density - to separate out the highly fluorescent CNTs.

"DGU allows us to separate highly fluorescent individual tubes from small aggregates which tend to have smaller photoluminescence quantum yields," explained Hertel. "The technique also allows us to get rid of metallic tubes which do not emit light and often diminish the ability of nearby semiconducting tubes to emit light."