Nanotech News

August 15, 2005

Carbon Nanotubes Deliver Payload, Cancer Cell-Killing Heat

Carbon nanotubes are proving to be a versatile tool for ferrying biological compounds into cells (see earlier story). Now, a team of investigators at Stanford University has shown that carbon nanotubes will both release a payload and turn into miniature thermal scalpels when irradiated with near-infrared light.

Writing in the journal Proceedings of the National Academy of Science USA, Hongjie Dai, Ph.D., and his colleagues describe how carbon nanotubes can be targeted to cancer cells growing in culture and used to either deliver DNA to the cell nucleus or kill the cell with high temperature. The key to either of these actions is that carbon nanotubes naturally absorb light in the near-infrared region of the light spectrum. Near-infrared light holds particular promise in nanotechnology-enabled cancer therapy because neither biological molecules nor water absorbs light in this frequency range, which enables near-infrared light to pass through tissues to reach tumor-targeted nanostructures such as carbon nanotubes.

"An interesting property of carbon nanotubes is that they absorb near-infrared light waves, which are slightly longer than visible rays of light and pass harmlessly through our cells," Dai says. But shine a beam of near-infrared light on a carbon nanotube, and the results are dramatic. Electrons in the nanotube become excited and begin releasing excess energy in the form of heat.

In their experiments, Dai’s team found that if they placed a solution of carbon nanotubes under a near-infrared laser beam, the solution would heat up to about 70° C (158° F) in two minutes. When nanotubes were placed inside cells and radiated by the laser beam, the cells were quickly destroyed by the heat. However, cells without nanotubes showed no effects when placed under near-infrared light.

The nanotubes were targeted to cancer cells by attaching folic acid to the surface of the nanotubes. Folic acid binds to a folic acid receptor protein found in abundance on the surfaces of many types of cancer cells. "Folate is just an experimental model that we used," Dai says. "In reality, there are more interesting ways we can do this. For example, we can attach an antibody to a carbon nanotube to target a particular kind of cancer cell."

One example is lymphoma, or cancer of the lymphatic system. Like many cancers, lymphoma cells have well-defined surface receptors that recognize unique antibodies. When attached to a carbon nanotube, the antibody would play the role of a Trojan horse. Dai and Dean Felsher, M.D., a lymphoma researcher in the Stanford School of Medicine, have begun a collaboration using laboratory mice with lymphoma. The researchers want to determine if shining near-infrared light on the animal's skin will destroy lymphatic tumors, while leaving normal cells intact.

The Proceedings of the National Academy of Science USA paper also showed that near-infrared radiation can trigger carbon nanotubes to release protein and DNA molecules that loosely bound to the nanotubes’ surface. This method uses pulses of near-infrared light, rather than continuous irradiation, to shake the therapeutic molecule loose from the nanotube after they have entered the cell.

Both sets of experiments are detailed in a paper titled, "Carbon nanotubes as multifunctional biological transporters and near-infrared agents for selective cancer cell destruction." This paper was posted online in advance of print publication. An abstract is available at the journal's website.
View abstract.