"One of the longstanding problems in medicine is how to cure cancer without harming normal body tissue," said Hongjie Dai, of Stanford University. "Standard chemotherapy destroys cancer cells and normal cells alike. That's why patients often lose their hair and suffer numerous other side effects. For us, the Holy Grail would be finding a way to selectively kill cancer cells and not damage healthy ones."

Dai and colleagues functionalized the nanotubes with a folate moiety. Unlike normal cells, cancer cells have receptors for folate on their surface. This means that in laboratory tests cancer cells took up the nanotubes by a process of endocytosis, but normal cells did not.

Irradiating the nanotubes with near-infrared light excited electrons in the structures, causing them to heat up and destroy the surrounding cancer cells. Typically, an 808 nm-wavelength beam with a power of 1.4 W/cm2 caused extensive cell death after two minutes, but such wavelengths passed harmlessly through healthy cells.

"We're using an intrinsic property of nanotubes to develop a weapon that kills cancer," said Dai. "The laser we used is a 3 cm beam that's held like a flashlight," said Dai. "We can take the beam and put it anywhere we want. We can shine it on a local area of the skin or inside an internal organ using a fibre-optic device."

And that's just the start. "Folate is just an experimental model that we used," said Dai. "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."

A team from Rice University, US, has carried out a similar technique using gold nanoshells in place of carbon nanotubes. But Dai and colleagues say their nanotube method compares favourably as it needs a lower laser power and shorter radiation times to destroy cancer cells.

Dai and colleagues also used nanotubes to transport molecules inside cells. They conjugated DNA to the nanotubes and allowed cells to take up the nanotubes by endocytosis. Then they used a pulsed near-infrared laser to break the membrane around the nanotube and detach the DNA. Pulsing the laser meant that it released the DNA without heating the cell sufficiently to cause its death. The result was delivery of the DNA to the cell nucleus. Dai says that delivering therapeutic molecules of DNA, RNA or protein directly into the cell nucleus could help fight various infections and diseases.

The researchers reported their work in PNAS.