"Carbon nanotubes naturally fluoresce in the near-infrared region of the spectrum, where human tissue and biological fluids are particularly transparent," said researcher Michael Strano. "We have developed molecular sheaths around the nanotube that respond to a particular chemical and modulate the nanotube's optical properties."

By exciting the nanotube with a laser beam, Strano and colleagues were able to monitor changes in its fluorescence, and so measure the concentration of glucose present.

To make the sensors, the team coated HiPco (high-pressure CO decomposition) nanotubes with a monolayer of the enzyme glucose oxidase. The enzyme both prevented the nanotubes from sticking together and acted as a binding site for glucose. Then the researchers added ferricyanide ions to the nanotube surface; these attached to the nanotube through the porous glucose oxidase monolayer.

"When glucose encounters the enzyme, hydrogen peroxide is produced, which quickly reacts with the ferricyanide to modulate the electronic structure and optical characteristics of the nanotube," said Strano. "The more glucose that is present, the brighter the nanotube will fluoresce."

The researchers sealed the nanotubes into a 200 µm diameter dialysis capillary tube and inserted it into human epidermal tissue. The capillary tube enabled the glucose molecules to diffuse in and out, but kept the nanotubes in place.

"The advantage of the near-infrared signalling to and from such a capillary device is its potential for implantation into thick tissue or whole blood media, where the signal may penetrate up to several centimetres," said Strano. "And because nanotubes won't degrade like organic molecules that fluoresce, these nanoparticle optical sensors would be suitable for long-term monitoring applications."

The researchers, who reported their work in Nature Materials, reckon they could use their technique to design sensors for many other chemicals.