Sweet glow: nanotube sensor brightens path to glucose detection.

Biomedical engineers have developed a new kind of glucose sensor, based on carbon nanotubes, that could free people with diabetes from the daily pinprick tests now required for monitoring blood sugar concentrations.

Michael S. Strano and his colleagues at the University of Illinois at Urbana-Champaign designed the new sensor. Although the first medical target is glucose sensing, Strano says the same basic design is widely applicable for such analytical tasks as detecting genes and proteins associated with diseases.

Over the years, researchers have sought to tailor carbon nanotubes to detect chemicals ranging from small gas molecules to large biomolecules. The tubes' small size and unique electronic properties make them especially adept at detecting minute changes in the environment. The nanotube-based chemical sensors developed so far generate an electric signal in the presence of a particular molecule.

However, designing a sensor that can function in the body for long periods and continually send out a signal poses many challenges. "Somehow, that signal has to be carried outside the body," says Strano.

It turns out that carbon nanotubes absorb and emit infrared light, which can penetrate tissues up to several centimeters. Strano wondered whether he could capitalize on that trait to create an optical sensor whose light signals would pass through the skin.

Strano and his colleagues coated carbon nanotubes with glucose oxidase, an enzyme that breaks down glucose molecules. The researchers then sprinkled ferricyanide, an electron-hungry molecule, onto the nanotubes' surfaces. Ferricyanide draws electrons from the nanotubes, quenching their capacity to glow when excited by infrared light.

When glucose is present, it reacts with the oxidase, producing hydrogen peroxide. In turn, the hydrogen peroxide reacts with ferricyanide in a way that reduces that molecule's hunger for electrons. The more glucose present, therefore, the greater the nanotube's infrared fluorescence. The researchers describe their sensor in the January Nature Materials.

"This is a very interesting new research direction for carbon nanotubes," says Jie Liu of Duke University in Durham, N.C. "Few materials emit light in this range."

To test the feasibility of implanting the sensors in the body, Strano's group placed oxidase- and ferricyanide-coated nanotubes inside a sealed glass tube a centimeter long and 200 microns thick. The tube was riddled with pores large enough to let glucose enter but small enough to keep the nanotubes inside. The researchers then implanted the tube in a sample of human skin and showed they could excite the sensor with infrared light and detect its fluorescence.

George Gruner of the University of California, Los Angeles lands the new glucose-detection scheme exciting but cautions that turning such a sensor into a commercial product can be a long, difficult process. "Many other things [in the blood] can attach to the sensor and cause a change in signal" he says. The challenge will be to ensure that the device responds only to glucose, he says.