Nanotechnology has become shorthand for gee-whiz miniature devices that may never find practical uses. At Rice University in Texas, however, tiny constructions called nanoshells have shown promise for fighting cancer and administering drugs. The devices are simple enough: beads about three millionths of an inch wide, with an outer metal wall and an inner silicon core. But by varying the size ratio between wall and core, electrical and computer engineer Naomi Halas and her Rice colleagues can tune the shells precisely to absorb or scatter specific wavelengths of light. "Small particles of gold absorb green light very strongly and look red," says Halas. "It's partly because of the metal and partly because of the shape, so we're controlling the shells' color by changing the shape."
Growth process of a nanoshell, as captured by an electron microscope.Photograph courtesy of Rice University
Working with Rice bioengineer Jennifer West, Halas found she could design gold-encased nanoshells to absorb infrared or visible light and convert it to heat. That raised the possibility of fighting cancer by selectively binding the shells to malignant cells. Infrared rays would pass harmlessly through soft tissue but generate lethal heat where they strike the nanoshells. In lab tests, Halas and West have used this selective heating to cook tumor cells without harming surrounding healthy ones. "This eliminates toxic chemotherapy drugs, so we predict it should result in lower side effects," Halas says. In a similar way, nanoshells could trigger implanted, temperature sensitive drug-delivery devices, releasing a dose only when illuminated with a specific infrared wavelength. Halas even envisions using the shells as miniature beacons for instant diagnostic tests, as they could emit telltale infrared signals in the presence of certain pathogens.