Gazing at an electrical meter, Yi Cui, a graduate student in the Harvard University lab of chemist Charles Lieber, waits for evidence of a remarkable feat in simple, ultrasensitive diagnostics. His target is prostate cancer. His new tool is a microchip bearing 10 silicon wires, each just 10 nanometers (billionths of a meter) wide. These nanowires have been slathered with biological molecules with an affinity for PSA, a protein all too familiar to men of a certain age as the telltale sign of prostate cancer. If the experiment works according to plan, when the PSA molecules bind to the nanowires, there will be a detectable electrical signal.

Cui washes a solution containing prostate cancer proteins over the chip. Immediately, the meter registers subtle changes, indicating not only that the device has detected the protein, but that it detected perhaps as few as three or four molecules, instantly and with minimal sample preparation-a previously unheard-of feat. The implications for diagnostics are enormous. A successful prostate cancer test must distinguish between normal and elevated protein levels. Ultrasensitive sensors like Lieber's could discern the slightest increase; what's more, they could do so in cheap, disposable tests that patients could use at home between visits to the doctor. "If I were at risk for a particular cancer, I wouldn't want to take a chance and wait for some cancer cells to grow wildly out of control over a year because the previous test missed it," says Lieber.