Sensor measures mass of one DNA molecule.

First, it was a lone bacterium; then, a solitary virus. Now, scientists have pushed the limits of ultrasensitive detection even farther to determine the mass of a single DNA molecule. Such precise biosensor measurements could lead to faster and more-accurate early screening for HIV infection, cancer, and other diseases.

The new biosensor, developed by Harold Craighead of Cornell University and his colleagues, consists of an array of micro cantilevers made of silicon nitride, each one spotted on the end with gold.

When the researchers bathed the cantilevers with DNA strands that have ends modified to bind to gold, they observed that the vibrational frequencies of the cantilevers changed. The frequency shifts were discernible even when only a single DNA strand, weighing about [10.sup.-18] gram, attached to a cantilever. For practical applications, customized DNA molecules could be attached to the cantilevers, where they would fish out particular DNA sequences in, say, a blood sample, from a patient, explains coinvestigator Rob Ilic.

"The sensitivity of this device is many orders of magnitude above that of other sensors," says Ilic. The next step, he says, is to try the device on proteins or antibodies, which weigh even less than the DNA strands.

The team reports its findings in the May Nano Letters.--A.G.