Detector measures single molecules.

Taking advantage of the atomic force microscope's extreme precision, researchers have adapted its highly sensitive detection mechanism for use in a hand-held sensor of biomolecules.

The new force-amplified biological sensor can detect molecules-including proteins and DNA-in concentrations as low as 1 particle per 100 microliters of fluid, or one molecule in a billion billion.

Chemists David R. Baselt, Gil U Lee, and Richard J. Colton of the Naval Research Laboratory in Washington, D.C., say that the new sensor can perform an assay in 10 to 15 minutes. Conventional bioassay techniques often take hours or days.

"We can now spot small numbers of viruses or bacteria," says Colton. "This device may prove useful . . . when there's not enough time to culture or amplify the sample." The group is designing the sensor to be small and rugged enough for use in clinics and emergency vehicles.

The device uses tiny magnetic beads coated with receptor molecules designed to latch onto target molecules in a sample, Colton explains. Beads that pick up the target molecules then stick to tiny cantilevers coated with receptors that bind elsewhere on th e target molecules. The device measures the force exerted on the cantilevers when a magnetic field acts on the beads.

Compared to current bioassay methods, the new sensor has the potential to increase detection sensitivity by a factor of more than 1 million, the researchers estimate. The scientists also say they can adapt the device to measure trace elements and heavy me tals in soil samples.