Number 263 (Story #2), March 22, 1996 by Phillip F. Schewe and Ben Stein SINGLE-MOLECULE BIOSENSOR. Atomic force microscopes (AFM) can directly measure forces at the nanoscopic level. For instance, scientists at the Naval Research Lab have measured the force between two complementary strands of DNA. The NRL researchers (Richard Colton (202-767-0801), Gil Lee, and David Baselt) now hope to use a device based on AFM technology to detect biomolecules. They have developed a "force amplified biological sensor" which will soon be capable of detecting atto- molar (10**-18 M) amounts of various biological species such as cells, proteins, viruses, and bacteria. Employed in a working device, an array of such biosensors would be able to perform immunoassays (the process by which the presence of antigens is detected) in about 10 minutes, much faster that other methods at these small concentrations. Currently, the prototype device works in this way: an antibody is attached to a sensitive cantilever beam. Next an antigen in solution binds to the antibody. A second antibody, mounted on a micron-sized magnetic bead, also binds to the antigen, forming an antibody-antigen-antibody-bead sandwich. What is measured is the deflection of the cantilever when a magnetic force is applied to the bead. By counting the beads one arrives at the antigen concentration in the solution. Colton and his colleagues reported on the sensor at the APS meeting.