Bioterrorism, once an abstract threat in the US, became reality in 2001. But even before the highly publicized outbreak of anthrax infections, scientists knew that they needed a sensitive, on-site technique for detecting trace amounts of deadly germs. Now a pair of researchers from the University of Rochester's Institute of Optics in New York has designed a biosensor that could offer a solution.Researchers have designed a biosensor based on a whispering-gallery-mode disc resonator. This simulation illustrates the change in the initial field distribution in the resonator (A) with the presence of an absorbing particle, such as a pathogen (B). Courtesy of Robert W. Boyd.The sensor is based on a high-finesse whispering-gallery-mode disc resonator, a 10- to 100-µm optical device that has applications in high-resolution spectroscopy and in add/drop filters for wavelength division multiplexing. Light from an optical waveguide is coupled to the resonator, and the spacing between them is manipulated so that coupling is weak, enabling the light to intensify in the cavity.Pathogens that fall onto the region where the light is confined act as absorbing species, and their presence produces a detectable change in the transfer characteristics of the resonator. The sensitivity of the detector depends on the amount of light in the structure, and the researchers' calculations suggest that the sensor may be able to detect as few as 100 molecules. Antibodies on the upper surface of the disc enable the team to determine the type of pathogen."Pathogens that fall onto this surface will then be bound to it, because this is how antibodies function -- by binding to specific pathogens," explained Robert W. Boyd, the principal researcher on the project.The researchers chose the near-IR for their initial studies with the biosensor, partly because many organic materials are highly absorbing in this region.Moreover, it enabled them to employ nano-fabrication methods that were developed for the telecommunications industry."We are writing this structure into a resist through use of electron-beam lithography and are then transferring the pattern into a GaAs waveguide using etching techniques," Boyd said.The researchers are cooperating with the Cornell Nanofabrication Facility at Cornell University in Ithaca, N.Y., to produce the sensors. "Fabrication is expected to be challenging," he added.While the sensor was still in the initial design phase, Boyd said that he hoped to have a working device soon. He also will explore techniques to deposit the binding layers for enhanced sensitivity."With the threat of biological warfare and biological terror, there is a great need to develop means to test for pathogens outside of the clinic -- for instance, on the battlefield or, in light of recent events, even in post offices," he said.