Even very small numbers of deadly infectious agents or allergenic pollen molecules can cause big problems for humans. But detecting such trace amounts is difficult to do fast enough to do any good. Current techniques -- for example, air sampling on filters or slides until enough molecules for a detectible signal or preparing specially tagged molecules for lab experiments -- are more suitable for general research than alerting people to an imminent threat.
Andrea Armani of the University of Southern California has adapted a clever optical microcavity resonance technique for rapidly detecting even individual unlabelled target molecules. The central element is a microtoroid resonator -- a ring of glass about 3 microns thick with a diameter of 100 microns (about the diameter of a human hair). The resonator is created using photolithography techniques developed for the semiconductor industry, but then Armani coats it with a protein that binds only to the target molecule. Finally, a tapered optical fiber is mated tangentially to the ring, enabling tunable laser light to be introduced into the ring so the waves match precisely with each circuit. Should even a single target molecule bind on the outside of the resonator, it will absorb a small amount of light from the "evanescent" field that extends a fraction of a micron beyond the glass ring, causing a change in resonance. Because the light continues to circulate through the ring, even the slight change due to a single molecule is strongly reinforced and can be detected.
Armani will report her successful detection of two Timothy grass pollen proteins, which are major human allergens. Her future efforts aim to adapt this technique to enable the rapid, remote detection of a wide variety of single molecules in the environment and in vitro.
Armani's talk, " Biophotonics: Resonant Detection of Single Molecules" is at 10:40 a.m. on Wednesday, October 22, 2008, in Room 202 of the Hynes Convention Center.