OCTOBER 24, 2008 -- Andrea Armani of the University of Southern California has adapted an optical microcavity resonance technique for rapidly detecting even individual unlabelled target molecules. Her technique could prove extremely valuable because even very small numbers of deadly infectious agents or allergenic pollen molecules can cause major problems for humans -- but detecting such trace amounts has been difficult to accomplish with enough speed to do any good.
The central element of Armani's detector is a microtoroid resonator -- a ring of glass about 3 microns thick with a diameter of 100 microns. 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 reported her successful detection of two Timothy grass pollen proteins, which are major human allergens, during this week's AVS Symposium. Her talk was titled "Biophotonics: Resonant Detection of Single Molecules. 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 technique could prove extremely valuable because even very small numbers of deadly infectious agents or allergenic pollen molecules can cause major problems for humans -- but detecting such trace amounts has been difficult to accomplish with enough speed to do any good. Currently popular 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.
"Biophotonics: Resonant Detection of Single Molecules" abstract