Andrea Armani, an assistant professor of chemical engineerin­g and materials science, has developed the first optical sensor that can detect single molecules without the use of labels such as fluorescent tags. No label-free detector previously developed has been sensitive enough to distinguish a single molecule.

Armani's sensor consists of a microscopic silica ring that sits on a pedestal atop a silicon wafer. "It's this little, tiny doughnut-shaped device," she says. The ring captures photons from a laser and holds them in orbit. Its surface is chemically treated to snag molecules of the target substance from the surrounding environment. As soon as even one molecule of the compound is ensnared, it creates a detectable change in the ring's optical properties.

Because it works in liquids, including blood, the sensor could be an ideal diagnostic device. Armani envisions, for instance, incorporating one into intravenous catheters that would monitor a patient for infection, picking up telltale molecules in minuscule quantities long before symptoms appeared. --Jocelyn Rice

Zeroing in: A tiny doughnut-shaped silica ring atop a silicon wafer acts as an extremely sensitive optical sensor. The ring's optical properties change when even a single molecule binds to it.
Credit: Courtesy Andrea Armani