Gold nanodots bring us closer to single-molecule sensors

The streptavidin protein, which set off a sensor once four molecules accumulated.

Washington.edu

Detection of substances on the single-molecule level could provide a helpful too for pharmacology, medicine, and product safety. But toxic or otherwise unwanted substances sometimes have a low molecular weight, making them hard to sense. To make a sensor for these molecules, you need materials with special properties, such that their interactions trigger an abrupt change in behavior.

V. G. Kravets and colleagues demonstrated the detection of tiny masses, on the order of a single biomolecule, using nanoscale optics. They fabricated a material that responded resonantly to light. When a tiny amount of mass was added to the surface, it caused a dramatic change in the amount of reflected light. This enabled the researchers to detect the presence of mass accumulation to the level of 10^-15 grams over a millimeter patch—equivalent to detecting a single human skin cell landing on a coffee table.

The key to sensitivity in this particular experiment is in the response of a special material in the presence of very few photons. When intensity is high—that is, a large number of photons are present—a small change in the reflectivity of a surface is hard to measure. (Reflectivity is simply the percentage of incoming light that gets reflected back.) However, when intensity is low, the same tiny change in reflectivity can result in no light being reflected at all, an abrupt, qualitative change in behavior.