Earlier sensors were developed around the ability to detect and respond to a single chemical, Sunshine said. An example is the sensor in a carbon monoxide detector. A variety of approaches emerged. In some sensors, such as the one being used in Athens to check on the freshness of poultry, the presence of airborne compounds changes the ability of electricity to flow through metal-oxide circuits, each tuned to a different group of compounds. At the Agriculture Department's laboratory in Winter Haven, individual plastic nodes have been doped with an electricity-conducting material such as graphite or carbon black.
Electrons move through the nodes at predictable rates.
But when the plastics absorb molecules of a vapor, they swell like sponges and the spacing between the conductive particles is changed. That changes the electrical conductivity. Each node reacts differently to different compounds. The computer analyzes the differences. No one expects the electronic sensors to match the range of a natural nose.