Sound sensing.

Acoustic wave detectors -- sensitive scientific instruments about the size of a microscope slide -- rely on ultrasonic waves to detect a variety of materials. The recent development by Maine researchers of a prototype to measure hydrogen-sulfide levels has sparked interest in the new devices. The team is now working to detect fragments of DNA and viruses.

These relatively simple sensors work quickly and could cost little, note John F. Vetelino and his colleagues at the University of Maine, Orono. Most acoustic wave detectors consist of a millimeter-thick substrate -- often a crystal such as lithium niobate -- with tiny electrodes attached that emit acoustic waves at frequencies beyond the range detectable by human ears.

Normally, wave characteristics such as frequency, amplitude and velocity would remain constant as the electrode-generated sound waves traveled from one end of the substrate to another. But Vetelino's team coats the crystal's surface with a material that will interact with whatever their sensor is designed to detect. This interaction alters the acoustic waves passing through the substrate, and monitors attached to the crystal note those acoustic wave changes.

For example, the Maine researchers coated the substrate of an earlier detector with a thin layer of tungsten trioxide. Because this coating chemically binds hydrogen sulfide, sound waves passing through the device differ in a characteristic way when the poisonous gas is present. Such a detector may one day monitor air in paper mills, which can emit the toxic gas.

Vetelino's group is now developing similar detectors whose substrates are coated with biological materials. In one, a coating of antibodies helped the researchers detect immunoglobulin G. Another sensor being tested by the Maine team searches DNA for specific genetic sequences. Vetelino, an electrical engineer, is also working with IBM to design acoustic wave detectors that can probe adhesive coatings between computer chips to determine why they sometimes fail.