Tiny acoustic wave sensors will find food applications.

Minute acoustic wave chemical sensors are being developed at the U.S. Department of Energy's Sandia National Laboratories (Microsensors Research and Development Department, PO Box 5800, MS-1425, Albuquerque, NM 87185). The devices will in the next two years be part of a hand-held chemical detection system as well as other integrated microsensor systems. The microsensors, each about the size of a grain of rice, will be able to detect the presence of chemicals in the environment.

There are food applications for these devices. Fundamentally, these acoustic wave chemical sensors are mass sensors. The trick is to develop a coating for the sensor that will cause a chemical or a bacterium, for example, to stick to the surface of the sensor. This changes the mass on the sensor, which in turn changes the measurable acoustic properties. These devices can be extremely sensitive, measuring picograms and potentially femtograms. Some researchers are developing coatings for detecting gas-phase organic chemicals. Sandia and other labs are developing biocoatings that can be immersed in aqueous media to detect bacteria, proteins, biotoxins and the like.

Sandia scientists are building a hand-held chemistry laboratory the size of a palm-top computer. The acoustic wave sensor is one piece of equipment in that laboratory. What's important here is that the chemical sensing transducer and microelectronics are integrated onto a single substrate. Monolithic integration has a lot of advantages but the principal one is the small size of the resulting devices. They can be so small that an array of as many as four or five miniature sensors could be built on a chip the size of a shirt button.

For the base material of the sensor the researchers chose gallium arsenide (GaAs) substrates instead of the usual quartz. Early on, researchers showed that GaAs made as sensitive a chemical detector as quartz. Like quartz, GaAs is piezoelectric, which is necessary to produce the surface acoustic waves for chemical sensing. At the same time, it is the best available semiconductor for fabricating the high-frequency microelectronics needed to operate the sensors.

Piezoelectric materials convert electric signals to surface acoustic waves that travel much like waves on the ocean's surface. To make a sensor, a thin selectively absorbing polymer layer is coated on the piezoelectric surface. When specific chemicals absorb into this layer, the acoustic waves travel more slowly. This change can be detected by the sensor microelectronics once the acoustic wave is converted back to an electric signal.

Further information. Steve Casalnuovo; phone: [phone]; fax: [phone]; email: [e-mail].