Dust-sized silicon chips may allow researchers at University of California, San Diego, to rapidly and remotely detect biological and chemical agents, including substances terrorists could dissolve in drinking water or spray into the atmosphere.

"Sensors as small as a piece of dust with built-in intelligence could inconspicuously stick to a wall or disperse into a cloud of gas to detect toxic chemicals or biological materials, says Michael Sailor, professor of chemistry and biochemistry at UCSD. "When the dust recognizes the chemicals or biological agents present, that information can be read like a series of bar codes by a laser similar to a grocery-store scanner. It could tell if a cloud is filled with anthrax bacteria or if the tank of drinking water into which we've sprinkled the smart dust is toxic," he adds.

The "bar code" is actually a wavelength of light reflected from the particle surface after thin films layered on the silicon chip chemically react to specific chemical or biological agents. Scientists begin with silicon wafers similar to those used to manufacture computer chips, then encode them by generating layers of nanometer-thick porous films using an electrochemical etch. The layered structure, created by breaking apart the wafer using ultrasound, imparts unusual optical properties to the particles. Called photonic crystals, these micron-sized particles reflect light in precise colors, each one of which can be thought of as a single bar in a bar code.

"When looking for chemical or biological warfare agents, you're searching for thousands of different chemicals," says Sailor. "Because the particles can be encoded for millions of potential reactions, it's possible to test for the presence of thousands of chemicals at the same time."The smart-dust crystals can detect potentially hazardous chemicals from a distance. The laser can see color changes in the dust 20 meters away, and the goal is one km using a more-powerful laser.