Building miniature 'noses' to sniff explosives.

• Author: Jean, Grace
• Position: INSIDE Science AND Technology

It's no secret that a dog's nose is hundreds of times more sensitive than our own olfactory systems. For this reason, humans have long relied upon their four-legged friends to track and detect odors for security, sustenance and survival.

While artificial "noses" have been built to sense malicious chemicals and substances, such as explosives, none has proven quite up to snuff with the canine's sniffer in terms of accuracy or ability.

"The best technology for detecting explosives is still the dog," says Ralph Whitten, president of Nevada Nanotech Systems Inc. But dogs have numerous drawbacks. They become tired or distracted, and they have a limited number of hours they can work.

For that reason, scientists are searching for the holy grail in chemical sensing--a mechanical device that is as good as or better than a dog's nose and can run nonstop to provide continuous monitoring of an environment.

In pursuit of that interminable nose, researchers have turned to nanoscience. They have uncovered ways to detect molecules using tiny sensors called microcantilevers.

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Made of silicon, microcantilevers look like miniature diving boards that are 100 microns long, one micron thick and 20 microns wide, says Thomas Thundat, senior scientist at the Oak Ridge National Laboratory who is credited with discovering how the devices can detect chemicals. Up to 40 microcantilevers can fit on a computer chip 3 mm across--smaller than the average pinkie fingernail.

The microcantilevers are coated with chemicals that react with certain molecules. Such reactions cause the sensors to bend or vibrate, much as when a person climbs onto a diving board. By monitoring how much each cantilever bends or how quickly it vibrates, scientists can tell how many molecules of a particular chemical are present.

Because the detectors are so tiny, they can sense minute quantities of material, says Whitten. Some of the fancier types of explosives give off few molecules in the air, so having a sensitive detector that can recognize those molecules is critical in the trace detection arena.

Whitten's company is working on a micro-electrical mechanical system to accomplish the task. The technology relies upon a computer-chip sensor composed of an array of microcantilevers coated with a variety of chemicals.

"By putting a bunch of coatings into an array in a sensor that have these more generic classifications, we find that that's still enough to allow you to discriminate...