Building miniature 'noses' to sniff explosives.

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 very carefully between different types of molecules," says Whitten.

In tests, the cell phone-sized system has detected not only explosives, but also a wide variety of toxic industrial chemicals and biological threats.

"We're very encouraged by the flexibility of our sensor because of that wide range," says Whitten.

But what distinguishes the technology from others is that it determines the number of specific molecules in the air and then translates the weight reading into an electrical signal, which few companies have done in the past with microcantilevers, he says.

"We add a special coating on there--a piezoelectric coating--and that film translates the mechanical motion of the cantilever into an electrical signal," he says.

The system requires a small amount of power to run the data analysis on the circuit board. It would consume only milliwatts of electricity, says Whitten. A watch battery would be sufficient to power the sensor in a limited duty cycle for several years.

The technology is being funded by the Defense Advanced Research Projects Agency for use as a mounted sensor inside shipping containers. Nevada Nanotech is building prototypes of the sensors and running field tests at a government site.

The Department of Homeland Security has been developing a "smartag" technology that will transmit information gathered by the sensor inside a shipping container to a central control computer.

Today's available sensors can determine whether the containers have been opened or tampered with.

Whitten says his company's technology would complement those systems by providing information on any hazardous materials inside the container.

The portable size of the detector makes it simple for security personnel to carry, whether they're customs officials or police officers. It could be incorporated into a wand device for screening at airport security checkpoints, says Whitten.

The low cost of producing computer chips could allow the sensors to uproot the high-priced explosives detection machines at airports, such as the air puffer machines, which have been criticized for having low sensitivity and high false alarm rates.

"They're not as selective as they could be," says Whitten. "Sometimes they get confused about actual chemicals in the air. Our approach allows us to be much more discriminating about the chemicals that are being detected."

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