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Analytical Chemistry

Laser Sensor Detects Tnt Vapor

Device based on a lasing semiconducting polymer has unprecedented sensitivity

by CELIA HENRY
April 18, 2005 | A version of this story appeared in Volume 83, Issue 16

EXPLOSIVES DETECTION

BRIGHT LIGHT
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Credit: NATURE 2005
Pumping the polymer film with 337-nm pulsed light above the threshold energy level (right) results in stimulated emission.
Credit: NATURE 2005
Pumping the polymer film with 337-nm pulsed light above the threshold energy level (right) results in stimulated emission.

A fluorescence sensor for explosives developed by researchers at Massachusetts Institute of Technology is 30 times more sensitive than its predecessors. The sensor can easily detect TNT vapor at sub-parts-per-billion levels (Nature 2005, 434, 876).

MIT chemistry professor Timothy M. Swager and coworkers have previously demonstrated sensitive detection based on analytes quenching the fluorescence of semiconducting organic polymers. Now, in collaboration with the group of MIT electrical engineering professor Vladimir Bulovic, they've shown that the sensitivity is amplified at least 30-fold when the organic polymer produces stimulated emission (lasing). The interaction between the analyte and the polymer reduces the lasing, signaling the analyte's presence. The principle should be broadly applicable to any system that can be made to lase, Swager says.

The system tested by the MIT team uses a polymer that lases at a low power threshold. "Instead of hitting it with a sledgehammer, we get to tickle it with a more modest intensity source," Swager says. By exciting the lasing just above that power threshold, sensitivity is improved, and a small dose of the analyte can be enough to stop the lasing completely.

The team used thin films of the polymer to make three types of devices: a waveguide, a diffraction grating, and a coated optical fiber. That any of these could be used to detect TNT demonstrates that the phenomenon doesn't depend on a specific device geometry, Swager says.

SHIELDED
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The poly(phenylene vinylene) backbone of the sensor's semiconducting organic polymer is protected by alkyl chains running parallel to it.
The poly(phenylene vinylene) backbone of the sensor's semiconducting organic polymer is protected by alkyl chains running parallel to it.

Michael J. Sailor, a chemistry professor at the University of California, San Diego, who also develops sensors, says: "In Swager's work, high sensitivity is realized in a few different optical structures that are easy to fabricate, demonstrating the ability to implement such devices in portable, low-power sensing units. The impact of this work should be quite broad in scope."

According to Swager, an unusual architecture improves the polymer's properties as a sensor. The attached alkyl chains run parallel to the conjugated backbone, shrouding and protecting it from degradation. "That's where I dust my knuckles on my shirt and say that's the power of making a darn good polymer," he says.

Like Swager's other polymer sensors, the new sensor has been licensed by Nomadics, a company in Stillwater, Okla., specializing in defense and homeland security applications. Swager is a consultant for the company.

Swager suspects that with additional device engineering, the polymer sensor will be capable of parts-per-quadrillion detection limits. "Devices that are designed solely to be sensors can be much better than what we're showing here," he says.

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