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  • Published: Dec 1, 2012
  • Channels: Raman
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Sniffing out explosives

Free-Surface Microfluidics/Surface-Enhanced Raman Spectroscopy for Real-Time Trace Vapor Detection of Explosives

A chip the size of a fingernail uses surface-enhanced Raman spectroscopy (SERS) to sniff out explosives and avoids the problem of bad dog days in airports and other security-sensitive environments. The system has the potential to be developed into a compact device resembling a smoke alarm to alert the authorities to a potentially explosive problem.

A highly sensitive and portable device that can sniff out explosives as well as a trained canine is being developed by scientists at University of California, Santa Barbara. Writing in the journal Analytical Chemistry, mechanical engineer Carl Meinhart and chemist Martin Moskovits explain how they have used microfluidics technology to emulate the dog's olfactory system in a chip. Not only is the device highly sensitive to specific vapours, such as 2,4-dinitrotoluene the vapour formed by loss of a nitro group, which emanates from TNT-based explosives and is present at much higher pressure than TNT itself, but can readily distinguish between different substances even if they are similar. 

My dog knows...

"Dogs are still the gold standard for scent detection of explosives. But like a person, a dog can have a good day or a bad day, get tired or distracted," explains Meinhart. In an airport setting that could mean the difference between explosives caught by security officers or a major incident. "We have developed a device with the same or better sensitivity as a dog's nose that feeds into a computer to report exactly what kind of molecule it's detecting."

The human nose cannot detect DNT at very low concentrations but dog's can. The new device has a microscale version of the canine olfactory mucus membrane, which absorbs and then concentrates airborne molecules, so that they can be analysed by the connected surface-enhanced Raman spectroscopy (SERS). A microscale channel of liquid absorbs and concentrates vapour molecules as a continuous stream absorbed from the air by up to six orders of magnitude. Once they enter the microchannel, nanoparticles give the necessary SERS boost to amplify the spectral lines under laser light. The resulting spectra are fed to a computer database of signatures to reveal what substance was trapped. 

Security, sampling

"The device is capable of real-time detection and identification of certain types of molecules at concentrations of 1 ppb (parts per billion) or below. Its specificity and sensitivity are unparalleled," explains Brian Piorek who worked in Meinhart's laboratory and is now Chief Scientist at Santa Barbara-based SpectraFluidics, Inc. The device has been patented and the technology is now being licensed exclusively to SpectraFluidics.

"System efficacy was demonstrated by routine detection of the vapour phase 2,4-DNT entrained in nitrogen at levels of 1 ppb, which is less than 1 percent of the saturated headspace concentration of neat 2,4-DNT," the team explains. "The FSF-SERS system responded within 2 min of exposure, reaching a signal-to-noise ratio that exceeded 20:1 within 10 min after initial analyte exposure."

The researchers focused on explosives in their Analytical Chemistry paper, but suggest that the same technology could be used to detect a wide range of substances in narcotics detection, forensics, breath-test disease diagnostics and environmental and workplace sampling. 

Related Links

Anal Chem, 2012, 84, 9700-9705: "Free-Surface Microfluidics/Surface-Enhanced Raman Spectroscopy for Real-Time Trace Vapor Detection of Explosives"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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