A handheld river may help detect explosives, toxins

A casual conversation between two professors on a train from Oxford to London has led to the development of a new type of chemical sensor that may be markedly better at sniffing out explosives, narcotics or environmental toxins than sensors now on the market.

The professors, Juan G. Santiago of Stanford University and Carl Meinhart of the University of California-Santa Barbara, both associate professors of mechanical engineering, had been at the University of Oxford for a conference. On the train, they were talking about a common interest, the control of tiny streams of water running through channels no wider than a human hair.

The sensor uses Raman spectroscopy to identify unknown substances by first dissolving them in water, then stimulating the mixture with laser light as it flows through the tube. The molecules of the unknown substances respond by emitting photons whose frequencies reveal the identity of the dissolved substances.

As the English countryside flashed by outside the train windows, the talk between the researchers turned to their recent refinement: running the mini-rivers through a miniature open-air canal instead of the glass tube. Santiago suggested that by exposing the water to the atmosphere in this fashion, the device might work as a detector rather than a laboratory instrument. Random molecules of the gases in the surrounding air would strike the water surface and be absorbed, making it possible to identify them. Meinhart understood the significance: "We were sitting on the train, and I said, 'Oh, I know how to do that.'"

The two professors then joined forces with Martin Moskovits, a professor of chemistry and biochemistry at UCSB who is an expert in surface-enhanced Raman spectroscopy. In conjunction with Sanjoy Banerjee, a UCSB professor of chemical engineering, the work was then carried out by graduate student Brian Piorek and post-doctoral researcher Seung Joon Lee, both of UCSB.

The team also has formed a company to take the detector to market. The company, SpectraFluidics Inc., has filed for patents. "The new aspect of this work is that you can have this interaction occur in real time," Santiago said. "As you're controlling the liquid, it's being exposed to the atmosphere." Detection of a specific airborne substance might take only two or three seconds, much quicker than current detectors, the researchers said. They described their work in an article in the online edition of the Proceedings of the National Academy of Sciences.

The basic detection technology, known as "surface-enhanced Raman spectroscopy," has been in use for some time. The "enhancement" comes from seeding the water with, typically, gold or silver nanoparticles. Certain target molecules will accumulate on the gold or silver; for complex reasons, this makes the target molecules much easier to detect. "It's a huge enhancement," Santiago said. In fact, the molecules may be a trillion times easier to see. As a practical matter, this enhancement means a substance may be detected if there are only a handful of molecules in the water.

Santiago and Meinhart both are exploring methods to realize and control the flow of micro “rivers” in channels just a few microns deep. Meinhart has been driving the flows with pressure. Santiago is exploring the use of electric fields to switch on and off and divert these open channel streams. In parallel, Moskovits is continuing to develop an understanding of the fundamental physics responsible for surface-enhanced Raman spectroscopy. “Professor Moskovits’ research in surface-enhanced Raman spectroscopy has provided the mathematical framework needed for our nano-scaled engineering effort to be so successful,” Meinhart said.

Meinhart predicts a handheld device in the near future and a much smaller model—an inch square, laser included—within five years. "We're not sure, but we think we're a few orders of magnitude more sensitive than commercially available sensors," he said. “Essentially the sensor is an optical nose. Light is used to smell things that otherwise only a dog can smell.”

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