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Intelligent Machines

A New Way to Make Ultrasensitive Explosives Detectors

HP researchers have developed a cheap way to make nanoparticle arrays that could lead to precise chemical sensors.

Ultrasensitive chemical sensors based on nanoparticles have the potential to detect a single molecule of an explosive or other hazardous chemical. But deploying such ultrasensitive detectors outside the lab will require manufacturing methods that are both highly precise and inexpensive. Now researchers at Hewlett-Packard and the University of California at Irvine say they have a process that uses basic semiconductor manufacturing to fabricate arrays of nanoparticles in minutes.

Gold nanowires on silicon (colorized for better contrast). A simple and inexpensive process, developed by Hewlett-Packard researchers, for making such nanowires and nanoparticles could be used to detect trace amounts of various explosives and dangerous chemicals. (Source: Regina Ragan, University of California at Irvine)

In the new method, described in an upcoming issue of Nano Letters (online), the HP researchers coated nanowires, initially formed by depositing rare-earth metals on a silicon crystal, with platinum. According to one of the researchers, Regina Ragan (formerly with HP and now a professor of chemical engineering at UC Irvine), at some concentrations of platinum, the metal seems to form clumps, leaving parts of the wire uncoated. After the researchers exposed the nanowires to plasma, the uncovered parts of the wires were etched away, leaving tightly spaced platinum nanoparticles each about eight nanometers across. The technique could be easy and inexpensive to scale up because it uses common commercial techniques for deposition and etching, and requires few steps, Ragan says.

The researchers believe the technique can also be used to create gold or silver nanoparticles – a key for single-molecule chemical detection – using a technique called Raman spectroscopy in which light scattered by molecules creates a telltale signature of a particular chemical. “The problem with regular Raman scattering is you need to have a very, very large sample of molecules,” says R. Stanley Williams, senior HP fellow and one of the researchers on the project. But, says William, if the targeted molecule “happens to be located between two silver nanoparticles,” the technique can be extremely sensitive. “You go from having to have a hundred trillion molecules to being able to see a Raman spectrum from only one molecule.”

Indeed, says David Rauh, president of EIC Laboratories, a research and development firm in Norwood, MA, the new fabrication technique could help researchers build sensors for airports and battlefields that can detect a host of different threats, possibly including liquid explosives.

Rauh says that a product could be available within five years, but it would probably not reach the one-molecule detection level possible in a lab. “Single molecule detection is done under highly specialized circumstances,” he says; but for airports, what the method offers “is the possibility of detecting a whole spectrum of different chemicals and identifying them at parts per billion levels.”

The work is one example in a growing field of research: developing new methods for creating carefully spaced nanostructures with well-controlled sizes, shapes, and spacings, which will be essential for the most sensitive devices. Williams says that in the past “having two silver nanoparticles that are exactly the right diameter and separated by only one or two nanometers has been an accident. It just simply hasn’t been possible to build such a thing. This [HP] process gives you almost for free the ability to do that.”

Chad Mirkin, professor of chemistry at Northwestern University, says the technique is “a clever way of arranging particles on a surface. And the ability to do that can impact many areas, ranging from catalysis to optics and electronics.” He notes, however, that the researchers have yet to demonstrate its versatility with various metals, and have not yet tested it for use in sensors. Furthermore, says Mirkin, the new method will have to compete with other experimental ways of creating arrays of nanoparticles of precise sizes and distributions.

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