The sensor array contains a collection of different chemiresistors to detect a range of volatile organic compounds (VOCs). To make the chemiresistors, Hughes first mixed commercial polymers that had been dissolved in a solvent with conductive carbon particles. He then painted this inklike mixture onto electrodes in specially designed microfabricated circuits. If VOCs are present, the polymers absorb the compounds and swell, which in turn changes the electrical resistance in the circuit. The swelling and change in resistance correspond to the concentration of the VOC. Once the chemical is removed, the polymers shrink back to normal. "By using four different kinds of polymers¿one for each sensor¿we think we can detect all solvents of interest," Hughes says.
Ho and other team members devised the weatherproof packaging for the chemiresistor chip¿without which the device could not be placed in water or underground. "The package is modular, like a watertight flashlight, and is fitted with O-rings," Ho explains. "It can be unscrewed, allowing for easy exchange of components." All told, the casing, constructed of stainless steel, measures a mere three centimeters in diameter. Chemical vapors pass through the casing to the chemiresistor array through a small window covered with a waterproof Gore-Tex membrane. When the device is placed in water, VOCs will partition across the membrane into the gas phase.
The scientists recently placed the sensor at Sandia's Chemical Waste Landfill to see how well the device works outside the lab. There it is suspended about 60 feet down a screened well and logs data every hour. This field test will last for several weeks or months, and others are planned at Edwards Air Force Base and the Nevada Test Site. From the experiments, Ho, Hughes and their colleagues hope to determine the sensor's life span, as well as its performance when temperature, pressure and humidity vary.
"Over the next few years I expect we will see this invention being applied to DOE sites that require monitoring, remediation and/or long-term stewardship of contaminated sites, which currently spend millions of dollars for off-site analysis of manual samples," Ho adds. "This device can also be applied to numerous commercial sites and applications, such as gas stations, which include more than two million underground storage tanks that require monitoring to satisfy the EPA requirements."
And the electronic sniffer may offer at least part of the solution toward safeguarding the national water infrastructure. "A low security level might mean hiring a security guard and installing some detection features around critical assets, and that won't cost a lot," Danneels says. "But to stop a fairly organized group from committing a terrorist act could be extremely expensive."