Nano Sensors Detect Volatile Compounds

Wednesday, June 29, 2011 @ 11:06 PM gHale

Nano-sized sensors that detect volatile organic compounds—harmful pollutants released from paints, cleaners, pesticides and other products—can offer advantages over today’s commercial gas sensors.

Modern commercial gas sensors consist of thin, conductive films of metal oxides. When a volatile organic compound like benzene interacts with titanium dioxide, a reaction alters the current running through the film, triggering an alarm. While thin-film sensors are effective, many must operate at temperatures of 200° C (392° F) or higher. Frequent heating can degrade the materials that make up the films and contacts, causing reliability problems.

In addition, most thin-film sensors work within a narrow range: One might catch a small amount of toluene in the air, but fail to sniff out a massive release of the gas. The range of the new nanowire sensors runs from just 50 parts per billion up to 1 part per 100, or 1 percent of the air in a room.

These new sensors, built using the same fabrication processes commonly used for silicon computer chips, operate using the same basic principle, but on a much smaller scale: The gallium nitride wires are less than 500 nanometers across and less than 10 micrometers in length, said researchers from National Institute of Standards and Technology (NIST), George Mason University and the University of Maryland. Despite their microscopic size, the nanowires and titanium dioxide nanoclusters they’re coated with have a high surface-to-volume ratio that makes them sensitive.

“The electrical current flowing through our nanosensors is in the microamps range, while traditional sensors require milliamps,” said NIST’s Abhishek Motayed. “So we’re sensing with a lot less power and energy. The nanosensors also offer greater reliability and smaller size. They’re so small that you can put them anywhere.” Ultraviolet light, rather than heat, promotes the titanium dioxide to react in the presence of a volatile organic compound.

Further, each nanowire is a defect-free single crystal, rather than the conglomeration of crystal grains in thin-film sensors, so they’re less prone to degradation. In reliability tests over the last year, the nano-sized sensors have not experienced failures. While the team’s current experimental sensors detect benzene as well as the similar volatile organic compounds toluene, ethylbenzene and xylene, their goal is to build a device that includes an array of nanowires and various metal oxide nanoclusters for analyzing mixtures of compounds. They plan to collaborate with other NIST teams to combine their ultraviolet light approach with heat-induced nanowire sensing technologies.



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