Thursday, June 19, 2008

Smarter, Faster Nano Sensor

A tiny detector can quickly sniff out dangerous gases.

By Prachi Patel-Predd

Small sensor: A close-up shows carbon nanotubes (bottom) spanning the space between interlocking gold electrodes in a new type of gas sensor. The nanotubes are coated with an amine through which gases adsorb on the nanotube surface and detach after a few milliseconds. Change in conductivity of the carbon nanotubes specifies which gas was adsorbed. Credit: Chang Young Lee, MIT

A tiny carbon-nanotube-based chemical sensor can detect low parts-per-billion concentrations of gases. It can also go from detecting one gas to another within half a minute. Typically, carbon-nanotube- or -nanowire-based sensors, which can be extremely sensitive in detecting gases, take hours to recover and be reused.

The researchers coat the carbon nanotubes with chemicals that allow the nanotubes to rapidly switch their response. A network of the sensors could be used to monitor the spread of toxic gases or the movement of various pollutants over a large area. "Instead of detecting whether a pollutant's there or not, you can detect its motion," says Michael Strano, a chemical-engineering professor at MIT, who led the work, which was presented in Angewandte Chemie. "Where is the wind moving it? Where is it most toxic?"

The new device is made of two main parts. The first is an ultrasmall gas chromatograph, an instrument commonly used in chemical analysis to separate mixtures of gases. To make a micro version of the instrument, the researchers etch a zigzagging, 35-centimeter-long channel on a silicon chip that is 800 micrometers on each side. As in conventional gas chromatography, different chemicals pass through the column at different rates, depending on their physical and chemical properties, so they exit the column at different times.

The output of the chromatograph feeds into the nanotube sensor. The sensor contains carbon nanotubes spanning the space between tiny gold electrodes. When various gases adsorb on the carbon nanotubes, the nanotubes' electrical conductivity changes by a different amount. By measuring the change in conductivity after the gas binds to the nanotubes, the researchers can identify the gas.

"The idea of incorporating a micro gas chromatograph with a carbon-nanotube sensor is probably the [best] way to go from a practical point of view," says Pulickel Ajayan, a mechanical-engineering and materials-science professor at Rice University. In a real-world setting, there would be a mixture of gases--air pollutants, say--which would need to be separated before the individual gases can be detected.

Otherwise, even with an extremely sensitive detector, "you can get very high sensitivity to chemicals, but usually you don't know what chemical it is," says Ray Baughman, director of the NanoTech Institute at the University of Texas at Dallas. "By coupling the micro gas chromatograph with the sensor, [researchers] . . . have a reasonable expectation of what the chemical is."

The researchers test the sensor with a chemical that mimics the nerve toxin sarin. They are able to detect a billion molecules of the gas, corresponding to a concentration of 150 parts per billion.