NanoTech-Electronics

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 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 new device, with its parts-per-billion sensitivity, might be less sensitive than others, but it could still find practical use, since parts-per-million levels of sarin can be lethal. More important, it presents the key advance of combining a micro chromatography column and the nanotube sensor into a tiny portable device, Baughman says.

The setup works because of a special coating on the carbon nanotubes. Many chemicals adsorb strongly on uncoated nanotubes, and they either take hours to detach or have to be removed. That is done by exposing the carbon nanotubes to ultraviolet light or heat, says Strano, who points out that "all those things are very slow and costly." So the researchers coat the carbon nanotubes with an amine, which reduces the strength of the bond between the tube and the chemical that is being detected. As gas molecules flow into the sensor from the chromatograph, they stick to the carbon nanotube but detach in a few milliseconds.