Others have obtained much higher sensitivities with nanosensors. Researchers at the Naval Research Laboratory have made carbon-nanotube sensors that detect 50 parts per billion of a sarin-like chemical. Jing Li and her colleagues at the NASA Ames Research Center have made carbon-nanotube- and metal-oxide nanowire-based sensor arrays that detect about four parts per billion of nitrogen dioxide.
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.
The chemical coating is a quick, low-power way to reuse the carbon nanotubes in the sensor again and again. It takes the carbon nanotubes about 26 seconds to go back to their original state and detect a new gas. That’s pretty fast, Baughman says, adding that the researchers “should be especially proud of their ability to simultaneous achieve ultra-high sensitivity and selectivity in a fast sensor system that is so small that 500 could be placed on the surface of a dime.”