Carbon nanotubes are rolled up sheets of graphite just nanometres in diameter that have very high mechanical strength and novel electronic properties. Recently, researchers found that individual semiconducting single-walled nanotubes show a large change in electrical resistance when exposed to certain gases. This property could be exploited in chemical sensors.

Snow and colleagues first grew an interconnected network of single-walled nanotubes in a tube furnace, and then patterned them into an array of sensor electrodes using optical lithography and metal lift-off techniques. The NRL team made its detector by coating the inner surface of a chemi-resistor flow cell – a quartz tube about 50 mm long and 3mm across – with the nanotube sensor material.

To test the device, Snow and co-workers exposed the tubes to DMMP (a chemical similar to the nerve agent Sarin), ammonia, water vapour and various hydrocarbons using air as the carrier gas. They observed a large increase in the resistance of the sensor as it adsorbed DMMP, but little or no change in resistance when it was exposed to water vapour or hydrocarbons. According to the team, this is because chemicals such as DMMP are strong electron donors and therefore reduce the hole density in the semiconducting nanotubes. This leads to an increase in their resistance. In contrast, water vapour and hydrocarbons do not possess these charge transfer properties.

The nerve gas detector is sensitive to one part in a billion of DMMP. The team now hopes to improve the device’s ability to distinguish between different chemicals by incorporating chemo-selective polymers into the sensors. Preliminary demonstrations with a hydrogen-bonding compound have shown that the device can effectively separate out signals from DMMP and ammonia.