"The team has developed a patterned growth approach to mass-produce nanotube sensors with high reliability and yield," Hongjie Dai of Stanford University told nanotechweb.org. "Nanotube sensors can now be made at a scale capable of meeting commercialization requirements. The ultimate goal would be an electronic nose based on nanotube sensor arrays for detecting numerous types of molecules in the gas phase."

To make the device, the researchers used chemical vapour deposition to grow single-walled carbon nanotubes onto catalyst areas patterned on an array of molybdenum electrodes. Because the catalyst regions were relatively wide at about 100 µm, between 20 and 30 nanotubes grew across each pair of electrodes. The scientists say that devices which use many tubes produce lower electrical noise than those with individual tubes.

When exposed to nitrogen dioxide (NO2) in argon, a typical multi-tube device was able to detect an NO2 concentration of around 10-50 ppb. But adsorbing the polymer polyethyleneimine (PEI) onto the devices meant they were able to respond to NO2 concentrations as low as 100 parts-per-trillion. Dai and colleagues claim that this is the first time electrical sensors have detected molecules at such low levels, which typically require spectroscopy techniques. That said, for reliable detection the modified devices required the presence of several ppb and showed a response time of one to two minutes.

What's more, the PEI-coated devices did not respond when exposed to ammonia (NH3) in concentrations of up to 1%, whereas untreated devices responded to around 100 ppm of the gas. The treated devices also showed no reaction to gases such as carbon monoxide, carbon dioxide, methane, hydrogen or oxygen. Coating as-grown multi-tube devices with Nafion, on the other hand, prevented NO2 and certain other molecules from reaching the nanotubes, allowing for more selective detection of ammonia.

"Polymer functionalization enhances the sensitivity of nanotube sensors, allowing for the detection of toxic gases at sub parts-per-billion concentrations," explained Dai. "Microspotting is used to functionalize different nanotube sensors with different polymers, and the multiplexed sensor arrays can selectively detect molecules in gas mixtures."

Dai says the work could result in a new generation of ultra-sensitive and selective nanoscale sensors for monitoring living and industrial environments, and for use in chemical, medical, military and homeland security applications. "The next step is to fully develop a set of generic functionalization chemistry for nanotube sensors and advance the multiplexation technology for large sensor arrays," he added.

The scientists reported their work in Nano Letters.