“Direct measurement of the capacitance of an individual single-walled carbon nanotube is difficult because it is so small - approximately 10-16 F,” researcher Eric Snow told nanotechweb.org. “Our devices were fabricated using two-dimensional networks of single-walled carbon nanotubes that contain many thousands of nanotubes. In this way, the capacitance is easily measured.”
Gas molecules adsorb onto the surface of the carbon nanotubes and become polarized by the electric field radiating from the nanotube electrodes. This increases the capacitance of the nanotubes.
To make their device, the scientists deposited a network of nanotubes on a 250 nm-thick thermal oxide layer on a heavily doped silicon substrate. The nanotubes act as one plate of the capacitor while the doped silicon substrate acts as the other. The researchers also created a 2 x 2 mm interdigitated array of palladium electrodes on top of the nanotube layer. These provided electrical contacts.
Applying a 30 kHz, 0.1 V AC voltage between the nanotubes and the substrate, and detecting the out-of-phase AC current with a lock-in amplifier enabled the scientists to measure the capacitance.
Other nanotube gas-detection techniques monitor changes in the resistance of the nanotubes, but these sensors are often troubled by noisy signals and only respond to a limited number of gases.
“The capacitance transduction mechanism eliminates the noise problem, is very sensitive, completely reversible and responds to a broad range of chemical vapours,” said Snow.
The team also coated the nanotubes with chemoselective materials to increase their capacitive response to particular gases. By adding a thin layer of hydrogen-bonding polymer to the tubes, the researchers decreased the minimum detectable level for dimethylmethylphosphonate - a simulant for the nerve agent sarin - to 0.5 ppb. Replacing the polymer with a hydrogen-bonding molecular monolayer gave an improved response time and a minimum detectable level of 50 ppb.
Snow and colleagues say their devices compared favourably with commercially available chemicapacitor sensors, having both higher sensitivity and faster response and recovery times. They believe the faster response times arose because their devices used a much thinner layer of chemoselective material than the commercial ones.
“We are currently working with a company that specializes in single-walled carbon nanotube sensors,” said Snow. “The potential applications include [detecting] chemical warfare agents, toxic industrial chemicals and explosives for defence and homeland security applications. There may also be some medical applications.”
Now Snow says he and his colleagues are improving the design of their sensors, developing chemoselective coatings that are custom-designed for single-walled carbon nanotubes, incorporating their sensors into a compact vapour delivery system, and starting to work on biosensing.
The researchers reported their work in Science.