Researchers in the US have used carbon nanotubes to make a miniature gas ionization sensor. Pulickel Ajayan and colleagues at the Rensselaer Polytechnic Institute in New York state say that their detector offers a low-cost, more practical alternative to conventional ionization sensors (A Modi et al. 2003 Nature 424 171).
Every gas has a unique breakdown voltage – the electric field at which it is ionized – and ionization sensors identify gases by measuring these voltages. The concentration of the gas can be determined by measuring the current discharged in the device. However, existing sensors are bulky, consume lots of power and require “risky” high voltages to operate.
Ajayan and colleagues made a simple discharge device in which the cathode is a thin-film array that contains billions of multiwall nanotubes. The anode is an aluminium sheet (see figure). Individual nanotubes in the film create very high electric fields near their tips, and the combined effect of all the nanotubes is to increase the overall field and so speed up the gas breakdown process. This means that the gases can be ionized at voltages that are up to 65% lower than in traditional sensors.
The researchers also found that the current discharged in the device was six times higher than in conventional electrodes, which makes the detector highly sensitive. It is able to detect concentrations of gas as low as 10-7 moles per litre. Moreover, it can distinguish between different gases in a mixture and is not affected by external factors such as temperature or humidity – unlike previous detectors.
The Rensselaer team says that its device could be incorporated into battery-operated portable sensors for use in environmental, industrial and even counter-terrorism applications. “We also intend to expand our technique to detect biomolecules, such as proteins, antibodies and DNA,” team member Nikhil Koratkar told PhysicsWeb.
