There is an urgent need to explore new chemical sensor technologies for emergency rescue crews and first responders that will enable “on-site” monitoring. In particular, the new sensors devices should provide definitive identification of contaminants in real-time mode with fast response. The sensors should be light-weight, compact, robust, affordable and should consume low power (battery operated). Existing sensor technologies are based on electrical conductivity measurements of gas responsive films, gas chromatography sensors with flame or photo ionization detectors and more complex mass spectrometry analysis. None of these technologies can meet all the requirements for field operation. For example mass spectrometry provides excellent performance but is limited by size and power considerations. Flame ionization detectors are hazardous for field operation since the carrier gas is typically a flammable agent. Photo ionization detectors show limited range of detectable gas species, while electrical conductivity based sensors display slow response times and are not suited for definitive identification of contaminants (i.e. high rate of false alarms). For these reasons there is a critical need to develop new sensor technologies to overcome these limitations.    

Koratkar’s research group at the Rensselaer Polytechnic Institute has pioneered a novel field-ionization gas detector that uses carbon nanotube (10-9 m in diameter and several micro-meters in length) arrays as the device electrodes. Carbon nanotubes, grown in vertical micro-arrays that resemble Lilliputian forests (see figure 1), have sharp, pointed tips with extremely high curvature. Because of this curvature effect, the local electrical field near the tip can be amplified by many orders of magnitude. Few volts applied to an electrode with such a nano-scale surface topology, would be amplified to several thousand volts at the tube tips.

Koratkar’s idea for sensing takes advantage of this extremely high electric field that is generated near sharp nanotube tips as a means of inducing ionization and electrical breakdown of the gas at low voltages. By monitoring the breakdown voltage, the gas can be identified (every gas has a characteristic breakdown behavior) and by monitoring the discharge current at breakdown, the gas concentration can be determined. Since all gases have a characteristic breakdown field, a broad range of analytes can be detected with enhanced selectivity. Moreover, a discharge is set up within micro-seconds enabling data to be gathered in a time-frame that enables operational decision-making. Koratkar’s group has tested a prototype of this sensor (figure 2) and fingerprinted the identity and concentrations of a range of gas species (NH3, N2, O2, CO2, He, Ar) using a simple, compact and low-power device. This work was recently reported in Nature, 424, 171 (2003).

While the principle of a field ionization detector is well known, the novelty in our approach lies in the innovative use of carbon nanotube arrays to amplify the electric field by imparting a nanoscale curvature to the electrode surface; this enables the efficient ionization of gas molecules at a fraction of the voltage needed for parallel plate electrodes. Lowering the device operating voltage is very important for safe operation of the sensor device. Reduction in the operating voltage can also enable battery-powered operation; which holds the key for compact, affordable nano-electronic sensor development.

Future work will focus on detection of easily ionizable analytes of greatest concern to the health and safety community, such as: chlorinated (or otherwise halogenated hydrocarbons), phenols, aromatic amines and common distillates in the presence of a carrier phase. Since these analytes ionize at a much lower electric field than a typical carrier gas, we expect to be able to selectively detect the target. It is also important to investigate sensor durability by studying the impact of cathode sputtering, tip damage and reactive reaction products and exploring strategies to minimize these effects. Research is on-going at the Rensselaer Polytechnic Institute to address these issues.

If successful this research can revolutionize gas sensing technology; these devices could be mounted on soldier’s uniforms or they can be used by first responders who rush to a site. Normally one would have to take samples, send them to a lab and wait several days for the results. Now, analysis can be performed in real-time mode at the site of an emergency. This type of sensor could also be deployed inexpensively for environmental monitoring, sensing in chemical processing plants, space missions as well as agricultural and medical applications.

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