A wireless, passive carbon nanotube-based gas sensor

A gas sensor, comprised of a gas-responsive multiwall carbon nanotube (MWNT)-silicon dioxide (SiO/sub 2/) composite layer deposited on a planar inductor-capacitor resonant circuit is presented here for the monitoring of carbon dioxide (CO/sub 2/), oxygen (O/sub 2/), and ammonia (NH/sub 3/). The absorption of different gases in the MWNT-SiO/sub 2/ layer changes the permittivity and conductivity of the material and consequently alters the resonant frequency of the sensor. By tracking the frequency spectrum of the sensor with a loop antenna, humidity, temperature, as well as CO/sub 2/, O/sub 2/ and NH/sub 3/ concentrations can be determined, enabling applications such as remotely monitoring conditions inside opaque, sealed containers. Experimental results show the sensor response to CO/sub 2/ and O/sub 2/ is both linear and reversible. Both irreversible and reversible responses are observed in response to NH/sub 3/, indicating both physisorption and chemisorption of NH/sub 3/ by the carbon nanotubes. A sensor array, comprised of an uncoated, SiO/sub 2/ coated, and MWNT-SiO/sub 2/ coated sensor, enables CO/sub 2/ measurement to be automatically calibrated for operation in a variable humidity and temperature environment.