A Wireless, Passive Carbon Nanotube-Based Gas Sensor

A gas sensor, comprised of a gas-responsive multiwall carbon nanotube (MWNT)-silicon dioxide (SiO2) composite layer deposited on a planar inductor-capacitor resonant circuit is presented here for the monitoring of carbon dioxide (CO2), oxygen (O-2), and ammonia (NH3). The absorption of different gases in the MWNT-SiO2 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 CO2, O-2 and NH3 concentrations can be determined, enabling applications such as remotely monitoring conditions inside opaque, sealed containers. Experimental results show the sensor response to CO2 and O-2 is both linear and reversible. Both irreversible and reversible responses are observed in response to NH3, indicating both physisorption and chemisorption of NH3 by the carbon nanotubes. A sensor array, comprised of an uncoated, SiO2 coated, and MWNT-SiO2 coated sensor, enables CO2 measurement to be automatically calibrated for operation in a variable humidity and temperature environment.