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 (O2), 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, O2 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 O2 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.