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 , 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