Microwave Gas Sensor Based on Carbon Nanotubes Loaded on Open Loop Ring Resonators

This letter presents a microwave gas sensor based on planar coupled open loop ring resonators (OLRRs) embedded with carbon nanotube sensing film for ammonia gas sensing. The sensing area adopts the interdigitated electrode patterns to provide a larger contact area for better sensitivity. By integrating carbon nanotube films as the local variable resistors into a microwave sensor platform, the platform can transduce the gas concentration to its electromagnetic properties. According to the step response of NH3 from 100 to 1000 ppm, the sensitivity is estimated as - 11~\mu dB/ppm and −232 Hz/ppm for the insertion losses and resonant frequency variations, respectively. The sensor repeatability and reversibility are also measured by maintaining the concentration from 300 to 700 ppm and repeating ten cycles with the average value of each concentration consistent with the step response. The results show a linearly response and a high repeatability of OLRRs via the real-time detection for gas sensing at high frequency and room temperature. Microwave gas sensors can be further integrated in the IoT or RF front-end system, offering low-cost, lightweight, reliable, and power efficient system for remote monitoring.