Microwave Sensor with Light-Assisted Enhancement Based on Ti₃C₂T_<i>x</i> MXene: Toward ppb-Level NO₂ Detection Limits

Chemiresistive sensors are currently the most popular gas sensors, and metal semiconductor oxides are often used as sensitive materials (SMs). However, their high operating temperature means that more energy is required to maintain normal operation of the SM, resulting in an increase in power consumption of the entire sensing system. In order to solve this problem, a microwave gas sensor embedded with multilayer Ti₃C₂T_<i>x</i> MXene and split ring resonator (SRR) for nitrogen dioxide (NO₂) detection was reported in this work. The sensor takes advantage of the weak coupling between the two SRRs to achieve a highly concentrated electric field and high <i>Q</i>-factor, in which the weak coupling region serves as the sensitive region to avoid damage to the resonator structure caused by the excessive conductivity of Ti₃C₂T_<i>x</i>. The sensor has good selectivity, a lower detection limit of 2 ppb, with an average detection sensitivity of 98.66 mdB ppm^-1 in the range of 2-10,000 ppb at room temperature. Additionally, the effect of different lighting source to the sensor performance is investigated, and the sensor reached the best response (1.54 dB) under blue light. Finally, the mechanism of the enhanced sensitivity is discussed in detail based on the results of simulations and tests. The sensor circuit designed in this work provides a new approach for MGSs and for the first time introduces the photocatalytic pathway into microwave sensors, which will contribute to the optimization of microwave gas sensors in the future.