Temperature-Regulated Reversible QCM and MEMS Dual-Mode Gas Sensing Based on a Thermo-/Electro-Responsive Viologen-Based Cu-MOF.

Metal-organic framework (MOF) materials are often used in gas sensing and detection due to their high porosity and designable structure. However, traditional MOFs are usually only suitable for quartz crystal microbalance (QCM) mass-responsive sensing due to their poor conductivity, and are not suitable for micro-electromechanical system (MEMS) chemiresistive sensing. Designing MOF materials that combine QCM and MEMS dual-mode gas sensing has become a challenge. Herein, we designed and synthesized a viologen-based Cu-MOF material that exhibits a sensitive thermo-/electro-responsive color change, accompanied by a decrease in the band gap from 2.106 eV at room temperature to 1.817 eV at 160 °C. The gas-sensing properties of Cu-MOF were explored using QCM and MEMS techniques. At room temperature, the QCM sensor exhibits high selectivity and a rapid response to chlorinated methanes (∼106 Hz). At 160 °C, Cu-MOF undergoes electron transfer to a free radical state, demonstrating a highly selective ethanol vapor response (235.56%) in the MEMS sensor, and has an ultrafast response speed (4-11 s) and cycling stability. Cu-MOF is the first material to realize reversible "mass-semiconductor" dual-mode gas sensing via temperature regulation, providing a novel strategy for designing multimodal gas-sensing materials.