A“Duet-Insurance” Sensing Strategy by Combining Janus TpMa COF/Eu-doped α-Fe2O3 and Variable-Temperature Excitation for Robust, Visual Indoor Pollution Gas Detection

Water competitive adsorption and limited reaction sites on the micro-scale sensing area of micro-electro-mechanical system (MEMS) sensors are two bottleneck problems that restrict the detection of trace indoor polluted gases, leading to drifting and weak signals. To address this, a “duet-insurance” sensing strategy is proposed to protect and amplify the sensing signal. For the first-level protection, the well-designed double-layer structure consisting of Janus TpMa covalent organic framework (COF)/Eu-doped α-Fe 2 O 3 can resist diffusion of water molecules and facilitate the electron transfer from formaldehyde (CH 2 O) to sensing layers, benefiting from the outer hydrophobic character of Janus TpMa and the strong CH 2 O adsorption capacity of Eu-doped Fe 2 O 3 . For the second-level amplification, the unique variable temperature excitation facilitates the deep penetration of CH 2 O into the active sites across COF layers, enabling signal amplification through thermally actuated carrier modulation. The sensor demonstrates high sensing ability to the CH 2 O, with an ultra-low limit of detection of 794 ppt. Leveraging the novel sensing strategy, a multi-functional, visual sensing system is designed to identify multi-indoor pollution gases by feature signal encoding and color mapping, showcasing the promising potential for MEMS sensors based on Janus TpMa COF/Eu-doped α-Fe 2 O 3 materials.