A Gravimetric Room-Temperature CO ₂ Sensing Platform Enabled by Mg ₂ (dobpdc) Functionalized with Diamines of Different Chain Lengths

The continuous rise in carbon dioxide (CO₂) concentrations has become a significant global issue, exacerbated by the rapid expansion of industrial activities. Due to the inherent stability of CO₂, conventional detection methods are often limited by their low sensitivity and selectivity. Inspired by the remarkable enrichment capabilities of metal-organic frameworks after diamine modification, which enables effective carbon dioxide adsorption through the specific binding between CO₂ and diamines, combined with the gravimetric-sensing properties of quartz crystal microbalances, allows for CO₂ detection at room temperature. Specifically, the diamine-functionalized Mg₂(dobpdc) exhibits switch-like adsorption behavior through the cooperative insertion of CO₂ into the Mg-N bond during a chain reaction, a process confirmed by in situ infrared spectroscopy and adsorption kinetic calculations. The gravimetric-based gas sensor, fabricated from Mg₂(dobpdc) functionalized with diamines of different chain lengths, can detect CO₂ as low as 7.5 ppm, exhibits a sensitivity of 355 toward 1000 ppm of CO₂, and demonstrates excellent selectivity in mixed gas environments. The chemical reactions and kinetic processes involved in the sensing mechanism were further analyzed through the shape of the adsorption isotherms (3.5 mmol/g) and the adsorption heat (45 kJ/mol), elucidating the influence of diamine chain length on sensing performance. Finally, simulations of CO₂ detection in human exhaled breath were conducted, offering a novel strategy for health management applications.