Room-Temperature CO₂ Monitoring Platform Enabled by Alkali Metal Functionalization of a Mg-MOF-74-Based QCM Sensor

Carbon dioxide (CO₂) detection is indispensable for monitoring climate change, ensuring air quality, managing industrial processes, and safeguarding human health. Nevertheless, the chemical inertness and stability of CO₂ pose significant challenges in advancing detection technologies in practical applications. In order to overcome these challenges, nanoscale MOF-74 metal-organic frameworks (MOFs) functionalized with alkali metals (Li, Na, and K) have been synthesized for the effective detection of the CO₂ gas. The sensing results indicate that the Li-Mg-MOF-74-based quartz crystal microbalance (QCM) CO₂ sensors demonstrate excellent properties, such as very high sensitivity, rapid response/recovery time (84 s/69 s), broad detection range (300-10000 ppm), and remarkable selectivity at room temperature. The enhanced performance benefits from the increased electrostatic force and Lewis's acidity resulting from alkali metal ions (Li⁺) and open metal sites (Mg²⁺). In addition, the equilibrium constant of CO₂ on the sensor surface was calculated by the Langmuir adsorption isotherm model, revealing spontaneous and robust adsorption behavior. These results indicate that alkali-metal-modified Mg-MOF-74 materials have great potential for practical CO₂ detection and provide a feasible solution for the design of high-performance, room-temperature CO₂ sensing platforms.