Spin-State Engineering in 2D Metal-Organic Frameworks for Ultrasensitive Room-Temperature Ammonia Sensing

Precise, noninvasive quantification of gaseous ammonia (NH3) in exhaled breath is a promising clinical diagnostic method with important applications for assessing liver and kidney metabolic function. A critical factor in NH3 detection is the electronic structure of metal active sites. However, the pivotal role of electron spin as a descriptor of these electronic properties is often overlooked. Here, we address this by synthesizing Co-HITP, CoNi-HITP, and CoNiFe-HITP gas-sensitive materials. These materials were developed by strategically modulating the spin state of Co metal sites, achieved through the synergistic coupling of Fe, Co, and Ni single atoms. Experimental investigations, complemented by density functional theory (DFT) calculations, revealed that the modulated spin states at the metal sites and their synergistic effects profoundly influence the adsorption and activation processes of NH3 and key reaction intermediates (e.g., *NH2, *HNO, and *NO), consequently altering the energy barriers. The optimized CoNiFe-HITP exhibited exceptional sensitivity (88.8%), selectivity, and a low detection limit (50 ppb) for 1000 ppm NH3 at room temperature. This study not only highlights the potential of 2D metal-organic frameworks as an ideal platform for spin regulation to enhance gas sensitivity but also provides novel design insights for developing highly efficient and low-power gas sensors.