Methyl functionalization of trimesic acid in copper-based metal-organic framework for ammonia colorimetric sensing at high relative humidity

Exhaled ammonia (NH3) can be used as a crucial biomarker of kidney and liver diseases. However, the high humidity in the detection conditions remains a challenge for accurate detection by gas sensors. Herein, a copper-based metal-organic framework (CH3-Cu-BTC) with methyl (CH3-) functionalization of trimesic acid was synthesized for NH3 colorimetric sensing. The CH3-Cu-BTC exhibited a strong response for 5 ppm NH3 with high selectivity under high relative humidity (75% RH). Density functional theory (DFT) simulations indicated that the NH3 molecules interacted more strongly with CH3-Cu-BTC than H2O molecules did, and the corresponding color switching was attributed to the lone-pair electron in NH3 changing the coordination environment of Cu2+ ions, leading to an obviously visible color switching response from ruby green to blue. Based on the tailor-made pore chemistry, the precise detection of trace amounts of NH3 in exhaled air was realized through functionalized MOF materials. The strategy used in this study not only offers a new pathway for the rapid detection of low concentration NH3 under humid conditions, but also shows a method for early respiration diagnosis of kidney and liver diseases.