Chemiresistive Sensing of Ambient CO₂ by an Autogenously Hydrated Cu₃(hexaiminobenzene)₂ Framework

A growing demand for indoor atmosphere monitoring relies critically on the ability to reliably and quantitatively detect carbon dioxide. Widespread adoption of CO₂ sensors requires vastly improved materials and approaches because selective sensing of CO₂ under ambient conditions, where relative humidity (RH) and other atmosphere contaminants provide a complex scenario, is particularly challenging. This report describes an ambient CO₂ chemiresistor platform based on nanoporous, electrically conducting two-dimensional metal-organic frameworks (2D MOFs). The CO₂ chemiresistive sensitivity of 2D MOFs is attained through the incorporation of imino-semiquinonate moieties, i.e., well-defined N-heteroatom functionalization. The best performance is obtained with Cu₃(hexaiminobenzene)₂, Cu₃HIB₂, which shows selective and robust ambient CO₂ sensing properties at practically relevant levels (400-2500 ppm). The observed ambient CO₂ sensitivity is nearly RH-independent in the range 10-80% RH. Cu₃HIB₂ shows higher sensitivity over a broader RH range than any other known chemiresistor. Characterization of the CO₂-MOF interaction through a combination of in situ optical spectroscopy and density functional theory calculations evidence autogenously generated hydrated adsorption sites and a charge trapping mechanism as responsible for the intriguing CO₂ sensing properties of Cu₃HIB₂.