Moisture-Insensitive and Highly Selective Detection of NO₂ by Ion-in-Conjugation Covalent Organic Frameworks

As a common toxic gas, nitrogen dioxide (NO₂) seriously threatens the environment and human respiratory system even at part per billion (ppb) level. Covalent organic frameworks (COFs) have gained widespread attention in sensing applications because of the benefits of designability, environmental stability, and a large number of active sites. However, the competitive adsorption of water molecules and the target gas molecules at room temperature as well as the weak interaction between COFs and gas molecules hinder their practical applications. Here, we introduce ion-in-conjugation (IIC) into a covalent organic framework (COF) by preparing a condensate of squaraine (SA) with 1,3,5-tris(4-aminophenyl)benzene (TAPB) to form a mesoporous macrocyclic material (SA-TAPB). Layers of SA-TAPB, drop cast onto interdigitated Ag-Pd alloy electrodes, show a statistically significant conductivity response to NO₂ at concentrations as low as 30 ppb and a theoretical detection limit of 10.9 ppb. The sensor displays a lower sensitivity to variations in humidity when operated at 80 °C compared to room temperature. The density functional theory (DFT) calculations indicated that the main adsorption site of NO₂ is dual hydrogen bonds formed between two amide hydrogen atoms of SA-TAPB and the NO₂ molecule. Gas adsorption experiments revealed that SA-TAPB has the largest adsorption capacity of NO₂ versus other interference gases, which were responsible for the excellent selectivity toward NO₂.