Intramolecular charge transfer-induced fluorochromic covalent organic frameworks for high speed on-line monitoring of volatile amines

Direct and visual evaluation of volatile amine molecules, a group of hazardous gases and indicators, remains challenge in the fields of chemical industry, environmental detection and food safety. Herein, a convenient and reversible intramolecular charge transfer (ICT)-induced fluorescence-colorimetric approach is proposed for on-line, visual and quantitative monitoring on volatile amines at molecular level through skeletal constructing of cationic pyridinium sites in a carbon-conjugated covalent organic framework (COF). In the presence of volatile amine molecules, the deprotonation of pyridinium sites causes a strong fluorescence enhancement (fluorescence quantum yield > 13 %) and a sharp color change. Specifically, with the reversable pyridinium sites, the COFs-based sensor first exhibits an ultrafast response time (τrise) of 0.65 s to ammonia gas (51.8 ppm), which outperforms reported nanostructure-based ammonia sensors. Meanwhile, through studying the relationship between the total volatile basic nitrogen (TVB-N) and Red/Green/Blue (RGB) outputs of home-made COFs membrane on the food spoilage (fish meat: R2 > 0.9987, shrimp meat: R2 > 0.9917), the protonated TPCH-mOBPy COF is fully verified to work as an on-line fluorochromic probe for visually, AI-intelligently and quantitatively monitoring the generation of volatile amines.