Dandelion-like covalent organic frameworks with high-efficiency fluorescence for ratiometric sensing and visual tracking-by-detection of Fe3+

Iron ions, one of the most common heavy metal pollutants in industrial waste materials, are continuously actively or passively delivered to the environment. Meanwhile, the importance of Fe3+ in biological processes in vivo can not be neglected due to its crucial role in maintaining normal physiological function. Therefore, a ratiometric fluorescence covalent organic framework (TD-COF) was constructed for tracking-by-detection of Fe3+. Alkynes-extended 1,3,6,8-tetrakis(4-ethynyl benzaldehyde)-pyrene (TEBPY) with complete planar structure and 2,5-dihydroxyterephthalohydrazide (DHTH) with functional group -OH were selected as the building blocks. The ratiometric fluorescence TD-COF with a dandelion-like structure exhibited its dual emission peaked at 510 nm and 630 nm. It displayed an obvious fluorescence color variation of yellow-red-black in the presence of Fe3+. Benefiting from the high luminescent efficiency (QY of 36.4%) and multiple identical binding sites, TD-COF exhibited a wide linear range to Fe3+ (0.005-50 μM) with a detection limit of 10.9 nM. Additionally, a smartphone visual sensing platform integrated with TD-COF was developed based on the color transformation and successfully applied to visual smart real-time monitoring Fe3+. More surprisingly, the maximum adsorption capacity of TD-COF towards Fe3+ was 833.3 mg/g due to the coordination interaction and cationic π-effect. The practicability of the smartphone-integrated ratiometric sensing platform for visual tracking-by-detection of Fe3+ was verified by choosing tap water as the actual sample, and the recoveries were calculated to be 98.71-100.88%. This work thus developed COF-based ratiometric sensing of Fe3+, which is an attractive candidate for further application in fluorescent sensing and visual monitoring.