Synthesis Of Two-dimensional Hydrazone Covalent Organic Frameworks (COFs) And The Fluorescence Sensing Study

Covalent Organic Frameworks（COFs）are porous structural crystalline materials formed by the reversible polymerization of light elements such as C,H,O,N,B under thermodynamic control.With the development of porous covalent organic framework material synthesis strategies and the diversification of applications,they have shown great advantages in gas storage,adsorption,drug delivery,separation,catalysis,sensing and other fields.The intrinsic porosity of COFs provides a transport channel for the analyte to respond quickly during sensing,the periodic structure and the introduction of specific binding sites make it highly selective.Compared with traditional small molecule chemical sensors,COFs are insoluble in water and common organic solvents,which are conducive to their separation,regeneration,reuse and further processing,so COFs have broad application prospects in the field of fluorescence sensing.In this study,four two-dimensional hydrazone fluorescent COFs were synthesized for the fluorescence detection of palladium ions in water,pH and NH₃.The main research contents are as follows:（1）The alkenyl functionalized hydrazone COF1 with good crystallinity was prepared by liquid-liquid interface synthesis method with two building blocks of2,5-bis（but-3-en-1-yloxy）terephthalohydrazide and 1,3,5-benzenetricarboxaldehyde.It can maintain good stability in organic solvents,acid and base solutions,which lays a good foundation for detection in different environments.COF1 has fluorescence characteristics.The optimal experimental conditions were determined by investigating the effects of organic solvent type,catalyst type and reaction time on COF1fluorescence intensity.COF1 was used for heavy metal ion fluorescence detection in water:the fluorescence intensity of COF1 was significantly reduced after the addition of Pd²⁺.COF1 showed good linearity for Pd²⁺in the concentration range of 1-80μM,and the limit of detection（LOD）is 0.15μM.COF1 fluorescent test paper enables visual detection of Pd²⁺under 365 nm UV lamps.COF1 also has a large specific surface area（676.1586 m²/g）.The adsorption capacity of COF1 on Pd²⁺in water was investigated,and the results showed that the maximum adsorption capacity could reach 109.2 mg/g.Finally,the detection and adsorption mechanism of Pd²⁺by COF1was preliminarily explored by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.（2）Three kinds of two-dimensional hydrazone fluorescent COFs were prepared via liquid-liquid interface synthesis method with aldehyde ligands of1,3,5-benzenetricarboxaldehyde and 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde,hydrazide ligands of 2,5-dimethoxyterephthalohydrazideand2,5-diethoxyterephthalohydrazide as building blocks.The synthesis,morphology,stability,crystallinity and fluorescence characteristics of these COFs were characterized by Fourier transform infrared spectroscopy,scanning electron microscope,thermogravimetric analysis,X-ray diffraction,and fluorescence spectroscopy.Among them,the crystallinity of COF3 and COF4 is better than COF2,and they have good chemical stability.The response of the fluorescence intensity of COF2 and COF3 to pH was investigated.COF2 and COF3 showed good linearity for pH in the range of 7-13,R² were 0.990 and 0.991,respectively.In addition,both materials offer good stability and repeatability,which provides a promising method for fluorescence detection of pH.The response of COF4 to NH₃ was investigated by exposure experiment.The deprotonation of-NH-in the COF4 structure changes the conjugated structure of skeleton,and it can respond quickly to NH₃ within 2 s,the fluorescence intensity is significantly reduced and the color changes from yellow-brown to reddish-brown.After COF4 is exposed to NH₃,it can restore fluorescence by re-protonation by immersing in acid solution,which has good reproducibility.COF4 aerogels prepared with chitosan as matrix also showed good detection ability for NH₃ gas.