Synthesis And Fluorescence Application Of Covalent Organic Frameworks Based On Schiff Base Reaction

Covalent Organic Frameworks（COFs）materials are a class of crystalline porous organic polymer materials formed by precise self-assembly of organic building blocks.They have low skeleton density,high specific surface area,and high crystallinity.By changing organic small molecular units,they can achieve orderly arrangement of framework structures and precise regulation of nano pore size at the atomic scale.This article selects covalent organic framework materials as the research object,and designs and synthesizes novel imine-based COFs materials with different pore structures and functional groups through functional modification using different monomers.Based on the successful preparation of COFs materials,their fluorescence detection capabilities for divalent copper ions(Cu²⁺)and divalent stannous ions(Sn²⁺)were investigated,and the recognition mechanism was analyzed.The method has been applied to the detection of Cu²⁺in water and Sn²⁺in canned food,and good application results have been achieved.The paper includes the following three parts:（1）Based on the Schiff base reaction,COF-300 and COF-OH were prepared by solvothermal synthesis using tetra（4-aminophenyl）methane,p-benzaldehyde,and 2,5-dihydroxyp-benzaldehyde as precursors.Through SEM,it was observed that COF-300 exhibits a rice grain shape in the microscopic state,while COF-OH exhibits a needle cluster shape.Structural simulation was conducted using Materials Studio（MS）to obtain two three-dimensional COFs material structures in an interpenetrating state.The PXRD patterns obtained from the experiment and the PXRD patterns of the simulated structure have a good overlap,which proves that COFs materials have been successfully prepared.The prepared COFs have good thermal stability,and the materials only begin to decompose at temperatures as high as 480℃and 390℃.Fluorescence properties show that both COFs have a specific response to Cu²⁺.Compared to the COF-300 structure,COF-OH has a Cu²⁺concentration of 30μM,the fluorescence is almost completely quenched,with better recognition performance and fast response time（40 s）.The linear equation is y=-80511x+2898010,and the linear relationship is good（R²=0.961）.The mechanism of ion recognition may be due to the complex effect of Cu²⁺with nitrogen and oxygen elements in COF-OH,resulting in fluorescence quenching.Based on its good fluorescence characteristics,it has been applied to the detection of Cu²⁺in tap water samples with good results.（2）Using tetra（4-aminophenyl）ethylene as one of the precursors,COF-ET and COF-ED were prepared by a one-step solvothermal method through a Schiff base reaction with p-benzaldehyde or 2,5-dihydroxyp-benzaldehyde.Both COFs have a high specific surface area（672.20 m²/g and 533.96 m²/g,respectively）,uniformly distributed structure,and good thermal stability（decomposition occurs only at 485℃and 378℃,respectively）.By comparing the PXRD patterns of the simulated structure with the experimental PXRD results,it was proved that the two COFs were both single-pore and AA stacked structural models,and the successful preparation of their structures was also confirmed.Through fluorescence performance testing,it was found that these two COFs materials have a specific response ability to Cu²⁺,and the response speed is fast（about 40 s）.In order to expand the application of COFs materials,C OF-ET and COF-ED doped polystyrene（PS）were used to prepare COFs composite films for solid phase detection of Cu²⁺.The test results show that the composite film still has a good recognition ability for Cu²⁺,which provides ideas for the application of its COFs materials to solid phase sensing materials.（3）Using 2,5-dimethoxy-1,4-dialdehyde and tetra（4-aminophenyl）ethylene as precursors,a novel COFs material（COF-ETTA-DMTA）with a high specific surface area（353.13 m²/g）was designed using a Schiff base reaction and solvent thermal synthesis method.Various possible structures of COFs were simulated,and it was found that it exhibited a single pore and AB stacked structural model.The detection of Sn²⁺by COF-ETTA-DMTA showed the advantages of short response time（about 50 s）and low detection limit（228 n M）,and the specificity recognition of Sn²⁺showed good linearity（R²=0.997）.In this work,we also simulated and verified the recognition mechanism of COFs for Sn²⁺by designing small molecules with the same functional units.The COFs have been successfully applied to the determination of Sn²⁺in solid canned foods（lunch pork,canned fish,canned red kidney beans）.Compared with the traditional ICP-MS method,the results obtained by this method are in line with expectations.This work provides a new method for identifying and determining metal ions using the rich reaction sites and large specific surface area of COFs materials,improving the detection sensitivity and capacity,and is expected to be a powerful tool for food safety assessment.