Green Synthesis Of Covalent Organic Framework And Application Of Modified Fluorescent Covalent Organic Framework In Cationic Dye Detection And Adsorption

Objective:（1）By using DESs based on Ch Cl-HFIP as the reaction solvent,a series of imine-linked COFs were prepared under the condition of catalyst free.COF-CH-1（TPB and TPDD as the reaction monomers）was used as the model to explore the synthesis mechanism of imine-linked COFs in DESs based on Ch Cl-HFIP.Moreover,the participation of HFIP as catalyst in the synthesis was investigated.（2）Based on the high-crystalline fluorescent COF-TPDD,COF-TPDD-COOH with stable fluorescence performance,strong interaction with target analytes and good dispersion in water system were prepared through post-modification.Further,a dual-function platform which can be used for selective fluorescence detection and enrichment of cationic dyes in water system was constructed by using the developed COF-TPDD-COOH.Methods:（1）By using DESs based on Ch Cl-HFIP as the reaction solvents,COFs which synthesized by using 1,3,5-tri（4-aminophenyl）benzene（TPB）and terephthalic acid（TPDD）as the model,the reaction temperature and reaction time were optimized with the crystallinity of COFs as the index.Further,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,N₂ adsorption-desorption isotherm experiment,thermogravimetric analysis（TGA）and Materials Studio analysis（MS）were performed to characterize the physicochemical properties of COFs.According to the characterization results,the synthesis mechanism of imine-linked COFs in Ch Cl-HFIP based DESs was studied.Next,in order to further verify the generality of the synthesis method to C₂+C₃imine-linked COFs,TPB was combined with 2,5-dimethoxy-benzene-1,4-diformaldehyde（DMBD）,2,5-dibromo-benzene-1,4-diformaldehyde（DBDA）and 2,3,5,6-tetrafluoroterephthalic acid（TFPD）as reaction monomers,respectively.A series of imine-linked COFs with high crystallinity were prepared by changing the molar ratio of Ch Cl and HFIP in DESs,and the physical and chemical properties of the COFs prepared under optimal conditions were characterized systematically.（2）By using1,3,5-tri（p-formylphenyl）benzene（TFPB）and 2,5-bis（allyl）p-benzoylhydrazine（DHz DAll）as the monomers of COFs,toluene as solvent and acetic acid as catalyst,fluorescent COF（Named as COF-TPDD）was prepared by freezing method combined with solvothermal method.The COF-TPDD with good crystallinity was prepared by optimizing the influence factors including reaction solvent,catalyst,amount of reaction solvent,reaction time and reaction temperature.The physicochemical properties of COF-TPDD were demonstrated by XRD,SEM,FT-IR,XPS,TGA and MS,further identifying the successful synethsis of COF-TPDD.Subsequently,to prepare COF-TPDD-COOH,glutathione（GSH）was modified to COF-TPDD by clicking reaction using AIBN as initiator.The fluorescence and adsorption properties of cationic dyes were studied by fluorescence analysis and adsorption experiments.The fluorescence quenching mechanism of cationic dyes on COF-TPDD-COOH was studied by analyzing the fluorescence lifetimes,Stern-Volmer values,UV-vis absorption spectra of the target substances and the excitation emission spectra of COF-TPDD-COOH.The adsorption mechanism of COF-TPDD-COOH for cationic dyes was studied by evaluating the adsorption properties of cationic dyes under different p H conditions and ESP diagrams.Finally,the fluorescence detection and enrichment dual-platform based on COF-TPD-COOH was used for the detection and enrichment of cationic dyes in actual water samples.Results:（1）By using TPB and TPDD as monomers,and DESs based on Ch Cl-HFIP（molar ratio of 1:4）as solvent,COF-CH-1 with high crystallinity was prepared by freezing method at 120℃for 72 h under the condition of no catalyst.SEM result showed that the morphology of COF-CH-1 was spherical.TEM result showed that COF-CH-1 had lattice fringe,which indicated that it had good crystallinity.FT-IR result showed that there was a C=N bond formed by the Schiff base reaction between TPB and TPDD at 1574.4 cm^-1,indicating that COF-CH-1 owned imine structure.XPS result showed that a peak attributed to the C=N bond appeared at 284.3 e V in the C 1s spectrum,and a peak which was attributed to the C=N bond appeared at 399 e V in the N 1s spectrum.These phenomena further proved the successful synthesis of COF-CH-1.The XRD and MS simulation results revealed that the XRD results were consistent with the MS simulation results,which further proved that COF-CH-1 was successfully prepared more likely with,an eclipsed AA stacking two-dimensional structure.BET result indicated that the specific surface area of COF-CH-1 was 377.29 m²/g and the pore volume was 0.3024 cm³/g.TGA data revealed that the weight loss of COF-CH-1 was very small（9%）before 450℃,while the significant weight loss（39%）caused by the structure collapse of COF-CH-1 was observed at 450-800℃,further indicating that the as-prepared COF-CH-1 had good thermal stability.Through the characterization of the physical and chemical properties of COFs which synthesized under different molar ratios of DESs,different reaction time and reaction temperature,it can be known that HFIP played the role of catalyst in the reaction process.Moreover,the amount of HFIP and the types of functional groups on the reaction monomer were closely related to the crystallinity of the as-prepared COFs.The XRD data of other four imine-linked COFs prepared by this synthesis strategy were consistent with those of the computer simulation,and the results showed that all of them had good crystallinity,large specific surface area and good thermal stability.These all results proved that the catalyst-free synthesis strategy by using DESs which based on Ch Cl-HFIP as general solvent had certain universality for the synthesis of imine-linked COFs.（2）By using toluene as solvent,acetic acid as catalyst,the bright yellow fluorescent COF-TPDD was successfully prepared through freezing method coupled with solvothermal method.The optimal synthesis parameters of COF-TPDD were as follows:ethanol as the solvent,acetic acid as the catalyst,solvent dosage as 2 m L,the reaction temperature as 140℃,and the reaction time as 72 h.SEM results showed that the morphology of COF-TPDD was curved strip.TEM results showed that COF-TPDD had lattice fringe.The FT-IR results of COF-TPDD revealed that there was a C=N bond which formed by the Schiff base reaction between TFPB and DHz DAll at 1600cm^-1.The XPS results showed that a peak which attributed to the C=N bond appeared at 285.89 e V in C 1s spectrum,and a peak belonged to the C=N bond appeared at 400.00 e V in N 1s spectrum.The results of FT-IR and XPS proved that the COF-TPDD was successful synthesized.The XRD and MS simulation results showed that the XRD results were consistent with the MS simulation results,which further proved that COF-TPDD was successfully prepared,and COF-TPDD had a crystal type and AA stack structure.BET results revealed that the specific surface area and pore volume of COF-TPDD were 174.43 m²/g and 0.147 cm³/g,respectively.The results of TGA data showed that the weight loss of COF-TPDD was very small（13.78%）before 350℃.When the temperature was higher than350℃,the structure of COF-TPDD collapsed significantly.These results indicated that the prepared COF-TPDD had good thermal stability.Further,by using AIBN as initiator,click reaction was carried out in nitrogen protection environment,and COF-TPDD-COOH was successfully synthesized by the post-modification of COF-TPDD using GSH.FT-IR characterization showed that there was a hydroxyl peak at 3488 cm^-1,which was in the same peak position of-OH in the FT-IR spectrum of GSH,indicating that GSH was successfully modified to COF-TPDD.Further,the fluorescence responses of COF-TPDD-COOH to cationic dyes were investigated.The results revealed that the response time for three cationic dyes（MG,MB,CV）was 30 s,and the limits of detection（LODs）was in range of 0.003-0.007μg/m L,with good linearity in the concentration range of 0-200μg/m L.In addition,in the presence of interfering substances（e.g.metal ions,antibiotics and dyes）,the fluorescence of COF-TPDD-COOH could still selectively quenched by three selected cationic dyes.Moreover,the quenching mechanism of cationic dyes on COF-TPDD-COOH proved that their quenching mechanism was a combination of internal rate effect（IFE）and static quenching.Subsequently,the adsorption performance of COF-TPDD-COOH was studied by static and dynamic adsorptions.The results showed that the maximum adsorption capacities of COF-TPDD-COOH for three dyes were in the range of 88.02-128.64 mg/g,and the Langmuir isothermal model was suitable for explaining the adsorption of cationic dyes by using COF-TPDD-COOH.The experimental data of dynamic adsorption showed that the adsorption of cationic dyes tended to be stable after 60 min,and the pseudo-second-order kinetic model was suitable for explaining the adsorption of cationic dyes by COF-TPDD-COOH.The results of the adsorption mechanism showed that COF-TPDD-COOH and three cationic dyes would be ionized in the water system.The quaternary ammonium nitrogen of cationic dyes would be positively charged,while the carboxyl oxygen atoms of COF-TPDD-COOH would be negatively charged,and thus cationic dyes can be adsorbed on COF-TPDD-COOH through electrostatic interaction.Finally,the actual sample analysis results showed that the recoveries of the three cationic dyes in aquaculture water were 88.11-111.11%with relative standard deviation less than 4.7%.Conclusion:（1）A series of imine-linked COFs with high crystallinity were prepared by using DESs which based on Ch Cl-HFIP（different molar ratio）as the reaction solvent in the absence of catalyst.This kind of synthesis strategy with certain universality not only uses green solvent,but also does not need additional catalyst,which is convenient for subsequent recycling.The imine-linked COFs prepared by using this method had high yield,which provided the possibility for the subsequent large-scale synthesis of imine-linked COFs.In addition,by using the COF-CH-1which was synthesized by using TPB and TPDD as precursors,as a model,the synthesis mechanism of COF-CH-1 by using Ch Cl-HFIP based DESs as reaction solvents was verified and explained,and the catalytic role of HFIP in the reaction was proved.The process of HFIP participating in the synthesis of imine-linked COFs was deduced,which would lay a foundation for the subsequent synthesis of other types of COFs using DESs containing HFIP.（2）Based on the COF-TPDD which prepared by using TFPB and Dhz DAll as precursors,the high-crystalline COF-TPDD-COOH functionalized with GSH with bright yellow fluorescence was successfully prepared through post-modification method.The introduction of GSH improved the dispersion and selectivity of the material without affecting the original properties of COF-TPDD.Cationic dyes can quench the fluorescence through internal filtration effect（IFE）and static quenching mechanism,and adsorbed on COF-TPDD-COOH through electrostatic force and size effect.Therefore,COF-TPDD-COOH can be used as a dual-function platform for sensing and adsorption of three cationic dyes,which can be used for the detection and enrichment of three dyes in actual samples.