Design And Synthesis Of Hierarchical Porous Covalent Organic Framework For Sample Pretreatment

Covalent Organic framework（COF）is a kind of porous crystalline polymers with long-range-order structure and topological geometry formed by covalent bonding of organic monomers containing light elements such as C,N,O,B,Si,and S.Due to the advantages of permanent porosity,considerable specific surface area,designable functionalization and good chemical stability,COF is an ideal adsorbent for sample pretreatment,and have been successfully used in solid phase extraction,dispersive solid phase extraction,magnetic solid phase extraction,stir bar sorptive extraction and solid phase microextraction to enrich various target analytes from complex samples such as environmental,food and biological samples.So far,most of COF are microporous structure,which slow down the mass transfer of analytes and restrict the active site exposure to large-size molecules,resulting in a decrease in the utilization of internal adsorption sites of COF.Constructing meso-or macropores in microporous COF to prepare hierarchical porous COF can effectively break through the bottleneck of COF in the application of sample pretreatment.However,owing to the synthetic difficulty and poor crystalline,the fabrication strategies of hierarchical porous COF are relatively scarce,and developing hierarchical porous COF as adsorbent for sample pretreatment is still in the dawn.Therefore,hierarchical porous COF was designed and synthesized by structural-defect or sacrificial-template protocol,and then was adopted for efficient separation and enrichment of target analytes.The specific research contents are as follows.1.Structural defect method has been widely used in the preparation of hierarchical porous MOF,but there was little report on the preparation of hierarchical porous COF.Both COF and MOF belong to porous organic framework,so the synthetic idea of hierarchical porous MOF is useful for the fabrication of hierarchical porous COF.Microporous Tp/TFB-Pa-1 with both imine bond andβ-ketoenamine bond were prepared by two mixed aldehyde monomers.Then,the imine bond was selectively etched by HCl based on different stabilities to acid of imine bond andβ-ketoenamine bond to produce structural defects,thus fabricating hierarchical porous HP-Tp Pa-1 with both micropores and mesopores.The hierarchical porosity of HP-Tp Pa-1 was confirmed by N₂ adsorption–desorption experiment.HP-Tp Pa-1 was used for the adsorption of cationic dye methylene blue（MB）in aqueous solution.Benefiting from the expansion of pore size,the adsorption capacity of HP-Tp Pa-1 for MB was 20.6,3.6 and 1.7 times that of microporous LZU1,Tp Pa-1 and Tp/TFB-Pa-1.The saturated adsorption capacity of HP-Tp Pa-1 on MB was 112.3 mg·g^-1.This work provided a novel method for the preparation of hierarchical porous COF.2.In addition to pore size,modifying COF according to the properties of the target analytes is another useful means to improve their adsorption capacity.Therefore,Tp EDA/DDS containing S–S bond in the skeleton was designed and prepared,and then S–S bond was reduced by glutathione（GSH）to generate–SH bond to realize functionalization.Meanwhile,the fracture of S–S bond resulted in skeleton defects and enlarged pore size.In this strategy,GSH played the dual role of modifying–SH group and breaking S–S bond,and the functionalized hierarchical porous HP-Tp EDA-SH was successfully constructed by a facile one-step reaction at ambient temperature.Due to the dual effects of electrostatic attraction and hierarchical porosity,the saturated extraction capacities of HP-Tp EDA-SH for MB,malachite green（MG）and crystal violet（CV）were 2.6,2.1 and 3.3 times that of microporous Tp EDA,respectively,and it also presented good extraction selectivity.The analytical method of cationic dyes based on dispersive solid phase extraction（DSPE）used HP-TPEDA-SH as adsorbent combined with HPLC-UV had high sensitivity with the limits of detection of 0.12?1.3μg·L^-1.The established method could be applied for the determination of trace MB,MG and CV in aquarium water,fishpond water and wastewater,and the spiked recoveries were 81.5?113.8%.This work not only established an effective method for detecting trace cationic dyes in water,but also opened a convenient and meaningful avenue for the preparation of functionalized hierarchical porous COF.3.Design and construction of a matrix according to the properties of target analytes can effectively improve the detection sensitivity of SALDI-TOF MS analysis.Sulfonic acid functionalized hierarchical porous H-COF-SO₃H was customized via defect-structure and post-synthesis modification method and used as adsorbents and matrix for detecting quaternary ammonium herbicides paraquat（PQ）and diquat（DQ）by SALDI-TOF MS.N₂adsorption–desorption experiments certified that the aperture of H-COF-SO₃H concentrated at 1.3,1.5,and 2.8 nm.The strong UV absorption of H-COF-SO₃H at 200?450 nm enabled it to transfer laser energy,thus could be used as the matrix of SALDI-TOF MS.Besides,H-COF-SO₃H could efficiently and specifically enrich PQ and DQ through electrostatic interaction and size complementarity,which improved the sensitivity of the analytical method.The adsorption mechanism of PQ and DQ by H-COF-SO₃H was investigated by the density functional theory（DFT）.The SALDI-TOF MS method based on H-COF-SO₃H presented the merits of simple pretreatment,low limits of detection(0.1?0.5 ng·m L^-1)and rapid analysis.The sensitivity of PQ and DQ enriched by H-COF-SO₃H was 20 and 60 times higher than that of directly determined without H-COF-SO₃H enrichment,respectively.The established method could be applied for the rapid determination of trace PQ and DQ in rice,apple and cabbage,and the spiked recoveries were 80.1?113.2%.This work provided an analyte-oriented approach for fabricating SALDI-TOF MS matrix.4.Photo-Fenton reaction possesses the virtues of strong oxidation ability and environmentally friendly,and its application in the preparation of hierarchical porous COF by structural defect method is likely to shorten the construction time.In this work,the photo-Fenton reaction was employed to selectively oxidize the active site–N=N–of microporous Tp EDA/Azo to produce skeleton defects,so as to elaborately synthesize hierarchical porous H-COF.Because this preparation tactics took full advantages of fast rate and high oxidation efficiency of photo-Fenton reaction,the rapid transformation from micropores to mesopores in Tp EDA/Azo was able to complete within 8 min,and its preparation efficiency was 3.75times that of Fenton reaction.The mesoporous volume ratio,surface morphology and hydrophilicity of H-COF could be controlled by light intensity and photon-Fenton reaction time.The formation mechanism of H-COF was discussed by DFT and X-ray photoelectron spectroscopy analysis.Thanks to the increased utilization of adsorption sites caused by enlarged aperture,the saturated adsorption capacity of H-COF for tetracycline（TC）was 3.52times that of Tp EDA/Azo.The maximum adsorption capacity of H-COF for TC was 162.1mg·g^-1,and the ionic strength and coexisting ions had little effect on the adsorption of TC.Therefore,H-COF was a potential adsorbent for removing TC from environmental water samples.This work provided a novel and intriguing idea for the rapid preparation of hierarchical porous COF.5.Perfluorinated substances（PFASs）are a kind of harmful pollutants with environmental persistence and high bioaccumulation.Due to their extremely low content in complex samples,effective sample pretreatment must be taken to detect trace PFASs in actual samples.COF has been applied in the efficient enrichment of PFASs,but its microporous structure limits its adsorption capacity and mass diffusion.Hierarchical porous COF is an effective strategy to overcome the above problem.Hierarchical porous H-COF was prepared with polystyrene microspheres（PS）as hard template,and then functionalized by quaternization reaction to obtain cationic hierarchical porous C-H-COF.Static adsorption experiments showed that the adsorption amounts of C-H-COF on five PFASs were superior to those of H-COF and microporous M-COF,confirming the effects of hierarchical porosity and electrostatic attraction on enhancing the enrichment of PFASs.The maximum adsorption capacity of C-H-COF for perfluorooctanoic acid（PFOA）calculated by Langmuir isothermal model was up to 400.0 mg·g^-1,which outperformed most of the reported PFOA adsorption materials.Under optimal extraction conditions,the analytical method of PFASs developed by introducing C-H-COF as DSPE adsorbent combined with UPLC-MS/MS detection possessed the virtues of high sensitivity and fast extraction,with the limits of detection were0.011?0.29 ng·L^-1.This method could be successfully used for the rapid quantitative analysis of five ultra-trace PFASs in milk and milk powder samples.6.Preparation of hierarchical porous COF by hard template method requires additional process to remove the template.In order to simplify the etching step of template,ionic liquid1-butyl-3-methylimidazolium tetrafluoroborate（[C₄Min][BF₄]）was used as a soft template to synthesize hierarchical porous HP-Tp BPy.[C₄Min][BF₄]also acted as a reaction solvent in this preparation strategy.Then,1,2-dibromoethane was reacted with bipyridine moiety in HP-Tp BPy to acquire positively charged HP-Tp BPy²⁺.The adsorption capacity of HP-Tp BPy²⁺on five PFASs was higher than that of HP-Tp BPy and microporous Tp BPy due to the augmentation of pore size and electrostatic attraction.The established DSPE-UPLC-MS/MS method based on HP-Tp BPy²⁺had the advantages of short extraction time（8 min）and low limits of detection(0.030?0.28 ng·L^-1),which could be applied for the quantitative analysis of five PFASs in urine samples of hyperuricemia patients and normouricemia people.The results indicated that the concentration of PFASs in hyperuricemia patients was higher than that in normouricemia people,and short-chain PFASs were more likely to accumulate in the body.This work was expected to provide an accurate and sensitive detection method for exploring the correlation between the exposure levels of PFASs and chronic diseases.