Synthesis method and application of zwitterionic covalent organic framework nanosheets

Abstract

The invention belongs to the technical field of food safety, and specifically relates to a synthesis method and application of a zwitterionic covalent organic framework nanosheet. The synthesis method of the zwitterionic covalent organic framework nanosheet comprises: step S1, synthesizing a sulfonic acid covalent organic framework matrix material through a condensation reaction between 2,4,6-trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid monomers; step S2, subjecting the sulfonic acid covalent organic framework matrix material synthesized in step S1 to a substitution reaction with methyl iodide, thereby preparing a zwitterionic covalent organic framework nanosheet material having a negatively charged sulfonic acid group and a positively charged quaternary ammonium group; the zwitterionic covalent organic framework nanosheet material prepared by the synthesis method of the zwitterionic covalent organic framework nanosheet has a regular pore size and a large aspect ratio, which is conducive to the rapid mass transfer of a target object in the material, and at the same time, high-density and regularly distributed zwitterionic active sites of sulfonic acid groups and quaternary ammonium groups enable the material to have a strong ion pair electrostatic attraction.

Description

Technical Field

[0001] The invention belongs to the technical field of food safety, and specifically relates to a synthesis method and application of a zwitterionic covalent organic framework nanosheet. Background Art

[0002] Quinolone antibiotics are a class of synthetic antibacterial drugs containing the basic structure of 4-quinolone. Due to their excellent antibacterial activity, they are widely used in the livestock and poultry disease prevention in the breeding industry to improve breeding efficiency. However, the abuse of quinolone antibiotics will lead to their accumulation in animals. The accumulated antibiotics are further transferred to animal-derived foods through food processing, which will undoubtedly affect food safety to a certain extent.

[0003] In order to ensure the safety of animal-derived food and eliminate the threat of quinolone antibiotic residues to human health, people have developed a variety of methods such as high-performance liquid chromatography and high-performance liquid chromatography-mass spectrometry to detect trace antibiotic residues in animal-derived food. However, due to the complex matrix of animal-derived food samples and the very low content of quinolone antibiotics, direct instrument detection cannot achieve the required sensitivity or accuracy. Therefore, it is necessary to use appropriate sample pretreatment techniques to purify samples before sample analysis to remove background interference and enrich quinolone antibiotics. Among them, solid phase extraction is widely used for rapid and selective enrichment of organic pollutants in food due to its advantages such as simple operation and rapidity, and has become one of the most competitive methods in sample pretreatment, among which solid phase extraction adsorbent is the core element of solid phase extraction.

[0004] In recent years, various materials such as inorganic nanomaterials, porous organic polymers, and metal organic frameworks have been used as solid phase extraction adsorbents for quinolone antibiotics in animal-derived foods. However, these materials themselves still generally have irregular porous structures that affect mass transfer effects and unclear or invalid functional groups, resulting in weak binding ability between the materials and quinolone antibiotics, making it difficult to achieve efficient enrichment of trace quinolone antibiotics in animal-derived foods, and also limiting the further practical application of solid phase extraction in the field of food safety. Covalent organic frameworks (COFs) are a class of crystalline porous organic materials with large specific surface area, regular pores, and clear functional groups. They have unique advantages in the adsorption and enrichment of organic pollutants, and show great application potential.

[0005] Therefore, taking advantage of the rich regular porous structure and designable functional groups of covalent organic framework materials, new covalent organic framework nanosheet materials with multiple functional groups are designed and synthesized to enhance the binding force to quinolone antibiotics. The structural advantages of covalent organic framework nanosheets are used to enhance the utilization rate and mass transfer efficiency of functional sites, thereby achieving efficient adsorption and enrichment of quinolone antibiotics in animal-derived foods. Summary of the invention

[0006] 1. Technical Problem to be Solved by the Invention

[0007] In view of the shortcomings of the prior art, the present invention proposes a method for synthesizing zwitterionic covalent organic framework nanosheets, which solves the problems that the current quinolone antibiotic solid phase extraction adsorption materials lack a regular porous structure, have a single type of functional sites, and have block materials that cause the active sites to be deeply embedded, affecting the utilization rate of the functional sites and the mass transfer efficiency of pollutants.

[0008] The present invention introduces high-density and regularly distributed sulfonic acid and quaternary ammonium zwitterionic functional sites into the material to enable the material to have strong ion-pair electrostatic attraction. At the same time, the introduction of the quaternary ammonium ion sites increases the interlayer repulsion of the material, so that the block material is peeled into a nanosheet structure. The material is effectively combined with quinolone antibiotics through strong electrostatic ion pair action and has excellent adsorption performance. At the same time, the nanosheet structure is beneficial to the utilization rate of the functional sites and enhances the mass transfer efficiency, and has good application prospects in the rapid and efficient enrichment of trace quinolone antibiotics in food.

[0009] Therefore, the present invention also claims protection for the use of the zwitterionic covalent organic framework nanosheet material prepared by the synthesis method in the solid phase extraction of quinolone antibiotics.

[0010] (ii) The technical solution adopted by the invention to solve its technical problem:

[0011] A method for synthesizing a zwitterionic covalent organic framework nanosheet material, the process comprising:

[0012] Step 1) dissolving 2,4,6-trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid monomers in a mixed solution of mesitylene, 1,4-dioxane and acetic acid, and subjecting the mixture to ultrasonic treatment to obtain a reaction mixture;

[0013] Step 2) degassing the glass reaction tube containing the reaction mixture through a freeze-pump-thaw cycle and flame sealing, reacting at 110-130° C. for 2-4 days, and cooling to room temperature;

[0014] Step 3) separating the reaction solid product, washing it with tetrahydrofuran and dimethylacetamide, and then vacuum drying it to obtain a sulfonic acid covalent organic framework matrix material;

[0015] Step 4) adding a sulfonic acid covalent organic framework matrix material to anhydrous ethanol to obtain a solid-liquid mixture a1; adding a dilute sodium hydroxide solution dropwise to the solid-liquid mixture to adjust the pH value of the solid-liquid mixture a1 to 8-9 to obtain a solid-liquid mixture a2; controlling the water bath temperature to 55-65° C. and stirring for 1-3 hours, adding iodomethane and sodium iodide, and stirring for 3-5 hours to obtain a solid-liquid mixture a3;

[0016] Step 5) separating the reaction solid product in the solid-liquid mixture a3 obtained in step 4), washing it with acetone and then vacuum drying it at 55-65° C. to obtain the zwitterionic covalent organic framework nanosheet.

[0017] In step 1), the weight ratio of the 2,4,6-trialdehyde phloroglucinol to 2,5-diaminobenzenesulfonic acid is 10:(13-15).

[0018] In step 1), the volume ratio of mesitylene to 1,4-dioxane in the reaction mixture is (3.5-4.5) : 1, and the volume ratio of the mixed solution of mesitylene and 1,4-dioxane to acetic acid is (4-6) : 1.

[0019] In step 4), the weight ratio of the sulfonic acid covalent organic framework matrix material: iodomethane: sodium iodide is 1273: (2900-4000): (600-800).

[0020] Iodomethane is a methylating agent in the synthesis, and quaternary ammonium salt groups are generated by SN2 substitution reaction with secondary amine groups in the material. Iodomethane is an ideal substrate for SN2 substitution reaction. On the one hand, due to its small steric effect, it is convenient for nucleophilic reagents to attack, and is particularly suitable for substitution reactions of polymers; on the other hand, since iodide ions are good leaving groups, it is conducive to the reaction to proceed in the positive direction, and a better synthesis effect is obtained. Therefore, the present invention uses sodium iodide as a catalyst and iodomethane as a nucleophilic reagent to efficiently prepare zwitterionic covalent organic framework nanosheets with N, N-dimethyl quaternary ammonium salt functional groups.

[0021] The zwitterionic covalent organic framework nanosheet material prepared by the synthesis method of the present invention is used as a solid phase extraction adsorbent for solid phase extraction of quinolone antibiotics: the zwitterionic covalent organic framework nanosheet material is used as a solid phase extraction efficient adsorbent to treat a food extract containing quinolone antibiotics to be processed.

[0022] The quinolone antibiotic is enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin or difloxacin. The concentration of the quinolone antibiotic is 0.1-100 m g / kg.

[0023] The food extract needs to be adjusted to a pH of 5-9 using a pH adjuster before solid phase extraction; preferably, the pH is 7.

[0024] (III) Beneficial effects of the invention:

[0025] The synthesis method of the zwitterionic covalent organic framework nanosheet material of the present invention, the prepared zwitterionic covalent organic framework nanosheet material has the characteristics of large adsorption capacity, high extraction efficiency, short adsorption equilibrium time, excellent stable and regenerable performance, etc. compared with traditional solid phase extraction adsorbents, and can be used for efficient adsorption and enrichment of trace quinolone antibiotics in food.

[0026] 1. The synthesis method of the zwitterionic covalent organic framework nanosheet of the present invention obtains a novel zwitterionic covalent organic framework nanosheet material by introducing sulfonic acid group and quaternary ammonium group zwitterionic functional groups, and solves the problem that the functional site type of the existing covalent organic framework nanosheet material is single, and the block material causes the active site to be deeply embedded, which affects the utilization rate of the functional site and the mass transfer efficiency of the pollutant. Based on the different target action sites of the sulfonic acid group and the quaternary ammonium group active groups, a strong ion pair electrostatic force is generated between the sulfonic acid group and the zwitterionic quinolone antibiotics, and at the same time, combined with the structural advantages of the regular pore size and large aspect ratio of the covalent organic framework nanosheet, the material has excellent enrichment and extraction performance for trace quinolone antibiotics in food matrices, and can be used as an efficient solid phase extraction adsorbent for quinolone antibiotics in complex food matrices.

[0027] 2. The synthesis method of the zwitterionic covalent organic framework nanosheet of the present invention, the prepared zwitterionic covalent organic framework nanosheet material has a regular pore size and a large aspect ratio, which is conducive to the rapid mass transfer of the target in the material, and at the same time, the high-density and regularly distributed sulfonic acid group and quaternary ammonium group zwitterionic active sites make the material have a strong ion pair electrostatic attraction. The present invention utilizes the regular nanoporous structure and large aspect ratio of the covalent organic framework nanosheet, makes full use of the material functional sites, and effectively improves the mass transfer efficiency, so that the enrichment process has rapid adsorption kinetics.

[0028] 3. The synthesis method of the zwitterionic covalent organic framework nanosheet of the present invention uses sodium iodide as a catalyst for the reaction and methyl iodide as a nucleophilic reagent, and promotes the reaction to move in the positive direction by reducing the solubility of iodine ions in the system, thereby improving the preparation efficiency of the zwitterionic covalent organic framework nanosheet. The prepared zwitterionic covalent organic framework nanosheet material introduces zwitterionic functional groups that provide electrostatic ion pair forces through directional design, and has a strong binding ability with the target quinolone antibiotic molecules.

[0029] 4. The synthesis method of the zwitterionic covalent organic framework nanosheet of the present invention is to introduce positive charge by quaternizing the secondary amine group in the sulfonic acid covalent organic framework matrix material, and at the same time promote the self-stripping of the two-dimensional covalent organic framework material to form a covalent zwitterionic covalent organic framework nanosheet. This synthesis preparation method has not been reported in zwitterionic covalent organic framework nanosheet materials and has good innovation. The zwitterionic covalent organic framework nanosheet material obtained by the synthesis method not only has good ion pair binding ability with quinolone antibiotics, but also further enhances the binding ability between the material and quinolone antibiotics through π-π interaction based on the structural advantages of the nanosheet, and achieves good application effect.

[0030] 5. The synthesis method of the zwitterionic covalent organic framework nanosheet of the present invention, the prepared zwitterionic covalent organic framework nanosheet material, eliminates the unstable secondary amine group in the structure by introducing quaternary ammonium ion groups, enhances the material stability, and uses charge repulsion to obtain a nanosheet structure that is conducive to rapid and efficient adsorption. The present invention also discloses an application of a zwitterionic covalent organic framework nanosheet material having a negatively charged sulfonic acid group and a positively charged quaternary ammonium group, and the material obtained by the synthesis method has an outstanding effect on the rapid selective enrichment of quinolone antibiotics. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 YesN + Schematic diagram of the synthesis of -TpPa-SO3H.

[0032] Figure 2 is TpPa-SO3H and N + -Powder diffraction pattern of TpPa-SO3H.

[0033] Figure 3 YesN + -Tyndall effect diagram of TpPa-SO3H and TpPa-SO3H solutions, where a is N + -TpPa-SO3H solution, b is TpPa-SO3H solution.

[0034] Figure 4 YesN + -Infrared spectra of TpPa-SO3H, TpPa-SO3H, Pa-SO3H and Tp.

[0035] Figure 5 is TpPa-SO3H and N + -ZETA potential of TpPa-SO3H.

[0036] Figure 6 YesN + -TpPa-SO3H extraction efficiency test bar graph.

[0037] Figure 7 is TpPa-SO3H and N + -TpPa-SO3H extraction efficiency comparison test.

[0038] Figure 8 is TpPa-SO3H and N + -Effect of TpPa-SO3H adsorption time on extraction efficiency.

[0039] In the figure, TpPa-SO3H is the zwitterionic covalent organic framework nanosheet matrix material, N+-TpPa- SO3H is the zwitterionic covalent organic framework nanosheet material, Pa- SO3H is a 2,5-diaminobenzenesulfonic acid monomer, Tp is 2,4,6-trialdehyde pyrogallol; ENR is enrofloxacin, CIP is ciprofloxacin, DAN is danofloxacin, SAR is sarafloxacin, and DIF is difloxacin. DETAILED DESCRIPTION

[0040] The technical scheme of the present invention is clearly and completely described below in conjunction with the embodiments. It should be understood that the embodiments described below are only a part of the present invention rather than all of it. The methods involved in this article are all conventional methods unless otherwise specified, and the raw materials can be obtained from public commercial channels unless otherwise specified. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work prerequisites belong to the scope of protection of the present invention.

[0041] Example 1

[0042] Synthesis and characterization of a zwitterionic covalent organic framework nanosheet material

[0043] Step 1) Preparation of zwitterionic covalent organic framework nanosheet matrix material: 63.1 mg of 2,4,6-triformylphloroglucinol, 84.7 mg of 2,5-diaminobenzenesulfonic acid, 2.4 mL of mesitylene, 0.6 mL of 1,4-dioxane and 0.6 mL of acetic acid were added to a 20 mL Pyrex tube, the mixture was sonicated for 10 minutes, flame-sealed after three freeze-pump-thaw cycles of degassing, and heated in a 120 ºC oven for 3 days. After the reaction mixture was cooled to room temperature, the solid product was separated by centrifugation, and the precipitate was washed several times with tetrahydrofuran and N,N-dimethylformamide. The solid product was Soxhlet extracted in tetrahydrofuran for 24 hours and vacuum dried at 90 ºC for 12 hours to prepare a zwitterionic covalent organic framework nanosheet matrix material (TpPa-SO3H);

[0044] Step 2) Preparation of zwitterionic covalent organic framework nanosheet material: 87.5 mg of the zwitterionic covalent organic framework nanosheet matrix material TpPa-SO3H prepared in step 1) was added to 20 mL of anhydrous ethanol, and a dilute sodium hydroxide solution was added dropwise to the solid-liquid mixture to adjust the pH to 8-9, and the water bath temperature was controlled to be 60°C and stirred for 2 hours, and 240 mg of iodomethane and 48 mg of sodium iodide were added, and stirred for 4 hours, and cooled to room temperature, and the solid product was separated by centrifugation, and the precipitate was washed with acetone several times, and the solid product was vacuum dried at 60°C to obtain a zwitterionic covalent organic framework nanosheet material (N + -TpPa-SO3H).

[0045] Figure 1 The figure shows the synthesis schematic diagram of zwitterionic covalent organic framework nanosheet materials. Among them, methyl iodide is a methylating agent in the synthesis, which generates a quaternary ammonium salt group through an SN2 substitution reaction with the secondary amine group in the material.

[0046] Iodomethane is an ideal substrate for SN2 substitution reaction. On the one hand, due to its small steric effect, it is convenient for nucleophilic reagents to attack, and is particularly suitable for substitution reactions of polymers; on the other hand, since iodide ion is a good leaving group, it is conducive to the reaction in the positive direction, and a better synthesis effect is obtained. Therefore, the present invention uses iodomethane as a nucleophilic reagent and sodium iodide as a catalyst to efficiently prepare zwitterionic covalent organic framework nanosheets through synthesis and substitution reactions.

[0047] The X-ray diffraction (XRD) method was used to characterize the TpPa-SO3H and N + -TpPa-SO3H crystal structure was studied, such as Figure 2 As shown in the XRD pattern of TpPa-SO3H, the strong diffraction peak at 4.8° is the characteristic peak of the (100) crystal plane of the crystal, indicating that the synthesized TpPa-SO3H has good crystallinity, and the weaker broad diffraction peak at 27.0° is the characteristic peak of the (001) crystal plane of the crystal, indicating that TpPa-SO3H is a two-dimensional nanostructure stacked layer by layer. + The decrease in the diffraction peak intensity at 4.8° and 27.0° of the -TpPa-SO3H material indicates that N + The introduction of destroys the original stacking structure and the material is transformed into a thin layer of nanosheet structure.

[0048] For TpPa-SO3H and N + -TpPa-SO3H mixed solution was tested, and the results were as follows Figure 3 As shown, N + -TpPa-SO3H solution (a) shows obvious Tyndall effect, while TpPa-SO3H solution (b) does not show Tyndall effect, further indicating that N +The introduction of destroys the original stacking structure and the material is transformed into a thin layer of nanosheet structure.

[0049] The TpPa-SO3H and N + -TpPa-SO3H chemical structure was characterized. Figure 4 As shown in the infrared spectrum of TpPa-SO3H, 1005 cm -1 The emergence of SO 3- The characteristic absorption peak at 1231 cm -1 With 3354 cm -1 The characteristic absorption peak of secondary amine at N + -TpPa-SO3H has a peak at 3354 cm -1 The characteristic absorption peak of -NH- at + -The successful introduction of sulfonic acid group and quaternary ammonium group into the TpPa-SO3H structure.

[0050] The effects of ZETA on TpPa-SO3H and N + The surface charge of -TpPa-SO3H was studied. Figure 5 As shown in Figure 2, the zeta potential of TpPa-SO3H is -22.2 mV, while that of N + The zeta potential of -TpPa-SO3H is 18.8 mV. The results show that the successful introduction of quaternary ammonium ions gives the material zwitterionic sites, achieving efficient adsorption of amphoteric quinolone antibiotics.

[0051] Example 2

[0052] The zwitterionic covalent organic framework nanosheet material prepared by the synthesis method of Example 1 is used as an adsorbent and applied to solid phase extraction of quinolone antibiotics.

[0053] Add 1.0 g of honey to a 15 mL centrifuge tube, add 10 mL of phosphate buffer, vortex for 1 min, centrifuge at 10,000 r / min for 5 min, and adjust the pH of the supernatant to 7.0 as the honey extract. At the same time, weigh appropriate amounts of enrofloxacin (ENR), ciprofloxacin (CIP), danofloxacin (DAN), sarafloxacin (SAR), and difloxacin (DIF), dissolve them in deionized water, accurately measure appropriate amounts and add them to honey, and prepare honey extracts with concentrations of 0.1, 10, and 100 m g / kg of positive honey extract. Take a 2 mL polypropylene tube and add 10 mg N +-TpPa-SO3H was added to a polypropylene tube to prepare a solid phase extraction column (10 mg / 2 mL). The extract was passed through the column at a speed of 4.0 mL / min, and then eluted with 2 mL of 0.5 mol / L hydrochloric acid solvent. The eluate was collected and 2 mL of 0.5 mol / L sodium hydroxide solution was added, and methanol was added to make the volume to 10 mL. The concentrations of enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin and difloxacin in the solution were determined by high performance liquid chromatography, and N + -Extraction efficiency of five quinolone antibiotics by TpPa-SO3H.

[0054] Figure 6 The shown is N + -TpPa-SO3H extraction efficiency bar graph for enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin and difloxacin. Figure 6 As shown, N + -TpPa-SO3H has an extraction efficiency of more than 91.5%, 96.8% and 98.6% for the three concentration levels of positive honey extracts, respectively. The results show that N + The uniformly and densely distributed sulfonic acid and quaternary ammonium functional groups in -TpPa-SO3H achieve efficient binding with the amphoteric quinolone antibiotics, thereby achieving efficient extraction of quinolone antibiotics from animal-derived foods.

[0055] Figure 7 is TpPa-SO3H and N + -TpPa-SO3H for 1 m g / kg of positive honey extract extraction efficiency. Figure 7 As shown, N + The extraction efficiencies of -TpPa-SO3H and TpPa-SO3H for enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin, and difloxacin were 96.8%-97.6% and 90.3%-91.5%, respectively. The results showed that the nanosheet structure with high-density and regularly distributed zwitterionic binding sites of sulfonic acid and quaternary ammonium groups gave the material a strong ion-pair binding force with quinolone antibiotics.

[0056] Figure 8 Is to investigate 1 m g / kg of positive honey extract passed N + -TpPa-SO3H and TpPa-SO3H solid phase extraction column time on the extraction efficiency. Figure 8 As shown, at a column flow rate of 8.0 mL / min, N +The extraction efficiencies of -TpPa-SO3H and TpPa-SO3H for enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin and difloxacin were 93.6%-95.4% and 71.3%-72.5%, respectively. The results showed that the nanosheet structure with high-density and regularly distributed sulfonic acid and quaternary ammonium zwitterionic binding sites was beneficial to the rapid mass transfer of target quinolone antibiotics, enabling quinolone antibiotics to quickly and effectively bind to the functional sites of the material, greatly improving the extraction efficiency.

[0057] Example 3

[0058] The present invention discloses a synthesis method based on zwitterionic covalent organic framework nanosheets, and the synthesis process comprises the following steps:

[0059] Step S1, synthesizing a sulfonic acid covalent organic framework matrix material through a condensation reaction between 2,4,6-trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid monomers;

[0060] Step S2, subjecting the sulfonic acid covalent organic framework matrix material synthesized in step S1 to a substitution reaction with methyl iodide to prepare a zwitterionic covalent organic framework nanosheet material, wherein the zwitterionic covalent organic framework nanosheet material has a negatively charged sulfonic acid group and a positively charged quaternary ammonium group, and the substitution reaction process comprises the following steps:

[0061] Step S21, adding a sulfonic acid covalent organic framework matrix material to anhydrous ethanol to obtain a solid-liquid mixture a1; adding a dilute sodium hydroxide solution dropwise to adjust the pH value of the solid-liquid mixture a1 to 8-9 to obtain a solid-liquid mixture a2; bathing the solid-liquid mixture a2 at a water bath temperature of 55-65° C., stirring for 1-3 hours, adding iodomethane and sodium iodide, and continuing stirring for 3-5 hours to obtain a solid-liquid mixture a3;

[0062] Step S22, separating the reaction solid product in the solid-liquid mixture a3 obtained in step S21, washing it with acetone and then vacuum drying it at 55-65°C to obtain the zwitterionic covalent organic framework nanosheet.

[0063] Wherein, in step S1, the process of synthesizing the sulfonic acid covalent organic framework matrix material by condensation reaction comprises the steps of:

[0064] Step S11, dissolving 2,4,6-trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid monomers in a mixed solution of mesitylene, 1,4-dioxane and acetic acid, and subjecting to ultrasonic treatment to obtain a reaction mixture; the weight ratio of 2,4,6-trialdehyde phloroglucinol to 2,5-diaminobenzenesulfonic acid is 10:(13-15); in the reaction mixture, the volume ratio of mesitylene to 1,4-dioxane is (3.5-4.5):1, and the volume ratio of the mixed solution of mesitylene and 1,4-dioxane to acetic acid is (4-6):1;

[0065] Step S12, degassing the glass reaction tube containing the reaction mixture through a freeze-pump-thaw cycle and flame sealing, reacting at 110-130° C. for 2-4 days, and cooling to room temperature to obtain a solid-liquid mixture b;

[0066] Step S13, separating the solid product of the solid-liquid mixture b, washing it with tetrahydrofuran and dimethylacetamide, and then vacuum drying it to obtain a sulfonic acid covalent organic framework matrix material.

[0067] In step S21, the weight ratio of the sulfonic acid covalent organic framework matrix material: iodomethane: sodium iodide is 1273: (2900-4000): (600-800).

[0068] The application of the zwitterionic covalent organic framework nanosheets obtained by the synthesis method of the present invention in the adsorption and enrichment of quinolone antibiotics includes: using the zwitterionic covalent organic framework nanosheets as solid phase extraction adsorbents for solid phase extraction of quinolone antibiotics in food. In the specific application process, the concentration of the quinolone antibiotics is 0.1-100m g / kg, the pH of the food extract is adjusted to 5-9 with a pH regulator before passing through the column. The quinolone antibiotics are enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin or difloxacin.

[0069] In summary, the present invention is based on the synthesis method of zwitterionic covalent organic framework nanosheets, with the secondary amine group in the sulfonic acid covalent organic framework matrix material as the target reaction site, using sodium hydroxide to remove the hydrogen on the secondary amine, using sodium iodide as a catalyst, and methyl iodide as a nucleophilic reagent, and the reaction generates a zwitterionic covalent organic framework nanosheet material with an N, N-dimethyl quaternary ammonium salt functional group. Sodium iodide is used as a catalyst for the reaction, and the solubility of iodide ions in the system is reduced to promote the reaction to move in the positive direction, thereby improving the preparation efficiency of zwitterionic covalent organic framework nanosheets.

[0070] The zwitterionic covalent organic framework nanosheet material obtained by this synthesis method has an outstanding effect on the rapid and selective enrichment of quinolone antibiotics. Based on the different target action sites of the sulfonic acid group and the quaternary ammonium group active groups, a strong ion-pair electrostatic force is generated between them and the zwitterionic quinolone antibiotics. At the same time, combined with the structural advantages of the regular pore size and large aspect ratio of the covalent organic framework nanosheet, this material has excellent enrichment and extraction performance for trace quinolone antibiotics in food matrices, and can be used as an efficient solid phase extraction adsorbent for quinolone antibiotics in complex food matrices.

[0071] The above is only a preferred embodiment of the present invention, which is only used to illustrate that the present invention can be implemented. It should be pointed out that for those skilled in the art, without departing from the principle of the present invention, several improvements and modifications can be made to obtain more specific embodiments. Undoubtedly, these improvements and modifications should also be considered to be included in the protection scope of the present invention.

Claims

1. A method for synthesizing zwitterionic covalent organic framework nanosheets, characterized in that: The steps include: Step S1, synthesizing a sulfonic acid covalent organic framework matrix material through a condensation reaction between 2,4,6-trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid monomers; In the condensation reaction, the weight ratio of 2,4,6-trialdehyde phloroglucinol to 2,5-diaminobenzenesulfonic acid is 10: (13-15); Step S2, subjecting the sulfonic acid covalent organic framework matrix material synthesized in step S1 to a substitution reaction with methyl iodide to prepare a zwitterionic covalent organic framework nanosheet material, wherein the zwitterionic covalent organic framework nanosheet material has a negatively charged sulfonic acid group and a positively charged quaternary ammonium group, and the substitution reaction process comprises: Step S21, adding a sulfonic acid covalent organic framework matrix material to anhydrous ethanol to obtain a solid-liquid mixture a1; adding a dilute sodium hydroxide solution dropwise to adjust the pH value of the solid-liquid mixture a1 to 8-9 to obtain a solid-liquid mixture a2; bathing the solid-liquid mixture a2 at a water bath temperature of 55-65° C., stirring for 1-3 hours, adding iodomethane and sodium iodide, and continuing stirring for 3-5 hours to obtain a solid-liquid mixture a3; The weight ratio of the sulfonic acid covalent organic framework matrix material: iodomethane: sodium iodide is 1273: (2900-4000): (600-800); Step S22, separating the reaction solid product in the solid-liquid mixture a3 obtained in step S21, washing it with acetone and then vacuum drying it at 55-65°C to obtain the zwitterionic covalent organic framework nanosheet.

2. The method for synthesizing zwitterionic covalent organic framework nanosheets according to claim 1, characterized in that: In step S1, the process of synthesizing the sulfonic acid covalent organic framework matrix material by condensation reaction includes: Step S11, dissolving 2,4,6-trialdehyde phloroglucinol and 2,5-diaminobenzenesulfonic acid monomers in a mixed solution of mesitylene, 1,4-dioxane and acetic acid, and performing ultrasonic treatment to obtain a reaction mixture; Step S12, degassing the glass reaction tube containing the reaction mixture through a freeze-pump-thaw cycle and flame sealing, reacting at 110-130° C. for 2-4 days, and cooling to room temperature to obtain a solid-liquid mixture b; Step S13, separating the solid product of the solid-liquid mixture b, washing it with tetrahydrofuran and dimethylacetamide, and then vacuum drying it to obtain a sulfonic acid covalent organic framework matrix material.

3. The method for synthesizing zwitterionic covalent organic framework nanosheets according to claim 2, characterized in that: In the reaction mixture described in step S11, the volume ratio of mesitylene to 1,4-dioxane is (3.5-4.5) : 1, and the volume ratio of the mixed solution of mesitylene and 1,4-dioxane to acetic acid is (4-6) :

1.

4. Application of the zwitterionic covalent organic framework nanosheets obtained by the synthesis method of zwitterionic covalent organic framework nanosheets according to claim 1 in the adsorption and enrichment of quinolone antibiotics.

5. The use according to claim 4, characterized in that: The zwitterionic covalent organic framework nanosheets are used as solid phase extraction adsorbents and applied to solid phase extraction of quinolone antibiotics in food.

6. The use according to claim 4, characterized in that: The concentration of the quinolone antibiotics is 0.1-100 mg / kg, and the pH of the food extract is adjusted to 5-9 with a pH regulator before passing through the column.

7. The use according to claim 4, characterized in that: The quinolone antibiotic is enrofloxacin, ciprofloxacin, danofloxacin, sarafloxacin or difloxacin.

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