A covalent organic framework composite membrane, its preparation method and application

Abstract

This invention discloses a covalent organic framework composite membrane, its preparation method, and its applications, belonging to the fields of materials chemistry and analytical chemistry. The covalent organic framework composite membrane comprises a porous polymer substrate membrane, a polydopamine-mediated layer, and a covalently bonded functional layer. The functional layer is formed by linking 2,5-dihydroxy-1,4-phenylenedialdehyde and 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine via imine bonds. This invention achieves controllable and robust growth of a novel COF on the surface of an inert polymer membrane through a "polydopamine-mediated layer + aldehyde pre-anchoring" strategy. As a solid-phase extraction medium, the composite membrane exhibits excellent enrichment performance and good reusability for trace amounts of carbamate pesticides in food and environmental samples, and has significant application value in sample pretreatment.

Description

Technical Field

[0001] This invention belongs to the field of materials chemistry and analytical chemistry, specifically relating to a covalent organic framework composite membrane, its preparation method, and its application. Background Technology

[0002] Carbamate pesticides are widely used in agricultural production due to their broad-spectrum and highly effective insecticidal properties. However, these pesticides are neurotoxic, and long-term low-dose intake may cause serious harm to human health. Given the complexity of food matrices and trace levels of pesticide residues, establishing efficient sample pretreatment methods to eliminate matrix interference and enrich target analytes is crucial for achieving accurate detection.

[0003] Currently, commonly used pretreatment techniques include liquid-liquid extraction and solid-phase extraction (SPE). Traditional SPE adsorbents such as C18 and HLB have limited selectivity and insufficient purification capacity for complex matrices. Covalent organic frameworks (COFs), as a new class of crystalline porous materials, have shown broad application prospects in adsorption and separation due to their high specific surface area, regular pore structure, and excellent chemical stability. However, existing COF materials mostly exist in the form of micro- and nano-powders, which can easily lead to problems such as excessive back pressure, adsorbent leakage, and agglomeration when directly filled into solid-phase extraction columns, severely restricting their practical application. Loading COFs onto polymer membrane substrates to prepare composite membranes is an effective way to overcome these limitations. Polyethersulfone (PES) membranes have become ideal substrate materials due to their excellent chemical stability and mechanical strength. However, PES surfaces are chemically inert, making it difficult for COFs to nucleate directly, and the interfacial bonding is weak, making them prone to detachment during operation. While there are reports of dopamine-modified substrates in the existing technology, most of the COFs used are of known structures. There is a lack of design and synthesis of novel COF materials, and even more so a lack of systematic strategies for achieving controllable, robust, and uniform growth of novel COFs on the membrane surface.

[0004] In summary, developing a COF composite membrane material with an original structure and achieving stable loading on the surface of an inert polymer membrane is of great significance for expanding the application of COF materials in the field of sample pretreatment. Summary of the Invention

[0005] To address the technical challenges of handling COF powder and its weak adhesion to the membrane substrate in existing technologies, this invention provides a covalent organic framework composite membrane with stable structure, uniform functional layers, and good reusability, along with its preparation method. This membrane is then applied to sample pretreatment to achieve efficient enrichment of trace organic pollutants. This invention is the first to combine 2,5-dihydroxy-1,4-phenylenedialdehyde (DHTA) with 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine (BAQD) to synthesize a novel COF material. This material was successfully loaded onto the polymer membrane surface using a "PDA-mediated layer + aldehyde pre-anchoring" strategy.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention discloses a covalent organic framework composite membrane, comprising:

[0008] Porous polymer substrate membrane;

[0009] A polydopamine-mediated layer formed on the surface of the basement membrane;

[0010] And a covalent organic framework functional layer that is covalently bonded to the surface of the polydopamine-mediated layer;

[0011] The covalent organic framework functional layer is formed by linking the aldehyde monomer 2,5-dihydroxy-1,4-phenylenedialdehyde (DHTA) and the amino monomer 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine (BAQD) via imine bonds, as shown in Formula I:

[0012] ;

[0013] The aldehyde monomer and the polydopamine mediator are pre-covalently anchored via a Schiff base reaction.

[0014] In some embodiments, the porous polymer substrate membrane is one of a polyethersulfone membrane, a polyvinylidene fluoride membrane, a nylon membrane, or a mixed cellulose ester membrane, and its average pore size can be controlled within the range of 0.1-0.45 μm. Preferably, the porous polymer substrate membrane is a polyethersulfone membrane with an average pore size of 0.22 μm. This preferred embodiment can ensure that the composite membrane has sufficient water flux while providing sufficient specific surface area and adhesion sites for the subsequent growth of functional layers.

[0015] In some embodiments, the water contact angle of the covalent organic framework composite membrane is 30°-50°, indicating excellent hydrophilicity. Compared to the original polyethersulfone membrane (water contact angle of approximately 71.3°), the hydrophilicity of the composite membrane of the present invention is significantly improved, which facilitates rapid wetting of the membrane surface by aqueous samples, thereby reducing membrane resistance and improving pretreatment efficiency. Preferably, the water contact angle of the composite membrane is 38.6°.

[0016] In some embodiments, the covalent organic framework composite membrane exhibits good reusability. Tests showed that after three consecutive reuses, the recovery rate of the target analyte remained above 85% of the initial recovery rate. This result indicates that the polydopamine-mediated layer imparts strong interfacial bonding to the composite membrane, enabling its functional layers to withstand repeated solvent rinsing and demonstrating good structural stability and application durability.

[0017] The covalent organic framework composite membrane of this invention employs a unique "substrate-mediating layer-functional layer" sandwich structure design, exhibiting superior comprehensive performance. The polyethersulfone membrane, with its high chemical stability and mechanical strength, serves as the substrate, providing a robust macroscopic support platform for the composite material and effectively overcoming the difficulties encountered in handling and recycling powdered COF materials. The polydopamine mediating layer possesses dual functions of interfacial adhesion and reactivity, firmly grafting the COF functional layer onto the substrate surface through covalent anchoring, fundamentally solving the problems of difficult COF growth and easy detachment on inert substrates. The COF functional layer achieves dense and uniform in-situ growth on the membrane surface, significantly increasing the specific surface area of ​​the composite membrane. Its abundant π-π conjugated system and polar functional groups also provide numerous specific adsorption sites for carbamate pesticides. Compared to traditional packed columns, this composite membrane significantly reduces mass transfer resistance, enabling rapid capture and efficient separation of target analytes. In addition, this composite membrane has high water flux, excellent hydrophilicity and good reusability, showing broad application prospects in the field of trace analysis of complex samples.

[0018] Secondly, the present invention provides a method for preparing the above-mentioned covalent organic framework composite membrane, comprising the following steps:

[0019] (1) Substrate modification: The porous polymer substrate membrane is placed in an alkaline buffer solution containing dopamine, and a polydopamine-mediated layer is formed on the surface of the substrate membrane through an oxidative self-polymerization reaction to obtain a PDA-modified membrane;

[0020] (2) Aldehyde monomer pre-anchoring: The PDA modified film is immersed in an organic solution containing aldehyde monomer 2,5-dihydroxy-1,4-phenylenedialdehyde and reacted under heating conditions to make the aldehyde monomer grafted onto the surface of the polydopamine mediator layer through Schiff base reaction.

[0021] (3) In-situ growth of COF functional layer: The pre-anchored membrane obtained in step (2) is placed in a mixed solution containing aldehyde monomer 2,5-dihydroxy-1,4-benzaldehyde, amino monomer 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine and catalyst, and a solvothermal reaction is carried out under heating conditions to grow the covalent organic framework functional layer with the structure of formula I in-situ on the membrane surface;

[0022] (4) Post-processing: After the reaction is completed, the membrane is washed and dried to obtain the covalent organic framework composite membrane.

[0023] The preparation method employed in this invention is an in-situ growth strategy of "polydopamine-mediated binding with aldehyde pre-anchoring". This strategy first utilizes the strong adhesion of polydopamine (PDA) to modify the surface of an inert polymer substrate film, introducing abundant amino and phenolic hydroxyl active sites. Then, through a "pre-anchoring" step, the aldehyde monomer undergoes a Schiff base reaction with the amino groups on the surface of the PDA layer, forming covalently linked nucleation sites on the film surface. Finally, amino monomers and a catalyst are added to guide the covalent organic framework crystals to undergo vertical and dense in-situ epitaxial growth on the film surface, starting from these pre-anchoring sites. This method abandons traditional physical coating or mixed matrix film-forming processes, effectively avoiding the clogging of film pores by binders, and fundamentally solving the problems of weak adhesion and easy detachment of covalent organic frameworks when directly grown on inert substrate surfaces.

[0024] In some embodiments, the alkaline buffer solution in step (1) is a Tris-HCl buffer solution with a pH of 8-9, a dopamine concentration of 2-6 mg / mL, and a reaction time of 2-6 h. Preferably, the volume of the Tris-HCl buffer solution is 40-80 mL. More preferably, the volume of the buffer solution is 50 mL, the dopamine concentration is 4 mg / mL, and the reaction is carried out at room temperature with light-protected shaking for 4 h. After the reaction is completed, the obtained PDA-modified membrane is thoroughly washed with ultrapure water and ethanol in sequence to remove non-specifically adsorbed particles and residual water on the surface, and finally immersed in ethanol for later use.

[0025] In some embodiments, the reaction in step (2) is carried out at 40-100°C for 0.5-4 h; the organic solution is selected from 1,4-dioxane, mesitylene, ethanol, acetonitrile, or mixtures thereof. Preferably, the organic solution is 1,4-dioxane; the concentration of the aldehyde monomer solution is 1-5 mg / mL. The PDA-modified film obtained in step (1) is immersed in the aldehyde monomer solution and a pre-anchoring reaction is carried out under heating conditions. During this process, the aldehyde monomer covalently bonds with the amino groups on the surface of the polydopamine layer through a Schiff base reaction, forming stable anchoring sites on the film surface. In a further preferred embodiment, the reaction temperature is controlled at 60°C, the reaction time is 1 h, and the aldehyde monomer concentration is 1.65 mg / mL. This pre-anchoring step lays a key structural foundation for the subsequent heterogeneous nucleation and dense growth of the covalent organic framework, ensuring a strong bond between the functional layer and the substrate.

[0026] In some embodiments, the catalyst in step (3) is a 3-12 mol / L aqueous solution of acetic acid, used in an amount of 1-5 mL; the solvothermal reaction is carried out at 40-100°C for 2-5 h. In a further preferred embodiment, the amino monomer BAQD is dissolved in 1,4-dioxane to prepare an amino monomer solution with a concentration of 1-5 mg / mL; preferably, the concentration of the amino monomer solution is 2 mg / mL. This solution is slowly added to the reaction system, and 2 mL of a 6 mol / L aqueous solution of acetic acid is added as a catalyst. The reaction is continued at 60°C for 3 h to allow the COF functional layer to grow fully in situ on the membrane surface.

[0027] In some embodiments, step (4) involves sequentially using 1,4-dioxane and ethanol to perform multi-stage solvent replacement cleaning on the composite membrane to thoroughly remove unreacted monomers remaining on the membrane surface and in the pores; then the membrane is placed in an oven at 50-80°C for 6-10 h to dry; preferably, the drying temperature is 60°C and the time is 8 h, thus obtaining the covalent organic framework composite membrane.

[0028] Thirdly, the present invention provides the application of the above-mentioned covalent organic framework composite membrane in sample pretreatment.

[0029] Specifically, the application involves using covalent organic framework composite membranes as solid-phase extraction media to enrich trace organic pollutants in food or environmental samples. Thanks to the composite membrane's unique "substrate-mediating layer-functional layer" sandwich structure, interconnected pore system, and excellent hydrophilic surface, sample solutions can rapidly pass through the membrane pores in a flow path, achieving efficient capture of target analytes and rapid removal of matrix interferences.

[0030] The trace organic pollutants include at least one of carbamate pesticides, organophosphate pesticides, pyrethroid pesticides, triazine herbicides, or antibiotics. Particularly in the application of enriching carbamate pesticides in food or environmental samples, the composite membrane of this invention exhibits excellent enrichment efficiency and selectivity.

[0031] In some embodiments, the carbamate pesticide includes at least one of carbaryl, isoprocarb, sec-butylcarbide, oxadiazon, pirimicarb, and propoxur.

[0032] Based on the above applications, the present invention further provides a method for enriching carbamate pesticides, comprising the following steps:

[0033] Stack 1-8 composite membranes in a solid-phase extraction apparatus, with a sieve plate on top and bottom to fix the membranes; adjust the sample solution to pH 5-9, and pass it through the composite membrane at a flow rate of 1-2 mL / min, so that the target analyte is adsorbed by the covalent organic framework functional layer on the membrane surface; then elute the adsorbed target analyte with 0.5-4 mL of organic solvent, and collect the eluent for subsequent instrument analysis.

[0034] In this enrichment method, the target analyte is rapidly "captured" onto the pore surface of the covalent organic framework through π-π conjugation, hydrogen bonding, and hydrophobic interactions, while matrix interferences are discharged with the sample solution, achieving a highly efficient integrated purification and enrichment process. This method overcomes the drawbacks of traditional powder solid-phase extraction (PSE) which involves cumbersome packing and is prone to clogging, as well as magnetic solid-phase extraction (MSE) which is prone to agglomeration. Enrichment can be completed with only a simple filtration operation, significantly shortening the pretreatment time. Furthermore, it eliminates the need for centrifugation or magnetic separation equipment, making it easy to automate and enable rapid on-site detection.

[0035] In a further preferred embodiment, based on a 5 mL liquid sample, five composite membranes were used, the pH of the sample solution was adjusted to 7.0, methanol was used as the elution solvent, and the elution volume was 3.0 mL. Under these optimized conditions, the enrichment and recovery rates of the six carbamate pesticides were consistently above 85%.

[0036] Furthermore, this invention also discloses a solid-phase extraction device, comprising a covalent organic framework composite membrane as described above, wherein the composite membrane is stacked within an extraction column, and sieve plates are respectively provided at the inlet and outlet ends of the extraction column to fix the composite membrane. This device has a simple structure, is easy to assemble, and is suitable for the efficient enrichment and rapid analysis of trace targets in complex matrices.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) This invention is the first to combine 2,5-dihydroxy-1,4-phenylenedialdehyde with 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine to synthesize a covalent organic framework material with a novel structure shown in Formula I. This COF material possesses a regular pore structure, abundant π-π conjugated system, and polar functional groups, exhibiting excellent selective adsorption capacity for carbamate pesticides.

[0039] (2) This invention achieves covalent bonding between the COF functional layer and the substrate membrane through a "PDA-mediated layer + aldehyde pre-anchoring" strategy. In the resulting composite membrane, the COF layer is tightly bonded to the substrate, exhibits excellent solvent erosion resistance, can be reused at least 3 times, and the target analyte recovery rate can still be maintained above 85%, demonstrating good structural stability and service life.

[0040] (3) The composite membrane of the present invention exhibits excellent enrichment efficiency for six carbamate pesticides, including carbaryl, isoprocarb, sec-butylcarbide, chlorpyrifos, pirimicarb, and propoxur, with a recovery rate of 85%-105% and a matrix effect of less than 11%, which fully meets the technical requirements of trace analysis and effectively ensures the sensitivity and accuracy of subsequent detection.

[0041] (4) The composite membrane of the present invention can be directly stacked in the solid phase extraction device in the form of membrane sheets. It does not require a complicated filling process or auxiliary equipment such as centrifugation or magnetic separation. The operation process is greatly simplified and it is easy to realize automated pretreatment and rapid on-site detection. Attached Figure Description

[0042] Figure 1 The chemical structural formulas of the aldehyde monomer DHTA and the amino monomer BAQD of this invention, and a structural schematic diagram of Formula I are shown.

[0043] Figure 2 This is a reaction pathway diagram for the preparation of the covalent organic frame composite membrane of the present invention;

[0044] Figure 3 (A) is a scanning electron microscope image of the PES microporous filter membrane provided in Comparative Example 1, and (B) is a scanning electron microscope image of the covalent organic framework composite membrane provided in Example 1.

[0045] Figure 4 (A) is a static contact angle diagram of the PES microporous filter membrane provided in Comparative Example 1, and (B) is a static contact angle diagram of the covalent organic framework composite membrane provided in Example 1.

[0046] Figure 5 Infrared spectra of the composite films provided in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0047] Figure 6 The results show the effects of membrane number, sample pH, elution solvent, and elution solvent volume on the solid-phase extraction recovery of carbamate pesticides.

[0048] Figure 7 This is a comparison chart of the adsorption performance of the embodiments and comparative examples of the present invention;

[0049] Figure 8 This diagram illustrates the reusability of the covalent organic framework composite membrane of this invention. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of this invention.

[0051] After reading the contents disclosed in this invention, those skilled in the art can make appropriate adjustments or substitutions to the process parameters of the methods and applications described in this invention without departing from the spirit and scope of this invention. Such obvious adjustments, substitutions or combinations should be included within the protection scope of this invention.

[0052] Unless otherwise specified, the materials, reagents, instruments and testing methods used in the following embodiments can be obtained commercially or prepared, operated and implemented with reference to conventional methods disclosed in the art.

[0053] It should be noted that all technical parameters described in this document as numerical ranges (such as temperature, ratio, time, content, etc.) should be understood as encompassing all possible sub-ranges and specific numerical points within that range, regardless of whether the specific numerical value or sub-range is explicitly listed. Unless otherwise specified, the technical terms used in this document have the meanings commonly understood by those skilled in the art.

[0054] In this invention, the terms "carbamate pesticides" and "carbamate compounds" have the same meaning and can be used interchangeably.

[0055] Example 1: A covalent organic framework composite membrane

[0056] This embodiment prepares a covalent organic framework composite membrane according to the following steps:

[0057] (1) Substrate modification: Polyethersulfone (PES) microporous filter membranes were used as porous polymer substrate membranes. The membranes were cut into circular pieces with a diameter of 16 mm and ultrasonically cleaned sequentially in anhydrous ethanol and ultrapure water for 10 min to remove surface oil and impurities. 50 mL of pH 8.5 Tris-HCl buffer was placed in a reaction vessel, and 200 mg of dopamine hydrochloride (PDA) (final concentration 4 mg / mL) was added. The mixture was shaken to dissolve and mix. The cleaned PES membrane pieces were completely immersed in the solution and reacted at room temperature in the dark for 4 h. A polydopamine-mediated layer was formed on the substrate membrane surface through oxidative self-polymerization. After the reaction, the membrane pieces were removed and repeatedly rinsed with ultrapure water to remove non-specifically attached particles. Then, they were rinsed with anhydrous ethanol to replace residual water, resulting in a wet PDA-modified membrane for later use.

[0058] (2) Pre-anchoring of aldehyde monomer: Accurately weigh 66.5 mg of aldehyde monomer 2,5-dihydroxy-1,4-phenylenedialdehyde (DHTA) (structural formula as shown in the figure). Figure 1 As shown), 40 mL of 1,4-dioxane was added, and the aldehyde monomer solution was prepared by ultrasonic-assisted dissolution. After draining off the excess solvent from the PDA-modified film prepared in step (1), it was immersed in the solution. The reaction vessel was sealed and placed in a constant temperature oil bath at 60°C for 1 h to allow the aldehyde monomer to be grafted onto the surface of the polydopamine-mediated layer via Schiff base reaction, forming a pre-anchored film.

[0059] (3) In-situ growth of COF functional layer: 103.7 mg of amino monomer 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine (BAQD) (structural formula as shown) was taken. Figure 1 The amino monomer solution was prepared by dissolving the pre-anchored membrane obtained in step (2) in 10 mL of 1,4-dioxane. The pre-anchored membrane was placed in a reaction vessel, the above amino monomer solution was added, and 2.0 mL of 6 mol / L acetic acid aqueous solution was added dropwise as a catalyst. After mixing evenly, the mixture was placed in a constant temperature oil bath at 60℃ and the reaction was continued for 3 h to carry out a solvothermal reaction, so that the membrane has the structure of formula I (structural formula as shown). Figure 1 The covalent organic framework functional layer (as shown) is grown in situ on the membrane surface.

[0060] (4) Post-treatment: After the reaction, the composite membrane was removed and subjected to multiple rounds of solvent replacement cleaning with 1,4-dioxane and ethanol to thoroughly remove unreacted monomers remaining on the membrane surface and in the pores. Finally, the membrane was dried in a 60°C oven for 8 h to obtain the covalent organic framework composite membrane. A schematic diagram of the reaction is shown below. Figure 2 As shown.

[0061] Comparative Example 1

[0062] This comparative example provides a polyethersulfone (PES) microporous filter membrane without any chemical modification. The membrane was only washed with ethanol and ultrapure water and used as a blank control material to evaluate the contribution of the COF functional layer to the adsorption performance.

[0063] Comparative Example 2

[0064] The difference between this comparative example and Example 1 is that only the substrate modification step was performed, without the pre-anchoring of aldehyde monomers and in-situ growth of the COF functional layer. That is, the PDA-modified film was prepared according to step (1) of Example 1, and was used to investigate the adsorption capacity of the polydopamine-mediated layer itself for the target analyte.

[0065] Comparative Example 3

[0066] The difference between this comparative example and Example 1 lies in the order of monomer addition: 103.7 mg of amino monomer BAQD was first added to 40 mL of 1,4-dioxane and reacted at 60°C for 1 h, followed by the addition of 66.5 mg of aldehyde monomer DHTA and a further reaction for 3 h. The remaining raw material ratios, reaction steps, and process parameters were exactly the same as in Example 1.

[0067] Example 1: Structural and performance characterization of composite membranes

[0068] 1. Morphological and structural analysis

[0069] The morphology of the covalent organic framework composite membrane prepared in Example 1 and the PES microporous filter membrane provided in Comparative Example 1 was characterized by scanning electron microscopy.

[0070] like Figure 3 As shown in (A), the PES microporous filter membrane exhibits a typical three-dimensional network porous structure. Its framework is composed of smooth, interwoven polymer fibers, with a clean and flat surface, clear and uniformly distributed pore structure, and no particulate matter observed. Figure 3 As shown in Figure (B), the surface morphology of the covalent organic framework composite membrane prepared by this invention undergoes a significant change, exhibiting a rough structure and being coated with a dense nanoparticle layer. This morphological transformation from smooth to rough greatly increases the specific surface area of ​​the membrane, providing abundant active sites for the adsorption of carbamate pesticides; at the same time, the dense coating layer confirms that the "PDA + pre-anchoring" strategy adopted in this invention achieves a firm loading of the COF functional layer on the PES surface.

[0071] 2. Static water contact angle analysis

[0072] The wetting performance of the covalent organic framework composite membrane prepared in Example 1 and the PES microporous filter membrane provided in Comparative Example 1 was evaluated using a static water contact angle meter.

[0073] like Figure 4As shown, the PES microporous filter membrane has a water contact angle of approximately 71.3°, exhibiting significant hydrophobicity. In contrast, the covalent organic framework composite membrane prepared in this invention has a significantly reduced water contact angle to 38.6°, achieving a fundamental shift from hydrophobic to hydrophilic. This improvement is mainly attributed to the introduction of numerous hydrophilic groups such as phenolic hydroxyl and amino groups into the PDA mediating layer and COF functional layer. This excellent hydrophilicity not only helps reduce the operating back pressure during solid-phase extraction and increase the throughput of aqueous samples, but also effectively resists the non-specific adsorption of hydrophobic impurities, extending the lifespan of the composite membrane.

[0074] 3. Infrared spectroscopy analysis

[0075] Fourier transform infrared spectroscopy was used to characterize the composite films provided in the examples and comparative examples to verify the covalent bonding between the COF layer and the PDA mediator layer.

[0076] like Figure 5 As shown, the PES microporous filter membrane provided in Comparative Example 1 at 1290 cm⁻¹ -1 and 1104 cm -1 The film exhibits asymmetric and symmetric stretching vibrations characteristic of sulfone groups at 3254 cm⁻¹. The PDA-modified film provided in Comparative Example 2 shows a peak at 3254 cm⁻¹. -1 A broad peak appeared at 1660 cm⁻¹, confirming the successful introduction of a PDA layer rich in amino and hydroxyl groups. Most importantly, in the spectrum of the covalent organic framework composite film prepared in Example 1 of this invention, a peak was observed at 1660 cm⁻¹. -1 A significant imine bond (-C=N-) characteristic peak was observed. Since unreacted aldehyde monomers had been washed away, the presence of this peak definitively proves that the pre-anchored aldehyde monomers underwent a Schiff base reaction with the amino groups on the PDA surface. The COF functional layer is firmly grafted onto the PDA layer via covalent bonds, rather than being a simple physical stacking or attachment. This chemically bonded structure provides the structural basis for the high stability of the composite membrane in flow-through solid-phase extraction operations.

[0077] Example 2: Optimization of solid-phase extraction conditions

[0078] This example demonstrates the establishment of an optimal solid-phase extraction method for six carbamate pesticides. A series of single-factor optimization experiments were conducted using the covalent organic framework composite membrane prepared in Example 1. The effects of the number of composite membrane sheets (1-8 sheets), the pH of the extraction solution (5-9), the type of elution solvent (methanol, ethanol, acetonitrile, isopropanol, ethyl acetate), and the elution volume (0.5-4.0 mL) on the extraction recovery rate were systematically investigated.

[0079] The specific operating steps are as follows:

[0080] (1) SPE column assembly and activation: Take 5 composite membranes prepared in Example 1, overlap and lay them flat at the bottom of a 12 mL SPE empty column tube, and fix them with a polyethylene sieve plate at the top and bottom. Activate with 2 mL of methanol and 2 mL of ultrapure water in sequence, keeping the membrane in a moist state.

[0081] (2) Sample loading and adsorption: Take the vegetable sample extract that has been extracted with organic solvent, blown with nitrogen to near dryness, and then reconstituted with ultrapure water, adjust the pH to 7.0, and pass 5 mL of sample solution through the SPE column at a flow rate of 1-2 mL / min, and discard the eluent.

[0082] (3) Elution: After the sample loading is completed, add 3.0 mL of methanol as eluent, and flow it through the membrane column three times to collect the eluent.

[0083] (4) Sample detection: The eluent was transferred to a liquid chromatography vial and analyzed by high performance liquid chromatography-mass spectrometry / mass spectrometry using a Shimadzu LC-30 AD high performance liquid chromatograph and a Qtrap 6500+ triple quadrupole tandem mass spectrometer.

[0084] The separation of carbamate pesticides was performed using an Agilent ZORBAX Eclipse Plus C18 column (2.1 mm × 100 mm, 3.5 μm) at 40 °C. Mobile phase A was acetonitrile, and mobile phase B was 0.1% formic acid aqueous solution. The flow rate was 0.4 mL / min, and the injection volume was 5 μL. The optimized gradient elution process was as follows: 0–3 min, 85–30% B; 3–8.5 min, 30%–20% B; 8.5–9.5 min, 20%–85% B; 9.5–11 min, 85% B.

[0085] Multiple reaction monitoring (MRM) mode was used, and mass spectrometry detection was performed using an electrospray ionization (ESI) source in positive ion mode (ESI+). Optimized MRM parameters for six carbamate pesticides, including parent ion mass-to-charge ratio, daughter ion mass-to-charge ratio, declustering voltage, and collision voltage, are shown in Table 1.

[0086] Table 1 Optimized MRM parameters for six carbamate pesticides

[0087]

[0088] In Table 1, * indicates quantitative ions. All experiments were conducted with three parallel samples, and the results were taken as the average. In the results charts, error bars represent the standard deviation between parallel data.

[0089] The results of the experimental condition optimization are as follows Figure 6As shown, the optimal solid-phase extraction conditions were determined to be: 5 composite membranes, sample solution pH 7.0, methanol as the elution solvent, and elution volume 3.0 mL. Under these optimized conditions, the extraction recoveries of all six target analytes remained consistently above 85%.

[0090] The method was validated under optimized conditions, examining its linear range and coefficient of determination (R²). 2 The limits of detection (LOD), quantitation (LOQ), matrix effect (ME), and precision (RSD) were evaluated. The linearity of the method was investigated, and the matrix effect was assessed by comparing the slopes of calibration curves obtained from analytes in the matrix and in the solvent. LOD and LOQ were determined using the baseline noise method, based on signal-to-noise ratios of 3 and 10, respectively. The ME and relevant parameters of the detection method are shown in Tables 2 and 3, respectively.

[0091] The results are shown in Tables 2 and 3: All target analytes exhibited good linearity within the range of 0.01–100.00 μg / kg, with R² ≥ 0.9949; the limits of detection were 0.008–0.050 μg / kg, and the limits of quantitation were 0.027–0.167 μg / kg; the absolute values ​​of matrix effects were all below 11%, indicating minimal matrix interference; the intra-day relative standard deviation was 2.79%–6.21%, and the inter-day relative standard deviation was 4.54%–8.76%, demonstrating good precision. These results indicate that the established method possesses high sensitivity, accuracy, and reliability, and is suitable for trace detection of carbamate pesticides in complex matrices such as vegetables.

[0092] Table 2 Matrix Effects (ME) of Solid Phase Extraction Methods

[0093]

[0094] Table 3. Linear range, LOD, and LOQ of the detection method

[0095]

[0096] Example 3: Adsorption Performance Comparison Measurement

[0097] Using the optimal solid-phase extraction process conditions, adsorption-elution experiments were conducted with Comparative Example 1 (PES microporous filter membrane), Comparative Example 2 (PDA modified membrane), Comparative Example 3 (monomer sequence inversion composite membrane), and Example 1 (covalent organic framework composite membrane) as adsorption media. The experiments were performed in triplicate, and the average recovery rate was calculated.

[0098] The results are as follows Figure 7As shown: Comparative Example 1 (PES microporous filter membrane) had extremely low recovery rate due to its surface chemical inertness; after PDA modification (Comparative Example 2), the recovery rate improved, but the effect was still limited due to non-specific adsorption; while the composite membrane prepared in this invention (Example 1) showed a significantly improved recovery rate, confirming that the COF functional layer plays a key role in adsorption. Its large specific surface area and strong π-π conjugation effect are the core factors for achieving efficient enrichment, fully demonstrating the necessity of the "substrate-mediating layer-functional layer" sandwich structure. Compared with the composite membrane prepared in Example 1, the composite membrane obtained by changing the monomer addition order in Comparative Example 3 showed a significant decrease in the extraction recovery rate of six carbamate pesticides. The reason is that the PDA layer surface is rich in amino groups, and the amino monomers added first cannot form chemical bonds with it, but can only adhere to the membrane surface in the form of physical adsorption. It cannot form a stable covalent anchor point, which leads to the subsequent polymerization reaction tending to homogeneously nucleate in the solution, and finally forming a loosely structured physical accumulation layer on the membrane surface, which is very easy to fall off during solid phase extraction. The results fully demonstrate that the "aldehyde pre-anchoring" strategy adopted in this invention is a key step in realizing the transformation of the COF functional layer from physical adhesion to chemical grafting and ensuring the stability of the composite membrane interface and efficient extraction performance.

[0099] Example 4: Reusability Study of Composite Membranes

[0100] To evaluate the application potential of the composite membrane, this effect example investigates the recyclability of the covalent organic framework composite membrane prepared in Example 1. After each adsorption-elution cycle, the used composite membrane was ultrasonically cleaned with methanol and acetonitrile to ensure complete removal of the target analyte and achieve material regeneration.

[0101] The results are as follows Figure 8 As shown, after three consecutive reuses, the extraction recovery rate of the covalent organic framework composite membrane for six carbamate pesticides did not decrease significantly, remaining at over 85% of the initial recovery rate. This result strongly demonstrates that the composite membrane prepared in this invention possesses excellent stability and regeneration capability as an adsorbent, can be reused at least three times, and exhibits good application value and cost-effectiveness.

[0102] The covalent organic framework composite membrane prepared by this invention can be used as a solid-phase extraction adsorbent for the enrichment and detection of trace pollutants in food, environmental, and biological samples. It is simple to operate, cost-effective, and reusable, making it suitable for food safety monitoring, environmental monitoring, clinical testing, and other fields, and has promising prospects for industrialization.

Claims

1. A covalent organic framework composite membrane, characterized in that, include: Porous polymer substrate membrane; A polydopamine-mediated layer formed on the surface of the basement membrane; And a covalent organic framework functional layer that is covalently bonded to the surface of the polydopamine-mediated layer; The covalent organic framework functional layer is formed by linking the aldehyde monomer 2,5-dihydroxy-1,4-phenylenedialdehyde and the amino monomer 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine via imine bonds, as shown in Formula I: ； The aldehyde monomer and the polydopamine mediator are pre-covalently anchored via a Schiff base reaction.

2. The covalent organic framework composite membrane according to claim 1, characterized in that, The porous polymer substrate membrane is one of polyethersulfone membrane, polyvinylidene fluoride membrane, nylon membrane or mixed cellulose ester membrane; the average pore size of the substrate membrane is 0.1-0.45 μm.

3. The covalent organic framework composite membrane according to claim 1, characterized in that, The water contact angle of the covalent organic framework composite membrane is 30°-50°; after being reused three times, the recovery rate of the target analyte remains above 85% of the initial recovery rate.

4. A method for preparing a covalent organic framework composite membrane according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Substrate modification: The porous polymer substrate membrane is placed in an alkaline buffer solution containing dopamine, and a polydopamine-mediated layer is formed on the surface of the substrate membrane through an oxidative self-polymerization reaction to obtain a PDA-modified membrane; (2) Aldehyde monomer pre-anchoring: The PDA modified film is immersed in an organic solution containing aldehyde monomer 2,5-dihydroxy-1,4-phenylenedialdehyde and reacted under heating conditions to make the aldehyde monomer grafted onto the surface of the polydopamine mediator layer through Schiff base reaction. (3) In-situ growth of COF functional layer: The pre-anchored membrane obtained in step (2) is placed in a mixed solution containing aldehyde monomer 2,5-dihydroxy-1,4-benzaldehyde, amino monomer 5',5''-bis(4-aminophenyl)-[1,1':3',1'':3'',1'''-tetraphenyl]-4,4'''-diamine and catalyst, and a solvothermal reaction is carried out under heating conditions to grow the covalent organic framework functional layer with the structure of formula I in-situ on the membrane surface; (4) Post-processing: After the reaction is completed, the membrane is washed and dried to obtain the covalent organic framework composite membrane.

5. The preparation method according to claim 4, characterized in that, The alkaline buffer solution mentioned in step (1) is a Tris-HCl buffer solution with pH 8-9, a dopamine concentration of 2-6 mg / mL, and a reaction time of 2-6 h.

6. The preparation method according to claim 4, characterized in that, The reaction in step (2) is carried out at 40-100℃ for 0.5-4 h; the organic solution is 1,4-dioxane, mesitylene, ethanol, acetonitrile or a mixture thereof.

7. The preparation method according to claim 4, characterized in that, The catalyst in step (3) is a 3-12 mol / L aqueous solution of acetic acid; the solvothermal reaction is carried out at 40-100℃ for 2-5 h.

8. The application of a covalent organic framework composite membrane according to any one of claims 1-3 or a covalent organic framework composite membrane prepared by the preparation method according to any one of claims 4-7 in sample pretreatment.

9. The application according to claim 8, characterized in that, The application involves using the covalent organic framework composite membrane as a solid-phase extraction medium to enrich trace organic pollutants in food or environmental samples; the trace organic pollutants include at least one of carbamate pesticides, organophosphorus pesticides, pyrethroid pesticides, triazine herbicides, or antibiotics.

10. The application according to claim 9, characterized in that, The carbamate pesticides include at least one of carbaryl, isoprocarb, sec-butylcarbide, oxadiazon, pirimicarb, and propoxur.

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