Magnetic composite material for detecting flavonoid compounds, preparation, detection method and application of preparing standard substance

Abstract

The application discloses a kind of detection flavonoid compound magnetic composite material, preparation, detection method and the application of preparation standard substance, belong to analytical chemistry technical field.Magnetic composite material includes magnetic nanoparticle core, double lanthanide series metal organic framework (MOF) first shell and covalent organic framework (COF) second shell successively coated on its surface, and phenylboronic acid group grafted on the surface of second shell.The material uses the specific covalent binding of phenylboronic acid and the characteristic ortho-diphenol hydroxyl group of flavonoid compound to realize efficient capture and enrichment;Through double lanthanide series MOF shell, promote energy transfer, COF shell provides high specific surface area and ordered diffusion channel, and magnetic core is convenient for rapid separation, which realizes high selectivity identification of flavonoid compound.The material has the advantages of high sensitivity, strong anti-matrix interference ability, rapid and simple detection, etc., and is suitable for accurate analysis of flavonoid compound in complex sample and development of related standard substance.

Description

Technical Field

[0001] This invention belongs to the field of analytical chemistry technology, specifically relating to a magnetic composite material for detecting flavonoid compounds, its preparation, detection method, and application in the preparation of standard substances. Background Technology

[0002] Flavonoids are widely found in fruits, vegetables, tea, and alcoholic beverages such as wine. Due to their antioxidant, anti-allergic, anti-cancer, antiviral, and anti-inflammatory properties, they can have beneficial effects on the health of the nervous, respiratory, and cardiovascular systems, as well as on skin tissue damage, thus attracting widespread attention.

[0003] Given these characteristics, simple, accurate, and reliable detection methods for flavonoids are crucial. Traditional methods for detecting flavonoids include high-performance liquid chromatography (HPLC), surface-enhanced Raman spectroscopy (SEPA), and capillary electrophoresis. While these methods each have their advantages, they all have significant limitations: they require expensive experimental equipment, complex pretreatment steps, specialized operators, and lengthy readout times. Furthermore, they may face potential limitations in sensitivity, selectivity, and cost, as well as interference from the complex matrix components of real samples. Therefore, there is an urgent need to develop efficient, accurate, sensitive, and convenient detection methods.

[0004] It should be noted that this part of the present invention only provides background technology related to the present invention, and does not necessarily constitute prior art or known technology. Summary of the Invention

[0005] This invention provides a magnetic composite material for detecting flavonoids, its preparation, detection method, and application of the prepared standard material, which at least solves the technical problems of existing flavonoid detection methods, such as reliance on large instruments, complex pretreatment, and weak resistance to matrix interference.

[0006] To achieve the above objectives, in a first aspect, the present invention provides a magnetic composite material for detecting flavonoid compounds, comprising a core, a first shell layer covering the surface of the core, a second shell layer covering the surface of the first shell layer, and phenylboronic acid groups grafted onto the surface of the second shell layer; the core is a magnetic nanoparticle; the first shell layer is a bis-lanthanide metal-organic framework compound, wherein the bis-lanthanide metal-organic framework compound contains two different lanthanide metal ions, and the first organic ligand of the bis-lanthanide metal-organic framework compound is an aromatic compound containing at least two carboxyl groups; the second shell layer is a covalent organic framework compound, wherein the covalent organic framework compound is formed by a second organic ligand and a third organic ligand connected by covalent bonds, wherein the second organic ligand is a polyaldehyde compound or a polyhydrazine compound, and the third organic ligand is an aromatic compound containing both amino and carboxyl groups.

[0007] Preferably, the two different lanthanide metal ions are selected from any two of terbium, europium, samarium, dysprosium, cerium, and gadolinium;

[0008] And / or, the first organic ligand is selected from at least one of 2-aminoterephthalic acid and 2,5-diaminoterephthalic acid;

[0009] And / or, the second organic ligand is selected from at least one of 1,3,5-tricarboxyloylphloroglucinol, pyromellitic pyroxenaldehyde, and 1,4-dihydrazidobenzene, and the third organic ligand is selected from at least one of 2,5-diphenylaminoterephthalic acid, 2-aminoterephthalic acid, and 3,5-diaminobenzoic acid;

[0010] And / or, the magnetic nanoparticles are selected from at least one of iron(II,III) oxide, cobalt ferrite, nickel ferrite, and manganese ferrite.

[0011] Preferably, the two different lanthanide metal ions are terbium and europium;

[0012] And / or, the first organic ligand is 2-aminoterephthalic acid;

[0013] And / or, the second organic ligand is 1,3,5-tricarboxyloyl-resorcinol, and the third organic ligand is 2,5-diphenylaminoterephthalic acid;

[0014] And / or, the magnetic nanoparticles are iron oxide nanoparticles.

[0015] Preferably, the average particle size of the kernel is 100nm~500nm;

[0016] And / or, the average thickness of the first shell is 10nm~50nm, and the average thickness of the second shell is 10nm~100nm;

[0017] And / or, both the first and second shells have porous structures, and the average pore size of the porous structures is 5 nm to 40 nm.

[0018] Secondly, the present invention provides a method for preparing the magnetic composite material of the first aspect, comprising the following steps:

[0019] S102. Provide magnetic nanoparticles and form a first shell of a lanthanide bimetallic organic framework compound on their surface to obtain a first intermediate.

[0020] S104. A second shell of a covalent organic framework compound is formed on the surface of the first intermediate to obtain the second intermediate;

[0021] S106. The phenylboronic acid group is grafted onto the surface of the second intermediate to obtain a magnetic composite material.

[0022] Preferably, step S102 includes: dispersing magnetic nanoparticles, a first lanthanide metal salt, a second lanthanide metal salt, and a first organic ligand in a mixed aqueous solution of N,N-dimethylformamide and ethanol, reacting in a high-pressure reactor at 100°C to 150°C for 12 to 72 hours, and after the reaction is completed and allowed to cool naturally to room temperature, separating them by adsorption using an external magnet, and then washing with deionized water and ethanol to obtain the first intermediate;

[0023] The molar ratio of the first lanthanide metal salt, the second lanthanide metal salt and the first organic ligand is (0.5~5):(0.5~5):1, and the molar ratio of the magnetic nanoparticles to the first organic ligand is (0.5~5) g:1 mmol.

[0024] The volume ratio of N,N-dimethylformamide, ethanol and water in the mixed aqueous solution is (1~3):(1~3):1, and the amount of mixed aqueous solution corresponding to each gram of magnetic nanoparticles is 100mL~200mL.

[0025] Preferably, step S104 includes: ultrasonically dispersing the first intermediate obtained in step S102 in N,N-dimethylformamide, then adding the second organic ligand, the third organic ligand and acetic acid solution, mixing evenly and performing 3 to 5 freeze-thaw degassing cycles, then reacting under sealed conditions at 100°C to 150°C for 48 to 96 hours, cooling to room temperature, collecting the product and washing it with tetrahydrofuran, then soaking it in acetone for 48 to 96 hours, and then drying it to obtain the second intermediate;

[0026] The ratio of the first intermediate to N,N-dimethylformamide is 1 g: (20~50) mL, the mass ratio of the first intermediate, the second organic ligand and the third organic ligand is 1: (1~5): (0.5~5), the volume ratio of N,N-dimethylformamide to acetic acid solution is (2~5): 1, and the concentration of acetic acid solution is 2 mol / L~5 mol / L.

[0027] Preferably, step S106 includes: dispersing the second intermediate obtained in step S104 in N,N-dimethylformamide, then adding 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, and then stirring for 10 to 30 minutes to activate the carboxyl group. After the carboxyl group is activated, 3-aminophenylboronic acid dispersed in N,N-dimethylformamide is added dropwise and the reaction is stirred for 8 to 24 hours. The separated solid is washed sequentially with N,N-dimethylformamide, ethanol and water, and then vacuum dried to obtain the magnetic composite material.

[0028] The ratio of the second intermediate to dispersed N,N-dimethylformamide is 1 g: 30 mL to 60 mL, the mass ratio of the second intermediate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide is 1:(0.1~3):(0.1~3), and the mass ratio of the second intermediate to 3-aminophenylboronic acid is 1:(0.1~5).

[0029] Thirdly, the present invention provides a method for detecting flavonoid compounds, comprising: dispersing the magnetic composite material of the first aspect or the magnetic composite material prepared by the preparation method of the second aspect in a solvent to form a composite material dispersion; mixing the composite material dispersion with a test solution containing flavonoid compounds to form a detection mixture; placing the detection mixture under an ultraviolet lamp and performing fluorescence measurement, and detecting flavonoid compounds by means of fluorescence response intensity.

[0030] Fourthly, the present invention provides a method for detecting flavonoid compounds, comprising: modifying the surface of a working electrode with the magnetic composite material of the first aspect or the magnetic composite material prepared by the preparation method of the second aspect to form a modified electrode; placing the modified electrode in a test solution containing flavonoid compounds for electrochemical measurement; and detecting flavonoid compounds by measuring the characteristic redox current signal of the flavonoid compounds.

[0031] Fifthly, the present invention provides the application of the magnetic composite material of the first aspect or the magnetic composite material prepared by the preparation method of the second aspect in the preparation of flavonoid compound standard substances.

[0032] The beneficial effects of this invention are as follows:

[0033] The magnetic composite material of this invention comprises a magnetic nanoparticle core, a first lanthanide metal-organic framework (MOF) shell coated thereon, a second covalent organic framework (COF) shell coated on the surface of the first shell, and phenylboronic acid (BA) groups grafted onto the surface of the second shell. When detecting flavonoids, under ultraviolet light excitation, the rare-earth ion energy levels in the lanthanide MOF shell synergistically interact with the electronic structure of the flavonoids, promoting efficient energy transfer. The high specific surface area and ordered channels of the COF shell provide ample space for diffusion and binding of flavonoid molecules. The phenylboronic acid groups can specifically and reversibly covalently bind to the characteristic ortho-dihydroxyl structure of flavonoids, achieving efficient capture and enrichment of the target molecules.

[0034] This invention uses lanthanide ions (such as Tb) 3+ and Eu 3+The MOF shell, constructed with organic ligands containing at least two carboxyl groups, achieves several advantages. Firstly, the bimetallic synergistic regulation of the MOF's energy level structure enhances its alignment with the highest occupied orbital (HOMO) of flavonoid molecules, strengthening electron transfer and improving fluorescence and electrochemical response sensitivity. Secondly, the hybrid metal clusters formed by the bimetallic centers create more regular and uniform hierarchical channels, significantly increasing the material's specific surface area and active site density. The introduction of the COF shell not only further expands the material's specific surface area and porosity through its highly ordered covalent framework, but its surface active groups also provide stable and uniform anchoring sites for the functionalization grafting of phenylboronic acid, preventing aggregation and blockage of recognition sites.

[0035] By grafting phenylboronic acid groups onto the surface of the COF shell through an amidation reaction, specific and highly selective recognition of flavonoids (especially flavonoids containing ortho-dihydroxyl structures such as quercetin) can be achieved. The borate anion formed by the dissociation of phenylboronic acid under suitable pH conditions can form stable five- or six-membered cyclic borate ester bonds with the ortho-dihydroxyl groups of flavonoids. Conversely, interfering substances such as sugars and proteins lacking this structure in the sample matrix cannot bind to the borate anion, thus significantly reducing interference from complex sample matrices and improving the selectivity and accuracy of detection. The magnetic nanoparticle core not only endows the composite material with rapid magnetic separation capabilities, simplifying pretreatment steps and improving detection efficiency, but also its own conductivity and Fe... 2+ / Fe 3+ Redox pairs can also serve as efficient electron transfer mediators, reducing the overpotential of redox reactions of flavonoids on the electrode surface and enhancing electrochemical signals.

[0036] The preparation method of this invention achieves controllable integration of functional layers by constructing a core-shell structure stepwise. First, a bis-lanthanide MOF shell is grown in situ on the surface of magnetic nanoparticles. Then, a second COF shell is constructed on the MOF surface via a covalent condensation reaction. Finally, phenylboronic acid is grafted onto the COF surface through an activation-amidation reaction. This process ensures strong bonding between layers, structural stability, and uniform distribution of functional groups. The resulting composite material possesses high specific surface area, abundant and specific recognition sites, excellent fluorescence / electrochemical response characteristics, and convenient magnetic separation performance.

[0037] The composite material of this invention integrates magnetic separation, porous enrichment, specific recognition, and dual-mode sensing. When used for the detection of flavonoids, it achieves highly sensitive fluorescence and electrochemical dual-mode responses. The two detection modes can be mutually verified, effectively improving the reliability and accuracy of the detection results. This method is simple to operate, has strong resistance to matrix interference, and a low detection limit, providing a new material basis and technical means for the rapid and accurate analysis of flavonoid functional components in complex matrices such as food and agricultural products, as well as the development of related standard substances. Attached Figure Description

[0038] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 The image shows a scanning electron microscope (SEM) image of Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1.

[0040] Figure 2 The image shows a transmission electron microscope (TEM) image of Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1.

[0041] Figure 3 The X-ray photoelectron spectroscopy (XPS) spectra of Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 are shown.

[0042] Figure 4 The high-resolution XPS spectra of C1s of Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 are shown.

[0043] Figure 5 The high-resolution XPS spectra of O1s of Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 are shown.

[0044] Figure 6 High-resolution XPS spectra of Tb3d of Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1.

[0045] Figure 7 The fluorescence response of Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 to different concentrations of quercetin is shown in the figure.

[0046] Figure 8 The differential pulse voltammetry (DPV) response of the Fe3O4@Tb / Eu-MOF@COF-BA modified electrode prepared in Example 1 to different concentrations of quercetin is shown. Detailed Implementation

[0047] In this invention, unless otherwise stated, directional terms such as "up," "down," "left," and "right" are generally understood in conjunction with the accompanying drawings and the directions shown in actual applications.

[0048] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0049] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0050] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The terms "optional" and "discretionary" mean that they may or may not be included (or may or may not be present).

[0051] In a first aspect, the present invention provides a magnetic composite material for detecting flavonoid compounds, comprising a core, a first shell layer covering the surface of the core, a second shell layer covering the surface of the first shell layer, and phenylboronic acid groups grafted onto the surface of the second shell layer; the core is a magnetic nanoparticle; the first shell layer is a bis-lanthanide metal-organic framework compound, wherein the bis-lanthanide metal-organic framework compound contains two different lanthanide metal ions, and the first organic ligand of the bis-lanthanide metal-organic framework compound is an aromatic compound containing at least two carboxyl groups; the second shell layer is a covalent organic framework compound, wherein the covalent organic framework compound is formed by a second organic ligand and a third organic ligand connected by covalent bonds, wherein the second organic ligand is a polyaldehyde compound or a polyhydrazine compound, and the third organic ligand is an aromatic compound containing both amino and carboxyl groups.

[0052] The core of this invention lies in constructing a multifunctional, highly selective core-shell magnetic composite material to solve the technical problems faced in the detection of flavonoids in complex sample matrices, such as cumbersome pretreatment, poor selectivity, insufficient sensitivity, and weak anti-interference ability. Traditional detection methods, such as high-performance liquid chromatography (HPLC), are accurate but rely on large instruments and are time-consuming; while some nanomaterial-based sensors are often subject to matrix interference due to non-specific adsorption and single recognition sites. This invention designs a layer-by-layer assembled functional material targeting the characteristic ortho-dihydroxyl structure of flavonoids.

[0053] This invention does not limit the types of flavonoid compounds, such as quercetin, kaempferol, apigenin, luteolin, myricetin, etc. In this invention, quercetin is used as an example for the detection of flavonoid compounds.

[0054] Choosing magnetic nanoparticles as the core endows the composite material with convenient magnetic response separation capabilities. In sample pretreatment, an external magnet can quickly separate the composite material dispersed in solution from the sample matrix, greatly simplifying the enrichment and washing steps and improving detection efficiency. Furthermore, magnetic nanoparticles (such as Fe3O4) themselves possess good electrical conductivity, and their surface Fe... 2+ / Fe 3+ Redox pairs can serve as highly efficient electron transfer media. In electrochemical detection mode, when flavonoid compounds undergo redox reactions on the electrode surface, the magnetic core effectively reduces the overpotential of electron transfer, enhances the current signal, and improves detection sensitivity. Furthermore, the rigid magnetic core provides a stable support framework for the growth of outer MOFs and COFs, preventing them from agglomerating due to van der Waals forces and maintaining the unobstructed pore structure.

[0055] The first shell of this invention employs a dual lanthanide metal-organic framework (MOF), which, compared to a single lanthanide MOF, introduces two different lanthanide metal ions (such as Tb). 3+ and Eu 3+ These two ions, together, construct the secondary structural units (SBUs) of the MOF, forming a "Ln1-O-Ln2" mixed metal-oxygen cluster. Due to the different ionic radii, coordination preferences, and electronegativity of the two ions, this synergistic effect induces the organic ligands to form a more regular, hierarchical pore structure with a more uniform pore size distribution. This optimized pore structure not only significantly increases the specific surface area of ​​the material, providing more space for the loading of subsequent functional sites and the diffusion of flavonoid molecules, but also enhances the crystallinity and chemical stability of the MOF framework, making it less prone to degradation in complex sample environments.

[0056] Unrestricted by fundamental principles, the contributions of lanthanide MOFs to flavonoid detection are multifaceted: First, at the recognition and enrichment level, the presence of bimetallic ions on the inner wall of MOF pores creates a unique microenvironment that is weakly hydrophobic and slightly locally positively charged. This environment facilitates the rapid diffusion of weakly polar flavonoid molecules into the pores while repelling hydrophilic macromolecular interferences (such as proteins and polysaccharides). More importantly, the bimetallic sites in the MOF backbone can coordinate with the characteristic ortho- and tho-dihydroxyl groups of flavonoids. This, combined with the covalent esterification reaction of phenylboronic acid on the COF layer, constitutes a multi-site synergistic recognition mechanism for target molecules, greatly enhancing the stability and specificity of the binding. Second, at the signal conversion and amplification level, lanthanide MOFs exhibit excellent dual-mode sensing capabilities. In fluorescence detection, the introduction of bimetallic ions modulates the energy level structure of the MOF (e.g., LUMO level), making it better matched with flavonoid molecules (HOMO level), which is conducive to effective energy transfer (e.g., FRET). Simultaneously, two lanthanide ions with different luminescent properties (e.g., Tb) can also interact with each other. 3+ With Eu 3+ These lanthanides can form fluorescent donor-acceptor pairs, enabling signal amplification, tuning, and even ratiometric detection, significantly improving the sensitivity and anti-interference capabilities of the fluorescence response. In electrochemical detection, bislanthanide MOFs provide two redox pairs with different potentials. They can serve as efficient multi-level electron transport bridges, forming a good match with the oxidation potential of the ortho- and tho-dihydroxyl groups of flavonoids. This significantly shortens the electron transport path from the flavonoid molecule to the electrode surface, reduces the reaction overpotential, and effectively avoids interference from other electroactive substances in the sample matrix, ultimately achieving highly selective amplification of the electrochemical signal.

[0057] The second shell of this invention employs a covalent organic framework (COF), which can further expand the specific surface area and porosity of the material and provide a highly ordered and stable covalently linked network. COFs are linked by strong covalent bonds (such as imine bonds), and their structure is more chemically and thermally stable than many MOFs, maintaining integrity under a wider range of pH and solvent conditions. Furthermore, through its highly programmable chemical structure and physical properties, the analytical performance for flavonoids can be comprehensively improved from multiple dimensions, including identification environment control, mass transfer pathway optimization, and signal conversion enhancement.

[0058] Unrestricted by fundamental principles, the introduction of a COF shell offers multiple advantages: First, COF plays a crucial role in constructing a specific recognition environment and improving mass transfer efficiency. By selecting specific building blocks, abundant amino and hydroxyl functional groups can be introduced into the COF backbone. These groups not only provide ideal anchoring sites for subsequent covalent grafting of phenylboronic acid via a robust amidation reaction, but they can also generate auxiliary interactions such as hydrogen bonds and π-π stacking with the phenolic hydroxyl groups and benzopyranone structures of flavonoid molecules. This multi-site recognition interface, formed by the functional groups of the COF backbone and the grafted phenylboronic acid, significantly enhances the binding affinity and selectivity of the composite material for flavonoids. Simultaneously, the highly ordered and tunable pore structure of COF, combined with the inherent hydrophobicity of its aromatic backbone, can create a weakly hydrophobic microenvironment within the pores. This environment is highly compatible with weakly polar flavonoid molecules, effectively reducing the interfacial resistance to flavonoid diffusion into the depths of the pores, promoting rapid transport to internal recognition sites, and achieving efficient bulk enrichment. In contrast, hydrophilic sugars, proteins, and other interfering substances in the sample are excluded from the pores because they are incompatible with this hydrophobic environment. This achieves spatial sieving of impurities in the first step of mass transfer, reducing non-specific adsorption at the source.

[0059] Secondly, COF contributes an indispensable dual function in enhancing signal conversion and amplifying detection signals. In electrochemical detection mode, the continuous, π-electron-rich conjugated framework of COF constitutes a highly efficient electron transport network. When flavonoid molecules are enriched on the surface of the composite material and undergo redox reactions, this network can significantly reduce the charge transfer resistance of electrons transferring from flavonoid molecules to the electrode surface, accelerating reaction kinetics. More importantly, by functionalizing the COF framework (such as introducing specific functional groups), its overall electron cloud density can be finely controlled to achieve a better match with the redox potential of flavonoid molecules, thereby further improving electron transfer efficiency and achieving a significant enhancement of the electrochemical response signal. In fluorescence detection mode, COF also plays an important role. On the one hand, the COF framework can create a microenvironmental isolation layer around flavonoid molecules (such as quercetin) through its highly ordered hydrophobic channels. When flavonoid molecules are specifically captured and enriched within the pores of COF by phenylboronic acid, this hydrophobic microenvironment reduces the contact between flavonoid molecules and polar water molecules, lowering solvent quenching effects and molecular thermal vibrations, thereby significantly improving the quantum yield of flavonoid autofluorescence. On the other hand, the regular pore structure of COF effectively confines flavonoid molecules to their specific positions. This spatial confinement effect reduces the probability of nonradiative transitions, resulting in a significant enhancement of the fluorescence signal of flavonoids at 445 nm. Furthermore, the aromatic structures in the COF layer can interact with flavonoid molecules through π-π stacking interactions, fine-tuning their electronic distribution and further optimizing their fluorescence properties.

[0060] This invention grafts phenylboronic acid groups onto the surface of the second shell, enabling highly selective recognition of flavonoid compounds. Flavonoids (such as quercetin) are characterized by the presence of an ortho-dihydroxyl group (catechol structure) on their B ring. Phenylboronic acid undergoes reversible dissociation under weakly alkaline conditions (pH approximately 8.5-10), forming a negatively charged tetrahedral borate anion. This anion can undergo a specific and reversible esterification reaction with the ortho-dihydroxyl group of flavonoids, forming a stable five- or six-membered cyclic borate ester bond. This is a covalent bond, with binding strength and specificity far exceeding that of ordinary hydrogen bonds or π-π stacking. Most interfering components in the sample matrix (such as sugars, amino acids, and common phenolic acids) do not possess this ortho-dihydroxyl structure and therefore cannot form stable complexes with phenylboronic acid. By grafting phenylboronic acid groups onto the surface of a COF shell with a high specific surface area, a high density of specific recognition sites can be constructed, enabling efficient capture and enrichment of target flavonoid molecules while minimizing matrix interference. This is the molecular basis for the high selectivity and high sensitivity detection achieved by this material.

[0061] The magnetic composite material for detecting flavonoids in this invention actually constructs a "magnetic core@double Ln-MOF@COF-phenylboronic acid" structure. The functions of each layer are not simply mechanically superimposed, but rather, through interfacial coupling and functional complementarity, a significant multi-level synergistic effect is generated. Unrestricted by principle, firstly, there is a "conduction-luminescence" synergy between the magnetic core and the double lanthanide MOF shell. Magnetic nanoparticles (such as Fe3O4) are not only separation carriers, but their surface Fe... 2+ / Fe 3+ Redox pairs and metallic properties can be considered as an electron transfer station. In electrochemical detection, when flavonoid molecules are captured and oxidized within the MOF channels, the released electrons can be transferred more efficiently to the conductive magnetic nucleus via lanthanide metal nodes in the MOF framework, and then rapidly transferred to the working electrode. This high-speed electron channel, jointly constructed by the MOF and the magnetic nucleus, significantly reduces charge transfer impedance and amplifies the electrochemical signal. Simultaneously, in fluorescence detection, magnetic nanoparticles may also modulate fluorescence intensity and stability by influencing the local crystal field or energy transfer pathways of lanthanide ions in the MOF on their surface.

[0062] Secondly, the lanthanide MOF shell and the COF shell form a close synergy of "structural interlocking and functional complementarity." Physically, while the MOF layer has a high specific surface area, its pores sometimes exhibit disorder or defects. Epitaxially growing a COF layer on its surface is equivalent to constructing a highly ordered, uniformly pore-sized secondary sieve and flow-conducting layer. The regular pores of the COF can form a certain degree of connection and communication with the underlying MOF porous structure, establishing a smoother and more coherent diffusion path from the outside of the material to the inner surface of the MOF. This greatly optimizes mass transfer kinetics, ensuring that flavonoid molecules can quickly enter the material interior, contacting more MOF active sites and phenylboronic acid recognition sites on the COF layer, thus solving the problem of limited adsorption capacity relying solely on the outer surface.

[0063] In fluorescence detection, the synergy between MOF and COF manifests as an indirect enhancement mechanism through pre-enrichment. The highly ordered hydrophobic channels of COF can rapidly capture and pre-enrich flavonoid molecules, effectively transporting them to regions close to the luminescent centers of the inner MOF. This spatial proximity effect increases the probability of interaction between flavonoid molecules and lanthanide luminescent ions in the MOF, thus creating the necessary conditions for subsequent efficient fluorescence resonance energy transfer (FRET) or luminescence quenching / enhancement effects.

[0064] In terms of electrochemical performance, the synergy between the two manifests as the integration and enhancement of the electron transport network. The bislanthanide MOF itself provides redox active sites, while the conjugated organic framework of the COF is an excellent electronic conductor. When the two are tightly composited, the conjugated framework of the COF acts as a bridge between MOF particles, forming a more continuous and efficient interlayer electron transport network within the composite material. In electrochemical detection, this integrated network significantly reduces the interfacial charge transfer resistance of the entire modified electrode, allowing electrons generated when flavonoid molecules undergo redox reactions at recognition sites to be collected and conducted to the electrode surface more quickly, thereby synergistically amplifying the electrochemical response signal. Furthermore, the COF layer's encapsulation of the MOF also provides physical protection, enhancing the structural stability of the composite material in the electrolyte.

[0065] Third, there is a "carrier-recognition" synergy between the COF shell and the surface phenylboronic acid. The high specific surface area and abundant active sites (carboxyl groups) of COF provide an ideal platform for achieving high-density, uniform, and robust covalent grafting of phenylboronic acid. This grafting method ensures that each phenylboronic acid recognition site is easily accessible to flavonoid molecules, avoiding site masking caused by physical adsorption or aggregation. More importantly, the hydrophobic channels of COF, after binding with phenylboronic acid, jointly create an affinity microenvironment for weakly polar flavonoid molecules. When flavonoid molecules are specifically captured by phenylboronic acid, the hydrophobic channels they occupy can shield against interference from external hydrophilic matrices (such as sugars and salts), stabilizing the flavonoid-boronic ester complex.

[0066] The stacking order of the "magnetic core@double Ln-MOF@COF-phenylboronic acid" in this invention is determined by the progressive logic of material synthesis chemistry, structural stability, and functional realization. It possesses inherent necessity and irreversibility, and is not limited by principles: First, placing magnetic nanoparticles in the innermost layer serves primarily as the structural basis for subsequent functional layers. From a synthetic chemistry perspective, magnetic nanoparticles are suitable as an initial solid substrate. The synthesis of both MOFs and COFs involves crystal growth in solution, requiring a well-defined and well-dispersed solid surface as a heterogeneous nucleation substrate. Magnetic nanoparticles, through their surface hydroxyl groups, provide initial sites for the adsorption and coordination of lanthanide metal ions and organic ligands, guiding the MOF layer to grow in situ and directionally on their surface. This is the foundation for forming a uniform and robust core-shell structure. Without this initial core, the MOF would undergo homogeneous nucleation in solution, forming a free powder that cannot be integrated with subsequent magnetic separation functions. Although the coating layer affects its magnetism to some extent, by optimizing the core size and coating layer thickness, sufficient magnetic responsiveness for rapid separation can be retained while ensuring structural integrity and chemical stability.

[0067] Secondly, this invention synthesizes the MOF layer first, then grows the COF layer, primarily based on the differences in synthesis conditions and stability between the two. The synthesis of lanthanide MOFs relies on metal-ligand coordination bonds, typically requiring prolonged solvothermal reactions at high temperatures in polar solvents, resulting in relatively harsh conditions. The synthesis of COFs, especially imine-linked COFs, while also requiring heating, involves more sensitive and precise reaction conditions (such as catalyst, monomer purity, and degassing requirements). If the COF is grown on a magnetic core first, and then placed in the high-temperature, high-pressure DMF environment of MOF synthesis, the imine bonds of the COF are likely to undergo hydrolysis or structural decomposition, leading to the destruction of the COF layer. Conversely, the first synthesized lanthanide MOF exhibits excellent thermal and chemical stability, effectively withstanding the mild solvothermal conditions of subsequent COF synthesis. In addition, the surface of the first-formed MOF layer is rich in active groups such as amino groups and unsaturated coordination sites, which have hydrogen bond and coordination interactions with COF monomers (such as aldehyde groups and amino groups). This significantly promotes the specific adsorption and nucleation of COF on the MOF surface, which is conducive to the formation of a dense, uniform and tightly bound second shell and avoids the spontaneous homogeneous nucleation of COF.

[0068] Third, this invention grafts phenylboronic acid onto the surface of the COF layer at the end of the entire synthesis process, primarily to protect the integrity and activity of the functional groups. Phenylboronic acid groups are pH sensitive and may be unstable under strong acids, strong bases, or prolonged high temperatures. If phenylboronic acid is grafted prematurely, this group is easily damaged or detached during the high-temperature reaction of MOF or COF synthesis, leading to functional failure. Therefore, after all high-temperature synthesis steps (MOF and COF construction) are completed, phenylboronic acid must be covalently grafted onto the carboxyl groups of the formed COF shell through an amidation reaction under mild conditions (room temperature or low-temperature stirring). This post-modification strategy ensures the survival rate and activity of the recognition sites. Grafting onto COF rather than MOF is chosen because the COF monomer used in this invention provides abundant and readily available carboxyl groups for amidation reactions; after activation, the grafting efficiency and density of these carboxyl groups may be higher. While the amino and carboxyl groups on the MOF surface can also react, their accessibility and quantity may be less than those on COF, and they may affect the stability of the MOF structure.

[0069] In summary, the layered structure sequence of this invention (magnetic core → MOF → COF → phenylboronic acid) is an interconnected engineering process. It follows a progressive principle from building a stable structural substrate to assembling a multifunctional carrier, and finally performing precise modification of active recognition sites. This sequential design is based on the inherent requirements of the synthetic chemistry and stability of each layer of material. If the sequence is reversed, it will be difficult to successfully prepare a composite material with a complete structure and full functionality.

[0070] Preferably, the two different lanthanide metal ions are selected from any two of terbium, europium, samarium, dysprosium, cerium, and gadolinium;

[0071] And / or, the first organic ligand is selected from at least one of 2-aminoterephthalic acid and 2,5-diaminoterephthalic acid;

[0072] And / or, the second organic ligand is selected from at least one of 1,3,5-tricarboxyloylphloroglucinol, pyromellitic pyroxenaldehyde, and 1,4-dihydrazidobenzene, and the third organic ligand is selected from at least one of 2,5-diphenylaminoterephthalic acid, 2-aminoterephthalic acid, and 3,5-diaminobenzoic acid;

[0073] And / or, the magnetic nanoparticles are selected from at least one of iron(II,III) oxide, cobalt ferrite, nickel ferrite, and manganese ferrite.

[0074] The preferred selection of the above components is based on their specific physicochemical properties and their synergistic effects in the composite system. The lanthanide ions (Ln) selected in this invention... 3+ ) such as Tb 3+ Eu 3+ 、Sm 3+ Dy 3+All of these possess abundant energy levels and unique luminescent properties, making them ideal metal centers for constructing fluorescent MOFs. Meanwhile, Ce... 3+ Although cerium emits light via the 5d→4f transition, it possesses a broad emission band and a large Stokes shift, which is beneficial for broadening the spectral response range; while Gd 3+ (Gadolinium) although ground state 8 The S7 / 2 transition to the excited state has a high energy level, but its higher energy levels allow for efficient energy transfer, enabling efficient energy transfer to other luminescent lanthanides (such as Tb) in bimetallic systems. 3+ Eu 3+ This has a significant sensitizing effect. Any two of them can be combined, for example, Tb. 3+ and Eu 3+ This is because of its different emission wavelengths and the good matching between its triplet energy level and the energy levels of common organic ligands and flavonoid molecules, making it easy to undergo the "antenna effect" (the ligand absorbs energy and transfers it to the metal center for emission) and energy transfer. Other combinations (such as those containing Ce) 3+ or Gd 3+ Bimetallic systems can also modulate the energy level structure, electron transport pathways, and local crystal field environment of MOFs through synergy between metals. This bimetallic combination not only optimizes MOF channels through the hybrid cluster effect but also provides multiple or tunable fluorescence responses in detection, enhancing signal discrimination and reliability. If only a single lanthanide metal is used, a sufficiently regular hybrid cluster structure may not be formed, channel uniformity may decrease, and the signal dimension may be singular, potentially weakening anti-interference and recognition capabilities.

[0075] The first organic ligand is preferably an aromatic compound containing both an amino group and at least two carboxyl groups, such as 2-aminoterephthalic acid or 2,5-diaminoterephthalic acid. The carboxyl group (-COOH) is a classic hard base, similar to the Ln group, which has hard acid properties. 3+ It possesses extremely strong coordination ability, enabling the formation of stable, multidentate MOF structures. Unrestricted by principle, the introduced amino group (-NH2) plays multiple roles: first, during MOF synthesis, it may participate in coordination or regulate the pH and charge environment of the reaction system, influencing crystal nucleation and growth; second, on the formed MOF framework, the amino group can serve as a potential active site, enhancing the hydrophilicity and chemical activity of the material surface, which is beneficial for interfacial bonding with the subsequent COF layer; third, the amino group itself can also form hydrogen bonds with the phenolic hydroxyl groups of flavonoids, providing auxiliary non-covalent interactions. If a polycarboxylic acid ligand without an amino group (such as terephthalic acid) is used, although MOFs can also be formed, the chemical diversity of the material surface and the interfacial bonding strength with the COF layer may be reduced.

[0076] The selection of the second and third organic ligands aims to form a stable COF backbone through dynamic covalent condensation reactions between their complementary functional groups. When the second organic ligand is a polyaldehyde compound (such as 1,3,5-tricarboxymethyl phloroglucinol or trimesoaldehyde), its aldehyde group undergoes a Schiff base reaction with the amino group simultaneously provided by the third organic ligand (such as 2,5-diphenylaminoterephthalic acid) to form an imine-linked COF. For example, 1,3,5-tricarboxymethyl phloroglucinol (Tp), as a trialdehyde monomer, can react with diamine monomers to form a stable COF with a β-ketoenamine structure. This structure exhibits excited-state intramolecular proton transfer (ESIPT) properties, potentially leading to unique fluorescence properties. When the second organic ligand is a polyhydrazine compound (such as 1,4-dihydrazinobenzene), its hydrazine group can condense with a carbonyl-containing component (aldehyde or ketone group) to form a COF linked by a hydrazone bond. In this case, the carbonyl-containing component can originate from the third organic ligand itself or other monomers in the reaction system. Both hydrazone bonds and imine bonds are typical dynamic covalent bonds, which are conducive to driving the formation of a well-crystallized, structurally ordered porous framework.

[0077] In any of the above-mentioned linkage methods, the carboxyl groups retained in the third organic ligand are crucial. These carboxyl groups are distributed on the surface or within the pores of the COF, providing covalent nodes for subsequent grafting of phenylboronic acid (in the form of 3-aminophenylboronic acid) via amidation. Without these carboxyl groups, phenylboronic acid can only be introduced through physical adsorption or weak interactions, resulting in weak grafting, uneven distribution, and easy detachment, severely affecting the stability and reusability of the recognition site. Ligands such as 2,5-diphenylaminoterephthalic acid, with their aniline structure, can further extend the π-conjugated system of the COF, further enhancing electron transport capabilities.

[0078] Magnetic nanoparticles are preferably spinel-type ferrites such as iron(III) oxide (Fe3O4) and cobalt ferrite (CoFe2O4). These materials have high saturation magnetization, facilitating rapid magnetic separation; they also exhibit good chemical stability and biocompatibility. Fe3O4 is particularly suitable due to its mature preparation process, low cost, and high Fe content. 2+ / Fe 3+ High redox activity makes them a preferred choice. Cobalt ferrites and similar materials may possess higher magnetic anisotropy and coercivity. These magnetic cores are chosen to ensure complete separation of the composite material under an applied magnetic field after enrichment, significantly improving pretreatment speed. Using materials with weak magnetic response results in longer separation times, potentially leading to desorption of some adsorbed target analytes, affecting enrichment efficiency and analytical accuracy.

[0079] Preferably, the two different lanthanide metal ions are terbium and europium;

[0080] And / or, the first organic ligand is 2-aminoterephthalic acid;

[0081] And / or, the second organic ligand is 1,3,5-tricarboxyloyl-resorcinol, and the third organic ligand is 2,5-diphenylaminoterephthalic acid;

[0082] And / or, the magnetic nanoparticles are iron oxide nanoparticles.

[0083] This is a particularly preferred embodiment of the present invention. Terbium (Tb) is selected. 3+ ) and europium (Eu) 3+ Tb is chosen as a bimetallic combination because it exhibits representative strong luminescence properties among lanthanide ions, and its excited-state energy levels match the triplet energy levels of flavonoid molecules quite well. 3+ of 5 D4→ 7 FJ transitions and emits green light, Eu 3+ of 5 D0→ 7 The FJ transition emits red light. The simultaneous presence of both can make the relationship between the fluorescence response of the composite material and the flavonoid concentration more sensitive and unique through energy transfer or competitive effects, thereby offsetting the interference of environmental factors and improving quantitative accuracy.

[0084] 2-Aminoterephthalic acid was chosen as the first organic ligand because of its symmetrical molecular structure, containing two carboxyl groups and one amino group, which aligns well with Tb. 3+ / Eu 3+ Coordination can form bimetallic MOFs with well-defined structures and high stability (such as structures similar to Ln-BDC-NH2). The introduction of amino groups brings additional chemical functionalities to MOFs.

[0085] The selection of 1,3,5-tricarboxyloylphloroglucinol (Tp) and 2,5-diphenylaminoterephthalic acid (Dhta) as the monomer pair for constructing the COF was based on several considerations, without being limited by principles: First, Tp and Dhta can form a COF with high crystallinity and high specific surface area through amine-aldehyde condensation. Second, the Dhta molecule contains two carboxyl groups, providing ample sites for subsequent phenylboronic acid grafting. Third, this COF system exhibits excellent chemical stability and porous properties. Fourth, the hydrophobic aromatic skeleton of this COF facilitates the creation of a weakly hydrophobic microenvironment, promoting the entry of weakly polar flavonoid molecules into the pores while repelling strongly polar impurities.

[0086] Ferric oxide nanoparticles, as magnetic cores, offer advantages such as mature and diverse synthesis methods (e.g., hydrothermal and coprecipitation methods), allowing for the controlled preparation of particles with uniform size. Furthermore, their superparamagnetic or ferromagnetic properties are suitable for most separation scenarios. The hydroxyl groups on their surface also readily interact with MOF precursors, promoting in-situ heterogeneous nucleation and growth of the MOF layer.

[0087] Preferably, the average particle size of the kernel is 100nm~500nm;

[0088] And / or, the average thickness of the first shell is 10nm~50nm, and the average thickness of the second shell is 10nm~100nm;

[0089] And / or, both the first and second shells have porous structures, and the average pore size of the porous structures is 5 nm to 40 nm.

[0090] The limitation of the structural dimensions of each layer in this invention is key to ensuring the optimal overall performance of the composite material. The preferred range for the average core particle size is 100 nm to 500 nm. This size range effectively coordinates the relationship between the material's magnetic responsiveness and functional specific surface area. Within this range, the magnetic nanoparticles possess a sufficiently large magnetic moment to ensure rapid magnetic separation of the composite material in complex sample matrices, avoiding material loss. Simultaneously, this particle size provides ample substrate surface area for the subsequent growth of MOF and COF shells, facilitating the formation of a core-shell composite system with high loading capacity and structural stability. The average core particle size can be 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 450 nm, 500 nm, or any value between them. A more preferred range is 100 nm to 300 nm, which better balances magnetic responsiveness, stability, and specific surface area.

[0091] Preferably, the average thickness of the first shell is 10 nm to 50 nm, and the average thickness of the second shell is 10 nm to 100 nm. This thickness range aims to achieve a balance between functional integrity and mass transfer efficiency. Shells grown within this range can form a continuous, dense coating with good mechanical strength, effectively protecting the core and supporting abundant recognition sites. Simultaneously, the appropriate thickness ensures that the diffusion path of flavonoid molecules into the recognition sites within the material is not too long, which is beneficial for maintaining a relatively fast enrichment kinetic rate. Furthermore, this thickness design also considers the overall magnetic responsiveness of the composite material, avoiding a significant decrease in magnetic separation performance due to excessively thick non-magnetic shells. The average thickness of the first shell (double lanthanide MOF) can be 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, or any value between them. The average thickness of the second shell (COF) can be 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, and any value in between.

[0092] The average pore size of the porous structure is preferably between 5 nm and 40 nm. This mesoporous range is set based on the physical size and diffusion behavior of flavonoid target molecules. This pore size is larger than the size of typical flavonoid molecules, providing low-resistance, rapid diffusion channels for flavonoids and their common glycoside derivatives, ensuring that molecules can smoothly reach the recognition sites on the inner wall of the pores, achieving efficient bulk enrichment. Simultaneously, this pore size range, based on size exclusion effect, can, to some extent, limit the entry of larger biomolecules such as proteins into the pores, thereby effectively reducing non-specific adsorption and improving detection selectivity. This mesoporous structure is an ideal choice for achieving high-capacity, high-selectivity enrichment. The average pore size of the first and second shells can be 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, or any value between them.

[0093] Preferably, the first shell and the second shell can have the same or different pore sizes to form a multi-level porous structure, which further optimizes the mass transfer path. It is only necessary to ensure that the average pore size of the porous structure of both is in the range of 5nm to 40nm.

[0094] Secondly, the present invention provides a method for preparing the magnetic composite material of the first aspect, comprising the following steps:

[0095] S102. Provide magnetic nanoparticles and form a first shell of a lanthanide bimetallic organic framework compound on their surface to obtain a first intermediate.

[0096] S104. A second shell of a covalent organic framework compound is formed on the surface of the first intermediate to obtain the second intermediate;

[0097] S106. The phenylboronic acid group is grafted onto the surface of the second intermediate to obtain a magnetic composite material.

[0098] The preparation method of this invention achieves controllable and precise integration of functional layers at the nanoscale through a stepwise construction strategy, exhibiting significant process advantages. This method first grows a bis-lanthanide MOF shell in situ on the surface of magnetic nanoparticles, then constructs a COF second shell on the MOF surface through a covalent condensation reaction, and finally grafts phenylboronic acid recognition groups onto the COF surface under mild conditions via an activation-amidation reaction. This process design ensures a strong bond and clear interface between the magnetic core, MOF signal layer, COF support layer, and phenylboronic acid recognition sites, with uniform distribution of each functional component. The entire process exhibits high reproducibility and controllability, enabling the stable preparation of composite materials possessing high specific surface area, abundant ordered pores, high-density specific recognition sites, excellent fluorescence / electrochemical response characteristics, and rapid magnetic separation performance, providing a reliable material basis for the highly sensitive and selective detection of flavonoid compounds.

[0099] Preferably, step S102 includes: dispersing magnetic nanoparticles, a first lanthanide metal salt, a second lanthanide metal salt, and a first organic ligand in a mixed aqueous solution of N,N-dimethylformamide and ethanol, reacting in a high-pressure reactor at 100°C to 150°C for 12 to 72 hours, and after the reaction is completed and allowed to cool naturally to room temperature, separating them by adsorption using an external magnet, and then washing with deionized water and ethanol to obtain the first intermediate;

[0100] The molar ratio of the first lanthanide metal salt, the second lanthanide metal salt and the first organic ligand is (0.5~5):(0.5~5):1, and the molar ratio of the magnetic nanoparticles to the first organic ligand is (0.5~5) g:1 mmol.

[0101] The volume ratio of N,N-dimethylformamide (DMF), ethanol and water in the mixed aqueous solution is (1~3):(1~3):1, and the amount of mixed aqueous solution corresponding to each gram of magnetic nanoparticles is 100mL~200mL.

[0102] The detailed process parameters for step S102 have been optimized to ensure the in-situ, uniform growth of a high-quality lanthanide MOF shell on the surface of magnetic nanoparticles. High-pressure reactors and solvothermal methods are common and effective methods for synthesizing crystalline MOFs. The high-pressure environment raises the boiling point of the solvent, allowing the reaction to proceed at higher temperatures, promoting the deprotonation, coordination, and crystal growth processes of metal ions and organic ligands, which is beneficial for forming MOFs with good crystallinity and stable structure.

[0103] Setting the reaction temperature within the range of 100℃ to 150℃ provides sufficient energy for the coordination reaction between metal ions and organic ligands. Within this temperature range, the deprotonation of the carboxyl groups of the organic ligands is facilitated, promoting the dynamic formation and recombination of coordination bonds, thereby guiding the formation of long-range ordered and structurally stable MOF crystals. Simultaneously, this temperature range is well-matched with the N,N-dimethylformamide (DMF) solvent system, maintaining suitable reaction kinetics, avoiding both slow reaction and poor crystallinity caused by excessively low temperatures, and preventing side reactions that may be triggered by excessively high temperatures.

[0104] The reaction time was controlled between 12 and 72 hours to ensure sufficient nucleation and growth of the MOF crystal to the desired thickness. A longer reaction time facilitates the repair of crystal defects and structural refinement, resulting in a MOF shell with a higher specific surface area and more regular pores.

[0105] The solvent system uses a mixture of DMF, ethanol, and water in a volume ratio controlled at (1~3):(1~3):1, forming an ideal reaction medium. DMF, as the main solvent, effectively dissolves organic ligands and metal salts. The addition of ethanol helps to adjust the polarity and viscosity of the system, positively affecting the morphology of MOF crystals. Maintaining a certain proportion of water is crucial, as it promotes the key step of ligand deprotonation without excessively diluting the system or causing instability in the MOF framework.

[0106] The molar ratio of the first and second lanthanide metal salts to the first organic ligand is maintained at (0.5~5):(0.5~5):1. This ratio ensures that the metal ions have a sufficient amount to fully coordinate with the ligand, which is beneficial for the formation of a structurally saturated bimetallic MOF. At the same time, the ratio of magnetic nanoparticles to the first organic ligand is controlled at (0.5~5) g:1 mmol. This ratio optimizes the relationship between the reactant concentration and the available substrate surface area, which can effectively promote heterogeneous nucleation and growth of MOF on the magnetic core surface and inhibit homogeneous nucleation of free MOF crystals, thereby obtaining a core-shell structure with high coverage and uniformity.

[0107] Using 100 to 200 ml of mixed solvent per gram of magnetic nanoparticles provides ample space for the liquid-phase reaction. Sufficient solvent volume facilitates the full dispersion of magnetic nanoparticles and prevents their aggregation during the reaction, which is an important physical guarantee for obtaining a uniform MOF coating.

[0108] Preferably, step S104 includes: ultrasonically dispersing the first intermediate obtained in step S102 in N,N-dimethylformamide, then adding the second organic ligand, the third organic ligand and acetic acid solution, mixing evenly and performing 3 to 5 freeze-thaw degassing cycles, then reacting under sealed conditions at 100°C to 150°C for 48 to 96 hours, cooling to room temperature, collecting the product and washing it with tetrahydrofuran, then soaking it in acetone for 48 to 96 hours, and then drying it to obtain the second intermediate;

[0109] The ratio of the first intermediate to N,N-dimethylformamide is 1 g: (20~50) mL, the mass ratio of the first intermediate, the second organic ligand and the third organic ligand is 1: (1~5): (0.5~5), the volume ratio of N,N-dimethylformamide to acetic acid solution is (2~5): 1, and the concentration of acetic acid solution is 2 mol / L~5 mol / L.

[0110] In step S104, the preferred ranges of each process parameter are intended to work synergistically to efficiently and controllably grow a COF shell with high crystallinity, ordered structure, and strong bond with the underlying layer on the surface of the constructed Fe3O4@doubleLn-MOF first intermediate.

[0111] Ultrasonic dispersion of the first intermediate in DMF is a crucial prerequisite for achieving subsequent uniform coating. This operation effectively deagglomerates particle clusters, forming a uniformly dispersed suspension system, thus creating conditions for the uniform adsorption and reaction of COF monomers on the MOF surface.

[0112] Performing three to five freeze-thaw degassing cycles is a crucial step in ensuring high-quality COF synthesis. This operation completely removes oxygen from the reaction system, creating an inert atmosphere for the amine-aldehyde condensation reaction, which is conducive to the formation of a COF framework with fewer defects and higher crystallinity through imine bond linkages.

[0113] Maintaining the reaction temperature between 100℃ and 150℃ provides suitable thermodynamic conditions for this type of amine-aldehyde condensation reaction. Within this temperature range, sufficient energy is provided to drive the efficient formation of imine bonds while maintaining the stability of the reaction system and avoiding excessive solvent evaporation or unnecessary side reactions.

[0114] A reaction time of 48 to 96 hours was set to ensure sufficient growth and self-assembly of the COF covalent framework. This timeframe allows covalent bonds to connect in an orderly manner and gradually refine its long-range ordered structure, thereby forming a stable COF shell with high specific surface area and well-developed pores.

[0115] Using an acetic acid solution with a concentration of 2 mol / L to 5 mol / L as a catalyst, and controlling its volume ratio with DMF to be (2~5):1, a moderate and uniform acidic environment can be provided for the reaction system. This condition can effectively catalyze the condensation of aldehyde and amino groups, and can also dynamically adjust network assembly through reversible reactions, leading to the formation of thermodynamically more stable crystalline products.

[0116] By controlling the mass ratio of the first intermediate, the second organic ligand, and the third organic ligand at 1:(1~5):(0.5~5), sufficient COF monomers were ensured to react on the MOF surface. This ratio range is conducive to the formation of a complete and appropriately thick COF coating layer, while also taking into account the stoichiometric balance between the two monomers, promoting the reaction to tend towards completion.

[0117] Cleaning with tetrahydrofuran after the reaction effectively removes residual monomers, oligomers, and DMF molecules from the pores. Subsequent immersion in acetone for 48 to 96 hours is a gentle solvent exchange process. Utilizing the low surface tension of acetone, the high-boiling-point solvent deep within the pores can be displaced, thus better preserving the porous structure and high specific surface area of ​​the COF material during subsequent drying.

[0118] Preferably, step S106 includes: dispersing the second intermediate obtained in step S104 in N,N-dimethylformamide, then adding 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, and then stirring for 10 to 30 minutes to activate the carboxyl group. After the carboxyl group is activated, 3-aminophenylboronic acid dispersed in N,N-dimethylformamide is added dropwise and the reaction is stirred for 8 to 24 hours. The separated solid is washed sequentially with N,N-dimethylformamide, ethanol and water, and then vacuum dried to obtain the magnetic composite material.

[0119] The ratio of the second intermediate to dispersed N,N-dimethylformamide is 1 g: 30 mL to 60 mL, the mass ratio of the second intermediate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide is 1:(0.1~3):(0.1~3), and the mass ratio of the second intermediate to 3-aminophenylboronic acid is 1:(0.1~5).

[0120] The process design of step S106 aims to covalently graft phenylboronic acid recognition groups onto the COF shell surface through an efficient and mild amidation reaction, thereby endowing the composite material with the ability to specifically recognize flavonoid compounds. The optimized range of each parameter together ensures the high efficiency, completeness, and controllability of the grafting reaction.

[0121] Proper dispersion of the second intermediate in DMF is fundamental to ensuring the uniformity of subsequent reactions. Maintaining a ratio of the second intermediate to DMF of 1 gram to 30 to 60 milliliters creates a suitable suspension system, providing both a sufficient liquid environment for the reaction and adequate exposure of the active sites.

[0122] Carboxyl activation is a crucial step. Carboxyl groups themselves have low reactivity and are difficult to react efficiently directly with amino groups. Therefore, a synergistic system of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) is required to activate the carboxyl groups first. EDC is a carbodiimide condensing agent that first reacts with the carboxyl groups to generate a highly reactive O-acylisourea intermediate. However, this intermediate is unstable and prone to hydrolysis or rearrangement. Upon addition of NHS, this intermediate reacts with NHS to generate a more stable and highly reactive NHS ester (succinimide ester). Maintaining a mass ratio of the second intermediate, EDC, and NHS of 1:(0.1~3):(0.1~3) provides sufficient and appropriate amounts of condensing agent to effectively activate the carboxyl groups on the COF surface, converting them into the highly reactive NHS ester intermediate. Setting an activation time of 10 to 30 minutes provides ample time for this conversion process, which is beneficial for the formation of stable active reaction sites.

[0123] After activation, 3-aminophenylboronic acid was added for grafting. 3-Aminophenylboronic acid was chosen over other phenylboronic acid derivatives because its amino group is the direct functional group for amidation, and the phenylboronic acid moiety has a simple structure and low steric hindrance, which is beneficial for its binding with flavonoid molecules. The mass ratio of the second intermediate to 3-aminophenylboronic acid was controlled at 1:(0.1~5). This ratio matches the number of carboxyl groups that can be grafted onto the COF surface, ensuring sufficient phenylboronic acid molecules participate in the reaction to achieve a high grafting density while avoiding waste and purification burden caused by excess reactants. A stirring reaction time of 8 to 24 hours provided sufficient progress for this type of nucleophilic substitution reaction to form amide bonds, promoting the complete grafting reaction.

[0124] After the reaction, the mixture was washed sequentially with DMF, ethanol, and water to effectively remove residual solvent, unreacted raw materials, and byproducts. The final vacuum drying process removed the solvent while yielding a structurally complete and functionally defined magnetic composite material.

[0125] Thirdly, the present invention provides a method for detecting flavonoid compounds, comprising: dispersing the magnetic composite material of the first aspect or the magnetic composite material prepared by the preparation method of the second aspect in a solvent to form a composite material dispersion; mixing the composite material dispersion with a test solution containing flavonoid compounds to form a detection mixture; placing the detection mixture under an ultraviolet lamp and performing fluorescence measurement, and detecting flavonoid compounds by means of fluorescence response intensity.

[0126] Fourthly, the present invention provides a method for detecting flavonoid compounds, comprising: modifying the surface of a working electrode with the magnetic composite material of the first aspect or the magnetic composite material prepared by the preparation method of the second aspect to form a modified electrode; placing the modified electrode in a test solution containing flavonoid compounds for electrochemical measurement; and detecting flavonoid compounds by measuring the characteristic redox current signal of the flavonoid compounds.

[0127] This invention provides two detection methods based on composite materials with the same function, namely fluorescence method and electrochemical method, forming a complementary dual-mode detection strategy, which effectively utilizes the specific enrichment and separation of target flavonoids by this composite material.

[0128] In fluorescence detection mode, without being limited by principle, the mechanism may be as follows: First, the composite material dispersion is mixed with the sample, and phenylboronic acid sites specifically capture flavonoid molecules. Under excitation at a specific wavelength (such as ultraviolet light), the composite material itself (especially the lanthanide MOF shell) may produce background fluorescence. When flavonoid molecules are captured and approach the MOF shell, one or more of the following processes may occur, leading to changes in fluorescence signal: (1) Flavonoid molecules absorb the excitation light energy, and their excited state energy is transferred to lanthanide ions (such as Tb) in the MOF through resonance energy transfer (such as FRET).3+ / Eu 3+ (1) Enhance the characteristic fluorescence of lanthanide ions; (2) Flavonoid molecules themselves can also produce fluorescence under specific conditions, and their intensity is proportional to the amount of enrichment. By measuring the intensity change (enhancement or quenching) at a specific wavelength in the fluorescence spectrum and comparing it with the standard curve, quantitative analysis of flavonoids can be achieved. The advantages of fluorescence method are high sensitivity, relatively simple operation, and the ability to achieve visual screening (observing color or brightness changes under ultraviolet light).

[0129] In electrochemical detection mode, the core is to fix the composite material onto the surface of the working electrode (such as a glassy carbon electrode) to make a modified electrode. The fixing method can be drop-coating and drying, or using film-forming agents such as Nafion. When the modified electrode is immersed in a solution containing flavonoids, the composite material on the electrode surface will enrich flavonoid molecules from the solution, making its local concentration much higher than that of the bulk solution. Then, electrochemical scanning (such as cyclic voltammetry CV, differential pulse voltammetry DPV) is performed. Flavonoid compounds, especially flavonoids containing ortho-diphenol hydroxyl groups, will undergo reversible redox reactions on the electrode surface (phenolic hydroxyl groups are oxidized to quinones). Without being limited by the principle, this composite material plays multiple roles in electrochemical detection: (1) The magnetic nanoparticle core and the conjugated framework of COF provide an excellent electronic conduction network, which significantly reduces the charge transfer resistance of the redox reaction of flavonoid molecules, thereby enhancing the oxidation peak current (ipa) and the oxidation potential (Epa) may shift negatively (i.e., the reaction is more likely to occur). (2) The high specific surface area of ​​lanthanide MOFs and COFs allows for the fixation of a large number of recognition sites (phenylboronic acid) on the electrode surface, enabling the enrichment of more flavonoid molecules to the electrode interface and further amplifying the current signal. (3) The specific recognition of phenylboronic acid ensures that the enriched target is flavonoids, rather than other electroactive interfering substances (such as ascorbic acid, uric acid, etc.), thus improving selectivity. Quantitative detection can be achieved by measuring the peak current of the characteristic oxidation peak of flavonoids and establishing a standard curve with the concentration. The advantages of electrochemical methods are that the instruments are relatively inexpensive and portable, have a fast response speed, and are easy to miniaturize and integrate.

[0130] A significant advantage of dual-mode detection lies in the mutual verification and complementarity of results. For the same sample, if two detection methods based on different physical principles (optical and electrical) can yield consistent results, the reliability and accuracy of the detection results are greatly improved. Furthermore, the two methods may be suitable for different application scenarios: fluorescence methods are more suitable for precise laboratory analysis and visual on-site screening; electrochemical methods may be more suitable for developing portable or online detection devices. The sharing of a single pretreatment material and process between the two modes also improves the versatility and cost-effectiveness of the method.

[0131] Fifthly, the present invention provides the application of the magnetic composite material of the first aspect or the magnetic composite material prepared by the preparation method of the second aspect in the preparation of flavonoid compound standard substances.

[0132] As the foundation of analytical measurements, the purity, accuracy, and homogeneity of standard reference materials are crucial. The core of this invention's application of the magnetic composite material in the development of flavonoid standard reference materials lies in leveraging its high selectivity, high enrichment efficiency, and convenient separation characteristics to achieve efficient, purified capture, and accurate quantification of target flavonoids in complex matrices. Specifically, in the extraction and purification stage of candidate standard reference materials, this composite material can selectively capture target flavonoid molecules from complex samples such as plant extracts and fermentation broths through the specific covalent binding of phenylboronic acid and the o-dihydroxyl groups of flavonoids. Rapid separation and washing are achieved using the magnetic core, efficiently removing a large number of matrix interfering substances such as sugars, proteins, and pigments, significantly improving the purity of the candidates. In the homogeneity assessment and value determination stage, the fluorescence and electrochemical dual-mode detection method based on this composite material, due to its high sensitivity, strong anti-interference ability, and good repeatability, can be used for precise and accurate content determination and homogeneity testing of purified flavonoid standard reference material candidates, ensuring the accuracy and reliability of value transfer. Therefore, the magnetic composite material provided by this invention is not only a high-performance detection tool, but its excellent selectivity, separation ability and quantitative performance also make it an ideal core material for developing high-purity and high-accuracy flavonoid compound standard substances, providing technical support for the verification and quality control of related food, pharmaceutical and agricultural product analysis methods.

[0133] To clarify the technical solution and advantages of this invention, the invention will be further described in detail with reference to implementation examples. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0134] Example

[0135] Example 1:

[0136] Step 1: Preparation of Fe3O4 magnetic nanoparticles:

[0137] 5g of FeCl3·6H2O, 12g of sodium acetate, and 3g of polyethylene glycol were weighed and dissolved sequentially in 100ml of ethylene glycol. The mixture was ultrasonically treated and stirred for 30min at room temperature. The homogeneous solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor and reacted at 200℃ for 8h. After the reaction, the obtained Fe3O4 nanoparticles were collected by adsorption using an external magnet. After being washed repeatedly with deionized water and ethanol, they were dried in a vacuum drying oven for later use.

[0138] Step 2: Preparation of Fe3O4@Tb / Eu-MOF first intermediate:

[0139] 1 mmol of Tb(NO3)3·6H2O, 1 mmol of Eu(NO3)3·6H2O, 1 mmol of 2-aminoterephthalic acid, and 0.5 g of Fe3O4 nanoparticles prepared in step 1 were dispersed in a mixed solvent consisting of 40 mL DMF, 20 mL ethanol, and 20 mL deionized water, and ultrasonicated to ensure uniform dispersion. The mixture was transferred to a high-pressure reactor, sealed, and heated in a vacuum oven at 120 °C for 48 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. After opening the high-pressure reactor, the product was collected by centrifugation and washed thoroughly with DMF and ethanol sequentially, with three parallel washes performed. Finally, the product Fe3O4@Tb / Eu-MOF was obtained by drying.

[0140] Step 3: Preparation of Fe3O4@Tb / Eu-MOF@COF second intermediate:

[0141] 45 mg of Fe3O4@Tb / Eu-MOF obtained in step 2 was ultrasonically dispersed in a glass tube containing 1.5 mL of DMF. Then, 1,3,5-tricarboxymethyl phloroglucinol (Tp, 48 mg) and 2,5-diphenylaminoterephthalic acid (Dhta, 32 mg) were added, and the mixture was ultrasonicated for another 30 min to ensure homogeneity. 0.5 mL of 3 mol / L acetic acid solution was added, and the mixture was thoroughly mixed before undergoing three freeze-thaw degassing cycles. The glass tube was sealed and placed in an oven at 120 °C for 72 h. After the reaction, the mixture was cooled to room temperature, and the product was collected by centrifugation and washed with tetrahydrofuran to remove unreacted monomers. The product was then immersed in acetone for 72 h for solvent exchange. Finally, it was vacuum dried at 120 °C for 12 h to obtain Fe3O4@Tb / Eu-MOF@COF.

[0142] Step 4: Preparation of the final product Fe3O4@Tb / Eu-MOF@COF-BA functionalized with phenylboronic acid:

[0143] 0.5 g of Fe3O4@Tb / Eu-MOF@COF obtained in step 3, 115 mg of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), and 69 mg of N-hydroxysuccinimide (NHS) were dispersed in 15 mL of DMF and sonicated to form a homogeneous suspension. The suspension was then mechanically stirred at room temperature for 15 min to activate the carboxyl groups on the COF surface. Separately, 50 mg of 3-aminophenylboronic acid (APBA) was dissolved in 15 mL of DMF. The APBA solution was added dropwise to the activated suspension, and the mixture was stirred at room temperature in the dark for 12 h. After the reaction was complete, the solid product was separated using an external magnet and washed sequentially with DMF, ethanol, and deionized water. The product was then vacuum dried to obtain the final product Fe3O4@Tb / Eu-MOF@COF-BA.

[0144] Example 2:

[0145] Referring to Example 1, the difference is that,

[0146] In step 1, the amount of FeCl3·6H2O was changed to 2g, the amount of sodium acetate was changed to 10g, and the reaction temperature was changed to 180℃.

[0147] In step 2, the amount of Tb(NO3)3·6H2O was changed to 2 mmol, the amount of Eu(NO3)3·6H2O was changed to 2 mmol, the amount of Fe3O4 nanoparticles was changed to 1 g, the amount of DMF was changed to 60 mL, and the reaction temperature was changed to 150℃.

[0148] In step 3, the amount of Fe3O4@Tb / Eu-MOF was changed to 60 mg, the amount of 1,3,5-tricarboxymethyl phloroglucinol (Tp) was changed to 100 mg, the amount of 2,5-diphenylaminoterephthalic acid (Dhta) was changed to 100 mg, the amount of acetic acid solution was changed to 1 mL, and the reaction temperature was changed to 100℃.

[0149] In step 4, the amount of Fe3O4@Tb / Eu-MOF@COF is changed to 1.0g, the amount of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) is changed to 500mg, the amount of N-hydroxysuccinimide (NHS) is changed to 100mg, the activation stirring time is changed to 30min, and the amount of 3-aminophenylboronic acid (APBA) is changed to 100mg.

[0150] The other steps and conditions are exactly the same as in Example 1.

[0151] Example 3:

[0152] Referring to Example 1, the difference is that in step 2, the lanthanide metal salts are replaced with Tb(NO3)3·6H2O (1 mmol) and Sm(NO3)3·6H2O (1 mmol), while the other steps and conditions are exactly the same as in Example 1.

[0153] Example 4:

[0154] Referring to Example 1, the difference is that in step 2, the first organic ligand is replaced with 2,5-diaminoterephthalic acid (1 mmol), while the other steps and conditions are exactly the same as in Example 1.

[0155] Example 5:

[0156] Referring to Example 1, the difference is that in step 3, the third organic ligand is replaced with 2-aminoterephthalic acid (32 mg), while the other steps and conditions are exactly the same as in Example 1.

[0157] Example 6:

[0158] Referring to Example 1, the difference is that in step 1, cobalt ferrite (CoFe2O4) nanoparticles with an average particle size of approximately 150 nm are prepared using a co-precipitation method instead of Fe3O4. All other steps and conditions are exactly the same as in Example 1.

[0159] Example 7:

[0160] Referring to Example 1, the difference is that in step 1, Fe3O4 nanoparticles with an average particle size of approximately 450 nm were prepared by adjusting the hydrothermal reaction parameters (increasing the amount of sodium acetate). All other steps and conditions were exactly the same as in Example 1.

[0161] Comparative Example

[0162] Comparative Example 1:

[0163] Referring to Example 1, the difference is that step 4 is omitted. That is, phenylboronic acid grafting is not performed, and Fe3O4@Tb / Eu-MOF@COF obtained in step 3 of Example 1 is directly used as the comparative material.

[0164] Comparative Example 2:

[0165] Referring to Example 1, the difference is that in step 2, only Eu(NO3)3·6H2O (2 mmol) is used, and Tb(NO3)3 is not used.

[0166] Comparative Example 3:

[0167] Referring to Example 1, the difference is that in step 4, equimolar amounts of 3-aminobenzoic acid are used instead of 3-aminophenylboronic acid for grafting. This group cannot form a specific borate ester bond with the ortho- and tho-dihydroxyl groups of flavonoids.

[0168] Test case

[0169] 1. Morphological and structural characterization:

[0170] The average particle size of the magnetic core and the average thickness of the first shell (double lanthanide MOF) and the second shell (COF) were measured and statistically analyzed using high-resolution TEM images. The specific surface area of ​​the composite material was determined by the BET method using a nitrogen adsorption-desorption apparatus, and its pore size distribution was analyzed using the BJH model to obtain the average pore size of the porous structure of the first and second shells.

[0171] 2. Detection of quercetin by fluorescence method (QCT):

[0172] Weigh 5.0 mg of the composite materials from each example and comparative example, disperse them in 10 mL of deionized water, and prepare a 0.5 mg / mL stock solution. Sonicate the solution for later use. Take 100 μL of the stock solution, add 50 μL of quercetin standard solutions of different concentrations, and dilute to 1 mL with deionized water. Mix well and let stand for 10 min for enrichment. Using a fluorescence spectrophotometer, measure the fluorescence emission intensity (F) of the solution at 445 nm at the optimal excitation wavelength. The fluorescence intensity of the sample without quercetin is defined as F0. The ratio of (F-F0) / F0 to the quercetin concentration (C) is calculated. QCT Plot the graphs to establish a standard curve, and calculate the linear range, limit of detection (LOD, signal-to-noise ratio S / N=3), and limit of quantitation (LOQ, signal-to-noise ratio S / N=10) of the method.

[0173] 3. Electrochemical detection of quercetin (QCT):

[0174] The glassy carbon electrode (GCE) was polished to a mirror finish using 0.3 μm and 0.05 μm Al₂O₃ suspensions, respectively, and then ultrasonically cleaned in water and ethanol before air-drying. 2 mg of MXene and 2 mg of the test composite material were ultrasonically dispersed in 1 mL of isopropanol / water (v:v=1:1) mixed solvent, with 10 μL of 0.5% Nafion solution added to each. First, 3 μL of the MXene dispersion was drop-coated onto the clean GCE surface and dried under an infrared lamp; then, 3 μL of the composite material dispersion was drop-coated and dried to obtain the modified electrode (MXene / composite material / GCE). Using this electrode as the working electrode, differential pulse voltammetry (DPV) was used to measure the oxidation peak current (I₀) of quercetin at approximately 0.12 V in PBS buffer (pH=7.4) containing different concentrations of quercetin. pa ). Take I pa For C QCT Plot the graph, establish a standard curve, and calculate the linear range, sensitivity (i.e., the slope of the standard curve), and limit of detection (LOD) of the method.

[0175] The examples and comparative examples were tested according to the methods described above, and the data obtained are summarized in Table 1.

[0176] Table 1

[0177]

[0178] Based on the experimental data in Table 1, the magnetic composite materials prepared in Examples 1 to 7 all exhibited excellent overall performance in the fluorescence and electrochemical detection of quercetin (QCT), with low detection limits, high sensitivity, and wide linear range, significantly outperforming all comparative examples. This indicates that the fully functional core-shell structure “Fe3O4@Tb / Eu-MOF@COF-BA” constructed in this invention is successful and efficient.

[0179] Comparing the data from Example 1 and Comparative Example 2, it can be found that using a dual lanthanide metal (Tb) 3+ / Eu 3+ Constructing a MOF shell using a single lanthanide metal (Comparative Example 2, single Eu) 3+ This significantly improves the detection performance of the material. The fluorescence detection limit and electrochemical sensitivity of Example 1 were significantly better than those of Comparative Example 2, confirming the crucial role of bimetallic synergy in optimizing pores, enhancing fluorescence energy transfer, and promoting electrochemical electron transport.

[0180] By comparing Example 1 with Comparative Example 1 (without phenylboronic acid) and Comparative Example 3 (with non-specific groups), the important role of the specific recognition group of phenylboronic acid can be clearly demonstrated. Comparative Examples 1 and 3, lacking specific interactions, exhibited severely degraded performance, with significantly increased detection limits, narrow linear ranges, and high initial concentrations. This proves that the formation of specific borate ester bonds between phenylboronic acid and the o-dihydroxyl groups of flavonoids is the molecular basis for achieving highly selective and sensitive trace analysis.

[0181] Furthermore, Examples 2 to 7 demonstrate that the technical solution of the present invention has good feasibility and fault tolerance by reasonably adjusting the type and size of the magnetic core, MOF metal pairs, organic ligands or COF monomers within the preferred parameter range. The preferred range of the present invention can effectively guide those skilled in the art to prepare high-performance detection materials.

[0182] In summary, the data in Table 1 demonstrate that by integrating magnetic nanoparticles, lanthanide metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and phenylboronic acid recognition groups in a specific core-shell structure, this invention generates a significant synergistic enhancement effect, thereby successfully preparing a magnetic composite material with high selectivity, high sensitivity, wide linear range, and rapid separation capability, effectively solving the problem of detecting flavonoid compounds in complex matrices.

[0183] To further demonstrate the technical effectiveness of the present invention, the following analysis will be conducted in conjunction with specific experimental test results and images.

[0184] Figure 1 The image shows a scanning electron microscope (SEM) image of Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1. The image shows that the magnetic nanoparticles prepared by this method have a particle size of approximately 400 nm and a relatively uniform spherical structure. Simultaneously, the nanomaterials have porous MOF and COF coatings on their surface, indicating that the nanomaterials can form a stable composite structure. Their layered porous structure further ensures the subsequent adsorption effect of quercetin and its fluorescence and electron transport efficiency, laying the foundation for high-accuracy fluorescence and electrochemical detection of quercetin in the later stages.

[0185] Figure 2This is a transmission electron microscope (TEM) image of the Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1. The image clearly shows a typical core-shell structure: darker-colored spherical magnetic Fe3O4 nanoparticles form the core, with a uniformly light-colored, uniformly thick double-shell structure covering its surface. The inner shell (first shell) is a bis-lanthanide MOF, and the outer shell (second shell) is a COF. The interface between the two is clear and tightly bonded, further confirming that the preparation method of this invention successfully constructed a stable Fe3O4@bis-Ln-MOF@COF-BA composite structure. This TEM image visually demonstrates the ordered integration of each functional layer at the nanoscale. The uniform and complete core-shell structure and porous features provide efficient diffusion pathways and abundant contact interfaces for flavonoid compounds, which is an important structural basis for achieving high-capacity enrichment and high-sensitivity detection of the material.

[0186] Figure 3 The X-ray photoelectron spectroscopy (XPS) spectra of Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 are shown. The characteristic peaks of Tb3d, Eu3d, O1s, N1s, and C1s are clearly visible in the spectra, confirming the presence of key elements (Tb, Eu, O, N, C) in the composite material and verifying the successful composite of the bis-lanthanide MOF and COF at the elemental composition level. Compared with Fe3O4@Tb / Eu-MOF, the peak intensities of O1s, N1s, and C1s in the Fe3O4@Tb / Eu-MOF@COF-BA spectrum are significantly enhanced, reflecting the successful introduction of the COF shell and phenylboronic acid groups.

[0187] Figure 4 The high-resolution XPS spectra of C1s of Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 are shown. The binding energies of 284.80 eV, 286.01 eV, 286.65 eV, and 288.69 eV correspond to CC / CH / C=C (the skeletal carbon from the benzene ring of the MOF, COF, and phenylboronic acid ligand), CO (the phenolic hydroxyl group of the Tp monomer in the COF), and C, respectively, in the nanomaterials. N / C=N (CN in MOF ligands and amide bonds and imine bonds in COF backbone), C=O (carboxylate groups in MOF and COF), and the introduction of COF and phenylboronic acid, which added a large number of benzene rings and carboxylate groups, enhanced the intensity of the CC / CH / C=C and C=O peaks.

[0188] Figure 5The high-resolution XPS spectra of O1s for Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 are shown. The peaks at 529.98 eV, 531.67 eV, and 534.75 eV are assigned to Tb-O / Eu-O / BO, C=O, and adsorbed water, respectively. A characteristic peak at 532.83 eV, belonging to the phenolic hydroxyl group (C–OH) in COF, appears, further proving that the COF layer has been successfully coated on the MOF surface.

[0189] Figure 6 High-resolution XPS spectra of Tb3d for Fe3O4@Tb / Eu-MOF and Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1. Tb3d 5 / 2 and Tb3d 3 / 2 Each region contains multiple peaks, corresponding to Tb-O, Tb-Tb, and Tb, respectively. 3+ The core-shell structure was confirmed to have a core-shell structure at the chemical state level, and the peak intensity decreased compared to pure MOF. This was because the outer COF coating had a shielding effect on photoelectrons.

[0190] Figure 7 The fluorescence response of Fe3O4@Tb / Eu-MOF@COF-BA prepared in Example 1 to different concentrations of quercetin is shown in the figure. The figure shows that increasing the concentration of QTC leads to an increase in the fluorescence intensity of the nanocomposite material (λ is the maximum value at 445 nm). A good linear relationship exists between the quercetin concentration and the fluorescence analysis signal in the concentration ranges of 0.1–0.8 μM and 1–65 μM, respectively. In the 0.1–0.8 μM range, the linear fitting equation is: (F-F0) / F0 = 0.07895 + 0.0498C QCT (R) 2 =0.9942), corresponding to a detection limit (LOD) of 0.096µM (SNR S / N = 3.3). In the range of 1–65µM, the linear fitting equation is: (F - F0) / F0 = 0.12979 + 0.00682C QCT (R) 2 =0.9935), corresponding to a limit of quantitation (LOQ) of 1.68 µM (signal-to-noise ratio S / N = 10). The results indicate that the material exhibits excellent fluorescence response to different concentrations of quercetin, and the method has a low detection limit, making it suitable for fluorescence detection of samples with varying concentrations.

[0191] Figure 8 The differential pulse voltammetry (DPV) response of the Fe3O4@Tb / Eu-MOF@COF-BA modified electrode prepared in Example 1 to different concentrations of quercetin is shown in the figure. Quercetin is oxidized at 0.12 V, and it is observed that I increases with increasing quercetin concentration.pa The current response gradually increases (0.3→30μA). For example... Figure 8 The peak DPV response increased with increasing quercetin concentration from 0.3 to 30 μM. Furthermore, a good linear relationship was observed between the oxidation peak current and the quercetin concentration. The linear equation can be expressed as I... pa (μA) = 0.04531C QTC (μM) -0.02361 (R) 2 =0.9987, 1μM≤C≤30μM) and I pa (μA) = 0.0509C QTC (μM) -0.01076 (R) 2 =0.9954, 0.3μM≤C≤1μM). The detection limit can be calculated using the equation LOD=3.3σ / S, where σ represents noise (standard deviation) and S represents sensitivity (slope of the calibration curve). The LOD of quercetin on the Fe3O4@MOF@COF-BA / MXene / GCE modified electrode is calculated to be 0.26μM, with a sensitivity as high as 0.04531μA / μM and a linear range of 0.3μM≤C≤30μM.

[0192] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A magnetic composite material for detecting flavonoid compounds, characterized in that, It includes a core, a first shell covering the surface of the core, a second shell covering the surface of the first shell, and phenylboronic acid groups grafted onto the surface of the second shell; The core is a magnetic nanoparticle; The first shell is a bis-lanthanide metal-organic framework compound, which contains two different lanthanide metal ions, and the first organic ligand of the bis-lanthanide metal-organic framework compound is an aromatic compound containing both an amino group and at least two carboxyl groups. The two different lanthanide metal ions are selected from any two of terbium, europium, samarium, dysprosium, cerium, and gadolinium; The second shell is a covalent organic framework compound, which is formed by a second organic ligand and a third organic ligand connected by covalent bonds. The second organic ligand is a polyaldehyde compound or a polyhydrazine compound, and the third organic ligand is an aromatic compound containing both amino and carboxyl groups.

2. The magnetic composite material according to claim 1, characterized in that, The first organic ligand is selected from at least one of 2-aminoterephthalic acid and 2,5-diaminoterephthalic acid; And / or, the second organic ligand is selected from at least one of 1,3,5-tricarboxyloylphloroglucinol, pyromellitic pyroxenaldehyde, and 1,4-dihydrazidobenzene, and the third organic ligand is selected from at least one of 2,5-diphenylaminoterephthalic acid, 2-aminoterephthalic acid, and 3,5-diaminobenzoic acid; And / or, the magnetic nanoparticles are selected from at least one of iron(II,III) oxide, cobalt ferrite, nickel ferrite, and manganese ferrite.

3. The magnetic composite material according to claim 2, characterized in that, The two different lanthanide metal ions are terbium and europium; And / or, the first organic ligand is 2-aminoterephthalic acid; And / or, the second organic ligand is 1,3,5-tricarboxyloyl-resorcinol, and the third organic ligand is 2,5-diphenylaminoterephthalic acid; And / or, the magnetic nanoparticles are iron(III) oxide nanoparticles.

4. The magnetic composite material according to claim 1, characterized in that, The average particle size of the core is 100nm~500nm; And / or, the average thickness of the first shell is 10nm~50nm, and the average thickness of the second shell is 10nm~100nm; And / or, both the first shell and the second shell have a porous structure, and the average pore size of the porous structure is 5nm~40nm.

5. A method for preparing a magnetic composite material as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S102. Provide magnetic nanoparticles and form a first shell of a lanthanide bimetallic organic framework compound on their surface to obtain a first intermediate. S104. A second shell of a covalent organic framework compound is formed on the surface of the first intermediate to obtain the second intermediate; S106. Graft phenylboronic acid groups onto the surface of the second intermediate to obtain the magnetic composite material.

6. The preparation method according to claim 5, characterized in that, Step S102 includes: dispersing magnetic nanoparticles, a first lanthanide metal salt, a second lanthanide metal salt, and a first organic ligand in a mixed aqueous solution of N,N-dimethylformamide and ethanol, reacting in a high-pressure reactor at 100℃~150℃ for 12~72 hours, and after the reaction is completed and allowed to cool naturally to room temperature, separating them by adsorption using an external magnet, and then washing with deionized water and ethanol to obtain the first intermediate; The molar ratio of the first lanthanide metal salt, the second lanthanide metal salt and the first organic ligand is (0.5~5):(0.5~5):1, and the molar ratio of the magnetic nanoparticles to the first organic ligand is (0.5~5) g:1 mmol. The volume ratio of N,N-dimethylformamide, ethanol and water in the mixed aqueous solution is (1~3):(1~3):1, and the amount of mixed aqueous solution corresponding to each gram of magnetic nanoparticles is 100mL~200mL.

7. The preparation method according to claim 5, characterized in that, Step S104 includes: ultrasonically dispersing the first intermediate obtained in step S102 in N,N-dimethylformamide, then adding the second organic ligand, the third organic ligand and acetic acid solution, mixing evenly and performing 3 to 5 freeze-thaw degassing cycles, then reacting under sealed conditions at 100℃ to 150℃ for 48h to 96h, cooling to room temperature, collecting the product and washing it with tetrahydrofuran, then soaking it in acetone for 48h to 96h, and then drying it to obtain the second intermediate; The ratio of the first intermediate to N,N-dimethylformamide is 1 gram to (20-50) milliliters, the mass ratio of the first intermediate, the second organic ligand, and the third organic ligand is 1:(1-5):(0.5-5), the volume ratio of N,N-dimethylformamide to acetic acid solution is (2-5):1, and the concentration of acetic acid solution is 2 mol / L to 5 mol / L.

8. The preparation method according to claim 5, characterized in that, Step S106 includes: dispersing the second intermediate obtained in step S104 in N,N-dimethylformamide, then adding 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, and then stirring for 10 to 30 minutes to activate the carboxyl group. After the carboxyl group is activated, 3-aminophenylboronic acid dispersed in N,N-dimethylformamide is added dropwise and the reaction is stirred for 8 to 24 hours. The separated solid is washed successively with N,N-dimethylformamide, ethanol and water, and then vacuum dried to obtain the magnetic composite material. The ratio of the second intermediate to dispersed N,N-dimethylformamide is 1 g: 30 mL to 60 mL, the mass ratio of the second intermediate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide is 1:(0.1~3):(0.1~3), and the mass ratio of the second intermediate to 3-aminophenylboronic acid is 1:(0.1~5).

9. A method for detecting flavonoid compounds, characterized in that, include: The magnetic composite material prepared by any one of claims 1 to 4 or by any one of claims 5 to 8 is dispersed in a solvent to form a composite material dispersion; the composite material dispersion is mixed with a test solution containing flavonoids to form a detection mixture; the detection mixture is placed under an ultraviolet lamp and fluorescence is measured, and the flavonoids are detected by fluorescence response intensity.

10. A method for detecting flavonoid compounds, characterized in that, include: A modified electrode is formed by modifying the surface of a working electrode with the magnetic composite material as described in any one of claims 1 to 4 or the magnetic composite material prepared by the preparation method described in any one of claims 5 to 8. The modified electrode is placed in a test solution containing flavonoids for electrochemical measurement. The flavonoids are detected by measuring the characteristic redox current signal of the flavonoids.

11. The use of a magnetic composite material according to any one of claims 1 to 4 or a magnetic composite material prepared by any one of claims 5 to 8 in the preparation of flavonoid compound standard substances.

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