Porphyrin-based COF based on double-bridging monomer, aerogel, preparation method and application

Abstract

The invention discloses porphyrinyl COF based on double-bridging monomers, aerogel, a preparation method and application, and belongs to the field of advanced functional materials. According to the invention, Tp-PDA-COF is prepared by using two bridging monomers, namely 2, 4, 6-triformyl phloroglucinol (Tp) and 2, 6-pyridinedicarboxaldehyde (PDA), on the basis of porphyrinyl COFs, and an electron withdrawing group in Tp optimizes the orbital energy level of a metal ligand and promotes electron transfer and formation of a coordinate bond; pDA enables COF to exist in a tetragonal topological structure, so that steric hindrance is reduced, coordination sites are exposed, selectivity and sensitivity to metal ions are improved, and rapid and sensitive visual fluorescence detection is realized; the Tp-PDA-COF aerogel is prepared by further combining polyvinyl alcohol, so that the problem of fluorescence quenching of porphyrin-based COFs in a solid state is solved, and the purpose of developing a COFs material with double functions of sensitive detection and efficient adsorption is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of advanced functional materials, and particularly relates to a porphyrin-based COF based on a bis-bridging monomer, an aerogel, a preparation method and an application, especially an application in the fields of heavy metal fluorescence sensing and cooperative adsorption. Background Art

[0002] Heavy metal pollution has become a major problem worldwide, posing a serious threat to human health and global food production. Developing materials with specific recognition and effective adsorption of heavy metals and dual-functional synergy is of great significance for environmental pollution control and resource recovery economy. It is estimated that the annual global heavy metal wastewater discharge exceeds 5 billion tons, and the recycling market value of only toxic metals such as lead and cadmium reaches 12 billion US dollars per year. However, currently about 87% of the research focuses on single-functional materials (such as only adsorption or only detection), and this functional fragmentation leads to serious bottlenecks in practical applications. Therefore, designing and developing a single material with dual functions of detection and adsorption is an inevitable choice to solve resource constraints, technical bottlenecks and complex application requirements.

[0003] Organic covalent framework materials (COFs) are two-dimensional or three-dimensional crystalline porous polymer materials formed by connecting organic structural units through covalent bonds. Due to their regular pores, high specific surface area, excellent chemical and thermal stability and other characteristics, they show potential application values in many fields such as adsorption, catalysis, separation, etc. In 2005, the team of American chemist Omar Yaghi successfully synthesized crystal materials with a two-dimensional honeycomb structure (such as COF-1 and COF-5) for the first time through borate ester condensation reaction, and this breakthrough filled the gap of pure organic porous materials. Subsequently, after years of development, the breakthrough of single-crystal X-ray diffraction technology realized the precise analysis of the COF structure, laying a foundation for the directional design of pore size and surface functional groups. Compared with other nanomaterials, the building units of COFs are organic molecules, which are widely sourced and diverse in types, making the building units diversified and facilitating the regulation of the structure and function of target materials through the building units. This characteristic makes COF show the potential of multifunctional materials. In particular, COF materials constructed with organic fluorescent molecules as monomers, through flexible monomer selection and corresponding functionalization modifications, not only have excellent adsorption performance and fluorescence visualization performance of COF, but also can have more outstanding selectivity and anti-interference ability.

[0004] Porphyrin is a large heterocyclic compound with bright red fluorescence and is usually used as a building monomer for COF. The COF constructed using porphyrin derivatives with red fluorescence as monomers not only has a larger framework structure but also has more potential coordination sites, greatly improving the application of COF materials in the field of fluorescence sensing. Especially in the field of heavy metals, the unique porous structure endows the porphyrin COF material with excellent adsorption properties. The porphyrin monomer can further bind and stabilize metal ions to achieve the visual detection of heavy metal ions. It should be noted that porphyrin is a highly conjugated system and can exist in coordination with different metal ions. This makes it difficult to specifically identify target heavy metal ions. Therefore, it is extremely challenging to design COF materials with specific recognition of target heavy metal ions by constructing monomers to regulate the structure and function of COF. In addition, due to the compositional heterogeneity and inherent rigidity of COF, some disadvantages are also exposed in practical applications. Porphyrin COF usually exists in the form of insoluble powder, with weak mechanical strength and poor processability. Due to its π-π stacking effect, porphyrin-based COF will undergo fluorescence quenching in the solid state. The above disadvantages limit the application of porphyrin-based COF in the field of analysis and detection.

[0005] Therefore, in order to achieve the dual goals of fluorescence sensing and co-adsorption of heavy metal ions, it is still necessary to construct and prepare new nanomaterials to solve the fluorescence quenching problem of solid COF. As is well known, excessive heavy metal content in the environment will pollute water, soil and food. Especially in the soil, acidic soil makes plants more likely to absorb trace metal elements, posing a threat to human safety from food sources. In the case of severe heavy metal pollution, copper pollution has always been an inevitable problem. Excessive Cu 2+ will seriously affect physical health and even lead to a series of diseases such as Wilson's disease, Alzheimer's disease and Menkes disease. Therefore, sensitive detection and efficient enrichment of heavy metal Cu in the environment 2+ can minimize human exposure. Summary of the Invention

[0006] The present invention provides a porphyrin-based COF and aerogel based on a double-bridged monomer, a preparation method and an application. The present invention uses two bridging monomers, 2,4,6-triformylphloroglucinol (Tp) and 2,6-pyridinedicarboxaldehyde (PDA), to construct a porphyrin-based COF (Tp-PDA-COF) with specific recognition ability for heavy metal ions to solve the limitation of porphyrin-based COF in specific recognition of heavy metals; and further combines polyvinyl alcohol to prepare a Tp-PDA-COF aerogel to solve the problem of fluorescence quenching of porphyrin-based COF in the solid state, and to achieve the purpose of developing a COF material with dual functions of sensitive detection and efficient adsorption.

[0007] To achieve the above object, the technical solution adopted by the present invention is: The present invention first prepared a porphyrin-based COF material with a square topological structure based on a bis-bridging monomer, namely Tp-PDA-COF. The square topological structure formed in Tp-PDA-COF is formed by two monomers, Tp and PDA, in a 1:1 molar ratio. Tp with strong electron-withdrawing ability can adjust the electron cloud density of the porphyrin ring, enhance the supply ability of the lone pair electrons of the N atom, and improve the coordination affinity. At the same time, the electron-withdrawing group in Tp optimizes the orbital energy level of the metal ligand, promoting electron transfer and the formation of coordination bonds. Adding PDA enables the COF to exist in a square topological structure, thereby reducing steric hindrance and fully exposing the coordination sites. This structure improves the selectivity and sensitivity to Cu 2+ and realizes its rapid and sensitive visual fluorescence detection. Tp-PDA-COF not only has Tp with strong electron-withdrawing ability, which can adjust the electron cloud density of the porphyrin ring and optimize the metal-ligand orbitals, thereby enhancing its coordination ability and specific recognition ability; but also fixes the square topological structure of the COF through PDA, reduces steric hindrance, fully exposes the coordination sites, and makes it easier to control the interlayer stacking state of the COF, providing favorable conditions for the preparation of fluorescent COF aerogels. Then, it is further combined with polyvinyl alcohol to obtain the product porphyrin-based COF aerogel (Tp-PDA-COF aerogel).

[0008] The present invention provides a preparation method of a porphyrin-based COF based on a bis-bridging monomer, comprising the following steps: In a pressure-resistant tube, porphyrin and two bridging monomers (2,6-pyridinedicarboxaldehyde and 2,4,6-triformylphloroglucinol) are dispersed in a mixed solvent composed of ortho-dichlorobenzene, n-butanol and acetic acid. After degassing the pressure-resistant tube through three freeze-vacuum-thaw cycles and sealing it, it is heated at 100-150 °C for 2-4 days. The precipitate is collected and purified by washing with tetrahydrofuran; finally, the product is dried overnight under vacuum at 30-50 °C to obtain a purple-black solid powder, namely the porphyrin-based COF based on the bis-bridging monomer, labeled as Tp-PDA-COF. Preferably, the porphyrin is tetrakis(4-aminophenyl)porphyrin (TAPP). TAPP has a symmetric rigid structure and active amino groups and is an ideal monomer for constructing high-performance COF; the molar ratio of 2,6-pyridinedicarboxaldehyde to 2,4,6-triformylphloroglucinol is 1:1, among which; the volume ratio of ortho-dichlorobenzene, n-butanol and 6M acetic acid in the mixed solvent is (1-10):(1-10):(1-2); more preferably 6:6:1.

[0009] The present invention also provides a method for preparing a porphyrin-based COF aerogel based on a double-bridged monomer, comprising the following steps: first, preparing Tp-PDA-COF according to the above content; stirring a polyvinyl alcohol aqueous solution at a temperature of 80-100°C for 2-4 hours to obtain a polyvinyl alcohol hydrogel, adding Tp-PDA-COF thereto and continuously stirring to obtain a uniform solution, and placing it in a freeze dryer for 1-3 days after three freeze-thaw cycles to obtain the desired aerogel.

[0010] With TAPP as the main monomer, a double-bridged porphyrin-based covalent organic framework (Tp-PDA-COF) with a tetragonal topology was prepared by adding two bridging monomers, 2,4,6-triformylphloroglucinol (Tp) and 2,6-pyridinedicarboxaldehyde (PDA) at a molar ratio of 1:1. The framework has the ability to selectively detect fluorescent and effectively capture heavy metal ions. The addition of bridging monomers Tp and PDA promotes electron transfer and the formation of coordination bonds, enhances the coordination ability of the porphyrin ring, fully exposes the coordination sites, reduces steric hindrance, and makes the interlayer stacking state of COFs easier to adjust. The establishment of the Tp-PDA-COF structure enables the porphyrin monomers in the framework to show resistance to heavy metal Cu 2+ In addition, TP-PDA-COF aerogel was constructed by combining polyvinyl alcohol polymer. 2+ The selective coordination of the Tp-PDA-COF and the cooperative adsorption of the gel structure further improved the adsorption effect. At the same time, the presence of the polyvinyl alcohol polymer increased the interlayer spacing of Tp-PDA-COF, blocked the π-π stacking effect, and solved the fluorescence quenching problem of Tp-PDA-COF in the solid state. Therefore, the designed Tp-PDA-COF can realize fluorescence sensing and cooperative adsorption of Cu 2+ The COF aerogel material provided by the present invention realizes the double goal of heavy metal Cu 2+ The specific adsorption and detection of heavy metals in the environment are further studied in this invention, which provides valuable guidance for further research in the field of heavy metal adsorption and detection in the environment.

[0011] The present invention also provides the use of the above-mentioned porphyrin-based COF aerogel based on double-bridged monomers, wherein the porphyrin-based COF aerogel can perform fluorescence sensing and synergistic adsorption of heavy metal ions in water, especially for Cu 2+ The Tp-PDA-COF aerogel prepared by the present invention has good sensitive detection and adsorption selectivity. Under 350-450 nm excitation, the overall fluorescence color of Tp-PDA-COF is red. 2+ concentration, the blue fluorescence of COF at 450-470 nm remains unchanged, while the red fluorescence at 600-700 nm is gradually quenched, showing a series of fluorescence changes from red to blue, thus achieving the Cu2+ Visual detection.

[0012] Preferably, the application of the Tp-PDA-COF is to prepare a mixture containing various Cu 2+ The prepared Tp-PDA-COF was dissolved in N,N-dimethylformamide (DMF) and the fluorescence intensity was detected under 350-450 nm ultraviolet light to establish the fluorescence intensity ratio I 691 / I 464 With Cu 2+ The standard curve of concentration (0-50μM) and the linear equation; the fluorescence intensity ratio I of the sample 691 / I 464 Detection, according to the standard curve and linear equation, the Cu content in the sample is obtained. 2+ concentration, thereby achieving Cu 2+ Quantitative detection.

[0013] Preferably, the Tp-PDA-COF aerogel is prepared based on the designed Tp-PDA-COF combined with polyvinyl alcohol polymer. The polyvinyl alcohol polymer is used to enlarge the interlayer spacing of Tp-PDA-COF, block the π-π stacking effect, and better retain the fluorescence characteristics. The overall fluorescence color of the Tp-PDA-COF aerogel is red, and the fluorescence of the aerogel increases with the increase of Cu 2+ concentration, the red fluorescence on the aerogel is gradually quenched, showing a series of fluorescence changes from red to dark blue, thus realizing the Cu 2+ The change of aerogel fluorescence further illustrates the effect of the prepared Tp-PDA-COF aerogel on Cu 2+ synergistic adsorption.

[0014] Preferably, the prepared Tp-PDA-COF aerogel is placed in 20-50 mL of a heavy metal solution (40-60 μM) to achieve the Tp-PDA-COF aerogel to Cu 2+ Evaluation of adsorption capacity (pH, temperature, time, heavy metal concentration, etc.).

[0015] It should be noted that the Tp-PDA-COF method for detecting Cu 2+ The principle is based on the charge transfer mechanism. Specifically, in combination with Cu 2+ Previously, the electrons of Tp-PDA-COF were transferred within the porphyrin ring under excitation, achieving local excitation (LE) and fluorescence generation. 2+ After binding, some electrons of Tp-PDA-COF-Cu are transferred from the porphyrin ring to the adjacent benzene ring in the excited state, resulting in fluorescence quenching. The entire fluorescence signal response can be completed within 3 min. By establishing the fluorescence intensity and Cu 2+The relationship of the concentration enables the quantitative detection of Cu 2+ .

[0016] Compared with the prior art, the beneficial effects of the embodiments of the present invention are as follows: 1. By adding two bridging monomers, Tp and PDA, the present invention constructs a covalent organic framework material (Tp-PDA-COF), realizing the ultra-sensitive detection of Cu 2+ and solving the limitations of some COF materials in the specific recognition of heavy metals.

[0017] 2. The detection principle of the Tp-PDA-COF of the present invention for Cu 2+ is explained in detail through simulation calculations.

[0018] 3. The detection limit of the Tp-PDA-COF of the present invention for Cu 2+ is 40.76 nM, which is much lower than the allowable limit of Cu 2+ in the soil.

[0019] 4. Based on the designed Tp-PDA-COF, the present invention combines polyvinyl alcohol to prepare a Tp-PDA-COF aerogel to solve the problem of fluorescence quenching of COFs in the solid state.

[0020] 5. In addition, under the synergistic adsorption of the selective coordination of the Tp-PDA-COF aerogel to Cu 2+ and the gel structure, the Tp-PDA-COF aerogel has an ultra-high adsorption capacity (1954 mg / g) for Cu 2+ .

[0021] 6. The prepared Tp-PDA-COF aerogel of the present invention is a material with dual functions of sensitive detection and efficient adsorption. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention.

[0023] Figure 1 It is a schematic diagram of the synthesis of Tp-PDA-COF in Embodiment 1 of the present invention.

[0024] Figure 2 It is a TEM image of the Tp-PDA-COF prepared in Embodiment 1 at different magnification scales.

[0025] Figure 3 It is an XRD pattern and a fine structure diagram of the Tp-PDA-COF prepared in Embodiment 1.

[0026] Figure 4 FTIR spectrum and solid-state 13C NMR spectrum of the material prepared in Example 1 of the present invention.

[0027] Figure 5 XPS spectrum of Tp-PDA-COF prepared in Example 1 of the present invention.

[0028] Figure 6 N2 adsorption experiment test results of Tp-PDA-COF prepared in Example 1 of the present invention.

[0029] Figure 7 Thermogravimetric analysis results of Tp-PDA-COF prepared in Example 1 of the present invention.

[0030] Figure 8 Excitation and emission spectrum analysis results of Tp-PDA-COF prepared in Example 1 of the present invention.

[0031] Figure 9 Quantum yield and fluorescence lifetime analysis results of Tp-PDA-COF prepared in Example 1 of the present invention.

[0032] Figure 10 Selectivity experiment results of Tp-PDA-COF prepared in Example 1 of the present invention towards different metals (50 μM).

[0033] Figure 11 In Figure A, it is the influence of adding different concentrations of Cu 2+ on the fluorescence spectrum of Tp-PDA-COF. The inset is the corresponding photo taken under 365 nm ultraviolet light; In Figure B, it is the linear relationship between the fluorescence intensity ratio (I 464 / I 691 ) of Tp-PDA-COF and the concentration of Cu 2+ .

[0034] Figure 12 Stability experiment results of Tp-PDA-COF prepared in Example 1 of the present invention before and after adding Cu 2+ for 2 hours.

[0035] Figure 13 The influence of different conditions on the fluorescence of Tp-PDA-COF prepared in Example 1.

[0036] Figure 14 Fluorescence reaction kinetics of Tp-PDA-COF prepared in Example 1 of the present invention towards Cu 2+ .

[0037] Figure 15 Three-mode coordination schemes of Tp-PDA-COF prepared in Example 1 of the present invention with Cu 2+ .

[0038] Figure 16 For the Tp-PDA-COF prepared in Example 1 of the present invention and Cu 2+ The mechanism of fluorescence quenching.

[0039] Figure 17 For the synthesis process of the Tp-PDA-COF aerogel prepared in Example 2 of the present invention and its fluorescence sensing and co-adsorption of Cu 2+ Schematic diagram.

[0040] Figure 18 Characterization spectra of the Tp-PDA-COF aerogel prepared in Example 2 of the present invention under different conditions.

[0041] Figure 19 For the XRD pattern and XPS spectrum of the Tp-PDA-COF aerogel prepared in Example 2 of the present invention after Cu 2+ Adsorption.

[0042] Figure 20 In Figure A, the effective adsorption of the Tp-PDA-COF aerogel on Cu 2+ at different pH values; in Figure B, the effective adsorption of the Tp-PDA-COF aerogel on Cu 2+ at different temperatures.

[0043] Figure 21 In Figure A, the adsorption kinetics study of the Tp-PDA-COF aerogel on Cu 2+ ; in Figure B, the adsorption kinetic curve of the pseudo-second-order model; in Figure C, the adsorption kinetic curve of the Weber-Morris model.

[0044] Figure 22 In Figure A, the adsorption isotherm of the Tp-PDA-COF aerogel on Cu 2+ ; in Figure B, the adsorption isotherm curve conforming to the Langmuir model.

[0045] Figure 23 In Figure A, the adsorption selectivity of the Tp-PDA-COF aerogel on Cu 2+ ; in Figure B, the distribution coefficient values of the Tp-PDA-COF aerogel for different metal ions. Detailed implementation manners

[0046] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clear and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments and drawings of the present invention. Obviously, the following embodiments will help those skilled in the art to further understand the embodiments of the present invention, but do not limit the embodiments of the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the embodiments of the present invention, several modifications and improvements can be made. These all belong to the protection scope of the embodiments of the present invention.

[0047] The following describes the specific implementation of the present invention in detail with reference to specific embodiments.

[0048] Example 1: A preparation method of a porphyrin-based COF based on a bis-bridged monomer, the synthesis schematic diagram of which is as Figure 1 shown, and includes the following steps: 1. In a pressure-resistant tube, disperse 101.5 mg (0.15 mmol) of tetraaminophenyl porphyrin (TAPP) and 0.1 mmol of ligands (the ligands include 2,6-pyridinedicarboxaldehyde and 2,4,6-triformylphloroglucinol, and the molar ratio of 2,6-pyridinedicarboxaldehyde to 2,4,6-triformylphloroglucinol is 1:1) in a mixed solvent composed of 6 ml of o-dichlorobenzene, 6 ml of n-butanol and 1 ml of 6M acetic acid.

[0049] 2. After degassing the pressure-resistant tube through three freeze-vacuum-thaw cycles and sealing it, heat it at 120 °C for 3 days.

[0050] 3. Collect the precipitate and wash and purify it with tetrahydrofuran. Finally, dry the product under vacuum at 40 °C overnight to obtain a purple-black solid powder, which is the target product, labeled as Tp-PDA-COF.

[0051] Example 2 A preparation method of a porphyrin-based COF aerogel based on a bis-bridged monomer, including the following steps: Stir the aqueous solution of polyvinyl alcohol at 90 °C for 3 h to obtain a 10% aqueous solution of polyvinyl alcohol hydrogel. Add 30 mg of the Tp-PDA-COF powder prepared in Example 1 thereto and continuously stir to obtain a uniform solution; the blank aerogel does not add Tp-PDA-COF as a control group. After three freeze-thaw cycles, put it into a freeze dryer for 2 d to obtain the required COF aerogel, denoted as Tp-PDA-COF aerogel.

[0052] Structure analysis of Tp-PDA-COF and its aerogel Using TAPP as the main monomer, a two-bridged porphyrin-based covalent organic framework (Tp-PDA-COF) with a square topological structure was prepared by adding 2,4,6-triformylphloroglucinol (Tp) and 2,6-pyridinedicarboxaldehyde (PDA) bridging monomers in a molar ratio of 1:1. This framework has the ability to selectively detect fluorescence and effectively capture heavy metal ions. The addition of the bridging monomers Tp and PDA promoted electron transfer and the formation of coordination bonds, enhanced the coordination ability of the porphyrin ring, fully exposed the coordination sites, reduced steric hindrance, and made it easier to adjust the interlayer stacking state of the COF. The establishment of the Tp-PDA-COF structure enabled the porphyrin monomers in the framework to exhibit specific recognition of heavy metal Cu 2+ Specific recognition of copper was achieved. In addition, a TP-PDA-COF aerogel was constructed by combining with polyvinyl alcohol polymer. The selective coordination of Cu 2+ and the synergistic adsorption of the gel structure further improved the adsorption effect. At the same time, the presence of the polyvinyl alcohol polymer increased the interlayer spacing of Tp-PDA-COF, blocked the π-π stacking effect, and solved the fluorescence quenching problem of Tp-PDA-COF in the solid state. Therefore, the designed Tp-PDA-COF can achieve the dual goals of fluorescence sensing and synergistic adsorption of Cu 2+

[0053] Visual fluorescence detection of Cu by Tp-PDA-COF 2+ Standard solutions containing various concentrations of Cu were prepared. The prepared Tp-PDA-COF was dissolved in N,N-dimethylformamide (DMF), and the fluorescence intensity was detected under ultraviolet light at 433 nm. Under excitation at 433 nm, the overall fluorescence color of the Tp-PDA-COF presented red. As the concentration of Cu increased continuously, the blue fluorescence of the COF at 464 nm remained unchanged, while the red fluorescence at 691 nm gradually quenched, showing a series of fluorescence changes from red to blue. Based on this, a standard curve and a linear equation of the fluorescence intensity ratio I 2+ / I 2+ versus the concentration of Cu (0 - 50 μM) were established; the fluorescence intensity ratio I 464 / I 691 of the sample was detected. According to the standard curve and the linear equation, the concentration of Cu in the sample was obtained, thereby realizing the quantitative detection of Cu 2+ 464 / I 691 2+ 2+

[0054] Fluorescence sensing and synergistic adsorption of Cu by Tp-PDA-COF aerogel 2+ ​​​​​​Tp-PDA-COF aerogel was prepared based on the designed Tp-PDA-COF combined with polyvinyl alcohol polymer. The polyvinyl alcohol polymer was used to enlarge the interlayer spacing of Tp-PDA-COF, blocking the π-π stacking effect, so that it can better retain the fluorescence characteristics. The overall fluorescence color of the Tp-PDA-COF aerogel was red. When the Cu 2+ concentration, the red fluorescence on the aerogel is gradually quenched, showing a series of fluorescence changes from red to dark blue, thus realizing the Cu 2+ The change of aerogel fluorescence further illustrates the effect of the prepared Tp-PDA-COF aerogel on Cu 2+ synergistic adsorption.

[0055] The various materials prepared in the examples of the present invention are characterized, and the specific description is as follows: Figure 2 TEM images of Tp-PDA-COF prepared in Example 1 at different magnifications, wherein: Figure A is a TEM image of Tp-PDA-COF at 100 nm, and Figure B is a TEM image of Tp-PDA-COF at 20 nm. It can be seen from the figures that a lattice fringe arrangement with a plane spacing of 2.62 nm can be observed, showing tetragonal crystal fringe diffraction.

[0056] Figure 3XRD patterns and refined structure diagrams of Tp-PDA-COF prepared in Example 1, where: Figure A is the XRD pattern of Tp-PDA-COF, and Figure B is the refined structure and pore size diagram of Tp-PDA-COF modeled by MS. In Figure A, the crystal structure of Tp-PDA-COF was elucidated by powder X-ray diffraction (PXRD) measurement, geometric optimization of the structural model using Materials Studio, Pawley refinement, and XRD fitting of experimental data. The calculated peak positions and intensities of the PXRD pattern of Tp-PDA-COF were consistent with the experimental results. The lattice parameters were determined to be a = 25.27 Å, b = 25.00 Å, c = 4.32 Å, α = β = γ = 90°. A good agreement factor (Rwp = 3.30% and Rp = 2.27%) was achieved between the experimental PXRD data and the Pawley refined model. Finally, the PXRD pattern showed several diffraction lines, including a unique peak centered at 2θ = 4.11°, which was attributed to the (110) diffraction plane of the Tp-PDA-COF network, followed by a broader signal spanning the 2θ range of 6-10°, mainly attributed to the (020), (220), and (310) planes. In addition, a relatively sharp and strong peak was observed at 2θ = 19.29°, corresponding to the (111) reflection. This broad diffraction peak was recorded as the conjugate superposition of two-dimensional layers. Figure B shows the refined pore size, and different colors in the figure represent different atoms, specifically as follows: N is blue, O is red, C is gray, and H is white. It can be seen that the pore size of Tp-PDA-COF is 26 Å, which matches the lattice fringe arrangement with an interplanar spacing of 2.62 nm observed by transmission electron microscopy. This fully demonstrates the rationality of the designed Tp-PDA-COF structure.

[0057] Figure 4 FTIR spectra and solid-state 13C NMR spectra of the materials prepared in Example 1 of the present invention. Among them: Figure A is the FTIR spectra of TAPP, PDA, and Tp-PDA-COF; Figure B is the solid-state 13C NMR spectrum of Tp-PDA-COF. It can be seen from the figure the formation of imine bonds (C=N bonds) in the structure, which fully illustrates the synthesis of Tp-PDA-COF.

[0058] Figure 5XPS spectrum of Tp-PDA-COF prepared in Example 1 of the present invention. Among them: Figure A is the total XPS spectrum of Tp-PDA-COF, and Figure B is the fine spectrum and peak fitting at the position of the C 1s peak in Figure A; Figure C is the fine spectrum and peak fitting at the position of the N 1s peak in Figure A; Figure D is the fine spectrum and peak fitting at the position of the O 1s peak in Figure A; It can be seen from the figure that the binding energy of 399.99 eV is attributed to the formation of C-C=N-C in Tp-PDA-COF, and the binding energy of 397.88 eV is the binding energy of R-C=N-R on the porphyrin ring, where R represents the C atom on the porphyrin ring. On the O 1s peak, the presence of C=O / O-H was observed at 531.20 eV. This is consistent with the results of IR and 13C, further proving the successful synthesis of Tp-PDA-COF.

[0059] Figure 6 The N2 adsorption experiment test results of Tp-PDA-COF prepared in Example 1 of the present invention. According to the nitrogen adsorption isotherm at 77 K, the Brunauer-Emmett-Teller (BET) specific surface area of Tp-PDA-COF was calculated to be approximately 1251.81 m² / g.

[0060] Figure 7 The thermogravimetric analysis results of Tp-PDA-COF prepared in Example 1 of the present invention; the thermogravimetric analysis (TGA) results show that Tp-PDA-COF is stable below 300 °C.

[0061] Figure 8 The excitation and emission spectrum analysis results of Tp-PDA-COF prepared in Example 1 of the present invention; it can be seen from the figure that the optimal excitation wavelength of Tp-PDA-COF is 433 nm. Under the excitation of 433 nm, Tp-PDA-COF exhibits strong red fluorescence (691 nm) and weak blue fluorescence (464 nm).

[0062] Figure 9 The quantum yield and fluorescence lifetime analysis results of Tp-PDA-COF prepared in Example 1 of the present invention; among them: Figure A is the quantum yield analysis result of Tp-PDA-COF; Figure B is the fluorescence lifetime analysis result of Tp-PDA-COF. After calculation, the fluorescence quantum yield of Tp-PDA-COF is 15.23%, and the fluorescence lifetime is 9.178 ns.

[0063] Figure 10 Selectivity experiment of Tp-PDA-COF prepared in Example 1 for different metals (metal concentration is 50 μM). Through the figure, Tp-PDA-COF for Cu 2+selectivity and quenching efficiency clearly indicate that the chemical structure of the Tp-PDA-COF design towards Cu 2+ exhibits good sensitivity and excellent selectivity.

[0064] Figure 11 In Figure A, the influence of adding different concentrations of Cu 2+ on the fluorescence spectrum of Tp-PDA-COF is shown. The inset is the corresponding photo taken under a 365 nm ultraviolet lamp; in Figure B, the fluorescence intensity ratio (I 691 / I 464 ) of Tp-PDA-COF and the linear relationship with the concentration of Cu 2+ are presented. As can be seen from Figure A, with the gradual increase in the concentration of Cu 2+ , the emission peak of Tp-PDA-COF at 691 nm continues to decrease, while the blue fluorescence at 464 nm remains unchanged, resulting in the fluorescence of Tp-PDA-COF changing from red to light blue. Figure B shows that the fluorescence intensity ratio (I 691 / I 464 ) of Tp-PDA-COF has a good linear relationship with the concentration of Cu 2+ (R 2 = 0.9964). By calculating 3δ / slope, the detection limit (LOD) of Tp-PDA-COF for Cu 2+ is as low as 40.76 nM.

[0065] Figure 12 This is the stability experiment of Tp-PDA-COF prepared in Example 1 of the present invention before and after adding Cu 2+ for 2 hours. Among them: Figure A is the stability experiment of Tp-PDA-COF within 2 hours before adding 50 µm Cu 2+ ; Figure B is the stability experiment of Tp-PDA-COF within 2 hours after adding 50 µm Cu 2+ . It can be seen from the figure that Tp-PDA-COF has good detection stability.

[0066] Figure 13 This shows the fluorescence influence of different conditions on Tp-PDA-COF prepared in Example 1; among them: Figure A shows the influence of pH value on the fluorescence of Tp-PDA-COF; Figure B shows the influence of temperature on the fluorescence of Tp-PDA-COF. As can be seen from Figure A, when the environmental pH value is less than 6, the inherent acid and alkali resistance of Tp-PDA-COF makes its detection performance very stable. When pH > 6, the presence of Cu(OH)2 in the solution will interfere with the detection results. Therefore, combined with the subsequent adsorption experiment, pH = 6 is the best choice. Figure B shows that Tp-PDA-COF is less affected by the environmental temperature during the detection process.

[0067] Figure 14 For the Cu of the Tp-PDA-COF prepared in Example 1 of the present invention 2+ fluorescence reaction kinetics. It can be seen from the figure that when 25 μM and 50 μM Cu are added respectively 2+ subsequently, the response of Tp-PDA COF to Cu 2+ is quickly completed within 3 minutes.

[0068] Figure 15 For the TP-PDA-COF and Cu prepared in Example 1 of the present invention 2+ three different mode coordination schemes. According to the metal coordination principle, they are respectively Mode 1 in Figure A (Cu coordinated in the porphyrin ring 2+ ), Mode 2 in Figure B (Cu coordinated with the Tp monomer 2+ ), and Mode 3 in Figure C (Cu coordinated with the PDA monomer 2+ ). Different colors in the figure represent different atoms, specifically as follows: N is blue, O is red, C is gray, H is white, and Cu is green. Structure optimization is carried out by the computational simulation package (VASP) and the PAW method. Structure optimizations are performed on the three configurations coordinated with Cu 2+ and the reaction free energy ΔG is calculated. The results are shown in Table 1 below: Table 1

[0069] ΔG = ΔE + ΔZPE - TΔS where E is the calculated total energy, ZPE is the zero-point energy, T is the temperature, and S is the entropy.

[0070] It can be seen that the reaction free energy ΔG of Mode 1 (Cu coordinated in the porphyrin ring 2+ ) is 0.207 eV, the action free energy ΔG of Mode 2 (Cu coordinated with the Tp monomer 2+ ) is 0.491 eV, and the free energy ΔG of Mode 3 (Cu coordinated with the PDA monomer 2+ ) is 2.317. Among the three modes, the reaction free energy of Mode 1 is the smallest, indicating that the reaction is most likely to occur in Mode 1 and the corresponding structure is the most stable.

[0071] Figure 16 For the fluorescence quenching mechanism of the TP-PDA-COF and Cu prepared in Example 1 of the present invention 2+ The blue and orange isosurfaces respectively represent the hole and electron distributions. The TP-PDA-COF and Cu 2+The substance formed after combination is labeled as TP-PDA-COF-Cu. The electron-hole density maps of the previously optimized corresponding structures were analyzed using Multiwfn software. It can be visually seen from the electron-hole density maps of Tp-PDA-COF and Tp-PDA-COF-Cu that before the combination of Cu 2+ , the electrons of Tp-PDA-COF transfer within the porphyrin ring under excitation (orange → blue), achieving local excitation (LE) and fluorescence generation. After the combination with Cu 2+ , some electrons of Tp-PDA-COF-Cu transfer from the porphyrin ring (blue) to the adjacent benzene ring (orange) in the excited state, resulting in the phenomenon of fluorescence quenching. This is because the orbits of electrons before and after the transition do not overlap spatially, and the electron-hole separation is relatively thorough, leading to a decrease in the oscillator strength during the transition process, which is called the transition barrier phenomenon, and realizing the fluorescence quenching of charge transfer excitation (CT).

[0072] The oscillator strengths of TP-PDA-COF and TP-PDA-COF-Cu were calculated, and the results are shown in Table 2: Table 2

[0073] Table 2 further verifies the fluorescence quenching effect and mechanism of Cu 2+ on Tp-PDA-COF. The oscillator strengths of the excited states of Tp-PDA-COF and Tp-PDA-COF-Cu were calculated using the electronic DFT of the ORCA quantum chemistry calculation software. The corresponding structures were optimized based on the r2SCAN-3c algorithm. For Tp-PDA-COF, the ground state of the structure is the singlet state S0. In the first excited state S1 of the singlet state, the excitation energy is 2.1310 eV, and the calculated oscillator strength f = 0.09597 (oscillator strength = experimental transition probability / ideal harmonic transition probability), and fluorescence can be generated. Generally, the smaller the oscillator strength, the higher the transition barrier. For Tp-PDA-COF-Cu, the ground state of the structure is the double-excited state D0. In the first excited state D1 of the double-excited state, the excitation energy is 2.2450 eV, and the calculated oscillator strength f is 0.01502. This substance does not generate fluorescence in the excited state. This also confirms our experimental phenomenon that the addition of Cu 2+ quenches the fluorescence of the Tp-PDA-COF solution.

[0074] Figure 17 Schematic diagram of the synthesis process for preparing Tp-PDA-COF aerogel in Example 2 of the present invention and its fluorescence sensing and co-adsorption of Cu 2+ . It can be seen from the figure that based on the π→π stacking effect, Tp-PDA-COF shows fluorescence quenching in the solid state (such as Figure 17(Figure I). However, the presence of polyvinyl alcohol polymer in the Tp-PDA-COF aerogel enables the Tp-PDA-COF aerogel to maintain its fluorescence characteristics ( Figure 17 (Figure II). When Cu 2+ is added, the fluorescence of the Tp-PDA-COF aerogel is quenched, as shown in Figure 17 (Figure III), indicating that the Tp-PDA-COF aerogel can absorb Cu 2+ in the solution and complete the detection of Cu 2+ .

[0075] Figure 18 Figure 14 shows the characterization spectra of the Tp-PDA-COF aerogel prepared in Example 2 of the present invention under different conditions; among them, Figure A is the SEM image of the Tp-PDA-COF aerogel before and after Cu 2+ adsorption; Figure B is the EDS spectrum of the Tp-PDA-COF aerogel before and after Cu 2+ adsorption; Figure C is the mapping spectrum of the Tp-PDA-COF aerogel after Cu 2+ adsorption, which are the mapping spectra of C, N, O, and Cu respectively. By comparing the SEM spectra (Figure A) of the Tp-PDA-COF aerogel before adsorption and the Tp-PDA-COF aerogel after Cu 2+ adsorption, it is found that the Tp-PDA-COF aerogel after adsorption has a relatively uniform and regular pore structure. Through the analysis of the EDS data before and after adsorption (Figure B), the Cu element is evenly distributed at different positions on the surface of the Tp-PDA-COF aerogel, fully indicating that Cu 2+ is successfully captured by the Tp-PDA-COF aerogel. Combining the mapping image analysis (Figure C), the appearance of Cu 2+ again proves that the Tp-PDA-COF aerogel can adsorb Cu 2+ well.

[0076] Figure 19 Figure 32 shows the XRD pattern and XPS spectrum of the Tp-PDA-COF aerogel prepared in Example 2 of the present invention after Cu 2+ adsorption; among them, Figure A is the XRD pattern of the Tp-PDA-COF aerogel after Cu 2+ adsorption; Figures B to F are the XPS spectra of the Tp-PDA-COF aerogel after Cu 2+ adsorption respectively. The appearance of the Cu characteristic peak in the XPS spectrum indicates that Cu 2+ has entered the interior of the Tp-PDA-COF aerogel.

[0077] The prepared Tp-PDA-COF aerogel was placed in 30 mL of heavy metal solution with a heavy metal ion concentration of 50 μM in the solution to evaluate the adsorption capacity of Tp-PDA-COF aerogel for different heavy metals (pH, temperature, time, heavy metal concentration, etc.). The results are as Figures 20 - 23 shown. The pH value is an important parameter for the adsorption of heavy metal ions during the adsorption process, which affects the surface charges of the adsorbent and metal ions. As Figure 20 shown in Figure A, copper was selected as the heavy metal 2+ . It can be seen that with the increase of the pH value, the adsorption capacity of Tp-PDA-COF aerogel for Cu 2+ gradually increases. When pH = 6, the adsorption capacity reaches the maximum value of 1942 mg / g. When pH > 6.0, part of Cu 2+ will exist in the form of Cu(OH)2, so the adsorption capacity of Tp-PDA-COF aerogel decreases. In addition, the effect of temperature on adsorption was studied through thermodynamic experiments. With the increase of temperature, the adsorption capacity of Tp-PDA-COF aerogel for Cu 2+ increases (as Figure 20 shown in Figure B), which may be due to the fact that the increase of temperature intensifies the intermolecular movement, thus obtaining stronger fluidity. However, when the temperature rises from room temperature of 25 °C to 40 °C, the adsorption capacity only increases by about 10%, which does not match the detection environment and cost. Therefore, in the subsequent experiments, the optimal pH value for Cu 2+ adsorption was designed to be 6.0, and the adsorption temperature was 25 °C. To study the effect of adsorption time on the adsorption behavior of Tp-PDA-COF aerogel, we carried out adsorption kinetics experiments. As Figure 21 shown in Figure A, the absorption rate of Tp-PDA-COF aerogel for Cu 2+ is the highest in the first 30 minutes, and the growth rate slows down in the next 30 - 180 minutes. After adsorption for more than 180 minutes, the adsorption amount of Cu 2+ almost remains unchanged. Through model fitting ( Figure 21 shown in Figure B), we found that the adsorption kinetics of Tp-PDA-COF aerogel can match the pseudo-second-order model, which well explains that the adsorption of Cu 2+ on Tp-PDA-COF aerogel is mainly dominated by chemisorption. In addition, we divided the adsorption process into two parts through the fitting of the W-M model ( Figure 21 shown in Figure C). Within 0 - 180 min, based on the lone pair electrons provided by the four nitrogen atoms on the porphyrin ring coordinating with Cu 2+ , Cu 2+ is adsorbed in the framework of COF through Tp-PDA-COF aerogel until the adsorption equilibrium is reached after 180 minutes.

[0078] In addition, to further understand the adsorption process, adsorption isotherm experiments were carried out in the concentration range of 0 - 50 mM (0 - 3200 mg / L) Cu 2+ . As shown in Figure A of Figure 22 , when the concentration of Cu 2+ is lower than 3200 mg / L, the absorption of Tp-PDA-COF aerogel for Cu 2+ continues to increase. However, due to the limited number of active sites of Tp-PDA-COF, the absorption of TP-PDA-COF aerogel for Cu 2+ remains almost unchanged when the concentration is higher than 3200 mg / L. Surprisingly, the Tp-PDA-COF aerogel can adsorb up to 1954 mg / g of Cu 2+ , which is better than most of the previously reported MOF or COF materials; Figure 22 Figure B in 2 is the adsorption isotherm curve that conforms to the Langmuir model, R 2 = 0.990. The fitting results of the Langmuir model show that monolayer adsorption plays a crucial role in the adsorption of Cu 2+ on the Tp-PDA-COF aerogel. To further explore whether the Tp-PDA-COF aerogel still maintains selectivity for Cu 2+ and evaluate the possibility of detecting and adsorbing Cu 2+ in practice, 10 Tp-PDA-COF aerogels were placed in a series of metal ion solutions (Ca 2+ , Pb 2+ , Hg 2+ , Mg 2+ , Cd 2+ , Zn 2+ , Mn 2+ , Fe 3+ , Al 3+ and Cu 2+ ) with a concentration of 3200 mg / L for adsorption experiments. As can be seen from Figure A of Figure 23 , the adsorption capacities of the Tp-PDA-COF aerogel for Ca 2+ , Pb 2+ , Hg 2+ , Mg 2+ , Cd 2+ , Zn 2+ , Mn 2+ , Fe 3+ , Al 3+ and Cu 2+ are 53.46, 197.83, 234.19, 46.34, 91.82, 85.60, 39.87, 76.62, 57.36 and 1936.94 mg / g respectively. The Tp-PDA-COF aerogel has the highest adsorption capacity for Cu2+ has a very high absorption rate, which may be due to the unique affinity between Tp-PDA-COF and Cu 2+ . In addition, by calculating the distribution coefficient (Kd) of each metal ion, the results are shown in Figure B of Figure 23 as follows. The Kd value of Tp-PDA-COF aerogel for Cu 2+ can reach 1085.20 mL / g. These results fully confirm that Tp-PDA-COF aerogel has excellent selectivity and adsorption performance, and can achieve the adsorption detection of Cu 2+ in the environment.

[0079] The above describes the preferred embodiments of the present invention in detail, describing the basic principles, main features and advantages of the present invention. However, those skilled in the art should understand that the embodiments of the present invention are not limited to the above embodiments. The above embodiments and descriptions in the specification are only preferred examples of the present invention and are not used to limit the present invention. Within the scope of knowledge possessed by ordinary technicians in the art, various changes can be made without departing from the purpose of the embodiments of the present invention. It is not necessary and impossible to enumerate all the embodiments here. And the obvious changes or variations derived therefrom are still within the protection scope of the embodiments of the present invention.

Claims

1. A preparation method of a porphyrin-based COF based on a bis-bridged monomer, characterized in that, It includes the following steps: Mix porphyrin, 2,6-pyridinedicarboxaldehyde and 2,4,6-triformylphloroglucinol evenly in a solvent to obtain a mixture. Place the mixture in a sealed environment for heating reaction. After the reaction is completed, purify the product to obtain a purple-black solid powder, which is the target product.

2. The preparation method of the porphyrin-based COF based on the bis-bridged monomer according to claim 1, wherein The molar ratio of the 2,6-pyridinedicarboxaldehyde to the 2,4,6-triformylphloroglucinol is 1:

1.

3. The preparation method of the porphyrin-based COF based on the bis-bridged monomer according to claim 1, wherein, The porphyrin is tetraaminophenyl porphyrin.

4. The preparation method of the porphyrin-based COF based on a bis-bridging monomer according to claim 1, characterized in that, The temperature of the heating reaction is 100 - 150 °C, and the time is 2 - 4 d.

5. The preparation method of the porphyrin-based COF based on a bis-bridging monomer according to claim 1, wherein The solvent is a mixed solvent composed of o-dichlorobenzene, n-butanol and acetic acid.

6. The preparation method of the porphyrin-based COF based on a bis-bridging monomer according to claim 5, characterized in that, The volume ratio of the o-dichlorobenzene, n-butanol and acetic acid is (1 - 10):(1 - 10):(1 - 2).

7. A porphyrin-based COF based on a bis-bridged monomer, characterized in that It is prepared by using the preparation method described in any one of claims 1 to 6.

8. A preparation method of a porphyrin-based COF aerogel based on a bis-bridged monomer, characterized in that, It includes the following steps: Prepare a porphyrin-based COF based on a bis-bridged monomer by using the preparation method described in any one of claims 1 to 6. Disperse the porphyrin-based COF based on the bis-bridged monomer in a polyvinyl alcohol hydrogel, and obtain the target product after freeze-drying.

9. A porphyrin-based COF aerogel based on a bis-bridged monomer, characterized in that, It is prepared by using the preparation method described in claim 8.

10. Use of the porphyrin-based COF aerogel based on a bis-bridged monomer according to claim 9, characterized in that: The porphyrin-based COF aerogel can perform fluorescence sensing and synergistic adsorption of heavy metal ions in water.

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