COFs as well as preparation method and application thereof

Abstract

The invention discloses COFs (covalent organic frameworks) as well as a preparation method and application thereof, and belongs to the technical field of preparation of covalent organic framework materials. The preparation method of the COFs comprises the following steps: adding a building unit containing an amino functional group and a building unit containing an aldehyde functional group into an acetonitrile-polyethylene glycol homogeneous solution, carrying out ultrasonic treatment until solids are completely dissolved, then adding diluted hydrochloric acid, uniformly mixing, carrying out a heating reaction for 4-36 hours at the temperature of 30-80 DEG C, separating precipitates, and drying to obtain the COFs. The detection sensitivity is high and the speed is high. The prepared COFs are spherical, the overall pore size distribution is uniform, and the porosity is high. The COFs disclosed by the invention not only have good crystallinity and morphology of the COFs, but also show good fluorescence characteristics, are used for sensitively detecting TNP in a water environment, generate extraordinary specific surface area and porosity, expose more active sites and generate strong adsorption capacity, and can be used as an adsorbent of TNP.

Description

Technical Field

[0001] The present invention belongs to the technical field of covalent organic framework material preparation, and in particular relates to COFs and a preparation method and application thereof. Background Art

[0002] Explosives are closely intertwined with human life, used in every aspect of life, such as building roads and paving the way. While they bring convenience, they also create a shadow. The threat that explosives pose to the safety of life and property has aroused profound alarm.

[0003] Currently, most explosives contain nitroaromatic compounds such as 2,4,6-trinitrophenol (TNP), 2,4,6-trinitrotoluene (TNT), dinitrotoluene (DNT), and 2,4,6-trinitrophenol (PA, picric acid). Among various nitro derivatives, TNP detection is a major concern. TNP is widely used in industrial applications such as fireworks, matches, dyes, leather, and glass. However, TNP is widely considered a deadly pollutant, exhibiting mutagenic properties when converted to picric acid (2-amino-4,6-dinitrophenol). Consuming water containing nitroaromatic explosives can cause a range of illnesses, including cancer, skin diseases, and liver dysfunction. Therefore, TNP detection is crucial for public health and environmental protection. However, TNP residues are present in relatively low concentrations or vapor conditions, necessitating extremely high sensitivity sensors for explosives detection in many scenarios.

[0004] With the rapid advancement of modern science and technology, instrumental detection technologies for chemicals and hazardous materials have also experienced rapid growth, driving the development of instrumental sensing technologies for explosives. Currently developed technologies primarily include spectral analysis, chemical sensors, and biosensors; spectral analysis techniques include mass spectrometry, chromatography, ion mobility analysis, spectroscopy, and hyphenated techniques. However, low detection limits and sensitivity have limited the development of some methods. Furthermore, mass spectrometry and chromatography require large, specialized, and expensive equipment, requiring specialized personnel for operation and pretreatment, and are unsuitable for real-time on-site detection. Therefore, developing fluorescent probes with simple preparation methods, high sensitivity, and rapid detection to replace traditional large-scale equipment is an effective means of achieving real-time monitoring.

[0005] Chinese invention patent publication number CN110790938A discloses a covalent organic framework membrane, its preparation, and its application in detecting nitro-explosives. The membrane is made by copolymerizing the covalent organic framework (COF) material and polyvinylidene fluoride (PVDF), and its application in the adsorption of organic pollutants. This method uses glacial acetic acid, a common catalyst for COF synthesis, and requires repeated freezing and storage in liquid nitrogen during the synthesis process, resulting in complex experimental steps. After synthesis, the membrane must be sealed at 100-120°C, which is a demanding experimental condition and consumes a lot of energy. The solid requires further Soxhlet extraction, resulting in a low yield. The resulting covalent organic framework membrane capable of detecting nitro-explosives is synthesized by first synthesizing the covalent organic framework and then reacting it with PVDF to form the membrane, which prevents a one-step membrane formation. Summary of the Invention

[0006] The present invention aims to provide a method for preparing COFs, which has simple steps, convenient operation, high detection sensitivity and fast speed.

[0007] To achieve the purpose of the present invention, the present invention provides a technical solution for preparing COFs, comprising the following steps: (1) Add the building blocks containing amino functional groups and the building blocks containing aldehyde functional groups to an acetonitrile-polyethylene glycol homogeneous solution, sonicate until the solids are completely dissolved and the solution turns light yellow, then add dilute hydrochloric acid and mix well to obtain a brown solution; (2) The obtained brown solution is heated to react for 4 to 36 hours at a heating temperature of 30 to 80°C, and a dark brown mixed solution is obtained after the reaction is completed; (3) The precipitate of the reaction mixture is separated and dried to obtain COFs.

[0008] Furthermore, in the step (1), the building block containing an amino functional group includes p-phenylenediamine or 1,3,5-tris(4-aminophenyl)benzene, and the building block containing an aldehyde functional group is a unit that can react under hydrothermal conditions; Furthermore, the building block containing an aldehyde functional group in step (1) is one of trimesicaldehyde, 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde, and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde; Furthermore, in the step (1), the molar ratio of the building block containing an amino functional group to the building block containing an aldehyde functional group is 1:1 to 1.5; Furthermore, in the step (1), in the homogeneous solution of acetonitrile-polyethylene glycol, the polyethylene glycol is PEG-200 to PEG-1000, and the volume ratio of acetonitrile to polyethylene glycol is 1:1 to 9; Among them, polyethylene glycol with a molecular weight below 200 cannot form a sufficient protective film to control the morphology of the molecule due to its low molecular weight. Polyethylene glycol with a high molecular weight has its water solubility, water absorption and solubility in organic solvents all decrease with increasing molecular weight, thereby resulting in a decrease in the solubility of the monomer and a decrease in the reaction rate. Therefore, the polyethylene glycol used in the present invention is PEG-200~PEG-1000.

[0009] Furthermore, in the step (2), the concentration of the dilute hydrochloric acid is 0.05M~0.3M, and the molar ratio of the added dilute hydrochloric acid to the building block containing the amino functional group is 1:0.2~1.6.

[0010] The present invention also provides COFs prepared by the above preparation method, which exhibit excellent fluorescence properties, have a porous spherical structure, and have a COF particle size of 300-400 nm.

[0011] The present invention also provides a technical solution for the application of the prepared COFs, and the prepared COFs are used for sensitive detection and adsorption of TNPs in an aqueous environment.

[0012] The beneficial effects of the present invention are:

[0013] (1) The present invention provides a technical solution for preparing COFs, which can produce COFs with good crystallinity, high porosity, and high specific surface area at 30-80°C for only 4-36 hours. Compared with the existing solvothermal synthesis method, this preparation method has mild reaction conditions, simple operation steps, short reaction time, and no need for subsequent treatment.

[0014] (2) The technical solution for preparing COFs provided by the present invention uses dilute hydrochloric acid as a catalyst for the reaction. Dilute hydrochloric acid, as a new type of COFs synthesis catalyst, has stable chemical properties. The selected concentration is between 0.05M and 0.3M to avoid the effects of high concentration acid. It plays a key role in COFs synthesis. As a catalyst, it accelerates the reaction time and shortens the experimental cycle. It plays a key role in controlling the morphology in COFs synthesis. Compared with the preparation method using glacial acetic acid as the reaction catalyst in the prior art, glacial acetic acid has a shorter catalyst life and is easily decomposed by temperature, which is not conducive to the reaction.

[0015] (3) The technical solution for preparing COFs provided in the present invention adopts acetonitrile-polyethylene glycol as the reaction medium, wherein polyethylene glycol is a non-toxic, non-irritating and odorless solvent with good water solubility, which can dissolve in various organic components. After being miscible with acetonitrile, a stable mixed solvent can be formed. As the reaction temperature increases, the mutual solubility increases. As a reaction medium, it affects the nucleation and growth process of the crystal by wrapping and connecting the particles. The polyethylene glycol attached to the surface acts as a non-ionic surfactant to maintain the pore structure formed by continuous growth during the crystal nucleation and growth process, and prevents the agglomeration of COFs by wrapping the entire particle, forming a better dispersion. In addition, polyethylene glycol also plays a solubilizing role, improving the versatility of COFs in aqueous solution and broadening its application range.

[0016] (4) The COFs prepared by the technical solution provided by the present invention are spherical as a whole. Compared with the layered 2DCOFs, they have higher chemical stability, thermal stability, salt stability and pH stability of fluorescence properties. The three-dimensional porous structure provides them with interlaced pores, which provide sufficient channels to ensure a large adsorption capacity of TNP. The larger specific surface area and the presence of a large number of open active sites can promote the rapid diffusion of TNP into the pores of COFs, promote the recognition and adsorption process, and improve the sensitivity and adsorption efficiency. The supported spherical network is a favorable framework that can be reused.

[0017] (5) The COFs prepared by the technical solution provided by the present invention have a particle size of 300-400 nm. Under this particle size condition, the fluorescence detection effect is good and the adsorption efficiency is high. The particle size affects the specific surface area and pore structure of COFs. COFs with a large specific surface area can provide more adsorption sites, but their pore structure will also be affected. Too large a particle size will lead to poor pore connectivity, affecting the adsorption rate and capacity, thereby improving the adsorption efficiency. However, too small a particle size will cause the particles to agglomerate and form clumps, reducing the effective specific surface area and reducing the adsorption effect.

[0018] (6) The COFs provided by the present invention are applied to the detection and adsorption technology of nitro explosive TNP in water. COFs can be used not only as a detector but also as an adsorbent, with high detection sensitivity and a detection limit as low as 1.6 nM; the adsorption effect is good, and its adsorption capacity is as high as 350~687 mg / g, which far exceeds the adsorption capacity of ordinary adsorbents. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 TEM image of COFs in Example 1 of the present invention; Figure 2 This is a high-magnification transmission electron microscope (TEM) image of COFs in Example 1 of the present invention; Figure 3This is a scanning electron microscope (SEM) image of COFs in Example 1 of the present invention; Figure 4 is the X-ray diffractometer (XRD) pattern of COFs in Example 1 of the present invention; Figure 5 This is a thermogravimetric (TGA) graph of COFs in Example 1 of the present invention; Figure 6 The maximum excitation spectrum and emission spectrum of COFs in Example 1 of the present invention; Figure 7 This is the standard curve for detecting TNP in COFs in Example 1 of the present invention; Figure 8 This is an investigation into the salt stability of COFs fluorescence in Example 1 of the present invention; Figure 9 The UV absorption spectra of COFs and TNPs and COFs in Example 1 of the present invention are shown; Figure 10 The ultraviolet absorption spectrum, fluorescence excitation and emission spectrum of the COFs fluorescent probe in Example 1 of the present invention; Figure 11 The BET surface area analysis of the COFs fluorescent probe in Example 1 of the present invention is performed by nitrogen adsorption-desorption experiment; Figure 12 is the number of times the COFs obtained in Example 1 of the present invention are reused when adsorbing TNP; Figure 13 TEM characterization image of COFs obtained in Comparative Example 1 of the present invention; Figure 14 TEM characterization image of COFs obtained in Comparative Example 2 of the present invention; Figure 15 TEM characterization image of COFs obtained in Comparative Example 3 of the present invention; Figure 16 This is a transmission electron microscope (TEM) characterization image of the COFs obtained in Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0020] The present invention is further described below with reference to the accompanying drawings and examples, but the protection scope of the present invention is not limited thereto. Example 1

[0021] Preparation of COFs 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 16.8 mg of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde were added to a 5 mL mixed solution of PEG-200 and acetonitrile (PEG-200 / acetonitrile = 1 mL:1 mL). Ultrasonication was performed until the solids were completely dissolved and the solution turned yellow. Subsequently, 0.05 M, 1000 uL of dilute hydrochloric acid was added as a catalyst and stirred evenly. The mixture was then added to a reactor and heated in an oven for 24 hours at 60°C to obtain a dark brown mixed solution. The mixture was filtered, the precipitate was separated, and dried to obtain COFs. The obtained COFs material had a particle size of 300-400 nm.

[0022] Experiments to identify TNP: Fluorescence titration experiments were conducted by adding 100 μL of TNPs of different concentrations into 900 μL of COFs solution prepared with 50 nM phosphate buffer solution (PBS, pH = 7). As the concentration of TNPs increased, the fluorescence intensity of COFs at 525 nm gradually decreased. When the concentration of TNPs increased to 2.2 x 10 2 When the fluorescence intensity at 525 nm was 3.0×10 μM, the intensity decreased by 91%, indicating that the fluorescent COFs had excellent fluorescence response performance to TNP. −3 −2.2×10 2 The linear relationship is excellent in the range of μM, log(F0 / F) = 0.00377c TNP +0.06763(R 2 = 0.994) and the limit of detection (LOD) was 1.6 nM based on the triplet signal-to-noise ratio. All optical tests were performed by setting the excitation / emission wavelengths to 397 / 525 nm and the slit widths to 15 / 15 nm.

[0023] TNP adsorption experiment: All adsorption experiments were performed at room temperature. A 10 mM TNP stock solution was first prepared and then diluted into standard solutions of different concentrations (ranging from 20 to 4.5 × 10 3μg / mL). COFs (4.0 mg) were added to 2.0 mL of the TNP standard solution prepared above. After shaking the solution for 6 hours, the supernatant was obtained by centrifugation and used to measure the UV-visible absorption to evaluate the adsorption capacity of TNP. Adsorption isotherm experiments showed that the adsorption capacity of COFs for TNP gradually increased with increasing initial TNP concentration. With further increase in initial concentration, the adsorption capacity of COFs for TNP gradually reached saturation, and the final measured adsorption capacity was 687 mg / g, which far exceeded the adsorption capacity of other adsorbents. These results indicate that COFs can serve as an ideal adsorbent for TNP. In addition, the reusability of COFs was also studied. The results showed that COFs maintained a high removal rate after 6 cycles, which can maximize economic benefits.

[0024] Table 1 is a table comparing the adsorption amount of TNP by COFs obtained in Example 1 of the present invention with other adsorbents. It can be seen from the table that the adsorption amount of TNP by COFs of the present invention far exceeds that of COFs-based composite materials (MCOFs@MIPs@CDs), MOFs (Zr-MOF) and some other materials reported in the literature, proving that the three-dimensionality of the porous structure enables it to have interlaced pores that provide sufficient channels to ensure a large adsorption amount of TNP. The larger specific surface area and the presence of a large number of open active sites can promote the rapid diffusion of TNP into the pores of COFs, thereby promoting the recognition and adsorption process.

[0025]

[0026] Table 2 shows the pH stability of COFs in Example 1. In the pH range of 3 to 10, the fluctuation of the normalized fluorescence intensity is not obvious and the normalized fluorescence intensity is always maintained at a high fluorescence level exceeding 80%, demonstrating that it can maintain good chemical stability in acidic and alkaline environments.

[0027] Example 2

[0028] 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 12.96 mg of trimesaldehyde were added to a 5 mL mixed solution of PEG-400 and acetonitrile (PEG-400 / acetonitrile = 1 mL:4 mL). Ultrasonication was performed until the solid was completely dissolved and the solution turned yellow. Subsequently, 0.1 M, 1500 uL of dilute hydrochloric acid was added as a catalyst and stirred evenly. The mixture was added to the reactor and heated in an oven for 4 hours at a heating temperature of 30°C to obtain a dark brown mixed solution. The mixture was filtered, the precipitate was separated, and dried to obtain COFs. The obtained COFs material had a particle size of 300-350 nm. Example 3

[0029] 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 31.47 mg of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde were added to a 5 mL mixed solution of PEG-600 and acetonitrile (PEG-400 / acetonitrile = 1 mL:6 mL). Ultrasonication was performed until the solid was completely dissolved and the solution turned yellow. Subsequently, 0.2 M, 2000 uL of dilute hydrochloric acid was added as a catalyst and stirred evenly. The mixture was added to the reactor and heated in an oven for 8 hours at a heating temperature of 50°C to obtain a dark brown mixed solution. The mixture was filtered, the precipitate was separated, and dried to obtain COFs. The obtained COFs material had a particle size of 350-400 μm. Example 4

[0030] 0.09mmol, 9.73mg of p-phenylenediamine and 0.06mmol, 9.73mg of trimesaldehyde were added to a 5mL mixed solution of PEG-800 and acetonitrile (PEG-800 / acetonitrile = 1mL:8mL). Ultrasonication was performed until the solid was completely dissolved and the solution turned yellow. Subsequently, 0.3M, 1000uL of dilute hydrochloric acid was added as a catalyst and stirred evenly. The mixture was added to the reactor and heated in an oven for 12h at 70°C to obtain a dark brown mixed solution. The mixture was filtered, the precipitate was separated, and dried to obtain COFs. The particle size of the obtained COFs material was 350-400nm. Example 5

[0031] 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 16.8 mg of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde were added to a 5 mL mixed solution of PEG-1000 and acetonitrile (PEG-1000 / acetonitrile = 1 mL:9 mL). Ultrasonication was performed until the solid was completely dissolved and the solution turned yellow. Subsequently, 0.05 M, 1000 uL of dilute hydrochloric acid was added as a catalyst and stirred evenly. The mixture was added to the reactor and heated in an oven for 36 hours at 80°C to obtain a dark brown mixed solution. The mixture was filtered, the precipitate was separated, and dried to obtain COFs. The obtained COFs material had a particle size of 300-350 nm. Comparative Example 1

[0032] 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 16.8 mg of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde were added to a mixed solution of 5 mL of acetonitrile and ultrasonicated until the solid was completely dissolved. Then, 0.05 M, 1000 uL of dilute hydrochloric acid was added as a catalyst and stirred evenly. The mixture was added to the reactor and heated in an oven for 24 hours at 60°C to obtain a dark brown mixed solution. The mixture was filtered to separate the precipitate and dried to obtain the COFs material.

[0033] Under the same experimental conditions as in Example 1, TNP detection and adsorption experiments were carried out. The experimental results showed that the COFs particle size was far greater than 400 nm, even reaching the micron level. As the particle size increased, its pore structure was affected, and the pore connectivity was poor. No obvious pore structure could be observed in the TEM image, which seriously affected the adsorption rate and capacity. The adsorption amount was only 190 mg / g. Comparative Example 2

[0034] 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 16.8 mg of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde were added to a 5 mL mixed solution of PEG-200 and acetonitrile (PEG-200 / acetonitrile = 1 mL:1 mL). Ultrasonication was performed until the solids were completely dissolved and the solution turned yellow. Subsequently, 0.1 M, 1000 μL of dilute hydrochloric acid was added as a catalyst, stirred evenly, and then added to a reactor. The mixture was heated in an oven at 150°C for 72 h to obtain a dark brown mixed solution. The mixture was filtered, the precipitate was separated, and dried to obtain COFs. The obtained COFs material overreacted, its morphology changed, and porous spherical COFs could not be obtained.

[0035] Under the same experimental conditions as in Example 1, TNP detection and adsorption experiments were carried out. The experimental results showed that the COFs synthesized in this comparative example did not have a significant fluorescence detection effect on TNP, but the adsorption of TNP was only about 200 mg / g, demonstrating the influence of the material structure on the adsorption amount. Comparative Example 3

[0036] 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 16.8 mg of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde were added to a 5 mL mixed solution of PEG-200 and acetonitrile (PEG-200 / acetonitrile = 1 mL:1 mL). Ultrasonication was performed until the solids were completely dissolved and the solution turned yellow. Subsequently, 1000 uL of 1 M dilute hydrochloric acid was added as a catalyst and stirred evenly. The mixture was then added to a reactor and the reaction was continued in an oven for 2 h at a temperature of 4°C and refrigerated to obtain a dark brown mixed solution. The mixture was filtered, the precipitate was separated, and dried to obtain the product, which did not have a uniform and controllable morphology.

[0037] Under the same experimental conditions as in Example 1, TNP detection and adsorption experiments were carried out. The experimental results showed that the COFs synthesized in this comparative example did not have a significant fluorescence detection effect on TNP, but the adsorption of TNP was only about 60 mg / g, demonstrating the influence of the material structure on the adsorption amount. Comparative Example 4

[0038] 0.08 mmol, 28 mg of 1,3,5-tris(4-aminophenyl)benzene and 0.08 mmol, 16.8 mg of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde were added to a 5 mL mixed solution of PEG-200 and acetonitrile (PEG-200 / acetonitrile = 1 mL:1 mL). Ultrasonication was performed until the solids were completely dissolved and the solution turned yellow. Subsequently, 0.3 M, 1000 μL of glacial acetic acid was added as a catalyst, stirred evenly, and the mixture was added to a reactor. The mixture was heated in an oven at 60°C for 24 h to obtain a light brown mixture. The mixture was filtered, the precipitate was separated, and dried to obtain COF material. The obtained COFs had unclear boundaries and irregular morphology, presumably because the low concentration of acetic acid gradually evaporated during the reaction heating process, making it impossible to accurately control the morphology.

[0039] Under the same experimental conditions as in Example 1, TNP detection and adsorption experiments were carried out. The experimental results showed that the COFs synthesized in this comparative example did not have a significant fluorescence detection effect on TNP, but the adsorption of TNP was only about 128 mg / g, proving the influence of the material structure on the adsorption amount.

[0040] Figure 1 The figure shows the transmission electron microscopy (TEM) characterization of COFs in Example 1 of the present invention. It can be seen that the synthesized material is evenly distributed and has a uniform particle size of about 300-400 nm. Under this particle size condition, the fluorescence detection effect is good and the adsorption efficiency is high.

[0041] Figure 2The figure shows a high-resolution transmission electron microscope (TEM) characterization image of COFs in Example 1 of the present invention. Figure 2 It can be clearly seen that the synthesized material has extremely high porosity, and the uniform and clear pore size distribution proves that COFs were successfully synthesized.

[0042] Figure 3 The figure shows a scanning electron microscope (SEM) characterization image of COFs in Example 1 of the present invention. From the scanning electron microscope, it can be seen from a three-dimensional perspective that pores are evenly distributed on the surface of COFs.

[0043] Figure 4 This is an X-ray diffraction (XRD) characterization diagram of COFs in Example 1 of the present invention, which describes the unique diffraction peaks contained in COFs. Among them, COFs has a strong characteristic peak at 2θ = ~2.77°, which can be corresponding to the (100) reflection plane of COFs. In addition, the characteristic peaks displayed near ~8.5° and ~10.3° can correspond to the Bragg peaks of the (110) and (001) crystal planes of COFs, respectively, proving the crystalline structure of the material. The successful synthesis of COFs is well verified by the diffraction peaks contained in XRD.

[0044] Figure 5 Shown is a thermogravimetric (TGA) graph of COFs from Example 1 of the present invention. As can be seen from the figure, the material maintains excellent stability at temperatures up to 400°C, with mass loss occurring in two stages. ① As the temperature rises within the 25°C to 230°C range, a portion of the mass loss is attributed to water and organic solvents. ② Subsequently, above 400°C, the COFs' hydroxyl groups and backbone decompose, resulting in a significant decrease in mass percentage, with attenuation visible at 460°C. However, the material maintains excellent stability within the 400°C range, demonstrating its high-temperature resistance.

[0045] Figure 6 Graphs 1 and 2 show the maximum excitation spectrum and emission spectrum of COFs in Example 1 of the present invention. It can be determined that COFs exhibit good fluorescence properties, with a maximum excitation wavelength of 397 nm and a maximum emission wavelength of 525 nm.

[0046] Figure 7 The gap between the COFs solution and TNP in Example 1 of the present invention is 3.0×10 −3 −2.2×10 2 The linear relationship is excellent in the range of μM, log(F0 / F) = 0.00377C TNP +0.06763(R 2 = 0.994) with a limit of detection (LOD) of 1.6 nM based on the triplet signal-to-noise ratio.

[0047] Figure 8This is the salt stability of the COFs in Example 1 of the present invention. The salt stability was tested in a PBS buffer solution with a pH of 7 containing different concentrations of NaCl. The experiment proved that the COFs of the present invention can maintain the stability of the normalized fluorescence intensity when the salt concentration is as high as 500 μM, and have good salt stability.

[0048] Figure 9 This is an exploration of the TNP detection mechanism of the COFs fluorescent probe in Example 1 of the present invention. In order to explore the possible detection mechanism, this experiment tested the UV-visible absorption spectra of the probe, TNP and the probe mixed solution. It was found through experiments that the experimental absorption spectrum of COFs with TNP is basically consistent with the absorption spectrum of COFs alone, proving that the interaction between COFs and TNP is weak and there is no possibility of forming a Meisenheimer complex. In subsequent explorations, the present invention further detected the excitation spectrum, emission spectrum and UV absorption spectrum of COFs. The experiment showed that the absorption spectrum of COFs has a large degree of overlap with its excitation spectrum, which meets the basic requirements of the internal filter effect reaction mechanism. The role of the fluorescence resonance energy transfer mechanism was studied. Similarly, the experiment showed that there was almost no overlap between the emission spectrum of COFs and the absorption of TNP, so the fluorescence resonance energy transfer mechanism played almost no role in this system. Therefore, according to the experiment, it can be proved that the internal filter effect reaction mechanism is the main mechanism that leads to the detection of TNP by COFs and is the main reason for the fluorescence quenching in the system.

[0049] Figure 10 Figures 2 and 3 show the UV absorption spectrum, fluorescence excitation, and emission spectra of the COFs fluorescent probe in Example 1 of the present invention. To explore possible mechanisms, the UV-vis absorption spectra of the probe T'NP and its mixed solution were studied. Experiments showed that the experimental absorption spectrum of the COFs with TNP added was essentially consistent with the UV absorption spectrum of the TNP itself, indicating a weak interaction between the COFs and TNP, thus ruling out fluorescence quenching due to the formation of a Meisenheimer complex. Subsequently, the present invention also investigated the role of the inner filter effect in this system. First, the present invention explored the mechanism of fluorescence quenching by studying the lifetime of fluorescence quenching. Experiments demonstrated that the absorption spectrum of TNP overlapped significantly with the excitation spectrum of COFs, meeting the basic requirements for the fluorescence quenching reaction mechanism reported in the literature due to the inner filter effect. Therefore, it was inferred that the inner filter effect was the primary mechanism of fluorescence quenching in this system. Next, the role of the fluorescence resonance energy transfer mechanism was investigated. Similarly, experiments demonstrated that there was little overlap between the emission spectrum of COFs and the absorption spectrum of TNP, indicating that the fluorescence resonance energy transfer mechanism played little role in this system. Therefore, in summary, the inner filter effect is the main mechanism causing fluorescence quenching when COFs detect TNP.

[0050] Figure 11 The specific surface area of COFs in Example 1 of the present invention is characterized by BET. The specific surface area of COFs is as high as 1735.36 m 2 / g, the high specific surface area means that there are more active sites on its surface, which can contact with more TNPs, and the ultra-high specific surface area makes COFs more efficient in the adsorption process, and can remove TNPs in water faster.

[0051] Figure 12 This is the number of times the COFs obtained in Example 1 of the present invention are reused when adsorbing TNP. Experiments have shown that a good removal rate can still be achieved after 6 cycles, and reuse is possible.

[0052] Figure 13 This is a transmission electron microscopy (TEM) image of the COFs obtained in Comparative Example 1 of the present invention. It shows that under the experimental conditions of solvent change and the absence of polyethylene glycol as a morphology control agent, solid spherical COFs were directly obtained. The COF particle size far exceeded 400 nm, even reaching the micron level. As the particle size increased, the pore structure was affected, resulting in poor pore connectivity. A clear pore structure was no longer visible in the TEM image, seriously affecting the adsorption rate and capacity. Application experiments showed that under the same experimental conditions, the detection sensitivity was significantly reduced, and the low-level TNP content detected in Example 1 could not be detected, and the adsorption capacity reached only 190 mg / g.

[0053] Figure 14 This is a transmission electron microscope (TEM) characterization image of the COFs obtained in Comparative Example 2 of the present invention. It can be seen that under the experimental conditions of increasing temperature and extending time, a COFs material with a porous spherical morphology cannot be obtained, and the obtained material is messy and has no obvious morphological rules.

[0054] Figure 15 This is a transmission electron microscope (TEM) characterization image of the COFs obtained in Comparative Example 3 of the present invention. When the concentration of the catalyst dilute hydrochloric acid is increased, a COFs material with a porous spherical morphology cannot be obtained. The obtained material is messy and has no obvious morphological rules. The too short reaction time and low temperature result in the reaction not being fully carried out, and a porous spherical COFs material cannot be obtained.

[0055] Figure 16 This is a transmission electron microscopy (TEM) characterization image of the COFs obtained in Comparative Example 4 of the present invention. When the catalyst is changed to low-concentration acetic acid to catalyze the reaction, the COFs material with a porous spherical morphology cannot be obtained. The boundaries of the obtained COFs are unclear and the morphology is irregular. It is speculated that this is because the low-concentration acetic acid gradually evaporates during the heating process of the reaction, and it is impossible to accurately control the morphology.

Claims

1. A method for preparing COFs, characterized in that: The following steps are involved: (1) Adding a building block containing an amino functional group and a building block containing an aldehyde functional group to an acetonitrile-polyethylene glycol homogeneous solution, ultrasonicating until the solid is completely dissolved, and then adding dilute hydrochloric acid to mix evenly to obtain a brown solution; (2) heating the brown solution obtained in step (1) at a temperature of 30-80°C for 4-36 hours to obtain a reaction-completed mixed solution; (3) Separating the precipitate from the reaction mixture obtained in step (2) and drying it to obtain COFs.

2. The method for preparing COFs according to claim 1, wherein In the step (1), the building block containing an amino functional group is p-phenylenediamine or 1,3,5-tris(4-aminophenyl)benzene; and the building block containing an aldehyde functional group is one of trimesaldehyde, 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde, and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde.

3. The method for preparing COFs according to claim 1, wherein In the step (1), the molar ratio of the building block containing the aldehyde functional group to the building block containing the amino functional group is 1:1 to 1.

5.

4. The method for preparing COFs according to claim 1, wherein In the step (1), in the homogeneous solution of acetonitrile-polyethylene glycol, the polyethylene glycol is PEG-200~PEG-1000, and the volume ratio of acetonitrile to polyethylene glycol is 1:1~9; the concentration of dilute hydrochloric acid is 0.05M~0.3M, and the molar ratio of dilute hydrochloric acid to the building unit containing an amino functional group is 1:0.2~1.

6.

5. A COFs prepared by the method for preparing COFs according to any one of claims 1 to 4, characterized in that: COFs have fluorescent properties and are porous spherical in shape with a particle size of 300~400nm.

6. A use of COFs according to claim 5, characterized in that: It was applied to the detection and adsorption of nitro explosive TNP in water.

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