A rare earth doped covalent organic framework, a preparation method and application thereof, and a dichloromethane concentration visual detection method

Abstract

The application belongs to the field of fluorescence detection of covalent organic framework materials, and discloses a rare earth doped covalent organic framework, a preparation method and application thereof, and a dichloromethane visual detection method. A hexagonal covalent organic framework (COF) matrix is constructed by subjecting 5,5'-diamino-2,2'-bipyridine and a triformylphloroglucinol monomer to Schiff base reaction, and then the matrix is immersed in a solution containing europium ions (Eu 3+ ) and terbium ions (Tb 3+ ), so as to realize loading of rare earth metals and obtain a rare earth doped covalent organic framework material with specific ratio fluorescence response performance. The rare earth doped covalent organic framework has a regular organic framework structure and rare earth doping characteristics, and can exhibit wavelength-resolvable fluorescence intensity ratio change and visual color change in the presence of dichloromethane. The material has high detection sensitivity, fast response speed and simple operation, and can realize rapid detection of trace dichloromethane in a surfactant without the need for expensive instruments and complex pretreatment.

Description

Technical Field

[0001] This invention belongs to the field of fluorescence detection of covalent organic frameworks, specifically relating to a rare earth-doped covalent organic framework, its preparation method and application, and a method for visual detection of dioxane concentration. Background Technology

[0002] Fatty alcohol polyoxyethylene ethers (AEOs) are a class of nonionic surfactants that can significantly reduce the interfacial tension between liquids / liquids, liquids / gases, or liquids / solids. Their molecules consist of hydrophilic polyoxyethylene segments and hydrophobic aliphatic hydrocarbon chains. These compounds are widely used in detergents, shampoos, and cosmetics. The production of fatty alcohol polyoxyethylene ethers requires an ethoxylation reaction, which adds ethylene oxide to the organic molecule to introduce a hydrophilic segment. Under catalytic and heating conditions, this reaction is prone to intramolecular cyclization, producing dioxane (1,4-dioxane), resulting in residual dioxane in the finished product. Dioxane is an organic pollutant with hepatotoxicity, nephrotoxicity, and environmental persistence, and is therefore subject to strict limits in cosmetics, pharmaceuticals, and food products.

[0003] Currently, the mainstream dioxane detection method is based on gas chromatography-mass spectrometry (GC-HS) equipped with a headspace sampler. This method requires sample pretreatment such as weighing, dilution, shaking, and heating equilibration. A single sample processing cycle takes more than 30 minutes. Oil samples also require the addition of N,N-dimethylformamide as a solubilizer, and complex samples such as shampoo require solid-phase extraction purification, which can extend the overall analysis cycle to several hours. GC-HS equipment is expensive, operation depends on professional technicians and constant temperature and humidity conditions, maintenance costs are high, sample throughput is low, and rapid detection is not supported.

[0004] Covalent organic frameworks (COFs) are a class of crystalline porous materials formed by covalently linked organic molecular units. They possess a highly ordered framework structure, large specific surface area, and the ability to introduce specific functional groups through molecular design. COFs exhibit excellent performance in adsorption, separation, and sensing. Introducing specific metal ions into the COF framework can create COF hybrid materials, which can form luminescent centers with characteristic emission peaks. When a target molecule enters the COF channels and forms a complex with the rare metal luminescent center, it alters the energy transfer efficiency between the ligand energy level and the rare earth ion, leading to changes in fluorescence intensity or emission wavelength. By monitoring fluorescence changes at specific wavelengths, rapid and sensitive identification and quantification of target analytes can be achieved.

[0005] Chinese patent application No. 202011298328.8, entitled "A Covalent Organic Framework Fluorescent Test Paper for Detecting Palladium Ions in Water and Its Preparation Method Thereof," discloses a COF material prepared by reacting the monomers tribenzaldehyde and 2,5-di(allyloxy)terephthalohydrazide via a Schiff base reaction. This material is then grown in situ on filter paper to form a fluorescent test paper. The fluorescent test paper emits yellow fluorescence under 365 nm ultraviolet light, and reacts with Pd... 2+ Fluorescence is significantly quenched upon contact, enabling rapid detection of Pd. 2+ It is suitable for rapid detection. Currently, COF is used as a fluorescent material for fluorescence detection; however, there is no application of COF materials for the detection of dioxane in fatty alcohol polyoxyethylene ethers.

[0006] In summary, the content of dioxane is closely related to the product quality of fatty alcohol polyoxyethylene ethers. However, existing GC-HS methods cannot meet the needs of rapid monitoring in terms of speed, cost, and portability. Therefore, the technical problem to be solved in this case is how to provide a rapid, simple detection method that does not require expensive equipment and complex pretreatment to achieve rapid quantitative or semi-quantitative detection of dioxane in surfactants. Summary of the Invention

[0007] The purpose of this invention is to provide a rare earth-doped covalent organic framework, its preparation method and application, as well as a method for visually detecting dioxane concentration. This rare earth-doped covalent organic framework can simplify the pretreatment process and reduce the dependence on detection equipment, thereby enabling rapid detection of dioxane in surfactants.

[0008] This invention provides a method for preparing rare-earth-doped covalent organic frameworks, the method comprising the following steps:

[0009] 1) The organic acid catalyst and 5,5'-diamino-2,2'-bipyridine were ground until uniform, and then trialdehyde pyrogallol was added and ground thoroughly. The product after thorough grinding was subjected to Schiff base reaction. After the reaction was completed, it was cooled to room temperature, washed, and dried to obtain bipyridine covalent organic framework powder.

[0010] 2) Add bipyridine covalent organic framework powder to a rare earth metal salt ethanol solution, disperse by ultrasonication, and after sufficient dispersion, perform impregnation treatment. After impregnation, centrifuge and wash the product to obtain rare earth doped covalent organic framework.

[0011] In step 1) of the above preparation method, the molar ratio of the organic acid catalyst, 5,5'-diamino-2,2'-bipyridine and trialdehyde phloroglucinol is 10:1.5~2.5:0.8~1.2, and the organic acid catalyst is one of p-toluenesulfonic acid or acetic acid.

[0012] In step 1) of the above preparation method, the term "fully ground" means grinding until the powder changes completely from yellow to orange, and the grinding time is 10-20 minutes.

[0013] In step 1) of the above preparation method, the Schiff base reaction is carried out at a temperature of 130-160°C, a reaction time of 50-80 min, and a reaction pressure of 500-800 Pa.

[0014] In step 1) of the above preparation method, the drying temperature is 60~80℃ and the drying time is 20~28h.

[0015] In step 2) of the above preparation method, the molar concentration of the rare earth metal salt in the rare earth metal salt ethanol solution is 0.02M, and the rare earth metal salt is terbium chloride hexahydrate and europium chloride hexahydrate, with a molar ratio of terbium chloride hexahydrate to europium chloride hexahydrate of 1:3.5~4.5.

[0016] In step 2) of the above preparation method, 1.9~2.1g of bipyridine covalent organic framework powder is added to every 1ml of the rare earth metal salt ethanol solution.

[0017] In step 2) of the above preparation method, the ultrasonic dispersion time is 20 min and the ultrasonic power is 250 W.

[0018] The washing refers to rinsing the product three times each with N,N-dimethylformamide, water, and acetone to remove impurities.

[0019] In step 2) of the above preparation method, the immersion treatment is carried out at a temperature of 70°C for 24 hours.

[0020] This invention provides a rare-earth-doped covalent organic framework prepared using the above-described method. The chemical structural formula of this rare-earth-doped covalent organic framework is as follows:

[0021] ;

[0022] In the formula, Ln is one or both of Eu and Tb.

[0023] The rare-earth-doped covalent organic framework emits fluorescence at 617 nm and 545 nm wavelengths under ultraviolet light excitation in the wavelength range of 254-285 nm. The ratio of the fluorescence intensity at 617 nm to that at 545 nm is linearly related to the concentration of dioxane in the range of 0-25 μM. This ratio response can be used to quantitatively determine the concentration of dioxane, with a detection limit of 7.81 nM.

[0024] This invention provides an application of rare earth-doped covalent organic frameworks in the visual detection of dioxane concentration.

[0025] This invention also provides a method for visually detecting the concentration of dioxane in fatty alcohol polyoxyethylene ethers, the specific details of which are as follows:

[0026] (1) Add dioxane standard solution to the DMF solution of the rare earth-doped covalent organic framework to make the final concentration of dioxane in the system 0, 10, 20, 30, 40, 50, 60 and 70 μM, respectively, and then test the RGB value of each system.

[0027] (2) Plot a standard curve with the ratio of the intensity of the red channel and the sum of the intensity of the green channel and the intensity of the blue channel [R / (G+B)] as the ordinate and the dioxane concentration as the abscissa to obtain the linear equation;

[0028] (3) Add the test solution to the DMF solution of the rare earth-doped covalent organic framework, test its RGB value, and substitute its R / (G+B) value into the linear equation to calculate the dioxane concentration in the test solution.

[0029] In the above detection method, the concentration of rare earth-doped covalent organic framework in the standard solution is 0.08~0.12 mg / mL. -1 .

[0030] In the above detection method, the test solution is a fatty alcohol polyoxyethylene ether with an addition number of 6 to 9.

[0031] In step (3) of the above detection method, the concentration of the rare earth-doped covalent organic framework in the DMF solution is 0.08~0.12 mg / mL. -1 .

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1. This invention prepares rare earth-doped covalent organic frameworks by a synthesis and impregnation method, which can be used as fluorescent sensors to achieve rapid ratio-based trace detection of dioxane.

[0034] 2. The ratio response of the material of the present invention is a fluorescence color shift from yellow to blue, which can be visually identified by the naked eye, and can achieve visualization and semi-quantitative detection of dioxane in the concentration range of 0~50μM with the help of a smartphone with color recognition software.

[0035] 3. The present invention is simple and easy to use, requiring no expensive instruments or complex sample pretreatment, and enables portable detection of dioxane without the need for professional laboratories and technicians. Attached Figure Description

[0036] Figure 1Comparison of powder X-ray diffraction (PXRD) patterns of bpy-COF, Eu / Tb-COF 1 and simulated bpy-COF;

[0037] Figure 2 The fluorescence spectra of COF materials before and after loading Eu and Tb are shown.

[0038] Figure 3 Fluorescence titration emission spectra of Eu / Tb-COF1 for detecting different concentrations of dioxane;

[0039] Figure 4 The fitted curve of Δ / Δ0 (Δ=I617 / I545) versus the linear range of dioxane concentration (0~25µM);

[0040] Figure 5 Figure (a) shows the CIE chromaticity coordinates of Eu / Tb-COF1 after adding different concentrations of dioxane; Figure (b) shows the linear relationship between dioxane concentration (0–50 μM) and R / (G+B); Figure (c) shows the color changes at dioxane concentrations of 0, 10, 20, 30, 40, 50, 60, and 70 μM, where λ ex =254nm;

[0041] Figure 6 A comparison of the response effects of Eu / Tb-COF 1 and Eu / Tb-COF 2 with different Tb:Eu loading ratios to dioxane; Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Unless otherwise specified, all test materials and reagents used in the following examples are commercially available.

[0044] Unless otherwise specified in the embodiments, the techniques or conditions described in the literature in this field or in accordance with the product manual may be followed.

[0045] The fluorescence detection in the embodiments and comparative examples of this invention was performed using a fluorescence spectrophotometer (Shimadzu RF-6000); the measurement conditions were: excitation wavelength 285 nm, emission wavelength range 400~800 nm, excitation and emission slit widths were both 5 nm, and the measurement was performed at a temperature of 25±5℃.

[0046] The HS-GC analysis conditions were based on GB / T 26388-2011. The instrument model was SHIMADZUNexis GC-2030 gas chromatograph combined with HS-20 headspace autosampler, using SH-I-5HT column, and N,N-dimethylformamide as solvent.

[0047] Example 1

[0048] Preparation of rare earth-doped covalent organic frameworks:

[0049] 1) 2.8 mmol of p-toluenesulfonic acid and 0.45 mmol of 5,5'-diamino-2,2'-bipyridine were added sequentially to a mortar and ground for 10 minutes until homogeneous. Then, 0.33 mmol of trialdehyde phloroglucinol was added, and grinding was continued for 10 minutes until the color changed from yellow to orange. The mixture was transferred to a reaction vessel and reacted with a Schiff base at 150 °C for 60 minutes under a vacuum oven at 500 Pa. After the reaction, the mixture was cooled to room temperature and then treated three times with N,N-dimethylformamide, water, and acetone to remove residues. After drying at 80 °C for 24 h, a bipyridine covalent organic framework was obtained, labeled as bpy-COF.

[0050] 2) Prepare 5 mL of a mixed ethanol solution with a concentration of 0.02 M and a molar ratio of terbium chloride hexahydrate to europium chloride hexahydrate of 1:4. Then add 10 g of bpy-COF powder to the solution, seal it, and ultrasonically disperse it at 250 W for 20 minutes. Then heat it at 70 °C for 24 hours for impregnation treatment. After completion, wash and centrifuge to obtain rare earth doped covalent organic framework, labeled as Eu / Tb-COF 1.

[0051] Depend on Figure 1 As can be seen, powder X-ray diffraction (PXRD) results show that the crystal structures of bpy-COF and Eu / Tb-COF remain unchanged before and after loading. Comparison of the measured experimental data with simulated X-ray diffraction patterns reveals high similarity and crystallinity, confirming the successful synthesis of bpy-COF.

[0052] The Brunauer−Emmett−Teller (BET) surface area of ​​Eu / Tb-COF1 is approximately 523.6–618.5 m² / g, with the pore size distribution of Eu / Tb-COF being most concentrated in the 2–4 nm range; Figure 2 It is evident that the COF material without rare earth elements did not exhibit a significant fluorescence emission peak, while the COF material showed a characteristic emission peak after the introduction of Eu / Tb elements. This proves that Eu / Tb metal ions have been successfully doped and immobilized in the COF framework.

[0053] Detection of dioxane by fluorescent titration of Eu / Tb-COF 1:

[0054] 5.0 mg of the Eu / Tb-COF 1 sample prepared in Example 1 was dispersed in 50 mL of N,N-dimethylformamide and sonicated for 15 minutes to obtain 0.1 mg / mL. -1 The stock solution was then transferred to a quartz cuvette, and freshly prepared 1 mM dioxane / DMF solution was added incrementally using a precision pipette. After each addition, the solution was incubated at room temperature for 2 min, and the fluorescence emission spectrum was recorded under 285 nm UV excitation. This process was repeated until a predetermined concentration gradient of 70 μM was achieved. All fluorescence measurements were performed under identical conditions to ensure consistency of results.

[0055] The results of the fluorescence titration are as follows Figure 3 As shown, as the dioxane concentration increased from 0 to 70 μM, the fluorescence intensity of the rare earth-doped covalent organic framework prepared in Example 1 gradually decreased at 545 nm, while the fluorescence intensity gradually increased at 617 nm.

[0056] according to Figure 3 The fluorescence intensity ratio (Δ) of Eu / Tb-COF 1 at 545 nm and 617 nm was used as an indicator. The results are plotted as follows: Figure 4 The curves showing Δ / Δ0 versus dioxane concentration are shown, where Δ0 represents the fluorescence intensity ratio of the blank sample (without dioxane) and Δ represents the fluorescence intensity ratio of the sample containing dioxane.

[0057] Experimental results show that, within the range of 0–25 μM, Δ / Δ0 exhibits a linear relationship with dioxane concentration, which can be described by the equation Δ / Δ0 = 3.739 × C + 0.9946 (C in μM). The linear fitting correlation coefficient R0 is [value missing]. 2 =0.999; The limit of detection (LOD) of dioxane was determined according to the IUPAC recommended method (3σ / K). The blank sample was measured three times. The standard deviation σ was substituted into the slope K of the standard curve to calculate the LOD, which was 7.81 nM.

[0058] Comparative Example 1

[0059] Synthesis of rare-earth-doped covalent organic frameworks:

[0060] The preparation method is the same as in Example 1, except that the molar ratio of terbium chloride hexahydrate to europium chloride hexahydrate in step 2) is changed to 4:1 to obtain a rare earth-doped covalent organic framework, labeled as Eu / Tb-COF 2.

[0061] Eu / Tb-COF2 was used for fluorescence detection of dioxane according to the method in Example 1. The experimental results were as follows: Figure 6 As shown, it is clear that Eu / Tb-COF 2 has a significantly lower sensitivity to dioxane compared to Eu / Tb-COF 1.

[0062] Application Example 1

[0063] A method for visually detecting dioxane concentration using rare-earth-doped covalent organic frameworks:

[0064] Rare earth-doped covalent organic frameworks with a concentration of 0.1 mg / mL were prepared using DMF as a solvent. -1 Furthermore, dioxane standard solutions with concentrations of 0, 10, 20, 30, 40, 50, 60, and 70 μM were used. Images of the test solutions were captured using a smartphone equipped with a color recognition application under 285 nm ultraviolet light irradiation. The values ​​of the R, B, and G channels in the fluorescence emission region were obtained. As the dioxane concentration increased from 0 μM to 70 μM, the Eu / Tb-COF samples exhibited a continuous change from pale yellow to red. Figure 5 In the middle (c), the CIE coordinates gradually redshift from (0.38917, 0.41182) to (0.47541, 0.31614). Figure 5 (a) is consistent with the emission color observed by the naked eye.

[0065] A standard curve was constructed by plotting the ratio of the red channel intensity to the sum of the green channel intensity and the blue channel intensity [R / (G+B)] on the ordinate and the dioxane concentration on the abscissa, yielding the linear equation y=0.156×x+0.766, where x is in μM. The limit of detection (LOD) was calculated using the 3σ / K method. The blank sample was measured three times, and the LOD was found to be 0.22 μM.

[0066] The test solution is added to a DMF solution containing a rare earth-doped covalent organic framework. An image of the test solution is taken using a smartphone with a color recognition application. The values ​​of the R, B, and G channels in the fluorescence emission region are obtained. The R / (G+B) value is substituted into a linear equation to calculate the dioxane concentration in the test solution.

[0067] Sensitivity and accuracy of spiked recovery detection of dioxane in surfactants using Eu / Tb-COF.

[0068] Fatty amine polyoxyethylene ether (AEO-9) was used as the real sample, and the recovery rate was determined by a spiked recovery experiment. 100 mg of Eu / Tb-COF 1 sample was dispersed in 1 L of N,N-dimethylformamide and sonicated for 15 minutes to obtain 0.1 mg / mL of the sample. -1The dispersion was prepared; 100g of AEO-9 sample was taken and diluted to 1L with the above stock solution. After sonication at 250W for 10min, a detection solution containing 10g / L AEO-9 was obtained; dioxane was added to the detection solution to make its final concentrations 5, 10, and 15μM, and the fluorescence spectra at 400~800nm ​​were measured respectively. Figure 4 The standard curve obtained was used to calculate the recovery rate of dioxane, and the recovery concentration was verified by HS-GC analysis. The results are shown in Table 1.

[0069] Table 1. Results of spiked recovery experiments of surfactant solutions determined by fluorescence spectroscopy.

[0070] 5 4.83 96.60 2.247 5.11 10 10.10 101.0 1.033 9.88 15 14.81 98.73 0.541 15.25

[0071] As can be seen, the recoveries of the probe for dioxane spiked in real samples ranged from 96.60% to 101.0%, with a relative standard deviation (RSD) of less than 2.5%. Furthermore, there was no significant difference between the results obtained by the HS-GC method and the fluorescence assay.

[0072] Under the same conditions, dioxane was added to the detection solution to achieve final concentrations of 10, 20, and 50 μM. The RGB values ​​of the solutions were then read under a 254 nm UV lamp. Figure 5 The standard curve obtained in b was used to calculate the recovery rate of dioxane, and the recovery concentration was verified by HS-GC analysis. The results are shown in Table 2.

[0073] Table 2 shows the experimental results of spiked recovery of surfactant solutions determined using RGB numerical values.

[0074] 10 9.24 92.40 3.428 9.89 30 32.1 107.00 2.547 30.81 50 53.01 106.02 3.451 50.35

[0075] As can be seen, the recoveries of the probe for dioxane spiked in real samples ranged from 92.40% to 107.0%, with a relative standard deviation (RSD) of less than 3.5%. Furthermore, there was no significant difference between the results obtained by the HS-GC method and the fluorescence assay.

[0076] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. The application of a rare-earth-doped covalent organic framework in the visual detection of dioxane concentration, characterized in that, The method for preparing the rare earth-doped covalent organic framework includes the following steps: 1) The organic acid catalyst and 5,5'-diamino-2,2'-bipyridine were ground until uniform, and then trialdehyde pyrogallol was added and ground thoroughly. The product after thorough grinding was subjected to Schiff base reaction. After the reaction was completed, it was cooled to room temperature, washed, and dried to obtain bipyridine covalent organic framework powder. 2) Add bipyridine covalent organic framework powder to a rare earth metal salt ethanol solution, disperse it by ultrasonication, and after it is fully dispersed, perform impregnation treatment. After impregnation, centrifuge and wash the product to obtain rare earth doped covalent organic framework. The rare earth metal salts are terbium chloride hexahydrate and europium chloride hexahydrate, with a molar ratio of terbium chloride hexahydrate to europium chloride hexahydrate of 1:3.5~4.5; The rare-earth-doped covalent organic framework emits fluorescence at wavelengths of 617 nm and 545 nm under ultraviolet light excitation in the wavelength range of 254–285 nm. The ratio of the fluorescence intensity at 617 nm to that at 545 nm is linearly related to the dioxane concentration in the range of 0–25 μM.

2. The application according to claim 1, characterized in that, The detection limit for dioxane concentration in visual detection is 7.81 nM.

3. The application according to claim 1, characterized in that, The molar ratio of the organic acid catalyst, 5,5'-diamino-2,2'-bipyridine, and trialdehyde-resorcinol is 10:1.5~2.5:0.8~1.2, and the organic acid catalyst is one of p-toluenesulfonic acid or acetic acid.

4. The application according to claim 1, characterized in that, The term "thorough grinding" refers to grinding until the powder completely changes from yellow to orange, with a grinding time of 10-20 minutes.

5. The application according to claim 1, characterized in that, The Schiff base reaction is carried out at a temperature of 130-160℃, a reaction time of 50-80 min, and a reaction pressure of 500-800 Pa.

6. The application according to claim 1, characterized in that, The rare earth metal salt ethanol solution has a rare earth metal salt molar concentration of 0.02 M, and 1.9~2.1 g of bipyridine covalent organic framework powder is added to each 1 ml of the rare earth metal salt ethanol solution.

7. A method for visually detecting dioxane concentration, characterized in that, The specific details of the detection method are as follows: (1) Add dioxane standard solution to the DMF solution of rare earth doped covalent organic framework to make the final concentration of dioxane in the system 0, 10, 20, 30, 40, 50, 60 and 70 μM, and then test the RGB value of each system. (2) Plot a standard curve with the ratio of the intensity of the red channel and the sum of the intensity of the green channel and the intensity of the blue channel [R / (G+B)] as the ordinate and the dioxane concentration as the abscissa to obtain the linear equation; (3) Add the test solution to the DMF solution of rare earth doped covalent organic framework, test its RGB value, and substitute its R / (G+B) value into the linear equation to calculate the dioxane concentration in the test solution. The method for preparing the rare earth-doped covalent organic framework includes the following steps: 1) The organic acid catalyst and 5,5'-diamino-2,2'-bipyridine were ground until uniform, and then trialdehyde pyrogallol was added and ground thoroughly. The product after thorough grinding was subjected to Schiff base reaction. After the reaction was completed, it was cooled to room temperature, washed, and dried to obtain bipyridine covalent organic framework powder. 2) Add bipyridine covalent organic framework powder to a rare earth metal salt ethanol solution, disperse it by ultrasonication, and after it is fully dispersed, perform impregnation treatment. After impregnation, centrifuge and wash the product to obtain rare earth doped covalent organic framework. The rare earth metal salts are terbium chloride hexahydrate and europium chloride hexahydrate, with a molar ratio of terbium chloride hexahydrate to europium chloride hexahydrate of 1:3.5~4.5; The rare-earth-doped covalent organic framework emits fluorescence at wavelengths of 617 nm and 545 nm under ultraviolet light excitation in the wavelength range of 254–285 nm. The ratio of the fluorescence intensity at 617 nm to that at 545 nm is linearly related to the dioxane concentration in the range of 0–25 μM.

8. The method for visually detecting dioxane concentration according to claim 7, characterized in that, The concentration of the rare-earth-doped covalent organic framework in the DMF solution is 0.08~0.12 mg / mL. -1 .

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