A Method for Detecting Cr(Ⅵ) Using a Dual-Lanthanide Metal-Organic Framework Eu / Tb MOF Ratio Fluorescence Sensor

Abstract

The present invention belongs to the technical field of nanomaterial detection, and specifically discloses a method for detecting Cr(Ⅵ) by a dual-lanthanide metal-organic framework Eu / Tb MOF ratio fluorescence sensor, which includes the following steps: 1. Screening and preparation of the lanthanide metal-organic framework Eu / Tb MOF with 3,5-dicarboxylphenylboronic acid as a ligand; 2. Detection of Cr(Ⅵ) by the Eu / Tb MOF ratio fluorescence sensor. Cr(Ⅵ) quenches the fluorescence absorption peak of Eu / Tb MOF at 545 nm, while the fluorescence absorption peak at 618 nm hardly changes. Based on this, a Eu / Tb MOF ratio fluorescence sensor is constructed to detect Cr(Ⅵ), and the concentration of Cr(Ⅵ) in the sample to be measured is judged by the fluorescence intensity, color change, and CIE chromaticity diagram degree. The Eu / Tb MOF ratio fluorescence sensor can generate more color ranges, detect different concentrations of Cr(Ⅵ) more precisely, and improve the sensitivity. The method of the present invention has the characteristics of low cost, fast response speed, low detection limit, high sensitivity and selectivity.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanomaterial detection, and particularly relates to a method for detecting Cr(VI) by a dual-lanthanide metal-organic framework Eu / Tb MOF ratio fluorescence sensor. Background Art

[0002] As a metal ion widely used in modern industrial and agricultural fields, chromium exhibits quite high stability in the ecological system. In nature, chromium mainly exists in two forms: trivalent and hexavalent. Among them, trivalent chromium (Cr(III)) belongs to the category of essential trace elements for humans, while hexavalent chromium (Cr(VI)) is quite different. Its toxicity is extremely strong, being more than 100 times higher than that of trivalent chromium. Moreover, Cr(VI) has stronger human absorbability and bioaccumulation, cannot be biodegraded in the water environment, and poses a major hazard to humans and most animals. Even at trace or ultra-trace concentrations, it still has the potential to induce serious consequences such as hereditary genetic defects, allergic reactions, and various cancers. Cr(VI) is defined as a toxic and harmful heavy metal, and extremely strict restrictions have been imposed on its content in drinking water. According to the "Hygienic Standards for Drinking Water" GB 5749-2022, the concentration threshold of Cr(VI) in drinking water is limited to 0.170 μM. Therefore, accurately detecting and determining Cr(VI) in the water environment is crucial for human safety and environmental protection.

[0003] Currently, the methods for detecting hexavalent chromium (Cr(VI)) mainly include atomic absorption spectrometry (AAS), ion chromatography (IC), and inductively coupled plasma mass spectrometry (ICP-MS), etc. Although these technologies have high sensitivity, they usually rely on large equipment, require complex sample pretreatment and professional knowledge, take a long time, and limit the feasibility of real-time detection. Therefore, there is an urgent need to develop a simple, rapid, sensitive, and accurate method for detecting Cr(VI) in the water environment. Fluorescence (FL) methods are suitable for the selective determination of trace chromium due to their excellent spatial and temporal resolution, high sensitivity, and simplicity. This method can be operated by personnel with less training using inexpensive spectrometers and can provide stable detection signals. The effective combination of these methods helps to improve the efficiency and accuracy of detecting hexavalent chromium (Cr(VI)). Summary of the Invention

[0004] Aiming at the technical problems existing in the prior art, the invention purpose of the present invention is to provide a method for detecting Cr(VI) by a dual-lanthanide metal-organic framework Eu / Tb MOF ratio fluorescence sensor. This method can not only detect Cr(VI) quickly and with high sensitivity, but also realize the visual detection of real samples such as tap water, lake water, and green tea.

[0005] To achieve the above object, the present invention provides a method for detecting Cr(Ⅵ) by Eu / Tb MOF ratio fluorescence sensing, which includes using Ln MOF modified with 3,5-dicarboxylphenylboronic acid as a raw material, and specifically binding it with Cr(Ⅵ) under a specific sample addition sequence and addition ratio to obtain a Eu / Tb MOF ratio fluorescence sensor.

[0006] The method includes the following steps:

[0007] Step (1), selection of ligands

[0008] (1.1) Using isophthalic acid as ligand A, weigh ligand A and Eu(NO)3·6H2O, dissolve them in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up; then centrifuge, discard the supernatant, and redisperse the obtained product after drying the residue in ultrapure water to obtain a Eu MOF solution;

[0009] According to the above process, replace isophthalic acid with 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid, and 3,5-dicarboxylphenylboronic acid as ligand A respectively, and keep the rest of the process unchanged. Correspondingly, Eu MOF solutions with 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid, and 3,5-dicarboxylphenylboronic acid as ligand A are obtained respectively;

[0010] (1.2) Using isophthalic acid as ligand A, weigh ligand A and Tb(NO)3·6H2O, dissolve them in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up; then centrifuge, discard the supernatant, and redisperse the obtained product after drying the residue in ultrapure water to obtain a Tb MOF solution;

[0011] According to the above process, replace isophthalic acid with 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid, and 3,5-dicarboxylphenylboronic acid as ligand A respectively, and keep the rest of the process unchanged. Correspondingly, Eu MOF solutions with 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid, and 3,5-dicarboxylphenylboronic acid as ligand A are obtained respectively;

[0012] (1.3) Using isophthalic acid as ligand A, weigh ligand A, Eu(NO)3·6H2O, and Tb(NO)3·6H2O, dissolve them in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up. Then, centrifuge and discard the supernatant. The residue after drying is redispersed in ultrapure water to obtain the Eu / Tb MOF solution. According to the above process, replace isophthalic acid with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid as ligand A respectively, and keep the rest of the process unchanged. Correspondingly, Eu / Tb MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid as ligand A are obtained respectively.

[0013] (1.4) Measure the Eu MOF solution, Tb MOF solution, and Eu / Tb MOF solution prepared in steps (1.1), (1.2), and (1.3) respectively, mix them with the same volume of Cr(Ⅵ) solution in sequence to obtain different mixed solutions, and compare the fluorescence absorption intensity ratios I 545 / I 618 of the mixed solutions and the fluorescence absorption intensity ratios I 545 / I 618 of the Eu MOF solution, Tb MOF solution, and Eu / Tb MOF solution prepared in steps (1.1), (1.2), and (1.3) respectively. Through comparative analysis, finally determine that 3,5-dicarboxyphenylboronic acid is the best ligand.

[0014] Step (2), Selection of the Eu / Tb ratio in Eu / Tb MOF

[0015] (2.1) Dissolve 3,5-dicarboxyphenylboronic acid and Eu(NO)3·6H2O and Tb(NO)3·6H2O with different molar ratios in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up. Then, centrifuge and discard the supernatant. The residue is dried and the obtained product is redispersed in ultrapure water to obtain the Eu / Tb MOF solution, which is stored in the refrigerator at 4℃ in the dark for standby. Among them, the total molar amount of Eu(NO)3·6H2O and Tb(NO)3·6H2O is in a ratio of 1:5 to the molar amount of 3,5-dicarboxyphenylboronic acid, and the molar ratios of Eu(NO)3·6H2O to Tb(NO)3·6H2O are 10:0, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 0:10 in sequence.

[0016] (2.2) Mix the Eu / Tb MOF solution prepared in step (2.1) with equal volumes of Cr(VI) solutions of different concentrations respectively to obtain a mixed solution after reaction; compare the fluorescence spectrum data of the Eu / Tb MOF solution prepared in step (2.1), compare the color change and fluorescence spectrum data of the mixed solution, and compare the CIE chromaticity diagram converted from the fluorescence spectrum data of the mixed solution; determine the Eu / Tb molar ratio based on the least overlapping part in the CIE chromaticity diagram by comparing the fluorescence spectrum data of the Eu / Tb MOF solution and comparing the color change and fluorescence spectrum data of the mixed solution, and obtain Eu 0.5 / Tb 0.5 MOF solution as a Eu / Tb MOF ratiometric fluorescence sensor.

[0017] Step (3), Eu 0.5 / Tb 0.5 MOF ratiometric fluorescence sensor for detecting Cr(VI)

[0018] Adjust the pH of the Eu / Tb MOF ratiometric fluorescence sensor obtained in step (2) to 7. Mix the Eu / Tb MOF ratiometric fluorescence sensor adjusted to pH = 7 with the Cr(VI) solution to obtain a mixed solution. By changing the concentration of the Cr(VI) solution, different mixed solutions mixed with Cr(VI) solutions of different concentrations are obtained. After the reaction is complete, collect the fluorescence spectrum data of the mixed solution at room temperature and normal pressure; according to the concentration of the Cr(VI) solution and the fluorescence intensity I 545 / I 618 value of it. Take the I 545 / I 618 value as the ordinate and the concentration of the Cr(VI) solution as the abscissa to establish a standard curve for detecting the concentration of Cr(VI), and based on this standard curve, determine the content of Cr(VI) in a complex matrix system.

[0019] Preferably, the Cr(VI) solution is prepared by adding pure water to K2Cr2O7.

[0020] Preferably, the magnetic stirrer stirring means using a magnetic stirrer to stir at a speed of 2000 rpm for 2 h.

[0021] Preferably, the heating is to transfer the mixed solution to a polytetrafluoroethylene container in a stainless steel autoclave, then place it in an oven and heat it to 150 °C for 12 h. The centrifugation is at 4000 rpm for 15 min, and the drying temperature is 60 °C and the time is 12 h.

[0022] Preferably, in the step (1.1), the molar ratio of isophthalic acid to Eu(NO)₃·6H₂O is 5:1. Similarly, the molar ratio of 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid to Eu(NO)₃·6H₂O is also 5:1; the volume ratio of N,N-dimethylformamide to ultrapure water is 7:3. More preferably, isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid is 0.5 mmol, Eu(NO)₃·6H₂O is 0.1 mmol, the volume of N,N-dimethylformamide is 7 ml, and the volume of ultrapure water is 3 ml.

[0023] Preferably, in the step (1.1), the concentration of the obtained Eu MOF solution is 0.3 mg / mL.

[0024] Preferably, in the step (1.2), the molar ratio of isophthalic acid to Tb(NO)₃·6H₂O is 5:1. Similarly, the molar ratio of 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid to Tb(NO)₃·6H₂O is also 5:1; the volume ratio of N,N-dimethylformamide to ultrapure water is 7:3. More preferably, isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid is 0.5 mmol, Tb(NO)₃·6H₂O is 0.1 mmol, the volume of N,N-dimethylformamide is 7 ml, and the volume of ultrapure water is 3 ml.

[0025] Preferably, in the step (1.2), the concentration of the obtained Tb MOF solution is 0.3 mg / mL.

[0026] Preferably, in the step (1.3), the molar ratio of the amount of isophthalic acid to the total molar amount of Eu(NO)₃·6H₂O and Tb(NO)₃·6H₂O is 5:1, and the molar ratio of Eu(NO)₃·6H₂O to Tb(NO)₃·6H₂O is 1:1; similarly, the molar ratio of the amount of 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid to the total molar amount of Eu(NO)₃·6H₂O and Tb(NO)₃·6H₂O is also 5:1; the volume ratio of N,N-dimethylformamide to ultrapure water is 7:3. More preferably, isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid is 0.5 mmol, both Eu(NO)₃·6H₂O and Tb(NO)₃·6H₂O are 0.05 mmol, the volume of N,N-dimethylformamide is 7 ml, and the volume of ultrapure water is 3 ml.

[0027] Preferably, in the step (1.3), the concentration of the obtained Eu / Tb MOF solution is 0.3 mg / mL.

[0028] Preferably, in the step (1.4), the concentration of the Cr(Ⅵ) solution is 10 μM, and the volume is 400 μL. The volumes of the Eu MOF solution, Tb MOF solution, and Eu / Tb MOF solution prepared in steps (1.1), (1.2), and (1.3) are all 400 μL.

[0029] Preferably, in the step (2.1), 3,5-dicarboxylphenylboronic acid is 0.5 mmol, the total molar amount of Eu(NO)3·6H2O and Tb(NO)3·6H2O is 0.1 mmol, the volume of N,N-dimethylformamide is 7 ml, and the volume of ultrapure water is 3 ml.

[0030] Preferably, in the step (2.2), the volumes of the Eu / Tb MOF solution prepared in the step (2.1) and the Cr(Ⅵ) solutions with different concentrations are both 400 μL. The Cr(Ⅵ) solutions with different concentrations refer to the Cr(Ⅵ) solutions with concentrations of 0.1 μM, 1 μM, and 10 μM in sequence.

[0031] Preferably, in the step (2.2), the CIE chromaticity diagram is obtained by conversion using the Chromaticity Diagram plug-in in Origin.

[0032] Preferably, in the step (3), the volume of the Eu / Tb MOF ratio fluorescence sensor is 320 μL, the volume of the Cr(Ⅵ) solution is 480 μL. The Cr(Ⅵ) solutions with different concentrations refer to the Cr(Ⅵ) solutions with concentrations of 0, 0.05, 0.06, 0.08, 0.1, 0.12, 0.15, 0.18, 0.22, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 1.8, 2.2, 3, 4, 5, 6, 7, 8, 9, 10 μM. The complete reaction means the reaction for 10 min.

[0033] Compared with the prior art, the beneficial effects and advantages of the present invention:

[0034] Compared with existing Cr(VI) detection methods, the method for detecting Cr(VI) using the dual lanthanide metal-organic framework (Eu / Tb MOF) ratio fluorescence sensor of the present invention has the characteristics of low cost, fast response speed, low detection limit, high sensitivity and high selectivity. The dual lanthanide metal-organic framework (Eu / Tb MOF) ratio fluorescence sensor of the present invention, compared with a single lanthanide metal-organic framework for detecting Cr(VI), has a linear detection range of 0 to 10 μM, and the color range can change from orange to red, with an increasing degree of color change and higher sensitivity. When using the Eu / Tb MOF ratio fluorescence sensor of the present invention to detect Cr(VI), the concentration of Cr(VI) in the sample to be tested can be judged by the fluorescence intensity and / or the degree of color change. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 In the figure, A is the chemical structure diagram of different ligands used in the present invention; B is the fluorescence spectrum diagram of Ln MOF obtained with 3,5-dicarboxylphenylboronic acid as the ligand; C is the fluorescence spectrum diagram of Ln MOF obtained with isophthalic acid as the ligand; D is the fluorescence spectrum diagram of Ln MOF obtained with 2-carboxylphenylboronic acid as the ligand; E is the fluorescence spectrum diagram of Ln MOF obtained with 3-carboxylphenylboronic acid as the ligand; F is the fluorescence spectrum diagram of Ln MOF obtained with 4-carboxylphenylboronic acid as the ligand;

[0036] Figure 2 In the figure, A is the fluorescence spectrum diagram of Eu MOF, Tb MOF and Eu / Tb MOF with different Eu / Tb ratios; B is the visualization result diagram of the mixed solution obtained after mixing EuMOF, Tb MOF and Eu / Tb MOF with different Eu / Tb ratios with Cr(Ⅵ) solutions with different concentrations; C and D are the fluorescence spectrum diagrams and their CIE chromaticity diagrams of the mixed solution obtained after mixing Eu 0.4 / Tb 0.6 MOF with Cr(Ⅵ) solutions with different concentrations; E and F are the fluorescence spectrum diagrams and their CIE chromaticity diagrams of the mixed solution obtained after mixing Eu 0.5 / Tb 0.5 MOF with Cr(Ⅵ) solutions with different concentrations; G and H are the fluorescence spectrum diagrams and their CIE chromaticity diagrams of the mixed solution obtained after mixing Eu 0.6 / Tb 0.4 MOF with Cr(Ⅵ) solutions with different concentrations;

[0037] Figure 3 In the figure, A is the fluorescence spectrum diagram of Eu 0.5 / Tb 0.5 MOF solutions with different volume ratios reacting with a 10 μM Cr(Ⅵ) solution. The inset is the corresponding I 545 and I 618Ratio. B is Eu adjusted to different pH values (pH = 3, 4, 5, 6, 7, or 8) 0.5 / Tb 0.5 Fluorescence spectra of the mixed solution after mixing the MOF solution (320 μL) and 10 μM Cr(Ⅵ) solution (480 μL). The inset is the corresponding I 545 and I 618 ratio. C is the fluorescence spectrum of the mixed solution obtained after mixing Eu MOF with Cr(Ⅵ) solutions at different concentrations at pH = 7; D is the fluorescence spectrum of the mixed solution obtained after mixing Tb MOF with Cr(Ⅵ) solutions at different concentrations at pH = 7; E is Eu 0.5 / Tb 0.5 Fluorescence spectrum of the mixed solution obtained after mixing MOF with Cr(Ⅵ) solutions at different concentrations at pH = 7;

[0038] Figure 4 Among them, A is the transmission electron microscopy (TEM) morphology map of Eu 0.5 / Tb 0.5 MOF, and B and C are the corresponding energy spectrum diagrams; D is the Fourier transform infrared spectroscopy (FT-IR) diagrams of 5-bop, Eu MOF, Tb MOF, and Eu 0.5 / Tb 0.5 MOF; E is the ultraviolet-visible absorption spectroscopy (UV-Vis) diagrams of 5-bop, Eu MOF, Tb MOF, and Eu 0.5 / Tb 0.5 MOF; F is the X-ray photoelectron spectroscopy (XPS) diagram of Eu 0.5 / Tb 0.5 MOF.

[0039] Figure 5 Among them, Figure A is the non-linear fitting curve graph in the range of 0 - 10 μM for the concentration of the Cr(Ⅵ) solution (the concentrations of the Cr(Ⅵ) solutions corresponding to the points in the figure are from left to right: 0, 0.05, 0.06, 0.08, 0.1, 0.12, 0.15, 0.18, 0.22, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 1.8, 2.2, 3, 4, 5, 6, 7, 8, 9, 10 μM. The ordinate is the fluorescence intensity I 545 and I 618 ratio of the mixed solution obtained by reacting with the Cr(Ⅵ) solution. I 545 is the fluorescence intensity of the reaction system at 545 nm, and I 618 is the fluorescence intensity of the reaction system at 618 nm; by transforming the non-linear fitting curve in Figure A, the linear fitting curve graph in Figure B is obtained; Figure C is Eu 0.5 / Tb0.5 Comparison diagram of the results of detecting Cr(Ⅵ) and other ions by the MOF ratio fluorescence sensor. In the figure, the vertical axis is the fluorescence intensity I of the solution 545 / I 618 value, and the horizontal axis from left to right is blank, Cr(Ⅵ), Cr 3 + , Ag + , Ca 2+ , Cd 2+ , Co 2+ , Cu 2+ , Fe 2+ , Fe 3+ , K + , Mg 2+ , Mn 2+ , Na + , NH4 + , Ni 2+ , Pb 2+ , Zn 2+ , Cl － , CO3 2－ , HCO3 － , I － , NO3 － , PO4 3－ , SO3 2－ , SO4 2－ . D is Eu 0.5 / Tb 0.5 Anti-interference experimental result diagram of the reaction after the MOF ratio fluorescence sensor is mixed with Cr(Ⅵ) solution and other cation or anion solutions. The vertical axis is the fluorescence intensity I of the solution 545 / I 618 value, and the horizontal axis from left to right is blank, Cr(Ⅵ)+Cr 3+ , Cr(Ⅵ)+Ag + , Cr(Ⅵ)+Ca 2+ , Cr(Ⅵ)+Cd 2+ , Cr(Ⅵ)+Co 2+ , Cr(Ⅵ)+Cu 2+ , Cr(Ⅵ)+Fe 2+ , Cr(Ⅵ)+Fe 3+ , Cr(Ⅵ)+K + , Cr(Ⅵ)+Mg 2+ , Cr(Ⅵ)+Mn 2+ , Cr(Ⅵ)+Na + , Cr(Ⅵ)+NH4 + , Cr(Ⅵ)+Ni 2+ , Cr(Ⅵ)+Pb 2+ , Cr(Ⅵ)+Zn 2+, Cr(Ⅵ) + Cl － , Cr(Ⅵ) + CO3 2－ , Cr(Ⅵ) + HCO3 － , Cr(Ⅵ) + I － , Cr(Ⅵ) + NO3 － , Cr(Ⅵ) + PO4 3－ , Cr(Ⅵ) + SO3 2－ , Cr(Ⅵ) + SO4 2－ . Specific Embodiments

[0040] To better understand the present invention, the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0041] The present invention designs a dual-lanthanide metal-organic framework Eu / Tb MOF ratio fluorescence sensor for detecting Cr(Ⅵ). Since Eu MOF does not react with Cr(Ⅵ), while Tb MOF will have a quenching reaction with Cr(Ⅵ). Based on this, Eu / Tb MOF ratio fluorescence sensors with different Eu / Tb ratios are designed, and the Eu / Tb MOF ratio fluorescence sensor with the optimal Eu / Tb ratio is selected to detect Cr(Ⅵ). The method for detecting Cr(Ⅵ) by the Eu / Tb MOF ratio fluorescence sensor of the present invention will be described in detail below through specific embodiments.

[0042] In the following examples: The Cr(Ⅵ) solution is prepared by adding potassium dichromate to ultrapure water. Through comparative experiments, Eu 0.5 / Tb 0.5 MOF is selected as the Eu / Tb MOF ratio fluorescence sensor to detect Cr(Ⅵ).

[0043] Example 1

[0044] Step (1), Preparation of Ln MOF with different ligands

[0045] Before synthesis, all glass instruments to be used are soaked in aqua regia (concentrated nitric acid: concentrated hydrochloric acid = 1:3, v / v) for 3 hours and then washed with ultrapure water.

[0046] (1.1) Preparation of Eu MOF: Dissolve 0.1050 mg (0.5 mmol) of 3,5-dicarboxylphenylboronic acid and 0.0446 g (0.1 mmol) of Eu(NO)₃·6H₂O in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml). Stir the solution with a magnetic stirrer at a speed of 2000 rpm for 2 h. Then transfer the mixed solution to a Teflon container in a stainless-steel autoclave and place it in an oven heated to 150 °C for 12 h. Then, centrifuge at 4000 rpm for 15 min, discard the supernatant, and dry it in a vacuum drying oven at 60 °C for 12 h. The obtained product is redispersed in ultrapure water to obtain a Eu MOF solution with a concentration of 0.3 mg / ml, and store it in a refrigerator at 4 °C in the dark for later use.

[0047] According to the above process, replace the above 3,5-dicarboxylphenylboronic acid with an equimolar amount of isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, and 4-carboxylphenylboronic acid in turn, and keep the rest of the process unchanged. Eu MOF solutions with a concentration of 0.3 mg / ml using isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, and 4-carboxylphenylboronic acid as ligands are prepared respectively.

[0048] (1.2) Preparation of Tb MOF: Dissolve 0.1050 mg (0.5 mmol) of 3,5-dicarboxylphenylboronic acid and 0.0453 g (0.1 mmol) of Tb(NO)₃·6H₂O in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml). Stir the solution with a magnetic stirrer at a speed of 2000 rpm for 2 h. Then transfer the mixed solution to a Teflon container in a stainless-steel autoclave and place it in an oven heated to 150 °C for 12 h. Then, centrifuge at 4000 rpm for 15 min, discard the supernatant, and dry it in a vacuum drying oven at 60 °C for 12 h. The obtained product is redispersed in ultrapure water to obtain a Tb MOF solution with a concentration of 0.3 mg / ml, and store it in a refrigerator at 4 °C in the dark for later use.

[0049] According to the above process, replace the above 3,5-dicarboxylphenylboronic acid with an equimolar amount of isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, and 4-carboxylphenylboronic acid in turn, and keep the rest of the process unchanged. Tb MOF solutions with a concentration of 0.3 mg / ml using isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, and 4-carboxylphenylboronic acid as ligands are prepared respectively.

[0050] (1.3) Preparation of Eu / Tb MOF: Dissolve 0.1050 mg (0.5 mmol) of 3,5-dicarboxylphenylboronic acid, 0.0223 g (0.05 mmol) of Eu(NO)3·6H2O, and 0.0227 g (0.05 mmol) of Tb(NO)3·6H2O in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml). Stir the solution with a magnetic stirrer at a speed of 2000 rpm for 2 h. Then transfer the mixed solution to a polytetrafluoroethylene container in a stainless-steel autoclave and place it in an oven heated to 150 °C for 12 h. Then, centrifuge at 4000 rpm for 15 min, discard the supernatant, and dry it in a vacuum drying oven at 60 °C for 12 h. The obtained product is redispersed in ultrapure water to obtain a Eu / Tb MOF solution with a concentration of 0.3 mg / ml, and store it in the dark at 4 °C in a refrigerator for standby.

[0051] According to the above process, replace the above-mentioned 3,5-dicarboxylphenylboronic acid with an equimolar amount of isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, and 4-carboxylphenylboronic acid in turn, and keep the rest of the process unchanged. Eu / Tb MOF with a concentration of 0.3 mg / ml using isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, and 4-carboxylphenylboronic acid as ligands is respectively prepared.

[0052] Selection of the optimal ligand

[0053] Figure 1 A shows the chemical structures of different ligands. ( Figure 1 In B - F, the concentration of the Cr(Ⅵ) solution is 10 μM and the volume is 400 μL, and the volume of each Ln MOF solution prepared in step (1) is 400 μL) Figure 1 In B, the Ln MOF prepared with 3,5-dicarboxylphenylboronic acid as the ligand has obvious fluorescence absorption peaks. Figure 1 In C, the Ln MOF prepared with isophthalic acid as the ligand also has obvious fluorescence absorption peaks, while Figure 1 in D, 2-carboxylphenylboronic acid, Figure 1 in E, 3-carboxylphenylboronic acid, and Figure 1 in F, the Ln MOF prepared with 4-carboxylphenylboronic acid as the ligand has only very weak fluorescence absorption peaks (the Eu MOF prepared with different ligands has no fluorescence absorption peak at 545 nm, and the Tb MOF prepared with different ligands has fluorescence absorption peaks at 545 nm and 622 nm). In addition, by comparison, compared with the Eu / Tb MOF prepared with isophthalic acid as the ligand, the Eu / Tb MOF prepared with 3,5-dicarboxylphenylboronic acid as the ligand, after reacting with Cr(Ⅵ), the fluorescence absorption intensity ratio I 545 / I 618More significant and easier to observe. Among them, after the Eu / Tb MOF prepared with isophthalic acid as the ligand reacts with Cr(Ⅵ), I 618 changes significantly. After the Eu / Tb MOF prepared with 3,5-dicarboxylphenylboronic acid as the ligand reacts with Cr(Ⅵ), I 618 shows no obvious change. Therefore, using isophthalic acid as the ligand for preparing Eu / Tb MOF is not conducive to the quantitative detection of Cr(Ⅵ); moreover, the fluorescence absorption peak of the Eu / Tb MOF prepared with 3,5-dicarboxylphenylboronic acid as the ligand is also stronger. Therefore, 3,5-dicarboxylphenylboronic acid is selected as the ligand for preparing Eu / Tb MOF in the subsequent experiment.

[0054] Step (2), Preparation of Eu / Tb MOF with different Eu / Tb ratios using 3,5-dicarboxylphenylboronic acid as the ligand

[0055] Dissolve 0.1050 mg (0.5 mmol) of 3,5-dicarboxylphenylboronic acid, and different ratios of Eu(NO)3·6H2O and Tb(NO)3·6H2O in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml). Among them, the molar ratio of Eu(NO)3·6H2O to Tb(NO)3·6H2O is 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8 in turn, and the sum of the amounts of substance of Eu(NO)3·6H2O and Tb(NO)3·6H2O is 0.1 mmol; then use a magnetic stirrer to stir at a speed of 2000 rpm for 2 h, then raise the temperature to 150 °C and heat for 12 h; subsequently, centrifuge at 4000 rpm for 15 min, discard the supernatant, dry the residue, and redisperse the obtained product in ultrapure water to obtain Eu 0.8 / Tb 0.2 MOF solutions, Eu 0.7 / Tb 0.3 MOF solutions, Eu 0.6 / Tb 0.4 MOF solutions, Eu 0.5 / Tb 0.5 MOF solutions, Eu 0.4 / Tb 0.6 MOF solutions, Eu 0.3 / Tb 0.7 MOF solutions, Eu 0.2 / Tb 0.8 MOF solutions, and store them in the refrigerator at 4 °C in the dark for later use.

[0056] Selection of Eu / Tb MOF ratiometric fluorescence sensor

[0057] Select the Eu / Tb MOF with the optimal Eu / Tb ratio through comparative experiments Figure 2 B, C, D, E, F, the blank sample is the mixed solution of Ln-MOF reacting with ultrapure water, and the Cr(Ⅵ) solution is not added), from Figure 2 Figure A, it can be seen that as the amount of Tb(NO)3·6H2O decreases and the amount of Eu(NO)3·6H2O increases, the fluorescence intensity of the Eu / Tb MOF solution gradually decreases. Combining Figure 2 Figure B showing the visualization results (color changes) of the Eu / Tb MOF ratiometric fluorescence sensor with different Eu / Tb ratios for detecting Cr(Ⅵ) and Figure 2 the fluorescence intensity changes of the Eu / Tb MOF ratiometric fluorescence sensor with different Eu / Tb ratios in Figure A, Eu 0.4 / Tb 0.6 MOF, Eu 0.5 / Tb 0.5 MOF and Eu 0.6 / Tb 0.4 MOF solutions (all with a volume of 400 μL) are reacted with Cr(Ⅵ) solutions with different concentrations (volume of 400 μL, and the concentrations of the Cr(Ⅵ) solutions are 0.1 μM, 1 μM, 10 μM) for 10 min, and the color changes of the resulting mixed solutions are the most obvious. Therefore, among Eu 0.4 / Tb 0.6 MOF, Eu 0.5 / Tb 0.5 MOF and Eu 0.6 / Tb 0.4 MOF, select the Eu / Tb MOF with the optimal Eu / Tb ratio. From Figure 2 Figures D, F, H showing the CIE chromaticity diagrams (the CIE chromaticity diagrams are obtained by converting with the Chromaticity Diagram plug-in in Origin), it can be seen that Eu 0.5 / Tb 0.5 MOF has the least overlapping part of the chromaticity of the mixed solution after being mixed and reacted with the 0.1 μM Cr(Ⅵ) solution. Therefore, Eu 0.5 / Tb 0.5 MOF solution is used as the Eu / Tb MOF ratiometric fluorescence sensor with the optimal Eu / Tb ratio.

[0058] Determination of the volume ratio of each solution when detecting Cr(Ⅵ): As can be seen from Figure 3 Figure A, by changing the volume ratio of the Eu 0.5 / Tb 0.5 MOF solution and the 10 μM Cr(Ⅵ) solution (volume ratio Eu 0.5 / Tb 0.5MOF:Cr(Ⅵ)=3:7, 4:6, 5:5, 6:4, 7:3. After mixing the two solutions and reacting for 10 min, the resulting mixed solution was subjected to fluorescence analysis), when Eu 0.5 / Tb 0.5 When the volume ratio of the MOF solution to the Cr(Ⅵ) solution was 4:6, the I 545 / I 618 ratio was the lowest. Considering the convenience of subsequent experimental operations, it was finally determined to select 320 μL of Eu 0.5 / Tb 0.5 MOF solution and 480 μL of Cr(Ⅵ) solution for subsequent experiments.

[0059] Determination of the pH value when detecting Cr(Ⅵ): As can be seen from Figure 3 B, when using 0.1 mM Tris-HCl buffer to adjust the pH value of the Eu 0.5 / Tb 0.5 MOF solution, under the conditions of pH = 7 and pH = 8, the mixed solution of 320 μL of the aforementioned Eu 0.5 / Tb 0.5 MOF solution and 480 μL of 10 μM Cr(Ⅵ) solution after mixing and reacting for 10 min had the lowest I 545 / I 618 ratio. Compared with the fluorescence intensity and I 545 / I 618 under the condition of pH = 8, the fluorescence intensity was higher and the I 545 / I 618 ratio was more obvious under the condition of pH = 7. Therefore, the Eu 0.5 / Tb 0.5 MOF ratio fluorescence sensor had the strongest detection ability under the condition of pH = 7.

[0060] Figure 3 C, 3D, and 3E are the fluorescence spectra of the mixed solutions obtained by mixing 320 μL of Eu MOF solution, 320 μL of Tb MOF solution, and 320 μL of Eu 0.5 / Tb 0.5 MOF solution with 480 μL of Cr(Ⅵ) solutions with different concentrations (the concentrations of the Cr(Ⅵ) solutions are 0.1 μM, 1 μM, and 10 μM in sequence) and reacting for 10 min ([[]] Figure 3 In C, D, and E, the blank samples are the mixed solutions of Ln-MOF and ultrapure water reacting without adding Cr(Ⅵ) solution). It can be seen from this that Eu MOF basically does not react with Cr(Ⅵ), Tb MOF will show a quenching reaction with Cr(Ⅵ), and Eu 0.5 / Tb 0.5The fluorescence intensity of the mixed solution obtained after the reaction of MOF with Cr(Ⅵ) changes significantly at 545 nm, while the change in fluorescence intensity at 618 nm is not obvious.

[0061] In summary, Eu 0.5 Tb 0.5 MOF is selected as the Eu / Tb MOF ratio fluorescence sensor, and qualitative and quantitative detection of Cr(Ⅵ) is carried out according to the ratio at I 545 / I 618 under pH = 7.

[0062] Figure 4 A is the TEM image of Eu 0.5 / Tb 0.5 MOF. It can be seen from the figure that it has an irregular hexagonal structure; Figure 4 B and C are the corresponding element energy spectrum diagrams. It can be seen from the figure that Eu 0.5 / Tb 0.5 elements in MOF are evenly dispersed in the material. From Figure 4 D, it can be seen that Ln MOF lacks the peak at 1702 cm -1 of the ligand 3,5-dicarboxylphenylboronic acid. This peak is generated by the C=O stretching vibration of 3,5-dicarboxylphenylboronic acid (abbreviated as "5-bop"), indicating that the carboxyl group coordinates with Eu 3+ and / or Tb 3+ to form LnMOF. The peaks at 1610 cm -1 and 1540 cm -1 may be generated by the cross-coordination of Eu 3+ and / or Tb 3+ with the carboxyl group of the ligand 3,5-dicarboxylphenylboronic acid. The peak at 1377 cm -1 is generated by the B-O stretching vibration in the ligand 3,5-dicarboxylphenylboronic acid. At the same time, the B-O characteristic peak is also observed at 1390 cm -1 , indicating that Ln MOF has free boric acid sites. Figure 4 E is the UV-visible spectrum diagram of 3,5-dicarboxylphenylboronic acid and Ln MOF. It can be seen from the figure that 3,5-dicarboxylphenylboronic acid and Ln MOF have similar absorption peaks, indicating that no new functional groups are generated. Figure 4 F is the XPS spectrum diagram of Eu 0.5 / Tb 0.5 MOF. It can be seen from the figure that C, O, B, Eu, and Tb characteristic elements exist in Eu 0.5 / Tb 0.5 MOF. The above results indicate the successful preparation of Eu 0.5 / Tb 0.5 MOF.

[0063] Step (3), Detection of Cr(Ⅵ) by Eu / Tb MOF ratio fluorescence sensor

[0064] Add 320 μL of the Eu 0.5 Tb 0.5 MOF solution prepared in step (2) into different 2 mL EP tubes respectively, adjust its pH value to 7, and then successively add 480 μL of Cr(Ⅵ) solutions with different concentrations into each of the said EP tubes, and mix well to obtain a mixed solution. After reacting for 10 min, under the conditions of room temperature and normal pressure, collect the fluorescence spectrum data of the mixed solution.

[0065] The volumes of the Eu / Tb MOF solution and the Cr(Ⅵ) solution remain unchanged, and only the concentration of the added Cr(Ⅵ) solution is changed: the concentrations of the Cr(Ⅵ) solution are 0, 0.05, 0.06, 0.08, 0.1, 0.12, 0.15, 0.18, 0.22, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 1.8, 2.2, 3, 4, 5, 6, 7, 8, 9, 10 μM respectively, to obtain mixed solutions mixed with Cr(Ⅵ) solutions of different concentrations. After reacting for 10 min, under the conditions of room temperature and normal pressure, collect the fluorescence spectrum data of the mixed solutions corresponding to the Cr(Ⅵ) solutions of each concentration. According to the fluorescence spectrum data of the mixed solutions corresponding to the Cr(Ⅵ) solutions of each concentration, establish a standard curve for detecting Cr(Ⅵ).

[0066] As Figure 5 shown in A and B, when the concentrations of the added Cr(Ⅵ) solutions are 0, 0.05, 0.06, 0.08, 0.1, 0.12, 0.15, 0.18, 0.22, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 1.8, 2.2, 3, 4, 5, 6, 7, 8, 9, 10 μM respectively, the fluorescence intensity I 545 / I 618 value (Y) has a good linear relationship with the concentration (X) of the Cr(Ⅵ) solution, and the detection limit is as low as 0.021 μM.

[0067] Verify the specificity of the Eu / Tb MOF ratio fluorescence sensor for detecting Cr(Ⅵ)

[0068] Verify the specificity of the reaction between Eu 0.5 Tb 0.5 MOF and Cr(Ⅵ) through a comparative experiment. Different metal ion standard solutions were selected to react with the Eu 0.5 / Tb 0.5 MOF solution, and the mixed solution after the reaction was tested. The specific process is as follows: According to the volume ratio of each raw material in step (3) of Example 1, in parallel Eu0.5 / Tb 0.5 Different kinds of high-concentration metal ion standard solutions were added to the MOF solution. Among them, the Cr(Ⅵ) standard solution (prepared by adding potassium dichromate to ultrapure water), the Cr 3+ standard solution (prepared by adding chromium(III) chloride hexahydrate to ultrapure water), the Ag + standard solution (prepared by adding silver nitrate to ultrapure water), the Ca 2+ standard solution (prepared by adding anhydrous calcium chloride to ultrapure water), the Cd 2+ standard solution (prepared by adding cadmium chloride dihydrate to ultrapure water), the Co 2+ standard solution (prepared by adding cobalt(II) chloride hexahydrate to ultrapure water), the Cu 2+ standard solution (prepared by adding copper(II) chloride dihydrate to ultrapure water), the Fe 2+ standard solution (prepared by adding iron(II) chloride tetrahydrate to ultrapure water), the Fe 3+ standard solution (prepared by adding iron(III) chloride hexahydrate to ultrapure water), the K + standard solution (prepared by adding potassium chloride to ultrapure water), the Mg 2+ standard solution (prepared by adding magnesium chloride to ultrapure water), the Mn 2+ standard solution (prepared by adding manganese(II) chloride tetrahydrate to ultrapure water), the Na + standard solution (prepared by adding sodium chloride to ultrapure water), the NH4 + standard solution (prepared by adding ammonium sulfate to ultrapure water), the Ni 2+ standard solution (prepared by adding nickel(II) chloride hexahydrate to ultrapure water), the Pb 2+ standard solution (prepared by adding lead chloride to ultrapure water), the Zn 2+ standard solution (prepared by adding zinc chloride to ultrapure water), the Cl - standard solution (prepared by adding sodium chloride to ultrapure water), the CO3 2- standard solution (prepared by adding sodium carbonate to ultrapure water), the HCO3 - standard solution (prepared by adding sodium bicarbonate to ultrapure water), the I - standard solution (prepared by adding potassium iodide to ultrapure water), the NO3 - standard solution (prepared by adding silver nitrate to ultrapure water), the PO4 3- standard solution (prepared by adding sodium phosphate dodecahydrate to ultrapure water), the SO3 2- standard solution (prepared by adding sodium sulfite to ultrapure water), the SO4 2- standard solution (prepared by adding ammonium sulfate to ultrapure water). As Figure 4 shown in C, the vertical axis is the fluorescence intensity I of the solution545 / I 618 Value. The concentrations of the above metal ion standard solutions are all 10 μM, Eu 0.5 / Tb 0.5 The volume of the MOF is 320 μL, and the volumes of the Cr(VI) standard solution and other metal ion standard solutions are both 480 μL. It can be seen that compared with other metal ions, the I 545 / I 618 ratio corresponding to Cr(VI) is the lowest. As Figure 4 shown in D, the concentration of the Cr(VI) standard solution is 1 μM, and the concentrations of other metal ion standard solutions are all 10 μM. Correspondingly, Eu 0.5 / Tb 0.5 The volume of the MOF is 320 μL, the volume of the 1 μM Cr(VI) solution is 240 μL, and the volumes of other metal ion standard solutions are also 240 μL. Compared with Figure 4 C, Figure 4 D shows that after the common addition of other metal ion standard solutions and the Cr(VI) standard solution, the I 545 / I 618 ratio decreases significantly, indicating that the Eu 0.5 / Tb 0.5 MOF ratio fluorescence sensor has excellent selectivity for Cr(VI).

[0069] Preparation of real samples and detection of Cr(VI) content

[0070] Preparation of real samples Tap water was taken from the laboratory and treated as follows: large particulate impurities were removed by filtration through a microporous filter membrane with a pore size of 0.22 μm; iced black tea and green tea were purchased from a supermarket (Wuhan, Hubei) and treated as follows: large particulate impurities were removed by filtration through a microporous filter membrane with a pore size of 0.22 μm and diluted 100 times; no other treatment was required. Different concentrations of Cr(VI) solutions were obtained using tap water, iced black tea, and green tea treated according to the above methods as solvents, respectively.

[0071] As shown in Table 1, when the concentrations of the obtained Cr(VI) solutions were 0.70 μM, 1.00 μM, and 5.00 μM after adding Cr(VI), the relative standard deviations (RSDs) of their addition amounts were all lower than 3%, and the recoveries were between 95.8% and 114.0%. This result indicates that this method has potential value in practical applications.

[0072] Table 1 Analytical results of Eu / Tb MOF for Cr(VI) in real samples

[0073]

[0074] The above-described embodiments merely represent the exact implementation modes of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the patent for the present invention shall be subject to the appended claims.

Claims

1. Method for detecting Cr(Ⅵ) by a dual-lanthanide metal-organic framework Eu / Tb MOF ratio fluorescence sensor, comprising the following steps: Step (1), selection of ligands (1.1)Using isophthalic acid as ligand A, weigh ligand A and Eu(NO)3·6H2O, dissolve them in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up. Then centrifuge, discard the supernatant, and redisperse the obtained product after drying the residue in ultrapure water to obtain a Eu MOF solution; According to the technological process of step (1.1), replace isophthalic acid with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, 3,5-dicarboxyphenylboronic acid as ligand A respectively, and keep the rest of the technological process unchanged. Correspondingly, Eu MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, 3,5-dicarboxyphenylboronic acid as ligand A are obtained respectively; (1.2) Using isophthalic acid as ligand A, weigh ligand A and Tb(NO)3·6H2O, dissolve them in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up; Then centrifuge, discard the supernatant, and redisperse the obtained product after drying the residue in ultrapure water to obtain a Tb MOF solution; According to the technological process of step (1.2), replace isophthalic acid with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, 3,5-dicarboxyphenylboronic acid as ligand A respectively, and keep the rest of the technological process unchanged. Correspondingly, Tb MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, 3,5-dicarboxyphenylboronic acid as ligand A are obtained respectively; (1.3) Using isophthalic acid as ligand A, weigh ligand A, Eu(NO)3·6H2O and Tb(NO)3·6H2O, dissolve them in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up; Then centrifuge, discard the supernatant, and redisperse the obtained product after drying the residue in ultrapure water to obtain a Eu / Tb MOF solution; According to the technological process of step (1.3), replace isophthalic acid with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, 3,5-dicarboxyphenylboronic acid as ligand A respectively, and keep the rest of the technological process unchanged. Correspondingly, Eu / Tb MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, 3,5-dicarboxyphenylboronic acid as ligand A are obtained respectively; (1.4)Measure the Eu MOF solution, Tb MOF solution, and Eu / Tb MOF solution prepared in steps (1.1), (1.2), and (1.3) respectively, mix them with the same volume of Cr(Ⅵ) solution in sequence to obtain different mixed solutions, and compare the fluorescence absorption intensity ratio I 545 / I 618 with the fluorescence absorption intensity ratio I 545 / I 618 of the Eu MOF solution, Tb MOF solution, and Eu / Tb MOF solution prepared in steps (1.1), (1.2), and (1.3) respectively for comparative analysis, and finally determine that 3,5-dicarboxylphenylboronic acid is the optimal ligand; Step (2), selection of Eu / Tb ratio in Eu / Tb MOF (2.1) Dissolve 3,5-dicarboxylphenylboronic acid with Eu(NO)₃·6H₂O and Tb(NO)₃·6H₂O in different molar ratios in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat up; then, centrifuge and discard the supernatant, dry the residue, and redisperse the obtained product in ultrapure water to obtain the Eu / Tb MOF solution, which is stored in the refrigerator at 4 °C in the dark for later use; among them, the total molar amount of Eu(NO)₃·6H₂O and Tb(NO)₃·6H₂O to the molar amount of 3,5-dicarboxylphenylboronic acid is 1:5, and the molar ratios of Eu(NO)₃·6H₂O and Tb(NO)₃·6H₂O are 10:0, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 0:10 in turn; (2.2) Mix the Eu / Tb MOF solution prepared in step (2.1) with equal volumes of Cr(Ⅵ) solutions with different concentrations respectively to obtain a mixed solution after reaction; compare the fluorescence spectral data of the Eu / Tb MOF solution prepared in step (2.1), compare the color changes and fluorescence spectral data of the mixed solution, and compare the CIE chromaticity diagram converted from the fluorescence spectral data of the mixed solution; By comparing the fluorescence spectral data of the Eu / Tb MOF solution, comparing the color change and fluorescence spectral data of the mixed solution, and then determining the Eu / Tb molar ratio based on the least overlapping part in the CIE chromaticity diagram, Eu 0.5 / Tb 0.5 MOF solution is used as a Eu / Tb MOF ratio fluorescence sensor; Step (3), Eu 0.5 / Tb 0.5 Cr(Ⅵ) detection by Eu / Tb MOF ratio fluorescence sensor Adjust the pH value of the Eu / Tb MOF ratio fluorescence sensor obtained in step (2) to 7, and then mix it with the Cr(Ⅵ) solution to obtain a mixed solution. By changing the concentration of the Cr(Ⅵ) solution, different mixed solutions mixed with different concentrations of the Cr(Ⅵ) solution are obtained; after the reaction is complete, under the conditions of room temperature and normal pressure, collect the fluorescence spectrum data of the mixed solution; according to the concentration of the Cr(Ⅵ) solution and the fluorescence intensity I 545 / I 618 value of the corresponding mixed solution, with I 545 / I 618 value as the ordinate and the concentration of the Cr(Ⅵ) solution as the abscissa, establish a standard curve for detecting the concentration of Cr(Ⅵ), and based on this standard curve, realize the determination of the content of Cr(Ⅵ) in a complex matrix system.

2. The method according to claim 1, characterized in that The Cr(Ⅵ) solution is prepared by adding pure water to K₂Cr₂O₇.

3. The method according to claim 1, characterized in that, The magnetic stirrer stirring means stirring with a magnetic stirrer at a speed of 2000 rpm for 2 h.

4. The method according to claim 1, wherein The heating up is to heat up to 150 °C and heat for 12 h, the centrifugation is at 4000 rpm for 15 min, and the drying is in a vacuum drying oven at a temperature of 60 °C for 12 h.

5. The method according to claim 1, characterized in that, In the said step (1.1), the molar ratio of isophthalic acid to Eu(NO)₃·6H₂O is 5:

1. Similarly, the molar ratio of 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid or 3,5-dicarboxylphenylboronic acid to Eu(NO)₃·6H₂O is also 5:1; the volume ratio of N,N-dimethylformamide to ultrapure water is 7:3; In the said step (1.1), the concentration of the Eu MOF solution obtained with isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid or 3,5-dicarboxylphenylboronic acid as ligand A is all 0.3 mg / mL.

6. The method according to claim 5, characterized in that, Isophthalic acid, 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid or 3,5-dicarboxylphenylboronic acid is 0.5 mmol, Eu(NO)₃·6H₂O is 0.1 mmol, the volume of N,N-dimethylformamide is 7 ml, and the volume of ultrapure water is 3 ml.

7. The method according to claim 1, wherein In the said step (1.2), the molar ratio of isophthalic acid to Tb(NO)₃·6H₂O is 5:

1. Similarly, the molar ratio of 2-carboxylphenylboronic acid, 3-carboxylphenylboronic acid, 4-carboxylphenylboronic acid or 3,5-dicarboxylphenylboronic acid to Tb(NO)₃·6H₂O is also 5:1; the volume ratio of N,N-dimethylformamide to ultrapure water is 7:3; In the step (1.2), the concentration of the Tb MOF solution obtained using isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid as ligand A is 0.3 mg / mL.

8. The method according to claim 7, wherein Isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid is 0.5 mmol, Tb(NO)3·6H2O is 0.1 mmol, the volume of N,N-dimethylformamide is 7 mL, and the volume of ultrapure water is 3 mL.

9. The method according to claim 1, wherein In the step (1.3), the molar ratio of isophthalic acid to the total molar amount of Eu(NO)3·6H2O and Tb(NO)3·6H2O is 5:1, and the molar ratio of Eu(NO)3·6H2O to Tb(NO)3·6H2O is 1:1; similarly, the molar ratio of 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid to the total molar amount of Eu(NO)3·6H2O and Tb(NO)3·6H2O is also 5:1; the volume ratio of N,N-dimethylformamide to ultrapure water is 7:

3. In the step (1.3), the concentration of the Eu / Tb MOF solution obtained using isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid as ligand A is 0.3 mg / mL.

10. The method according to claim 9, wherein Isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, or 3,5-dicarboxyphenylboronic acid is 0.5 mmol, both Eu(NO)3·6H2O and Tb(NO)3·6H2O are 0.05 mmol, the volume of N,N-dimethylformamide is 7 mL, and the volume of ultrapure water is 3 mL.

11. The method according to claim 1, characterized in that In the step (1.4), the concentration of the Cr(Ⅵ) solution is 10 µM and the volume is 400 µL, and the volumes of the Eu MOF solution, Tb MOF solution, and Eu / Tb MOF solution prepared in steps (1.1), (1.2), and (1.3) are all 400 µL.

12. The method according to claim 1, characterized in that, In step (2.1), 3,5-dicarboxyphenylboronic acid is 0.5 mmol, the total molar amount of Eu(NO)3·6H2O and Tb(NO)3·6H2O is 0.1 mmol, the volume of N,N-dimethylformamide is 7 mL, and the volume of ultrapure water is 3 mL.

13. The method according to claim 1, wherein In step (2.2), the volumes of the Eu / Tb MOF solution prepared in step (2.1) and the Cr(Ⅵ) solutions with different concentrations are all 400 µL. The Cr(Ⅵ) solutions with different concentrations refer to those with Cr(Ⅵ) concentrations of 0.1 µM, 1 µM, and 10 µM in sequence.

14. The method according to claim 1, characterized in that In step (3), the volume of the Eu / Tb MOF ratio fluorescence sensor is 320 μL, the volume of the Cr(VI) solution is 480 μL, and the different concentrations of the Cr(VI) solution refer to the concentrations of the Cr(VI) solution being 0, 0.05, 0.06, 0.08, 0.1, 0.12, 0.15, 0.18, 0.22, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 1.8, 2.2, 3, 4, 5, 6, 7, 8, 9, 10 μM respectively, and the completion of the reaction means that the reaction lasts for 10 min.

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