Method for detecting Cr (VI) by using double-lanthanide metal organic framework Eu / Tb MOF ratio fluorescence sensor

Abstract

The invention belongs to the technical field of nano material detection, and particularly discloses a method for detecting Cr (VI) by using a double-lanthanide metal organic framework Eu / Tb MOF ratio fluorescence sensor, which comprises the following steps: screening and preparing a lanthanide metal organic framework Eu / Tb MOF using 1, 3, 5-dicarboxyphenylboronic acid as a ligand; and 2, detecting Cr (VI) by using an Eu / Tb MOF ratio fluorescence sensor. The Cr (VI) enables the fluorescence absorption peak of the Eu / Tb MOF to be quenched at 545nm and the fluorescence absorption peak of the Eu / Tb MOF to be almost unchanged at 618nm, so that the Eu / Tb MOF ratio fluorescence sensor is constructed to detect the Cr (VI), and the concentration of the Cr (VI) in a to-be-detected sample is judged through fluorescence intensity, color change and CIE chromaticity diagram degree. The Eu / Tb MOF ratio fluorescence sensor can generate more color intervals, Cr (VI with different concentrations can be detected more accurately, and the sensitivity is improved. The method disclosed by the invention has the characteristics of low cost, high response speed, low detection limit and high sensitivity and selectivity.

Description

Technical Field

[0001] The invention belongs to the technical field of nano material detection, and in particular relates to a method for detecting Cr(VI) using a dual lanthanide metal organic framework Eu / TbMOF ratio fluorescence sensor. Background Art

[0002] As a metal ion widely used in modern industry and agriculture, chromium has shown a high degree of stability in the ecosystem. In nature, chromium mainly exists in trivalent and hexavalent forms. Trivalent chromium (Cr(III)) belongs to the category of essential trace elements for humans, while hexavalent chromium (Cr(VI)) is completely different. Its toxicity is extremely strong, 100 times higher than that of trivalent chromium. In addition, Cr(VI) has stronger human absorption and bioaccumulation, and cannot be biodegraded in the water environment, posing a major hazard to humans and most animals. In trace or even ultra-trace concentrations, it may still induce serious consequences such as hereditary genetic defects, allergic reactions, and various types of cancer. Cr(VI) is defined as a toxic and harmful heavy metal, and its content in drinking water is subject to extremely strict restrictions. According to GB 5749-2022 "Sanitary Standards for Drinking Water", the concentration threshold of Cr(VI) in drinking water is limited to 0.170μM. Therefore, accurate detection and determination of Cr(VI) in the water environment is crucial for human safety and environmental protection.

[0003] At present, the methods for detecting hexavalent chromium (Cr(VI)) mainly include atomic absorption spectroscopy (AAS), ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS). Although these technologies are highly sensitive, they usually rely on large equipment and require complex sample pretreatment and professional knowledge, which is time-consuming and limits the feasibility of real-time detection. Therefore, it is urgent to develop simple, rapid, sensitive and accurate methods for detecting Cr(VI) in water environments. The fluorescence (FL) method is suitable for the selective determination of trace chromium due to its excellent spatial and temporal resolution, high sensitivity and simplicity. This method can be operated by less trained personnel using an inexpensive spectrometer and can provide a stable detection signal. The effective combination of these methods helps to improve the efficiency and accuracy of hexavalent chromium (Cr(VI)) detection. Summary of the invention

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

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

[0006] The method comprises the following steps:

[0007] Step (1): Selection of ligand

[0008] (1.1) Using isophthalic acid as ligand A, weighing ligand A and Eu(NO)3·6H2O, dissolving them in a mixed solution of N,N-dimethylformamide and ultrapure water, stirring with a magnetic stirrer, and then heating; then centrifuging, discarding the supernatant, and re-dispersing the residue after drying in ultrapure water to obtain a Eu MOF solution;

[0009] According to the above process, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid are used to replace isophthalic acid as ligand A, and the rest of the process remains unchanged, and Eu MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid as ligand A are obtained respectively;

[0010] (1.2) Using isophthalic acid as ligand A, weighing ligand A and Tb(NO)3·6H2O, dissolving them in a mixed solution of N,N-dimethylformamide and ultrapure water, stirring with a magnetic stirrer, and then heating; then centrifuging, discarding the supernatant, and re-dispersing the residue after drying in ultrapure water to obtain a Tb MOF solution;

[0011] According to the above process, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid are used to replace isophthalic acid as ligand A, and the rest of the process remains unchanged, and Eu MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid as ligand A are obtained respectively;

[0012] (1.3) Using isophthalic acid as ligand A, weighing ligand A, Eu(NO)3·6H2O, and Tb(NO)3·6H2O, dissolving them in a mixed solution of N,N-dimethylformamide and ultrapure water, stirring with a magnetic stirrer, and then heating; then centrifuging, discarding the supernatant, and re-dispersing the residue after drying in ultrapure water to obtain Eu / Tb MOF solution; according to the above process, using 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid, and 3,5-dicarboxyphenylboric acid to replace isophthalic acid as ligand A, and the rest of the process remains unchanged, and correspondingly obtaining Eu / Tb MOF solutions with 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid, and 3,5-dicarboxyphenylboric acid as ligand A, respectively;

[0013] (1.4) The Eu MOF solution, Tb MOF solution, and Eu / TbMOF solution prepared in steps (1.1), (1.2), and (1.3) are respectively measured and mixed with an equal volume of Cr(VI) solution in turn to obtain different mixed solutions. The fluorescence absorption intensity ratio of the mixed solutions is calculated. 545 / I 618 The fluorescence absorption intensity ratio I of the Eu MOF solution, Tb MOF solution and Eu / TbMOF solution prepared in steps (1.1), (1.2) and (1.3) is 545 / I 618 Comparative analysis was performed, and 3,5-dicarboxyphenylboronic acid was finally determined to be the best ligand;

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

[0015] (2.1) 3,5-dicarboxyphenylboronic acid and different molar ratios of Eu(NO)3·6H2O and Tb(NO)3·6H2O were dissolved in a mixed solution of N,N-dimethylformamide and ultrapure water, stirred with a magnetic stirrer, and then heated; then, centrifuged and the supernatant was discarded, the residue was dried, and the resultant was redispersed in ultrapure water to obtain a Eu / Tb MOF solution, which was stored in a refrigerator at 4°C in the dark for future use; wherein the ratio of the total molar amount of Eu(NO)3·6H2O and Tb(NO)3·6H2O to the molar amount of 3,5-dicarboxyphenylboronic acid was 1:5, and the molar ratios of Eu(NO)3·6H2O to Tb(NO)3·6H2O were 10:0, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 0:10, respectively.

[0016] (2.2) mixing the Eu / Tb MOF solution obtained in step (2.1) with equal volumes of Cr(VI) solutions of different concentrations, respectively, to obtain a mixed solution after reaction; comparing the fluorescence spectrum data of the Eu / Tb MOF solution obtained in step (2.1), comparing the color change and the fluorescence spectrum data of the mixed solution, and comparing the CIE chromaticity diagram obtained by converting the fluorescence spectrum data of the mixed solution; by comparing the fluorescence spectrum data of the Eu / Tb MOF solution, comparing the color change and the fluorescence spectrum data of the mixed solution, and then determining the Eu / Tb molar ratio based on the minimum overlap in the CIE chromaticity diagram, Eu is obtained. 0.5 / Tb 0.5 MOF solution as Eu / Tb MOF ratiometric fluorescence sensor.

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

[0018] The pH of the Eu / Tb MOF ratio fluorescence sensor obtained in step (2) is adjusted to 7, and the Eu / TbMOF ratio fluorescence sensor adjusted to pH 7 and the Cr(VI) solution are mixed 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, the fluorescence spectrum data of the mixed solution is collected at room temperature and normal pressure; according to the concentration of the Cr(VI) solution and the fluorescence intensity I of the mixed solution corresponding to the concentration of the Cr(VI) solution, 545 / I 618 Value, in I 545 / I 618 The value is taken as the vertical axis and the concentration of Cr(VI) solution is taken as the horizontal axis. A standard curve for detecting the concentration of Cr(VI) is established. Based on this standard curve, the determination of Cr(VI) content in a complex matrix system is realized.

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

[0020] Preferably, the stirring with a magnetic stirrer refers to stirring with a magnetic stirrer at a rotation speed of 2000 rpm for 2 hours.

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

[0022] Preferably, in the step (1.1), the molar ratio of isophthalic acid to Eu(NO)3·6H2O is 5:1, and similarly, the molar ratio of 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric 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, the amount of isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric acid is 0.5 mmol, the amount of 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, and similarly, the molar ratio of 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric 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, the amount of isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric acid is 0.5 mmol, the amount of 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 ratio of the molar 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 ratio of the molar amount of 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric 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, the amount of isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid or 3,5-dicarboxyphenylboronic acid is 0.5 mmol, the amount of Eu(NO)3·6H2O and the amount of Tb(NO)3·6H2O are both 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 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.

[0029] Preferably, in step (2.1), the amount of 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.

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

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

[0032] Preferably, 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, the Cr(VI) solutions of different concentrations refer to 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, and the complete reaction refers to the reaction for 10 min.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] Compared with the existing Cr(VI) detection method, the method of detecting Cr(VI) by 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 selectivity. Compared with the detection of Cr(VI) by using a single lanthanide metal organic framework, the dual lanthanide metal organic framework (Eu / Tb MOF) ratio fluorescence sensor of the present invention has a linear range of detection of 0 to 10 μM, a color range from orange to red, a greater degree of color change, and higher sensitivity. When the Eu / Tb MOF ratio fluorescence sensor of the present invention is used to detect Cr(VI), the concentration of Cr(VI) in the sample to be tested can be determined by the fluorescence intensity and / or the degree of color change. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

[0038] Figure 4 In the equation, A is Eu 0.5 / Tb 0.5 Transmission electron microscopy (TEM) morphology of MOF, B and C are the corresponding energy spectra; D is 5-bop, Eu MOF, Tb MOF and Eu 0.5 / Tb 0.5 Fourier transform infrared (FT-IR) spectrum of MOF; E is 5-bop, Eu MOF, TbMOF and Eu 0.5 / Tb 0.5 UV-Vis absorption spectrum (UV-Vis) of MOF; F is Eu 0.5 / Tb 0.5 X-ray photoelectron spectroscopy (XPS) graph of MOF.

[0039] Figure 5 Figure A is a nonlinear fitting curve of the concentration of Cr(VI) solution in the range of 0 to 10 μM (the concentrations of Cr(VI) solution corresponding to each point 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 of the mixed solution obtained by adding the Cr(VI) solution. 545 with I 618 Ratio, I 545 is the fluorescence intensity of the reaction system at 545 nm, I 618 is the fluorescence intensity of the reaction system at 618nm; the nonlinear fitting curve in Figure A is transformed to obtain the linear fitting curve in Figure B; Figure C is Eu 0.5 / Tb0.5 Comparison of the results of MOF ratio fluorescence sensor detecting Cr(VI) and other ions. In the figure, the ordinate is the fluorescence intensity I of the solution 545 / I 618 Value, 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 The anti-interference experimental results of the reaction of MOF ratio fluorescence sensor mixed with Cr(VI) solution and other cation or anion solution. The vertical axis is the fluorescence intensity I of the solution. 545 / I 618 Value, 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－ . DETAILED DESCRIPTION

[0040] In order 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 to detect Cr(VI). Since Eu MOF does not react with Cr(VI), while Tb MOF will undergo 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 best Eu / Tb ratio is selected to detect Cr(VI). The method of 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: Cr(VI) solution is prepared by adding potassium dichromate to ultrapure water. 0.5 / Tb 0.5 MOF as Eu / Tb MOF ratiometric fluorescence sensor for detection of Cr(VI).

[0043] Example 1

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

[0045] Before the synthesis, all glassware required were 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: 0.1050 mg (0.5 mmol) of 3,5-dicarboxyphenylboronic acid and 0.0446 g (0.1 mmol) of Eu(NO)3·6H2O were dissolved in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml), and stirred at 2000 rpm for 2 h using a magnetic stirrer. The mixed solution was then transferred to a polytetrafluoroethylene container in a stainless steel reactor and then placed in an oven and heated to 150°C for 12 h. The mixture was then centrifuged at 4000 rpm for 15 min, the supernatant was discarded, and the mixture was dried in a vacuum oven at 60°C for 12 h. The resulting mixture was redispersed in ultrapure water to obtain a EuMOF solution with a concentration of 0.3 mg / ml, and stored in a refrigerator at 4°C away from light for later use.

[0047] According to the above process, the above 3,5-dicarboxyphenylboric acid is replaced with isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid and 4-carboxyphenylboric acid in equal amounts, and the rest of the process remains unchanged, to prepare Eu MOF solutions with a concentration of 0.3 mg / ml using isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid and 4-carboxyphenylboric acid as ligands, respectively.

[0048] (1.2) Preparation of Tb MOF: 0.1050 mg (0.5 mmol) of 3,5-dicarboxyphenylboronic acid and 0.0453 g (0.1 mmol) of Tb(NO)3·6H2O were dissolved in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml), and stirred at 2000 rpm for 2 h using a magnetic stirrer. The mixed solution was then transferred to a polytetrafluoroethylene container in a stainless steel reactor and then placed in an oven and heated to 150°C for 12 h. The mixture was then centrifuged at 4000 rpm for 15 min, the supernatant was discarded, and the mixture was dried in a vacuum oven at 60°C for 12 h. The resulting solution was redispersed in ultrapure water to obtain a TbMOF solution with a concentration of 0.3 mg / ml, and stored in a refrigerator at 4°C away from light for later use.

[0049] According to the above process, the above 3,5-dicarboxyphenylboronic acid is replaced with isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid and 4-carboxyphenylboronic acid in equal amounts in sequence, and the rest of the process remains unchanged, to prepare Tb MOF solutions with a concentration of 0.3 mg / ml using isophthalic acid, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid and 4-carboxyphenylboronic acid as ligands, respectively.

[0050] (1.3) Preparation of Eu / Tb MOF: 0.1050 mg (0.5 mmol) of 3,5-dicarboxyphenylboronic acid, 0.0223 g (0.05 mmol) of Eu(NO)3·6H2O, and 0.0227 g (0.05 mmol) of Tb(NO)3·6H2O were dissolved in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml), and stirred at 2000 rpm for 2 h using a magnetic stirrer. The mixed solution was then transferred to a polytetrafluoroethylene container in a stainless steel reactor and then placed in an oven and heated to 150°C for 12 h. The mixture was then centrifuged at 4000 rpm for 15 min, the supernatant was discarded, and the mixture was dried in a vacuum oven at 60°C for 12 h. The resulting solution was redispersed in ultrapure water to obtain a Eu / Tb MOF solution with a concentration of 0.3 mg / ml, and stored in a refrigerator at 4°C in the dark for later use.

[0051] According to the above process, the above 3,5-dicarboxyphenylboric acid is replaced with isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid and 4-carboxyphenylboric acid in equal amounts, and the rest of the process remains unchanged, to prepare Eu / Tb MOFs with a concentration of 0.3 mg / ml using isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid and 4-carboxyphenylboric acid as ligands.

[0052] Selection of the best ligand

[0053] Figure 1 A shows the chemical structures of different ligands. ( Figure 1 In BF, the concentration of Cr(VI) solution is 10 μM and the volume is 400 μL. 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-dicarboxyphenylboronic acid as a ligand has an obvious fluorescence absorption peak. Figure 1 In C, the Ln MOF prepared with isophthalic acid as a ligand also has an obvious fluorescence absorption peak. Figure 1 2-Carboxyphenylboronic acid in D, Figure 1 3-Carboxyphenylboronic acid and Figure 1 The Ln MOF prepared with 4-carboxyphenylboronic acid as the ligand in F has only a weak fluorescence absorption peak (Eu MOF prepared with different ligands has no fluorescence absorption peak at 545nm, and Tb MOF prepared with different ligands has fluorescence absorption peaks at 545nm and 622nm). In addition, it can be seen from the comparison that compared with Eu / Tb MOF prepared with isophthalic acid as the ligand, the fluorescence absorption intensity ratio I of Eu / Tb MOF prepared with 3,5-dicarboxyphenylboronic acid as the ligand after reacting with Cr(VI) is 545 / I 618It is more obvious and easier to observe. Among them, after the Eu / Tb MOF prepared with isophthalic acid as a ligand reacts with Cr(VI), I 618 After the Eu / Tb MOF prepared with 3,5-dicarboxyphenylboronic acid as a ligand reacted with Cr(VI), I 618 There is no obvious change, so the use of isophthalic acid as a ligand for the preparation of Eu / Tb MOF is not conducive to the quantitative detection of Cr(VI); and the fluorescence absorption peak of Eu / Tb MOF prepared with 3,5-dicarboxyphenylboronic acid as a ligand is also stronger. Therefore, 3,5-dicarboxyphenylboronic acid was selected as the ligand for the preparation of Eu / TbMOF in subsequent experiments.

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

[0055] 0.1050 mg (0.5 mmol) of 3,5-dicarboxyphenylboronic acid and different ratios of Eu(NO)3·6H2O and Tb(NO)3·6H2O were dissolved in a mixed solution of N,N-dimethylformamide (7 ml) and ultrapure water (3 ml), wherein the molar ratios of Eu(NO)3·6H2O to Tb(NO)3·6H2O were 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, the sum of the amounts of Eu(NO)3·6H2O and Tb(NO)3·6H2O was 0.1 mmol; then the mixture was stirred at 2000 rpm for 2 h using a magnetic stirrer, and then heated to 150°C for 12 h; then, the mixture was centrifuged at 4000 rpm for 15 min, the supernatant was discarded, the residue was dried, and the obtained mixture was redispersed in ultrapure water to obtain Eu(NO)3·6H2O with a concentration of 0.3 mg / ml, respectively. 0.8 / Tb 0.2 MOF solution, Eu 0.7 / Tb 0.3 MOF solution, Eu 0.6 / Tb 0.4 MOF solution, Eu 0.5 / Tb 0.5 MOF solution, Eu 0.4 / Tb 0.6 MOF solution, Eu 0.3 / Tb 0.7 MOF solution, Eu 0.2 / Tb 0.8 The MOF solution was stored in a refrigerator at 4°C away from light until use.

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

[0057] The Eu / Tb MOF with the best Eu / Tb ratio was selected by comparative experiments. Figure 2 The blank samples in B, C, D, E, and F are the mixed solutions of Ln-MOF and ultrapure water, without the addition of Cr(VI) solution). Figure 2 A shows that with the decrease of Tb(NO)3·6H2O amount and the increase of Eu(NO)3·6H2O amount, the fluorescence intensity of Eu / Tb MOF solution gradually decreases. Figure 2 B shows the visualization results (color change) of the Eu / Tb MOF ratio fluorescence sensor with different Eu / Tb ratios for detecting Cr(VI) and Figure 2 Fluorescence intensity changes of Eu / Tb MOF ratio fluorescence sensors with different Eu / Tb ratios in A. 0.4 / Tb 0.6 MOF, Eu 0.5 / Tb 0.5 MOF and Eu 0.6 / Tb 0.4 The color change of the mixed solution obtained after the MOF solution (volume of each solution was 400 μL) was reacted with different concentrations of Cr(VI) solution (volume of each solution was 400 μL, and the concentrations of Cr(VI) solution were 0.1 μM, 1 μM, and 10 μM, respectively) for 10 min was the most obvious. 0.4 / Tb 0.6 MOF, Eu 0.5 / Tb 0.5 MOF and Eu 0.6 / Tb 0.4 Select the best Eu / Tb ratio in MOF. Figure 2 From the CIE chromaticity diagrams presented in D, F, and H (the CIE chromaticity diagram is converted using the Chromaticity Diagram plug-in in Origin), we can see that Eu 0.5 / Tb 0.5 The chromaticity overlap of the mixed solution after the MOF and 0.1μM Cr(VI) solution were mixed and reacted was the least, so Eu 0.5 / Tb 0.5 MOF solution as the Eu / Tb MOF ratio fluorescence sensor with the optimal Eu / Tb ratio.

[0058] Determination of the volume ratio of each solution when detecting Cr(VI): Figure 3 It can be seen from A that by changing Eu 0.5 / Tb 0.5 The volume ratio of MOF solution to 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, the two solutions were mixed and reacted for 10 minutes, and the resulting mixed solution was analyzed by fluorescence. 0.5 / Tb 0.5 When the volume ratio of MOF solution to Cr(VI) solution is 4:6, 545 / I 618 The ratio is the lowest. Considering the convenience of subsequent experimental operations, 320 μL of Eu 0.5 / Tb 0.5 MOF solution, and 480 μL of Cr(VI) solution for subsequent experiments.

[0059] Determination of pH value when detecting Cr(VI): Figure 3 B shows that 0.1 mM Tris-HCl buffer was used to adjust Eu 0.5 / Tb 0.5 The pH value of the MOF solution, under the conditions of pH = 7 and pH = 8, the aforementioned Eu 0.5 / Tb 0.5 320 μL of MOF solution was mixed with 480 μL of 10 μM Cr(VI) solution and the mixed solution was reacted for 10 min. 545 / I 618 The ratio is the lowest, which is consistent with the fluorescence intensity and I 545 / I 618 Compared with the above, the fluorescence intensity was higher at pH = 7. 545 / I 618 The ratio is more obvious, so Eu 0.5 / Tb 0.5 The MOF ratio fluorescence sensor has the strongest detection ability at pH = 7.

[0060] Figure 3 C, 3D, and 3E are 320 μL Eu MOF solution, 320 μL Tb MOF solution, and 320 μL Eu MOF solution at pH = 7, respectively. 0.5 / Tb 0.5 The fluorescence spectra of the mixed solution obtained after 320 μL of MOF solution and 480 μL of Cr(VI) solution with different concentrations (the concentration of Cr(VI) solution is 0.1 μM, 1 μM, and 10 μM, respectively) were mixed and reacted for 10 minutes ( Figure 3 The blank samples in C, D, and E are mixed solutions of Ln-MOF and ultrapure water, without the addition of Cr(VI) solution). It can be seen that Eu MOF and Cr(VI) basically do not react, Tb MOF and Cr(VI) have a quenching reaction, and Eu 0.5 / Tb 0.5The fluorescence intensity of the mixed solution obtained after the reaction of MOF and Cr(VI) changed significantly at 545nm, while the fluorescence intensity at 618nm did not change significantly.

[0061] In summary, Eu 0.5 Tb 0.5 MOF as Eu / Tb MOF ratio fluorescence sensor, according to the pH = 7 I 545 / I 618 The ratio at the position is used to qualitatively and quantitatively detect Cr(VI).

[0062] Figure 4 A is Eu 0.5 / Tb 0.5 TEM image of MOF, from which it can be seen that it has an irregular hexagonal structure; Figure 4 B and C are the corresponding element energy spectra. It can be seen from the figure that Eu 0.5 / Tb 0.5 The Eu and Tb elements of MOF are evenly dispersed in the material. Figure 4 D shows that Ln MOF lacks the ligand 3,5-dicarboxyphenylboronic acid at 1702 cm -1 The peak at , which is caused by the C=O stretching vibration of 3,5-dicarboxyphenylboronic acid (abbreviated as "5-bop"), indicates that the carboxyl group is 3+ and / or Tb 3+ Coordinated to form LnMOF. At 1610cm -1 and 1540cm -1 The peak at 3+ and / or Tb 3+ The cross-coordination with the carboxyl group of the ligand 3,5-dicarboxyphenylboronic acid resulted. -1 The peak at 1390 cm is caused by the BO stretching vibration of the ligand 3,5-dicarboxyphenylboronic acid. -1 The BO characteristic peaks were also observed at , indicating that Ln MOF has free boronic acid sites. Figure 4 E is the UV-visible spectrum of 3,5-dicarboxyphenylboronic acid and Ln MOF. It can be seen from the figure that 3,5-dicarboxyphenylboronic acid and Ln MOF have similar absorption peaks, indicating that no new functional groups are generated. Figure 4 F is Eu 0.5 / Tb 0.5 XPS spectrum of MOF. It can be seen from the figure that Eu 0.5 / Tb 0.5 MOF contains characteristic elements such as C, O, B, Eu, and Tb. The above results show that Eu 0.5 / Tb 0.5 Successful preparation of MOF.

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

[0064] Add the Eu prepared in step (2) into different 2 mL EP tubes. 0.5 Tb 0.5 320 μL of MOF solution was added and its pH value was adjusted to 7, and then 480 μL of Cr(VI) solution of different concentrations was added to each of the EP tubes in turn and mixed to obtain a mixed solution. After reacting for 10 minutes, the fluorescence spectrum data of the mixed solution was collected at room temperature and normal pressure.

[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 a mixed solution mixed with Cr(VI) solutions of different concentrations. After reacting for 10 minutes, the fluorescence spectrum data of the mixed solution corresponding to each concentration of Cr(VI) solution is collected at room temperature and normal pressure. According to the fluorescence spectrum data of the mixed solution corresponding to each concentration of Cr(VI) solution, a standard curve for detecting Cr(VI) is established.

[0066] like Figure 5 As shown in A and B, when the concentration of the added Cr(VI) solution is 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, and 10 μM, the fluorescence intensity I 545 / I 618 There is a good linear relationship between the value (Y) and the concentration (X) of Cr(VI) solution, and the detection limit is as low as 0.021 μM.

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

[0068] Verify Eu through comparative test 0.5 Tb 0.5 The specificity of the reaction between MOF and Cr(VI) was studied by selecting different metal ion standard solutions and Eu 0.5 / Tb 0.5 The MOF solution was reacted and the mixed solution after the reaction was tested. The specific process was as follows: according to the volume ratio of each raw material in step (3) of Example 1, Eu0.5 / Tb 0.5 Different types of high-concentration metal ion standard solutions were added to the MOF solution, including Cr(VI) standard solution (prepared by adding potassium dichromate to ultrapure water), Cr 3+ Standard solution (prepared by adding chromium chloride hexahydrate to ultrapure water), Ag + Standard solution (prepared by adding silver nitrate to ultrapure water), Ca 2+ Standard solution (prepared by adding anhydrous calcium chloride to ultrapure water), Cd 2+ Standard solution (prepared by adding cadmium chloride dihydrate to ultrapure water), Co 2+ Standard solution (prepared by adding cobalt chloride hexahydrate to ultrapure water), Cu 2+ Standard solution (prepared by adding cupric chloride dihydrate to ultrapure water), Fe 2+ Standard solution (prepared by adding ferrous chloride tetrahydrate to ultrapure water), Fe 3+ Standard solution (prepared by adding ferric chloride hexahydrate to ultrapure water), K + Standard solution (prepared by adding potassium chloride to ultrapure water), Mg 2+ Standard solution (prepared by adding magnesium chloride to ultrapure water), Mn 2+ Standard solution (prepared by adding manganese chloride tetrahydrate to ultrapure water), Na + Standard solution (prepared by adding sodium chloride to ultrapure water), NH4 + Standard solution (prepared by adding ammonium sulfate to ultrapure water), Ni 2+ Standard solution (prepared by adding nickel chloride hexahydrate to ultrapure water), Pb 2+ Standard solution (prepared by adding lead chloride to ultrapure water), Zn 2+ Standard solution (prepared by adding zinc chloride to ultrapure water), Cl - Standard solution (prepared by adding sodium chloride to ultrapure water), CO3 2- Standard solution (prepared by adding sodium carbonate to ultrapure water), HCO3 - Standard solution (prepared by adding sodium bicarbonate to ultrapure water), I - Standard solution (prepared by adding potassium iodide to ultrapure water), NO3 - Standard solution (prepared by adding silver nitrate to ultrapure water), PO4 3- Standard solution (prepared by adding sodium phosphate dodecahydrate to ultrapure water), SO3 2- Standard solution (prepared by adding sodium sulfite to ultrapure water), SO4 2- Standard solution (prepared by adding ammonium sulfate to ultrapure water). Figure 4 C, the ordinate is the fluorescence intensity I of the solution545 / I 618 The concentration of the above metal ion standard solutions is 10 μM, Eu 0.5 / Tb 0.5 The volume of MOF is 320 μL, and the volume of Cr(VI) standard solution and other metal ion standard solutions are 480 μL. It can be seen that compared with other metal ions, the I corresponding to Cr(VI) 545 / I 618 The ratio is the lowest. Figure 4 As shown in D, the concentration of Cr(VI) standard solution is 1 μM, and the concentration of other metal ion standard solutions is 10 μM. 0.5 / Tb 0.5 The volume of MOF is 320 μL, the volume of 1 μM Cr(VI) solution is 240 μL, and the volume of other metal ion standard solutions is also 240 μL. Figure 4 Compared with C, Figure 4 D shows that after adding other metal ion standard solutions and Cr(VI) standard solution, I 545 / I 618 The ratio of Eu 0.5 / Tb 0.5 The MOF ratiometric fluorescence sensor exhibits excellent selectivity for Cr(VI).

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

[0070] Preparation of real samples: Tap water was obtained from the laboratory and processed as follows: large particles were removed by filtering through a microporous membrane with a pore size of 0.22 μm; iced black tea and green tea were purchased from a supermarket (Wuhan, Hubei Province) and processed as follows: large particles were removed by filtering through a microporous membrane with a pore size of 0.22 μm and diluted 100 times; no other processing was required. The tap water, iced black tea, and green tea treated according to the above method were used as solvents to obtain Cr(VI) solutions of different concentrations.

[0071] As shown in Table 1, when the concentrations of the Cr(VI) solutions obtained after adding Cr(VI) were 0.70 μM, 1.00 μM, and 5.00 μM, the relative standard deviations (RSDs) of the added amounts were all less 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 Analysis results of Eu / Tb MOF on Cr(VI) in real samples

[0073]

[0074] The above-mentioned embodiments only express the accurate implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present invention. It should be pointed out that, for those of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the attached claims.

Claims

1. A method for detecting Cr(VI) using a dual lanthanide metal organic framework Eu / Tb MOF ratio fluorescence sensor, comprising the following steps: Step (1): Selection of ligand (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; Then centrifuge, discard the supernatant, and redisperse the residue after drying in ultrapure water to obtain a Eu MOF solution; According to the above process, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid are used to replace isophthalic acid as ligand A, and the rest of the process remains unchanged, and Eu MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 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; Then centrifuge, discard the supernatant, and redisperse the residue after drying in ultrapure water to obtain a Tb MOF solution; According to the above process, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid are used to replace isophthalic acid as ligand A, and the rest of the process remains unchanged, and Tb MOF solutions with 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 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; Then centrifuge, discard the supernatant, and redisperse the residue after drying in ultrapure water to obtain Eu / Tb MOF solution; According to the above process, 2-carboxyphenylboronic acid, 3-carboxyphenylboronic acid, 4-carboxyphenylboronic acid, and 3,5-dicarboxyphenylboronic acid are used to replace isophthalic acid as ligand A, and the rest of the process remains unchanged, and 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; (1.4) The Eu MOF solution, Tb MOF solution, and Eu / Tb MOF solution prepared in steps (1.1), (1.2), and (1.3) are respectively measured and mixed with an equal volume of Cr(VI) solution in turn to obtain different mixed solutions. The fluorescence absorption intensity ratio of the mixed solutions is calculated. 545 / I 618 The fluorescence absorption intensity ratio I of the Eu MOF solution, Tb MOF solution and Eu / Tb MOF solution prepared in steps (1.1), (1.2) and (1.3) is 545 / I 618 Comparative analysis was performed, and 3,5-dicarboxyphenylboronic acid was finally determined to be the best ligand; Step (2): Selection of Eu / Tb ratio in Eu / Tb MOF (2.1) Dissolve 3,5-dicarboxyphenylboronic acid and different molar ratios of Eu(NO)3·6H2O and Tb(NO)3·6H2O in a mixed solution of N,N-dimethylformamide and ultrapure water, stir with a magnetic stirrer, and then heat; then, centrifuge and discard the supernatant, dry the residue, and redisperse the obtained solution in ultrapure water to obtain a Eu / Tb MOF solution, which is stored in a refrigerator at 4°C in the dark for future use; wherein the ratio of the total molar amount of Eu(NO)3·6H2O and Tb(NO)3·6H2O to the molar amount of 3,5-dicarboxyphenylboronic acid is 1:5, 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, respectively; (2.2) mixing the Eu / Tb MOF solution obtained in step (2.1) with equal volumes of Cr(VI) solutions of different concentrations, respectively, to obtain mixed solutions after reaction; comparing the fluorescence spectrum data of the Eu / Tb MOF solution obtained in step (2.1), comparing the color change and the fluorescence spectrum data of the mixed solution, and comparing the CIE chromaticity diagram obtained by converting the fluorescence spectrum data of the mixed solution; By comparing the fluorescence spectrum data of Eu / Tb MOF solution, comparing the color change and fluorescence spectrum data of the mixed solution, and then determining the Eu / Tb molar ratio based on the minimum overlap in the CIE chromaticity diagram, the Eu 0.5 / Tb 0.5 MOF solution as Eu / Tb MOF ratiometric fluorescence sensor; Step (3), Eu 0.5 / Tb 0.5 MOF ratiometric fluorescence sensor for detection of Cr(VI) The pH value of the Eu / Tb MOF ratio fluorescence sensor obtained in step (2) is adjusted to 7, and then mixed with the Cr(VI) solution to obtain a mixed solution, and different mixed solutions mixed with Cr(VI) solutions of different concentrations are obtained by changing the concentration of the Cr(VI) solution; after the reaction is complete, the fluorescence spectrum data of the mixed solution is collected at room temperature and normal pressure; according to the concentration of the Cr(VI) solution and the fluorescence intensity I of the mixed solution corresponding to it, 545 / I 618 Value, in I 545 / I 618 The value is taken as the vertical axis and the concentration of Cr(VI) solution is taken as the horizontal axis. A standard curve for detecting the concentration of Cr(VI) is established. Based on this standard curve, the determination of Cr(VI) content in a complex matrix system is realized.

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

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

4. The method according to claim 1, characterized in that: The heating step is heating at 150° C. for 12 hours, the centrifugation step is centrifugation at 4000 rpm for 15 minutes, and the drying step is drying in a vacuum drying oven at 60° C. for 12 hours.

5. The method according to claim 1, characterized in that In the step (1.1), the molar ratio of isophthalic acid to Eu(NO)3·6H2O is 5:1, and similarly, the molar ratio of 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric 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, the amount of isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric acid is 0.5 mmol, the amount of 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; Preferably, in step (1.1), the concentration of the obtained Eu MOF solution is 0.3 mg / mL; Preferably, in the step (1.2), the molar ratio of isophthalic acid to Tb(NO)3·6H2O is 5:1, and similarly, the molar ratio of 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric 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, the amount of isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric acid is 0.5 mmol, the amount of 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; Preferably, in step (1.2), the concentration of the obtained Tb MOF solution is 0.3 mg / mL; Preferably, in the step (1.3), the molar ratio of isophthalic acid to the total molar ratio 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 ratios of 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric acid to the total molar ratios of Eu(NO)3·6H2O and Tb(NO)3 are 5:1 and 1:1, respectively. The total molar ratio of Eu(NO)3·6H2O is also 5:1; the volume ratio of N,N-dimethylformamide and ultrapure water is 7:3; more preferably, isophthalic acid, 2-carboxyphenylboric acid, 3-carboxyphenylboric acid, 4-carboxyphenylboric acid or 3,5-dicarboxyphenylboric acid is 0.5mmol, Eu(NO)3·6H2O and Tb(NO)3·6H2O are both 0.05mmol, the volume of N,N-dimethylformamide is 7ml, and the volume of ultrapure water is 3ml; Preferably, in step (1.3), the concentration of the obtained Eu / Tb MOF solution is 0.3 mg / mL; Preferably, in 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.

6. The method according to claim 1, characterized in that In step (2.1), the amount of 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.

7. The method according to claim 1, characterized in that In step (2.2), the volume of the Eu / TbMOF solution prepared in step (2.1) and the volume of the Cr(VI) solution of different concentrations are both 400 μL, and the Cr(VI) solution of different concentrations refers to the Cr(VI) solution having a concentration of 0.1 μM, 1 μM, and 10 μM, respectively.

8. 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, the Cr(VI) solutions of different concentrations refer to the concentrations of the Cr(VI) solutions 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, and 10 μM, respectively, and the complete reaction refers to the reaction for 10 min.

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