Eu-NH2BDC / OX MOF material as well as preparation method and application thereof

Abstract

The invention relates to an Eu-NH2BDC / OX MOF material as well as a preparation method and application thereof, and belongs to the technical field of fluorescent materials. According to the invention, 2-fluorobenzoic acid is used as a core cluster structure guiding agent, oxalic acid and 2-aminoterephthalic acid are used as organic ligands, and the Eu-NH2BDC / OX MOF material is synthesized based on a double-ligand strategy. The invention relates to application as a sulfur dioxide ratio fluorescence sensor, application in detection of sulfur dioxide and derivatives thereof in food, and development of fluorescence detection test paper for sulfur dioxide and derivatives thereof. The material has the dual-wavelength emission characteristic, the light emitting color changes from red to blue along with the SO3 < 2-> concentration change, visual inspection is facilitated, and the material has the advantages of quick response, high stability and high sensitivity; as a sulfur dioxide ratiometric fluorescence sensor, high-sensitivity and anti-interference detection on SO3 < 2-> can be realized, and the precision requirement of food safety detection can be met; the fluorescence detection test paper prepared from the Eu-NH2BDC / OX MOF material is simple, convenient and rapid to apply. The method realizes visual rapid detection of sulfur dioxide and derivatives thereof, and has important application value and market potential.

Description

Technical Field

[0001] The invention belongs to the technical field of fluorescent materials, and specifically relates to a Eu-NH2BDC / OX MOF material and a preparation method and application thereof. Background Art

[0002] Sulfur dioxide and its derivatives (sulfite / bisulfite / sulfur dioxide) are commonly used to protect foods and beverages from oxidation, browning and microbial reactions. However, high concentrations of SO3 2- It is harmful to human health and food safety. In most countries, SO3 2- The threshold level of SO2 is strictly limited. According to the standards issued by the World Health Organization Expert Committee, its ADI limit is 0.7 mg / kg. Therefore, the development of effective SO2 detection and quantitative methods is of great significance in food safety and quality control.

[0003] Fluorescence analysis has received increasing attention in sulfur dioxide detection in recent years due to its advantages such as fast response speed, strong anti-interference ability, high sensitivity and convenient operation.

[0004] The metal organic framework (Ln-MOFs) fluorescent probe based on lanthanide elements is suitable for fluorescent detection of various substances. The existing technology uses 2-fluorobenzoic acid as a core cluster structure director, designs and uses an organic ligand 2-aminoterephthalic acid, and synthesizes a six-core cluster rare earth metal-organic framework material Eu-BDC-NH2 with europium ion rare earth salt through a core clustering preparation strategy to detect SO2. However, the Ln-MOFs synthesized by 2-fluorobenzoic acid (2-FBA) have low thermal stability and can only exist stably within 500°C. In addition, the introduction of only a single organic ligand makes the response to the detected substance low, with a minimum detection limit of 0.65 mg / L, low luminous efficiency, poor sensitivity, and poor anti-interference.

[0005] Therefore, the development of a Eu-NH2BDC / OX MOF material synthesized by a dual-ligand strategy to achieve rapid response, high stability, and high sensitivity to sulfur dioxide and its derivatives, and its application as a sulfur dioxide ratio fluorescence sensor has important research significance and application value. Summary of the invention

[0006] The present invention aims to overcome the deficiencies of the prior art, and the primary purpose of the present invention is to provide a Eu-NH2BDC / OXMOF material. The Eu-NH2BDC / OX MOF material provided by the present invention uses 2-fluorobenzoic acid as a core cluster structure directing agent, oxalic acid and 2-aminoterephthalic acid as organic ligands, and based on the above dual ligand strategy, a Eu-NH2BDC / OX MOF material that can not only specifically identify sulfur dioxide and its derivatives but also has good stability is synthesized.

[0007] Another object of the present invention is to provide a method for preparing the Eu-NH2BDC / OX MOF material.

[0008] Another object of the present invention is to protect the application of the above Eu-NH2BDC / OX MOF material as a sulfur dioxide ratio fluorescence sensor.

[0009] Another object of the present invention is to protect the application of the above-mentioned Eu-NH2BDC / OX MOF material in the detection of sulfur dioxide and its derivatives.

[0010] Another object of the present invention is to protect a method for fluorescent detection of sulfur dioxide and its derivatives.

[0011] Another object of the present invention is to protect the use of the above-mentioned Eu-NH2BDC / OX MOF material in the preparation of test paper for the detection of sulfur dioxide and its derivatives.

[0012] Another object of the present invention is to protect a fluorescent detection test paper for sulfur dioxide and its derivatives.

[0013] Another object of the present invention is to provide a method for preparing the fluorescent detection test paper of sulfur dioxide and its derivatives.

[0014] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme:

[0015] The present invention provides a method for preparing a Eu-NH2BDC / OX MOF material, comprising the following steps:

[0016] The Eu-NH2BDC / OX MOF material is prepared by hydrothermally reacting a DMF-nitric acid-water mixed solution of europium salt, oxalic acid, 2-aminoterephthalic acid and 2-fluorobenzoic acid at 115-125° C. for 20-30 hours;

[0017] The molar ratio of the europium salt, oxalic acid, 2-aminoterephthalic acid and 2-fluorobenzoic acid is (4-5): (1-3): (3-3.5): (40-50).

[0018] The present invention uses 2-fluorobenzoic acid (2-FBA) as a core cluster structure directing agent, oxalic acid (H2OA) and 2-aminoterephthalic acid (H2BDC-NH2) as organic ligands, and based on the above dual ligand strategy, synthesizes a Eu-NH2BDC / OX MOF material that can not only specifically recognize sulfur dioxide and its derivatives but also has good stability.

[0019] Eu 3+ The -COOH group is chelated on the ligand, which increases the rigidity of the skeleton and enhances the fluorescence properties. 3+ It has the characteristics of high coordination number and flexible structure. The three-dimensional rigid structure formed by the interaction between Eu and ligands in Eu-NH2BDC / OX has good thermal stability and can exist stably within 800℃.

[0020] Oxalic acid (H2OA) is a rigid organic ligand, and -COOH participates in the coordination and connection between the metal center to form an extended structure from 0 to 3D. At the same time, it shows a more flexible skeleton and coordination mode, which can connect metal ions into a high-dimensional framework. Using oxalic acid as a synthesis regulator can regulate crystal growth and make Eu-NH2BDC / OXMOF have good crystallinity without affecting the structure and composition of the material.

[0021] Further preferably, in the preparation method of the Eu-NH2BDC / OX MOF material, the hydrothermal reaction is carried out at 120°C for 24 hours.

[0022] Preferably, the concentration of HNO3 is 3-4 mol / L.

[0023] Further preferably, the concentration of HNO3 is 3.5 mol / L.

[0024] Preferably, the volume ratio of DMF to water is 1:7 to 1:8. More preferably, the volume ratio of DMF to water is 1:7.8.

[0025] Preferably, the europium salt is europium nitrate.

[0026] The present invention protects the Eu-NH2BDC / OX MOF material prepared according to the above preparation method.

[0027] The present invention protects the use of the above Eu-NH2BDC / OX MOF material as a sulfur dioxide ratio fluorescence sensor.

[0028] The present invention protects the use of the above-mentioned Eu-NH2BDC / OX MOF material in detecting sulfur dioxide and its derivatives.

[0029] The present invention protects a method for fluorescent detection of sulfur dioxide and its derivatives, which adopts the Eu-NH2BDC / OXMOF material as a sulfur dioxide ratio fluorescence sensor.

[0030] Specifically, the method comprises the following steps:

[0031] The solution to be tested is added to the Eu-NH2BDC / OX MOF material dispersion to obtain a mixed solution, the pH is adjusted to 7-11, and fluorescence spectrophotometry is performed for detection;

[0032] Fluorescence detection conditions: λex: 300 nm, λem: 450 nm / 615 nm, voltage: 400 V, slit width: 5 nm, scanning speed: 3000 nm / min.

[0033] Preferably, the adjusted pH value is 10.

[0034] Preferably, the method uses sodium sulfite to prepare a standard solution, the amount of the standard solution added is 0.4-0.8 mL, and the SO3 2- The concentration is 5-50μg / mL, and the detection limit is 0.25mg / kg.

[0035] Preferably, the method uses the concentration of sodium sulfite as the abscissa, I 450 / I 615 A standard curve was drawn as the ordinate, and the r of the standard curve was 0.9939.

[0036] Preferably, the dispersant of the Eu-NH2BDC / OX MOF material dispersion is one of methanol, ethanol, acetonitrile, isopropanol or N,N-dimethylformamide.

[0037] Preferably, the detection amount of the Eu-NH2BDC / OX MOF material dispersion is 1-3 mL, preferably 1.5 mL; the concentration is 0.3-1 mg / mL, and more preferably the concentration is 0.5 mg / mL.

[0038] Preferably, the method for preparing the sample solution of the method comprises the following steps: mixing the sample to be tested with the extracting solution, performing ultrasonic extraction, and taking the supernatant after separation; the extracting solution is selected from a sodium hydroxide or potassium hydroxide solution with a concentration of 0.1% to 1%.

[0039] Preferably, the concentration of the extract is 0.5%.

[0040] Preferably, the extraction time is 20 to 40 minutes, preferably 30 minutes.

[0041] Preferably, the sample to be tested is candied food, selected from at least one of candied dates, raisins, preserved plums and dried mangoes.

[0042] Preferably, the amount of the sample to be tested and the extract is 1-10 g:5-15 mL, preferably 5 g:10 mL.

[0043] The present invention protects the use of the above Eu-NH2BDC / OX MOF material in preparing a test paper for detecting sulfur dioxide and its derivatives.

[0044] The invention protects a fluorescent detection test paper for sulfur dioxide and its derivatives, wherein the detection test paper contains the Eu-NH2BDC / OX MOF material.

[0045] The present invention provides a method for preparing the above-mentioned fluorescent detection test paper, comprising the following steps:

[0046] The glass fiber is immersed in the Eu-NH2BDC / OX MOF material dispersion and dried to obtain the fluorescence detection test paper.

[0047] Specifically, the Eu-NH2BDC / OX MOF material dispersion is prepared by dispersing the Eu-NH2BDC / OX MOF material in an alcohol solvent, wherein the dispersion is ultrasonic dispersion, and the alcohol solvent is selected from at least one of methanol, ethanol, and propanol.

[0048] Specifically, the soaking is performed with the assistance of ultrasound, and the ultrasound time is 5 to 20 minutes.

[0049] Preferably, in the preparation method of the fluorescent test paper, the amount of alcohol solvent added is 1 mL, and the amount of Eu-NH2BDC / OX MOF added is 1 mg.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1. The present invention uses 2-fluorobenzoic acid as a core cluster structure directing agent, oxalic acid and 2-aminoterephthalic acid as organic ligands to synthesize a Eu-NH2BDC / OX MOF material that can specifically identify sulfur dioxide and its derivatives, has good stability within 800°C and good recyclability, and has the characteristics of rapid response, high stability and high sensitivity to sulfur dioxide and its derivatives.

[0052] 2. The present invention uses Eu-NH2BDC / OX MOF material as a sulfur dioxide fluorescence ratio sensor, and realizes the quantitative detection of sulfur dioxide and its derivatives in candied foods such as candied dates, raisins, plums, and dried mangoes through fluorescence spectrophotometry. It has a low detection limit and high recovery rate. The detection limit is 0.25 mg / kg and the recovery rate is 87.28% to 106.3%.

[0053] 3. The present invention uses Eu-NH2BDC / OX MOF material to prepare fluorescent detection test paper for sulfur dioxide and its derivatives. The change of fluorescence intensity can be observed by naked eyes and has good stability. The practical application is simple and fast, and the color development takes 30 seconds. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure (1) Preparation process of Eu-NH2BDC / OX material.

[0055] Figure (2) Schematic diagram of the fluorescence reaction process.

[0056] Figure (3) SEM image of Eu-NH2BDC / OX material.

[0057] Figure (4) TEM image of Eu-NH2BDC / OX material.

[0058] Figure (5) Particle size distribution of Eu-NH2BDC / OX material.

[0059] Figure (6) EDS energy spectrum of Eu-NH2BDC / OX material.

[0060] Figure (7) EDS graph of C, N, O, and Eu on the surface of Eu-NH2BDC / OX material.

[0061] Figure (8) Infrared spectrum of Eu-NH2BDC / OX material.

[0062] Figure (9) Thermal gravimetric loss curve of Eu-NH2BDC / OX powder.

[0063] Figure (10) XRD spectrum of Eu-NH2BDC / OX material.

[0064] Figure (11) XPS spectrum of Eu-NH2BDC / OX material: N1s (a), Eu3d (b), C1s (c), and total spectrum (d).

[0065] Figure (12) UV-visible absorption spectrum of Eu-NH2BDC / OX material.

[0066] Figure (13) Excitation and emission spectra of Eu-NH2BDC / OX and emission spectrum of Eu-NH2BDC. Figure (14) Effect of solvent type on the fluorescence intensity of the system.

[0067] Figure (15) The effect of material solution dosage on the fluorescence intensity of the system.

[0068] Figure (16) Effect of the amount of sodium sulfite solution on the fluorescence intensity of the system.

[0069] Figure (17) Effect of pH on the fluorescence intensity of the system.

[0070] Figure (18) shows the fluorescence intensity change of Eu-NH2BDC / OX dissolved in anhydrous ethanol and stored at 4°C for 3 days under 275nm excitation.

[0071] Figure (19) tests the Eu-NH2BDC / OX material and detects the changes in sodium sulfite solution multiple times.

[0072] Figure (20) The influence of different interfering ions on the detection results of the system.

[0073] Figure (21) Fluorescence spectrum and standard curve of Eu-NH2BDC / OX material detecting 0-50μg / ml sodium sulfite.

[0074] Figure (22) Linear relationship between different concentrations of sodium sulfite and green value detected by Eu-NH2BDC / OX. Figure (23) CIE chromaticity diagram of sodium sulfite detected by Eu-NH2BDC / OX material.

[0075] Figure (24) Standard curve of RGB value and sodium sulfite concentration. DETAILED DESCRIPTION

[0076] The present invention is further described below in conjunction with examples. These examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples that do not specify specific conditions are usually carried out according to conventional conditions in the art or according to the conditions recommended by the manufacturer; the raw materials, reagents, etc. used, unless otherwise specified, are all raw materials and reagents that can be obtained from commercial channels such as conventional markets. Any non-substantial changes and substitutions made by those skilled in the art on the basis of the present invention belong to the scope of protection claimed by the present invention.

[0077] Embodiments 1 to 3 of the present invention include the following:

[0078] Example 1 Preparation and Application of a Sulfur Dioxide Ratio Fluorescence Sensor - Preparation of a Sulfur Dioxide Ratio Fluorescence Sensor

[0079] (1) Synthesis of Eu-NH2BDC / OX MOF (diaminoterephthalic acid to oxalic acid ratio 0.33:0.166 = 2:1)

[0080] Accurately weigh 223 mg of europium nitrate hexahydrate, 59.8 mg of 2-aminoterephthalic acid, 21.415 mg of oxalic acid (the amount of dihydrated oxalic acid) and 604.8 mg of 2-fluorobenzoic acid, dissolve in 7.8 mL of DMF, then add 1 mL of H2O and 0.6 mL of HNO3 (DMF dilutes HNO3 to 3.5 mol / L) to the solution, seal in a 25 mL reactor and react at 120°C for 24 hours, and cool to room temperature. The product needs to be washed three times with DMF and ethanol to remove excess ligands and metal ions. Finally, dry at 40°C for 12 hours for use (Figure 1 ).

[0081] (2) Characterization method of Eu-NH2BDC / OX materials

[0082] The surface morphology of Eu-NH2BDC / OX MOF was investigated by scanning electron microscopy (SEM) ( Figure 2 ) and transmission electron microscopy (TEM) ( Figure 3 , 4 )observe; Figure 5 is the particle size distribution diagram of Eu-NH2BDC / OX MOF; the types and contents of the micro-component elements of Eu-NH2BDC / OX material were analyzed by energy dispersive spectrometer (EDS) ( Figure 6 , 7 ) for analysis; the surface chemical properties of the material were analyzed by using Fourier transform infrared spectrometer (FTIR) ( Figure 8 ) were analyzed; the thermal stability of Eu-NH2BDC / OX MOF materials was explored using thermogravimetric analysis (TGA) ( Fig. 9 ).

[0083] (3) Characterization experimental results-morphology analysis

[0084] SEM test of Eu-NH2BDC / OX MOF ( Figure 3 ), and disperse the prepared Eu-NH2BDC / OX MOF powder in an ethanol solution. Figure 3 It can be seen that the crystals of Eu-NH2BDC / OX MOF are relatively regular polyhedral, with clear shape and size, and a certain granularity. The particle size is about 54.5±15.8nm, which is significantly smaller than that of Eu-NH2BDC, and thus has more superior stability and solubility ( Figure 3 , 4 , 5).

[0085] At the same time, the successful preparation of Eu-NH2BDC / OX MOF material was further proved. The impurity phase formed during the preparation process may be the main reason for the appearance of a small number of spherical particles on the crystal surface. Therefore, the dual ligand strategy realizes the dual emission, specific recognition and rapid response of sulfur dioxide and its derivatives, as well as the regulation of the size and morphology of MOF.

[0086] At the same time, according to the EDS spectrum data of Eu-NH2BDC / OX MOF material ( Figure 6 ), not only the signal peaks of C, N and O elements can be observed, but also the signal peaks of Eu 3+ Element signal peak, proving Eu 3+ It was successfully introduced into the system, indicating the successful preparation of Eu-NH2BDC / OX MOF material.

[0087] Table 1 C, O, Zr, and Eu contents in EDS measurement

[0088]

[0089] In order to further explore the actual distribution of the Eu-NH2BDC material surface, EDS mapping was used to characterize the C, N, O and Eu elements on the surface of Eu-NH2BDC / OX MOF ( Figure 7 ), it can be seen from the EDS image that the four elements C, N, O and Eu are uniformly distributed on the surface of each Eu-NH2BDC / OX MOF, which reflects the successful preparation of Eu-NH2BDC / OX MOF materials (Table 1).

[0090] (4) Characterization experimental results-infrared spectroscopy analysis

[0091] The FT-IR spectra of Eu-NH2BDC / OX MOF and oxalic acid were compared ( Figure 8 ), it was found that carbonyl and rare earth ions (oxalic acid is 1700cm -1 , Eu-NH2BDC / OX is 1640cm -1 ) is obviously lower in coordination. -1 A new band appears at 1320 cm-1, which is the region where the Eu-O bond occurs, corresponding to a new mode, which is formed by the coordination of Eu carboxylic acid bonds with oxalic acid. -1 Another shift is also observed at 722 cm-1, which corresponds to the stretching of the CO bond. -1 (Oxalic acid) shifted to 802 cm -1 (Eu-NH2BDC / OX) is part of the binding site, further supporting the chelation of the carboxylic acid group to the metal site.

[0092] On the other hand, CC of oxalic acid (1380 cm -1 ) remains constant in all FT-IR spectra. Eu 3+ The -COOH group chelates on the ligand, which increases the rigidity of the skeleton and enhances the fluorescence performance. In addition, by comparing the infrared spectra of Eu-NH2BDC / OX MOF with oxalic acid and H2BDC-NH2, it can be found that the position and intensity of the spectral bands have changed significantly, which indicates that Eu 3+ There is a coordination effect between the ligand and the ligand, that is, a complex is formed.

[0093] (5) Characterization Experimental Results - Thermogravimetric Analysis

[0094] The structural stability of MOFs is very important in practical applications. Lanthanide (III) ions have the characteristics of high coordination number and flexible structure, so their thermal stability is relatively significant.

[0095] Depend on Fig. 9 It can be seen that the weight loss of Eu-NH2BDC / OX MOF is 10% from 40 to 336°C, which may be caused by the volatilization of water and DMF molecules on the surface and in the pores of Eu-NH2BDC / OX MOF crystals. From 336 to 397°C, it is 20.2%, corresponding to the release of two lattice water molecules and four coordinated water molecules. From 397 to 600°C, it is estimated that the collapse of the main skeleton. It can be seen from the figure that the three-dimensional rigid structure formed by the interaction between Ln and ligands in Eu-NH2BDC / OX has good thermal stability and can exist stably within 800°C.

[0096] (6) Characterization experimental results - X-ray diffraction

[0097] Using oxalic acid as a synthesis regulator can regulate crystal growth and make Eu-NH2BDC / OX MOF have good crystallinity without affecting the structure and composition of the material. The results are shown in ( Fig.10 ), obvious peaks can be observed at 12.86°, 14.34° and 19.02°, and these three peaks have a slight shift compared with the positions of their corresponding standard simulation peaks (ccd number: 2061887), indicating its isomorphous skeleton topology. PXRD confirmed the structural stability of Eu-NH2BDC / OX during the sensing process, excluding the possibility of structural collapse in the sensing experiment.

[0098] (7) Characterization Experimental Results - X-ray Photoelectron Spectroscopy

[0099] X-ray photoelectron spectroscopy (XPS) can be used in MOFs to elucidate the identity of elements and their chemical states, such as their coordination environments. 3+ To determine whether it was really introduced into the system, XPS characterization was performed on Eu-NH2BDC / OX to observe the bonding conditions of each element.

[0100] The results are as follows Fig.11As shown, the obvious 3d peak of Eu can be seen, confirming the presence of the Eu element. Eu, N, O and C elements were observed in Eu-NH2BDC / OX MOF. Three peaks at 284.78, 285.98 and 288.58 eV were observed in C1s, which can be attributed to O=C-OH, C=O and NC=O. The Eu3d of Eu-NH2BDC / OX, the separated peaks are shown at 1164.78, 1154.38, 1135.08 and 1124.78 eV, which are related to Eu3d3 and Eu3d5.

[0101] Example 2 Preparation and Application of a Sulfur Dioxide Ratio Fluorescence Sensor - Fluorescence Detection of Sulfur Dioxide and Its Derivatives

[0102] (1) Preparation of main solutions

[0103] Sodium sulfite standard solution: Dilute the Na2SO3 standard (concentration of 100μg / mL) to a Na2SO3 stock solution with a concentration range of 5-50μg / mL and place it in a 50mL volumetric flask.

[0104] (2) Calculation of sulfur dioxide residue

[0105]

[0106] Where X is the sulfur dioxide content (mg / kg); V 提取 is the volume of ultrapure water used to extract the sample (mL); C is the SO2 concentration calculated based on the standard curve (μg / mL); V 定容 is the constant volume of the reaction (mL); M 称量 is the actual sample mass weighed (g); V 吸取 is the volume of sample extract aspirated (mL).

[0107] (3) Fluorescence detection conditions

[0108] λex: 300nm, λem: 450nm / 615nm. Voltage: 400V. Slit width: 5nm. Scanning speed: 3000nm / min.

[0109] (4) Fluorescence spectrophotometry detection process

[0110] Weigh a certain amount of Eu-NH2BDC / OX MOF, add an appropriate amount of detection dispersant to make it into a 0.5 mg / mL material solution, add sodium sulfite standard solution to adjust the pH to 7-11, and make it to 5 mL. Ultrasonicate for 5 minutes before detection. The whole experimental operation process is as follows: Figure 2 shown.

[0111] (5) Preparation of actual samples

[0112] Use a balance to accurately weigh 5.00 g of candied dates, raisins, plums and dried mangoes cut into small pieces by scissors, add 10 mL of 0.5% sodium hydroxide solution, ultrasonically treat for 30 min, centrifuge (8000 r / min) for 5 min, and draw 1.0 mL of the supernatant and add deionized water to dilute to 10 mL for later use.

[0113] The obtained dilution was tested according to the method (4).

[0114] (6) Fluorescence performance analysis

[0115] UV-visible absorption: Sodium sulfite and H2OA have absorption peaks at 226nm and 232nm respectively. Fig.12 , sodium sulfite and Eu-NH2BDC / OX MOF have an effective overlapping region in the ultraviolet visible (UV-vis) absorption range, indicating that there is a competitive absorption of light source between sodium sulfite and Eu-NH2BDC / OX MOF.

[0116] The fluorescence excitation and emission spectra of Eu-NH2BDC / OX MOF and the fluorescence emission spectrum of Eu-NH2BDC were recorded in outdoor conditions. Fig.13 The excitation spectrum measured at EM = 615 nm shows a broad absorption band centered at 260. Eu-NH2BDC / OX MOF has a peak absorption band at 596 nm ( 5 D0→ 7 F2) and 615nm( 5 D0→ 7 F2) has typical red emission, which is consistent with Eu 3+ The f-f electron transfer indicates that the antenna effect occurs, that is, the ligand absorbs energy and then conducts it to the lanthanide metal ions. The excitation spectra of Eu-NH2BDC / OX MOF and Eu-NH2BDC are the same, while the fluorescence spectrum is obviously Eu 3+ The emission is higher, which may be due to the further sensitization of the luminescence by the introduced oxalic acid.

[0117] Specifically, H2BDC-NH2 acts as an "antenna" molecule. When excited by ultraviolet light, Eu-NH2BDC / OX MOF produces an "antenna effect", absorbing energy through H2BDC-NH2 and transferring it to the central Eu 3+ ,Eu 3+ The luminescence of Eu 3+ emission, since the emission peak of H2BDC-NH2 at 450nm is similar to that of Eu 3+ Corresponds to 5 D0→ 7The excitation peaks of the F2 energy level transitions overlap highly, and fluorescence resonance energy transfer occurs at this time ( Fig.13 ).

[0118] A large number of exposed selective recognition sites are located on the surface and internal pores of Eu-NH2BDC / OX MOF crystals. 2- Later, it interacts specifically with H2BDC-NH2 at the site to form a stable 1:1 charge transfer complex, which can lead to charge transfer and hinder the transfer of H2BDC-NH2 to Eu 3+ The electron transition (ligand to metal charge transfer, LMCT) process, at this time H2BDC-NH2 and Eu 3+ The FRET between Eu 3+ The fluorescence intensity of H2BDC-NH2 decreased, while that of I 450 / I 615 Increase.

[0119] (7) Optimization of detection conditions - solvent type

[0120] In order to effectively detect sulfur dioxide and its derivatives, the effect of solvents on fluorescence intensity was studied. Six commonly used solvents, methanol, ethanol, acetonitrile, isopropanol and N, N-dimethylformamide (DMF), were selected to study the solvent effect ( Fig.14 ).

[0121] The emission spectra of Eu-NH2BDC / OX MOF suspensions in five solvents excited at 300nm showed that different solvents had a significant effect on the luminescence intensity of Eu-NH2BDC / OX MOF. The order of luminescence intensity of the system in different solvents was: DMF, ethanol, isopropanol, methanol and acetonitrile. The results showed that ethanol was the best Eu-NH2BDC / OX MOF solvent for the detection of sulfur dioxide and its derivatives because it had a higher I 450 / I 615 value, and in addition, ethanol has a relatively low toxicity compared to other selected solvents.

[0122] (8) Optimization of testing conditions - material usage

[0123] The amount of Eu-NH2BDC / OX MOF material used is an important factor affecting the fluorescence intensity of the system during the reaction. In order to reduce the waste of Eu-NH2BDC / OX MOF material, the volume of Eu-NH2BDC / OX material solution should be as small as possible while achieving the best fluorescence intensity. The fluorescence intensity of the system will be different if the volume of Eu-NH2BDC / OX MOF material is different. If the amount of Eu-NH2BDC-OX MOF material solution does not make the fluorescence intensity of the system reach the highest level, the reaction is incomplete.

[0124] This study conducted a series of tests on material volumes of 1, 1.5, 2, 2.5, and 3 mL. The results are as follows Fig.15 The results show that when the material volume is 1.5 mL, the fluorescence intensity of the system is the strongest. Therefore, in subsequent experiments, 1.5 mL (RSD = 0.085%) was selected as the most suitable amount of Eu-NH2BDC / OX MOF material solution.

[0125] (9) Optimization of test conditions - dosage of sodium sulfite solution

[0126] The amount of sodium sulfite used is an important factor affecting the fluorescence intensity of the Eu-NH2BDC / OX MOF system during the experimental reaction, so its dosage needs to be optimized. This study conducted a series of 0.1, 0.3, 0.5, 0.7, and 0.9 mL volume tests of 15ug / mL sodium sulfite solution, and then used anhydrous ethanol to dilute to 5 mL, and measured their fluorescence intensities respectively.

[0127] like Fig.16 The results showed that when the volume of sodium sulfite solution added was 0.5 mL, the fluorescence intensity of the system was the strongest. Therefore, in subsequent experiments, 0.5 mL (RSD = 0.02%) was selected as the most suitable amount of sodium sulfite solution.

[0128] (10) Optimization of detection conditions - pH

[0129] It has been reported that acid regulators (such as trifluoroacetic acid, hydrochloric acid, formic acid, etc.) can be used to improve the crystallinity of metal oxides in solvothermal reactions. When the pH value is different, free ion sulfate can be in HSO3 - 、SO3 2- and sulfur dioxide.

[0130] pH value is an important factor affecting the application of probes. The experiment studied the detection of SO3 by Eu-NH2BDC / OX probe in the pH range of 2 to 9. 2- The fluorescence ratio (I 450 / I 615 ), when the pH is between 4 and 9 ( Fig.17), the fluorescence ratio of the probe (I 450 / I 615 )<1, indicating that the probe remains stable in a wide pH range. 2- , the fluorescence ratio of the probe (I 450 / I 615 ) rapidly increased to a peak value at pH = 10, indicating that the probe can reliably detect SO3 2- . And from Fig.17 It can be seen that the fluorescence intensity of the Eu-NH2BDC / OXMOF system is the strongest when pH = 10. Therefore, the most suitable pH is 10 in subsequent experiments.

[0131] (11) Optimization of test conditions - stability test

[0132] The stability of Eu-NH2BDC / OX MOF materials is also very important in the experimental process, and will affect the final results of the experiment to a certain extent. In order to explore the stability of Eu-NH2BDC / OX MOF materials, this study conducted a series of stability tests on Eu-NH2BDC / OX MOF materials to detect sulfur dioxide at different times. As can be seen from the figure, it has a good detection effect within 72h (RSD<2.05%). Eu-NH2BDC / OX MOF has relatively good stability, which may be because Eu 3+ The three-dimensional rigid structure produced by the interaction with the ligand lays the foundation for subsequent tests.

[0133] After 96 hours in ethanol solution, the fluorescence intensity of the Eu-NH2BDC / OX MOF system becomes about 60% of the initial fluorescence intensity. This is because after 96 hours of exposure to ethanol, the Eu-NH2BDC / OX-SO3 2- Aggregation and degradation occurred at an increased rate, with a loss of nearly 40% of the initial fluorescence within 24 h. The strong red fluorescence, good structural stability, and fluorescence stability indicate the feasibility of the synthesized Eu-NH2BDC / OX MOF as a fluorescent probe in aqueous media ( Fig.18 ).

[0134] (12) Optimization of detection conditions - cycling experiment and interference experiment

[0135] In practical applications, easily recyclable sensors are also important. The Eu-NH2BDC / OX MOF suspension containing sodium sulfite was subjected to a recycling test, centrifuged under ultrasound and washed with ethanol several times. After the fifth cycle (RSD < 3.60%), the framework remained intact. At the same time, the quenching efficiency of each cycle remained above 90% ( Fig.19). The results showed that Eu-NH2BDC / OX MOF has high recyclability and stability for the detection of sodium sulfite in ethanol medium.

[0136] Considering the complexity of the actual samples, this study selected different metal ions as interfering ions to analyze the anti-interference ability of the probe. Under the optimal detection conditions, a certain amount of Na2SO3 solution (15 μg / mL) and interfering substances (HPO4 2- 、HCO3 - 、P2O7 2- , S 2- , Cl - The concentration was 150 μg / mL and Br - ,I - 、F - 、SO4 2- The concentration of the probe was 300 μg / mL) solution, and the anti-interference ability of the probe in the detection of sulfur dioxide and its derivatives was analyzed.

[0137] Depend on Fig. 20 It can be seen that even if the interfering substance is introduced at a concentration 10 or even 20 times that of Na2SO3, its effect on the fluorescence signal is very small. The above results show that Eu-NH2BDC / OX MOF is an excellent luminescent sensor with excellent anti-interference performance, high sensitivity, and recyclability that can be applied to the analysis of sulfur dioxide residues in actual samples.

[0138] (13) Optimization of detection conditions-Analysis method characteristics

[0139] Based on the above results, anhydrous ethanol was selected as the dispersant of Eu-NH2BDC / OX MOF material, and the sensing performance of Eu-NH2BDC / OX MOF for sulfur dioxide and its derivatives was studied with 300nm as the excitation wavelength. 450 / I 615 With the addition of sodium sulfite aqueous solution, the Fig.21 ), which agrees well with the first-order exponential equation.

[0140] In the range of 5-50 μg / mL, the intensity changes follow a linear I 450 / I 615 =0.1179x+2.3043c equation, c is the concentration of sodium sulfite, r is 0.9939, and the detection limit is 0.25 mg / kg, which shows that Eu-NH2BDC / OX MOF material can be used as an excellent sensing material for high-precision quantitative detection of sulfur dioxide and its derivatives in candied foods.

[0141] The above results show that Eu-NH2BDC / OX MOF material can be used to detect SO3 in water phase. 2- ion.

[0142] In order to obtain a more obvious visual detection effect, the RGB curve was drawn according to the difference in fluorescence intensity caused by the change in sodium sulfite concentration. There is a good linear relationship between the two (r = 0.9883), as shown in Figure 2. Fig. 22 The above results indicate that the Eu-NH2BDC / OX MOF ratiometric fluorescence sensor exhibits a wide range of color changes during the reaction with the analyte.

[0143] When excited at 300 nm, the emission peak of Eu-NH2BDC / OX MOF is at 615 nm, and the blue emission peak at 450 nm increases with the increase of SO3 2- The content increased from 5μg / mL to 50μg / mL and gradually became significant, while the 615nm red emission peak gradually decreased. The corresponding CIE chromaticity diagram was calculated based on the emission spectrum. CIE also increased with SO3 2- The content increases from (0.45, 0.20) to (0.17, 0.06), and the position of the corresponding point also changes from the initial red area to the final blue area ( Fig.23 ).

[0144] (14) Testing of actual samples

[0145] In order to verify the practicality of the method, four samples of plum, mango, grape and candied date were weighed for testing. The actual samples were processed according to the above (5) and tested. The test was repeated three times. Under the optimal experimental conditions, two concentration levels (15 μg / mL and 20 μg / mL) were simultaneously tested and the spike recovery rate was calculated to verify the accuracy of the method.

[0146] The results are shown in Table 2. The recovery rates of the four groups of samples ranged from 87.28% to 106.3%, and the relative standard deviations of the recovery rates ranged from 1.03% to 9.29%. The detection method has good accuracy, and the recovery rate and precision meet the quality control requirements of daily experimental analysis. The test results of dried mango and candied dates are both lower than the national standard of 350 mg / kg, while grapes and plums exceed the national allowable range.

[0147] The samples were measured according to the national standard method and the Eu-NH2BDC / OX MOF fluorescence spectrophotometry was verified. The relative deviations of the parallel sample results all met the precision of the first method acid-base titration method in GB5009.34-2022 "National Food Safety Standard Determination of Sulfur Dioxide in Foods". The absolute difference between the two independent measurement results obtained under repeated conditions should not exceed 10% of the arithmetic mean. The sample's 2.68% ≥ RSD ≥ 1.03%, indicating that the Eu-NH2BDC / OX MOF method has good precision. At the same time, the results of the two methods were statistically analyzed, and the analysis results showed significant differences (P>0.05), indicating that the results of the established method are accurate.

[0148] Table 2 Results of actual candied fruit food testing and spike recovery experiments (n=3)

[0150]

[0151] Example 3 Preparation and application of a sulfur dioxide ratio fluorescence sensor - sulfur dioxide and its derivatives fluorescence detection test paper

[0152] Preparation of Eu-NH2BDC / OX MOF ratio probe test strip: Eu-NH2BDC / OX MOF ethanol solution (1 mg / ml) was ultrasonically dispersed to obtain a uniform and stable suspension. Glass fiber was immersed in the solution and ultrasonicated for 10 min. The glass fiber was taken out and dried in an oven to obtain a test strip that showed red light under ultraviolet light.

[0153] According to the above method, a series of test papers for sulfur dioxide and its derivatives were prepared, and sodium sulfite solutions with concentrations of 0, 10, 20, 30, 40, and 50 μg / mL were dripped onto the test papers at the same time for detection. The fluorescence intensity was observed under ultraviolet light in a dark environment. It took about 30 seconds from the addition of the sodium sulfite solution to the color change. After about 1 minute, the color change was stable and no longer deepened. The experiment found that the concentration of the sodium sulfite solution had a certain effect on the fluorescence color development of Eu-NH2BDC / OX MOF. With the increase of the sodium sulfite concentration, the fluorescence color change intensity of the Eu-NH2BDC / OX MOF material under ultraviolet light became higher. The color block values ​​at the same site on each test paper were extracted to calculate the ratio of its RGB value to the concentration to make a linear curve as shown below. Fig.24 As shown, the linear relationship is good, r = 0.9978, indicating that the sulfur dioxide detection test paper has been successfully prepared and has the characteristics of being visible, simple and rapid.

[0154] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing Eu-NH2BDC / OX MOF material, characterized in that: The following steps are involved: The Eu-NH2BDC / OX MOF material is prepared by hydrothermally reacting a DMF-nitric acid-water mixed solution of europium salt, oxalic acid, 2-aminoterephthalic acid and 2-fluorobenzoic acid at 115-125° C. for 20-30 hours; The molar ratio of the europium salt, oxalic acid, 2-aminoterephthalic acid and 2-fluorobenzoic acid is (4-5): (1-3): (3-3.5): (40-50).

2. A Eu-NH2BDC / OX MOF material, characterized in that: Prepared by the preparation method described in claim 1.

3. Use of the Eu-NH2BDC / OX MOF material according to claim 2 as a sulfur dioxide ratio fluorescence sensor.

4. Use of the Eu-NH2BDC / OX MOF material according to claim 2 in detecting sulfur dioxide and its derivatives.

5. A method for fluorescent detection of sulfur dioxide and its derivatives, characterized in that: The Eu-NH2BDC / OX MOF material described in claim 2 is used as a sulfur dioxide ratio fluorescence sensor.

6. The method according to claim 5, characterized in that The following steps are involved: The solution to be tested is added to the Eu-NH2BDC / OX MOF material dispersion to obtain a mixed solution, the pH is adjusted to 7-11, and fluorescence spectrophotometry is performed for detection; Fluorescence detection conditions: λex: 300 nm, λem: 450 nm / 615 nm, voltage: 400 V, slit width: 5 nm, scanning speed: 3000 nm / min.

7. The method according to claim 5, characterized in that The dispersant of the Eu-NH2BDC / OX MOF material dispersion is one of methanol, ethanol, acetonitrile, isopropanol or N,N-dimethylformamide.

8. Use of the Eu-NH2BDC / OX MOF material according to claim 2 in preparing a test paper for detecting sulfur dioxide and its derivatives.

9. A fluorescent test paper for sulfur dioxide and its derivatives, characterized in that: The fluorescent detection test paper contains the Eu-NH2BDC / OX MOF material according to claim 2.

10. A method for preparing the fluorescent detection test paper for sulfur dioxide and its derivatives according to claim 9, characterized in that: The following steps are involved: The glass fiber is immersed in the Eu-NH2BDC / OX MOF material dispersion and dried to obtain the fluorescence detection test paper.

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