Preparation and Application of Bimetallic Lanthanide Metal-Organic Frameworks

Abstract

The present invention discloses a preparation and application of a bimetallic lanthanide metal organic framework, using a dual emission fluorescence chemical sensor based on Ln-MOFs (Eu (1.4μM) ‑Tb‑MOF) has successfully developed a smartphone-assisted platform for on-site detection of OTC and DOX. By adjusting the ratio of lanthanide ions, the sensor has excellent selectivity and dual-off response to OTC and DOX, and other types of antibiotics will not cause obvious interference, achieving highly sensitive, accurate and visualized detection of OTC and DOX. The sensing mechanism was revealed by electrostatic potential analysis and density functional theory theoretical calculations. More importantly, Ln‑MOFs were successfully developed for latent fingerprint recognition, and can produce multiple colors of fluorescence emission under 330 nm excitation, providing more reliable materials for criminal investigation. In addition, the sensor has also been successfully applied to the recovery detection of OTC and DOX in milk, beef and pork. It is believed that this efficient strategy can expand the application range of Ln‑MOFs in smartphone-based fluorescence sensors, paving the way for portable and high-precision fluorescence sensors to sense other pollutants.

Description

Technical Field

[0001] The present invention relates to the technical field of metal organic framework preparation, and in particular to the preparation and application of a bimetallic lanthanide metal organic framework. Background Art

[0002] Tetracycline antibiotics are a type of antibiotic with broad-spectrum antibacterial properties and chemical stability, and are widely used in the fields of medicine, animal husbandry, and food industry. The residues of such antibiotics directly or indirectly lead to increased water pollution, animal-derived food risks, increased human morbidity and mortality, etc. Therefore, it is necessary for us to develop effective and convenient technical means to monitor and remove the risks brought by tetracycline antibiotic residues. For the sake of water sources and human health, many countries have banned the use of such antibiotics and have formulated regulations on the maximum residues of tetracycline antibiotics in animal-derived foods. For example, the maximum residue of tetracycline antibiotics in milk is 100 μg / kg, and the maximum residue of tetracycline antibiotics in beef and pork is 200 μg / kg. Up to now, the traditional detection methods for tetracycline antibiotic residues mainly include microbiological methods, high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), immunoassay, etc. These methods currently have the disadvantages of long time cost, poor stability, low sensitivity, and poor anti-interference, and can no longer meet the detection needs of trace or trace tetracycline antibiotics. The emerging fluorescence sensing analysis method has attracted widespread attention due to its advantages such as low cost, simple operation and high sensitivity.

[0003] Metal-organic frameworks (MOFs) are crystalline porous materials formed by self-assembly of metal ions or metal clusters and organic linkers. They are programmable materials. Due to the flexible and changeable spatial structure of MOFs, they are applied in many fields, such as gas storage and adsorption separation, energy storage, catalysis, sensing and biomedicine. By adjusting the ratio of different metals or ligands, sensors with different structures and properties can be constructed. Among them, lanthanide metal-organic frameworks (Ln-MOFs) exhibit excellent properties such as large Stokes shift, long fluorescence lifetime and sharp emission band due to their tunable multi-luminescence centers. Therefore, Ln-MOFs are widely used in fluorescence sensors for various analytes. However, Ln-MOFs fluorescence sensors based on single-emission luminescence centers limit the sensitivity, anti-interference and accuracy of the method. Therefore, the selection of Ln-MOFs fluorescence sensors with multiple luminescence centers is the key to solving the above problems in the high-precision ratio detection of tetracycline antibiotics. However, there are still great challenges for fluorescence-based tetracycline antibiotic sensing methods to achieve rapid response, high sensitivity and accuracy. Summary of the invention

[0004] The purpose of the present invention is to disclose a method for preparing a bimetallic lanthanide metal organic framework, to determine Tb3+ By adjusting the doping amount of Eu 3+ The doping amount of Eu was used to prepare Ln-MOFs with different luminescent colors, and finally the most suitable Eu for the selective detection of tetracycline antibiotics (OTC and DOX) was determined. (1.4μM) -Tb-MOF.

[0005] 1. Eu-MOF, Tb-MOF and Eu x -Synthesis of Tb-MOF

[0006] Synthesis of Eu-MOF: Eu(NO3)3·6H2O and H3TATB were dissolved in a mixed solvent of N,N-dimethylformamide, N-methylpyrrolidone and water, and ultrasonically stirred evenly. Then, they were placed in a Teflon-lined high-pressure container and kept warm in an oven at 150~190℃ for 48~72h. Then, the mixture was cooled to room temperature at a rate of 15°C / 2h to obtain colorless and transparent crystals, which were washed and dried for later use.

[0007] Synthesis of Tb-MOF: The synthesis of Tb-MOF was similar to that of Eu-MOF, except that Eu(NO3)3·6H2O was replaced by Tb(NO3)3·6H2O.

[0008] Eu x -Synthesis of Tb-MOF:Eu x The synthesis of -Tb-MOF is similar to that of Eu-MOF, except that Eu(NO3)3·6H2O is replaced by co-doping of Eu(NO3)3·6H2O and Tb(NO3)3·6H2O. Taking the doping amount of Tb(NO3)3·6H2O as the unit value, by adjusting the doping amount of Eu(NO3)3·6H2O, Eu (0.2μM) -Tb-MOF, Eu (0.6μM) -Tb-MOF, Eu (1.0μM) -Tb-MOF, Eu (1.4μM) -Tb-MOF, Eu (1.8μM) -Tb-MOF and Eu (2.2μM) -Tb-MOF hybrid materials.

[0009] In the synthesis process of the above Eu-MOF and Tb-MOF: the molar ratio of Eu(NO3)3·6H2O or Tb(NO3)3·6H2O to H3TATB is 0.3:1 to 3:1, preferably 1:1. The volume ratio of N,N-dimethylformamide, N-methylpyrrolidone and water is 1:2:6 to 6:2:1, preferably 5:2:3.

[0010] 2. Eu-MOF, Tb-MOF and Eu x -Characterization of Tb-MOF

[0011] 1. PXRD and FT-IR analysis

[0012] Eu-MOF, Tb-MOF and their Eu x The structure of the -Tb-MOF composite material was confirmed by PXRD measurement. As shown in Figure 1, Eu x -The PXRD spectrum of Tb-MOF is in good agreement with the simulation data of Tb-MOF single crystal structure ( Figure 1a ), indicating that Eu x -Tb-MOF has high purity and intact crystalline phase. In addition, Eu-MOF, Tb-MOF and Eu x The PXRD patterns of -Tb-MOFs are consistent, indicating their isomorphism, confirming that Eu x -Successful synthesis of Tb-MOF composites.

[0013] Eu-MOF, Tb-MOF and their Eu x The composition and chemical element status of the -Tb-MOF composites were confirmed by FT-IR spectroscopy. Figure 1b As shown, in Eu-MOF, Tb-MOF and their Eu x The non-ionized carboxyl characteristic peak of the ligand H3TATB was not observed in the FT-IR spectrum of the -Tb-MOF composite. -1 The broad absorption band at 1699 cm-1 is attributed to the OH stretching vibration of the carboxylic acid ligand. -1 The absorption peaks at 1283 and 1580 cm-1 are attributed to the C=O stretching vibration of the carboxylic acid ligand. -1 The characteristic peaks at 932 cm-1 are attributed to the symmetric and antisymmetric stretching vibrations of the -COO group of the ligand. -1 There is an out-of-plane bending vibration absorption peak at 3455 cm -1 The broad absorption peak at is related to the OH stretching vibration of the adsorbed H2O molecules. These results indicate that the prepared Eu-MOF, Tb-MOF and Eu x -Tb-MOF composite materials were successfully synthesized and contain abundant carboxyl and hydroxyl functional groups, which can easily combine with hydrophilic and hydrophobic sites in organic matter to form stable compounds, greatly increasing the adsorption kinetics of the material, which is beneficial to the analysis of sensor performance.

[0014] 2. Fluorescence spectroscopy

[0015] The fluorescence spectra were recorded at room temperature with an excitation wavelength of 330 nm. (1.4μM)-Tb-MOF (2.0 mg) was dissolved in water (5.0 mL) and ultrasonically stirred for 30 min to obtain a uniform suspension solution. 250 μL of antibiotic aqueous solutions of different concentrations were dispersed in 1.0 mL Eu (1.4μM) -Tb-MOF suspension (0.4 mgmL -1 ) in a cuvette, stirred evenly, and the fluorescence spectrum of the reaction system was measured by a fluorescence spectrophotometer. Subsequently, the fluorescence spectrum data was converted using the Origin plug-in ChromaticityDiagram CIE 1931 to obtain the corresponding coordinates (x, y) on the color coordinates. The chromaticity shift value was calculated based on the (x, y) converted from the experimental group (containing OTC and DOX) and the control group (excluding OTC and DOX).

[0016] Eu-MOF, Tb-MOF and Eu x -The excitation and emission spectra of Tb-MOF are shown in Figure 2a -h. Their excitation spectra are mainly concentrated in the range of 200-390 nm, with the optimal excitation wavelength being 330 nm. x -Tb-MOF shows the characteristic emission band of lanthanide ions at the optimal excitation wavelength, and the energy of the ligand is almost completely transferred to the lanthanide ions through the "antenna effect". In addition, the four emission peaks observed near 594, 620, 670, and 703 nm are attributed to the Eu(III) in Eu-MOF. 5 D0→ 7 F J (J = 1, 2, 3, 4) transitions. 5 D4→ 7 F J (J = 6, 5, 4, 3) transitions, Tb-MOF shows characteristic emission bands at 490 and 546 nm. As expected, the bimetallic Eu x -Tb-MOF simultaneously shows the characteristic emission peaks of Eu(Ⅲ) and Tb(Ⅲ). It is worth noting that with the increase of Eu(Ⅲ) doping amount, x The characteristic emission of Eu(Ⅲ) in Tb-MOF gradually increases compared with that in Eu-MOF, while the characteristic emission of Tb(Ⅲ) gradually decreases compared with that in Tb-MOF, which may be attributed to the x -Energy transfer from Tb(Ⅲ) to Eu(Ⅲ) in Tb-MOF.

[0017] The doping amount of Eu(Ⅲ) and Tb(Ⅲ) ions in bimetallic MOF is the most critical factor affecting the fluorescence sensing performance of MOF and needs to be carefully edited. First, the doping ratio of Eu(Ⅲ) and Tb(Ⅲ) ions was adjusted. In order to achieve high-sensitivity monitoring of MOF materials, different fluorescence colors can be achieved by controlling the doping amount of Tb(Ⅲ) ions and doping different amounts of Eu(Ⅲ) ions. x -Tb-MOF (x = 0.2, 0.6, 1.0, 1.4, 1.8, 2.2) (unit: μM) can show the characteristic emission peaks of Eu(Ⅲ) and Tb(Ⅲ) ions. After the introduction of oxytetracycline (OTC) and doxycycline (DOX), Eu x -Tb-MOF's characteristic emission peaks are all quenched, and the quenching degree is >90%. The inset in the upper right corner of the figure can intuitively show the experimental phenomenon, and the CIE coordinate diagram in the upper left corner also shows that Eu (1.4μM) -The chromaticity shift value of Tb-MOF (0.17247, 0.14486) is much larger than that of other Eu (0.2μM，0.6μM，1.0μM、1.8μM、2.2μM) -Chromaticity shift value of Tb-MOF ( Figure 3a -f). In summary, Eu (1.4μM) -Tb-MOF is most conducive to achieving high-sensitivity and high-precision fluorescence detection of OTC and DOX. At the same time, the sensing performance of Eu-MOF and Tb-MOF as fluorescence sensors for OTC and DOX was also monitored ( Figure 3g -h). The results show that although the fluorescence intensity of Eu-MOF and Tb-MOF is quenched, the quenching degree is less than 85%. The illustration in the upper right corner of the figure can intuitively show the experimental phenomenon, and the chromaticity shift values ​​of Eu-MOF and Tb-MOF are much smaller than that of Eu (1.4μM) -Tb-MOF chromaticity shift value. In summary, compared with single-emission fluorescent probes, ratiometric fluorescent probes have the advantages of larger chromaticity shift value, low detection line, and high sensitivity.

[0018] 3. Thermal stability

[0019] Thermogravimetric analysis (TGA) was used to investigate the (1.4μM) -Tb-MOF thermal stability. As shown in Figure 4, Eu-MOF and Tb-MOF have small weight losses in the range of 438.46 and 477.57 °C, respectively, indicating that the two monomer MOFs have good thermal stability. (1.4μM)The weight loss of -Tb-MOF in the range of 366.77 °C may be attributed to the removal of water molecules and free solvent molecules. The second weight loss occurred in the temperature range of 366.77-474.06 °C, which may be attributed to the removal of coordinated solvent molecules. As the temperature continued to rise, the skeleton began to collapse. The TGA curve shows that Eu (1.4μM) -Tb-MOF has good thermal stability.

[0020] 4. Liquid resistance and stability

[0021] Then we studied Eu (1.4μM) -Water stability of Tb-MOF in water for 30 days. Figure 5a As shown, Eu (1.4μM) -Tb-MOF has almost no change in its PXRD pattern before and after immersion in water, indicating that the skeleton has not collapsed and has good water stability. (1.4μM) -The fluorescence lifetime of Tb-MOF (546 nm and 620 nm) Figure 5b ), and their fitting functions show lifetimes of 0.692 ms, 0.725 ms, 0.402 ms, and 1.938 ms, respectively. In addition, regarding the pH effect, we chose sodium hydrogen phosphate-citric acid buffer solution to test Eu (1.4μM) -Acid and alkali resistance of Tb-MOF ( Figure 6a ). (1.4μM) -The fluorescence intensity of Tb-MOF ( I 546 / I 620 ) showed the smallest change in the pH range of 2.6 to 8.0, which indicates that Eu (1.4μM) -Tb-MOF is suitable for use in strong acid and weak alkaline media.

[0022] 5. SEM and XPS analysis

[0023] The Eu (1.4μM) -Tb-MOF morphology and structure. Figure 7 As shown in ab, SEM shows Eu (1.4μM) -Tb-MOF has an irregular rectangular morphology. (1.4μM) -Energy dispersion spectrometry (EDS) of Tb-MOF shows the presence of five elements: C, N, O, Eu and Tb ( Figure 7 ch). SEM results show that Eu (1.4μM) -Successful synthesis of Tb-MOF. XPS was used to study Eu (1.4μM) -Elemental composition and chemical structure of the surface of Tb-MOF composites. Figure 6b As shown, Eu (1.4μM) The presence of Eu, Tb, C, and O elements in the XPS spectrum of -Tb-MOF indicates that the composite material was successfully synthesized. (1.4μM) -Eu in Tb-MOF 3+ The doping level is extremely low ( Figure 6c ), which is less than 5%. Therefore, the Eu 3d high-resolution spectrum shows four characteristic peaks with very low intensity of 1165.25 eV, 1155.13 eV, 1132.85 eV, and 1121.73 eV, which are respectively attributed to Eu 3+ 3d 3 / 2 、Eu 2+ 3d 3 / 2 、Eu 3+ 3d 5 / 2 、Eu 2+ 3d 5 / 2 . Figure 6d The high-resolution spectrum of Tb 3d is described, showing two peaks at 1274.89 eV and 1240.08 eV, respectively. 3 / 2 、Tb 3d 5 / 2 The three peaks at 286.88 eV, 284.80 eV and 283.08 eV in the XPS spectrum of C 1s are attributed to CN, CC and C=O( Figure 6e The three peaks at 530.70 eV, 529.81 eV and 529.17 eV in the XPS spectrum of O 1s are attributed to C=O, Eu-O / Tb-O and CO( Figure 6f ).

[0024] 3. Eu x -Application research of Tb-MOF

[0025] 1. Fluorescence detection of OTC and DOX

[0026] In order to explore Eu (1.4μM) -Tb-MOF’s ability to detect OTC and DOX was studied, and the effects of different concentrations of OTC and DOX on Eu (1.4μM) -Tb-MOF sensor fluorescence spectrum. We prepared a 1×10 -3 Common antibiotic aqueous solutions (NMS, LCM, STR, CAP, Pen G, CFX, PCG, ROX, SDZ, OTC and DOX) were used as interfering substances to study Eu (1.4μM) -Selectivity and anti-interference ability of Tb-MOF sensors ( Figure 8a -c). The results showed that the sensor was only sensitive to tetracycline antibiotics (OTC and DOX), and the presence of interfering substances did not affect the accurate response of OTC and DOX, indicating that Eu(1.4μM) -Tb-MOF sensor has excellent selectivity and anti-interference ability for OTC and DOX. Figure 8d As shown in Figure 2, with the gradual increase of OTC and DOX, Eu (1.4μM) The fluorescence intensity of -Tb-MOF gradually quenched. There was a good linear relationship between the chromaticity shift value (y) and the concentration of OTC (x, 0-60 μM) and DOX (x, 0-172 μM) in the low concentration range ( R 2 = 0.998, R 2 = 0.998)( Figure 8f -i), the equations are y = 0.00034x-0.000176, y = 0.00063x-0.000522. According to the formula LOD =3 σ / k , the detection limits were calculated to be 8.6 nM and 4.8 nM, respectively.

[0027] For practical applications, time response is an important factor that must be considered in the test. After adding OTC and DOX, Eu (1.4μM) -Ratiometric fluorescence intensity of Tb-MOF ( I 546 / I 620 ) increases immediately and remains stable for a long time after 12 and 15 seconds ( Figure 8l -m). Indicates Eu (1.4μM) -Tb-MOF can be used as a fast-response probe for OTC and DOX. (1.4μM) -Tb-MOF stability and economic cost-effectiveness for Eu (1.4μM) -The reusability of Tb-MOF was studied ( Figure 8j -k). (1.4μM) After -Tb-MOF recognized OTC and DOX, it was washed with DMF:H2O (V:V = 1:1) to remove OTC and DOX, centrifuged, dried and reused. (1.4μM) After 4 and 5 cycles, the fluorescence intensity of -Tb-MOF remained almost stable, indicating that Eu (1.4μM) -Tb-MOF has high stability and recyclability.

[0028] In summary, compared with previous reports using various materials for OTC and DOX sensing, our proposed sensing strategy has the advantages of lower cost, ratiometric detection properties, visualization, excellent specific selectivity, wide detection range and low detection line.

[0029] 2. Sensing mechanism of OTC and DOX

[0030] First, the quenching caused by framework collapse was explored, and PXRD results showed that Eu (1.4μM) -Tb-MOF structure hardly changes during the whole sensing process, indicating that there is no structural collapse after the interaction between MOF, OTC and DOX ( Figure 9a ). We take OTC as an example and measure Eu 3+ , Tb 3+ 、H3TATB、Eu 3+ +H3TATB,Tb 3+ +H3TATB,Eu 3+ +Tb 3+ +H3TATB and Eu (1.4μM) -Fluorescence spectrum in the presence of Tb-MOF. Figure 9g As shown, only Eu (1.4μM) -Tb-MOF has a significant fluorescence quenching effect on the OTC reaction system, which cannot be achieved by other mixtures. Figure 9b As shown, OTC and DOX were (1.4μM) After the reaction of -Tb-MOF, the absorption spectrum showed an obvious red shift and the absorption intensity increased significantly, indicating that the conjugation degree between them increased and Eu (1.4μM) The excitation spectrum of -Tb-MOF covers a large range of the absorption spectra of OTC and DOX, indicating that OTC and DOX absorb Eu (1.4μM) -Tb-MOF excitation light leads to an enhanced inner filter effect (IFE), which quenches Eu (1.4μM) -Tb-MOF fluorescence. (1.4μM) The emission spectrum of -Tb-MOF has almost no overlap with the absorption spectra of OTC and DOX, which excludes the quenching caused by fluorescence resonance energy transfer. FTIR spectroscopy further explored the (1.4μM) -Interactions between Tb-MOF, OTC and DOX. Fig.9c As shown, with Eu (1.4μM) Compared with Tb-MOF, OTC and DOX, Eu (1.4μM) The FT-IR spectrum of -Tb-MOF showed no new functional groups, which also proved that no new complexes were generated. Fig.9e -f describes the Eu before and after adding OTC and DOX respectively (1.4μM) The fluorescence lifetime of -Tb-MOF at 546 and 620 nm, and the transient fluorescence spectrum is plotted according to the detection results. The transient fluorescence spectrum is fitted according to the equation to obtain the fluorescence lifetime. ave = (A1τ1 2 +A2τ2 2) / (A1τ1+A2τ2) where τ ave Represents the average lifetime, τ1 and τ2 represent the fitted lifetime, and A1 and A2 represent the relative occupancy of τ1 and τ2. The results show that there is no obvious change in the fluorescence lifetime, indicating that it is a static quenching process. In addition, we used the DFT method to calculate the relative energy of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of H3TATB, OTC and DOX. Figure 9d As shown, OTC and DOX have lower unoccupied molecular orbitals (LUMO: −0.20878 eV and −0.25098 eV), which can effectively enhance the photoinduced electron transfer (PET) process from H3TATB (LUMO: −0.09453 eV) to OTC and DOX, resulting in fluorescence quenching.

[0031] We simulated Eu from the analytical level (electrostatic potential analysis) (1.4μM) -Host-guest interaction between Tb-MOF and OTC and DOX. First, the electrostatic potential surfaces of H3TATB, OTC and DOX were analyzed. The ESP around the carboxyl oxygen atom of H3TATB is more negative, which means that this area has a strong attraction to OTC and DOX, so the carboxyl oxygen atom is most likely the binding site for OTC and DOX. Secondly, we found that the positrons of OTC are concentrated around the hydroxyl oxygen atom and the -NH2 group, and the positrons of DOX are concentrated on the hydroxyl oxygen atom of the central six-membered ring, which means that this area contains empty orbitals, which will combine with the negatively charged carboxyl oxygen atom of H3TATB to form a strong hydrogen bond, thereby causing electron transfer to the analytes OTC and DOX, which in turn leads to Eu (1.4μM) -Fluorescence quenching of Tb-MOF. In summary, the fluorescence quenching mechanism is the result of the combined effects of IFE, static quenching and PET.

[0032] 3. Application in real food samples

[0033] Milk pretreatment: 1.0 mL trichloroacetic acid was added to 5.0 mL milk to remove proteins and lipids, and ultrasonic treatment was performed uniformly. The mixture was centrifuged at 3000 rpm for 10 min, and the supernatant was filtered with a 0.22 μm pore size filter membrane and refrigerated at 4 °C until use.

[0034] Pretreatment of beef and pork: Take an appropriate amount of homogenized beef and pork in a test tube, add 40.0 mL of Na2EDTA buffer solution, and evenly extract the supernatant by ultrasonic in an ice water bath, then centrifuge at 3000 rpm for 10 min and filter with a 0.22 μm pore size filter membrane.

[0035] In order to verify that Eu (1.4μM)To investigate the practicality of the Tb-MOF, the residual amounts of OTC and DOX in milk, beef, and pork samples were tested. Different concentrations of OTC and DOX (0, 30, 60, and 90 μM) were added using the standard addition method to obtain a series of samples to be tested, and the accuracy of the probe was evaluated by the recovery of OTC and DOX. The sample recovery results are summarized in Table 1. OTC and DOX were not detected in the real samples, and the recoveries of the probes were satisfactory. The recoveries of OTC and DOX in milk were 100.3-101.39% and 99.67-101.77%, respectively, with RSDs lower than 3.21% and 2.69%, respectively; the recoveries in beef were 95.17-103.45% and 101.69-109.63%, respectively, with RSDs lower than 3.54% and 4.05%, respectively; the recoveries in pork were 92.0-103.08% and 101.1-104.23%, respectively, with RSDs lower than 4.56% and 3.46%, respectively. The above results show that Eu (1.4μM) -Tb-MOF probes have good accuracy and precision and can be used for the detection of OTC and DOX in real samples.

[0036] Table 1 Determination results of OTC and DOX in actual samples

[0037]

[0038] To simplify the analytical detection method, we use smartphones to replace bulky and expensive fluorescence spectrophotometers to improve overall efficiency. Fig.10 As shown, different concentrations of OTC and DOX samples were added to the fluorescent dish and incubated with Eu (1.4μM) -Tb-MOF mixture. Under 365nm UV light, the fluorescence image of the sample was captured by a smartphone. The smartphone is equipped with an analysis program that specifically recognizes the fluorescence image. The RGB value is extracted by capturing the color of the fluorescence image, and the concentration of OTC and DOX can be calculated by calculating the ratio of red (R) and blue (B). From the figure, we can see that with the addition of OTC and DOX, Eu (1.4μM) -Tb-MOF fluorescence is gradually quenched. As shown in the figure, the concentrations of OTC and DOX maintain a good linear relationship with the R / B value ( R 2 =0.998, R 2 =0.995). The corresponding LODs were calculated to be 9.8 nM and 14.2 nM. The results showed that the smartphone-based sensor can effectively and quickly detect different concentrations of OTC and DOX with satisfactory stability and accuracy.

[0039] 4. Latent fingerprint visualization

[0040] Latent fingerprints (LFPs) are unique features of individuals and are of great reference value for identity recognition and criminal investigation. Fingers are covered with subcutaneous fat, and even clean hands will leave fingerprints on various objects. However, LFPs are difficult to observe with the naked eye in sunlight, which requires chemical methods to make them visible. In this work, Eu-MOF, Tb-MOF and Eu x -Tb-MOF is used to develop latent fingerprint visualization due to its relatively small nano-size and tunable fluorescence. First, we print the fingerprint of a finger soaked in oil on a smooth glass substrate, then grind the Ln-MOFs into fine powder, spray it evenly on the printed fingerprint, and gently blow off the excess material with a hair dryer. In order not to affect the pattern details of the fingerprint, a soft brush is used to gently sweep the fingerprint ridges and grooves of the LFP. Fig.11 As shown, we collected LFP from different volunteers, and the fluorescent ridges and colorless ridge grooves of different Ln-MOFs can be clearly seen. Ln-MOFs only dyed the fingerprint ridges and did not dye the fingerprint grooves and background, eliminating background interference and having high selectivity and accuracy. In addition, we can clearly observe the three-level detailed features of fingerprints with the naked eye, including primary details (fingerprint shape, whorls and rings), secondary details (fingerprint terminal ridges, bifurcations and islands) and tertiary details (scars, ridge widths and microscopic features of pores). Detailed analysis of these fingerprint features plays a vital role in criminal case investigations.

[0041] In addition, to confirm the applicability of Ln-MOFs in daily life, we developed LFP on the surfaces of various substrates such as wrenches, playing cards, cigarette boxes, and water cups. Fig.12 As shown in Figure 2, under 365 nm UV light, the fingerprint patterns on various substrates showed bright fluorescence, and the ridges and grooves on the fingerprints could be observed with the naked eye. These unique characteristic information demonstrates the great potential of Ln-MOFs as LFP development materials.

[0042] In summary, in the present invention, a dual-emission fluorescence chemical sensor based on Ln-MOFs (Eu (1.4μM)-Tb-MOF) was successfully developed for the on-site detection of OTC and DOX using a smartphone-assisted platform. By adjusting the ratio of lanthanide ions, the sensor has excellent selectivity and dual-off response to OTC and DOX, and other antibiotics do not cause significant interference, achieving highly sensitive, accurate and visualized detection of OTC and DOX. The sensing mechanism was revealed by electrostatic potential analysis and density functional theory theoretical calculations. More importantly, Ln-MOFs were successfully developed for latent fingerprint recognition, and were able to produce multiple colors of fluorescence emission under 330 nm excitation, providing more reliable materials for criminal investigation. In addition, the sensor was successfully applied to the recovery detection of OTC and DOX in milk, beef and pork. We believe that this efficient strategy can expand the application range of Ln-MOFs in smartphone-based fluorescence sensors, paving the way for portable and high-precision fluorescence sensors to sense other pollutants. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 (a) Eu-MOF, Tb-MOF and their Eu x -PXRD of Tb-MOF composites. (b) PXRD of Eu-MOF, Tb-MOF and their Eu x -FT-IR spectrum of Tb-MOF composite.

[0044] In Figure 2, (a) Eu-MOF, (b) Tb-MOF, (c) Eu (0.2μM) -Tb-MOF, (d) Eu (0.6μM) -Tb-MOF, (e)Eu (1.0μM) -Tb-MOF, (f)Eu (1.4μM) -Tb-MOF, (g)Eu (1.8μM) -Tb-MOF and (h)Eu (2.2μM) -Excitation and emission spectra of Tb-MOF.

[0045] In Figure 3 (a) Eu (0.2μM) -Tb-MOF, (b) Eu (0.6μM) -Tb-MOF, (c) Eu (1.0μM) -Tb-MOF, (d)Eu (1.4μM) -Tb-MOF, (e)Eu (1.8μM) -Tb-MOF, (f)Eu (2.2μM) -Tb-MOF, (g) Eu-MOFand (h) Tb-MOF fluorescence spectra of OTC and DOX; the inset in the upper left corner is the chromaticity shift value before and after detection, and the inset in the upper right corner is the fluorescence photograph under 356 nm ultraviolet light before and after detection.

[0046] In Figure 4, (a) Eu-MOF, (b) Tb-MOF, (c) Eu (1.4μM) -TGA of Tb-MOF.

[0047] In Figure 5 (a) Eu (1.4μM) -PXRD patterns of Tb-MOF before and after immersion in water for 30 days; (b) PXRD patterns of Eu-MOF, Tb-MOF and Eu (1.4μM) -Fluorescence lifetime of Tb-MOF (546 nm and 620 nm).

[0048] In Figure 6 (a) Eu (1.4μM) -The fluorescence intensity of Tb-MOF ( I 546 / I 620 ) changes in pH range from 2.6 to 12.8; (b) Eu (1.4μM) -Tb MOF XPS total spectrum; (c) Eu3d, (d) Tb3d, (e) C1s and (f) O1s high-resolution spectra.

[0049] Figure 7 (a) SEM image of Eu(1.4μM)-Tb-MOF; (b) magnified SEM image of Eu(1.4μM)-Tb-MOF; (ch) element distribution map of Eu(1.4μM)-Tb-MOF.

[0050] In Figure 8 (a) Eu (1.4μM) -Tb-MOF selectivity for detecting OTC and DOX. (1.4μM) -Tb-MOF detects (b) OTC and (c) DOX. With the addition of (d) OTC and (e) DOX, Eu (1.4μM) -Tb-MOF fluorescence spectrum changes. (f) OTC and (g) DOX concentration gradients converted from fluorescence spectra. (e) Relationship between chromaticity shift value and (h) OTC and (i) DOX concentration. Eu (1.4μM) -Tb-MOF tests the recyclability of (j) OTC and (k) DOX. (1.4μM) -Temporal response of Tb-MOF in detecting (l) OTC and (m) DOX.

[0051] In Figure 9 (a) Eu (1.4μM) -Tb-MOF PXRD patterns before and after OTC and DOX detection; (b) Eu (1.4μM) -Tb-MOF, OTC, DOX and Eu (1.4μM) -Tb-MOF absorption spectra after detecting OTC and DOX, Eu (1.4μM)-Excitation and emission spectra of Tb-MOF; (c) Eu (1.4μM) -Tb-MOF, OTC, DOX and Eu (1.4μM) -FT-IR spectra of Tb-MOF after detection of OTC and DOX; (d) Eu (1.4μM) - Details of the fluorescence shut-off mechanism of the Tb-MOF sensor; Eu before and after the addition of OTC and DOX (1.4μM) -Tb-MOF fluorescence lifetime at (e) 546 nm and (f) 620 nm; (g) Eu 3+ , Tb 3+ 、H3TATB、Eu 3+ +H3TATB、Tb 3+ +H3TATB、Eu 3+ +Tb 3+ +H3TATB and Eu (1.4μM) -Fluorescence spectra of Tb-MOF for detecting OTC and DOX.

[0052] Fig.10 Under 365nm UV light, the RGB values ​​of the fluorescence color of Eu(1.4μM)-Tb-MOF containing different concentrations were extracted using a smartphone. The red channel (R value) and blue channel (B value) were used to calculate the fluorescence color of Eu(1.4μM)-Tb-MOF containing different concentrations of OTC and DOX.

[0053] Fig.11 Under 365 nm UV light, (a) Eu-MOF, (b) Tb-MOF, (c) Eu (0.2μM) -Tb-MOF, (d) Eu (0.6μM) -Tb-MOF, (e)Eu (1.0μM) -Tb-MOF and (f) Eu (1.4μM) -Tb-MOF collects fingerprint patterns of different volunteers on a glass petri dish. (g) Eu (1.0μM) -Magnified fingerprint and partial magnified image of Tb-MOF.

[0054] Fig.12 Under natural light and 365nm ultraviolet light, using Eu (1.4μM) -Tb-MOF (a, a') fingerprint pattern on a wrench. Fingerprint pattern on a playing card using Eu-MOF (b, b'). Fingerprint pattern on a cigarette box using Tb-MOF (c, c'). (0.2μM) -The fingerprint pattern formed by Tb-MOF (d, d') on the water cup. DETAILED DESCRIPTION

[0055] The present invention will be further explained below in conjunction with the accompanying drawings.

[0056] Reagents and instruments

[0057] In the present invention, all reagents were purchased commercially and used directly without purification. Eu(NO3)3·6H2O, Tb(NO3)3·6H2O, N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP) and H3TATB were purchased from Energy Chemical (Anhui, China). Neomycin sulfate (NMS), lincomycin hydrochloride (LCM), streptomycin sulfate (STR), chloramphenicol (CAP), penicillin sodium (Pen G), oxytetracycline hydrochloride (OTC), cephalexin (CFX), penicillin potassium (PCG), doxycycline monohydrochloride hemiethanol hemihydrate (DOX), roxithromycin (ROX) and sulfadiazine (SDZ) were purchased from Aladdin (Shanghai, China).

[0058] The following instruments were used for characterization and analysis: scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) (ZEISSGeminiSEM 500), powder X-ray diffraction (PXRD) (Philips PW 1710-BASED), single crystal X-ray diffraction (XRD) (Rigaku-Oxford), thermogravimetric analysis (TGA) (PerkinElmer TG-7analyzer), Fourier transform infrared spectroscopy (FT-IR) (VERTEX 70, Germany), fluorescence spectroscopy (F-7000 fluorescence spectrophotometer, Japan), UV-visible spectroscopy (Hitachi UV-3900 spectrometer, Japan), fluorescence lifetime (FLSP920 steady-state / transient fluorescence spectrometer, UK), K-Alpha X-ray photoelectron spectrometer (ThermoKalpha, USA). Example

[0059] Synthesis of Eu-MOF

[0060] Briefly, Eu(NO3)3·6H2O (0.01 mmol, 4.6 mg) and H3TATB (0.01 mmol, 4.4 mg) were dissolved in DMF (5.0 mL), H2O (3.0 mL) and NMP (2.0 mL), and the mixture was ultrasonically stirred for 10 min, then placed in a 25.0 mL Teflon-lined high-pressure container, kept in an oven at 170 °C for two days, and then cooled to room temperature at a rate of 15 °C / 2h. Colorless and transparent crystals were obtained. They were washed with DMF and H2O several times and then dried in an oven at 60 °C.

[0061] Synthesis of Tb-MOF

[0062] The synthesis of Tb-MOF is similar to that of Eu-MOF, except that Eu(NO3)3·6H2O is replaced by Tb(NO3)3·6H2O. The crystal data, bond length and bond angle data of Tb-MOF are summarized in Appendix 4.1, Appendix 4.2 and Appendix 4.3.

[0063] Eu x -Synthesis of Tb-MOF

[0064] Eu x The synthesis of -Tb-MOF is similar to that of Eu-MOF, except that Eu(NO3)3·6H2O is replaced by co-doping of Eu(NO3)3·6H2O and Tb(NO3)3·6H2O. By controlling the doping amount of Tb(NO3)3·6H2O, Eu (0.2μM) -Tb-MOF, Eu (0.6μM) -Tb-MOF, Eu (1.0μM) -Tb-MOF, Eu (1.4μM) -Tb-MOF, Eu (1.8μM) -Tb-MOF and Eu (2.2μM) -Tb-MOF hybrid materials.

[0065] Structural characterization and performance evaluation are described above.

Claims

1. A method for preparing a bimetallic lanthanide metal-organic framework, characterized in that: Eu(NO3)3·6H2O, Tb(NO3)3·6H2O and H3TATB are dissolved in a mixed solvent of N,N-dimethylformamide, N-methylpyrrolidone and water, and ultrasonically stirred to be uniform. Then, the mixture is placed in a Teflon-lined high-pressure container, kept in an oven at 150-190°C for 48-72h, and then cooled to room temperature at a rate of 15°C / 2h to obtain colorless and transparent crystals, which are washed and dried for standby use. The molar ratio of Eu(NO3)3·6H2O to Tb(NO3)3·6H2O is 0.2:1-2.2:1, and the molar ratio of Tb(NO3)3·6H2O to H3TATB is 0.3:1-3:

1.

2. A method for preparing a bimetallic lanthanide metal organic framework as claimed in claim 1, characterized in that: The molar ratio of Eu(NO3)3·6H2O to Tb(NO3)3·6H2O is 1.4:

1.

3. A method for preparing a bimetallic lanthanide metal organic framework as claimed in claim 1, characterized in that: The volume ratio of the N,N-dimethylformamide, N-methylpyrrolidone and water is 1:2:6 to 6:2:

1.

4. A bimetallic lanthanide metal organic framework prepared by the method of claim 1, characterized in that: The bimetallic lanthanide metal-organic framework includes Eu (0.2μM) -Tb-MOF, Eu (0.6μM) -Tb-MOF, Eu (1.0μM) -Tb-MOF, Eu (1.4 μM) -Tb-MOF, Eu (1.8μM) -Tb-MOF and Eu (2.2μM) -Tb-MOF.

5. Application of the bimetallic lanthanide metal organic framework prepared by the method as claimed in claim 1 in the detection of tetracycline antibiotics and latent fingerprint recognition.

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