A three-dimensional pillared Co-MOF, a two-dimensional Co-MOF nanosheet, a preparation method thereof and application thereof in cyclohexylamine detection

Abstract

The application discloses a three-dimensional pillar-supported Co-MOF, a two-dimensional Co-MOF nanosheet, a preparation method of the two-dimensional Co-MOF nanosheet and application of the three-dimensional pillar-supported Co-MOF and the two-dimensional Co-MOF nanosheet in cyclohexylamine detection. The preparation method of the two-dimensional Co-MOF nanosheet is as follows: a DMF solution in which 4,4',4''-(1H-imidazole-2,4,5-triyl)tripyrrole and 2,5-furan dicarboxylic acid are dissolved is added into a deionized water solution of a cobalt salt, mixed uniformly, reacted at 80-105 DEG C for 65-72 hours, then centrifuged and dried to obtain the three-dimensional pillar-supported Co-MOF; the three-dimensional pillar-supported Co-MOF is soaked in pyridine and subjected to ultrasonic treatment, then centrifuged, the supernatant is collected, and the supernatant is concentrated and dried to obtain the two-dimensional Co-MOF nanosheet; the three-dimensional pillar-supported Co-MOF and the two-dimensional Co-MOF nanosheet are used as fluorescent probes, and high-sensitivity detection of cyclohexylamine can be realized.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterials technology, specifically relating to a three-dimensional pillared Co-MOF, a two-dimensional Co-MOF nanosheet, their preparation methods, and their application in the detection of cyclohexylamine. Background Technology

[0002] Cyclohexylamine (CHA) is a potentially harmful organic amine that may originate from the metabolism of food additives (such as cyclamate) or industrial pollution. As early as 1996, Groenewold et al. used static secondary ion mass spectrometry (SIMS) to detect cyclohexylamine adsorbed on the soil surface, with a detection limit of 100 ppb (1 μM) (Groenewold et al., 1996, DOI:10.1016 / 1044-0305(95)00638-9). SIMS technology obtains mass spectrometry signals, which requires ultra-high vacuum and complex instruments, and data processing is also very troublesome.

[0003] Fluorescence sensing technology based on metal-organic framework nanosheets has become an emerging detection method due to its high sensitivity, rapid response and tunability.

[0004] Metal-organic frameworks (MOFs) have been extensively studied over the past decade due to their diverse structures, high porosity, large specific surface area, ease of modification, abundant luminescence sources, and tunable fluorescence properties. They are considered one of the most promising fluorescent detection materials. As fluorescent probes, they have shown excellent performance in the detection of metal ions, anions, small organic molecules, nitro explosives, antibiotics, pesticides, and more.

[0005] With the rapid development of two-dimensional nanomaterials, represented by graphene, many researchers have begun to attempt to prepare ultrathin two-dimensional MOF nanosheets as fluorescent detection materials. Compared with bulk MOFs, MOF nanosheets have ultrathin thickness and larger lateral dimensions, larger specific surface area, more exposed active sites, better dispersibility, and superior fluorescence properties. This allows more recognition sites to come into contact with the analyte faster and more fully, thus often exhibiting higher sensitivity and faster response speed.

[0006] Methods for preparing two-dimensional MOF nanosheets can be broadly classified into two categories: top-down and bottom-up. The top-down exfoliation method is currently the most widely used method for preparing two-dimensional MOF nanosheets due to its advantages such as simplicity, low cost, large-scale production capability, and compatibility with solution processing. Top-down exfoliation almost always uses two-dimensional bulk MOFs as precursors. This is because two-dimensional MOFs are formed by stacking two-dimensional coordination layers through weak non-covalent interactions such as hydrogen bonding, CH…π, and π…π stacking. The coordination and covalent bonds within the two-dimensional coordination layers are usually much stronger than the interlayer interactions, allowing the interlayer interactions to be broken by external forces such as ultrasound, mechanical manipulation, or ion insertion, thus achieving successful exfoliation of the nanosheets. Although this method can effectively prepare two-dimensional MOF nanosheets, it also significantly limits the scope of MOF nanosheet research.

[0007] Existing technologies have never focused on how to use chemical methods to exfoliate three-dimensional MOFs into two-dimensional MOF nanosheets and apply them to the field of fluorescence detection technology for cyclohexylamine. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a three-dimensional pillared Co-MOF, a two-dimensional Co-MOF nanosheet, and a method for preparing the same. This method is simple and can rapidly exfoliate three-dimensional pillared MOFs into monolayer MOF nanosheets. The synthesized nanosheets have large lateral dimensions, ultrathin thickness, and good crystallinity.

[0009] This invention also provides the application of the three-dimensional pillared Co-MOF or two-dimensional Co-MOF nanosheets in the detection of cyclohexylamine. Using the three-dimensional pillared Co-MOF or two-dimensional Co-MOF nanosheets of this invention as fluorescent probes, high-sensitivity detection of cyclohexylamine is achieved. The two-dimensional Co-MOF nanosheets have higher detection sensitivity than the three-dimensional pillared Co-MOF.

[0010] The present invention also provides a method for detecting cyclohexylamine, which has higher sensitivity and lower detection limit.

[0011] The technical solution adopted in this invention is as follows:

[0012] This invention provides a method for preparing two-dimensional Co-MOF nanosheets, the method comprising the following steps:

[0013] (1) Add a DMF solution containing 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine and 2,5-furandicarboxylic acid to a deionized aqueous solution of cobalt salt, mix well, react at 80~105 °C for 65~72 h, then centrifuge and dry to obtain three-dimensional pillared Co-MOF;

[0014] (2) The three-dimensional pillared Co-MOF was immersed in pyridine for ultrasonic treatment, then centrifuged, and the supernatant was collected. The supernatant was then concentrated and dried to obtain two-dimensional Co-MOF nanosheets.

[0015] In step (1), the molar ratio of cobalt salt, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine and 2,5-furandicarboxylic acid is 11~20:4.5~8:5~10.

[0016] In step (1), the cobalt salt is CoCl2·6H2O.

[0017] In step (1), the volume ratio of DMF to deionized water is 11~14:1.

[0018] In step (2), the ratio of the amount of three-dimensional pillared Co-MOF to pyridine is 5~15 mg:1 mL.

[0019] In step (2), the ultrasonic treatment time is 2.5~3.5 h.

[0020] The present invention also provides three-dimensional pillared Co-MOF and / or two-dimensional Co-MOF nanosheets prepared by the preparation method described above.

[0021] The present invention also provides the application of the three-dimensional pillared Co-MOF or two-dimensional Co-MOF nanosheets in the detection of cyclohexylamine.

[0022] The present invention also provides a method for detecting cyclohexylamine, the method comprising the following steps:

[0023] 1) Disperse three-dimensional pillared Co-MOF or two-dimensional Co-MOF nanosheets in deionized water to prepare a fluorescent probe solution with a concentration of 50-52 mg / L;

[0024] 2) Prepare a series of cyclohexylamine standard solutions;

[0025] 3) After mixing the fluorescent probe solution with cyclohexylamine standard solutions of different concentrations, the fluorescence intensity of each detection system was tested at an excitation wavelength of 320 nm. A standard curve was constructed with the concentration of cyclohexylamine standard solution as the abscissa and the ratio of the fluorescence intensity of the fluorescent probe solution to the fluorescence intensity of the detection system at 400 nm as the ordinate, and the linear equation was obtained.

[0026] 4) Mix the cyclohexylamine solution to be tested with the fluorescent probe solution prepared in step 1), detect the fluorescence intensity of the probe at an excitation wavelength of 320 nm, and then determine the concentration of cyclohexylamine in the cyclohexylamine solution to be tested by the standard curve or linear equation obtained in step 3).

[0027] The method for preparing two-dimensional Co-MOF nanosheets provided by this invention first involves coordinating 2,4,5-tris(4-pyridyl)imidazolium (TPIM) and 2,5-furandicarboxylic acid (FDCA) with cobalt ions to form a three-dimensional pillared Co-MOF crystal. Then, the three-dimensional pillared Co-MOF crystal is immersed in pyridine, and pyridine is used as a substitution ligand to partially substitute the multidentate pyridine ligand TPIM under ultrasonic conditions, resulting in ultrathin two-dimensional Co-MOF nanosheets. These nanosheets possess ultrathin thickness, large lateral dimensions, and good crystallinity. As a fluorescent probe, they exhibit good stability, are easy to store, and show good selectivity and sensitivity for the detection of cyclohexylamine.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The method for preparing two-dimensional Co-MOF nanosheets provided by this invention is simple, rapid, and uses inexpensive and readily available materials. A ligand exchange strategy successfully exfoliates three-dimensional pillared Co-MOF into two-dimensional Co-MOF nanosheets (Co-MOF-NS). The resulting nanosheets exhibit ultrathin thickness, large lateral dimensions, and good crystallinity, making them stable and easy to store as fluorescent probes. Cyclohexylamine quenches the fluorescence of organic ligands in both three-dimensional pillared Co-MOF and two-dimensional Co-MOF nanosheets. Based on this characteristic, three-dimensional pillared Co-MOF and two-dimensional Co-MOF nanosheets can be used as fluorescent probes for the quantitative detection of cyclohexylamine, with good selectivity, high sensitivity, and low detection limit. Attached Figure Description

[0030] Figure 1 (a) Co coordination environment of Co in Co-MOF, drawn as an ellipsoid with a 50% probability level. Hydrogen atoms are omitted for clarity. Symmetry opcodes: #1: 1 / 2-x, 1 / 2+y, 1 / 2-z; #2: -x, -y, -z; #3: 1 / 2+x, 1 / 2-y, 1 / 2+z; (b) Dinuclear Co2 unit in Co; (c) 2D structural diagram; (d) 3D structural diagram;

[0031] Figure 2 A simplified representation of binuclear Co2 units and ligands in a three-dimensional pillared Co-MOF, along with a schematic diagram of the topology of the coordination framework.

[0032] Figure 3(a) PXRD patterns of three-dimensional pillared Co-MOF and Co-MOF-NS; (b) FT-IR patterns of three-dimensional pillared Co-MOF, Co-MOF-NS, ligand TPIM, and H2FDCA; (c, d) PXRD patterns of three-dimensional pillared Co-MOF under different pH conditions and in different solvents; (e) TG curves of three-dimensional pillared Co-MOF and Co-MOF-NS; (f) Structures of ligand TPIM and H2FDCA;

[0033] Figure 4 (a) SEM image of a three-dimensional columnar Co-MOF; (b) TEM image of Co-MOF-NS, with the inset showing a photograph of the Tyndall effect;

[0034] Figure 5 (a) AFM image of Co-MOF-NS; (b) thickness of nanosheets; (c) thickness of Py-Co-FDCA-TPIM coordination layer formed after one side of the TPIM molecule is replaced by pyridine (Py).

[0035] Figure 6 (a) Comparison of fluorescence intensity between three-dimensional pillared Co-MOF and Co-MOF-NS; (b) Stability of fluorescence intensity of Co-MOF-NS;

[0036] Figure 7 (a) Fluorescence spectrum of Co-MOF-NS suspension in the presence of various organic amine small molecules and (b) bar chart of relative intensity; (c) graph of relative intensity of Co-MOF-NS suspension over time and fluorescence spectrum after addition of CHA;

[0037] Figure 8 (a) Fluorescence titration curve of Co-MOF-NS and (b) Fitted curve obtained according to Stern-Volmer formula; (c) Fluorescence titration curve of three-dimensional pillared Co-MOF and (d) Fitted curve obtained according to Stern-Volmer formula;

[0038] Figure 9 Bar chart of relative fluorescence intensity (I / I0) of Co-MOF-NS probe in the presence of various interfering substances: (a) antibiotics, (b) aromatic compounds, (c) cations and (d) anions;

[0039] Figure 10 (a) UV-Vis absorption spectrum of cyclohexylamine and fluorescence excitation spectrum of Co-MOF-NS, (b) UV-Vis absorption spectra of cyclohexylamine with different concentrations added in the presence of Co-MOF-NS. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the embodiments.

[0041] Example 1

[0042] A method for preparing two-dimensional Co-MOF nanosheets includes the following steps:

[0043] (1) 26.17 mg (0.11 mmol) of CoCl2·6H2O was dissolved in 0.5 mL of deionized water; 7.8 mg (0.045 mmol) of 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine and 26.17 mg (0.05 mmol) of 2,5-furandicarboxylic acid were dissolved in 5.5 mL of N,N-dimethylformamide; the two solutions were mixed evenly and placed in a 20 mL transparent screw-top glass bottle. The mixture was reacted at 85 °C for 72 h. The precipitate was collected by centrifugation and washed with DMF and deionized water in sequence. The precipitate was then dried in an oven at 60 °C for 6 h to obtain a three-dimensional pillared Co-MOF with a yield of 63.86%; Tables 1 and 2 list the corresponding crystallographic data and bond lengths, respectively.

[0044] 2) 80 mg of three-dimensional pillared Co-MOF was soaked in 8 mL of pyridine and sonicated at room temperature for 3 hours. Then, it was centrifuged at 10,000 rpm for 5 minutes. The supernatant colloidal suspension with Tyndall effect was collected, concentrated under reduced pressure and dried to obtain two-dimensional Co-MOF nanosheets (Co-MOF-NS).

[0045] Table 1 Crystal structure data and refinement parameters

[0046]

[0047] Table 2. Partial Bond Lengths (Å) and Bond Angles (°)

[0048]

[0049] Example 2

[0050] The rest is the same as in Example 1, except that the mass ratio of CoCl2·6H2O and deionized water in step (1) is adjusted to 1:25, and the yield of the three-dimensional pillared Co-MOF obtained in step (1) is 64.01%.

[0051] Example 3

[0052] The rest is the same as in Example 1, except that the mass ratio of CoCl2·6H2O and deionized water in step (1) is adjusted to 1:18, and the yield of the three-dimensional pillared Co-MOF obtained in step (1) is 60.57%.

[0053] Example 4

[0054] The rest is the same as in Example 1, except that the volume ratio of DMF to deionized water in step (1) is adjusted to 14:1, and the yield of the three-dimensional pillared Co-MOF obtained in step (1) is 63.86%.

[0055] Example 5

[0056] The rest is the same as in Example 4, except that the reaction conditions in step (1) are adjusted to 105 °C for 72 h. The yield of the three-dimensional pillared Co-MOF obtained in step (1) is 59.24%.

[0057] Example 6

[0058] The rest is the same as in Example 4, except that the molar ratio of CoCl2·6H2O, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine, and 2,5-furandicarboxylic acid in step (1) is adjusted to 18:6:6. The yield of the three-dimensional pillared Co-MOF obtained in step (1) is 63.61%.

[0059] Example 7

[0060] The rest is the same as in Example 4, except that the molar ratio of CoCl2·6H2O, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine, and 2,5-furandicarboxylic acid in step (1) is adjusted to 20:5:5. The yield of the three-dimensional pillared Co-MOF obtained in step (1) is 63.41%.

[0061] Example 8

[0062] The rest is the same as in Example 4, except that the molar ratio of CoCl2·6H2O, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine, and 2,5-furandicarboxylic acid in step (1) is adjusted to 20:4.5:5. The yield of the three-dimensional pillared Co-MOF obtained in step (1) is 61.37%.

[0063] Example 9

[0064] The rest is the same as in Example 4, except that the molar ratio of CoCl2·6H2O, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine, and 2,5-furandicarboxylic acid in step (1) is adjusted to 11:8:5. The yield of the three-dimensional pillared Co-MOF obtained in step (1) is 43.71%.

[0065] Example 10

[0066] The rest is the same as in Example 4, except that the molar ratio of CoCl2·6H2O, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine, and 2,5-furandicarboxylic acid in step (1) is adjusted to 11:4.5:10. The yield of the three-dimensional pillared Co-MOF obtained in step (1) is 32.71%.

[0067] Example 11

[0068] The rest is the same as in Example 4, except that the molar ratio of CoCl2·6H2O, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine, and 2,5-furandicarboxylic acid in step (1) is adjusted to 11:8:10. The yield of the three-dimensional pillared Co-MOF obtained in step (1) is 26.53%.

[0069] Test case

[0070] The single-crystal structure of the three-dimensional pillared Co-MOF in Example 1 was determined using single-crystal X-ray diffraction (SC-XRD). The SC-XRD results indicate that the obtained Co-MOF crystal belongs to the monoclinic crystal system, space group P21 / n, and its asymmetric unit cell consists of two crystallographically independent Co atoms. 2+ A cation, one pyridine ligand TPIM, and two carboxylic acid ligands FDCA 2- The compound consists of an anion, a coordinated water molecule, a coordinated DMF molecule, and a free DMF molecule, along with some highly disordered, free solvent molecules. By removing the disordered components using PLATON's SQUEEZE program and combining the number of electrons removed with thermogravimetric analysis results, its molecular formula is calculated to be [Co₂(FDCA)₂(TPIM)(DMF)(H₂O)]·3DMF·3H₂O. Figure 1 As shown in Figure a, the central metal atoms Co1 and Co2 both adopt a six-coordinate octahedral configuration. Co1 is associated with pyridine nitrogen atoms (N1, N2#2) from two different TPIM ligands and three different FDCA ligands. 2- The carboxyl oxygen atom (O1#1, O4, O6) on the ligand is coordinated with a water molecule (O11), while Co2 is coordinated with a pyridine nitrogen atom (N5#4) on a TPIM ligand and three different FDCA molecules. 2- The ligand is coordinated with a carboxyl oxygen atom (O3, O5, O8#3), a water molecule (O11), and an oxygen atom (O12) from a DMF molecule. Two adjacent metal ions, Co1 and Co2, are connected by a bridging water molecule and two bridging carboxyl groups, forming a binuclear cobalt cluster (Co2) node. Each binuclear cluster node is associated with four FDCA molecules. 2- The ligand is linked to three TPIM ligands, such as Figure 1As shown in Figure b. Further analysis of the Co-MOF structure reveals that although the carboxylic acid ligand FDCA... 2- The two carboxyl groups in the formula have different coordination modes, namely, one carboxyl group is associated with two Co groups. 2+ Coordination, one is only with one Co 2+ They coordinate, but each is only linked to a single binuclear cobalt cluster (Co2), therefore the carboxylic acid ligand FDCA 2- Coordination with binuclear Co2 clusters forms a two-dimensional coordination polymer network structure, such as... Figure 1 As shown in Figure c, in the pyridine ligand TPIM, only the pyridine nitrogen atom participates in coordination, while the imidazole nitrogen atom does not. Furthermore, the three pyridine nitrogen atoms each coordinate with one of the three Co2 clusters from two different FDCA-Co layers. 2+ Coordination, therefore the ligand TPIM acts like a supporting pillar, assembling these FDCA-Co two-dimensional layers to form the final three-dimensional pillared coordination polymer framework structure, such as... Figure 1 The d-plot in the diagram. Furthermore, from a topological perspective, the binuclear Co2 cluster, ligand TPIM, and FDCA in Co-MOFs... 2- Each node can be viewed as a 7-connected, 3-connected, and 2-connected node, respectively. Using TOPOSPRO software to analyze its topology, we can find that its three-dimensional structure can be simplified to a 2, 2, 3, 7-connected Schläfli, notated as {3.5}. 2}{3 2 0.5 2 0.6 2 5.4 2 6.4.6.4.6 2 0.5 4 The topology of .7.6}{3}{4}, such as Figure 2 As shown.

[0071] like Figure 3 As shown in Figure a, the PXRD diffraction pattern of the three-dimensional pillared Co-MOF is almost identical to the diffraction pattern obtained by fitting the single-crystal structure, indicating that the prepared Co-MOF is a pure phase. Meanwhile, infrared spectroscopy results show that the ligand H2FDCA exhibits high activity at 1695 cm⁻¹. -1 The vibrational peak belonging to the carboxyl group shifted to 1652 cm⁻¹ in Co-MOF. -1 At 1599 cm, the ligand TPIM was present. -1 The vibrational peak belonging to the pyridyl group shifted to 1615 cm⁻¹ in Co-MOF. -1 Place, such as Figure 3 Figure b further illustrates the relationship between the two ligands in Co-MOF and Co. 2+Coordination occurred. Furthermore, after immersing the three-dimensional pillared Co-MOFs in aqueous solutions of different pH values ​​and different organic solvents for two days, their PXRD diffraction patterns remained essentially unchanged, indicating that Co-MOFs possess good chemical stability. Figure 3 As shown in Figures c and d.

[0072] Thermogravimetric analysis was also conducted to investigate the thermal stability of Co-MOFs. For example... Figure 3 As shown in Figure e, Co-MOF lost 4.47% of its weight in the 30-110 °C range, corresponding to the departure of free water molecules from Co-MOF (theoretical value: 4.86%); then it lost another 27.81% of its weight in the 250 °C range, corresponding to the loss of free DMF molecules and coordinated water and DMF molecules (theoretical value: 28.48%); thereafter it continued to lose weight slowly, corresponding to the collapse of the Co-MOF structure, indicating that the prepared Co-MOF has good thermal stability.

[0073] In Example 1, the three-dimensional pillared Co-MOF was immersed in pyridine and sonicated. The supernatant suspension exhibited a significant Tyndall effect, such as... Figure 4 The inset in Figure b illustrates that it may have formed MOF nanosheets. Figure 3 As shown in Figures a and b, PXRD and infrared spectroscopy tests indicate that the PXRD diffraction peak positions and vibrational peak positions of the sample obtained from the upper suspension remained almost unchanged, indicating that its framework structure remained intact. Furthermore, some diffraction peaks showed significant broadening and attenuation, suggesting the possible formation of two-dimensional nanosheets; some diffraction peaks also shifted towards smaller angles, which may be related to the interlayer sliding expansion of the two-dimensional coordination layers during the exfoliation process.

[0074] To more intuitively illustrate the formation of nanosheets, they were characterized using SEM, TEM, and AFM. Figure 4 As shown in the SEM image of Figure a, the three-dimensional pillared Co-MOF in Example 1 (before peeling) exhibits an irregular blocky morphology with a size of several micrometers; while after peeling, the obtained Co-MOF-NS has a distinct lamellar structure, such as... Figure 4 Figure b in the diagram shows that, according to AFM measurements, the thickness of the Co-MOF-NS formed after exfoliation is less than 2.12 nm. Figure 5As shown, this thickness is basically consistent with the thickness of the Py-Co-FDCA-TPIM coordination layer formed by replacing one side of the TPIM molecule with pyridine (Py) in the TPIM-Co-FDCA-TPIM coordination layer (the thickness of a single layer is 1.92 nm without considering CH bonds). This not only proves the successful preparation of Co-MOF-NS ultrathin nanosheets, but also shows that the Co-MOF-NS formed is a single-layer nanosheet.

[0075] Application examples

[0076] Application of the three-dimensional pillared Co-MOF and two-dimensional Co-MOF nanosheets prepared in Example 1 in the detection of cyclohexylamine

[0077] The three-dimensional pillared Co-MOF and two-dimensional Co-MOF nanosheets prepared in Example 1 were dispersed in deionized water to form Co-MOF suspensions and Co-MOF-NS suspensions with a concentration of 50 mg / L, respectively. The fluorescence intensity of the solutions was tested at an excitation wavelength of 320 nm. The test results are as follows: Figure 6 As shown in Figure a, both Co-MOF and Co-MOF-NS exhibit characteristic fluorescence emission peaks around 400 nm, and the fluorescence emission peak intensity of Co-MOF-NS is more than twice that of Co-MOF.

[0078] In addition, the stability of the Co-MOF-NS suspension was investigated. The Co-MOF-NS suspension was left to stand for 12 days, and its fluorescence intensity was measured daily. The test results are as follows: Figure 6 As shown in Figure b, the fluorescence emission peak intensity of the colloidal solution of Co-MOF-NS in water remained almost unchanged after 12 days, indicating that the Co-MOF-NS suspension has good stability.

[0079] Subsequently, Co-MOF-NS was used as a probe to detect small organic amine molecules. Solutions of different small organic amine molecules, including N,N,N',N'-tetramethylethylenediamine (TMED), triethanolamine (TEOA), tetraethylammonium chloride (TEAC), triethylamine (TEA), o-phenylenediamine (OPD), oleylamine (OAM), ethylenediamine (EDA), N,N-diisopropylethylamine (DIEA), acrylamide (AM), 4-nitroaniline (4-NA), and cyclohexylamine (CHA), were added to Co-MOF-NS suspensions at a volume ratio of 1:100. The fluorescence intensity of each system was measured, and the results are shown below. Figure 7As shown in Figures a and b, it can be seen from the figures that only the introduction of CHA caused a sharp decrease in the fluorescence intensity of Co-MOF-NS, with a quenching efficiency as high as 97.4%. This phenomenon indicates that Co-MOF-NS can selectively recognize CHA molecules.

[0080] Simultaneously, the response rate of Co-MOF-NS to CHA was investigated. Cyclohexylamine was added to the Co-MOF-NS suspension at a final concentration of 1 μM / L, and its fluorescence intensity was measured at regular intervals. The detection results are as follows: Figure 7 As shown in Figure c, the fluorescence intensity of the Co-MOF-NS suspension was almost quenched within 15 seconds after the addition of CHA and remained unchanged for the next 10 minutes, indicating that Co-MOF-NS has a fast response speed.

[0081] Finally, to further investigate the sensitivity of three-dimensional pillared Co-MOF and Co-MOF-NS for CHA detection, a fluorescence titration experiment was conducted. Different concentrations of CHA were added to the Co-MOF and Co-MOF-NS suspensions, varying the final concentration between 0 and 37.5 μM, and the fluorescence intensity of each system was measured. The results are as follows: Figure 8 As shown, it can be seen that with the continuous increase of CHA concentration in the system, the fluorescence emission peak intensities of both Co-MOF and Co-MOF-NS gradually decrease. When plotting the relative intensity I0 / I of the Co-MOF-NS fluorescence emission peak (I0 and I represent the fluorescence emission peak intensities of Co-MOF-NS before and after the addition of the analyte, respectively) against the concentration of the analyte CHA, it can be seen that it is almost a straight line in the low concentration region. Therefore, using the Stern-Volmer equation I0 / I=K SV [C]+1(K SV [C] and [Quarantine constant] and analyte concentration, respectively, were linearly fitted, and their correlation coefficient R was obtained. 2 The values ​​are as high as 0.9834 and 0.9957 respectively, indicating that both have a very good linear relationship with CHA. The quenching constant K of CHA on Co-MOF-NS can be obtained from the fitting results. SV 1.78×10 5 M -1 Furthermore, the corresponding limit of detection (LOD) can be calculated to be 0.10 μM using the formula LOD = 3σ / m (σ is the standard deviation, and m is the slope). Compared to the LOD of 4.69 μM when the three-dimensional pillared Co-MOF crystal is used directly as a probe, the LOD value of Co-MOF-NS is lower. Compared to some previously reported CHA fluorescent sensors, Co-MOF-NS exhibits comparable or even better sensing performance as a CHA fluorescent probe, as shown in Table 3.

[0082] Table 3. Comparison of sensing performance with existing cyclohexylamine (CHA) fluorescence sensors

[0083]

[0084] Since anti-interference and reusability are important parameters for evaluating a fluorescence sensing material, the anti-interference and reusability properties of the Co-MOF-NS probe were investigated. When common interfering molecules such as antibiotics, aromatic compounds, cations, and anions were sequentially added to the Co-MOF-NS suspension, it was observed that the fluorescence emission peak intensity of Co-MOF-NS showed little change after the addition of these interfering substances. However, upon subsequent addition of CHA, its fluorescence was almost completely quenched. The final concentration of both the interfering molecules and the CHA in the system was 1 μM / L. Figure 9 As shown, the presence of these interfering substances has virtually no impact on the fluorescence recognition performance of Co-MOF-NS.

[0085] Furthermore, the fluorescence mechanism of CHA quenching of Co-MOF-NS was investigated. For example... Figure 10 As shown in Figure a, the UV-Vis absorption peak of CHA significantly overlaps with the fluorescence excitation peak of Co-MOF-NS, while its fluorescence emission peak shows almost no overlap. This suggests that the internal filtering effect (IFE) of energy-competitive absorption may be the main cause of fluorescence quenching in Co-MOF-NS. To further verify this hypothesis, a UV-vis titration experiment was also performed, as shown in Figure a. Figure 10 As shown in Figure b, with the gradual addition of CHA solution to the Co-MOF-NS suspension, the UV-Vis absorption peak of Co-MOF-NS significantly increased with increasing CHA concentration. This indicates that after CHA is added to the Co-MOF-NS suspension, it competitively absorbs the excitation light of Co-MOF-NS, thus causing fluorescence quenching.

[0086] To further verify the practicality of Co-MOF-NS as a fluorescent probe for detecting CHA in real samples, samples from the Huajin River, tap water, and Yangtze River were used as actual samples. After pretreatment, a certain concentration of CHA solution was injected into the Huajin River, tap water, and Yangtze River water, respectively, and the corresponding three real samples were extracted and purified using the QuEChERS method. The fluorescence detection results of Co-MOF-NS in these samples are shown in Table 4. The recoveries and RSDs were in the range of 94.29%–105.34 and 0.9–4.31, respectively, indicating that the prepared Co-MOF-NS can be used as a fluorescent probe for the detection of CHA in common water systems.

[0087] Table 4. Detection of cyclohexylamine (CHA) in real samples.

[0088]

[0089] The above detailed description of a two-dimensional Co-MOF nanosheet, its preparation method, and its application in cyclohexylamine detection, with reference to the embodiments, is illustrative rather than limiting. Several embodiments may be listed within the defined scope. Therefore, variations and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A method for preparing two-dimensional Co-MOF nanosheets, characterized in that, The preparation method includes the following steps: (1) Add a DMF solution containing 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine and 2,5-furandicarboxylic acid to a deionized aqueous solution of cobalt salt, mix well, react at 80~105 °C for 65~72 h, then centrifuge and dry to obtain three-dimensional pillared Co-MOF; (2) The three-dimensional pillared Co-MOF was immersed in pyridine for ultrasonic treatment, then centrifuged, and the supernatant was collected. The supernatant was then concentrated and dried to obtain two-dimensional Co-MOF nanosheets.

2. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of cobalt salt, 4,4',4''-(1H-imidazol-2,4,5-triyl)tripyridine and 2,5-furandicarboxylic acid is 11~20:4.5~8:5~10.

3. The preparation method according to claim 1 or 2, characterized in that, In step (1), the cobalt salt is CoCl2·6H2O.

4. The preparation method according to claim 1 or 2, characterized in that, In step (1), the volume ratio of DMF to deionized water is 11~14:

1.

5. The preparation method according to claim 1, characterized in that, In step (2), the ratio of the amount of three-dimensional pillared Co-MOF to pyridine is 5~15 mg:1 mL.

6. The preparation method according to claim 1, characterized in that, In step (2), the ultrasonic treatment time is 2.5~3.5 h.

7. Three-dimensional pillared Co-MOF and / or two-dimensional Co-MOF nanosheets prepared by the preparation method according to any one of claims 1-6.

8. The application of the three-dimensional pillared Co-MOF or two-dimensional Co-MOF nanosheets as described in claim 7 in the detection of cyclohexylamine.

9. A method for detecting cyclohexylamine, characterized in that, The method includes the following steps: 1) Disperse the three-dimensional pillared Co-MOF or two-dimensional Co-MOF nanosheets as described in claim 7 in deionized water to prepare a fluorescent probe solution with a concentration of 50-52 mg / L; 2) Prepare a series of cyclohexylamine standard solutions; 3) After mixing the fluorescent probe solution with cyclohexylamine standard solutions of different concentrations, the fluorescence intensity of each detection system was tested at an excitation wavelength of 320 nm. A standard curve was constructed with the concentration of cyclohexylamine standard solution as the abscissa and the ratio of the fluorescence intensity of the fluorescent probe solution to the fluorescence intensity of the detection system at 400 nm as the ordinate, and the linear equation was obtained. 4) Mix the cyclohexylamine solution to be tested with the fluorescent probe solution prepared in step 1), detect the fluorescence intensity of the probe at an excitation wavelength of 320 nm, and then determine the concentration of cyclohexylamine in the cyclohexylamine solution to be tested by the standard curve or linear equation obtained in step 3).

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