A fluorescent probe based on metal organic framework and application thereof in detecting hexavalent chromium ion in water body

Abstract

This invention belongs to the field of fluorescent materials and discloses a fluorescent probe based on a metal-organic framework (MOF) and its application in detecting hexavalent chromium ions in water. The MOF-based fluorescent probe is CDs@NH2-MIL-101(Cr). This MOF fluorescent probe is a composite material with high fluorescence properties constructed by embedding carbon quantum dots into a MOF through low-temperature calcination. This invention uses chromium nitrate nonahydrate solutions of different concentrations as detection reagents. It then utilizes the fact that hexavalent chromium ions can cause fluorescence quenching of CDs@NH2-MIL-101(Cr) to perform parallel fluorescence signal testing, thereby accurately obtaining the linear relationship between hexavalent chromium ion concentration and fluorescence signal. Based on the linear curve, the concentration of hexavalent chromium ions in the water body can be accurately determined. This invention benefits from the synergistic effect of the functional components in CDs@NH2-MIL-101(Cr), achieving rapid detection with high sensitivity and strong anti-interference capabilities, and has broad application prospects in practical water treatment.

Description

Technical Field

[0001] This invention belongs to the field of fluorescent materials, and specifically relates to a fluorescent probe based on a metal-organic framework and its application in detecting hexavalent chromium ions in water. Background Technology

[0002] Cr(VI) is a serious environmental pollutant that poses a direct threat to human health. Cr(VI) may cause digestive, respiratory, and immune system problems, and more seriously, it has potential carcinogenicity; long-term exposure may increase the risk of cancer. Currently, techniques for detecting Cr(VI) include colorimetry, surface-enhanced Raman scattering, electrochemistry, adsorption, reduction precipitation, ion exchange, and fluorescence methods. Compared with other methods, fluorescence analysis offers advantages such as fast analysis speed, good selectivity, high sensitivity, low cost, and simple instrument operation.

[0003] Metal-organic frameworks (MOFs), as a novel type of microporous hybrid organic-inorganic crystal material, have wide applications in the detection and removal of heavy metals due to their tunable pore structure, large specific surface area, and good stability. However, MOF-based fluorescent probes suffer from low sensitivity, while carbon quantum dot-based fluorescence spectroscopy offers significantly enhanced sensitivity for detecting environmental Cr(VI).

[0004] Carbon quantum dots (CDs) are widely used in fluorescence detection, photocatalysis, and biosensors due to their excellent biocompatibility, high chemical stability, and low toxicity. The application of ultra-small and uniform carbon nanodot (CD) arrays in optical devices has attracted great interest. To improve their photoelectric properties and stability, metal-organic frameworks (MOFs) are used as ideal carriers, resulting in CDs with uniform and controllable sizes, tunable photoluminescence emission, and significant nonlinear optics. CDs@MOFs composites possess the advantages of both CDs and MOFs, specifically interacting with target analytes to produce fluorescence quenching, showing great application potential.

[0005] Against this backdrop, the present invention addresses the aforementioned problems by providing a fluorescent probe based on a metal-organic framework and its technique for detecting hexavalent chromium ions in water. Summary of the Invention

[0006] In order to overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide a fluorescent probe based on a metal-organic framework.

[0007] Another objective of this invention is to provide a method for preparing the above-mentioned fluorescent probe based on a metal-organic framework.

[0008] Another objective of this invention is to provide the application of the above-mentioned metal-organic framework-based fluorescent probe in the detection of hexavalent chromium ions in water.

[0009] The objective of this invention is achieved through the following solution:

[0010] A fluorescent probe based on a metal-organic framework, namely CDs@NH2-MIL-101(Cr).

[0011] A method for preparing the above-mentioned metal-organic framework-based fluorescent probe includes the following steps:

[0012] (1) Add Cr(NO3)3·9H2O, NH2-H2BDC and NaOH to water and mix evenly. Then transfer the mixture to a reaction vessel to carry out a hydrothermal reaction.

[0013] (2) After the hydrothermal reaction is completed, the reaction vessel is cooled to room temperature. The product is washed and then transferred to another reaction vessel with ethanol or DMF and heated for reaction. After the reaction is completed, the green powder NH2-MIL-101(Cr) is obtained by purification.

[0014] (3) The green powder NH2-MIL-101(Cr) was immersed in an ethanol aqueous solution containing a carbon source so that the carbon source was evenly distributed in the pores of NH2-MIL-101(Cr) to obtain a composite.

[0015] (4) The composite was calcined in a tube furnace, and the resulting solid was CDs@NH2-MIL-101(Cr).

[0016] The mass ratio of Cr(NO3)3·9H2O, NH2-H2BDC and NaOH in step (1) is 1:0.43-0.46:0.24-0.26; preferably 1.6:072:0.4.

[0017] The amount of water used in step (1) is such that the concentration of Cr(NO3)3·9H2O in the mixture is 51.00-55.00 mg / mL, preferably 53.33 mg / mL;

[0018] The mixing process described in step (1) is preferably ultrasonicated for 30 minutes to ensure uniform mixing.

[0019] The hydrothermal reaction mentioned in step (1) refers to heating to 148-152℃ and reacting for 11.5-12.5 hours, preferably heating to 150℃ and reacting for 12 hours.

[0020] The washing described in step (2) is preferably performed using ethanol or DMF;

[0021] The heating reaction mentioned in step (2) refers to heating to 98-102℃ for 23.5-24.5 hours, preferably heating to 100℃ for 24 hours. The reaction mentioned in step (2) is to displace the unreacted raw material NH2-H2BDC in the pores.

[0022] The purification described in step (2) refers to cooling the obtained reaction solution to room temperature, washing it with ethanol, centrifuging it, and then drying it.

[0023] The preferred carbon source in step (3) is glucose;

[0024] The concentration of carbon source in the ethanol aqueous solution containing carbon source in step (3) is 0.98-1.02 mmol / L, preferably 1 mmol / L; the volume ratio of ethanol to water in the ethanol aqueous solution is 8.8-1:9.2-1, preferably 9:1; the amount of green powder used in step (3) satisfies the following: the mass ratio of green powder to carbon source is 0.1-1.5:0.1-1.8;

[0025] The soaking time in step (3) is 6-24 hours, preferably 12 hours;

[0026] The soaking in step (3) is preferably agitated soaking so that the carbon source can be evenly dispersed and penetrate into the pores of NH2-MIL-101(Cr).

[0027] After soaking in step (3), the soaked product is centrifuged to obtain a precipitate, which is then washed with ethanol and centrifuged again. The resulting solid is dried to obtain the complex.

[0028] The calcination mentioned in step (4) refers to calcination in an inert atmosphere (preferably a nitrogen atmosphere), with a calcination temperature of 200-250℃, preferably 200℃, and a calcination time of 2-4h.

[0029] The preparation method described in this invention involves embedding carbon quantum dots into the pores of NH2-MIL-101(Cr) through low-temperature calcination, thereby achieving uniform distribution of CDs on the NH2-MIL-101(Cr) support and constructing a CDs@NH2-MIL-101(Cr) composite material with high fluorescence properties.

[0030] The above-mentioned application of the metal-organic framework-based fluorescent probe in the detection of hexavalent chromium ions in water.

[0031] A method for detecting hexavalent chromium ions in water includes the following steps:

[0032] 1) CDs@NH2-MIL-101(Cr) was dissolved in water to prepare a CDs@NH2-MIL-101(Cr) stock solution. Then, different concentrations of Cr(VI) solution were added to the stock solution, and a buffer solution was added to adjust the pH of the mixture to 7.8-8.2 to obtain a variety of mixtures with different Cr(VI) concentrations. The fluorescence intensity F of the mixtures with different Cr(VI) concentrations was obtained by fluorescence testing.

[0033] 2) Replace the Cr(VI) solutions of different concentrations in step 1) with an equal volume of water, and keep everything else the same as in step 1). Perform fluorescence testing on the mixture to obtain the fluorescence intensity F0 of the mixture without Cr(VI) concentration; then, plot F / F0-1 as the ordinate and Cr(VI) concentration as the abscissa to obtain a linear relationship curve.

[0034] 3) Based on step 1), replace the Cr(VI) solution added in step 1) with an equal volume of the water to be tested, and then repeat step (1). Convert the obtained fluorescence intensity into the corresponding ordinate F / F0-1, and then substitute it into the linear relationship curve in step 2). Convert the corresponding concentration to obtain the Cr(VI) concentration in the water to be tested.

[0035] The buffer solution mentioned in step 1) is a phosphate buffer solution (PBS) with a pH of 8;

[0036] The concentration of the CDs@NH2-MIL-101(Cr) stock solution mentioned in step 1) is 25-400 mg / L, preferably 100 mg / L;

[0037] The concentration of the Cr(VI) solution mentioned in step 1) is 1-200 μmol / L;

[0038] The pH value of the mixture described in step 1) is preferably 8.0;

[0039] In step 1), the fluorescence test conditions are: excitation wavelength 340 nm, slit width 10 nm, and 900 V.

[0040] In step 1), the linear relationship curve has two linear ranges in the range of 1 to 150 μmol / L, namely 1-20 μmol / L and 20-150 μmol / L.

[0041] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0042] This invention utilizes a fluorescent probe constructed by embedding carbon quantum dots into a metal-organic framework through low-temperature calcination, which quenches the fluorescence of Cr(VI). This probe exhibits excellent detection sensitivity and strong anti-interference capability. When used for actual sample detection, the recovery rate is 101%–105%, and the RSD is 0.32%–1.5%. Attached Figure Description

[0043] Figure 1 These are SEM images of the present invention, wherein (a) is a SEM image of NH2-MIL-101(Cr) and (b) is a SEM image of CDs@NH2-MIL-101(Cr).

[0044] Figure 2 This is the FTIR diagram of the present invention.

[0045] Figure 3 This is the XRD pattern of the present invention.

[0046] Figure 4 The present invention relates to the ultraviolet-visible absorption spectrum and fluorescence excitation and emission spectrum.

[0047] Figure 5 This is the emission spectrum of the present invention at different Cr(VI) concentrations.

[0048] Figure 6 It is the variation of (F0-F) / F with Cr(VI) concentration in the range of 1 to 200 μmol / L.

[0049] Figure 7 This invention provides fluorescence spectra with selectivity for different ions.

[0050] Figure 8 This is a graph showing the fluorescence quenching effect of the present invention on interfering ions. Detailed Implementation

[0051] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0052] Unless otherwise specified, all reagents used in the examples are commercially available.

[0053] Example 1

[0054] (1) Preparation of CDs@NH2-MIL-101(Cr)

[0055] Weigh 1.60 g of Cr(NO3)3·9H2O and dissolve it in 30 mL of deionized water. Add 0.72 g of NH2-H2BDC and 0.40 g of NaOH to the above solution. After sonicating for 30 minutes, transfer the solution to a reaction vessel and heat it at 150 °C for 12 h. After the reaction vessel cools to room temperature, wash the product three times with DMF solution. Then transfer the product to a stainless steel reaction vessel with an appropriate volume of ethanol and heat it at 100 °C for 24 h to displace the NH2-H2BDC in the channels. After cooling to room temperature, wash with ethanol and centrifuge. Dry the obtained product in a vacuum drying oven at 80 °C for 12 h to obtain the green powder NH2-MIL-101(Cr).

[0056] 100 mg of the above powder was placed in 100 mL of glucose (G) ethanol aqueous solution (1 mmol / L, ethanol to water volume ratio 9:1) and shaken for 12 h. The precipitate was obtained by centrifugation, and the sample was washed three more times with ethanol aqueous solution and centrifuged. The resulting solid was dried in a vacuum drying oven for 6 h, and the resulting powder was G@NH2-MIL-101(Cr). The prepared powder was heated to 200 °C in a tube furnace under a nitrogen atmosphere and held for 2 h, and the resulting solid was CDs@NH2-MIL-101(Cr).

[0057] NH2-MIL-101(Cr) powder was heated to 200°C in a tube furnace under a nitrogen atmosphere to obtain NH2-MIL-101(Cr)-200 powder; glucose was heated to 200°C in a tube furnace under a nitrogen atmosphere to obtain G-200 solid.

[0058] The surface morphology and structure of the obtained CDs@NH2-MIL-101(Cr) were characterized using scanning electron microscopy and Fourier transform infrared spectroscopy, as follows: Figure 1 , Figure 2 and Figure 3 As shown.

[0059] Figure 1 The image shows a scanning electron microscope (SEM) image of CDs@NH2-MIL-101(Cr). The synthesized composite material CDs@NH2-MIL-101(Cr) retains the original microstructure of NH2-MIL-101(Cr), indicating that the entry of CDs into the MOF pores did not change its morphology.

[0060] Figure 2 This is the infrared spectrum of CDs@NH2-MIL-101(Cr), 1496 cm⁻¹. -1 and 1434cm -1 This represents the stretching vibration peak of -OCO- in the MOF framework. It occurs at 1336 cm⁻¹. -1 and 1260cm -1The two characteristic peaks at the point are attributed to the stretching vibration of CN on the benzene ring of the MOF.

[0061] Figure 3 The images show the XRD patterns of glucose (G), CDs@NH2-MIL-101(Cr), G@NH2-MIL-101(Cr), NH2-MIL-101(Cr)-200, and NH2-MIL-101(Cr). The XRD pattern of NH2-MIL-101(Cr) shows the same topological structure as reported in the literature, and no new peaks appeared, indicating that the MOF lattice was not affected by glucose. The calcination results of G@NH2-MIL-101(Cr) are significantly different from those of direct glucose G, indicating that the MOF host mainly affects the thermal decomposition of glucose. The sample changed from colorless to black, proving that glucose is easily calcined into carbon materials and can be successfully embedded in the pores of the MOF. The relative intensity of the XRD peaks changes with the load, and the increase in the relative intensity of the peaks indicates that the MOF pores carry glucose.

[0062] Figure 4 These are the UV-Vis absorption and fluorescence excitation and emission spectra of CDs@NH2-MIL-101(Cr). The synthesized CDs@NH2-MIL-101(Cr) shows a large absorption at 285 nm in the UV-Vis absorption spectrum, and the excitation and emission wavelengths in the fluorescence spectrum are 340 nm and 438 nm, respectively. The UV-Vis absorption and fluorescence emission spectra of CDs@NH2-MIL-101(Cr) show little overlap, and both exhibit a large Stokes shift. The material has low self-absorption, which effectively eliminates fluorescence quenching caused by self-absorption, thereby improving detection accuracy.

[0063] (2) Detection of Cr(VI)

[0064] Take 0.4 mL of CDs@NH2-MIL-101(Cr) stock solution (100 mg / L) into a 5 mL centrifuge tube, and add 0.4 mL of Cr(VI) solutions of different concentrations (1–200 μmol / L). Vortex mix with 3.2 mL of phosphate-buffered saline (PBS) at pH 8 for 1 min. Acquire fluorescence signals at an excitation wavelength of 340 nm, a slit width of 10 nm, and 900 V, and record the signal as F. Perform the test three times in parallel. Simultaneously, take 0.4 mL of CDs@NH2-MIL-101(Cr) stock solution (100 mg / L) into a 5 mL centrifuge tube, and add 0.4 mL of pure water. Vortex mix with 3.2 mL of phosphate-buffered saline (PBS) at pH 8 for 1 min. Acquire fluorescence signals at an excitation wavelength of 340 nm, a slit width of 10 nm, and 900 V, and record the signal as F0. Perform the test three times in parallel. Record the values ​​and plot a standard curve for quantitative analysis.

[0065] like Figure 5 As shown, the fluorescence intensity of CDs@NH2-MIL-101(Cr) decreases significantly with the introduction of Cr(VI), and the fluorescence intensity at 438 nm gradually decreases with the increase of Cr(VI) concentration (1-200 μmol / L).

[0066] Fluorescence quenching efficiency was quantitatively assessed using the equation F / F0=1+K[C]. For example... Figure 6 As shown, there are two linear ranges within the range of 1–150 μmol / L, namely 1–20 μmol / L and 20–150 μmol / L, with linear equations of y = 0.0095c + 0.1523(R) and y = 0.0095c + 0.1523(R) respectively. 2 =0.9974) and y =0.00104c + 0.3262(R) 2 =0.9856), where y represents (F / F0)⁻¹ and x represents the Cr ion concentration in the test solution. Within the range of 1–200 μmol / L, there is a good linear relationship between Cr(VI) concentration and (F / F0)⁻¹.

[0067] (3) Selectivity of CDs@NH2-MIL-101(Cr)

[0068] Mix 0.4 mL of a 100 mg / L CDs@NH2-MIL-101 (Cr) solution with 0.4 mL of a 20 mmol / L solution of Cr(VI) ions or other ions (Li). + Al 3+ K + Na + Zn 2+ Ca 2+ Co 2+ Mg2+ NH4 + Ni 2+ SO3 2- HSO3 - PO4 3- HPO4 2- H2PO4 - CO3 2- C2O4 2- CH3COO - BF4 - Cr2O7 2- F - Cl - ,Br - S 2- I - NO3 - NO2 - S2O3 2- SCN - SO4 2- The solutions were mixed in equal volumes and then brought to a final volume of 4 mL with pH 8 PBS solution. After vortexing for 1 min, fluorescence signals were acquired at an excitation wavelength of 340 nm, a slit width of 10 nm, and a voltage of 900 V. The assay was performed in triplicate to detect the selectivity of CDs@NH2-MIL-101(Cr) for Cr(VI) ions. A blank control group was prepared by adding only CDs@NH2-MIL-101(Cr) without any other ions. The results are as follows: Figure 7 As shown.

[0069] from Figure 7 As can be seen, Cr(VI) ions exhibit the best fluorescence quenching ability. Under weakly alkaline conditions at pH 8, Cr... 3+ Ag + Cd 2+ Fe 3+ Hydrolysis will occur, Ba 2+ It will react with PO4 in the buffer solution 3- Precipitation forms, causing the solution to become turbid and resulting in fluorescence quenching; therefore, Cr is not selected among the cations. 3+ Ag + Cd 2+ Fe 3+ Ba 2+ Isocations. Most cations showed little change in fluorescence intensity after reacting with CDs@NH2-MIL-101(Cr), among which Ni... 2+ This reduces the fluorescence intensity of CDs@NH2-MIL-101(Cr) by approximately 7%. (The C2O4 anion...) 2- CH3COO -NO2 - S2O3 2- The fluorescence intensity of CDs@NH2-MIL-101(Cr) was reduced by 10%, 16%, 19%, and 25%, respectively, by BF4. - The fluorescence intensity of the composite material decreased by 52%, possibly due to BF4. - This is due to hydrolysis. However, upon reaction with the target ion Cr(VI), the fluorescence intensity of the composite material was completely quenched, demonstrating that CDs@NH2-MIL-101(Cr) is selective for the detection of Cr(VI).

[0070] (4) Interference immunity of CDs@NH2-MIL-101(Cr)

[0071] Mix equal volumes of 0.4 mL of 100 mg / L CDs@NH2-MIL-101(Cr) solution and 0.4 mL of 20 mmol / L Cr(VI) ion solution. Then add various interfering ions (F) at a final concentration (referring to the final concentration in the test solution) four times the final concentration of Cr(VI) ions (referring to the final concentration in the test solution). - ,Br - I - CO3 2- HCO3 - CH3COO - SO3 2- HSO3 - SCN - S2O3 2- NO3 - NO2 - PO4 3- HPO4 2- H2PO4 - BF4 - Mg 2+ Co 2+ Ni 2+ Na + Zn 2+ Li + NH4 + The solution was diluted to 4 mL with PBS solution at pH 8, vortexed for 1 min to obtain the test solution. The fluorescence intensity of each test solution was collected under the conditions of excitation wavelength 340 nm, slit width 10 nm, and 900 V, and denoted as F. A blank control group without interfering ions and containing Cr(VI) was also set up, and the fluorescence intensity obtained was denoted as F0. The test was performed in triplicate to detect the anti-interference ability of CDs@NH2-MIL-101(Cr) for Cr(VI) detection. The results are as follows: Figure 8 As shown.

[0072] Simultaneously, interfering ions (4.0 equiv, final concentration of 8 mmol / L in the test solution) were added to 0.4 mL of a 100 mg / L CDs@NH2-MIL-101(Cr) solution. The solution was then diluted to 4 mL with pH 8 PBS solution and vortexed for 1 min to obtain the test solution. Fluorescence signals and intensities of each test solution were collected under conditions of 340 nm excitation wavelength, 10 nm slit width, and 900 V, denoted as F. A blank control group without interfering ions was also set up, and its fluorescence intensity was denoted as F0. The test was performed in triplicate to detect the anti-interference ability of CDs@NH2-MIL-101(Cr) against Cr(VI) detection. The results are as follows: Figure 8 As shown.

[0073] from Figure 8 As can be seen, Cr(VI) can still be detected by CDs@NH2-MIL-101(Cr) in the presence of other metal ions or anions. Furthermore, the quenching effect of Cr(VI) ions on the composite material is significant, proving that the presence of interfering ions does not affect the detection of Cr(VI) by CDs@NH2-MIL-101(Cr), further demonstrating that the prepared CDs@NH2-MIL-101(Cr) sensor has good anti-interference capabilities.

[0074] (5) Testing of Cr(VI) concentration in actual water bodies

[0075] Take 0.4 mL of CDs@NH2-MIL-101(Cr) stock solution (100 mg / L) into a 5 mL centrifuge tube, and add 0.4 mL of tap water solution. Vortex mix with 3.2 mL of phosphate buffered saline (PBS) at pH 8 for 1 min. Acquire fluorescence signals at an excitation wavelength of 340 nm, a slit width of 10 nm, and 900 V, and record the signal as F. Perform the test three times in parallel. Substitute the recorded values ​​into the linear curve equation to obtain the concentration value after dilution with tap water. The concentration value after dilution with tap water is too low to be measured.

[0076] 0.4 mL of CDs@NH2-MIL-101(Cr) stock solution (100 mg / L) was added to a 5 mL centrifuge tube, followed by 0.4 mL of tap water solution, and then 0.4 mL of 50, 100, and 300 μmol / L Cr(VI) solutions, respectively. The mixture was vortexed with 2.8 mL of phosphate-buffered saline (PBS) at pH 8 for 1 min to obtain the test solution. Fluorescence signals were collected at an excitation wavelength of 340 nm, a slit width of 10 nm, and a voltage of 900 V, and denoted as F. The test was performed in triplicate. The fluorescence intensity of the obtained mixed solution was measured, and the fluorescence intensity was substituted into the linear curve to obtain the Cr(VI) concentrations in the test solutions as 5.1, 10.3, and 31.6 μmol / L, respectively. The recoveries of the tap water samples were 101%, 103%, and 105%, respectively, with RSDs of 1.5%, 0.89%, and 0.32%, respectively. This demonstrates that the prepared CDs@NH2-MIL-101(Cr) fluorescent probe can be applied to the detection of Cr(VI) content in water.

[0077] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. The application of a metal-organic framework-based fluorescent probe in the detection of hexavalent chromium ions in water, characterized in that... The aforementioned metal-organic framework-based fluorescent probe is CDs@NH2-MIL-101(Cr); The aforementioned metal-organic framework-based fluorescent probe is prepared by the following steps: (1) Add Cr(NO3)3·9H2O, NH2-H2BDC and NaOH to water and mix evenly. Then transfer the mixture to a reaction vessel to carry out a hydrothermal reaction. (2) After the hydrothermal reaction is completed, the reaction vessel is cooled to room temperature. The product is washed and then transferred to another reaction vessel with ethanol or DMF and heated for reaction. After the reaction is completed, the green powder NH2-MIL-101(Cr) is obtained by purification. (3) The green powder NH2-MIL-101(Cr) was immersed in an ethanol aqueous solution containing a carbon source so that the carbon source was evenly distributed in the pores of NH2-MIL-101(Cr) to obtain the composite. (4) The composite was calcined in a tube furnace, and the resulting solid was CDs@NH2-MIL-101(Cr); The carbon source mentioned in step (3) is glucose.

2. The application of the metal-organic framework-based fluorescent probe according to claim 1 in the detection of hexavalent chromium ions in water, characterized in that: The mass ratio of Cr(NO3)3·9H2O, NH2-H2BDC and NaOH mentioned in step (1) is 1:0.43-0.46:0.24-0.26; The amount of water used in step (1) is such that the concentration of Cr(NO3)3·9H2O in the mixture is 51.00-55.00 mg / mL; The hydrothermal reaction mentioned in step (1) refers to heating to 148-152℃ and reacting for 11.5-12.5h.

3. The application of the metal-organic framework-based fluorescent probe according to claim 1 in the detection of hexavalent chromium ions in water, characterized in that: The washing described in step (2) is performed with ethanol or DMF; The heating reaction mentioned in step (2) refers to heating to 98-102℃ and reacting for 23.5-24.5h.

4. The application of the metal-organic framework-based fluorescent probe according to claim 1 in the detection of hexavalent chromium ions in water, characterized in that: The concentration of carbon source in the ethanol aqueous solution containing carbon source in step (3) is 0.98-1.02 mmol / L; the volume ratio of ethanol to water in the ethanol aqueous solution is 8.8-1:9.2-1; the amount of green powder used in step (3) satisfies the following: the mass ratio of green powder to carbon source is 0.1-1.5:0.1-1.8; The soaking time described in step (3) is 6-24 hours; After soaking in step (3), the soaked product is centrifuged to obtain a precipitate, which is then washed with ethanol and centrifuged again. The resulting solid is dried to obtain the complex.

5. The application of the metal-organic framework-based fluorescent probe according to claim 1 in the detection of hexavalent chromium ions in water, characterized in that: The calcination mentioned in step (4) refers to calcination under an inert atmosphere, with a calcination temperature of 200-250 ℃ and a calcination time of 2-4 h.

6. A method for detecting hexavalent chromium ions in water, characterized in that... Includes the following steps: 1) The metal-organic framework-based fluorescent probe CDs@NH2-MIL-101(Cr) was dissolved in water to prepare a CDs@NH2-MIL-101(Cr) stock solution. Then, different concentrations of Cr(VI) solution were added to the stock solution, and a buffer solution was added to adjust the pH of the mixture to 7.8-8.2 to obtain a variety of mixtures with different Cr(VI) concentrations. The fluorescence intensity F of the mixtures with different Cr(VI) concentrations was obtained by fluorescence testing. 2) Replace the Cr(VI) solutions of different concentrations in step 1) with an equal volume of water, and keep everything else the same as in step 1). Perform fluorescence testing on the mixture to obtain the fluorescence intensity F0 of the mixture without Cr(VI) concentration; then, plot F / F0-1 as the ordinate and Cr(VI) concentration as the abscissa to obtain a linear relationship curve. 3) Based on step 1), replace the Cr(VI) solution added in step 1) with an equal volume of water to be tested, and then repeat step 1). Calculate the corresponding ordinate F / F0-1 of the obtained fluorescence intensity, and then substitute it into the linear relationship curve in step 2). Convert the corresponding concentration to obtain the Cr(VI) concentration in the water to be tested. The aforementioned metal-organic framework-based fluorescent probe CDs@NH2-MIL-101(Cr) was prepared by the following steps: (1) Add Cr(NO3)3·9H2O, NH2-H2BDC and NaOH to water and mix evenly. Then transfer the mixture to a reaction vessel to carry out a hydrothermal reaction. (2) After the hydrothermal reaction is completed, the reaction vessel is cooled to room temperature. The product is washed and then transferred to another reaction vessel with ethanol or DMF and heated for reaction. After the reaction is completed, the green powder NH2-MIL-101(Cr) is obtained by purification. (3) The green powder NH2-MIL-101(Cr) was immersed in an ethanol aqueous solution containing a carbon source so that the carbon source was evenly distributed in the pores of NH2-MIL-101(Cr) to obtain the composite. (4) The composite was calcined in a tube furnace, and the resulting solid was CDs@NH2-MIL-101(Cr); The carbon source mentioned in step (3) is glucose.

7. The method for detecting hexavalent chromium ions in water according to claim 6, characterized in that: The buffer solution mentioned in step 1) is a phosphate buffer solution with a pH of 8; The concentration of the CDs@NH2-MIL-101(Cr) stock solution mentioned in step 1) is 25-400 mg / L; The concentration of the Cr(VI) solution mentioned in step 1) is 1-200 μmol / L; The pH value of the mixture described in step 1) is 8.0; In step 1), the fluorescence test conditions are: excitation wavelength 340 nm, slit width 10 nm, and 900 V.

8. The method for detecting hexavalent chromium ions in water according to claim 6, characterized in that: In step 2), the linear relationship curve has two linear ranges within the range of 1-150 μmol / L, namely 1-20 μmol / L and 20-150 μmol / L.

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