Ratio fluorescent composite material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of ratiometric fluorescent probe detection, and particularly discloses a ratiometric fluorescent composite material as well as a preparation method and application thereof. The ratiometric fluorescent composite material comprises a copper nanocluster CuNCs and a europium metal organic framework Eu-MOFs, wherein the europium metal organic framework is coated on the surface of the copper nanocluster; a ligand of the europium metal organic framework is a tricarboxyl aromatic ligand. The fluorescent probe material provided by the invention has excellent luminescence properties of CuNCs and Eu-MOFs, CuNCs and Eu-MOFs can be respectively used as a reference unit and a recognition unit for ratio fluorescence detection of Hg < 2 + >, and the fluorescent probe material shows good structural stability and fluorescence stability in an aqueous solution, and has good application prospects. The intensity ratio of the two fluorescence emission peak values and the concentration of Hg < 2 + > present a good linear relationship in a range of 0-360 nM, and the sensitivity is high, so that a new idea and a new method are provided for detection of Hg < 2 + >.

Description

Technical Field

[0001] The present invention relates to the technical field of ratio fluorescence probe detection, and in particular to a ratio fluorescence composite material, a preparation method thereof, and an application thereof. Background Art

[0002] Mercury (Hg) is a highly toxic heavy metal element, which mainly exists in three chemical forms in the human activity environment: elemental mercury (Hg 0 ), organic mercury compounds (such as CH3Hg + , C2H5Hg + etc.), and inorganic mercury compounds (such as Hg 2+ , Hg + etc.). Under natural conditions, Hg 2+ is difficult to be biodegraded, and will continuously cycle, migrate and accumulate in environmental media such as the atmosphere, water bodies and soil, so that Hg 2+ can exist in the environment for a long time, and enter organisms through the food chain or respiratory pathway. Hg 2+ has persistence, low density, high toxicity, carcinogenicity and bioaccumulation. Even at relatively low concentrations, Hg 2+ will pose a serious threat to the health of organisms. Exposure to mercury can cause permanent damage to the main organs of the human body such as the liver, kidneys and lungs, and lead to disorders of the nervous and immune systems, and even induce serious diseases, such as Minamata disease, acrodynia (pink disease), Alzheimer's disease, etc. Therefore, it is crucial to develop new methods to detect Hg 2+ simply, with high sensitivity and high selectivity.

[0003] Currently, the traditional analytical methods for detecting Hg 2+ mainly include atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma mass spectrometry (ICP-MS) and high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) techniques. These methods have high sensitivity and low detection limits, but they usually require complex sample pretreatment and high instrument maintenance costs, while the repeatability of the emerging electrochemical method needs to be improved. Fluorescent probes have many advantages, such as diverse materials, low cost, rapid response, easy operation, high sensitivity, etc., and have gradually become a more attractive alternative method. Currently, most fluorescent sensors mainly rely on the change in the fluorescence intensity of a single color to indicate the presence of Hg 2+ . This single-color fluorescence detection method is extremely vulnerable to external environmental factors, thus affecting the accuracy and reliability of detection. Therefore, designing a simple and easily available ratio fluorescence probe for visual and efficient detection of Hg 2+ has become an urgent need in current research. Summary of the Invention

[0004] For the detection of Hg 2+ which has problems such as complex detection methods, weak anti-interference ability, and low sensitivity, the present invention provides a ratio fluorescence composite material, a preparation method thereof, and an application. Through the design of the ratio fluorescence composite material, ultrafast fluorescence quantitative detection of Hg 2+ is realized, improving the efficiency, selectivity, and sensitivity of the existing Hg 2+ detection.

[0005] To solve the above technical problems, the technical solution provided by the present invention is as follows: In a first aspect, the present invention provides a ratio fluorescence composite material, including copper nanoclusters and europium metal-organic framework, and the europium metal-organic framework is coated on the surface of the copper nanoclusters; The ligand of the europium metal-organic framework is a tricarboxyl aromatic ligand.

[0006] Compared with the prior art, the ratio fluorescence composite material provided by the present invention (denoted as CuNCs@Eu-MOFs) not only combines the excellent luminescence properties of copper nanoclusters (CuNCs) and europium metal-organic framework (Eu-MOFs), but also CuNCs and Eu-MOFs can be used as a reference unit and an identification unit respectively for ratio fluorescence detection of Hg 2+ It also shows good structural stability and fluorescence stability in aqueous solution, and shows good application prospects in the detection of actual sample Hg 2+ providing new ideas and methods for the detection of Hg 2+ .

[0007] In the present invention, the tricarboxylic acid structure in the tricarboxyl aromatic ligand can coordinate with both Eu 3+ and the surface sites of CuNCs, bridging Eu 3+ in a μ2 or μ3 manner to form a stable three-dimensional network structure, encapsulating CuNCs in the pores of the metal-organic framework (MOF), providing more binding sites with Hg 2+ ; the tricarboxyl aromatic ligand has a rigid aromatic structure, and the rigid structure of the benzene ring can prevent the collapse of the MOF, avoiding the loose structure of the MOF caused by flexible chain ligands (such as EDTA), maintaining the porosity of the material, and being beneficial to subsequent applications (such as sensing, catalysis).

[0008] Experimental results show that CuNCs@Eu-MOFs has two obvious characteristic fluorescence emission peaks at λem = 618 nm and λem = 450 nm, and can specifically recognize Hg 2+ . When Hg 2+After adding the CuNCs@Eu-MOFs system, the fluorescence emission peak at λem = 450 nm increases, while the fluorescence emission peak at λem = 618 nm decreases. The intensity ratio of the two fluorescence emission peaks shows a good linear relationship with the concentration of Hg 2+ in the range of 0 - 360 nM (R = 0.9986), and the detection limit is 0.94 nM. Common metal ions have no obvious response to the CuNCs@Eu-MOFs detection system. When the ratiometric fluorescence composite material provided by the present invention is applied to the content analysis of Hg 2+ in river water and the aqueous extract of traditional Chinese medicine Atractylodes lancea, the spiked recovery rate is between 95.04% and 107.61%, and the relative standard deviation (RSD) is less than 4.47%. This solves the key problems of traditional Hg 2+ detection techniques, such as poor selectivity and weak anti-interference ability, and has the potential application value for visual detection of Hg 2+ in complex matrices.

[0009] Preferably, the tricarboxylic acid aromatic ligand includes 1,3,5-benzenetricarboxylic acid.

[0010] In the present invention, 1,3,5-benzenetricarboxylic acid (BTC) has symmetry (C3 symmetry, and the 3 carboxyl groups are distributed at 120°), which can form uniform pores, making the MOF pores more uniform. At the same time, the carboxyl groups of BTC can coordinate with Cu + / Cu 0 on the surface of CuNCs. Combining with the physical isolation of the pore structure, it can prevent its oxidation or aggregation and improve stability. The conjugated benzene ring of BTC can effectively absorb ultraviolet light and transfer the energy to Eu 3+ , enhancing its characteristic fluorescence emission (such as 3 D0→ 7 F2, red light around 614 nm), and the energy transfer efficiency is high. As a bridge, BTC can promote the energy transfer between CuNCs and Eu 3+ , forming a dual-emission fluorescence material, which is suitable for ratiometric sensing. Through a large number of experiments, the present invention found that if BTC is replaced with other ligands, the performance of the product will decrease.

[0011] Preferably, the copper nanoclusters exhibit blue fluorescence, and the europium metal-organic framework exhibits red fluorescence.

[0012] Preferably, the ratiometric fluorescence composite material is in the shape of crystal rods, with a diameter of 10 nm - 180 nm and a length of 1 μm - 30 μm.

[0013] In the second aspect, the present invention provides a preparation method of the ratiometric fluorescence composite material, including the following steps: S1. Mix the copper salt solution with the L-cysteine solution, conduct a pre-coordination reaction, adjust the pH of the system to slightly alkaline, and carry out a reduction reaction at 50 °C to 65 °C to obtain a copper nanocluster solution; S2. Add the copper nanocluster solution, europium salt, and tricarboxyl aromatic ligand into an organic solvent, conduct a coordination reaction, adjust the pH of the system to alkaline, and crystallize to obtain a ratiometric fluorescence composite material.

[0014] The preparation method of the ratiometric fluorescence composite material provided by the present invention involves a pre-coordination reaction between a copper salt and L-cysteine (L-Cys). Through coordination, the reduction potential of Cu 2+ is lowered, facilitating its subsequent reduction to Cu + and Cu 0 . At the same time, the uniform dispersion and stability of Cu 2+ are ensured, thereby improving the uniformity and yield of the ratiometric fluorescence composite material. Then, under the action of a strong alkaline pH regulator, Cu 2+ is reduced to Cu + and Cu 0 , and the nucleation growth of Cu atomic clusters is promoted to form CuNCs wrapped by L-cysteine. The -SH on the surface of CuNCs can achieve specific recognition of Hg 2+ . Then, a coordination reaction is carried out between CuNCs, europium salt, and tricarboxyl aromatic ligand. The carboxyl groups coordinate with Eu 3+ and the surface of CuNCs to form a ternary precursor complex, laying a structural foundation for subsequent MOF crystallization, ensuring molecular-level mixing of reactants, and avoiding heterogeneous aggregation caused by direct addition of alkali. Finally, under the action of a weak alkaline pH regulator, Eu-MOFs are crystallized and CuNCs are coated therein to form CuNCs@Eu-MOFs.

[0015] In the present invention, L-cysteine contains a mercapto group (-SH), which can be used to reduce Cu 2+ and stabilize CuNCs. L-cysteine contains an amino group (-NH2) and a carboxyl group (-COOH), enhancing its water solubility and coordination ability. L-cysteine also has a suitable steric hindrance, which can keep the size of the nanoclusters uniform and avoid excessive aggregation.

[0016] Preferably, in S1, the copper salt solution includes at least one of a copper sulfate solution, a copper nitrate solution, or a copper chloride solution.

[0017] More preferably, in S1, the copper salt solution is a copper sulfate solution.

[0018] Preferably, in S1, the concentration of the copper salt solution is 0.8 mM to 1.2 mM.

[0019] Preferably, in S1, the concentration of the L-cysteine solution is 30 mg / mL to 40 mg / mL.

[0020] Preferably, in S1, the volume ratio of the copper salt solution to the L-cysteine solution is 1:(8 - 12).

[0021] Preferably, in S1, the temperature of the pre-coordination reaction is 0°C to 40°C, and the time of the pre-coordination reaction is 9 min to 12 min.

[0022] Preferably, in S1, the slightly alkaline to neutral refers to pH = 7 - 7.5.

[0023] Preferably, in S1, a strong alkaline pH regulator is used to adjust the pH of the system to slightly alkaline to neutral, and the strong alkaline pH regulator includes at least one of an inorganic strong base solution or an organic strong base solution.

[0024] Further preferably, in S1, the inorganic strong base solution includes at least one of a sodium hydroxide solution, a potassium hydroxide solution, or a lithium hydroxide solution.

[0025] Further preferably, in S1, the concentration of the inorganic strong base solution is 0.35 M to 0.45 M.

[0026] Further preferably, in S1, the organic strong base solution includes a tetramethylammonium hydroxide (TMAH) solution.

[0027] More preferably, in S1, the strong alkaline pH regulator is a 0.35 M to 0.45 M sodium hydroxide solution.

[0028] In the present invention, the strong alkaline pH regulator can provide a slightly alkaline to neutral environment. At pH = 7 - 7.5, each functional group of L-cysteine is in a suitable dissociation state, enabling it to better coordinate with copper ions to form a stable complex; meanwhile, under slightly alkaline to neutral conditions, L-cysteine is more likely to reduce Cu 2+ to Cu + or Cu 0 , and promote the nucleation of CuNCs, avoiding the formation of Cu(OH)2 precipitate by Cu 2+ under alkaline pH.

[0029] Preferably, in S1, the time of the reduction reaction is 4 h to 5 h.

[0030] Exemplarily, in S1, after the reduction reaction, it further includes: filtration and dialysis to obtain a copper nanocluster solution. Filtration is carried out using a 0.22 μm filter membrane, dialysis is carried out using a dialysis membrane with a molecular weight cut-off of 1000 MWCO, and the dialysis time is 22 h to 26 h. Filtration can remove excess impurities in the reaction solution, and dialysis can remove unreacted small molecules.

[0031] Preferably, in S2, the europium salt includes at least one of europium chloride, europium nitrate, europium acetate or europium sulfate.

[0032] More preferably, in S2, the europium salt is europium chloride.

[0033] Preferably, in S2, the organic solvent includes at least one of methanol, ethanol, isopropanol, DMF, acetonitrile or acetone.

[0034] More preferably, in S2, the organic solvent is methanol.

[0035] In the present invention, the medium-polar organic solvent has good solubility, helps the coordination reaction to proceed, and at the same time avoids the too-fast precipitation or loose structure of MOF caused by strong-polar solvents (such as water). Methanol can weakly bind to Eu 3+ through -OH without interfering with the main coordination of BTC.

[0036] Preferably, in S2, the dosage ratio of the copper nanocluster solution, the europium salt, the tricarboxyl aromatic ligand to the organic solvent is (0.8~1.2) mL: 0.1 mmol: (18~24) mg: (8~12) mL.

[0037] Preferably, in S2, the temperature of the coordination reaction is 0 °C to 40 °C, and the time of the coordination reaction is 25 min to 45 min.

[0038] Preferably, in S2, the alkalinity refers to pH = 9~11.

[0039] Preferably, in S2, a weak alkaline pH regulator is used to adjust the pH of the system to alkaline. The weak alkaline pH regulator includes at least one of triethylamine, triethanolamine or pyridine.

[0040] More preferably, in S2, the weak alkaline pH regulator is triethylamine.

[0041] In the present invention, the weak alkaline pH regulator can neutralize the carboxyl group of BTC to completely deprotonate it (-COOH → -COO - ), promote the more complete coordination reaction between Eu 3+ and BTC 3- , and avoid the hindrance of MOF crystallization under acidic conditions; the N atom of the weak alkaline pH regulator can weakly coordinate with Eu 3+ , temporarily occupy the coordination site, prevent the too-fast precipitation of MOF, thereby controlling the crystallization rate, and optimize the pore structure of MOF through steric hindrance effect; in addition, the organic base can be miscible with the organic solvent to ensure the homogeneity of the reaction.

[0042] Preferably, in S2, the crystallization temperature is 0°C to 40°C, and the crystallization time is 1.8 h to 2.5 h.

[0043] Exemplarily, after crystallization in S2, it further includes: solid-liquid separation, washing, and drying to obtain the ratio fluorescence composite material. Centrifugation is used for solid-liquid separation, the centrifugation speed is 8000 rpm, and the centrifugation time is 10 min; ethanol is used for washing, and vacuum drying is carried out at 40°C to 50°C.

[0044] In a third aspect, the present invention provides an application of the ratio fluorescence composite material in the detection of Hg by dual-emission ratio fluorescence method 2+ therein.

[0045] Preferably, the detection result can be presented on a smartphone through a fluorescence color photo and a color recognition APP.

[0046] The ratio fluorescence composite material of the present invention can detect Hg through a smartphone platform 2+ , and utilize the portability, image processing ability, and wide popularity of the smartphone to achieve the purpose of low-cost and rapid detection of mercury ions, and the operation is simple, and it can realize on-site visual real-time monitoring of Hg in samples such as water bodies and foods 2+ . Description of the Drawings

[0047] Figure 1 is the SEM image of CuNCs in Example 1 of the present invention; Figure 2 is the TEM image of CuNCs in Example 1 of the present invention; Figure 3 is the SEM image of Eu-MOFs in Comparative Example 1 of the present invention; Figure 4 is the TEM image of Eu-MOFs in Comparative Example 1 of the present invention; Figure 5 is the SEM image of CuNCs@Eu-MOFs in Example 1 of the present invention; Figure 6 is the diameter size distribution diagram of CuNCs@Eu-MOFs in Example 1 of the present invention; Figure 7EDS mapping diagrams corresponding to CuNCs@Eu-MOFs in Example 1 of the present invention; wherein, Figure a shows the SEM image of CuNCs@Eu-MOFs, Figure b shows the EDS mapping diagrams of all elements corresponding to Figure a, Figure c shows the EDS mapping diagram of C element corresponding to Figure a, Figure d shows the EDS mapping diagram of N element corresponding to Figure a, Figure e shows the EDS mapping diagram of O element corresponding to Figure a, Figure f shows the EDS mapping diagram of S element corresponding to Figure a, Figure g shows the EDS mapping diagram of Eu element corresponding to Figure a, and Figure h shows the EDS mapping diagram of Cu element corresponding to Figure a; Figure 8 XRD diagrams of CuNCs@Eu-MOFs, CuNCs in Example 1 of the present invention, and Eu-MOFs in Comparative Example 1; Figure 9 FT-IR spectra of CuNCs@Eu-MOFs, CuNCs in Example 1 of the present invention, BTC, L-Cys, and Eu-MOFs in Comparative Example 1; Figure 10 Emission spectra of CuNCs@Eu-MOFs in Example 1 of the present invention at different excitation wavelengths; wherein, Figures a - d respectively show the emission spectra at the corresponding excitation wavelengths in the figure; Figure 11 Fluorescence spectra of CuNCs@Eu-MOFs, CuSO4, CuNCs in Example 1, EuCl3, Eu-MOFs in Comparative Example 1, L-Cys, and BTC in Example 1 of the present invention at an excitation wavelength λex = 360 nm; Figure 12 Fluorescence spectra of CuNCs@Eu-MOFs, CuSO4, CuNCs in Example 1, EuCl3, Eu-MOFs in Comparative Example 1, L-Cys, and BTC in Example 1 of the present invention at an excitation wavelength λex = 270 nm; Figure 13 For CuNCs@Eu-MOFs in Example 1 of the present invention and Eu-MOFs in Comparative Example 1 in the presence and absence of Hg 2+ Fluorescence spectra at an excitation wavelength λex = 350 nm; Figure 14 For CuNCs@Eu-MOFs in Example 1 of the present invention and Eu-MOFs in Comparative Example 1 in the presence and absence of Hg 2+ Fluorescence spectra at an excitation wavelength λex = 250 nm; Figure 15 Comparison diagram of the detection performance differences of CuNCs@Eu-MOFs (BTC) in Example 1 of the present invention and CuNCs@Eu-MOFs (DPA) in Comparative Example 2 for Hg 2+ ; Figure 16 For verification example 6 of the present invention, at different Hg 2+ detection concentrations, the fluorescence emission spectrum of CuNCs@Eu-MOFs when the excitation wavelength λex = 350 nm; Figure 17 For verification example 6 of the present invention, at different Hg 2+ detection concentrations, the fluorescence emission spectrum of CuNCs@Eu-MOFs when the excitation wavelength λex = 250 nm; Figure 18 For verification example 6 of the present invention, the fluorescence intensity ratio (F 450 / F 618 ) and the linear relationship diagram of the Hg 2+ concentration; Figure 19 For verification example 6 of the present invention, the CIE chromaticity change diagram of the CuNCs@Eu-MOFs suspension; Figure 20 For verification example 6 of the present invention, the correlation between the fluorescence intensity of the CuNCs@Eu-MOFs suspension and the Hg 2+ concentration; Figure 21 For verification example 6 of the present invention, the detection results of the CuNCs@Eu-MOFs suspension for different metal ions; where Blank means no metal ions are added; Figure 22 For verification example 8 of the present invention, the fluorescence color images of the ratiometric fluorescence composite material at different concentrations of Hg 2+ , under 254 nm ultraviolet light, and the flow schematic diagram of obtaining the ratio function relationship between red (R), blue (B) and the Hg 2+ concentration through the Color Picker APP of the smartphone. Detailed implementation manners

[0048] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0049] All reagents in this invention are of analytical pure grade, and all solutions are prepared with ultrapure water. Europium(III) chloride hexahydrate (EuCl3·6H2O), L-cysteine (L-Cys), copper(II) sulfate pentahydrate (CuSO4·5H2O), sodium hydroxide (NaOH), 1,3,5-benzenetricarboxylic acid (BTC), mercury(II) sulfate (HgSO4), mercury(I) chloride (Hg2Cl2), magnesium chloride hexahydrate (MgCl2·6H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), and disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; sodium chloride (NaCl) was purchased from Beijing Solarbio Science & Technology Co., Ltd.; barium chloride (BaCl2) and lead(II) acetate trihydrate (Pb(CH3COO)2·3H2O) were purchased from Tianjin Biaozhun Technology Co., Ltd.; potassium hydroxide (KOH) and iron(III) chloride (FeCl3) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd.; methanol and ethanol were purchased from Fuchen (Tianjin) Chemical Reagent Co., Ltd.; acetonitrile was purchased from Thermo Fisher Scientific (China) Co., Ltd.; isopropyl alcohol was purchased from Tianjin Kermel Chemical Reagent Co., Ltd.; dialysis bags (molecular weight cut-off MWCO: 1000) were purchased from Hunan Yibo Biotechnology Co., Ltd. In this invention, materials without special instructions are all commercially available products.

[0050] Example 1 This example provides a ratiometric fluorescent composite CuNCs@Eu-MOFs, which includes copper nanoclusters CuNCs and europium metal-organic framework Eu-MOFs, and the europium metal-organic framework Eu-MOFs is coated on the surface of the copper nanoclusters CuNCs.

[0051] The preparation method of the above ratiometric fluorescent composite includes the following steps: S1. Mix 2.5 mL of 1 mM copper sulfate solution with 25 mL of 35 mg / mL L-cysteine solution, carry out a pre-coordination reaction at room temperature, after stirring for 10 min, add 0.4 M sodium hydroxide solution to adjust the pH of the system to 7.2, carry out a reduction reaction at 55 °C, keep stirring for 4.5 h, and the color of the mixed solution turns light yellow; filter with a 0.22 μm filter membrane, and dialyze with a dialysis membrane with a molecular weight cut-off of 1000 MWCO in deionized water. After dialysis for 24 h, a copper nanocluster solution is obtained.

[0052] S2. Add 1 mL of copper nanocluster solution, 36.7 mg (0.1 mmol) of europium chloride, and 21 mg of BTC into 10 mL of organic solvent (methanol), conduct a coordination reaction at room temperature. After stirring for 30 min, add a weak alkaline pH regulator (triethylamine) to adjust the pH of the system to 9.8, let it stand for crystallization at room temperature for 2 h, perform solid-liquid separation, washing, and drying to obtain the ratiometric fluorescence composite material CuNCs@Eu-MOFs.

[0053] Example 2 This example provides a ratiometric fluorescence composite material CuNCs@Eu-MOFs, which includes copper nanoclusters CuNCs and europium metal-organic framework Eu-MOFs. The europium metal-organic framework Eu-MOFs is coated on the surface of the copper nanoclusters CuNCs.

[0054] The preparation method of the above ratiometric fluorescence composite material includes the following steps: S1. Mix 2.5 mL of 0.8 mM copper sulfate solution with 20 mL of 40 mg / mL L-cysteine solution, conduct a pre-coordination reaction at 0 °C to 40 °C. After stirring for 9 min, add 0.35 M sodium hydroxide solution to adjust the pH of the system to 7, conduct a reduction reaction at 50 °C, keep stirring for 5 h, and the color of the mixed solution turns light yellow; filter with a 0.22 μm filter membrane, and perform dialysis in deionized water using a dialysis membrane with a molecular weight cut-off of 1000 MWCO. After dialysis for 22 h, obtain the copper nanocluster solution.

[0055] S2. Add 0.8 mL of copper nanocluster solution, 36.7 mg (0.1 mmol) of europium chloride, and 18 mg of BTC into 8 mL of organic solvent (ethanol). The volume ratio of the copper nanocluster solution to the organic solvent is 1:8. Conduct a coordination reaction at room temperature. After stirring for 25 min, add a weak alkaline pH regulator (triethanolamine) to adjust the pH of the system to 11, let it stand for crystallization at 0 °C to 40 °C for 2.5 h, perform solid-liquid separation, washing, and drying to obtain the ratiometric fluorescence composite material CuNCs@Eu-MOFs.

[0056] Example 3 This example provides a ratiometric fluorescence composite material CuNCs@Eu-MOFs, which includes copper nanoclusters CuNCs and europium metal-organic framework Eu-MOFs. The europium metal-organic framework Eu-MOFs is coated on the surface of the copper nanoclusters CuNCs.

[0057] The preparation method of the above ratiometric fluorescence composite material includes the following steps: S1. Mix 2.5 mL of 1.2 mM copper sulfate solution with 30 mL of 30 mg / mL L-cysteine solution, carry out a pre-coordination reaction at 0 °C to 40 °C. After stirring for 12 min, add 0.45 M sodium hydroxide solution to adjust the pH of the system to 7.5, and carry out a reduction reaction at 65 °C. After keeping warm and stirring for 4 h, the color of the mixed solution turns light yellow. Filter with a 0.22 μm filter membrane, and carry out dialysis in deionized water using a dialysis membrane with a molecular weight cut-off of 1000 MWCO. After dialysis for 26 h, a copper nanocluster solution is obtained.

[0058] S2. Add 1.2 mL of copper nanocluster solution, 36.7 mg (0.1 mmol) of europium chloride, and 24 mg of BTC to 12 mL of an organic solvent (isopropanol). The volume ratio of the copper nanocluster solution to the organic solvent is 1:12. Carry out a coordination reaction at room temperature. After stirring for 45 min, add a weak alkaline pH regulator (pyridine) to adjust the pH of the system to 9, and let it stand for crystallization at 0 °C to 40 °C for 2.2 h. Separate the solid and liquid, wash, and dry to obtain the ratio fluorescence composite material CuNCs@Eu-MOFs.

[0059] Example 4 This example provides a ratio fluorescence composite material CuNCs@Eu-MOFs, which includes copper nanoclusters CuNCs and europium metal-organic framework Eu-MOFs. The europium metal-organic framework Eu-MOFs is coated on the surface of the copper nanoclusters CuNCs.

[0060] The preparation method of the above ratio fluorescence composite material includes the following steps: S1. Mix 2.5 mL of 1 mM copper sulfate solution with 25 mL of 35 mg / mL L-cysteine solution, carry out a pre-coordination reaction at 0 °C to 40 °C. After stirring for 11 min, add 0.4 M sodium hydroxide solution to adjust the pH of the system to 7.4, and carry out a reduction reaction at 60 °C. After keeping warm and stirring for 4.5 h, the color of the mixed solution turns light yellow. Filter with a 0.22 μm filter membrane, and carry out dialysis in deionized water using a dialysis membrane with a molecular weight cut-off of 1000 MWCO. After dialysis for 24 h, a copper nanocluster solution is obtained.

[0061] S2. Add 1 mL of copper nanocluster solution, 36.7 mg (0.1 mmol) of europium chloride, and 22 mg of BTC to 10 mL of an organic solvent (acetone). Carry out a coordination reaction at 0 °C to 40 °C. After stirring for 25 min, add a weak alkaline pH regulator (triethylamine) to adjust the pH of the system to 10, and let it stand for crystallization at room temperature for 1.8 h. Separate the solid and liquid, wash, and dry to obtain the ratio fluorescence composite material CuNCs@Eu-MOFs.

[0062] Comparative Example 1 This comparative example provides a europium metal-organic framework Eu-MOFs, and its preparation method includes the following steps: Add 36.7 mg (0.1 mmol) of europium chloride and 21 mg of BTC to 10 mL of methanol, carry out a coordination reaction at room temperature. After stirring for 30 min, add a weakly basic pH regulator (triethylamine) to adjust the pH of the system to 10, let it stand for crystallization at room temperature for 2 h, carry out solid-liquid separation, washing, and drying to obtain the europium metal-organic framework Eu-MOFs.

[0063] Comparative Example 2 This comparative example provides a ratiometric fluorescence composite material CuNCs@Eu-MOFs, which is similar to Example 1, except that: BTC is replaced with 2,6-pyridinedicarboxylic acid (DPA) of the same mass. The other conditions are the same as those in Example 1 and will not be elaborated here.

[0064] Verification Example 1 1. In order to observe the microscopic morphology and size of CuNCs@Eu-MOFs, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize and analyze CuNCs@Eu-MOFs, CuNCs of Example 1, and Eu-MOFs of Comparative Example 1, and the results are as Figures 1 to 6 shown.

[0065] It can be seen from Figure 1 that CuNCs present an irregular nanoparticle morphology and are effectively encapsulated by L-Cys. Figure 2 This further confirms that CuNCs have good dispersibility in solution and show a uniform distribution. It can be seen from Figures 3 to 6 that there are obvious differences in morphology between Eu-MOFs and CuNCs@Eu-MOFs: Eu-MOFs show a typical rod-like crystal structure, while CuNCs@Eu-MOFs present a grass-like crystal structure, with a diameter of 10 nm to 180 nm and a length of 1 μm to 30 μm.

[0066] 2. Use energy dispersive spectroscopy (EDS) to analyze the elemental composition of CuNCs@Eu-MOFs of Example 1, and the results are as Figure 7 shown. The N, S, and Cu elements detected in the EDS spectrum confirm that CuNCs are successfully encapsulated in the CuNCs@Eu-MOFs structure.

[0067] 3. To further understand the crystal structure characteristics of CuNCs@Eu-MOFs, CuNCs in Example 1, and Eu-MOFs in Comparative Example 1, X-ray diffraction (XRD) was used for characterization analysis. Meanwhile, Fourier transform infrared spectroscopy (FT-IR) was used to characterize CuNCs@Eu-MOFs, CuNCs, BTC, L-Cys in Example 1, and Eu-MOFs in Comparative Example 1. The results are as Figures 8 to 9 shown.

[0068] As can be seen from Figure 8 , CuNCs exhibit several distinct sharp diffraction peaks, indicating its high crystallinity. In contrast, the diffraction peak intensity of Eu-MOFs appears weaker, and its pattern is smoother. This difference reveals the difference in crystal structure between Eu-MOFs and CuNCs. The crystallinity of Eu-MOFs is relatively low, which is due to the complex structural characteristics of metal-organic frameworks (MOFs) or because its crystal structure is relatively disordered. The XRD pattern of CuNCs@Eu-MOFs is similar to that of Eu-MOFs and is even smoother, without obvious characteristic peaks of CuNCs. The characteristics of CuNCs@Eu-MOFs are closer to Eu-MOFs rather than CuNCs, indicating that CuNCs may be effectively encapsulated by Eu-MOFs, resulting in the masking of its characteristic peaks in the XRD pattern. On the other hand, CuNCs are relatively dispersed in the pores of Eu-MOFs, reducing the overall crystallinity.

[0069] As can be seen from Figure 9 , in the spectra of L-Cys and CuNCs, the broad absorption peak at 3300 cm -1 ~3400 cm -1 is attributed to the stretching vibration of hydroxyl (-OH) or amine (-NH). The absorption peak of L-Cys at 2551.82 cm -1 corresponds to the stretching vibration of -SH, and this peak disappears in CuNCs, indicating that -SH forms a Cu-S bond with Cu atoms. In the spectrum of BTC, the strong absorption peak at 1628 cm -1 corresponds to the stretching vibration of C=O, the region of 1500 cm -1 ~1600 cm -1 is the vibration of the aromatic ring C=C skeleton, 1282 cm -1 is the stretching vibration of C-O, and the peak around 1000 cm -1 is caused by the bending vibration of C-H. Due to the coordination of carboxylic acid groups with Eu 3+ , the peaks at 1628 cm -1 and 1282 cm -1 disappear, and 1552 cm-1 The appearance of the peak indicates that Eu-MOFs are formed by the coordination of Eu 3+ and carboxyl groups. In the region of 500 cm -1 ~600 cm -1 , the vibration peak of Eu-O appears, which does not appear in the spectrum of BTC, further indicating that Eu 3+ coordinates with the oxygen atoms of the carboxylic acid groups. In the spectrum of CuNCs@Eu-MOFs, the C=O stretching vibration peak at 1600 cm -1 is similar to that of Eu-MOFs, but there is a slight shift. In the region of 500 cm -1 ~600 cm -1 , not only the vibration of the Eu-O bond is observed, but also the characteristics of the Cu-S bond are included. Through infrared spectroscopy analysis, it can be seen that CuNCs@Eu-MOFs retains the main structural characteristics of Eu-MOFs, and in the low wavenumber region, due to the addition of copper nanoclusters, the vibration characteristics of Cu-S or Cu-O are shown, and these characteristics further enhance the properties of the composite material (CuNCs@Eu-MOFs).

[0070] Verification Example 2 1. To verify the luminescence properties of CuNCs@Eu-MOFs, the emission spectra of CuNCs@Eu-MOFs in Example 1 were tested at different excitation wavelengths, and the test results are as Figure 10 shown.

[0071] From Figure 10 , it can be seen that under the excitation of λex = 350 nm, the fluorescence intensity of CuNCs@Eu-MOFs is the largest; under the excitation of λex = 250 nm, CuNCs@Eu-MOFs shows 3 characteristic peaks, corresponding to the 3+ 5 D0→ 7 F1, 5 D0→ 7 F2, 5 D0→ 7 F4 transitions; under the excitation of λem = 620 nm, the excitation spectrum of CuNCs@Eu-MOFs mainly shows a peak at 250 nm; at different excitation wavelengths, the fluorescence emission intensity of CuNCs@Eu-MOFs has obvious changes, and under the excitation of 250 nm, the fluorescence emission intensity is the largest.

[0072] 2. To verify the fluorescence properties of each material, the present invention analyzed the fluorescence spectra of CuNCs@Eu-MOFs in Example 1, CuSO4, CuNCs in Example 1, EuCl3, Eu-MOFs in Comparative Example 1, L-Cys and BTC, and the results are as Figures 11 to 12 ​as shown

[0073] As can be seen from Figures 11 to 12 it that before being assembled into the composite material, CuNCs exhibit strong blue fluorescence with an emission peak at 460 nm; Eu-MOFs show characteristic red fluorescence due to the antenna effect, with an emission peak at 618 nm; when CuNCs are encapsulated inside Eu-MOFs to form CuNCs@Eu-MOFs, the fluorescence intensity decreases. In the spectrum of CuNCs@Eu-MOFs, in addition to the strong emission peak of CuNCs (450 nm), three strong characteristic emission peaks located at 590 nm, 619 nm, and 696 nm respectively appear, and these peaks belong to the characteristic emission of Eu 3+ . It should be noted that the high-intensity fluorescence at 450 nm and 618 nm does not come from the simple mixture of CuSO4, L-Cys or EuCl3, BTC, but the characteristics of CuNCs and Eu-MOFs themselves.

[0074] In Figure 10 the fluorescence spectrum of c, when the excitation wavelength is λex = 350 nm, CuNCs@Eu-MOFs shows a weak emission peak at 450 nm, indicating that the strong emission peak originally at 460 nm ( Figure 11 the emission peak of CuNCs at λex = 360 nm in

[0075] Verification Example 3 Accurately weigh 4 mg of CuNCs@Eu-MOFs of Example 1 and Eu-MOFs of Comparative Example 1, disperse them as samples in 4 mL of deionized water, and ultrasonically treat for 10 min to ensure that the samples form a uniform suspension with a concentration of 1 mg / mL; then, in a clean quartz cuvette, add 400 μL of the above-prepared CuNCs@Eu-MOFs suspension and 600 μL of deionized water, and mix well. Record the fluorescence emission spectra at λex = 350 nm and λex = 250 nm respectively, and then add 10 μL of 50 mM Hg 2+ aqueous solution, record the fluorescence emission spectrum, and set the widths of the excitation slit and the emission slit to 10 nm. The test results are as Figures 13 to 14 shown

[0076] As can be seen from Figures 13 to 14 it that CuNCs@Eu-MOFs simultaneously exhibit CuNCs and Eu 3+The emission characteristics of CuNCs are significantly reduced after encapsulation in Eu-MOFs, which is due to aggregation-induced quenching, ligand effects, or energy transfer. The addition of mercury ions leads to an enhancement of the characteristic peak of CuNCs@Eu-MOFs, which may be caused by the interaction between mercury ions and CuNCs, competition with Eu-MOFs, or the introduction of new energy transfer pathways. These interactions change the local environment of Eu-MOFs and the optical properties of CuNCs, thus enabling the detection of mercury ions.

[0077] To visually demonstrate the fluorescent properties of CuNCs@Eu-MOFs, the researchers also observed the color changes of the suspension under different UV light conditions. Under 365nm UV light, the suspension appeared blue, while under 254nm UV light, it exhibited a bright red color. This further confirmed the successful encapsulation of CuNCs within the Eu-MOF cavities.

[0078] Verification Example 4 To verify the difference in mercury ion detection performance between CuNCs@Eu-MOFs (BTC) of Example 1 and CuNCs@Eu-MOFs (DPA) of Comparative Example 2, 4 mg of CuNCs@Eu-MOFs (BTC) of Example 1 and CuNCs@Eu-MOFs (DPA) of Comparative Example 2 were accurately weighed and dispersed in 4 mL of deionized water as samples. Ultrasonic treatment was performed for 10 minutes to ensure that the sample formed a uniform suspension with a concentration of 1 mg / mL. Then, 400 μL of the above-prepared CuNCs@Eu-MOFs suspension and 600 μL of water were added to a clean quartz cuvette and mixed. Fluorescence emission spectra were measured at λex = 350 nm and λex = 250 nm, respectively. 10 μL of 10 mM Hg 2+ The fluorescence emission spectrum of aqueous solution was recorded, the width of the excitation slit and the emission slit were set to 10nm, and the fluorescence emission spectrum of the two was calculated after adding Hg 2+ The fluorescence ratio change before and after (F 450 / F 618 ), the results are as follows Figure 15 shown.

[0079] from Figure 15 It can be seen that the F of Example 1 450 / F 618 Ratio when adding Hg 2+ The results show that the ratio fluorescence composite material constructed by BTC ligand has a significant effect on Hg 2+ Has higher response sensitivity.

[0080] Verification Example 5 In order to evaluate the feasibility of the material as a fluorescent probe in aqueous media, Hg2+ Detection system. In this invention, the luminescence and structural stability of CuNCs@Eu-MOFs were studied under different solvents, concentrations, pH values, temperatures, and storage times.

[0081] The results showed that the ratio fluorescence intensity (F 450 / F 618 ) was larger under neutral conditions, and CuNCs@Eu-MOFs was suitable for neutral environments; the dual emission intensity was closely related to the addition amount of CuNCs. When the molar ratio of CuNCs to EuCl3 was 1:(35 - 45), the intensity ratio of CuNCs to Eu-MOFs (F 450 / F 618 ) reached the maximum value. With the increase in the concentration of CuNCs during the synthesis process, the emission band of Eu-MOFs weakened slightly, which was due to the coordination of carboxyl groups in CuNCs with Eu 3+ centers. According to the antenna effect, the energy transfer efficiency from the initial ligand BTC to Eu 3+ was reduced, thus reducing the characteristic emission intensity of Eu 3+ .

[0082] Verification Example 6 In the CuNCs@Eu-MOFs detection system of Verification Example 3, different concentrations of Hg 2+ were added for detection, and the test results are as Figures 16 to 20 shown.

[0083] It can be seen from Figures 16 to 18 that as the concentration of Hg 2+ increased from 0 to 360 μM, the fluorescence intensity of CuNCs@Eu-MOFs at λem = 618 nm gradually decreased, while the fluorescence intensity at λem = 450 nm gradually increased, which proved that CuNCs@Eu-MOFs was very sensitive to Hg 2+ . The ratio fluorescence intensity (F 450 / F 618 ) showed a good linear relationship with the concentration of Hg 2+ . In the concentration range of 0 - 360 μM, the response value showed a linear positive correlation, and the regression equation was Y = 0.00113X + 0.08735 (R = 0.9986), where X was the concentration of Hg 2+ . In addition, according to 3σ / s and 10σ / s (σ was the standard deviation of blank samples (n = 8), s was the slope of the linear equation), the detection limit (LOD) and quantification limit (LOQ) reached 0.94 nM and 3.12 nM respectively. This indicated that this method had a wider detection range, higher sensitivity, and a wider application range, further proving the ratio fluorescence sensor based on CuNCs@Eu-MOFs for detecting Hg 2+ in aqueous solutions.has good application prospects in this aspect.

[0084] As can be seen from Figures 19 to 20 , with the increase of Hg 2+ concentration, under 254 nm UV light, CuNCs@Eu-MOFs shows obvious color changes. According to the fluorescence spectra corresponding to different Hg 2+ concentrations, the CIE chromaticity coordinates are calculated. When the Hg 2+ concentration increases from 0 μM to 360 μM, the CIE coordinates change from red (0.6253, 0.2782) to pink-purple (0.4153, 0.1615), providing a basis for the visual detection of Hg 2+ .

[0085] Verification Example 7 To investigate the selectivity of the detection system, common ions were selected to verify the detection method provided by the present invention. 10 times the Hg 2+ concentration of Mg 2+ , Cu 2+ , Al 3+ , Hg + , Na + , Fe 3 + , Ba 2+ , K + , Pb 2+ were respectively added to the CuNCs@Eu-MOFs suspension in Verification Example 3, and the test results are as Figure 21 shown.

[0086] As can be seen from Figure 21 , except for Cu 2+ , Al 3+ , Fe 3+ , other metal ions have little effect on the fluorescence intensity ratio (F 450 / F 618 ). This indicates that the CuNCs@Eu-MOFs provided by the present invention has excellent anti-interference ability against most common metal ions, effectively resists the interference of metal ions, and demonstrates its ability to accurately detect specific Hg 2+ in a complex environment through its specific fluorescence enhancement response to Hg 2+ . It should be noted that the addition of Cu 2+ , Al 3+ , Fe 3+ leads to the weakening of the characteristic fluorescence peak of CuNCs and reduces the ratio of F 450 / F 618 , which is due to the fluorescence quenching effect caused by the interaction between these metal ions and CuNCs.

[0087] Verification Example 8 When Hg 2+ After the addition of the CuNCs@Eu-MOFs system, the fluorescence emission peak at λem = 450nm increased, while the peak at λem = 618nm decreased, resulting in a gradual change in the solution's fluorescence color and a corresponding increase in the fluorescence intensity ratio. Under 254nm UV light, ratiometric fluorescence color photographs of the CuNCs@Eu-MOFs suspension were captured using a smartphone. The RGB values were then collected using the smartphone's Color Picker app. To confirm the accuracy of the results, the average B / R ratio was calculated for each image using three randomly selected locations.

[0088] The different concentrations of Hg 2+ The solution is added to the prepared CuNCs@Eu-MOFs suspension. After the reaction is complete at room temperature, the reaction solution is placed in a cuvette. The smartphone camera can take corresponding photos and automatically identify the RGB values with the help of the application. Figure 22 As shown, it exhibits red fluorescence under 254nm UV light. 2+ As the concentration increases, the fluorescence gradually changes from red to pink. Then, using the color recognition app on the mobile phone, different concentrations of Hg 2+ The color of the Hg@Eu-MOFs was collected and converted into RGB values. The results showed that in the range of 0~360μM, Hg 2+ The concentration of Hg was linearly related to its blue-red ratio (B / R) (R=0.9963), and the detection limit was 2.77 μM, which can be used for Hg 2+ Visual qualitative and semi-quantitative detection.

[0089] Verification Example 9 The spike recovery method was used to evaluate the detection performance of CuNCs@Eu-MOFs in real samples. River water and the water extract of Chinese herbal medicine Atractylodes lancea were used as real sample models to simulate the detection of Hg in a complex matrix environment. 2+ The test results are shown in Table 1.

[0090] Before the spike recovery experiment, the samples were treated as follows: (1) Preparation of river water samples: Detection of Hg in river water using the CuNCs@Eu-MOFs of Example 1 2+ The river water samples were collected from Wulie River in Chengde City. The water samples were filtered with a water filter membrane (0.22μm pore size microporous filter membrane) to remove suspended particles, and then different concentrations of Hg 2+ Solution. CuNCs@Eu-MOFs and Hg 2+Incubate it in the river water sample for 10 min, record the fluorescence spectrum under the same conditions as those for the standard solution detection, and obtain the F 450 / F 618 value. Test it three times and take the average value; and determine the content of Hg 2+ in the real sample of river water according to the obtained working curve, and repeat it three times.

[0091] (2) Preparation of traditional Chinese medicine samples: Use the CuNCs@Eu-MOFs of Example 1 to detect the content of Hg 2+ in Atractylodes lancea (purchased from Shangluo, Shaanxi) of traditional Chinese medicine. Put the dried Atractylodes lancea medicinal materials into a pulverizer for sufficient pulverization, collect the powder after passing through a 100-mesh sieve. Weigh 50 mg of Atractylodes lancea powder precisely, add 5 mL of deionized water, ultrasonicate for 10 min, then centrifuge at 10000 rpm for 10 min, and then take the supernatant and filter it with a hydrophilic filter membrane (a microporous filter membrane with a pore size of 0.22 μm). The final solution is diluted with water at a ratio of 1:100 (v / v) as the traditional Chinese medicine sample. Finally, after incubating CuNCs@Eu-MOFs and Hg 2+ at different concentrations in the traditional Chinese medicine sample for 10 min, record the fluorescence spectrum under the same conditions as those for detecting Hg 2+ in the standard solution, and repeat it three times.

[0092] Table 1 Test results of the recovery rate of CuNCs@Eu-MOFs for Hg 2+ spiked samples

[0093] As can be seen from Table 1, the recovery rate of Hg 2+ is 95.04% - 107.61%, and the relative standard deviation is relatively low (RSD = 0.37% - 4.47%). From this, it can be seen that when CuNCs@Eu-MOFs detects Hg²⁺ in complex matrices such as river water and traditional Chinese medicine Atractylodes lancea, it not only shows excellent sensitivity and accuracy, but also exhibits good anti-interference ability and practical application potential, providing a certain experimental basis for the application of CuNCs@Eu-MOFs in the fields of environmental monitoring, food safety detection, etc.

[0094] From Verification Examples 1 - 8, it can be seen that in the present invention, water-soluble CuNCs are encapsulated into Eu-MOFs through a self-assembly technique, and a novel dual-emission ratio fluorescence composite material CuNCs@Eu-MOFs is successfully developed, retaining the properties of CuNCs and Eu 3+optical properties, showing excellent structural and fluorescence stability in an aqueous solution environment. CuNCs@Eu-MOFs can effectively resist the interference of external environmental factors such as pH value changes and extended storage time, ensuring the accuracy and reliability of detection results. This property is particularly important in the field of heavy metal ion detection, especially when detecting highly toxic Hg 2+ ions. The experimental results show that the ratiometric fluorescence sensor based on CuNCs@Eu-MOFs exhibits excellent performance in Hg 2+ detection, with a wide linear range (0 - 360 μM) and a low detection limit (0.94 nM), enabling accurate detection of Hg 2+ even at low concentrations. Applying this sensor to the detection of Hg 2+ in river water and the traditional Chinese medicine Atractylodes lancea obtained good recovery rates, verifying its feasibility and effectiveness in detecting Hg 2+ in complex matrices. In addition, based on the multicolor fluorescence changes of CuNCs@Eu-MOFs in this invention, a portable visual fluorescence probe was developed. By analyzing and processing the data of a smartphone color recognition App, the correlation between the Hg 2+ concentration and the B / R value was obtained, providing a basis for the development of on-site rapid detection Hg 2+ devices. This invention not only broadens the design ideas of ratiometric fluorescence composite materials but also provides strong support for the popularization of visual sensors in multiple fields such as environmental monitoring, bioanalysis, and food safety.

[0095] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, or improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A ratio fluorescence composite material, characterized in that, It includes copper nanoclusters and europium metal-organic frameworks, and the europium metal-organic frameworks are coated on the surface of the copper nanoclusters; The ligand of the europium metal-organic framework is a tricarboxyl aromatic ligand.

2. The ratio fluorescence composite material according to claim 1, characterized in that, The tricarboxyl aromatic ligand includes 1,3,5-benzenetricarboxylic acid.

3. The ratio fluorescence composite material according to claim 1, characterized in that The ratio fluorescent composite material is in the shape of a crystal rod, with a diameter of 10 nm to 180 nm and a length of 1 μm to 30 μm.

4. The preparation method of the ratiometric fluorescence composite material according to any one of claims 1 to 3, characterized in that, It includes the following steps: S1. Mix a copper salt solution and an L-cysteine solution, conduct a pre-coordination reaction, adjust the pH of the system to slightly alkaline to neutral, and conduct a reduction reaction at 50 °C to 65 °C to obtain a copper nanocluster solution; S2. Add the copper nanocluster solution, europium salt, and tricarboxyl aromatic ligand into an organic solvent, conduct a coordination reaction, adjust the pH of the system to alkaline, and crystallize to obtain the ratio fluorescent composite material.

5. The preparation method of the ratio fluorescence composite material according to claim 4, characterized in that, In S1, the copper salt solution includes at least one of a copper sulfate solution, a copper nitrate solution, or a copper chloride solution, and the concentration of the copper salt solution is 0.8 mM to 1.2 mM; In S1, the concentration of the L-cysteine solution is 30 mg / mL to 40 mg / mL; In S1, the volume ratio of the copper salt solution to the L-cysteine solution is 1:(8 to 12).

6. The preparation method of the ratiometric fluorescence composite material according to claim 4, characterized in that, In S1, the slightly alkaline to neutral refers to pH = 7 to 7.5; In S2, the alkaline refers to pH = 9 to 11.

7. The preparation method of the ratiometric fluorescence composite material according to claim 4, characterized in that, In S2, the europium salt includes at least one of europium chloride, europium nitrate, europium acetate, or europium sulfate; In S2, the organic solvent includes at least one of methanol, ethanol, isopropanol, DMF, acetonitrile, or acetone; In S2, the dosage ratio of the copper nanocluster solution, europium salt, tricarboxyl aromatic ligand to the organic solvent is (0.8 to 1.2) mL:0.1 mmol:(18 to 24) mg:(8 to 12) mL.

8. The preparation method of the ratio fluorescence composite material according to claim 4, characterized in that, In S1, the temperature of the pre-coordination reaction is 0 °C to 40 °C, and the time of the pre-coordination reaction is 9 min to 12 min; In S1, the time of the reduction reaction is 4 h to 5 h; In S2, the temperature of the coordination reaction is 0 °C to 40 °C, and the time of the coordination reaction is 25 min to 45 min; In S2, the temperature of the crystallization is 0 °C to 40 °C, and the crystallization time is 1.8 h to 2.5 h.

9. Use of the ratio fluorescent composite material according to any one of claims 1 to 3 or the ratio fluorescent composite material prepared by the preparation method of the ratio fluorescent composite material according to any one of claims 4 to 8 in the dual-emission ratio fluorescence method for detecting Hg 2+ in it.

10. Application of the ratiometric fluorescence composite material as described in claim 9 in the detection of Hg by dual-emission ratiometric fluorescence method 2+ wherein, The detection results can be presented on a smartphone through a fluorescent color photo and a color recognition APP.

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