Preparation of star-shaped ZIF-8 and modified electrode and method for detecting heavy metal cadmium

Abstract

The invention relates to a method for preparing a star-shaped ZIF-8 and a modified electrode and detecting heavy metal cadmium. The method for preparing the star-shaped ZIF-8 comprises: (1) mixing a zinc salt aqueous solution with a mixed aqueous solution of a surfactant and 2-methylimidazole, stirring the mixture for reaction, centrifuging, washing, and vacuum drying to obtain a precursor; (2) grinding the mixture, performing three-stage heating pyrolysis under a protective atmosphere, and cooling the mixture. The invention also discloses a method for preparing a star-shaped ZIF-8 modified electrode and a method for detecting heavy metal cadmium using the star-shaped ZIF-8 modified electrode. The star-shaped ZIF-8 obtained by the method of the invention is metal-free, has a large specific surface area, high porosity, good conductivity, simple process, low cost, and is environmentally friendly. The modified electrode obtained by the method of the invention has a wide detection range, high sensitivity, low background current, and high selectivity. The method for detecting heavy metals of the invention has high stability, anti-interference ability, and accuracy.

Description

Technical Field

[0001] The invention relates to a method for preparing ZIF-8 and a modified electrode and detecting heavy metal cadmium, and particularly relates to a method for preparing star-shaped ZIF-8 and a modified electrode and detecting heavy metal cadmium. Background Art

[0002] The water environment is the foundation of human survival and development and a vital natural resource. With the booming electroplating, nonferrous metal, and smelting industries, heavy metal ions generated by industrial wastewater continue to accumulate in ecosystems and are difficult to degrade naturally. Heavy metal pollution in water bodies ultimately poses a significant threat to human health, with cadmium pollution being particularly prominent. When considerable amounts of cadmium enter the environment through wastewater, its high mobility and intense biological toxicity can severely impact the growth and development of plants and animals. It accumulates in humans through the food chain, causing renal insufficiency and disrupting the reabsorption of amino acids and proteins. Furthermore, it can lead to bone damage, including low bone mineralization, osteomalacia, and severe bone pain, resulting in irreversible damage. Therefore, monitoring cadmium levels in drinking water and food is crucial.

[0003] Given the environmental and human hazards of cadmium ions, numerous reliable methods for detecting trace amounts of cadmium have been introduced. Examples include inductively coupled plasma mass spectrometry (ICP-MS), flame atomic absorption spectrometry (FAAS), neutron activation analysis (NAA), and capillary electrophoresis (CE). Compared to these traditional methods, electrochemical methods offer the advantages of fast response, ease of operation, high selectivity, and low cost. Key to achieving these advantages is the need for electrodes modified with high-performance electrosensing materials.

[0004] Zeolitic imidazolate framework (ZIF-8) is a functional material with a rich, ordered pore structure and high specific surface area. Its flexible framework and controllable morphology make it an ideal electrosensor material for rapid electron transfer, strong electrocatalytic activity, and abundant active sites. However, leaching of toxic metal ions from ZIF-8 and oxidation of the modified materials can lead to secondary contamination of the aqueous environment and electrode irreproducibility, making it unsuitable for modified electrodes with high sensitivity and low detection limits. Therefore, modifying ZIF-8 to address these issues is of great significance for its practical applications. ZIF-8-derived metal-free nanomaterials, a type of metal-free carbon-based material with Zn removed, are promising carbon templates that offer high specific surface area and porous structures. However, carbonized ZIF-8 often exhibits unsatisfactory microstructures and low conductivity in acidic electrolytes, limiting the development of ZIF-8-derived metal-free catalysts.

[0005] In summary, it is urgent to find a preparation method for star-shaped ZIF-8, in which the obtained material is metal-free, has a large specific surface area, high porosity, good conductivity, simple process, low cost, and is environmentally friendly; a preparation method for the obtained modified electrode has a wide detection range, high sensitivity, low background current, and high selectivity; and a method for detecting heavy metal cadmium with high stability, anti-interference, and accuracy. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for preparing star-shaped ZIF-8, which has the advantages of being metal-free, large specific surface area, high porosity, good conductivity, simple process, low cost and environmentally friendly.

[0007] The technical problem to be further solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for preparing a star-shaped ZIF-8 modified electrode having a wide detection range, high sensitivity, low background current and high selectivity.

[0008] A further technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for detecting heavy metal cadmium using a star-shaped ZIF-8 modified electrode with high stability, anti-interference and accuracy.

[0009] The technical solution adopted by the present invention to solve the technical problem is as follows: a method for preparing star-shaped ZIF-8 comprises the following steps:

[0010] (1) mixing a zinc salt aqueous solution with a mixed aqueous solution of a surfactant and 2-methylimidazole, stirring for reaction, centrifuging, washing, and vacuum drying to obtain a precursor;

[0011] (2) After grinding the precursor obtained in step (1), the precursor was subjected to three-stage heating and pyrolysis under a protective atmosphere, and then naturally cooled to room temperature to obtain star-shaped ZIF-8.

[0012] The invention idea of ​​the preparation method of the star-shaped ZIF-8 of the present invention is: a zeolite imidazole framework (ZIF-8) / surfactant-derived metal-free carbon-based nanomaterial star-shaped ZIF-8 is prepared by chemical co-precipitation and thermal decomposition in an aqueous solution, which is a ZIF-8-derived metal-free carbon-based material.

[0013] Preferably, in step (1), the molar ratio of zinc element to 2-methylimidazole in the zinc salt is 1:100-500 (more preferably 1:200-400). Zeolite imidazolate framework material (ZIF-8) is composed of zinc ions (Zn 2+ ) and 2-methylimidazole to form a metal-organic framework that can bind zinc ions (Zn 2+) and 2-methylimidazole are dissolved in water. Excess 2-methylimidazole deprotonates to form a linking unit with the zinc ion, existing in a neutral, stable form. By adjusting the type and ratio of the ligands, ZIFs with different structures can be obtained. Metal-free carbon-based materials derived from ZIF-8, as ZIFs without the metal, inherit the advantages of ZIF-8, such as high porosity and strong catalytic activity, while exhibiting improved stability and ORR catalytic activity.

[0014] Preferably, in step (1), the concentration of the zinc salt aqueous solution is 10 to 15 mmol / L.

[0015] Preferably, in step (1), the zinc salt includes one or more soluble inorganic zinc salts selected from zinc nitrate, zinc sulfate, zinc chloride, and hydrates thereof.

[0016] Preferably, in step (1), in the mixed aqueous solution of the surfactant and 2-methylimidazole, the concentration of the surfactant is 0.1 to 1.0 mmol / L, and the concentration of 2-methylimidazole is 200 to 1000 mmol / L. In order to improve the redox activity of the carbon material, the addition of the surfactant induces the morphological evolution of the crystals, thereby imparting strong catalytic activity to the electrode interface, providing a large number of binding sites that can be captured for the analyte, thereby forming a sensor with high sensitivity.

[0017] Preferably, in step (1), the preparation method of the mixed aqueous solution of the surfactant and 2-methylimidazole is: adding the surfactant and 2-methylimidazole into water and ultrasonically treating the water.

[0018] Preferably, the ultrasonic treatment has a frequency of 30 to 50 kHz, a power of 140 to 160 W, and a duration of 5 to 15 minutes.

[0019] Preferably, in step (1), the surfactant includes cetyltrimethylammonium bromide. Cetyltrimethylammonium bromide (CTAB) is a quaternary ammonium salt-type cationic surfactant, which is composed of a hydrophilic head compatible with water and a hydrophobic tail compatible with oil, and has a special structure of amphiphilic molecules. Therefore, CTAB can not only provide specific molecules to the electrode interface through electrostatic interaction, so as to promote the material to exhibit strong adsorption and high accumulation of target ions and improve sensitivity, but also form a surfactant film on the electrode surface, enhance the electron transfer rate and amplify the electrochemical signal. CTAB is easy to combine with the negatively charged perfluorosulfonic acid polymer solution to improve the dispersibility of the solution. The long hydrophobic hydrocarbon chain of CTAB can be adsorbed by the hydrophobic surface of ZIF-8 in the aqueous medium to form a more stable structure, promoting electron exchange between the sensor and the analyte, which will give the electrode interface strong catalytic activity to provide a large number of analyte-capable binding sites, forming a sensor with high sensitivity.

[0020] Preferably, in step (1), the stirring reaction temperature is room temperature, the rotation speed is 800-1000 r / min, and the time is 3-5 h.

[0021] Preferably, in step (1), the washing refers to washing with water and methanol alternately for ≥1 times.

[0022] Preferably, in step (1), the vacuum drying temperature is 60-80°C, the vacuum degree is 0.03-0.05 MPa, and the time is 10-15 h.

[0023] Preferably, in step (2), the grinding is performed to a powdery state.

[0024] Preferably, in step (2), the three-stage heating pyrolysis refers to: first heating to 300-500°C at a rate of 4-6°C / min, maintaining for 1-3 hours, then heating to 500-700°C at a rate of 4-6°C / min, maintaining for 1-3 hours, and finally heating to 850-950°C at a rate of 4-6°C / min, maintaining for 1-3 hours. If the star-shaped ZIF-8 is directly heated to a higher temperature, the framework of the material is not easy to be maintained, it is difficult to maintain the star-shaped structure, and the skeleton is easy to melt and form agglomerates. At a lower temperature, the star-shaped ZIF-8 will first be shaped, maintaining the skeleton of the material, and the skeleton is not easy to collapse when the temperature is gradually increased. The boiling point of Zn in the material is 907°C. During the third-stage heating pyrolysis process, the ZIF framework is reduced to Zn metal, and then evaporates with the flow of the protective atmosphere. Therefore, the material has been converted into a metal-free carbon matrix through the three-stage heating pyrolysis. The method of the present invention solves the shortcomings of the ZIF-8 carbon matrix, which is easily volatile due to high-temperature calcination of metallic zinc, low porosity and poor electrical conductivity, thereby improving the performance and application potential of the catalyst.

[0025] Preferably, in step (2), the protective atmosphere is argon and / or nitrogen. The protective atmosphere used in the present invention is a high-purity atmosphere with a purity of ≥99.9%.

[0026] The present invention further solves the technical problem by adopting the following technical solution: a method for preparing a star-shaped ZIF-8 modified electrode, comprising the following steps:

[0027] (1) The Al2O3 slurry is polished in descending order of particle size using the “8” method until the surface of the glassy carbon electrode has a mirror effect;

[0028] (2) ultrasonically cleaning the glassy carbon electrode polished in step (1) in water and then in anhydrous ethanol, and then drying it in a protective atmosphere;

[0029] (3) The surface of the glassy carbon electrode dried in step (2) is evenly drop-coated with the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 and allowed to air-dry naturally to obtain a star-shaped ZIF-8 modified electrode.

[0030] Preferably, in step (1), the mass concentration of Al2O3 slurry is 70-90 g / L.

[0031] Preferably, in step (1), the particle size ranges from 3 μm to 30 nm.

[0032] Preferably, in step (1), the specific operation of the polishing treatment is: first, the Al2O3 slurry with a particle size of 1 μm to 3 μm is polished on the surface of the glassy carbon electrode 180 to 220 times using the "drawing 8" method, and then the Al2O3 slurry with a particle size of 30 to 100 nm is polished on the surface of the glassy carbon electrode 180 to 220 times using the "drawing 8" method.

[0033] Preferably, in step (2), the frequency of the ultrasonic cleaning is 30 to 50 kHz, the power is 80 to 120 W, and the time is 10 to 15 s.

[0034] Preferably, in step (2), the protective atmosphere comprises nitrogen or argon. The protective atmosphere used in the present invention is a high-purity atmosphere with a purity of ≥99.9%.

[0035] Preferably, in step (3), the concentration of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is 0.5 to 1.5 mg / mL.

[0036] Preferably, in step (3), the amount of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 to be applied is 0.5 to 2.5 μL / mm 2 The star-shaped ZIF-8 was modified by coating the electrode surface with perfluorosulfonic acid dispersion droplets as a new generation of electrode material for the determination of trace Cd(II) concentration.

[0037] Preferably, in step (3), the preparation method of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is as follows: adding the star-shaped ZIF-8 into a perfluorosulfonic acid aqueous solution and ultrasonically dispersing the dispersion.

[0038] Preferably, the mass volume ratio of the star-shaped ZIF-8 to the perfluorosulfonic acid aqueous solution is 0.5 to 1.5:1 in mg / mL.

[0039] Preferably, in the perfluorosulfonic acid aqueous solution, the volume ratio of perfluorosulfonic acid to water is 1:8-10.

[0040] Preferably, the frequency of the ultrasonic dispersion is 30 to 50 kHz, the power is 150 to 300 W, and the time is 30 to 40 minutes.

[0041] The present invention further solves the technical problem by adopting the following technical solution: a method for detecting heavy metals using a star-shaped ZIF-8 modified electrode, comprising the following steps:

[0042] (1) When drawing a standard curve, the cadmium standard solution is added dropwise to the electrolyte of the electrolytic cell to obtain a linear concentration of 0.5 to 230 μg / L; or when detecting weak acid or neutral cadmium-containing wastewater, the cadmium-containing wastewater is directly placed in the electrolytic cell; or when detecting acidic cadmium-containing wastewater, the acidic cadmium-containing wastewater is added dropwise to the electrolyte of the electrolytic cell and the pH value is adjusted to 3.5 to 5.5; then, the star-shaped ZIF-8 modified electrode is used as the working electrode, Ag / AgCl is used as the reference electrode, and Pt wire is used as the counter electrode to connect to the electrochemical workstation;

[0043] (2) Under stirring conditions, the heavy metal cadmium ion content of the solution in the electrolytic cell is detected by square wave anodic stripping voltammetry.

[0044] Preferably, in step (1), the concentration of the cadmium standard solution is 10 to 500 μg / mL.

[0045] Preferably, in step (1), the volume ratio of the electrolyte to the cadmium standard solution is 50 mL:2 μL~100 μL.

[0046] Preferably, in step (1), the electrolyte is an acetic acid-sodium acetate aqueous solution. The electrolyte has the following two functions: 1) as a conductive medium to transmit current; 2) to carry out electrochemical reactions under the action of the electric field, so that the anodic dissolution can proceed smoothly and in a controlled manner.

[0047] Preferably, in step (1), the mixed concentration of the acetic acid-sodium acetate aqueous solution is 0.05-0.15 mol / L, and the pH value is 3.5-5.5. If the pH value is too low, the hydrophilic head of CTAB is protonated, and more H is ionized to react with Cd 2+ Coexistence; heavy metal ions on the electrode surface are easily dissolved. If the pH value is too high, OH − In solution, it is easy to react with Cd 2+ Metal hydroxides are formed as a result of precipitation, and the peak current decreases. Therefore, determining the appropriate pH is crucial for testing.

[0048] Preferably, in step (1), the pH value of the weakly acidic or neutral cadmium-containing wastewater is 4.5 to 8.0. The weakly acidic or neutral heavy metal ion-containing wastewater includes natural water bodies, etc.

[0049] Preferably, in step (1), the pH value of the acidic cadmium-containing wastewater is less than 4.5. The acidic heavy metal ion-containing wastewater includes digestion solution, etc.

[0050] Preferably, in step (1), the volume ratio of the electrolyte to the acidic cadmium-containing wastewater is 50 mL: 2 μL to 100 μL. The pH value is adjusted to 3.5 to 5.5 with 5 to 10 mol / L sodium hydroxide. The method of the present invention is divided into two Cd stages: 0.5 to 30 μg / L and 30 to 230 μg / L according to the degree of fit. 2+ The working curve of concentration was finally obtained, and the linear concentration range of 0.5 to 230 μg / L was obtained. These two concentration ranges are different concentrations of Cd 2+ On the modified electrode, redox behavior occurs through adsorption. The reaction rates are different at different concentrations, and different calibration curves are selected for calculation.

[0051] Preferably, in step (1), when the cadmium ion concentration in the acidic, weakly acidic or neutral cadmium-containing wastewater is too high or too low, or even exceeds the detection limit, its concentration is adjusted by concentrating, diluting or adding dropwise to a buffer solution so that its detection concentration in the electrolytic cell is 0.5 to 230 μg / L.

[0052] Preferably, in step (1), the electrochemical workstation model is RST5000; the internal solution of the reference electrode Ag / AgCl is a saturated KCl solution.

[0053] Preferably, in step (2), the stirring speed is 200-400 r / min.

[0054] Preferably, in step (2), the detection parameters of the square wave anodic stripping voltammetry are: accumulation potential: -0.9 to -1.3 V, accumulation time: 50 to 500 s, rest time: 5 to 20 s, starting potential: -1.3 to -1.0 V, ending potential: 0.2 to 0.6 V, square wave amplitude: 0.02 to 0.03 V, potential increment: 0.002 to 0.005 V, and square wave frequency: 20 to 30 Hz. Square wave anodic stripping voltammetry (SWASV) has two advantages: 1) square wave anodic stripping voltammetry performs current sampling at the end of the forward and reverse potential steps, and independent measurements eliminate the charging current effect; 2) the current difference between the two measurements is used as the voltage for plotting, which can eliminate the influence of capacitance. Therefore, square wave anodic stripping voltammetry has a faster scan rate and higher sensitivity, and can effectively suppress background current, and can be widely used in the study of electrode materials and electrochemical detection systems. Applying an enrichment voltage can drive electron transfer during the enrichment process, and negative voltage is beneficial to Cd 2+ However, when the deposition potential is too negative, the release of H2 in the hydrogen evolution reaction occupies the active sites of heavy metal ions on the electrode surface and easily damages the surface of the modified material, resulting in a sharp drop in the stripping peak current. Therefore, determining the appropriate enrichment potential is the key to the test; enrichment time is also one of the important parameters. In the process of gradually increasing enrichment time, Cd2+ When ions diffuse on the electrode surface over a certain period of time, the current signal typically rises significantly. However, after reaching a certain level, the current signal no longer increases significantly. This is because at the critical time, the chemical adsorption sites on the electrode reach saturation. Extending the deposition time also makes it difficult to maintain the accumulation of heavy metal ions, resulting in a constant peak current. Therefore, the selection of the enrichment time parameter will have a certain impact on the detection performance. The purpose of setting the rest time after enrichment is to stop stirring and ensure that the mass transfer between the electrode surface and the bulk phase of the solution reaches a relatively stable state.

[0055] The beneficial effects of the present invention are as follows:

[0056] (1) The material obtained by the preparation method of the star-shaped ZIF-8 of the present invention is metal-free and has a specific surface area of ​​up to 589.3922 m 2 / g, an average pore size of 3.83 nm, good conductivity, simple process, low cost, environmentally friendly, and in compliance with the principle of sustainable development;

[0057] (2) The modified electrode prepared by the method of the present invention has good electrochemical activity and conductivity, thus having a wide detection range, high sensitivity, low background current, and high selectivity;

[0058] (3) The present invention uses star-shaped ZIF-8 modified electrodes to detect heavy metals, which has the characteristics of stability, anti-interference and high accuracy, especially for Cd 2+ It has good electrochemical response, showing advantages such as high sensitivity, low background current, and high selectivity. 2+ The recovery rate was as high as 95.00-104.96%, with high accuracy and a detection limit as low as 0.48 μg / L, indicating that the use of star-shaped ZIF-8 modified electrodes for the detection of heavy metals provides a new path for the rapid detection of trace heavy metal ions in water environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 This is a SEM image of the precursor obtained in step (1) of Example 1 of the preparation method of star-shaped ZIF-8 of the present invention;

[0060] Figure 2 1 is a SEM image of the star-shaped ZIF-8 obtained in Example 1 of the preparation method of the star-shaped ZIF-8 of the present invention;

[0061] Figure 3 This is the XRD pattern of the star-shaped ZIF-8 obtained in Example 1 of the preparation method of the star-shaped ZIF-8 of the present invention;

[0062] Figure 4 This is a pore size distribution diagram of the star-shaped ZIF-8 obtained in Example 1 of the preparation method of the star-shaped ZIF-8 of the present invention;

[0063] Figure 5 is the isothermal adsorption-desorption curve of the star-shaped ZIF-8 obtained in Example 1 of the preparation method of the star-shaped ZIF-8 of the present invention;

[0064] Figure 6 Cyclic voltammograms of the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the star-shaped ZIF-8 modified electrode of the present invention, the carbonized ZIF-8 modified electrode obtained in Comparative Example 1, the Nafion modified electrode obtained in Comparative Example 2, the CTAB modified electrode obtained in Comparative Example 3, and a bare electrode;

[0065] Figure 7 Square wave anodic stripping voltammetry curves of the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the star-shaped ZIF-8 modified electrode of the present invention, the carbonized ZIF-8 modified electrode obtained in Comparative Example 1, the Nafion modified electrode obtained in Comparative Example 2, the CTAB modified electrode obtained in Comparative Example 3, and a bare electrode;

[0066] Figure 8 In 0.1 mol / L acetic acid-sodium acetate aqueous solution (pH=4.5), the star-shaped ZIF-8 modified electrode prepared in Example 1 of the present invention was subjected to the following tests: 2+ Response curve of square wave anodic stripping voltammetry (0.5-230 μg / L);

[0067] Figure 9 The peak current of the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the present invention in 0.1 mol / L acetic acid-sodium acetate aqueous solution (pH = 4.5) corresponds to different concentrations of Cd 2+ (0.5~30 μg / L) working curve;

[0068] Figure 10 The peak current of the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the present invention in 0.1 mol / L acetic acid-sodium acetate aqueous solution (pH = 4.5) corresponds to different concentrations of Cd 2+ (30~230 μg / L) working curve. DETAILED DESCRIPTION

[0069] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0070] The original standard solution used in the embodiment of the present invention is a commercially available cadmium standard solution with a concentration of 1000 μg / mL, which is prepared into standard solutions with concentrations of 10 μg / mL, 40 μg / mL, and 400 μg / mL for use; the diameter of the glassy carbon electrode core used in the embodiment of the present invention is 3 mm; the protective atmosphere used in the embodiment of the present invention is a high-purity atmosphere with a purity of ≥99.9%; the raw materials and chemical reagents used in the embodiment of the present invention, unless otherwise specified, are obtained through conventional commercial channels.

[0071] Example 1 of the preparation method of star-shaped ZIF-8

[0072] (1) 80 mL of Zn(NO3)2·6H2O aqueous solution (12 mmol / L) was mixed with 320 mL of a mixed aqueous solution of CTAB and 2-methylimidazole (CTAB: 0.34 mmol / L, 2-methylimidazole: 685.1 mmol / L). The mixture was stirred at room temperature and a speed of 900 r / min for 4 h, centrifuged, and washed alternately with water and methanol twice. The precursor was vacuum dried at 70 °C and a vacuum degree of 0.04 MPa for 12 h to obtain the precursor.

[0073] The mixed aqueous solution of CTAB and 2-methylimidazole is prepared by adding 40 mg (0.11 mmol) of CTAB and 18 g (219.22 mmol) of 2-methylimidazole to 320 mL of deionized water, and ultrasonically treating the mixture at a frequency of 40 kHz and a power of 150 W for 10 minutes.

[0074] (2) After the precursor obtained in step (1) is ground into powder, it is first heated to 400 °C at a rate of 5 °C / min under the protection of high-purity argon atmosphere, maintained for 2 h, then heated to 600 °C at a rate of 5 °C / min, maintained for 2 h, and finally heated to 910 °C at a rate of 5 °C / min, maintained for 2 h, and then subjected to three-stage heating pyrolysis. The mixture is naturally cooled to room temperature to obtain star-shaped ZIF-8.

[0075] like Figure 1 As shown, the precursor obtained in step (1) of the embodiment of the present invention has a smooth surface and a regular hexagonal shape with six equal branches.

[0076] like Figure 2 As shown, the star-shaped ZIF-8 obtained after the three-stage heating pyrolysis in step (2) of the embodiment of the present invention exhibits uneven textures, with each branch being approximately 220 nm in diameter, and its morphology remains intact after high-temperature calcination.

[0077] like Figure 3As shown, the star-shaped ZIF-8 obtained in the embodiment of the present invention has carbon properties, among which only two relatively broad XRD diffraction peaks appear at 25° and 43°, belonging to the (002) and (100) crystal planes respectively, and are amorphous; since there are no diffraction peaks of other impurities, it shows that the residual Zn has been completely removed.

[0078] like Figure 4 、 5 As shown in the figure, the BET results reveal that the star-shaped ZIF-8 obtained in the embodiment of the present invention has a rich pore structure; the mesoporous characteristics of the star-shaped ZIF-8 have a diameter of 589.3922 m 2 / g large specific surface area, 0.0312 cm 3 / g pore volume and an average pore diameter of 3.83nm; smaller mesopores are conducive to the adsorption of N2, which is conducive to exposing more adsorption and reaction catalytic sites for the detection of heavy metal ion cadmium in water.

[0079] Example 2 of the preparation method of star-shaped ZIF-8

[0080] (1) 80 mL of Zn(NO3)2·6H2O aqueous solution (12 mmol / L) was mixed with 320 mL of a mixed aqueous solution of CTAB and 2-methylimidazole (CTAB: 0.63 mmol / L, 2-methylimidazole: 937.4 mmol / L). The mixture was stirred at room temperature and a rotation speed of 1000 r / min for 5 h, centrifuged, and washed alternately with water and methanol once. The precursor was vacuum dried at 80 °C and a vacuum degree of 0.05 MPa for 15 h to obtain the precursor.

[0081] The mixed aqueous solution of CTAB and 2-methylimidazole is prepared by adding 72.89 mg (0.2 mmol) of CTAB and 24.63 g (300.0 mmol) of 2-methylimidazole to 320 mL of deionized water, and ultrasonically treating the mixture at a frequency of 50 kHz and a power of 160 W for 15 minutes.

[0082] (2) After the precursor obtained in step (1) is ground into powder, it is first heated to 500 °C at a rate of 6 °C / min under the protection of high-purity argon atmosphere, maintained for 3 h, then heated to 700 °C at a rate of 6 °C / min, maintained for 3 h, and finally heated to 950 °C at a rate of 6 °C / min, maintained for 3 h, and then naturally cooled to room temperature to obtain star-shaped ZIF-8.

[0083] Example 3 of the preparation method of star-shaped ZIF-8

[0084] (1) 80 mL of Zn(NO3)2·6H2O aqueous solution (10 mmol / L) was mixed with 320 mL of a mixed aqueous solution of CTAB and 2-methylimidazole (CTAB: 0.31 mmol / L, 2-methylimidazole: 625.0 mmol / L). The mixture was stirred at room temperature and a speed of 800 r / min for 3 h, centrifuged, and washed alternately with water and methanol twice. The precursor was vacuum dried at 60 °C and 0.03 MPa for 10 h to obtain the precursor.

[0085] The mixed aqueous solution of CTAB and 2-methylimidazole is prepared by adding 36.445 mg (0.1 mmol) of CTAB and 16.42 g (200.0 mmol) of 2-methylimidazole to 320 mL of deionized water, and ultrasonically treating the mixture at a frequency of 30 kHz and a power of 140 W for 5 minutes.

[0086] (2) After the precursor obtained in step (1) is ground into powder, it is first heated to 300 °C at a rate of 4 °C / min under the protection of high-purity argon atmosphere, maintained for 1 h, then heated to 500 °C at a rate of 4 °C / min, maintained for 1 h, and finally heated to 850 °C at a rate of 4 °C / min, maintained for 1 h, and then subjected to three-stage heating pyrolysis. The mixture is naturally cooled to room temperature to obtain star-shaped ZIF-8.

[0087] Example 1 of the preparation method of star-shaped ZIF-8 modified electrode

[0088] (1) First, the Al2O3 slurry with a particle size of 2 μm (mass concentration of 80 g / L) was used to polish the surface of the glassy carbon electrode 200 times using the "8" method. Then, the Al2O3 slurry with a particle size of 50 nm (mass concentration of 80 g / L) was used to polish the surface of the glassy carbon electrode 200 times using the "8" method until the electrode surface showed a mirror effect.

[0089] (2) The glassy carbon electrode polished in step (1) was ultrasonically cleaned in deionized water and anhydrous ethanol at a frequency of 40 kHz and a power of 100 W for 12 s each, and then dried in a high-purity argon atmosphere;

[0090] (3) Evenly drop-coat 10 μL of the perfluorosulfonic acid dispersion (1 mg / mL) of star-shaped ZIF-8 obtained in Example 1 of the preparation method of star-shaped ZIF-8 on the surface of the glassy carbon electrode dried in step (2), and air-dry naturally to obtain a star-shaped ZIF-8 modified electrode;

[0091] The preparation method of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is as follows: 1 mg of the star-shaped ZIF-8 obtained in Example 1 of the preparation method of the star-shaped ZIF-8 is added to 1 mL of a perfluorosulfonic acid aqueous solution (the volume ratio of perfluorosulfonic acid to water is 1:9), and ultrasonic dispersion is performed at a frequency of 40 kHz and a power of 200 W for 35 minutes.

[0092] Example 2 of Preparation Method of Star-Shaped ZIF-8 Modified Electrode

[0093] (1) First, the surface of the glassy carbon electrode was polished 180 times using the “8” method with a 3 μm Al2O3 slurry (mass concentration of 90 g / L). Then, the surface of the glassy carbon electrode was polished 180 times using the “8” method with a 100 nm Al2O3 slurry (mass concentration of 90 g / L) until the electrode surface showed a mirror effect.

[0094] (2) The glassy carbon electrode polished in step (1) was ultrasonically cleaned in deionized water and anhydrous ethanol at a frequency of 50 kHz and a power of 120 W for 15 s each, and then dried in a high-purity argon atmosphere;

[0095] (3) Evenly drop-coat 15 μL of the perfluorosulfonic acid dispersion of star-shaped ZIF-8 (1.5 mg / mL) obtained in Example 2 of the preparation method for star-shaped ZIF-8 on the surface of the glassy carbon electrode dried in step (2), and air-dry naturally to obtain a star-shaped ZIF-8 modified electrode;

[0096] The preparation method of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is as follows: 1.5 mg of the star-shaped ZIF-8 obtained in Example 2 of the preparation method of the star-shaped ZIF-8 is added to 1 mL of a perfluorosulfonic acid aqueous solution (the volume ratio of perfluorosulfonic acid to water is 1:9), and ultrasonic dispersion is performed at a frequency of 50 kHz and a power of 300 W for 40 minutes.

[0097] Example 3 of Preparation Method of Star-Shaped ZIF-8 Modified Electrode

[0098] (1) First, the Al2O3 slurry with a particle size of 1 μm (mass concentration of 70 g / L) was polished 220 times using the "8" method on the surface of the glassy carbon electrode. Then, the Al2O3 slurry with a particle size of 30 nm (mass concentration of 70 g / L) was polished 220 times using the "8" method on the surface of the glassy carbon electrode until the electrode surface showed a mirror effect.

[0099] (2) The glassy carbon electrode polished in step (1) was ultrasonically cleaned in deionized water and anhydrous ethanol at a frequency of 30 kHz and a power of 80 W for 10 s each, and then dried in a high-purity argon atmosphere;

[0100] (3) Evenly drop-coat 5 μL of the perfluorosulfonic acid dispersion (0.5 mg / mL) of star-shaped ZIF-8 obtained in Example 3 of the preparation method of star-shaped ZIF-8 on the surface of the glassy carbon electrode dried in step (2), and air-dry naturally to obtain a star-shaped ZIF-8 modified electrode;

[0101] The preparation method of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is as follows: 0.5 mg of the star-shaped ZIF-8 obtained in Example 3 of the preparation method of the star-shaped ZIF-8 is added to 1 mL of a perfluorosulfonic acid aqueous solution (the volume ratio of perfluorosulfonic acid to water is 1:9), and ultrasonic dispersion is performed at a frequency of 30 kHz and a power of 150 W for 30 minutes.

[0102] Comparative Example 1 of Preparation Method of Carbonized ZIF-8 Modified Electrode

[0103] (1) The same preparation method as step (1) of Example 1 for the star-shaped ZIF-8 modified electrode;

[0104] (2) The same preparation method of the star-shaped ZIF-8 modified electrode as step (2) in Example 1;

[0105] (3) Evenly apply 10 μL of carbonized ZIF-8 aqueous solution (1 mg / mL) on the surface of the glassy carbon electrode after drying in step (2) and air dry it to obtain a carbonized ZIF-8 modified electrode;

[0106] The carbonized ZIF-8 aqueous solution is prepared by adding 1 mg of carbonized ZIF-8 to 1 mL of water, and performing ultrasonic dispersion at a frequency of 40 kHz and a power of 200 W for 35 minutes.

[0107] The preparation method of the carbonized ZIF-8 comprises the following steps:

[0108] (1) 80 mL of Zn(NO3)2·6H2O aqueous solution (12 mmol / L) was mixed with 320 mL of 685.1 mmol / L 2-methylimidazole aqueous solution. The mixture was vigorously stirred at room temperature and a speed of 900 r / min for 4 h, centrifuged, and washed alternately with water and methanol twice. The precursor was vacuum dried at 70 °C and a vacuum degree of 0.04 MPa for 12 h.

[0109] (2) The same as step (2) of Example 1 for the preparation of star-shaped ZIF-8.

[0110] Comparative Example 2 of Preparation Method of Nafion Modified Electrode

[0111] (1) The same as step (1) of Example 1 for preparing the star-shaped ZIF-8 modified electrode;

[0112] (2) The same preparation method of the star-shaped ZIF-8 modified electrode as step (2) in Example 1;

[0113] (3) Evenly apply 10 μL of perfluorosulfonic acid dispersion (the volume ratio of perfluorosulfonic acid to water is 1:9) on the surface of the glassy carbon electrode after drying in step (2), and air dry it to obtain a Nafion modified electrode.

[0114] Comparative Example 3 of Preparation Method of CTAB Modified Electrode

[0115] (1) The same as step (1) of Example 1 for preparing the star-shaped ZIF-8 modified electrode;

[0116] (2) The same preparation method of the star-shaped ZIF-8 modified electrode as step (2) in Example 1;

[0117] (3) Evenly apply 10 μL of CTAB aqueous solution (1 mg / mL) on the surface of the glassy carbon electrode after drying in step (2) and let it air dry to obtain a CTAB modified electrode.

[0118] In order to evaluate the electrochemical performance of the star-shaped ZIF-8 modified electrode (star-shaped ZIF-8-Nafion / GCE) obtained in Example 1 of the preparation method of the star-shaped ZIF-8 modified electrode of the present invention, it was compared with the carbonized ZIF-8 modified electrode (ZIF-8 / GCE) obtained in Comparative Example 1, the Nafion modified electrode (Nafion / GCE) obtained in Comparative Example 2, the CTAB modified electrode (CTAB / GCE) obtained in Comparative Example 3, and the bare electrode (GCE).

[0119] like Figure 6 As shown, at 5 mmol / L Fe (CN)6 3- / 4-Cyclic voltammetry curves of different electrodes in 0.1 mol / L KCl solution show that peaks and valleys in the cyclic voltammetry curves represent redox reactions. The electrochemical activity of the electrodes improved to varying degrees after modification with ZIF-8, Nafion, and CTAB. The redox peak areas of CTAB / GCE, star-shaped ZIF-8-Nafion / GCE, ZIF-8 / GCE, Nafion / GCE, and GCE decreased in order. Doping ZIF-8 with CTAB significantly increased the redox peak area and electron transfer rate. The main reasons for the increased redox peak area of ​​ZIF-8-Nafion / GCE are: 1) The introduction of CTAB creates a more mesoporous structure in the modified electrode material, significantly increasing its specific surface area and conductivity. Furthermore, the six epitaxial branches of the material provide abundant active sites and conductive pathways. 2) The combination of CTAB and ZIF-8 forms a rich CN network in the material. Nitrogen, with its isolated electrons and higher electronegativity than carbon, facilitates the formation of a conjugated structure between carbon and nitrogen, facilitating charge transport. Therefore, the star-shaped ZIF-8-Nafion / GCE exhibits good electrical conductivity and is suitable for electrochemical detection of heavy metal Cd. 2+ .

[0120] like Figure 7 As shown, 50 μg / L Cd was enriched in 0.1 mol / L acetic acid-sodium acetate aqueous solution (pH = 4.5) buffer. 2+ The square wave anodic stripping voltammetric responses of different electrodes were obtained, and the detection parameters were the same as those in Example 1 of the method for detecting heavy metals using a star-shaped ZIF-8 modified electrode. 2+ Oxidation reaction occurs on the electrode surface (Cd-2e - →Cd 2+ ), the star-shaped ZIF-8-Nafion / GCE has the most negative oxidation potential and the strongest stripping peak current, and its peak potential (-0.77 V) and peak current (18.10 μA) represent Cd 2+ The good electrochemical behavior of star-shaped ZIF-8-Nafion / GCE can be attributed to the following factors: 1) The large specific surface area of ​​star-shaped ZIF-8 and the three-dimensional structure formed by imidazole ligands facilitate the Cd 2+ 1) CTAB is rapidly deposited at the electrode sensing interface; 2) the long hydrophobic hydrocarbon chain of CTAB can be adsorbed by the hydrophobic surface of ZIF-8 in aqueous medium to form a more stable structure, which is beneficial to the electron exchange between the electrode sensing interface and the analyte.

[0121] Example 1 of a method for detecting heavy metal cadmium using a star-shaped ZIF-8 modified electrode (drawing a standard curve)

[0122] (1) Add 0 μL, 2.5 μL, 10 μg / mL, 5 μL, 10 μg / mL, 2.5 μL, 10 μg / mL, 2.5 μL, 10 μg / mL, 2.5 μL, 10 μg / mL, 5 μL, 10 μg / mL, 2.5 μL, 40 μg / mL, 2.5 μL, 40 μg / mL, 2.5 μL, 400 μg / mL, 2.5 μL, 400 μg / mL, 2.5 μL, 400 μg / mL, 5 μL, 400 μg / mL, 5 μL, 400 μg / mL, 5 μL, 400 μg / mL, 5 μL, 400 μg / mL of cadmium standard solution to 50 mL of acetic acid-sodium acetate aqueous solution (0.1 μL) in the electrolytic cell. mol / L, pH value is 4.5), so that after each drop, the Cd 2+ The concentrations were: 0, 0.5, 1.5, 2, 2.5, 3, 4, 6, 8, 10, 30, 50, 70, 110, 150, 190, 230 μg / L, and then, the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the star-shaped ZIF-8 modified electrode was used as the working electrode, Ag / AgCl (the internal solution was a saturated KCl solution) was used as the reference electrode, and Pt wire was used as the counter electrode, which were connected to the electrochemical workstation RST5000;

[0123] (2) Under stirring conditions at a speed of 300 r / min, square wave anodic stripping voltammetry was used to detect Cd in the solution of the electrolytic cell. 2+ content (detection parameters are: enrichment potential: -1.2V, enrichment time: 400s, standing time: 10s, starting potential: -1.2V, ending potential: 0.4V, square wave amplitude: 0.025V, potential increment: 0.004V, square wave frequency: 25Hz).

[0124] like Figure 8 As shown, by Cd 2+ The response curve of square wave anodic stripping voltammetry with a concentration of 0.5 to 230 μg / L shows that with the increase of Cd(II) concentration, the stripping peak current gradually increases and the stripping peak potential gradually shifts positively. This is a normal phenomenon, which may be caused by the specific interaction between the modified film and the analyte and will not affect the detection of heavy metal ions. Figure 8 As can be seen from the black dotted line part, the star-shaped ZIF-8 modified electrode has an extremely low background current (as low as 1.5 μA), which is conducive to the detection of target ion dissolution and obtains an extremely low detection limit.

[0125] like Figure 9 、 10 As shown, Cd2+ The sensitivity of detection in the low concentration range (0.5-30 μg / L) was 0.305 μA / μM·cm 2 , Cd 2+ The sensitivity of detection in the high concentration range (30-230 μg / L) was 0.231 μA / μM·cm 2 Using the formula: LOD = 3Sb / S, the detection limit LOD was calculated to be 0.48 μg / L (S / N = 3). The calculated LOD is far lower than the recommended values ​​for drinking water given by WHO and EPA (3 μg / L and 5 μg / L). Through calculation, the linear relationship corresponding to the star-shaped ZIF-8 modified electrode is divided into two parts, expressed as follows:

[0126] 0.5-30 μg / L: I p (μA) = 0.305x + 0.018, R 2 = 0.991;

[0127] 30-230 μg / L: I p (μA) = 0.231x + 5.390, R 2 = 0.995.

[0128] The above two concentration ranges are for different concentrations of Cd adsorbed on the star-shaped ZIF-8 modified electrode. 2+ The redox behavior occurs through adsorption, and the reaction rate varies at different concentrations, so different calibration curves are selected for calculation. Therefore, the star-shaped ZIF-8 modified electrode performs well in the electrochemical detection of heavy metal ions.

[0129] Example 2 of the method for detecting heavy metal cadmium using a star-shaped ZIF-8 modified electrode (detection of neutral simulated cadmium-containing wastewater)

[0130] (1) 50 mL of tap water sample (pH = 7) from Yunnan University was placed in an electrolytic cell, and 5 μL, 10 μg / mL, and 5 μL, 40 μg / mL cadmium standard solutions were added in sequence to obtain Cd 2+ The simulated cadmium-containing tap water sample with a concentration of 5 μg / L was added with 5 μL, 10 μg / mL and 5 μL, 40 μg / mL cadmium standard solutions three times to obtain Cd 2+ A simulated cadmium-containing tap water sample with a concentration of 20 μg / L was added with 2.5 μL of a 400 μg / mL cadmium standard solution to obtain Cd 2+A simulated cadmium-containing tap water sample with a concentration of 40 μg / L was prepared. Then, the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the star-shaped ZIF-8 modified electrode was used as the working electrode, Ag / AgCl (the internal solution was a saturated KCl solution) was used as the reference electrode, and Pt wire was used as the counter electrode, which were connected to the electrochemical workstation RST5000.

[0131] (2) Under stirring conditions at a speed of 300 r / min, square wave anodic stripping voltammetry was used to detect Cd in the water samples in the electrolytic cell. 2+ Content (test parameters are: the same as in Example 1), that’s it.

[0132] Example 3 of the method for detecting heavy metal cadmium using a star-shaped ZIF-8 modified electrode (detection of neutral simulated cadmium-containing wastewater)

[0133] (1) 50 mL of water sample from Laoyu River in Kunming (pH = 6.5) was placed in an electrolytic cell, and 5 μL, 10 μg / mL and 5 μL, 40 μg / mL cadmium standard solutions were added twice in sequence to obtain Cd 2+ The simulated cadmium-containing river water sample with a concentration of 10 μg / L was added with 2.5 μL of 400 μg / mL cadmium standard solution to obtain Cd 2+ A simulated cadmium-containing river water sample with a concentration of 30 μg / L was prepared. Then, the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the star-shaped ZIF-8 modified electrode was used as the working electrode, Ag / AgCl (the internal solution was a saturated KCl solution) was used as the reference electrode, and Pt wire was used as the counter electrode, which were connected to the electrochemical workstation RST5000.

[0134] (2) Under stirring conditions at a speed of 300 r / min, square wave anodic stripping voltammetry was used to detect Cd in the water samples in the electrolytic cell. 2+ Content (test parameters are: the same as in Example 1), that’s it.

[0135] Example 4 of the method for detecting heavy metal cadmium using a star-shaped ZIF-8 modified electrode (detection of acidic cadmium-containing wastewater)

[0136] (1) 15 μL and 30 μL of tobacco wastewater sample digestion solution (Cd 2+ The concentration was 12.8 μg / mL, pH = 0.6) was added dropwise to 50 mL of acetic acid-sodium acetate aqueous solution (0.1 mol / L, pH 4.5) in the electrolytic cell, and the pH value was adjusted to 4.5 with 5 mol / L sodium hydroxide each time to obtain Cd 2+The digestion solution samples of tobacco wastewater samples containing cadmium at concentrations of 3.84 μg / L and 11.52 μg / L were then connected to an electrochemical workstation RST5000 using the star-shaped ZIF-8 modified electrode obtained in Example 1 of the preparation method of the star-shaped ZIF-8 modified electrode as a working electrode, Ag / AgCl (the internal solution was a saturated KCl solution) as a reference electrode, and a Pt wire as a counter electrode;

[0137] (2) Under stirring conditions at a speed of 300 r / min, square wave anodic stripping voltammetry was used to detect Cd in the solution of the electrolytic cell. 2+ Content (test parameters are: the same as in Example 1), that’s it.

[0138] In addition to the quantitative analysis of water samples according to the method of detecting heavy metal cadmium using star-shaped ZIF-8 modified electrode in Examples 1 to 4, inductively coupled plasma mass spectrometry (ICP-MS) was used as a verification method to detect Cd 2+ Among them, the extremely small amount of Cd in tap water samples and Laoyu River water samples in Kunming City 2+ The test results are shown in Table 1.

[0139] Table 1 Detection of Cd in actual water by star-shaped ZIF-8 modified electrode 2+ Measured value of content, recovery rate and comparison table with ICP-MS

[0140]

[0141] Note: The recovery rate in the table is calculated as follows: (Three parallel measurements of Cd 2+ Average content) / Cd 2+ concentration.

[0142] From Table 1, we can see that in Cd 2+ In the range of 0-40 μg / L, Cd 2+ The recovery rate was 95.00-104.96%, which proved that the star-shaped ZIF-8 modified electrode can show good performance in both tap water and river water, and can effectively remove Cd in complex environmental systems. 2+ It also has good selectivity; at the same time, it is proved that the modified electrode has good stability, anti-interference and accuracy when used in practical environments.

Claims

1. A method for preparing star-shaped ZIF-8, characterized in that: The following steps are involved: (1) A zinc salt aqueous solution is mixed with a mixed aqueous solution of a surfactant and 2-methylimidazole, stirred for reaction, centrifuged, washed, and vacuum dried to obtain a precursor; the molar ratio of zinc element in the zinc salt to 2-methylimidazole is 1:100-500; the surfactant includes hexadecyltrimethylammonium bromide; the stirring reaction temperature is room temperature, the rotation speed is 800-1000 r / min, and the time is 3-5 hours; (2) After grinding the precursor obtained in step (1), the precursor is subjected to three-stage heating pyrolysis under a protective atmosphere and naturally cooled to room temperature to obtain star-shaped ZIF-8; the three-stage heating pyrolysis refers to: first heating to 300-500 °C at a rate of 4-6 °C / min, maintaining for 1-3 h, then heating to 500-700 °C at a rate of 4-6 °C / min, maintaining for 1-3 h, and finally heating to 850-950 °C at a rate of 4-6 °C / min, maintaining for 1-3 h.

2. The preparation method of star-shaped ZIF-8 according to claim 1, wherein: In step (1), the concentration of the zinc salt aqueous solution is 10 to 15 mmol / L; the zinc salt includes one or more soluble inorganic zinc salts selected from zinc nitrate, zinc sulfate, zinc chloride, and hydrates thereof; in the mixed aqueous solution of the surfactant and 2-methylimidazole, the concentration of the surfactant is 0.1 to 1.0 mmol / L, and the concentration of the 2-methylimidazole is 200 to 1000 mmol / L; the preparation method of the mixed aqueous solution of the surfactant and 2-methylimidazole is as follows: adding the surfactant and 2-methylimidazole to water and ultrasonically treating the solution; the frequency of the ultrasonic treatment is 30 to 50 kHz, the power is 140 to 160 W, and the time is 5 to 15 min; the washing refers to washing with water and methanol alternately for ≥1 times; the temperature of the vacuum drying is 60 to 80 ° C, and the time is 10 to 15 h.

3. The preparation method of star-shaped ZIF-8 according to claim 1 or 2, wherein: In step (2), the protective atmosphere is argon and / or nitrogen.

4. A method for preparing a star-shaped ZIF-8 modified electrode, characterized in that: The following steps are involved: (1) The Al2O3 slurry is polished in descending order of particle size using the "8" method until the surface of the glassy carbon electrode has a mirror effect; (2) ultrasonically cleaning the glassy carbon electrode polished in step (1) in water and then in anhydrous ethanol, and then drying it in a protective atmosphere; (3) The surface of the glassy carbon electrode dried in step (2) is evenly coated with a perfluorosulfonic acid dispersion of the star-shaped ZIF-8 obtained according to any one of claims 1 to 3, and the mixture is naturally air-dried to obtain a star-shaped ZIF-8 modified electrode.

5. The method for preparing the star-shaped ZIF-8 modified electrode according to claim 4, wherein: In step (1), the mass concentration of the Al2O3 slurry is 70 to 90 g / L; the particle size range is 3 μm to 30 nm; the specific operation of the polishing treatment is: first, the Al2O3 slurry with a particle size of 1 μm to 3 μm is polished on the surface of the glassy carbon electrode by the "drawing 8" method for 180 to 220 times, and then the Al2O3 slurry with a particle size of 30 to 100 nm is polished on the surface of the glassy carbon electrode by the "drawing 8" method for 180 to 220 times.

6. The method for preparing the star-shaped ZIF-8 modified electrode according to claim 4 or 5, wherein: In step (2), the frequency of the ultrasonic cleaning is 30 to 50 kHz, the power is 80 to 120 W, and the time is 10 to 15 s; the protective atmosphere includes nitrogen or argon.

7. The method for preparing the star-shaped ZIF-8 modified electrode according to claim 4 or 5, characterized in that: In step (3), the concentration of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is 0.5 to 1.5 mg / mL; the amount of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 used for drop coating is 0.5 to 2.5 μL / mm 2 ; The preparation method of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is: adding the star-shaped ZIF-8 obtained according to any one of claims 1 to 3 to a perfluorosulfonic acid aqueous solution and ultrasonically dispersing the dispersion; the mass volume ratio of the star-shaped ZIF-8 to the perfluorosulfonic acid aqueous solution is 0.5 to 1.5:1 in mg / mL; in the perfluorosulfonic acid aqueous solution, the volume ratio of perfluorosulfonic acid to water is 1:8 to 10; the frequency of the ultrasonic dispersion is 30 to 50 kHz, the power is 150 to 300 W, and the time is 30 to 40 min.

8. The method for preparing the star-shaped ZIF-8 modified electrode according to claim 6, wherein: In step (3), the concentration of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is 0.5 to 1.5 mg / mL; the amount of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 used for drop coating is 0.5 to 2.5 μL / mm 2 ; The preparation method of the perfluorosulfonic acid dispersion of the star-shaped ZIF-8 is: adding the star-shaped ZIF-8 obtained according to any one of claims 1 to 3 to a perfluorosulfonic acid aqueous solution and ultrasonically dispersing the dispersion; the mass volume ratio of the star-shaped ZIF-8 to the perfluorosulfonic acid aqueous solution is 0.5 to 1.5:1 in mg / mL; in the perfluorosulfonic acid aqueous solution, the volume ratio of perfluorosulfonic acid to water is 1:8 to 10; the frequency of the ultrasonic dispersion is 30 to 50 kHz, the power is 150 to 300 W, and the time is 30 to 40 min.

9. A method for detecting heavy metal cadmium using the star-shaped ZIF-8 modified electrode obtained according to any one of claims 4 to 8, characterized in that: The following steps are involved: (1) When drawing a standard curve, the cadmium standard solution is added dropwise to the electrolyte of the electrolytic cell to obtain a linear concentration of 0.5 to 230 μg / L; or when detecting weak acid or neutral cadmium-containing wastewater, the cadmium-containing wastewater is directly placed in the electrolytic cell; or when detecting acidic cadmium-containing wastewater, the acidic cadmium-containing wastewater is added dropwise to the electrolyte of the electrolytic cell and the pH value is adjusted to 3.5 to 5.5; then, the star-shaped ZIF-8 modified electrode obtained according to any one of claims 4 to 8 is used as a working electrode, Ag / AgCl is used as a reference electrode, and Pt wire is used as a counter electrode to connect to an electrochemical workstation; (2) Under stirring conditions, the heavy metal cadmium ion content of the solution in the electrolytic cell is detected by square wave anodic stripping voltammetry.

10. The method for detecting heavy metal cadmium using the star-shaped ZIF-8 modified electrode according to claim 9, wherein: In step (1), the concentration of the cadmium standard solution is 10 to 500 μg / mL; the volume ratio of the electrolyte to the cadmium standard solution is 50 mL:2 μL to 100 μL; the electrolyte is an acetic acid-sodium acetate aqueous solution; the mixed concentration of the acetic acid-sodium acetate aqueous solution is 0.05 to 0.15 mol / L, and the pH value is 3.5 to 5.5; the pH value of the weakly acidic or neutral cadmium-containing wastewater is 4.5 to 8.0; the pH value of the acidic cadmium-containing wastewater is less than 4.5; the volume ratio of the electrolyte to the acidic cadmium-containing wastewater is 50 mL:2 μL to 100 μL; when the cadmium ion concentration in the acidic, weakly acidic or neutral cadmium-containing wastewater is too high or too low, or even exceeds the detection limit, its concentration is adjusted by concentrating, diluting or adding dropwise to a buffer solution so that its detection concentration in the electrolytic cell is 0.5 to 230 μg / L.

11. The method for detecting heavy metal cadmium using the star-shaped ZIF-8 modified electrode according to claim 9 or 10, characterized in that: In step (2), the stirring speed is 200 to 400 r / min; the detection parameters of the square wave anodic stripping voltammetry are: enrichment potential: -0.9 to -1.3 V, enrichment time: 50 to 500 s, standing time: 5 to 20 s, starting potential: -1.3 to -1.0 V, ending potential: 0.2 to 0.6 V, square wave amplitude: 0.02 to 0.03 V, potential increment: 0.002 to 0.005 V, and square wave frequency: 20 to 30 Hz.

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