Composite sensing electrode and preparation method and application thereof

Abstract

The invention discloses a composite sensing electrode and a preparation method and application thereof, and belongs to the technical field of environmental detection.The preparation method comprises the steps that CNC is activated, then the activated CNC is modified with a CdS cluster material, a CdS-CNC composite electrode material is obtained, then a GCE electrode is modified with the CdS-CNC composite electrode material, and a CdS-CNC-GCE composite sensing electrode is obtained; the invention also discloses the CdS-coated CNC-GCE composite sensing electrode prepared by the method and application of the composite sensing electrode in detection of heavy metal elements in an aqueous solution. The CdS-coated CNC-GCE composite sensing electrode has the beneficial effects that an effective technical method and way are provided for improving the electrode performance of a heavy metal sensor, the prepared CdS-coated CNC-GCE composite sensing electrode is high in anti-interference performance and good in stability, the content of various heavy metals can be timely and rapidly detected, and the purpose of dynamically monitoring the change of the content of the heavy metals in the production activity period is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental monitoring, in particular to a composite sensing electrode, a preparation method thereof and an application thereof. Background Art

[0002] Heavy metal pollution is mainly caused by human factors such as mining, waste gas emissions in production activities, sewage irrigation and the use of products with excessive heavy metals, resulting in environmental pollution of the atmosphere, soil, water bodies, etc., and has become a global environmental problem that has attracted much public attention. Toxic heavy metals often refer to the "five toxic elements" with biological toxicity headed by cadmium, mercury, arsenic, lead and chromium. Once these heavy metals and harmful elements enter the human body, they will continuously accumulate, causing DNA damage and chronic poisoning.

[0003] To achieve trace detection of HMIs in water samples, a variety of conventional techniques have been adopted, including atomic absorption spectrometry, neutron activation analysis, ion chromatography, ultraviolet-visible spectroscopy, inductively coupled plasma mass spectrometry and other methods. These detection techniques have high selectivity and sensitivity, but they require the use of complex and expensive instrument equipment, are time-consuming and cannot be used for on-site determination. Electrochemical detection methods, especially differential pulse stripping voltammetry (DPSV), stand out for their simple sample preparation, low equipment cost, easy miniaturization, high precision, long stability and fast response speed, and do not require the use of large-scale instruments. The detection results can be qualitatively and quantitatively analyzed through the peak change of the response current curve, and it has been widely used in the rapid detection of heavy metals.

[0004] In recent years, great progress has been made in the research of detecting heavy metals using electrochemical detection techniques. However, the currently proposed heavy metal electrochemical sensors have a single detection object and usually can only detect one or two heavy metals. Simultaneously detecting multiple heavy metal ions with high sensitivity and strong anti-interference ability is still a major challenge faced by the development of electrochemical methods. We need to develop an electrochemical sensing electrode with high sensitivity, high selectivity, high stability and capable of simultaneously detecting multiple heavy metals to achieve the purpose of dynamically monitoring the change of heavy metal content during production activities and providing reference value for controlling the heavy metal content. Summary of the Invention

[0005] The purpose of the present invention is to provide a composite sensing electrode, a preparation method thereof and an application thereof, so as to solve the problem that the existing methods for detecting heavy metals have high requirements for equipment and detection techniques and it is difficult to detect the content of multiple heavy metals in the process of production activities in a timely and rapid manner.

[0006] The purpose of the present invention is achieved through the following technical solutions: Preparation method of composite sensing electrode. The preparation method activates CNC, then modifies the activated CNC with CdS cluster material to obtain CdS@CNC composite electrode material, and then modifies the GCE electrode with the CdS@CNC composite electrode material to prepare the CdS@CNC-GCE composite sensing electrode.

[0007] Further, the activation of CNC includes the following steps: X1. Add CNC and concentrated nitric acid into a container in sequence, and immerse the CNC in the concentrated nitric acid to obtain a CNC dispersion; X2. Seal the container and place it in the dark. After the CNC is activated, separate the activated CNC; X3. Wash the separated CNC with deionized water and anhydrous ethanol in sequence, and then dry it for later use.

[0008] Further, the modification of the activated CNC with CdS cluster material includes the following steps: Y1. Prepare a Cd precursor solution and an S precursor solution respectively; Y2. Take the Cd precursor solution, the S precursor solution, deionized water and butylamine, mix them evenly to obtain a cluster growth mixed solution; Y3. Immerse the activated CNC in the cluster growth mixed solution, grow a cluster structure on the surface of the CNC at room temperature, and separate the CdS@CNC cluster material; Y4. Wash the CdS@CNC cluster material with deionized water and anhydrous ethanol in sequence, and then dry it to obtain the CdS@CNC composite electrode material.

[0009] Further, the modification of the GCE electrode with the CdS@CNC composite electrode material includes the following steps: Z1. Disperse the CdS@CNC composite electrode material in deionized water and perform ultrasonic treatment to obtain a CdS@CNC dispersion; Z2. Take the CdS@CNC dispersion and a Nafion solution, mix them and perform ultrasonic treatment to obtain a mixed solution, drop the mixed solution on the surface of the polished and ground GCE electrode, and dry it to obtain the CdS@CNC-GCE composite sensing electrode.

[0010] Further, in step X1, control the concentration of the CNC dispersion to be 2 mg / ml.

[0011] The present invention also provides a composite sensing electrode, and the composite sensing electrode is the CdS@CNC-GCE composite sensing electrode prepared by the above preparation method of the composite sensing electrode.

[0012] The present invention also provides the application of the composite sensing electrode, specifically the application of the CdS@CNC-GCE composite sensing electrode in the detection of heavy metal elements in aqueous solution.

[0013] Furthermore, the heavy metal elements include: Zn, Cd, Pb, Cu, and Hg.

[0014] Furthermore, the steps for the CdS@CNC-GCE composite sensing electrode to detect the heavy metal content in aqueous solution are as follows: a. Place the CdS@CNC-GCE composite sensing electrode in an electrolyte solution containing an ABS buffer solution and standard solutions of heavy metal ions with different concentrations, and use the DPSV test method of the three-electrode system for testing. During the current-voltage scanning process, record the current values generated by the corresponding heavy metal ions on the electrode surface as the voltage changes. b. According to the concentrations of the heavy metal ion standard solutions and the corresponding current response peaks, make a standard curve and construct a linear equation. c. Replace the standard solutions of heavy metal ions with different concentrations in step a with the aqueous solution to be tested, and perform the test according to the test method in step a to obtain the corresponding current response value, and substitute it into the linear equation constructed in step b to calculate the content of heavy metal ions in the aqueous solution to be tested.

[0015] Furthermore, in step b, the constructed linear equation is: Ⅰp = aXmeal + b, where Xmetal is the concentration of the heavy metal ion standard solution, and Ⅰp is the current response peak.

[0016] The present invention has the following advantages: 1. By modifying and activating CNC with CdS cluster materials, the CdS@CNC composite electrode material is obtained, and then the GCE electrode is modified to prepare the CdS@CNC-GCE composite sensing electrode, which provides an effective technical method and approach for improving the performance of heavy metal sensor electrodes.

[0017] 2. The prepared CdS@CNC-GCE composite sensing electrode has strong anti-interference ability and good stability, can detect the contents of multiple heavy metals in a timely and rapid manner, and achieves the purpose of dynamically monitoring the changes in heavy metal contents during production activities. Description of the Drawings

[0018] Figure 1 It is the SEM diagram of the unmodified CNC and the CdS@CNC composite electrode material.

[0019] Figure 2 It is the elemental map of the CdS@CNC composite electrode material.

[0020] Figure 3 It is the TEM diagram and the elemental distribution diagram of the CdS@CNC composite electrode material.

[0021] Figure 4 It is the XRD pattern of the CdS@CNC composite electrode material.

[0022] Figure 5 It is the performance detection diagram of the CdS@CNC-GCE composite sensing electrode.

[0023] Figure 6 It is the standard curve of the heavy metal concentration and the peak value of the current response of the CdS@CNC-GCE composite sensing electrode.

[0024] Figure 7 It is the verification diagram of the anti-interference property (A) and stability (B) of the CdS@CNC-GCE composite sensing electrode. Specific implementation mode

[0025] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and shown in the accompanying drawings here can be arranged and designed in various different configurations.

[0026] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

[0027] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0028] It should be noted that: similar reference numerals and letters denote similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0029] In the description of the present invention, it should also be noted that, unless otherwise clearly defined and limited, the terms "set", "installed", "connected", "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0030] Example 1

[0031] This embodiment provides a method for preparing a composite sensing electrode. The preparation method comprises activating CNC (carbon nanocoil with a spatial spiral structure), then modifying the activated CNC with a CdS (cadmium sulfide) cluster material to obtain a CdS@CNC composite electrode material, and then modifying a GCE electrode (glassy carbon electrode) with the CdS@CNC composite electrode material to obtain a CdS@CNC-GCE composite sensing electrode.

[0032] Among them, CNC activation includes the following steps: X1. Add 100 mg CNC into a 100 ml transparent glass reagent bottle, then add 50 ml concentrated nitric acid into the transparent glass reagent bottle containing CNC, and immerse the CNC in the concentrated nitric acid to prepare a CNC dispersion with a concentration of 2 mg / ml; X2. Seal the transparent glass reagent bottle and place it away from light for 3 days. After the CNC is activated, separate the activated CNC from the concentrated nitric acid using a vacuum filter; X3. Wash the separated CNC with deionized water and anhydrous ethanol for 5-10 times, and then dry the CNC in an oven at 60°C for later use.

[0033] Carbon nanocoils are helical carbon nanotubes. Unlike the line contact between carbon nanotubes and the surface contact between graphene, carbon nanocoils are in point contact, which minimizes the intermolecular force. Therefore, they are basically monodispersed in water or other organic solvents, solving the problem of carbon nanomaterial agglomeration and making the largest use of the large specific surface area advantage of carbon nanomaterials. At the same time, carbon nanocoils also inherit the excellent properties of carbon nanotubes, such as good conductivity, excellent mechanical properties and physical and chemical stability. In addition, the helicity of carbon nanocoils brings advantages that carbon nanotubes do not have. First, the spatial porosity brought by the helix is conducive to the diffusion and adsorption of the HMIs to be tested, which can effectively improve the sensitivity of detection. Second, due to the defects introduced by the helical structure, the coil surface has a large number of dangling bonds, which increases the chemical activity of the surface and is conducive to the composite of nanometals.

[0034] In this embodiment, CNC is acidified by concentrated nitric acid, which can introduce a large number of oxygen-containing functional groups such as carboxyl, hydroxyl, and nitro groups on its surface, which not only significantly improves its hydrophilicity and surface activity, but also effectively improves its dispersion stability in polar solvents and avoids agglomeration. At the same time, the strong oxidizing property of concentrated nitric acid can be used to remove impurities and amorphous carbon on the surface of CNC, thereby improving the purity and structural integrity of the material.

[0035] By controlling the concentration of the CNC dispersion to 2 mg / ml, while ensuring sufficient oxidation, the dispersion uniformity and surface activity of CNC in the reaction system can be effectively improved, which helps to introduce functional groups, enhance the interfacial binding ability, and improve the efficiency and controllability of subsequent functional modification.

[0036] The modification and activation of CNC by CdS cluster materials include the following steps: Y1. Respectively prepare a Cd precursor solution containing cadmium chloride, 3-mercaptopropionic acid, deionized water, and potassium hydroxide, and an S precursor solution containing thiourea and deionized water; Y2. Take 300 μL of the Cd precursor solution, 300 μL of the S precursor solution, 6 ml of deionized water, and 6 ml of butylamine, and mix them evenly to obtain a cluster growth mixed solution; Y3. Immerse the activated CNC in the cluster growth mixed solution, let it stand for 3 days, grow cluster structures on the surface of CNC at room temperature, and separate the CdS@CNC cluster material from the solution by a vacuum filtration machine; Y4. Wash the separated CdS@CNC cluster material 5 - 10 times successively with deionized water and absolute ethanol, and then dry it in an oven at 60 °C to obtain the CdS@CNC composite electrode material.

[0037] SEM characterization is as Figure 1 shown, Figure 1 (A), Figure 1 (B) is the unmodified electrode CNC, Figure 1 (C), Figure 1 (D) is the CdS@CNC composite electrode material. The element maps are as Figure 2 shown, Figure 2 (B), Figure 2 (C), Figure 2 (D) are the element maps of C, S, and Cd respectively.

[0038] It can be seen from Figure 1 that the CdS@CNC composite electrode material is formed by depositing CdS clusters on the surface of the unmodified carbon nanocoil CNC, which can prove the effective synthesis of the composite electrode material. It can be seen from Figure 2 (B), Figure 2 (C), Figure 2 (D) that the S element and the Cd element are evenly distributed on the CNC, which also verifies the successful synthesis of the CdS@CNC composite electrode material again.

[0039] TEM characterization is as Figure 3 shown, which characterizes the TEM image of the CdS@CNC composite electrode material. Figure 3 (C), Figure 3 (D) are the distribution situations of the S and Cd elements respectively.

[0040] It can be observed that S and Cd elements are evenly distributed on the CNC, indicating the successful synthesis and uniform distribution of the CdS@CNC composite electrode material. Figure 3 It can be observed that S and Cd elements are evenly distributed on the CNC, indicating the successful synthesis and uniform distribution of the CdS@CNC composite electrode material.

[0041] The XRD characterization is as Figure 4 shown. The figure shows the XRD pattern of the CdS@CNC composite electrode material, with the diffraction angle on the abscissa and the diffraction intensity on the ordinate.

[0042] It can be obtained that the XRD peaks of the CdS@CNC composite electrode material are basically consistent with the peaks of the CdS standard card (PDF#89-0440), further proving the effective synthesis of the CdS@CNC composite electrode material. Figure 4 It can be obtained that the XRD peaks of the CdS@CNC composite electrode material are basically consistent with the peaks of the CdS standard card (PDF#89-0440), further proving the effective synthesis of the CdS@CNC composite electrode material.

[0043] When constructing a heavy metal ion electrochemical sensor, introducing CdS clusters as a modification material has significant advantages. First, CdS clusters are composed of nanoparticles, with a high specific surface area and abundant surface active sites, which are beneficial to the adsorption and enrichment of heavy metal ions. Second, CdS is a typical II-VI group semiconductor material with good electron transport performance, which can effectively promote the charge transfer process at the electrode interface. In addition, the sulfide ions (S²⁻) on the surface of CdS can form stable complex structures with heavy metal ions such as Pb²⁺ and Hg²⁺, enhancing the selective recognition ability of the material. Modifying CdS clusters on the well-conductive carbon nanocoils CNC not only improves the electrochemical activity of the electrode but also realizes the interfacial synergistic effect, contributing to the construction of a sensitive, stable and highly selective heavy metal detection platform.

[0044] After the CdS clusters are combined with the CNC, on the one hand, a fast electron channel is constructed through the CNC to improve the overall conductivity, and on the other hand, CdS is used to enrich and identify the target ions, thus significantly enhancing the response current and selective detection ability of the electrode. This configuration combines structural innovation and functional synergy, breaking through the performance bottleneck of traditional electrode materials and having great application potential.

[0045] The modification of the GCE electrode with the CdS@CNC composite electrode material includes the following steps: Z1. Disperse 1 mg of the CdS@CNC composite electrode material in 1 ml of deionized water and ultrasonically treat it for 10 min to obtain a CdS@CNC dispersion; Z2. Take 180 μL of the CdS@CNC dispersion and 20 μL of the Nafion solution, mix them and ultrasonically treat for 10 min to obtain a mixed solution. Take 5 μL of the mixed solution and drop-coat it on the surface of the polished GCE electrode to prepare a CdS cluster-modified N-CNT / CC composite electrode (CdS@CNT / CC). Then dry it in an oven at 60 °C for 10 min to obtain a CdS@CNC-GCE composite sensing electrode.

[0046] Among them, the polishing method of the GCE electrode is as follows: Sprinkle a small amount of alumina polishing powder on the suede, hold the electrode vertically and polish it in an "8" shape. Polish it successively with two specifications of alumina polishing powder of 0.3 μm and 0.05 μm until the surface of the GCE electrode is as smooth as a mirror. Ultrasonically clean it with deionized water and absolute ethanol successively for 3 minutes, and dry it in an oven at 60 °C for later use.

[0047] Example 2

[0048] This example provides a CdS@CNC-GCE composite sensing electrode prepared by the method described in Example 1.

[0049] Example 3

[0050] This example also provides the application of the CdS@CNC-GCE composite sensing electrode as in Example 2, specifically the application of the CdS@CNC-GCE composite sensing electrode in the detection of heavy metal elements in aqueous solution.

[0051] Among them, the heavy metal elements include: Zn, Cd, Pb, Cu, and Hg.

[0052] The specific steps for rapidly detecting the heavy metal content in aqueous solution using the CdS@CNC-GCE composite sensing electrode are as follows: a. Add 0 μL, 20 μL, 40 μL, 100 μL, 200 μL, 300 μL, 400 μL, 500 μL, 600 μL, 700 μL, 800 μL of heavy metal ion standard solutions to 100 ml of electrolyte (prepared with ABS acetate buffer: 1 mol / L acetic acid, 1 mol / L sodium acetate, adjust the pH of the electrolyte to 6) respectively, so that the heavy metal concentration range in the detection system is 0 - 8 μg / mL. Use the DPSV test method of the three-electrode system, place the CdS@CNC-GCE composite sensing electrode in the above standard solutions with different concentrations for current-voltage scanning, and record the current values generated by the corresponding heavy metal ions on the electrode surface with voltage change during the whole scanning process; Such as Figure 5As shown in the figure, the sub-figures are the DPSV curves for the simultaneous detection of heavy metal ions Zn(Ⅱ), Cd(Ⅱ), Pb(Ⅱ), Cu(Ⅱ) and Hg(Ⅱ), the DPSV curve for the separate gradient detection of Zn(Ⅱ), the DPSV curve for the separate gradient detection of Cd(Ⅱ), the DPSV curve for the separate gradient detection of Pb(Ⅱ), the DPSV curve for the separate gradient detection of Cu(Ⅱ), and the DPSV curve for the separate gradient detection of Hg(Ⅱ), which reflect the current values generated by heavy metal ions on the electrode surface with the change of voltage. Figure 5 (A) It shows that the CdS@CNC-GCE composite sensing electrode can simultaneously detect heavy metal ions Zn(Ⅱ), Cd(Ⅱ), Pb(Ⅱ), Cu(Ⅱ) and Hg(Ⅱ) without mutual interference. Figure 5 (B), Figure 5 (C), Figure 5 (D), Figure 5 (E), Figure 5 (F) It shows that the CdS@CNC-GCE composite sensing electrode can separately detect heavy metal ions Zn(Ⅱ), Cd(Ⅱ), Pb(Ⅱ), Cu(Ⅱ) and Hg(Ⅱ), and the response values for separate detection will increase with the increase of the concentration of heavy metal ions.

[0053] b. According to the concentration of the heavy metal ion standard solution and the corresponding current response peak value, the heavy metal concentration and the current response peak value are corresponded, and the Origin software is used to fit a linear equation to make a standard curve, and a linear equation is constructed: Ⅰp = aXmeal + b, where Xmetal is the concentration of the heavy metal ion standard solution, and Ⅰp is the current response peak value, and the standard curve of the heavy metal concentration and the current response peak value of the CdS@CNC-GCE composite sensing electrode as shown in Figure 6 is obtained; As shown in Figure 6As shown in the figure, the sub-figures in the figure are the linear relationships between the concentrations and current peaks of heavy metal ions Zn(Ⅱ), Pb(Ⅱ), Cd(Ⅱ), Cu(Ⅱ) and Hg(Ⅱ) when detected separately. Among them: the regression equation of the current response change value for detecting Zn(Ⅱ) is Ⅰ = 0.697665XZn + 2.71, and the correlation coefficient is 0.9942; the regression equation of the current response change value for detecting Pb(Ⅱ) is Ⅰ = 1.85XPb + 2.36, and the correlation coefficient is 0.99252; the regression equation of the current response change value for detecting Cd(Ⅱ) is Ⅰ = 3.43XCd + 1.01, and the correlation coefficient is 0.99367; the regression equation of the current response change value for detecting Cu(Ⅱ) is Ⅰ = 0.53045XCu + 3.06, and the correlation coefficient is 0.99239; the regression equation of the current response change value for detecting Hg(Ⅱ) is Ⅰ = 0.905369XHg + 1.53, and the correlation coefficient is 0.99383. According to the above regression equations, it can be shown that the CdS@CNC-GCE composite sensing electrode has good detection performance for heavy metal ions Zn(Ⅱ), Cd(Ⅱ), Pb(Ⅱ), Cu(Ⅱ) and Hg(Ⅱ).

[0054] c. Detection of heavy metals in the sample: Replace the standard solutions of different concentrations of heavy metal ions in step a with the aqueous solution to be tested, and perform tests according to the test method in step a to obtain the corresponding current response values, and substitute them into the linear equation constructed in step b to calculate the content of heavy metal ions in the aqueous solution to be tested.

[0055] Anti-interference test of CdS@CNC-GCE composite sensing electrode: Place the CdS@CNC-GCE composite sensing electrode in 100 mL of electrolyte, and 400 μl of standard solutions of heavy metal ions Zn(Ⅱ), Cd(Ⅱ), Pb(Ⅱ), Cu(Ⅱ) and Hg(Ⅱ) are pre-added to this electrolyte. Then, add interference ions Al(Ⅲ), As(Ⅱ), Bi(Ⅱ), Ca(Ⅱ), Co(Ⅱ), Cr(Ⅱ), Fe(Ⅲ), Mg(Ⅱ), Mn(Ⅱ), Na(Ⅰ), Ni(Ⅱ) to the above electrolyte in turn. Each time a heavy metal ion is added, perform two DPSV tests to obtain the detection curve under the interference state, as Figure 7 It can be seen from the relationship between different interference ions and the current response peak value of the CdS@CNC-GCE composite sensing electrode shown in (A) that the composite sensing electrode of the present application has good anti-interference ability.

[0056] Stability test of CdS@CNC-GCE composite sensing electrode: Fifteen CdS@CNC-GCE composite sensing electrodes were prepared and subjected to DPSV detection respectively. 400 μl of standard solutions of heavy metal ions Zn(II), Cd(II), Pb(II), Cu(II) and Hg(II) were added to 100 mL of electrolyte, as Figure 7 The peak current responses of multiple CdS@CNC-GCE composite sensing electrodes shown in (B) were obtained by testing in the standard solution, indicating that the composite sensing electrodes prepared by this method have good stability.

[0057] In the verification of anti-interference and stability, the electrolyte was an acetate buffer solution of ABS; the concentration of heavy metal ions was 0.4 μg / mL.

[0058] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent substitution on some of the technical features. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. Preparation method of composite sensing electrode, characterized in that: The preparation method activates CNC, then modifies the activated CNC with CdS cluster material to obtain a CdS@CNC composite electrode material, and then modifies a GCE electrode with the CdS@CNC composite electrode material to prepare a CdS@CNC-GCE composite sensing electrode.

2. The preparation method of the composite sensing electrode according to claim 1, characterized in that: The activation of the CNC includes the following steps: X1. Add CNC and concentrated nitric acid into a container in sequence, and immerse the CNC in the concentrated nitric acid to obtain a CNC dispersion; X2. Seal the container and place it in the dark. After the CNC is activated, separate the activated CNC; X3. Wash the separated CNC with deionized water and anhydrous ethanol in sequence, and then dry it for standby.

3. The preparation method of the composite sensing electrode according to claim 2, characterized in that: The modification of the activated CNC with the CdS cluster material includes the following steps: Y1. Prepare a Cd precursor solution and an S precursor solution respectively; Y2. Take the Cd precursor solution, the S precursor solution, deionized water and butylamine, mix them evenly to obtain a cluster growth mixed solution; Y3. Immerse the activated CNC in the cluster growth mixed solution, grow a cluster structure on the surface of the CNC at room temperature, and separate the CdS@CNC cluster material; Y4. Wash the CdS@CNC material with deionized water and anhydrous ethanol in sequence, and then dry it to obtain a CdS@CNC composite electrode material.

4. The preparation method of the composite sensing electrode according to claim 3, characterized in that: The modification of the GCE electrode with the CdS@CNC composite electrode material includes the following steps: Z1. Disperse the CdS@CNC composite electrode material in deionized water and perform ultrasonic treatment to obtain a CdS@CNC dispersion; Z2. Take the CdS@CNC dispersion and a Nafion solution, mix them and perform ultrasonic treatment to obtain a mixed solution, drop the mixed solution on the surface of the polished and buffed GCE electrode, and dry it to obtain a CdS@CNC-GCE composite sensing electrode.

5. The preparation method of the composite sensing electrode according to claim 2, characterized in that: In the step X1, the concentration of the CNC dispersion is controlled to be 2 mg / ml.

6. Composite sensing electrode, characterized in that: A CdS@CNC-GCE composite sensing electrode is prepared by the preparation method of the composite sensing electrode according to any one of claims 1-5.

7. Application of the composite sensing electrode, characterized in that: Use of the CdS@CNC-GCE composite sensing electrode according to claim 6 in the detection of heavy metal elements in an aqueous solution.

8. The application of the composite sensing electrode according to claim 7, characterized in that: The heavy metal elements include: Zn, Cd, Pb, Cu and Hg.

9. The application of the composite sensing electrode according to claim 8, characterized in that: The steps for the CdS@CNC-GCE composite sensing electrode to detect the heavy metal content in an aqueous solution are as follows: a. Place the CdS@CNC-GCE composite sensing electrode in an electrolyte containing an ABS buffer solution and standard solutions of heavy metal ions with different concentrations, and perform testing using the DPSV testing method of a three-electrode system. During the current-voltage scanning process, record the current values generated by the corresponding heavy metal ions on the electrode surface with the change of voltage; b. According to the concentrations of the standard solutions of heavy metal ions and the corresponding current response peaks, make a standard curve and construct a linear equation; c. Replace the standard solutions of heavy metal ions with different concentrations in step a with the aqueous solution to be tested, perform testing according to the testing method in step a, obtain the corresponding current response value, and substitute it into the linear equation constructed in step b to calculate the content of heavy metal ions in the aqueous solution to be tested.

10. The application of the composite sensing electrode according to claim 9, characterized in that: In the step b, the constructed linear equation is: Ip = aXmeal + b, where Xmetal is the concentration of the heavy metal ion standard solution, and Ip is the peak value of the current response.

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