Preparation Method and Application of an Efficient and Sensitive NHDC Electrochemical Sensor

Abstract

The present invention relates to the field of electrochemical sensing technology, and particularly to a method for preparing an efficient and sensitive NHDC electrochemical sensor and its application. A two-step drop-coating method is adopted. First, the CuO dispersion is drop-coated on the electrode surface, and then the ZIF-8@CNTs dispersion is continuously drop-coated on the electrode surface to prepare a CuO / ZIF-8@CNTs double-layer composite film modified electrode, thereby constructing a rapid and sensitive NHDC electrochemical sensor. The present invention utilizes the high electrocatalytic activity of the CuO material, the high specific surface area and high porosity of the ZIF-8 material, and the high conductivity of the CNTs to prepare a CuO / ZIF-8@CNTs double-layer composite film, and then compositely modifies it on the surface of the glassy carbon electrode. The synergistic effect of the three enhances the electrochemical response signal of NHDC on its electrode surface, and there is a good linear relationship between the oxidation current of NHDC and the concentration within a certain concentration range, with a low detection limit.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical sensing technology, and particularly relates to a preparation method and application of an efficient and sensitive NHDC electrochemical sensor. Background Art

[0002] As a low-calorie artificial sweetener, NHDC has been widely used in various foods and beverages. And it has many biological activities such as inhibiting liver oxidative damage, antioxidant effect, anti-inflammatory effect and anti-apoptosis effect, which has attracted wide attention in the field of biomedicine. Despite the extensive application and development of NHDC in various fields, the standard of its dosage is also an important link in the preparation and production process. Electrochemical analysis technology is widely used in fields such as environmental monitoring, food quality control, medical diagnosis and chemical process detection. This method is different from traditional analytical detection methods and has advantages such as sensitive response signal, low detection concentration and convenient operation.

[0003] Electrochemical analysis technology has become a platform for the application and development of nanomaterials, and nanomaterial-modified electrodes also provide good reliability and efficient real-time diagnostic analysis capabilities for electrochemical sensing. Metal oxide nanostructures have attracted wide attention due to their excellent properties such as high specific surface area and electrocatalytic activity. Copper oxide (CuO) nanomaterials belong to P-type semiconductors. Due to their high specific surface area and quantum size effect, nanoscale CuO has good catalytic performance.

[0004] The high porosity and specific surface area of ZIFs can load guest molecules and / or target substances with high catalytic activity, providing inherent sensitivity for electrochemical detection. The multi-porosity of ZIFs materials allows guest molecules to diffuse into the bulk structure, and the shape and size of the pores produce selectivity for the shape and size of the guest. Due to the porous topological structure and high specific surface area characteristics of ZIF-8 materials, they are widely used in electrochemical analysis. Summary of the Invention

[0005] The present invention aims to combine CuO with high electrocatalytic activity, ZIF-8 with porous structure and CNTs with high conductivity, and the three act synergistically for more sensitive detection and analysis of NHDC content. The materials are modified on the surface of a glassy carbon electrode by a simple drop-casting method to construct a fast and sensitive NHDC electrochemical sensor.

[0006] The present invention provides a preparation method of an efficient and sensitive NHDC electrochemical sensor. The steps of the preparation method are as follows:

[0007] A two-step drop-coating method is adopted. First, the CuO dispersion is drop-coated on the surface of the glassy carbon electrode. After natural drying, the ZIF-8@CNTs dispersion is continuously drop-coated on the surface of the above electrode. After natural drying, a CuO / ZIF-8@CNTs double-layer composite film modified electrode, namely the electrochemical sensor, is prepared.

[0008] Furthermore, the CuO dispersion is prepared by dispersing CuO nanomaterials in double-distilled water, with a concentration of 0.5 - 3 mg / ml, preferably 1 mg / ml, a volume of 4 - 12 μL, and preferably 8 μL.

[0009] Furthermore, the ZIF-8@CNTs dispersion is prepared by dispersing ZIF-8@CNTs materials in solvent A, with a concentration of 0.5 - 3 mg / ml, preferably 1 mg / ml, a volume of 4 - 12 μL, and preferably 8 μL. The ZIF-8@CNTs materials are prepared by the following method:

[0010] CNTs and polyvinylpyrrolidone PVP are added to solvent B, and ultrasonic dispersion is carried out to form a uniform suspension. Then, Zn(CH 3 COO) 2 ·2H 2 O and 2-methylimidazole MeIM are added and mixed evenly. After standing (preferably standing for 12 h), suction filtration, washing, and drying (preferably, the obtained solid is washed 3 times with absolute ethanol and double-distilled water respectively, and finally dried in an oven at 80 °C for 12 h) are carried out to obtain ZIF-8@CNTs.

[0011] Even further, the solvent B is absolute methanol; the solvent A is a solution formed by a 0.5% chitosan solution and double-distilled water with a volume ratio of 1:9.

[0012] Furthermore, after the glassy carbon electrode (GCE) is polished, it is ultrasonically treated in dilute nitric acid solution, ethanol aqueous solution, and double-distilled water for 1 min respectively, and then left to dry naturally before being put into use.

[0013] The present invention also provides the NHDC electrochemical sensor obtained by the above preparation method, which is prepared by the above method.

[0014] The present invention also provides the application of the above NHDC electrochemical sensor in the detection of NHDC content.

[0015] Furthermore, the application is to use the electrochemical sensor for the detection of NHDC content in products containing NHDC (such as white liquor).

[0016] Furthermore, the specific steps of the above application are as follows:

[0017] (1) Use the prepared electrochemical sensor as the working electrode, a saturated calomel electrode as the reference electrode, and a platinum wire electrode as the counter electrode to form a three-electrode system;

[0018] (2) Place the above three-electrode system in the electrolyte. The electrolyte is a pH = 3.0 MBS solution formed by mixing 0.1 M citric acid and 0.2 M disodium hydrogen phosphate solution. Add NHDC standard solutions with different concentrations to it. Use the DPV method to measure the DPV curves of NHDC standard solutions in the concentration range of 0.03 - 30 μM, record the corresponding peak currents and peak potentials, and then plot the i pa1 -c NHDC standard curve, and further obtain the linear relationship of detection and its corresponding fitting linear equation.

[0019] Further, the electrochemical sensor has a good linear relationship for detecting NHDC. In the low concentration range of 0.03 - 10 μM, its fitting linear equation: i pa1 (μA) = 0.0638c NHDC (μM) + 0.0553, R 2 = 0.9938, and the detection limit is calculated to be 16 nM.

[0020] Further, the conditions for the above DPV method test are: open circuit stirring for 150 s, potential window 0.4 - 0.8 V, amplitude: 0.05 V, pulse width: 0.05 s, sampling width: 0.0167 s, pulse period: 0.5 s. The advantages and beneficial effects of the present invention are as follows:

[0021] CuO and ZIF-8@CNTs have the best dispersion effect in the mixture of double-distilled water and 0.5% chitosan solution. After ultrasonic treatment, the dispersion liquid will not layer when standing, and the 0.5% chitosan solution is beneficial to fixing the CuO and ZIF-8@CNTs materials on the surface of the glassy carbon electrode to prevent the CuO and ZIF-8@CNTs from falling off.

[0022] Through experimental research, the CuO modified electrode has a response to the detection of NHDC, which is due to the electrocatalytic activity of CuO; the ZIF-8@CNTs modified electrode also has a response to the detection of NHDC, which is due to the adsorption of NHDC molecules by the porous structure of ZIF-8, increasing the accumulation amount of NHDC on the electrode surface and playing a sensitizing role.

[0023] The present invention utilizes the high electrocatalytic activity of CuO material, the high specific surface area, high porosity of ZIF-8 material, and the high electrical conductivity of CNTs to prepare a CuO / ZIF-8@CNTs double-layer composite film, and then compositely modifies it on the surface of a glassy carbon electrode. The three act synergistically (the oxidation peak potential of NHDC detected by the CuO / ZIF-8@CNTs modified electrode is relatively low, (a relatively low oxidation peak potential has a relatively high apparent rate constant and high catalytic performance on the surface of the modified electrode, which is more conducive to the rapid response and catalytic oxidation of NHDC), and the oxidation peak current value is the largest, where i pa1 are respectively: CuO: 0.3912 μA; ZIF-8@CNTs: 0.9707 μA; CuO / ZIF-8@CNTs: 2.186 μA , the current value 2.186 μ A > 0.9707 μA + 0.3912 μA, showing an effect of 1 + 1 > 2. This result indicates that CuO in the CuO / ZIF-8@CNTs modified electrode has a strong catalytic oxidation effect on NHDC, ZIF-8 has an adsorption effect on NHDC to increase the accumulation amount of NHDC on the surface of the modified electrode, CNTs improve the electrical conductivity of the overall composite material, and enhance the current signal response of the redox of NHDC) enhances the electrochemical response signal of NHDC on its electrode surface, and there is a good linear relationship between the oxidation current of NHDC and the concentration within a certain concentration range, with a low detection limit. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the preparation of the CuO / ZIF-8@CNTs composite film modified electrode and the process for NHDC detection;

[0025] Figure 2 (a) CNTs; (b) CuO; (c) ZIF-8@CNTs; (d) SEM images of CuO / ZIF-8@CNTs on the surface of a glassy carbon sheet substrate;

[0026] Figure 3 CV response curves of CuO, CNTs, CuO / ZIF-8@CNTs, and ZIF-8@CNTs modified electrodes in a pH = 3.0 MBS solution containing 20 μM NHDC;

[0027] Figure 4 CuO, CNTs, CuO / ZIF-8@CNTs, and ZIF-8@CNTs modified electrodes in 5 mM Fe[(CN) 6 3- / 4- and 0.1 M KCl solution; the inset is the equivalent circuit diagram fitted to the impedance curve;

[0028] Figure 5 ​(a) DPV curves of CuO / ZIF-8@CNTs modified electrode for detecting different concentrations of NHDC in pH = 3.0 MBS solution. The detection concentrations of NHDC from bottom to top are 0.03 μM, 0.3 μM, 2 μM, 6 μM, 10 μM, 14 μM, 18 μM, 26 μM, and 30 μM respectively; (b) Linear fitting relationship diagram between i pa1 and c NHDC ;

[0029] Figure 6 Reproducibility test of CuO / ZIF-8@CNTs GCE; "Same" means the same CuO / ZIF-8@CNTs GCE was continuously tested 7 times; "different" means 7 groups of the same CuO / ZIF-8@CNTs GCE were prepared according to the above preparation method of CuO / ZIF-8@CNTs GCE and tested 7 times, and the oxidation peak current values of NHDC were obtained to calculate the relative average standard deviation. Detailed implementation mode

[0030] The technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] In the following examples, the particle size of the CuO material used is 64 nm. Through characterization, the crystal morphology and size of ZIF-8 in Examples 3 and 4 are about 150 nm.

[0032] In the following examples, all NHDC used is the purchased NHDC standard solution.

[0033] In the following examples, the 0.5% chitosan solution is prepared by dissolving 0.02 g of chitosan (deacetylation degree > 90.0%) in 4 mL of 1% acetic acid solution.

[0034] Example 1: Preparation of CuO modified electrode

[0035] The glassy carbon electrode (GCE) was polished successively with 0.3 μm and 0.05 μm 2 Al 3 powder on suede, and then the glassy carbon electrode was ultrasonically treated in dilute nitric acid solution (a solution prepared by mixing 68% concentrated nitric acid and water in a volume ratio of 1:1), ethanol aqueous solution (a solution prepared by mixing 75% ethanol and water in a volume ratio of 1:1), and double-distilled water for 1 min respectively, and then air-dried naturally to obtain the pretreated electrode.

[0036] 1 mg of CuO material was uniformly dispersed in a solution formed by 100 μL of 0.5% chitosan solution and 900 μL of double-distilled water to obtain a CuO dispersion. Then, 8 μL of the CuO dispersion was uniformly dropped onto the surface of the above-pretreated electrode and dried. This was used as the working electrode, and a three-electrode system was assembled with a saturated calomel electrode and a platinum electrode. The electrolyte was a pH = 3.0 MBS solution formed by mixing 0.1 M citric acid and 0.2 M disodium hydrogen phosphate solution, and it was connected to an electrochemical workstation for the electrochemical detection of NHDC.

[0037] Example 2: Preparation of CNTs Modified Electrode

[0038] The glassy carbon electrode was polished successively with 0.3 μm and 0.05 μm Al 2 O 3 powder on suede, and then the glassy carbon electrode was ultrasonically treated for 1 min in dilute nitric acid solution (a solution obtained by mixing 68% concentrated nitric acid and water in a volume ratio of 1:1), ethanol aqueous solution (a solution obtained by mixing 75% ethanol and water in a volume ratio of 1:1), and double-distilled water respectively, and dried naturally to obtain a pretreated electrode.

[0039] First, 1 mg of CNTs material was dispersed in a solution formed by 100 μL of 0.5% chitosan solution and 900 μL of double-distilled water to obtain a CNTs dispersion. Then, 8 μL of the CNTs dispersion was uniformly dropped onto the surface of the above-pretreated electrode. After drying, it was used as the working electrode, and a three-electrode system was assembled with a saturated calomel electrode and a platinum electrode. The electrolyte was a pH = 3.0 MBS solution formed by mixing 0.1 M citric acid and 0.2 M disodium hydrogen phosphate solution, and it was connected to an electrochemical workstation for the electrochemical detection of NHDC.

[0040] Example 3: Preparation of CuO / ZIF-8@CNTs Composite Film Modified Electrode

[0041] (1) Preparation of ZIF-8@CNTs Material

[0042] 20.00 mg of CNTs and 0.2000 g of polyvinylpyrrolidone (PVP) were added to 40 mL of anhydrous methanol, and ultrasonically dispersed to form a uniform suspension. Then, 0.1750 g of zinc acetate dihydrate (Zn(CH 3 COO) 2 .2H 2 O) and 0.2630 g of 2-methylimidazole (MeIM) were mixed evenly, stirred for 5 min, allowed to stand for 12 h, filtered by suction, and the obtained solid was washed successively with anhydrous ethanol and double-distilled water 3 times each. Finally, it was dried in an oven at 80 °C for 12 h to obtain ZIF-8@CNTs material.

[0043] (2) Preparation of CuO / ZIF-8@CNTs Composite Film Modified Electrode

[0044] Prepare the pretreated electrode according to the method of Example 1.

[0045] First, disperse 1 mg of ZIF-8@CNTs material in a solution formed by 100 μL of 0.5% chitosan solution and 900 μL of double-distilled water to obtain Solution A; then disperse 1 mg of CuO in a solution formed by 100 μL of 0.5% chitosan solution and 900 μL of double-distilled water to obtain Solution B.

[0046] Step I: First, pipette 8 μL of Solution B and evenly drop it on the surface of the pretreated glassy carbon electrode, and wait for natural drying.

[0047] Step II: Then, drop 8 μL of Solution A on the surface of the dried electrode, and after natural drying, obtain the CuO / ZIF-8@CNTs composite modified film electrode. Use it as the working electrode, assemble it with a saturated calomel electrode and a platinum electrode into a three-electrode system. The electrolyte is a pH = 3.0 MBS solution formed by mixing 0.1 M citric acid and 0.2 M disodium hydrogen phosphate solution, and connect it to an electrochemical workstation for the electrochemical detection of NHDC.

[0048] Example 4: Preparation of ZIF-8@CNTs Composite Film Modified Electrode

[0049] First, disperse 1 mg of the ZIF-8@CNTs material prepared in step (1) of Example 3 in a solution formed by 100 μL of 0.5% chitosan solution and 900 μL of double-distilled water to obtain Solution A, and then pipette 8 μL of Solution A and evenly drop it on the surface of the pretreated glassy carbon electrode, and wait for natural drying to obtain the ZIF-8@CNTs composite modified film electrode. Use it as the working electrode, assemble it with a saturated calomel electrode and a platinum electrode into a three-electrode system. The electrolyte is a pH = 3.0 MBS solution (the pH = 3.0 MBS solution used in the following examples is prepared by this method), and connect it to an electrochemical workstation for the electrochemical detection of NHDC.

[0050] Perform SEM morphology characterization, study on the electrochemical behavior of NHDC, electrochemical characterization, and anti-interference and reproducibility performance tests for the different modified electrodes prepared in Examples 1-4.

[0051] Detection 1: SEM Characterization

[0052] As Figure 2As shown, the SEM characterization test results of CNTs, CuO, CuO / ZIF-8@CNTs, and ZIF-8@CNTs on the surface of a glassy carbon substrate, where (a) CNTs; (b) CuO; (c) ZIF-8@CNTs; (d) CuO / ZIF-8@CNTs. (a) It can be observed that the CNTs material forms a tightly intertwined network structure film on the surface of the glass slide substrate; (b) is spherical nano-scale CuO material with uniform specifications. However, after ultrasonic dispersion treatment in water, it agglomerates severely when modified on the surface of the glass slide substrate, and the spherical structure collapses, forming an approximately flat flake structure with a size of about 50 - 100 nm; (c) It can be observed that the ZIF-8@CNTs material is that the tightly intertwined network of CNTs wraps the ZIF-8 crystals, and there is local agglomeration. Among them, the crystal structure of ZIF-8 is not a regular rhombic dodecahedron structure, and the edges on the crystal surface disappear. This may be due to the influence of the synthesis process of ZIF-8 material in an aqueous system on the morphology, as well as the influence of the polyvinylpyrrolidone PVP polymer material wrapped on the surface of the ZIF-8 material, forming an irregular structure with a size of about 100 - 200 nm; (d) is a composite modified film of nano-scale CuO and ZIF-8@CNTs stepwise drop-coated on the substrate surface. The CNTs wind around the ZIF-8 crystals and are loaded on the CuO. Moreover, the nano CuO has a smaller size and is in the gap of the ZIF-8@CNTs composite material. In the CuO / ZIF-8@CNTs composite material, the electrocatalytic activity of nano CuO, the porous structure of ZIF-8, and the high conductivity of CNTs act synergistically to promote the catalytic oxidation-reduction of NHDC and enhance the oxidation-reduction peak current signal response of NHDC.

[0053] Detection Two: Study on the Electrochemical Behavior of NHDC

[0054] As Figure 3 shown, the CV response curves of CNTs, CuO, CuO / ZIF-8@CNTs, and ZIF-8@CNTs composite film electrodes in a pH = 3.0 MBS solution containing 20 μM NHDC (potential window 0 - 0.8 V, scan rate 0.05 V / s, scan 4 cycles, open-circuit stirring enrichment for 150 s), and the CV response curve of the CuO / ZIF-8@CNTs modified electrode in a blank pH = 3.0 MBS buffer solution (without adding 20 μM NHDC) (CuO / ZIF-8@CNTs(Blank)). It can be seen from the figure that the four material-modified electrodes have obvious electrochemical responses for detecting NHDC. Among them, the oxidation peak current i corresponding to the detection of NHDC by the four material-modified electrodes pa1Respectively: CNTs: 0.7170 μA; CuO: 0.3912 μA; CuO / ZIF-8@CNTs: 2.186 μA; ZIF-8@CNTs: 0.9707 μA. NHDC obtained an oxidation peak P on the surface of CuO / ZIF-8@CNTs a1 The current is 3.0 times that of CNTs, 5.6 times that of CuO, and 2.3 times that of the ZIF-8@CNTs composite film. The oxidation peak potentials of NHDC during the oxidation process on the surfaces of the four modified electrodes are respectively

[0055] CNTs: 0.630 V, CuO: 0.676 V, CuO / ZIF-8@CNTs: 0.640 V, ZIF-8@CNTs: 0.657 V. The lower oxidation peak potential indicates a higher apparent rate constant (k s ) on the surface of the modified electrode and a stronger electrocatalytic ability; when comparing ZIF-8@CNTs with CNTs, the presence of ZIF-8 inhibits the electrocatalytic activity of CNTs, but the oxidation peak current of NHDC generated on the surface of the ZIF-8@CNTs modified electrode is significantly greater than that of CNTs. The reason is that ZIF-8 has a large specific surface area and porous structure, which promotes the adsorption of NHDC on the electrode surface; compared with ZIF-8@CNTs, CuO / ZIF-8@CNTs has a significant enhancement effect on the oxidation and sensing signals of NHDC. The oxidation peak current on its electrode surface is significantly stronger and the oxidation peak potential is lower, which may be due to the synergistic effect of the strong electrocatalytic activity of CuO and ZIF-8@CNTs

[0056] Test three: Electrochemical characterization

[0057] Electrochemical impedance spectroscopy is usually used to study the interfacial properties of different materials on the surface of modified electrodes Figure 4 As shown, the electrochemical impedance EIS curves of CNTs GCE, CuO GCE, CuO / ZIF-8@CNTs GCE, and ZIF-8@CNTs GCE were respectively tested. The conditions for impedance testing were: in a 10 mL mixed solution composed of 5 mM Fe[(CN) 6 3- / 4- and 0.1 M KCl, tested under open circuit voltage, with a frequency range of 0.01 - 100000 HZ and an amplitude of 0.005 V

[0058] Results Figure 4 showed that the characteristic trend of the EIS curves of each modified electrode was a characteristic impedance semicircle in the high-frequency region. The diameter of the semicircular arc represents the charge transfer resistance (R ct ) between the solution and the surface of the modified electrode. Among them, the charge transfer resistance (R ct ​)Sorted in ascending order: CNTs GCE < CuO / ZIF-8@CNTs GCE < ZIF-8@CNTs GCE < CuO GCE; A characteristic impedance straight line appears in the low-frequency region, indicating Fe in the solution 3+ / Fe 4+ The redox reaction on the electrode surface is a diffusion-controlled process. The ZIF-8 in the yixZIF-8@CNTs composite has poor electrical conductivity. After the combination of CuO and ZIF-8@CNTs, their synergistic effect enhances the conductivity and accelerates the electron conduction on the electrode surface. The impedance test was carried out in a solution of 5 mM Fe[(CN) 6 3- / 4- and 0.1 M KCl. For the same CuO electrocatalytic Fe 3+ / Fe 4+ redox process, after the combination of CuO and ZIF-8@CNTs, the Fe 3+ oxidation peak potential is reduced, and it has a high apparent electrode rate constant. The porous structure of ZIF-8 in ZIF-8@CNTs adsorbs Fe 3+ / Fe 4+ ions on the electrode surface, accelerating the electron transfer in the Fe 3+ / Fe 4+ redox process and reducing the impedance value.

[0059] Test Four: Anti-interference Detection

[0060] (1) Add a certain amount of phenolic organic compounds and inorganic salts to the pH = 3.0 MBS solution containing 10 μM NHDC to test the anti-interference ability of the CuO / ZIF-8@CNTs composite modified film electrode. The potential window is 0 - 0.8 V, the scanning rate is 0.05 V / s, scan 4 cycles, and open-circuit stirring enrichment for 150 s.

[0061] (2) Take the organic salts in step (1) as NaCl, MgCl 2 , FeCl 3 , KNO 3 , CuSO 4 aqueous solutions with a molar concentration 500 times that of NHDC, and detect the i pa1 value of NHDC under this condition according to step (1). The experimental results show that the relative deviation of the i pa1 value is less than 5%, indicating that the detection of NHDC with this modified electrode is basically not interfered by the above-mentioned ions; in the same way, take the phenolic organic compounds in step (1) as sucrose and glucose with a molar concentration 20 times that of NHDC, and the results are also basically not interfered; take the phenolic organic compounds in step (1) as dopamine, ascorbic acid, and p-nitrophenol with a molar concentration 10 times that of NHDC for detection. The experimental results are shown in Table 1, and the results show that the i​pa1 The relative deviation of the values is less than 5%, indicating that the above substances have basically no interference on the detection results of NHDC. Therefore, it shows that CuO / ZIF-8@CNTs GCE has good anti-interference ability.

[0062] Table 1 Interference of different substances on i in 10 μM NHDC pa1 Value interference

[0063]

[0064] Detection Five: Reproducibility Test

[0065] (1) The CuO / ZIF-8@CNTs GCE modified electrode was repeatedly tested 7 times by the DPV method. After each test, the modified electrode was placed in a blank pH = 3 MBS buffer solution and subjected to 20 cycles of CV scanning (the specific parameters of the scanning were: potential range 0 - 0.8 V, scan rate 0.1 V / s) until the NHDC adsorbed on the electrode surface was desorbed, and then the next test was carried out. After the detection, the i pa1 value was recorded. The specific test conditions were: carried out in a pH = 3.0 MBS solution containing 10 μM NHDC, stirred open circuit for 150 s, tested by the DPV method, potential window 0.4 - 0.8 V, amplitude: 0.05 V, pulse width: 0.05 s, sampling width: 0.0167 s, pulse period: 0.5 s. The results are as Figure 6 shown (Same, where the abscissa is the 1st - 7th detection), and the relative average standard deviation of the i pa1 value is 1.70%, and its value is within the error allowance of 5%, indicating that the reproducibility of this modified electrode is good;

[0066] (2) According to the preparation method of CuO / ZIF-8@CNTs GCE in the above embodiment, 7 groups of CuO / ZIF-8@CNTs GCE of the same batch were prepared, and the DPV method was used to test each group of electrodes. After the detection, the i pa1 value was recorded. The specific test conditions were the same as those in the above step (1). The results are as Figure 6 shown (different, where the abscissa is the 1st - 7th group of electrodes).

[0067] Example 5 Effect Example: Working Curve and Detection of NHDC in Actual Samples The following are the conditions for NHDC detection: stirred open circuit for 150 s, tested by the DPV method, potential window 0.4 - 0.8 V, amplitude: 0.05 V, pulse width: 0.05 s, sampling width: 0.0167 s, pulse period: 0.5 s.

[0068] Working Curve:

[0069] Based on the CuO / ZIF-8@CNTs composite modified film electrode, the DPV method was used to measure the NHDC solution in the range of 0.03 - 30 μM. The electrochemical sensor detected the NHDC solution with different concentrations added dropwise in the MBS solution at pH = 3.0. The corresponding oxidation peak current increased gradually to obtain the DPV curve, and the corresponding peak current and peak potential were recorded. Then, the i pa1 -c NHDC standard curve was made according to the experimental data, and the linear relationship of the detection and its corresponding fitting linear equation were further obtained.

[0070] As Figure 5 (a) shown, under the condition of 3 times signal-to-noise ratio (S / N), according to the formula the detection limit was calculated, where: LOD represents the detection limit, σ represents the relative standard deviation of the peak current value at the lowest concentration under the working curve; R represents the slope of the fitting curve.

[0071] Figure 5 (b) is the linear relationship diagram of different concentrations (c NHDC ) and the measured i pa1 of NHDC. c NHDC and i pa1 showed a good linear relationship. In the range of low concentration 0.03 - 10 μM, its fitting linear equation: i pa1 (μA) = 0.0638c NHDC (μM) + 0.0553, R 2 = 0.9938, and the calculated detection limit was 16 nM.

[0072] Detection of NHDC in actual samples:

[0073] Taking the purchased ordinary white liquor as the sample to be measured, based on the CuO / ZIF-8@CNTs composite modified film electrode, 1 kg of white liquor sample was added in the MBS solution at pH = 3.0, and the content of NHDC in it was measured 3 times by the DPV method (after detecting the i pa1 value, its content was calculated according to the above linear equation, and the average value of three groups of data was taken for each measurement). As a result, NHDC was not detected in the white liquor sample or the content of NHDC in this sample was extremely low. Subsequently, the standard addition recovery method was used. After adding 1.0 μM, 2.0 μM, and 3.0 μM of NHDC standard solutions to the sample respectively, it was detected three times under the same conditions. As shown in Table 2, the recovery rate of the test results was between 99.1% and 107.8%, and the result reliability was good. Therefore, this method can be used for the content detection of NHDC in white liquor. g / kg in the table represents the mass of NHDC contained in 1 kg of white liquor sample.

[0074] Table 2 Detection of NHDC in white liquor by standard addition recovery method (n = 3)

[0075]

Claims

1. Method for detecting NHDC by an efficient and sensitive electrochemical sensor for NHDC, characterized in that, it is used for quantitatively detecting the concentration of NHDC at 0.03 - 10 μmol / L, and judging whether the sample to be tested contains NHDC with a concentration exceeding 16 nmol / L; the method comprises the following steps: The prepared electrochemical sensor was used as the working electrode, the saturated calomel electrode was used as the reference electrode, and the platinum wire electrode was used as the counter electrode to form a three-electrode system. The above three-electrode system was placed in the electrolyte to form a detection system. The electrolyte was a pH = 3.0 MBS solution mixed by 0.1 M citric acid and 0.2 M disodium hydrogen phosphate solution. Different concentrations of NHDC standard solutions were added to the above detection system, and the DPV curves of the NHDC standard solutions in the concentration range of 0.03 - 30 μM were measured by the DPV method. The corresponding peak currents and peak potentials were recorded, and then the standard curve was made according to the experimental data, and the linear relationship of the detection and its corresponding fitting linear equation were further obtained; In the low concentration range of 0.03 - 30 μM, its fitting linear equation is: , R 2 = 0.9938; Add the sample to be detected to the detection system, and use the DPV method to measure its i pa1 value, and then calculate the content of NHDC in the sample to be detected according to the fitting linear equation described above; The conditions for testing by the DPV method are: open - circuit stirring for 150 s, potential window of 0.4 - 0.8 V, amplitude of 0.05 V, pulse width of 0.05 s, sampling width of 0.0167 s, and pulse period of 0.5 s; The steps of the preparation method of the electrochemical sensor are as follows: Add CNTs and polyvinylpyrrolidone PVP to anhydrous methanol, and ultrasonically disperse them to form a uniform suspension. Then add Zn(CH 3 COO) 2 ·2H 2 O and 2-methylimidazole MeIM, mix them evenly, let it stand, filter by suction, wash and dry to obtain ZIF-8@CNTs; Disperse 1 mg of ZIF-8@CNTs material in solvent A to prepare a ZIF-8@CNTs dispersion. The solvent A is a solution formed by 100 μL of 0.5% chitosan solution and 900 μL of double-distilled water; Disperse 1 mg of CuO nanomaterial in solvent A to obtain a CuO dispersion liquid. First, drop - coat the CuO dispersion liquid on the surface of a glassy carbon electrode. After natural drying, continue to drop - coat the ZIF - 8@CNTs dispersion liquid on the surface of the above - mentioned electrode. After natural drying, a CuO / ZIF - 8@CNTs double - layer composite film - modified electrode is prepared, that is, the electrochemical sensor. The nano - CuO of the modified electrode forms an approximately flat flake structure, and the size of the nano - CuO is between 50 - 100 nm and is in the gap of the ZIF - 8@CNTs composite material with a size of 100 - 200 nm.

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