Zif-67 / cnhs-based carbendazim electrochemical sensor and preparation method thereof

Abstract

The application discloses a carbendazim electrochemical sensor based on ZIF-67 / CNHs and a preparation method thereof. The sensor is prepared by modifying a zeolite imidazole framework-67 / carbon nanohorn (ZIF-67 / CNHs) composite material on a glassy carbon electrode. The ZIF-67 / CNHs composite material is synthesized by using cobalt nitrate hexahydrate and 2-methyl imidazole to synthesize ZIF-67, and then adding CNHs to combine on the surface of the ZIF-67 to form the ZIF-67 / CNHs composite material. The electrochemical sensor can realize high-sensitivity detection of carbendazim. The linear range of the sensor is 3-3000 ng / mL, and the detection limit is 1 ng / mL. The sensor has the advantages of fast detection speed, high sensitivity, high precision and the like.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical sensing technology, and relates to a carbendazim electrochemical sensor based on ZIF-67 / CNHs and its preparation method. Background Technology

[0002] In modern agricultural production, the use of pesticides plays a vital role in preventing pests and diseases and increasing grain yield. Carbendazim (CBZ) is a broad-spectrum fungicide widely used in my country due to its effectiveness in controlling various fungal diseases in agriculture. Although the widespread use of CBZ has increased grain production, its long half-life of 12 months means that residues in agricultural products may have long-term potential impacts on humans and the environment. Therefore, it is necessary to develop effective detection technologies to achieve efficient detection and analysis of CBZ in agricultural products.

[0003] Currently, the main detection technologies for carbendazim (CBZ) include gas chromatography, high-performance liquid chromatography-mass spectrometry, and spectrophotometry. However, these technologies suffer from drawbacks such as expensive equipment, long processing times, and complex operation, failing to meet the requirements for rapid on-site detection and causing inconvenience for food regulatory authorities in inspecting agricultural products. Therefore, there is a need to develop a simple, rapid, and efficient detection method. Compared to the aforementioned methods, electrochemical sensing technology, with its advantages of low cost, fast response, and simple operation, has attracted increasing attention from researchers. However, the sensitivity and accuracy of existing electrochemical sensors for detecting carbendazim still need improvement. Therefore, developing a high-performance electrochemical sensor capable of achieving efficient detection of CBZ is of significant research importance. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a carbendazim electrochemical sensor based on ZIF-67 / CNHs and its preparation method. This preparation method constructs an electrochemical sensor by modifying a glassy carbon electrode with a zeolite imidazole framework-67 / carbon nano-angle (ZIF-67 / CNHs) composite material, thereby achieving highly sensitive detection of carbendazim. It has advantages such as fast speed, high sensitivity, and high accuracy.

[0005] The technical solution adopted by the present invention to solve the aforementioned technical problem is as follows:

[0006] In a first aspect, the present invention provides a method for preparing a carbendazim electrochemical sensor based on ZIF-67 / CNHs, the method comprising the following steps:

[0007] (1) The glassy carbon electrode GCE was polished to a smooth finish with aluminum oxide powder of different particle sizes, and then ultrasonicated in ethanol and pure water to remove surface residues. It was then dried at room temperature.

[0008] (2) ZIF-67 powder was added to the methanol dispersion of carbon nanoparticles, and ultrasonic treatment was performed for 30 to 60 minutes. Then, the mixture was stirred at room temperature for 2 to 4 hours. After centrifugation, washing and drying, ZIF-67 / CNHs composite material was obtained. The mass ratio of ZIF-67 powder to carbon nanoparticles CNHs was ZIF-67:CNHs = 1:1 / 16 to 1:1 / 45.

[0009] (3) The ZIF-67 / CNHs composite material was dispersed in dimethylformamide (DMF) to obtain a ZIF-67 / CNHs solution with a concentration of 0.05-5 mg / mL;

[0010] (4) The ZIF-67 / CNHs solution was dropped onto the glassy carbon electrode GCE treated in step (1), and dried at room temperature to obtain the ZIF-67 / CNHs / GCE electrode. Then, an electrochemical sensor for detecting carbendazim was made using this electrode.

[0011] Furthermore, in step (1), the diameter of the glassy carbon electrode is 3 mm; the particle sizes of the aluminum oxide powder used are 0.3 μm and 0.05 μm, respectively;

[0012] In step (4), the amount of ZIF-67 / CNHs solution used is 4 to 8 μL, and preferably the concentration of ZIF-67 / CNHs solution is 0.5 to 1 mg / mL.

[0013] Furthermore, the preparation process of the ZIF-67 / CNHs composite material is as follows:

[0014] Dissolve 0.2933–2.933 g of cobalt nitrate hexahydrate in 10–100 mL of methanol and sonicate for 5 minutes to obtain solution A;

[0015] Dissolve 0.6491–6.491 g of 2-methylimidazole in 10–100 mL of methanol and sonicate for 5 minutes to obtain solution B;

[0016] An equal volume of solution A was slowly added to solution B. After stirring at room temperature for 2–4 hours, ZIF-67 powder was obtained by centrifugation, washing, and drying.

[0017] Disperse 1–10 mg of carbon nanoparticles in 32–320 mL of methanol and sonicate for 50 minutes to obtain solution C;

[0018] Add 16-160 mg of the ZIF-67 powder to solution C, sonicate for 30-60 minutes, stir at room temperature for 2-4 hours, centrifuge, wash and dry to obtain ZIF-67 / CNHs composite material.

[0019] Furthermore, the ZIF-67 / CNHs solution is obtained by dispersing 2-20 mg of the ZIF-67 / CNHs composite material in 4-40 mL of dimethylformamide (DMF).

[0020] Secondly, the present invention provides an electrochemical sensor for detecting carbendazim based on ZIF-67 / CNHs, wherein the electrochemical sensor is obtained using the aforementioned preparation method.

[0021] Thirdly, the present invention provides an electrochemical sensor for detecting carbendazim based on ZIF-67 / CNHs, the electrochemical sensor including a working electrode, the working electrode being a glassy carbon electrode modified with ZIF-67 / CNHs.

[0022] The detection limit of the electrochemical sensor is 1 ng / mL, and the concentration range of the carbendazim is 3 × 10⁻⁶. -9 ~3×10 -6 g / mL.

[0023] Fourthly, the present invention provides a method for detecting carbendazim using the aforementioned electrochemical sensor, wherein the steps for detecting carbendazim are:

[0024] (1) Carbendazim was diluted with PBS buffer to prepare standard solutions of different concentrations. Multiple prepared electrochemical sensors were placed in these standard solutions of different concentrations, with each sensor corresponding to a specific concentration. In a three-electrode system, the ZIF-67 / CNHs / GCE electrode was used as the working electrode, the Ag / AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. Differential pulse voltammetry (DPV) was used for detection, and the peak current I of carbendazim was measured. p ;

[0025] (2) With carbendazim concentration as the x-axis, the peak current I in the DPV curve is... p Using the ordinate as the vertical axis, establish the peak current I corresponding to the carbendazim concentration. p The standard curve;

[0026] (3) Place the electrochemical sensor in the solution to be tested, use DPV to detect it in the three electrodes, record the peak value of the DPV current of the sample solution, and obtain the concentration of carbendazim in the sample by referring to the standard curve of the peak value of the current corresponding to the concentration of carbendazim.

[0027] Peak current I p With carbendazim concentration C CBZ The standard linear curve between them is I p =0.100+2.551C CBZ (R 2 =0.998); When testing the solution to be tested, adjust the pH value of the solution to 5.5-6.5.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) In the preparation method of this invention, ZIF-67 is used as a carrier. CNHs are coated on the surface of ZIF-67 and form a stable composite material through physical adsorption, chemical bonding, electron transfer and other interactions. Then, the ZIF-67 / CNHs composite material is used to modify the working electrode, which enhances the conductivity and adsorption capacity of the glassy carbon working electrode. The combination of CNHs and ZIF-67 not only makes up for the disadvantage of poor conductivity of ZIF-67, but also the CNHs coating on the surface of ZIF-67 further increases the specific surface area. The synergistic effect of CNHs and ZIF-67 makes the ZIF-67 / CNHs composite material exhibit excellent performance, which is beneficial for the high-sensitivity detection of carbendazim.

[0030] (2) The electrochemical sensor for detecting carbendazim in this invention has the advantages of fast detection speed, high sensitivity, wide linear range, high accuracy and low detection limit. It also has anti-interference ability and good repeatability, and has potential application prospects in the field of agricultural product quality testing.

[0031] The electrochemical sensor described in this invention can achieve highly sensitive detection of carbendazim. The sensor exhibits a wide linear range of 3-3000 ng / mL and an extremely low detection limit of 1 ng / mL. Attached Figure Description

[0032] Figure 1 This is a schematic diagram illustrating the construction and detection process of the electrochemical sensor of the present invention.

[0033] Figure 2 (A) shows the DPV response for different concentrations of CBZ, where the concentrations of CBZ are 3 × 10⁻⁶. -9 1×10 -8 3×10 -8 1×10 -7 3×10 -7 1×10 -6 and 3×10 -6 g / mL; (B) is the standard linear curve between the peak DPV current and the CBZ concentration.

[0034] Figure 3 GCE, CNHs / GCE, ZIF-67 / GCE, and ZIF67 / CNHs / GCE were respectively prepared in mL of 3.0 μg / mL -1 Comparison of DPV current response in 0.1M PBS (pH 6.0) for CBZ.

[0035] Figure 4To change the compound ratio of ZIF-67 to CNHs in a solution containing 3.0 μg / mL -1 The effect of CBZ on the DPV current response in 0.1M PBS (pH 6.0). Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this is not intended to limit the scope of protection of this application.

[0037] The present invention relates to a method for preparing an electrochemical sensor for detecting carbendazim, comprising the following steps:

[0038] (1) The glassy carbon electrode GCE was polished to a smooth finish with aluminum oxide powder of different particle sizes, and then ultrasonicated in ethanol and pure water to remove surface residues. It was then dried at room temperature.

[0039] (2) Dissolve 0.2933–2.933 g of cobalt nitrate hexahydrate in 10–100 mL of methanol and sonicate for 5 minutes to obtain solution A;

[0040] (3) Dissolve 0.6491–6.491 g of 2-methylimidazole in 10–100 mL of methanol and sonicate for 5 minutes to obtain solution B;

[0041] (4) Slowly add an equal amount of solution A to solution B, stir at room temperature for 2 to 4 hours, centrifuge, wash and dry to obtain ZIF-67 powder;

[0042] (5) Disperse 1-10 mg of carbon nanoparticles in 32-320 mL of methanol and sonicate for 50 minutes to obtain solution C;

[0043] (6) Add 16-160 mg of ZIF-67 powder to solution C, sonicate for 30-60 minutes, stir at room temperature for 2-4 hours, centrifuge, wash and dry to obtain ZIF-67 / CNHs composite material. The mass ratio of ZIF-67 powder to carbon nanotubes (CNHs) is ZIF-67:CNHs = 1:1 / 16 to 1:1 / 45.

[0044] (7) Disperse 2-20 mg of the above ZIF-67 / CNHs composite material in 4-40 mL of dimethylformamide (DMF) to obtain a ZIF-67 / CNHs solution with a concentration of 0.05-5 mg / mL; using dimethylformamide (DMF) as a solvent ensures the detection effect and stability of carbendazim.

[0045] (8) The ZIF-67 / CNHs solution was dropped onto the glassy carbon electrode GCE treated in step (1) and dried at room temperature to obtain the ZIF-67 / CNHs / GCE working electrode. Then, an electrochemical sensor for detecting carbendazim was made using this working electrode.

[0046] The steps for detecting carbendazim in this invention are as follows:

[0047] (1) Carbendazim was diluted with PBS buffer to prepare standard solutions of different concentrations. Multiple prepared electrochemical sensors were placed in these standard solutions of different concentrations, with each sensor corresponding to a specific concentration. In a three-electrode system, the ZIF-67 / CNHs / GCE working electrode was used as the working electrode, the Ag / AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. Differential pulse voltammetry (DPV) was used for detection, and the peak current I of carbendazim was measured. p ;

[0048] (2) With carbendazim concentration as the x-axis, the peak current I in the DPV curve is... p Using the ordinate as the vertical axis, establish the peak current I corresponding to the carbendazim concentration. p The standard curve;

[0049] (3) Place the electrochemical sensor in the solution to be tested, use DPV to detect it in the three electrodes, record the peak value of the DPV current of the sample solution, and obtain the concentration of carbendazim in the sample by referring to the standard curve of the peak value of the current corresponding to the concentration of carbendazim.

[0050] Preferably, the concentration range of the carbendazim is 3 × 10⁻⁶. -9 ~3×10 -6 g / mL, the dosage of carbon nanoparticles and ZIF-67 is 1-10 mg and 16-160 mg, respectively.

[0051] The working principle of the electrochemical sensor of this invention is as follows:

[0052] ZIF-67 possesses a large specific surface area, high porosity, and adsorption capacity for CBZ, which is significant for improving CBZ detection capabilities. Using ZIF-67 as a substrate, carbon nanotubes (CNHs) are coated onto the ZIF-67 surface and form a composite material through physical adsorption, chemical bonding, and electron transfer. The combination of CNHs and ZIF-67 not only compensates for the poor conductivity of ZIF-67 but also further increases the specific surface area, resulting in excellent conductivity and abundant active sites. The synergistic effect of ZIF-67 and CNHs significantly improves the conductivity and electrochemical active surface area of ​​the entire electrochemical sensor. When carbendazim undergoes a redox reaction, thanks to the excellent performance of the modified electrode, a significant redox peak appears during electrochemical detection, enabling qualitative detection of carbendazim. As the concentration of carbendazim increases, the current intensity also increases significantly, allowing for comparison with the peak current I corresponding to the carbendazim concentration. p The concentration of carbendazim in the sample was obtained by using a standard curve.

[0053] Example 1

[0054] This embodiment describes a method for preparing an electrochemical sensor for detecting carbendazim. The construction and detection process of the electrochemical sensor are shown in the attached figure. Figure 1 As shown;

[0055] Specifically, the steps include:

[0056] (1) Preparation of ZIF-67: First, 2.933 g of cobalt nitrate hexahydrate was dissolved in 100 mL of methanol and sonicated for 5 minutes to obtain solution A; 6.491 g of 2-methylimidazole was dissolved in 100 mL of methanol and sonicated for 5 minutes to obtain solution B; an equal amount of solution A was slowly added to solution B, and after stirring at room temperature for 2 hours, ZIF-67 powder was obtained after centrifugation, washing and drying.

[0057] (2) Preparation of ZIF-67 / CNHs solution: 1 mg of carbon nanoparticles were dispersed in 64 mL of methanol and sonicated for 50 minutes to obtain solution C; 32 mg of ZIF-67 powder was added to solution C and sonicated for 30 minutes. Then, the mixture was stirred at room temperature for 2 hours, centrifuged, washed and dried to obtain ZIF-67 / CNHs composite material; 2 mg of the above ZIF-67 / CNHs composite material was dispersed in 4 mL of dimethylformamide (DMF) to obtain ZIF-67 / CNHs solution.

[0058] (3) The glassy carbon electrode GCE with a diameter of 3 mm was polished with 0.3 μm and 0.05 μm aluminum oxide powder in sequence, and then ultrasonicated in ethanol and water in sequence to remove surface residues, and then dried at room temperature.

[0059] (4) Add 6 μL of ZIF-67 / CNHs solution with a concentration of 0.5 mg / mL to the treated glassy carbon electrode GCE, and dry it at room temperature to obtain the ZIF-67 / CNHs / GCE electrode. Then use this working electrode to make an electrochemical sensor for detecting carbendazim.

[0060] Carbendazim (CBZ) was diluted with PBS buffer to prepare standard solutions of different concentrations. The electrochemical sensor used for carbendazim detection was then sequentially added with these different concentrations of carbendazim standard solutions, with CBZ concentrations of 3 × 10⁻⁶ each time. -9 1×10 -8 3×10 -8 1×10 -7 3×10 -7 1×10 -6 and 3×10 -6 g / mL; In a three-electrode system, the ZIF-67 / CNHs / GCE working electrode was used as the working electrode, the Ag / AgCl (saturated KCl) electrode as the reference electrode, and the platinum wire as the counter electrode. Differential pulse voltammetry (DPV) was used to measure the oxidation peak current of carbendazim; with carbendazim concentration as the abscissa, the peak current I in the DPV curve was... p Using the ordinate as the vertical axis, establish the peak current I corresponding to the carbendazim concentration. p The standard curve;

[0061] from Figure 2 As can be seen from (A), the peak current gradually increases with the increase of CBZ concentration.

[0062] from Figure 2 From (B), it can be seen that the peak current I p The standard linear curve between CBZ concentration and I is p =0.100+2.551C CBZ (R 2 =0.998), the linear range was 3 ng / mL to 3 μg / mL, and the limit of detection was 1 ng / mL.

[0063] An electrochemical sensor was applied to the spiked detection of different concentrations of carbendazim in actual samples of lettuce and tomatoes. The detection process was as follows: First, lettuce and tomatoes purchased from a local market were chopped and filtered through a 0.2 μm nylon membrane. After centrifugation, the mixture was diluted 100 times with 0.1 M PBS solution, and 5 mL of the resulting solution was used for detection. In a three-electrode system, the modified ZIF-67 / CNHs / GCE electrode was used as the working electrode, the Ag / AgCl (saturated KCl) electrode as the reference electrode, and the platinum wire as the counter electrode. The three electrodes were placed in the sample-treated solution, and differential pulse voltammetry (DPV) was used to measure the peak current I.p Substitute into the standard linear curve I p =0.100+2.551C CBZ The concentration information of carbendazim in lettuce and tomatoes was calculated.

[0064] Table 1: Detection results of CBZ in lettuce and tomato samples by the constructed electrochemical sensor

[0065]

[0066] The results in Table 1 show that the constructed electrochemical sensor can achieve highly sensitive and accurate detection of carbendazim, with a relative standard deviation controlled within 5%.

[0067] Figure 3 GCE, CNHs / GCE, ZIF-67 / GCE, and ZIF67 / CNHs / GCE were respectively prepared in mL of 3.0 μg / mL -1 A comparison of the DPV current response of CBZ in 0.1M PBS (pH 6.0). GCE, CNHs / GCE, ZIF-67 / GCE, and ZIF67 / CNHs / GCE represent the corresponding electrochemical sensors prepared by glassy carbon electrode, carbon nanoparticle-modified glassy carbon electrode, ZIF-67-modified glassy carbon electrode, and ZIF-67 / CNHs composite material-modified glassy carbon electrode, respectively. Figure 3 As can be seen, the current response of ZIF-67 and CNHs combined is much higher than that of ZIF-67 and CNHs alone, which significantly improves the detection effect.

[0068] Example 2

[0069] This embodiment modifies the mass ratio of ZIF-67 powder to carbon nanotubes (CNHs), such as 1:1 / 8, 1:1 / 32, 1:1 / 64, and 1:1 / 128, to prepare a ZIF-67 / CNHs-based carbendazim electrochemical sensor according to the method in Example 1. Figure 4 As shown, the DPV response is highest and the detection effect is best when the mass ratio of ZIF-67 powder to carbon nanotubes (CNHs) is 1:1 / 32; the detection effect is poor when the mass ratio of ZIF-67 powder to carbon nanotubes (CNHs) is greater than 1:1 / 64.

[0070] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

[0071] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A method for preparing a carbendazim electrochemical sensor based on ZIF-67 / CNHs, characterized in that, The preparation method includes the following steps: (1) The glassy carbon electrode GCE was polished to smoothness with aluminum oxide powder of different particle sizes in turn, and then ultrasonicated in ethanol and pure water in turn to remove surface residues. It was then dried at room temperature. (2) ZIF-67 powder was added to the methanol dispersion of carbon nanoparticles, and ultrasonic treatment was performed for 30 to 60 minutes. Then, the mixture was stirred at room temperature for 2 to 4 hours. After centrifugation, washing, and drying, ZIF-67 / CNHs composite material was obtained. The mass ratio of ZIF-67 powder to carbon nanoparticles CNHs was ZIF-67:CNHs = 1:1 / 16 to 1:1 / 45. (3) The ZIF-67 / CNHs composite material was dispersed in dimethylformamide (DMF) to obtain a ZIF-67 / CNHs solution with a concentration of 0.05-5 mg / mL; (4) The ZIF-67 / CNHs solution was dropped onto the glassy carbon electrode GCE treated in step (1), and dried at room temperature to obtain the ZIF-67 / CNHs / GCE electrode. Then, an electrochemical sensor for detecting carbendazim was made using this electrode.

2. The preparation method according to claim 1, characterized in that, In step (1), the diameter of the glassy carbon electrode is 3 mm; the particle size of the aluminum oxide powder used is 0.3 mm and 0.05 mm respectively. In step (4), the amount of ZIF-67 / CNHs solution used is 4 to 8 µL.

3. The preparation method according to claim 2, characterized in that, In step (4), the concentration of the ZIF-67 / CNHs solution is 0.5 to 1 mg / mL.

4. The preparation method according to claim 1, characterized in that, The preparation process of the ZIF-67 / CNHs composite material is as follows: Dissolve 0.2933~2.933g of cobalt nitrate hexahydrate in 10~100mL of methanol, and sonicate for 5 minutes to obtain solution A; Dissolve 0.6491~6.491g of 2-methylimidazole in 10~100mL of methanol and sonicate for 5 minutes to obtain solution B; An equal volume of solution A was slowly added to solution B. After stirring at room temperature for 2–4 hours, ZIF-67 powder was obtained by centrifugation, washing, and drying. Disperse 1-10 mg of carbon nanoparticles in 32-320 mL of methanol and sonicate for 50 minutes to obtain solution C; Add 16-160 mg of the ZIF-67 powder to solution C, sonicate for 30-60 minutes, stir at room temperature for 2-4 hours, centrifuge, wash and dry to obtain ZIF-67 / CNHs composite material.

5. The preparation method according to claim 1, characterized in that, The ZIF-67 / CNHs solution was obtained by dispersing 2-20 mg of the ZIF-67 / CNHs composite material in 4-40 mL of dimethylformamide (DMF).

6. An electrochemical sensor for detecting carbendazim based on ZIF-67 / CNHs, characterized in that, The electrochemical sensor is obtained using any one of the preparation methods described in claims 1-5.

7. The electrochemical sensor for detecting carbendazim based on ZIF-67 / CNHs according to claim 6, characterized in that, The electrochemical sensor includes a working electrode, which is a ZIF-67 / CNHs modified glassy carbon electrode.

8. The electrochemical sensor according to claim 6 or 7, characterized in that, The detection limit of the electrochemical sensor is 1 ng / mL, and the concentration range of the carbendazim is 3 × 10⁻⁶. -9 ~3×10 -6 g / mL.

9. A method for detecting carbendazim using the electrochemical sensor described in claim 6, characterized in that, The steps for testing carbendazim are: (1) Carbendazim was diluted with PBS buffer to prepare standard solutions of different concentrations. Multiple prepared electrochemical sensors were placed in these standard solutions of different concentrations, with each sensor corresponding to a specific concentration. In a three-electrode system, the ZIF-67 / CNHs / GCE electrode was used as the working electrode, the Ag / AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. Differential pulse voltammetry (DPV) was used for detection, and the peak current I of carbendazim was measured. p ; (2) With carbendazim concentration as the x-axis, the peak current I in the DPV curve p Using the ordinate as the vertical axis, establish the peak current I corresponding to the carbendazim concentration. p The standard curve; (3) Place the electrochemical sensor in the solution to be tested, use DPV to detect it in the three electrodes, record the peak value of the DPV current of the sample solution, and obtain the concentration of carbendazim in the sample by referring to the standard curve of the peak value of the current corresponding to the concentration of carbendazim.

10. The method according to claim 9, characterized in that, Peak current I p With carbendazim concentration C CBZ The standard linear curve between them is I p = 0.100 + 2.551C CBZ (R 2 =0.998); When testing the solution to be tested, adjust the pH value of the solution to 5.5-6.5.

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