Preparation method and application of electrochemical sensor for detecting mancozeb

Abstract

The invention discloses a preparation method and application of an electrochemical sensor for detecting mancozeb. Electrode modification is carried out by using SQDs to prepare a portable electrochemical sensor with SPCE or GCE as a substrate electrode, and the electrochemical sensor is applied to detection and analysis of MCZ in a water environment. An SQDs nano material is obtained through a top-down method, modification is carried out on the surface of SPCE or GCE through a dispensing method, modified SPCE or modified GCE of the SQDs nano material is constructed, then a portable electrochemical workstation controlled by a smart phone is combined, an electrochemical sensor technology platform with immediate response is constructed, and in-situ and on-site efficient detection of MCZ is achieved. The SQDs-based electrochemical sensor disclosed by the invention has the advantages of strong specificity, simplicity in preparation, high sensitivity and the like, and has a wide application prospect in MCZ detection.

Description

Technical Field

[0001] The present invention relates to the technical field of mancozeb detection, and in particular to a preparation method and application of an electrochemical sensor for detecting mancozeb. Background Art

[0002] Mancozeb (MCZ) is a complex of manganese zinc ethylenedithiocarbamate. It is a commonly used fungicide for the protection of fruits, vegetables, and crops. In recent years, MCZ has been recognized as a novel environmental endocrine disruptor, posing serious risks to the environment and organisms. MCZ residues in fruits and vegetables have been reported to enter mammals through the oral cavity, and humans can also be exposed to MCZ through drinking water. As a typical pesticide residue, even extremely low levels of MCZ can induce significant hepatotoxicity, neurotoxicity, reproductive toxicity, cytotoxicity, and endocrine disruption in experimental mammals upon long-term exposure. Studies have shown that MCZ can cause various diseases. For example, MCZ induces apoptosis and neuroinflammation in hypothalamic neurosecretory cells and causes oxidative stress in lymphoid organs. MCZ may also induce apoptosis in cells of the male reproductive system. The main metabolite of MCZ is classified as a carcinogen and teratogen in humans. Therefore, trace detection of MCZ is important to reduce the risk of human exposure and aquatic contamination. Currently, a variety of analytical techniques have been used for the determination of MCZ, including advanced technologies such as flow injection, chromatography, GC-flame photometric detection, MS, and LC-MS / MS. Although these methods are highly sensitive, they are generally time-consuming and costly, and the operation process requires experienced professionals to perform. In addition, the widespread use of MCZ has led to an increasingly urgent need for rapid quantitative detection methods at trace levels. Therefore, there is an urgent need to develop a portable and rapid method to determine the content of MCZ pesticides, especially for the accurate detection of low concentrations of MCZ.

[0003] Compared to other methods, electrochemical sensor detection technology does not require complex pretreatment processes and expensive large instruments. It has the advantages of simple operation, short reaction time, high sensitivity, low cost, real-time in-situ detection, and easy portability. It is widely used in the detection of new pollutants. With the design and development of miniaturized devices, the volume of intelligent micro-electrochemical workstations is only the size of a mobile phone. The workstation has a simple structure and is easy to operate. It also has a wire interface and "Bluetooth" function. Smart phones have become an indispensable tool in today's daily life. Due to their convenient transmission and portability and intelligent operation, they can be used as smart terminal devices to process and read information. Intelligent micro-electrochemical workstations can be combined with smartphones and flexible electrodes to integrate portable integrated electrochemical sensors. As a new method for on-site sampling and analysis, portable integrated electrochemical sensors have become a trendsetter in the development of on-site rapid detection due to their simplicity, portability, and freedom from site restrictions. The core of electrochemical sensing technology lies in the introduction of specific modifying materials or chemicals onto the electrode surface to alter its surface properties and functionality, thereby changing its electrochemical performance, enhancing analytical sensitivity, and expanding its application range. However, electrochemical sensors constructed using bare glassy carbon electrodes (GCEs) for the detection of organic compounds in aqueous environments are ineffective and cannot detect trace amounts of the analytes. To enhance the specific detection and sensitivity of the analytes, the sensor performance must be improved through careful design of the electrode material composition and structure. The use of nanomaterials, such as non-metallic nanomaterials, metal or metal oxide nanoparticles, and two-dimensional materials such as graphene or graphene oxide, can significantly enhance the selectivity and sensitivity of electrochemical sensors. For example, Karimi-Maleh et al. modified a glassy carbon electrode with a single-walled carbon nanotube nanocomposite modified with Pt and Pd-doped ZnO nanoparticles and a biorecognition element, ds-DNA, to develop an ultrasensitive and efficient electroanalytical biosensor for idarubicin (IDR) monitoring. Moghadam et al. successfully modified glassy carbon electrodes (F-MWCNT / GCE) for the detection of tryptophan by improving electrochemical activity through functionalized carbon nanotubes.

[72] Wusuozhu et al. constructed an electrochemical sensor for sunset yellow detection using a copper oxide-carboxylated graphene (CuO-CG) nanocomposite modified electrode. Therefore, finding suitable electrode modification materials is of great significance for achieving high-specificity electrochemical sensor detection of MCZ.

[0004] Sulfur quantum dots (SQDs) are a type of nanostructured material composed of sulfur atoms. As an emerging and highly active non-metallic element nanomaterial, they are green, low-toxic, have strong adhesion, chemical stability, are easy to synthesize, and have good water solubility. Their unique optical, electrical, and chemical properties have attracted great interest from researchers in the field of quantum dots, and they have good application prospects in nanoelectronics, optics, catalytic chemistry, biomedicine, and sensors. Due to the special chemical properties of sulfur itself, SQDs have high oxidative activity and are easily oxidized by Zn in MCZ. 2+ 、M n2+ There is a strong coordination effect between them. Summary of the Invention

[0005] The purpose of the present invention is to provide a preparation method and application of an electrochemical sensor for detecting mancozeb.

[0006] A portable electrochemical sensor based on SPCE or GCE substrate electrodes was fabricated using SQDs for electrode modification and applied to the detection and analysis of MCZ in aqueous environments. SQD nanomaterials were prepared via a top-down approach and then modified on the SPCE or GCE surface using a drop-coating method to construct SQD-nanomaterial-modified SPCE or GCE. Furthermore, a portable electrochemical workstation controlled by a smartphone was used to construct an instant-response electrochemical sensor technology platform, enabling efficient in situ and on-site detection of MCZ. The electrochemical sensor exhibited a detection limit of 432.7 nM and a linear range of 3 μM to 1 mM. This SQD-based electrochemical sensor offers advantages such as strong specificity, simple preparation, and high sensitivity, and holds great promise for the detection of MCZ.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing an electrochemical sensor for detecting mancozeb comprises the following steps:

[0009] 1) 1 mg / mL SQDs dispersion and nafion solution were mixed at a volume ratio of 3.8-4.2:1 and ultrasonicated for 30-40 min to obtain a SQDs-nafion mixed solution.

[0010] 2) dropping the SQDs-nafion mixed solution onto the pretreated substrate electrode surface and drying it at room temperature to obtain an SQDs-modified electrode;

[0011] 3) According to conventional methods, the SQDs-modified electrodes are assembled into electrochemical sensors.

[0012] Furthermore, the substrate electrode is a GCE electrode, and a SQDs-modified GCE electrode is obtained accordingly, wherein the pretreatment method of the GCE electrode is as follows: the exposed GCE surface is finely polished using Al2O3 powder with a particle size of 0.05 μm, and the polished GCE electrode surface is thoroughly cleaned using anhydrous ethanol and ultrapure water in sequence to remove residues and impurities.

[0013] Furthermore, the assembly method of the electrochemical sensor whose substrate electrode is a GCE electrode is as follows: connect the SQDs-modified GCE electrode to the electrochemical workstation through a connecting wire, connect the platinum wire electrode and the AgCl electrode to the electrochemical workstation, ensure that the communication and data transmission between the devices are normal, insert the three electrodes into the pretreated sample solution to be tested, ensure that the sample is evenly immersed in the electrode surface, and then start the electrochemical workstation application installed on the computer, and set the parameters of the electrochemical test through the program.

[0014] Furthermore, the substrate electrode is an SPCE electrode, and accordingly, an SQDs-modified SPCE electrode is obtained. The SPCE electrode pretreatment method is as follows: the SPCE electrode is placed in PBS at pH 7.0, and the screen-printed electrode is activated by cyclic voltammetry with a scan range of -0.5 to 1 V, a scan speed of 200 mV / s, and 40 scan cycles.

[0015] Furthermore, the assembly method of the electrochemical sensor whose substrate electrode is the SPCE electrode is as follows: the SQDs-modified SPCE electrode is connected to a smartphone via a connecting cable and a portable electrochemical workstation to ensure normal communication and data transmission between the devices, and a drop of pretreated and tested sample is added to the working surface of the SQDs-modified SPCE electrode to ensure that the sample evenly covers the electrode surface, and then the HY-1550 mobile application installed on the smartphone is started, and the parameters of the electrochemical test are set through the program.

[0016] Furthermore, the volume of the mixed solution dripped onto the pre-treated surface of the substrate electrode was 3.1 μL.

[0017] Furthermore, the synthesis method of the SQDs dispersion is as follows: 50 mL of ultrapure water, 1.4 g of sublimed sulfur, 3 mL of PEG-400 and 4 g of NaOH are mixed in a round-bottom flask, and then continuously stirred and reacted at 70° C. for 48 hours, and a transparent yellow SQDs dispersion is obtained by centrifugation.

[0018] Beneficial effects of the present invention: The present invention successfully prepared an electrochemically modified sensor based on SQDs, aiming to achieve ultra-sensitive detection of mancozeb pesticide. By comprehensively applying advanced characterization techniques such as infrared spectroscopy, X-ray diffraction analysis and high-resolution transmission electron microscopy, the structural characteristics and fine morphology of SQDs were systematically analyzed. The study found that the electrode modified with SQDs showed significantly improved selectivity for MCZ, which was mainly attributed to the interaction between S and Zn on its surface. 2+ 、Mn 2+ Strong coordination interactions are formed between ions. In the DPV detection mode, the sensor achieved high-sensitivity determination of MCZ in a wide linear dynamic range of 3μM to 1mM, demonstrating its excellent electrochemical activity. Even at extremely low concentration levels, it can effectively identify MCZ. In addition, the SQDs electrode not only inherits high conductivity and excellent selectivity, but also integrates into a small electrochemical workstation, further seamlessly connecting with smartphones and customized applications, building a portable, real-time, in-situ electrochemical sensing platform. This platform not only has a wide linear detection range, extremely low detection limit and excellent selectivity, but also exhibits high stability, making it suitable for long-term monitoring tasks. It provides strong technical support for the on-site rapid detection of MCZ pesticides and has broad application prospects and important practical significance in the fields of environmental monitoring, biomedicine, food safety, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the preparation and detection of the electrochemical sensor of the present invention.

[0020] Figure 2 Pretreatment of real samples for MCZ electrochemical sensor.

[0021] Figure 3 Figure 2 is the fluorescence spectrum of SQDs under 365 nm ultraviolet excitation, and the inset is the image under visible light (left) and 365 nm ultraviolet light (right).

[0022] Figure 4 Zeta potential diagram of SQDs.

[0023] Figure 5 TEM and HRTEM (a) and particle size distribution histogram (b) of SQDs.

[0024] Figure 6 FT-IR spectrum of SQDs.

[0025] Figure 7 This is the full XPS spectrum of SQDs.

[0026] Figure 8 XPS S2p spectrum of SQDs.

[0027] Figure 9 Raman spectrum of SQDs.

[0028] Figure 10 The scanning rate of SQDs and screen-printed electrodes in K3[Fe(CN)6] / K4[Fe(CN)]6 solution containing 0.1M KCl was 100mV·s -1 Cyclic voltammetry curves when .

[0029] Figure 11 Nyquist plots of SQDs and screen-printed electrodes.

[0030] Figure 12 SQDs (a) and PEG-400 / screen-printed electrodes and screen-printed electrodes (b) in 0.01 M MCZ solution at a scan rate of 100 mV·s -1 Cyclic voltammetry curves when .

[0031] Figure 13 HR-XPS spectra of Zn(a) and Mn(b).

[0032] Figure 14 CV diagram of MCZ on SQDs modified electrode as the scan rate changes from 1 to 100 mV s-1.

[0033] Figure 15 is the Current-υ curve.

[0034] Figure 16 CV curves (a) and peak separation potentials (b) of SQDs with different modification amounts.

[0035] Figure 17 The effect of different concentrations of MCZ on the pulse voltammetry of SQDs-3.1.

[0036] Figure 18 is the linear fit between MCZ concentration and the inverse of the current response value.

[0037] Figure 19 Detection of MCZ selectivity for SQDs sensors.

[0038] Figure 20 Detecting the stability of MCZ for SQDs sensors.

[0039] Figure 21 The reproducibility of MCZ detection for SQDs sensors was investigated. DETAILED DESCRIPTION

[0040] Preparation of main solution

[0041] Preparation of Sodium Hydroxide Solution: To prepare a 0.1 M sodium hydroxide solution, calculate the required mass based on the target volume (e.g., 4.0 g NaOH for 1 L of solution). Slowly dissolve the weighed solid in 800 mL of deionized water, dilute to 1 L, and store in a polyethylene bottle.

[0042] Preparation of phosphate buffered saline (PBS, pH 7.4): Prepare 0.1 M PBS using potassium dihydrogen phosphate and potassium dihydrogen phosphate in a 1.58:1 molar ratio. Weigh 21.5 g of K₂HPO₄·2H₂O and 12.6 g of KH₂PO₄, dissolve them, and dilute to 1 L. Calibrate the pH to 7.4 using a pH meter and sterilize by autoclaving or filtering with a 0.22 μm filter.

[0043] Prepare a potassium ferrocyanide / potassium chloride solution: Weigh 1.646 g of potassium ferrocyanide and 7.455 g of potassium chloride, dissolve them, and dilute to 1 L. Adjust the concentration by increasing or decreasing the reagent amounts proportionally.

[0044] Preparation of MCZ solution: Weigh 1.0g of MCZ into a 50mL beaker, add 30mL of anhydrous ethanol as solvent, and stir until completely dissolved. Transfer the dissolved solution to a 1L volumetric flask and dilute to the mark with deionized water. Stir the solution continuously on a magnetic stirrer for 2 hours and homogenize it using an ultrasonic cleaner for 30 minutes. Then, dilute the solution stepwise to form standard solutions of varying concentrations and store at 4°C.

[0045] The main equipment used in the embodiment of Table 1 is as follows

[0046]

[0047]

[0048] Example 1 Synthesis of SQDs

[0049] 50 mL of ultrapure water, 1.4 g of sublimed sulfur, 3 mL of PEG-400, and 4 g of NaOH were mixed in a round-bottom flask and then reacted with continuous stirring at 70° C. for 48 h. Subsequently, a transparent yellow SQDs dispersion was obtained by centrifugation.

[0050] Example 2 Preparation of SQDs-based sensor with GCE as substrate electrode

[0051] The exposed GCE surface was finely polished with 0.05 μm Al2O3 powder to ensure a highly smooth surface. The polished GCE surface was then thoroughly cleaned with anhydrous ethanol and then ultrapure water to remove residues and impurities, providing a clean, uniform substrate for subsequent modification steps. 80 μL of 1 mg / mL SQDs and 20 μL of nafion solution were mixed, stirred, and sonicated for 30 minutes. Subsequently, 3.1 μL of the uniformly dispersed solution was pipetted onto the GCE electrode surface and dried at room temperature to obtain a sulfur quantum dot-modified GCE electrode.

[0052] The sulfur quantum dot-modified GCE electrode (working electrode) was connected to an electrochemical workstation (model CHI-660) via a connecting cable. The platinum wire electrode (counter electrode) and the AgCl electrode (reference electrode) were connected to the electrochemical workstation to ensure normal communication and data transmission between the devices. Next, the three electrodes were inserted into the pretreated sample solution to be tested, ensuring that the sample evenly submerged the electrode surface. Then, the electrochemical workstation application installed on the computer was started, and the parameters of the electrochemical test were set through the program.

[0053] Example 3 Preparation of SQDs-based sensor using SPCE as substrate electrode

[0054] The screen-printed electrode was placed in PBS with a pH of 7.0 and activated by cyclic voltammetry (scan range -0.5 to 1 V, scan speed 200 mV / s, and 40 scan cycles). 80 μL of 1 mg / mL SQDs and 20 μL of nafion solution were mixed, stirred, and ultrasonicated for 30 minutes. Subsequently, 3.1 μL of the mixed solution of SQDs and nafion was drop-coated on the working electrode area of ​​the screen-printed electrode. After natural drying, a sulfur quantum dot-modified screen-printed electrode was obtained.

[0055] The sulfur quantum dot-modified screen-printed electrode was connected to a smartphone (iPhone 11) via a cable to the HY-E100X portable electrochemical workstation, ensuring proper communication and data transmission between the devices. Next, a drop of the pretreated sample to be tested was added to the working surface of the screen-printed electrode, ensuring even coverage. The HY-1550 mobile app installed on the smartphone was then launched to set the electrochemical test parameters.

[0056] Example 4: Sample Pretreatment

[0057] In the following embodiments, the sensor of the present invention is further described by taking an SQDs-based sensor with SPCE as the substrate electrode as an example.

[0058] In the real sample pretreatment and calibration process, tap water is first used as the medium and passed through a 0.22μm pore size membrane filter to effectively remove suspended sediment particles in the water sample, ensuring initial sample purification. Next, deep filtration is performed through a 0.22μm pore size membrane to further purify the sample and reduce potential interference factors.

[0059] The samples were analyzed using the standard addition method, and standard substances were added to the pretreated samples. Figure 2 As shown, a sulfur quantum dot-modified screen-printed electrode was connected to a smartphone (iPhone 11) via a cable to the HY-E100X portable electrochemical workstation, ensuring proper communication and data transmission between the devices. Next, a drop of the pretreated sample to be tested was dripped onto the working surface of the screen-printed electrode, ensuring even coverage. The HY-1550 mobile app installed on the smartphone was then launched to set the parameters for the electrochemical test.

[0060] Set the voltage range to 0 to 0.5 V and select DPV as the sampling test method to obtain the electrochemical response signal of the sample. Finally, run the DPV test and record the resulting DPV signal curve. Substituting the DPV response signal of MCZ in different samples into the calibration curve, the MCZ concentration in the actual sample is obtained.

[0061] Apple samples were purchased from a local supermarket in Fuzhou, Fujian Province, China, and underwent the following pretreatment process: First, 35 g of crushed apple sample was mixed with 80 mL of an extraction solution consisting of methanol and phosphate buffered saline (PBS) in a volume ratio of 3:7 and shaken on a vortex shaker for 5 minutes. Next, the mixture was centrifuged at 11,000 rpm for 15 minutes. 1 mL of the supernatant was diluted with 4 mL of 0.1 M PBS (pH 6.0) and filtered through a 0.22 μm filter. Finally, various concentrations of MCZ were spiked into the diluted apple solution for quantitative analysis, and the results were reported as recoveries.

[0062] Example 5 Result Analysis

[0063] 1. Morphological characterization of different electrodes

[0064] SQDs were successfully prepared by using PEG-400 as a passivating agent and alkali treatment of sublimated sulfur powder. Figure 3 As shown in Figure 2, the SQDs sample is well dispersed in aqueous solution without aggregation. It appears transparent yellow under natural light and emits blue light under 365nm ultraviolet light.

[0065] The surface charge characteristics of the synthesized SQDs were systematically characterized by dynamic light scattering technology. The Zeta potential of the SQDs aqueous dispersion system was measured at a constant temperature of 25°C. Figure 4 As shown, after three independent parallel experiments, the Zeta potential of SQDs was stabilized at -20.3±4.4mV, which significantly exceeds the empirical threshold of colloidal system stability (±30mV). The experimental results show that the highly dissociated negatively charged groups on the surface of SQDs form an effective charge shield in the diffuse double layer. Through the synergistic effect of the Stern layer and the Gouy-Chapman diffusion layer, a sufficient electrostatic repulsion barrier is generated, successfully suppressing the van der Waals-dominated particle aggregation behavior. The high negative potential value on the surface of the quantum dots confirms the existence of strong electrostatic repulsion between particles. This interaction can effectively prevent the aggregation tendency of nanoparticles, thereby endowing the SQDs colloidal solution with excellent physical stability. It can be seen that the prepared SQDs have the characteristics of uniform size and uniform dispersion.

[0066] By HRTEM Figure 5 (a), clear SQDs lattice stripes can be observed, and the lattice spacing is 0.22nm. Figure 5 (b) It can be seen that the average particle size of SQDs is about 7.4±0.2nm.

[0067] Fourier transform infrared spectrophotometry was used to analyze the SQDs to further confirm the chemical structure of SQDs ( Figure 6 ). At 3410cm -1 The absorption peak at 2890 cm is the stretching vibration of hydroxyl -OH. -1 The absorption peak at 1660 cm is the stretching vibration of alkyl CH. -1 The absorption peak at 1450 cm is attributed to C=O. -1 The absorption peak at 1130 cm is the bending vibration of alkyl CH. -1 The absorption at 1060 and 997 cm is assigned to the stretching vibration of COH or CN bonds. -1 The absorption peak at is sulfate SO4 2- Symmetric stretching vibration, the absorption peak at 749 is sulfate SO4 2- The asymmetric angle vibration of SO bond is at 670cm -1 456cm -1 The absorption peak at is sulfate SO4 2- The above features indicate that the surface of SQDs is covered by PEI.

[0068] The chemical composition and valence state of SQDs were analyzed by X-ray photoelectron spectroscopy. Figure 7It can be seen that the three elements C, O, and S are present in the full XPS spectrum of SQDs. The presence of carbon again indicates that PEG-400 has successfully covered the SQDs, while oxygen indicates the oxidized form of sulfur in the core structure of SQDs.

[0069] At the same time, sulfur (S 2p ) was further confirmed by high-resolution XPS spectral analysis, such as Figure 8 The spectrum reveals binding energy peaks at 160.88 eV and 162.18 eV, corresponding to the zero valence or atomic state of sulfur (S0). In addition, peaks are observed at binding energies > 166 eV, particularly at 166.68 eV and 168.18 eV, corresponding to SO 3- (2p 2 / 3 ) and SO2 2- (2p 1 / 2 ), or SO3 2- (2p 2 / 3 ), confirming the presence of oxidized forms of sulfur in SQDs. The SQDs are mainly composed of atomic sulfur and abundant sulfonyl / sulfonic acid groups on the surface, which is consistent with previously reported SQDs.

[0070] Finally, SQDs were measured using Raman spectroscopy, and the results were as follows: Figure 9 , at 152, 218 and 472cm -1 There is a clear peak at 472 cm -1 The peaks are attributed to zero-valent sulfur. This indicates that the single-cell structure of SQDs is similar to that of sublimed sulfur, proving that the synthesized material contains zero-valent sulfur. These characterization results confirm the successful synthesis of SQDs.

[0071] 2. Cyclic voltammetry characterization of different electrodes

[0072] During the electrode modification process, Nafion was used to uniformly mix the SQDs solution with its excellent film-forming properties, and then the mixed solution was drop-coated on the electrode surface. This process not only achieved uniform distribution of SQDs on the electrode, but also significantly enhanced the stability and conductivity of the electrode with the formation of the Nafion film. In order to explore the electrochemical activity and catalytic performance of the electrode before and after modification, the electrodes before and after modification were placed in 5 mmol L containing 1 M KCl. -1 Electrochemical properties were detected by CV in K3[Fe(CN)6] / K4[Fe(CN)]6 solution. Figure 10As shown, CV experiments were conducted in a K3[Fe(CN)6] / K4[Fe(CN)]6 solution at a scan rate of 0.1 V / s, and the results for different electrodes were analyzed. The results showed that the SQD-modified electrode significantly outperformed the bare screen-printed electrode in terms of oxidation peak current, and the potential difference between the two peaks was reduced. This indicates that SQDs effectively promote electron transport and have higher electrochemical activity. They can provide a pathway for electron conduction at the electrode interface, accelerate electron conduction, enable redox reactions to proceed at lower potentials, and enhance the oxidation peak of the reaction. In addition, SQDs exhibited the best electrochemical response and reversibility.

[0073] 3. Impedance characterization of different electrodes

[0074] The EIS results reveal the interface characteristics of the modified electrode. The semicircle part in the high frequency band corresponds to the electron transfer limitation process, and the semicircle diameter reflects the charge transfer resistance. The line in the low frequency region represents the ion diffusion resistance. The higher the slope value of the line, the smaller the impedance value. Figure 11 The results show that the impedance of the SQDs-modified electrode is significantly lower than that of the bare screen-printed electrode, indicating that it has higher conductivity. This shows that the modified SQDs effectively improve the electron transfer capacity, which may be attributed to the presence of more redox mediators on the SQDs surface, which can transfer electrons from the electrode surface faster, thereby improving the response rate.

[0075] 4. Redox characteristics of MCZ by different electrodes

[0076] like Figure 12 As shown, the electrochemical behavior of MCZ was evaluated using cyclic voltammetry in the range of -1.0 to +1.0 V. Clear oxidation peaks were observed at the SQD-modified electrode. The first oxidation peak (+0.10 V) is likely associated with the formation of sulfur bridges. The second oxidation process (+0.60 V) is associated with the functional amine groups and is electrochemically active. The current signal obtained at the SQD-modified electrode was significantly greater than that obtained using the bare screen-printed electrode. No reduction peak was observed during the cyclic voltammetric regression, indicating that a completely irreversible electrocatalytic oxidation of MCZ occurred at the electrode. Furthermore, the SQD-modified electrode exhibited a higher current density than the bare screen-printed electrode, confirming its excellent electrocatalytic activity for MCZ oxidation. Due to the strong chemical activity of SQDs, a strong chemical interaction between metal ions and sulfur atoms occurs. Compared to weaker physical interactions, this strong chemical interaction facilitates electron transfer and significantly promotes the in situ reduction-reoxidation reaction of MCZ, thereby significantly improving the detection sensitivity of the electrochemical sensor. Further experimental verification showed that when the screen-printed electrode was modified with PEG-400, its conductive properties in the test experiment were not affected. This conclusion was supported by Figure 12 intuitive display.

[0077] During the electrochemical reduction process, mancozeb follows a two-electron-two-proton transfer mechanism to generate thiocarbamic acid products through continuous disulfide bond cleavage reactions. Experimental results show that this electrochemical reduction process will induce a significant reconstruction of the molecular configuration: at the electrode interface, the spatial distance between the two dithiocarbamic acid groups increases due to the reorganization of molecular orbitals, resulting in the formation of a thiocarbamic acid product with Zn 2+ and Mn 2+ The coordination ability of Zn is significantly reduced. Because this structural reconstruction destroys the original tetrahedral coordination environment, 2+ and Mn 2+ The chelation between the ligand and the Zn is dissociated and finally enters as a free ion, namely 2+ Will be freed from CS2, the reaction mechanism is:

[0078]

[0079] In order to verify the principle of SQDs' specific selection of MCZ, the present invention characterized the chemical bonds on the electrode surface after the reaction by HR-XPS spectroscopy, such as Figure 13 In the XPS spectrum of Zn 2p shown in (a), there is an obvious Zn2p 3 / 2 Peak, this signal indicates the existence of Zn-S bond. Figure 13 (b) shows an obvious Mn 2p at 640.8 eV. 3 / 2 , which indicates the presence of Mn-S bonds. The results show that the presence of SQDs can convert Zn in MCZ into 2+ 、Mn 2+ enriched on the electrode surface, thereby causing signal enhancement, SQDs and Zn 2+ 、Mn 2+ A strong coordination effect is generated between the two, which significantly improves the specificity of detection.

[0080] 5. Dynamic characteristics of MCZ with different electrodes

[0081] In order to evaluate the kinetics of MCZ oxidation reaction on SQDs modified electrode, different scan rates (10-100 mV s -1 ) to study the electrochemical behavior of MCZ. The superimposed CV curves are shown in Figure 14 As shown, it can be seen that the peak current gradually increases with the increase of scan rate, and the oxidation peak current increases from 4.6μV to 19μV.

[0082] According to linear data Figure 15It can be seen that the peak current is linearly related to the square root of the scan rate, and the equation is Ip=6.9727C-0.20424(R 2 = 0.98814, Ip in μA, C in μmol L -1 These results confirm that the electrochemical oxidation of MCZ on SQD-modified electrodes is an adsorption-controlled process. Increasing the scan rate causes a surge of ions from the electrolyte solution toward the electrode surface. If the deposition is uneven, this can lead to ion adhesion. Therefore, SQD deposition exhibits uniformity, and the deposition process is primarily controlled by diffusion.

[0083] The amount of electrode surface modifier used significantly affects the electrochemical performance of the electrode. Even for highly conductive modifiers, excessive amounts can lead to lengthy electron transfer pathways and diminished sensing performance. Therefore, the amount of modifier on the electrode is a critical factor. Figure 16 (a) shows the CV curves of SQDs modified with an amount ranging from 1 μL to 10 μL. As expected, the MCZ sensing performance is highly correlated with the amount of SQDs modified. Figure 16 (b) Summary of the relationship between the MCZ oxidation current and the SQD modification amount. The current response initially increases with the addition of modified SQDs. This is because the SQD nanostructure effectively enriches MCZ molecules through π-π stacking. Simultaneously, the quantum confinement effect creates edge active sites that significantly enhance electron transfer kinetics, causing the oxidation peak current to increase with the modification amount, thereby providing an electrochemical signal. Notably, during the MCZ oxidation process, an anomalous decrease in the oxidation peak current was observed after the SQD modification amount reached a critical value of 3.1 μL. Further increases in the SQD addition resulted in a continued decrease in the current. This is attributed to the formation of a multilayered stacking structure with excessive SQDs, resulting in a nanolayer thickness exceeding the diffusion layer thickness of the MCZ molecules, extending the electron conduction path, hindering mass transfer, and thus weakening the electrode response sensitivity. Therefore, for effective analysis and determination of MCZ, the optimal SQD modification amount should be 3.1 μL.

[0084] 6. Detection and analysis of MCZ

[0085] Under the optimized experimental conditions, we exposed the electrode to a series of MCZ with different concentrations to further explore the analytical detection performance of SQDs for MCZ. Figure 17 As shown, for the SQDs-3.1μL system, in the concentration range of 1μmol / L-100μmol / L, as the MCZ concentration gradually increased, the peak current measured by DPV increased significantly and steadily.

[0086] Furthermore, Figure 18The linear relationship between the current value and the MCZ concentration is clearly shown. In the concentration range of 1μmol / L to 100μmol / L, this linear relationship is particularly significant, and the linear equation is Ip = -1.14896E -4 C+0.4737(R 2 = 0.99839, Ip in μA, C in μmol L -1 Based on statistical rigor, the present invention used seven test samples with a reasonable distribution in the low concentration range and calculated three times the standard deviation of these samples to determine the detection limit of the detection method to be 10nM. This result highlights the excellent ability of this method in detecting extremely low concentrations of MCZ.

[0087] 7. Sensor selectivity, reproducibility and repeatability

[0088] The comprehensive evaluation system of electrochemical sensor performance mainly covers core indicators such as reproducibility, repeatability, anti-interference performance and long-term stability. It is worth noting that in actual water environment monitoring scenarios, complex matrix effects often pose a severe challenge to sensor detection performance. Alkali metal / alkaline earth metal ions (such as K + 、Na + , Ca 2+ Mg 2+ ) and transition metal ions (such as Zn 2+ 、Mn 2+ ), together with electrochemically active phenolic compounds (such as pentachlorophenol, trichlorophenol, nonylphenol), nitrofurans (such as furazolidone) and organophosphorus pesticides (such as methyl parathion) among organic pollutants, may produce synergistic interference effects on the detection signal through mechanisms such as competitive adsorption, redox reaction interference or electrode surface passivation.

[0089] In order to systematically evaluate the selective recognition ability of SQDs / screen-printed electrodes for the target compound MCZ, a multi-component interference experimental system was designed in this study. DPV tests were carried out in a 0.1 M phosphate buffer solution (PBS, pH = 7.0) matrix. Six inorganic metal ions (K + 、Na + , Ca 2+ Mg 2+ 、Zn 2+ 、Mn 2+ ) and six typical organic pollutants (ascorbic acid, pentachlorophenol, trichlorophenol, nonylphenol, furazolidinone, methyl parathion), such as Figure 19As shown, the SQDs-modified screen-printed electrode exhibited only a single significant oxidation peak in the presence of MCZ, clearly demonstrating its highly specific detection capability for MCZ, while exhibiting no significant response to other ions or molecules. These results confirm that the SQDs / screen-printed electrode possesses excellent specific selectivity for MCZ in complex matrices.

[0090] Next, the long-term stability of the SQDs-modified electrode was evaluated. By recording the DPV current value of 0.1 mmol L-1 MCZ five times every other day for two consecutive weeks, it was found that the oxidation peak current could still maintain about 95% of the original value after two weeks ( Figure 20 ), which fully demonstrates that the sensor has excellent long-term stability.

[0091] In order to more comprehensively investigate the stability of the sensor, we also prepared five independent SQDs modified sensors and used them to detect the oxidation peak current of 0.1mmolL-1MCZ. Figure 21 As shown, the oxidation peak current values ​​measured by these sensors were 2.09, 2.06, 2, 2.01, 2.02, 2.06 and 2.03 μA, respectively, and the calculated relative standard deviation was 1.10%, indicating that the sensor had good reproducibility during preparation and use, which is crucial to ensure the reliability and consistency of the detection results.

[0092] To evaluate the anti-interference performance of the chemically modified SQDs / screen-printed electrodes, the recovery rate and relative standard deviation (RSD) of the added interferon were calculated, as well as the recovery value of interferon, as shown in Table 2. However, after the addition of foreign substances to the analytical solution, there was no significant deviation in the Ipc and peak potential of the metabolites, indicating that the prepared SQDs have a special selectivity for the metabolites. The RSD values ​​in the table are <5%, indicating that the SQDs have good selectivity for the target analytes.

[0093] Table 2 Effects of various interferents on the oxidation peak current.

[0094]

[0095]

[0096] Interference studies were performed in phosphate buffer at pH 7 at a scan rate of 100 mV / s in the presence of 20 μM interfering agents. The RSD values ​​from the final actual sample analysis, as shown in Table 3, were all below 5%, further confirming the strong selectivity of SQDs for the target analytes. However, after the addition of foreign substances to the analytical solution, there was no significant deviation in the Ipc and peak potential of the metabolites, indicating that the prepared SQDs have exceptional selectivity for the metabolites. The RSD values ​​in the table are <5%, indicating that the SQDs have good selectivity for the target analytes. Interference studies were performed in phosphate buffer at pH 7 at a scan rate of 100 mV / s in the presence of 20 μM interfering agents. The RSD values ​​from the final actual sample analysis, as shown in Table 3, were all below 5%, further confirming the strong selectivity of SQDs for the target analytes.

[0097] Table 3 Detection of MCZ in real samples (n=3)

[0098]

[0099] 8. Analysis of MCZ recovery rate in actual samples

[0100] To verify the effectiveness of the constructed SQDs electrochemical sensor in practical applications, specifically its ability to detect MCZ pesticides, the authors conducted systematic experimental investigations. In the experiments, standard solutions of MCZ pesticide at varying concentrations (0.1 ng / mL, 0.2 ng / mL, and 0.3 ng / mL) were accurately added to equal volumes of tap water samples to simulate a real-world detection scenario. These spiked tap water samples were then tested using the sensor, and detailed test results were recorded.

[0101] Based on a systematic detection performance evaluation system, the sensor demonstrated excellent detection performance for MCZ pesticides at various concentrations. Specifically, the sensor's recovery rates ranged from 98.7% to 100.6%, fully demonstrating the sensor's accuracy and reliability during detection. The corresponding relative standard deviations (RSDs) were also low, at 4.7%, 4.4%, and 5.1%, respectively. These results confirm the SQDs electrochemical biosensor's high sensitivity and selectivity, making it suitable for the detection of MCZ pesticides in real samples. To further validate the detection advantages of the SQDs / screen-printed electrode, this study systematically compared its performance parameters with recently reported MCZ electrochemical detection methods. As shown in Table 4, the proposed sensor exhibits significant advantages in key performance indicators: a low LOD of 0.43 μM (S / N = 3), which is 40-fold and 17.5-fold higher than those reported for PEDOT-modified electrodes (LOD = 10 μM) and SWAdSV detection (LOD = 7 μM), respectively. The linear dynamic range (3-1000 μM) is 2-3 orders of magnitude wider than that of the comparative methods. The data demonstrate that the method developed in this paper is more sensitive, stable, selective, and economical than the reported sensors. Furthermore, the detection limit of this test is lower than that of the two conventional tests listed in the table, and its performance stability significantly outperforms most of the tests reported in the literature.

[0102] Table 4 Comparison of the prepared SQDs / screen-printed electrodes with reported detection methods

[0103]

[0104] Based on these experimental results, the SQDs electrochemical biosensor constructed in this paper not only has high sensitivity and selectivity, but can also be effectively applied to the detection of MCZ pesticide in real samples. This discovery provides a new and powerful detection tool for environmental monitoring, biomedicine, food safety and other fields.

[0105] The effectiveness of the developed sensor was investigated by performing peak addition and recovery tests (standard addition method) on MCZ in various samples. The results are shown in Table 5 for recovery. The 100% recovery demonstrates that the developed sensor has good tolerance to complex matrix interference in all tested samples and exhibits high accuracy in MCZ detection.

[0106] Table 5 Mancozeb addition and recovery test in apple samples

[0107]

Claims

1. A method for preparing an electrochemical sensor for detecting mancozeb, characterized in that: The following steps are involved: 1) Mix 1 mg / mL SQDs dispersion and nafion solution at a volume ratio of 3.8-4.2:1 and sonicate for 30-40 min to obtain an SQDs-nafion mixed solution; 2) The SQDs-nafion mixed solution was dropped onto the pretreated substrate electrode surface and dried at room temperature to obtain an SQDs-modified electrode; 3) According to conventional methods, the SQDs-modified electrodes were assembled into electrochemical sensors.

2. The method for preparing an electrochemical sensor for detecting mancozeb according to claim 1, wherein: The substrate electrode is a GCE electrode or an SPCE electrode, and a SQDs-modified GCE electrode or a SQDs-modified SPCE electrode is obtained accordingly.

3. The method for preparing an electrochemical sensor for detecting mancozeb according to claim 2, wherein: The pretreatment method of the GCE electrode is as follows: the exposed GCE surface is finely polished using Al2O3 powder with a particle size of 0.05 μm, and the polished GCE electrode surface is thoroughly cleaned using anhydrous ethanol and ultrapure water in sequence to remove residues and impurities.

4. The method for preparing an electrochemical sensor for detecting mancozeb according to claim 3, wherein: The assembly method of the electrochemical sensor is as follows: connect the SQDs-modified GCE electrode to the electrochemical workstation via a connecting wire, connect the platinum wire electrode and the AgCl electrode to the electrochemical workstation, ensure normal communication and data transmission between the devices, insert the three electrodes into the pretreated sample solution to be tested, ensure that the sample is evenly immersed in the electrode surface, and then start the electrochemical workstation application installed on the computer to set the parameters of the electrochemical test through the program.

5. The method for preparing an electrochemical sensor for detecting mancozeb according to claim 2, wherein: The pretreatment method of the SPCE electrode is as follows: the SPCE electrode is placed in PBS with a pH of 7.0, and the screen-printed electrode is activated by cyclic voltammetry, with a scanning range of -0.5 to 1 V, a scanning speed of 200 mV / s, and a scanning number of 40 circles.

6. The method for preparing an electrochemical sensor for detecting mancozeb according to claim 5, wherein: The electrochemical sensor was assembled as follows: the SQDs-modified SPCE electrode was connected to a smartphone via a cable and connected to a portable electrochemical workstation to ensure normal communication and data transmission between the devices. A drop of pretreated sample to be tested was added to the working surface of the SQDs-modified SPCE electrode to ensure that the sample evenly covered the electrode surface. The HY-1550 mobile application installed on the smartphone was then launched to set the parameters for the electrochemical test.

7. The method for preparing an electrochemical sensor for detecting mancozeb according to claim 2, wherein: The volume of the mixed solution added dropwise to the surface of the pretreated substrate electrode was 3.1 μL.

8. The method for preparing an electrochemical sensor for detecting mancozeb according to claim 2, wherein: The synthesis method of the SQDs dispersion is as follows: 50 mL of ultrapure water, 1.4 g of sublimed sulfur, 3 mL of PEG-400 and 4 g of NaOH are mixed in a round-bottom flask, and then continuously stirred at 70°C for 48 hours, and a transparent yellow SQDs dispersion is obtained by centrifugation.

9. Use of the electrochemical sensor obtained according to the preparation method according to any one of claims 1 to 8 in detecting mancozeb.

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