CuO-C nanofilm electrode, preparation method thereof and application of CuO-C nanofilm electrode in passion fruit juice detection

Abstract

The application belongs to the cross field of nanometer material preparation, electrochemical sensing technology and food detection technology, discloses a CuO-C nano film electrode and a preparation method and application thereof in passion fruit juice detection, and the electrode is prepared through the following steps: grinding a Cu-C electrode to a mirror surface, immersing in anhydrous ethanol for impurity removal, and then placing in a 0.5-3.0 M H2SO4 solution, and pretreating for 20-40 min under an alternating electromagnetic field of 50-200 Hz and 0.5-1.0 T; then adopting a chronocoulometry method to anodize for 50-120 s under a 0.8-1.4 V potential, and controlling the pH value at 4-8; after the reaction, immersing in deionized water, ultrasonic treating in N,N-dimethylformamide solution, and airing to dryness to obtain the electrode. The obtained electrode has a sheet-like cluster structure, the carbon content is 95-97%, the copper content is 0.01-0.4%, and the oxygen content is 1-5%, and the electrode has high conductivity and catalytic activity, can efficiently detect the content of citric acid in passion fruit juice, is simple and convenient to operate, and has low cost, and is suitable for the field of food component analysis.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary field of nanomaterial preparation, electrochemical sensing technology and food detection technology, and relates to a CuO-C nanofilm electrode, its preparation method and its application in passion fruit juice detection. Background Technology

[0002] Passion fruit is a perennial vine belonging to the Passiflora genus of the Passifloraceae family. The fruit is rich in citric acid, vitamin C, flavonoids, and other substances, offering benefits such as beauty enhancement, metabolism promotion, and lipid reduction. Citric acid is one of the main organic acids contributing to its unique flavor, and its content is closely related to fruit ripeness, variety, and storage conditions. Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid) is a ternary organic weak acid widely found in natural fruits (such as citrus fruits, lemons, and passion fruit) and vegetables. Accurately determining the citric acid content in passion fruit juice is not only a key indicator for evaluating its sensory quality, nutritional value, and processing suitability, but also an important step in achieving standardized production and quality control. Therefore, accurate determination of its content is of great significance for product quality control and nutritional value assessment. Currently, traditional methods for determining citric acid have drawbacks such as cumbersome operation, long processing time, complex sample pretreatment, and long detection cycles, making it difficult to meet the needs of rapid on-site testing.

[0003] Among existing detection electrodes, precious metal electrodes (such as platinum and gold) are expensive and scarce, pure copper electrodes are prone to oxidation and corrosion leading to performance degradation, glassy carbon electrodes are brittle and easily contaminated, and stainless steel electrodes have insufficient catalytic activity in certain electrochemical reactions. These materials often face shortcomings in terms of economy, stability or activity under long-term operation or high-requirement scenarios. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a CuO-C electrode that combines the high conductivity of copper with the chemical stability and specific surface area of ​​carbon, exhibiting excellent catalytic activity, corrosion resistance, and mechanical strength, while remaining relatively inexpensive. Carbon-based nanomaterials, due to their good electrochemical activity, abundant resources, and low cost, are often used for electrode modification, significantly enhancing their catalytic redox capabilities for organic acids. Copper materials, on the other hand, are inexpensive, readily available, capable of complexing with citric acid, and exhibit high sensitivity. Therefore, a rapid detection method can be used to determine the citric acid content in passion fruit juice by preparing CuO-C nanofilms.

[0005] The technical solution adopted by this invention to solve its technical problem is as follows: A CuO-C nanofilm electrode and its preparation method are disclosed. The polished Cu-C electrode is pretreated in a magnetic field in an H2SO4 electrolyte solution. It is then anodized for 50-120 s at a potential of 0.8-1.4 V using a chronoamperometry (it) method. After the anodic reaction is completed, the electrode is removed and washed with deionized water to remove residual liquid on the electrode surface. An N,N-dimethylmethylamine solution is added, and the electrode is ultrasonically cleaned for 5-20 s. After removal and drying, the CuO-C nanofilm electrode is obtained.

[0006] Furthermore, before the electrode is prepared, it needs to be polished, then soaked in anhydrous ethanol for more than 3 minutes to remove impurities, and then dried.

[0007] The preferred method is to first sand the surface with 8000# sandpaper, then sand it with 5000# sandpaper until it becomes a smooth mirror surface, then soak it in anhydrous ethanol for more than 3 minutes to remove impurities, and then let it dry for later use.

[0008] Preferably, the concentration of H2SO4 in the H2SO4 electrolyte solution is 0.5-3.0 M.

[0009] Preferably, the alternating electromagnetic field in the magnetic field pretreatment is 50-200Hz, the magnetic field strength is 0.5-1.0T, and the electrode is immersed in H2SO4 electrolyte solution for 20-40min under the alternating electromagnetic field.

[0010] Preferably, the pH value during the anodizing reaction is 4-8.

[0011] Furthermore, you can soak it for 3-8 minutes before washing with deionized water.

[0012] The CuO-C nanofilm electrode is in the form of sheets and clusters, with a carbon content of 95-97%, a copper content of 0.01-0.4%, and an oxygen content of 1-5% by mass fraction.

[0013] An electrode prepared by the method described above can be used as a sensor for detecting passion fruit juice.

[0014] Furthermore, it is mainly used to detect the citric acid content in passion fruit juice.

[0015] The prepared CuO-C modified electrode exhibits excellent repeatability, reproducibility, and stability, with a wide linear detection range (0.1-0.5 mM) and high sensitivity (12.1 μA mM) for citric acid. -1 cm -2 The detection limit is low (0.633 μM). Furthermore, the sensor fabricated with the electrode exhibits good performance in the presence of ascorbic acid, uric acid, glucose, and calcium. 2+ Na +In the absence of interfering substances such as sucrose, it exhibits excellent selectivity for the detection of citric acid and has been successfully applied to the accurate determination of citric acid in passion fruit juice.

[0016] Compared with the prior art, the beneficial effects of the present invention include: Among electrode materials, carbon materials possess advantages such as high specific surface area, low cost, better stability, and conductivity, making them a research hotspot in materials, energy, and environmental fields, and the most widely used electrode material choice. A rich variety of carbon materials are available, primarily including carbon nanotubes, activated carbon, and graphene. Carbon nanomaterials have attracted widespread research interest due to their electronic, magnetic, and optical properties, chemical versatility and ease of handling, biocompatibility, and chemical stability. Biomass carbon-based electrodes possess high specific surface area and high microporosity, providing ample adsorption sites to adsorb electrolyte ions and form an electric double-layer capacitor. Currently, research on carbon materials mainly focuses on improving their oxidation catalysis and conductivity. This goal can be achieved through various methods, such as depositing metallic materials on the carbon material surface to increase its specific surface area, or introducing heteroatom doping such as nitrogen, oxygen, phosphorus, sulfur, and boron. Heteroatom doping not only improves electrode wettability and pore structure, increasing specific surface area, but also triggers redox reactions, thereby synergistically improving capacitance performance. This scheme uses the chronoamperometry (it) method to shape the electrode morphology. By shaping a layered, sheet-like microstructure, the contact surface of the electrode is increased, the specific surface area is increased, the electrode capacitance is improved, the electron transfer rate and quantity are increased, and the detection sensitivity and detection limit are improved.

[0017] Copper nanoparticles refer to copper metal particles with at least one dimension ranging from 1 to 100 nm in size. At this scale, the material exhibits physical and chemical properties distinctly different from macroscopic bulk copper. At the nanoscale, it possesses a high specific surface area, with copper oxide nanoparticles extensively exposed on the surface, exhibiting superparamagnetic properties and providing abundant active sites. Compared to noble metal oxides such as gold and silver, copper oxide particles offer advantages such as low cost, excellent electrical and thermal conductivity, and superior catalytic performance, making them widely applicable in electronics, catalysis, medicine, and energy fields. The surface chemical composition of this material in this scheme is based on Cu... 2+ The material is predominantly composed of oxides (CuO), accompanied by surface-adsorbed carbon and oxygen-containing functional groups. The successful formation of CuO and the presence of carbon species suggest that this material may have potential in applications such as catalysis and sensing that rely on surface active sites and electronic structures.

[0018] (1) Optimization of preparation process: By combining magnetic field pretreatment with anodizing reaction with specific potential and time, the morphology and composition of CuO-C nanofilm are precisely controlled to form a sheet-like cluster structure, which increases the electrochemical active surface area of ​​the electrode and enhances the detection sensitivity.

[0019] (2) Significantly improved performance: The carbon content in the electrode is as high as 95-97%, which gives it high conductivity; the copper element exists in trace form and works with the oxygen element to form a highly efficient catalytic site, realizing the highly selective detection of citric acid in passion fruit juice with low detection limit and fast response speed.

[0020] (3) Simple and efficient operation: The preparation process does not require complex equipment or high temperature conditions. The combination of magnetic field pretreatment and ultrasonic assistance shortens the preparation cycle. The electrodes can be used after simple grinding and polishing, which is inexpensive and suitable for large-scale production.

[0021] (4) Strong application target: It is designed specifically for passion fruit juice, especially suitable for the detection of citric acid content, filling the gap of existing sensors in the analysis of tropical fruit juice components, and has broad market application prospects. Attached Figure Description

[0022] Figure 1 (A) is the DPV diagram of CuO-C nanofilm electrodes deposited in H2SO4 at different concentrations for 100 s in 0.1 M PBS + 0.1 mMCA; (B) is the DPV diagram of CuO-C nanofilm electrodes deposited in H2SO4 at different times in 0.1 M PBS + 0.1 mM. Figure 2 This is a DPV diagram showing the change in deposition potential of a CuO-C nanofilm electrode in 0.1 M PBS + 0.1 mM CA; Figure 3 This is a DPV diagram of a CuO-C nanofilm electrode in phosphate buffer solutions at different pH values; Figure 4 (A) is the SEM morphology characterization of the CuO-C nanofilm electrode at 5 μm, (B) is the SEM morphology characterization of the CuO-C nanofilm electrode at 5 μm, and (C) is the X-ray energy dispersive spectroscopy (EDS) image of the CuO-C electrode. Figure 5 These are XPS analysis images of CuO-C nanofilms. (A) is the full spectrum, (B) is the C 1s spectrum, (C) is the Cu 2p fine spectrum, and (D) is the O 1s spectrum. Figure 6 The figures show the electrochemical impedance curves of the C electrode and the CuO-C nanofilm electrode in 0.1 M K3[Fe(CN)6] + 0.1 mM CA solution. Figure 7 The cyclic voltammetry (CV) curves of the C electrode and the CuO-C electrode in 0.1 M K3[Fe(CN)6] + 0.1 mM CA solution are shown. Figure 8(A) is the CV curve of CuO-C nanofilm electrode in 0.1 M PBS + 0.1 mM CA solution at different scan rates; (B) is the oxidation peak current, reduction peak current and scan rate υ. 1 / 2 Relationship Figure 9 (A) is the DPV current response diagram of CuO-C nanofilm electrode with anti-interference material added; (B) is the DPV current bar chart of CuO-C nanofilm electrode with anti-interference material added. Figure 10 (A) and (B) are the DPV current response diagram and calibration curve of CuO-C nanofilm electrode with varying citric acid concentration; Figure 11 (A) is the repeatability of the CuO-C nanofilm electrode measured in 0.1 M PBS + 0.1 M CA solution; (B) is the reproducibility of the CuO-C nanofilm electrode measured in 0.1 M PBS + 0.1 M CA solution; (C) is the stability of the CuO-C nanofilm electrode measured in 0.1 M K3[Fe(CN)6] + 0.1 M CA solution. Figure 12 (A) is the DPV curve for determining the concentration of citric acid in the sample using the standard addition method, and (B) is the linear regression graph for determining the concentration of citric acid in the sample using the standard addition method. Detailed Implementation

[0023] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0025] The CuO-C electrode used in this embodiment was purchased from Zouping Sanjing Carbon Co., Ltd., with a diameter of 3mm. The graphite electrode is coated with a layer of copper with a thickness of 0.04-0.15mm.

[0026] Example 1: Optimization of anodizing time and H2SO4 concentration The effects of electrode anodizing time and electrolyte H2SO4 concentration on the catalytic oxidation capacity of citric acid were investigated. Figure 1(A) shows the DPV diagrams of CuO-C nanofilm electrodes deposited in 0.5 M H2SO4, 1.0 M H2SO4, 2.0 M H2SO4, and 3.0 M H2SO4 for 100 s. With increasing H2SO4 concentration, the oxidation peak current initially increases and then decreases. The peak current reaches its maximum value at 1.0 M H2SO4 concentration, but decreases further when the concentration increases to 3.0 M. Therefore, 1.0 M H2SO4 can be used as the base solution for subsequent experiments. Figure 1 (B) shows the DPV diagrams of CuO-C nanofilm electrodes deposited in 1 M H2SO4 solution for 50 s, 80 s, 100 s, and 120 s, respectively. With increasing anodizing time, the oxidation peak current first increases and then decreases. The peak current reaches its maximum value at 80 s anodizing time, and decreases further at 120 s. Therefore, the response current is best at 80 s anodizing. Based on the above description, anodizing time of 80 s and 1.0 mM H2SO4 were selected for subsequent experimental studies.

[0027] Example 2 Optimization of Anodic Potential The anodic potential is a key factor determining the nucleation, growth, and final morphology of metal nanoparticles, directly affecting the catalytic performance of modified electrodes. Therefore, we systematically investigated the effect of anodic potential (from 0.8 V to 1.4 V) on the performance of CuO-C nanofilm electrodes and evaluated it by measuring its DPV response in 0.1 mM citric acid solution.

[0028] The results are as follows Figure 2 As shown, the anodic potential significantly affects the catalytic activity of the electrode. At an anodic potential of 0.8 V, the oxidation peak current is relatively small, indicating insufficient copper deposition and limited active sites under this condition. With a negative shift in deposition potential, the peak current gradually increases, reaching a maximum at 1.2 V. This is attributed to the moderate reduction driving force of copper ions at this potential, which is conducive to the formation of nanostructures with high specific surface area and abundant active sites. However, when the potential shifts further negatively to 1.4 V, the peak current decreases significantly. This is because the excessively rapid deposition rate leads to the aggregation of copper nanoparticles, reducing the effective catalytic area.

[0029] In summary, 1.2 V was determined to be the optimal deposition potential, and CuO-C nanofilm electrodes prepared under this condition were used in all subsequent experiments.

[0030] Example 3: Optimization of pH of phosphate buffer solution By changing the pH of the solution while keeping other conditions constant, the effects of phosphate buffer solutions with different pH values ​​on the electrochemical behavior of citric acid on the electrode were investigated. Figure 3These are the differential pulse voltammetry curves of a CuO-C nanofilm electrode under different pH conditions, derived from... Figure 3 It is evident that pH has a significant impact on the electrochemical behavior of citric acid. The peak current of citric acid oxidation first increases and then decreases with increasing pH, indicating that protons are involved in the electrochemical oxidation. The maximum peak current is observed at pH 5.0, which represents the optimal pH for the experiment. Subsequently, the peak current decreases with further increases or decreases in pH. Therefore, phosphate-propagated saline (PBS) with a pH of 5.0 is the preferred buffer solution for the test.

[0031] Example 4: SEM morphology characterization of CuO-C nanofilm electrodes The morphology of CuO-C nanofilms was systematically characterized using scanning electron microscopy (SEM) at different magnifications. At low magnification... Figure 4 (A) The material exhibits a uniform microstructure with continuous sheet-like pores. As the magnification increases... Figure 4 (B) It can be clearly observed that the basic unit constituting this architecture is a nanoscale sheet. This unique multi-level structure has advantages. The structure composed of nanounits can provide a huge specific surface area, which is crucial for catalytic reactions because it can significantly increase the number of active sites. Figure 4 (C) shows the X-ray energy dispersive spectroscopy (EDS) pattern of the CuO-C electrode, indicating the presence of three elements: C, O, and Cu. This corroborates the excellent electrochemical performance observed in this study. The high catalytic activity exhibited by this material can be attributed to the abundant active sites resulting from its high specific surface area.

[0032] Example 5 XPS analysis of CuO-C nanofilm electrode To investigate the surface chemical state and composition of the material, we performed X-ray photoelectron spectroscopy (XPS) analysis. Figure 5 (A) The full spectrum shows that the material surface is mainly composed of three elements: copper (Cu), oxygen (O), and carbon (C). The Cu 2p fine spectrum is shown in Figure 1. Figure 5 (C) shows that its main peak is Cu 2p 3 / 2 Located at 933.5 eV, Cu 2p 1 / 2 Located at 953.5 eV, and accompanied by a series of distinct satellite peaks, this is Cu 2+ The typical characteristics of the ions clearly confirmed that CuO was the main copper substance present on the sample surface. Meanwhile, the O 1s spectrum... Figure 5 (D) The peak at 529.8 eV further confirms the presence of lattice oxygen (O) in CuO. 2- The existence of ). C 1s spectrum Figure 5 (B) The main peak at 284.8 eV is attributed to the carbon matrix, and the weak peaks in the range of 286-290 eV indicate the presence of a small number of oxygen-containing functional groups.

[0033] XPS analysis showed that the surface chemical composition of this material was dominated by Cu. 2+ The material is predominantly composed of oxides (CuO), accompanied by surface-adsorbed carbon and oxygen-containing functional groups. The successful formation of CuO and the presence of carbon species suggest that this material may have potential in applications such as catalysis and sensing that rely on surface active sites and electronic structures.

[0034] Example 6 Electrochemical impedance spectroscopy (EIS) is a frequency-domain electrochemical characterization technique based on the theory of linear electrical response, widely used in the study of interfacial processes. By applying a small-amplitude sinusoidal AC voltage or current perturbation, the impedance response of the system is measured within different frequency ranges, thus obtaining the system's frequency response characteristics. In a 0.1 M K₃[Fe(CN)₆] solution, with a frequency range of 0.1 Hz–100 kHz and an amplitude of 5 mV, the impedance characteristics of modified and unmodified electrodes were tested. Figure 6 As can be seen, the impedance curve of the CuO-C electrode is located significantly to the lower left of the C electrode, indicating a significant reduction in its impedance value. The semi-circular diameter of the CuO-C electrode in the high-frequency region is significantly smaller than that of the C electrode, proving that its charge transfer resistance is smaller. This indicates that the CuO-C electrode has superior electron transport dynamics at the electrode-electrolyte interface, and the charge transfer process is more rapid.

[0035] Furthermore, the slope of the CuO-C electrode is steeper in the low-frequency region than that of the C electrode, indicating better ion diffusion behavior and more desirable capacitance characteristics. This is typically associated with a higher specific surface area or superior pore structure in the electrode material. Introducing Cu nanoparticles may enhance the electrode's conductivity and provide more active sites, thereby improving overall electrochemical performance.

[0036] In summary, electrochemical impedance spectroscopy analysis shows that CuO-C nanocomposite thin film electrodes have lower interfacial impedance and faster charge transport capabilities compared to pure carbon electrodes, indicating that they have better potential in electrochemical energy storage or sensing applications.

[0037] Example 7: Catalytic oxidation performance of CuO-C nanofilm electrode for citric acid Figure 7 shows the cyclic voltammetry (CV) response curves of the C electrode and the CuO-C nanofilm electrode in 0.1 M K3[Fe(CN)6] + 0.1 mM CA solution, illustrating the differences in cyclic voltammetry (CV) curves between the C electrode and the CuO-C nanofilm electrode in citric acid solution. The CV curve of the C electrode has a flat peak and a weak current response (peak current absolute value <0.5 mA), exhibiting only a weak redox signal, indicating that the C electrode has low electrochemical catalytic activity for citric acid.

[0038] In contrast, the CV curve of the CuO-C nanofilm electrode showed significant oxidation and reduction peaks, with a substantial increase in the absolute value of the peak current (up to approximately 2.0 mA), and the peak shape was sharper and the response signal more stable. This phenomenon may be attributed to the introduction of abundant active sites on the electrode surface by CuO modification, while the synergistic interface formed between CuO and the C substrate accelerates the electron transfer process of citric acid molecules: citric acid can specifically adsorb onto the active sites of CuO, and the interaction between its carboxyl functional groups and CuO promotes the transfer of protons or electrons, thereby enhancing the kinetics of the electrochemical reaction.

[0039] This result directly demonstrates that CuO modification can significantly improve the electrochemical catalytic performance of carbon electrodes for citric acid, laying the foundation for the subsequent construction of a highly sensitive citric acid detection sensor.

[0040] Example 8 Electrochemical behavior of CuO-C nanofilm electrode Figure 8 Figure A presents the cyclic voltammetry (CV) curves of a CuO-C modified electrode in 0.1 M PBS + 0.1 mM CA solution at different scan rates (10 mV / s ~ 140 mV / s). With increasing scan rate, the absolute values ​​of the currents of both the oxidation and reduction peaks gradually increase, and the peak potentials shift, with the oxidation peak shifting positively and the reduction peak shifting negatively. Figure 8 From B, we can know that the peak current (i) p ) and the square root of the scan rate (V 1 / 2 The correlation coefficient R shows a good linear relationship. 2 > 0.99), which indicates that the electrochemical reaction process of citric acid on the CuO-C modified electrode surface is diffusion-controlled: that is, the diffusion rate of citric acid molecules from the bulk solution to the electrode surface is the rate-limiting step of the reaction. When the scan rate increases, the number of citric acid molecules diffused to the electrode surface per unit time increases, thereby leading to an increase in peak current.

[0041] Meanwhile, when the scanning rate is too high, the peak shape becomes broadened and the symmetry decreases. This is due to the increased mass transfer resistance on the electrode surface and the increased proportion of the double-layer charging current under rapid scanning.

[0042] Example 9: Sensor Anti-interference Performance Test In practical sample analysis, sensor selectivity is crucial. To evaluate the selectivity of CuO-C nanofilm electrode for citric acid detection, we investigated the potential interference of common coexisting substances in passion fruit juice on the measurement signal. High concentrations of various interfering substances (including sugars, other organic acids, inorganic ions, etc.) were introduced into PBS (pH=5.0) containing 0.1 mM citric acid, and their DPV responses were recorded.

[0043] The results are as follows Figure 9 As shown in (A), when 10 times the concentration of glucose, fructose, sucrose, ascorbic acid, uric acid, and potassium is added... + Na + Ca 2+ After the removal of inorganic ions, the oxidation peak current and peak potential of citric acid did not change significantly. More importantly, even in the presence of 10 times the concentration of the highly electroactive substance ascorbic acid and structurally similar uric acid, its DPV curve highly overlapped with that of pure citric acid, without the appearance of a new oxidation peak or significant fluctuations in the original peak current.

[0044] We quantitatively calculated the peak current values, and the relative error of the signal caused by all tested interfering substances was less than ±4%, far below the acceptable standard of ±5%. This excellent anti-interference performance can be attributed to two aspects: First, under the optimized test potential, the CuO-C nanofilm provides specific catalytic sites and reaction pathways for the oxidation of citric acid; second, the electrode modification layer may have altered the oxidation overpotential of the interfering substances themselves, thereby avoiding signal overlap. Figure 9 The bar chart in (B) further shows that the current response corresponding to CA is much stronger than that of other interfering substances, indicating that the sensor has good anti-interference ability for the detection of CA in complex systems. These experimental results fully demonstrate that this sensor can be directly used for the highly selective determination of citric acid in complex samples such as passion fruit juice without complex sample pretreatment.

[0045] Example 10: Effect of CuO-C nanofilm electrode on the response of different concentrations of citric acid To evaluate the quantitative analytical performance of the constructed CuO-C nanofilm electrode for citric acid, we systematically measured its DPV response in citric acid solutions of different concentrations. The results are as follows: Figure 10 As shown in (A), as the citric acid concentration increased from 0.1 mM to 0.5 mM, the characteristic oxidation peak current at approximately -0.13 V significantly increased, while the reduction peak potential remained stable. This indicates that CA underwent an oxidation reaction during differential pulse voltammetry analysis in a buffer solution at pH=5. Meanwhile, no significant Faraday current was observed in the blank control group (pH=5 PBS buffer) within this potential range, indicating that the oxidation peak originates from the electrocatalytic oxidation of citric acid, resulting in a low background signal and good selectivity for the sensor. To establish the calibration curve for the CA sensor, a graph showing the relationship between peak current and concentration was plotted as follows. Figure 10 (B) This curve shows a linear relationship in the concentration range of 0.1-0.5 mM, and its linear equation is i p =-0.0121 C(mM)-0.15623, correlation coefficient R 2=0.99537, confirming the linearity of the calibration curve. Within the CA concentration range of 0.1–0.5 mM, the peak current for CA analysis exhibited good linearity, defined as the linear range for CA detection in a pH 5 buffer solution. The obtained linear dynamic range is quite broad for CA detection. The sensitivity of the CA sensor can be expressed as a slope of 12.1 μA mM. -1 cm -2 .

[0046] Example 11 Reliability assessment of CuO-C nanofilm electrodes To evaluate the reliability of the constructed CuO-C nanofilm electrode, we systematically investigated its repeatability, reproducibility, and stability. Repeatability refers to the consistency of multiple consecutive measurements of the same sample using the same electrode within a certain time period. The same CuO-C nanofilm electrode was used to perform five consecutive DPV scans on a 0.1 mM citric acid solution for evaluation. The results are as follows: Figure 11 As shown in (A), the DPV curves obtained from the five scans almost completely overlap, and the characteristic oxidation peak current value of citric acid fluctuates very little. The relative standard deviation of the oxidation peak current was calculated to be 0.94% (n=5). This result indicates that the nanofilm electrode prepared by this method possesses excellent operational stability and measurement repeatability, and can provide reliable signals for continuous multiple detections.

[0047] Five CuO-C nanofilm electrodes were independently prepared using the same method, and the DPV response of the films was measured under the same conditions using 0.1 mM citric acid solution to evaluate the reproducibility of the films. Results 11(B) show that the DPV curves measured from the five different electrodes exhibited consistent shapes, and the oxidation peak potential remained essentially unchanged, demonstrating the reliability of the preparation method. However, there were some fluctuations in the oxidation peak current values ​​among the electrodes, with a relative standard deviation of 1.42% (n=5). This fluctuation may stem from minor differences in the morphology, thickness, and number of active sites of the nanofilm during each electrodeposition process. Nevertheless, the 1.42% RSD indicates that the electrode preparation method has good reproducibility and can meet the requirements of routine analytical testing.

[0048] The long-term stability of the sensor is one of the key indicators for its practical application. Therefore, we wrapped the prepared CuO-C nanofilm electrode in plastic wrap and stored it at 4°C. Every two days, we tested its DPV response to a 0.1 mM citric acid solution to examine its stability. The results are as follows: Figure 10 As shown in (C).

[0049] During the 8-day testing period, although the current response of the electrode showed a slow decreasing trend with the extension of storage time, it still maintained 90.5% of the initial current response on the 8th day. The performance retention rate of over 85% fully demonstrates that the CuO-C nanofilm electrode prepared by this invention has excellent long-term stability and can meet the requirements for sensor storage stability in actual sample analysis.

[0050] Table 1 compares the linear range, sensitivity, and detection limit of the CuO-C nanofilm electrode proposed in this experiment with those of other carbon-based nanomaterials. The comparison of data in the figure shows that the CuO-C nanofilm electrode proposed in this experiment has advantages in terms of linear range, sensitivity, and detection limit.

[0051] The above results demonstrate that the CuO-C nanofilm electrode preparation method established in this study is stable and reliable. The obtained electrodes exhibit both good single-use repeatability and satisfactory batch-to-batch reproducibility, laying the foundation for the practical application of sensors.

[0052] Table 1. Comparison of electrochemical performance with sensors constructed from other carbon-based nanomaterials. Example 12: Analysis of Actual Samples To verify the practicality of the constructed sensor, we applied it to the determination of citric acid content in passion fruit juice samples extracted from golden passion fruit harvested from a passion fruit orchard in Beiliu City. The juice samples were diluted 3000 times and filtered. Using the standard addition method, 0.1–0.5 mM citric acid standard solution was added dropwise to eliminate matrix effects. DPV was measured under optimal conditions, with each sample measured in triplicate and the average value plotted. Figure 12 (A)

[0053] like Figure 12 As shown in (B), in the diluted passion fruit juice sample matrix, the oxidation peak current of citric acid showed a good linear relationship with its concentration in the range of 0.1 to 0.5 mM, with a linear regression equation of Y = -0.01508x - 0.00204 and a coefficient of determination (R²). 2 The concentration of citric acid in the passion fruit juice sample was as high as 0.9976. Based on the standard curve, the citric acid content was calculated to be 42.27 mM / L. The calculated recoveries ranged from 80.5% to 98% at the calculated concentrations. These results fully demonstrate that the detection method based on the CuO-C nanofilm modified electrode developed in this study is accurate and reliable, and can be used for the rapid and highly sensitive determination of citric acid in actual passion fruit juice samples.

[0054] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A CuO-C nanofilm electrode and its preparation method, characterized in that, The polished Cu-C electrode was pretreated in a magnetic field in an H2SO4 electrolyte solution. It was then anodized for 50-120 s at a potential of 0.8-1.4 V using a chronoamperometry method. After the anodic reaction was completed, the electrode was removed and washed with deionized water to remove any residual liquid on the electrode surface. An N,N-dimethyl methylamine solution was added, and the electrode was ultrasonically cleaned for 5-20 s. After cleaning, the electrode was removed and dried to obtain the CuO-C nanofilm electrode.

2. The CuO-C nanofilm electrode and its preparation method according to claim 1, characterized in that, Before the electrodes are prepared, they need to be polished, then soaked in anhydrous ethanol for more than 3 minutes to remove impurities, and then dried.

3. The CuO-C nanofilm electrode and its preparation method according to claim 2, characterized in that, For polishing, first use 8000# sandpaper to smooth the surface, then use 5000# sandpaper to polish until a smooth mirror finish is achieved.

4. The CuO-C nanofilm electrode and its preparation method according to claim 1, characterized in that, The concentration of H2SO4 in the H2SO4 electrolyte solution is 0.5-3.0 M.

5. The CuO-C nanofilm electrode and its preparation method according to claim 1, characterized in that, In the magnetic field pretreatment, the alternating electromagnetic field is 50-200Hz and the magnetic field strength is 0.5-1.0T. Under the alternating electromagnetic field, the electrode is immersed in H2SO4 electrolyte solution for 20-40 minutes.

6. The CuO-C nanofilm electrode and its preparation method according to claim 1, characterized in that, The pH value is 4-8 during the anodizing process.

7. The CuO-C nanofilm electrode and its preparation method according to claim 1, characterized in that, Soak for 3-8 minutes before washing with deionized water.

8. The CuO-C nanofilm electrode and its preparation method according to claim 1, characterized in that, The CuO-C nanofilm electrode is in the form of sheets and clusters, with a carbon content of 95-97%, a copper content of 0.01-0.4%, and an oxygen content of 1-5% by mass fraction.

9. An electrode prepared by the method according to any one of claims 1-8, characterized in that, Used as a sensor to detect passion fruit juice.

10. An application of the electrode as described in claim 9, characterized in that, Used to detect the citric acid content in passion fruit juice.

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