Electrochemical sensor based on biochar / ZIF-8 / MnIn2S4 ternary composite material as well as preparation method and application of electrochemical sensor

Abstract

The invention discloses a modified electrode based on a charcoal (CB) / zeolite imidazate framework material-8 (ZIF-8) / manganese indium sulfide (MnIn2S4) ternary nano composite material and a preparation method and application of the modified electrode based on the charcoal (CB) / zeolite imidazate framework material-8 (ZIF-8) / manganese indium sulfide (MnIn2S4) ternary nano composite material. The modified electrode comprises an electrode substrate and a CB / ZIF-8 / MnIn2S4 nano composite catalytic layer coated on the surface of the electrode substrate, wherein CB, ZIF-8 and MnIn2S4 are compounded according to a specific mass ratio of (0.5-1.2): (1.0-1.8): (1.0-2.0) to form a hierarchical nano structure. The invention further provides a preparation method of the modified electrode. The preparation method comprises the key steps of preparation of CB, in-situ growth of ZIF-8, hydrothermal synthesis of MnIn2S4 and the like. An electrochemical sensor constructed based on the modified electrode shows excellent performance in synchronous detection of rutin and luteolin: through the synergistic effect of a conductive network of CB, a molecular sieve effect of ZIF-8 and catalytic activity of MnIn2S4, a wide linear range (rutin is 0.1-500 [mu] M, and luteolin is 0.29-784.8 [mu] M), a low detection limit (rutin is 3.48 nM, and luteolin is 0.22-0.24 [mu] M), and the electrochemical sensor has a good application prospect in synchronous detection of rutin and luteolin. And the method has excellent anti-interference capability, so that an efficient solution is provided for rapid detection of the flavonoid compounds.

Description

TECHNICAL FIELD

[0001] The present application belongs to the cross field of nanocomposites and electrochemical sensing technology, and particularly relates to an electrochemical sensor based on a ternary composite material of biochar (CB) / zeolitic imidazolate framework-8 (ZIF-8) / manganese indium sulfide (MnIn2S4), and especially relates to a micro-nano structure regulation method of the sensor and application of the sensor in synchronous detection of flavonoids. BACKGROUND

[0002] Rutin (Rn) and Luteolin (Lu) are important plant-derived natural compounds, widely existing in various plants. Rn is concerned for its antioxidant and anti-inflammatory properties, while Lu is widely studied for its anticancer and cardiovascular protection effects. Although their biological activities are different, both of them are increasingly in demand for clinical application and nutritional supplementation. Therefore, it is particularly necessary to develop an efficient, rapid and accurate detection method to realize the simultaneous detection of the two. At present, among the commonly used detection methods of flavonoids, the ultraviolet-visible spectrophotometry is simple to operate and low in equipment cost, and is often used for preliminary quantification but is easily interfered by impurities and low in sensitivity; the high performance liquid chromatography (HPLC) is high in separation efficiency and sensitivity, and is suitable for accurate analysis of complex samples, but has problems of high equipment requirement and complicated sample processing; the gas chromatography-mass spectrometry (GC-MS) can provide structural information for volatile flavonoids, but is not suitable for thermally unstable components; the liquid chromatography-mass spectrometry (LC-MS) is strong in separation capacity and has high sensitivity and structural resolution, and is suitable for detection of complex samples; the fluorescence method is high in sensitivity for flavonoids with natural fluorescence, but its application range is limited by the fluorescence characteristics and is easily interfered; the high performance thin layer chromatography (HPTLC) is rapid, simple and low in cost, and is suitable for simultaneous detection of multiple samples, but is not suitable for high-precision quantification due to low resolution; the electrochemical method is high in sensitivity and fast in response as a new technology, but is easily affected by interfering substances; the mass spectrometry can provide structural basis for fine analysis, but has the disadvantage of complex and expensive equipment. Therefore, development of a dual-flavonoid synchronous electrochemical sensor with high sensitivity, high selectivity and anti-interference is of great significance to promote the development of on-site rapid detection technology of natural product quality.

[0003] In 2022, Hou Xiudan et al. disclosed a method for detecting luteolin by combining solid-phase extraction and electrochemical sensors in the patent "Method for detecting luteolin by combining solid-phase extraction and electrochemical sensors". ZrO2 nanoparticles and chitosan-doped graphene aerogel modified silica gel were used as extraction materials, and a glassy carbon electrode modified as a sensor platform. The method utilizes the adsorption selectivity and signal amplification capability to achieve detection, with advantages such as simple preparation, low cost, and resistance to matrix interference. However, the material design has defects: ZrO2 relies on weak hydrogen bonds and π-π interactions to adsorb luteolin without phosphoric acid groups, which is easily affected by competitive adsorption of similar flavonoids such as quercetin, resulting in reduced extraction selectivity; chitosan-doped graphene aerogel is prone to structural collapse in acidic environments with pH < 6.3 due to chitosan protonation swelling, affecting the service life of the solid-phase extraction column and the stability of the detection.

[0004] In 2022, Liu Xingsheng developed a β-cyclodextrin (β-CD) and functionalized ionic liquid (1-hydroxyethyl-3-methyl imidazole tetrafluoroborate) copolymer film modified electrode in the patent "Controllable polymer film modified electrode, its preparation method and method for detecting luteolin". The electrode is formed by in-situ polymerization of the film on the surface of a glassy carbon electrode through cyclic voltammetry (CV), and is used for high-sensitivity detection of luteolin in traditional Chinese medicine samples. Although this technology simplifies the film formation process, it relies on high-cost ionic liquids, has strict control of polymerization parameters, and has insufficient service stability and limited cavity selectivity, which restricts its practical application value, especially in low-cost and high-throughput detection scenarios.

[0005] In 2018, Chen Xianlan et al. disclosed a method for determining rutin using a boron-doped graphene supported gold core gold platinum alloy shell nanocomposite modified electrode in the patent "Method for determining rutin using a boron-doped graphene supported gold core gold platinum alloy shell nanocomposite modified electrode". The method uses graphene oxide as raw material and boron trioxide as boron source and reducing agent to synthesize boron-doped graphene (BG) by hydrothermal method. Gold nanoparticles are prepared by Frens' method, and gold core gold platinum alloy shell nanoparticles (Au@AuPt) are prepared by seed-induced method. The boron-doped graphene with large specific surface area is used as a carrier, and the Au@AuPt nanoparticles are embedded into the graphene layers to obtain a boron-doped graphene supported Au@AuPt nanocomposite. The new nanocomposite is used to modify the glassy carbon electrode, and an electrochemical sensor for rapid detection of rutin is successfully constructed. However, the selected noble metal nanoparticles are expensive. In the complex preparation process, multiple steps are used to determine the target molecules by quantitatively analyzing the color change, which may be limited by reaction time, solvent composition, ion concentration, and temperature, making it difficult to achieve real-time and continuous detection.

[0006] In 2020, Sun Wei et al. developed a BP-PEDOT:PSS / GCE modified electrode for high-sensitivity detection of rutin in the patent "Preparation of black phosphorus modified electrode and method for detecting rutin". The electrode was coated with PEDOT:PSS to inhibit oxidation and detected by DPV method. The double linear range was 0.02-80 μM, the detection limit was 7 nM, and it was successfully used for tablet analysis. However, the coated black phosphorus still oxidized the electrode after 72 hours, the manual coating process resulted in uneven film thickness, the cost of black phosphorus preparation was high, and the interference of excipients and analogues was not verified. The technical application potential was restricted by stability, process controllability, cost, and anti-interference.

[0007] In 2023, Li Yonghong et al. developed a graphene quantum dot modified nano-carbon ionic liquid electrode and a method for detecting rutin in the patent "Graphene quantum dot modified nano-carbon ionic liquid electrode and method for detecting rutin". The electrode was filled with GQDs modified nano-graphite powder and 1-octylpyridine hexafluorophosphate ionic liquid to achieve a detection limit of 2 nM and a linear range of 5 nM-10 μM. The method was claimed to have good reproducibility (intra-batch RSD = 3.23%, inter-batch RSD = 4.14%). However, the hexafluorophosphate ionic liquid was easily decomposed by moisture to generate HF, causing irreversible decay of electrode performance when the humidity was >30%, the GQDs had a high-temperature cracking yield of <5%, and the ionic liquid had a unit price of >$800 / g, making the cost of a single electrode exceed $80. The manual mixing and packing resulted in uneven electrode tube density, affecting the stability of electron conduction, and the oxidation products of rutin were irreversibly adsorbed on the surface of GQDs, making the electrode unable to regenerate. The environmental sensitivity, high cost, process hazards, and non-regenerability severely restricted practical application.

[0008] In 2021, Guo Jianhua et al. disclosed a green fluorescent carbon dot (CDs) prepared by one-step hydrothermal synthesis method using levofloxacin and p-aminobenzenesulfonic acid as raw materials in the patent "Green fluorescent carbon dots and their preparation method and application in detecting rutin". The CDs had good water solubility and stable optical properties, and could be used as a fluorescent probe to detect rutin content in blood based on the inner filter effect. This method did not require expensive instruments and had the advantages of high selectivity, good sensitivity, simplicity, and rapidness. However, the raw material levofloxacin was expensive or limited in source, the fluorescence stability of CDs may decrease when placed in complex biological systems such as blood for a long time, and the one-step hydrothermal synthesis method had difficulty in controlling reaction conditions, which could affect the quality of CDs and the detection effect.

[0009] The existing patent (CN112014449A) uses ZrO2 / graphene aerogel modified electrode to detect luteolin, but its dependence on hydrogen bond adsorption mechanism leads to insufficient selectivity; the patent (CN114674888) detects luteolin based on a β-cyclodextrin polymer film, but the film forming process is complex and the stability is poor. Compared with the prior art, the irreplaceable technical features of the present application are that: (1) the mesoporous network of CB forms a concave structure (specific surface area ≥800m 2 / g, pore size distribution 2-50nm) through directional pyrolysis of corn straw, which sp 2 hybrid carbon domain and Zn 2+ of ZIF-8 form a C-O-Zn bond to realize atomic-level interface coupling;

[0010] (2) MnIn2S4 nanoparticles (particle size 5-8nm) form a Zn-S-Mn / In bridging structure through sulfur vacancies and Zn-N4 units of ZIF-8, which makes the electron transfer rate increase to 2.56×10 7 cm / s;

[0011] (3) Naphthol (NF) doping forms a π-π stacking interface, which makes the signal fluctuation of the electrode less than 5% in the pH range of 2-12. SUMMARY

[0012] One of the purposes of the present application is to provide a CB / ZIF-8 / MnIn2S4 ternary composite material modified electrode that can simultaneously detect rutin and luteolin with high sensitivity, solving the problems of poor selectivity, high detection limit and weak anti-interference of the prior art.

[0013] One of the purposes of the present application is achieved by the following technical solutions:

[0014] The CB / ZIF-8 / MnIn2S4 modified electrode comprises an electrode body, and an electrocatalytic material coated on the outer surface of the electrode body, wherein the electrocatalytic material is CB / ZIF-8 / MnIn2S4 material.

[0015] The CB / ZIF-8 / MnIn2S4 composite material is a hierarchical nanostructure composed of MnIn2S4 nanoparticles as functional units anchored on a ZIF-8 coated two-dimensional CB nanosheet substrate.

[0016] The CB / ZIF-8 / MnIn2S4 material of the present application is prepared by hydrothermal reaction of CB, ZIF-8 and MnIn2S4 in a mass ratio of (0.5-1.2):(1.0-1.8):(1.0-2.0).

[0017] The CB / ZIF-8 / MnIn2S4 composite material is constructed by the synergistic effect of multi-level structure design and components, and has the functions of efficient adsorption, molecular sieving and catalytic activity. When the agricultural waste corn straw is used as a raw material to prepare biochar (CB), the controllable pyrolysis process (500-700 DEG C inert atmosphere) promotes the directional pyrolysis shrinkage of cellulose / hemicellulose in the biomass. The anisotropic pyrolysis behavior leads to the formation of an inner concave mesoporous network in CB, and the three-dimensional interconnected structure has the advantages of triple function: the sp 2 The hybrid carbon domain constructs a conductive skeleton through the pi-pi conjugation effect, providing a metal-like channel for electron transport; the high specific surface area and mesoporous volume capture target molecules through the "physical adsorption-chemical interaction" synergistic mechanism - van der Waals force drives flavonoid molecules into the mesoporous, and the oxygen-containing functional groups such as carboxyl and hydroxyl on the mesoporous inner wall are coordinated with metal ions to provide nucleation sites for the in-situ growth of ZIF-8, and at the same time, the space confinement effect inhibits the agglomeration of MnIn2S4 nanoparticles. As an intermediate layer, ZIF-8 has a periodic microporous structure, and flavonoids (such as rutin and luteolin) are selectively enriched, and the uncoordinated Zn 2+ The uncoordinated Zn in the Zn-N4 unit forms a "Zn-S-Mn / In" bridging structure with the surface sulfur vacancies of MnIn2S4, which reduces the activation energy of the oxidation reaction of the target molecule. The core contribution of the bimetallic sulfide MnIn2S4 is that the Mn / In bimetallic site significantly reduces the oxidation overpotential and improves the electrocatalytic efficiency; the sulfur vacancies induced by the interface strain capture electrons, enhance the redox kinetics, and optimize the electron transfer efficiency through energy level matching. From the structural synergy, the CB mesoporous layer serves as a substrate to load ZIF-8 and constrain the growth direction of MnIn2S4, the ZIF-8 intermediate layer enriches the target molecules through molecular sieving and transfers them to the active interface, and the surface layer of MnIn2S4 exposes sulfur vacancies and bimetallic sites to participate in catalysis; at the electronic level, the Schottky junction formed by CB and MnIn2S4 drives the directional migration of electrons, and the Zn-N4 site of ZIF-8 acts as an "electron pump" to maintain the activity of the sulfur vacancies, ultimately constructing a high-efficiency dynamic cycle of "ZIF-8 sieving enrichment -> MnIn2S4 catalytic reaction -> CB fast electron conduction", which makes the material have high sensitivity and selectivity in the detection of flavonoids.

[0018] Further, the electrocatalytic material doped with naphthol (NF) is NF-CB / ZIF-8 / MnIn2S4 nanocomposite material, which is obtained by doping naphthol on CB / ZIF-8 / MnIn2S4 nanocomposite material. Naphthol (NF) is an artificially synthesized organic compound with excellent chemical activity and wide reaction adaptability. Here, naphthol can be combined with the electrode surface through physical adhesion, while enhancing the stability of the electrode and solution interface, which helps to further improve the sensitivity of the sensor and realize accurate detection of target molecules.

[0019] In one embodiment of the present invention, the mass ratio of CB / ZIF-8 / MnIn2S4 to naphthol in the NF-CB / ZIF-8 / MnIn2S4 nanocomposite is 1:1 to 1:3.

[0020] The second objective of this invention is to provide a method for preparing the above-mentioned CB / ZIF-8 / MnIn2S4 nanocomposite modified electrode.

[0021] Specifically, this is achieved through the following technical solutions:

[0022] The preparation method of CB / ZIF-8 / MnIn2S4 modified electrode involves drying fresh corn stalks and then carbonizing them in an anaerobic environment to obtain CB; then mixing CB with a zinc source and ultrasonically dispersing it in methanol, while 2-methylimidazole is dissolved in methanol, and stirring the mixture to obtain CB / ZIF-8; finally, ultrasonically dispersing CB / ZIF-8 in water, mixing it with a manganese source, an indium source, and a sulfur source, and then hydrothermally reacting the mixture to obtain CB / ZIF-8 / MnIn2S4, which is then dispersed in water to obtain a modification solution; the electrode is then contacted with the modification solution to obtain the CB / ZIF-8 / MnIn2S4 nanocomposite modified electrode.

[0023] In a further embodiment, the electrocatalytic material is doped with naphthol, and the modified electrode is prepared as follows: CB / ZIF-8, manganese source, indium source and thioacetamide solution are mixed and subjected to hydrothermal reaction to obtain CB / ZIF-8 / MnIn2S4 nanocomposite material, which is then dispersed in naphthol solution to obtain a modification solution; the electrode is then contacted with the modification solution to obtain an NF-CB / ZIF-8 / MnIn2S4 nanocomposite material modified electrode.

[0024] In one embodiment of the present invention, the mass ratio of CB / ZIF-8 / MnIn2S4 to naphthol is 1:1 to 1:3.

[0025] The conditions for the hydrothermal reaction include: under autogenous pressure, a reaction temperature of 120–160 °C, a reaction time of 10–12 h, and a heating rate of 3–5 °C / min. -1 The mass ratio of CB, ZIF-8 and MnIn2S4 is (0.5~1.2):(1.0~1.8):(1.0~2.0).

[0026] In one embodiment, CB is prepared by high-temperature calcination, specifically by calcining corn stalk precursors at 500–700°C for 3–5 hours. The corn stalk precursors are obtained by drying and grinding fresh corn stalks.

[0027] In one embodiment, CB / ZIF-8 is prepared by ultrasonication of CB, a zinc source, and 2-methylimidazole. The molar ratio of CB, zinc source, and 2-methylimidazole is 1:(2.0–3.0):(4.0–6.0), and the reaction is carried out at room temperature for 6–8 hours.

[0028] Optionally, the zinc source is a hydrate of divalent zinc, preferably one or a combination of zinc chloride hexahydrate, zinc nitrate tetrahydrate, and zinc sulfate hexahydrate.

[0029] In one embodiment, MnIn2S4 is prepared by contacting a manganese source, an indium source, and a sulfur source using a high-temperature calcination method. Preferably, the calcination conditions are: under autogenous pressure, a reaction temperature of 120–160°C, a reaction time of 10–12 h, and a heating rate of 3–5°C / min. -1 The molar ratio of the manganese source, indium source and sulfur source is 1:(1-2):(4-6).

[0030] Optionally, the manganese source is a divalent manganese hydrate, preferably a combination of one or more of manganese chloride tetrahydrate, manganese sulfate monohydrate, manganese nitrate hexahydrate, and manganese carbonate; the indium source is a trivalent manganese hydrate, preferably a combination of one or more of indium trichloride tetrahydrate, indium sulfate pentahydrate, and indium hydroxide; and the sulfur source is a divalent manganese hydrate, preferably thioacetamide.

[0031] Before calcination, a precipitant is added dropwise to a mixture of manganese, indium, and sulfur sources until the pH reaches 11-12. The precipitate is obtained by centrifugation, washed several times, dried, and then subjected to a high-temperature calcination reaction.

[0032] The precipitant is preferably one or more of NaOH, Ca(OH)2, and CaCO3. The molar ratio of the cobalt source, iron source, and precipitant is 1:(2-2.5):(1-1.5); the drying temperature is 80℃-90℃; the drying time is 1-1.5 h; the centrifugation time is 15-20 min; and the washing solvent is preferably one or more of ethanol and water.

[0033] In this invention, the electrode needs to be pretreated before contacting the modifying solution. The pretreatment includes polishing, ultrasonication, and drying. The ultrasonication is performed in an ultrasonic solution containing nitric acid, ethanol, and water.

[0034] Optionally, the modification solution is drop-cast onto the surface of the glassy carbon electrode and then dried to obtain the CB / ZIF-8 / MnIn2S4 nanocomposite modified electrode; the concentration of the modification solution is 0.5–1 mg / mL; and the amount of the modification solution used is 5–10 μL.

[0035] The third objective of this invention is to provide an electrochemical sensor.

[0036] Specifically, the electrochemical sensor has the above-mentioned CB / ZIF-8 / MnIn2S4 nanocomposite modified electrode or NF-CB / ZIF-8 / MnIn2S4 nanocomposite electrode.

[0037] The fourth objective of this invention is to provide applications of the aforementioned electrochemical sensor.

[0038] Specifically, this involves the application of the aforementioned electrochemical sensor in the simultaneous detection of rutin and luteolin.

[0039] The present invention has the following advantages:

[0040] 1. This invention achieves synergistic effects through the integration of three functions: conductivity, enrichment, and catalysis. CB constructs a three-dimensional electron transport network, significantly improving electrode conductivity; the microporous structure of ZIF-8 physically sieves rutin and luteolin, achieving selective enrichment of target molecules; MnIn2S4 activates catalytic sites through defect engineering, efficiently promoting electrochemical reaction kinetics.

[0041] 2. Naphthol is doped into the CB / ZIF-8 / MnIn2S4 nanocomposite material. Naphthol binds the electrocatalytic material to the electrode surface through physical adhesion, while enhancing the stability of the electrode-solution interface. This helps to further improve the sensitivity of the sensor and achieve accurate detection of target molecules.

[0042] 3. The NF-CB / ZIF-8 / MnIn2S4 nanocomposite modified electrode exhibits high conductivity and excellent catalytic performance in the electrochemical detection of ciprofloxacin, demonstrating the synergistic effect between CB, ZIF-8, MnIn2S4, and NF. The NF-CB / ZIF-8 / MnIn2S4 nanocomposite modified electrode also achieved a wide linear range (0.1 × 10⁻⁶) for the simultaneous detection of rutin and luteolin. -6 ~500×10 -6 and 0.29×10 -6 ~784.8×10 -6 mol L -1 ) and a low detection limit (3.48 nmol L). -1 and 2.41 nmol L -1 (LOD, S / N = 3), the detection method has good stability and high sensitivity; the two-component oxidation peaks have high separation, effectively avoiding signal overlap; the dynamic detection range is wide, covering the full range of needs from trace residues to constant analysis.

[0043] 4. It has strong anti-interference ability and strong tolerance to electroactive substances (such as ascorbic acid and dopamine) in complex matrices; the actual sample detection recovery rate is high (97.4-105.2%) and the reproducibility is good (RSD<3.72%), meeting the pharmacopoeia standards; using biomass such as corn stalks as raw materials, the cost is only 1 / 20 of commercial materials, supporting on-site portable rapid testing. Attached Figure Description

[0044] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the following detailed description to explain the invention, but do not constitute a limitation thereof. In the drawings:

[0045] Figure 1 The FT-IR spectra of CB, ZIF-8, CB / ZIF-8, MnIn2S4, and CB / ZIF-8 / MnIn2S4 in Embodiment 1 of the present invention are shown.

[0046] Figure 2 The images show the XRD patterns of CB, ZIF-8, CB / ZIF-8, MnIn2S4, and CB / ZIF-8 / MnIn2S4 in Embodiment 1 of the present invention.

[0047] Figure 3 The images show cyclic voltammetry diagrams of different modified electrodes in the preparation process of Example 1 of the present invention in a standard three-electrode system containing an electrolyte solution of 5 mM potassium ferricyanide + 0.1 M potassium chloride (pH = 7.0 PBS).

[0048] Figure 4 An electrochemical sensor using the NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrode prepared in Example 1 of this invention as the working electrode detected a series of differential pulse voltammetry diagrams of different concentrations of rutin and luteolin.

[0049] Figure 5 This is a selective detection diagram of the electrochemical sensor with the NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrode prepared in Example 1 of the present invention as the working electrode.

[0050] Figure 6 This is a stability test diagram of the electrochemical sensor with the NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrode prepared in Example 1 of the present invention as the working electrode.

[0051] Figure 7 This is a reproducibility test diagram of the electrochemical sensor with the NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrode as the working electrode, prepared in Example 1 of this invention. Figure 8This is a reproducibility test diagram of the electrochemical sensor with six NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrodes as working electrodes, prepared in Example 1 of this invention. Detailed Implementation

[0052] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0053] In the following examples, β-naphthol (2-naphthol) is selected as the naphthol. β-naphthol, chemical name 2-hydroxynaphthalene, molecular formula C 10 H7OH is a phenolic compound formed by replacing one hydrogen atom on a naphthalene ring with a hydroxyl group. It was purchased from Shanghai Aladdin Biotechnology Co., Ltd., CAS No. 135-19-3, with a purity of ≥99%, a melting point of 121-123℃, and a molecular weight of 144.17 g / mol.

[0054] Example 1

[0055] Preparation of CB:

[0056] Selected fresh corn stalks were chopped into small pieces and dried to reduce moisture content. Then, under anaerobic conditions, they were heated to 600°C for 4 hours to promote carbonization. After cooling, the biochar was crushed and ground uniformly, passing through a 100-mesh sieve to ensure consistent particle size.

[0057] S1. Preparation of CB / ZIF-8 material using in-situ growth method:

[0058] First, CB (0.5 g) and zinc nitrate tetrahydrate (0.02 mol) were mixed in a methanol solution (60 mL) and thoroughly dispersed by ultrasonication. Then, 2-methylimidazole (0.12 mol) was dissolved in another methanol solution (60 mL) and stirred vigorously to form a homogeneous mixture. This mixture was stirred at room temperature for 24 h. The solution was allowed to stand for 6 h. The supernatant was carefully discarded, and the precipitate was washed three times with a methanol-water mixture. Subsequently, the product was vacuum dried at 80 °C for 8 h to obtain the CB / ZIF-8 product.

[0059] S2. Preparation of CB / ZIF-8 / MnIn2S4 nanocomposite materials

[0060] First, 300.0 mg of CB / ZIF-8 was completely dispersed in 50 mL of deionized water using ultrasound. Then, 2 mmol of MnCl₂·4H₂O, 4 mmol of InCl₃·4H₂O, and 12 mmol of thioacetamide were dissolved in 60 mL of ultrapure water under stirring at room temperature, followed by magnetic stirring for 30 min. The synthesized solution was then transferred to a Teflon-lined stainless steel autoclave. The autoclave was heated at 140 °C for 12 h and then allowed to cool naturally to room temperature. The product was then obtained by centrifugation, washed with deionized water and ethanol, and dried overnight at 60 °C.

[0061] S3. Preparation of NF-CB / ZIF-8 / MnIn2S4 composite material

[0062] Weigh 0.2g of naphthol, dissolve it in 50mL of anhydrous ethanol, and add 0.1g of CB / ZIF-8 / MnIn2S4 nanocomposite material. Disperse the mixture by ultrasonication for 30min to obtain a brownish-yellow homogeneous suspension. This is the NF-CB / ZIF-8 / MnIn2S4 aqueous solution, which is the modification solution.

[0063] S4. Preparation of NF-CB / ZIF-8 / MnIn2S4 modified electrode

[0064] A bare glassy carbon electrode (GCE) with a diameter of 5 mm was pretreated by polishing its surface to a mirror finish and then ultrasonically treating it in nitric acid solution (V / V = 1:1), ethanol, and deionized water. 8 μL of an NF-CB / ZIF-8 / MnIn2S4 aqueous solution (4 mg / mL) that had been ultrasonically treated for 15 min was then drop-cast onto the pretreated GCE surface. The solution was then dried under an infrared lamp for 10 min to obtain an NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrode.

[0065] Example 2

[0066] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1. The only difference was that in step S1, 0.5 g of activated CB and 0.02 mol Zn(NO3)2·4H2O were ultrasonically dispersed in 60 mL of methanol, and then a methanol solution containing 0.12 mol 2-methylimidazole was added dropwise (controlling the dropping rate to 1 mL / min). The mixture was stirred at room temperature for only 6 h (shortening the time by 50%). The macropore confinement effect was used to suppress the excessive growth of ZIF-8, and ultra-small ZIF-8 particles were obtained.

[0067] Example 3

[0068] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1, except that in step S1, 0.5 g of CB was dispersed in 60 mL of methanol, and 0.02 mol Zn(NO3)2 solution was slowly added (0.5 mL / min), and the mixture was stirred and adsorbed for 2 h; then, a methanol solution containing 0.12 mol 2-methylimidazole was added dropwise, and the reaction was carried out at 4 °C for 48 h (ultra-slow growth ensures that ZIF-8 uniformly coats the CB surface).

[0069] Example 4

[0070] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1. The only difference was that in step S1, CB (0.3 g) and zinc nitrate tetrahydrate (0.03 mol) were mixed in 60 mL of methanol solution, ultrasonically dispersed, and then 60 mL of methanol solution of 2-methylimidazole (0.15 mol) was added. The mixture was stirred at room temperature for 36 h. The remaining steps were the same as in S1 of Example 1, resulting in a CB / ZIF-8 material with a higher ZIF-8 loading.

[0071] Example 5

[0072] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1. The only difference was that in step S2, 300 mg of CB / ZIF-8 was dispersed in a mixed solvent of 40 mL water and 20 mL ethylene glycol. After adding 2 mmol MnCl2·4H2O, 4 mmol InCl3·4H2O, and 12 mmol thioacetamide, the mixture was pre-reacted at 60 °C for 30 min, and then transferred to an autoclave and reacted at 130 °C for 16 h (the extended reaction time at low temperature improved the crystallinity).

[0073] Example 6

[0074] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1. The only difference was that in step S2, CB / ZIF-8 (300 mg) was mixed with the metal salt solution and then transferred to a microwave reactor. The mixture was microwave-irradiated at 140°C for 30 min, replacing the traditional hydrothermal treatment for 12 h. The remaining steps were the same as in S2 of Example 1, but the reaction time was shortened.

[0075] Example 7

[0076] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1, except that in step S3, 5 μL of an aqueous solution of NF-CB / ZIF-8 / MnIn2S4, which had been ultrasonically treated for 15 min, was drop-cast onto the surface of the treated glassy carbon electrode. The concentration of the aqueous solution of NF-CB / ZIF-8 / MnIn2S4 was 5 mg / mL. The electrode was then dried under an infrared lamp for 10 min to obtain the NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrode.

[0077] Example 8

[0078] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1, except that in step S2, 0.5 mmol of thiourea was added as a sulfur vacancy regulator during the hydrothermal reaction. The remaining steps were the same as in S3 of Example 1, resulting in a composite material with a higher surface defect density.

[0079] Example 9

[0080] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1, except that CB (0.5 g), zinc nitrate tetrahydrate (0.02 mol), and 2-methylimidazole (0.12 mol) were simultaneously added to 120 mL of methanol solution, ultrasonically dispersed, and stirred at room temperature for 48 h. The remaining steps were the same as S1 in Example 1, simplifying the preparation process.

[0081] Example 10

[0082] The NF-CB / ZIF-8 / MnIn2S4 modified electrode was prepared using the same method as in Example 1, except that a distributed hydrothermal method was employed in step S2: First, 300.0 mg of CB / ZIF-8 was completely dispersed in 50 mL of deionized water using ultrasound. Then, 2 mmol of MnCl2·4H2O, 4 mmol of InCl3·4H2O, and 12 mmol of thioacetamide were dissolved in 60 mL of ultrapure water under stirring at room temperature, followed by magnetic stirring for 30 min. In the first stage, the synthesized solution was transferred to a Teflon-lined stainless steel autoclave and reacted at 100 °C for 2 h to form MnIn2S4 nanocrystal nuclei. In the second stage, the temperature was raised to 140 °C and reacted for 4 h to oriented and assemble into ultrathin nanosheets, which were then naturally cooled to room temperature. The product was then obtained by centrifugation, washed with deionized water and ethanol, and dried overnight at 60 °C.

[0083] Comparative Example 1: CB / ZIF-8 Composite Nanomaterial Modified Electrode

[0084] Selected fresh corn stalks were chopped into small pieces and dried to reduce moisture content. Then, under anaerobic conditions, they were heated to 600°C for 4 hours to promote carbonization. After cooling, the biochar was crushed and ground uniformly, passing through a 100-mesh sieve to ensure consistent particle size. CB (0.5 g) and zinc nitrate tetrahydrate (0.02 mol) were mixed in a methanol solution (60 mL) and thoroughly dispersed using ultrasound. Then, 2-methylimidazole (0.12 mol) was dissolved in another methanol solution (60 mL) and vigorously stirred to form a homogeneous mixture. This mixture was stirred at room temperature for 24 hours. The solution was allowed to stand for 6 hours. The supernatant was carefully discarded, and the resulting precipitate was washed three times with a methanol-water mixture. Subsequently, the product was vacuum dried at 80°C for 8 hours to obtain the CB / ZIF-8 product.

[0085] A bare glassy carbon electrode (GCE) with a diameter of 5 mm was polished to a mirror finish and then ultrasonically treated in nitric acid solution (V / V = 1:1), ethanol, and deionized water. 8 μL of an aqueous solution of CB / ZIF-8 composite nanomaterials, which had been ultrasonically treated for 15 min, was drop-cast onto the treated GCE surface and dried under an infrared lamp for 10 min to obtain a CB / ZIF-8 composite nanomaterial-modified electrode.

[0086] Comparative Example: Electrode Modified with 2CB / MnIn2S4 Composite Nanomaterials

[0087] First, 0.5 g of CB prepared according to the method in Example 1 was dispersed in 60 mL of deionized water and sonicated for 30 min. Then, 2 mmol of MnCl2·4H2O and 4 mmol of InCl3·4H2O were added and the mixture was stirred at 60 °C for 1 h for adsorption. Then, 20 mL of aqueous solution containing 12 mmol of thioacetamide was added dropwise and the mixture was stirred at room temperature for 6 h (without high-temperature hydrothermal treatment). The precipitate was collected by centrifugation and washed three times with deionized water / ethanol. The precipitate was dried at 60 °C for 12 h to obtain CB / MnIn2S4. Next, 0.14 g of naphthol was dissolved in 50 mL of 2% acetic acid solution and 0.06 g of CB / MnIn2S4 was added and sonicated for 30 min to obtain the modification solution. Finally, 8 μL of the modification solution with a concentration of 4 mg / mL was drop-coated onto the pretreated GCE surface and dried with an infrared lamp to obtain the modified electrode.

[0088] Comparative Example 3: ZIF-8 / MnIn2S4 Composite Nanomaterial Modified Electrode

[0089] First, 0.02 mol Zn(NO3)2·4H2O and 0.12 mol 2-methylimidazole were dissolved in 60 mL of methanol and mixed. The mixture was stirred at room temperature for 24 h, centrifuged and washed, and then vacuum dried at 80 °C for 8 h to obtain pure ZIF-8 with a particle size of approximately 100 nm. Next, 300 mg of ZIF-8 was dispersed in 50 mL of water and sonicated for 30 min. Then, 2 mmol MnCl2·4H2O, 4 mmol InCl3·4H2O and 12 mmol thioacetamide were added. After magnetic stirring for 30 min, the mixture was hydrothermally reacted at 140 °C for 12 h to obtain the ZIF-8 / MnIn2S4 complex. Then, 0.14 g of naphthol was dissolved in 50 mL of 2% acetic acid solution, and 0.06 g of ZIF-8 / MnIn2S4 was added and sonicated to prepare a modification solution. Finally, 8 μL of the modification solution with a concentration of 4 mg / mL was drop-coated onto the surface of a glassy carbon electrode (GCE) to complete the preparation of the modified electrode.

[0090] Test case

[0091] Material characterization

[0092] Through FT-IR spectroscopy ( Figure 1 The functional group evolution and interfacial bonding characteristics of CB, ZIF-8, CB / ZIF-8, MnIn2S4, and CB / ZIF-8 / MnIn2S4 were systematically analyzed. All samples were analyzed at 3435 cm⁻¹. -1 All regions exhibit broad absorption bands, attributed to the stretching vibrations of the OH bonds in the adsorbed water molecules, at 1612.2 cm⁻¹. -1 The weak peak at this point corresponds to a C=N bond. HB is at 1044.3 cm⁻¹. -1 The characteristic peaks at this location originate from the stretching vibrations of the COC ether bonds, confirming the presence of oxygen-containing functional groups on its surface, which is consistent with the analysis results of CO bonds in XPS. In the infrared spectrum of pure ZIF-8, the peaks at 500-1400 cm⁻¹... -1 The interval exhibits multiple characteristic peaks, 745.6 cm⁻¹ -1 Attributable to in-plane bending vibration of the imidazole ring; 994.5 cm -1 Corresponding to CN stretching vibration; 1480.6cm -1 : Attributable to the deformation vibration of the NH bond in the imidazole ring; 1384.2 cm -1 : Reflects the respiratory vibration pattern of the imidazole ring. Additionally, 2200cm -1 The weak peak at this point may originate from residual cyano (-C≡N) precursors or coordination unsaturated sites from the synthesis process. After recombination (CB / ZIF-8), the characteristic peak intensity of ZIF-8 significantly decreased, indicating that CB interacts with Zn in ZIF-8 through its surface oxygen-containing functional groups (-COOH, -OH). +The coordination effect of the imidazole ring restricts the vibrational freedom of the imidazole ring, thus inhibiting the ordered growth of the crystal. In the infrared spectrum of MnIn2S4, at 520.4 cm⁻¹... -1 With 760.6cm -1 The strong absorption bands at this point correspond to the antisymmetric stretching vibration of the Mn-S bond and the bending vibration of the In-S bond, respectively, confirming the formation of its hexagonal phase structure (corresponding to the (311) crystal plane diffraction peak at 2θ = 27.7° in XRD). 1142.8cm -1 The shoulder peak at the point is attributed to the CC / C=C skeletal vibration, originating from residual organic precursors during synthesis. In the CB / ZIF-8 / MnIn2S4 ternary composite material, the characteristic peaks of both ZIF-8 and MnIn2S4 are retained. In summary, FT-IR analysis reveals the evolution of functional groups, interfacial chemical bonding, and structural integrity in the composite material, providing molecular vibrational evidence for the synergistic effect of multiple components.

[0093] Characterized by XRD Figure 2The crystal structures and phase compositions of CB, ZIF-8, CB / ZIF-8, MnIn2S4, and CB / ZIF-8 / MnIn2S4 composites were systematically analyzed. Pure CB exhibited broad (002) and (101) diffraction peaks at 2θ = 27.6° and 43.8°, respectively, corresponding to the short-range ordered structure of partially graphitized carbon, which originates from the gradient carbonization of corn stalks and the pore-forming process activated by KOH. The diffraction peak positions of pure ZIF-8 (2θ = 5.7°, 11.8°, 14.5°, 17.4°, 27.8°, 30.2°, 38.7°) precisely matched the standard rhombohedral crystal system structure (CCDC 602542), and were assigned to the (011), (002), (112), (222), (114), (233), and (004) crystal planes, respectively, confirming its high crystallinity and phase purity. After composite formation, the intensity of the characteristic peaks of ZIF-8 in the XRD spectrum of CB / ZIF-8 decreased significantly (e.g., the intensity of the (011) peak decreased by 45%), indicating that CB suppresses the long-range ordered diffraction signal of ZIF-8 crystal through physical coating and chemical anchoring of the ZIF-8 crystal by surface oxygen-containing functional groups (-COOH, -OH). The XRD spectrum of MnIn2S4 shows sharp diffraction peaks at 2θ = 14.7°, 27.7°, 33.5°, 43.5° and 48.3°, corresponding to the (111), (311), (400), (511) and (440) crystal planes of hexagonal MnIn2S4 (JCPDS 65-7474), respectively, and there are no impurity phases, confirming its high purity and complete crystal structure. In the CB / ZIF-8 / MnIn2S4 ternary composite material, the characteristic peaks of ZIF-8 and MnIn2S4 are clearly visible, indicating that the composite process did not destroy the intrinsic crystal structure of each component. Notably, the intensity of the (511) diffraction peak (2θ = 43.5°) of MnIn2S4 was approximately 1.8 times higher than that of the pure phase, attributed to the preferred orientation growth of the (511) crystal plane induced by ZIF-8. This indicates that the CB / ZIF-8 heterojunction inhibits sulfide grain coarsening and introduces lattice strain. In summary, XRD analysis verifies the phase purity, multi-component structural compatibility, and effectiveness of defect engineering of the composite material from a crystallographic perspective, providing crucial evidence for heterojunction interface design and electrochemical performance optimization.

[0094] Electrochemical characterization analysis

[0095] Electrochemical tests were performed on the NF-CB / ZIF-8 / MnIn2S4 composite nanomaterial modified electrode of Example 1. The test steps are as follows: [The electrode was subjected to an atmosphere containing 5 mM [Fe2CN)6] 3- / 4-In a standard three-electrode system with +0.1M KCl (pH=7.0PBS) electrolyte solution, using NF-CB / ZIF-8 / MnIn2S4 as the working electrode, the electrochemical behavior of glassy carbon electrodes modified with different materials was studied by CV in the potential range of -0.4 to 0.8V; the electrochemical behavior of rutin and quercetin modified with NF-CB / ZIF-8 / MnIn2S4 composite nanomaterials was also investigated; and the selectivity, stability, and reusability of the electrochemical sensor were also assessed.

[0096] (1) Electrochemical characterization of glassy carbon electrodes modified with different materials

[0097] like Figure 3 As shown, the bare GCE at 0.42V (I pa =40.5μA) and -0.05V(I pc A symmetrical redox peak was observed at -38.8 μA, indicating that the interfacial reaction was highly reversible. After CB modification, the redox peak current was significantly increased (I0.05). pa =72.3 μA, improvement rate 78.5%, attributed to the three-dimensional porous structure of CB accelerating ion diffusion and providing abundant electroactive sites. The ZIF-8 modified electrode suffers from a decrease in peak current due to its poor intrinsic conductivity (I...). pa =41.4 μA), while the CB / ZIF-8 composite electrode promotes electron tunneling through interfacial CO-Zn coordination bonds, restoring the current to 58.7 μA. The MnIn2S4 modified electrode exhibits excellent catalytic activity (I = 41.4 μA), while the CB / ZIF-8 composite electrode promotes electron tunneling through interfacial CO-Zn coordination bonds, restoring the current to 58.7 μA. pa =81.5μA), originating from its sulfur vacancy defects and multi-metal sites (Mn 3+ / In 3+ The ternary composite material CB / ZIF-8 / MnIn2S4 exhibits the best performance in synergistically reducing the reaction energy barrier. pa =115.2 μA), which is 1.8 times higher than that of the bare electrode, verifying the multi-component synergistic mechanism: the three-dimensional conductive network of CB accelerates charge transport, the micropore confinement effect of ZIF-8 enriches probe molecules, and the defect sites of MnIn2S4 enhance catalytic oxidation kinetics.

[0098] (2) Electrochemical behavior of rutin and luteolin in electrodes modified with NF-CB / ZIF-8 / MnIn2S4 composite nanomaterials

[0099] Differential pulse voltammetry (DPV) is a sensitive technique that can detect low concentrations of analytes over a wide linear range. Figure 4(A) shows the DPV curve, representing the linear range of simultaneous detection with inserted analytes. The peak values ​​of the analytes are added for better understanding. Individual analysis revealed enhanced response after consecutive additions of Rn and Lu, indicating excellent electrochemical performance of the electrode material. DPV measurements were performed using a CB / ZIF-8 / MnIn2S4 / GCE sensor in 0.1M PBS solution (pH 7.0), with a potential scan range of 0V to +0.9V and a scan rate of 100mV s. -1 For Rn, the regression equation for the concentration range is: I = 0.388C + 21.032(Rn). 2 =0.997). For example... Figure 4 (B) is shown. Similarly, the regression equation for the low concentration range of Lu (0.29 μM-50 μM) is: I 0.29-50 =0.648C + 20.309(R) 2 =0.986). In the higher concentration range (50 μM-784.8 μM), the equation is: I 50-784.8 =0.289C + 29.052(R) 2 =0.996), such as Figure 4 As shown in (C). The LODs of Rn and Lu, calculated using the detection limit (LOD), are 3.48 nM and 2.41 nM, respectively. Furthermore, the sensor's sensitivity was determined to be 5.54 μA·μM based on sensitivity calculations. -1 ·cm -2 These results demonstrate that the developed sensor has a wide concentration range, low detection limit, and high sensitivity.

[0100] (3) Selective detection by electrochemical sensors

[0101] like Figure 5 As shown, common biomolecules or inorganic ions (such as K+) are used. + Al 3+ Na + SO4 2- Cl - PO 3- After mixing glucose, sucrose, dopamine, ascorbic acid, glycine, tryptophan, and urea with 5 μM Rn and 10 μM Lu (pH=7), interfering substances were added periodically and DPV curves were recorded. The results showed that the interfering substances had little effect on the electrode, indicating that the sensor has high selectivity for Rn and Lu.

[0102] (4) Stability detection of electrochemical sensors

[0103] like Figure 6As shown, the CB / ZIF-8 / MnIn2S4 / GCE electrode was stored in a 4°C refrigerator, and a mixed solution of 5 μM Rn and 10 μM Lu was tested every three days by DPV. The results showed that after 21 days, the current responses of Rn and Lu retained 82.76% and 85.89% of their initial values, respectively. This indicates that the electrode exhibits good stability, mainly attributed to the inherent stability of the CB / ZIF-8 / MnIn2S4 / GCE composite material.

[0104] (5) Reusability testing of electrochemical sensors

[0105] like Figure 7 As shown, a single electrode underwent seven consecutive tests at room temperature, resulting in small variations in the current response, with relative standard deviations (RSDs) of 1.93% and 2.54% for Rn and Lu, respectively. Seven independent electrodes were fabricated and their current responses were recorded. Figure 8 The results showed that the current responses were similar, with RSDs of 1.71% and 3.04%, respectively. This indicates that the fabricated sensor has acceptable repeatability and reproducibility. The test results for each embodiment are shown in Table 1.

[0106] Table 1

[0107]

[0108] In summary, this invention utilizes a high-temperature calcination method to prepare CB nanosheets and ZIF-8, as well as their composite material (CB / ZIF-8). A CB / ZIF-8 / MnIn2S4 composite nanomaterial is then prepared via a hydrothermal method. Using NF as a binder, a modified electrode is fabricated using a drop-coating method, enabling trace detection of rutin and luteolin. Through a multi-level heterostructure design, the three-dimensional conductive network of CB significantly accelerates electron transport, the micropore confinement effect of ZIF-8 selectively enriches target molecules, and the sulfur vacancy defects of MnIn2S4 synergistically reduce oxidation overpotential and enhance catalytic reaction kinetics with the bimetallic active sites. This sensor exhibits excellent linear responses to rutin and luteolin in the range of 0.1–784.4 μM, with detection limits as low as 3.48 nM and 2.41 nM, respectively, and demonstrates strong anti-interference capabilities. The spiked recoveries of actual samples (honeysuckle and ginkgo leaf extracts) were 96.4%–104.3% (RSD<3.3%), verifying their reliability in complex matrices.

[0109] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0110] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0111] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A biochar (CB) / ZIF-8 / MnIn2S4 modified electrode, characterized in that, The electrocatalytic material coating the electrode surface is prepared by the following method: (1) Carbonize corn stalks at 500-700℃ for 2-4 hours under anaerobic conditions, and grind them through a 200-mesh sieve to obtain CB; (2) Disperse CB and zinc nitrate in methanol at a mass ratio of 1:(0.5-1.2), sonicate for 10-30 minutes, and then add 2-methylimidazolium methanol solution, wherein Zn 2+ The molar ratio of CB / ZIF-8 to 2-methylimidazole was 1:(4-6). The mixture was stirred at room temperature for 6-12 hours, centrifuged, washed with methanol, and dried under vacuum at 60°C to obtain CB / ZIF-8. (3) Disperse CB / ZIF-8 in deionized water, and add manganese chloride tetrahydrate, indium trichloride tetrahydrate, and thioacetamide, wherein... The molar ratio of Mn:In:S is 1:(1-2):(4-6). The mixture is transferred to a polytetrafluoroethylene-lined reactor and hydrothermally reacted at 120-160℃ for 10-16 hours. After centrifugation and washing, it is dried at 60℃ to obtain CB / ZIF-8 / MnIn2S4. (4) Dissolve β-naphthol in anhydrous ethanol to prepare a 1-3 mg / mL solution, add CB / ZIF-8 / MnIn2S4 to make a mass ratio of 1:(1-3), and ultrasonically disperse for 30 minutes to obtain the modified solution; (5) Drop 5-10 μL of modification solution onto the surface of the pretreated glassy carbon electrode and dry it under an infrared lamp to form a film.

2. The method for preparing the modified electrode according to claim 1, characterized in that, In step (2), the growth of ZIF-8 adopts any of the following optimized processes: (a) Control the dropping rate of 2-methylimidazole solution to 0.5-1 mL / min and stir at room temperature for 4-6 hours; (b) First, mix Zn(NO3)2 solution with CB and adsorb for 1-2 hours, then react at 4℃ for 48-72 hours. (c) Add CB, Zn(NO3)2 and 2-methylimidazole to the reaction system simultaneously and stir at room temperature for 36-48 hours.

3. The preparation method according to claim 1, characterized in that, In step (3), the synthesis of MnIn2S4 adopts any of the following improved processes: (a) Use a mixed solvent of water and ethylene glycol in a volume ratio of (1-2):1, pre-react at 60°C for 30 minutes, and then hydrothermally react at 130°C for 16-20 hours. (b) Add 0.5-1 mmol thiourea as a sulfur vacancy regulator; (c) Stepwise hydrothermal reaction: First react at 100℃ for 2 hours, then raise the temperature to 140℃ and react for 4-6 hours.

4. The preparation method according to claim 1, characterized in that, In step (3), microwave-assisted synthesis is used instead of hydrothermal reaction. The reaction conditions are: microwave power 300-500W, temperature 140-160℃, and reaction time 20-30 minutes.

5. The preparation method according to claim 1, characterized in that, In step (5), the amount of the modification solution dropped is 5-8 μL and the concentration is 4-5 mg / mL.

6. A CB / ZIF-8 / MnIn2S4 composite nanomaterial prepared by any one of claims 1-5, characterized in that: ZIF-8 particles exhibit a polyhedral morphology with a particle size of 50-100 nm, and are uniformly loaded on the CB surface; MnIn2S4 grows on the ZIF-8 surface in the form of nanosheets with a thickness of 5-10 nm or crystal nuclei with a diameter of 3-5 nm.

7. An electrochemical sensor, characterized in that, The working electrode comprises the biochar (CB) / ZIF-8 / MnIn2S4 modified electrode as described in any one of claims 1-6.

8. The application of the electrochemical sensor according to claim 7 in the simultaneous detection of rutin and luteolin, characterized in that: The detection range is 0.01-784.4 μmol·L⁻¹ -1 The detection limits were rutin ≤ 3.48 nM and luteolin ≤ 2.41 nM (S / N = 3).

9. The application according to claim 8, characterized in that, The tested substances are rutin and luteolin in extracts of traditional Chinese medicinal materials, including but not limited to at least one of honeysuckle, ginkgo leaves, and sophora japonica buds.

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