FeNiOOH / S-coated N-MWCNT / CC composite electrode and preparation method and application thereof

Abstract

The invention discloses a FeNiOOH / S-coated N-MWCNT / CC composite electrode as well as a preparation method and application thereof, and belongs to the technical field of food safety detection. In order to solve the problems that an existing electrochemical sensor cannot detect urea in one step and is not suitable for an ethanol-containing sample, the invention provides a preparation method of a FeNiOOH / S (at) N-MWCNTs / CC composite electrode, and the preparation method comprises the following steps: repeatedly soaking activated carbon cloth in an N-MWCNTs aqueous dispersion liquid for ultrasonic treatment to obtain an N-MWCNTs / CC electrode, then synthesizing the FeNiS (at) N-MWCNTs / CC electrode by a hydrothermal method, and finally obtaining the FeNiOOH / S (at) N-MWCNTs / CC composite electrode. And finally, synthesizing the FeNiOOH / S (at) N-MWCNTs / CC composite electrode rich in Ni (III) through a chronopotentiometric method. Urea decomposition can be catalyzed in one step, the detection time is short, the detection limit is low, the sensitivity is high, and on-site rapid detection of urea in vinasse and fermented grains can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of food safety testing technology, specifically relating to a FeNiOOH / S@N-MWCNT / CC composite electrode, its preparation method, and its application. Background Technology

[0002] Studies have shown that ethyl carbamate, a Group 2A carcinogen in alcoholic beverages, is mostly a final product of the reaction between ethanol and urea. Urea in baijiu (Chinese white liquor) mainly comes from two sources: firstly, urea is introduced into the raw materials during baijiu brewing; secondly, it is produced by the metabolism of arginine by microorganisms during baijiu fermentation. The increase in ethyl carbamate content during fermentation is mainly due to the reaction between urea and ethanol in the mash. The increase or decrease in ethyl carbamate is basically synchronized with the change in urea, and the correlation between the two is very obvious. Urea is the main precursor of ethyl carbamate during baijiu fermentation. Early monitoring of urea content not only helps reduce the formation of ethyl carbamate but is also crucial for ensuring the safety of baijiu production. Therefore, developing a rapid detection technology for urea content in baijiu mash is imperative.

[0003] Currently, traditional detection methods include spectrophotometry and chromatography. Spectrophotometry mainly determines the corresponding spectrophotometric value through direct or indirect colorimetry. Commonly used direct colorimetric methods include the diacetyl monooxime method, the o-phthalaldehyde method, and the p-dimethylaminobenzaldehyde method. Commonly used indirect colorimetric methods include the urease-Bohrer method, the enzyme-coupled method, and the cresol red method. Chromatography is mainly based on techniques related to liquid chromatography. Currently, the main methods include high-performance liquid chromatography-fluorescence detector method and liquid chromatography-isotope dilution tandem mass spectrometry. Besides traditional methods, there are also some emerging technologies for urea detection, such as test strips and kits, portable colorimeters, and electrochemical sensors. Traditional methods are cumbersome to operate, and some require expensive and large instruments, resulting in high costs and making them unsuitable for on-site testing. Therefore, they are gradually being replaced by various rapid detection products that are easy to operate, provide accurate results, and are portable. However, test strips and kits are mainly qualitative and semi-quantitative, primarily used for clinical diagnosis. Portable sensors, such as electrochemical sensors, can directly read urea concentrations and enable large-scale on-site detection. They are rapid, convenient, and adaptable to different matrices, making them promising for urea detection in spirits and fermented grains. Electrochemical sensors are generally divided into enzyme sensors and non-enzyme sensors. Enzyme activity in enzyme sensors is easily affected by environmental interference, requiring refrigeration for storage. Non-enzyme sensors are more environmentally adaptable than enzyme sensors, but currently, most use the urease active center, metal Ni(II), as the reaction site. This requires two steps to catalyze the decomposition of urea, namely, converting it into Ni(III) before reacting with urea to achieve urea detection. Furthermore, they are not widely used for detecting urea in actual samples containing ethanol, such as fermented grains.

[0004] CN108690556A discloses a reduced graphene oxide / multi-walled carbon nanotube / nickel ferrite ternary nanocomposite microwave absorbing material (RGO / MWCNTs / NiFe2O4), which is composed of two-dimensional sheet-like RGO, one-dimensional tubular MWCNTs and magnetic NiFe2O4 nanoparticles, and the composite material contains a local three-dimensional conductive network, which enhances the attenuation loss of electromagnetic waves; it is prepared by the following method: (1) Take three 100mL beakers, add 60mL of deionized water to each, and then... Then, 36 mg of graphene oxide was added, and the mixture was sonicated for 30 min and stirred vigorously for 2 h to obtain an aqueous dispersion of graphene oxide (GO) with a concentration of 0.60 mg / mL. Then, 0, 10 and 20 mg of MWCNTs were added, and the mixture was sonicated for 30 min to make it uniformly dispersed in the aqueous solution. (2) A certain amount of ferric nitrate nonahydrate (Fe(NO3)3·9H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) were added, and the mixture was stirred vigorously for 30 min and sonicated for 30 min. 3+ with Ni 2+ The molar ratio satisfies nFe 3+ :nNi 2+ =2:1; (3) Add ammonia dropwise to adjust the pH of the mixed dispersion to 11; (4) Pour the mixed dispersion into a 100mL reaction vessel and hydrothermally react at 180℃ for 24h; (5) After the reaction is complete, cool to room temperature, collect the solid product by magnetic separation, wash with deionized water until neutral, and then wash three times with anhydrous ethanol; (6) Place in a vacuum drying oven and dry at 60℃ for 24h, then grind to obtain the final product. This method prepares a magnetic NiFe2O4 nanoparticle electromagnetic composite material for absorbing electromagnetic waves, but it cannot be used for food safety testing.

[0005] CN119023768A discloses a method for preparing an N-CNT / CC enzyme-modified electrode and its application in the rapid detection of ethyl carbamate (EC) content in alcoholic beverages. The preparation method involves activating carbon cloth, immersing the activated carbon cloth in an N-CNT solution, allowing it to react fully, and then washing and drying to obtain an N-CNT / CC electrode. The N-CNT / CC electrode is then modified with a biocomposite material containing glutamate dehydrogenase and ethyl carbamate degrading enzyme to obtain the N-CNT / CC enzyme-modified electrode. However, this method can only detect ethyl carbamate content and cannot detect urea. Therefore, it cannot monitor urea content at the source and strictly control urea content to reduce ethyl carbamate formation.

[0006] Therefore, it is necessary to develop a new, simple, sensitive method for detecting urea in actual samples of distiller's grains and mash, in order to monitor changes in urea content during the fermentation stage of baijiu, thereby controlling the formation of ethyl carbamate and providing a technical reference for reducing the ethyl carbamate content in baijiu. Summary of the Invention

[0007] The technical problem to be solved by the present invention is that the existing electrochemical sensors for detecting urea require Ni(II) to be converted into Ni(III) before urea electro-oxidation can be performed, and are not applicable to samples containing ethanol, i.e., distiller's grains.

[0008] To address the aforementioned technical problems, this invention first provides a method for preparing a FeNiOOH / S@N-MWCNT / CC composite electrode, which includes the following steps:

[0009] A. Activated carbon cloth;

[0010] B. Preparation of N-MWCNTs / CC electrode: The activated carbon cloth is repeatedly immersed in the aqueous dispersion of N-MWCNTs and subjected to ultrasonic treatment to obtain the N-MWCNTs / CC electrode.

[0011] C. Preparation of FeNiS@N-MWCNTs / CC electrode: The N-MWCNTs / CC electrode is immersed in a composite aqueous solution of soluble inorganic nickel salt, soluble inorganic iron salt or ferrous salt, fluorine source, pH adjuster, sulfiding agent and dispersant, and FeNiS@N-MWCNTs / CC electrode is obtained by hydrothermal method.

[0012] D. Preparation of FeNiOOH / S@N-MWCNTs / CC composite electrode: FeNiS@N-MWCNTs / CC electrode was obtained by chronopotentiometric method.

[0013] In the above preparation method, step A, the specific method for activating the carbon cloth is as follows: concentrated sulfuric acid, concentrated nitric acid, and water are mixed in a volume ratio of 1~3:1~3:1~3, and then a mixture with an area of ​​1~5 cm² is prepared. 2 The raw carbon cloth is immersed in it, sealed, and subjected to hydrothermal reaction at 100~140℃ for 6~12 h. It is then washed with ultrapure water 5~10 times and dried.

[0014] Preferably, in the above preparation method, when activating the carbon cloth in step A, the volume ratio of concentrated sulfuric acid, concentrated nitric acid and water is 1:1:1, the area of ​​the raw carbon cloth is 2 cm*2 cm, and the hydrothermal reaction is carried out at 140℃ for 12 h.

[0015] In the preparation method described above, step B specifically involves immersing the activated carbon cloth in an aqueous dispersion of N-MWCNTs while simultaneously performing ultrasonic treatment. After drying the carbon cloth, it is immersed and ultrasonically treated again. Finally, it is washed and dried to obtain the N-MWCNTs / CC electrode.

[0016] In the above preparation method, in step B, the concentration of the aqueous dispersion of N-MWCNTs is 0.5~2.5 mg / mL.

[0017] In the above preparation method, in step B, the aqueous dispersion of N-MWCNTs is obtained by ultrasonically dispersing N-MWCNTs and water for 1-5 h.

[0018] In the above preparation method, in step B, the duration of each ultrasonic treatment is 5 to 20 minutes.

[0019] Preferably, in the above preparation method, the duration of each ultrasound session in step B is 10 minutes.

[0020] In the above preparation method, step B involves a total of 4 to 6 soaking and ultrasonic treatments.

[0021] Preferably, in the above preparation method, the total number of soaking and ultrasonic treatments in step B is 4.

[0022] In the above preparation method, in step C, the soluble inorganic nickel salt is selected from at least one of nickel nitrate, nickel chloride, nickel acetate, or nickel sulfate.

[0023] In the above preparation method, in step C, the soluble inorganic ferric salt or ferrous salt is selected from at least one of ferric nitrate, ferric chloride, ferric sulfate, ferrous nitrate, ferrous chloride, or ferrous sulfate.

[0024] In the above preparation method, in step C, the fluorine source is selected from at least one of NH4F, NaF, or KF.

[0025] In the above preparation method, in step C, the pH adjuster is selected from at least one of urea, ammonia, or ammonium carbonate.

[0026] In the above preparation method, in step C, the sulfiding agent is selected from at least one of thiourea, thioacetamide, sodium sulfide, or potassium sulfide.

[0027] In the above preparation method, in step C, the dispersant is selected from at least one of hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, or polyethylene glycol.

[0028] In the preparation method described above, in step C, the concentration of the soluble inorganic nickel salt in the composite aqueous solution is 24-64 mM, the concentration of the soluble inorganic iron salt or ferrous salt is 12-52 mM, the concentration of the fluorine source is 100-200 mM, the concentration of the pH adjuster is 300-400 mM, the concentration of the vulcanizing agent is 24-64 mM, and the concentration of the dispersant is 0.1-1 mM.

[0029] In the above preparation method, step C, the method for preparing the composite aqueous solution is as follows: first, mix the soluble inorganic nickel salt, the soluble inorganic iron salt or ferrous salt, the fluorine source, the pH adjuster and water, dissolve by ultrasonication, stir for 10~60 min, then add the sulfiding agent and the dispersant, dissolve by ultrasonication, and the solution is obtained.

[0030] In the above preparation method, in step C, the temperature of the hydrothermal method is 100~180 ℃.

[0031] Preferably, in the above preparation method, the temperature of the hydrothermal method in step C is 120°C.

[0032] In the above preparation method, in step C, the hydrothermal process takes 3 to 15 hours.

[0033] Preferably, in the above preparation method, the hydrothermal process in step C takes 6 hours.

[0034] In the above preparation method, step D, the specific method of the chronopotential method is as follows: the FeNiS@N-MWCNTs / CC electrode is used as the working electrode of electrochemistry, immersed in 0.5~2M KOH solution, and electrochemical oxidation is performed under a constant current of 5~20 mA for 5~20 min by chronopotential scanning. Then, it is cleaned and dried to obtain the FeNiOOH / S@N-MWCNTs / CC composite electrode.

[0035] Preferably, in the above preparation method, in step D, the chronopotentiometric parameters are 10 mA constant current treatment for 10 min.

[0036] The present invention also provides a FeNiOOH / S@N-MWCNTs / CC composite electrode, which is prepared by the above preparation method.

[0037] This invention also provides the application of the above-mentioned FeNiOOH / S@N-MWCNTs / CC composite electrode in urea detection.

[0038] Among the above applications, the FeNiOOH / S@N-MWCNTs / CC composite electrode of the present invention is more suitable for use in detecting the urea content in distiller's grains or mash.

[0039] This invention also provides a method for detecting urea content in distiller's grains or mash using the above-mentioned FeNiOOH / S@N-MWCNTs / CC composite electrode, which includes the following steps:

[0040] a. Place the FeNiOOH / S@N-MWCNTs / CC composite electrode in a 0.05~0.15 M KOH solution, add a series of urea standard solutions of different concentrations, apply a voltage of -0.4~0.6 V, perform differential pulse voltammetry scanning, record the peak current, and calculate the corresponding current response change value;

[0041] b. Based on the concentration of the urea standard solution and the corresponding change in current response, a standard curve is prepared and a linear equation is constructed.

[0042] c. Replace the series of urea standard solutions of different concentrations in step a with the pretreated sample of distiller's grains or mash, and perform the detection according to the method in step a to obtain the corresponding current response change value. Substitute the value into the linear equation constructed in step b to calculate the urea content in the distiller's grains or mash.

[0043] In the above method for detecting urea content in distiller's grains or mash, in step a, the concentration range of the series of urea standard solutions of different concentrations is 1~17000 μM.

[0044] In the method for detecting urea content in distiller's grains or mash, step b shows a three-segment linear relationship between urea concentration and current response within the ranges of 1~10 μM, 10~100 μM, and 100~17000 μM.

[0045] In the method for detecting urea content in distiller's grains or mash, step b involves obtaining the current response change value based on a series of urea standard solutions of different concentrations. Construct a linear equation: ;in, denoted as the concentration of the urea standard solution in μM, a is the slope of the linear equation, and b is the intercept of the linear equation.

[0046] In the above method for detecting urea content in distiller's grains or mash, step c involves the following pretreatment operation: based on the national standard method GB / T 36859-2018 Determination of Urea Content in Feed, the final filtrate is filtered through a 0.22 μm needle filter to obtain the sample to be tested.

[0047] The beneficial effects of this invention are:

[0048] This invention provides a FeNiOOH / S@N-MWCNTs / CC composite electrode and a method for detecting urea content in distiller's grains and mash using this FeNiOOH / S@N-MWCNTs / CC composite electrode. The method for detecting urea content in distiller's grains and mash provided by this invention does not require large and complex instruments; only an electrochemical workstation is needed to detect the electrical signal. The reaction does not require two steps to catalyze the decomposition of urea (Ni(III) directly catalyzes urea). The detection time is short, with the current response value detectable in only 125 s, thus achieving urea detection. The method of this invention has a low detection limit and high sensitivity, which can meet the actual needs of urea detection in samples, especially distiller's grains and mash. It can realize rapid on-site detection of urea in distiller's grains and mash, and has practical significance. Attached Figure Description

[0049] Figure 1 The image shows the SEM characterization of Example 1; where a is activated carbon cloth, b is FeNiOOH / S@CC, c is FeNiOOH@N-MWCNTs / CC, d is FeNiS@N-MWCNTs / CC, and e is FeNiOOH / S@N-MWCNTs / CC.

[0050] Figure 2 The elemental spectrum of the FeNiOOH / S@N-MWCNTs / CC composite electrode obtained in Example 1 is shown.

[0051] Figure 3 This is a comparison chart of the electrochemical performance of different electrodes.

[0052] Figure 4 This is a stability verification diagram of the electrochemical method for rapid detection of urea content using the FeNiOOH / S@N-MWCNTs / CC composite electrode obtained in Example 1.

[0053] Figure 5 This is a standard curve showing the relationship between urea concentration and current response. Detailed Implementation

[0054] In a first aspect, the present invention provides a method for preparing a FeNiOOH / S@N-MWCNT / CC composite electrode, comprising the following steps:

[0055] (1) Activated carbon cloth (CC):

[0056] First, concentrated sulfuric acid, concentrated nitric acid, and water are added to the reactor liner in a volume ratio of 1-3:1-3:1-3; then, a 1-5 cm² area is added. 2 The raw carbon cloth was immersed in this solution, sealed, and subjected to hydrothermal reaction at 100~140 ℃ for 6~12 h; finally, it was washed 5~10 times with ultrapure water and dried.

[0057] Preferably, the volume ratio of concentrated sulfuric acid, concentrated acid and water is 1:1:1, the area of ​​raw carbon cloth is 2 cm*2 cm, and the hydrothermal reaction is carried out at 140℃ for 12 h.

[0058] (2) Preparation of N-MWCNTs / CC electrode:

[0059] First, an aqueous dispersion of N-MWCNTs with a concentration of 0.5~2.5 mg / mL was prepared using ultrapure water. After ultrasonic dispersion for 1~5 h, a uniformly dispersed aqueous dispersion of N-MWCNTs was obtained. Then, the activated carbon cloth (CC) was immersed in the uniformly dispersed aqueous dispersion of N-MWCNTs and ultrasonically treated for 5~20 min. The carbon cloth was then removed, dried, and immersed and ultrasonically treated again. This process was repeated 3~5 times (i.e., a total of 4~6 times). Finally, the unattached N-MWCNTs were washed with ultrapure water and dried to obtain the N-MWCNTs / CC electrode.

[0060] Preferably, each ultrasound treatment lasts for 10 minutes; this is repeated 3 times (i.e., a total of 4 times).

[0061] (3) Preparation of FeNiS@N-MWCNTs / CC electrode:

[0062] First, a composite aqueous solution containing 24–64 mM soluble inorganic nickel salt, 12–52 mM soluble inorganic iron or ferrous salt, 100–200 mM fluorine source, and 300–400 mM pH adjuster was prepared using ultrapure water. The solution was dissolved by ultrasonication and stirred for 10–60 min. Then, 24–64 mM sulfiding agent and 0.1–1 mM dispersant were added and dissolved by ultrasonication. The N-MWCNTs / CC electrode was immersed in the solution. Finally, the electrode was placed in an oven for a hydrothermal reaction at 100–180 °C for 3–15 h. After the reaction was completed, the electrode was removed, rinsed with ultrapure water, and dried to obtain the FeNiS@N-MWCNTs / CC electrode.

[0063] Preferably, the hydrothermal reaction temperature is 120℃ and the time is 6 h;

[0064] (4) Preparation of FeNiOOH / S@N-MWCNTs / CC composite electrode:

[0065] First, the FeNiS@N-MWCNTs / CC electrode was used as the working electrode for electrochemistry and immersed in 0.5~2 M KOH solution. Then, electrochemical oxidation was performed by chronopotential scanning, i.e., under a constant current of 5~20 mA for 5~20 min. Finally, it was cleaned and dried to obtain the FeNiOOH / S@N-MWCNTs / CC composite electrode.

[0066] Preferably, the timing potentiometric method parameters are 10 mA constant current treatment for 10 min.

[0067] In step C of this invention, the soluble inorganic nickel salt is selected from at least one of nickel nitrate, nickel chloride, nickel acetate, or nickel sulfate; the soluble inorganic ferric salt or ferrous salt is selected from at least one of ferric nitrate, ferric chloride, ferric sulfate, ferrous nitrate, ferrous chloride, or ferrous sulfate; the fluorine source is selected from at least one of NH4F, NaF, or KF; the pH adjuster is selected from at least one of urea, ammonia, or ammonium carbonate; the sulfiding agent is selected from at least one of thiourea, thioacetamide, sodium sulfide, or potassium sulfide; and the dispersant is selected from at least one of hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, or polyethylene glycol. The soluble inorganic nickel salt, soluble inorganic ferric salt, or ferrous salt provides nickel ions and (ferrous) ferrous ions; sulfur atoms are introduced through the sulfiding agent; fluorine sources such as NH4F mainly regulate crystal growth and control material morphology; urea and the like play a pH adjustment role; and hexadecyltrimethylammonium bromide and the like act as dispersants to prevent particle agglomeration and improve material dispersibility.

[0068] In step D of this invention, through experimental screening, it was found that the FeNiS@N-MWCNTs / CC electrode can be converted into a Ni(III)-rich FeNiOOH / S@N-MWCNTs / CC composite electrode within 5-20 min under a constant current of 5-20 mA. Urea electro-oxidation process: ; This invention promotes the production of more Ni(III) active sites through electrochemical oxidation treatment, reduces the conversion process of Ni(II) to Ni(III), accelerates the catalytic cycle, reduces the reaction energy potential, and achieves efficient urea detection.

[0069] Secondly, the present invention provides a FeNiOOH / S@N-MWCNTs / CC composite electrode prepared by the above preparation method.

[0070] Thirdly, the present invention provides the application of the above-mentioned FeNiOOH / S@N-MWCNTs / CC composite electrode in urea detection, and the composite electrode of the present invention is particularly suitable for the detection of urea content in ethanol-containing samples such as distiller's grains or mash.

[0071] Fourthly, the present invention provides a method for detecting urea content in distiller's grains or mash using the above-mentioned FeNiOOH / S@N-MWCNTs / CC composite electrode, comprising the following steps:

[0072] (1) Measure the change in the current response of the sample:

[0073] The FeNiOOH / S@N-MWCNTs / CC composite electrode was placed in a 0.05~0.15 M KOH solution, and a series of urea standard solutions with concentrations ranging from 1 to 17000 μM were added. A voltage of -0.4~0.6 V was applied, and differential pulse voltammetry (DPV) scan was performed. The peak current was recorded, and the corresponding current response change was calculated.

[0074] (2) Establish a standard curve and construct a linear equation relating urea concentration to the change in current response:

[0075] A standard curve was constructed based on the concentration of the urea standard solution and the corresponding change in current response, and a linear equation was established.

[0076] More specifically, the change in urea concentration and current response exhibits three linear segments within the ranges of 1–10 μM, 10–100 μM, and 100–17000 μM. The change in current response was obtained by testing urea standard solutions of a series of concentrations. Construct a linear equation: ;in, denoted as the concentration of the urea standard solution in μM, where a is the slope of the linear equation and b is the intercept of the linear equation.

[0077] (3) Detection of urea in the sample to be tested:

[0078] According to the national standard method GB / T 36859-2018 Determination of Urea Content in Feed, the distiller's grains were pretreated and the final filtrate was filtered through a 0.22 μm needle filter to obtain the sample to be tested. The sample to be tested was used to replace the urea standard solution of different concentrations in step (1), and the detection was carried out according to the method in step (1) to obtain the corresponding current response change value. The value was substituted into the linear equation to calculate the urea content in the distiller's grains to be tested.

[0079] The technical solution and effects of the present invention will be further illustrated below through practical examples. Unless otherwise specified, all experimental materials used in the following embodiments were purchased from conventional biochemical reagent stores.

[0080] Example 1: SEM characterization and elemental mapping of FeNiOOH / S@N-MWCNTs / CC composite electrode

[0081] The preparation method of the FeNiOOH / S@N-MWCNTs / CC composite electrode in this embodiment includes the following steps:

[0082] (1) Activated carbon cloth (CC):

[0083] First, sulfuric acid, nitric acid, and water were added to the liner of the reactor in a volume ratio of 1:1:1. Then, a piece of raw carbon cloth with an area of ​​2 cm * 2 cm was immersed in the solution, sealed, and subjected to hydrothermal reaction at 140 °C for 12 h. Finally, it was washed 6 times with ultrapure water and dried.

[0084] (2) Preparation of N-MWCNTs / CC electrode:

[0085] First, an N-MWCNTs solution with a concentration of 1.5 mg / mL was prepared using ultrapure water and ultrasonically dispersed for 3 h to obtain a uniformly dispersed N-MWCNTs solution. Then, the activated carbon cloth (CC) was immersed in the uniformly dispersed N-MWCNTs solution and ultrasonically treated for 10 min. The carbon cloth was then removed, dried, and immersed and ultrasonically treated again, repeating the process 3 times. Finally, the unattached N-MWCNTs were washed with ultrapure water and dried to obtain an N-MWCNTs / CC electrode.

[0086] (3) Preparation of FeNiS@N-MWCNTs / CC electrode:

[0087] First, a composite aqueous solution of 44 mM Ni(NO3)2·6H2O, 22 mM Fe(NO3)3·9H2O, 173 mM NH4F and 339 mM urea was prepared using ultrapure water, dissolved by sonication, and stirred for 30 min. Then, 44 mM thiourea and 0.5 mM hexadecyltrimethylammonium bromide (CTAB) were added, dissolved by sonication, and the N-MWCNTs / CC electrode was immersed in it. Finally, it was placed in an oven for hydrothermal reaction at 120℃ for 6 h. After the reaction was completed, it was taken out, rinsed with ultrapure water, and dried to obtain the FeNiS@N-MWCNTs / CC electrode.

[0088] (4) Preparation of FeNiOOH / S@N-MWCNTs / CC composite electrode:

[0089] First, the FeNiS@N-MWCNTs / CC electrode was used as the working electrode for electrochemistry and immersed in 1 M KOH solution. Then, electrochemical oxidation was performed within 10 min under a constant current of 10 mA by chronopotential scanning. Finally, the electrode was cleaned and dried to obtain the FeNiOOH / S@N-MWCNTs / CC composite electrode.

[0090] The SEM characterization of the FeNiOOH / S@N-MWCNTs / CC composite electrode obtained in this embodiment is as follows: Figure 1 As shown, Figure 1In the image, (a) is activated carbon cloth, (b) is FeNiOOH / S@CC, (c) is FeNiOOH@N-MWCNTs / CC, (d) is FeNiS@N-MWCNTs / CC, and (e) is FeNiOOH / S@N-MWCNTs / CC.

[0091] Depend on Figure 1 It can be seen that FeNiOOH / S@CC forms a nanoflower structure on carbon fiber, FeNiOOH@N-MWCNTs / CC without S element exhibits a velvety structure, and the FeNiOOH / S@N-MWCNTs / CC composite electrode forms an N-MWCNT root-and-stem nanoflower structure, increasing the contact sites with the target urea. Figure 2 It can be seen that C, N, O, S, Fe, and Ni elements are all present and uniformly distributed in the FeNiOOH / S@MWCNTs / CC composite electrode.

[0092] Example 2: Comparison of electrochemical performance of different electrodes

[0093] The activated carbon cloth (CC), N-CNT / CC, FeNiOOH / S@CC, FeNiOOH@N-MWCNTs / CC, FeNiS@N-MWCNTs / CC, and FeNiOOH / S@N-MWCNTs / CC from Example 1 were placed in the same electrolyte (5 mM [Fe(CN)6)). 3- / 4- Cyclic voltammetry and electrochemical impedance spectroscopy were performed using 0.1 M KCl. Figure 3 As shown, FeNiOOH / S@N-MWCNTs / CC exhibits a higher oxidation peak and lower impedance compared to FeNiOOH / S@CC, FeNiOOH@N-MWCNTs / CC, and FeNiS@N-MWCNTs / CC, indicating a higher electron transfer rate for the FeNiOOH / S@N-MWCNTs / CC composite electrode. Although N-MWCNTs / CC and pure CC show higher CV oxidation peaks and lower impedances, their primary functions are to increase conductivity and act as a substrate, not to detect the active components of urea.

[0094] In the above verification steps, the test conditions are to apply a voltage of -0.4~0.9 V and a scan rate of 100 mV / s.

[0095] Example 3: Stability verification of an electrochemical method for rapid detection of urea content using FeNiOOH / S@N-MWCNTs / CC composite electrode

[0096] Repeatability: The FeNiOOH / S@N-MWCNTs / CC composite electrode obtained in Example 1 was placed in a 0.1 M KOH solution, and differential pulse voltammetry (DPV) was performed to detect the peak current generated on the electrode surface. This was repeated 10 times. See [link to relevant documentation]. Figure 4 The relative standard deviation was 0.18%.

[0097] Reproducibility: Ten FeNiOOH / S@N-MWCNTs / CC composite electrodes were prepared using the same process as in Example 1. Differential pulse voltammetry (DPV) was performed on each electrode to detect the peak current generated on the electrode surface. See [link to relevant documentation]. Figure 4 The relative standard deviation was 0.39%, indicating good method stability.

[0098] In the above verification steps, the differential pulse voltammetry (DPV) test parameters are -0.4~0.6 V.

[0099] Example 4: Detection of urea standard using FeNiOOH / S@N-MWCNTs / CC composite electrode

[0100] First, a 0.1 M KOH alkaline electrolyte was prepared using ultrapure water. Then, different volumes of urea standard solution were added to 100 mL of the electrolyte to achieve urea concentrations ranging from 1 to 17000 μM (1, 2.5, 5, 7.5, 10, 30, 50, 70, 100, 500, 1000, 5000, 9000, 13000, 17000 μM). A voltage range of -0.4 to 0.6 V was applied, and differential pulse voltammetry (DPV) scans were performed sequentially using the FeNiOOH / S@N-MWCNTs / CC composite electrode obtained in Example 1. The peak current was recorded, and the corresponding current response change was calculated. A linear equation was fitted using Origin software with urea concentration as the x-axis and current response change as the y-axis.

[0101] like Figure 5 As shown, the change in urea concentration and current response exhibits a three-segment linear relationship: 1~10 μM (sensitivity 7.863 μA / μM), 10~100 μM (sensitivity 0.441 μA / μM), and 100~17000 μM (sensitivity 0.005 μA / μM). Based on the formula LOD=3SD / k, the detection limit is calculated to be 0.398 μM.

[0102] Example 5: Spiked recovery rate of urea in distiller's grains and mash

[0103] To verify the sensing performance of the FeNiOOH / S@N-MWCNTs / CC composite electrode method for urea detection in distillers' grains, the distillers' grains were first pretreated according to the national standard method GB / T 36859-2018 "Determination of Urea Content in Feed". The resulting filtrate was filtered through a 0.22 μm needle filter and stored at 4 °C for later testing. Then, the FeNiOOH / S@N-MWCNTs / CC composite electrode obtained in Example 1 was used to detect urea spikes (30 μM, 50 μM, 70 μM) in the test samples. The recoveries ranged from 97.22% to 99.58%, and the RSDs were between 2.64% and 3.56%.

Claims

1. A method for preparing a FeNiOOH / S@N-MWCNT / CC composite electrode, characterized in that: Includes the following steps: A. Activated carbon cloth; B. Preparation of N-MWCNTs / CC electrode: The activated carbon cloth is repeatedly immersed in the aqueous dispersion of N-MWCNTs and subjected to ultrasonic treatment to obtain the N-MWCNTs / CC electrode. C. Preparation of FeNiS@N-MWCNTs / CC electrode: The N-MWCNTs / CC electrode is immersed in a composite aqueous solution of soluble inorganic nickel salt, soluble inorganic iron salt or ferrous salt, fluorine source, pH adjuster, sulfiding agent and dispersant, and FeNiS@N-MWCNTs / CC electrode is obtained by hydrothermal method. D. Preparation of FeNiOOH / S@N-MWCNTs / CC composite electrode: FeNiS@N-MWCNTs / CC electrode was obtained by chronopotentiometric method.

2. The method for preparing the FeNiOOH / S@N-MWCNT / CC composite electrode according to claim 1, characterized in that: In step A, the specific method for activating the carbon cloth is as follows: sulfuric acid, nitric acid, and water are mixed in a volume ratio of 1~3:1~3:1~3, and then a mixture with an area of ​​1~5 cm² is prepared. 2 The raw carbon cloth is immersed in it, sealed, and subjected to hydrothermal reaction at 100~140℃ for 6~12 h, washed with ultrapure water 5~10 times, and dried; preferably, the volume ratio of sulfuric acid, nitric acid and water is 1:1:1, the area of ​​raw carbon cloth is 2 cm*2 cm, and the hydrothermal reaction is carried out at 140℃ for 12 h.

3. The method for preparing the FeNiOOH / S@N-MWCNT / CC composite electrode according to claim 1, characterized in that: In step B, at least one of the following must be satisfied: The specific operation is as follows: the activated carbon cloth is immersed in the aqueous dispersion of N-MWCNTs and ultrasonically treated at the same time. After the carbon cloth is taken out and dried, it is immersed and ultrasonically treated again. Finally, it is washed and dried to obtain the N-MWCNTs / CC electrode. The concentration of the aqueous dispersion of N-MWCNTs is 0.5~2.5 mg / mL; The aqueous dispersion of N-MWCNTs was obtained by ultrasonically dispersing N-MWCNTs and water for 1-5 h. The duration of each ultrasound session is 5 to 20 minutes; preferably 10 minutes. The total number of soaking and ultrasonic treatments is 4 to 6 times; preferably 4 times.

4. The method for preparing the FeNiOOH / S@N-MWCNT / CC composite electrode according to claim 1, characterized in that: In step C, at least one of the following must be satisfied: The soluble inorganic nickel salt is selected from at least one of nickel nitrate, nickel chloride, nickel acetate, or nickel sulfate; The soluble inorganic ferric salt or ferrous salt is selected from at least one of ferric nitrate, ferric chloride, ferric sulfate, ferrous nitrate, ferrous chloride, or ferrous sulfate. The fluorine source is selected from at least one of NH4F, NaF, or KF; The pH adjuster is selected from at least one of urea, ammonia, or ammonium carbonate; The vulcanizing agent is selected from at least one of thiourea, thioacetamide, sodium sulfide or potassium sulfide; The dispersant is selected from at least one of hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, or polyethylene glycol; In the composite aqueous solution, the concentration of soluble inorganic nickel salt is 24-64 mM, the concentration of soluble inorganic iron salt or ferrous salt is 12-52 mM, the concentration of fluorine source is 100-200 mM, the concentration of pH adjuster is 300-400 mM, the concentration of vulcanizing agent is 24-64 mM, and the concentration of dispersant is 0.1-1 mM. The method for preparing the composite aqueous solution is as follows: first, mix the soluble inorganic nickel salt, soluble inorganic iron salt or ferrous salt, fluorine source, pH adjuster and water, dissolve by ultrasonication, stir for 10~60 minutes, then add the sulfiding agent and dispersant, dissolve by ultrasonication, and the solution is obtained.

5. The method for preparing the FeNiOOH / S@N-MWCNT / CC composite electrode according to claim 1, characterized in that: In step C, at least one of the following must be satisfied: The temperature of the hydrothermal method is 100~180℃; preferably 120℃. The hydrothermal method takes 3 to 15 hours; preferably 6 hours.

6. The method for preparing the FeNiOOH / S@N-MWCNT / CC composite electrode according to claim 1, characterized in that: In step D, the specific method of the chronopotentiometric method is as follows: the FeNiS@N-MWCNTs / CC electrode is used as the working electrode of the electrochemistry, immersed in 0.5~2M KOH solution, and electrochemical oxidation is performed at a constant current of 5~20 mA for 5~20 min by chronopotentiometric scanning. Then, it is cleaned and dried to obtain the FeNiOOH / S@N-MWCNTs / CC composite electrode. Preferably, the chronopotentiometric method parameters are 10 mA constant current treatment for 10 min.

7. The FeNiOOH / S@N-MWCNT / CC composite electrode is prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the FeNiOOH / S@N-MWCNT / CC composite electrode according to claim 7 in urea detection; preferably, its application in detecting urea content in distiller's grains or mash.

9. A method for detecting urea content in distiller's grains or mash using the FeNiOOH / S@N-MWCNT / CC composite electrode as described in claim 7, characterized in that: Includes the following steps: a. Place the FeNiOOH / S@N-MWCNTs / CC composite electrode in a 0.05~0.15 M KOH solution, add a series of urea standard solutions of different concentrations, apply a voltage of -0.4~0.6 V, perform differential pulse voltammetry scanning, record the peak current, and calculate the corresponding current response change value; b. Based on the concentration of the urea standard solution and the corresponding change in current response, a standard curve is prepared and a linear equation is constructed. c. Replace the series of urea standard solutions of different concentrations in step a with the pretreated sample of distiller's grains or mash, and perform the detection according to the method in step a to obtain the corresponding current response change value. Substitute the value into the linear equation constructed in step b to calculate the urea content in the distiller's grains or mash.

10. The method for detecting urea content in distiller's grains or mash according to claim 9, characterized in that: At least one of the following must be met: In step a, the concentration range of the series of urea standard solutions of different concentrations is 1~17000 μM; In step b, the changes in urea concentration and current response exhibit three linear segments within the ranges of 1–10 μM, 10–100 μM, and 100–17000 μM. In step b, the current response change values ​​are obtained by testing with a series of urea standard solutions of different concentrations. Construct a linear equation: ;in, denoted as the concentration of the urea standard solution in μM, where a is the slope of the linear equation and b is the intercept of the linear equation. In step c, the pretreatment operation is as follows: based on the national standard method "GB / T 36859-2018 Determination of Urea Content in Feed", the final filtrate is filtered through a 0.22 μm needle filter to obtain the sample to be tested.

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