Electrochemical sensor and electrochemical detection method for detecting hydrazine in environmental water sample

Abstract

The invention discloses an electrochemical sensor and an electrochemical detection method for detecting hydrazine in an environmental water sample, the electrochemical sensor is a three-electrode system, and a working electrode is prepared by mixing N-GQDs and an rGO solution, carrying out ultrasonic treatment and centrifugation to obtain a compound, dispensing the compound onto the surface of the electrode, drying, immersing into a gold precursor solution, and carrying out electro-deposition. The working electrode in the electrochemical sensor prepared by the invention is a ternary nano composite material, the ternary nano composite material is used as an electrode sensitizing material and integrates the advantages of three nano materials, and the sensor is used for high-sensitivity detection of hydrazine in an environmental water sample, has the advantages of low detection limit, wide detection range, low detection cost and simple operation, and is suitable for large-scale application.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical detection, and particularly relates to an electrochemical sensor and an electrochemical detection method for detecting hydrazine in environmental water samples. Background Art

[0002] Hydrazine (N 2 H 4 ) is a colorless liquid compound, which is widely used in the fields of space shuttle fuel, fuel cells, pesticides, herbicides, plastic foams, photographic chemical intermediates, and deoxidizers for water heaters. However, N 2 H 4 has strong neurotoxicity and can also cause serious harm to human health at low concentrations, such as temporary blindness, headache, dizziness, and damage to the liver, kidneys, and central nervous system. The US Environmental Protection Agency (USEPA) classifies N 2 H 4 as a genotoxic carcinogen, and its threshold in drinking water is 10 ppb (0.313 μM). With the increasing demand for N 2 H 4 , the environmental and health risks it brings are becoming more and more serious. Therefore, it is of great significance to develop a highly sensitive and effective analytical method for monitoring the content of hydrazine in environmental water samples.

[0003] Currently, the analytical methods for N 2 H 4 include spectrophotometry, chromatography, colorimetry, flow injection analysis, and electrochemistry. These methods can all obtain satisfactory analytical results, but they require expensive instruments or complex pretreatment processes for samples. In contrast, electrochemical analysis has the advantages of high sensitivity, low cost, fast response, and simple operation. However, unmodified electrodes usually face the problem of high oxidation overpotential when detecting N 2 H 4 . Therefore, introducing nanomaterials on the electrode surface can achieve highly sensitive detection of hydrazine in environmental water samples.

[0004] The method for detecting hydrazine by electrochemical detection provided by the disclosure of CN 114740062 A includes the following steps: S1. Prepare a platinum-polyaniline-reduced graphene oxide nanocomposite dispersion; S2. Drop the platinum-polyaniline-reduced graphene oxide nanocomposite dispersion onto the surface of a glassy carbon electrode to modify the glassy carbon electrode, forming a platinum-polyaniline-reduced graphene oxide glassy carbon electrode; S3. Use the platinum-polyaniline-reduced graphene oxide glassy carbon electrode as the working electrode of a hydrazine sensor, use a phosphate buffer solution as the supporting electrolyte for electrochemical detection, detect the peak current values of hydrazine at different concentrations, and use the concentration of the hydrazine standard solution as the abscissa and the peak current value as the ordinate to fit a standard curve; S4. Detect hydrazine in the test solution according to the standard curve. However, the preparation method of its working electrode is complex, the detection process is cumbersome, and the detection limit is also relatively high, only 3.3 μmol.

[0005] Gold nanoparticles (AuNPs) have high electrical conductivity and excellent catalytic effects, and can promote the electron transfer ability at the sensing electrode interface, and are widely used in the field of analytical sensing. The electrocatalytic performance of AuNPs is closely related to their size and dispersion degree. Therefore, it is particularly important to develop a simple method to prepare AuNPs with small size and uniform dispersion on the electrode surface. Reduced graphene oxide (rGO) is a two-dimensional planar carbon material and can be used as a supporting material for AuNPs to further improve the conductivity of the electrode. In current reports, when preparing the AuNPs / rGO composite material, it is mostly necessary to use a protective agent to prevent the aggregation of AuNPs on the rGO nanosheets. However, the introduction of the protective agent inevitably blocks some active sites on the surface of AuNPs, thereby reducing the sensing performance of the electrode. There is no report yet on introducing nitrogen-doped graphene quantum dots (N-GQDs) on the surface of rGO, using the amino groups in N-GQDs as reducing agents and stabilizers to synthesize AuNPs and applying them to the detection of hydrazine in environmental water samples. Summary of the Invention

[0006] Aiming at the problems of complex detection equipment, low detection sensitivity, and high detection limit for hydrazine in environmental water samples, the present invention provides an electrochemical sensor for detecting hydrazine in environmental water samples. The working electrode of the sensor is a ternary nanocomposite material, which is used for highly sensitive detection of hydrazine in environmental water samples, and has the advantages of low detection limit, wide detection range, low detection cost, simple operation, and suitability for large-scale applications.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] An electrochemical sensor for detecting hydrazine in environmental water samples, characterized in that the electrochemical sensor is a three-electrode system, and the preparation of the working electrode includes the steps:

[0009] Step 1: Mix the N-GQDs and rGO solutions, perform ultrasonic treatment and centrifugation to obtain the composite N-GQDs@rGO;

[0010] Step 2: Drop-coat the composite N-GQDs@rGO onto the electrode surface and dry it to obtain an electrode modified with the N-GQDs@rGO composite;

[0011] Step 3: Immerse the electrode modified with the N-GQDs@rGO composite into the solution of the gold precursor and perform electrodeposition to obtain a modified electrode with AuNPs deposited on the surface as the working electrode.

[0012] In the present invention, N-GQDs are adsorbed onto the surface of rGO through π-π stacking interactions to obtain the N-GQDs@rGO composite; the N-GQDs@rGO composite is drop-coated onto the electrode surface to obtain an electrode modified with the N-GQDs@rGO composite; finally, using the amino groups in N-GQDs as a reducing agent and a protective agent, uniformly sized AuNPs are in-situ prepared on the surface of N-GQDs@rGO by a controllable electrodeposition method. N-GQDs can not only prevent the aggregation of rGO but also provide abundant anchoring sites for the synthesis of AuNPs, contributing to the formation of small-sized and uniformly distributed AuNPs. In addition, N-GQDs can further improve the conductivity and catalytic activity of the composite material. The prepared AuNPs / N-GQDs@rGO ternary nanocomposite as an electrode sensitizing material integrates the advantages of the three nanomaterials, having a high specific surface area, good water solubility, excellent conductivity and catalytic activity, a simple preparation process, and excellent stability. The electrochemical sensor prepared by the present invention is used for the highly sensitive detection of hydrazine in environmental water samples, with a low detection limit, a wide detection range, low detection cost, simple operation, and suitability for large-scale applications.

[0013] The electrolyte solution of the electrochemical sensor includes phosphate buffer solution, aqueous sodium chloride solution, aqueous sodium sulfate solution, aqueous potassium chloride solution, aqueous potassium hydrogen phthalate solution, or acetic acid-sodium acetate buffer solution. The molar concentration of the electrolyte is preferably 0.005 - 0.5 mol / L, and more preferably 0.008 - 0.2 mol / L.

[0014] The electrode in Step 2 includes any one of a glassy carbon electrode, a fluorine-doped tin oxide electrode, an indium tin oxide electrode, a gold electrode, a screen-printed electrode, a graphite electrode, and a carbon fiber electrode. Further preferably, it is a glassy carbon electrode and an indium tin oxide electrode.

[0015] Preferably, the concentration of the N-GQDs solution is 0.5-5 mg / mL, more preferably 1.2-2 mg / mL; the concentration of the rGO solution is 0.02-0.5 mg / mL, more preferably 0.14-0.2 mg / mL; the ultrasonic time is 30 min-6 h, more preferably 1-4 h; the centrifugation speed is 10,000-15,000 rpm, more preferably 11,000-13,000 rpm.

[0016] The mass ratio of the N-GQDs to the rGO is 8:1-1:5, more preferably 5:1-1:3; preferably, the dropping volume of the N-GQDs@rGO composite is 4-10 μL; the drying temperature is 40-80 °C, more preferably 50-70 °C.

[0017] The gold precursor in step 3 includes one or more of chloroauric acid, gold nitrate, gold cyanide, and gold complexes.

[0018] The molar concentration of the gold precursor in the gold precursor solution is 0.01-1 mmol / L.

[0019] In step 3, the electro-deposition is -1.0 to -0.1 V, more preferably -0.6 to -0.4 V; the electro-deposition time is 2-20 s, more preferably 2-8 s, and the temperature is room temperature.

[0020] Preferably, the rGO can be purchased or prepared by oneself. The synthesis method for self-preparation is any one of chemical reduction method, thermal reduction method, and photochemical reduction method, more preferably the chemical reduction method;

[0021] The basic steps of the chemical reduction method include: mixing an aqueous solution of graphene oxide (GO) with ammonia water and reducing agent hydrazine hydrate, and after the water bath reaction is completed, centrifuging to obtain rGO.

[0022] The N-GQDs can be purchased or prepared by oneself. The synthesis method for self-preparation is any one of hydrothermal method, microwave method, and chemical oxidation method, more preferably the hydrothermal method.

[0023] The basic steps of the hydrothermal method include: dissolving 1-aminopyrene in an ammonia water solution, placing it in a high-pressure reaction kettle, reacting at a high temperature, filtering the product through a filter membrane and a dialysis bag, and freeze-drying to obtain a solid product of N-GQDs.

[0024] The present invention also provides an electrochemical detection method for detecting hydrazine in environmental water samples. The environmental water sample is diluted with an electrolyte solution to be used as a test solution, and the hydrazine in the environmental water sample is monitored by using the electrochemical sensor described above.

[0025] The pH of the liquid to be detected is 4 - 8. Preferably, the pH of the detection liquid is 6 - 8, and the detection sensitivity and accuracy are higher in this range.

[0026] The working electrode of the electrochemical sensor contacts the liquid to be detected for more than 10 s, preferably more than 20 s, and further preferably 20 - 35 s. The sensor has an extremely fast detection speed, does not require a long enrichment time, and can quickly obtain the detection result.

[0027] The concentration of hydrazine in the liquid to be detected is 3 nmol / L - 20 μmol / L. Preferably, the concentration of hydrazine in the liquid to be detected is 10 nmol / L - 20 μmol / L. The detection concentration of hydrazine by the sensor shows a linear relationship in the range of 10 nmol / L - 20 μmol / L, and the detection limit is 4.98 nmol / L. This detection method has a wide detection linear range, high selectivity, high detection sensitivity, and low detection limit. Moreover, the detection does not require complex pretreatment and separation processes, reduces the detection cost, is easy to operate, and is suitable for large-scale applications.

[0028] The liquid to be detected further includes one or more interfering substances such as hydroquinone, o-dihydroxybenzene, naphthol, uric acid, potassium bromide, copper sulfate, calcium chloride, magnesium sulfate, and zinc nitrate, and the concentration of each interfering substance is below 50 μmol / L. This sensor has excellent anti-interference ability and is not affected by the interfering substances in the liquid to be detected in terms of sensitivity and accuracy.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) The present invention prepares the AuNPs / N-GQDs@rGO composite, and obtains AuNPs with a nanoscale size and uniform distribution on the surface of N-GQDs@rGO. No additional reducing agent and protective agent are added during the synthesis process. Compared with the AuNPs with a protective agent, the AuNPs synthesized by the method of the present invention retain the active sites on the surface and have high catalytic activity.

[0031] (2) The AuNPs / N-GQDs@rGO composite prepared by the present invention combines the advantages of three nanomaterials, has a high specific surface area, good water solubility, excellent conductivity and catalytic activity, a simple preparation process, and excellent stability.

[0032] (3) The detection method of the present invention shows a linear relationship for the detection concentration of hydrazine in the range of 10 nmol / L - 20 μmol / L, and the detection method has a wide detection linear range and high sensitivity.

[0033] (4) The detection method of the present invention has a fast response and rapid detection, and the detection result can be obtained in only a few seconds. Description of the Drawings

[0034] Figure 1 Schematic diagram for the electrochemical detection of hydrazine using the AuNPs / N-GQDs@rGO ternary nanocomposite modified glassy carbon electrode (GCE) prepared in Example 1.

[0035] Figure 2 UV-visible spectra of GO and rGO in Example 1, high-resolution C 1s XPS spectra of GO (b) and rGO (c).

[0036] Figure 3 UV-visible and fluorescence spectra of N-GQDs in Example 1. The inset in (a) is the fluorescence image of the N-GQDs solution excited under a 365 nm UV lamp; high-resolution C 1s (b), N 1s (c) and O 1s spectra (d) of N-GQDs.

[0037] Figure 4 Transmission electron microscope images of rGO (a) and N-GQDs@rGO (b) prepared in Example 1.

[0038] Figure 5 Scanning electron microscope images of N-GQDs@rGO (a) and AuNPs / N-GQDs@rGO (b) in Example 1. (c) CV curves of bare GCE, N-GQDs@rGO / GCE and AuNPs / N-GQDs@rGO / GCE in potassium hydrogen phthalate solution (0.05 mol / L) containing K 3 Fe(CN) 6 (0.5 M). (d) Electrochemical impedance spectra of bare GCE, N-GQDs@rGO / GCE and AuNPs / N-GQDs@rGO / GCE in potassium chloride solution (0.1 M) containing K 3 Fe(CN) 6 / K 4 Fe(CN) 6 (2.5 mM).

[0039] Figure 6 CV curves of bare GCE, rGO / GCE, N-GQD / GCE, N-GQDs@rGO / GCE and AuNPs / N-GQDs@rGO / GCE in PBS (0.1 M, pH = 7) containing 3 μM N 2 H 4 .

[0040] Figure 7 For Application Example 2, AuNPs / N-GQDs@rGO / GCE for N-containing 2 H 4CV curves of (3 μM) in PBS (0.1 M) with different pH values (pH = 4, 5, 6, 7, 8) (a), relationship diagram between oxidation peak current and solution pH (b), AuNPs / N-GQDs@rGO / GCE in the presence of N 2 H 4 CV curves obtained by stirring and enriching for different times (10, 15, 20, 25, 30, 35 s) in PBS (0.1 M, pH = 7) containing (3 μM) (c), relationship diagram between oxidation peak current and stirring enrichment time (d).

[0041] Figure 8 For Application Example 3, CV curves of AuNPs / N-GQDs@rGO / GCE in PBS (0.1 M, pH = 7) containing different concentrations of N 2 H 4 are shown in (a), and the inset is an enlarged view of the low-concentration region; linear relationship diagram between oxidation peak current and N 2 H 4 concentration, and the inset is an enlarged view of the calibration curve in the low-concentration region (b).

[0042] Figure 9 For Application Example 4, oxidation peak current signals of AuNPs / N-GQDs@rGO / GCE in PBS (0.1 M, pH = 7) containing N 2 H 4 (10 μM), possible co-existing environmental interfering substances (50 μM), and the mixture are shown. Detailed implementation manners

[0043] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Those skilled in the art who make modifications or equivalent replacements based on the understanding of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered within the protection scope of the present invention.

[0044] Unless otherwise specified, the raw materials used in the following detailed implementation manners are all purchased through commercial channels and used directly without special treatment. The main raw materials used in the embodiments are as follows:

[0045] Monolayer GO aqueous dispersion (10 mg / g) is purchased from Hangzhou Graphene Technology Co., Ltd.;

[0046] Hydrazine hydrate: Hangzhou Shuanglin Chemical Reagent Co., Ltd.;

[0047] Potassium ferricyanide (K 3 [Fe(CN) 6, 99.5%): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0048] Potassium ferrocyanide (K 4 [Fe(CN) 6 , 99.5%): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0049] Potassium chloride (KCl, AR): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0050] Chloroauric acid (HAuCl 4 ·3H 2 O, 99.9%): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0051] 1-Aminopyrene (C 16 H 11 N, 98%): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0052] Ammonia water (NH 3 ·H 2 O, AR): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0053] Sodium dihydrogen phosphate (NaH 2 PO 4 ·2H 2 O, AR): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0054] Disodium hydrogen phosphate (Na 2 HPO 4 ·12H 2 O, AR): Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0055] Ethanol (99.8%): Hangzhou Gaojing Fine Chemical Co., Ltd.;

[0056] In the following examples, the graphene oxide rGO and nitrogen-doped graphene quantum dots N-GQDs used were prepared by ourselves, and products purchased from the market can also be used. For example, rGO can be purchased from Suzhou Carbon Feng Technology Co., Ltd., Hangzhou Hangdan Optoelectronic Technology Co., Ltd., Shenzhen Guoheng Qihang Technology Co., Ltd., etc.; N-GQDs can be purchased from Xianfeng Nano Co., Ltd., Suzhou Carbon Feng Technology Co., Ltd., Puquality Materials Co., Ltd., etc.

[0057] Example 1

[0058] Preparation method of AuNPs / N-GQDs@rGO composite modified glassy carbon electrode (GCE), comprising the following steps:

[0059] (1) Preparation of rGO by chemical reduction method: 8 mL of graphene oxide (GO) solution with a concentration of 1 mg / mL was added to 32 mL of deionized water. After mixing evenly, a GO solution with a concentration of 0.2 mg / mL was obtained. Subsequently, the GO solution was ultrasonically treated for about 24 h to ensure complete dispersion of GO in water and form a uniform suspension. After the GO solution was dispersed evenly, 135 μL of ammonia solution with a mass fraction of 25% and 14 μL of reducing agent hydrazine hydrate with a mass fraction of 40% were added in sequence. Then, the mixed solution was placed in a water bath at 85 °C and stirred for 3 hours. After the reaction, the solution was centrifuged at 3000 rpm for 30 min, and finally a well-dispersed rGO solution was obtained.

[0060] (2) Preparation of N-GQDs by hydrothermal method: 40 mg of 1-aminopyrene was dissolved in 20 mL of ammonia solution (0.4 M) to form a uniform mixture, which was placed in a high-pressure reaction kettle and reacted at a temperature of 200 °C for 8 h. After the reaction was completed, the solution was filtered through a 0.22 μm membrane to remove large particle impurities. Subsequently, the filtrate was dialyzed using a dialysis bag with a molecular weight cut-off of 500 Da for 24 h to effectively remove unreacted small molecules and salts. Finally, the dialysate was freeze-dried to obtain a solid product of N-GQDs.

[0061] (3) The rGO (1) and N-GQDs (2) solutions were mixed at a mass ratio of 1:1 and ultrasonically treated for 2 h to ensure full fusion of the two materials and form a uniform composite system. Then, the mixture was centrifuged at 12,000 rpm for 30 minutes to collect the precipitate. The precipitate was washed three times each with ethanol and deionized water to remove residual impurities, and then redispersed in 1 mL of deionized water to obtain an aqueous dispersion of N-GQDs@rGO composite;

[0062] (4) 4 μL of the above N-GQDs@rGO composite dispersion was dropped onto the polished GCE surface and dried in an oven at 60 °C to make the composite adhere evenly to the electrode surface, and finally N-GQDs@rGO / GCE was obtained.

[0063] (5) The N-GQDs@rGO / GCE prepared in (4) was immersed in a 0.5 mM chloroauric acid (HAuCl 4 ) solution and electro-deposited at a constant voltage of -0.5 V for 2 seconds. The electrode surface was carefully rinsed with ultrapure water to remove the undeposited HAuCl 4 solution to ensure that the electrode surface was clean and free of contamination. Finally, an AuNPs-modified N-GQDs@rGO / GCE composite electrode (named AuNPs / N-GQDs@rGO / GCE) was obtained.

[0064] Application Example 1

[0065] Using the AuNPs / N-GQDs@rGO / GCE of Example 1 as the working electrode, the electrochemical detection process for detecting the hydrazine content in tap water is shown in the schematic diagram of the test process as Figure 1 shown below:

[0066] First, prepare a hydrazine stock solution. Then, dilute the tap water 10 times with a 0.1 mmol / L phosphate buffer solution (pH 7) and add different volumes of the hydrazine stock solution to obtain a series of test solutions (the concentrations of hydrazine are 10 nmol / L, 100 nmol / L, 400 nmol / L, 700 nmol / L, 1 μmol / L, 3 μmol / L, 5 μmol / L, 7 μmol / L, 10 μmol / L, 15 μmol / L, and 20 μmol / L). Perform cyclic voltammetry curve tests on the test solutions. Using a simple three-electrode system, use the AuNPs / N-GQDs@rGO / GCE prepared in Example 1 as the working electrode, a platinum electrode as the counter electrode, and a silver / silver chloride electrode as the reference electrode.

[0067] Performance Observation Test

[0068] 1. rGO Characterization

[0069] Characterize the rGO in step (1) of Example 1 using ultraviolet-visible spectroscopy (UV-vis) and X-ray photoelectron spectroscopy (XPS). Figure 2 a shows the UV-vis spectra of GO and rGO. The ultraviolet absorption spectrum of GO shows an obvious peak at about 230 nm, which results from the π→π* transition of conjugated C=C bonds, indicating its aromatic structure. In addition, a shoulder peak appears near 300 nm, which is contributed by oxygen-containing groups such as carbonyl groups. The ultraviolet absorption spectrum of rGO obtained after reduction treatment shows a peak red shift, and the main absorption peak moves from about 230 nm to about 260 nm. At the same time, the shoulder peak at 300 nm decreases significantly. This change indicates that through the reduction process, the oxygen-containing functional groups of rGO decrease, and its sp 2 structure is restored, thus enhancing the ability of π→π* transition. At the same concentration, rGO usually shows a higher maximum absorption intensity, indicating that the reduced carbon network has a stronger light absorption ability. These characterization results clearly verify the successful reduction of GO to rGO.

[0070] Figure 2 b and 2c show the high-resolution C1s spectra of GO and rGO. In these spectra, we can identify four types of carbon bonds: C-C / C=C (285.4 eV), C-O (287.2 eV), C=O (288.5 eV), and O-C=O (289.0 eV), which correspond to sp in graphite respectively 2Carbon, phenol / alcohol / ether, carbonyl, and carboxyl structures. After reduction treatment, the proportion of C-C / C═C bonds in rGO increased, indicating an increase in the sp 2 structure of graphene. At the same time, compared with GO, the content of oxygen-containing groups in rGO decreased significantly. This indicates that the chemical reduction process effectively removed surface oxides, restored the conjugated structure of the material, and improved its electronic conductivity. However, rGO still retains some oxygen-containing groups, which can serve as active sites in electrochemical reactions.

[0071] 2. Characterization of N-GQDs and N-GQDs@rGO

[0072] UV-vis, fluorescence spectroscopy, XPS, and transmission electron microscopy (TEM) were used to characterize N-GQDs in step (2) of Example 1 and N-GQDs@rGO in step (3). Figure 3 a is the UV-vis diagram of N-GQDs, with an obvious ultraviolet absorption peak near 350 nm, which results from the n→π* transition of C═O. In addition, under the excitation of 465 nm, the maximum emission wavelength of N-GQDs is 520 nm. As Figure 3 shown in b-d, N-GQDs are mainly composed of carbon (C), oxygen (O), and nitrogen (N) elements. In the high-resolution C1s XPS spectrum, the binding energy peaks at 285.9 eV, 286.3 eV, and 288.5 eV correspond to C═C, C-N, and C-OH bonds, respectively. In the O1s XPS spectrum, the peaks at 531.4 eV and 532.0 eV indicate the presence of C-O and C═O bonds, respectively. The N 1s spectrum further confirms the presence of amino nitrogen, graphitic nitrogen, and pyrrolic nitrogen. These results together verify the success of nitrogen doping.

[0073] Figure 4 are the TEM diagrams of rGO (a) and N-GQDs@rGO (b). It can be seen that rGO has a wrinkled structure; N-GQDs can be observed on the wrinkled rGO structure after being combined with N-GQDs, and its average size is 3.1 nm. This proves the successful combination of rGO and N-GQDs.

[0074] 3. Characterization of AuNPs / N-GQDs@rGO / GCE

[0075] Scanning electron microscopy (SEM) and electrochemical methods were used to characterize AuNPs / N-GQDs@rGOs / GCE in step (5) of Example 1. As Figure 5 shown in a, the N-GQDs@rGO composite material shows a wrinkled texture on the GCE surface. This structure significantly increases the specific surface area of the electrode and provides rich active sites for the subsequent electrodeposition of gold nanoparticles. Figure 5b further confirmed the uniform distribution and tight anchoring of gold nanoparticles on the N-GQDs@rGO substrate, with an average particle size of about 50 nm. These characterization results of morphology and composition fully confirmed the successful synthesis of the composite material.

[0076] To evaluate the electrochemical performance of the electrode, we employed cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Figure 5 c shows the CV curves of different electrodes in a potassium hydrogen phthalate (KHP, 0.05 mM) solution containing ferricyanide (Fe(CN) 6 3- , 0.5 mM). Figure 5 d shows the typical Nyquist plots (EIS) and their corresponding equivalent circuits of different electrodes in a ferricyanide / ferrocyanide (Fe(CN) 6 3- / 4- , 2.5 mM) probe solution. In the EIS plot, the semicircle in the high-frequency region represents the electron transfer process, and its diameter reflects the electron transfer resistance (R ct ). For the unmodified bare GCE, redox peaks can be detected, with a peak-to-peak difference (ΔE p ) of 82 mV and an impedance of 376 Ω. After modification with N-GQDs@rGO, the redox peak current increased significantly, and ΔE p decreased to 80 mV, and the impedance decreased to 352 Ω. This performance improvement is mainly attributed to the increase in the electroactive surface area and conductivity of the electrode by N-GQDs@rGO, while playing a catalytic role in the redox reaction. AuNPs / N-GQDs@rGO / GCE exhibited the highest redox peak current and the lowest impedance, indicating its excellent electrochemical activity. This result is attributed to the synergistic effect of AuNPs and N-GQDs@rGO on the electrode surface, significantly improving the interfacial conductivity, expanding the effective electrochemical surface area, and accelerating the reaction of the redox probe at the electrode interface.

[0077] 4. Electrochemical detection of hydrazine

[0078] The electrocatalytic activity of AuNPs / N-GQDs@rGO / GCE towards hydrazine was evaluated using the CV method. As Figure 6 shown, almost no oxidation peak current was observed on the bare GCE. On rGO / GCE, N-GQDs / GCE, and rGO-N-GQDs@rGO / GCE, the oxidation peak current of hydrazine was also not obvious. However, AuNPs / N-GQDs@rGO / GCE showed a significant oxidation current towards hydrazine, which is attributed to the higher specific surface area of AuNPs / N-GQDs@rGO and its high catalytic ability towards hydrazine.

[0079] Application Example 2: Optimization of Detection Conditions

[0080] The effects of the pH value of the detection solution and the stirring enrichment time on the detection performance were investigated respectively. The pH value of the detection solution in Application Example 1 (4 - 8) was changed, and the effect of AuNPs / N-GQDs@rGO / GCE on the detection performance of hydrazine was tested. As Figure 7 shown in a-b, when the solution pH value changed from 4 to 8, the catalytic peak current (I pa ) of hydrazine oxidation reached the maximum value at pH = 7.0. When the pH value decreased or increased, the oxidation current decreased. At the same time, we observed that the oxidation peak potential shifted positively with the decrease of the pH value and negatively with the increase of the pH value. We selected the PBS solution with pH = 7.0 as the optimal condition for hydrazine determination. As Figure 7 shown in c-d, with the extension of the stirring time, the oxidation peak current of hydrazine gradually increased and finally reached stability at 30 seconds. Therefore, we selected 30 seconds as the optimal enrichment time for subsequent research on hydrazine.

[0081] Application Example 3: Electrochemical Detection of Hydrazine

[0082] The AuNPs / N-GQDs@rGO / GCE prepared in Example 1 was placed statically in a buffer system containing hydrazine at a standard concentration, and tested by the CV method. As Figure 8 shown, as the concentration of hydrazine gradually increased from 10 nmol / L to 20 μmol / L, its oxidation peak current showed a continuous upward trend, presenting two linear ranges: 10 nmol / L - 3 μmol / L and 3 - 20 μmol / L respectively, and the detection limit was 4.98 nM.

[0083] Application Example 4: Selectivity of the Sensor

[0084] Figure 9 The anti-interference data of the constructed electrochemical sensor are shown. Hydroquinone, o-dihydroxybenzene, naphthol, uric acid, potassium bromide, copper sulfate, calcium chloride, magnesium sulfate, and zinc nitrate were used as interfering substances. The AuNPs / N-GQDs@rGO / GCE was used to test the test solution of hydrazine (10 μM) and coexisting interfering substances. The concentration of the interfering substances was 50 μM, and the mixture refers to the mixture of hydrazine and the above interfering substances. It can be found that the presence of interfering substances does not affect the electrochemical signal of hydrazine, proving that the constructed electrochemical sensor has good anti-interference ability.

[0085] The above results prove that AuNPs / N-GQDs@rGO / GCE has excellent catalytic oxidation effect on hydrazine and can achieve highly sensitive detection of hydrazine in environmental water samples.

Claims

1. An electrochemical sensor for detecting hydrazine in environmental water samples, characterized in that: The electrochemical sensor is a three-electrode system, wherein the preparation of the working electrode comprises the steps of: Step 1, N-GQDs and rGO solution were mixed, ultrasonicated and centrifuged to obtain a composite N-GQDs@rGO; Step 2, drop-coating the composite N-GQDs@rGO onto the electrode surface, and drying to obtain an electrode modified with the N-GQDs@rGO composite; Step 3, immersing the electrode modified with the N-GQDs@rGO composite into a solution of a gold precursor, and electro-depositing to obtain a modified electrode with AuNPs deposited on the surface as a working electrode.

2. The electrochemical sensor for detecting hydrazine in environmental water samples according to claim 1, characterized in that: The electrolyte solution of the electrochemical sensor includes a phosphate buffer solution, a sodium chloride aqueous solution, a sodium sulfate aqueous solution, a potassium chloride aqueous solution, a potassium hydrogen phthalate aqueous solution or an acetic acid-sodium acetate buffer solution.

3. The electrochemical sensor for detecting hydrazine in environmental water samples according to claim 1, characterized in that: The electrode in step 2 includes any one of a glassy carbon electrode, a fluorine-doped tin oxide electrode, an indium tin oxide electrode, a gold electrode, a screen-printed electrode, a graphite electrode, and a carbon fiber electrode.

4. The electrochemical sensor for detecting hydrazine in environmental water samples according to claim 1, characterized in that: The concentration of the N-GQDs solution in step 1 is 0.5-5 mg / mL, and the concentration of the rGO solution is 0.02-0.5 mg / mL; the ultrasonic time is 30 min-6 h, and the centrifugal speed is 10000-15000 rpm; The mass ratio of the N-GQDs to rGO is in the range of 8:1 to 1:

5.

5. The electrochemical sensor for detecting hydrazine in environmental water samples according to claim 1, characterized in that: In step 2, the volume of the composite N-GQDs@rGO drop-coated on the electrode is 4 to 10 μL, and the drying temperature is 40 to 80°C.

6. The electrochemical sensor for detecting hydrazine in environmental water samples according to claim 1, characterized in that: In step 3, the gold precursor includes one or more of chloroauric acid, gold nitrate, gold cyanide, and a gold complex; the molar concentration of the gold precursor in the gold precursor solution is 0.01 to 1 mmol / L.

7. The electrochemical sensor for detecting hydrazine in environmental water samples according to claim 1, characterized in that: In step 3, the electrodeposition voltage is -1.0 to -0.1 V, the electrodeposition time is 2 to 20 s, and the temperature is room temperature.

8. An electrochemical detection method for detecting hydrazine in environmental water samples, characterized in that: The environmental water sample is diluted with an electrolyte solution as a test solution, and the hydrazine in the environmental water sample is monitored using the electrochemical sensor according to any one of claims 1 to 7.

9. The electrochemical detection method for detecting hydrazine in environmental water samples according to claim 8, characterized in that: The pH of the liquid to be tested is 4-8.

10. The electrochemical detection method for hydrazine in environmental water samples according to claim 8, characterized in that: The concentration of hydrazine in the test solution is 3 nmol / L to 20 μmol / L; And / or, the time that the working electrode in the electrochemical sensor contacts the liquid to be detected is more than 10 seconds.

11. The electrochemical detection method for hydrazine in environmental water samples according to claim 8, characterized in that: The test solution also includes any one or more interfering substances selected from the group consisting of hydroquinone, o-diphenol, naphthol, uric acid, potassium bromide, copper sulfate, calcium chloride, magnesium sulfate, and zinc nitrate, and the concentration of each interfering substance is below 50 μmol / L.

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