A modified carboxymethyl chitosan-based conductive composite material modified electrode and its preparation method and application

Abstract

The application provides a modified carboxymethyl chitosan-based conductive composite material modified electrode and a preparation method and application thereof, and belongs to the technical field of heavy metal detection sensors. The preparation method of the modified carboxymethyl chitosan-based conductive composite material modified electrode comprises the following steps: after carboxymethyl chitosan is dissolved, an activator is added for activation, then L-methionine is added for coupling reaction, and an L-methionine grafted modified carboxymethyl chitosan derivative is obtained; the L-methionine grafted modified carboxymethyl chitosan derivative, polypyrrole and multi-walled carbon nanotubes are dispersed in a solvent to obtain a dispersion liquid; the dispersion liquid is coated on the surface of a base electrode, and dried to obtain the modified carboxymethyl chitosan-based conductive composite material modified electrode. The electrochemical sensor constructed by the modified carboxymethyl chitosan-based conductive composite material modified electrode realizes simultaneous determination of Cd 2+ , Pb 2+ and Cu 2+ .

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heavy metal detection sensors, in particular to a modified carboxymethyl chitosan-based conductive composite modified electrode and a preparation method and application thereof. BACKGROUND

[0002] Heavy metal ions (HMIs) refer to ions of metal elements with a density greater than 4.5 grams per cubic centimeter, which can gradually accumulate in the human body through the food chain and cause various adverse effects on human health. For example, Cd 2+ Once absorbed by the human body, it is difficult to remove and can cause lesions in the kidneys, lungs, liver, bones and reproductive organs of the human body. Pb 2+ After being ingested by the human body, it can cause multiple damages to the nervous system, digestive system, urinary system, blood circulation system, etc. of the human body. Similarly, when the intake of Cu 2+ exceeds the normal level required by the body, it can cause serious diseases such as gastrointestinal bleeding, hematuria, hemolysis, methemoglobinemia, liver poisoning and acute renal failure. In order to avoid the harm of heavy metal ions to the environment and the human body, it is of great significance to develop a high-sensitivity and high-reliability heavy metal ion detection technology. Considering that heavy metal ions in pollutants are usually coexisting, it is necessary to develop a simultaneous detection technology for multiple heavy metal ions.

[0003] The methods commonly used for detecting HMIs include atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, inductively coupled plasma atomic emission spectroscopy, fluorescence determination and colorimetry, etc. However, these technologies have high equipment cost, require high-skilled professionals, and are time-consuming, which limits their application in simultaneous detection of HMIs. Therefore, there is an urgent need to develop a portable, low-cost, high-precision in-situ analysis method for sensitive and selective monitoring of multiple HMIs in water resources. In recent years, anodic stripping voltammetry (ASV) has attracted widespread attention in HMIs monitoring due to its fast response, low cost, simplicity, portability and high reliability. The performance of ASV mainly depends on the working electrode, and the modified electrode material is a core component of electrochemical methods, so it is crucial to select a suitable electrode modification material. SUMMARY

[0004] Therefore, the present application aims to provide a modified carboxymethyl chitosan-based conductive composite modified electrode and a preparation method and application thereof.

[0005] In order to achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0006] One of the technical solutions of the present application is a preparation method of a modified carboxymethyl chitosan-based conductive composite material modified electrode, comprising the following steps:

[0007] After the carboxymethyl chitosan is dissolved, an activator is added for activation, and then L-methionine is added for coupling reaction to obtain an L-methionine grafted modified carboxymethyl chitosan derivative;

[0008] The L-methionine grafted modified carboxymethyl chitosan derivative, polypyrrole and multi-walled carbon nanotube are dispersed in a solvent to obtain a dispersion liquid;

[0009] The dispersion liquid is coated on the surface of a base electrode and dried to obtain the modified carboxymethyl chitosan-based conductive composite material modified electrode.

[0010] The second technical solution of the present application is a modified carboxymethyl chitosan-based conductive composite material modified electrode prepared by the above preparation method.

[0011] The third technical solution of the present application is an electrochemical sensor, and the working electrode is the modified carboxymethyl chitosan-based conductive composite material modified electrode.

[0012] The fourth technical solution of the present application is an application of the modified carboxymethyl chitosan-based conductive composite material modified electrode or the electrochemical sensor in heavy metal ion detection; the heavy metal ion is at least one of cadmium ion, lead ion and copper ion.

[0013] The fifth technical solution of the present application is a method for detecting heavy metal ions, wherein the modified carboxymethyl chitosan-based conductive composite material modified electrode, a reference electrode and a counter electrode are assembled into a three-electrode system, and then placed in a buffer solution containing heavy metal ions for SWASV electrochemical testing.

[0014] The present application discloses the following technical effects:

[0015] The present application uses L-methionine as a grafting group to graft and modify carboxymethyl chitosan, and then composites the modified carboxymethyl chitosan with polypyrrole and multi-walled carbon nanotube to prepare a modified carboxymethyl chitosan-based conductive composite material, and constructs an electrochemical sensor based on the conductive composite material modified electrode for simultaneously determining cadmium, lead and copper ions. 2+ 2+ 2+ 2+ ​​​, Pb 2+ and Cu 2+ The linear determination range is 0.01-5μmol / L, and the detection limits are 5, 4, and 6nmol / L, respectively. In addition, the sensor has good stability and selectivity, and the actual river water sample was spiked and recovered for Cd 2+ , Pb 2+ and Cu 2+ The recovery rates of the three were between 96.36% and 103.35%, indicating that this sensor is suitable for the detection of Cd in actual water samples. 2+ , Pb 2+ and Cu 2+ Simultaneous electrochemical analysis monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 This is the schematic diagram of L-methionine grafted onto carboxymethyl chitosan;

[0018] Figure 2 The infrared spectra of CMCS and CMCS-Met in the effect verification example;

[0019] Figure 3 CV curves of bare GCE and M-GCE in the effect verification example;

[0020] Figure 4 The EIS curves of bare GCE and M-GCE in the effect verification example are shown;

[0021] Figure 5 For the effect verification example, bare electrode GCE simultaneously measures Cd 2+ , Pb 2+ and Cu 2+ SWASV diagram;

[0022] Figure 6 For the verification of the effect, M-GCE was used to simultaneously determine Cd 2+ , Pb 2+ and Cu 2+ SWASV diagram;

[0023] Figure 7 To optimize the amount of conductive composite material used in the effect verification example;

[0024] Figure 8Optimization of enrichment potential in the effect verification example

[0025] Figure 9 Optimization of enrichment time in the effect verification example

[0026] Figure 10 Optimization of pH of the buffer solution containing heavy metal ions in the effect verification example

[0027] Figure 11 Graph of simultaneous determination of different concentrations of Cd 2+ , Pb 2+ and Cu 2+ by SWASV method in the effect verification example

[0028] Figure 12 Standard curve for detection of Cd 2+ in the effect verification example

[0029] Figure 13 Standard curve for detection of Pb 2+ in the effect verification example

[0030] Figure 14 Standard curve for detection of Cu 2+ in the effect verification example DETAILED DESCRIPTION

[0031] Various exemplary embodiments of the present application will now be described in detail, without intent to limit the application, which is only limited by the claims. Understanding that these embodiments are given for purposes of illustration and not for purposes of limitation, the specific embodiments disclosed are understood to be illustrative of certain aspects, features, and / or embodiments of the present application.

[0032] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. In addition, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict, the present specification will control.

[0033] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict, the present specification will control.

[0034] Many modifications and variations to the illustrative embodiments described herein will be apparent to those of ordinary skill in the art from this specification, which is to be regarded in an illustrative manner. Other embodiments will be apparent to those of ordinary skill in the art from consideration of the specification and practice of the embodiments disclosed herein. The specification and examples given are exemplary only.

[0035] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", "contains", "containing", or variations thereof, are intended to be open-ended terms that mean including, but not limited to.

[0036] The first aspect of the present application provides a preparation method of a modified carboxymethyl chitosan-based conductive composite modified electrode, comprising the following steps:

[0037] After the carboxymethyl chitosan is dissolved, an activating agent is added for activation, and then L-methionine is added for coupling reaction, to obtain an L-methionine grafted modified carboxymethyl chitosan derivative (denoted as CMCS-Met).

[0038] The L-methionine grafted modified carboxymethyl chitosan derivative, polypyrrole and multi-walled carbon nanotube are dispersed in a solvent to obtain a dispersion liquid.

[0039] The dispersion liquid is coated on the surface of a base electrode and dried to obtain the modified carboxymethyl chitosan-based conductive composite modified electrode (denoted as CMCS-Met-PPy-CNT / GCE modified electrode, abbreviated as M-GCE).

[0040] In some embodiments of the present application, the solvent for dissolving the carboxymethyl chitosan is water.

[0041] In some embodiments of the present application, the activating agent is EDC and NHS; and the mass ratio of the chitosan, EDC, NHS and L-methionine is (0.8-1.5):(0.7-1.2):(0.5-0.8):(0.6-0.9), preferably 1:0.9:0.6:0.7.

[0042] In some embodiments of the present application, the parameters of the activation are set as: stirring at room temperature for 20-40 min, preferably stirring at room temperature for 30 min; the parameters of the coupling reaction are set as: reaction at room temperature for 10-18 h; and when the coupling reaction is performed, the pH value of the reaction system is 6.5-7.5. In the present application, a dilute alkali solution (for example, a sodium hydroxide solution) is selected to adjust the pH value to be close to neutral, so as to better perform the coupling reaction and graft L-methionine to the carboxymethyl chitosan.

[0043] In some embodiments of the present application, after the coupling reaction is completed, the product is further subjected to dialysis in ultrapure water for two days, and then freeze-dried.

[0044] In some embodiments of the present application, the preparation method of the dispersion liquid is as follows: CMCS-Met, polypyrrole and multi-walled carbon nanotubes are weighed and dissolved in a solvent (the solvent is preferably DMF), then ultrasonic dispersion is performed to form suspensions, and finally the suspensions are mixed uniformly.

[0045] In some embodiments of the present application, the mass-volume ratio of the L-methionine grafted modified carboxymethyl chitosan derivative, polypyrrole, multi-walled carbon nanotube and solvent is 2-4 mg:2-4 mg:2-4 mg:3 mL, and preferably 3 mg:3 mg:3 mg:3 mL.

[0046] In some embodiments of the present application, before the dispersion liquid is coated on the surface of the base electrode, the base electrode is polished on 0.3 μm and 0.05 μm Al2O3 paste in sequence, then ultrasonic cleaning is performed in a mixed solution of nitric acid, anhydrous ethanol and water at a volume ratio of 1:1:1, and nitrogen blowing drying is performed.

[0047] In some embodiments of the present application, the base electrode is a glassy carbon electrode (GCE). In the present application, the shape of the glassy carbon electrode used is a cylinder, and the bottom surface (i.e. the active surface in contact with the electrolyte) is a circle with a diameter of 3 mm.

[0048] In some embodiments of the present application, when the dispersion liquid is coated on the surface of the base electrode, the amount of the dispersion liquid is set to be 3-6 μL per 7.07 mm 2 The amount of the active working area of the base electrode is 3-6 μL. 7.07 mm 2 The area of the active working area of the base electrode is calculated according to that the bottom surface (i.e. the active surface in contact with the electrolyte) of the base electrode is a circle with a diameter of 3 mm. The present application does not have special requirements for the method of coating the dispersion liquid on the surface of the base electrode, and the conventional technical means of those skilled in the art can be selected, for example, drop coating.

[0049] In some embodiments of the present application, the drying method is infrared light drying.

[0050] The second aspect of the present application provides a modified carboxymethyl chitosan-based conductive composite material modified electrode prepared by the preparation method of the above technical solution.

[0051] The third aspect of the present application provides an electrochemical sensor, and the working electrode is the modified carboxymethyl chitosan-based conductive composite material modified electrode.

[0052] The fourth aspect of the present application provides an application of the modified carboxymethyl chitosan-based conductive composite material modified electrode or the electrochemical sensor in heavy metal ion detection; the heavy metal ion is at least one of cadmium ion, lead ion and copper ion.

[0053] The fifth aspect of the present application provides a method for detecting heavy metal ions, after a modified carboxymethyl chitosan-based conductive composite electrode is assembled into a three-electrode system with a reference electrode and a counter electrode, the three-electrode system is placed in a buffer solution containing heavy metal ions for SWASV electrochemical testing; the heavy metal ions are at least one of cadmium ions, lead ions and copper ions.

[0054] In some embodiments of the present application, the pH value of the buffer solution containing heavy metal ions is 4-6.5; the parameter settings of the SWASV electrochemical testing are: the enrichment voltage is-0.9V to-1.3V, and the enrichment time is 250s.

[0055] In some embodiments of the present application, the parameter settings of the SWASV electrochemical testing are: the enrichment voltage is-1.0V to-1.2V, and the enrichment time is 250s.

[0056] In some embodiments of the present application, the parameter settings of the SWASV electrochemical testing are: the enrichment voltage is-1.2V, and the enrichment time is 250s.

[0057] In order to better understand the present application, the content of the present application is further illustrated below in combination with examples, but the content of the present application is not limited to the following examples.

[0058] The technical solutions described in the present application are conventional solutions in the art, unless otherwise specified.

[0059] Instruments and reagents:

[0060] O-carboxymethyl chitosan (CMCS), degree of carboxylation≥80%, molecular weight 150-800 thousand, Shanghai Yuanye Biological Technology Co., Ltd.; L-methionine (Met), polypyrrole (PPy), acetic acid, sodium acetate, DMF, Shanghai Maikelin Biological Technology Co., Ltd.; Multi-walled carbon nanotubes (MWCNT) were purchased from Jiangsu Xianfeng Nanometer Material Technology Co., Ltd. Cadmium nitrate standard solution (1mg / mL), lead nitrate standard solution (0.1mol / L), copper nitrate standard solution (0.1mol / L) were purchased from Beijing Beifang Weiye Measurement Technology Research Institute. All drugs are analytical pure and are not further treated before use. The water used in the experiment is deionized water provided by Aikou A2S-05-CE.

[0061] Infrared spectrometer (FT-IR), Spectrum 3, PerkinElmer, USA; electrochemical workstation, CHI660E type, Shanghai Chenhua Instrument Co., Ltd. Glassy carbon electrode (3mm in diameter), silver-silver chloride electrode and platinum plate electrode, purchased from Shanghai Yuemai Electronics Technology Co., Ltd.

[0062] Example 1

[0063] The preparation of the modified carboxymethyl chitosan-based conductive composite material modified electrode is as follows:

[0064] Step 1. Preparation of modified chitosan

[0065] 1g of carboxymethyl chitosan was weighed and dissolved in 100mL of water, then 0.9g of EDC and 0.6g of NHS were added, and the system was activated under stirring at room temperature for 30min, then 0.7g of L-methionine was added, and then the pH of the system was adjusted to 6.5 with dilute alkali solution, and then the reaction was carried out for 12 hours. The product was then dialyzed in ultrapure water for two days, and finally freeze-dried to obtain the L-methionine grafted modified carboxymethyl chitosan derivative, denoted as CMCS-Met.

[0066] Step 2. Preparation of the modified carboxymethyl chitosan-based conductive composite material modified electrode

[0067] Electrode pretreatment: The glassy carbon electrode (GCE) was polished on 0.3μm and 0.05μm Al2O3 paste in turn, and then ultrasonically cleaned in a mixed solution of nitric acid, anhydrous ethanol and water in a volume ratio of 1:1:1, and then dried with nitrogen for standby use.

[0068] Preparation of the modified carboxymethyl chitosan-based conductive composite material modified electrode: 3mg of CMCS-Met, 3mg of polypyrrole and 3mg of multi-walled carbon nanotubes were weighed respectively and dissolved in 1mL of DMF, then ultrasonically dispersed into suspensions respectively, and finally the three suspensions were mixed uniformly to obtain a composite material mixture. Then 4.5uL of the composite material mixture was dropped onto the surface of the treated GCE, and then dried under infrared light to obtain the CMCS-Met-PPy-CNT / GCE modified electrode (modified GCE), abbreviated as M-GCE.

[0069] Effect verification example

[0070] 1. Material characterization

[0071] The schematic diagram of L-methionine grafted carboxymethyl chitosan is as shown in Figure 1 Under the synergistic action of EDC and NHS, the amino group on Met is coupled to the carboxyl group of CMCS. In order to prove the successful grafting of the material, the present application adopts infrared spectrum test to characterize the grafted product, and the test result is as shown in Figure 2 Figure 2 The middle curve a is the infrared spectrum of CMCS, 1587cm -1 -1 is the absorption band of symmetric stretching vibration of carboxyl and bending vibration of N-H, 1417cm -1 -1 is the absorption band of asymmetric stretching vibration of carboxyl, 1311cm -1 ​The wave number vicinity is C-N stretching vibration absorption band. Figure 2 Curve b is the infrared spectrum of CMCS-Met, compared with the spectrum of CMCS, the infrared spectrum of CMCS-Met appears characteristic absorption band of Met at 2130cm -1 , 1640cm -1 , and 1575cm -1 , which is the stretching vibration absorption band of C=O, the asymmetric stretching vibration of carboxyl group moves from 1417cm -1 to 1402cm -1 , and the stretching vibration of C-N moves from 1311cm -1 to 1322cm -1 , which indicates that after L-methionine is grafted to carboxymethyl chitosan, the carboxyl group on Met and the amino group on CMCS have electrostatic interaction, and the above indicates that the synthesis of L-methionine modified carboxymethyl chitosan derivative is successful.

[0072] 2. Electrochemical test

[0073] The three-electrode system is placed in 10mL 0.1mol / L HAc-NaAc buffer solution, after the system is stable, the heavy metal ions cadmium, lead and copper standard sample are added, first enriched at a certain potential, and then electrochemical test is carried out by square wave anodic stripping voltammetry (SWASV). The electrochemical test parameters and conditions: the cyclic voltammetry (CV) and the alternating current impedance test (EIS) are carried out in 5mmol / L [Fe(CN)6] 3- / 4- electrolyte solution, wherein the solution contains 0.1mol / L KCl to enhance the conductivity of the electrolyte. The scanning range of CV is-0.4-0.6V, and the scanning speed is 100mV / s; the frequency range of EIS test is 0.1Hz-10kHz, the amplitude is 50mV, and the bias voltage is open circuit voltage. The SWASV test parameters: the scanning range is-1.0-0.2V, the amplitude is 25mV / s, the enrichment potential is-1.2V, and the enrichment time is 250s.

[0074] 2.1 Electrochemical characterization of different electrodes: the present application uses two kinds of electrochemical test methods of cyclic voltammetry (CV) and electrochemical alternating current impedance method (EIS) to study the modification of the conductive composite material on the surface of GCE, and the test results are shown in Figure 3 and Figure 4 .

[0075] The CV results of GCE and M-GCE are shown in curve a and curve b of Figure 3 , it can be seen that whether the CV graph of bare GCE or M-GCE has a pair of obvious [Fe(CN)6] 3- / 4-However, in the CV curve of M-GCE, [Fe(CN)6] 3- / 4- The redox peaks of the modified GCE are wider, that is, the oxidation peak current value is larger and the reduction peak current value is smaller, indicating that the conductivity of the modified GCE becomes stronger, which is conducive to the redox couple approaching the electrode surface and promoting the transfer of electrons on the electrode surface.

[0076] In the Nyquist plot of electrochemical impedance spectroscopy (EIS), the semicircle observed at higher frequencies corresponds to the limiting process of charge transfer, and the diameter of the semicircle reflects the electron transfer resistance value (Rct) of the electrode surface. The EIS results of GCE and M-GCE are shown in Figure 2. Figure 4 As shown in curve a and curve b, the interfacial charge transfer resistance Rct of GCE is 211Ω, while that of M-GCE is 49Ω. It can be seen that compared with bare GCE, the EIS graph of M-GCE shows a smaller semicircle, which indicates that after modification of the conductive composite material, the charge transfer resistance on the electrode surface is reduced. This is because multi-walled carbon nanotubes and polypyrrole have strong conductivity, which is conducive to the transfer of redox couples on the modified electrode surface. It can be seen that the EIS test results are consistent with the CV test results, and both prove that the M-GCE modified electrode is successfully constructed.

[0077] 3. Feasibility study of modified electrodes

[0078] Square wave anodic stripping voltammetry (SWASV) includes two electrochemical processes, namely, enrichment and stripping. It is a highly sensitive electrochemical method for determining trace metal ions. Therefore, the present invention selects SWASV for the simultaneous determination of Cd in water. 2+ , Pb 2 + and Cu 2+ The electrochemical response of bare GCE and M-GCE to heavy metal ions was compared to evaluate the binding capacity of M-GCE to heavy metal ions. Figure 5 and Figure 6 shown.

[0079] from Figure 5 It is concluded that Cd 2+ , Pb 2+ and Cu 2+The stripping peak potentials of the three metal ions (the test concentration is 1 μM) on the bare GCE electrode are -0.78 V, -0.48 V and -0.10 V respectively, and the stripping peak currents ΔI (the peak current minus the baseline current) are 1.05 μA, 1.56 μA and 1.34 μA respectively, while the stripping peak potentials of the three metal ions on the M-GCE electrode are -0.79 V, -0.55 V and -0.04 V respectively, and the stripping peak currents ΔI are 3.88 μA, 4.48 μA and 3.85 μA respectively, which shows that the peak current is significantly improved, and increases by 2.83 μA, 2.92 μA and 2.51 μA respectively. In addition, compared with the baseline current value 3.92 μA of the GCE, the baseline current value of the M-GCE is 141.26 μA, which is obviously higher than that of the GCE, which further proves that the M-GCE surface modified by the conductive composite has better conductivity. Another obvious test result is that the stripping peaks of the three heavy metal ions on the M-GCE are three sharp peaks, while on the GCE they are wide peaks, which also shows that the M-GCE has better distinguishability for Cd 2+ , Pb 2+ and Cu 2+ three heavy metal ions, which may be due to the conductive composite having active groups such as amino, carboxyl, sulfur elements and π bonds, which play a key role in the enrichment and stripping of heavy metal ions. Therefore, the electrochemical response of the M-GCE electrode modified by the conductive composite to Cd 2+ , Pb 2+ and Cu 2+ three heavy metal ions is effectively improved, and the M-GCE electrode can be used as a sensing platform for simultaneous determination of the three.

[0080] 4. Optimization of the drop coating amount of the conductive composite

[0081] The amount of the modified chitosan-based conductive composite directly affects the deposition of metal ions on the electrode surface, and further affects the electrochemical test results, so the present application first optimizes the drop coating amount of the modified chitosan-based conductive composite on the GCE. Here, the effects of the material modification amounts of 3, 3.5, 4, 4.5, 5 and 6 μL on the electrochemical detection of heavy metal ions are studied, and the detection concentration of Cd 2+ , Pb 2+ and Cu 2+ is 1 μM, and the test results are as follows Figure 7As shown. It can be seen that when the modification amount of the composite material is increased from 3 μL to 4.5 μL, the electrochemical response current of the three metal ions gradually increases and reaches a maximum value. On the contrary, when the modification amount continues to increase, the electrochemical response currents of the three do not increase but decrease. This is because when the modification amount is less than 4.5 μL, the composite material is not completely covered with the electrode surface and can not provide enough active sites. Therefore, when the modification amount of the material is gradually increased from 3 μL to 4.5 μL, the electrochemical response currents of the three gradually increase. When the modification amount of the material continues to increase, the material is stacked on the electrode surface, causing the material to easily fall off from the electrode surface, and too much modification material may also hinder electron transmission, so excessive modification will cause the electrochemical response current to decrease. Therefore, the present invention selects 4.5 μL as the optimal composite material drop coating amount.

[0082] 5. Optimization of Enrichment Potential

[0083] The enrichment potential is another key factor affecting the accuracy of electrochemical detection. This paper discusses the effect of enrichment potential from -0.9V to -1.3V on the Cd 2+ , Pb 2+ and Cu 2+ The test concentration of the three was 1 μM. The experimental results are as follows Figure 8 As shown. It can be seen that when the enrichment potential is in the range of -0.9V to -1.3V, when the enrichment potential gradually becomes negative, the stripping peak current values ​​of the three first increase and then decrease, reaching a maximum value at -1.2V. When the potential continues to increase negatively to -1.3V, the stripping peak current value decreases. This is because when the enrichment potential increases negatively from -0.9V to -1.2V, as the potential increases negatively, it is more conducive to the electrodeposition of metal ions, and therefore the stripping peak current of the metal ions is also larger. When the enrichment potential continues to shift negatively, a hydrogen evolution reaction will occur on the surface of the modified electrode, hindering the electrochemical deposition of metal ions, so the stripping peak current is correspondingly reduced. Therefore, the present invention selects -1.2V as the optimal electrochemical enrichment potential.

[0084] 6. Optimization of enrichment time

[0085] The principle of the SWASV electrochemical test method is to first electrochemically reduce metal ions to the electrode surface and then electrochemically dissolve them. Therefore, the choice of enrichment time in this process will directly affect the test results. 2+ , Pb 2+ and Cu 2+ The electrochemical tests of the three showed that as the electrochemical deposition time increased, the peak current values ​​of the three gradually increased. The test results are as follows Figure 9The maximum current value is obtained at 250 s, and then tends to be flat, indicating that 250 s is sufficient for the electrochemical deposition of the three, and thus 250 s is selected as the optimal electrochemical enrichment time.

[0086] 7. Optimization of pH of electrochemical test solution

[0087] Because the stability of heavy metal ions in solutions at different pH values is different, the pH of the test system also affects the electrochemical test results. The test buffer solution selected by the present application is a 0.1 mol / L HAc-NaAc buffer solution. The effects of test buffer solutions with pH values of 4, 4.5, 5, 5.5, 6, and 6.5 on SWASV tests of 1 μM Cd 2+ , Pb 2+ , and Cu 2+ are investigated. The test results are shown in Figure 10 . It can be seen that when the pH of the test system increases from 4.0 to 5.5, the stripping current values of the three gradually increase, and when the pH of the test solution continues to increase, the stripping current values slightly decrease. Therefore, the pH of the test solution is selected to be 5.5.

[0088] 8. Establishment of standard curve and detection limit

[0089] A three-electrode test system is adopted, M-GCE is used as the working electrode, Ag / AgCI is used as the reference electrode, and a platinum wire electrode is used as the counter electrode. Under the above-optimized test conditions, SWASV electrochemical test technology is used to simultaneously electrochemically test Cd 2+ , Pb 2+ , and Cu 2+ . The test results are shown in Figure 11 . Curves a to f correspond to concentrations of 0, 0.01, 0.1, 1, 2, and 5 μmol / L, respectively. As can be seen from the figure, with the increase of the test concentration of heavy metal ions, the electrochemical response current values of the three increase. In the range of 0.01-5 μmol / L, the stripping peak current values of the three and their concentrations present a good linear relationship. The linear regression equations are ΔI = 2.76 × C(Cd 2+ ) + 1.08, the linear correlation coefficient R 2 = 0.998, see Figure 12 ; ΔI = 3.10 × C(Pb 2+ ) + 1.03, the linear correlation coefficient R 2 = 0.998, see Figure 13 ; and ΔI = 2.07 × C(Cu 2+ ) + 1.83, the linear correlation coefficient R 2 = 0.998, see Figure 14The calculation method of the detection limit is as follows: 10 parallel determinations of the blank sample give the standard deviation (s), and the ratio of 3 times the standard deviation s to the slope k of the standard curve (LOD = 3s / k) gives the detection limits of Cd 2+ , Pb 2+ and Cu 2+ , which are 5, 4 and 6 nmol / L respectively. The detection limits of the three heavy metal ions in the application meet the detection requirements of the World Health Organization [WHO. Guidelines for drinking-water quality, World Health Organization, 2004, vol. 1.] and the national drinking water standard [Drinking Water Health Standards, GB 5749-2006, National Standard of the People's Republic of China] for Cd 2+ , Pb 2+ and Cu 2+ in water.

[0090] 9. Stability and selectivity of the electrode

[0091] In order to study the stability of the M-GCE electrode, the composite modified electrode was placed in a refrigerator at 4℃ for one month, and then SWASV was used to simultaneously determine 1 μM of Cd 2+ , Pb 2+ and Cu 2+ three heavy metal ions. Compared with the test results of the newly prepared M-GCE, the stripping peak current value of Cd 2+ was 92.36% of the current of the newly prepared electrode, the stripping peak current value of Pb 2+ was 93.45% of the current of the newly prepared electrode, and the stripping peak current value of Cu 2+ was 93.69% of the current of the newly prepared electrode, indicating that the activity of the modified electrode remained good and could be stably stored for a short period of time.

[0092] The selectivity of the M-GCE was explored by the method of testing the interference of coexisting ions. Specifically, in order to investigate the selectivity of the electrode, when there were 10 μM of Ca 2+ , K + , Mg 2+ , Fe 2+ , Zn 2+ , SO4 2- , Cl - , NO3 - and other ions, 1 μM of Cd 2 + , Pb 2+ and Cu 2+SWASV test was performed. The SWASV test results show that even if there are 10 times the concentration of the above interfering ions, the dissolution peak current response of the three is more than 92.54% of the current without interfering ions, that is, the coexisting interfering ions have little effect on the response current of the target ions, indicating that the sensor has good selectivity.

[0093] 10. Actual sample analysis

[0094] The river water of a certain freshwater river was filtered through a 0.45 μm filter membrane, diluted 5 times with 0.1M HAc-NaAc solution, and then the standard addition recovery experiment was performed under the optimal test conditions. After 3 parallel tests each time, the average value was taken, and the test results are shown in Table 1. The standard addition recovery rate is between 96.36% and 103.35%, and the variation coefficient RSD is between 2.61% and 3.41%, indicating that the standard addition recovery rate of the sensor is good, and it can be used for analysis of heavy metal ions in actual water samples.

[0095] Table 1: Standard addition recovery experiment results

[0096]

[0097] In summary, the L-methionine modified carboxymethyl chitosan derivative is synthesized by chemical coupling method, the conductive composite material is prepared by compounding the L-methionine modified carboxymethyl chitosan derivative, polypyrrole and multi-walled carbon nanotubes, then the conductive composite material is modified on the surface of a glassy carbon electrode, and an electrochemical sensor for simultaneously determining cadmium, lead and copper ions is constructed. The glassy carbon electrode modified by the conductive composite material improves the distinguishability of the three heavy metal ions. In the concentration range of 0.01-5 μM, the electrochemical sensor has good linear correlation with the three heavy metal ions. The standard addition recovery results show that the sensor is suitable for simultaneous electrochemical monitoring of cadmium, lead and copper ions in water.

[0098] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for preparing a modified carboxymethyl chitosan-based conductive composite material modified electrode, characterized in that: The following steps are involved: After dissolving the carboxymethyl chitosan, an activator is added for activation, and then L-methionine is added for coupling reaction to obtain an L-methionine-grafted carboxymethyl chitosan derivative; dispersing the L-methionine grafted modified carboxymethyl chitosan derivative, polypyrrole and multi-walled carbon nanotubes in a solvent to obtain a dispersion; The dispersion is coated on the surface of a base electrode and dried to obtain the modified carboxymethyl chitosan-based conductive composite material modified electrode.

2. The method for preparing a modified carboxymethyl chitosan-based conductive composite material modified electrode according to claim 1, wherein: The activators are EDC and NHS; the mass ratio of the carboxymethyl chitosan, EDC, NHS and L-methionine is (0.8-1.5):(0.7-1.2):(0.5-0.8):(0.6-0.9).

3. The method for preparing a modified carboxymethyl chitosan-based conductive composite material modified electrode according to claim 1, wherein: The activation parameters are set as: stirring at room temperature for 20-40 minutes; the coupling reaction parameters are set as: reaction at room temperature for 10-18 hours; when performing the coupling reaction, the pH value of the reaction system is 6.5-7.

5.

4. The method for preparing a modified carboxymethyl chitosan-based conductive composite material modified electrode according to claim 1, wherein: The mass volume ratio of the L-methionine grafted modified carboxymethyl chitosan derivative, polypyrrole, multi-walled carbon nanotubes and solvent is 2-4 mg: 2-4 mg: 2-4 mg: 3 mL.

5. The method for preparing a modified carboxymethyl chitosan-based conductive composite material modified electrode according to claim 1, wherein: When the dispersion is applied to the surface of the base electrode, the amount of the dispersion is set to: 2 The amount used for the active working area of ​​the substrate electrode is 3-6 μL.

6. A modified carboxymethyl chitosan-based conductive composite material modified electrode prepared by the preparation method according to any one of claims 1 to 5.

7. An electrochemical sensor, characterized in that The working electrode is the modified carboxymethyl chitosan-based conductive composite material modified electrode according to claim 6.

8. Use of the modified carboxymethyl chitosan-based conductive composite modified electrode according to claim 6 or the electrochemical sensor according to claim 7 in the detection of heavy metal ions; the heavy metal ions are at least one of cadmium ions, lead ions and copper ions.

9. A method for detecting heavy metal ions, characterized in that: After assembling the modified carboxymethyl chitosan-based conductive composite material modified electrode described in claim 7, a reference electrode and a counter electrode into a three-electrode system, the system is placed in a buffer solution containing heavy metal ions for SWASV electrochemical testing.

10. The method for detecting heavy metal ions according to claim 9, wherein The pH value of the buffer solution containing heavy metal ions is 4-6.5; the parameters of the SWASV electrochemical test are set as follows: the enrichment voltage is -0.9V to -1.3V, and the enrichment time is 250s.

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