CN113325053A

CN113325053A - Cadmium ion electrochemical sensor working electrode and preparation method, detection method and application thereof

Info

Publication number: CN113325053A
Application number: CN202110638458.XA
Authority: CN
Inventors: 薛强; 刘月华; 王群; 单筱寒
Original assignee: China University of Geosciences Beijing
Current assignee: China University of Geosciences Beijing
Priority date: 2021-06-08
Filing date: 2021-06-08
Publication date: 2021-08-31
Anticipated expiration: 2041-06-08
Also published as: CN113325053B

Abstract

The invention discloses a working electrode of a cadmium ion electrochemical sensor and its preparation, detection method and application, and solves the problems of high cost and inability to perform in-situ real-time monitoring of heavy metal Cd(II) in the water environment in the prior art. The working electrode of the cadmium ion electrochemical sensor includes a LIG electrode and a tin film modified on its surface; the main body of the tin film is metal tin, and a tin dioxide metal oxide film is formed on the surface of the metal tin; the LIG electrode can be made of The polyimide film is used as the substrate, and is prepared by etching the polyimide film by laser induction technology; the tin film can be formed by immersing the LIG electrode in Sn(II) solution and depositing by the potentiostatic method. It is verified by experiments that the invention has better anti-interference ability, reproducibility, detection stability and accuracy; and the working electrode of the cadmium ion electrochemical sensor can be made into a product in the form of a "test strip", which is convenient for It is also conducive to the standardization of detection.

Description

Cadmium ion electrochemical sensor working electrode and preparation method, detection method and application thereof

Technical Field

The invention relates to the technical field of heavy metal ion electrochemical sensors, in particular to a working electrode of an electrochemical sensor for detecting Cd (II), a preparation method of the working electrode and a method for detecting Cd (II) by using the electrochemical sensor.

Background

In general, any substance having conductivity may be used as the working electrode. When an electrochemical method is adopted to detect heavy metals in a water environment, commonly used working electrodes mainly comprise mercury electrodes, gold electrodes, glassy carbon electrodes and the like. The mercury electrode is a specific electrode when the polarography is adopted to detect the heavy metals in the water environment, has the advantages of quick surface updating and continuous measurement, but has the defects of great harm to organisms and environment and the like because of the environmental unfriendliness of mercury; therefore, gold electrodes and glassy carbon electrodes have been developed; the gold electrode and the glassy carbon electrode have good conductivity and high chemical stability, and can be made into different shapes such as a cylinder shape, a sheet shape and the like according to requirements, but the two electrodes are expensive and complicated to modify, and modified substances are easy to fall off and are not suitable to be used as electrode substrates for in-situ real-time monitoring.

In recent years, researchers find that a Polyimide (PI) film and lignin can be etched by Laser, a Graphene-like material with good conductivity, namely Laser-Induced Graphene (LIG), can be Induced at an instant high temperature, and the Laser-Induced technology has the advantages of simple preparation, low cost, high efficiency and no pollution.

The polyimide film is a fiber mesh polymer with special five-membered imide groups, has good thermal stability, can rapidly and uniformly absorb energy generated by laser instantaneous high-temperature etching, and can well control the etching shape through a computer program. The principle of laser-induced graphene is that infrared laser or CO is used₂The laser acts on the surface of the material, and the three-dimensional carbon atoms hybridized by sp3 on the PI film are converted into the planar carbon atoms hybridized by sp2 through high-temperature catalysis, so that the graphene material with good electrical conductivity is obtained, and can be used for constructing a flexible electrode sensor.

At present, sensors based on LIG electrodes mainly modify gold layers or silver layers, are mostly used for detecting antibiotics, pathogens and the like, and are not suitable for detecting Cd (II) in water environments.

Disclosure of Invention

The first purpose of the invention is to provide a working electrode of a cadmium ion electrochemical sensor based on a relatively mature electrochemical sensor technology, and solve the problems that the prior art is high in cost and cannot carry out in-situ real-time monitoring on heavy metal Cd (II) in a water environment and the like.

The second purpose of the invention is to provide a method for preparing the working electrode of the cadmium ion electrochemical sensor.

The third purpose of the invention is to provide a method for detecting the content of heavy metal Cd (II) in the water environment, which can realize in-situ real-time monitoring, is simple and convenient to operate and has lower cost.

In order to achieve the purpose, the invention provides the following technical scheme:

in a first aspect, a working electrode of a cadmium ion electrochemical sensor comprises an LIG electrode and a tin film modified on the surface of the LIG electrode; the tin film is mainly made of metal tin, and a tin dioxide metal oxide film is formed on the surface of the metal tin; the overall thickness of the tin film is 50nm to 1 μm.

Preferably, the LIG electrode is prepared by using a polyimide film as a substrate and etching the polyimide film by using a laser induction technology. In addition, other materials such as lignin can be adopted, and the graphene-like material with good conductivity can be induced by instant high temperature and is mainly used as a substrate for depositing metal tin.

Preferably, the tin film is formed by potentiostatic deposition by immersing the LIG electrode in a Sn (ii) solution.

In a second aspect, a method for preparing the working electrode of the cadmium ion electrochemical sensor comprises the following steps:

the method comprises the following steps of (1) etching a polyimide film by using the polyimide film as a substrate through a laser induction technology to prepare a naked LIG electrode;

immersing the naked LIG electrode into a Sn (II) solution, and depositing a metal film on the surface of the LIG electrode by adopting a potentiostatic method deposition mode to form an Sn/LIG electrode;

and after the deposition is finished, taking out the Sn/LIG electrode for cleaning, and then placing the Sn/LIG electrode in a vacuum anaerobic box for storage for later use.

Preferably, the metal film is deposited on the surface of the LIG electrode by adopting a potentiostatic deposition mode, wherein the concentration of the prepared Sn (II) solution is 5-7 mg/L, the deposition time is 150-300 s, and the deposition potential is-1.0-1.5V.

The optimal parameters after comprehensive consideration are as follows: the concentration of the Sn (II) solution is 6mg/L, the deposition time is 180s, and the deposition potential is-1.2V.

In a third aspect, a method for detecting cadmium ion content in an aqueous environment comprises the following steps:

1) reduction and enrichment

Immersing the working electrode of the cadmium ion electrochemical sensor into a water sample to be detected, stirring the water sample to be detected, applying a set deposition voltage on the working electrode in a potentiostatic deposition mode, and continuing for a set deposition time;

2) standing still

After the reduction enrichment process is finished, stopping stirring to keep the solution in a stable state;

3) oxidative digestion

And applying forward scanning potential on the working electrode in a differential pulse voltammetry scanning mode, obtaining a differential pulse anode dissolution voltammogram after scanning is finished, and then analyzing and calculating the differential pulse anode dissolution voltammogram to obtain the cadmium ion content in the water sample to be detected.

According to the method, the electrochemical sensor which is commercialized can simply, conveniently and accurately obtain the actually measured concentration value of the cadmium ions by pre-calibrating the electrochemical sensor (establishing a standard curve/function relation of current-concentration).

Preferably, in the step 1), the water sample to be detected is prepared into NaAc-HAc buffer solution (as electrolyte solution); the optimum pH value is 4.4.

Preferably, in the step 1), the set deposition voltage is-1.1 to-1.4V, and the set deposition time is 200 to 300 s. The optimal parameters after comprehensive consideration are as follows: the deposition voltage was set at-1.2V and the deposition time was set at 210 s.

Preferably, in step 3), a forward scanning potential is applied to the working electrode, which is scanned from-1.0V to-0.5V, with an amplitude: 25mV, step voltage 4mV, frequency: 25 Hz.

In addition, the invention also discloses a further product development prospect, namely: the working electrode of the cadmium ion electrochemical sensor can be used as a component unit to construct or assemble a water body in-situ real-time monitoring device and the like.

Compared with the prior art, the invention has at least the following beneficial effects:

1. the method can be used for carrying out in-situ real-time monitoring on the heavy metal Cd (II) in the water environment, and is simple and convenient to operate and low in cost.

2. Experiments prove that the invention has better anti-interference capability, reproducibility, detection stability and accuracy.

3. The working electrode of the cadmium ion electrochemical sensor can be made into products similar to a test strip form, is convenient to take and use, and is also beneficial to detection standardization.

Drawings

FIG. 1 is a schematic diagram of one embodiment of a working electrode of a cadmium ion electrochemical sensor of the present invention; this figure is intended to illustrate the basic hierarchical structure in principle and does not represent a physical product form; in the figure, 1 is a flexible substrate (polyimide film), 2 is metallic tin, and 3 is a passivation layer (tin dioxide metal oxide film).

FIG. 2 is an SEM image of front and rear electrodes of an electrodeposited Sn film.

FIG. 3 is a graph showing EIS comparison before and after electrodeposition of Sn film.

FIG. 4 is a graph showing the variation trend of the oxidation peak current of Cd (II) detected by the working electrode deposited at different Sn (II) concentrations.

FIG. 5 shows the variation trend of the oxidation peak current of Cd (II) detected by the working electrode obtained after different deposition times of Sn (II).

FIG. 6 is a graph showing the variation trend of the oxidation peak current of Cd (II) detected by a working electrode obtained by using different deposition potentials of Sn (II).

FIG. 7 is a graph showing the trend of the oxidation peak current of Cd (II) detected by a working electrode deposited by electrolyte solutions with different pH values.

FIG. 8 shows the variation of oxidation peak current under different deposition potentials when detecting Cd (II) content.

FIG. 9 shows the variation of oxidation peak current under different deposition time conditions when detecting Cd (II) content.

FIG. 10 is a current-concentration linear relationship diagram of different Cd (II) concentrations detected by the Sn/LIG electrode.

Fig. 11 is a test result graph of the interference resistance test.

Detailed Description

While certain embodiments (including reference examples) will now be described in detail to illustrate certain preferred embodiments and advantages of the invention, other advantages and benefits of the invention will become apparent to those skilled in the art from the disclosure herein, and it should be understood that the described embodiments are only some of the possible embodiments of the invention, rather than all of them.

According to one embodiment of the invention, a polyimide film is used as a substrate, a PI film is etched at high temperature to prepare a naked LIG electrode, a tin film is electrodeposited on the surface of the electrode to construct a novel flexible electrochemical sensor Sn/LIG electrode, heavy metal ions Cd (II) in a water environment are detected by adopting a differential pulse stripping voltammetry (DPASV), and conditions for preparing the electrode (including deposition potential during electrodeposition of Sn (II), concentration of Sn (II) during electrodeposition, deposition time of Sn (II) during electrodeposition) and detection conditions (including deposition potential during electrodeposition of Cd (II), deposition time during electrodeposition of Cd (II) and solution pH) are respectively optimized by controlling a variable method, so that in-situ real-time monitoring of heavy metal Cd (II) in the water environment is realized.

The main experimental instruments in this example are an electrochemical workstation and a laser etcher. Both of these laboratory instruments are conventional in the art.

For example, the electrochemical workstation used in this embodiment is a model CHI660E electrochemical workstation manufactured by shanghai chenhua instruments ltd, and the workstation can implement various operations such as Cyclic Voltammetry (CV), Linear Sweep Voltammetry (LSV), square wave pulse voltammetry (SWV), Differential Pulse Voltammetry (DPV), current-time curves (i-t method), alternating current impedance-time curves (a.c. impedance), and the like, which are commonly used in electrochemical experiments; the laser etcher used in this embodiment is a Nano Pro-iii laser etcher produced by tianjin jia silver Nano-technology limited, and different powers and depths can be controlled by a computer program, so that the polyimide film is subjected to laser etching to induce and generate a graphene-like conductive material, and the graphene-like conductive material is used as a working electrode substrate. In addition, various characterization equipment, material storage equipment and the like are used in the experimental process, wherein the contact angle of the material is measured by using a video contact angle tester and is used for characterizing the hydrophilicity and hydrophobicity of the front surface and the rear surface of the electrode before modification; scanning electron microscope is used for carrying out SEM appearance, particle size characterization and energy spectrum analysis on the modified electrode; obtaining the appearance of a sample through a field emission transmission electron microscope, and performing micro analysis and component analysis on a nanometer scale, wherein the micro analysis and component analysis comprises high resolution lattice fringe image (HRTEM), diffraction pattern, energy spectrum surface scanning (mapping), energy spectrum spot measurement (EDS) and the like; performing XPS test by an X-ray photoelectron spectrometer, and analyzing the distribution and valence of surface substance elements; and a vacuum moisture-proof box is adopted to store the electrode material so as to prevent the electrode material from being oxidized by oxygen in the air. Other common laboratory instruments also include analytical balances, pH meters, magnetic stirrers, ultrasonic cleaners, and the like.

The reagents used in this example are shown in table 1.

TABLE 1 reagents used in the experiments

During electrochemical detection, a three-electrode system is adopted, namely a working electrode (an LIG electrode is used as an electrode substrate and is obtained by inducing a PI film by laser), a reference electrode (an Ag/AgCl electrode which is stored in a saturated KCl solution to protect the electrode when the electrode is idle) and a counter electrode (a platinum wire electrode).

Method for manufacturing Sn/LIG electrode

And controlling proper power and depth, generating a graphene-like electrode through laser etching, and performing surface modification by using the graphene-like electrode as a substrate.

There are generally two methods for modifying the surface of the electrode with a metal film:

one is pre-plating, namely immersing the LIG electrode to be modified into a salt solution containing electroplated metal (Sn (II) solution with proper concentration in the experiment), depositing a metal film on the surface of the electrode by adopting a constant potential method (i-t method) deposition mode, taking out the electrode, cleaning, placing the electrode in a vacuum anaerobic box for storage, and then carrying out electrochemical detection;

the other method is an in-situ coating method, namely soluble salt of a metal film to be modified is added into a solution to be detected, and then the modified film and a detected substance are simultaneously electrodeposited on the surface of a working electrode in a constant potential deposition mode in the detection process.

Therefore, in this embodiment, it is preferable to modify the surface of the electrode with a tin film by plating in advance (the former method).

The obtained cadmium ion electrochemical sensor working electrode hierarchical structure is shown in fig. 1, and sequentially comprises a flexible substrate (polyimide film) 1, metal tin 2 and a passivation layer (tin dioxide metal oxide film) 3.

Second, detection method of heavy metal cadmium ion

In the experiment, 0.2M NaAc-HAc acetate buffer solution with proper volume is used as electrolyte solution, and the NaAc-HAc buffer solution is used as the electrolyte solution, so that the method has the following advantages that firstly, the ion concentration is increased, and compared with the method without adding the electrolyte solution, the electrochemical signal can be effectively enhanced; secondly, the pH of the solution is easy to adjust, and compared with a salt solution such as a KCl solution serving as an electrolyte solution, the pH value of the solution to be detected is more convenient to control to be at a proper level; and thirdly, the concentration of the Cd (II) solution can be kept unchanged, and compared with a PB phosphate buffer solution, the acetic acid buffer solution cannot generate a complex reaction with heavy metal ions, so that the concentration constancy of the Cd (II) is ensured.

CdCl diluted with electrolyte solution (NaAc-HAc buffer)₂Mother liquor prepared into CdCl with different concentrations₂The electrochemical determination is carried out on the solution under stirring, and the detection of Cd (II) in the solution is carried out by adopting Differential Pulse Anodic Stripping Voltammetry (DPASV), and the specific steps are as follows:

1. reduction and enrichment processes: under the condition of stirring the solution, a proper voltage is applied to the working electrode for a period of time, the Cd (II) in the solution can undergo a reduction reaction, two electrons are lost, and zero-valent Cd is converted into zero-valent Cd and deposited on the surface of the electrode. The reduction and enrichment method is generally constant potential deposition, and the experiment adopts two methods, namely i-t adopts a chronoamperometry, and the deposition potential and the deposition time are set, but a differential pulse voltammetry method is still required to set and detect the dissolution current during the dissolution experiment; the other method is to start the striping mode and set the deposition voltage and the deposition time when setting the differential pulse voltammetry DPV. The two methods have the same effect and can be selected according to different requirements of actual experiments. The selection of deposition potential during deposition generally includes overpotential deposition and underpotential deposition;

2. standing, stopping stirring after the enrichment process is finished, keeping the solution in a stable state, and setting the standing time of the solution to be 10 s;

3. and (3) oxidation and dissolution processes: is differential pulse voltammetric scanning, a forward scanning potential is applied to a working electrode, the scanning is from-1.0V to-0.5V, the amplitude: 25mV, step voltage 4mV, frequency: 25Hz, obtaining a differential pulse anode dissolution voltammogram (a current-voltage curve in the oxidation process) which is a required data graph after the scanning is finished, and then analyzing the voltage-current curve in the oxidation process.

4. Cleaning: after the dissolution experiment is finished, under the condition of stirring the solution, applying forward voltage +0.4V as the cleaning voltage of the working electrode, keeping the time for 30s, and removing the metal pollutants Cd and other impurities remained on the surface of the electrode to prepare for the next use.

5. And (3) placing the electrode under an infrared lamp or naturally drying, and placing the electrode in a vacuum drying mode to be stored inwards for later use.

In practical application, the working electrode is usually used as a detection tool in the form of a "test strip", and the

steps

4 and 5 are not required to be carried out.

And (3) analyzing an experimental result:

a. interface characterization of different working electrodes

And performing interface characterization on the bare LIG electrode and the modified Sn/LIG electrode prepared in the experiment, and performing morphology, mapping analysis, EDS analysis, high-resolution lattice fringe image (HRTEM) analysis and diffraction pattern analysis by using a Scanning Electron Microscope (SEM) and a Transmission Electron Microscope (TEM) respectively.

a1)SEM

The scanning electron microscope can clearly see the morphological characteristics of the electrode surface, which is one of the commonly used characterization means in material preparation. FIG. 2 shows the topographic profiles before and after modification of the LIG electrode, respectively, wherein the surfaces of the bare LIG electrode are shown as (A) and (B); the modified Sn/LIG electrode is shown as (C) and (D). As can be seen from (a) and (B), the surface of the bare LIG electrode is relatively smooth, and the surface structure is in a layered sheet shape, which is consistent with the graphene structure, and thus it can be seen that the laser-induced conductive graphene material electrode material is successful; it can be seen from (C) and (D) that after the tin film is electrodeposited, the existence of particles can be clearly seen on the surface of the Sn/LIG electrode, the surface morphology of the Sn/LIG electrode is changed, the surface is rougher, and compared with the bare LIG electrode before modification, the surface has uniform wrinkle protrusions, which indicates that the electrodeposited Sn film can successfully change the roughness of the electrode surface, increase the surface area of redox reaction, provide more active sites for redox reaction, and facilitate the adsorption of heavy metals.

a2)XPS

The XPS analysis is the X-ray photoelectron spectroscopy analysis, can carry out qualitative analysis and semi-quantitative analysis on the element composition and the valence state of a substance, can check the element composition, the valence and the structural information of the surface of the substance at a corresponding peak position by analyzing the XPS spectrum and contrasting a manual, and can judge the content of elements according to the height of the peak position.

Through tests, the main elements contained on the surface of the experimentally prepared LIG electrode are C and O, and the main elements contained on the surface of the prepared Sn/LIG electrode are C, O and Sn after the tin film is modified through electrodeposition. In addition, the content of Cd in the metal to be detected, which is gathered on the surface of the modified Sn/LIG electrode, is obviously increased in the enrichment process, which shows that the tin film is favorable for enhancing the reduction enrichment of the metal to be detected, and the enhancement of the dissolution peak current is influenced to a certain extent.

The valence state of tin on the surface of the modified Sn/LIG electrode is as follows: the valence state distribution of the tin film on the surface of the electrode is single, the tin film mainly contains tetravalent tin, and the Sn 3d can be known through analysis_5/2、Sn 3d_3/2The binding energies of (A) are 486.4eV and 495.9eV respectively, indicating that SnO is used₂Mainly comprises the following steps. This is because, during the electrochemical deposition process, Sn (ii) in the solution undergoes a reduction reaction to become metallic tin, which is uniformly deposited on the surface of the LIG electrode to form a tin film, but since tin is passivated at room temperature to form a dense oxide film, the stability of the electrode surface is enhanced, and SnO₂The method has a catalytic effect on heavy metal detection. It is known from a review of published literature that Cd is in SnO₂The diffusion barrier of the crystal face is small, and the migration conversion effect is better, so that the performance of the electrode is greatly improved, and the dissolution of Cd is easier, so that the modified Sn/LIG electrode has higher oxidation peak current when detecting Cd (II).

b. Electrochemical impedance analysis (EIS)

EIS is also called AC impedance spectrogram analysis, which is a particularly important characterization means for detecting the conductivity of an electrode in the electrochemical analysis and test process and is used for obtaining the electron transfer rates of different working electrodes by comparing EIS spectrograms. In the electrochemical workstation, this embodiment employs the a.c. impedance Parameters technique, setting the open circuit voltage, setting the detection frequency range from high frequency to low frequency to 0.1Hz-1000kHz, at 1mM K containing 0.5M KCl₃[Fe(CN)₆]And carrying out an electrochemical impedance detection process in the solution. Electrochemical impedance spectra of the LIG electrode and the Sn/LIG electrode before and after modification are shown in FIG. 3, the naked LIG electrode before modification has a semicircle with a larger radius (Rct ≈ 650 Ω), which indicates the electrode pair in the KCl-containing electrolyte solutionFe(CN)₆ ^3-The redox electron transfer resistance of (2) is large; compared with a bare LIG, the modified Sn/LIG electrode has a semicircle with a smaller radius (Rct ≈ 550 Ω). This indicates that the Sn film has been successfully modified on the surface of the electrode, and the modified Sn/LIG electrode has lower impedance and smaller resistance to electron transfer, which further proves that when the modified Sn/LIG electrode is used for detecting Cd (ii) in a water environment, the electrochemical signal is effectively improved, probably due to the increased conductivity of the electrode and the faster electron transfer rate.

c. Contact angle

The contact angle refers to an included angle formed by a tangent line of a tangent of an interface of the gas and the liquid drop and a tangent line of an interface of the solid and the liquid drop when the liquid drop is dropped on the surface of the solid by a needle, and the included angle is the contact angle. The size of the contact angle indicates the wettability of the liquid to the solid, namely the hydrophilicity and hydrophobicity of the surface of the solid, and the larger the contact angle is, the poorer the hydrophilicity of the surface of the material is, and the stronger the hydrophobicity is; the smaller the contact angle, the more hydrophilic and less hydrophobic the material surface. The instrument used for measuring the contact angle is a contact angle tester, and the detection result is as follows: the contact angle θ of the bare LIG electrode before modification is 83.21 °, and the contact angle is relatively large, which indicates that the LIG electrode before modification has relatively poor hydrophilicity, which affects the relatively poor wettability of the electrode when contacting with a solution, because when graphene is induced by laser, the degree of graphitization of the polyimide film is not high, and the polyimide is hydrophobic, so the hydrophilicity of the electrode surface is relatively poor. However, after the Sn/LIG electrode is modified by the electrodeposited tin film, the contact angle of the electrode becomes small, and θ is 39.28 °, which indicates that the modified Sn/LIG electrode changes the surface property of the electrode, and the hydrophilicity is stronger, which indicates that the wettability of the electrode is improved to a certain extent when the electrode is in contact with a metal solution to be measured, because the modified Sn/LIG electrode increases that the tin oxide film is a hydrophilic substance, which causes the hydrophilicity of the electrode to be enhanced.

Parameter optimization:

after the naked LIG electrode is prepared, the electrode is modified, namely a Sn film is electrodeposited, and the Sn/LIG electrode is prepared for standby application, wherein the main parameters influencing the Cd (II) detection effect of the Sn/LIG electrode in the process comprise three conditions of Sn (II) concentration, Sn (II) deposition time and Sn (II) deposition potential, so that in the optimization experiment, a controlled variable method is adopted to optimize the preparation conditions.

In the electrochemical detection process, mainly an oxidation-reduction reaction occurs, and specifically in this embodiment: in the deposition and enrichment process, reduction reaction occurs, and Cd (II) +2e^-→ Cd; cd-2e occurs mainly during dissolution^-→ Cd (II), when detecting Cd (II) by using a DPASV method, the detected result is the oxidation current in the dissolution process, so as to reflect the content of Cd (II) in the solution.

Potassium ferricyanide solution is a commonly used solution to characterize the performance of an electrode. This example was performed at 1mM K with 0.5M KCl₃[Fe(CN)₆]Cyclic Voltammetry (CV) scans were performed in the solution to compare the electrode electrochemical activity of the glassy carbon electrode, LIG electrode, and modified Sn/LIG electrode. Respectively using a bare Glassy Carbon Electrode (GCE), a bare LIG electrode and a modified Sn/LIG electrode, carrying out electrochemical detection on the same Cd (II) solution by adopting a differential pulse stripping voltammetry (DPASV), setting the deposition time during detection to be 120s, setting the deposition potential to be-1.2V, scanning the voltage to be-0.5V from-1.0V, setting the amplitude to be 25mV, setting the stepping voltage to be 4mV and the frequency to be 25Hz, obtaining a differential pulse anodic stripping voltammogram respectively after the scanning is finished, namely a current-voltage relation curve in the oxidation process, and then analyzing the voltage-current curve in the oxidation process, wherein the detection result is as follows: the oxidation peak current value of the bare Glassy Carbon Electrode (GCE) for detecting Cd (II) is minimum (about 0.5 muA), the LIG electrode is slightly higher (about 1.2 muA), and the oxidation peak current of the bare LIG electrode prepared by direct laser induction is higher (about 5 muA) than that of the glassy carbon electrode for detecting Cd (II); compared with a bare Glassy Carbon Electrode (GCE) and a bare LIG electrode, the oxidation peak current of the modified Sn/LIG electrode is obviously improved when Cd (II) is detected. Therefore, the modified Sn/LIG electrode can effectively improve the detection effect when detecting Cd (II), which indicates that the method for preparing the Sn/LIG electrode by electrodepositing tin is effectiveIn (1). The reason is that a layer of tin oxide film is arranged in the Sn/LIG electrode and can form a multi-component mixture with metal cadmium, so that the release of cadmium ions in the dissolving process is easy, and therefore, a peak current signal is obviously improved, and the Sn/LIG has better response and improved electrochemical signals to the cadmium ions.

a. Optimization of Sn (II) concentration

The modifier used in the LIG electrode in this embodiment is a Sn (ii) solution, and therefore, the concentration of the Sn (ii) solution and the deposition time of the Sn (ii) solution are two important factors that determine the content of the modifier. Acetic acid buffer solutions containing 50 μ g/L of Cd (ii) and having a pH of 4.4 were prepared, and the tendency of the oxidation peak current of Cd (ii) was investigated by a controlled variation method when the concentration of the Sn (ii) solution was 1mg/L, 2mg/L, 3mg/L, 4mg/L, 5mg/L, 6mg/L, 7mg/L, and 8mg/L, respectively. As shown in FIG. 4, the oxidation peak current gradually increased as the concentration of the electrodeposited Sn (II) solution increased from 1mg/L to 6mg/L, and the oxidation peak current did not increase any more but showed a decreasing trend as the concentration of the electrodeposited Sn (II) solution continued to increase again from 6 mg/L. This is because, as the concentration of Sn (ii) increases, the tin dioxide film electrodeposited on the surface of the LIG electrode gradually becomes thicker and thicker, and more Cd binding sites are provided, so that the oxidation peak current is increased; however, since the elution of metal Cd is hindered by the tin dioxide thin film after the treatment, the current is reduced to some extent. Therefore, the optimum deposition concentration of the Sn (II) solution is selected to be 6mg/L in terms of the magnitude of the peak current and the quality of the detection effect.

b. Optimization of Sn (II) deposition time

Similarly, an acetic acid buffer solution containing 50 μ g/L of Cd (ii) and having a pH of 4.4 was prepared, and changes in the oxidation peak current of cadmium ions when the deposition time of the Sn (ii) solution was 30s, 60s, 90s, 120s, 150s, 180s, 210s, 240s, 270s, and 300s, respectively, were examined by a controlled variable method. As shown in FIG. 5, the current of cadmium elution peak gradually increased as the deposition time of Sn (II) solution increased from 30s to 210s, and the current of oxidation peak was in a steady state with a slow decrease tendency as the deposition time of electrodeposited Sn (II) solution was further increased from 210 s. This is because the tin film thickness generated by electrodeposition continues to increase with the increase in the deposition time of the Sn (ii) solution, and Cd is hardly eluted from the thick tin film layer, so that the current is decreased to some extent. And comprehensively considering the magnitude of the peak current and the length of the electrode preparation time, and selecting the deposition time of the Sn (II) solution as 180s as the optimal deposition time of the Sn (II) solution.

c. Optimization of Sn (II) deposition potential

The deposition voltage has a great influence on the detection result of the anodic stripping voltammetry, and directly influences whether the tin film can be modified on the surface of a naked LIG electrode. According to the principle of electrochemical deposition, the embodiment adopts overpotential deposition, that is, when a certain heavy metal element is electrodeposited, the deposition potential is at least more negative than the oxidation potential of the metal, in the embodiment, the oxidation peak potential of the Sn (ii) solution to be deposited is about-0.6V, according to the principle, the deposition potential to be selected should be less than-0.6V, when the deposition potential is more positive and less than the oxidation potential, the electrode cannot be effectively modified, so that the peak current value of the dissolution peak is relatively small and the reproducibility is poor. In this example, in the preparation of an acetic acid buffer solution containing 50 μ g/L of Cd (ii) and having a pH of 4.4, the concentration of the Sn (ii) solution was 6mg/L, the deposition time of the Sn (ii) solution was 180s, and the change in the oxidation peak current of cadmium ions when the deposition potential of the Sn (ii) solution was-0.6V, -0.7V, -0.8V, -0.9V, -1.0V, -1.1V, -1.2V, -1.3V, -1.4V, and-1.5V, respectively, was examined by a controlled variable method, and the results of detection are shown in fig. 6. As can be seen from the figure, no significant peak of the stripping current appears at the deposition potential of-0.6V, which demonstrates that Sn (II) can be effectively reduced to the surface of the electrode only by overpotential deposition, and the electrode is equivalent to a bare LIG electrode. The oxidation peak current of cadmium gradually increases when the deposition potential of the Sn (II) solution is changed from-0.6V to-1.2V along with the gradual leftward movement of the deposition potential, the oxidation peak current of Cd (II) reaches the maximum value when the deposition potential is-1.2V, but the oxidation peak current of Cd (II) gradually decreases along with the further reduction of the deposition potential. This is because tin can be reduced better at less than-0.6V on the electrode surface, but after the deposition potential is lowered to-1.2V, although the deposition potential continues to decrease, the dissolution peak current of Cd (ii) tends to decrease, which is mainly because the deposition potential decreases to promote the formation of a tin film, but if the tin film thickness is too large, dissolution of metal Cd is hindered, and oxidation dissolution from a thicker tin film is difficult, thereby decreasing the peak current, so that the deposition potential of the Sn (ii) solution is selected to be-1.2V as the optimum deposition potential of the Sn (ii) solution.

d. Optimization of solution pH

The acetic acid buffer solution in the embodiment is used as an electrolyte solution, so that the ion concentration in the solution can be increased, and the ion transmission rate in the solution can be enhanced. Therefore, the pH value of the solution can greatly influence the detection effect, the pH adjusting range of the acetic acid buffer solution is 3.6 to 5.6, and the acetic acid solution and the sodium acetate solution can be added dropwise when the pH value exceeds the range. In this example, the concentration of Sn (II) solution was controlled at 6mg/L, the deposition potential and deposition time of Sn (II) solution were controlled at-1.2V and 180s, and then the change of the oxidation peak current of cadmium ion was examined when the pH of the solution containing 50. mu.g/L Cd (II) was 3.2, 3.6, 4.0, 4.4, 4.8, 5.2, 5.6 and 6.0, respectively, and the results of the measurement are shown in FIG. 7. As can be seen from the figure, when the pH of the Sn/LIG electrode is in the range of 3.2-4.4, the oxidation current peak of Cd gradually rises, and the peak potential tends to move from right to left. When the pH value of the solution is 4.4, the oxidation peak current of Cd (II) reaches the maximum value, but with the gradual increase of the pH value of the solution, the oxidation peak current of Cd (II) is in a descending trend, because when the pH value is lower, the electrode is easy to generate hydrogen evolution reaction under the peracid solution environment, so that the deposition process of cadmium ions on the surface of the electrode is influenced, the deposition amount is reduced, and the peak current during dissolution is reduced. When the pH value of the solution is too high, the dissolution current peak of cadmium is also lower, mainly because when the pH value is too high, tin and cadmium are not easy to form a mixture, the detection effect of the sensor is influenced, and the detection effect of the sensor is influenced. Therefore, the pH value of the Cd (II) solution is selected to be 4.4 as the optimal pH value of the Cd (II) solution, and when in-situ monitoring is realized, the pH value can be adjusted by setting automatic pH adjusting equipment.

e. Optimization of deposition potential at the time of detection

Similar to the optimization experiment of Sn (II) deposition potential, in this embodiment, the oxidation peak potential of the Cd (II) solution to be deposited is about-0.8V, and according to the principle, the deposition potential to be selected should be less than-0.8V, and when the deposition potential is more positive and less than the oxidation potential, Cd (II) cannot be completely reduced at the electrode, so that the peak current value of the subsequent dissolution peak is smaller. In this example, the concentration of the Sn (II) solution was 6mg/L, the deposition potential and deposition time of the Sn (II) solution were-1.2V and 180s, respectively, in an acetic acid buffer solution of pH 4.4 containing 50. mu.g/LCd (II), and the change in oxidation peak current was examined when the deposition potential of the Cd (II) solution was-0.8V, -0.9V, -1.0V, -1.1V, -1.2V, -1.3V, -1.4V and-1.5V, respectively, by the controlled variable method, and the results are shown in FIG. 8. As can be seen from FIG. 8, no significant peak of dissolution current appears when the deposition potential is-0.8V, which demonstrates that only overpotential deposition can effectively reduce Cd (II) to the electrode surface, and as the deposition potential gradually moves negatively, the oxidation peak current gradually increases when the deposition potential of Cd (II) changes from-0.8V to-1.2V, and as the deposition potential is-1.2V, the peak current reaches the maximum value, but as the deposition potential further decreases, the oxidation peak current of Cd (II) shows a gradually decreasing trend. This is mainly due to the fact that cadmium can be reduced better at the electrode surface at negative potentials. However, when the deposition potential exceeds-1.2V, hydrogen evolution reaction may be caused along with the further reduction of the deposition potential, or the excessive negative deposition potential causes some impurities in the solution to be deposited on the surface of the electrode together, so that the dissolution peak current of cadmium is gradually reduced, and negative influence is caused on detection, therefore, the deposition potential of the Cd (II) solution is selected to be-1.2V as the optimal deposition potential of the Cd (II) solution.

f. Optimization of deposition time at detection

Similar to the optimization experiment of Sn (ii) deposition potential, this example prepares an acetic acid buffer solution with pH 4.4 containing 50 μ g/L of Cd (ii), prepares an electrode under the conditions that the Sn (ii) concentration is 6mg/L, the deposition time and the deposition potential are 180s, -1.2V, and studies the change of oxidation peak current of Cd (ii) when the deposition time of Cd (ii) solution is 30s, 60s, 90s, 120s, 150s, 180s, 210s, 240s, 270s, and 300s, respectively, by controlling the pH and the deposition potential of Cd (ii) solution. As can be seen from FIG. 9, the current of the cadmium dissolution peak gradually increased as the deposition time of the Cd (II) solution increased from 30s to 300s, and the current of the oxidation peak appeared to be in a steady state and increased more slowly as the deposition time of the Sn (II) electrodeposition solution continued to increase from 210 s. The reason is that the Cd electrodeposited on the electrode gradually reaches saturation along with the extension of the deposition time of the Cd (II) solution, the deposition amount of the Cd on the electrode surface is not obviously increased any more, and the current response tends to be stable. And comprehensively considering the magnitude of the peak current and the length of the detection time, and selecting the deposition time of the Cd (II) solution as the optimal deposition time of the Cd (II) solution, wherein the deposition time of the Cd (II) solution is 210 s.

Performance evaluation:

a. linear range and detection limit

Preparing an electrode under the conditions that the concentration of Sn (II) is 6mg/L, the deposition time and the deposition potential of the concentration of Sn (II) are respectively 180s and-1.2V, selecting the pH value of an acetic acid buffer solution of Cd (II) solution to be 4.4, and setting the deposition potential and the deposition time of the Cd (II) solution to be-1.2V and 210s respectively. The detection results of the solutions of Cd (II) with different concentrations were determined by DPASV and are shown in FIG. 10. As can be seen from FIG. 10, as the concentration of Cd (II) in the acetic acid buffer solution of the Cd (II) solution continuously increases, the oxidation peak current of Cd (II) in the electrode detection also shows a linear increasing trend, and the linear working range of the Sn/LIG electrode in the detection of Cd (II) solution is 0.6-100. mu.g/L. The linear regression equation obtained by software linear fitting is as follows: ip 0.1395C_Cd(Ⅱ)+0.0684，R²The linear relationship is good when the detection limit LOD of the Sn/LIG electrode in the solution for detecting Cd (II) is 0.1 mu g/L when the detection limit LOD is 0.9978.

b. Anti-interference test of Sn/LIG electrode

The Sn/LIG electrode is expected to be used for carrying out in-situ real-time monitoring analysis on an actual water sample in a field environment. Coexisting ions inevitably exist in the actual water environment, possibly influence the detection effect of the electrode, to evaluate the interference resistance of Sn/LIG in the detection of Cd (II) solutions, under optimized experimental conditions, firstly, the DPASV detection is carried out on cadmium ions, the purpose of preparing the Sn/LIG electrode is to carry out the DPASV detection on the cadmium ions under the field environment, the in-situ real-time monitoring and analysis of the actual water sample can cause other coexisting ions in the actual water environment to possibly influence the detection effect of the electrode, to assess the interference resistance of Sn/LIG in the detection of Cd (II) solutions, under the optimized experimental conditions, firstly, the DPASV detection is carried out on cadmium ions, the detection result is shown as Cd2+ column in figure 11, then adding other common interfering ions with certain mass ratio into the solution, and carrying out DPASV detection on Cd (II). Under optimized experimental conditions, the result shows that the oxidation dissolution peak current of Cd (II) has NO obvious change after adding 200 times of K +, Na +, Mg2+, Zn2+, Ca2+, SO42-, NO3-, Cl-and Pb2 +. The Sn/LIG electrode prepared by the experiment has better anti-interference capability.

c. Reproducibility and stability of Sn/LIG electrodes

(1) Reproducibility: under the optimized experimental conditions, a solution containing 50 mu g/L of Cd (II) is selected as a detection sample, the same electrode is used for carrying out 5 times of DPASV detection on the sample, and the detection result shows that the repeatability detection results of each time are basically consistent, the relative standard deviation between the results is only 3.125 percent, which shows that the Sn/LIG electrode prepared by the experiment has better repeatability.

(2) Stability: under the optimized experimental conditions, a prepared Sn/LIG electrode is selected, DPASV detection is carried out on a Cd (II) solution containing 50 mu g/L, within 30 days, every 6 days, the electrode is used for measuring the Cd (II) solution containing 50 mu g/L again under the same conditions, and the detection result shows that the maximum relative standard deviation between two measurements is 4.28 percent, which shows that the Sn/LIG electrode prepared by the experiment has better detection stability.

d. Determination of actual water sample by Sn/LIG electrode

The practical application of the Sn/LIG electrode to the detection of Cd (II) in a water environment is verified by the concentration of Cd (II) in tap water and underground water by using the prepared Sn/LIG modified electrode through a standard addition recovery test. The method comprises the steps of accurately preparing solutions with Cd (II) concentrations of 20 mug/L, 50 mug/L and 80 mug/L by using underground water and tap water respectively, measuring the solutions by using differential pulse stripping voltammetry respectively, measuring oxidation peak current, substituting the measured current value into a calibrated current-concentration linear equation, obtaining calculated concentrations according to the current, and obtaining the content measurement results of Cd (II) in the underground water and the tap water according to a table 2.

TABLE 2 determination of actual water sample by Sn/LIG electrode

As can be seen from Table 2, the recovery rates of the added standard are close to 100%, which also proves that the Sn/LIG modified electrode prepared by the experiment has certain reliability in the application of the actual water sample. The invention can also be used for developing and utilizing a later-stage on-site water sample determination sensor, and the electrode can be used as a component unit for constructing or assembling a water body in-situ real-time monitoring device.

The products (cadmium ion electrochemical sensor working electrodes) with different tin film thicknesses are obtained by adjusting the process parameters of the preparation method, and tests show that the overall thickness of the tin film is 50 nm-1 mu m, which can basically meet the working requirements and is preferably 200 nm-300 nm; if the tin film thickness is too large, the elution of cadmium is not facilitated.

Having thus described the invention by way of illustration in general terms and by way of example only, it will be apparent to those skilled in the art that certain changes and modifications may be made thereto without departing from the scope of the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.

Claims

1. a cadmium ion electrochemical sensor working electrode, is characterized in that: comprise LIG electrode and the tin film modified on its surface; The main body of described tin film is metal tin, and the surface of metal tin is formed with tin dioxide metal oxide film; the overall thickness of the tin film is 50 nm to 1 μm.

2 . The working electrode of the cadmium ion electrochemical sensor according to claim 1 , wherein the LIG electrode is prepared by using a polyimide film as a base and etching the polyimide film by using a laser-induced technology. 3 . .

3 . The working electrode of the cadmium ion electrochemical sensor according to claim 1 , wherein the tin film is formed by immersing the LIG electrode in a Sn(II) solution by potentiostatic deposition. 4 .

4. a method for preparing the described cadmium ion electrochemical sensor working electrode of claim 1, is characterized in that, comprises the following steps:

Using the polyimide film as the substrate, the bare LIG electrode was prepared by etching the polyimide film by laser induction technology;

The bare LIG electrode is immersed in the Sn(II) solution, and the metal film is deposited on the surface of the LIG electrode by means of potentiostatic deposition to form a Sn/LIG electrode;

After the deposition, the Sn/LIG electrode was taken out for cleaning, and then placed in a vacuum anaerobic box for storage for later use.

5 . The method according to claim 4 , wherein the metal film is deposited on the surface of the LIG electrode by the method of potentiostatic deposition, wherein the concentration of the Sn(II) solution prepared is 5-7 mg/L, and the deposition The time is 150～300s, and the deposition potential is -1.0～-1.5V.

6 . The method according to claim 5 , wherein the concentration of the prepared Sn(II) solution is 6 mg/L, the deposition time is 180 s, and the deposition potential is -1.2 V. 7 .

7. a method for detecting cadmium ion content in water environment, is characterized in that, comprises the following steps:

1) Reduction and enrichment

Immerse the working electrode of the cadmium ion electrochemical sensor according to claim 1 in the water sample to be tested, stir the water sample to be tested, and apply a set deposition voltage on the working electrode by means of potentiostatic deposition for a set deposition time. ;

2) Let stand

After the reduction and enrichment process is over, stop stirring to keep the solution in a stable state;

3) Oxidative dissolution

Using the differential pulse voltammetry scanning method, a positive scanning potential is applied to the working electrode. After the scanning, the differential pulse anodic stripping voltammogram is obtained, and then the differential pulse anodic stripping voltammogram is analyzed and calculated to obtain the water to be tested. The cadmium ion content in the sample.

8. The method for detecting cadmium ion content in a water environment according to claim 7, wherein in step 1), the water sample to be tested is prepared as a NaAc-HAc buffer.

9 . The method for detecting cadmium ion content in a water environment according to claim 8 , wherein the pH of the NaAc-HAc buffer solution is 4.4. 10 .

10. The method for detecting cadmium ion content in a water environment according to claim 7, wherein in step 1), the set deposition voltage is -1.1 to -1.4V, and the set deposition time For 200 to 300s.

11. The method for detecting cadmium ion content in a water environment according to claim 10, wherein in step 1), the set deposition voltage is -1.2V, and the set deposition time is 210s.

12. The method for detecting cadmium ion content in a water environment according to claim 7, wherein in step 3), a forward scanning potential is applied on the working electrode, which is scanned from -1.0V to -0.5V, and the amplitude is : 25mV, step voltage 4mV, frequency: 25Hz.

13. The application of the cadmium ion electrochemical sensor working electrode as claimed in claim 1 in constructing or assembling a water body in-situ real-time monitoring device as a constituent unit.