CN110095520B - Working electrode for electrochemical sensor based on Cs/Ce-MOF - Google Patents
Working electrode for electrochemical sensor based on Cs/Ce-MOF Download PDFInfo
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Abstract
The invention belongs to the technical field of electrochemical sensors, and particularly relates to a preparation method of a working electrode for an electrochemical sensor based on a Cs/Ce-MOF composite material. The invention aims to solve the problems of narrow linear range, high detection limit, poor stability and slow response time of the existing electrochemical sensor for detecting tryptophan. The product is as follows: the GCE electrode consists of a GCE electrode and a Cs/Ce-MOF modified membrane wrapped outside the GCE electrode; the electrochemical sensor constructed by the working electrode has good detection performance on tryptophan. Its linear detection range is 2.5X 10‑7M–3.31×10‑4M, detection limit of 1.4 × 10‑7M。
Description
Technical Field
The invention belongs to the technical field of electrochemical sensors, and particularly relates to a working electrode for a Cs/Ce-MOF-based electrochemical sensor.
Background
Tryptophan (Trp) is one of essential amino acids for human body, is a natural nutrient, widely exists in various grains, fruits and vegetables, has various physiological and biochemical functions, and plays an important role in growth, development and metabolism of human and animals. Meanwhile, it is also a raw material for the human body to synthesize serotonin (serotonin) by itself. Because the animal body can not synthesize tryptophan and must draw from food, the establishment of an accurate, rapid and simple analysis and detection method of tryptophan has important significance for tryptophan research. Spectrophotometry, fluorescence, chromatography and capillary are common methods for tryptophan analysis, but spectrophotometry and fluorescence usually require a lot of experimental work, and the experimental process is slow; chromatography and capillary methods generally require complicated and time-consuming derivatization of tryptophan, require relatively long analysis periods, and involve the use of large amounts of solvents, which can cause environmental pollution. Electrochemical analysis has the advantages of simplicity, rapidness, low cost, sensitivity and environmental protection, and the determination of tryptophan by using an electrochemical method is widely reported. Among them, electrochemical methods, particularly enzymatic electrochemical sensors, are widely used for the measurement of tryptophan due to their characteristics of simplicity, high sensitivity, and the like. However, in the development of enzymatic electrochemical sensors, since enzymes are easily inactivated by external factors (such as temperature, humidity, pH, etc.), resulting in poor stability and reproducibility, their application is greatly limited. However, the currently developed enzyme-free electrochemical sensor often has the problems of narrow linear range, low detection limit, slow response time and the like. Therefore, in order to minimize or eliminate these disadvantages, it is very popular to prepare an enzyme-free electrochemical sensor having excellent performance to realize direct electrocatalysis of tryptophan.
Metal-Organic Frameworks (MOFs), which are Organic-inorganic hybrid materials with intramolecular pores formed by self-assembly of Organic ligands and Metal ions or clusters through coordination bonds. The MOFs material has the characteristics of typical adjustable structure, larger specific surface area and pore volume, highly ordered porous structure, functional pore space, good thermal stability, chemical stability and the like. Therefore, the large specific surface area and the catalytic activity of the MOFs material are utilized, and the response performance of the electrochemical sensor can be improved. The Ce-MOF prepared by the method can generate mixed valence Ce after partial oxidation3+/Ce4+Mixed valence Ce in Ce-MOF3+/Ce4+Can spontaneously circulate in oxidation-reduction, has enzyme-like activity and has good catalytic action on tryptophan oxidation. This is also one reason that other MOFs cannot be compared with.
The Cs/Ce-MOF is the chitosan modified Ce-MOF, the catalytic oxidation current of the Cs/Ce-MOF composite material modified electrode to tryptophan is obviously increased, and the single Cs and Ce-MOF modified electrode have catalytic action to the oxidation of tryptophan. When the two are compounded, the catalytic oxidation current of the electrode modified by the Cs/Ce-MOF composite material to tryptophan is obviously increased, and the charge transfer resistance of the electrode modified by the Cs/Ce-MOF composite material is reduced. Compared with other sensors, the sensor has wider linear range and detection limit ratio butzylcholine sensor, nanoAu/MWCNT sensor and Nafion/TiO2-graphene sensor low. Therefore, the sensor is a promising tryptophan electrochemical sensor.
Disclosure of Invention
The invention aims to solve the problems of narrow linear range, high detection limit, slow response time, expensive preparation cost and difficult operation technology of the existing electrochemical sensor for detecting tryptophan, and provides a working electrode for the electrochemical sensor based on Cs/Ce-MOF.
The working electrode for the electrochemical sensor based on the Cs/Ce-MOF is composed of a GCE electrode and a Cs/Ce-MOF modified membrane wrapped outside the GCE electrode.
The invention has the beneficial effects that:
compared with the traditional tryptophan sensor, the working electrode for the electrochemical sensor based on the Cs/Ce-MOF, which is constructed by the invention, solves a series of problems of slow response time, slow detection speed, high detection limit, complex operation, high cost and the like in tryptophan detection in the current environment, food, medicine and industry. The linear range of the tryptophan electrochemical sensor of the working electrode for the electrochemical sensor prepared on the basis of the Cs/Ce-MOF composite material prepared by the invention after various tests is 2.5 multiplied by 10-7M–3.31×10-4M, detection limit of 1.4 × 10-7M, sensitivity of 0.665. mu.A.. mu.M-1·cm-2The linear range is wide, the detection speed is high, and the detection limit is low. And the recovery rate obtained by the experiment result and calculation is in the range of 98.40-100.97%, which shows that the prepared sensor can be suitable for the detection of Trp in human serum.
Drawings
FIG. 1 is an SEM image of Ce (III) -MOF prepared in example 1 of the present invention under a scanning electron microscope;
FIG. 2 is an SEM image of Ce (III, IV) -MOF prepared in example 1 of the present invention under a scanning electron microscope;
FIG. 3 is an SEM image under a scanning electron microscope of the Cs/Ce (III, IV) -MOF prepared in example 1 of the present invention;
FIG. 4 is an XPS spectrum of Ce-MOF prepared in example 1 of the present invention;
FIG. 5 is a high resolution XPS spectrum of C1s of Ce-MOF prepared in example 1 of the present invention;
FIG. 6 is a high resolution XPS spectrum of O1s of Ce-MOF prepared in example 1 of the present invention;
FIG. 7 is a high resolution XPS spectrum of Ce3d of Ce-MOF prepared in example 1 of the present invention;
FIG. 8 is a Fourier infrared spectrum of a different material prepared in example 1 of the present invention;
FIG. 9 is a graph of chitosan concentration versus Trp peak current for Cs/Ce-MOF prepared in example 1 of the present invention;
FIG. 10 is a PXRD spectrum of various materials prepared in example 1 of the present invention;
FIG. 11 is a DPV graph of Cs/Ce-MOF modified electrode prepared in example 1 of the present invention in 0.1M PB (pH 3.0) buffer solution with different concentrations of xanthines (0,0.01,0.02,0.03,0.04 and 0.05 mM);
FIG. 12 is a DPV graph of Cs/Ce-MOF modified electrode prepared in example 1 of the present invention in 0.1M PB (pH 3.0) buffer solution, different electrodes including bare glassy carbon electrode, Cs/Ce-MOF and Cs/Ce-MOF modified electrode vs. 0.05mM tryptophan;
FIG. 13 is a graph of the current response of Cs/Ce-MOF modified electrode prepared in example 1 of the present invention in 0.1M PB (pH 3.0) buffer solution to simultaneous addition of different concentrations of DA, UA and Trp;
FIG. 14 is a graph of the current response of a Cs/Ce-MOF-modified electrode prepared in accordance with example 1 of the present invention to continuously added Trp in a 0.1M PB (pH 3.0) buffer solution (in the presence of 10. mu.M DA and UA);
FIG. 15 is a graph showing the current response of Cs/Ce-MOF modified electrode prepared in example 1 of the present invention to different concentrations of tryptophan in 0.1M PB (pH 3.0) buffer solution. Illustration is shown: a linear relation graph of oxidation peak current and tryptophan concentration under the applied potential of 0.88V (vs. Ag/AgCl);
Detailed Description
The first embodiment is as follows: the working electrode for the electrochemical sensor based on the Cs/Ce-MOF is composed of a GCE electrode and a Cs/Ce-MOF modified membrane wrapped outside the GCE electrode.
The second embodiment is as follows: the preparation method of the working electrode for the electrochemical sensor based on the Cs/Ce-MOF is carried out according to the following steps:
synthesis of ce (iii) -MOF: solution A: 1,3, 5-benzenetricarboxylic acid is dissolved in a mixed solvent of ethanol and water (V: V ═ 1: 1). Solution B: ce (NO)3)3·6H2O is dissolved in deionized water. And dropwise adding the solution B into the solution A under the condition of vigorous stirring of the solution A, and continuously stirring for 4-7 min. Centrifuging the obtained product for 5min at the rotating speed of 4000-6000rpm, washing the product for 5 times by water, and performing vacuum drying at the temperature of 60-80 ℃ to obtain white solid powder;
the molar concentration of the 3, 5-benzene tricarboxylic acid in the first step is 0.2mmol-0.5 mmol;
the volumes of the ethanol and the water in the step one are respectively VAnhydrous ethanol:VDeionized water＝1:1,1:2,1:3；
Ce (NO) in step one3)3·6H2The molar concentration of O is 0.2mmol-0.5 mmol;
the volume of the deionized water in the first step is 1-5 mL;
II, synthesis of mixed valence Ce (III, IV) -MOF: taking Ce (III) -MOF, ultrasonically dispersing the Ce (III) -MOF in distilled water, and then adding newly prepared NaOH and H2O2The mixed solution is subjected to in-situ partial oxidation. And (3) carrying out reaction for 5min, changing the suspension from white to yellow, then centrifuging for many times, washing with water until the supernatant is neutral, and drying the product in a vacuum oven at the temperature of 50-60 ℃ for 10-12 h. The product obtained after oxidation is mixed valence Ce (III, IV) -MOF.
NaOH and H in step two2O2The mixed solution is 50-75 μ L;
thirdly, synthesizing the Cs/Ce-MOF: dissolving chitosan with certain mass in acetic acid solution, and then adding Ce-MOF according to certain mass proportion to form a chitosan solution with the concentration of X mg.mL-1And (3) carrying out ultrasonic treatment on the Cs/Ce-MOF turbid liquid under the conditions that the ultrasonic frequency is 60-80 Hz and the ultrasonic time is 15-25 min, so that the turbid liquid is uniformly dispersed.
The concentration of the immobilized Ce-MOF in step three is 2mg mL-1The concentration of chitosan is 1mg mL respectively-1，2mgmL-1，3mg mL-1And 4mg mL-1；
In the third step, the volume of the solution of the acetic acid is 1mL, and the mass fraction is 1%;
fourthly, preparing a working electrode for the electrochemical sensor based on the Cs/Ce-MOF: 5-10 mu L of Cs/Ce-MOF suspension is prepared to be dripped on the surface of the polished glassy carbon electrode, and the glassy carbon electrode is dried for 5-8min in an oven at the temperature of 60-80 ℃ and then used for testing.
In the fourth step, the volume of the Cs/Ce-MOF turbid liquid is 5-10 mu L;
the following experiments and characterization were performed to verify the effects of the present invention
The first test is that the preparation method of the working electrode for the electrochemical sensor based on the Cs/Ce-MOF is carried out according to the following steps:
synthesis of ce (iii) -MOF: solution A: 1,3, 5-benzenetricarboxylic acid is dissolved in a mixed solvent of ethanol and water (V: V ═ 1: 1). Solution B: ce (NO)3)3·6H2O is dissolved in deionized water. And dropwise adding the solution B into the solution A under the condition of vigorous stirring of the solution A, and continuously stirring for 4-7 min. Centrifuging the obtained product for 5min at the rotating speed of 4000-6000rpm, washing the product for 5 times by water, and performing vacuum drying at the temperature of 60-80 ℃ to obtain white solid powder;
the molar concentration of the 1,3, 5-benzene tricarboxylic acid in the step one is 0.5 mmol;
the volumes of ethanol and water in step one are 40mL (V: V ═ 1: 1);
ce (NO) in step one3)3·6H2The molar concentration of O is 0.5 mmol;
the volume of the deionized water in the step one is 1 mL;
II, synthesis of mixed valence Ce (III, IV) -MOF: taking Ce (III) -MOF to disperse in distilled water by ultrasound, and thenPost-addition of freshly prepared NaoH and H2O2The mixed solution is subjected to in-situ partial oxidation. And (3) carrying out reaction for 5min, changing the suspension from white to yellow, then centrifuging for many times, washing with water until the supernatant is neutral, and drying the product in a vacuum oven at the temperature of 50-60 ℃ for 10-12 h. The product obtained after oxidation is mixed valence Ce (III, IV) -MOF.
NaOH and H in step two2O2The mixed solution was 75. mu.L. (specific experimental drug parameters were 9.5mL,2.5M NaOH and 0.5mL of 30 wt% H2O2)；
Thirdly, synthesizing the Cs/Ce-MOF: dissolving chitosan with certain mass in acetic acid solution, and then adding Ce-MOF according to certain mass proportion to form a solution with the concentration of X mg.mL-1And (3) carrying out ultrasonic treatment on the Cs/Ce-MOF turbid liquid under the conditions that the ultrasonic frequency is 60-80 Hz and the ultrasonic time is 15-25 min, so that the turbid liquid is uniformly dispersed.
In the third step, the mass of the chitosan is 2mg, and the concentration of the chitosan is 2 mg-mL-1；
In the third step, the volume of the solution of the acetic acid is 1mL, and the mass fraction is 1%;
fourthly, preparing a working electrode for the electrochemical sensor based on the Cs/Ce-MOF: and (3) dropwise adding 10 mu L of Cs/Ce-MOF suspension on the surface of the polished glassy carbon electrode, drying in an oven at the temperature of 60-80 ℃ for 5-8min, and then testing.
The volume of the Cs/Ce-MOF suspension in the four steps is 10 mu L;
characterization test
The morphology of the synthesized materials with different components is characterized by utilizing a Scanning Electron Microscope (SEM):
FIG. 1 is an SEM image of Ce (III) -MOF prepared by the invention under a scanning electron microscope; FIG. 2 is an SEM image of Ce (III, IV) -MOF prepared by the invention under a scanning electron microscope; FIG. 3 is an SEM image of the Cs-Ce (III, IV) -MOF prepared by the invention under a scanning electron microscope; as can be observed from FIG. 1, Ce (III) -MOF is in the shape of wheat bundle, and the length of the single wheat bundle is in the range of 4-5 μm, and the middle diameter is 1-2 μm. FIG. 2 is a scanning electron microscope image of the Ce (III, IV) -MOF obtained after partial oxidation, from which the bundle-like morphology can still be clearly seen, which shows that the morphology of the material after oxidation is not changed. Fig. 3 is a scanning electron microscope image of the composite material obtained after loading chitosan, and it can be seen from the image that the chitosan film coats the Ce (III, IV) -MOF therein to form a firm and stable composite material, so that the composite material is not easy to fall off from the surface of the electrode.
(II) an X-ray photoelectron spectrometer is utilized to characterize the Ce (III, IV) -MOF composite material obtained by the experiment, and the Ce (III, IV) -MOF composite material is proved to be oxidized3+And Ce4+Presence of (a):
as can be seen from fig. 4, the compound obtained after the partial oxidation contains C, O, Ce elements. Figure 5 shows a high resolution XPS spectrum of C1s with characteristic peaks at 284.8, 286.3 and 288.7eV assigned to C-C, C-O and C ═ O, respectively. FIG. 6 shows a high resolution XPS spectrum of O1s with characteristic peaks at 529.5, 531.5 and 533.5 assigned to Ce-O, adsorbed oxygen and C-O, respectively. FIG. 7 shows a high resolution XPS spectrum of the element Ce3d, with u and v labeled to represent spin-orbit 3d, respectively3/2And 3d5/2Wherein u is1And v1Represents Ce3+，u0，u2，u3，v0，v2And v and3represents Ce4+. From the XPS test results, it is known that MOF obtained by partial oxidation contains Ce at the same time3+And Ce4+。
And (III) characterizing the synthesized material by utilizing Fourier infrared spectroscopy:
as shown in fig. 8, the ir spectra of Ce (III) -MOF and Ce (III, IV) -MOF have similar functional group characteristics and are consistent with literature reports, indicating that Ce (III) -MOF and Ce (III, IV) -MOF contain the same chemical bond. Wherein is located at 3398cm-1The broad peak is the expansion vibration peak of the O-H bond in free water and is positioned at 1613-1554 cm-1Peaks within the range of (-COO) in the ligand-) The antisymmetric telescopic vibration peak is positioned at 1433--1Peaks within the range of (-COO) in the ligand-) 531cm of the stretching vibration peak of-1The peak at (A) is the stretching vibration peak of the Ce-O bond.
(IV) characterizing the relationship of chitosan-loaded Ce-MOF modified electrodes with different concentrations on tryptophan catalytic current:
the concentration of the immobilized Ce-MOF was 2mg mL-1The concentration of chitosan is 1mg mL respectively-1，2mg mL-1，3mg mL-1And 4mg mL-1. As shown in FIG. 9, when the chitosan concentration was 2mg mL-1The response current is maximum. This may be due to too low a concentration of chitosan resulting in too little modification, reducing the catalytic activity. The concentration is too high, so that the thickness of the modified film is too thick, and the conductivity of the electrode is influenced. Thus, the concentration was 2mg mL-1The chitosan of (a) was used to modify the electrode.
(V) the synthesized material is characterized by X-ray powder diffraction:
as shown in FIG. 10, the PXRD spectrum of the synthesized Ce (III) -MOF is consistent with the main diffraction peak alignment of the simulated spectrum, and is consistent with the reported diffraction peak position alignment in the literature, which proves that the Ce (III) -MOF is successfully prepared. The PXRD of the Ce (III, IV) -MOF generated after partial oxidation is consistent with the main diffraction peak ratio of the Ce (III) -MOF, which shows that the valence of the metal Ce is only changed after the partial oxidation, and the original structure of the metal Ce is not changed.
And (VI) testing the electrocatalytic activity of the material by using differential pulse voltammetry:
FIG. 11 shows the current response of the Cs/Ce-MOF modified electrode to different concentrations of tryptophan, and it can be seen from FIG. 11 that the response current shows linear increase with the increase of the tryptophan concentration, indicating that the Cs/Ce-MOF modified electrode has good catalytic effect on the tryptophan oxidation. FIG. 12 shows a graph comparing the current response of Cs, Ce-MOF and Cs/Ce-MOF modified electrodes to 0.05mM tryptophan. As can be observed from the figure, the Cs and the Ce-MOF modified electrode have catalytic effects on the oxidation of tryptophan. The catalytic oxidation current of the electrode modified by the Cs/Ce-MOF composite material formed after the two materials are compounded to tryptophan is obviously increased mainly due to the following reasons: mixed valence state of Ce in first, Ce-MOF3+/Ce4+Spontaneous circulation can be realized in oxidation reduction, and the tryptophan oxidase has enzyme-like activity, so that the tryptophan oxidase has good catalytic action on the tryptophan oxidation; second, due to metal organicThe porosity of the frame is favorable for increasing active sites and enriching more target detection objects to generate larger response current; thirdly, the Ce-MOF is dispersed into the Cs solution and is modified on the glassy carbon electrode, so that the aggregation of the Ce-MOF is prevented; fourthly, the Cs has good film-forming property and permeability, so that the surface of the modified electrode is not easy to fall off, and high and stable response current is obtained; fifthly, Cs has a catalytic effect on tryptophan oxidation, and the enhanced interaction of hydrogen bonds and pi-pi bonds with tryptophan enables the composite material modified electrode to have larger response current. Therefore, the catalytic oxidation effect on tryptophan is effectively improved due to the synergistic effect of the excellent properties of the Cs and the Ce-MOF.
And (seventhly), testing the selectivity and the interference resistance of the material:
as shown in FIG. 13, by adding DA, UA and Trp at different concentrations simultaneously, the catalytic oxidation current was increased uniformly at the same time. Their oxidation potentials were 0.4V, 0.55V, and 0.88V, respectively, with peak differences of 150(DA-UA) and 330mV (UA-Trp), respectively, and potential differences of greater than 100mV, indicating that the composite Cs/Ce-MOF modified electrode can detect DA, UA, and Trp simultaneously with no interference between them. In addition, in the case where DA and UA were present simultaneously, with addition of Trp at different concentrations, the oxidation peak current increased uniformly with the increase in Trp concentration, as shown in fig. 14, and this result indicates that Trp could be detected selectively even in the presence of interferents. From fig. 13 and 14, it is seen that the sensor has good selectivity for detecting Trp, and has good anti-interference performance in practical application.
(eighthly), testing the linear range, detection limit and sensitivity of the sensor by adopting a DPV method:
in 0.1M PB (pH 3.0) buffer solution, different concentrations of Trp were added, and the current response of the sensor to Trp is shown in FIG. 15, and in the range of 0.25. mu.M to 331. mu.M, the oxidation peak current linearly increases with increasing Trp concentration. The linear regression equation is as follows: i ispa(μA)＝0.594+0.047CTrp(μM)(R20.997). According to the calculation formula of the sensitivity: sensitivity is S/a, where S is the slope of the regression equation and a is the surface area of the electrode. The sensor can be obtained by calculationThe sensitivity of (A) was 0.665. mu.A. mu.M-1·cm-2. The detection limit can be calculated according to 3Sd/S, where Sd is the blank signal standard deviation (n ═ 5), and S is the slope of the linear equation, and the detection limit of the sensor can be calculated to be 0.14 μ M.
Claims (1)
1. A working electrode for a tryptophan electrochemical sensor based on Cs/Ce-MOF is characterized in that the working electrode is composed of a GCE electrode and a Cs/Ce-MOF modified membrane wrapped outside the GCE electrode; the Ce-MOF is in a wheat bundle shape, the length of a single wheat bundle is within the range of 4-5 mu m, and the middle diameter is 1-2 mu m; Ce-MOF is mixed valence Ce (III, IV) -MOF;
the preparation method of the working electrode for the tryptophan electrochemical sensor based on the Cs/Ce-MOF is completed according to the following steps:
synthesis of ce (iii) -MOF:
solution A: dissolving 1,3, 5-benzene tricarboxylic acid in a mixed solvent of ethanol and water (V: V ═ 1: 1);
solution B: ce (NO)3)3·6H2Dissolving O in deionized water;
under the condition that the solution A is stirred vigorously, the solution B is added into the solution A drop by drop, and stirring is continued for 4-7 min; centrifuging the obtained product for 5min at the rotating speed of 4000-6000rpm, washing the product for 5 times by water, and performing vacuum drying at the temperature of 60-80 ℃ to obtain white solid powder;
in the first step, the molar concentration of the 1,3, 5-benzene tricarboxylic acid is 0.2mmol-0.5 mmol;
the volume of the absolute ethyl alcohol and the volume of the water in the step one are respectively VAnhydrous ethanol:VDeionized water＝1:1,1:2,1:3；
Ce (NO) in step one3)3·6H2The molar concentration of O is 0.2mmol-0.5 mmol;
in the first step, the volume of the deionized water is 1-5 mL;
II, synthesis of mixed valence Ce (III, IV) -MOF:
taking Ce (III) -MOF, ultrasonically dispersing the Ce (III) -MOF in distilled water, and then adding newly prepared NaOH and H2O2The mixed solution is subjected to in-situ partial oxidation, and the reaction is carried out 5min, changing the white color of the suspension into yellow color, centrifuging for multiple times, washing with water until the supernatant is neutral, and drying the product in a vacuum oven at the temperature of 50-60 ℃ for 10-12 h; the product obtained after oxidation is Ce (III, IV) -MOF with mixed valence state, the shape is wheat bundle-shaped, the length of a single wheat bundle is within the range of 4-5 mu m, and the diameter of the middle part is 1-2 mu m;
NaOH and H in step two2O2The mixed solution is 50-75 μ L;
thirdly, synthesizing the Cs/Ce-MOF:
dissolving chitosan with certain mass in acetic acid solution, and then adding Ce-MOF according to certain mass proportion to form a solution with the concentration of X mg.mL-1Carrying out ultrasonic treatment on the Cs/Ce-MOF suspension under the conditions that the ultrasonic frequency is 60-80 Hz and the ultrasonic time is 15-25 min, so that the suspension is uniformly dispersed;
in the third step, the Ce-MOF is Ce (III, IV) -MOF;
the concentration of Ce-MOF in the third step is 2mg mL-1The concentration of chitosan is 2mg mL-1；
In the third step, the volume of the solution of acetic acid is 1mL, and the mass fraction is 1%;
fourthly, preparing a working electrode for the electrochemical sensor based on the Cs/Ce-MOF:
5-10 mu L of Cs/Ce-MOF turbid liquid is prepared to be dripped on the surface of the polished glassy carbon electrode, and the glassy carbon electrode is dried for 5-8min in an oven at the temperature of 60-80 ℃ and then used for testing;
in the fourth step, the volume of the Cs/Ce-MOF suspension is 5-10 mu L.
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