Preparation Method and Application of a Surface-Enhanced Raman Substrate Based on Ag@ZIF-8 Core-Shell Nanochains

Abstract

The present invention provides a preparation method and application of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanochains, belonging to the field of chemical analysis and detection. The method comprises the following steps: (1) preparing silver nanowires with a high aspect ratio by a solvothermal method; (2) in-situ synthesizing ZIF-8 on the surface of the silver nanowires to obtain bead-like Ag@ZIF-8 core-shell nanochains; (3) enriching the analyte solution on the surface of the Ag nanowires by using the adsorption and concentration effect of ZIF-8; (4) placing the above-mentioned SERS substrate adsorbed with the analyte under a Raman spectrometer for detection. The advantages of the present invention are as follows: in-situ and simultaneous detection of a variety of water-insoluble pesticides is realized. And the modification of ZIF-8 overcomes the problems of poor stability such as easy oxidation in air and agglomeration in solution of metal nanoparticles. This SERS analysis method has the advantages of high sensitivity, good stability and high accuracy.

Description

Technical Field

[0001] The present invention belongs to the technical field of Raman spectroscopy substrates, and particularly relates to the preparation and application of a SERS substrate of bead-shaped Ag@ZIF-8 core-shell structured nanowires. Background Art

[0002] Surface-enhanced Raman spectroscopy (SERS), as a rapid, non-destructive, real-time, and ultrasensitive detection method, can detect the chemical structure and molecular composition of substances, and thus shows strong application prospects in the fields of environmental monitoring, chemical analysis, food safety monitoring, life science, etc. According to the Raman enhancement mechanism, the enhancement of Raman intensity requires the target molecule to be on or near the surface of noble metals in nanostructures (<3-5 nm) and be related to the local surface plasmon resonance (LSPR) induced on the metal surface. Silver nanoparticles have a high interband transition frequency, can generate surface plasmon resonance in the entire visible range, and have a large dielectric constant, which can produce a strong SERS enhancement effect. They are currently a hot topic in SERS substrate research. However, silver nanoparticles are vulnerable to corrosion by oxygen and sulfur in the air, and the formation of a thin corrosion layer will seriously affect the optical properties of the silver structure, leading to drastic changes or even disappearance of its surface plasmon resonance characteristics. In addition, silver nanowires have poor stability, and the physical or chemical adsorption of analytes on the metal surface is difficult to control. Therefore, in order to protect the exposed silver nanowires from oxidation and improve the affinity of the substrate material for target molecules, various coated, core-shell silver-based composite nanomaterials have been continuously prepared. Existing research has coated a molecularly imprinted layer on the surface of silver microspheres to enrich and specifically adsorb target molecules on the substrate, but the SERS signal enhancement effect of this method is limited. Therefore, it is very necessary to develop advanced composite silver-based nanomaterials to achieve highly sensitive SERS detection.

[0003] Metal-organic framework materials (MOF) are a class of crystalline inorganic-organic hybrid materials composed of organic ligands and metal nanoclusters, with a high specific surface area, uniformly adjustable nanoscale cavities, and good performance in adsorbing and capturing targets. It has been found that MOF can stabilize noble metal nanoparticles and enrich analytes at positions close to the SERS detection "hot spots" and establish a local concentration of analytes. Currently, different morphologies of metal nanostructures, including nanospheres, nanorods, and core / shell nanoparticles, have been embedded in the MOF matrix to obtain highly sensitive SERS signals. Due to the synergistic effect, metal nanoparticle-MOF composites will significantly amplify the SERS signal. However, due to the protective effect of the MOF layer, the heterogeneous structure with completely buried nanoparticles is not conducive to energy collection and exchange. For example, in patent CN202010119698.4, the constructed Ag@ZIF-8@ZIF-67 nanostructure has silver nanowires completely wrapped by ZIF-8, and the size of the surface ZIF-8 is small and the crystal structure is incomplete. Summary of the Invention

[0004] Aiming at the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a preparation method and application of a surface-enhanced Raman substrate based on Ag@ZIF-8. The present invention constructs bead-shaped Ag@ZIF-8 core-shell nanowires with a semi-encapsulated structure, and through the synergistic effect of silver nanowires and ZIF-8, realizes the function of "1 + 1 > 2". The specific mechanism is as follows: Silver nanowires have a high interband transition frequency, can generate surface plasmon resonance in the entire visible range, and have a large dielectric constant, which can produce a strong SERS enhancement effect. ZIF-8 has a high specific surface area, uniformly adjustable nanostructured cavities, good performance of adsorbing and capturing targets, can enrich analytes at positions close to the SERS detection "hot spots" and establish the local concentration of analytes, improving the detection sensitivity; and can stabilize silver nanowires and improve the stability of the composite material. The Ag@ZIF-8 SERS Raman substrate has excellent properties: (1) The surface ZIF-8 nanoparticles retain the advanced porous structure available for trace target enrichment, can capture water-insoluble molecules on the surface of silver nanowires, and improve the Raman enhancement effect. (2) The modification of ZIF-8 makes the Ag nanowires have good stability and are not easily corroded and aggregated into clusters. (3) The semi-encapsulated core-shell nanowire structure is more conducive to the energy collection and exchange of nanoparticles. This Raman substrate can achieve high-precision, high-sensitivity, and multi-target detection of target substances.

[0005] The purpose of the present invention is to provide a preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanowires, and the substrate is prepared by in-situ growth of ZIF-8 on the surface of silver nanowires.

[0006] Another purpose of the present invention is to provide the application of the surface-enhanced Raman substrate obtained by the above preparation method.

[0007] The above object of the present invention is achieved by the following technical solutions:

[0008] The present invention provides a preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanowires, including the following steps:

[0009] The present invention provides a preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanowires. The metal substrate used is silver nanowires with a high aspect ratio. Porous ZIF-8 polyhedra are in-situ synthesized in a solution containing silver nanowires to form bead-shaped Ag@ZIF-8 core-shell nanowires with a semi-encapsulated structure, improving the stability of the composite material; due to the synergistic effect of the two, Ag@ZIF-8 exhibits good adsorption performance and SERS activity. The preparation method includes the following steps:

[0010] First, silver nanowires with a high aspect ratio are prepared. During the reaction process of silver nanowire synthesis, ethylene glycol decomposes into acetaldehyde under heating conditions. Acetaldehyde reduces silver ions to silver and aggregates to form nuclei. PVP binds strongly to the {100} plane of silver and weakly to the {111} plane, enabling the nuclei to grow anisotropically towards the {111} plane, ultimately forming silver nanowires. In some embodiments, the diameter of the silver nanowires is approximately 100 nm, and the aspect ratio > 400.

[0011] Utilize the property that PVP is insoluble in acetone to selectively precipitate silver nanowires. The silver nanowires after multiple precipitations and washings are vacuum-dried at 60 °C and redispersed in methanol to prepare a 10 mg / mL silver nanowire-methanol dispersion.

[0012] By using the in-situ growth method, with 2-methylimidazole and zinc nitrate as raw materials, ZIF-8 crystals are grown in-situ on the surface of silver nanowires to prepare bead-like Ag@ZIF-8 core-shell structure nanochains. This substrate has a unique semi-encapsulated structure with unique physical and chemical advantages.

[0013] In some embodiments, the volume ratio of the silver nanowire dispersion to the zinc nitrate solution is 1 - 8:100, and the particle size of the ZIF-8 encapsulated on its surface changes from 500 to 175 nm. By adjusting the volume ratio of the silver nanowire dispersion to the zinc nitrate solution in the present invention, while maintaining the good crystallinity of the ZIF-8 particles on the composite material, the effective regulation of their size can be achieved, and the ZIF-8 forms a discontinuous distribution and a semi-encapsulated core-shell structure on the surface of the silver nanowires. ZIF-8 still maintains a good crystal structure, and its size can be easily regulated. Therefore, the feature of the present invention lies in the unique structure of the synthesized product.

[0014] In some specific embodiments, as the volume ratio of the silver nanowire dispersion to the zinc nitrate solution increases, the particle size of the ZIF-8 polyhedra on the Ag@ZIF-8 core-shell nanochains gradually decreases. Moreover, as the amount of silver nanowires added increases, the SERS activity of Ag@ZIF-8 shows a trend of first increasing and then decreasing.

[0015] Preferably, the volume ratio of the silver nanowire dispersion to the zinc nitrate solution is 1:50. When the volume ratio of the silver nanowire dispersion to the volume of the zinc nitrate hexahydrate solution is 1:50, the size of the ZIF-8 polyhedra on Ag@ZIF-8 is 475 nm, and at this time, Ag@ZIF-8 has the strongest Raman enhancement effect.

[0016] The bead-like Ag@ZIF-8 core-shell structure nanochains prepared by the above method are within the protection scope of the present invention.

[0017] In addition, the application of the Ag@ZIF-8 Raman substrate obtained by the above method is also within the protection scope of the present invention.

[0018] Meanwhile, without departing from the essence, ideas, and spirit of the present invention, combinations, substitutions, and improvements made by those skilled in the art should all be within the protection scope of the present invention.

[0019] The main innovation points of the present invention are as follows:

[0020] Silver nanowires have a relatively high interband transition frequency, can generate surface plasmon resonance throughout the visible range, and have a relatively large dielectric constant, which can produce a strong SERS enhancement effect. Due to the poor stability of silver nanowires and the difficulty in controlling the adsorption of analytes on their surfaces, ZIF-8 with a high specific surface area, uniformly adjustable nanostructured cavities, and good performance in adsorbing and capturing targets is encapsulated on their surfaces, and bead-like Ag@ZIF-8 core-shell nanochains with a unique semi-encapsulated structure are designed and synthesized. Using this composite material as a SERS detection substrate to detect two target substances, parathion-methyl and carbaryl, not only has high sensitivity but also realizes the simultaneous detection of two mixed pesticides with different concentration ratios.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] The Raman substrate solves the problems of easy oxidation corrosion and poor stability of silver nanoparticles in an air environment. And the encapsulation of ZIF-8 endows it with the performance of adsorbing and capturing targets, enabling in-situ detection of target substances. In addition, at different stages of agricultural production, different types of pesticides are mixed and used, bringing many difficulties to actual detection. This Ag@ZIF-8 core-shell nanochain as a Raman substrate can realize on-site and simultaneous detection of multiple pesticide residues on the surface of crops, and shows high sensitivity and accuracy. The detection limit is more than 3 orders of magnitude lower than the national limit standard, and the detection limits for parathion-methyl and carbaryl reach 7.6×10 -9 mol / L and 5.7×10 -9 mol / L respectively. This bead-like Ag@ZIF-8 core-shell nanochain Raman SERS substrate provides a simple, sensitive, rapid, and accurate method for detecting multiple pesticide residues. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0024] Figure 1 It is a schematic diagram for preparing surface-enhanced Raman spectroscopy in Example 1

[0025] Figure 2 Scanning electron microscope image of the silver nanowires prepared in Example 1

[0026] Figure 3 Characterization diagrams of different Ag@ZIF-8 surface-enhanced Raman substrates prepared in Examples 2-5

[0027] Figure 4 Raman spectra of different Ag@ZIF-8 surface-enhanced Raman substrates prepared in Examples 2-5 after adsorbing the same amount of R6G

[0028] Figure 5 Raman spectra (A) of the surface-enhanced Raman substrate in Example 3 after adsorbing different concentrations of R6G, and the relationship diagram (B) between the relative intensity at 1511 cm -1 and the logarithm of the concentration.

[0029] Figure 6 Raman spectra of different concentrations of parathion-methyl (A) and carbaryl (A) obtained on the Raman substrate in Example 3, and the corresponding relationship diagrams (C) between the relative intensity at 1347 cm -1 and the logarithm of the concentration, and (D) at 1376 cm -1 .

[0030] Figure 7 Raman spectra of different concentration ratios of parathion-methyl and carbaryl obtained on the surface-enhanced Raman substrate in Example 3 Specific embodiments

[0031] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present disclosure. Unless otherwise specified, any technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present disclosure pertains.

[0032] As introduced in the background art, silver nanomaterials are common surface-enhanced Raman substrates, which have a high interband transition frequency, can generate surface plasmon resonance in the entire visible range, and have a large dielectric constant, so they can produce strong SERS enhancement. However, silver nanoparticles are vulnerable to corrosion by oxygen and sulfur in the air, and the formation of a thin corrosion layer will seriously affect the optical properties of the silver structure, resulting in a drastic change or disappearance of its surface plasmon resonance characteristics. In addition, the physical or chemical adsorption of analytes on the surface of silver nanoparticles is difficult to control. Therefore, in order to protect the exposed silver nanowires from oxidation and improve the affinity of the substrate material for target molecules, the silver nanowires were modified, and a semi-encapsulated bead-like Ag@ZIF-8 core-shell nanocable was developed.

[0033] In order to enable those skilled in the art to clearly understand the technical solution of the present disclosure, the following will refer to the accompanying drawings and combine with embodiments to detail the present invention.

[0034] Example 1

[0035] A preparation method of a bead-like Ag@ZIF-8 core-shell nanochain SERS substrate includes the following steps:

[0036] The preparation process of the surface-enhanced Raman substrate provided in this example is as Figure 1 shown. First, silver nanowires are prepared. Weigh 1.4 g of polyvinylpyrrolidone with a molecular weight of 13,000, vigorously stir and dissolve it in 30 mL of ethylene glycol, and add 0.06 mmol of sodium chloride. Separately, dissolve 3 mmol of silver nitrate in 50 mL of ethylene glycol. After both are completely dissolved, under magnetic stirring, the solution containing polyvinylpyrrolidone and sodium chloride is added dropwise to the silver nitrate solution. Transfer the mixture to a reaction kettle and react at 160 °C for 6 hours to obtain an off-white suspension with a silk luster. Add 20 mL of deionized water and 80 mL of acetone, stir and then let it stand for 1 hour. Suck off the upper layer solution (nano silver particles and impurities), and the bottom precipitate is the silver nanowires. Then repeat the above steps 3 times. After drying in vacuum at 60 °C, redissolve it in a methanol solution (10 mg / mL). The scanning electron microscope images of the prepared silver nanowires are as Figure 2 shown in A and B. The diameter of the prepared silver nanowires is about 100 nm, and the aspect ratio is above 400.

[0037] Example 2

[0038] By in-situ synthesis method, ZIF-8 is in-situ grown on the surface of silver nanowires to prepare bead-like Ag@ZIF-8 core-shell nanochains. First, take 0.25 mL of the silver nanowires (10 mg / mL) prepared in Example 1 in a 100 mL round-bottom flask, and sequentially add 25 mL of zinc nitrate hexahydrate (25 mmol L -1 ), 25 mL of 2-methylimidazole (25 mmol L -1 ) under magnetic stirring, and let it stand and react at room temperature for 2 hours. Centrifuge and collect the product at 5000 rpm, wash it several times with methanol, and dry it in vacuum at 60 °C for subsequent use.

[0039] The scanning electron microscope images of the obtained Ag@ZIF-8 core-shell nanochains are as Figure 3 shown in A. ZIF-8 polyhedra (650 ± 70 nm) are stacked on the Ag nanowires, and there is almost no exposed Ag nanowire.

[0040] Example 3

[0041] Take 0.50 mL of the silver nanowires (10 mg / mL) prepared in Example 1 in a 100 mL round-bottom flask, and sequentially add 25 mL of zinc nitrate hexahydrate (25 mmol L-1 ),25 mL of 2-methylimidazole (25 mmol L -1 ), and the reaction was allowed to stand at room temperature for 2 hours. The product was collected by centrifugation at 5000 rpm to obtain Ag@ZIF-8 core-shell nanochains. The product was washed several times with methanol and dried in vacuo at 60 °C for subsequent use.

[0042] The SEM image of the obtained Ag@ZIF-8 is shown as Figure 3 shown in B. The size of the ZIF-8 particles decreased (475 ± 60 nm), and the spacing on the AgNWs increased, forming a beaded and discontinuous distribution.

[0043] Example 4

[0044] Take 1.0 mL of the silver nanowires prepared in Example 1 (10 mg / mL) in a 100 mL round-bottom flask. Under magnetic stirring, 25 mL of zinc nitrate hexahydrate (25 mmol L -1 ), 25 mL of 2-methylimidazole (25 mmol L -1 ) were added successively, and the reaction was allowed to stand at room temperature for 2 hours. The product was collected by centrifugation at 5000 rpm to obtain Ag@ZIF-8 core-shell nanochains. The product was washed several times with methanol and dried in vacuo at 60 °C for subsequent use.

[0045] The SEM image of the obtained Ag@ZIF-8 is shown as Figure 3 shown in C. The size of the ZIF-8 particles on the composite material decreased to 285 ± 35 nm.

[0046] Example 5

[0047] Take 2.0 mL of the silver nanowires prepared in Example 1 (10 mg / mL) in a 100 mL round-bottom flask. Under magnetic stirring, 25 mL of zinc nitrate hexahydrate (25 mmol L -1 ), 25 mL of 2-methylimidazole (25 mmol L -1 ) were added successively, and the reaction was allowed to stand at room temperature for 2 hours. The product was collected by centrifugation at 5000 rpm to obtain Ag@ZIF-8 core-shell nanochains. The product was washed several times with methanol and dried in vacuo at 60 °C for subsequent use.

[0048] The SEM image of the obtained Ag@ZIF-8 is shown as Figure 3 shown in D. The size of the ZIF-8 particles decreased to 175 ± 20 nm and could hardly completely cover and wrap the silver nanowires.

[0049] Example 6

[0050] To compare the enhancement activities of the SERS substrates prepared in Examples 2-5, rhodamine 6G was used as the probe molecule. 1 mg of the SERS substrates (Ag@ZIF-8) obtained in Examples 2-5 was immersed in 1 mL of 10-6 mmol L -1 into the rhodamine 6G standard solution of -1 . After stirring and adsorbing for 1 hour, 20 μL of the mixed solution was transferred onto a silicon wafer for SERS detection. The test parameters were a 785 nm laser, a 50× objective lens, an integration time of 10 s, and a laser power of 5%. The obtained SERS spectra are as shown in Figure 4 and the intensity of the SERS characteristic peak at 1511 cm -1 was selected for analysis and comparison. As can be seen from the figure, the SERS substrate obtained in Example 3 has the strongest Raman enhancement effect.

[0051] To accurately calculate the SERS effect of the Ag@ZIF-8 substrate obtained in Example 3, its SERS enhancement factor was used for measurement. The SERS enhancement factor of the Ag@ZIF-8 core-shell nanocable obtained in Example 3 was estimated using the peak value with the strongest SERS effect of rhodamine 6G on the surface of Ag@ZIF-8, and its calculation formula is as shown in Formula 1:

[0052] Enhancement factor (EF) = (I SERS / I Raman ) × (N Raman / N SERS ) (1)

[0053] The calculated enhancement factor of the Ag@ZIF-8 substrate is 4.6×10 7 , indicating that the bead-like Ag@ZIF-8 substrate prepared by the in-situ growth method of the present invention has good SERS activity.

[0054] Example 7

[0055] To further prove that the Ag@ZIF-8 substrate provided in Example 3 of the present invention has excellent SERS activity, the detection limit of rhodamine 6G was studied. 1 mg of the SERS substrate (Ag@ZIF-8) obtained in Example 3 was immersed in 1 mL of rhodamine 6G standard solutions with different concentrations (10 -10 -10 -4 mol L -1 ). After stirring for 1 hour, 20 μL of the mixed solution was transferred onto a silicon wafer for SERS detection. The obtained SERS spectra of the rhodamine 6G solution on Ag@ZIF-8 are as shown in Figure 5A. As the concentration of rhodamine 6G decreases, the relative intensity of the characteristic peak gradually decreases. Figure 5 Figure 5B is a graph showing the relationship between the intensity of the characteristic peak of rhodamine 6G at 1511 cm -1 and the logarithm of the concentration. The results show that in the range of 10 -10 -10 -4 mol L -1In the range, the characteristic peak intensity has a linear relationship with the logarithm of the concentration, the correlation coefficient is 0.993, and the detection limit for rhodamine 6G is 5.76×10 -11 mol L -1 . Therefore, the bead-like Ag@ZIF-8 core-shell nanocables prepared in the present invention have excellent SERS enhancement effect.

[0056] Example 8

[0057] In order to further prove the application potential of the Ag@ZIF-8 SERS substrate provided in Example 3 of the present invention in pesticide detection. Immerse 1 mg of the SERS substrate (Ag@ZIF-8) obtained in Example 3 into 1 mL of pesticide standard solutions with different concentrations. After stirring for 1 hour, transfer 20 μL of the mixed solution onto a silicon wafer for SERS detection, and the obtained SERS spectra are as Figure 6 shown. Figure 6 A is the SERS spectrum of parathion-methyl with different concentrations on the Ag@ZIF-8 core-shell nanocables. Figure 6 B is the relationship diagram between the characteristic peak intensity of parathion-methyl at 1347 cm -1 and the logarithm of the concentration. The results show that in the range of 10 -10 -10 -4 mol L -1 , the characteristic peak intensity has a linear relationship with the logarithm of the concentration, the correlation coefficient is 0.987, and the detection limit for parathion-methyl is 7.6×10 -9 mol L -1 . Figure 6 C is the SERS spectrum of carbaryl with different concentrations on the Ag@ZIF-8 core-shell nanocables. Figure 6 D is the relationship diagram between the characteristic peak intensity of carbaryl at 1376 cm -1 and the logarithm of the concentration. The results show that in the range of 10 -10 -10 -4 mol L -1 , the characteristic peak intensity has a linear relationship with the logarithm of the concentration, the correlation coefficient is 0.990, and the detection limit for carbaryl is 5.7×10 -9 mol L -1 . Comparing the above detection limits with the maximum residue limit values of parathion-methyl drug and carbaryl stipulated at home and abroad, the detection limits of the above two pesticides are far lower than the stipulated maximum residue limit values.

[0058] Example 9

[0059] For the detection of multiple pesticide residues, it is similar to the above steps. Immerse 1 mg of the SERS substrate (Ag@ZIF-8) obtained in Example 3 into 1 mL of a mixed solution of parathion-methyl and carbaryl at different concentration ratios. After stirring for 1 hour, transfer 20 μL of the mixed solution onto a silicon wafer for SERS detection. Figure 7 Figure 7 is the SERS spectra of the mixed solution of parathion-methyl and carbaryl at different concentration ratios on the Ag@ZIF-8 core-shell nanocables. It fully demonstrates that the surface-enhanced Raman spectroscopy substrate provided by the present invention can achieve accurate and sensitive detection of parathion-methyl and carbaryl, and can simultaneously detect the two mixed pesticides.

[0060] The above description is only the preferred embodiment of the present disclosure and is not intended to limit the present disclosure. For those skilled in the art, the present disclosure can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure shall be included within the protection scope of the present disclosure.

Claims

1. A preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanochains, characterized in that, The detailed synthesis steps of the method are as follows: (1) Synthesis of silver nanowires: Polyvinylpyrrolidone was added to 30.0 mL of ethylene glycol and stirred until completely dissolved. Then sodium chloride was added to obtain solution A. Silver nitrate was added to 50.0 mL of ethylene glycol and stirred in the dark until completely dissolved to obtain solution B. Under magnetic stirring, solution A was added dropwise to solution B. The mixture was transferred to a reaction kettle with a polytetrafluoroethylene inner lining and reacted at 160 °C for 6 hours to obtain a suspension. 20 mL of ultrapure water and 80 mL of acetone were added to the suspension. After stirring, it was left standing for 1 hour to precipitate the silver nanowires to the bottom of the cup. The upper nanoparticles and impurity solution were sucked off, leaving the silver nanowire precipitate at the bottom. The above steps were repeated 3 times. After vacuum drying at 60 °C, it was redissolved in methanol at 10 mg / mL. (2) Synthesis of Ag@ZIF-8 core-shell nanochains: A certain volume of the silver nanowire methanol dispersion was taken in a 100 mL round-bottom flask. Under stirring, 25 mmol / L zinc nitrate hexahydrate methanol solution and 25 mmol / L 2-methylimidazole methanol solution were added successively. The reaction was allowed to stand at room temperature for 2 hours. The product AgNWs@ZIF-8 was collected by centrifugation at 5000 rpm, washed with methanol 3 times, and dried under vacuum at 60 °C for later use. The prepared Ag@ZIF-8 has a unique bead-like semi-encapsulated structure. Several ZIF-8 polyhedra are strung on each silver nanowire. The size of the ZIF-8 polyhedra is 175 nm to 500 nm. The silver nanowire passes through the ZIF-8 polyhedra and connects multiple ZIF-8s together to form a semi-encapsulated structure.

2. The preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanochains according to claim 1, wherein: In step (1), the molecular weight of the polyvinylpyrrolidone is 130,000; the molar mass ratio of silver nitrate, polyvinylpyrrolidone, and sodium chloride is 1:4.2:0.02; the concentration of silver nitrate in the reaction system is 0.0375 mol / L.

3. The preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanochains according to claim 1, characterized in that: In step (2), the volume ratio of the silver nanowire dispersion to the zinc nitrate hexahydrate solution is 1 - 8:100; the molar ratio of zinc nitrate hexahydrate to 2-methylimidazole is 1:

1.

4. The preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanochains according to claim 3, characterized in that, The volume ratio of the silver nanowire dispersion to the zinc nitrate hexahydrate solution is 1:

50.

5. The preparation method of a surface-enhanced Raman substrate based on Ag@ZIF-8 core-shell nanochains according to claim 1, characterized in that: The prepared silver nanowires have a uniform diameter, between 95 and 105 nm, and an aspect ratio > 400.

6. Use of the Ag@ZIF-8 core-shell nanochains prepared by the preparation method according to any one of claims 1-5 in SERS detection, characterized in that, Using the said Ag@ZIF-8 as a substrate, the rhodamine 6G solution was detected by SERS technology.

7. Application of the Ag@ZIF-8 core-shell nanochains prepared by the preparation method described in any one of claims 1 - 5 in the in-situ and simultaneous detection of multiple pesticide residues on the surface of fruits and vegetables.

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