A high-performance SERS substrate based on Ag@ZIF-8 and its preparation method and application

Abstract

The application discloses an Ag@ZIF-8-based high-performance SERS substrate and a preparation method and application thereof, which comprises a substrate from bottom to top, ZIF-8 nanocrystals, a silver layer on the substrate and a silver nanocap on the ZIF-8 nanocrystals. A metal nanogap is formed between the silver layer on the substrate and the silver nanocap on the ZIF-8 nanocrystals, the electromagnetic field intensity in the gap is enhanced by using the coupling resonance enhancement effect of the metal nanogap structure, the ZIF-8 material has a rich pore structure, a great specific surface area and adsorption capacity, can adsorb the molecules to be detected and has a pre-concentration effect, the coupling resonance enhancement effect of the metal nanogap and the pre-concentration effect of the ZIF-8 material on the molecules to be detected are combined to significantly improve the activity of the SERS, the molecular detection limit (LOD) is low, the enhancement factor is high, the SERS substrate is synthesized by a two-step method, the cost is low, the SERS substrate is easy to prepare, the sensitivity is high and the SERS substrate has excellent repeatability.

Description

Technical Field

[0001] The present invention belongs to the field of surface enhanced Raman spectroscopy (SERS) detection technology, and particularly relates to a high-performance SERS substrate based on Ag@ZIF-8, a preparation method thereof, and an application thereof. Background Art

[0002] Surface-enhanced Raman spectroscopy (SERS) is a spectroscopic detection technique that reveals molecular fingerprints. It boasts rapidity, convenience, high sensitivity, and nondestructive detection. It has broad applications in fields such as food safety, environmental monitoring, pharmaceutical analysis, and public safety. SERS substrates are crucial for SERS detection, as the Raman signals of molecules are typically very weak. These signals must be enhanced and amplified through the substrate's electromagnetic enhancement mechanism (EM) and chemical enhancement mechanism (CM), enabling detection of these weak signals. EM primarily originates from the excitation and coupling of localized surface plasmon resonance (LSPR) within noble metal (such as Au or Ag) nanostructures, often referred to as plasmon "hotspots." CM is typically associated with charge transfer (CT) between the probe molecule and the substrate. Therefore, SERS sensitivity depends primarily on the structural design of the "hotspot" and the number of target molecules that enter the vicinity of the "hotspot." To achieve high-performance SERS substrates, it is necessary to develop novel plasmonic nanostructures and improve the physical or chemical adsorption properties of molecules on the substrate surface to simultaneously enhance both EM and CM, thereby improving the detection performance of the SERS substrate.

[0003] Metal-organic frameworks (MOFs) are a class of periodic network structures composed of metal cations or metal clusters and organic linkers assembled through coordination bonds. They are a new type of porous material with ultra-high specific surface area, adjustable pore structure, high crystallinity, and designable organic ligands. In addition, zeolitic imidazolate frameworks (ZIFs) are a subclass of MOF materials with zeolite or zeolite-like topologies. They have very high chemical and thermal stability, making MOF materials suitable for SERS substrates. Currently, most MOF-metal nanoparticle composites are core-shell structures, primarily utilizing the protective effect of the MOF layer on the nanoparticles. However, the complete embedding of metal nanoparticles into the MOF structure is not conducive to the energy collection and exchange of the metal nanoparticles, which greatly affects the performance of the composite SERS substrate, resulting in low sensitivity and poor reproducibility. Summary of the Invention

[0004] Technical problem to be solved: In order to overcome the deficiencies in the prior art, this application proposes a high-performance SERS substrate based on Ag@ZIF-8 and its preparation method and application, so as to solve the technical problems of complex SERS substrate preparation process, high cost and low sensitivity in the prior art, and provide a simple, easy-to-operate and high-performance technical route for constructing a ZIF-8-based nanostructure as a SERS platform for molecular detection.

[0005] Technical solution:

[0006] A high-performance SERS substrate based on Ag@ZIF-8 comprises a substrate, ZIF-8 nanocrystals, a silver layer on the substrate, and silver nanocaps on the ZIF-8 nanocrystals from bottom to top; a metal nanogap is formed between the silver layer on the substrate and the silver nanocaps on the ZIF-8 nanocrystals.

[0007] As a preferred technical solution of the present application, the ZIF-8 nanocrystals are truncated rhombic dodecahedron structures.

[0008] As a preferred technical solution of the present application, the diameter of the ZIF-8 nanocrystals with truncated rhombic dodecahedron structure is 300±20 nm.

[0009] As a preferred technical solution of the present application, the thickness of the silver layer on the substrate and the silver nanocap on the ZIF-8 nanocrystal is 130-150 nm.

[0010] As a preferred technical solution of the present application, the height of the metal nanogap formed by the silver layer on the substrate and the silver nanocaps on the ZIF-8 nanocrystals is 0-20 nm.

[0011] As a preferred technical solution of the present application, the material of the substrate is ITO.

[0012] This application also discloses a method for preparing a high-performance SERS substrate based on Ag@ZIF-8, comprising the following steps:

[0013] Step 1: Synthesis of ZIF-8 nanocrystals with truncated rhombic dodecahedral structure:

[0014] a) First, prepare a CTAB (0.4 mM) methanol solution: take 0.15 g of CTAB and add it to 100 ml of methanol solution and fully dissolve it to obtain a CTAB (0.41 mM) methanol solution;

[0015] b) Weighing Zn(NO₃)₂·6H₂O (0.8128 g) and 2-methylimidazole (2.233 g), then adding 10 mL of CTAB (0.41 mM) methanol solution to the 2-methylimidazole and stirring to dissolve, then adding 10 mL of methanol to the Zn(NO₃)₂·6H₂O and stirring to dissolve;

[0016] c) mixing the two solutions, stirring with a glass rod for 5 minutes, and allowing to stand at room temperature for 3 hours;

[0017] d) centrifuging to obtain a ZIF-8 precipitate, and re-dispersing and washing the ZIF-8 precipitate with methanol, repeating 2-3 times, and adding methanol solution to the obtained ZIF-8 precipitate to prepare a 50 mg / ml ZIF-8 methanol dispersion, i.e., a ZIF-8 nanocrystal solution;

[0018] Step 2: Clean the substrate using a plasma cleaner;

[0019] Step 3: Spin-coat the ZIF-8 nanocrystal solution on the substrate using a spin coater at room temperature;

[0020] Step 4: Use a sputtering machine to sputter a silver layer on the substrate and a silver nanocap on the ZIF-8 nanocrystals.

[0021] This application also discloses the application of the above-mentioned Ag@ZIF-8-based high-performance SERS substrate Raman scattering spectrum detection, which includes the following steps:

[0022] Step 1: Soak the Ag@ZIF-8-based high-performance SERS substrate in the test solution for 30 minutes, then take it out and dry it in air;

[0023] Step 2: Perform Raman scattering spectrum test on the dried Ag@ZIF-8-based high-performance SERS substrate.

[0024] Beneficial effects:

[0025] 1. The Ag@ZIF-8-based nanogap structure prepared in the present invention utilizes its localized surface plasmon coupling resonance enhancement effect to significantly enhance the local electromagnetic field within the nanogap, effectively improving EM and obtaining more hotspots.

[0026] 2. At the same time, the ZIF-8 material selected in the present invention has excellent stability, rich pore structure, large specific surface area, and strong molecular adsorption capacity. It can adsorb the molecules to be tested on the substrate and has a pre-concentration effect on the molecules to be tested, thereby improving CM.

[0027] 3. Combination of the two, the detection sensitivity of the SERS substrate is greatly improved, and the detection limit can reach 10 - 9 mol / L order of magnitude, the enhancement factor is as high as 2.84*10 8 , and the relative standard deviation is 6.99%.

[0028] 4. The application has the advantages of simple structure, small size, high sensitivity, low cost and high repeatability. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The preparation process of the high-performance SERS substrate based on Ag@ZIF-8 provided in the application is shown in the figure;

[0030] Figure 2 The 45° scanning electron microscope images of the high-performance SERS substrate based on Ag@ZIF-8 and a single Ag@ZIF-8 structure of the application are shown in the figure, wherein the left image is the 45° scanning electron microscope image of the high-performance SERS substrate based on Ag@ZIF-8, and the right image is the 45° scanning electron microscope image of the single Ag@ZIF-8 structure.

[0031] Figure 3 The XRD pattern of Ag@ZIF-8 of the application is shown in the figure.

[0032] Figure 4 The nitrogen adsorption graph of ZIF-8 of the application is shown in the figure.

[0033] Figure 5 The Raman spectrum of 4-aminothiophenol (4-ATP) absorbed on the ITO substrate and the single dispersed layer of ZIF-8 spin-coated on the substrate is shown in the figure.

[0034] Figure 6 The experimental analysis graph of the SERS performance of Ag@ZIF-8 of the application and the scanning electron microscope images of Ag@ZIF-8 structures with different thicknesses of Ag layer are shown in the figure, wherein a is the enhancement factor of the SERS substrate with different nanogap sizes, b-d are the SEM images of Ag@ZIF-8 with 60nm nanogap, 10nm nanogap and no nanogap, respectively.

[0035] Figure 7 The SERS spectrum of the application is shown in the figure, wherein a is the SERS spectrum of 4-ATP adsorbed on Ag@ZIF-8, b is the SERS spectrum from different samples of Ag@ZIF-8, c is the SERS spectrum of 10 -4 M 4-ATP on 25 random points on the Ag@ZIF-8 substrate, and d is the intensity distribution graph corresponding to 1090cm -1 of c.

[0036] Explanation of the accompanying figures: 1: substrate; 2: ZIF-8 nanocrystal; 3-1: silver layer on the substrate; 3-2: silver nanocap on the ZIF-8 nanocrystal; 4: metal nanogap. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present invention will be described clearly and completely below, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0038] Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0039] Example 1:

[0040] like Figure 1 As shown, a high-performance SERS substrate based on Ag@ZIF-8 includes a substrate 1, ZIF-8 nanocrystals 2, a silver layer 3-1 on the substrate, and silver nanocaps 3-2 on the ZIF-8 nanocrystals from bottom to top: a metal nanogap 4 is formed between the silver layer 3-1 on the substrate and the silver nanocaps 3-2 on the ZIF-8 nanocrystals.

[0041] The material used in the ZIF-8 nanocrystals 2 is a truncated rhombic dodecahedron structure; the material of the substrate 1 is selected from ITO.

[0042] In the Ag@ZIF-8 high-performance SERS substrate, the nanogap structure formed by the silver layer 3-1 on the substrate and the silver nanocaps on the ZIF-8 nanocrystals significantly enhances the local electric field within the nanogap through the plasmon coupling resonance effect. The ZIF-8 nanocrystals adsorb the target molecules, achieving pre-concentration. This combination significantly improves the sensitivity of the SERS substrate.

[0043] The thickness of the silver layer 3-1 on the substrate and the silver nanocap 3-2 on the ZIF-8 nanocrystal is 140 nm; the particle size of the ZIF-8 nanocrystal is 300 nm, and the metal nanogap 4 of the prepared Ag@ZIF-8 structure is 10 nm;

[0044] Based on the high-performance Ag@ZIF-8 SERS substrate, the ZIF-8 nanocrystals used are a novel porous material with an ultra-high surface area, adjustable pore structure, high crystallinity, and designable organic ligands, resulting in exceptionally high chemical and thermal stability. The ZIF-8 substrate's powerful molecular adsorption capacity allows it to adsorb target molecules onto the substrate, creating a pre-concentration effect for these molecules.

[0045] like Figure 4The nitrogen adsorption isotherm of ZIF-8 is a typical type I nitrogen adsorption-desorption isotherm. The amount of nitrogen adsorbed increases dramatically at lower relative pressures, demonstrating the material's microporous and adsorption properties.

[0046] like Figure 5 As shown in the figure, the Raman spectra of ZIF-8 and ITO after adsorption of 4-ATP were studied. After adsorption of 4-ATP, ZIF-8 exhibited a Raman peak that matched the probe molecule 4-ATP, showing a certain Raman enhancement effect, while the ITO substrate showed no Raman enhancement. The results indicate that the porosity and adsorption properties of ZIF-8 particles pre-concentrate the target molecules, which increases the sensitivity of SERS.

[0047] Figure 6 The SERS enhancement factor (EF) for different silver layer thicknesses is described. It can be seen that as the thickness of the Ag layer increases, the EF gradually increases. When the Ag layer thickness reaches a certain value, the EF drops sharply. This indicates that the plasma coupling resonance effect of the metal nanogap increases as the metal nanogap decreases. When the metal nanogap disappears, the coupling resonance effect disappears as well. The metal nanogap has a good coupling effect below 10 nanometers. The corresponding SEM images prove that the metal nanogap provided by the case of the present invention can be controlled and is low-cost.

[0048] In summary, the high-performance SERS substrate based on Ag@ZIF-8 provided by the embodiments of the present invention has a simple structure, common and readily available materials, a small size, is easy to prepare, has low cost, has a wide range of applications, and has high detection sensitivity, and has important application prospects in food safety and environmental monitoring.

[0049] Example 2:

[0050] A method for preparing a high-performance SERS substrate based on Ag@ZIF-8, which can be used to prepare the high-performance SERS substrate based on Ag@ZIF-8 provided in Example 1, comprises the following steps:

[0051] Step 1: Preparation of ZIF-8 nanocrystals with truncated rhombic dodecahedron structure:

[0052] a) First, 0.15 g of CTAB was added to 100 ml of methanol solution and fully dissolved to obtain a CTAB (0.41 mM) methanol solution;

[0053] b) Weigh Zn(NO₃)₂·6H₂O (0.8128 g) and 2-methylimidazole (2.233 g). Then, add 10 mL of CTAB (0.41 mM) methanol solution to the 2-methylimidazole and stir to dissolve. Then, add 10 mL of methanol to the Zn(NO₃)₂·6H₂O and stir to dissolve.

[0054] c) mixing the two solutions, stirring with a glass rod for 5 minutes, and allowing to stand at room temperature for 3 hours;

[0055] d) centrifuging to obtain a ZIF-8 precipitate, and re-dispersing and washing the ZIF-8 precipitate with methanol, repeating 2-3 times, and adding methanol solution to the obtained ZIF-8 precipitate to prepare a 50 mg / ml ZIF-8 methanol dispersion, i.e., a ZIF-8 nanocrystal solution;

[0056] Step 2: Preparation of Ag@ZIF-8 SERS substrate

[0057] Step 1: using a plasma cleaning machine to clean the substrate layer 1, wherein the material of the substrate layer 1 is ITO;

[0058] Step 2: Spin-coat the ZIF-8 nanocrystal 2 solution on the substrate layer 1 at room temperature using a spin coater;

[0059] Step 3, using a sputtering machine to sputter the silver layer 3-1 on the substrate and the silver nanocap 3-2 on the ZIF-8 nanocrystal on the substrate 1 and the ZIF-8 nanocrystal 2;

[0060] Specifically, the size of the ZIF-8 nanocrystal is 300 nm, the thickness of the silver layer on the substrate and the silver nanocap on the ZIF-8 nanocrystal is 140 nm, and the metal nanogap 4 of the prepared Ag@ZIF-8 structure is 10 nm.

[0061] Example 3:

[0062] The testing principle of the surface enhanced Raman spectroscopy of the present invention is to immerse the SERS substrate in the sample solution to be tested for 30 minutes and then dry it in air. The SERS signal is obtained under the laser scanning of the surface enhanced Raman spectrometer and output and displayed in the computer.

[0063] The uniformity and repeatability of the Ag@ZIF-8 high-performance SERS substrate prepared in Example 2 were tested and evaluated:

[0064] Using 4-ATP as the target, the Ag@ZIF-8SERS substrate was immersed in a solution with a concentration of 10 -4 , 10 -5 , 10 -6 , 10 -7 , 10 -8 and 10 -9The SERS response of 25 different positions on the same substrate was tested in 4-ATP solution of 1 M. Five different batches of SERS substrates were prepared according to the method of Example 2, and the SERS response of different batches of Ag@ZIF-8 SERS substrates was tested. The SERS test of different positions on the same substrate and the SERS test of different batches of substrates were both performed using a 633 nm laser as the light source with an integration time of 10 s. The obtained SERS spectra are shown in Figure 2. Figure 7 As shown. -1 The relative standard deviation (RSD) value was calculated by taking the characteristic peak intensity at as reference. Figure 7 The results show that Figure 7 a in the figure is the SERS response of Ag@ZIF-8SERS substrate at different concentrations. -4 ~10 -9 mol / L, b is the SERS response of different batches of Ag@ZIF-8 SERS substrates, c and d are the SERS responses at different locations on the same substrate, and the relative standard deviation of the analytical signal on the same Ag@ZIF-8 SERS substrate is 6.99% (n=25). This shows that the Ag@ZIF-8 SERS substrate of the present invention has good uniformity and repeatability, which can meet the requirements of SERS quantitative analysis precision.

[0065] The embodiment of the present invention provides a high-performance SERS substrate based on Ag@ZIF-8, and provides a method for preparing a high-performance SERS substrate based on ZIF-8 material, which has the advantages of simple structure, high sensitivity, low cost, and high repeatability.

[0066] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A high-performance SERS substrate based on Ag@ZIF-8, characterized by: The high-performance SERS substrate based on Ag@ZIF-8 comprises a substrate (1), ZIF-8 nanocrystals (2) on the substrate, and a silver layer covering the substrate and the ZIF-8 nanocrystals; wherein the silver layer forms a continuous silver layer (3-1) in the substrate region and forms a silver nanocap (3-2) on the top of the ZIF-8 nanocrystals; a metal nanogap (4) is formed between the silver layer (3-1) on the substrate and the silver nanocap (3-2) on the ZIF-8 nanocrystals, and the height of the metal nanogap (4) formed by the silver layer (3-1) on the substrate and the silver nanocap (3-2) on the ZIF-8 nanocrystals is ≤20nm.

2. A high-performance SERS substrate based on Ag@ZIF-8 according to claim 1, characterized in that: The ZIF-8 nanocrystal (2) has a truncated rhombic dodecahedron structure.

3. The high-performance SERS substrate based on Ag@ZIF-8 according to claim 2, characterized in that: The diameter of the ZIF-8 nanocrystal with a truncated rhombic dodecahedron structure is 300±20 nm.

4. The high-performance SERS substrate based on Ag@ZIF-8 according to claim 1, characterized in that: The thickness of the silver layer (3-1) on the substrate and the silver nanocap (3-2) on the ZIF-8 nanocrystal is 130-150 nm.

5. The high-performance SERS substrate based on Ag@ZIF-8 according to claim 1, characterized in that: The material of the substrate (1) is ITO.

6. A method for preparing a high-performance SERS substrate based on Ag@ZIF-8 according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: Step 1: Synthesis of ZIF-8 nanocrystals with truncated rhombic dodecahedral structure: First, prepare a CTAB (0.4 mM) methanol solution: take 0.15 g of CTAB and add it to 100 ml of methanol solution to fully dissolve it to obtain a CTAB (0.41 mM) methanol solution; Weigh Zn(NO3)2·6H2O (0.8128 g) and 2-methylimidazole (2.233 g). Then, take 10 mL of CTAB (0.41 mM) methanol solution and add it to 2-methylimidazole and stir to dissolve. Then, take 10 mL of methanol and add it to Zn(NO3)2·6H2O and stir to dissolve. The two solutions were mixed, stirred with a glass rod for 5 minutes, and allowed to stand at room temperature for 3 hours; The ZIF-8 precipitate was obtained by centrifugation and re-dispersed and washed with methanol. After repeating 2-3 times, the obtained ZIF-8 precipitate was added with methanol solution to prepare a 50 mg / ml ZIF-8 methanol dispersion, i.e., a ZIF-8 nanocrystal solution. Step 2: Cleaning the substrate using a plasma cleaner (1); Step 3: Spin-coat the ZIF-8 nanocrystal (2) solution on the substrate (1) using a spin coater at room temperature; Step 4: using a sputtering machine to sputter a silver layer (3-1) on the substrate and a silver nanocap (3-2) on the ZIF-8 nanocrystal on the substrate (1) and the ZIF-8 nanocrystal (2).

7. A use of the high-performance SERS substrate based on Ag@ZIF-8 according to any one of claims 1 to 5 in Raman scattering spectrum detection, characterized in that: The application comprises the following steps: Step 1: Soak the Ag@ZIF-8-based high-performance SERS substrate in the test solution for 30 min, then take it out and dry it in air; Step 2: Perform Raman scattering spectrum test on the dried Ag@ZIF-8-based high-performance SERS substrate.

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