Preparation method of SERS (Surface Enhanced Raman Scattering) substrate of L-Au NPs (at) ZIF-67 (at) S-Au NPs with bulk heterostructure

Abstract

The invention provides a preparation method of an SERS (Surface Enhanced Raman Scattering) substrate of L-Au NPs (at) ZIF-67 (at) S-Au NPs of a bulk heterostructure. The method adopts a three-step method, and comprises the following steps: firstly, preparing L-Au NPs and S-Au NPs solutions with different sizes by a reduction method, and modifying by adopting PVP (Polyvinyl Pyrrolidone); then cobalt nitrate hexahydrate and 2-methylimidazole are mixed and stirred in a methanol solution, centrifugation, washing and drying are carried out, and L-Au NPs (at) ZIF-67 powder is obtained; finally, dissolving the L-Au NPs (at) ZIF-67 powder in methanol, performing ultrasonic treatment on the L-Au NPs (at) ZIF-67 powder and an S-Au NPs solution, centrifuging, washing and drying to form L-Au NPs (at) ZIF-67 (at) S-Au NPs powder; and further dispersing in a methanol solution and depositing on a silicon substrate to form the SERS substrate of L-Au NPs (at) ZIF-67 (at) S-Au NPs. According to the method, an SERS substrate capable of keeping an SERS hot spot generated between L-Au NPs and S-Au NPs consistent with an enrichment detection point on a shallow surface layer of ZIF-67 is constructed by utilizing the orientation relationship between ZIF-67 and L-Au NPs and S-Au NPs, and the substrate is good in SERS signal repeatability, high in uniformity, high in sensitivity and good in structural stability and has the advantages of being simple in preparation process, low in cost and the like.

Description

Technical Field

[0001] The invention relates to a SERS substrate preparation method, in particular to a MOF-based bulk heterostructure SERS substrate preparation method, belonging to the technical field of Raman spectroscopy. Background Art

[0002] Surface enhanced Raman scattering (SERS) technology is a spectral analysis technology that uses the surface plasmon resonance effect of metal nanoparticles (MNPs) to significantly enhance the Raman scattering signal of molecules. It has the characteristics of high sensitivity and good selectivity, and can achieve accurate detection of trace substances. Traditional SERS technology is based on the local surface plasmon resonance (LSPR) generated by single metal nanoparticles (MNPs) to achieve surface Raman effect enhancement. However, traditional MNPs have poor adsorption performance, uneven dispersion and high ease of agglomeration, resulting in unstable Raman signals and poor reliability of qualitative and quantitative analysis. In recent years, metal organic frameworks (MOFs) to form MNPs / MOF heterostructures have been proven to be an effective strategy to prevent MNPs aggregation and improve the adsorption performance of SERS substrates. For MNPs-modified MOF nanocomposites, additional active sites are given to work synergistically, thereby obtaining excellent SERS performance. For example, Liu Yaqing and others invented a patent with the patent number: 201710486565.9, and the patent name is: A MOF-precious metal composite SERS substrate and its preparation method. Through electrostatic action in an alcohol solution, CTAB-modified positively charged precious metal nanoparticles are loaded on the surface of the MOF material to form a MOF-precious metal nanocomposite SERS substrate, which achieves precise control of the morphology, size and density of metal nanoparticles on the MOF surface, prepares a large number of Raman active points, and further effectively improves the SERS performance of the composite substrate. At the same time, a large number of studies have shown that the SERS performance of MNPs-modified MOF nanomaterials obviously depends on the position of MNPs relative to MOF. The MOF / MNPs nanocomposite structure exhibits high activity, but there are still MNPs electrostatically adsorbed on the outer surface of MOF, which are easily affected by the environment and fall off, thereby reducing its stability. Summary of the invention

[0003] In order to overcome the shortcomings of the prior art, the present invention provides a method for preparing a SERS substrate of L-Au NPs@ZIF-67@S-Au NPs with a bulk heterogeneous structure. The method introduces two L-Au NPs and S-Au NPs of different sizes, wherein the L-Au NPs are coated inside the ZIF-67, and the S-Au NPs are adsorbed on the surface of the ZIF-67 and are evenly distributed, and the L-Au NPs and the S-Au NPs form a "satellite" structure to improve stability. The orientation relationship between ZIF-67 and gold nanoparticles is utilized to construct a SERS substrate with a bulk heterogeneous structure in which the SERS "hot spots" generated between the L-Au NPs and the S-Au NPs and the enrichment detection points of the shallow layer of the ZIF-67 are kept consistent. The preparation process is simple and the cost is low. The prepared SERS substrate has the advantages of more SERS "hot spots", a larger specific surface area, a higher adsorption capacity, a lower detection limit, a high sensitivity, and the like.

[0004] The present invention comprises the following steps:

[0005] Step 1: Preparation of large-Au nanoparticles (L-Au NPs): First, heat and stir a chloroauric acid solution with a mass concentration of 1 part per ten thousand in an oil bath until boiling, quickly add 1 ml of a sodium citrate aqueous solution with a mass concentration of 1%, and continue heating for 15 minutes after the solution gradually turns wine red. Turn off the oil bath, and after cooling naturally to room temperature, add a PVP solution with an average molecular weight of 55,000 and a concentration of 25 mg / ml to the solution, stir, centrifuge, wash, and then disperse in a methanol solution to obtain an L-Au NPs solution.

[0006] Step 2: Preparation of small-Au nanoparticles (S-Au NPs): First, heat and stir a chloroauric acid solution with a mass concentration of 1 part per ten thousand in an oil bath until boiling, quickly add 2.5 ml of a sodium citrate aqueous solution with a mass concentration of 1%, and continue heating for 15 minutes after the solution gradually turns wine red. Turn off the oil bath, and after cooling naturally to room temperature, add a PVP solution with an average molecular weight of 55,000 and a concentration of 25 mg / ml to the solution, stir, centrifuge, wash, and then disperse in a methanol solution to obtain a L-Au NPs solution.

[0007] Step 3: Preparation of L-Au NPs@ZIF-67 powder: A methanol solution of cobalt nitrate hexahydrate and a methanol solution of 2-methylimidazole at a molar ratio of 1:6 were placed in a beaker and stirred for 6 hours, and then the L-Au NPs solution described in step 1 was added. The color of the solution changed from transparent purple to opaque blue-purple. The blue-purple product was then centrifuged, washed, and dried to obtain L-Au NPs@ZIF-67 powder;

[0008] Step 4: Preparation of L-Au NPs@ZIF-67@S-Au NPs powder: dissolve the L-Au NPs@ZIF-67 powder described in step 3 in a methanol solution to obtain a L-Au NPs@ZIF-67 methanol solution, and then add the S-Au NPs solution described in step 2 in a volume ratio of 1:1 between the L-Au@ZIF-67 methanol solution and the S-Au NPs solution, mix and ultrasonicate, and then let stand. Then, centrifuge, wash, and dry the mixed solution to obtain L-Au NPs@ZIF-67@S-Au NPs powder;

[0009] Step 5: The L-Au NPs@ZIF-67@S-Au NPs powder described in step 5 is dissolved in a methanol solution, dispersed evenly by ultrasonic oscillation, and suspended on a silicon wafer. After drying at 60 degrees Celsius for 30 minutes, a SERS substrate of L-Au NPs@ZIF-67@S-AuNPs is formed.

[0010] The large-particle nano-gold in step one has a particle size of 30 nanometers to 45 nanometers, and the small-particle nano-gold in step two has a particle size of 15 nanometers to 20 nanometers.

[0011] The methanol volume of the methanol solution of 2-methylimidazole in step 3 and step 4 is 100 ml, and the methanol volume of the methanol solution of cobalt nitrate hexahydrate is 15 ml; the centrifugal speed is 9000 rpm, and the centrifugal time is 15 minutes.

[0012] The L-Au NPs@ZIF-67 methanol solution in step 4 is 8 mg of L-Au NPs@ZIF-67 powder dissolved in 8 ml of methanol solution; the ultrasonic time is 1 hour, the standing time is 20 minutes; the centrifugal speed is 5000 rpm, and the centrifugal time is 10 minutes.

[0013] The solvent used for washing in steps 1 to 4 is a methanol solution; the drying temperature in steps 3 to 5 is 60 degrees Celsius and the drying time is 12 hours.

[0014] The beneficial effects of the present invention are:

[0015] 1. The present invention introduces two different sizes of L-Au NPs and S-Au NPs through modification, L-Au NPs are coated inside ZIF-67, S-Au NPs are adsorbed on the surface of ZIF-67 and are evenly distributed, and L-Au NPs and S-Au NPs form a similar "satellite" structure, which can produce more SERS "hot spots" through the synergistic effect of the two on the rough shallow surface of ZIF-67. The intensity of the Raman signal of the prepared SERS substrate is enhanced, and the structure has the characteristics of good stability.

[0016] 2. The SERS substrate prepared by the present invention utilizes the positional relationship between ZIF-67, L-Au NPs and S-Au NPs to construct a SERS substrate of bulk heterostructure L-Au NPs@ZIF-67@S-Au NPs, which can keep the SERS "hot spots" generated between L-Au NPs and S-Au NPs consistent with the enrichment detection points in the shallow layer of ZIF-67.

[0017] 3. The free volume in the pores of the ZIF-67 material of the SERS substrate prepared by the present invention is large, the porosity is high, the adsorption efficiency is high, and the adsorption capacity is strong, so more analytes can be enriched in the adsorption layer of ZIF-67. At the same time, the modified L-AuNPs and S-Au NPs will not change the crystal structure of ZIF-67. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the SERS substrate of bulk heterostructure L-Au NPs@ZIF-67@S-Au NPs in an embodiment of the present invention;

[0019] Figure 2 Schematic diagram of bulk heterostructure L-Au NPs@ZIF-67@S-Au NPs in an embodiment of the present invention;

[0020] Figure 3 This is a SEM image of large-particle gold nanoparticles (L-Au NPs) prepared in an embodiment of the present invention;

[0021] Figure 4 This is a SEM image of small-particle gold nanoparticles (S-Au NPs) prepared in an embodiment of the present invention;

[0022] Figure 5 This is a SEM image of the bulk heterostructure L-Au NPs@ZIF-67@S-Au NPs prepared in the examples of the present invention;

[0023] Figure 6 This is an EDS image of bulk heterostructure L-Au NPs@ZIF-67@S-Au NPs powder in an embodiment of the present invention;

[0024] Figure 7 This is a Raman spectrum of the SERS substrate labeled R6G test with different concentrations in the embodiment of the present invention;

[0025] Figure 8 is a waterfall diagram of the Raman signal uniformity of the SERS substrate in an embodiment of the present invention;

[0026] Fig. 9 In the embodiment of the present invention, the SERS substrate has a concentration of 10 -6 Raman spectra of R6G at different adsorption times of M;

[0027] Fig.10 In the embodiment of the present invention, the SERS substrate has a concentration of 10 -6 Adsorption rate diagram of R6G at different adsorption times of M.

[0028] Figure 1 Middle: 1 is the SERS substrate of bulk heterostructure L-Au NPs@ZIF-67@S-Au NPs, Figure 2 Middle: 2 is L-AuNPs, 3 is S-Au NPs, and 4 is ZIF-67. DETAILED DESCRIPTION

[0029] A method for preparing a SERS substrate of bulk heterostructured L-Au NPs@ZIF-67@S-Au NPs comprises the following steps:

[0030] Step 1: Preparation of large-Au nanoparticles (L-Au NPs): First, heat and stir a chloroauric acid solution with a mass concentration of 1 part per ten thousand in an oil bath until boiling, quickly add 1 ml of a sodium citrate aqueous solution with a mass concentration of 1%, and continue heating for 15 minutes after the solution gradually turns wine red. Turn off the oil bath, and after cooling naturally to room temperature, add a PVP solution with an average molecular weight of 55,000 and a concentration of 25 mg / ml to the solution, stir, centrifuge, wash, and then disperse in a methanol solution to obtain an L-Au NPs solution.

[0031] Step 2: Preparation of small-Au nanoparticles (S-Au NPs): First, heat and stir a chloroauric acid solution with a mass concentration of 1 part per ten thousand in an oil bath until boiling, quickly add 2.5 ml of a sodium citrate aqueous solution with a mass concentration of 1%, and continue heating for 15 minutes after the solution gradually turns wine red. Turn off the oil bath, and after cooling naturally to room temperature, add a PVP solution with an average molecular weight of 55,000 and a concentration of 25 mg / ml to the solution, stir, centrifuge, wash, and then disperse in a methanol solution to obtain a L-Au NPs solution.

[0032] Step 3: Preparation of L-Au NPs@ZIF-67 powder: A methanol solution of cobalt nitrate hexahydrate and a methanol solution of 2-methylimidazole at a molar ratio of 1:6 were placed in a beaker and stirred for 6 hours, and then the L-Au NPs solution described in step 1 was added. The color of the solution changed from transparent purple to opaque blue-purple. The blue-purple product was then centrifuged, washed, and dried to obtain L-Au NPs@ZIF-67 powder;

[0033] Step 4: Preparation of L-Au NPs@ZIF-67@S-Au NPs powder: dissolve the L-Au NPs@ZIF-67 powder described in step 3 in a methanol solution to obtain a L-Au NPs@ZIF-67 methanol solution, and then add the S-Au NPs solution described in step 2 in a volume ratio of 1:1 between the L-Au@ZIF-67 methanol solution and the S-Au NPs solution, mix and ultrasonicate, and then let stand. Then, centrifuge, wash, and dry the mixed solution to obtain L-Au NPs@ZIF-67@S-Au NPs powder;

[0034] Step 5: The L-Au NPs@ZIF-67@S-Au NPs powder described in step 5 is dissolved in a methanol solution, dispersed evenly by ultrasonic oscillation, and suspended on a silicon wafer. After drying at 60 degrees Celsius for 30 minutes, a SERS substrate of L-Au NPs@ZIF-67@S-AuNPs is formed.

[0035] The large-particle nano-gold in step one has a particle size of 30 nanometers to 45 nanometers, and the small-particle nano-gold in step two has a particle size of 15 nanometers to 20 nanometers.

[0036] The methanol volume of the methanol solution of 2-methylimidazole in step 3 and step 4 is 100 ml, and the methanol volume of the methanol solution of cobalt nitrate hexahydrate is 15 ml; the centrifugal speed is 9000 rpm, and the centrifugal time is 15 minutes.

[0037] The L-Au NPs@ZIF-67 methanol solution in step 4 is 8 mg of L-Au NPs@ZIF-67 powder dissolved in 8 ml of methanol solution; the ultrasonic time is 1 hour, the standing time is 20 minutes; the centrifugal speed is 5000 rpm, and the centrifugal time is 10 minutes.

[0038] The solvent used for washing in steps 1 to 4 is a methanol solution; the drying temperature in steps 3 to 5 is 60° C. and the drying time is 12 hours.

[0039] Example:

[0040] The present invention will be specifically described below in conjunction with specific embodiments:

[0041] Example 1: Preparation of L-Au@ZIF-67 methanol solution and S-Au NPs solution in a volume ratio of 1:1

[0042] Step 1: Preparation of large-Au nanoparticles solution (L-Au NPs): First, weigh 0.1 g of sodium citrate and completely dissolve it in 10 ml of deionized water to obtain a sodium citrate aqueous solution with a mass concentration of 1%; secondly, weigh 1 g of chloroauric acid and completely dissolve it in 100 ml of deionized water to obtain a chloroauric acid solution with a mass concentration of 1%; take 1 ml of the chloroauric acid solution with a mass concentration of 1% and add it to 99 ml of deionized water to obtain a chloroauric acid solution with a mass concentration of 1 / 10,000; then heat and stir in an oil bath until boiling, quickly add 1 ml of a sodium citrate aqueous solution with a mass concentration of 1%, and continue heating for 15 minutes after the color of the solution gradually turns wine red, turn off the oil bath, and after naturally cooling to room temperature, add 10 ml of a PVP solution with an average molecular weight of 55,000 and a concentration of 25 mg / ml to the solution, then stir at 600 rpm for 2 hours, centrifuge at 9500 rpm for 30 minutes, wash with methanol 3 times, and finally disperse in a methanol solution to obtain L-Au NPs solution; it was titrated on a silicon wafer for SEM testing, and the results were as follows Figure 3 As shown, from the SEM image of the prepared large-particle gold nanoparticles, it can be seen that the prepared L-Au NPs are spherical, with uniform particle size, ranging from 30 nm to 45 nm, and are evenly distributed;

[0043] Step 2: Preparation of small-Au nanoparticles solution (S-Au NPs): First, weigh 0.1 g of sodium citrate and completely dissolve it in 10 ml of deionized water to obtain a sodium citrate aqueous solution with a mass concentration of 1%; secondly, weigh 1 g of chloroauric acid and completely dissolve it in 100 ml of deionized water to obtain a chloroauric acid solution with a mass concentration of 1%; take 1 ml of the chloroauric acid solution with a mass concentration of 1% and add it to 99 ml of deionized water to obtain a chloroauric acid solution with a mass concentration of 1 / 10,000; then heat and stir in an oil bath until boiling, quickly add 2.5 ml of a sodium citrate aqueous solution with a mass concentration of 1%, and continue heating for 15 minutes after the color of the solution gradually turns wine red, turn off the oil bath, and after naturally cooling to room temperature, add 10 ml of a PVP solution with an average molecular weight of 55,000 and a concentration of 25 mg / ml to the solution, then stir at 600 rpm for 2 hours, centrifuge at 9500 rpm for 30 minutes, wash with methanol 3 times, and finally disperse in a methanol solution to obtain L-Au NPs solution; it was titrated on a silicon wafer for SEM testing, and the results were as follows Figure 4 As shown, from the SEM image of the prepared small-particle gold nanoparticles, it can be seen that the prepared S-Au NPs are spherical, with uniform particle size, about 15 nm to 20 nm, and are evenly distributed;

[0044] Step 3: Preparation of L-Au NPs@ZIF-67 powder: A methanol solution of cobalt nitrate hexahydrate and a methanol solution of 2-methylimidazole in a molar ratio of 1:6 were placed in a beaker and stirred, and then the L-Au NPs solution described in step 1 was added and stirred at 600 rpm for 5 minutes. The color of the solution changed from transparent purple to opaque blue-purple. The blue-purple product was then centrifuged at 9000 rpm for 15 minutes, washed twice with methanol, and then dried at 60 degrees Celsius for 12 hours to obtain L-Au NPs@ZIF-67 powder;

[0045] Step 4: Preparation of L-Au NPs@ZIF-67@S-Au NPs powder: Take 8 mg of the L-Au@ZIF-67 powder described in step 3 and dissolve it in 8 ml of methanol solution. Then take the S-Au NPs solution described in step 2, mix the L-Au NPs@ZIF-67 methanol solution and the S-Au NPs solution in a volume ratio of 1:1, and ultrasonically treat it for 1 hour, let it stand for 20 minutes, and then centrifuge the mixed solution at 5000 rpm for 10 minutes, wash it with methanol solution 3 times, and then dry it at 60 degrees Celsius for 12 hours to obtain L-Au NPs@ZIF-67@S-Au NPs powder; then perform SEM test, the results are as follows Figure 5As shown, from the SEM image of L-AuNPs@ZIF-67@S-Au NPs, it can be seen that the prepared L-Au NPs@ZIF-67@S-Au NPs are dodecahedral with good crystallinity, and the S-Au NPs attached to the surface are evenly distributed;

[0046] Step 5: Take 5 mg of L-Au NPs@ZIF-67@S-Au NPs powder in step 5 and dissolve it in 1 ml of methanol solution, and disperse it evenly by ultrasonic oscillation to form a L-Au NPs@ZIF-67@S-Au NPs methanol solution; take 1 μl of the L-Au NPs@ZIF-67@S-Au NPs methanol solution and drop it on a 3×3×1 mm silicon wafer, and dry it at 60 degrees Celsius for 20 minutes to form a SERS substrate of L-Au NPs@ZIF-67@S-Au NPs;

[0047] Step 6: Take R6G powder and mix it into 10 -10 ~10 -4 mol / L of 7 solutions with different concentrations. The SERS substrate of L-Au NPs@ZIF-67@S-Au NPs obtained in step 6 was immersed in R6G solutions of different concentrations for 20 minutes, dried at 60 degrees Celsius for 30 minutes, and then taken out for R6G labeling Raman test. The test conditions were: confocal Raman spectrometer, 785 nm laser excitation for 10 seconds, ND filter set to 25%, scanning spectral range 550-1800 cm -1 .

[0048] Figure 1 Schematic diagram of the SERS substrate of the bulk heterostructure L-Au NPs@ZIF-67@S-Au NPs in an embodiment of the present invention.

[0049] Figure 2 This is a schematic diagram of L-Au NPs@ZIF-67@S-Au NPs in an embodiment of the present invention. It can be seen that ZIF-67 wraps L-Au NPs, S-Au NPs are attached to the shallow layer of ZIF-67, and L-Au NPs and S-Au NPs form a "satellite"-like structure, which can generate more SERS "hotspots" on the rough shallow layer of ZIF-67 through the synergistic effect of the two.

[0050] from Figure 6 It can be seen from the EDS graph that the prepared L-Au NPs@ZIF-67@S-Au NPs contain elements such as Co, Au, C, N, and O. The successful synthesis of L-Au NPs@ZIF-67@S-Au NPs is confirmed by the EDS graph.

[0051] Figure 7From the Raman spectra of the SERS substrate labeled with different concentrations of R6G, it can be seen that the characteristic peak wave number of the Raman test spectrum of the R6G labeled SERS substrate of L-Au NPs@ZIF-67@S-Au NPs is 610 cm -1 , 772 cm -1 , 1360 cm -1 , 1509 cm -1 , these Raman spectral characteristic peak wave numbers completely correspond to the R6G vibration mode and belong to the R6G Raman spectral characteristic peak. At the same time, it can be seen that when the concentration is lower than 10 -10 mol / L, the characteristic peak wave number of R6G is 610 cm -1 and 1360 cm -1 There is still a strong Raman spectral signal at the prepared L-Au NPs@ZIF-67@S-Au NPs SERS substrate for R6G detection limit concentration of 10 -10 mol / L, proving that the substrate has high sensitivity.

[0052] In order to illustrate the reproducibility of the SERS signal of the substrate, the L-Au NPs@ZIF-67@S-Au NPs substrate was used to detect the concentration of 10 -4 The Raman spectrum of R6G at mol / L was collected, and the waterfall diagram of SERS signal intensity was drawn by randomly collecting 20 different test areas. Figure 8 As shown, it is calculated that at 610 cm -1 The relative standard deviation (RSD) of the typical characteristic peak intensity is about 1.66%, indicating that the SERS substrate of L-Au NPs@ZIF-67@S-Au NPs has high uniformity and excellent signal reproducibility.

[0053] Fig. 9 , Fig.10 The SERS substrate of L-Au NPs@ZIF-67@S-Au NPs in the embodiment of the present invention is 10 -6 Comparison of Raman spectra and adsorption rate results of different adsorption times of R6G at mol / L. It shows that R6G reaches a rapid adsorption equilibrium within 10 min, which may be attributed to the rapid enrichment ability of ZIF-67 on the SERS substrate of L-Au NPs@ZIF-67@S-Au NPs for target analytes, including hydrogen bonding, π-π interaction and electrostatic interaction. The Raman characteristic peak is at 610 cm -1 and 1360 cm -1The Raman intensity at shows that with the increase of immersion time, the amount of R6G adsorbed on the substrate gradually increases, resulting in a gradual increase in the stability of the Raman signal. It can be seen that R6G can reach adsorption and desorption equilibrium within 30 minutes, indicating that the SERS substrate of L-Au NPs@ZIF-67@S-Au NPs has good adsorption and high adsorption rate.

Claims

1. A method for preparing a SERS substrate of bulk heterostructured L-Au NPs@ZIF-67@S-Au NPs, the method comprising the steps of: Step 1: Preparation of large-Au nanoparticles (L-Au NPs) solution: First, heat and stir a chloroauric acid solution with a mass concentration of 1 part per 10,000 in an oil bath until boiling, and quickly add 1 ml of a sodium citrate aqueous solution with a mass concentration of 1% until the solution gradually turns wine red and continues to heat for 15 minutes. Turn off the oil bath, and after cooling naturally to room temperature, add a drop of PVP solution to the solution, stir, centrifuge, wash, and then disperse in a methanol solution to obtain a L-AuNPs solution. Step 2: Preparation of small-Au nanoparticles (S-Au NPs): First, heat and stir a chloroauric acid solution with a mass concentration of 1 part per 10,000 in an oil bath until boiling, and quickly add 2.5 ml of a sodium citrate aqueous solution with a mass concentration of 1% until the solution gradually turns wine red and continues to heat for 15 minutes. Turn off the oil bath, and after cooling naturally to room temperature, add a drop of PVP solution to the solution, stir, centrifuge, wash, and then disperse in a methanol solution to obtain a L-AuNPs solution. Step 3: Preparation of L-Au NPs@ZIF-67 powder: A methanol solution of cobalt nitrate hexahydrate and a methanol solution of 2-methylimidazole were placed in a beaker and stirred, and then the L-Au NPs solution described in step 1 was added. The color of the solution changed from transparent purple to opaque blue-purple. The blue-purple product was then centrifuged, washed, and dried to obtain L-Au NPs@ZIF-67 powder. Step 4: Preparation of L-Au NPs@ZIF-67@S-Au NPs powder: dissolve the L-Au NPs@ZIF-67 powder described in step 3 in a methanol solution to obtain a L-Au NPs@ZIF-67 methanol solution, and then add the S-Au NPs solution described in step 2, mix and ultrasonicate, and then let stand. Then, centrifuge, wash, and dry the mixed solution to obtain L-Au NPs@ZIF-67@S-Au NPs powder; Step 5: The L-Au NPs@ZIF-67@S-Au NPs powder described in step 5 is dissolved in a methanol solution, dispersed evenly by ultrasonic oscillation, and suspended on a silicon wafer. After drying at 60 degrees Celsius for 30 minutes, a SERS substrate of L-Au NPs@ZIF-67@S-Au NPs is formed; It is characterized in that: in the step 3, the molar ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:6, and the mixing and stirring time is 6 hours; in the steps 1 and 2, the average molecular weight of PVP is 55,000, and the concentration is 25 mg / ml; in the step 4, the volume ratio of L-Au NPs@ZIF-67 methanol solution to S-Au NPs solution is 1:

1.

2. The method for preparing a SERS substrate of bulk heterostructured L-Au NPs@ZIF-67@S-Au NPs according to claim 1, characterized in that: The large-particle nano-gold in step one has a particle size of 30 nanometers to 45 nanometers, and the small-particle nano-gold in step two has a particle size of 15 nanometers to 20 nanometers.

3. The method for preparing a SERS substrate of bulk heterostructured L-Au NPs@ZIF-67@S-Au NPs according to claim 1, characterized in that: The methanol volume of the methanol solution of 2-methylimidazole in step 3 and step 4 is 100 ml, and the methanol volume of the methanol solution of cobalt nitrate hexahydrate is 15 ml; the centrifugal speed is 9000 rpm, and the centrifugal time is 15 minutes.

4. The method for preparing a SERS substrate of bulk heterostructured L-Au NPs@ZIF-67@S-Au NPs according to claim 1, characterized in that: The L-Au NPs@ZIF-67 methanol solution in step 4 is 8 mg of L-Au NPs@ZIF-67 powder dissolved in 8 ml of methanol solution; the ultrasonic time is 1 hour, the standing time is 20 minutes; the centrifugal speed is 5000 rpm, and the centrifugal time is 10 minutes.

5. The method for preparing a SERS substrate of bulk heterostructured L-Au NPs@ZIF-67@S-Au NPs according to claim 1, characterized in that: The solvent used for washing in steps 1 to 4 is a methanol solution; the drying temperature in steps 3 to 5 is 60 degrees Celsius and the drying time is 12 hours.

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