Flexible three-dimensional MOF array SERS (Surface Enhanced Raman Scattering) substrate and preparation method

Abstract

The invention discloses a flexible three-dimensional MOF (Metal Organic Framework) array SERS (Surface Enhanced Raman Scattering) substrate and a preparation method thereof. The flexible three-dimensional MOF array SERS substrate comprises a flexible substrate, a three-dimensional MOF periodic array layer and a gold nanoparticle film which are sequentially arranged from bottom to top. The three-dimensional ordered MOF periodic array structure on the substrate can provide more attachment points for the gold nanoparticles, and high-density hot spots are formed; meanwhile, the MOF material has the advantages of high porosity, large specific surface area, adjustable aperture and the like, and can adsorb and capture a target analyte, so that to-be-detected molecules are concentrated and enriched in a hot spot region, and finally, the SERS activity is improved. The SERS substrate is a flexible substrate and can be directly attached to the surface of a complex to-be-detected object, and in-situ detection is achieved. The method has the characteristics of high sensitivity, good repeatability and wide applicability, and provides important support for rapid detection in food safety and environment monitoring.

Description

Technical Field

[0001] The invention belongs to the field of surface enhanced Raman spectroscopy (SERS) detection technology, and specifically relates to a flexible three-dimensional MOF array SERS substrate and a preparation method thereof. Background Art

[0002] Surface enhanced Raman spectroscopy (SERS) is an advanced analytical technique that can achieve rapid, sensitive and specific trace molecule detection, complex sample analysis and real-time monitoring by detecting the characteristic vibration information of target molecules. It is widely used in food safety, environmental testing, materials science, biomedicine and other fields. The enhancement mechanism of SERS is generally believed to be derived from two principles: electromagnetic field (EM) enhancement and chemical (CM) enhancement. EM enhancement comes from the excitation of localized surface plasmon resonances (LSPRs) inside plasma micro-nanostructures. Especially in the narrow gaps of some plasma structures, strong local electromagnetic field enhancement is generated by the concentration of light, which is called the "hot spot" effect; CM enhancement is usually related to the charge transfer (CT) interaction between the target analyte and the substrate. According to this principle, the design of a special substrate can strongly amplify the Raman signal of the analyte.

[0003] The sensitivity of the SERS substrate mainly depends on the number and intensity of the "hot spots" and the number of target molecules entering the "hot spots". In order to obtain high-performance SERS substrates, it is necessary to develop new structures and improve the physical or chemical adsorption properties of molecules on the substrate surface in order to simultaneously improve EM and CM, thereby improving the detection performance of the SERS substrate. Summary of the invention

[0004] Purpose of the invention: In order to solve the technical problems of the SERS substrate in the prior art, such as complex preparation process, high cost, low sensitivity, and unsuitability for in-situ testing, the present invention provides a flexible three-dimensional MOF array SERS substrate. The three-dimensional ordered MOF periodic array structure on the substrate can provide more attachment points for gold nanoparticles to form a high-density "hot spot". At the same time, the MOF material has the advantages of high porosity, large specific surface area, and adjustable pore size. It can adsorb and capture target analytes, thereby concentrating and enriching the molecules to be detected in the "hot spot" area, and ultimately improving the activity of SERS.

[0005] Technical solution: To achieve the above purpose, the technical solution adopted by the present invention is:

[0006] A flexible three-dimensional MOF array SERS substrate comprises a flexible substrate, a three-dimensional MOF periodic array layer and a gold nanoparticle film which are sequentially arranged from bottom to top.

[0007] Preferably, the flexible substrate is a PDMS flexible film with a thickness of 2-5 mm.

[0008] Preferably, the three-dimensional MOF periodic array layer comprises a base film and micro-hemispherical particles, wherein the micro-hemispherical particles are disposed on the base film and arranged in an array on the base film.

[0009] Preferably, the basement membrane and the micro-hemispherical are made of ZIF-8 nanoparticles.

[0010] Preferably, the diameter of the micro-hemisphere is 2-3 μm.

[0011] Preferably, the gold nanoparticle film is made of gold nanoparticles, and the diameter of the gold nanoparticles is 15-135 nm.

[0012] The present invention also provides a method for preparing the flexible three-dimensional MOF array SERS substrate as described above, comprising the following steps:

[0013] Step 1: Preparation of PDMS mold: Design the required three-dimensional periodic array sapphire substrate, use the three-dimensional periodic array sapphire substrate as a mold, pour polydimethylsiloxane (PDMS) on top to cover the sapphire substrate, and after it solidifies, form a concave micro-pit array on the surface of the polydimethylsiloxane (PDMS) to make a PDMS mold.

[0014] Step 2: Preparation of three-dimensional MOF periodic arrays: Add the prepared ZIF-8 colloidal solution onto the flexible substrate, cover it with a PDMS mold, place it on a heating table and heat it. After the solvent in the solution evaporates, uncover the PDMS mold to obtain the intermediate product.

[0015] Step 3: Preparation of gold nanoparticle film: The intermediate product is placed upside down on the surface of the gold nanoparticle solution to transfer the gold film to the surface of the patterned three-dimensional MOF array to obtain a flexible three-dimensional MOF array SERS substrate.

[0016] Preferably: the method for preparing the surface of the gold nanoparticle solution comprises the following steps:

[0017] Step 31, preparation of sodium citrate solution: adding sodium citrate crystals into a beaker, and then adding deionized water thereto to fully dissolve, thereby preparing a sodium citrate solution.

[0018] Step 32, preparation of tetrachloroauric acid solution: adding tetrachloroauric acid crystals into a beaker, and then adding deionized water thereto to fully dissolve the tetrachloroauric acid solution to obtain a tetrachloroauric acid solution.

[0019] Step 33, preparation of gold nanoparticle solution: reducing the gold ions of tetrachloroauric acid into gold nanoparticles by sodium citrate solution to obtain a gold nanoparticle solution.

[0020] Preferably: the method for preparing gold nanoparticles of different diameters in a gold nanoparticle solution comprises the following steps:

[0021] Step 331, add deionized water into a three-necked flat-bottomed flask, place it on a heating table, heat it to boiling, and then cool it down. Then immediately inject sodium citrate solution. Then add tetrachloroauric acid solution. After the reaction, move it to a beaker and cool it naturally to obtain a gold nanoparticle solution with a diameter of 15nm.

[0022] Step 332, taking out a portion of the gold nanoparticle solution prepared in step 331 and adding it to a three-necked flat-bottom flask, adding deionized water at the same time, and heating to boiling. Immediately inject the sodium citrate solution. Then add the tetrachloroauric acid solution. After that, add the tetrachloroauric acid solution at regular intervals, repeat n times. After the last addition of the tetrachloroauric acid solution, the reaction obtains a gold particle solution with a diameter of 25nm.

[0023] Step 333, repeat step 332, and then add tetrachloroauric acid solution every few minutes, and the number of repetitions is reduced to 6-7 times, and finally gold nanoparticles with a diameter ranging from 55nm to 135nm are obtained.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The three-dimensional MOF micro-hemispherical array structure prepared by the present invention and the gold nanoparticle film covered thereon. Compared with the two-dimensional planar structure, the three-dimensional MOF micro-hemispherical array structure provides more sites for gold nanoparticles and can form high-density "hot spots". By utilizing the localized surface plasmon coupling resonance enhancement effect between gold nanoparticles, the localized electromagnetic field on the film surface is significantly enhanced, effectively improving EM.

[0026] 2. The ZIF-8 material selected in the present invention has the advantages of high porosity, large specific surface area, and adjustable pore size, and has good ability to adsorb and capture target analytes, enabling it to significantly enrich target analytes in the "hotspot" area. The properties of MOFs can also be customized by using different metal centers and organic linkers, which can greatly improve the CT behavior between the SERS substrate and the analyte.

[0027] 3. The flexible three-dimensional MOF array SERS substrate provides more attachment sites for gold nanoparticles through a three-dimensional array structure, and the localized surface plasmon resonance effect of a large number of gold nanoparticles generates high-density "hot spots". At the same time, MOF materials have efficient pre-concentration and enrichment capabilities for target molecules. The close contact between MOF materials and gold nanoparticles increases the probability of the molecules to be tested entering the "hot spots", achieving Raman signal enhancement, thereby improving the detection sensitivity of the substrate. In addition, the flexible PDMS substrate can be adhered to complex surfaces for in-situ detection, combining high sensitivity, high adaptability and convenience, providing an innovative solution for practical applications. The combination of the two greatly improves the detection sensitivity of the SERS substrate, and the detection limit can reach 10-15 mol / L order of magnitude, with an enhancement factor as high as 3.965×10 7 , the relative standard deviation is 10.68%.

[0028] 4. The substrate used in the present invention is a transparent and flexible PDMS substrate, which can be directly pasted on the surface of a complex object to be tested in actual applications to achieve in-situ detection.

[0029] 5. The present invention has a simple structure, good flexibility, and high sensitivity. At the same time, the preparation method has low cost and high repeatability, and has great practical application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The present invention provides a preparation process of a flexible three-dimensional MOF array SERS substrate.

[0031] Figure 2 These are scanning electron microscope images of the flexible three-dimensional MOF array SERS substrate and a single ZIF-8 micro-hemisphere of this application. The left image is a scanning electron microscope image of the flexible three-dimensional MOF array SERS substrate, and the right image is a scanning electron microscope image of a single ZIF-8 micro-hemisphere.

[0032] Figure 3 A scanning electron microscope image of a single micro-hemisphere covered with a gold film.

[0033] Figure 4 This is a simulation diagram of the electric field of the surface plasmon resonance effect of gold nanoparticles.

[0034] Figure 5 This is the XRD pattern of the flexible three-dimensional MOF array SERS substrate of the present application.

[0035] Figure 6 The flexible three-dimensional MOF array SERS substrate with a diameter of 85 nm gold nanoparticles was used to adsorb 4-ATP (10 -8 -10 -15 M) after Raman spectrum.

[0036] Figure 7 1072cm -1 Linear fitting plot of Raman signal intensity and concentration of 4-ATP.

[0037] Figure 8 20 random positions (10 -8 M) SERS spectrum of 4-ATP.

[0038] Fig. 9 Flexible three-dimensional MOF array SERS substrate 1072cm -1 Raman signal intensity distribution at . DETAILED DESCRIPTION

[0039] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0040] Example 1

[0041] This embodiment provides a flexible three-dimensional MOF array SERS substrate, such as Figure 1 As shown, it includes a flexible substrate, a three-dimensional MOF periodic array layer, and a gold nanoparticle film arranged in sequence from bottom to top, and the gold nanoparticle film covers the surface of the three-dimensional MOF periodic array layer. The flexible substrate is a PDMS flexible film with a thickness of 2-5 mm.

[0042] The three-dimensional MOF periodic array layer includes a base film and micro-hemisphericals, the micro-hemisphericals are arranged on the base film, and the micro-hemisphericals are arranged in an array on the base film. The base film and the micro-hemisphericals are made of ZIF-8 nanoparticles. The diameter of the micro-hemisphericals is 2-3μm. The surface of the hemispherical particles is not completely symmetrical, especially at the top and edge, and local "hot spots" are often formed. These areas can significantly enhance the local electric field and become the key position for SERS signal enhancement. In particular, the MOF materials we use form irregular hemispherical shapes, and the "hot spots" formed are more and denser. The curvature of the hemispherical structure is large, especially near the top and bottom. Due to the sharp change in geometric morphology, this will make the electron density and local electric field on the metal surface more concentrated in these areas, thereby enhancing the Raman signal. The curvature of the hemispherical structure enables light to be better focused on the sharp parts of the particles (such as the top and edge) when it is scattered or reflected on the surface of the particle. This focusing effect increases the interaction time between light and molecules, thereby increasing the intensity of the SERS signal. When light hits the hemispherical particles, the light can be reflected and refracted multiple times inside the particles, thereby enhancing the interaction with the molecules.

[0043] The gold nanoparticle film is made of gold nanoparticles, and the diameter of the gold nanoparticles is 15-135nm.

[0044] The flexible three-dimensional MOF array SERS substrate provides more attachment sites for gold nanoparticles through the three-dimensional array structure, and the localized surface plasmon resonance effect of a large number of gold nanoparticles generates high-density "hot spots". At the same time, MOF materials have efficient pre-concentration and enrichment capabilities for target molecules. The close contact between MOF materials and gold nanoparticles increases the probability of the molecules to be tested entering the "hot spots", achieving the enhancement of Raman signals, thereby improving the detection sensitivity of the substrate. In addition, the flexible PDMS substrate can be attached to complex surfaces for in-situ detection, combining high sensitivity, high adaptability and convenience, and providing innovative solutions for practical applications. The diameter of the gold nanoparticles used on the substrate is 15-135nm.

[0045] like Figure 2-5 As shown, they are the scanning electron microscope images of the flexible three-dimensional MOF array SERS substrate and a single ZIF-8 micro-hemisphere, the scanning electron microscope image of a single micro-hemisphere covered with a gold film, the electric field simulation diagram of the surface plasmon resonance effect of gold nanoparticles, and the XRD spectrum of the flexible three-dimensional MOF array SERS substrate. Figure 6 As shown in the figure, the flexible three-dimensional MOF array SERS substrate with gold nanoparticles of 85 nm in diameter is used to adsorb 4-ATP (10 -8 -10 -15 The results show that even at very low concentrations (10 -15 M), the unique peak of 4-ATP can also be clearly identified. The surface plasmon resonance effect of gold nanoparticles significantly enhances the electric field.

[0046] The three-dimensional periodic array is an effective microstructure for constructing a large number of “hot spots”, which can provide additional attachment sites for gold nanoparticles to form abundant “hot spots” and provide a large number of binding sites for detecting analyte molecules.

[0047] Metal-organic frameworks (MOFs) are porous crystalline frameworks composed of organic ligands and metal ions. They have the advantages of high porosity, large specific surface area, and adjustable pore size. They have good ability to adsorb and capture target analytes, enabling them to significantly enrich target analytes in "hotspot" areas. The properties of MOFs can also be customized by using different metal centers and organic linkers, which can greatly improve the CT behavior between the SERS substrate and the analyte.

[0048] The three-dimensional MOF array of the flexible three-dimensional MOF array SERS substrate not only achieves efficient pre-concentration and enrichment of target molecules, but also provides more attachment sites for gold nanoparticles. Through the localized surface plasmon resonance effect of gold nanoparticles, the two work together to generate high-density "hot spots" to significantly enhance the Raman signal, further improving the detection sensitivity. The flexible PDMS substrate can be attached to complex surfaces for in-situ detection (the flexible SERS substrate can be directly attached to the rough sample surface to achieve in-situ detection), breaking through the limitations of traditional substrates in practical applications, integrating high sensitivity, high adaptability and convenience, thereby achieving convenient and rapid detection in practical applications. The combination of the three-dimensional periodic array structure and the flexible substrate not only greatly improves the detection sensitivity, but also provides direct and portable detection.

[0049] In summary, the flexible three-dimensional MOF array SERS substrate provided by the embodiment of the present invention has a simple structure, common and easily available materials, small size, easy preparation, low cost, wide application field, high detection sensitivity, and has important application prospects in food safety and environmental monitoring.

[0050] Example 2

[0051] This embodiment provides a method for preparing a flexible three-dimensional MOF array SERS substrate, which can be used to prepare the flexible three-dimensional MOF array SERS substrate provided in Example 1, comprising the following steps:

[0052] Step 1: Preparation of PDMS mold: Design the required three-dimensional periodic array sapphire substrate, use the customized three-dimensional periodic array sapphire substrate as the mold, pour polydimethylsiloxane (PDMS) on top to cover the sapphire substrate, and after it solidifies, form a concave micro-pit array on the surface of the polydimethylsiloxane (PDMS) to make a PDMS mold.

[0053] Step 2: Preparation of three-dimensional MOF periodic arrays: Add the prepared ZIF-8 colloidal solution onto the flexible substrate, cover it with a PDMS mold, place it on a heating table and heat it at 30°C. After the solvent in the solution evaporates, uncover the PDMS mold to obtain the intermediate product.

[0054] Step 3: Preparation of gold nanoparticle film: The intermediate product is placed upside down on the surface of the gold nanoparticle solution to transfer the gold film to the surface of the patterned three-dimensional MOF array to obtain a flexible three-dimensional MOF array SERS substrate.

[0055] In another embodiment, a method for preparing the surface of a gold nanoparticle solution is provided, comprising the following steps:

[0056] Step 31, preparation of sodium citrate solution: weigh 0.2 g of sodium citrate crystals and add them into a beaker, then add 19.8 mL of deionized water to fully dissolve the crystals to prepare 20 mL of 1 wt% sodium citrate solution.

[0057] Step 32, preparation of tetrachloroauric acid solution: weigh 0.0537 g of tetrachloroauric acid crystals and add them into a beaker, then add 15 mL of deionized water to fully dissolve the crystals to prepare 15 mL of 10 mM tetrachloroauric acid solution.

[0058] Step 33, preparation of gold nanoparticle solution: reducing the gold ions of tetrachloroauric acid into gold nanoparticles by sodium citrate solution to obtain a gold nanoparticle solution.

[0059] In another embodiment, a method for preparing gold nanoparticles of different diameters in a gold nanoparticle solution is provided, comprising the following steps:

[0060] Step 331, add 100 mL of deionized water into a three-necked flat-bottom flask, place it on a heating table (280°C), heat it to boiling, and then cool it to 180°C; then immediately inject 4 mL of sodium citrate solution; after 3 minutes, add 3.2 mL of tetrachloroauric acid solution; after reacting for 25 minutes, move it to a beaker and cool it naturally to obtain a gold nanoparticle solution with a diameter of 15 nm.

[0061] Step 332, take out 20 mL of the Au seed solution prepared in step 331 and add it to a three-necked flat-bottom flask (the Au seed solution can be taken out when it is not completely cooled to reduce the boiling time), and add 80 mL of deionized water at the same time, and heat to boiling; then immediately inject 2 mL of sodium citrate solution; after 3 minutes, add 0.2 mL of tetrachloroauric acid solution; thereafter, add 0.2 mL of tetrachloroauric acid solution every 8 minutes, and repeat 9 times; after the last addition of tetrachloroauric acid solution, react for 25 minutes to obtain a gold particle solution with a diameter of 25 nm.

[0062] Step 333, repeat step 332, and then add 0.2mL of tetrachloroauric acid solution every 8 minutes, and the number of repetitions is reduced to 6-7 times, and finally gold nanoparticles with a diameter ranging from 55nm to 135nm are obtained.

[0063] Example 3

[0064] 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 the air, obtain the SERS signal under the laser scanning of the surface enhanced Raman spectrometer, and output and display it in the computer.

[0065] The uniformity and repeatability of the flexible three-dimensional MOF array SERS substrate prepared in Example 2 were tested and evaluated:

[0066] Using 4-ATP as the target, the flexible three-dimensional MOF array SERS substrate was immersed in a concentration of 10 -8 , 10 -10 , 10 -12 , 10 -14 and 10 -15 The SERS response of 20 different positions on the same substrate was tested in 4-ATP solution of 100 M. Five different batches of SERS substrates were prepared according to the method of Example 2, and the SERS response of different batches of flexible three-dimensional MOF array SERS substrates was tested. The SERS test of different positions on the same substrate and the SERS test of different batches of substrates both used 633 nm laser as the light source, and the integration time was 10 s. The obtained SERS spectra are shown in the figure. Figure 8 As shown. At 1072cm -1 The Raman signal intensity distribution diagram obtained by calculating the relative standard deviation (RSD) value is as follows: Fig. 9 shown. Figure 6 The SERS response of the flexible three-dimensional MOF array SERS substrate at different concentrations. The detection concentration is 10 -8 -10 -15 mol / L, Figure 7 The SERS responses of different batches of flexible three-dimensional MOF array SERS substrates. Figure 8 , 9 The relative standard deviation of the analysis signal on the same flexible three-dimensional MOF array SERS substrate is 10.68% (n=20) for SERS responses at different positions on the same substrate. It can be seen that the flexible three-dimensional MOF array SERS substrate of the present invention has good uniformity and repeatability, and can meet the requirements of SERS quantitative analysis precision.

[0067] The flexible three-dimensional MOF array SERS substrate provided in the embodiment of the present invention provides a high-performance SERS substrate preparation method based on ZIF-8 material, which has the advantages of simple structure, high sensitivity, low cost, high repeatability, etc.

[0068] 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 principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A flexible three-dimensional MOF array SERS substrate, characterized in that: It includes a flexible substrate, a three-dimensional MOF periodic array layer, and a gold nanoparticle film which are arranged in sequence from bottom to top.

2. The flexible three-dimensional MOF array SERS substrate according to claim 1, characterized in that: The flexible substrate is a PDMS flexible film with a thickness of 2-5 mm.

3. The flexible three-dimensional MOF array SERS substrate according to claim 2, characterized in that: The three-dimensional MOF periodic array layer comprises a base film and micro-hemispherical particles, wherein the micro-hemispherical particles are arranged on the base film and are arranged in an array on the base film.

4. The flexible three-dimensional MOF array SERS substrate according to claim 3, characterized in that: The basement membrane and micro-hemispheres are made of ZIF-8 nanoparticles.

5. The flexible three-dimensional MOF array SERS substrate according to claim 4, characterized in that: The micro-hemisphere has a diameter of 2-3 μm.

6. The flexible three-dimensional MOF array SERS substrate according to claim 5, characterized in that: The gold nanoparticle film is made of gold nanoparticles, and the diameter of the gold nanoparticles is 15-135nm.

7. A method for preparing a flexible three-dimensional MOF array SERS substrate as claimed in any one of claims 4 to 6, characterized in that: The following steps are involved: Step 1: Preparation of PDMS mold: Design the required three-dimensional periodic array sapphire substrate, use the three-dimensional periodic array sapphire substrate as a mold, pour polydimethylsiloxane (PDMS) on it to cover the sapphire substrate, and after it solidifies, form a concave micro-pit array on the surface of the polydimethylsiloxane (PDMS) to make a PDMS mold; Step 2: Preparation of three-dimensional MOF periodic array: drip the prepared ZIF-8 colloidal solution on the flexible substrate, cover it with a PDMS mold, place it on a heating table and heat it. After the solvent in the solution evaporates, uncover the PDMS mold to obtain an intermediate product. Step 3: Preparation of gold nanoparticle film: The intermediate product is placed upside down on the surface of the gold nanoparticle solution to transfer the gold film to the surface of the patterned three-dimensional MOF array to obtain a flexible three-dimensional MOF array SERS substrate.

8. The preparation method according to claim 7, characterized in that: The method for preparing a gold nanoparticle solution comprises the following steps: Step 31, preparation of sodium citrate solution: adding sodium citrate crystals into a beaker, and then adding deionized water thereto to fully dissolve the sodium citrate crystals to prepare a sodium citrate solution; Step 32, preparation of tetrachloroauric acid solution: adding tetrachloroauric acid crystals into a beaker, and then adding deionized water thereto to fully dissolve the crystals to obtain a tetrachloroauric acid solution; Step 33, preparation of gold nanoparticle solution: reducing the gold ions of tetrachloroauric acid into gold nanoparticles by sodium citrate solution to obtain a gold nanoparticle solution.

9. The preparation method according to claim 8, characterized in that: The method for preparing gold nanoparticles with different diameters in a gold nanoparticle solution comprises the following steps: Step 331, add deionized water into a three-necked flat-bottom flask, place it on a heating table, heat it to boiling, and then cool it down; then immediately inject the sodium citrate solution; then add the tetrachloroauric acid solution; after the reaction, transfer it to a beaker and cool it naturally to obtain a gold nanoparticle solution with a diameter of 15 nm; Step 332, taking out a portion of the gold nanoparticle solution prepared in step 331 and adding it to a three-necked flat-bottom flask, adding deionized water at the same time, and heating to boiling; then immediately injecting the sodium citrate solution; then adding the tetrachloroauric acid solution; then adding the tetrachloroauric acid solution at regular intervals, repeating n times; after the last addition of the tetrachloroauric acid solution, reacting to obtain a gold particle solution with a diameter of 25 nm; Step 333, repeat step 332, and then add tetrachloroauric acid solution every few minutes, and the number of repetitions is reduced to 6-7 times, and finally gold nanoparticles with a diameter ranging from 55nm to 135nm are obtained.

Images