SERS (Surface Enhanced Raman Scattering) substrate constructed based on micro-droplet technology and construction method of SERS substrate

Abstract

The invention discloses an SERS (Surface Enhanced Raman Scattering) substrate constructed based on a micro-droplet technology and a construction method thereof.The construction method comprises the steps that a chloroauric acid solution and a sodium borohydride solution are prepared, and then the chloroauric acid solution and the sodium borohydride solution are mixed according to the volume ratio of 1: (1-4) to form a gold nanoparticle reduction solution; uniformly spraying the gold nanoparticle reducing solution on the surface of the substrate for a certain time by adopting a micro-droplet device to form the substrate with gold nanoparticles distributed on the surface; and washing the substrate with an ethanol solution, and blow-drying with inert gas to prepare the SERS substrate. According to the method, the SERS substrate can be simply and rapidly prepared under the normal temperature condition by adopting a mode of combining a gold nano reducing solution formed according to a specific ratio with a micro-droplet technology, and the relatively good SERS enhancement activity of the SERS substrate is ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface enhanced Raman scattering (SERS) substrate construction, and particularly relates to an SERS substrate constructed based on microdroplet technology and a construction method thereof. Background Art

[0002] Surface enhanced Raman scattering (SERS) mainly includes two enhancement mechanisms, namely electromagnetic enhancement (EM) and chemical enhancement (CM). The enhancement effect of SERS mainly depends on EM, which is specifically mainly manifested as the local surface plasmon resonance (LSPR) characteristics and lightning rod effect of noble metal nanostructures. The EM enhancement mechanism is that incident light excites the collective oscillation of free electrons on the surface of nanoparticles, generating local electric field enhancement (up to 10 6 -10 10 times) in the particle gaps ("hot spots"). The CM enhancement mechanism is that the adsorbed molecules undergo charge transfer with the metal surface, changing the molecular polarizability. Gold nanoparticles are regarded as ideal SERS substrate materials due to their excellent chemical stability and tunable optical properties, and have important application values in the fields of analytical chemistry, surface and interface chemistry, and life science.

[0003] Traditional preparation methods mainly include high-temperature liquid-phase reduction method and seed growth method, etc. The high-temperature liquid-phase reduction method requires using sodium citrate to reduce chloroauric acid at 80-100°C, and has problems such as high energy consumption and wide particle size distribution. Although the seed growth method can control the morphology, it involves multiple steps of reaction, the process is complex and cumbersome, and the yield is low. In recent years, room-temperature synthesis strategies have made certain progress, but there are still key defects: (1) It is difficult to achieve morphology control; (2) Some biological reducing agents (such as plant extracts) have complex components and are prone to cause particle aggregation; (3) The prepared particles are difficult to uniformly self-assemble on the substrate. These problems result in weak and uneven SERS enhancement signals and poor stability. (4) The existing technologies generally rely on complex temperature control and cumbersome preparation processes, the synthesized nanoparticles have uneven and large particle sizes, and limited by the nano-size effect, the coverage of the SERS enhancement signal is relatively low.

[0004] Therefore, how to provide a new method to simply and quickly prepare an SERS substrate under room-temperature conditions and ensure its good SERS enhancement activity is the research direction required by the present invention. Summary of the Invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a SERS substrate constructed based on microdroplet technology and a method for constructing the same. By combining the formation of a gold nanoparticle reduction solution with a specific ratio and microdroplet technology, a SERS substrate can be simply and rapidly prepared under normal temperature conditions, and its good SERS enhancement activity can be ensured.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a method for constructing a SERS substrate based on microdroplet technology, comprising the following steps:

[0007] A. Preparation of gold nanoparticle reduction solution: Prepare a chloroauric acid (HAuCl4) solution and a sodium borohydride (NaBH4) solution, and then mix the chloroauric acid solution and the sodium borohydride solution according to a volume ratio of 1:(1-4) to form a gold nanoparticle reduction solution;

[0008] B. Microdroplet spraying: Use a microdroplet device to evenly spray the gold nanoparticle reduction solution in step A on the surface of the substrate for a certain period of time to form a substrate with gold nanoparticles distributed on the surface; The present invention is based on the chemical reduction principle combined with SERS technology. Through a chemical reducing agent, Au in chloroauric acid 3+ is reduced to metallic Au 0 , forming nanoparticles at the nanoscale. The key reaction is as follows:

[0009] AuCl4 - +3e - →Au 0 +4Cl - ;

[0010] C. Cleaning of the SERS substrate: Rinse the substrate prepared in step B with an ethanol solution and dry it with an inert gas to make a SERS substrate.

[0011] Furthermore, both steps A and B are carried out under normal temperature conditions.

[0012] Furthermore, the microdroplet device includes a microinjector and a microinjection pump. The microinjector is used to suck the gold nanoparticle reduction solution and spray it onto the surface of the substrate through a connecting capillary, and the microinjection pump provides the spraying power for the microinjector.

[0013] Furthermore, the flow rate of the microinjection pump for spraying the gold nanoparticle reduction solution is controlled at 30 μL / Min, the sheath gas is an inert gas, the air pressure is 60 psi, and the distance between the capillary and the surface of the substrate is 1.5 cm.

[0014] Furthermore, the chloroauric acid solution is made by mixing chloroauric acid and ultrapure water to form a 1 mmol chloroauric acid solution; the sodium borohydride solution is made by mixing sodium borohydride and ultrapure water to form a 1 mmol sodium borohydride solution.

[0015] Further, the inert gas is nitrogen (N2).

[0016] Further, the substrate is a glass slide or a silicon wafer.

[0017] For the SERS substrate prepared by the above method for constructing an SERS substrate, gold nanoparticles are uniformly distributed on the surface of the SERS substrate, and the particle size of the gold nanoparticles is between 3 and 5 nm, presenting a granular shape.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. The present invention forms a gold nanoparticle reducing solution with a specific ratio and sprays it onto the surface of the substrate by using the microdroplet technology to make an SERS substrate, and the surface thereof has highly dispersed nanoparticles with a particle size between 3 and 5 nm, thereby forming an ultra-high density of SERS hot spots, and further effectively ensuring that the SERS substrate has good SERS enhancement activity.

[0020] 2. The whole process of the present invention does not require additional heating and is carried out under normal temperature conditions without precise temperature control and cumbersome synthesis steps; by adjusting the ratio of different reducing agents in the reducing solution and the air pressure and flow rate during microdroplet spraying, the solution enters the microdroplet device for reaction, so as to directly obtain gold nanoparticles with different dispersions on the substrate, which not only has high preparation efficiency, but also has low cost and is convenient for popularization and use.

[0021] 3. The present invention only uses two reagents, chloroauric acid and sodium borohydride, and the surface of the prepared nanoparticles is clean, greatly reducing the background interference peaks and effectively ensuring the subsequent use effect of the SERS substrate. Description of the Drawings

[0022] Figure 1 is a schematic flow chart of constructing an SERS substrate according to the present invention;

[0023] Figure 2 is a transmission electron microscope (TEM) image of the surface of an SERS substrate prepared in different embodiments;

[0024] where (a) is Example 1; (b) is Example 4;

[0025] Figure 3 is an SERS spectrum of 4-NTP molecules adsorbed on the surface of gold nanoparticles during performance testing;

[0026] where (a) is a comparison diagram of the signal enhancement effect of 4-NTP with or without gold nanoparticles on the surface; (b) is the SERS signal of Examples 1 to 4; (c) is the SERS signal of Example 4 and different preparation methods. Detailed Embodiments

[0027] The present invention will be further described below.

[0028] Example 1: It includes the following steps:

[0029] A. Preparation of gold nanoparticle reducing solution: Under normal temperature conditions, a 1 mmol chloroauric acid (HAuCl4) solution is prepared by mixing chloroauric acid and ultrapure water; a 1 mmol sodium borohydride (NaBH4) solution is prepared by mixing sodium borohydride and ultrapure water; then the chloroauric acid solution and the sodium borohydride solution are mixed in a volume ratio of 1:1 to form a gold nanoparticle reducing solution; A glass slide is selected as the substrate material, which is rinsed with ethanol and water respectively and dried with N2 gas for standby.

[0030] B. Micro-droplet spraying: Under normal temperature conditions, as Figure 1 shown, the gold nanoparticle reducing solution in step A is evenly sprayed on the surface of the substrate by using a micro-droplet device for 10 minutes to form a substrate with gold nanoparticles distributed on the surface; The micro-droplet device is an existing device, which includes a microsyringe and a micro-injection pump. A 500 μL microsyringe is used to suck 300 μL of the gold nanoparticle reducing solution and spray it on the surface of the substrate through a connecting capillary. The micro-injection pump provides the spraying power for the microsyringe; Among them, the flow rate of spraying the gold nanoparticle reducing solution is controlled at 30 μL / Min, the sheath gas is N2 gas, the air pressure is 60 psi, and the distance between the capillary and the surface of the substrate is 1.5 cm; The capillary size has an inner diameter of 74 μm and an outer diameter of 195 μm.

[0031] C. Cleaning of the SERS substrate: The substrate prepared in step B is repeatedly rinsed with ethanol and water for 3 minutes and dried with N2 gas to make the SERS substrate of Example 1.

[0032] Example 2: Its preparation process is basically the same as that of Example 1, except that the chloroauric acid solution and the sodium borohydride solution are mixed in a volume ratio of 1:2 to form a gold nanoparticle reducing solution, thereby making the SERS substrate of Example 2.

[0033] Example 3: Its preparation process is basically the same as that of Example 1, except that the chloroauric acid solution and the sodium borohydride solution are mixed in a volume ratio of 1:3 to form a gold nanoparticle reducing solution, thereby making the SERS substrate of Example 3.

[0034] Example 4: Its preparation process is basically the same as that of Example 1, except that the chloroauric acid solution and the sodium borohydride solution are mixed in a volume ratio of 1:4 to form a gold nanoparticle reducing solution, thereby making the SERS substrate of Example 4.

[0035] Performance test:

[0036] 1. Transmission electron microscope (TEM) images: The SERS substrates prepared in Examples 1 to 4 were all imaged using a transmission electron microscope (TEM). As Figure 2 shown in a, the particle size of the nanoparticles in Example 1 (i.e., the reducing solution formed by mixing chloroauric acid solution and sodium borohydride solution in a volume ratio of 1:1) was 3 - 5 nm, and the dispersibility was good; as the volume ratio of the sodium borohydride solution increased, the particle size of the nanoparticles on the surface of the prepared SERS substrate continued to increase and the dispersibility became worse and worse. As Figure 2 shown in b, the particle size of the nanoparticles in Example 4 (i.e., the reducing solution formed by mixing chloroauric acid solution and sodium borohydride solution in a volume ratio of 1:4) was up to 7 - 12 nm, and agglomeration occurred.

[0037] 2. SERS activity test:

[0038] The SERS activity of the gold nanoparticles on the surface of the SERS substrate was characterized by detecting the SERS signal of 4-NTP molecules adsorbed on the surface of the gold nanoparticles.

[0039] (1) Probe molecule adsorption: The SERS substrates prepared in Examples 1 to 4 were successively immersed in an ethanol solution of 1 mmol of 4-NTP for 5 min, taken out, rinsed with ethanol and water, and then dried with N2 gas.

[0040] (2) SERS result analysis: As Figure 3 shown in a, after the 4-NTP molecules were adsorbed on the surface of the gold nanoparticles, the Raman signal was significantly enhanced, indicating that the prepared gold nanoparticles had SERS enhancement activity. In addition, as Figure 3 shown in b, the SERS signals of different examples gradually weakened with the ratio of the reducing solution (the volume ratio of chloroauric acid solution and sodium borohydride solution from 1:1 to 1:4), indicating that the intensity of the SERS signal was closely related to the particle size and dispersion degree of the nanoparticles, that is, the smaller the particle size of the nanoparticles and the better the dispersion degree, the higher the intensity of the SERS signal. Example 1 was the optimal example.

[0041] (3) Verification experiment: To exclude the formation of Au nanoparticles with SERS effect when the chloroauric acid solution and sodium borohydride solution were mixed, gold nanoparticles were prepared by different methods (dropwise sampling, soaking) and SERS tests were carried out, and compared with the gold nanoparticles on the surface of the SERS substrate prepared in Example 4 of the present invention. The specific steps are as follows:

[0042] Dropwise sampling experiment operation: The reducing solution prepared with the same volume ratio as in Example 4 was mixed in a micro syringe for 1 min and 10 min respectively, and then dropped on a substrate made of a glass slide (for one minute), and then rinsed 3 times with ethanol and set aside.

[0043] Soaking experiment operation: The glass slide was used as a substrate and directly soaked in the reducing solution prepared with the same volume ratio as in Example 4 (for ten minutes), and then rinsed three times with ethanol and reserved.

[0044] The above SERS activity tests were carried out on the mixed drop samples of 1 min and 10 min, and the soaked substrates. Finally, a comparison was made with Example 4. As Figure 3 shown in c, the results show that the gold nanoparticles prepared in Example 4 have the strongest SERS activity. However, Example 4 has the worst effect among all the examples. Therefore, it shows that the micro-droplet technology adopted in the present invention can simply and quickly prepare the SERS substrate under normal temperature conditions and ensure its good SERS enhancement activity.

[0045] The above are only the preferred embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for constructing a SERS substrate based on microdroplet technology, characterized in that, The following steps are involved: A. Preparation of gold nanoparticle reduction solution: prepare chloroauric acid solution and sodium borohydride solution, then mix the chloroauric acid solution and the sodium borohydride solution in a volume ratio of 1:(1-4) to form a gold nanoparticle reduction solution; B. Micro-droplet spraying: using a micro-droplet device to evenly spray the gold nanoparticle reduction solution of step A on the surface of the substrate for a certain period of time to form a substrate with gold nanoparticles distributed on the surface; C. SERS substrate cleaning: The substrate prepared in step B is rinsed with an ethanol solution and blown dry with an inert gas to form a SERS substrate.

2. The method for constructing the SERS substrate based on the microdroplet technology according to claim 1, characterized in that, The steps A and B are both carried out at room temperature.

3. The method for constructing the SERS substrate based on the microdroplet technology according to claim 1, wherein The micro-droplet device comprises a micro-injector and a micro-injection pump. The micro-injector is used to absorb the gold nanoparticle reduction solution and spray it on the substrate surface through the connecting capillary, and the micro-injection pump provides spraying power for the micro-injector.

4. The method for constructing the SERS substrate based on the microdroplet technology according to claim 3, characterized in that, The flow rate of the gold nanoparticle reducing solution sprayed by the microinjection pump was controlled at 30 μL / Min, the sheath gas was an inert gas, the gas pressure was 60 psi, and the capillary was 1.5 cm away from the substrate surface.

5. The method for constructing the SERS substrate based on the microdroplet technology according to claim 1, wherein, The chloroauric acid solution is prepared by mixing chloroauric acid and ultrapure water to form a 1mmol chloroauric acid solution; the sodium borohydride solution is prepared by mixing sodium borohydride and ultrapure water to form a 1mmol sodium borohydride solution.

6. The method for constructing the SERS substrate based on the microdroplet technology according to claim 1 or 3, characterized in that, The inert gas is nitrogen.

7. The method for constructing the SERS substrate based on the microdroplet technology according to claim 1, wherein The substrate is a glass slide or a silicon wafer.

8. A SERS substrate prepared by the method for constructing a SERS substrate according to claim 1, characterized in that, Gold nanoparticles are evenly distributed on the surface of the SERS substrate, and the gold nanoparticles have a particle size between 3 and 5 nm and are in a granular shape.

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