Method of synthesizing triangular plate-formed gold nano-plate narrow in side-length distribution using microfluidic device

Abstract

To provide a method of synthesizing a gold nano-plate having an intense light-absorption peak by mixing solutions uniformly to control the side length of the gold nano-plate in a state of normal temperature and normal pressure.SOLUTION: A method of synthesizing a gold nano-plate using two kinds of reducing agents in a gold ion-containing solution, includes the steps of: preparing solutions each containing two kinds of reducing agents and a protecting agent; preparing a solution optimized in blending of the two kinds of reducing agents and a solution containing a gold ion and a protecting agent; and synthesizing a gold nano-plate by mixing the prepared solutions using a microfluidic device and keeping the same stand.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for synthesizing gold nanoplates having a narrow side length distribution using a microfluidic device. [Background technology]

[0002] Gold nanoparticles have traditionally been used in many fields, including as biosensors, catalysts, therapeutic drugs, and conductive materials, due to their excellent stability and biocompatibility.

[0003] The light absorption caused by the localized surface plasmon resonance (LSPR) of gold nanoparticles is used in antigen tests by immunochromatography such as influenza tests, pregnancy tests, and allergy tests. There are several methods for synthesizing gold nanoparticles, such as a liquid-phase reduction method and a method of leaving the plate at room temperature and pressure. In order to increase the sensitivity of antigen tests, which are pathogens, in these test drugs, it is necessary to sharpen the light absorption peak of the gold nanoparticles. A triangular flat gold nanoplate has an absorption peak due to the length of the sides of the plate and an absorption peak due to the thickness of the plate, so it appears blue and shows a sharp absorption peak.

[0004] For example, Patent Document 1 discloses a method for synthesizing gold nanoplates, in which suspension A in the document is prepared as gold nanoplates for use in a biosensor by stirring a 50 mM aqueous solution of hexadecyltrimethylammonium chloride (CTAC), adding a 50 mM aqueous solution of chloroauric acid, a 10 mM aqueous solution of sodium iodide, and a 100 mM aqueous solution of L-ascorbic acid, stirring for 30 seconds, adding a 10-fold diluted solution of a suspension of gold nanoplate seed particles, and stirring the mixture, and allowing the resulting suspension to stand in an incubator (30°C) for 12 hours to synthesize gold nanoplates.

[0005] Furthermore, Experiment A in Non-Patent Document 1 discloses a method for synthesizing gold nanoplates by adding 100 mM trisodium citrate, 1.25 mM chloroauric acid, and hexadecyltrimethylammonium bromide (CTAB) to a container kept at 90°C and mixing them to synthesize gold nanoplates.

[0006] Non-Patent Document 2 discloses a method for synthesizing gold nanoplates that exhibit a blue transparent color by simply leaving them at room temperature and pressure for 24 hours or more without precise temperature control of the solution, by optimizing the mixture of two reducing agents, trisodium citrate and tannic acid.

[0007] In the synthesis of spherical gold nanoparticles, gold nanoparticles with an average particle size of several tens of nm can be synthesized by heating and reducing with citric acid. Furthermore, it has been found that the particle size decreases by using citric acid and tannic acid as reducing agents. The particle size depends on the number of nuclei generated in the early stages of the reaction, and tannic acid reduces gold ions in the early stages of the reaction to generate nuclei. Therefore, increasing the amount of tannic acid added generates more nuclei. Furthermore, citric acid acts as a reducing agent that forms a shell around the nuclei after they are generated.

[0008] The synthesis of gold nanoplates requires the formation of nuclei first. Therefore, in the seed method, gold nanoparticles with a particle diameter of several nm that have been synthesized in advance are used as nuclei, and chloroauric acid, cetyltrimethylammonium bromide (hereinafter simply referred to as "CTAB"), and citric acid are added to protect the (111) face of the gold crystal with CTAB, while forming a shell on the (100) or (110) face with citric acid, thereby synthesizing gold nanoplates. In addition, the method of thermal reduction with citric acid requires the formation of nuclei by heating, but unless the temperature is strictly controlled, the size of the nuclei cannot be controlled, making it difficult to synthesize blue translucent gold nanoplates with side lengths of tens to hundreds of nm.

[0009] The gold nanoplates obtained by these methods are suspensions exhibiting red, magenta, purple, dark blue, blue, cyan, or light blue tones. In antigen testing using the immunochromatography method, an antibody bound to a blue gold nanoplate (hereinafter simply referred to as a gold nanoplate antibody complex) is used in a conjugation pad, and a sandwich-structured complex is formed on the test line of the membrane in which the antibody, antigen, and gold nanoplate antibody complex that have been applied to the test line in advance are bound, resulting in a blue line on the test line. However, it is disclosed that the visibility of the detection line can be significantly improved compared to when spherical gold nanoparticles that can only be prepared in red are used, and therefore a blue transmission color is better than a red transmission color. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Re-table 2017 / 154989 [Non-patent literature]

[0011] [Non-Patent Document 1] DVRavi Kumar, et al., Synthesis of triangular gold nanoplates: Role of bromide ion and temperature (2013), Vol. 422, pp.181-190 [Non-Patent Document 2] M. Hamamoto, A. Kimura, H. Yagyu, Seedless and non-thermal liquid-phase reduction synthesis of gold nanotriangles. MRS Advances (2023). https: / / doi.org / 10.1557 / s43580-023-00587-1 Summary of the Invention [Problem to be solved by the invention]

[0012] However, the gold nanoplates synthesized using the beaker described in Non-Patent Document 2 have the problem that the side length of the gold nanoplates cannot be made constant because it is difficult to mix the solution uniformly. In addition, there is a need to increase the sensitivity of optical sensors using gold nanoplates by synthesizing gold nanoplates with a constant side length and generating gold nanoplates that show a strong light absorption peak at a wavelength of around 600 nm.

[0013] The present invention has been made in consideration of these circumstances, and aims to provide a method for synthesizing gold nanoplates with strong optical absorption peaks by using a microfluidic device to uniformly mix solutions and control the size of the gold nanoplates, thereby controlling the side length of the gold nanoplates. [Means for solving the problem]

[0014] In order to solve this problem, the present inventors have conducted extensive research and have come up with the following invention.

[0015] This is a method for synthesizing gold nanoplates with a narrow side length distribution, and is a method for synthesizing gold nanoplates using a microfluidic device, characterized by including the following steps 1 to 6. Step 1: Ion-exchanged water and trisodium citrate are mixed to prepare a first aqueous reducing agent solution having a trisodium citrate concentration of 50 mM or more and 150 mM or less. Step 2: Ion-exchanged water and tannic acid are mixed to prepare a second reducing agent aqueous solution having a tannic acid concentration of 2 mM or more and 10 mM or less. Step 3: A cationic surfactant that selectively adsorbs to the (111) crystal face of gold is mixed with ion-exchanged water to prepare a solution containing the protective agent at a concentration of 5 mM to 15 mM. Step 4: The first reducing agent aqueous solution, the second reducing agent aqueous solution, and ion-exchanged water are mixed to prepare solution A having a citric acid concentration of 1 mM or more and 5 mM or less and a tannic acid concentration of 0.1 mM or more and 5 mM or less. Step 5: The protective material-containing solution is added to an aqueous solution of chloroauric acid with a molar concentration of 10 mM to 40 mM while stirring to prepare solution B with a chloroauric acid concentration of 0.1 mM to 10 mM and a protective material-containing concentration of 1 mM to 15 mM. Step 6: Using a microfluidic device equipped with two inlets, either solution A or solution B is delivered through one inlet and the other solution is delivered through the other inlet. Solutions A and B are then merged and reacted in a hollow flow path with a flow path length of 100 mm or more. The solution is then recovered at the outlet of the microfluidic device and allowed to stand at room temperature and pressure for 24 hours or more to synthesize gold nanoplates with a narrow edge length distribution.

[0016] In the step 1, it is preferable to prepare the first aqueous reducing agent solution by mixing 2 g or more and 3 g or less of trisodium citrate with 50 mL or more and 200 mL or less of ion-exchanged water.

[0017] Next, in the step 2, it is preferable to prepare a second reducing agent aqueous solution by mixing 0.5 g to 1.5 g of tannic acid and 50 mL to 150 mL of ion-exchanged water.

[0018] In step 3, it is preferable to prepare a protective agent-containing solution by mixing 0.01 g to 0.02 g of a cationic surfactant that selectively adsorbs to the (111) crystal face of gold as a protective agent with 4 mL to 6 mL of ion-exchanged water.

[0019] In the step 4, it is preferable to prepare solution A by mixing 120 μL to 130 μL of the first reducing agent aqueous solution, 0.1 mL to 0.3 mL of the second reducing agent aqueous solution, and 7 mL to 8 mL of ion-exchanged water.

[0020] In the step 5, it is preferable to prepare solution B by adding 3.0 mL to 5.0 mL of the protective agent-containing solution while stirring 0.1 mL to 0.3 mL of an aqueous chloroauric acid solution with a stirrer.

[0021] In step 6, it is preferable that solution A and solution B are delivered from each delivery port of the microfluidic device at a flow rate of 0.1 mL / mm or more and 1.0 mL / mm using a syringe pump, solutions A and B are joined, and reacted in a hollow flow path with a flow length of 100 mm or more. The solution recovered from the outlet of the microfluidic device is prepared so that the molar ratio of each component to gold is 1.5 or more and 2.6 or less for citric acid, 6.0 or more and 8.0 or less for CTAB, and 0.1 or more and 1.0 or less for tannic acid, and allowed to stand at room temperature and pressure for 24 hours or more. Effect of the Invention

[0022] The method for synthesizing gold nanoplates according to the present invention uses two types of reducing agents, trisodium citrate and tannic acid. By optimizing the mixture of these two types of reducing agents, heating is no longer necessary to reduce the solution at an appropriate rate. Furthermore, when using this method for synthesizing gold nanoplates, precise temperature control of the solution is not required, and by using a microfluidic device to react the solution in a flow channel, the side length of the gold nanoplates can be controlled, and gold nanoplates that exhibit stronger optical absorption peaks can be synthesized by simply leaving them at room temperature and pressure for 24 hours or more. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a schematic diagram of a microfluidic device. [Diagram 2] This is a TEM image of gold nanoplates synthesized using a microfluidic device. [Diagram 3] This is a TEM image of gold nanoplates synthesized using a beaker. [Figure 4] 1 is a graph showing the visible light absorption spectrum of synthesized gold nanoplates. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, a method for synthesizing gold nanoplates using the microfluidic device according to the present invention will be described.

[0025] 1. Gold Nanoplate Synthesis The gold ions used in the present invention are chloroauric acid tetrahydrate. The two types of reducing agents used are trisodium citrate and tannic acid. A cationic surfactant that selectively adsorbs to the (111) crystal face of gold is mixed as a protective agent. In particular, it is more preferable to use CTAB as the protective agent. Normally, when chloroauric acid, trisodium citrate, and tannic acid are mixed while heating, tannic acid acts as a reducing agent and trisodium citrate acts as a dispersing agent, and gold nanoparticles are synthesized. However, in this synthesis method, the combination of two types of reducing agents is optimized without heating, and the gold ions and protective agent are mixed. As a result, gold nanoplates can be synthesized by leaving it at room temperature and normal pressure. In the present invention, gold nanoplates are formed by the protective agent protecting the (111) crystal face of the gold, the reducing agent tannic acid forming nuclei, and citric acid forming shells on the (100) or (110) faces, but by optimizing the amounts of tannic acid and citric acid added as reducing agent solutions, it is possible to obtain gold nanoplates that exhibit a stable blue transmission color. Each process will be explained below.

[0026] 1-1. Process 1 The first reducing agent aqueous solution according to the present invention is preferably a mixture of ion-exchanged water and trisodium citrate, with a trisodium citrate concentration of 50 mM or more and 150 mM or less. If the trisodium citrate concentration is less than 50 mM, the gold ions cannot be sufficiently reduced, and the average side length of the gold nanoplates becomes excessively short. On the other hand, if the trisodium citrate concentration is higher than 150 mM, a large amount of unreacted reducing agent remains, and the average side length of the gold nanoplates becomes long and varies greatly. As a result, it is difficult to stably synthesize gold nanoplates that exhibit a blue transparent color, which is not preferable.

[0027] Furthermore, the first reducing agent aqueous solution according to the present invention preferably has a trisodium citrate concentration of 90 mM or more and 110 mM or less. In the synthesis method of the present invention, reduction is performed using two types of reducing agents, and as a result of considering the optimization of the combination with the other reducing agent, a more stable success rate of gold nanoplate synthesis and a high yield can be obtained.

[0028] 1-2. Process 2 The second reducing agent aqueous solution according to the present invention is preferably a mixture of ion-exchanged water and tannic acid, with a tannic acid concentration of 2 mM or more and 10 mM or less. Since gold nanoparticles synthesized in advance are used as nuclei to synthesize gold nanoplates, nuclei must first be formed. Tannic acid is used as a reducing agent that plays a role in forming the nuclei. When the tannic acid concentration is less than 2 mM, the reduction of gold ions cannot be performed sufficiently, nuclei are not formed sufficiently, and the average side length of the gold nanoplates becomes excessively short and the side length variation becomes large. On the other hand, when the tannic acid concentration is higher than 10 mM, the average side length of the gold nanoplates becomes excessively long and the side length variation becomes large. As a result, it becomes difficult to synthesize gold nanoplates that exhibit a blue transparent color with small side length variation, which is not preferable.

[0029] Furthermore, the second reducing agent aqueous solution according to the present invention preferably has a molar concentration of tannic acid of 5.88 mM. This is because the synthesis method of the present invention uses two types of reducing agents for reduction, and by considering the optimization of the combination with the other reducing agent, it is possible to obtain a more stable success rate and high yield of gold nanoplate synthesis.

[0030] 1-3.Process 3 The protective agent-containing aqueous solution according to the present invention is preferably a mixture of CTAB and ion-exchanged water, with a protective agent concentration of 5 mM to 15 mM. If the protective agent concentration is less than 5 mM, the amount of protective agent adsorbed to the (111) crystal face of the gold is reduced, which is undesirable as it may cause the gold to aggregate and reduce the yield. On the other hand, if the protective agent concentration is more than 15 mM, the amount of chloride ions in the chloroauric acid substituted for the bromine in the protective agent increases, which may stabilize the bromoauric acid and slow the reaction rate, which is undesirable.

[0031] 1-4.Process 4 The solution A according to the present invention is preferably prepared by mixing a reducing agent aqueous solution 1 of 100 μL to 150 μL, a reducing agent aqueous solution 2 of 0.2 mL to 0.3 mL, and ion-exchanged water of 5 mL to 10 mL to prepare a solution A with a citric acid concentration of 1.0 mM to 2.0 mM and a tannic acid concentration of 0.1 mM to 1.0 mM. When the citric acid concentration is less than 10 mM, reduction cannot be performed sufficiently, making it difficult to synthesize gold nanoplates. On the other hand, when the citric acid concentration is higher than 2.0 mM, the average side length of the synthesized gold nanoplates is excessively long and the side length variation becomes large, making it difficult to synthesize gold nanoplates with a blue transmission color. Also, when the tannic acid concentration is less than 0.1 mM, gold ions cannot be sufficiently reduced, and gold nanoplates with a red transmission color are synthesized, which is not preferable. On the other hand, if the tannic acid concentration is higher than 1.0 mM, the reducing agent will remain, the average side length of the synthesized gold nanoplates will be excessively long and the side length will vary greatly, making it difficult to synthesize gold nanoplates that exhibit a blue transparent color, which is undesirable.

[0032] Furthermore, solution A according to the present invention is prepared by mixing 116 μL to 140 μL of reducing agent aqueous solution 1 and 0.18 mL to 0.22 mL of a second reducing agent aqueous solution. It is more preferable to add 7.4 mL to 7.6 mL of ion-exchanged water to the mixed solution to prepare solution A with a citric acid concentration of 1.5 mM to 1.7 mM and a tannic acid concentration of 0.13 mM to 0.17 mM. As a result, gold ions can be appropriately reduced, and gold nanoplates with a bluer transmitted color can be obtained.

[0033] 1-5.Process 5 The solution B according to the present invention is preferably prepared by first mixing ion-exchanged water and chloroauric acid tetrahydrate to prepare a chloroauric acid aqueous solution having a chloroauric acid concentration of 20 mM to 40 mM. Next, it is preferable to prepare a solution B having a chloroauric acid concentration of 0.1 mM to 10 mM and a protective agent-containing aqueous solution having a concentration of 1 mM to 15 mM by mixing 0.1 mL to 0.3 mL of a chloroauric acid aqueous solution and 3.0 mL to 6.0 mL of a protective agent-containing aqueous solution. If the chloroauric acid concentration is less than 20 mM, the amount of reducing agent and protective agent is increased relative to the gold ions, which is not preferable because many spherical gold nanoparticles are generated. On the other hand, if the chloroauric acid concentration is higher than 40 mM, the nuclei generated by tannic acid will aggregate, which impairs particle dispersibility, increases the average side length of the gold nanoplates, and makes it difficult to synthesize gold nanoplates that have excellent particle dispersibility and exhibit a blue transparent color, which is not preferable.

[0034] Furthermore, it is more preferable to prepare solution B according to the present invention by mixing ion-exchanged water and chloroauric acid tetrahydrate to prepare an aqueous solution of chloroauric acid with a chloroauric acid concentration of 26 mM to 33 mM. Then, it is more preferable to prepare solution B with a chloroauric acid concentration of 1.05 mM to 1.45 mM and a protective agent concentration of 7.86 mM to 9.66 mM by mixing 0.18 mL to 0.24 mL of an aqueous solution of chloroauric acid and 4.3 mL to 5.3 mL of an aqueous solution containing a protective agent.

[0035] 1-6.Process 6 In the method for synthesizing gold nanoplates according to the present invention, either solution A or solution B is pumped to one of the two liquid feed ports shown in FIG. 1, and the other solution is pumped to the other liquid feed port, and these solutions A and B are mixed at the confluence. This mixed solution has a molar ratio to gold (molar number of reducing agent or protecting agent in the mixed solution / molar number of chloroauric acid in the mixed solution) of 1.5 to 2.6 for citric acid, 6.0 to 8.0 for CTAB, and 0.1 to 1.0 for tannic acid. It is preferable to collect the solution through the reaction path of the microfluidic device and collect it from the outlet into a glass bottle and leave it at room temperature and pressure for 24 hours or more. However, if the time to leave it is less than 24 hours, the side length of the synthesized gold nanoplates is too short, which is not preferable. Also, if the time to leave it is more than 240 hours, the color of the solution changes from a blue transparent color, which is not preferable. Any pump may be used as long as it can stably deliver a specified amount of solution.

[0036] In addition, it is more preferable that the mixed solution has a composition in which the molar ratios of citric acid to gold are 1.8 to 2.3, CATB is 6.9 to 7.3, and tannic acid is 0.15 to 0.2. Furthermore, if the reducing agent or protective agent remains after standing, the reduction will proceed and the particle size variation of the gold nanoplates will increase, so it is preferable to remove the reducing agent or protective agent by centrifugation or filtration. It is also preferable to perform centrifugation or filtration in order to improve the yield. By preparing the mixed solution with the above molar ratios and standing it for 24 hours or more, the gold ions can be appropriately reduced, and gold nanoplates with a bluer transmission color and a small side length distribution can be efficiently synthesized.

[0037] 1-7. Synthesis conditions for blue transparent gold nanoplates The concentrations of each aqueous solution described in the embodiment of the present invention are guideline concentrations, and the synthesis conditions for gold nanoplates exhibiting a blue transparent color are considered to be determined by the molar ratio of the reducing agent and protective material to gold. If this molar ratio deviates from the optimum value, the variation in the side length distribution becomes small, and it tends to be difficult to synthesize gold nanoplates exhibiting a blue transparent color, which is undesirable. It is particularly preferable that the optimal molar ratio to gold is 0.188 for the first reducing agent aqueous solution, 2.06 for the second reducing agent aqueous solution, and 7.03 for the protective material-containing aqueous solution.

[0038] 1-8. Microfluidic device conditions The microfluidic device used in the present invention preferably has a hollow channel with a channel width of 400 μm to 500 μm, a depth of 100 μm to 200 μm, and a channel length from the Y-shaped mixing section to 100 mm to 200 mm. In addition, it is preferable to deliver the solution at a flow rate of 0.1 mL / min to 0.5 mL / min. Furthermore, it is particularly preferable to appropriately adjust the channel width, depth, channel length from the Y-shaped mixing section, and flow rate depending on the time the solution flows through the microfluidic device channel. EXAMPLES

[0039] In the examples, gold nanoplates were synthesized by fixing the molar ratio of protective material to gold at 7.0 in the following steps. Step 1: An aqueous solution of a first reducing agent having a trisodium citrate concentration of 100 mM was prepared. Step 2: An aqueous solution of a second reducing agent having a tannic acid concentration of 5.88 mM was prepared. Step 3: Using CTAB as a protecting agent, an aqueous solution containing the protecting agent with a protecting agent concentration of 9.15 mM was prepared. Step 4: The amount of the second reducing agent solution added was set to 0.2 mL, and a solution A having a trisodium citrate concentration of 1.64 mM and a tannic acid concentration of 0.15 mM was prepared. Step 5: The protective agent-containing aqueous solution was added to the chloroauric acid aqueous solution, and the mixture was stirred at 1500 rpm for 10 minutes to prepare solution B having a chloroauric acid concentration of 0.12 mM and a protective agent concentration of 8.76 mM. Step 6: Solution A was fed to one of the liquid feed ports of the microfluidic device, and solution B was fed to the other liquid feed port of the microfluidic device at a flow rate of 0.2 mL / mm using a syringe pump. The solutions that flowed through the flow paths and mixed were collected in a glass bottle from the outlet of the microfluidic device and left to stand for 3 days at room temperature and pressure. Comparative Example

[0040] In the comparative example, gold nanoplates were synthesized by fixing the molar ratio of protective material to gold at 7.0 in the following steps. Step 1: An aqueous solution of a first reducing agent having a trisodium citrate concentration of 100 mM was prepared. Step 2: An aqueous solution of a second reducing agent having a tannic acid concentration of 5.88 mM was prepared. Step 3: Using CTAB as a protecting agent, an aqueous solution containing the protecting agent with a protecting agent concentration of 9.15 mM was prepared. Step 4: The amount of the second reducing agent aqueous solution added was 0.2 mL, and solution A having a trisodium citrate concentration of 1.64 mM and a tannic acid concentration of 0.15 mM was prepared. Step 5: The protective agent-containing aqueous solution was added to the chloroauric acid aqueous solution, and the mixture was stirred at 1500 rpm for 10 minutes to prepare solution B having a chloroauric acid concentration of 0.12 mM and a protective agent concentration of 8.76 mM. Step 6: While stirring solution A with a stirrer, solution B was added, and the mixture was mixed at a rotation speed of 700 rpm for 10 minutes, and then allowed to stand at room temperature and pressure for 3 days.

[0041] 〔evaluation〕 In the examples and comparative examples, the mixed solution was mixed by stirring using a stirrer (AS ONE, RS-1AN). The microfluidic device used in Example 1 is the device shown in FIG. 1, which is made of glass and has a hollow channel with a channel width of 430 μm, a depth of 150 μm, and a channel length of 130 mm after the Y-shaped mixing part. The solutions used in the examples and comparative examples were chloroauric acid tetrahydrate (Fujifilm Wako Pure Chemical Industries, Ltd., 077-00931, 99%), sodium citrate (Fujifilm Wako Pure Chemical Industries, Ltd., 203-13605, 99%), tannic acid (Fujifilm Wako Pure Chemical Industries, Ltd., 201-06332), CTAB (Sigma-Aldrich, Ltd., H5882-100G, 98% or more), and ion-exchanged water (pure water: conductivity 1 μs / cm or less).

[0042] A. Evaluation of Gold Nanoplates To confirm the gold nanoplates synthesized in the examples and comparative examples, TEM photographs of the obtained gold nanoplates are shown in Figures 2 and 3. From the TEM photographs shown in Figures 2 and 3, the average side length of the gold nanoplates, the coefficient of variation, and the yield (number of triangular flat gold nanoplates / total number of particles) were determined, and the results are shown in Table 1 below. Furthermore, Figure 4 shows the visible light absorption spectra of the gold nanoplates synthesized in the examples and comparative examples. From these results, it was evaluated whether control of the side length of the gold nanoplates had improved.

[0043] 2 and 3, it can be seen that the results of the Example in Fig. 2 show less variation in the size of the gold nanoplates compared to the results of the Comparative Example in Fig. 3. This shows that the side length of the synthesized gold nanoplates can be controlled by using a microfluidic device.

[0044] Furthermore, from the results shown in Table 1, it can be seen that the average side length of the Example is shorter than that of the Comparative Example, and the coefficient of variation is also smaller in the Example than in the Comparative Example, and the variation in the average side length of the gold nanoplates is reduced. This shows that the side length of the gold nanoplates can be controlled to be short and with little variation by synthesizing them using a microfluidic device.

[0045] In addition, from the results shown in Figure 4, the visible light absorption spectrum of the Example showed a strong light absorption peak at a peak wavelength of 580 nm. The visible light absorption spectrum of the Comparative Example showed a strong light absorption peak at a peak wavelength of 598 nm, but the peak intensity was smaller than that of the Example. This shows that by using a microfluidic device for synthesis, it is possible to control the side length of the gold nanoplates and synthesize gold nanoplates with large peak intensity.

[0046] [Table 1] [Industrial Applicability]

[0047] According to the present invention, a gold ion-containing solution is reacted with two types of reducing agents with an optimized composition and a protective agent in a hollow channel using a microfluidic device, thereby mixing the solutions and synthesizing gold nanoplates at room temperature and pressure. Furthermore, this method of synthesizing gold nanoplates does not require heating, and does not require precise temperature control of the solution. For this reason, it is suitable for synthesizing gold nanoplates with large peak intensity, which is suitable for use as a biosensor with easier and more stable control of the success rate and side length of the gold nanoplate synthesis. [Explanation of symbols]

[0048] 1. Microfluidic Device 2 Liquid inlet 3 Hollow Channel 4 Outlet

Claims

1. A method for synthesizing gold nanoplates having a narrow side length distribution, comprising the following steps 1 to 6, using a microfluidic device: Step 1: Ion-exchanged water and trisodium citrate are mixed to prepare a first reducing agent aqueous solution having a trisodium citrate concentration of 50 mM or more and 150 mM or less. Step 2: Ion-exchanged water and tannic acid are mixed to prepare a second reducing agent aqueous solution having a tannic acid concentration of 2 mM or more and 10 mM or less. Step 3: A cationic surfactant that selectively adsorbs to the (111) crystal plane of gold is mixed with ion-exchanged water to prepare a solution containing the protective agent at a concentration of 5 mM to 15 mM. Step 4: The first reducing agent aqueous solution, the second reducing agent aqueous solution, and ion-exchanged water are mixed to prepare solution A having a citric acid concentration of 1 mM or more and 5 mM or less and a tannic acid concentration of 0.1 mM or more and 5 mM or less. Step 5: The protective material-containing solution is added to an aqueous solution of chloroauric acid having a molar concentration of 10 mM to 40 mM while stirring to prepare solution B having a chloroauric acid concentration of 0.1 mM to 10 mM and a protective material-containing concentration of 1 mM to 15 mM. Step 6: Using a microfluidic device equipped with two inlets, either solution A or solution B is delivered through one inlet and the other solution is delivered through the other inlet. Solutions A and B are then merged and reacted in a hollow flow path with a flow path length of 100 mm or more. The solution is then recovered at the outlet of the microfluidic device and allowed to stand at room temperature and pressure for 24 hours or more to synthesize gold nanoplates with a narrow edge length distribution.

2. 2. The method for synthesizing gold nanoplates using a microfluidic device according to claim 1, wherein in step 1, the first reducing agent aqueous solution is prepared by mixing 2 g to 3 g of trisodium citrate and 50 mL to 200 mL of ion-exchanged water.

3. 2. The method for synthesizing gold nanoplates using a microfluidic device according to claim 1, wherein in the step 2, the second reducing agent aqueous solution is prepared by mixing 0.5 g to 1.5 g of tannic acid and 50 mL to 150 mL of ion-exchanged water.

4. 2. The method for synthesizing gold nanoplates using a microfluidic device according to claim 1, wherein in step 3, a protective agent-containing solution is prepared by mixing 0.01 g to 0.02 g of a cationic surfactant that selectively adsorbs to the (111) crystal face of gold as the protective agent with 4 mL to 6 mL of ion-exchanged water.

5. 2. The method for synthesizing gold nanoplates using a microfluidic device according to claim 1, wherein in step 4, solution A is prepared by mixing 120 μL to 130 μL of the first reducing agent aqueous solution, 0.1 mL to 0.3 mL of the second reducing agent aqueous solution, and 7 mL to 8 mL of ion-exchanged water.

6. 2. The method for synthesizing gold nanoplates using a microfluidic device according to claim 1, wherein in step 5, solution B is prepared by adding 3.0 mL to 5.0 mL of the protective agent-containing solution while stirring 0.1 mL to 0.3 mL of the aqueous chloroauric acid solution with a stirrer.

7. 2. The method of claim 1, wherein in step 6, solution A and solution B are fed from each liquid feed inlet of the microfluidic device at a flow rate of 0.1 mL / mm or more and 1.0 mL / mm using a syringe pump, and solutions A and B are joined and reacted in a hollow flow path having a flow path length of 100 mm or more. The solution recovered from the outlet of the microfluidic device is prepared so that the molar ratio of each component to gold is 1.5 or more and 2.6 or less for citric acid, 6.0 or more and 8.0 or less for CTAB, and 0.1 or more and 1.0 or less for tannic acid, and the solution is allowed to stand at room temperature and normal pressure for 24 hours or more.

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