CN115178748B

CN115178748B - Method for improving stability of monodisperse gold nanoparticles

Info

Publication number: CN115178748B
Application number: CN202211107059.1A
Authority: CN
Inventors: 张志阳; 陈燕; 陈令新
Original assignee: Yantai Institute of Coastal Zone Research of CAS
Current assignee: Yantai Institute of Coastal Zone Research of CAS
Priority date: 2022-09-13
Filing date: 2022-09-13
Publication date: 2022-12-20
Anticipated expiration: 2042-09-13
Also published as: CN115178748A

Abstract

The invention discloses a method for improving the stability of monodisperse gold nanoparticles, and belongs to the technical field of nanoparticle preparation. The method comprises the following two steps: reducing chloroauric acid by using sodium citrate to synthesize AuNPs, regulating the pH value of the AuNPs obtained by using 0.1M NaOH aqueous solution, and regulating the pH value to 12. The invention has the advantages that: (1) The charge quantity and conformation of the citrate are changed after the pH value is regulated, so that the stability of the citrate to AuNPs is improved, and stable monodisperse AuNPs are obtained; (2) The AuNPs obtained by the method provided by the invention not only realize the separation and concentration of nano particles, directly modify sulfydryl molecules or ions and convert from a conventional water phase to an organic solvent under the condition of not adding a surfactant, but also slow down the coffee ring effect, and do not introduce a surfactant capable of generating a background signal.

Description

Method for improving stability of monodisperse gold nanoparticles

Technical Field

The invention relates to a method for improving the stability of monodisperse gold nanoparticles, belonging to the technical field of nanoparticle preparation.

Background

The synthesis of gold nanoparticles (AuNPs) by reducing chloroauric acid with sodium citrate is a classical method for AuNPs synthesis.

Because the AuNPs are simple in preparation method and adjustable in size, the AuNPs are widely applied to the fields of sensing analysis, catalysis, cell imaging and the like.

However, auNPs are less stable, highly susceptible to self-aggregation, and often require the addition of surfactants for centrifugation, ligand modification, and solvent switching, such as:

(1) In the article of Viscosity gradient as a novel mechanism for the centralized differentiation of nanoparticles (Qiu P, mao C. Advanced Materials, 2011, 23 (42): 4880-4885.), in order to improve the stability of AuNPs during centrifugal concentration, a surfactant polyvinylpyrrolidone is added to the AuNPs;

(2) In the article of Adsorbed Tween 80 is unique in its ability to enhance the stability of gold nanoparticles in solutions of biolecules (Yuyuun Zhao, zhuo Wang, et al Nanoscale, 2010, 2 (10): 2114-2119.), in order to improve the stability of AuNPs, surfactant Tween 80 is added to the AuNPs;

(3) In the article of furniture phase transfer of gold nanoparticles from aqueous solution to organic solvents with a chlorinated poly (ethylene glycol) (M. Alkilay, A.I. Bani Yaesen, et al. RSC Advances, 2014, 4 (95): 52676-52679), surfactant thiolated polyethylene glycol (PEG-SH) and dodecanethiol were added to AuNPs in order to improve the stability of the AuNPs when solvent switching was performed.

The addition of surfactants is the most common method for solving the self-aggregation of AuNPs, but the addition of surfactants brings certain problems while improving the stability of AuNPs, such as:

(1) The article of High-sensitivity SERS based sensing on the labeling side of glass slides using low branched nanoparticles with surfactant-free synthesis (TubaTezcan, chia-Hsien Hsu. RSC Advances, 2020, 10;

(2) Both of the thermally-ordered mesotopous carbon nanoparticies with high porosity for lithium-sulfur batteries (Schuster J, guang He, et al. Angew. Chem. 2012, 51 (124): 3651-3655.) and the Controlled synthesis of mesotopous carbon nanoparticies via "Silica-dispersed" energy (Zhennan Qiao, bingkun Guo, et al. Nano Lett. 2013, 13 (1): 207-212.) indicate that Tween 80 is toxic to the cells;

(3) Both the simple synthesis of platinum Pt-Pd nanosheets for enhanced formation of acids and oxygen reduction reaction (Yang Qian, shi Lijie, et al. Journal of Materials Chemistry, 2019, 7 (32): 18846-18851.) and the Self-supported 3D PdCu alloy nano sheets as a binary catalyst for electrochemical reduction of alcohols (ZHao X, dai L, qin Q, et al. Small, 2017, 13 (12)) indicate that surfactants hinder the exposure of catalytically active centers and thereby reduce the catalytic activity of NPs.

Disclosure of Invention

In order to solve the problems of the prior art, the present invention aims to provide a method for improving the stability of monodisperse AuNPs, wherein the AuNPs obtained by the method are not easy to self-aggregate even without adding a surfactant when centrifugal concentration, ligand modification and solvent conversion are carried out.

In order to achieve the above object, the present invention adopts the following technical solutions:

a method for improving the stability of monodisperse gold nanoparticles is characterized by comprising the following steps:

step 1: reducing chloroauric acid by using sodium citrate to synthesize AuNPs;

step 2: and (3) regulating the pH value of the AuNPs obtained in the step (1) by using an alkaline solution, and regulating the pH value to 12 to obtain stable monodisperse AuNPs.

Preferably, in step 1, the process for synthesizing AuNPs by reducing chloroauric acid with sodium citrate is as follows: HAuCl with the mass concentration of 0.01 percent ₄ Heating the water solution to boiling, and rapidly adding 1% sodium citrate water solution and HAuCl ₄ The volume ratio of the aqueous solution to the sodium citrate aqueous solution is 1000: and 7, stopping the reaction after slightly boiling for 40min, and cooling in a water bath to obtain AuNPs.

Preferably, in step 2, the alkaline solution is a 0.1M NaOH aqueous solution; the particle size of the stable monodisperse AuNPs is 55nm.

The invention has the advantages that:

(1) According to the method, the pH value of AuNPs synthesized by reducing chloroauric acid with sodium citrate is regulated, the pH value of the AuNPs is regulated to 12, the charge quantity and conformation of citrate are changed after the pH value is regulated, so that the stability of the citrate to the AuNPs is improved, and stable monodisperse AuNPs are obtained;

(2) The AuNPs obtained by the method provided by the invention realize separation and concentration of nanoparticles without adding a surfactant, and provide a new idea for purification of nanoparticles;

(3) The AuNPs obtained by the method provided by the invention slow down the coffee ring effect, and a surfactant capable of generating a background signal is not introduced, so that the AuNPs have a promoting effect on the development of various fields including SERS;

(4) The AuNPs obtained by the method realize direct modification of sulfhydryl molecules or ions without adding a surfactant, and provide a basis for functionalization of nanoparticles, preparation of probe molecules and removal of an SERS background;

(5) The AuNPs obtained by the method provided by the invention realize the conversion of nanoparticles from a conventional water phase to an organic solvent under the condition of not adding a surfactant, and provide a new visual angle for the research of reaction in the organic solvent.

Drawings

FIG. 1 is an SEM image of AuNPs obtained by the method provided by the invention;

FIG. 2 is a diagram of ultraviolet absorption spectra of AuNPs before pH adjustment and AuNPs after pH adjustment after centrifugal concentration;

FIG. 3 is a graph of ultraviolet absorption spectra of AuNPs before pH adjustment and AuNPs after pH adjustment after ligand modification;

FIG. 4 is a diagram of UV absorption spectra of AuNPs before pH adjustment and AuNPs after pH adjustment after solvent conversion;

FIG. 5 is a graph of the aggregation levels of AuNPs before and after pH control, after centrifugal concentration, ligand modification, and solvent conversion, respectively;

FIG. 6 is a diagram of aggregation of AuNPs added with different surfactants and AuNPs after pH regulation after centrifugal concentration, ligand modification and solvent conversion, respectively;

FIG. 7 is a SERS comparison graph of AuNPs after pH regulation and AuNPs with CTAB added to modify 4-mercaptobenzoic acid;

FIG. 8 is a microscope image of AuNPs after pH adjustment and concentration by centrifugation and observing the presence or absence of coffee ring effect;

FIG. 9 is a graph of the aggregation of AuNPs before and after pH control, which have been ligand-modified;

FIG. 10 is a diagram of the UV absorption spectra of AuNPs after pH adjustment and solvent conversion;

FIG. 11 is a reaction kinetics diagram of AuNPs after pH regulation for catalyzing NaBH4 to reduce p-nitrophenol in solvent systems with different ethanol contents;

FIG. 12 is a Zeta potential diagram of AuNPs under different pH conditions;

FIG. 13 is Zeta potential diagrams of AuNPs before pH adjustment and AuNPs after pH adjustment after centrifugal concentration, ligand modification and solvent conversion, respectively;

FIG. 14 is a SERS graph of AuNPs under different pH conditions;

fig. 15 is a schematic diagram of pH-regulated stability of AuNPs in the absence of surfactants.

Detailed Description

The invention is described in detail below with reference to the figures and the embodiments.

1. Preparation of Stable monodisperse AuNPs

The method provided by the invention can improve the stability of the monodisperse AuNPs and obtain stable monodisperse AuNPs, and the method specifically comprises the following two steps:

1. AuNPs synthesized by reducing chloroauric acid with sodium citrate

200mL of 0.01% (w/v) HAuCl ₄ Heating the aqueous solution to boiling, then quickly adding 1.4mL of 1% (w/v) sodium citrate aqueous solution, changing the color of the mixed solution from light yellow to black after about 1min, changing the color of the mixed solution from black to brownish red after 2-3 min, continuously keeping slight boiling for 40min, finally stopping the reaction, cooling in water bath to obtain AuNPs, and storing for later use.

2. Regulating and controlling the pH value of AuNPs

And (3) regulating the pH value of the AuNPs obtained in the step (1) by using a NaOH aqueous solution with the concentration of 0.1M, and regulating the pH value to 12 to obtain stable monodisperse AuNPs.

The AuNPs after pH regulation are subjected to electron microscope scanning, and the obtained SEM image is shown in figure 1.

As can be seen from fig. 1: auNPs with uniform size are successfully prepared, and the particle size of the AuNPs is 55nm.

2. Comparing the stability of AuNPs before and after pH control

1. Centrifugal concentration

Centrifuging and concentrating AuNPs before pH regulation (AuNPs obtained in step 1) and AuNPs after pH regulation (AuNPs obtained in step 2) at 6000r/min for 10min, detecting the ultraviolet absorption spectrum of the AuNPs, and recording the absorbance values of the AuNPs at 700nm and 530 nm.

The ultraviolet absorption spectrum of the AuNPs obtained by detection is shown in figure 2.

2. Ligand modification

Modifying AuNPs before pH regulation (AuNPs obtained in step 1) and AuNPs after pH regulation (AuNPs obtained in step 2) by using 4-mercaptobenzoic acid (MBA), wherein the modification process is as follows:

adding MBA into 1mL of AuNPs before pH regulation and 1mL of AuNPs after pH regulation respectively, wherein the final concentration of the MBA is 0.01mM, then fully reacting in a shaking table for 1.5h, then centrifuging at 6000r/min for 10min, and centrifuging and washing the obtained solid with NaOH aqueous solution with the pH =12 twice (centrifuging at 6000r/min for 10 min), thus finishing ligand modification.

And after ligand modification is finished, detecting the ultraviolet absorption spectrum of the AuNPs, and recording the absorbance values of the AuNPs at 700nm and 530 nm.

The ultraviolet absorption spectrum of the AuNPs obtained by detection is shown in FIG. 3.

3. Solvent conversion

The AuNPs before pH regulation (AuNPs obtained in step 1) and the AuNPs after pH regulation (AuNPs obtained in step 2) are converted into an organic solvent (ethanol) from an aqueous phase, and the process of solvent conversion is as follows:

and respectively centrifuging 1mL of AuNPs before pH regulation and 1mL of AuNPs after pH regulation at 6000r/min for 10min, discarding supernatant, and re-dispersing the solid into ethanol with the same volume to complete solvent conversion.

And after the solvent conversion is finished, detecting the ultraviolet absorption spectrum of the AuNPs, and recording the absorbance values of the AuNPs at 700nm and 530 nm.

The ultraviolet absorption spectrum of the AuNPs obtained by detection is shown in FIG. 4.

The aggregation degree of AuNPs is reflected by the ratio (A700/A530) of the absorbance value of AuNPs at 700nm to the absorbance value at 530nm, the aggregation degree of AuNPs before pH regulation and the aggregation degree of AuNPs after pH regulation after centrifugal concentration, ligand modification and solvent (ethanol) conversion are respectively obtained by calculation, and the calculation result is shown in figure 5.

As can be seen from fig. 2 to 5: when the AuNPs are subjected to centrifugal concentration, ligand modification and solvent conversion, the AuNPs before pH regulation are easy to aggregate, and the AuNPs after pH regulation are good in dispersion, which shows that the stability of the AuNPs is enhanced by pH regulation.

3. Comparing stability of AuNPs after adding surfactant and after pH regulation

Polyvinylpyrrolidone (PVP), tween-20 and cetyltrimethylammonium bromide (CTAB) were added to AuNPs before pH control (AuNPs obtained in step 1), and the final concentrations of PVP, tween-20 and CTAB were 1% (w/v), 0.1% (w/v) and 1mM, respectively.

1. Centrifugal concentration

And (3) centrifugally concentrating the AuNPs added with different surfactants and the pH-regulated AuNPs (AuNPs obtained in step (2)) at 6000r/min for 10min, and observing the aggregation condition of the AuNPs in each tube.

2. Ligand modification

The AuNPs added with different surfactants and the AuNPs after pH regulation (AuNPs obtained in step 2) are modified by MBA, and the modification process is the same as before, and is not described again. And after ligand modification is finished, observing the aggregation condition of AuNPs in each tube.

3. Solvent conversion

The AuNPs added with different surfactants and the pH-regulated AuNPs (AuNPs obtained in step 2) are subjected to solvent (ethanol) conversion, and the process of solvent conversion is the same as that described above, and is not described again. And after the solvent conversion is finished, observing the aggregation condition of the AuNPs in each tube.

The aggregation of each group of AuNPs is finished, and the finishing result is shown in FIG. 6 and Table 1, wherein x represents aggregation of AuNPs, and v represents non-aggregation of AuNPs.

TABLE 1 statistical table of aggregation for groups of AuNPs

As can be seen from fig. 6 and table 1:

(1) During centrifugal concentration, ligand modification and solvent conversion, the addition of the surfactant can only ensure that AuNPs do not aggregate in one or two operations, and cannot ensure that AuNPs do not aggregate in three operations, while the AuNPs after pH regulation (AuNPs obtained in step 2) do not aggregate in three operations;

(2) Among three surfactants of PVP, tween-20 and CTAB, CTAB has the best protective effect on AuNPs, and can ensure that AuNPs do not aggregate when centrifugal concentration and ligand modification are carried out.

After the AuNPs added with CTAB and the pH-regulated AuNPs (AuNPs obtained in step 2) are modified by MBA, SERS detection is performed, and the resulting SERS contrast graph is shown in fig. 7.

As can be seen from fig. 7: compared with AuNPs without CTAB, the AuNPs with CTAB are 1075cm in SERS detection ^-1 、1141cm ^-1 、1272cm ^-1 、1344cm ^-1 、1374cm ^-1 、1449cm ^-1 And 1484cm ^-1 Generates obvious SERS background signal, wherein, 1075cm ^-1 And 1141cm ^-1 The characteristic peak is attributed to C-C stretching vibration, 1272cm ^-1 Characteristic peak of (A) is attributed to-CH ₂ −N ⁺ −(CH ₃ ) ₃ Delta (C-H) vibration of the radical, 1344cm ^-1 And 1374cm ^-1 Characteristic peak of (A) and CH ₂ Correlation, 1449cm ^-1 And 1484cm ^-1 Characteristic peak of (A) and CH ₂ Shear and CH ₃ Related to the deformation of (1), introduction of CTABThe method has a certain restriction effect on the development of AuNPs in the SERS field, and because CTAB is not added to the AuNPs after pH regulation, the SERS background signal is not introduced during SERS detection, which shows that the AuNPs (namely the AuNPs after pH regulation) obtained by the method provided by the invention have certain superiority when applied to the SERS field.

In conclusion, the AuNPs obtained by the method provided by the invention have good stability, are not easy to self-aggregate even in the absence of a surfactant when centrifugal concentration or separation, ligand modification and solvent conversion are carried out, the self-aggregation problem of the AuNPs in the centrifugal concentration or separation, ligand modification and solvent conversion is solved, and the further application of the AuNPs is widened.

4. Specific effect of pH-regulated AuNPs in practical application

1. Centrifugal concentration

Concentration of nanoparticles by centrifugation is a common process for nanoparticle sensing and catalytic applications.

And (3) centrifugally concentrating 1mL of pH-regulated AuNPs (AuNPs obtained in step (2)) at 6000r/min for 10min, dripping the centrifugally concentrated AuNPs on a silicon wafer, and finally carrying out microscope detection on the silicon wafer to obtain an optical photo as shown in figure 8.

As can be seen from fig. 8: the AuNPs after pH regulation have good stability, are not easy to aggregate after centrifugal concentration, and can still be uniformly dispersed on a silicon wafer, so that the coffee ring effect is obviously reduced.

Therefore, the stability of AuNPs is regulated and controlled by pH, so that the method has important significance for centrifugal concentration of nanoparticles, and has a promoting effect on development of various fields related to coffee ring effect.

2. Ligand modification

Ligand modified AuNPs are common experiments in various application fields such as nanoparticle self-assembly, chemistry, biosensing and the like.

Respectively adding positive charge mercapto molecules (p-aminophenol, 4-acetamino thiophenol, 4-mercaptopyridine, o-aminobenzothiophenol) and neutral mercapto molecules (mercaptoethanol, 3-mercapto-1-propanol, 1, 3-propanedithiol, 1-hexanethiol, N-octadecyl into 1mL of AuNPs after pH regulationThiols, perfluorodecyl thiol), negatively charged sulfhydryl molecules (4-mercaptophenylboronic acid, 4-mercaptobenzoic acid, 3-mercaptopropionic acid, L-cysteine) and anions (SCN) ^- 、I ^- 、Br ^- 、S ₂ O ₃ ^2- ) The final concentration of the sulfhydryl ligand is 0.01mM, the final concentration of the anionic ligand is 10mM, the shaking table is fully reacted for 1.5h, then the centrifugation is carried out for 10min at 6000r/min, and finally the obtained solid is centrifugally washed twice by NaOH aqueous solution with the pH =12 (the centrifugation is carried out for 10min at 6000 r/min), thus finishing the ligand modification.

The aggregation conditions of AuNPs before pH regulation (AuNPs obtained in step 1) and AuNPs after pH regulation (AuNPs obtained in step 2) after modification by thiol molecules and ions with different charges are shown in FIG. 9.

As can be seen from fig. 9: compared with AuNPs before pH regulation, the AuNPs after pH regulation can successfully modify sulfhydryl molecules and ions without significant aggregation.

Therefore, the aggregation problem caused by directly modifying the ligand can be well solved by regulating the stability of the AuNPs through pH, and the application range of the AuNPs is enlarged.

3. Solvent conversion

The pH-controlled AuNPs (AuNPs obtained in step 2) were converted from the aqueous phase into dimethyl sulfoxide (DMSO), N-Dimethylformamide (DMF), methanol and ethanol, respectively, and the process of solvent conversion was as follows:

and (3) centrifuging and concentrating 1mL of pH-regulated AuNPs at 6000r/min for 10min, and re-dispersing the concentrated solid in DMSO, DMF, methanol and ethanol with the same volume to complete solvent conversion.

After the solvent conversion is finished, the ultraviolet absorption spectrum of the AuNPs is detected, and the detection result is shown in figure 10.

As can be seen from fig. 10: the pH-regulated AuNPs achieved the conversion of AuNPs from water to four common organic solvents (methanol, ethanol, DMF, and DMSO).

The AuNPs (AuNPs obtained in step 2) after pH regulation are converted from the water phase into ethanol water solutions with different concentrations, the volume concentrations of the ethanol are respectively 0%, 20%, 50% and 100%, and then the AuNPs are used for catalyzing NaBH ₄ Reducing p-nitrophenol, wherein the catalytic reduction process is as follows:

1mL of a p-nitrophenol ethanol solution with a concentration of 0.1mM, 50. Mu.L of AuNPs (the volume concentrations of ethanol are 0%, 20%, 50%, and 100%, respectively) after the conversion of the solvent (ethanol), and 100. Mu.L of an aqueous NaBH4 solution with a concentration of 0.1M are sequentially mixed for catalytic reaction.

And in the catalytic reduction process, detecting ultraviolet absorption spectra at different reaction time points, and drawing a reaction kinetic diagram. The resulting reaction kinetics are plotted in FIG. 11.

As can be seen from fig. 11: catalytic ability of AuNPs with Water (H) ₂ O) is present in a large relationship, probably due to the organic solvent and H ₂ The formation of hydrogen bonds between O and thus the surface reaction (BH) is influenced ₄ ^- +H ₂ O+2Au→BH ₃ OH ^- +2 Au-H) which will provide new ideas and directions for research in the relevant fields involving reactions that must be carried out in organic solvents.

5. Explores the principle of pH regulation and control of the stability of AuNPs

1. Detecting Zeta potential of AuNPs under different pH conditions

Adding 0.1M NaOH aqueous solution into AuNPs synthesized by reducing chloroauric acid with sodium citrate (namely the AuNPs obtained in step 1), adjusting the pH values of the AuNPs to 8, 10 and 12 respectively, and detecting the Zeta potentials of the AuNPs under different pH conditions.

The results of the Zeta potential measurements of AuNPs at different pH are shown in FIG. 12.

Zeta potentials at pH 7 to pH 12 indicate that the enhancement of stability of AuNPs can be attributed to a significant increase in negative charge of the AuNPs.

2. Detecting the Zeta potential of AuNPs after centrifugal concentration, ligand modification and solvent conversion before and after pH regulation

And (3) centrifugally concentrating the AuNPs (AuNPs before pH regulation) obtained in the step (1) and the AuNPs (AuNPs after pH regulation) obtained in the step (2) at 6000r/min for 10min, and then detecting the Zeta potential of the AuNPs.

The detection results of Zeta potentials of AuNPs before and after pH regulation after centrifugal concentration, ligand modification and solvent conversion are shown in figure 13.

Comparing Zeta potentials measured after centrifugal concentration, ligand modification and solvent conversion of AuNPs (pH = 7) before pH regulation and AuNPs (pH = 12) after pH regulation respectively, the negative charge amount of the AuNPs after pH regulation is still higher after three operations.

3. SERS spectrogram for detecting AuNPs under different pH conditions

Adding 0.1M HCl aqueous solution or 0.1M NaOH aqueous solution into the AuNPs obtained in the step 1, respectively adjusting the pH values of the AuNPs to 3, 10 and 12, and detecting SERS spectrograms of the AuNPs under different pH conditions.

The SERS measurements of AuNPs at different pH conditions are shown in fig. 14.

1360cm ^-1 Corresponding to a symmetrical stretching of Vs (COO-), 1539cm ^-1 And 1597cm ^-1 Corresponding to an asymmetric stretching of Va (COO-). 1539cm ^-1 And 1597cm ^-1 In relation to the terminal and central carboxylates, respectively.

From the comprehensive analysis of fig. 14, it can be seen that: at alkaline pH-COO-due to reaction with H ⁺ The competition is less (acid effect), the gold surface is more prone to be combined, so that a stable short fiber structure is formed (Au-O = C-O-Au), the formation of the stable short fiber structure can selectively enhance the symmetrical stretching of-COO-and hinder the asymmetrical stretching of-COO-, and the pH regulation influences the combination mode and conformation of Au and COO-, and the mechanism has guiding significance for the research of AuNPs.

4. Conclusion

Through a series of previous researches, the principle of pH regulation and control of the stability of AuNPs is as follows:

as shown in fig. 15, 0.1M NaOH aqueous solution was added to AuNPs synthesized by reducing chloroauric acid with sodium citrate, and the pH of the AuNPs was adjusted to alkaline, specifically to 12, and the alkaline condition was able to suppress H ⁺ And the competition with AuNPs increases the negative quantity of citrate on the surface of the AuNPs, enhances the binding capacity of the citrate and the AuNPs, and further enhances the stability of the AuNPs.

It should be noted that the above examples of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. Not all embodiments are exhaustive. Obvious changes or modifications of the invention are within the scope of the invention.

Claims

1. A method for improving the stability of monodisperse gold nanoparticles is characterized by comprising the following steps:

step 1: reducing chloroauric acid by using sodium citrate to synthesize AuNPs;

step 2: and (3) regulating the pH value of the AuNPs obtained in the step (1) by using an alkaline solution under the condition of not adding a surfactant, and regulating the pH value to 12 to obtain stable monodisperse AuNPs.

2. The method for improving the stability of monodisperse gold nanoparticles according to claim 1, wherein in step 1, the process for synthesizing AuNPs by reducing chloroauric acid with sodium citrate is as follows:

HAuCl with the mass concentration of 0.01 percent ₄ Heating the water solution to boiling, and rapidly adding 1% sodium citrate water solution and HAuCl ₄ The volume ratio of the aqueous solution to the sodium citrate aqueous solution is 1000: and 7, stopping the reaction after slightly boiling for 40min, and cooling in a water bath to obtain AuNPs.

3. The method for improving the stability of monodisperse gold nanoparticles according to claim 1, wherein in step 2, the alkaline solution is 0.1M NaOH aqueous solution; the particle size of the stable monodisperse AuNPs is 55nm.