Preparation method of positively charged and LSPR wavelength controllable aggregated nano-silver

Abstract

The application discloses a preparation method of positively charged and LSPR wavelength controllable aggregated nano silver, which is a one-step method for preparing aggregated nano silver with controllable and adjustable LSPR wavelength in the range of 532 nm to 785 nm by using cetyltrimethylammonium bromide as a special surface coating agent and a connecting agent, adjusting the pH of a solution, and matching three commonly used Raman lasers (532, 633 and 785 nm) so that the electromagnetic field enhancement effect reaches the best state. Meanwhile, the chemical enhancement effect is coordinated, so that the detection sensitivity of the measured molecules is improved. The preparation method is simple in operation, environment-friendly, low in technical requirement and high in operability. The material obtained by the application can keep a long-term stable aggregation state in a solution, so that the material can be directly used as a liquid substrate for surface enhanced Raman and applied to sensitive detection of some pesticide residues and illegal food additives.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of preparation of surface-enhanced Raman substrates, and relates to a preparation method of positively charged aggregated nanosilver with adjustable LSPR wavelength. BACKGROUND

[0002] Surface-enhanced Raman scattering (SERS) technology combines traditional Raman spectroscopy with nanofabrication technology, which not only has the characteristics of traditional Raman spectroscopy fingerprint identification, but also uses the surface plasmon resonance effect of noble metal nanoparticles to significantly improve the sensitivity. Therefore, SERS technology has been widely used in the fields of food safety, biomedicine, environmental monitoring, etc. The enhancement effect of SERS depends largely on the preparation of the substrate. In a long period of time, it is generally believed that EM is the main reason for the SERS effect, and its contribution is much greater than CM. Therefore, people often pay more attention to the construction of SERS substrates with high electromagnetic "hot spot" density. Silver matrix nanostructures have strong localized surface plasmon (LSPR) absorption in the visible and near-infrared light regions, which can produce a very strong local electromagnetic field, ultimately leading to strong EM effect. Therefore, silver matrix nanostructures have become a popular choice for SERS substrates. However, in recent years, more and more evidence shows that the contribution of CM to SERS is also not negligible. Recently, from our related research, it has been found that even for gold and silver matrix SERS substrates, CM cannot be ignored. Therefore, to construct a SERS substrate material with high sensitivity response, the contributions of EM and CM need to be considered simultaneously to play their synergistic effect. When detecting different molecules, according to the energy interval between the Fermi level of the silver-based SERS substrate and the molecular energy level, a laser with energy matching is selected to obtain the strongest CM effect. At the same time, the LSPR wavelength of the SERS substrate needs to be adjusted to obtain the strongest EM effect. At present, aggregated silver nanoparticles (a-AgNPs) usually have a large number of electromagnetic field "hot spots", so they are the most commonly used SERS substrate. However, at present, the aggregated silver nanoparticles are usually obtained by adding cationic inducers to the solution of carboxyl-functionalized silver nanoparticles to induce aggregation. However, the material obtained by this method has a short stable time in solution, and the LSPR wavelength is uncontrollable. In addition, aggregated nanosilver can also be prepared by direct preparation or self-assembly. At present, many self-assembly strategies have been developed, such as using electrostatic interaction to induce nanoparticle aggregation, DNA hybridization driven method, and molecular interaction method, etc. to prepare aggregated silver nanoparticles. However, due to the complexity of the operation and the difficulty in controlling the degree of aggregation, the ideal effect is often not achieved.

[0003] Cetyltrimethylammonium bromide is a kind of cationic surfactant, which is well known as a kind of morphology directing agent, capping agent and linker to assemble nanoparticles into aggregates. On the other hand, the presence of surfactant can protect nanoparticles from excessive aggregation and stabilize them in aqueous solution. With the increase of aggregation degree of nanoparticles, more SERS hot spots can be generated, and the Raman signal of the analyte can be significantly enhanced when the analyte is located in the hot spot. In addition, unlike most existing silver-based SERS substrates, the substrate obtained by the method has a positive charge on the surface, which is more conducive to the detection of analytes with negative charges. The sol SERS substrate prepared can be directly used for rapid detection of trace pollutants in food. SUMMARY

[0004] The present application aims at the deficiencies of the prior art and provides a preparation method of aggregated nano-silver with positive charge and adjustable LSPR wavelength, which is simple in operation and low in cost. The cetyltrimethylammonium bromide-coated silver-based nanomaterial obtained has good surface-enhanced Raman activity, and the LSPR adjustable nanomaterial prepared can be matched with three common Raman lasers (532 nm, 633 nm and 785 nm), so that the EM effect reaches the strongest. The nanomaterial sol can be applied to rapid detection of certain pesticide residues.

[0005] To achieve the above object, the present application adopts the following technical scheme:

[0006] A preparation method of aggregated nano-silver with positive charge and adjustable LSPR wavelength, comprising the following specific steps:

[0007] 1) Add an appropriate amount of deionized water into a container and start heating;

[0008] 2) Add silver nitrate solution and cetyltrimethylammonium bromide solution into the solution of step 1);

[0009] 3) Heat the mixed solution obtained in step 2) to about 50 DEG C, add an alkaline solution under stirring to adjust the pH of the solution to about 8-10, and fully react for 3-5 minutes;

[0010] 4) Add glucose solution (the final concentration of glucose is 0.003 mol / L) into the solution of step 3) and continuously heat at 65 DEG C for 20 minutes.

[0011] 5) Naturally cool the solution obtained in step 4) to room temperature and centrifugally wash and collect the precipitate;

[0012] 6) Redisperse the precipitate obtained after centrifugation in step 5) in deionized water to obtain aggregated silver nanoparticles.

[0013] The cetyltrimethylammonium bromide in step (1) is a cationic active agent, which can be used as a coating agent and a connecting agent. In step 1), the concentration of the cetyltrimethylammonium bromide is 0.05-0.2 mol / L, the concentration of the silver nitrate is 0.05-0.1 mol / L, and the molar ratio of the silver nitrate solution to the cetyltrimethylammonium bromide solution is 3:1-8:1.

[0014] In step 3), the base includes sodium hydroxide, potassium hydroxide, ammonia water, lithium hydroxide and the like, the concentration of the base is 0.1-0.2 mol / L, and the pH value of the solution is 8-10.

[0015] In step 4), the final concentration of the glucose is 0.003 mol / L.

[0016] In step 5), the centrifugal speed is 12000-15000 rpm, the centrifugal time is 10-20 min, the centrifugal washing time is 2 times, and the precipitate is collected.

[0017] The significant advantages of the present application are:

[0018] 1) The preparation method of the positively charged and LSPR wavelength controllable aggregated state nano-silver surface enhanced Raman substrate adopted in the present application is simple to operate, does not need complex synthesis steps, is environmentally friendly, has good product performance and good stability;

[0019] 2) The cetyltrimethylammonium bromide coated silver-based nanomaterial LSPR wavelength prepared in the present application is controllable, can be matched with three commonly used Raman lasers (532 nm, 633 nm and 785 nm) wavelengths, the EM effect reaches the strongest, and thus a stronger SERS enhancement effect is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 TEM images of the three different aggregation degree cetyltrimethylammonium bromide coated silver nanoparticles prepared in the examples; wherein figures a, b and c are a-532nm-AgNPs / CTAB 8.75 , a-633nm-AgNPs / CTAB9, a-785nm-AgNPs / CTAB 9.75 TEM images of the three different aggregation degree materials;

[0021] Figure 2 UV-visible absorption spectrum of the different aggregation degree cetyltrimethylammonium bromide coated silver nanoparticles;

[0022] Figure 3 Infrared absorption spectrum of the cetyltrimethylammonium bromide solid and the three different aggregation degree cetyltrimethylammonium bromide coated silver nanoparticles prepared in the examples;

[0023] Figure 4 For the optimal example, the Raman signal intensity of 4-mercaptobenzoic acid was detected on a sol substrate containing silver nanoparticles coated with hexadecyltrimethylammonium bromide. Figure 4 a represents different concentrations of 4-mercaptobenzoic acid (1×10⁻⁶). -11 ~1×10 -7 mol / L) at a-785nm-AgNPs / CTAB 9.75 Typical characteristic peak (1073 cm⁻¹) -1 The strength of ); Figure 4 b represents different concentrations of 4-mercaptobenzoic acid (1×10⁻⁶). -11 ~1×10 -7 mol / L) at a-785nm-AgNPs / CTAB 9.75 Typical SERS spectra. Detailed Implementation

[0024] To better understand the present invention, examples are provided for further illustration, but the present invention is not limited thereto.

[0025] Example 1

[0026] Dissolve 0.1 mL of cetyltrimethylammonium bromide solution (0.1 mol / L) in 17–19 mL of water, then add 0.5 mL of silver nitrate solution (0.1 mol / L) to the mixture. Heat the resulting solution to approximately 50 °C, add 1 mL of sodium hydroxide to adjust the pH to approximately 8.75, and then react uniformly for 3–5 min. While stirring, rapidly add 600 μL of glucose solution (0.1 mol / L), and continue the reaction at 65 °C for 20 min. Collect the supernatant, and then centrifuge the collected solution twice at 12,000 rpm for 10 min each time. Redisperse the resulting precipitate in 5 mL of deionized water to obtain silver nanoparticles coated with cetyltrimethylammonium bromide, with an SPR absorption peak around 532 nm. Store at 4 °C.

[0027] Example 2

[0028] Take 0.1 mL of cetyltrimethylammonium bromide solution (0.1 mol / L) in 17-19 mL of water, then add 0.5 mL of silver nitrate solution (0.1 mol / L) to the above mixture, heat the resulting mixture to about 50°C, add 1.5 mL of sodium hydroxide to adjust the pH to about 9, then uniformly react for 3-5 min, quickly add 600 μL of glucose solution (0.1 mol / L) under stirring, and continue to react at 65°C for 20 min. Collect the supernatant, then centrifuge the collected solution at 12000 rpm for 10 min twice; re-disperse the resulting precipitate solution in 5 mL of water, to obtain the cetyltrimethylammonium bromide-coated silver nanoparticle material with SPR absorption peak at about 633 nm, and store it in a 4°C refrigerator.

[0029] Example 3

[0030] Take 0.1 mL of cetyltrimethylammonium bromide solution (0.1 mol / L) in 17-19 mL of water, then add 0.5 mL of silver nitrate solution (0.1 mol / L) to the above mixture, heat the resulting mixture to about 50°C, add 2 mL of sodium hydroxide to adjust the pH to about 10, then uniformly react for 3-5 min, quickly add 600 μL of glucose solution (0.1 mol / L) under stirring, and continue to react at 65°C for 20 min. Collect the supernatant, then centrifuge the collected solution at 12000 rpm for 10 min twice; re-disperse the resulting precipitate solution in 5 mL of water, to obtain the cetyltrimethylammonium bromide-coated silver nanoparticle material with SPR absorption peak at about 785 nm, and store it in a 4°C refrigerator.

[0031] Example 4

[0032] The sol material obtained in Example 3 is directly used as a surface-enhanced Raman substrate, and 4-mercaptobenzoic acid is used as a probe molecule for Raman enhancement test. First, clean the glass slides by ultrasonic cleaning in ethanol and water for 5-10 min, and then dry them under N2 atmosphere. Add 50 μL of the sol material obtained in Example 3 and 50 μL of 4-mercaptobenzoic acid solution to a 1.5 mL centrifuge tube, mix them thoroughly, and incubate at room temperature for about 10-15 min. Finally, take 50 μL of the mixed solution and drop it on a clean glass slide, spread it evenly, and measure the Raman enhancement effect using a Raman instrument.

[0033] Figure 1 Transmission electron microscopy images of the three different aggregation levels of cetyltrimethylammonium bromide-coated silver nanoparticles prepared in the examples. Figure 1The results show that the size of silver nanoparticles prepared by this method is uniform, about 28 nm. The results show that the size of silver nanoparticles prepared by this method is uniform, about 28 nm. The results show that the size of silver nanoparticles prepared by this method is uniform, about 28 nm.

[0034] Figure 2 The UV-Vis absorption spectra of silver nanoparticles coated with cetyltrimethylammonium bromide with different aggregation degrees. Figure 2 The results show that the silver nanoparticles coated with cetyltrimethylammonium bromide with different aggregation degrees all have a sharp peak at about 395 nm, corresponding to the characteristic absorption peak of silver nanoparticles. Another absorption peak appears in the range of 532 nm to 785 nm, reflecting the formation of aggregates. As the pH increases, the LSPR red shifts, and the aggregation degree increases. Combined with the results of transmission electron microscopy, it is more certain that silver nanoparticles with different aggregation degrees are synthesized.

[0035] Figure 3 The infrared absorption spectra of cetyltrimethylammonium bromide solid and silver nanoparticles with three different aggregation degrees in the examples. From Figure 3 It can be seen that the cetyltrimethylammonium bromide solid and silver nanoparticles with different aggregation degrees all contain the same N-CH3 symmetric and anti-symmetric stretching vibration peaks and carbon chain skeleton vibration peaks. The position and intensity of the absorption peaks have changed, the position of the absorption peaks has shifted slightly, and the intensity of the absorption peaks has weakened significantly. This indicates that there is an interaction between silver nanoparticles and cetyltrimethylammonium bromide, and it is more certain that silver nanoparticles coated with cetyltrimethylammonium bromide are synthesized.

[0036] Figure 4 The Raman signal intensity of 4-mercaptobenzoic acid is detected by the silver nanoparticle sol-gel substrate material coated with cetyltrimethylammonium bromide with a LSPR absorption peak wavelength near 785 nm. The detection concentration range is 10 -7 ~ 10 -11 ( Figure 4 a). From Figure 4 b can be seen that the silver nanoparticle substrate material coated with cetyltrimethylammonium bromide with a LSPR near 785 nm has high detection sensitivity for 4-mercaptobenzoic acid, and the SERS signal intensity shows a good linear relationship with the logarithmic value of the concentration of the measured substance. Compared with other surface-enhanced Raman substrates, the silver nanoparticle substrate material with positive charge and adjustable LSPR wavelength has relatively better Raman signal.

[0037] The raw material used in the application is cheap and easy to obtain, the experimental operation is simple and convenient, no special experimental instrument is needed (only oil bath device and condensation reflux device are needed), it is environment-friendly, and the product has good dispersity and stability, when used as a SERS substrate, it has good sensitivity, uniformity, reproducibility and stability.

[0038] The above merely describes preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the present application should be included in the scope of the present application.

Claims

1. A method for preparing positively charged and LSPR wavelength-controllable aggregated nano-silver, characterized by comprising the following steps: The preparation method comprises the following steps: ​ 1) adding an appropriate amount of deionized water into a container and starting heating; 2) adding a cetyltrimethylammonium bromide solution and a silver nitrate solution into the solution of step 1); 3) when the mixed solution of step 2) is heated to about 50℃, adding a base solution to adjust the pH to 8-10 under the condition of continuous heating and stirring, and fully reacting for 3-5 minutes; the LSPR wavelength of the aggregated nano-silver is controlled by adjusting the pH of the solution; 4) adding a glucose solution into the solution of step 3) and reacting at a temperature of 65℃ for 20 minutes; 5) naturally cooling the solution of step 4) to room temperature and centrifuging and washing to collect the precipitate; 6) dispersing the precipitate obtained after centrifugation of step 5) in deionized water to obtain silver nanoparticles with different aggregation degrees.

2. The method according to claim 1, wherein the method for preparing the positively charged and LSPR wavelength-controllable aggregated nano-silver is characterized by: In step 2), the concentration of cetyltrimethylammonium bromide is 0.05-0.2 mol / L, the concentration of silver nitrate is 0.05-0.1 mol / L, and the molar ratio of the silver nitrate solution to the cetyltrimethylammonium bromide solution is 3:1-8:

1.

3. The method according to claim 1, wherein the method is characterized by: In step 3), the base includes sodium hydroxide, potassium hydroxide, lithium hydroxide and ammonia.

4. The method according to claim 1, wherein the method is characterized by: In step 4), the final concentration of the glucose solution is 0.003 mol / L.

5. The method according to claim 1, wherein the method is characterized by: In step 5), the centrifugal speed is 12000-15000 rpm, the centrifugal time is 10-20 min, the centrifugal washing times are 2, and the precipitate is collected.

6. Aggregated nano-silver with positive charge and adjustable LSPR wavelength prepared by the preparation method of any one of claims 1-5.

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