Two-dimensional heterogeneous self-assembly film and preparation method thereof

Abstract

The invention discloses a two-dimensional heterogeneous self-assembled film and a preparation method thereof. The preparation method comprises the following steps: successfully synthesizing Fe3O4-COOH nanoparticles which are uniform in morphology and stable in size and have carboxyl modified surfaces by adopting an improved solvothermal method; then, the surface of the Fe3O4-COOH is modified with a layer of positively charged polymer BPEI through the electrostatic adsorption effect, and Fe3O4 (at) BPEI is formed; the preparation method comprises the following steps: preparing Fe3O4 (at) BPEI (at) Au, further uniformly adsorbing (12nm) gold nanoparticles on the surface of the Fe3O4 (at) BPEI (at) Au through electrostatic adsorption, finally forming a stable gold shell structure, constructing Fe3O4 (at) BPEI (at) Au with a magnetic core gold shell structure, modifying sulfydryl polystyrene (SH-PS) on the surface of the Fe3O4 (at) BPEI (at) Au, successfully preparing a two-dimensional heterogeneous self-assembled film by taking the Fe3O4 (at) BPEI (at) Au as a construction unit, and constructing the two-dimensional heterogeneous self-assembled film by taking the Fe3O4 (at) BPEI (at) Au as the construction unit. The two-dimensional heterogeneous self-assembled thin film shows good SERS enhancement performance and uniformity, and shows SERS signal sensitization capacity under the action of an external magnetic field.

Description

Technical Field

[0001] This invention relates to the field of two-dimensional ordered nanofilm preparation technology, specifically to a two-dimensional heterogeneous self-assembled thin film and its preparation method. Background Technology

[0002] Multifunctional heterostructured micro / nanostructures have attracted significant attention due to their superior performance in biomedical detection. Plasmon materials, with their unique optical properties, can excite locally enhanced electromagnetic fields at the nanoscale, significantly enhancing Raman signals; while magnetic materials possess excellent external field response capabilities, facilitating the efficient enrichment and separation of target molecules. Combining these two technologies to construct plasmon / magnetic heterocomposites holds promise for achieving synergistic regulation of structure and properties, providing a novel strategy for highly sensitive and selective biosensing detection.

[0003] The performance of plasmonic micro / nanomaterials typically depends on the fine-tuning of their structural morphology and the enrichment of their nanostructures. Introducing magnetic materials allows plasmonic / magnetic micro / nanomaterial heterostructures to enhance their performance through various possible sensitization pathways. However, in current surface-enhanced Raman scattering (SERS) sensitization mechanisms, the crucial scientific question of whether an applied magnetic field can intrinsically modulate SERS performance remains unexplored. Constructing plasmonic / magnetic heterostructures using plasmonic / magnetic micro / nanomaterials as basic building blocks can provide a new entry point for this research direction and open up new pathways for revealing the SERS sensitization mechanism.

[0004] The SERS signal of a composite substrate of plasmonic materials and magnetic nanomaterials may be enhanced or weakened under the influence of an external magnetic field. However, to date, there is no definitive principle report proving that an external magnetic field can intrinsically and significantly enhance or degrade the SERS performance of a SERS substrate containing magnetic nanoparticles. Existing substrates often exhibit inhomogeneity, and their structure changes under an external magnetic field, altering the interparticle spacing and affecting the reproducibility and reliability of experiments. Therefore, if plasmonic materials and magnetic nanomaterials can be combined to form a single building block, and further fabricated into ordered two-dimensional heterogeneous self-assembled films, it will contribute to a deeper understanding of the modulation mechanism of the SERS signal by an external magnetic field, providing a new direction for research in this field. Summary of the Invention

[0005] To address the problems mentioned in the background section, this invention discloses a two-dimensional heterogeneous self-assembled thin film and its preparation method. First, Fe3O4-COOH magnetic nanobeads with carboxyl groups on their surface are synthesized, and a layer of strongly positively charged branched polyethyleneimine (BPEI) is uniformly coated onto their surface via electrostatic adsorption. Subsequently, a gold shell is uniformly adsorbed onto the Fe3O4@BPEI surface again using electrostatic adsorption, forming a Fe3O4@BPEI@Au nanostructure. Further, thiolated polystyrene (SH-PS) is modified onto its surface to balance and regulate the complex interactions between nanoparticles during the two-step drying-mediated self-assembly process, thereby ultimately constructing a single-layer ordered two-dimensional heterogeneous self-assembled thin film.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention discloses a method for preparing a two-dimensional heterogeneous self-assembled thin film, characterized by comprising the following steps:

[0008] S01, AuNPs preparation: 25 mM chloroauric acid solution is added to deionized water, stirred, and then 1 wt.% sodium citrate solution is added. After the reaction is completed, the obtained gold nanoparticles AuNPs are stored in a refrigerator at 4°C for later use; preferably, the particle size of the gold nanoparticles AuNPs is 5 nm, 12 nm, and 20 nm.

[0009] The preparation of SO2 and Fe3O4-COOH involves dissolving ferric chloride hexahydrate in a mixed solution of diethylene glycol and ethylene glycol, then adding sodium acetate and sodium acrylate, alternating between vigorous magnetic stirring at 1200-1500 rpm and ultrasonic treatment, and then reacting at 200℃ for 10 h to obtain carboxyl-functionalized iron tetroxide nanoparticles Fe3O4-COOH.

[0010] Preparation of SO3, Fe3O4@BPEI@Au, using 2 mg / mL -1 Fe3O4-COOH aqueous solution, 2 mg / mL -1 A mixture of branched polyethyleneimine aqueous solution and mechanically stirred at 600 rpm for 30 min at room temperature, followed by separation and washing, was dispersed in deionized water to obtain 2 mg / mL. -1 Fe3O4@BPEI solution was mixed with AuNPs solution prepared in step S01 and mechanically stirred at 600 rpm for 3 h to obtain Fe3O4@BPEI@Au.

[0011] Preparation of S04, two-dimensional heterogeneous self-assembled thin films, 2 mg / mL of Fe3O4@BPEI@Au was added. -1A tetrahydrofuran solution of mercaptopolystyrene was shaken overnight, washed sequentially with tetrahydrofuran and chloroform, and centrifuged to obtain Fe3O4@BPEI@Au with high concentration of mercaptopolystyrene ligands. This Fe3O4@BPEI@Au was then dispersed on a copper mesh or a clean silicon wafer surface with added deionized water. After natural evaporation, a two-dimensional heterogeneous self-assembled film was obtained.

[0012] Further, in step S01, the volume ratio of chloroauric acid solution to 1 wt.% sodium citrate solution is 0.66-3.9:1. Further, the volume ratio of 5 nm chloroauric acid solution to 1 wt.% sodium citrate solution is 1.35-1.37:1; the volume ratio of 12 nm chloroauric acid solution to 1 wt.% sodium citrate solution is 3.7-3.8:1; and the volume ratio of 20 nm chloroauric acid solution to 1 wt.% sodium citrate solution is 0.66-0.68:1.

[0013] Further, in step S02, the mass-to-volume ratio of ferric chloride hexahydrate to diethylene glycol is 1:17-19, the volume ratio of diethylene glycol to ethylene glycol is 1:1, and the mass ratio of ferric chloride hexahydrate, sodium acetate, and sodium acrylate is 0.3-0.4:1:1.

[0014] Further, in step S03, the volume ratio of Fe3O4-COOH solution to branched polyethyleneimine solution is 1:5, and the volume ratio of Fe3O4@BPEI solution to AuNPs solution is 1:25.

[0015] Furthermore, in step S04, the mass ratio of Fe3O4@BPEI@Au to mercaptopolystyrene is 0.15:1.

[0016] Further, in step S04, the volume ratio of high-concentration mercaptopolystyrene ligand-stabilized Fe3O4@BPEI@Au to deionized water is 1:4, and the concentration of high-concentration mercaptopolystyrene ligand-stabilized Fe3O4@BPEI@Au is 1500-1800 mg / mL. -1 .

[0017] The present invention also provides a two-dimensional heterogeneous self-assembled thin film prepared by the preparation method described above.

[0018] The present invention also provides an application of the two-dimensional heterogeneous self-assembled film as a SERS substrate. Before use, the prepared two-dimensional heterogeneous self-assembled film is placed in a plasma cleaner for cleaning. The cleaning parameters are as follows: the cleaning voltage is set to 850V and the cleaning time is 5min.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] This invention synthesizes Fe3O4-COOH magnetic nanoparticles with carboxyl groups modified on their surface via a solvothermal reaction. Subsequently, a positively charged BPEI film is deposited on their surface using electrostatic adsorption. Next, negatively charged AuNPs of different sizes are adsorbed onto their surface via electrostatic interactions. To modulate the interaction forces between the nanoparticles, SH-PS are further modified onto the AuNPs surface as ligands. Finally, a two-step drying-mediated self-assembly method is used to construct a structurally ordered two-dimensional heterogeneous self-assembled film on the substrate surface.

[0021] The 12nm AuNPs-modified Fe3O4@BPEI@Au nanoparticles of this invention exhibit good micro-assembly, forming a relatively regular and tightly packed two-dimensional monolayer film. Furthermore, they demonstrate superior signal enhancement across various Raman shift ranges, achieving a detection limit (LOD) of 10 for 4-MBA. -8 Furthermore, the Raman signal intensity exhibited a good linear relationship with the 4-MBA concentration across various concentration ranges. Simultaneously, substrate homogeneity testing revealed a high degree of overlap in the Raman spectra of 20 randomly selected points, indicating good stability of the film.

[0022] The Fe3O4@BPEI@Au two-dimensional heterogeneous self-assembled thin film constructed in this invention exhibits good SERS enhancement performance, uniformity and long-term stability, and shows SERS signal enhancement capability under the action of an external magnetic field, providing an experimental basis for subsequent magnetically controlled SERS research. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the construction of a two-dimensional heterogeneous self-assembled thin film;

[0024] Figure 2 Morphology table of Fe3O4-COOHNPs (ac) TEM and SEM images and particle size analysis of Fe3O4-COOHNPs;

[0025] Figure 3 The results show the zeta potential of Fe3O4-COOH, BPEI-coated Fe3O4-COOH, and gold nanoparticles of different sizes.

[0026] Figure 4 TEM images of (a) 5nm; (b) 12nm; (c) 20nm AuNPs, scale bar: 20nm;

[0027] Figure 5 HRTEM and EDS elemental analysis plots of Fe3O4@BPEI@(a) 5nm Au; (b) 12nm Au; (c) 20nm Au, scale bar: 50nm;

[0028] Figure 6 TEM and SEM images of two-dimensional heterogeneous self-assembled films formed by Fe3O4@BPEI@Au nanoparticles with different sizes as building blocks; (a1, a2) 5nm AuNPs; (b1, b2) 12nm AuNPs; (c1, c2) 20nm AuNPs; where a1, b1, c1 are TEM images, and a2, b2, c2 are SEM images, scale bar: 500nm;

[0029] Figure 7 The concentration of (a)4-MBA is 1×10 -1 (b) Raman spectrum of M on silicon substrate; (b) 4-MBA concentration of 1×10 -4 SERS spectra of M on two-dimensional ordered monolayer self-assembled films constructed from 5 nm and 12 nm AuNPs;

[0030] Figure 8 (a) Two-dimensional heterogeneous self-assembled thin film SERS substrates constructed using 12 nm AuNPs at different concentrations of 4-MBA (10 -4 M to 10 -8 Raman spectrum at (a) 1587 cm⁻¹; (b) Raman spectrum at (c) 1587 cm⁻¹ -1 Linear fitting curve of SERS signal intensity at characteristic peak and 4-MBA concentration;

[0031] Figure 9 For (a) from a single sample (4-MBA concentration of 10) -4 (a) 4-MBA SERS spectra of 20 randomly selected different points in M); (b) the corresponding 4-MBA characteristic peak (~1587 cm⁻¹). -1 Raman intensity distribution histogram;

[0032] Figure 10 For (a) measuring a single sample (4-MBA concentration of 10) at specific time points (day 1, day 3, day 5, day 7, day 14, day 21, and day 28). -4 (a) 4-MBA SERS spectrum of M; (b) corresponding 4-MBA characteristic peak (~1587 cm⁻¹) -1 Raman intensity distribution histogram;

[0033] Figure 11 For (a) the concentration of 4-MBA at different applied magnetic field strengths (0 mT, 10 mT, 25 mT, 50 mT and 75 mT) is 10 -4 (b) SERS spectrum of two-dimensional heterogeneous self-assembled thin film M; -1 Linear fitting curve of SERS signal intensity versus applied magnetic field intensity. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Tetrachloroauric acid trihydrate (HAuCl4·3H2O, ≥49.0 Aubasis), trisodium citrate (C6H5Na3O7, ≥98%), 4-mercaptobenzoic acid (4-MBA, ≥99%), and branched polyethyleneimine (BPEI, Mw 25kDa) were purchased from Sigma-Aldrich. Ferric chloride hexahydrate (FeCl3·6H2O, ≥99%) was purchased from Aladdin. Diethylene glycol ((CH2OH)2, >98%) was purchased from Macklin. Sodium acetate (CH3COONa, ≥99.5%) and ethylene glycol (C4H2O, >99%) were also purchased from Macklin. 10 O3 (≥99.5%) was purchased from General-reagent. Sodium acrylate (C3H3O2Na, ≥98%) was purchased from Adamas. Tetrahydrofuran (THF, ≥99.9%) was purchased from Meryer. Chloroform (CHCl3, ≥99.0%) was purchased from Sinopharm Shanghai Co., Ltd. Mercaptopolystyrene (SH-PS, Mn: 50000, Mw: 53000, Mw / Mn: 1.06, f>0.80) was purchased from Polymer Source.

[0036] The preparation process of two-dimensional heterogeneous self-assembled thin films is as follows: Figure 1 As shown, Fe3O4-COOH magnetic nanoparticles with carboxyl groups modified on their surface were synthesized via a solvothermal reaction. Subsequently, a positively charged BPEI film was modified onto their surface using electrostatic adsorption. Next, negatively charged AuNPs were adsorbed onto their surface via electrostatic interaction. To modulate the interaction forces between nanoparticles, SH-PS were further modified onto the AuNPs surface as ligands. Finally, a two-step drying-mediated self-assembly method was used to construct a structurally ordered two-dimensional heterogeneous self-assembled film on the substrate surface.

[0037] Example 1

[0038] Synthesis of gold nanoparticles (AuNPs) of different sizes

[0039] (a) Preparation of 5nm AuNPs

[0040] Add 500 μL of 25 mM chloroauric acid (HAuCl4) solution to 50 mL of deionized water, and add 367.5 μL of 1% sodium citrate solution while stirring at 500 rpm at room temperature. After stirring for 10 min, add 1.5 mL of chilled 0.1 M sodium borohydride (NaBH4) solution, and stir for another 15 min. After the reaction is complete, store the AuNPs in a refrigerator at 4 °C for later use.

[0041] (b) Preparation of 12nm AuNPs

[0042] Add 2 mL of 25 mM chloroauric acid (HAuCl4) solution to 48 mL of deionized water and bring the solution to a boil at room temperature with stirring at 800 rpm. After boiling, add 7.5 mL of 1% sodium citrate solution all at once, and then stir and boil for another 15 min. After the reaction is complete, store the AuNPs in a refrigerator at 4°C for later use.

[0043] (c) Preparation of 20nm AuNPs

[0044] Add 588 μL of 25 mM chloroauric acid (HAuCl4) solution to 50 mL of deionized water and bring the solution to a boil at room temperature with stirring at 800 rpm. After boiling, add 880 μL of 1% sodium citrate solution in one go, and then stir and boil for another 15 min. After the reaction is complete, store the AuNPs in a refrigerator at 4°C for later use.

[0045] TEM images of 5nm, 12nm, and 20nm AuNPs are shown below. Figure 4 As shown in the figure, AuNPs of different sizes all exhibit a uniform spherical structure with a relatively consistent size distribution. Subsequently, Zeta potential analysis was performed on these AuNPs, and the results showed that the Zeta potential of 5nm AuNPs was -15.5mV, 12nm AuNPs was -23.0mV, and 20nm AuNPs was -43.8mV. Figure 3 These negative charges mainly originate from the sodium citrate stabilizer used in the synthesis process, which gives the AuNPs a negatively charged citrate ion on their surface. In subsequent synthesis, due to electrostatic adsorption, these negatively charged AuNPs can naturally adsorb onto the strongly positively charged Fe3O4@BPEI surface, eventually forming a stable gold shell on its surface.

[0046] Example 2

[0047] Preparation of carboxyl-functionalized iron(III) oxide nanoparticles (Fe3O4-COOH)

[0048] Superparamagnetic carboxyl-functionalized iron oxide nanoparticles (Fe3O4-COOH) were synthesized using a one-step solvothermal reaction.

[0049] 0.54 g of ferric chloride hexahydrate (FeCl3·6H2O) was completely dissolved in 10 mL of diethylene glycol ((CH2OH)2) and 10 mL of ethylene glycol (C4H) 10 A mixed solution of sodium acetate (CH3COONa) and sodium acrylate (C3H3O2Na) was prepared. Then, 1.50 g of sodium acetate (CH3COONa) and 1.50 g of sodium acrylate (C3H3O2Na) were added to the solution, and the solution was completely dissolved by alternating vigorous magnetic stirring at 1200-1500 rpm and ultrasonic treatment. The resulting homogeneous solution was transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene and reacted at 200 °C for 10 h. After the reaction, the black precipitate was collected by magnetic separation and washed three times each with deionized water (DI water) and ethanol, and finally dried under vacuum at 60 °C for 12 h.

[0050] The morphology and magnetic properties of the carboxyl-functionalized iron oxide nanoparticles prepared in Example 2 are as follows: Figure 2 As shown in the SEM and TEM images, Fe3O4-COOHNPs exhibit a uniform spherical structure with a good size distribution and an average particle size of 164.1 ± 25.4 nm. Figure 2 (a, b, c)

[0051] The Zeta potential results for Fe3O4-COOH are as follows: Figure 3 As shown, Fe3O4-COOH has a Zeta potential of -47.24 mV due to the carboxyl group modification on its surface.

[0052] Example 3

[0053] Preparation of Fe3O4@BPEI@Au

[0054] First, prepare 5 mL of 2 mg / mL solution using the Fe3O4-COOH obtained in Example 2. -1 Fe3O4-COOH aqueous solution, and simultaneously prepare 25 mL of 2 mg / mL solution. -1 Aqueous solutions of branched polyethyleneimine (BPEI) were prepared. Two aliquots of the prepared solutions were mixed and mechanically stirred at 600 rpm for 30 min at room temperature. The supernatant was removed by magnetic separation, and the precipitate was washed three times with deionized water. Finally, the product was redispersed in 5 mL of deionized water (2 mg / mL). -1 Take 1 mL of the previously prepared Fe3O4@BPEI solution and add it to 25 mL of the 5 nm, 12 nm, or 20 nm AuNPs solution prepared in Example 1. Stir mechanically at 600 rpm for 3 h. After the reaction is complete, magnetically separate the supernatant and wash the precipitate three times with deionized water. Finally, redisperse the product (Fe3O4@BPEI@Au) in 10 mL of deionized water (0.2 mg / mL). -1 Store in a 4°C refrigerator for later use.

[0055] The zeta potential results for Fe3O4@BPEI are as follows: Figure 3 As shown, the strongly positively charged polymer BPEI adsorbs onto the Fe3O4 surface through electrostatic self-assembly, significantly altering its surface potential and resulting in nanoparticles exhibiting a uniform +33.97 mV. This strong positive potential characteristic plays a crucial role in the subsequent adsorption of negatively charged AuNPs and the formation of a uniform gold shell.

[0056] The morphology and elemental composition of the obtained magnetic core-gold shell nanoparticles Fe3O4@BPEI@Au were characterized using high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS). Figure 5 As shown, 5 nm AuNPs can uniformly coat the Fe3O4@BPEI surface, and the Au (red) and Fe (blue) elements in the EDS elemental distribution map highly overlap, indicating good bonding between AuNPs and the magnetic core. For 12 nm AuNPs, they still effectively cover the Fe3O4@BPEI surface, with a relatively dense particle arrangement and a uniform Au element signal distribution, indicating that AuNPs of this size can still effectively adsorb onto the magnetic core surface. However, when the AuNP size increases to 20 nm, the surface coverage of the particles decreases significantly, and the Au element distribution becomes more dispersed, indicating a significant reduction in adsorption capacity. Although the 20 nm AuNPs have the lowest Zeta potential (-43.8 mV) among all sizes, their distribution on the Fe3O4@BPEI surface is the most sparse, which may be related to the weakened electrostatic adsorption effect caused by the larger particle size. This further illustrates that the size of AuNPs has a significant impact on the morphology of the formed magnetic core-gold shell nanostructure.

[0057] Example 4

[0058] Construction of two-dimensional heterogeneous self-assembled thin films

[0059] Take 9 mL of the Fe3O4@BPEI@Au solution prepared in Example 3 and add it to 6 centrifuge tubes (1.5 mL each). Then, magnetically separate the supernatant. Add 1 mL of 2 mg / mL solution to each centrifuge tube. -1A solution of thiol-polystyrene (SH-PS) in tetrahydrofuran (THF) was shaken overnight at 500 rpm. After shaking, magnetic separation was performed, the supernatant was removed, and 1 mL of THF was added for washing, repeated three times. Then, 1 mL of chloroform (CHCl3) was added, and magnetic separation was performed to remove the supernatant, repeated three times. Finally, the precipitate from the six washed centrifuge tubes was concentrated and transferred to one centrifuge tube, and resuspended in 1 mL of CHCl3. First, the SH-PS ligand-stabilized Fe3O4@BPEI@Au solution resuspended in 1 mL of CHCl3 was magnetically separated to remove the supernatant, yielding a concentrated high-concentration Fe3O4@BPEI@Au (the volume of the concentrated liquid after removing the supernatant was 7.6-9.2 μL; therefore, the concentration of the high-concentration thiol-polystyrene ligand-stabilized Fe3O4@BPEI@Au was calculated to be 1500-1800 mg / mL). -1 Subsequently, 10 μL of deionized water was dropped onto the surface of a copper mesh or a clean silicon wafer, allowing it to spread naturally into a crescent shape. Next, 2.5 μL of concentrated Fe3O4@BPEI@Au was taken and gently touched to the surface of the crescent-shaped droplet, causing it to instantly disperse on the droplet surface and spontaneously form nanosheets. As the liquid evaporated naturally, under the action of surface tension, the nanoparticles on the droplet surface gradually aggregated and eventually adhered to the substrate, forming a monolayer structure that exhibits an ordered arrangement of two-dimensional heterogeneous self-assembled thin film at the microscale.

[0060] From SEM ( Figure 6 Images a2, b2, and c2 show that Fe3O4@BPEI@Au nanoparticles of different sizes successfully formed a two-dimensional heterogeneous self-assembled film, but the particle density and uniformity varied. Combined with TEM images (…),… Figure 6 As shown in Figures a1 and b1, the 5nm AuNPs-modified Fe3O4@BPEI@Au nanoparticles are relatively well-organized, with close packing between particles, forming a somewhat ordered monolayer film structure. This indicates that the 5nm AuNPs are uniformly adsorbed on the Fe3O4 surface, and the ligand modification is sufficient, resulting in adequate repulsive forces between particles, which is conducive to the formation of an ordered monolayer film. For the sample modified with 12nm AuNPs (Figure a1 and b1),... Figure 6 In the a2, b2 group, the film exhibits good properties, with a tight overall arrangement and uniformity comparable to the 5nm group, maintaining a high level of order. However, when the AuNPs size increases to 20nm, the particle arrangement becomes significantly disordered, unevenly distributed, and the gaps between particles increase. Figure 6(c1, c2). This may be because the 20nm AuNPs have a lower surface coverage when forming the gold shell, resulting in insufficient subsequent SH-PS ligand modification, which weakens the interparticle repulsion and ultimately forms a non-uniform, non-monolayer film. In summary, the two-dimensional heterogeneous self-assembled films constructed with 5nm and 12nm AuNPs exhibit better uniformity and order, while the film constructed with 20nm AuNPs has a loose lattice arrangement due to the lack of surface ligands, limiting its application potential in highly ordered film structures. Therefore, future research will focus on the SERS performance of films formed with 5nm and 12nm AuNPs as gold shells and explore the influence of internal magnetic nuclei on the SERS signal of such films under an applied magnetic field.

[0061] Example 5

[0062] Investigation of SERS properties of two-dimensional heterogeneous self-assembled thin films

[0063] Two-dimensional heterogeneous self-assembled thin film SERS substrate treatment

[0064] Before performing micro Raman spectroscopy characterization, the two-dimensional heterogeneous self-assembled thin film substrate prepared in Example 4 was placed in a plasma cleaner for cleaning. The cleaning parameters were as follows: cleaning voltage was set to 850V, and cleaning time was 5min. This effectively removed the SH-PS ligands remaining on the Fe3O4@BPEI@Au surface, avoiding interference from Au-S bonds in SERS detection, and effectively eliminating potential fluorescence interference in SERS detection, thus improving the accuracy and sensitivity of the signal.

[0065] 4-MBA, a common molecule with a large Raman cross section, was selected as the Raman probe molecule for testing.

[0066] Subsequently, the enhanced performance of two-dimensional heterogeneous self-assembled thin film SERS substrates constructed with gold shells at the same 4-MBA concentration, using 5 nm AuNPs and 12 nm AuNPs as the comparison, was investigated, and the performance was mainly achieved through 1077 cm⁻¹. -1 and 1587cm -1 The Raman intensities corresponding to the two characteristic peaks are evaluated. Figure 7 As can be clearly seen from b, the SERS performance of the 12nm AuNPs-coated gold film is significantly better than that of the 5nm AuNPs-coated gold film across all Raman cross sections, and even better at 1077cm. -1 and 1587cm -1 The two main characteristic peaks are most pronounced. Subsequently, the SERS enhancement factor (EF) for different film substrates was calculated using the SERS results. The calculation process is shown below:

[0067]

[0068] Among them, I SERS To determine the Raman signal intensity of 4-MBA under SERS enhancement on a thin film substrate, I Raman The intensity of the spontaneous Raman signal in 4-MBA; N Raman N represents the number of molecules on the thin film substrate that affect the SERS intensity. SERS The number of molecules representing the spontaneous SERS intensity. During SERS detection, the experimental conditions were set as follows: integration time 30 s, integration count 2, and laser power 0.26 mW. The concentration of 4-MBA on a clean silicon substrate was 10. -1 M, while the 4-MBA concentration on the thin film substrates constructed by 5nm and 12nm AuNPs was 10. -4 M, the final SERS test results are as follows Figure 7 As shown, both types of two-dimensional ordered monolayer self-assembled films significantly enhanced the Raman signal of 4-MBA, with the film constructed from 12 nm AuNPs showing better enhancement than the film constructed from 5 nm AuNPs across almost all Raman shifts. According to the EF calculation formula, the EF of the 5 nm group is 3.58 × 10⁻⁶. 2 The EF of the 12nm group is 1.51 × 10⁻⁶. 3 Although both the 5nm and 12nm AuNPs-constructed films have relatively regular monolayer structures, the 12nm group showed a superior enhancement effect in SERS performance. Therefore, the two-dimensional ordered monolayer self-assembled film constructed with 12nm AuNPs was ultimately selected as the substrate for subsequent SERS research.

[0069] The 12nm group was selected as the target for subsequent SERS sensitivity studies. The results of the SERS sensitivity investigation of the two-dimensional heterogeneous self-assembled thin films are as follows: Figure 8 As shown, the SERS signal intensity gradually weakens with decreasing 4-MBA concentration, especially at 1077 cm⁻¹. -1 and 1587cm -1 The characteristic peak attenuation was particularly pronounced at this location. The final measured limit of detection (LOD) for this SERS substrate was 10. -8 M. Furthermore, at 1587cm -1 Linear regression analysis was performed on the characteristic peak intensity at 10. -8 M to 10 -4 Within the M4-MBA concentration range, the fitted correlation coefficient (R) was obtained. 2 The correlation coefficient (LOD) of 0.998 indicates a good linear correlation between the SERS signal and the target molecule concentration, suggesting high detection reliability. The theoretical LOD calculated based on this regression curve is 10. -8.21 M, and experimentally measured 10-8 The similarity of the M results further validates the high sensitivity of this SERS substrate. In summary, the constructed two-dimensional heterogeneous self-assembled thin-film SERS substrate exhibits excellent enhancement effects and can serve as a high-efficiency SERS sensing platform for trace analysis and detection.

[0070] Signal uniformity of the SERS substrate is a key factor affecting the reproducibility of SERS studies. To further verify this, a 4-MBA concentration of 10... -4 On the thin film substrate of M, 20 different points were randomly selected for SERS testing. Figure 9 The Raman spectra obtained from the test are highly overlapping, at 1587 cm⁻¹. -1 The average Raman intensity of the characteristic peak at 1587 cm⁻¹ is 4332.1, which is close to the calculated value of 4180.2 from the linear regression equation. -1 The relative standard deviation (RSD) of the characteristic peak signal intensity was 14.68%, which meets the standard of SERS substrate signal uniformity RSD < 20%, indicating that the substrate has good signal uniformity.

[0071] To evaluate the SERS signal stability of the two-dimensional heterogeneous self-assembled thin film constructed in this invention, a 4-MBA concentration of 10 was selected. -4 A single thin film sample of M was stored in a sealed container, and SERS analysis was performed on it at specific time points within 28 days to observe signal changes. Figure 10 The results showed that the Raman signals of 4-MBA were highly overlapping and the signal intensity remained almost unchanged within 1 to 28 days. Meanwhile, at 1587 cm⁻¹... -1 Statistical analysis of the characteristic peak Raman intensity at the specified location showed that the signals remained essentially consistent. This excellent SERS signal stability is mainly attributed to the fact that gold not only possesses strong SERS enhancement properties but also exhibits excellent chemical stability. Therefore, under appropriate storage conditions, this type of film can maintain excellent SERS enhancement properties for a long period of time.

[0072] SERS signal enhancement analysis of two-dimensional heterogeneous self-assembled thin films under external magnetic field control.

[0073] Under the same detection conditions, external static magnetic fields of different intensities were applied, and the results were compared with those of a single 4-MBA concentration of 10. -4 SERS analysis was performed on sample M. The results showed that as the applied magnetic field increased from 0 mT to 75 mT, the Raman signal of 4-MBA gradually increased (e.g., ...). Figure 11 (As shown in Figure a). Among them, 1587cm -1 The Raman characteristic peak intensity at the location shows a possible linear relationship with the applied magnetic field (e.g., Figure 11(As shown in b). However, it should be noted that when the applied magnetic field exceeds 25 mT, the amplification of the Raman signal fluctuates significantly, and at 75 mT, a certain degree of vibration was observed in the film under a micro-Raman microscope. This vibration phenomenon significantly affects the uniformity of the film's SERS signal, leading to increased signal fluctuations. This phenomenon may stem from the large building block size of the film (150–200 nm), resulting in poor stability under an applied magnetic field and limiting the feasibility of further increasing the magnetic field strength.

[0074] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0075] It should be noted that the above content merely illustrates the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. For those skilled in the art, various improvements and modifications can be made without departing from the principle of the present invention, and all such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a two-dimensional heterogeneous self-assembled thin film, characterized in that, Includes the following steps: To prepare S01 and AuNPs, 25 mM chloroauric acid solution was added to deionized water, and after stirring, 1 wt.% sodium citrate solution was added. After the reaction was completed, the obtained gold nanoparticles AuNPs were stored in a refrigerator at 4°C for later use. The preparation of SO2 and Fe3O4-COOH involves dissolving ferric chloride hexahydrate in a mixed solution of diethylene glycol and ethylene glycol, then adding sodium acetate and sodium acrylate, alternating between magnetic stirring at 1200-1500 rpm and ultrasonic treatment, and then reacting at 200℃ for 10 h to obtain carboxyl-functionalized iron tetroxide nanoparticles Fe3O4-COOH. Preparation of SO3, Fe3O4@BPEI@Au, using 2 mg / mL -1 Fe3O4-COOH aqueous solution, 2 mg / mL -1 The branched polyethyleneimine aqueous solution was mixed and mechanically stirred at 600 rpm for 30 min at room temperature. After separation and washing, it was dispersed in deionized water to obtain 2 mg / mL. -1 Fe3O4@BPEI solution was mixed with AuNPs solution prepared in step S01 and mechanically stirred at 600 rpm for 3 h to obtain Fe3O4@BPEI@Au. Preparation of S04, two-dimensional heterogeneous self-assembled thin films, 2 mg / mL of Fe3O4@BPEI@Au was added. -1 A tetrahydrofuran solution of mercaptopolystyrene was shaken overnight, washed sequentially with tetrahydrofuran and chloroform, and centrifuged to obtain Fe3O4@BPEI@Au with high concentration of mercaptopolystyrene ligands. This Fe3O4@BPEI@Au was then dispersed on a copper mesh or a clean silicon wafer surface with added deionized water. After natural evaporation, a two-dimensional heterogeneous self-assembled film was obtained.

2. The method for preparing a two-dimensional heterogeneous self-assembled thin film according to claim 1, characterized in that, In step S01, the volume ratio of chloroauric acid solution to 1 wt.% sodium citrate solution is 0.66-3.8:

1.

3. The method for preparing a two-dimensional heterogeneous self-assembled thin film according to claim 1, characterized in that, In step S02, the mass-to-volume ratio of ferric chloride hexahydrate to diethylene glycol is 1:17-19, the volume ratio of diethylene glycol to ethylene glycol is 1:1, and the mass ratio of ferric chloride hexahydrate, sodium acetate, and sodium acrylate is 0.3-0.4:1:

1.

4. The method for preparing a two-dimensional heterogeneous self-assembled thin film according to claim 1, characterized in that, In step S03, the volume ratio of Fe3O4-COOH solution to branched polyethyleneimine solution is 1:5, and the volume ratio of Fe3O4@BPEI solution to AuNPs solution is 1:

25.

5. The method for preparing a two-dimensional heterogeneous self-assembled thin film according to claim 1, characterized in that, In step S04, the mass ratio of Fe3O4@BPEI@Au to mercaptopolystyrene is 0.15:

1.

6. The method for preparing a two-dimensional heterogeneous self-assembled thin film according to claim 1, characterized in that, In step S04, the volume ratio of Fe3O4@BPEI@Au stabilized by high concentration of mercaptopolystyrene ligands to deionized water is 1:

4.

7. A two-dimensional heterogeneous self-assembled thin film prepared by the preparation method according to any one of claims 1-6.

8. The use of a two-dimensional heterogeneous self-assembled thin film prepared by the preparation method according to any one of claims 1-6 as a SERS substrate, characterized in that, Before use, the prepared two-dimensional heterogeneous self-assembled film was placed in a plasma cleaner for cleaning. The cleaning parameters were as follows: the cleaning voltage was set to 850V and the cleaning time was 5min.

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