Preparation method and application of LSPR enhanced SERS (Surface Enhanced Raman Scattering) substrate based on critical spacing regulation and control

Abstract

The invention belongs to the technical field of nano photonics and spectrum detection, and particularly relates to a preparation method and application of an LSPR enhanced SERS substrate based on critical spacing regulation and control. The core of the LSPR enhanced SERS substrate disclosed by the invention is that the strongest coupling LSPR effect can be excited by accurately controlling the shortest surface distance between the silver nanoparticle pairs on the substrate carrier to be 0.5-5.0 nm, especially when the shortest surface distance is close to a critical value of 3.0 nm, so that an electromagnetic field enhancement factor reaches a peak value. According to the method, the technical bottlenecks of'hot spot 'random and low enhancement efficiency of a traditional SERS substrate are broken through, and the sensitivity, reproducibility and stability of the SERS substrate are synergistically improved through quantitative spacing design. According to the preparation method, precise control over the shortest surface spacing is achieved by means of regulating and controlling self-assembly conditions, using molecular spacer arms and the like. The SERS substrate can be used for high-sensitivity and rapid detection of trace substances such as environmental pollutants and biomolecules.

Description

Technical Field

[0001] This invention belongs to the field of nanophotonics and spectroscopic detection technology, specifically relating to a surface-enhanced Raman scattering (SERS) substrate, and more particularly to a SERS substrate that maximizes the localized surface plasmon resonance (LSPR) effect and electromagnetic field enhancement by precisely controlling the critical spacing between metal nanoparticles, its controllable preparation method, and its application in trace substance detection. Background Technology

[0002] Surface-enhanced Raman scattering (SERS) is a powerful analytical technique capable of amplifying the Raman signal of molecules adsorbed on rough metal surfaces or nanostructures by millions of times or more. The enhancement mechanism primarily stems from two aspects: electromagnetic enhancement (EM) and chemical enhancement (CM). Electromagnetic enhancement is the dominant factor, its physical essence being the localized surface plasmon resonance (LSPR) effect. Specifically, when the incident light frequency matches the collective oscillation frequency of free electrons in the metal nanostructure, a highly localized enhanced electromagnetic field is generated near the nanostructure surface; these regions are called "hot spots."

[0003] Among numerous SERS substrate materials, silver nanoparticles (Ag NPs) have been extensively studied due to their strong and tunable LSPR effect in the visible light region, high electric field enhancement factor, and relatively mature fabrication process. Both theory and practice have demonstrated that when two or more metal nanoparticles approach each other, a coupled LSPR effect occurs in their interstitial region, resulting in an order-of-magnitude increase in electromagnetic field strength. This interstitial region is one of the most efficient "hot spots."

[0004] However, existing technologies suffer from significant bottlenecks: the control of nanoparticle gaps largely relies on empirical attempts, lacking a deep understanding and precise control of the quantitative and nonlinear relationship between gap size and electromagnetic field enhancement. Most studies only qualitatively state that "smaller is better," but fail to reveal its inherent nonlinear laws and critical thresholds. This leads to unstable "hot spot" density and enhancement efficiency in the prepared SERS substrates, poor reproducibility, and performance that cannot reach the theoretical optimal value. Furthermore, excessively pursuing extremely small gaps (e.g., <1 nm) often triggers uncontrolled nanoparticle aggregation, resulting in LSPR peak broadening, increased energy dissipation, and poor stability in practical applications.

[0005] Therefore, there is an urgent need in this field for a method that can accurately guide and realize the controllable construction of nanoparticle spacing, as well as a SERS substrate with known, optimal and stable gap "hot spots" developed based on this method, in order to solve the problem that the sensitivity, reproducibility and stability of existing SERS technology are difficult to balance. Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, the primary objective of the present invention is to provide a SERS substrate with ultra-high and stable electromagnetic field enhancement, which is achieved by constructing metal nanoparticle pairs or clusters with a specific critical spacing.

[0007] Another objective of this invention is to provide a method for preparing the above-mentioned SERS substrate, which enables precise and controllable adjustment of the spacing between nanoparticles.

[0008] Another object of the present invention is to provide the application of the SERS substrate in the detection of trace chemicals, biomolecules and environmental pollutants.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A critical spacing-controlled LSPR-enhanced SERS substrate includes a substrate carrier and metal nanostructure units fixed to the surface of the substrate carrier. Each metal nanostructure unit comprises at least one pair of metal nanoparticles. When the metal nanoparticles are silver nanoparticles, the shortest surface spacing d between the two metal nanoparticles in each pair satisfies: 0.5 nm ≤ d ≤ 5.0 nm; when the metal nanoparticles are silver-gold core-shell structured nanoparticles, the shortest surface spacing d between the two metal nanoparticles in each pair satisfies: 8 nm ≤ d ≤ 15 nm; and within the spacing d range, the electromagnetic enhancement factor |E / E0| of the metal nanoparticle pair... 4 As d decreases, it exhibits a nonlinear relationship of first increasing and then decreasing.

[0011] Preferably, the substrate is a silicon wafer, glass, indium tin oxide (ITO) conductive glass, or a flexible polymer film.

[0012] Preferably, the metal nanoparticles are spherical, cubic, rod-shaped, or polyhedral in shape, and their particle size is between 50 nm and 150 nm.

[0013] Preferably, when the metal nanoparticles are silver nanoparticles, the shortest surface spacing d between the two metal nanoparticles in each pair is 3.0 nm ± 0.5 nm. The inventors, through systematic finite-difference time-domain (FDTD) simulations, discovered that for spherical silver nanoparticles with a radius of 100 nm, when the shortest surface spacing d decreases from 5.0 nm to 3.0 nm, the electromagnetic enhancement factor |E / E0| at the center of the gap remains unchanged. 4 The electromagnetic enhancement factor |E / E0| increases dramatically nonlinearly from approximately 75.7 to 224, corresponding to the electromagnetic enhancement factor |E / E0| 4This represents an improvement of approximately two orders of magnitude. However, when the shortest surface spacing d is further reduced to less than 3.0 nm, the electromagnetic enhancement factor actually decreases. Therefore, d = 3.0 nm has been confirmed as a critical spacing where the electromagnetic enhancement factor is maximized, and the coupled LSPR effect reaches its strongest point.

[0014] When the metal nanoparticles are silver-gold core-shell structured nanoparticles, due to the change in their dielectric environment and coupling mechanism, the optimal shortest surface spacing d is expanded to the range of 8-15 nm, and the electromagnetic enhancement factor can be further improved by about 40% compared with the silver nanoparticle system, and it has better chemical stability.

[0015] The above-mentioned method for preparing an LSPR-enhanced SERS substrate based on critical spacing control includes the following steps:

[0016] 1) Substrate pretreatment: The substrate carrier is cleaned and surface activated to enhance its bonding force with metal nanoparticles;

[0017] 2) Modification of metal nanoparticles: Metal nanoparticles are fixed on the surface of the pretreated substrate carrier through chemical self-assembly, electrostatic adsorption or template-assisted method to form a monolayer or sub-monolayer distribution.

[0018] 3) Precise control of the shortest surface spacing d: The shortest surface spacing d between metal nanoparticles is controlled by precisely controlling the process parameters in step 2). When the metal nanoparticles are silver nanoparticles, d is made to fall within the range of 0.5-5.0 nm. When the metal nanoparticles are silver-gold core-shell structured nanoparticles, d is made to fall within the range of 8-15 nm.

[0019] Preferably, when the metal nanoparticles are silver nanoparticles, the shortest surface distance d between the two metal nanoparticles in a pair of metal nanoparticles is 3.0 nm ± 0.5 nm.

[0020] Furthermore, the process parameters include, but are not limited to: the concentration, solvent properties, and dispersion stability of the nanoparticle suspension; ionic strength, pH value, reaction temperature, and time; precise control using bifunctional molecules (such as DNA, alkyl thiols, etc.) as spacer arms; and the geometric dimensions of the template, followed by mild annealing to induce particle spacing rearrangement to achieve a stable state with the lowest energy.

[0021] The core parameter of the chemical self-assembly method is "bifunctional molecule (spacer arm)," which is the most direct and effective method to achieve sub-nanometer precision control; the core parameters of the electrostatic adsorption method are "ionic strength" and "pH value." This method is suitable for rapid and large-scale preparation, but it controls the statistical average spacing, resulting in relatively low precision; the core parameter of the template-assisted method is "the geometric dimensions of the template itself."

[0022] The application of any of the above-described LSPR-enhanced SERS substrates based on critical spacing regulation in the detection of trace substances, including environmental pollutants (such as pesticide residues, dye molecules such as rhodamine B), biomarkers (such as DNA, proteins), explosives, and drugs.

[0023] Furthermore, in the above application, the detection method is as follows: the LSPR-enhanced SERS substrate based on critical spacing control is immersed in or the sample solution to be tested is added. After the molecules are adsorbed in the "hot spot" region, the SERS signal is acquired and analyzed using a Raman spectrometer.

[0024] Compared with the prior art, the present invention has the following significant advantages:

[0025] 1. Performance Breakthrough: This invention, for the first time, explicitly reveals and experimentally verifies, through FDTD simulation and theoretical calculation, that there exists a critical optimal spacing (approximately 3 nm) in the interparticle coupling, rather than a monotonically "the smaller the better" approach. Based on this discovery, the SERS substrate constructed exhibits electromagnetic field enhancement capabilities far exceeding those of traditional randomly distributed nanoparticle substrates, achieving a quantitative design and breakthrough in performance.

[0026] 2. Extremely high sensitivity and reproducibility: Due to the precise control and optimization of the size and enhancement factor of the "hot spot," the substrate of this invention exhibits extremely high detection sensitivity to probe molecules, reaching 10-10. -9 M, or even lower concentrations. At the same time, due to the controllable spacing, the reproducibility of SERS signals (RSD usually <15%) between different batches or different sites within the same batch is greatly improved.

[0027] 3. Excellent stability: By stabilizing the spacing near the critical value, uncontrollable aggregation of particles due to excessive proximity is avoided, thereby significantly improving the long-term storage and use stability of the substrate.

[0028] 4. Controllable and versatile preparation: The preparation method described is based on a clear principle, with controllable process parameters, making it easy to standardize and scale up. This technical solution is not only applicable to silver nanospheres, but can also be extended to other precious metals (gold, copper), different morphologies (rods, cubes), and core-shell structures, demonstrating broad applicability.

[0029] 5. Broad application prospects: This type of high-performance and highly stable SERS substrate can be widely used in many high-precision analytical fields such as food safety monitoring, environmental water quality analysis, public safety inspection, and early disease diagnosis, and has great social and economic value. Attached Figure Description

[0030] Figure 1This is a schematic diagram of the structure of the SERS substrate prepared according to the present invention.

[0031] Figure 2 The graph shows the variation of the gap electric field intensity |E / E0| of spherical silver nanoparticles (radius 100 nm) with the surface spacing d obtained by FDTD simulation, where (a) d=0.5 nm, (b) d=1.5 nm, and (c) d=3.0 nm.

[0032] Figure 3 The simulated electric field distribution |E / E0| is shown for different spacings ((a) d=3.5 nm, (b) d=4 nm, (c) d=4.5 nm).

[0033] Figure 4 This is a comparison of the SERS spectra of the SERS substrate of this invention and the comparative example (random aggregated substrate without spacing control) when detecting the same concentration of Rhodamine B solution.

[0034] Figure 5 The SERS spectra of the SERS substrate of this invention for detecting analytes of different concentrations are shown to demonstrate its excellent sensitivity. Detailed Implementation

[0035] The present invention will be further described in detail below with reference to embodiments, but the scope of protection of the present invention is not limited thereto.

[0036] Example 1: Preparation and characterization of a silver nanosphere SERS substrate based on a critical spacing (3 nm)

[0037] Substrate pretreatment: The monocrystalline silicon wafer was ultrasonically cleaned in acetone, ethanol and ultrapure water for 15 minutes each, dried with nitrogen, and then placed in an oxygen plasma cleaner for 5 minutes to increase surface hydrophilicity.

[0038] Nanoparticle self-assembly: A commercially available colloidal solution of spherical silver nanoparticles with a particle size of 100 nm ± 5 nm (concentration 0.02 mg / mL) was purchased. The solution pH was adjusted to near the isoelectric point, and an appropriate amount of NaCl (final concentration 1 mM) was added to shield electrostatic repulsion, promoting the self-assembly of particles on the silicon wafer surface to form a sub-monolayer film. Particle density was initially controlled by adjusting the adsorption time (5-30 minutes).

[0039] Precise spacing control: The self-assembled substrate was placed in a sealed container at room temperature containing a dodecyl mercaptan ethanol solution of a specific concentration, allowing it to slowly evaporate. Dodecyl mercaptan molecules form a self-assembled monolayer film on the surface of silver nanoparticles, and their chain length serves as an effective spacer arm, helping to lock the shortest distance between particles at approximately 2-3 nm. Subsequently, the substrate was annealed at 150°C for 1 hour under an inert atmosphere to relax the particle spacing to a more stable state.

[0040] Characterization and Validation:

[0041] Structural characterization: A schematic diagram of the SERS substrate is shown below. Figure 1 This study demonstrates pairs of metal nanoparticles with a specific spacing d on a substrate.

[0042] Simulation verification and mechanism analysis: To reveal the enhancement mechanism of this substrate and verify its optimality, FDTD simulations were performed.

[0043] Figure 2 The results show that the electric field strength |E / E0| at the center of the gap nonlinearly increases as the interparticle spacing increases from 0.5 nm (a) to 3.0 nm (c). Specifically, the electric field strength is 116 when the spacing is 0.5 nm; it increases to 151 when the spacing is 1.5 nm; and it reaches a peak of 156 when the spacing reaches 3.0 nm. This variation clearly indicates that there exists an optimal critical spacing (approximately 3 nm), rather than a non-monotonic "the smaller the better" approach.

[0044] Figure 3 The following diagrams further illustrate the situation when the spacing exceeds the critical value: electromagnetic field enhancement diagrams at spacings of 3.5 nm, 4 nm, and 4.5 nm. With a spacing of 3.5 nm, the electromagnetic field strength reaches 73 in the middle of the gap and 24.4 around the gap. With a spacing of 4 nm, the electromagnetic field strength reaches 74.7 in the middle of the gap and 24.9 around the gap. With a spacing of 4.5 nm, the electromagnetic field strength reaches 75.6 in the middle of the gap and 25.2 around the gap.

[0045] When the spacing between silver nanoparticles is less than 3 nm, their electromagnetic field enhancement effect exhibits significant nonlinear enhancement characteristics. However, if the spacing exceeds 3 nm, the electromagnetic field enhancement effect rapidly decays and gradually stabilizes, demonstrating the extreme sensitivity of near-field coupling to changes in spacing. This is consistent with... Figure 2 The results together form a complete chain of evidence, proving that 3 nm is the critical spacing for the strongest coupling LSPR effect. FDTD simulations verify that at this spacing, |E / E0| at the center of the gap... 4 The theoretical value exceeds 2.5 × 10 6.

[0046] SERS performance testing: The prepared substrate was immersed in 10... -9 The sample was immersed in an aqueous solution of Rhodamine B (RB) for 10 minutes, then rinsed gently with deionized water and dried under nitrogen. SERS signals were acquired under 532 nm laser excitation. Figure 4 The results showed that RB was at 1650 cm. -1 The characteristic peak intensity is high, and the signal-to-noise ratio is excellent. As a comparison, using a randomly aggregated silver sol substrate without spacing control, the peak intensity measured under the same conditions is lower, and the signal fluctuation is huge.

[0047] Example 2: Application in the detection of trace pollutants in environmental water samples

[0048] A water sample from a lake was filtered through a 0.22 μm filter membrane, and then Rhodamine B (RB) standard was added to bring the concentration to 10. -4 M, 10 -5 M, 10 -6 M. Using the SERS substrate prepared in Example 1, the SERS substrate was immersed in the sample solution to be tested. After the molecules were adsorbed into the "hot spot" region, the signal was acquired and analyzed using a Raman spectrometer. The characteristic Raman peak of RB could still be clearly detected. Figure 5 This indicates that the substrate of the present invention is suitable for rapid and highly sensitive detection of trace organic pollutants in complex real-world environmental samples.

[0049] Example 3: Detection of analytes at different concentrations using a SERS substrate

[0050] To further verify the sensitivity and quantitative detection capability of the SERS substrate of this invention, a series of Rhodamine B (RB) aqueous solutions of different concentrations (concentration range: 10) were prepared. -4 M to 10 -6 M). The SERS substrate prepared in Example 1 was used for detection: the substrate was immersed in RB solutions of different concentrations for 10 minutes, then gently rinsed with deionized water and dried with nitrogen. SERS signals were acquired under 532 nm laser excitation, and the results are as follows. Figure 5 As shown.

[0051] like Figure 5 As shown, the SERS signal intensity exhibits a regular decay as the RB concentration decreases, but at 10 -6 Characteristic peaks can still be clearly detected at concentrations of M. This embodiment fully demonstrates the excellent performance and practical application potential of the SERS substrate of this invention in trace substance detection.

Claims

1. A LSPR-enhanced SERS substrate based on critical spacing control, characterized in that, The system includes a substrate and metal nanostructure units fixed to the surface of the substrate; each metal nanostructure unit contains at least one pair of metal nanoparticles; when the metal nanoparticles are silver nanoparticles, the shortest surface distance d between the two metal nanoparticles in each pair satisfies: 0.5 nm ≤ d ≤ 5.0 nm; when the metal nanoparticles are silver-gold core-shell structured nanoparticles, the shortest surface distance d between the two metal nanoparticles in each pair satisfies: 8 nm ≤ d ≤ 15 nm; and within the distance d, the electromagnetic enhancement factor of the metal nanoparticle pair has a maximum value as the distance d changes.

2. The LSPR-enhanced SERS substrate based on critical spacing control according to claim 1, characterized in that, The substrate carrier is a silicon wafer, glass, indium tin oxide conductive glass, or a flexible polymer film.

3. The LSPR-enhanced SERS substrate based on critical spacing control according to claim 1, characterized in that, The metal nanoparticles are spherical, cubic, rod-shaped, or polyhedral in shape, and have a particle size of 50 nm to 150 nm.

4. The LSPR-enhanced SERS substrate based on critical spacing control according to claim 1, characterized in that, When the metal nanoparticles are silver nanoparticles, the shortest surface distance d between the two metal nanoparticles in each pair is 3.0 nm ± 0.5 nm.

5. A method for preparing an LSPR-enhanced SERS substrate based on critical spacing control as described in any one of claims 1-4, characterized in that, Includes the following steps: 1) Substrate pretreatment: The substrate carrier is cleaned and surface activated to enhance its bonding force with metal nanoparticles; 2) Modification of metal nanoparticles: Metal nanoparticles are fixed on the surface of the pretreated substrate carrier through chemical self-assembly, electrostatic adsorption or template-assisted method to form a monolayer or sub-monolayer distribution. 3) Precise control of the shortest surface spacing d: The shortest surface spacing d between metal nanoparticles is controlled by precisely controlling the process parameters in step 2). When the metal nanoparticles are silver nanoparticles, d is made to fall within the range of 0.5-5.0 nm. When the metal nanoparticles are silver-gold core-shell structured nanoparticles, d is made to fall within the range of 8-15 nm.

6. The preparation method according to claim 5, characterized in that, The controlled process parameters include: the concentration of the nanoparticle suspension, ionic strength, pH value, reaction temperature and time, the use of bifunctional molecules as spacer arms, and the geometry of the template.

7. The preparation method according to claim 6, characterized in that, The bifunctional molecule is DNA or an alkyl thiol.

8. The application of the LSPR-enhanced SERS substrate based on critical spacing control as described in any one of claims 1-4 in the detection of trace substances.

9. The application according to claim 8, characterized in that, The trace substances include environmental pollutants, biomarkers, explosives, and drugs.

10. The application according to claim 8, characterized in that, The detection method is as follows: the LSPR-enhanced SERS substrate based on critical spacing control is brought into contact with the sample containing the analyte, and then the SERS signal is acquired and analyzed using a Raman spectrometer.

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