Preparation method of Au@4-mba@ag surface enhanced raman probe

Abstract

The application discloses a kind of Au@4-MBA@Ag surface-enhanced Raman probe preparation methods, belong to nanomaterial and spectroscopic detection technical field.The probe is Au nanoparticle as core, 4-mercapto benzoic acid as intermediate layer, outside coating Ag shell, through the Au-S bond of 4-MBA is fixed in gold core, carboxyl induces Ag+ Coordination deposition, realize Ag shell controllable growth, form core-shell structure.Preparation contains Au nanoparticle, Au@4-MBA nanoparticle, Au@4-MBA@Ag probe three-step core technology, reaction condition is mild, process is easy to control.The probe uses Ag shell plasmon resonance to form electromagnetic enhancement zone, combined with the adsorption enrichment effect of 4-MBA, realize trace detection of paraquat, diquat and its free radical such as dipyridyl herbicide, with high sensitivity, high selectivity and good repeatability, detection limit is low and the requirement of sample pretreatment is low, can also be extended to other pesticide and pollutant detection, suitable for environmental and food safety on-site rapid detection.

Description

Technical Field

[0001] This invention relates to the fields of nanomaterial preparation technology, spectroscopic analysis technology, and environmental and food safety detection technology. Specifically, it relates to a chemical preparation method of a core-shell structured nanoprobe based on surface enhanced Raman scattering (SERS). In particular, it relates to a SERS core-shell nanoprobe for trace detection of bipyridine herbicides such as paraquat, diquat, and their free radicals by constructing an Au@4-MBA@Ag surface enhanced Raman probe, as well as its preparation method and application. Background Technology

[0002] Paraquat and diquat are structurally similar quaternary ammonium salt contact herbicides, known for their rapid action and broad weed control, and have long been widely used in agricultural production. However, both exhibit high toxicity: they rapidly kill plants by inhibiting photosynthesis and can cause severe damage to organs such as the liver, kidneys, and lungs in humans and animals. Due to their stable chemical structures and slow environmental degradation, these two herbicides easily leave residues in water bodies, soil, and agricultural products, posing persistent ecological and food safety risks. Structurally, paraquat is a para-substituted bispyridine cation containing a positively charged pyridine ring, while diquat is a bipyridine cation, readily interacting with negatively charged functional groups via electrostatic reactions. This characteristic leads to their easy adsorption onto particulate matter surfaces in the environment and also provides possibilities for molecular recognition methods based on interfacial interactions, such as surface-enhanced Raman spectroscopy (SERS). Therefore, developing rapid, accurate, and field-applicable detection methods for paraquat and diquat is of great significance.

[0003] In terms of applications, SERS technology has gained widespread attention in fields such as food safety, environmental monitoring, and biomedical analysis due to its advantages such as high sensitivity, strong fingerprint specificity, and fast detection speed. Especially in the detection of trace substances such as pesticide residues and environmental pollutants, SERS technology is considered a detection method with significant application prospects. SERS is an enhanced spectroscopy technique based on Raman scattering. Its core advantage lies in retaining the "fingerprint recognition" characteristic of Raman spectroscopy for molecular vibrational modes while significantly improving signal intensity through the enhancement effect of nanoscale metal structures, thus overcoming the problems of small cross-sections, weak signals, and difficulty in achieving trace detection in traditional Raman scattering. Since the discovery in the 1970s that rough metal surfaces can significantly enhance the Raman signal of adsorbed molecules, SERS has become an important tool for highly sensitive chemical and biological detection, showing broad application prospects in areas such as environmental pollutants, food additives, drug residues, and pathogen detection. The SERS enhancement mechanism can generally be summarized into two categories: electromagnetic enhancement (EM) and chemical enhancement (CM), with electromagnetic enhancement considered the dominant factor. Electromagnetic enhancement originates from localized surface plasmon resonance (LSPR) generated by the light-induced excitation of metallic nanostructures, resulting in a significant enhancement of the local electromagnetic field near the metal surface. When analyte molecules are located in regions of enhanced local electric field, their Raman scattering intensity can increase exponentially. Particularly in regions such as nano-gap areas, tips, or edges between nanoparticles, the electric field becomes highly concentrated, forming "hot spots," whose enhancement capacity is typically much higher than that of ordinary surface areas, becoming crucial for achieving ultrasensitive detection.

[0004] In existing technologies, researchers have proposed various SERS substrate construction schemes to enhance the Raman signal of target materials. Common schemes include using rough surfaces of noble metals, aggregates of metal nanoparticles, and metal thin film structures to form localized surface plasmon resonance, thereby enhancing the Raman signal. For example, Wang Siyu et al. disclosed a patent (CN202210084593.9) entitled "A method for preparing a SERS substrate with surface-loaded silver nanoparticles." This invention discloses a method for preparing a SERS substrate with surface-loaded silver nanoparticles. The steps involve preparing a silver-plated quartz substrate, placing it in a muffle furnace, maintaining the heating temperature at 350-450℃, controlling the heating rate at 1-10℃ / min, and controlling the heating time at 2-60min; then allowing the obtained silver-plated quartz substrate to cool naturally in air, thus obtaining the SERS substrate loaded with silver nanoparticles. The substrate prepared by this invention has high sensitivity, high pressure resistance, and increases the "hot spots" of the SERS.

[0005] In addition, Li Qiujin et al. published a patent (CN202311644197.8) entitled "A method for fabricating a lactic acid sensor based on the SERS effect of bilayer gold nanorods." This invention provides a method for fabricating a lactic acid sensor based on the SERS effect of bilayer gold nanorods. The main features of this sensor are good reproducibility and high stability, which can meet the needs of continuous and real-time monitoring of sweat in different scenarios.

[0006] To balance enhanced performance and structural stability, some existing technologies have further proposed gold-silver composite structures as SERS substrates. For example, Feng Jun et al. disclosed patent (CN201911095635.3) "A method for detecting trace amoxicillin using Ag@Au as a SERS substrate." This invention relates to a method for detecting trace amoxicillin using Ag@Au as a SERS substrate. This method employs a simple seed-mediated approach to prepare chemically stable bimetallic Ag@Au nanoparticles, ultimately transforming triangular Ag NPs into disk-shaped Ag@Au nanoparticles. These disk-shaped Ag@Au nanoparticles are then used as the SERS substrate for detecting trace amoxicillin, thereby achieving trace detection of amoxicillin. However, in the actual preparation process, the growth of Ag shell layers is often difficult to control precisely, easily leading to uneven coating or local detachment, resulting in an incomplete core-shell structure and affecting the final SERS enhancement effect and signal consistency.

[0007] In recent years, SERS technology has developed rapidly in the field of pesticide residue detection. Due to the complexity of pesticide molecules, low residue concentrations, and strong matrix interference, traditional detection methods often rely on high-precision analytical methods such as chromatography-mass spectrometry. SERS, however, is rapid, non-destructive, and requires no complex labeling, giving it significant advantages in on-site pesticide screening and rapid early warning. Current research mainly focuses on three aspects: SERS substrate structure design, target molecule enrichment and selective recognition strategies, and intelligent analysis methods for spectral data.

[0008] In terms of substrate structure design, researchers have developed various nanostructures to construct high-density hotspots, such as nanoflowers, nanostars, nanorods, and three-dimensional porous structures, to enhance local electromagnetic field enhancement. However, these structures are often complex to synthesize, exhibit large morphological dispersion, and the hotspots rely on random tips or random aggregations, leading to insufficient batch-to-batch consistency and limiting the reliability of quantitative detection. In contrast, core-shell structures, due to their synergistic optimization of enhancement and stability, are gradually becoming one of the mainstream directions for SERS substrates in pesticide detection.

[0009] In summary, while existing SERS substrate technologies have achieved Raman signal enhancement to some extent, they generally suffer from insufficient controllability in the fabrication process, limited structural stability, and poor repeatability of the enhancement effect. Therefore, there is an urgent need to provide a SERS nanoprobe with a rationally designed structure, a stable and controllable fabrication method, and high sensitivity and good repeatability in practical detection, in order to overcome the shortcomings of the existing technologies. Summary of the Invention

[0010] Purpose of the invention: To address the aforementioned problems in the prior art, this invention provides an Au@4-MBA@Ag core-shell type surface-enhanced Raman scattering nanoprobe, its preparation method, and its application. By introducing a specific organic molecule intermediate layer between the gold core and the Ag shell, the Ag shell can be grown in a controlled manner, thereby obtaining a SERS detection substrate with excellent enhancement performance, high structural stability, and good repeatability.

[0011] The present invention adopts the following technical solution: a core-shell type surface-enhanced Raman scattering nanoprobe, comprising an Au nanoparticle core, an organic molecular intermediate layer coated on the surface of the Au nanoparticle core, and an Ag shell layer coated on the outside of the organic molecular intermediate layer, wherein the organic molecular intermediate layer is a 4-mercaptobenzoic acid molecular layer.

[0012] Furthermore, the 4-mercaptobenzoic acid molecule forms a stable coordination bond with the surface of the Au nanoparticle core through its thiol groups, and its carboxyl functional groups are distributed outwards to induce the adsorption and reduction deposition of silver ions. The Ag shell is a continuous and dense structure, and its thickness can be controlled by adjusting the silver salt concentration, the amount of reducing agent, and the reaction time. The preparation process of this invention includes the following steps: The first step is the preparation of Au nanoparticles: First, 50–150 mL of Au nanoparticles with a concentration of 0.5–2 mmol•L were prepared. -1 An aqueous solution of tetrachloroauric acid was placed in a 200–500 mL single-necked flask and heated to boiling with a magnetic stirrer at 400–800 rpm. Then, 5–15 mL of a solution with a concentration of 20–50 mmol·L⁻¹ was added. -1 The sodium citrate solution was used for reduction reaction for 10-20 min, and then cooled to room temperature to obtain Au nanoparticle dispersion solution. The second step is the preparation of Au@4-MBA nanoparticles: 1–3 mL of a concentration of 1–10 mmol•L was added to the Au nanoparticle dispersion obtained above. -1An ethanol solution of 4-MBA was reacted with magnetic stirring at 400–600 rpm at room temperature for 6–18 h. The resulting reaction mixture was centrifuged at 8000–12000 rpm for 10–15 min to separate the particles. The supernatant was discarded, and the particles were ultrasonically washed with deionized water and ethanol, centrifuged three times, and redispersed in deionized water to obtain Au@4-MBA nanoparticle dispersion. The third step is the preparation of Au@4-MBA@Ag surface-enhanced Raman probes: 1–10 mL of a concentration of 0.5–5 mmol•L was added sequentially to the above Au@4-MBA nanoparticle dispersion. -1 Add 1-3 mL of silver nitrate aqueous solution with a concentration of 1-10 mmol / L. -1 Reducing agent solution, carboxyl groups in 4-MBA on the surface of Au nanoparticles induce Ag + The Ag nanoparticles are coated with Ag and reacted at 400–600 rpm at room temperature for 10–30 min. The mixture is then centrifuged at 8000–10000 rpm for 10–15 min, the supernatant is discarded, and the mixture is washed with deionized water, centrifuged three times, and then redispersed in deionized water to obtain the Au@4-MBA@Ag surface-enhanced Raman probe solution, which is used for the detection of paraquat, diquat and their free radicals.

[0013] The prepared Au@4-MBA@Ag solution was uniformly dispersed and dropped onto the surface of a 2 cm × 2 cm silicon wafer, and allowed to stand at room temperature until naturally dried to obtain a SERS substrate for later use. During the detection process, prepared paraquat, diquat, and their free radical solutions were dropped onto the dried Au@4-MBA@Ag substrate, respectively. During the detection, different concentrations (10... -3 ~ 10 -10 mol·L -1 Paraquat, diquat and their free radical solutions were dropped onto the dry Au@4-MBA@Ag substrate surface and allowed to stand for a while to ensure full adsorption. Then, their SERS signals were collected using a Raman spectrometer.

[0014] The present invention further provides the application of the above-mentioned core-shell type surface-enhanced Raman scattering nanoprobe in the detection of target objects, including but not limited to pesticide molecules, environmental pollutants and other chemical or biological molecules that can be identified by Raman spectroscopy.

[0015] Through the above technical solution, the present invention not only realizes the controllable growth of Ag shell on the surface of gold core, but also makes full use of the role of organic intermediate layer in structural stability and enhanced hot spot construction, providing a new technical approach for obtaining high-performance SERS detection substrate.

[0016] In recent years, in 2023, Qiu Rongliang et al. issued an invention patent (CN202310830712.5) entitled "A method and application for quantitative detection of hydroxyl radicals." This invention provides a method and application for quantitative detection of hydroxyl radicals. The method involves first adding a surfactant and a thiol compound to a colloidal solution of metal nanoparticles, allowing it to stand to obtain a nanoprobe solution, and then adding different concentrations of hydroxyl radicals to the nanoprobe solution to measure characteristic Raman signals. A curve showing the relationship between the characteristic Raman signal and the concentration of hydroxyl radicals is plotted, achieving quantitative detection of hydroxyl radicals. This invention is low-cost, environmentally friendly, biocompatible, highly sensitive, and selective, and can detect hydroxyl radical concentrations in real time, showing broad application prospects in environmental, biological, and medical fields. The invention also provides applications of the method in environmental monitoring and biomedical testing. In 2025, Zhu Jinfeng et al. issued an invention patent (CN202510101930.4) entitled "A Nanoflower SERS Chip Integrating Ordered and Disordered Structures and Its Preparation Method." This invention employs a nanoflower SERS chip integrating ordered and disordered structures and its preparation method to solve the problem of the mutual constraint between the detection repeatability and sensitivity of traditional SERS chips, and achieve a high degree of unity between repeatability and high sensitivity in SERS detection.

[0017] However, these literature reports merely describe the preparation of different metal substrates using different methods, without any surface modification of the resulting substrates. Therefore, the Au@4-MBA@Ag surface-enhanced Raman probe preparation method provided by this invention, used for trace detection of paraquat, diquat, and their free radicals, has the following beneficial effects: Firstly, this invention constructs a stable Au@4-MBA@Ag surface-enhanced Raman probe by introducing an organic intermediate layer containing thiol and carboxyl groups onto the surface of Au nanoparticles and depositing an Ag shell structure on its outer layer. This structure, while ensuring the high surface-enhanced Raman scattering performance of Ag nanomaterials, fully leverages the excellent chemical stability of Au nanoparticles, effectively overcoming the problems of easy oxidation, easy aggregation, and poor signal stability of existing single Ag nanomaterials, thus improving the structural stability and lifespan of the SERS substrate.

[0018] Secondly, the core-shell structure employed in this invention can form effective interfacial coupling between the Au core and Ag shell, and generate nanoscale electromagnetic enhancement regions between adjacent core-shell nanoprobes, thereby significantly enhancing the Raman scattering signals of paraquat, diquat, and their free radicals. Compared with existing single Au or Ag nanomaterials, the core-shell structure of this invention has significant advantages in Raman signal enhancement and detection sensitivity, and can meet the needs of trace and even ultra-trace detection of target analytes.

[0019] Thirdly, this invention introduces 4-mercaptobenzoic acid as an organic intermediate layer in the core-shell structure. This organic intermediate layer not only forms a stable chemical bond with Au nanoparticles through thiol groups, but also provides coordination sites for the deposition of the Ag shell through exposed carboxyl groups, thereby achieving effective control over the formation process of the core-shell structure, avoiding the problem of uneven growth of the Ag shell, and improving the consistency and repeatability of Au@4-MBA@Ag core-shell type surface-enhanced Raman probes.

[0020] Fourth, unlike existing SERS substrates that mainly rely on physical enhancement mechanisms, the organic intermediate layer of this invention serves as both an adsorption and enrichment interface for target molecules, enabling paraquat, diquat, and their free radicals to preferentially adsorb onto the surface of the Au@4-MBA@Ag probe or the nano-gap region, thereby significantly improving the selectivity of the detection system for target molecules and reducing the influence of coexisting interfering substances on the detection results.

[0021] Fifth, due to the similarity in molecular structure between paraquat, diquat, and their free radicals, traditional detection methods face certain difficulties in distinguishing between the two. This invention, through the synergistic effect of the core-shell structure and the organic intermediate layer, combined with the molecular fingerprinting capability of surface-enhanced Raman scattering (SERS), achieves effective identification and differentiation of paraquat, diquat, and their free radicals, thereby improving the accuracy and reliability of the detection results.

[0022] Sixth, the Au@4-MBA@Ag probe preparation method provided by this invention has a simple process flow, mild reaction conditions, readily available raw materials, good repeatability of the preparation process, and is easy to control. It is suitable for preparation under laboratory conditions and also has the potential for further scale-up and large-scale production, which is conducive to practical application and promotion.

[0023] Seventh, the Au@4-MBA@Ag probe of the present invention has low requirements for sample pretreatment during actual detection, and the detection process is simple to operate and has a fast response speed. It can meet the needs of rapid on-site detection and real-time monitoring, and overcome the shortcomings of existing detection methods that rely on large precision instruments and have long detection cycles.

[0024] Eighth, the Au@4-MBA@Ag probe provided by this invention is not only suitable for the detection of paraquat, diquat and their free radicals, but can also be extended to the surface-enhanced Raman scattering detection of other pesticide residues or organic pollutants by adjusting the type of organic intermediate layer and the core-shell structure parameters, and has good versatility and scalability.

[0025] Ninth, compared with existing fluorescence detection, electrochemical detection and other methods, the detection method based on surface-enhanced Raman scattering technology of this invention has the advantages of stable signal, good repeatability and is not easily affected by photobleaching or environmental electrochemical conditions, thus improving the reliability of the detection results.

[0026] Tenth, in summary, this invention, through structural design and functional integration, achieves a synergistic improvement in core-shell structure stability, Raman enhancement performance, and molecular recognition ability, providing a new technical approach for the highly sensitive and selective detection of paraquat, diquat, and their free radicals, and has significant technological advancements and application value. Attached Figure Description

[0027] Figure 1 This is a schematic diagram illustrating the preparation method of the Au@4-MBA@Ag surface-enhanced Raman probe described in the invention.

[0028] Figure 2 This is the UV-Vis absorption spectrum of the Au@4-MBA@Ag probe described in this invention.

[0029] Figure 3 The image shows the XRD pattern of the Au@4-MBA@Ag probe described in this invention.

[0030] Figure 4 SEM and TEM images of the Au@4-MBA@Ag probe described in this invention.

[0031] Figure 5 The surface-enhanced Raman scattering spectrum changes of the Au@4-MBA@Ag probe described in this invention when detecting different concentrations of paraquat, diquat and their free radicals.

[0032] Figure 6 The results show the sensitivity and repeatability of the Au@4-MBA@Ag probe described in this invention for detecting paraquat, diquat, and their free radicals.

[0033] Figure 1 This is a schematic diagram of the preparation method of the Au@4-MBA@Ag core-shell surface-enhanced Raman probe of the present invention. This method uses Au nanoparticles as the core and achieves controllable growth of the Ag shell through an organic molecular intermediate layer, thereby obtaining a structurally stable core-shell SERS substrate with significant enhancement effects.

[0034] First, Au nanoparticles were prepared as the core structure. Specifically, tetrachloroauric acid was selected as the gold source and dissolved in deionized water to form a homogeneous and transparent gold salt solution. Under continuous stirring, a reducing agent was added to induce a reduction reaction of gold ions, gradually generating Au nanoparticles. During this process, the reaction temperature, reaction time, and the rate of addition of the reducing agent were controlled to maintain a high degree of uniformity in the particle size distribution of the obtained Au nanoparticles. Studies have shown that if the Au nanoparticle particle size distribution is too wide, it will lead to uneven growth of the subsequent Ag shell, thereby affecting the repeatability and stability of the final SERS signal. Therefore, the reaction conditions were optimized and controlled in this embodiment.

[0035] After preparing the Au nanoparticles, the resulting solution was centrifuged, and the precipitate was washed multiple times with deionized water to remove unreacted precursors and excess reducing agent. Finally, it was redispersed in deionized water for later use. The Au nanoparticles obtained through these steps exhibited good dispersibility and stability, providing a reliable foundation for subsequent intermediate layer modification.

[0036] Secondly, 4-mercaptobenzoic acid (4-MBA) was introduced as an organic intermediate layer molecule onto the surface of Au nanoparticles. A 4-MBA solution of a certain concentration was added to the above Au nanoparticle dispersion, and the reaction was allowed to proceed statically at room temperature. This allowed the thiol groups in the 4-MBA molecule to form a strong coordination bond with the surface of the Au nanoparticles, thereby forming a stable organic molecular layer on the gold core surface. Because the 4-MBA molecule has a thiol group at one end that can form a strong Au-S bond with gold, and a carboxyl functional group at the other end, this structural feature not only ensures the stable fixation of the molecule on the gold surface, but also provides an effective binding site for the subsequent growth of the Ag shell.

[0037] In this step, the coverage density of 4-MBA on the Au nanoparticle surface can be controlled by adjusting the amount of 4-MBA added and the reaction time. Too low a coverage density leads to insufficient nucleation sites during Ag shell growth, affecting the continuity of the Ag shell; too high a coverage density may cause intermolecular stacking, hindering further deposition of metal ions. Therefore, in this embodiment, suitable 4-MBA modification conditions were obtained through experimental screening, enabling it to function as both a Raman signal molecule and a bridge for Ag shell growth.

[0038] After 4-MBA modification, the reaction system was centrifuged and washed again to remove unbound 4-MBA molecules, yielding Au@4-MBA composite nanostructures. This structure maintains good dispersibility, and the uniformly distributed carboxyl functional groups on its surface provide favorable conditions for subsequent metal deposition.

[0039] Next, an Ag shell was constructed on the surface of the Au@4-MBA composite nanostructure. A silver salt solution was slowly added to the Au@4-MBA dispersion, and a reducing agent was introduced under appropriate conditions to induce a reduction deposition reaction of silver ions on the gold core surface. Due to the presence of the 4-MBA molecular layer, silver ions preferentially adsorbed and reduced near their carboxyl groups, thereby inducing uniform growth of silver on the gold core surface, ultimately forming a continuous and dense Ag shell structure.

[0040] During the Ag shell growth process, the thickness of the Ag shell can be effectively controlled by adjusting the silver salt concentration, reducing agent dosage, and reaction time. If the Ag shell is too thin, the plasma coupling effect is insufficient, resulting in limited SERS enhancement; if the Ag shell is too thick, it may mask the "hot spot" effect generated at the core-shell interface, thus reducing the enhancement performance. Therefore, in this embodiment, the above parameters were optimized to achieve a good balance between enhancement performance and structural stability in the obtained Au@4-MBA@Ag core-shell nanostructure. After the Ag shell deposition is completed, residual silver ions and byproducts in the reaction system are removed through centrifugation and washing steps, ultimately obtaining the Au@4-MBA@Ag core-shell SERS nanoprobe. The nanoprobe obtained in this embodiment has a clear core-shell structure, good dispersibility, and stable surface chemical properties, enabling it to provide stable and reproducible Raman enhancement signals in subsequent detection applications.

[0041] Figure 2 This is the UV-Vis absorption spectrum of Au@4-MBA@Ag described in this invention. The inset shows the typical localized surface plasmon resonance (LSPR) absorption peak of pure Au nanoparticles in the visible region, located around 520 nm, which is a common optical characteristic of well-dispersed spherical Au nanoparticles. In contrast, 4-MBA molecules mainly exhibit absorption response in the UV region, with almost no significant absorption in the visible region. After Au surface modification with 4-MBA (Au@4-MBA), its LSPR absorption peak shows a slight change compared to the unmodified Au sample, accompanied by a certain degree of peak broadening. This is usually related to the change in local refractive index environment caused by surface molecular modification, indicating that 4-MBA has been successfully modified on the Au particle surface and affects its optical response. After further coating with an Ag shell (Au@4-MBA@Ag), the absorption curve of the sample underwent a more significant adjustment: the overall absorption intensity in the visible light region was enhanced and the band was wider, while the absorption response in the short-wavelength region was more prominent. This indicates that the outer Ag deposition altered the plasmonic behavior of the particles and enhanced the optical coupling effect of the composite structure. This result optically demonstrates that the introduction of the Ag shell is not a simple mixing process, but rather a significant modulation of the LSPR response of the Au system, consistent with the characteristics of core-shell composite structure formation.

[0042] Figure 3The XRD pattern of the Au@4-MBA@Ag core-shell nanoprobe described in this invention is shown in the inset. The inset shows that the XRD pattern of pure silver nanoparticles also exhibits characteristic diffraction peaks at approximately 38.1°, 44.3°, 64.4°, and 77.3° at 2θ, corresponding to the (111), (200), (220), and (311) crystal planes of face-centered cubic silver. These peak positions are consistent with the standard silver crystal diffraction card, indicating that the prepared Ag nanoparticles have high crystallinity. The XRD pattern of the Au@4-MBA@Ag core-shell nanoprobe simultaneously displays characteristic diffraction peaks of both gold and silver. The main peak near approximately 38° at 2θ shows a significant increase in intensity, indicating that the Ag shell successfully coats the gold core surface and forms a well-crystallized metallic shell structure. Since both gold and silver are face-centered cubic structures and their characteristic peak positions are close, there is a certain degree of overlap in their diffraction peaks within the core-shell structure. However, the formation of the core-shell composite structure can still be confirmed by comparison with individual gold and silver samples. Figure 3 The illustrations show the standard card positions corresponding to the main diffraction peaks of each sample, further corroborating the crystal structure characteristics of the Au core and Ag shell in the obtained Au@4-MBA@Ag core-shell nanoprobe. The XRD results above demonstrate that the Au@4-MBA@Ag nanoprobe prepared in this invention possesses a clear Au core and Ag shell crystal structure, providing a reliable structural basis for its excellent surface-enhanced Raman performance.

[0043] Figure 4 The images show the SEM and TEM results of the Au@4-MBA@Ag probe described in this invention. The Au@4-MBA@Ag core-shell nanoprobe exhibits a relatively uniform particle distribution with a relatively concentrated particle size. No obvious large-scale aggregation was observed. Each particle has a clear outline and exhibits an approximately spherical or near-spherical structure. Significant contrast differences exist within the particles, indicating that it possesses typical core-shell structure characteristics. This demonstrates that the prepared core-shell nanoprobe has good dispersibility and morphological consistency, which is beneficial for obtaining stable and consistent surface-enhanced Raman signals during subsequent detection.

[0044] Figure 5 The Raman spectral changes of the Au@4-MBA@Ag probe described in this invention when detecting different concentrations of paraquat, diquat, and their free radicals are shown. Figure 5 (A) During the paraquat detection process, when the paraquat concentration increased from 10... -4 mol•L -1 Gradually reduce to 10 -10 mol•L -1 At that time, it was approximately 857 cm -1 1176 cm -1 and 1614 cm -1The characteristic Raman peaks at the target concentration can still be effectively identified. As the concentration decreases, the Raman signal intensity shows a regular downward trend, but it remains detectable at lower concentrations, indicating that the prepared Au@4-MBA@Ag core-shell surface-enhanced Raman nanoprobe also has a good enhancement effect and a low detection limit for diquat. Figure 5 (B) During the detection of paraquat free radicals, the concentration was 10 -4 ~ 10 - 10 mol•L -1 Within the concentration range, clear characteristic peaks of free radicals can still be observed in the spectrum, mainly located at 1073, 1388, 1580, and 1647 cm⁻¹. -1 The spectra also exhibit a monotonically decreasing peak intensity and essentially unchanged peak position with decreasing concentration, indicating that under controlled sampling timing (critical metastable state stage), the substrate can effectively retain the free radical signal and achieve a usable concentration response. Compared to the paraquat parent molecule, the paraquat free radical at 1388 cm⁻¹... -1 1647 cm -1 Isopeakers exhibit more significant differences. Figure 5 (C) The concentration of dichlorvos was reduced from 10 -4 mol•L -1 Gradually decrease to 10 -8 mol•L -1 Throughout the process, the position of its characteristic Raman peak remained basically consistent, at approximately 1072 cm. -1 1170 cm -1 1300 cm -1 1375 cm -1 1575 cm -1 and 1614 cm -1 Clear characteristic peak signals were observed at all locations. As the paraquat concentration decreased, the peak intensities of each characteristic peak gradually weakened, but the main characteristic peaks could still be distinguished under lower concentration conditions, indicating that the Au@4-MBA@Ag core-shell nanoprobe has significant Raman enhancement ability and high detection sensitivity for paraquat molecules. Figure 5 (D) The concentration of dichlorvos free radicals in the medium decreased from 10 -4 mol•L -1 Gradually decrease to 10 -9 mol•L -1 Throughout the process, the position of its characteristic Raman peak remained basically consistent, at approximately 1157cm. -1 1211 cm -1 343 cm -1 1530 cm -1 1560 cm -1Clear characteristic peak signals were observed at all locations. Compared to diquat, the peak shape and position of its free radical spectrum showed significant differences, indicating that the Au@4-MBA@Ag substrate can effectively distinguish the fingerprint characteristics of free radical morphologies. As the concentration decreased, the intensity of the characteristic peaks gradually decreased while the peak positions stabilized, indicating that the substrate has the ability to effectively detect free radical species.

[0045] The above results demonstrate that the Au@4-MBA@Ag surface-enhanced Raman probe of this invention can achieve highly sensitive and repeatable surface-enhanced Raman detection of target pesticide molecules such as paraquat, diquat, and their free radicals. It can maintain stable characteristic peak positions and good signal response under different concentration conditions, verifying the application potential of this core-shell nanoprobe in the field of rapid detection of pesticide residues.

[0046] Figure 6 The results show the sensitivity and repeatability of the Au@4-MBA@Ag core-shell nanoprobe described in this invention for detecting diquat, paraquat, and their free radicals. The Au@4-MBA@Ag substrate maintained stable spectral peak positions and identifiable characteristic signals for diquat, paraquat, and their free radicals during tests at time intervals of 30–330 s, demonstrating good temporal stability and applicability for continuous detection. This provides a reliable signal basis for subsequent concentration gradient detection and spectral identification analysis.

[0047] exist Figure 6 (A) indicates that the peak positions of paraquat in the 30–330 s range are well consistent, and the characteristic peaks appear stably at each time point, indicating that the substrate's enhanced response to paraquat has good repeatability. The intensity changes show some fluctuations, but the overall spectral structure remains consistent, indicating that no obvious substrate failure or signal decay occurred during the measurement process.

[0048] exist Figure 6 (B) indicates that the spectral peak distribution remained stable across different time points, with no significant shift in the main peak positions and minimal changes in spectral shape, suggesting that the substrate exhibits good temporal stability in its enhanced response to paraquat free radicals. Although peak intensities varied across different time points, the characteristic peaks remained consistently visible, meeting the basic requirements for subsequent identification and analysis.

[0049] exist Figure 6 (C) indicates that the enhanced response of diquat on the substrate surface has good temporal stability. The peak intensity fluctuates with time, but there is no continuous decay or abrupt decrease, indicating that the signal output can be maintained at a usable level within this time scale.

[0050] exist Figure 6(D) indicates that the diquat radical exhibits relatively stable peak positions and spectral shapes at different time points, and the characteristic peaks remain identifiable throughout the spectrum. Compared with the parent molecule, the intensity fluctuations in the radical spectrum are relatively more pronounced, which is usually related to the radical's greater sensitivity to the interfacial environment and local hot spots. However, these fluctuations do not lead to the disappearance or change of characteristic peak positions, indicating that the substrate also has a stable signal retention capability for the radical morphology.

[0051] In conclusion, Figure 6 The results show that the Au@4-MBA@Ag substrate can maintain stable spectral peak positions and identifiable characteristic signals for diquat, paraquat and their free radicals in the time interval test of 30~330s, demonstrating good temporal stability and applicability for continuous detection, and providing a reliable signal basis for subsequent concentration gradient detection and spectral identification analysis.

[0052] Detailed Implementation: A method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe, characterized in that: the Au@4-MBA@Ag surface-enhanced Raman probe uses Au nanoparticles as the core, with 4-mercaptobenzoic acid (4-MBA) modified on its surface as an intermediate layer, the surface carboxyl groups coordinating with silver ions, and Ag... + Enriched on the surface of Au nanoparticles, an Ag nanoshell is formed on the outer surface of the intermediate layer through a redox reaction. The target analyte adheres to the surface of the Ag nanoshell, and the Ag shell surface enhances the Raman signal of the target analyte, thus realizing the detection of the target analyte. The above-mentioned surface-enhanced Raman probe preparation method includes the following steps: The first step is the preparation of Au nanoparticles: First, 50–150 mL of Au nanoparticles with a concentration of 0.5–2 mmol•L were prepared. -1 An aqueous solution of tetrachloroauric acid was placed in a 200–500 mL single-necked flask and heated to boiling with a magnetic stirrer at 400–800 rpm. Then, 5–15 mL of a solution with a concentration of 20–50 mmol·L⁻¹ was added. -1 The sodium citrate solution was used for reduction reaction for 10-20 min, and then cooled to room temperature to obtain Au nanoparticle dispersion solution. The second step is the preparation of Au@4-MBA nanoparticles: 1–3 mL of a concentration of 1–10 mmol•L was added to the Au nanoparticle dispersion obtained above. -1An ethanol solution of 4-MBA was reacted with magnetic stirring at 400–600 rpm at room temperature for 6–18 h. The resulting reaction mixture was centrifuged at 8000–12000 rpm for 10–15 min to separate the particles. The supernatant was discarded, and the particles were ultrasonically washed with deionized water and ethanol, centrifuged three times, and redispersed in deionized water to obtain Au@4-MBA nanoparticle dispersion. The third step is the preparation of Au@4-MBA@Ag surface-enhanced Raman probes: 1–10 mL of a concentration of 0.5–5 mmol•L was added sequentially to the above Au@4-MBA nanoparticle dispersion. -1 Add 1-3 mL of silver nitrate aqueous solution with a concentration of 1-10 mmol / L. -1 Reducing agent solution, carboxyl groups in 4-MBA on the surface of Au nanoparticles induce Ag + The Ag nanoparticles are coated with Ag and reacted at 400–600 rpm at room temperature for 10–30 min. The mixture is then centrifuged at 8000–10000 rpm for 10–15 min, the supernatant is discarded, and the nanoparticles are washed with deionized water, centrifuged three times, and then redispersed in deionized water to obtain the Au@4-MBA@Ag surface-enhanced Raman probe solution for the detection of target analytes.

[0053] Example: Au nanoparticles were prepared as the core and 4-MBA as the modified intermediate layer. A reduction reaction was used to obtain Au@4-MBA@Ag core-shell surface-enhanced Raman nanoprobes.

[0054] A method for preparing Au@4-MBA@Ag surface-enhanced Raman probes, characterized in that: the Au@4-MBA@Ag surface-enhanced Raman probe uses Au nanoparticles as the core, with 4-mercaptobenzoic acid (4-MBA) modified on its surface as an intermediate layer, and its surface carboxyl groups coordinated with silver ions, Ag... + Enriched on the surface of Au nanoparticles, an Ag nanoshell is formed on the outer surface of the intermediate layer through a redox reaction. The target analyte adheres to the surface of the Ag nanoshell, and the Ag shell surface enhances the Raman signal of the target analyte, thus realizing the detection of the target analyte. The above-mentioned surface-enhanced Raman probe preparation method includes the following steps: The first step is the preparation of Au nanoparticles: First, 100 mL of 1.0 mmol•L... -1 An aqueous solution of tetrachloroauric acid was placed in a 250 mL single-necked flask and heated to boiling with a magnetic stirrer at 600 rpm. Then, 10 mL of a 35 mmol•L solution was added. -1The sodium citrate solution was used for reduction reaction for 15 min, and then cooled to room temperature to obtain Au nanoparticle dispersion solution. The second step is the preparation of Au@4-MBA nanoparticles: 2 mL of a 5 mmol•L solution was added to the Au nanoparticle dispersion obtained above. -1 An ethanol solution of 4-MBA was reacted at room temperature for 12 h with magnetic stirring at 500 rpm. The resulting reaction mixture was centrifuged at 10000 rpm for 12 min to separate the particles. The supernatant was discarded, and the particles were ultrasonically washed with deionized water and ethanol, centrifuged three times, and redispersed in deionized water to obtain Au@4-MBA nanoparticle dispersion. The third step is the preparation of the Au@4-MBA@Ag surface-enhanced Raman probe: 5 mL of a 3 mmol•L solution was added sequentially to the above Au@4-MBA nanoparticle dispersion. -1 Add 2 mL of silver nitrate aqueous solution with a concentration of 5 mmol•L -1 Reducing agent solution, carboxyl groups in 4-MBA on the surface of Au nanoparticles induce Ag + The Ag nanoparticles were coated with Ag and reacted at 500 rpm at room temperature for 20 min. The mixture was then centrifuged at 9000 rpm for 10-15 min, the supernatant was discarded, and the nanoparticles were washed with deionized water, centrifuged three times, and then redispersed in deionized water to obtain the Au@4-MBA@Ag surface-enhanced Raman probe solution for the detection of target analytes.

Claims

1. A method for preparing Au@4-MBA@Ag surface-enhanced Raman probes, characterized in that: The Au@4-MBA@Ag surface-enhanced Raman probe uses prepared Au nanoparticles as the core, with 4-mercaptobenzoic acid (4-MBA) modified on its surface as an intermediate layer. The surface carboxyl groups coordinate with silver ions, and Ag... + Enriched on the surface of Au nanoparticles, an Ag nanoshell is formed on the outer surface of the intermediate layer through a redox reaction. The target analyte adheres to the surface of the Ag nanoshell, and the Ag shell surface enhances the Raman signal of the target analyte, thus realizing the detection of the target analyte. The above-mentioned surface-enhanced Raman probe preparation method includes the following steps: 1.1 The first step is the preparation of Au nanoparticles: First, 50–150 mL of Au nanoparticles with a concentration of 0.5–2 mmol•L were prepared. -1 An aqueous solution of tetrachloroauric acid was placed in a 200–500 mL single-necked flask and heated to boiling with a magnetic stirrer at 400–800 rpm. Then, 5–15 mL of a solution with a concentration of 20–50 mmol·L⁻¹ was added. -1 The sodium citrate solution was used for reduction reaction for 10-20 min, and then cooled to room temperature to obtain Au nanoparticle dispersion solution. 1.2 The second step is the preparation of Au@4-MBA nanoparticles: 1–3 mL of a 1–10 mmol•L solution of Au nanoparticle dispersion was added to the above-prepared Au nanoparticle dispersion. -1 An ethanol solution of 4-MBA was reacted with magnetic stirring at 400–600 rpm at room temperature for 6–18 h. The resulting reaction mixture was centrifuged at 8000–12000 rpm for 10–15 min to separate the particles. The supernatant was discarded, and the particles were ultrasonically washed with deionized water and ethanol, centrifuged three times, and redispersed in deionized water to obtain Au@4-MBA nanoparticle dispersion. 1.3 The third step is the preparation of Au@4-MBA@Ag surface-enhanced Raman probes: 1–10 mL of a 0.5–5 mmol•L solution was added sequentially to the above Au@4-MBA nanoparticle dispersion. -1 Add 1-3 mL of silver nitrate aqueous solution with a concentration of 1-10 mmol / L. -1 Reducing agent solution, carboxyl groups in 4-MBA on the surface of Au nanoparticles induce Ag + The Ag nanoparticles are coated with Ag and reacted at 400–600 rpm at room temperature for 10–30 min. The mixture is then centrifuged at 8000–10000 rpm for 10–15 min, the supernatant is discarded, and the nanoparticles are washed with deionized water, centrifuged three times, and then redispersed in deionized water to obtain the Au@4-MBA@Ag surface-enhanced Raman probe solution for the detection of target analytes.

2. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The Au nanoparticles have a particle size of 10–20 nm.

3. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The reducing agent is ascorbic acid.

4. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The Ag shell forms a continuous or semi-continuous coating structure on the surface of Au nanoparticles, creating nanoscale gaps between adjacent core-shell nanoprobes.

5. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The Au@4-MBA@Ag surface-enhanced Raman probe includes an Au nanoparticle core, 4-MBA nanoparticles on its surface, and a silver shell covering the 4-MBA nanoparticles.

6. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The 4-MBA in the intermediate layer is fixed to the surface of Au nanoparticles via Au–S bonds, and its exposed carboxyl groups are used to regulate the deposition of the Ag shell and provide adsorption sites for target analyte molecules.

7. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The target analytes are bipyridine herbicides such as paraquat, diquat, and their free radicals.

8. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The Ag shell in the probe exhibits a surface region plasmon resonance effect, which can form an electromagnetic enhancement region on the probe surface and in its nano-gap, thereby enhancing the Raman scattering signal of the target analyte.

9. The method for preparing an Au@4-MBA@Ag surface-enhanced Raman probe according to claim 1, characterized in that: The Ag in the probe interacts with the target analyte, causing the target analyte to adhere to the probe surface. Qualitative or quantitative detection of the target analyte is achieved through surface-enhanced Raman scattering signal.

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