Preparation method of MF (at) NH2 (at) Ag SERS (Surface Enhanced Raman Scattering) sensor for detecting paraquat and aquacide

Abstract

The invention discloses a preparation method of an MF (at) NH2 (at) Ag SERS (Surface Enhanced Raman Scattering) sensor for detecting paraquat and diquat, belongs to the field of nano material synthesis and spectral analysis, and solves the problems that a traditional detection method is complicated in process and cannot detect pesticides and free radicals thereof at the same time, and a traditional SERS substrate is poor in enhancement effect and low in stability. The preparation method comprises the following steps: firstly segmenting melamine foam, cleaning and drying, soaking in hydrochloric acid, soaking in a polyethyleneimine aqueous solution to modify amino, drying, soaking in a silver nitrate solution for ultrasonic treatment, adding a sodium borohydride solution under an ice bath, stirring and reducing, and finally cleaning and drying to obtain the sensor. The sensor takes three-dimensional melamine foam as a support, silver nanoparticles are uniformly loaded on the surface of the three-dimensional melamine foam, paraquat, aquacide and free radicals thereof are enriched through electrostatic interaction based on a localized surface plasma resonance principle, and Raman signal enhancement is realized. The detection limit is 10 <-9 > mol / L, the signal stability and repeatability are excellent, the applicability to actual water sample detection is good, the detection of other pesticides can be expanded, and the method is suitable for on-site rapid trace detection.

Description

Technical Field

[0001] This invention relates to the fields of nanomaterial synthesis and spectral analysis, and in particular to a method for preparing an MF@NH2@Ag SERS sensor capable of detecting trace amounts of paraquat, diquat, and their free radicals. Background Technology

[0002] From a social safety perspective, paraquat and diquat belong to the bipyridine class of non-selective herbicides. Due to their high weeding efficiency and rapid action, they are widely used in agricultural production. However, these herbicides pose high biotoxicity and environmental risks. Paraquat, in particular, can continuously generate reactive oxygen species (ROS) (such as superoxide anion radicals) in the human body and other organisms through redox cycles, leading to lipid peroxidation, cell damage, and even multi-organ failure. It has become one of the high-risk pesticides of global concern. While diquat is slightly less toxic than paraquat, it also poses a potential hazard of oxidative damage through the free radical pathway. Therefore, effective detection of paraquat, diquat, and their free radicals is of significant practical importance for pesticide residue monitoring, environmental pollution assessment, and toxicological research.

[0003] In terms of application, current detection methods for paraquat and diquat mainly focus on the quantitative analysis of their parent molecules. Commonly used techniques include high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), electrochemical analysis, and spectrophotometry. While these methods offer high sensitivity and accuracy, they generally suffer from complex procedures, demanding sample pretreatment requirements, long detection cycles, expensive equipment, and difficulty in achieving rapid on-site detection. Furthermore, these methods often rely on large-scale laboratory instruments, making it difficult to meet the practical needs of grassroots regulatory departments or emergency on-site testing.

[0004] On the other hand, paraquat and diquat are often accompanied by the generation and transformation of free radicals in the environment and biological systems, and free radicals play a key role in their toxic mechanisms. However, due to the short lifetime, low concentration, and high reactivity of free radicals, traditional detection techniques are difficult to achieve direct, rapid, and in-situ detection. Existing studies mostly rely on indirect characterization methods, which cannot fully reflect their true state and reaction processes. Therefore, developing a method that can detect both paraquat and diquat molecules themselves and their related free radicals with high sensitivity has become a pressing technical challenge in this field.

[0005] Surface-enhanced Raman scattering (SERS) is a spectroscopic analysis technique based on the localized surface plasmon resonance effect of noble metal nanostructures. It can significantly enhance the Raman scattering signal of analyte molecules, showing broad application prospects in pesticide residue detection, environmental monitoring, and bioanalysis. Since Fleichmann et al. discovered the surface-enhanced Raman spectrum of pyridine adsorbed on the rough surface of a silver electrode in 1974, SERS spectroscopy has been widely used in drug analysis, residue detection, and bioanalysis due to its advantages such as high sensitivity, extremely low detection limit, fast detection speed, and non-destructive analysis capabilities. However, the practical application effect of SERS technology largely depends on the performance of the reinforcing substrate material. Existing silver or gold nanoparticle SERS substrates are mostly two-dimensional structures, prone to aggregation, resulting in uneven distribution of enhancement "hot spots," leading to poor signal repeatability and stability. Furthermore, their ability to enrich target molecules and their free radicals in complex systems is limited, restricting their further application.

[0006] Compared to existing detection methods based on two-dimensional planar substrates or colloidal systems, using substrate materials with three-dimensional porous structures provides more favorable structural conditions for surface-enhanced Raman scattering (SERS) detection. Three-dimensional substrates typically possess a large specific surface area and interconnected pore structures, allowing for the introduction of more effective enhancement regions on a spatial scale. This increases the loading capacity of metal nanostructures and the number of electromagnetic enhancement hotspots, facilitating the enrichment of target molecules and signal amplification. Simultaneously, the three-dimensional support framework effectively restricts the migration and aggregation of metal nanoparticles, improving their dispersion uniformity and structural stability, thereby enhancing the repeatability and reliability of detection results. Furthermore, three-dimensional substrates generally exhibit good mechanical stability and ease of handling in practical applications, allowing for effective interaction with the sample through simple immersion or contact methods, making them more suitable for complex systems and in-situ detection conditions. Based on these characteristics, three-dimensional porous substrates can, to some extent, replace traditional two-dimensional substrates or colloidal detection systems, providing a more promising technical approach for the detection of multi-target, trace substances using SERS.

[0007] Existing SERS substrates mainly include colloidal metal substrates and immobilized metal nanostructure substrates. Colloidal substrates are relatively simple to prepare, but they are prone to aggregation or sedimentation during use, resulting in uneven distribution of enhancement hotspots and poor repeatability and stability of detection results. While immobilized substrates offer improved structural stability, the effective enhancement area of ​​traditional two-dimensional planar substrates is limited, and the loading of metal nanostructures is low, which is not conducive to the sufficient enrichment of target molecules, especially limiting detection performance in trace analysis and complex sample systems. Among commonly used SERS-enhancing metal materials, gold, silver, and copper are some of the most widely used. Silver nanostructures exhibit strong localized surface plasmon resonance in the visible light band, and their electromagnetic enhancement ability is generally superior to that of gold and copper, achieving higher Raman signal enhancement factors, which is beneficial for achieving low detection limit analysis. Furthermore, silver materials have good surface adsorption capacity for various small organic molecules, which helps to improve the interaction between target molecules and the enhancing substrate, thereby improving detection sensitivity. Compared to copper, silver has higher chemical stability and is less prone to rapid oxidation, making it suitable for practical detection applications.

[0008] In recent years, due to the excellent SERS detection performance of silver nanomaterials, silver nanomaterials have attracted a large number of researchers' interest. In 2020, Kong Xianming et al. published a patent (CN202011093406.0) entitled "A method for preparing and applying a SERS substrate with filtration function". It adopts the following steps: Step 1: Prepare an HCl-SnCl2 mixed solution; Step 2: Contact one side of the filter paper with the pre-prepared HCl-SnCl2 mixed solution at the solid-liquid interface; Step 3: Take out the filter paper, rinse it with ultrapure water and acetone in sequence, let the filter paper air dry, soak the dried filter paper in AgNO3 solution for a certain period of time, take out the filter paper with one side white and the other side dark brown, and rinse it with ultrapure water; Step 4: Soak the filter paper at this time in AgNO3 solution, then add ascorbic acid solution, soak for a certain period of time, take it out and wash it again with ultrapure water to obtain a SERS substrate with filtration function. In 2020, Yu Zhi et al. published a patent (CN202010552706.4) entitled "Graphene-Silver SERS Composite Substrate and its Preparation Method". It employs the following steps: Step 1: Add a first-concentration graphene oxide dispersion, a second-concentration silver nitrate solution, and deionized water to a container in first, second, and third preset volumes, respectively, and stir thoroughly; Step 2: Add a fourth preset volume of ascorbic acid solution with a third concentration to the container and stir thoroughly to form a reaction product; Step 3: Separate the reaction product from the solvent by centrifugation, wash with deionized water a preset number of times, and then dry under a vacuum atmosphere and at a preset temperature to form the substrate; Step 4: Titrate the solution to be tested onto the substrate, and then dry under a vacuum atmosphere and at a preset temperature to enable Raman detection on the substrate. In 2019, Yu Qian et al. published a patent (CN201910027758.7) entitled "A Preparation Method of a Flexible SERS Sensor for Detecting Bisphenol A and its Application". It employs the following steps: Step 1: Preparation of cellulose fibers from recycled waste paper; Step 2: Surface modification of cellulose fibers; Step 3: Preparation of flexible fiber-silver SERS substrate; Step 4: Detection of bisphenol A in bottled beverages.

[0009] However, these methods only produce silver nanoparticles and two-dimensional material composites. Due to the limited effective reinforcement region of the two-dimensional planar substrate and the low loading of the metal nanostructure, it is not conducive to the sufficient enrichment of target molecules. Furthermore, there are no literature reports on the trace detection of paraquat, diquat, and their free radicals using silver nanoparticles and three-dimensional material composites. Therefore, it is necessary to develop a method for synthesizing a highly stable and sensitive MF@NH2@Ag SERS sensor to achieve ultra-trace detection of paraquat, diquat, and their free radicals.

[0010] In this invention, we report a method for detecting trace amounts of paraquat, diquat, and their free radicals by composite silver nanoparticles on the surface of melamine foam based on the principle of plasmon resonance. A surface-enhanced Raman spectroscopy (SERS) sensor based on melamine foam (MF) was constructed by reacting silver ions with the amino groups at the ends of polyethyleneimine. Silver ions were reduced to silver nanoparticles using an in-situ chemical reduction method and uniformly loaded into the three-dimensional network structure of melamine foam. First, the foam was immersed in hydrochloric acid (HCl) and polyethyleneimine (PEI) solutions to functionalize the foam surface and introduce amino groups. Then, the foam was immersed in AgNO3 solution to enrich the foam surface with Ag. + By adding NaBH4 solution, the Ag on the foam surface was removed. + The process involves reduction, resulting in the formation of uniform and dense silver nanoparticles on the foam surface. After compositing, trace amounts of pesticides are first enriched onto the surface of the silver nanoparticles using the physicochemical properties of the foam. Then, the Raman signal of the target analyte is amplified through the regional plasmon resonance effect of the silver nanoparticles. The resulting MF@NH2@Ag composite substrate exhibits abundant "hot spot" regions and a good specific surface area, significantly enhancing the Raman signal. No literature reports on the composite of silver nanoparticles on melamine foam. Through systematic optimization of experimental conditions, the constructed SERS substrate can achieve trace detection of diquat and paraquat, as well as their free radicals, with a detection limit (LOD) as low as 10. -9 mol•L -1 The substrate demonstrated excellent signal consistency and stability in multiple batches of repeated detections, and also showed good applicability and anti-interference ability for the detection of two pesticides in actual water samples. Summary of the Invention

[0011] Objective: To address the shortcomings of existing technologies, this invention proposes for the first time a SERS detection method for paraquat, diquat, and their free radicals based on MF@Ag. By in-situ loading silver nanoparticles onto a three-dimensional porous melamine foam framework, a stable three-dimensional SERS substrate is constructed, enabling highly sensitive and rapid trace detection of paraquat, diquat, and their free radicals. The method is a chemical synthesis approach. First, functional modification of the foam surface introduces amino groups to effectively adsorb silver ions, serving as anchoring points for the in-situ reduction preparation of silver nanoparticles, ensuring uniform growth and firm adhesion of the nanoparticles. Subsequently, a reducing agent is used to synthesize silver nanoparticles in situ on the foam surface, preparing a SERS substrate for trace detection of paraquat, diquat, and their free radicals.

[0012] The technical solution of this invention is: a method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat, characterized in that: the MF@NH2@Ag SERS sensor uses a three-dimensional mesh framework of melamine foam as a support, the surface of which is functionalized with polyethyleneimine to modify an amino molecular layer. The amino groups coordinate with silver ions, resulting in the enrichment of silver ions on the surface of the melamine foam. The silver ions are reduced to silver nanoparticles, which adhere to the surface of the melamine foam. Paraquat, diquat, and their free radicals adhere to the surface of the silver nanoparticles, enhancing the Raman signals of paraquat, diquat, and their free radicals, thereby achieving the detection of paraquat, diquat, and their free radicals. The preparation process of the above sensor includes the following steps: First, the melamine foam is cut into pieces with a volume of 1 to 8 cm. 3 The cubic blocks were soaked in deionized water and anhydrous ethanol for 4-6 minutes each, then removed and placed in a vacuum oven to dry at room temperature. They were then placed in 80-120 mL of 0.5-1.5 mol•L⁻¹ solution. -1 After soaking in HCl solution for 20-40 min, remove and wash with deionized water until neutral. Dry in an oven at 50-70 °C for 1-3 h. Then, immerse the acid-treated melamine foam in a 0.5-1.5% polyethyleneimine aqueous solution and react at room temperature for 1-3 h to modify the foam surface with amino groups. Wash three times with deionized water and dry in a vacuum oven at 40-60 °C for 2-4 h. Immerse the dried melamine foam in 90-110 mL of 0.001 mol•L⁻¹ solution. -1 In an AgNO3 solution, sonicate for 9–11 min, then slowly add 80–100 mL of a 0.01 mol•L⁻¹ solution. - 1 The NaBH4 solution was stirred at 200-400 rpm for 10 min in an ice bath. The resulting MF@NH2@Ag particles were then washed three times with deionized water and anhydrous ethanol to remove excess impurities adsorbed on the surface. The particles were then vacuum dried at a certain temperature to obtain a highly sensitive and trace detection MF@NH2@AgSERS sensor for paraquat, diquat, and their free radicals.

[0013] As a further improvement to the existing technology, the SERS sensor is MF@NH2@Ag; the MF is a flexible, porous three-dimensional material, melamine foam; the particle size of the silver nanoparticles is controllable and can be controlled by adjusting the reaction time and the amount of reducing agent; the surface of the MF is modified to be rich in amino groups; the amino-modified MF surface forms coordination bonds with silver ions, and silver nanoparticles are obtained by in-situ reduction and adhering to the surface of the MF; in the SERS sensor, the silver nanoparticles automatically accumulate on the surface of the silver nanoparticles through electrostatic interaction with paraquat, diquat, and their free radicals; the MF@NH2@Ag SERS sensor is based on the principle of localized surface plasmon resonance to detect paraquat, diquat, and their free radicals.

[0014] In recent years, due to the excellent SERS detection performance of silver nanomaterials, silver nanomaterials have attracted a large number of researchers' interest. In 2020, Kong Xianming et al. published a patent (CN202011093406.0) entitled "A method for preparing and applying a SERS substrate with filtration function". It adopts the following steps: Step 1: Prepare an HCl-SnCl2 mixed solution; Step 2: Contact one side of the filter paper with the pre-prepared HCl-SnCl2 mixed solution at the solid-liquid interface; Step 3: Take out the filter paper, rinse it with ultrapure water and acetone in sequence, let the filter paper air dry, soak the dried filter paper in AgNO3 solution for a certain period of time, take out the filter paper with one side white and the other side dark brown, and rinse it with ultrapure water; Step 4: Soak the filter paper at this time in AgNO3 solution, then add ascorbic acid solution, soak for a certain period of time, take it out and wash it again with ultrapure water to obtain a SERS substrate with filtration function. In 2020, Yu Zhi et al. published a patent (CN202010552706.4) entitled "Graphene-Silver SERS Composite Substrate and its Preparation Method". It employs the following steps: Step 1: Add a first-concentration graphene oxide dispersion, a second-concentration silver nitrate solution, and deionized water to a container in first, second, and third preset volumes, respectively, and stir thoroughly; Step 2: Add a fourth preset volume of ascorbic acid solution with a third concentration to the container and stir thoroughly to form a reaction product; Step 3: Separate the reaction product from the solvent by centrifugation, wash with deionized water a preset number of times, and then dry under a vacuum atmosphere and at a preset temperature to form the substrate; Step 4: Titrate the solution to be tested onto the substrate, and then dry under a vacuum atmosphere and at a preset temperature to enable Raman detection on the substrate. In 2019, Yu Qian et al. published a patent (CN201910027758.7) entitled "A Preparation Method and Application of a Flexible SERS Sensor for Detecting Bisphenol A". It employs the following steps: Step 1: Preparation of cellulose fibers from recycled waste paper; Step 2: Surface modification of cellulose fibers; Step 3: Preparation of flexible fiber-silver SERS substrate; Step 4: Detection of bisphenol A in bottled beverages.

[0015] However, these methods only produce silver nanoparticles and two-dimensional material composites. Due to the limited effective reinforcement region of the two-dimensional planar substrate and the low loading of the metal nanostructure, it is not conducive to the sufficient enrichment of target molecules. Furthermore, there are no literature reports on the trace detection of paraquat, diquat, and free radicals using silver nanoparticles and three-dimensional material composites. Therefore, it is necessary to develop a method for synthesizing a highly stable and sensitive SERS probe MF@NH2@Ag to achieve ultra-trace detection of paraquat, diquat, and free radicals.

[0016] This invention first involves cutting melamine foam into pieces with a volume of 1 to 8 cm³. 3The cubic blocks were soaked in deionized water and anhydrous ethanol for 4-6 minutes each, then removed and placed in a vacuum oven to dry at room temperature. They were then placed in 80-120 mL of 0.5-1.5 mol•L⁻¹ solution. -1 After soaking in HCl solution for 20-40 min, remove and wash with deionized water until neutral. Dry in an oven at 50-70 °C for 1-3 h. Then, immerse the acid-treated melamine foam in a 0.5-1.5% polyethyleneimine aqueous solution and react at room temperature for 1-3 h to modify the foam surface with amino groups. Wash three times with deionized water and dry in a vacuum oven at 40-60 °C for 2-4 h. Immerse the dried melamine foam in 90-110 mL of 0.001 mol•L⁻¹ solution. -1 In an AgNO3 solution, sonicate for 9–11 min, then slowly add 80–100 mL of a 0.01 mol•L⁻¹ solution. - 1 The NaBH4 solution was stirred at 200-400 rpm for 10 min in an ice bath. The resulting MF@NH2@Ag particles were then washed three times with deionized water and anhydrous ethanol to remove excess impurities adsorbed on the surface. The particles were then vacuum dried at a certain temperature to obtain the MF@NH2@Ag SERS sensor for the detection of paraquat, diquat, and their free radicals.

[0017] Secondly, melamine foam possesses a rich porous structure and a large specific surface area. After surface modification with amino groups, its surface contains a large number of amino functional groups, exhibiting strong adsorption for polar molecules such as paraquat and diquat. Furthermore, the composite with silver nanoparticles further enhances the enrichment effect of target molecules and their free radicals on the substrate surface, making the analytes more easily distributed in the SERS "hotspot" region, thereby significantly improving detection efficiency and signal-to-noise ratio. It can also be selectively used to detect other pesticide molecules, such as thiram and chlorpyrifos. Therefore, the method provided by this invention is universal and has a wide range of applications.

[0018] Thirdly, the MF@NH2@Ag SERS substrate described in this invention possesses both a high specific surface area and a high-density electromagnetic enhancement "hot spot," which can significantly amplify the Raman scattering signals of paraquat and diquat molecules. Compared with traditional two-dimensional silver nanoparticle substrates, the three-dimensional foam structure allows the analyte molecules to make multi-point and multi-angle contact with the silver nanostructure in space, thereby obtaining clear and identifiable characteristic Raman peaks even under extremely low concentration conditions, enabling trace detection of paraquat and diquat.

[0019] Fourthly, in the method described in this invention, the particle size distribution of Ag NPs is controllable and can be controlled by adjusting the reduction reaction time and the reduction dosage.

[0020] Fifthly, the purpose of choosing silver nanoparticles to composite melamine foam is because it has the following advantages: (1) Melamine foam has a continuous three-dimensional porous skeleton structure. After silver nanoparticles are loaded on its surface, a large number of spatially distributed nanoscale gaps can be formed at the intersection of the foam skeleton and on the surface of the pore wall, thereby constructing a high-density, three-dimensional electromagnetic enhancement "hot spot". Compared with traditional two-dimensional substrates, MF@NH2@Ag substrates can provide more effective enhancement sites in space, which is beneficial to improve the Raman signal intensity of paraquat, diquat and its free radicals; (2) Melamine foam provides a stable carrier and fixed interface for silver nanoparticles, so that silver nanoparticles are dispersed and anchored on the surface of the foam skeleton, avoiding the problems of easy aggregation and sedimentation of traditional colloidal silver nanoparticles during use, thus ensuring the stability and repeatability of SERS enhancement effect, and is suitable for trace detection and multiple tests; (3) MF@NH2@Ag substrates organically combine the physical adsorption of melamine foam and the electromagnetic enhancement effect of silver nanostructures, so that paraquat, diquat and its free radicals are enriched and located near the SERS hot spot, forming a synergistic enhancement mechanism, so that a high signal-to-noise ratio Raman signal can still be obtained under low concentration conditions, significantly reducing the detection limit. It has chemical and thermal stability during the reaction process and does not react with organic solvents; (4) Free radicals have the characteristics of short life and high reactivity, and traditional substrates are difficult to detect effectively. The micro-nano confinement environment provided by the MF@NH2@Ag substrate can adsorb and transiently stabilize free radicals in the early stage of their generation, allowing them to enter the SERS enhancement region before rapid quenching occurs, thereby enabling trace detection of paraquat and diquat free radicals. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the preparation of the silver nanoparticle composite melamine foam SERS substrate used in this invention and the detection of the target analytes.

[0022] Figure 2 These are SEM images (a), (b), and (c) of the melamine foam used in this invention, magnified from low to high magnification; and SEM images (d), (e), and (f) of the silver nanoparticle composite melamine foam, magnified from low to high magnification.

[0023] Figure 3 This is the UV-Vis absorption spectrum of the aqueous solutions of Ag NPs (black line), PEI (red line), and Ag NPs + PEI (blue) used in this invention.

[0024] Figure 4 These are the XRD patterns of Ag and MF@NH2@Ag used in this invention.

[0025] Figure 5 These are (a) the particle size distribution diagram of the Ag nanosolution used in this invention; and (b) the potential diagram of the Ag nanosolution.

[0026] Figure 6 The following are the XPS summary plots of (a) MF@NH2@Ag, (b) individual element plots of Ag, (c) individual element plots of C, and (d) individual element plots of N used in this invention.

[0027] Figure 7 These are the Raman spectra of different concentrations of paraquat (a), diquat (b), paraquat free radical (c), and diquat free radical (d) used in this invention.

[0028] Figure 8 The linear fitting method used in this invention was used to plot the Raman spectral intensity diagrams of paraquat (a), diquat (b), paraquat free radical (c), and diquat free radical (d) at different concentrations.

[0029] Figure 9 The Raman spectra of paraquat (a), diquat (b), paraquat free radical (c), and diquat free radical (d) used in this invention are obtained as the time interval increases from 90 seconds to 360 seconds. Detailed Implementation

[0030] Figure 1 This is a schematic diagram illustrating the preparation and detection of target analytes using the silver nanoparticle composite melamine foam SERS substrate employed in this invention. First, melamine foam is functionalized by immersion in HCl and PEI solutions to introduce amino groups. Then, it is immersed in AgNO3 solution to enhance the amino groups on Ag... + The coordination and electrostatic attraction of Ag make it uniformly adsorbed on the foam skeleton. Subsequently, NaBH4 is added to the AgNO3 solution as a reducing agent to enrich the foam surface with Ag. + As a precursor, Ag undergoes a reduction reaction. + The silver nanoparticles were reduced to silver nanoparticles and adhered to the surface of melamine foam. Paraquat, diquat, and their free radicals adhered to the surface of the silver nanoparticles through electrostatic interaction. The Raman signal output of the target analytes was enhanced by the plasma resonance of the surface region of the silver nanoparticles, thus realizing the trace detection of target analytes distributed on the Ag-based surface by the MF@NH2@Ag SERS sensor.

[0031] Figure 2These are SEM images (a), (b), and (c) of the melamine foam used in this invention, magnified from low to high magnification; and SEM images (d), (e), and (f) of the silver nanoparticle composite melamine foam, magnified from low to high magnification. The results show that the silver nanoparticles adhere to and are uniformly and densely distributed on the surface of the melamine foam. The silver nanoparticles are monodisperse, with a size of approximately 25 nm, and are uniformly and densely distributed on the surface of the melamine foam framework, forming a nanostructure on the surface of the silver nanoparticle composite melamine foam.

[0032] Figure 3 This is the UV-Vis absorption spectrum of the aqueous solutions of Ag NPs (black line), PEI (red line), and Ag NPs + PEI (blue) used in this invention. Conductive electrons undergo surface plasmon resonance (SPR) on the surface of the silver nanoparticles, and the synthesized silver nanoparticles exhibit the following characteristics: Figure 3 The absorption characteristics shown are as follows: the silver nanoparticle solution exhibits a characteristic light absorption peak at about 382 nm, and the PEI solution exhibits a characteristic light absorption peak at about 220 nm. When the two solutions are mixed, the nitrogen atoms on the PEI chain will have a strong coordination interaction with the surface of the silver nanoparticles, which will cause a slight red shift of the SRP peak.

[0033] Figure 4 These are the XRD patterns of Ag and MF@NH2@Ag used in this invention. The structure of silver nanoparticles under the SERS effect was characterized using X-ray diffraction patterns. Typical XRD patterns of MF@NH2@Ag and silver nanoparticles show that the maximum peaks at 2θ angles of 38.19°, 44.56°, 64.18°, and 77.45° correspond to the crystal planes of the (111), (200), (220), and (311) silver structures (JCPDS No. 87-0597), revealing that silver nanoparticles with a crystalline structure are obtained after the reduction of silver ions, and the structure of the silver nanoparticles attached to the surface of melamine foam remains unchanged.

[0034] Figure 5 These are (a) the particle size distribution diagram of the Ag nanosolution used in this invention; and (b) the potential diagram of the Ag nanosolution. To further investigate the essential characteristics of the obtained material, Zeta potential and particle size distribution tests were performed. Ag NPs with a Zeta potential greater than ±30 mV were considered the most stable. The potential value of Ag NPs was -27.1 mV, indicating that its aqueous dispersion has good stability and a negatively charged surface. The average size of the silver nanoparticles was approximately 22.5 nm, which matches the scanning electron microscope images.

[0035] Figure 6These are the XPS summary images of (a) MF@NH2@Ag, (b) elemental spectra of Ag, (c) elemental spectra of C, and (d) elemental spectra of N used in this invention. The chemical bonding interaction between MF and silver nanoparticles was studied using XPS. As shown in Figure (a), the XPS spectrum of MF@NH2@Ag shows that, in addition to the characteristic peaks of C (1s) and N (1s), there are additional characteristic peaks at 374.08 and 368.08 eV, corresponding to Ag3d and N, respectively. 3 / 2 and Ag3d 5 / 2 The deposition of silver nanoparticles is further demonstrated in Figure (b). The peaks at 284.8 eV and 401.3 eV respectively confirm the presence of C1s and N1s. Figures (c) and (d) show the high-resolution spectra of C1s and N1s, respectively. Figure (c) shows the binding energies of CC, CN, and OC=O bonds, which are characteristic peaks in melamine foam molecules. In Figure (d), N1s shows three peaks at 403 eV, 400 eV, and 399 eV, corresponding to NH, N-Ag, and CN=C, respectively. Among them, N-Ag is a very important characteristic peak, indicating that a coordination bond has been formed between the nitrogen atom and the silver nanoparticle. The appearance of this peak is direct evidence that the silver nanoparticles have successfully bonded to the melamine foam substrate. The NH bond at a binding energy of 403 eV indicates the presence of amino groups on the surface of the melamine foam.

[0036] Figure 7 The concentrations of the four target analytes used in this invention (from bottom to top) are 10. -9 10 -8 10 -7 10 -6 10 -5 10 -4 mol•L -1 Raman spectra of paraquat (a), diquat (b), paraquat free radical (c), and diquat free radical (d). The concentrations of 10... -9 10 -8 10 -7 10 -6 10 -5 10 -4 mol•L -1 Paraquat, diquat, and their free radicals were dropped onto the surface of the SERS sensor, and detection was initiated both after (the first two types) and before (the latter two types) the sensor was completely dry. Paraquat was detected at 1187, 1220, 1524, and 1634 cm⁻¹, respectively. -1 Its Raman characteristic peak appears at [location], with a detection limit as low as 10. -9 mol•L -1 As shown in Figure (a). Figure (b) shows the values ​​from 10... -9 Up to 10-4 Different concentrations of diquat showed Raman peaks at 660, 1171, 1319, 1486, 1559, 1579, and 1602 cm⁻¹. -1 The Raman shifts were also observed. Furthermore, Figures (c) and (d) show that the Raman characteristic peaks of paraquat and diquat radicals, which possess true herbicidal activity, are at 825, 983, 1192, 1544, and 1666 cm⁻¹, respectively. -1 And 1168, 1309 and 1638 cm -1 The location was detected.

[0037] Figure 8 The linear fitting method used in this invention was used to plot the Raman spectral intensity diagrams of paraquat (a), diquat (b), paraquat free radical (c), and diquat free radical (d) at different concentrations. Figure 8 (a) shows a linear relationship between the Raman peak intensities of different concentrations of paraquat, with a Raman shift of 1634 cm⁻¹. -1 (from) Figure 7 (a)); Figure 8 (b) shows a linear relationship between the Raman spectral peak intensities of different concentrations of diquat, with a Raman shift of 1486 cm⁻¹. -1 (from) Figure 7 (b)); Similarly, at 1544 and 1309 cm -1 At the Raman shift, using Figure 7 Data from (c) and (d) were obtained through linear fitting. Figure 8 (c) and (d) show the linear relationship between the Raman spectral peak intensities of paraquat and diquat radicals at different concentrations, respectively. By linearly fitting the intensity evolution of the Raman spectral peaks of different concentrations of paraquat, diquat, paraquat radical, and diquat radical, the slopes and root mean square errors were obtained. The slopes for paraquat, diquat, paraquat radical, and diquat radical were 60.71, 90.28, 111.54, and 293.04 L•, respectively. mol -1 The correlation coefficients between the standard curve and the intensity evolution of different concentrations of the target species were 0.9712, 0.9687, 0.9680 and 0.9620, respectively.

[0038] Figure 9The Raman spectra of paraquat (a), diquat (b), paraquat radical (c), and diquat radical (d) used in this invention were obtained as the time interval increased from 90 seconds to 360 seconds. Whether the developed MF@NH2@Ag SERS sensor can obtain a sensitive detection signal is an important criterion. The surface plasmon resonance (SPR) peak of silver nanoparticles is precisely located at the wavelength of the excitation light due to the aggregation-induced local SPR redshift, exhibiting a critical state with the strongest SERS signal. After evaporation of the immobilized droplet for 30 min, the SERS spectrum was recorded near the critical metastable state. The optimal parameters were used to detect herbicide analytes via SERS substrates. The detection results for diquat and paraquat using the developed SERS sensor were consistent. However, for paraquat and diquat radicals, to avoid the quenching of these radicals, the SERS spectrum was detected in the near-critical metastable state before the immobilized droplet was completely vaporized. Figures (a, b, c, d) show the surface-enhanced Raman spectra of paraquat, diquat, paraquat, and diquat radicals at 90-second intervals from after complete drying (a, b) and before complete drying (c, d). The peak intensity of the SERS spectra remained constant over time. The most important criteria for evaluating the quality of a sensor are its high and low sensitivity and selectivity.

[0039] Detailed Implementation: A method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat, characterized in that: the MF@NH2@Ag SERS sensor uses a three-dimensional mesh framework of melamine foam as a support, the surface of which is functionalized with polyethyleneimine to create an amino molecular layer. The amino groups coordinate with silver ions, resulting in the enrichment of silver ions on the surface of the melamine foam. The silver ions are reduced to silver nanoparticles, which adhere to the surface of the melamine foam. Paraquat, diquat, and their free radicals adhere to the surface of the silver nanoparticles, enhancing the Raman signals of paraquat, diquat, and their free radicals, thus achieving the detection of paraquat, diquat, and their free radicals. The preparation process of the above sensor includes the following steps: First, the melamine foam is cut into pieces with a volume of 1 to 8 cm. 3 The cubic blocks were soaked in deionized water and anhydrous ethanol for 4-6 minutes each, then removed and placed in a vacuum oven to dry at room temperature. They were then placed in 80-120 mL of 0.5-1.5 mol•L⁻¹ solution. -1After soaking in HCl solution for 20-40 min, remove and wash with deionized water until neutral. Dry in an oven at 50-70 °C for 1-3 h. Then, immerse the acid-treated melamine foam in a 0.5-1.5% polyethyleneimine aqueous solution and react at room temperature for 1-3 h to modify the foam surface with amino groups. Wash three times with deionized water and dry in a vacuum oven at 40-60 °C for 2-4 h. Immerse the dried melamine foam in 90-110 mL of 0.001 mol•L⁻¹ solution. -1 In an AgNO3 solution, sonicate for 9–11 min, then slowly add 80–100 mL of a 0.01 mol•L⁻¹ solution. - 1 The NaBH4 solution was stirred at 200-400 rpm for 10 min in an ice bath. The resulting MF@NH2@Ag particles were then washed three times with deionized water and anhydrous ethanol to remove excess impurities adsorbed on the surface. The particles were then vacuum dried at a certain temperature to obtain the MF@NH2@Ag SERS sensor for the detection of paraquat, diquat, and their free radicals.

[0040] The MF@NH2@Ag substrate organically combines the physical adsorption of melamine foam with the electromagnetic enhancement effect of silver nanostructures. This allows paraquat, diquat, and their free radicals to be enriched while being located near SERS hotspots, forming a synergistic enhancement mechanism. As a result, a high signal-to-noise ratio Raman signal can still be obtained under low concentration conditions, significantly reducing the detection limit and achieving trace detection of the target analyte.

[0041] Example: Using sheared melamine foam as a support, polyethyleneimine is functionalized on its surface to modify an amino molecular layer. Silver ions are coordinated with the amino groups and adhere to the surface of MF. An in-situ chemical reduction reaction is used to prepare an MF@NH2@Ag SERS sensor with a surface rich in silver nanoparticles.

[0042] A method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat, characterized in that: the MF@NH2@Ag SERS sensor uses a three-dimensional mesh framework of melamine foam as a support, the surface of which is functionalized with polyethyleneimine to create an amino molecular layer. The amino groups coordinate with silver ions, resulting in the enrichment of silver ions on the surface of the melamine foam. The silver ions are reduced to silver nanoparticles, which adhere to the surface of the melamine foam. Paraquat, diquat, and their free radicals adhere to the surface of the silver nanoparticles, enhancing the Raman signals of paraquat, diquat, and their free radicals, thus enabling the detection of paraquat, diquat, and their free radicals. The preparation process of the sensor includes the following steps: First, the melamine foam was cut into pieces with a volume of 1.5 cm. 3 The cube blocks were soaked in deionized water and anhydrous ethanol for 5 min each, then removed and placed in a vacuum oven to dry at room temperature. They were then placed in 100 mL of a 1.0 mol•L⁻¹ solution. -1 After soaking in HCl solution for 30 min, the foam was removed and washed with deionized water until neutral. It was then dried in an oven at 60 °C for 2 h. The acid-treated melamine foam was then immersed in a 1.0% polyethyleneimine aqueous solution and reacted at room temperature for 2 h to modify the foam surface with amino groups. The foam was then washed three times with deionized water and dried in a vacuum oven at 50 °C for 3 h. The dried melamine foam was then immersed in 100 mL of a 0.001 mol•L⁻¹ solution. -1 In an AgNO3 solution, sonicate for 10 min, then slowly add 90 mL of a 0.01 mol•L⁻¹ solution. -1 The NaBH4 solution was stirred at 300 rpm for 10 min in an ice bath. The resulting MF@NH2@Ag particles were then washed three times with deionized water and anhydrous ethanol to remove excess impurities adsorbed on the surface. The particles were then vacuum dried at a certain temperature to obtain the MF@NH2@Ag SERS sensor for the detection of paraquat, diquat, and their free radicals.

Claims

1. A method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat, characterized in that: The MF@NH2@Ag SERS sensor uses a three-dimensional mesh framework of melamine foam as a support. Its surface is functionalized with polyethyleneimine to create an amino molecular layer. The amino groups coordinate with silver ions, resulting in silver ion enrichment on the surface of the melamine foam. The silver ions are then reduced to silver nanoparticles, which adhere to the surface of the melamine foam. Paraquat, diquat, and their free radicals adhere to the surface of the silver nanoparticles, enhancing the Raman signals of paraquat, diquat, and their free radicals, thus enabling the detection of paraquat, diquat, and their free radicals. The fabrication process of the sensor includes the following steps: First, the melamine foam is cut into pieces with a volume of 1 to 8 cm. 3 The cubic blocks were soaked in deionized water and anhydrous ethanol for 4-6 minutes each, then removed and placed in a vacuum oven to dry at room temperature. They were then placed in 80-120 mL of 0.5-1.5 mol•L⁻¹ solution. -1 After soaking in HCl solution for 20-40 minutes, remove and wash with deionized water until neutral. Dry in an oven at 50-70℃ for 1-3 hours. Then, immerse the acid-treated melamine foam in a 0.5-1.5% polyethyleneimine aqueous solution and react at room temperature for 1-3 hours to modify the foam surface with amino groups. Wash three times with deionized water and dry in a vacuum oven at 40-60℃ for 2-4 hours. Immerse the dried melamine foam in 90-110 mL of 0.001 mol·L⁻¹ solution. -1 In an AgNO3 solution, sonicate for 9–11 min, then slowly add 80–100 mL of a 0.01 mol•L⁻¹ solution. -1 The NaBH4 solution was stirred at 200-400 rpm for 10 min in an ice bath. The resulting MF@NH2@Ag particles were then washed three times with deionized water and anhydrous ethanol to remove excess impurities adsorbed on the surface. The particles were then vacuum dried at a certain temperature to obtain the MF@NH2@Ag SERS sensor for the detection of paraquat, diquat, and their free radicals.

2. The method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat according to claim 1, characterized in that: The SERS sensor mentioned is MF@NH2@Ag.

3. The method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat according to claim 1, characterized in that: The MF is a flexible, porous three-dimensional material called melamine foam.

4. The method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat according to claim 1, characterized in that: The particle size of the silver nanoparticles is controllable and can be controlled by adjusting the reaction time and the amount of reducing agent.

5. The method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat according to claim 1, characterized in that: The MF surface is enriched with amino groups after modification.

6. The method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat according to claim 1, characterized in that: The modified amino group forms coordination bonds with silver ions on the surface of the MF, and silver nanoparticles are obtained by in-situ reduction and adherence to the surface of the MF.

7. The method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat according to claim 1, characterized in that: In the SERS sensor, silver nanoparticles automatically accumulate on the surface of silver nanoparticles through electrostatic interactions with paraquat, diquat, and their free radicals.

8. The method for preparing an MF@NH2@Ag SERS sensor for detecting paraquat and diquat according to claim 1, characterized in that: The MF@NH2@Ag SERS sensor described above is based on the principle of localized surface plasmon resonance to detect paraquat, diquat, and their free radicals.

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