A repeatable water-washing composite SERS substrate and a preparation method and application thereof

Abstract

The present application belongs to the technical field of chemical analysis, and particularly relates to a composite SERS substrate capable of repeated water washing and a preparation method and application thereof. The present application takes a flexible carbon cloth as a carrier, in-situ grows ZIF-8 on the surface of the flexible carbon cloth, and loads gold nanoparticles on the surface of the ZIF-8 to form a hierarchical composite structure. The present application solves the problems of easy peeling and easy instability of MOF on a flexible substrate from the aspects of a preparation method and a structure, so that the substrate can still maintain good structural integrity and flexible adaptability under the conditions of bending, washing and liquid immersion. Meanwhile, under the premise of not damaging the pore structure of MOF and the enhancement performance of noble metal, the present application introduces a hydrophobic interface to improve the water phase stability and washability, so as to effectively improve the anti-interference ability and detection reliability of the composite SERS substrate in complex liquid samples.

Description

Technical Field

[0001] This invention belongs to the field of chemical analysis technology, specifically relating to a reusable water-washable composite SERS substrate, its preparation method, and its application. Background Technology

[0002] Surface-enhanced Raman scattering (SERS) is a highly sensitive molecular detection technique based on the localized surface plasmon resonance effect of noble metal nanostructures. It can significantly enhance the Raman scattering signal of molecules adsorbed on the surface of metal nanostructures such as gold and silver, thereby enabling trace and even ultra-trace detection of target analytes. Due to its advantages of high sensitivity, high selectivity, and no need for complex pretreatment, SERS technology has significant application value in fields such as food safety, environmental monitoring, and bioanalysis.

[0003] Existing SERS substrates mostly use noble metal nanoparticles such as gold and silver as the main reinforcing units. However, in actual preparation and use, noble metal nanoparticles are prone to aggregation, leading to uneven distribution of local electromagnetic "hot spots," resulting in insufficient SERS signal stability and reproducibility. Therefore, metal-organic frameworks (MOFs) have begun to be introduced as carriers for noble metal nanoparticles. MOFs have the characteristics of large specific surface area, tunable pore structure, and abundant surface functional sites, which can inhibit the aggregation of noble metal nanoparticles to a certain extent and achieve the enrichment of target molecules through the pore structure, thereby improving the SERS detection sensitivity and signal uniformity.

[0004] However, existing MOF-based SERS substrates still have many shortcomings. Most existing MOF-based SERS substrates are constructed on rigid carriers such as glass and silicon wafers, which have poor flexibility and limited application scenarios, making it difficult to meet the integration requirements of curved surface detection or portable detection devices. At the same time, MOF materials are prone to structural hydrolysis or surface contamination in aqueous environments. After actual liquid sample detection, it is often difficult to restore their SERS activity through simple cleaning. As a result, the relevant substrates can usually only be used once, making it difficult to achieve repeated detection and increasing detection costs.

[0005] Especially when MOF materials are introduced into flexible SERS substrate systems, they still face a series of technical challenges that differ from traditional metal or polymer-based flexible SERS substrates. MOFs are typically crystalline porous structures, which are prone to detachment, structural damage, or interface failure with the flexible substrate during bending, ultrasonic cleaning, or repeated liquid immersion on flexible supports, making it difficult to maintain stable microstructural integrity while ensuring flexibility. Simultaneously, while the pore structure of MOFs enriches target molecules, it also easily traps impurity molecules or interfering substances after detection. Conventional water washing is insufficient to effectively clean the pores, leading to irreversible degradation of SERS activity. Furthermore, many MOF materials are inherently sensitive to water molecules, easily undergoing framework hydrolysis or performance degradation in aqueous environments. Water-stable MOFs typically require high-temperature, long-duration reactions during preparation, making them difficult to grow on flexible substrates. For water-sensitive MOF materials, improper conventional hydrophobic modification methods may clog MOF pores or weaken their enrichment function, thus affecting the SERS enhancement effect. Therefore, how to stably construct MOF-based SERS structures on flexible carriers and achieve their structural stability, washability, and reusability in aqueous environments without damaging the MOF pore structure and noble metal reinforcement properties remains a technical problem that has not yet been effectively solved in the existing technology.

[0006] Furthermore, in complex water or food samples, a large number of coexisting water molecules and matrix interfering substances easily cover or competitively adsorb onto the active sites of noble metals, further weakening the stable output of the SERS signal. Currently, there is a lack of flexible MOF-based SERS substrates that can maintain structural stability in aqueous and complex matrix environments, have recoverable SERS activity, and be reusable after washing. Therefore, there is an urgent need to develop a novel SERS substrate that combines flexible structure, high SERS enhancement performance, aqueous stability, and reusability after washing to meet the comprehensive requirements of reliability, durability, and economy in practical applications. Summary of the Invention

[0007] Based on this, this invention uses flexible carbon cloth as a carrier to grow ZIF-8 in situ on its surface, and then loads gold nanoparticles onto the ZIF-8 surface to form a hierarchical composite structure. Furthermore, without compromising the MOF pore structure and noble metal reinforcement properties, a hydrophobic interface is introduced to improve aqueous stability and washability. Thus, this invention addresses the problems of easy peeling and instability of MOFs on flexible substrates from both the preparation method and structural perspectives, enabling the substrate to maintain good structural integrity and flexibility under bending, cleaning, and liquid wetting conditions.

[0008] The first objective of this invention is to provide a method for preparing a reusable, washable composite SERS substrate.

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

[0010] A method for preparing a reusable washable composite SERS substrate involves using flexible carbon cloth as a carrier, growing ZIF-8 in situ on its surface, and loading gold nanoparticles onto the ZIF-8 surface to obtain a CC / ZIF-8@Au composite structure; then, dipping in n-dodecyl mercaptan to obtain a reusable washable composite SERS substrate.

[0011] It is worth noting that this invention utilizes the ZIF-8 porous structure to confine and disperse noble metal nanoparticles, achieving a high-density and stable SERS enhancement interface, effectively suppressing the aggregation of noble metal nanoparticles and improving detection performance. Specifically, the porous structure of ZIF-8 provides a stable loading and confinement environment for gold nanoparticles, effectively avoiding the aggregation problem of noble metal nanoparticles during use. At the same time, it can enrich the target analyte into the electromagnetic enhancement "hot spot" region near the gold nanoparticles, thereby significantly improving the SERS signal intensity and spatial uniformity.

[0012] Furthermore, without compromising the MOF pore structure and noble metal reinforcement properties, this invention introduces a hydrophobic interface by dipping in n-dodecyl mercaptan to enhance aqueous stability and washability. By hydrophobizing the CC / ZIF-8@Au composite structure, a hydrophobic barrier is constructed on its surface, reducing the direct erosion of the MOF framework and noble metal active sites by water molecules and impurities. This alleviates the hydrolysis or deactivation of MOFs in aqueous environments and avoids significant blockage of MOF pores by the hydrophobic layer, thus ensuring stable SERS activity of the substrate even after multiple water washes or ultrasonic cleanings. Compared to MOF-based SERS substrates without hydrophobic treatment, this substrate maintains good structural integrity and SERS activity after multiple water washes or ultrasonic cleanings, enabling stable cyclic use and significantly reducing actual detection costs.

[0013] Furthermore, the preparation method of the present invention specifically includes the following steps:

[0014] S1. Preparation of CC / ZIF-8 composite material: The carbon cloth was ultrasonically cleaned with ethanol and deionized water in sequence, and then dried for later use; 50-70 mg / mL zinc acetate dihydrate aqueous solution and 200-250 mg / mL 2-methylimidazole aqueous solution were mixed in a beaker at a volume ratio of about 1:1; the carbon cloth was immersed in the mixture and reacted at a constant temperature for a period of time, and then allowed to stand at room temperature; finally, the sample surface was thoroughly rinsed with deionized water and vacuum dried overnight at 50-70°C to obtain CC / ZIF-8 composite material;

[0015] S2. Preparation of CC / ZIF-8@Au composite SERS substrate: The CC / ZIF-8 composite material obtained in S1 is mixed with a certain amount of PVP-modified gold nanoparticle methanol solution and subjected to ultrasonic reaction. After ultrasonication, the CC / ZIF-8@Au is quickly placed in an oven at 50~70℃ and dried for 8~15 minutes.

[0016] S3. Hydrophobic treatment of CC / ZIF-8@Au: Briefly immerse the CC / ZIF-8@Au composite material obtained in step S2 in a 0.5~2% (w / w) solution of n-dodecyl mercaptan ethanol, remove it and dry it in an oven at 50~70℃ for 8~15 minutes to obtain the washable composite SERS substrate.

[0017] It is worth noting that in step S2, the gold nanoparticles are stably loaded onto ZIF-8 through interaction with the surface and pore structure of ZIF-8, rather than being directly deposited on the carbon cloth surface. The porous structure of ZIF-8 confines and disperses the gold nanoparticles, effectively preventing the aggregation of noble metal nanoparticles, thereby forming a high-density and stable SERS electromagnetic enhancement hotspot.

[0018] In step S3, this invention introduces a hydrophobic layer onto the surface of the CC / ZIF-8@Au composite material, constructing a hydrophobic barrier without compromising the SERS activity of the noble metal nanoparticles. This hydrophobic barrier effectively prevents direct contact between the aqueous environment and the ZIF-8 framework, reducing the irreversible adsorption of water molecules and impurities. This allows the composite substrate to maintain good structural integrity and SERS enhancement performance even after multiple ultrasonic water washes, thus enabling reusability.

[0019] Furthermore, in steps S1 and S2, the ratio of carbon cloth area to mixed liquid volume to the volume of the PVP-modified gold nanoparticle methanol solution is 2.25~4 cm³. 2 : 8~12mL : 4~5 mL.

[0020] Furthermore, in step S1, after immersing the carbon cloth in the mixture, it is gently stirred for a few seconds to fifteen seconds, and then the beaker containing the carbon cloth and the mixture is kept at a constant temperature of 45~55°C for 0.5~2 hours; then the beaker is removed and left to stand at room temperature for 14~16 hours.

[0021] It is worth noting that, in order to achieve in-situ growth of ZIF-8 on the carbon cloth (CC) surface, this invention employs stirring for several to fifteen seconds. This facilitates thorough wetting of the carbon cloth surface by the mixed solution and promotes initial contact between metal ions and the active sites on the carbon cloth surface, thereby inducing effective nucleation of ZIF-8 on the carbon cloth surface. If the stirring time is too short, the contact between the mixed solution and the carbon cloth surface will be insufficient, easily leading to a large number of ZIF-8 spontaneously nucleating in the solution, making it difficult to form a stable adhesion on the carbon cloth surface, thus reducing the loading and uniformity of ZIF-8 on the carbon cloth. On the other hand, excessive stirring may destroy the initial nucleation structure, which is not conducive to the formation of a continuous MOF capping layer.

[0022] This invention employs a reaction at 45–55 °C for 0.5–2 hours, followed by a standing period at room temperature for 14–16 hours, to balance the formation rate and structural stability of ZIF-8. If the reaction time is too short, the crystallization and growth process of ZIF-8 is insufficient, resulting in inadequate formation, incomplete crystals, and difficulty in forming a continuous and effective porous framework structure. Conversely, if the reaction time is too long, it can easily induce partial hydrolysis or structural defects in the ZIF-8 framework in an aqueous environment, which is detrimental to the subsequent stable loading of gold nanoparticles and the maintenance of SERS performance.

[0023] Therefore, by synergistically limiting the stirring time and reaction time, this invention achieves controllable in-situ growth of ZIF-8 on the surface of carbon cloth (CC), laying a structural foundation for the uniform loading of gold nanoparticles and the stability and reusability of the composite SERS substrate.

[0024] Furthermore, in step S2, the conditions for the ultrasonic reaction are a power of 30~50 kHz and an ultrasonic time of 45~50 min.

[0025] It is worth noting that, in order to achieve effective loading of gold nanoparticles on the ZIF-8 surface, this invention limits the ultrasonication time to 45-50 min. Under ultrasonication, the methanol solution of PVP-modified gold nanoparticles can maintain good dispersion in the solution and fully contact the ZIF-8 surface, achieving effective loading of the ZIF-8 surface. When the ultrasonication time is too short, the contact and assembly process between the gold nanoparticles and ZIF-8 is insufficient, resulting in a low gold nanoparticle loading and difficulty in forming a sufficient number of electromagnetic enhancement "hot spots" on the ZIF-8 surface, thus limiting the SERS signal intensity of the composite substrate. When the ultrasonication time is too long, continuous ultrasonication can easily induce secondary migration and local aggregation of the loaded gold nanoparticles, and even enter and block the pore structure of ZIF-8. This not only weakens the adsorption and enrichment capacity of ZIF-8 for target molecules, but may also disrupt the reasonable spacing between gold nanoparticles, adversely affecting the SERS detection performance. Therefore, by limiting the ultrasonic time to a suitable range of 45 to 50 minutes, this invention ensures sufficient and uniform loading of gold nanoparticles while avoiding nanoparticle aggregation and pore blockage, thus achieving synergistic optimization of ZIF-8 adsorption performance and electromagnetic enhancement effect of gold nanoparticles.

[0026] Furthermore, the method for preparing the PVP-modified gold nanoparticle methanol solution in step S2 includes:

[0027] 1) Preparation of aqueous solution of gold nanoparticles (AuNPs): First, weigh 0.1 g of sodium citrate and dissolve it completely in 9.9 mL of deionized water to obtain a 1% sodium citrate aqueous solution; then weigh 0.1 g of chloroauric acid and dissolve it completely in 9.9 mL of deionized water to obtain a 1% chloroauric acid aqueous solution; add 99 mL of deionized water and 1 mL of 1% chloroauric acid solution to a three-necked flask, heat in an oil bath and keep stirring at 120 °C at a stirring speed of 600 rpm; after the solution in the three-necked flask boils, add 1.4 mL of 1% sodium citrate aqueous solution and continue stirring until the solution turns wine red. Start timing at this point and continue stirring for 15 minutes. Then remove the flask and cool it to 25 °C to obtain the aqueous solution of gold nanoparticles.

[0028] 2) Preparation of PVP-modified gold nanoparticle methanol solution: Weigh 200~300 mg of polyvinylpyrrolidone (PVP) and dissolve it in 8~12 mL of water to obtain PVP aqueous solution; slowly pour the PVP aqueous solution into the gold nanoparticle aqueous solution prepared in step (1), place it on a magnetic stirring table, stir at 300~600 rpm for 2~3 hours, then wash it three times with methanol, and finally adjust the volume to 8~12 mL with methanol to obtain PVP-modified gold nanoparticle methanol solution.

[0029] It is worth noting that, in order to achieve the assembly of gold nanoparticles onto the surface of ZIF-8, this invention limits the volume of the PVP-modified gold nanoparticle methanol solution to 8-12 mL. After completing the PVP surface modification of the gold nanoparticles, this invention uses methanol as the dispersion solvent for volume adjustment, rather than directly using an aqueous system. Methanol has a weaker hydrolytic effect on the ZIF-8 framework than water, which can effectively reduce the direct contact between ZIF-8 and water molecules during subsequent ultrasonic assembly, thereby maintaining the integrity of the ZIF-8 porous structure. Furthermore, this invention further limits the volume of the PVP-modified gold nanoparticle methanol solution to 8-12 mL to achieve a balance between the gold nanoparticle concentration and loading behavior. When the volume is too large, the concentration of gold nanoparticles is too low, making it difficult to provide enough nanoparticles for effective loading on the ZIF-8 surface during ultrasonic assembly. This results in insufficient gold nanoparticle density in the composite structure, thus limiting the formation of SERS electromagnetic enhancement "hot spots." Conversely, when the volume is too small, the concentration of gold nanoparticles is too high, making them prone to aggregation and agglomeration before assembly or during ultrasonication. This not only weakens their dispersibility but may also affect their uniform assembly effect on the ZIF-8 surface. Therefore, by synergistically limiting the type of dispersion solvent and the volume, this invention achieves stable and efficient loading of gold nanoparticles on the ZIF-8 surface while maintaining good dispersion. This provides a crucial guarantee for constructing a structurally stable and reproducible CC / ZIF-8@Au composite SERS substrate.

[0030] Therefore, considering the high sensitivity of MOF materials to reaction conditions during nucleation and growth, and the complex surface structure and abundant pores of flexible carbon cloth, improper control of reaction conditions can easily lead to homogeneous nucleation of ZIF-8 in solution, making in-situ growth on the carbon cloth surface difficult and affecting the uniformity and stability of the composite structure. This invention effectively induces heterogeneous nucleation and continuous growth of ZIF-8 on the carbon cloth fiber surface by synergistically limiting the initial stirring time, isothermal reaction time, and subsequent settling process, thus constructing a stable MOF hierarchical structure attached to a flexible carrier.

[0031] However, while the porous structure of MOFs is beneficial for the enrichment of target molecules, improper selection of loading methods or conditions during the loading of noble metal nanoparticles can easily lead to aggregation or pore blockage, thereby weakening the structural advantages and SERS performance of MOFs. This invention utilizes PVP-modified gold nanoparticles, combined with specific ultrasonic conditions and solvent systems, to achieve uniform assembly of gold nanoparticles on the ZIF-8 surface. This ensures the formation of high-density SERS electromagnetic enhancement "hot spots" while maintaining the integrity of the ZIF-8 pore structure.

[0032] Finally, addressing the issue of structural instability of ZIF-8 under aqueous environment and ultrasonic cleaning conditions, this invention does not simply rely on thickening or densifying the coating. Instead, based on the surface chemical properties of ZIF-8, a specific concentration of dodecyl mercaptan hydrophobic layer with good compatibility with gold nanoparticles is introduced. Without significantly hindering the target molecules from approaching the SERS active sites, an effective hydrophobic protective interface is constructed, thereby significantly improving the structural stability and reusability of the composite substrate under multiple water washing conditions.

[0033] The second objective of this invention is to provide a reusable, washable composite SERS substrate prepared by the method described above.

[0034] A reusable washable composite SERS substrate, wherein the composite SERS substrate has a sandwich structure, with flexible carbon cloth (CC) as the carrier, a ZIF-8@Au composite structure in the middle layer, and a dodecyl mercaptan hydrophobic layer on top.

[0035] Furthermore, in the ZIF-8@Au composite structure, gold nanoparticles are assembled on the surface of ZIF-8 to form a metal nanocomposite structure with ZIF-8 as the supporting framework.

[0036] A third objective of this invention is to provide an application of the reusable washable composite SERS substrate as described above.

[0037] Application of a reusable, washable composite SERS substrate in liquid sample detection.

[0038] Furthermore, the composite SERS substrate is reused at least several times, with a maximum of 5 times.

[0039] It is worth noting that this invention utilizes a hydrophobic layer to partially block direct contact between the aqueous environment and the ZIF-8 framework, which helps to exclude interference from non-target water molecules and slows down the hydrolysis or deactivation process of the MOF structure in the aqueous phase. Simultaneously, by leveraging the adsorption effect of the MOF and the synergistic effect of the hydrophobic interface, the analyte molecules are pre-enriched, enabling the substrate to stably output clear SERS signals for the analyte molecules even in complex matrix samples. Therefore, the reusable washable composite SERS substrate exhibits excellent anti-interference capability and detection reliability in complex liquid samples.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. To address the issue of ZIF-8 materials being prone to hydrolysis in aqueous environments and their pore structure being easily contaminated, leading to performance degradation, this invention introduces a hydrophobic layer onto the surface of the CC / ZIF-8@Au composite structure. By rationally selecting hydrophobic materials, specific concentrations, and treatment methods, effective protection of the ZIF-8 structure is achieved. Specifically, based on the structural characteristics of ZIF-8, which relies on its pore structure to achieve pre-enrichment of target molecules and is not suitable for thick-layer coating to avoid pore blockage, this invention uses n-dodecathiol for dip-coating. Utilizing its small-molecule hydrophobic properties, a hydrophobic interface is constructed without significantly affecting the MOF pore structure. Simultaneously, the thiol groups in the n-dodecathiol molecule preferentially form stable Au–S bonds with the surface of gold nanoparticles, anchoring the hydrophobic modification to the noble metal reinforcing unit and its surrounding region. This reduces direct contact between water molecules and the ZIF-8 framework while avoiding damage to the MOF pore function and SERS activity. The hydrophobic layer ensures the long-term structural integrity and functional stability of CC / ZIF-8@Au, enabling the composite substrate to maintain good structural integrity and stable SERS enhancement performance after multiple ultrasonic water washings, thus achieving reusability.

[0042] 2. Compared with traditional rigid MOF-based SERS substrates, this invention uses flexible carbon cloth (CC) as a carrier, which gives the substrate the characteristics of being flexible and easy to cut into any shape and size as needed, to meet the needs of micro-sampling or integration into miniaturized detection equipment.

[0043] 3. Compared with existing flexible non-MOF-based SERS substrates, this invention introduces ZIF-8 as a porous metal-organic framework structure to construct a composite enhancement system on a flexible substrate that combines target molecule enrichment with noble metal confinement. Unlike non-MOF flexible SERS substrates that mainly rely on surface-localized electromagnetic enhancement, the regular pore structure of ZIF-8 can pre-enrich and spatially confine the target analyte during detection, making it more effectively close to the electromagnetic enhancement "hotspot" region formed by gold nanoparticles. This improves detection sensitivity and signal stability without significantly increasing the amount of noble metal used. At the same time, this porous structure helps improve the consistency of SERS response between different regions of the flexible substrate, enhancing the reproducibility and reliability of detection results.

[0044] 4. Compared with the polymer films, elastomers, or flexible metal foil carriers commonly used in existing flexible SERS substrates, this invention selects carbon cloth (CC) as a flexible support substrate, which has the advantages of low cost, wide availability, and simple processing technology. CC is composed of interwoven carbon fibers, forming a stable three-dimensional porous network structure with good overall flexibility and strong bendability. It can also be easily cut into different shapes and sizes as needed, making it suitable for detection scenarios on irregular surfaces or in confined spaces. At the same time, carbon cloth has good heat resistance, solvent resistance, and chemical stability. It is not prone to deformation or performance degradation under repeated ultrasonic cleaning treatments or complex environmental conditions, providing reliable support for the stable construction and long-term use of flexible MOF-based SERS structures.

[0045] 5. This invention enriches the target analyte through the ZIF-8 porous structure and works synergistically with gold nanoparticles to achieve high-sensitivity detection. At the same time, the hydrophobic treatment of the substrate surface can effectively repel water molecule interference, and can still output signals stably and clearly in humid or complex matrix environments. It has both high sensitivity and strong anti-interference ability, which significantly improves the reliability and practicality of detection. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0047] Figure 1 This is a SEM image of ZIF-8@Au on carbon fiber CC in Embodiment 1 of the present invention.

[0048] Figure 2 This is a magnified SEM image of CC / ZIF-8@Au in Embodiment 1 of the present invention.

[0049] Figure 3 The composite SERS substrate pair 10 in Embodiment 2 of the present invention -5 Results of cyclic analysis of M crystal violet.

[0050] Figure 4 The composite SERS substrate pair 10 in Embodiment 2 of the present invention -5 M crystal violet cyclic detection 1170 cm -1 Intensity histogram at the location.

[0051] Figure 5 The composite SERS substrate pair 10 in Embodiment 3 of the present invention -3 ~10 -7 Raman spectrum of ofloxacin (OFL) ethanol solution of M.

[0052] Figure 6 The composite SERS substrate pair 10 in Embodiment 3 of the present invention -3 ~10 -7 M of ofloxacin (OFL) ethanol solution 1391 cm -1 Linear fitting plot at [location].

[0053] Figure 7 The composite SERS substrate pair 10 in Embodiment 4 of the present invention -3 ~10 -7 Raman spectrum of ofloxacin (OFL) in milk solution.

[0054] Figure 8 The composite SERS substrate pair 10 in Embodiment 4 of the present invention -3 ~10 -7 M of ofloxacin (OFL) milk solution 1388 cm -1 Linear fitting plot at [location]. Detailed Implementation

[0055] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0056] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance testing in the embodiments of this application, unless otherwise specified, employs conventional testing methods in the art. It should be understood that the terminology used in this application is merely for describing particular implementations and is not intended to limit the scope of this disclosure.

[0057] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; other experimental methods and technical means not specifically mentioned herein refer to experimental methods and technical means commonly used by one of ordinary skill in the art.

[0058] To better illustrate the content of this application, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this application can be implemented even without certain specific details. In the embodiments, some methods, means, instruments, and devices well-known to those skilled in the art are not described in detail in order to highlight the main points of this application.

[0059] Without conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the resulting technical solution belongs to the content disclosed in the embodiments of this application.

[0060] This invention discloses a reusable washable composite SERS substrate, its preparation method, and its applications, belonging to the field of chemical analysis technology. The invention uses flexible carbon cloth as a carrier, grows ZIF-8 in situ on its surface, and loads gold nanoparticles onto the ZIF-8 surface to form a hierarchical composite structure. This addresses the problems of easy peeling and instability of MOFs on flexible substrates from both the preparation method and structural perspectives, ensuring the substrate maintains good structural integrity and flexibility under bending, washing, and liquid wetting conditions. Furthermore, without damaging the MOF pore structure and noble metal reinforcement properties, this invention introduces a hydrophobic interface to improve aqueous phase stability and washability, thereby effectively enhancing the anti-interference ability and detection reliability of the composite SERS substrate in complex liquid samples.

[0061] To better understand the present invention, the following embodiments are provided for further detailed description of the present invention, but they should not be construed as limiting the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are also considered to fall within the protection scope of the present invention.

[0062] Example 1: Preparation and hydrophobic treatment of a CC / ZIF-8@Au composite SERS substrate

[0063] Step 1: Preparation of gold nanoparticle aqueous solution (AuNPs): First, weigh 0.1 g of sodium citrate and dissolve it completely in 9.9 mL of deionized water to obtain a 1% sodium citrate aqueous solution; then weigh 0.1 g of chloroauric acid and dissolve it completely in 9.9 mL of deionized water to obtain a 1% chloroauric acid aqueous solution; add 99 mL of deionized water and 1 mL of 1% chloroauric acid solution to a three-necked flask, heat in an oil bath and keep stirring at 120 °C and 600 rpm; after the solution in the three-necked flask boils, add 1.4 mL of 1% sodium citrate aqueous solution and continue stirring until the solution turns wine red. Start timing at this point and continue stirring for 15 minutes. Then remove the flask and cool it to 25 °C to obtain the gold nanoparticle aqueous solution.

[0064] Step 2: Preparation of PVP-modified gold nanoparticle methanol solution: Weigh 250 mg of polyvinylpyrrolidone (PVP) and dissolve it in 10 mL of water to obtain a PVP aqueous solution with a concentration of 25 mg / mL; Slowly pour the PVP aqueous solution into the gold nanoparticle aqueous solution prepared in Step 1, place it on a magnetic stirrer, stir at 600 rpm for 2 hours, then wash it three times with methanol at 8000 rpm, and finally make up to 10 mL with methanol to obtain the PVP-modified gold nanoparticle methanol solution;

[0065] Step 3: Preparation of CC / ZIF-8 composite material: Take a piece of carbon cloth (2 cm × 2 cm), ultrasonically clean it for 15 minutes each with ethanol and deionized water, and then dry it at 60°C for later use; weigh 300 mg of zinc acetate dihydrate powder, and dilute it to 5 mL with deionized water to obtain an aqueous solution of zinc acetate dihydrate with a concentration of 60 mg / mL; weigh 1.1166 g of 2-methylimidazole powder, and dilute it to 5 mL with deionized water. mL, to obtain an aqueous solution of 2-methylimidazole with a concentration of 223 mg / mL; mix the aqueous solution of zinc acetate dihydrate and the aqueous solution of 2-methylimidazole powder in a beaker, immerse the dried carbon cloth in it, stir gently for 15 seconds, and then place the beaker containing the carbon cloth and the mixture in a water bath at 50°C for 1 hour; then remove the beaker and let it stand at room temperature for 14 hours to promote the growth of ZIF-8 on the surface of the carbon cloth; after the growth is completed, rinse the sample surface thoroughly with deionized water and vacuum dry overnight at 60°C to obtain the CC / ZIF-8 composite material;

[0066] Step 4: Preparation of CC / ZIF-8@Au composite SERS substrate: The CC / ZIF-8 composite material obtained in Step 3 was placed in a glass bottle. 4 mL of the methanol solution of PVP-modified gold nanoparticles prepared in Step 2 was dropped into the glass bottle. The glass bottle was then placed in an ultrasonic machine and sonicated at 40 kHz for 45 minutes to assemble the gold nanoparticles onto CC / ZIF-8 to obtain CC / ZIF-8@Au. After sonication, CC / ZIF-8@Au was quickly placed in a 60 ℃ oven to dry for 10 minutes.

[0067] Step 5: Hydrophobic treatment of CC / ZIF-8@Au: Take 0.1 g of n-dodecyl mercaptan and dissolve it in 9.9 g of anhydrous ethanol to prepare a 1% n-dodecyl mercaptan ethanol solution; immerse the CC / ZIF-8@Au composite material dried in Step 4 into the solution, briefly wet it, and then take it out and dry it in a 60 ℃ oven for 10 minutes.

[0068] Figure 1 Here is a SEM image of ZIF-8@Au on carbon fiber. Figure 2 This is a magnified SEM image of a CC / ZIF-8@Au image. Figure 1 It can be observed that ZIF-8@Au grows uniformly and densely on the fibers of the carbon cloth; by Figure 2 It can be observed that ZIF-8 exhibits a regular dodecahedral structure, and gold nanoparticles are uniformly distributed on ZIF-8.

[0069] Example 2

[0070] The washable composite SERS substrate prepared in Example 1 was immersed for 10 minutes. -5 After soaking CC / ZIF-8@Au in crystal violet (CV) solution for 5 minutes and drying it in an oven at 60 °C for 10 minutes, Raman detection was performed on CC / ZIF-8@Au under a laser with a wavelength of 785 nm. Five points were taken for each detection, and the average of the five obtained spectral data was taken as the Raman spectral data of that detection.

[0071] After each round of testing, the used composite SERS substrate was immersed in 30 mL of deionized water, ultrasonically cleaned for 10 minutes to remove residual CV molecules, dried in an oven at 60 °C for 10 minutes, and the Raman spectrum of the composite SERS substrate after cleaning and drying was detected.

[0072] The cleaned and dried composite SERS substrate was immersed in 10... -5After soaking CC / ZIF-8@Au in the CV solution of M for 10 minutes and drying it in an oven at 60°C for 10 minutes, Raman detection was performed on CC / ZIF-8@Au using a laser with a wavelength of 785 nm. Five points were collected for each detection, and the average of the five spectra was taken as the Raman spectrum for that detection. After five cycles of detection, the results were as follows: Figure 3 The spectrum shown is illustrated. Raman spectra A, B, C, D, and E correspond to the first, second, third, fourth, and fifth measurements, respectively; Raman spectra A1, B2, C3, D4, and E5 correspond to the spectra measured after the first, second, third, fourth, and fifth ultrasonic cleaning and drying processes, respectively. Observation. Figure 3 Raman spectra before and after cleaning (1170 cm⁻¹) -1 The strength at the location indicates that after each ultrasonic cleaning, the CV reaches 1170 cm⁻¹ on the composite SERS substrate. -1 The strength at that point is almost nonexistent, indirectly proving that ultrasonic cleaning can effectively remove CV molecules from the composite SERS substrate. For example... Figure 4 As shown Figure 3 In five CV tests, 1170 cm -1 The histogram of intensity at a certain point shows that in the fifth test after cleaning, the intensity was 1170 cm. -1 The strength remains high, indicating that the composite SERS substrate has good reusability.

[0073] Example 3

[0074] The washable composite SERS substrate prepared in Example 1 was immersed for 10 minutes. -3 ~10 -7 After soaking CC / ZIF-8@Au in an aqueous solution of ofloxacin (OFL) for 5 minutes and drying it in an oven at 60 °C for 10 minutes, Raman detection was performed on CC / ZIF-8@Au under a laser with a wavelength of 785 nm. Five points were taken for each detection, and the average of the five obtained spectral data was taken as the Raman spectral data of that detection.

[0075] Take 1391 cm -1 A linear fit was performed using the intensity at the Raman characteristic peak as the ordinate and the concentration as the ordinate. 10 -3 ~10 -7 The detection results of the Raman spectrum of ofloxacin ethanol solution of M are as follows Figure 5 As shown, it can be found that when the concentration of ofloxacin is 10... -7 At M, it can still reach 1391 cm -1 and 1615 cm -1 A relatively obvious Raman peak was observed at this point. The line fitting results are as follows: Figure 6As shown, the fitted equation is Y=403.57 Log(C) + 3326.71, R 2 It is 0.99.

[0076] Example 4

[0077] Centrifuge 50 mL of milk sample at 5000 rpm for 10 minutes, and collect the supernatant to remove fat; then add 50 mL of acetonitrile to the skimmed milk sample, mix and stir at 600 rpm until homogeneous, then centrifuge at 8000 rpm for 10 minutes, and collect the supernatant as pretreated milk. Use the pretreated milk to prepare 10... -3 ~10 -7 M ofloxacin milk solution.

[0078] The washable composite SERS substrate prepared in Example 1 was immersed for 10 minutes. -3 ~10 -7 After soaking the substrate in an ofloxacin-milk solution for 5 minutes and drying it in an oven at 60°C for 10 minutes, the composite SERS substrate was subjected to Raman detection under a laser with a wavelength of 785 nm. Five points were taken for each detection, and the average of the five spectral data was taken as the Raman spectral data of that detection.

[0079] Take 1388 cm -1 A linear fit was performed using the intensity at the Raman characteristic peak as the ordinate and the concentration as the ordinate. 10 -3 ~10 -7 The Raman spectroscopy results of ofloxacin in milk solution of M are as follows: Figure 7 As shown, it can be found that when the concentration of ofloxacin is 10... -7 At M, it can be 1388 cm -1 and 1609 cm -1 A more pronounced Raman peak was observed at this location, similar to the 1391 cm peak observed in Example 3. -1 and 1615 cm -1 Compared to the characteristic peak positions, the positions have shifted slightly due to changes in the chemical environment of the target being detected. The line fitting results are as follows: Figure 8 As shown, the fitted equation is Y=416.35 Log(C) + 3432.07, R 2 It is 0.99.

[0080] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a reusable washable composite SERS substrate, characterized in that, Using flexible carbon cloth as a carrier, ZIF-8 was grown in situ on its surface, and gold nanoparticles were loaded onto the ZIF-8 surface to obtain a CC / ZIF-8@Au composite structure. Then, n-dodecyl mercaptan is dip-coated to obtain a reusable washable composite SERS substrate, specifically including the following steps: S1. Preparation of CC / ZIF-8 composite material: The carbon cloth was ultrasonically cleaned with ethanol and deionized water in sequence, and then dried for later use; 50-70 mg / mL of zinc acetate dihydrate aqueous solution and 200-250 mg / mL of 2-methylimidazole aqueous solution were mixed; the carbon cloth was immersed in the mixture and stirred gently, and then the beaker containing the carbon cloth and the mixture was reacted at a constant temperature of 45-55°C for 0.5-2 hours; the beaker was then removed and allowed to stand at room temperature for 14-16 hours; finally, the sample surface was thoroughly rinsed with deionized water and vacuum dried overnight to obtain the CC / ZIF-8 composite material. S2. Preparation of CC / ZIF-8@Au composite SERS substrate: The CC / ZIF-8 composite material obtained in S1 was mixed with a certain amount of PVP-modified gold nanoparticle methanol solution, subjected to ultrasonic reaction, and then dried to obtain CC / ZIF-8@Au. S3. Hydrophobic treatment of CC / ZIF-8@Au: The CC / ZIF-8@Au composite material obtained in step S2 is immersed in a 0.5~2% n-dodecyl mercaptan ethanol solution and dried to obtain the reusable washable composite SERS substrate.

2. The preparation method according to claim 1, characterized in that, In steps S1 and S2, the ratio of carbon cloth area to mixed liquid volume to the volume of the PVP-modified gold nanoparticle methanol solution is 2.25–4 cm³. 2 : 8~12mL : 4~5 mL.

3. The preparation method according to claim 1, characterized in that, In step S2, the conditions for the ultrasonic reaction are a power of 30-50 kHz and an ultrasonic time of 45-50 min.

4. The preparation method according to claim 1, characterized in that, The preparation method of the PVP-modified gold nanoparticle methanol solution in step S2 includes: 1) Preparation of aqueous solution of gold nanoparticles; 2) Preparation of PVP-modified gold nanoparticle methanol solution: Weigh 200~300 mg of polyvinylpyrrolidone (PVP) and dissolve it in 8~12 mL of water to obtain PVP aqueous solution; slowly pour the PVP aqueous solution into the gold nanoparticle aqueous solution prepared in step (1), place it on a magnetic stirring table, stir at 300~600 rpm for 2~3 hours, then wash it three times with methanol by centrifugation, and finally adjust the volume to 8~12 mL with methanol to obtain PVP-modified gold nanoparticle methanol solution.

5. The washable composite SERS substrate prepared by the preparation method according to any one of claims 1-4, characterized in that, The composite SERS substrate has a sandwich structure, with flexible carbon cloth (CC) as the carrier, a ZIF-8@Au composite structure in the middle layer, and a dodecyl mercaptan hydrophobic layer on top.

6. The washable composite SERS substrate according to claim 5, characterized in that, In the ZIF-8@Au composite structure, gold nanoparticles are assembled on the surface of ZIF-8 to form a metal nanocomposite structure with ZIF-8 as the supporting framework.

7. The application of the washable composite SERS substrate as described in any one of claims 5-6, characterized in that, Application of the composite SERS substrate in liquid sample detection.

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