High-single-molecule-dispersity SiO2-coated ZIF-8 core-shell SERS (Surface Enhanced Raman Scattering) substrate as well as preparation and application thereof

Abstract

The invention discloses a SiO2 (at) ZIF-8 core-shell SERS (Surface Enhanced Raman Scattering) substrate with high monomolecular dispersity as well as preparation and application thereof. The SERS substrate prepared by the method disclosed by the invention has relatively high monomolecular dispersity and core-shell uniformity, and a uniform negative charge layer is formed around the SiO2 spheres by modifying the SiO2 spheres so as to promote the coating of a ZIF-8 shell, so that a SiO2-coated ZIF-8 core-shell structure is formed. The substrate synthesized by the method has the characteristics of high repeatability, high sensitivity, good stability, low price and the like; the method has lower detection limit and wider detection range on volatile organic compounds (VOCs) molecules (such as methylbenzene, styrene, orthodichlorobenzene and aniline); the rapid and accurate detection of the volatile organic compounds is realized, and the method has a good application prospect.

Description

Technical Field

[0001] The present invention relates to a preparation method of a SiO2@ZIF-8 core-shell SERS substrate and its application in the detection of volatile organic compounds (VOCs), belonging to the field of nanomaterials and chemical analysis detection. Background Art

[0002] Volatile organic compounds (VOCs) in the atmosphere are a very complex class of pollutants that have a variety of negative impacts on climate change, plant growth, and human health. Therefore, how to identify the characteristic information of different VOCs has become the focus of today's researchers; current VOC sensors are usually coated with various sensing materials to achieve targeted detection and real-time monitoring of VOC molecules in the environment.

[0003] Surface enhanced Raman scattering (SERS) technology is widely used in biology, chemistry, medicine, food safety and environmental monitoring. It is an analytical method that can detect quickly, with high sensitivity and high accuracy. Based on the wide application of this technology, the development of low-cost and highly active SERS substrates has become a research focus in this field. At present, the most common SERS substrate is a noble metal SERS substrate. The noble metal SERS substrate usually has extremely high detection sensitivity, but its chemical properties are active, the preparation is cumbersome and expensive, which is a major difficulty that troubles researchers. In comparison, semiconductor SERS substrates have the advantages of stable chemical properties, convenient preparation and low cost. However, traditional semiconductor substrates mainly rely on the charge transfer resonance between the molecule to be detected and the semiconductor to enhance the Raman signal of the molecule to be detected. From a mechanistic point of view, it has very demanding detection requirements. Not only does it require the molecule to be detected to have a functional group that is affinity with the semiconductor surface, but it also requires the energy level difference between the molecule to be detected and the semiconductor to match the Raman laser energy. And similar to the metal substrate, the size, morphology and preparation of semiconductor nanomaterials greatly affect the SERS response. Therefore, traditional semiconductor substrates are only suitable for the detection of a few standard analytes with amino, thiol, and carboxyl groups, and have almost no practical application value. Currently, researchers are working on modifying the properties of semiconductors to further improve their SERS performance.

[0004] Metal-organic framework materials (MOFs) are self-assembled from metal ions and complexes, and have the characteristics of large specific surface area, adjustable pore structure, and easy functionalization. Among them, zeolitic imidazolate framework (ZIF-8) has been widely used due to its mild preparation conditions and good chemical stability. Due to its rich surface chemical properties and highly ordered framework structure, it can effectively stabilize noble metal nanoparticles and can effectively enrich analytes, and has also been used as a SERS substrate by researchers in recent years. However, due to the high cost of the noble metals used and the cumbersome and harsh preparation process, in recent years, researchers have been committed to combining semiconductor particles with MOF materials to prepare highly active SERS substrates and widely applying them in various fields.

[0005] Therefore, based on the above research background, considering and combining the advantages of semiconductor substrates and MOFs, the present invention develops a semiconductor@MOFs core-shell SERS substrate material, and at the same time conducts a series of comparisons on the prepared core-shell materials SiO2 and the substrate materials of ZIF-8. The SiO2@ZIF-8 core-shell structure designed and prepared in the present invention improves the electromagnetic enhancement ability of the inner-core semiconductor by coating a MOF shell layer on the semiconductor surface. Summary of the Invention

[0006] Surface-enhanced Raman scattering has unique advantages different from traditional detection techniques, such as simple and fast detection process, low requirements for the treatment of samples to be detected, and ability to provide fingerprint spectra of molecules, etc., and is widely used in many fields such as biomedical analysis, chemical reaction monitoring, and physical characterization. Nevertheless, the SERS technology still faces some problems in the actual application process, such as the detection performance of the SERS substrate being lower than expected, poor universality, and high preparation cost. Among them, the optimization of the detection performance of the substrate is the key problem that needs to be solved urgently in the SERS technology, which is usually closely related to the sensitivity of the substrate. At present, there are mainly two effective strategies to improve the sensitivity of the SERS substrate: one is to develop and prepare plasmonic nanostructures with rich "hot spots" to enhance the SERS activity of the substrate; the other is to improve its adsorption ability on the substrate surface and increase the contact efficiency between the target analyte and the SERS substrate. Since the development of SERS for decades, various substrates have been discovered and explored by researchers. At present, several types of substrates that are relatively popular in research are: noble metal substrates, semiconductor substrates, MOF-related substrates, and SERS composite substrates.

[0007] The action mechanism of noble metal substrates is electromagnetic enhancement, which has extremely high detection sensitivity. Therefore, it is the mainstream SERS substrate product on the market at present. However, its disadvantages are that it has active chemical properties, unstable properties, cumbersome preparation, and high price. Semiconductor substrates have stable chemical properties, convenient preparation, and low cost. However, because their action principle is chemical enhancement, they can only be applied to the SERS detection of a very small number of chemisorbed molecules, and there are few related products on the market. Some studies have shown that some semiconductor substrates can also enhance the SERS signal of detected molecules through the electromagnetic enhancement mechanism, which is also effective for non-adsorbing molecules, but their electromagnetic enhancement effect is extremely weak. The semiconductor@MOFs substrate developed in this invention enhances the electromagnetic enhancement ability of semiconductor substrates, and at the same time has advantages such as high sensitivity, good stability, and low price, which will be the key to expanding the application prospects of semiconductor SERS technology.

[0008] In order to overcome the disadvantages and deficiencies of the prior art, the present invention aims to provide a preparation method of a SiO2@ZIF-8 core-shell SERS substrate. This substrate has characteristics such as strong sensitivity, high repeatability, good stability, and low price. Another object of the present invention is to provide the application of the SiO2@ZIF-8 core-shell SERS substrate obtained by the above synthesis strategy.

[0009] The present invention synthesizes solid SiO2 spheres with uniform morphology and good dispersibility by an improved Stöber method. Compared with the traditional method, highly dispersed solid SiO2 spheres can be easily obtained by the improved Stöber method based on the hydrolysis-condensation mechanism under ammonia catalysis.

[0010] In the present invention, after modifying the surface of SiO2 spheres with amino groups and reacting with PVP, a uniform negatively charged layer is formed around them. By using the coordination effect, a physical layer barrier ZIF-8 protective layer is constructed on its outer layer, which can effectively expand the action range of the electromagnetic field on the semiconductor surface, enhance the action effect of the electromagnetic field mechanism, and greatly expand the applicable range of the semiconductor SERS substrate.

[0011] It should be noted that during the synthesis of the SiO2@ZIF-8 core-shell: when the oxygen atom of the carbonyl group in PVP (pyrrolidone ring) binds to some coordinated Zn 2+ ions on the surface of ZIF-8, a part of the PVP molecules will be hydrated and thus form a larger hydrated shell, which stabilizes in space and prevents agglomeration between individual particles; in addition, during the formation of the ZIF-8 shell layer, the thickness of the layer mainly depends on the initial nucleation rate, which is controlled by the deprotonation of 2-MeIM.

[0012] The present invention utilizes the enrichment effect of ZIF-8 on the VOCs molecules to be detected and its electromagnetic enhancement effect, combined with the high stability, easy availability, etc. of the SiO2 substrate, to form a SERS-active composite substrate, effectively expanding the action range of the electromagnetic field on the semiconductor surface and enhancing the effect of the electromagnetic field mechanism. Different from the charge transfer resonance mechanism of traditional semiconductors, the electromagnetic field mechanism does not require the molecules to be detected to be adsorbed on the semiconductor surface, and can act on the molecules at a few nanometers to more than ten nanometers from the semiconductor surface, and has no selectivity for the adsorption ability and energy level structure of the molecules. This greatly expands the applicable range of the semiconductor SERS substrate and lays a foundation for further realizing the practical application of the semiconductor SERS substrate.

[0013] The SERS substrate prepared by the present invention has good repeatability, stability and low detection limit for the detection of VOCs molecules: it can reach 250 ppm for toluene; it can reach 10 ppm for styrene; it can reach 50 ppm for o-dichlorobenzene; it can reach 1 ppm for aniline. Specific implementation examples

[0014] The present invention will be described in more detail below through specific examples, but the protection scope of the present invention is not limited to these examples. Examples

[0015] Synthesis of SiO2@ZIF-8 core-shell structure Disperse 10 mg of SiO2 nanostructures in 10 mL of deionized water. After ultrasonic treatment for 1 h, add 130 μL of 3-aminopropyltriethoxysilane (APTES), and stir overnight at room temperature. Centrifuge the resulting suspension and wash it three times with absolute ethanol to remove the residues. Then add 10 mL of an aqueous solution of poly(sodium 4-styrenesulfonate) (PSS) with a concentration of 0.3 wt %, and ultrasonic treatment for 30 min to make it evenly dispersed. After centrifugation and washing, transfer it to 10 mL of a methanol solution of polyvinylpyrrolidone (PVP) (0.02 g). After stirring at room temperature for 1 h 30 min, add 1 mL of a methanol solution of 0.0672 M zinc nitrate hexahydrate Zn(NO3)2·6H2O, and stir at room temperature for 10 min. After completion, add 4 mL of 0.1096 M 2-methylimidazole (2-MeIM), stir at room temperature for 2 h, centrifuge to collect the precipitate, wash it 3 times with methanol, and place it in a vacuum oven to dry to obtain SiO2@ZIF-8 core-shell.

[0016] Synthesis of SiO2 nanostructures Synthesis of SiO2 nanostructures using an improved Stöber method: Take 75 mL of ethanol in a beaker, add 10 mL of deionized water, then add 6 mL of tetraethyl orthosilicate (TEOS), stir for 10 minutes, then add 3.15 mL of ammonium hydroxide solution, and stir in an oil bath at 50 °C. After 1 h, a white precipitate is formed, collected by centrifugation, and washed several times with distilled water and absolute ethanol. Finally, the SiO2 nanoparticles are dried in a vacuum dryer at 60 °C for 8 h before use.

[0017] Synthesis of ZIF-8 Dissolve Zn(NO3)2·6H2O (1.116 g) in a CH3OH (30 mL) solution and sonicate for 3 min to form solution A. Subsequently, dissolve 2-methylimidazole (1.232 g) in a CH3OH (30 mL) solution and sonicate for 3 min to form solution B. Then mix solutions A and B and sonicate for 10 min. Pour the well-mixed solution into a 100 mL Teflon reactor, and then react in a forced-air constant-temperature oven at 120 °C for 2 h. After natural cooling, the catalyst at the bottom of the reactor is centrifugally washed 3 times with CH3OH to obtain a white powder. Then the obtained white powder is placed in a vacuum oven at 60 °C and dried overnight. The finally obtained powder is ZIF-8.

[0018] The method for detecting the SERS activity of different substrates provided by the present invention in different VOCs is as follows: Disperse 5 mg of SiO2, SiO2@ZIF-8 or ZIF-8 particles evenly in 1 mL of ethanol by sonication. Take 1 μL and spread it on a quartz sheet covered with tin foil and dry it to prepare a blank SERS substrate. Then transfer the dried quartz sheet to a customized quartz reactor and place it in a sealed manner, and evacuate it. Prepare VOCs solutions with different concentrations, mix them evenly, and then aspirate 10 μL and inject it into the sealed and evacuated reactor. Subsequently, incubate it with pure air for 1 h, then open the reactor and perform SERS detection on the SiO2 / SiO2@ZIF-8 / ZIF-8 substrate. Continuously dilute the concentration of VOCs to determine the lowest detection limit of the SiO2@ZIF-8 substrate for this kind of VOCs. The SiO2@ZIF-8 core-shell SERS substrate synthesized by the present invention realizes sensitive and stable detection of volatile organic compound (VOCs) molecules. Description of the Drawings

[0019] Figure 1a is the scanning electron microscope (SEM) image of SiO2 in Comparative Example 1. The particle size of SiO2 particles is approximately around 265 nm, and the particles are distinct and of uniform size, providing a good precursor for the preparation of SiO2@ZIF-8 in Example 1 later. Figure 1 b is the transmission electron microscope (TEM) image of ZIF-8 in Comparative Example 2. It is observed that ZIF-8 has a typical regular dodecahedron structure. Figure 1 c is the transmission electron microscope (TEM) image of SiO2@ZIF-8. At a high magnification electron microscope, it is photographed that the surface of the particles is wrinkled, and the particle diameter has increased to 336 nm. Compared with Figure 1 a, there is an apparent change indeed.

[0020] Figure 2 a and Figure 2 b are the EDX energy spectrum and elemental analysis mapping of SiO2@ZIF-8 particles, which can further prove the elemental composition of the prepared core-shell structured particles. Through the EDX energy spectrum of SiO2@ZIF-8 particles, it can be found that the main elements of such substrates include five elements: C, N, O, Zn, and Si. Through the elemental analysis of mapping, it is found that C, N, and Zn elements are evenly distributed on the outer shell, while Si and O elements are distributed in the center of the inner core, proving that the inner core remains unchanged as SiO2 and the outer shell part is the ZIF-8 structure.

[0021] Figure 3 are the SERS activity spectra of 4 kinds of 1000 ppm VOC molecules on SiO2, SiO2@ZIF-8, and ZIF-8 substrates respectively, as well as the intrinsic Raman spectra of VOCs. As Figure 3 shown in a-d, among them, all 4 kinds of VOCs have SERS characteristic peaks at 1000 cm -1 (toluene at 1002 cm -1 , styrene at 1001 cm -1 , o-dichlorobenzene at 1039 cm -1 , aniline at 998.5 cm -1 ). This is the stretching vibration peak of the benzene ring, proving that such a substrate has a good SERS response to monosubstituted benzene rings. In addition to the stretching vibration peak around 1000 cm -1 , we found that there are also corresponding SERS characteristic peaks at 1300 - 1600 cm -1 , and the intensity of the peaks is proportional to the Raman peak intensity of the VOC molecules themselves. Among them, for o-dichlorobenzene, its own Raman characteristic peaks are relatively few, so there is a relatively weak SERS active peak at 1036 cm -1 , and its characteristic peak has a certain displacement compared with monosubstituted benzene ( Figure 3c). Therefore, the SiO2@ZIF-8 substrate has a certain universality for detecting different VOC gases, which has important research significance in the field of SERS.

[0022] Figures 4 - 7 They are the SERS spectra of toluene, styrene, o-dichlorobenzene, and aniline at different concentrations on the SiO2@ZIF-8 substrate. We detected the SERS activity of 4 VOCs at different concentrations on this substrate to illustrate that the ZnO@ZIF-8 substrate can exhibit good SERS responses (1000 ppm) to the above 4 VOC molecules. As shown by the marks in the shaded areas of the figure, as the concentration of VOCs increases, the corresponding SERS active peaks also increase accordingly, indicating that the intensity of the characteristic peaks is related to the concentration. And their minimum detection limits can be obtained: 250 ppm for toluene; 10 ppm for styrene; 50 ppm for o-dichlorobenzene; 1 ppm for aniline.

Claims

1. A method for preparing a SiO2@ZIF-8 core-shell SERS substrate; comprising the following steps: (1) Preparation of solid SiO2 spheres by improved Stöber method; (2) Preparation of amino-modified SiO2 spheres; (3) coating the outer layer of the SiO2 sphere obtained in step (2) with a layer of anionic polymer to form a modified SiO2 sphere with a uniform negative charge layer; (4) A specific organic modifier is used to functionalize the surface of the material matrix, and a uniform ZIF-8 protective layer is constructed on the outside of the SiO2 sphere through the coordination reaction of zinc ions and polyvinylpyrrolidone (PVP; as a surfactant molecule) to prepare the SiO2@ZIF-8 core-shell SERS substrate.

2. The preparation method according to claim 1, characterized in that: The diameter of the SiO2 spheres described in step (1) is about 265±15 nm; The SiO2@ZIF-8 core-shell diameter described in step (4) is about 336±6nm; The thickness of the ZIF-8 protective layer described in step (4) is about 44±11 nm.

3. The preparation method according to claim 1, characterized in that: The step (1) specifically includes the following steps: (1.1) Place 75 mL of ethanol in a beaker, add 10 mL of deionized water, and then add 6 mL of tetraethyl orthosilicate (TEOS), and stir for 10 minutes. (1.2) After stirring, immediately add 3.15 mL of ammonium hydroxide solution and stir in an oil bath at 50 °C for 1 h. (1.3) A white precipitate is formed, which is collected by centrifugation and washed several times with distilled water and anhydrous ethanol; (1.4) The SiO2 nanoparticles were then dried in a vacuum dryer at 60°C for 8 hours to obtain solid SiO2 nanostructured spheres; The step (2) specifically includes the following steps: (2.1) Disperse 10 mg of SiO2 spheres prepared in step (1) in 10 mL of deionized water and ultrasonicate for 1 h; (2.2) Add 130 μL 3-aminopropyltriethoxysilane (APTES) and stir at room temperature overnight; (2.3) The resulting suspension was centrifuged and washed three times with anhydrous ethanol to remove the residue, thereby obtaining amino-modified SiO2 spheres; The step (3) specifically includes the following steps: (3.1) Add 10 mL of 0.3 wt % poly(sodium 4-styrene sulfonate) (PSS) aqueous solution to the SiO2 spheres prepared in step (2) and ultrasonicate for 30 min to make them evenly dispersed; (3.2) Centrifuge and wash the suspension obtained in (3.1) twice; (3.3) Transfer the precipitate collected in (3.2) into 10 mL of polyvinyl pyrrolidone (PVP) methanol solution (0.02 g) and stir at room temperature for 1 h 30 min to obtain a SiO2 sphere suspension with a uniform negative charge layer; The step (4) specifically includes the following steps: (4.1) Add 1 mL of 0.0672 M zinc nitrate hexahydrate Zn(NO3)2·6H2O methanol solution to the suspension prepared in (3.3) and stir at room temperature for 10 min; (4.2) Add 4 mL of 0.1096 M 2-methylimidazole (2-MeIM) to the suspension obtained in step (4.1) and stir at room temperature for 2 h; (4.3) After the reaction, the suspension was centrifuged and washed with methanol three times, and then dried in a vacuum oven to obtain the SiO2@ZIF-8 core-shell SERS substrate.

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