A SERS substrate based on silver-covalent organic framework heterojunction and its preparation method and application

Abstract

The present invention relates to a SERS substrate based on a silver-covalent organic framework heterojunction and a preparation method and application thereof, belonging to the technical field of analysis and detection. The SERS substrate based on a silver-covalent organic framework heterojunction of the present invention comprises an amidoxime-modified covalent organic framework material and silver nanoparticles adsorbed on the amidoxime-modified covalent organic framework material. COF-TpDb-AO with semiconductor characteristics can improve the Raman signal intensity through the chemical enhancement effect, and the silver nanoclusters produce a strong electromagnetic enhancement effect. The SERS substrate significantly improves the detection sensitivity (the detection limit is as low as 7.4×10 ‑9 mol / L), and the enhancement factor (EF) was 1.26×10 6 .

Description

Technical Field

[0001] The invention belongs to the technical field of analysis and detection, and in particular relates to a SERS substrate based on a silver-covalent organic framework heterojunction and a preparation method and application thereof. Background Art

[0002] As fossil energy is becoming increasingly depleted, nuclear energy accounts for an increasingly high proportion of energy in countries around the world. The development of nuclear energy has led to an increasing demand for nuclear fuel. As an important nuclear fuel, uranium will inevitably enter the environment from uranium mining to the treatment of radioactive spent fuel. Uranium is radioactive and chemically toxic, which will not only affect the ecosystem, but also migrate through underground and surface water systems and enter the human body. The most stable form of uranium in water is uranyl ion (UO2 2+ ), which easily accumulates in the human bones, kidneys, lungs and liver, thereby interfering with body functions and endangering health. The World Health Organization and the US Environmental Protection Agency stipulate that the uranium concentration in drinking water cannot exceed 30µg·L -1 ,Therefore, trace detection of uranyl ions in environmental water is particularly important.

[0003] Traditional detection methods, such as X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry, and high performance liquid chromatography, are highly sensitive, but the equipment is expensive and the operation is complicated. It is difficult to achieve rapid on-site detection, which is not conducive to responding to emergencies. At present, some emerging detection methods, such as electrochemiluminescence, fluorescence spectroscopy, and surface enhanced Raman scattering spectroscopy (SERS) have gradually developed. These methods are fast, simple, and low-cost, and have high selectivity and sensitivity. Among them, SERS is a powerful in situ analysis technology that can significantly enhance the Raman signal of molecules, with an enhancement factor (EF) of 10 8 Or larger, it can achieve highly sensitive detection of uranyl ions. In addition, SERS can respond within seconds, with fast analysis speed and portable equipment. Therefore, SERS is an effective means to achieve rapid and on-site monitoring of trace uranyl ion concentrations.

[0004] Common SERS substrates are mainly composed of precious metal nanomaterials (gold, silver, copper, etc.) with different morphologies, such as nanospheres, nanorods, nanosheets, and nanowires. S. Dai et al. studied the Raman spectra of uranyl ions on silver-doped sol-gel films and found for the first time that silver nanoparticles enhance the Raman signal of uranyl ions. The detection limit (LOD) of uranyl ions on this substrate is 8.5×10 -8 mol / L. Wang Xiaolin and others from the China Academy of Engineering Physics prepared a uniform and dense nanosilver substrate by self-assembly. The Raman detection of uranyl ions adsorbed on the substrate showed an EF of 1.24×10 4 , the detection limit is as low as 10 -7However, single noble metal nanoparticles are prone to aggregation and clustering during preparation and use, and lack the ability to specifically bind to uranyl, resulting in poor stability, poor selectivity, and low sensitivity, which seriously hinders the practical application of SERS.

[0005] In order to improve the selectivity and stability of the substrate, the SERS substrate has been further developed. The plasmonic nanostructure was functionalized to effectively capture uranyl ions. Haes et al. used 2-[4-(2-hydroxyethyl)-1-piperazinyl]propanesulfonic acid (EPPS) to synthesize gold nanostars. EPPS acts as both a reducing agent and a stabilizer, and the sulfonate group can specifically bind to uranyl ions, significantly improving the sensitivity and stability. Teiten and Burneau et al. used N-(2-mercaptopropenyl)glycine (MPG) and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) to modify silver nanoparticles and enrich uranyl ions, with a detection limit (LOD) as low as 10 -9 mol / L. However, since the organic groups modified on the substrate surface usually have high Raman activity, there is strong background interference, which reduces the accuracy of uranyl ion detection. In addition, when faced with real samples with low concentrations and complex system interference, there are still problems such as poor anti-interference ability and poor stability.

[0006] Heterojunction SERS substrate combines the advantages of surface plasmons and other materials, significantly improving the stability, repeatability and sensitivity of SERS substrates, and has become a common SERS substrate. Specifically, it is to combine surface plasmons with other nanomaterials, take advantage of the advantages of the two materials, specifically capture uranyl ions, and reduce the self-aggregation of nanoparticles, thereby enhancing the sensitivity and stability of detection. For example, surface plasmons form composite structures with metal organic framework materials, graphene oxide, porous silica, etc., especially porous nanostructures, which can effectively capture and enrich analytes, use pore confinement to form local high concentrations, and improve sensitivity. Haea et al. at the University of Iowa constructed a core-shell structure of Ag@Au@SiO2 nanostructures. Uranyl ions diffuse into the area of ​​strong electromagnetic field through the nanopores of SiO2, effectively capture and enrich uranyl ions, and the detection limit is as low as 110nM. Dutta et al. designed a SERS substrate that loads silver nanoparticles on graphene oxide materials, and enriches uranyl ions through the hydroxyl and carboxyl functional groups on the graphene material. However, the structural instability of these substrates under acidic or alkaline conditions affects their application. Covalent organic frameworks (COFs) are a new type of crystalline porous nanomaterials composed of organic ligands connected by covalent bonds. COFs show remarkable stability in acidic or alkaline environments, and also have the characteristics of high specific surface area, adjustable pore size and easy functionalization. Therefore, hybrid SERS substrates based on COF and plasmons can combine the advantages of both materials, effectively detect trace uranyl ions, and maintain structural stability in complex sample matrices. Although many studies have reported hybrid SERS substrates combining COF with plasmons, there are few reports specifically for uranyl ion detection. Liao et al. first proposed an Ag / Ag2O–COF composite SERS substrate for detecting uranyl ions. The substrate enriches uranyl ions through COF, and then reduces AgNO3 to generate Ag / Ag2O through Raman laser. Ag / Ag2O nanoparticles may fall off the substrate in complex samples and lack the stability of the substrate. In addition, this substrate requires 100 mL of sample solution, limiting the detection of trace amounts of actual samples.

[0007] Therefore, it is crucial to develop a novel heterojunction SERS substrate combining COF and surface plasmon for the detection of uranyl ions. Summary of the invention

[0008] Therefore, the technical problem to be solved by the present invention is to overcome the problems of poor stability, poor selectivity and low sensitivity of the SERS substrate in the prior art in real water body measurement.

[0009] In order to solve the above technical problems, the present invention provides a SERS substrate based on a silver-covalent organic framework heterojunction and its preparation method and application. The SERS substrate can effectively enrich uranyl ions through amine oxime groups, bringing them close to silver nanoparticles. Then, the electromagnetic enhancement effect of silver nanoparticles is coordinated with the chemical enhancement effect of non-metallic nanomaterials to significantly improve the sensitivity of SERS detection. In addition, the covalent organic framework (COF) in the SERS substrate has high stability both in acidic and alkaline conditions, which can not only prevent surface plasmon aggregation, but also meet the needs of detection in complex water bodies.

[0010] The first object of the present invention is to provide a SERS substrate based on a silver-covalent organic framework heterojunction, characterized in that the SERS substrate based on a silver-covalent organic framework heterojunction comprises a covalent organic framework material modified with an amidoxime group, and silver nanoparticles adsorbed on the covalent organic framework material modified with an amidoxime group.

[0011] In one embodiment of the present invention, the covalent organic framework material modified with an amine oxime group is COF-TpDb-AO. COF is a type of periodic porous non-metallic nanomaterial assembled by organic ligands through covalent bonds, and has the characteristics of porosity, large specific surface area, chemical stability and easy post-modification. COF-TpDb is a ketimine-based COF material, which is formed by an irreversible enol-keto tautomerization reaction, and then the cyano group is post-modified into an amine oxime COF (COF-TpDb-AO). The amine oxime group (AO) is an excellent uranium bidentate ligand with excellent adsorption performance and selectivity for uranyl ions.

[0012] In one embodiment of the present invention, the electromagnetic enhancement effect of silver nanoparticles is two orders of magnitude higher than that of other metal nanoparticles.

[0013] In one embodiment of the present invention, the mass ratio of the amidoxime-modified covalent organic framework material to the silver nanoparticles is (5-10): (300-400).

[0014] The second object of the present invention is to provide a method for preparing the SERS substrate based on the silver-covalent organic framework heterojunction, comprising the following steps:

[0015] S1, mixing the amidoxime-modified covalent organic framework material and the reducing agent in water to obtain a mixed solution;

[0016] S2. Adding silver nitrate solution to the mixed solution described in S1 to carry out heating reaction, and obtaining the SERS substrate based on the silver-covalent organic framework heterojunction through centrifugation and washing.

[0017] In one embodiment of the present invention, in S1, the reducing agent is selected from polyvinyl pyrrolidone and / or sodium citrate. The silver nitrate is reduced to silver nanoparticles on the surface of COF-TpDb-AO by the reducing agent, and the reducing agent not only plays a reducing role, but also plays a protective and anti-oxidative role on the surface of the silver nanoparticles.

[0018] In one embodiment of the present invention, in S1, the concentration of the amidoxime-modified covalent organic framework material is 0.2 mg / mL-2 mg / mL; the mass ratio of the amidoxime-modified covalent organic framework material to the reducing agent is (5-50): (5000-50000).

[0019] In one embodiment of the present invention, in S2, the concentration of the silver nitrate solution is 0.2 mol / L-0.4 mol / L; the volume ratio of the mixed solution to the silver nitrate solution is (10-50): (5-50).

[0020] In one embodiment of the present invention, in S2, the heating reaction is carried out under a protective atmosphere at 55° C.-65° C. for 8 h-14 h.

[0021] Furthermore, the protective atmosphere is a nitrogen atmosphere.

[0022] In one embodiment of the present invention, in S2, the centrifugation is performed at a speed of 5800 rpm-6200 rpm for 4 min-6 min.

[0023] The third object of the present invention is to provide an application of the SERS substrate based on silver-covalent organic framework heterojunction in detecting uranyl ions.

[0024] The technical solution of the present invention has the following advantages over the prior art:

[0025] The SERS substrate based on the silver-covalent organic framework heterojunction of the present invention is prepared from COF-TpDb-AO material and silver nanoparticles. COF-TpDb-AO with semiconductor properties can improve the Raman signal intensity through chemical enhancement effect, and silver nanoclusters can enhance the Raman signal through electromagnetic enhancement effect. The SERS substrate significantly improves the detection sensitivity through electromagnetic synergistic chemical enhancement effect, and the enhancement factor (EF) is 1.26×10 6 .

[0026] The COF-TpDb-AO material in the SERS substrate based on the silver-covalent organic framework heterojunction of the present invention greatly improves the specific surface area of ​​the SERS substrate, which not only increases the contact area with uranyl, but also provides a larger space for the loading of silver nanoparticles; in addition, since the amidoxime groups modified on its surface have excellent adsorption capacity for uranyl ions, the selectivity and sensitivity of the SERS substrate are greatly improved, and the detection limit is as low as 7.4×10 -9 mol / L.

[0027] The preparation method of the present invention aims at the characteristic that silver nanoparticles are easily oxidized and inactivated. When reducing silver nitrate to synthesize silver nanoparticles in situ, materials such as polyvinyl pyrrolidone are selected as reducing agents and stabilizers, thereby greatly improving the stability of the silver nanoparticles.

[0028] The SERS substrate based on the silver-covalent organic framework heterojunction of the present invention has excellent actual detection performance. It shows great application potential in the detection of uranyl ions in actual water bodies, and the spike recovery rate in tap water, river water, and lake water is greater than 90%. In particular, it has broad application prospects in the field of rapid on-site monitoring when used in conjunction with a portable Raman spectrometer. In addition, the SERS substrate also has excellent stability. In the reproducibility experiment, the relative standard deviation (RSD) value is as low as 6.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0030] Figure 1 The present invention is a structural characterization of the SERS substrate based on the silver-covalent organic framework heterojunction of Test Example 1; wherein, (a) is a scanning electron microscope image of the Ag@COF-TpDb-AO substrate, (b) is a transmission electron microscope image of the Ag@COF-TpDb-AO substrate, (c) is a high-resolution transmission electron microscope image of silver nanoparticles, (d) is a powder X-ray diffraction pattern of COF-TpDb, COF-TpDb-AO and Ag@COF-TpDb-AO, (e) is a high-resolution X-ray photoelectron spectrum of silver nanoparticles, and (f) is an infrared spectrum of COF-TpDb, COF-TpDb-AO and Ag@COF-TpDb-AO;

[0031] Figure 2 This is a schematic diagram of the detection of uranyl ions by the SERS substrate based on the silver-covalent organic framework heterojunction in Test Example 2 of the present invention;

[0032] Figure 3The test results of test example 2 of the present invention are as follows; wherein (a) is the Raman signal intensity of uranyl ions at different concentrations, (b) is a relationship diagram between uranyl ions at different concentrations and Raman signal intensity, (cd) are test diagrams for the repeatability of uranyl ion detection, (e) is a Raman response diagram of uranyl ions under different pH conditions, and (f) is a test diagram for the ion selectivity of uranyl ion detection;

[0033] Figure 4 The Raman signal diagram of uranyl ion detection in actual water body based on the silver-covalent organic framework heterojunction SERS substrate in Test Example 3 of the present invention; wherein (a) is the Raman signal of uranyl ions of different concentrations diluted by river water, (b) is the Raman signal of uranyl ions of different concentrations diluted by lake water, (c) is the Raman signal of uranyl ions of different concentrations diluted by tap water, and (d) is a comparison diagram of the Raman signal of uranyl ions prepared with pure water and the Raman signal of uranyl ions prepared with river water, lake water and tap water;

[0034] Figure 5 The different incubation volumes 10 in Test Example 3 of the present invention -8 Raman signal diagram of 1 mol / L uranyl solution. DETAILED DESCRIPTION

[0035] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0036] In the present invention, unless otherwise explained, technical and scientific terms used in the present invention have the same meanings as commonly understood by those skilled in the art to which the present invention belongs.

[0037] In the present invention, unless otherwise stated, the term "and / or" used in the present invention includes any and all combinations of one or more of the associated listed items.

[0038] In the present invention, unless otherwise stated, the experimental methods used in the embodiments of the present invention are conventional methods unless otherwise stated, and the materials, reagents, etc. used are all commercially available unless otherwise stated. Example 1

[0039] The SERS substrate based on the silver-covalent organic framework heterojunction and the preparation method thereof of this embodiment specifically include the following steps:

[0040] Preparation of COF-TpDb: In a 10 mL capped bottle, 1,3,5-trialdehyde phloroglucinol (21 mg, 0.1 mmol), 2-cyano-p-phenylenediamine (19.9 mg, 0.15 mmol), 1,4-dioxane (0.5 mL), mesitylene (0.5 mL) and glacial acetic acid solution (6 mol / L, 0.1 mL) were added in order; after capping, ultrasonic treatment was performed for 15 min; after ultrasonic treatment, the glass bottle was immediately quickly frozen in liquid nitrogen at 77 K, and then vacuumed, and the cycle was repeated 3 times after thawing; after the temperature returned to room temperature, the mixture was heated at 120°C in an oven for 72 h to form an orange-red precipitate; the precipitate was separated by filtration, and Soxhlet extraction was performed with anhydrous tetrahydrofuran, and washed for 2 days; finally, vacuum drying was performed at 50°C to obtain COF-TpDb;

[0041] Preparation of COF-TpDb-AO: COF-TpDb (200 mg) was mixed with hydroxylamine hydrochloride (NH2OH·HCl, 82 mg) and trimethylamine (120 mg) in methanol (5 mL), and heated to 70°C in an oil bath for 2 h. After heating, the reaction mixture was cooled to room temperature, filtered, washed with water and dried in vacuo at 50°C to obtain COF-TpDb-AO.

[0042] Preparation of SERS substrate (Ag@COF-TpDb-AO) based on silver-covalent organic framework heterojunction: 8 mg COF-TpDb-AO and 4 g polyvinyl pyrrolidone were dissolved in 16 mL aqueous solution, heated in an oil bath with stirring to 60 °C under a nitrogen atmosphere; then silver nitrate aqueous solution (0.3 mol / L, 6 mL) was quickly added to the above solution and stirred for 5 min to ensure sufficient mixing; and heated with stirring at 60 °C for 10 h; after the reaction was completed, centrifuged at 6000 rpm for 5 min, and washed with deionized water 3 times to remove excess silver nanoparticles and polyvinyl pyrrolidone to obtain a SERS substrate based on silver-covalent organic framework heterojunction, namely Ag@COF-TpDb-AO.

[0043] Test Example 1

[0044] The SERS substrate based on the silver-covalent organic framework heterojunction of Example 1 was characterized. Figure 1 As shown. Figure 1 (a)- Figure 1 (c) It can be seen that the silver nanoparticles are uniformly loaded on the surface of the COF-TpDb-AO material and do not self-aggregate. Figure 1 (e) It can be seen that the Ag 3d spectrum shows the same 5 / 2 and Ag 3d 3 / 2The double peaks corresponding to the electronic binding energy are located at 367.7eV and 373.6eV, respectively. The distance between the Ag 3d double peaks is about 6eV, indicating that elemental silver rather than silver oxide is formed on COF-TpDb-AO, which also shows that polyvinyl pyrrolidone plays a protective and antioxidant role. Figure 1 (d) and Figure 1 (f) It can be seen that COF-TpDb, COF-TpDb-AO and Ag@COF-TpDb-AO were successfully synthesized, and silver nanoparticles were successfully loaded without changing the crystal structure of COF-TpDb-AO.

[0045] Test Example 2

[0046] Raman test: 10 -2 The uranyl stock solution was diluted to 10 mol / L with deionized water. -4 mol / L-10 -8 mol / L uranyl solution; reference Figure 2 As shown, 4mL of uranium ion solution with different concentrations was mixed with Ag@COF-TpDb-AO dispersion (1mg / mL, 100µL) and incubated at 25℃ for 2h to adsorb enough uranyl; the resulting mixture was then centrifuged at 6000rpm to remove the supernatant, and the concentrated solution was adjusted to a final volume of 50µL; finally, 10µL of Ag@COF-TpDb-AO adsorbed with uranium ions was deposited on a microscope slide and slowly dried to obtain a uniform SERS substrate, followed by Raman testing. Using a 50x objective lens, a grating of 1200 fringes / mm, a laser power of 5.9mW, and a data acquisition time of 30s, the test results are shown in Figure 3 (a)- Figure 3 (b). Figure 3 (a)- Figure 3 (b) It can be seen that the Raman signal intensity of uranyl ions increases first and then decreases with the increase of concentration. This is because at 5µmol / L, uranyl ions are close to reaching monolayer adsorption saturation. Higher concentrations of uranyl ions will affect the detection of Raman signals. In the low concentration range of 0.1µmol / L to 0.75µmol / L, the symmetric stretching vibration peak of uranyl ions appears at 715cm -1 When the concentration increases from 1µmol / L to 10µmol / L, the symmetric stretching vibration peak of uranyl ion at 720cm -1This shift is attributed to the increased charge transfer from the COF-TpDb-AO substrate to the uranyl ions at lower concentrations, resulting in a decrease in the frequency of the Raman peak. In addition, the intensity of the characteristic peak of uranyl ions is linearly related to the concentration between 0.1µmol / L and 2.5µmol / L, with the equation y=18092.39x+1738.92 and a correlation coefficient (R²) of 0.98. The limit of detection (LOD) was calculated to be 7.4×10 -9 mol / L, and the specific calculation is shown in Equation S1. Finally, based on the slope ratio of concentration-dependent surface enhanced Raman scattering (SERS) and normal Raman measurement, the enhancement factor was calculated to be 1.26×10 6 , indicating that the detection sensitivity of the substrate to uranyl ions is significantly improved. The specific calculation is shown in Equation S2.

[0047]

[0048] Among them, S b It is 720cm in the blank control −1 The standard deviation of the signal intensity at , and k represents the slope of the fitting equation.

[0049]

[0050] Among them, I SERS and I Raman Represents the surface enhanced Raman (SERS) and normal Raman signal intensities of uranyl ions, respectively. SERS and c Raman Representative uranyl ion concentrations for SERS and normal Raman spectra.

[0051] Repeatability test: The uranyl ions at 20 points were randomly recorded on the SERS substrate (10 -6 mol / L) SERS spectrum and calculate the relative standard deviation. The results are as follows Figure 3 (c)- Figure 3 (d). Figure 3 (c)- Figure 3 (d) It can be seen that the relative standard deviation is small (6.5%), indicating that the SERS signal has good repeatability on this substrate and the silver nanoparticles and “hot spots” are evenly distributed.

[0052] Acid-base stability test: The SERS substrate was incubated in uranyl solutions with pH values ​​ranging from 3 to 11.7, and Raman tests were performed. The results are shown in Figure 2. Figure 3 (e) is shown. Figure 3(e) It can be seen that the signal of the SERS substrate has excellent repeatability under strong acid and weak alkaline conditions. Under strong alkaline conditions, uranyl ions are positively charged and repel the SERS substrate, resulting in a weakening of the uranyl Raman signal, indicating that the SERS substrate has excellent acid-base stability within a certain pH range.

[0053] Ion selectivity: In 4 mL of common metal ion solution, including K + 、Na + , Ba 2+ , Ca 2+ , Cu 2+ 、Co 2+ , Fe 3+ Mg 2+ , Mn 2+ 、Zn 2+ , Sr 2+ , Cs + and VO3 － , the concentrations of which are all uranyl ions (10 -6 mol / L), added 100µL of Ag@COF-TpDb-AO dispersion, incubated for 2h, and then performed Raman testing at 720cm -1 The Raman signals on the left and right are as follows Figure 3 (f). Figure 3 (f) It can be seen that other ions have almost no interference with uranyl. In addition, in the case of a mixture of various metal cations and uranyl ions, the observed surface enhanced Raman scattering intensity is very close to that of uranyl ions alone, indicating that the Ag@COF-TpDb-AO composite material has a strong anti-interference ability.

[0054] In summary, the SERS substrate based on the silver-covalent organic framework heterojunction of this embodiment has high sensitivity, high repeatability and high selectivity to uranyl ions.

[0055] Test Example 3

[0056] To further evaluate the application of the SERS substrate based on the silver-covalent organic framework heterojunction in actual environmental samples, the uranyl ion mother solution was diluted with tap water, river water and lake water to 0.1µmol / L, 0.25µmol / L and 0.5µmol / L, respectively, with a pH value of 5.0. Then, the corresponding concentrations of uranyl ions in different water samples were incubated with the SERS substrate based on the silver-covalent organic framework heterojunction and Raman tests were performed. The results are shown in Figure 2. Figure 4 As shown. Figure 4 It can be seen that at the same concentration, there is no significant difference in the Raman intensity of uranyl ions in tap water, lake water and river water. This shows that this method has good application potential for detecting uranyl ions in actual water samples.

[0057] Test Example 4

[0058] The linear range (mol / L) and detection limit (mol / L) of the SERS substrate based on the silver-covalent organic framework heterojunction prepared in Example 1 and the existing different types of SERS substrates for detecting uranyl ions are compared, as shown in Table 1:

[0059] Table 1

[0060]

[0061] It can be seen from Table 1 that the SERS substrate based on the silver-covalent organic framework heterojunction of Example 1 has a lower detection limit (7.4×10 -9 mol / L), which is superior to most SERS substrates for uranyl ion detection. Although the Ag / Ag2O-COF composite substrate exhibits a lower detection limit, it requires a large sample volume of up to 100 mL.

[0062] When using different volumes (4mL, 20mL and 80mL), the -8 mol / L concentration to detect uranate ions, Raman signals such as Figure 5 As shown. Figure 5 It can be seen that the Raman intensity increases with the increase of sample volume. Therefore, as the sample volume increases, the detection limit decreases and more uranyl ions can be enriched on the substrate. In addition, this SERS substrate is synthesized by in situ reduction of AgNPs on COF-TpDb-AO, which is more stable and reproducible than the Ag / Ag2O-COF substrate that relies on Raman laser irradiation to form Ag / Ag2O composite nanoparticles on COF.

[0063] In summary, the SERS substrate based on silver-covalent organic framework heterojunction of the present application exhibits good stability and sensitivity, demonstrating its great potential in uranyl ion detection.

[0064] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A SERS substrate based on a silver-covalent organic framework heterojunction, the SERS substrate being used for uranyl ion detection, characterized in that: The SERS substrate based on silver-covalent organic framework heterojunction comprises a covalent organic framework material modified by amidoxime groups and silver nanoparticles adsorbed on the covalent organic framework material modified by amidoxime groups; the covalent organic framework material modified by amidoxime groups is COF-TpDb-AO.

2. The SERS substrate based on silver-covalent organic framework heterojunction according to claim 1, characterized in that: The mass ratio of the amidoxime-modified covalent organic framework material to the silver nanoparticles is (5-10): (300-400).

3. The method for preparing a SERS substrate based on a silver-covalent organic framework heterojunction according to any one of claims 1 to 2, characterized in that: The following steps are involved: S1, mixing the amidoxime-modified covalent organic framework material and the reducing agent in water to obtain a mixed solution; S2. Adding silver nitrate solution to the mixed solution described in S1 to carry out heating reaction, and obtaining the SERS substrate based on the silver-covalent organic framework heterojunction through centrifugation and washing.

4. The method for preparing a SERS substrate based on a silver-covalent organic framework heterojunction according to claim 3, characterized in that: In S1, the reducing agent is selected from polyvinyl pyrrolidone and / or sodium citrate.

5. The method for preparing a SERS substrate based on a silver-covalent organic framework heterojunction according to claim 3, characterized in that: In S1, the concentration of the amidoxime-modified covalent organic framework material is 0.2 mg / mL-2 mg / mL; the mass ratio of the amidoxime-modified covalent organic framework material to the reducing agent is (5-50): (5000-50000).

6. The method for preparing a SERS substrate based on a silver-covalent organic framework heterojunction according to claim 3, characterized in that: In S2, the concentration of the silver nitrate solution is 0.2 mol / L-0.4 mol / L; the volume ratio of the mixed solution to the silver nitrate solution is (10-50): (5-50).

7. The method for preparing a SERS substrate based on a silver-covalent organic framework heterojunction according to claim 3, characterized in that: In S2, the heating reaction is carried out under a protective atmosphere at 55° C.-65° C. for 8 h-14 h.

8. The method for preparing a SERS substrate based on a silver-covalent organic framework heterojunction according to claim 3, characterized in that: In S2, the centrifugation is performed at a speed of 5800 rpm-6200 rpm for 4 min-6 min.

9. Use of the SERS substrate based on silver-covalent organic framework heterojunction according to any one of claims 1 to 2 in detecting uranyl ions.

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