Preparation of a metal-organic framework-molecularly imprinted paper-based Raman sensor and its application in the detection of thiabendazole

Abstract

The present invention relates to a metal-organic framework-molecularly imprinted polymer paper-based surface-enhanced Raman scattering (SERS) sensor for highly specific, efficient, and multi-sample analysis of thiabendazole residues in fruits, vegetables, and juices. Using an in situ growth method, a metal-organic framework material, Ag / ZIF-67, with a high specific surface area and porous structure, was synthesized in situ on a paper sheet, and a molecularly imprinted polymer (MIPs) layer was synthesized on the chip. Simultaneously, gold nanostars, which exhibit a lightning rod effect, were doped into the MIPs layer. Due to the lightning rod effect, the anisotropic structure can concentrate the electric field around the tips ("hotspots"). These hotspots lead to SERS enhancement, constructing a Raman enhancement system for target molecules using the Ag / ZIF-67-MIPs (gold nanostar) structure. The Au layer provides abundant "hotspots," enabling rapid, on-site detection of thiabendazole in fruits and vegetables. This metal-organic framework-modified paper-based SERS-enhanced sensor was applied to the detection of thiabendazole in fruits, vegetables, and juices, demonstrating good selectivity and high sensitivity.

Description

Technical Field

[0001] The present invention relates to a type of Raman enhanced sensor based on metal organic framework materials, and in particular to a paper-based surface enhanced Raman sensor of metal organic framework materials and molecularly imprinted polymers, a preparation method thereof, and application in thiabendazole detection. Background Art

[0002] Benzimidazole fungicides are a class of organic fungicides based on the fungicidal benzimidazole ring. Thiabendazole belongs to this class of benzimidazole fungicides and is a systemic fungicide with inhibitory activity against Ascomycetes, Basidiomycetes, and Deuteromycetes. It is used to control a variety of crop fungal diseases and preserve fruits and vegetables. Thiabendazole is a highly effective, broad-spectrum fungicide used internationally. However, due to its high stability at room temperature and pressure and its slow natural degradation rate, it easily remains on the surfaces of fruits and vegetables. Thiabendazole is also toxic and can cause harm to the human body. The presence of thiabendazole residues in fruits, vegetables, and fruit juices is a matter of great concern both domestically and internationally, with strict limits established for its residue levels. Currently, the primary methods for detecting thiabendazole are chemical methods such as gas chromatography (GC), high-performance liquid chromatography (HPLC), and solid-phase extraction ion-exchange chromatography. While these methods offer high sensitivity, the expensive equipment and complex pretreatment processes make them unsuitable for on-site, real-time detection.

[0003] Surface-enhanced Raman spectroscopy (SERS) is a powerful spectroscopic technique capable of detecting trace amounts of chemicals and identifying them based on their unique vibrational signatures. Typically, SERS can enhance the Raman signal by several orders of magnitude by amplifying the electron cloud density surrounding metallic nanostructures. SERS provides highly specific and sensitive spectroscopic information about the chemical structure and composition of molecules through enhanced single-molecule signals. SERS is currently widely used in immunoassays, medicine, biological and chemical sensing, and food safety monitoring. Precious metals such as gold (Au), silver (Ag), and copper (Cu) and semiconductor materials are considered effective SERS-active materials. Currently, precious metals and semiconductors are typically coated onto substrates such as glass and silicon via nanolayer etching, electrochemical deposition, and redox methods. However, these substrates are often expensive, complex, time-consuming, and inflexible, making them unsuitable for large-scale processing and on-site analytical applications. Summary of the Invention

[0004] Paper substrates have a porous structure and a large specific surface area, making them suitable for the deposition and growth of nanomaterials with various properties. Furthermore, using paper as a substrate material is low-cost and economically viable, making it a promising flexible surface-enhanced Raman spectroscopy substrate. The present invention provides a paper-based SERS sensor modified with a metal-organic framework (MOF) material, a preparation method thereof, and its application in the detection of thiabendazole. This sensor, a MOF-MIP paper-based SERS sensor, is designed for highly specific, efficient, and multi-sample analysis of thiabendazole residues in fruits, vegetables, and juices. Using an in situ growth method, Ag / ZIF-67, a MOF material with a high specific surface area and porous structure, was synthesized on a paper sheet. A molecularly imprinted polymer (MIP) layer was then synthesized on the chip. Furthermore, the MIPs layer was doped with gold nanostars, which exhibit a lightning rod effect. Due to this lightning rod effect, the anisotropic structure can concentrate the electric field around the tip ("hotspot") of the MIPs. These hotspots lead to SERS enhancement, creating a Raman enhancement system for target molecules within the Ag / ZIF-67-MIPs (gold nanostar) structure. The Au layer provides abundant "hotspots," enabling rapid on-site detection of thiabendazole in fruits and vegetables. This SERS-enhanced paper sensor is simple to fabricate, low-cost, and can be manufactured in large quantities. It can be tailored to specific needs, making it portable and exhibiting excellent selectivity and high sensitivity.

[0005] The technical solution adopted by the present invention is:

[0006] A method for preparing a paper-based surface-enhanced Raman sensor of a metal-organic framework material comprises the following steps:

[0007] (1) Each functional area was drawn using the graphics software Adobe Illustrator CS6. Then, 30 preliminary paper chip models were simultaneously manufactured using solid wax printing on chromatography paper. The wax prints were baked to allow the wax to penetrate the back of the paper, forming hydrophobic areas on both sides of the chromatography paper.

[0008] (2) cutting the paper chip model obtained in step (1) into individual paper chips;

[0009] (3) Soak the paper chip in 2-methylimidazole solution, then add the cobalt nitrate hexahydrate solution containing silver nitrate to the above solution under stirring, let the mixed system stand for a period of time, then take out the paper chip and dry it;

[0010] (4) Synthesizing gold seed solution by citric acid reduction method, and synthesizing gold nanostars by seed-mediated method using the gold seed solution;

[0011] (5) Dissolve thiabendazole and acrylamide in methanol, pass nitrogen gas through, add EGDMA and benzoin ethyl ether, and the gold nanostars obtained in step (4);

[0012] (6) dropping the solution obtained in step (5) onto the paper chip obtained in step (3), and polymerizing the solution at 254 nm using a portable UV lamp under nitrogen flow to prepare a thiabendazole molecular imprinted membrane (MIPs) on the paper chip;

[0013] (7) The paper chip obtained in step (6) was eluted with a mixed solution of methanol and acetic acid and air-dried for later use;

[0014] (8) Add the sample solution dropwise to the functional area of ​​the paper chip obtained in step (7), adsorb for a certain period of time, and then rinse the unadsorbed sample;

[0015] (9) Using a Raman spectrometer under 785 nm laser excitation, a standard curve of Raman intensity versus thiabendazole concentration was drawn to determine the thiabendazole concentration in the actual sample.

[0016] In the above preparation method, the molar concentration ratio of 2-methylimidazole and cobalt nitrate hexahydrate described in step (3) is 20-25:1-3, mmol / L: mmol / L; the volume ratio of 2-methylimidazole and cobalt nitrate hexahydrate solution is 0.5-1.5:0.8-1.2, mL: mL; the amount of silver nitrate is in the range of 1-2 mmol; the immersion temperature is in the range of 30-40°C; the immersion time is 18-28 h; the drying temperature is in the range of 40-60°C, and the drying time is 9-15 h.

[0017] In the above step (5), the molar concentration ratio of thiabendazole and acrylamide is 0.8~1.2:3~5, mmol / L:mmol / L; the volume range of EGDMA is 0.05~0.2 mL; and the amount of benzoin ethyl ether is 0.01~0.05 mmol.

[0018] The polymerization time in the above step (6) ranges from 2 to 5 h.

[0019] In the above step (7), the volume ratio of methanol to acetic acid is 8-10:0.8-1.2, mL:mL.

[0020] The volume of the adsorption sample in step (8) above ranges from 30 to 50 μl; the adsorption time ranges from 50 to 250 s.

[0021] The above method obtains a paper-based SERS sensor modified with a metal organic framework material.

[0022] The application of the above-mentioned paper-based SERS sensor in the detection of thiabendazole in fruit, vegetable and juice samples.

[0023] The outstanding features of the present invention are: 1) in the present invention, Ag / ZIF-67 is grown in situ on a paper substrate, giving the substrate a large specific surface area and a porous structure; 2) a molecular imprinting membrane is polymerized on the substrate to form a specific recognition component, which can improve the sensor's selectivity for target analytes; 3) gold nanostars are doped in the MIPs layer to enhance and amplify the Raman signal of the thiabendazole molecule, and the signal enhancement effect is stronger than the signal intensity when the two materials are used alone; 4) the paper-based Raman sensor has a simple preparation process, low cost, and fast detection time, and has great application potential for rapid on-site detection of thiabendazole content in fruits, vegetables and fruit juices. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 . The modification process of the paper chip in the present invention.

[0025] Figure 2 SEM images of (A) bare paper and (B) paper chip modified with Ag / ZIF-67.

[0026] Figure 3 . TEM image of gold nanostars. DETAILED DESCRIPTION

[0027] Example 1:

[0028] 1.5 mmol of Co(NO₃)₂·6H₂O was dissolved in 20 mL of methanol, and 1.5 mmol of AgNO₃ was added to this solution, designated Solution A. 22.5 mmol of 2-methylimidazole was dissolved in 20 mL of methanol, designated Solution B. The paper chip was immersed in Solution B for 5 minutes. Then, Solution A was added to Solution B with vigorous stirring. The mixture was incubated at 35°C for 24 hours and washed three times with ethanol and deionized water. After drying at 50°C for 12 hours, the Ag / ZIF-67-modified paper chip was obtained.

[0029] To prepare the gold seed solution, first, heat 144 mL of ultrapure water to boiling (approximately 100°C) in a three-necked flask. Then, add a mixture of 3.5 mL of sodium citrate (60 mM) and 1.5 mL of citric acid (60 mM) and maintain under vigorous stirring for 30 minutes. Next, inject 0.1 mL of sodium ethylenediaminetetraacetic acid (30 mM), followed by the rapid addition of 1 mL of 25 mM HAuCl₄ aqueous solution. After 60 seconds, turn off the heat while maintaining the solution under magnetic stirring. When the temperature drops to 95°C (approximately 5 minutes), immerse the three-necked flask in an ice-cold water bath to prepare the gold seed solution. Then, mix 20 mL of 0.25 mM HAuCl₄ aqueous solution with 0.03 mL of 1 M HCl and 1.5 mL of the previously prepared gold seeds, and stir the solution at room temperature. 0.15 mL of 1 mM AgNO₃ and 0.15 mL of ascorbic acid (66.67 mM) were simultaneously added to the mixture. After 10 minutes, 5 mL of 0.2 M CTAB solution was added. The solution was stirred for 2 minutes, and then the flask was immersed in an ice-cold water bath for 10 minutes. Finally, the gold nanostars were collected by centrifugation.

[0030] 0.1 mmol of thiabendazole and 0.4 mmol of acrylamide were dissolved in 1.4 mL of methanol and purged with nitrogen for 2 min. 0.15 mL of EGDMA, 0.03 mmol of benzoin ethyl ether and the obtained gold nanostars were added under stirring. The mixture was dropped onto a paper chip modified with Ag / ZIF-67 and polymerized at 254 nm using a portable UV lamp for 3 h under nitrogen. The mixture was then eluted with a mixed solution of 4.5 mL of methanol and 0.5 mL of acetic acid, and then adsorbed with a thiabendazole standard solution. The SERS signal of thiabendazole was detected using a portable Raman spectrometer.

[0031] Example 2:

[0032] 1.2 mmol of Co(NO₃)₂·6H₂O was dissolved in 16 mL of methanol, and 1.2 mmol of AgNO₃ was added to this solution, designated Solution A. 18 mmol of 2-methylimidazole was dissolved in 16 mL of methanol, designated Solution B. The paper chip was immersed in Solution B for 5 minutes. Then, Solution A was added to Solution B with vigorous stirring. The mixture was incubated at 35°C for 24 hours and washed three times with ethanol and then with deionized water. After drying at 50°C for 12 hours, the Ag / ZIF-67-modified paper chip was obtained.

[0033] To prepare the gold seed solution, first dilute a 1% aqueous HAuCl₄ solution to 90 mL and add 2 mL of a 38.8 mM sodium citrate solution. Then, slowly add 1 mL of freshly prepared 0.075% NaBH₄ solution (38.8 mM sodium citrate). The reaction is stirred overnight at room temperature to form the seed solution. Next, 50 mL of the gold seed solution is mixed with 10 mM PVP-10 at room temperature for 24 hours to prepare the PVP-coated gold seed solution. 82 µL of a 50 mM HAuCl₄ solution and 15 mL of 10 mM PVP-10 are mixed in DMF. The PVP-coated gold seed solution is then quickly added and stirred at room temperature for 3 hours. The gold nanostars are washed with ethanol and then centrifuged in water to obtain the gold nanostars.

[0034] 0.05 mmol of thiabendazole and 0.2 mmol of acrylamide were dissolved in 1 mL of methanol. 0.075 mL of EGDMA, 4 mL of toluene, 4 mL of styrene, 0.015 mmol of benzoin ethyl ether, and the obtained gold nanostars were added under stirring and in the absence of oxygen. The mixture was dropped onto a paper chip modified with Ag / ZIF-67 and polymerized at 254 nm for 3 h using a portable UV lamp under nitrogen. The resulting product was then eluted with a mixed solution of 4.5 mL of methanol and 0.5 mL of acetic acid. The product was then adsorbed onto a thiabendazole standard solution, and the SERS signal of thiabendazole was detected using a portable Raman spectrometer.

Claims

1. Application of a metal organic framework-molecularly imprinted paper-based Raman sensor in the detection of thiabendazole, characterized in that: The steps include: (1) Each functional area was drawn using the graphics software Adobe Illustrator CS6. Then, 30 preliminary paper chip models were simultaneously manufactured using solid wax printing on chromatography paper. The wax prints were baked to allow the wax to penetrate the back of the paper, forming hydrophobic areas on both sides of the chromatography paper. (2) cutting the paper chip model obtained in step (1) into individual paper chips; (3) Soak the paper chip in 2-methylimidazole solution, then add the cobalt nitrate hexahydrate solution containing silver nitrate to the above solution under stirring, let the mixed system stand for a period of time, then take out the paper chip and dry it; The concentration of 2-methylimidazole in step (3) is 1.125 mol / L, the concentration of cobalt nitrate hexahydrate is 0.075 mol / L, the amount of 2-methylimidazole solution is 20 mL, the amount of cobalt nitrate hexahydrate solution is 20 mL, the amount of silver nitrate is 1.5 mmol, the immersion temperature is 35°C, the immersion time is 24 h, the drying temperature is 50°C, and the drying time is 12 h; (4) Synthesizing gold seed solution by citric acid reduction method, and synthesizing gold nanostars by seed-mediated method using the gold seed solution; (5) Dissolve thiabendazole and acrylamide in methanol, pass nitrogen gas through, add EGDMA and benzoin ethyl ether, and the gold nanostars obtained in step (4); In step (5), the amount of thiabendazole is 0.1 mmol, the amount of acrylamide is 0.1 mmol, the volume of methanol is 1.4 mL, the volume of EGDMA is 0.15 mL, and the amount of benzoin ethyl ether is 0.03 mmol; (6) dropping the solution obtained in step (5) onto the paper chip obtained in step (3), and polymerizing the solution at 254 nm using a portable UV lamp under nitrogen flow to prepare a thiabendazole molecular imprinted membrane (MIPs) on the paper chip; The polymerization time range in step (6) is 3 h; (7) The paper chip obtained in step (6) was eluted with a mixed solution of methanol and acetic acid and air-dried for later use; The volume ratio of methanol to acetic acid in step (7) is 9:1, mL:mL; (8) Add the sample solution dropwise to the functional area of ​​the paper chip obtained in step (7), adsorb for a certain period of time, and then rinse the unadsorbed sample; The volume of the sample solution in step (8) is 30 μL; the adsorption time is 150 s; (9) Using a Raman spectrometer under 785 nm laser excitation, a standard curve of Raman intensity versus thiabendazole concentration was drawn to determine the thiabendazole concentration in the actual sample.

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