COF material for detecting heavy metal cadmium as well as synthesis method and application of COF material

Abstract

The invention discloses a COF material for detecting heavy metal cadmium as well as a synthesis method and application of the COF material. The covalent organic framework material for detecting the heavy metal cadmium is synthesized from pyrrole and terephthalaldehyde through an acid solvent thermal reaction method, and the volume mass ratio of the pyrrole to the terephthalaldehyde is (20-25) L: (20-25) mg during synthesis.

Description

Technical Field

[0001] The present invention belongs to the technical field of food safety detection and new materials, and particularly relates to a COF material for detecting heavy metal cadmium, a synthesis method thereof, and an application thereof. Background Art

[0002] Cadmium ions (Cd 2+ ) poses multiple threats to human health, including kidney toxicity, liver damage, cardiovascular system effects, nervous system disturbances and potential carcinogenicity. These hazards have made it one of the important risk factors in environmental and food pollution. The Food and Agriculture Organization of the United Nations and the World Health Organization jointly listed Cd 2+ It is listed as a Category III pollutant requiring key monitoring in the food safety field. Long-term exposure to cadmium can cause yellow-brown gums and loss of smell. Once absorbed, cadmium is not metabolized in the intestines, but accumulates in the liver and kidneys over time, particularly damaging renal function. Currently, activities such as ore mining and industrial waste gas and waste discharge have led to serious cadmium contamination in water. According to GB 2762-2022, the maximum residue limit (MRL) for cadmium in food is 0.01-3.0 mg / kg, including 0.01 mg / L for tap water and tea beverages. According to GB 8538-2022, cadmium detection methods include flame atomic absorption spectrometry and graphite furnace atomic absorption spectrometry. While these methods offer the advantages of high sensitivity, they suffer from high instrument costs and demanding operator requirements. Rapid detection methods are highly efficient, simple, and environmentally friendly.

[0003] In view of the advantages of COFs materials such as designability and structural controllability, they have begun to be used in the removal of Cd in food. 2+ The rapid detection of Cd 2+ Specific optical recognition and efficient adsorption. Porphyrin structure is a luminescent group with excellent fluorescence properties and has also been applied to Cd 2+ It has excellent fluorescence intensity in the visible light region (380-750nm). The tetrasulfonated phenylporphyrin is combined with the test paper to construct a rapid detection of Cd 2+ At the same time, the central hole of the porphyrin macrocycle is more likely to capture Cd 2+ Combined with it, the fluorescence intensity of porphyrin itself changes specifically. Therefore, the COFs material containing rich porphyrin structure is constructed to be used as a target for Cd 2+ The specific fluorescent probe has good application prospects.

[0004] According to GB 8538-2022, the detection methods for cadmium include flame atomic absorption spectrometry and graphite furnace atomic absorption spectrometry. Although these methods have the advantage of high sensitivity, they have the disadvantages of high instrument purchase costs and high operator requirements. Therefore, the market urgently needs to establish a fast, efficient, highly sensitive and easy-to-use detection method for Cd in water. 2+ Rapid detection.

[0005] Wang Yanan et al. have established a rapid detection method for cadmium ions in food by colloidal gold immunochromatography. Cd was synthesized by chelating cadmium ions with 1-(4-isothiocyanobenzyl)ethylenediamine-N,N,N',N'-tetraacetic acid. 2+ -iEDTA hapten, isothiocyanate method to prepare immunogen Cd 2+ -iEDTA-bovine serum albumin and coating original Cd 2+ -iEDTA-chicken ovalbumin, applied Cd 2+ -EDTA mAb to establish Cd 2+ Rapid detection method of residual colloidal gold immunochromatography (Cd 2+ -Strip), and tested its performance. The results showed that: Cd 2+ -BSA and Cd in iEDTA-BSA 2+ The contents of Cd and Cd were 7.1 mg / mL and 191.7 μg / mL respectively; 2+ The half-maximal inhibitory concentration of -iEDTA was 16.3 μg / L, which was similar to that of Hg 2+ The cross-reaction rate (CR) of -EDTA was 18.6%, and there was no CR with other heavy metal ions; Cd 2+ -Strip detection time is 10min, the detection limit is 5μg / L, and its detection results are comparable to those of the competitive ELISA kit (Cd 2+ ELISA-Kit) and inductively coupled plasma emission spectrometry with a 100% coincidence rate. 2+ mAb, a sensitive, specific, rapid and simple colloidal gold immunochromatographic rapid detection method for food cadmium ions was established. The disadvantage of this method is that for liquid samples such as water, urine, blood, milk, etc., 10% of the volume fraction of the substance with a concentration of 0.1 mol / L EDTA chelating agent is added to the liquid sample and the mixture is reacted for 30 minutes to make Cd 2+ After fully chelating with EDTA, centrifuge at 5,000 r / min for 10 minutes, and take the supernatant for detection. The pretreatment method is relatively complicated.

[0006] Wu Feng et al. have studied the use of fluorescent microspheres to label mouse polyclonal antibodies against heavy metal mercury and cadmium ions, respectively. The heavy metal mercury ions and cadmium ions chelated with bovine serum albumin and EDTA were coated on nitrocellulose membranes to prepare fluorescent rapid detection strips. The heavy metal ions in water samples were detected by chromatography competition method. The sensitivity of the heavy metal mercury ion fluorescent detection strip for heavy metal mercury ion was 5.5 ng / mL, with a detection limit of 0.03 ng / mL. The sensitivity of the heavy metal cadmium ion fluorescent detection strip for heavy metal cadmium ion was 0.51 ng / mL, with a detection limit of 0.04 ng / mL. The cross-reaction rate with other metal ions was low. The established method is simple to operate, rapid, highly sensitive, and has good specificity. It can be used as an effective means for screening heavy metal residues in water samples. The disadvantage of this method is that the established fluorescent labeling technology and immunochromatographic competition method are suitable for cadmium ion detection in water, but the test strips need to be prepared using reagents such as antigens and antibodies, which has the disadvantage of high cost.

[0007] Tu Dongkun et al. investigated the effects of different pH values, thiazole orange-aptamer concentration ratios, thiazole orange-aptamer binding time, and aptamer-cadmium ion binding time on the fluorescence intensity of the system. The results showed that the optimal conditions for fluorescence detection were pH 7, a thiazole orange-aptamer concentration ratio of 7:3, a thiazole orange-aptamer binding time of 15 minutes, and a aptamer-cadmium ion binding time of 20 minutes. Under these conditions, the fluorescence intensity exhibited a good linear relationship with the cadmium ion concentration, with a linear range of 2.00 to 80.00 ng / mL and a detection limit of 0.23 ng / mL. This method does not require fluorescent labeling of the aptamer and is simple to operate. However, the method's disadvantages are that the established fluorescent method for cadmium ion detection, based on the cadmium ion aptamer and thiazole orange dye, requires the preparation of test strips using reagents such as antigens and antibodies, making it suitable for cadmium ion detection in water.

[0008] Sun Jingwei et al. used fluorescent microspheres to label antibodies and constructed a heavy metal cadmium fluorescent immunochromatographic detection card. They verified the linearity, stability, and specificity of the card and tested water samples. The results showed that the linear range of this method was 1-40 μg·L -1 , with a sensitivity of 8.7 μg·L -1 The minimum detection limit was 1.5 μg·L -1 . The pretreatment process of this method is simple and convenient, and the results are highly consistent with those of traditional instrumental analysis methods, making it suitable for rapid on-site detection at the grassroots level. The disadvantages of this method are: the fluorescent microspheres are used to label antibodies, and a new, rapid, practical, and sensitive nano-fluorescent immunochromatographic detection method for heavy metal cadmium is established, but it requires the use of reagents such as antigens and antibodies to prepare test strips, which has the disadvantage of high cost, and in the presence of heavy metal ions, it may lead to antibody inactivation. Summary of the Invention

[0009] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to synthesize a new COF-PPR material and construct a Cd 2+ Fluorescence detection method to detect Cd in food 2+ The efficient and specific detection method aims to solve the problems of low sensitivity, complex operation and high cost in existing heavy metal detection methods, and provide a more convenient, accurate and economical means for food safety monitoring.

[0010] In order to achieve the above object, the technical solution provided by the present invention is:

[0011] The covalent organic framework material for detecting heavy metal cadmium is synthesized by an acid-solvent thermal reaction method with pyrrole and terephthalaldehyde. During the synthesis, the volume mass ratio of pyrrole to terephthalaldehyde is (20-25) μL: (20-25) mg, preferably 22.5 μL: 23.4 mg.

[0012] Preferably, the acid solvent during the synthesis is acetic acid and trifluoroacetic acid, and the volume ratio of acetic acid to trifluoroacetic acid is 2000:1 to 2200:1, preferably 2000:1.

[0013] Preferably, nitrobenzene is also added during the synthesis, and the volume mass ratio of pyrrole, nitrobenzene, and terephthalaldehyde is (20-25) μL: (250-350) μL: (20-25) mg, preferably 22.5 μL: 300 μL: 23.4 mg.

[0014] The above-mentioned synthesis method of the covalent organic framework material for detecting heavy metal cadmium comprises the following steps:

[0015] (1) Add acetic acid and trifluoroacetic acid to a reaction flask, then add pyrrole, nitrobenzene, and terephthalaldehyde and mix thoroughly; then add a magnetic rotor and connect a vacuum oil pump to evacuate the flask until the pressure gauge shows -0.85 MPa, then stop, close the vacuum valve of the reaction flask, remove the reaction flask, seal and reinforce the reaction flask, and place it in a temperature-controlled magnetic stirrer and stir at 80°C for 72 hours;

[0016] (2) After the reaction is completed, the temperature gradient is lowered: the temperature is set to 70°C and magnetic stirring is continued for 1 hour. After completion, the temperature is set to 60°C and magnetic stirring is continued for 1 hour. This operation is continued until the temperature drops to 30°C and then cooled to room temperature. The reaction solution is filtered several times to obtain a solid powder product, which is placed in a -80°C refrigerator overnight. The frozen product is placed in a vacuum freeze dryer and vacuum freeze-dried for 24 hours to obtain a covalent organic framework material COF-PPR. The temperature is set to -80°C and the vacuum degree is -0.09MPa during the vacuum freeze drying.

[0017] The method for detecting cadmium based on the covalent organic framework material for detecting heavy metal cadmium is to add the sample to be tested to the covalent organic framework material solution and react for 1 to 90 minutes, the volume ratio of the sample to be tested and the covalent organic framework material solution is 1:2, the fluorescence intensity of the covalent organic framework material is measured, and then the cadmium concentration value is calculated according to the functional relationship formula, the functional relationship formula is: Y = 4161.76-3.97X, R 2 The fluorescence intensity of the covalent organic framework material is 0.9958, wherein Y is the fluorescence intensity of the covalent organic framework material, X is the concentration value of cadmium, and the concentration unit is μg / L; the covalent organic framework material solution is an acetonitrile, ethanol, DMF, methanol or ethyl acetate solution of the covalent organic framework material, wherein the concentration of the covalent organic framework material is 10 to 1000 μg / L, and the pH value of the covalent organic framework material solution is 2.0 to 12.0.

[0018] Preferably, the reaction time of adding the sample to be tested to the covalent organic framework material solution is 5 minutes; the covalent organic framework material solution is an acetonitrile solution of the covalent organic framework material, wherein the concentration of the covalent organic framework material is 100 μg / L; the pH value of the covalent organic framework material solution is 7.0.

[0019] Preferably, the detection concentration range of cadmium by the method is 5 to 500 μg / L.

[0020] Preferably, the storage time of the covalent organic framework material in acetonitrile is 1 to 28 days, preferably 7 to 14 days, and more preferably 7 days.

[0021] The present invention will be further described below:

[0022] The present invention provides a novel covalent organic framework material (COF-PPR), a synthesis method thereof, and a method for detecting Cd based on COF-PPR material. 2+ This method not only solves the existing Cd 2+ The detection method has the problems of low sensitivity, complicated operation and high cost, and it also significantly increases the concentration of Cd in water. 2+ The detection efficiency is high, which provides a new way to achieve efficient, convenient and economical heavy metal detection. By preparing COF-PPR materials, a fluorescence detection system for Cd 2+ The linearity, specificity and anti-interference of the method were verified by the method, and the water sample was tested. The present invention synthesized COF-PPR by acid solvent thermal reaction of pyrrole and terephthalaldehyde. The fluorescence properties of COF-PPR material were studied and it was found that COF-PPR exhibited strong fluorescence emission at 368nm excitation wavelength and was sensitive to Cd 2+ It has high selectivity and anti-interference. Subsequently, a COF-PPR fluorescence detection method for Cd 2+The method was found to have the advantages of wide linear range, low detection limit and reasonable quantification limit. To optimize this detection method, we used the control variable method to systematically investigate the type of material solvent, COF-PPR concentration, material storage period, solution pH value and the relationship between COF-PPR and Cd 2+ The reaction time and other factors affected the test results. The optimal reaction conditions were finally determined as follows: COF-PPR solvent was acetonitrile, storage time was 7 days, concentration was 100.0 μg / L, pH value was 7.0, and reaction time was 5 min. Finally, the COF-PPR material was applied to the spiked recovery experiment of water and tea beverages. The results showed that the recovery rate was high (94.6-106.0%) and the reproducibility was good (RSD ≤ 6.9%), which indicated that the detection of Cd based on COF-PPR can be realized. 2+ The method has good practical application performance.

[0023] In summary, the present invention successfully synthesized a new covalent organic framework material COF-PPR. 2+ The fluorescence detection method has the advantages of wide linear range, low detection limit and reasonable quantification limit. 2+ The high selectivity and anti-interference of Cd 2+ The specific fluorescence detection of Cd based on nanoclusters, quantum dots and COF is significantly more sensitive than that reported in the literature in the past five years. 2+ Rapid detection method for Cd in water, tea and other foods 2+ This invention has opened up a new path for the application of COFs materials in the field of heavy metal detection, and also provided valuable theoretical support and practical guidance for the advancement of food safety detection technology. 2+ The detection mechanism and adsorption mechanism of COFs provide a theoretical basis and technical support for the subsequent application of COFs materials in food safety testing and environmental governance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 : Field emission scanning electron microscopy images of COF-PPR (magnifications of images a, b, c, and d are 3000, 8500, 9500, and 15000, respectively);

[0025] Figure 2 : Transmission electron microscopy images (ac) and particle size distribution (d) of COF-PPR;

[0026] Figure 3 : FT-IR spectra of terephthalaldehyde (a), pyrrole (b), COF-PPR (c) and their Y-shift stacking diagram (d);

[0027] Figure 4 : XPS spectra of COF-PPR, C 1s and N1s;

[0028] Figure 5 : XRD pattern of COF-PPR;

[0029] Figure 6 : TGA curve (a) and DTG curve (b) of COF-PPR;

[0030] Figure 7 : XRD patterns of COF-PPR treated with 1 M HCl (a) and 1 M NaOH (b) for 1 day (red), 3 days (blue), and untreated (black);

[0031] Figure 8 :Fluorescence excitation and emission spectra of COF-PPR (a) and images under fluorescent light and UV light (b);

[0032] Figure 9 :COF-PPR fluorescence detection of various heavy metal pollutants (Hg 2+ 、As 3+ 、Cu 2+ 、Mn 2+ 、Co 2+ , Pb 2+ 、Cd 2+ )’s spectra and fluorescence ratios;

[0033] Figure 10 :COF-PPR fluorescence detection of various common metal elements (Fe 3+ , K + 、Na + 、Zn 2+ Mg 2+ )’s spectra and fluorescence ratios;

[0034] Figure 11 :COF-PPR fluorescence detection of different concentrations of Cd 2+ Fluorescence spectrum of Cd 2+ The concentrations from top to bottom are: 0, 2, 5, 10, 100, 200, 250, 300, 350, 400, 450, 500, 550 μg / L) and the linear fitting curve;

[0035] Figure 12 :Simultaneous detection of Cd in COF-PPR materials 2+ Fluorescence ratio diagram with any other metal ion;

[0036] Figure 13: Optimization of the pH (a), material storage time (b), solvent (c), material concentration (d) and reaction time (e) of the constructed method. DETAILED DESCRIPTION

[0037] 1. Experimental Methods

[0038] 1.1 Experimental Reagents

[0039] Sulfuric acid (≥98%) and hydrochloric acid (33%–36%) were purchased from Huachengda Chemical Co., Ltd. Potassium dichromate (analytical grade) and anhydrous ethanol (analytical grade) were purchased from Best Chemical Co., Ltd. Trisodium citrate (analytical grade) and chloroauric acid (48–50% Aubasis) were purchased from MacLean Biochemical Technology Co., Ltd. Sodium borohydride (analytical grade), cetyltrimethylammonium bromide (CTAB) (analytical grade), ascorbic acid (analytical grade), glycine (analytical grade), silver nitrate (>98%), sodium hydroxide (analytical grade), PEGDA (average Mn 7000 Da), Irgacure 2959, chitosan oligosaccharide lactate (degree of deacetylation ≥90%, Mn 5 kDa), PEG600 (Mn 600 Da), calcium chloride, hexadecane, and other reagents were purchased from Aladdin Biochemical Technology Co., Ltd.

[0040] 1.2 Experimental equipment and materials

[0041] Analytical balance (Sartorius BS110S) was purchased from Sartorius, Germany; micropipette was purchased from Thermo Fisher Scientific (Shanghai) Instrument Co., Ltd.; temperature-controlled ultrasonic cleaning machine (JP-040S) was purchased from Shenzhen Jiemeng Cleaning Equipment Co., Ltd.; temperature-controlled magnetic stirrer (DF-101S) was purchased from Shanghai Xiniu Technology Co., Ltd.; -80℃ ultra-low temperature freezer (MDF-382E(N)) was purchased from Sanyo Technology Co., Ltd., Japan; electric constant temperature air drying box (DHG 9070A) was purchased from Shanghai Yiheng Technology Co., Ltd., the pure water and ultrapure water all-in-one machine (Milli-Q) was purchased from Donghong International Co., Ltd., the vacuum freeze dryer (SCIENTZ-12N) was purchased from Ningbo Xinzhi Biotechnology Co., Ltd., the vacuum oven (DZF-6050) was purchased from Shanghai Jinghong Experimental Equipment Co., Ltd., the vacuum oil pump (2XZ-2) was purchased from Linhai Tanshi Vacuum Equipment Co., Ltd., the constant temperature oscillator (SHA-B) was purchased from Changzhou Zhiborui Instrument Manufacturing Co., Ltd., the pH meter (PHS-25-3C-3E-2F) was purchased from Shanghai Leici Technology Co., Ltd., the UV lamp (120W-365nm) was purchased from Han Yan Equipment Co., Ltd., and the fluorescence spectrometer (F-7000) was purchased from Hitachi Technologies Co., Ltd., Japan.

[0042] 1.3 Preparation of COF-PPR

[0043] Measure 100 mL of acetic acid and 50 μL of trifluoroacetic acid into a 500 mL reaction flask as an acidic catalyst. Subsequently, add 22.5 μL of pyrrole, 300 μL of nitrobenzene, and 23.4 mg of terephthalaldehyde to the suspension and mix thoroughly. Add a magnetic rotor and connect a vacuum pump. Vacuum the reaction until the pressure gauge reads -0.85 MPa. Close the vacuum valve on the reaction flask. Remove the reaction flask, seal it, and place it in a temperature-controlled magnetic stirrer. Stir at 80°C for 72 hours. After the reaction is complete, do not cool rapidly; instead, reduce the temperature gradient. Set the temperature to 70°C and continue magnetic stirring for 1 hour. After completion, set the temperature to 60°C and continue magnetic stirring for 1 hour. Continue this process until the temperature drops to 30°C, then cool to room temperature. Filter the reaction solution three times to obtain a solid powder product, which is stored in a -80°C refrigerator overnight. The pre-frozen product is freeze-dried in a vacuum freeze dryer. The temperature was set to -80°C, the vacuum level to -0.09 MPa, and the drying time to 24 hours. After vacuum freeze-drying, the resulting solid powder product was washed three times with acetone, isopropanol, and deionized water, respectively. The washed solid powder was dried in a vacuum oven at 70°C, a vacuum level of 0.09 MPa, and a drying time of 12 hours. The final product, COF-PPR, was obtained after drying and stored in a desiccator until material characterization and application.

[0044] 1.4 Experimental determination method used in the present invention

[0045] 1.4.1 Characterization of COF-PPR Materials

[0046] (1) Field emission scanning electron microscopy

[0047] The shape and appearance of the material were observed using a field emission scanning electron microscope. The electron microscope voltage was 5.0 kV and the magnification was 3000 to 15000 times.

[0048] (2) Transmission electron microscopy

[0049] Place a small amount of material in a 2mL centrifuge tube and add 1.5mL of anhydrous ethanol. Pour the solution onto a precision copper mesh and allow to dry at room temperature for 15 minutes before testing.

[0050] (3) Fourier transform infrared spectroscopy

[0051] Fourier transform infrared spectroscopy was used to obtain characteristic group information of the material. The scanning range was 400-4000 nm and the resolution was 8.

[0052] (4) X-ray photoelectron spectroscopy

[0053] Prepare a clean aluminum foil and cut a piece of double-sided tape to adhere to it. Take a small amount of material powder and evenly spread it across the tape. Fold the aluminum foil in half and place it in a tablet press. The pressed sample should be attached to the sample stage of the X-ray photoelectron spectrometer.

[0054] (5) Powder X-ray diffraction

[0055] The scanning range was 3° to 50°, the tube current and voltage were 100 mA and 40 kV, respectively, the scanning speed was 10° per minute, the step size was 0.01°, and Cu-Kα radiation with a wavelength of 0.15406 nm was used.

[0056] (6) Thermogravimetric analysis

[0057] After measuring the mass of the empty crucible, weigh 1.5 mg of the material sample into the crucible. After starting the test, reset the instrument, load the sample, and calibrate the temperature. The data is then analyzed by differential thermogravimetric analysis.

[0058] Study on the fluorescence properties of 1.5COF-PPR materials

[0059] (1) Weigh 1 mg of COF-PPR material and add it to 100 mL of acetonitrile. Ultrasonicate at room temperature for 5 min. Measure 2.5 mL of the COF-PPR solution into a four-way quartz cuvette and measure the fluorescence excitation and emission spectra of the COF-PPR. The slit is 5.0 nm.

[0060] (2) Weigh 1 mg of COF-PPR material and add it to 100 mL of acetonitrile. Ultrasonicate at room temperature for 5 min. Measure the COF-PPR solution and add 7 heavy metal ion solutions (Hg 2+ 、As 3+ 、Cu 2+ 、Mn 2+ 、Co 2+ , Pb 2+ 、Cd 2+ ) to a final COF-PPR concentration of 100 μg / L and a 10 μg / L concentration of each ion. Vortex mixing was followed by sonication for 30 seconds. Fluorescence spectra were measured with an excitation wavelength of 368 nm and a slit of 5.0 nm.

[0061] (3) Measure the COF-PPR solution and add 5 common metal ion solutions (Fe 3+ , K + 、Na + 、Zn 2+ Mg 2+ ) to mix, so that the final concentration of COF-PPR is 100μg / L and the concentration of each ion is 10μg / L. Vortex

[0062] After mixing, ultrasonication was performed for 30 seconds, and the fluorescence spectrum was measured with the excitation wavelength set to 368 nm and the slit set to 5.0 nm.

[0063] 1.6 COF-PPR fluorescence detection of Cd 2+ Method development

[0064] 1.6.1 Establishment of standard curve

[0065] 2.0 mL of COF-PPR solution was measured and placed in 12 5 mL centrifuge tubes, and Cd 2+ The final ion concentrations were 2, 5, 10, 100, 200, 250, 300, 350, 400, 450, 500, and 550 μg / L, respectively. After vortex mixing, the mixture was sonicated for 30 s and the fluorescence spectra were measured. The excitation wavelength was set to 368 nm and the slit was set to 5.0 nm.

[0066] 1.6.2 Sensitivity

[0067] The limit of detection (LOD) and the limit of quantitation (LOQ) were determined using a signal-to-noise ratio of 3 and 10, respectively. The LOD and LOQ are the concentrations of the sample at 3 and 10 times the signal-to-noise ratio, respectively.

[0068] 1.6.3 Specificity

[0069] 2.0 mL of COF-PPR solution was measured and placed in 12 5 mL centrifuge tubes, and Cd 2+ , the final ion concentration was 10μg / L. Hg 2+ 、As 3+ 、Cu 2+ 、Mn 2+ 、Co 2+ , Pb 2+ 、Fe 3+ , K + 、Na + 、Zn 2+ Mg 2+ The ions were added to a centrifuge tube at a concentration of 10 μg / L. After vortex mixing, the mixture was sonicated for 30 seconds. The fluorescence spectra were measured with an excitation wavelength of 368 nm and a slit of 5.0 nm.

[0070] 1.6.4 Experimental conditions for optimizing the construction method

[0071] The control variable method was used to control the material solvent, material concentration, material storage time, pH value, COF-PPR and Cd 2+The reaction time was optimized. The material solvents selected were DMF, ethanol, methanol, acetonitrile and ethyl acetate, the material concentrations were 10, 100, 200, 500 and 1000 μg / L, the material storage time was set to 1, 7, 14, 21 and 28 days, the pH values ​​were set to 2.0, 4.0, 6.0, 7.0, 8.0, 10.0 and 12.0, and the reaction time was set to 1, 5, 30, 60 and 90 min, respectively.

[0072] 1.6.5 Recovery and relative standard deviation

[0073] 2.0 mL of COF-PPR solution was added to the spiked tap water and tea beverage respectively. The Cd content in tap water was 2+ The concentrations were 3, 10 and 15 μg / L, respectively, and the Cd content in tea beverages 2+ The concentrations were 5, 7, and 12 μg / L, respectively. Vortex mixing was followed by sonication for 30 seconds. The fluorescence spectra were measured, and the recovery and standard deviation were calculated.

[0074] 1.7 Characterization and evaluation of COF-PPR material preparation

[0075] 1.7.1 Field Emission Scanning Electron Microscopy

[0076] The FESEM results of COF-PPR are as follows Figure 1 As shown. At the microscopic level, COF-PPR exhibits a spherical structure with a relatively regular shape and a very smooth surface. Furthermore, it was observed that the spherical structures of COF-PPR vary in size and exhibit distinct clustering. This is consistent with the results for COF-DhaTph synthesized using tetraaminophenylporphyrin and dihydroxyterephthalaldehyde as monomers, and is attributed to the rich hydrogen bonding structure within and between the molecular layers within the material.

[0077] 1.7.2 Transmission electron microscopy

[0078] TEM images and particle size distribution results of COF-PPR are shown in Figure 2. Figure 2 TEM image analysis reveals that the COF-PPR exhibits a three-dimensional spherical structure with a regular shape and smooth surface. The spherical particles are well dispersed and uniform in color. Furthermore, the particle size distribution shows an average particle size of 1.48 μm, with 63.43% ranging from 1.0 to 2.0 μm, 22.36% from 0 to 1.0 μm, and 14.21% from 2.0 to 3.0 μm, demonstrating a uniform and moderately sized COF-PPR particle size distribution.

[0079] 1.7.3 Fourier transform infrared spectroscopy

[0080] FT-IR spectra of COF-PPR, terephthalaldehyde and pyrrole Figure 3The characteristic group of terephthalaldehyde is the aldehyde group, which is at 1698 cm -1 A strong stretching vibration peak of C=O can be observed at 1615 cm -1 The characteristic C=C stretching vibration of pyrrole can be observed at 3133 cm -1 and 3336cm -1 There are also obvious characteristic NH peaks at two locations. In the COF-PPR spectrum, 1698 cm -1 The stretching vibration peak of C=O at 1645 cm -1 A strong C=N stretching vibration peak appears at 1615cm -1 Pyrrole, terephthalaldehyde, and COF-PPR all exhibit C=C stretching vibration peaks of varying intensities, with the COF-PPR peak being the most pronounced, indicating the formation of a porphyrin structure. Therefore, the FT-IR spectrum confirms that a dehydration condensation reaction occurred between pyrrole and terephthalaldehyde during material synthesis.

[0081] 1.7.4 X-ray Photoelectron Spectroscopy

[0082] The X-ray photoelectron spectroscopy results of COF-PPR are as follows Figure 4 The XPS spectrum of COF-PPR was analyzed using Avantage software, and two strong peaks appeared at 284.08eV and 399.08eV, representing C1s and N1s respectively. The XPS spectra of C1s and N1s were peak fitted, and the XPS spectrum of C 1s was divided into four partial peaks, located at 283.20 eV, 283.75 eV, 285.10 eV and 287.55 eV, respectively. The position of 283.20 eV corresponds to the C=C of the benzene ring in the COF-PPR skeleton and the C=C inside the porphyrin cavity, the position of 283.75 eV corresponds to the C–C on the entire COF-PPR material skeleton, the position of 285.10 eV corresponds to the CN on the pyrrole ring inside the porphyrin cavity, and the position of 287.55 eV corresponds to the C=N generated when the porphyrin cavity in the COF-PPR material skeleton is formed. The XPS spectrum of N 1s is divided into three peaks, located at 397.10eV, 398.92eV and 399.93eV respectively. The 397.10eV position corresponds to the C=N in the COF-PPR porphyrin hole, and the 398.92eV position corresponds to the C=N in the porphyrin ring.

[0083] The CN at the center of the porphyrin cavity corresponds to the NH at 399.93 eV. These results indicate that a dehydration condensation reaction occurs between pyrrole and terephthalaldehyde to form a porphyrin structure, which is consistent with the FT-IR results.

[0084] 1.7.5 Powder X-ray Diffraction

[0085] The XRD pattern of COF-PPR is shown in Figure 5 As shown in the figure, there are obvious diffraction peaks at 6.389°, 7.372°, 21.287° and 23.569°, which are consistent with the reference standard spectrum of spherical crystals and literature reports, indicating that COF-PPR has good spherical crystals.

[0086] 1.7.6 Thermogravimetric analysis

[0087] The TGA and DTG results of COF-PPR are as follows Figure 6 The TGA spectrum shows that when the temperature rises to 200°C, the material loses 9.6% of its weight. At 300°C, the loss increases by 4.8%, for a total loss of 14.4%. Further heating to 600°C increases the material loss by 18.3%, for a total loss of 32.7%. Above 600°C, the weight loss decreases sharply and then levels off. At 1000°C, the total weight loss is 40%. The DTG spectrum indicates that the rate of mass change within 200°C is within the applicable range. These results demonstrate that COF-PPR has good thermal stability.

[0088] 1.7.7 Analysis of COF-PPR Chemical Stability Experimental Results

[0089] The chemical stability results of COF-PPR are as follows Figure 7 As shown in the figure, the XRD patterns of COF-PPR materials did not change significantly after being treated in 1M HCl or NaOH solutions for 1 or 3 days, indicating that the synthesized COF-PPR materials have excellent chemical stability and are suitable for special chemical environments with little impact on performance. It is worth noting that compared with NaOH treatment, the XRD patterns of COF-PPR after HCl treatment change less, and this trend is maintained regardless of whether the treatment is 1 or 3 days, indicating that the material's acid resistance is better than its alkali resistance.

[0090] 1.9 Study on the Fluorescence Properties of COF-PPR Materials

[0091] The fluorescence excitation and emission spectra of COF-PPR and the pictures under fluorescent light or UV light are as follows: Figure 8 As shown in Figure 1, the maximum excitation wavelength of COF-PPR is 368 nm, and the maximum emission wavelength is 435 nm. The COF-PPR solution appears colorless under fluorescent light, but emits bright blue fluorescence under ultraviolet light (excitation wavelength is 365 nm).

[0092] COF-PPR detection of Hg 2+ 、As 3+、Cu 2+ 、Mn 2+ 、Co 2+ , Pb 2+ and Cd 2+ The fluorescence spectrum and fluorescence ratio results are as follows Figure 9 The fluorescence spectrum shows that COF-PPR detects Hg 2+ and As 3+ The fluorescence intensity is significantly enhanced when 2+ and Mn 2+ , these two elements also significantly enhanced the fluorescence of COF-PPR, but the increase was slightly less than the previous two; 2+ and Pb 2+ The fluorescence intensity of COF-PPR was slightly improved. 2+ When the fluorescence intensity of COF-PPR is significantly quenched, this change is in sharp contrast to the fluorescence enhancement of other ions, indicating that COF-PPR has a strong effect on the fluorescence of Cd. 2+ It has specific detection capability. Hg 2+ 、As 3+ 、Cu 2+ 、Mn 2+ 、Co 2+ , Pb 2+ The fluorescence ratio changes of Cd were 0.7049, 0.6682, 0.4823, 0.3650, 0.1066 and 0.0388, all showing positive values. 2+ The fluorescence ratio change was -0.1027, which was a negative value.

[0093] COF-PPR detection of Fe 3+ , K + 、Na + 、Zn 2+ and Mg 2+ The fluorescence spectrum and fluorescence ratio diagram results are as follows Figure 10 As shown in the fluorescence spectrum, when Fe 3+ , K + 、Na + When the Zn 2+ and Mg 2+ Compared with the detection of heavy metal ions, the effect of these common metal elements on the fluorescence intensity of COF-PPR is not significant, which provides a basis for the detection of Cd 2+ The fluorescence ratio diagram clearly reflects the influence of common metal ions on the fluorescence intensity of COF-PPR, among which Fe 3+ , K+ 、Na + 、Zn 2+ and Mg 2+ The fluorescence ratio changes of Cd were 0.0152, 0.0124, 0.008, -0.007 and -0.0193 respectively. 2+ Compared with the fluorescence ratio change of Cd(-0.1027), these changes are significantly smaller, indicating that the common metal elements have a significant effect on Cd 2+ The impact of specificity testing was minimal.

[0094] 1.10 COF-PPR fluorescence detection of Cd 2+ The establishment of the method

[0095] 1.10.1 Establishment of standard curve

[0096] Detection of Cd at different concentrations using COF-PPR 2+ The fluorescence spectrum and linear fitting curve are shown in Figure 2. Figure 11 The peak data corresponding to all the concentrations are shown in Table 1. The fluorescence intensity of COF-PPR increases with the increase of Cd 2+ The concentration of Cd decreases linearly. 2+ The concentration of COF-PPR was taken as x, and the fluorescence intensity of COF-PPR material was taken as Y. The linear fitting analysis was performed on the two, and the functional relationship was F = 4161.76-3.97x. The detection range was 5-500 μg / L, R 2 The linear equation has an excellent fit, with a LOD of 1.4 μg / L and a LOQ of 4.6 μg / L, which meet the MRL (0.01 mg / L).

[0097] Table 1 Concentration-peak comparison

[0098]

[0099] 1.10.2 Method Specificity

[0100] Simultaneous detection of Cd in COF-PPR materials 2+ The fluorescence ratio of any other metal ion is as follows Figure 12 As shown. When Hg 2+ 、As 3+ 、Cu 2+ 、Mn 2+ 、Co 2+ , Pb 2+ Heavy metal ions such as Cd 2+When they coexist, their fluorescence ratios are 0.0204, 0.0103, -0.0326, -0.0630, -0.0401, and -0.0857, respectively. These values ​​are significantly lower than when they are detected alone. 3+ , K + 、Na + 、Zn 2+ Mg 2+ The fluorescence ratios of common metal ions such as Cd in the coexistence system were -0.0802, -0.0775, -0.0901, -0.0818, and -0.0994, respectively. These values ​​also showed a significant decrease and were close to those of Cd. 2+ The fluorescence ratio of -0.1027. The above results are consistent with the detection of Cd constructed with tetrabenzyl porphyrin. 2+ The results show that COF-PPR can accurately distinguish and detect Cd even in a complex environment with multiple metal ions. 2+ , indicating that COF-PPR is effective in detecting Cd 2+ It has excellent specificity.

[0101] 1.10.3 Optimization of experimental conditions

[0102] Detection of Cd based on COF-PPR materials 2+ The results of experimental condition optimization are as follows Figure 13 As shown in the figure, a comparison of different solvents shows that acetonitrile is the optimal solvent for COF-PPR, with a fluorescence intensity significantly higher than that of ethanol, DMF, methanol, and ethyl acetate. Different solvents have different polarities and solubility for the material. Selecting an appropriate solvent significantly enhances the fluorescence intensity of the material. Conversely, an inappropriate solvent has a significant negative impact on the fluorescence intensity of the material.

[0103] Comparison of material concentrations showed that fluorescence intensity gradually increased within the 10-100 μg / L concentration range and decreased within the 100-1000 μg / L concentration range. Based on the principles of material conservation, economy, and environmental protection, 100 μg / L was selected as the optimal concentration for COF-PPR. Choosing the right concentration can save material and also significantly impact fluorescence intensity; higher material concentrations do not necessarily lead to increased intensity.

[0104] The comparison results of material storage time show that the fluorescence intensity of COF-PPR material decreases slightly from 1 to 7 days after synthesis, and stabilizes between 4300 and 4500 a.u. from 7 to 28 days, indicating that COF-PPR material is not sensitive to storage time and is relatively stable within 28 days. Finally, 7 to 14 days was selected as the optimal material storage time.

[0105] The results of pH optimization of the material showed that the fluorescence intensity of the COF-PPR material is related to pH, with the maximum fluorescence intensity at pH 6.0-7.0. The fluorescence intensity of the COF-PPR material is significantly higher in acidic environments compared to alkaline environments, which is consistent with literature reports that COF materials tend to favor acidic environments in pH optimization experiments. The stronger fluorescence of the COF-PPR in acidic environments is also consistent with the results of chemical stability experiments.

[0106] The experimental results of the optimal detection time showed that the fluorescence intensity increased from 1 to 5 minutes and stabilized after 5 minutes, indicating that the COF-PPR material can react with Cd in a short time (5 minutes). 2+ The reaction time between COF material and target will directly affect the accuracy of the test results, so choose the appropriate reaction time to ensure that Cd 2+ Completely adsorbed by COF-PPR.

[0107] 1.10.4 Food sample spike experiment

[0108] The results of the spiked experiment are shown in Table 2. 2+ The recovery rate of spiked tap water was 95.8-98.3% with RSD ≤ 6.9%, and the recovery rate of spiked tea beverages was 94.6-106.0% with RSD ≤ 5.3%. According to GB 27417-2017, the recovery rate was 80-120% with RSD < 10.0%. The recovery rate and RSD of this method met the methodological requirements. This shows that the detection of Cd based on COF-PPR is feasible. 2+ The method has good practical application performance.

[0109] Table 2 Detection of Cd in real food samples 2+ (n=3)

[0110]

Claims

1. A covalent organic framework material for detecting heavy metal cadmium, characterized in that: The covalent organic framework material for detecting heavy metal cadmium is synthesized by an acid-solvent thermal reaction method between pyrrole and terephthalaldehyde, wherein the volume-to-mass ratio of pyrrole to terephthalaldehyde during the synthesis is (20-25) μL: (20-25) mg.

2. The covalent organic framework material for detecting heavy metal cadmium according to claim 1, characterized in that: The volume-to-mass ratio of pyrrole to terephthalaldehyde during the synthesis was 22.5 μL:23.4 mg.

3. The covalent organic framework material for detecting heavy metal cadmium according to claim 1, characterized in that: The acid solvents used in the synthesis are acetic acid and trifluoroacetic acid, and the volume ratio of the acetic acid to the trifluoroacetic acid is 2000:1 to 2200:

1.

4. The covalent organic framework material for detecting heavy metal cadmium according to claim 3, characterized in that: The acid solvents used in the synthesis are acetic acid and trifluoroacetic acid, and the volume ratio of the acetic acid to the trifluoroacetic acid is 2000:

1.

5. The covalent organic framework material for detecting heavy metal cadmium according to claim 1, characterized in that: Nitrobenzene is also added during the synthesis, and the volume mass ratio of pyrrole, nitrobenzene, and terephthalaldehyde is (20-25) μL: (250-350) μL: (20-25) mg.

6. The covalent organic framework material for detecting heavy metal cadmium according to claim 5, characterized in that: Nitrobenzene was also added during the synthesis, and the volume-to-mass ratio of pyrrole, nitrobenzene, and terephthalaldehyde was 22.5µL: 300µL: 23.4mg.

7. The method for synthesizing a covalent organic framework material for detecting heavy metal cadmium according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: (1) Add acetic acid and trifluoroacetic acid to the reaction bottle, then add pyrrole, nitrobenzene and terephthalaldehyde and mix thoroughly; then add a magnetic rotor and connect the vacuum oil pump to evacuate the solution until the pressure gauge shows -0.85 MPa, then stop, close the vacuum valve of the reaction bottle, remove the reaction bottle, seal and reinforce the reaction bottle, and place it in a temperature-controlled magnetic stirrer and stir at 80 °C for 72 h; (2) After the reaction is completed, the temperature gradient is lowered: the temperature is set to 70 °C and magnetic stirring is continued for 1 hour. After completion, the temperature is set to 60 °C and magnetic stirring is continued for 1 hour. This operation is continued until the temperature drops to 30 °C and then cooled to room temperature. The reaction solution is filtered several times to obtain a solid powder product, which is placed in a -80 °C refrigerator overnight. The frozen product is placed in a vacuum freeze dryer and vacuum freeze-dried for 24 hours to obtain the covalent organic framework material COF-PPR. The temperature is set to -80 °C and the vacuum degree is -0.09 MPa during the vacuum freeze drying.

8. A method for detecting cadmium based on the covalent organic framework material for detecting heavy metal cadmium according to any one of claims 1 to 6, characterized in that: The method comprises adding a sample to be tested into a covalent organic framework material solution and reacting for 1 to 90 minutes, wherein the volume ratio of the sample to be tested to the covalent organic framework material solution is 1:2, measuring the fluorescence intensity of the covalent organic framework material, and then calculating the cadmium concentration value according to a functional relationship, wherein the functional relationship is: Y=4161.76-3.97X, R 2 is 0.9958, wherein Y is the fluorescence intensity of the covalent organic framework material, X is the concentration value of cadmium, and the concentration unit is μg / L; the covalent organic framework material solution is an acetonitrile, ethanol, DMF, methanol or ethyl acetate solution of the covalent organic framework material, wherein the concentration of the covalent organic framework material is 10-1000 μg / L, and the pH value of the covalent organic framework material solution is 2.0-12.

0.

9. The method according to claim 8, wherein The reaction time of adding the sample to be tested to the covalent organic framework material solution is 5 minutes; the covalent organic framework material solution is an acetonitrile solution of the covalent organic framework material, wherein the concentration of the covalent organic framework material is 100 μg / L; the pH value of the covalent organic framework material solution is 7.

0.

10. The method according to claim 8, wherein The detection concentration range of cadmium by the method is 5~500 μg / L.

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