Preparation method of COFs fluorescent probe for nitroaromatic explosive detection

Abstract

The application relates to a preparation method of a COF fluorescent probe for nitroaromatic explosive (NACs) detection. Through a Schiff base reaction, three two-dimensional covalent organic frameworks COF1, COF2 and COF3 are successfully prepared. The COFs have high specific surface area, surface electron-rich amino groups and excellent fluorescence characteristics. The electron-rich amino groups on the surface of the COFs and the electron-deficient nitro groups in the NACs are connected through electrostatic action, the fluorescence emission spectrum of the COFs is overlapped with the ultraviolet-visible absorption spectrum of the NACs, fluorescence resonance energy transfer occurs, and high-sensitivity trace detection of the NACs is realized. In addition, the large pi conjugated system and the pore size structure of the COFs enable the COFs to have good adsorption capacity for nitroaromatic compounds such as 2,4,6-trinitrophenol (TNP). The COF fluorescent probe prepared by the application can realize high-selectivity and high-sensitivity trace detection of the NACs based on the fluorescence resonance energy transfer mechanism, and has wide application prospects in the fields of environmental monitoring, explosive detection and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of material science, in particular to a preparation method of a two-dimensional covalent organic framework fluorescent probe for ultra-trace nitroaromatic compound detection. BACKGROUND

[0002] From the perspective of social safety, nitroaromatic compounds (NACs) have extremely high explosiveness and environmental hazards, therefore, in recent years, the detection of ultra-trace nitroaromatic compounds (NACs) and the exploration of related sensor arrays have attracted widespread attention and fruitful exploration of social research institutions. The laboratory detection of specific NAC explosives and their signals has been widely carried out by methods such as gas chromatography-mass spectrometry, ion mobility spectrometry, and neutron activity analysis. These traditional analysis techniques can meet the basic requirements of analysis, such as selectivity, reliability, accuracy, and repeatability, but these detection methods are expensive, time-consuming, and cumbersome, because the sample must be sent to the laboratory for analysis away from the detection site, and real-time and on-site detection cannot be achieved. In summary, it is necessary to seek a method that can quickly and conveniently detect NACs. In view of this, it is crucial to develop a method that is simple to operate and has high performance, can accurately determine and effectively remove NACs, and is related to ecology, health, and industry development. Currently, researchers have developed various NACs detection methods such as high-performance liquid chromatography, colorimetric method, fluorescence quenching method, and electrochemical reduction method, among which the fluorescence method is highly favored due to its outstanding advantages. In terms of application, fluorescent materials are ideal sensitive materials for constructing nano-scale structures, and semiconductor-doped nanocrystals are high-efficiency and ideal fluorescent materials. In order to solve the problem of explosive detection, there is an urgent need for new strategies to provide a high selectivity, high sensitivity, high response, fast, low-cost, and in-situ detection of target analytes in the environment.

[0003] In various signal detection, optical addressable chemical sensors based on fluorescence "off" or "on" mechanism have been proven to be a new strategy for researchers to detect various small molecule target analytes in many challenging environments, due to the high signal output and reliable detection results of this detection method. Chemical sensors based on fluorescence "off" mechanism are extremely advantageous for detecting nitrophenol compounds by fluorescence method. NAC explosive with an electron-deficient aromatic ring is an electron acceptor, which shows high affinity to the surface of electron-rich fluorescent materials. This photoluminescence is directly quenched by electron transfer π-complex mechanism between electron acceptor and donor, and this quenching mainly depends on the electron accepting ability of nitroaromatic compounds.

[0004] In the detection of fluorescence signals, it is easy to operate, ordinary personnel can be trained in a short period of time; high sensitivity, can capture trace NACs signal; strong practicability, adapt to a variety of complex detection scene; good accuracy, small detection error. At the same time, the development of fluorescent materials such as molecular probes, quantum dots, hydrogels, molecularly imprinted polymers, polymer fibers, metal organic frameworks (MOF) is booming. Liu Lu et al. published invention patent (CN116253896A) "a one-dimensional cadmium MOF material for fluorescence recognition and its preparation method", the invention discloses a one-dimensional cadmium MOF material for fluorescence recognition and its preparation method. It contains two key ligands: dbim as bridging ligand, with open active site, can construct complex containing Lewis site; m-H2bdc is used to adjust the structural properties. The synthesized complex 1 has obvious advantages, has strong fluorescence recognition ability to p-nitrophenol, 2,6-dichloro-4-nitroaniline, and has good application prospect in water ion detection.

[0005] In recent years, COFs materials stand out among the crowd. With the advantages of flexible structure regulation, regular pore size, and high specific surface area, they are widely used in gas storage and separation, catalysis, chemical sensing, and other frontier fields. In the field of environmental protection, they have also achieved remarkable results in the removal of heavy metal ions, antibiotics, and uranium elements. COFs materials have strong designability and can specifically bind to NACs, providing a new way to solve NACs problems. The invention patent (CN115078314A) "Porous covalent organic framework material gas fluorescence sensor and its preparation method" disclosed by Wang Taoping et al. aims to solve the problem of low sensitivity and small specific surface area of existing fluorescence sensors. The gas fluorescence sensor is a porous covalent organic framework material compounded on a substrate. The porous covalent organic framework material is a condensation product of aryl aldehyde and imine compounds. The condensation product is a Schiff base type covalent organic framework material. The skeleton structure of the Schiff base COFs contains N atoms, which can form hydrogen bonds with nitro explosives, causing changes in HOMO and LUMO orbitals, i.e., electron transfer. The aryl aldehyde as one of the condensation raw materials can improve the sensitivity of the condensation product in detecting TNT explosives. The invention patent (CN110554016A) disclosed by Wang Hong et al. discloses a self-driven micromotor for fluorescence detection of explosives and its synthesis and use method. The invention discloses a self-driven micromotor based on covalent organic framework (COFs). Fe3O4 nanoparticles, MnO2 microspheres, and COFs are wrapped in polycaprolactone balls through an oil-in-water emulsion synthesis method. The COFs used for fluorescence detection contain a-C=N-N=C- conjugated structure that can form hydrogen bonds with guest molecules, leading to fluorescence quenching. MnO2 microspheres can catalyze the decomposition of hydrogen peroxide to produce oxygen, driving the autonomous movement of the micromotor and accelerating the detection process. Fe3O4 nanoparticles provide the micromotor with magnetic properties, allowing its movement to be controlled by an external magnetic field. Due to the self-driving and self-stirring action of the micromotor, the contact mass transfer process is enhanced, and the detection efficiency of explosives is improved. In a solution containing trace amounts of explosives, the fluorescence of the micromotor will be quenched within a few minutes, achieving the detection purpose. By eliminating sample pretreatment and reliance on large analytical instruments, the method can be performed on-site, with the characteristics of low cost, high efficiency, and strong sensitivity.

[0006] NACs is difficult to be biodegraded, and tends to accumulate in soil and spread with water flow, which destroys the soil ecology and affects the vegetation. Meanwhile, a large number of studies have confirmed that NACs is highly toxic to organisms, can damage and induce DNA mutation, cause cell carcinogenesis and genetic diseases, and threaten the biological reproduction and ecological balance. The regular porous structure and large specific surface area of COFs material make it have great advantages in gas storage and solution macromolecule interception. In recent years, the adsorbent based on COFs material has made significant progress, and has been effectively used to remove various environmental pollutants. Focusing on the adsorption and separation application of COFs material in gas phase or solution, the potential is explored, and innovative breakthrough is strived for. In view of the great harm of nitroaromatic compounds (NACs) in the application of industry and public security field, and the limitation of existing treatment methods, COFs material can rely on its designability and existing achievements to solve the problem of NACs, and help the coordinated development of industry and ecology.

[0007] Therefore, it is necessary to select a high selectivity and high sensitivity fluorescent probe for detecting trace NACs. The present application focuses on the high selectivity and high sensitivity trace detection of 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenol (TNP) nitroaromatic compounds (NACs), and a two-dimensional covalent organic framework fluorescent probe for detecting trace nitroaromatic compounds is prepared. Three two-dimensional covalent organic frameworks (COFs) with fluorescence characteristics are prepared. The three COFs have a relatively large specific surface area, and a large number of aminos are densely distributed on the periphery. The aminos on the surface can precisely and effectively react with nitroaromatic compounds (NACs) molecules. Especially when the COFs are uniformly dispersed in dimethyl sulfoxide (DMSO) medium, the electron-donating aminos on the surface of the COFs act as electron donors, charge transfer occurs between the electron-accepting NACs and the COFs, and the fluorescence emission spectrum of the COFs is absorbed by the NACs adhered to the surface of the COFs through electrostatic interaction, and fluorescence resonance energy transfer occurs. The intensity of the fluorescence emission spectrum of the COFs decreases, realizing the selective recognition and trace detection of ultra-trace explosive NACs molecules. In addition, the high specific surface area of the COFs endows it with strong adsorption capacity, which can effectively adsorb 2,4,6-trinitrophenol, further expanding the application scope of the COFs. SUMMARY

[0008] The present application designs three two-dimensional covalent organic frameworks (COFs), utilizes Schiff base reaction of aldehyde group and amino group, and prepares a two-dimensional covalent organic framework fluorescent probe for detecting trace nitroaromatic compounds, so that the surface of the COFs has electron-rich aminos, the aminos interact with electron-deficient nitroaromatic compounds (NACs) through electrostatic interaction, and the NACs adhere to the surface of the COFs. Meanwhile, the COFs have high specific surface area and surface empty structure.

[0009] The application is implemented by the following technical scheme: a preparation method of a COFs fluorescent probe for trace nitroaromatic explosive detection, first Schiff base synthesis, synthesis of three two-dimensional covalent organic frameworks (COFs), the products are COF1, COF2 and COF3, characterized in that: the three COFs have relatively high specific surface area, and the COFs surface has amino groups, fluorescence characteristics and large specific surface area, the electron-rich amino groups on the COFs surface and the electron-deficient nitro groups in NACs interact through electrostatic attraction, the fluorescence emission spectrum of COFs overlaps with the UV-visible absorption spectrum of NACs, the fluorescence emission spectrum of COFs is absorbed by NACs, fluorescence resonance energy transfer occurs, and high selectivity recognition and high sensitivity trace detection of NACs molecules are realized by using fluorescence intensity change. In addition, the COFs have high specific surface area and pore structure, and can selectively adsorb 2,4,6-nitrophenol. The preparation method of the COFs fluorescent probe for nitroaromatic explosive detection comprises the following steps:

[0010] Synthesis of Schiff base COFs by solvothermal method: 0.05 ~ 0.2 g Y1 and 0.05 ~ 0.2 g X are mixed in a 250 mL round-bottom flask, then 40 ~ 50 mL of o-dichlorobenzene, 10 ~ 14 mL of anhydrous ethanol and 0.46 ~ 050 mL of 5 ~ 7 mol / L acetic acid are added, ultrasonic treatment for 10 ~ 20 min, 77 K refrigeration under liquid nitrogen atmosphere, three refrigeration-pumping-thawing cycles for degassing, nitrogen atmosphere, oil bath heating at 110 ~ 130℃ for 70 ~ 74 h, the reaction product is washed with anhydrous ethanol, anhydrous acetone, anhydrous dichloromethane, and finally washed with anhydrous methanol three times, dried in a 120℃ oven for 2 ~ 4 h, to obtain two-dimensional covalent organic framework COF1 fluorescent probe, Y2 and Y3 are mixed with X to prepare COF2 fluorescent probe and COF3 fluorescent probe, respectively, and the preparation method is similar to that of COF1 fluorescent probe.

[0011] The invention patent (CN115078314A) "porous covalent organic framework material gas fluorescent sensor and its preparation method" disclosed by Wang Taoping et al. The fluorescent sensor is a porous covalent organic framework material compounded on a substrate, and the condensation product is a Schiff base type fluorescent covalent organic framework material. The use of aryl aldehyde as one of the condensation raw materials can improve the sensitivity of the condensation product in detecting TNT explosives.

[0012] However, the patent report only prepared a porous two-dimensional covalent organic framework material, and the prepared fluorescent material only has detection effect on TNT explosives, does not mention that multiple nitroaromatic explosives can be detected, the detection range is small, and there is no report based on the principle of fluorescence resonance energy transfer to detect target analytes. Therefore, it is necessary to synthesize a high selectivity and high sensitivity COFs fluorescent probe for the detection of nitroaromatic explosives, and the preparation method of the present application is as follows:

[0013] Synthesis of Schiff base COFs by solvothermal method: 0.05 ~ 0.2 g Y1 and 0.05 ~ 0.2 g X are mixed in a 250 mL round-bottom flask, then 40 ~ 50 mL of o-dichlorobenzene, 10 ~ 14 mL of anhydrous ethanol and 0.46 ~ 050 mL of 5 ~ 7 mol / L concentrated acetic acid are added, respectively, ultrasonic 10 ~ 20 min, liquid nitrogen atmosphere 77 K freezing, three times of freezing pump-thaw cycle degassing, nitrogen atmosphere, oil bath 110 ~ 130℃ heating 70 ~ 74 h, the reaction product is washed with anhydrous ethanol, anhydrous acetone, anhydrous dichloromethane, and finally washed with anhydrous methanol three times, dried in a 120℃ oven for 2 ~ 4 h, to obtain two-dimensional covalent organic framework COF1 fluorescent probe, the yield is 80%; similarly, Y2 and Y3 are mixed with X to prepare COF2 and COF3, the yield is 78% and 90%, respectively.

[0014] Fluorescence spectrum detection: 1 mg of COFs powder is dispersed into 50 mL of dimethyl sulfoxide, and ultrasonic dispersion is performed, 0.1 mL of different concentrations of nitrophenol (10 -13 M -1 ~10 -4 M -1 ) of different concentrations are added into 2 mL of covalent organic framework suspension to record the fluorescence spectrum. (Excitation wavelength 300 nm, absorption spectrum 300 to 700 nm, slit value 10 nm and PMT=700 V)

[0015] Compared with the prior art, the advantages of the present application are that three imine type covalent organic frameworks (COFs) are successfully prepared. The unique feature is to build a layer-by-layer stacked large pi conjugated system, which endows the COFs with excellent fluorescence characteristics and makes them stand out in the field of fluorescent materials. The electron-rich amino group on the surface of the COFs and the electron-deficient nitro aromatic compound are adhered to the surface of the COFs through electrostatic attraction, the fluorescence emission spectrum of the COFs overlaps with the ultraviolet absorption spectrum of the target nitro aromatic compound, and the COFs and NACs are in contact with each other in space, so that fluorescence resonance energy transfer occurs, resulting in a decrease in fluorescence intensity, thereby realizing selective recognition and sensitive detection of NACs. Therefore, the present application prepares a NACs sensing platform with high selectivity and high sensitivity, which can accurately detect nitrophenol and has potential applications in many fields such as scientific research, environmental monitoring and industrial detection, and provides reliable technical support for nitrophenol recognition. In terms of adsorption performance, the COFs and TNP synergistically enhance the adsorption efficiency of nitrophenol through hydrogen bonding and pi-pi stacking, which can efficiently remove nitrophenol and reduce the pollution risk. The three COFs have excellent comprehensive performance, good chemical stability, chemical inertness in complex environments, multiple functions, large specific surface area, and excellent thermal stability, which can maintain stable structure and performance at high temperature, thereby effectively guaranteeing the wide application of the COFs in many fields and promoting the development of the industry.

[0016] In summary, the COFs fluorescent probe for detecting nitroaromatic explosive prepared by the present application has the following advantages.

[0017] Firstly, X, Y1, Y2 and Y3 in the above are 4', 4''' and 4'''''-(1, 3, 5-triazine-2, 4, 6-triazyl)tris([[1, 1-biphenyl]-4-amine]), 4, 4', 4''-nitrobenzaldehyde, 1, 3, 5-tris (4-formylphenyl) benzene) and 4, 4', 4''-(1, 3, 5-triazine-2, 4, 6-triazyl) benzaldehyde, respectively.

[0018] Secondly, the branched chain of the monomer X contains two benzene rings, and the size of the two-dimensional covalent organic framework can be changed by changing the number of benzene ring chains.

[0019] Thirdly, the two-dimensional covalent organic framework COFs in the above has a fluorescent group on the surface.

[0020] Fourthly, the two-dimensional covalent organic framework COFs in the above has an electron-rich amino group and an electron-deficient nitro group on the surface through electrostatic interaction.

[0021] Fifthly, the two-dimensional covalent organic framework COFs in the above has pore size selectivity matching TNP adsorption.

[0022] Sixthly, the fluorescence emission spectra of the above-mentioned two-dimensional covalent organic frameworks COFs dispersed in dimethyl sulfoxide solution are blue and green light emitting bands.

[0023] Seventhly, the above-mentioned COFs fluorescence probe for detecting nitroaromatic explosives is based on the principle of fluorescence resonance energy transfer. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is the preparation of COFs fluorescence probe.

[0025] Figure 2 is the SEM images of COF1 (A), COF2 (B) and COF3 (C) at different magnifications.

[0026] Figure 3 is the UV-Vis (a) and fluorescence emission (b) spectra of COF1 (A), COF2 (B) and COF3 (C). 13 CNMR spectra.

[0027] Figure 4 is the specific surface area, pore size distribution and TG curve of COF1 (A, D, G), COF2 (B, E, H) and COF3 (C, F, I).

[0028] Figure 5 is the normalized UV-Vis (a) and fluorescence emission (b) spectra of COF1, COF2 and COF3 (A-C), and the images show the photos of COF1, COF2 and COF3 under natural light (c) and purple light (d) at 365 nm wavelength.

[0029] Figure 6 are the normalized UV-Vis spectra of target analytes DNP (a), TNP (b), TNT (c) and DNT (d), and the normalized fluorescence emission spectra of COF1 (e), COF2 (e) and COF3 (e) and COFs.

[0030] Figure 7 is the detection limit (LOD) of COF1 (A, D), COF2 (B, E) and COF3 (C, F) for two different analytes DNP and TNP.

[0031] Figure 8 is the quenching constant of COF1 (A), COF2 (B) and COF3 (C) for DNP and TNP.

[0032] Figure 9 is the linear curve of the maximum absorbance of TNP and the concentration (A), the absorbance of p-nitrophenol before and after the adsorption of COFs (B), and the asterisk indicates the adsorption rate of p-nitrophenol.

[0033] Figure 1The application relates to preparation of COF fluorescent probes. Three novel electron-rich covalent organic framework materials (COFs) are successfully designed and synthesized; the three COF materials are used for fluorescence quenching detection and high-efficiency capture adsorption of p-nitrophenol; the three COF materials have good chemical stability, can maintain structural stability in various chemical environments, have excellent thermal stability and can withstand certain high-temperature conditions, and simultaneously have excellent acid and alkali resistance and can still maintain inherent properties in acid and alkali environments.

[0034] Figure 2 The application relates to preparation of COF fluorescent probes. Three novel electron-rich covalent organic framework materials (COFs) are successfully designed and synthesized; the three COF materials are used for fluorescence quenching detection and high-efficiency capture adsorption of p-nitrophenol; the three COF materials have good chemical stability, can maintain structural stability in various chemical environments, have excellent thermal stability and can withstand certain high-temperature conditions, and simultaneously have excellent acid and alkali resistance and can still maintain inherent properties in acid and alkali environments.

[0035] Figure 3 The application relates to preparation of COF fluorescent probes. Three novel electron-rich covalent organic framework materials (COFs) are successfully designed and synthesized; the three COF materials are used for fluorescence quenching detection and high-efficiency capture adsorption of p-nitrophenol; the three COF materials have good chemical stability, can maintain structural stability in various chemical environments, have excellent thermal stability and can withstand certain high-temperature conditions, and simultaneously have excellent acid and alkali resistance and can still maintain inherent properties in acid and alkali environments. 13 C NNR spectrum. Further by means of solid-state 13 C NMR spectrum Figure 3 A-C) in-depth analysis, in COF1, COF2 and COF3, the C=N bond carbon atom signals in the X triazine ring are 172.7ppm and 170.7ppm respectively. 172.7ppm is particularly key, and the newly synthesized imine bond carbon atom signals are observed at 160.0 ppm, 159.8 ppm and 159.5 ppm (corresponding Figure 3 A-C) in-depth analysis, in COF1, COF2 and COF3, the C=N bond carbon atom signals in the X triazine ring are 172.7ppm and 170.7ppm respectively. 172.7ppm is particularly key, and the newly synthesized imine bond carbon atom signals are observed at 160.0 ppm, 159.8 ppm and 159.5 ppm (corresponding

[0036] Figure 4 The application relates to preparation of COF fluorescent probes. Three novel electron-rich covalent organic framework materials (COFs) are successfully designed and synthesized; the three COF materials are used for fluorescence quenching detection and high-efficiency capture adsorption of p-nitrophenol; the three COF materials have good chemical stability, can maintain structural stability in various chemical environments, have excellent thermal stability and can withstand certain high-temperature conditions, and simultaneously have excellent acid and alkali resistance and can still maintain inherent properties in acid and alkali environments. Figure 4A-C) can be seen that the pore characteristics of COF1, COF2 and COF3 are different; the BET specific surface area of COF1 is 284.751 m²•g⁻¹, and the pore size is accurately distributed at 2.17 nm, which belongs to mesoporous. The mesoporous pore size distribution of COF1 is wider than that of COF2 and COF3. Since the diameter of TNP molecule is usually 0.8-1.2 nm, it can provide a convenient path for TNP to smoothly diffuse into the pore, trigger efficient adsorption, and ensure the high efficiency of the application of the present application; the specific surface area of COF3 is as high as 1429.180 m²•g⁻¹, which theoretically provides a large number of adsorption sites, but its pore size is 1.83 nm, which belongs to microporous, and the size of TNP molecule is relatively large, close to the upper limit of microporous, which makes it difficult to enter and the diffusion rate is greatly reduced. Although it has a large area advantage, the actual adsorption efficiency is difficult to fully play, which indicates the direction for optimizing COF3; the specific surface area of COF2 is 458.718 m²•g⁻¹, and the pore size is close to 2.04 nm. It avoids the adsorption capacity limitation of COF1 and the problem of too small pore size of COF3. The BET is better than that of COF1, which can provide more sites, and the pore size distribution is more suitable for TNP adsorption than that of COF3, which makes TNP shuttle through the pore and interact closely with the site, which is probably the source of its high adsorption force. Thermogravimetric analysis (TGA) is the key to explore the thermal stability of COFs Figure 4 G-I), in the experiment, when the environmental temperature rises to 485℃, COF1 loses weight significantly first; at 500℃, COF2 also shows similar situation; at 540℃, COF3 loses weight significantly. After analysis, these weight losses are all caused by the progressive disintegration of the COFs framework. The above findings lay a solid foundation for mastering the thermal properties of the material and expanding the application scenarios.

[0037] Figure 5 After grinding and ultrasonic stripping, the prepared three kinds of COFs were dispersed in dimethyl sulfoxide, and the ultraviolet-visible absorption spectrum and fluorescence spectrum were recorded. It can be seen that COF1 has three absorption peaks at 260, 291 and 360 nm Figure 5 A-a), the COF1 and dimethyl sulfoxide suspension solution irradiated under the ultraviolet lamp at 316 nm shows blue-green color Figure 5 A-d), under the excitation of λ=300 nm, the fluorescence absorption spectrum (λmax = 470 nm) of COF1 has strong fluorescence Figure 5 A-b). The ultraviolet absorption of COF2 has two peak values at 298 and 355 nm Figure 5 B-a), its color dispersed in dimethyl sulfoxide under the ultraviolet lamp at 316 nm is also blue-green Figure 5 B-d), under the excitation of λ=300 nm, its dispersed solution also has strong fluorescence (λmax = 472 nm) Figure 4 B-b). However, the dispersed solution of COF3 is blue under the ultraviolet lamp (λ=316 nm) Figure 5Cd), the ultraviolet absorption has two absorption peaks at 290 nm and 358 nm. Figure 5 (Ca), when excited at λ=300nm, exhibits strong fluorescence at λmax = 455nm.

[0038] Figure 6 AC is a comparison of the UV absorption spectra of four nitrophenols dissolved in dimethyl sulfoxide and the fluorescence spectra of COFs. It can be seen that the order of the overlap area between the fluorescence of COFs and the UV absorption of TNT, DNT, TNP and DNP dissolved in dimethyl sulfoxide is DNP > TNP > TNT > DNT. Figure 6 A) These overlapping areas are the reason for fluorescence quenching: fluorescence resonance energy transfer (FRET).

[0039] Figure 7 The detection limit in this invention is determined by adding 200 μL of nitrophenol solution of different concentrations to 2 mL of COFs and dimethyl sulfoxide suspension, where the nitrophenol concentration is 10. -5 mol / L~10 -13 mol / L, through Figure 7 The decrease in fluorescence intensity shows that even ultra-low nitrophenol concentrations can cause a decrease in fluorescence intensity, demonstrating the high sensitivity of this invention. Furthermore, the figure shows that COF1, COF2, and COF3 all exhibit better detection performance for DNP than for TNP. Figure 6 The overlap area of ​​DNP is greater than that of TNP, and the two results match perfectly.

[0040] Figure 8 This is a graph of the quenching constant. From the graph, it can be seen that the quenching constant of COF1 for DNP is 26315 M. -1 The quenching constant for TNP is 7307 M. -1 The quenching constants of COF2 for DNP and TNP are 12321 M. -1 and 12074M -1 The quenching constants of COF3 for DNP and TNP are 49611 M. -1 and 15466M -1 Comparing these results, it can be seen that the quenching constant of COFs for DNP is greater than that for TNP, indicating that the quenching effect on DNP is greater than that on TNP. Figure 6 and Figure 7 The results are consistent.

[0041] Figure 9 It is a standard curve for determining TNP, using a specific wavelength λ. TNP The absorbance was measured at 355 nm, and a good correlation R was obtained after linear fitting. 2= 0.9943 (A) Figure 9 A), which laid the foundation for further study. The adsorption kinetics was determined by putting 10 mg of COF1 and COF3 into 30 mL of TNP solution with a concentration of 5*10 -4 M -1 , and 10 mg of COF2 was placed in 30 mL of TNP solution with a concentration of 7*10 -4 M -1 . By using the corresponding relationship between absorbance and concentration, we successfully calculated the change of TNP concentration by carefully measuring the absorbance of the solution before and after adsorption, and then accurately calculated the removal efficiency of COF1, COF2 and COF3 to TNP, which reached 77.92, 86.54 and 77.65% (B), respectively. These data provide key basis for further understanding the performance of the adsorption system, and help to further optimize the adsorption material and process. Figure 9 B), which laid the foundation for further study. The adsorption kinetics was determined by putting 10 mg of COF1 and COF3 into 30 mL of TNP solution with a concentration of 5*10 -4 M -1 , and 10 mg of COF2 was placed in 30 mL of TNP solution with a concentration of 7*10 -4 M -1 . By using the corresponding relationship between absorbance and concentration, we successfully calculated the change of TNP concentration by carefully measuring the absorbance of the solution before and after adsorption, and then accurately calculated the removal efficiency of COF1, COF2 and COF3 to TNP, which reached 77.92, 86.54 and 77.65% (B), respectively. These data provide key basis for further understanding the performance of the adsorption system, and help to further optimize the adsorption material and process. DETAILED DESCRIPTION

[0042] The Schiff base COF1 was synthesized by solvothermal method: 0.1 g of Y1 and 0.1 g of X were mixed in a 250 mL round-bottom flask, and then 48 mL of o-dichlorobenzene, 12 mL of anhydrous ethanol and 0.48 mL of concentrated acetic acid with a concentration of 6 mol / L were added, respectively. The mixture was ultrasonically treated for 15 min, frozen at 77 K under liquid nitrogen atmosphere, degassed by three cycles of freezing-pumping-thawing, and heated at 120℃ for 72 h under nitrogen atmosphere. The reaction product was washed with anhydrous ethanol, anhydrous acetone, anhydrous dichloromethane, and finally washed with anhydrous methanol three times. The product was dried in an oven at 120℃ for 3 h to obtain the two-dimensional covalent organic framework COF1 fluorescent probe with a yield of 80%. Similarly, Y2 and Y3 were mixed with X to prepare COF2 and COF3, respectively, with yields of 78% and 90%, respectively.

[0043] The electron-rich amino group on the surface of the above-synthesized COFs fluorescent probe and the electron-deficient nitro aromatic compound are attracted by electrostatic attraction and adhere to the surface of the COFs. The fluorescence emission spectrum of the COFs overlaps with the ultraviolet absorption spectrum of the target NACs. In space, the COFs and the NACs contact each other. The fluorescence emission spectrum of the COFs is absorbed by the target NACs, and fluorescence resonance energy transfer occurs, resulting in a decrease in fluorescence intensity, thereby realizing high-selectivity recognition and high-sensitivity trace detection of NACs. Therefore, the COFs fluorescent probe with high selectivity and high sensitivity for trace detection of NACs is prepared.

Claims

1. A method for preparing a COFs fluorescent probe for the detection of nitroaromatic explosives, comprising synthesizing three two-dimensional covalent organic frameworks (COFs) by reacting monomers X with Y1, Y2, and Y3 using Schiff bases, the products being COF1, COF2, and COF3, characterized in that: The three types of COFs possess fluorescent properties, have amino groups on their surface, and have a large specific surface area. The electron-rich amino groups on the surface act as electron donors and adhere to the surface of the COFs via electrostatic interactions with the electron-deficient nitroaromatic explosive acceptors. The fluorescence emission spectrum of the COFs overlaps with the ultraviolet-visible absorption spectrum of the target nitroaromatic explosive, resulting in fluorescence resonance energy transfer, thereby enabling the identification and detection of nitroaromatic explosives. The preparation process of the above fluorescent probes is as follows: First, 0.05–0.2 g of Y1 and 0.05–0.2 g of X were mixed in a 250 mL round-bottom flask. Then, 40–50 mL of o-dichlorobenzene, 10–14 mL of anhydrous ethanol, and 0.46–0.50 mL of 5–7 mol / L acetic acid were added. The mixture was sonicated for 10–20 min, frozen at 77 K under liquid nitrogen atmosphere, and degassed using a three-stage freeze-thaw cycle. Under nitrogen atmosphere, the mixture was heated in an oil bath at 110–130 °C for 70–74 h. The reaction product was washed with anhydrous ethanol, anhydrous acetone, and anhydrous dichloromethane, respectively. Finally, it was washed three times with anhydrous methanol and dried in an oven at 120 °C for 2–4 h to obtain a two-dimensional covalent organic frame. COF1 fluorescent probe was prepared with a yield of 80%. COF2 and COF3 were prepared by reacting Y2 with X and Y3 with X under the same solvothermal conditions, with yields of 78% and 90%, respectively. Among them, the monomer X is 4',4",4"'-(1,3,5-triazine-2,4,6-triyl)tri(([1,1-biphenyl]-4-amine)), Y1 is 4,4',4"-nitrotribenzaldehyde, Y2 is 1,3,5-tris(4-formylphenyl)benzene, and Y3 is 4,4',4"-(1,3,5-triazinecyclo-2,4,6-triyl)tribenzaldehyde.

2. The method for preparing a COFs fluorescent probe for detecting nitroaromatic explosives according to claim 1, characterized in that: The nitro aromatic explosives are 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, 2,4,6-trinitrophenol, and 2,4-dinitrophenol.

3. The method for preparing a COFs fluorescent probe for detecting nitroaromatic explosives according to claim 1, characterized in that: The two-dimensional covalent organic framework (COFs) has electron-rich amino groups on its surface that interact electrostatically with electron-deficient nitro groups in nitro aromatic explosives.

4. The method for preparing a COFs fluorescent probe for detecting nitroaromatic explosives according to claim 1, characterized in that: The two-dimensional covalent organic framework (COFs) is used for TNP adsorption with pore size matching.

5. The method for preparing a COFs fluorescent probe for detecting nitroaromatic explosives according to claim 1, characterized in that: When the two-dimensional covalent organic frameworks (COFs) are dispersed in a dimethyl sulfoxide solution, their fluorescence emission spectrum exhibits blue and green emission bands.

6. The method for preparing a COFs fluorescent probe for detecting nitroaromatic explosives according to claim 1, characterized in that: The COFs fluorescent probe described above is based on the principle of fluorescence resonance energy transfer for the detection of nitroaromatic explosives.

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