A dual-mechanism fluorescent probe for distinguishing between aniline and benzylamine based on efficient gas-curing chemistry
Aniline and benzylamine are the intermediates of chemical industry and precursor chemicals, respectively. The rapid on-site detection of the two is of great significance in environmental detection and combating drug production. We designed and synthesized a fluorescent probe with benzothiadiazole as backbone and aldehyde group as active unit. The probe film exhibited fluorescence quenching for aniline vapor and enhanced fluorescence for benzylamine vapor at room temperature. In air, the material has good photostability and no photobleaching within 300 s. After contact with aniline gas, the reaction was completed within 300 s, the fluorescence was quenched by 55%, and the detection limit reached 5.6 ppm; after contact with benzylamine gas, the fluorescence was enhanced by 272%, and the detection limit reached 0. 63 ppm. For the first time, this fluorescent probe realizes the highly selective recognition of aniline and benzylamine by two ways of fluorescence quenching and enhancement, and other common organic amines and solvents have little influence on the detection. NMR and mass spectrometry analysis showed that the aldehyde group of the probe reacted with aniline and benzylamine at the solid-gas interface at room temperature, respectively, to form the corresponding Schiff bases. Quantitative calculations show that the differences in the charge transfer and spatial structure of Schiff bases lead to their different fluorescence signal changes. The probe is expected to be applied in the detection of aniline-type environmental pollutants and the detection of benzylamine raw materials in drug-making dens, providing technical support for environmental monitoring and drug detection.
Key words: fluorescent probe; aldehyde group; aniline; benzylamine; gas phase detection
Organic amine compounds are widely used in organic synthesis and chemical technology. They bring a lot of convenience to people’s lives, but they also bring many problems such as environmental pollution and food safety. As one of the most important amines, aniline is widely used in dye industry, pesticide industry and rubber auxiliaries production. The leakage of aniline will pollute the air and water, and aniline is highly toxic and volatile, and can enter the body through the respiratory tract, digestive tract and skin, seriously threatening human health . At present, different detection methods for aniline in aqueous phase have been developed, such as high performance liquid chromatography [2-3], electrochemical method [4-5], gas-mass spectrometry [6-7] and fluorescence spectroscopy Law [8-10] and so on. However, these methods cannot meet the rapid on-site detection of gas-phase aniline in the environment. Benzylamine is a colorless to pale yellow liquid with a typical ammonia-like odor and strong basicity. It is an important part of the chemical , pharmaceutical  and polymer  industries. In synthetic organic chemistry, it is often used as a substrate for the production of various nitrogen heterocycles , benzonitrile  and benzamide . Benzylamine, as a drug-making intermediate, is also particularly necessary in combating illegal crimes, so it has become more urgent to develop new and distinguishable detection methods.
In recent years, fluorescent sensing materials with real-time response, high sensitivity and excellent selectivity have received extensive attention from scientists [17-18]. At present, the application of fluorescent materials in gas-phase aniline detection has also been widely studied. For example, Shi used benzothiadiazole-pyridine branched triphenylamine derivatives to detect aniline vapor with excellent sensitivity, selectivity and reproducibility by photo-induced electron transfer (PET). Zhang et al.  used nanofiber fluorescent materials to detect aniline. The material shows high sensitivity while also possessing excellent sensing properties for other organic amines and pyridines. Liu et al.  used the gel films of organic fluorescent materials to rapidly and selectively detect gas-phase aniline with a detection limit of 8. 6 ppm. Wang  used ZnS/PTCDA film to reduce the detection limit of aniline to 100 ppb. Jiang and his research group  used the π-stacking effect to further reduce the detection limit of aniline to 80 ppb. Although great progress has been made in the detection of aniline, there is currently a lack of fluorescent probes that can clearly distinguish aniline from benzylamine and other analogs. Efficient distinction between the two is very important for environmental monitoring and drug control. Regarding the method that the two can be distinguished, for example, Zhang et al.  used vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS) combined with a new doping technique to distinguish organic amines. However, this method is complicated to operate and requires high equipment.
The detection mechanisms of most fluorescent probes for gas-phase organic amines either rely solely on PET or gas-phase chemical reactions. The PET mechanism and the consequences of inhibiting it often result in weaker or stronger fluorescence, making it difficult to effectively distinguish the two amines based on changes in intensity alone. To explore efficient gas-phase chemical reactions, it is easier to achieve effective differentiation according to their reactivity or the properties of the products after action. In this paper, based on benzothiadiazole, a fluorescent gas-phase probe was designed that interacts with aldehyde groups and two amines. The detection and differentiation of aniline and benzylamine using the turn off and turn on dual mechanisms in one molecule provides a new idea for the design of fluorescent probes for the detection of aniline.
1.1 Experimental drugs and instruments
4,7-dibromo-2,1,3-benzothiadiazole, catalyst tetrakis(triphenylphosphine) palladium were from McLean Company, 4-formylphenylboronic acid was from Sukai Road Company, and other solvents were purchased from Explore the platform, all chemical reagents and pharmaceuticals are purchased from commercial sources, and organic reagents are analyzed
pure level. The fluorescence properties were characterized using a Fluoromax Plus fluorescence spectrometer from HORIBA. The probe was dissolved in tetrahydrofuran to prepare its solution. The solution was dropped on a 1 cm × 3 cm filter paper, and dried under vacuum for 1 h to remove the solvent for later use. All spectral data are ensured that the probe test strip is at a 60° angle to the incident light to ensure the accuracy and comparability of the data. Quantum chemical calculations were performed using Material studio software, and the optimized molecular configurations and energy level charge distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were given.
1.2 Synthesis of fluorescent probe molecules
4-Bromo-7-benzaldehyde-2,1,3-benzothiadiazole (referred to as The synthesis method of B1) is shown in Fig.1.
Synthesis of B1: 4,7-dibromo-2,1,3-benzothiadiazole (0.59 g, 2 mmol), 4-formylbenzeneboronic acid ( 0.33 g, 2.2 mmol) and catalyst tetrakis(triphenylphosphine)palladium (115 mg, 0.1 mmol), deoxygenated, protected by argon, and then injected into deoxygenated toluene (40 mL) with a syringe and K2CO3 aqueous solution (10 mL, 2 M). Under the protection of argon, it was heated to 100 °C and reacted for 12 h to complete the reaction. The reaction solution was washed with saturated brine, and extracted with dichloromethane to obtain an organic phase, which was then dried over anhydrous magnesium sulfate. The filtered solution was spin-dried and separated by column chromatography (eluting with
Reagent: petroleum ether: dichloromethane = 1: 1). After drying in vacuo, a pale yellow solid powder (0.51 g, 52%) was obtained. 1H NMR (500 M, CDCl3, ppm): δ 10.12 (s, 1H), 8.16-8.02 (m, 4H), 7.98 (d, J = 7.5 Hz, 1H), 7.66 (d, J = 7.6 Hz, 1H). MS: m/z 317.9. MS for C13H7BrN2OS: m/z 317.9; A- nal. Calcd: 317.95.
2.Results and discussion
Through the above synthesis steps, the fluorescent probe molecule B1 was synthesized. Its structure was confirmed by means of nuclear magnetic resonance and mass spectrometry. The probe is prepared by a one-step synthesis method with simple steps. Moreover, the probe has good solubility and is soluble in common solvents such as dichloromethane, tetrahydrofuran (THF) and toluene and other organic solvents.
2.1 Optical properties
The excitation and emission spectra of the fluorescent probe molecule B1 in THF (1.5 mg/mL) solution and film are shown in Fig. 2. In solution, its absorption maxima and emission peaks are 405 nm and 457 nm, respectively. In the thin film state, the maximum absorption peak is 379 nm, and the maximum emission peak is 468 nm. From solution to thin film, the absorption and emission of probe molecules are broadened, indicating that B1 molecules have self-aggregation in the solid state.
2.2 Sensing properties
We first investigated the response of B1 to two amines in solution. It was found that 0.14 mg/mL of B1 in tetrahydrofuran was essentially unresponsive to 0.5, 1, 2, 3, 4, 5, and 6 equivalents of aniline and benzylamine.
Further, we investigated the gas-phase sensing properties of the B1 thin films. The concentration of the probe has an important effect on the sensing performance of the prepared films, so we first optimized the solution concentration of the probe. Taking the filter paper as the base, the probe solutions of different concentrations are added dropwise to the surface of the filter paper, and the sensing film can be obtained after dissolving and volatilizing. The reasons for using filter paper as the substrate here are: the preparation method is simple, and the filter paper has a porous structure, which is conducive to the permeation of gas molecules and is beneficial to sensing. Fig. 3(a, b) are 0 respectively. 5 mg/mL, 1. 0 mg/mL, 1. Sensing performance of test paper made of 5 mg/mL and 2.0 mg/mL probe molecule solutions in saturated vapor of aniline and benzylamine. Fig. 3(a) shows that within 300 s, in aniline, the light of the films prepared with the four concentrations of the probe molecules is attenuated by 41.3%, 48.2%, 54.4% and 55% sequentially. From Fig. 3(b), it can be seen that in benzylamine, the fluorescence of the four membranes is enhanced by 131%, 150%, 234% and 272%, respectively, with increasing concentration. When B1 reacted with aniline, its fluorescence peak did not change, but after reacting with benzylamine, its fluorescence peak had a red shift of 5 nm. Combining the aforementioned solution-phase unresponsiveness and its efficient sensing performance in the thin-film state, its gas-phase sensing performance originates from the nature of the aggregated state. In addition, synthesizing the effect of the probe concentration on the sensing performance of benzylamine and aniline shown in the two figures, we choose 1.5 mg/mL as the appropriate concentration, and the test paper used in all subsequent tests adopts 1.5 mg /mL solution of this concentration was prepared.
Stability and response rate are important parameters to measure whether the fluorescent probe material is excellent.
Fig.4 presents the stability and response curves of B1 test paper in air and two amines. It can be seen from the figure that under the air condition, the probe film does not have any attenuation within 300 s. In aniline (AN), the quenching is close to saturation within 100 s, reaching about 50%; at 300 s, the fluorescence quenching is the largest, and the quenching value is about 55%. In benzylamine (BZA), the fluorescence enhancement is about 125% within 100 s, and the fluorescence enhancement reaches saturation in 300 s, and the maximum enhancement is more than 270%. The photostability and response curves show that the probe has excellent photostability and excellent sensing performance for two amines.
The selectivity of the probe is another important indicator of its sensing performance.
First, we tested the effect of other organic amines on fluorescent probes. Fig. 5 shows the change of fluorescence color after the interaction of B1 with organic amine vapor for 300 s. The atmosphere of the probe is air, benzylamine, aniline, n-propylamine, N-methylbenzylamine, diethylamine from left to right. amine and triethylamine. It can be seen from the figure that in saturated aniline vapor, the fluorescence is quenched to basically disappear; in benzylamine, the fluorescence is greatly enhanced, and the test paper is bright blue; while in N-methylbenzylamine, diethylamine and tris There is basically no change in ethylamine. Therefore, the probe has good selectivity. After the reaction, the test paper was evacuated for 0.5 h to remove the excess adsorbed amine, and the fluorescence did not change, indicating that the probe may have an efficient gas-curing chemical reaction with aniline and benzylamine.
Fig. 6 shows the fluorescence intensity changes of B1 fluorescent probe in different saturated vapors of organic amines and common volatile solvents (tetrahydrofuran, dichloromethane, methanol, and toluene) within 300 s. It can be seen from the figure that the probe test paper is only significantly quenched in aniline, with a quenching rate of 55%; it is only significantly enhanced in benzylamine, with an enhancement rate of 272%. In other amines and organic solvents, there is basically no response. Among them, there is a 17% enhancement in N-methylbenzylamine, and N-methylbenzylamine can also be used as an intermediate for drug generation, and the probe also has application value for its response. Therefore, the probe has good anti-interference ability against other organic amines or common volatile solvents.
The detection limits of the probes for the two amines can be obtained by measuring the intensity changes of the fluorescence signals before and after the sensing film was exposed to different concentrations of AN and BZA vapors. Fig.7 shows the relationship between the vapor pressure of the probe and the rate of fluorescence change. Fig.7(a) shows the relationship between the quenching degree of the fluorescence intensity at 468 nm of the sensing film by different concentrations of AN vapor. The AN vapor was continuously diluted, and even at a concentration of 27 ppm, the fluorescence quenching response was still 3% (as shown in the inner panel of Fig. 7). Taking the signal-to-noise ratio of the fluorescence spectrometer as 1%, the fluorescence quenching function can be well fitted by the Langmuir equation. The detection limit of the fitted probe for AN vapor is as low as 5.6 ppm, which is far lower than the immediate harm to the human body. Concentration 200 ppm. Similarly, according to the relationship between the fluorescence enhancement rate of B1 test paper and the vapor pressure of BZA within 300 s in saturated BZA, the detection limit of the probe for BZA vapor can be obtained as 0.63 ppm. Therefore, the probe has high sensitivity to both AN and BZA.
2.3 Sensing mechanism
Based on the probe design, we expected to achieve the distinction between the two amines through an efficient gas-curing chemical reaction. Combined with the previous probes in the two amine vapors, the fluorescence of the formed film remains unchanged after being evacuated. We speculate that an efficient chemical reaction occurs between the two. In this regard, the mechanism of the interaction between the probe and the two amines was studied by 1H NMR and mass spectrometry. As shown in Fig. 8, the aldehyde group peak of the probe was at 10.13 ppm before the action, and the peak disappeared after the action. The peak shifted to 8.7 ppm with aniline and 8.2 ppm with benzylamine. Correspondingly, the formation of peaks at 396 and 407 in its mass spectrum MS (m/z) (Fig. 9) further confirmed that the probe molecule had chemical reactions with aniline and benzylamine. After the probe interacts with aniline and benzylamine, its carbonyl group is cleaved, and then a C=N bond is formed with both amines to obtain their corresponding Schiff base compounds. The mechanism is shown in Fig. 10.
Quantum chemical calculations were performed using MS software, and Fig. 11 shows the optimized molecular configuration and the energy level charge distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO). The calculation results show that the HOMO and LUMO charges of the B1 molecule before the reaction are delocalized in the whole molecule. After the reaction, both B1-AN and B1-BZA experienced a localized state caused by the charge transfer from the delocalized state of HOMO to LUMO. The difference is: after the interaction with AN, the benzene ring and the molecular system are coplanar; after the interaction with BZA, the benzene ring is perpendicular to the molecular system and is in a twisted state. On the one hand, the coplanar structure of B1-AN is more conducive to the intramolecular charge transfer, and on the other hand, it enhances the intermolecular !-! stacking in the aggregated state, both of which will lead to its fluorescence quenching. The twisted state of B1-BZN not only inhibits the intramolecular charge transfer to a certain extent, but also helps to reduce the intermolecular interaction in its aggregated state, so its fluorescence is enhanced.
In conclusion, in this paper, a fluorescent probe molecule B1 containing an aldehyde group was designed and synthesized. Using the aldehyde group functional group, it can have a high-efficiency gas-solid reaction with aniline and benzylamine, and realize the gas-phase detection of the two. Fluorescence sensing performance studies showed that its fluorescence was quenched by 55% after contact with aniline gas and enhanced by 272% within 300 s in benzylamine. Thus, an efficient differentiation of the fluorescence turn-on and turn-off of the two amines using one reaction mechanism is achieved. The probe has high photostability, response rate, and sensitivity, and the detection limits for aniline and benzylamine vapors are 5.6 ppm and 0.63 ppm, respectively, which are well below the minimum human immediate harmful concentration of 200 ppm. H NMR and mass spectrometry showed that the chemical reaction to form Schiff base occurred between them. Quantitative calculations showed that the probes formed a charge transfer state when they interacted with them; but when they interacted with aniline, a coplanar system was formed; when they interacted with benzylamine, a distorted molecular structure was formed. The different structures determine the fluorescence quenching and enhancement in the aggregated state, respectively. The probe realizes the distinction between two types of amines through a mechanism, which is related to
The design of the probe provides an efficient strategy, while the probe provides technical support for environmental pollutant monitoring and drug detection.