English

• The increasing use of nitroaromatic explosives by terrorists poses a challenge to effectively detect trace nitroaromatic compounds (NACs) in a variety of settings, including airports, seaports, and international borders [1, 2]. Moreover, NACs are also widely used in leather, pharmaceuticals, dyes, fireworks, and other industries. Once released into the environment, they are easily polluted by groundwater and soil, posing a serious threat to the environment [3-6]. From the perspective of environmental pollution and life health, obviously, it is highly desirable to fast, selectively, and sensitively detect NACs [7, 8]. Lots of spectroscopic and analytical techniques, such as gas-mass spectrometry, Raman spectroscopy, and ion mobility spectrometry, have been used to detect NACs, but the high expense, complicated operation, and portability restrict their wide applications [9-11]. In contrast, fluorescence-based chemical sensing may be a more competent candidate. Developing environmentally-friendly, low-cost, and portable probes with high selectivity, sensitivity and rapid reaction in various environments is of great importance both to fundamental analytic science and practical applications.

  Metal-organic frameworks (MOFs) are notable among materials of the same class owing to their versatility of function and design, which provide almost infinite potential candidates for solid-state sensors [12-17]. Compared with other traditional luminescent sensing materials, one of its advantages is that its structure and performance can be reasonably designed to accurately adjust the fluorescence intensity of MOFs by introducing specific functional fluorophores, including metal ions or organic molecules [18-22], to achieve functional sensing MOFs materials for better detection of target analytes [23-26]. Meanwhile, its clear crystal structure makes it easier to study the sensing mechanism of the frame at the molecular level [27]. So far, many MOFs with fluorescent properties have been synthesized and used for the detection of various explosives [28-32], however, most of the fluorescent sensor has limitations, such as the low selectivity and sensitivity, and lack of portability in use and a single applicable detection environment. The practical application of MOFs sensors remains a huge challenge for us.

  In this paper, we selected an aggregation induced-emission (AIE) [33-35] chromophore H₄TCPP as an organic ligand and, nontoxic and low-cost Mg²⁺ [7, 36] as metal nodes to design and synthesize a new 3D MOFs, that is, [Mg₂(H₂O)₄TCPP]·DMF·5CH₃CN (Mg-TCPP). Through the restriction of intramolecular rotation (RIR) by AIE fluorophore [37-40], Mg-TCPP shows bright blue fluorescent emission. As a sensor material, Mg-TCPP can be used as a sensor material with good selectivity and sensitivity for NACs detection and the K_SV is as low as 3.63×10⁵ L/mol. The detection limit can reach 1.12×10⁻⁷ mol/L (25.6 ppb), beyond most of the previously reported fluorescent materials [41-44]. It is worth noting that the made luminescent films of Mg-TCPP have a wider range of applications in 2,4,6-trinitrophenol (TNP) sensing and detection. For example, Mg-TCPP films can immediately detect TNP in ethanol solution and water but also can detect TNP vapor, the brightness change in 3 min. At the same time, Mg-TCPP film is obtained and can be recycled after a simple cleaning, and has the advantages of better portability and easy visual detection.

  The synthesis of [Mg₂(H₂O)₄TCPP]·DMF·5CH₃CN (Mg-TCPP) is as follows. Mg(NO₃)₂·6H₂O (60 mg, 0.23 mmol) and H₄TCPP (10 mg, 0.017 mmol) were dissolved in the mixture of N, N-dimethylformamide (DMF 3 mL), acetonitrile (1 mL) and nitric acid (60 µL). The as-obtained mixture was transferred to a stainless steel Teflon-lined autoclave of 20 mL capacity. Stir at room temperature for 1 h and heat in a preheated oven at 130 ℃ for 30 h. After the colorless block crystals were cooled to room temperature, they were washed in DMF about 3 times and dried in room temperature air to obtain colorless and transparent pure Mg-TCPP. The yield was 62% based on Mg(NO₃)₂·6H₂O. The detection of NACs is that the synthesized Mg-TCPP crystal (0.5 mg) was uniformly dispersed in DMF (2 mL) for 10 min of ultrasonic, and then different concentrations of nitro explosives (all 0.2 mL) were dropped into a suspension containing the same concentration of Mg-TCPP crystal DMF (2 mL) suspension for fluorescence detection experiments. Preparation of an explosive sensor based on the Mg-TCPP films is that a suspension was made by dispersing 5 mg of Mg-TCPP in 10 mL of DMF and kept under stirring for 1 h. Then add 20 mg polyvinylidene fluoride (PVDF) to the suspension and stir for 1 h to form an emulsion. Finally, put the mixed emulsion in an oven for 2 h to dry, forming Mg-TCPP films.

  According to the analysis of single-crystal X-ray diffraction revealed that Mg-TCPP in the monoclinic space group of p2/c. In the asymmetric unit, there are two Mg²⁺ ions, two half of a fully deprotonated TCPP⁴⁻ ligand, and four water molecules. Each Mg²⁺ ions have six-coordinated environments and is connected to each other to form octahedral patterns (Fig. 1a). Mg1 is bonded to four carboxyl oxygen atoms from four different TCPP⁴⁻, one coordinated water molecule, and one bridged oxygen atom. Mg2 is coordinated by three carboxyl oxygen atoms from two TCPP⁴⁻ ligands, two coordinated water molecules and one bridged oxygen atom (Fig. 1b). Two Mg²⁺ ions are linked together by one carboxylate group to constitute the polyhedral Mg₂(COO)₄ secondary building units (SBUs). These SBUs are connected to TCPP⁴⁻, forming a porous framework, as shown in Fig. 1c viewed along the c axis. The Mg-O bond is in the range of 1.835–2.693 Å, which is within the reasonable bond length range. A summary of the crystallographic data and refinement parameters is given in Table S1 (Supporting information).

  Figure 1

  

  Figure 1.  (a) The coordination-environment of Mg-TCPP. (b) The secondary building unit of Mg-TCPP. (c) 3D crystal structures of Mg-TCPP. (d) Topology structure of Mg-TCPP. (e) 1D microporous channel viewed along the a-axis in Mg-TCPP. Color code: C gray; N blue; O red; Mg yellow; H atoms are omitted.

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  There are one-dimensional (1D) open channels in the framework: a quadrangular shape channel with a diameter of 22.914 × 20.907 Ų (Fig. 1e). The SQUEEZE program of PLATON [45] was used to remove the disordered solvent. Calculation by PLATON reveals that the effective pore volume of Mg-TCPP is approximately 43.0% (4792.5 ų) after removing the guest molecules from the channels. To better understand the crystal structure of Mg-TCPP, it is further simplified from the perspective of topology [46]. When simplifying the Mg₂(COO)₄ SBUs and the organic ligand to be a 4-connected node, the entire crystal can be described as a 2-nodal(4-c)(4-c) connected topological structure (Fig. 1d). Topological analysis suggests the point symbol of {4².8⁴}.

  PXRD shows the peak position of the Mg-TCPP synthesized in different environments, and the simulated single crystal data is consistent, indicating that the Mg-TCPP crystal has good purity and stability (Fig. 2a). The functional groups of the crystals were characterized by an infrared spectrum. The absorption bands around 1400(s) cm⁻¹ and 1600(s) cm⁻¹ represent the C=O bond of the carboxyl, and the absorption bands around 1670(s) cm⁻¹ are caused by the C=N (Fig. 2b). At the same time, its solid UV–visible spectrum was detected (Fig. 2c). UV-vis spectra show that absorption bands appear in the UV region of 300–400 nm in Mg-TCPP, which may be caused by π-π^* interaction and charge transfer between ligands. Thermogravimetric analysis shows the thermal stability of the crystals (Fig. S2 in Supporting information). For Mg-TCPP, the mass is reduced by 20% between 30 ℃ and 200 ℃ due to loss of guest molecules(including water molecules, DMF and acetonitrile molecules) and then begins to collapse gradually at 500 ℃. At 77 K, the N₂ sorption isotherm of the activated Mg-TCPP was examined, and evaluated Brunauer Emmett Teller (BET) surface areas was 209.05 m²/g (Fig. S3 in Supporting information).

  Figure 2

  

  Figure 2.  (a) PXRD of Mg-TCPP under different conditions. (b) IR spectra of Mg-TCPP and H₄TCPP. (c) UV–vis spectrum of Mg-TCPP in the solid-state. (d) Luminescent spectra of H₄TCPP and Mg-TCPP in the solid-state.

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  In AIE-gen based MOF materials, the AIE effect of organic ligands and the matrix coordination-induced emission effect will cause crystal luminescence. Compared with H₄TCPP ligand, Mg- TCPP displayed a much brighter blue emission at 432 nm with a slight blue shift (Fig. 2d). The enhancement of crystal fluorescence is mainly on account of the coordination between the carboxyl group and Mg ion, which limits the free rotation vibration of benzene rings, the restriction of intramolecular rotation of AIE of TCPP⁴⁻, and makes the crystal show stronger fluorescence emission [40, 47]. In addition, Mg-TCPP has a high porosity and unique structure allow a certain distance between H₄TCPP to reduce the fluorescence quenching caused by intermolecular interactions [48]. At the same time, the quantum yield of Mg-TCPP was detected. The experimental results showed that the quantum yield of Mg-TCPP was 10.3%, which was higher than that 7.2% of the H₄TCPP. This shows that MOFs constructed from AIE-based ligands have better fluorescence performance than organic ligand itself.

  The excellent luminescence properties of Mg-TCPP and the rich π-electron environment of the ligands stimulate our study on the sensing behavior of Mg-TCPP to NACs. Subsequently, Mg-TCPP was used to explore the detection of TNP, and its emission spectrum was measured. With the increase of TNP, the fluorescence intensity of Mg-TCPP decreased significantly (Fig. 3a). Subsequently, to further study the sensing ability of Mg-TCPP, the fluorescence spectra of different NACs, such as 1,2-dinitrobenzene (1,2-DNB), 1,3-dinitrobenzene (1,3-DNB), 1,4-dinitrobenzene (1,4-DNB), 2,6-dinitrobenzene (2,6-DNB), nitrobenzene (NB), were measured in the same way as described above and compared with TNP (Figs. S4-S8 in Supporting information). When adding 50 ppm nitro explosives, TNP has the best quenching efficiency of 94% among these NACs, while for other explosives, the quenching efficiency is very small, only 25%−35% (Fig. S9 in Supporting information). The studies indicate that Mg-TCPP has a high selectivity for TNP.

  Figure 3

  

  Figure 3.  (a) Fluorescence profiles of Mg-TCPP responding to TNP in DMF with different concentrations (inset: the luminescent photograph of before and after quenching). (b) SV plot of Mg-TCPP in the DMF (inset: SV plot of Mg-TCPP at low concentration). (c) Three-dimensional SV histogram of various nitro-explosives. (d) Five recycle experiments of Mg-TCPP for sensing TNP. (e) Fluorescence profiles of Mg-TCPP responding to TNP in H₂O with different concentrations. (f) SV plot of Mg-TCPP in the H₂O (inset: SV plot of Mg-TCPP at low concentration).

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  The Stern-Volmer (SV) equation of relative luminescent intensity, I₀/I = K_SV[A] + 1 [49-51], clearly shows the relationship between the two (Fig. 3b). The SV curve shows a linear upward trend with the increase of concentration at lower concentrations, while the curve deviates from the linear trend and presents an upward curve at higher concentrations. According to the SV equation, we can calculate that K_SV is 3.63×10⁵ L/mol at low concentrations, ranking among the highest value of reported fluorescent materials (Table S2 in Supporting information). By formula calculation [52], the limit of detection is 1.12×10⁻⁷ mol/L (25.6 ppb). Therefore, it can be shown that Mg-TCPP has high sensitivity to be used as an ideal sensor to detect nitro explosives. Meanwhile, the three-dimensional SV histogram of other nitro explosives was plotted and compared with TNP. The results showed that Mg-TCPP had high sensitivity for the detection of TNP (Fig. 3c).

  Moreover, recycling experiments of Mg-TCPP for sensing TNP were studied. After washing with DMF several times, the quenching degree of fluorescence intensity of Mg-TCPP-DMF suspension did not significantly change (Fig. 3d). After five cycles, the fluorescent response results indicate that Mg-TCPP can be recycled. The PXRD of the recovered product are completely consistent with the original one, indicating that the product has good stability and recyclability (Fig. S10 in Supporting information).

  In addition, nitro explosive drugs are widely present in water [53-55], we measured the emission spectra of TNP at Mg-TCPP in the water system. The experimental results were similar to those in DMF solvent, and the fluorescence intensity of Mg-TCPP decreased rapidly with the increase of TNP content. When TNP concentration reaches 50 ppm, the fluorescence quenching efficiency can reach 91% (Fig. 3e). The Stern-Volmer (SV) equation for relative luminescence intensity is calculated. Fig. 3f shows the relationship. According to the Stern-Volmer equation, we can calculate the K_SV of 2.06×10⁵ L/mol at low concentrations. We can find Mg-TCPP showed excellent selectivity and sensitivity in water conditions and has a rapid fluorescence quenching response to TNP.

  In order to facilitate the detection of TNP vapor, the films of Mg-TCPP crystal were fabricated by using stable and corrosion-resistant PVDF (Fig. 4a). In the experiment, all the films used for TNP vapor detection contain 20 wt% Mg-TCPP, and the films exhibit good water stability and ductility. As shown in Fig. S11 (Supporting information), the pure PVDF films are a high grade of transparency, while Mg-TCPP films exhibit milk white under sunlight. The bright blue photoluminescence is visible from Mg-TCPP films; nevertheless, no fluorescence phenomenon is found from the pure PVDF films under 365 nm UV lamp.

  Figure 4

  

  Figure 4.  (a) Schematic of fabrication and fluorescence detection of Mg-TCPP films. (b) Fluorescence profiles of Mg-TCPP films responding to TNP vapor with different times. (c) Three recycle experiments of Mg-TCPP films for sensing TNP vapor. (d) Photographs of Mg-TCPP of the quenching of Mg-TCPP film in TNP vapor over time.

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  Then the detected Mg-TCPP film was sealed in a glass container containing 0.5 mL TNP ([TNP] = 10 mg/mL). After a certain interval, the fluorescence spectrum was recorded for TNP detection (Fig. 4b). The results showed that Mg-TCPP films had excellent fluorescence quenching response to TNP vapor with time, the fluorescence intensity of Mg-TCPP films decreased significantly. Mg-TCPP film initially appears bright blue, but the color gradually shifts to darken with the increase of time and is identified by the naked eye at 3 min (Fig. 4d). Therefore, the portability of detection and ease of operation are greatly improved by visualizing changes in light and shade. In order to show the quenching performance more easily, we calculated the quenching efficiency (QE) (Fig. S12 in Supporting information) [32]. In the initial period of Mg-TCPP films exposure to TNP vapor, the QE increases rapidly, but with the exposure time, the increase rate gradually flattens out.

  Additionally, the recyclable performance of Mg-TCPP film in TNP vapor was also investigated, quenched Mg-TCPP films in TNP vapor can be restored to fluorescence by simple acetone cleaning and drying for continued. The fluorescence response results show that after five recycles, the fluorescence intensity was not significantly reduced, which means that the Mg-TCPP films could be reused by simple cleaning (Fig. 4c). The response of Mg-TCPP films to TNP in different solvents was also studied. It was showed that fluorescence quenching response of Mg-TCPP films in both 1 mg/mL TNP aqueous solution and ethanol solution (Figs. S13 and S14 in Supporting information).

  Next, the quenching mechanism of TNP by Mg-TCPP was investigated. On the one hand, most nitro explosives are electrically deficient compounds with low LUMO energy, which are easy to receive exciting electrons from electron-rich groups, leading to photoinduced electron transfer (PET) fluorescence quenching [56, 57]. Since the photoluminescence of Mg-TCPP originates from the H₄TCPP ligand [58, 59], the HOMO/LUMO energies of H₄TCPP and all nitro explosives at B3LYP/6–31+G^* levels were calculated using density functional theory (DFT) (Fig. S15 in Supporting information) [60, 61]. The LUMO energy of nitro explosives is low and can accept fluorescence quenching by electrons. Compared with other nitro explosives, TNP has the lowest LUMO energy and the highest fluorescence quenching. However, the calculated LUMO energy level is not completely consistent with the order of the experimental quenching efficiency, suggesting that PET is not the only mechanism of fluorescence quenching. This phenomenon may be caused by the Resonant Energy Transfer Mechanism (RET). To demonstrate this mechanism, we have recorded the UV–vis spectrum for all the nitroaromatics (Fig. S16 in Supporting information). According to the mechanism of RET when the greater the overlap between the emission spectrum of Mg-TCPP and the ultraviolet absorption band of nitro explosives, Mg-TCPP is more easily transferred to nitro explosives, and the fluorescence quenching efficiency is higher. We can find that the spectrum of TNP has the greatest overlap with the emission spectrum of Mg-TCPP, while all other nitrocellulose explosives have little overlap. Therefore, the combination of PET and RET is the main cause of fluorescence quenching.

  In conclusion, we successfully designed and synthesized AIE-based MOF (Mg-TCPP) based on AIE chromophore H₄TCPP, and nontoxic and low-cost Mg²⁺ ions. For RIR of AIE fluorophore and coordination effect, Mg-TCPP has stronger blue fluorescence emission than the ligand. As a luminescent probe, Mg-TCPP shows excellent selectivity and sensitivity in the detection of TNP, with K_SV as low as 3.63×10⁵ L/mol. It is worth noting that the luminescent films of Mg-TCPP are made and could be used to detect TNP in a wide range of conditions including organic solvents, aqueous solution, and vapor. Particularly, TNP vapor can be fast detected within a few minutes by the naked eye and the film could be regenerated under simple solvent cleaning. The quenching mechanism was investigated by DFT calculations, illuminating efficient electron transfer and energy transfer cause the fluorescence quenching. This work not only provides an optional efficient material for practical detection of nitro explosives in various environments but paves the way for the design of MOFs-based fluorescent sensors.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22175033), Science and Technology Development Plan of Jilin Province (Nos. YDZJ202101ZYTS063, 20210508022RQ), Research Foundation of Education Department of Shaanxi Province (No. 18JS009).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.014.

  
  
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  Figure 1  (a) The coordination-environment of Mg-TCPP. (b) The secondary building unit of Mg-TCPP. (c) 3D crystal structures of Mg-TCPP. (d) Topology structure of Mg-TCPP. (e) 1D microporous channel viewed along the a-axis in Mg-TCPP. Color code: C gray; N blue; O red; Mg yellow; H atoms are omitted.

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  Figure 2  (a) PXRD of Mg-TCPP under different conditions. (b) IR spectra of Mg-TCPP and H₄TCPP. (c) UV–vis spectrum of Mg-TCPP in the solid-state. (d) Luminescent spectra of H₄TCPP and Mg-TCPP in the solid-state.

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  Figure 3  (a) Fluorescence profiles of Mg-TCPP responding to TNP in DMF with different concentrations (inset: the luminescent photograph of before and after quenching). (b) SV plot of Mg-TCPP in the DMF (inset: SV plot of Mg-TCPP at low concentration). (c) Three-dimensional SV histogram of various nitro-explosives. (d) Five recycle experiments of Mg-TCPP for sensing TNP. (e) Fluorescence profiles of Mg-TCPP responding to TNP in H₂O with different concentrations. (f) SV plot of Mg-TCPP in the H₂O (inset: SV plot of Mg-TCPP at low concentration).

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  Figure 4  (a) Schematic of fabrication and fluorescence detection of Mg-TCPP films. (b) Fluorescence profiles of Mg-TCPP films responding to TNP vapor with different times. (c) Three recycle experiments of Mg-TCPP films for sensing TNP vapor. (d) Photographs of Mg-TCPP of the quenching of Mg-TCPP film in TNP vapor over time.

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