A Zn(II)-based metal-organic framework (MOF) decorated with amine and azine functionalities, TMU-17-NH2 (formulated as [Zn(H2ata)(L)].2DMF; L = 1,4-bis(4-pyridyl)-2,3-diaza-2,3-butadiene and H2ata = 2-aminoterephthalic acid) has been successfully synthesized via a solvothermal method. According to crystallographic studies, the synthesized TMU-17-NH2 has three dimensional cuboidal structure with the pore surface decorated with free amine (-NH2) and azine (=N-N=) functional groups. The photoluminescence investigations proved that the synthesized MOF can be effectively utilized for selective detection of 2,4,6-trinitrophenol (TNP) in water with an apparent turn-off quenching response. Its limits of detection (LOD) for TNP was 9.4 ppb and competitive nitro explosive testing confirmed its higher selectivity towards TNP (over other nitro explosives). Calculations based on density functional theory (DFT) and spectrum overlap were utilized to evaluate the sensing mechanisms. This MOF-based fluorescence sensing technique for TNP had a high sensitivity (Ksv = 3.26 × 104 M-1).
Research Article
Hazardous 2,4,6-trinitrophenol (TNP) detection in water by amine and azine functionalized metal-organic framework
https://doi.org/10.21203/rs.3.rs-3231664/v1
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Metal-organic framework (MOF)
Schiff base ligand
Fluorescent sensor
2
4
6-Trinitrophenol (TNP)
Density functional theory (DFT)
Spectral overlap
- TMU-17-NH2 sensor decorated with amine and azine functionalities was synthesized.
- TMU-17-NH2 identified the TNP in water via turn-off quenching response.
- Turn-off response was explained both experimentally and theoretically.
- LOD of TMU-17-NH2 for TNP sensing was 9.4 ppb.
Nitro explosives (NEs) such as 2,4-dinitrotoluene (2,4-DNT), nitrophenols (NPs), 2,4,6-trinitrophenol (TNP), nitrobenzene (NB), 2,4,6-trinitrotoluene (TNT), and 2,6-dinitrotoluene (2,6-DNT) are recognized to have a great potential to cause harm to human health and environment. TNP is extremely toxic and non-biodegradable among these dangerous NEs and causes significant health risks (Chen et al. 2017, Shyamal et al. 2019). Therefore, it is critically important to develop an ultrafast, sensitive, and selective detection systems for such NEs and other pollutants in the context of protecting human health, the environment, and national security (Dalapati &Biswas 2017, Singh et al. 2022). Up to now various techniques have been developed for the determination of TNP, including high performance liquid chromatography (Rahimi-Nasrabadi et al. 2012), voltammetry (Yan et al. 2015), and ion mobility spectroscopy (IMS) to satisfy such environmental and/or societal demands (Guerra-Diaz et al. 2010). These existing analytical detection techniques suffer from numerous restrictions (such as their high cost, need of sophisticated instruments, long response time, and complicated sample preparation) which limits their utility. Recently, fluorescence-based sensing techniques have gained a lot of interest in this regard due to its advantageous qualities (such as high sensitivity, selectivity, low-cost, straightforward detection, short response time, and portability) (Wang et al. 2018). Besides TNP, fluorescence-based sensing technologies are suitable for detecting various other targets such as heavy metal ions, anions, insecticides, and biomolecules. Most of fluorescent sensors detect these targets via fluctuations in fluorescence peak intensity or position which is quantified as the detection response. In this study, we focused primarily on the development of fluorescent sensor for TNP detection in order to address the potential dangerous effects on human health that they caused when released into environmental systems.
To date, a variety of fluorescent materials have been developed and used for TNP detection, such as carbon dots (Siddique et al. 2018), conjugated polymers (Jiang et al. 2021, Mehta &Kaith 2021), nanoparticles (Rong et al. 2015), and covalent organic frameworks (Zhu et al. 2018). However, majority of them have issues like poor stability, high cost, poor sensitivity, a lack of particular interaction sites, and restricted reusability. To address these issues, researchers have been concentrating on creating new fluorescence sensors. Metal-organic frameworks (MOFs) have aroused tremendous attention as fluorescent sensors during recent decades owing to their ability to adsorb and pre-concentrate the analytes around recognition sites to impart the high sensitivity (e.g., through host-guest interactions). To ensure the high sensing capability of MOF sensors, it is essential to choose the organic ligands with appropriate sensing sites. However, expensive materials and a number of time-consuming reactions are required for the synthesis of the organic ligands having appropriate sensing sites that are frequently used in MOF sensors preparation.
Different kinds of organic ligands having appropriate sensing sites have been researched recently due to the rising demand for low-cost starting reagents and time-saving methods. In this context, researchers have paid a lot of attention towards Schiff base ligands because of their high stability, cost-effectiveness, availability of starting materials (such as aldehydes and ketones), high yield, and ease with which one can modify their properties through the careful choice of reactants (Halder et al. 2018). Schiff base ligands can be used to build high-performance MOF-based sensors as they provide the target-specific recognition sites (such as hydrazone, azine-methyl, imine, and azine groups) (Farahani &Safarifard 2019a, b, Parmar et al. 2017). In Schiff bases MOFs, the nitrogen atoms of Schiff base sites function as Lewis basic sites to interact with the target analytes during sensing via hydrogen bonding, coordination, and/or acid-base interactions. A framework created employing Schiff base ligands and amine-decorated ligands provide a highly effective platform for detecting TNP up to ppb levels (Parmar et al. 2017, Parmar et al. 2016). To the best of our knowledge, there are numerous papers in the literature that focus on Schiff base and amine-ligand-based MOFs for TNP detection in water (Parmar et al. 2017, Parmar et al. 2016).
Enlightened by above thoughts, we selected 2-aminoterephthalic acid (H2ata) and 1,4-bis(4-pyridyl)-2,3-diaza-2,3-butadiene (L) as an organic linker for the preparation of amine- and azine-functionalized MOF, namely TMU-17-NH2. The detection of TNP at ppb levels in an aqueous media was then accomplished using this MOF in a sensitive and focused manner. In addition to the Schiff base group, this MOF also contains amine groups, which worked together to boost its sensitivity and selectivity for detecting TNP in the presence of interfering NEs. Among the chosen NEs, TNP had the lowest LUMO energy value (-4.0511 eV) and electrons were easily transferred from the HOMO of MOF to the LUMO of the electron-poor TNP. This was in line with the TMU-17-NH2 observed selectivity towards TNP. Moreover, the spectral overlap between the TNP absorption band and the TMU-17-NH2 emission profile was very large. As a result, energy transfer from TMU-17-NH2 to TNP occurred in accordance with the results that were shown. The other NEs, on the other hand, showed essentially no such overlap. Until now, the use of TMU-17-NH2 has only been limited to catalysis, adsorption, and sensing of inorganic species. This research has looked into the viability of employing TMU-17-NH2 for the detection of TNP in water in an effort to broaden its usefulness.
2.1. Materials
All reagents and chemicals were procured from diverse chemical suppliers as follows: Loba Chemie (Maharashtra, India), Avra synthesis Pvt. Ltd. (Bangalore, India), TCI Chemicals (Tamil Nadu, India), and Sigma-Aldrich. 2-nitrophenol (2-NP, 99%), nitrobenzene (NB, 98%), 1,3-dinitrobenzene (1,3-DNB, 98%), isonicotinic acid hydrazide (99%), 2,4,6-trinitrophenol (TNP, 99%), 4-nitrotoluene (4-NT, 98%) were purchased from Loba Chemie (Maharashtra, India). Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, 98%), 4-pyridine carboxaldehyde, (98%), 4-nitrophenol (4-NP, 99%),2,4-dinitrophenol (2,4-DNP, 96%), 3-nitrotoluene (2-NT, 98%), 2-nitrotoluene (2-NT, 98%) were supplied by the Avra synthesis. 2-aminoterphthalic acid (H2ata, 99%) was obtained from Sigma-Aldrich. TCI Chemicals (Tamil Nadu, India) supplied the 2,6-dinitrotoluene (2,4-DNT, > 99%), and 2,4-dinitrotoluene (2,6-DNT, > 99%). For the stock solution preparation and synthetic manipulation, triply distilled water was used.
2.2. Characterization
A FTIR Spectrophotometer (RXIFT, Perkin Elmer) was used to scan the FTIR spectra of the Schiff base ligand L and TMU-17-NH2 in the spectral range of 450–4000 cm− 1. The 1H and 13C nuclear magnetic resonance (NMR) spectra for the ligand L in CDCl3 solvent were obtained using a Bruker Avance NEO 500 MHz spectrometer calibrated with tetramethylsilane (TMS), a reference material. For the purpose of determining the crystal structure, powder X-ray diffraction (PXRD) patterns were acquired using an XPERT PRO Powder X-ray Diffractometer [CuK X-ray (= 1.5406); 1,800W (45 kV, 40 mA)]. For morphological and topographical examinations of TMU-17-NH2, a field emission scanning electron microscope (FE-SEM) (Hitachi SU8010 Series, Japan) was employed. Under a pure nitrogen atmosphere, a TGA (Hitachi STA7300, Japan) was conducted at a heating rate of 10oC/min in the temperature range of 35 to 700oC. A spectrophotometer (Shimadzu UV 1800 model) and a spectrofluorophotometer (Shimadzu RF-5301PC) were used for the absorption and photoluminescence studies, respectively.
2.3. Synthesis
2.3.1. Synthesis of 1,4-bis(4-pyridyl)-2,3-diaza-2,3-butadiene (L)
In a 100-mL round bottom flask, a combination of 4-pyridine carboxaldehyde (1.88 mL, 20 mmol) and hydrazine hydrate (80 in H2O) (0.490 mL, 10 mmol) were dissolved in methanol (30 mL) (Rachuri et al. 2018). For 24 hours, the contents of the flask were continuously stirred at room temperature. The mixture was allowed to cool to room temperature after the reaction. To get a pure product, gleaming yellow needle-shaped crystals were collected, filtered, washed with diethyl ether, and recrystallized using a chloroform/methanol (1/1) mixture. Yield = 89%.
2.3.2. Synthesis of TMU-17-NH2
The TMU-17-NH2 was prepared in accordance with the published protocol (Farahani &Safarifard 2019b). As shown in Fig. 1, the Zn(II) salt reacted with 2-aminoterphthalic acid (H2ata) ligand and Schiff base ligand (L) to synthesize the crystalline material, namely TMU-17-NH2. In a 15 ml DMF, H2ata (0.181 g, 1 mmol), L (0.210 g, 1 mmol), and Zn(NO3)2·6H2O (0.297 g, 1 mmol) were dissolved. In a Teflon reactor, the mixture was added and heated for three days at 80°C before being progressively cooled to ambient temperature over the course of 24 h. The crystals were obtained in a 33% yield.
2.4. Procedures for fluorescence measurements
A 2 mM standard aqueous stock solutions of 2,4-DNT, 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT), TNP, 2,6-DNT, 3-nitrotoluene (3-NT), 1,3-dinitrobenzene (1,3-DNB), and NB were prepared for the NEs sensing tests. The TMU-17-NH2 water suspensions were made by ultrasonically dispersing 3 mg of powdered samples of TMU-17-NH2 into 3 mL of water for 30 min. To determine the TMU-17-NH2 selectivity for TNP, 2 mM aqueous stock solutions of various NEs were gradually added from 75 to 375 µL to water suspensions of TMU-17-NH2 (i.e., 3 mg MOF dispersed in 3 mL water), yielding final concentrations ranging from 0.05 mM to 0.25 mM (via ultrasonication for 30 min). The emission measurements of the above generated suspensions with different concentrations were carried out in the spectral range of 350 − 650 nm. At the same time, the fluorescence intensity value of TMU-17-NH2 was monitored at 473 nm at an excitation wavelength of 325 nm. The quenching efficiency of fluorescence intensity was calculated by [(Io-I)/Io] ×100%, where Io and I denote the fluorescence intensity of TMU-17-NH2 without and with NE addition, respectively.
2.5. Computational methods
The GaussView version 5.08 programme was used to perform the theoretical calculations of numerous NEs used in the sensing experiment and the ligands of TMU-17-NH2 to support the experimental results with theoretical results for understanding the sensing mechanisms. Gaussian 09 Version was used to execute the theoretical calculations on the prepared input files. The Becke, 3-parameter, and Lee-Yang-Parr (B3LYP)-6311G(d,p) method was used to calculate the geometrical optimization, molecular electrostatic potential (MEP), Mulliken charges, the highest occupied molecular orbitals (HOMO), and lowest unoccupied molecular orbitals (LUMO) at DFT levels.
3.1. Characterizations of ligand L and TMU-17-NH2
3.1.1. 1H NMR
The formation of L ligand was established by the 1H NMR results and was recorded in CDCl3 solvent. The 4-bpdb ligand being a symmetrical molecule, its proton count comes out to be half because all the protons at symmetrical position resonates at same position in 1H NMR. As shown in Fig. 2 and Table 1, a sharp singlet appearing at 8.57 ppm was attributed to the imine –CH = N- proton and confirmed the L ligand formation (Pal et al. 2017). Two proton double doublet located at 8.76 ppm was ascribed to the H-2 and H-6 protons of the pyridine ring directly connected to the nitrogen of the pyridine ring. Another two proton double doublet appeared at 7.70 ppm was attributed to the H-3 and H-5 protons of the pyridine ring (Pal et al. 2017). Sharp peak located at 7.27 ppm was ascribed to the solvent CDCl3. The absence of extra peaks confirmed the high purity of the synthesized ligand.
|
Structure of 4-bpdb |
Spectral data |
|---|---|
|
|
1H-NMR (500 MHz, CDCl3): δ ppm = 7.70 (2 H, dd, Jo = 5.05 Hz, H-3, -5), 8.57 (1H, s, -CH = N-), 8.76 (2H, dd, Jo = 4.6 Hz, H-2, -6). |
|
13C-NMR (500 MHz, CDCl3): δ ppm = 122.11 (4 C, s, C-3, -5, -3’, -5’), 140.56 (1 C, s, C-4, -4’), 150.57 (4 C, s, C-2, -6, -2’, -6’), 160.34 (1 C, s, -CH = N-). |
3.1.2. 13C NMR
The L synthesis was also verified by 13C NMR and carried out in CDCl3 solvent at 500 MHz. As shown in Fig. 3 and Table 1, the imine carbon of –CH = N- group of the Schiff base ligand appeared as a sharp singlet at 160.34 ppm, which confirmed the formation of L. Four carbon atoms of the two pyridine rings directly connected to the pyridyl nitrogen atoms appeared as a sharp singlet at 150.57 ppm (Pal et al. 2017). Two carbon atom of the pyridyl rings linked with the imine carbon atoms seemed as a singlet at 140.56 ppm (Pal et al. 2017). The rest of the four carbon atoms of the pyridyl rings appeared as a singlet at 122.11 ppm (Pal et al. 2017). The carbon of CDCl3 solvent appeared as a triplet at 77.41 ppm (Pal et al. 2017).
3.1.3. FTIR
The FTIR analysis was performed to check the formation of TMU-17-NH2 using Zn(II) metal node with H2ata ligand and Schiff base ligand L. The FTIR spectra of L displayed a sharp signal at 1615 cm− 1 ascribed to C = N stretching of the azine (–CH = N-N = CH-) group, demonstrating the effective synthesis of L (Fig. 4a). This azine band remain intact in the FTIR spectra of TMU-17-NH2 indicates that azine group remain free in the pore cages of MOF. Moreover, the FTIR spectra of TMU-17-NH2 showed the two signal near 3,436 and 3,488 cm–1 which were ascribed to v –NH2 stretching vibrations. As such, this confirmed the presence of free amine groups in the pore cages of this MOF. Further, symmetric and antisymmetric metal carboxylate stretching bands at 1,380 and 1,587 cm–1 are also detected in the FTIR spectra of TMU-17-NH2 (Fig. 4b). The difference of these bands was found to be 193 which suggests a chelating bidentate coordination mode of the carboxylate moieties of H2ata ligand in this MOF. The presence of Zn-N band at 480 cm− 1 showed the coordination of L with Zn through pyridyl nitrogen atom. All these FTIR results confirmed the successful synthesis of TMU-17-NH2 from L and H2ata.
3.1.4. PXRD
The phase purity and crystallinity of the synthesized material were assessed using PXRD analysis. The highly crystalline structure of the synthesized MOF was evident from the strong peaks that were seen in the recorded PXRD pattern, as shown in Fig. 5a. The successful synthesis of TMU-17-NH2 was confirmed by the exact matching of the diffraction peaks at the 2θ scale of the experimentally synthesized MOF with the standard diffraction pattern of its single crystal structure. The lack of additional impurity peaks further supports the phase purity of the synthesized material. PXRD results also revealed that the TMU-17-NH2 has a three dimensional cubic lattice structure. Each cubic structure length and width made by the 8 H2ata ligands while height made by the eight L ligands. These cubic structures aggregate together results in the formation of three dimensional network. In three dimensional network, each Zn metal node exhibits a distorted octahedral coordination geometry (ZnN2O4) (Fig. 6). Two oxygen atoms of two bridging bidentate carboxylate groups and two oxygen atoms of one chelating bis-bidentate carboxylate group coordinates with the Zn center resulting the formation of two dimensional layers. These layers are further pillared by the two nitrogen atoms of L ligand in the axial direction (Fig. 7). As amine and azine groups of H2ata and L are not involved in framework formation, the pore surface of TMU-17-NH2 is decorated with pendant amine (-NH2) and azine (= N-N=) functional groups.
3.1.5. Thermogravimetric analysis (TGA)
To evaluate the thermal stability for further utilization, thermogravimetric analysis (TGA) of TMU-17-NH2 was carried out under flowing N2 atmosphere with the temperature range from 35 to 600 ℃ (Fig. 5b). The TGA curve of TMU-17-NH2 showed two-stage weight loss with 22% residual. The first one is the elimination of the trapped solvents in the pores over the temperature range of ∼35–270 ℃ to form the guest-free form, [Zn(H2ata)(L)]. And the significant mass reduction after 300 ℃ may occur due to the decomposition of organic ligand of MOF framework. These results indicate that TMU-17-NH2 material is thermally stable up to 300 ℃, above which the thermal degradation of MOF begins.
3.1.6. FE-SEM
FE-SEM micrographs were recorded to examine the topographical and morphological characteristics of the TMU-17-NH2. Figures 8a-d represents the FE-SEM micrographs of TMU-17-NH2 at different magnifications of × 22,000, × 60,000, × 70,000, and × 80,000. From these micrographs, it was observed that the TMU-17-NH2 had a smooth surface with plate-like morphology.
3.1.7. Photoluminescent (PL) properties
MOFs assembled from d10 metal ions and conjugated organic ligands are known to exhibit excellent photoluminescent (PL) properties (Farahani &Safarifard 2019a). The water suspensions of TMU-17-NH2 showed emission intensities at 473 nm under the same excitation wavelength of 325 nm.
3.2. Fluorescence detection of NEs by TMU-17-NH2
3.2.1. Selectivity of TMU-17-NH2
To examine the potential of TMU-17-NH2 as a fluorescent sensor for NEs in the aqueous phase, the luminescent emission profile of TMU-17-NH2 upon subsequent addition of different NEs into their aqueous suspensions has been investigated at an excitation wavelength of 325 nm (Figs. 9a-f and 10a-d). According to the luminescent quenching test, only TNP causes the significant turn-off quenching effect on the fluorescence intensity of TMU-17-NH2 (Figs. 11a). In contrast, the rest of other NEs had a low or negligible quenching effect on the luminescence of TMU-17-NH2 (Table 2). It was found that TNP displayed the highest quenching in TMU-17-NH2. When TNP was gradually added to TMU-17-NH2 water suspensions (3 mg/3 mL) at concentrations of 0.05, 0.10, 0.15, 0.20, and 0.25 mM, the corresponding fluorescence quenching efficiencies were 62, 69, 72, 77, and 85%, respectively. To assess the impact of interferences, the fluorescence emission intensity of TMU-17-NH2 for TNP was also evaluated in the presence of different NEs. Notably, the presence of other NEs had no impact on TNP detection ability of TMU-17-NH2 (Figs. 11b). In the case of NPs, the quenching efficiencies declined in the order: TNP > 2,4-DNP > 4-NP > 2-NP, which was exactly in line with the acidity of these NEs acidic protons (Fig. 12).
|
Order |
Analyte |
TMU-17-NH2 |
|
|---|---|---|---|
|
Quenching (%) |
Quenching constant (KSV) |
||
|
1. |
TNP |
85 |
3.26 × 104 |
|
2. |
2,4-DNP |
45 |
3.62 × 103 |
|
3. |
4-NP |
28 |
3.11 × 103 |
|
4. |
2-NP |
22 |
2.48 × 103 |
|
5. |
2,6-DNT |
24 |
1.75 × 103 |
|
6. |
2,4-DNT |
29 |
1.62 × 103 |
|
7. |
4-NT |
26 |
2.66 × 103 |
|
8. |
3-NT |
29 |
1.67 × 103 |
|
9. |
2-NT |
23 |
1.21 × 103 |
|
10. |
1,3-DNB |
21 |
1.34 × 103 |
|
11. |
NB |
27 |
1.23 × 103 |
3.2.2. Optimization of parameters for the TMU-17-NH2 sensor for highly selective detection of TNP
It is crucial to optimize the crucial parameters (such as response time and pH) that could exert controls on the interaction of TNP with TMU-17-NH2 sensor in order to achieve the extremely sensitive detection of TNP. To access the kinetics of TMU-17-NH2 sensor, the luminescence intensity of TMU-17-NH2 (3 mg) dispersed in a 2 mM standard aqueous solution of TNP (3 mL) was examined at different time intervals. As shown in Figs. 13a, the change in fluorescence intensity (I0-I) reached a constant value after 10 min. Hence, 10 min was selected as incubation time. Therefore, for further analysis of TNP sensitivity, a minimum of 10 min ultrasonication was done to achieve the maximum interactions of TNP with this MOF sensor to induce the maximum fluorescence quenching.
The pH of the solution is important for the detection of TNP because it controls the ionization of TNP in the solution as well as the surface charge of the MOF sensor. To fully understand the impact of pH on the efficacy of the TMU-17-NH2 sensor fluorescence quenching, the pH was varied in the range of 3 to 12. Using 0.1 M NaOH or HCl, 3 mL of a 2 mM standard aqueous solutions of TNP containing 3 mg of TMU-17-NH2 of pH 3–12 were created (Figs. 13b). The free amine and azine sites of the TMU-17-NH2 sensor are positively charged in acidic circumstances (pH > 7), and the hydroxyl groups of TNP molecules are likewise positively charged via protonation. As a result, the interactions between TNP and MOF sensor are weakened because the positively charged surfaces of the sensor or TNP molecules repel one another. These weakened interactions suppress the quenching response of TMU-17-NH2 towards TNP. On the other hand, the TMU-17-NH2 surface is negatively charged at pH (< 7) due to proton abstraction from the amine sites by the base. Moreover, TNP molecules also become negatively charged due to abstraction of the highly acidic proton of hydroxyl group of TNP by the base. It is worth noting that the negatively charged TMU-17-NH2 repel the negatively charged TNP molecules. As such, it can prevent the entry of TNP molecules within pore cages of TMU-17-NH2 where binding sites are present. This may further suppress the quenching response of TMU-17-NH2 for TNP. Therefore, the solution of pH was maintained at 7 to obtain the maximum fluorescence quenching.
3.2.4. Fluorescence stability
To examine the storage ability of the fluorescent sensors, the fluorescence stability is significant parameter. The fluorescence stability of TMU-17-NH2 sensor was investigated by measuring the luminescence intensity of its aqueous suspension at regular interval of 5 min. As shown in Fig. 13c, TMU-17-NH2 sensor showed a stable emission within 25 min. This reveals a good physical stability and uniform dispersion of TMU-17-NH2 in the aqueous solution. Additionally, the frequent determination of the fluorescence intensity was also verified after one week. It was found that TMU-17-NH2 sensor retained the 93.6% of its fluorescence emission intensity to confirm their reliable storage stability.
3.2.3. Sensitivity of TMU-17-NH2 towards TNP
The sensitivity of TMU-17-NH2 towards TNP ions was studied by the measurement of their fluorescence quenching effects as a function of TNP concentration in the range of 0.05–0.25 mM (Figs. 9a and 11c). It was found that the as the concentration of TNP increased, the fluorescence intensity of TMU-17-NH2 steadily reduced. The Stern-Volmer equation, I0/I = 1 + Ksv[A], was used to further simplify and compute the fluorescence quenching response of TMU-17-NH2 for TNP. In this instance, I0 and I stand for the fluorescence intensities of sample before and after TNP addition (having molar concentration [A]), respectively. Stern − Volmer constant/quenching constant is denoted by the Ksv. For TMU-17-NH2, Stern-Volmer plot was linear (Fig. 11c). The considerable quenching impact of TNP on the fluorescence intensity of TMU-17-NH2 is indicated by high value of quenching constant (Ksv) computed as 3.26 × 104 M− 1. According to Section S1 summary, the LOD of TMU-17-NH2 for TNP was calculated with mathematical formula LOD = 3σ/ m, where m is the calibration slope of the linear curve and σ is the standard deviation from five blank measurements for TMU-17-NH2. The LOD of TMU-17-NH2 was estimated as 9.4 ppb (0.041 µM), suggesting its excellent potential for sensing TNP in aqueous media (Section S1 and Figure S1). TMU-17-NH2 potential as a turn-off sensor for quick detection of TNP in aqueous solutions is shown by a number of its beneficial characteristics, including outstanding Ksv, low LOD values, and little interference effects.
3.2.5. Mechanism of fluorescence detection
TMU-17-NH2 sensing mechanisms towards TNP were investigated to determine their suitability for sensing applications. The following are the basic mechanisms that regulate the turn-off quenching between these electron-rich MOFs and the electron-deficient TNP: (1) crystalline MOF structure collapse; (2) photo-induced electron transfer (PET); and (3) fluorescence resonance energy transfer (FRET). To understand the quenching mechanism, the PXRD data of TMU-17-NH2 sample was collected after TNP sensing. Note that the TNP-treated TMU-17-NH2 PXRD pattern remain unaltered (e.g., in comparison to their experimentally synthesized form). This finding suggests that in the process of TNP sensing, the fundamental structure of TMU-17-NH2 MOF should be preserved. Therefore, the collapse of the framework is not the cause of the fluorescence quenching of TMU-17-NH2 (Figure S2). Since the crystallinity was preserved even after the sensing experiment, the addition of the TNP is unlikely to compromise the MOF structural integrity.
PET has been suggested as the cause of several other MOFs strong selectivity against electron-deficient NEs (Dalapati &Biswas 2017). Due to its abundance of electrons, TMU-17-NH2 can readily transfer electrons to electron-deficient NEs. As a result, such electron transfer is thought to be responsible for the quenching of TMU-17-NH2 fluorescence. In general, the LUMOs of electron-deficient NEs are located between the TMU-17-NH2 conduction band (CB or HOMO) and the valence band (VB or LUMO) (Dalapati &Biswas 2017). Therefore, an electron transfer process from the HOMO of TMU-17-NH2 to the LUMO of the electron-deficient NEs could have happened quickly. This could have resulted in a decrease in fluorescence intensity upon excitation. To validate the PET mechanism relevant for this turn-off quenching, EHOMO and ELUMO energies were calculated using Gaussian software. First, using B3LYP/6-311G(d, p) basis set and the Gaussian 09 package program, the structures of ligands (L and H2ata) and diverse NEs used in this study were optimized at DFT level. Figure 14 displays the ligands optimized structures along with those of several NEs. These optimized structures were used to calculate the EHOMO and ELUMO energies as well as the energy gaps (eV) of several NEs with the L and H2ata ligands (Fig. 15 and Table S1). PET easily occurred from the HOMO of TMU-17-NH2 to the LUMO of the electron-deficient TNP (e.g., relative to the other NEs) as the TNP molecule had the lowest LUMO energy value (-4.0511 eV) among all the selected NEs. This result was consistent with the greatest quenching efficiency for TNP found. Thus, it is concluded that TMU-17-NH2 can be used for the selective detection of TNP.
For FRET to occur, there must be an effective spectral overlap between the fluorophore emission spectrum and the analyte absorption spectrum. To further understand the FRET mechanism, the normalized absorption spectra of several NEs and the emission spectra of TMU-17-NH2 in water suspensions were examined. As a result, there was a superb spectrum overlap between the TNP absorption band and the TMU-17-NH2 emission profiles. The other NEs, however, showed essentially no such overlap (Fig. 16). The best quenching efficacy of TNP was correlated with its largest spectrum overlap. Accordingly, the fluorescence quenching by TNP should take place under the FRET mechanism. Generally speaking, this mechanism is a long range process that can cause TMU-17-NH2 to lose its fluorescence. However, to function such long-range FRET, the relative or appropriate orientation of the donor and acceptor sites of TMU-17-NH2 and NEs, respectively, is essential. An appropriate relative orientation of donor amine and azine sites of TMU-17-NH2 and acceptor phenolic site of TNP in this study likely contributed to the achievement of the strong hydrogen bonding interactions between the phenolic group of TNP and the azine moiety of L/amine sites of H2ata, as reported previously in the literature (Figs. 17a-d) (Parmar et al. 2017, Parmar et al. 2016).
To create H-bonds between the MOF and the analyte, the ligands used in its production must have such bonding sites. It is well known that Mulliken charges, or the net atomic charges on the atoms of a molecule, are essential for predicting the locations of H-bonding interaction sites. (Govindasamy et al. 2014). H-bonding interactions can occur between heteroatoms having a strong negative charge. To investigate the H-bonding sites in TMU-17-NH2, the Mulliken charges of the ligands (L and H2ata) employed in the TMU-17-NH2 synthesis were computed (Figs. 18a,b). The large negative Mulliken charge was found to be mostly centered on the nitrogen atoms of the azine groups of L and amine groups of H2ata that are freely distributed in the pore cages of TMU-17-NH2. We deduced from these Mulliken charges that the negatively charged nitrogen atoms in L azine groups or H2ata amine groups are most suited for hydrogen bonding interactions with the positively charged and highly acidic phenolic protons in TNP. However, as reported in the literature, the positively charged acidic phenolic protons are absent in other NEs (Nagarkar et al. 2015). As a result of hydrogen-bonding interactions, the extremely acidic TNP can selectively and strongly engage with the pendant Lewis basic amine and azine group to produce an enhanced quenching response. Because these interactions do not exist for the other nitro analytes, they have a low quenching effect.
MEP can also be used to predict the locations of H-bonding interactions. Each colour indicates a different electrostatic potential over the molecular surface on the MEP mapped surface of molecules. A blue MEP plot, for example, depicts a zone of positive electrostatic potential, whereas red and yellow colors suggest a region of negative electrostatic potential. DFT calculations were used to study the MEP areas for the L and H2ata utilizing the optimized geometry of B3LYP/6-311G(d, p) (Figs. 18c,d). The DFT results showed that the red and yellow sites in the L were primarily situated on the nitrogen atoms of the azine groups to represent the electron-acceptor behavior of these atoms in H-bonding. Based on these findings, we inferred that TMU-17-NH2 Lewis basic amine and azine capabilities should serve as TNP recognition sites. A combination of electron-transfer and energy-transfer via PET, FRET, and H-bonding quenching mechanisms, on the other hand, is likely to contribute to the exceptional and synergistic selectivity for TNP in the aqueous phase.
3.2.6. Performance evaluation
As documented in Table 3, the majority of MOF-based sensors for TNP have been made using luminescent d10- and lanthanide-based MOFs. However, the majority of these MOFs are employed in organic solvents like acetonitrile (ACN), methanol (MeOH), and dimethyl amide (DMA). This is mostly due to their weak aquatic environment stability, which can limit their practical usage (Bagheri et al. 2016, Hong et al. 2017, Zhang et al. 2016). It is crucial to note that just a few MOFs have been utilized to detect TNP in aqueous conditions (Table 3). The fundamental QA metrics like LOD can be used to access the sensing performance of these luminous sensors. When compared to other MOF sensors that had previously been used for the detection of TNP in the aqueous phase, the LOD of the TMU-17-NH2 used in this study was significantly lower (Table 3). As shown in Table 3, the LOD value of TMU-17-NH2 (0.04 µM) is better than those reported from other studies such as [Cd(L10)(L11)]n (0.27 µM) (Rachuri et al. 2016), [(CH3)2NH2]3[Zn4Na(L7)3]·4CH3OH·2DMF (5 µM) (Zhou et al. 2015), {[Zn(L1)(L2)]}n (0.12 µM) (Parmar et al. 2017), [Zn4(DMF)(urotropine)2(L12)4] (7.1 µM) (Mukherjee et al. 2015), {[Cd(L1)(L2)]}n (0.06 µM) (Parmar et al. 2017), and [Eu3(L8)3(HCOO(µ3-OH)2(H2O)].x solvent (4.98 µM) (Song et al. 2014),. As such, TMU-17-NH2 appear as excellent sensor for sensitive identification of TNP in aqueous systems.
|
Sr. No. |
Material (LCP/LMOF) |
Solvent |
Ksv, M− 1 |
LOD (µM) |
Reference |
|---|---|---|---|---|---|
|
1. |
{[Zn(L1)(L2)]}n |
H2O |
1.16 × 104 |
0.12 |
(Parmar et al. 2017) |
|
2. |
{[Cd(L1)(L2)]}n |
H2O |
1.35 × 104 |
0.06 |
(Parmar et al. 2017) |
|
3. |
[Cd4(L3)2(L4)3(DMA)2].(H2O)4 |
DMA |
4.8 × 104 |
0.29 ppm |
(Hong et al. 2017) |
|
4. |
[Cd4(L3)2(L5)3(DMA)2].(H2O)4 |
DMA |
2.5 × 104 |
1.3 ppm |
(Hong et al. 2017) |
|
5. |
{[Eu2(L6)3(CH3OH)2]·CH3OH} |
MeOH |
1.6 × 104 |
9.5 |
(Zhang et al. 2016) |
|
6. |
TMU-5S |
CAN |
- |
- |
(Bagheri et al. 2016) |
|
7. |
[(CH3)2NH2]3[Zn4Na(L7)3]·4CH3OH·2DMF |
H2O |
3.2 × 104 |
5 |
(Zhou et al. 2015) |
|
8. |
[Eu3(L8)3(HCOO(µ3-OH)2(H2O)]·(x solvent) |
H2O |
2.1 × 104 |
4.98 |
(Song et al. 2014) |
|
9. |
Zr6O4(OH)4(L9)6 |
H2O |
5.8 × 104 |
1.7 |
(Nagarkar et al. 2015) |
|
10. |
[Cd(L10)(L11)]n |
H2O |
2.68 × 104 |
0.27 |
(Rachuri et al. 2016) |
|
11. |
[Zn4(DMF)(urotropine)2(L12)4] |
H2O |
10.83 × 104 |
7.1 |
(Mukherjee et al. 2015) |
|
12. |
TMU-17-NH2 |
H2O |
3.26 × 104 |
0.041 |
This work |
|
H2L1 = isophthalic acid; L2 = (E)-N'-(pyridin-3-ylmethylene)nicotinohydrazide; L3 = 4-(2- hydroxy-3-methoxy-benzyli-denehydrazino-carbonyl)-N-pyridin-4-yl-benzamide, H2L4 = 2-aminoterphthalic acid, H2L5 = 1,4-benzene-dicarboxylic acid, L6 = thiophene2,5-dicarboxylate; L7 = biphenyl-3,3',5,5'- tetracarboxylate; L8 = 4,4′-(4,4′-bipyridine-2,6-diyl)dibenzoate; L9 = 2′-amino-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylate; L10 = 5-bromo isophthalate; L11 = 1,3,5-tris- (imidazol-1-ylmethyl)benzene; L12 = 2,6-naphthalenedicarboxylate. |
|||||
In this work, the chemically and thermally stable amine/azine functionalized Zn(II) metal-organic framework namely TMU-17-NH2 was successfully synthesized by solvothermal approach. Notably, the water suspensions of TMU-17-NH2 displayed extremely sensitive quenching responses toward TNP to reflect the powerful role of the PET/FRET processes and the electrostatic interactions (i.e., H-bonding). As a result, even in the presence of competing nitro explosives, the fluorescence quenching remained essentially unaffected. The LOD of TMU-17-NH2 for TNP was estimated as 9.4 ppb (0.04 µM). By carefully choosing ligand moieties with the appropriate functional properties, we anticipate that our work will open up new possibilities for the design and synthesis of strong MOFs with chemical stability, enabling sensitive and precise sensing applications towards TNP in aqueous conditions.
Acknowledgements
The authors thank the Chemistry Department, Punjabi University, Patiala, for providing the laboratory and instrument facilities. Manpreet Kaur is also grateful for the UGC-SRF fellowship, UGC, New Delhi, India.
Ethical approval
There is no ethical approval required.
Consent to participate
All authors give consent to participate in the revision of the manuscript.
Consent to publish
All authors give consent to publish the paper.
Author Contributions
Manpreet Kaur has performed the experiment and written the paper. Ashok Kumar Malik has supervised the whole work.
Funding
No Funding available to support this work.
Competing Interests
“The authors have no relevant financial or non-financial interests to disclose.”
Availability of Data and materials
All relevant data will be provided on request.
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