Article pubs.acs.org/IC
H‑Bonding Interactions Induced Two Isostructural Cd(II) Metal− Organic Frameworks Showing Different Selective Detection of Nitroaromatic Explosives Zhong-Jie Wang,† Ling Qin,† Jin-Xi Chen,‡ and He-Gen Zheng*,† †
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China ‡ School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China S Supporting Information *
ABSTRACT: Two luminescent Cd(II) metal−organic frameworks (MOFs) were prepared from electron-rich π-conjugated fluorescent ligands. They are isostructural with sql nets. Their strong luminescences can be quenched by a series of nitroaromatic explosives. Notably, MOF 1 shows highly selective and sensitive detection of 4-nitrophenol (4-NP), while MOF 2 exhibits good responses toward picric acid (PA) compared with other nitroaromatic explosives. This different order of quenching efficiency is because there are H-bonding interactions between MOF 1 and 4-NP, while MOF 2 lacks these H-bonding interactions. MOF 1 displays highly selective and sensitive detection of 4-NP with the high quenching constant (6.74× 104 M−1) and low detection limit (34.48 ppb), which is better than those of known MOFs. MOF 1 and MOF 2 have highly sensitive and selective detection of 4-NP and PA in the practical application, respectively.
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detection of 4-NP with very high Ksv values (4.2 × 104 and 4.7 × 104 M−1, respectively). However, they did not study the quenching mechanism and PA, which may show higher photoluminescence quenching than 4-NP. So, it has become more pressing to discover new MOFs for selectively detecting 4-NP from other nitroaromatic explosives and to study the quenching mechanism. In most cases, the mechanism of detecting nitroaromatic explosives is an oxidative quenching mechanism.4 It is an effective strategy to synthesize MOFs utilizing electron-rich π-conjugated fluorescent ligands used in luminescent sensors. But there are few researches about the role of H-bonding interactions, one kind of electrostatic interaction, in the quenching mechanism. Herein, two luminescent MOFs, [Cd3(BPPA)3(aba)3]n (MOF 1) and [Cd3(BPPA)3(oba)3]n (MOF 2; BPPA = bis(4-(pyridine-4-yl)phenyl)amine), H2aba = 4,4′-azanediyldibenzoic acid, H2oba = 4,4′-oxidibenzoic acid) were synthesized. MOF 1 has highly sensitive detection of 4-NP from other nitroaromatic explosives, and MOF 2 exhibits good responses toward PA compared to others. Choosing these materials to construct MOFs is based on four major considerations: (1) BPPA is an electron-rich π-conjugated fluorescent ligand with good electron-transferring ability. (2) “V-shape” semirigid ligands have advantages to build MOFs with varied charming
INTRODUCTION Rapid and selective detection of nitroaromatic explosives has become a pressing issue because of the increasing concern about environment security.1 Many ways are been developed to detect nitroaromatic explosives, such as gas chromatography coupled with mass spectrometry, plasma desorption mass spectrometry, X-ray imaging, energy-dispersive X-ray diffraction, ion mobility spectrometry, surface-enhanced Raman spectroscopy, and other analysis methods.2 These methods usually have high selectivity and sensitivity, but the shortcomings are high cost, time-consuming, not easy to operate, and assemble in a small and low-power package. Therefore, another new technology needs to be developed so that we may inexpensively and rapidly complete detection. Recently, significant progress3 has been made in the application of luminescent metal−organic frameworks (MOFs) for detection explosives, due to their easy synthesis, low cost, and easy modification. Many MOFs have been employed for detection of nitroaromatic explosives, but most of them have only selective and sensitive detection of picric acid (PA).4,5 To our knowledge, there are few MOFs applied to effectively detect 4nitrophenol (4-NP). In the previous works, selectivity and sensitivity of detecting 4-nitrophenol from other nitroaromatic explosives by luminescent MOFs are unsatisfactory.6 For example, Sun et al. reported a luminescent Tb MOF, which exhibited selective sensing of 4-NP with a high Ksv value (7.4 × 103 M−1). Zhou et al. reported two stable Zr MOFs for the © XXXX American Chemical Society
Received: June 26, 2016
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DOI: 10.1021/acs.inorgchem.6b01521 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic views of 2D framework of MOF 1; (b) schematic representation of H-bonding interactions Symmetry code: #1 = x, −y + 2, z − 0.5; (c) views of H-bonding interactions in MOF 1. The H-bonding interactions are shown with red dash lines. Symmetry code: #1 = x − 1, y, z.
with Cd centers to form two-dimensional (2D) nets (Figure S3). To better understand the nature of this framework, a TOPOS analysis is provided by software TOPOS 4.0.7 The Cd centers act as four-connected nodes, and BPPA ligands act as linkers. MOF 1 represents four-connected sql net with point symbol of 44·62. The 2D nets are stacked in an ABCD··· manner (Figure 1a). A and C represent 2D interpenetrating layers (2D + 2D → 2D), which are both formed by ligands, Cd2, and Cd3, while B and D represent 2D nets formed by ligands and Cd1. Importantly, A and C are in mirror symmetry, and so are B and D. As shown in Figure 1 and Table S3, these neighboring 2D layers are extended into three-dimensional (3D) supramolecular networks when considering H-bonding interactions (N2−H2A···O1#1 = 2.840(6) Å, N6−H6A···O9#2 = 2.833(6) Å, N10−H10A···O5#3 = 2.881(6) Å, N4−H4A··· O10#4 = 3.231(5) Å, #1: x, −y + 2, z − 0.5; #2: x − 1, y, z; #3: x, −y + 2, z − 0.5; #4: x − 1, y, z). N2, N6, and N10 are all from BPPA, N4, and the O atoms from H2aba. BPPA more easily acts as electron donors to form H-bonding interactions because electron-withdrawing effects of pyridine groups are stronger than those of carboxylic groups. Crystal Structure of [Cd3(BPPA)3(oba)3]n (MOF 2). Single-crystal structure analysis reveals that MOF 2 and MOF 1 are isostructural with sql nets and crystallize in the monoclinic crystal system with Cc space group. Adjacent layers are packed into 3D supramolecular structure by hydrogenbonding interactions (N2−H2A···O2#5 = 2.838(6) Å, #5: x , − y + 1, z − 0.5; N5−H5A···O12#6 = 2.851(5) Å, #6: x − 0.5, −y + 1.5, z + 0.5; N8−H8A···O6#7= 2.837(4) Å, #7: x, −y + 1, z + 0.5). Interestingly, there are six kinds of catenation: 6−6 Hopf, 6−7 Hopf, 6−8 Hopf, 7−7 Hopf, 7−8 Hopf, and 8−8 Hopf in
structure, because the rotation of the C−N or C−O single bonds between benzene rings and amino group or ether group can adjust the coordination orientations. (3) N atoms from amino group in BPPA and H2aba can act as electron donors to form H-bonding interactions, which not only benefit to construct MOFs with charming structure and good stability but also provide possibility to form H-bonding interaction with nitroaromatic explosives to improve the ability of detecting them. (4) MOFs built from π-conjugated ligands and d10 metal centers, such as Cd, are promising to have excellent photoluminescence properties. As we expected, the two MOFs display fascinating structures and ability of nitroaromatic explosives detection.
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RESULTS AND DISCUSSION Crystal Structure of [Cd3(BPPA)3(aba)3]n (MOF 1). Single-crystal structure analysis reveals that MOF 1 crystallizes in the monoclinic crystal system with Cc space group. The asymmetric unit of MOF 1 contains three crystallographically independent Cd cations, three BPPA ligands, and three aba2− anions. It is interesting that the three crystallographically Cd cations cannot connect with each other without considering other interactions, such as H-bonding or π−π stacking interactions. And this structure has rarely been reported before. Cd1, Cd2, and Cd3 are five-coordinated/six-coordinated/fourcoordinated by three/four/two oxygen atoms from two H2aba ligands and two nitrogen atoms belonging to two BPPA ligands. There are three uncoordinated oxygen atoms (O1, O9, and O12) in carboxylic groups. The Cd−O distances range from 2.210(3) to 2.595(3) Å, and Cd−N distances range from 2.239(3) to 2.294(4) Å. The BPPA and H2aba ligands connect B
DOI: 10.1021/acs.inorgchem.6b01521 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. (a) Photoluminescence spectra and (b) SV plot of MOF 1 by gradual addition of 4-NP in DMF (0.5 mM). (inset) Emission quenching linearity relationship at low concentrations of 4-NP; (c) percentage of photoluminescence quenching of MOF 1 and MOF 2 upon addition of different explosive analytes.
trations of nitroaromatic explosives in DMF solution (5 × 10−4 M) were prepared to monitor the photoluminescence response. As shown in Figure 2a, the photoluminescence intensities decrease sharply with increasing 4-NP in MOF 1. To our surprise, only upon gradual addition of 120 μL of 4-NP solution (5 × 10−4 M), the photoluminescence intensity decreases to 14.59% for MOF 1. The photoluminescence quenching by 4NP could be determined at very low concentration (0.25 μM) for MOF 1 (Figure 2). In other words, the detection limit of 4NP is 34.48 ppb, which is lower than those of known MOFs.6 It is more interesting that other nitroaromatic explosives show minor effects on the photoluminescence intensity of MOF 1, compared with 4-NP (Figures S9−S16). These results indicate that MOF 1 has high sensitivity and selectivity to detect 4-NP. The photoluminescence quenching efficiency can be quantitatively explained by the Stern−Volmer (SV) equation: (I0/I) = 1 + Ksv [Q], in which Ksv is the quenching constant (M−1), [Q] is the molar concentration of the nitroaromatic analyte, and I0 and I are the photoluminescence intensities before and after adding the nitroaromatic analyte, respectively. At low concentrations, the SV plots of 4-NP display nearly linear (R2 = 0.9981), as shown in Figure 2b. However, at higher concentrations, the SV plots subsequently deviate from linearity and bend upward, which may be due to self-absorption.8 The SV plots of all the other nitroaromatic explosives are also nearly linear with gradual addition of corresponding solution (Figures
MOF 2 considering H-bonding interactions (Figure S4e). The N atoms acting as electron donors to form H-bonding interactions are from BPPA, which show the O atoms from H2oba are hardly as electron donors in MOF 2. There are some differences between the two MOFs. (1) MOF 1 owns three uncoordinated oxygen atoms (O1, O9, and O12) in carboxylic groups, while MOF 2 is totally coordinated. (2) There are two free amino groups (N8 and N12) in MOF 1, which can act as electron donors to form H-bonding interactions with others. MOF 2 has no free amino groups. Detection of Nitroaromatic Explosives. To examine their luminescence properties, the solid-state luminescence of MOF 1, MOF 2, and free BPPA ligand are investigated at room temperature. The solid-state luminescences of MOF 1 and MOF 2 are very similar (Figure S8). The emission peak center of MOF 2 is at 514 nm, which is slightly blue-shifted compared with MOF 1 (λem = 525 nm). This can be assigned to the weaker conjugative effect of H2oba than H2aba. Considering environmental security and the strong emission of MOF 1 and MOF 2, we investigate their possibilities of sensing nitroaromatic explosives, including 4-NP, PA, 2,4-dinitrophenol (2,4-DNP), 3-nitrophenol (3-NP), nitrobenzene (NB), 1,3dintrobenzene (1,3-DNB), 1,4-dintrobenzene (1,4-DNB), 4nitrotoluene (NT), and 2,4-dinitrotoluene (2,4-DNT); in detail, a series of suspensions of MOF 1 and MOF 2 in dimethylformamide (DMF) with gradually increasing concenC
DOI: 10.1021/acs.inorgchem.6b01521 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. (a) Photoluminescence spectra and (b) SV plot of MOF 2 by gradual addition of PA in DMF (0.5 mM).
Figure 4. (a) UV−vis spectra of MOF 1 in DMF and upon addition of 4-NP; (b) spectral overlap between normalized absorbance spectra of explosive analytes and the normalized emission spectra of MOF 1 and MOF 2 in DMF.
S9−S16). MOF 1 shows the highest Ksv values (6.74 × 104 M−1) with 4-NP in the nitroaromatic analytes. The quenching constant of 4-NP is ca. 5 times greater than that of PA and at least an order greater than others. To the best of our knowledge, these values are the highest for the known luminescent MOF-based sensors for 4-NP.6 Low detection concentration and high quenching constant of 4-NP show MOF 1 can easily detect a trace quantity of 4-NP. In addition, the emission intensity of MOF 1 decreases continuously upon exposure to the 4-NP vapor by reported methods.9 Within 10 min, 4-NP quenches the emission by as much as 78.3%, indicating that MOF 1 can detect trace 4-NP in the vapor (Figure S17). When gradually adding 600 μL of PA solution (5 × 10−4 M), the photoluminescence intensity decreases to 11.28% for MOF 2 (Figure 3a). The photoluminescence quenching by PA could be determined at very low concentration (2.5 μM) for MOF 2 (Figure 3b). It indicates that MOF 2 can detect PA, which is comparable with known organic polymers.4 The coexistence of electron transfer and resonance energy transfer makes PA exhibit high photoluminescence quenching compared to others. As shown in Figure 2c, the order of quenching efficiency for MOF 2 is PA > 2,4-DNP > 4-NP > 1,4-DNB > 3-NP > 2,4DNT > 1,3-DNB > NT > NB, which is different from that for MOF 1.
The Mechanism of MOF 1 for Sensing Nitroaromatic Explosives. In most cases, MOFs have the highest selective and sensitive detection of PA compared with other nitroaromatic explosives. To date, there are no MOFs showing the highest Ksv values with 4-NP. Thus, the quenching mechanism of MOF 1 for sensing 4-NP must be investigated. Generally, the possible mechanisms of luminescent MOFs for sensing nitroaromatic explosives are based on electron transfer,10 energy transfer process,11 electrostatic interaction (for example, forming hydrogen bond between MOFs and nitro analytes),6b,12 or a combination of them.4b,5b First, whether the electron transfer is one of the quenching mechanisms is studied. MOFs built from d10 metal are usually characterized by narrow energy bands due to highly localized electronic states. Therefore, their energy levels of valence band (VB) and conduction band (CB) can be described in a way similar to molecular orbitals (MOs).13 As shown in Figure S18, the energy levels and isodensity surfaces of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are computed by density functional theory at the B3LYP/6-31G(d) level for BPPA and nitro analytes. It is obvious that BPPA ligand has the higher LUMO energy than those of nitro analytes, which can force the excited electrons transferring from CB of MOFs to LUMOs of nitroaromatic explosives. And this can lead to fluorescence quenching upon D
DOI: 10.1021/acs.inorgchem.6b01521 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Images (under UV illumination at 365 nm) of MOF 1 and MOF 2 on test strips after soaked in nitroaromatic explosives solution (0.5 mM in DMF). The interfaces soaked in nitroaromatic explosives diluted solution are shown with red lines. (a, b) Images (under UV illumination at 365 nm) of MOF 1 and MOF 2 on test strips after soaked in nitroaromatic explosives diluted solution (from left to right: blank, 4-NP, PA, 2,4-DNP, 3NP, 1,4-DNB, 1,4DNT, NT, NB, 1,3-DNB), respectively. (c, d) Images (under UV illumination at 365 nm) of MOF 1 and MOF 2 on test strips after soaked in nitroaromatic explosives mixed solutions (from left to right: blank, 4-NP + PA, 4-NP + 2,4-DNP, 4-NP + 3-NP, 4-NP + 1,4-DNB, 4NP + 1,4DNT, 4-NP + NT, 4-NP + NB, 4-NP + 1,3-DNB, mixed solutions of all the nitroaromatic explosives), respectively.
interactions play important roles in MOF 1 showing the highest Ksv values with 4-NP. The phenolic hydroxyl groups and nitro groups can form intramolecular hydrogen bonding in PA, which results in the quenching efficiency of PA being lower than that of 4-NP (Figure S21b). Thus, the most possible mechanism of highly sensitive sensing 4-NP for MOF 1 is a combination of electron transfer and H-bonding interactions. Although MOF 1 and MOF 2 are isostructural, the quenching efficiency of 4-NP for MOF 2 is lower than that of MOF 1, which can be attributed to lacking uncoordinated oxygen atoms and free amino groups to form H-bonding interactions with 4NP. MOF 2 has the highest selective and sensitive detection of PA; the quenching mechanism is conjugative effect of electron transfer and the resonance energy transfer as reported.4 Practical Application. Nitroaromatic explosives detection in the solid state was also performed for exploring the practical application. Test strips were prepared by dip-coating the MOFs solution into filter paper and then drying under vacuum. The test strips all show strong emission in solid state. After the MOF 1 test strips were dropped into 4-NP solution (5 × 10−4 M in DMF), no emission was observed by the naked eyes under 365 nm UV light, while the MOF 1 test strips dropped into other nitroaromatic explosives show minor changes on the luminescence intensity (Figure 5a). In addition, MOF 2 test strips exhibit good responses toward PA (Figure 5b). To better study the effects of other nitro explosives on the sensing property of MOFs, we performed a series of competition experiments by testing the fluorescence changes in the presence of other nitro explosives. As shown in Figure 5c,d, no emission could be observed by the naked eyes under 365 nm UV light in all nine possible combinations of 4-NP and other nitro explosives (4-NP + PA, 4-NP + 2,4-DNP, 4-NP + 3-NP, 4NP + 1,4-DNB, 4-NP + 1,4DNT, 4-NP + NT, 4-NP + NB, 4NP + 1,3-DNB, mixed solutions of all the nitroaromatic explosives). It shows that MOF 1 and MOF 2 have highly sensitive and selective detection of 4-NP and PA in the practical application, respectively.
excitation. To confirm whether the electron transfer is one of the quenching mechanisms, UV−vis spectra of 4-NP, MOF 1 in DMF, and MOF 1 upon addition of 4-NP are made. Compared with the absorption bands intensities of 4-NP and MOF 1 in DMF, these of MOF 1 upon addition of 4-NP at 319 and 356 nm are reduced, while new absorption band appears at 328 nm (Figure 4a). This indicates that the excited electrons transfer from CB of MOF 1 to LUMOs of 4-NP. But the order of observed quenching efficiency is not in agreement with the corresponding LUMO energies of the nitro analytes, showing that it is not the only mechanism for quenching MOF 1. Another possible reason is resonance energy transfer. It is generally known that the resonance energy can transfer from MOFs to nitro analytes if there is an effective overlap between the emission spectrum of MOFs and nitro analytes. The absorption band of PA has the biggest degree of overlap with the photoluminescence spectra of MOF 1 in DMF, while all the others almost have no overlap (Figure 4b). It indicates that the quenching mechanism of 4-NP cannot be the resonance energy transfer. As mentioned above, it explains why MOF 1 has higher Ksv values with PA than others, except 4-NP. So there must be another quenching mechanism. The possibilities of sensing nitroaromatic explosives for free ligands are also investigated in same method. As shown in Figure S20, the order of quenching efficiency for ligand BPPA is 2,4-DNP > PA > 4-NP > 1,4-DNB > NT > 2,4-DNT > 3-NP > NB > 1,3-DNB, and the order of it for H2aba is 2,4-DNP > 4NP > PA > 1,4-DNB > 2,4-DNT > NT > 3-NP > NB > 1,3DNB. Ligands BPPA and H2aba all show highly sensitive detection of 2,4-DNP, PA, and 4-NP, but selectivity is unsatisfactory. And the order of quenching efficiency for MOF 1 is not in accordance with them for free ligands. Considering that there are three uncoordinated oxygen atoms (O1, O9, and O12) in carboxylic groups and two free amino groups (N8 and N12) having no contribution to forming hydrogen bonding in MOF 1, the supposition that forming hydrogen bonding between MOF 1 and 4-NP was proposed. O1, O9, and O12 act as electron acceptor to form H-bonding interactions with phenolic hydroxyl groups from 4-NP, and N8 and N12 can provide open sites to form H-bonding interactions with nitro groups from 4-NP (Figure S21a). These H-bonding
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CONCLUSIONS In summary, two MOFs have been synthesized from the selfassembly of “V-shaped” BPPA ligands and H2aba (or H2oba) E
DOI: 10.1021/acs.inorgchem.6b01521 Inorg. Chem. XXXX, XXX, XXX−XXX
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with Cd. They are isostructural and are all 2D sql nets. The 2D layers are stacked in an ABCD··· manner. A and C represent 2D interpenetrating layers (2D + 2D → 2D), which are both formed by ligands, Cd2, and Cd3, while B and D represent 2D nets formed by ligands and Cd1. Interestingly, A and C are in mirror symmetry, as are B and D. MOF 1 exhibits exceptionally selective and sensitive detection of 4-NP. This can be owing to the excited electrons transferring from CB of MOF 1 to LUMO of 4-NP and H-bonding interactions between MOF 1 and 4NP. Because of the coexistence of electron transfer and resonance energy transfer, PA exhibits high photoluminescence quenching compared to others for MOF 2. In other words, the two MOFs showing different order of quenching efficiency depend on forming H-bonding interactions between MOFs and nitroaromatic explosives, or not. MOF 1 and MOF 2 have highly sensitive and selective detection of 4-NP and PA in the practical application, respectively. Our work provides a rational strategy for design and synthesis of MOF-based sensors for selective and sensitive detection of one nitro explosive from other nitro explosives.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01521. selected bond lengths and angles, experimental details, FT-IR spectra, TGA curves, PXRD, photoluminescence spectra and SV plots. CCDC (1479235) and (1479236). (PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: 86-25-89682309. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by grants from the National Natural Science Foundation of China (Nos. 21371092, 91022011) and National Basic Research Program of China (2010CB923303).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b01521 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01521 Inorg. Chem. XXXX, XXX, XXX−XXX
