Rational Design and Functionalization of a Zinc MetalâOrganic

Article

Rational Design and Functionalization of a ZnMOF for highly selective detection of TNP Shanghua Xing, Qi-Ming Bing, Hui Qi, Jingyao Liu, Tianyu Bai, Guanghua Li, Zhan Shi, Shouhua Feng, and Ruren Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06482 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 29, 2017

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ACS Applied Materials & Interfaces

Rational Design and Functionalization of a Zn-MOF for highly selective detection of TNP Shanghua Xinga, Qiming Bingb, Hui Qic, Jingyao Liub, Tianyu Baia, Guanghua Lia*, Zhan Shia, Shouhua Fenga, and Ruren Xua a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun 130012, P. R. China. b

c

Institute of Theoretical Chemistry, Jilin University, Changchun, 130023.

The Second Hospital of Jilin University, Changchun 130041, PR China

* Corresponding author: Prof. Guanghua Li E-mail address: [email protected] KEYWORDS: metal organic framework, 2,4,6-trinitrophenol (TNP), sensor, experimental synthesis, theoretical simulation ABSTRACT To develop potential MOFs in TNP detection, an amino-functionalized Zn-MOF, [NH2(CH3)2][Zn4O(bpt)2(bdc-NH2)0.5]∙5DMF(H3bpt

=

biphenyl-3,4’,5-tricarboxylate,

H2bdc-NH2 = 2-aminoterephthalic acid), has been designed theoretically and synthesized experimentally. Its structure is composed of Zn4O(CO2)7 SBU linked by mixed ligands,

ACS Paragon Plus Environment

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exhibiting three dimensional framework. The fluorescence exploration revealed that the amino-functionalized Zn-MOF shows high selectivity and sensitivity for TNP, which agrees well with the predictions of theoretical simulations. This work provides a suitable means to develop new potential MOFs for TNP detection performance with the combination of the experimental and theoretical perspectives. INTRODUCTION Detection of nitro explosives including nitromethane (NM), nitrobenzene (NB), 2,6dinitrotoluene (2,6-DNT), dinitrobenzene (DNB), and 2,4,6-trinitrophenol (TNP), which are the common ingredients used in industrial explosives, plays a crucial role in human security, thus has been widely concerned in recent years1-7. Of all the nitro explosives, TNP has been paid the most attention due to their high explosive power. Additionally, TNP is also recognized as a highly toxic pollutant. TNP contamination in ground water and soil not only induce the severe health problems, but also result in miserable environmental pollution8-12. As a consequence, it is an urgent need to detect TNP with high selectivity and sensitivity. Current explosive detection methods (canines or sophisticated instruments) are high cost, complicated and have portability problems during in practice use13-15. Recently, it is noteworthy that fluorescence-based chemical sensor has obtained intense attention because of its speed, high sensitivity, and easily detectable signals. The materials like conjugated polymers, nanoparticles and metal complexes have been utilized for fluorescence based nitro explosives detection16-18. Among the diverse fluorescence-based sensors, metal–organic frameworks (MOFs) have been considered as the leading sensors due to their tunable porous nature and detectable fluorescence changes19-26. To date, a number of MOF-based sensors have employed for sensing towards nitro explosives27-35. Li et al. firstly reported a luminescent microporous MOF which was able to


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detect trace explosives in the vapor phase36. Ghosh and coworkers reported a luminescent CdMOF showing highly selective detection of TNP37. To obtain efficient sensor for nitro explosives, the rational design and preparation of targeted MOF become a necessity. Generally, two design strategies are usually utilized to make materials for ideal properties. (i) Through the careful design of raw materials, such as ligand and metal clusters, the chemical and physical properties of MOFs can be finely modulated and materials with desired performance be presented38-39. However, this method exits an uncertainty by empirical exploratory syntheses owing to much possible chemistries. (ii) The theoretical simulation is an efficient means to design the most promising MOFs candidates for detection prior to extensive experiments. This method has been successfully used in other fields such as MOFs gas adsorption simulation, while their utilization for nitro explosives detection has been much less explored40-42. New MOFs can be designed theoretically and their performance predicted by simulation. Thus the theoretical simulation methods are able to further guide experiments through the design and prepare of the potential MOFs for sensing nitro explosives. In this work, with the combination of theoretical prediction and experimental synthesis, two MOFs, [NH2(CH3)2][Zn4O(bpt)2(bdc)0.5]∙5DMF(1) and

[NH2(CH3)2][Zn4O(bpt)2(bdc-

NH2)0.5]∙5DMF(2), H3bpt = biphenyl-3,4’,5-tricarboxylate, H2bdc = terephthalic acid, have been designed and synthesized. Their luminescent properties of sensing nitro explosives are studied systemically. The compound 2 exhibits high selectivity and sensitivity for TNP compared to the compound 1. EXPERIMENTAL SECTION Synthesis of compound 1


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Zn(NO3)2·6H2O (60 mg, 0.2 mmol), H3bpt (26 mg, 0.1 mmol), and H2bdc (3 mg, 0.015 mmol) in 5 mL DMF was stirred for 30 min in a vial (20 mL) with the addition of 100µL HBF4 (40 %). The vial was sealed into a Teflon-lined stainless steel vessel, heated at 120 °C for 3 days. After slowly cooling to room temperature, the colorless crystals of compound 1 were collected, followed by washing with DMF, and dried in air. Yield: 61% (based on H3bpt). Anal. calcd (%) for [NH2(CH3)2][Zn4O(bpt)2(bdc)0.5]∙5DMF: C, 45.80; H, 4.45; N, 6.28. Found: C, 45.54; H, 4.29; N, 6.01. Synthesis of compound 2 Zn(NO3)2·6H2O (60 mg, 0.2 mmol), H3bpt (26 mg, 0.1 mmol), and H2bdc–NH2 (3 mg, 0.015 mmol) in 5 mL DMF was stirred for 30 min in a vial (20 mL) with the addition of 50µL HBF4 (40 %). The vial was sealed into a Teflon-lined stainless steel vessel, heated at 120 °C for 3 days. After slowly cooling to room temperature, the colorless crystals of compound 2 were collected, followed by washing with DMF, and dried in air. Yield: 57% (based on H3bpt). Anal. calcd (%) for [NH2(CH3)2][Zn4O(bpt)2(bdc-NH2)0.5]∙5DMF: C, 45.54; H, 4.46; N, 6.77. Found: C, 45.40; H, 4.71; N, 6.55. Results and discussion We define a scheme for designing target MOF with superior detection performance for nitro explosives by combining the theoretical prediction and experimental synthesis method. In order to predict the performance of the target MOF, we first attempt to model the hypothetical frameworks which are favorable for nitro explosives detection. We herein take a strategy aimed at modifying the reported or synthesized structures to fabricate the target MOF for nitro explosives detection. In general, the nitro explosives detection performance of MOFs is predominantly governed by four factors: (i) MOFs possessing luminescent properties are


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essential for sensing as a result of the detectable changes in luminescent characteristics; (ii) Using high conjugated linkers is an effective method to construct MOF that are considered to be crucial with the detection of nitro explosives because of electron deficiency of nitro explosives known as fluorescence quenchers; (iii) Suitable channels also play a key role in the nitro explosives sensing performance, permitting easy diffusion of analyte inside the channel; (iv) Absolutely, the role of non-covalent interactions (e.g. electrostatic interactions, π-π interactions) cannot be ignored. The possible interaction between analyte and the internal surface of the framework can give rise to improved selective recognition. Through judicious selection of our previous synthesized structure, we choose the Zn-MOF acted as a parent MOF to explore nitro explosives detection, although the structure is similar to that reported recently by Kitigawa and Su et al.43-44. We were motivated to use this Zn-MOF as the parent MOF owing to the following beneficial properties: (i) its mixed high conjugated linkers can enhance the electron density of the frameworks; (ii) its inherent porosity matches well with the TNP analyte; (iii) its outstanding luminescence properties are beneficial to recognize fluorescence response. However, the fluorescence exploration of Zn-MOF reveals that it has general detection ability for TNP. The possible reason might be the interaction between the TNP analyte and the host framework is weak. In other words, there has no special binding site within the framework to interact with the TNP analyte. For further improve the TNP detection performance, it is possible to introduce the functional groups within the MOF structures. Alternatively, -NO2, -NH2, -OH or -CH3 group can be acted as binding sites in parent Zn-MOF. We consider the NH2 group might be the best binding site, because of its electronic donor, as well as hydrogen-bonding interactions with TNP. To predict the TNP detection performance for amino-functionalized Zn-MOF, we carried out periodic density functional theoretical calculation to determine the adsorption energies of TNP.


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For comparison, the TNP detection performance for the parent Zn-MOF was also predicted. The adsorption energies of TNP in two MOFs are calculated as follows: Eads= Etotal- (EMOF + Emol) where Etotal is the total energy of adsorption system, EMOF is the total energy of Zn-MOF, Emol is the energy of adsorbate, and Eads is the calculated adsorption energy. According to this formula, a smaller Eads indicates the greater adsorption strength of MOF to molecule. The calculation results show that the Eads of TNP in the amino-functionalized Zn-MOF (2) is -0.99 eV, which is much lower than that of -0.21 eV in the parent Zn-MOF (1), suggesting that compound 2 has stronger adsorption ability to TNP than compound 1. In the optimized structure of TNP in compound 2, as shown in Figure 1, the proton from the hydroxyl group of TNP binds to the amino group, and thus the strong hydrogen-bond interactions between the protonated amino group of MOFs and the phenolic oxygen atom of TNP molecule can contribute to the good adsorption ability of compound 2 to TNP. The theory predicts excellent TNP detection performance for compound 2 over 1, thus we plan to focus such experiments on these systems.

Figure 1. Optimized adsorption structure of TNP in compound 2.


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Motivated by the theoretical calculation results, the compound 2 have been synthesized in a similar synthesis condition. At low concentration of HBF4, its presence is in favor to promote the crystallinity and inhibit the formation of amorphous precipitate. The PXRD pattern indicated that the crystal structure of compound 1 and 2 are isostructural (Fig. S1). Therefore, the structure of compound 2 was described representatively. Compound 2 crystallizes in the monoclinic crystal system of the P21/n space group. The host framework of compound 2 is anionic and charge balanced by the (CH3)2NH2+, resulting from the decomposition of DMF solvent. The asymmetric unit contains one Zn4 (µ4-O) cluster, two deprotonated bpt and 1/2 deprotonated bdc-NH2 ligands. Each Zn4(µ4- O) cluster is surrounded by seven carboxylate groups from six H3bpt linkers and one bdc-NH2 to form a Zn4O(CO2)7 secondary building unit (SBU). Four H3bpt and two bdc-NH2 ligands link six different Zn4O(CO2)7 SBUs at the corners to form a octahedral cage. As a result, the octahedral cages were linked by H3bpt ligands into a 3D porous framework (Fig. 2a-b). From a topological point of view, the Zn4O(CO2)7 SBU can be simplified to be a 7-connected node, and the bpt linker can be regarded as 3-connected node. Thus the structure of compound 2 can be described as a (3, 7)-connected network with a Schläfli symbol (4∙52)2∙(42∙55∙610∙84) ( Fig. 2c). As represented in Fig. 2d, there are three kinds of tiles of [62∙82], [4∙52∙62] and [42∙54]. After eliminating all the guest molecules, the total accessible volume in compound 2 is 64.9% using the PLATON/VOID routine45-46.


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Figure 2. Structure for compound 2: (a) the octahedral cages. (b) The space-filling model of the framework. (c) The (3, 7)-connected topology network. (d) Topological features of the compound displayed by tiles. The experimental PXRD pattern of two compounds agrees well with the simulated one according to their single crystal X-ray data, showing the purity of the synthesized products (Fig. S1). Because compound 1 is isostructural with 2, thus we describe the thermogravimetric analyses of compound 2 representatively. The thermogravimetric analyses of compound 2 showed a weight loss of 31.41% before 250oC, corresponding to the loss of all guest molecules (calculated 30.56%) (Fig. S2). The framework started to collapse and decompose with increasing temperature. The final residue was ZnO (experimental: 25.01%; calculated: 24.20%). Compound 2 was rinsed with DMF followed by methanol exchanged experiment to get methanol exchanged compound. The methanol exchanged compound was dried at 120 oC under high vacuum for 10 hours to get activated


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2. The PXRD pattern indicated that the original framework of activated compound 2 was retained after the activation (Fig. S3). The characteristic peak of C=O of DMF at 1666 cm-1 was disappeared for the activated compound 2, suggesting all DMF molecules are removed completely (Fig. S4). The permanent porosity of activated compound 2 can be confirmed by the N2 sorption at 77K, showing a type-I sorption behavior (Fig. S5). The BET surface area for compound 2 was calculated to be 407 m2g-1. The fluorescent spectra of two compounds in solid state have been investigated at room temperature (Fig. S6). Upon the excitation of 360 nm, compound 1 shows strong emission at 402 nm with a quantum yield of 7.64%. The fluorescent emission maxima of compound 2 at 434nm upon excitation at 360nm. The quantum yield of compound 2 is 45.17%, which is higher than the reported transition MOFs sensor for nitro explosives12, 33, 47-49

. To check the nitro explosives sensing properties of two MOFs, the fluorescence

quenching titration experiments have been performed with the dispersed compound 1 and 2 by gradual addition of 1mM DMF solutions of 2,4,6-trinitrophenol (TNP). The fluorescence spectra for activated 1 reveal a quenching efficiency (63%) for TNP on incremental addition of 1mM (300 µL) analyte (Fig. 3a), while activated 2 shows high quenching efficiency (94 %) after adding the same amount of analyte (Fig. 3c). The quenching efficiency of compound 2 is amongst the highest reported MOF sensing TNP1, 3, 25, 49-50

. Furthermore, upon addition of TNP solution to the dispersion of activated 2 in

DMF causes fluorescence intensity quenching with red-shifted of emission maxima (Fig. 3b). The emission spectra of activated compound 2 exhibit red-shifts of 30 nm compared to the pure DMF. The red-shift becomes higher along with increasing TNP concentration (Fig. 4a, Fig. S9). While addition of the same amount of TNP only alter the initial


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emission intensity of activated 1. Up to now, although some fluorescence MOFs have been synthesized and explored for sensing nitro aromatics51-54, most sensors are solely monitored optical signal “turn-off” by emission quenching. Such MOF sensors reveal intrinsic limitations, such as lacking sufficient chemical selectivity and accuracy for a specific analyte. A change in spectral shift can be utilized

Figure 3 (a) The fluorescence spectra upon adding 300 µL TNP solution (1mM) in DMF. (b) The normalized emission spectra of compound 2 in DMF solution upon adding 300 µL TNP solution (1mM). (c) The fluorescence quenching efficiencies upon incremental addition of TNP solution (d) SV plots of I0/I versus the TNP concentration. as another sensing parameter. While utilizing this parameter for TNP detection has been much less reported. The compound 2 was able to detect TNP by virtue of the changes in both emission intensity and spectra shift. Additionally, the BET surface area for activated compound 2 was


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calculated to be 407 m2g-1. After sensing TNP, there is almost no adsorption of N2 molecules, indicating that TNP analyte were adsorbed into the pores of compound 2.

Figure 4 (a) The change in emission intensity of compound 2 with incremental addition TNP. (b) Digital photographs of solutions of compound 2 in the presence of different nitro analytes under portable UV light. The quenching efficiency can be quantitatively analyzed using the Stern–Volmer equation: (I0/I) = Ksv[A] + 155-56, where I0 and I are the emission intensities before and after adding nitro analyte, respectively, [A] is the concentration of the nitro analyte, and Ksv is the quenching constant. The Ksv is calculated via luminescent data. The Ksv for compound 1 is 1.69×104 M-1. But the Ksv for compound 2 was calculated to be 6.19×104 M-1 (Fig. 3d). To best of our knowledge, the value is higher than those found for MOFbased sensing TNP (Table S2). Notably, the Ksv of compound 2 is about 3.6 times of that of 1. The above experiment results show that compound 2 shows high selective detection towards TNP compared to the parent Zn-MOF, which is well consistent with the theoretical calculation results. In addition, the detection limit of compound 2 sensing TNP was estimated to be 5.6 × 10-7 M (Fig. S7, Table S3-4), which is moderate among the


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previously reported MOFs sensor for TNP (Table S2). Enthused from the low detection limit and high quenching constant, the trace quantity detection of TNP was explored. Compound 2 respond rapidly to the TNP vapor. The quenching maximum percentage nearly reached 31% (Fig. S8). Fluorescence quenching titrations for compound 2 were also performed with other nitro explosives such as 2,4-dinitrotoluene (2,4-DNT), 2,6-DNT, DNB, nitrobenzene (NB) , 4nitrotoluene (NT). Some other aliphatic nitro compounds like nitromethane (NM) and nitroethane (NE) were also examined. The changes in emission intensity for other nitro explosives were extraordinarily low (Fig. 4b, Fig. S10-16). Additionally, PXRD pattern indicated that compound 2 can maintain its crystallinity treated by the different nitro explosives solutions (Fig. S17). The results demonstrate that activated 2 shows high selective detection towards TNP over other nitro explosives. Enthused from the above outcomes for compound 2, we checked the selectivity for TNP in the presence of other nitro explosives. The emission spectrum for activated compound 2 dispersed in DMF solution was monitored upon sequential addition of 2, 4-DNT solution (40 µL in two equal portions), resulted in negligible emission quenching. And then adding the equal amount of TNP solution gave rise to the significant fluorescence intensity quenching. The trend was also repeated in the following addition cycles and the fluorescence intensity quenching phenomenon of TNP was unchanged (Fig. 5, Fig. S19). Other nitro explosives have the similar results and phenomenon (Fig. S18-25). The high selective and sensitive detection in the presence of other nitro explosives makes compound 2 a reliable and efficient sensor for TNP.


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We also investigate the recyclability of compound 2 that can be recovered by centrifugation, followed by washing methanol several times. The emission intensity and quenching efficiency of 2 exhibited little changes after being used for four cycles. As demonstrated by the PXRD measurements, the framework of recovered sample maintained its crystallinity compared with the original one. The above results indicate that the compound 2 have the recyclable ability (Fig. S26).

Figure 5 Fluorescence quenching of compound 2 upon addition of different nitro analyte solutions followed by TNP. To further insight into the outstanding sensing mechanism of activated 2, the electronic properties of both the nitro analytes and MOF were analyzed. The emission quenching mechanism through electron transfer from the conduction band of MOFs to the LUMOs of the electron-deficient nitro explosives has been well established57-60. MOFs, especially in the present case involving d10 metal ions, have narrow energy band owing to their extended network structure and highly localized electronic system. The electron-rich MOFs lie at higher energies than the LUMOs of the electron-deficient nitro explosives,


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allowing the excited state electron from conduction band (CB) transfer to the LUMOs of the nitro explosives. This can result in emission quenching upon excitation. The observed maximum emission quenching is due to the lowest LUMO energy level of TNP (Table. S5). However, the order of observed quenching response (TNP >NT > 2, 4-DNT > 2, 6DNT > 1, 3-DNB > NB > NM > NE) is not in full agreement with the trend of electron deficiency. This clearly indicates that electron transfer process is not the sole emission quenching mechanism. Another possible reason for emission quenching maybe the long range energy transfer mechanism61. The degree of energy transfer heavily relies on the spectra overlap between

Figure 6 spectral overlap extent between the analytes absorption band and the emission band of the compound 2 the adsorption band of nitro explosives and the emission spectrum of the MOFs. The greater the extent of spectral overlap, the higher the degree of energy transfer. Fig. 6 shows that TNP and compound 2 have the greatest spectral overlap, thus leading to red shift in the emission maxima with increasing TNP, whereas almost no overlap was found


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for other nitro analytes. As the nonlinear SV plot for TNP suggests that energy transfer between the TNP molecule and MOF or the combination of static and dynamic quenching process involved in emission quenching process (Fig. S27). The decay time of MOFs adding different amount of TNP can differentiate the static and dynamic quenching process. In static mechanism, the decay time remains unchanged with increasing TNP concentration. However, in dynamic mechanism, the decay time will be reduced. We examined the decay time of compound 2 before and after adding TNP analyte (Fig. S28). The decay time remained unchanged after adding 300 µL TNP, suggesting the presence of static quenching mechanism. To get deeper insight, we measured the UV adsorption spectrum of compound 2 before and after adding TNP analyte (Fig. S29). The results show that a new band at 430 nm appeared, which further supports the static mechanism. Additionally, the strong hydrogen-bond interactions between the amino of bdc-NH2 ligand and hydroxyl of TNP also are the reason of emission quenching, as confirmed by the above theoretical calculation results. Some other possessing hydroxyl group analytes like 2, 4-dinitrophenol (2, 4-DNP) and 4-nitrophenol (NP) or even phenol were also examined (Fig. S30-32). The order of observed quenching response was: TNP > 2, 4-DNT > NP > phenol, which is well consistent with the sequence of acidity. However, other nitro analytes have no acidic hydroxyl group to interact with amino within the framework, so they show very low quenching effect. Hence, compound 2 shows highly selective and sensitive detection for TNP in the presence of other nitro explosives owing to the occurrence of electron transfer and energy transfer processes in addition to hydrogen-bond interactions. CONCLUSION


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A microporous amino-functionalized MOF constructed by non-toxic Zn metal centre and mixed ligands has been designed and synthesized by combining experimental and theoretical method. The fluorescence explorations showed that the amino-functionalized Zn-MOF can be used to detect TNP with fast response, highly effectivity and sensitivity, even in the presence of other interfering nitro explosives, and simultaneously shows emission quenching with red-shifted of the emission maxima. The outstanding selective detection of TNP can be attributed to the presence of energy and electron transfer processes as well as hydrogen-bond interactions between the hydroxyl group of TNP and the amino group of the bdc-NH2 ligand. Clearly, the amino functionalized MOF show great potential as a promising fluorescence sensor for selectively sensing TNP. The result highlights the capabilities of theoretical simulation methods to identify the potential MOFs material which are more favorable for nitro explosives detection, providing valuable implications for the experimental synthesis of new MOFs.

ASSOCIATED CONTENT Supporting Information Experimental section, structure Information for MOF 1-2, PXRD pattern, IR spectra, N2 adsorption, and additional luminescent measurements. X-ray crystallographic files (CIF)

ACKNOWLEDGMENTS This work was supported by the Foundation of the Natural Science Foundation of China (no. 21571077, 21371069, 21373098 and 21301065).


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