- Email: [email protected]
High-sensitivity detection of nitroaromatic compounds (NACs) by the pillared-layer metal-organic framework synthesized via ultrasonic method
Accepted Manuscript High-sensitivity Detection of Nitroaromatic Compounds (NACs) by the pillared-layer metal- organic frameworksynthesized viaultrasonic method Azar Hakimifar, Ali Morsali PII: DOI: Reference:
S1350-4177(18)30802-2 https://doi.org/10.1016/j.ultsonch.2018.11.002 ULTSON 4371
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
27 May 2018 3 November 2018 3 November 2018
Please cite this article as: A. Hakimifar, A. Morsali, High-sensitivity Detection of Nitroaromatic Compounds (NACs) by the pillared-layer metal- organic frameworksynthesized viaultrasonic method, Ultrasonics Sonochemistry (2018), doi: https://doi.org/10.1016/j.ultsonch.2018.11.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High-sensitivity Detection of Nitroaromatic Compounds (NACs) by the pillared-layer metal- organic framework synthesized via ultrasonic method
Azar Hakimifar and Ali Morsali*
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-4838, Tehran, Islamic Republic of Iran, Tel: +98-21-82884416, Email: [email protected]
1
Abstract Nanorods of zinc(II) based metal-organic framework (MOF) were prepared via ultrasonic method without any surfactants at room temperature and atmospheric pressure. Control of particle size and morphology was enhanced
in
this
synthesis
method.
Nanorods
of
pillared-layer
metal
organic
framework,
[Zn2(ubl)2(bipy)]·DMF (TMU-18), where ubl (urea-based ligand) is 4,4'-carbonylbis(azanediyl)dibenzoic acid,
4,4'-Bipyridine (bipy)
DMF = N,N-dimethyl formamide), was synthesized under ultrasound
irradiation in different concentrations of initial precursor. The nano structure and morphology of the synthesized MOF were characterized by Field Emission Scanning Electron Microscopy (FE-SEM), powder X-ray diffraction, elemental analysis and FT-IR spectroscopy. Moreover, Fluorescence emissions of nanorods have been studied. Luminescent MOFs (LMOFs) have shown great potential as sensor for various nitro explosives by modulating the luminescence behavior in presence of nitro explosives.
Urea-
functionalized MOF shows high selectivity for sensing of the nitro explosive 2,4,6-trinitrophenol (TNP) even in the presence of other nitroaromatic compounds in methanol solution. Fluorescence intensity decreased with increasing contents of nitroaromatics in organic solution due to fluorescence quenching effect. The ultrasound method has some advantages such as short duration time of reaction, no need to high temperatures and pressures for synthesis nano-materials and low costs in comparison to other methods. Considering these advantages we used ultrasonic method to produce these nanorods which show high sensitivity in detecting nitroaromatics.
Keywords:
Metal-organic
Framework;
Nanorods;
trinitrophenol.
2
Ultrasonic
irradiation;
Luminescence;
2,4,6-
1.
Introduction Three-dimensional (3D) metal–organic frameworks (MOFs) are a new class of porous materials which
have attracted extensive attention due to their potential applications as functional materials in structuredependent applications, such as gas storage and separation, ion exchange, sensing, catalysis and drug delivery [1–5].The MOFs can be designed by choosing appropriate organic ligands and inorganic secondary building units (SBUs) [6–9]. Synthesis of nanoscale MOFs has also been attractive [10-12]. Traditional techniques, like slow vapor diffusion methods and solvothermal techniques, are commonly applied to prepare MOFs [13]. However, the reaction processes using these custom techniques usually have to be implemented at high reaction temperature and pressure which could take long time in many cases. Synthesis of nano MOFs (NMOFs) has also been attractive. One of the simplest and most effective methods for preparation of nano or microstructures MOFs is ultrasound irradiation [14–17]. In this simple, fast and green method molecules undergo a chemical reaction because of powerful ultrasound radiation in the range of 20 KHz to 10 MHz [18]. The important part of sonochemistry is the generation, growth, and collapse of a bubble which is produced in aquatic ambience. This process is called acoustic cavitation which presents very high local temperatures from 5000 to 25000 K and pressures with a very higher rate of cooling and heating [19 -21]. The results show that ultrasonic synthesis is one of the simplest and most effective method for the construction of nano MOFs [22]. Nitroaromatic compounds such as 2,4,6-trinitrophenol (TNP), nitrobenzene (NB), and TNT are explosive in nature. Therefore, fast and highly selective detection of them is very important [23-27]. One of such toxic ordinance molecule is TNP, a yellow crystalline solid commonly known as picric acid [28]. Also, TNP is widely used in aniline synthesis, oxidative metal itching, dyes, matches, fireworks, glass, and
3
leather industries [29]. During commercial production and use, TNP is released into the environment and impart serious health and environmental issues. Thus, there is an urgent need for efficient and reliable sensors for the detection of TNP. Recently, fluorescence based detection systems are gaining increasing attention for detection of NACs pertaining to their high sensitivity, ability to detect even single molecule, short response time and flexibility to employ in both solid as well as liquid phase applications [30]. Detectable variations in luminescence by tuning the host–guest chemistry along with tailorable porosity and a high surface area makes MOFs excellent candidates for sensing [31]. Variety of π-electron rich fluorescent organic molecules has been employed for the fluorescence based sensing of explosive molecules. NACs are oxidizers due to a low-lying unoccupied π* orbital, which can accept an electron from the excited state fluorophore, thus efficiently quenching the fluorescence emission of this compound. Interacting with these electron rich moieties though π-π stacking interaction and subsequently lead to decrease in fluorescence character. High sensitivity, differential response towards pool of analytes and feasibility in device integration have already been demonstrated [32]. In this work, we synthesized [Zn2(ubl)2(bipy)]·DMF(TMU-18) as pillared-layer MOF by the sonochemical method, which have shown fluorescence properties. Also in this report we study ultrasonic synthesis of TMU-18 in different time and concentrations of initial reagent to understand the effect of them on size and shape of particles. Moreover, we have investigated the fluorescence behavior of TMU-18 with various nitro aromatic analytes. The study reveals that urea functional groups inside the pore cavity of MOFs can detect TNP in a very fast, selective and sensitive way even in presence of other aromatic compounds.
2. Experimental
4
All reagents for the syntheses and analysis were commercially available and used as received. IR spectra were recorded using Nicolet Fourier Transform IR, Nicolet 100 spectrometer in the range 500-4000 cm-1 using the KBr disk technique. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert diffractometer with mono chromated Co-Ka (1.78897 Å) radiation. Ultrasonic generation was carried out in an ultrasonic bath SONICA-2200 EP (frequency of 40 KHz). The sonicator used in this study was a SONICA-2200 EP with an adjustable power output (maximum 305 W at 40/60 kHz). A horn type tube Pyrex reactor was custom-made and fitted to the sonicator bath. The samples were characterized with a field emission scanning electron microscope (FE-SEM) ZEISS SIGMA VP (Germany) with gold coating.
2.1. Synthesis of compound TMU-18 nano-structures using sonochemical method To prepare nano-sized TMU-18, 10 mL solution of zinc (II) acetate dihydrate (0.5 mmol, 0.065 g) in DMF was positioned in a high-density ultrasonic bath at ambient temperature and atmospheric pressure, operating with a power output of 305 W. Into this solution 0.5 mmol (0.150 g) and 1 mmol (0.078 g) of the ligands ubl and bipy, respectively, were added and sonicated for 60 min. The obtained precipitates were filtered off, washed with DMF and then dried in air. FT-IR (KBr pellet , cm-1): 3317 (br), 1673 (vs), 1596 (vs), 1400 (vs), 1302 (s), 1227 (m), 1178 (m), 862 (w), 775 (m), 651 (w), 549 (w). For studying the effect of initial reagent concentrations on size and morphology of nanostructure TMU-18, the above processes were done with different concentrations (0.02, 0.01, and 0.005 M).
3. Results and discussion
5
Our previous study showed that the reaction between ‘‘ubl’’ ligand with a mixture of Zn(NO3)2, bipy and DMF led to the formation the single crystal of zinc(II) porous coordination metal-organic framework, [Zn2(ubl)2(bipy)]·DMF (TMU-18) crystallizes in the Triclinic Pī space group. In this compound, the coordination geometry around the Zn(II) can be described as distorted octahedral, with four sites occupied by oxygen atoms of four different urea ligand carboxylate groups in an approximately square configuration and the fifth site occupied by a nitrogen atom of bipy ligand (Fig.1) [ 34 ].The remaining coordination site of each metal center is located inside the zinc paddle-wheel cluster. In this study we prepare nanorod of TMU-18 by ultrasonic irradiation in DMF solution with different concentrations (0.02, 0.01 and 0.005 M). The best morphology was observed at concentration 0.01M. Single crystalline materials were obtained using a heat gradient applied to a solution of the reagents [33]. Scheme 1 gives an overview of the methods used for the synthesis of TMU-18 using the two different paths. The IR spectra of both bulk material and nanorods of TMU-18 produced by conventional heating and sonochemical method show the amide NH stretching absorption can be observed at 3317cm -1. The amide C=O stretching absorption is 1673cm
-1
(Fig. 2). Fig 3. shows the simulated XRD pattern from single
crystal X-ray data of TMU-18 (Fig. 3a) in comparison with the XRD pattern of TMU-18 prepared by the sonochemical method (Fig. 3b). Desirable agreement was observed between the simulated and experimental XRD patterns. This demonstrates that the compound which is acquired by the sonochemical process as nanostructures is modification to that obtained by single crystal diffraction. The significant broadening of the peaks shows that the particles have nanometer dimensions. To examine the eff ect of the initial concentration on the morphology and size, experiments were performed by changing the concentration of reagents in the range of 0.02–0.005 mol.L−1 (Table1). Fig. 5 shows the FE-SEM images provided as nanorods of TMU-18 were obtained for all two sonication time (30 and 60 min) at concentrations 0.01M (Fig. 4b). For this MOF, with a concentration of [0.02M] of
6
initial reagents prepare the agglomerate nanorod morphology (Fig.4a). Also, when the current concentration goes up to 0.005 M the nanoparticles were obtained (Fig. 4c). In order to investigate the effect of sonication time on morphology and size of TMU-18, the synthesis was also performed at two diff erent ultrasonic reaction times. FE-SEM images demonstrated that in 30 min, nanorods were generated and increasing the time of sonication has the same eff ect as the concentration on the size and morphology of these compounds (Fig 5). The results display that the size and morphology of the TMU-18 nanostructure strongly depends on concentration and time. To explore the ability of TMU-18 nanostructure for discover a small quantity of analytes such as nitro aromatics (NAC), fluorescence titrations were performed with the incremental addition of analytes to TMU-18 nanorods dispersed in methanol. After NMOF TMU-18 dispersed in methanol shows typical photoluminescence attributes with emission maximum at 423 nm upon excitation at 330 nm, that can be specified to the luminescence from intra-ligand emission excited state. To illustrate the potential of NMOF TMU-18 nanorods for the detection of nitroaromatic compounds, luminescence quenching experiments were carried out. The solutions of nitrobenzene and 1,3dinitrobenzene in methanol (0.2 M) were dropped into 3 mL of methanol in which NMOF TMU-18 was dispersed, respectively. The effect of the sample pH was tested in different range, the maximum quenching were obtained at pH= 6, which is the pH of the isolate MOF in water, so it was chosen as the optimum value at further studies. It was not found good absorption in other pH ranges due to the involvement of the urea functional group. The decrease of the fluorescence intensity of NMOF TMU-18 with the volume of solutions of nitroaromatic compounds show efficient quenching effects (Fig.6a-e). Among them, the most effective quencher is TNP and the order of quenching efficiency is 2,4,6-trinitrophenol(TNP)˃>2,4dinitrophenol(DNP)>4-nitrophenol(NP)>1,3–dinitrobenzene
(DNB)>nitrobenzene(NB)
(Fig.7).
The
fluorescence quenching efficiency is calculated using [(I0-I)/I0] × 100%,where I0 is the initial fluorescence
7
intensity of soaked MOF sample in methanol and I is the fluorescence intensity in the presence of desired nitro aromatic compounds [34]. Comparison of KSV of TMU-18 nanorods towards different nitro aromatic compounds (NACs) indicates a highly selective response to nitrobenzene compound (Fig. 8). Our results show that one of the reasons for high Ksv values for the nitro aromatic compounds especially TNP is probably due to the strongly electron-withdrawing -NO2 groups [35-36]. Fig.10 illustrate the O– H⋯O interaction between TNP and the urea ligand which explains the high selectivity for TNP compared to other nitro-aromatics. Moreover, the linear correlation coefficient (R) in the Ksv curve suggests that the quenching effect of analytes on the fluorescence of TMU-18 fit the Stern-Volmer mode well (Fig. 9). These results reveal that NMOF TMU-18 has a sensitive detection of nitro aromatic explosives easily, environmentally and rapidly. Furthermore, UV-Vis plot for analytes and fluorescence spectrum of MOF indicate that the significant sensitivity of TNP originates from overlap with the excitation and emission bands. The greater the spectral overlap between the absorbance spectrum of the analyte and the emission spectrum of the MOF, the higher the probability of energy transfer and hence fluorescence quenching. Fig. 11. As a result, TNP can adsorb excitation and emission energy which decrease the fluorescence intensity. This might suggest energy transfer mechanism for the fluorescence quenching by TNP [37].
4. Conclusions In summary, pillared-layer metal-organic framework, TMU-18, [Zn2(ubl)2(bipy)]·DMF has been synthesized by sonochemical irradiation. The eff ects of various parameters such as various concentrations of initial reagents and diff erent times of irradiation were also tested for study the obtained morphologies. The results show that nanorods morphology of this MOF can be obtained in 0.01 M concentration with, 305 W. Moreover, this compound exhibited selective detection of TNP, even in the presence of other nitro compounds in methanol solution. The high quenching efficiency and good selectivity of TMU-18 for TNP
8
make it a potential multifunctional MOF for detection of explosives. This study has confirmed that the multifunctional MOF materials combine optical-sensing, and sensitization properties, thus is very useful in many potential applications.
Acknowledgements Support of this investigation by Tarbiat Modares University, Iran University of Science and Technology and the Iran National Science Foundation (INSF) are gratefully acknowledged.
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for
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13
Scheme 1. Ultrasonic and solvothermal synthesis of TMU-18.
14
Fig. 1. The packing diagram of the structure of TMU-18.
15
\ Fig. 2. FT-IR spectrum of (a) nanostructures of TMU-18 produced by sonochemical method and (b) bulk materials of TMU-18.
Fig. 3. XRD patterns; (a) simulated pattern based on single crystal data of compound TMU-18, (b) nanostructure of compound TMU-18 prepared by sonochemical process.
16
Fig. 4. Field-emission scanning electron microscopy (FE-SEM) images of TMU-18 samples obtained with various concentrations: (a) 0.02M, (b) 0.01M, (c) 0.005M.
Fig. 5. Field-emission scanning electron microscopy (FE-SEM) images of TMU-18 samples obtained with various time: (a) 30 min (0.01 M), (b) 1h (0.01 M).
17
Table 1. Experimental details for synthesis of TMU-18 nanorod in 10 ml DMF and sonication power 12W. Sample name
Molar ratio (ubl:bipy:Zn(OAc)2) mmol
Concentration
Time
[ubl]/[bipy]/
(min)
Morphology
[Zn(OAc)2] (M)
i
1:1:1
[0.02]/[0.02]/[0.02]
30
Agglomerated
ii
1:1:1
[0.02]/[0.02]/[0.02]
60
Agglomerated
iii
1:1:1
[0.01]/[0.01]/[0.01]
30
Nano-rods
iv
1:1:1
[0.01]/[0.01]/[0.01]
60
Nano-rods
v
1:1:1
[0.005]/[0.005]/[0.005]
30
Nanoparticles
vi
1:1:1
[0.005]/[0.005]/[0.005]
60
Nanoparticles
18
Fig. 6. Variation of emission spectra of NMOF 1 nanorods with different concentration of nitroaromatic compounds: a) nitrobenzene; (b) 1,3-dinitrobenzene; (c) 4- nitrophenol: (d) 2,4-dinitrophenol ; (e) 2,4,6trinitrophenol in 3 mL methanol. The excitation wavelength is 330 nm, fluorescence emission was monitored from 380 nm to 580 nm. 19
Fig. 7. Percentage of fluorescence quenching of TMU-18 obtained for different analytes in methanol solution at room temperature.
20
Fig. 8. Comparison of KSV of TMU-18 nanorods towards different nitroaromatic compounds (NACs).
21
Fig. 9. Corresponding KSV plots of the analytes in methanol.
22
Fig 10. Schematic image of O–H⋯O interaction between TNP and urea ligand.
23
Fig 11. UV-Vis absorption spectra of analytes and the excitation and emission spectra of TMU-18.
24
Highlights
Pillared-layer MOF have been synthesized by ultrasonic method and characterized.
Concentration of initial reagents influences the size and morphology of MOFs.
High concentrations of initial reagents increases rods size of TMU-18.
Low concentrations of initial reagents lead to nanoparticles.
High-sensitivity detection of TNP even in the presence of other nitro compound.
25
S1350-4177(18)30802-2 https://doi.org/10.1016/j.ultsonch.2018.11.002 ULTSON 4371
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
27 May 2018 3 November 2018 3 November 2018
Please cite this article as: A. Hakimifar, A. Morsali, High-sensitivity Detection of Nitroaromatic Compounds (NACs) by the pillared-layer metal- organic frameworksynthesized viaultrasonic method, Ultrasonics Sonochemistry (2018), doi: https://doi.org/10.1016/j.ultsonch.2018.11.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High-sensitivity Detection of Nitroaromatic Compounds (NACs) by the pillared-layer metal- organic framework synthesized via ultrasonic method
Azar Hakimifar and Ali Morsali*
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-4838, Tehran, Islamic Republic of Iran, Tel: +98-21-82884416, Email: [email protected]
1
Abstract Nanorods of zinc(II) based metal-organic framework (MOF) were prepared via ultrasonic method without any surfactants at room temperature and atmospheric pressure. Control of particle size and morphology was enhanced
in
this
synthesis
method.
Nanorods
of
pillared-layer
metal
organic
framework,
[Zn2(ubl)2(bipy)]·DMF (TMU-18), where ubl (urea-based ligand) is 4,4'-carbonylbis(azanediyl)dibenzoic acid,
4,4'-Bipyridine (bipy)
DMF = N,N-dimethyl formamide), was synthesized under ultrasound
irradiation in different concentrations of initial precursor. The nano structure and morphology of the synthesized MOF were characterized by Field Emission Scanning Electron Microscopy (FE-SEM), powder X-ray diffraction, elemental analysis and FT-IR spectroscopy. Moreover, Fluorescence emissions of nanorods have been studied. Luminescent MOFs (LMOFs) have shown great potential as sensor for various nitro explosives by modulating the luminescence behavior in presence of nitro explosives.
Urea-
functionalized MOF shows high selectivity for sensing of the nitro explosive 2,4,6-trinitrophenol (TNP) even in the presence of other nitroaromatic compounds in methanol solution. Fluorescence intensity decreased with increasing contents of nitroaromatics in organic solution due to fluorescence quenching effect. The ultrasound method has some advantages such as short duration time of reaction, no need to high temperatures and pressures for synthesis nano-materials and low costs in comparison to other methods. Considering these advantages we used ultrasonic method to produce these nanorods which show high sensitivity in detecting nitroaromatics.
Keywords:
Metal-organic
Framework;
Nanorods;
trinitrophenol.
2
Ultrasonic
irradiation;
Luminescence;
2,4,6-
1.
Introduction Three-dimensional (3D) metal–organic frameworks (MOFs) are a new class of porous materials which
have attracted extensive attention due to their potential applications as functional materials in structuredependent applications, such as gas storage and separation, ion exchange, sensing, catalysis and drug delivery [1–5].The MOFs can be designed by choosing appropriate organic ligands and inorganic secondary building units (SBUs) [6–9]. Synthesis of nanoscale MOFs has also been attractive [10-12]. Traditional techniques, like slow vapor diffusion methods and solvothermal techniques, are commonly applied to prepare MOFs [13]. However, the reaction processes using these custom techniques usually have to be implemented at high reaction temperature and pressure which could take long time in many cases. Synthesis of nano MOFs (NMOFs) has also been attractive. One of the simplest and most effective methods for preparation of nano or microstructures MOFs is ultrasound irradiation [14–17]. In this simple, fast and green method molecules undergo a chemical reaction because of powerful ultrasound radiation in the range of 20 KHz to 10 MHz [18]. The important part of sonochemistry is the generation, growth, and collapse of a bubble which is produced in aquatic ambience. This process is called acoustic cavitation which presents very high local temperatures from 5000 to 25000 K and pressures with a very higher rate of cooling and heating [19 -21]. The results show that ultrasonic synthesis is one of the simplest and most effective method for the construction of nano MOFs [22]. Nitroaromatic compounds such as 2,4,6-trinitrophenol (TNP), nitrobenzene (NB), and TNT are explosive in nature. Therefore, fast and highly selective detection of them is very important [23-27]. One of such toxic ordinance molecule is TNP, a yellow crystalline solid commonly known as picric acid [28]. Also, TNP is widely used in aniline synthesis, oxidative metal itching, dyes, matches, fireworks, glass, and
3
leather industries [29]. During commercial production and use, TNP is released into the environment and impart serious health and environmental issues. Thus, there is an urgent need for efficient and reliable sensors for the detection of TNP. Recently, fluorescence based detection systems are gaining increasing attention for detection of NACs pertaining to their high sensitivity, ability to detect even single molecule, short response time and flexibility to employ in both solid as well as liquid phase applications [30]. Detectable variations in luminescence by tuning the host–guest chemistry along with tailorable porosity and a high surface area makes MOFs excellent candidates for sensing [31]. Variety of π-electron rich fluorescent organic molecules has been employed for the fluorescence based sensing of explosive molecules. NACs are oxidizers due to a low-lying unoccupied π* orbital, which can accept an electron from the excited state fluorophore, thus efficiently quenching the fluorescence emission of this compound. Interacting with these electron rich moieties though π-π stacking interaction and subsequently lead to decrease in fluorescence character. High sensitivity, differential response towards pool of analytes and feasibility in device integration have already been demonstrated [32]. In this work, we synthesized [Zn2(ubl)2(bipy)]·DMF(TMU-18) as pillared-layer MOF by the sonochemical method, which have shown fluorescence properties. Also in this report we study ultrasonic synthesis of TMU-18 in different time and concentrations of initial reagent to understand the effect of them on size and shape of particles. Moreover, we have investigated the fluorescence behavior of TMU-18 with various nitro aromatic analytes. The study reveals that urea functional groups inside the pore cavity of MOFs can detect TNP in a very fast, selective and sensitive way even in presence of other aromatic compounds.
2. Experimental
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All reagents for the syntheses and analysis were commercially available and used as received. IR spectra were recorded using Nicolet Fourier Transform IR, Nicolet 100 spectrometer in the range 500-4000 cm-1 using the KBr disk technique. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert diffractometer with mono chromated Co-Ka (1.78897 Å) radiation. Ultrasonic generation was carried out in an ultrasonic bath SONICA-2200 EP (frequency of 40 KHz). The sonicator used in this study was a SONICA-2200 EP with an adjustable power output (maximum 305 W at 40/60 kHz). A horn type tube Pyrex reactor was custom-made and fitted to the sonicator bath. The samples were characterized with a field emission scanning electron microscope (FE-SEM) ZEISS SIGMA VP (Germany) with gold coating.
2.1. Synthesis of compound TMU-18 nano-structures using sonochemical method To prepare nano-sized TMU-18, 10 mL solution of zinc (II) acetate dihydrate (0.5 mmol, 0.065 g) in DMF was positioned in a high-density ultrasonic bath at ambient temperature and atmospheric pressure, operating with a power output of 305 W. Into this solution 0.5 mmol (0.150 g) and 1 mmol (0.078 g) of the ligands ubl and bipy, respectively, were added and sonicated for 60 min. The obtained precipitates were filtered off, washed with DMF and then dried in air. FT-IR (KBr pellet , cm-1): 3317 (br), 1673 (vs), 1596 (vs), 1400 (vs), 1302 (s), 1227 (m), 1178 (m), 862 (w), 775 (m), 651 (w), 549 (w). For studying the effect of initial reagent concentrations on size and morphology of nanostructure TMU-18, the above processes were done with different concentrations (0.02, 0.01, and 0.005 M).
3. Results and discussion
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Our previous study showed that the reaction between ‘‘ubl’’ ligand with a mixture of Zn(NO3)2, bipy and DMF led to the formation the single crystal of zinc(II) porous coordination metal-organic framework, [Zn2(ubl)2(bipy)]·DMF (TMU-18) crystallizes in the Triclinic Pī space group. In this compound, the coordination geometry around the Zn(II) can be described as distorted octahedral, with four sites occupied by oxygen atoms of four different urea ligand carboxylate groups in an approximately square configuration and the fifth site occupied by a nitrogen atom of bipy ligand (Fig.1) [ 34 ].The remaining coordination site of each metal center is located inside the zinc paddle-wheel cluster. In this study we prepare nanorod of TMU-18 by ultrasonic irradiation in DMF solution with different concentrations (0.02, 0.01 and 0.005 M). The best morphology was observed at concentration 0.01M. Single crystalline materials were obtained using a heat gradient applied to a solution of the reagents [33]. Scheme 1 gives an overview of the methods used for the synthesis of TMU-18 using the two different paths. The IR spectra of both bulk material and nanorods of TMU-18 produced by conventional heating and sonochemical method show the amide NH stretching absorption can be observed at 3317cm -1. The amide C=O stretching absorption is 1673cm
-1
(Fig. 2). Fig 3. shows the simulated XRD pattern from single
crystal X-ray data of TMU-18 (Fig. 3a) in comparison with the XRD pattern of TMU-18 prepared by the sonochemical method (Fig. 3b). Desirable agreement was observed between the simulated and experimental XRD patterns. This demonstrates that the compound which is acquired by the sonochemical process as nanostructures is modification to that obtained by single crystal diffraction. The significant broadening of the peaks shows that the particles have nanometer dimensions. To examine the eff ect of the initial concentration on the morphology and size, experiments were performed by changing the concentration of reagents in the range of 0.02–0.005 mol.L−1 (Table1). Fig. 5 shows the FE-SEM images provided as nanorods of TMU-18 were obtained for all two sonication time (30 and 60 min) at concentrations 0.01M (Fig. 4b). For this MOF, with a concentration of [0.02M] of
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initial reagents prepare the agglomerate nanorod morphology (Fig.4a). Also, when the current concentration goes up to 0.005 M the nanoparticles were obtained (Fig. 4c). In order to investigate the effect of sonication time on morphology and size of TMU-18, the synthesis was also performed at two diff erent ultrasonic reaction times. FE-SEM images demonstrated that in 30 min, nanorods were generated and increasing the time of sonication has the same eff ect as the concentration on the size and morphology of these compounds (Fig 5). The results display that the size and morphology of the TMU-18 nanostructure strongly depends on concentration and time. To explore the ability of TMU-18 nanostructure for discover a small quantity of analytes such as nitro aromatics (NAC), fluorescence titrations were performed with the incremental addition of analytes to TMU-18 nanorods dispersed in methanol. After NMOF TMU-18 dispersed in methanol shows typical photoluminescence attributes with emission maximum at 423 nm upon excitation at 330 nm, that can be specified to the luminescence from intra-ligand emission excited state. To illustrate the potential of NMOF TMU-18 nanorods for the detection of nitroaromatic compounds, luminescence quenching experiments were carried out. The solutions of nitrobenzene and 1,3dinitrobenzene in methanol (0.2 M) were dropped into 3 mL of methanol in which NMOF TMU-18 was dispersed, respectively. The effect of the sample pH was tested in different range, the maximum quenching were obtained at pH= 6, which is the pH of the isolate MOF in water, so it was chosen as the optimum value at further studies. It was not found good absorption in other pH ranges due to the involvement of the urea functional group. The decrease of the fluorescence intensity of NMOF TMU-18 with the volume of solutions of nitroaromatic compounds show efficient quenching effects (Fig.6a-e). Among them, the most effective quencher is TNP and the order of quenching efficiency is 2,4,6-trinitrophenol(TNP)˃>2,4dinitrophenol(DNP)>4-nitrophenol(NP)>1,3–dinitrobenzene
(DNB)>nitrobenzene(NB)
(Fig.7).
The
fluorescence quenching efficiency is calculated using [(I0-I)/I0] × 100%,where I0 is the initial fluorescence
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intensity of soaked MOF sample in methanol and I is the fluorescence intensity in the presence of desired nitro aromatic compounds [34]. Comparison of KSV of TMU-18 nanorods towards different nitro aromatic compounds (NACs) indicates a highly selective response to nitrobenzene compound (Fig. 8). Our results show that one of the reasons for high Ksv values for the nitro aromatic compounds especially TNP is probably due to the strongly electron-withdrawing -NO2 groups [35-36]. Fig.10 illustrate the O– H⋯O interaction between TNP and the urea ligand which explains the high selectivity for TNP compared to other nitro-aromatics. Moreover, the linear correlation coefficient (R) in the Ksv curve suggests that the quenching effect of analytes on the fluorescence of TMU-18 fit the Stern-Volmer mode well (Fig. 9). These results reveal that NMOF TMU-18 has a sensitive detection of nitro aromatic explosives easily, environmentally and rapidly. Furthermore, UV-Vis plot for analytes and fluorescence spectrum of MOF indicate that the significant sensitivity of TNP originates from overlap with the excitation and emission bands. The greater the spectral overlap between the absorbance spectrum of the analyte and the emission spectrum of the MOF, the higher the probability of energy transfer and hence fluorescence quenching. Fig. 11. As a result, TNP can adsorb excitation and emission energy which decrease the fluorescence intensity. This might suggest energy transfer mechanism for the fluorescence quenching by TNP [37].
4. Conclusions In summary, pillared-layer metal-organic framework, TMU-18, [Zn2(ubl)2(bipy)]·DMF has been synthesized by sonochemical irradiation. The eff ects of various parameters such as various concentrations of initial reagents and diff erent times of irradiation were also tested for study the obtained morphologies. The results show that nanorods morphology of this MOF can be obtained in 0.01 M concentration with, 305 W. Moreover, this compound exhibited selective detection of TNP, even in the presence of other nitro compounds in methanol solution. The high quenching efficiency and good selectivity of TMU-18 for TNP
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make it a potential multifunctional MOF for detection of explosives. This study has confirmed that the multifunctional MOF materials combine optical-sensing, and sensitization properties, thus is very useful in many potential applications.
Acknowledgements Support of this investigation by Tarbiat Modares University, Iran University of Science and Technology and the Iran National Science Foundation (INSF) are gratefully acknowledged.
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Scheme 1. Ultrasonic and solvothermal synthesis of TMU-18.
14
Fig. 1. The packing diagram of the structure of TMU-18.
15
\ Fig. 2. FT-IR spectrum of (a) nanostructures of TMU-18 produced by sonochemical method and (b) bulk materials of TMU-18.
Fig. 3. XRD patterns; (a) simulated pattern based on single crystal data of compound TMU-18, (b) nanostructure of compound TMU-18 prepared by sonochemical process.
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Fig. 4. Field-emission scanning electron microscopy (FE-SEM) images of TMU-18 samples obtained with various concentrations: (a) 0.02M, (b) 0.01M, (c) 0.005M.
Fig. 5. Field-emission scanning electron microscopy (FE-SEM) images of TMU-18 samples obtained with various time: (a) 30 min (0.01 M), (b) 1h (0.01 M).
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Table 1. Experimental details for synthesis of TMU-18 nanorod in 10 ml DMF and sonication power 12W. Sample name
Molar ratio (ubl:bipy:Zn(OAc)2) mmol
Concentration
Time
[ubl]/[bipy]/
(min)
Morphology
[Zn(OAc)2] (M)
i
1:1:1
[0.02]/[0.02]/[0.02]
30
Agglomerated
ii
1:1:1
[0.02]/[0.02]/[0.02]
60
Agglomerated
iii
1:1:1
[0.01]/[0.01]/[0.01]
30
Nano-rods
iv
1:1:1
[0.01]/[0.01]/[0.01]
60
Nano-rods
v
1:1:1
[0.005]/[0.005]/[0.005]
30
Nanoparticles
vi
1:1:1
[0.005]/[0.005]/[0.005]
60
Nanoparticles
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Fig. 6. Variation of emission spectra of NMOF 1 nanorods with different concentration of nitroaromatic compounds: a) nitrobenzene; (b) 1,3-dinitrobenzene; (c) 4- nitrophenol: (d) 2,4-dinitrophenol ; (e) 2,4,6trinitrophenol in 3 mL methanol. The excitation wavelength is 330 nm, fluorescence emission was monitored from 380 nm to 580 nm. 19
Fig. 7. Percentage of fluorescence quenching of TMU-18 obtained for different analytes in methanol solution at room temperature.
20
Fig. 8. Comparison of KSV of TMU-18 nanorods towards different nitroaromatic compounds (NACs).
21
Fig. 9. Corresponding KSV plots of the analytes in methanol.
22
Fig 10. Schematic image of O–H⋯O interaction between TNP and urea ligand.
23
Fig 11. UV-Vis absorption spectra of analytes and the excitation and emission spectra of TMU-18.
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Highlights
Pillared-layer MOF have been synthesized by ultrasonic method and characterized.
Concentration of initial reagents influences the size and morphology of MOFs.
High concentrations of initial reagents increases rods size of TMU-18.
Low concentrations of initial reagents lead to nanoparticles.
High-sensitivity detection of TNP even in the presence of other nitro compound.
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