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A pillar-layer metal-organic framework as a turn-on luminescent sensor for highly selective and sensitive detection of Zn(II) ion
Journal of Solid State Chemistry 279 (2019) 120968
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
A pillar-layer metal-organic framework as a turn-on luminescent sensor for highly selective and sensitive detection of Zn(II) ion Hossein Shayegan, Yeganeh Davoudabadi Farahani, Vahid Safarifard * Department of Chemistry, Iran University of Science and Technology, Tehran, 16846-13114, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal-organic frameworks Luminescent Sensing Zn2þ
The intriguing architectures of metal-organic frameworks (MOFs), makes them an emerging class of promising materials in various application fields ranging from gas adsorption to sensing. In the present work, a threedimensional Zn-based MOF, Zn2(BDC)2(DABCO), (H2BDC ¼ 1,4-benzene dicarboxylic acid, DABCO ¼ 1,4-diazabicyclo[2.2.2]octane) was prepared and its sensing behavior to metal ions was investigated by fluorescence assays. Our results show that the luminescence intensity of Zn2(BDC)2(DABCO) is strongly sensitive to Zn2þ ions. The effects of selectivity, initial concentration, the limit of detection, response time and regeneration were studied in the case of Zn2þ detection operations. The linear response to Zn2þ was obtained across the concentration range of 50–200 μM with the detection limit of 0.7 μM. The sensor also displayed excellent performance in selectivity for Zn2þ over other metal ions including Fe3þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ and Cd2þ ions. The luminescence enhancement and recovery cycle were reiterated multiple times without much effect on the sensitivity. All the above features showed that this sensor as a sensing platform holds great potential for detection of Zn2þ with a response time of <1 min that has been rarely reported.
1. Introduction In recent years, the study of essential metal ions in the human body like zinc, iron, and copper have gained considerable attention due to their significance in many fundamental physiological processes [1]. Zinc ion (Zn2þ) is the second most abundant transition metal in the human body cells and plays a key role in many physiological and pathological processes including gene transcription, immune function, cellular metabolism, apoptosis, and neurotransmission [2]. Though it’s a relatively nontoxic element, both its deficiency or overload can cause various disorders [3]. The accumulation of free zinc ions leads to diabetes, epilepsy, ischemia, prostate cancer, and Alzheimer’s disease, while its deficiency induces growth retardation, diarrhea, and impotence [4]. Furthermore, Zn2þ can act as an environmental pollutant, because it’s excess amounts may reduce the soil microbial activity causing phytotoxic effects [5]. What’s worse, due to the Zn2þ closed-shell 3d10 configuration, it has no optical spectroscopic signature, which limits the types of methods that can be used for its study [6]. Therefore, the development of a selective and sensitive detecting method for sensing zinc in clinical, medicinal, environmental samples analysis is highly desirable. A variety of detection strategies have been developed for zinc metal
ion, such as ultraviolet–visible spectroscopy [7], atomic absorption spectroscopy [8], electrochemical [9]. However, a common constraint of all the above-mentioned is that their analytical results are easily impressed by the presence of metal ions with similar chemical properties (such as Cd2þ) [10]. Among the techniques employed, zinc metal ion sensing by fluorescence spectrometry has received much importance due to its simple equipment, rapid detection, and very high sensitivity [11–13]. In a fluorescence sensing system, a sensor is employed to the recognition of analytes and the fluorophore section converts the recognition events into electrical signals [14,15]. Therefore, the design of the sensor is the most important factor in fluorescence sensing systems. Some sensors already specified in the literature for Zn2þ ions using peptides [16], quinoline [17], rhodamine [18], fluorescein [19], polymers [20]. These sensors for detection Zn2þ still have drawbacks: i) some Zn2þ sensors involve boring synthetic procedures. ii) Some sensors are prepared from highly toxic precursors which would confine their applications in biological and environmental systems. iii) The detection processes of these sensors towards Zn2þ ion are relatively long. Hence, the design of fast, highly sensitive and selective fluorescent sensors for Zn2þ detection is one of the most important objectives.
* Corresponding author. E-mail address: [email protected] (V. Safarifard). https://doi.org/10.1016/j.jssc.2019.120968 Received 10 August 2019; Received in revised form 12 September 2019; Accepted 18 September 2019 Available online 19 September 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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Journal of Solid State Chemistry 279 (2019) 120968
Metal-organic frameworks (MOFs) are a class of well-known porous materials with widespread applications as fluorescent sensors due to their high structural diversity, tunable pores size, and excellent photoluminescence properties [21–26]. These sensors can show not only clear fluorescence intensity variations towards some specific metal ions, but also cause an obvious color change during the sensing event, facilitating "naked eye" detection [27,28]. Moreover, the introduction of functional groups into MOF will favor new progress of efficient sensors for detecting cations, anions, small molecules and explosives [29]. In this context, although a large number of MOFs have been designed and synthesized for detecting various chemical species, very few reports display sensing features for zinc metal ion. In the year 2018, Fan and co-workers synthesized a Tb-based MOF containing triazine groups that could use as a sensor for sensing and imaging of Zn2þ ions simultaneously [30]. Most recently, Majee et al. have reported a Cd-based MOF for selective sensing of Zn2þ with a limit detection of 0.3 μM [31]. On the other, most of the MOF-based sensors can sense various analytes through fluorescence quenching. For instance, Chen and colleagues reported europium–organic framework with tubular channels based on the semi-rigid carboxylate ligand for sensing Fe3þ/Al3þ by luminescent quenching/enhancing and found that the luminescent quenching/enhancing mechanism was related to the cation exchange process [32]. Xiang and co-workers synthesized an amino group functionalized MOF exhibiting selective sensing of Fe3þ through fluorescence quenching [33]. Hao et al. developed a three-dimensional europium MOF with highly detectable luminescence response for Cu2þ ion and acetone molecule [34]. In this article, a sensitive and selective turn-on fluorescence sensor, Zn2(BDC)2(DABCO), was further developed for the detection of Zn2þ metal ion in hexanol solution. Mixed-linker microporous MOF, Zn2(BDC)2(DABCO) have been constructed from Zn(II) ions and H2BDC (1,4-benzendicarboxylic acid) and DABCO (1,4-diazabicyclo[2.2.2]octane) ligands under solvothermal conditions. The sensing behavior of Zn2(BDC)2(DABCO) to various metal ions was studied by fluorescence spectra. The investigated results displayed that Zn2(BDC)2(DABCO) exhibited high selectivity, sensitivity, and rapid fluorescence increasing response to Zn2þ over other metal ions such as Fe3þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ and Cd2þ ions. The detection limit of this approach for Zn2þ reaches as low as 0.7 μM, which is lower than the maximum limit of Zn2þ (76 μM) in drinking water (World Health Organization). The possible sensing mechanism towards to Zn2þ was also discussed in detail. 2. Experimental section 2.1. Chemicals, reagents and apparatus Starting reagents for the synthesis were purchased and used without further purification from commercial suppliers (Merck, Sigma-Aldrich and others). Zn(NO3)2⋅6H2O and 1,4-benzenedicarboxylic acid (H2BDC), 1,4-diazabicyclo[2.2.2]octane (DABCO) were used to synthesize Zn2(BDC)2(DABCO). N,N-Dimethylformamide (DMF) was used as the solvent to purify Zn2(BDC)2(DABCO). Aqueous solutions of Cd2þ, Zn2þ, Pb2þ, Co2þ, As3þ, Ni2þ, Al3þ, Cu2þ, Mn2þ and Fe3þ were prepared from CdCl2⋅2.5H2O, Zn(NO3)2⋅6H2O, Pb(NO3)2, Co(NO3)2⋅6H2O, NaAsO2, Ni(OAc)2⋅4H2O, Al(NO3)3⋅9H2O, Cu(NO3)2⋅3H2O, MnCl2⋅6H2O, and Fe(NO3)3⋅9H2O, respectively. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert diffractometer with monochromated Cu-kα radiation (λ¼1.54056 Å). The simulated XRD powder pattern based on single crystal data was prepared using Mercury software [35]. The infrared spectra were recorded on a Nicolet Fourier Transform IR, Nicolet 100 spectrometer in the range 500–4000 cm-1 using the KBr disk technique. The fluorescence experiments were performed at room temperature on a Shimadzu RF-6000 fluorescence spectrometer (kyoto, Japan) with a photomultiplier voltage of 700 V, scan speed of 60,000 nm min1, excitation slit width of 900 nm, emission slit width of 200–800 nm, and a 380 nm optical filter. The fluorescent
Fig. 1. (a) Perspective view of Zn2(BDC)2(DABCO) structure as viewed along the c axis to show three-dimensional structure (Zn, blue; C, black; O, red). All disordered guest molecules are omitted for clarity. (b) PXRD of Zn2(BDC)2(DABCO): simulated (black), as-synthesized (red); Zn2(BDC)2(DABCO) @Zn2þ (blue). (c) FT-IR spectra of Zn2(BDC)2(DABCO) (red), Zn2(BDC)2(DABCO) @Zn2þ (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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temperature during 24 h. FT-IR (cm-1): 1676 (s), 1620 (s), 1573 (m), 1392 (vs), 1087 (m), 817 (m), 750 (m), 563 (m). Before the luminescence process, the crystals were washed with 3 5 mL of DMF and then dried at 100 C under vacuum for at least 24 h. Luminescent Experiments. In a typical experiment, equal volumes (20 μL) of metal ions (Mnþ ¼ Fe3þ, Zn2þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ, and Cd2þ) aqueous solution (0.01 M) and hexanol suspension of Zn2(BDC)2(DABCO) (1 mg, 4 mL) were mixed for the sensing studies. After stirred and ultrasonic treatment for 15 min, the emission spectra of these solutions were measured. Moreover, different amounts of Zn2þ ions (up to 360 μL) was gradually added to a 4 mL solution of Zn2(BDC)2(DABCO) dispersed in hexanol for luminescent titration measurements. The fluorescence enhancing spectra were recorded with excitation at 360 nm. 3. Results and discussion 3.1. Characterization of Zn2(BDC)2(DABCO) The solvothermal reaction of the organic ligands H2-BDC and DABCO with Zn(NO3)2⋅6H2O in N,N-dimethylformamide (DMF) afforded colorless block crystals of Zn2(BDC)2(DABCO). The guest-free MOF Zn2(BDC)2(DABCO) is formed of a Zn(II) paddle-wheel unit as a node, and BDC and DABCO as a linker and pillar, respectively. The Zn(II) paddle-wheel units tethered by terephthalate ligands (BDC) forms a distorted two-dimensional square lattices Zn2(1,4-BDC)2, which was further extended by DABCO into a 3D porous structure (Fig. 1a). The powder XRD pattern of the as-synthesized Zn2(BDC)2(DABCO) is shown in Fig. 1b, which is in good agreement with the simulated one. The asprepared Zn2(BDC)2(DABCO) was also monitored by FTIR spectroscopy. As shown in Fig. 1c the peak at ca. 3450 cm-1 is ascribed to O–H vibrations of uncoordinated water molecules. Besides the peak at 29502850 cm-1 is related to weak C–H bending vibration (aliphatic). The peaks with significant intensity at 1600 cm-1 and 1400 cm-1 can be assigned to νas(C–O) and νs(C–O) vibration of COO- groups, respectively. The XRD and FTIR results indicate that Zn2(BDC)2(DABCO) has been synthesized successfully. 3.2. Fluorescent properties of Zn2(BDC)2(DABCO) The MOFs containing transition metals with filled d-orbitals are very fascinating as luminescent material because they can yield strong linkerbased luminescence emission [31]. Therefore, the Zn2(BDC)2(DABCO) was explored for the application in the detection of metal ions in the liquid suspension state based on fluorescent sensing. Typically, to achieve the results correct and accurate from the detection of metal ions, the material should have a relatively strong emission intensity in the selected exciting wavelength. Here, we examined the excitation dependent emission of the Zn2(BDC)2(DABCO) by changing the excitation wavelength (Fig. 2a). The emission wavelength displayed no shift while the excitation wavelength was changed from 250 nm to 380 nm, indicating that the Zn2(BDC)2(DABCO) show the excitation independent fluorescence properties. Upon excitation with 360 nm beam, the fluorescence spectrum of Zn2(BDC)2(DABCO) displays a strong peak at ~436 nm. Next, the luminescence behavior of Zn2(BDC)2(DABCO) towards various solvents was investigated. The finely ground samples of activated Zn2(BDC)2(DABCO) (1 mg) were dispersed in different solvents (4 mL), including N,N0 -dimethylformamide (DMF), acetonitrile (MeCN), methanol (MeOH), ethanol (EtOH), tetrahydrofuran (THF), water and hexanol. Then, the luminescence spectra of these solutions were recorded in Fig. 2b and c at λex ¼ 360 nm. The luminescence intensity depends on the nature of the solvent molecule with the sequence of hexanol > THF > ethanol > water > methanol > DMF > acetonitrile. It was observed that maximum enhancing in emission intensity of Zn2(BDC)2(DABCO) took place only in hexanol solvent. For sensor Zn2(BDC)2(DABCO), hexanol can be used as a good dispersion solvent for
Fig. 2. (a) Fluorescence emission spectra (excitation wavelength (λex) from 250-380 nm) of Zn2(BDC)2(DABCO) (1mg) in hexanol solution. (b) The photoluminescence spectra of Zn2(BDC)2(DABCO) introduced into various pure solvents (solvents ¼ hexanol, THF, ethanol, water, methanol, DMF, acetonitrile). (c) Relative luminescence intensities at 436 nm of the Zn2(BDC)2(DABCO) @Solvent solutions.
emission spectra were recorded in the wavelength range of 300–800 nm upon excitation at 360 nm. Preparation of Zn2(BDC)2(DABCO). Zn(NO3)2⋅6H2O (0.245 g, 0.82 mmol), H2BDC (0.136 g, 0.97 mmol), DABCO (0.073 g, 0.65 mmol) were dissolved in 15 ml DMF. The mixture was placed in a Teflon reactor and heated at 120 C for 2 days, and then it was gradually cooled to room 3
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its liquid-phase luminescence sensing of metal ions. In order to estimate the special recognition ability of Zn2(BDC)2(DABCO), we investigated the fluorescence spectra of Zn2(BDC)2(DABCO) (1 mg) towards a series of metal ions (50 μM) including Fe3þ, Zn2þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ, and Cd2þ in hexanol solution. As shown in Fig. 3a, the suspension of Zn2(BDC)2(DABCO) in the absence of metal ions exhibited an excellent fluorescence peak at 436 nm. In contrast, the luminescence intensity of Zn2(BDC)2(DABCO)@Mnþ solutions shows the enhancing effect for all metal ions but the enhancing degree is strongly dependent on the species of metal ions. The most striking phenomenon is that Zn2þ ions have a very significant enhancing effect on the luminescence of Zn2(BDC)2(DABCO). Also, the fluorescence relative intensity of Zn2(BDC)2(DABCO) shows significant enhancing by 144% as compared to its initial solution in the presence of Zn2þ ions (Fig. 3b). Therefore, the primary metal ion selectivity experiment indicated that Zn2(BDC)2(DABCO) can sense Zn2þ ions via fluorescence turn-on behavior. Due to the fact that the reported fluorescent sensors for Zn2þ ions often have some interference induced by other metal ions (like Cu2þ), one basic challenge for the sensor is achieve the detection of Zn2þ in the presence of a wide range of competing ions [2,5]. The fluorescence responses of Zn2(BDC)2(DABCO) as a result of the addition of other metal ions, including Fe3þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ, and Cd2þ were also investigated. Fluorescence intensity changes of solutions of Zn2(BDC)2(DABCO) (1 mg, 4 ml), recorded in the presence of 250 μM Zn2þ and 250 μM of each of these metal ions under the identical condition, were illustrated in Fig. 3c. As you see, the competitive metal ions have a significant effect on the fluorescence of Zn2(BDC)2(DABCO) (the red bars). It is very encouraging that the enhancing effects by the Zn2þ ions on the fluorescence of the suspension of Zn2(BDC)2(DABCO) are almost not be influenced by the interfering metal ion (the blue bars). These results clearly indicated that the Zn2(BDC)2(DABCO) sensor possesses a highly selective sensing ability to Zn2þ ions. The concentration-dependent fluorescence measurements were conducted to examine how the concentration of Zn2þ influences the luminescence of the MOF. The finely ground sample of Zn2(BDC)2(DABCO) was dispersed in hexanol solution containing different concentrations of Zn2þ ions to monitor the fluorescent response. As shown in Fig. 4a, upon incremental addition of Zn2þ (50 μM) to the suspension of Zn2(BDC)2(DABCO), the fluorescence emission was gradually increased and reached the saturation state when 900 μM of Zn2þ was introduced. Also, the calibration plot displays a good linear relationship between the fluorescence intensity and the Zn2þ concentrations (Fig. 4b). To further research about the fluorescence enhancing effect of Zn2þ on fluorescence intensity of Zn2(BDC)2(DABCO), the fluorescence enhancing data of Zn2(BDC)2(DABCO) towards Zn2þ were analyzed by the Stern–Volmer equation: I0/I¼1 þ KSV[M], where I0 and I correspond to the luminescence intensity for Zn2(BDC)2(DABCO) in absence and presence of Zn2þ ions, respectively, [M] is the Zn2þ concentration, and KSV is the Stern–V€ olmer constant. The Stern–Volmer plot for Zn2þ ions are linear at low concentrations and with increase concentration gradually deviated from linearity. On the basis of the experimental data in Fig. 4b, a good linear relationship for the plot of I0/I-1 vs. [Zn2þ] in the range of 50–200 μM with correlation coefficient R2 ¼ 0.9939 was observed and the value of Ksv is estimated to be 1610.6 M1 for Zn2þ (Fig. 4c). The detection limit of 0.7 μM was obtained based on a 3σ/k (σ denotes the standard deviation of the blank signal and k refers to the slope of the intensity versus zinc ion concentration), which was comparable or superior than most of the previous reported assays for Zn2þ detection, indicating that the Zn2(BDC)2(DABCO) possessed excellent sensitivity in the detection of trace Zn2þ than other Zn2þ-sensing materials [36,37]. Besides high sensitivity and selectivity, fast response towards specific analyte is essential for any sensors. The fluorescence intensity of Zn2 -loaded sample was monitored at different time intervals to determine the response rate of the fluorescence enhancing upon the addition of
Fig. 3. (a) Fluorescence spectra and (d) Relative luminescence intensities of Zn2(BDC)2(DABCO) @hexanol suspension in presence of different metal ions with the concentration of 50 μM (λex ¼ 360 nm). (c) Comparison of the luminescence intensity of Zn2(BDC)2(DABCO) (1 mg) upon addition of various metal ions in the presence Zn2þ in hexanol solvent. The gray bars represent the emission of Zn2(BDC)2(DABCO) suspension in hexanol solvent. The red bars represent the emission of Zn2(BDC)2(DABCO) in the presence of 250 μ M of metal ions. The blue bars represent the change of the emission that occurs upon the subsequent addition of 250 μM Zn2þ to a solution containing of Zn2(BDC)2(DABCO) and 250 μM metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 4. (a) Increases in fluorescence emission of sensor Zn2(BDC)2(DABCO) in hexanol upon continuous addition of 0–950 μM of Zn2þ ions. (b) Ksv curve of Zn2(BDC)2(DABCO) in hexanol solutions in the presence of various concentrations of Zn2þ under excitation at 360 nm. (c) Linear relationship between I0/I-1 and Zn2þ concentration at 50–200 μM.
Fig. 5. (a) Fluorescence enhancing of Zn2(BDC)2(DABCO) by 100 μM Zn2þ in hexanol as a function of time (λex ¼ 360 nm). (b) Enhancing and recovery tests of Zn2(BDC)2(DABCO) for 50 μM Zn2þ in hexanol.
Zn2þ. To an optical quartz cell with a 1 cm path length containing Zn2(BDC)2(DABCO) (1mg) in a hexanol solution (4 ml), Zn2þ (100 μM) was added. The change in the fluorescence intensity of Zn2(BDC)2(DABCO) over a period of 14 min was recorded. Just as illustrated in Fig. 5a, the fluorescence intensity enhanced obviously within 1 min after the introduction of Zn2þ and the fluorescence intensity remained almost unchanged after 2 min, suggesting that the reaction between Zn2þ and Zn2(BDC)2(DABCO) is very fast. Thus the emission spectral studies revealed that 1 min reaction time is required to form metal ion-incorporated MOF, which is very much shorter than the previously reported sensors. Such a short response time of the Zn2(BDC)2(DABCO) towards the presence of Zn2þ ions can be ascribed to the quick interaction of the metal ion with luminophores on the pore surface. According to the above studies, we tried to explain the possible sensing mechanism of fluorescence increasing for Zn2þ. At present, the following three approaches are usually used to explain the mechanisms of fluorescence intensity of MOFs enhanced by metal ions. They are i) ion exchange with the targeted analyte, ii) the strong interaction between metal ions and the frameworks, and iii) the collapse of the main framework structure by ions [38,39]. In the present case, the possible reasons for the luminescence enhancing of Zn2(BDC)2(DABCO) are analyzed as follows: the PXRD study was first performed to check the structure changes during Zn2þ solution treatment. 5 mg of Zn2(BDC)2(DABCO) has dispersed in 5 ml hexanol solution of 50 μM Zn2þ. After immersion for 24 h, the solid compound was collected from the mixture through
centrifugation and then dried in an oven. Just as shown in Fig. 1b, the PXRD pattern of Zn2þ-loaded sample fitted well in the synthesized one, which indicates the crystalline state maintain during the enhancing process. Moreover, the FT-IR spectra of Zn2(BDC)2(DABCO)@Zn2þ was almost similar to that of Zn2(BDC)2(DABCO) (Fig. 1c). This indicates that the luminescence intensity effect was not caused by the framework collapse. Second, short response time can rule out that the fluorescence enhancing phenomena resulted from the ion exchange between the framework metal ions and Zn2þ ions. Although a donor-acceptor electron transfer mechanism has been previously reported to explain the change in luminescence intensity in MOFs [40,41], a deep understanding of such enhancement effects is still unclear. According to the above results, we speculate that the strong enhancing effect of Zn2þ on the fluorescence intensity of Zn2(BDC)2(DABCO) may originates from the strong interaction between the ligands and analyte. For a chemical sensor to be widely used to detect specific analytes, reversibility is a momentous aspect. We also studied the reversibility of Zn2þ sensing operation based on the Zn2(BDC)2(DABCO) sensor. The recognition reversibility of MOF@Zn2þ to initial MOF was performed by centrifuging the suspension after detecting selected analyte and washing several times with hexanol. The result was shown in Fig. 5b. When Zn2þloaded MOF was washed with hexanol, the fluorescence intensity at 436 nm was decreased and further addition of 50 μM Zn2þ could recover the fluorescence again. This process could be repeated at least four times without loss of sensitivity, which clearly demonstrates the high degree of reversibility of the Zn2(BDC)2(DABCO) sensor. Thus, 5
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Zn2(BDC)2(DABCO) can be classified as a reversible sensor for Zn2þ in the solution of hexanol. Such reversibility and regeneration are important for the design of sensors to sense the Zn2þ ions.
[16]
4. Conclusion
[17]
In summary, a highly luminescent Zn(II)-based MOF, Zn2(BDC)2(DABCO) has been successfully synthesized under solvothermal condition by employing 1,4-benzene dicarboxylic acid (H2BDC) and 1,4 diazabicyclo[2.2.2]octane (DABCO) as ligands. Sensor Zn2(BDC)2(DABCO) shows remarkable capabilities for identification of Zn2þ ion, even in the presence of other metal ions. This Zn2þ sensing system had a good linear relationship between fluorescence intensity and concentration of Zn2þ ions within a wide concentration range between 50 and 200 μM and the detection limit is calculated to be 0.7 μM which is below the acceptable level of Zn2þ in drinking water mandated by WHO. Results obtained in this study have opened a pave route for the development of high-performance sensing probes for the detection of other chemical species in environmental and biomedical applications.
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Acknowledgments [25]
Support of this investigation by Iran University of Science and Technology, Iran National Science Foundation: INSF and Iran’s National Elites Foundation is gratefully acknowledged.
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A pillar-layer metal-organic framework as a turn-on luminescent sensor for highly selective and sensitive detection of Zn(II) ion Hossein Shayegan, Yeganeh Davoudabadi Farahani, Vahid Safarifard * Department of Chemistry, Iran University of Science and Technology, Tehran, 16846-13114, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal-organic frameworks Luminescent Sensing Zn2þ
The intriguing architectures of metal-organic frameworks (MOFs), makes them an emerging class of promising materials in various application fields ranging from gas adsorption to sensing. In the present work, a threedimensional Zn-based MOF, Zn2(BDC)2(DABCO), (H2BDC ¼ 1,4-benzene dicarboxylic acid, DABCO ¼ 1,4-diazabicyclo[2.2.2]octane) was prepared and its sensing behavior to metal ions was investigated by fluorescence assays. Our results show that the luminescence intensity of Zn2(BDC)2(DABCO) is strongly sensitive to Zn2þ ions. The effects of selectivity, initial concentration, the limit of detection, response time and regeneration were studied in the case of Zn2þ detection operations. The linear response to Zn2þ was obtained across the concentration range of 50–200 μM with the detection limit of 0.7 μM. The sensor also displayed excellent performance in selectivity for Zn2þ over other metal ions including Fe3þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ and Cd2þ ions. The luminescence enhancement and recovery cycle were reiterated multiple times without much effect on the sensitivity. All the above features showed that this sensor as a sensing platform holds great potential for detection of Zn2þ with a response time of <1 min that has been rarely reported.
1. Introduction In recent years, the study of essential metal ions in the human body like zinc, iron, and copper have gained considerable attention due to their significance in many fundamental physiological processes [1]. Zinc ion (Zn2þ) is the second most abundant transition metal in the human body cells and plays a key role in many physiological and pathological processes including gene transcription, immune function, cellular metabolism, apoptosis, and neurotransmission [2]. Though it’s a relatively nontoxic element, both its deficiency or overload can cause various disorders [3]. The accumulation of free zinc ions leads to diabetes, epilepsy, ischemia, prostate cancer, and Alzheimer’s disease, while its deficiency induces growth retardation, diarrhea, and impotence [4]. Furthermore, Zn2þ can act as an environmental pollutant, because it’s excess amounts may reduce the soil microbial activity causing phytotoxic effects [5]. What’s worse, due to the Zn2þ closed-shell 3d10 configuration, it has no optical spectroscopic signature, which limits the types of methods that can be used for its study [6]. Therefore, the development of a selective and sensitive detecting method for sensing zinc in clinical, medicinal, environmental samples analysis is highly desirable. A variety of detection strategies have been developed for zinc metal
ion, such as ultraviolet–visible spectroscopy [7], atomic absorption spectroscopy [8], electrochemical [9]. However, a common constraint of all the above-mentioned is that their analytical results are easily impressed by the presence of metal ions with similar chemical properties (such as Cd2þ) [10]. Among the techniques employed, zinc metal ion sensing by fluorescence spectrometry has received much importance due to its simple equipment, rapid detection, and very high sensitivity [11–13]. In a fluorescence sensing system, a sensor is employed to the recognition of analytes and the fluorophore section converts the recognition events into electrical signals [14,15]. Therefore, the design of the sensor is the most important factor in fluorescence sensing systems. Some sensors already specified in the literature for Zn2þ ions using peptides [16], quinoline [17], rhodamine [18], fluorescein [19], polymers [20]. These sensors for detection Zn2þ still have drawbacks: i) some Zn2þ sensors involve boring synthetic procedures. ii) Some sensors are prepared from highly toxic precursors which would confine their applications in biological and environmental systems. iii) The detection processes of these sensors towards Zn2þ ion are relatively long. Hence, the design of fast, highly sensitive and selective fluorescent sensors for Zn2þ detection is one of the most important objectives.
* Corresponding author. E-mail address: [email protected] (V. Safarifard). https://doi.org/10.1016/j.jssc.2019.120968 Received 10 August 2019; Received in revised form 12 September 2019; Accepted 18 September 2019 Available online 19 September 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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Metal-organic frameworks (MOFs) are a class of well-known porous materials with widespread applications as fluorescent sensors due to their high structural diversity, tunable pores size, and excellent photoluminescence properties [21–26]. These sensors can show not only clear fluorescence intensity variations towards some specific metal ions, but also cause an obvious color change during the sensing event, facilitating "naked eye" detection [27,28]. Moreover, the introduction of functional groups into MOF will favor new progress of efficient sensors for detecting cations, anions, small molecules and explosives [29]. In this context, although a large number of MOFs have been designed and synthesized for detecting various chemical species, very few reports display sensing features for zinc metal ion. In the year 2018, Fan and co-workers synthesized a Tb-based MOF containing triazine groups that could use as a sensor for sensing and imaging of Zn2þ ions simultaneously [30]. Most recently, Majee et al. have reported a Cd-based MOF for selective sensing of Zn2þ with a limit detection of 0.3 μM [31]. On the other, most of the MOF-based sensors can sense various analytes through fluorescence quenching. For instance, Chen and colleagues reported europium–organic framework with tubular channels based on the semi-rigid carboxylate ligand for sensing Fe3þ/Al3þ by luminescent quenching/enhancing and found that the luminescent quenching/enhancing mechanism was related to the cation exchange process [32]. Xiang and co-workers synthesized an amino group functionalized MOF exhibiting selective sensing of Fe3þ through fluorescence quenching [33]. Hao et al. developed a three-dimensional europium MOF with highly detectable luminescence response for Cu2þ ion and acetone molecule [34]. In this article, a sensitive and selective turn-on fluorescence sensor, Zn2(BDC)2(DABCO), was further developed for the detection of Zn2þ metal ion in hexanol solution. Mixed-linker microporous MOF, Zn2(BDC)2(DABCO) have been constructed from Zn(II) ions and H2BDC (1,4-benzendicarboxylic acid) and DABCO (1,4-diazabicyclo[2.2.2]octane) ligands under solvothermal conditions. The sensing behavior of Zn2(BDC)2(DABCO) to various metal ions was studied by fluorescence spectra. The investigated results displayed that Zn2(BDC)2(DABCO) exhibited high selectivity, sensitivity, and rapid fluorescence increasing response to Zn2þ over other metal ions such as Fe3þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ and Cd2þ ions. The detection limit of this approach for Zn2þ reaches as low as 0.7 μM, which is lower than the maximum limit of Zn2þ (76 μM) in drinking water (World Health Organization). The possible sensing mechanism towards to Zn2þ was also discussed in detail. 2. Experimental section 2.1. Chemicals, reagents and apparatus Starting reagents for the synthesis were purchased and used without further purification from commercial suppliers (Merck, Sigma-Aldrich and others). Zn(NO3)2⋅6H2O and 1,4-benzenedicarboxylic acid (H2BDC), 1,4-diazabicyclo[2.2.2]octane (DABCO) were used to synthesize Zn2(BDC)2(DABCO). N,N-Dimethylformamide (DMF) was used as the solvent to purify Zn2(BDC)2(DABCO). Aqueous solutions of Cd2þ, Zn2þ, Pb2þ, Co2þ, As3þ, Ni2þ, Al3þ, Cu2þ, Mn2þ and Fe3þ were prepared from CdCl2⋅2.5H2O, Zn(NO3)2⋅6H2O, Pb(NO3)2, Co(NO3)2⋅6H2O, NaAsO2, Ni(OAc)2⋅4H2O, Al(NO3)3⋅9H2O, Cu(NO3)2⋅3H2O, MnCl2⋅6H2O, and Fe(NO3)3⋅9H2O, respectively. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert diffractometer with monochromated Cu-kα radiation (λ¼1.54056 Å). The simulated XRD powder pattern based on single crystal data was prepared using Mercury software [35]. The infrared spectra were recorded on a Nicolet Fourier Transform IR, Nicolet 100 spectrometer in the range 500–4000 cm-1 using the KBr disk technique. The fluorescence experiments were performed at room temperature on a Shimadzu RF-6000 fluorescence spectrometer (kyoto, Japan) with a photomultiplier voltage of 700 V, scan speed of 60,000 nm min1, excitation slit width of 900 nm, emission slit width of 200–800 nm, and a 380 nm optical filter. The fluorescent
Fig. 1. (a) Perspective view of Zn2(BDC)2(DABCO) structure as viewed along the c axis to show three-dimensional structure (Zn, blue; C, black; O, red). All disordered guest molecules are omitted for clarity. (b) PXRD of Zn2(BDC)2(DABCO): simulated (black), as-synthesized (red); Zn2(BDC)2(DABCO) @Zn2þ (blue). (c) FT-IR spectra of Zn2(BDC)2(DABCO) (red), Zn2(BDC)2(DABCO) @Zn2þ (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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temperature during 24 h. FT-IR (cm-1): 1676 (s), 1620 (s), 1573 (m), 1392 (vs), 1087 (m), 817 (m), 750 (m), 563 (m). Before the luminescence process, the crystals were washed with 3 5 mL of DMF and then dried at 100 C under vacuum for at least 24 h. Luminescent Experiments. In a typical experiment, equal volumes (20 μL) of metal ions (Mnþ ¼ Fe3þ, Zn2þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ, and Cd2þ) aqueous solution (0.01 M) and hexanol suspension of Zn2(BDC)2(DABCO) (1 mg, 4 mL) were mixed for the sensing studies. After stirred and ultrasonic treatment for 15 min, the emission spectra of these solutions were measured. Moreover, different amounts of Zn2þ ions (up to 360 μL) was gradually added to a 4 mL solution of Zn2(BDC)2(DABCO) dispersed in hexanol for luminescent titration measurements. The fluorescence enhancing spectra were recorded with excitation at 360 nm. 3. Results and discussion 3.1. Characterization of Zn2(BDC)2(DABCO) The solvothermal reaction of the organic ligands H2-BDC and DABCO with Zn(NO3)2⋅6H2O in N,N-dimethylformamide (DMF) afforded colorless block crystals of Zn2(BDC)2(DABCO). The guest-free MOF Zn2(BDC)2(DABCO) is formed of a Zn(II) paddle-wheel unit as a node, and BDC and DABCO as a linker and pillar, respectively. The Zn(II) paddle-wheel units tethered by terephthalate ligands (BDC) forms a distorted two-dimensional square lattices Zn2(1,4-BDC)2, which was further extended by DABCO into a 3D porous structure (Fig. 1a). The powder XRD pattern of the as-synthesized Zn2(BDC)2(DABCO) is shown in Fig. 1b, which is in good agreement with the simulated one. The asprepared Zn2(BDC)2(DABCO) was also monitored by FTIR spectroscopy. As shown in Fig. 1c the peak at ca. 3450 cm-1 is ascribed to O–H vibrations of uncoordinated water molecules. Besides the peak at 29502850 cm-1 is related to weak C–H bending vibration (aliphatic). The peaks with significant intensity at 1600 cm-1 and 1400 cm-1 can be assigned to νas(C–O) and νs(C–O) vibration of COO- groups, respectively. The XRD and FTIR results indicate that Zn2(BDC)2(DABCO) has been synthesized successfully. 3.2. Fluorescent properties of Zn2(BDC)2(DABCO) The MOFs containing transition metals with filled d-orbitals are very fascinating as luminescent material because they can yield strong linkerbased luminescence emission [31]. Therefore, the Zn2(BDC)2(DABCO) was explored for the application in the detection of metal ions in the liquid suspension state based on fluorescent sensing. Typically, to achieve the results correct and accurate from the detection of metal ions, the material should have a relatively strong emission intensity in the selected exciting wavelength. Here, we examined the excitation dependent emission of the Zn2(BDC)2(DABCO) by changing the excitation wavelength (Fig. 2a). The emission wavelength displayed no shift while the excitation wavelength was changed from 250 nm to 380 nm, indicating that the Zn2(BDC)2(DABCO) show the excitation independent fluorescence properties. Upon excitation with 360 nm beam, the fluorescence spectrum of Zn2(BDC)2(DABCO) displays a strong peak at ~436 nm. Next, the luminescence behavior of Zn2(BDC)2(DABCO) towards various solvents was investigated. The finely ground samples of activated Zn2(BDC)2(DABCO) (1 mg) were dispersed in different solvents (4 mL), including N,N0 -dimethylformamide (DMF), acetonitrile (MeCN), methanol (MeOH), ethanol (EtOH), tetrahydrofuran (THF), water and hexanol. Then, the luminescence spectra of these solutions were recorded in Fig. 2b and c at λex ¼ 360 nm. The luminescence intensity depends on the nature of the solvent molecule with the sequence of hexanol > THF > ethanol > water > methanol > DMF > acetonitrile. It was observed that maximum enhancing in emission intensity of Zn2(BDC)2(DABCO) took place only in hexanol solvent. For sensor Zn2(BDC)2(DABCO), hexanol can be used as a good dispersion solvent for
Fig. 2. (a) Fluorescence emission spectra (excitation wavelength (λex) from 250-380 nm) of Zn2(BDC)2(DABCO) (1mg) in hexanol solution. (b) The photoluminescence spectra of Zn2(BDC)2(DABCO) introduced into various pure solvents (solvents ¼ hexanol, THF, ethanol, water, methanol, DMF, acetonitrile). (c) Relative luminescence intensities at 436 nm of the Zn2(BDC)2(DABCO) @Solvent solutions.
emission spectra were recorded in the wavelength range of 300–800 nm upon excitation at 360 nm. Preparation of Zn2(BDC)2(DABCO). Zn(NO3)2⋅6H2O (0.245 g, 0.82 mmol), H2BDC (0.136 g, 0.97 mmol), DABCO (0.073 g, 0.65 mmol) were dissolved in 15 ml DMF. The mixture was placed in a Teflon reactor and heated at 120 C for 2 days, and then it was gradually cooled to room 3
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its liquid-phase luminescence sensing of metal ions. In order to estimate the special recognition ability of Zn2(BDC)2(DABCO), we investigated the fluorescence spectra of Zn2(BDC)2(DABCO) (1 mg) towards a series of metal ions (50 μM) including Fe3þ, Zn2þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ, and Cd2þ in hexanol solution. As shown in Fig. 3a, the suspension of Zn2(BDC)2(DABCO) in the absence of metal ions exhibited an excellent fluorescence peak at 436 nm. In contrast, the luminescence intensity of Zn2(BDC)2(DABCO)@Mnþ solutions shows the enhancing effect for all metal ions but the enhancing degree is strongly dependent on the species of metal ions. The most striking phenomenon is that Zn2þ ions have a very significant enhancing effect on the luminescence of Zn2(BDC)2(DABCO). Also, the fluorescence relative intensity of Zn2(BDC)2(DABCO) shows significant enhancing by 144% as compared to its initial solution in the presence of Zn2þ ions (Fig. 3b). Therefore, the primary metal ion selectivity experiment indicated that Zn2(BDC)2(DABCO) can sense Zn2þ ions via fluorescence turn-on behavior. Due to the fact that the reported fluorescent sensors for Zn2þ ions often have some interference induced by other metal ions (like Cu2þ), one basic challenge for the sensor is achieve the detection of Zn2þ in the presence of a wide range of competing ions [2,5]. The fluorescence responses of Zn2(BDC)2(DABCO) as a result of the addition of other metal ions, including Fe3þ, Mn2þ, Cu2þ, As3þ, Ni2þ, Pb2þ, Co2þ, Al3þ, and Cd2þ were also investigated. Fluorescence intensity changes of solutions of Zn2(BDC)2(DABCO) (1 mg, 4 ml), recorded in the presence of 250 μM Zn2þ and 250 μM of each of these metal ions under the identical condition, were illustrated in Fig. 3c. As you see, the competitive metal ions have a significant effect on the fluorescence of Zn2(BDC)2(DABCO) (the red bars). It is very encouraging that the enhancing effects by the Zn2þ ions on the fluorescence of the suspension of Zn2(BDC)2(DABCO) are almost not be influenced by the interfering metal ion (the blue bars). These results clearly indicated that the Zn2(BDC)2(DABCO) sensor possesses a highly selective sensing ability to Zn2þ ions. The concentration-dependent fluorescence measurements were conducted to examine how the concentration of Zn2þ influences the luminescence of the MOF. The finely ground sample of Zn2(BDC)2(DABCO) was dispersed in hexanol solution containing different concentrations of Zn2þ ions to monitor the fluorescent response. As shown in Fig. 4a, upon incremental addition of Zn2þ (50 μM) to the suspension of Zn2(BDC)2(DABCO), the fluorescence emission was gradually increased and reached the saturation state when 900 μM of Zn2þ was introduced. Also, the calibration plot displays a good linear relationship between the fluorescence intensity and the Zn2þ concentrations (Fig. 4b). To further research about the fluorescence enhancing effect of Zn2þ on fluorescence intensity of Zn2(BDC)2(DABCO), the fluorescence enhancing data of Zn2(BDC)2(DABCO) towards Zn2þ were analyzed by the Stern–Volmer equation: I0/I¼1 þ KSV[M], where I0 and I correspond to the luminescence intensity for Zn2(BDC)2(DABCO) in absence and presence of Zn2þ ions, respectively, [M] is the Zn2þ concentration, and KSV is the Stern–V€ olmer constant. The Stern–Volmer plot for Zn2þ ions are linear at low concentrations and with increase concentration gradually deviated from linearity. On the basis of the experimental data in Fig. 4b, a good linear relationship for the plot of I0/I-1 vs. [Zn2þ] in the range of 50–200 μM with correlation coefficient R2 ¼ 0.9939 was observed and the value of Ksv is estimated to be 1610.6 M1 for Zn2þ (Fig. 4c). The detection limit of 0.7 μM was obtained based on a 3σ/k (σ denotes the standard deviation of the blank signal and k refers to the slope of the intensity versus zinc ion concentration), which was comparable or superior than most of the previous reported assays for Zn2þ detection, indicating that the Zn2(BDC)2(DABCO) possessed excellent sensitivity in the detection of trace Zn2þ than other Zn2þ-sensing materials [36,37]. Besides high sensitivity and selectivity, fast response towards specific analyte is essential for any sensors. The fluorescence intensity of Zn2 -loaded sample was monitored at different time intervals to determine the response rate of the fluorescence enhancing upon the addition of
Fig. 3. (a) Fluorescence spectra and (d) Relative luminescence intensities of Zn2(BDC)2(DABCO) @hexanol suspension in presence of different metal ions with the concentration of 50 μM (λex ¼ 360 nm). (c) Comparison of the luminescence intensity of Zn2(BDC)2(DABCO) (1 mg) upon addition of various metal ions in the presence Zn2þ in hexanol solvent. The gray bars represent the emission of Zn2(BDC)2(DABCO) suspension in hexanol solvent. The red bars represent the emission of Zn2(BDC)2(DABCO) in the presence of 250 μ M of metal ions. The blue bars represent the change of the emission that occurs upon the subsequent addition of 250 μM Zn2þ to a solution containing of Zn2(BDC)2(DABCO) and 250 μM metal ions. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 4. (a) Increases in fluorescence emission of sensor Zn2(BDC)2(DABCO) in hexanol upon continuous addition of 0–950 μM of Zn2þ ions. (b) Ksv curve of Zn2(BDC)2(DABCO) in hexanol solutions in the presence of various concentrations of Zn2þ under excitation at 360 nm. (c) Linear relationship between I0/I-1 and Zn2þ concentration at 50–200 μM.
Fig. 5. (a) Fluorescence enhancing of Zn2(BDC)2(DABCO) by 100 μM Zn2þ in hexanol as a function of time (λex ¼ 360 nm). (b) Enhancing and recovery tests of Zn2(BDC)2(DABCO) for 50 μM Zn2þ in hexanol.
Zn2þ. To an optical quartz cell with a 1 cm path length containing Zn2(BDC)2(DABCO) (1mg) in a hexanol solution (4 ml), Zn2þ (100 μM) was added. The change in the fluorescence intensity of Zn2(BDC)2(DABCO) over a period of 14 min was recorded. Just as illustrated in Fig. 5a, the fluorescence intensity enhanced obviously within 1 min after the introduction of Zn2þ and the fluorescence intensity remained almost unchanged after 2 min, suggesting that the reaction between Zn2þ and Zn2(BDC)2(DABCO) is very fast. Thus the emission spectral studies revealed that 1 min reaction time is required to form metal ion-incorporated MOF, which is very much shorter than the previously reported sensors. Such a short response time of the Zn2(BDC)2(DABCO) towards the presence of Zn2þ ions can be ascribed to the quick interaction of the metal ion with luminophores on the pore surface. According to the above studies, we tried to explain the possible sensing mechanism of fluorescence increasing for Zn2þ. At present, the following three approaches are usually used to explain the mechanisms of fluorescence intensity of MOFs enhanced by metal ions. They are i) ion exchange with the targeted analyte, ii) the strong interaction between metal ions and the frameworks, and iii) the collapse of the main framework structure by ions [38,39]. In the present case, the possible reasons for the luminescence enhancing of Zn2(BDC)2(DABCO) are analyzed as follows: the PXRD study was first performed to check the structure changes during Zn2þ solution treatment. 5 mg of Zn2(BDC)2(DABCO) has dispersed in 5 ml hexanol solution of 50 μM Zn2þ. After immersion for 24 h, the solid compound was collected from the mixture through
centrifugation and then dried in an oven. Just as shown in Fig. 1b, the PXRD pattern of Zn2þ-loaded sample fitted well in the synthesized one, which indicates the crystalline state maintain during the enhancing process. Moreover, the FT-IR spectra of Zn2(BDC)2(DABCO)@Zn2þ was almost similar to that of Zn2(BDC)2(DABCO) (Fig. 1c). This indicates that the luminescence intensity effect was not caused by the framework collapse. Second, short response time can rule out that the fluorescence enhancing phenomena resulted from the ion exchange between the framework metal ions and Zn2þ ions. Although a donor-acceptor electron transfer mechanism has been previously reported to explain the change in luminescence intensity in MOFs [40,41], a deep understanding of such enhancement effects is still unclear. According to the above results, we speculate that the strong enhancing effect of Zn2þ on the fluorescence intensity of Zn2(BDC)2(DABCO) may originates from the strong interaction between the ligands and analyte. For a chemical sensor to be widely used to detect specific analytes, reversibility is a momentous aspect. We also studied the reversibility of Zn2þ sensing operation based on the Zn2(BDC)2(DABCO) sensor. The recognition reversibility of MOF@Zn2þ to initial MOF was performed by centrifuging the suspension after detecting selected analyte and washing several times with hexanol. The result was shown in Fig. 5b. When Zn2þloaded MOF was washed with hexanol, the fluorescence intensity at 436 nm was decreased and further addition of 50 μM Zn2þ could recover the fluorescence again. This process could be repeated at least four times without loss of sensitivity, which clearly demonstrates the high degree of reversibility of the Zn2(BDC)2(DABCO) sensor. Thus, 5
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Zn2(BDC)2(DABCO) can be classified as a reversible sensor for Zn2þ in the solution of hexanol. Such reversibility and regeneration are important for the design of sensors to sense the Zn2þ ions.
[16]
4. Conclusion
[17]
In summary, a highly luminescent Zn(II)-based MOF, Zn2(BDC)2(DABCO) has been successfully synthesized under solvothermal condition by employing 1,4-benzene dicarboxylic acid (H2BDC) and 1,4 diazabicyclo[2.2.2]octane (DABCO) as ligands. Sensor Zn2(BDC)2(DABCO) shows remarkable capabilities for identification of Zn2þ ion, even in the presence of other metal ions. This Zn2þ sensing system had a good linear relationship between fluorescence intensity and concentration of Zn2þ ions within a wide concentration range between 50 and 200 μM and the detection limit is calculated to be 0.7 μM which is below the acceptable level of Zn2þ in drinking water mandated by WHO. Results obtained in this study have opened a pave route for the development of high-performance sensing probes for the detection of other chemical species in environmental and biomedical applications.
[18] [19] [20]
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Acknowledgments [25]
Support of this investigation by Iran University of Science and Technology, Iran National Science Foundation: INSF and Iran’s National Elites Foundation is gratefully acknowledged.
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