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Colorimetric detection of heavy metal ions in water via metal-organic framework
Accepted Manuscript Research paper Colorimetric detection of heavy metal ions in water via Metal-Organic Framework Jiwon Kim, Jung Suk Oh, Kyoung Chul Park, Gajendra Gupta, Chang Yeon Lee PII: DOI: Reference:
S0020-1693(18)31386-0 https://doi.org/10.1016/j.ica.2018.10.025 ICA 18571
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
30 August 2018 16 October 2018 17 October 2018
Please cite this article as: J. Kim, J.S. Oh, K.C. Park, G. Gupta, C. Yeon Lee, Colorimetric detection of heavy metal ions in water via Metal-Organic Framework, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica. 2018.10.025
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.
Colorimetric detection of heavy metal ions in water via Metal-Organic Framework Jiwon Kima†, Jung Suk Oha†, Kyoung Chul Parka, Gajendra Guptaa,b*, Chang Yeon Leea,b* aDepartment
of Energy and Chemical Engineering and bInnovation Center for Chemical
Engineering, Incheon National University, Incheon 22012, Republic of Korea. * Corresponding author. Email address: [email protected] (G. G.); [email protected] (C. Y. L). †These
authors contributed equally to this work
ABSTRACT: Interest in the design and use of metal-organic frameworks (MOFs) for the selective detection of heavy metal ions is increasing rapidly. Herein, a new SALI-MAA-3eq MOF was synthesized by incorporating mercaptoacetic acid into NU-1000 via a solvent-assisted ligand incorporation (SALI) approach. SALI-MAA-3eq was fully characterized by common analytical techniques and further tested for sensing applications. SALI-MAA-3eq was found to selectively detect Hg2+ over a range of metal ions in an aqueous environment. Its selectivity was further confirmed by fluorescence and X-ray photoelectron spectroscopy (XPS) studies.
KEYWORDS: metal-organic frameworks, solvent-assisted ligand incorporation, heavy metal detection, luminescent properties. 1. INTRODUCTION
Mercury, one of the heavy metals, can damage the nervous and respiratory systems, causing symptoms such as muscle weakness, poor memory, paralysis of the limbs, and weakened lung function, among others [1-4]. Therefore, mercury sensing in aqueous media, which is one of its major routes of exposure, is becoming more and more important worldwide [5]. Thus, rapid and selective detection of mercury ions using several techniques has been widely studied, such as colorimetric detection, electrochemiluminescence, atomic absorption spectrometry, and fluorescence detection. Among these methods, investigation of the use of metal-organic frameworks for fluorescence detection methods is actively being carried out [6]. Metal-organic frameworks (MOFs), formed by self-assembly of organic ligands with metal ions or metal clusters, are porous materials suitable for various applications including gas storage, catalysis, sensing, drug delivery, and electrocatalysis [7-11]. NU-1000, one of the Zr-based MOFs which is known to be stable in water, is composed of Zr clusters and pyrene derivative ligands [12]. Pyrene, a polycyclic aromatic hydrocarbon (PAH), has a high quantum yield and long lifetime. This important feature of pyrene makes its complexes useful for detection purposes. Previous studies have also shown that the fluorescence of pyrene derivative ligands can be deactivated by mercury [13-14]. Moreover, versatile tools such as post-synthetic modification (PSM), solvent-assisted ligand exchange (SALE), and solvent-assisted ligand incorporation (SALI) can be used to synthesize MOFs which would otherwise be difficult due to limited linker solubility, stability, or problems with functional groups. Among these, SALI is a method in which the -OH group of a metal cluster of a MOF and the -COOH group of a newly included ligand are coupled through an acid-base reaction [15]. SALI has the advantage that it allows easy control over the properties of MOFs by combining functional groups with MOFs using solvents, thereby keeping the overall structure of
the MOFs unaltered and minimizing the generation of unwanted byproducts which can occur during the synthesis process [16].
Scheme. 1. Schematic representation of SALI using NU-1000 and mercaptoacetic acid. Thioglycolic acid, often called mercaptoacetic acid (MAA), is a ligand with an -SH functional group known to have strong binding affinity for mercury. In this short communication, we describe the successful incorporation of MAA into NU-1000 via SALI. The new SALI-MAA-3eq MOF was fully characterized by different analytical techniques. The SALI-MAA-3eq MOF was further utilized for the selective detection of metal ions in water and showed promising activity in selectively detecting mercury over a series of metal ions in water. 2. EXPERIMENTAL SECTION 2.1 Materials and reagents: 1,3,6,8-Tetrabromopyrene (97%), tetrakis(triphenylphosphine) palladium(0) (>98%), potassium tribasic phosphate, zirconyl chloride octahydrate (98%), benzoic acid (>99.5%), thioglycolic acid (>99%), aluminum nitrate nonahydrate (>98%), zinc nitrate hexahydrate (>98%), cadmium nitrate tetrahydrate (>98%), cobalt(II) nitrate hexahydrate (>98%), lead(II) nitrate (>98%), nickel(II) nitrate hexahydrate, and magnesium nitrate hexahydrate were purchased from Aldrich chemicals and used as received. Mercury(II) nitrate hydrate (98%) was purchased from Alfa Aesar and used as received. 1,4-Dioxane (>99.0%) and N,N-
dimethylformamide (>99.5%) were purchased from TCI and used as received. Hydrochloric acid (37%) was purchased from Acros Organics and used as received. Tetrahydrofuran (99%), chloroform (99.5%), sodium hydroxide (97%), and acetone (99.5%) were purchased from Daejung and used as received. Magnesium sulfate (anhydrous) was purchased from Junsei and used as received. (4-(Methoxycarbonyl)phenyl)boronic acid (97%) was purchased from Frontier Scientific and used as received. NU-1000 was synthesized by a previously reported method [12]. 2.2 Synthesis of SALI-MAA-3eq from NU-1000: A 45 mg (0.021 mmol) portion of activated NU-1000 was loaded into a 5-mL vial. Subsequently, 3 mL of a 0.021 M (0.063 mmol) solution of mercaptoacetic acid in DMF was added to the reaction vial. The vial was then placed in a 60 °C oven for 24 h to ensure good dispersion. The supernatant of the reaction mixture was exchanged with fresh DMF for washing. The process was repeated twice using DMF, dichloromethane, and acetone and the resulting material was finally dried under vacuum. 2.3 Experiments for metal ion detection: First, 3 mg of SALI-MAA-3eq was loaded into an 8mL vial. Then, 3 mL of a different metal solution was added into the vial containing SALI-MAA3eq. The vial was shaken well for around 3 min at room temperature and experiments were performed thereafter with 2 mL of the solution. 2.4 Characterization: Powder X-ray diffraction (PXRD) measurements were carried out on a Rigaku Smartlab with Cu Kα radiation over a range of 2° < 2θ < 30° in 0.02° steps with a counting time of 1 s per step. 1H NMR spectra were recorded using an Agilent 400-MR spectrometer. SEM images were obtained using a field-emission scanning electron microscope (JEOL, JSM-7800F) operated at an acceleration voltage of 15.0 kV. Samples were coated with a layer of Au (~3 nm thickness) prior to imaging. N2 adsorption/desorption isotherms were measured volumetrically at 77 K in the range of 7.0 × 10-6 < P/P0 < 1.00 with an Autosorb-iQ outfitted with the micropore
option from Quantachrome Instruments (Boynton Beach, Florida USA) using the Autosorb-iQ Win software package. The samples were activated at 120 °C for 12 h using the outgas port of the Autosorb-iQ instrument. The specific surface areas for N2 were calculated using the BrunauerEmmett-Teller (BET) model in the linear range, as determined using the consistency criteria. Xray photoelectron spectroscopy (XPS) was performed on a PHI 5000 VersaProbe II X-ray photoelectron spectrometer. Fluorescence lifetime and emission spectra were analyzed using a time-correlated single photon counting spectrophotometer (FluoTime300). 3. RESULTS AND DISCUSSION SALI-MAA-3eq was synthesized using NU-1000 and mercaptoacetic acid. NU-1000 was added into a vial containing mercaptoacetic acid (3 equivalents per NU-1000) in dimethylformamide. The mixture was sonicated for around 5 min and then the vial was placed in an oven at 60 °C for 24 h and cooled to room temperature. The yellow crystalline solids obtained were washed with dimethylformamide, methylene chloride, and then with acetone and dried in vacuum. The details of its synthesis are described in the Experimental section. Scanning electron microscopy (SEM) was applied to confirm the shapes of the as-synthesized NU-1000 and SALI-MAA-3eq MOFs. The parent NU-1000 was observed to have rod-shaped crystals with an individual size of 5–7 μm. The shape and size of the SALI-MAA-3eq were also found to be similar to those of the parent NU-1000 (Fig. 1a). It is interesting to observe that the physical morphology of the parent NU-1000 was retained even after the inclusion of mercaptoacetic acid in the new SALI-MAA-3eq through the solvent-assisted ligand incorporation method. This was also further confirmed by PXRD data. The PXRD patterns confirmed the phasepurity of both the NU-1000 as well as the SALI-MAA-3eq MOFs. It was observed that the PXRD patterns of the as-synthesized NU-1000 and SALI-MAA-3eq are consistent with that of the
simulated NU-1000 (Fig. 1b). Therefore, it was confirmed that the overall structure did not change even after SALI. Furthermore, 1H NMR analysis was performed to determine the ratio of mercaptoacetic acid incorporated into the Zr6 node of NU-1000. The 1H NMR spectra of both the MOFs were measured using a DMSO-d6/D2SO4 (9:1 ratio) solution. In addition to the proton peaks of the parent NU1000, a new peak at 3.5 ppm was observed in the spectrum of SALI-MAA-3eq (Fig. 1c, Fig. S4 and S5). This peak at 3.5 ppm can be assigned to the proton attached to the alpha carbon of the mercaptoacetic acid. Further integration assignment of these peaks showed that around 3.6 molecules of mercaptoacetic acid were incorporated per Zr6 node. The porosities of the as-synthesized NU-1000 and SALI-MAA-3eq MOFs were analyzed based on nitrogen adsorption-desorption isotherms at 77 K. For the N2 sorption isotherm analysis, each MOF was activated for 12 hours at 120 °C. At the low pressure of the isotherm graph of each MOF, a “knee” was observed, which is characteristic of type IV N2 isotherms [17]. The BET results show that the surface area of NU-1000 is 2253 m2/g and that of SALI-MAA-3eq is 1906 m2/g. This reduction in surface area of about 350 m2/g after SALI treatment on NU-1000 is reasonable due to the incorporation of mercaptoacetic acid into NU-1000 (Fig. 1d). In addition, the emissive properties of the MOF were not changed after SALI treatment under UV light at a wavelength of 365 nm in the solid state (Fig. S1).
Fig. 1. (a) Scanning electron microscopy images of NU-1000 and SALI-MAA-3eq; (b) Powder XRD patterns of simulated NU-1000 and as-synthesized NU-1000 and SALI-MAA-3eq; (c) 1H NMR spectra of NU-1000 and SALI-MAA-3eq in DMSO-d6/D2SO4 (9:1) solution and (d) N2absorption-desorption isotherms for NU-1000 and SALI-MAA-3eq. Because mercury is one of the most toxic heavy metals, its presence can cause severe health issues even at very low concentrations. Mercury ions in its +2 state are highly soluble in water which makes it much easier for them to get into living organisms. Therefore, several methods are being used by researchers for sensitive and selective mercury detection [6]. Among these methods, those based on absorption and emission spectroscopy are the most common. To this end, we applied our new SALI-MAA-3eq to determine if it can selectively detect mercury in solution.
Photoluminescence (PL) spectroscopy was applied to analyze the selective mercury detection ability of SALI-MAA-3eq. In order to confirm its selective sensing ability, SALI-MAA-3eq was tested over a wide range of metal ions.
Fig. 2. (a) Emission spectra showing the effect of different metal ions on SALI-MAA-3eq when treated with 1 mM aqueous solutions; (b) Normalized fluorescence intensity of SALI-MAA-3eq in the presence of different metal ions. To set up a sensing experiment, 3 mg of SALI-MAA-3eq was added to 3 mL of a 1 mM aqueous solution of various metal ions (Cd2+, Co2+, Mg2+, Zn2+, Ni2+, Hg2+, Pb2+, Al3+). The solution was mixed properly by shaking for 3 minutes and the experiments were performed thereafter. Fig. 2a shows the emission spectra of SALI-MAA-3eq when treated with several metal ions in an aqueous medium. The fluorescence intensity peak at 470 nm showed a significant quenching in the presence of Hg2+. However, with various other metal ions, it did not show any such response. This indicates that SALI-MAA-3eq was very selective in detecting Hg2+ over the range of metal ions studied (Fig. 2a and 2b). In order to further verify its selectivity, which is important for the practical detection of Hg2+, experiments involving the addition of different concentrations of Hg2+ metal ions (0.1-10.0 mM) and different time intervals (0-900 s) were performed (Fig. S2 and S3). As a result, it was confirmed that the fluorescence intensity was decreased with increasing concentration
of mercury and further increases in time had no significant effect on the fluorescence intensity. Fig. 3 shows that SALI-MAA-3eq displayed a significant change in fluorescence upon mercury detection which is clearly visible under UV-light. This further confirms the selectivity of SALIMAA-3eq towards Hg2+ detection.
Fig. 3. (a) Photographs of SALI-MAA-3eq bare, Hg, Co, Cd, Pb in white light (dispersed in water). (b) Photographs of SALI-MAA-3eq bare, Hg, Co, Cd, Pb in UV light (365 nm) (dispersed in water). Furthermore, the PL lifetime of the SALI-MAA-3eq before and after Hg treatment was measured. The average fluorescence lifetime showed a considerable decrease from 1.694 ns to 0.912 ns for Hg@SALI-MAA-3eq as compared to that of free SALI-MAA-3eq, which is due to the occurrence of fluorescence quenching (Table 1). Thus, fluorescence quenching in the pyrene ligands of SALI-MAA-3eq could be due to the heavy atom effect. The heavy atom effect due to mercury causes an intersystem crossing that changes from a singlet state to a triplet state through
a phenomenon called spin-orbit coupling [18-19]. Therefore, it can be confirmed that this quenching phenomenon was caused by mercury. Table 1: Fluorescence lifetime of SALI-MAA-3eq and Hg@ SALI-MAA-3eq. To further understand its
Sample
λex (nm)
λem (nm)
τAv (ns)
SALI-MAA-3eq
405
470
1.694
detection mechanism, X-
Hg@SALI-MAA-3eq
405
470
0.912
ray
photoelectron
spectroscopy analysis was performed on SALI-MAA-3eq and Hg@SALI-MAA-3eq. XPS analysis will help to determine the valence states of the metals associated with the system [20]. After the mercury detection experiments in solution, the remaining solid SALI-MAA-3eq was recovered and XPS analysis was performed. Fig. 4 shows the XP spectra of SALI-MAA-3eq before and after Hg2+ detection. Fig. 4d clearly indicates the presence of Hg on the surface of Hg@SALI-MAA-3eq after Hg2+ detection, in contrast to the spectrum in Fig. 4b which was taken before the addition of Hg2+ ions. The new sharp peaks at 103.6 eV and 99.6 eV in Fig. 4d, which belong to the Hg 4f peak in Hg@SALI-MAA-3eq, appeared due to the interaction between S and Hg, further confirming the detection of Hg2+ ions [20].
Fig. 4. X-ray photoelectron spectroscopy of SALI-MAA-3eq before and after Hg2+ detection. (a) S 2p and (b) Hg 4f of SALI-MAA-3eq; (c) S 2p and (d) Hg 4f of Hg@SALI-MAA-3eq 4. CONCLUSIONS In summary, we have designed a new fluorescent sensor for Hg2+ detection in water by incorporating mercaptoacetic acid into NU-1000 using a SALI approach. The formation of the new SALI-MAA-3eq MOF was confirmed by different analytical techniques. The SALI-MAA-3eq has shown promising sensing activity in selectively detecting Hg2+ ions among a range of metal ions in water. Our results suggest that thiol-based MOFs can be utilized for practical applications in the removal of heavy metal ions, specifically mercury. Further challenges remain in the detection and simultaneous removal of heavy atoms from water, which will be the focus of future research. AUTHOR INFORMATION
Corresponding Author *E-mail: [email protected] (G. G); [email protected] (C.Y. L). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by an Incheon National University Research Grant in 2015. REFERENCES [1] X. Zhang, T.F. Xia, K. Jiang, Y.J. Cui, Y. Yang, G.D. Qian, Highly sensitive and selective detection of mercury (II) based on a zirconium metal-organic framework in aqueous media, J. Solid State Chem. 253 (2017) 277-281. [2] E. Ha, N. Basu, S. B-O’Reilly, J.G. Dórea. E. McSorley, M. Sakamoto, H.M. Chan, Current progress on understanding the impact of mercury on human health. Environ. Res. 152 (2017) 419-433. [3] K.-H. Kim, E. Kabir, S. A. Jahan, A review on the distribution of Hg in the environment and its human health impacts. J. Hazard. Mater. 306 (2016) 376-385. [4] P. Holmes, K.A.F. James, L.S. Levy, Is low-level environment mercury exposure of concern to human health? Sci. Total Environ. 408 (2009) 171-182. [5] C. Li, J. Huang, H. Zhu, L. Liu, Y. Feng, G. Hu, X. Yu, Dual-emitting fluorescence of Eu/ZrMOF for ratiometric sensing formaldehyde, Sens. Actuator B-Chem. 253 (2017) 275-282. [6] X. Sun, P. Yang, G. Hou, J. Wei, X. Wang, D. Yang, X. Zhang, H. Dong, F. Zhang, Luminescent Functionalised Supramolecular Coordination Polymers Based on an
Aromatic Carboxylic Acid Ligand for Sensing Hg2+ Ions, Aust. J. Chem. 70 (2017) 786791. [7] J.-R. Li, R.J. Kuppler, H.-C. Zhou, Selective Gas Adsorption and Separation in Metal−Organic Frameworks, Chem. Soc. Rev. 38 (2009) 1477−1504. [8] J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Metal-Organic Framework Materials as Catalysts, Chem. Soc. Rev. 38 (2009) 1450−1459. [9] L.E. Kreno, K. Leong, O.K. Farha, M. Allendorf, R.P. Van Duyne, J.T. Hupp, Metal−Organic Framework Materials as Chemical Sensors, Chem. Rev. 112 (2012) 1105−1125. [10] P. Horcajada, T. Chalati, C. Serre, B. Gillet, C. Sebrie, T. Baati, J.F. Eubank, D. Heurtaux, P. Clayette, C. Kreuz, Porous metal-organic-framework nanoscale carriers as a potential platform for drug delivery and imaging, Nat. Mater. 9 (2010) 172-178. [11] A. Morozan, F. Jaouen, Metal organic frameworks for electrochemical applications. Energy Environ, Sci. 5 (2012) 9269-9290. [12] J.E. Mondloch, W. Bury, D. Fairen-Jimenez, S. Kwon, E.J. DeMarco, M.H. Weston, A.A. Sarjeant, S.T. Nguyen, P.C. Stair, R.Q. Snurr, O.K. Farha, J.T. Hupp, Vapor-Phase Metalation by Atomic Layer Deposition in a Metal-Organic Framework, J. Am. Chem. Soc. 135 (2013) 10294-10297. [13] Y. Wang, B. Li, L. Zhang, L. Liu, Q. Zuo, P. Li, A highly selective regenerable optical sensor for detection of mercury(II) ion in water using organic-inorganic hybrid nanomaterials containing pyrene, New J. Chem. 34 (2010) 1946-1953. [14] A.C. Gonçalves, J.L. Capelo, C. Lodeiro, A.A.D. Santos, A seleno-pyrene selective probe for Hg2+ detection in either aqueous or aprotic systems, Sensors and Actuators B, 239 (2017) 311-318.
[15] P. Deria, J.E. Mondloch, O. Karagiaridi, W. Bury, J.T. Hupp, O. K. Farha, Beyond postsynthesis modification: evolution of metal-organic frameworks via building block replacement, Chem. Soc. Rev. 43 (2014) 5896-5912. [16] P. Deria, J.E. Mondloch, E. Tylianakis, P. Ghosh, W. Bury, R.Q. Snurr, J.T. Hupp, O.K. Farha, Perfluoroalkane Functionalization of NU-1000 via Solvent-Assisted Ligand Incorporation: Synthesis and CO2 Adsorption Studies, J. Am. Chem. Soc. 135 (2013) 16801-16804. [17] K.C. Park, C. Seo, G. Gupta, J. Kim, C. Y. Lee, Efficient Energy Transfer (EnT) in Pyreneand Porphyrin-Based Mixed-Ligand Metal-Organic Frameworks, ACS. Appl. Mater. Interfaces 9 (2017) 38670-38677. [18] T. Yogo, Y. Urano, Y. Ishitsuka, F. Maniwa, T. Nagano, Highly Efficient and Photostable Photosensitizer Based on BODIPY Chromophore, J. Am. Chem. Soc. 127 (2005) 1216212163. [19] J. H. Gibbs, L. T. Robins, Z. Zhou, P. B. Parvanova, M. Cottam, G. T. McCandless, F. R. Fronczek and M. G. H. Vicente, Spectroscopic, computational modeling and cytotoxicity of a series of meso-phenyl and meso-thienyl-BODIPYs, Bioorg. Med. Chem. 21 (2013) 5770-5781. [20] M. Sun, G. Cheng, X. Ge, M. Chen, C. Wang, L. Lou, X. Xu, Aqueous Hg(II) immobilization by chitosan stabilized magnetic iron sulfide nanoparticles, Sci. Total Environ. 621 (2018) 1074-1083.
Graphical Abstract for:
Colorimetric detection of heavy metal ion from water via Metal-Organic Framework Jiwon Kima†, Jung Suk Oha†, Kyoung Chul Parka, Gajendra Guptaa,b*, Chang Yeon Leea,b* aDepartment
of Energy and Chemical Engineering and bInnovation Center for Chemical
Engineering, Incheon National University, Incheon 22012, Republic of Korea. A new mercaptoacetic acid based NU-1000 MOF, SALI-MAA-3eq, obtained via solvent-assisted ligand incorporation (SALI) was designed and used for selective detection of Hg2+ ion in aqueous medium.
Highlights:
New mercaptoacetic acid incorporated NU-1000 Metal Organic Frameworks (MOF) is presented.
Fluorescence spectroscopy is used for detection of Hg2+ ions in water.
The new MOF can selectively detect Hg2+ ions in water.
S0020-1693(18)31386-0 https://doi.org/10.1016/j.ica.2018.10.025 ICA 18571
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
30 August 2018 16 October 2018 17 October 2018
Please cite this article as: J. Kim, J.S. Oh, K.C. Park, G. Gupta, C. Yeon Lee, Colorimetric detection of heavy metal ions in water via Metal-Organic Framework, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica. 2018.10.025
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.
Colorimetric detection of heavy metal ions in water via Metal-Organic Framework Jiwon Kima†, Jung Suk Oha†, Kyoung Chul Parka, Gajendra Guptaa,b*, Chang Yeon Leea,b* aDepartment
of Energy and Chemical Engineering and bInnovation Center for Chemical
Engineering, Incheon National University, Incheon 22012, Republic of Korea. * Corresponding author. Email address: [email protected] (G. G.); [email protected] (C. Y. L). †These
authors contributed equally to this work
ABSTRACT: Interest in the design and use of metal-organic frameworks (MOFs) for the selective detection of heavy metal ions is increasing rapidly. Herein, a new SALI-MAA-3eq MOF was synthesized by incorporating mercaptoacetic acid into NU-1000 via a solvent-assisted ligand incorporation (SALI) approach. SALI-MAA-3eq was fully characterized by common analytical techniques and further tested for sensing applications. SALI-MAA-3eq was found to selectively detect Hg2+ over a range of metal ions in an aqueous environment. Its selectivity was further confirmed by fluorescence and X-ray photoelectron spectroscopy (XPS) studies.
KEYWORDS: metal-organic frameworks, solvent-assisted ligand incorporation, heavy metal detection, luminescent properties. 1. INTRODUCTION
Mercury, one of the heavy metals, can damage the nervous and respiratory systems, causing symptoms such as muscle weakness, poor memory, paralysis of the limbs, and weakened lung function, among others [1-4]. Therefore, mercury sensing in aqueous media, which is one of its major routes of exposure, is becoming more and more important worldwide [5]. Thus, rapid and selective detection of mercury ions using several techniques has been widely studied, such as colorimetric detection, electrochemiluminescence, atomic absorption spectrometry, and fluorescence detection. Among these methods, investigation of the use of metal-organic frameworks for fluorescence detection methods is actively being carried out [6]. Metal-organic frameworks (MOFs), formed by self-assembly of organic ligands with metal ions or metal clusters, are porous materials suitable for various applications including gas storage, catalysis, sensing, drug delivery, and electrocatalysis [7-11]. NU-1000, one of the Zr-based MOFs which is known to be stable in water, is composed of Zr clusters and pyrene derivative ligands [12]. Pyrene, a polycyclic aromatic hydrocarbon (PAH), has a high quantum yield and long lifetime. This important feature of pyrene makes its complexes useful for detection purposes. Previous studies have also shown that the fluorescence of pyrene derivative ligands can be deactivated by mercury [13-14]. Moreover, versatile tools such as post-synthetic modification (PSM), solvent-assisted ligand exchange (SALE), and solvent-assisted ligand incorporation (SALI) can be used to synthesize MOFs which would otherwise be difficult due to limited linker solubility, stability, or problems with functional groups. Among these, SALI is a method in which the -OH group of a metal cluster of a MOF and the -COOH group of a newly included ligand are coupled through an acid-base reaction [15]. SALI has the advantage that it allows easy control over the properties of MOFs by combining functional groups with MOFs using solvents, thereby keeping the overall structure of
the MOFs unaltered and minimizing the generation of unwanted byproducts which can occur during the synthesis process [16].
Scheme. 1. Schematic representation of SALI using NU-1000 and mercaptoacetic acid. Thioglycolic acid, often called mercaptoacetic acid (MAA), is a ligand with an -SH functional group known to have strong binding affinity for mercury. In this short communication, we describe the successful incorporation of MAA into NU-1000 via SALI. The new SALI-MAA-3eq MOF was fully characterized by different analytical techniques. The SALI-MAA-3eq MOF was further utilized for the selective detection of metal ions in water and showed promising activity in selectively detecting mercury over a series of metal ions in water. 2. EXPERIMENTAL SECTION 2.1 Materials and reagents: 1,3,6,8-Tetrabromopyrene (97%), tetrakis(triphenylphosphine) palladium(0) (>98%), potassium tribasic phosphate, zirconyl chloride octahydrate (98%), benzoic acid (>99.5%), thioglycolic acid (>99%), aluminum nitrate nonahydrate (>98%), zinc nitrate hexahydrate (>98%), cadmium nitrate tetrahydrate (>98%), cobalt(II) nitrate hexahydrate (>98%), lead(II) nitrate (>98%), nickel(II) nitrate hexahydrate, and magnesium nitrate hexahydrate were purchased from Aldrich chemicals and used as received. Mercury(II) nitrate hydrate (98%) was purchased from Alfa Aesar and used as received. 1,4-Dioxane (>99.0%) and N,N-
dimethylformamide (>99.5%) were purchased from TCI and used as received. Hydrochloric acid (37%) was purchased from Acros Organics and used as received. Tetrahydrofuran (99%), chloroform (99.5%), sodium hydroxide (97%), and acetone (99.5%) were purchased from Daejung and used as received. Magnesium sulfate (anhydrous) was purchased from Junsei and used as received. (4-(Methoxycarbonyl)phenyl)boronic acid (97%) was purchased from Frontier Scientific and used as received. NU-1000 was synthesized by a previously reported method [12]. 2.2 Synthesis of SALI-MAA-3eq from NU-1000: A 45 mg (0.021 mmol) portion of activated NU-1000 was loaded into a 5-mL vial. Subsequently, 3 mL of a 0.021 M (0.063 mmol) solution of mercaptoacetic acid in DMF was added to the reaction vial. The vial was then placed in a 60 °C oven for 24 h to ensure good dispersion. The supernatant of the reaction mixture was exchanged with fresh DMF for washing. The process was repeated twice using DMF, dichloromethane, and acetone and the resulting material was finally dried under vacuum. 2.3 Experiments for metal ion detection: First, 3 mg of SALI-MAA-3eq was loaded into an 8mL vial. Then, 3 mL of a different metal solution was added into the vial containing SALI-MAA3eq. The vial was shaken well for around 3 min at room temperature and experiments were performed thereafter with 2 mL of the solution. 2.4 Characterization: Powder X-ray diffraction (PXRD) measurements were carried out on a Rigaku Smartlab with Cu Kα radiation over a range of 2° < 2θ < 30° in 0.02° steps with a counting time of 1 s per step. 1H NMR spectra were recorded using an Agilent 400-MR spectrometer. SEM images were obtained using a field-emission scanning electron microscope (JEOL, JSM-7800F) operated at an acceleration voltage of 15.0 kV. Samples were coated with a layer of Au (~3 nm thickness) prior to imaging. N2 adsorption/desorption isotherms were measured volumetrically at 77 K in the range of 7.0 × 10-6 < P/P0 < 1.00 with an Autosorb-iQ outfitted with the micropore
option from Quantachrome Instruments (Boynton Beach, Florida USA) using the Autosorb-iQ Win software package. The samples were activated at 120 °C for 12 h using the outgas port of the Autosorb-iQ instrument. The specific surface areas for N2 were calculated using the BrunauerEmmett-Teller (BET) model in the linear range, as determined using the consistency criteria. Xray photoelectron spectroscopy (XPS) was performed on a PHI 5000 VersaProbe II X-ray photoelectron spectrometer. Fluorescence lifetime and emission spectra were analyzed using a time-correlated single photon counting spectrophotometer (FluoTime300). 3. RESULTS AND DISCUSSION SALI-MAA-3eq was synthesized using NU-1000 and mercaptoacetic acid. NU-1000 was added into a vial containing mercaptoacetic acid (3 equivalents per NU-1000) in dimethylformamide. The mixture was sonicated for around 5 min and then the vial was placed in an oven at 60 °C for 24 h and cooled to room temperature. The yellow crystalline solids obtained were washed with dimethylformamide, methylene chloride, and then with acetone and dried in vacuum. The details of its synthesis are described in the Experimental section. Scanning electron microscopy (SEM) was applied to confirm the shapes of the as-synthesized NU-1000 and SALI-MAA-3eq MOFs. The parent NU-1000 was observed to have rod-shaped crystals with an individual size of 5–7 μm. The shape and size of the SALI-MAA-3eq were also found to be similar to those of the parent NU-1000 (Fig. 1a). It is interesting to observe that the physical morphology of the parent NU-1000 was retained even after the inclusion of mercaptoacetic acid in the new SALI-MAA-3eq through the solvent-assisted ligand incorporation method. This was also further confirmed by PXRD data. The PXRD patterns confirmed the phasepurity of both the NU-1000 as well as the SALI-MAA-3eq MOFs. It was observed that the PXRD patterns of the as-synthesized NU-1000 and SALI-MAA-3eq are consistent with that of the
simulated NU-1000 (Fig. 1b). Therefore, it was confirmed that the overall structure did not change even after SALI. Furthermore, 1H NMR analysis was performed to determine the ratio of mercaptoacetic acid incorporated into the Zr6 node of NU-1000. The 1H NMR spectra of both the MOFs were measured using a DMSO-d6/D2SO4 (9:1 ratio) solution. In addition to the proton peaks of the parent NU1000, a new peak at 3.5 ppm was observed in the spectrum of SALI-MAA-3eq (Fig. 1c, Fig. S4 and S5). This peak at 3.5 ppm can be assigned to the proton attached to the alpha carbon of the mercaptoacetic acid. Further integration assignment of these peaks showed that around 3.6 molecules of mercaptoacetic acid were incorporated per Zr6 node. The porosities of the as-synthesized NU-1000 and SALI-MAA-3eq MOFs were analyzed based on nitrogen adsorption-desorption isotherms at 77 K. For the N2 sorption isotherm analysis, each MOF was activated for 12 hours at 120 °C. At the low pressure of the isotherm graph of each MOF, a “knee” was observed, which is characteristic of type IV N2 isotherms [17]. The BET results show that the surface area of NU-1000 is 2253 m2/g and that of SALI-MAA-3eq is 1906 m2/g. This reduction in surface area of about 350 m2/g after SALI treatment on NU-1000 is reasonable due to the incorporation of mercaptoacetic acid into NU-1000 (Fig. 1d). In addition, the emissive properties of the MOF were not changed after SALI treatment under UV light at a wavelength of 365 nm in the solid state (Fig. S1).
Fig. 1. (a) Scanning electron microscopy images of NU-1000 and SALI-MAA-3eq; (b) Powder XRD patterns of simulated NU-1000 and as-synthesized NU-1000 and SALI-MAA-3eq; (c) 1H NMR spectra of NU-1000 and SALI-MAA-3eq in DMSO-d6/D2SO4 (9:1) solution and (d) N2absorption-desorption isotherms for NU-1000 and SALI-MAA-3eq. Because mercury is one of the most toxic heavy metals, its presence can cause severe health issues even at very low concentrations. Mercury ions in its +2 state are highly soluble in water which makes it much easier for them to get into living organisms. Therefore, several methods are being used by researchers for sensitive and selective mercury detection [6]. Among these methods, those based on absorption and emission spectroscopy are the most common. To this end, we applied our new SALI-MAA-3eq to determine if it can selectively detect mercury in solution.
Photoluminescence (PL) spectroscopy was applied to analyze the selective mercury detection ability of SALI-MAA-3eq. In order to confirm its selective sensing ability, SALI-MAA-3eq was tested over a wide range of metal ions.
Fig. 2. (a) Emission spectra showing the effect of different metal ions on SALI-MAA-3eq when treated with 1 mM aqueous solutions; (b) Normalized fluorescence intensity of SALI-MAA-3eq in the presence of different metal ions. To set up a sensing experiment, 3 mg of SALI-MAA-3eq was added to 3 mL of a 1 mM aqueous solution of various metal ions (Cd2+, Co2+, Mg2+, Zn2+, Ni2+, Hg2+, Pb2+, Al3+). The solution was mixed properly by shaking for 3 minutes and the experiments were performed thereafter. Fig. 2a shows the emission spectra of SALI-MAA-3eq when treated with several metal ions in an aqueous medium. The fluorescence intensity peak at 470 nm showed a significant quenching in the presence of Hg2+. However, with various other metal ions, it did not show any such response. This indicates that SALI-MAA-3eq was very selective in detecting Hg2+ over the range of metal ions studied (Fig. 2a and 2b). In order to further verify its selectivity, which is important for the practical detection of Hg2+, experiments involving the addition of different concentrations of Hg2+ metal ions (0.1-10.0 mM) and different time intervals (0-900 s) were performed (Fig. S2 and S3). As a result, it was confirmed that the fluorescence intensity was decreased with increasing concentration
of mercury and further increases in time had no significant effect on the fluorescence intensity. Fig. 3 shows that SALI-MAA-3eq displayed a significant change in fluorescence upon mercury detection which is clearly visible under UV-light. This further confirms the selectivity of SALIMAA-3eq towards Hg2+ detection.
Fig. 3. (a) Photographs of SALI-MAA-3eq bare, Hg, Co, Cd, Pb in white light (dispersed in water). (b) Photographs of SALI-MAA-3eq bare, Hg, Co, Cd, Pb in UV light (365 nm) (dispersed in water). Furthermore, the PL lifetime of the SALI-MAA-3eq before and after Hg treatment was measured. The average fluorescence lifetime showed a considerable decrease from 1.694 ns to 0.912 ns for Hg@SALI-MAA-3eq as compared to that of free SALI-MAA-3eq, which is due to the occurrence of fluorescence quenching (Table 1). Thus, fluorescence quenching in the pyrene ligands of SALI-MAA-3eq could be due to the heavy atom effect. The heavy atom effect due to mercury causes an intersystem crossing that changes from a singlet state to a triplet state through
a phenomenon called spin-orbit coupling [18-19]. Therefore, it can be confirmed that this quenching phenomenon was caused by mercury. Table 1: Fluorescence lifetime of SALI-MAA-3eq and Hg@ SALI-MAA-3eq. To further understand its
Sample
λex (nm)
λem (nm)
τAv (ns)
SALI-MAA-3eq
405
470
1.694
detection mechanism, X-
Hg@SALI-MAA-3eq
405
470
0.912
ray
photoelectron
spectroscopy analysis was performed on SALI-MAA-3eq and Hg@SALI-MAA-3eq. XPS analysis will help to determine the valence states of the metals associated with the system [20]. After the mercury detection experiments in solution, the remaining solid SALI-MAA-3eq was recovered and XPS analysis was performed. Fig. 4 shows the XP spectra of SALI-MAA-3eq before and after Hg2+ detection. Fig. 4d clearly indicates the presence of Hg on the surface of Hg@SALI-MAA-3eq after Hg2+ detection, in contrast to the spectrum in Fig. 4b which was taken before the addition of Hg2+ ions. The new sharp peaks at 103.6 eV and 99.6 eV in Fig. 4d, which belong to the Hg 4f peak in Hg@SALI-MAA-3eq, appeared due to the interaction between S and Hg, further confirming the detection of Hg2+ ions [20].
Fig. 4. X-ray photoelectron spectroscopy of SALI-MAA-3eq before and after Hg2+ detection. (a) S 2p and (b) Hg 4f of SALI-MAA-3eq; (c) S 2p and (d) Hg 4f of Hg@SALI-MAA-3eq 4. CONCLUSIONS In summary, we have designed a new fluorescent sensor for Hg2+ detection in water by incorporating mercaptoacetic acid into NU-1000 using a SALI approach. The formation of the new SALI-MAA-3eq MOF was confirmed by different analytical techniques. The SALI-MAA-3eq has shown promising sensing activity in selectively detecting Hg2+ ions among a range of metal ions in water. Our results suggest that thiol-based MOFs can be utilized for practical applications in the removal of heavy metal ions, specifically mercury. Further challenges remain in the detection and simultaneous removal of heavy atoms from water, which will be the focus of future research. AUTHOR INFORMATION
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Graphical Abstract for:
Colorimetric detection of heavy metal ion from water via Metal-Organic Framework Jiwon Kima†, Jung Suk Oha†, Kyoung Chul Parka, Gajendra Guptaa,b*, Chang Yeon Leea,b* aDepartment
of Energy and Chemical Engineering and bInnovation Center for Chemical
Engineering, Incheon National University, Incheon 22012, Republic of Korea. A new mercaptoacetic acid based NU-1000 MOF, SALI-MAA-3eq, obtained via solvent-assisted ligand incorporation (SALI) was designed and used for selective detection of Hg2+ ion in aqueous medium.
Highlights:
New mercaptoacetic acid incorporated NU-1000 Metal Organic Frameworks (MOF) is presented.
Fluorescence spectroscopy is used for detection of Hg2+ ions in water.
The new MOF can selectively detect Hg2+ ions in water.