Automatic In-Syringe Dispersive Microsolid Phase Extraction Using

Jul 3, 2015 - ... to build the flow-based setup are described in detail in the Supporting ... Procedure for Automatic In-Syringe D-μ-SPE Using Magnet...
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Automatic in-syringe dispersive micro-solid phase extraction using magnetic metal-organic frameworks Fernando Maya, Carlos Palomino Cabello, José Manuel Estela, Víctor Cerdà, and Gemma Turnes Palomino Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b01993 • Publication Date (Web): 03 Jul 2015 Downloaded from http://pubs.acs.org on July 5, 2015

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Analytical Chemistry

Automatic in-syringe dispersive micro-solid phase extraction using magnetic metal-organic frameworks Fernando Maya, Carlos Palomino Cabello, Jose Manuel Estela, Víctor Cerda, Gemma Turnes Palomino* Department of Chemistry, University of the Balearic Islands, Cra. de Valldemossa km 7.5, Palma de Mallorca, E-07122, Spain; Fax: (+34) 971 173426; E-mail: [email protected]. ABSTRACT: A novel automatic strategy for the use of micro- and nanomaterials as sorbents for dispersive micro-solid phase extraction (D-µ-SPE) based on the lab-in-syringe concept is reported. Using the developed technique, the implementation of magnetic metal-organic framework (MOF) materials for automatic solid-phase extraction has been achieved for the first time. A hybrid material based on submicrometric MOF crystals containing Fe3O4 nanoparticles was prepared and retained in the surface of a miniature magnetic bar. The magnetic bar was placed inside the syringe of an automatic bi-directional syringe pump, enabling dispersion and subsequent magnetic retrieval of the MOF hybrid material by automatic activation/deactivation of magnetic stirring. Using malachite green (MG) as a model adsorption analyte, a limit of detection of 0.012 mg/L and a linear working range of 0.04-2 mg/L were obtained for a sample volume equal to the syringe volume (5 mL). MG preconcentration was linear up to a volume of 40 mL, obtaining an enrichment factor of 120. The analysis throughput is 18 h-1, and up to 3000 extractions/g of material can be performed. Recoveries ranging between 95-107% were obtained for the analysis of MG in different types of water and trout fish samples. The developed automatic D-µ-SPE technique is a safe alternative for the use of small-sized materials for sample preparation, and is readily implementable to other magnetic materials independently of their size and shape and can be easily hyphenated to the majority of detectors and separation techniques.

Introduction Metal-organic frameworks (MOFs)1,2 are receiving widespread attention as materials to meet current analytical challenges.3 MOFs are based on linking metal ions or clusters with organic ligands, giving porous structures with high surface area, good thermal stability and uniform nanoscale cavities, making them good candidates as advanced sorbents for solid-phase extraction (SPE).4,5 However, due to their unsuitable particle size and morphology, alternatives to the classic column packings need to be engineered for the application of MOFs as SPE materials. With this purpose, MOFs have been integrated in flow-through devices like monolithic columns6 or polymer disks,7 or directly used implementing dispersive micro-solidphase extraction (D-µ-SPE)8,9 or nanoparticle enhanced dispersive liquid-liquid microextraction.10 However, the troublesome recovery and reusability of the sorbent material, as well as the lack of automation of the overall procedure are the main bottlenecks of the two previous approaches. A recently reported approach to facilitate the use of MOFs for SPE is their magnetization.11-14 Between SPE techniques, D-µ-SPE15 is enormously facilitated by the use of magnetic materials,16,17 which enables magnetic sorbent retrieval in the preconcentration and elution steps. To our knowledge, the potential of magnetic MOFs has not been explored yet for automatic SPE, although other magnetic materials have been used for this purpose immobilizing them in a micro-column format by the action of external magnets, concomitantly avoiding the use of frits.18-20 Nonetheless, the properties of magnetic sorbents are not fully exploited in a micro-column format, since the particles are compressed within the micro-column by the external magnetic field, limiting the contact between the particles and the analytes. This drawback could be solved by developing an automatic D-µ-SPE methodology to fully exploit the potential of magnetic MOFs, among other magnetic sorbent materials. To date, just one

approach for automatic magnetic D-µ-SPE has been described, which is based on the use of dissolvable Fe3O4-layered double hydroxide core-shell microspheres as sorbent, and a dedicated autosampler as automation tool.21 We report, in the present work, a novel approach for the automation of D-µ-SPE using magnetic materials based on the recently described lab-in-syringe concept,22-25 which allows to fully benefit of the properties of the sorbent. Using this approach, the implementation of magnetic MOFs for automatic SPE has been carried out for the first time. As a proof-ofconcept we have developed an automatic method to exploit the advantageous features of the magnetic MIL-100(Cr) MOF for the extraction of malachite green (MG),26 an acutely toxic compound widely used as dye,27 fungicide and anti-septic.28 The developed approach could be easily extended to other kind of magnetic materials, and readily implementable with the majority of detectors, or hyphenated with other techniques.

EXPERIMENTAL SECTION The chemicals and samples used, the preparation of the trout fish samples, the synthesis of the original MOF and the magnetic hybrid material, and the instruments and experimental conditions used for the characterization of the materials prepared and to build the flow-based set-up are described in detail in the Supporting Information. Procedure for automatic in-syringe D-µ-SPE using magnetic MOFs The analytical procedure for automatic D-µ-SPE is based on the system depicted in Figure 1A and is detailed in Table S1 (see supporting information). In a preliminary step, 10 mg of the magnetic MOF material were loaded in the syringe already 1

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containing the magnet (Figure 1B). The extraction procedure is based on the loading of the sample into the syringe at a high flow rate while stirring, achieving the total dispersion of the magnetic MOF (Figure 1C). In order to increase the extraction yield the mixing time was increased by stirring the sample and the MOF in stop flow mode. The sample matrix was then discarded while the MOF containing the extracted analyte was gradually deposited onto the magnet. Once the sample was discarded, the procedure was repeated with pure water in order to wash the MOF and remove the remaining sample matrix. Finally, the procedure was repeated again loading this time a small amount of the eluent. A volume of 0.5 mL of eluent was set for all experiments, since it was the minimum volume of solvent required to perform the extraction in a reproducible manner. After elution a volume of the eluent was loaded into the loop of the injection valve and subsequently injected towards the detector by the action of a second syringe installed into the multisyringe pump. A schematic depiction summarizing the procedure is shown in Figure 1D.

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Cr3O(F/OH)(H2O)2[C6H3(CO2)3]2⋅nH2O which is known to exhibit very high air, water, and thermal stability. The powder X-ray diffractogram of the prepared MIL-100(Cr) sample (Figure 2A) showed good crystallinity and was in good agreement with the powder diffraction pattern reported by Ferey et al.29 SEM micrographs showed that the metal-organic framework was formed by microcrystals having a globular shape with an average size of 0.20-0.25 µm (Figure 2B). After magnetization, the original green color of MIL100(Cr) turned out to brown due to the incorporation of Fe3O4 nanoparticles onto the surface of the metal-organic framework crystals. The structure of the MOF was maintained as demonstrated by the presence of the corresponding diffraction peaks in the XRD pattern of the Fe3O4@MIL-100(Cr) (Figure 2A). The X-ray diffractogram of the hybrid material also showed additional lines at high angles which could be assigned to crystalline magnetite confirming the co-existence of both phases, MIL-100(Cr) and Fe3O4. The morphology of the Fe3O4@MIL-100(Cr) sample did not apparently change after magnetization either (Figure 2C). A closer view of the hybrid material using transmission electron microscopy showed that it was formed by MIL-100(Cr) microcrystals, presenting aggregates of Fe3O4 nanoparticles (5-10 nm) on their surface (Figure 2D). The presence of Cr and Fe on Fe3O4@MIL-100(Cr) particles was demonstrated by energy dispersive X-ray spectroscopy (EDS) (Figure 2E). The Fe content of Fe3O4@MIL100(Cr) was quantified using ICP-OES after acid decomposition of the material and resulted to be of a 30% wt. Elemental EDS mapping (Figure 2F) showed the homogeneous distribution of both metals, Cr and Fe, in the Fe3O4@MIL-100(Cr) particles suggesting that Fe3O4 nanoparticles were uniformly deposited on the surface of the MOF.

Automatic in-syringe D-µ-SPE using Fe3O4@MIL-100(Cr)

Figure 1. A) Scheme of the flow-based instrumental set-up. V1-V2, solenoid valves. S1-S2, 5 mL glass syringes. W, waste reservoir. B) Magnet coated with 10 mg of magnetic MOF placed inside a 5 mL glass syringe. C) In-syringe dispersion of a magnetic MOF facilitated by magnetic stirring. D) Schematic depiction of the extraction procedure in syringe 1 (washing step between extraction and elution is not included).

RESULTS AND DISCUSSION Characterization of Fe3O4@MIL-100(Cr) For the preparation of the Fe3O4@MIL-100(Cr) magnetic MOF, first we synthesized the pristine MIL-100(Cr) metalorganic framework,29-30 followed by magnetization by in-situ growth of Fe3O4 nanoparticles in an aqueous suspension of the MIL-100(Cr).31 MIL-100(Cr) is a crystalline porous green solid of chemical composition

The extraction of MG was studied using the prepared Fe3O4@MIL-100(Cr) hybrid material and the developed set-up for fully-automatic in-syringe D-µ-SPE. One of the crucial factors involved in the extraction process is the selection of the eluent used to desorb the analyte once the extraction has been accomplished. Methanol, ethanol, acetone and acetonitrile were tested for the elution of MG from the magnetic Fe3O4@MIL-100(Cr) hybrid material. MG recovery was significant with all solvents tested. Highest MG recoveries were obtained by using methanol and acetone, although the elution of MG was more reproducible using methanol (RSD= 3%, n= 5) than acetone (RSD= 7%, n= 5). Therefore methanol was selected as the eluent for subsequent experiments. A further improvement of the MG recovery was obtained by acidifying methanol to pH= 2.5 (adjusted with a 10 mmol/L HCl solution), obtaining a sensitivity increase of a 13% and a concomitant enhancement of signal reproducibility (RSD= 2%, n= 5).

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Figure 2. A) X-ray diffractograms of Fe3O4 nanoparticles, MIL-100(Cr) and Fe3O4@MIL-100(Cr). Scanning electron micrographs of B) the MIL-100(Cr) material and C) the Fe3O4@MIL-100(Cr) hybrid material. D) Transmission electron micrograph of a Fe3O4@MIL-100(Cr) microcrystal. E) Energy dispersive X-ray spectra of Fe3O4 nanoparticles, MIL-100(Cr) and Fe3O4@MIL100(Cr). F) Energy dispersive X-ray mapping of Fe and Cr of the Fe3O4@MIL-100(Cr) hybrid material. The mapped area is highlighted in Figure 2C. The D-µ-SPE technique requires a good dispersion of the solid phase throughout the sample. In this case, the Fe3O4@MIL-100(Cr) sorbent was initially magnetically immobilized on the magnet placed inside the extraction syringe. In order to ensure a good dispersion of the material, the aqueous phase containing the analyte was loaded at the highest flow rate possible (15 mL/min) while stirring, taking 20 s to load the whole syringe volume. Once the syringe was completely filled with the aqueous phase and a good dispersion of the MOF was achieved, additional stirring time under stopflow conditions is required to enhance the extraction efficiency. This parameter was also evaluated, and as shown in Figure 3A, the mass transfer of MG from the aqueous phase to the sorbent was increased, requiring less than 1 minute of additional stirring to obtain the maximum extraction performance under the detailed conditions. The stirring time is also critical in order to obtain an efficient desorption of the MG from the sorbent, allowing to maximize the enrichment factor and to increase the reproducibility of consecutive measurements. As shown in Figure 3B, the maximum MG recovery was obtained after only 30 s of additional stirring once the eluent was loaded into the syringe. Therefore, in order to obtain the maximum extraction efficiency without decreasing drastically the analysis throughput, stirring extraction and desorption times of 30 s in each step were adopted for further experiments. The pH of the extraction medium plays an essential role in the extraction process, as it influences not only the

stability and the ionization state of the analyte, but also the charge of the sorbent surface, thus affecting the extraction efficiency. The effect of the pH on the extraction of MG using

Figure 3. Effect on the measured absorbance of 1 mg/L malachite green of the A) stirring time in stop flow mode for the extraction step, B) stirring time in stop flow mode for the elution step, C) pH of the extraction medium and D) sample volume. E) Elution peak profiles for the direct injection of 0.25 mg/L malachite green, and after preconcentrating 5 or 40 mL of sample. 3

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Fe3O4@MIL-100(Cr) as sorbent is shown in Figure 3C. Best extraction performance is obtained at pH 5-7. The lower extraction yield of MG observed at pH values lower than 5 and higher than 7 is probably due to the repulsive interaction between the positively charged MG molecules and the negative surface charge of the hybrid sorbent at low pH values as well as the instability of MG molecules at higher pH values. 26 Thus a pH value of 6.0 was adopted for further experiments. In the developed system, a glass syringe with a volume of 5 mL is used as the container of the Fe3O4@MIL100(Cr) crystals, being this one the maximum volume of sample that can be preconcentrated in one syringe loading. However, since the preconcentration step can be automatically repeated several times prior to the elution of the analyte, the preconcentration factor can be increased. Figure 3D shows the linearity of the analytical response when several MG preconcentration cycles were performed. Up to 40 mL of a standard solution containing 0.25 mg/L of MG could be preconcentrated without loss of linearity and with good signal reproducibility (RSD= 5%, n= 3), reaching an enrichment factor of 120 in comparison with the direct injection of MG without preconcentration. The signal peak profiles for the direct injection of a standard solution containing 0.25 mg/L of MG, and after preconcentrating 5 and 40 mL of standard solution are shown in Figure 3E.

Features of the automatic in-syringe D-µ-SPE using Fe3O4@MIL-100(Cr) The analytical features of the proposed in-syringe automatic D-µ-SPE using the Fe3O4@MIL-100(Cr) hybrid material were studied using a sample volume of 5 mL. The limit of detection (LOD) and the limit of quantification (LOQ) were calculated as three and ten times the standard deviation of the blank (n= 10) divided by the slope of the calibration curve and resulted to be 0.012 mg/L and 0.040 mg/L, respectively. The LOD and LOQ can be improved by repeating the preconcentration step up to 8 times, as it has been shown in Figure 3D, reaching limits of detection for MG at the µg/L level. The signal reproducibility for 10 consecutive measurements using 0.25 mg/L MG was 1.9%. Five different calibration curves spanning a linear dynamic range from 0.05-2 mg/L of MG were performed at different days and using fresh Fe3O4@MIL-100(Cr) material daily, obtaining an RSD of the slope of the calibration curve of a 3.5%. The slope of the calibration curve was 0.204 L/mg, with an average R2 of 0.9980. The sorbent could be reused at least 30 times without loss of extraction capacity, meaning that at least 3000 extractions can be performed with 1 g of Fe3O4@MIL-100(Cr). The analysis throughput was 18 h-1. Using the maximum sample volume that provided a reproducible extraction signal (40 mL), an injection throughput of 3 h-1 was obtained. The production of waste products was low, since just 0.5 mL of methanol were used per extraction. Both in natural or wastewaters, different concentrations of various salts coexist increasing the ionic strength, potentially interfering on the adsorption of the MG dye onto

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the Fe3O4@MIL-100(Cr). The effect of the concentration of different ions on the extraction of MG was studied using a 1 mg/L MG solution containing a 1000-fold high concentration of a given ion. We considered a significant interference a deviation of the recovery value higher than a ±5%. Results showed no significant interference when 1000-fold high concentration of NO2-, NO3-, SO42-, SCN-, Ca2+, Mg2+, K+, Cu2+ or Zn2+ were studied. The effect of NaCl was studied to a higher concentration level, since high NaCl concentrations are typically found in water samples. Concentrations up to 50 g/L NaCl were tolerated with no appreciable interference. The determination of MG using the proposed technique was applied to drinking water, tap water and groundwater. It was also applied to two trout fish samples obtained at different grocery stores. All samples were analyzed by triplicate using a preconcentration volume of 10 mL. For the trout samples, the washing step prior to the elution was repeated twice. MG was spiked to all the studied samples at 25.0 and 50.0 µg/L levels. The results obtained are shown in Table 1. No detectable amount of MG was initially found in any of the analyzed samples, and recovery values for the spiked samples ranged between 95-107% in all instances. The RSD values obtained for all samples ranged between 1.4 – 4.3%, demonstrating the applicability and reproducibility of the proposed automatic in-syringe D-µ-SPE technique using Fe3O4@MIL100(Cr) as sorbent material for the determination of low levels of MG dye in water and fish samples. Table 1. Analytical results for the determination of malachite green in water and fish samples obtained applying the developed automatic in-syringe D-µ-SPE technique using Fe3O4@MIL-100(Cr) as sorbent Sample Drinking water

Spiked (µg/L) -

Tap water

Groundwater

Fish 1

Fish 2

25.0 50.0 25.0 50.0 25.0 50.0 25.0 50.0 25.0 50.0

Found (µg/L, n= 3)

Recovery (%)

n.d.

-

24.3 ± 0.5 48.2 ± 1.2 n.d. 23.9 ± 0.7 52.7 ± 0.8 n.d. 25.5 ± 0.4 51.6 ± 2.1 n.d. 26.7 ± 0.7 52.3 ± 1.1 n.d. 25.6 ± 1.1 51.9 ± 2.0

97 96 95 105 102 103 107 105 103 104

The performance of the proposed approach for the automatic, simple and safe solid-phase extraction of MG using magnetic materials as sorbent is on par with other recently reported manual methods.32,33 Since the sorbent particles are placed in a closed syringe container, risks inherent to the manipulation of very small particles are minimized. In comparison with manual procedures for MG extraction using magnetic graphene,32 or maghemite nanoparticles33 comparable enrichment factors and detection limits at the µg/L level were obtained, concomitantly providing an automatic tool for the 4

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Analytical Chemistry

miniaturized and safer handling of micro- and nanomaterials to perform D-µ-SPE procedures. The developed approach is not only limited to the use of MOFs as sorbent materials, and is readily implementable to any other magnetic materials independently of their size and shape. In the case of MOF materials, more than 20.000 structures have been reported in the last decade,2 enabling a plethora of additional applications using the presented technique.

CONCLUSIONS A novel strategy for the automation of the D-µ-SPE technique has been developed using an in-syringe, multisyringe flowbased approach. The developed strategy enabled for the first time the development of a fully automatic D-µ-SPE with reusable materials, as well as the first application of automatic Dµ-SPE using magnetic MOFs. The integrated system enabled the dispersion of the magnetic MOF microcrystals throughout the sample volume, and the subsequent magnetic collection, washing, elution and online spectrophotometric detection of the analyte. It has been applied to the determination of the dye malachite green at µg/L levels, as a proof of concept, to describe the technique. Future work will be directed towards the combination of the developed automatic D-µ-SPE technique with highperformance liquid chromatography or gas chromatography, in order to enhance the sensitivity and selectivity of the technique towards the determination of low levels of mixtures of analytes of interest. Additionally, the preparation of other magnetic water-stable MOF hybrids materials with different metals centers or organic ligands will be crucial in order to enhance the selectivity towards the selected analytes for each application of interest.

AUTHOR INFORMATION

(3) Gu, Z.Y.; Yang, C.X.; Chang, N.; Yan, X.P. Acc. Chem. Res. 2012, 45, 734-745. (4) Ge, D.; Lee, H.K. J. Chromatogr. A 2011, 1218, 8490-8495. (5) Ciu, X-Y.; Gu, Z-Y.; Jiang, D-Q.; Li, Y.; Wang, H-F.; Yan, X-P. Anal. Chem. 2009, 81, 9771-9777. (6) Saeed, A.; Maya, F.; Xiao, D.J.; Najam-ul-Haq, M.; Svec, F.; Britt, D.K. Adv. Funct. Mater. 2014, 24, 5790-5797. (7) Maya, F.; Cabello, C.P.; Clavijo, S.; Estela, J.M.; Cerdà, V.; Palomino, G.T. Chem. Commun. 2015, 51, 8169-8172. (8) Li, N.; Wang, Z.; Zhang, L.; Nian, L.; Lei, L.; Yang, X.; Zhang, H.; Yu, A. Talanta 2014, 128, 345-353. (9) Rocio-Bautista, P.; Martinez-Benito, C.; Pino, V.; Pasan, J.; Ayala, J.H.; Ruiz-Perez, C.; Afonso, A.M. Talanta 2015, 139, 13-20. (10) Maya, F.; Cabello, C.P.; Clavijo, S.; Estela, J.M.; Cerdà, V.; Palomino, G.T. RSC Adv. 2015, 5, 28203-28210. (11) Huo, S-H.; Yan, X.P. Analyst 2012, 137, 3445-3451. (12) Silvestre, M.E.; Franzreb, M.; Weidler, P.G.; Shekhah, O.; Woll, C. Adv. Funct. Mater. 2013, 23, 1210-1213. (13) Wang, Y.; Xie, J.; Wu, Y.; Ge, H.; Hu, X. J. Mater. Chem. A 2013, 1, 8782-8789. (14) Wang, Y.; Chen, H.; Tang, J.; Ye, G.; Ge, H.; Hu, X. Food. Chem. 2015, 181, 191-197. (15) Anastassiades, M.; Lehotay, S.J.; Stajnbaher, D.; Schenck, F.J. J. AOAC Int. 2003, 86,412– 431. (16) Asgharinezhad, A.K.; Mollazadeh, N.; Ebrahimzadeh, H.; Mirbabaei, F.; Shekari, N. J. Chromatogr. A 2014, 1338, 1-8. (17) Benede, J.L.; Chisvert, A.; Giokas, D.L.; Salvador, A. J. Chromatogr. A 2014, 1362, 25-33. (18) Huang, Y-F.; Li, Y.; Jiang, Y.; Yan, X-P. Analyst 2010, 25, 1467-1474. (19) Wang, Y.; Wang, L.; Tian, T.; Hu, X.; Yang, C.; Xu, Q. Analyst 2012, 137, 2400-2405. (20) Giakisikli, G.; Anthemidis, A.N. Talanta 2013, 110, 229-235. (21) Tang, S.; Chia, G.H.; Chang, Y.; Lee, H.K. Anal. Chem. 2014, 86, 11070-11076. (22) Maya, F.; Horstkotte, B.; Estela, J.M.; Cerdà, V. Anal. Bioanal. Chem. 404, 2012, 909-917.

Corresponding Author

* E-mail: [email protected]; Fax: (+34) 971 173426; Tel: (+34) 971 173250.

ACKNOWLEDGMENT The Spanish Ministerio de Economía y Competitividad (MINECO) and the European Funds for Regional Development (FEDER) are gratefully acknowledged for financial support through Project CTQ2013-47461-R. F.M. acknowledges the support of the Government of the Balearic Islands and the European Social Fund (postdoctoral fellowship).

(23) Horstkotte, B.; Suarez, R.; Solich, P.; Cerdà, V. Anal. Chim. Acta 788, 2013, 52-60. (24) Giakisikli, G.; Miro, M.; Anthemidis, A.N. Anal. Chem. 85, 2013, 8968-8972. (25) Maya, F.; Horstkotte, B.; Estela, J.M.; Cerdà, V. Trends Anal. Chem. 59, 2014, 1-8. (26) Huo, S-H.; Yan, X-P. J. Mater. Chem. 2012, 22, 7449. (27) Santhi, T.; Manonmani, S. Clean: Soil, Air, Water 2011, 39, 162170. (28) Saha, P.; Chowdhury, S.; Gupta, S.; Kumar, I.; Kumar, R. Clean: Soil, Air, Water 2010, 38, 437-445.

SUPPORTING INFORMATION AVAILABLE

(29) Férey, G.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; Surblé, S.; Dutour, J.; Margiolaki, I. Angew. Chem. 2004, 116, 6450–6456.

Additional information as noted in the text. This information is available free of charge via the Internet at http://pubs.acs.org/.

(30) Llewellyn, P.L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.; Hamon, L.; De Weireld, G.; Chang, J-S.; Hong, D-Y.; Hwang, Y.K.; Jhung, S.H.; Férey, G. Langmuir 2008, 24, 7245–7250.

REFERENCES

(31) Saikia, M.; Bhuyan, D.; Saikia, L. New J. Chem 2015, 38, 64-67. (32) Sergi, A.; Shemirani, F.; Alvand, M.; Tajbakhshian, A. Anal. Methods 2014, 6, 7744-7751. (33) Afkhami, A.; Moosavi, R.; Madrakian, T. Talanta 2010, 84, 785-789.

(1) Kitagawa, S.; Kitaura, R.; Noro, S.I. Angew. Chem., Int. Ed. 2004, 43, 2334-2375. (2) Furukawa, H.; Cordova, K.E.; O'Keeffe, M.; Yaghi, O.M. Science 2013, 341, 1230444.

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