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Thermo-responsive polymer tethered metal-organic framework core-shell magnetic microspheres for magnetic solid-phase extraction of alkylphenols from environmental water samples
Accepted Manuscript Title: Thermo-responsive polymer tethered metal-organic framework core-shell magnetic microspheres for magnetic solid-phase extraction of alkylphenols from environmental water samples Author: Yuqian Jia Hao Su Y.-L. Elaine Wong Xiangfeng Chen T.-W. Dominic Chan PII: DOI: Reference:
S0021-9673(16)30748-8 http://dx.doi.org/doi:10.1016/j.chroma.2016.06.004 CHROMA 357631
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
4-3-2016 1-6-2016 2-6-2016
Please cite this article as: Yuqian Jia, Hao Su, Y.-L.Elaine Wong, Xiangfeng Chen, T.-W.Dominic Chan, Thermo-responsive polymer tethered metal-organic framework core-shell magnetic microspheres for magnetic solid-phase extraction of alkylphenols from environmental water samples, Journal of Chromatography A http://dx.doi.org/10.1016/j.chroma.2016.06.004 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.
Journal of Chromatography A
Thermo-responsive polymer tethered metal-organic framework core-shell magnetic microspheres for magnetic solid-phase extraction of alkylphenols from environmental water samples Yuqian Jiaa, Hao Sua, Y.-L. Elaine Wongb, Xiangfeng Chenab*, T.-W. Dominic Chanb* a
Key Laboratory for Applied Technology of Sophisticated Analytical Instruments,
Shandong Academy of Sciences, Jinan, Shandong, P. R. China b
Department of Chemistry, The Chinese University of Hong Kong, Hong Kong SAR
Address correspondence to: Dr. Xiangfeng Chen, Shandong Academy of Sciences, Jinan, China. E-mail: [email protected]. Professor T.-W. D. Chan, Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, China. E-mail: [email protected].
1
Highlights
A thermo-responsive polymer tethered core-shell magnetic microspheres was synthesized.
The microspheres was used as sorbent for MSPE of alkylphenols from water samples.
High extraction efficiency was achieved using the developed method.
Abstract In
this
work,
the
thermo-responsive
polymer
PNIPAM
tethered
to
Fe3O4@SiO2@MOF core-shell magnetic microspheres was first synthesized by a surface-selective post-synthetic strategy and underwent highly efficient magnetic solid-phase extraction (MSPE) of alkylphenols from aqueous samples. Alkylphenols, including 4-tert-octylphenol (OP) and 4-n-nonylphenol (NP), were selected as target compounds. The sample quantification was carried out using LC-MS/MS in multiple reaction monitor (MRM) mode. Under optimal working conditions, the developed method showed good linearity in the range of 5 to 1000 ng L-1, a low limit of detection (1.5 ng L-1), and good repeatability (relative standard deviation, <8%, n=5) for NP and OP. Owning to the hydrophilic/hydrophobic switchable properties of the nanocomposite, high recoveries (78.7-104.3%) of alkylphenols were obtained under different extraction conditions. The levels of OP and NP in environmental samples collected from local river, lake and pond waters were analyzed using the developed 2
method. It was believed that the synthesized material with the thermo-responsive coating, large surface areas and magnetic properties should have great potential in the extraction and removal of alkylphenols from environmental samples. Keywords: magnetic solid-phase extraction, alkylphenol, thermo-responsive polymer, metal-organic frameworks, environmental water samples
1 Introduction Solid-phase sorption-based extraction techniques have been widely used in the sample pretreatment process [1]. The interactions between the sorbent and the target analytes play a critical role in the extraction process [2]. Due to the designable nature of nanomaterials, many novel nanomaterial-based sorbents have been fabricated and used to improve the performance of the extraction techniques. Recently, metal-organic frameworks (MOFs) have attracted great attention and have been explored as a sorbent in extraction techniques [3-7]. For example, Yan and coworkers have systemically investigated the performance of MOFs as SPE sorbents and stationary phases of GC capillaries and HPLC columns [8-11]. Lee et al. used MIL-101 and ZIF-8 as sorbents for micro-solid-phase extraction (µ-SPE) of acid drugs and organochlorine pesticides from water samples [12,13]. Ouyang and coworkers have utilized MIL-101 (Cr) and isoreticular bio-MOF 100-102 as the fiber coating of solid-phase microextraction (SPME) for determination of organic pollutants [14,15]. We have explored the application of MIL-53(Al) as a sorbent for SPME of PAHs and membrane funnel-based spray ionization mass spectrometry analysis of bovine serum albumin tryptic digest [16,17]. 3
Alkylphenols
have
hydrophilic/hydrophobic
changeable properties
charge under
properties
different
and
conditions
switchable [18].
Taking
4-n-nonylphenol (NP) as an example, the phenolic hydroxyl group may be deprotonated or protonated, which makes NP negatively charged or electron neutral at different pHs. The pristine form of NP is more hydrophobic due to the nature of the nonyl group, while the NP becomes more hydrophilic with deprotonation of the phenolic group under alkaline conditions. The alkylphenols are endocrine disruptors and have been detected in water samples. Therefore, it is quite interesting and necessary to develop a sorbent that could satisfy the requirement of alkylphenol extraction under different conditions. One of the important properties of MOFs is their availability of in-pore functionality and outer-surface modification [19,20]. In this work, a thermo-responsive polymer tethered to Fe3O4@SiO2@MOF core-shell magnetic microspheres is synthesized by surface-selective post-synthetic strategy and undergoes highly efficient MSPE of alkylphenols from environmental water samples. As representatives of alkylphenols, 4-tert-octylphenol (OP) and NP, which have been reported to be widespread, are selected as target compounds. Structurally, they both have alkyl and phenolic hydroxyl groups; bisphenol A and phenol, which lack the hydrophobic groups and have smaller log KOW, are used for comparison. 2 Experimental 2.1 Reagents and materials All reagents were analytical grade. Iron (III) chloride hexahydrate (Fe3Cl·6H2O) (99%), ethyl alcohol, ethylene glycol, N,N-dimethylformamide (DMF) and acetic acid were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). 4
Sodium acetate trihydrate (NaAc) was obtained from Kermel Chemical Reagent Company(Tianjin, China). Ammonia solution (25%) and chloroform were obtained from Fuyu Fine Chemical Co., Ltd (Tianjin, China). Tetraethyl orthosilicate (TEOS), succinic anhydride (99%) and zirconium chloride (ZrCl4) (98%) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). 2-Aminoterephthalic acid (99%), poly (N-isopropylacrylamide) (PNIPAM) and ester-terminated N-hydroxy-succinimide (NHS)
were
obtained
from
Sigma-Aldrich
Co.,
Ltd.
(Shanghai,
China).
(3-Aminopropyl) triethoxysilane (98%) (APTES) was supplied by Macklin Biochemical Co., Ltd (Shanghai, China). De-ionized water (18.2 MΩ cm−1) obtained from a Millipore Milli-Q system (Millipore, Bedford, MA, USA) was used to prepare aqueous solutions for further experiments. The standards of bisphenol A (BPA), 4-tert-octylphenol (CAS No. 140-66-9) and 4-n-nonylphenol (CAS No. 140-40-5) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). 4-Nonylphenol 2,3,5,6-d4 and bisphenol A-d16 (Sigma-Aldrich, USA) were used as internal standards of BPA and NP. A working standard (100 ng mL-1 in methanol) was prepared weekly. Methanol, acetone, acetonitrile and n-hexane were obtained from Tedia Company Inc.(USA). 2.2 Instrumentation The morphology of the synthesized crystals was observed with scanning electron microscopy
(SEM)(SWPRATM55,
Carl
Zeiss
Micro
Imaging
Co.,
Ltd.,
Germany).Transmission electron microscopy (TEM) was observed on a JEOL Ltd. JEM-2010 spectrometer. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet Magna 750 FTIR spectrometer. Powder X-ray diffraction (PXRD) pattern was acquired at room temperature (298 K) on a Bruker SMART APEX CCD-based diffractometer. 1H nuclear magnetic resonance (NMR) was performed on a Bruker 5
DRX-600 spectrometer. The Brunauer-Emmett-Teller specific surface areas of typical products were measured using an ASAP 2020 porosimeter (Micromeritics, USA). The magnetization curves were obtained at room temperature on an MPMS-SQUID-VSM (Quantum Design, USA). Thermogravimetric analysis (TGA) was performed on ~25− 30 mg of powdered sample loaded inside a platinum crucible on a Stanton Red croft TG-DSC instrument. All samples were heated at a ramp of 5°C per minute to 700°C under N2 and O2 with flow rates at 20 and 5 mL per minute, respectively. 2.3 Synthesis procedure For the synthesis of PNIPAM-tethered Fe3O4@SiO2@MOF core-shell magnetic microspheres, UiO-66, was used as a model MOF. To allow the coupling of ligand with the polymer, 2-amino-benzenedicarboxylic (H2N-H2BDC) with an amine group was selected as the ligand [21]. The synthetic procedure of the nano-composite was shown in Fig. 1. Fe3O4 and Fe3O4@SiO2 magnetic particles were synthesized according to the previously reported method with a minor modification; the detailed procedure was described in the supplementary information [22]. Synthesis of Fe3O4@SiO2@UiO-66-NH2 core-shell microspheres Fe3O4@SiO2@UiO-66-NH2 microspheres were fabricated via facile hydrothermal synthesis. Typically, carboxylate-terminated Fe3O4@SiO2 microspheres (10 mg), ZrCl4 (46.61 mg, 0.20 mmol), 2-aminoterephthalic acid (36.23 mg, 0.20 mmol) and acetic acid (30 μL) were added in sequence to the DMF solution (20 mL) and mixed under ultrasonication. The resulting mixture was transferred to a round-bottomed flask and placed in an oil bath at 130°C for 4 hrs under mechanical stirring. Finally, the obtained Fe3O4@SiO2@UiO-66-NH2 microspheres were washed several times with DMF and ethanol and then dried at 60°C for 1 hr. 6
Synthesis of Fe3O4@SiO2@UiO-66-PNIPAM In a 2-mL screw vial, Fe3O4@SiO2@UiO-66-NH2 (3.0 mg) and PNIPAM-NHS (0.01 g) in chloroform (500 μL) were mixed, and the mixture was placed in an oil bath at 60°C for 24hrs. After cooling to room temperature, the solid was collected by a magnet and washed repeatedly with chloroform and methanol. 2.4 Extraction procedure All extraction experiments were carried out in 50-mL glass bottles. To carry out the extraction, 20 mL of an aqueous standard solution or water samples was added to the bottle, and then 5.0 mg of Fe3O4@SiO2@UiO-66-PNIPAM was added and mixed to homogeneity. The extraction was simultaneously carried out under ultrasonication for 6 min. After adsorption, an external magnet was placed aside of the glass bottle to collect the sorbents. The temperature in the extraction process was controlled by the water bath. After removing the aqueous solution, 1 mL of eluent was added into the glass bottle under ultrasonication, and the magnetic microspheres were collected by magnetic separation. The collected eluent was concentrated using a gentle stream of nitrogen at 30°C. The obtained extract was diluted to 100 µL, from which 10µL was used for LC-MS/MS analysis. 2.5 LC-MS analysis Sample analysis was carried out using Thermo U-3000 (Dionex, CA, USA) liquid chromatography coupled with AB Sciex QTrap 5500 (AB SCIEX, USA) mass spectrometry. The separation was performed using an AcclaimTMC18 column (150 mm x 2.1 mm i.d. 3 μm, Thermo Scientific, USA). The mobile phase was methanol and water (v:v 90:10) with 0.1% ammonium hydroxide for isocratic elution. The flow rate was 0.3 mL/min. The injection volume was 10 μL. The mass spectrometer was 7
operated in negative electrospray ionization (ESI) mode. Analysis was performed in multiple reaction monitoring (MRM)mode. The source temperature was 500C. N2 was used as the collision gas. The capillary voltage was -4500 V. The curtain gas was set to 25 psi. The ion source gas 1 and 2 were set to 40 psi. The parameters such as MRM transition (precursor ion → production) and collision energy are presented in Table S1. 2.6 Method validation The developed method was validated by determining the limit of detection (LOD), limit of quantification (LOQ), linearity, selectivity, precision (intra- and inter-day) and recovery. The LOD and LOQ were estimated at a signal-to-noise ratio of 3:1 and 10:1, respectively. The linearity was evaluated at six concentration levels (5, 10, 50, 100, 500 and 1000 ng L-1). The selectivity was evaluated by comparing the recovery of NP and OP with that of BPA under the same working conditions. NP and OP contain an alkyl group and thus could interact with the grafted polymer branches through hydrophobic interactions, while BPA and pehnol lack the hydrophobic group, and their log KOW were also smaller than that of NP and OP. The intra-day and inter-day precisions were evaluated using quality control points prepared at three concentration levels. The residual standard deviation (RSD%) was used to evaluate the precision. The recovery was assessed by calculating the ratio of calculated and spiked concentrations of the analytes and internal standards (IS). The recovery (R) was calculated based on the equation shown below: R (%) =
Cmeasured − Cblank Cspiked
where Cmeasured isis the measured concentration of the spiked samples, Cblank isis the concentration of the sample before spiking and Cspiked isis the spiked concentration. 8
2.7 Environmental water samples Real water samples, including river water (Yellow River), spring water (Baotu), pond water and drinking water, were collected locally. The samples were filtered through a 0.45-μm membrane (Tianjin Jinteng Experiment Equipment Co., Ltd., Tianjin, China), and stored in brown glass bottles at 4 °C prior to analysis. 3 Results and discussion 3.1 Characterization of Fe3O4@SiO2@UiO-66-PNIPAM Figs.
2(a)
and
2(c)
show
the
SEM
images
of
Fe3O4@SiO2
and
Fe3O4@SiO2@UiO-66-PNIPAM. The microspheres were composed of many octahedral crystal shapes with MOF doping on the surface of the Fe3O4@SiO2 with diameters of approximately 200-300 nm. As shown in the TEM images (Figs. 2(b) and 2(d)), the core-shell structures of Fe3O4 and the coating were clearly revealed by the dark center and bright rim. The shell thickness of the MOFs was approximately 30 to 50 nm. Fig. 2(e) shows the magnetic properties of the microspheres obtained by vibrating sample magnetometry within the field range of −20 to 20 KOe. The magnetization saturation values of Fe3O4
microspheres,
Fe3O4@SiO2
and
Fe3O4@SiO2@UiO-66-PNIPAM core shell magnetic microspheres were shown to be 80.15 emu g−1, 60.20 emu g−1 and 45.60 emu g−1, respectively. According to the 1H NMR, two new peaks of protons, methyl and isopropyl, were attributed to the introduction of PNIPAM on the MOF. The modification rate of PNIPAM on the organic ligand was estimated to be approximately 10.2%, which clearly indicated a surface-selected modification [21]. The nitrogen isotherm, the pore size distributions and thermogravimetric analysis are illustrated in Fig. S1. The BET surface area was 262.5 m2 g-1, and the pore width was ~6 Å. As shown in Fig. 2(g), UiO-66-PNIPAM 9
had a stretching band at 1235 cm-1 attributable to the amino group on an organic ligand and another stretching band at 1652 cm-1 that was assigned to the amide I band derived from PNIPAM-modification through amidation of the activated ester. Powder X-ray diffraction images of the Fe3O4, UiO-66-NH2 and Fe3O4@SiO2@UiO-66-NH2 core–shell magnetic microspheres are shown in Fig. 2(h). We found that all diffraction peaks of the as-synthesized microspheres were readily indexed to Fe3O4 and UiO-66-NH2, respectively. The PNIPAM tethered on the surface of the MOF was a thermo-responsive polymer with hydrophilic/hydrophobic switchable properties [23]. When the extraction temperature was lower than the lower critical solution temperature (LCST) of the polymer, the PNIPAM branches of the as-synthesized material were vertically extended. The surface of the nanocomposite was hydrophilic. When the working temperature was higher than the LCST of the polymer, the branches of the PNIPAM collapsed and cover the surface of the MOF. The collapsed PNIPAM coils formed a polymer layer and the material became more hydrophobic. In other words, the hydrophilic/hydrophobic was readily switched by “open” (LCST) states of the polymer-MOF through conformational change of the PNIPAM. In addition, the unmodified amine group on the surface of synthesized MOF might be protonated under acidic conditions and the formed ammonium ions might be deprotonated with an increase in pH. Therefore, the PNIPAM tethered MOF should be pH- and thermo-dual responsive and suitable for extraction of alkylphenols under different conditions. 3.2 Optimization of the conditions for MSPE of alkylphenols The extraction and elution conditions were optimized separately. The concentration of 10
the target compounds and the amount of sorbent were kept constant. The elution step was critical to guarantee complete desorption of the target compounds from the sorbent. Three types of solvent, namely, methanol, acetone and n-hexane, were considered. The initial elution experiments were performed using 5.0 mg of sorbent and a spiked amount of OP and NP (1×103 ng L-1) to select the best elution solvent. Three replicates of each experiment were performed to optimize the solvent selection. The results of solvent selection are shown in Fig. 3(a). It was found that the desorption efficiency of the solvent was higher than 85% and that methanol gave the best elution results. The effect of desorption time on the recoveries of NP and OP is shown in Fig. S2(a). The target compounds were completely desorbed by two consecutive ultrasonication (4 min). Thus, methanol was selected as the elution solvent and the final solvent. No further solvent-exchange steps were required for the following instrumental analysis. The extraction conditions, including amount of sorbent, extraction time, ionic strength and solution pH, were optimized. The amount of sorbent was important in determining the extraction efficiency. The amounts of sorbent tested were 1.0, 3.0, 5.0, 7.0 and 10.0 mg. As shown in Fig. 2S(b), it was found that 5 mg of sorbent was enough to ensure an adequate extraction efficiency and thus was selected as the optimum value. Increasing the amount of sorbent should increase the adsorption capacity of the method. However, as a magnetic-assisted separation technique, increasing the amount of sorbent of MSPE would result in a longer separation time. Therefore, higher amounts of sorbent (>10 mg) were not considered to ensure a short separation time. Because the LCST of PNIPAM was in the range of 30 to 35°C, two extraction temperatures (25 and 40°C) were used to tune the hydrophilic/hydrophobic state of the surface. Considering the structural forms of NP and OP in the water 11
samples, the solution pH was selected to be in the range of 5 to 9. NaCl (0-2%, w/v) was included to test the effect of ionic strength on the extraction efficiency. As shown in Fig. 3(b), the extraction efficiency increased until 5 min and then remained almost constant. Further increasing the extraction time had no significant effect on the recoveries of the target analytes. Thus, 6 min was chosen as the operational extraction time. As shown in Fig. 3(c), a slight increase in recoveries for OP and NP was observed upon NaCl addition, which was a result of the salt-out effect. Fig. 3(d) showed the effect of pH on the extraction efficiency. It was interesting to note that the extraction efficiencies decreased with an increase in pH at 40°C, while the extraction efficiencies increased with a decrease in pH at 25°C for both NP and OP. Therefore, the effect of solvent pH on the extraction efficiency changed with temperature. Based on these results, the optimal conditions for the MSPE of NP and OP were determined as follows: extraction time, 6 min by ultrasonication; extraction temperature, 25°C (pH>7) or 40°C (pH<7); ionic strength, 3% NaCl (w/v); desorption time, 3 min with methanol as the solvent. The extraction behavior of the sorbent towards NP and OP at different pH values could be understood by considering the interaction between the analyte and the sorbent. As shown in Fig. 4, at 25°C and pH 9, the vertically extended polymer allowed the deprotonated phenolic hydroxyl group to pass the branches and interact with the unmodified amine group on the surface of the MOF. The PNIPAM branches of the polymer were hydrogen bonded with water molecules or wrapped with the nonyl groups of the alkylphenol. When the pH decreased to less than 7, the molecules were neutral and became more hydrophobic. Because the surface was hydrophilic at this condition, the extraction efficiencies decreased. At 40°C and pH 5, NP and OP molecules in pristine form were more hydrophobic. The collapsed polymer formed a 12
hydrophobic layer and covered the surface of the MOF. The nonyl chains were adsorbed on the surface through hydrophobic interaction. When the pH increased to over 7, deprotonation of the phenolic hydroxyl groups of NP and OP occurred, and the molecule became hydrophilic. Therefore, the extraction efficiencies decreased. These properties made the PNIPAM tethered magnetic MOF suitable for extraction and removal of alkylphenol from water samples under different conditions. The selectivity of PNIPAM-tethered magnetic MOF was studied by extraction of BPA, phenol and the target compounds (NP and OP) under the same conditions. NP and OP contain alkyl-groups and can thus interact with the grafted polymer branches through hydrophobic interaction, while BPA and phenol lacked the hydrophobic group, and their log KOW values were also smaller than that of NP and OP. It was believed that the interaction between alkylphenol and the sorbent should be stronger than that of BPA and phenol. Therefore, BPA and phenol were chosen to test the selectivity of PNIPAM tethered magnetic MOF. As shown in Fig. 3(d), the extraction efficiency in terms of recovery for BPA was much lower than that of alkylphenols. Moreover, the extraction efficiency of BPA remained almost constant at different pH values. The behavior of phenol (data not shown) was similar to that of BPA. These results indicated that the sorbent possessed a high selectivity towards the OP and NP. The lgKow values of BPA and phenol (3.32 and 1.46, respectively) were smaller than that of OP (4.12) and NP (4.48) [24]. In addition, the tethered polymer might obstruct the interaction between BPA and the surface due to the bi-phenol groups. It was also important to note that the selectivity of this type of material was affected by the structures of the polymer and MOF. The effects of molecular structure on sorbent selectivity required additional investigation and would be valuable to study further. 3.3Analytical performance and application 13
The analytical performance of the developed MSPE-LC-MS/MS method under optimal experimental conditions was evaluated. The parameters for analytical performance of the developed method are shown in Table 1. The linear correlation coefficients (r) for the calibration curves were above 0.989 for the range of 5 to 1000 ng L-1. The limit of detection (LOD, S/N=3) and limit of quantification (LOQ, S/N=10) for OP and NP were 1.5 ng L-1 and 5 ng L-1, respectively. Three concentration levels (10, 50 and 500 ng L-1) were selected to study the intra-day and inter-day precisions of the developed method. Three replicates were performed for recovery studies. The results of repeatability and reproducibility were varied between 5.4% and 7.8% (relative standard deviation of recoveries, %RSD) for OP and NP. The accuracy of the developed method was tested by analysis of local water samples, including the river water, spring water, pond water and drinking water. The results are summarized in Table 2. The water samples spiked with OP and NP at concentrations of 10 ng L-1, 50 ng L-1 and 500 ng L-1 were further tested to demonstrate the applicability of the method. Good spike recoveries between 78.7% and 104.3% for these water samples were achieved, which further confirmed the accuracy and applicability of the developed method. The extracted ion chromatogram LOD, blank and spiked water samples are shown in Fig. S3. Table S2 shows comparisons between the analytical parameters of the developed method and previous methods. Compared with previous methods for the extraction of alkylphenols [25-28], the advantages of the developed method were (i) simple operation, which avoided filtration and centrifugation steps with the aid of an external magnetic field; (ii) high sensitivity (low LODs and LOQs) and comparable recovery while requiring low sample volume and low organic solvent consumption; and (iii) high extraction efficiency and applicable under both acidic and alkaline conditions. 14
4 Conclusions In summary, a thermo-responsive polymer-tethered core-shell magnetic MOF microsphere was synthesized using a surface-selective post-synthetic strategy and applied for MSPE of alkylphenols from water samples prior to determination with LC-MS/MS. Owing to the benefit of thermo- and pH-responsive nanocomposites, alkylphenols could be efficiently adsorbed on the microspheres. The synthesized material with thermo-responsive coating, large surface areas and magnetic properties should have great potential in the extraction and removal of alkylphenols from environmental samples. Acknowledgements
The authors would like to acknowledge the financial support from National Natural Science Foundation of China (NSFC 21205071, 21477068 and 21407099), Natural Science Foundation of Shandong Province (ZR2012BQ009), Fundamental Research Funds and Funds for Fostering Distinguished Young Scholar of Shandong Academy of Sciences.
15
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Turnes-Carou,
S.
Muniategui-Lorenzo,
P.
Lopez-Mahia, D. P. Rodriguez, Membrane assisted solvent extraction coupled with liquid chromatography tandem mass spectrometry applied to the analysis of alkylphenols in water samples, J. Chromatogr. A 1281 (2013) 46-53. [28] N.
Salgueiro-Gonzalez,
E.
Concha-Grana,
I.
Turnes-Carou,
S.
Muniategui-Lorenzo, P. Lopez-Mahia, D. Prada-Rodriguez, Determination of alkylphenols and bisphenol A in seawater samples by dispersive liquid-liquid microextraction and liquid chromatography tandem mass spectrometry for compliance with environmental quality standards (Directive 2008/105/EC), J. Chromatogr. A 1223 (2012) 1-8.
18
Figure Captions Fig. 1 Synthetic procedures for Fe3O4@SiO2@UiO-66-PNIPAM core–shell magnetic microspheres.
Fig. 2 (a) SEM of the Fe3O4@SiO2; (b) TEM of the Fe3O4@SiO2; (c) SEM of the Fe3O4@SiO2@UiO-66-PNIPAM; (d) TEM of the Fe3O4@SiO2@UiO-66-PNIPAM; (e) Magnetic curves of Fe3O4 and Fe3O4@SiO2@UiO-66-PNIPAM; (f) 1H NMR spectra of digested solution of Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-PNIPAM; (g) FT-IR spectra of PNIPAM-NHS, Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-PNIPAM; (h) PXRD of Fe3O4, Fe3O4@SiO2, UiO-66-NH2 and Fe3O4@SiO2@UiO-66-PNIPAM.
19
Fig. 3. Effect of the experimental conditions (a) elution solvent, (b) extraction time, (c) ionic strength and (d) pH value on the extraction efficiency of alkylphenols. Error bars indicate the standard deviation of the mean (n=3).
Fig. 4 Schematic diagram of the adsorption of NP on the surface of the material.
20
Table 1 Analytical parameters of Fe3O4@SiO2@UiO-66-PNIPAM as MSPE sorbent for LC-MS/MS determination of OP and NP. Linear range (ng L-1)
Correlation coefficient (r)
LOD (ng L-1)
LOQ (ng L-1)
OP
5-1000
0.9892
1.5
NP
5-1000
0.9895
1.5
Analyte
a
n means six replicates. concentration at 10 ng L-1. c concentration at 50 ng L-1. d concentration at 500 ng L-1. b
21
Precision (RSD, n=6)a (%) Intra-day repeatability
Inter-day reproducibility
5
7.3b
5.8c
6.5d
7.5b
7.0c
7.3d
5
6.9b
7.2c
6.2d
7.3b
6.5c
6.9d
Table 2 Analytical results for determination of NP and OP in water samples Analytes
River water
Spring water
Pond water
Drinking water
a Recovery
OP
NP
Found (ng L-1) Recoverya (%) Recoveryb (%) Recoveryc (%) Found (ng L-1) Recoverya (%) Recoveryb (%) Recoveryc (%) Found (ng L-1) Recoverya (%) Recoveryb (%) Recoveryc (%) Found (ng L-1)
Recoverya (%)
85.65.2
84.55.8
Recoveryb (%)
84.84.9
90.14.6
Recoveryc
92.75.6
96.14.0
(%)
ng·L-1
of spiked 10 Recovery of spiked 50 ng·L-1 c Recovery of spiked 500 ng·L-1 b
22
S0021-9673(16)30748-8 http://dx.doi.org/doi:10.1016/j.chroma.2016.06.004 CHROMA 357631
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
4-3-2016 1-6-2016 2-6-2016
Please cite this article as: Yuqian Jia, Hao Su, Y.-L.Elaine Wong, Xiangfeng Chen, T.-W.Dominic Chan, Thermo-responsive polymer tethered metal-organic framework core-shell magnetic microspheres for magnetic solid-phase extraction of alkylphenols from environmental water samples, Journal of Chromatography A http://dx.doi.org/10.1016/j.chroma.2016.06.004 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.
Journal of Chromatography A
Thermo-responsive polymer tethered metal-organic framework core-shell magnetic microspheres for magnetic solid-phase extraction of alkylphenols from environmental water samples Yuqian Jiaa, Hao Sua, Y.-L. Elaine Wongb, Xiangfeng Chenab*, T.-W. Dominic Chanb* a
Key Laboratory for Applied Technology of Sophisticated Analytical Instruments,
Shandong Academy of Sciences, Jinan, Shandong, P. R. China b
Department of Chemistry, The Chinese University of Hong Kong, Hong Kong SAR
Address correspondence to: Dr. Xiangfeng Chen, Shandong Academy of Sciences, Jinan, China. E-mail: [email protected]. Professor T.-W. D. Chan, Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, China. E-mail: [email protected].
1
Highlights
A thermo-responsive polymer tethered core-shell magnetic microspheres was synthesized.
The microspheres was used as sorbent for MSPE of alkylphenols from water samples.
High extraction efficiency was achieved using the developed method.
Abstract In
this
work,
the
thermo-responsive
polymer
PNIPAM
tethered
to
Fe3O4@SiO2@MOF core-shell magnetic microspheres was first synthesized by a surface-selective post-synthetic strategy and underwent highly efficient magnetic solid-phase extraction (MSPE) of alkylphenols from aqueous samples. Alkylphenols, including 4-tert-octylphenol (OP) and 4-n-nonylphenol (NP), were selected as target compounds. The sample quantification was carried out using LC-MS/MS in multiple reaction monitor (MRM) mode. Under optimal working conditions, the developed method showed good linearity in the range of 5 to 1000 ng L-1, a low limit of detection (1.5 ng L-1), and good repeatability (relative standard deviation, <8%, n=5) for NP and OP. Owning to the hydrophilic/hydrophobic switchable properties of the nanocomposite, high recoveries (78.7-104.3%) of alkylphenols were obtained under different extraction conditions. The levels of OP and NP in environmental samples collected from local river, lake and pond waters were analyzed using the developed 2
method. It was believed that the synthesized material with the thermo-responsive coating, large surface areas and magnetic properties should have great potential in the extraction and removal of alkylphenols from environmental samples. Keywords: magnetic solid-phase extraction, alkylphenol, thermo-responsive polymer, metal-organic frameworks, environmental water samples
1 Introduction Solid-phase sorption-based extraction techniques have been widely used in the sample pretreatment process [1]. The interactions between the sorbent and the target analytes play a critical role in the extraction process [2]. Due to the designable nature of nanomaterials, many novel nanomaterial-based sorbents have been fabricated and used to improve the performance of the extraction techniques. Recently, metal-organic frameworks (MOFs) have attracted great attention and have been explored as a sorbent in extraction techniques [3-7]. For example, Yan and coworkers have systemically investigated the performance of MOFs as SPE sorbents and stationary phases of GC capillaries and HPLC columns [8-11]. Lee et al. used MIL-101 and ZIF-8 as sorbents for micro-solid-phase extraction (µ-SPE) of acid drugs and organochlorine pesticides from water samples [12,13]. Ouyang and coworkers have utilized MIL-101 (Cr) and isoreticular bio-MOF 100-102 as the fiber coating of solid-phase microextraction (SPME) for determination of organic pollutants [14,15]. We have explored the application of MIL-53(Al) as a sorbent for SPME of PAHs and membrane funnel-based spray ionization mass spectrometry analysis of bovine serum albumin tryptic digest [16,17]. 3
Alkylphenols
have
hydrophilic/hydrophobic
changeable properties
charge under
properties
different
and
conditions
switchable [18].
Taking
4-n-nonylphenol (NP) as an example, the phenolic hydroxyl group may be deprotonated or protonated, which makes NP negatively charged or electron neutral at different pHs. The pristine form of NP is more hydrophobic due to the nature of the nonyl group, while the NP becomes more hydrophilic with deprotonation of the phenolic group under alkaline conditions. The alkylphenols are endocrine disruptors and have been detected in water samples. Therefore, it is quite interesting and necessary to develop a sorbent that could satisfy the requirement of alkylphenol extraction under different conditions. One of the important properties of MOFs is their availability of in-pore functionality and outer-surface modification [19,20]. In this work, a thermo-responsive polymer tethered to Fe3O4@SiO2@MOF core-shell magnetic microspheres is synthesized by surface-selective post-synthetic strategy and undergoes highly efficient MSPE of alkylphenols from environmental water samples. As representatives of alkylphenols, 4-tert-octylphenol (OP) and NP, which have been reported to be widespread, are selected as target compounds. Structurally, they both have alkyl and phenolic hydroxyl groups; bisphenol A and phenol, which lack the hydrophobic groups and have smaller log KOW, are used for comparison. 2 Experimental 2.1 Reagents and materials All reagents were analytical grade. Iron (III) chloride hexahydrate (Fe3Cl·6H2O) (99%), ethyl alcohol, ethylene glycol, N,N-dimethylformamide (DMF) and acetic acid were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). 4
Sodium acetate trihydrate (NaAc) was obtained from Kermel Chemical Reagent Company(Tianjin, China). Ammonia solution (25%) and chloroform were obtained from Fuyu Fine Chemical Co., Ltd (Tianjin, China). Tetraethyl orthosilicate (TEOS), succinic anhydride (99%) and zirconium chloride (ZrCl4) (98%) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). 2-Aminoterephthalic acid (99%), poly (N-isopropylacrylamide) (PNIPAM) and ester-terminated N-hydroxy-succinimide (NHS)
were
obtained
from
Sigma-Aldrich
Co.,
Ltd.
(Shanghai,
China).
(3-Aminopropyl) triethoxysilane (98%) (APTES) was supplied by Macklin Biochemical Co., Ltd (Shanghai, China). De-ionized water (18.2 MΩ cm−1) obtained from a Millipore Milli-Q system (Millipore, Bedford, MA, USA) was used to prepare aqueous solutions for further experiments. The standards of bisphenol A (BPA), 4-tert-octylphenol (CAS No. 140-66-9) and 4-n-nonylphenol (CAS No. 140-40-5) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). 4-Nonylphenol 2,3,5,6-d4 and bisphenol A-d16 (Sigma-Aldrich, USA) were used as internal standards of BPA and NP. A working standard (100 ng mL-1 in methanol) was prepared weekly. Methanol, acetone, acetonitrile and n-hexane were obtained from Tedia Company Inc.(USA). 2.2 Instrumentation The morphology of the synthesized crystals was observed with scanning electron microscopy
(SEM)(SWPRATM55,
Carl
Zeiss
Micro
Imaging
Co.,
Ltd.,
Germany).Transmission electron microscopy (TEM) was observed on a JEOL Ltd. JEM-2010 spectrometer. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet Magna 750 FTIR spectrometer. Powder X-ray diffraction (PXRD) pattern was acquired at room temperature (298 K) on a Bruker SMART APEX CCD-based diffractometer. 1H nuclear magnetic resonance (NMR) was performed on a Bruker 5
DRX-600 spectrometer. The Brunauer-Emmett-Teller specific surface areas of typical products were measured using an ASAP 2020 porosimeter (Micromeritics, USA). The magnetization curves were obtained at room temperature on an MPMS-SQUID-VSM (Quantum Design, USA). Thermogravimetric analysis (TGA) was performed on ~25− 30 mg of powdered sample loaded inside a platinum crucible on a Stanton Red croft TG-DSC instrument. All samples were heated at a ramp of 5°C per minute to 700°C under N2 and O2 with flow rates at 20 and 5 mL per minute, respectively. 2.3 Synthesis procedure For the synthesis of PNIPAM-tethered Fe3O4@SiO2@MOF core-shell magnetic microspheres, UiO-66, was used as a model MOF. To allow the coupling of ligand with the polymer, 2-amino-benzenedicarboxylic (H2N-H2BDC) with an amine group was selected as the ligand [21]. The synthetic procedure of the nano-composite was shown in Fig. 1. Fe3O4 and Fe3O4@SiO2 magnetic particles were synthesized according to the previously reported method with a minor modification; the detailed procedure was described in the supplementary information [22]. Synthesis of Fe3O4@SiO2@UiO-66-NH2 core-shell microspheres Fe3O4@SiO2@UiO-66-NH2 microspheres were fabricated via facile hydrothermal synthesis. Typically, carboxylate-terminated Fe3O4@SiO2 microspheres (10 mg), ZrCl4 (46.61 mg, 0.20 mmol), 2-aminoterephthalic acid (36.23 mg, 0.20 mmol) and acetic acid (30 μL) were added in sequence to the DMF solution (20 mL) and mixed under ultrasonication. The resulting mixture was transferred to a round-bottomed flask and placed in an oil bath at 130°C for 4 hrs under mechanical stirring. Finally, the obtained Fe3O4@SiO2@UiO-66-NH2 microspheres were washed several times with DMF and ethanol and then dried at 60°C for 1 hr. 6
Synthesis of Fe3O4@SiO2@UiO-66-PNIPAM In a 2-mL screw vial, Fe3O4@SiO2@UiO-66-NH2 (3.0 mg) and PNIPAM-NHS (0.01 g) in chloroform (500 μL) were mixed, and the mixture was placed in an oil bath at 60°C for 24hrs. After cooling to room temperature, the solid was collected by a magnet and washed repeatedly with chloroform and methanol. 2.4 Extraction procedure All extraction experiments were carried out in 50-mL glass bottles. To carry out the extraction, 20 mL of an aqueous standard solution or water samples was added to the bottle, and then 5.0 mg of Fe3O4@SiO2@UiO-66-PNIPAM was added and mixed to homogeneity. The extraction was simultaneously carried out under ultrasonication for 6 min. After adsorption, an external magnet was placed aside of the glass bottle to collect the sorbents. The temperature in the extraction process was controlled by the water bath. After removing the aqueous solution, 1 mL of eluent was added into the glass bottle under ultrasonication, and the magnetic microspheres were collected by magnetic separation. The collected eluent was concentrated using a gentle stream of nitrogen at 30°C. The obtained extract was diluted to 100 µL, from which 10µL was used for LC-MS/MS analysis. 2.5 LC-MS analysis Sample analysis was carried out using Thermo U-3000 (Dionex, CA, USA) liquid chromatography coupled with AB Sciex QTrap 5500 (AB SCIEX, USA) mass spectrometry. The separation was performed using an AcclaimTMC18 column (150 mm x 2.1 mm i.d. 3 μm, Thermo Scientific, USA). The mobile phase was methanol and water (v:v 90:10) with 0.1% ammonium hydroxide for isocratic elution. The flow rate was 0.3 mL/min. The injection volume was 10 μL. The mass spectrometer was 7
operated in negative electrospray ionization (ESI) mode. Analysis was performed in multiple reaction monitoring (MRM)mode. The source temperature was 500C. N2 was used as the collision gas. The capillary voltage was -4500 V. The curtain gas was set to 25 psi. The ion source gas 1 and 2 were set to 40 psi. The parameters such as MRM transition (precursor ion → production) and collision energy are presented in Table S1. 2.6 Method validation The developed method was validated by determining the limit of detection (LOD), limit of quantification (LOQ), linearity, selectivity, precision (intra- and inter-day) and recovery. The LOD and LOQ were estimated at a signal-to-noise ratio of 3:1 and 10:1, respectively. The linearity was evaluated at six concentration levels (5, 10, 50, 100, 500 and 1000 ng L-1). The selectivity was evaluated by comparing the recovery of NP and OP with that of BPA under the same working conditions. NP and OP contain an alkyl group and thus could interact with the grafted polymer branches through hydrophobic interactions, while BPA and pehnol lack the hydrophobic group, and their log KOW were also smaller than that of NP and OP. The intra-day and inter-day precisions were evaluated using quality control points prepared at three concentration levels. The residual standard deviation (RSD%) was used to evaluate the precision. The recovery was assessed by calculating the ratio of calculated and spiked concentrations of the analytes and internal standards (IS). The recovery (R) was calculated based on the equation shown below: R (%) =
Cmeasured − Cblank Cspiked
where Cmeasured isis the measured concentration of the spiked samples, Cblank isis the concentration of the sample before spiking and Cspiked isis the spiked concentration. 8
2.7 Environmental water samples Real water samples, including river water (Yellow River), spring water (Baotu), pond water and drinking water, were collected locally. The samples were filtered through a 0.45-μm membrane (Tianjin Jinteng Experiment Equipment Co., Ltd., Tianjin, China), and stored in brown glass bottles at 4 °C prior to analysis. 3 Results and discussion 3.1 Characterization of Fe3O4@SiO2@UiO-66-PNIPAM Figs.
2(a)
and
2(c)
show
the
SEM
images
of
Fe3O4@SiO2
and
Fe3O4@SiO2@UiO-66-PNIPAM. The microspheres were composed of many octahedral crystal shapes with MOF doping on the surface of the Fe3O4@SiO2 with diameters of approximately 200-300 nm. As shown in the TEM images (Figs. 2(b) and 2(d)), the core-shell structures of Fe3O4 and the coating were clearly revealed by the dark center and bright rim. The shell thickness of the MOFs was approximately 30 to 50 nm. Fig. 2(e) shows the magnetic properties of the microspheres obtained by vibrating sample magnetometry within the field range of −20 to 20 KOe. The magnetization saturation values of Fe3O4
microspheres,
Fe3O4@SiO2
and
Fe3O4@SiO2@UiO-66-PNIPAM core shell magnetic microspheres were shown to be 80.15 emu g−1, 60.20 emu g−1 and 45.60 emu g−1, respectively. According to the 1H NMR, two new peaks of protons, methyl and isopropyl, were attributed to the introduction of PNIPAM on the MOF. The modification rate of PNIPAM on the organic ligand was estimated to be approximately 10.2%, which clearly indicated a surface-selected modification [21]. The nitrogen isotherm, the pore size distributions and thermogravimetric analysis are illustrated in Fig. S1. The BET surface area was 262.5 m2 g-1, and the pore width was ~6 Å. As shown in Fig. 2(g), UiO-66-PNIPAM 9
had a stretching band at 1235 cm-1 attributable to the amino group on an organic ligand and another stretching band at 1652 cm-1 that was assigned to the amide I band derived from PNIPAM-modification through amidation of the activated ester. Powder X-ray diffraction images of the Fe3O4, UiO-66-NH2 and Fe3O4@SiO2@UiO-66-NH2 core–shell magnetic microspheres are shown in Fig. 2(h). We found that all diffraction peaks of the as-synthesized microspheres were readily indexed to Fe3O4 and UiO-66-NH2, respectively. The PNIPAM tethered on the surface of the MOF was a thermo-responsive polymer with hydrophilic/hydrophobic switchable properties [23]. When the extraction temperature was lower than the lower critical solution temperature (LCST) of the polymer, the PNIPAM branches of the as-synthesized material were vertically extended. The surface of the nanocomposite was hydrophilic. When the working temperature was higher than the LCST of the polymer, the branches of the PNIPAM collapsed and cover the surface of the MOF. The collapsed PNIPAM coils formed a polymer layer and the material became more hydrophobic. In other words, the hydrophilic/hydrophobic was readily switched by “open” (
the target compounds and the amount of sorbent were kept constant. The elution step was critical to guarantee complete desorption of the target compounds from the sorbent. Three types of solvent, namely, methanol, acetone and n-hexane, were considered. The initial elution experiments were performed using 5.0 mg of sorbent and a spiked amount of OP and NP (1×103 ng L-1) to select the best elution solvent. Three replicates of each experiment were performed to optimize the solvent selection. The results of solvent selection are shown in Fig. 3(a). It was found that the desorption efficiency of the solvent was higher than 85% and that methanol gave the best elution results. The effect of desorption time on the recoveries of NP and OP is shown in Fig. S2(a). The target compounds were completely desorbed by two consecutive ultrasonication (4 min). Thus, methanol was selected as the elution solvent and the final solvent. No further solvent-exchange steps were required for the following instrumental analysis. The extraction conditions, including amount of sorbent, extraction time, ionic strength and solution pH, were optimized. The amount of sorbent was important in determining the extraction efficiency. The amounts of sorbent tested were 1.0, 3.0, 5.0, 7.0 and 10.0 mg. As shown in Fig. 2S(b), it was found that 5 mg of sorbent was enough to ensure an adequate extraction efficiency and thus was selected as the optimum value. Increasing the amount of sorbent should increase the adsorption capacity of the method. However, as a magnetic-assisted separation technique, increasing the amount of sorbent of MSPE would result in a longer separation time. Therefore, higher amounts of sorbent (>10 mg) were not considered to ensure a short separation time. Because the LCST of PNIPAM was in the range of 30 to 35°C, two extraction temperatures (25 and 40°C) were used to tune the hydrophilic/hydrophobic state of the surface. Considering the structural forms of NP and OP in the water 11
samples, the solution pH was selected to be in the range of 5 to 9. NaCl (0-2%, w/v) was included to test the effect of ionic strength on the extraction efficiency. As shown in Fig. 3(b), the extraction efficiency increased until 5 min and then remained almost constant. Further increasing the extraction time had no significant effect on the recoveries of the target analytes. Thus, 6 min was chosen as the operational extraction time. As shown in Fig. 3(c), a slight increase in recoveries for OP and NP was observed upon NaCl addition, which was a result of the salt-out effect. Fig. 3(d) showed the effect of pH on the extraction efficiency. It was interesting to note that the extraction efficiencies decreased with an increase in pH at 40°C, while the extraction efficiencies increased with a decrease in pH at 25°C for both NP and OP. Therefore, the effect of solvent pH on the extraction efficiency changed with temperature. Based on these results, the optimal conditions for the MSPE of NP and OP were determined as follows: extraction time, 6 min by ultrasonication; extraction temperature, 25°C (pH>7) or 40°C (pH<7); ionic strength, 3% NaCl (w/v); desorption time, 3 min with methanol as the solvent. The extraction behavior of the sorbent towards NP and OP at different pH values could be understood by considering the interaction between the analyte and the sorbent. As shown in Fig. 4, at 25°C and pH 9, the vertically extended polymer allowed the deprotonated phenolic hydroxyl group to pass the branches and interact with the unmodified amine group on the surface of the MOF. The PNIPAM branches of the polymer were hydrogen bonded with water molecules or wrapped with the nonyl groups of the alkylphenol. When the pH decreased to less than 7, the molecules were neutral and became more hydrophobic. Because the surface was hydrophilic at this condition, the extraction efficiencies decreased. At 40°C and pH 5, NP and OP molecules in pristine form were more hydrophobic. The collapsed polymer formed a 12
hydrophobic layer and covered the surface of the MOF. The nonyl chains were adsorbed on the surface through hydrophobic interaction. When the pH increased to over 7, deprotonation of the phenolic hydroxyl groups of NP and OP occurred, and the molecule became hydrophilic. Therefore, the extraction efficiencies decreased. These properties made the PNIPAM tethered magnetic MOF suitable for extraction and removal of alkylphenol from water samples under different conditions. The selectivity of PNIPAM-tethered magnetic MOF was studied by extraction of BPA, phenol and the target compounds (NP and OP) under the same conditions. NP and OP contain alkyl-groups and can thus interact with the grafted polymer branches through hydrophobic interaction, while BPA and phenol lacked the hydrophobic group, and their log KOW values were also smaller than that of NP and OP. It was believed that the interaction between alkylphenol and the sorbent should be stronger than that of BPA and phenol. Therefore, BPA and phenol were chosen to test the selectivity of PNIPAM tethered magnetic MOF. As shown in Fig. 3(d), the extraction efficiency in terms of recovery for BPA was much lower than that of alkylphenols. Moreover, the extraction efficiency of BPA remained almost constant at different pH values. The behavior of phenol (data not shown) was similar to that of BPA. These results indicated that the sorbent possessed a high selectivity towards the OP and NP. The lgKow values of BPA and phenol (3.32 and 1.46, respectively) were smaller than that of OP (4.12) and NP (4.48) [24]. In addition, the tethered polymer might obstruct the interaction between BPA and the surface due to the bi-phenol groups. It was also important to note that the selectivity of this type of material was affected by the structures of the polymer and MOF. The effects of molecular structure on sorbent selectivity required additional investigation and would be valuable to study further. 3.3Analytical performance and application 13
The analytical performance of the developed MSPE-LC-MS/MS method under optimal experimental conditions was evaluated. The parameters for analytical performance of the developed method are shown in Table 1. The linear correlation coefficients (r) for the calibration curves were above 0.989 for the range of 5 to 1000 ng L-1. The limit of detection (LOD, S/N=3) and limit of quantification (LOQ, S/N=10) for OP and NP were 1.5 ng L-1 and 5 ng L-1, respectively. Three concentration levels (10, 50 and 500 ng L-1) were selected to study the intra-day and inter-day precisions of the developed method. Three replicates were performed for recovery studies. The results of repeatability and reproducibility were varied between 5.4% and 7.8% (relative standard deviation of recoveries, %RSD) for OP and NP. The accuracy of the developed method was tested by analysis of local water samples, including the river water, spring water, pond water and drinking water. The results are summarized in Table 2. The water samples spiked with OP and NP at concentrations of 10 ng L-1, 50 ng L-1 and 500 ng L-1 were further tested to demonstrate the applicability of the method. Good spike recoveries between 78.7% and 104.3% for these water samples were achieved, which further confirmed the accuracy and applicability of the developed method. The extracted ion chromatogram LOD, blank and spiked water samples are shown in Fig. S3. Table S2 shows comparisons between the analytical parameters of the developed method and previous methods. Compared with previous methods for the extraction of alkylphenols [25-28], the advantages of the developed method were (i) simple operation, which avoided filtration and centrifugation steps with the aid of an external magnetic field; (ii) high sensitivity (low LODs and LOQs) and comparable recovery while requiring low sample volume and low organic solvent consumption; and (iii) high extraction efficiency and applicable under both acidic and alkaline conditions. 14
4 Conclusions In summary, a thermo-responsive polymer-tethered core-shell magnetic MOF microsphere was synthesized using a surface-selective post-synthetic strategy and applied for MSPE of alkylphenols from water samples prior to determination with LC-MS/MS. Owing to the benefit of thermo- and pH-responsive nanocomposites, alkylphenols could be efficiently adsorbed on the microspheres. The synthesized material with thermo-responsive coating, large surface areas and magnetic properties should have great potential in the extraction and removal of alkylphenols from environmental samples. Acknowledgements
The authors would like to acknowledge the financial support from National Natural Science Foundation of China (NSFC 21205071, 21477068 and 21407099), Natural Science Foundation of Shandong Province (ZR2012BQ009), Fundamental Research Funds and Funds for Fostering Distinguished Young Scholar of Shandong Academy of Sciences.
15
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[13] D. Ge, H.K. Lee, Zeolite imidazolate frameworks 8 as sorbent and its application to sonication-assisted emulsification microextraction combined with vortex-assisted porous membrane-protected micro-solid-phase extraction for fast analysis of acidic drugs in environmental water samples, J. Chromatogr. A 1257 (2012) 19-24. [14] L. Xie, S. Liu, Z. Han, R. Jiang, H. Liu, F. Zhu, F. Zeng, C. Su, G. Ouyang, Preparation
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[22] S. Qin, W. Cai, X. Tang, L. Yang, Sensitively monitoring photodegradation process of organic dye molecules by surface-enhanced raman spectroscopy based on Fe3O4@SiO2@TiO2@Ag particle, Analyst 139 (2014) 5509-5515. [23] Z. Yang, N. Zhuo, S. Zhang, Y. Dong, X. Zhang, J. Shen, W. Yang, Y. Wang, J. Chen, A pH- and temperature-responsive magnetic composite adsorbent for targeted removal of nonylphenol, ACS Appl. Mater. Interfaces 7 (2015) 24446-24457. [24] G-G. Ying, B. Williams, R. Kookana, Environmental fate of alkylphenols and alkylphenol ethoxylates-a review, Environ Int. 28 (2002) 215-226. [25] Y. Cai, G. Jiang, J. Liu, Q. Zhou, Multiwalled carbon nanotubes as a solid-phase extraction adsorbent for the determination of bisphenol A, 4-n-nonylphenol, and 4-tert-octylphenol, Anal. Chem. 75 (2003) 2517-2521. [26] S. Luo, L. Fang, X. Wang, H. Liu, G. Ouyang, C. Lan, T. Luan, Determination of octylphenol and nonylphenol in aqueous sample using simultaneous derivatization and dispersive liquid-liquid microextraction followed by gas chromatography-mass spectrometry, J. Chromatogr. A 1217 (2010) 6762-6768. [27] N.
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Figure Captions Fig. 1 Synthetic procedures for Fe3O4@SiO2@UiO-66-PNIPAM core–shell magnetic microspheres.
Fig. 2 (a) SEM of the Fe3O4@SiO2; (b) TEM of the Fe3O4@SiO2; (c) SEM of the Fe3O4@SiO2@UiO-66-PNIPAM; (d) TEM of the Fe3O4@SiO2@UiO-66-PNIPAM; (e) Magnetic curves of Fe3O4 and Fe3O4@SiO2@UiO-66-PNIPAM; (f) 1H NMR spectra of digested solution of Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-PNIPAM; (g) FT-IR spectra of PNIPAM-NHS, Fe3O4@SiO2@UiO-66-NH2 and Fe3O4@SiO2@UiO-66-PNIPAM; (h) PXRD of Fe3O4, Fe3O4@SiO2, UiO-66-NH2 and Fe3O4@SiO2@UiO-66-PNIPAM.
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Fig. 3. Effect of the experimental conditions (a) elution solvent, (b) extraction time, (c) ionic strength and (d) pH value on the extraction efficiency of alkylphenols. Error bars indicate the standard deviation of the mean (n=3).
Fig. 4 Schematic diagram of the adsorption of NP on the surface of the material.
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Table 1 Analytical parameters of Fe3O4@SiO2@UiO-66-PNIPAM as MSPE sorbent for LC-MS/MS determination of OP and NP. Linear range (ng L-1)
Correlation coefficient (r)
LOD (ng L-1)
LOQ (ng L-1)
OP
5-1000
0.9892
1.5
NP
5-1000
0.9895
1.5
Analyte
a
n means six replicates. concentration at 10 ng L-1. c concentration at 50 ng L-1. d concentration at 500 ng L-1. b
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Precision (RSD, n=6)a (%) Intra-day repeatability
Inter-day reproducibility
5
7.3b
5.8c
6.5d
7.5b
7.0c
7.3d
5
6.9b
7.2c
6.2d
7.3b
6.5c
6.9d
Table 2 Analytical results for determination of NP and OP in water samples Analytes
River water
Spring water
Pond water
Drinking water
a Recovery
OP
NP
Found (ng L-1) Recoverya (%) Recoveryb (%) Recoveryc (%) Found (ng L-1) Recoverya (%) Recoveryb (%) Recoveryc (%) Found (ng L-1) Recoverya (%) Recoveryb (%) Recoveryc (%) Found (ng L-1)
Recoverya (%)
85.65.2
84.55.8
Recoveryb (%)
84.84.9
90.14.6
Recoveryc
92.75.6
96.14.0
(%)
ng·L-1
of spiked 10 Recovery of spiked 50 ng·L-1 c Recovery of spiked 500 ng·L-1 b
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