Adsorptive removal of trace sulfonamide antibiotics by water-dispersible magnetic reduced graphene oxide-ferrite hybrids from wastewater

Journal of Chromatography B, 1029–1030 (2016) 106–112

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Journal of Chromatography B journal homepage: www.elsevier.com/locate/chromb

Adsorptive removal of trace sulfonamide antibiotics by water-dispersible magnetic reduced graphene oxide-ferrite hybrids from wastewater Jianrong Wu a,1 , Hongyan Zhao b,1 , Rong Chen a , Chuong Pham-Huy c , Xuanhong Hui a , Hua He a,d,e,∗ a

Department of Analytical Chemistry, China Pharmaceutical University, 24 Tongjia Lane, Nanjing 210009, China Department of Hygienic Analysis and Detection, School of Public Health, Nanjing Medical University, Nanjing, Jiangsu 211166, China c Faculty of Pharmacy, University of Paris V, 4 Avenue de l’Observatoire, 75006 Paris, France d Key Laboratory of Biomedical Functional Materials, China Pharmaceutical University, Nanjing 210009, China e Key Laboratory of Drug Quality Control and Pharmacovigilance, Ministry of Education, China Pharmaceutical University, Nanjing 210009, China b

a r t i c l e

i n f o

Article history: Received 19 May 2016 Received in revised form 6 July 2016 Accepted 8 July 2016 Available online 9 July 2016 Keywords: Magnetic reduced graphene oxide Sulfonamide antibiotics Adsorption Environmental wastewater samples

a b s t r a c t A one-pot solvothermal synthesis method was developed to prepare reduced graphene oxide (RGO) supported ferrite hybrids using graphite oxide and metal ions (Fe3+ ) as starting materials. The as-prepared composites were characterized by transmission electron microscopy(TEM), Fourier transform infrared spectrophotometer (FT-IR), X-ray powder diffraction pattern(XRD) and vibrating sample magnetometer (VSM). It was shown that Fe3 O4 nanoparticles with a uniform size of ∼35 nm were anchored on RGO nanosheets. Importantly, the obtained nanocomposites are effective adsorbents for the determination of three sulfonamides in wastewater. Several parameters affecting the extraction efficiency were optimized, including amount of adsorbent, extraction time, pH and desorption conditions. Compared with other adsorbents, the as-prepared RGO-Fe3 O4 showed the better extraction efficiencies for the SAS due to the large surface area of RGO. A linear range from 1 to 200 ng/mL was obtained with a high correlation coefficient (R2 > 0.9987), and the limits of detection for three SAs ranged from 0.43 to 0.57 ng/mL. This method was successfully applied to the analysis of SAs in environmental wastewater samples, the recoveries in different sample matrices were in the range from 89.1 and 101.7% with relative standard deviations less than 8.6%. It is believed that such composites will find wide applications in the water pretreatment area. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Graphene, a monolayer of carbon atoms densely packed into a two dimensional honeycomb crystal lattice, has recently sparked much research interest. It combines unique electronic properties and intriguing quantum effects with exceptional thermal and mechanical properties [1]. Notably, graphene is a double-sided polyaromatic scaffold with an ultrahigh specific surface area (theoretical value 2630 m2 /g, compared to 10 m2 /g of graphite and 1315 m2 /g of nanotubes) [2]. On the other hand, rapid mass transfer can be obtained due to the sufficiently large contact area

∗ Corresponding author at: Department of Analytical Chemistry, China Pharmaceutical University, 24 Tongjia Lane, Nanjing 210009, China. E-mail addresses: [email protected], [email protected] (H. He). 1 These authors should be considered co-first authors. http://dx.doi.org/10.1016/j.jchromb.2016.07.018 1570-0232/© 2016 Elsevier B.V. All rights reserved.

between the sorbents and the analytes, which is beneficial for rapid equilibrium [3,4], making it a promising candidate for sorption material with high loading capacity. Its large delocalized p-electron system also endows graphene a strong affinity for carbon-based ring structures, which are widely present in drugs, pollutants, and biomolecules. Graphene-based materials such as graphene and chemically modified graphene including graphene oxide have shown many applications in analytical chemistry [5–8]. Feng et al. reported the application of graphene as a sorbent for SPE and revealed the great potential of graphene in analytical process [9]. Although graphene has recently been used as the adsorbent for the preconcentration of analytes, graphene is an ultralight material, so it is usually hard to retrieve from a suspension. To solve these problems, magnetic adsorbent has been a solution. The retrieval of the adsorbent could be realized by external magnetic field [10–13]. In this way magnetic particles were loaded

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onto graphene to fabricate magnetic graphene composite is a superior choice, which can ensure the convenient magnetic separation after adsorption. Magnetic graphene (RGO-Fe3 O4 ) are of considerable interest in materials chemistry because of their unique physical properties and non-toxicity, much more convenient, efficient and economic and many interesting applications nowadays [14]. Also, it can modified with different functional groups to prevent aggregation and extend their application. These functional groups include silica, metal oxides, polymers and so on. Up to now, graphene based Fe3 O4 nanocomposites have become a hot topic of research and exhibit attractive application prospects in magnetic resonance imaging [15], environmental remediation [16–18], drug delivery [19,20] and photocatalyst [21,22]. An increase in the number of publications concerning the use of magnetic graphene was observed in recent years. On the basis of the advantages of graphene and Fe3 O4 , the RGO-Fe3 O4 composites have been developed and widely applied in sample pretreatment fields. At present, a variety of methods, such as solvothermal reaction, the reduction of GO and Fe3+ in a NaBH4 solution, electrostatic interactions [3], chemical precipitation [23,24], hydrothermal reaction [25], physical adsorption [9] and covalent bonding [26,27], have been applied to prepare graphene–Fe3 O4 . Chemical methods offer potentially low cost and large scale production of graphene-based hybrid materials. In this work, an easy-to-handle approach was presented to prepare RGO-Fe3 O4 . Antibiotics have been investigated as sources of emerging environmental contaminants. As a major class of antibiotics, sulfonamide antibiotics (SAs) are widely used for the treatment of diseases and as prophylaxis [28–30]. However, SAs cannot be effectively eliminated in conventional wastewater treatment plants because of their anionic characteristics [31]. SAs have been gained more and more concerns for the residues in the natural water system or in the water treatment facilities [32] and their potential carcinogenicity [33]. As a result, it is fundamental to develop adequate analytical methods for their determination in water samples. Nowadays, the study of nanomaterial based composites for water treatment is still at the primary stage. To design a cost effective method for the fabrication of nanomaterials is still a challenge. Here, we report a novel magnetic composite based on GO synthesized in situ at low temperatures (<100 ◦ C). In a solvothermal strategy of the mixture of iron(III) and GO in alkaline condition and the reduction of GO by the addition of hydrazine hydrate could proceed simultaneously. The as-prepared RGO-Fe3 O4 can also be dispersed in water due to the retained hydrophilic moieties, which were applied for the first time as a novel adsorbent for the enrichment of three sulfonamide antibiotics (SAs) from water samples. Coupling this MSPE technique with HPLC separation and detection, a highly simple and sensitive analytical method to determine SAs in environmental water samples was established.

2. Materials and methods 2.1. Chemicals and materials Graphite powder was obtained from the Nanjing XFNANO Materials Tech Co. Ltd. (Nanjing, China). Iron(III/II) chloride (99.9+ %) and hydrazine hydrate (99%) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Sulphuric acid (H2 SO4 , 98%), phosphoric acid (H3 PO4 ), hydrochloric acid (HCl), Ammonia solution, H2 O2 (30%), KMnO4 , sodium acetate (NaAc) and actone were purchased from Aladdin. Gradient grade ACN, MeOH for LC and acetic acid were purchased from Merck (Germany). Analytical standards of sulfadiazine (SDZ), sulfamerazine (SMR), sulfamethoxazole (SMX) were provided by Sigma-Aldrich Co. LLC. (USA) and their chemical structures are shown in Fig. S1. The stock

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solutions (1 mg mL−1 ) of the analytes were prepared in methanol and stored in the dark at 4 ◦ C. The primary and final sewage effluent samples were taken from Hospital of Nanjing Medical University (Nanjing, China). The lake water sample was collected from Xuanwu Lake (Nanjing, China). The tap water sample came from our laboratory. All water samples were collected randomly and stored at 4 ◦ C. The spiked water samples were made by adding certain amounts of SAs standard solution to the real water samples of fixed volume and stored at room temperature. 2.2. Instrumentation The TEM image was performed on a FEI Tecnai G2 F20 transmission electron microscope. Phase identification was done by the X-ray powder diffraction pattern (XRD), using X’TRA X-ray diffractometer with Cu Ka irradiation at c = 1.540562. FT-IR spectrum was collected by using a 8400s FTIR spectrometer in KBr pellet at room temperature (Shimadzu Corporation, Japan). The magnetic properties were studied using an LDJ 9600-1 vibrating sample magnetometer (VSM) operating at room temperature with applied fields of up to 10 kOe. Deionized water was acquired from MilliQ50SP Reagent system (Millipore Corporation, MA, USA). 2.3. Synthesis of graphene oxide (GO) GO was fabricated according to the method previously reported in the literature with minor modifications [34]. The details of synthesis steps were described in Supporting information. 2.4. Synthesis of magnetite graphene (RGO-Fe3 O4 ) composites Fig. 1 displays the synthetic scheme of RGO-Fe3 O4 composites prepared in this work. Typically, 0.3 g GO was dispersed in 220 mL of deionized water and ultrasonicated for 2 h to produce a suspension of GO sheets. The mixed solution of 5.3 g FeCl3 ·6H2 O and 1.99 g FeCl2 ·4H2 O (Fe3+ and Fe2+ with a mole ratio of 2:1) was added slowly to the GO solution at RT. 30% Ammonia solution was added to this solution until the pH = 10. The temperature of solution raised to 90 ◦ C and 5 mL of hydrazine hydrate was added. After that, the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 190 ◦ C for a certain period of time to ensure complete reduction of GO, and then cooled naturally to room temperature. The black solution was filtered, washed with water and ethanol several times, and finally dried in vacuum at 70 ◦ C to obtain the RGO-Fe3 O4 composites. 2.5. MSPE procedure The extraction procedure was similar to our previous work with minor modification [35–37]. 20 mg of sorbent (RGO-Fe3 O4 ) was added into 5 mL spiked water sample by ultrasonicating to form a homogeneous suspension. Then 5 mL phosphate buffer solution (PBS, 0.02 M) were added to adjust the suited pH. Subsequently, the mixture was homogenized and the extraction was performed under an oscillator for 15 min. Then, the magnetic adsorbent was collected using an external magnet and supernatant solution was decanted. Desorption of the target analytes was accomplished by washing the sorbent with 1.5 mL acetonitrile containing 5% ammonium (0.5 mL + 1.0 mL). The collected sorbents adsorping the target analytes were eluted with 1.5 mL acetonitrile containing 5% ammonium to desorb the analytes. Then evaporated to dryness under a mild nitrogen stream. The residue was dissolved in 0.5 mL methanol and 10 ␮L of the eluate was injected into the HPLC system for analysis.

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Fig. 1. The synthetic scheme of RGO-Fe3 O4 composites prepared in this work.

Fig. 2. TEM of GO (A) and RGO-Fe3 O4 (B).

3. Results and discussion 3.1. Characterization of RGO-Fe3 O4 The morphologies of GO and RGO-Fe3 O4 were characterized by TEM, which is shown in Fig. 2. In Fig. 2A, the GO consisting of ultrathin, semi-transparent and crumpled nanosheets with lateral size ranged from dozens of nanometers to several micrometers, which maintained a large surface area. Fig. 2B and the inset in Fig. 2B show the low and high magnification TEM image of RGOFe3 O4 , respectively, and the graphene sheets showing the folding nature are clearly visible. It is clear that the graphene nanosheets are well loaded by Fe3 O4 NPs and the diameters of the Fe3 O4 NPs are 30–40 nm. Compared with the GO, the significant morphology changes after hydrazine hydrate reaction also indicate that Fe3 O4 nanoparticles can be successfully attached to the surface of the graphene. To obtain the phase and structure information about graphite oxide and as synthesized RGO-Fe3 O4 , powder XRD patterns were conducted. It was shown in Fig. S2, Supporting information. The obtained results confirmed the successful synthesis of RGO-Fe3 O4 nanocomposites by solvothermal reaction. Further, FT-IR spectra were used to characterize the chemical structure of GO and RGOFe3 O4 nanocomposites as shown in Fig. S3. These absorption bands are indicated successful synthesis of RGO-Fe3 O4 . The field dependence of magnetization for the Fe3 O4 NPs and RGO-Fe3 O4 composites was measured by a vibrating sample mag-

Fig. 3. VSM magnetization curves of Fe3 O4 (a) and RGO-Fe3 O4 (b) at room temperature.

netometer (VSM) at room temperature. As shown in Fig. 3, all curves had no magnetic hysteresis loops. The saturation magnetization values were 57.06 emu/g for Fe3 O4 (Fig. 3a) and 46.81 emu/g for RGO-Fe3 O4 (Fig. 3b), respectively. The magnetic intensities are lower than bulk Fe3 O4 due to the presence of graphene and the small size of Fe3 O4 nanoparticles. The RGO-Fe3 O4 composites dispersed in water solution (1 mg/mL) can be separated from water by using a magnet (inset in Fig. 3), which makes them a promising can-

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pounds, most of SAs were deprotonated and charged negatively, which made the SAs less hydrophobic and also suppressed the ␲–␲ electron. As a result, the SAs could not be efficiently adsorbed onto the sorbent and consequently decreased the extraction recoveries. Although the satisfactory recovery was achieved at pH 7.0, the pH was adjusted to 6.0 for the following experiments. 3.2.3. Effect of extraction time In order to realize complete extraction, the effect of extraction time on the adsorption was investigated. The results are shown in Fig. S4, which demonstrates that the extraction efficiency increased with the increased extraction time from 5 to 15 min, and then remained almost constant after 15 min. Therefore, 15 min was selected for extraction time. Fig. 4. Effect of amount of RGO-Fe3 O4 NPs.

3.2.4. Desorption conditions The desorption capabilities of these solvents are depicted in Table S1, it was found that 1.5 mL acetonitrile containing 5% ammonium hydroxide had the best desorption ability. Since the eluent cannot match to HPLC, it was evaporated to dryness and reconstituted with the mobile phase for HPLC. 3.3. Comparison of SAs sorption capacity of RGO–Fe3 O4 with other adsorbents

Fig. 5. Effect of pH on the adsorption of sulfonamides.

didate for practical applications in the field of pollutant adsorption with great separability and recyclability. 3.2. Extraction optimization 3.2.1. Effect of the amount of RGO-Fe3 O4 NPs Fewer amounts of nano-adsorbents may be achieved more satisfactory results than micro-adsorbents because of their greater surface areas [12]. To achieve good recovery, the adsorbent amount was investigated. The different amounts of the magnetic graphene (5, 10, 15, 20, 25, 30 mg) were tested. As shown in Fig. 4, the recoveries of three SAs increased with increasing sorbent doses from 5 to 20 mg, and then stabilized with further increases. Thus 20 mg was employed used in the next experiments. 3.2.2. Effect of sample pH The pH of the sample solution plays an important role for the adsorption of the analytes to the sorbents. The pH not only changes the formation of the analytes, but also alters the interaction between the sorbents and the analytes. In our experiment, the pH optimization was performed in 20 mM phosphate buffer solution over the pH range from 3.0 to 9.0. As seen from Fig. 5, the highest extraction efficiency for these sulfonamides is obtained at pH 6.0. The adsorption recovery of three SAs decreased when the pH increased greatly from 6.0 to 9.0. This may be attributed to the fact that the adsorption of SAs was mainly predominated by ␲–␲ electron coupling. The pKa values of SDZ, SMX and SMR are 6.5, 5.9, 7.0, respectively. At the pH of 6.0, most SAs existed in neutral forms and a few in protonated forms, which could interact with polymer backbone and by ␲–␲ electron coupling and hydrophobic interaction. However, when the sample pH is higher than the pKa of the com-

In order to evaluate the potential application prospect of RGO–Fe3 O4 , a sequence of experiments was carried out under the optimal conditions to compare the extraction efficiencies of reduced graphene oxide, Fe3 O4 nanoparticles, RGO–Fe3 O4 , magnetic graphene oxide and magnetic multiwall carbon nanotubes (MMWCNTs). As can be seen from Table S2, the extraction efficiency of RGO–Fe3 O4 was better than other magnetic adsorbents. This may be attributed to the fact that the main interaction between sorbent and analyte is ␲-␲ force, and the surface area of RGO is more large than other adsorbent. It is necessary to point out that the iron oxide nanoparticles presented in the surface of RGO–Fe3 O4 decrease surface charge sorption area of RGO, further leading to less sorption capacity than reduced graphene oxide. Considering all aspects, the magnetic composite of RGO–Fe3 O4 would be a prospective adsorbent for the preconcentration of SAs in the complex matrix. 3.4. Validation of the method Under the optimized conditions, a series of quantitative parameters with regard to the linear range, correlation coefficient, limit of detection (LOD), limits of quantification (LOQ), recovery and reproducibility were examined to validate the proposed MSPE–HPLC method. The linear regression, the LOD and LOQ

Fig. 6. Chromatograms obtained from the analysis of sulfonamides by MSPE: (a) blank water sample, (b) spiked with 50 ng mL−1 and (c) spiked with 10 ng mL−1 . (1. SDZ; 2. SMZ; 3. SMX).

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Table 1 The linear range, limits of detection (LOD), limits of quantification (LOQ) for the detection of 3 SAs from water samples. Analytes

Calibration curves Range (ng/mL)

SMX SMR SDZ

1.0–200.0 2.0–200.0 1.0–200.0

Equation

R2 value

Y = 393.17X + 225.5 Y = 417.17X − 48.3 Y = 499X − 54.7

0.9987 0.9991 0.9992

LOD (ng/mL)

LOQ (ng/mL)

0.43 0.57 0.46

1.36 1.94 1.45

Table 2 Method precisions for the determination of the SAs from water samples. Analytes

SMX SMR SDZ

Intra-day precision (RSD%, n = 6)

Inter-day precision (RSD%, n = 6)

1 ng/mL

10 ng/mL

100 ng/mL

1 ng/mL

10 ng/mL

100 ng/mL

5.1 4.7 5.4

3.2 3.9 4.8

3.4 4.1 3.2

4.8 3.9 4

4.1 4.7 6.1

2.6 3.4 3.7

Table 3 Recoveries and precisions of the SAs in the analysis of three environmental water samples.a Sample

Analytes

Founded (ng/mL)

Recovery (%)

RSD (%,n = 4)

The tap water

SMX SMR SDZ

9.67 9.46 10.17

96.7 94.6 101.7

4.5 3.1 5.4

SMX SMR SDZ

8.91 9.01 9.70

89.1 90.1 97.0

7.3 8.6 7.7

SMX SMR SDZ

9.28 10.09 9.11

92.8 109.9 91.1

5.6 5.1 7.8

Sewage outfall of a hospital

The XuanWu Lake

a

Real water samples were spiked at 10 ng/mL.

data are listed in Table 1. Linear regression analysis was performed using the recoveries against the concentrations of the respective analytes and each SAs exhibited good linearity with correlation coefficient R2 > 0.9987 in the studied range. The LOD and LOQ were calculated as the concentration corresponding to the signals of 3 and 10 times the standard deviation of the baseline noise, respectively. The LOD and LOQ for three SAs were found to be 0.43–0.57 ng/mL and 1.36–1.94 ng/mL, respectively. Precision was assessed by testing intra-day and inter-day variations and the results are listed in Table 2. Repeatability was evaluated by analysis of spiked water samples at three different concentrations (1, 10 and 100 ng/mL). The intra- and inter-day RSD were less than 5.4% and 6.1%. These results demonstrated that the MSPE method had good precision.

3.5. Analysis of environmental water samples After the method was established, the proposed method was applied to the determination of the SAs in three kinds of environmental water samples including lake water, sewage outfall of a hospital and tap water, the results are outlined in Table 3. It can be seen that the recoveries for the spiked samples at 10 ng/mL were between 89.1 and 96.7%. The HPLC chromatograms of the blank butter sample and spiked sample are shown in Fig. 6. No interference from the matrix was observed after the MSPE–HPLC analysis of a real water sample (Fig. 6a), a spiked water sample with 50 ng mL−1 standard solution of three SAs (Fig. 6b) and 10 ng mL−1 standard solution of three SAs (Fig. 6c).

3.6. Comparison with other methods In the previous work, many methods were successfully developed for the analysis of antibiotics in some matrices. In order to further demonstrate the superiority of our proposed method, we compared our method with previous reported [9,29,32,38–41]. As seen from Table S3, in comparison with other methods, MSPE method had higher extraction ability, thereby it illustrated our method had better extraction efficiency. Moreover, our method provided a relative wider linear range and a comparable detection limit. The results revealed that the proposed method for the analysis of flavonoids in different matrices was simple, rapid and sensitive. 4. Conclusion In summary, we present an easy and general solvothermal method to fabricate RGO–Fe3 O4 hybrids with magnetite particle size average of ∼35 nm. RGO–Fe3 O4 composites are superparamagnetic at room temperature and can be separated by an external magnetic field. It was used as an extraction media for the enrichment of trace amount of three sulfonamide antibiotics in environmental water samples. Low detection limit and satisfactory recoveries were achieved and good recoveries was obtained, indicating that the proposed MSPE-HPLC-DAD method is very efficient and sensitive. Compared with other adsorbents, the as-prepared RGO-Fe3 O4 showed the better extraction efficiencies for the SAS due to the large surface area of RGO. The results suggest that RGO–Fe3 O4 has a potential application for the extraction and determination of antibacterials from complex sample matrices.

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Acknowledgements This work were supported by the Open Project of Key Laboratory of Modern Toxicology of the Ministry of Education (Grant No. NMUMT201404), the Jiangsu Province Science Foundation for Youths (BK20130644), the Natural Science Foundation of China (81502848), the Fundamental Research Funds for the Central Universities (JKQZ2013006). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jchromb.2016. 07.018. References [1] C.N. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Graphene: the new two-dimensional nanomaterial, Angew. Chem. Int. Ed. Engl. 48 (2009) 7752–7777. [2] Q. Ye, L. Liu, Z. Chen, L. Hong, Analysis of phthalate acid esters in environmental water by magnetic graphene solid phase extraction coupled with gas chromatography-mass spectrometry, J. Chromatogr. A 1329 (2014) 24–29. [3] Q. Han, Z. Wang, J. Xia, S. Chen, X. Zhang, M. Ding, Facile and tunable fabrication of Fe3 O4 /graphene oxide nanocomposites and their application in the magnetic solid-phase extraction of polycyclic aromatic hydrocarbons from environmental water samples, Talanta 101 (2012) 388–395. [4] Y. Huang, Y. Wang, Q. Pan, Y. Wang, X. Ding, K. Xu, N. Li, Q. Wen, Magnetic graphene oxide modified with choline chloride-based deep eutectic solvent for the solid-phase extraction of protein, Anal. Chim. Acta 877 (2015) 90–99. [5] E. Ziaei, A. Mehdinia, A. Jabbari, A novel hierarchical nanobiocomposite of graphene oxide–magnetic chitosan grafted with mercapto as a solid phase extraction sorbent for the determination of mercury ions in environmental water samples, Anal. Chim. Acta (2014) 49–56. [6] X. Ding, Y. Wang, Y. Wang, Q. Pan, J. Chen, Y. Huang, K. Xu, Preparation of magnetic chitosan and graphene oxide-functional guanidinium ionic liquid composite for the solid-phase extraction of protein, Anal. Chim. Acta (2015) 36–46. [7] Y. Wang, S. Gao, X. Zang, J. Li, J. Ma, Graphene-based solid-phase extraction combined with flame atomic absorption spectrometry for a sensitive determination of trace amounts of lead in environmental water and vegetable samples, Anal. Chim. Acta 716 (2012) 112–118. [8] N. Sun, Y. Han, H. Yan, Y. Song, A self-assembly pipette tip graphene solid-phase extraction coupled with liquid chromatography for the determination of three sulfonamides in environmental water, Anal. Chim. Acta 810 (2014) 25–31. [9] Y.B. Luo, Z.G. Shi, Q. Gao, Y.Q. Feng, Magnetic retrieval of graphene: extraction of sulfonamide antibiotics from environmental water samples, J. Chromatogr. A 1218 (2011) 1353–1358. [10] Z.-G. S, H.K. Lee, Dispersive liquid-liquid microextraction coupled with dispersive␮-solid-phase extraction for the fast determination of polycyclic aromatic hydrocarbons in environmental water samples, Anal. Chem. 82 (2010) 1540–1545. [11] Z. He, D. Liu, R. Li, Z. Zhou, P. Wang, Magnetic solid-phase extraction of sulfonylurea herbicides in environmental water samples by Fe3 O4 @dioctadecyl dimethyl ammonium chloride@silica magnetic particles, Anal. Chim. Acta 747 (2012) 29–35. [12] Q. Cheng, F. Qu, N.B. Li, H.Q. Luo, Mixed hemimicelles solid-phase extraction of chlorophenols in environmental water samples with 1-hexadecyl-3-methylimidazolium bromide-coated Fe3 O4 magnetic nanoparticles with high-performance liquid chromatographic analysis, Anal. Chim. Acta 715 (2012) 113–119. [13] H. Bagheri, O. Zandi, A. Aghakhani, Extraction of fluoxetine from aquatic and urine samples using sodium dodecyl sulfate-coated iron oxide magnetic nanoparticles followed by spectrofluorimetric determination, Anal. Chim. Acta 692 (2011) 80–84. [14] W. Lü, Y. Wu, J. Chen, Y. Yang, Facile preparation of graphene–Fe3 O4 nanocomposites for extraction of dye from aqueous solution, CrystEngComm 16 (2014) 609–615. [15] L.-Z. Bai, D.-L. Zhao, Y. Xu, J.-M. Zhang, Y.-L. Gao, L.-Y. Zhao, J.-T. Tang, Inductive heating property of graphene oxide–Fe3 O4 nanoparticles hybrid in an AC magnetic field for localized hyperthermia, Mater. Lett. 68 (2012) 399–401. [16] W. Wang, R. Ma, Q. Wu, C. Wang, Z. Wang, Fabrication of magnetic microsphere-confined graphene for the preconcentration of some phthalate esters from environmental water and soybean milk samples followed by their determination by HPLC, Talanta 109 (2013) 133–140.

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