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reduced graphene oxide nanoparticles for determination of trace isocarbophos residues in different matrices
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Magnetic solid phase extraction based on magnetite/reduced graphene oxide nanoparticles for determination of trace isocarbophos residues in different matrices Shan Yan a , Ting-Ting Qi a , De-Wen Chen b , Zhao Li a , Xiu-Juan Li a,∗ , Si-Yi Pan a a Key Laboratory of Environment Correlative Dietology (Ministry of Education), College of Food Science & Technology, Huazhong Agricultural University, Wuhan 430070, China b National Center of Quality Supervision and Inspection for Commodity, Yiwu, Zhejiang 322000, China
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 20 April 2014 Accepted 22 April 2014 Available online xxx Keywords: Graphene Magnetite Magnetic solid phase extraction Isocarbophos Nanoparticles Gas chromatography
a b s t r a c t A simple one-step solvothermal method was applied for the preparation of magnetite/reduced graphene oxide (MRGO), and the synthetic nanocomposites with a magnetic particle size of ∼8 nm were used as an adsorbent for magnetic solid phase extraction of isocarbophos (ICP) in different sample matrices prior to gas chromatography (GC) detection. The identity of the nanomaterial was confirmed using Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was shown that Fe3 O4 nanoparticles with a uniform size were homogeneously anchored on RGO nanosheets. Increased oxidation degrees of graphite oxide, big particle sizes and large loading amounts of Fe3 O4 on the surface of RGO led to a decrease of adsorption capacity of MRGO to ICP. The adsorption behavior of this adsorbent was better fitted by the pseudo-second-order kinetic model. Several parameters affecting the extraction efficiency were investigated and optimized, including adsorbent dosage, extraction time, ionic strength and desorption conditions. And then, a rapid and effective method based on MRGO combined with GC was developed for the determination of ICP in aqueous samples. A linear range from 0.05 to 50 ng mL−1 was obtained with a high correlation coefficient (R2 ) of 0.9995, and the limit of detection was found to be 0.0044 ng mL−1 . This method was successfully applied to the analysis of ICP in five kinds of samples, including apple, rice, lake water, cowpea and cabbage. The recoveries in different sample matrices were in the range from 81.00% to 108.51% with relative standard deviations less than 9.72%. It can be concluded that the proposed analytical method is highly-efficient, sensitive, precise, accurate and practicable. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Sample pretreatment prior to instrumental analysis is a crucial step in a whole analytical process, especially in the analysis of trace analytes in complex matrices. To date, a variety of methods have been developed for the separation and preconcentration of the target compounds from various samples [1]. Among these technologies, solid phase extraction (SPE) is one of the most welcomed technologies because it is cost-effective, eco-friendly, little by-product, high performing, and simple to operate [2]. However, the process in SPE is also time-consuming. In recent years, a new mode of SPE, based on the use of magnetic or magnetically modified adsorbents termed as magnetic solid-phase extraction (MSPE),
∗ Corresponding author. Tel.: +86 27 87282111; fax: +86 27 8728 8373. E-mail addresses: [email protected], [email protected] (X.-J. Li).
has been developed [3]. In comparison with traditional SPE, MSPE has several advantages. The separation process in MSPE can be performed directly in crude samples containing suspended solid materials by applying an external magnet without the need of additional centrifugation, filtration or gravitational separation, which greatly facilitates the extraction process [4]. Magnetic separation based on the superparamagnetic ferriferrous oxide (Fe3 O4 ) is obviously much more convenient, economic and efficient [5]. Fe3 O4 nanoparticles have been widely used because of their good biocompatibility, strong superparamagnetic property, catalytic activity, eco-friendliness, low toxicity and low cost. However, Fe3 O4 nanoparticles have drawbacks of severe aggregation between particles and poor cyclic stability, which restrict their application. In addition, it is necessary to further improve the selectivity and extraction capacity of Fe3 O4 nanoparticles to adapt to various targets. To alleviate these problems, great efforts have been made to modify Fe3 O4 nanoparticles, such as
http://dx.doi.org/10.1016/j.chroma.2014.04.073 0021-9673/© 2014 Elsevier B.V. All rights reserved.
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chitosan/Fe3 O4 [6], carbon nanotube/Fe3 O4 [7], graphene/Fe3 O4 [8,9] and core–shell structured Fe3 O4 [10–12]. Among these materials, graphene is one of the most excellent candidates in constructing new types of magnetic composite materials. Graphene, a single-atom-thick sheet of honeycomb carbonlattice, possesses extraordinary properties, such as large reversible specific capacity, remarkable surface area, excellent conductivity and many other unique physical and chemical properties, which provide it with a wide range of applications in many technological fields [13]. However, pristine graphene sheets with a high specific surface area tend to form irreversible agglomerates or even restack to form graphite through -stacking and van der Waals interactions if the sheets are not well separated from each other, which will reduce the surface area, and is not beneficial for the adsorption of contaminants. Therefore, chemical modification of graphene is the most efficient means to fully develop its excellent properties. Graphene oxide (GO) and reduced graphene oxide (RGO) play important roles in the preparation of graphene-based materials. The properties of one-atom-thick and two-dimensional plane structure endow GO and RGO with high specific surface area, which can immobilize large amounts of Fe3 O4 nanoparticles, prevent their aggregation and further bestow on Fe3 O4 some new properties and function. Up to now, graphenebased Fe3 O4 nanocomposites have become a hot topic of research and exhibit attractive application prospects in magnetic resonance imaging [14], drug delivery [15], environmental remediation [16], electrode material [17], and magnetic controlled switches [18]. In the past few years, the application based on graphene/Fe3 O4 nanocomposites in sample preparation to sorb trace analytes has also received more and more attention. Sun et al. [19] developed a simple one-step solvothermal strategy to prepare magnetite/reduced graphene oxide (MRGO) nanocomposites. The MRGO was fabricated by simultaneously forming superparamagnetic Fe3 O4 nanoparticles and reducing GO. In this process, Fe3 O4 nanoparticles were homogeneously anchored onto the surface of the graphene sheets with the aid of –COOH on GO, which prevented the agglomeration of Fe3 O4 nanoparticles and inhibited their growth. The average particle size of Fe3 O4 was 9 nm. These MRGO nanocomposites exhibited excellent removal efficiency (over 94% for malachite green and over 91% for rhodamine B) and rapid separation from aqueous solution by an external magnetic field. Chandra et al. [20] synthesized MRGO composites with magnetite particle size of 10 nm. The composites showed near complete (over 99.9%) arsenic removal within 1 ppb. Ai et al. [21] investigated the adsorption abilities of the graphene nanosheets (GNS)/Fe3 O4 composite, GNS and Fe3 O4 . The results demonstrated that Fe3 O4 had much poorer ability to sorb methylene blue molecule comparing with GNS/Fe3 O4 and GNS, indicating that the introduction of graphene greatly increased the adsorption capacity of the composites. As far as we know, MRGO is usually applied as adsorbents to the disposal of metal ions [22–24] and benzenoid compounds from wastewater [25–28]. On the one hand, with a large delocalized -electron system, graphene can interact with the benzene ring compounds through strong – stacking interaction, so it can serve as a good adsorbent for aromatic compounds. On the other hand, the oxygen-containing functional groups on RGO have strong complexation capacities with metal ions, and therefore, it is appropriate to remove radionuclides and heavy metal ions from wastewater. What’s more, Fe3 O4 particles can also adsorb contaminants due to their surface property, large specific surface area and small internal diffusion resistance. Hence, it is significant to study the effect of the oxidation levels of graphite oxide, the size and distribution of Fe3 O4 on RGO on adsorption. However, to our best knowledge, the research on these aspects is very few.
As a common and broad-spectrum organophosphorus pesticide (OPP), isocarbophos (ICP) has been considered as one of the most frequently detected OPPs residues in rice, cotton, fruit and other crops [29]. The excessive use has a deleterious effect on humans and the environment and its presence in food is particularly dangerous. Hence, it is of great importance to develop an eco-friendly, simple, sensitive and economical method for the separation and determination of ICP. In this paper, a simple one-step solvothermal strategy using non-toxic and cost-effective precursors has been developed to prepare MRGO nanocomposites for separation of ICP from different samples. The as-prepared MRGO was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The effects of the oxidation levels of graphite oxide, the particle sizes and distribution of Fe3 O4 on RGO on adsorption performance were investigated. The adsorption mechanism of magnetic nanocomposites applying kinetic models was discussed. The impacts of some experimental factors on their adsorption power were figured out. Coupling this MSPE technique with GC separation and detection, a highly simple and sensitive analytical method to determine ICP was established. 2. Experimental 2.1. Chemicals, standard solutions and real samples Graphite powder, potassium permanganate, sulphuric acid (H2 SO4 ), hydrogen peroxide (H2 O2 , 30%), hydrochloric acid (HCl), anhydrous ferric chloride (FeCl3 ), sodium acetate (NaOAc), diethylene glycol (DEG), ethanol, acetonitrile, methanol, sodium chloride (NaCl), acetone, and dichloromethane were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Graphite powder was of spectroscopically pure grade, and all of the other reagents were of analytical grade and used without further purification. Deionized water was used throughout the work. ICP with the purity of 99% was purchased from Helishun Technology Co., Ltd. (Beijing, China). The stock solution of ICP was prepared in methanol at a concentration of 1 mg mL−1 . Working standard solutions of ICP were prepared by diluting the stock solution with methanol. All of the solutions were stored at 4 ◦ C in a refrigerator. Four kinds of real samples (apple, cabbage, cowpea and rice) were randomly purchased from a local market (Wuhan, China); lake water was collected from the South Lake (Wuhan, China). 50 g of cabbage, apple and cowpea were homogenized along with 50, 50 and 100 mL of deionized water using a juice extractor (Philips China Co., Guangzhou, China), respectively. The rice sample was powdered using a cyclone mill (Taisite Instrument Co., Tianjin, China) and passed through an 80 mesh sieve. Subsequently, 20 g of processed rice were diluted with 100 mL of deionized water. The water sample was filtered through a 0.45-m membrane to eliminate particulate matters before analysis. The processed samples were then placed in separate amber glass bottles and stored in a freezer at 4 ◦ C until analysis. 2.2. Apparatus and chromatographic conditions XRD measurements were carried out on a Bruker D8-Advance X-ray diffractometer (Bruker, Germany) using Cu K␣ radiation ( = 0.1514 nm) at a generator voltage of 40 kV and a generator current of 40 mA. The scattering angles (2) were in the range of 5–80◦ with a scanning velocity of 10◦ min−1 . TEM analysis was obtained using a Philip CM 12 transmission electron microscope (Philip, Netherlands) operated at 120 kV. SEM images were recorded on
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a Zeiss supra55 field-emission SEM system. Infrared analyses were conducted with a Nicolet Nexus 470 FTIR spectrometer (Thermo Nicolet, USA) where KBr was used to prepare the sample tablets. The chromatographic analysis was performed on an SP-6890A capillary GC system (Shandong Lunan Ruihong Chemical Engineering Instrument Co., Ltd., Tengzhou, China) equipped with a capillary split/splitless injector system and a nitrogen phosphorus detector (NPD) system. Separations were accomplished on an SE54 capillary column (30 m × 0.32 mm × 0.33 m, Lanzhou ATECH technologies Co., Ltd., Lanzhou, China). The oven temperature was held at 150 ◦ C and then increased to 230 ◦ C at a rate of 10 ◦ C min−1 , where the temperature was held for 9 min. The temperatures of the injector and the detector were set at 250 ◦ C and 270 ◦ C, respectively. Nitrogen (99.999%) was used as carrier gas at a flow rate of 12–15 cm s−1 for all the analyses. 2.3. Preparation of graphite oxide with different degrees of oxidation Graphite oxide was synthesized from natural graphite powder according to a modified Hummers method [30]. The procedure is described as follows. Graphite powder (2 g) was put into a 1000 mL three-neck flask containing cold (0 ◦ C) concentrated H2 SO4 (80 mL) under vigorous stirring to avoid agglomeration inside an ice bath. After the graphite powder was well dispersed, the required amount of KMnO4 was added slowly to the mixture. Meanwhile, the reaction temperature was controlled to below 10 ◦ C by cooling. The mixture continued to react in this condition for 4 h. The ice bath was then removed and the solution was transferred to a (35 ± 5)◦ C water bath and stirred for about 1 h. After that, 160 mL of deionized water was gradually added and the solution was stirred at a temperature of 90 ◦ C for a period of time. Afterwards, the mixture was further diluted with 480 mL of deionized water and then a 30 wt% solution of H2 O2 was added, turning the color of the solution from dark brown to bright yellow. Finally, the resulting graphite oxide suspension was washed by repeated centrifugation, first with 5 % HCl aqueous solution and then with deionized water until the pH of the solution became neutral. The resulted solid was dried under freeze vacuum for 24 h. The amount of oxygenated functional groups in graphite oxide was varied by changing the amount of KMnO4 from 3 to 6 g with an increment of 1.5 g per oxidation level, and setting the reaction time of high-temperature stage at 15, 30 and 60 min, respectively, while the other parameters in the reaction were kept constant. 2.4. Synthesis of MRGO nanocomposites with different particle sizes and loading amounts of Fe3 O4 nanoparticles Typical synthesis of MRGO nanocomposites was carried out in a solvothermal system using FeCl3 and GO as precursors and DEG as a solvent and reducing agent. Graphite oxide (34 mg) was exfoliated by ultrasonication in DEG (10 mL) for 3 h to produce GO dispersion. Certain quantities of FeCl3 and NaOAc were dissolved in DEG (24 mL) via ultrasonication for 10 min to produce a transparent solution. Then, the GO dispersion was added into the solution, followed by ultrasonication for 1 h to obtain a homogeneous suspension. 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, and then cooled naturally to ambient temperature. The resulting nanocomposites were collected by applying an external magnetic field. Finally, the black product was thoroughly rinsed with ethanol and deionized water for several times, and dried in vacuum at 60 ◦ C for 24 h. For comparison, Fe3 O4 nanoparticles were synthesized via the same method without adding graphite oxide.
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Table 1 Different synthesis conditions for MRGO nanocomposites. MRGO
MRGO-1 MRGO-2 MRGO-3 MRGO-4 MRGO-5 MRGO-6 MRGO-7 MRGO-8 MRGO-9 MRGO-10
Degrees of oxidation of graphite oxidea
Solvothermal reactionb
m1 :m2
T1 (min)
T2 (h)
m3 :m4
1:6 1:6 1:6 1:3 1:4.5 1:3 1:3 1:3 1:3 1:3
30 30 30 30 30 15 60 30 30 30
4 6 8 4 4 4 4 4 4 4
1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1 1:2 1:3
a T1 , the reaction time of high temperature stage; m1 , the mass of graphite; m2 , the mass of KMnO4 . b T2 , the solvothermal reaction time; m3 , the mass of graphite oxide; m4 , the mass of FeCl3 .
MRGO nanocomposites with different Fe3 O4 particle sizes and loadings were synthesized by simply varying the solvothermal reaction time (4, 6, and 8 h) and the amount of FeCl3 (34, 68, and 102 mg) while the other experimental conditions were kept unchanged. Different conditions are listed in Table 1. 2.5. MSPE procedure MSPE was carried out by using MRGO as an adsorbent to extract ICP from aqueous samples. Typically, 20 mL of ICP solution of desired initial concentration (or sample) and 20 mg of MRGO were added into a 50-mL centrifuge tube and then the tube was shaken on a platform shaker under ambient temperature. At the completion of preset time intervals, the solid and liquid phases were separated by the aid of a permanent magnet and the supernatant was discarded. After that, the residual solution and MRGO were totally transferred to a 2-mL centrifuge tube. The adsorbent was gathered again by depositing a magnet at the outside of tube wall in order to completely remove the residual solution. After being washed with deionized water to remove impurities that may exist, the preconcentrated target analytes were desorbed from the isolated MRGO with 0.5 mL of mixed solvents of methanol and acetonitrile (1:1, v/v) by vigorously vortexing for 1 min. Afterwards, the desorption solution was collected by placing a magnet to the outside of the centrifuge tube. The same desorption procedure was operated another three times. The supernatant solutions were combined together and totally transferred to a 4-mL centrifuge tube, and then evaporated to dryness under a gentle stream of nitrogen. The residue was reconstituted in 50 L of methanol, and 1 L was injected into the GC system for analysis. Ultrasound was also introduced to investigate the effect of the mode of elution on desorption efficiency. It was proceeded by replacing vortex processing mode with ultrasound (100 W, 1 min). 3. Results and discussion 3.1. Characterization of MRGO Infrared spectra were performed to characterize graphite oxide and MRGO. Fig. 1 shows the FTIR spectra of graphite, graphite oxide and MRGO. In graphite oxide spectrum, the strong and broad absorption at 3396.61 cm−1 corresponds to O H stretching vibration, the absorption peak appearing at 1723.39 cm−1 is attributed to C O stretching of COOH groups, the absorption at 1619.89 cm−1 is due to O H bending vibration, epoxide groups and skeletal ring vibrations, and the other three vibrational bands located at 1409.56 cm−1 , 1225.65 cm−1 and 1058.78 cm−1 can be
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Fig. 3. TEM images of GO (a) and MRGO (b). The synthesis conditions were described below: the mass ratio of graphite oxide to FeCl3 was 1:1, and the solvothermal reaction time was 4 h.
Fig. 1. Infrared spectra of graphite, graphite oxide and MRGO.
assigned to COO− symmetric vibration, epoxy C O and alkoxy C O stretching vibrations, respectively [31]. The characteristic IR features of graphite oxide indicate the presence of abundant oxygen-containing functional groups on the surface of graphite oxide, confirming the successful oxidation of graphite. The IR spectrum of MRGO differs from that of graphite oxide as evidenced by the disappearance of C O and epoxy C O stretching vibrations and the dramatic decrease in the intensities of the characteristic absorption bands of O H and alkoxy C O. The absorption band that appears at 1570.84 cm−1 may be attributed to the skeletal vibration of the graphene sheets. It can be concluded that GO had been reduced to graphene sheets. In addition, a sharp peak around 579.4 cm−1 can be ascribed to lattice absorption of iron oxide, indicating that Fe3 O4 was successfully grafted on RGO. To obtain the phase and structure information about graphite, graphite oxide and as synthesized MRGO, powder XRD patterns were conducted. As shown in Fig. 2, the graphite powder exhibits a typical sharp (0 0 2) diffraction peak at 26.7◦ corresponding to an interlayer distance of 0.316 nm. After oxidation treatment, the graphite peak disappears and is replaced by a sharp diffraction
peak at 2 = 11.6◦ (d = 0.76 nm), resulting from the typical diffraction peak of graphite oxide nanosheets. When in situ conversion of FeCl3 to Fe3 O4 and simultaneous reduction of GO into RGO in DEG solution were achieved, the graphite oxide peak disappears and seven diffraction lines are observed in the representative XRD pattern of Fe3 O4 at 2 = 30.2◦ , 35.5◦ , 43.2◦ , 53.5◦ , 57.2◦ , 62.6◦ , 74.1◦ . These diffraction peaks can be indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) planes, respectively, of the cubic phase Fe3 O4 (JCPDS card NO. 19-0629) with a face-centered cubic structure [32]. These results suggest the presence of Fe3 O4 on RGO, which is consistent with the FTIR results. In addition, the broad diffraction peaks are indications of the nanoparticles with very small sizes. The morphology and structure of GO and MRGO were investigated by SEM and TEM. In Fig. 3a, the copper grid substrates were covered with large flakes of GO with a few layers thick, confirming that the GO sheets had been exfoliated. Furthermore, the GO sheet exhibited a smooth surface, an irregular shape and some wrinkles, which maintained a large surface area. From Fig. 3b, TEM image of MRGO showed significant changes in the surface morphology and roughness, visually demonstrating the presence of Fe3 O4 nanoparticles. The Fe3 O4 nanoparticles with an average size of 8 nm were homogeneously assembled on the surface of RGO sheets, and no big conglomeration of Fe3 O4 particles or large vacancy on RGO is observed. In addition, it can be observed that the Fe3 O4 nanoparticles are still strongly deposited on the surface of RGO sheets even after a long time of sonication during the preparation of the TEM specimen, indicating the strong interaction between Fe3 O4 nanoparticles and RGO sheets. The SEM measurement (Fig. 4) showed that the nanocomposites possessed a three-dimensional morphology. The large RGO flakes exhibited slightly wrinkled surface and the Fe3 O4 nanoparticles appeared as bright dots, densely and evenly deposited on both sides of RGO sheets. 3.2. Optimization of the synthesis conditions of MRGO
Fig. 2. XRD patterns of graphite (a), graphite oxide (b) and MRGO (c).
The oxidation level of graphite oxide was varied by changing the amount of KMnO4 and the reaction time of high temperature. As shown in Fig. 5a and b, the adsorption ability decreased as the degrees of oxidation of graphite oxide increased. In general, increased oxidation degrees will result in more oxygenated functional groups on the surface of GO, which leads to more loading sites for Fe3 O4 nanoparticles. That is to say, the adsorption ability of MRGO to ICP decreased as Fe3 O4 loadings increased. Fig. 6a–c shows the TEM images of MRGO with different amounts of FeCl3 . As can be seen, the Fe3 O4 loadings increased as the amount of FeCl3
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Fig. 4. SEM image of MRGO. The synthesis conditions were the same as Fig. 3.
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increased. But the increased loadings led to a decline of adsorption ability to ICP, as shown in Fig. 5c. In addition, the effect of particle sizes of Fe3 O4 was also studied, and the results are shown in Fig. 5d. The size of the magnetite nanocrystals can be easily tuned by different reaction time [33]. The average sizes of the obtained magnetite on RGO were 8, 11 and 15 nm after reaction at 190 ◦ C for 4 h, 6 h and 8 h, respectively. As shown in Fig. 5d, the adsorption capacity of MRGO to ICP decreased with the increasing of the diameters of Fe3 O4 . Figs. 5 and 6 demonstrated that increased oxidation degrees of graphite oxide, big particle sizes and large loading amounts of Fe3 O4 on the surface of RGO led to a decrease of adsorption capacity of MRGO to ICP. It seemed that Fe3 O4 contributed less to the adsorption than graphene, and mainly dedicated to the separation process in MSPE. Fig. 7 compares the adsorption ability of GO, MRGO and Fe3 O4 . During the separation process, GO was collected by centrifugation.
Fig. 5. Adsorption ability of MRGO synthesized under different conditions.
Fig. 6. TEM images of MRGO-8 (a), MRGO-9 (b) and MRGO-10 (c).
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The pseudo-first-order kinetic model is generally represented by the following equation: ln(qe − qt ) = ln qe − k1 t
(1)
The pseudo-second-order rate equation is expressed as follows: 1 t t = + qt qe k2 q2e
Fig. 7. Adsorption ability of GO, MRGO and Fe3 O4 to ICP from aqueous solution. The dosage of each adsorbent was 20 mg.
(2)
where qe is the amount of ICP adsorbed per unit mass of adsorbent at equilibrium (mg g−1 ), and qt is the amount of ICP adsorbed at time t (mg g−1 ). The parameters k1 and k2 represent the pseudofirst-order and pseudo-second-order rate constants for the kinetic models, respectively. Based on the two models, the parameters for the two kinetic models are obtained and given in Table 2. For the pseudo-firstorder model, the calculated qe value (qe,cal ) (96.5537 mg g−1 ) is not consistent with the experimental data (qe,exp ) (217.2694 mg g−1 ). In contrast, the qe,cal value (208.3333 mg g−1 ) for the pseudo-secondorder model agrees well with the experimental one, and a good linear relationship between t/qt and t is obtained with R2 being 0.9932. These results illustrated that the experimental data for ICP sorbed by MRGO followed the pseudo-second-order kinetic model better than the pseudo-first-order model. Based on the assumption of the pseudo-second-order model, the adsorption process is mainly predominated by chemisorption. 3.4. Optimization of adsorption and desorption conditions 3.4.1. Effect of the amount of MRGO The amounts of MRGO in the range of 5–45 mg were investigated to extract ICP. As shown in Fig. 9a, the peak area of ICP increased rapidly as the amount of adsorbent increased, and the maximum plateau of the adsorption was achieved when the amount of MRGO increased to 20 mg. After that, the extracted amount was almost constant, indicating that 20 mg of sorbent was sufficient to extract ICP from the aqueous solution. According to the result, 20 mg of MRGO was employed in the following studies.
Fig. 8. Effect of adsorption time on the adsorbed amount of ICP by MRGO. Extraction conditions: initial concentration of ICP, 0.25 g L−1 ; MRGO, 20 mg; concentration of NaCl, 2%; extraction temperature, 20 ◦ C; eluent, methanol and acetonitrile (1:1, v/v); desorption mode, vortex.
It can be seen that GO got the highest adsorption amounts, a little higher than that of MRGO, while Fe3 O4 obtained the least, proving that graphene played a major role in the adsorption process and MRGO nanocomposites combined both the merits of graphene and Fe3 O4 nanoparticles. Above all, MRGO-8 was chosen as a model to further investigate extraction of ICP from aqueous solution. 3.3. Evaluation of adsorption kinetics The effect of adsorption time on the removal of ICP by MRGO was studied. Fig. 8 shows the adsorption data by MRGO at different time intervals. The results indicated that a pretty fast adsorption process of ICP occurred during the first few minutes and the adsorbed amount of ICP reached its equilibrium value very quickly. As shown in Fig. 8, the equilibrium time for ICP was about 10 min. Kinetic study provides important information about the mechanism of ICP adsorption onto MRGO. Specifically, the kinetic data were analyzed employing the pseudo-first-order and pseudosecond-order rate equations [34].
3.4.2. Effect of ionic strength The addition of salt has both positive and negative impacts on the extraction process. On the one hand, the occurring of saltingout effect can decrease the solubility of organic analytes, which is conductive to the improvement of extraction efficiency; on the other hand, the addition of salt can also increase the viscosity of the solution, which is against the adsorption of ICP by MRGO due to the reduction of extraction capability and diffusion coefficient. In this study, the addition of different amounts of NaCl into the sample solutions (i.e., 0, 0.5, 2, 5, 10 and 18% (w/v)) was studied. Fig. 9b shows that the highest peak area was achieved when the concentration of NaCl was 2%. However, the extraction efficiency decreased with further increasing the concentration of NaCl. As a result, the concentration of NaCl added to the solution was set at 2%. 3.4.3. Desorption conditions Desorption of the analytes from the magnetic nanocomposites is a crucial part in the whole process. In this work, organic solvents, the amount of eluent, desorption time and the mode of elution used for MSPE procedure were optimized. In addition, before the target analytes were desorbed from MRGO, the nanocomposites were washed with deionized water to remove any possible impurities. It was found that no loss of analytes occurred after the washing procedure. Fig. 10 represents the desorption efficiency of different eluents in the mode of vortex. The results showed that the mixture of methanol and acetonitrile with the volume ratio of 1:1 had the
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Fig. 9. Effect of the amounts of MRGO (a) and the ionic strength (b) on MSPE efficiency. Extraction conditions: initial concentration of ICP, 0.25 g L−1 ; extraction time, 10 min; extraction temperature, 20 ◦ C; eluent, methanol and acetonitrile (1:1, v/v); desorption mode, vortex.
In summary, after extraction and separation, the MRGO was washed with deionized water, and the ICP was desorbed from MRGO by vortexing for 1 min with 0.5 mL of mixed eluent of methanol and acetonitrile (1:1, v/v) for four times. 3.5. Reusability of MRGO In order to investigate the reusability of MRGO, the adsorbent was reused for extraction of ICP after desorption of the analytes from MRGO. The peak areas of the first adsorption-desorption cycle and the tenth cycle were 174,658.1 and 156,193.297, respectively, indicating that MRGO can be reused at least 10 times without a significant decrease of the sorption capacity. It can be seen that MRGO nanocomposites have the advantage of reuse stability, which can significantly reduce the cost in sample preparation. 3.6. Validation of the MSPE-GC method
Fig. 10. The effect of desorption solvents on the extraction efficiency of ICP. Extraction conditions: initial concentration of ICP, 0.25 g L−1 ; MRGO, 20 mg; concentration of NaCl, 2%; extraction time, 10 min; extraction temperature, 20 ◦ C; desorption mode, vortex.
strongest desorption power among these eluents. The effect of desorption solution volume was also investigated. Results revealed that a complete desorption of ICP from MRGO could be achieved by rinsing the adsorbent with each 0.5 mL of eluent for four times. Meanwhile, the effect of desorption time was studied. The peak areas after vortexing for 1 min and 3 min were 6102.5 and 6118.950, respectively, suggesting that desorption time had no obvious influence on the desorption efficiency. The modes of elution were compared using ultrasound and vortex. The amount of ICP desorbed by vortexing was 2.4 times bigger than that by the mode of ultrasound, which could be attributed to the fact that oscillation vortex was more intense than ultrasound.
Several important parameters including linearity, correlation coefficient (R2 ), limit of detection (LOD) and limit of quantification (LOQ) were determined to validate the method. Under the optimized conditions, a series of standard solutions containing ICP at different concentration levels were prepared to establish the calibration curve and a linear range from 0.05 to 50 ng mL−1 was obtained with a high R2 value of 0.9995, indicating good linearity. The LOD and LOQ data were calculated at a signal-to-noise ratio of three (S/N = 3) and a signal-to-noise ratio of ten (S/N = 10), respectively. It was found to be 0.0044 and 0.0147 ng mL−1 , respectively. In addition, the precision of the MSPE-GC method, expressed as relative standard deviation (RSD), was assessed by three parallel extractions of ICP, and it was calculated as 1.77 %. Table 3 compares the performance of the proposed method in this study and other methods reported in literatures. It can be observed that the MSPE-GC-NPD method has a lower detection limit and RSD in contrast with the methods mentioned above and the samples used in this study have significant matrix difference, confirming that this method has high sensitivity, good repeatability and practicability.
Table 2 Parameters based on the pseudo-first-order and pseudo-second-order kinetic equations for the adsorption of ICP on MRGO. Analyte
qe,exp (mg g−1 )
Pseudo first-order −1
qe,cal (mg g ICP
217.2694
96.5537
)
Pseudo second-order −1
K1 (min 0.2324
)
R
qe,cal (mg g−1 )
K2 (g mg−1 min−1 )
R2
0.9906
208.3333
0.0092
0.9932
2
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Table 3 Comparison of the proposed method with other methods for the determination of ICP. Linear range (ng mL−1 )
Methods a
DSPE -GC–MS USAEMEb -HPLC-DAD DSMEc -GC-FPD QuEChERSd -GC-FPD SDMEe -GC-FPD QuEChERS-SPE-LC–MS/MS MSPE-GC-NPD a b c d e
7–200 1.0–200 0.1–100 20–1280 10–500 2–125 0.05–50
LOD (ng mL−1 )
R2
RSD (%)
Matrices
Ref.
1.6 0.1 0.03 15.0 2.2 0.08–1.8 0.0044
0.9947 0.9998 0.9995 0.9994 0.9919 0.9993 0.9995
2.68–8.48 3.30 9.20 3.43 6.30 3.9–9.2 1.77
Peanut oil Well, rain, reservoir water Wine Morinda roots Orange juices Soil, water Apple, cabbage, lake water, cowpea, rice
[35] [36] [37] [38] [39] [40] This study
DSPE: dispersive solid phase extraction. USAEME: ultrasound-assisted emulsification microextraction. DSME: dispersive suspended microextraction. QuEChERS: quick, easy, cheap, effective, rugged and safe. SDME: single drop microextraction.
Table 4 Determination of ICP residues and recoveries in real samples. Real samples
Spiking level (ng g−1 )
ICP Found (ng g−1 )
RSD (%)
Recovery (%)
a
Apple (n = 3)
0.00 0.1 1.0
nd 0.099 0.925
Cabbage (n = 3)
0.00 0.1 1.0
nd 0.110 0.863
7.25 9.34
108.51 86.32
Lake water (n = 3)
0.00 0.1 1.0
1.275 0.081 0.923
7.41 2.00 1.84
81.00 92.33
Cowpea (n = 3)
0.00 0.1 1.0
nd 0.094 1.067
4.03 6.53
93.81 106.72
Rice (n = 3)
0.00 0.1 1.0
nd 0.103 0.994
9.72 4.69
103.33 99.39
a
very efficient and sensitive. In addition, different real samples were also treated using MRGO, and the results demonstrated that sample matrices had little interference with the performance of MRGO. The results suggest that MRGO has a potential application prospect in the determination of trace analytes.
2.97 6.14
99.20 92.49
nd, not detected.
3.7. Analysis of real samples The method was used for the determination of ICP in five kinds of real samples including apple, cabbage, cowpea, lake water and rice. The results are summarized in Table 4. Only a low concentration of ICP (1.275 ng g−1 ) was found in the lake water sample and no residues were detected in the other four samples. To estimate the effect of the matrices, all of the samples were spiked at two concentration levels of ICP (0.1 and 1.0 ng g−1 ) to determine the recovery of the targeted analyte. For each concentration level, three replicates were conducted. As shown in the table, satisfactory recoveries of ICP from five real samples, ranging from 81.00% to 108.51% with RSDs less than 9.72%, were observed. The results demonstrated that these real sample matrices had little interference with the performance of MRGO and it can also be concluded that the MSPE-GC analytical method is highly-efficient, precise and accurate. 4. Conclusions MRGO was synthesized via a simple one-step solvothermal strategy and charaterized by FTIR, XRD, SEM and TEM. The two components of Fe3 O4 and graphene endow the material with manipulative convenience as well as high adsorption capacity. Thus, MRGO exhibited excellent extraction capability for ICP and could be conveniently separated from aqueous samples by an external magnet. Low detection limit and satisfactory recoveries were achieved, indicating that the proposed MSPE-GC-NPD method is
Acknowledgements This work was kindly supported by the Scientific and Technological Project of Wuhan City of China (Program No. 2013020501010171) and the National Natural Science Foundation of China (Program No. 30901007). References [1] J. Fenik, M. Tankiewicz, M. Biziuk, Trends Anal. Chem. 30 (2011) 814. [2] I. Ferrer, J.F. García-Reyes, M. Mezcua, E.M. Thurman, A.R. Fernández-Alba, J. Chromatogr. A 1082 (2005) 81. ˇ ˇ [3] M. Safaˇ rı´ıková, I. Safaˇ rı´ık, J. Magn. Magn. Mater. 194 (1999) 108. [4] L.M. Ravelo-Pérez, A.V. Herrera-Herrera, J. Hernández-Borges, M.Á. RodríguezDelgado, J. Chromatogr. A 1217 (2010) 2618. [5] Q. Han, Z.H. Wang, J.F. Xia, S. Chen, X.Q. Zhang, M.Y. Ding, Talanta 101 (2012) 388. [6] A. Kaushik, P.R. Solanki, A.A. Ansari, S. Ahmad, B.D. Malhotra, Electrochem. Commun. 10 (2008) 1364. [7] S. Song, R. Rao, H. Yang, H. Liu, A. Zhang, Nanotechnology 21 (2010) 185602. [8] J.H. Deng, X.R. Zhang, G.M. Zeng, J.L. Gong, Q.Y. Niu, J. Liang, Chem. Eng. J. 226 (2013) 189. [9] D.N. Huang, X.Y. Wang, C.H. Deng, G.X. Song, H.F. Cheng, X.M. Zhang, J. Chromatogr. A 1325 (2014) 65. [10] J.R. Meng, J. Bu, C.H. Deng, X.M. Zhang, J. Chromatogr. A 1218 (2011) 1585. [11] J.R. Meng, C.Y. Shi, B.W. Wei, W.J. Yu, C.H. Deng, X.M. Zhang, J. Chromatogr. A 1218 (2011) 2841. [12] Z.B. Li, D.N. Huang, C.F. Fu, B.W. Wei, W.J. Yu, C.H. Deng, X.M. Zhang, J. Chromatogr. A 1218 (2011) 6232. [13] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Prog. Mater. Sci. 56 (2011) 1178. [14] L.Z. Bai, D.L. Zhao, Y. Xu, J.M. Zhang, Y.L. Gao, L.Y. Zhao, J.T. Tang, Mater. Lett. 68 (2012) 399. [15] X.Y. Yang, X.Y. Zhang, Y.F. Ma, Y. Huang, Y.S. Wang, Y.S. Chen, J. Mater. Chem. 19 (2009) 2710. [16] W.N. Wang, R.Y. Ma, Q.H. Wu, C. Wang, Z. Wang, Talanta 109 (2013) 133. [17] Q.H. Wang, L.F. Jiao, H.M. Du, Y.J. Wang, H.T. Yuan, J. Powder Sour. 245 (2014) 101. [18] J.J. Liang, Y.F. Xu, D. Sui, L. Zhang, Y. Huang, Y.F. Ma, F.F. Li, Y.S. Chen, J. Phys. Chem. C 114 (2010) 17465. [19] H.M. Sun, L.Y. Cao, L.H. Lu, Nano Res. 4 (2011) 550. [20] V. Chandra, J. Park, Y. Chun, J.W. Lee, I. Hwang, K.S. Kim, ACS Nano 4 (2010) 3979. [21] L.H. Ai, C.Y. Zhang, Z.L. Chen, J. Hazard. Mater. 192 (2011) 1515. [22] P.F. Zong, S.F. Wang, Y.L. Zhao, H. Wang, H. Pan, C.H. He, Chem. Eng. J. 220 (2013) 45. [23] X.B. Luo, C.C. Wang, S.L. Luo, R.Z. Dong, X.M. Tu, G.S. Zeng, Chem. Eng. J. 187 (2012) 45. [24] L. Zhou, H.P. Deng, J.L. Wan, J. Shi, T. Su, Appl. Surf. Sci. 283 (2013) 1024. [25] Q.H. Wu, G.Y. Zhao, C. Feng, C. Wang, Z. Wang, J. Chromatogr. A 1218 (2011) 7936. [26] Y.B. Luo, Z.G. Shi, Q. Gao, Y.Q. Feng, J. Chromatogr. A 1218 (2011) 1353. [27] W.N. Wang, R.Y. Ma, Q.H. Wu, C. Wang, Z. Wang, J. Chromatogr. A 1293 (2013) 20. [28] Q. Ye, L.H. Liu, Z.B. Chen, L.M. Hong, J. Chromatogr. A 1329 (2014) 24. [29] D.H. Chen, Z.H. Song, H.R. Lv, Food Chem. 135 (2012) 2549.
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Magnetic solid phase extraction based on magnetite/reduced graphene oxide nanoparticles for determination of trace isocarbophos residues in different matrices Shan Yan a , Ting-Ting Qi a , De-Wen Chen b , Zhao Li a , Xiu-Juan Li a,∗ , Si-Yi Pan a a Key Laboratory of Environment Correlative Dietology (Ministry of Education), College of Food Science & Technology, Huazhong Agricultural University, Wuhan 430070, China b National Center of Quality Supervision and Inspection for Commodity, Yiwu, Zhejiang 322000, China
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 20 April 2014 Accepted 22 April 2014 Available online xxx Keywords: Graphene Magnetite Magnetic solid phase extraction Isocarbophos Nanoparticles Gas chromatography
a b s t r a c t A simple one-step solvothermal method was applied for the preparation of magnetite/reduced graphene oxide (MRGO), and the synthetic nanocomposites with a magnetic particle size of ∼8 nm were used as an adsorbent for magnetic solid phase extraction of isocarbophos (ICP) in different sample matrices prior to gas chromatography (GC) detection. The identity of the nanomaterial was confirmed using Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was shown that Fe3 O4 nanoparticles with a uniform size were homogeneously anchored on RGO nanosheets. Increased oxidation degrees of graphite oxide, big particle sizes and large loading amounts of Fe3 O4 on the surface of RGO led to a decrease of adsorption capacity of MRGO to ICP. The adsorption behavior of this adsorbent was better fitted by the pseudo-second-order kinetic model. Several parameters affecting the extraction efficiency were investigated and optimized, including adsorbent dosage, extraction time, ionic strength and desorption conditions. And then, a rapid and effective method based on MRGO combined with GC was developed for the determination of ICP in aqueous samples. A linear range from 0.05 to 50 ng mL−1 was obtained with a high correlation coefficient (R2 ) of 0.9995, and the limit of detection was found to be 0.0044 ng mL−1 . This method was successfully applied to the analysis of ICP in five kinds of samples, including apple, rice, lake water, cowpea and cabbage. The recoveries in different sample matrices were in the range from 81.00% to 108.51% with relative standard deviations less than 9.72%. It can be concluded that the proposed analytical method is highly-efficient, sensitive, precise, accurate and practicable. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Sample pretreatment prior to instrumental analysis is a crucial step in a whole analytical process, especially in the analysis of trace analytes in complex matrices. To date, a variety of methods have been developed for the separation and preconcentration of the target compounds from various samples [1]. Among these technologies, solid phase extraction (SPE) is one of the most welcomed technologies because it is cost-effective, eco-friendly, little by-product, high performing, and simple to operate [2]. However, the process in SPE is also time-consuming. In recent years, a new mode of SPE, based on the use of magnetic or magnetically modified adsorbents termed as magnetic solid-phase extraction (MSPE),
∗ Corresponding author. Tel.: +86 27 87282111; fax: +86 27 8728 8373. E-mail addresses: [email protected], [email protected] (X.-J. Li).
has been developed [3]. In comparison with traditional SPE, MSPE has several advantages. The separation process in MSPE can be performed directly in crude samples containing suspended solid materials by applying an external magnet without the need of additional centrifugation, filtration or gravitational separation, which greatly facilitates the extraction process [4]. Magnetic separation based on the superparamagnetic ferriferrous oxide (Fe3 O4 ) is obviously much more convenient, economic and efficient [5]. Fe3 O4 nanoparticles have been widely used because of their good biocompatibility, strong superparamagnetic property, catalytic activity, eco-friendliness, low toxicity and low cost. However, Fe3 O4 nanoparticles have drawbacks of severe aggregation between particles and poor cyclic stability, which restrict their application. In addition, it is necessary to further improve the selectivity and extraction capacity of Fe3 O4 nanoparticles to adapt to various targets. To alleviate these problems, great efforts have been made to modify Fe3 O4 nanoparticles, such as
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chitosan/Fe3 O4 [6], carbon nanotube/Fe3 O4 [7], graphene/Fe3 O4 [8,9] and core–shell structured Fe3 O4 [10–12]. Among these materials, graphene is one of the most excellent candidates in constructing new types of magnetic composite materials. Graphene, a single-atom-thick sheet of honeycomb carbonlattice, possesses extraordinary properties, such as large reversible specific capacity, remarkable surface area, excellent conductivity and many other unique physical and chemical properties, which provide it with a wide range of applications in many technological fields [13]. However, pristine graphene sheets with a high specific surface area tend to form irreversible agglomerates or even restack to form graphite through -stacking and van der Waals interactions if the sheets are not well separated from each other, which will reduce the surface area, and is not beneficial for the adsorption of contaminants. Therefore, chemical modification of graphene is the most efficient means to fully develop its excellent properties. Graphene oxide (GO) and reduced graphene oxide (RGO) play important roles in the preparation of graphene-based materials. The properties of one-atom-thick and two-dimensional plane structure endow GO and RGO with high specific surface area, which can immobilize large amounts of Fe3 O4 nanoparticles, prevent their aggregation and further bestow on Fe3 O4 some new properties and function. Up to now, graphenebased Fe3 O4 nanocomposites have become a hot topic of research and exhibit attractive application prospects in magnetic resonance imaging [14], drug delivery [15], environmental remediation [16], electrode material [17], and magnetic controlled switches [18]. In the past few years, the application based on graphene/Fe3 O4 nanocomposites in sample preparation to sorb trace analytes has also received more and more attention. Sun et al. [19] developed a simple one-step solvothermal strategy to prepare magnetite/reduced graphene oxide (MRGO) nanocomposites. The MRGO was fabricated by simultaneously forming superparamagnetic Fe3 O4 nanoparticles and reducing GO. In this process, Fe3 O4 nanoparticles were homogeneously anchored onto the surface of the graphene sheets with the aid of –COOH on GO, which prevented the agglomeration of Fe3 O4 nanoparticles and inhibited their growth. The average particle size of Fe3 O4 was 9 nm. These MRGO nanocomposites exhibited excellent removal efficiency (over 94% for malachite green and over 91% for rhodamine B) and rapid separation from aqueous solution by an external magnetic field. Chandra et al. [20] synthesized MRGO composites with magnetite particle size of 10 nm. The composites showed near complete (over 99.9%) arsenic removal within 1 ppb. Ai et al. [21] investigated the adsorption abilities of the graphene nanosheets (GNS)/Fe3 O4 composite, GNS and Fe3 O4 . The results demonstrated that Fe3 O4 had much poorer ability to sorb methylene blue molecule comparing with GNS/Fe3 O4 and GNS, indicating that the introduction of graphene greatly increased the adsorption capacity of the composites. As far as we know, MRGO is usually applied as adsorbents to the disposal of metal ions [22–24] and benzenoid compounds from wastewater [25–28]. On the one hand, with a large delocalized -electron system, graphene can interact with the benzene ring compounds through strong – stacking interaction, so it can serve as a good adsorbent for aromatic compounds. On the other hand, the oxygen-containing functional groups on RGO have strong complexation capacities with metal ions, and therefore, it is appropriate to remove radionuclides and heavy metal ions from wastewater. What’s more, Fe3 O4 particles can also adsorb contaminants due to their surface property, large specific surface area and small internal diffusion resistance. Hence, it is significant to study the effect of the oxidation levels of graphite oxide, the size and distribution of Fe3 O4 on RGO on adsorption. However, to our best knowledge, the research on these aspects is very few.
As a common and broad-spectrum organophosphorus pesticide (OPP), isocarbophos (ICP) has been considered as one of the most frequently detected OPPs residues in rice, cotton, fruit and other crops [29]. The excessive use has a deleterious effect on humans and the environment and its presence in food is particularly dangerous. Hence, it is of great importance to develop an eco-friendly, simple, sensitive and economical method for the separation and determination of ICP. In this paper, a simple one-step solvothermal strategy using non-toxic and cost-effective precursors has been developed to prepare MRGO nanocomposites for separation of ICP from different samples. The as-prepared MRGO was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The effects of the oxidation levels of graphite oxide, the particle sizes and distribution of Fe3 O4 on RGO on adsorption performance were investigated. The adsorption mechanism of magnetic nanocomposites applying kinetic models was discussed. The impacts of some experimental factors on their adsorption power were figured out. Coupling this MSPE technique with GC separation and detection, a highly simple and sensitive analytical method to determine ICP was established. 2. Experimental 2.1. Chemicals, standard solutions and real samples Graphite powder, potassium permanganate, sulphuric acid (H2 SO4 ), hydrogen peroxide (H2 O2 , 30%), hydrochloric acid (HCl), anhydrous ferric chloride (FeCl3 ), sodium acetate (NaOAc), diethylene glycol (DEG), ethanol, acetonitrile, methanol, sodium chloride (NaCl), acetone, and dichloromethane were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Graphite powder was of spectroscopically pure grade, and all of the other reagents were of analytical grade and used without further purification. Deionized water was used throughout the work. ICP with the purity of 99% was purchased from Helishun Technology Co., Ltd. (Beijing, China). The stock solution of ICP was prepared in methanol at a concentration of 1 mg mL−1 . Working standard solutions of ICP were prepared by diluting the stock solution with methanol. All of the solutions were stored at 4 ◦ C in a refrigerator. Four kinds of real samples (apple, cabbage, cowpea and rice) were randomly purchased from a local market (Wuhan, China); lake water was collected from the South Lake (Wuhan, China). 50 g of cabbage, apple and cowpea were homogenized along with 50, 50 and 100 mL of deionized water using a juice extractor (Philips China Co., Guangzhou, China), respectively. The rice sample was powdered using a cyclone mill (Taisite Instrument Co., Tianjin, China) and passed through an 80 mesh sieve. Subsequently, 20 g of processed rice were diluted with 100 mL of deionized water. The water sample was filtered through a 0.45-m membrane to eliminate particulate matters before analysis. The processed samples were then placed in separate amber glass bottles and stored in a freezer at 4 ◦ C until analysis. 2.2. Apparatus and chromatographic conditions XRD measurements were carried out on a Bruker D8-Advance X-ray diffractometer (Bruker, Germany) using Cu K␣ radiation ( = 0.1514 nm) at a generator voltage of 40 kV and a generator current of 40 mA. The scattering angles (2) were in the range of 5–80◦ with a scanning velocity of 10◦ min−1 . TEM analysis was obtained using a Philip CM 12 transmission electron microscope (Philip, Netherlands) operated at 120 kV. SEM images were recorded on
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a Zeiss supra55 field-emission SEM system. Infrared analyses were conducted with a Nicolet Nexus 470 FTIR spectrometer (Thermo Nicolet, USA) where KBr was used to prepare the sample tablets. The chromatographic analysis was performed on an SP-6890A capillary GC system (Shandong Lunan Ruihong Chemical Engineering Instrument Co., Ltd., Tengzhou, China) equipped with a capillary split/splitless injector system and a nitrogen phosphorus detector (NPD) system. Separations were accomplished on an SE54 capillary column (30 m × 0.32 mm × 0.33 m, Lanzhou ATECH technologies Co., Ltd., Lanzhou, China). The oven temperature was held at 150 ◦ C and then increased to 230 ◦ C at a rate of 10 ◦ C min−1 , where the temperature was held for 9 min. The temperatures of the injector and the detector were set at 250 ◦ C and 270 ◦ C, respectively. Nitrogen (99.999%) was used as carrier gas at a flow rate of 12–15 cm s−1 for all the analyses. 2.3. Preparation of graphite oxide with different degrees of oxidation Graphite oxide was synthesized from natural graphite powder according to a modified Hummers method [30]. The procedure is described as follows. Graphite powder (2 g) was put into a 1000 mL three-neck flask containing cold (0 ◦ C) concentrated H2 SO4 (80 mL) under vigorous stirring to avoid agglomeration inside an ice bath. After the graphite powder was well dispersed, the required amount of KMnO4 was added slowly to the mixture. Meanwhile, the reaction temperature was controlled to below 10 ◦ C by cooling. The mixture continued to react in this condition for 4 h. The ice bath was then removed and the solution was transferred to a (35 ± 5)◦ C water bath and stirred for about 1 h. After that, 160 mL of deionized water was gradually added and the solution was stirred at a temperature of 90 ◦ C for a period of time. Afterwards, the mixture was further diluted with 480 mL of deionized water and then a 30 wt% solution of H2 O2 was added, turning the color of the solution from dark brown to bright yellow. Finally, the resulting graphite oxide suspension was washed by repeated centrifugation, first with 5 % HCl aqueous solution and then with deionized water until the pH of the solution became neutral. The resulted solid was dried under freeze vacuum for 24 h. The amount of oxygenated functional groups in graphite oxide was varied by changing the amount of KMnO4 from 3 to 6 g with an increment of 1.5 g per oxidation level, and setting the reaction time of high-temperature stage at 15, 30 and 60 min, respectively, while the other parameters in the reaction were kept constant. 2.4. Synthesis of MRGO nanocomposites with different particle sizes and loading amounts of Fe3 O4 nanoparticles Typical synthesis of MRGO nanocomposites was carried out in a solvothermal system using FeCl3 and GO as precursors and DEG as a solvent and reducing agent. Graphite oxide (34 mg) was exfoliated by ultrasonication in DEG (10 mL) for 3 h to produce GO dispersion. Certain quantities of FeCl3 and NaOAc were dissolved in DEG (24 mL) via ultrasonication for 10 min to produce a transparent solution. Then, the GO dispersion was added into the solution, followed by ultrasonication for 1 h to obtain a homogeneous suspension. 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, and then cooled naturally to ambient temperature. The resulting nanocomposites were collected by applying an external magnetic field. Finally, the black product was thoroughly rinsed with ethanol and deionized water for several times, and dried in vacuum at 60 ◦ C for 24 h. For comparison, Fe3 O4 nanoparticles were synthesized via the same method without adding graphite oxide.
3
Table 1 Different synthesis conditions for MRGO nanocomposites. MRGO
MRGO-1 MRGO-2 MRGO-3 MRGO-4 MRGO-5 MRGO-6 MRGO-7 MRGO-8 MRGO-9 MRGO-10
Degrees of oxidation of graphite oxidea
Solvothermal reactionb
m1 :m2
T1 (min)
T2 (h)
m3 :m4
1:6 1:6 1:6 1:3 1:4.5 1:3 1:3 1:3 1:3 1:3
30 30 30 30 30 15 60 30 30 30
4 6 8 4 4 4 4 4 4 4
1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1 1:2 1:3
a T1 , the reaction time of high temperature stage; m1 , the mass of graphite; m2 , the mass of KMnO4 . b T2 , the solvothermal reaction time; m3 , the mass of graphite oxide; m4 , the mass of FeCl3 .
MRGO nanocomposites with different Fe3 O4 particle sizes and loadings were synthesized by simply varying the solvothermal reaction time (4, 6, and 8 h) and the amount of FeCl3 (34, 68, and 102 mg) while the other experimental conditions were kept unchanged. Different conditions are listed in Table 1. 2.5. MSPE procedure MSPE was carried out by using MRGO as an adsorbent to extract ICP from aqueous samples. Typically, 20 mL of ICP solution of desired initial concentration (or sample) and 20 mg of MRGO were added into a 50-mL centrifuge tube and then the tube was shaken on a platform shaker under ambient temperature. At the completion of preset time intervals, the solid and liquid phases were separated by the aid of a permanent magnet and the supernatant was discarded. After that, the residual solution and MRGO were totally transferred to a 2-mL centrifuge tube. The adsorbent was gathered again by depositing a magnet at the outside of tube wall in order to completely remove the residual solution. After being washed with deionized water to remove impurities that may exist, the preconcentrated target analytes were desorbed from the isolated MRGO with 0.5 mL of mixed solvents of methanol and acetonitrile (1:1, v/v) by vigorously vortexing for 1 min. Afterwards, the desorption solution was collected by placing a magnet to the outside of the centrifuge tube. The same desorption procedure was operated another three times. The supernatant solutions were combined together and totally transferred to a 4-mL centrifuge tube, and then evaporated to dryness under a gentle stream of nitrogen. The residue was reconstituted in 50 L of methanol, and 1 L was injected into the GC system for analysis. Ultrasound was also introduced to investigate the effect of the mode of elution on desorption efficiency. It was proceeded by replacing vortex processing mode with ultrasound (100 W, 1 min). 3. Results and discussion 3.1. Characterization of MRGO Infrared spectra were performed to characterize graphite oxide and MRGO. Fig. 1 shows the FTIR spectra of graphite, graphite oxide and MRGO. In graphite oxide spectrum, the strong and broad absorption at 3396.61 cm−1 corresponds to O H stretching vibration, the absorption peak appearing at 1723.39 cm−1 is attributed to C O stretching of COOH groups, the absorption at 1619.89 cm−1 is due to O H bending vibration, epoxide groups and skeletal ring vibrations, and the other three vibrational bands located at 1409.56 cm−1 , 1225.65 cm−1 and 1058.78 cm−1 can be
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Fig. 3. TEM images of GO (a) and MRGO (b). The synthesis conditions were described below: the mass ratio of graphite oxide to FeCl3 was 1:1, and the solvothermal reaction time was 4 h.
Fig. 1. Infrared spectra of graphite, graphite oxide and MRGO.
assigned to COO− symmetric vibration, epoxy C O and alkoxy C O stretching vibrations, respectively [31]. The characteristic IR features of graphite oxide indicate the presence of abundant oxygen-containing functional groups on the surface of graphite oxide, confirming the successful oxidation of graphite. The IR spectrum of MRGO differs from that of graphite oxide as evidenced by the disappearance of C O and epoxy C O stretching vibrations and the dramatic decrease in the intensities of the characteristic absorption bands of O H and alkoxy C O. The absorption band that appears at 1570.84 cm−1 may be attributed to the skeletal vibration of the graphene sheets. It can be concluded that GO had been reduced to graphene sheets. In addition, a sharp peak around 579.4 cm−1 can be ascribed to lattice absorption of iron oxide, indicating that Fe3 O4 was successfully grafted on RGO. To obtain the phase and structure information about graphite, graphite oxide and as synthesized MRGO, powder XRD patterns were conducted. As shown in Fig. 2, the graphite powder exhibits a typical sharp (0 0 2) diffraction peak at 26.7◦ corresponding to an interlayer distance of 0.316 nm. After oxidation treatment, the graphite peak disappears and is replaced by a sharp diffraction
peak at 2 = 11.6◦ (d = 0.76 nm), resulting from the typical diffraction peak of graphite oxide nanosheets. When in situ conversion of FeCl3 to Fe3 O4 and simultaneous reduction of GO into RGO in DEG solution were achieved, the graphite oxide peak disappears and seven diffraction lines are observed in the representative XRD pattern of Fe3 O4 at 2 = 30.2◦ , 35.5◦ , 43.2◦ , 53.5◦ , 57.2◦ , 62.6◦ , 74.1◦ . These diffraction peaks can be indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) planes, respectively, of the cubic phase Fe3 O4 (JCPDS card NO. 19-0629) with a face-centered cubic structure [32]. These results suggest the presence of Fe3 O4 on RGO, which is consistent with the FTIR results. In addition, the broad diffraction peaks are indications of the nanoparticles with very small sizes. The morphology and structure of GO and MRGO were investigated by SEM and TEM. In Fig. 3a, the copper grid substrates were covered with large flakes of GO with a few layers thick, confirming that the GO sheets had been exfoliated. Furthermore, the GO sheet exhibited a smooth surface, an irregular shape and some wrinkles, which maintained a large surface area. From Fig. 3b, TEM image of MRGO showed significant changes in the surface morphology and roughness, visually demonstrating the presence of Fe3 O4 nanoparticles. The Fe3 O4 nanoparticles with an average size of 8 nm were homogeneously assembled on the surface of RGO sheets, and no big conglomeration of Fe3 O4 particles or large vacancy on RGO is observed. In addition, it can be observed that the Fe3 O4 nanoparticles are still strongly deposited on the surface of RGO sheets even after a long time of sonication during the preparation of the TEM specimen, indicating the strong interaction between Fe3 O4 nanoparticles and RGO sheets. The SEM measurement (Fig. 4) showed that the nanocomposites possessed a three-dimensional morphology. The large RGO flakes exhibited slightly wrinkled surface and the Fe3 O4 nanoparticles appeared as bright dots, densely and evenly deposited on both sides of RGO sheets. 3.2. Optimization of the synthesis conditions of MRGO
Fig. 2. XRD patterns of graphite (a), graphite oxide (b) and MRGO (c).
The oxidation level of graphite oxide was varied by changing the amount of KMnO4 and the reaction time of high temperature. As shown in Fig. 5a and b, the adsorption ability decreased as the degrees of oxidation of graphite oxide increased. In general, increased oxidation degrees will result in more oxygenated functional groups on the surface of GO, which leads to more loading sites for Fe3 O4 nanoparticles. That is to say, the adsorption ability of MRGO to ICP decreased as Fe3 O4 loadings increased. Fig. 6a–c shows the TEM images of MRGO with different amounts of FeCl3 . As can be seen, the Fe3 O4 loadings increased as the amount of FeCl3
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Fig. 4. SEM image of MRGO. The synthesis conditions were the same as Fig. 3.
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increased. But the increased loadings led to a decline of adsorption ability to ICP, as shown in Fig. 5c. In addition, the effect of particle sizes of Fe3 O4 was also studied, and the results are shown in Fig. 5d. The size of the magnetite nanocrystals can be easily tuned by different reaction time [33]. The average sizes of the obtained magnetite on RGO were 8, 11 and 15 nm after reaction at 190 ◦ C for 4 h, 6 h and 8 h, respectively. As shown in Fig. 5d, the adsorption capacity of MRGO to ICP decreased with the increasing of the diameters of Fe3 O4 . Figs. 5 and 6 demonstrated that increased oxidation degrees of graphite oxide, big particle sizes and large loading amounts of Fe3 O4 on the surface of RGO led to a decrease of adsorption capacity of MRGO to ICP. It seemed that Fe3 O4 contributed less to the adsorption than graphene, and mainly dedicated to the separation process in MSPE. Fig. 7 compares the adsorption ability of GO, MRGO and Fe3 O4 . During the separation process, GO was collected by centrifugation.
Fig. 5. Adsorption ability of MRGO synthesized under different conditions.
Fig. 6. TEM images of MRGO-8 (a), MRGO-9 (b) and MRGO-10 (c).
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The pseudo-first-order kinetic model is generally represented by the following equation: ln(qe − qt ) = ln qe − k1 t
(1)
The pseudo-second-order rate equation is expressed as follows: 1 t t = + qt qe k2 q2e
Fig. 7. Adsorption ability of GO, MRGO and Fe3 O4 to ICP from aqueous solution. The dosage of each adsorbent was 20 mg.
(2)
where qe is the amount of ICP adsorbed per unit mass of adsorbent at equilibrium (mg g−1 ), and qt is the amount of ICP adsorbed at time t (mg g−1 ). The parameters k1 and k2 represent the pseudofirst-order and pseudo-second-order rate constants for the kinetic models, respectively. Based on the two models, the parameters for the two kinetic models are obtained and given in Table 2. For the pseudo-firstorder model, the calculated qe value (qe,cal ) (96.5537 mg g−1 ) is not consistent with the experimental data (qe,exp ) (217.2694 mg g−1 ). In contrast, the qe,cal value (208.3333 mg g−1 ) for the pseudo-secondorder model agrees well with the experimental one, and a good linear relationship between t/qt and t is obtained with R2 being 0.9932. These results illustrated that the experimental data for ICP sorbed by MRGO followed the pseudo-second-order kinetic model better than the pseudo-first-order model. Based on the assumption of the pseudo-second-order model, the adsorption process is mainly predominated by chemisorption. 3.4. Optimization of adsorption and desorption conditions 3.4.1. Effect of the amount of MRGO The amounts of MRGO in the range of 5–45 mg were investigated to extract ICP. As shown in Fig. 9a, the peak area of ICP increased rapidly as the amount of adsorbent increased, and the maximum plateau of the adsorption was achieved when the amount of MRGO increased to 20 mg. After that, the extracted amount was almost constant, indicating that 20 mg of sorbent was sufficient to extract ICP from the aqueous solution. According to the result, 20 mg of MRGO was employed in the following studies.
Fig. 8. Effect of adsorption time on the adsorbed amount of ICP by MRGO. Extraction conditions: initial concentration of ICP, 0.25 g L−1 ; MRGO, 20 mg; concentration of NaCl, 2%; extraction temperature, 20 ◦ C; eluent, methanol and acetonitrile (1:1, v/v); desorption mode, vortex.
It can be seen that GO got the highest adsorption amounts, a little higher than that of MRGO, while Fe3 O4 obtained the least, proving that graphene played a major role in the adsorption process and MRGO nanocomposites combined both the merits of graphene and Fe3 O4 nanoparticles. Above all, MRGO-8 was chosen as a model to further investigate extraction of ICP from aqueous solution. 3.3. Evaluation of adsorption kinetics The effect of adsorption time on the removal of ICP by MRGO was studied. Fig. 8 shows the adsorption data by MRGO at different time intervals. The results indicated that a pretty fast adsorption process of ICP occurred during the first few minutes and the adsorbed amount of ICP reached its equilibrium value very quickly. As shown in Fig. 8, the equilibrium time for ICP was about 10 min. Kinetic study provides important information about the mechanism of ICP adsorption onto MRGO. Specifically, the kinetic data were analyzed employing the pseudo-first-order and pseudosecond-order rate equations [34].
3.4.2. Effect of ionic strength The addition of salt has both positive and negative impacts on the extraction process. On the one hand, the occurring of saltingout effect can decrease the solubility of organic analytes, which is conductive to the improvement of extraction efficiency; on the other hand, the addition of salt can also increase the viscosity of the solution, which is against the adsorption of ICP by MRGO due to the reduction of extraction capability and diffusion coefficient. In this study, the addition of different amounts of NaCl into the sample solutions (i.e., 0, 0.5, 2, 5, 10 and 18% (w/v)) was studied. Fig. 9b shows that the highest peak area was achieved when the concentration of NaCl was 2%. However, the extraction efficiency decreased with further increasing the concentration of NaCl. As a result, the concentration of NaCl added to the solution was set at 2%. 3.4.3. Desorption conditions Desorption of the analytes from the magnetic nanocomposites is a crucial part in the whole process. In this work, organic solvents, the amount of eluent, desorption time and the mode of elution used for MSPE procedure were optimized. In addition, before the target analytes were desorbed from MRGO, the nanocomposites were washed with deionized water to remove any possible impurities. It was found that no loss of analytes occurred after the washing procedure. Fig. 10 represents the desorption efficiency of different eluents in the mode of vortex. The results showed that the mixture of methanol and acetonitrile with the volume ratio of 1:1 had the
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Fig. 9. Effect of the amounts of MRGO (a) and the ionic strength (b) on MSPE efficiency. Extraction conditions: initial concentration of ICP, 0.25 g L−1 ; extraction time, 10 min; extraction temperature, 20 ◦ C; eluent, methanol and acetonitrile (1:1, v/v); desorption mode, vortex.
In summary, after extraction and separation, the MRGO was washed with deionized water, and the ICP was desorbed from MRGO by vortexing for 1 min with 0.5 mL of mixed eluent of methanol and acetonitrile (1:1, v/v) for four times. 3.5. Reusability of MRGO In order to investigate the reusability of MRGO, the adsorbent was reused for extraction of ICP after desorption of the analytes from MRGO. The peak areas of the first adsorption-desorption cycle and the tenth cycle were 174,658.1 and 156,193.297, respectively, indicating that MRGO can be reused at least 10 times without a significant decrease of the sorption capacity. It can be seen that MRGO nanocomposites have the advantage of reuse stability, which can significantly reduce the cost in sample preparation. 3.6. Validation of the MSPE-GC method
Fig. 10. The effect of desorption solvents on the extraction efficiency of ICP. Extraction conditions: initial concentration of ICP, 0.25 g L−1 ; MRGO, 20 mg; concentration of NaCl, 2%; extraction time, 10 min; extraction temperature, 20 ◦ C; desorption mode, vortex.
strongest desorption power among these eluents. The effect of desorption solution volume was also investigated. Results revealed that a complete desorption of ICP from MRGO could be achieved by rinsing the adsorbent with each 0.5 mL of eluent for four times. Meanwhile, the effect of desorption time was studied. The peak areas after vortexing for 1 min and 3 min were 6102.5 and 6118.950, respectively, suggesting that desorption time had no obvious influence on the desorption efficiency. The modes of elution were compared using ultrasound and vortex. The amount of ICP desorbed by vortexing was 2.4 times bigger than that by the mode of ultrasound, which could be attributed to the fact that oscillation vortex was more intense than ultrasound.
Several important parameters including linearity, correlation coefficient (R2 ), limit of detection (LOD) and limit of quantification (LOQ) were determined to validate the method. Under the optimized conditions, a series of standard solutions containing ICP at different concentration levels were prepared to establish the calibration curve and a linear range from 0.05 to 50 ng mL−1 was obtained with a high R2 value of 0.9995, indicating good linearity. The LOD and LOQ data were calculated at a signal-to-noise ratio of three (S/N = 3) and a signal-to-noise ratio of ten (S/N = 10), respectively. It was found to be 0.0044 and 0.0147 ng mL−1 , respectively. In addition, the precision of the MSPE-GC method, expressed as relative standard deviation (RSD), was assessed by three parallel extractions of ICP, and it was calculated as 1.77 %. Table 3 compares the performance of the proposed method in this study and other methods reported in literatures. It can be observed that the MSPE-GC-NPD method has a lower detection limit and RSD in contrast with the methods mentioned above and the samples used in this study have significant matrix difference, confirming that this method has high sensitivity, good repeatability and practicability.
Table 2 Parameters based on the pseudo-first-order and pseudo-second-order kinetic equations for the adsorption of ICP on MRGO. Analyte
qe,exp (mg g−1 )
Pseudo first-order −1
qe,cal (mg g ICP
217.2694
96.5537
)
Pseudo second-order −1
K1 (min 0.2324
)
R
qe,cal (mg g−1 )
K2 (g mg−1 min−1 )
R2
0.9906
208.3333
0.0092
0.9932
2
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Table 3 Comparison of the proposed method with other methods for the determination of ICP. Linear range (ng mL−1 )
Methods a
DSPE -GC–MS USAEMEb -HPLC-DAD DSMEc -GC-FPD QuEChERSd -GC-FPD SDMEe -GC-FPD QuEChERS-SPE-LC–MS/MS MSPE-GC-NPD a b c d e
7–200 1.0–200 0.1–100 20–1280 10–500 2–125 0.05–50
LOD (ng mL−1 )
R2
RSD (%)
Matrices
Ref.
1.6 0.1 0.03 15.0 2.2 0.08–1.8 0.0044
0.9947 0.9998 0.9995 0.9994 0.9919 0.9993 0.9995
2.68–8.48 3.30 9.20 3.43 6.30 3.9–9.2 1.77
Peanut oil Well, rain, reservoir water Wine Morinda roots Orange juices Soil, water Apple, cabbage, lake water, cowpea, rice
[35] [36] [37] [38] [39] [40] This study
DSPE: dispersive solid phase extraction. USAEME: ultrasound-assisted emulsification microextraction. DSME: dispersive suspended microextraction. QuEChERS: quick, easy, cheap, effective, rugged and safe. SDME: single drop microextraction.
Table 4 Determination of ICP residues and recoveries in real samples. Real samples
Spiking level (ng g−1 )
ICP Found (ng g−1 )
RSD (%)
Recovery (%)
a
Apple (n = 3)
0.00 0.1 1.0
nd 0.099 0.925
Cabbage (n = 3)
0.00 0.1 1.0
nd 0.110 0.863
7.25 9.34
108.51 86.32
Lake water (n = 3)
0.00 0.1 1.0
1.275 0.081 0.923
7.41 2.00 1.84
81.00 92.33
Cowpea (n = 3)
0.00 0.1 1.0
nd 0.094 1.067
4.03 6.53
93.81 106.72
Rice (n = 3)
0.00 0.1 1.0
nd 0.103 0.994
9.72 4.69
103.33 99.39
a
very efficient and sensitive. In addition, different real samples were also treated using MRGO, and the results demonstrated that sample matrices had little interference with the performance of MRGO. The results suggest that MRGO has a potential application prospect in the determination of trace analytes.
2.97 6.14
99.20 92.49
nd, not detected.
3.7. Analysis of real samples The method was used for the determination of ICP in five kinds of real samples including apple, cabbage, cowpea, lake water and rice. The results are summarized in Table 4. Only a low concentration of ICP (1.275 ng g−1 ) was found in the lake water sample and no residues were detected in the other four samples. To estimate the effect of the matrices, all of the samples were spiked at two concentration levels of ICP (0.1 and 1.0 ng g−1 ) to determine the recovery of the targeted analyte. For each concentration level, three replicates were conducted. As shown in the table, satisfactory recoveries of ICP from five real samples, ranging from 81.00% to 108.51% with RSDs less than 9.72%, were observed. The results demonstrated that these real sample matrices had little interference with the performance of MRGO and it can also be concluded that the MSPE-GC analytical method is highly-efficient, precise and accurate. 4. Conclusions MRGO was synthesized via a simple one-step solvothermal strategy and charaterized by FTIR, XRD, SEM and TEM. The two components of Fe3 O4 and graphene endow the material with manipulative convenience as well as high adsorption capacity. Thus, MRGO exhibited excellent extraction capability for ICP and could be conveniently separated from aqueous samples by an external magnet. Low detection limit and satisfactory recoveries were achieved, indicating that the proposed MSPE-GC-NPD method is
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