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Shuaihua Zhang Zhi Li Chun Wang Zhi Wang Department of Chemistry, College of Science, Agricultural University of Hebei, Baoding, Hebei, China Received December 2, 2014 Revised January 25, 2015 Accepted February 14, 2015

Research Article

Cyclodextrin-functionalized reduced graphene oxide as a fiber coating material for the solid-phase microextraction of some volatile aromatic compounds A novel solid phase microextraction fiber was prepared for the first time by using a sol– gel technique with hydroxypropyl-␤-cyclodextrin-functionalized reduced graphene oxide as the fiber coating material. The results verified that the ␤-cyclodextrin was successfully grafted onto the surface of reduced graphene oxide and the coating possessed a uniform folded and wrinkled structure. The performance of the solid phase microextraction fiber was evaluated by using it to extract nine volatile aromatic compounds from water samples before determination with gas chromatography and flame ionization detection. Some important experimental parameters that could affect the extraction efficiency such as the extraction time, extraction temperature, desorption temperature, desorption time, the volume of water sample solution, stirring rate, as well as ionic strength were optimized. The new method was validated to be effective for the trace analysis of some volatile aromatic compounds, with the limits of detection ranging from 2.0 to 8.0 ng/L. Single fiber repeatability and fiber-to-fiber reproducibility were in the range of 2.5–9.4 and 5.4–12.9%, respectively. The developed method was successfully applied to the analysis of three different water samples, and the recoveries of the method were in the range from 77.9 to 113.6% at spiking levels of 10, 100, and 1000 ng/L, respectively. Keywords: Gas chromatography / Hydroxypropyl-␤-cyclodextrin / Reduced graphene oxide / Solid-phase microextraction / Volatile aromatic compounds DOI 10.1002/jssc.201401363



Additional supporting information may be found in the online version of this article at the publisher’s web-site

1 Introduction SPME, first developed by Pawliszyn et al. [1, 2], is an efficient and solvent-free sample preparation technique, which integrates sampling, isolation, and concentration into one step [3]. Due to its convenient manipulation of on-line coupling to GC and HPLC with MS, SPME has been considered as one of the most attractive sample preparation methods applied in environmental, food, pharmaceutical, and biological sample analysis, especially for the preconcentration of volatile and semivolatile organic compounds [4]. The mechanism of SPME is based on the equilibrium of Correspondence: Dr. Zhi Wang, College of Science, Agricultural University of Hebei, Baoding 071001, Hebei, China E-mail: [email protected]; [email protected] Fax: +86-312-7521513

Abbreviations: APTES, 3-aminopropyltriethoxysilane ; DVB, divinylbenzene; EF, enrichment factor; FID, flame ionization detection; HP-␤-CD, hydroxypropyl-␤-cyclodextrin; HS-SPME, headspace solid-phase microextraction; MTMOS, methyltrimethoxysilane; PA, polyacrylate; PDMS, polydimethylsiloxane; RGO, reduced graphene oxide; VAC, volatile aromatic compound  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the target analytes between the sample and fiber coating. Thus, the stationary phase coated onto the fiber is critical for improving the SPME performance [5]. To this end, several commercial fused-silica-based polymeric coating fibers have been developed, such as nonpolar polydimethylsiloxane (PDMS), carboxen/PDMS, semipolar PDMS/divinylbenzene (PDMS/DVB) and polar polyacrylate (PA), carbowax/PDMS, PEG, and carbowax/templated resin, etc. However, these fibers are fragile and need to be handled with great care during use [3, 4]. Moreover, some shortcomings of the commercialized fibers, such as the relatively low operating temperatures, stripping of the fiber coating, and swelling in organic solvents, have in some degree limited their wide applications [3, 6]. Therefore, it is of great interest to develop novel, low cost, and robust SPME fibers with long service life and simple fabrication method for the desired analytical characteristics [4]. The recent research for the supporting substrates of SPME fibers has mainly focused on the mechanically resistant metal wires, including aluminum [7, 8], zinc [9], copper [10, 11], gold [12, 13], platinum [14, 15], silver [16], titanium [17,18], nitinol alloy [19], and stainless steel wire [20,21]. Among them, the stainless steel wire, with the advantages

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of durability, rigidity, nontoxicity and low cost, has gained great interest in SPME metal fiber modifications. Several stainless-steel-wire-based SPME fibers have been prepared by using novel fiber coating materials, such as carbon nanomaterials [22] (e.g., carbon nanotubes [23], carbon nanocone or nanodisks [24], etc.), metal–organic frameworks [25], porous materials [26], and polymeric ionic liquids [21, 27] through electrochemical procedures [19, 28], physical coating [4], chemical bonding [21, 29, 30], and sol–gel techniques [17, 31]. Reduced graphene oxide (RGO), or graphene, as a single layer of sp2 -hybridized carbon atoms in a closely packed honeycomb 2D lattice, has attracted enormous attention since its discovery in 2004 [32]. Because of its high specific surface area, remarkable thermal and chemical stability, high mechanical strength, and high affinity for some kinds of organic compounds, RGO or graphene has been widely explored as SPME coatings to extract pyrethroid pesticides [33], organochlorine pesticides [4, 34], polycyclic aromatic hydrocarbons [29], polybrominated diphenyl ethers [31], phenols [35], halogenated aromatic hydrocarbons [36], and triazine and acetanilideherbicides [37, 38] from water and soil samples. However, the strong hydrophobic character of the RGO may cause its irreversible self-agglomeration due to its large specific surface area and strong tendency to van der Waals and ␲–␲ interactions [39]. Thus, an introduction of new functional molecules into RGO through covalent or noncovalent strategies for effectively dispersing RGO sheets and meanwhile bringing new or enhanced functions to RGO is highly desirable and important [40]. Cyclodextrins (CDs) are cyclic oligosaccharides consisting of six, seven, or eight (␣-1,4)-linked ␣-D-glucopyranose units (␣, ␤, or ␥-CD, respectively), which are toroidal in shape with a hydrophobic inner cavity and a hydrophilic exterior [40, 41]. Such characteristics can make them selectively bind many kinds of inorganic, organic, and biological molecules into their cavities, even if only partially, to form supramolecular complexes without structural changes [41]. Once RGO is functionalized with CDs, it is likely to gain a new material simultaneously having the properties of both RGO (high surface-to-weight ratio and large delocalized ␲-electron system) and CDs (high supramolecular recognition and enrichment capability) through combining their individual characteristics. CDs-functionalized RGO has been used for the extraction of compounds such as dopamine [42], carbendazim [43], imidacloprid [44], rutin [45], phenolic pollutants [41], and heavy metal ions [46]. However, as far as we know, there has been no report yet for the application of the CDs-functionalized RGO as a fiber coating material in SPME for the analysis of organic molecules especially for volatile or semivolatile organic pollutants. In this work, a hydroxypropyl-␤-cyclodextrin (HP-␤-CD) decorated RGO was prepared under the conditions of microwave irradiation. The HP-␤-CD macromolecules were covalently grafted onto the surface of RGO nanosheets through ester bonds. A novel HP-␤-CD-RGO-coated SPME fiber was prepared by immobilizing the HP-␤-CD-RGO through a

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sol–gel technique onto a prefunctionalized stainless-steel wire that was first coated by microstructured silver layer through silver mirror reaction to enlarge its surface area [3]. The extraction performance of the HP-␤-CD-RGO-coated fiber was evaluated with some volatile aromatic compounds (VACs) as the model analytes because of their toxicity and widespread environmental occurrence. The main experimental parameters including the extraction time and temperature, desorption temperature and time, the ratio of the volume of water solution to headspace (HS), stirring rate, and ionic strength were investigated to achieve a good extraction efficiency. In addition, the fiber lifetime was also evaluated. Finally, a HS-SPME with the HP-␤-CD-RGO coated fiber followed by GC with flame ionization (GC–FID) detection for the determination of some VACs was developed. The method was applied to the determination of the VACs in tap, mineral and pond water samples.

2 Materials and methods 2.1 Reagents and materials Natural graphite powders (50 mesh) used for the preparation of GO were purchased from Boaixin (Baoding, China). 2-HP-␤-CD was purchased from Fluka (Sigma–Aldrich, Steinheim, Switzerland). Methyltrimethoxysilane (MTMOS), 3-aminopropyltriethoxysilane (APTES), and TFA (99%) were obtained from Energy Chemical (Sahn Chemical Technology, Shanghai, China). Other chemical reagents including acetone, ethanol, DMF, dichloromethane, hydrochloric acid (HCl), sodium chloride (NaCl), sodium hydroxide (NaOH), hydrazine hydrate, and ammonia, all of analytical grade, were from Kermel Chemical Reagent (Tianjin, China). The stainless-steel wires of 350 ␮m od used for SPME fiber support were obtained from Shanghai Gaoge Industrial and Trade (Shanghai, China). Standard VACs including ethylbenzene, m-xylene, o-xylene, chlorobenzene, mesitylene, bromobenzene, m-dichlorobenzene, o-dichlorobenzene, and 1,2,4-trichlorobenzene were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). A mixture stock solution containing 1.0 mg/L each of the VACs was prepared in acetone and stored in stoppered glass bottles at 4⬚C. A series of different concentrations of the standard solutions were prepared by serially diluting the stock solution with acetone. The water used throughout the work was double-distilled on a SZ-93 automatic double-distiller purchased from Shanghai Yarong Biochemistry Instrumental Factory (Shanghai, China). Tap water sample was collected freshly from our laboratory (Baoding, China), mineral water sample was purchased from the local supermarket (Baoding, China), and pond water samples were collected from a pond located in our campus.

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Sample Preparation

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2.2 Apparatus GC analyses were carried out on a FULI GC-9790II system (Fuli, http://www.cnfuli.com.cn/) equipped with a split/splitless injector and a FID for the analysis of the VACs. The analytes were separated on a KB-Wax fusedsilica capillary column (30 m × 0.32 mm × 0.25 ␮m) coated with PEG (Kromat, http://www.kromat.com.cn/). Chromatographic separations were programmed as follows: initial temperature from 50⬚C, held for 2.0 min, followed by increasing the temperature at 5⬚C/min to 80⬚C, and finally programmed at 20⬚C/min to 240⬚C. The injector and detector temperatures were set at 250 and 260⬚C, respectively. The ultrapure nitrogen (99.999%) was served as the carrier gas at a flow rate of 2.0 mL/min. The SPME fiber desorptions were carried out in the split mode with a split radio of 1:10. A laboratory-made SPME fiber was put on the manual SPME fiber holder. The commercial 100 ␮m PDMS, 85 ␮m PA, and 65 ␮m PDMS/DVB SPME fibers (Supelco, St. Louis, MO, USA) were used for the comparison study. All of the extractions were performed in 25.0 mL glass vials with Teflon-lined caps to prevent sample evaporation. A model DF-101S temperature-controlled magnetic stirrer bought from the High-tech Zone Sunshine Science Instrument (Baoding, China) was employed for stirring the sample during the extraction. The NJL07-3 microwave oven was obtained from Nanjing Jiequan Microwave Development (Nanjing, China). The SKS200H ultrasonic instrument was purchased from Shanghai Kudos Ultrasonic Cleaning Instrument (Shanghai, China). SEM (Hitachi SU8010, Hitachi, Japan) was applied to investigate the morphology of the fiber coating material HP-␤CD-RGO. The thermal properties of the HP-␤-CD-RGO coating were measured by thermal gravimetric analyses (TGA) with a TG209 F3 instrument (Netzsch, Germany). FTIR spectra were performed on WQF-520 FTIR spectrometer (Beijing Rayleigh Analytical Instruments, Beijing, China). The Brunauer–Emmett–Teller surface area was measured by using a V-Sorb 2800P volumetric adsorption analyzer (Jinaipu, China).

2.3 Preparation of GO, RGO, and HP-␤-CD-RGO Graphene oxide (GO) was prepared from nature graphite by a modified Hummers’ method based on our early report [47]. RGO was prepared by the reduction of the GO directly with hydrazine hydrate and ammonia [46]. HP-␤-CD-RGO was prepared according to the reference method with some modifications [41]. The synthetic procedures for the HP-␤CD-RGO composite are shown in Supporting Information Fig. S1. First, 40 mg GO and 1.0 g HP-␤-CD powders were dispersed in 80 mL double-distilled water under mild sonication for 30 min to get a visually homogeneous solution. The reaction flask was irradiated by microwave (450 W) at 50⬚C for 30 min. Then, hydrazine hydrate (100 ␮L) and ammonia (500 ␮L) were added into the above solution under  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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microwave (450 W) at 75⬚C for 30 min. After the reaction, the mixture was centrifuged at 10 000 rpm for 15 min and then, the resultant precipitate was washed by anhydrous ethanol to remove the unreacted HP-␤-CD and finally dried under vacuum at 70⬚C to yield the HP-␤-CD-RGO.

2.4 Functionalization of stainless steel wires One end (2.0 cm) of the 18 cm long stainless-steel wire (od 350 ␮m) was first washed in 1 M NaOH at 80⬚C for 5 min to remove the organic pollutants and then rinsed with distilled water. The wire was dipped in nitrohydrochloric acid (HCl/HNO3 = 3:1, v/v) for several minutes to remove the stable oxide on its surface and corrode the end to the diameter of about 190 ␮m. The pretreated stainless-steel wire was immersed into 0.1 M [Ag(NH3 )2 ]+ solution and left at room temperature for about 2.5 h to form a microstructured silver layer by silver mirror reaction [48]. Then, the wire was taken out and rinsed with distilled water. After being dried at room temperature, a firm and porous coating of silver was formed on the surface of the stainless-steel wire (od 200 ␮m).

2.5 Preparation of HP-␤-CD-RGO, RGO, HP-␤-CD, and pure APTES- and MTMOS-coated SPME fibers by the sol–gel method For the preparation of the HP-␤-CD-RGO-coated SPME fiber, 30 mg of the HP-␤-CD-RGO was weighed in a microtube. Subsequently, 50 ␮L of APTES and 200 ␮L of MTMOS were added and sonicated for 30 min. Finally, 100 ␮L of TFA (an acidic catalyst with purity of 95%) was added and mixed thoroughly for 5 min. After that, the sol solution of the HP-␤CD-RGO composite coating material was obtained. The functionalized stainless-steel wires were immersed (1.5 cm) in the sol solution for 30 min. After drying, the coated wire was reimmersed into the sol solution and pulled out. This coating process was repeatedly operated until the desired thickness of the coating was obtained. Such prepared fibers were then placed in a desiccator for 24 h at room temperature. The fibers were assembled to a 5 ␮L microsyringe and conditioned in the GC injector at 100⬚C for 1 h, 200⬚C for 1 h, and 280⬚C for 1 h under a stream of nitrogen before their use for SPME. After the conditioning process, a fiber blank was run to confirm that there were no extraneous interfering peaks from the fiber. The preparation procedures for RGO, HP-␤-CD, and pure APTES- and MTMOS-coated fibers are described in detail in the Supporting Information Experimental Section.

2.6 HS-SPME procedure For HS-SPME process, water samples (17.5 mL) were put in 25.0 mL glass vials containing a small magnetic bar and 6.125 g NaCl (35%, w/v). The needle of the SPME device www.jss-journal.com

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Figure 1. Scanning electron micrographs of (a) the surface of the HP-␤-CD-RGO-coated fiber at the magnification of 200, (b) the surface of the bare stainless steel wire at the magnification of 200; (c) the cross section of the HP-␤-CD-RGO-coated fiber at the magnification of 200; (d) the surface of the HP-␤CD-RGO-coated fiber at the magnification of 20 000.

in which the HP-␤-CD-RGO-coated fiber was housed, was passed through the cap, and then the fiber was pushed out and exposed to the HS above the sample solution. After the extraction under stirring at 1400 rpm for 40 min, the needle was removed from the vial and then immediately injected into the GC injector for thermal desorption at 250⬚C for GC analysis. To eliminate the fiber carryover, the fiber was held in the injector for the whole run before the next extraction. Before the first use each day, the fiber was first activated by keeping it in the injection port at 250⬚C for 30 min and then a blank analysis was made to verify that no interference peaks exist in the fiber.

3 Results and discussion 3.1 Characterization of the HP-␤-CD-RGO-coated SPME fiber The morphology of the obtained HP-␤-CD-RGO-coated SPME fiber was investigated by SEM under different magnifications (Fig. 1). Figure 1A was a low-magnification SEM image of the surface of the HP-␤-CD-RGO-coated fiber, which had a rough tree-bark-like structure with striped appearance compared with the surface of the bare stainless-steel wire (Fig. 1B). As shown in Fig. 1C, the thickness of the fiber coating was estimated to be about 30 ␮m. The high-magnification SEM image of the fiber coating in Fig. 1D shows that the HP␤-CD-RGO coating possessed a uniform, folded, and wrinkled structure, which would significantly increase the available surface area on the fiber, thus enhancing the extraction efficiency. The Brunauer–Emmett–Teller surface area of the HP-␤-CD-RGO powder from the fiber was determined to be 89.3 m2 /g, which is much larger than that of commercial PDMS (0.9946. The LODs, calculated at a S/N of 3, ranged from 2.0 to 8.0 ng/L, indicating that the developed method is sensitive and can be used to analyze trace level of the analytes. For the repeatability, the one same fiber was used for six replicate extractions of an aqueous sample containing 0.1 ␮g/L each of the VACs under the same conditions, and the resultant RSDs ranged from 2.5 to 9.4%. Five HP-␤-CD-RGO-coated fibers prepared in the same batch were used to evaluate the fiber-to-fiber reproducibility and the RSDs were 150 replicate extractions without a significant loss of performance. Compared with the commercial PA, PDMS, and PDMS/DVB SPME fibers, the HP␤-CD-RGO-coated fiber showed a higher extraction efficiency for the VACs. A wide linear range, low LODs, and good recoveries of the analytes for the three environmental water samples indicated that the HP-␤-CD-RGO-coated fiber-based HS-SPME–GC–FID method could satisfy the analysis of the VACs in environmental water samples. The HP-␤-CD-RGOcoated fiber may have a great potential for the extraction of more organic contaminants in different samples.

Figure 4. The chromatograms of (a) pond water, (b) the pond water spiked with each of the VACs at 0.1 ␮g/L. Peak identifications: (1) ethylbenzene, (2) m-xylene, (3) o-xylene, (4) chlorobenzene, (5) mesitylene, (6) bromobenzene, (7) m-dichlorobenzene, (8) odichlorobenzene, (9) 1,2,4-trichlorobenzene, and (u) unidentified peak.

tap, mineral, and pond water samples, respectively. At 100.0 and 1000.0 ng/L, the recoveries fell in the range from 87.1 to 113.6%, and from 77.9 to 110.8% for different water samples. Figure 4 displays the typical GC–FID chromatograms of blank and spiked pond water samples extracted with the HP␤-CD-RGO-coated fiber. These results demonstrate a good applicability of the established method for the analysis of trace-level VACs in environmental water samples.

4 Concluding remarks

The authors acknowledge the financial support of the National Natural Science Foundation of China (No. 31171698, 31471643), the Scientific Research Program of Hebei Education Department (QN2014133) and the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009), respectively. The authors have declared no conflict of interest.

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Here, we report the first example of the preparation of a HP-␤-CD-RGO-coated fiber by sol–gel method. The adhesion between the HP-␤-CD-RGO-MTMOS coating and the modified stainless steel wire makes the SPME fiber robust.  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Cyclodextrin-functionalized reduced graphene oxide as a fiber coating material for the solid-phase microextraction of some volatile aromatic compounds.

A novel solid phase microextraction fiber was prepared for the first time by using a sol-gel technique with hydroxypropyl-β-cyclodextrin-functionalize...
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