Layer-by-Layer Fabrication of Chemical-Bonded Graphene Coating

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Layer-by-Layer Fabrication of Chemical-Bonded Graphene Coating for Solid-Phase Microextraction Suling Zhang, Zhuo Du,* and Gongke Li* School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, China ABSTRACT: A new fabrication strategy of the graphenecoated solid-phase microextraction (SPME) fiber is developed. Graphite oxide was first used as starting coating material that covalently bonded to the fused-silica substrate using 3-aminopropyltriethoxysilane (APTES) as cross-linking agent and subsequently deoxidized by hydrazine to give the graphene coating in situ. The chemical bonding between graphene and the silica fiber improve its chemical stability, and the obtained fiber was stable enough for more than 150 replicate extraction cycles. The graphene coating was wrinkled and folded, like the morphology of the rough tree bark. Its performance is tested by headspace (HS) SPME of polycyclic aromatic hydrocarbons (PAHs) followed by GC/MS analysis. The results showed that the graphene-coated fiber exhibited higher enrichment factors (EFs) from 2-fold for naphthalene to 17-fold for B(b)FL as compared to the commercial polydimethylsioxane (PDMS) fiber, and the EFs increased with the number of condensed rings of PAHs. The strong adsorption affinity was believed to be mostly due to the dominant role of π π stacking interaction and hydrophobic effect, according to the results of selectivity study for a variety of organic compounds including PAHs, the aromatic compounds with different substituent groups, and some aliphatic hydrocarbons. For PAHs analysis, the graphene-coated fiber showed good precision ( 6300) than other analytes (EFs < 2300) studied here. PAHs are common environmental contaminants generated by a wide variety of incomplete combustion sources such as plastics, coal, gasoline, and fossil fuel. Recent studies indicated their nondegradation and potential carcinogenic and mutagenic effects.31 33 Therefore, a sensitive method is desired to preconcentrate these analytes prior to detection. On the basis of the strong adsorption affinity of the graphene-coated fiber to PAHs, its extraction performance was evaluated for the headspace (HS)-SPME of eight PAHs in water and soil samples.

’ EXPERIMENTAL SECTION Reagents and Materials. o-Xylene (g98.0) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 1,2-Dichlorobenzene, 1,2,4-trichlorobenzene (99%), 2-phenylethanol, acetophenone, and n-undecane (99%) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Chlorobenzene (g99.5%), nitrobenzene (g99.0%), phenol, and benzaldehyde (g98.5%) were from Guangzhou Chemical Reagents (Guangzhou, China). 3-Octanone, n-octanol, and nonanal were from Aldrich (St. Louis, MO). The PAH standards (naphthalene (NAP), acenaphthene (ANE), fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), pyrene (PYR), and benzo(b)fluoranthene (B(b)FL) were purchased from Chem Service (America). HPLC grade isooctane was from Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). HPLC grade n-hexane was obtained from Fuchen Chemical Reagents (Tianjin, China). Water was doubly distilled. The fused-silica fibers (120 μm i.d.) were obtained from Fiber Home Telecommunication Technologies (Wuhan, China). Graphite powder (99.95%, particle size e30 μm) was purchased from Alfa Aesar. 3-Aminopropyltriethoxysilane (APTES) was purchased from Aladdin Chemistry. All other chemicals were of analytical grade. Instruments. An Agilent HP 6890 gas chromatography/5973 mass detector system (GC/MS; Palo Alto, CA) was used for all

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experiments. A DB-5MS (Agilent Scientific, USA) capillary column (30 m long 0.25 mm i.d. 0.25 μm film thickness) was used for the chromatographic separation. For PAHs analysis, the instrumental conditions were as follows: splitless mode; injector temperature, 280 °C; transfer line temperature, 280 °C; energy of electron, 70 eV; ion source temperature, 230 °C; initial oven temperature, 60 °C (held for 2 min) and then increased to 200 °C at a rate of 20 °C/min, finally increased to 260 °C at a rate of 20 °C/min, and held for 8 min. The total running time was 23 min. The mass spectrometer was operated in the electron ionization (EI) mode with quadrupole temperature of 150 °C. Date acquisition was carried out in the selected ion monitoring (SIM) mode. Helium (99.999% purity, Guangzhou Xicheng Industrial Gas And Equipment Co., Ltd.) was used as carrier gas at a constant flow rate of 1.0 mL/min. Scanning electron microscopy (SEM) images were conducted by an S-4300 SEM instrument (HITACHI, Japan). The surface images of the graphene coating were obtained at 10.0 KV, and the cross-section image was obtained at 5.0 KV. X-ray photoelectron spectroscopy (XPS) experiments were performed on an ESCALAB 250 XPS with monochromated Al Kα radiation (hυ = 1486.6 eV), 45° photoelectron takeoff angle, and a 500 μm beam size. The X-ray was emitted using a 15 kV acceleration voltage at 2 10 9 mbar vacuum. IR spectra were done on a NICOLET AVATAR 330 Fourier transform infrared (FT-IR) spectrometer. A thermogravimetric (TG) analyzer (Netzsch-209, Bavaria, Germany) was applied to evaluate the thermal stability of the coating from room temperature to 700 °C in flowing N2 at heating rate of 10 °C/min. A 200 W ultrasonicator (KQ 5200, Kunshan, China) was used for the preparation of GO dispersions. An IKA RET magnetic stirrer (Germany) was employed for stirring the sample. Commercial 100 μm polydimethylsioxane (PDMS) SPME fiber (Supelco, St. Louis, MO) was used for the comparison study. Synthesis of Graphite Oxide. Graphite oxide was synthesized from natural graphite according to the modified Hummers method.34,35 In brief, powered graphite was treated with a mixture of concentrated H2SO4, K2S2O8, and P2O5 at 80 °C. The obtained preoxidized graphite was put into cold concentrated H2SO4, and then, KMnO4 was added slowly. Then, H2O2 and distilled water were added to terminate the reaction. The resulting solution was filtrated, washed with HCl solution, and dried finally to give a brown solid. The obtained GO could be exfoliated by 30 min ultrasonication in water to form stable 0.2 wt % GO dispersions, which were used for the graphene fiber coating preparation. Preparation of Graphene-Coated SPME Fiber. The fabrication of graphene-coated fiber involved the following five processes (Figure 1). (a) The silica fiber was cut into 17 cm in length. Its one tip (1.0 cm) was immersed in acetone for 20 min to remove the protecting polyimide layer. Subsequently, the fiber was dipped into 1 mol/L NaOH solution for 1 h to expose the maximum number of silanol, finally washed with water thoroughly and dried. (b) The alkaline treatment fiber was immersed into the APTES solution for 12 h at room temperature, which mainly reacted with hydroxyl groups on the silica fiber surface, forming Si O Si bonding. Then, the fiber was pulled out and immediately placed into an oven at 70 °C to complete the silanization reaction. (c) For coating, the silylated fiber was inserted into the 0.2 wt % aqueous GO dispersion for 2 h in a 70 °C water bath. Then, it was taken out and dried in air to give a thin layer of GO coating. The two operations (b) and (c) described 7532

dx.doi.org/10.1021/ac201864f |Anal. Chem. 2011, 83, 7531–7541

Analytical Chemistry

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Figure 1. Schematic demonstrating of the fabrication processes of graphene-coated SPME fiber.

above were repeated four times to give a homogeneous coating of about 20 μm in thickness. (d) The GO-coated fiber was conditioned in the GC injector under nitrogen at 60 °C for 2 h. This aging process was essential to avoid the GO coating stripping off from the silica fiber during the following reduction process. (e) The GO-coated fiber was deoxidized in the mixture of distilled water (10 mL), hydrazine solution (35 wt % in water, 40 μL), and ammonia solution (28 wt % in water, 36 μL) at 70 °C for 18 h to give the graphene-coated SPME fiber.36 HS-SPME Procedures. Concentrations of 5, 10, 50, 100, 300, and 500 μg/L of PAH solutions were prepared by diluting the stock solution (500 mg/L in isooctane) step-by-step with hexane.37 An aliquot (25 μL) of each of the above solutions was added to 25 mL of ultrapure water, giving the final concentration of PAH working solutions ranging from 5 to 500 ng/L. All the stock solutions were stored at 20 °C for further use. Prior to use, the obtained fiber coated with GO or graphene was mounted into a 5 μL microsyringe to make up a homemade SPME device38 and then was conditioned in the GC injector under nitrogen at 60 280 °C with gradual rising temperature and held at 280 °C for 2 h. For PAHs analysis, all extraction experiments were carried out in a 25 mL working solution, which was introduced to a 40 mL amber vial capped with PTFE-coated septa. Magnetic stirring with a Teflon-coated stir bar was used to agitate the solution at 1500 rpm. A thermostatic water bath was used to control the extraction temperature. To perform the extraction, the graphenecoated SPME fiber or the commercial fiber was exposed to the headspace above water for a certain time. After extraction, the fiber was removed from the vial and immediately inserted into GC inlet for thermal desorption. Before each use, the fiber was conditioned at 280 °C for 15 min. Sample Preparation. The water samples were collected from a local river (Guangzhou, China) and a pond in our campus. These samples were analyzed immediately after sampling without any pretreatment process. A soil sample was collected from a petrol station. After taken, it was air-dried to constant weight at room temperature and sieved to a particle size of 0.45 mm. Soil (10 g) was extracted with 30 mL of acetone for 30 min using a rotary stirrer and then centrifuged for 10 min. The extract was filtered and evaporated to dryness at 30 °C using a rotary evaporator. Afterward, the dry residue was redissolved in 500 μL of hexane, and 25 μL of this solution was diluted with 25 mL of water for HS-SPME. For the analytical performance assessment, a 5 mL standard acetone solution of 5 μg/L PAH mixture was added to 10 g of soil to give a spiked level of 2.50 ng/g for each of the target compounds. Extractions were performed after 2 h to ensure the solvent had evaporated.

Determination of Enrichment Factor. The enrichment factor (EF) was defined as the ratio of the analyte concentration after extraction to that in the original water sample. The chromatographic peak area, which was obtained by direct injection of 1 μL of standard solution, was used for quantification of the final analyte concentration after extraction.18

’ RESULTS AND DISCUSSION Stability of Graphene-Coated Fiber. Graphene sheets tend to form irreversible agglomerates because of their hydrophobic nature and the strong π π interactions. In contrast, GOs, possessing a considerable amount of hydroxyl and epoxide functional groups on both surfaces of each sheet and carboxyl groups mostly at the sheet edges, are strongly hydrophilic and can form well-dispersed aqueous colloids. Generally, large amounts of graphene are most easily produced via the reduction of GO, which offers tremendous opportunities for chemicalbonded graphene SPME coating preparation. According to that, we explored GO as starting coating material chemically bonded to the fused-silica fiber surface and subsequently deoxidized to give the graphene coating in situ. The chemical bonding between the graphene coating and the fused-silica substrate improve its chemical stability. This proposed preparation method was reproducible, and the obtained graphene-coated fiber was stable enough for more than 150 replicate extraction cycles. Morphological Structure of Graphene Coating. Figure 2a was a low-magnification surface SEM image which revealed a rough tree bark-like structure with striped appearance, while the high magnification SEM image in Figure 2b showed a continuous folded and winkled structure. For the cross-sectional image, the fiber was broken off and mounted on the SEM sample holder parallel to the electron beam. The thickness of the coating was approximately 20 μm after four coating cycles. Hundreds of thin winkled and scrolled graphene sheets provided a loose structure, which is beneficial for extraction performance. Characteristics of Graphite Oxide and Graphene Coatings. The chemical compositions of both the graphite oxide and graphene materials were analyzed by X-ray photoelectron spectroscopy (XPS). Deconvolution of the C 1s peak in Figure 3C showed the presence of C C, C O, and OdC OH for graphite oxide. After reduction with hydrazine, oxygen content decreased from 36.20% to 11.19% and the C 1s XPS spectra of graphene only showed the presence of C C and C O, indicating that deoxygenation has occurred. In addition, a small amount of incorporated nitrogen (0.91%) was attributed to exposure to hydrazine. According to the FT-IR spectrum of GO in Figure 4, the most characteristic features were the adsorption bands corresponding to the CdO of carboxyl 7533



Figure 2. Scanning electron micrographs of a SPME fiber coated with graphene. The surface images at magnifications of (a) 350 and (b) 5000; the cross-section image at magnifications of (c) 3000.

stretching at 1621 cm 1, the O H deformation vibration at 1401 cm 1, and the C OH stretching of epoxide group at 1070 cm 1. The O H stretching vibrations of the OH group of GO and water appeared at 3165 cm 1 and 3401 cm 1 as broad adsorption bands, respectively.39,40 Hydrazine-assisted chemical reduction eliminated most oxide groups of GO, which was accordant with the results of XPS datas. However, the

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epoxide groups remained on the surfaces of graphene due to its lower reactivity compared to OH and COOH.39,41 The thermal resistance of the fiber coating is a very important parameter for SPME-GC applications. The thermal stability of the graphene coating was evaluated by the thermogravimetric analysis (Figure 5). The weight loss near 100 °C was attributed to the physically adsorbed water. After aging at high temperature, most unstable oxygen-containing groups were removed.There was no obvious weight loss of graphene between 100 and 300 °C and 90.0% of the weight remained at 700 °C, indicating that it could endure high temperature in the GC injector. Effect of Experimental Conditions on Extraction Efficiency of the Graphene-Coated Fiber. Some physical chemical parameters of PAHs are listed in Table 1. Eight PAHs were selected as target compounds based on their different hydrophobic properties and different electron polarizabilities. The high KOW values indicate their strong hydrophobic properties that increase with their molecular weights. The low KAW values indicate that these compounds tend to vaporize in headspace from the water sample by increasing temperature or saturating with sodium chloride, especially for the ones with high molecular weights. Considering that, headspace extraction above the aqueous solution was selected. Different experimental parameters that affect extraction efficiency including extraction temperature, extraction time, ion strength, and desorption time were, respectively, investigated and optimized in sequential order using 25 mL of aqueous solution spiked with 100 ng/L of PAHs. Extraction Temperature. In general, increasing the extraction temperature can enhance mass transfer of analytes from water to headspace and further to the fiber coating, thereby increasing extraction efficiency of HS-SPME. Extraction temperature profiles for PAHs ranging from 30 to 70 °C were investigated. As shown in Figure 6a, the extraction efficiency increased with temperature, especially for the four- and five-ring PAHs (FLA, PYR, and B(b)FL) with lower KAW. The extraction temperature of 70 °C was chosen for the following experiments. Extraction Time. SPME is an equilibrium-based technique, and there is a direct relationship between the extraction amount and the extraction time. Extraction time profiles for the eight PAHs are shown in Figure 6b. The results indicated that the extraction efficiency greatly increased as the exposure time was raised from 10 to 50 min, and the adsorption equilibrium time for PAHs increased with molecular weight because of the relative low diffusion coefficients of higher molecular weight compounds. Extraction equilibrium was reached in 10 min for ANE, 30 min for FLU, 40 min for PHE; there was no equilibrium in the extraction period of 50 min for FLA and PYR. However, 10 40 min was sufficient to achieve high extraction efficiency for the extraction of NAP. Therefore, 40 min was ultimately chosen as the preferred extraction time. Ionic Strength and Desorption Time. Ionic strength is another parameter increasing the partial vapor pressure of the analytes in the headspace volume. Adding salt to the water sample would decrease the solubility of some analytes in aqueous phase, which increases their concentrations in the headspace. For that reason, the effect of ionic strength was studied by adding different amounts of NaCl (0 35% m/v) as salting-out agent (Figure 6c). The results indicated that the extraction efficiency increased with the ionic strength for most of the PAHs, but a small decrease was observed for NAP when the NaCl concentration was above 20% (w/v). It might result from the competitive adsorption of PAHs to graphene coating. NAP adsorption was partially replaced by 7534



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Figure 3. XPS spectra of (A) GO and (B) graphene materials and C 1s XPS spectra of (C) GO and (D) graphene materials. The contents of C and O are 61.17% and 36.20% for GO and 82.85% and 11.19% for graphene, respectively.

Figure 4. FT-IR spectra of (a) GO, (b) graphene, and (c) graphene after aging.

other PAHs while higher ionic strength enhanced the partial vapor pressure. Therefore, 20% (w/v) NaCl concentration was chosen in the following extractions. The desorption time profiles ranging from 3 to 7 min were investigated (Figure 6d). The peak area reached the maximum at 5 min for all the studied analytes. On the basis of the observations mentioned above, the conditions for the HS-SPME of PAHs were determined as

Figure 5. TG curve of graphene in nitrogen gas atmosphere; heating rate: 10 °C min 1.

follows: extraction temperature, 70 °C; extraction time, 40 min; desorption temperature, 280 °C; desorption time, 5 min; 20% (w/v) NaCl. Comparison of Graphene-Coated Fiber with GO-Coated Fiber. As illustrated in Figure 7, the extraction efficiency of the fiber with graphene coating increased dramatically when GO 7535



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Table 1. Physical Chemical Properties of Eight PAHs and Enrichment Factors (EFs) for PAHs Obtained with the GrapheneCoated Fiber

a KOW: n-octanol water partition coefficients, indicator for hydrophobicity. Data taken from ref 42. b KAW: Air-to-water partitioning coefficients. Data taken from R. Sander: Henry’s law constants (http://www.syrres.com/what-we-do/databaseforms.aspx?id=386). 43

Figure 6. Effect of the experimental conditions on the extraction efficiency of graphene-coated fiber for 100 ng/L PAHs, including (a) extraction temperature, (b) extraction time, (c) ionic strength, and (d) desorption time. Errors bars show the standard deviation of the mean (n = 3).

coating was deoxidized by hydrazine (minimum 5-fold for NAP and maximum 18-fold for B(b)FL as compared to the GO-coated

fiber of the same thickness). This phenomenon implied the strong π π stacking interaction and the hydrophobic effect 7536



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Figure 7. Chromatograms of the PAH standard solution containing 100 ng/L obtained with the graphene and GO-coated fibers: 1-NAP, 2-ANE, 3-FLU, 4-PHE, 5-ANT, 6-FLA, 7-PYR, 8-B(b)FL.

Figure 8. Enrichment factors (EFs) per coating thickness obtained with the graphene-coated fiber and commercial PDMS fiber for the PAH HSSPME. Error bars show the standard deviation of the mean (n = 3).

between PAHs and the highly delocalized conjugate system of the π-electron on the graphene surface, since the π π interaction and the hydrophobic effect increased considerably during the reduction process. Comparison of Graphene-Coated Fiber with Commercial PDMS Fiber. The general extraction performance of the graphene-coated fiber was compared with a commercial 100 μm PDMS fiber, which is proved to be the most efficient for PAH enrichment due to its typically nonpolar polymer phase. In view of the different thickness of the commercial coating, the extraction time profile of the PDMS fiber was investigated to ensure that the comparison was tested under equilibrium. The results showed that the PDMS fiber reached equilibrium in 50 min for extraction of the eight PAHs. Therefore, extraction time of 50 min was the optimum condition for commercial fiber. The comparison of graphene-coated fiber with commercial fiber for HS-SPME of 100 ng/L PAH solution under their own optimum conditions was performed by calculating EFs of analytes per coating thickness. The results are represented in Figure 8. The relative response of graphene-coated fiber could be roughly 6 and 17 times higher than that of the commercial fiber for the four- and five-ring PAHs, respectively. Furthermore, a significant trend could be seen that the adsorption affinity increased with the increasing number of condensed rings of PAHs (the electron polarizability) and hydrophobicity (KOW, Table 1). Apparently, graphene showed a complete different adsorption mechanism from the polymer material (PDMS) for PAHs. Selectivity of the Graphene-Coated Fiber. Mechanism research of the organic compounds graphene interaction is critical to extend the potential environmental applications of graphene as adsorbent. To further understand the adsorptive interaction, a series of organic compounds with different physical chemical properties (hydrophobicity, electron polarizability, polarity, etc.) were evaluated as analytes, which could be classified into three groups: (a) polycyclic aromatic hydrocarbons with rich π-electrons; (b) aromatic compounds with different substituent groups (chlorinated aromatics, nitrobenzene, benzaldehyde, acetophenone, phenol, and 2-phenylethanol); (c) aliphatic hydrocarbons (3-octanone, n-octanol, nonanal, and n-undecane). The adsorption affinity of the graphene coating to different compounds was evaluated by the EFs for the SPME with headspace. The results are listed in Tables 1 and 2.

The adsorption affinity of graphene coating to PAHs (EFs, 6354 71872, Table 1) was much higher than the other compounds (EF e 2260, Table 2). It increased with the number of the condensed rings of PAHs (except semivolatile benzo(b)fluoranthene with lower KAW value) and was consistent with their hydrophobicity (KOW) and electron polarizability strength, implying the potential main role of the hydrophobic effect and π π stacking interaction. To determine the more important one, o-xylene was chosen because of its similar hydrophobicity to naphthalene. The EF value of naphthalene with higher electron polarizalibity was much higher. Therefore, we suggested that the adsorption affinity was controlled primarily by π π stacking and then the hydrophobic effect. Likewise, the result that graphene coating gave a higher EF value for phenanthrene than 1,2,4trichlorobenzen with the similar hydrophobicity further verified the dominant role of the π π stacking interaction between analytes and the flat conformation of graphene. Among the aromatic compounds with different substituent groups, it appeared that the adsorption affinity to chlorinated aromatics was much higher than the other ones, including nitrobenzene, benzaldehyde, acetophenone, phenol, and 2-phenethanol. As nonpolar compounds, chlorinated aromatic adsorption to the graphene surface mainly relied on the hydrophobic effect, and the EFs were proportional to the KOW values. For the other functional groups, substituent aromatic compounds with the similar KOW values (1.34 1.65), nitrobenzene was highly polar and was adsorbed much strongly to the graphene surface. It was indicated that, besides the hydrophobic effect, there were other mechanisms controlling the adsorption of different aromatics, in which the physical chemical properties of different substituents must be considered. In a previous study, Chen et al.44 proposed a mechanism of electron donor acceptor (EDA) interaction to explain the strong adsorption of nitroaromatic compounds by CNTs. The π π EDA interaction is a specific force existing between electron-rich and electron-poor moieties, which involves one electron transfer from the highest occupied molecular orbital (HOMO) of electron-rich moieties to the lowest unoccupied molecular orbital (LUMO) of electron-poor moieties to form a weak covalent bond.45,46 Nitro group was strongly electron withdrawing and could cause the nitro-substituted benzene ring to be electron-depleted and hence act as π-electon acceptor, which could interact strongly 7537



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Table 2. Enrichment Factors (EFs) for the Different Compounds on Graphene-Coated Fiber

a

Data taken from http://logkow.cisti.nrc.ca/logkow/search.html.43 b The same as log KAW in Table 1. HS-SPME conditions: extraction temperature, 50 °C; extraction time, 30 min; desorption temperature, 280 °C; desorption time, 5 min; saturated NaCl solution.

with the π-electron-rich region of graphene surface as π-electon donor via π π EDA interaction. Aldehyde and ketone groups both were weaker electron-withdrawing, which lead to their relatively lower EF values compared to nitro group. In addition, phenol and 2-phenylethanol were not electron withdrawing, and the adsorption to graphene surface was correspondingly weaker. As such, the adsorption affinity of graphene coating to different functional groups substituent aromatic compounds could be explained reasonably. Besides that, H-bonding between the substituents ( CHO, COR, OH) and epoxide groups on graphene sheets as an alternative mechanism might also enhance the adsorption affinity to polar aromatic compounds. Graphene also showed strong adsorption affinity to the aliphatic analytes. The polar aliphatic compounds (3-octanone, n-octanol, nonanal), with even lower KOW values, were adsorbed more strongly than the nonpolar aromatic compounds, supporting the presence of the H-bonding interaction between the polar substituents of aliphatics and epoxy groups. The adsorption interaction of nonpolar aliphatics (n-undecane) mostly relied on the hydrophobic effect.

From that mentioned above, we concluded that the extraction ability of the graphene-coated fiber was due to the combination interaction following the order of π π stacking, hydrophobic effect, and π π EDA interaction, if existing, via polar interaction (mainly H-bonding). The high adsorption affinity to aromatics and hydrophobic aliphatic analytes makes graphene an excellent adsorbent in sample preparation. Quantitative Analysis. The figures of merit of the calibration curves, limits of detection, correlation coefficients, and the repeatability were performed under the optimized conditions for PAH analysis. The results are summarized in Table 3. The linearity with good correlation coefficients (R2) higher than 0.9968 for all the tested analytes varied from 5 to 500 ng/L. The limits of detection (LODs), calculated as three times the standard deviation of the obtained peak area at the lowest sample concentration divided by the slope of the calibration curve, were in the interval between 1.52 and 2.72 ng/L. That efficient extraction allows trace analysis in real samples to be performed. For the repeatability study, one fiber was used for five replicate extractions of an aqueous sample containing 7538



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Table 3. Analytical Performance for GC/MS Determination of PAHs Using Graphene-Coated Fiber RSD compound

regression equation

linear range (ng/L)

R2

detection limit (ng/L)

one fiber (%, n = 5)

fiber-to-fiber (%, n = 3)

NAP

y = 1031x + 916

5 500

0.9998

2.70

9.3

10.8

ANE

y = 3370x + 1812

5 500

0.9970

1.64

4.9

6.5

FLU

y = 7116x + 1561

5 500

0.9968

2.15

6.5

4.0

PHE

y = 9417x + 23846

5 500

0.9994

2.14

9.3

9.0

ANT

y = 10185x + 9012

5 500

0.9980

1.92

5.1

4.3

FLA

y = 14041x + 7069

5 500

0.9974

1.52

6.4

6.1

PYR

y = 12760x

5 500

0.9974

2.32

6.2

5.3

B(b)FL

y = 1015x

5 500

0.9976

2.72

7.6

9.0

16589 831

Table 4. Analytical Results for the Determination of PAHs in Water and Soil Samples river water

pond water

soil sample

compound concentration (ng/L) recoverya RSD (%, n = 3) concentration (ng/L) recoverya RSD (%, n = 3) concentration (ng/g) recoveryb RSD (%, n = 3)

a

NAP ANE

6.8 9.8

92.2 85.3

4.1 6.0

nd nd

99.0 95.1

3.4 3.0

2.77 nd

72.7 79.4

9.2 9.2

FLU

8.1

87.7

0.6

PHE

ndc

83.8

5.9

nd

95.9

7.8

0.78

74.7

4.3

nd

101.7

5.7

5.22

81.0

ANT

nd

92.7

7.7

6.9

nd

100.8

4.9

nd

91.6

FLA

9.6

7.0

89.5

4.5

nd

95.1

6.1

2.64

94.6

PYR

3.9

12.8

85.6

5.0

nd

94.2

5.4

2.37

90.1

B(b)FL

3.8

nd

91.1

6.0

nd

91.4

2.0

nd

84.0

7.2

Spiked with 50.0 ng/L PAH solution. b Spiked at the 2.50 ng/g level of PAHs. c Not detected.

100 ng/L PAHs under the same conditions, and the relative standard deviation (RSD) was below 10%. The RSD for fiberto-fiber reproducibility study was less than 10.8% using three different fibers prepared in the same way. In addition, the graphene-coated fiber allowed more than 150 replicate extractions without measurable loss of performance. These results proved that the coating property was quite stable and the fabrication of graphene coating based on chemical bonding was reproducible. Sample Analysis. The optimized graphene-based SPME GC/MS method is validated for the enrichment of PAHs in two natural water samples including river water and pond water on campus and a soil sample from petrol station. The results are shown in Table 4. Five PAHs were detected in river water, ranging from 6.8 12.8 ng/L, while the concentrations of PHE, ANT, and B(b)FL were below the LODs. No PAHs were founded in pond water. The solution was spiked with 50.0 ng/L PAHs to evaluate the accuracy of the method. Good recoveries were obtained by graphene-coated fiber, which ranged from 84% to 102% for all the studied analytes. For HS-SPME of analytes from soil sample, exposing the fiber in the headspace above the slurry mixture of soil sample and distilled water is a convenient method.47,48 However, some papers report that SPME of the aqueous solution containing organic extract obtained by solid liquid extraction is the most sensitive method.49 51 Therefore, the latter approach is chosen for the analysis of PAHs in soil sample. The preparation process of the soil sample is described in the Experimental Section. The results are shown in Table 4 and Figure 9. The contents of five PAHs were in the range of 0.78 2.77 ng/g. Recoveries obtained

Figure 9. Chromatograms of extracts of PAHs from (a) blank soil sample and (b) spiked soil sample at 2.50 ng/g spiked level, (c) 50.0 ng/L PAHs standard solution using graphene-coated fiber: 1 NAP, 2 ANE, 3 FLU, 4 PHE, 5 ANT, 6 FLA, 7, PYR, and 8 B(b)FL.

by spiking at the 2.50 ng/g level with PAHs ranged from 72% to 95%, which were relatively lower than that obtained from the water samples. This could be ascribed to the slight loss in pretreatment process including solid liquid extraction, filtration, and evaporation, especially for the low-ring PAHs. On the other hand, the fact is that the complex soil matrix, mainly the natural organic matrix and clay, involved in the absorption of lipophilic compounds might also contribute to the recovery loss.52,53 7539



’ CONCLUSION We reported the first example of a chemical-bonded method for the preparation of a graphene-coated fiber. The obtained fiber exhibited a uniform structure after several coating cycles. The chemical bonding between the graphene coating and the fusedsilica substrate makes it a robust SPME fiber, which allowed more than 150 replicate extractions without measurable loss of performance. Besides that, good precision indicated the repeatability and reproducibility of the proposed fiber preparation method. The graphene-coated fiber was successfully used to determine trace PAHs in water and soil samples prior to GC/MS analysis. A wide linear range, low LODs, and good recoveries for three real samples indicated that the graphene-coated fiber-based GC/MS method exhibits acceptable extraction efficiency for the studied analytes under the optimal experimental conditions. The selectivity study indicated that the strong π π stacking, π π EDA interaction, and hydrophobic interaction between the compounds and graphene surface resulted in the high retention for the analytes with high electron polarizability, high hydrophobicity, or electron withdrawing groups. Therefore, graphene has great potential for the preconcentration of aromatics and hydrophobic aliphatic analytes in complicated samples. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86-20-84110922. Fax: +86-20-84115107. E-mail: [email protected] (G.L.); [email protected] (Z.D.).

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21127008, 21105132, and 90817012), the Guangdong Provincial Natural Science Foundation (9251027501000004), and the China Postdoctoral Science Foundation (20100480051), respectively. ’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (2) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Angew. Chem., Int. Ed. 2009, 48, 7752–7777. (3) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385–388. (4) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Graighead, H. G.; McEuen, P. L. Science 2007, 315, 490–493. (5) Hao, L.; Sheng, L. Solid State Commun. 2009, 149, 1962–1966. (6) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394–3398. (7) Yang, W.; Ratinac, K. R.; Ringer, S. P.; Thordarson, P.; Gooding, J. J.; Braet, F. Angew. Chem., Int. Ed. 2010, 49, 2114–2138. (8) Hu, Y. H.; Wang, H.; Hu, B. ChemSusChem 2010, 3, 782–796. (9) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Nano Lett. 2008, 8, 3498–3502. (10) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652–655. (11) Wang, Y.; Li, Y.; Tang, L.; Lu, J.; Li, J. Electrochem. Commun. 2009, 11, 889–892. (12) Chen, G.; Weng, W.; Wu, D.; Wu, C.; Lu, J.; Wang, P.; Chen, X. Carbon 2004, 42, 753–759. (13) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145–2148. (14) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993, 65, 1843–1852.

ARTICLE

(15) Djozan, D.; Assadi, Y.; Haddadi, S. H. Anal. Chem. 2001, 73, 4054–4058. (16) Hu, X.; Hu, Y.; Li, G. J. Chromatogr., A 2007, 1147, 1–9. (17) Liu, J.-F.; Li, N.; Jiang, G.-B.; Liu, J.-M.; Åke J€onsson, J.; Wen, M.-J. J. Chromatogr., A 2005, 1066, 27–32. (18) Cui, X.-Y.; Gu, Z.-Y.; Jiang, D.-Q.; Li, Y.; Wang, H.-F.; Yan, X.-P. Anal. Chem. 2009, 81, 9771–9777. (19) Giardina, M.; Olesik, S. V. Anal. Chem. 2001, 73, 5841–5851. (20) Sun, T.; Jia, J.; Fang, N.; Wang, Y. Anal. Chim. Acta 2005, 530, 33–40. (21) Yu, J.; Dong, L.; Wu, C.; Wu, L.; Xing, J. J. Chromatogr., A 2002, 978, 37–48. (22) Wang, J.-X.; Jiang, D.-Q.; Gu, Z.-Y.; Yan, X.-P. J. Chromatogr., A 2006, 1137, 8–14. (23) Haberhauer-Troyer, C.; Crnoja, M.; Rosenberg, E.; Grasserbauer, M. Fresenius J. Anal. Chem. 2000, 366, 329–331. (24) Rastkari, N.; Ahmadkhaniha, R.; Yunesian, M. J. Chromatogr., B 2009, 877, 1568–1574. (25) Liu, X.; Ji, Y.; Zhang, Y.; Zhang, H.; Liu, M. J. Chromatogr., A 2007, 1165, 10–17. (26) L€u, J.; Liu, J.; Wei, Y.; Jiang, K.; Fan, S.; Liu, J.; Jiang, G. J. Sep. Sci. 2007, 30, 2138–2143. (27) Chen, J.; Zou, J.; Zeng, J.; Song, X.; Ji, J.; Wang, Y.; Ha, J.; Chen, X. Anal. Chim. Acta 2010, 678, 44–49. (28) Liu, Q.; Shi, J.; Zeng, L.; Wang, T.; Cai, Y.; Jiang, G. J. Chromatogr., A 2011, 1218, 197–204. (29) Luo, Y. -B.; Shi, Z. -G.; Gao, Q.; Feng, Y.-Q. J. Chromatogr., A 2011, 1218, 1353–1358. (30) Zhang, H; Lee, H. K. J. Chromatogr., A 2011, 1218, 4509–4516. (31) Sagiv, S. K.; Gaudet, M. M.; Eng, S. M.; Abrahamson, P. E.; Shantakumar, S.; Teitelbaum, S. L.; Bell, P.; Thomas, J. A.; Neugut, A. I.; Santella, R. M.; Gammon, M. D. Environ. Res. 2009, 109, 287–291. (32) Mersch-Sundermann, V.; Mochayedi, S.; Kevekordes, S.; Kern, S.; Wintermann, F. Anticancer Res. 1993, 13, 2037–2043. (33) Li, D.; Wang, M.; Dhingra, K.; Hittelman, W. N. Cancer Res. 1996, 56, 287–293. (34) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339–1339. (35) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771–778. (36) Li, D.; M€uller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101–105. (37) Zhang, W.; Sun, Y.; Wu, C.; Xing, J.; Li, J. Anal. Chem. 2009, 81, 2912–2920. (38) Zeng, J.; Yu, B.; Chen, W.; Lin, Z.; Zhang, L.; Lin, Z.; Chen, X.; Wang, X. J. Chromatogr., A 2008, 1188, 26–33. (39) Ju, H.-M.; Huh, S.-H.; Choi, S.-H.; Lee, H.-L. Mater. Lett. 2010, 64, 357–360. (40) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Carbon 2006, 44, 3342–3347. (41) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (42) Doong, R.; Chang, S. Anal. Chem. 2000, 72, 3647–3652. (43) Xu, H.-L.; Li, Y.; Jiang, D.-Q.; Yan, X.-P. Anal. Chem. 2009, 81, 4971–4977. (44) Chen, W.; Duan, L.; Zhu, D. Environ. Sci. Technol. 2007, 41, 8295–8300. (45) Zhu, D.; Pignatello, J. J. Environ. Sci. Technol. 2005, 39, 2033–2041. (46) Hunter, C A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525–5534. (47) Llompart, M.; Li, K.; Fingas, M. Talanta 1999, 48, 451–459. (48) Fernandez-Alvarez, M.; Llompart, M.; Pablo Lamas, J.; Lores, M.; Garcia-Jares, C.; Cela, R.; Dagnac, T. J. Chromatogr., A 2008, 1188, 154–163. 7540



ARTICLE

(49) Bouaid, A.; Ramos, L.; Gonzalez, M. J.; Fernandez, P.; Camara, C. J. Chromatogr., A 2001, 939, 13–21. (50) Lambropoulou, D. A.; Albanis, T. A. Anal. Chim. Acta 2004, 514, 125–130. (51) ûrovic, R. D.; Gajic Umiljendic, J. S.; Cupac, S. B.; Ignjatovic, L. M. J. Braz. Chem. Soc. 2010, 21, 985–994. (52) Eisert, R.; Pawliszyn, J. Crit. Rev. Anal. Chem. 1997, 27, 103–135. (53) King, A. J.; Readman, J. W.; Zhou, J. L. Environ. Geochem. Health 2003, 25, 69–75.

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Layer-by-Layer Fabrication of Chemical-Bonded Graphene Coating

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