Fabrication of Graphene Oxide Nanosheets Incorporated Monolithic

Letter pubs.acs.org/ac

Fabrication of Graphene Oxide Nanosheets Incorporated Monolithic Column via One-Step Room Temperature Polymerization for Capillary Electrochromatography Man-Man Wang†,‡ and Xiu-Ping Yan*,† †

State Key Laboratory of Medicinal Chemical Biology, and Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin 300071, China ‡ College of Public Health, Hebei United University, Tangshan 063000, Hebei, China ABSTRACT: Graphene oxide (GO) has received great interest for its unique properties and potential diverse applications. Here, we show the fabrication of GO nanosheets incorporated monolithic column via one-step room temperature polymerization for capillary electrochromatography (CEC). GO is attractive as the stationary phase for CEC because it provides not only ionized oxygen-containing functional groups to modify electroendoosmotic flow (EOF) but also aromatic macromolecule to give hydrophobicity and π−π electrostatic stacking property. Incorporation of GO into monolithic column greatly increased the interactions between the tested neutral analytes (alkyl benzenes and polycyclic aromatics) and the stationary phase and significantly improved their CEC separation. Baseline separation of the tested neutral analytes on the GO incorporated monolithic column was achieved on the basis of typical reversedphase separation mechanism. The precision (relative standard deviation (RSD), n = 3) of EOF was 0.3%, while the precision of retention time, peak area, and peak height for the tested neutral analytes were in the range of 0.4−3.0%, 0.8−4.0%, and 0.8−4.9%, respectively. In addition, a set of anilines were well separated on the GO incorporated monolith. The GO incorporated monolithic columns are promising for CEC separation.

G

Considering the superior qualities of GO, novel stationaryphase materials based on GO for chromatography are expected. However, there is difficulty in the packing of nanothickness GO into the cartridge/column format.17 Furthermore, it is also difficult to use GO directly to fabricate uniform separation matrix and to retain it in the column, especially under high pressure in chromatographic systems. To avoid the abovementioned problems and still take advantage of the specific features of GO, fabrication of GO incorporated monolithic column is a good choice. Monolithic materials with a porous “single particle” structure have been extensively evolved as the legitimate member of the large family of separation media because of their available preparation, fast mass transfer, and enhanced efficiency.20−22 Monolithic columns not only overcome the difficulties associated with standard packed column technology but also eliminate the need for end frits to retain the stationary phase in capillary electrochromatography (CEC).23,24 CEC is a hybrid separation technique that combines the capillary column format and electroendoosmotic flow (EOF) typical of high-performance capillary electrophoresis with the use of a solid stationary phase and a separation mechanism based on specific

raphene oxide (GO), as a two-dimensional nanomaterial, has attracted tremendous attention for its novel properties and potential applications in eletronics, energy research, catalysis, and biomedical research.1−10 GO is a chemically modified graphene sheet with a giant aromatic macromolecule containing reactive oxygen functional groups on their basal planes and edges such as epoxide, hydroxyl, and carboxylic acid. Owing to its unique structure, GO exhibits outstanding physicochemical properties such as exceptional thermal and mechanical properties, high electrical conductivity, superior dispersibility, and facile modification,1,11−14 which renders and promotes its popularity extensively. In particular, the ultrahigh specific surface area and π−π electrostatic stacking property of GO given by its electron-rich double-sided polyaromatic scaffold makes it a promising candidate as an extraordinarily wonderful adsorbent.15−17 GO has been reported as a high efficient preconcentration and matrix in direct surface-enhanced laser desorption/ionization analysis or mass spectrometry.15,16 Most recently, a platform of graphene and GO sheets supported on silica as versatile and high-performance adsorbents for solidphase extraction has been reported for various analytes ranging from small molecules of pollutants to biomolecules such as proteins and peptides.17 In addition, graphene and GO have been explored for solid-phase microextraction of pyrethroid pesticides in natural water samples18 and polycyclic aromatic hydrocarbons in water and soil samples.19 © 2011 American Chemical Society

Received: October 29, 2011 Accepted: December 11, 2011 Published: December 11, 2011 39

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CEC Separation. Electrochromatographic experiments were carried out on a P/ACE MDQ capillary electrophoresis system (Beckman, Fullerton, CA, USA) equipped with a DAD detector. Data acquisition and processing was controlled by Beckman ChemStation software. The mobile phases prepared from the mixture of CH3CN and 12.5 mM acetic acid (HAc)− sodium acetate (NaAc) buffer at pH 5.6 (70/30, v/v) for CEC separation of neutral analytes and from the mixture of CH3CN and 5 mM phosphate buffer solution (PBS) at pH 8.0 (55/45, v/v) for the separation of polar compounds were filtered before use. Prior to CEC experiments, a detection window was created by burning off a 2 mm segment of the protecting polymer layer at the end of the monolithic bed. The monolithic capillary column (total length, 31.2 cm; effective length, 20.0 cm) was then installed in the CEC instrument and equilibrated at 15 kV until a stable current and baseline was achieved. To avoid the generation of bubbles during separation, a pressure of 20 psi was applied to both inlet and outlet vials simultaneously and 15 kV voltage was performed if not otherwise stated. DAD detection was set at 214 nm for neutral compounds and 235 nm for anilines. The temperature was kept at 25 °C. The samples were degassed under ultrasonication and injected electrokinetically by applying a voltage of 1 kV for 3 s. Characterization. Tapping-mode atomic force microscopy (AFM) was conducted on a Multimode SPM with a Nanoscope IIIa Controller from Digital Instruments. Fourier transforminfrared (FT-IR) spectra (4000−400 cm−1) were obtained using a Magna-560 spectrometer (Nicolet, Madison, WI, USA) in KBr plate. Solid UV spectra (200−800 nm) were recorded on a V-550 spectrometer (JASCO, Japan). The morphologies and microstructures of the GO and GO incorporated monolith were characterized on a JEOL-100CXII transmission electron microscopy (TEM) with an accelerating voltage of 100 kV and on a SS-550 scanning electron microscope (Shimadzu, Japan) at 15.0 kV.

interactions of solutes with a stationary phase characteristic of HPLC.25−27 The stationary phase in CEC plays a dual role: providing sites for the desired interactions as in HPLC and generating EOF.26 Here, we report the fabrication of GO incorporated capillary monolithic column for CEC. GO not only possesses the stability and large surface area necessary for separation media but also provides ionized oxygen-containing functional groups to modify EOF in CEC and aromatic macromolecule to give hydrophobicity and π−π electrostatic stacking property. In this work, GO incorporated capillary monolithic column was fabricated via a room temperature strong inorganic acid initiated methacrylate polymerization strategy28 and evaluated as a new stationary phase for CEC application.

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EXPERIMENTAL SECTION Materials and Chemicals. All reagents used were of analytical grade unless otherwise stated. Ultrapure water (Wahaha, Hangzhou, China) was used throughout all experiments. γ-Methacryloxypropyltrimethoxysilane (γ-MAPS), ethylene glycol dimethacrylate (EDMA), and methacrylic acid (MAA) were purchased from Acros (Geel, Belgium), Alfa Aesar (Ward Hill, MA, USA), and Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China), respectively. Cyclohexanol was from Medical Material Supplier of the Academy of Military Medical Sciences (Beijing, China), and HNO3 was obtained from Tianjin No. 3 Chemical Reagent Plant (Tianjin, China). Thiourea, benzene, toluene, ethylbenzene, and isopropylbenzene were supplied by Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Naphthalene, acenaphthene, fluorene, and anthrancene were from Tianchang Chemical Co. Ltd. (Anshan, China). Acetanilide, aniline, 4-methylaniline, 2-nitroaniline, and 1-naphthylamine were purchased from Tianjin No. 1 Chemical Reagent Plant (Tianjin, China), Tianjin Huayue Chemical Reagent Plant (Tianjin, China), Beijing Changcheng Chemical Reagent Plant (Beijing, China), Shanghai No. 3 Chemical Reagent Plant (Shanghai, China), and Beijing Chemical Plant (Beijing, China), respectively. HPLC-grade of CH3OH and CH3CN were purchased from Burdick & Jackson (Muskegon, MI, USA). Fused silica capillary (375 μm o.d × 75 μm i.d.) was supplied by Yongnian Optic Fiber Plant (Handan, China). Preparation of GO Sheets Incorporated Monolithic Materials. GO was synthesized from natural graphite powder based on a modified Hummers method.29,30 The resulting purified GO powders was collected by centrifugation and air-dried. For the fabrication of GO incorporated monolith, GO sheets were dissolved in cyclohexanol to create a brown and homogeneous dispersion at a concentration of 0.2 mg mL−1 under ultrasonication for 0.5 h. Cyclohexanol was used as the porogen in the preparation of the monolithic stationary phase. MAA (10 μL), EDMA (100 μL), and HNO3 (0.14 mmol) were added into the resulting mixture (600 μL). After further sonication for 15 min, the monophasic solution was introduced into the preconditioned fused silica capillary to an appropriate length by syringe injection. Both ends of the filled capillary were plugged, and the capillary was placed at room temperature for 24 h. The obtained column was then rinsed with CH3OH to remove the unreacted monomers and porogen. For comparison, monolithic column without adding GO was also prepared in the same way. The materials were also synthesized in stainless steel columns (4.1 mm i.d.) and were dried under vacuum for 12 h after Soxhlet extraction with CH3OH for solid-UV spectrophotometric and TEM characterization.

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RESULTS AND DISCUSSION Characterization of GO. GO is a layered material with a wide range of oxygen functional groups including hydroxyl, epoxy, and carboxylic acid groups located on their basal planes and the sheet edges (Figure 1A).1 The FT-IR spectra of the prepared GO material reveal the characteristic bands of the carbon−carbon double bonds at 1620 cm−1 and hydroxyl in GO appearing at 3430 cm−1 (Figure 1B). The band around 1725 cm−1 corresponds to CO stretching vibrations from carbonyl and carboxylic groups. The bands around 1230 cm−1 and 1070 cm−1 are attributed to C−OH stretching vibrations and C−O stretching vibrations, respectively. The FT-IR spectra provide the evidence of the presence of different types of oxygen functionalities on the GO material. AFM image of GO illustrates their flakelike shapes with the thickness of GO being about 1.15 nm (Figure 1C). Fabrication of GO Incorporated Monolithic Capillary Column Based on HNO3 Initiated Polymerization at Room Temperature. The presence of functional groups in GO sheets improves their solubility,11 thus providing convenience for the fabrication of monolithic materials. At the same time, the oxygen-containing functional groups provide active sites for the desired interactions for separation as in HPLC and modify EOF in CEC. A good dispersion of GO in the polymerization mixture is required to ensure a uniform monolithic matrix in the capillary column. The GO sheets without any treatment were well dispersed in cyclohexanol via gentle ultrasonication, 40

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Figure 1. (A) Schematic model of a sheet of GO. (B) FT-IR spectrum of GO. (C) AFM height image of GO sheets.

immobilized via physical adsorption rather than covalent bonding (Figure 3D). Figure 4 shows the SEM images of the GO incorporated poly MAA−EDMA monolithic column. The polymer matrix with the incorporation of GO sheets exhibited a porous and uniform structure. CEC Separation on GO Incorporated Capillary Monolithic Column. CEC is a powerful separation technique that essentially combines the advantages of HPLC and capillary electrophoresis. As in HPLC, solutes partition between the mobile and stationary phases. However, instead of a pressure gradient to pump the mobile phase, application of an electric field to the capillary induces EOF, which is responsible for bulk transport.27,31 Thus, the elution order of the analytes depends not only on their partition between the mobile phase and stationary phase but also on their mobility. For neutral species, their separation is only controlled by their partition between the stationary phase and mobile phase. It is well-known that EOF generated by the ionized functionalities located on the surface of the stationary phase is the driving force in CEC. Generally, neutral compounds are used as test models to evaluate the performance of GO incorporated column in CEC. The migration velocity (umig) of neutral component is given as follows, where ueo is the migration velocity of a neutral unretained marker and k′ is the chromatographic retention factor of the test neutral compound.27

forming brown transparent solution (Figure 2A-b). At concentrations lower than 0.2 mg mL−1, the dispersions are

Figure 2. (A) Photographs of cyclohexanol (a), GO suspension in cyclohexanol (b), and GO suspension in cyclohexanol after 5 weeks (c). (B) TEM image of GO suspension in cyclohexanol (c) after 5 weeks of storage.

very stable, and no precipitation was observed under a microscope even after storage for 5 weeks (Figure 2A-c). Figure 2B shows the TEM image of GO suspension in cyclohexanol deposited for 5 weeks (Figure 2A-c), revealing the good solubility and stability of GO in cyclohexanol. Addition of the monomer mixture and HNO3 into the cyclohexanol suspension of GO resulted in a room temperature polymerization; thus, GO incorporated monolith was prepared. Figure 3A shows the brown GO incorporated monolith after CH3OH washing in comparison with the white rod of the polymer of MAA−EDMA without incorporation of GO (4.1 mm in diameter). The TEM image of GO incorporated monolith after extraction with CH3OH obviously shows the presence of GO sheets in the monolith (Figure 3B). Furthermore, comparison of the solid state UV spectra of GO−MAA−EDMA polymers (before and after extraction with CH3OH) and MAA− EDMA polymers shows that a significant absorbance of GO appears at 270 nm in the GO incorporated polymers while the main absorbance in the polymers is still present in the polymers after extraction (Figure 3C). The above results confirm the successful incorporation of the GO into the polymers. Comparison of the FT-IR spectra of the GO, the polymers produced from MAA and EDMA, and the GO incorporated poly MAA−EDMA monolith shows that immobilization of GO into poly MAA−EDMA monolith did not result in new absorption peaks and significant peak shifts, indicating that GO was

umig = ueo/(1 + k′) The chromatographic behaviors of neutral compounds in CEC depend on their k′ values, which can be regarded as the same results performed in HPLC. In other words, the chromatographic behaviors of neutral compounds on the conditioned column reflect the interactions between the GO incorporated polymeric monolith matrix and the solutes directly. To study the influence of GO on column retention, thiourea was used as the EOF marker, and a series of alkyl benzenes and polycyclic aromatic hydrocarbons were separated on a GO incorporated monolithic column and a poly(MAA−EDMA) monolithic column with 70% CH3CN/30% HAc−NaAc buffer (12.5 mM, pH 5.6) as the mobile phase. Alkyl benzenes of benzene, toluene, ethylbenzene, isopropylbenzene, and polycyclic 41

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Figure 3. (A) Photographs of poly MAA−EDMA monolith and GO incorporated poly MAA−EDMA monolith. (B) TEM image of GO incorporated poly MAA−EDMA monolith. (C) Solid-UV spectra of poly MAA−EDMA monolith (control) after extraction, and GO incorporated poly MAA−EDMA monoliths before and after extraction. (D) Comparison of FT-IR spectra of the GO, the polymers produced from MAA and EDMA, and the GO incorporated poly MAA−EDMA monolith.

Figure 4. SEM image of the GO incorporated poly MAA−EDMA monolith.

aromatic hydrocarbons of acenaphthene and fluorene were almost coeluted on the poly MAA−EDMA column (Figure 5A). In contrast, GO incorporated monolithic column gave a baseline separation of all the compounds with much longer retention times but a reduction of the retention time for the EOF marker thiourea (Figure 5B). Chromatographic parameters including retention time (t), retention factor, and EOF mobility on GO incorporated monolithic column and poly MAA−EDMA monolithic column are compared in Table 1 to examine the role of incorporated GO in CEC separation. There was an enhanced EOF mobility (μEOF) on the GO incorporated monolith which was attributed to the increasing ionized groups of GO in monolithic matrix. The intrinsic characteristics of GO such as carbon-based ring structures and oxygen functional groups affected CEC separation in view of the changes in EOF and interactions with solutes. Both EOF and the interaction between solutes and column matrix decide the retention behaviors of the test neutral components. EOF is the driving force in CEC separation, and higher EOF should give shorter retention time. However, the retention times and retention factors of all the test compounds on the monolithic column incorporated with GO were significantly higher than those on the monolithic column without GO. The above results

Figure 5. Electrochromatograms for the separation of test neutral compounds on (A) poly MAA−EDMA monolith; (B) poly GO− MAA−EDMA monolith. Separation conditions: mobile phase, 70% CH3CN/30% HAc−NaAc buffer (12.5 mM, pH 5.6); applied voltage, 15 kV; DAD detection at 214 nm; temperature, 25 °C. Peak identity: 0, thiourea; 1, benzene; 2, toluene; 3, ethylbenzene; 4, isopropylbenzene; 5, naphthalene; 6, acenaphthene; 7, fluorene; 8, anthracene.

reveal that the interactions between GO present in the stationary phase and the solutes should be dominant in the separation. It is also noted that the retention time of these neutral analytes follows the order of thiourea < benzene < toluene < ethylbenzene < isopropylbenzene < naphthalene < acenaphthene 42

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Table 1. Comparison of CEC Performance on the GO Incorporated Monolithic Column and Poly MAA−EDMA Monolithic Column poly (MAA−EDMA)

poly (GO−MAA− EDMA)

column

t (min)

k′

t (min)

k′

benzene toluene ethylbenzene isopropylbenzene naphthalene acenaphthene fluorene anthracene thiourea (min) μEOF (10−8 m2V−1s−1)

5.30 5.48 5.60 5.79 6.20 6.58 6.92 7.78 5.05 1.38

0.05 0.08 0.11 0.15 0.23 0.30 0.37 0.54

7. 60 8.21 9.13 10.1 10.9 12.9 13.9 18.1 4.65 1.49

0.63 0.77 0.96 1.18 1.35 1.78 2.00 2.90

Figure 7. Electrochromatograms for the separation of anilines on poly GO−MAA−EDMA monolith. Separation condition: mobile phase, a mixture of 55% CH3CN and 45% PBS (5 mM, pH 8.0); applied voltage, 15 kV; DAD detection at 235 nm; temperature, 25 °C. Peak identity: 1, acetanilide; 2, aniline; 3, 4-methylaniline; 4, o-nitroaniline; 5, 1-naphthylamine.

< fluorene < anthracene, which is corresponding to an increasing order of their hydrophobicity. Furthermore, the effect of the content of CH3CN in the mobile phase on the k′ value of the neutral compounds was investigated on the GO incorporated monolithic column. As illustrated in Figure 6, the k′ value of the

Figure 6. Effect of the content of CH3CN in HAc−NaAc buffer (12.5 mM, pH 5.6) on the retention factor (k) of the neutral compounds selected. Separation conditions: mobile phase, a mixture of CH3CN and HAc−NaAc buffer (12.5 mM, pH 5.6) with various contents of CH3CN (%, v/v); applied voltage, 15 kV; DAD detection at 214 nm; temperature, 25 °C. 1, benzene; 2, toluene; 3, ethylbenzene; 4, isopropylbenzene; 5, naphthalene; 6, acenaphthene; 7, fluorene; 8, anthracene.

neutral analytes decreased with an increase of CH3CN content in the mobile phase. Both the elution order and the changes of k′ with CH3CN content are consistent with the typical reversedphase separation mechanism. GO is an electron-rich material with hydrophobicity and π−π electrostatic stacking property, and it gives a strong affinity to carbon-based ring structure molecules, which facilitates the separation as stationary phase in CEC. To further demonstrate the applicability of the GO incorporated monoliths, CEC separation of polar compounds anilines was performed. A set of anilines, including acetanilide, aniline, 4-methylaniline, o-nitroaniline, and 1-naphthylamine were also well separated without significant peak tailing (Figure 7).

Figure 8. Electrochromatograms of three replicate injections of the tested neutral compounds for CEC separation on the GO incorporated monolith. The CEC separations were performed at the applied voltage of 15 kV at 25 °C using a mixture of CH3CN and HAc−NaAc buffer (12.5 mM, pH 5.6) = 70/30 (v/v) as the mobile phase. Peak identity: 0, thiourea; 1, benzene; 2, toluene; 3, ethylbenzene; 4, isopropylbenzene; 5, naphthalene; 6, acenaphthene; 7, fluorene; 8, anthracene.

Reproducibility and Loading Capacity of GO Incorporated Monolithic Column. Figure 8 shows the reproducibility for three replicate CEC separations of the tested neutral compounds on the GO incorporated monolith in terms of electrochromatograms. The precision (relative standard deviation 43

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(18) Chen, J. M.; Zou, J.; Zeng, J. B.; Song, X. H.; Ji, J. J.; Wang, Y. R.; Ha, J.; Chen, X. Anal. Chim. Acta 2010, 678, 44−49. (19) Zhang, S. L.; Du, Z.; Li, G. K. Anal. Chem. 2011, 83, 7531− 7541. (20) Hjertén, S.; Li, Y. M.; Liao, J. L.; Nakazato, K.; Mohammad, J.; Pettersson, G. Nature 1992, 356, 810−811. (21) Svec, F.; Huber, C. G. Anal. Chem. 2006, 78, 2100−2107. (22) Miller, S. Anal. Chem. 2004, 76, 99 A−101A. (23) Peters, E. C.; Petro, M.; Svec, F.; Fréchet, J. M. J. Anal. Chem. 1997, 69, 3646−3649. (24) Hilder, E. F.; Svec, F.; Fréchet, J. M. J. J. Chromatogr., A 2004, 1044, 3−22. (25) Pretorius, V.; Hopkins, B. J.; Schieke, J. D. J. Chromatogr., A 1974, 99, 23−30. (26) Deyl, Z., Svec, F., Eds. Capillary electrochromatography, Elsevier: New York, 2001. (27) Zou, H.-F.; Liu, Z.; Ye, M.-L.; Zhang, Y.-K. Capillary electrochromatography and applications; Science Press: Beijing, China, 2001. (28) Wang, M.-M.; Wang, H.-F.; Jiang, D.-Q.; Wang, S.-W.; Yan, X.-P. Electrophoresis 2010, 31, 1666−1673. (29) Hummers, W. S.; Offeman, J. R. E. J. Am. Chem. Soc. 1958, 80, 1339−1339. (30) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2008, 130, 5856−5857. (31) Bartle, K. D.; Myers, P. Capillary Electrochromatography; The Royal Society of Chemistry: Cambridge, U.K., 2001.

(RSD), n = 3) of EOF was 0.3%. For the selected model compounds, the precision of retention time, peak area, and peak height were in the range of 0.4−3.0%, 0.8−4.0%, and 0.8−4.9%, respectively. The loading capacity of the GO-incorporated monolithic column was 0.45 mg mL−1 for alkyl benzenes, 0.30 mg mL−1 for naphthalene and acenaphthene, and 0.26 mg mL−1 for fluorene and anthracene.

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CONCLUSIONS In summary, we have fabricated a GO incorporated monolithic column via one-step room temperature polymerization for CEC. The prepared GO incorporated monolithic column gives excellent performance for CEC separation of both neutral and polar compounds. The intrinsic characteristic of GO such as carbon-based ring structures and oxygen functional groups makes the GO incorporated monolithic column attractive for CEC separation.

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AUTHOR INFORMATION Corresponding Author *Fax: (86)22-23506075. E-mail: [email protected]. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 20935001, 21077057) and the Fundamental Research Funds for the Central Universities.

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REFERENCES

(1) Zhu, Y. W.; Murali, S.; Cai, W. W.; Li, X. S.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Adv. Mater. 2010, 22, 3906−3924. (2) Zhou, M.; Zhai, Y. M.; Dong, S. J. Anal. Chem. 2009, 81, 5603− 5613. (3) Robinson, J. T.; Zalalutdinov, M.; Baldwin, J. W.; Snow, E. S.; Wei, Z. Q.; Sheehan, P.; Houston, B. H. Nano Lett. 2008, 8, 3441− 3445. (4) Wang, L.; Lee, K.; Sun, Y. Y.; Lucking, M.; Chen, Z.; Zhao, J. J.; Zhang, S. B. ACS Nano 2009, 3, 2995−3000. (5) Liu, Y.; Liu, J. Y.; Deng, C. H.; Zhang, X. M. Rapid Commun. Mass Spectrom. 2011, 25, 3223−3234. (6) Wilson, N. R.; Pandey, P. A.; Beanland, R.; Young, R. J.; Kinloch, I. A.; Gong, L.; Liu, Z.; Suenaga, K.; Rourke, J. P.; York, S. J.; Sloan, J. ACS Nano 2009, 3, 2547−2556. (7) Pyun, J. Angew. Chem., Int. Ed. 2011, 50, 46−48. (8) Zhang, L. M.; Xia, J. G.; Zhao, Q. H.; Liu, L. W.; Zhang, Z. J. Small 2009, 6, 537−544. (9) Wang, Y.; Li, Z. H.; Hu, D. H.; Lin, C.-T.; Li, J. H.; Lin, Y. H. J. Am. Chem. Soc. 2010, 132, 9274−9276. (10) Sun, X. M.; Liu, Z.; Welsher, K.; Robinson, J. T.; Goodwin, A.; Zaric, S.; Dai, H. J. Nano Res. 2008, 1, 203−212. (11) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228−240. (12) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A.; Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Nano Lett. 2009, 9, 1058−1063. (13) Paredes, J. I.; Villar-Rodil, S.; Martínez-Alonso, A.; Tascón, J. M. D. Langmuir 2008, 24, 10560−10564. (14) Hontoria-Lucas, C.; Lopez-Peinado, A. J.; Lopez-Gonzalez, J. D. D.; Rojas-Cervantes, M. L.; Martin-Aranda, R. M. Carbon 1995, 33, 1585−1592. (15) Tang, L.; Wang, J. Z.; Loh, K. P. J. Am. Chem. Soc. 2010, 132, 10976−10977. (16) Gulbakan, B.; Yasun, E.; Shukoor, M. I.; Zhu, Z.; You, M. X.; Tan, X. H.; Sanchez, H. N.; Powell, D. H.; Dai, H. J.; Tan, W. H. J. Am. Chem. Soc. 2010, 132, 17408−17410. (17) Liu, Q.; Shi, J. B.; Sun, J. T.; Wang, T.; Zeng, L. X.; Jiang, G. B. Angew. Chem., Int. Ed. 2011, 50, 5913−5917. 44

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