Preparation of the Hybrid Film of Poly(allylamine hydrochloride

Feb 7, 2012 - The gold/GO nanohybrid films were thoroughly investigated using various analytical methods, including Raman spectroscopy and atomic forc...
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Preparation of the Hybrid Film of Poly(allylamine hydrochloride)Functionalized Graphene Oxide and Gold Nanoparticle and Its Application for Laser-Induced Desorption/Ionization of Small Molecules Young-Kwan Kim† and Dal-Hee Min*,‡ †

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Korea Department of Chemistry, Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 151-747, Korea

‡

S Supporting Information *

ABSTRACT: Hybrid films of gold nanoparticles and graphene oxides (GOs) were prepared by directly growing gold nanoparticles on supported thin layers of GO films on a glass slide. The gold/GO nanohybrid films were thoroughly investigated using various analytical methods, including Raman spectroscopy and atomic force microscopy. The hybrid film was then applied to laser desorption/ionization (LDI) of small molecules, which enabled mass spectrometric analysis of analytes. After a series of detailed mechanistic studies and systematic investigations, we found that the gold/GO hybrid films serve as a successful LDI platform for small-molecule analysis because of the high desorption efficiency of analytes from the hybrid films without inducing significant fragmentation of analytes. We suggest that the underlying GO films may effectively dissipate excess thermal energy generated by laser irradiation of Au to prevent undesirable analyte fragmentation.

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INTRODUCTION Graphene is a promising material in various research and industrial fields because of its unique mechanical1,2 and optoelectrical properties that stem from its chemical structure consisting of a sp2 carbon network.3 Recently, graphene oxide (GO), an oxidized form of graphene with a negatively charged surface at physiological pH, has emerged as an important derivative of graphene that exhibits high water solubility and an entire suite of unique opto-electrical properties, such as fluorescence4 and surface-enhanced Raman scattering.5 Therefore, GO has been used in various applications, including the development of biosensors,6−8 enzyme activity assay systems,9 and surface-enhanced Raman scattering platforms.10 Additionally, GO derivatives are known to absorb laser light and to efficiently transfer the energy to surface-bound analytes. Thus, graphene,11 GO,12 and reduced GO (RGO) films13 have been used as matrices in laser desorption/ionization−mass spectrometry (LDI−MS) of small molecules and biomolecules. Conventional matrix-assisted laser desorption/ionization−mass spectrometry (MALDI−MS) is generally not compatible with small organic molecule analysis because of interference from matrix molecules (aromatic small molecules, such as dihydroxy benzoic acid, that are necessary in MALDI−MS for transfer of laser energy to analytes) in the low mass region.14 It also requires much trial and error to optimize analysis conditions because sample preparation using traditional organic matrices generates inhomogeneous signal intensity because of irregular co-crystallization of the analyte and matrix.15,16 For these reasons, LDI−MS is an important analytical tool for analyses of © 2012 American Chemical Society

small organic molecules and biomolecules with molecular weights of less than 500 Da. Our group previously reported that the supported GO film on glass slides can serve as a substrate for LDI−MS, the LDI efficiency of the supported GO films can be greatly enhanced by incorporating multiwalled carbon nanotubes (MWCNTs) onto the GO films, and the GO/MWCNT hybrid films can be harnessed for monitoring phospholipase activity,17 smallmolecule detection, and tissue imaging.18 In this study, the LDI efficiency of GO derivatives was found to be dependent upon the chemical structure of GO, which determines its optoelectrical properties. Recently, we have also reported that the opto-electrical properties, such as electrical conductivity and Raman scattering intensity, of RGO can be enhanced by growing gold nanostructures on its surface.19,20 The enhancement factor varied with the shape and density of the gold nanostructures on the RGO film, which could be controlled through surface functionalization of the RGO films. Herein, we report the isotropic growth of gold nanostructures on the supported GO film surfaces coated with poly(allylamine hydrochloride) (PAA−GO) for the fabrication of gold/GO nanohybrid (Au/PAA−GO) films and their application as a LDI−MS platform. Although GO, RGO, and gold nanomaterials are known as alternatives of organic matrices for LDI−MS analysis of small molecules, when those materials were immobilized on glass substrates as thin Received: October 25, 2011 Revised: February 2, 2012 Published: February 7, 2012 4453

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Scheme 1. (a) Fabrication of Gold Nanostructures on PAA-Functionalized GO Films and (b) LDI of Analytes on Au/PAA−GO Films

at the target plate) in positive reflective modes. The accelerating voltage was 19 kV, and all spectra were recorded by averaging 500 laser shots. Repeat Analysis of Small Molecules on Au/PAA−GO Films. A total of 1 μL of 1 mM cellobiose and Leu-enkephalin solution was spotted on Au/PAA−GO films, allowed to dry under vacuum to minimize the interval time for each repeat analysis cycle, and subjected to LDI−MS analysis. After finishing each LDI−MS analysis cycle, the used Au/PAA−GO films were washed with water and ethanol and dried under a stream of nitrogen. The cleaned Au/PAA−GO films were repeatedly applied to the same LDI−MS analysis procedure by using 1 nmol of cellobiose and Leu-enkephalin as model compounds. Estimation of the Survival Yield and Desorption Efficiency. Benzylpyridinium salt (BP) was synthesized by a previously reported method,18 dissolved in methanol to 1 mM, spotted onto the prepared substrates, allowed to dry under ambient conditions, and subjected to LDI−MS analysis under the same experimental conditions. The intensities of the precursor and fragment peaks from BP were used to calculate survival yields and desorption efficiencies. Characterization. The surface morphologies and line profiles of GO sheets, GO and gold, and PEA- and PAA-functionalized GO nanohybrid films were obtained with an XE-100 (Park System, Korea). The surface morphology changes of gold and gold and GO nanohybrid films induced by Piranha treatment were confirmed by S-4800 field emission scanning electron microscopy (Hitachi, Japan). Raman characterization was carried out by LabRAM HR UV/vis/NIR (Horiba Jobin Yvon, France) using an Ar ion CW laser (514.5 nm) as an excitation source focused through a BXFM confocal microscope equipped with an objective (50×, numerical aperture of 0.50). The ultraviolet−visible (UV−vis) analysis was performed by a UV-2550 (Shimadzu, Japan). Fourier transform infrared (FTIR) spectra of GO were recorded with an EQUINOX55 (Bruker, Germany) using the KBr pellet method.

films, LDI efficiency was very low for LDI−MS analysis. In contrast, the Au/PAA−GO hybrid films composed of gold nanoparticles and GO showed excellent applicability as a LDI− MS platform because of possible synergistic effects arising from the hybrid junctions in the Au/PAA−GO films, which were expected to enhance the LDI efficiency of analytes.

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EXPERIMENTAL SECTION

Preparation of 5 nm Gold Nanoparticles. A total of 20 mL of 0.25 mM HAuCl4 and 0.25 mM trisodium citrate was prepared in a conical tube. A total of 0.6 mL of 0.1 M NaBH4 (ice-cooled) was added to the solution at once with gentle shaking. The suspension was aged for 2 h and used for the seed-mediated growth reaction. Preparation of Gold and GO Nanohybrid Films. GO was prepared by Hummers method21 and exfoliated in water by sonication for 30 min to make a suspension of GO at 1.5 mg/mL. The 3aminopropyltriethoxysilane (APTES)-treated glass substrates were immersed in the suspension for 1 h, washed with water and ethanol, dried under a stream of nitrogen, and heated at 125 °C for 10 min under a nitrogen atmosphere. Pyrene ethyleneglycol amine (PEA) and PAA functionalization of prepared GO films was performed by immersing GO film-coated glass substrates into a 1 mM ethanolic solution of PEA or a 1 mg/mL aqueous solution of PAA for 12 h and 30 min, respectively. After washing with water and ethanol and drying under a stream of nitrogen, the PEA- and PAA-functionalized GO films were immersed in a suspension of 5 nm gold nanoparticles for 1 h, washed with water and ethanol, and dried under a stream of nitrogen. The seed-loaded PEA- and PAA-functionalized GO films were immersed in a growing solution containing 9 mL of 100 mM cetyltrimethyl ammonium bromide (CTAB), 450 μL of 10 mM hydrogen tetrachloroaurate, and 100 μL of 100 mM ascorbic acid for 1 h, washed with water and ethanol, and dried under a stream of nitrogen. Prior to the LDI−MS analysis of small molecules, the prepared gold and GO nanohybrid films were immersed in Piranha solution for 3 min, washed with water and ethanol, and dried under a stream of nitrogen to remove any surface-adsorbed organic molecules during the growth process. LDI−MS and Imaging Analysis of Small Molecules. The various small molecules, glucose, mannitol, cellobiose, leucine, phenylalanine, glutamine, and Leu-enkephalin, were dissolved separately in water to a concentration of 10 mM and then diluted to 1 mM. The prepared analyte solutions (1 μL) were spotted on the prepared substrates, allowed to dry under ambient conditions, and subjected to LDI−MS analysis under the same experimental conditions. All mass spectrometric experiments were performed using a Bruker Autoflex III (Bruker Daltonics, Germany) with a Smartbeam laser (Nd:YAG, 355 nm, 100 Hz, 50 μm in spot diameter

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RESULTS AND DISCUSSION To prepare the hybrid film, gold nanoparticles were grown directly onto the surface of GO films supported on a glass slide (Scheme 1). First, GO was prepared by a modified Hummers method (see Figure S1 of the Supporting Information)21 and dispersed in water (1.5 mg/mL) by sonication for 30 min. Then, APTES-treated glass substrates were immersed in an aqueous suspension of GO sheets. This process resulted in the immobilization of GO on the glass slide via electrostatic interactions between the negatively charged GO sheets and the protonated amine groups of the APTES-treated glass substrates.22 The surfaces of the GO films were then 4454

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Figure 1. (a) AFM images and line profiles and (b) UV−vis spectra of GO film, PAA-functionalized GO film with surface-adsorbed gold seeds, and gold nanostructures grown on PAA-functionalized GO film are shown. (c) SEM image shows the gold nanostructures grown on PAA-functionalized GO films after Piranha treatment. (d) Raman spectra are given for GO and PAA-functionalized GO films with gold nanostructures.

Figure 2. (a) Mass spectra of glucose, mannitol, cellobiose, leucine, phenylalanine, glutamine, and Leu-enkephalin were obtained on Au/PAA−GO films. (b) Mass spectrometric images show spots of glucose and cellobiose obtained on the Au/PAA−GO films. Chemical structures of all of the small molecules and corresponding exact molecular weights are presented in the Supporting Information.

functionalized with PAA via electrostatic interactions.23 PAA functionalization served to introduce amine groups onto the surface of the GO films through multiple electrostatic interactions. The PAA-functionalized GO films were then immersed in a suspension of gold nanoparticles (5 nm in diameter) for 1 h. This process immobilized small gold nanoparticle seeds onto the PAA-coated GO films. The seedloaded films were then immersed for 1 h in a growing solution consisting of 9 mL of 100 mM CTAB, 450 μL of 10 μM chloroauric acid, and 100 μL of 100 mM ascorbic acid. This seed-mediated Au growth on the PAA-coated GO films resulted in the formation of isotropic gold nanostructures on the GO films with high density, because of highly dense loading of gold seeds (Scheme 1), in contrast with previous reports that resulted in anisotropic gold nanostructure growth.19,20 An atomic force microscopy (AFM) image of the GO films on glass substrates showed 1−3 layers of densely packed GO sheets (average of 2.17 nm in thickness; Figure 1a).19 AFM

measurements of gold seed-immobilized PAA−GO films (Au seed/PAA−GO) revealed that the gold nanoparticles were homogeneously immobilized on the GO film-coated glass substrates with high coverage. Gold nanostructures grown on the seed-loaded GO films mostly consisted of isotropic, average of 30 nm diameter spheres (Figure 1a). UV−vis spectra of these nanohybrid films are shown in Figure 1b. A characteristic absorption peak corresponding to the gold nanoparticle was observed around 530 nm from hybrid films containing gold nanoparticles (Figure 1b). Prior to further application and characterization, the Au/PAA−GO hybrid films were briefly immersed in Piranha solution to remove surface-adsorbed organic molecules. The gold nanostructures remained on the GO surface without observable damage after the Piranha treatment (Figure 1c). GO and Au/PAA−GO films were characterized by Raman spectroscopy to understand the effect of gold nanostructure growth on the chemical structure of the underlying GO. A 4455

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Figure 3. (a) Mass spectra of BP were obtained on GO, Au/PAA−GO, Au/PAA, and Au/PEA−GO films. (b) Fragmentation process of BP during LDI−MS experiment. (c) Calculated survival yields and desorption efficiencies are given for BP on GO, Au/PAA−GO, Au/PAA, and Au/PEA−GO films.

Na]+ and m/z 219 [M + K]+; leucine, m/z 154 [M + Na]+ and m/z 170 [M + K]+; phenylalanine, m/z 188 [M + Na]+ and m/ z 204 [M + K]+; glutamine, m/z 169 [M + Na]+; and Leuenkephalin, m/z 576 [M + Na]+ and m/z 592 [M + K]+).28 The origin of cations was attributed to the residual cations of oxidative reagents used for synthesis of GO.18 The detection limit of small molecules on Au/PAA−GO films was estimated as 100 pmol when cellobiose, Leu-enkephaline, glucose, lysine, and phenylalanine were applied as model compounds for disaccharide, small peptide, monosaccharide, and amino acid (see Figure S4 of the Supporting Information). The bare GO films omitting gold nanoparticles yielded no observable peaks corresponding to the analytes under the same experimental conditions (see Figure S5 of the Supporting Information). This was likely because the films were only composed of 1−3 layers of GO and not sufficient to induce LDI. These results demonstrate that gold nanostructure growth enables GO films to be used as an efficient LDI−MS platform for small-molecule analyses. The relatively poor detection sensitivity of Au/PAA− GO films compared to colloidal Au nanoparticles might be attributed to the relatively low amount of Au nanoparticles involved in LDI−MS analysis because the Au nanoparticles in the Au/PAA−GO films were immobilized as a single-layered structure. For further control experiments, two additional substrates were prepared, in which the gold nanostructures were grown on the PEA-functionalized GO films19,29 and PAA-functionalized glass substrates omitting the GO layer. In the first control substrate, PEA was used in place of PAA to give relatively

Raman spectrum of the GO film showed typical D and G peaks around 1350 and 1607 cm−1, which are derived from sp3 and sp2 carbon domains, respectively (Figure 1d).24 These bands were both shifted to higher frequencies (1359 and 1609 cm−1), and their intensities were enhanced by 4.77- and 4.71-fold following PAA functionalization and the growth of the gold nanostructures, respectively (Figure 1d). The enhancement was due to localized surface plasmon resonance at the surface of the gold nanostructures.25 These changes in peak intensities and positions suggest the formation of chemically and electronically strong, interactive nanohybrid interfaces (Figure 1d).26,27 The presence of the gold nanostructures did not result in any significant change in the intensity ratio between the D and G peaks relative to that of the bare GO films. This suggests that the immobilization of gold seeds and the growth of gold nanoparticles did not alter the chemical structure of the underlying GO film. Next, the applicability of Au/PAA−GO films on glass substrates as an LDI−MS platform was evaluated using 1 nmol of various small molecules, including glucose, mannitol, cellobiose, leucine, phenylalanine, glutamine, and Leu-enkephalin (see Figure S2 of the Supporting Information). A total of 1 μL of each solution (1 mM) was deposited onto the Au/ PAA−GO films and allowed to dry under ambient conditions. Figure 2a shows that clear and clean mass peaks with low background noise peaks were obtained on the Au/PAA−GO films, corresponding to sodium or potassium adducts of all of the analytes (cellobiose, m/z 365 [M + Na]+; mannitol, m/z 205 [M + Na]+ and m/z 221 [M + K]+; glucose, m/z 203 [M + 4456

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nanoparticles by dissipating local, laser-induced thermal energy through its graphitic structure. A well-balanced survival yield and desorption efficiency explain the high efficiency of the Au/ PAA−GO hybrid film as a LDI−MS platform. LDI−MS images of small molecules spotted on Au/PAA− GO films were also obtained (with a 50 μm laser beam diameter and a 100 μm raster width). These images show the compatibility of the present platform with small-molecule array formats. The mass images of glucose and cellobiose showed signal intensity distributions depending upon the shape of the sample spot, suggesting that mass intensity may be used to determine relative amounts of analytes at specific locations that are spatially distributed on a single substrate (Figure 2b). The salt tolerance was evaluated next by measuring the detection limits of small molecules (cellobiose, Leu-enkephalin, glucose, lysine, and phenylalanine) dissolved in phosphatebuffered saline (PBS) solutions. For direct analysis of biological samples without the desalting process, it is important to yield mass peaks of analytes even in the presence of a high salt concentration. Although the detection limit of glucose, lysine, and phenylalanine was decreased by 1 order of magnitude, that of cellobiose and Leu-enkephalin remained as 100 pmol. Therefore, Au/PAA−GO films were shown applicable to direct analyses of small molecules dissolved in PBS, which had an ionic strength similar to that of human body fluids, without requiring a purification or enrichment process (see panels a and b of Figure S10 of the Supporting Information). The structural integrity of Au/PAA−GO films was next examined after the LDI−MS application to know whether laser irradiation damages the surface. No significant changes in the surface structure were observed after LDI−MS analyses (see Figure S11 of the Supporting Information). Next, the repeated LDI−MS analysis of cellobiose and Leu-enkephaline was carried out on Au/PAA−GO films. The possible repeat cycles were suggested as 10 cycles because the mass signal corresponding to each analyte decreased significantly after 10 repeat cycles (see panels a and b of Figure S11 of the Supporting Information), and the burned spot was observed on the region of Au/PAA−GO films where repeated laser irradiation was applied (see Figure S11c of the Supporting Information). These results show the Au/PAA−GO films are durable up to 10 repeated LDI−MS analysis cycles with clean mass spectra of small molecules and low background signals.

sparse distribution of amines, inducing a lower density of gold nanostructures. The second control substrate (gold particles on a PAA glass slide without the GO layer) was prepared to investigate the role of GO in LDI application. The density of gold nanostructures grown on the PEA-functionalized GO films was lower than that on the PAA-functionalized GO films, as expected (see Figure S6a of the Supporting Information). The gold nanostructures grown on the PAA-functionalized glass substrates, without the GO layer, were of similar density and shape as those grown on Au/PAA−GO films (see Figure S6b of the Supporting Information). Thin films of gold particles and PEA-functionalized GO (Au/PEA−GO) and gold nanostructures on PAA-functionalized glass substrates (Au/PAA) exhibited the characteristic UV−vis absorption peak of gold nanoparticles around 530 nm (see panels a and b of Figure S7 of the Supporting Information). The same small molecules analyzed above were then analyzed on the Au/PEA−GO and Au/PAA films. Only phenylalanine was detected on the Au/ PEA−GO film with a relatively weak signal intensity, and no other molecules could be detected on either control substrate (see Figure S8 of the Supporting Information). These control experiments suggest that the presence of GO films lying underneath the gold nanostructures and an appropriate density of gold nanoparticles are pivotal to the success of smallmolecule LDI analyses. To investigate the mechanism underlying the enhanced LDI performance of small molecules on Au/PAA−GO films, BP was synthesized and used to obtain survival yields and desorption efficiencies for LDI analyses on each of the prepared substrates.30 These parameters are useful in understanding the complex energy transfer process during LDI by decoupling analyte ionization and surface desorption. This is accomplished by considering a molecule, such as BP, that is already ionized and tends to be fragmented into smaller ions under intense irradiation (panels a and b of Figure 3).31 For successful LDI− MS analysis of analytes, fragmentation should be avoided to measure intact ion peaks, if possible. Thus, a combination of high survival yield and high desorption efficiency would be preferred in LDI platforms. However, under general circumstances, high desorption efficiency, which tends to be obtained at high laser intensities and energy transfer, results in a low survival yield, giving fragmentation of precursor ions. Low survival yield (38%) and low desorption efficiency (174) were obtained with BP on GO films, which explains the low LDI efficiency of small molecules on bare GO films (Figure 3c). These values were considerably higher on Au/PAA−GO films: 74% and 59 234, respectively (Figure 3c). These higher values could be attributed to the densely grown gold nanoparticles on GO films, which efficiently absorb laser energy and transfer to analytes for LDI−MS analysis.32 This result explains the observed high LDI efficiency of small molecules on Au/PAA− GO films. Mass spectra of BP were also acquired on the control substrates, Au/PEA−GO and Au/PAA films. The survival yield and desorption efficiency on Au/PEA−GO films were relatively low (52% and 23 101; Figure 3c), probably because of the low density of gold nanoparticles. Interestingly, the desorption efficiency of BP was highest (128 538) on the Au/PAA films with a relatively low survival yield (56%) compared to that of the Au/PAA−GO films (Figure 3c). The only difference between the Au/PAA−GO and Au/PAA films is the existence of the underlying GO film. Thus, the difference in the performance may be attributed to the GO film, which may prevent rapid heating and/or overheating of the gold

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CONCLUSION In conclusion, Au/PAA−GO hybrid films were prepared by the direct growth of gold nanostructures on PAA-functionalized GO films, and their applicability as a LDI−MS platform was investigated thoroughly. The synthesized gold nanostructures on PAA-functionalized GO films were mostly an average of 30 nm in diameter and spherical, resulting from isotropic growth. These nanostructures interacted strongly with the underlying GO films. The LDI−MS analysis on Au/PAA−GO films was performed using various small molecules and BP, an analytical standard for the analysis of the LDI process. The LDI efficiency of GO films could be greatly enhanced by the direct surface growth of dense gold nanostructures, which yielded strongly interactive nanohybrid surfaces that exhibited a balanced survival yield and desorption efficiency. We suggest that the GO films may dissipate excess thermal energy generated by laser irradiation of the gold nanoparticles, thereby reducing the tendency of undesirable analyte fragmentation. We believe that the developed LDI−MS platform will be an effective tool for 4457

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(21) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (22) Kim, Y.-K.; Min, D.-H. Langmuir 2009, 25, 11302. (23) Kong, B.-S.; Yoo, H.-W.; Jung, H.-T. Langmuir 2009, 25, 11008. (24) Wang, H.; Robinson, J. T.; Li, X.; Dai, H. J. Am. Chem. Soc. 2009, 131, 9910. (25) Jasuja, K.; Berry, V. ACS Nano 2009, 3, 2358. (26) Lin, Y.; Watson, K. A.; Fallbach, M. J.; Ghose, S.; Smith, J. G. Jr.; Delozier, D. M.; Cao, W.; Crooks, R. E.; Connell, J. W. ACS Nano 2009, 3, 871. (27) Kim, K. K.; Bae, J. J.; Park, H. K.; Kim, S. M.; Geng, H. Z.; Park, K. A.; Shin, H. J.; Yoon, S. M.; Benayad, A.; Choi, J. Y.; Lee, Y. H. J. Am. Chem. Soc. 2008, 130, 12757. (28) The mass spectrum of Au/PAA−GO films supported on glass substrates showed strong characteristic Au cluster peaks (Au+, m/z 197; Au2+, m/z 394; and Au3+, m/z 591) with a weak unknown mass peak at m/z 198, which might be derived from laser-induced partial fragmentation of GO sheets. The Au cluster and GO derived unknown peaks were often observed during LDI−MS analysis of small molecules on Au/PAA−GO, Au/PEA−GO, and Au/PAA films. For the mass spectrum of Au/PAA−GO films, see Figure S3 of the Supporting Information. (29) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. J. Am. Chem. Soc. 2008, 130, 5856. (30) The survival yield and desorption efficiency are calculated by dividing the precursor ion intensity by the total intensity of the fragmented and precursor ions and by summing the absolute intensities of the fragment and parent ions, respectively. (31) Tang, H. W.; Ng, K. M.; Lu, W.; Che, C. M. Anal. Chem. 2009, 81, 4720. (32) McLean, J. A.; Stumpo, K. A.; Russell, D. H. J. Am. Chem. Soc. 2005, 127, 5304.

small-molecule analyses because of its excellent performance, cost-effectiveness, and simple fabrication process. In addition, this hybrid film would offer an interesting platform to study the mechanism of the complicated LDI process and, thereby, provide a fundamental mechanistic understanding required to design an improved LDI platform in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Materials, APTES modification of a glass substrate, preparation of GO, synthesis of BP, and Figures S1−S11. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-2-880-4338. Fax: +82-2-875-6636. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea, funded by the Ministry of Education, Science, and Technology (MEST) of the Korea government (Grants 313-2008-2-C00538, 2008-0062074, and 2011-0017356), and the Nano R&D program of NRF of Korea, funded by the MEST of the Korea government (Grant 2008-2004457).

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