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Langmuir 2006, 22, 6956-6960
Fluoro-N,N,N′,N′-Tetramethylformamidinium Hexafluorophosphate: A Reagent for Formation of Interchain Carboxylic Anhydrides on Self-Assembled Monolayers Young Shik Chi and Insung S. Choi* Department of Chemistry and School of Molecular Science (BK21), Center for Molecular Design and Synthesis, KAIST, Daejeon 305-701, Korea ReceiVed March 23, 2006. In Final Form: May 19, 2006 In this paper, we report the reactivity of fluoro-N,N,N′,N′-tetramethylformamidinium hexafluorophosphate (TFFH), a reagent for transformation of carboxylic acids into acid fluorides in solution, toward self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid on gold. Contrary to the solution-based reactions, we found that only interchain carboxylic anhydrides (ICAs), not acid fluorides (AFs), were obtained at surfaces by the facile interchain reaction under most reaction conditions studied. AFs were found to be formed only when tetrabutylammonium fluoride, a reagent inducing fast decomposition of ICAs, was added to the reaction mixture. The reactivity of TFFH toward carboxylic acid-terminated SAMs was different from that of cyanuric fluoride, which has been reported previously (Langmuir 2005, 21, 11765-11772). This study provides more insight into the role of the proximity effect in SAMbased reactions as well as another approach to the formation of ICAs from carboxylic acid-terminated SAMs.
Introduction There are many interesting and challengeable phenomena in two-dimensional, interfacial reactions. Recently, self-assembled monolayers (SAMs) were introduced as an ideal platform for studying the rules that govern “reactions in two dimensions”. SAMs are highly ordered molecular assemblies which are formed spontaneously by chemisorption of functionalized surfactants onto solid surfaces.1 The well-defined, highly controllable structures of SAMs provide great advantages for the design of two-dimensional systems for investigating interfacial phenomena or reaction behaviors.2,3 Reactions on SAMs are also crucial for the design of surfaces for further utilizations, such as the construction of biochips via the tethering of biologically active molecules.2-4 Therefore, it is also practically important for efficient surface-tailoring to understand characteristic behaviors in SAM-based reactions. Numerous characteristic phenomena of two-dimensional reactions, which have no analogues to solution-based reactions, have been found in the reactions on SAMs.5 For example, because of densely packed and highly ordered structures, sterically demanding reactions are often hindered in the SAM-based reactions.6-8 In other words, the steric effect is inherently engaged in twodimensional reactions. The changes in pKasa characteristic nature of ionizable moleculessare general trends induced by electrostatic interactions among functional groups at surfaces.9-11 In addition, * To whom correspondence should be addressed. E-mail: ischoi@ kaist.ac.kr. (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (3) Chi, Y. S.; Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Bull. Korean Chem. Soc. 2005, 26, 361. (4) Sullivan, T. P.; Huck, W. T. S. Eur. J. Org. Chem. 2003, 17. (5) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. AdV. Mater. 2000, 12, 1161. (6) Houseman, B. T.; Mrksich, M. Angew. Chem., Int. Ed. 1999, 38, 782. (7) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (8) Vanryswyk, H.; Turtle, E. D.; Watson-Clark, R.; Tanzer, T. A.; Herman, T. K.; Chong, P. Y.; Waller, P. J.; Taurog, A. L.; Wagner, C. E. Langmuir 1996, 12, 6143. (9) Shimazu, K.; Teranishi, T.; Sugihara, K.; Uosaki, K. Chem. Lett. 1998, 669.
Figure 1. Chemical structures of cyanuric fluoride (1) and TFFH (2).
it was reported that even the reaction pathway of SAM-based reactions could be tuned by controlling local environments at surfaces.12,13 In the course of studying SAM-based reactions, we reported that the proximity effect, one of the unique phenomena at interfaces, controlled the product distributions and reaction pathways of interfacial reactions.14 We formed the SAMs of 16-mercaptohexadecanoic acid (MHDA) on gold and treated the SAMs with cyanuric fluoride (CyF) (1 in Figure 1) and pyridine. CyF and pyridine are generally used for generating acid fluorides (AFs) from carboxylic acids in solution-based reactions,15 but in the SAM-based reaction, two different products, AFs and interchain carboxylic anhydrides (ICAs), were controllably obtained at surfaces under different reaction conditions while keeping the reagents (CyF and pyridine) the same. In that paper, we proposed that proximity-driven interchain reactions played a crucial role in the formation of ICAs at the surface. However, a detailed reaction mechanism still remains to be seen. In particular, we did not confirm whether the formation of ICA was caused by the reaction between a “transient” AF and a carboxylate anion or between an intermediate, leading to the formation of AF, and a carboxylate anion. Another question was whether the proximity-driven interchain reaction could occur in other reaction systems or was a specific phenomenon only in that case. In this study, we investigated the effect of reagents to tackle these (10) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (11) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (12) Yousaf, M. N.; Chan, E. W. L.; Mrksich, M. Angew. Chem., Int. Ed. 2000, 39, 1943. (13) Gawalt, E. S.; Mrksich, M. J. Am. Chem. Soc. 2004, 126, 15613. (14) Chi, Y. S.; Choi, I. S. Langmuir 2005, 21, 11765. (15) Olah, G. A.; Nojima, M.; Kerekes, I. Synthesis 1973, 487.
10.1021/la060777r CCC: $33.50 © 2006 American Chemical Society Published on Web 07/01/2006
TFFH in Formation of ICAs on SAMs
questions. Specifically, we applied a different reagent, fluoroN,N,N′,N′-tetramethylformamidinium hexafluorophosphate (TFFH) (2 in Figure 1), to the SAMs of MHDA and characterized the resulting product distributions. In the solution-based reactions, TFFH is also used for the conversion of carboxylic acids into AFs with reactivities different from that of CyF.16 Therefore, the comparison of reaction behaviors between CyF and TFFH would provide more insight into proximity effects and reaction mechanisms in the reaction of the SAMs of MHDA. Materials and Methods Materials. Si〈100〉 wafers were obtained from Prolog Semicor, Ltd., Ukraine. Absolute ethanol (EtOH; 99.8+%, Merck), dichloromethane (CH2Cl2; 100.0%, J.T. Baker, HPLC grade), TFFH (99.0+%, Fluka), anhydrous pyridine (99.8%, Aldrich), MHDA (90%, Aldrich), tetrabutylammonium fluoride hydrate (TBAF; 98%, Aldrich), undecylamine (UDA; 98%, Aldrich), sulfuric acid (H2SO4; 95.0+%, Junsei), acetic acid (99.0+%, Junsei), and hydrogen peroxide (H2O2; 30-35%, Junsei) were used as received. [((6Aminohexyl)amino)carbonyl]ferrocene (Fc-NH2) was synthesized by following the reported procedure.14 Ultrapure water (18.3 MΩ/ cm) from the Human Ultra Pure System (Human Corp., Korea) was used. Preparations of Self-Assembled Monolayers. The gold substrates were prepared by thermal evaporation of 5 nm of titanium and 100 nm of gold onto silicon wafers. Prior to use, gold substrates were cleaned for 1 min in piranha solution (3:7 by volume of 30% H2O2 and H2SO4; caution: piranha solution reacts Violently with most organic materials and must be handled with extreme care!), rinsed with H2O and ethanol, and dried under a stream of argon. The SAMs of MHDA were prepared by immersing the gold substrates in a 1 mM ethanol/water/acetic acid (80:10:10, v/v/v) solution of MHDA overnight according to the previously reported procedure.17 After the formation of the SAMs, the substrates were rinsed with ethanol several times and then dried under a stream of argon. Reactions of the SAMs with TFFH and Pyridine. In a 20 mL scintillation vial, a 10 mL solution of TFFH and pyridine in CH2Cl2 was prepared. Precleaned substrates of the SAMs terminating in carboxylic acids were immersed in the freshly prepared solution of TFFH and pyridine without stirring at room temperature, taken from the solution, rinsed thoroughly with CH2Cl2 and EtOH, and dried in a stream of argon. When ineffaceable precipitates remained on the surface after washing with organic solvents, simple washing with water was necessary. The substrates were characterized by FT-IR spectroscopy and X-ray photoelectron spectroscopy. Reactions of the Activated SAMs with Amines. A 1 mM solution of UDA or Fc-NH2 in CH2Cl2 was prepared in a 20 mL scintillation vial. Precleaned substrate presenting activated products was immersed in the solution without stirring for 1 h at room temperature, taken from the solution, rinsed thoroughly with ethanol, and dried in a stream of argon. Grazing Angle FT-IR Spectroscopy. IR spectra were obtained in a single reflection mode using a dry N2-purged Thermo Nicolet Nexus FT-IR spectrophotometer equipped with the smart SAGA (smart apertured grazing angle) accessory. The p-polarized light was incident at 80° relative to the surface normal of the substrate, and a narrow band mercury-cadmium-telluride (MCT) detector cooled with liquid nitrogen was used to detect the reflected light. We averaged 2000 scans to yield the spectra at a resolution of 2 cm-1, and all the spectra were reported in the absorption mode relative to that of a clean gold surface. X-ray Photoelectron Spectroscopy (XPS). The XPS study was performed with a VG-Scientific ESCALAB 250 spectrometer (U.K.) with a monochromatized Al KR X-ray source. Emitted photoelectrons were detected by a multichannel detector at a take-off angle of 90° relative to the surface. During the measurements, the base pressure (16) Carpino, L. A.; El-Faham, A. J. Am. Chem. Soc. 1995, 117, 5401. (17) Lee, J. K.; Kim, Y.-G.; Chi, Y. S.; Yun, W. S.; Choi, I. S. J. Phys. Chem. B 2004, 108, 7665.
Langmuir, Vol. 22, No. 16, 2006 6957 was 10-9-10-10 Torr. Survey spectra were obtained at a resolution of 1 eV from 3 scans, and high-resolution spectra were acquired at a resolution of 0.05 eV from 5-20 scans. All binding energies were determined with the Au 4f7/2 core level peak at 84 eV as a reference. Cyclic Voltammetry. Cyclic voltammograms (CVs) were acquired using an Autolab potentiostat 10 (Ecochemie, Netherlands). The three-electrode electrochemical cell consisted of a modified Au electrode, a Pt wire counter electrode, and a Hg/Hg2SO4 (mercury sulfate electrode (MSE), saturated K2SO4) reference electrode. Experiments were carried out in degassed H2O containing 0.1 M HClO4 as a carrier electrolyte. The active area of the gold electrode was 0.283 cm2, and the surface coverage values were corrected for surface roughness, assuming a roughness factor of 1.2.18,19
Results and Discussion Reaction of TFFH with the SAM of MHDA. TFFH (2 in Figure 1) was designed for the transformation of carboxylic acids into AFs in the course of developing peptide coupling reagents.16 TFFH is a benign substitute for CyF (1), a corrosive agent, and appears to be an ideal coupling reagent for solid-phase synthesis of peptides. Its reaction pathway is very similar to that of CyF: after the deprotonation of carboxylic acids by base, TFFH allows for the conversion of carboxylic acids into AFs by its reaction with carboxylate anions. The basic condition of the reaction provides environments similar to that of the CyF system for the action of the proximity effect because a neighboring carboxylate anion, that is, a deprotonated form of carboxylic acid, could play a crucial role in the formation of ICAs via interchain reactions.14 On the basis of the similarity, if differences in product distributions are found between the reaction systems using CyF and TFFH, the differences would be caused by different reaction courses with different reactivities in the AF-forming reactions. Therefore, we expected that the TFFH system would provide important information on interchain reactions that occur in the course of the formation of AFs. In this work, we treated the SAM of MHDA with TFFH to compare the reaction behaviors with the reactions using CyF. Specifically, we investigated the similarities and differences of the reactions with an emphasis on the proximitydriven formation of ICAs via interchain reactions. After the formation of the SAM of MHDA (the SAM was characterized by grazing angle infrared (IR) spectroscopy; see Figure 2a and ref 20), the gold substrate was treated with a 10 mL CH2Cl2 solution containing 12 mg (45.4 µmol) of TFFH and 100 µL of pyridine at room temperature for 1 h. A 12 mg sample of TFFH corresponds to 4 µL of CyF in moles, and in the previous study, where CyF was used for the reaction, only AFs were formed under the reaction conditions (4 µL of CyF and 100 µL of pyridine, Figure 2b).14 After the treatment, the two IR peaks of the carboxylic acid groups at 1742 and 1719 cm-1 disappeared and two new peaks appeared at 1821 and 1754 cm-1 (Figure 2c), not around 1840 cm-1 from AFs. The two new peaks were characteristic of the CdO stretching absorption of ICAs and were assigned as in-phase and out-of-phase stretching modes of the two coupled carbonyl groups of carboxylic anhydrides, respectively. In contrast to the distinct F 1s signal detected from the AF-rich surface (Figure 2e), the XPS spectrum showed no observable F 1s signal (Figure 2f), which proved that the surface products were ICAs. The comparison experiments show that the use of the same number of moles (or concentration) of TFFH (18) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (19) Oh, S.-K.; Baker, L. A.; Crooks, R. M. Langmuir 2002, 18, 6981. (20) The IR spectrum of the SAM of MHDA showed two CdO stretching bands at 1742 and 1719 cm-1 from free and hydrogen-bonded carboxylic acids, respectively. The presence of C-H asymmetric and symmetric stretching bands at 2919 and 2850 cm-1 confirmed that the SAM was formed as a highly ordered structure (Figure 2a).
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Figure 3. IR spectra of MHDA SAMs after reaction with (a) 3 mg of TFFH and 2 µL of pyridine, (b) 30 mg of TFFH and 2 µL of pyridine, (c) 30 mg of TFFH and 10 µL of pyridine, and (d) 30 mg of TFFH and 100 µL of pyridine in 10 mL of CH2Cl2 for 1 h at room temperature. (e) IR spectra of MHDA SAMs after reaction with 30 mg of TFFH and 10 µL of pyridine in 10 mL of CH2Cl2 for 3 h at room temperature. (f) IR spectra of MHDA SAMs after reaction with 30 mg of TFFH and 10 µL of pyridine in 10 mL of CH2Cl2 containing 2 mM TBAF for 1 h at room temperature.
Figure 2. IR spectra of (a) intact MHDA SAMs and (b, c) MHDA SAMs after reactions with (b) 4 µL of CyF and 100 µL of pyridine and (c) 12 mg of TFFH and 100 µL of pyridine in 10 mL of CH2Cl2 for 1 h at room temperature. (d) IR spectrum of the substrate after reaction of the ICA-activated surface with undecylamine. Highresolution XPS spectra of the F 1s region acquired from SAMs presenting (e) AFs and (f) ICAs.
gave a product (ICA) different from the product (AF) obtained with CyF. As mentioned above, the observed difference would be caused by different reactivities of TFFH toward the AF formation from CyF. Therefore, it was necessary to study reaction behaviors of TFFH at surfaces in detail, and the results would provide more information on the effects of interchain reactions on the SAM in conjunction with the CyF system.14 Effects of Various Factors on Product Distributions. In the previous study using CyF, product distributionssratios of AFs and ICAssat surfaces were highly dependent on the reaction conditions.14 Various factors, such as the concentrations of reagents, reaction times, and additives, affected the product distribution at surfaces: in brief, higher concentrations of reagents, longer reaction times, and addition of fluoride anions induced AF-rich surfaces. First, we investigated the effect of the concentration of each reagent in the TFFH system. When the amount of pyridine was fixed to be 2 µL and the amount of TFFH increased to 30 mg from 3 mg, we still observed that ICAs formed predominantly at the surface with AFs as a minor product (Figure 3a,b). As a comparison, the same concentrations of CyF induced AF-rich surfaces in the CyF system.14 The increase of the amount of pyridine also did not affect the product distribution apparently. When the amount of TFFH was fixed to be 30 mg and the amount
of pyridine was varied to 2, 10, or 100 µL, ICAs were still formed at the surface as a major product and no changes in the product distribution were observed (Figure 3b-d). It was also unsuccessful to form AF-rich surfaces by changing the reaction times. We increased the reaction time to 3 h (with 30 mg of TFFH and 10 µL of pyridine), but a major product was still ICA (Figure 3e). Although TFFH is a reagent for the formation of AFs in solution, the results show that it was very difficult to form AFs at the surface presenting the SAMs of MHDA. In contrast, ICAs were easily formed by the interchain reaction in the system. The only method to form AF-rich surfaces was the addition of TBAF, a reagent added to the reaction mixture for the effective formation of AFs via decomposition of ICAs in the previous study.14 The addition of TBAF dramatically changed the surface products into AFs. The surface was fully covered with AFs only after 1 h of reaction in the solution containing 2 mM TBAF (and 30 mg of TFFH and 10 µL of pyridine). Only one IR peak from AFs was observed at 1846 cm-1 (Figure 3f). Mechanistic Considerations. In our previous paper, we proposed a reaction mechanism for the formation of ICAs at the surface under the reaction conditions for forming AFs in solution. We suggested that ICAs were formed by the substitution reaction between an “initially formed” acid fluoride and an adjacent carboxylate, and AFs formed by following the mechanism suggested for the solution-based reactions. Because of the similar reaction pathways, we expected that ICAs were generated at the surface by following a mechanism similar to that of the CyF system (Figure 4). We still could not completely exclude the possibility that ICA was formed by a reaction between an intermediate, leading to the formation of AF, and an adjacent carboxylate anion. However, it is more probable that ICA was formed by a reaction between AF, not an intermediate, and an adjacent carboxylate anion because ICAs were effectively formed at surfaces when different reagents inducing the formation of AFs via different intermediates were used. In the aspect of the control of product distributions, one major difference between the CyF system and the TFFH system was that, in the TFFH system, the transition from ICA-rich surfaces to AF-rich surfaces was not easily detected by changing the concentrations of reactants or reaction times in contrast to the CyF system. We believe that the observed behavior was caused
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Figure 4. Proposed mechanism for the formation of ICAs.
by relatively slow reaction rates for the formation of AFs in the TFFH system, although no kinetic studies have been reported yet. If the formation of AFs was much slower than the formation of ICAs, sparsely formed AFs would be transformed into ICAs by facile interchain reactions. In this case, the transformation of ICAs into AFs would be unfavorable because the hydrolysis of ICAs, a key reaction for the transformation, generates two carboxylic acids and the two adjacent carboxylic acids would be recombined to ICA when one of the carboxylic acids is reacted with TFFH. In the solution-based reaction, the formation of AFs using TFFH was highly dependent on steric environments.16 On the basis of that paper, we expected that the reaction was relatively hindered at the SAM environment because of its inherent steric effect as mentioned in the Introduction. Comparative Studies with Amines. We compared the reactivity of ICAs generated by TFFH/pyridine with the reactivity of ICAs generated by other methods (that is, the trifluoroacetic anhydride/triethylamine (TFAA/TEA)21 and CyF/pyridine methods14) by reaction with UDA and Fc-NH2. First, the ICA-activated surface (generated by TFFH/pyridine) was incubated in a 1 mM CH2Cl2 solution of amines for 1 h at room temperature. After the reaction between the surface and UDA, the two CdO stretching bands of the ICAs disappeared completely and two new absorption bands appeared at 1736 and 1561 cm-1 (Figure 2d). The IR band at 1736 cm-1 was assigned as the CdO stretching band of the carboxylic acids, and that at 1561 cm-1 was assigned as the amide II band. The two bands indicated that the reaction between the ICA-activated surface and UDA provided a surface presenting a mixture of amide compounds and carboxylic acids. We also observed two new bands at 2962 and 2886 cm-1 assigned as the asymmetric C-H stretching and the symmetric C-H stretching of the terminal methyl group of the UDA, respectively. Using UDA as a probe, we could not find any differences among three ICA-forming methods regarding the reactivity because the IR spectra showed the complete reaction of ICAs with UDA apparently. For a quantitative comparison, we estimated the number of reactive sites after the reaction between the ICA-activated surfaces and Fc-NH2. Figure 5 shows the CV obtained from the ICAactivated surface prepared by TFFH after the reaction with FcNH2. The inset shows the anodic peak current as a function of scan rate, which exhibited a linear dependence as expected for a surface-bound species.22,23 The number of surface-bound ferrocenes was calculated by the integration of the oxidation peak in the obtained CVs, and the surface coverage was calculated to be 1.57 × 1014 cm-2. The value was about 36% of 4.32 × 1014 cm-2, a reported coverage of MHDA molecules in the SAM on gold.17 We also obtained the surface coverage of ferrocenes (21) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 113, 6704. (22) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (23) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301.
Figure 5. Cyclic voltammograms of the ICA-activated gold surface after the reaction with Fc-NH2 with scan rates of (a) 50 mV/s, (b) 100 mV/s, (c) 200 mV/ s, (d) 400 mV/s, and (e) 800 mV/s. A plot of anodic peak currents as a function of scan rate is shown as an inset.
from the TFAA/TEA and CyF/pyridine systems, and the obtained values were 2.02 × 1014 (47% coverage with respect to the surface density of MHDA) and 1.90 × 1014 cm-2 (44% coverage with respect to the surface density of MHDA), respectively. The value of 50% means that ICAs were generated at the surface in a high yield and a surface presenting a 1:1 mixture of amide compounds and carboxylic acids was generated under the reaction systems. However, in the TFFH system, a relatively larger portion of carboxylic acids seemed to remain at their intact form. We thought that the results also reflected that the reaction by TFFH was more hindered and slower than the reaction by CyF.
Conclusions We reported the reactivity of TFFH, a reagent for the transformation of carboxylic acids into AFs in solution, toward carboxylic acid-terminated SAMs. In typical reaction conditions, only ICAs were obtained by interchain reactions and AFs were not formed effectively at the surface. We think that the observed reactivity was caused by the combination of slow AF formation and fast interchain reaction. The prepared ICAs showed similar reaction behaviors toward amine compounds compared with ICAs generated by other methods. However, from the quantitative analysis, we found that the number of ICAs was less than that of ICAs prepared by other methods. Although the comparison experiments indicated that ICAs were not fully generated by TFFH/pyridine, this method is also valuable as a simple route for the ICA activation of surfaces, which has been used as one useful method for surface functionalizations.21,24-29 (24) Chapman, R. G.; Ostuni, E.; Yan, L.; Whitesides, G. M. Langmuir 2000, 16, 6927. (25) Yan, L.; Huck, W. T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208. (26) Chapman, R. G.; Ostuni, E.; Liang, M. N.; Meluleni, G.; Kim, E.; Yan, L.; Pier, G.; Warren, S.; Whitesides, G. M. Langmuir 2001, 17, 1225.
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The result is our second finding of the formation of unexpected products in SAM-based reactions. With this finding, we concluded that proximity-effect-driven interchain reactions must be considered seriously in the design of surface reactions with other characteristic aspects. Especially, when a desired product is reactive to neighboring groups, interchain reactions would change the reaction pathways. (27) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605. (28) Lee, K. B.; Kim, D. J.; Yoon, K. R.; Kim, Y.; Choi, I. S. Korean J. Chem. Eng. 2003, 20, 956. (29) Chi, Y. S.; Choi, I. S. AdV. Funct. Mater. 2006, 16, 1031.
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Acknowledgment. This work was supported by a Korea Research Foundation grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2005-070C00078). The FT-IR spectrophotometer was purchased with research funds from the Center for Molecular Design and Synthesis. We thank Dr. Myoung-gyu Ha and Dr. Mi-Sook Won of the Korea Basic Science Institute (KBSI) for the XPS analysis. We also thank Professor Juhyoun Kwak and Dr. Seongpil Hwang of the Dapartment of Chemistry at KAIST for help with cyclic voltammetry experiments. LA060777R
