Aqueous Dispersion, Surface Thiolation, and Direct Self-Assembly of

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Langmuir 2007, 23, 3363-3371

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Aqueous Dispersion, Surface Thiolation, and Direct Self-Assembly of Carbon Nanotubes on Gold Natalia Kocharova,*,† Timo A ¨ a¨ritalo,† Jarkko Leiro,‡ Jouko Kankare,† and Jukka Lukkari† Laboratory of Materials Chemistry, Department of Chemistry, and Department of Physics, UniVersity of Turku, 20014 Turku, Finland ReceiVed October 27, 2006. In Final Form: December 21, 2006 We report the efficient aqueous dispersion of pristine HiPco single-walled carbon nanotubes (SWNTs) with ionic liquid (IL)-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), the thiolation of nanotube sidewalls with 2, and the controlled self-assembly of positively charged SWNT-1,2 composites on gold. Optical absorption spectra and resonance Raman (RR) data of obtained aqueous SWNT-1,2 dispersions are consistent with debundled and noncovalently functionalized nanotubes whose electronic properties have not been disturbed. Additionally, the dispersion of pristine nanotube material with surfactants 1 and 2 leads to a high degree of purification from carbonaceous particles. The chiralities of the 14 smallest semiconducting HiPco SWNTs in resonance with Raman excitation at 1064 nm (1.165 eV) were determined in SWNT-2 aqueous dispersion using UV-vis-NIR and RR spectra. X-ray photoelectron spectroscopy (XPS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopy of SWNT-2 submonolayers on gold verified the encapsulation of individualized SWNTs with IL surfactants, the cleavage of S-S disulfide bonds formed in aqueous SWNT-2 suspensions, and the direct chemisorption of the SWNT-2 composite on bare gold via the Au-S bond. Aqueous dispersions of SWNTs with IL-based surfactants add biofunctionality to carbon nanotubes by imparting the positive surface charge necessary for interactions with cell membranes. Our technique, which purifies pristine nanotube material and produces watersoluble, positively charged nanotubes with pendent surface-active thiol groups, may also be translated to other carbon nanotubes and carbon nanostructures. Self-assembled, positively charged submonolayers of SWNTs can be further used for applications in cell biology and sensor technology.

Introduction Single-walled carbon nanotubes (SWNTs), because of their unique physical, electronic,1 and optical2 properties, are novel, exciting nanomaterials for a wide range of applications in materials science and sensor technology3 and for a number of biological and biomedical applications including biochemical sensing4and substrates for nerve cell growth in vitro.5,6 Possible applications set strict criteria for nanotube materials. Nanotubes have to be modified to achieve solubility and to enable their integration in ordered structures. In particular, the use of nanotubes for biomedical and biological applications requires their processing in aqueous media. One strategy for SWNT modification/functionalization is the use of covalent chemistry7,8 that, however, alters the intrinsic electronic properties of the nanotubes.9 Therefore, noncovalent modification of the outer SWNT sidewalls in order to preserve * Corresponding author. E-mail: [email protected]. Tel: +3582-333 6718. Fax: +358-2-333 6700. † Department of Chemistry. ‡ Department of Physics. (1) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. B: Condens. Matter Mater. Phys. 1992, 46, 1804. (2) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (3) (a) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. Science 2000, 287, 622. (b) Tans, S.; Verschueren, A.; Dekker, C. Nature 1998, 393, 49. (c) Kong, J.; Dai, H. J. Phys. Chem. B 2001, 105, 2890. (4) (a) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (b) Star, A.; Gabriel, J. P.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 459. (c) So, H.-M.; Won, L.; Kim, B.-K.; Ryu, B. H.; Na, P. S.; Kim, H.; Lee, J.-O. J. Am. Chem. Soc. 2005, 127, 11906. (d) Portney, N. G.; Ozkan, M. Anal. Bioanal. Chem. 2006, 384, 620. (5) Mattson, M. P.; Haddon, R. C.; Rao, A. M. J. Mol. Neurosci. 2000, 14, 175. (6) Gheith, M.; Sinani, V. A.; Wicksted, J. P.; Matts, R. L.; Kotov, N. A. AdV. Mater. 2005, 17, 2663. (7) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853.

their extended π-electron networks by the adsorption of ordered and charged architectures seems more attractive.10 Nowadays, noncovalent modification with common surfactants is widely used to obtain aqueous SWNT dispersions.2,7,11-14 Water-soluble polymers,15,16 proteins,17 and DNA18 have also been shown to disperse nanotubes in a noncovalent way and in aqueous media. The noncovalent approaches are based on interaction of the hydrophobic part of the adsorbed molecule with nanotube sidewalls through van der Waals, π-π, CH-π, and other interactions, and aqueous solubility is provided by the hydrophilic part of the molecule. The charging of the nanotube surface by adsorbed ionic molecules additionally prevents nanotube aggregation by the Coulombic repulsion forces between modified SWNTs. A vast majority of the above-mentioned noncovalent (8) (a) Huang, W.; Fernando, S.; Lin, Y.; Zhou, B.; Allard, L. F.; Sun, Y.-P. Langmuir 2003, 19, 7084. (b) Dyke, C. A.; Tour, J. M. J. Phys. Chem. A 2004, 108, 11151. (9) Tasis, D.; Tagmatarchis, N.; Geogakilas, V.; Prato, M. Chem.sEur. J. 2003, 9, 4001. (10) Hirsch, A.; Vostrowsky, O. Top Curr. Chem. 2005, 245, 193. (11) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269. (12) Matarredona, O.; Rhoads, H.; Li, Z.; Harwell, J. H.; Balzano, L.; Resasco, D. E. J. Phys. Chem. B 2003, 107, 13357. (13) Wenseleers, W.; Vlasov, I. I.; Goovaerts, Obraztsova, E. D.; Lobach, A. S.; Bouwen, A. AdV. Funct. Mater. 2004, 14, 1105. (14) Tan, Y.; Resasco, D. E. J. Phys. Chem. B 2005, 109, 14454. (15) O’Connel, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (16) Sinani, V. A.; Gheith, M. K.; Yaroslavov, A. A.; Rakhnyanskaya, A.; Sum, K.; Mamedov, A.; Wicksted, J. P.; Kotov, N. A. J. Am. Chem. Soc. 2005, 127, 3463. (17) Karajanagi, S. S.; Yang, H.; Asuri, P.; Sellitto, E.; Dordick, J. S.; Kane, R. S. Langmuir 2006, 22, 1392. (18) (a) Zheng, M.; Jagota, A.; Senke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338. (b) Strano, M. S.; Zheng, M.; Jagota, A.; Onoa, B.; Heller, D. A.; Barone, P. W.; Usrey, M. L. Nano Lett. 2004, 4, 543. (c) Barisci, J. N.; Tabhan, M.; Wallace, G. G.; Badaire, S.; Vaugien, T.; Maugey, M.; Poulin, P. AdV. Mater. 2004, 14, 133.

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aqueous solubilization schemes have been applied to purified nanotube material. Only recently as-produced SWNTs have been solubilized by several surfactants (including bile salts),13 nucleotides,19 and designed amphiphilic peptides20 The introduction of charged functionalities by nanotubes modification is an essential step for making nanoscale electronic devices because electrical conductance in semiconducting SWNTs was found to be highly sensitive to the surface adsorption of various molecules3a,21 and electrostatic charging.22 In addition, the availability of both negatively and positively charged carbon nanotubes is also needed for the fabrication of highly organized nanotube films on solid substrates where the main driving force for building multilayer structures by the layer-by-layer (LbL) self-assembly technique is ionic electrostatic interactions. The formation of a self-assembled submonolayer of carbon nanotubes on gold or another metal surface and the deposition of gold or platinum nanoparticles or atoms onto nanotube walls requires nanotube thiolation for providing chemisorption via the free thiol groups.10,23 For this purpose, the covalent binding of thiol group-containing molecules to carbon nanotubes has been mainly proposed.24 The noncovalent approach has been much less explored.25 A vast majority of both covalent and noncovalent thiolation modifications of carbon nanotubes have been performed in organic media. Room-temperature ionic liquids, and, in particular, ionic liquids (ILs) based on the alkyl-substituted imidazolium cation, are amphiphilic molecules and may be considered to be cationic surfactants.26 In nanotube chemistry, ILs for the first time were used for the synthesis of a gel of polymerizable ILs and SWNTs,27 as an electrolyte for electrochemical applications of SWNTs,28 (19) Ikeda, A.; Hamano, T.; Hayashi, K.; Kikuchi, J.-I. Org. Lett. 2006, 8, 1153. (20) (a) Dieckmann, G. R.; Dalton, A. B.; Johnson, P. A.; Razal, J.; Chen, J.; Giordano, G. M.; Munoz, E.; Musselman, I. H.; Baughman, R. H.; Draper, R. K. J. Am. Chem. Soc. 2003, 123, 1770. (b) Dalton, A. B.; Ortiz-Acevedo, A.; Zorbas, V.; Brunner, E.; Simpson, W. M.; Collins, S.; Razal, J. M.; Yoshida, M. M.; Baughman, R. H.; Draper, R. K.; Musselman, I. H.; Jose-Yacaman, M.; Dieckmann, G. R. AdV. Funct. Mater. 2004, 14, 1147. (c) Ortiz-Acevedo, A.; Xie, H.; Zorbas, V.; Sampson, W. M.;Dalton, A. B.; Baughman, R. H.; Draper, R. K.; Musselman, I. H.; Dieckmann, G. R. J. Am. Chem. Soc. 2005, 127, 9512. (d) Zorbas, V.; Ortiz-Acevedo, A.; Dalton, A. B.; Yoshida, M. M.; Dieckmann, G. R.; Draper, R. K.; Baughman, R. H.; Jose-Yacaman, M.; Musselman, I. H. J. Am. Chem. Soc. 2004, 126, 7222. (e) Zorbas, V.; Smith, A. L.; Xie, H.; Ortiz-Acevedo, A.; Dalton, A. B.; Dieckmann, G. R.; Draper, R. K.; Baughman, R. H.; Musselman, I. H. J. Am. Chem. Soc. 2005, 127, 12323. (21) (a) Collins, P. G.; Bradley, K.; Ishigami, M.; Zetti, A. Science 2000, 287, 1801. (b) Li, C.; Curreli, M.; Lin, H.; Lei, B.; Ishikawa, F. N.; Datar, R.; Cote, R. J.; Thompson, M. E.; Zhou, C. J. Am. Chem Soc. 2005, 127, 12484. (22) Leonard, F.; Tersoff, J. Phys. ReV. Lett. 2000, 84, 4693. (23) Isaacs, L.; Chin, D. N.; Bowden, N.; Xia, Y.; Whitesides, G. M. In Supramolecular Materials and Technologies; Reinhoudt, D. N., Ed.; Wiley & Sons: New York, 1999; p 14. (24) (a) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J. Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (b) Liu, Z.; Shen, Z.; Zhu, T.; Hou, S.; Ying, L. Langmuir 2000, 16, 3569. (c) Hu, J.; Shi, J.; Li, S.; Qin, Y.; Guo, Z.-X.; Song, Y.; Zhu, D. Chem. Phys. Lett. 2005, 401, 352. (d) Zhang, L.; Zhang, J.; Schmandt, N.; Cratty, J.; Khabashesku, V. N.; Kelly, K. F.; Barron, A. R. Chem. Commun.(Cambridge, U.K.) 2005, 43, 5429. (e) Smorodin, T.; Beierlein, U.; Ebbecke, J.; Wixforth, A. Small 2005, 1, 1188. (f) Profumo, A.; Fagnoni, M.; Merli, D.; Quartarone, E.; Protti, S.; Dondi, D.; Albini, A. Anal. Chem. 2006, 78, 4194. (g) Curran, S. A.; Cech, J.; Zhang, D.; Dewald, J. L.; Avadhanula, A.; Kandadai, M.; Roth, S. J. Mater. Res. 2006, 21, 1012. (25) (a) Liu, L.; Wang, T.; Li, J.; Guo, Z.-X.; Dai, L.; Zhang, D.; Zhu, D. Chem. Phys. Lett. 2003, 367, 747. (b) Han, L.; Wu, W.; Kirk, L.; Luo, J.; Maye, M. M.: Kariuki, N. N.; Lin, Y.; Wang, C.; Zhong, C.-J. Langmuir 2004, 20, 6019. (c) Baskaran, D.; Mays, J. W.; Bratcher, M. S. Chem. Mater. 2005, 17, 3389. (d) Tzitzios, V.; Georgakilas, V.; Oikonomou, E.; Karakassides, M.; Petridis, D. Carbon 2006, 44, 848. (26) Structure-Performance Relationships in Surfactants, 2nd ed. Esumi, K., Ueno, M., Eds.; Marcel Dekker: New York, 2003. (27) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072. (28) Barisci, J. N.; Wallace, G. G.; MacFarlane, D. R.; Baughman, R. H. Electrochem. Commun. 2004, 6, 22.

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in the electrochemical functionalization of SWNTs with Nsuccinimidyl acrylate,29 as a medium in the covalent modification of SWNTs,30 and for a preliminary dispersion of multiwalled carbon nanotubes for further melt-blending with polystyrene.31 ILs containing hydrophilic imidazolium cations and hydrophobic PF6-, CF3SO3, and [CF3SO2]2N- anions generally do not mix with water, but imidazolium-based ILs with hydrophilic halide anions are water-soluble.32 Recently we obtained33 water-soluble negatively and positively charged SWNTs by noncovalent modification of the nanotube sidewalls with several anionic and cationic pyrene and naphthalene derivatives and used them34 for the layer-by-layer (LbL) electrostatic deposition of polyelectrolyte multilayers. In this article, we demonstrate a novel approach to the noncovalent functionalization of pristine SWNTs for the preparation of highly stable aqueous dispersions of positively charged nanotube composites including thiolated nanotubes and the self-assembly of prepared composites on gold. In short, the as-produced SWNTs were briefly sonicated in aqueous solutions of two ILss1-alkyl3-methylimidazolium bromides consisting of 1-(12-(dodecyl)3-methylimidazolium bromide (1) and 1-(12-(mercaptododecyl)3-methylimidazolium bromide (2) (Chart 1)swith further highspeed centrifugation. The supernatants contained a high concentration of individually dispersed positively charged nanotubes. The term “pristine” relates here to as-produced, not purified, SWNTs whose physical and electronic properties have not been disturbed upon chemical purification and/or covalent modification. The incorporation of thiol group in surfactant 2 enables direct self-assembly of the SWNT-2 composite on gold. The high efficiency of the 1-alkyl-3-methylimidazolium cations as dispersing agents is due to their amphiphilic nature. As with common ionic surfactants and polymers, the hydrophobic long alkyl chains of the IL cations adsorb on the nanotube surface via van der Waals and CH-π25c interactions, minimizing the interfacial energy of the nanotube-water interface,2,35 likely following the nanotube graphene structure in the groove sites between adjacent nanotubes in bundles.36 The hydrophilic cationic imidazolium groups are oriented toward the aqueous phase, providing water solubility. Thus, nanotubes become positively charged and can be used for LbL deposition with polyelectrolytes and for biological or sensor applications. In the aqueous phase of the SWNT-2 composite, thiol groups are exposed to the exterior, providing direct chemisorption on gold via S-Au bonding. Importantly, the proposed approach successfully solves some basic challenges in nanotube chemistry: the formation of a stable aqueous dispersion of positively charged nanotubes directly from asproduced nanotube material with the simultaneous purification and preservation of the nanotube electronic structure and direct self-assembly of individualized nanotubes on the metal substrate. To the best of our knowledge, this is the first report on the direct self-assembly of positively charged pristine SWNTs on gold from aqueous dispersions and the characterization of their self-assembled submonolayers by surface-sensitive spectroscopic (29) Zhang, Y.; Shen, Y.; Li, J.; Niu, Li, Dong, S.; Ivaska, A. Langmuir 2005, 21, 4797. (30) Price, B. K.; Hudson, J. L.; Tour, J. M. J. Am. Chem. Soc. 2005, 127, 14867. (31) Bellayer, S.; Gilman, J. W.; Eidelman, N.; Bourbigot, S.; Flambard, X.; Fox, D. M.; De Long, H. C.; Trulove, P. C. AdV. Funct. Mater. 2005, 15, 910 (32) Ple´net, J. S.; Gaillon, L.; Letellier, P. Talanta 2004, 63, 979. (33) Paloniemi, H.; A ¨ a¨ritalo, T.; Liuke, H.; Kocharova, N.; Haapakka, K.; Terzi, F.; Seeber, R.; Lukkari, J. J. Phys. Chem. B 2005, 109, 8634. (34) Paloniemi, H.; Lukkarinen, M.; A ¨ a¨ritalo, T.; Areva, S.; Leiro, J.; Heinonen, M.; Haapakka, K.; Lukkari, J. Langmuir 2006, 22, 74. (35) Kang, Y. J.; Taton, T. A. J. Am. Chem. Soc. 2003, 125, 5650. (36) Kondratyuk, P.; Wang, Y.; Johnson, J. K.; Yates, J. T., Jr. J. Phys. Chem B 2005, 109, 20999.

Dispersion, Thiolation, and Self-Assembly of SWNTs

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Chart 1. Schematic Illustration of Noncovalent Functionalization, Thiolation, and Self-Assembly of SWNTs with 1-Alkyl-3-methylimidazolium Bromides

techniques. The availability of water-soluble and positively charged SWNTs, which is very limited, is vital for the layerby-layer self-assembly of SWNTs and for several biological and biomedical applications that involve interactions with negatively charged living cells.6,16 We also point out that the simple and versatile scheme of aqueous dispersion, thiolation, and selfassembly of SWNTs on gold, shown in this work, can be useful for the preparation of thin films on gold substrates with the use of other types of nanotubes and carbon nanostructures. Experimental Section Materials. Raw SWNTs made by high-pressure catalytic decomposition of carbon monoxide (HiPco) were purchased from Carbon Technologies, Inc. (Houston, TX; lot no. PO184) and were used as received. 1-(12-Mercaptododecyl)-3-methylimidazolium bromide (2) was synthesized according to Lee et al.37 The synthesis of 1-dodecyl-3-methylimidazolium bromide (1) was performed in the following way. Dodecylbromide (10 g, 20 mmol) and Nmethylimidazole (6.6 g, 80 mmol) were dissolved in acetonitrile (100 mL), and the mixture was refluxed for 20 h. The mixture was evaporated to dryness and washed with diethyl ether (300 mL). 1-Dodecyl-3-methylimidazolium bromide was purified by dissolving the solid residue in acetonitrile (50 mL) and adding a mixture of 4:1 (v/v) diethylether and petroleum ether. The product was crystallized at -16 °C and filtered. 1H NMR (DMSO): 9.17 (1H, s), 7.80 (1H, s), 7.73 (1H, s), 4.16 (2H, t), 3.86 (3H, s), 1.78 (2H, m), 1.24 (18H, m), 0.86 (3H, t). The procedure for the point of zero charge (PZC) determination of as-produced SWNTs is available in the Supporting Information. Modification of Pristine SWNTs. The successful SWNT dispersion in water included ultrasonic sonication of 0.73 mg/mL pristine SWNT in 26 × 95 mm vials in a low-energy ultrasonic bath sonicator (Bandelin Sonorex TK52) for 10 min. After adding IL 1 or 2 with a final concentration of 15 or 14 mM, respectively, the SWNT suspensions were further sonicated for 1 h and left overnight for stabilization and sedimentation of undispersed SWNTs and carbonaceous impurities. During all sonications, the vials with SWNT dispersions were immersed in an ice water sonicator bath to prevent heating. The next morning, the upper part of the dispersions was carefully decanted and centrifuged (Biofuge Stratos, Heraeus Instruments) at 50 000g for 1 h to separate nanotube bundles, carbon aggregates, and insoluble material from dispersed SWNTs. In each (37) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S.-g. J. Am. Chem. Soc. 2004, 126, 480.

run, the upper 70% of the transparent dark-gray supernatant was carefully decanted for characterization. Self-Assembly of SWNT-1,2 Composites on Gold. Preparation of Gold-Plated and Colloidal Gold Substrates. The procedure of the gold substrates preparation for the self-assembly of SWNT-1,2 composites is available in Supporting Information. Self-Assembly of SWNT-1,2 on Gold. The gold substrates were directly used for the adsorption of SWNT-2. For the adsorption of SWNT-1, gold substrates were initially negatively primed with a mercaptoethanesulfonic acid (MESA) monolayer by immersing them in aqueous 1 mM MESA solution. For the self-assembly of SWNT1,2, the prepared substrates were immersed overnight in SWNT-1,2 aqueous dispersions (concentrations of ∼0.2 mg/mL SWNT and deaerated with Ar) in tightly closed vessels. The substrates were then thoroughly rinsed with running deionized water and dried with nitrogen gas. Characterization. The solubility of the noncovalently modified nanotubes was estimated by UV-vis-NIR spectroscopy using the extinction coefficient reported earlier for pristine HiPco SWNTs (500 ) 28.6 L g-1 cm-1 at 500 nm).38 The UV-vis-NIR spectra of the SWNT-1,2 aqueous dispersions were recordered with a Varian Cary SE UV-vis-NIR spectrophotometer using quartz cuvettes with a path length of 1 mm. Raman spectra were obtained with 1:100 diluted (with water) SWNT-1,2 stock solutions. In the case of SWNT-2, the stock solution was also diluted 1:100 with 0.75 M NaBH4 for chemical reduction of the created disulfides (Results and Discussion section). Raman and SERRS spectra were recordered using the Nicolet Nexus 870 FT-Raman spectrometer equipped with an Nd:YAG laser (laser excitation at 1064 nm or 1.165 eV) and a liquid N2-cooled Ge detector. To avoid damaging the nanotubes upon laser irradiation, RR and SERRS spectra were collected with the lowest possible laser power and with the 10-fold optical density filter. Spectra were averaged for 2048 and 1068 scans for RR and SERRS spectra collection, respectively, and with a spectral resolution of 4 cm-1. Raman spectral deconvolution was started with the set of pure Lorentzian peaks, initially selected and fixed in number, frequency, and fwhm, and continued with further optimization of the number of lines, their frequencies, fwhm values, and background.39 The natural line width (fwhm) for isolated SWNTs in solution was recently reported to be 5 cm-1; 40 therefore, the fwhm was initially (38) Bahr, J.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 193. (39) Meier, R. J. Vib. Spectrosc. 2005, 39, 266. (40) Jorio, A.; Santos, A. P.; Ribeiro, H. B.; Fantini, C.; Souza, M.; Vieira, J. P. M.; Furtado, C. A.; Jiang, J.; Saito, R.; Balzano, L.; Resasco, D. E.; Pimenta, M. A. Phys. ReV. B 2005, 72, 075207.

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chosen to be 5 cm-1 for all RBM lines. The fitting procedure gave optimized fwhm values for RBMs in the range of 4-6 cm-1. The accuracy of the obtained frequencies in this work (the coefficient of determination) was g0.999. For XPS and SERRS characterization, the prepared gold-plated and gold-colloid substrates (Supporting Information) were directly used for the self-assembly of SWNT-2. The gold substrates were incubated overnight in a highly diluted SWNT-2 aqueous dispersion (∼0.2 mg/mL SWNT) under an argon atmosphere. The substrates were then rinsed and dried as described above. The XPS spectra of SWNT-2 were recordered with a PerkinElmer PHI 5400 spectrometer equipped with a concentric hemispherical electron analyzer using a monochromatic Al KR X-ray source (hν ) 1486.6 eV). XPS spectra were measured with a takeoff angle 45° and an analyzer pass energy of 35.75 or 89.45 eV (for narrow-scan spectra and wide-scan spectra, respectively). The binding-energy scale was calibrated with the Au 4f7/2 peak position at 84.00 eV. XPS peaks have been fitted using UNIFITWI software.

Results and Discussion Dispersion of SWNTs in Water with the Ionic LiquidBased Surfactants. To enhance the adsorption of ionic surfactants on the SWNT surface without possible oxidation of nanotubes, it is important to control the surface charge of the nanotubes by the pH of the medium.12 The point of zero charge (PZC) of solid SWNTs is the pH at which the nanotube surface has a net zero charge. At pH values below the PZC, the nanotube surface becomes protonated and positively charged and strong adsorption of anionic surfactants is expected, whereas above the PZC a strong adsorption of cationic surfactants can be achieved. The procedure for the determination of the PZC is available in Supporting Information. In this work, the PZC of pristine HiPco SWNTs was found to be 6.2. The suspension of SWNTs in water with 1 and 2 was successful at pristine pH 8 of SWNT-1,2 that is well above the observed PZC. The aqueous dispersions of SWNT-1,2 are transparent and black, without any visible particulates or precipitation for almost a year, implying that they consist mainly of individual nanotubes. We assume that the SWNT-1,2 stock solutions are saturated because solid SWNT material was present at all preparation stages (after sonication and centrifugation). Characterization of SWNT-1,2 Dispersions. Optical Absorption Spectra. To verify the noncovalent nature of the SWNT functionalization and the ability of the surfactants to encapsulate and isolate individual nanotubes, we characterized the SWNT1,2 dispersions by UV-vis-NIR spectroscopy. Figure 1 shows the absorption spectra of the SWNT-1,2 supernatant aqueous dispersions. To account for the difference in nanotube concentration, spectra were normalized at 925 nm, a wavelength outside the main nanotube absorption bands. It is important to note the difference between true solubilization of individual nanotubes and the dispersion of small bundles of tubes. The dissolution of SWNTs in ionic surfactants, to which ILs 1 and 2 belong, has been well studied. There is common agreement that upon separation by ultrasonication, SWNTs become encapsulated in surfactant micelles and repulsive forces between similarly charged surfactant molecules prevent further nanotube aggregation and precipitation.13,16 O’Connel et al. first demonstrated that although after sonication step small nanotube bundles can also be encapsulated by surfactant molecules the following ultracentrifugation step effectively removes the bundles, carbonaceous impurities, and residual catalyst, leaving mainly individually dissolved nanotubes in the supernatant as evidenced by highly resolved optical absorption bands.2 Although the absorption spectra of dispersed SWNTs are a superposition of distinct electronic transitions from a variety of (n, m) tubes, the spectra of well-separated SWNTs from bundles are expected to

Figure 1. UV-vis-NIR spectra of supernatant fractions collected after centrifugation of pristine SWNTs ultrasonicated in aqueous solutions of surfactants: (a) 1 and (b) 2. Spectra have been normalized at 925 nm and the SWNT-1 spectrum has been shifted down for clarity. EMii and ESii denote the interband transitions between the ith van Hove singularities in metallic and semiconducting HiPco SWNTs, respectively.

be dominated by relatively sharp van Hove singularity (vHs) bands. The spectra of SWNT-1,2 shown in Figure 1 are consistent with this expectation, showing the first ES11 and the second ES22 vHs transitions of semiconducting SWNTs in the ranges from 873 to ∼1300 nm and from 556 to 860 nm, respectively, which are characteristic of HiPco semiconducting SWNTs.41 The lowestenergy vHs transitions of metallic nanotubes, situated between ∼400 and 600 nm, are also well resolved in the spectra of SWNT1,2. The sharp vHs absorption peaks suggest a high degree of individually dispersed SWNTs in the surfactant solutions.42 In addition, the retention of vHs features ES11, ES22, and EM11 after the nanotube sidewall functionalization with surfactants 1 and 2 is a signature of the noncovalent nature of the modification.33,43 It is important to determine whether the noncovalently modified SWNT-1,2 nanotubes remain unperturbed and individually suspended in their dispersions/solutions. There are several effects that are known to bleach the optical absorbance of unperturbed individual nanotubes and affect the radial breathing modes (RBMs) region in the resonance Raman spectra. They include the aggregation (bundling) effect,44-46 electron-transfer reactions,47 and, in particular, the acidification of the SWNT suspension/solution.48 In nanotube bundles, intertube interactions lead to a decrease in the energy separation between the valence and conduction band states with concomitant broadening of the transition bandwidths,44 which leads to red shifts and the broadening of absorption features in the absorption spectra.45 A comparison of the best resolved vHs band positions in our SWNT1,2 suspensions with known experimental data for thin semi(41) Weisman, B. B.; Bachilo, S. M. Nano Lett. 2003, 3, 1235. (42) (a) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2003, 10, 1379. (b) Paredes, J. I.; Burghard, M. Langmuir 2004, 20, 5149. (43) (a) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (b) Dyke, C. A.; Tour, J. M. Chem.sEur. J. 2004, 10, 81230. (44) Reich, S.; Thomsen, C, Ordejon, P. Phys. ReV. B 2002, 65, 155411. (45) O’Connel, M. J.; Sivaram, S.; Doorn, S. K. Phys. ReV. B 2004, 69, 235415. (46) Fantini, C.; Jorio, A.; Souza, M.; Strano, M. S.; Dresselhaus, M. S.; Pimenta, M. S. Phys. ReV. Lett. 2004, 93, 147406. (47) Zheng, M.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 15490. (48) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connel, M. J.; Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh, S.; Hauge, R. H.; Smalley, R. E. J. Phys. Chem B 2003, 107, 6979.

Dispersion, Thiolation, and Self-Assembly of SWNTs

conducting41,49 and metallic49 HiPco SWNTs shows practically no red shifts, an indication that the SWNT-1,2 suspensions mainly consist of individually dispersed nanotubes. Broadening and disappearance of the vHs transitions of the semiconducting and metallic nanotubes may be also caused by charge-transfer processes, which are chirality-sensitive.47,50 Acidification (up to pH 5) of the surfactant-dispersed SWNTs solution in the presence of preadsorbed oxygen can lead to nanotube sidewall protonation, which tends to localize the nanotube valence electrons.48 Because the sidewall protonation of the nanotube side walls is band gap-sensitive,47,50 acidification leads to selective decreases in various EM11 and ES11 features.2 Metallic nanotubes are the most reactive toward H+, and the smallest-diameter semiconducting tubes are the least reactive.48 In our case, the high pH 8 of the aqueous media of SWNT-1,2 and minimum sonication times (∼1 h), kept for dispersions of SWNTs, practically rule out any possible acidification effect, which is evidenced by the retention of highly resolved metallic EM11 and semiconducting ES11 vHSs transitions. Therefore, the absorption spectra of SWNT-1,2, imply that the electronic properties of SWNTs have not been disturbed upon nanotube sidewall encapsulation with surfactants 1 and 2. On the basis of the optical absorption measurements, the nanotube water solubility was estimated to be 0.68 g/L (ca. 80% of individual SWNT) and 0.17 g/L (ca 25%) for SWNT-1 and SWNT-2, respectively. These values were obtained with short sonication and centrifugation times. Perhaps longer processing times or multiple sonication and centrifugation cycles may improve these values; however, one of our goals was to simplify and shorten the dispersion process as much as possible. The water solubility with surfactant 1 is much higher than that reported earlier with the best surfactant known for SWNT dispersion, SDDBS, which is up to ∼63 or 62.4% of the individual SWNT yield11,12 and the SWNT solubility in organic media with pyrenefunctionalized block copolymers (0.057 g/L).51 However, the obtained water solubility is lower than the recently reported remarkable value of 3.5 g/L reached by the noncovalent modification of purified SWNTs with diazo dye Congo red using a physical grinding treatment.52 We note also that the water solubility obtained with surfactant 1 is more than twice the highest one achieved in our previous work33 using naphthalene derivatives (0.30 g/L). However, those values were obtained after dialysis and are not directly comparable with the solubility values in the present work. The lower solubility with surfactant 2 can be explained by cross-linking of the thiol end groups, thus forming disulfide bonds20 that tend to bind the modified nanotubes together in small aggregates. Raman Characterization of SWNT-1,2. For further characterization of SWNT-1,2 aqueous dispersions and to probe the extent of intertube aggregation, we used Raman spectroscopy. Although HiPco semiconducting SWNTs (s-SWNTs) have diameter distributions between ∼0.7 and ∼1.2 nm53 and can contain up to 36 (n, m) structures,54 our Raman analysis was restricted to thin semiconducting SWNTs owing to instrumental limitations. However, we were able to detect and assign the 14 (49) (a) Telg, H.; Maultzsch, J.; Reich, S.; Hennrich, F.; Thomsen, C. Phys. ReV. Lett. 2004, 93, 177401. (b) Fantini, C.; Jorio, A.; Souza, M.; Strano, M. S.; Dresselhaus, M. S.; Pimenta, M. A. Phys. ReV. Lett. 2004, 93, 147406. (50) Okazaki, K.; Nakato, Y.; Murakoshi, K. Phys. ReV. B 2003, 68, 035434. (51) Bahun, C. J.; Wang, C.; Adronov, A. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1941. (52) Hu, C.; Chen, Z.; Shen, A.; Shen, X.; Li, J.; Hu, S. Carbon 2006, 44, 428. (53) Kavan, L.; Kalbac, M.; Zukalova, M.; Dunsch, L. J. Phys. Chem. B 2005, 109, 19613. (54) Luo, Z.; Pfefferie, L.; Haller, G. L.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2006, 128, 15511.

Langmuir, Vol. 23, No. 6, 2007 3367

Figure 2. Pristine RR spectra of the stock supernatant SWNT-2 aqueous dispersion diluted 1:100 with (a) water and (b) 0.75 M NaBH4. Spectrum 2b is shown with account of the RR spectrum of the 0.75 M solution of NaBH4. The inset shows the solid-phase RR spectrum of IL surfactant 2.

thinnest HiPco semiconducting nanotubes with diameters in the range of 0.69-1.0 nm in the SWNT-2 dispersion. In this work, major emphasis was given to nanotube composite SWNT-2 because of the presence of the thiol group that can enable the chemisorption of nanotubes on the gold surface. Thiols and disulfides are known to adsorb directly on noble metals and, in particular, on gold surfaces through Au-S bonds. However, the lower water solubility of SWNT-2 implied the oxidation of some thiol groups of surfactant 2 and the formation of S-S bonds. Methods for the chemical reduction of disulfides involve the reduction of disulfide bonds by thiols, which are the most commonly used reagents for this purpose. The disadvantage of these methods is the necessity of removing the excess thiol used as a reductant before it is possible to assay the newly generated -SH groups. Therefore, in this work, for the chemical reduction of created in stock aqueous dispersions of SWNT-2 disulfides, we have used the most convenient reagent for this purpose, sodium borohydride.55 Figure 2 shows the RR spectra of the SWNT-2 dispersion diluted with water and with a strong reductant, sodium borohydride. The RR spectra of SWNT-2 in Figure 2a,b contain all of the Raman features that are characteristic of SWNTs, namely, the first-order RR scattering A1g-symmetry radial breathing modes (RBMs) and (A1g + E1g + E2g)-symmetry G band of the axial C-C stretch in graphene at ∼200-300 and at ∼1590 cm-1, respectively, the double-resonance Raman modes, the disorderinduced D band in the vicinity of 1280 cm-1, the overtone G′ bands of the D band, and overtone M bands of infrared-active SWNT modes in the vicinities of 2550 and ∼1750 cm-1, respectively.56 In addition to these well-studied Raman features, the RR spectra of SWNT-1,2 also contain so-called intermediate frequency modes (IFMs) situated between 600 and 1100 cm-1.57 The spectrum of SWNT-2 in water (Figure 2a) also displays an additional wide Raman band attributed to S-S stretching vibrations in the range of 462-555 cm-1, which indicates that (55) Jocelyn, P. C. Methods Enzymol. 1987, 134, 246. (56) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005, 409, 47. (57) (a) Fantini, C.; Jorio, A.; Souza, M.; Ladeira, L. O.; Souza Filho, A. G.; Samsonidze, G. G.; Dresselhaus, G.; Dresselhaus, M. S.; Pimenta, M. A. Phys. ReV. Lett. 2004, 93, 087401. (b) Fantini, C.; Jorio, A.; Souza, M.; Saito, R.; Samsonidze, Ge.; Dresselhaus, M. S.; Pimenta, M. A. Phys. ReV. B 2005, 72, 085446.

3368 Langmuir, Vol. 23, No. 6, 2007

Figure 3. Resonance Raman spectra of (a) SWNT-1 and (b) SWNT-2 diluted 1:100 with (a) water and (b) 0.75 M NaBH4. The inset shows the D-mode spectral region indicated by a star. Raw spectra are background subtracted to remove Rayleigh scattering and normalized to the maximum of the G+ mode at 1590.0 cm-1. The marked wavenumber values correspond to peaks attributed to the imidazolium cation of 2.

part of the thiol groups have been oxidized and have formed disulfide linkages between the tubes.58 The addition of sodium borohydride in the dispersion of SWNT-2 reduced the S-S bonds, which is evidenced by the disappearance of the wide S-S stretching (Figure 2b). The RR spectra obtained for the SWNT-1,2 dispersions are shown in Figure 3. Although the Raman modes of surfactants 1 and 2 do not show resonance enhancement, their weak Raman bands can also be distinguished in the spectra. The Raman bands of the 1-alkyl-3-methylimidazolium cation of 2, indicated by arrows in the inset of Figure 3 at 1230 and 1253 cm-1, result from the in-plane asymmetric stretching of the imidazolium ring59 and the combination vibration of imidazolium ring stretching with the alkyl -CH2 bend, respectively. The band at 1325 cm-1 corresponds to the alkyl in-plane -CH2 bend, and the feature at 1114 cm-1 is the combination band of the imidazolium ring HCCH symmetric bend with the ring in-plane asymmetric stretch.59 The bands at 725-746 cm-1, corresponding to the asymmetric C-S stretch58b in the RR spectrum in Figure 3b, confirm the functionalization of s-SWNT with the thiol groupcontaining imidazolium cation of 2. In the RR spectra of SWNT1,2 dispersions, the maximum of SWNT G band was observed at the same wavenumber (1590.9 cm-1), which is characteristic of indiVidual pristine SWNTs.56 Very small intensities of the disorder-induced D band in the RR spectra of SWNT-1,2 verify the dispersion of SWNTs on the individual nanotube level.60 The parameter (1-D/G), where D and G are intensities of the D- and G-modes, respectively, can be used as a semiquantitative measure12 of the nanotube sample quality. For SWNT-1 and SWNT-2, this parameter was 0.945 and 0.95, respectively, indicating negligible concentrations of carbonaceous impurities and imperfections in the graphene lattice of SWNTs in these samples, thus implying efficient purification of the pristine nanotube material from carbonaceous particles with surfactants (58) (a) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 2000, 104, 62186224. (b) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley: New York, 1974. (59) Talaty, E.; Raja, S.; Storhaug, V. J.; Do¨lle, A.; Carper, W. R. J. Phys. Chem. B 2004, 108, 13177. (60) Anglaret, E.;Dragin, F.; Pe´nicaud, A.; Martel, R. J. Phys. Chem B 2006, 110, 3949.

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Figure 4. Lorentzian peak fit of the RBM spectral envelope in the RR spectra of the SWNT-2 dispersion revealing 14 (n,m) chirality types (in parenthesis) of s-SWNTs, which are in resonance with the 1064 nm laser excitation. Experimental and resolved spectra are shown in blue and in red, respectively

1 and 2. This result also indicates noncovalent functionalization with no damage done to the nanotube wall structure and, therefore, their electronic properties because in covalently modified SWNTs the D-mode intensity is comparable to the G-band intensity.7c, 61

Evaluation of the SWNT Population in SWNT-2. Morphological changes in the bundling of SWNTs can cause the relative peak intensity changes in the Raman spectra62 and frequency shifts for RBMs that have been attributed to a red shift of the resonant electronic transitions for bundled nanotube.45 In addition, charge-transfer reactions lead to a large-scale blue shifting and attenuation of the RBM intensities. Depending on the accuracy of fulfilling resonance conditions, different (n, m) structures exhibit different sensitivities to doping.53 Therefore, in an effort to more accurately clarify the SWNT individualization and the noncovalent nature of the functionalization and also to gain information about the chirality and diameter distribution of individualized thin s-SWNTs, we have deconvoluted the spectral envelope of diameter-dependent RBMs in the Raman spectrum of SWNT-2 (Figure 3b). Our Raman excitation (1.165 eV or 1064 nm) is in resonance with the ES11 vHs transitions of exclusively semiconducting nanotubes. We have deconvoluted the SWNT-2 RBM spectral range with a carefully selected set of RBM frequencies for 14 HiPco s-SWNTs using the empirically based “Kataura plot” 41,49a that gives reliable optical transition frequencies versus chiral angle and/or diameter. On choosing these (n, m) chirality types, we have considered only those tubes in the HiPco diameter range for which their first vHs absorption ES11 bands are within a window of (0.1 eV63 around the excitation energy. The deconvolution results and RBM assignment are presented in Figure 4 and Table 1. One can see that for the vast majority of SWNT-2 RBMs the obtained frequency values are practically identical, with deviations of less than 1%, with the earlier reported experimental data on RBMs for 12 individual HiPco s-SWNTs dispersed in SDS49b and DNA.18b Additionally, Telg et al.49a also (61) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6356. (62) (a) Heller, D. A.; Barone, P. W.; Swanson, J. P.; Mayrhover, R. M.; Strano, M. S. J. Phys. Chem. B 2004, 108, 6905. (b) Baik, S.; Usrey, M.; Rotkina, L.; Strano, M. J. Phys. Chem. B 2004, 108, 15560. (63) Ku¨rti, J.; Zo´lyomi, V.; Gru¨neis, A.; Kuzmany, H. Phys. ReV. B 2002, 65, 165433.

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Table 1. Deconvoluted RBM Frequencies for s-SWNTs Dispersed in 1-(12-Mercaptododecyl)-3-methylimidazolium Bromide Solution and having Resonance with Laser Excitation at 1064 nm (1.165 eV) (n, m)

dta

fwhm (cm-1) this work

ωRBM (cm-1) this work

ωRBM49(b)

(n, m)

dta

fwhm (cm-1) this work

ωRBM this work

ωRBM49b

(6, 4) (7, 3) (7, 5) (7, 6) (8, 3) (8, 4) (8, 6)

0.692 0.706 0.829 0.895 0.782 0.840 0.966

4.6 6 6 4 4 6 4.6

337.5 330.9 284.0 261.3 297.2 278.0 246.5

337.5 329.2b 282.3 263.0 297.9 280.0 245.0

(9, 1) (9, 2) (9, 4) (10, 2) (10, 3) (11, 0) (12, 1)

0.757 0.806 0.916 0.884 0.936 0.873 0.995

5.2 6 4 5 4.6 6 4

309.7 289.9 257.5 265.6 250.0 268.7 236.9

307.0 290.0 257.0 265.0 252.7 266.7c 236.9

a SWNT diameter values in nanometers. b Predicted41 RBM frequency for the (7, 3) s-SWNT. c Experimental49a RBM frequency for the (11, 0) s-SWNT.

reported an RBM frequency for (11, 0) tubes observed at 266.7 cm-1 for HiPco s-SWNTs dispersed with SDS or SDDBS. Our fitting of the RBM spectral envelope gave us the RBM frequency for this tube at 268.7 cm-1, which is consistent with the predicted (268.4 cm-1) value for individual (11, 0) tubes.41 The small discrepancies between the deconvoluted and earlier reported experimental RBM frequency values of the same (n, m) structures can be related to differences between surfactants used for the SWNT dispersion.41 Interestingly, in addition to earlier reported experimental RBM values for 13 thin HiPco s-SWNTs,49 we found in our HiPco SWNT sample dispersed with 2 and 1 (the RBM peak fit for SWNT-1 is not shown) an additional peak at 330.9 that we believe belongs to the (7, 3) structure. Indeed, (7, 3) tubes have a diameter within the HiPco SWNT diameter range, and the vHs emission ES11 of this tubes lies at 1.250 eV (992 nm),41 which falls into the resonance window around laser excitation used. The (7, 3) RBM frequency has been predicted41 but not yet experimentally observed in the Raman spectra of HiPco s-SWNTs. We have not observed any attenuation or considerable blue shift of the RBMs in the RR spectra of SWNT-2, which also implies the absence of charge-transfer processes and the noncovalent nature of the SWNT functionalization. The strongest RBM intensities of the (11, 0) and (10, 2) tubes are caused by the closeness of their ES11 transitions (1037 nm or 1.196 eV and 1053 nm or 1.177 eV, respectively) to the exciting Elaser. We cannot entirely rule out the presence of some small bundling in SWNT-2. However, the observed similarity in the determined RBM frequencies between SWNT-2 and SWNT-1 (not shown) and their coincidence with previously reported experimental and predicted values imply the correctness of the spectral assignments made, the noncovalent nature of functionalization, and the absence of significant bundling of thin semiconducting tubes in the SWNT-2 dispersion. Characterization of the Self-Assembled SWNT-1,2 Submonolayers. We adsorbed the SWNT-2 composite on evaporated gold and both SWNT-1,2 composites on a gold colloid surface from their highly diluted aqueous dispersions and characterized the obtained SWNT submonolayers with surface-sensitive spectroscopic techniques, X-ray photoelectron spectroscopy, and surface-enhanced resonance Raman scattering spectroscopy. XPS. To confirm the coating of SWNTs with surfactant 2 and the direct chemisorption of the SWNT-2 composite on gold, we performed XPS spectroscopic measurements. Figure 5 displays the band-deconvoluted core-level XPS spectra of N 1s, C 1s, and S 2p emission. The accuracy of the highest binding-energy (BE) values obtained in this work is about (0.2 eV. For convenience, XPS characterization results are summarized in Table 2. N 1s Region. The N 1s core-level spectrum showing binding energies of the nitrogen atoms (N 1s at 400.1 and 402.0 eV) suggests the presence of an imidazole ring in the SWNT-2

Figure 5. Narrow-scan XPS spectra of the (a) N (1s), (b) C (1s), and (c) S (2p) regions of the SWNT-2 SAM on gold. Table 2. Binding Energies (eV) in the XPS Spectra of the SWNT-2 Submonolayer

C 1s

S 2p3/2,1/2 unbounded sulfur

S 2p3/2,1/2 bounded sulfur

S 2p oxidized sulfur

N 1s

Au 4f7/2

284.7 285.3 286.7

163.4 164.6

162.0 163.2

168.9

400.1 402.0

84.0

composite.37,64 The N 1s core-level spectrum of the surfactant 2 SAM on gold is available in Supporting Information. C 1s Region. The best fitting of the C 1s core-level spectrum was obtained by introducing three components, which resulted in BEs at 284.7 (C1), 285.3 (C2), and 286.7 eV (C3). The BE of C1 is consistent with the previously reported C 1s emission of SWNTs65 and is typical for graphite-like C atoms with sp2 hybridization as in HOPG. Here it should be noted that in the (64) Park, M. J.; Lee, J. K.; Lee, B. S.; Lee, Y.-W.; Choi, I. S.; Lee, S-g. Chem. Mater. 2006, 18, 1546. (65) (a) Yudasaka, M.; Kilouchi, R.; Ohki, Y.; Yoshimura, S. Carbon 1997, 35, 195. (b) Wang, Y. Q.; Sherwood, P. M. A. Chem. Mater. 2004, 16, 5427.

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fullerene-based systems the inelastic mean free path of photoelectrons at this kinetic energy is only about 1/3 of the nanotube diameter.66 Therefore, only half of the SWNT carbons, which belong to the top layer of the nanotube submonolayer, are probed with XPS. This partially explains the relatively small intensity of the SWNT C1 peak observed. Two nitrogen atoms in the imidazolium ring of 2, as a result of their high positive charge, have a strong inductive effect on neighboring carbons. Therefore, we attribute the highest BE in the C 1s core spectrum (peak C3) to the carbon atoms bonded to nitrogens in the imidazolium ring. SAMs of alkanethiols67 and imidazolium thiols37 on gold display a C 1s peak at ca. 285 eV; therefore, we assign the C2 peak to carbon atoms in the alkyl chain of 2. We note that the SWNT-2 dispersion contains free molecules of 2, which also adsorbs on the gold surface via their thiolate bonds. This fact can additionally explain higher intensities of the C2 and C3 carbon peaks compared to that of the C1 peak of SWNTs. S 2p Region. To fit the S 2p3/2,1/2 spectrum, we used the standard S 2p spin-orbit doublet in thiols with ∆E ) 1.2 eV between the S p3/2 and S p1/2 components and a fixed intensity ratio of 2:1 (Table 1).68 The S 2p spin-orbit doublet of the unbounded thiol group was observed with maxima at 163.4 and 164.6 eV. Upon chemisorption of thiol on gold, it shifts to BE values of 162 and 163.2 eV, respectively, indicating chemical binding of SWNT-2 nanotubes to the Au surface via the S-Au bond.69 The S 2p spectrum of SWNT-2 SAM shows not only bound thiolates and unbounded thiols but also a smaller S 2p peak of some oxidized sulfur species with the BE at 168.9 eV. This BE can be attributed to several overlapping doublets of oxidized sulfur species, which are usually observed at BEs between 167.8 and 169.01 eV.69(a) We note here that this peak was also observed in the S 2p corelevel spectrum of pure surfactant 2 adsorbed on gold (Supporting Information) and can be attributed to the oxidation of some thiol groups at the monolayer-liquid interface during the self-assembly process. As was shown above, the thiol groups in cations of IL 2 can create disulfide S-S bonds in an aqueous medium of SWNT-2. According to the literature, S-S is present in XPS spectra as a high-intensity band at 163.8 eV.69b The absence of this band in the S 2p core-level spectrum of the SWNT-2 submonolayer, adsorbed on gold from its aqueous dispersion, implies sulfursulfur bond cleavage resulting in two thiolates when disulfide binds on the gold surface69a,70 This cleaving of the S-S bond upon adsorption of SWNT-2 on gold is also expected to be confirmed by SERRS. SERRS. We also used SERRS spectroscopy to acquire spectroscopic signatures of SWNT-2 self-assembly on gold. For this purpose, both SWNT-1 and SWNT-2 have been adsorbed on the gold colloid substrate in the form of self-assembled submonolayers. In case of SWNT-1, the surface was first negatively primed with a monolayer of MESA. In addition to conventional resonance Raman enhancement for SWNTs, there are also two specific enhancement mechanisms characteristic of (66) Goldoni, A.; Sangaletti, L.; Parmigiani, F.; Conelli, G.; Paolucci, G. Phys. ReV. Lett. 2001, 87, 076401. (67) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8. (68) (a) Castner D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (b) Lu, H. B.; Campbell, C. T.; Castner D. G. Langmuir 2000, 16, 1711. (c) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8. (d) Laiho, T.; Leiro, J. A.; Lukkari, J. Appl. Surf. Sci. 2003, 212-213, 525. (69) (a) Vance, A. L.; Willey, T. M.; Nelson, A. J.; van Buuren, T.; Bostedt, C.; Terminello, L. J.; Fox, G. A. Langmuir 2002, 18, 8123. (b) Munro, J. C.; Frank, C. W. Polymer 2003, 44, 6335. (c) Wang, H.; Chen, S.; Li, L.; Jiang, S. Langmuir 2005, 21, 2633. (70) (a) Ulman, A. Chem. ReV. 1996, 96, 1533. (b) Chung, Y. C.; Chiu, Y. H.; Wu, Y. W.; Tao, Y. T. Biomaterials 2005, 26, 2313.

KocharoVa et al.

Figure 6. SERRS spectra of self-assembled submonolayers of (a) SWNT-1 and (b) SWNT-2 on gold. The SWNT-1 SERRS spectrum was corrected for the presence of the primer MESA SAM. The inset shows the RBM and Au-S stretch spectral region.

molecular structures adsorbed on noble metals in SERS that are responsible for the anomalous Raman intensity rise. The electromagnetic (EM) enhancement mechanism is associated with the enhancement of the electromagnetic field of the incident light at the surface under conditions of surface plasmon resonance for free-electron metals Cu, Ag, and Au.71 For the chemical species in closest proximity or bound directly to these metal surfaces, another mechanism, charge transfer (CT) enhancement, based on charge transfer between the metal substrate and adsorbate, has also been proposed.72 The EM enhancement is relatively long-range whereas the “chemical” or CT mechanism is applicable only for molecules adsorbed directly onto or in very close proximity to the metal surface. Figure 6 shows the SERRS spectrum of the SWNT-2 submonolayer on gold. For comparison, the SERRS spectrum of SWNT-1 adsorbed on the gold/MESA surface is also shown. The SWNT-1,2 SERRS spectra contain all of the characteristic features of SWNTs: RBMs in the lower frequency region, the G band at 1592.9 cm-1, and also D, G, and M bands in the vicinity of 1280-1300, 2550, and ∼1750 cm-1, respectively. Additionally, one can also see the IFMs in the 728-1058 cm-1 spectral range, which were also observed in the SWNT-1,2 RR spectra (Figure 3). The origin of the SERRS bands at 474-492 cm-1 and at 561 cm-1, not previously observed in RR spectra of SWNT-1,2 dispersions, remains unclear. The up-shifted position of the G-mode band in SERRS spectra of SWNT-1,2 (1592.9 cm-1), compared with that in the RR spectra (1590.9 cm-1) of the corresponding SWNT-1,2 aqueous dispersions (Figures 2 and 3), can be understood by taking into account the CT enhancement mechanism. Within CT theory, Otto et al.72 proposed the concept of “adsorption sites of lowered affinity level”, termed E(xtra)-sites. These E-sites are present on rough metal surfaces as metal nanoparticles or electrochemically processed electrodes, and they are responsible for the high electromagnetic field enhancement and the vibrational band shift of the adsorbed or chemisorbed species. The increased intensities of the D band in the SERRS spectra of SWNT-1,2 compared with those in their RR spectra (Figures 2 and 3) are most likely a result of some nanotube bending upon adsorption on substrates. (71) Moskovits, M. J. Ram. Spectr. 2005, 36, 485. (72) Otto, A. J. Raman Spectrosc. 1991, 22, 743.

Dispersion, Thiolation, and Self-Assembly of SWNTs

The presence of all RR SWNT features in both SERRS spectra undoubtedly confirms the SWNT-1,2 self-assembly on gold. However, there are some evident differences between the SERRS spectra of SWNT-1 and SWNT-2 submonolayers, which help to illuminate the different nature of the self-assembly of these nanotube composites on gold. First, the encapsulation of nanotubes with 1-alkyl-3-methylimidazolium cations 1 and 2 is confirmed in the SERRS spectra by the alkyl C-H asymmetric stretch at 2916 and 2920 cm-1 for SWNT-1 and SWNT-2, respectively, and by the Raman-active combination band of the imidazolium ring HCCH symmetric bend with the ring in-plane asymmetric stretch observed at 111459 and at 1118 cm-1 for SWNT-1 and SWNT-2, respectively. The differences in the latter frequencies between the SERRS spectra of SWNT-1 and SWNT-2 may be a consequence of different SERRS enhancement mechanisms working for SWNT-1,2 submonolayers that reside at different distances from the gold substrate. Second, the increased Rayleigh background in the SERRS spectrum of SWNT-2 is the signature of a larger number of hydrocarbon groups,73 which is also evidenced by a more intense alkyl C-H asymmetric stretch and a higher intensity of hydrocarbon CH bending bands at 2920 cm-1 and 1329 cm-1, respectively. This is not surprising taking into account that the SWNT-2 submonolayer also contains free surface-bound molecules of 2 that are not adsorbed on the SWNT sidewalls. In the SERRS spectrum of adsorbed SWNT-2 (Figure 6b) deposited from a pure aqueous dispersion, the absence of the S-S stretching vibrational band at 508 cm-1 and the C-S stretching vibrational band at 746 cm-1, previously observed in the RR spectra of SWNT-2 (Figures 2 and 3, respectively), implies the cleavage of disulfide bonds upon SWNT-2 chemisorption on gold and direct binding of SWNT-2 to gold via S-Au bonds. Most importantly, the SERRS spectrum of the SWNT-2 submonolayer shows evidence of direct chemisorption of SWNT-2 on gold via the thiolate group by the presence of a high-intensity shoulder with a maximum at ∼300 cm-1 resulting from the stretching vibration of the Au-S bond.74 This feature is absent in the SERRS spectrum of SWNT-1 because in this case self-assembly has been achieved by electrostatic interactions between the negatively charged MESA film and the positively charged SWNT-1 composite. The surface analysis of SWNT-2 submonolayers with XPS and SERRS reveals that the disulfide S-S bonds formed in an aqueous dispersion of SWNT-2 are cleaved upon adsorption on gold and the SWNT-2 composite binds to the surface via goldthiolate bonds, indicating that the majority of thiol groups of 2 in the SWNT-2 composite are free and exposed to the exterior. Recently, Bottini et al. have suggested that aliphatic thiols may react with the pentagon and other defect sites on the nanotube sidewall.75 However, their conclusion about thiol binding to the nanotube wall is not supported by spectral evidence. We have found no evidence of covalent binding of thiols to the nanotube surface in our work. Because of nearly perfect hexagonal structure and strong sp2 bonding, pristine SWNTs are chemically stable and do not form chemical bonds with most molecules. The very (73) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61, 14095. (74) (a) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 2000, 104, 6318. (b) Hope, G. A.; Woods, R.; Watling, K. Colloids Surf., A 2003, 214, 87. (75) Bottini, M.; Magrini, A.; Rosato, N.; Bergamaschi, A.; Mustelin, T. J. Phys. Chem. B 2006, 110, 13685.

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similar UV-vis-NIR spectra of SWNT-1 and SWNT-2 (Figure 1) and the small D-mode intensities observed in the Raman spectra of both composites (Figure 3) indicate that the interactions between the sidewalls of pristine SWNTs and surfactants 1 and 2 are similar and noncovalent in nature. In addition, the SWNT solubility achieved with surfactant 1 (without the pending thiol group) was even higher than that with surfactant 2. Spectroscopic characterization of the nanotube ordering and orientation on the gold substrate and also an infrared spectroscopic characterization of the SWNT-1,2 submonolayers is in progress and will be reported in a future publication.

Conclusions We have demonstrated that amphiphilic cationic imidazolium derivatives, ionic liquid-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-mercaptododecyl-3-methylimidazolium bromide (2), are highly efficient for noncovalent cationic functionalization, yielding stable aqueous dispersions and thiolation (with 2) of pristine HiPco SWNTs. Long-chain imidazolium derivatives 1 and 2 can be considered to be multifunctional surfactants because they provide not only efficient dispersions of SWNTs in water by the encapsulation of nanotubes but also a high degree of nanotube purification from carbonaceous particles and impart positive charge to dispersed SWNTs. Additionally, the pendent thiol group of surfactant 2 enables direct self-assembly of the SWNT-2 composite on the gold surface from its aqueous dispersion. The UV-vis-NIR and Raman spectra of the SWNT-1,2 composites in aqueous media and the XPS and SERRS spectra of self-assembled SWNT-1,2 films confirm that the sidewall functionalization is noncovalent in nature and does not alter the nanotube electronic structure but provides a high degree of SWNT individualization and thiolation with surfactant 2. Surface analysis of SWNT-2 submonolayers with XPS and SERRS shows that the disulfide S-S bonds formed in an aqueous dispersion of SWNT-2 are cleaved upon adsorption on gold and the SWNT-2 composite binds to the surface via the S-Au bond. The imparted positive surface charge of modified SWNTs is very important for the preparation of the layer-bylayer self-assembled nanotube multilayers and for applications in cell biology and sensor technology. The reported approach to preparing water-soluble, positively charged self-assembling pristine SWNTs with the long-chain 1-mercaptoalkyl-3-methylimidazolium-based surfactant can also be translated to other carbon nanotubes and carbon nanostructures. We believe that this work will significantly advance the incorporation of carbon nanotubes into electronically and biologically related nanostructures. Acknowledgment. Financial support from the Academy of Finland (grant no. 106215) is gratefully acknowledged. Supporting Information Available: Determination procedure of the SWNT point of zero charge, preparation procedure of gold substrates for adsorption of SWNT-1,2 composites, vis-NIR spectra of the gold colloid nanoparticles in aqueous solution and of the gold colloid monolayer adsorbed on a (3-aminopropyl)triethoxysilane-coated glass substrate, Raman spectrum of solid-state 1-(12-mercaptododecyl)-3methylimidazolium bromide (IL surfactant 2), N 1s and S 2p core-level spectra of the IL surfactant 2 SAM on gold, XPS survey spectrum, and O 1s core-level spectrum of SWNT-2 adsorbed on gold. This material is available free of charge via the Internet at http://pubs.acs.org. LA0631522