Chiral Ionic Liquid Monolayer-Stabilized Gold Nanoparticles

May 25, 2010 - Chiral ionic liquid monolayer-stabilized gold nanoparticles were synthesized in a two-phase liquid−liquid system and found to self-as...
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Chiral Ionic Liquid Monolayer-Stabilized Gold Nanoparticles: Synthesis, Self-Assembly, and Application to SERS Xiangtao Bai, Xinwei Li, and Liqiang Zheng* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Received March 17, 2010. Revised Manuscript Received May 15, 2010 Chiral ionic liquid monolayer-stabilized gold nanoparticles were synthesized in a two-phase liquid-liquid system and found to self-assemble into ringlike structures at the air/water interface. Control experiments with long-chain ILs revealed that the molecular structure of the CIL significantly affects the formation of the gold nanoparticle ring structures. A possible mechanism based on Marangoni-Benard convection in evaporating droplets was proposed. These gold nanoparticle structures were shown to yield a large SERS enhancement for Rhodamine 6G.

Introduction In recent years, monolayer-protected nanoparticles have attracted a tremendous amount of research attention. These molecular structures often have versatile properties that are useful in both fundamental science1 and technological applications, such as catalysis,2 sensing,3 drug or DNA delivery,4 electrochemistry,5 and nanotechnology.6 This kind of nanoparticle is a core/shell hybrid nanomaterial composed of a metal core and an organic shell. The physical properties of these hybrid materials may be tuned by changing either the metal core or the monolayer ligand shell.7 Mention about the core, gold, and silver has always been the most studied because of their unique optical properties for the design of optoelectronic analytical devices. The organic shell can also be functionalized in various ways. In 1994, Schiffrin’s group developed a reliable way to prepare gold nanoparticles terminated by alkanethiols for the first time.8 Since then, much effort has been made on the monolayer functionalization. The most used capping agents are thiols or other compounds with thiol groups. Recently, chiral versions of many of these materials have been of interest to researchers, and the application of such chiral molecular structures in many scientific areas and applied technologies is a promising area of inquiry.9 The property of chirality is a common feature in materials of both synthetic and natural origin. Of interest here, it has been reported that metal surfaces can *Corresponding author: e-mail [email protected]; Ph þ86-53188366062; Fax þ86-531-88564750. (1) Gies, A. P.; Hercules, D. M.; Gerdon, A. E.; Cliffel, D. E. J. Am. Chem. Soc. 2007, 129, 1095–1104. (2) Hickey, N.; Larochette, P. A.; Gentilini, C.; Sordelli, L.; Olivi, L.; Polizzi, S.; Montini, T.; Fornasiero, P.; Pasquato, L.; Graziani, M. Chem. Mater. 2007, 19, 650–651. (3) Watanabe, S.; Sonobe, M.; Arai, M.; Tazume, Y.; Matsuo, T.; Nakamura, T.; Yoshida, K. Chem. Commun. 2002, 2866–2867. (4) Zheng, M.; Davidson, F.; Huang, X. Y. J. Am. Chem. Soc. 2003, 125, 790– 7791. (5) Nobusada, K. J. Phys. Chem. B 2004, 108, 11904–11908. (6) Nagaraju, D. H.; Lakshminarayanan, V. Langmuir 2008, 24, 13855–13857. (7) Hu, Y.; Uzun, O.; Dubois, C.; Stellacci, F. J. Phys. Chem. C 2008, 112, 6279– 6284. (8) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801–802. (9) (a) Gautier, C.; Burgi, T. ChemPhysChem 2009, 10, 483–492. (b) Qi, H.; Hegmann, T. J. Am. Chem. Soc. 2008, 130, 14201–14206. (c) Ha, J.-M.; Solovyov, A.; Katz, A. Langmuir 2009, 25, 153–158. (10) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483– 2487.

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exhibit intrinsically chiral structures.10 Furthermore, achiral metal surfaces can be modified to present a chiral surface by adsorption of suitable chiral molecules.11 However, chirality in metal nanoparticles has been rarely reported, mainly because of the lack of reliable preparative procedures. Recently, some publications have related the preparation of chiral gold,9,12 silver,13 Pd, and Rh nanoparticles.14 The most often used chiral molecules used to modify metal surfaces are DNA,15 polypeptides,16 dendrimers,14 or other small organic molecules;9,12,17 similar systems based on chiral ionic liquids (CILs) have not yet been reported. The self-assembly of functionalized nanoparticles into controlled architectures or patterns yields a new route to feature these nanocomposites with useful chemical, electronic, and physical properties.18 Gold nanoparticle aggregated structures have been most studied mainly because they may have potential applications in a variety of fields, for example, electronics, photonic, biological labeling, chemical sensing and imaging, and surface-enhanced Raman spectroscopy (SERS).19 However, fabrication such assemblies over extended areas are still a major challenge, and an effective method still needs to be developed. In the past few years, (11) Barlow, S. M.; Raval, R. Curr. Opin. Colloid Interface Sci. 2008, 13, 65–73. (12) Nuruzzaman, M.; Prestona, T. C.; Mittler, S.; Jones, N. D. Synlett 2008, 207–212. (13) Nishida, N.; Yao, H.; Kimura, K. Langmuir 2008, 24, 2759–2766. (14) Pittelkow, M.; Brock-Nannestad, T.; Moth-Poulsen, K.; Christensen, J. B. Chem. Commun. 2008, 2358–2360. (15) Shemer, G.; Krichevski, O.; Markovich, G.; Molotsky, T.; Lubitz, I.; Kotlyar, A. B. J. Am. Chem. Soc. 2006, 128, 11006–11007. (16) Chen, C. L.; Zhang, P. J.; Rosi, N. L. J. Am. Chem. Soc. 2008, 130, 13555– 13557. (17) Gual, A.; Godard, C.; Philippot, K.; Chaudret, B.; Denicourt-Nowicki, A.; Roucoux, A.; Castillon, S.; Claver, C. ChemSusChem 2009, 2, 769–779. (18) (a) Fan, H.; Pan, Z. Q.; Gu, H. Y. Microchim. Acta 2010, 168, 239–244. (b) Li, B.; Liu, J. L. Thin Solid Films 2010, 518, S262–S265. (c) Cheng, Z. G.; Wang, S. Z.; Wang, Q.; Geng, B. Y. CrystEngComm 2010, 12, 144–149. (d) Xiang, J. Y.; Tu, J. P.; Zhang, L.; Zhou, Y.; Wang, X. L.; Shi, S. J. J. Power Sources 2010, 195, 313–319. (e) Nadagouda, M. N.; Polshettiwar, V.; Varma, R. S. J. Mater. Chem. 2009, 19, 2026– 2031. (19) (a) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (b) Feng, H. J.; Yang, Y. M.; You, Y. M.; Li, G. P.; Guo, J.; Yu, T.; Shen, Z. X.; Wu, T.; Xing, B. G. Chem. Commun. 2009, 1984–1986. (c) Klein, D. L.; McEuen, P. L.; Katari, J. E. B.; Roth, R.; Alivisatos, A. P. Appl. Phys. Lett. 1996, 68, 2574–2576. (d) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G. H.; Atwater, A. Adv. Mater. 2001, 13, 1501–150. (e) Katz, E.; Willner, I. Angew. Chem, Int. Ed. 2004, 43, 6042–6108. (f) Schultz, D. A. Curr. Opin. Biotechnol. 2003, 14, 13–22. (g) Salem, A. K.; Searson, P. C.; Leong, K. W. Nat. Mater. 2003, 2, 668–671. (h) Nie, S. M.; Emery, S. R. Science 1997, 275, 1102–1106.

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amphiphilic molecules assembly at the air/water interface has been extensively studied, as it provides a perfect environment for transfer of the structures onto solid substrates.20 This method was first demonstrated by Fendler and co-workers.21 In their work, compact and ordered arrays of monolayer protected clusters of different chemical compositions were first formed by spreading the nanoparticle organic solution at air/water interface, and then these nanoparticle monolayers were transferred onto suitable substrates using the LB technique.21 From then on, this approach has been extensively used by many other groups to form superlattices of gold and other nanoparticles stabilized with different passivating agents.22 Well-ordered two-dimensional gold nanoparticle network has been obtained by Bjornholm’ group at the air/water interface by the use of the alkanethiol and an amphiphilic poly(p-phenylene).19b Hao and co-workers have demonstrated the fabrication of gold nanoparticle-based macroporous structures based on solvent evaporation at the air/water interface.23 However, patterns with other morphologies, for example, rings, which is easily obtained on the solid substrate, have not been reported at the air/water interface.24 Herein we present our latest efforts to prepare well-ordered patterns of gold nanoparticles at the air/water interface. The use of a new chiral, long-chain ionic liquid (IL), S-3-hexadecyl-1-(2hydroxy-1-methylethyl)imidazolium bromide ([C16hmim]Br), to prepare monolayer-covered gold nanoparticles in a two-phase liquid-liquid system8 is reported for the first time. Gold nanoparticles prepared in this manner were found to self-assemble in a number of unusual patterns at the air/water interface. Both the nanoparticles and the patterns can be used to fabricate SERS substrates.

Experimental Section Chemicals. High-purity [C16hmim]Br was synthesized according to the literature (Supporting Information).25 The product was further purified by column chromatography. All chemicals related to the synthesis were purchased from Acros and used as received. HAuCl4 was purchased from Shanghai Chemical Reagent Co. Ltd. and used as received. All other chemicals were analytical grade and used as received. Synthesis of Gold Nanoparticles. AuCl4- was first transferred from the aqueous solution to the chloroform solution. Typically, 0.22 g (0.50 mmol) of [C16hmim]Br was dispersed in 50 mL of ethyl acetate. After [C16hmim]Br was dissolved completely, 50 mL of HAuCl4 aqueous solution of 10-3 M was added, mixed vigorously, and left standing. The organic layer was collected, and ethyl acetate was removed by a rotary evaporator. The goldencolored [C16hmim]-AuCl4 complex and the excess [C16hmim]Br were obtained and redispersed in 50 mL of chloroform. The final concentrations of [C16hmim]þ and AuCl4- were 10-2 and 10-3 M, respectively. Gold nanoparticles were directly synthesized in a two-phase liquid-liquid system of chloroform and water.8 Typically, 10 mL of [C16hmim]-AuCl4 chloroform solution and 1.0 mL of 0.10 M ice-cold NaBH4 aqueous solution were added to a 20 mL glass (20) (a) Norgaard, K.; Bjornholm, T. Chem. Commun. 2005, 1812–1823. (b) Hansen, C. R.; Westerlund, F.; Moth-Poulsen, K.; Ravindranath, R.; Valiyaveettil, S.; Bjørnholm, T. Langmuir 2008, 24, 3905–3910. (c) Hassenkam, T.; Norgaard, K.; Iversen, L.; Kiely, C. J.; Brust, M.; Bjornholm, T. Adv. Mater. 2002, 14, 1126–1130. (21) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607–632. (22) Lefebure, S.; Menager, C.; Cabuil, V.; Assenheimer, M.; Gallet, F.; Flament, C. J. Phys. Chem. B 1998, 102, 2733–2738. (23) Ma, H. M.; Hao, J. C. Chem.;Eur. J. 2010, 16, 655–660. (24) (a) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. 2003, 36, 1078–1080. (b) Khanal, B. P.; Zubarev, E. R. Angew. Chem., Int. Ed. 2007, 46, 2195–2198. (25) (a) Bao, W.; Wang, Z.; Li, Y. J. Org. Chem. 2003, 68, 591–599. (b) Li, X. W.; Gao, Y. A.; Liu, J.; Zheng, L. Q.; Chen, B.; Wu, L. Z.; Tung, C.-H. J. Colloid Interface Sci. 2010, 343, 94–101.

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vial, followed by rapid mixing. After 3 h, 1.0 mL of water was added to extract the water-soluble components. The organic phase was collected and used for further characterization. Nanoparticle Characterization. The gold nanoparticles were characterized by transmission electron microscopy (TEM) (JEM-100CX II (JEOL)) and UV-vis spectroscopy (Hitachi U-4100).

Self-Assembly of Gold Nanoparticles at the Air/Water Interface. Two different concentrations of gold nanoparticle solutions were used in the formation of larger nanoparticle structures: the original solution (10-2 M for [C16hmim]þ) and a 10-fold concentrated solution (10-1 M for [C16hmim]þ). An appropriate amount of these two solutions was spread onto the surface of water with a microsyringe, and the solvent was allowed to evaporate for 15 min, resulting in the self-assembled structures of the gold nanoparticles. The patterns formed at the air/water interface were transferred onto both Formvar-covered and carbon-coated copper grids for characterization by TEM. The patterns were also transferred onto quartz slides for the fabrication of the SERS substrate. SERS Measurements. SERS measurements were carried out on a confocal microprobe Raman spectrometer (Jobin-Yvon HR800). Samples for SERS were prepared by mixing the analyte solution (10-2 M for [C16hmim]þ and 10-3 M for AuCl4-) with an Rhodamine 6G (R6G) solution to a final concentration of 10-5 or 10-6 M. After thermodynamic equilibrium was reached, thin films were prepared by drop-casting an appropriate amount of the resulting dispersions on quartz slides. For the gold nanoparticle pattern modified substrates, the samples was prepared by drop casting 10 μL of 10-4 or 10-5 M R6G aqueous solution onto a the quartz substrate. The reference sample was prepared by drop-casting 100 μL of 10-4 M R6G aqueous solution onto a quartz substrate and allowing the solvent to evaporate. The SERS measurements were performed with an excitation wavelength of 633 nm and power of 20 mW. Spectra were collected by focusing the laser line onto the sample using a 50 objective, providing a spatial resolution of about 1 μm. The data acquisition time was 10 s for one accumulation. In order to test the reproducibility, measurements at different positions were carried out for each sample.

Results and Discussion Gold Nanoparticles. Parts a and b of Figure 1 show the typical TEM images of the as-prepared gold nanoparticles and their size distribution analyses, respectively. A large quantity of small gold nanoparticles with a mean diameter of 6.0 ( 1.4 nm can be observed and a relatively narrow size distribution. Figure 1c shows the UV-vis spectra of [C16hmim]Br, [C16hmim]-AuCl4 complex, and the as-prepared Au nanoparticles. As expected, there is no absorption of [C16hmim]Br CIL in the range of 300-800 nm (curve 1); thus, the absorption maximum at about 393 nm (curve 2) indicates the formation of the [C16hmim]-AuCl4 complexes. The as-prepared gold nanoparticles exhibit a single strong surface plasmon peak around 524 nm (curve 3); this peak is caused by dipole plasmon resonance and indicates the formation of Au nanoparticles.26 Gold nanostructures are excellent substrates for surfaceenhanced Raman scattering (SERS), as demonstrated by the significant work by Alvarez-Puebla and co-workers.27 They found that various gold nanostructures can sustain surface plasmon resonances, making them suitable for plasmonic applications and optical enhancements such as SERS. The gold nanoparticles described here were also used for fabricating SERS substrates with (26) Heo, J.; Kim, D. S.; Kim, Z. H.; Lee, Y. W.; Kim, D.; Kim, M.; Kwon, K.; Park, H. J.; Yun, W. S.; Han., S. W. Chem. Commun. 2008, 6120–6122. (27) Baigorri, R.; Garcia-Mina, J. M.; Aroca, R. F.; Alvarez-Puebla, R. A. Chem. Mater. 2008, 20, 1516–1521.

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Figure 1. (a) TEM image of the as-prepared gold nanoparticles. (b) Size histogram. (c) UV-vis spectra. (d) SERS spectra of R6G at different concentrations: 10-6 M (1), 10-5 M (2 and 3).

enhanced response in this analytical technique. SERS activity of the gold nanoparticles has been successfully demonstrated with the dye molecule, R6G. Here, the SERS response of R6G was tested with the [C16hmim]-Au nanoparticles (Figure 1d). Specifically, vibrations assigned to C-H in-plane bending (1191 cm-1), CO-C stretching (1314 cm-1), and C-C stretching of the aromatic ring (1366, 1511, 1577, and 1653 cm-1) are enhanced greatly, as seen in other experimental reports.28 When the concentration of R6G increases from 10-6 M to 10-5 M, the SERS intensity of these vibrations in the R6G-Au nanoparticle mixture increased. Curves 2 and 3 show that this SERS substrate exhibits good reproducibility. Self-Assembled Patterns at the Air/Water Interface. Surface patterning at the nanometer scale is important because of its potential in the design of surfaces exhibiting new functionalities.29 The CIL-stabilized gold nanoparticles formed here were found to self-assemble into ringlike structures at the air/water interface. Figure 2a,b shows the transmission electron micrograph of a typical ringlike structure formed at the air/water interface. The diameters of the rings are generally in the micrometer range, and the typical width is about 100 nm. Most of the rings are not regular; other researchers have reported well-defined nanoparticle rings on amorphous carbon substrates, but the drawback in this case was the presence of many singly dispersed nanoparticles both inside and outside the rings. In the present example, it is difficult to find any singly dispersed nanoparticles either inside the rings or in the areas between the rings. In other words, it appears that all of the nanoparticles are generally involved in the formation of the (28) Guo, S. J.; Dong, S. J.; Wang, E. K. Cryst. Growth Des. 2009, 9, 372–377. (29) Wang, L.; Montagne, F.; Hoffmann, P.; Pugin, R. Chem. Commun. 2009, 3798–3800.

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ring structures. Finally, in the ring structures illustrated here, the orientation of the nanoparticles is random, rather than having an orderly arrangement. Figure 2a,b illustrates that some smaller aggregates also can be seen among the microscale rings (arrows). Upon magnification (inset Figure 2b), the smaller structures are also ringlike in shape and appear to be more regular than the larger rings. Rings with other features are also observed; for example, rings containing nanoparticle clusters or uniformly dispersed nanoparticles (Figure 2d,e) are also found in different preparations. The rings containing nanoparticle clusters (Figure 2d) are most like a “core-shell” structure, but the composition of the core and the shell is the same, only exhibiting a different packing density. The aggregation of nanoparticles in the core is irregular resulting in no uniform morphology. In contrast, Figure 2e shows the rings that contain uniformly dispersed nanoparticles, similar to the “core” in Figure 2d that was redispersed and formed a uniform dispersion. Many reports indicate that gold nanoparticles can form various self-assembled structures on solid substrates.24,30 To distinguish the patterns formed at the air/water interface, rather than formation on TEM grid upon drying a series of validation experiments were conducted. First, samples were prepared by leaving the TEM grids on the surface for different times (from several seconds to 1 min) and then dried by different methods: absorption with filter paper or evaporation at room temperature or under infrared light irradiation. Under all conditions the resulting patterns on the grids exhibited no significant difference. Second, both Formvarand carbon-coated TEM grids were used and exhibited similar (30) (a) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 1057– 1060. (b) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846–3851. (c) Ji, Q. M.; Acharya, S.; Hill, J. P.; Richards, G. J.; Ariga, K. Adv. Mater. 2008, 20, 4027–4032. (d) Heo, J.; Lee, Y. W.; Kim, M.; Yun, W. S.; Han, S. W. Chem. Commun. 2009, 1981–1983.

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Figure 2. Various structures observed to form at the air/water interface: (a, b) rings; (c) rings containing Au-nanoparticle clusters; (d) rings with uniformly dispersed Au-nanoparticles.

patterns for the prepared films. Although the surfaces of both of these kinds of grids are hydrophobic, their surface properties are not the same, potentially leading to a different type of pattern if formed on the surface. Third, nanoparticle solutions were dropped directly onto carbon-coated copper grids (dry or treated with water vapor), resulting in only a few irregular domains (data not shown) instead of the patterns obtained at the air/water interface. These results reveal that the patterns are not influenced by either the treatment of the TEM grids or the surface properties of the TEM grids. Thus, the patterns are formed at the air/water interface rather than on the TEM grid upon drying. The shape of the rings is influenced by the solution conditions, specifically the concentration of the nanoparticles and the temperature of the water phase. Here we also investigated these two mainly conditions. When the nanoparticle solution is dispersed on a hot water surface, no rings are observed, as shown in Figure 3a. The resulting particles in this case are ordered and close-packed in mono- or multilayer forms. When a lower concentration nanoparticle solution is spread on either cold or hot water, only irregular, closed aggregates are obtained (Figure 3b-d). These structures appear to be the resulting tubes after a collapse of the ring structures; a spiral may even be observed at the end of some tubes (Figure 2b, arrow). A Reasonable Mechanism. Heath et al.24a have proposed that holes in solution thin films lead to the ringed arrangement of nanoparticles, which was termed a “hole-nucleation” mechanism. Alternatively, Zubarev et al. have proposed a “water droplet” mechanism for nanoparticle ring formation;24b specifically, they assumed that the nanoparticles assemble around water droplets that condense from the air when the volatile solvent evaporates (also known as a “breath figure”). These two mechanisms are not suitable for the present case since the present “substrate” for the patterning is water, and the spreading solution does not wet it. We propose that the ring patterning of the gold nanoparticles (31) (a) Nguyen, V. X.; Stebe, K. J. Phys. Rev. Lett. 2002, 88, 164501.1– 164501.4. (b) Nikolov, A. D.; Wasan, D. T. Ind. Eng. Chem. Res. 2009, 48, 2320–2326.

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is caused by a “surfactant-enhanced Marangoni-Benard instability”.31 The trajectories of polystyrene microspheres in water inside the wedge film were studied by Wasan’s group using advanced microinterferometric techniques.31b They also monitored the temperature gradient over the drying droplet surface. For our situation, this mechanism may be explained as follows (Figure 4). When the chloroform solution is spread on the surface of the water, the chloroform is not miscible with water and denser; thus, small chloroform droplets will be formed that tend to sink. However, due to their surface tension, the chloroform droplets are suspended and form a concave surface (Figure 4(1)). The chloroform will evaporate from the surface and cool the top surface of the droplet, leading to a vertical temperature gradient in the chloroform droplet. At the same time, chloroform at the edge of the droplet evaporates faster than in the center, causing a horizontal temperature gradient. The presence of both gradients in turn may lead to Marangoni-Benard convection (Figure 4(2)). As evaporation continues, the colloidal suspension becomes more concentrated and some nanoparticles may coalesce (Figure 4(3)). This would lead to a pair of results. First, a pinned contact line drives some of the nanoparticles into the wedge region (at the air/ water/chloroform three-phase interface) due to the interactions between the water phase and the hydroxyl group of the CIL. Second, some nanoparticles inside the droplet may settle in the concave water surface (at the chloroform/water interface). With the decrease in the size of the droplet, the convection cell will gradually disappear. Since there is a small amount of surfactant remaining in the droplet, a secondary Marangoni flow is generated, caused by the surface tension gradient. This secondary Marangoni flow is opposite to the primary one (generated by the temperature gradient)31b and causes the nanoparticles to move from the margin to the center. Simultaneously, there exists various interactions (particle-particle, CIL-water, buoyancy, gravity, etc.) that act together with the two opposite Marangoni flows to cause the nanoparticles to keep moving, until forming an equilibrium structure. This leads to the following possibilities: first, nanoparticles at the concave surface will move to air/water Langmuir 2010, 26(14), 12209–12214

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Figure 3. Structures form at the air/water interface under different conditions: (a) higher concentration nanoparticles solution/hot water; (b-d) lower concentration nanoparticles solution/cold water.

Figure 4. Schematic illustration of the proposed spontaneous pattern formation during evaporation: (1-3) cross-sectional view of the proposed Marangoni-Benard convection flow pattern during evaporation; (a-c) formation of different patterns.

interface and assemble with nanoparticles at the interface (Figure 4a) to form rings with no nanoparticles inside (Figure 2a,b); second, the nanoparticles may move to the bottom of the concave surface (Figure 4b) and aggregate, which would lead to the clustering of nanoparticles inside the rings (Figure 2c); third, these nanoparticles may remain fixed in place (Figure 4c) and lead to uniformly dispersed nanoparticles inside the rings (Figure 2d). Because of the fluidity of the water, the first two types of rings are not uniformly round (Figure 2a-c). However, if nanoparticles fill the ring structure, the resulting rings are much more round due to the tension arising from the packing of the nanoparticles inside them (Figure 2d). Similarly, due to the tension, small rings will exhibit more uniformly round structures (Figure 2b, inset). When hot water is used as a “substrate”, the evaporation rate will increase, and the fine balance of forces mentioned above will be disrupted due to increased thermal Langmuir 2010, 26(14), 12209–12214

motion. As a result, rings can no longer be form and are replaced by singly dispersed nanoparticles (Figure 3a). Finally, ring structures formed using a more dilute concentration of nanoparticle suspension requires a larger volume of solution to be comparable to the rings formed using the concentrated solution. In this case, subsequently added solution affects the rings that have been formed earlier as follows: nanoparticles are added to the earlier formed rings, and tubular structures are formed (Figure 3b,c). Under these conditions, several rings may even become connected together to form a necklace-like structure (Figure 3d). As one test of our hypothesis, [C12mim]Br and [C16mim]Br (Figure S1) stabilized Au nanoparticles (denoted as C12-Au and C16-Au) was also prepared using the same procedure. Although [C16mim]Br may form a Langmuir monolayer on surface of water,32 both C12-Au and C16-Au poorly spread on the water interface. Under any conditions, ringlike structures are not observed to be formed from solutions of gold nanoparticles containing these two ionic liquids; rather, ordered and closepacked monolayers were observed to form (Figure S2). The main difference between [Cnmim]þ (n = 12, 16) and [C16hmim]þ is the hydrophilic group. Since there is no hydroxyl in [Cnmim]þ, they lack a group that can allow the nanoparticles to interact with the water surface. As a result, the contact line can not be pinned. The nanoparticles suspended with the hydrophobic ILs will be scattered on the water surface, and an ordered and close-packed mono- or multilayer structure will form due to the hydrophobic interactions between the alkyl chains.33 Nanoparticle Patterns as SERS Substrates. Figure 5 shows the Raman spectrum of solid R6G and the SERS spectra of R6G on the films of different patterns as shown in Figure 2. (32) Bai, X. T.; Zheng, L. Q.; Li, N.; Dong, B.; Liu, H. G. Cryst. Growth Des. 2008, 8, 3840–3846. (33) (a) Kanehara, M.; Kodzuka, E.; Teranishi, T. J. Am. Chem. Soc. 2006, 128, 13084–13094. (b) Santhanam, V.; Liu, J.; Agarwal, R.; Andres, R. P. Langmuir 2003, 19, 7881–7887. (c) Zhao, S.-Y.; Wang, S. H.; Kimura, K. Langmuir 2004, 20, 1977– 1979.

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were calculated using the following expression (a detailed calculation is in the Supporting Information):28 EF ¼ ðISERS =Nads Þ=ðIbulk =Nbulk Þ

Figure 5. Raman spectrum of solid R6G (curve 1) and SERS spectra on different gold nanoparticle patterns (10-5 M (curves 2-4) and 10-4 M (curve 5)).

Few studies have shown SERS of R6G from gold substrates because R6G does not strongly interact with gold surfaces.34 Zhang et al.34a reported that R6G molecule may readily bind to the gold nanoparticle aggregates and exhibit strong SERS activity due to the unique surface chemistry afforded by sulfur species on the surface. R6G on the quartz substrate gave very weak signals (Figure 5, curve 1); however, R6G on the gold nanoparticle patterned substrate gives a much stronger SERS response (Figure 5, curves 2-5). This is primarily due to the interaction between the aromatic rings in R6G and the CIL imidazole groups, allowing the R6G molecules to readily bind to the gold nanoparticles. The SERS activities of the three typical patterns (Figure 2) do not have any significant difference under the same conditions (Figure 5, curves 2-4). As the concentration of R6G increases, the SERS intensities progressively increase (curve 5). The surface enhancement factors (EF) of R6G on the pattern film (34) (a) Schwartzberg, A. M.; Grant, C. D.; Wolcott, A.; Talley, C. E.; Huser, T. R.; Bogomolni, R.; Zhang, J. Z. J. Phys. Chem. B 2004, 108, 19191–19197. (b) Gupta, R.; Weimer, W. A. Chem. Phys. Lett. 2003, 374, 302–306.

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where ISERS is the intensity of a vibrational mode in the surfaceenhanced spectrum, Ibulk is the intensity of the same mode in the Raman spectrum, Nads is the number of molecules adsorbed and sampled on the SERS-active substrate, and Nbulk is the number of molecules sampled in the bulk. Taking curve 5 as an example, the EF at the pattern film for the band located at 1366 cm-1 can be calculated to be ∼7.28  104. In addition, SERS spectra of R6G on the patterned films are reproducible at different sites on a substrate (data not shown). Thus, the nanoparticle patterns uniformly distributed on a quartz substrate may be used as reliable SERS substrates. In conclusion, chiral ionic liquid monolayer-stabilized gold nanoparticles were successfully synthesized in a two-phase liquid-liquid system. The as-synthesized nanoparticles were found to self-assemble into ringlike structures at the air/water interface. The ring formation is suggested to be a result of Marangoni-Benard convection in evaporating droplets of the nanoparticle suspension. Control experiments with long-chain ILs revealed that the molecular structure of the CIL significantly affects the formation of the gold nanoparticle ring structures. Both the gold nanoparticles and the nanoparticle patterns were found to yield a large SERS enhancement for R6G; this substantial increase in the SERS signal is partly attributed to nanoparticle aggregation. Acknowledgment. The authors are grateful for funding from the National Natural Science Foundation of China (No. 20773081 and 50972080) and the National Basic Research Program (2007CB808004 and 2009CB930101). We also thank Dr. J. David Van Horn (Visiting Professor, Shandong University) for editorial assistance. Supporting Information Available: Synthesis details, molecular formula and structures, and enhancement factor calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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