Carbon Nanotube Microspheres Produced by Surfactant-Mediated

Feb 18, 2011 - Center for Paralysis Research, Department of Basic Medical Science, School of Veterinary Medicine, Purdue University, West Lafayette, ...
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Carbon Nanotube Microspheres Produced by Surfactant-Mediated Aggregation Mahvash Zuberi,† Debra M. Sherman,‡ and Youngnam Cho*,† †

Center for Paralysis Research, Department of Basic Medical Science, School of Veterinary Medicine, Purdue University, West Lafayette, Indiana 47907, United States ‡ Life Science Microscopy Facility, Purdue University, West Lafayette, Indiana 47907, United States ABSTRACT: Using the inherent self-organizing nature of nanotubes, we have prepared mechanically robust, stable, surfactant-mediated carbon nanotube microspheres (CNMs). This work reveals the effects of a series of surfactants with cationic, anionic, and nonionic charges on the formation of CNMs. This occurs through the spherical entanglement of carboxylic acid-terminated multiwalled carbon nanotubes (CNTs). The morphology, electrochemical and electrical performance, and the surface charge of CNMs were systemically observed with SEM, focused ion beam (FIB) milling, four-point probe, cyclic voltammetry, and ζ-potential analysis. The SEM results revealed that surfactant-wrapped CNMs were densely packed with relatively smooth exterior surfaces possessing large reactive areas. In addition, FIB milling observation provided cross-sectional views of CNMs and demonstrated the similarity to those of the external surface. The surfactants' concentration (0.01-1% w/v) influenced the properties of CNMs, leading to a slight variation in ζ-potential, resistance, and electrochemical properties. The water-stabilized CNMs, with a range of diameters and porosities, can offer new approaches to various potential new applications.

1. INTRODUCTION Carbon nanotubes have many interesting properties, including remarkable electrical properties, high mechanical strength, and stable thermal characteristics.1-3 Moreover, distinct structural features due to their hollow structure permit extraordinarily high aspect ratios and large surface areas that would be useful in various areas as diverse as electronic devices, environmental fields, and the biotechnology industry.4-8 However, despite enormous progress in their synthesis, characterization, and attempts at possible applications, carbon nanotubes have been victims of inherent problems inhibiting further advancement. One of the challenges arises from strong inherent π-π interactions between the nanotubes that make them insoluble in many types of solvents and consequently causes them to form large agglomerates. Taking this into consideration, a number of studies have investigated methods to solve these problems with the use of novel organic solvents, surfactants, strong acids, or through the functionalization of the surface of the nanotubes relying on the interfacial interactions in an effort to ultimately engineer the selfassembly nature of nanotubes.9-13 These approaches have shown some utility stabilizing dispersions of the nanotubes. Alternately and additionally, considerable efforts have been devoted to the fabrication of structurally well-defined micro/ nanoarchitectures with morphological diversity based on the assembly of CNT bundles.14-18 In most of these studies, versatile approaches, such as layer-by-layer (LBL) templating techniques or a water-in-oil emulsion system using colloidal templates, were adopted for the production of CNT-based capsules.19-25 However, there were some drawbacks in the process because intricate r 2011 American Chemical Society

fabrication steps and the collapse of the spherical architecture as a result of subsequent removal of core templates are unavoidable. Here, we provide a simple and direct approach for the large-scale construction of surfactant-mediated, spherical, water-stabilized CNT microspheres (CNMs). The carboxylic acid-terminated multiwalled CNTs readily interacted with positively, negatively, or nonionically charged surfactant molecules to create stable spherical colloids under continuous moderate mechanical agitation. To our knowledge, this is the first report describing the preparation and characterization of CNT colloidal structures using common surfactants, such as CTAB, SC, and Triton X-100. These can induce the nanotubes to aggregate in colloidal structures while maintaining their structural integrity. As an unexpected dividend, iterative experiments have definitely demonstrated the preferential formation of particle-like CNT bundles even though the exact mechanism underlying their formation is not clear. In addition to morphological features, we evaluated structural and electrical properties of CNMs, including their size distribution, packing density, surface charge, electrical conductance, and electrochemical features. It was very important to present the formation of surfactant-mediated CNT microspheres as well as their colloidal properties relative to the similarity to those of other polymer or metal particles. In contrast to conventional approaches that focus on separating the nanotubes, this procedure rather takes advantage of the spontaneous self-aggregation nature of nanotubes to form microspheres by adjusting Received: October 19, 2010 Revised: January 27, 2011 Published: February 18, 2011 3881

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Figure 1. (a) The illustration showing the preparation of CNMs with the aid of surfactants. (b) Chemical structures of the surfactants, such as cetyl trimethylammonium bromide (CTAB), sodium cholate (SC), and Triton X-100.

the concentration of the surfactant. Indeed, surfactant-aided fabrication of CNMs has provided new insights into the discovery and design of colloidal structures composed of carbon nanotubes.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Surfactant-Mediated Carbon Nanotube Microspheres. Carboxyl (COOH) group-decorated multi-

walled carbon nanotubes (MWCNTs) were purchased from Nanolab (Newton, MA) with a diameter of 30 nm and length of 1-5 μm. These carboxylated nanotubes have 2-7 wt % COOH after functionalization, and metal catalyst impurities would be around 3-4%. All chemicals were obtained from Sigma Aldrich unless otherwise specified. The fabrication of surfactant-mediated CNMs was prepared as follows. Three different surfactants, cetyltrimethylammonium bromide (CTAB), sodium cholate (SC), and Triton X-100, were employed in the process of surfactant-assisted CNMs. Nanotubes were sonicated in an aqueous solution of 0.01, 0.1, and 1% (w/v) CTAB, SC, Triton X-100 for 1 h at various chosen pH values, followed by three centrifugation cycles and redispersions to yield well-formulated CNMs. After repeating the process three to five times, CNMs were dried at room temperature. 2.2. Characterization Methods. The morphology of CNMs was examined by an FEI NOVA nanoSEM (FE Company, Hillsboro, OR) when the sample of CNMs was dried onto glass coverslips and sputter-coated with AuPd (Hummer II sputtercoater, Anatech USA, Union City, CA). Imaging was done with an Everhart-Thornley (ET) detector or Through-the-Lens highresolution detector (TLD) using a 5 kV accelerating voltage. Pt deposition protected the CNM surface during FIB milling. Milling parameters were 30 kV, 10 mm WD, and 52° tilt. Images were captured using the ET detector at a 5 kV accelerating voltage. To analyze the inner structure of CNMs, the focused ion beam (FIB) ablation was used using an FEI Quanta 3D FEG Dualbeam FESEM. Pt deposition protected the CNM surface during FIB milling. Milling parameters were 30 kV, 10 mm WD, and 52° tilt. Images were captured using the ET detector at a 5 kV accelerating voltage. CNMs were further characterized with a ζ-potential/particle size analyzer (Zetasizer) to measure the particle size and surface charge. To begin, samples were diluted in deionized water and measured in an automatic mode. All measurements were performed in three to five repetitions. The direct conductivity of various types of CNMs was measured using

a standard four-probe technique (micromanipulator’s model 6000) at room temperature. CNMs were then pressed into a pellet using standard sample preparation tools for FTIR spectroscopy. The in situ measurement of voltage when applying a constant current across the surface further correlated to determine resistance. The Van der Pauw equation calculated the sheet conductivity of films of each sample. Measurements were performed four times in different directions from the center of the film. Cyclic voltametry (CV) experiments were carried out using a 604 model potentiostat (CH Instruments) at room temperature. In a three-electrode cell, platinum gauze, a saturated silver/ silver chloride, and a CNM-coated ITO surface were used as counter, reference, and working electrodes, respectively, to observe the electrochemical properties of the surface. Cyclic voltammograms were recorded within the potential range from -200 to 1000 mV in 0.1 M PBS containing 5.0 mM Fe(CN)64-/3 at a scan rate of 50 mV/s.

3. RESULTS AND DISCUSSION 3.1. Spontaneous Formation of Surfactant-Mediated CNMs. The inherent self-aggregation nature of carbon nanotubes

caused practical difficulties for further processing, and effective methods for breaking the large agglomerates into individual nanotubes have been extensively investigated. Indeed, surfactant-assisted dispersion of the carbon nanotubes has proven helpful to minimize the intertube interactions resulted from van der Waals and hydrophobic bonds through the nonspecific adsorption of amphiphilic molecules on the surface of the nanotubes, but this has not been completely achieved.26,27 In a different strategy, one can manipulate the degree of entanglement of the nanotubes and precisely engineer the morphology into numerous shapes coupled with the ability to accurately tailor the surface characteristics. Ideally, these nanotube structures would willingly participate in a wide range of applications as fundamental components. To this end, we present a surfactantmediated method to prepare the microspheres composed of only carbon nanotubes, rather than relying on the geometrical features defined by using a template, such as polystyrene beads. Given the unique advantages of carbon nanotubes, these novel CNMs fabricated by the assembling of individual nanotubes into microspheres can offer more technical opportunities with exciting applications and benefits over conventional micro- or nanoparticles. As illustrated in Figure 1, the large-scale construction of spherical, water-stabilized CNT microspheres (CNMs) can be 3882

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Figure 2. High-resolution SEM images of CNMs produced from (a, b) 0.01% CTAB solution, (d, e) 0.01% SC solution, and (g, h) 0.01% Triton X-100. Panels (c), (f), and (i) show the histograms of the size distribution of these colloidal particles prepared in a solution of CTAB, SC, and Triton X-100 at pH 7.0.

achieved through the interaction of carboxylic acid-terminated nanotubes with different surfactant types and concentrations, and under continuous ultrasonic power. Because the acidic treatment of pristine multiwalled CNTs (30 nm diameter, 1-5 μm length) produces numerous anions, the bundles of nanotubes are supported to enable agglomeration into spherical shapes with diameters of up to several micrometers. The transition of irregular agglomerates to regular colloidal structures may be attributed to the combined forces of the van der Waals and electrostatic interactions between negatively charged nanotubes and hydrophilic/hydrophobic molecules of the surfactants. However, it should be noted that the formation of a spherical structure does rely on the density of the carboxyl groups present on the nanotubes. Figure 2 shows SEM images of CNMs produced using various types of surfactants at pH 7. CNMs with large populations were constructed by adding negatively charged nanotubes in three different concentrations of surfactant solutions (e.g., 0.01, 0.1, and 1% w/v). First, we observed the effect of a cationic surfactant, cetrimonium bromide (CTAB), on the processing of CNMs. As shown in Figure 2a,b, microspheres prepared by adding 0.01% CTAB solution were quasi-spherical with a smooth surface morphology. However, the microspheres showed a relatively wide range of size distribution with a mean diameter of 15.6 μm (Figure 2c). The interactions between the negatively charged nanotubes and the cationic surfactant

favorably promoted the assembly of individual nanotubes into microspheres, strengthening a spherical geometry in an effort to neutralize the net surface charge. In fact, coagulation factors appear to be linearly related to the positive charge density present on CTAB and their steric bulk. Indeed, surface features of CNMs turned out to be dependent on the concentration of the surfactant. Our results indicated that, when excessive amounts of surfactant were used (more than 1% CTAB), CNMs tended to be crystallized and fully covered with surfactant monomers, whereas when less than 0.01% CTAB was employed, the particles turned out to be unstable. Thus, the particles lying in the 0.011% surfactant range appeared to possess a uniform and rigid spherical shape with a high yield of production. Subsequently, we constructed large populations of CNT microspheres by adding negatively charged nanotubes in 0.01% anionic SC aqueous solutions (Figure 2d,e). Interestingly, as anionic surfactants were added into nanotubes, they induced a spherically aggregated state. We have also observed the size distribution of the resulting CNMs in an effort to systematically optimize the experimental conditions. Our results indicated that the size and shape of the resulting particles were not sensitive to the types or the concentration of the surfactants used. Similarly, Figure 2g,h also shows the successful synthesis of CNMs by the addition of a nonionic surfactant, Triton X-100. Our study has demonstrated that the charge of surfactants 3883

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Figure 3. SEM images of self-supporting CNMs: (a-c) 0.01% CTAB-based CNMs and (d-f) 0.01% SC-based CNMs. (b, e) High-magnification SEM images of the surface of a CNM, demonstrating the densely packed and entangled network of nanotubes. (c, f) High-magnification cross-sectional images of a CNM after focused ion beam ablation of a portion of the CNM.

had little effect on the gross microsphere morphology and size. Rather, the surfactant concentrations play a more critical role in providing favorable assembly into stable colloids. Indeed, the rigidity of CNMs produced at even relatively low concentrations of surfactants was shown to be much more stable than those produced in aqueous suspensions. Although the major role of surfactants in the formation of CNMs is difficult to clarify, it can be inferred the possible evidence based on several studies detailing the process of surfactant-metal catalyst-mediated particle formation. Wang et al proposed the formation of metal oxide particles prepared by a surfactant-assisted method, where the chelation of metal ions by a surfactant in an aqueous solution facilitated the particulate assemblies process, including nucleation and growth.28 In addition, Chen el al has reported the in situ fabrication of chestnut-like carbon nanotube spheres by mixing nickel catalysts that were reduced to nanoparticles within composites during synthesis.29 Thus, we assumed two important features in processing stable colloidal CNM particles would be the metal catalyst and the surfactant. Metal catalysts present during nanotube synthesis, but not completely removed, might serve as seed particles. As surfactant-derived micelles decorated on individual nanotubes act as templates, the interaction between surfactant crystallites and metal catalysts may accelerate the microsphere formation in a shape-controlled fashion at a particular concentration. In addition, consistent with a previous report in which Yang et al proved the effect of ultrasonication on the formation of carbon nanotube capsules, CNM formation was also accomplished by mechanical agitation/resuspension processes, assuring that the ultrasonication process was one of the primary causes for determining controllable colloidal shapes.30 The interior morphology of surfactant-based CNMs is displayed in Figure 3. The ion-milling process was carried out to confirm the detailed inner structure of these CTAB- and SC-mediated particles. First, CTAB-based CNMs showed the presence of a precipitate corresponding to the bright spots in the internal microstructure,

as shown in Figure 3c. We assumed that such defective structures would be due to surfactant micelle self-assembly in aqueous solutions, resulting in the formation of the macromolecular precipitations or crystallization inside the particle. In contrast, SC-mediated CNMs displayed an obvious similar exterior and interior morphology. This might be explained by the relatively small aggregation number of aqueous sodium cholate.31 The small size of this micelle allows uniform distribution and saturation on individual nanotubes. 3.2. Effect of Surfactant Types and Concentrations on the Physicochemical Properties of CNMs. The CNMs synthesized through a surfactant-mediated approach were evaluated by ζpotential, cyclic voltammetry, and four-point probe to assess the surface charge, electrochemical property, and surface resistance. Figure 4 shows the ζ-potential distributions of CNMs produced by three types of surfactants at different concentrations and pHs. Knowing the surface charge of CNMs reliably enabled us to predict and understand the interaction between the nanotubes and surfactant molecules and permitted control over the colloidal stability. As shown in Figure 4a, the different responses in ζ-potential values of surfactant-wrapped CNMs reflected the charge of surfactants used because the adsorption of the anionic or cationic species imparts a special characteristic, including charge, onto the coated nanotubes. The CTAB-mediated CNMs showed an obvious positive ζ-potential value with a peak at þ19.6 mV, whereas SC- and Triton X-100-based CNMs showed relatively similar negative ζ-potential values corresponding to -46.2 and -39.8 mV, respectively. In the case of SC- and Triton X-100-based CNMs, a slight negative shift was recorded when compared with carboxylic acid-terminated carbon nanotubes (-23.4 mV) due to the presence of negatively charged ionic surfactant species. Figure 4b-d shows the dependence of ζ-potential values on the concentration of the surfactant used. There appears to be a linear correlation between ζ-potential values and the concentration and pH of surfactant 3884

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Figure 4. (a) ζ-potential distribution of carboxylic acid-funtionalized nanotubes and CNMs produced in a series of surfactants (0.01% w/v) at pH 7.0. ζ-potential of CNMs prepared in a solution of (b) CTAB, (c) SC, and (d) Triton X-100 at different concentrations and pHs.

Figure 5. Cyclic voltammograms of 5 mM Fe(CN)63-/4- at (a) CTAB-based CNMs, (b) SC-based CNMs, and (c) Triton X-100-based CNMs at a scan rate of 100 mV sec-1. (d) Electrical resistance (Ω cm) as a function of the surfactant concentration of the CNMs.

solutions. Because the spontaneous adsorption of surfactant molecules on the surface of nanotubes occurs through van der Waalse interactions, it is possible that surfactant-nanotube interactions alter the surface charge by the adsorbed ionic surfactants. With increasing amounts of CTAB, the CNMs become more positive with þ47.9 mV as the surfactant coverage on the surface

of nanotubes increases. In this experiment, we documented the effect of solution pH on ζ-potential values. Notably, the ζ-potential decreases as the pH increases, indicating the transition of the carboxyl group (COOH) present on the nanotubes into ionized carboxylic acid species (COO-). Similarly, negatively charged SC-wrapped CNMs showed a surfactant concentration 3885

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The Journal of Physical Chemistry C and a pH-dependent decrease in ζ-potential values. Expectedly, the addition of a nonionic surfactant tends to slightly increase the surface charge of CNMs in the magnitude of the ζ-potential, resulting in the adsorption of Triton X-100 around nanotubes. We note that the surface charge of CNMs produced from a series of anionic, cationic, and nonionic surfactants was virtually identical to those observations of surfactant-assisted carbon nanotube dispersions.12,32 Figure 5 shows the electrochemical properties relative to surfactant concentration at pH 7.0. In general, the CV was described as irreversible oxidation and reduction waves, where positions and redox peak currents were strongly influenced by the nature of the electrolyte solution. Figure 5a shows that bare indium tin oxide (ITO) surfaces coated with CNT-COOHs exhibited the magnitude of the oxidation and reduction current. On the other hand, the addition of CTAB decreased the redox current magnitude conducted in ferricyanide probe solutions as an indicator. The deposition of surfactant-coated CNMs was likely to restrict the efficient electron transfer, ultimately acting as a barrier between the ferricyanide species in solution and the ITO/ CTAB-CNM. Likely with CTAB-based CNMs, SC- and Triton X-100-based CNMs showed the decreased redox current of ferricyanide, indicating that the electrons of Fe(CN)64-/3- are hard to reach the ITO surface due to the presence of surfactant. Subsequently, we explored the resistivity of CNMS using a four-point probe method. Figure 5d plots the resistance of the CNMs as a function of the concentration of surfactants. Appreciable changes in electrical resistance suggested the presence of insulating layers of surfactant molecules. These observations are in a good agreement with previous reports, where surfactant-coated dispersions of nanotubes hindered the electron mobility and subsequently resulted in higher resistivity values.33 Indeed, the resistivity values obtained in our study strongly represented the properties of the surfactants, especially the aggregation number. As Triton X-100 forms relatively large micelles at a critical micelle concentration (cmc) when compared to CTAB or SC, CNMs produced from Triton X-100 might possess a nonuniform coating, allowing charge transfer through bare nanotube surfaces.34-36 CNMs prepared in SC that had a smaller aggregation number than CTAB and Triton X-100 displayed high resistance values, which clearly corresponded to sufficient coating of SC on the surface of the nanotubes. On the basis of the plot of resistivity versus the loading of the surfactant (0-1%) in the microspheres, the electrical conductivity of the CNMs is likely governed by precise manipulation of surfactant concentrations.

4. CONCLUSIONS Contrary to current concepts that focus on breaking bundles into individual nanotubes, we offer the spontaneous selfaggregating nature of the nanotubes to merge them into stable colloidal structures. The use of dispersants, such as cationic, anionic, and nonionic surfactants, facilitated self-assembly into carbon nanotube microspheres (CNMs). The conductivity, surface charge, and electrochemical properties of CNMs varied as a function of the surfactant concentration and pHs. Further investigations using the CNM complexes conjugated with biological species or inorganic particles will highlight the possibilities for new applications.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We appreciate the excellent illustrations and graphics by Michel Schweinsberg, and the administrative assistance of Jennifer Danaher for manuscript preparation. We acknowledge financial support from the General Funds of the Center for Paralysis Research, The State of Indiana, and a generous endowment from Mrs. Mari Hulman George. ’ REFERENCES (1) Bethune, D. S.; Klang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (2) Iijima, S. Nature 1991, 354, 56. (3) Zhang, Y.; Ichihashi, T.; Landree, E.; Nihey, F.; Iijima, S. Science 1999, 285, 1719. (4) Avouris, P.; Chen, Z.; Perebeinos, V. Nat. Nanotech. 2007, 2, 605. (5) Bradley, K.; Gabriel, J.-C. P.; Gruner, G. Nano Lett. 2003, 3, 1353. (6) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (7) Collins, P. G.; Arnold, M. Science 2001, 292, 706. (8) Star, A.; Han, T.-R.; Gabriel, J.-C. P.; Bradley, K.; Gruner, G. Nano Lett. 2003, 3, 459. (9) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (10) Holzinger, M.; Chem, D.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. 2001, 40, 4002. (11) Ramesh, S.; Ericson, L. M.; Davis, V. A.; Saini, R. K.; Pasquali, C. K. M.; Billups, W. E.; Adams, W. W.; Hauge, R. H.; Smalley, R. E. J. Phys. Chem. B 2004, 108, 8794. (12) Sun, Z.; Nicolosi, V.; Rickard, D.; Bergin, S. D.; Aherne, D.; Coleman, J. N. J. Phys. Chem. C 2008, 112, 10692. (13) Tummala, N. R.; Striolo, A. ACS Nano 2009, 3, 595. (14) Ericson, L. M.; Fan, H.; Peng, H.; Davis, V. A.; Zhou, W.; Sulpizio, J.; Wang, Y.; Booker, R.; Vavro, J.; Guthy, C.; Parra-Vasquez, A. N. G.; Kim, M. J.; Ramesh, S.; Saini, R. K.; Kittrell, C.; Lavin, G.; Schmidt, H.; Adams, W. W.; Billups, W. E.; Pasquali, M.; Hwang, W.; Hauge, R. H.; Fischer, J. E.; Smalley, R. E. Science 2004, 305, 1447. (15) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987. (16) Richard, C.; Balavoine, F.; Schultz, P.; Ebbesen, T. W.; Mioskowski, C. Science 2003, 300, 775. (17) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Science 2001, 293, 1299. (18) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331. (19) Correa-Duarte, M. A.; Kosioreck, A.; Kandulski, W.; Giersig, M.; Salgueirini-Maceira, V. Small 2006, 2, 220. (20) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; Liz-Marzan, L. M. Chem. Mater. 2005, 17, 3268. (21) Ji, L.; Ma, J.; Wei, W.; Ji, L.; Wang, X.; Yang, M.; Lu, Y.; Yang, Z. Chem. Commun. 2006, 1206. (22) Panhuis, M. i. h.; Paunov, V. N. Chem. Commun. 2005, 1726. (23) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Nano Lett. 2002, 2, 531. (24) Paunov, V. N.; Panhuis, M. i. h. Nanotechnology 2005, 16, 1522. (25) Yi, H.; Song, H.; Chen, X. Langmuir 2007, 23, 3199. (26) Vaisman, L.; Wagnet, J. D.; Marom, G. Adv. Colloid Interface Sci. 2006, 128, 37. (27) Wang, H. Curr. Opin. Colloid Interface Sci. 2009, 14, 364. 3886

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