Letter pubs.acs.org/Langmuir
Hydrophilic Gold Nanoparticles Adaptable for Hydrophobic Solvents Shota Sekiguchi,‡ Kenichi Niikura,*,§,† Yasutaka Matsuo,§,† and Kuniharu Ijiro§,† ‡
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita13, Nishi8, Kita-Ku, Sapporo 060-8628, Japan Research Institute for Electronic Science (RIES), Hokkaido University, Kita21, Nishi10, Kita-Ku, Sapporo 001-0021, Japan † JST-CREST, Sanban-cho 5, Chiyoda-ku, Tokyo 102-0075, Japan §
S Supporting Information *
ABSTRACT: Surface ligand molecules enabling gold nanoparticles to disperse in both polar and nonpolar solvents through changes in conformation are presented. Gold nanoparticles coated with alkyl-head-capped PEG derivatives were initially well dispersed in water through exposure of the PEG residue (bent form). When chloroform was added to the aqueous solution of gold nanoparticles, the gold nanoparticles were transferred from an aqueous to a chloroform phase through exposure of the alkyl-head residue (straight form). The conformational change (bent to straight form) of immobilized ligands in response to the polarity of the solvents was supported by NMR analyses and water contact angles.
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INTRODUCTION The utility of metal inorganic nanoparticles is a topic of increasing interest in biomedical science,1 and nanoparticles are commonly used as diagnostic2 and therapeutic3 tools both in vitro and in vivo. Since surface ligands on the nanoparticles play an important role in the determination of their colloidal properties4 and biological functions,5 their display has opened a wide range of new applications for nanoparticles. The addition of a hydrophobic feature to nanoparticles by modification with hydrophobic surface ligands allows high penetration through the cellular membrane via hydrophobic interactions.6 Hydrophobicity is also known to be an important factor for passage through nuclear pores, which are filled with gel-like hydrophobic proteins.7 On the other hand, increases in the hydrophobicity of the nanoparticle surface leads to a decrease in nanoparticle dispersibility in aqueous solutions. Thus, the potential applications of nanoparticles will be widened if the surface of the nanoparticles can be designed so as to allow the nanoparticles to cross the hydrophobic barrier while retaining high dispersibility in aqueous solutions. Although colloidal nanoparticles thermodynamically prefer to accumulate at the aqueous/organic interface,8 unlike small molecules they cannot easily cross the interface as the surface energy of the nanoparticles cannot be easily changed to be suitable for each phase. The aqueous to organic phase transfer of nanoparticles has often been carried out by ligand exchange reactions using alkylthiol derivatives,9 which induce the nanoparticles to move into the organic phase. However, these methods require the external addition of surface-reactive ligand molecules. Recently, several methods for creating amphiphilic nanoparticles, which can disperse in both aqueous and organic solvents, have been developed,10 and there have been some reports on the © 2012 American Chemical Society
aqueous/organic phase transfer of nanoparticles. In these studies, the aqueous/organic phase transfer of nanoparticles was triggered by external stimuli that altered the surface properties of the nanoparticles.9a,11−13 Wang and co-workers reported that polylactide (PLA)-coated gold nanoparticles were transferred from hydrophobic gels to hydrophilic water phase upon the alkaline hydrolysis of PLA.11 Zhao and co-workers reported that poly(N-isopropylacrylamide) (NIPAm)-coated particles can be transferred between organic and aqueous phases dependent upon temperature change around cloud point.12 We focus on the flexibility of ligand molecules with the aim of making amphiphilic nanoparticles that can pass the aqueous/ organic interface through conformational change rather than by external stimulation. Here, we demonstrate that small and flexible ligands, alkyl-head poly(ethylene glycol) (PEG) derivatives, can provide an amphiphilic feature to the surface of nanoparticles, thereby allowing nanoparticles to cross from a water to an organic phase. These ligands have a flexible hydrophilic PEG region sandwiched between two hydrophobic alkyl chains (Scheme 1). In nonpolar solvents, such as chloroform, the nanoparticles are expected to be well-dispersed through the exposure of their terminal alkylchains (alkyl-head) to solvents. On the contrary, in polar solvents, exposure of the PEG region by bending the molecules should allow the nanoparticles to be dissolved. The conformational change of the surface ligands can elicit a macroscopic change in the dispersibility of the nanoparticles in solvents. As far as we know, Received: January 20, 2012 Revised: March 2, 2012 Published: March 19, 2012 5503
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Scheme 1. Chemical Structures of Alkyl-Head PEG Derivatives Used in This Studya
a
The molecular weight of the PEG region is 600 g/mol (average degree of polymerization; n = 13).
this is the first paper to report the phase transfer of nanoparticles induced by the conformational change of small surface ligands.
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RESULTS AND DISCUSSION We designed alkyl (C2, C4, C6, and C8)-head PEG derivatives (alkyl-head PEG) as flexible molecules responsive to solvent change (Scheme 1). The alkyl-head PEGs were synthesized according to our previous reports14 with some modifications (see details in Supporting Information). First, we examined the water contact angles of self-assembled monolayers (SAMs) of PEG derivatives on a gold substrate. The C8-head and PEGhead were immobilized on gold substrates from ethanol solutions of various concentrations (total 1 mM) by dilution with octanethiol (OT) (percentage of PEG derivatives in the mixture: 100%, 90%, 70%, 50%, 30%, 10%, and 0%). After 24 h immobilization, the substrates were washed with ethanol three times and dried under a N2 atmosphere. The water contact angles (θ) of each SAM are shown in Figure 1. For C8-head, the 100% C8-head SAM showed a similar contact angle to OT SAM (θ = 86° for both 100% and 0%), suggesting that the terminus C8-head region forms a packed layer as OT SAM (see illustration in Figure 1). Interestingly, the diluted C8-head SAMs with OT generated more hydrophilic SAM than the fully covered C8-head SAM. The water contact angle of the C8-head/OT mixed SAMs showed a minimum value at 50−70% C8-head ratio. On the contrary, for PEG-head, the increases in mixing ratio of PEGhead to OT lead to decreases in the contact angle of the SAMs. The model to explain this phenomenon of C8-head/OT SAMs is shown in Figure 1. It is expected that the dilution of C8-head with OT weakens the packing of the terminus C8 residue, allowing the free movement of the inner flexible PEG region. Next, we applied these SAMs to the surface of nanoparticles. PEG derivatives were immobilized on AuNPs (20 nm in diameter) via a surface exchange reaction from citric acid. Briefly, citric acid-coated AuNPs dispersed in methanol were added to a 40 mM methanolic solution of a PEG derivative/OT mixture at the desired ratio. Previous literature showed that the display ratio of two thiolate ligands on gold substrates corresponds to the mixing ratio in the solution.15 Based on the data, we assume that the display ratio of SAMs on AuNPs would correspond to that of the prepared solution. After sonication, the mixture was left to stand for 24 h at room
Figure 1. Water contact angles of PEG-head and C8-head immobilized gold substrates and conceptual diagram of the effect of dilution with OT. Error bars represent standard deviations of three measurements.
temperature. The excess ligands were removed by centrifuge (8000 rpm, 15 min, twice) and redispersed in methanol, chloroform, or water. Figure 2a,b shows photographs of PEGhead and C8-head AuNP dispersions, respectively. The observable red-to-blue color change of the AuNP dispersions is often used to judge soluble state. Red (absorbance peak is around 520−530 nm) and blue (absorbance peak shifts to 560−650 nm) indicate dispersion and aggregation, respectively. In water, the color of AuNP solution with a high ratio of PEGhead (90% or 100%) was red (Figure 2c), and the color of the AuNP solutions changed to purple and then blue as the ratio of PEG-head was decreased. Similar trends in color changes were observed for methanol and chloroform. The AuNPs with low PEG-head ratios (below 30%) were in an aggregated state in all solvents tested. In the case of C8-head AuNPs, the full coverage of AuNPs with C8-head (100%) resulted in an aggregated state in water and methanol, whereas in chloroform, they were welldispersed. Free C8-head ligands were soluble in methanol; however, 100% C8-head AuNPs were not well-dispersed in methanol. This difference is thought to be due to the orientation of the ligands on the AuNPs, in which only the terminal alkyl-heads are exposed to the solvent, resulting in their low dispersibility in methanol. On the other hand, 90% C8-head led to good dispersions of the AuNPs in all the solvents examined: methanol, chloroform, and even water (Figure 2d). In methanol, particularly, C8-head AuNPs were well-dispersed at a wide range of ratios (50−90%). These data for nanoparticle dispersity, as well as the data for SAMs on the flat substrate, support the notion that the co-display of OT with C8-head allows the exposure of the PEG region, giving higher hydrophilicity. 5504
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Figure 2. Photographs of (a) PEG-head and (b) C8-head AuNPs dissolved in water, methanol, and chloroform at each mixing ratio with OT. Absorbance spectra of 100% and 90% (c) PEG-head and (d) C8-head AuNPs dissolved in water.
straightened and that the alkyl-head moiety was dissolved in CDCl3 (Figure 3b). In CDCl3/CD3OD, a decrease in the signal intensity of the terminal methyl group (signal a) and an increase in that of the alkyl (signal b) and PEG (signal c) were observed compared to that in CDCl3. These results support the notion that the alkyl-head moiety of the C8-head ligand was exposed in chloroform, whereas it was hidden in methanol within the inner SAM, thereby exposing the PEG region to the solvent (Figure 3b). This means that the conformational change (i.e., molecular bending) in the PEG residue enables the dispersal of AuNPs in both polar and nonpolar solvents. Finally, we tested the phase-transfer of 90% C8-head AuNPs from water to chloroform (Figure 4). As a control, 90% PEGhead was used. The 90% C8-head AuNPs, dissolved in water, were slowly transferred to a chloroform phase, but the AuNPs dissolved in chloroform were not transferred to a water phase (data not shown). This means that the surface energy of 90% C8-head AuNPs in chloroform is lower than that in water. On the contrary, 90% PEG-head AuNPs in either water or chloroform did not cross the interface. PEG-head AuNPs in chloroform seemed to be accumulated at the interface of the two solvents after 24 h. As suggested in Figure 2a, 90% PEGhead Au NPs were well-dispersed in both chloroform and water. However, in a two-phase system, it has been reported that nanoparticles tend to accumulate at the interface due to thermodynamic stability.8 These data suggest that the flexible alkyl-head is effective in decreasing the surface energy of nanoparticles when crossing the aqueous−organic interface through a change in conformation.
We further explored the effect of alkyl chain length of the head residue (C2-, C4-, and C6-head) on AuNPs dispersion. The dispersity of C2- and C4-head AuNPs showed a similar trend to PEG-head AuNPs, whereas that of C6-head AuNPs followed a similar trend to C8-head AuNPs (Figure S1). These data indicate that an alkyl-head of C6 is long enough to allow the flexible movement of the PEG region upon change of solvents. To study the effect of nanoparticle size, 5 nm AuNPs were also used. Photographs of ligand-coated 5 nm AuNPs in water are shown in Figure S2. Not only 100% PEG-, but also 100% C8-head AuNPs were clearly dispersed in water without the addition of OT. Since 5 nm AuNPs have larger curvature than 20 nm AuNPs, it is expected that there is enough space for the PEG bending even at a ratio of 100%. On the other hand, the addition of 10% OT caused the aggregation of 5 nm AuNPs in water. It was considered that the inner OT layer on the 5 nm AuNPs was more easily exposed than that of the 20 nm AuNPs. This suggests that the optimal density of surface ligands for molecular movement is needed to set for each nanoparticle size. The solvent-dependent conformational change in ligands on 20 nm AuNPs was confirmed by proton nuclear magnetic resonance (1H NMR) analysis. Figure 3a shows NMR spectra of free C8-head ligands in CDCl3 and 90% C8-head AuNPs in CDCl 3, CDCl3 /CD3OD (v/v = 50/50), and CD3 OD, respectively. In the case of 90% C8-head AuNPs, the signals corresponding to the proton neighboring the thiol residue (signal d: 2.5 ppm) was not observed in all solvents tested. This suggests that the thiol-terminal alkyl part was not dissolved in solvents due to tight packing on the AuNPs. In CDCl3, the signal of the terminal methyl group of the alkyl-head (signal a: 0.88 ppm) was clearly observed, whereas those of PEG residues (signal c: 3.7 ppm) and inner alkyl (signal b: 1.3 ppm) were weakened compared to that for the free ligand. This indicates that the structure of the C8-head ligand on AuNPs is
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CONCLUSION We demonstrated that 90% C8-head AuNPs act as amphiphilic nanoparticles. The 90% C8-head AuNPs can be dispersed in water, methanol, and chloroform via the conformational change (bent or straight) of the surface ligands. Importantly, these 5505
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Figure 4. Water−chloroform phase transfer of (a) 90% C8-head and (b) 90% PEG-head AuNPs (20 nm). reagents were purchased from Wako Pure Chemical Industries or Aldrich and used without further purification. The gold substrate was purchased from KENIS. Water contact angles were measured using a DropMaster300 system (Kyowa Interface Science). Absorbance spectra were measured using a UV 1650-PC spectrometer (Shimazu). NMR spectra were measured with an ECX-400 device (JEOL). MALDI-TOF-MS spectra were measured with a Voyager-DE STR-H spectrometer (Applied BioSystems) using 2,5-dihydroxybenzoic acid (Bluker) as the matrix. Photograph images were obtained using a D60 digital camera (Nikon). Contact Angle Measurement. The C8-head and PEG-head were immobilized on gold substrates from ethanol solutions of various concentrations (total 1 mM) by dilution with octanethiol (OT) (percentage of PEG derivatives in the mixture: 100%, 90%, 70%, 50%, 30%, 10%, and 0%). After 24 h immobilization, the substrates were washed with ethanol three times and dried under a N2 atmosphere. A drop of water (2 μL) was deposited on the surface at room temperature and water contact angles were measured with a DropMaster300 system. The error represents the standard deviation for three repeated experiments. Surface Modification of Gold Nanoparticles. The 20 nm citricacid-coated AuNPs (1.2 nM, 1 mL) were purified by centrifugation (8000 rpm, 15 min, twice) and redispersed in 0.5 mL methanol. This methanol solution was added to a 40 mM methanol solution of a PEG derivative/OT mixture (40 mM, 33.5 μL) at the desired ratio. After sonication, the mixture was left to stand for 24 h at room temperature. The excess ligands were removed by centrifugation (8000 rpm, 15 min, twice) and redispersed in 0.5 mL methanol, chloroform, or water (final conc. 2.4 nM). Phase Transfer of Gold Nanoparticles. An aqueous solution of 90% C8- or PEG-head AuNPs (2.4 nM, 200 μL) was added to 200 μL chloroform in a 1.7 mL tube. The tubes were then slowly shaken with a shaker for 24 h at room temperature.
Figure 3. (a) 1H NMR spectra (400 MHz) of free C8-head ligand and 90% C8-head AuNPs dissolved in CDCl3, a mixture of CDCl3/ CD3OD (v/v = 50/50) or CD3OD. (b) A conceptual diagram showing the conformational change in the C8-head on AuNPs in chloroform and methanol.
particles can cross the interface from water to chloroform without external stimulus. Since these translocations are applicable to crossing biological interfaces, such as cell membranes and nuclear pores, our findings expand the potential uses of amphiphilic nanoparticles in drug delivery systems (DDSs). We are currently engaged in research toward the penetration of the cellular membrane using these watersoluble amphiphilic nanoparticles.
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ASSOCIATED CONTENT
S Supporting Information *
Syntheses of PEG derivatives and absorbance spectra of AlkylHead AuNPs. This material is available free of charge via the Internet at http://pubs.acs.org.
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EXPERIMENTAL SECTION
AUTHOR INFORMATION
Corresponding Author
General Information. Citric-acid-coated gold nanoparticles (5 and 20 nm in diameter) were purchased from BBI. All other chemical
*E-mail:
[email protected]. 5506
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Notes
self-assembly of charged nanoparticles at the water/oil interface. Phys. Chem. Chem. Phys. 2006, 8, 3828−3835. (9) (a) Yang, J.; Lee, J. Y.; Ying, J. Y. Phase transfer and its applications in nanotechnology. Chem. Soc. Rev. 2011, 40, 1672−1696. (b) Sperling, R. A.; Parak, W. J. Surface modification, functionalization and bioconjugation of colloidal inorganic nanoparticles. Philos. Trans. R. Soc. London, Ser. A 2010, 368, 1333−1383. (c) Mayya, K. S.; Caruso, F. Phase transfer of surface-modified gold nanoparticles by hydrophobization with alkylamines. Langmuir 2003, 19, 6987−6993. (10) (a) Zubarev, E. R.; Xu, J.; Sayyad, A.; Gibson, J. D. Amphiphilicity-driven organization of nanoparticles into discrete assemblies. J. Am. Chem. Soc. 2006, 128, 4958−4959. (b) Prasad, B. L.; Arumugam, S. K.; Bala, T.; Sastry, M. Solvent-adaptable silver nanoparticles. Langmuir 2005, 21, 822−826. (c) Duan, H.; Kuang, M.; Wang, D.; Kurth, D. G.; Möhwald, H. Colloidal stable amphibious nanocrystals derived from poly[(2-dimethylamino)ethyl methacrylate] capping. Angew. Chem., Int. Ed. 2005, 44, 1717−1720. (11) Mao, Z.; Guo, J.; Bai, S.; Nguyen, T.-L.; Xia, H.; Huang, Y.; Mulvaney, P.; Wang, D. Hydrogen bonding selective phase transfer of nanoparticles across liquid/gel interfaces. Angew. Chem., Int. Ed. 2009, 48, 4953−4956. (12) Li, D.; Zhao, B. Temperature-induced transport of thermosensitive hairy hybrid nanoparticles between aqueous and organic phases. Langmuir 2007, 23, 2208−2217. (13) (a) Edwards, E. W.; Chanana, M.; Wang, D.; Möhwald, H. Stimuli-responsive reversible transport of nanoparticles across water/ oil interfaces. Angew. Chem., Int. Ed. 2008, 47, 320−323. (b) Imura, Y.; Morita, C.; Endo, H.; Kondo, H.; Kawai, T. Reversible phase transfer and fractionation of Au nanoparticles by pH change. Chem. Commun. 2010, 46, 9206−9208. (c) Cheng, L.; Liu, A.; Peng, S.; Duan, H. Responsive plasmonic assemblies of amphiphilic nanocrystals at oilwater interfaces. ACS Nano 2010, 4, 6098−6104. (d) Dorokhin, D.; Tomczak, N.; Han, M.; Reinhoudt, D. N.; Velders, A. H.; Vancso, G. J. Reversible phase transfer of (CdSe/ZnS) quantum dots between organic and aqueous solutions. ACS Nano 2009, 3, 661−667. (e) Qin, B.; Zhao, Z.; Song, R.; Shanbhag, S.; Tang, Z. A temperature-driven reversible phase transfer of 2-(diethylamino)ethanethiol-stabilized CdTe nanoparticles. Angew. Chem., Int. Ed. 2008, 8, 9875−9878. (14) (a) Niikura, K.; Nishio, T.; Akita, H.; Matsuo, Y.; Kamitani, R.; Kogure, K.; Harashima, H.; Ijiro, K. Accumulation of O-GlcNAcdisplaying CdTe quantum dots in cells in the presence of ATP. ChemBioChem 2007, 8, 379−384. (b) Sekiguchi, S.; Niikura, K.; Matsuo, Y.; Yoshimura, S. H.; Ijiro, K.; Ijiro, K. Nuclear transport facilitated by the interaction between nuclear pores and carbohydrates. RSC Adv. 2012, 2, 1656−1662. (15) Nelson, K. E.; Gamble, L.; Jung, L. S.; Boeckl, M. S.; Naeemi, E.; Golledge, S. L.; Sasaki, T.; Castner, D. G.; Campbel, C. T.; Stayton, P. S. Surface characterization of mixed self-assembled monolayers designed for streptavidin immobilization. Langmuir 2001, 17, 2807− 2816.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JST-CREST and KAKENHI 22655050: Grant-in-Aid for challenging Exploratory Research. Sekiguchi appreciates the financial support provided by JSPS. The analysis of NMR was carried out at the OPEN FACILITY, Hokkaido University Sousei Hall. Measurement of water contact angles were carried out at Industrial Research Institute, Hokkaido Research Organization (HRO).
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REFERENCES
(1) (a) Jonas, M. R.; Osberg, K. D.; Macfarlane, R. J.; Langille, M. R.; Mirkin, C. A. Templated techniques for the synthesis and assembly of plasmonic nanostructures. Chem. Rev. 2011, 111, 3736−3827. (b) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591− 3605. (c) Cobley, C. M.; Chen, J.; Cho, E. C.; Wang, L. V.; Xia, Y. Gold nanostructures: a class of multifunctional materials for biomedical applications. Chem. Soc. Rev 2011, 40, 44−56. (d) Algar, W. R.; Prasuhn, D. E.; Stewart, M. H.; Jennings, T. L.; Blanco-Canosa, J. B.; Dawson, P. E.; Medintz, I. L. The controlled display of biomolecules on nanoparticles: A challenge suited to bioorthogonal chemistry. Bioconjugate Chem. 2011, 22, 825−858. (2) (a) Rosi, N. L.; Mirkin, C. A. Nanostructures in biodiagnostics. Chem. Rev. 2005, 105, 1547−1562. (b) Fortina, P.; Kricka, L. J.; Graves; Jason Park, D. J.; Hyslop, T.; Tam, F.; Halas, N.; Surrey, S.; Waldman, S. A. Applications of nanoparticles to diagnostics and therapeutics in colorectal cancer. Trends Biotechnol. 2007, 25, 145− 152. (c) Na, H. B.; Song, I. C.; Hyeon, T. Inorganic nanoparticles for MRI contrast agents. Adv. Mater. 2009, 21, 2133−2148. (d) Medintz, I. L.; Mattoussi, H. Quantum dot-based resonance energy transfer and its growing application in biology. Phys. Chem. Chem. Phys. 2009, 11, 17−45. (e) Niikura, K.; Nagakawa, K.; Ohtake, N.; Suzuki, T.; Matsuo, Y.; Sawa, H.; Ijiro, K. Gold nanoparticle arrangement on viral particles through carbohydrate recognition: A non-cross-linking approach to optical virus detection. Bioconjugate Chem. 2009, 20, 1848−1852. (3) (a) Bardhan, R.; Lal, S.; Joshi, A.; Halas, N. Theranostic nanoshells: From probe design to imaging and treatment of cancer. Acc. Chem. Res. 2011, 44, 936−946. (b) Kelkar, S. S.; Reineke, T. M. Theranostics: Combining imaging and therapy. Bioconjugate Chem. 2011, 22, 1879−1903. (4) Moyano, D. F.; Rotello, V. M. Nano meets biology: Structure and function at the nanoparticle interface. Langmuir 2011, 27, 10376− 10385. (5) Nel, A. E.; Madler, L.; Velegol, D.; Xia, T.; Hoek, E. M. V.; Somasundaran, P.; Klaessig, F.; Castranova, V.; Thompson, M. Understanding biophysicochemical interactions at the nano−bio interface. Nat. Mater. 2009, 8, 543−557. (6) (a) Tan, S. J.; Jana, N. R.; Gao, S.; Pastra, P. K.; Ying, J. Y. Surface-ligand-dependent cellular interaction, subcellular localization, and cytotoxicity of polymer-coated quantum dots. Chem. Mater. 2010, 22, 2239−2247. (b) Maity, A. R.; Jana, N. R. Chitosan−cholesterolbased cellular delivery of anionic nanoparticles. J. Phys. Chem. C 2011, 115, 137−144. (7) Naim, B.; Zbaida, D.; Dagan, S.; Kapon, R.; Reich, Z. Cargo surface hydrophobicity is sufficient to overcome the nuclear pore complex selectivity barrier. EMBO J. 2009, 28, 2697−2705. (8) (a) Binks, B. P. Particles as surfactants-similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (b) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Nanoparticle assembly and transport at liquid-liquid Interfaces. Science 2003, 299, 226−229. (c) Duan, H.; Wang, D.; Kurth, D. G.; Möhwald, H. Directing selfassembly of nanoparticles at water/oil interfaces. Angew. Chem., Int. Ed. 2004, 43, 5639−5642. (d) Reincke, F.; Kegel, W. K.; Zhang, H.; Nolte, M.; Wang, D.; Vanmaekelbergh, D.; Möhwald, H. Understanding the 5507
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