NANO LETTERS
Tailoring the Interfaces between Nematic Liquid Crystal Emulsions and Aqueous Phases via Layer-by-Layer Assembly
2006 Vol. 6, No. 10 2243-2248
Elvira Tjipto,† Katie D. Cadwell,‡ John F. Quinn,† Angus P. R. Johnston,† Nicholas L. Abbott,*,‡ and Frank Caruso*,† Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, Victoria 3010, Australia, and Department of Chemical and Biological Engineering, UniVersity of Wisconsin-Madison, 1415 Engineering DriVe, Madison, Wisconsin 53706 Received July 12, 2006; Revised Manuscript Received August 16, 2006
ABSTRACT We report the assembly of polyelectrolyte multilayer (PEM) films at the interfaces of thermotropic liquid crystal (LC) droplets dispersed in an aqueous phase. Exposure of PEM-coated droplets to surfactant slowed the bipolar-to-radial ordering transition of the LCs by 2 orders of magnitude relative to naked droplets. This shows that PEMs can be used to influence the interactions of analytes with the LC cores of the droplets, allowing tuning of the LC emulsion sensing properties.
Introduction. The ordering of molecules within liquid crystals (LCs) makes them attractive for a variety of sensing and interfacial applications.1 The orientations established at the LC interface are able to propagate into the bulk of the LC (up to 100 µm), thereby leading to optical changes that are observable using polarized microscopy. This approach has been used previously to detect the adsorption and/or organization of lipids,2 proteins,3 and surfactants4 at LC interfaces. Such a method may ultimately be useful for the detection of trace compounds, including toxins, or in other biological assays. To modulate and optimize LC interfacial characteristics, strategies for tailoring the chemical functionality and nanoscale structure of the interfaces of LCs need to be explored. To this end, we have recently shown that it is possible to prepare layer-by-layer (LbL) films on an approximately planar interface between the nematic LC 4′-pentyl-4-cyanobiphenyl (5CB) hosted in a gold grid, and an aqueous phase (water).5 The LbL films preserved the planar anchoring of 5CB in contact with water.5 Furthermore, the multilayer films were observed to mediate the interactions between solutes dissolved in the aqueous phase and the LC. One attractive feature of using LbL assembly to modify the surface properties of LCs is the flexibility of the approach: different * Corresponding author. N. L. Abbott (E-mail:
[email protected]; fax: 608-262-5434) or F. Caruso (E-mail:
[email protected]; fax: + 61 3 8344 4153). † The University of Melbourne. ‡ University of Wisconsin-Madison. 10.1021/nl061604p CCC: $33.50 Published on Web 09/12/2006
© 2006 American Chemical Society
materials can be assembled, including polymers,6 proteins,7 DNA,8 multivalent ions,9 and nanoparticles.10 The technique can also be transferred to three-dimensional substrates, such as colloidal particles,11 biomolecule crystals,12 macroporous membranes,13 and porous beads.14 Despite these studies, very little effort has focused on the LbL coating or modification of emulsions. Recently, three-layered polymer membranes (consisting either of lecithin, chitosan, and pectin or lactoglobulin, carrageenan, and gelatin) were deposited on oilin-water emulsions of corn oil.15,16 These polymers were deposited without intermediate washing steps, and, in some cases, droplet aggregation was observed. Subsequent sonication of these emulsions resulted in individual droplets. Emulsions formed from LCs are particularly promising candidates for the creation of LC-based sensors because they have a much higher surface area than LCs formed at planar interfaces, they are mobile, and frustrated states assumed by LCs within droplets provide an additional opportunity to tune the response of LCs to interfacial events.5 Previous studies on LC emulsion droplets have shown their usefulness for studying a variety of phenomena, including the rotational motion of particles,17 the effect of hydrodynamic flows,18 and the effect of electric fields.19 Such emulsions have been prepared by methods including photopolymerization,19 dispersion polymerization,20 shearing of droplets and subsequent crystallization fractionation,17,21 ultrasonication22 and droplet break-off in a co-flowing stream.23 Notably, Weitz and coworkers have made LC emulsions with monodisperse droplet
Scheme 1.
Structures of the Molecules Used in This Study
sizes23,24 and Terentjev and co-workers have studied the droplet size selection and influence of topological defects on the stability of LC emulsions in water.22,25 Herein, we report the formation of polyelectrolyte multilayers (PEMs) on thermotropic LC (oil)-in-water emulsions. The growth of the poly(styrene sulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) multilayers was characterized with microelectrophoresis, flow cytometry, and fluorescence microscopy. Up to eight PSS/PAH bilayers were deposited, and the effect of the PEM formation on the orientation of the LC was examined using polarized light microscopy. We also examined the influence of the multilayer coating on the interaction between an analyte (surfactant) and the LC core. A bipolar-to-radial ordering transition triggered by exposure of PEM-coated droplets to surfactant was observed to be slowed by 2 orders of magnitude relative to naked LC droplets, thus providing evidence that PEMs can be used to influence the interactions of analytes with the LC core of the emulsion droplets. Finally, PSS/PAHcoated LC droplets were treated with ethanol to dissolve the LC cores and thereby form hollow capsules. These results are significant in that (a) they demonstrate the ability to prepare stable LC emulsions with an easily modifiable interface and (b) they show that LC emulsions can be used to template polyelectrolyte multilayer formation. These developments are important because of the potential benefits in sensing and encapsulation applications. Results and Discussion. Emulsion Preparation. The LCin-water emulsions were formed by sonicating a mixture of 1 vol % 5CB in a dispersant at 10 W for 60 s. Three types of dispersants were used: (a) water, (b) an aqueous solution of a strongly charged polyelectrolyte, poly(styrene sulfonate) (PSS) (1 mg mL-1), and (c) an aqueous solution of a cationic phospholipid, 1,2-dilauroyl-sn-glycero-3-ethylphosphocholine (DLEPC) (700 µM). The resulting emulsions were denoted as 5CB-H2O, 5CB-PSS, and 5CB-DLEPC, respectively. The molecular structures of the materials used are shown in Scheme 1. The LC droplets in all three types of emulsions were spherical, with a size range of 1-10 µm and were visually observed to be stable against coalescence for months. The 5CB-PSS and 5CB-DLEPC emulsions are stable because PSS and DLEPC can associate with the interface of the water-immiscible droplets of 5CB, effectively acting like surfactants to electrostatically stabilize the droplets. 2244
Figure 1. (a) pH dependence of the ζ-potential of uncoated LC emulsions. (b) ζ-potential of (PAH/PSS)-coated 5CB-PSS and 5CBDLEPC emulsions as a function of layer number. The ζ-potential measurements were taken in water (pH ≈ 5.7). Hollow and filled symbols indicate the PSS and PAH adsorption steps, respectively. The error in ζ-potential measurements is (5 mV.
The ζ-potentials of 5CB-H2O, 5CB-PSS, and 5CB-DLEPC droplets in water at pH ∼5.7 were measured to be -40, -45, and +40 mV, respectively. The ζ-potentials were also measured over a wide pH range to determine the isoelectric point (IEP) of the droplets. The IEP for the 5CB-H2O droplets was found to be 5, and the IEP shifted to 3.5 and 7.0 for the 5CB-PSS and 5CB-DLEPC droplets, respectively (Figure 1a). This result is indicative of the PSS and DLEPC providing charge stabilization for the emulsions. Notably, 5CB did not form a stable emulsion in water when PAH was used instead of PSS. This result is similar to our previous findings; PSS can render uncharged pyrene crystals water-dispersible, but PAH does not.26 This suggests that successful emulsification of 5CB with PSS may be attributed to the amphiphilic nature of PSS (the aromatic group is hydrophobic while the charged sulfonate group is hydrophilic) and/or the π-π interactions between the phenyl groups of 5CB and PSS. The stability of the 5CB-H2O emulsion is likely due to the spontaneous adsorption of hydroxide ions at the oilwater interface, which has been reported for surfactant-free emulsions of linear alkanes such as hexadecane and dodecane.27,28 In those studies, the oil droplets were negatively charged and the magnitude of their ζ-potential depended on Nano Lett., Vol. 6, No. 10, 2006
the pH and the ionic strength of the aqueous phase. This phenomenon has been reported to be specific to the hydroxide ion; other cations and anions are not preferentially adsorbed at the oil-water interface. This specificity has led to the suggestion that adsorption of the hydroxide ion may involve hydrogen bonding with the water molecules at the interface.29,30 The 5CB-H2O emulsion exhibits a similar behavior; it is negatively charged as prepared (-40 mV, measured in water at pH ∼ 5.7), and this charge is pH-dependent. The IEP of the 5CB-H2O emulsion (5) is slightly higher than those reported for oil-in-water emulsions of linear alkanes (IEP ca. 3-4).28 Multilayer Coating of Emulsions. The polyelectrolytes for LbL assembly were deposited from 1 mg mL-1 solutions containing 0.1 M NaCl. The electrolyte is added to facilitate polyelectrolyte adsorption. In the absence of salt, the polyelectrolytes adopt a stretched conformation and adsorb as thinner, more rigid layers,31 which might result in incomplete and/or less homogeneous coverage of the LC droplets. The pH of the PAH solution was adjusted to pH 7 (pKa of PAH is ∼8.5)32 while the pH of the PSS solution was not adjusted (PSS is a strong polyelectrolyte). The 5CB-H2O emulsions were negatively charged as prepared; hence, the cationic PAH was deposited as the first layer. Despite the stability of the 5CB-H2O emulsions, the LbL assembly with PAH and PSS did not proceed as well as that with the 5CB-DLEPC or 5CBPSS emulsions. ζ-potential measurements of these samples show inconsistent magnitudes and a broad distribution of charges (data not shown). Flow cytometry was also used to quantify the fluorescence intensity of the 5CB-H2O emulsion droplets after alternating adsorption of fluorescein isothiocyanate-labeled PAH (PAH-FITC) and PSS. Although there was a general increase in fluorescence intensity over 14 adsorption steps, the intensity was less than half of that observed with 5CB-PSS and 5CB-DLEPC emulsions (data not shown). We also observed desorption of PAH-FITC after coating; the fluorescence measured in the aqueous phase of 5CB-H2O emulsions with a PAH-FITC-terminated layer increased slowly after the adsorption and washing steps. These results indicate that PAH-FITC was only weakly bound to the 5CB-H2O core, resulting in incomplete polymer coverage of the droplets and that the surface of these droplets have both positive and negative charges, accounting for the poorer stability of these emulsions. Alternatively, the 5CB-PSS and 5CB-DLEPC emulsions were coated successfully with at least 16 layers of PAH and PSS. The alternating ζ-potentials for both the 5CB-PSS and 5CB-DLEPC emulsions during LbL assembly (between +40 mV and -50 mV) are consistent with stepwise adsorption of cationic PAH and anionic PSS (Figure 1b). These values are similar to the ζ-potentials of PSS and PAH alternately deposited on charged solid particles.11,26,33 As each layer is adsorbed, the surface charge is reversed, allowing the adsorption of the next layer via electrostatic interactions. However, ζ-potential measurements only serve as a qualitative indication of multilayer growth. As such, PAHFITC was used to further confirm multilayer growth on 5CBPSS and 5CB-DLEPC emulsions using flow cytometry Nano Lett., Vol. 6, No. 10, 2006
Figure 2. (a) Fluorescence intensity of (PAH-FITC/PSS)-coated 5CB-PSS and 5CB-DLEPC emulsions as a function of layer number, as measured by flow cytometry. The measurements were taken after deposition of each bilayer (after each PAH-FITC layer). (b) Fluorescence image of the 5CB-PSS emulsion coated with seven bilayers of PAH-FITC/PSS. The scale bar is 10 µm.
(Figure 2a). The growth in fluorescence intensity was approximately linear after the third and first layer for 5CBPSS and 5CB-DLEPC emulsions, respectively. For the 5CBPSS system, it takes a few deposition steps for the PEM buildup to become regular. This can be explained by substrate effects, which are commonly observed in the first few layers in LbL systems.34,35 A fluorescence image of 5CB-PSS coated with seven bilayers of PAH-FITC/PSS shows uniform fluorescence around the droplets, confirming the flow cytometry results (Figure 2b). Similar images were obtained for 5CB-DLEPC emulsions coated with seven bilayers of PSS/PAH-FITC (data not shown). The results obtained from microelectrophoresis, flow cytometry, and fluorescence microscopy demonstrate that PEM growth at the mobile interface of 5CB emulsion droplets is comparable to similar multilayers assembled on solid, charged colloidal particles. Orientation of 5CB within PEM-Coated Emulsion Droplets. Polarized light microscopy was used to study the orientation of the 5CB within the droplets before and after deposition of the PEM films. Past studies have established that the orientations of a LC within a droplet depend on factors such as the bulk elasticity of the LC, the orientation of the easy axis of the LC at the interface of the droplet, and the anchoring energy of the LC. For sufficiently large droplets, surface anchoring dominates, which results in droplets that contain topological defects at equilibrium.36 We observed the orientation of 5CB in the emulsion droplets to 2245
Figure 3. Director profiles and optical micrographs of (a) 5CB-DLEPC emulsions and (b) 5CB-H2O emulsions. The scale bars are 2 µm. Homeotropic anchoring of 5CB at the interface of a 5CB-DLEPC emulsion droplet results in a radial director profile with a point defect at the droplet center (Figure 3a), whereas planar anchoring of 5CB at the interface of a 5CB-H2O emulsion droplet results in a bipolar configuration with two surface defects at the droplet interface (Figure 3b). The droplets shown in the top and bottom rows of Figure 3b possess two surface defects that lie in the image plane: the droplets differ in the orientation of the defects with respect to the polarization of the incident light.
be independent of droplet size for the size range produced (1-10 µm). This implies that the droplets are at the limit of strong anchoring. Previous studies have also shown that planar anchoring of 5CB at the interface results in a bipolar configuration where each droplet contains two or more point defects at the interface, known as “boojums”.36 However, homeotropic anchoring of 5CB at the interface results in a radial configuration, where each droplet has a point defect at its center, known as a “hedgehog”.36 Optical images (crossed polars) of 5CB-H2O and 5CB-DLEPC emulsion droplets are shown in Figure 3 along with illustrations of the director profiles of the 5CB within the droplets. The 5CBH2O emulsion droplets have a bipolar configuration with two point defects at the surface, while the 5CB-DLEPC emulsion droplets have a radial configuration, with characteristic crosslike appearances. The appearance of the 5CB-PSS emulsion droplets was similar to the 5CB-H2O droplets (data not shown). The orientations of the LC at the interfaces of the droplets are similar to the orientations of 5CB and DLEPCdecorated 5CB reported previously at planar LC-aqueous interfaces.5 Figure 4 shows optical images (crossed polars) of 5CB-PSS and 5CB-DLEPC emulsion droplets coated with five bilayers of PAH/PSS. The results reveal that the bipolar and radial configurations of the LC within the 5CB-PSS and 5CB-DLEPC emulsion droplets are preserved during multilayer coating. Influence of PEMs on the Interaction of Surfactant with LC Emulsion Droplets. PEM-coated LC emulsion droplets appear to be promising systems for the creation of biological 2246
and chemical sensors because the presence of the PEMs provides a means to tune the interaction of analytes with the LC core. As a first step in this direction, we explored how the presence of a PEM influences the interaction of a model analyte (a surfactant) with LC droplets. In previous studies, the exposure of 5CB to surfactants such as sodium dodecyl sulfate (SDS, anionic) has been shown to cause a rapid ordering transition from a planar to a homeotropic orientation of 5CB.4,5 Our preliminary studies on the uncoated and (PAH/PSS)7-coated LC emulsion droplets also showed that 5 mM SDS triggered an ordering transition of 5CB from a bipolar to radial configuration within the droplet. This transition was reported by the appearance of crosses on the emulsion droplets when viewed with cross-polarizers (Figure 5). Although the ordering transition within the uncoated droplets occurred within a few seconds, the change took about 5 min to occur for the (PAH/PSS)7-coated emulsions. This result indicates that the SDS can permeate the PEM to interact with the LC core of the droplets but the rate of arrival of SDS at the LC core is slowed by approximately 2 orders of magnitude by the PEM. Further studies on the interactions of other analytes with PEM-coated LC emulsion droplets will be reported in a forthcoming publication. Hollow Capsules. To further verify the structural integrity of the PEM coatings on LC emulsions, we selectively removed the LC core of the PEM-coated emulsion droplets by dissolution of the LC with ethanol to yield hollow polyelectrolyte capsules. We found that hollow capsules could only be obtained from multilayer-coated emulsions Nano Lett., Vol. 6, No. 10, 2006
Figure 4. Cross-polarized images of (a) 5CB-PSS emulsions coated with (PAH/PSS)5 and (b) 5CB-DLEPC emulsions coated with (PSS/ PAH)5. The scale bars are 5 µm.
Figure 6. (a) TEM images of hollow capsules obtained from 5CBPSS emulsions coated with (PAH/PSS)5 and (b) an AFM image of a hollow capsule obtained from 5CB-DLEPC emulsions coated with (PSS/PAH)5/PSS. The z scale is 245 nm.
Figure 5. Cross-polarized images of 5CB-PSS emulsions coated with (PAH/PSS)7 after exposure to 5 mM SDS. The scale bar is 10 µm.
with a PSS-terminated layer. Emulsions coated with a PAHterminated layer aggregated upon treatment with ethanol. It is possible that the ionization of PAH at the surface decreases (via conversion of the ammonium groups to amines) when exposed to ethanol, thereby destabilizing the emulsion. Decreasing solvent polarity (e.g., increasing ethanol content) of the PAH deposition solutions can result in a dramatic increase of layer thickness as a consequence of the polyNano Lett., Vol. 6, No. 10, 2006
electrolytes adopting a more coiled conformation.37 The PSSterminated multilayer-coated emulsions turned clear immediately after the addition of ethanol. This is a common indication of core dissolution in other multilayer systems where solid core particles are used.11,38 The hollow capsules were characterized with transmission electron microscopy (TEM) and atomic force microscopy (AFM). The capsule walls have a grainy texture, which is probably due to slight rearrangement of polyelectrolytes when they were exposed to ethanol.37 When dried, the capsules collapse and the folds are visible from TEM and AFM images (Figure 6). AFM was used to obtain the thickness of the capsule walls by taking a cross-sectional profile of the capsules where it was folded only once. Hence, the measured thickness corresponds to twice the multilayer wall thickness. The average thickness per layer was deter2247
mined by taking the average thickness of several crosssectional profiles and dividing by twice the total number of layers deposited. The layer thickness was calculated to be approximately 1.4 nm for both multilayer-coated 5CB-PSS and 5CB-DLEPC emulsions, which corresponds well to typical layer thicknesses of the same polyelectrolyte multilayers assembled on planar and colloidal supports.11,33,38-40 The current study also demonstrates that thermotropic LCs, despite having a mobile interface, can be used as cores in core-shell systems. Our future work will concentrate on the formation of monodisperse emulsions using microfluidics.23 The ability to synthesize monodisperse emulsions at a desired size will overcome difficulties currently faced when using solid particles as templates of polyelectrolyte capsules. For example, it is often difficult to completely remove melamine formaldehyde (MF) cores; residual MF typically remains in the capsule walls.38 Conclusions. Stable emulsions of nematic 5CB in water were formed by sonication. Layer-by-layer growth of multilayers of PAH and PSS on micrometer-sized 5CB-PSS and 5CB-DLEPC emulsion droplets was confirmed with ζ-potential measurements, flow cytometry, and fluorescence microscopy. The LC in the 5CB-PSS emulsion droplets assumes a bipolar configuration, whereas the LC in the 5CBDLEPC emulsion droplets adopts a radial configuration, and these configurations were preserved after PEM assembly. In addition, the multilayer can be used to influence the interaction of analyte with the LC core. When the 5CB cores of the multilayer-coated emulsion droplets were dissolved with ethanol, hollow capsules were formed, indicating homogeneous surface coverage of polymers on the emulsion droplets. The results reported are significant for a number of reasons. First, the results demonstrate that LbL assembly can be applied to the mobile interfaces between LC droplets and aqueous phases, which represents an important development toward the preparation of chemically tailored interfaces for use in micrometer-sized biological and chemical sensors. In addition, this approach should be applicable to other oils, allowing a versatile route for producing hollow polymeric capsules. Such a development offers considerable advantages over solid templates. Acknowledgment. We thank Jugal K. Gupta (University of Wisconsin), Nathan A. Lockwood (University of Wisconsin), and George V. Franks (University of Melbourne) for helpful discussions and Yajun Wang (University of Melbourne) for TEM measurements. This work was supported by the Australian Research Council under the Federation Fellowship and Discovery Project schemes, by the Victorian State Government under the STI Initiative, and partially supported by the NSF (DMR-0602570, NSEC program DMR-0425880 and CTS-0327489) and the U.S. Army Research Office (DAAD190210299). The Particulate Fluids Processing Centre at the University of Melbourne is acknowledged for infrastructure support. Supporting Information Available: Materials and methods. This material is available free of charge via the Internet at http://pubs.acs.org. 2248
References (1) Shah, R. R.; Abbott, N. L. Science 2001, 293, 1296. (2) Brake, J. M.; Daschner, M. K.; Luk, Y. Y.; Abbott, N. L. Science 2003, 302, 2094. (3) Lockwood, N. A.; Abbott, N. L. Curr. Opin. Colloid Interface Sci. 2005, 10, 111. (4) Brake, J. M.; Mezera, A. D.; Abbott, N. L. Langmuir 2003, 19, 6436. (5) Lockwood, N. A.; Cadwell, K. D.; Caruso, F.; Abbott, N. L. AdV. Mater. 2006, 18, 850. (6) Hammond, P. T. Curr. Opin. Coll. Interface Sci. 2000, 4, 430. (7) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427. (8) Johnston, A. P. R.; Read, E. S.; Caruso, F. Nano Lett. 2005, 5, 953. (9) Hatzor, A.; Moav, T.; Cohen, H.; Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13469. (10) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101. (11) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (12) Caruso, F.; Trau, D.; Mo¨hwald, H.; Renneberg, R. Langmuir 2000, 16, 1485. (13) Liang, Z. J.; Susha, A.; Yu, A. M.; Caruso, F. AdV. Mater. 2003, 15, 1849. (14) Wang, Y. J.; Caruso, F. Chem. Mater. 2005, 17, 953. (15) Ogawa, S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2004, 52, 3595. (16) Gu, Y. S.; Decker, A. E.; McClements, D. J. Langmuir 2005, 21, 5752. (17) Hsu, P.; Poulin, P.; Weitz, D. A. J. Colloid Interface Sci. 1998, 200, 182. (18) Tong, P.; Xia, K. Q.; Ackerson, B. J. J. Chem. Phys. 1993, 98, 9256. (19) Cairns, D. R.; Sibulkin, M.; Crawford, G. P. Appl. Phys. Lett. 2001, 78, 2643. (20) Vennes, M.; Zentel, R. Macromol. Chem. Phys. 2004, 205, 2303. (21) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474. (22) Tixier, T.; Heppenstall-Butler, M.; Terentjev, E. M. Langmuir 2006, 22, 2365. (23) Umbanhowar, P. B.; Prasad, V.; Weitz, D. A. Langmuir 2000, 16, 347. (24) Utada, A. S.; Lorenceau, E.; Link, D. R.; Kaplan, P. D.; Stone, H. A.; Weitz, D. A. Science 2005, 308, 537. (25) Heppenstall-Butler, M.; Williamson, A. M.; Terentjev, E. M. Liq. Cryst. 2005, 32, 77. (26) Caruso, F.; Yang, W. J.; Trau, D.; Renneberg, R. Langmuir 2000, 16, 8932. (27) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045. (28) Beattie, J. K.; Djerdjev, A. M. Angew. Chem., Int. Ed. 2004, 43, 3568. (29) Franks, G. V.; Djerdjev, A. M.; Beattie, J. K. Langmuir 2005, 21, 8670. (30) Beattie, J. K.; Djerdjev, A. M.; Franks, G. V.; Warr, G. G. J. Phys. Chem. B 2005, 109, 15675. (31) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (32) Ochiai, H.; Anabuki, Y.; Kojima, O.; Tominaga, K.; Murakami, I. J. Polym. Sci., Part B: Polym. Phys. 1990, 28, 233. (33) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (34) Bosio, V.; Dubreuil, F.; Bogdanovic, G.; Fery, A. Colloids Surf., A 2004, 243, 147. (35) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249. (36) Lavrentovich, O. D. Liq. Cryst. 1998, 24, 117. (37) Poptoshev, E.; Schoeler, B.; Caruso, F. Langmuir 2004, 20, 829. (38) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (39) Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (40) Leporatti, S.; Voigt, A.; Mitlohner, R.; Sukhorukov, G.; Donath, E.; Mo¨hwald, H. Langmuir 2000, 16, 4059.
NL061604P Nano Lett., Vol. 6, No. 10, 2006
