Characterization of the Growth of Polyelectrolyte Multilayers Formed at

DOI: 10.1021/la800013f. Publication Date (Web): April 18, 2008. Copyright © 2008 American Chemical Society. * Corresponding authors. (F.C.) E-mail: ...
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Characterization of the Growth of Polyelectrolyte Multilayers Formed at Interfaces between Aqueous Phases and Thermotropic Liquid Crystals Jugal K. Gupta,† Elvira Tjipto,‡ Alexander N. Zelikin,‡ Frank Caruso,*,‡ and Nicholas L. Abbott*,† Department of Chemical & Biological Engineering, UniVersity of WisconsinsMadison, 1415 Engineering DriVe, Madison, Wisconsin 53706, and Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, Victoria 3010, Australia ReceiVed January 2, 2008. ReVised Manuscript ReceiVed February 17, 2008 Polyelectrolyte multilayers (PEMs) formed at interfaces between aqueous solutions and thermotropic (waterimmiscible) liquid crystals (LCs) offer the basis of a new method to tailor the nanometer-scale structure and chemical functionality of these interfaces. Toward this end, we report a study that compares the growth of PEMs formed at mobile and deformable interfaces defined by LCs relative to growth observed at model (rigid) solid surfaces. Experiments aimed at determining if polyelectrolytes such as poly(sodium-4-styrenesulfonate) (PSS) can partition from the aqueous phase into the bulk of the LC yielded no evidence of such partitioning. Whereas measurements of the growth of PEMs formed from poly(allylamine hydrochloride) (PAH) and PSS at the aqueous-LC interface revealed growth characteristics similar to those measured at both hydrophobic and hydrophilic interfaces of solids, the growth of PEMs from PAH and poly(acrylic acid) (PAA) at the aqueous-LC interface was found to differ substantially from the solids investigated: (i) the linear growth of PEMs of PAH/PAA that was measured at the aqueous-LC interface under conditions that did not lead to the growth of PEMs at the interface of octadecyltrichlorosilane (OTS)-treated glass (a hydrophobic solid surface), and (ii) in comparison to the growth of PEMs of PAH/PAA at the surface of glass (a hydrophilic charged surface), a higher rate of growth was observed at the aqueous-LC interface. The finding that the growth rate of PEMs of PAH/PAA at aqueous-LC interfaces is greater than on solid surfaces is supported by additional measurements of growth as a function of pH. Finally, the pH-triggered reorganization of PAH/PAA PEMs supported at the aqueous-LC interface led to changes in the order and optical properties of the LC. These data are discussed in light of the nature of aqueous-LC interfaces, including the mobility and deformability of the interface and recent measurements of the zeta-potentials of aqueous-LC interfaces.

Introduction Recent studies have demonstrated that highly cooperative and long-range ordering transitions that propagate from the interfaces of liquid-crystalline materials can provide methods to amplify and transduce molecular and biomolecular interactions at these interfaces.1–4 For example, at interfaces between aqueous phases and water-immiscible (thermotropic) liquid crystals (LCs), the self-assembly of surfactants and lipids,5–7 enzymatic reactions,1 and protein binding events2,3 have been shown to trigger ordering transitions in micrometer-thick films of LCs. The realization of general and facile methods to control the nanoscopic structure and chemical functionality of such interfaces will provide additional means to exploit LCs as amplifiers of chemical and biological phenomena at interfaces. * Corresponding authors. (F.C.) E-mail: [email protected]. Fax: +61 3 8344 4153. (N.A.) E-mail: [email protected]. Fax: 1 +608262-5434. † University of WisconsinsMadison. ‡ University of Melbourne. (1) Brake, J. M.; Daschner, M. K.; Luk, Y.-Y.; Abbott, N. L. Science 2003, 302, 2094–2097. (2) Gupta, V. K.; Skaife, J. J.; Dubrovsky, T. B.; Abbott, N. L. Science 1998, 279, 2077–2080. (3) Brake, J. M.; Abbott, N. L. Langmuir 2007, 23, 8497–8507. (4) Jang, C. H.; Cheng, L. L.; Olsen, C. W.; Abbott, N. L. Nano Lett. 2006, 6, 1053–1058. (5) Brake, J. M.; Abbott, N. L. Langmuir 2002, 16, 6101–6109. (6) Brake, J.; Daschner, M.; Abbott, N. Langmuir 2005, 21, 2218–2228. (7) Gupta, J. K.; Meli, M. V.; Teren, S.; Abbott, N. L. Phys. ReV. Lett. 2008, 100, 048301–048304.

Toward this end, our recent studies8,9 have demonstrated that it is possible to grow polyelectrolyte multilayers (PEMs) by the sequential deposition of poly(sodium-4-styrenesulfonate) (PSS)/ poly(allylamine hydrochloride) (PAH) at interfaces formed between thermotropic LCs and aqueous interfaces. In one study,8 an approximately planar interface between the nematic LC 4′pentyl-4-cyanobiphenyl (5CB) and an immiscible aqueous solution (Figure 1a) was stabilized by hosting the LC within an electron microscopy grid. Sequential contact of this interface with PSS and fluorescein isothiocyanate-labeled PAH (FITCPAH) provided the first evidence that PEMs can be formed at these interfaces. In an another study,9 the formation of PEMs from PSS and PAH at the curved interfaces of micrometer-sized LC emulsion droplets was demonstrated. The growth of the PEMs on the emulsion droplets was confirmed by microelectrophoresis, flow cytometry, and fluorescence microscopy (using FITC-PAH). In addition to providing evidence for the growth of PEMs at aqueous-LC interfaces, both prior studies demonstrated that the PEMs can be exploited to mediate the interactions of analytes (surfactants) added to the aqueous phase with the LC, thus tailoring anchoring transitions within the LC. Dissolution of the LC was also demonstrated to lead to unsupported films and capsules of PEMs. The current article provides the results of a comprehensive study of the growth of PEMs at aqueous-LC interfaces. Whereas most (8) Lockwood, N. A.; Cadwell, K. D.; Caruso, F.; Abbott, N. L. AdV. Mater. 2006, 18, 850–854. (9) Tjipto, E.; Cadwell, K. D.; Quinn, J. F.; Johnston, A. P. R.; Abbott, N. L.; Caruso, F. Nano Lett. 2006, 6, 2243–2248.

10.1021/la800013f CCC: $40.75  2008 American Chemical Society Published on Web 04/18/2008

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Figure 1. (A) Schematic illustration of the experimental geometry used to prepare interfaces between aqueous phases and immiscible thermotropic LCs. (B) Structures of the molecules used in this study.

past reports provide a detailed characterization of the growth of PEMs at solid surfaces, few studies have examined the formation of PEMs at mobile interfaces such as air–water10 or water–oil.8,9 Whether PEMs generally grow at these mobile and deformable interfaces in a manner comparable to that of aqueous interfaces of solids is not known. In this article, we compare and contrast the growth of PSS/PAH and PAH/poly(acrylic acid) (PAA) PEMs formed at aqueous-LC interfaces to growth at several hydrophilic and hydrophobic interfaces of solids. By using ellipsometry and fluorescently labeled polyelectrolytes, we characterize the incorporation of polyelectrolytes into the PEMs formed at these various interfaces as well as provide estimates of the thicknesses of PEMs formed at the aqueous-LC interfaces. For PSS/PAH PEMs, by varying the thickness of the film of the LC used to support the multilayer, we address the possibility of partitioning of amphiphilic polyelectrolytes such as PSS from the aqueous phase into the bulk of the LC. In this study, we also demonstrate that it is possible to form PEMs from two weak polyelectrolytes (PAA and PAH) at the aqueous interface of a thermotropic LC. In contrast to PEMs formed from PSS and PAH, we observe the growth of the PAA/PAH PEMs at the interface of the LC to differ substantially from the growth of the same PEMs at hydrophobic and hydrophilic interfaces of solids. In particular, we observe the growth of the PEMs at the aqueous interface of (10) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871– 8878.

the LC under conditions that do not lead to the growth of PEMs at solid interfaces. These results are discussed in light of the known properties of aqueous-LC interfaces. Past studies performed on solid-supported PEMs have demonstrated that PAH/ PAA PEMs (deposited from solutions at pH 7.5 and 3.5, respectively) reorganize and become highly porous upon treatment with acidic solutions.11,12 We sought to determine whether comparable reorganization of PEMs takes place at aqueous-LC interfaces and the impact of any such reorganization on the ordering of LC in contact with the PEM. These results, when combined, provide an understanding of the factors that govern the assembly of PEMs at LC interfaces, which is necessary for the adaptation of the assembly process to LC materials that hold promise in chemical and biological sensing.

Materials and Methods Materials. Poly(sodium-4-styrenesulfonate) (PSS, Mw 70 kDa) and poly(allylamine hydrochloride) (PAH, Mw 70 kDa) were purchased from Sigma-Aldrich (Milwaukee, WI) and used without further purification. Poly(acrylic acid) (PAA, Mw 70 kDa) sodium salt was purchased from Polysciences, Inc. (Warrington, PA) and used without further purification. Fluorescein isothiocyanate-labeled (11) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017–5023. (12) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59–63.

5536 Langmuir, Vol. 24, No. 10, 2008 PAH (FITC-PAH, Mw 70 kDa),13 methacryloxyethyl thiocarbamoyl rhodamine B-labeled PSS (Rh-PSS, Mw 45 kDa),14 and rhodamine isothiocyanate-labeled PAH (Rh-PAH, Mw 45 kDa)15 were prepared as described elsewhere. The nematic LC 5CB was purchased from EMD Chemicals (Hawthorne, NY) and used without further purification. Gold specimen grids (bars with thicknesses of 5, 10, 15 or 20 µm, a width of 55 µm, and a spacing of 283 µm, Figure 1a) were obtained from Electron Microscopy Sciences (Fort Washington, PA). The deionization of a distilled water source was performed with a Milli-Q system (Millipore, Bedford, MA) to give water with a resistivity of 18.2 MΩ cm. Glass microscope slides were Fisher’s finest premium grade obtained from Fisher Scientific (Pittsburgh, PA). Octadecyltrichlorosilane (OTS) was obtained from Fisher Scientific. Epoxy resin and hardener (2 Ton Clear) was obtained from ITW Devcon (Danvers, MA). Figure 1b shows the molecular structures of the LC and polymers used in this study. Preparation of Optical Cells. A detailed description of the methods used to prepare and optically examine the LC can be found in a previous publication.5 Briefly, glass microscope slides were cleaned according to published procedures and coated with octadecyltrichlorosilane (OTS).5 The quality of each OTS layer was assessed by checking the alignment of 5CB within the LC-filled grid prior to the immersion of the grid under water. A small square of OTS-coated glass (ca. 5 mm × 5 mm) was fixed to the bottom of each well of an eight-well chamber slide (Nalge Nunc International, Rochester, NY) with epoxy and cured overnight at 60 °C. The wells were rinsed several times with ethanol to remove uncured monomer and subsequently dried. Gold specimen grids that were cleaned sequentially in ethanol, methanol, and methylene chloride were placed on the surface of the OTS-treated glass slides, one per well. Approximately 1 µL of 5CB was dispensed onto each grid, and the excess LC was removed with a syringe (Figure 1a). Layer-by-Layer Formation of PEMs at Aqueous-LC Interfaces. Aqueous solutions used to form PEM films were 1 mg mL-1 (PSS, PAH, Rh-PAH, FITC-PAH, and PAA) or 0.2 mg mL-1 (FITCPAH and Rh-PSS) in 0.5 M NaCl. The pH values of solutions of PAA, PAH, Rh-PAH and FITC-PAH used to form PSS/PAH and PAA/PAH multilayers were adjusted to the desired value by using NaOH or HCl; we confirmed the pH of each polyelectrolyte solution before and after the deposition of polyelectrolyte multilayers. The pH was not adjusted for PSS. PEM films were prepared by exposing the 5CB interfaces to the appropriate solutions in the wells of an eight-well chamber slide for 15 min. The formation of PSS/PAH multilayers was initiated by the adsorption of PSS, and PAH/PAA multilayer growth was commenced by the adsorption of PAH. After incubation with a polyelectrolyte, the solution in the sample well was exchanged three times with water (1, 2, and 2 min) to remove free polyelectrolyte from the well.8 This procedure removed all measurable free polyelectrolyte in the wells, as indicated by the lack of fluorescence in the final rinsewater following FITC-PAH incubations. Layer-by-Layer Formation of PEMs on Glass and Silicon Substrates with or without an OTS Coating. OTS-coated glass or silicon substrates were prepared as described above. Glass and silicon substrates used for growing multilayers were RCA cleaned.16 Solutions used for PEM film formation were identical to those described above for PEM formation at the aqueous-LC interface. To prepare samples for ellipsometric measurements, PEM films were prepared by placing a silicon substrate in 2 mL of the appropriate solution for 15 min. After incubation with polyelectrolyte, the substrate was removed and rinsed three times in water (incubated for 1 min in each rinse) before moving the slide to the next polyelectrolyte solution. For fluorescence measurements, glass substrates were glued to the chamber slide as described above, and (13) Caruso, F.; Yang, W. J.; Trau, D.; Renneberg, R. Langmuir 2000, 16, 8932–8936. (14) Johnston, A. P. R.; Zelikin, A. N.; Lee, L.; Caruso, F. Anal. Chem. 2006, 78, 5913–5919. (15) Ge, L. Q.; Mohwald, H.; Li, J. B. Biochem. Biophys. Res. Commun. 2003, 303, 653–659. (16) Kern, W. RCA ReV. 1978, 39, 278–308.

Gupta et al. the multilayers were grown in a manner similar to those grown on the aqueous-LC interface. Fluorescence Microscopy of PEM Films. Fluorescence images were obtained using an Olympus IX-71 inverted microscope at 4× magnification equipped with a Chroma 41001 fluorescence filter cube (HQ480/40x excitation filter, G505LP dichroic, and HQ535/ 50m emission filter) and a Chroma 41004 rhodamine filter cube (HQ560/55x excitation filter, Q595LP dichroic, and HQ645/75m emission filter). Images were acquired with a monochrome Hamamatsu digital camera controlled with SimplePCI software. The fluorescein and rhodamine exposure times were 2 and 0.01 s, respectively. Fluorescence images of aqueous-LC interfaces were taken with the LC/OTS-treated glass interface facing toward the objective. Ellipsometry of PEMs on OTS-Coated Silicon. Ellipsometry measurements were made with a Gaertner LSE ellipsometer (λ ) 632.8 nm, Ψ ) 70°). The substrate parameters (ns ) 3.85, ks ) -0.02) were determined by averaging three measurements for a cleaned silicon wafer. The reported ellipsometric thickness of the PEM layers is the average of four points on each of eight samples. In this study, all experimental measurements obtained using LCs are accompanied by error bars calculated as the standard deviation obtained from 16 measurements.

Results and Discussion Growth Behavior of PEMs Formed from PSS and PAH at Aqueous-LC Interfaces. The first experiments reported in this article characterize the growth of PEMs formed from PAH and PSS at aqueous-LC interfaces. The LC used in these studies was 5CB, which forms a nematic phase under the conditions of the experiments reported below. These studies were performed by using PSS labeled with methacryloxyethyl thiocarbamoyl rhodamine B (Rh-PSS) or PAH labeled with FITC (FITC-PAH). Past studies have demonstrated that high growth rates of PEMs are obtained when the charge density of a weak polyelectrolyte is 70–90% of the fully charged state.17 In view of this past result, the solutions of the weak polyelectrolyte PAH used in our studies were adjusted to pH 8 (pKa between 8 and 9).17 PSS is a strong polyelectrolyte,8 and thus the pH of solutions from which PSS was adsorbed was not adjusted. In each of the experiments reported in Figure 2, the interface of the LC was first contacted with PSS (or Rh-PSS) because PSS is known to adsorb to hydrophobic surfaces.13 In experiments described below, however, we report that PAH also adsorbs directly to the interface of 5CB in the absence of a PSS layer. Figure 2A shows that the fluorescence intensity from Rh-PSS (measured after the deposition of each Rh-PSS layer) grows linearly with the number of layers of Rh-PSS deposited within the PEM. It is also evident, however, that the increment in fluorescence intensity associated with the deposition of the first layer of Rh-PSS (8.5 AU) is substantially larger than the increment in fluorescence intensity associated with deposition of subsequent layers of Rh-PSS (1.6 AU/layer). Possible explanations for this phenomenon include that (i) the amount of Rh-PSS adsorbed onto the interface of the 5CB (first layer) is greater than the amount of Rh-PSS deposited in subsequent layers of the PEM or (ii) Rh-PSS dissolves into the bulk of the film of LC upon initial contact with the aqueous solution of Rh-PSS. To distinguish between these two possibilities, we performed measurements on the adsorption of Rh-PSS onto the interfaces of films of LC that each possessed a thickness of 5, 10, 15, or 20 µm (Figure 3). The thickness of each LC film was determined by the thickness of the TEM grid used in the experiment. We initially performed these measurements using the same imaging conditions that were used to obtain the data in Figure 2. In an attempt to provide a (17) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116–124.

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Figure 3. Fluorescence intensity due to the adsorption of Rh-PSS (1 mg mL-1, 0.5 M NaCl) at the aqueous-LC interface. The LC film thickness was either 5, 10, 15, or 20 µm.

Figure 2. Characterization of the growth of PSS/PAH multilayers at the aqueous-5CB interfaces. Fluorescence intensity of (A) Rh-labeled PSS during the growth of PSS-Rh (0.2 mg mL-1)/PAH (1 mg mL-1, pH 8) PEMs and (B) fluorescence intensity of FITC-PAH during the growth of PSS (1 mg mL-1)/FITC-PAH (0.2 mg mL-1, pH 8) PEMs at the aqueous-5CB interface. Fitted lines show the linear growth of the fluorescence intensity in both cases. PEMs were formed in the presence of 0.5 M NaCl.

more stringent test of the amount of Rh-PSS adsorbed onto the interface, we also collected the fluorescence intensity using imaging conditions (an exposure time of 0.02 s and a gain of 120) that led to high fluorescence intensities (also shown in Figure 3). An inspection of Figure 3 reveals that there is no statistically significant dependence of the fluorescence intensity on the thickness of the films of LC, indicating the absence of measurable

partitioning of the Rh-PSS into the bulk of the LC. This result is consistent with past studies18 of mixtures of polymers and low molecular weight LCs, which have reported that flexible polymers are excluded (phase separate) from the LCs. Flexible polymers are typically excluded from nematic LCs (at equilibrium) because of the constraint of the order that the LC imposes on the configurational degrees of freedom of the polymer. Hence, we conclude from the data in Figure 2A that the amount of Rh-PSS adsorbed onto the fresh interface of 5CB is greater than that incorporated into subsequent layers of the PEM. Figure 2B reveals that the growth of the fluorescence intensity of the FITC-PAH in the PEM (measured after the deposition of each FITC-PAH layer) is also linear with the layer number. We note that the absolute values of fluorescence intensity cannot be directly compared between parts A and B of Figure 2 because the measurements were performed with different fluorophores as well as imaging conditions. The first layer of FITC-PAH was deposited onto the PSS-laden interface of the LC, and an extrapolation of the growth curve passes through the origin. This contrasts with the deposition of the first layer of Rh-PSS, where the first layer that was deposited was substantially greater than subsequent layers (Figure 2A). Overall, with the exception of the deposition of the initial Rh-PSS layer, we note linear increases in the fluorescence for the multilayers incorporating either RhPSS or FITC-PAH. These results obtained at the aqueous-LC interface contrast with past reports of the growth of PEMs formed from PSS and PAH at air–water interfaces.10 At an air–water interface that was decorated with a cationic lipid (dimethyldioctadecylammonium bromide, DODAB), the ellipsometric thickness of the PSS/PAH PEM was observed to grow exponentially for the seven layers deposited. In contrast, we observe a linear increase in fluorescence intensity for 20 layers. We note that the pH values of the PAH solutions used in the studies at the air–water interfaces were not defined and that variations in pH can influence the growth behavior. In our experiments, the pH of PAH was adjusted to 8. For the purposes of comparison to the above-described measurements of PEM growth at aqueous-LC interfaces, we (18) Kempe, M. D.; Scruggs, N. R.; Verduzco, R.; Lal, J.; Kornfield, J. A. Nat. Mater. 2004, 3, 177–182.

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Figure 4. Comparison of the growth of PSS/FITC-PAH multilayers at the aqueous-5CB interface to growth of the multilayers on hydrophobic (OTS-treated glass) and hydrophilic solids (glass and native oxide on silicon wafers). Fluorescence intensity of FITC-PAH shown (left axis) during the growth of PEMs from PSS (1 mg mL-1, 0.5 M NaCl)/FITCPAH (0.2 mg mL-1, 0.5 M NaCl, pH 8). The right y-axis indicates the ellipsometric thickness of the PEMs grown on native oxide on a silicon wafer.

next measured the growth of PEMs formed from PSS and FITCPAH under identical solution conditions at the interfaces of several model solids: (i) hydrophobic surfaces, prepared using OTStreated glass and OTS-treated silicon wafers and (ii) charged and hydrophilic surfaces, using glass and native oxide-coated silicon wafers. In these experiments, PSS is the first polyelectrolyte deposited onto the OTS-treated glass whereas FITC-PAH is the first polyelectrolyte deposited onto the hydrophilic surfaces. An inspection of Figure 4 shows that the growth of PEMs from PSS and FITC-PAH is largely independent of the nature of the interface (LC or any of the solids) on which the PEM is grown. Only for bilayer numbers greater than eight is there a statistically significant higher rate of growth of the FITC fluorescence on the interface of the LC (as compared to the solids). This result (similarity of growth of PEMs at aqueous-LC interface and interfaces of solids), however, should not be interpreted as a general one. As reported below, the growth of PEMs from PAA and PAH at aqueous-LC interfaces differs substantially from the growth at interfaces of both the hydrophilic and hydrophobic solids. The results described above using PSS and PAH likely reflect the amphiphilic nature of the PSS, which readily adsorbs to hydrophobic surfaces, and the cationic nature of PAH, which drives adsorption to negatively charged surfaces. We also note that the subtle differences in the rate of growth of PEMs of PSS and PAH at aqueous-LC interfaces as compared to that at solid interfaces (as seen in Figure 4) were also evident upon lowering the pH at which the PAH was deposited. As shown in Figure 5, the growth of PSS/PAH multilayers is higher on the LC as compared to that on OTS-treated glass when the pH of the PAH solution was either 5 or 8. As reported previously for PEMs deposited onto solid interfaces17 (see also Figure S1 in Supporting Information for additional data), because PAH carries a higher charge density at pH 5 than at pH 8, thinner layers of PAH adsorb at the lower pH.17 We hypothesize that roughening of the

Gupta et al.

Figure 5. pH-dependent growth of PEMs of PSS/FITC-PAH on hydrophobic OTS-coated silicon substrates and at aqueous-5CB interfaces. The fluorescence intensity of FITC-PAH measured during the growth of PEMs from PSS (1 mg mL-1, 0.5 M NaCl)/FITC-PAH (0.2 mg mL-1, 0.5 M NaCl, pH 5 or 8) is shown.

deformable aqueous-LC interface during deposition of the PEMs on the LC leads to higher effective interfacial areas as compared to the rigid interfaces of the solids described above. As described in detail later in this article, roughening of the LC interface during the growth of PEM is further supported by phase-contrast images of the aqueous-LC interface with PEMs formed from FITC-PAH(7.5)/PAA(3.5). It is important to note that although the fluorescence of FITC-PAH is known to vary with pH19 all fluorescence measurements reported above were obtained after rinsing the PEMs with water for 5 min. The fluorescence measurements reported above provide a useful means to characterize the rate of incorporation of polymers into the PEMs at the interface of the LC. To date, however, the thicknesses of the PEMs formed at the interfaces of the LCs have not been established. To estimate the thickness of the PEMs, we combined measurements of fluorescence intensity in Figure 4 (data obtained with the deposition of PAH at pH 8) with ellipsometric measurements. Although ellipsometry has been successfully used to characterize the growth of PEMs at the surface of water (using Langmuir films10), the anisotropic optical properties of the LC complicate the interpretation of ellipsometric measurements performed at aqueous-LC interfaces. Because of this complexity, we adopted an approximate methodology that combined ellipsometric measurements of the thicknesses of PEMs formed from PSS and PAH on native oxide surfaces and OTStreated surfaces (Supporting Information, Figure S2) with the data in Figure 4. By assuming that growth of the PEMs is similar on glass and native oxide-coated silicon and by using the observation in Figure 4 that similar fluorescence intensities are measured when PEMs are formed on the LCs as well as solid interfaces (for a given number of bilayers deposited), we arrived at estimates of the thicknesses of the PEMs formed on the interfaces of the LC (Figure 4). We note here that the assumed (19) Klugerma, Mr. J. Immunol. 1965, 95, 1165–1173.

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similarity of the growth of PEMs on glass and native oxide is reasonable in light of past measurements of contact angles of liquids on these surfaces.20 From these measurements, we conclude that the thicknesses of PEMs of PSS and PAH formed on aqueous interfaces of 5CB range from 2.5 to 50 nm as the number of bilayers deposited increases from 1 to 10. Growth of PEMs Formed from PAH and PAA at Aqueous-LC Interfaces. The results of previous studies,8,13,21,22 when combined with the measurements described above, lead to the conclusion that PEMs of PSS and PAH grow at aqueous-LC interfaces in a manner that is similar (in some cases quantitatively similar) to that for solid surfaces. To establish the generality of these conclusions, we sought to determine if PEMs of PAA and PAH would form at the aqueous-LC interface and, if so, to characterize their growth behavior. In contrast to the PSS/PAH system discussed above, neither PAA nor PAH is amphiphilic, and both PAA and PAH are weak polyelectrolytes. The PAA/PAH system is also interesting because PEMs formed from PAA and PAH are known to reorganize upon exposure to solutions that differ in pH.11,12 We sought to prepare PEMs at aqueous-LC interfaces from FITC-PAH and PAA using solutions adjusted to the following pHs (in parentheses): (a) FITC-PAH(7.5)/PAA(3.5) and (b) FITC-PAH(6.5)/PAA(6.5). The former solution condition leads to PEMs on some solid surfaces that have been reported to undergo changes in thickness and porosity upon subsequent exposure to low pH:11,12 the latter solution conditions lead to very thin PEMs as a result of the highly charged states of the two polyelectrolytes.23 In both experiments, FITC-PAH was the first polyelectrolyte to be adsorbed to the interface of the LC. For comparison, we also characterized the growth of PAH/PAA PEMs formed under identical conditions at the surfaces of glass and OTS-treated glass. Figure 6 shows fluorescence intensity measurements of aqueous-LC interfaces treated repeatedly with PAA and FITCPAH. We make several observations regarding these measurements. First and most importantly, the results shown in Figure 6A provide evidence that PEMs of PAA and FITC-PAH can be formed at interfaces of aqueous phases and LCs. When PAA and FITC-PAH were deposited at pH values of 7.5 and 3.5, respectively, the growth in fluorescence at the aqueous-LC interface was linear with the number of layers deposited (open diamonds in Figure 6A). This result is interesting because under the same set of conditions we measured a negligible increase in the fluorescence of FITC-PAH at the OTS-treated surface of cleaned glass (hydrophobic interface, data indicated by stars in Figure 6A). We did find evidence of the growth of PEMs at the surface of glass under these deposition conditions (closed diamonds in Figure 6A), although the rate of growth of fluorescence was still less than that recorded at the aqueous-LC interface. The observation that PEMs of PAA and FITC-PAH do not grow at hydrophobic surfaces is consistent with the point noted above that neither PAA nor PAH is substantially amphiphilic: the adsorption of both PAA and PAH is expected to be minimal or weak at hydrophobic surfaces. This contrasts with the growth of the PEMs from PSS and PAH, where the amphiphilic nature of the PSS can seed the growth of PEMs from hydrophobic (20) Daoud, W. A.; Xin, J. H.; Tao, X. M. Appl. Surf. Sci. 2006, 252, 5368– 5371. (21) Ahrens, H.; Baltes, H.; Schmitt, J.; Mohwald, H.; Helm, C. A. Macromolecules 2001, 34, 4504–4512. (22) Ahrens, H.; Buscher, K.; Eck, D.; Forster, S.; Luap, C.; Papastavrou, G.; Schmitt, J.; Steitz, R.; Helm, C. A. Macromol. Symp. 2004, 211, 93–105. (23) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213–4219.

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Figure 6. Quantification of the growth of PEMs from FITC-PAH/PAA at the aqueous-5CB interface on hydrophilic glass or on hydrophobic OTS-coated glass. (A) Fluorescence intensity of FITC-PAH shown during the growth of PEMs from FITC-PAH (1 mg mL-1, pH 7.5)/PAA (1 mg mL-1, pH 3.5) or FITC-PAH(1 mg mL-1, pH 6.5)/PAA(1 mg mL-1, pH 6.5). (B) Fluorescence intensity of Rh-PAH during the growth of PEMs from Rh-PAH (1 mg mL-1, pH 6.5)/PAA (1 mg mL-1, pH 6.5) at the aqueous-5CB interface or the hydrophilic glass interface.

surfaces. A recent study by Park and Hammond24 has reported the adsorption of pyrene-labeled PAH on OTS-treated surfaces. In the absence of high electrolyte (>0.1 M) concentrations it was observed that the pyrene-labeled PAH is weakly bound and was displaced upon subsequent exposure to solutions of PAA and PAH, resulting in the incomplete growth of PEMs.24 It is possible (24) Park, J.; Hammond, P. T. Macromolecules 2005, 38, 10542–10550.

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that a similar phenomenon (displacement of weakly adsorbed PAH) underlies our observation of the absence of growth of the PEMs formed from FITC-PAH/PAA on OTS-treated glass. The observation of the growth of PEMs formed from FITCPAH/PAA at the surface of cleaned glass (not OTS-treated) suggests that the negatively charged surface of glass leads to the adsorption of PAH and thereby seeds the growth of the PEMs. This result is interesting in light of recent measurements of the ζ-potentials of aqueous-LC interfaces,9 which have revealed that the ζ-potential of the aqueous-5CB interface at pH 7.5 is approximately -60 mV. This result may indicate that it is the charged nature of the aqueous-LC interface (which is likely due to the adsorption of hydroxyl ions at the interface25,26) that seeds the growth of the PEM of PAA and FITC-PAH. Indeed, the fluorescence measurements shown in Figure 6A provide evidence of the adsorption of FITC-PAH onto the interface of the LC upon initial contact with FITC-PAH (3 AU, layer no. ) 1). The rates of growth of PEM on the LC and on the surface of the glass, however, are not identical. A substantially higher rate of growth was observed on the LC as compared to that on glass. Figure 6A also shows the FITC fluorescence intensity measured during attempts to grow PEMs of FITC-PAH and PAA at pH 6.5 (for both FITC-PAH and PAA) at the aqueous interfaces of either LC, glass, or OTS-treated glass. Similar to the pH 7.5/3.5 conditions, the growth of PEMs was observed on the LC (open squares) and glass (solid squares) but not on OTS-treated glass (solid circles). However, in comparison to the results discussed above (obtained at pH 7.5/3.5), the rate of growth of fluorescence is lower at pH 6.5 for both the LC and glass interfaces (also see Figures S4 and S5).23 The lower rate of growth of the PEM of PAA/PAH at the aqueous-LC interface at pH 6.5 is qualitatively consistent with the expected effects of the charge density of the polyelectrolytes on the mass of polyelectrolyte deposited in each adsorption step. (Similar effects are seen at solid surfaces; see Figure S3.) Surprisingly, the rate of increase in FITC fluorescence intensity was observed to plateau after the deposition of approximately five bilayers on the LC interface (open squares in Figure 6A). Such a plateau in the growth of these PEMs is not observed on glass or in the ellipsometric measurements obtained using silicon wafers (Figure S3). To investigate if this apparent plateau might be caused by the influence of the microenvironment of the PEM on the fluorescence of the FITC, we performed growth measurements using RhB-labeled PAH (Rh-PAH). Rhodamine is known to be less sensitive to local pH than FITC. The growth of a PEM composed of 10 bilayers of Rh-PAH(6.5)/PAA(6.5) is shown in Figure 6B. The fluorescence data in Figure 6B (filled diamonds) shows that with every layer of Rh-PAH deposited on the 5CB interface there is a decrease in the rate of increase in fluorescence intensity, thus confirming the trend suggested by FITC-PAH fluorescence measurements in Figure 6A. These results suggest that the trends observed in Figure 6A are not artifacts of the use of FITC. The results in Figure 6B also confirm the linear growth of the Rh-PAH(6.5)/PAA(6.5) multilayer on glass. These results, when combined, indicate that the growth of PAH/PAA PEMs on the LC at pH 6.5 differs from that on both hydrophobic and charged/hydrophilic solid surfaces. We next investigated the extent to which the orientational ordering of the LC was coupled to the formation and pH-induced reorganization11,12 of PEMs of PAH/PAA formed at the interface of the LC. Multilayers of PAH/PAA undergo pH-induced (25) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045–2051. (26) Beattie, J. K.; Djerdjev, A. M. Angew. Chem., Int. Ed. 2004, 43, 3568– 3571.

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reorganization (leading to the formation of nanoporous films) and thus may enable approaches leading to aqueous-LC interfaces with tunable properties.11,12 We used polarized light microscopy to characterize the orientational order within the film of LC in contact with the PEM. The optical appearance of 5CB under water (no polyelectrolyte in solution) was bright with pale-yellow interference colors (Figure 7A). Because 5CB is anchored perpendicular to the OTS-treated glass slide that supports the LC film (Figure 1A), we interpret the bright optical appearance of the LC in Figure 7A to result from anchoring of the LC parallel to the aqueous interface (as shown in Figure 1A).27 The in-plane birefringence of the film of LC rotates that plane of polarization of the incident light, thus leading to the bright, smoothly varying optical appearance of the LC in Figure 7A (crossed polars, transmission mode). The formation of a PEM film consisting of up to 10 bilayers of FITC-PAH (7.5)/PAA (3.5) at the interface of 5CB introduced graininess into the polarized light image of the film of LC (Figure 7B), as compared to the naked interface (Figure 7A). Phase-contrast images obtained at high magnification before and after deposition of the PEM also revealed the introduction of a grainy structure at the interface with a characteristic feature size of ∼5 µm (compare parts E and D of Figure 7). Interestingly, for PEMs formed under identical conditions on glass substrates, we observed no evidence of microscale structure in phase-contrast or polarized light images. These results lead us to conclude that the inhomogeneous and grainy appearance of the phase-contrast image of the LC interface after PEM deposition likely indicates a roughening of the LC interface, a conclusion that is consistent with the observed higher rates of growth of the PEMs on the LCs as compared to that on solid surfaces. We note here that PEMs formed from PSS and PAH on the LC, which grew at rates similar to those on solids (Figure 4), did not exhibit a grainy texture when imaged using polarized light or phase-contrast microscopy (data not shown). To determine if a pH-induced reorganization of the PAH/ PAA multilayer formed on the LC would lead to an ordering transition in the LC, we treated 10 bilayers of FITC-PAH(7.5)/ PAA(3.5) with a pH of 2.3 for 1 min. Control experiments performed with FITC-PAH/PAA multilayers grown on a silicon wafer under identical conditions indicated a thickness increase with treatment at pH 2.3, as reported by Rubner and co-workers.11 An inspection of Figure 7C,F reveals that treatment at pH 2.3 does lead to changes in both the polarized light and phase contrast micrographs of the LC. An increase in the graininess is evident in the image in Figure 7C and the features evident in phasecontrast images also increased in size (Figure 7F). We also observed that treatment at pH 2.3 led to the formation of wrinkled regions of the interface of the LC (Figure S6 of Supporting Information). In summary, we observe the pH-induced reorganization of PEMs of PAA/PAH to be reflected in changes in the ordering and thus the optical appearance of the LC in contact with the PEM. The coarsening of the optical texture of the LC is consistent with past observations on solid surfaces where treatment at pH 2.3 results in the formation of a coarse, porous film.11,12

Conclusions The main conclusions of this study are threefold. First, we have demonstrated that the formation of PEMs at aqueous-LC interfaces is not restricted to amphiphilic polyelectrolytes such as PSS. We have reported for the first time that it is possible to grow PEMs from PAH and PAA at the aqueous-LC interface. (27) Perez, E.; Proust, J. E. J. Phys. Lett. 1977, 38, L117–L120.

Growth of Polyelectrolyte Multilayers

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Figure 7. Optical images of PEMs of PAA and PAH formed at the aqueous-LC interface. (A) Polarized light micrographs (crossed polars) of 5CB in contact with water and (B) after the deposition of 10 bilayers of FITC-PAH (1 mg mL-1, pH 7.5)/PAA (1 mg mL-1, pH 3.5). (C) Polarized light micrograph after treatment with an aqueous solution at pH 2.3 for 1 min. (D-F) Phase-contrast micrographs corresponding to the samples shown in A-C. Scale bars: (A-C) 150 µm and (D–F) 40 µm.

Results presented in this article suggest that PAH adsorbs to the aqueous-LC interface to initiate the growth of the PEM. The driving force for adsorption is likely electrostatic in origin and is associated with the negative ζ-potential of the aqueous-LC interface. The second major conclusion arising from this work is that the growth of PEMs at aqueous-LC interfaces can differ substantially from that observed at solid surfaces. Although the PEMs formed from PSS and PAH exhibited growth characteristics that were similar to those found on model solid surfaces (hydrophobic and hydrophilic), the growth characteristics of the PEMs formed from PAA and PAH at the aqueous-LC interface

were substantially different from those of the solid surfaces used in our study. In particular, we observe the growth of PAA and PAH PEMs at the aqueous-LC interface under conditions for which the PEMs do not grow on solid, hydrophobic surfaces. In addition, we observed the rate of growth of the PEMs at the LC interface to be substantially greater than for PEMs formed at solid, hydrophilic surfaces. The higher growth rate is likely due to the roughness of the deformable interface of the LC during the growth of the PEM: this conclusion was supported by optical microscopy (phase contrast and polarized light), which revealed the presence of heterogeneity at the aqueous-LC interface but

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not at the solid interfaces. The third conclusion emerging from this study is that pH-induced ordering transitions in PEMs of PAA/PAH lead to changes in the optical properties of LC in contact with these PEMs. The reorganization of these films into microporous structures may provide us with the capability to tune the films for selective and controlled adsorption/transport of analytes to the interface of the LC. Acknowledgment. We thank Dr. John Quinn and Dr. Anthony Quinn from the University of Melbourne, Australia, and Dr. Katie D. Cadwell and Dr. Nathan A. Lockwood from the University of WisconsinsMadison for helpful comments and discussion. This work was partially supported by the National Science Foundation through the Materials World Network Program (DMR-0602570),

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the MRSEC program (DMR-0079983), the U.S. Army Research Office (WQ11NF-06-1-0314 and WQ11NF-07-1-0446), the Australian Research Council under the Linkage International Materials World Network Grant, Federation Fellowship and Discovery Project schemes, and the Victorian State Government under the STI Initiative. Supporting Information Available: Ellipsometric thicknesses of PSS/FITC-PAH and FITC-PAH/PAA multilayers grown on solid substrates. Fluorescence intensity measurements of FITC-PAH/PAA film growth as a function of polyelectrolyte concentration. Phase-contrast images of FITC-PAH/PAA multilayers before and after pH-induced reorganization. This material is available free of charge via the Internet at http://pubs.acs.org. LA800013F