Preparation of Ultrathin Self-Standing Polyelectrolyte Multilayer

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NANO LETTERS

Preparation of Ultrathin Self-Standing Polyelectrolyte Multilayer Membranes at Physiological Conditions Using pH-Responsive Film Segments as Sacrificial Layers

2006 Vol. 6, No. 4 592-598

Shoko Sugiyama Ono†,‡ and Gero Decher*,†,§ Institut Charles Sadron, 6, Boussingault, F-67083, Strasbourg, France, R&D Center, Mitsui Chemicals, Inc, 580-32, Nagaura, Sodegaura, Chiba, 299-0265, Japan, and UniVersite´ Louis Pasteur, Faculte´ de Chemie, 1, rue Blaise Pascal, F-67008, Strasbourg Cedex, France Received August 8, 2005; Revised Manuscript Received January 20, 2006

ABSTRACT A new system to obtain ultrathin self-standing polyelectrolyte multilayer membranes at physiological conditions is introduced. On the surface of a substrate, a hybrid film structure composed of two compartments, (1) a pH-responsive film segment formed via hydrogen bonds and (2) a polyelectrolyte multilayer film on top of 1, was assembled. The pH-responsive polymer multilayer segments disintegrate at a neutral pH and release self-standing polyelectrolyte multilayer films. The obtained self-supporting polyelectrolyte multilayer membranes had thicknesses of 55 to several hundred nanometers and areas of a few square centimeters, approximately. The preparation method introduced here avoids harsh release conditions and thus broadens the choice of materials that can be incorporated into the self-standing film.

1. Introduction. Thin films with well-defined architecture at the nanometer level have great potential for multifunctionalization, miniaturization, and integration of devices in a broad range of applications.1 Layer-by-layer deposition is a simple and green technology for fabricating multimaterial thin films with nanoscale precision by consecutive adsorptions of positively and negatively charged species from their aqueous solutions onto substrates of almost any size and shape.2-8 Multicomposite thin films without substrates are expected to broaden further the field of applications, such as separation membranes, sensors, catalytic film, micromechanical devices, wound dressing, or even artificial organs. Therefore, it is desirable to develop a simple approach that would release the thin films from a substrate using a single trigger event, resulting in “self-standing” thin-film membranes. So far, several groups succeeded in releasing thin films9-11 from a substrate using stimuli-responsive materials that cover the substrate surface, so-called “sacrificial layers”.12-23 Okano et al.12-14 reported that the cultured cells were readily detached from the surfaces grafted with poly(n-isopropyl* Corresponding author. E-mail: [email protected]. † Institut Charles Sadron. ‡ Mitsui Chemicals, Inc. § Universite ´ Louis Pasteur. 10.1021/nl0515504 CCC: $33.50 Published on Web 03/03/2006

© 2006 American Chemical Society

acrylamide), exhibiting a lower critical solution temperature (LCST) by lowering the incubation temperature without the usual damage associated with trypsinization. Kotov et al. demonstrated that multilayer films of montmorillonite and polyelectrolyte, which show extraordinary tensile strength, were released from the substrate whose surface was covered with acetyl cellulose using its solubility in acetone15,16 or with the thin layer of SiO2 using its solubility in 0.5 wt % HF solution.17 Schlenoff et al.18-22 reported that polyelectrolyte multilayer films were released from a substrate whose surface was covered with multilayers formed via electrostatic interaction between poly(acrylic acid)/poly(diallyldimethylammonium), which decompose at high salt concentrations or at a high pH. Although the integration of biomacromolecules such as proteins or DNA in layer-by-layer assembled films is very interesting,24-45 these biological materials are quite sensitive and therefore the conditions for releasing layer-by-layer films in which such molecules are embedded should take place as close to physiological conditions as possible. Therefore, a system that releases thin films at physiological conditions, does not contain or liberate any nonbiocompatible material, and can be used on almost any kind of substrate surface would be highly desirable. Recently, the formation of pH-responsive polymer multilayers via hydrogen bonds was reported.46-58 Granick and

Figure 1. Schematic of the principle for releasing of self-standing polyelectrolyte membranes using disintegration of a pH-responsive multilayer film segment. (a) Top: a hybrid structure composed of two components: a pH-responsive multilayer film segment and an electrostatically assembled polyelectrolyte film. Bottom: obtained self-standing polyelectrolyte multilayer membrane. (b) pH response of hydrogen bonds between PAA and PEG by dissociation of carboxylic acid (-COOH) groups to carboxylate ions (-COO-) of PAA. (c) The layer-by-layer alternating spray depositions of hydrogen donor/acceptor or polycation/polyanion (top) and the immerse of the substrates in a neutral pH medium (bottom).

Sukhishvili fabricated several types of multilayer films containing poly(vinylpyrrolidone)/poly(methylacrylic acid or poly(ethylene oxide)/poly(methylacrylic acid) or poly(ethylene oxide)/poly(acrylic acid) at low pH and investigated the disintegration of such films at pH levels above 6.9, 4.6, and 3.6, respectively.51-56 They proposed that the critical pH for the decomposition is controlled by a balance of internal ionization, especially the fraction of carboxylic groups. Caruso et al. reported the pH-induced film deconstruction characteristics of hydrogen-bonded layers consisting of poly(4-vinylpylidine)/poly(acrylic acid)57 in the electrostatically assembled layer and showed that the thickness of the hydrogen-bonded layers plays an important role for controlling the film dissolution. To our knowledge, self-standing films obtained by the disintegration of hydrogen bonds have not yet been described in the literature. Here, we report on a new system for obtaining selfstanding polyelectrolyte multilayer membranes at close to physiological conditions. In this system, we use poly(acrylic acid), PAA, and poly(ethyleneglycol), PEG, for constructing the pH-responsive film segment, which is a similar film as reported by Sukhishvili. This film covers the substrate surface and disintegrates by a pH change in order to release the polyelectrolyte multilayer film constructed on top of the pHresponsive multilayer.59 The principle of this study is depicted in Figure 1. The advantages of using PAA and PEG as components for a sacrificial layer in this system are as follows: First of all, PAA and PEG are biocompatible, biotolerated, or bioinert. The release of components of the sacrificial layer would not lead to adverse effects in vitro, nor would a trace of the sacrificial components in the freely suspended film circumvent its use in, for example, therapeutic devices and aids. Further, the use of PAA and PEG minimizes the damage of the properties of target membranes Nano Lett., Vol. 6, No. 4, 2006

(released by the degradation of the pH-responsive multilayer), which could diffuse into or remain at the interface of the target film. In this study, we used PAA and PEG as model compounds. It is to be expected that the use of similar macromolecules such as poly(glutamic acid) or poly(lactic acid) would further enhance the biotolerance of the sacrificial layer without drastically changing the disintegration conditions. Second, the combination of a neutral PEG and a PAA enable assembly via a hydrogen-bonding interaction at pH < 3.5 and trigger their decomposition at pH > 3.5.52 Therefore, the thin target film would be able to be released under physiological conditions, such as at neutral pH. Of course, it is required that the assembly of the target film on top of the sacrificial layer, which naturally takes place at physiological conditions, can be finished before the onset of dissolution of the pH-responsive layer. Thus, the control of the dissolution kinetics of the sacrificial layer is of prime importance. In this context we use spray deposition for the multilayer assembly, which has been shown recently to be capable of speeding up the film assembly process by a factor of 50-150.60 Especially the deposition time for a single layer can be as fast as 6 s for the case of PSS and PAH. Another reason for using PAA and PEG is that the studies on the combination of PAA and PEG are an important subject because this combination is a prospective pH-responsive system for controlled release and has a potential as a candidate for solid polymer electrolyte,58 not only as a sacrificial layer for releasing the self-standing film. As a target polyelectrolyte multilayer film, we chose poly(allylaminehydrochloride), PAH and poly(sodium 4-styrenesulfonate), PSS as a first model because the (PAH/PSS) multilayer is one of the most studied polyelectrolyte multilayer systems. Moreover, the obtained self-standing (PAH/ PSS) multilayer film can be functional itself, for example, 593

Table 1. Solutions Used in This Study

PAA PEG PAH PSS PEI rinsing solution water

MW (g/mol)

supplier

250 000 15 000 70 000 70 000 25 000

Aldrich Farbwerke Hoechst AG Aldrich Aldrich Lupasol, BASF from Milli Q system

as a selective semipermeable membrane.61-65 In this first manuscript, we describe the thinnest (55 to several hundred nanometers) films that can be obtained by the approach described above. 2. Materials and Methods. The molecular weight and the concentrations of all of the polymers used in this study are described in Table 1. PAA and PEG aqueous solutions were filtered through 1.0 µm pore size filters, especially because undissolved PEG was reported as a possible source of aggregates in PEG solution.66 The concentrations of PAH and PSS in the aqueous solutions were 3 × 10-3 mol‚ repetitive unit/L, resulting in 27.4 mg/100 mL and 61.4 mg/ 100 mL. The pH of the solutions was adjusted by HCl using pH meters. The ultrapure water used for all of the experiments and all of the cleaning steps were obtained by reverse osmosis (Milli-RO 3 plus; Millipore GmbH) followed by a filtration step (Milli-Q RG, Millipore GmbH). The resistivity was better than 18 M cm and the total oxidizable carbon is less than 10 ppb (according to the manufacturer). Multilayers of polymers were prepared by alternatively spraying polymer solutions using commercially available spray bottles, “AIR-BOY” (Carl Roth GmbH). The solutions were sprayed onto the vertically set silicon substrate from a distance of about 10 cm. A typical sequence for one layer consists of (1) spraying the polymer solution for 3 s and admitting for 27 s contact time (partial drainage and evaporation), (2) spraying of rinsing solution for 20 s and admitting for 10 s (waiting time). In continuation, different polymer solutions were then sprayed in a similar way (3, 4). The standard procedure used for the formation of multilayer films by alternating spray deposition was represented graphically in Figure 1c. Details for the alternative spraying method are described elsewhere.60 The hybrid films depicted in Figure 1a, top, were constructed from the solutions whose pH values were 2. To disintegrate the pHresponsive layer in the hybrid films, we immeresed the obtained hybrid films in milliQ water (pH 5.6-6.3) or in isotonic aqueous solution with 0.154 M NaCl at pH 7.00. The total thickness of the multilayers on the substrate was measured with ellipsometry. The measurements were performed using PLASMOS SD2300 with He/Ne laser (632.8 nm) illumination at a 70° angle of incidence. A refractive index for the polymer multilayer was assumed to be 1.465. Although this procedure will lead to slightly incorrect values with respect to the absolute film thicknesses, it allows for the quick and precise determination of the relative film thicknesses. Thickness values obtained with the assumption 594

polymer concn (mg/100 mL)

NaCl concn (mol/L)

pH

100.0 100.0 27.4* 61.4* 250.0 0.0 0.0

0.0 0.0 0.5 0.5 0.0 0.0 0.0

2.00 2.00 2.00 2.00 10.92 2.00 5.6-6.3

of a fixed refractive index for all films are of better precision than those required for the comparison of film growth data as in this report. The measurement was carried out on the sample after blow-drying with argon (ex situ measurement). All of the measurements were done on selected homogeneous regions of the substrates because the drainage of solution during the deposition processes may generate some slight heterogeneities. For measuring the thickness with ellipsometry, we chose silicon wafers as substrates with a size of 24 × 76 mm that was cut from silicon disks, Si〈100〉, 700775 µm thick, 8 in. diameter, polished on one side (Wafer Net, Inc.). The polished surfaces of all of the wafers were cleaned by water and acetone and then treated with at least 2 h immersion in a 1:1 mixture of methanol and hydrochloric acid, followed by an extensive rinse with milli-Q water and a second immersion in concentrated sulfuric acid for at least 4 h and another extensive rinse with milli-Q water. All of the substrates were prepared for multilayer deposition by adsorption of a single layer of PEI followed by rinsing and drying. The thicknesses of the released membranes were measured with atomic force microscopy, Nanoscope IIIa (Digital Instruments, Santa Barbara, CA), in air using standard Si tips (Nanosensors, NCH-10V) in tapping mode. The scan rate was between 0.02 and 0.05 Hz. 3. Results and Discussion. 3.1. Formation of a pHResponsiVe Film Segment. A pH-responsive multilayer film segment was typically constructed by alternating spray deposition of PAA and PEG onto a substrate covered with PEI. The total thickness of multilayers was followed by ellipsometry and plotted as a function of the number of layer pairs. The growth profile was shown in Figure 2 in the case that the molecular weights of PAA and PEG are 250 000 and 15 000, respectively. The total thickness increased gradually and showed a linear growth after a certain induction period. The growth increment in this linear growth was approximately 24 (nm/layer pair). In the initial stage whose number of layer pairs is less than five, the increment of the thickness was relatively small. The reason for this induction period is not yet completely clear; however, it seems safe to assume that in the initial depositions less than five layer pairs correspond to a situation in which the assembled layers are somewhat influenced by the underlying substrate. For the assembly of PAA/PEO or PMAA/PEO, similar “superlinear growth”, “avalanche-growth”, or ”runaway growth” have been reported previously until 10 layer pairs.52,58 In this study, a linear growth behavior after five-layer pair was found for Nano Lett., Vol. 6, No. 4, 2006

Figure 2. Growth profile of the pH-responsive layer consisting of PAA and PEG, whose molecular weights are 250 000 and 15 000, respectively, constructed by alternating spray deposition.

Figure 3. Response of pH-responsive film segment, (PAA/PEG)n, to milli Q water. n ) 11 (b), 9 (O), 7 (9), 5 (0), 3 ([), and 1 (]). Total thicknesses were measured in air by ellipsometry.

the first time. Furthermore, it should be noted that Figure 2 indicates that the thickness of the pH-responsive layer is precisely controlled simply by varying the number of deposition cycles. 3.2. Disintegration of the pH-ResponsiVe Film Segment (Sacrificial Layer). The next step was to evaluate the disintegration of the pH-responsive multilayer segment with an architecture of (PAA/PEG)n at a neutral pH. Six samples of the pH-responsive film segments whose architecture is represented as SiO2/PEI/(PAA/PEG)nPAA, (n ) 11, 9, 7, 5, 3, 1) were prepared simply by varying the number of deposition cycles. After certain times of the immersion in Milli-Q water (pH 5.6-6.3), the substrates were taken out from the solutions and their total thickness was measured by ellipsometry in air. As shown in Figure 3, the total thickness of pH-responsive film decreased drastically in minutes and almost disappeared in several hours. This Nano Lett., Vol. 6, No. 4, 2006

Figure 4. Growth profile of polyelectrolyte multilayer films. Open circle: (PAH/PSS)40-80 formed on the pH-responsive layer segment at pH 2.00 in the architecture of SiO2/PEI/(PAA/PEG)9 PAA (PAH/ PSS)80. Filled circle: (PAH/PSS)0-20 without the pH-responsive layer segment at pH 2.00 in the architecture of SiO2/PEI/(PAH/ PSS)20.

gradual decrease shows that the hydrogen-bonding interactions between PAA and PEG in the sacrificial multilayer disintegrated at a neutral pH in aqueous solution. 3.3. Formation of Polyelectrolyte Multilayer Films on top of the pH-ResponsiVe Film Segments. To construct a hybrid structure composed of two compartments as outlined schematically in Figure 1, a second film compartment, a polyelectrolyte multilayer film, was constructed by alternating spray deposition of electrostatically interacting polymers (PSS and PAH) on top of a pH-responsive film segment whose growth profile is already shown in Figure 2. The increment of the total thickness of the targeted polyelectrolyte multilayer films whose number of layer pairs were between 40 and 80 was followed by ellipsometry and plotted as a function of the number of layer pairs (Figure 4, open circle). Here, we do not elaborate on the deposition behaviors in the transition region between the (PAA/PEG) film and the (PAH/PSS) film. The interface between parts of the multilayer film is quite complex and will be discussed in detail in a later manuscript. In general, polyelectrolyte multilayer films with an architecture of (PAH/PSS)40-80 grow linearly. The thickness increased 3.316 nm par layer pair. The average interval of the growth increment for at least 20 samples was 3.1-3.6 nm per layer pair. This value corresponds to the growth increment of polyelectrolyte multilayer films consisting of PAH and PSS without an underlying sacrificial layer at pH 2.0 (3.5 nm per layer pair, Figure 4 (filled circles)). It means that the linear growth of (PAH/PSS)40-80 shown in Figure 4 (open circle) corresponds to the intrinsic growth increment of polyelectrolyte multilayer film consisting of PAH and PSS at pH 2.00, and the film growth is independent of the substrate and such independent of the underlying sacrificial film. 3.4. Release of Polyelectrolyte Multilayer Membrane by Disintegration of Sacrificial Layers. To obtain self-standing 595

Figure 5. Photos of self-floating electrostatically assembled polyelectrolyte membranes in a milli Q water (a) and self-standing electrostatically assembled polyelectrolyte membranes in air (b).

polyelectrolyte film membranes, the hybrid multilayer film SiO2/PEI/(PAA/PEG)9PAA (PAH/PSS)80 was prepared and immersed in milliQ water (pH 5.6-6.3). Before the immersion into water, the multilayer film was partially cut with straight-scratches by a diamond pen for facilitating of the release of controlled areas of the film. During immersion, small bubbles started to be visible probably at the interface between the substrate and the film and the scratched lines became gradually visible. After several minutes of immersion, the multilayer film started to float freely in the water. Figure 5a shows one example of self-floating film in milliQ water. The size of the self-floating film was 150 mm2 in Figure 5a. In the solution, this sheetlike structure of the films was stable for at least several days, and no obvious folding of the film was observed, if solutions were at rest. The size of the self-standing multilayer film is easily controllable just by varying the size of the area scratched by the diamond pen. The self-floating multilayer films are taken out of the water using tweezers and then left to dry, leading to the selfstanding multilayer films, such films obtained with PSS and PAH being fairly transparent. They are mechanically strong enough to be handled with tweezers, if some care is taken. Figure 5b shows one example of this self-standing multilayer film in air. The size of the self-standing multilayer film in Figure 5b was approximately 20 mm2. The thickness of the self-standing polyelectrolyte film membrane released from the hybrid multilayer film SiO2/PEI/(PAA/PEG)9PAA (PAH/ PSS)80 was measured directly with atomic force microscopy, AFM. For the case of 80 (PAH/PSS) layer pairs, the thickness of the membrane was 209-235 nm, approximately (Supporting Information). The self-standing membranes were also obtained in isotonic aqueous solution (0.154 M NaCl) at pH 7.00, which corresponds to physiological conditions. These results demonstrate that our new system, the pH-responsive multilayers formed via hydrogen bonds release the polyelectrolyte multilayer constructed on top of the sacrificial multilayers, is a prospective and general approach for obtaining selfstanding thin polyelectrolyte multilayer films under physiological conditions. As a consequence, the released film may contain substances of biological or medical interest including biomacromolecules that would denature at nonphysiological conditions. 3.5. Controls of the Release Parameters and the Properties of the Released Membranes. The detailed film architecture allows one to control the release parameters and the 596

properties of the released membranes. It was found that the thickness of the pH-responsive segment, which is regulated by varying the layer numbers, controls the release of the upper polyelectrolyte membranes. The hybrid multilayer films SiO2/PEI/(PAA/PEG)nPAA (PAH/PSS)40 (n ) 1, 3, 5, 7, 9, 11) were immersed in water. In the case when n is 7, 9, or 11, the upper polyelectrolyte membranes were released within 30 min. On the contrary, in the case when n is 1, 3, or 5, the upper membranes were not released even after 3 months. This result shows that there is a critical thickness in the pH-responsive segment that is necessary for releasing the upper polyelectrolyte membranes, and the critical thickness in this system is around 117 nm. The deposition cycles, n, in film architectures with a composition SiO2/PEI/(PAA/PEG)9PAA (PAH/PSS)n control the properties of the upper polyelectrolyte membranes. When n is 20, an ultrathin film was freely floated from the substrate. The thickness of the single sheet of the membrane was approximately 55 nm, according to the direct observation with AFM.67 (Figure 6C, single sheet: 55 nm, folded double sheets: 116 nm, folded triple sheets: 156 nm) A 209-235 nm film, whose n is 80, is released slower than a 55 nm film, whose n is 20. The effect of the thickness of the (PAH/ PSS) film on the release indicates that the release proceeds probably by two different mechanisms: (1) water entering from the scratch mark and (2) water entering through the upper film fragment. Although films composed of (PAH/PSS) can be regulated as soft and flexible, the incorporation of clay platelets, for example, montmorillonites, as one of the components for the polyelectrolyte multilayer film brought about an effect of increasing mechanical properties, mainly stiffness, of the film. In addition, the incorporation of clay platelets may also show the effect on the pathway of water, which controls the releasing parameter. 4. Conclusions. We investigated if our strategy depicted in Figure 1a represents a simple approach to release thin films from a substrate, resulting in self-standing polyelectrolyte multilayer membranes. The pH-responsive multilayer segment formed via hydrogen bonds consists of PAA and PEG, which cover the substrate surface disintegrated at a neutral pH and released the polyelectrolyte multilayer membrane. As a result, we succeeded in obtaining ultrathin self-standing polyelectrolyte multilayer membranes under physiological conditions. We show by AFM and simple photography that freely floating films whose thicknesses are Nano Lett., Vol. 6, No. 4, 2006

Figure 6. Measurement of the thickness of the self-standing membrane released from the hybrid film whose architecture is SiO2/PEI/ (PAA/PEG)9PAA (PAH/PSS)20 with AFM. (A) Optical microscope image of the edge of the membrane refixed on the substrate. (B) Topview tapping-mode AFM image (scan size: 80 µm) (C) Cross-sectional profile along the line in B. (D) Illustrations of preparations of samples for AFM observations (left), the folded structure of the membrane in C, and step heights revealed with AFM (right). Table 2. Release Property of Upper Layers Depending on the Thickness of the pH-Responsive Layera

n

thickness of the pH-responsive layer (Å)

released or not

1 3 5 7 9 11

88 296 674 1173 1662 2077

no* no* no* released# released# released#

a The original architecture of the hybrid film is SiO /PEI/(PAA/ 2 PEG)nPAA (PAH/PSS)40

controlled by the number of layer pairs (typically 50-200 nm) can be obtained easily in the size range of square centimeters. The detailed film architecture allows one to control the release parameters and the properties of the released membrane. This system allows one to release selfNano Lett., Vol. 6, No. 4, 2006

standing multilayer films under physiological conditions; as a consequence, the released membrane may contain substances of biological or medical interest including biomacromolecules that would denature at nonphysiological conditions. A subsequent paper with more details is in preparation. Supporting Information Available: Measurement of the thickness of the self-standing membrane released from the hybrid film whose architecture is SiO2/PEI/(PAA/PEG)9PAA (PAH/PSS)80 with AFM. (A) Optical microscope image of the edge of the membrane refixed on the substrate. (B) Top-view tapping-mode AFM image. (C) Cross-sectional profile along the line in B. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) For example, Okamura, T.; Fukuda, S.; Koike, K.; Saigou, H.; Yoshikai, M.; Koyama, M.; Misawa, T.; Matsuzaki, Y. J. Vac. Sci. Technol., A 2001, 19. 597

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Nano Lett., Vol. 6, No. 4, 2006