© Copyright 2001 American Chemical Society
JUNE 26, 2001 VOLUME 17, NUMBER 13
Letters Nanoporous Thin Films Formed by Salt-Induced Structural Changes in Multilayers of Poly(acrylic acid) and Poly(allylamine) Andreas Fery, Bjo¨rn Scho¨ler, Thierry Cassagneau, and Frank Caruso* Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany Received February 20, 2001. In Final Form: April 18, 2001 We report here on the influence of changes in the solution salt concentration on the structure of multilayers of weak polyelectrolytes. For poly(acrylic acid) (PAA) and poly(allylamine) (PAH) multilayers assembled by the layer-by-layer process in the presence of sodium chloride, washing with pure water after deposition of each layer produces films with considerable surface roughness (root-mean-squared (rms) roughness ∼17 nm for a 10 layer film), as assessed by scanning force microscopy. In contrast, relatively smooth (rms roughness ∼1 nm) and homogeneous PAH/PAA multilayer films are formed when the salt concentration is kept constant both during the assembly process and in the washing steps. For such smooth films, subsequent exposure to pure water leads to the introduction of regular, discrete, nanometer-sized pores, thus providing a means of introducing lateral structure into the PAH/PAA multilayer films. Electrochemical measurements revealed that the pores formed in less than 10 min. The sensitivity of the multilayer films to salt as well as the subsequent creation of nanopores potentially makes them attractive candidates for use in controlled-release applications where defined permeability characteristics are desired.
Introduction In recent years, the layer-by-layer deposition of polyelectrolytes on charged surfaces has been established as a highly successful and versatile technique for tailoring surface properties of thin films.1 The electrostatic interaction between charged polyelectrolytes and surfaces of opposite charge causes adsorption of the polyelectrolyte from aqueous solution and leads to surface charge reversal, enabling further alternate adsorption of positively and negatively charged polyelectrolytes. By replacing one of the polyelectrolytes with a similarly charged species, the incorporation of proteins, inorganic nanoparticles, or multivalent dyes in the multilayers is also possible.2,3 * To whom correspondence should be addressed. Fax: +49 331 567 9202. E-mail:
[email protected]. (1) For a review, see: Decher, G. Science 1997, 277, 1232. (2) Lvov, Y. In Protein Architecture: Interfacing Molecular Assemblies and Immobilization Biotechnology; Lvov, Y., Mo¨hwald, H., Eds; Marcel Dekker: New York, 2000; pp 125-167.
Furthermore, the technique is not limited to planar substrates but can be used to coat colloid particles with multilayers of various components or to form hollow multilayer capsules by subsequent dissolution of the templated particles.4,5 While originally multilayers were exclusively formed from strong polyelectrolytes (that is, under conditions where the polyelectrolyte is essentially fully dissociated), recently, increasing attention has been paid to multilayers consisting of weak polyelectrolytes. Rubner et al. showed that the incomplete dissociation of the polyelectrolytes used for the layer buildup can lead to chemical reactivity of the resulting multilayer, because only a fraction of the reactive groups are utilized for the polyelectrolyte inter(3) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (4) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (5) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2201.
10.1021/la0102612 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/25/2001
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layer complexation.6,7 Furthermore, structural changes in the layer architecture of multilayers formed from weak polyelectrolytes can be induced by changes in the environmental pH to produce microporous thin films when the layers are prepared from polyelectrolyte solutions containing no added salt.6 However, monovalent salts (e.g., sodium chloride) are typically added to polyelectrolyte solutions to promote the adsorption process.1 The main effect of the salt is to screen the electrostatic charges.8 By screening, the repulsion of charges of the same sign on the polymer chain is reduced, and hence the polyelectrolyte in solution changes from a stretched to coiled conformation.8 In addition, the repulsion between polyelectrolytes of the same species is reduced. Both of these effects explain, qualitatively, the observed larger thicknesses of polyelectrolyte multilayers assembled in the presence of salt.1 Recent work has suggested that in the presence of salt polyelectrolyte multilayers are thicker (than those in the absence of salt), primarily due to an increase in extrinsically compensated charges; i.e., the layers are swollen with salt ions and more water.9,10 In this study, we examine the influence of environmental salt concentration on the structure of multilayer films made of poly(acrylic acid) (PAA) and poly(allylamine) (PAH) by scanning force microscopy and cyclic voltammetry. We demonstrate a simple, one-step process for the formation of nanoporous polyelectrolyte multilayer films through salt-induced structural changes: This is achieved by exposing PAH/PAA multilayers prepared from saltcontaining polyelectrolyte solutions to pure water. The creation of such porous films is of interest in various technological applications, particularly for the controlled release of substances. For example, we have recently applied the layer-by-layer process to encapsulate enzymes11 and low molecular weight compounds (model drug systems)12 in polyelectrolyte multilayer capsules. Their encapsulation was achieved by the stepwise assembly of polyelectrolytes onto colloid particles comprised of crystallized components. The ability to introduce well-defined pores in the multilayer coatings would provide an attractive means of controlling the release characteristics of the encapsulated compounds, thus further extending the usefulness of polyelectrolyte multilayer films prepared by the layer-by-layer technique. Experimental Section Both poly(acrylic acid) sodium salt (PAA) of average molecular weight 30 000 and poly(allylamine hydrochloride) (PAH) of average molecular weight 70 000 were purchased from Aldrich. PAA was obtained as a 40 wt % percent aqueous solution, while PAH was supplied in the form of a powder. Both polyelectrolytes were used as received. The concentration of the polyelectrolyte dipping solutions was 10-2 M based on the molecular repeat unit of the monomer. The pH of the solutions was adjusted to 5.0 by adding NaOH or HCl. (These conditions were chosen as they were employed to coat colloid particles with PAH/ (6) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (7) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (8) Dautzenberg, H.; Jaeger, W.; Koetz, J.; Philipp, W.; Seidel, Ch.; Stscherbina, D. In Polyelectrolytes: Formation, Characterization and Application; Hanser Verlag: Mu¨nchen, 1994. (9) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (10) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (11) Caruso, F.; Trau, D.; Mo¨hwald, H.; Renneberg, R. Langmuir 2000, 16, 1485. (12) Caruso, F.; Yang, W.; Trau, D.; Renneberg, R. Langmuir 2000, 16, 8932.
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PAA multilayers.13) The salt concentration was adjusted by adding NaCl. Both dipping and washing solutions were prepared from Millipore water (18.2 MΩ cm, pH 5.5-6.0). Silicon wafers were received from Wacker (Germany) and cleaned by a modified RCA method.14 Indium-tin oxide (ITO)-coated glass (polished float glass), R ) 10 ( 2 Ω, was purchased from Delta Technologies, Ltd., and was used as a working electrode. Potassium ferricyanide (photographic grade) was purchased from Sigma and used as received. Layer-by-layer dipping was carried out manually. Each adsorption step consisted of exposing the substrate to the polyelectrolyte solution for 20 min, followed by rinsing in three separate beakers of washing solution (1-2 min each). The substrates were not dried between the adsorption steps but were blown dry with nitrogen after the final step of film formation. In the case of multilayers prepared by washing in salt solutions, the substrates were shortly (3 s) rinsed with Millipore (pure) water to remove salt crystals, prior to morphology measurements. All multilayers were grown on silicon substrates (PAH deposited first) with the exception of films prepared for electrochemical studies. The surface morphology of the deposited multilayers was investigated by tapping mode scanning force microscopy (TM-SFM) on a Nanoscope III Multimode SFM (Digital Instruments, USA) using point-probe silicon cantilevers from Nanosensors (Germany) (20-50 N/m nominal spring constant). NIH-image on a PC was used for image analysis. The data were plane-fitted to correct for nonlinearity of the scanner; such a correction does not introduce periodicity into the data. Cyclic voltammetry was carried out with an Autolab PGSTAT30 using an Ag/AgCl reference electrode, a Pt counter electrode, and a film-supporting ITO-coated working electrode at a scan rate of 20 mV/s. A 0.005 M K3Fe(CN)6 solution (with 0.2 M NaCl or without salt) was purged with a flow of Ar for 5 min prior to any measurement and was maintained thereafter. The PAH/PAAcoated (working) electrodes were left in 0.2 M NaCl for 1 h prior to measurement. The cathodic current gave an additional contribution originating from the reduction of tin oxide (at the ITO surface) and/or the presence of oxygen near or within the film (water reduction). This phenomenon was not observed in the case of salt-free electrolytic solution (i.e., no added NaCl). Results and Discussion PAH/PAA multilayer films were prepared from solutions of pH 5.0 with or without NaCl. When pure water is used to wash the films after deposition of each polyelectrolyte layer, significant differences are observed in the surface morphology of PAH/PAA multilayers prepared from polyelectrolyte solutions containing no added salt and those assembled in the presence of NaCl. The surface roughness for the multilayers prepared from salt-free polyelectrolyte solutions is of the order of 1 nm and is independent of the number of polyelectrolyte layers adsorbed (up to 10 layers) (data not illustrated). This film is laterally homogeneous with no distinct surface features.6,7 In contrast, a remarkable increase in surface roughness is found for the multilayers (comprising five or more layers) deposited from salt-containing solutions and washed with pure water after deposition of each layer: Figure 1 shows the morphology of a 10-layer polyelectrolyte (13) Schuetz, P.; Fery, A.; Caruso, F. Manuscript in preparation. (14) Riegler, H.; Engel, M. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1424.
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Figure 1. SFM image of a five-bilayer PAH/PAA film prepared from polyelectrolyte solutions containing 0.2 M NaCl, followed by washing with pure water after deposition of each layer. The maximum z-range (height) of the image is 50 nm.
film prepared from PAH and PAA solutions containing 0.2 M NaCl and with pure water washing after each layer is adsorbed. This film shows strong thickness modulations with a distinct lateral periodicity. To investigate this phenomenon in more detail, PAH/PAA films of 5-10 layers were assembled in the presence of 0.2 M NaCl (with intermittent washing with pure water), and their surface morphology was examined as a function of the different number of layers adsorbed by scanning force microscopy (SFM). The total (average) film thickness was determined by scratching the film partly off the wafer surface and measuring the step height between the bare wafer and film. The lateral periodicity of the film roughness, determined by a two-dimensional fast Fourier transform (FFT) of the surface thickness profiles, depends on the total film thickness of the multilayer, i.e., the number of adsorbed layers (Figure 2). The lateral periodicity of the surface roughness reaches an asymptotic value around 600 nm at a film thickness of about 45 nm. To investigate the multilayer film instability to salt, the salt concentration of the washing solution was varied and the salt influence on the film structure was examined. It was found that the roughness development could be suppressed if a solution of salt concentration equal to that used for the deposition of the PAH/PAA layers is used for washing the films between each polyelectrolyte assembly step. Figure 3 shows a SFM image of a film that was prepared from PAH and PAA solutions containing 0.2 M salt and washed with a solution of salt concentration equal to that from which the polymers were adsorbed after each layer was deposited. The morphology of this film is significantly different to that observed for films prepared under the same conditions but washed with pure water following deposition of each layer (see Figure 1). These images suggest that the roughness observed is caused by changes of the salt concentration during the washing cycles rather than by the presence of salt during the PAH/PAA adsorption process. In a related study, Harris and Bruening15 reported that the permeability of PAH/PAA multi(15) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006.
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Figure 2. Relationship between the lateral fluctuation wavelength and the total film thickness for PAH/PAA multilayers prepared from polyelectrolyte-containing solutions with 0.2 M NaCl, followed by washing with pure water after each adsorption step. The inset shows the corresponding fluctuation wavelength versus the number of polyelectrolyte layers.
Figure 3. SFM image of five-bilayer PAH/PAA film prepared from polyelectrolyte solutions containing 0.2 M NaCl, followed by washing with 0.2 M NaCl solution after deposition of each layer. The maximum z-range (height) of the image is 25 nm.
layers assembled in the presence of salt were found to be highly permeable as compared to films of the same thickness assembled without added salt. Since intermittent water washing steps were used in the preparation of the films, salt-induced changes in the film structure (as observed in the current work) may, in part, be responsible for the different permeability characteristics of the layers. To further examine the influence of salt-induced structural changes on PAH/PAA multilayers, films similar to those shown in Figure 3 (i.e., PAH/PAA layers assembled from solutions containing 0.2 M NaCl and washed with 0.2 M NaCl after deposition of each layer) were immersed in pure water for 30 min. The film morphology changed
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Figure 4. SFM image of a five-bilayer PAH/PAA film prepared from polyelectrolyte solutions containing 0.2 M NaCl, washing with 0.2 M NaCl after each layer was deposited, and subsequently exposing the film to pure water for 30 min. The maximum z-range (height) of the image is 10 nm. The inset displays a higher magnification of the pores produced in the multilayer film. The lateral dimensions of the inset are 300 nm × 300 nm, and the maximum z-range (height) is 10 nm.
drastically, as shown in Figure 4. (Increased immersion times did not yield further morphological changes, as observed by SFM.) Pores with typical diameters of 20-30 nm and (apparent) depths of 7-10 nm formed in the film, which had a total thickness of 22 nm (10-layer film). The same pore diameter range and typical separation length of 200-300 nm were found on different regions of the film, suggesting a uniform, large-scale change of film structure. The depth of the pores cannot be determined unambiguously from the SFM measurements, as the pore dimensions are of the order of the SFM tip size. To examine the pore formation in more detail, the multilayer film permeability was electrochemically evaluated using Fe(CN)63- as a probe on the system ITO-(PAH/ PAA)5 (10 layers). It was expected that upon exposure to pure water the formation of nanopores would favor the diffusion of ferricyanide complexes to the surface of the electrode, resulting in a higher redox current. Two series of experiments were conducted to follow this phenomenon. In one set, a freshly prepared electrode coated with a (PAH/ PAA)5 film (prepared from solutions containing 0.2 M NaCl and washed with 0.2 M NaCl between adsorption steps) was first dipped into a 0.005 M K3Fe(CN)6 solution containing 0.2 M NaCl. Both reversible anodic and cathodic currents (peak current 0.87 µA/cm2) were detected, with a small hysteresis, which corresponded to redox events occurring within the film prior to exposure to pure water. To quantify the influence of water on the multilayer film structure, the same electrode was kept in water for at least 30 min and reused to monitor the film permeability with the same electrolytic solution. Figure 5 shows that a dramatic increase in current (a 14-15-fold increase for the anodic current, ipa ) -12.6 µA/cm2) was measured during the first few minutes, slightly decreasing to a 12fold increase after about 20 min. This higher current was indicative of a better accessibility of the complexes to the ITO surface, suggesting the presence of nanopores throughout the film (as shown by SFM, Figure 4). Furthermore,
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Figure 5. Cyclic voltammograms of ITO-coated electrodes coated with a five-bilayer PAH/PAA film in a 0.2 M NaCl and 0.005 M K3Fe(CN)6 solution, prior to (a) and after (b) exposure to water (up to 30 min). Scan rate is 20 mV/s.
it can be indirectly concluded that the pore morphology remained relatively stable on the time scale of the experiments (at least 20 min; longer exposure times were not investigated). In the second experiment, another (PAH/ PAA)5-coated ITO electrode (film prepared under the same conditions used for that deposited on the other electrode, see above) was dipped into a 0.005 M ferricyanide solution without additional NaCl. Though the electrolyte concentration was very low (i.e., no added NaCl), a small irreversible anodic current was detected after about 3-4 min (ipa ) -0.12 µA/cm2) of exposure (probably due to the release of salt trapped within the multilayers and the potassium ferricyanide salt itself). However, the redox processes became reversible beyond 8 min, with no further change in the current observed upon prolonged exposure (up to 20 min), suggesting that the pore formation occurred within 10 min upon exposing the multilayer film to water. The same electrode was removed and kept for 30 min in water prior to reusing in the same cell. Reversible redox events were still observed with a 1.6-fold increase in the anodic (and cathodic) peak current, in contrast with the first set of experiments. This observation was consistent with the results of the first experiment by showing the effect of deionized water on the permeability of the films. In a related study, PAH/PAA films prepared from saltfree solutions and immersed in a solution of pH ∼ 2.4 followed by exposure to neutral water for times greater than 1 h have been shown to exhibit throughpores of diameter 50-200 nm.6 Structural changes in polyelectrolyte multilayers due to changes in environmental salt concentration are also known for multilayers of other polyelectrolytes. Swelling has been reported for films of poly(styrene sulfonate) (PSS)/ PAH (PAH was assembled as a strong polyelectrolyte) upon exposure to solutions of increased salt concentration.16 In this particular case the structural changes were reversible in these systems and no pronounced increase of film roughness was observed.15 However, a more recent study dealing with PSS/PAH multilayers has reported no (16) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 948.
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change in the film thickness upon rinsing with salt solution after completion of the adsorption process.17 For the weak polyelectrolyte systems investigated here, we find that film architectures stable with respect to structural changes can only be achieved by keeping the salt concentration constant in both the polyelectrolyte and the rinsing solutions. Qualitatively, this can be understood due to the smaller number of dissociated groups; hence the polyelectrolyte interlayer interactions are on the whole weaker, rendering the system more unstable toward perturbations of the layer architecture. This occurs upon water rinsing PAH/PAA multilayers prepared from saltcontaining solutions. Presumably, upon rinsing, salt ions originally present compensating charges in the multilayer films are removed, thereby inducing structural rearrangements within the layers. It is also noted that an additional possible influence of pH on film structure (due to ionization of some of the carboxylic acid groups to COO-)6,7 cannot be ruled out since the PAH/PAA multilayers were assembled at pH 5.0 and subsequently rinsed with pure water (pH 5.5-6.0).18 A striking feature of the morphological changes in the PAH/PAA films is the appearance of a preferred wavelength in the film roughness (Figure 2), as described earlier. For PAH/PAA films assembled from polyelectrolyte solutions containing no added salt, the development of similar surface morphologies was attributed to spinodal decomposition.6 However, in the current work the strong dependency of the induced wavelength on the film thickness cannot be readily explained only by a spinodal decomposition mechanism, as spinodal decomposition is a bulk effect and the film morphology should in this case be independent of the film thickness. Rather, the dependency of the film morphology on the film thickness points toward a dewetting mechanism. The transition from a flat morphology to a rough (dewetted) film with increasing film thickness is typical for pseudo partial wetting.19 The (17) Steitz, R.; Leiner, V.; Siebrecht, R.; v. Klitzing, R. Colloids Surf., A 2000, 163, 63. (18) Buffered pH 5.0 solutions were not used for washing due to the inherent salt content of these solutions.
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roughness of preferred wavelength that is observed for thick films could be explained by a spinodal dewetting mechanism. Spinodal dewetting results in film morphologies similar to those observed in this study and for this situation it is known that the wavelength of film roughness depends strongly on the total film thickness.20,21 However, an interplay of both dewetting and spinodal decomposition cannot be ruled out as a reason for the film morphology observed. It can be concluded that salt-induced structural changes in PAH/PAA multilayers can be utilized to introduce lateral structure into the multilayer films after their preparation. Changes in the salt concentrations in both the adsorbing and washing solutions should be taken into account as a destabilizing factor, if homogeneous layer structures are desired for PAH/PAA multilayers (under the conditions assembled, pH 5.0). Discrete nanopores in the multilayer films are accessible simply by immersing PAH/PAA films prepared from, and washed with, saltcontaining (0.2 M NaCl) solutions, in pure water. These multilayers are expected to find applications as delivery systems and porous coatings. To this end, we are currently extending this approach to colloid particle templates to encapsulate biologically significant compounds within multilayers of weak polyelectrolytes with controlled porosity. Furthermore, with respect to targeting applications of the encapsulated materials, the carboxyl moieties of PAA are also attractive reactive groups for the linking of proteins. Such reactivity can be exploited when PAA forms the outermost layer of multilayers used for the encapsulation of various substances. Acknowledgment. This work was supported by the BMBF as part of its BioFuture research initiative. LA0102612 (19) 19. Phase Transitions and Critical Phenomena; Dietrich, S., Domb, C., Lebovitz, J. L., Eds; Academic Press: London, 1988. (20) Brochard-Wyart, F.; Daillant, J. J. Can. Phys. 1990, 68, 1084. (21) Herminghaus, S.; Jacobs, K.; Mecke, K.; Bischof, J.; Fery, A.; Ibn-Elhaj, M.; Schlagowski, S. Science 1998, 282, 916.
