Methods of Loading and Releasing Low Molecular Weight Cationic

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Methods of Loading and Releasing Low Molecular Weight Cationic Molecules in Weak Polyelectrolyte Multilayer Films A. J. Chung and M. F. Rubner* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 11, 2001. In Final Form: September 21, 2001 This work studied the loading capabilities and release behavior of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) multilayer films, using methylene blue dye (MB) as an indicator. PAA/ PAH multilayers assembled at pH ) 2.5/2.5 loaded MB throughout the multilayers due to available binding sites and a permeable structure. The amount of MB loaded increased linearly with increasing film thickness. Use of buffered MB solutions enabled faster and more uniform MB loading into the multilayers, because ions within the MB solution caused further swelling of the multilayer film. Release studies demonstrated a pH-sensitive release mechanism, where lower pH environments increased the release rate. Buffered environments also increased the release rate of MB from the multilayer films. By assembling capping layers of PAA/PAH at pH 6.5/6.5 on top of 2.5/2.5 PAA/PAH multilayers, the diffusion of MB through the multilayers was better controlled under nonbuffered conditions. Under buffered conditions, however, the permeability of the 6.5/6.5 PAA/PAH capping layers increased due to swelling, and MB diffusion was not hindered. Further work needs to be done to slow release of MB under buffered conditions. The controlled loading capabilities and pH-sensitive release behavior of weak polyelectrolyte multilayer films demonstrated here may be useful toward drug delivery applications.

Introduction Over the past several years, great advances have been made toward new sustained and controlled drug delivery systems. Sustained and controlled release mechanisms offer greater effectiveness, lower toxicity, and improved patient convenience over conventional formulations.1-3 Furthermore, localized drug delivery reduces systemic side effects and increases drug efficiency. The new development of layer-by-layer assembled polyelectrolyte (PE) films over the past decade may prove useful in the arena of novel drug delivery systems. These multilayer films consist of alternating layers of polycations and polyanions that are adsorbed to a substrate via electrostatic interactions.4 With nanoscale level control over each deposited layer, the composition, molecular architecture, and surface properties of these films may be adjusted through different process parameters to form highly tailored PE films.4,5 In the area of biomaterials, the ability to control film properties at a nanoscale level has led to studies of PE multilayers for applications such as biosensors, microcapsules, membranes, and coatings to improve biocompatibility.6-13 While much of this work is still being (1) Baker, R. Controlled Release of Biologically Active Agents; John Wiley & Sons: New York, 1987. (2) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Chem. Rev. 1999, 99, 3181-3198. (3) Langer, R. Science 1990, 249, 1527-1533. (4) Decher, G. Science 1997, 277, 1232-1237. (5) Hammond, P. T. Colloid Interface Sci. 2000, 4, 430-442. (6) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427-3433. (7) Caruso, F.; Schuler, C. Langmuir 2000, 16, 9595-9603. (8) Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 5355-5362. (9) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023. (10) Decher, G.; Lahr, B.; Lowack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 677-684. (11) Rilling, P.; Walter, T.; Pommersheim, R.; Vogt, W. J. Membr. Sci. 1997, 129, 283-287.

researched, multilayer films will be introduced in industry as a coating for contact lenses, demonstrating the relevance of these films in the biomaterials field. Caruso and others have been able to form hollow microcapsules by depositing polyelectrolyte layers onto colloids and subsequently extracting the colloidal template. This technique has also been shown to work for encapsulating enzymes, while preserving the enzymatic activity.7,14,15 This microcapsule technique may be one method for drug delivery applications, in which the polyelectrolyte layers act as a shell to encapsulate and release different agents. The release behavior would be dependent on the permeability or breakdown of the multilayer shell. These multilayer shells have been found to be permeable to small molecular weight dyes and ions,16,17 and recent studies have been done on the release behavior of low molecular weight compounds from these microcapsules.17-19 The focus of this work is to study the loading and release capabilities of the multilayer films for drug delivery applications. While the microcapsules use the multilayers as a shell to encapsulate different agents, this paper offers novel ways to load small molecules within the layers of the film for sustained or triggered release. These thin films may be used for localized drug delivery in the form of a coating for medical devices and implants. They may (12) Gaserod, O.; Sannes, A.; Skjak-Braek, G. Biomaterials 1999, 20, 773-783. (13) Bartkowiak, A.; Hunkeler, D. Chem. Mater. 2000, 12, 206-212. (14) Caruso, F.; Trau, D.; Mohwald, H.; Renneberg, R. Langmuir 2000, 16, 1485-1488. (15) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 11111114. (16) Sukhorukov, G. B.; Brumen, M.; Donath, E.; Mohwald, H. J. Phys. Chem. B 1999, 103, 6434-6440. (17) Shi, X.; Caruso, F. Langmuir 2001, 17, 2036-2042. (18) Caruso, F.; Yang, W.; Trau, D.; Renneberg, R. Langmuir 2000, 16, 8932-8936. (19) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Mohwald, H. J. Phys. Chem. B 2001, 105, 2281.

10.1021/la010873m CCC: $22.00 © 2002 American Chemical Society Published on Web 01/18/2002

Cationic Molecules in Weak Polyelectrolyte Films

Figure 1. Molecular structures of (a) poly(acrylic acid) (PAA), (b) poly(allylamine hydrochloride) (PAH) (counterions not shown), and (c) methylene blue (MB).

also add a further drug releasing function to other biomaterial applications that are currently being explored for polyelectrolyte multilayers. In the case of microcapsules, being able to incorporate drugs within the shell, in addition to any drug encapsulated within the microcapsule, can result in a multilayering of different drugs to be released at different stages. Furthermore, applications need not be limited to drug delivery, as these studies offer insights into the general loading and release behavior of polyelectrolyte multilayers. In addition, they address fundamental issues concerning the penetration of small charged molecules into multilayers. Our methods take advantage of the ability to manipulate the molecular architecture of weak polyelectrolyte films by changing the charge density along the polymer chains.9,20,21 By varying the pH environment during multilayer assembly, the degree of ionization of weak polyelectrolytes can be controlled. This was found to affect multilayer properties such as layer thickness, the degree of interpenetration between layers, surface wettability, and number of unbound functional groups.20,21 Therefore, by choosing the right pH conditions, a platform may be found with properties that are advantageous for loading charged small molecules into the film via electrostatic interactions. Because weak polyelectrolytes become less ionized under different pH environments, a pH-sensitive trigger can be used to release the charged molecules. Of the several different pH combinations studied using weak polyelectrolytes,20 the use of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) at a dipping pH of 2.5 was selected as a model for loading and release of small molecules. This platform was selected because of its permeability and availability of binding sites. The chemical structures of PAA and PAH are given in Figure 1. With a pKa of ∼5.5 in solution, PAA is only partially ionized at pH ) 2.5. PAH, however, with a pKa ∼ 8-9 in solution, is fully ionized at pH ) 2.5. This results in a nonstoichiometric pairing of repeat units, creating thick and loopy layers. Past studies show the average bilayer thickness to be ∼40-50 Å.20 Because the loopy conformation contains uncharged segments that are not ionically cross-linked, the structure is more susceptible to swelling under aqueous conditions to increase permeability. The uncharged segments along the PAA chains contain carboxylic acid groups. Under higher pH conditions, these acid groups may be converted to negatively charged carboxylate groups, which can act as binding sites for charged small molecules. While these were the initial assumptions, this study worked to verify the suitability of 2.5/2.5 PAA/PAH for loading and release applications. (20) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 42134219. (21) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309-4318.

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Similar loading work has been done with PAA/PAH multilayer platforms to bind inorganic ions that were subsequently converted to nanoparticles.22 Methylene blue dye (MB), a monovalent cation, was used as a probe and indicator for possible drug behavior. MB has been used in the past to study multilayer surface properties, using its ability to bind to available negatively charged functional groups.20,21 Furthermore, extensive work has been done looking at MB interactions with clay suspensions and polyanion solutions.23-27 The chemical structure of MB is given in Figure 1. The richness of the MB absorbance spectrum also helps us understand the aggregation behavior of the dye under different conditions. It should be noted that we have observed similar behavior to that reported here with MB when using the anesthetic drug procaine, a cationic molecule, in the multilayers. This study examines the loading capability of 2.5/2.5 PAA/PAH multilayers, the release behavior under environments of varying pH and salt concentration, and the addition of capping layers to control the rate of release. Experimental Section Materials. The polyanion, poly(acrylic acid) (PAA), Mw 90 000, was obtained from Polysciences. The polycation, poly(allylamine hydrochloride) (PAH), Mw 70 000, and methylene blue dye (MB) were purchased from Aldrich. Dulbecco’s phosphate buffer solution (PBS), pH 7.4, was obtained from Gibco/Life Technologies for use in buffered MB solutions. The PBS included 0.2 g/L KCl, 0.2 g/L KH2PO4, 8 g/L NaCl, and 1.15 g/L Na2HPO4. The ionic strength of the PBS was ∼0.15 M. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to adjust the pH of the polyelectrolyte and MB solutions. All water was filtered through a Millipore Milli-Q A10 purification system. Sample Preparation. Glass and silicon substrates were sonicated, using a Branson 2210 sonicator, for 15 min in a bath of Micro-90 solution, 1.5% concentration. Samples were then rinsed and sonicated for another 15 min in deionized Millipore Milli-Q water. They were rinsed once again with deionized water and dried with compressed air before mounting onto a sample holder. Assembly of Multilayer Films. Multilayer films were prepared by alternating deposition of PAH and PAA onto the glass and silicon substrates. The concentration of both polyelectrolytes was 10-2 M, and the pH of both solutions was adjusted to pH 2.5. The substrates were first immersed in PAH for 15 min, followed by rinsing in three separate baths of DI water for 2, 1, and 1 min. The substrates were then immersed in PAA for 15 min, followed by the same rinse cycle. This entire process was repeated, using an automated dipper (Zeiss), with the final deposited layer being PAH, unless otherwise noted. Films were assembled with a total of 5.5, 10.5, 20.5, and 30.5 bilayers. Addition of Capping Layers. To test the effectiveness of capping layers, additional layers of PAA/PAH at pH 6.5/6.5 were assembled onto the 2.5/2.5 PAA/PAH multilayers. These layers were assembled using the same procedure and cycle time as stated above. The number of 6.5/6.5 PAA/PAH multilayers deposited ranged from 0.5 to 4.5 bilayers, with PAH as the outermost layer. As a reference, 10.5 and 40.5 bilayer films of 6.5/6.5 PAA/PAH were assembled on glass. Loading of Methylene Blue. All MB solutions were at a concentration of 10-3 M. The buffered MB solutions were composed of 50 vol % of the stock PBS solution mixed with DI (22) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354-1359. (23) Bujdak, J.; Komadel, P. J. Phys. Chem. B 1997, 101, 90659068. (24) Jacobs, K. Y.; Schoonheydt, R. A. J. Colloid Interface Sci. 1999, 220, 103-111. (25) Neumann, M. G.; Hioka, N. J. Appl. Polym. Sci. 1987, 34, 28292836. (26) Shirai, M.; Nagatsuka, T.; Tanaka, M. Makromol. Chem. 1978, 179, 173-179. (27) Duffel, B. c.; Verbiest, T.; Elshocht, S. V.; Persoons, A.; Schryver, F. C. D.; Schoonheydt, R. A. Langmuir 2001, 17, 1243-1249.

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Figure 2. Absorbance of MB in 2.5/2.5 PAA/PAH multilayer films with different loading times, under (a) nonbuffered MB conditions and (b) buffered MB conditions. PAH is the outermost layer. water and adjusted to pH 7.0 with HCL. Using an automated dipping system, films were immersed in either nonbuffered or buffered MB solutions at pH 7.0 for varying times. They were then rinsed in three separate baths of DI water for 2, 1, and 1 min for loading times of 15 min or less. Rinsing times of 2, 2, and 1 min were used for loading times of 1 h or more. It should be noted that, over the range of pHs examined in this study (3-7.5), methylene blue does not undergo any pH induced color changes. Release of Methylene Blue. All films used for release studies were assembled using 10.5 bilayers of 2.5/2.5 PAA/PAH on glass substrates, with PAH as the outermost layer. MB was loaded into the film under buffered conditions at pH 7.0 for 15 min. MB loaded samples were placed in a volume of 40 mL of deionized water that was being stirred constantly and gently with a magnetic stir bar. As MB was released into the environment, the water would be periodically replaced with fresh water to ensure accurate absorbance readings, where the concentration of MB remained dilute enough to follow Beer’s law. Furthermore, the continual replacement of the aqueous environment better resembles the environment in which a drug is released, where the agent diffuses away from the film and is carried to other surrounding areas. In studying the effects of pH, the pH of the DI water was adjusted to pH 3.0, 5.0, and 7.0 using HCl, before immersing the sample for release. The stock phosphate buffer solution (PBS) adjusted to pH 7.0 was used for examining buffered environments. Samples were placed in release environments for a total of 1 h, with the exception of the sample released in nonbuffered pH 7.0, which was immersed for 20 h. Thickness Measurements. Thickness measurements of films assembled on silicon were taken using a Gaertner ellipsometer. On the basis of previous studies, a refractive index of 1.52 was used to estimate thickness. Absorption Measurements. UV/vis spectra of MB loaded films on the glass substrates were taken using an Oriel UV/vis spectrophotometer. To obtain release profiles, absorbance measurements were taken of MB released into the surrounding environment. After each time interval, 4 mL of the 40 mL solution was placed into a cuvette to measure absorbance. For release experiments, measurements were also taken of the MB absorbance of the film on the glass substrates before and after release of MB into aqueous solution.

Results Loading of MBsBuffered versus Nonbuffered Loading. The 2.5/2.5 PAA/PAH multilayer platform was selected for loading and releasing of small cationic molecules because of its permeable film structure and high content of free carboxylic acid groups. In these studies, the average thickness contributed by a PAA/PAH 2.5/2.5 bilayer (measured in the dry state) was ∼55 Å. After the first few bilayers were deposited, the thickness of the multilayer film increased linearly. The initial nonlinear

growth observed for the first few bilayers is attributed to surface effects of the substrate, which influence the amount of polymer adsorbed and its conformation. The linear growth in thickness after the first few bilayers implies that the layers were absorbed to the surface in a consistent and constant manner. Therefore, the molecular architecture through the bulk of the film should be uniform, in terms of density. In comparing films loaded from buffered and nonbuffered MB solutions, both solutions allowed for a significant amount of loading (release data indicate ∼165 µg of MB in 10.5 bilayers covering an area of 2000 mm2). However, using a buffered MB solution allowed for a faster and more uniform loading. As will be discussed, this is a result of the higher ionic strength of the buffered solutions. Parts a and b of Figure 2 show the MB absorbance profiles as a function of loading times in nonbuffered and buffered solutions, respectively. The films loaded in a buffered MB environment saturate after about 15 min. However, films loaded in a nonbuffered environment took at least 1 h to reach a saturation point. In addition, as seen in Figure 2a, at greater thicknesses, the amount of MB loaded continued to increase up to at least 3 h. During this lengthy loading time, the pH of the nonbuffered solution is likely to decrease. Due to the change in degree of ionization of PAA with decreasing pH, a shift in pH would affect the amount of MB loaded (our previous studies21 found decreases in MB absorbance at lower pH). More erratic loading of MB was observed using nonbuffered conditions, with larger variations in the amount of MB loaded into multilayer films under the same conditions. Therefore, it was difficult to rely on loading in nonbuffered MB solution to determine the amount of MB that could be loaded into the films. Figure 3 shows a linear increase in MB absorbance with increasing film thickness, from ∼115 to 1730 Å, with loading under buffered conditions (note the similar MB absorbances for both 15 and 60 min loading times). This implies that MB was able to diffuse throughout the bulk of the film. Furthermore, increasing the number of layers assembled increased the amount of MB loaded. A higher proportion of MB absorbance was observed for the first five bilayers (115 Å), compared to the incremental increase in absorbance with increasing thickness. This greater amount of MB loading for the first few layers may be attributed to greater loading occurring either in the outermost region or near the substrate surface, where the molecular structure of the film is affected by the solution environment and substrate surface effects, re-

Cationic Molecules in Weak Polyelectrolyte Films

Figure 3. Increase of MB absorbance with increasing thickness in 2.5/2.5 PAA/PAH multilayer films. PAH is the outermost layer, and the MB solution was buffered. The dotted line and squares indicate a loading time of 15 min. The solid line and triangles indicate a loading time of 60 min.

Figure 4. UV/vis spectra of MB in a polyelectrolyte multilayer film and in PBS solution. For both spectra, the film used was 2.5/2.5 PAA/PAH, 10.5 bilayers, with a MB loading time of 15 min under buffered conditions. The spectrum of MB in PBS solution was taken after immersion of the film in PBS solution for 30 s.

spectively.28 As a multilayer film is built, the bulk region increases in thickness, while the thicknesses of the outermost layer region and the region close to the substrate remain constant.28 With increasing thickness of this bulk region, MB absorbance in 2.5/2.5 PAA/PAH multilayers increases linearly as well. Release of MB. In examining the release of MB from multilayers, the effect of the environment on both the amount released and the rate of release is of interest. Mass amounts of MB released into solution were calculated using Beer’s law. MB was found to follow Beer’s law for dilute concentrations of 10-6 to 10-5 M, with an extinction coefficient of ∼7850 M-1 mm-1, which is in good agreement with values cited in the literature.29 Figure 4 compares UV/vis spectra of MB released into solution and MB within the film. The maximum absorbance of MB was observed for all films within the wavelength range 570-600 nm, with a shoulder around 665 nm. MB absorbance bands in the range 570-600 nm indicate that most of the MB within the multilayer film forms molecular aggregates of differing sizes; some monomeric MB is also observed at 665 nm.23 However, when MB is released from the film in aqueous solution, the primary absorbance peak of MB in solution occurs around 665 nm, with a shoulder at ∼605 nm. These (28) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249-1255. (29) Norden, B.; Tjerneld, F. Biopolymers 1982, 21, 1713-1734.

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Figure 5. Release profiles of MB from 2.5/2.5 PAA/PAH films in aqueous environments of varying pH, under nonbuffered conditions. The inset shows the release profiles during the first 60 min

absorbance bands indicate that MB is in its monomer and dimer forms, respectively. The UV spectrum of MB in aqueous solution shown in Figure 4 was taken after the film had been immersed in PBS solution at pH 7.0 for 30 s. The release profile of MB from 10.5 bilayers of 2.5/2.5 PAA/PAH immersed in nonbuffered solutions of different pH is given in Figure 5. Under all pH conditions shown, the total amount of MB released was about the same and was close to the total amount loaded into the film (about 95% released). However, the rate of release increased significantly as the pH of the surrounding environment decreased. Others have studied the effects of pH on polyelectrolyte complexes, as well. For example, Kono found increased permeability under more acidic conditions for cross-linked PAA-poly(ethylenimine) (PEI) complex capsules.30 Recent studies on multilayer microcapsules have also demonstrated a pH-controlled encapsulation and release of FITC-labeled dextran (MW ∼75 000).31 All of these results clearly indicate the usefulness of a pHcontrolled trigger for changes in permeability of polyelectrolyte multilayers. Release profiles of MB from 10.5 bilayers of 2.5/2.5 PAA/ PAH at pH 7.0 are shown in Figure 6, where the use of buffered and nonbuffered solutions is compared. In both cases, the total amount of MB released was about the same. At pH 7.0, under nonbuffered conditions, the release of MB took place over several hours. However, when the solution was buffered using PBS, the rate of MB release was much more rapid, with release of all of the MB occurring within about 5-10 min. To better quantify the rate of release, the multilayer system was modeled after a monolithic device.1 A monolithic device is one where the agent is dispersed uniformly throughout a polymer medium. From the loading data shown in Figure 3, it can be assumed that MB was evenly dispersed throughout the bulk of the multilayer film. Furthermore, for the concentrations used, MB does not appear to be limited in its solubility. Therefore, release profiles were constructed on the basis of a monolithic solution system. The release rate profiles constructed from this analysis are given in Figure 7. Under nonbuffered conditions, lower pH solutions cause higher initial release rates than higher (30) Kono, K.; Tabata, F.; Takagishi, T. J. Membr. Sci. 1993, 76, 233-243. (31) Sukhorukov, G. B.; Antipov, A. A.; Voigt, A.; Donath, E.; Mohwald, H. Macromol. Rapid Commun. 2001, 22, 44-46.

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Figure 6. Release profiles of MB from 2.5/2.5 PAA/PAH films in buffered and nonbuffered environments at pH 7.0. The inset shows the release profiles during the first 60 min.

Figure 7. Release rate of MB from 2.5/2.5 PAA/PAH, with varying pH and buffered conditions. The inset shows the release rate profile under a pH 7.0 environment.

pH solutions, with a more rapid decrease in release rate, as well. Under buffered conditions, the release rate is initially much higher than that for nonbuffered conditions, and it quickly drops down to zero as the MB concentration is depleted. This approach can also be used to estimate diffusion coefficients. Because the film is known to swell under aqueous conditions, it is difficult to know the exact thickness of the film during release. However, dried films are measured to be ∼540 Å for 10.5 bilayers. With an estimated swelling of 20-50% taken into account, diffusion coefficients were estimated to range from 10-15 to 10-16 cm2/s (nonbuffered, pH 5 and 7 solutions, respectively) to 10-14 cm2/s for the pH 7 buffered solution. It should be noted that, for the release of MB into a pH 3.0 solution, a quantitative analysis is not given. The release of MB occurred very rapidly, with the majority being released in under 30 s. Therefore, it was difficult to attain detailed data within this short time period that could be fit to the monolithic solution model. However, under all other conditions, release profiles estimated by the monolithic solution model appeared to correspond well with experimental data. 6.5/6.5 PAA/PAH Capping Layers. The structure of 2.5/2.5 PAA/PAH multilayers enabled relatively fast release of MB in low pH or buffered environments. To

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Figure 8. Effect of 6.5/6.5 PAA/PAH capping layers on loading 2.5/2.5 PAA/PAH multilayers under nonbuffered and buffered MB conditions. The MB loading time is 15 min. 2.5/2.5 PAA/ PAH multilayers are 10.5 or 20.5 bilayers thick, with PAH as the outermost layer.

further control the diffusion of MB from 2.5/2.5 PAA/PAH multilayer films, capping layers of 6.5/6.5 PAA/PAH were added. As fully ionized polyelectrolytes, PAA and PAH at pH 6.5 form highly ionically cross-linked multilayers, with little or no free carboxylic acid groups available as binding sites for MB loading. For example, when 6.5/6.5 PAA/ PAH multilayers were assembled directly onto a planar substrate (10.5 bilayers, thickness ∼ 41 Å and 40.5 bilayers, thickness ∼ 193 Å), no MB was loaded into either platform under either buffered or nonbuffered conditions for immersion times as long as 20 h. Furthermore, this tightly knit structure should be less permeable to MB, hindering the diffusion of MB in to and out of the 2.5/2.5 PAA/PAH multilayers. In these studies, loading was studied as an indicator for release behavior. Slower loading of MB into the films due to the addition of capping layers would imply slower release as well. The thickness contributed by the 6.5/6.5 PAA/PAH capping layers was in the range 20-50 Å per layer. As suggested above, the thickness of 6.5/6.5 PAA/PAH layers deposited onto flat substrates is in the range only 3-5 Å per layer. In a pH 6.5 aqueous environment, the high charge density created on the surface of 2.5/2.5 PAA/PAH multilayers by ionized acid groups enables thicker layers of 6.5/6.5 PAA/PAH multilayers to be deposited. In addition, it has been found that the average thickness of PAA/PAH layers deposited at pH 6.5 increases when they are assembled onto polyelectrolyte multilayers of greater surface roughness.20 Eventually, as more layers are deposited, a smoothing out occurs and the incremental thickness decreases to more typical values. Figure 8 shows the MB absorbance of multilayer thin films containing 10.5 and 20.5 bilayers of 2.5/2.5 PAA/ PAH, that were capped with 0.5 or 4.5 bilayers of 6.5/6.5 PAA/PAH. These platforms were immersed in either buffered or nonbuffered MB solutions for 15 min. It should be noted that when PAH is the outermost layer of the 2.5/2.5 multilayer film (i.e.,10.5 or 20.5 bilayer films), it is necessary to start the pH 6.5 capping process with a PAH layer. This is because when the 2.5/2.5 multilayers are immersed into a polyelectrolyte solution at pH 6.5, the plentiful free acid groups within the multilayer film become charged, thereby creating a net negatively charged surface that will attract polycations and repel polyanions. A more complete description of a similar phenomenon can be found in ref 20. As shown in Figure 8, capped multilayers immersed in nonbuffered MB solutions showed little or no MB absorbance after this short loading time, suggesting the

Cationic Molecules in Weak Polyelectrolyte Films

effectiveness of the 6.5/6.5 capping layers in slowing the diffusion of MB into the multilayer film. Even a single capping layer of PAH deposited at pH 6.5 dramatically decreases the amount of MB loaded during this time. However, the capped platforms immersed in buffered MB solution looked very similar to uncapped films; i.e., they picked up about the same amount of MB regardless of how many capping layers were added. When placed in a buffered environment, the capping layers became ineffective in blocking the diffusion of MB into the multilayer film. The implications of these results are covered in the Discussion section. Discussion The molecular architecture of PAA/PAH multilayer films assembled at pH 2.5/2.5 allowed for loading of MB throughout the bulk of the film. As mentioned before, at pH 2.5, PAH is fully ionized, while PAA is only partially ionized (30-40% ionized within the film).21 The nonstoichiometric pairing of chain segments results in relatively thick and interpenetrated bilayers, with each succeeding bilayer contributing about equal thickness, as evidenced by the linear increase in film thickness. Previous studies show that PAA accounts for about 70% of the average bilayer thickness.21 The uncharged segments of the PAA chains are entangled but not electrostatically bound to PAH segments. When the film is placed in aqueous solution, the structure swells to a more open structure, as the unbound segments along the PAA chains extend. Furthermore, at higher pH conditions, the unbound carboxylic acid groups will become ionized. The repelling of like charges along the PAA segments will also act to extend and swell the film. Therefore, this platform provides a relatively open structure that allows for facile MB diffusion. In addition to a permeable structure, binding sites are necessary in order for MB to be loaded into the films. Because PAA is only partially charged at pH 2.5, there are many free carboxylic acid groups within the film that can act as binding sites. When the 2.5/2.5 PAA/PAH films are immersed into pH 7.0 solutions, PAA becomes essentially fully charged. MB then diffuses into the film and binds to the free carboxylate groups. In contrast, 2.5/ 2.5 films immersed in MB solutions at pH 3.0 at loading times up to 24 h show little or no MB absorbance (