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pH-Responsive Properties of Hollow Polyelectrolyte Microcapsules Templated on Various Cores Christophe De´jugnat* and Gleb B. Sukhorukov Max Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, 14424 Potsdam, Germany Received February 3, 2004. In Final Form: June 2, 2004 Hollow polyelectrolyte microcapsules made of poly(allylamine hydrochloride) and sodium poly(styrene sulfonate), templated on various cores, manganese and calcium carbonate particles or polystyrene latexes, were investigated. The polyelectrolyte multilayers respond to a change of pH, leading to a swelling of the capsules in basic conditions and a further shrinking when the pH is reduced to acidic. The nature of the core and the subsequent dissolution process have an influence on this pH responsiveness, and the structuring effect of tetrahydrofuran on the multilayers has been demonstrated. Increasing the molecular weight of the polymers or the number of layers causes also a rigidification of the structure and modifies the pH response.
Introduction Polymeric capsules with a nanoengineered shell composed of polyelectrolyte (PE) multilayers were introduced a few years ago.1-3 They have been prepared using the layer-by-layer (LbL) technique,4 the sequential adsorption of oppositely charged PEs, onto the surface of sacrificial colloidal templates, followed by core dissolution. Wide ranges of capsules have been prepared, using various cores and polyelectrolyte couples, and they were extensively characterized.5-9 Moreover, these polyelectrolyte microcapsules have attracted great interest due to their multiple potential uses as microreactors for inorganic synthesis,10,11 carriers,12 or sensors as temperature-responsive systems.13 The design of smart systems based on stimuli-responsive properties of polymers is very attractive, especially for drug delivery applications. In addition, many of the PEs used in the LbL technique are weak polybases or polyacids, like poly(allylamine hydrochloride); hence their charge depends on pH. The response of weak PEs as a function of the pH has been widely studied.14-19 * Corresponding author. E-mail: Christophe.Dejugnat@ mpikg-golm.mpg.de. Tel: +49 331 567 9437. Fax: +49 331 567 9202. (1) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202-2205. (2) Sukhorukov, G. B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Caruso, F.; Popov, V. I.; Moehwald, H. Polym. Adv. Technol. 1998, 9, 759-767. (3) Sukhorukov, G. B. In Novel Methods to Study Interfacial Layers; Moebius, D., Miller, R., Eds.; Elsevier Science B. V.: Amsterdam, 2001; p 383. (4) Decher, G. Science 1997, 277, 1232-1237. (5) Sukhorukov, G. B.; Antipov, A. A.; Voigt, A.; Donath, E.; Moehwald, H. Macromol. Rapid Commun. 2001, 22, 44-46. (6) Ibarz, G.; Daehne, L.; Donath, E.; Moehwald, H. Adv. Mater. 2001, 13, 1324-1327. (7) Ibarz, G.; Daehne, L.; Donath, E.; Moehwald, H. Macromol. Rapid Commun. 2002, 23, 474-478. (8) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Moehwald, H. J. Phys. Chem. B 2001, 105, 2281-2284. (9) Gao, C.; Donath, E.; Moya, S.; Dudnik, V.; Moehwald, H. Eur. Phys. J. E 2001, 5, 21-27. (10) Antipov, A. A.; Shchukin, D. G.; Fedutik, Y.; Zanaveskina, I.; Klechkovskaya, V.; Sukhorukov, G. B.; Moehwald, H. Macromol. Rapid. Commun. 2003, 24, 274-277. (11) Shchukin, D. G.; Radtchenko, I. L.; Sukhorukov, G. B. Mater. Lett. 2003, 57, 1743-1747. (12) Volodkin, D. V.; Balabushevitch, N. G.; Sukhorukov, G. B.; Larionova, N. I. Biochem. Moscow 2003, 68, 236-241. (13) Glinel, K.; Sukhorukov, G. B.; Moehwald, H.; Khrenov, V.; Tauer, K. Macromol. Chem. Phys. 2003, 204, 1784-1790.
In this paper, we report on the preparation and the properties of pH-responsive PE hollow capsules made of sodium poly(styrene sulfonate) and poly(allylamine hydrochloride) multilayers. Previous studies have shown that most of the wall properties (thickness, permeability, homogeneity, etc.) depend on the fabrication history, such as assembly conditions, concentration, and so on, and, especially, the core used and its dissolution process. For example, capsules templated on slightly cross-linked melamine-formaldehyde (MF) latex particles might contain some MF residues (oligomers) depending on the age of the cores and the dissolution process,20 and hollow capsules templated on erythrocytes underwent chemical modification due to the use of a strong oxidative agent for the destruction of the sacrificial cell.21 For these reasons, we investigated two series of capsules, prepared either on inorganic (MnCO3) or on organic (polystyrene) templates. The influence of the nature of the template on the capsules’ pH sensitivity is presented. Experimental Section Materials. Sodium poly(styrene sulfonate) (PSS, Mw ≈ 70 kDa or 1000 kDa), poly(allylamine hydrochloride) (PAH, Mw ≈ 70 kDa), boric acid, sodium hydrogenphosphate, citric acid monohydrate, and Rhodamine 6G were obtained from SigmaAldrich (Germany). Tetrahydrofuran (THF), sodium chloride, sodium hydroxide, and hydrochloric acid were purchased from Roth (Germany). Monodisperse polystyrene (PS) particles, diameter ) 10.55 ( 0.09 µm, were obtained from Microparticles GmbH (Berlin). Monodisperse MnCO3 template particles with a diameter of 3.6 ( 0.2 µm were synthesized from manganese chloride and ammonium hydrogencarbonate,10 and CaCO3 template particles with an average diameter of 9 µm (size (14) Sui, Z. J.; Schlenoff, J. B. Langmuir 2003, 19, 7829-7831. (15) Hiller, J.; Rubner, M. F. Macromolecules 2003, 36, 4078-4083. (16) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 12351243. (17) Rmaile, H. H.; Schlenoff, J. B. Langmuir 2002, 18, 8263-8265. (18) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550-9551. (19) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 42134219. (20) Gao, C.; Moya, S.; Lichtenfeld, H.; Casoli, A.; Fiedler, H.; Donath, E.; Moehwald, H. Macromol. Mater. Eng. 2001, 286, 355-361. (21) Donath, E.; Moya, S.; Neu, B.; Sukhorukov, G. B.; Georgieva, R.; Voigt, A.; Baeumler, H.; Kiesewetter, H.; Moehwald, H. Chem.sEur. J. 2002, 8, 5481-5485.
10.1021/la049706n CCC: $27.50 © 2004 American Chemical Society Published on Web 07/24/2004
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distribution, 8-10 µm) were prepared from calcium chloride and sodium carbonate, as described elsewhere.22 The water used in all experiments was prepared in a threestage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18 MΩ cm. Formation of Hollow Polyelectrolyte Capsules. Hollow PAH/PSS capsules were prepared in two stages. First the assembly of PAH/PSS multilayers on the surface of MnCO3, CaCO3, or PS particles (initially washed three times with water) was performed by the LbL technique from 5 mg mL-1 PSS (70 kDa) and PAH solutions in NaCl (0.5 M), starting from PAH. The pH of the PAH dipping solution was 6, ensuring the protonation of more than 90% of the amino groups (NH3+ form).22 Washing out nonadsorbed polymer molecules followed each adsorption step. Centrifugation of the samples at 1500g for 2 min was used to easily separate the supernatant. After the desired number of layers was reached, the sacrificial core was removed to obtain hollow microcapsules. The decomposition and removal of the MnCO3 cores were achieved by a 0.2 M citric acid solution (acting as both acid and strong complexant for Mn2+ ions) in 0.1 M HCl for 1 h (CaCO3 cores were dissolved using 0.2 M EDTA at pH ) 7).22 This procedure was repeated at least four times (until the CO2 formation stopped); then the resulting hollow capsules were extensively washed with water until the pH reached 6. For this second stage, the supernatants were removed after centrifugation at 1000g for 15 min. In the case of covered PS particles, the core was not destroyed but only dissolved overnight using THF. Multiple washing with THF and then water followed the dissolution process. Centrifugation was no longer used for these PS-templated systems because it seemed to increase the amount of broken or deformed capsules, especially for the thinner ones (capsules with only 8 layers were also prepared, but at least 50% of them were deformed or destroyed; they were not included in this study). The capsules were thus let to sediment for several hours before removal of the supernatant. Characterization. Optical images of polyelectrolyte capsules in solution were obtained on a Leica TCS SP confocal scanning system (Leica, Germany) equipped with a 100×/1.4-0.7 oil immersion objective. The PE multilayers were visualized either by incorporated labeled polymer (PAH-TRITC)6 used during the capsule preparation or by mixing 1 µM dye (Rhodamine 6G) externally. To study the influence of pH and/or salt on the capsule size, 3 µL of the capsule suspension was deposited on glass for observation by the microscope; then 10µL of buffer and/or salt solution was added to adjust the pH value in this drop. Atomic force microscopy (AFM) measurements were performed in air at ambient temperature (20-25 °C) using a Nanoscope III Multimode SFM (Digital Instruments Inc., Santa Barbara, CA) operating in tapping mode.
Results and Discussion Preparation of Hollow Capsules: Dissolution Process. Three series of hollow capsules were prepared. MnCO3 templates were covered by 8, 12, or 16 layers [(PAH/PSS)4, (PAH/PSS)6, and (PAH/PSS)8, below named Mn8L, Mn12L, and Mn16L respectively] or by 12+4 layers [(PAH/PSS)6(PAH/PSS)2, Mn12+4L], this latter structure being obtained by an additional LbL covering of the 12 layer structure after core dissolution. CaCO3 particles were used to prepare (PAH/PSS)4 capsules, below named Ca8L. Hollow capsules templated on PS particles were prepared by adsorption of 10-16 layers [(PAH/PSS)n, n ) 5-8, below named PS10L to PS16L]. The dissolution process was followed directly by confocal microscopy. In the case of capsules templated on carbonate cores, the decomposition was fast ( 11. They behaved as true PE complexes.22 Nevertheless, not all capsules dissolved when the pH was around the critical value of 11.50. For the solutions at pH 11.25 and 11.50, these surviving capsules start to shrink dramatically after their huge swelling (Figure 4), ending in a diameter of about 2 µm. The total duration of the process (swelling and then shrinking) was fast as it took less than 5 s. This phenomenon was observed whatever the number of layers in the capsule wall. As in the case of NaOH, the swelling might be explained by electrostatic repulsion between the accumulated negative excess charges. In this particular case where the pH is not high enough to ensure the total deprotonation of the PAH, there are some remaining positive charges that can enable the cohesion of the system. The subsequent shrinking should correspond to a rearrangement of the polymeric network that leads to a thermodynamically more stable closer packed state. This (30) Leporatti, S.; Gao, C.; Voigt, A.; Donath, E.; Mo¨hwald, H. Eur. Phys. J. E 2001, 5, 13-20.
Figure 5. Influence of the number of layers on the final size of the capsules after swelling and relaxation in basic solutions (PS-templated capsules), as recorded by CLSM.
Figure 6. Final diameter of PS capsules after relaxation, as a function of the pH and the number of layers.
significant shrinking requires flexible enough polyelectrolyte multilayers, for whom the resistant force (elasticity) is not strong enough to fight against the shrinking process. In the case of PS-templated capsules, the change in size was only observed for pH values higher than 11. The swelling occurred within 1-2 s starting for pH ) 11.25 and was always followed by a more or less strong shrinking due to the relaxation of the structure. The fact that the capsules relaxed after swelling in 0.1 M buffer for pH ) 12.5 and did not relax after swelling in 0.1 M NaOH (pH ) 12.5) can be attributed to a different ionic strength. In fact, the buffer used was based on phosphate (H2PO4-, HPO42-, PO43-) and then the ionic strength of the solution was higher than in the case of NaOH at the same concentration. The presence of multiply charged species increased the screening effect by these ions and reduced the Debye length, enabling a relaxation of the structure. The fact that the final size of the capsules depended on the number of polyelectrolyte layers in their walls is very interesting (Figures 5 and 6). The thinner capsules (PS10L and PS12L) shrunk to very small sizes (below 7 µm) as the thicker ones only relax to 12-16 µm. The final sizes did not change within minutes. In Figure 6 are reported the final sizes of the capsules after relaxation (the diameter was measured at least 10 min after adding buffer and mixing the solution) as a function of the pH value and the number of layers. It is clear that the number of layers influences the shrinking of the capsules during the relaxation process. There are two opposite driving forces: first the thermodynamically favored shrinking leads to a closer and more compact
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arrangement of the polyelectrolytes, and the elastic resistance of the capsule wall opposes this. With a lot of layers, the wall is more rigid. Capsules containing only a few layers may not be flexible enough to undergo such a rearrangement. This rigidity can be partially reduced by screening the electrostatic interactions between the oppositely charged polyelectrolytes, using a salt solution. In the presence of 0.5 M NaCl, added after the relaxation, all capsules reduced their size by about 2 µm due to the thermodynamic driving force. Another way was explored to confirm this hypothesis: capsules were prepared using PSS with high molecular weight. To check the influence of the strength of the wall on the elastic properties, hollow capsules templated on PS particles were prepared using high molecular weight PSS: 1000 kDa instead of 70 kDa. In this case, capsules with 8 layers were easily obtained without significant deformation, showing already that these systems can resist the mechanical stress occurring during the dissolution process. Capsules made of 10 layers were also prepared and compared to the corresponding capsules containing low molecular weight PSS. Significant changes were observed: in the presence of NaOH, the capsules swelled dramatically (up to 22 µm in diameter), and in the presence of buffers the swelling occurred at the same limit (pH > 11) but the shrinking due to the relaxation was weak. Using 1000 kDa PSS, the polymeric network became stronger and these new capsules behave like capsules with 16 layers (PS16L). Conclusions We reported in this paper the pH-responsive properties of polyelectrolyte microcapsules made of sodium poly(styrene sulfonate) and poly(allylamine hydrochloride) and templated on various sacrificial cores such as inorganic particles (MnCO3, CaCO3) or latex particles (polystyrene). In basic solutions, when the pH increased above 11, all structures prepared underwent significant swelling due to the deprotonation of PAH and the electrostatic repulsion between the remaining sulfonate moieties. Several factors
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influenced this pH response. We have first shown that the dissolution of the core might significantly modify the compactness and the strength of the polymeric network constituting the capsule wall. Consequently, capsules subjected to THF treatment resisted the dissolution which occurred during the base-induced swelling of non-THFtreated capsules. We also reported that increasing the number of layers or the molecular weight of PSS increased the strength and the resistance of the polymeric network. Finally we observed that in the presence of 0.5 M NaCl, the interactions between the oppositely charged polyelectrolytes were reduced, leading to a more flexible structure. The remarkable swelling observed with polyelectrolyte microcapsules can be correlated with similar observations seen on free-standing LbL films from clays and carbon nanotubes,31,32 for which the initial part of the stretching curves clearly demonstrates exceptional “stretchability” of the LbL films. As their permeability should change between the swollen and the shrunken state, these pH-responsive capsules are promising systems for controlled uptake and release of substrates. Work is currently in progress to investigate this property and to cover other ranges of pH response, using different weak polyelectrolytes. Acknowledgment. This work was supported by the Sofja Kovalevskaja Program funded by the Alexander von Humboldt Foundation and the German Ministry of Education and Research. Dr. D. G. Shchukin and Dr. A. I. Petrov are thanked for providing the MnCO3 and CaCO3 cores, respectively. Professor Dr. H. Mo¨hwald is greatly thanked for reading the manuscript and for helpful discussions. LA049706N (31) Tang, Z.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2 (6), 413-416. (32) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guldi, D.; Wicksted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190-194.
