Nanoengineering of Polymeric Capsules with a Shell-in-Shell

Langmuir 2002, 18, 9533-9538

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Nanoengineering of Polymeric Capsules with a Shell-in-Shell Structure Zhifei Dai and Helmuth Mo¨hwald Max-Planck Institute of Colloids and Interfaces, D-14424 Golm/Potsdam, Germany

Brigitte Tiersch Department of Colloid Chemistry, Institute of Physical and Theoretical Chemistry, University of Potsdam, 14476 Golm, Germany

Lars Da¨hne* Capsulution Nanoscience AG, Volmer Strasse 7b, 12 489 Berlin, Germany Received August 29, 2002 Polymeric capsules possessing a shell-in-shell structure were fabricated by assembling layers of polyelectrolytes with interlayers of silica nanoparticles onto microspheres of melamine formaldehyde. Subsequently the cores were removed with hydrochloric acid and the silica layers with hydrofluoric acid. Removal of the silica layers leaves an aqueous phase of a poly(allylamine hydrochloride) solution between an outer and an inner polyelectrolyte shell. The interlayer thickness could be varied by the diameter of silica particles or by the number of deposition steps. The double-shell structure of the microcapsules was determined by scanning force microscopy, confocal laser scanning microscopy, and transmission electron microscopy on ultramicrotome sections in detail. In contrast to one-shell capsules of polyelectrolytes, the permeability for small molecules does not change with time and temperature. In addition, the shell-in-shell capsules exhibit a remarkably higher mechanical stability as shown by experiments of osmotically induced deformations.

Introduction For the advancement of nanoscience, there has been a major interest in designing and assembling polymer materials with novel morphology possessing special properties and functions in various application fields.1-8 In recent years hollow capsules with nanoscaled shells have been fabricated by means of electrostatic deposition of oppositely charged species layer by layer (LbL) onto colloidal templates. Subsequently the cores were removed by dissolution.10-12 “Ideal” capsules with high mechanical stability and controllable permeability have a wide range of applications, such as constrained environments for the preparation of nanostructured materials, the encapsulation of guest molecules, drug delivery, catalysis, and host containers for nucleic acid * Corresponding author. Telephone: 49-30-6392-3600. E-mail: [email protected]. (1) Stucky, G. D. Nature 2001, 410, 885. (2) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III; House A. B.; Edeki, E. M.; Feldhaus, J. C.; Felheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (3) Huang, H. Y.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (4) He, X. H.; Song, M.; Liang, H. J.; Pan, C. Y. J. Chem. Phys. 2001, 114, 10510. (5) Bo¨ltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391, 26. (6) Mazumdar, S. Science 2000, 288, 630. (7) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1999, 82, 2602. (8) Wang, Q.; Yan, Q.; Nealey, P. F.; Pablo, J. J. Macromolecules 2000, 33, 4512. (9) Dai, Z. F.; Da¨hne, L.; Mo¨hwald, H. Angew. Chem., in press. (10) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (11) Caruso, F.; Caruso, R.; Mo¨hwald, H. Science 1998, 282, 1111. (12) Dai, Z. F.; Voigt, A.; Leporatti, S.; Donath, E.; Da¨hne, L.; Mo¨hwald, H. Adv. Mater. 2001, 13, 1339.

storage and transport.13-16 To our knowledge, only oneshell capsules have been prepared up to now. For that, usually 7-10 layers of oppositely charged polyelectrolytes were deposited, which yields walls of 14-20 nm in thickness.17 Their mechanical stability against physical stress or osmotic pressure is small,16 which reduces their applicability remarkably. The stability can be increased slightly by deposition of additional layers, but simultaneously the permeability decreases and the excellent high ratio between capsule volume and wall material diminishes. To increase the mechanical stability and to maintain the wall material and the permeability, some ideas can be obtained from nature. The green plants consist of only 5-15% solid material,18 but the few percent of material yields very stable stalks, leaves, and fruits. The basic principle is the subdivision of the plant in small substructures having only thin walls but high osmotic pressure inside.18 Osmotic pressure has already been applied to the stabilization of capsules. They were filled by unbounded polyelectrolytes.13,20 The counterions induced osmotic (13) Da¨hne, L.; Leporatti, S.; Donath, E.; Mo¨hwald, H. J. Am. Chem. Soc. 2001, 123, 5431. (14) Ibarz, G.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. Adv. Mater. 2001, 13, 1324. (15) Sukhorukov, G.; Da¨hne, L.; Hartmann, J.; Donath, E.; Mo¨hwald, H. Adv. Mater. 2000, 12, 112. (16) Gao, C. Y.; Donath, M.; Moya, S.; Dudnik, V.; Mo¨hwald, H. Eur. Phys. J. E 2001, 5, 21. (17) Leporatti, S.; Gao, C.; Voigt, A.; Donath, E.; Mo¨hwald, H. Eur. Phys. J. E 2001, 5, 13. (18) Strasburger, E.; et al. Lehrbuch der Botanik, 33rd ed.; G. Fischer Verlag: Stuttgart, Jena, New York, 1991; pp 89-95. (19) (a) Schirmer, T.; Keller, T. A.; Wang, Y. F.; Rosenbusch, J. P. Science 1995, 267, 512. (b) Winterhalter, M.; Hilty, C.; Bezrukov, S. M.; Nardin, C.; Meier, W.; Fournier, D. Int. J. Pure Appl. Anal. Chem. 2001, 55, 965.

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pressure which yielded at high concentration a remarkable swelling of the capsules. However, for most applications the interior has to be filled by other materials. A way out is the stabilization of the wall by use of a shell-in-shell (SiS) structure, whereas the space between the shells of few nanometers has to have a high osmotic pressure. This principle is also found in nature. The gram-negative bacteria possess two separate cell walls, separated from each other by an aqueous phase filled with the watersoluble polymer peptidoglycan.19 By means of this arrangement the bacteria can control the permeation of molecules through the cell wall better.19 We describe here the design and the fabrication of such novel capsules with a SiS structure from silica particles and polyelectrolytes. Removal of the silica particles in the wall yielded the desired structure of a thin interlayer filled by a concentrated polyelectrolyte solution. The SiS capsules were characterized by scanning force microscopy (SFM), confocal laser scanning microscopy (CLSM), and ultramicrotome transmission electron microscopy (UMTEM). The influence of the SiS structure on the mechanical stability and the permeability of capsules was studied. A brief communication on part of the results has been published recently.9 Experimental Section Poly(styrenesulfonate sodium salt) (PSS; MW ∼70 000 g/mol) and poly(allylamine hydrochloride) (PAH; MW ∼70 000) were obtained from Aldrich. Silica particles (50 nm) were prepared according to the method reported by Sto¨ber et al.21 Silica particles (average diameter 30 nm, Ludox TM 40) were purchased from Aldrich. Monodisperse melamine formaldehyde resin particles (MF; 4.0 and 4.77 µm) and polystyrene (PS; 5 µm) latex particles were purchased from Microparticles GmbH, Berlin Germany. Fluorescein-labeled human serum albumin (MW 69 000, 10 dye molecules per albumin molecule) was purchased from Aldrich. PAH labeled with fluorescein isothiocyanate and rhodamine isothiocyanate was denoted as PAH-Fl and PAH-Rho, respectively.13 The degree of labeling was about 1 fluorescein molecule per 118 PAH units and 1 rhodamine molecule per 245 PAH units, determined by absorption spectroscopy. The procedure employed for the deposition of polyelectrolyte (PE) and silica particles is as follows: 1.5 mL of the solution of the species with charge opposite that of the templates or the last layer deposited (1 mg mL-1 PE in aqueous 0.5 M NaCl or 2 wt % silica particles) was added to 0.3 mL of a template latex solution, allowing 15 min for adsorption. The excess of the added species was removed by three repeated centrifugation (2000g, 4 min)/ washing/redispersion cycles with diluted aqueous 0.1 M NaCl after each layer was deposited. Following the final washing step, the particles were redispersed in 0.3 mL of water. Subsequent layers were deposited until the desired number of multilayers was achieved. Hollow capsules were prepared by dissolving the MF cores with 0.1 M HCl solution. The silica particles in the capsule walls were removed with 0.1 M hydrofluoric acid. Similar SiS structures with a solid core inside were prepared by using polystyrene templates, which are not soluble at low pH. Absorption spectra were measured using a Varian Cary 4E UV-visible spectrophotometer. Fluorescence spectra were obtained by a Spex Fluorolog 1680 double spectrometer. All (20) Radtchenko, I. L.; Sukhorukov, G. B.; Leporatti, S.; Khomutov, G. B.; Donath, E.; Mo¨hwald, H. J. Colloid Interface Sci. 2000, 230, 272. (21) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (22) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. Da¨hne, L.; Ibarz, H.; Vieira, E.; Mo¨hwald, H. Submitted for publication in J. Phys. Chem. B. (23) Ibarz, H.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. Chem. Mater., in press. (24) Edelmann, P.; Esa, A.; Hausmann, M.; Cremer, C. Optik 1999, 110, 194. (25) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249.

Dai et al. Scheme 1. General Procedure for the Construction of Multishell Capsules with a SiS Structure

measurements were performed on air-equilibrated solutions at room temperature. Confocal micrographs were taken with a confocal laser scanning microscope “Aristoplan” from Leica, equipped with a 100× oil immersion objective. The interlayer thickness was determined by using two color channels of rhodamine and fluorescein simultaneously. The overlay of both images gave the rainbow-like picture (Figure 3). The interlayer distance between the fluorescein-labeled outer shell and the rhodamine-labeled inner shell could be determined by the fluorescence profile along lines through the center of the capsule. The difference between the capsule diameter at both channels yielded the interlayer distance D:

D ) 0.5(dFl - dRho) The average of five different profile lines was taken as the value. Annealing was conducted by heating the capsule suspension for 6 min at 70 °C in water. After cooling to room temperature, the permeability tests were performed. The mechanical stability of the capsules was investigated by applying an osmotic pressure from the outer solution to the capsules by means of a 12% PSS solution corresponding to 0.75 M Na+ ions. SFM images were taken with a Digital Instrument Nanoscope IIIa in the tapping mode. Samples were prepared by applying a drop of the capsule solution onto a freshly cleaved mica substrate. After the capsules were allowed to settle, the substrate was extensively rinsed with Millipore water and dried under a gentle stream of nitrogen. The morphology of the capsule wall was investigated by transmission electron microscopy of ultrathin sections. The capsules were embedded in a 3:2 mixture of methyl methacrylate and n-butyl methacrylate by stepwise solvent exchange from water via ethanol to the monomer solution. After polymerization of the matrix by heating at 60 °C overnight, the sample was cut in 50 nm thick sections on an ultramicrotome “Ultracut E”. The sections were investigated in a Zeiss EM 902 transmission electron microscope.

Results and Discussion 1. Capsule Preparation. Different SiS capsules were constructed through LbL deposition of PSS, PAH, and silica nanoparticles onto colloidal MF or PS cores (Scheme 1). To obtain stable inner shells, two bilayers of PSS/PAH were deposited. Then, different layer numbers of SiO2/ PAH were assembled on top to vary the thickness of the aqueous phase between the shells and the concentration of PAH solution in this space. At last a stable outer shell was deposited using (PAH/PSS)PAH. The MF cores were dissolved in 0.1 M HCl. The silica nanoparticles in the wall were removed in a subsequent dissolution step by HF. The removal of the silica particles leaves behind PAH

Polymeric Capsules with a Shell-in-Shell Structure

Figure 1. Fluorescence spectra of hollow capsules of [(PSS/ PAH)2(silica/PAH-Rho)n(PSS/PAH)] (n ) 0, 1, 2, 3, 4).

polymer, which dissolves in the resulting aqueous phase because it cannot penetrate the inner or outer shell. Some residues of SiF62- counterions, formed by the reaction of the silica with HF might be bound to PAH, because PAH(SiF6)n is a weakly soluble complex. To obtain evidence for film growth using silica particles, PAH-Rho was deposited as cationic counterpart between the silica layers on top of the (PAH/PSS)2 layer. The fluorescence spectra of the hollow capsules of [(PSS/PAH)2(silica/PAH-Rho)n(PSS/PAH)] with different silica/PAHRho bilayers (n ) 0, 1, 2, 3, 4) are shown in Figure 1. Three layers of silica particles/PAH could be assembled with high efficiency, but the deposited amount of further double layers dropped. Comparative studies of the deposition of pure PAH/silica layers on MF cores showed that no capsules could be obtained after the dissolution of the core, although it is reported for similar systems.11 Probably the recharging of the surface decreased after each silica deposition step, which reduced the adsorption of the next PAH layer. 2. Scanning Force Microscopy (SFM). The wall thickness of the capsules was measured before (silica capsules) and after silica removal (SiS capsules) by SFM in the dried state. Twelve-layer capsules (PSS/ PAH/PSS/PAH-Rho)(silica/PAH/silica)(PAH-Fl/PSS/ PAH/PSS/PAH) were prepared. For the assembling 30 nm SiO2 nanoparticles were used. The images of air-dried hollow capsules exhibit a big difference in thickness for capsules before and after the silica dissolution (Figure 2a,b). From the profiles (Figure 2c,d) a thickness of 7280 nm was determined for the capsule wall with silica particles inside considering a minimum height comparable to about two superimposed walls. The removal of the silica layers left a wall thickness of 20-24 nm, which is slightly more than the thickness of 18 nm of usual capsules consisting of nine layers of PAH/PSS. This could be caused by some cavities in the wall that are not fully collapsed. Hence, the average thickness increment of approximately 54 nm for the two silica layers agrees well with the expectations of two monolayers of silica nanoparticles of 30 nm in diameter. The use of silica particles of 50 nm in size or the assembling of more than two layers yielded thicker walls, but the homogeneity of the capsules decreased remarkably. This might be due to agglomeration effects between the nanoparticles as well as due to an incomplete covering. Nevertheless, the overall thickness of the silica/PAH interlayers can be varied by means of the particle diameter and the number of silica layers. 3. High-Resolution Confocal Two-Color Fluorescence Microscopy. After the silica layers were removed, an interlayer between the inner shell of (PSS/PAH/PSS/

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PAH-Rho) and the outer shell of (PAH-Fl/PSS/PAH/PSS) should be formed. The thickness of the inner and outer shells of four layers each was estimated to be approximately 9 nm. Hence, if the capsules are not dried, an interlayer of approximately 54 nm thickness filled by a PAH solution should remain, if the space occupied by the silica layers before does not change during the dissolution. To detect changes in the capsule wall caused by the removal of the silica particles, the wall thickness in solution was investigated by means of spectrally resolved high-resolution fluorescence microscopy26 as well as by means of Fo¨rster resonance energy transfer (FRET). For this purpose, the layers next to the silica particles were prepared using labeled PAH. The CLSM image of capsules of (PSS/PAH/PSS/PAH-Rho)(silica/PAH/silica)(PAH-Fl/PSS/PAH/PSS) shows the occurrence of two colocalized fluorescent shells (Figure 3), which are attributed to an inner shell of (PSS/PAH/PSS/PAH-Rho) and an outer shell of (PAH-Fl/PSS/PAH/PSS) separated by two silica interlayers. Although the spatial resolution of the light microscopy is restricted to half the wavelengths, shorter distances can be detected by using two colors simultaneously.24 The profiles in Figure 3b show the fluorescence distribution of both dyes detected simultaneously at different channels. The difference between both maxima yielded an average distance taken from five profiles of 50 ( 4 nm between the PAH-Rho on the inner shell and the PAH-Fl on the outer shell. This distance agrees well with the 54 nm derived for the silica layer thickness by SFM. After removal of the silica particles a thickness of 38 ( 4 nm was determined for the distance between outer and inner shell. Although the distance is slightly reduced, this result reveals that the dissolution of the silica leaves a distinct interspace which is filled by water and eventually by remaining PAH. This was proved in a parallel experiment: Capsules with the same constitution were prepared, but labeled PAH-Rho had been used only for the PAH layer between the silica layers. The dissolution of the silica layers caused only a slight decrease of the fluorescence intensity of the capsules. Hence, the PAH-Rho remained mainly in the capsule wall. The SFM and the two-color experiments showed that the structure of the hollow SiS capsules is essentially shape persistent in solution while the air-drying induces a collapse of the capsules and the interlayer. 4. Fo1 rster Resonance Energy Transfer (FRET). Another possibility for detecting changes in the wall configuration offers the measurement of the FRET between the rhodamine-labeled inner shell and the fluorescein-labeled outer shell. Figure 4 shows characteristic fluorescence spectra of hollow capsules with (PSS/ PAH/PSS/PAH-Rho)(silica/PAH/silica/PAH/silica)(PAHFl/PSS/PAH/PSS). An excitation wavelength of 470 nm was chosen where rhodamine absorbs only a little light. If rhodamine molecules are located near the fluorescein, FRET occurs. The relative transfer efficiency EFRET was defined as

EFRET ) (IRho - 0.4IFl)/(IRho + 0.6IFl)

(1)

In this equation IFl and IRho are the fluorescence intensities at 533 and 582 nm corresponding to the emissions of fluorescein and rhodamine, respectively. The factor of 0.4 takes into account the resonance integral of donor fluorescence and acceptor absorption. From eq 1 we calculated FRET efficiencies of 0.43 and 0.49 for capsules with and without silica layers between (PSS/PAH/PSS/ PAH-Fl) and (PAH-Rho/PSS/PAH/PSS/PAH), respectively.

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Figure 2. SFM images of air-dried hollow capsules of (PSS/PAH/PSS/PAH-Rho)(silica/PAH/silica)(PAH-Fl/PSS/PAH/PSS) (a) before and (b) after removal of silica layers in the walls. (c, d) Profiles of the capsules in (a) and (b), respectively

Figure 3. (a) CLSM fluorescence image of capsule of (PSS/ PAH/PSS/PAH-Rho)(silica/PAH/silica)(PAH-Fl/PSS/PAH/ PSS). The red color results from the rhodamine in the inner shell, and the green-yellow color results from fluorescein in the outer shell. (b) Fluorescence profiles along a line through the capsule center: green, fluorescein channel; red, rhodamine channel. Insert: magnification of the left peaks.

FRET can occur only at distances below approximately 6 nm. Hence, the clear FRET signal, found already in the presence of the silica particles, reveals a strong interpenetration between the silica and the PAH layers. There is little increase of EFRET after the removal of silica particles, indicating a small decrease of the distance between the rhodamine and fluorescein molecules as measured by two-color CLSM. A collapse of the water interlayer can be definitely excluded by the FRET measurements because the transfer efficiency would increase strongly. 5. Transmission Electron Microscopy on Ultramicrotome Sections (UM-TEM). The wall structure could be visualized by electron microscopy. Up to now electron microscopy was only applied to capsules in the dried state. However, embedding of the capsules in polymers allows the preparation of ultramicrotome (UM) sections of capsules. Figure 5 shows UM-TEM images of hollow capsules (PSS/PAH/PSS/PAH-Fl)(silica/PAH/silica/ PAH/silica)(PAH-Rho/PSS/PAH/PSS/ PAH) before (a, b) and after (c) the dissolution of the silica particles. The

Figure 4. Fluorescence spectra (λexc. 490 nm) of hollow capsules (PSS/PAH/PSS/PAH-Fl)(silica/PAH/silica)(PAH-Rho/PSS/PAH/ PSS) before and after removal of silica particles in the wall. The intensity is normalized to the rhodamine fluorescence.

Figure 5. Ultramicrotome transmission electron microscopy of capsules of (PSS/PAH/PSS/PAH-Fl)(silica/PAH/silica/PAH/ silica)(PAH-Rho/PSS/PAH/PSS/PAH) (a, b) before and (c) after the dissolution of the silica particles.

capsules are almost spherical in shape, which is also observed in the CLSM images. Some of them were invaginated, caused by the embedding technique. The silica particles in the wall of the capsules are dark points (Figure 5a). After they were removed, the resulting water interlayer is distributed between the polymer shells seen as bright parts, whereas the center of about 4 µm in

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Figure 7. CLSM images of capsules in 12 wt % PSS solution: (PSS/PAH)2(silica/PAH/silica/PAH/PSS)2/PAH (a) before and (b) after removal silica particles with HF; (c) normal capsules of (PSS/PAH)6.

Figure 6. CLSM images of capsules of (PSS/PAH)2(silica/PAH/ silica/PAH/PSS)2/PAH (a) with and (b) without silica particles (SiS capsules) in a solution of rhodamine-labeled PAH (MW 70 000 g/mol). (c,d) Capsules annealed at 70 °C in a solution of fluorescein. (c) Capsules with silica particles. (d) SiS capsules.

diameter is totally empty (Figure 5c). The two-shell layering for this sample was not as good as expected from the other results. Maybe the use of three silica layers caused this problem. Obviously the silica particles formed aggregates in the layers and did not cover the particles completely, which prevents the preparation of an ideal ringlike intermediate layer. In addition, it is not quite clear if some residues of PAH(SiF6)n complex remained in the interlayer as mentioned before. 6. Investigation of the Wall Permeability. The permeability of capsules of (PSS/PAH)2(silica/PAH/silica/ PAH/PSS)2/PAH was studied by confocal microscopy for positively (PAH) and negatively (PSS) charged fluorescent polymers, as well as for low molecular weight species, such as fluorescein and rhodamine. Representative CLSM images of capsules in solutions of PAH-Fl and pure fluorescein are shown in Figure 6. Without further treatment, the capsules exhibit the same behavior as usual (PSS/PAH)6 capsules irrespective of whether the silica particles are in the wall or not. They are impermeable to the polymers (Figure 6a,b), but permeable to the small dye molecules fluorescein and rhodamine 6G. Recently it was found that a remarkable reduction of the permeability of PSS/PAH capsules can be achieved by annealing the capsules at temperatures above 50 °C.22,23 After elongated heating even small dye molecules cannot penetrate the capsule walls. The decrease of the diffusion coefficient in the wall was ascribed to a rearrangement of the macromolecular layer constituents after annealing.22 The increase of temperature induced a stronger bonding of the polyelectrolytes to each other in the center of the wall connected with a decreasing number of free charges in the wall and decreasing hydrophilicity. Macroscopically, the rearrangement reduced the capsule diameter by more than 20%. Capsules of (PSS/PAH)6 and of (PSS/PAH)2(silica/PAH/ silica/PAH/PSS)2/PAH before and after removal of silica particles were heated to 70 °C for 6 min and then cooled. Capsules of (PSS/PAH)6 as well as the capsules with silica layers in the walls became impermeable to small dye molecules such as fluorescein (Figure 6c). In contrast, the SiS capsules remained permeable to the dyes (Figure 6d), although the overall wall material is almost the same as for the (PSS/PAH)6 capsules. However, the wall is divided

into three subshells of about 5 nm thickness. It is proposed in the literature that the outer layers of LbL films have a different structure than the inner ones due to the surface charge.25 Presumably, annealing does not affect the SiS capsule wall strongly because it consists only of layers in close proximity to highly charged surfaces. This assumption is confirmed by the capsules that still contain the silica particles. There are larger inner parts of fully equalized charges in which annealing can lead to impermeability for fluorescein (Figure 6c). This finding is important for the application of such capsules as drug delivery systems. Because of the increase of hydrophobicity during storage of thick capsules at higher temperatures, the release time for the encapsulated material can increase with the storage time.23 The twoshell system could be one alternative that is able to solve this problem. 7. Osmotic Properties of the SiS Capsules. The mechanical stability of capsules was elucidated by applying osmotic pressure from the outside. Tunable osmotic pressure to the capsules can be adjusted by addition of a Na+nPSSn- solution because the PSS (MW 70 000) is not permeable to the capsule wall. The high Na+ concentration outside the capsules leads to water permeation from inside to outside and to the deformation of the capsules into a crescent-shaped form (Figure 7c). This can easily be observed by CLSM. If usual (PSS/PAH)6 capsules are incubated in a 12% PSS solution, they transform within a few minutes to this form, which was already reported by Gao et al.16 The same PSS solution was applied to capsules of (PSS/PAH)2(silica/PAH/silica/PAH/PSS)2/ PAH before and after removal of silica (Figure 7a,b). No deformation was observed for both samples even after 1 h, indicating that the capsules are much more resistant against mechanical pressure. Permeation of the PSS molecules can be excluded because of the impermeability of the capsules for high molecular weight polymers (Figure 6a,b). The different stability of usual capsules and shell-inshell capsules must be attributed to differences in their shell structure. It is easily understandable that the rigid organic-inorganic silica capsules with the high wall thickness are more deformation resistant. However, after removal of the inorganic layers, the shell-in-shell capsules have almost the same chemical composition and the same amount of polymer material as the (PSS/PAH)6 capsules. Nevertheless, they possess a higher mechanical stability. This has the following reason: the excess PAH resulting from removal of silica layers has been released into the water interlayer. There it builds up an osmotic pressure. The concentration of PAH in the interlayer can roughly be estimated by the surface of the silica nanoparticles, the amount of PAH in one layer covering the silica particles, and the thickness of the interlayer. Taking into account the thickness of one PAH/PSS double layer increment of 4 nm in the dried state,13 a density of 1.1

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g/cm3 for the PAH/PSS complex, and a 1:1 ratio, one layer contains 2 × 10-9 mol/cm2 polyelectrolyte complex. Hence, a two silica layer system results in a concentration of about 1 monomol/L PAH solution in an interlayer of 40 nm, which exceeds the sodium ion concentration of 0.75 M in the applied 12% PSS solution. The chloride counterions in the interlayer cause a pressure of 1.34 × 106 N/m2, while the outside pressure is only 106 N/m2.16 Nevertheless, the overall pressure to the interior of the water-filled capsules from the outside does not change by the interlayer. Hence, the shell-in-shell structure with the high-pressure interlayers is able to stabilize the wall very effectively. Summary The assembling of silica particle interlayers between usual polyelectrolyte complex films on colloids allows the construction of new nanostructured materials. The dissolution of the silica layers by hydrofluoric acid yields capsules with a shell-in-shell structure. Scanning force microscopy proved complete removal of the silica particles. The remaining wall thickness corresponds to the expected 4 nm layer thickness per PAH/PSS increment. The thickness of the interlayers could be controlled by the size of the silica particles and by the number of silica layers assembled alternately with PAH. The average distance between the two shells was determined by two-color CLSM in accordance with expectations based on the particle size. Fo¨rster resonance energy transfer proved that the water interlayer distance between the shells remains after the dissolution of the silica layers. TEM on ultramicrotome sections of the capsules showed that the interlayer is not

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a homogeneous ring as expected from a monoparticle layer deposition. Obviously, the multifold SiO2 particle/PAH deposition leads to perturbations in the layer quality. This results in a final level of PAH/silica assembling, too. The SiS capsules show the same permeability as usual PAH/PSS capsules. However, a decrease of permeability after annealing at higher temperatures was not observed. The remarkably larger charged surface of the SiS capsules prevents the decrease of permeability during the capsule aging. Furthermore, despite the same chemical composition of SiS capsules as the one-shell capsules, a remarkably higher degree of mechanical stability was obtained. This stability is based on the high osmotic pressure between the onion shells. This principle is adapted from plants and bacteria in nature. We expect the novel SiS capsules to be of particular interest for applications such as catalysis, chemical sensing, and separation, for which structural features such as selective permeability, cavity shape, and a remarkable mechanical stability are essential. High stability and ageindependent permeability distinguish these novel capsules from usual one-shell capsules. Acknowledgment. This work was supported by Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie (BMBF) and BASF. We thank Prof. Dr. Edwin Donath for scientific discussions, Heidemarie Zastrow for the preparation of silica particles and Anne Heilig for assistance with the SFM measurements. LA026491D