Inorganic Particle Synthesis in Confined Micron-SizedPolyelectrolyte Capsules

Igor L. Radtchenko,† Michael Giersig,‡ and Gleb B. Sukhorukov*,†

Max Planck Institute of Colloids & Surfaces, D-14476 Potsdam, Germany,and Hahn Meitner Inst Kernforsch Berlin GmbH, D-14109 Berlin, Germany

Received February 12, 2002. In Final Form: July 15, 2002

Iron oxide nanoparticles were synthesized inside 5 µm polyelectrolyte capsules. The capsule wall, madeof polyelectrolyte multilayers of poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH),is permeable for small ions but not for polymers. To perform particle synthesis exclusively in the capsuleinterior, capsules were loaded with PAH. Encapsulation of PAH was done by assembly of a double-shellstructure on a decomposable 5 µm template. The outer shell is composed of stable PAH/PSS multilayers,while the inner shell is a complex of PAH with multivalent anions. This complex can be dissolved, releasingsmall ions outside and keeping PAH inside the capsules. This was proved by fluorescent confocal microscopy.The presence of polycations inside the capsules maintains a pH gradient across the capsule wall accordingto a Donnan equilibrium. By use of fluorescent markers covalently bound to PAH, pH measurements inthe capsule interior revealed a basic pH inside the capsule. Fe3+ ions penetrating capsule walls face ahigher pH and precipitate in the interior, forming iron oxide particles. These particles were studied bytransmission electron microscopy.

Introduction

A novel method of micron- and submicron-sized capsulefabrication has been recently introduced.1,2 This approachconsists of alternative adsorption of oppositely chargedmacromolecules on the surfaces of colloidal templates.The alternate adsorption or so-called layer-by-layer (LbL)assembly method was introduced for flat planar sur-faces;3-5 for a review see ref 6. LbL makes use of electro-static interaction at each step of adsorption and can involvemany substances as layer constituents, such as syntheticpolyelectrolytes, proteins, nucleic acids, lipids, inorganicnanoparticles, and multivalent dyes. In the past few years,this layer-by-layer technology was transferred from flatsubstrates to surfaces of submicron-sized colloidal par-ticles. Different colloidal cores were used to template LbLpolyelectrolyte assembly on their surfaces. Some examplesare organic latex particles,7 inorganic particles,8 dye anddrug nanocrystals,9 compact forms of DNA, proteinaggregates,10 and biological cells.11 In certain cases, thecolloidal core might be decomposed at conditions where

the polyelectrolyte multilayers are stable, leaving a hollowpolyelectrolyte capsule (for a review see ref 12).

The most important feature of the polyelectrolytemultilayer shells is the controllable wall permeability,which makes them promising as microcontainers forperforming chemical reactions in restricted volumes.Basically, small solutes such as ions or dye molecules canreadily penetrate polyelectrolyte multilayers, while largemacromolecules cannot. At the moment, there are twomain approaches on how to deliver macromolecules intothe capsule interior to be kept there. One method is basedon switchable capsule wall permeability. Polyelectrolytemultilayers made of weak polyelectrolytes are verysensitive to the pH value, and therefore this parametermay be used to control shell permeability. In ref 13, thecapsule permeability demonstrated a transition betweenopen and closed states as the pH value approached the pKvalue of the polyelectrolyte used for shell assembly.Capsule treatment in water/ethanol mixtures also causesa reversible polyelectrolyte segregation. This segregationforms defects in the shell, opening the multilayer formacromolecule uptake. Soaking these capsules again inwater provides capturing the uptaken macromolecules.The second approach consists of a double-shell structureformation, where the inner shell can later be decomposed,forming polymer components in the capsule interior.14,15

The outer shell remains stable and allows polymercapturing.

Rather flexible variation of the chemical compositionin the capsule interior gives an opportunity to perform

* Corresponding author. Tel: (049)-0331-567-9429. Fax: (049)-0331-567-9202. E-mail: Gleb@MPIKG-GOLM.MPG.DE.

† Max Planck Inst Colloids & Surfaces.‡ Hahn Meitner Inst Kernforsch Berlin GmbH.(1) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S.; Mohwald,

H. Angew. Chem. 1998, 110, 2323; Angew. Chem., Int. Ed. 1998, 37,2201.

(2) Voigt, A.; Lichtenfeld, H.; Sukhorukov, G. B.; Zastrow, H.; Donath,E.; Baumler, H.; Mohwald, H. Ind. Eng. Chem. Res. 1999, 38, 4037.

(3) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569.(4) Lee, H.; Kepley, L. J.; Hong, H. G.; Akhter, S.; Mallouk, T. E. J.

Phys. Chem. 1988, 92, 2597.(5) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991,

46, 321.(6) (a) Decher, G. Science 1997, 277, 1232. (b) Bertrand, P.; Jonas,

A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21,319.

(7) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.;Knippel, M.; Budde, A.; Mohwald, H. Colloids Surf., A 1998, 137, 253.

(8) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111.(9) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Mohwald, H. J.

Phys. Chem. B 2001, 105, 2281.(10) Balabushevitch, N. G.; Sukhorukov, G. B.; Moroz, N. A.; Volodkin,

D. V.; Larionova, N. I.; Donath, E.; Mohwald, H. Biotechnol. Bioeng.2001, 76, 207.

(11) Neu, B.; Voigt, A.; Mitlohner, R.; Leporatti, S.; Donath, E.; Gao,C. Y.; Kiesewetter, H.; Mohwald, H.; Meiselman, H. J.; Baumler, H. J.Microencapsulation 2001, 18, 385.

(12) Sukhorukov, G. B. Designed Nano-engineered Polymer Filmson Colloidal Particles and Capsules. In Novel Methods to StudyInterfacial Layers; Mobius, D., Miller, R., Eds.; Elsevier Science B.V.:Amsterdam, 2001; p 384.

(13) Sukhorukov, G. B.; Antipov, A. A.; Voigt, A.; Donath, E.;Mohwald, H. Macromol. Rapid Commun. 2001, 22, 44.

(14) Radtchenko, I. L.; Sukhorukov, G. B.; Leporatti, S.; Khomutov,G. B.; Donath, E.; Mohwald, H. J. Colloid Interface Sci. 2000, 230, 272.

(15) Radtchenko, I. L.; Sukhorukov, G. B.; Mohwald, H. ColloidsSurf., A 2002, 202, 127.

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chemical reactions only inside where the reagents easilydiffuse from the bulk to the interior but the product staysin the interior. As demonstrated in ref 16, when thepolyions are placed either in the interior or in the bulk apH gradient can be established. Therefore a pH-dependentchemical reaction should occur according to a certain pHvalue. In this paper, we demonstrate the synthesis of ironoxide particles in the capsule interior where the pH iskept more basic than in the bulk.

Experimental DetailsMaterials. Sodium poly(styrene sulfonate) (PSS, MW ∼

70 000), poly(allylamine hydrochloride) (PAH, MW ∼ 50 000),and citric acid were obtained from Aldrich. 4-Pyrene sulfate (4-PS) was purchased from Molecular Probes. For confocal fluo-rescence microscopy, PAH was labeled with rhodamine isothio-cyanate (TRITC) and fluorescein isothiocyanate (FITC) formingPAH-Rh and PAH-Fl, respectively, according to ref 7. Allcommercial polyelectrolytes were used without further purifica-tion except for PSS which was dialyzed against Milli-Q water(MW cutoff, 14 000) and lyophilized.

Latex Particles. Dispersions of monodisperse weakly cross-linked melamine formaldehyde (MF) particles with a diameterof 5.67 µm (product name MF-R-5.7, CV 1.9%) were purchasedfrom Microparticles GmbH (Berlin, Germany). These particlesdissolve at pH values less than 1.6 and have been used astemplates for the production of hollow polyelectrolyte capsules.1

Methods. Layer-by-Layer Assembly on Colloid Templates.Assembly on MF particles was done using a filtration setup.2 Asuspension of the particles with a concentration of about 0.2 vol% was placed above the cellulose acetate filters with a pore sizeof 1 µm. The following solutions were used for alternatingadsorption of PSS/PAH multilayers: 2 mg/mL PSS in 0.5 M NaCland 1 mg/mL PAH in 0.5 M NaCl. Each adsorption step wasfollowed by passing water through the filter to wash outnonadsorbed molecules. Hollow capsules were obtained bydecomposing the MF core with 0.1 M HCl solution according toref 1.

Fluorescence Spectroscopy. Fluorescence spectra were recordedby means of a Fluorolog, Spex. Excitation was set to 492 nm.

Confocal Laser Scanning Microscopy. Confocal micrographswere taken with a Leica TCS SP, equipped with a 100× oilimmersion objective. The excitation wavelength was chosenaccording to the two labels, which were fluorescein (488 nm) orrhodamine (525 nm).

Transmission Electron Microscopy. Transmission electronmicroscopy (TEM) measurements were performed on a PhilipsCM-300 microscope operating at 300 kV.

Results and DiscussionControlled Precipitation of PAH on MF Templates

with Following Encapsulation of the Polycation.Controlled precipitation of PAH on colloidal particles wasobtained by the following approach (Scheme 1). Thesuspension of MF particles was mixed with 4-PS4- ions

(a), and then the polycation PAH was added, which leadsto the formation of precipitates 4-PS4-/PAH (b). Onemilliliter of MF particle suspension with a concentrationof 5 × 108 cm-3 with 5 × 10-3 M 4-PS4- was continuouslystirred during dropping (10 µL each droplet) of PAH-Flsolution (1 mg/mL) until the final PAH-Fl concentrationreached a certain value. The insoluble 4-PS4-/PAH-Flcomplex is slowly formed and MF particles are collectingthe precipitated complex. This amount of PAH-Fl asestimated is sufficient to coat MF particles with ap-proximately 80 monolayers of complex. After 10-15 min,the particles were centrifuged (c), and the amount of PAH-Fl molecules, which were not bound to the particles, wasmeasured by supernatant fluorescence. Remarkably, theamount of PAH-Fl that was not precipitated onto MFparticles is less than 20%. It should also be noted that wealways have an excess of the 4-PS4- charge, to form acomplex with the PAH-Fl molecules and counterions. TheMF particles were observed by confocal fluorescentmicroscopy, and the fluorescence coverage of MF particlesis rather smooth (Figure 1), with very few fluorescentspecies outside the particles. Fluorescent PAH-Fl washomogeneously distributed on the surface of MF particles.Similar results were obtained with citric acid (instead of4-PS) and PAH-Rh (instead of PAH-Fl).17

Such complexes of multicharged ions and PAH (4-PS4-/PAH and citrate/PAH) are not absolutely stable. 4-PS4-/PAH complexes could be destroyed by high ionic strength.14

For dissolution of the PAH/citrate complex, a pH of higherthan 10 or lower than 2 could be used.

Loading of PAH into polyelectrolyte capsules consistsof assembling stable polyelectrolyte multilayers, forinstance, PAH/PSS, on top of the obtained core/shellstructures (Scheme 1, part d) by means of the layer-by-layer technique. After colloidal core decomposition (e), thecapsules are composed of two-shell structures (f). Theprecipitated polymer might be dissolved in the capsuleinterior at a condition where the outer shell is stable (g).Multivalent ions are small enough to penetrate through

(16) Sukhorukov, G. B.; Brumen, M.; Donath, E.; Mohwald, H. J.Phys. Chem. B 1999, 103, 6434.

(17) Dudnik, V.; Sukhorukov, G. B.; Radtchenko, I. L.; Mohwald, H.Macromolecules 2001, 34, 2329.

Scheme 1. Schematic Illustration of the Preparationof Capsules Loaded with Polyelectrolytes

Figure 1. Typical confocal microscopy image of MF particlescovered with PAH labeled with a fluorescent marker by meansof controlled precipitation (secondary rings on the picture arediffraction-caused artifacts).

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polyelectrolyte multilayers comprising the outer shell,while polymers used for inner shell buildup cannot beexpelled due to their high molecular weight. Thus, thesepolymers are captured inside the capsule as freely floatingmolecules (h).

The described idea has been used to load the polyelec-trolyte capsules with PAH. MF particles covered with4-PS4-/PAH or citrate/PAH complexes were additionallycovered by four PSS/PAH layer pairs. Then the MFparticles were dissolved in 0.1 M HCl leaving hollowcapsules. Capsules with 4-PS4-/PAH complexes weretreated in 2 M NaCl. The capsules containing citrate/PAH were exposed to basic pH conditions. As mentionedabove, by these conditions the PAH/multivalent-ioncomplexes are dissolved. The ions are expelled out of thecapsules and can be removed. The confocal fluorescenceobservations of these capsules after inner shell decom-position revealed the filling of the capsule interior withfree polymers labeled with fluorescent marker and theiruniform concentration over the capsule interior (Figure2). Complete dissolution of 4-PS4-/PAH complexes can beassumed because there was no 4-PS excimer peak foundin the fluorescence spectra of the solution containing thecapsules, which would be expected for complex formationwith PAH.18

The concentration of free polymer molecules inside thecapsules was estimated by the integrated fluorescenceintensity from the interior revealed by confocal microscopy.It gives a value of approximately 0.1 M PAH monomerconcentration in rather good agreement with the estima-tions based on the assumption that all polymer moleculesare finally dissolved in the interior. This means that wehave about 1.2 pg of polymer per capsule. Actually, theamount of loaded polymer is determined by the ratiobetween polymer and colloidal particle concentrationsduring the controlled precipitation.15

Measuring of the pH Gradient between Bulk andInterior of the Polycation Loaded Shells. At normalconditions, the capsule wall is permeable for water,hydrogen ions, hydroxyl ions, and small salt ions and it

is impermeable for polymeric solutes, such as the poly-cations inside.13 As a result, entrapping the polyelectro-lytes inside the capsules leads to a pH gradient across thecapsule wall mediated by a Donnan equilibrium.16 In ourcase, the presence of PAH (polybase) only inside thecapsules shifts the pH in their interiors to basic valueswhen the pH in the bulk solution is neutral (Figure 3).

Fluorescence of rhodamine derivatives used for PAHlabeling has a reasonable pH sensitivity and gave us away to measure the pH inside the loaded capsules. Toestimate the pH shift, the fluorescence intensity of thesuspension of polyelectrolyte capsules loaded with PAH-Rh was titrated versus the pH value in the systemestablished by means of a Na2CO3/NaHCO3 buffer.Because small molecules of the buffer can easily penetratethrough the capsule wall, we assumed that the buffer withionic strength 0.3 M equalizes the pH value outside andinside of the polycation capsules, where the PAH monomerconcentration is 0.1 M. The titration curve (Figure 4) showsthe typical behavior for the polycation charged groupsprotonated at pH values close to their pK.19 The influenceof the salt concentration on the fluorescence intensity isnegligible in comparison with the pH effect, and thetitration curve could be used to calibrate the following pHcalculations from the known fluorescence intensities. Bythis method, the pH inside the modified capsules in awater suspension (bulk pH ) 7) was calculated and gavea value of 8.8 equivalent to a gradient of 1.8 pH unitsacross the capsule wall. Deprotonation of PAH aminegroups, used for loading, takes place at values higher than8 pH units. Above this pH level, charged PAH groups lose

(18) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mohwald, H. Macro-molecules 1999, 32, 2317. (19) Borkovec, M.; Koper, G. J. M. Macromolecules 1997, 30, 2151.

Figure 2. Fluorescent confocal microscopy image of thepolyelectrolyte capsules loaded with PAH-Rd by means of innershell decomposition.

Figure 3. Schematic illustration of the pH gradient establishedacross the wall of the capsule loaded with polycation.

Figure 4. Dependence of the maximum fluorescence intensityof capsules modified with PAH-Fl against the bulk pH.

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protons, become neutral, and as a result release negativelycharged molecules (counterions) associated with stoppinga further pH gradient increase.

Selective Synthesis of Iron Oxide Particles insidethe Polycation-Modified Shells. One of the mostinteresting chemical reactions is pH-induced precipitationand crystal formation of inorganic materials. A pHgradient across the capsule wall enables us to performpH-sensitive processes only in the restricted volumes ofthe polyelectrolyte capsules or to select certain materialsfrom the mixture. To verify this, capsules modified withPAH were mixed with a 0.2 M solution of FeCl3, storedin such conditions for 2 h, and then washed by thecentrifugation technique. The pH shift in the capsuleinterior provided by polycations was high enough for Fe3+

ions to form an insoluble hydroxyl Fe(OH)3 and precipitate.With time, this substance changes structure and formsFe2O3 crystals. A typical confocal image in transmissionmode of the water suspension of capsules loaded with PAH

is presented in Figure 5a. In Figure 5b, there are capsulesexposed to FeCl3 solution. The presence of the precipitate,only in capsules after the mixing with FeCl3, is clearlyseen. Precipitates remained in the capsule interior evenafter the washing.

Visualization and characterization of the precipitateswere performed by TEM. A typical image of capsules filledwith the precipitated material at low magnification ispresented in Figure 6. The other part of this figure showsthe precipitates at high magnification; the elongated shapewith the average dimensions of 225 nm in length and 50nm thickness and the polyelectrolyte capsule are clearlyseen.

The ultrastructure of the precipitate is presented inFigure 7. Typical lattice plane distances of 0.36 and 0.25

Figure 5. (a) Transmission confocal microscopy image of thepolyelectrolyte capsules loaded with PAH by means of innershell decomposition. (b) Transmission confocal microscopyimage of the polyelectrolyte capsules loaded with PAH afterthe treatment with FeCl3 solution and following washing.

Figure 6. Low-magnification TEM image of Fe-based pre-cipitates in capsules modified with PAH.

Figure 7. TEM image of the precipitate ultrastructure incapsules modified with PAH with the inserted high-resolutionimage.

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nm for the R-Fe2O3 modification could be detected in theinserted high-resolution image, and their power spectrumof the 0.36 nm distance is shown.

To check the sensitivity of capsules modified with PAH,they were exposed to a mixture of 0.2 M FeCl3 and 0.2 MNiCl2 for 2 h. As a result, Fe-containing crystals of thesame type and structure were formed only inside thecapsules modified with PAH. After the washing, no Niwas found associated with capsules, which was checkedby elemental analysis made from single capsules. Thenecessary pH to form insoluble precipitates for Ni2+ ismore than 10, that is, higher than capsules filled withPAH can maintain. That is why as a result only Federivatives selectively precipitate in the restricted capsulevolume while Ni keeps its ionic form.

ConclusionEncapsulation of polycations with a selective concen-

tration provides us with the ability to control the phys-icochemical properties of the polyelectrolyte capsuleinterior. The presence of different polycations only insidethe capsules leads to different pH shifts in these capsule-restricted volumes and to the stabilization of the interiorpH around the pK value of the chosen polyelectrolyte.

The result is the establishment of pH gradients acrossthe capsule walls. This gradient is a driving force forchemical reactions such as pH-dependent precipitationand crystal formation, which was demonstrated on Federivatives. Crystals of Fe2O3 were formed only insidethe capsules modified by polycations. We could also useion selective chemistry; only the processes with a cutoffpH lower than the pH inside the loaded capsules couldtake place in the interior. Such modified capsules aremicrocontainers and microreactors and are supposed toperform chemical reactions in their restricted volumeoffering unique controlled conditions on the micron-scalerange.

Acknowledgment. The authors thank Professor Dr.H. Mohwald for helpful discussions and support and Dr.A. Susha for valuable comments (both at the Max-PlanckInstitute of Colloids and Interfaces, Potsdam). Ms. MichellePrevot (Louisiana Tech University, USA) is acknowledgedfor reading the manuscript and corrections. The work wassupported by the Sofia Kovalevskaya Program of theAlexander von Humboldt Foundation, German ministryof research and education.

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