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Nanoparticle Synthesis in Engineered Organic Nanoscale Reactors** By Dmitry G. Shchukin and Gleb B. Sukhorukov* In this review, the recent achievements in the synthesis of inorganic nanomaterials inside the spatially confined volume of individual microand submicroreactors (emulsions, micelles, organized thin films, polyelectrolyte capsules, etc.) are presented. The advantages and shortcomings of each type of microreactor are discussed. Particular attention is paid to polyelectrolyte capsules as confined microreactors with controlled shell permeability and the possibility of shell engineering on the nanolevel, thus tailoring different functionalities. Nanomaterials synthesized inside a confined multifunctional microreactor have several advantages: i) absence of particle aggregates, ii) amorphous or metastable crystal phases, and iii) unique composite inorganic/inorganic and inorganic/organic structures.

1. Introduction The investigation of new inorganic and inorganic/organic composite nanomaterials and nanostructures is a rapidly developing and expanding area of research with tremendous potential for industrial applications. The main advantages of these nanomaterials are their specific physical (optical, mechanic, magnetic), chemical (reaction activity, catalytic), and biomedical (curing, delivery, release) properties. Different approaches for the fabrication of nanoscale materials based on physical or chemical principles have been exploited (for example wet synthesis,[1] evaporation,[2] lithography[3]). One of the most interesting and sophisticated methods is to mimic biomineralization processes, especially for the synthesis

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[*] Dr. G. B. Sukhorukov, Dr. D. G. Shchukin Max Planck Institute of Colloids and Interfaces Am Mühlenberg 1, D-14424 Potsdam (Germany) E-mail: [email protected] [**] This work was supported by the Sofja Kovalevskaja Program funded by the Alexander von Humboldt Foundation. Prof. Dr. Helmuth Möhwald is gratefully acknowledged for continuous support and stimulating discussions. The authors thank Rona Pitschke, Dr. Jürgen Hartmann, and Dr. Vera Klechkovskaya for scanning and transmission electron microscopy analysis, and Dr. Alexei Antipov, Dr. Igor Radtchenko, Dr. Alexander Petrov, Wenfei Dong, and Yuri Fedutik for their help in conducting the experimental work.

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of complex inorganic nanomaterials with dimensional, structural, and morphological specificity inside spatially confined cubic micrometer or submicrometer volumes of a defined shape. An organized reaction microenvironment is employed to constrain and pattern the inorganic precursors and to perform the synthesis of inorganic nanocomposites in a controllable and preselected way. The virtues of this synthetic approach are: d the confined cubic micrometer or submicrometer volume enables one to carry out chemical synthesis in a highly organized solvent (water) structure, which can result in new composite nanomaterials that are impossible or difficult to synthesize in conventional, bulk media; d the in-situ fabrication of nanoreactors filled with catalytically active components; d a diminishing of the effect of overconcentration and overheating in the reaction vessel upon adding reagents; d the tailoring of different functionalities to the microenvironment as a result of synthesis in one reaction; d the modeling and mimicking of biochemical (particularly biomineralization) processes in living cells and their compartments by means of nanoscale chemistry. There are several different types of micrometer- and submicrometer-scale reactors, which are either individual (vesicles, micelles, emulsion droplets, liposomes, capsules) or connected to neighbors (ªopenº or ªclosedº micro- and mesoporous hosts, lab-on-a-chip approach). The latter type of microreac-

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tor has been well studied, and there is plenty of published research on this topic. For more detailed information on nanosynthesis inside micro- and mesoporous hosts made of block copolymers,[4] polymer gels,[5] etc., and their application, we refer the reader to several recent papers.[6] However, there have been only a few demonstrations of the idea of synthetic reactions inside spatially confined individual micro- and/or nanoreactors so far. Systematic, continuous studies of the mechanisms and reaction kinetics, the influence of different parameters (size of the microreactor, solvent structure, etc.) on the synthetic processes, as well as a comparison of the properties of the resulting nanomaterials with those obtained by conventional methods in solution, have not been performed. Such detailed investigations will help, not only in the fabrication of new inorganic composite materials and the development of spatially confined synthetic approaches, but also in acquiring additional understanding of the chemical, physicochemical, and biochemical processes occurring in a confined environment on the sub-micrometer scale. Despite the fact that a size range a hundred times smaller than the range discussed here is required to exert an influence on elementary chemical kinetics (1 nm), the effect of confined geometry together with controllable diffusion of the reagents could still lead to results not achievable in bulk synthesis. This review is an attempt to summarize the current knowledge on

spatially confined nanosynthesis inside individual micro(submicro)reactors (microemulsions, micelles, liposomes, organized thin films, polyelectrolyte capsules) and bring to light the main hindrances in this new and interesting area of preparative nanochemistry.

2. Micro- and Submicroreactors 2.1. Emulsions Microemulsions were one of the first types of individual microreactors explored for nanosynthesis. Different preparation procedures to fabricate both direct and reverse emulsions have been developed and studied.[7] These procedures allow the preparation of monodisperse, stable microemulsions of different sizes (between 50 nm and tens of micrometers). In general, during nanosynthesis in an emulsion microreactor, either both of the initial reagents are separately dissolved in disperse media,[8] or one is a constituent of the surfactant.[9] Another method is based on the reaction, in a so-called micellar exchange process, between two or more emulsion microdroplets containing dissolved reactants.[10] As a result, the microdroplets aggregate and form a bigger drop filled with the reaction product.

Dmitry Shchukin is currently a Humboldt Fellow at the Max Planck Institute of Colloids and Interfaces (Interfacial Department headed by Prof. Helmuth Möhwald), Potsdam, Germany. In 1998 he graduated from the Belarusian State University (Minsk, Belarus). In January 2002 he received his doctorate degree in Inorganic and Physical Chemistry. He was a DAAD (German Academic Exchange Service) fellow at the Max Planck Institute of Colloids and Interfaces (2001± 2002), an INTAS fellow at the Ecole Centrale de Lyon, Lyon, France (2001), and a postdoctoral research fellow at the Louisiana Tech University, Ruston, USA (2002±2003). His main scientific interests concern the template synthesis of composite nanomaterials, the study of chemical processes in confined micro- and nanoenvironment, biomimetic synthesis, and biomineralization. Gleb Sukhorukov is currently running a group ªMultifunctional Nanoengineered Polymer Capsulesº at the Max-Planck Institute of Colloids and Interfaces, Potsdam, Germany. He graduated from Department of Physics, Lomonosov Moscow State University, where he received his doctorate degree in 1994 in the area of biophysics. He was a scientific researcher at the Institute of Crystallography, Russian Academy of Sciences, and a postdoctoral researcher at the Institute of Physical Chemistry, University of Mainz, Germany, working on layer-by-layer assembly of polyelectrolytes. Since 1997 he has been a postdoctoral researcher at the Max-Planck Institute of Colloids and Interfaces in the department headed by Prof. Helmuth Möhwald. In 2000 he co-founded the start-up company ªCapsulution NanoScienceº based on encapsulation technology developed in the Institute. He worked in the company as project manager until he was awarded a Sofja Kovalevskaja scholarship of the Alexander von Humboldt Foundation in 2001, which allowed him to return to the Institute as group leader. His research field encompasses physical chemistry, biophysics, and materials science, comprising physics and (bio)chemistry on submicrometer dimensions, the design of multifunctional colloidal particles and capsules, and nano-engineered biomaterials. 672

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A wide range of inorganic nanoparticles has been obtained in microemulsion systems, although most of the research papers involve the preparation of metal oxides (SiO2,[11] TiO2,[12] complex titanates,[13] ZrO2,[14] Fe2O3 hematite and Fe3O4 magnetite,[15] zeolites,[16] CaCO3[17]) in reverse waterin-oil emulsions. Metal oxides are usually synthesized by a micellar exchange process or by dissolving a metal salt in an aqueous phase inside the microdroplet and adding a base (pyridine, methoxy ethyl amine) to the oil phase. Dense, mesoporous oxide particles and hollow spheres can thus be synthesized. The size of the oxide nanoparticles varies from 4 to 20 nm depending on the material and the diameter of the emulsion microdroplet. Other model nanomaterials made in reverse microemulsions are calcium carbonate[17] and barium sulfate.[18] In this case, the metal salts are dissolved in dispersed aqueous microdroplets, while the CO2 or sulfate come from organic media. Two polymorph modifications of the resulting CaCO3 (calcite, vaterite) were formed, whereas under the same conditions in bulk solution only the calcite modification forms.[17b] This is apparently due to a confined reaction microenvironment, water structure, and the presence of surfactants stabilizing the emulsion microdroplets. The vaterite/calcite ratio can be controlled by the pH of the aqueous media or the type of emulsifier, and ranges from individual crystalline nanoparticles to hierarchically ordered aggregates. Changing the concentration of CO2 in the oil phase allows hollow or dense calcium carbonate agglomerates to be formed. Inorganic nanomaterials inside microemulsion droplets can also be prepared by physical crystallization from concentrated salt solutions and molten salts (FeCl3, ice, hexane);[19] metallic nanoparticles with low melting points (gallium alloys) can also be obtained.[15a] Mini-emulsification of low melting salts and metals enables the direct synthesis of nanoparticles of high homogeneity with diameters of between 150 and 400 nm. The crystallization temperature of such emulsion microdroplets is significantly lower than the bulk material, which has been attributed to a very effective suppression of heterogeneous nucleation.[20] Microemulsions can serve as excellent microreactors for enzyme-driven hydrolysis, as this medium enables a high substrate concentration and a high yield of the products.[21] Garti et al. have demonstrated the feasibility of such a microreactor with the hydrolysis of water-insoluble phosphatidylcholine by a water-soluble enzyme, phospholipase A2, trapped inside a water-in-oil emulsion.[22] The polar region of the phosphatidylcholine is hydrolyzed within the aqueous microdroplet of the reverse emulsion; the rate of hydrolysis is dependent on the structure of the microemulsion particles. The main advantage of microemulsions as spatially confined microreactors is their easy, one-step fabrication without sophisticated, time-consuming synthetic stages. However, there are several considerable shortcomings: the narrow range of nanomaterials that are possible to synthesize inside the microemulsion droplet; lack of mild control (changing pH, ionic strength) over the reaction process; and the absence of a

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stable, rigid shell of the emulsion microreactor. In addition, the reaction usually occurs in the dilute solution of precursors, which does not result in a high yield of nanomaterial.

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2.2. Micellar Systems The structure and composition of micelles (a typical reversed micellar system includes the amphiphilic compound, water, and the bulk organic solvent) differ considerably from those of emulsions, despite the fact that both of them are formed by amphiphilic hydrocarbons.[23] Micelles result from the thermodynamically stable organization of surfactants, which encapsulate a nanosized water core (1±20 nm) in contact with their hydrophilic head groups. The hydrophobic tails of the surfactant are solvated by a bulk, continuous-phase solvent. When water-soluble polar and ionic substances are dissolved within the water core, the continuous-phase (usually liquid alkanes) solvent is left essentially unaltered. As a result, reactions involving the dissolved substances occur directly in the micelle cores. The small size of the micelle core opens an avenue to employ micelle nanoreactors for the synthesis of narrowly distributed semiconductors with marked quantum-size (Q-size) effects. This micellar approach has been demonstrated in a number of papers where Q-sized semiconductors (ZnSe,[24] CdS,[25] CdSe,[26] CdTe,[25b,27] PbS,[28] ZnS[29]) were fabricated inside AOT (sodium bis(2-ethylhexyl)sulfosuccinate) reverse micelles using ion-exchange reactions. This anionic surfactant has received particular attention because of its ability to solubilize relatively large amounts of water in a variety of hydrophobic organic solvents.[30] The average diameter of the resulting nanoparticles is between 3 and 6 nm. The specific synthetic concept here is to entrap a metal salt solution (e.g., Zn(ClO4)2) inside a micellar core, followed by instant addition of the anion (e.g., Na2Se) to the solvent phase. Besides Q-dots, individual TiO2 and SiO2 nanoparticles or those in combination with metals (Pt/SiO2, Pd/SiO2) have been obtained.[31] The synthesis of nanosized metal particles inside a micellar nanoreactor has been shown with Co, Cu, Pd, Fe, Ag, and Au nanoparticles, or their polymer composites, as examples.[32] Metal ions were introduced into micelle core either as salts (complexes) with the surfactant or by dissolving them inside prior to the reaction. The embedded metal ions were reduced by a strong, soluble reducing agent (e.g., NaBH4, LiB(C2H5)3H, N2H4) or gaseous H2. The size of the resulting metal nanoparticles and their catalytic activity depend on the micelle diameter, and the type and concentration of the reducing agent. Micelles of large diameter where H2 is used as reducing agent lead to large metal particles (10±15 nm), whereas small nanoparticles are formed when NaBH4 is used. These metal nanoparticles exhibit a higher catalytic stability than classical metal catalysts.[32e] Micelles are of great interest in enzymology, since enzymes solubilized in the aqueous interiors of reversed micelles may

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show enhanced activity and changes in substrate specificity.[33] This enhanced activity can be attributed to various factors, including the increased conformational rigidity of the enzyme, a reduction of substrate inhibition, and stabilization of the transition state by interaction with the surfactant head groups. Solubilization of several enzymes, including lysozyme, chymotrypsin, lipase, lipoxidase, carbonic anhydrase, and ribonuclease A has been discussed previously.[34] The shell of the reverse micelles significantly alters the conformation of the solubilized enzymes in order to accommodate the micellar structure, through interactions with the micellar interface. Some authors[35] have demonstrated the enzyme-catalyzed formation of conductive polyaniline inside sodium dodecylbenzenesulfonate and Triton X100 reverse micelles. Horseradish peroxidase, capable of catalyzing the polymerization of anilines and phenols in the presence of hydrogen peroxide, was used as the enzyme catalyst. The ability to control the properties of inorganic (organic) materials at the nanometer scale has also been shown using biological objects (protein crystals,[36] viral capsids,[37] a hollow microenvironment made up of enzymes,[38] polypeptide cages,[39] and liposomes[40]) as confined media for biomimetic synthesis. Lumazine synthase, a hollow enzyme found in bacteria and fungi, and phospholipid microlamellar vesicles of approx. 30 nm diameter were used as a mineralization environment in the fabrication of nanocrystalline iron oxide.[38] The mediating factors for this process include vesicle shape and size, the presence of positively and negatively charged channels in the capsid shell, and diffusion-limited processes of ion-transport plus ion-binding at the curved lipid head-group surface. The biomineralized nanodeposits differ in their structure, morphology, and size from precipitates made in bulk aqueous solution. Micelles, as confined microreactors, have very definite characteristics, such as a small micro(nano)reactor volume (micellar core) and the possibility of broad control over the permeability of the reactor shell by changing the concentration of the amphiphilic compound or by modification of the micellar shell by third components. Nevertheless, a nanoengineered reactor shell cannot be achieved for micelles because their size and permeability cannot be tuned with sufficient precision.

2.3. Multilayer Organic Films Despite the significant success in spatially confined nanosynthesis inside emulsions and micelles, these types of microand submicroreactors have a number of significant drawbacks that prevent them from being used in the same way as conventional ªmacroreactorsº to precisely control the reactions and properties of the resulting nanomaterials: it is hard to control the composition and properties of their shell and microvolume on the nanolevel. Moreover, it is impossible to carry out sequential reactions inside the same micelle or emulsion droplet, in order, for example, to obtain layered composite materi-

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als, such as the deposition of an SiO2 shell on the surface of already synthesized nano-TiO2. Nanoengineering of a polymer microreactor shell becomes possible by applying the recently developed layer-by-layer (LbL) approach,[41] comprising adsorption of oppositely charged polyelectrolytes on a solid surface of variable shape. This technique permits the step-wise adsorption of various components, as the layer growth is governed by competitive charge compensation, and allows the formation of multilayer shells with nanometer precision.[42] LbL assembly allows the facile, cheap, and environmentally friendly processing of both simple and complex structures. By varying the pH of the medium during multilayer assembly, the layer thickness, permeability, surface wettability, and number of unbound functional groups can be controlled.[43] Therefore, by choosing the right assembly conditions, a polyelectrolyte film can be fabricated with desired properties that are advantageous for synthesizing composite nanomaterials inside. Multilayer thin films of adsorbed polyelectrolytes have been utilized as nanoreactors for both metallic (Ag, Ni, Pd, Cu) and semiconductor nanoparticles (PbS).[44] Polyelectrolyte multilayers with a controlled content of free carboxylic acid binding groups have been fabricated with weak polyelectrolytes such as poly(allylamine hydrochloride) and poly(acrylic acid). These groups were used to bind various inorganic ions (Ag+, Pd(NH3)42+, PdCl42±, Cu2+), which were then converted into nanoparticles by a reducing agent. Spatial control (on the nanoscale) over the growth of the nanoparticles was achieved by the use of multilayer heterostructures containing bilayer blocks that are not able to bind inorganic ions (Fig. 1A).[45] These ªnon-bindingº bilayers were fabricated from strong polyacids such as poly(styrene sulfonic acid). The synthetic reaction also regenerates the carboxylic acid sites, allowing the nanoreactors to be ªreloadedº with the same metal. The primary advantages of this approach are the ability to fabricate large area, highly uniform nanocomposite thin films, and control of the supramolecular organization and molecular environment of the nanoparticles by a simple layerby-layer deposition process. Erokhin and co-workers have used a similar technique for the fabrication of semiconductor nanoparticles in Langmuir±Blodgett thin films.[46] Lvov et. al.[47] have demonstrated a new approach for building three-dimensional nanoscale structures using charged nanoparticles and 500 nm diameter lipid tubules. Silica or gold nanoparticle structures were assembled onto the lipid tubules by sequential adsorption with oppositely charged polymers. For tubules of the zwitterionic diacetylenic lipid DC8,11PC, this process leads to the formation of caps on the ends of the tubules, with 50±100 silica spheres in each cap (Fig. 1B). For tubules of DC8,11PC mixed with 2 % of the charged lipids, the sequential adsorption of PEI/PSS/PEI (PEI: poly(ethyleneimine); PSS: poly(styrene sulfonate)) nanoparticles leads to both end caps and helices of nanoparticles winding around the interior of the tubules (Fig. 1C). These results gave new insight into the charge distribution in the lipid tubules. Therefore, the LbL technique allowed the

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Figure 1. A) Cross-sectional TEM image of a multilayer thin film comprised of poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) bilayer blocks alternating with poly(allylamine hydrochloride)/poly(styrene sulfonic acid) (PAH/PSS) bilayer blocks. Silver nanoparticles are created only within the two PAH/PAA bilayer blocks. The inset shows a higher magnification of a region of the film showing the diffuse interfaces between the multilayers (reprinted with permission from [45]. Copyright American Chemical Society, 2000). B) TEM images of zwitterionic diacetylenic DC8,11PC microtubes treated with poly(ethyleneimine)/poly(styrene sulfonate) + 40 nm silica. C) Images of the microtubules formed upon addition of 2 % of charged lipids after the same treatment. The formation of nanoparticulate silica caps and helices is clearly seen. (Reprinted with permission from Prof. Dr. Yuri Lvov).

exploration of very small underlying charge distributions that were previously not observable. Each layer of polyion multiplies the charge of the layer below, yielding a significant amplification of the initial charge. This offers an approach to the formation of complex three-dimensional composite nanostructures.

2.4. Polyelectrolyte Capsules In 1998, a protocol to construct a novel type of microcontainer was introduced by Sukhorukov and Donath et al.[48] This procedure consists in the templating of LbL polyelectrolyte films on the surface of micrometer- and submicrometer-sized colloidal particles.[41] Core decomposition leads to the formation of hollow structures whose size and shape are determined by the initial colloidal core, and whose shell, composed of polyelectrolyte multilayers, is tunable in the nanometer range. Due to the tremendous possibilities to vary the permeability of polyelectrolyte multilayers,[43] this approach opens an avenue to control the flux into and out of the shell and to regulate the interior of the formed capsules. The capsule shell is usually permeable for macromolecules and nanoparticles at low pH (< 3) or high ionic strength, while it is ªclosedº at high pH (> 9). Currently there are several ways to introduce different classes of molecules into the capsules: i) capturing macromolecules by reversibly opening the pores in polyelectrolyte multilayers at different pH[49] or by varying the salt[50] or solvent;[51] ii) pre-precipitation of macromolecules on cores, followed by polyelectrolyte assembly (the macromolecules dissolve into the capsule after sacrificing the template core);[52] iii) embedding molecules in a porous core also followed by polyelectrolyte assembly;[53] and iv) synthesis of polymers in the capsule interior.[54] Recent reviews are also available on this topic.[55] Polyelectrolyte capsules can be employed as promising microreactors for carrying out chemical synthesis in their

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restricted volume. The possibility of varying the shell components and defining the capsule size provides diversity in the synthetic approaches and initial reagents, while selective and controllable permeability of the capsule shell allows the control of diffusion of the reagents and reaction kinetics. Depending on the nature of the compounds captured inside the capsule volume or in the shell, a great variety of chemical, biochemical, and physicochemical processes (nanoparticle precipitation, biomineralization, biomimetic synthesis, photocatalytic synthesis, etc.) can be performed in the capsule's interior. Moreover, polyelectrolyte capsules can be attained with defined characteristics (e.g., luminescent, magnetic) by chemical synthesis in the capsule shell or volume for further usage in drug delivery and storage systems, as catalytic or enzymatic microreactors, or as microcontainers for removing metal ions from industrial effluents. In this review we focus on some examples of spatially confined synthesis in polyelectrolyte capsules fabricated by the LbL approach. Other types of organic microcapsules (dendrimer, polymer) as reaction microenvironments have been described elsewhere.[56] One of the significant advantages of the polyelectrolyte capsules over other types of microreactors is their selective permeability. Obviously, in the simplest way the capsules are permeable for small ions but not permeable for polymers.[57] Because many polymers suppress the growth of inorganic crystals, the idea of inorganic particle formation in the lumen of the capsules with simultaneous presence of polymers outside was proposed.[58] This approach allowed the growth of calcium carbonate and barium carbonate particles inside polyelectrolyte capsules. Another way to initiate the growth of particles in the capsule interior is to change the pH of the whole system slightly over the threshold of solubility. The inner shell of the capsule might serve for nucleation.[59] It is possible to synthesize functional materials in the microcapsule (e.g., luminescent polymers), and to modify the permeation properties of the microcapsule walls by depositing polymers within the microcapsule walls.[59b]

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2.4.1. Chemical Reactions in Capsules Driven by a pH Gradient Selective permeability of the capsule wall can lead to the setting up of a concentration gradient, even for permeable solutes. Indeed, the presence of charged polymers exclusively either inside or outside the capsule contributes, according to the Donnan equilibrium, to the distribution of all ions for which the shell is permeable, meaning that small ions, including H+ and OH±, may have a concentration gradient across the capsule wall (Fig. 2). The polyelectrolytes' role is to act as a buffer so that the area surrounding their location is main-

Figure 2. Schematic illustration of the pH gradient established across the wall of a capsule loaded with polyanion (due to electroneutrality [Poly±] » [H+]).

tained around their own pK. A theoretical consideration, together with experimental data on pH measurements inside the capsules,[57] has yielded a convincing description of the distribution of protons inside and outside the capsules. A macroscopic model of these micrometer-sized capsules might look like a dialysis sac filled with polyions. In this case the pH shift can reach 4±5 units.[60] The influence of pH gradient on the chemical reaction in the capsule interior has been used for the precipitation of acidic dyes in a capsule filled with negatively charged poly(styrene sulfonate) (PSS) buffering the pH around 2±2.5.[55a,60] The shell-permeable carboxylic dyes experience a low pH inside the capsule, and precipitate. As result the capsules are filled with dye, as observed by scanning electron microscopy (SEM).[60] A remarkable observation is that these precipitates do not form a crystalline structure as they do when they precipitate at low pH in the bulk. Molecular dynamics simulations of the precipitation process for dye molecules in the capsule interior showed very limited possibilities for crystal formation, as also proven by X-ray analysis.[61] The decreased pH inside capsules loaded with PSS opens up the possibility to precipitate inorganic materials in acidic media. Among the substances, tungstate and molybdate ions can undergo chemical polymerization (polycondensation) in acidic media, forming different polytungstates (polymolyb676

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dates) with interesting catalytic, electrocatalytic, and electrochromic properties.[62] To synthesize polytungstic acids and nanosized WO3, PSSloaded capsules were added to NaWO4 solution and kept for 24 h.[63] Such a long reaction time (12±24 h) is required to complete the reaction, because of the possible diffusion or equilibrium limitations found in the capsule microreactor. The synthesis of polytungstate anions inside the polyelectrolyte capsules leads to the appearance of Raman signals at 390, 670, and 940 cm±1 corresponding to the vibration of the W±O and W±O±W bonds in the inorganic polyanion; the presence of the latter confirms the formation of polytungstate ions (Fig. 3A). The polymerization of polytungstate ions can be continued inside polyelectrolyte capsules to give nano-WO3. Dried capsules with WO3 particles inside have a rough surface that reproduces the bulky shape of the initial polyelectrolyte capsules in solution. The diameter of WO3-containing dried capsules is twice as small as that of the initial capsules,[55a] indicating partial shrinkage and densification of the capsule. The transmission electron microscopy (TEM) photograph (Fig. 3B) shows that WO3 particles completely fill the inner capsule volume and clearly demonstrates the uniformity of the WO3 nanodeposit (25±30 nm in diameter), which is in good agreement with the particle size estimated from X-ray diffraction (XRD) broadening if the formation of aggregates composed of several crystallites is considered. Cationic poly(allyl amine hydrochloride) (PAH), with OH± as counterions, creates an alkaline environment in the capsules;[64] PAH maintains the OH± concentration inside the capsule at a constant level. The presence of PAH in a 0.1 M monomer-chain concentration within the capsule interior causes a pH gradient from 6 outside the capsule to 9 inside. This pH shift is in good agreement with the theoretical value derived from the Donnan equation.[57] The alkaline pH in the capsule interior is a major force to selectively deposit nanosized Fe2O3, TiO2 gel or hollow spheres, magnetic ferrites, and Fe3O4 exclusively inside the capsules from the corresponding inorganic salts.[64,65] The resulting magnetic nanomaterials possess a magnetic susceptibility of around 1 mT. The presence of dissolved PAH inside the capsules is an essential condition for enabling the reaction; precipitation does not occur when using hollow capsules without PAH. A typical SEM image of polyelectrolyte capsules filled with Fe3O4 is shown in Figure 3C. The inorganic nanomaterial in the capsule interior prevents capsule collapse during drying, which was found to occur for empty polyelectrolyte capsules without inorganic or organic interior frameworks,[55a] thus maintaining the original bulky shape. The deposition of the magnetic ferrites and magnetite starts on the inner surface of the PAH/ PSS shell (see TEM image, Fig. 3D) where PAH molecules, which form the first layer of the PAH/PSS shell, are at their highest concentration. If the initial concentration of precursor salts is increased both the quantity and size of the formed magnetic particles also increase. Almost complete filling of

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the capsule interior with Fe3O4 aggregates of particles of average size 25±30 nm was observed when a concentrated (0.66 M FeSO4 + 0.62 M Fe(NO3)3) precursor solution was used for the precipitation.[65a] 2.4.2. Synthesis of Nanocomposites by Ion-Exchange Reactions Exposing PAH-loaded capsules to a solution of other anions (e.g., 0.1 M H3PO4 or 0.1 M HF) leads to rapid substitution of the citrate anions with PO43± or F± and formation of PAH/ PO43±±PAH/PSS or PAH/F±±PAH/PSS capsules. Thus, PAH/ PSS capsules containing precipitant inside can be obtained in a simple procedure (Fig. 4). Adding F±-loaded capsules to a water solution containing Y3+ ions results in formation of YF3 inside the capsules only, with no traces of precipitate in the surrounding media.[66] The yttrium compound precipitated inside the capsules is weakly crystallized YF3 with traces of Y(OH)3 as a minor component. Formation of the latter can be explained by hydrolysis of the yttrium salt in the presence of PAH molecules (see above). The crystallite size for YF3 inside the capsules, derived from

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XRD data, is around 7 nm, which is lower than that of YF3 precipitated from water solution (50±100 nm). The thickness and particle size of the YF3 layer (TEM data, Fig. 5A,B) depends greatly on the concentration of yttrium salt in solution. For high concentrations (10±3±5 ” 10±5 M [Y3+]) a continuous, 50±100 nm layer made up from 7±10 nm particles is observed, while separate agglomerates or individual particles attached to the inner capsule wall are formed at Y3+ concentrations below 10±6 M. Dried, polyelectrolyte capsules filled with yttrium fluoride have a morphology reminiscent of the original form of the polyelectrolyte capsule in solution (Fig. 5C). Folds and creases spreading from one YF3-loaded capsule to another are observed, thus demonstrating the partial shrinkage of the capsule shell. Moreover, part of the polyelectrolyte shell is exfoliated, exhibiting an inner YF3 layer. The deposition of insoluble rare-earth fluorides inside polyelectrolyte capsules permits their use as microcontainers for rare-earth-element separation and recovery from water solutions (wastewater, seawater, etc.[67]). A study has been carried out on the removal of traces of yttrium from both pure

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Y(NO3)3 aqueous solution and its mixtures with other metal salts. Adding F±-loaded capsules to a Y3+-containing solution in a 10±4±10±5 M concentration range results in quantitative (> 99 %) extraction of yttrium ions to the capsule interior (Table 1). An excess amount of other metals in the yttriumcontaining solution generally decreases the yttrium recovery yield. However, despite the negative effect of multicharged cations, the recovery yield remains high and Y3+-filled polyelectrolyte capsules can be easily collected from solution by a simple sedimentation or centrifugation procedure. A biomimetic synthesis of the most typical apatiteÐcalcium hydroxyapatite Ca10(PO4)6(OH)2Ðexclusively inside PAH/ PSS polyelectrolyte capsules has been accomplished by employing a PAH/PO43± complex captured in the capsule volume as a source of phosphate anions (Fig. 4).[68] Thus loaded polyelectrolyte capsules were added to a solution containing 1 M CaCl2 at pH 9 (adjusted by NaOH) for 48 h (Fig. 4). During the reaction no hydroxyapatite precipitate was observed in the outer solution. The average weight of a polyelectrolyte capsule after finishing the reaction was about 30 pg; the average weight of the initial PAH-loaded PAH/PSS is around 10 pg. TEM analysis established the preferential formation of hydroxyapatite nanoparticles on the inner side of the PAH/ PSS shell, resulting in empty hydroxyapatite spheres (Fig. 6). The thickness of the Ca10(PO4)6(OH)2 layer was 100±120 nm, made up of particles with a diameter of 12±16 nm. The formed hydroxyapatite particles have a shape and surface morphology different from the particles synthesized by common methods in solution. Other special properties of hydroxyapatite composite hollow shells, such as surface acid-

(A)

500 nm

Figure 4. Schematic illustration of the ionexchange nanosynthesis inside polyelectrolyte capsules. A ® B) Replacement of citrate anions in the PAH/citrate complex with desired anions (F±, PO43±). B ® C) Addition of metal ions to the capsule suspension.

(B)

Table 1. The efficiency of yttrium recovery from a 0.1 M solution containing other metal salts. Conc. of Y3+ [M]

±

Fe3+

ZrO2+

Na+

K+

10±6 10±5 10±4 10-3

92.3% 99.2% 99.4% 94.1%

78.4% 81.3% 83.4% 90.0%

82.4% 85.2% 86.7% 91.8%

91.8% 99.1% 99.2% 94.2%

92.0% 99.0% 99.5% 94.2%

ity, catalytic and biological activity, and bone-repairing effects can be expected. Composite YF3±polyelectrolyte or apatite±polyelectrolyte capsules have a higher mechanical stability than the initial polyelectrolyte ones. Besides the shape persistence after drying, the capsules are also stable against ultrasonic treatment. A mechanical force applied to the composite capsule leads, at first, to their reversible elastic deformation; further increase of the force results in irreversible plastic deformation of the inorganic capsules.

2.4.3. Photocatalytic Reactions in Polyelectrolyte Capsules Entrapped PSS or polyaniline (in emeraldine form) molecules can act as electron donors for photoinduced silver reduction both inside the capsule volume and in the capsule shell.[69] Figure 7 shows the schematic procedure of this process occurring within the capsule. At first, polyelectrolyte capsules were mixed with 0.01 M AgNO3 solution[69a] (or Ag+ ions were introduced during shell assembly[69b]) and kept for

(C)

5 µm

500 nm

Figure 5. TEM images of ultramicrotomed polyelectrolyte capsules filled with YF3 deposited from 5 ” 10±6 M Y(NO3)3 (A), 10±3 M Y(NO3)3 (B). C) SEM image of polyelectrolyte capsules filled with YF3 (reprinted with permission from [66]. Copyright American Chemical Society, 2003).

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Figure 6. TEM image of Ca10(PO4)6(OH)2 spheres precipitated inside polyelectrolyte capsules (reprinted with permission from [68]. Copyright American Chemical Society, 2003).

30 min in the dark to ensure homogeneous distribution of the silver ions (Fig. 7A,B). The capsules were then exposed to polychromatic irradiation (350±700 nm, 5 mW cm±2) or a reducing agent (acetaldehyde) (Fig. 7B,C); Agn clusters (n = 3± 8) were formed after only 10±20 min of irradiation. After 8 h of irradiation, the size of the silver particles stabilized and both stable Agn clusters (n > 10) and silver metal nanoparticles of about 8 nm diameter were observed. A TEM image of polyelectrolyte capsules filled with PSS/Ag composite is depicted in Figure 8A, in which the silver nanoparticles are seen as dark spots (see insert in Fig. 8A) and small Ag clusters and the PSS matrix form the dark background inside the polyelectrolyte shell. Varying the quantity of polyelectrolyte layers and initial Ag+ concentration leads to different morphology, stability, and properties of the silver nanocomposites.

A

B

(A)

1000 nm

In principal, the size of the irradiated area depends only on the wavelength of the light and, if laser-induced, the reduction area can be confined to several nanometers. To prove this, an experiment similar to dye bleaching was performed on a confocal fluorescence microscope.[69b] The laser beam was focused for 1 s onto the targeted areas of the capsules containing PSS±Ag+, resulting in a nanowritten image (Fig. 8B). The size of each spot is around 300 nm. It is important to point out that the photoinduced reduction of silver under these conditions occurs exclusively inside polyelectrolyte capsules or on the shell. For such spatially selective formation of silver particles it is necessary to maintain a low intensity of irradiation as well as a ~ 10±2 M concentration of silver ions in the surrounding solution. Increasing either the light intensity or the AgNO3 concentration induces the formation of elemental silver outside the capsules, whereas a decrease of either of these parameters leads to a longer reaction time. The excellent catalytic properties of the silver capsule system were observed by performing the silver-mediated reduction of 4-nitrophenol into 4-aminophenol in the presence of sodium borohydride.[69b] Other metal ions (Pd2+, Cu2+, Ag+) can also be reduced inside polyelectrolyte capsules by a heterogeneous, TiO2mediated photocatalytic process.[70] Here, the microreactor consists of hollow PAH/PSS polyelectrolyte capsules of micrometer scale with photoactive TiO2 nanoparticles incorporated into the polyelectrolyte shell. These TiO2 nanoparticles act as microheterogeneous photocatalysts, while the polyelectrolyte layers are good irreversible electron donors for photogenerated holes. Additional improvement of the metal photoreduction efficiency was achieved by engineering the micro-

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D. G. Shchukin, G. B. Sukhorukov/Nanosynthesis in Confined Microvolumes

Figure 7. Schematic illustration of photoinduced synthetic reactions inside polyelectrolyte capsules. A ® B) Adsorption of Ag+ or nanoTiO2 in the capsule shell and the volume. B ® C) Photoinduced formation of metal nanoparticles.

C (B)

2 µm

100 nm

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Figure 8. A) TEM image of ultramicrotomed, Ag-filled polyelectrolyte capsules (reprinted from [69a]). B) Silver nanopainting. Each single silver dot was photoreduced by a laser beam (reprinted with permission from [69b]. Copyright American Chemical Society, 2002).

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capsule volume and shell with a more effective electron donor (e.g., poly(vinyl alcohol)). Reduced metal nanoparticles precipitate inside the capsule shell, forming hollow metal microspheres.

2.4.4. CaCO3 Biomineralization in Polyelectrolyte Capsules Different enzymatic reactions can proceed in the capsule interior, such as the decomposition of urea by encapsulated urease[51] or hydrolysis of N-benzoyl-L-tyrosine ethyl ester by entrapped a-chymotrypsin.[71] A biomimetic synthesis of calcium carbonate exclusively inside urease-loaded PAH/PSS capsules has also been reported.[72] The deposition of CaCO3 was performed from water solution containing CaCl2 and urea. The presence of urease inside the polyelectrolyte capsule stimulates the catalytic decomposition of urea, forming carbonate anions exclusively in the capsule interior[51] where they react with metal cations from the surrounding solution. The formation of CaCO3 precipitate inside the capsules starts immediately after introducing urease-containing polyelectrolyte capsules into a mixture of 0.5 M urea and 1 M CaCl2. Calcium carbonate formation inside the polyelectrolyte capsules after different reaction times is depicted in Figure 9. Crystal growth begins on the inner surface of the capsule shell, and it proceeds until the capsule is completely filled. It can be seen from the SEM image (Fig. 9C) that calcium carbonate forms dense, completely filled crystals. The resulting CaCO3 particles have a surface morphology similar to that obtained by biomineralization reactions in solution.[73] Electron diffraction analysis of individual CaCO3-loaded polyelectrolyte capsules detected only the vaterite modification of CaCO3; the more stable calcite phase was not found. The formation of only the metastable vaterite phase has also been observed during biomimetic CaCO3 precipitation in the presence of different block copolymers, while only calcite crystals are formed in water solutions.[73]

(A)

(B)

3. Conclusions and Outlook The templated, spatially confined synthesis of inorganic nanomaterials enables the formation of unusual crystal modifications and morphological forms. A particular advantage of this approach is the absence of particle aggregates, so individual, randomly distributed nanoparticles are formed, which are difficult to prepare by common methods. The nanomaterials synthesized inside the confined cubic micrometer and submicrometer volume are expected to have several advantages, including: 1) a high catalytic activity due to the nanoparticle morphology and large surface area; 2) a high stability of nanoparticles against aggregation; 3) a microreactor shell that protects the nanomaterial from impurities; 4) the formation of metastable and amorphous modifications; 5) the possibility to carry out multistep synthesis and to obtain composite, hierarchically architectured nanomaterials. Polyelectrolyte capsules as individual microreactors have advantages due to the semi-permeable properties of the polyelectrolyte shell, its nanoengineering and tailoring of different multifunctionality, and a wide range of available capsule sizes (from 20 nm to 20 lm). The drawbacks of polyelectrolyte capsules are their complicated and time-consuming fabrication, the difficulties in controlling permeability of the shell for small molecules and ions,[74] the limitation of only being able to work in liquid (mainly water) media, and their instability at elevated temperature (at 80 C capsules undergo shrinkage and shell bending[75]). In situ modification of polyelectrolyte capsules by synthesizing them inside inorganic nanoparticles has created a new class of multifunctional capsules that combine the properties of inorganic nanomaterials (magnetic, fluorescent, optical, mechanic) and a conventional polyelectrolyte shell.[76] The possibility of switching the open/closed state of the inorganic/ organic capsules, together with their stability upon drying or mechanical deformation and the possibility of attaining magnetic or fluorescent properties, is a powerful tool for control-

(C)

1 µm

Figure 9. A,B) CaCO3 crystal growth inside a polyelectrolyte capsule at different reaction times. C) SEM image of the resulting CaCO3 crystal (reprinted from [72]).

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ling the release of the organic materials. These multifunctional composite capsules may find applications for protection, delivery, and storage of biochemical compounds that are unstable in solution or under UV/vis irradiation where capsules made only from polymeric components may not be useful. However, a lot of research work still remains to be done, especially in understanding the detailed mechanism of the chemical reactions in the capsule microvolume, determining the influence of this microvolume on solvent structure, diffusion limitations and phase modifications of the resulting nanomaterials, and identifying the parameters of crucial importance for performing physical and chemical processes inside polyelectrolyte capsules in a desired way. Received: November 13, 2003 Final version: February 16, 2004

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