Formation of hollow capsule

Capsule Permeability The most widely characterized polyelec-trol3 es in the formation of hollow capsules have been alternating layers of polystyrenesulphonate (PSS, negative) and polyallylamine hydrochloride (PAH, positive), and therefore the following permeability discussion will be based on that system. (PAH/PSS) will refer to a film of n bilayers. 

Use of the LbL assembly is not limited to fabrication of thin films on fiat substrates. It can be applied to a microsized colloidal particle core to prepare hollow capsules, which are expected to be highly useful for DBS applications. In this innovative strategy, LbL films are assembled sequentially on a colloidal core similar to the conventional LbL assemblies on a fiat substrate. Destruction of the central particle core after completion of the LbL assembly results in formation of hollow capsule structures. Figure 2.2.7 illustrates one example to prepare a biocompatible polyelectrolyte microcapsule with DNA encapsulation by Lvov and coworkers [15]. In their approach, water-insoluble DNA/spermidine complexes were... 

Colloidal core decomposition and formation of hollow capsule.395... 

It was found that the nanocapsules are formed in a miniemulsion process by a variety of monomers in the presence of larger amounts of a hydrophobic oil. Hydrophobic oil and monomer form a common miniemulsion before polymerization, whereas the polymer is immiscible with the oil and phase-separates throughout polymerization to form particles with a morphology consisting of a hollow polymer structure surrounding the oil. The differences in the hydro-philicity of the oil and the polymer turned out to be the driving force for the formation of nano capsules. 

Few years ago this concert of LbL assembling of charged species was transferred to coat micron and sub-micron sized colloidal particles [24-29]. The idea is to employ the nano-engineered properties of multilayers as shell structures formed on colloidal particles. This paper outlines the recent works on step-wise shell formation on various colloidal cores, fabrication and properties of hollow capsules, regulation of capsule wall permeability and approaches to encapsulate different materials into these capsules. 

Different colloidal cores can be decomposed after multilayers are assembled on their surface. If the products of core decomposition are small enough to expel out of polyelectrolyte multilayer the process of core dissolution leads to formation of hollow polyelectrolytes shells (Fig. 2.1, d-f). Up to now, various colloidal templates such as organic and inorganic cores, like MF-particles, organic crystals, carbonate particles and biological cells were used as templates for hollow capsule fabrication. Decomposition can be done by different means, such as low pH for MF- and carbonate particles [43], organic water miscible solvents for organic crystals [44] and... 

Use of the LbL technique is not restricted to the preparation of planar thin films. One of the most outstanding strategy modifications of the LbL technique involves assembly on colloidal particles followed by hollow capsule formation. For example, Caruso and co-workers reported the formation of hollow silica vesicles through LbL assembly on colloidal nanoparticle templates (Fig. 14). Polyelectrolytes and smaller silica particles were initially formed on a larger colloidal core, which was subsequently selectively destroyed. Calcination of the hybrid vesicles resulted in a hollow vesicle composed of silica. Formation of controlled organic-inorganic layer structures on colloidal particles by LbL assembly also provides media appropriate for investigation of fundamental phenomena. 

As will be shown, model systems for cells employing lipids or composed of polymers have been in existence for some time. Model systems for coccolith-type structures are well known on the nanoscale in inorganic and materials chemistry. Indeed, many complex metal oxides crystallize into approximations of spherical networks. Often, though, the spherical motif interpenetrates other spheres making the formation of discrete spheres rare. Inorganic clusters such as quantum dots may appear as microscopic spheres, particularly when visualized by scanning electron microscopy, but they are not hollow, nor do they contain voids that would be of value as sites for molecular recognition. All these examples have the outward appearance of cells but not all function as capsules for host molecules. 

Hollow microcapsules have been obtained after dissolution of inorganic matrix at pH 1-2. During processing of firm insoluble cores by HCL, the osmotic pressure inside capsules tends to increase due to formation of soluble CaCl2 [8]. To avoid breaks of the microcapsule walls the process of dissolution was optimized by smoothly decreasing the pH value (incubating coated cores for 1 min successively at pH 4, 3, 2 and 1). 

The amine-based Henry reaction catalyst was encapsulated via the interfacial polymerization of oil-in-oil emulsions. PEI was encapsulated by dispersing a methanolic PEI solution into a continuous cyclohexane phase. Upon emulsification, 2,4-tolylene diisocyanate (TDI) was added to initiate crosslinking at the emulsion interface, forming polyurea shells that contain free chains of PEI. The microcapsules crenate when dry and swell when placed in solvents such as methanol and dimethylformamide, suggesting a hollow capsule rather than a solid sphere formation. The catalyst loading was determined to be 1.6 mmol g . ... 

Figure 1. Schematic illustration of hollow polyelectrolyte capsule formation (a-c) followed by the selective inorganic synthesis inside (d-e). a-b layer-by-layer precipitation of poly(styrene sulfonate), poly(allylamine hydrochloride) monolayers b-c dissolution of template core c-d loading of polyelectrolyte capsules with corresponding anions d-e precipitation of inorganic material from... 

Fig. 10 Hollow capsule formation using enzyme crystal as a core material for high loading of enzyme...

Abstract Nanoparticles (NPs, diameter range of 1-100 nm) can have size-dependent physical and electronic properties that are useful in a variety of applications. Arranging them into hollow shells introduces the additional functionalities of encapsulation, storage, and controlled release that the constituent NPs do not have.This chapter examines recent developments in the synthesis routes and properties of hollow spheres formed out of NPs. Synthesis approaches reviewed here are recent developments in the electrostatics-based tandem assembly and interfacial stabilization routes to the formation of NP-shelled structures. Distinct from the well-established layer-by-layer (LBL) synthesis approach, the former route leads to NP/polymer composite hollow spheres that are potentially useful in medical therapy, catalysis, and encapsulation applications. The latter route is based on interfacial activity and stabilization by NPs with amphiphilic properties, to generate materials like colloidosomes, Pickering emulsions, and foams. The varied types of NP shells can have unique materials properties that are not found in the NP building blocks, or in polymer-based, surfactant-based, or LBL-assembled capsules. 

Another recent development features the in situ formation of liquid colloidal templates. The assembly of NPs at the periphery of these templates is driven by electrostatics, resulting in the formation of robust NP-sheUed hoUow spheres, originally termed nanoparticle-assembled capsules (NACs). This scheme is called tandem assembly , nanoparticle-polymer tandem assembly , or polymer-aggregate tern-plating and presents an alternate, simple and non-destructive route for formation of NP-shelled hollow spheres [6,32-35,40,80,81]. 

Kadali et al. demonstrated another useful application of NACs prepared by tandem assembly in the formation of catalyst support materials. NACs can be calcined to remove the polymer without collapsing the hollow sphere structure (Fig. 10). Such stability is more difficult to achieve with LBL-assembled capsules. On calcination,... 

This chapter describes the non-LBL approaches of tandem assembly and interfacial stabilization for the formation of closed shell structures, with an emphasis on ensembles in which NPs constitute the shell. Tandem assembly is a versatile and environmentally friendly route to the formation of useful NP-shelled capsules. In contrast to sacrificial core templating and LBL assembly methods, tandem assembly has the important differentiating feature that it avoids the incineration or solvent dissolution step to generate the hollow interior of the capsule. Enhancements in optical, mechanical, catalytic, and release properties of such materials hold great promise for their application in photoresponsive delivery systems, catalysis, and encapsulation. Interfacial stabilization routes are found to yield NP-shelled structures in the form of emulsions and foams that have enhanced stability over those from conventional, surfactant-based approaches. Unusual interactions of the NP with fluid interfaces have made possible new structures, such as water-in-air foams, colloidosomes, and anisotropic particles. 

Hydrophilic materials can be encapsulated with the inverse minianulsions by using interfacial polymerization such as polyaddition and polycondensation, radical, or anionic polymerization. Crespy et al. reported that silver nitrate was encapsulated and subsequently reduced to give silver nanoparticles inside the nanocapsules. The miniemulsions were prepared by anulsilying a solution of amines or alcohols in a polar solvent with cyclohexane as the nonpolar continuous phase. The addition of suitable hydrophobic diisocyanate or diisothiocyanate monomers to the continuous phase allows the polycondensation or the cross-linking reactions to occur at the interface of the droplets. By using different monomers, polyurea, polythiourea, or polyurethane nanocapsules can be formed. The waU thickness of the capsules can be directly tuned by the quantity of the reactants. The nature of the monomers and the continuous phase are the critical factors for the formation of the hollow capsules, which is explained by the interfacial properties of the systan. The resulting polymer nanocapsules could be subsequently dispersed in water. 

PAH/P04 -PAH/PSS capsules were employed for the biomimetic synthesis of calcium hydroxyapatite, CaiQ(P04)g(0H)2, inside polyelectrolyte capsules [53]. Transmission electron microscopy (TEM) analysis established preferable formation of the hydroxyapatite nanoparticles on the iimer side of the PAH/PSS shell, and this resulted in empty hydroxyapatite spheres. The thickness of the CajQ(P04)g(0H)2 layer is 100-120 nm, and is composed of 12- to 16-nm particles. The hydroxyapatite particles formed have shape and surface morphology which is different from the particles synthesized by common methods in solution. Other special properties of hydroxyapatite composite hollow shells, including surface acidity, catalytic and biological activity, as well as bone-repairing effects, can also be expected. 

Sukhorukov, A Kornowski, H. Mohwald, M. Giersig, A. Eychmiiller, H. Weller H, Formation of luminescent spherical coreshell particles by the consecutive adsorption of polyelectrolyte and CdTe(S) nanocrystals on latex colloids. Colloid Surface A 2000, 163, 39-44 (c) F. Caruso, Hollow capsule processing through colloidal templating and self-assembly, Chem. Eur. J. 

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