Sacrificial particle template

Codeposition of the colloidal template together with the matrix material, typically in the form of nanoparticles, while the colloidal particles form a highly ordered structure, which can be further processed to remove the sacrificial particle template [25,35]. 

The benefit of the LbL technique is that the properties of the assemblies, such as thickness, composition, and function, can be tuned by varying the layer number, the species deposited, and the assembly conditions. Further, this technique can be readily transferred from planar substrates (e.g., silicon and quartz slides) [53,54] to three-dimensional substrates with various morphologies and structures, such as colloids [55] and biological cells [56]. Application of the LbL technique to colloids provides a simple and effective method to prepare core-shell particles, and hollow capsules, after removal of the sacrificial core template particles. The properties of the capsules prepared by the LbL procedure, such as diameter, shell thickness and permeability, can be readily adjusted through selection of the core size, the layer number, and the nature of the species deposited [57]. Such capsules are ideal candidates for applications in the areas of drug delivery, sensing, and catalysis [48-51,57]. 

The pore size and distribution in the porous particles play essential roles in NPS synthesis. For example, only hollow capsules are obtained when MS spheres with only small mesopores (<3 nm) are used as the templates [69]. This suggests that the PE has difficulty infiltrating mesopores in this size range, and is primarily restricted to the surface of the spheres. The density and homogeneity of the pores in the sacrificial particles is also important to prepare intact NPSs. In a separate study, employing CaC03 microparticles with radial channel-like pore structures (surface area 8.8 m2 g 1) as sacrificial templates resulted in PE microcapsules that collapse when dried, which is in stark contrast to the free-standing NPSs described above [64]. 

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. 

Fig. 27 Top SEM images of CaCOs particles grown on a glass slip in the early reaction stage, PEO-6-PMAA, [CaCb] = 10 mM, lgL , 5h. a CaCOs particles with either spherical or hollow structures, b Zoom showing the calcite rhombohedral subunits grown on the surface of the hollow structure and the inner part consisting of tiny primary nanocrystals with a grain size of about 320 nm as indicated by the arrow (sacrificial vaterite template). Bottom Proposed formation mechanism of the calcite hollow spheres a polymer-stabilized amorphous nanoparticles b Formation of spherical vaterite precursors c aggregation of the vaterite nanoparticles d vaterite-calcite transformation starting on the outer sphere of the particles e formation of calcite hollow spheres under consumption of the sacrificial vaterite precursors. Reproduced in part from [61] with permission of the American Chemical Society...

Another method to synthesize hollow nanocapsules involves the use of nanoparticle templates as the core, growing a shell around them, then subsequently removing the core by dissolution [30-32]. Although this approach is reminiscent of the sacrificial core method, the nanoparticles are first trapped and aligned in membrane pores by vacuum filtration rather than coated while in aqueous solution. The nanoparticles are employed as templates for polymer nucleation and growth Polymerization of a conducting polymer around the nanoparticles results in polymer-coated particles and, following dissolution of the core particles, hollow polymer nanocapsules are obtained. 

Recently, we proposed an alternative process for encapsulating biomacromolecules within PE microcapsules. This approach involves using nanoporous particles as sacrificial templates for both enzyme immobilization and PE multilayer capsule formation (Figure 7.2, route (I)) [66,67]. Unlike previous LbL encapsulation strategies, this approach is not limited to species that undergo crystallization, and is not dependent upon adjustments in electrostatic interactions within PE microcapsules to alter shell permeability characteristics. The salient feature of this method is that it is applicable to a wide range of materials for encapsulation. 

Capsules with high enzyme loading and activity prepared by templating BMS spheres can also be used as biomimetic reactors [89]. For example, PGA/ PLL capsules with pre-loaded urease (prepared via a BMS sacrificial template) are capable of catalyzing the hydrolysis of urea and have been shown to induce the exclusive formation of CaC03 particles inside the capsules [89]. 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

The materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

Kim et al fabricated SWNT composite films with ordered voids by using sacrificial 2D silica colloidal template (Figure 6.7). Experimentally, SWNTs stabilized with water-soluble polythiophene were mixed with a suspension of silica colloidal particles, and then assembled into 2D colloid-SWNT array on flexible substrate. Selective removal of silica colloids led to a composite film... 

Spherical colloids can also be employed as templates. The deposition of MOX films onto the outer surface of colloidal particles, which are then chemically or thermally removed, further gives the possibility to design hollow spherical structures like small capsules (Fig. 3.16). This method permits good control over the properties of the hollow spheres (e.g., size, composition, thickness, permeability, function) by proper choice of the sacrificial colloids and the film components (Wang et al. 2008). 

An example was reported for hollow spheres composed of calcite rombo-hedra, which where grown on a sacrificial spherical vaterite template particle grown in the presence of PEO-fo-PMAA at an acidic starting pH [61]. Initially, amorphous nanoparticles were formed and stabilized by the DHBC. Subse-... 

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