Particle core/shell

Nonporous particles, with a solid core and a thin porous surface layer (pellicular particles), have occasionally been used for macromolecules. The reason is that the slow diffusion velocity of macromolecules results in less band broadening in combination with the thin layers. However, the nonporous particles have a much smaller loading capacity than the totally porous particles. 

The most recent core shdl particles consist of a solid 1.3-2.6 im core with a relatively thick porous 0.2-0.5 pm shell. The homogeneous porous shell is grown on a solid silica core by sol-gel techniques. The current standard pore size is 10 nm and the surface area is about 200 m g. Due to the small particle sizes and the short diffusion paths, 5 cm columns may give up to 15 000 plates, equivalent to 300000 plates per meter. The 1.7 pm particles are available in 5, 10, and 15 cm columns, with inner diameters of 2-4.6mm. The smallest particles need ultrahigh-pressure systems (600-1000 bar). Core-shell particles allow high flow rates due to the fast mass transfer. 

The potential of SAXS for a precise analysis of the radial structure of latexes can be discussed best when considering model particles consisting of a well-defined core and a closed shell of a second polymer. The particles analyzed by SAXS [45-49] have been prepared recently [45] by a seeded emulsion polymerization [97] of PMMA onto a polystyrene core having a narrow size distribution. The alteration effected by seeded emulsion polymerization can be seen directly in the analysis of the size distribution by ultracentrifugation [87], the resulting distributions are shown in Fig. 17. Besides the increase in radius when going from the 

PS-core particles to the core-shell latices, Fig. 17 also shows the slightly asymmetric size distribution found for rather narrowly distributed latex systems. For a highly precise analysis of the SAXS-data the directly measured distributions should be used instead of the Gaussian distribution often used in the analysis of colloidal particles [47]. 

The pronounced alterations effected through change of the electron density of the medium are evident. In particular, the isoscattering point indicated in Fig. 18b by an arrow leads to an outer radius of 91 nm in good agreement with the value obtained by ultracentrifugation (92.3 nm). 

The analysis [47,49] of the scattering intensities reveals a well-defined concentric core-shell particle. The data taken near the match point (filled circles in Fig. 18b) show furthermore that there is a finite contribution to the SAXS-inten-sity at q=0 even near vanishing contrast. This points directly to a polydispersity of the average contrast, mainly caused by the variation of the thickness of the shell [47]. Another important point of this analysis is the interface between the core and the shell. Here the interfacial region between the two immiscible polymers was found to be very small ( 4 nm [49]). 

The fit shown in Fig. 18 is restricted by a number of experimental parameters such as electron density of the polymers, and the concentration of the particles. It must be noted that absolute intensities have been used here. Hence, the number density of the particles is fixed and cannot be used as a fit parameter. These constraints lead to the elucidation of the radial structure of the particles with a resolution of a few nm. 

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Liz-Marzan L M, Giersig M and Mulvaney P 1996 Synthesis of nanosized gold-silica core-shell particles Langmuir 12 4329-35... 

When monomers of drastically different solubiUty (39) or hydrophobicity are used or when staged polymerizations (40,41) are carried out, core—shell morphologies are possible. A wide variety of core—shell latices have found appHcation ia paints, impact modifiers, and as carriers for biomolecules. In staged polymerizations, spherical core—shell particles are made when polymer made from the first monomer is more hydrophobic than polymer made from the second monomer (42). When the first polymer made is less hydrophobic then the second, complex morphologies are possible including voids and half-moons (43), although spherical particles stiU occur (44). 

Tamil S.S., Spatz, J.P., Klok H.A., and Martin M. Gold-polypyrrole core-shell particles in diblock copolymer micells, Aiiv. Mater., 10, 132, 1998. 

For the characterization of Langmuir films, Fulda and coworkers [75-77] used anionic and cationic core-shell particles prepared by emulsifier-free emulsion polymerization. These particles have several advantages over those used in early publications First, the particles do not contain any stabihzer or emulsifier, which is eventually desorbed upon spreading and disturbs the formation of a particle monolayer at the air-water interface. Second, the preparation is a one-step process leading directly to monodisperse particles 0.2-0.5 jim in diameter. Third, the nature of the shell can be easily varied by using different hydrophilic comonomers. In Table 1, the particles and their characteristic properties are hsted. Most of the studies were carried out using anionic particles with polystyrene as core material and polyacrylic acid in the shell. 

Engineering of Core-Shell Particles and Hollow Capsules... 

An important class of materials that originates from the precursor core-shell particles is hollow capsules. Hollow capsules (or shells ) can be routinely produced upon removal of the core material using chemical and physical methods. Much of the research conducted in the production of uniform-size hollow capsules arises from their scientific and technological interest. Hollow capsules are widely utilized for the encapsulation and controlled release of various substances (e.g., drugs, cosmetics, dyes, and inks), in catalysis and acoustic insulation, in the development of piezoelectric transducers and low-dielectric-constant materials, and for the manufacture of advanced materials [14],... 

Hollow gold spheres or core-shell particles consisting of a gold shell on a core of some other material have recently attracted attention. This is because they have interesting and tunable optical extinction properties [56]. These can be readily calculated using Mie theory [57], and there had been some scattered early interest in these shapes as a result [58, 59], but the versatility and properties of these particles only became widely... 

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]. 

Graf C., Van Blaaderen A., Metallodielectric colloidal core-shell particles for photonic applications, Langmuir 2002 18 524-534. 

The following protocol is based on the creation of fluorescent silica core/shell particles using the method of van Blaaderen and Vrij (1992). 

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