Silica particles encapsulation

FIGURE 4.5 Chromatograms of a-phenylethanol enantiomers nsing (a) SFC and (b) open tubular column GC. Conditions (a) 12 cmx250 p.m ID capillary packed with 5-p.m porous (300 A) silica particles encapsulated with fS-CD polymethylsiloxane (10% w/w) and end-capped with HMDS, 30°C, 140 atm, CO2, FID, 10 cmxl2 p.m ID restrictor, (b) 25 mx250 p.m ID cyano-deactivated capillary cross-linked with fi-CD polymethylsiloxane (0.25 xm df) 130°C He FID. (Reprinted from Wu, N. et al. 2000. J. Microcol. Sep. 12 454-461. With permission.)... 

Jain TK, Roy I, De TK, Maitra A (1998) Nanometer silica particles encapsulating active compounds a novel ceramic drug carrier. J Am Chem Soc 120 11092-11095... 

Lee MH, Oh SG, Moon SK, Bae SY. 2001. Preparation of silica particles encapsulating retinol using O/W/O multiple emulsions. J Colloid Interface Sci 240 83-89. 

Ow et al. (2005) developed an improved method of incorporating fluorescent molecules into silica particles using a modified Stober synthesis, which resulted in both enhanced fluorescence and photostability of the encapsulated dyes. In this two-stage procedure, reactive organic dyes... 

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

Also, here, the effect of the adsorption layer of HPC on encapsulation of silica particles in polymerization of styrene in the presence of silica particles has been investigated. Encapsulation is promoted greatly by the existence of the adsorption layer on the silica particles, and the dense adsorption layer formed at the LCST makes composite polystyrene latices with silica particles in the core (7.). This type of examination is entirely new in polymer adsorption studies and we believe that this work will contribute not only to new colloid and interface science, but also to industrial technology. 

It was apparent that the dense adsorption layer of HPC which was formed on the silica particles at the LCST plays a part in the preparation of new composite polymer latices, i.e. polystyrene latices with silica particles in the core. Figures 10 and 11 show the electron micrographs of the final silica-polystyrene composite which resulted from seeded emulsion polymerization using as seed bare silica particles, and HPC-coated silica particles,respectively. As may be seen from Fig.10, when the bare particles of silica were used in the seeded emulsion polymerization, there was no tendency for encapsulation of silica particles, and indeed new polymer particles were formed in the aqueous phase. On the other hand, encapsulation of the seed particles proceeded preferentially when the HPC-coated silica particles were used as the seed and fairly monodisperse composite latices including silica particles were generated. This indicated that the dense adsorption layer of HPC formed at the LCST plays a role as a binder between the silica surface and the styrene molecules. 

All these results indicate that the dense adsorption layer of HPC formed on silica particles at the LCST plays a very important role in the area of particle encapsulation. 

Sol-gel microencapsulation in silica particles shares the versatility of the sol-gel molecular encapsulation process, with further unique advantages. Sol-gel controlled release formulations are often more stable, potent and tolerable than currently available formulations. The benefits of microencapsulation can be customized to deliver the maximum set of benefits for each active ingredient. Overall, these new and stable combinations of active pharmaceutical ingredients (APIs) result in improved efficacy and usability. 

Choi, H.-H., Park, J., and Singh, R.K., Nanosized titania encapsulated silica particles using an aqueous TiC solution, Appl. Surf. Sci., 240, 7, 2005. 

We demonstrated that a naturally derived polysaccharide, chitosan, is capable of forming composite nanoparticles with silica. For encapsulated particles, we used silicification and biosilicification to encapsulate curcumin and analyzed the physicochemical properties of curcumin nanoparticles. It proved that encapsulated curcumin nanoparticles enhanced stability toward ultraviolet (UV) irradiation, antioxidation and antitumor activity, enhanced/added function, solubility, bioactivities/ bioavailability, and control release and overcame the immunobarrier. We present an in vitro study that examined the cytotoxicity of amorphous and composite silica nanoparticles to different cell lines. These bioactives include curcumin mdAntrodia cinnamomea. It is hoped that by examining the response of multiple cell lines to silica nanoparticles more basic information regarding the cytotoxicity as well as potential functions of silica in future oncological applications could become available. 

Another recent report describes the large scale synthesis of ahgned carbon nanotubes, of uniform length and diameter, by passage of acetylene over iron nanoparticles embedded in mesoporous silica [107]. The latter two methods, based on the pyrolysis of organic precursors over templated/catalysts supports, are by far superior by comparison with plasma arcs, since other graphitic structures such as polyhedral particles, encapsulated particles and amorphous carbon are notably absent (Fig. 16). 

Silica nanoparticles are also hydrophilic and have therefore to be functionalized prior to encapsulation. Without functionalization, the negatively charged silica particles can be used as Pickering stabilizers, leading to hybrid nanoparticles with silica located on the surface of the nanoparticles (see Fig. 13a) [100]. 

Besides styrene, MMA, BA, or their copolymers, and also less commonly used polymers such as poly(styrenesulfonic acid) (PSSA), poly (hydroxyethylmethacrylate) (PHEMA), poly(aminoethylmethacrylate) PAEMA [111], polyethylene (PE) [112], or polyamides [113], were used for the encapsulation of the silica, as reported in the literature. Polyethylene [112] could also be obtained as encapsulating polymer if a nickel-based catalyst which is dispersed in the aqueous continuous phase is used. Here, lentil-shaped hybrid particles with semicrystalline polyethylene or isotropic hybrid particles with amorphous polyethylene are detected. Silica/polyamide hybrid nanoparticles were synthesized by miniemulsifying a dispersion consisting of 3-aminopropyl triethoxysilane (APS) modified silica particles and sebacoylchloride [113] in an aqueous continuous phase where hexamethylene diamine is dropwise added. 

Immobilized-enzyme reactors were considered in Chapter 20. It is easy to extend the concept to immobilization on the walls of a membrane tube. What is even more practical is to immobilize the enzyme in the usual manner on solid particles such as silica and encapsulate the particles in a ribbed sheet of a microporous plastic such as PVC. Then this sheet can be rolled in a jelly-roll configuration inside a spiral reactor (Figure 24.2i). The consequent large surface area of immobilized enzyme available per unit volume of reactor space makes such a spiral reactor an attractive choice. 

Early examples of the precipitation approach include the aqueous solution polymerizations reported by Chaimberg et al. [53] for the graft polymerization of polyvinylpyrrolidone onto silica. The nonporous silica particles were modified with vinyltriethoxysilane in xylene, isolated and dispersed in an aqueous solution of vinylpyrrolidone. The reaction was performed at 70°C and initiated by hydrogen peroxide, after which precipitation on the surface occurred, leading to encapsulation. Nagai et al. [54] in 1989 reported on the aqueous polymerization of the quaternary salt of dimethylaminoethyl methacrylate with lauryl bromide, a surface-active monomer, on silica gel. Although the aim was to polymerize only on the surface, separate latex particles were also formed. 

Inverse emulsions were also prepared, and (the otherwise difficult) encapsulation with water-soluble monomers like acrylamide was performed [58]. In a first step, a colloidal dispersion was prepared by dispersing the silica particles in an aqueous solution of acrylamide containing a water-soluble dispersant, a crosslinking agent like Af,Af-methylene bisacrylamide and an initiator. The colloidal system was dispersed in decane containing a suitable surfactant. 

Another relevant system involves oleic acid (OA) adsorption at the silica-water interface. This method was first demonstrated by Ding et al. [45] and was next used by Mahdavian and coworkers to encapsulate very small silica nanoparticles [46]. In the latter case, a core-shell structure with a core composed of aggregated silica particles and a shell made of MMA, styrene and acrylic acid (AA), was formed. The authors suggest that the polymerization proceeds through oligoradical entry into the OA admicelles. 

They rely on vigorous stirring or ultrasonication, either to help the dispersion of iron oxides in water or in the monomer, or to achieve a fine dispersion of the monomer droplets before starting the polymerization. They also imply an intermediate step consisting in the encapsulation of several iron oxides in a silica particle, or the use of a magnetic emulsion as a seed instead of iron oxide nanoparticles. These processes are described below. 

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