A general method to coat colloids with silica is described. The amphiphilic, nonionic polymer poly(vinylpyrrolidone) (PVP) was adsorbed to various colloidal particles such as small gold colloids, gold-shell silica-core particles, small and large silver colloids, boehmite rods, gibbsite platelets, and positively or negatively charged polystyrene. After this functionalization the stabilized particles could be transferred to a solution of ammonia in ethanol and directly coated with smooth and homogeneous silica shells of variable thickness by addition of tetraethoxysilane in a seeded growth process. The length of the polymer used strongly influences the stability of the colloids and the homogeneity and smoothness of the initial silica coating. This method is especially useful for colloidal particles that cannot be covered directly with SiO2 by a Stober-like growth process. Compared to methods known from the literature for the coating of such particles, this new method is faster and requires neither the use o...

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A General Method To Coat Colloidal Particles with Silica

The aim of this article is to provide a comprehensive review of current research activities that center on superparamagnetic colloids. We begin with an overview of synthetic strategies that have been developed for generating both nanoscale and mesoscale superparamagnetic colloids, with a focus on those systems that can be prepared as monodisperse samples and in relatively large quantities. We then discuss a variety of techniques that have been exploited for modifying surface properties, as well as for controlling the assembly and patterning of these magnetically active colloids. Towards the end, we highlight a range of innovative applications enabled by the unique combination of superparamagnetism and colloidal suspension. We conclude this review article with personal remarks and perspectives on the directions toward which future research in this area might be directed.

Superparamagnetic Colloids: Controlled Synthesis and Niche Applications**

We report a strategy for the production of materials with structural hierarchy. The approach employs polymer microgels as templates for the synthesis of semiconductor, metal, or magnetic nanoparticles (NPs). We show that NPs with predetermined dimensions and size-dependent properties can be synthesized by using a very delicate balance between the reaction conditions, the composition and the structure of microgel templates, and the concentration of NPs in the microgel. Postheat treatment of microgels doped with semiconductor nanoparticles reduces NP polydispersity and allows control of their photoluminescence. Microgel templates are particularly beneficial in the synthesis of polymer microspheres heavily loaded with monodisperse superparamagnetic Fe3O4 NPs. Hybrid submicrometer-size microgels have promising potential applications in photonics, catalysis, and separation technologies.

Polymer microgels: reactors for semiconductor, metal, and magnetic nanoparticles.

Sol-gel based materials for biomedical applications

Formation of highly efficient dye-sensitized solar cells by hierarchical pore generation with nanoporous TiO2spheres

Monodisperse spherical titania particles of variable sizes are produced in a sol−gel synthesis from Ti(EtO)4 in ethanol with addition of a salt or a polymer solution. The influence of different salt ions or polymer molecules on the size and the size distribution of the final particles was investigated. The amorphous hydrous titania particles were characterized by electron microscopy, thermogravimetry, 1H-MAS NMR, X-ray absorption spectroscopy, and electrophoresis. Nitrogen absorption measurements revealed that the addition of polymers yields hollow and porous titania colloids.

Synthesis and Characterization of Porous and Nonporous Monodisperse Colloidal TiO2 Particles

Monodisperse spherical ZrO2 particles were prepared by controlled hydrolysis of zirconium tetraalkoxide in alcoholic solution. Ageing the reaction solution at 60 °C for 4 h yields particles with sizes in the range between 200 and 2000 nm. The influence of various ions and surfactants on the final product was investigated. Stable suspensions of coated, monodisperse, zirconia particles were prepared by hydrolysis and subsequent polymerisation of tetraethoxysilane in an alkaline ethanol solution. Thermal analysis revealed that the weight loss of bound water occurred below 400 °C and is about 35 %. The phase transition from amorphous zirconia to the tetragonal phase proceeded at approximately 450 °C, the transition to the monoclinic phase began at about 600 °C and was completed at about 800 °C. The crystallinity of the powders was investigated by X-ray diffraction. Electrophoretic measurements were carried out to determine the charges on the particle surface. Elementary analyses were used for further investigations. On the basis of the results obtained, the mechanism of particle formation was discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Synthesis and characterisation of monodisperse zirconia particles

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Silver particles tailoring of shapes and sizes

The synthesis of colloidal particles with tailored shapes and controlled chemical composition and physical properties is one prerequisite for the formation of self-assembled photonic crystals. Three-dimensional photonic crystals can be seen as the optical analog of semiconductors. Owing to their periodic order, both photonic bandgap materials and electronic semiconductors possess a bandgap. In the case of photonic crystals this bandgap is mainly defined by the index of refraction and the structure of the colloidal crystals used. An important focus of the research on photonic bandgap materials is, therefore, the fabrication of periodic structures that have been predicted to show a suitable bandgap. [1,2] The tailored colloidal crystallization of such structures can be achieved either by a layer-by-layer method [3] or by bulk crys-tallization. [4] The former method may offer the possibility of better control of the crystal growth. However, both methods still yield crystals with unwanted defects. Therefore, as the latter pathway is less complicated and faster, it warrants further development. In this respect, the self-assembly of strongly interacting particles might be a promising approach to optimize the colloidal crystal structure. [5] Previous reports on two-dimensional systems [6,7] suggest that the use of magnetic colloids may open the way to tailored bulk crystallization assisted by an external magnetic field. Magnetic colloidal particles of the required quality and a size between 500 and 700 nm are not commercially available. Therefore, an easily reproducible synthetic procedure had to be developed in order to obtain particles that are monodis-perse in size and in their distribution of the magnetite nano-particles. Our synthetic pathway is based on the preparation of commercially available particles 2.8 and 4.5 lm in diameter [8] and our previous investigations of polystyrene colloids. Here we describe the synthesis of tailored magnetic polymer colloids 400±800 nm in diameter. The synthesis involves the preparation of parent methyl/glycidyl methacrylate co-polymer particles, their chemical modification with ethylene-diamine in order to obtain internal anchor groups, and the generation of superparamagnetic magnetite nanoparticles by impregnation and subsequent hydrolysis of ferric and ferrous chloride inside the methacrylate matrix. The synthesis of the parent polymer colloids yields particles with very narrow size distribution as illustrated by the scanning electron microscopy (SEM) image shown in Figure 1a. Depending on the amount of monomer in the reaction mixture , the particle size can be varied between 400 and 700 nm. Due to the presence of the glycidyl methacrylate comonomer, the reaction …

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Stefanie Eiden-Assmann

Papers

Synthesis and Characterization of Porous and Nonporous Monodisperse Colloidal TiO2 Particles

Synthesis and characterisation of monodisperse zirconia particles

Silver particles tailoring of shapes and sizes

Synthesis of Monodisperse Magnetic Methacrylate Polymer Particles

CeF3 nanoparticles: synthesis and characterization