Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors

Rapid preparation and characterization of uniform, large, spherical Ag particles through a simple wet-chemical route

In this article we report the rapid preparation of uniform, large, spherical Ag particles through a simple wet-chemical route. The formation of Ag particles with about 750 nm in diameter occurs in a single process, carried out by direct mix of AgNO3 aqueous solution and ortho-phenylenediamine N-methyl-2-pyrrolidone solution at room temperature, producing a relatively low-polydispersity product. It is also found that raising the temperature results in larger particles. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectra have been used to characterize the resulting product.

Preparation and the mechanisms of formation of silver particles of different morphologies in homogeneous solutions

Uniform, well-dispersed silver particles of various morphologies have been prepared by reducing highly acidic silver nitrate solutions with ascorbic acid in the presence of a sodium naphthalene sulfonate-formaldehyde copolymer as dispersing agent. By varying the temperature of the reaction, the free acid content, the addition rate of the reductant, and the aging time, both isometric and anisotropic silver particles could be obtained. It was found that the latter were formed by aggregation of nanosize subunits, which were identified by electron microscopy and X-ray diffractometry.

Formation of Uniform CuO Nanorods by Spontaneous Aggregation: Selective Synthesis of CuO, Cu2O, and Cu Nanoparticles by a Solid−Liquid Phase Arc Discharge Process

Uniform and monodisperse CuO nanorods have been synthesized by directional aggregation and crystallization of tiny CuO nanoparticles generated from a solid-liquid arc discharge process under ambient conditions in the absence of any surfactants. Uniform CuO nanorods with sharp ends are formed from tiny nanoparticles via a process that involves the rapid oxidation of Cu nanoclusters, the spontaneous aggregation of CuO nanoparticles, and the Ostawald ripening process. The spontaneous aggregation and oriented attachment of tiny CuO nanoparticles contributed obviously to the formation of these kinds of nanostructures. By choice of suitable reducing agent to prevent the oxidation of Cu nanoclusters, Cu and Cu2O nanoparticles can be selectively synthesized.

Size-controlled preparation of Cu2O octahedron nanocrystals and studies on their optical absorption

We report herein the size-controlled preparation of monodispersed cuprous oxide octahedron nanocrystals smaller than 100 nm. The method is based on the reduction of copper nitrate in Triton X-100 water-in-oil (w/o) microemulsions by gamma-irradiation. The average edge length of the octahedron-shaped nanocrystals varies from 45 to 95 nm as a function of the dose rate. The quantum confinement effect was illustrated by the blueshift in the optical absorption. In addition, the growth process was also traced by absorption spectra.

Micron-size drug particles: common and novel micronization techniques

Drug powders containing micron-size drug particles are used in several pharmaceutical dosage forms. Many drugs, especially newly developed substances, are poorly water soluble, which limits their oral bioavailability. The dissolution rate can be enhanced by using micronized drugs. Small drug particles are also required in administration forms, which require the drug in micron-size size due to geometric reasons in the organ to be targeted (e.g., drugs for pulmonary use). The common technique for the preparation of micron-size drugs is the mechanical comminution (e.g., by crushing, grinding, and milling) of previously formed larger particles. In spite of the widespread use of this technique, the milling process does not represent the ideal way for the production of small particles because drug substance properties and surface properties are altered in a mainly uncontrolled manner. Thus, techniques that prepare the drug directly in the required particle size are of interest. Because physicochemical drug powder properties are decisive for the manufacturing of a dosage form and for therapeutic success, the characterization of the particle surface and powder properties plays an important role. This article summarizes common and novel techniques for the production of a drug in small particle size. The properties of the resulting products that are obtained by different techniques are characterized and compared.

Mechanism of growth of colloidal silver nanoparticles stabilized by polyvinyl pyrrolidone in γ-irradiated silver nitrate solution

Silver nanoparticles were prepared by using polyvinyl pyrrolidone (PVP) as a stabilizer and gamma-irradiation. Transmission electron microscopy (TEM) results showed that both the amount and the molecular weight of PVP in the irradiated solution considerably affect the average size of the silver nanoparticles. The average size of the silver nanoparticles decreases with increasing the amount of PVP in the solution, but increases with increasing its molecular weight. Further, TEM showed that the silver nanoparticles become disassembled into smaller nanoparticles after dilution with distilled water and sonication. Since the processes of dilution and sonication are not expected to result in chemical reactions or to split the silver nanoparticles, we conclude that each silver nanoparticle prepared by [Formula: see text] -irradiation consists of several smaller nanoparticles surrounded by PVP. Thus, based on these observations, we propose a three-step mechanism for the growth of the silver nanoparticles under the conditions considered here. In the first step, the silver ions interact with PVP, then in the second step the silver ions that are exposed to gamma-irradiation are reduced to silver atoms; nearby silver atoms then aggregate at close range. These aggregates are the primary nanoparticles. Finally, these primary nanoparticles coalesce with other nearby primary nanoparticles or interact with PVP to form larger aggregates which are the secondary (final) nanoparticles.

Preparation and coating of finely dispersed drugs

The preparation of colloidal particles of different morphologies, including spheres, of two drugs, loratadine and danazol, is described. In principle these particles were obtained by precipitation when nonsolvents (water or aqueous surfactant solutions) were added to ethanol solutions of the drug. In addition, procedures were developed that made it possible to use the drug particles thus obtained as cores to be then coated with either silica or aluminum (hydrous) oxide layers. The presence of these inorganic shells was confirmed by electron microscopy, energy dispersive spectroscopy, and electrophoresis.

Preparation of Finely Dispersed Drugs

Two different precipitation processes are described, which produced dispersions of spherical particles of cyclosporine ranging in diameter from approximately 10 nm to several micrometers. This drug is of interest for its immunosuppressive activity in the antirejection of transplanted organs. The effects of several experimental parameters on the average particles size and uniformity have been investigated. Aging of spherical particles resulted in large crystalline-type aggregates.

Homogeneous Precipitation of Calcium Carbonates by Enzyme Catalyzed Reaction

Catalytic decomposition of urea by urease in aqueous calcium chloride solutions was used to rapidly prepare calcium carbonate polymorphs at room temperature. The nature of the resulting particles depended on the concentration of the enzyme and, in a strong manner, on the agitation of the reacting solutions. In an undisturbed system an amorphous precipitate is formed first, which readily crystallized to vaterite and upon aging changed to calcite. Under the influence of magnetic stirring, the amorphous phase could be not observed; instead smaller particles were initially obtained, which aggregated to vaterite and calcite. Similarly, the application of ultrasonic energy produced small vaterite particles at the early stages. It is apparent that enzyme macromolecules are important in the development of calcite faces and, as such, they exert significant influence on calcite morphology, without being present in detectable amounts in the resulting solids. Copyright 2001 Academic Press.

Aggregation-based crystal growth and microstructure development in natural iron oxyhydroxide biomineralization products

Crystals are generally considered to grow by attachment of ions to inorganic surfaces or organic templates. High-resolution transmission electron microscopy of biomineralization products of iron-oxidizing bacteria revealed an alternative coarsening mechanism in which adjacent 2- to 3-nanometer particles aggregate and rotate so their structures adopt parallel orientations in three dimensions. Crystal growth is accomplished by eliminating water molecules at interfaces and forming iron-oxygen bonds. Self-assembly occurs at multiple sites, leading to a coarser, polycrystalline material. Point defects (from surface-adsorbed impurities), dislocations, and slabs of structurally distinct material are created as a consequence of this growth mechanism and can dramatically impact subsequent reactivity.