Metal particle composition and size dependence

Immunoassays can be performed on the surface of beads or other particles. The composition and size of the beads may vary depending on the application. Surface area available for the reactions is greatly increased by using beads instead of a flat surface as the solid phase. Additionally, beads can be easily transferred from one reagent to another and washing steps are rapid and convenient (e.g. beads containing metal can be transferred magnetically). 

The properties of nanometric particles strictly depend on their microscopical structure (ie, chemical composition, shape, size, percentage of defects, microstrain concentration, etc). For example, the characteristic surface plasmon absorption of a system of metal nanoparticles dispersed into a dielectric matrix is related to the particle shape and size (34). To prepare a color filter, identical particles should be used, otherwise the material will appear black. The presence of a single type of microscopic structure allows each particle to provide the same contribution to the composite properties. From a theoretical point of view, an ideal nanostructin-ed composite should be made of identical metal domains uniformly dispersed into the polymeric matrix. However, since it is very difficult to prepare a sample of... 

While the microemulsion method has been widely applied to the production and stabilization of spherical metal particles with various sizes and compositions, shape control of noble metallic particles using this procedure has only been demonstrated in a handful of studies to date. Pileni and co-workers demonstrated that it is possible to control nanocrystal shape to some extent within microemulsions.Although the shape of the templates plays a role during the growth of the nanocrystals, these authors showed that the particle shape can be controlled even if the microscopic structure of the self-assembled surfactant system used as a template remains unchanged and that addition of salt to the templates can induce drastic changes in the particle shape. Recently, the same group also reported the synthesis of silver nanodisks in reverse micellar solution by reduction of Ag(AOT) with hydrazine, with various sizes that depended on the relative amount of hydrazine, but with constant aspect ratio. 

Due to their high surface-to-volume ratio and size-dependent electronic properties nanostructured materials like NPs are good as catalysts. NPs of different sizes and structures can show significantly different catalytic activities and thus provides an opportunity to understand the structure-function relationship. NPs prepared usually in ensembles of NPs immobilized on an electrode. Thus the electrocatalytic property result of the average properties of the ensemble. Optimization of the catalyst requires increasing the number of sites available for the reaction to occur, shape and size effect of NP and composition of particles (in case of mixed metal... 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

The A1 AI2O3 composite grown at low temperatures (450-500 °C) and low pressure (10 -10 mbar) consists of aluminum particles (diameters ranging from 1-50 nm depending on reaction time), which are embedded in an almost amorphous AI2O3 matrix. The sizes of the particles seem to follow a fractal distribution with a fractal exponent of 2.4 [24] which we have already found for other metal/metal-oxide composites grown by similar CVD processes [22,29]. The amorphous aliuninum oxide is transformed to the crystalhne 7-AI2O3 at temperatures aroimd 550-600 °C. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

Titania and silica glass thin films Au, Pt Photoreduction of HAuCl and K.2PtCl4 in ethanol-water in the presence of poly(N-vinyl-2-pyrrolidone) or poly(methyl vinyl ether led to metal particles (sizes depended on solvent composition the smallest, 2.8 nm in diameter, was obtained in 100% alcohol) which were mixed with Ti(i-OC3H7)4 and acetylacetone under N2. Subsequent to 30 minutes of stirring, exposure to moisture produced Ti02-embedded metal particles 74... 

Quantitative simulation of spectra as outlined above is complicated for particle films. The material within the volume probed by the evanescent field is heterogeneous, composed of solvent entrapped in the void space, support material, and active catalyst, for example a metal. If the particles involved are considerably smaller than the penetration depth of the IR radiation, the radiation probes an effective medium. Still, in such a situation the formalism outlined above can be applied. The challenge is associated with the determination of the effective optical constants of the composite layer. Effective medium theories have been developed, such as Maxwell-Garnett 61, Bruggeman 62, and other effective medium theories 63, which predict the optical constants of a composite layer. Such theories were applied to metal-particle thin films on IREs to predict enhanced IR absorption within such films. The results were in qualitative agreement with experiment 30. However, quantitative results of these theories depend not only on the bulk optical constants of the materials (which in most cases are known precisely), but also critically on the size and shape (aspect ratio) of the metal particles and the distance between them. Accurate information of this kind is seldom available for powder catalysts. 

The constant coefficient, b, which is related to late surface deposition, is independent of particle composition hence, it is the same for the soil and the metal sphere populations. The coefficients, au are related to volume deposition and depend on chemical composition of the particles hence, they would be expected to be different for soil and metal sphere constituents. It would also be expected that the individual types of particles which make up a particular soil (e.g., quartz and feldspar) would exhibit different radionuclide compositions however, in the soil case the composition variation is independent of particle size so that a single average coefficient, 02,3, applies. However, the sphere/soil ratio in general varies with size, so that the radionuclide composition of the sphere population needs to be considered separately. 

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