The factors affecting the nucleation and growth of gold nanorods, (Jana et al., Adv. Mater. 2001, 13, 1389) have been investigated. It is shown that the size and aspect ratio can be controlled through the use of different sized seed particles. The length of the rods can be tuned from 25–170 nm, while the width remains almost constant at 22–25 nm. The formation of rods requires the presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Lower temperature favors rod formation, although this reduces CTAB solubility. The addition of chloride ions or the use of dodecyltrimethylammonium bromide (DTAB) leads to shorter‐aspect rods. AuIII and AuI are shown to be quantitatively bound to the CTAB micelles. We propose an electrochemical mechanism for rod formation, whereby the flux of AuI bound to cationic micelles to the seed surface is maximized at points of highest curvature, where the electrical double layer gradient is highest. Initial numerical solutions to the electric potential and field around an ellipsoid in a 1:1 electrolyte are provided, which indicate that the field at the particle tip scales linearly with the aspect ratio. Mean free passage times for ions are found to be shortest at the tips. The results provide a general explanation for the formation of non‐equilibrium crystal habits and a mechanism for controlling crystal growth.

In recent years, new strategies for silica coating of inorganic nanoparticles and organic nanomaterials, which differ from the classical methodologies, have emerged at the forefront of materials science. Silica as a coating material promises an unparalleled opportunity for enhancement of colloidal properties and functions by using core-shell rational designs and profiting from its synthetic versatility. This contribution provides a brief overview of recent progress in the synthesis of silica-coated nanomaterials and their significant impact in different areas such as spectroscopy, magnetism, catalysis, and biology.

Gold nanoparticles are readily oxidized by Au(III) in the presence of cetyl-trimethylammonium bromide (CTAB). Oxidation occurs preferentially at surface sites with higher curvature. Conversely, oxidation with cyanide ions in the absence of CTAB leads to uniform oxidation over the whole surface. Examples of the spatially directed oxidation are provided using large, irregular spheres, nanocubes, and nanorods. We conclude that the mechanism of oxidation depends on whether the oxidant is attached to CTAB micelles. It is postulated that the CTAB micelles approach the nanoparticles preferentially at the tips, leading to spatially directed oxidation.

A modified seeded growth process has been used for the controlled synthesis of quasispherical, CTAB-stabilized gold nanoparticles from 12 up to 180 nm with narrow size distributions. The UV-visible spectra of the aqueous colloids show distinct bands corresponding to dipole and quadrupole plasmon modes, for diameters above 100 nm, in close agreement with predictions based on Mie theory. The assignment of the modes is demonstrated by calculation of near field enhancement maps based on the boundary element method. Apart from other applications, since absorption is drastically reduced above 600 nm, while scattering is largely increased, these particles open new possibilities for construction of highly efficient photonic structures.