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.

Surfactant-assisted seeded growth of metal nanoparticles (NPs) can be engineered to produce anisotropic gold nanocrystals with high chiroptical activity through the templating effect of chiral micelles formed in the presence of dissymmetric cosurfactants. Mixed micelles adsorb on gold nanorods, forming quasihelical patterns that direct seeded growth into NPs with pronounced morphological and optical handedness. Sharp chiral wrinkles lead to chiral plasmon modes with high dissymmetry factors (~0.20). Through variation of the dimensions of chiral wrinkles, the chiroptical properties can be tuned within the visible and near-infrared electromagnetic spectrum. The micelle-directed mechanism allows extension to other systems, such as the seeded growth of chiral platinum shells on gold nanorods. This approach provides a reproducible, simple, and scalable method toward the fabrication of NPs with high chiral optical activity.

Noble-metal nanoparticles [1] with localized surface-plasmon resonances (LSPR) have been recently used to prepare new materials with improved optical circular dichroism.[2] This interest stems from a wide range of applications in biology and physics, including the structural determination of proteins and DNA [3] and the pursuit of negative refraction.[4] Surfaceplasmon-mediated circular dichroism (SP-CD) in solution has been explored to date using small spherical metal particles, invariably resulting in moderate signals over a narrow spectral range. [5][6][7][8][9][10] In contrast, we present herein a novel class of metamaterial consisting of gold nanorods (NRs) organized in three-dimensional (3D) chiral structures and yielding a record circular dichroism anisotropy factor for metal nanoparticles (> 0.02) across visible and near-infrared (Vis-NIR) wavelengths (600-900 nm). The fabrication process can be easily upscaled, as it involves the self-assembly of gold nanorods on a fiber backbone with chiral morphology. Our measurements are fully supported by theoretical modeling based on coupled dipoles, unraveling the key role of gold nanorods in the chiroptical response.Three major strategies have been considered for the generation of SP-CD responses: synthesis of metal clusters with an intrinsically chiral surface, [11,12] adsorption of chiral molecules onto achiral metal nanoparticles, [5,13] and organization of nanoparticles into three-dimensional chiral arrangements.[5-10] Herein, we present a record level of optical activity with a chiral assembly of NRs, which we interpret as SP-CD. Known as plasmonic nanoantennas, [14] these particles are characterized by the resonant collective interaction of their conduction electrons with light in the form of both scattering and absorption, with resonance frequencies that can be tuned across the Vis-NIR spectrum by simply changing the aspect ratio of the nanocrystals.[15] Moreover, the LSPR of NRs is very sensitive to the presence and relative orientation of neighboring particles.[16] These two properties combined make NRs promising building blocks for intense and tunable SP-CD. We have designed a new nanocomposite using a self-assembly strategy [17] with NRs adsorbed onto a scaffold of supramolecular fibers with chiral morphology through specific non-covalent interactions.NRs with an average length of 45 nm and average width of 17 nm were prepared by a seeding growth method, [18] and subsequently coated with the amphiphilic polymer poly(vinylpyrrolidone) (PVP) in ethanol. [19] Fibers having a chiral morphology were obtained by adding water to a DMF/ ethanol solution of anthraquinone-based oxalamide 1 (Figure 1 a), [20] forming a fluid dispersion (see the Supporting Information for experimental details). In Figure 1 b,c, we present scanning electron microscopy (SEM) images of twisted fibers with right-(P) and left-handedness (M), corresponding to (R)-1 and (S)-1, respectively, with widths in the hundred-nanometer range and lengths of several micrometers.For the preparation o...

The irradiation of gold nanorod colloids with a femtosecond laser can be tuned to induce controlled nanorod reshaping, yielding colloids with exceptionally narrow localized surface plasmon resonance bands. The process relies on a regime characterized by a gentle multishot reduction of the aspect ratio, whereas the rod shape and volume are barely affected. Successful reshaping can only occur within a narrow window of the heat dissipation rate: Low cooling rates lead to drastic morphological changes, and fast cooling has nearly no effect. Hence, a delicate balance must be achieved between irradiation fluence and surface density of the surfactant on the nanorods. This perfection process is appealing because it provides a simple, fast, reproducible, and scalable route toward gold nanorods with an optical response of exceptional quality, near the theoretical limit.

We investigate theoretically the optical activity of a dimer of plasmonic nanoantennas, mimicking the geometry of a molecule with two isolated chromophores, a situation commonly described as exciton coupling in organic chemistry. As the scale of the system increases and approaches the wavelength of visible light, a rich variety of effects arise that are unique to the plasmonic case. Scattering of light by the particles, negligible in very small clusters, strongly perturbs, and eventually dominates, the optical activity. Additionally, retardation effects in dimers with an interparticle separation commensurate with the wavelength of the incident light affect the electromagnetic coupling between the particles and lead to an asymmetric circular dichroism spectrum. We identify conditions for efficient interaction and predict remarkably large anisotropy factors.

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Recent reports have illustrated the promising potential of chiral metal nanostructures, which exploit the characteristic localized surface plasmon resonance of metal colloids, to produce intense optical activity. In this article we review the concepts, synthetic methods, and theoretical predictions underlying the chirality of metal colloids with a particular emphasis on the size range of 10-100 nanometers. The formation of individual colloidal nanoparticles with a chiral morphology and a plasmonic response remains elusive; however, collective chirality and the associated optical activity in nanoparticle assemblies is a promising alternative that has seen a few recent experimental demonstrations. We conclude with a perspective on chiral nanostructures built up from achiral anisotropic metal particles.

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