Metal@dielectric composite nanostructures are of high demand for their vast technological applications. The Stöber method, either in its original form or by modification, has been utilized for the fabrication of silica shells over metal, semiconductor, or even dielectric nanostructures, with the aim to protect them from degradation, enhance their biocompatibility, or use them for molecular anchoring. However, the stability of silica shells and the dispersion of core–shell nanostructures remain the main limitations for their efficient applications. Here we demonstrate that utilization of ultrasound during hydrolysis and condensation of the metal–organic silicon precursor in Stöber process can produce stable and uniform silica shell layers around gold nanoparticles, enhancing both their stability and dispersion. Through transmission electron microscopy and infrared spectroscopy techniques, we demonstrate that the Au@SiO2 nanoparticles fabricated with ultrasound treatment during silica shell growth contain a lower content of silanol (Si–OH) groups, which are principally responsible for the instability of silica-coated metal nanostructures. The core–shell structures fabricated by ultrasound-assisted hydrolysis using prefabricated Au nanoparticles are well-dispersed, uniform in size, and protected from further hydrolysis in aqueous media, including simulated body fluid. The method applied to fabricate silica-coated Au nanoparticles can be utilized to fabricate other silica-coated metal nanoparticles to enhance their chemical and thermal stability.

Growth of anisotropic nanostructures enables the manipulation of optical properties across the electromagnetic spectrum by fine morphological tuning of the nanoparticles. Among them, stellated metallic nanostructures present enhanced properties owing to their complex shape, and hence, the control over the final morphology becomes of great importance. Herein, a seedmediated method for the high-yield production of gold rich − copper concave branched nanostructures and their structural and optical characterization is reported. The synthesis protocol enabled excellent control and tunability of the final morphology, from concave pentagonal nanoparticles to five-fold branched nanoparticles, named "nanostars". The anisotropic shape was achieved via kinetic control over the synthesis conditions by selective passivation of facets using a capping agent and assisted by the presence of copper chloride ions, both having a crucial impact over the final structure. Optical extinction measurements of nanostars in solution indicated a broad spectral response, hiding the properties of the individual nanostars. Hence, single-particle scattering measurements of individual concave pentagonal nanoparticles and concave nanostars were performed to determine the origin of the multiple plasmon bands by correlation with their morphological features, following their growth evolution. Finite-difference time-domain calculations delivered insights into the geometry-dependent plasmonic properties of concave nanostars and their packed aggregates. Our results uncover the intrinsic scattering properties of individual nanostars and the origin of the broad spectral response, which is mostly due to z-direction packed aggregates.

Application of core−shell plasmonic nanostructures in fluorescence enhancement and surface-enhanced Raman scattering (SERS) strongly depends on their near-field electrodynamical environments. A nonradiative energy transfer takes place between fluorescent molecules and surface plasmon when they are too close. However, for a dielectric shell, the SERS intensity of analytes decreases exponentially beyond 2 nm thickness. Although electromagnetic-field enhancement due to surface plasmon still occurs at longer distances from the metal core, it needs a proper design of the composite nanostructure to exploit this advantage, and an optimal distance between the metal-core and analyte/fluorescent molecule still seems necessary. We analyze, both theoretically and experimentally, the near-electric-field (NEF) distributions in the proximity of the core−shell and shell−medium interfaces of Au@ SiO 2 and Ag@SiO 2 core−shell structures immersed in common dispersing media such as air, water, and DMSO to investigate the effects of surrounding medium and particle geometry on them. Through Mie-based theoretical calculations, we demonstrate that the NEF distributions near core−shell and shell−medium interfaces depend not only on the geometrical parameters, but also on the dielectric constant gradient at these interfaces. For each of the dispersion media and a wide range of metal-core radii, we calculate the optimum shell thickness for obtaining the maximum near-field enhancement at the core−shell and shell−medium interfaces, the essential requirements for applying these nanostructures in fluorescence enhancement and SERS. Theoretically obtained results have been qualitatively verified with experiments.

Technological application of polyhedral plasmonic nanoparticles (NP) is closely associated with their strong ability of localizing electric field at and near the surface. In this study, we calculate the optical properties of most common regular polyhedral Au nanoparticles such as the cube, rhombic dodecahedron, pentagonal bipyramid, and octahedron and compare with the optical properties of spherical particles to investigate the plasmonic behavior of metallic nanoparticles due to the breakdown of spherical symmetry. We demonstrate that the highest electric field enhancement (1 order magnitude higher than the spherical NP) occurs in octahedral Au NPs as the consequence of strong localization of incident electromagnetic field, decreasing the diffraction limit of the optical microscopies, which depends both on their size and orientation with respect to the polarization of incident light, increasing the system resolution. Higher order modes (quadrupole, hexapole, octupole, etc.) appear along the dipole mode as the shape of the NPs deviates from their regular spherical shape, even though their size remains much smaller than the incident wavelength. Irrespective of their shape, the major contribution of small NPs comes from the light absorption. Although the scattering efficiency of Au NPs becomes significant with the increase of their size, the NPs with sharper borders and vertices scatter higher fraction of light than the NPs of spherical shape.

The near electric field enhancement around plasmonic nanoparticles (NPs) is very important for applications like surface enhanced spectroscopies, plasmonic dye-sensitized solar cells and plasmon-enhanced OLEDs, where the interactions occur close to the surface of the NPs. In this work we have calculated the near-field enhancement around solid and core-shell alloy NPs as a function of their geometrical parameters and composition. We have found that the field enhancement is lower in the AuxAg1-x alloys with respect to pure Ag NPs, but it is still high enough for most near-field applications. The higher order modes have a stronger influence over the near-field due to a sharper spatial decay of the near electric field with the increase of the order of multipolar modes. For the same reason, in AuxAg1-x@SiO2 core-shell structures, the quadrupolar mode is dominant around the core, whereas the dipolar mode is predominant around the shell. The LSPR modes can have different behaviours in the near- and the far-field, particularly for larger particles with high Ag contents, which indicates that caution must be exercised for designing plasmonic nanostructures for near-field applications, as the variations of the LSPR in the near-field cannot be inferred from those observed in the far-field. These results have important implications for the application of gold-silver alloy NPs in surface enhanced spectroscopies and in the fabrication of plasmon-based optoelectronic devices, like dye-sensitized solar cells and plasmon-enhanced organic light-emitting diodes.