Branched plasmonic nanoparticles (NPs) composed of noble metals are an interesting subclass of plasmonic NPs due to their unique properties that arise from the strong electric field enhancements that occur at their tips. The plasmonic properties of branched metal NPs can be manipulated through altering their symmetry and structural features such as branch sharpness, composition, and their surroundings. In this Featured Article, the unique optical properties that arise from branched plasmonic NPs are introduced, which were revealed through pioneering studies of Au nanostars and related structures. Next, branched NPs with high symmetry are discussed as a model system to explore more fully how parameters such as NP size, shape, and composition impact their properties, enabling applications in chemical sensing and beyond. These studies provide design criteria and synthetic strategies toward new nanostructures with increasing structural and compositional complexity.

Metallic nanoparticles (NPs) display interesting optical and catalytic properties that depend on NP composition, size, shape, and architecture. These NPs and their properties are often defined by the symmetry of the NPs themselves, with many seed-mediated syntheses for metal NPs maintaining the same symmetry between the seed and the resulting NP. However, recent research has shown that the symmetry of NPs can be reduced in a defined manner during seed-mediated syntheses through judicious control of reaction conditions. This ability offers unique and tunable optoelectronic and catalytic properties. In this Perspective, we outline general pathways to obtain NPs with reduced symmetry by seed-mediated methods and the interesting optical and catalytic properties that result from such a reduction in symmetry.

Chiral plasmonic nanocrystals with varied symmetries were synthesized by l-glutathione-guided overgrowth from Au tetrahedra, nanoplates, and octahedra, highlighting the importance of chiral molecule adsorption at transient kink sites.

Plasmonic metal nanoparticles (NPs) show promise in a variety of applications, ranging from theranostics to chemical sensing, with chemical sensing made possible by the sensitivity of the localized surface plasmon resonance (LSPR) to the surrounding dielectric environment. Au NPs have been the standard for LSPR sensing applications due to their narrow plasmon band, tunable LSPR maximum, and relative chemical stability; however, the comparatively low refractive index sensitivity (RIS) and morphological instability of Au nanostructures are limiting factors. Recent research has found that incorporating Pd into Au systems can increase RIS and impart multifunctionality, but how the distribution of Pd within Au-based nanostructures affects LSPR sensing is unclear.Here, Au−Pd heterostructures with different Au−Pd distributions were prepared to systematically study the effect of Pd distribution on RIS. Specifically, symmetrically branched Au nanocrystals with O h symmetry (i.e., octopods) were selected as building blocks as their branch tips concentrate E-fields locally. Using these nanocrystals as seeds, Pd-tipped Au octopods and core@shell Au@Pd octopods were synthesized for comparison to alloy Au−Pd octopods and all-Au octopods. Through experiment and simulation, we show that RIS depends both on Pd loading and location in Au−Pd heterostructures, with the Pd-tipped Au octopods displaying the highest RIS while maintaining a moderate figure of merit. This systematic analysis highlights that localization of Pd at LSPR hotspots is critical to achieving the highest RIS, with this insight intended to guide design of future LSPR sensors that move beyond the all-Au standard.

Use of asymmetrically passivated seeds provides a pathway to regioselective overgrowth and nanoparticles with exciting plasmonic properties.