Importance of Pd Distribution to Au–Pd Nanocrystals with High Refractive Index Sensitivity

Abstract

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 Oₕ 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.