Understanding shell coating effects to overcome quenching in single anisotropic upconversion nanoparticles.

Shell coating is known to suppress luminescence quenching in spherical upconversion nanoparticles. However, the emergence of anisotropic nanoparticles with facet-selective, directional growth complicates the coating process, and the use of traditional active, inert, or polymer coatings on such structures remains largely unexplored. Here, we synthesize a series of nanorods with designed geometries, enabling quantitative spectral analysis at the single-particle level. We observe that directional growth of inert or active shells at the rod tips enhances emission relative to the parent core, with their relative effectiveness governed by power density and shell thickness. Ligand presence-polymer or oleate-quenches upconversion relative to bare nanorods. Although local heating is observed at the single-particle level, it does not affect spectroscopic observations, ligand stability, or data reproducibility. Our findings reveal how directionally grown shells influence the optical properties of upconversion nanorods, providing essential insights for their future applications in bioimaging, sensing, and photonics.