Toward Ultranarrow Surface Plasmon Resonances: Multiwave-length Analytical Ultracentrifugation Assisted Fractionation of Gold Nanorods

Slight non-uniformity in nanoparticles size or shape can lead to deviations in the observed properties from predictions based on ideal monodisperse nanoparticles. For reducing nanoparticle polydispersity, post-synthesis treatment serves as a com-plementary approach to synthesis optimization, yet it is often overlooked. Centrifugation is theoretically a versatile method for fractionating all types of polydisperse nanoparticles. However, due to the challenges of precisely controlling fractionation in the centrifugal field and the complex interactions between nanoparticles and ligands, centrifugation is largely confined to a washing step following nanoparticle synthesis. In this work, we utilize our custom-built multiwavelength analytical ultracen-trifugation (MWL-AUC), which can simultaneously capture sedimentation profiles and in-situ UV-vis spectra of gold nano-rods, to investigate their sedimentation behavior in a centrifugal field. Our findings indicate that the bilayer model of cetyltrimethylammonium bromide (CTAB) requires modification. The gold nanorods should be considered as being envel-oped by a significantly thick outer layer of CTAB, while only a small amount of free CTAB remains in the aqueous solvent. Gold nanorods at the sedimentation boundary possess a significantly thinner repulsive layer than those concentrated at the bottom of the centrifuge tube. Consequently, the increased polydispersity in the repulsive layer counteracts the fractionation effect along the centrifugal field, ultimately leading to a “fractionation saturation”. To achieve effective fractionation, sufficient equilibrium time for ligand self-assembly on the outer layer of gold nanorods should be allowed between each centrifugation round. Our work not only demonstrates the feasibility of producing highly monodisperse gold nanorods, but also provides a new avenue to reassess the surfactant layer structure of other sterically stabilized nanoparticles, ultimately enabling the production of monodisperse functional nanoparticles across a wide range of types.