Narrowing Surface Plasmon Resonances of Gold Nanorods via Multiwavelength Analytical Ultracentrifugation-Assisted Fractionation

Slight nonuniformity in the size or shape of nanoparticles can lead to deviations in the observed properties from predictions based on ideal monodisperse nanoparticles. For reducing nanoparticle polydispersity, postsynthesis treatment serves as a complementary 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 a 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 ultracentrifugation (MWL-AUC), which can simultaneously capture sedimentation profiles and in situ UV-vis spectra of gold nanorods to investigate their sedimentation behavior in a centrifugal field. Our findings indicate that relying solely on the bilayer model of cetyltrimethylammonium bromide (CTAB) is inadequate for explaining steric repulsion. The gold nanorods should be considered to be encapsulated by a CTAB bilayer along with an additional 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 repulsive layer significantly thinner than the layer of 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, additional CTAB should be introduced to replenish the depleted CTAB and an equilibrium period should be allowed between centrifugation rounds to facilitate ligand self-assembly at the outer layer of gold nanorods. 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.