Shape Separation of Gold Nanostars using Density Gradient Centrifugation

Abstract

Gold nanostars (NS) synthesized using Good’s buffers are desirable due to their ability to produce NS with tunable branch size and number. The NS synthesized using good’s buffer are bio-compatible, monodispersed, and stable over many weeks. These NS, due to their absorption in NIR and smaller size (~50 nm), have potential in imaging, bio-medicinal and surface enhanced Raman spectroscopy (SERS) based applications. This work focuses on NS synthesized using Good’s buffer MOPS which has high heterogeneity and instability. Here, we found by washing and re-dispersing the NS in DI water after synthesis, provided higher stability to the MOPS NS for over a month. Moreover, we explored the use of density gradient centrifugation for the shape separation of as-synthesized NS. Density gradient, centrifugation time and speed, and gradient steepness were optimized to separate the NS by increasing size and branch number. The sorted NS were analyzed using TEM, UV-Vis, and Dynamic light scattering. Over ~300 NS per sorted fraction were manually analyzed using ImageJ to determine the quality of separation. We found an overall decrease in the spherical NP population from 20 % to ~1% whereas an increase in NS population with high branch number from ~1% to 67% over the fractions. We also observed an increase in the average NS size and branch length from 18nm to 34nm over fractions.