Controlled reshaping and plasmon tuning mechanism of gold nanostars

Abstract

Anisotropic multi-branched gold nanoparticles exhibit intense localized electromagnetic fields at their tips/edges and hence have attracted significant attention in surface enhanced Raman scattering (SERS), as well as in bio-sensing applications. Our quest for such complex hyper-branching in gold nanostructures has revealed that even the addition of a simple base (like NaOH) to the precursor reaction mixture enhances the fine tuning/reshaping of the 3D star/flower-like gold nanostars with controlled precision right from the nucleation stage. With increasing the basicity of the reaction mixture, the two strongly localized surface plasmon resonance (LSPR) peaks of the gold nanostars essentially merge into a broad singular peak, effectively indicating the steady transition from (non)planar structures to conventional spheroidal nanostructures, as confirmed by the transmission electron microscopy (TEM) measurements. Such pH induced size/shape transitions of gold nanostructures were monitored kinetically in detail through correlated molecular spectroscopic measurements nuclear magnetic resonance, fourier transformation infrared spectroscopy and X-ray photoelectron spectroscopy (NMR, FTIR and XPS), for the first time to the best of our knowledge, which ascertains a rational paradigm in better understanding the complex polyvinylpyrrolidone (PVP) functionality for its simultaneous reducing, as well as stabilizing action, in precisely controlling the anisotropic gold nanostructure growth mechanism and further exploiting this functionality in utilizing these as-formed extremely stable colloidal gold dispersions for various specific plasmonic applications.