Conformational changes in the intrinsic chemical structure of the polymer poly(N-vinyl-2-pyrrolidone) (PVP) in aqueous as well as (in)organic solvents essentially dictates the novel room temperature seedless synthetic procedure for the reduction of hydro-chloroauric acid (HAuCl 4 .3H 2 O) leading to the formation of different anisotropic size/ shaped gold nanoparticles. The interaction between gold metal ions and PVP at the given specific monomer to metal ratio leads to sequential metal ligand exchange, thereby simultaneously utilizing the mild reducing property as well as distinct structure-directing/capping ability of PVP in different (in)organic solvents, the synchronized features of which have been carefully explored through NMR and FTIR measurements identifying the foolproof signatures of the polymer coordination interaction for the first time in designing the systematic nucleation and growth/stabilization procedures of anisotropic metal gold nanostructures. Furthermore, the complementary XPS data evaluates the quantitative role of coupled oxygen and nitrogen components of the pyrrolidone ring in the PVP−solvent complex in asserting seedless surface mediation as well as the morphology driven localized surface plasmon suitable for wide range of plasmonic as well as photonic applications.

Recently, seed-mediated approach has been widely utilized for synthesizing monodisperse gold nanostars well-known for plasmonic and sensing applications. Herein, we report a single step seedless/templateless organic synthesis procedure for the complex 3D gold nanostars with a high degree of size/shape control, involving the complexation of polyvinylpyrrolidone (PVP) in polar solvent N,N-dimethylformamide (DMF) and their reduction of AuCl4 – ions along with hydrochloric acid (HCl) as an effective mediator. On the basis of the kinetic optical absorption and Fourier transform infrared (FTIR) spectroscopy measurements, a convenient ligand exchange mechanism has been proposed for the first time to the best of our knowledge to understand the evolution of these complex shaped gold nanostructures. The coordination interaction among PVP and DMF as well as PVP–DMF–AuCl4 has been identified as the major driving factors influencing the temporal evolution of the size/shape-controlled gold nanostars.

We performed cathodoluminescence (CL) spectroscopy and imaging in a high-resolution scanning electron microscope to locally and selectively excite and investigate the plasmonic properties of a multi-branched gold nanostar on a silicon substrate. This method allows us to map the local density of optical states from the nanostar with a spatial resolution down to a few nanometers. We resolve, both in the spatial and spectral domain, different plasmon modes associated with the nanostar. Finite-difference time-domain (FDTD) numerical simulations are performed to support the experimental observations. We investigate the effect of the substrate on the plasmonic properties of these complex-shaped nanostars. The powerful CL-FDTD combination helps us to understand the effect of the substrate on the plasmonic response of branched nanoparticles.

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.

Differential role of PVP modified with halide ions has been meticulously studied for in situ tuning of Au nanoparticle growth utilizing XRD measurements together with FTIR data, thus quantifying their catalysis and SERS applications.