Abhitosh Kedia

Controlled reshaping and plasmon tuning mechanism of gold nanostars

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.

Local electron beam excitation and substrate effect on the plasmonic response of single 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.

Halide ion induced tuning and self-organization of gold nanostars

The simple addition of different halide ions to the precursor polymer–solvent solution for the synthesis of gold nanostars drastically changes their optical properties owing to the blue/red shifting of the longitudinal surface plasmon resonance (LSPR) bands as well as the emergence of new plasmon bands, aesthetically illustrating the simultaneous size/shape tuning enhanced with intriguing self organization, as confirmed from the TEM measurements. Such a synchronized morphology tuning has been very well understood in terms of the distinct reaction kinetics imparted by the different adsorption affinities of inorganic halide ions towards metal ions/surfaces. The systematic results of which are discussed rigorously in the present work in conjunction with the uncommon/unusual assemblies of anisotropic/elongated gold nanostructures, the first of its kind to the best of our knowledge, that are envisioned as ‘hot spots’ for exotic applications in surface enhanced Raman scattering (SERS) as well as plasmonic waveguiding.

Tweaking anisotropic gold nanostars: covariant control of a polymer–solvent mixture complex

A binary solvent mixture (N,N-dimethylformamide (DMF)–propanol in the present case) has been employed for the first time in a simple yet versatile approach for the in situ intuitive and highly tweaked formation of complex anisotropic gold nanostructures. In this novel approach, the uncommon usage of a polar/non-polar binary solvent mixture provides extra degrees of freedom in tailoring the intrinsic solution properties via simultaneously modifying dipole–dipole and H-bonding interactions with the universally soluble amphiphilic polymer PVP. Interactions are carefully monitored through NMR, which further establishes a rational paradigm in better understanding the kinetic control over both nucleation and size/shape transition/evolution of reproducible anisotropic Au nanostructures, as revealed/correlated through optical absorption and TEM measurements respectively. This unusual size/shape transformation strategy for gold nanostructures allows precise tuning of their respective plasmonic characteristics over the entire visible/NIR spectral range, significantly enabling them to serve as excellent candidates/substrates for tunable surface enhanced Raman spectroscopy (SERS), the preliminary measurements of which are systematically illustrated.

Gold nanostars reshaping and plasmon tuning mechanism

Au nanostars are multi-branched nanoparticles with sharp tips which display enhanced plasmonic applications in SERS and nanophotonics. It has already been well documented that polyvinylpyrrolidone (PVP) dispersed in DMF solvent medium act as a unique candidate for realization of this 3-D complex branched metal nanostructures even under normal conditions. Interestingly, controlled addition of propanol to DMF brings about significant changes in the morphology of these gold nanostars visualized through gradual blue shifting of the localized surface plasmon resonance (LSPR) from 920 to 600 nm. Modified interaction between DMF-PVP arising due to introduction of alcohol results in fine tuning of LSPR correlated with corresponding aesthetic changes as clearly evidenced from TEM images. Thus, our ability in synthesizing anisotropic metal nanoparticles with wavelength tunable LSPRs through a simple yet elegant chemical solution synthesis procedure opens up a gamut of new applications in both linear and nonlinear optical spectroscopy.

Pertinent Shape Effects of Gold Nanospheres and Nanostellates

Modulating optical resonances in plasmonic resonators were easily achieved by virtue of their constituent composition, size as well as shape. Such confinement of electric field in the nano-regime enhances extra-ordinarily the light-matter interaction, further enabling wide range of applications in nanophotonics, SERS etc. Herein, we compare and contrast the surface plasmon modes of Au nanospheres and nanostellates, the visibly distinct morphologies of which not only correlates exceptionally well with their structure-property functional relationships, but also articulate their respective growth mechanisms in a rather meticulous manner. Higher amplification of analyte Raman signals visibly demonstrates the increased number of hot-spots with reference to their surface morphology and roughness.

Gold-Copper alloy “nano-dumplings” with tunable compositions and plasmonic properties

The unique yet tunable optical properties of plasmonic metal nanoparticles have made them attractive targets for a wide range of applications including nanophotonics, molecular sensing, catalysis etc. Such diverse applications that require precisely stable / reproducible plasmonic properties depend sensitively on the particle morphology ie. the shape, size and constituents. Herein, we systematically study the size / shape controlled synthesis of gold-copper “dumpling” shaped alloy nanoparticles by simultaneous reduction of gold and copper salts in the PVP-methanol solute-solvent system, by effectively utilizing the efficient but mild reduction as well as capping abilities of Poly (N-vinylpyrrolidone). Introduction of copper salts not only yielded the alloy nanoparticles, but also slowed down the growth process to maintain high mono-dispersity of the new shapes evolved. Copper and gold has different lattice constants (0.361 and 0.408 nm respectively) and hence doping/addition/replacement of copper atoms to...

NVP Encapsulated Gold Nanoclusters by In Situ Polymerization of Monomer

We propose a simple versatile environmental friendly approach for the synthesis of stable gold nanoclusters via utilizing N-vinyl pyrrolidone (NVP) both as a solvent and reaction medium in systematically reducing the metal precursor, chloroauric acid, further radically oxidizing itself to form oligomers which simultaneously encapsulates the as-formed gold nanoclusters, the evidence of which can be clearly seen from TEM and other molecular spectroscopic measurements as illustrated herewith.

Bromide ion induced formation of PVP-capped anisotropic gold nanoplates/nanotriangles

Anisotropic gold nanoparticles are particularly important owing to their exciting applications in plasmonics as well as nanophotonics and biosensing. Herein, we have synthesized gold nanoplates/nanotriangles with an average side-length varying from 80 to 150 nm via a single step room-temperature solution-phase chemical reduction method utilizing predominantly the PVP-bromine interaction. The concentration of bromine as well as the surrounding reaction medium/environment plays an important role in determining the yield of gold nanoplates/nanotriangles at a given PVP to metal ratio. In the presence of bromine ions, the distinctive binding of PVP with metal salt changes owing to its conformational variations, as illustrated successfully through FTIR, optical absorption and TEM, leading to the formation of anisotropic gold nanoplates/nanotriangles.

Solvent Induced Kinetic Growth of Shape Controlled Gold Nanostructures

Shape controlled gold nanoparticles were synthesized using a novel one step process involving the reduction of chloroauric acid (HAuCl4.3H2O) in the presence of polyvinylpyrrolidone (PVP) in different solvents at room temperature. PVP acts both as a reducing agent as well as stabilizer. Shape, size and plasmon resonance of the as‐prepared gold nanoparticles were determined using TEM and optical absorption measurements. Conformational changes in the PVP polymer when dissolved in different solvents results in distinctive binding with gold salts, thereby initiates the formation of size/shape controlled gold nanostructures. In water and ethanol solvents, we get planar gold nanostructures but in N, N‐Dimethylformamide (DMF) we get 3‐D star like gold nanostructures. Henceforth, we have taken advantage of the multifunctional physico‐chemical aspects of PVP including its high solubility in most of the common organic/inorganic solvents, high stability and weak reducing power for preparation of metal nanoparticles.