Partha Pratim Patra

Plasmofluidic single-molecule surface-enhanced Raman scattering from dynamic assembly of plasmonic nanoparticles.

Single-molecule surface-enhanced Raman scattering (SM-SERS) is one of the vital applications of plasmonic nanoparticles. The SM-SERS sensitivity critically depends on plasmonic hot-spots created at the vicinity of such nanoparticles. In conventional fluid-phase SM-SERS experiments, plasmonic hot-spots are facilitated by chemical aggregation of nanoparticles. Such aggregation is usually irreversible, and hence, nanoparticles cannot be re-dispersed in the fluid for further use. Here, we show how to combine SM-SERS with plasmon polariton-assisted, reversible assembly of plasmonic nanoparticles at an unstructured metal-fluid interface. One of the unique features of our method is that we use a single evanescent-wave optical excitation for nanoparticle assembly, manipulation and SM-SERS measurements. Furthermore, by utilizing dual excitation of plasmons at metal-fluid interface, we create interacting assemblies of metal nanoparticles, which may be further harnessed in dynamic lithography of dispersed nanostructures. Our work will have implications in realizing optically addressable, plasmofluidic, single-molecule detection platforms.

Single-Molecule Surface-Enhanced Raman Scattering Sensitivity of Ag-Core Au-Shell Nanoparticles: Revealed by Bi-Analyte Method

Single-molecule surface-enhanced Raman scattering (SM-SERS) is an important application of localized surface plasmons in metallic nanostructures. Conventionally, Ag nanoparticles are used in solution-based SM-SERS experiments, but their usage is limited due to toxicity and oxidation issues. Au nanoparticle solutions are relatively biocompatible and SERS-active, but they do not facilitate large-scale SERS enhancement factors, which is an important prerequisite for SM-SERS. Under such constraints, silver-core gold-shell nanoparticles can be an excellent alternative for SM-SERS. Motivated by this, herein we report on the experimental evidence of SM-SERS sensitivity of Ag-core Au-shell nanoparticles by employing bianalyte method. Additionally, by detecting resonant molecules at femtomolar concentrations, we show that Ag-core Au-shell nanoparticle can be harnessed for ultrasensitive detection of molecules. The provided evidence will further motivate usage of such gold-shell-based bimetallic nanostructures for SM-SERS in biological environments.

Microbially-induced flocculation and flotation for pyrite separation from oxide gangue minerals

Selective separation of pyrite from quartz and calcite was achieved through microbiologically induced flotation and flocculation in presence of Bacillus polymyxa. Adsorption behavior of bacterial cells onto pyrite, calcite and quartz was established. Surface chemical changes on various minerals brought about by bacterial interaction were established through zeta-potential measurements as a function of pH. It has been shown that pyrite can be separated from quartz and calcite through either selective flocculation or flotation after interaction with cells of B. polymyxa or bioproteins separated from the bacterial metabolite. The results of this study are significant from the viewpoint of desulphurisation of flotation tailings with respect to environmental control.

Surfactant-free synthesis of anisotropic gold nanostructures: can dicarboxylic acids alone act as shape directing agents?

The present study describes the synthesis of gold nanostructures using different dicarboxylic (viz. oxalic, malonic, succinic, glutaric and adipic) acids as reducing agents in the absence of any other additives or surfactants. Various anisotropic structures such as kites, tadpoles, triangular/hexagonal plates, and twinned particles were seen to evolve depending on the molar ratio of dicarboxylic acid to HAuCl4 used. It was also demonstrated that gold nanostructure formation is hampered as the chain length/distance between two carboxylic acid groups increases. Among the various structures obtained the kite like structures displayed the largest surface enhanced Raman enhancement factors.

Geometry-dependent anti-Stokes SERS radiation patterns from gold nanorod dimers

The geometry of individual plasmonic nanostructures and their collective arrangement plays a critical role in electromagnetic enhancement of surface-enhanced Raman scattering (SERS). Concomitantly, the same attributes also have a direct implication on plasmon resonance line-shapes that further affect the far-field radiation pattern of SERS in Rayleigh, Stokes and anti-Stokes spectral regimes. Here we numerically show how Fano-type plasmon resonance lineshapes in certain geometrical configurations of gold nanorod dimers can selectively influence the far-field radiation pattern in the anti-Stokes spectral region of SERS. We explored a variety of dimer angles (0° to 180°) between the gold nanorods, and compared their far-field radiation patterns in the anti-Stokes, Rayleigh and Stokes regions of their plasmon resonance. We found that angles 30° to 60° exhibited greater in-plane forward-to-backward intensity ratios in the anti-Stokes region, compared to other configurations. The results discussed herein highlight a three-way connection between the geometrical arrangement of plasmonic nanostructures, Fano-type lineshapes and far-field radiation patterns in SERS, especially in the anti-Stokes region. Our work will have implications not only in designing optical antennas for linear and nonlinear Raman scattering, but also in creating directional light sources based on nonlinear optical processes such as two-photon fluorescence and photon frequency upconversion.