Janardan Kundu

Metallic nanoparticle arrays: a common substrate for both surface-enhanced Raman scattering and surface-enhanced infrared absorption.

Nanoshell arrays have recently been found to possess ideal properties as a substrate for combining surface enhanced raman scattering (SERS) and surface enhanced infrared absorption (SEIRA) spectroscopies, with large field enhancements at the same spatial locations on the structure. For small interparticle distances, the multipolar plasmon resonances of individual nanoshells hybridize and form red-shifted bands, a relatively narrow band in the near-infrared (NIR) originating from quadrupolar nanoshell resonances enhancing SERS, and a very broadband in the mid-infrared (MIR) arising from dipolar resonances enhancing SEIRA. The large field enhancements in the MIR and at longer wavelengths are due to the lightning-rod effect and are well described with an electrostatic model.

Fano Resonances in Plasmonic Nanoclusters: Geometrical and Chemical Tunability

Clusters of plasmonic nanoparticles and nanostructures support Fano resonances. Here we show that this spectral feature, produced by the interference between bright and dark modes of the nanoparticle cluster, is strongly dependent upon both geometry and local dielectric environment. This permits a highly sensitive tunability of the Fano dip in both wavelength and amplitude by varying cluster dimensions, geometry, and relative size of the individual nanocluster components. Plasmonic nanoclusters show an unprecedented sensitivity to dielectric environment with a local surface plasmon resonance figure of merit of 5.7, the highest yet reported for localized surface plasmon resonance sensing in a finite nanostructure.

Tailoring plasmonic substrates for surface enhanced spectroscopies

Our understanding of how the geometry of metallic nanostructures controls the properties of their surface plasmons, based on plasmon hybridization, is useful for developing high-performance substrates for surface enhanced spectroscopies. In this tutorial review, we outline the design of metallic nanostructures tailored specifically for providing electromagnetic enhancements for surface enhanced Raman scattering (SERS). The concepts developed for nanoshell-based substrates can be generalized to other nanoparticle geometries and scaled to other spectroscopies, such as surface enhanced infrared absorption spectroscopy (SEIRA).

Interactions of ibuprofen with hybrid lipid bilayers probed by complementary surface-enhanced vibrational spectroscopies

The incorporation of small molecules into lipid bilayers is a process of biological importance and clinical relevance that can change the material properties of cell membranes and cause deleterious side effects for certain drugs. Here we report the direct observation, using surface-enhanced Raman and IR spectroscopies (SERS, SEIRA), of the insertion of ibuprofen molecules into hybrid lipid bilayers. The alkanethiol-phospholipid hybrid bilayers were formed onto gold nanoshells by self-assembly, where the underlying nanoshell substrates provided the necessary enhancements for SERS and SEIRA. The spectroscopic data reveal specific interactions between ibuprofen and phospholipid moieties and indicate that the overall hydrophobicity of ibuprofen plays an important role in its intercalation in these membrane mimics.

Nanoshell-based substrates for surface enhanced spectroscopic detection of biomolecules

Nanoshells are optically tunable core-shell nanostructures with demonstrated uses in surface enhanced spectroscopies. Based on their ability to support surface plasmons, which give rise to strongly enhanced electromagnetic fields at their surface, nanoshells provide simple, scalable, high-quality substrates. In this article, we outline the development and use of nanoshell-based substrates for direct, spectroscopic detection of biomolecules. Recent advances in the use of these nanostructures lead to improved spectroscopic quality, selectivity, and reproducibility.

Mesoscopic nanoshells: Geometry-dependent plasmon resonances beyond the quasistatic limit

The plasmon response of a spherical metallic shell becomes significantly more complex as its size is increased beyond the quasistatic limit. With increasing size and decreasing aspect ratio (r1/r2), higher order multipolar modes contribute in a more dominant manner, and two distinct core-shell geometries exist that provide the same dipole plasmon resonance, with differing relative multipolar contributions in their overall spectral response. With further increase in particle size, the geometric tunability of the core-shell structure disappears, and in the infinite radius limit the plasmon response is consistent with that of a thin metallic film.

Matching Solid-State to Solution-Phase Photoluminescence for Near-Unity Down-Conversion Efficiency Using Giant Quantum Dots.

Efficient, stable, and narrowband red-emitting fluorophores are needed as down-conversion materials for next-generation solid-state lighting that is both efficient and of high color quality. Semiconductor quantum dots (QDs) are nearly ideal color-shifting phosphors, but solution-phase efficiencies have not traditionally extended to the solid-state, with losses from both intrinsic and environmental effects. Here, we assess the impacts of temperature and flux on QD phosphor performance. By controlling QD core/shell structure, we realize near-unity down-conversion efficiency and enhanced operational stability. Furthermore, we show that a simple modification of the phosphor-coated light-emitting diode device-incorporation of a thin spacer layer-can afford reduced thermal or photon-flux quenching at high driving currents (>200 mA).

Nanoscale plasmonics for molecular recognition and light-triggered molecular release

Metallic nanostructures designed to provide plasmon resonances at specific optical frequencies and strong yet uniform near-field electromagnetic enhancements are useful nanodevices for light-driven sensing and actuation. The large local fields on the surface of these structures support surface-enhanced spectroscopies such as surface enhanced Raman spectroscopy (SERS). To use plasmonic nanostructures for molecular recognition, their properties must be exploited in combination with molecular layers that provide an optical signature that corresponds to capture of a target molecule. DNA oligomers bound to the surface of plasmonic nanostructures provide an optical signal that is sensitive to the conformational changes in the DNA itself due to interaction with other molecules, as would occur in binding events. This type of optical detection is label-free and reporter-free, that is, it does not depend upon the presence of a dye molecule bound to the DNA to provide an optical signal. DNA-drug interactions can be directly detected in this manner: the binding kinetics of chemotherapy drugs such as cisplatin can be directly monitored by this method, providing a streamlined spectroscopic approach to drug discovery.