Satish Rao

Raman Study of Mechanically Induced Oxygenation State Transition of Red Blood Cells Using Optical Tweezers

Raman spectroscopy was used to monitor changes in the oxygenation state of human red blood cells while they were placed under mechanical stress with the use of optical tweezers. The applied force is intended to simulate the stretching and compression that cells experience as they pass through vessels and smaller capillaries. In this work, spectroscopic evidence of a transition between the oxygenation and deoxygenation states, which is induced by stretching the cell with optical tweezers, is presented. The transition is due to enhanced hemoglobin-membrane and hemoglobin neighbor-neighbor interactions, and the latter was further studied by modeling the electrostatic binding of two of the protein structures.

Single DNA molecule detection in an optical trap using surface-enhanced Raman scattering

Raman spectra from single DNA molecules in their natural aqueous environment are presented. A DNA molecule that is anchored between two optically trapped dielectric beads is suspended in a solution with nanosized silvercolloid particles. The nonspecific binding of the metal to the DNA enhances the Raman scattering that is excited by a near-infrared beam. A Raman spectrum is first recorded followed by a force-extension curve that verifies the presence of a single DNA molecule.

Diffusion and cellular uptake of drugs in live cells studied with surface-enhanced Raman scattering probes.

An understanding of the mechanisms of drug diffusion and uptake through cellular membranes is critical for elucidating drug action and in the development of effective drug delivery systems. We study these processes for emodin, a potential anticancer drug, in live cancer cells using surface-enhanced Raman scattering. Micrometer-sized silica beads covered by nanosized silver colloids are passively embedded into the cell and used as sensors of the drug. We demonstrate that the technique offers distinct advantages: the possibility to study the kinetics of drug diffusion through the cellular membrane toward specific cell organelles, the detection of lower drug concentrations compared to fluorescence techniques, and less damage imparted on the cell.

Detection of plasmon-enhanced luminescence fields from an optically manipulated pair of partially metal covered dielectric spheres

Using optical tweezers combined with luminescence measurements we detected the optical field around two optically trapped silica microspheres partially covered by metal. By monitoring the luminescence of rhodamine 6G we were able to observe an increase of the local field intensity owing to the coupling of the local surface plasmons at the surfaces of two spheres.

Tip-enhanced Raman scattering from bridged nanocones.

We present two silver nanocones separated by 450 nm, well beyond the typical gap spacing of coupled nanoantennas, and connected by a metal bridge to facilitate plasmonic coupling between them. The tip-enhanced Raman scattering from crystal violet molecules is found to be almost an order of magnitude higher from the bridged cones than from individual cones. This result is supported by local-field calculations of the two types of structures. The bridged nanocones are easily fabricated by a nanoimprint-based process, thus offering a faster and simpler approach compared to other fabrication techniques.

Nonlinear optical response from single spheres coated by a nonlinear monolayer

We detected the second-order nonlinear response from single isolated spheres comprised from a centrosymmetric material but covered by a layer of a material with strong second-order nonlinear properties and isolated from an ensemble by the optical trapping technique. We show that when large size parameter spheres are used, the measured second-harmonic efficiency deviates strongly from the prediction of the nonlinear Rayleigh scattering theory. Our results are in very good agreement with the predictions from the exact nonlinear Mie scattering theory.

Tip-Enhanced Raman Scattering from Bridged Metal Nanocones

Tip-enhanced Raman scattering (TERS) is a powerful near-field spectroscopic tool to measure Raman spectra of materials with nanometer spatial resolution and even with sensitivity down to single-molecule level [1]. The optical enhancement in TERS relies on lightning-rod effect and plasmonic coupling of input electric field into sharp metal tips. In general, the presence of strong electric field component along the tip axis (i.e., longitudinal field) is required for efficient coupling and several coupling schemes (e.g., use of radial polarization) have been suggested to enhance the interaction [1, 2].

Using 2D correlation and multivariate analysis combined with plasmonic effects to expand the use of Raman microspectroscopy in biomedical applications

We show that Surface Enhanced Raman spectroscopy (SERS) combined with 2D correlation and multivariate analysis provides considerable progress in using Raman microspectroscopy for cutting edge biomedical research applications such as treatment delivering in cancer living cells, the diagnosis of retina neuroinflamed tissue and the study of elastic properties of single DNA molecules.

Extending the applicability of Raman microspectroscopy in biomedicine using statistical analysis and plasmonic effects

We show that plasmonic effects combined with statistical analysis provides considerable progress in using Raman microspectroscopy for cutting edge biomedical research applications of samples such as cancer living cells and single DNA molecules.

Detection of momentum transfer from the emission of Raman photons

The momentum transfer to a scatterer from Raman photons was detected using an optical system that permits one to simultaneously measure the radiation forces exerted on, and the Raman emission from the scatterer.