Ravisubhash Tangirala

On the kinetics of nanoparticle self-assembly at liquid/liquid interfaces

We investigate the concentration and size dependent self-assembly of cadmium selenide nanoparticles at an oil/water interface. Using a pendant drop tensiometer, we monitor the assembly kinetics and evaluate the effective diffusion coefficients following changes in the interfacial tension for the early and late stages of nanoparticle adsorption. Comparison with the coefficients for free diffusion reveals the energy barrier for particle segregation to the interface. The formation of a nanoparticle monolayer at the oil/water interface is characterised by transmission electron microscopy.

Self-Assembly of Tobacco Mosaic Virus at Oil/Water Interfaces

The oil/water interfacial assembly of tobacco mosaic virus (TMV) has been studied in situ by tensiometry and small-angle X-ray and neutron scattering (SAXS and SANS). TMV showed different orientations at the perfluorodecalin/water interface, depending on the initial TMV concentration in the aqueous phase. At low TMV concentration, the rods oriented parallel to the interface, mediating the interfacial interactions at the greatest extent per particle. At high TMV concentrations, the rods were oriented normal to the interface, mediating the interfacial interactions and also neutralizing inter-rod electrostatic repulsion. We found that the inter-rod repulsive forces between TMVs dominated the in-plane packing, which was strongly affected by the ionic strength and the bulk solution but not by the pH in the range of pH = 6-8.

Connecting quantum dots and bionanoparticles in hybrid nanoscale ultra-thin films

Ligand-functionalized CdSe quantum dots and nanorods, and horse spleen ferritin bionanoparticles, were co-assembled at an oil-water interface, then used in polymerization at the interface, effectively cross-linking the assembled mixtures of nanoparticles into robust structures. Both ring-opening metathesis polymerization (ROMP), and imine formation, proved suitable for preparation of the desired ultra-thin films in the form of capsules and sheets. The nanoparticle-based films prepared by ROMP exhibit chemical stability, while those prepared by aldehyde-amine coupling could be disrupted by addition of acid. Characterization of these hybrid nanoparticle-based materials, using transmission electron microscopy (TEM) and fluorescence confocal microscopy, confirmed the presence of both synthetic and naturally derived nanoparticles in the hybrid materials.

Modification of Blinking Statistics in Solid State Quantum Dot/Conjugated Organic Polymer Composite Nanostructures

Fluorescence intermittency, or “blinking” in quantum dot systems has been the subject of great interest since the first observation of this phenomenon nearly 10 years ago. The stability of quantum dot fluorescence emission is especially important in the context of photovoltaic, optoelectronic, and biological applications, where device performance, or the ability to track labeled particles, is affected adversely by fluorescence intermittency. Single-molecule spectroscopy combined with atomic force microscopy measurements reveal that CdSe quantum dots functionalized with oligo(phenylene vinylene), OPV, ligands exhibit modified optical properties such as suppression of blinking when compared to conventional TOPO covered or ZnS-capped CdSe quantum dots. The blinking suppression is shown to be highly sensitive to the degree of ligand coverage on the quantum dot surface and this effect is interpreted as resulting from charge transport from photoexcited OPV into vacant trap sites on the quantum dot surface. This direct surface derivatization of quantum dots with organic ligands also enables a “tunable” quantum dot surface that allows dispersion of quantum dots in a variety of polymer supported thin films without phase segregation. This facilitates straightforward inclusion of these new hybrid materials into solid state formats and suggests exciting new applications of composite quantum dot/organic systems in optoelectronic systems.