Investigation of nanoparticle-based self-assembly and excitation energy transfer processes

Abstract

Nanotechnology is an emerging scientific field having great potential for fundamental research and future technology. As the name implicit, nanomaterials are condensed matters having sizes in the nanometer range i.e., larger than atoms, but smaller than bulk solids. They are too big to behave like atoms but too small to act like the bulk solid. In the last two decades, various functional nanomaterials have been fabricated. These materials can be zero (0D), one (1D), two (2D), and three (3D) dimensional. Quantum dots (QDs) are theoretically described as a point like, or a 0D entity. QDs are typically composed of either single-element (Silicon (Si) or Germanium (Ge)) or binary compounds formed from combinations of II-VI (CdSe, CdTe, CdS, ZnO, ZnS, etc.), III-V (InP, GaAs), or IV-VI (PbS, PbSe) group elements. They are very important nanomaterials as they exhibit broad excitation spectra, size- and composition-tunable narrow emission from ultraviolet to near-infrared wavelength range, high luminescence quantum yield, and better stability against photobleaching compared to organic dyes. Similarly, metal nanoparticles (NPs) are nanoscopic materials with an overall dimension below 100 nm. Several transition metal NPs including gold (Au), silver (Ag), iron oxides (Fe2O3), copper (Cu) have been designed for various applications. Metal NCs are another class of nanomaterials that consist of few to several metal atoms with a core size in the range of 1-2 nm. In recent times, there has been growing interest in designing new multifunctional self-assembled nanocomposites such as hybrid vesicles, polymersomes, hydrogels, nanocapsules, and colloidal coacervate droplets because of their vast potential in basic research as well as in technology-related fields. Self-assembly among the charged molecules such as synthetic and natural polymers, fatty acids, biological macromolecules, poly or oligopeptides are reported in literature and provide great details of the mechanism, structure, stability, and sequestration properties of a wide range of coacervate droplets prepared mainly from organic molecules but there are no reports on the fabrication of organic-inorganic hybrid droplets. Vesicles from other amphiphilic molecules such as block copolymers, dendrimers, polypeptides, surfactants, and fatty acid have been fabricated successfully with enhanced colloidal and mechanical stability compared to conventional phospholipid vesicles. Although these recent studies highlight the multifunctional properties of organic-inorganic hybrid plasmonic vesicles, there is a growing interest in the fabrication of stable stimuliresponsive inherently luminescent hybrid vesicles for in vivo bioimaging applications. There are various applications of QD based self-assembled nanoassemblies including chemical and biological sensing, energy harvesting, and excitation energy transfer (EET) processes. EET from various photoexcited donors to ground-state acceptors have been studied thoroughly in the recent past due to its importance in photovoltaics, light-emitting diodes, sensors, and bioimaging. The EET from a photoexcited donor to a nearby acceptor is mainly described by Förster resonance energy transfer (FRET) and nanometal surface energy transfer (NSET) mechanisms. The tuning of intermolecular FRET has been demonstrated in various systems such as micelles, reverse micelles, polymer matrices, spherical vesicles, and proteins. Among these, FRET processes across the liposome bilayer are of particular interest due to its potential in exploring various membrane related biophysical processes such as membrane fusion, trafficking, and receptor-ligand interactions. On the other hand, metal and semiconductor NPs based donor-acceptor nanocomposite systems have attracted considerable attention due to their size-dependent optoelectronic properties, which allow easy tuning of various energy transfer related parameters. Several studies have shown that the intrinsic emission properties of QDs/dyes can be efficiently tuned near a plasmonic NP as a consequence of long-range electromagnetic coupling between exciton and localized surface plasmon resonance (LSPR) of NPs. Extensive experimental and theoretical studies have been performed to understand the fundamental mechanism behind this long-range electromagnetic coupling and subsequent fluorescence quenching at the metal NP surface. The quenching of excited donors in the presence of metal surface is demonstrated earlier by Chance et al. and later extended by Persson and Lang by using a Fermi golden rule. Modification of the CPS-Kuhn expression by including the size-dependent electronic terms proved to be an efficient method to explain the size-dependent NSET. These EET theories have been extensively utilized to understand the various complex chemical and biological processes across lipid bilayers of liposomes and cell membranes. Various cell membrane mimicking systems such as micelles, liposomes, and polymersomes with biocompatible interfaces have been utilized to understand the fundamental interactions with various NPs. In this thesis, the photophysical properties of MSA-capped CdTe QDs have been investigated using various spectroscopic, microscopic techniques and utilized them as a luminescent marker to understand the self-assembly process in the presence of poly (diallyldimethylammonium chloride) (PDADMAC) polymer and hexadecyltrimethylammonium bromide (CTAB) surfactant. Furthermore, the detailed mechanism and tuning of EET in the presence of liposome from various photoexcited donors such as MSA-capped CdTe QDs and 4´,6-diamidino- 2-phenylindole (DAPI) to gold (Au) NPs, silver (Ag) NPs and Ag NC as acceptors have been demonstrated. The overall aim of the work described in the thesis is to explore the interaction of luminescent QDs with oppositely charged polymers and surfactants as well as to understand EET across the lipid bilayers of liposomes.