Synthesis and Structure-Assembly Relationships of Metal Nanoparticles and their Coupling with Two-Photon Organic Dyes

Abstract

The size-dependent properties of nanoparticles offer many opportunities for the creation of new functional materials. The goal of the work presented here is to utilize metal nanoparticles for new applications in linear and nonlinear spectroscopy. This is centered on the synthesis and characterization of metal nanoparticle composites for metal microfabrication and biosensing. In particular, the structures of nanoparticle composites and their optical properties are explored via chemically tailoring the nature of the organic layer surrounding the nanoparticles.The majority of these studies are concerned with discrete, thiol-passivated silver nanoparticles and elucidation of the structure of these alkylthiol-coated nanoparticles. New interpretations of the thermodynamic nature of nanoparticles and nanoparticle assemblies are made. An effective structure-property relationship that correlates the nature of the ligands with the temperature and energy of the order-disorder phase transition is determined. Consequently, the solubility of silver nanoparticles in organic solvents was optimized in a rational manner, allowing for the preparation of high quality polymer composites containing nanoparticles and utilized for two-photon induced writing of 3D silver microstructures. Alkylthiol-capped copper nanoparticles were also prepared, with a particular focus on the influence of oxygen on the composition of the nanoparticles and a structural model of copper nanoparticles is developed.Silver nanoparticles were also utilized as "concentrators" via functionalizion with a dense layer of two-photon absorbing chromophores. Despite having a high concentration of dyes attached to metal surfaces, these composites exhibit a high fluorescence quantum yield, illustrating the first functional use of nanoparticles as tools for concentrating fluorophores in a small volume around a metal core. Moreover, these particles exhibit an exceedingly large two-photon absorption cross section. Subsequently, these nanoparticles were polyfunctionalized with hydrophilic ligands and biotin for application in biological imaging and sensing. Water solubility may successfully be induced, allowing for use of a hydrophobic fluorophore in an aqueous environment.In addition to discrete nanoparticles, the formation of fractal aggregates of metal nanoparticles is described. Enhancement of the two-photon fluorescence intensity of dye-labeled DNA located near these silver nanoparticle fractal clusters is shown, and hence a proof of concept for potential ultrasensitive detection of DNA is outlined.