Ultra-Bright Metal-Fluorophore Aggregates by Metal-Enhanced Fluorescence of Dye-Doped Silica Nano Particles

Abstract

Enhancement of molecular fluorescence is of great interest due to the widespread popularity of fluorescence-based detection techniques available today. Although fluorescence-based detection is considered to be more sensitive than other optical approaches, there is still an intense need for more photo stable, high quantum yield fluorophores. In this regard, metal-enhanced fluorescence (MEF) has opened novel pathways for the development of brighter, fluorescent markers with enhanced stability. Surface plasmon resonance of metal nanoparticles can modify the radiative properties of nearby fluorophores in ways not seen in classical fluorescence. Trapping fluorophores between coupled plasmons can further enhance the near-field interactions leading to even larger enhancements. The aim of this research was to a develop a solution based approach to study MEF by aggregation of fluorophore tethered gold nanoparticles, which would ultimately lead to the development of ultrabright fluorescent probes. Here, we report a simple method for aggregating multiple gold nanoparticles (GNPs) on Rhodamine B (RhB)-doped silica nanoparticles (SiNPs) utilizing dithiocarbamate (DTC) chemistry to produce MEF in solution. Dye was covalently incorporated into the growing silica framework via co-condensation of a 3-aminopropyltriethoxysilane (APTES) coupled RhB precursor using the Stober method. Electron microscopy imaging revealed that these mainly non-spherical particles were relatively large (80 nm on average) and not well defined. Spherical core-shell particles were prepared by physisorbing a layer of RhB around a small spherical silica particle (13 nm) before condensing an outer layer of silica onto the surface. The core-shell method produced nanospheres (~30 nm) that were well defined and monodispersed. Both dye-doped SiNPs were functionalized with pendant amines that readily reacted with carbon disulfide (CS2) under basic conditions to produce DTC ligands that have exhibited a high affinity for gold surfaces. GNPs were produced via citrate reduction method and the resulting 13 nm gold nanospheres were then recoated with an ether-terminated alkanethiol to provide stability in ethanol. Fluorescent enhancement was observed when excess GNPs were added to DTC coated dye-doped SiNPs to form nanoparticle aggregates. Optimization of this system gave a fluorescence brightness enhancement of over 200 fold. Samples that gave fluorescence enhancement were characterized through Transmission Emission Micrograph (TEM) to reveal a pattern of multiple aggregation of GNPs on the dyedoped SiNPs. This study was performed in University o f Oklahoma, USA, and the results were included in a dissertation with two published papers for a PhD degree with the University o f Oklahoma and defended the dissertation on 2nd o f May 2012.