Plasmonic enhancement using core-shell nanoparticles

Abstract

In this paper, we report on a strategy, which produces enhancement of fluorescence using the so-called plasmonic effect whereby the presence of adjacent metallic nanoparticles can dramatically alter the fluorescence emission and absorption properties of a fluorophore. The effect, which is a result of the surface plasmon resonance of the metal surface, can lead to increases in quantum efficiency, radiative decay rates and photostability of the fluorophore, and depends very sensitively on parameters such as geometry of the nanoparticles, nanoparticle-fluorophore separation and fluorophore type. The work is aimed at improving the efficiency of optical biochips. Key benefits from this enhancement include lower limits of detection, reduced reagent requirements and better resolution. This study is part of a comprehensive investigation of plasmonic enhancement using a range of metal nanoparticle (NP) fabrication techniques and a range of measurement configurations. The focus here is on the fabrication of chemically prepared silver-gold alloy spherical NP with a variable thickness silica shell on the surface of which is immobilised a layer of fluorescent dye molecules. The variable thickness shell serves to control the dye-NP separation, which plays a key role in the enhancement mechanism. Transmission electron microscopy (TEM) was used to characterise the NP. The dye used here was the ruthenium polypyridyl complex [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline)], abbreviated to [Ru(dpp)₃]²⁺. This paper reports the tuning of the NP plasmon resonance via NP size and alloy composition. The wavelength of the plasmon peak as a function of NP size and composition correlated very well with theoretical predictions based on the Mie scattering theory. Preliminary fluorescence enhancement measurements on this system yielded an enhancement factor of approximately 5.