Single Nanoparticle Surface Enhanced Fluorescence

Abstract

It is well established that when in close proximity to gold nanoparticles the optical properties of local fluorescent molecules are dramatically altered. When the localised surface plasmon resonance (LSPR), tuned to the fluorophore absorption band is excited a strong optical enhancement is observed near the nanoparticle due to enhancement in the excitation rate. Both the radiative and non-radiative decay rates undergo significant modification, resulting in either quantum efficiency enhancement, or fluorophore quenching, and a corresponding reduction in the fluorescence lifetime. These effects depend on fluorophore and nanoparticle separation, the fluorophore quantum efficiency, and the alignment of fluorophore excitation and emission wavelength with the LSPR.

Fluorescence lifetime imaging microscopy (FLIM) is used to create high-resolution spatial maps of molecular lifetime and intensity values of single gold nanoparticles deposited on a thin fluorescent-doped polymer film, separated by a SiO spacer layer. A strong enhancement in emission intensity is observed in the region of a single nanoparticle. The fluorescence lifetime images are described well using two contributions to fluorescence decay; an unmodified term, allowing for the fact that the diffraction limited focus is significantly larger than the nanoparticle, and some average modified term, accounting for the reduction in fluorescence lifetime. Large numbers of nanoparticles are interrogated, giving a statistical distribution of intensity enhancement and lifetime reduction, associated with varying nanoparticle size and shape. These nanoparticle populations are measured for a variety of excitation wavelengths, LSPRs, and dyes, allowing analysis of the relationship between LSPR, fluorophore excitation and emission wavelength, and quantum efficiency.