Metallic nanoparticles (NPs) are exploited for their ability to interact with organic compounds and to increase significantly the fluorescence intensity and the photostability of many fluorescent dye molecules. Metal enhanced fluorescence (MEF) is therefore widely investigated for biosensing applications. When used in immunoassays, silver island films (SIFs) could augment the fluorescence intensity of fluorescein by a factor of seventeen; SIFs were also able to double or triple the emission intensity of cyanine dyes which are commonly used in (deoxyribonucleic acid) DNA microarrays. The emission intensity of indocyanine green — widely used as a contrast agent in medical imaging — was about twenty times higher in the proximity of SIFs.

This enhancement phenomenon — due to the surface plasmon polaritons associated with the metallic NPs — can be explained by energy transfer from the metal NPs to the fluorescent dye molecules or by a modified local electromagnetic field experienced by the fluorophores in the vicinity of metal surfaces.

Our research focused on the optical characterization of colloidal gold-silver alloy NPs containing different ratios of gold and silver (Au_1.00-Ag_0.00, Au_0.75-Ag_0.25, Au_0.50-Ag_0.50, and Au_0.25-Ag_0.75), as well as their interaction with three fluorophores: rose bengal, rhodamine B, and fluorescein sodium. Depending upon the dye quantum yield and its concentration in solution, enhancement or quenching of fluorescence was obtained. Thus, a three to five times increase in fluorescence intensity was observed in a 2.0 mM solution of rose bengal with all nanoalloys, a slight enhancement of fluorescence (1.2 – 1.6 times) was noticed in a 0.13 mM solution of rhodamine B with all four types of NPs, and fluorescence quenching occurred in all the fluorescein-NP solutions regardless of the dye concentration.