Emission enhancement and lifetime modification of phosphorescence on silver nanoparticle aggregates

Silver nanoparticle aggregates have been shown to support very large enhancements of fluorescence intensity from organic dye molecules coupled with an extreme reduction in observed fluorescence lifetimes. Here we show that for the same type of aggregates, similar enhancement factors (~75× in intensity and ~3400× in lifetime compared to the native radiative lifetime) are observed for a ruthenium-based phosphorescent dye (when taking into account the effect of charge and the excitation/emission wavelengths). Additionally, the inherently long native phosphorescence lifetimes practically enable more detailed analyses of the distribution of lifetimes (compared with the case with fluorescence decays). It was thus possible to unambiguously observe the deviation from mono-exponential decay which we attribute to emission from a distribution of fluorophores with different lifetimes, as we could expect from a random aggregation process. We believe that combining phosphorescent dyes with plasmonic structures, even down to the single dye level, will offer a convenient approach to better characterize plasmonic systems in detail.