Metal-enhanced fluorescence of gold nanoclusters adsorbed onto Ag@SiO2 core–shell nanoparticles

Abstract

The static and time-resolved metal-enhanced fluorescence (MEF) of Au₂₅-adsorbed Ag@SiO₂ core–shell nanoparticles (NPs) has been studied systematically with the variation of shell thicknesses, core sizes, and excitation wavelengths. The emission of Au₂₅-adsorbed Ag@SiO₂ NPs is blue-shifted and highly enhanced compared with that of free Au₂₅ clusters. The blue shift arises from the plasmonic coupling of Au₂₅ clusters with Ag NPs, which decreases exponentially with the increase of separation between Au₂₅ clusters and Ag NPs. The photoluminescence (PL) intensity of Au₂₅-adsorbed Ag@SiO₂ NPs is higher by a factor of 7.4 than that of free Au₂₅ clusters. The PL enhancement of Au₂₅-adsorbed Ag@SiO₂ NPs is determined by two competing processes of near-field enhancement and fluorescence resonance energy transfer (FRET). The increase of the radiative decay rate constant with separation is identical to that of PL enhancement, suggesting that the MEF of Au₂₅-adsorbed Ag@SiO₂ NPs arises from the increase of the radiative decay rate constant induced by the near-field enhancement of plasmonic Ag NPs. Au₂₅-adsorbed Ag@SiO₂ NPs have also been found to be a highly sensitive and selective ‘turn-off’ sensor for Cu²⁺ ions.