Aggregation-Induced Plasmon Coupling-Enhanced One- and Two-Photon Excitation Fluorescence by Silver Nanoparticles

Plasmon coupling-induced intense local electrical field in the gap of closely packed metal nanoparticles (NPs) has been known capable of significantly enhancing optical properties of chromophores. Here, we have investigated aggregation-induced plasmon coupling-enhanced one-photon excitation (1PE) and two-photon excitation (2PE) fluorescence of dyes using Ag NPs of three different sizes (20, 36, and 48 nm). The fluorescence of a model dye, Rhodamine B isothiocyanate (RiTC), was prequenched by attaching to Ag NPs and subsequently enhanced upon forming aggregates of Ag NPs. It was found that aggregates of larger sized Ag NPs gave larger 1PE and 2PE fluorescence enhancement on the basis of free dyes, while aggregates of smaller counterparts displayed larger enhancement on the basis of the corresponding prequenched ones. 1PE and 2PE fluorescence were enhanced by 2.5- and 10.2-fold by aggregated 48 nm Ag NPs compared to free dyes and by 8.0- and 22.5-fold by aggregated 20 nm Ag NPs compared to the quenched ones, respectively. This scheme achieved fluorescence enhancement significantly beyond the level of fluorescence recovery, much larger than conventional turn-on fluorescence probes, which is attractive for developing sensitive fluorescence turn-on-based detection with reduced background.