Size-dependent two-photon excitation photoluminescence enhancement in coupled noble-metal nanoparticles.

Plasmon coupling is known to enhance the two-photon excitation photoluminescence of metal nanoparticles significantly. Here, Au and Ag nanospheres of different sizes were prepared to systematically investigate the effects of particle size on plasmon coupling enhanced two-photon excitation photoluminescence. An oppositely charged polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA), was used to induce the coupling of Au and Ag nanospheres. The two-photon excitation photoluminescence enhancement factor was found to first increase and then decrease with the increasing particle size for both Au and Ag nanospheres. Optimum enhancement factors of 25-fold and 14-fold were obtained for coupled 55-nm Au nanospheres and 50-nm Ag nanospheres, respectively. The coupled Au and Ag nanospheres displayed two-photon action cross sections of up to 9 × 10(4) GM per particle (where 1 GM = 10(-50) cm(4) s/photon). Similar to Ag nanoparticles, Au nanoparticles also displayed large coupling induced enhancement of two-photon excitation photoluminescence. Considering their excellent biocompatibility, high inertness, and easy preparation, Au nanoparticles are expected to find many new applications in two-photon biosensing and bioimaging.