Surface-Enhanced Phosphorescence Measurement by an Optically Trapped Colloidal Ag Nanoaggregate on Anionic Thiacarbocyanine H-Aggregate

Yasutaka Kitahama, Masato Kashihara, Tamitake Itoh, and Yukihiro Ozaki
A citrate-reduced Ag nanoaggregate was optically trapped on a fiber-shaped H-aggregate of an anionic thiacarbocyanine dye against Coulomb repulsion by focusing a near-infrared (NIR) laser beam. As the NIR laser power increased, namely, as the Ag nanoaggregate approaches the H-aggregate, phosphorescence from the H-aggregate with the Ag nanoaggregate excited moderately at 514 and 647 nm was strengthened, although that at 568 nm was weakened. By excitation at 568 nm, which was close to a surface plasmon resonance peak of the Ag nanoaggregate, surface-plasmon-enhanced optical trapping potential well might have deepened, and then the Ag nanoaggregate might have approached the H-aggregate too closely to enhance the phosphorescence because of energy transfer to the metal. As the excitation laser intensity increased, namely, as the surface-plasmon-enhanced optical trapping potential well was deepened, the phosphorescence enhancement factor trended upward and then downward by enhancement due to plasmon at a close distance from the Ag surface and the energy transfer at the closer distance, respectively.
DOI