Analytical Sciences

Versatile Gap Mode Plasmon under ATR Geometry towards Single Molecule Raman, Laser Trapping and Photocatalytic Reactions

Masayuki FUTAMATA, Keitaro AKAI, Chiaki IIDA, and Natsumi AKIBA

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura, Saitama 338-8570, Japan

We have investigated various aspects of a gap mode plasmon to establish it as an analytical tool. First, markedly large (10⁷ – 10⁹) enhancement factors for the Raman scattering intensity from a thiophenol (TP) monolayer sandwiched by Ag films on a prism and silver nanoparticles (AgNPs) were obtained under attenuated total reflection (ATR) geometry. Second, AgNPs with a radius of ∼20 nm were optically trapped and immobilized on TP-covered Ag films under a gap mode resonance with extremely weak laser power density of ∼1 μW/μm² at 532 nm. The observed optical trapping and immobilization were theoretically rationalized using a dipole-dipole coupling and van der Waals interaction between AgNPs and Ag films. Third, p-alkyl TP molecules such as p-methyl TP, p-ethyl TP, p-isopropyl TP, and p-tertiary butyl TP were photocatalytically oxidized into p-carboxyl TP, whereas o- and m-methyl TP did not show such reactions.

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