Lateral diffusion dynamics for single molecules of fluorescent cyanine dye at the free and surfactant-modifted dodecane-water interface

The present study proposed a single molecule probing of transport properties of the nanoregion of liquid-liquid interfaces. Fluorescence from single cyanine dye molecules (DiI) adsorbed at a dodecanewater interface was detected in the absence and presence of surfactants by total internal reflection fluorescence microscopy with a single photon counting devise. Intermittent photon bundles from single DiI molecules were observed in time-resolved photon counting measurements, when the average number of interfacial DiI molecules was less than I in the observation area (830 run in diameter). Photon signals emitted by the same DiI molecule in the observation area were discriminated with the time interval between two photon signals. From the analyses of the photon bundles, the following properties of the interfacial region were obtained: (1) the lateral diffusion coefficient of single DiI molecules from the maximum duration of the photon bundle, (2) the interfacial viscosity from the diffusion coefficient of the single DiI molecules, and (3) the fluorescence quantum yield of single DiI molecules from the density of the photon bundles. The adsorption of anionic or zwitterionic surfactant at the interface reduced the lateral diffusion coefficient of single DiI molecules by an increase in the interfacial viscosity.