Single-molecule lactonization of octadecylrhodamine B at a liquid-liquid interface.

The total internal reflection laser fluorescence microscope method was used to observe the lateral diffusion of a single octadecylrhodamine B (C18RB) molecule at the toluene–water interface. The interfacial diffusion constant of single fluorescent cation C18RB+ was obtained from the maximum residence time in a small observation area with pH 3, the maximum residence time was remarkably shortened, indicating that single fluorescent zwitterion C18RB± rapidly converted to the nonfluorescent lactone at the interface. The lactonization rate was completed within 0.13 ms at the toluene–water interface but slowed to 67 ms at an interface saturated with dimyristoylphosphatidylcholine.