Single Molecule Probing at the Interface

A demonstration of the single molecule detection at the liquid-liquid interface was reported for the fluorescent probe of l,l/-dioctadecyl-3,3,3/,. 3 -tetramethyI i ndoearboeyan i ne (Dil), which is a monovalent cation with two Cig alkyl chains. Thus, it has high adsorptivity at the dodecane-water interface. 

Aqueous phase (2.7 mm3) was placed in the thin lower compartment of the microcell and the Dil dodecane solution (63 mm3) was added on top of the aqueous layer. Fluorescence of the interfacial Dil was observed in the range of 571-575 nm. The influence of two kinds of surfactants, sodium dodecyl sulfate (SDS) and dimyristoyl phosphatidylcholine (DMPC), on the lateral diffusion dynamics of single molecules at the interface was investigated. DMPC was dissolved in chloroform, and the solution was mixed with pure diethyl ether at a ratio of 1 19 (chloroform diethyl ether) by volume. Pure water was placed in the lower container, and the DMPC solution was subsequently (5 mm3) spread carefully on the water. After evaporation of chloroform and diethyl ether, the Dil dodecane solution was added on the DMPC layer. Since Dil has a high 

The lateral diffusion coefficient (Dj) of Dil was calculated using the equation  

The radius (r) and the diffusion coefficient (D) of a spherical molecule are related to the viscosity (rj) of the medium surrounding the molecule by the Ein-stein-Stokes equation  

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