Combined use of two membrane-potential-sensitive dyes for determination of the Galvani potential difference across a biomimetic oil/water interface

Determination of the Electrostatic Potential of Oil-in-Water Emulsion Droplets by Combined Use of Two Membrane Potential-Sensitive Dyes

The fluorescence behaviors of potential-sensitive dyes including anionic DiBAC₄(3) (denoted by dye A), DiSBAC₂(3) (dye B), and zwitterionic di-4-ANEPPS (dye C) were studied in oil-in-water (O/W) emulsions. In this study, the equilibrium Galvani potential difference (Δ_O^Wφ_eq) of the O/W-emulsion droplets was controlled by changing the ratio of the concentrations of electrolytes added to the O (=1,2-dichloroethane) and W phases. When using an adequate combination of the dyes, i.e., B and C, we could observe that the ratio of their fluorescence peak intensities was changed from 1.08 to 1.38, depending on the change of (Δ_O^Wφ_eq from 26 to 73 mV. It is desirable to apply this method to study the potential-dependent ion or electron-transfer reactions occurring at vesicles or liposomes, and also to biomembranes.

Potential-Induced Aggregation of Anionic Porphyrins at Liquid|Liquid Interfaces

The adsorption and self-aggregation of anionic porphyrins were studied at the polarized water|1,2-dichloroethane (DCE) interface by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin diacid (H₄TPPS^2-) and protoporphyrin IX (H₂PP^2-) exhibited high surface activities at the interface. The selective excitation of interfacial species in PM-TIRF measurements elucidated the potential-induced aggregation mechanism of the porphyrins. The J-aggregates of H₄TPPS^2- were reversibly formed only at the water|DCE interface by applying appropriate potentials even when the porphyrins exist as monomers in the aqueous and organic solutions. In the H₂PP^2- system, the slow aggregation process was found in the negative potential region. The spectral characteristics and the signal phase of PM-TIRF indicated that the H₂PP^2- monomers were adsorbed with relatively standing orientation and that the long axis of the J-aggregates was nearly in plane of the interface. H₂PP^2- was also investigated at the biomimetic phospholipid-adsorbed water|DCE interface. The competitive adsorption of neutral glycerophospholipids effectively inhibited the potential-dependent adsorption and interfacial aggregation processes of H₂PP^2-. The results demonstrated that the aggregation state of the charged species can reversibly be controlled at liquid|liquid interfaces as a function of externally applied potential.