It is possible that a defined curvature at the membrane interface controls its pH/polarity to exhibit specific bioactivity. By utilizing an interface-interacting spiro-rhodamine pH probe and the Schiff base polarity probe, we have shown that the pH deviation from the bulk phase to the interface (ΔpH)/interfacial dielectric constant (κ(i)) for amphiphilic self-assemblies can be regulated by the curvature geometry (positive/negative) and its radius. According to H NMR and fluorescence anisotropy investigations, the probes selectively interact with an anionic interfacial Stern layer. The ΔpH/κ(i) values for the Stern layer are estimated by UV-vis absorption and fluorescence studies. For the anionic sodium bis-2-ethylhexyl-sulfosuccinate (AOT) inverted micellar (IM) negative interface, the highly restricted water and proton penetration into the Stern layer owing to tight surfactant packing or a reduced water-exposed headgroup area may be responsible for the much lower ΔpH ≈ -0.45 and κ(i) ≈ 28 in comparison to ∼-2.35 and ∼44, respectively, for the anionic sodium dodecyl sulfate (SDS) micellar positive interface with a close similar Stern layer. With increasing AOT IM water-pool radius (1.7-9.5 nm) or [water]/[AOT] ratio ( w) (8.0-43.0), the ΔpH and κ(i) increase maximally up to ∼-1.22 and ∼45, respectively, due to a greater water-exposed headgroup area. However, the unchanged ΔpH ≈ -0.65 and κ(i) ≈ 53.0 within radii ∼3.5-8.0 nm for the positive interface of a mixed Triton X-100 (TX-100)/SDS (4:1) micelle justify its packing flexibility. Interestingly, the continuously increasing ΔpH trend for IM up to its largest possible water-pool radius of ∼9.5 nm may rationalize the increase in ΔpH (∼-1.4 to -1.6) with the change in the curvature radii (∼15 to 50 nm) for sodium 1,2-dimyristoyl- sn-glycero-3-phosphorylglycerol (DMPG)/1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) (2:1) large unilamellar vesicles (LUV) owing to its negative interface. Whereas, similar to the micellar positive interface, the unchanged ΔpH at the positive LUV interface was confirmed by fluorescence microscopic studies with giant unilamellar vesicles of identical lipids composition. The present study offers a unique and simple method of monitoring the curvature-radius-dependent interfacial pH/polarity for biologically related membranes.

A newly synthesised Schiff base molecule (PMP) existing in equilibrium between non-ionic and zwitterionic forms displays solvent polarity induced ratiometric interconversion from one form to another, such novelty being useful to detect the medium polarity. The specific interface localisation of PMP in versatile amphiphilic self-assembled systems has been exploited to monitor their interfacial polarity by evaluating such interconversion equilibrium with simple UV-Vis spectroscopy. In spite of the large differences in pH and/or viscosity between the bulk and interface, the unchanged equilibrium between the two molecular forms on varying the medium pH or viscosity provides a huge advantage for the exclusive detection of interfacial polarity.

A simple pH-sensing method for cationic micelle and vesicle interfaces is introduced, utilizing a Schiff-base molecule, 2-((4H-1,2,4-triazol-4-ylimino)methyl)-6-(hydroxymethyl)-4-methylphenol (AH). AH containing a phenolic moiety was obtained by the reaction between 4-amino-4H-1,2,4-triazole containing polar O- and N-centres with opposite polarity to the cationic interface and 2-hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde. The acid/base equilibrium of AH was investigated at the interfaces of cetrimonium bromide (CTAB) micelles, tri-block-copolymeric micelles (TBPs) and large unilamellar vesicles (LUVs) of different lipid compositions using steady state UV-Vis absorption spectroscopy. AH interacted strongly with the micelle and vesicle interfaces, according to the binding studies with LUV. A larger amount of AH proton dissociation was observed when localized at the interface of micelles and vesicles compared to that in the bulk phase, indicating that the pH values at the cationic interfaces are higher than in the bulk phase. The pH values were about 2.2 and 1.6 units higher at the CTAB and TBP micelle interfaces, respectively, than the bulk pH. The pH variation decreased from 2.4 to 1.5 units by increasing the neutral 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid content from 0 to 50% in the cationic dimethyldioctadecylammonium (DDAB) LUV, indicating that the interfacial positive charges are responsible for the higher interfacial pH. Detailed structural and absorption characteristics of neutral AH and its anionic A(-) forms were investigated by fluorescence spectroscopic measurements and DFT based theoretical calculations. The present simple pH detection method may be applied to various biological micelle and vesicle interfaces.

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