A simple interfacial pH detection method for cationic amphiphilic self-assemblies utilizing a Schiff-base molecule

Area of Science:

• Supramolecular Chemistry
• Physical Chemistry
• Analytical Chemistry

Background:

• Cationic interfaces of micelles and vesicles play vital roles in biological processes.
• Accurate pH determination at these interfaces is challenging but essential.
• Schiff-base molecules offer potential as responsive probes for interfacial environments.

Purpose of the Study:

• To develop and validate a simple Schiff-base molecule (AH) as a pH sensor for cationic micelle and vesicle interfaces.
• To investigate the acid/base equilibrium of AH at various cationic interfaces.
• To quantify the pH difference between interfacial and bulk phases.

Main Methods:

• Synthesis of the Schiff-base molecule 2-((4H-1,2,4-triazol-4-ylimino)methyl)-6-(hydroxymethyl)-4-methylphenol (AH).
• Steady-state UV-Vis absorption spectroscopy to study AH's acid/base equilibrium at interfaces.
• Binding studies with large unilamellar vesicles (LUVs) to confirm interfacial interaction.
• Fluorescence spectroscopy and DFT calculations for structural and absorption characterization.

Main Results:

• AH effectively senses pH at the interfaces of cetrimonium bromide (CTAB) micelles, tri-block-copolymeric micelles (TBPs), and dimethyldioctadecylammonium (DDAB) LUVs.
• Interfacial pH was significantly higher than bulk pH (e.g., ~2.2 units higher for CTAB micelles).
• Increased neutral lipid content in DDAB LUVs reduced the interfacial pH elevation, confirming the role of positive charges.

Conclusions:

• The Schiff-base molecule AH provides a straightforward method for detecting pH variations at cationic interfaces.
• Cationic interfaces exhibit elevated pH compared to the bulk solution.
• This pH-sensing approach holds promise for applications in studying biological micelle and vesicle systems.