pH response of carboxy-terminated colorimetric polydiacetylene vesicles

Carboxy-terminated polydiacetylene vesicles are known to undergo dramatic color transitions in response to exposure to external stimuli such as pH, temperature, and receptor-ligand binding. FTIR spectroscopy was used to identify the breakdown in the interfacial hydrogen-bonding interactions of the carboxylic acid headgroups of polymerized 10,12-tricosadiynoic acid (TRCDA) vesicles in aqueous solution during pH chromic transition. The headgroup structure was monitored as the chromic transition takes place and the dissociation dependence of the pKa was determined. Due to the attenuated acidity of the interfacially confined carboxy groups, which exhibit pKa values in the range 9.5-9.9, it was found that the deprotonation-triggered blue-red chromic transition occurred in the pH range 9.0-10.1 and that the mechanism of the transition required interaction with the surface carboxyl group, which is of importance in the design of a biochromic mechanism using PDA assemblies. Transmission electron microscopy and FTIR spectroscopy revealed that the surface ionization and the pH-induced chromogenic transition was also accompanied by a dramatic vesicle-planar morphological transition alongside subtle changes to the alkyl chain conformation and packing. A two-step mechanism was implicated as causing the chromic transition that first involves surface deprotonation and then specific cation binding, which can aid the design of sensitive surface-ligand chemistry for new PDA structures.