Unveiling Migration Dynamics of Acid Molecules in Microenvironments via Fluorescence Signal: A Case Study in Polymer Emulsion Film Formation

Ionic dynamics in microenvironments profoundly impact
entire systems;
however, their complexity and subtlety hinder real-time monitoring
and correlation with macroscopic behaviors. Herein, with polymer emulsions
as a case study, we demonstrate the feasibility of fluorescence-based
techniques for tracking intricate migration dynamics of acid molecules
in microenvironments. Specifically, an acid-sensitive fluorescent
probe with a strong donor–acceptor effect, namely, TPE-DMA-MN,
is developed. Protonation and deprotonation by hydrochloric acid molecules
tune the charge transfer extent in probe molecules, resulting in a
distinct fluorescence shift from near-infrared to visible yellow light.
By integrating this acid-sensitive chromophore into a cationic polyurethane
dispersion, we achieve real-time observation of acid migration caused
by electric double-layer disruption and acid evaporation with a fluorescence
wavelength shift exceeding 200 nm, visible to the naked eye. By correlating
dynamic ionic processes in microenvironments with the macroscopic
film formation process, we develop a simple yet efficient platform
for digitally monitoring the drying and postcuring processes of polymer
emulsions with high contrast and precision. This work provides a novel
and versatile approach for real-time tracking of intricate ionic dynamics
with broad potential applications in life and material sciences.