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.