Polymer-based coacervates as microenvironmental acid-base indicators.

Liquid-liquid phase separation (LLPS) is a ubiquitous physicochemical phenomenon that has garnered significant attention in the fields of biomaterials and soft matter science in recent years. Synthetic polymer systems, owing to their programmability, offer new design strategies for constructing smart responsive coacervate-based materials. In this work, sulfobetaine methacrylate (SB) and spiropyran (SP) monomers are selected as functional building blocks for coacervate formation. Electrostatic interaction-mediated SB polymer chains induce the formation of coacervate microdroplets with organelle-like structures. As a classic stimuli-responsive, acidochromic compound, SP endows the polymer material with the ability to sense and respond to external environmental changes. Notably, the coacervates formed by the PSB-co-PSP copolymer undergo rapid colour switching in response to pH variations and enable visual reporting of the acid-base microenvironment. In addition, the conformational transition from SP to merocyanine (MC) is accompanied by alterations in the internal micro-network of the coacervates, thereby allowing the regulation of coacervate properties. In summary, this work not only provides a model system for understanding molecular conformational changes within the coacervate microenvironment but also opens new avenues for the development of label-free visual microenvironmental acid-base indicators.