Caffeic acid, a natural extract, as an activatable molecular probe for viscosity detection in a liquid system

Liquids, functioning as nutrients and energy systems, regulate various functions during storage programs. Microenvironmental viscosity is one of the most important physical parameters associated with the extent of deterioration, and it is crucial to monitor the mutation of viscosity at a molecular level. Herein, we utilized caffeic acid (CaC), a natural product extracted from thistles, as a molecular probe for viscosity sensing. CaC contains phenol hydroxyl (electron-donor) and carboxyl (electron-acceptor) groups, with both moieties connected by conjugated single and double bonds, forming a typical twisted intramolecular charge transfer system. The fluorescent probe CaC, obtained from a natural product without any chemical processing, exhibits high sensitivity (x = 0.43) toward viscosity, with an obvious visualized turn-on signal. Moreover, it displays good photostability, selectivity, and wide universality in commercial liquids. Utilizing CaC, we have successfully visualized viscosity enhancement during the spoilage process, with a positive correlation between the degree of liquid spoilage and microenvironmental viscosity. Thus, this study will provide a convenient and efficient molecular probe for food safety inspection across the boundaries of traditional biological applications.