A coumarin-based fluorescent probe for viscosity imaging: photophysical characterization and applications in living...

As an essential parameter of biological microenvironment, viscosity is closely associated with diffusion, transport and interaction of numerous biological molecules. Abnormal viscosity levels are recognized as a hallmark of various diseases and have emerged as the potential target for clinical diagnosis. Thus, developing an effective tool of monitoring viscosity dynamics plays a crucial role in the study of physiological and pathological processes.

Based on the classical coumarin scaffold, this study designed a series of readily synthesized fluorescent probes with different viscosity sensitivity by means of altering the length of alkyl chain and the size of molecular rotor. Compared with other probes, FXP-A3 with high fluorescence quantum yield exhibited a superior viscosity sensitivity, with a 120-fold fluorescence enhancement at 476 nm from PBS (pH = 7.2) to 90% glycerol. Delightfully, FXP-A3 also displayed favorable biocompatibility, excellent photostability, and lower cytotoxicity. Moreover, FXP-A3 was successfully used for bioimaging in living cells, demonstrating not only a rapid response within 5 min but also the ability to distinguish between cancer cells and normal cells. More significantly, a significant fluorescence enhancement was observed in Hela cells upon treatment with Nystatin or LPS. The results indicate that the molecule gains enhanced rotational freedom after the elongation of the alkyl chain, leading to a significant improvement in viscosity sensitivity.

The excellent detection performance endows FXP-A3 with the ability to track viscosity changes under pathological conditions, thus making it a promising imaging tool for disease diagnosis.