Synergistic effects of multiple rotors and hydrogen-bond interactions lead to sensitive near-infrared viscosity probes for live-cell microscopy

Changes in cellular viscosity are associated with various physiological processes and pathological conditions. To study these cellular processes and functions, highly sensitive fluorescent probes that detect subtle changes in viscosity are urgently needed but remain lacking. In this study, we present a series of viscosity-responsive near-infrared (NIR) fluorescent probes based on styrene-coated boron dipyrromethene (BODIPY). The probe modified with dimethylaminostyrene and piperazine at the two terminals of the BODIPY scaffold showed extremely high viscosity sensitivity values (x, around 1.54), with excellent performance for detecting viscosity below 20 cP. This outstanding property is attributed to the synergistic effects of multiple rotatable bonds and hydrogen-bond interactions. Additionally, this probe has been successfully deployed to monitor viscosity changes in various cellular compartments (i.e., cytoplasm) and processes (such as during autophagy). This work provides a rational molecular design strategy to construct fluorescent probes with high viscosity sensitivity for exploring cell functions.