Enhancing tumor lysosomal viscosity imaging by modifying the meso-substituents in BODIPY dyes

Viscosity-sensitive fluorescent probes are receiving considerable attention in the field of bioimaging research, primarily due to their significant potential for monitoring dynamic viscosity changes within organelles such as lysosomes. These innovative probes are crucial for the biophysical properties of the cellular microenvironment and their implications in complex biological processes and disease pathogenesis. Here, we have designed four viscosity-responsive BODIPY-based fluorescent probes that incorporate meso-carboxylate functioning as fluorescent rotors. These probes exhibit exceptional sensitivity to viscosity, both in solution and within tumor cells. We systematically verified that the fluorescence intensity of the probes can be effectively modulated by altering the steric hindrance effects of the substituents at the meso position. Furthermore, it is noteworthy that the probes ^HB-Me and ^HB-Bn are capable of spontaneously forming J-aggregates in solution, which is associated with a redshift in UV absorption. Intriguingly, ^HB-Bn and ^EtB-Bn demonstrated exceptional lysosomal targeting capabilities and the ability to differentiate between tumor and normal cells, presumably due to the viscosity variations inherent to these cell types. This discovery expands the use of viscosity-sensitive fluorescent probes and introduces new possibilities for early tumor diagnosis and precision therapy research.