A facilely synthesized near-infrared fluorescent probe for selective lysosomal viscosity imaging.

Lysosomal viscosity is critical for cellular homeostasis, but real-time monitoring of its fluctuations remains challenging. We developed TCF-V-Lyso, a photostable and pH-tolerant fluorescent probe, via a concise one-step procedure for selective lysosomal viscosity imaging. The probe exhibits a 480-fold fluorescence turn-on enhancement at 755 nm upon viscosity increase, with good linearity, high selectivity, and low cytotoxicity. It successfully tracked viscosity changes induced by nystatin, monensin, and H2O2 in living cells, and also visualized elevated lysosomal viscosity in tumor-bearing mice, demonstrating its potential for investigating lysosome-associated pathology.