A lysosome‐targeted ultra‐sensitive viscosity probe for monitoring viscosity alterations during chemotherapy

Lysosomal viscosity is a key biomarker of cancer progression and chemotherapy response, but real‐time, precise monitoring remains challenging. To address this challenge, we develop a novel chemosensing platform based on a molecular rotor architecture for specific, dynamic detection of lysosomal viscosity. The platform operates via an “off‐on” switching mechanism: in low‐viscosity environments, rapid rotor rotation through the twisted intramolecular charge transfer effect quenches fluorescence; in high‐viscosity conditions, restricted rotation triggers strong emission, enabling an ultra‐sensitive and selective response. Using systematic molecular engineering and screening within this platform, the probe PMA‐H is identified as the optimal candidate, demonstrating a remarkable 187‐fold fluorescence enhancement in response to viscosity (from 0.54 to 1410 cP), excellent environmental stability with minimal interference from pH, polarity, or biomolecules, and precise lysosomal targeting. Subsequently, PMA‐H is employed to track lysosomes in HeLa cells, and it reveals alterations in lysosomal viscosity, morphology, and abundance during apoptosis, ferroptosis, cuproptosis, and zinc‐induced cell death. In general, this platform allows real‐time tracking of lysosomal viscosity fluctuations induced by various chemotherapeutic agents, highlighting its significant potential as a powerful tool for early cancer diagnostics and fundamental lysosomal research.