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.