Quinolinium-based viscosity probes for lysosome imaging and tracing lysosomal viscosity changes in living cells

Lysosomal viscosity fluctuation is related to various diseases such as diabetes, neurodegenerative diseases, and cancer. We designed and synthesized four novel pH-insensitive fluorescent viscosity probes (Lyso-QAP1–4) using a quinolinium as the fluorophore and acceptor group while an internal amino benzene ring was incorporated as a donor group and a targeting group for monitoring the viscosity change of lysosomes. Lyso-QAP1–4 possessed a highly sensitive response toward viscosity change with red emission at around 625–640 nm (excited around 570–580 nm), excellent water solubility, good photostability, favorable membrane permeabilization, and moderate cytotoxicity. Colocalization study proved that Lyso-QAP1–4 probes exhibited fast lysosomal detection (within 15 min) without influencing effect from other microenvironments like pH, polarity, and interferent species but Lyso-QAP4 (without a phenyl ring on C2 of quinolinium) also localized in nucleus. Importantly, we demonstrated that a phenyl ring on C2 of quinolinium and an internal amino benzene played an important role in the lysosome specificity. Lyso-QAP1–4 probes can be applied for intracellular viscosity detection. Moreover, Lyso-QAP3 was successfully applied to living cell imaging for cellular and lysosomal viscosity changes. These results suggest that Lyso-QAP3 would provide new opportunities for biomedical diagnosis and imaging applications.