A fluoran-based viscosity probe with high-performance for lysosome-targeted fluorescence imaging

A new fluorescent probe Lyso-Fl has been facilely prepared by an esterification reaction of spironolactone fluoran dye Rdi with ethanol, which shows viscosity-selective response by fluorescence. The new probe delivers obvious fluorescence signal enhancement when environmental viscosity changes from 1.01 cP (water) to 1256 cP (98% glycerol). And, both the emission intensity (575 nm) and fluorescence lifetime of Lyso-Fl exhibit individually good linear relationships with the solution viscosity. Besides, Lyso-Fl gives a selective response to viscosity among various biological species and exhibits pH-independent (1–10) fluorescent signals towards viscosity. More importantly, Lyso-Fl shows low cytotoxicity and can be utilized for monitoring of dexamethasone-stimulated viscosity enhancement by cell imaging with excellent lysosome-targeted performance, promoting it a promising fluorescent probe for lysosomal viscosity detection.