A water-soluble probe with p-hydroxybenzyl quaternary ammonium linker for selective imaging in senescent cells

A water-soluble probe with p-hydroxybenzyl quaternary ammonium linker, FR-2a, for selective imaging in senescent cells is reported. Probe FR-2a integrated water-soluble fluorophore (HT-4a) and β-galactosidase (β-gal) trigger into one entity by a p-hydroxybenzyl quaternary ammonium linker. HT-4a is a styryl-based push–pull benzothiazole fluorophore with attractive properties, including excellent water-solubility, intense fluorescence emission and a large Stokes shift (161 nm), characterized by an intramolecular charge transfer (ICT) excited state. The formation of quaternary ammonium deactivated the ICT state, resulting in fluorescence quenching of FR-2a. In the presence of β-gal, the glycosidic bond was hydrolyzed and fluorophore HT-4a was released through self-immolative process, resulting in effective fluorescence recovery. FR-2a shows high affinity to β-gal (K_m = 1.33 μM), exhibiting good sensitivity, selectivity and stability for imaging in senescent cells.