A selective and sensitive fluorescence probe for imaging endogenous zinc in living cells

A carboxamidoquinoline-based fluorescent Zn2+ probe 1 containing N/S/S heteroatoms as a receptor was designed and readily synthesized, which was featured by the Zn2+-induced red-shift of emission (45 nm) based on internal charge transfer (ICT) in an aqueous HEPES buffer (pH=7.4). Moreover, spectroscopic studies indicated that the composition of the complex 1-Zn2+ was 1:1, which was also confirmed by X-ray crystallography. In addition, 1H NMR titration experiment suggested that probe 1 transformed from the amide tautomer to imidic acid tautomer after binding with Zn2+. The binding of zinc with 1 was easily reversed by addition of N,N,N′,N′-tetrakis(2-picolyl)ethylenediamine (TPEN) or EDTA. Furthermore, probe 1 was successfully applied to image both exogenous and endogenous zinc in living HeLa cells.