Zinc(II)-selective ratiometric fluorescent sensors based on inhibition of excited-state intramolecular proton transfer

To develop a zinc(II)-selective emission ratiometric probe suitable for biological applications, we explored the cation-induced inhibition of excited-state intramolecular proton transfer (ESIPT) with a series of 2-(2'-benzene-sulfonamidophenyl)benzimidazole derivatives. In the absence of Zn-II at neutral pH, the fluorophores undergo ESIPT to yield a highly Stokes' shifted emission from the proton-transfer tautomer. Coordination of Zn-II inhibits the ESIPT process and yields a significant hypsochromic shift of the fluorescence emission maximum. Whereas the paramagnetic metal cations Cu-II, Fe-II, Ni-II, Co-II, and Mn-II result in fluorescence quenching, the emission response is not altered by millimolar concentrations of Ca-II or Mg-II, rendering the sensors selective for Zn-II among all biologically important metal cations. Due to the modular architecture of the fluorophore, the Zn-II binding affinity can be readily tuned by implementing simple structural modifications. The synthesized probes are suitable to gauge free Zn-II, concentrations in the micromolar to picomolar range under physiological conditions.