Zn2+ Binding-Enabled Excited State Intramolecular Proton Transfer: A Step toward New Near-Infrared Fluorescent Probes for Imaging Applications

In order to facilitate the in vivo study of zinc-related biology, it is essential to develop a zinc-selective sensor that exhibits both near-infrared (NIR) emission and larger Stokes shift. A fluorescent sensor, Zinhbo-5, has been constructed by using bis(benzoxazole) ligand with 2,2'-dipicolylamine (DPA) as metal ion receptor. In aqueous solution, Zinhbo-5 exhibits high sensitivity (K-d = 2.58 nM(2)) and selectivity for Zn2+ cation, revealing about 14-fold fluorescence enhancement upon zinc binding to give green emission. Remarkably, Zn2+ binding to Zinhbo-5 switches on the excited state intramolecular proton transfer (ESIPT), producing the desirable near-infrared region (over 710 nm) with large Stokes shift (ca. 240 nm). The new probe is demonstrated to be useful for in vivo imaging of the intracellular Zn2+ ion. The Zinhbo-5 is also useful for detecting zinc ion distribution during the development of living zebrafish embryos.