Zn²⁺-triggered amide tautomerization produces a highly Zn ²⁺-selective, cell-permeable, and ratiometric fluorescent sensor

It is still a significant challenge to develop a Zn²⁺-selective fluorescent sensor with the ability to exclude the interference of some heavy and transition metal (HTM) ions such as Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, and Hg²⁺. Herein, we report a novel amide-containing receptor for Zn²⁺, combined with a naphthalimide fluorophore, termed ZTRS. The fluorescence, absorption detection, NMR, and IR studies indicated that ZTRS bound Zn²⁺ in an imidic acid tautomeric form of the amide/di-2-picolylamine receptor in aqueous solution, while most other HTM ions were bound to the sensor in an amide tautomeric form. Due to this differential binding mode, ZTRS showed excellent selectivity for Zn²⁺ over most competitive HTM ions with an enhanced fluorescence (22-fold) as well as a red-shift in emission from 483 to 514 nm. Interestingly, the ZTRS/Cd²⁺ complex showed an enhanced (21-fold) blue-shift in emission from 483 to 446 nm. Therefore, ZTRS discriminated in vitro and in vivo Zn²⁺ and Cd²⁺ with green and blue fluorescence, respectively. Due to the stronger affinity, Zn²⁺ could be ratiometrically detected in vitro and in vivo with a large emission wavelength shift from 446 to 514 nm via a Cd²⁺ displacement approach. ZTRS was also successfully used to image intracellular Zn²⁺ ions in the presence of iron ions. Finally, we applied ZTRS to detect zinc ions during the development of living zebrafish embryos.