An Off–On–Off Fluorescent Sensor for pH Windows Based on the 13aneN4–Zn2+System

The new ligand L was prepared and features a 13-membered tetraaza macrocyclic ring with a 1,8-naphthalimide fluorophore appended to a C atom of its backbone. The protonation constants of L as well as its complexation constants with Zn2+ ions were determined in 1:1 water/methanol solutions by potentiometric titrations. Fluorimetric pH titrations were performed with L alone and L in the presence of Zn2+ ions (1:1), and the species distributions (%) versus pH were compared. A window-shaped fluorescence trend was observed with pH for the L/Zn2+ system, which behaves as an off-on-off pH sensor. The on window is centred in the 6.5-7.5 pH range, in correspondence with the formation of [ZnL](2+). The ligand L also works as a sensor for Zn2+ ions in this pH window, which includes the physiological pH value (7.4). Accordingly, L was used as a probe for the visualization of the Zn2+ pools in Saccharomyces cerevisiae yeast cells at pH 7.4 and has the additional ability to distinguish between living and dead cells.