Solvent-dependent selective fluorescence sensing of Al³⁺ and Zn²⁺ using a single Schiff base

The selective assay of Al^{3 +} and Zn^{2 +} ions is reported using fluorescence enhancement of a simple Schiff base receptor 1 (1 = 1-((E)-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-ylimino)methyl)naphthalen-2-ol) in aqueous solvents. The Schiff base receptor 1 showed a turn-on fluorescence in the presence of Al^{3 +} in a mixture of methanol-water, as a result of a restricted CN isomerization mechanism. The ion selectivity of 1 could be switched for Zn^{2 +} by swapping the solvent from methanol-water to DMF-water mixture. In particular, this chemosensor could clearly distinguish Zn^{2 +} from Cd^{2 +}. The binding properties of 1 with the metal ions were investigated by UV-vis, fluorescence, electrospray ionization mass spectroscopy and ¹H NMR titration.