ZnAF-1F and ZnAF-2F do not fluoresce in the presence of other biologically important cations such as Ca2+ and Mg2+, and are insensitive to change of pH. The complexes with Zn2+ of previously developed ZnAFs, ZnAF-1, and ZnAF-2 decrease in fluorescence intensity below pH 7.0 owing to protonation of the phenolic hydroxyl group of fluorescein, whose pKa value is 6.2. On the other hand, the Zn2+ complexes of ZnAF-1F and ZnAF-2F emit stable fluorescence around neutral and slightly acidic conditions because the pKa values are shifted to 4.9 by substitution of electron-withdrawing fluorine at the ortho position of the phenolic hydroxyl group. Spectral data: lambdaex 492nm; lambdaem 517nm in PBS.