The first generation of
ZnAF probes, ZnAF-1 and ZnAF-2, featured low quantum
yields in the absence of Zn2+ (0.022 and 0.023, respectively)
and high turn-on responses to Zn2+ (17-fold and 51-fold
increases in fluorescence intensity, respectively) and a pKa value
of 6.2 for the phenolic hydroxyl group of the fluorophore.
Although the original probes were not able to cross the cell
membrane, diacetyl derivatives masked the negative charge on