Zinquin, Free Acid is a Zn2+ specific fluorophore that forms zinquin-Zn2+ complexes in either 1:1 or 2:1 ratios (Kd of 370 ± 60 nM and 850 ± 160 nM, respectively). Intracellular zinc mobilization has been shown to be an early indicator of apoptosis. As a zinc probe, it is UV-excitable and emits in the blue region of the spectrum.