Treatment of aqueous zinc solutions with incremental additions of a ditopic fluorescent sensor of the Zinpyr family, based on pyridine/pyrazine-containing metal recognition units, affords a fluorescence titration curve with a sharp maximum at a sensor:Zn2+ ratio of 0.5 (Zhang, X-a.; Hayes, D.; Smith, S. J.; Friedle, S.; Lippard, S. J. J. Am. Chem. Soc. 2008, 130, 15788-15789). This fluorescence response enables the quantification of readily chelatable zinc in biological samples by a simple titration protocol. In the present work a new set of ditopic fluorescence zinc sensors functionalized with pyridine/pyrazine-containing metal chelating units is described, and through detailed studies the principles governing the characteristic OFF ON OFF fluorescence behavior and quantification capabilities of the family are delineated. Incorporation of carboxylate/ester groups in the 6 position of the fluorescein allows for control of the spatial distribution of the sensor for selective extra- or intracellular imaging of mobile zinc, without introducing significant changes in zinc-binding properties. A combination of spectrophotometric and potentiometric measurements provided a complete description of the H+- and Zn2+-binding properties of the compounds and their correlation with the observed fluorescence profile. The first zinc-binding event has an apparent affinity, K-1', of 1.9 x 10(9)-3.1 x 10(9) M-1, whereas for coordination of the second Zn2+ ion, responsible for fluorescence turn-on, the apparent formation constant, K-2', is 5.5 x 10(7)-6.9 x 10(7) M-1. A detailed chemical and mathematical analysis of the system demonstrated that the difference in emission efficiencies of the dimetalated (LZn2) vs monometalated (LZn) and metal-free (L) forms, a consequence of the combined quenching effects of the two metal-chelating units, significantly influences the shape of the titration curve. The scope of the titration method was investigated mathematically, and a lower boundary for the range of concentrations that can be determined was established as a function of the magnitude of K-2'. Our results suggest that the principles governing the response of the ZPP1 series are applicable to other analogues of the Zinpyr family. Moreover, they may guide the design of other ditopic sensors suitable for determining the concentrations of a wide range of mobile metal ions and other chemical signaling agents of relevance in biological systems.