Several classic examples of fluorescent viability and cytotoxicity assays are known [58] . These methods rely on an impermeant DNA intercalating dye which labels cells with damaged membranes and a permeable dye that is sequestered inside viable cells after esterase cleavage of a labeling group. Variations of these traditional chemistries are rarely employed where total fluorescence from assay wells is measured, because of poor sensitivity from simple stoichiometric dye binding events in the non-viable cell population and requisite washes required for removal of unbound dye and serum-derived esterases. Stable, non-viable cell esterase activity is particularly problematic for assay utility as it greatly impacts assay background values and constricts signal windows.