The cell-impermeant ratiometric pH indicator SNARF™-4F 5-(and-6)-carboxylic acid (pK_a of ∼6.4) exhibits a significant pH-dependent emission shift from yellow-orange to deep red fluorescence under acidic and basic conditions, respectively. This pH dependence allows the ratio of the fluorescence intensities from the dye at two emission wavelengths - typically 580 nm and 640 nm - to be used for quantitative determinations of pH.

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