5-(and-6)-Carboxy SNARF-1 Maleimide is an excellent thiol-reactive building block that can be used for developing SNARF-1 pH probes. It can be readily conjugated to peptides, antibodies and other thiol-containing molecules such as thiol-modified oligos. SNARF-1 has pKa of ≤7.5, thus extremely useful for monitoring pH changes in the range of 6.5 and 8.5. It exhibits a significant pH-dependent emission shift from yellow orange to deep red fluorescence under acidic and basic conditions, respectively. The ratioable pH dependent fluorescence intensity change 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 in cells and other biological media.