Widely-used fluorescent indicator for estimating intracellular pH. This membrane-permeant compound is modified by intracellular esterases to produce BCECF, a polar fluorescein derivative that is retained by cells. Intracellular BCECF is stable and has an efflux half-life of greater than two hours. Intracellular pH measurements are made by ratio imaging, which involves determining the pH-dependent ratio of emission intensity (detected at 535nm) when the dye is excited at 490nm vs. the emission intensity when excited at 440nm. This approach is amenable to either spectrofluorometry or flow cytometry. BCECF-AM ester can also be used to investigate intracellular changes in other ions, including potassium. Spectral properties: lambdaex 490nm; lambdaem 535nm; lambdaex=505nm; lambdaem=520nm in DMSO.