LysoSensor Yellow/Blue dextran is a ratiometric probe that can be used to measure the pH of acidic organelles, such as lysosomes. This dextran conjugate is taken up by the cells through endocytosis. The LysoSensor dye produces blue fluorescence in neutral environments, but changes to yellow fluorescence in more acidic environments. The pKa of LysoSensor Yellow/Blue dextran is ∼3.9.