Fluorescent pH Indicator. Spectral Changes of 4-Methylumbelliferone

Abstract

4-Methylumbelliferone is a useful acid-base fluorescent indicator with a midpoint of its transition interval at pH 7.6. Both anionic and neutral forms fluoresce in the blue with high quantum yield (70%). The indicator properties are based on a blue shift of the absorption spectrum in acid. Depending on the excitation wavelength employed, one has the option of following either an increase or a decrease in fluorescence as the pH changes.