8-Anilino-1-naphthalene Sulfonic Acid is a hydrophobic polarity sensitive fluorescent dye useful as a site probe to detect conformational changes in cell and micelle membranes and molecules such as proteins. ANSA is also used as a fluorescent probe for the estimation of the critical micelle concentration (CMC) of surfactants. It is a hydrophobic fluorescent probe for undenatured proteins on polyacrylamide gels.