Toluidine Blue O (Toluidine Blue) is a thiazine dye of the quinone-imine family and is cationic in nature. Toluidine Blue O along with light-emitting diode has been used as a photosensitizer to control in vitro biofilms and to kill gram-negative bacteria. Toluidine Blue has wide applications in vital staining in living tissues and used as a special stain.