Fast Red KL Salt is a synthetic dye with a wide range of applications in scientific research, including in the synthesis of nanoparticles and as a fluorescent probe for imaging. It is a water-soluble, red azo dye that is used in histological staining and immunohistochemistry to stain proteins and nucleic acids. Fast Red KL Salt works by penetrating cells and tissues. It binds to proteins and nucleic acids through electrostatic interactions and hydrogen bonding. The dye has a high affinity for certain amino acid residues, such as lysine and arginine. The binding results in a red color that can be visualized under a microscope.