AF 647 carboxylic acid is a derivative of the near-infrared dye AF 647. It is frequently used as an alternative to the Cy5 dye. The excitation wavelength (λex) is 635 nm for conventional fluorescence detection and 620 nm for transient detection. AF 647 carboxylic acid contains a carboxyl group, allowing conjugation with amine groups on biomolecules for dye labeling.