IRDye® 800CW is an NHS-ester and can be attached at the 5'-end of an oligonucleotide via conjugation to a primary amine. This infrared dye is made by LI-COR Biosciences and is more hydrophillic than any of the other infrared dyes available. Combined with excellent tissue penetration seen in the 800 nm wavelength sprectra, it is ideal for in vivo fluorescence imaging applications.