Bis-(N,N'-carboxyl-PEG4)-Cy5 is a PEG linker containing a cyanine dye with excitation/emmission maximum 649/667 nm and a free terminal carboxyl acid. Terminal carboxylic acid can react with primary amine groups in the presence of of activators (e.g. EDC, or HATU) to form a stable amide bond. The hydophilic PEG spacer increases solubility in aqueous media.
