Fluorescein-PEG4-Acid is a xanthene dye with excitation/emmission maximum 494/517 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 DCC) to form a stable amide bond. It be used as quantum yield standard. Hydophilic PEG spacer increases solubility in aqueous media and reduces steric hindrance during binding.