Fluorescein-PEG5-Acid is a xanthene dye with excitation/emission maximum of 494/517 nm and a free terminal carboxyl acid. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond. It be used as quantum yield standard. Hydrophilic PEG spacer increases solubility in aqueous media and reduces steric hindrance during binding.