N-(Azido-PEG3)-N-Fluorescein-PEG3-acid contains azide, fluorescein and carboxylic acid moieties. The azide can react with alkynes in copper-catalyzed Click Chemistry. Fluorescein is a universal dye which is used for dye labeling. The carboxylic acid can react with primary amines in the presence of activators such as EDC and DCC. The hydrophilic PEG linker increases the compounds water solubility in aqueous media.