N-(Mal-PEG4)-N-Fluorescein-PEG4-NHS ester is a dye-labeling reagent containing a fluorescein dye, maleimide and an activated NHS esters. The NHS esters is easily linked with nucleophiles such as primary amines, while the maleimide group can be used as a cysteine-specific covalent linker. Fluorescein is a near-UV/Vis spectrum dye that can be used for in vivo fluorescence assays such as FRET.