Fluorescein-PEG3-(N-Boc)-amine is a heterobiofunctional PEG derivative containing a fluorescein dye and a Boc-protected amine group, which can be regenerated by mild acidic conditions and then be reactive with cabroxylic acid, activated NHS ester, carbonyl (ketone, aldehyde) etc. Fluorescein is a common fluorescent tracer (494nm/521nm) for staining cells, tissues, biomarkers or nanoparticles and this reagent can be used to tag biomolecules and nanoparticles.