Folate-PEG-Mal (Folic acid-PEG-maleimide) is one of the heterobifunctional folic acid-PEG derivatives. When the pH is 6.5-7.5, the maleimide group can react rapidly with the sulfhydryl group. The PEG moiety provides water solubility, biocompatibility and flexibility. Folic acid-functionalized substrates have been used for targeted drug delivery, imaging, and bioassay development.