SPDP-PEG24-NHS ester is a PEG derivative containing an SPDP crosslinker and an NHS ester. The SPDP crosslinker is reactive toward amine and thiol groups and contains a cleavable disulfide bond. This crosslinker is also membrane permeable which allows for intracellular reactions to take place. The NHS ester can react with primary amines to form stable amide bonds. The hydrophilic PEG linker increases the water solubility of compounds in aqueous media.