HOOC-PEG-SH (MW 40000) [HOOC-PEG-Thiol (MW 40000)] is a thiol PEG derivative featuring a terminal carboxyl group. The carboxyl group can react with amine or hydroxyl groups to form stable amide bonds or less stable ester bonds. This reactivity allows the conversion of amine or hydroxyl groups into free thiols linked via a linear PEG chain. The resulting thiol group is useful for modifying the surface of gold nanoparticles or participating in other PEGylation reactions. The PEG linker between the thiol and carboxyl groups provides excellent water solubility, flexible linker distance, and increased stability.