Poly(ethylene glycol)2-mercaptoethyl ether acetic acid is a heterobifunctional PEG linker with a thiol (-SH) group and a carboxylic acid (-COOH) terminus, used as a hydrophilic crosslinker in bioconjugation and drug delivery systems. The thiol group enables covalent attachment via Michael addition to maleimide-functionalized molecules or binding to gold and other metal nanoparticles. The PEG spacer enhances water solubility, reduces non-specific interactions, and minimizes aggregation in biological environments. The carboxylic acid allows further conjugation to amines on proteins, peptides, or surfaces using standard coupling reagents like EDC/NHS. Commonly applied in PEGylated therapeutics, diagnostic probes, functionalized surfaces for biosensors, and targeted drug delivery. Also utilized in controlled release formulations requiring a biocompatible, potentially cleavable linker.