mPEG-COOH is widely used in bioconjugation and drug delivery systems due to its ability to improve solubility, stability, and biocompatibility of therapeutic agents. It is commonly employed to modify proteins, peptides, and nanoparticles, enhancing their circulation time in the body and reducing immunogenicity. In pharmaceutical formulations, it serves as a linker to attach drugs or targeting molecules, enabling controlled release and targeted delivery. Additionally, it is utilized in the development of hydrogels and biomaterials for tissue engineering, where it provides functional groups for cross-linking and modification. Its applications also extend to diagnostics, where it is used to label biomolecules for imaging and detection purposes.