mPEG-COOH is widely used in bioconjugation and drug delivery systems due to its ability to improve the solubility, stability, and biocompatibility of therapeutic agents. It is commonly employed to modify proteins, peptides, and nanoparticles, enhancing their pharmacokinetic properties and reducing immunogenicity. In drug delivery, it serves as a linker to attach drugs or targeting molecules, enabling controlled release and targeted therapy. Additionally, it is utilized in surface modification of biomaterials to create non-fouling surfaces, preventing unwanted protein adsorption and improving biocompatibility in medical devices. Its carboxyl group allows for easy functionalization, making it a versatile tool in biomedical research and applications.