mPEG-COOH is widely used in bioconjugation to modify proteins, peptides, and nanoparticles, enhancing their stability and solubility while reducing immunogenicity. Its carboxylic acid end group allows for easy coupling with amine-containing molecules using standard activation reagents like EDC/NHS, making it ideal for creating stable amide bonds. It is commonly employed in drug delivery systems to prolong circulation time of therapeutic agents. In diagnostics, mPEG-COOH is used to functionalize surfaces of sensors and imaging agents, improving biocompatibility and reducing non-specific binding. It also serves in tissue engineering and hydrogel formation by acting as a linker or surface modifier to control interactions between biomolecules and synthetic materials.