Used in bioconjugation and surface modification, Carboxy-PEG4-phosphonic acid serves as a heterobifunctional linker molecule. The carboxylic acid group allows covalent attachment to biomolecules like proteins or peptides via amide bond formation, while the phosphonic acid group enables strong binding to metal oxides. This makes it valuable for creating stable coatings on surfaces or nanoparticles for medical devices or biosensors. The PEG4 spacer enhances solubility, reduces steric hindrance, and minimizes non-specific interactions, improving efficiency in drug delivery systems. It is also employed in nanotechnology for functionalizing materials to enhance biocompatibility and targeting capabilities.