Used in bioconjugation to link biomolecules such as peptides, proteins, or antibodies to surfaces or nanoparticles via thiol-reactive chemistry. The PEG spacer enhances solubility and reduces non-specific binding, while the phosphonate group (PO3H) enables strong binding to metal oxides or calcium-containing surfaces, making it useful in biosensors, diagnostic coatings, and targeted drug delivery systems. Also applied in surface functionalization of implants or medical devices to improve biocompatibility and stability in physiological environments.