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 phosphonic acid group enables stable attachment to metal oxide surfaces like titanium dioxide or iron oxide. Commonly applied in biosensors, diagnostic devices, and targeted drug delivery systems where controlled orientation and stability of biomolecules are required. Also used in surface modification of implants to improve biocompatibility and reduce fouling.