Used in bioconjugation to link biomolecules such as peptides, proteins, or antibodies to surfaces or nanoparticles. The thiol group (HS) reacts selectively with maleimide, gold surfaces, or disulfide compounds, enabling stable covalent attachment. The PEG spacer improves solubility, reduces aggregation, and minimizes non-specific binding in biological environments. The terminal phosphonate group (PO3H) binds strongly to metal oxides, hydroxyapatite, or calcium-containing surfaces, making it useful in bone-targeting drug delivery, dental materials, or implant coatings. Commonly applied in biosensors, targeted therapeutics, and surface functionalization of medical devices.