Thiol-C9-PEG4-acid is widely used in bioconjugation and surface modification due to its reactive thiol group and hydrophilic PEG spacer. It is commonly employed to functionalize gold nanoparticles, enabling stable and biocompatible coatings for applications in drug delivery, biosensing, and diagnostics. The thiol group facilitates strong binding to gold surfaces, while the PEG4 spacer enhances solubility and reduces non-specific interactions. Additionally, the terminal carboxylic acid group allows for further conjugation with biomolecules such as proteins, peptides, or antibodies, making it valuable in creating targeted therapeutic or diagnostic systems. Its properties also make it suitable for use in hydrogel formation and as a linker in bioconjugate chemistry for developing advanced biomedical materials.