Carboxy-PEG4-sulfonic acid is widely used in bioconjugation and surface modification due to its water solubility and reactive functional groups. Its carboxylic acid group allows for covalent attachment to biomolecules like proteins, peptides, or antibodies, making it valuable in drug delivery systems and targeted therapies. The sulfonic acid group enhances hydrophilicity and stability, making it suitable for applications in coatings, hydrogels, and materials requiring improved biocompatibility. It is also employed in the development of biosensors and diagnostic tools, where its ability to modify surfaces and interact with biological molecules is crucial. Additionally, it serves as a linker or spacer in the synthesis of complex molecules, facilitating controlled release or improved pharmacokinetics in pharmaceutical formulations.