Mal-PEG-COOH is widely used in bioconjugation and drug delivery systems due to its dual functional groups. The maleimide (Mal) group selectively reacts with thiol groups (-SH) in proteins, peptides, or antibodies, enabling site-specific conjugation. This is especially valuable in the development of antibody-drug conjugates (ADCs), where precise attachment of cytotoxic agents improves therapeutic efficacy and reduces side effects. The carboxylic acid (COOH) terminus allows for further coupling to amine-containing molecules via EDC/NHS chemistry, offering versatility in linking payloads, fluorescent labels, or targeting moieties. The PEG spacer enhances solubility, reduces aggregation, and increases the circulation time of conjugated drugs by minimizing immune recognition. It is also employed in surface modification of nanoparticles, hydrogels, and biosensors, where controlled immobilization of biomolecules is required. The PEG chain provides a hydrophilic linker that helps maintain bioactivity and reduces non-specific binding in diagnostic applications. Overall, Mal-PEG-COOH is a key reagent in pharmaceutical research, targeted therapy, and the engineering of bioactive materials.