Mal-amido-PEG9-NHS ester is a heterobifunctional crosslinker used in bioconjugation to link biomolecules such as proteins, peptides, and antibodies with other molecules or surfaces. The maleimide (Mal) group enables specific reaction with thiol (-SH) groups on cysteine residues, while the NHS ester group forms stable covalent bonds with primary amine-containing compounds. The PEG9 spacer enhances solubility, reduces steric hindrance, and minimizes non-specific interactions, thereby improving overall reaction efficiency. It is commonly applied in drug delivery systems to develop targeted therapies and in diagnostic assays for biomolecule labeling and detection purposes.