Maleimide-NHS ester is a UV-Vis traceable, hetero bifunctional crosslinker with N-hydroxysuccinimide (NHS) ester and maleimide group that enable covalent conjugation of amine- and sulfhydryl- containing molecules. NHS esters acts with primary amines at pH 7-9 to form amide bonds, while maleimides react with sulfhydryl groups at pH 6.5-7.5 to form stable thioether bonds. It is a viscous pale yellow liquid that is difficult to weigh and dispense. NHS-ester reactive group is susceptible to hydrolysis so minimize the exposure.