The environment sensitive NBD methylhydrazine can be reversibly coupled to aldehydes and ketones to form a Schiff base - which can be reduced to generate a stable amine derivative by sodium borohydride (NaBH4) or sodium cyanoborohydride (NaCNH3). Carboxylic acids of proteins and other water-soluble biopolymers can be coupled to this molecule in aqueous solution using water-soluble carbodiimides such as EDAC (E2247). NBD is a functional analog of the dinitrophenyl hapten, and its fluorescence is quenched upon binding to anti-dinitrophenyl (anti-DNP) antibodies.