QSY 21 NHS, a dark quencher is an efficient energy transfer acceptor of the far red and NIR fluorescent probes. QSY 21 NHS works in the wavelength range of 540-750 nm, and is frequently used in FRET applications. QSY 21 NHS does not emit fluorescence in normal conditions. NHS esters can be used to label the primary amines (R-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules[1].