QSY 21 NHS is an effective energy transfer acceptor for far-red and near-infrared fluorescent probes. This quencher operates within the wavelength range of 540-750 nm, typically utilized in FRET applications. QSY 21 NHS does not emit fluorescence under normal conditions. The NHS ester form of QSY 21 allows for the labeling of proteins, amine-modified oligonucleotides, and other primary amine-containing molecules (R-NH2).