H-FluNox is a selective fluorescent probe designed to detect labile heme by triggering fluorescence through a biomimetic deoxygenation reaction with nitrogen oxides. Its selectivity for labile heme is over 100 times higher than for Fe(II), enabling it to distinguish labile heme from the unstable Fe(II) pool in live cells. H-FluNox is sensitive enough to detect subcellular labile heme. (λex=490 nm, λem=535 nm)