A turn-on fluorescent probe to detect hypoxia; transformed into a highly fluorescent molecule in the presence of nitroreductase, a marker of hypoxia; selective for nitroreductase over a variety of bioanalytes, including glutathione, ascorbic acid, and H2O2; has been used to detect hypoxia in A549 human lung carcinoma cells at 1 μM; ex/em = 454/520 nm, respectively