Abstract: Provided is a compound having the structure: (SIG)-(SI-MOD)m where SIG is a signaling molecule, SI is a self-immolative structure bound to SIG such that SIG has a reduced signal relative to the signal of SIG without SI, MOD is a moiety bound to SI that is subject to modification by an activator, and m is an integer from 1 to about 10. When MOD is modified by an activator, SI is destabilized and self-cleaved from SIG such that SIG generates an increased signal. Also provided are methods of determining whether a sample, such as a cell, comprises an activator, such as a nitroreducase, using the compound. Further provided are methods of determining whether a mammalian cell is hypoxic using the compound where nitroreductase is the activator. A method of detecting a microorganism that comprises a nitroreductase using the compound where nitroreductase is the activator is also provided.