A compound comprising the structure: (SIG)-(SI-MOD)m. is provided in which 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 comprises an activator such as a nitroreductase, and methods of determining whether a mammalian cell is hypoxic using a compound of the formula (SIG)-(SI-MOD)m.