IQ-R is a novel hypoxia-sensitive fluorescent probe that consists of an indolequinone unit coupled with a rhodol fluorophore.

IQ-R is activated and reduced by endogenous reductase preferentially under hypoxic conditions when administered to A549 cells, thereby visualizing hypoxic cancer cells by robust fluorescence. While the fluorescence of rhodol in the IQ-R conjugate is quenched by the function of the intramolecular indolequinone unit, it is restored under hypoxic conditions through the enzymatic one-electron reduction of IQ-R by NADPH: cytochrome P450 reductase to release the nonconjugated free rhodol. IQ-R has good solubility in water and longer wavelength for absorption and emission, which are favorable for cellular bioimaging.