Superoxide anion((O2))serves as a precursor of other reactive oxygen species(ROS).Aberrant production or accumulation of (O2) is closely connected to many diseases.Therefore,imaging of (O2) fluctuations is of great concern,especially at the subcellular level.To address this issue,we herein have developed a new endoplasmic reticulum-targeted ratiometric fluorescent probe(ER-NAPC)for imaging of superoxide anion in live cells.Experimental data demonstrate that ER-NAPC can sensitively and selectively detect (O2) in endoplasmic reticulum.Moreover,the probe exhibits excellent photostability and is insensitive to pH changes.In confocal fluorescence imaging,ER-NAPC has been successfully applied to monitor (O2) in the condition of endoplasmic reticulum stress induced by different stimuli.The presented probe provides powerful opportunities for uncovering the chemical biology roles of (O2) in endoplasmic reticulum-associated disease.