A chemiluminescent probe; used to detect superoxide production by enzymatic and cellular sources; produces similar chemiluminescence in isolated aortic and cardiac tissues from wild-type and Nox1-Nox2-Nox4 triple knockout mice, suggesting that it is not selective for NADPH-based ROS production; reacts with hydrogen peroxide without generating free radical intermediates; a fluorescent chloride-sensitive indicator, with its fluorescence being quenched by chloride (ex/em = 455/505 nm, respectively)