Used primarily as a fluorescent probe to detect superoxide radicals in biological systems. It penetrates cells and, upon reaction with superoxide, is oxidized to ethidium, which intercalates into DNA and emits strong red fluorescence. This property makes it valuable in cellular and mitochondrial oxidative stress studies. Commonly applied in flow cytometry, fluorescence microscopy, and live-cell imaging to assess reactive oxygen species production in various cell types, including vascular cells, neurons, and cancer cells.