Lucigenin is a commercially available aromatic bisacridinium compound, which has the appearance of an orange powder and forms a yellow-green solution when dissolved in water. The maximum of the main absorption band of lucigenin is observed at 368 nm (Ɛ = 34200 M -1 cm -1 ), and minor bands are present at 352 nm, 430 nm, and 455 nm (Figure ). Lucigenin exhibits blue-green fluorescence in solution. The emission spectrum has a broad band of which the maximum is located around 505 nm (Figure ). The shape of the emission spectrum does not change when using different excitation wavelengths. It is noteworthy that the fluorescence intensity of lucigenin is temperature dependent, being higher at the lower temperatures (see Section 3.5 and Figure ). Lucigenin was reported by Decker and Dunant in 1906 and was first investigated for its chemiluminescence. The chemiluminescent properties of lucigenin are currently used for the detection of various biologically relevant substances and the detection of superoxide. Applications of lucigenin as chloride-sensitive probe emerged originally in the study of anion transport in cells and later for the optical sensing of chloride. The fluorescence of lucigenin is quenched in presence of chloride, bromide, and iodide, and several other anions. The quenching is very rapid and faster than other processes taking place during transport experiments.