Acridine orange is a cell-permeable, nucleic acid-selective fluorescent cationic dye useful for cell cycle determination and detection of cellular autophagy. [1][2][3] It exhibits excitation/emission spectra of 502/525, 460/650, and 475/590 nm when bound to dsDNA, bound to ssDNA or RNA, and under acidic conditions, respectively. [1] [2] The ratio of acridine orange emission at 590 to emission at 525 nm can be used to quantify the increase in the number of acidic vesicular organelles observed during cellular autophagy.

Reference:
[1]. Virant-Klun, I., Tomazevic, T., and Meden-Vrtovec, H. Sperm single-stranded DNA, detected by acridine orange staining, reduces fertilization and quality of ICSI-derived embryos. J.Assist.Reprod.Genet. 19(7), 319-328 (2002).
[2]. Han, J., and Burgess, K. Fluorescent indicators for intracellular pH. Chem. Revs. 110(5), 2709-2728 (2010).
[3]. Thomé, M.P., Filippi-Chiela, E.C., Villodre, E.S., et al. Ratiometric analysis of Acridine Orange staining in the study of acidic organelles and autophagy. J. Cell Sci. 129(24), 4622-4632 (2016).