DDAO is a promising near-infrared (NIR) red fluorescent probe with a long emission wavelength (λem = 656 nm) and a tunable excitation wavelength (600-650 nm). It can be designed for detecting the activities of various enzymes such as β-galactosidase, sulfatase, protein phosphatase 2A, carboxylesterase 2, human albumin, and esterases.