A reactivity-based fluorescent probe 1 for Cu²⁺ was synthesized. The structure of this probe was characterized by infrared spectroscopy (IR), mass spectrometry (MS), X-ray crystallography, and ¹H NMR and ¹³C NMR spectroscopy. Its photophysical as well as binding properties towards various metal ions were studied. Probe 1 showed highly sensitive and selective “Off-On” fluorescence changes with Cu²⁺ among various metal ions when excited at 350 nm. These selective changes were attributed to an oxidative cyclization reaction transforming nonemissive azoanilines into highly fluorescent benzotriazoles. Furthermore, fluorescence imaging experiments of Cu²⁺ ions in living cells demonstrated its value of practical applications in biological systems.