The commercially available fluorescent probe, 1,8-diamino naphthalene with a naphthalene ring as a chromophore and two amino groups as binding sites has been developed for sensitive detection of copper ions by fluorescence spectrometer in aqueous solution. It displayed an excellent selectivity and sensitivity for Cu 2+ over other metal ions. The mechanism is that the fluorescence of probe was quenched by the paramagnetic effect from spin–orbit coupling of the Cu 2+ after probe coordinated with Cu 2+ , and mechanism was proved by ESI-MS. The probe can be used to react to Cu 2+ in live cells, providing a potentially powerful approach for probing Cu 2+ chemistry in biological system.