A highly selective and sensitive OFF-ON fluorescent sensor 1, employing the PET mechanism, was designed and synthesized. It could be used to detect Cd2+ ion in aqueous solution and to image Cd2+ ion in living cells. The fluorescence intensity significantly enhanced about 195-fold and the quantum yield increased almost 100-fold. Moreover the fluorescence intensity of 1 increased linearly with high sensitivity (0-1 mu M) toward Cd2+.