Two copper(II) complexes containing dansylated ligands were investigated as turn-on fluorescence-based nitric oxide (NO) sensors. Upon addition of NO (g), the quenched fluorescence of both complexes was restored in both organic and buffered aqueous solutions, which is caused by the formation of a diamagnetic Cu(I) species and protonation of the sulfonamide functionality of the ligands. The NO detection limit of these Cu(II) complexes is 10 nM.close767