Copper is a key metal ion both in environment monitoring and in biology, and exposure to high concentration of copper can cause adverse health effects. Although significant progresses have been made in designing fluorescent sensors for diamagnetic metal ions, few effective Cu2+ sensors are known because of the paramagnetic nature of the metal ion. We herein report a highly sensitive and selective “turn-on” fluorescent Cu2+ sensor based on an in vitro selected DNAzyme (also known as catalytic DNA or deoxyribozyme). The substrate strand of the DNAzyme was labeled with a fluorophore on the 3‘-end and a quencher on the 5‘-end, and the enzyme strand was labeled with a second quencher on the 5‘-end. Initially, the fluorescence was quenched. Addition of Cu2+ induced oxidative cleavage of the substrate, and the fluorescence intensity increased by 13-fold. The sensor has a detection limit of 35 nM and a dynamic range up to 20 μM. The sensor selectivity is more than 2000-fold for Cu2+ over Fe2+ and UO22+ and more t...