A label-free fluorescent molecular switch for Cu2+ based on metal ion-triggered DNA-cleaving DNAzyme and DNA intercalator

A label-free fluorescent sensor for the detection of Cu2+ was constructed based on internal DNA cleavage and DNA intercalator SYBR Green I. The DNAzyme complex formed by the hybridization of the enzyme strand and substrate strand has two base-pairing regions including a DNA duplex and a DNA triplex. The copper ion-induced catalytic reaction makes the substrate strand irreversibly break at the cleavage site, which disturbs the formation of DNA strands. The weakly fluorescent dye SYBR Green I served as a fluorescence indicator that intercalated into the base-pairing regions of DNAzyme, resulting in a strong fluorescence emission. In the presence of Cu2+ in the test system, the complementary regions were reduced due to the fracture of the DNA strand. The fluorescence intensity was decreased because SYBR Green I was released from the cleaved regions. Other metal ions had no notable "on-off" phenomenon even at 50 times Cu2+ concentration. Under the optimal conditions, a linear response with Cu2+ concentration ranging from 4 x 10(-8) to 120 x 10(-8) M was obtained. The detection limit reached as low as 10 nM. Moreover, the cost-effective sensor was successfully applied to the determination of Cu2+ in drinking water.