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

Abstract

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 × 10⁻⁸ to 120 × 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.