Sensitive dual DNAzymes-based sensors designed by grafting self-blocked G-quadruplex DNAzymes to the substrates of metal ion-triggered DNA/RNA-cleaving DNAzymes.

A universal label-free metal ion sensor design strategy was developed on the basis of a metal ion-specific DNA/RNA-cleaving DNAzyme and a G-quadruplex DNAzyme. In this strategy, the substrate strand of the DNA/RNA-cleaving DNAzyme was designed as an intramolecular stem-loop structure, and a G-rich sequence was caged in the double-stranded stem and could not form catalytically active G-quadruplex DNAzyme. The metal ion-triggered cleavage of the substrate strand could result in the release of the G-rich sequence and subsequent formation of a catalytic G-quadruplex DNAzyme. The self-blocking mechanism of the G-quadruplex DNAzyme provided the sensing system with a low background signal. The signal amplifications of both the DNA/RNA-cleaving DNAzyme and the G-quadruplex DNAzyme provided the sensing system with a high level of sensitivity. This sensor design strategy can be used for metal ions with reported specific DNA/RNA-cleaving DNAzymes and extended for metal ions with unique properties. As examples, dual DNAzymes-based Cu(2+), Pb(2+) and Hg(2+) sensors were designed. These "turn-on" colorimetric sensors can simply detect Cu(2+), Pb(2+) and Hg(2+) with high levels of sensitivity and selectivity, with detection limits of 4 nM, 14 nM and 4 nM, respectively.