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Highly sensitive electrochemical detection of mercuric ions based on sequential nucleic acid amplification and guanine nanowire formation

The selective and sensitive detection of mercuric ions (Hg2+) is of great significance in environmental protection and disease prevention. By integrating target induced strand displacement amplification (SDA) and metal ion-dependent DNAzyme cyclic amplification to grow guanine nanowires (G-nanowires) as the signal output component, we demonstrate here a label-free and sensitive strategy for Hg2+ detection. The target induced SDA can amplify a large amount of Mg2+-dependent enzymatic sequences, which can be further hybridized with a hairpin-structured substrate that contains a G-quadruplex sequence to form Mg2+-dependent DNAzymes. In the presence of Mg2+, the Mg2+-dependent DNAzymes exhibit high catalytic activity and excellent cleavage specificity toward the substrate sequences, releasing the enzymatic sequences to trigger another cleavage cycle and leaving the G-quadruplex fragment on the electrode surface for the formation of G-quadruplex/hemin repetitive units. With the enzyme/DNAzyme for signal amplification and the G-quadruplex/hemin for signal generation, the electrochemical assay for Hg2+ detection shows excellent sensitivity for a concentration variation from 0.2 pM to 100 nM with a detection limit of 0.097 pM and is successfully used in the direct detection of real water samples with high sensitivity and selectivity.