A colorimetric zinc(II) assay based on the use of hairpin DNAzyme recycling and a hemin/G-quadruplex lighted DNA nanoladder

A method is described for sensitive colorimetric determination of zinc(II) ions that uses (a) a Zn(II)-responsive hairpin DNAzyme that assists target recycling, (b) hybridization chain reaction, and (c) hemin/G-quadruplex DNA nanoladder. The Zn(II)-responsive split of the hairpin DNAzyme (HD) acts as the recognition and transformation probe. Upon addition of Zn (II) and enzyme strand, a duplex is formed in the loop region of hairpin. The caged initiator sequence is subsequently liberated from the HD by the Zn(II)-selective split of the substrate strand. This cleavage induces an enzyme strand recycling for the next round of cleavage. As a result, the initiator DNA is accumulated and cross-opened H1 and H2 to start a hybridization chain reaction (HCR). The caged G-quadruplex is released after the HCR to recruit hemin to form the hemin/G-quadruplex that is inserted into the DNA nanoladder. Once formed in the DNA nanoladder, these act as catalytic labels for the ABTS, resulting a green color change. This cascade amplification strategy allows 10 nM to 100 mu M of Zn(II) to be linearly quantified by colorimetry at 415 nm with a detection limit of 3.5 nM. The recoveries ranged from 97.7 to 108.3% were obtained, confirming high reliability of the method for Zn2+ analysis in lake water samples.