G-quadruplex–hemin DNAzyme-amplified colorimetric detection of Ag+ ion

A G-quadruplex–hemin DNAzyme-amplified Ag+-sensing method was developed based on the ability of Ag+ to stabilize C–C mismatches by forming C–Ag+–C base pairs. In this method, only one unlabelled oligonucleotide strand was used. In the absence of Ag+, the oligonucleotide strand formed an intramolecular duplex. The G-rich sequence in the oligonucleotide was partially caged in this duplex structure and cannot fold into the G-quadruplex structure. The addition of Ag+ promoted the formation of another intramolecular duplex in which C–C mismatches were stabilized by C–Ag+–C base pairs, leading to the release of the G-rich sequence which can fold into a G-quadruplex capable to bind hemin to form a catalytically active G-quadruplex–hemin DNAzyme. As a result, a UV–vis absorbance increasing was observed in the H2O2–ABTS (2,2′-azinobis(3-ethylbenzothiozoline)-6-sulfonic acid) reaction system. This “turn-on” process allowed the detection of aqueous Ag+ at concentrations as low as 6.3 nM using a simple colorimetric technique, showing a high selectivity over a range of other metal ions.