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.