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 H(2)O(2)-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.