Quantitative detection of Ag+ and cysteine using G-quadruplex-hemin DNAzymes

A highly sensitive and selective Ag+ detection method was developed based on the Ag+-mediated formation of G-quadruplex-hemin DNAzymes. In this method, two unlabelled oligonucleotides with different lengths are used. In the absence of Ag+, the two oligonucleotides hybridize to each other to form an intermolecular duplex. The addition of Ag+ can disrupt the intermolecular duplex and promote a part of the sequence of the longer oligonucleotide to fold into an intramolecular duplex, in which cytosine-cytosine (C-C) mismatches are stabilized by C-Ag+-C base pairs. As a result, the G-rich sequence of the same oligonucleotide can fold into a G-quadruplex, which is able to bind hemin to form a catalytically active G-quadruplex-hemin DNAzyme. This can be reflected by an absorbance increase when monitored in the H2O2-ABTS (2,2'-azinobis(3-ethylbenzothiozoline)-6-sulfonic acid) reaction system by using UV-vis absorption spectroscopy. This 'turn-on' process allows the detection of aqueous Ag+ at concentrations as low as 20 nM using a simple colorimetric technique. Considering that Cysteine (Cys) is a strong binder of Ag+, the presence of Cys may disrupt the C-Ag+-C base pairs in the intramolecular duplex, resulting in the reformation of the intermolecular duplex and the decrease of the catalytic activity of the sensing system. Therefore, the Ag+-sensing system can be further developed as a Cys-sensing system. This method allows the detection of Cys with a detection limit of 25 nM. With the development of the studies on DNA-metal base pairs, this Ag+-sensing method can be easily extended to the analysis of other metal ions.