(A) fluorescent detection of silver ion based on the target-catalyzed hairpin assembly

Abstract

In this study, we have developed a novel fluorescent strategy to detect silver ion ($Ag^{+}$) by utilizing $Ag^{+}$-mediated formation of C-C mismatched base pairs. In this system, the single stranded (ss) trigger DNA is designed to hybridize with the first hairpin probe through the formation of C-C mismatched base pairs only in the presence of $Ag^{+}$, consequently opening the hairpin probe. The exposed ss region within the first hairpin probe subsequently opens the second hairpin probe, forming complex and displacing the $Ag^{+}$ and ss trigger DNA from the first hairpin probe. Released $Ag^{+}$ and ss trigger DNA are recycled to initiate the next cycle of hairpin assembly and produce many copies of short double stranded (ds) DNA complexes, which would be resistant to the adsorption and quenching by graphene oxide (GO) and consequently produce highly enhanced fluorescence signal from ds specific fluorescent dye, SYBR green I. On the other hand, in the absence of $Ag^{+}$, the ss trigger DNA cannot promote the hairpin assembly and consequently leads to the low fluorescence signal, which is attributed to the adsorption of DNA probes and SYBR Green I and consequent quenching of fluorescence signal by GO. Importantly, GO contributes to the reduction of background signal from intact or intercalated SYBR green I, which enable the highly sensitive detection of $Ag^{+}$. The present detection system exhibits high sensitivity and selectivity toward Ag+ as expected. In addition, present strategy has a potential for applications of other metal ion biosensors by rational design of the probe sequences.