A Sensitive Fluorescence Biosensor for Silver Ions (Ag) Detection Based on C-Ag-C Structure and Exonuclease III-Assisted Dual-Recycling Amplification.

A C-Ag-C structure-based fluorescence biosensor with novel combination design of exonuclease III (Exo III) dual-recycling amplification is proposed for the application of silver ions (Ag) detection. Since oligo-1 involves C-C mismatches, the presence of Ag can be captured to form C-Ag-C base pairs, which results in a double-helix structure with a blunt terminus. The double-helix structure can be cleaved by EXO III to release short mononucleotide fragments (trigger DNA) and Ag. Released Ag can form new bindings with oligo-1, and other trigger DNA can be produced in the digestion cycles. Hybridization with the signal DNA (oligo-2) transforms a trigger DNA into double-stranded DNA with blunt terminus which can be cleaved by Exo III to reproduce the trigger DNA and form guanine- (G-) quadruplex DNA. The trigger DNA returns free to the solution and hybridizes with another signal DNA, which realizes the dual-recycling amplification. The G-quadruplex DNA can be reported by -methylmesoporphyrin IX (NMM), a specific G-quadruplex DNA fluorochrome. This method allows Ag to be determined in the 5 to 1500 pmol/L concentration range, with a 2 pmol/L detection limit, and it has been successfully applied to the detection of Ag in real samples.