Oligonucleotide-based fluorescence system for sensitive and selective detection of Ag(I) ions in aqueous solution.

In this letter, a new homogeneous assay with high sensitivity and selectivity was developed for the detection of silver ions in aqueous media at room temperature by using EvaGreen (EG), polycytosine oligonucleotide C₂₀ and single-walled carbon nanotubes (SWNTs). The fluorescence of EG is weak in the absence or presence of randomly coiled C₂₀ After C₂₀ interacted specifically with Ag(+) ions through C-Ag(+)-C bonding, C₂₀ would form a folded structure that preferentially binds to EG. As a result, the fluorescence of the mixture of C₂₀ and EG increased in the presence of Ag(+). As having a higher affinity with ssDNA and organic dye, SWNTs could easily form a complex with EG and C₂₀, which led to fluorescence quenching of the dye. Additionally, they can also effectively reduce background-signal fluorescence of the dye and greatly improve its specificity. Under the optimum conditions, the EG/C₂₀ probe exhibited a high selectivity towards Ag(+) over other metal ions. The detection limit was up to 7.4 nM (0.8 ppb). This method is low cost, label-free, and easy to use.