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.
Keywords: biosensor, EvaGreen, polycytosine oligonucleotide, SWNTs