Fluorescent detection of silver-specific oligonucleotide

We report on a novel method for the determination of silver ion (Ag.sup.+) and cysteine (Cys) by using the probe SYBR Green I (SGI) and an Ag+-specific cytosine-rich oligonucleotide (C-DNA). The fluorescence of SGI is very weak in the absence or presence of randomly coiled C-DNA. If, however, C-DNA interacts with Ag.sup.+ through the formation of cytosine-Ag.sup.+-cytosine (C-Ag.sup.+-C) base pairs, the randomly coiled C-DNA undergoes a structural changes to form a hairpin-like structure, thereby increasing the fluorescence of SGI. This fluorescence turn-on process allows the detection of Ag.sup.+ in the 10-600 nM concentration range, with a detection limit of 4.3 nM. Upon the reaction of Ag.sup.+ with Cys, Cys specifically removes Ag.sup.+ from the C-Ag.sup.+-C base pairs and destroys the hairpin-like structure. This, in turn, results in a decrease in fluorescence intensity. This fluorescence turn-off process enables the determination of Cys in the 8-550 nM concentration range, with a detection limit of 4.5 nM. The method reported here for the determination of either Ag.sup.+ or Cys is simple, sensitive, and affordable, and may be applied to other detection systems if appropriately selected DNA sequences are available.