Nucleic acid G-quadruplex based label-free fluorescence turn-on potassium selective sensing

We report a label-free fluorescence turn-on approach for the selective sensing of potassium. A properly selected G-rich oligonucleotide (oligo-Y) folded into stable quadruplex structure when mixed with potassium in an aqueous solution. Single-stranded nucleic acid specific nuclease was subsequently added. Since an oligonucleotide in quadruplex structure is markedly more resistant to nuclease digestion than in its random coil conformation, oligo-Y digestion by nuclease was considerably slow. On the other hand, oligo-Y mixed with other common mono-or divalent ions was completely digested in 5 min under our experimental conditions because no quadruplex or less stable quadruplex was formed. Oligo-Y in potassium was subsequently mixed with a positively charged pyrene probe. Electrostatic interactions between oligo-Y (a polyanion) and the probe induced aggregation of the probe, which in turn induced strong pyrene excimer emission. The intensity of the induced excimer emission was directly proportional to the amount of potassium added. Our method shows good sensitivity, and good selectivity against other common interference ions.