Detection of Pb<sup>2+</sup> based on label-free fluorescence probe
A simple and sensitive detection method for lead ions was developed, based on the G-quadruplex and label-free fluorescence probe. A appropriate G-rich oligonucleotide folded into stable quadruplex structure when mixed with lead in an aqueous solution. Subsequently, single-stranded nucleic acid specific nuclease was added. An oligonucleotide in quadruplex structure is markedly resistant to nuclease digestion . However, G-rich oligonucleotide mixed with other common mono- or divalent ions was completely digested in 5 min under optimized experimental conditions because no quadruplex or less stable quadruplex was formed. Finally, a positively charged pyrene probe was mixed with oligonucleotide in lead. Electrostatic interactions between oligonucleotide (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 lead added. The detection limit of our method was 500 nM.