A label-free fluorescent DNA sensor for the detection of lead ions (Pb 2+ ) based on lead(II)-stabilized G-quadruplex formation is proposed in this article. A guanine (G)-rich oligonucleotide, T30695, was used as a recognition probe, and a DNA intercalator, SYBR Green I (SG), was used as a signal reporter. In the absence of Pb 2+ , the SG intercalated with the single-stranded random-coil T30695 and emitted strong fluorescence. While in the presence of Pb 2+ , the random-coil T30695 would fold into a G-quadruplex structure and the SG could barely show weak fluorescence, and the fluorescence intensity was inversely proportional to the involving amount of Pb 2+ . Based on this, a selective lead ion sensor with a limit of detection of 3.79 ppb (parts per billion) and a detection range from 0 to 600 ppb was constructed. Because detection for real samples was also demonstrated to be reliable, this simple, low-cost, sensitive, and selective sensor holds good potential for Pb 2+ detection in real environmental samples. [ABSTRACT FROM AUTHOR]