A label-free biosensor for selective detection of DNA and Pb2+ based on a G-quadruplex

A highly sensitive and selective label-free biosensor was designed for DNA determination based on target-induced opening of the hairpin DNA and forming a G-quadruplex with protoporphyrin IX (PPIX) as a signal reporter. In the absence of target-DNA (T-DNA), the probe-DNA (P-DNA) forms a hairpin structure. A G-rich sequence in the strand is partially caged in the stem-loop structure and cannot fold into a G-quadruplex. In the presence of T-DNA, the specific combination of targets with the sensing sequence (loop sequence) triggers the release of the G-rich sequence and allows it to fold properly and bind with PPIX resulting in the fluorescence of the G-quadruplex–PPIX providing the readout signal for the sensing event (detection limit of 3.5 nM). Based on the higher efficiency of Pb2+ at stabilizing G-quadruplexes than K+ and the Pb2+-stabilized G-quadruplexes not binding to PPIX, which resulted in fluorescence decrease, the proposed DNA-sensing system can be further exploited as a Pb2+-sensing method. The detection limit for Pb2+ is 2.6 nM.