Label-free DNA sensor for Pb2+ based on a duplex–quadruplex exchange

A simple label-free Pb2+ fluorescent aptasensor was proposed in this work. It is known that SYBR Green1 can embed into double-stranded DNA resulting in fluorescence enhancement, but it cannot bind to the G-quadruplex formed by Pb2+ and PS2.M. In the absence of Pb2+, SYBR Green1 intercalated into the double-stranded DNA hybridized between PS2.M and its complementary strand and the fluorescence was greatly enhanced. In the presence of Pb2+, a compact G-quadruplex structure was formed and the double-stranded structure was destroyed, releasing SYBR Green1 and causing the fluorescence intensity to significantly decrease. The detection limit of the sensor was as low as 3 nM. This proposed assay strategy was a low-cost detection method for Pb2+ with high sensitivity and good selectivity.