Target-receptive structural switching of ssDNA as selective and sensitive biosensor for subsequent detection of toxic Pb

A structural switching in Single-stranded DNA (ssDNA) fluorescence biosensor for quick turn-on/off detection of Pb2+ ions and pesticide was reported. The design strategy of Hex-labelled ssDNA consists of two types of aptamer probe, G-rich base pair sequence forms G-quadruplex confirmation with Pb2+ ions. While other part of base pair sequence exhibits affinity to fold isocarbophos pesticide. MoS2 nanosheets were identified as quick quencher of Hex fluorescence intensity via Vander-Waals interaction and its significance was compared with other nanomaterials. This sensing mechanism proposes a specific affinity of GA-rich ssDNA with Pb2+ to form G-quadruplex via G-Pb2+-G sequences. Consequently, ssDNA relived from MoS2 nanosheets to restore the fluorescence intensity (turn-on). Subsequent addition of pesticide shows stronger affinity towards unfolded aptamer probe to form a random coil like structure. This causes Hex-labelled 5' end closer to the G-quadruplex connected at the 3' end of ssDNA resulting in a remarkable fluorescence quenching (turn-off) owing to PET process. Moreover, the sensing probe (Hex-labelled GA-rich ssDNA) was recycled by introducing acetylcholinesterase enzyme and thiocoline into the reaction mixture. The detection limits of Pb2+ and isocarbophos pesticide was estimated to be 0.6 nM and 0.018 μg/L respectively. Moreover, this study reveals a high sensitivity and selectivity towards target molecules in environmental samples.