A fluorescent biosensor for Cd2+ detection in water samples based on Cd2+-fueled wheel DNAzyme walker and its logic gate applications

A fluorescent biosensor was developed for Cd2+ detection based on a Cd2+-fueled wheel DNAzyme walker. Cd2+ can activate the wheel to roll along the DNA walking tracks through DNAzyme cleavage and toehold-mediated strand displacement. The substrate strand was modified with BHQ and Cy5. Through continuous cleavage reactions toward the substrate strands, a high fluorescence signal can be obtained. The biosensor is ultrasensitive, and the detection limit is 0.2 pM (S/N = 3). The fluorescent assay is robust and has been applied to the determination of Cd2+ in real water samples with good accuracy and reliability. Using Cd2+, Pb2+, and Hg2+ as the three inputs, we also construct a concatenated AND logic gate. The input combination of (111) can produce an output of 1. Other input combinations produce an output of 0. Our proposed detection platform and logic system hold great promise for the ultrasensitive and intelligent sensing of different heavy metal ions in water samples.