In this paper, we propose a fluorescent biosensor for the sequential detection of Pb2+ ions and the cancer drug epirubicin (Epn) using the interactions between label-free guanine-rich ssDNA (LFGr-ssDNA), acridine orange (AO), and a metal–phenolic nanomaterial.

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Reliable, label-free, and ultraselective detection of Pb2+ and Ag+ ions is of paramount importance for toxicology assessment, human health, and environmental protection. Herein, we present a novel recyclable fluorometric aptasensor based on the Pb2+ and Ag+-induced structural change of the GC-rich ssDNA (guanine cytosine-rich single-strand DNA) and the differences in the fluorescence emission of acridine orange (AO) from random coil to highly stable G-quadruplex for the detection of Pb2+ and Ag+ ions. More interestingly, the construction and principle of the aptasensor explore that the GC-rich ssDNA and AO can be strongly adsorbed on the CaSnO3@PDANS surface through the π–π stacking, hydrogen-bonding, and metal coordination interactions, which exhibit high fluorescence quenching and robust holding of the GC-rich ssDNA. However, in the presence of Pb2+, the specific G-rich ssDNA segment could form a stable G-quadruplex via G4–Pb2+ coordination and capture of AO from the CaSnO3@PDANS surface resulting in fluorescence recovery (70% enhancement). The subsequent addition of Ag+ ion induces coupled cytosine base pairs in another segment of ssDNA to get folded into a duplex structure together with the G-quadruplex, which highly stabilizes the G-quadruplex resulting in the maximum recovery of AO emission (99% enhancement). When the Cys@Fe3O4Nps are added to the above solution, the sensing probe was restored by complexation between the Cys in the Cys@Fe3O4Nps and target metal ions, resulting in the fabrication of a highly sensitive recyclable Pb2+ and Ag+ assay with detection limits of 0.4 and 0.1 nM, respectively. Remarkably, the Cys@Fe3O4Nps can also be reused after washing with EDTA. The utility of the proposed approach has great potential for detecting the Pb2+ and Ag+ ions in environmental samples with interfering contaminants.

In this work, we have proposed a novel DNAzyme/MnCoPBAs-PDANCs complex-based fluorescence biosensor for subsequent detection of Pb 2+ and Ag + ions. The GR-5/GC-rich DNAzymes are strongly anchored or quenched on the surface of polydopamine hybridized 3D metal–organic framework MnCoPBAs-PDANCs by π–π stacking interaction. Addition of Pb 2+ ions has exhibited a catalytic inner cleavage of DNAzyme complex and disturbs to release shorter GC-rich sequence over the surface of MnCoPBAs-PDANCs complexes. Later on, addition of intercalating dye ThT interacts with free GC-rich substrate strand to form a G-quadruplex-ThT structure and thereby effectively enhanced the fluorescence intensity (“turn-on”). Interestingly, subsequent addition of Ag + ions has an uncoiled GQ-ThT structure to provide a robust double-stranded DNA featuring C–Ag + –C, which diminishes (“turn-off”) the fluorescence intensity. This improved hybrid sensor exhibited a linear response in a concentration range of 3–9 nM for Pb 2+ , while 4–20 nM for Ag + ions with a lower detection limit of 1.6 and 4.2 nM, respectively. Further, the method was successfully implemented for the analysis of Pb 2+ and Ag + ions in real water samples with a good regaining and high efficacy for practical analysis.

In this study, we have designed a three-fluorophore-labeled Y-shaped DNAzyme with a high catalytic cleavage activity and a three-dimensional (3D) MOF-MoS 2 NB (metal−organic framework fused with molybdenum disulfide nanobox), which was synthesized as an efficient quencher of the fluorescent biosensor. The synthesized porous 3D MOF-MoS 2 NBs and Y-shaped DNAzyme exhibited a good analytical response toward the simultaneous multiple detections of Hg 2+ , Ni 2+ , and Ag + ions over the other coexisting metal ions. More specifically, the three kinds of enzyme aptamer and substrate aptamer (SA) were hybridized and annealed to form the Y-shaped DNAzyme structure and labeled with three different fluorophores such as FAM, TAMRA, and ROX over the 3′-end of SA. When the targets were induced, the DNAzyme was triggered to cleave the fluorophore-labeled SAs. Then, the cleaved SAs (FAM-SA, TAMRA-SA, and ROX-SA) were adsorbed on the 3D MOF-MoS 2 NB surface to quench the fluorescence signal due to a noncovalent interaction (van der Waals and π−π stacking interaction), which transmuted the fluorescence on-state to off-state. As a result, the fluorescence assay confiscated the high selectivity and sensitivity for the target analytes of Hg 2+ , Ni 2+ , and Ag + ions achieved for the detection limits of 0.11 nM, 7.8 μM, and 0.25 nM, respectively. Accordingly, the sensitivity of the developed sensor was explored with a better lower detection limit than the previously reported biosensors. The utility of the designed Y-shaped DNAzyme may find a broad field of application in real water sample analysis with interfering contaminants.