Exceptional peroxidase-like activity of an iron and copper based organic framework nanosheet for consecutive colorimetric biosensing of glucose and kanamycin in real food samples.

Two-dimensional metal-organic framework nanosheets are attractive as peroxidase mimicking nanocatalysts due to their rich chemical functional groups, large surface area, high porosity, and accessible active sites. In this study, we synthesized FeCu bifunctional 2D MOF nanosheets using a solvothermal method. Fe and Cu ions were added as metal precursors, while organic amine and acid served as the organic ligands to construct the FeCu-MOF nanosheets. These nanosheets demonstrated robust peroxidase-like catalytic activities and were employed to develop a visual detection system for multiple targets, such as glucose and kanamycin. In the detection mechanism, glucose was oxidized into gluconic acid by glucose oxidase (GOx), leading to the generation of H2O2. When H2O2 is present, the FeCu-MOF NSs demonstrate high intrinsic peroxidase-like activity, which might catalytically oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into a blue-coloured oxTMB product with a strong UV absorption at 654 nm. Subsequently, kanamycin was added to the above sensing system. The kanamycin strongly interacted with the FeCu-MOF NSs through H-bonding and blocked electron transfer, resulting in a colour change of the solution from blue to colourless with a weak UV absorption at 654 nm. Under the optimal conditions, the proposed colorimetric sensor exhibits an excellent linear response to glucose and kanamycin over the 0.25-5 µM and 0.02-0.1 µM ranges, respectively. The proposed colorimetric assay detection limits for glucose and kanamycin were found to be as low as 0.1 µM and 8 nM, respectively, and such a sensor shows excellent selectivity and sensitivity against different potential interferents. Thus, our proposed colorimetric assay was satisfactory when applied to glucose and kanamycin detection in agricultural and livestock husbandry samples.

Highly efficient peroxidase-like activity of a metal-oxide-incorporated CeO2-MIL(Fe) metal-organic framework and its application in the colorimetric detection of melamine and mercury ions via induced hydrogen and covalent bonds.

The illegal addition of melamine to dairy products and the contamination of water with mercury (Hg2+) are serious threats to human health. Hence, herein, a highly sensitive colorimetric sensor for the visual detection of melamine and Hg2+ ions has been developed using a metal-oxide-in-MOF nanomaterial (CeO2-MIL (Fe)) as a peroxidase mimic. Highly mono-dispersed CeO2-MIL (Fe) was synthesised via a facile hydrothermal process. The CeO2-MIL (Fe) exhibited outstanding peroxidase activity, and can catalyze the oxidation of TMB (3,3',5,5'-tetramethylbenzidine) by H2O2, resulting in the development of blue-coloured oxidation products within 5 min. In the presence of melamine, the H2O2 interacts with melamine to form melamine-H2O2via H-bonding. Due to the uptake of H2O2 by melamine, the catalytic oxidation reaction was halted, and the blue TMB oxidation product became pale. The relative change in the absorption intensity at 652 nm was proportional to the concentration of melamine in the linear range of 0-0.1 µM and the detection limit was found to be 8 nM. Subsequently, when Hg2+ ions were added to the above solution, the Hg2+ ions reacted with melamine via strong covalent bonding to form a Hg2+-melamine covalent complex, causing the release of H2O2, which again strongly oxidised the TMB to give the blue-coloured oxidation product. Furthermore, the comparative change in the absorption intensity at 652 nm was dependent on the concentration of Hg2+ ions in the linear range of 0-6 nM, and a detection limit of 2 nM was achieved. The suggested system has several advantages including greater simplicity, good selectivity, naked-eye detection and cost-effectiveness without using any complicated detection procedure. This technique was successfully utilized to identify melamine in real foods and Hg2+ ions in real water samples, yielding high recovery rates.

Label-free DNAzyme for highly sensitive detection of multiple biomolecules in real samples through target-triggered catalytic cleavage reactions with auramine O's discriminated fluorescence emission.

A universal enzyme strand (E-DNA) recyclable L-histidine (L-His), melamine (MA), and cisplatin (CP) biosensor was fabricated on the basis of a target-specific RNA-cleaving DNAzyme with specific auramine O (AuO) dye instead of thioflavin T. In this strategy, the substrate strand (S-DNA) of the RNA-cleaving site was constructed as an intramolecular stem-loop structure, and a GT-rich sequence was imprisoned in the double-stranded stem which inhibits the formation of stable G-quadruplex (G4cpx) with AuO. The presence of L-His initiates a catalytic reaction for cleaving the RNA site of the S-DNA hydrolytically releasing the GT-rich portion, which subsequently combines with AuO and forms a G4cpx for enhanced fluorescent signal. The subsequent addition of MA uncoils the G4cpx to form T-MA-T dsDNA, or addition of CP unwound the G4cpx to form CP-DNA leading to an intensive decrease of AuO emission. Remarkably, the liberated L-His can ultimately cause several rounds of cleavage, and the liberated E-DNA can catalyze the subsequent reaction with the other S-DNA. The use of L-His and E-DNA repeatedly induces S-DNA cleavage and intensifies the emission signal. The results show that the proposed biosensor is extremely sensitive to L-His, MA, and CP with a detection limit of 0.98, 10, and 3.4 nM respectively. To the best of our knowledge, the utilization of AuO as the G4cpx inducer and stabilizer for L-His, MA, and CP detection in real milk and urine samples has never been reported.

Recyclable Target Metal-Enhanced Fluorometric Naked Eye Aptasensor for the Detection of Pb2+ and Ag+ Ions Based on the Structural Change of CaSnO3@PDANS-Constrained GC-Rich ssDNA.

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.

High Catalytic Activity of Fluorophore-Labeled Y-Shaped DNAzyme/3D MOF-MoS2NBs as a Versatile Biosensing Platform for the Simultaneous Detection of Hg2+, Ni2+, and Ag+ Ions.

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-MoS2NB (metal-organic framework fused with molybdenum disulfide nanobox), which was synthesized as an efficient quencher of the fluorescent biosensor. The synthesized porous 3D MOF-MoS2NBs and Y-shaped DNAzyme exhibited a good analytical response toward the simultaneous multiple detections of Hg2+, Ni2+, 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-MoS2NB 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 Hg2+, Ni2+, 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.