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In this study, a paper-based enzyme biosensor for hypoxanthine (Hx) was developed, enabling visual and one-step fish freshness detection. Xanthine oxidase and horseradish peroxidase were immobilized on nitrocellulose membranes with 3,3',5,5'-tetramethylbenzidine to output the colour signal. Chitosan oligosaccharide lactate-modified nitrocellulose membranes entrapped the dual-enzyme system and exhibited excellent microfluidic aggregation effect. The developed enzyme biosensor produced a linear response of 0.01-0.16 mmolL -1 with a detection limit of 8.22 μmolL -1 , and was selective for Hx with recoveries of 96.13-103.11 % for fish samples. These biosensors were attached directly to the surface of fish samples and the colour was revealed within 3 min. Colour signals can be judged by the naked-eye to distinguish between fresh and spoiled fish samples and analyzed by a smartphone for quantitative analysis. The biosensor shows great potential as a powerful pattern- and reagent-free device for on-site freshness evaluation of fish., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Motivated by the desire for simple, rapid and highly sensitive DNA detection, we presented a signal-on electrochemical DNA (E-DNA) sensing strategy utilizing cooperative proximity hybridization based on a G-quadruplex (G4) probe labeled with the SH, which could specifically hybridize with its target DNA in homogenous solution. In the presence of target DNA, proximity hybridization was triggered to form a Y-shaped complex and the SH was released from G4 probe stem, companied by chemisorption on the electrode surface through Au-S binding when applied a positive potential, which brought Fc labeled on the signal probe close to the electrode surface. Thus, electrochemical signal dramatically increased, ensuring the highly sensitive "signal-on" assay. Such an E-DNA sensing strategy allows for ultrasensitive DNA detection with a detection limit as low as 2.82 × 10 -15  M and a wide linear response from 1.0 × 10 -9 to 1.0 × 10 -15  M. In addition, the powerful discriminating ability of the mismatched DNA from the perfect matched target DNA was also demonstrated. More importantly, this homogenous proximity hybridization strategy could expand to colorimetric assay by incorporating G4 probe with hemin to form DNAzyme, which could effectively catalyze ABTS to generate a visual color change. Taking the joint advantages of G4 stem-loop probe and homogenous proximity hybridization, this sensing strategy exhibits greatly enhanced sensitivity and excellent specificity, making it a promising strategy for point-of-care testing., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The DNA tetrahedron has developed a broad spectrum of applications in biosensor construction thanks to its excellent mechanical rigidity and structural stability. However, how to construct a highly sensitive biosensor using a DNA tetrahedron is still a challenge. In this work, an ultrasensitive electrochemical biosensor based on a DNA tetrahedral nanostructure was developed with the help of synergy from proximity-dependent hybridization. To decrease the steric hindrance of DNA tetrahedra to proximity-dependent hybridization, the detection signal was set on the inclined side chain structure of a DNA tetrahedral sensing system. Additionally, when the target hybridized with the DNA probe, the ferrocene (Fc) labeled on the end of the DNA probe was driven close to the surface of the biosensor, providing a sensitive faradaic current. The experimental results exhibited a good linear relationship from 1 fM to 10 pM with a linear correlation coefficient of 0.9977, and a high sensitivity with a detection limit of 0.2 fM. Our DNA biosensor also showed good stability according to electrode characterization and target detection at different time scales and the anti-jamming capabilities in a complicated biological extraction environment were excellent. The electrochemical sensing system established here has greatly improved the detection sensitivity of a DNA biosensor based on a DNA tetrahedron, which will further promote its practical applications.

Meat screening plays a significant role in human health and religion. But the identification methods for beef were little reported. In this work, a simple colorimetric method based on denaturation bubble-mediated strand exchange amplification (SEA) was developed for the rapid and sensitive identification of beef. The whole strategy was performed on a portable metal bath and the distinguishable color between positive and negative controls was observed directly by the naked eyes. The feasibility using crude extraction samples by a heating treatment in PBS for 2 min was evaluated in duck spiked by beef. The result demonstrated that the developed method could identify as low as 1% (w/w) beef/duck within 50 min. Meanwhile, the results showed the method had a good repeatability and specificity. Therefore, this assay allows for the rapid, sensitive, specific detection of beef, and can be recommended as an effective, promising strategy for on-site meat identification.