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In this study, a novel, rapid and ultrasensitive fluorescence strategy using the three-dimensional (3D) dynamic DNA walker (DW)-induced branched hybridization chain reaction (bHCR) has been proposed for the detection of ampicillin (AMP). The sensing system was composed of an Nt·Bbvcl-powered DNA walker blocked by an AMP aptamer, hairpin-shaped DNA track probe (TP) and four kinds of metastable hairpin probes as the substrates of bHCR, which triggered the formation of the split G-quadruplex as the signal molecule. Due to the reasonable design, the specific binding between AMP and its aptamer activated the DW, and the DW moved on the surface of the gold nanoparticles (AuNPs) with the help of Nt·Bbvcl to produce primer probes (PPs), which induced bHCR. The products of the bHCR gathered two split G-quadruplex sequences together to form one complete G-quadruplex. The formed G-quadruplex emitted a strong fluorescence signal in the presence of thioflavin-T (ThT) to achieve the purpose of detecting AMP. The sensitivity of this method was greatly improved by the use of the 3D DNA walker and bHCR. The split G-quadruplex enhanced the signal-to-noise ratio (SNR). Under the optimal experimental conditions, a good correlation was obtained between the fluorescence intensity of the sensing system and the concentration of AMP ranging from 5 pM to 500 nM with a limit of detection (LOD) of 3.68 pM. Simultaneously, the method has been applied to the detection of antibiotics in spiked milk samples with satisfactory results. [ABSTRACT FROM AUTHOR]

A novel electrochemical immunosensor assay (EIA) for highly sensitive and specific detection of Escherichia coli O157:H7 has been developed. This immunosensor is constructed by the assembly of capture antibody on SG-PEDOT-AuNPs composites modified glass carbon electrode. In the presence of target E. coli O157:H7, horseradish peroxidase (HRP)-labeled antibody is captured on the electrode surface to form a sandwich-type system via the specific identification. As a result, E. coli O157:H7 detection is realized by outputting a redox current from electro-reduction of hydrogen peroxide reaction catalyzed by HRP. In our assay, the combination of the unique properties of sulfonated graphene (SG) and gold nanoparticles (AuNPs) can not only accelerate electron transfer on electrode interface, but also provide an excellent scaffold for the conjugation of capture antibody that significantly improves the target capture efficiency and enhances the sensitivity of the biosensor. The results reveal the calibration plot obtained for E. coli O157:H7 is approximately linear from 7.8 x 10-7.8 x 106 colony-forming unit (cfu) mL-1 with the limit of detection of 3.4 x 10 cfu mL"1. In addition, the biosensor has been successfully applied to the quantitative assay of E. coli O157:H7 in synthetic samples (spring water and milk). Hence, the developed electrochemical-based immunosensor might provide a useful and practical tool for E. coli O157:H7 determination and related food safety analysis and clinical diagnosis. [ABSTRACT FROM AUTHOR]