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In this study, an electrochemical biosensor was proposed for microRNA-21 detection based on Fe3O4/CeO2 @Au magnetite nanoparticles (Fe3O4/CeO2 @Au MNPs) as nanocatalyst and catalytic hairpin assembly (CHA) for signal application. Firstly, target microRNA-21 hybridized with hairpin H2 to form H2-T duplex stranded DNA (dsDNA), which could further open the hairpin H1 for the formation of H1-H2 dsDNA. Simultaneously, the Fe3O4/CeO2 @Au-S1 not only hybridized with single stranded fragment of H1-H2 dsDNA with producing long dsDNA to absorb a large amount of electroactive substances of methylene blue (MB), but also acted as nanocatalyst to directly catalyze the reduction of MB for amplifying the electrochemical signal. Herein, compared with pure Fe3O4 nanoparticles, Fe3O4/CeO2 @Au MNPs exhibited excellent catalytic performance since the cerium oxide (CeO2) nanoparticles and Au nanoparticles can greatly improve the catalytic activity of Fe3O4 nanoparticles and effectively prevent the agglomeration of Fe3O4 nanoparticles. Owing to the signal amplification strategy, the proposed biosensor provided a wide linear range of 1 fM to 1 nM with a low detection limit of 0.33 fM (defined as S/N = 3) for microRNA-21 detection, and exhibited excellent specificity and sensitivity. This strategy provided a novel avenue for the detection of other biomarkers in electrochemical biosensors.

The portable and rapid detection of biomolecules via pH meters to monitor the concentration of hydrogen ions (H+) from biological reactions (e.g. loop-mediated isothermal amplification, LAMP) has attracted research interest. However, this assay strategy suffered from inherent drawback of low sensitivity, resulting in great limitations in practical applications. Herein, a novel electrochemical biosensor was constructed for highly sensitive detection of Nosema bombycis gene DNA (PTP1) through transducing chemical stimuli H+ from PTP1-based LAMP into electrochemical output signal of electroactive ferrocene (Fc). With use of target PTP1 as the template, the H+ from LAMP induced the conformational switch of pH-responsive DNA nanostructures (DNA NSs, Fc-Sp@Ts) that was assembled by the hybridization of Fc-labeled signal probe (Fc-Sp) with DNA-based receptor (Ts). Due to the folding of Ts into stable triplex structure at decreased pH, the configuration change of Fc-Sp@Ts led to the releasing of Fc-Sp, which was subsequently immobilized in the electrode interface through the hybridization with the capture probe modified with -SH (SH-Cp), generating amplified electrochemical signal from Fc. The developed biosensor for PTP1 exhibited a reliable linear range of 1 fg µL−1 to 50 ng µL−1 with the limit of detection of 0.31 fg µL−1. Thus, by the regulation of H+ from LAMP reaction on DNA NSs allostery, this novel and simple transduction scheme would be interesting and promising to open up a novel analytical route for sensitive monitoring of different target DNAs in related disease diagnosis.

In this work, a nanohybrid of platinum nanoparticles-porous ZnO spheres-hemin (Pt-pZnO-hemin) was synthesized for construction of alkaline phosphatase-based immunosensor for detection of influenza. Briefly, porous ZnO spheres (pZnO) were prepared using soluble starches as the capping agent, followed by surface functionalization of platinum nanoparticles via a hydrothermal method (Pt-pZnO). Then, hemin with carboxylic functionality was spontaneously adsorbed onto Pt-pZnO by ester-like binding between carboxylic group of hemin and ZnO. Compared with platinum nanoparticles and hemin, the resulting Pt-pZnO-hemin nanohybrid showed more excellent electrocatalysis activity toward 1-naphthol (1-NP). Taking advantage of the Pt-pZnO-hemin, we have developed an amplified electrochemical immunosensor based on in situ generation of redox probe by alkaline phosphatase (ALP) and Pt-pZnO-hemin as signal enhancer. Herein, electrochemically active 1-NP was generated by enzymatic hydrolysis of inactive 1-naphthyl phosphate by ALP, then Pt-pZnO-hemin was used as catalyst to catalytically oxidize 1-NP, resulting in electrochemical signal amplification. Furthermore, in comparison with other nanomaterials including Au-pZnO, Pt-pZnO and Au-pZnO-hemin, the excellent catalytical property of Pt-pZnO-hemin make it a promising nanohybrid material for ALP-based immunosensor for signal amplification.

In this work, an amplified electrochemical immunosensor based on 1-naphthol as electroactive substance and Pt/CeO 2 /GO composites as catalytic amplifier was constructed for sensitive detection influenza. Through “sandwich” reaction, the Pt/CeO 2 /GO functionalized bioconjugates were captured on electrode surface and the electrochemical signal directly originated from 1-naphthol, which was in situ produced with high local concentration though the hydrolysis of 1-naphthyl phosphate catalyzed by ALP. Then, 1-naphthol as new reactant was oxidized by Pt/CeO 2 /GO composites with outstanding catalytic performance, resulting in detection signal amplification. In addition, as compared to label electroactive substance to antibodies, a simplified preparative step of immunosensor could be achieved because the signal probe get rid of introducation other electroactive substances. The proposed immunosensor achieved a linear range of 1.0×10 −3 –1.0 ng mL −1 and 5.0 to 1.0×10 2 ng mL −1 with a detection limit of 0.43 pg mL −1 (defined as S / N =3).

A facile and feasible sandwich-type electrochemical immunosensor for ultrasensitive determination of Carbohydrate antigen 19-9 (CA19-9) was designed by using Au nanoparticles functionalized porous graphene (Au-PGO) as sensing platform and Au@Pd core/shell bimetallic functionalized graphene nanocomposites (Au@Pd-Gra) as signal enhancers. Herein, Au@Pd-Gra with a large surface area was prepared for immobilizing plentiful of redox probe-thionine (Thi), horseradish peroxidase (HRP) and secondary antibodies (Ab2), leading to the formation of Au@Pd-Gra/Thi-Ab2/HRP bioconjugate which exhibited satisfying electrochemical redox activity, high electrocatalytic activity and friendly biocompatibility. With the synergistic effect between Au@Pd-Gra and HRP, almost triple amplified detection signal was achieved in the presence of H2O2, so as to improve the detection limit of the proposed immunosensor effectively. Furthermore, Au-PGO was utilized as the biosensor platform which could greatly enhance the surface area to immobilize a large amount of captured primary antibodies (Ab1) leading a further enhancement in the sensitivity of immunosensor. Under optimal conditions, the electrochemical immunosensor exhibited desirable performance for determination of CA19-9 with a wide linearity in the range from 0.015 to 150 U mL–1 and a relatively low detection limit of 0.006 U mL–1. Importantly, the resulted immunosensor displayed good specificity and high sensitivity, implying potential applications in clinical research.