An enzyme-free electrochemical biosensor combining target recycling with Fe3O4/CeO2@Au nanocatalysts for microRNA-21 detection

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.