A label-free electrochemical magnetic aptasensor based on exonuclease III-assisted signal amplification for determination of carcinoembryonic antigen.

A novel label-free and exonuclease III (Exo III)-assisted signal amplification electrochemical aptasensor was constructed for the determination of carcinoembryonic antigen (CEA) via magnetic field-induced self-assembly of magnetic biocomposites (Fe3O4@Au NPs-S1-S2-S3). The magnetic biocomposites were acquired by modifying double-stranded DNA (S1-S2-S3) on the surface of Fe3O4@Au nanoparticles (Fe3O4@Au NPs). Among them, Fe3O4@Au NPs were used as carriers for magnetic separation, thiolated single-stranded DNA (S1) provided signal sequence, CEA aptamer (S2) worked as a recognition element, and complementary strand (S3) was used to form double strands. In the presence of CEA, S2 bonded with CEA competitively; the exposed S1 could not be cleaved since Exo III was inactive against ssDNA. The G-quadruplex/hemin complexes finally formed with the existence of K+, and the high electrochemical signal of G-quadruplex/hemin complexes was recorded by differential pulse voltammetry (DPV) at - 0.6 V. Conversely, in the absence of CEA, dsDNA was cleaved from the 3' blunt end by Exo III; the disappearance of G-rich sequence blocked the generation of the signal. This method exhibited good selectivity and sensitivity for the determination of CEA; the linear range was from 0.1 to 200 ng mL-1 and the limit of detection was 0.4 pg mL-1. Graphical abstract.

A label-free electrochemical biosensor based on magnetic biocomposites with DNAzyme and hybridization chain reaction dual signal amplification for the determination of Pb2.

A highly sensitive and selective electrochemical biosensor for Pb2+ with a dual-amplification strategy is proposed. The first amplification step was realized by the cycle of Pb2+ and 8-17 DNAzyme (S2), and the hybridization chain reaction (HCR) triggered by S1 further amplified the electrochemical signal. Fe3O4@Au NPs, as a multifunctional magnetic carrier, is not only manifested in the construction of a magnetically controlled electrochemical response interface, but also has significant contribution in the purifying system, reducing interference, increasing the specific surface area, and the DNA loading. The magnetic nanocomposites were characterized by TEM as spheres with particle size of around 39 nm. When there was no Pb2+, long double-strand DNA (dsDNA) is formed on the surface of Fe3O4@Au NPs by the S1-triggered HCR; in the presence of Pb2+, S2 is activated and S1 on the surface of magnetic biocomposites (Fe3O4@Au NPs-S1) is continuously cleaved with the cycle of Pb2+ and S2, leading to a significant decrease of methylene blue (MB) absorbed on dsDNA. Such reverse dual-signal amplification strategy effectively increased the current difference and improved the sensitivity of the proposed sensor. The electrochemical signal of MB was obtained by differential pulse voltammetry (DPV) with preconcentration, showing a linear response toward Pb2+ ranging from 50 pM to 1 µM with a detection limit of 15 pM. The proposed method has feasible applications in detecting other heavy metal ions based on other metal-dependent DNAzyme. Graphical Abstract.

A biosensor based on the biomimetic oxidase Fe3O4@MnO2 for colorimetric determination of uric acid.

High plasma urate is closely related to gout, cardiovascular and other diseases. Therefore, monitoring the content of uric acid (UA) in plasma is of great significance for the treatment of gout and the prevention of other related diseases. Herein, a biosensor based on the biomimetic oxidase Fe3O4 nanoparticles (NPs) @MnO2 nanosheets (Fe3O4@MnO2 NS) was constructed for colorimetric determination of UA. MnO2 NS is an efficient biomimetic oxidase, and we found that the intrinsic oxidase activity of MnO2 NS doped with Fe3O4 NPs can be significantly enhanced. The chromogenic substrate TMB can be catalyzed by Fe3O4 @MnO2 NS to generate blue oxidized TMB, and UA can decompose the MnO2 NS to inhibit the color reaction of TMB selectively, thereby realizing the quantitative detection of UA. In addition, the UA biosensor can perform colorimetric analysis of UA level through three methods: naked eye, smartphone and ultraviolet-visible (UV-vis) spectrophotometer. The linear ranges of UV-vis spectrophotometry and colorimetry with smartphone were 1-70 µM and 200-650 µM, respectively, and the limits of detection (LOD) were 0.27 µM and 21 µM. The analysis results of human plasma samples showed that the method had good selectivity and practicability.

Electrochemical Biosensor Based on HRP/Ti3C2/Nafion Film for Determination of Hydrogen Peroxide in Serum Samples of Patients with Acute Myocardial Infarction.

Hydrogen peroxide (H2O2) has been reported to mediate a variety of physiological and pathological processes in living systems. In this work, a biosensor for determination of H2O2 was prepared by using an HRP/Ti3C2/Nafion film-modified glassy carbon electrode (GCE). Ti3C2 nanosheets with remarkable conductivity and high specific surface area were chosen as carriers for HRP. Moreover, this biosensor modified with HRP has a specific catalytic effect on H2O2. The difference in peak current could reflect the quantitative change of H2O2. The linear range of the biosensor is 5-8000 µM, and the detection limit is 1 µM (S/N = 3). This biosensor was used to detect H2O2 in clinical serum samples of normal controls and patients with acute myocardial infarction (AMI) before and after percutaneous coronary intervention (PCI). The results showed that the difference between normal controls and patients is significant (P < 0.05), as well as the difference for patients before and after PCI (P < 0.01), but no significant difference existed between postoperative patients and normal controls. This biosensor has the advantages of simple preparation, high sensitivity, and quick detection, showing potential application in clinical diagnosis.