Ascorbic acid-triggered electrochemical–chemical–chemical redox cycling for design of enzyme-amplified electrochemical biosensors on self-assembled monolayer-covered gold electrodes

Overview on the Development of Electrochemical Immunosensors by the Signal Amplification of Enzyme- or Nanozyme-Based Catalysis Plus Redox Cycling

Enzyme-linked electrochemical immunosensors have attracted considerable attention for the sensitive and selective detection of various targets in clinical diagnosis, food quality control, and environmental analysis. In order to improve the performances of conventional immunoassays, significant efforts have been made to couple enzyme-linked or nanozyme-based catalysis and redox cycling for signal amplification. The current review summarizes the recent advances in the development of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling for signal amplification. The special features of redox cycling reactions and their synergistic functions in signal amplification are discussed. Additionally, the current challenges and future directions of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling are addressed.

Optical Bioassays Based on the Signal Amplification of Redox Cycling

Optical bioassays are challenged by the growing requirements of sensitivity and simplicity. Recent developments in the combination of redox cycling with different optical methods for signal amplification have proven to have tremendous potential for improving analytical performances. In this review, we summarized the advances in optical bioassays based on the signal amplification of redox cycling, including colorimetry, fluorescence, surface-enhanced Raman scattering, chemiluminescence, and electrochemiluminescence. Furthermore, this review highlighted the general principles to effectively couple redox cycling with optical bioassays, and particular attention was focused on current challenges and future opportunities.

Chemical-Chemical Redox Cycle Signal Amplification Strategy Combined with Dual Ratiometric Immunoassay for Surface-Enhanced Raman Spectroscopic Detection of Cardiac Troponin I

Improving the sensitivity and reproducibility of surface-enhanced Raman spectroscopy (SERS) methods for the detection of bioactive molecules is crucial in biological process research and clinical diagnosis. Herein, we designed a novel SERS platform for cardiac troponin I (cTnI) detection by a chemical-chemical redox cycle signal amplification strategy combined with a dual ratiometric immunoassay. First, ascorbic acid (AA) was generated by enzyme-assisted immunoreaction with a cTnI-anchored sandwich structure. Then, oxidized 4-mercaptophenol (ox4-MP) was reacted with AA to produce 4-mercaptophenol (4-MP). Quantitative analysis of cTnI was realized by a Raman signal switch between ox4-MP and 4-MP. Specifically, AA could be regenerated by reductant (tris(2-carboxyethyl) phosphine, TCEP), which in turn produced more signal indicator 4-MP, causing significant signal amplification for cTnI analysis by SERS immunosensing. Moreover, a dual ratiometric-type SERS method was established with the intensity ratio I1077/I822 and I633/I822, which improved the reproducibility of the cTnI assay. The excellent performance of the chemical-chemical redox cycle strategy and ratio-type SERS assay endows the method with high sensitivity and reproducibility. The linear ranges of cTnI were 0.001 to 50.0 ng mL-1 with detection limits of 0.33 pg mL-1 (upon I1077/I822) and 0.31 pg mL-1 (upon I635/I822), respectively. The amount of cTnI in human serum samples yielded recoveries from 89.0 to 114%. This SERS method has remarkable analytical performance, providing an effective approach for the early diagnosis of cardiovascular diseases, and has great latent capacity in the sensitive detection of bioactive molecules.

Comparative Studies of CPEs Modified with Distinctive Metal Nanoparticle-Decorated Electroactive Polyimide for the Detection of UA

In this present work, an electrochemical sensor was developed for the sensing of uric acid (UA). The sensor was based on a carbon paste electrode (CPE) modified with electroactive polyimide (EPI) synthesized using aniline tetramer (ACAT) decorated with reduced nanoparticles (NPs) of Au, Pt, and Ag. The initial step involved the preparation and characterization of ACAT. Subsequently, the ACAT-based EPI synthesis was performed by chemical imidization of its precursors 4,4′-(4.4′-isopropylidene-diphenoxy) bis (phthalic anhydride) BPADA and ACAT. Then, EPI was doped with distinctive particles of Ag, Pt and Au, and the doped EPIs were abbreviated as EPIS, EPIP and EPIG, respectively. Their structures were characterized by XRD, XPS, and TEM, and the electrochemical properties were determined by cyclic voltammetry and chronoamperometry. Among these evaluated sensors, EPI with Au NPs turned out the best with a sensitivity of 1.53 uA uM⁻¹ UA, a low limit of detection (LOD) of 0.78 uM, and a linear detection range (LDR) of 5–50 uM UA at a low potential value of 310 mV. Additionally, differential pulse voltammetric (DPV) analysis showed that the EPIG sensor showed the best selectivity for a tertiary mixture of UA, dopamine (DA), and ascorbic acid (AA) as compared to EPIP and EPIS.

Ultrasensitive electrochemical sensing platform based on graphene wrapping SnO2 nanocorals and autonomous cascade DNA duplication strategy

In this work, a sensitive, universal and reusable electrochemical biosensor based on stannic oxide nanocorals-graphene hybrids (SnO₂ NCs-Gr) is developed for target DNA detection by using two kinds of DNA enzymes for signal amplification through an autonomous cascade DNA duplication strategy. A hairpin probe is designed composing of a projecting part at the 3'-end as identification sequence for target, a recognition site for nicking endonuclease, and an 18-carbon shim to stop polymerization process. The designed DNA duplication-incision-replacement process is handled by KF polymerase and endonuclease, then combining with gold nanoparticles as signal carrier for further signal amplification. In the detection system, the electrochemical-chemical-chemical procedure, which uses ferrocene methanol, tris(2-carboxyethyl)phosphine and l-ascorbic acid 2-phosphate as oxidoreduction neurogen, deoxidizer and zymolyte, separately, is applied to amplify detection signal. Benefiting from the multiple signal amplification mechanism, the proposed sensor reveals a good linear connection between the peak current and logarithm of analyte concentration in range of 0.0001-1 × 10^-11molL^-1 with a detection limit of 1.25 × 10^-17molL^-1 (S/N=3). This assay also opens one promising strategy for ultrasensitive determination of other biological molecules for bioanalysis and biomedicine diagnostics.