Liposome-assisted enzyme catalysis: toward signal amplification for sensitive split-type electrochemiluminescence immunoassay

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.

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

A novel DNA detection using spherical identification probe and strand displacement reaction-initiated silver nanocluster switch

Herein, we developed a novel fluorescent assay using spherical identification probes and toehold-mediated strand displacement reaction-initiated silver nanoclusters (AgNCs) "on-off" signal switch. In this strategy, the target was captured by the spherical probes to induce the activity of exonuclease III (Exo III), catalyzing the cyclic cleavage of substrates to produce a mass of trigger strands. After magnetic bead separation, the intermediates in the supernatant activated downstream toehold-mediated strand displacement reaction to change the structure of silver nanocluster templates, leading to fluorescence intensity reduction. Furthermore, it is demonstrated that the application of spherical identification probes could reduce the signal leakage and the limit of detection. In addition, AgNCs with perfect optical property were ingeniously combined to realize signal output, which reduced the cost and time of synthesis. Under the optimal conditions, the sensing method displayed a good linear range from 250 pM to 25 nM with a detectable minimum concentration of 250 pM. And the practical application potential in complex biological matrices was also evaluated. Considering these advantages, this constructed strategy opens a new path for nucleic acid detection with better performance. A simple, label- and hairpin-free fluorescent system based on spherical identification probe and toehold-mediated strand displacement reaction-initiated silver nanoclusters (AgNCs) "on-off" signal switch was successfully constructed to detect target DNA.

Liposome-assisted chemical redox cycling strategy for advanced signal amplification: A proof-of-concept toward sensitive electrochemiluminescence immunoassay

This work presents a novel signal amplification strategy for electrochemiluminescence (ECL) biosensor based on liposome-assisted chemical redox cycling for in situ formation of Au nanoparticles (Au NPs) on TiO₂ nanotubes (TiO₂ NTs) electrode. The system was exemplified by ascorbic acid (AA)-loaded liposome, the redox cycling of AA utilizing tris (2-carboxyethyl) phosphine (TCEP) as reductant, and the use of Au nanoclusters (Au NCs)/TiO₂ NTs as working electrode to implement the ECL detection of prostate specific antigen (PSA). Specifically, the AA-loaded liposomes were used as tags to label the captured PSA through a sandwich immunoreaction. After the lysate of the liposome was transferred onto the interface of Au NCs/TiO₂ NTs in the presence of Au³⁺ and TECP, the chemical redox cycling was triggered. In the cycling, Au³⁺ was directly reduced in situ by AA to form Au NPs on Au NCs/TiO₂ NTs electrode, whereas the oxidation product of AA was reduced by TCEP to regenerate AA. The large loading capacity of the liposome and chemical redox cycling resulted in the incomplete reduction of the Au NCs to Au NPs on the TiO₂ NTs electrode, enhancing the ECL intensity greatly. The multiple signal amplification strategy achieved an ultrasensitive detection for PSA with a detection limit down to 6.7 × 10^-15 g mL^-1 and a wide linear concentration range from 1.0 × 10^-14 to 1.0 × 10^-8 g mL^-1. It is believed that this work is anticipated to extend the employment of advanced chemical redox cycling reaction in the field of ECL bioassays.