Combined Signal Amplification Using a Propagating Cascade Reaction and a Redox Cycling Reaction for Sensitive Thyroid-Stimulating Hormone Detection

Enzymatic Precipitation of Highly Electroactive and Ion-Transporting Prussian Blue for a Sensitive Electrochemical Immunosensor

Sensitive and/or multiplex electrochemical biosensors often require efficient (bio)catalytic conversion of substrates into insoluble electroactive products. The enzymatic formation and precipitation of coordination polymers under mild conditions offers a promising solution for this purpose. Herein, we report the enzymatic precipitation of Prussian blue (PB), a highly electroactive and ion-transporting coordination polymer, on an immunosensing electrode for application in a sensitive electrochemical immunosensor for detecting thyroid-stimulating hormone (TSH). Five pairs of redox enzymes and their specific reductants were examined to achieve rapid PB precipitation and electrochemical oxidation. Among these pairs, O2-insensitive flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) paired with glucose yielded the highest electrochemical signal-to-background (S/B) ratio. FAD-GDH catalyzed the conversion of Fe(CN)63- to Fe(CN)64-, which coordinated with Fe3+, leading to PB formation and subsequent precipitation through repeated conversions. The resulting PB precipitate, with its close proximity to the electrode, facilitated rapid electrochemical oxidation and generated a strong electrochemical signal. Notably, the precipitation and electrochemical oxidation of PB were more effective than those of its analogues. When applied to a sandwich-type immunosensor for TSH detection, the enzymatic PB precipitation achieved a calculated detection limit of approximately 2 pg/mL in artificial serum, covering the clinically relevant range. These findings indicate the potential widespread utility of PB precipitation and electrochemical oxidation for sensitive multiplex biomarker detection.

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.

Signal Amplification by Spatial Concentration for Immunoassay on Cellulose Media

Immunoassay is one of the most common bioanalytical techniques from lab-based to point-of-care settings. Over time, various approaches have been developed to amplify signals for greater sensitivity. However, the need for effective, versatile, and simple signal amplification methods persists yet. This paper presents a novel signal amplification method for immunoassay that utilizes spatial concentration of a cellulose-based plate possessing sensor transducers, specifically gold nanoparticles. By modifying the dimensions of the plate, the density of nanoparticles increased, resulting in intensified color signals. The coating material, polydopamine, which is utilized to protect the gold nanoparticles. Chemical changes in nanocomposites are characterized using scanning electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The application of this method to colorimetric quantification demonstrated great consistency across various concentrations of nanoparticles, with better reliability at lower concentration ranges. A model immunoassay is designed to evaluate the analytical performance. As a result, this method successfully corrected a false-negative result with a lowered Kd of 0.509 pmol per zone. This method shows strong signal enhancement capability that can correct false-negative signals in the immunoassays, with potential benefits including versatility, simplicity, low cost, and the ability to operate multiple plates simultaneously.

Chemical Redox Cycling in an Organic Photoelectrochemical Transistor: Toward Dual Chemical and Electronic Amplification for Bioanalysis

The organic photoelectrochemical transistor (OPECT) has been proven to be a promising platform to study the rich light-matter-bio interplay toward advanced biomolecular detection, yet current OPECT is highly restrained to its intrinsic electronic amplification. Herein, this work first combines chemical amplification with electronic amplification in OPECT for dual-amplified bioanalytics with high current gain, which is exemplified by human immunoglobulin G (HIgG)-dependent sandwich immunorecognition and subsequent alkaline phosphatase (ALP)-mediated chemical redox cycling (CRC) on a metal-organic framework (MOF)-derived BiVO4/WO3 gate. The target-dependent redox cycling of ascorbic acid (AA) acting as an effective electron donor could lead to an amplified modulation against the polymer channel, as indicated by the channel current. The as-developed bioanalysis could achieve sensitive HIgG detection with a good analytical performance. This work features the dual chemical and electronic amplification for OPECT bioanalysis and is expected to stimulate further interest in the design of CRC-assisted OPECT bioassays.

ALP-assisted chemical redox cycling signal amplification for ultrasensitive fluorescence detection of DNA methylation

Affinity assays allow direct detection of DNA methylation events without requiring a special sequence. However, the signal amplification of these methods heavily depends on nanocatalysts and bioenzymes, making them suffer from low sensitivity. In this work, alkaline phosphatase (ALP)-assisted chemical redox cycling was employed to amplify the sensitivity of fluorescence affinity assays for DNA methylation detection using Ru@SiO2@MnO2 nanocomposites as fluorescent probes. In the ALP-assisted chemical redox cycling reaction system, ALP hydrolyzed 2-phosphate-L-ascorbic acid trisodium salt (AAP) to produce AA, which could reduce MnO2 nanosheets to form Mn2+, making the fluorescence recovery of Ru@SiO2 nanoparticles possible. Meanwhile, AA was oxidized to dehydroascorbic acid (DHA), which was re-reduced by tris(2-carboxyethyl) phosphine (TCEP) to trigger a redox cycling reaction. The constantly generated AA could etch large amounts of MnO2 nanosheets and greatly recover Ru@SiO2 fluorescence, amplifying the signal of the fluorescence assay. Employing the proposed ALP-assisted chemical redox cycling signal amplification strategy, a sensitive affinity assay for DNA methylation detection was achieved using ALP encapsulated liposomes that were linked with the 5mC antibody (Ab) to bind with methylated sites. A detection limit down to 2.9 fM was obtained for DNA methylation detection and a DNA methylation level as low as 0.1% could be distinguished, which was superior to conventional affinity assays. Moreover, the affinity assays could detect DNA methylation more specifically and directly, implying their great potential for the analysis of tumor-specific genes in liquid biopsy.

Sensitive electrochemical immunosensor via amide hydrolysis by DT-diaphorase combined with five redox-cycling reactions

Amide hydrolysis using enzyme labels, such as proteases, occurs at a slower rate than phosphoester and carboxyl ester hydrolysis. Thus, it is not very useful for obtaining high signal amplification in biosensors. However, amide hydrolysis is less sensitive to nonenzymatic spontaneous hydrolysis, allowing for lower background levels. Herein, we report that amide hydrolysis by DT-diaphorase (DT-D) occurs rapidly and that its combination with five redox-cycling reactions allows for the development of a highly sensitive electrochemical immunosensor. DT-D rapidly generates ortho-aminohydroxy-naphthalene (oAN) from its amide substrate via amide hydrolysis, which not even trypsin, a highly catalytic protease, can achieve. NADH, which is required for amide hydrolysis, advantageously acts as a reducing agent for rapid electrooxidation-based redox-cycling reactions. In the presence of oAN, DT-D, and NADH, two redox-cycling reactions rapidly occur. In the additional presence of an electron mediator, Ru(NH₃)₆³⁺ [Ru(III)], three more redox-cycling reactions occur because Ru(III) reacts rapidly with oAN and DT-D. Although the O₂-related redox-cycling reactions and redox reaction decrease electrochemical signals, this signal-decreasing effect is not significant in air-saturated solutions. The slow electrooxidation of NADH at an indium tin oxide electrode and sluggish reaction between NADH and Ru(III) allow for low electrochemical backgrounds. When the developed signal amplification scheme is tested for the sandwich-type electrochemical detection of parathyroid hormone (PTH), a detection limit of ∼1 pg/mL is obtained. The detection method is highly sensitive and can accurately measure PTH in serum samples.

Highly Sensitive Electrochemical Immunoassay Using Signal Amplification of the Coagulation Cascade

Here, we report a highly sensitive immunoassay for human immunoglobulin G (IgG) that uses signal amplification of the coagulation cascade. Z-Phe-Pro-Lys-p-nitroaniline (FPK-pNA) was used as a substrate for thrombin activation in the last step of the coagulation cascade. During the coagulation cascade, pNA is liberated from FPK-pNA and can be detected electrochemically. Using square wave voltammetry with a glassy carbon electrode, we demonstrated that pNA can be quantified in a solution modeling the coagulation cascade prepared by mixing FPK-pNA and pNA. Characterization of the reactivity of thrombin toward FPK-pNA revealed that thrombin efficiently reacted with FPK-pNA. Subsequent characterization of factor XIa activity of factor XIa-labeled antibody revealed that factor XIa was not inactivated during labeling. Finally, a coagulation cascade-based immunoassay for human IgG was performed using a factor XIa-labeled antibody on magnetic beads. The limit of detection for human IgG was 5.0 pg/mL (33 fM) indicating that the coagulation cascade can amplify the immunoassay sensitivity compared to immunoassay using a thrombin-labeled antibody as a condition without a coagulation cascade. Coagulation cascade-based immunoassay was also highly selective. In the near future, we will report a highly sensitive immunoassay for the simultaneous detection of multiple analytes using a coagulation cascade-based immunoassay and Limulus amebocyte lysate reaction-based immunoassay we previously reported.

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.

Bio-Nanohybrid Cell Based Signal Amplification System for Electrochemical Sensing

A signal amplification system for electrochemical sensing was established by bio-nanohybrid cells (BNC) based on bacterial self-assembly and biomineralization. The BNC was constructed by partially encapsulating a Shewanella oneidensis MR-1 cell with the self-biomineralized iron sulfide nanoparticles. The iron sulfide nanoparticle encapsulated BNCs showed high transmembrane electron transfer efficiency and was explored as a superior redox cycling module. Impressively, by integrating this BNC redox cycling module into the electrochemical sensing system, the output signal was amplified over 260 times compared to that without the BNC module. Uniquely, with this BNC redox cycling system, ultrasensitive detection of riboflavin with an extremely low LOD of 0.2 nM was achieved. This work demonstrated the power of BNC in the area of biosensing and provided a new possibility for the design of a whole cell redox cycling based signal amplification system.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Digital immunoassay for biomarker concentration quantification using solid-state nanopores

Single-molecule counting is the most accurate and precise method for determining the concentration of a biomarker in solution and is leading to the emergence of digital diagnostic platforms enabling precision medicine. In principle, solid-state nanopores—fully electronic sensors with single-molecule sensitivity—are well suited to the task. Here we present a digital immunoassay scheme capable of reliably quantifying the concentration of a target protein in complex biofluids that overcomes specificity, sensitivity, and consistency challenges associated with the use of solid-state nanopores for protein sensing. This is achieved by employing easily-identifiable DNA nanostructures as proxies for the presence (“1”) or absence (“0”) of the target protein captured via a magnetic bead-based sandwich immunoassay. As a proof-of-concept, we demonstrate quantification of the concentration of thyroid-stimulating hormone from human serum samples down to the high femtomolar range. Further optimization to the method will push sensitivity and dynamic range, allowing for development of precision diagnostic tools compatible with point-of-care format.

The concentration of a biomarker in solution can be determined by counting single molecules. Here the authors report a digital immunoassay scheme with solid-state nanopore readout to quantify a target protein and use this to measure thyroid-stimulating hormone from human serum.

Sensitive electrochemical immunosensor using a bienzymatic system consisting of β-galactosidase and glucose dehydrogenase

Bienzymatic systems are often used with electrochemical affinity biosensors to achieve high signal levels and/or low background levels. It is important to select two enzymes whose reactions do not exhibit mutual interference but have similar optimal conditions. Here, we report a sensitive electrochemical immunosensor based on a bienzymatic system consisting of β-galactosidase (Gal, a hydrolase enzyme) and flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH, a redox enzyme). Both enzymes showed high activities at neutral pH, the reactions catalyzed by them did not exhibit mutual interference, and the electrochemical-enzymatic redox cycling based on FAD-GDH coupled with enzymatic amplification by Gal enabled high signal amplification. Among the three amino-hydroxy-naphthalenes and 4-aminophenol (potential Gal products), 4-amino-1-naphthol showed the highest signal amplification. Glucose, as an electro-inactive, stable reducing agent for redox cycling, helped in achieving low background levels. Our bienzymatic system could detect parathyroid hormone at a detection limit of ∼0.2 pg mL-1, implying that it can be used for highly sensitive electrochemical detection of parathyroid hormone and other biomarkers in human serum.

Fenton reaction-mediated dual-attenuation of signal for ultrasensitive amperometric immunoassay

In order to alter the complexion of immunoprobe with large impedance as negative factor in sensitivity of amperometric immunosensor, a strategy of Fenton reaction-mediated dual-attenuation of signal was proposed. Herein, metal-polydopamine-Fe³⁺ composite with the ability of Fenton reaction was initially prepared as immunoprobe for an ultrasensitive immunoassay. The polymerization of dopamine occurred on the surface of ZIF-67 to gain the metal-polydopamine shell, which possessed rich functional groups, negative charge and high specific surface. Then the prepared functional shell was further used to absorb Fe³⁺ and immobilize labeling antibody as immunoprobe, which was used to construct a sandwich type immunosensor. With addition of H₂O₂ and aniline, Fenton reaction was triggered to produce hydroxyl radicals, which can not only decrease the current value by degrading methylene blue molecules, but also further initiate aniline to polymerize into non-conductive polyaniline for successive abatement of signal intensity. Therefore, the dual-attenuation of signal model rendered the immunoprobe into a favorable factor and synchronously enhance sensitivity. Expectedly, the detection performance with a linear range from 1.0 × 10^-4-100 ng mL^-1 and ultralow detection limit of 9.07 × 10^-5 ng mL^-1 toward neuron-specific enolase was obtained under optimal conditions. This work offered a novel tactic for enhancing sensitivity of immunosensor through the preparation of functional immunoprobe and its rational utilization as signal enhancer.

A fluorometric and colorimetric dual-signal nanoplatform for ultrasensitive visual monitoring of the activity of alkaline phosphatase

Considering the limited sensitivity and accuracy of single-signal assay strategies, the multi-signal assay strategy has sparked significant excitement in recent years. In this study, for the first time, we reported a one-pot method in situ synthesis of carbon-containing nanoparticles (CNPs) via p-aminophenol (AP) and diethylenetriamine (DETA). The CNP solution exhibits yellow and light blue fluorescence under UV-light. Moreover, the CNPs exhibited excellent photoluminescence stability even under extreme conditions. Inspired by the alkaline phosphatase (ALP)-triggered specific catalytic reaction, we constructed an ultrasensitive fluorescence and colorimetric two-channel strategy for monitoring the ALP activity. By optimizing the detection parameters, the detection limits for both fluorometric and colorimetric were 0.05 mU mL^-1. Moreover, the strategy showed high specificity and was successfully applied to monitor the ALP activity level in human serum samples. The analytical strategy opened a new window for the detection of the ALP activity, screening of the ALP inhibitor, and disease diagnosis.

Washing- and Separation-Free Electrochemical Detection of Porphyromonas gingivalis in Saliva for Initial Diagnosis of Periodontitis

Indirect detection of Porphyromonas gingivalis in saliva, based on proteolytic cleavage by an Arg-specific gingipain (Arg-gingipain), has traditionally been used for simple, initial diagnosis of periodontitis. To accurately detect P. gingivalis using a point-of-care format, development of a simple biosensor that can measure the exact concentration of P. gingivalis is required. However, electrochemical detection in saliva is challenging due to the presence of various interfering electroactive species in different concentrations. Here, we report a washing- and separation-free electrochemical biosensor for sensitive detection of P. gingivalis in saliva. Glycine-proline-arginine conjugated with 4-aminophenol (AP) was used as an electrochemical substrate for a trypsin-like Arg-gingipain, and glycylglycine was used to increase the Arg-gingipain activity. The electrochemical signal of AP was increased using electrochemical-chemical (EC) redox cycling involving an electrode, AP, and tris(2-carboxyethyl)phosphine, and the electrochemical charge signal was corrected using the initial charge obtained before a 15 min incubation period. The EC redox cycling combined with the matrix-corrected signal facilitated a high and reproducible signal without requiring washing and separation steps. The proteolytic cleavage of the electrochemical substrate was specific to P. gingivalis. The calculated detection limit for P. gingivalis in artificial saliva was 5 × 10⁵ colony-forming units/mL, and the concentration of P. gingivalis in human saliva could be measured. The developed biosensor can be used as an initial diagnosis method to distinguish between healthy people and patients with periodontal diseases.