A Tyrosinase-Responsive Nonenzymatic Redox Cycling for Amplified Electrochemical Immunosensing of Protein

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.

Fluorometric Assay of Tyrosinase and Atrazine Based on the Use of Carbon Dots and the Inhibition of Tyrosinase Activity

Sensitive and convenient strategy of tyrosinase (TYR) and its inhibitor atrazine is in pressing demand for essential research as well as pragmatic application. In this work, an exquisite label-free fluorometric assay with high sensitivity, convenience and efficiency was described for detecting TYR and the herbicide atrazine on the basis of fluorescent nitrogen-doped carbon dots (CDs). The CDs were prepared via one-pot hydrothermal reaction starting from citric acid and diethylenetriamine. TYR catalyzed the oxidation of dopamine to dopaquinone derivative which could quench the fluorescence of CDs through a fluorescence resonance energy transfer (FRET) process. Thus, a sensitive and selective quantitative evaluation of TYR can be constructed on the basis of the relationship between the fluorescence of CDs and TYR activity. Atrazine, a typical inhibitor of TYR, inhibited the catalytic activity of TYR, leading to the reduced dopaquinone and the fluorescence was retained. The strategy covered a broad linear range of 0.1-150 U/mL and 4.0-80.0 nM for TYR and atrazine respectively with a low detection limit of 0.02 U/mL and 2.4 nM/mL. It is also demonstrated that the assay can be applied to detect TYR and atrazine in spiked complex real samples, which provides infinite potential in application of disease monitoring along with environmental analysis.

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.

Redox Cycling Amplified Electrochemical Lateral-Flow Immunoassay: Toward Decentralized Sensitive Insulin Detection

While paper-based lateral-flow immunoassays (LFA) offer considerable promise for centralized diagnostic applications, the analytical capability of conventional LFA remains constrained due to the low sensitivity of its common optical detection strategy. To address these issues, we report a simple electrochemical LFA (eLFA) with nanocatalytic redox cycling for decentralized insulin detection. Simultaneous binding of insulin with detection antibodies and capture antibodies through the capillary flow at the LFA platform and signal amplification through the rapid nanocatalytic reduction of [Fe(CN)6]3- (Fe3+) with Au nanoparticles (AuNP) and ammonia-borane (AB), coupled to electrochemical redox cycling reactions involving Fe3+, AuNP, and AB on the carbon working electrode, offer higher sensitivity than conventional colorimetric LFA and enzymatic redox cycling. The resulting integrated eLFA strip allows the detection of low insulin concentrations (LOD = 12 pM) and offers considerable promise for highly sensitive decentralized assays of different biological fluids (saliva and serum) without additional pretreatment or washing steps.

Novel Green Fluorescent Probe Stem From Carbon Quantum Dots for Specific Recognition of Tyrosinase in Serum and Living Cells

Tyrosinase (TYR), an important biomarker for melanoma, offered significant information early detection of melanoma and may decrease the likelihood of mortality. Therefore, this article constructed a highly sensitive and selective green fluorescent functionalized carbon quantum dots (TYR-CQDs) for tyrosinase (TYR) activity detection by one-step hydrothermal protocol utilizing catechol, citric acid and urea as precursors. The prepared TYR-CQDs illustrated excellent linear relationship and broad linear range with a low detection limit, which exhibited high accuracy and recovery in quantitative determination of TYR in human serum samples. Furthermore, the TYR-CQDs had successfully realized intracellular TYR detection owing to excellent biocompatibility, high anti-interference ability and good cellular imaging capability, suggesting the potential biomedical applications in early diagnosis of melanoma and other tyrosinase-related diseases.

Disposable Paper-Based Biosensors: Optimizing the Electrochemical Properties of Laser-Induced Graphene

Laser-induced graphene (LIG) on paper substrates is a desirable material for single-use point-of-care sensing with its high-quality electrical properties, low fabrication cost, and ease of disposal. While a prior study has shown how the repeated lasing of substrates enables the synthesis of high-quality porous graphitic films, however, the process-property correlation of lasing process on the surface microstructure and electrochemical behavior, including charge-transfer kinetics, is missing. The current study presents a systematic in-depth study on LIG synthesis to elucidate the complex relationship between the surface microstructure and the resulting electroanalytical properties. The observed improvements were then applied to develop high-quality LIG-based electrochemical biosensors for uric acid detection. We show that the optimal paper LIG produced via a dual pass (defocused followed by focused lasing) produces high-quality graphene in terms of crystallinity, sp² content, and electrochemical surface area. The highest quality LIG electrodes achieved a high rate constant k₀ of 1.5 × 10^-2 cm s^-1 and a significant reduction in charge-transfer resistance (818 Ω compared with 1320 Ω for a commercial glassy carbon electrode). By employing square wave anodic stripping voltammetry and chronoamperometry on a disposable two-electrode paper LIG-based device, the improved charge-transfer kinetics led to enhanced performance for sensing of uric acid with a sensitivity of 24.35 ± 1.55 μA μM^-1 and a limit of detection of 41 nM. This study shows how high-quality, sensitive LIG electrodes can be integrated into electrochemical paper analytical devices.

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.

Phenolic Tyrosinase Substrate with a Formal Potential Lower than That of Phenol to Obtain a Sensitive Electrochemical Immunosensor

The high and selective catalytic activities of tyrosinase (Tyr) have frequently led to its application in sensitive biosensors. However, in affinity-based biosensors, the use of Tyr as a catalytic label is less common compared to horseradish peroxidase and alkaline phosphatase owing to the fact that phenolic Tyr substrates have yet to be investigated in detail. Herein, four phenolic compounds that have lower formal potentials than phenol were examined for their applicability as Tyr substrates, and three reducing agents were examined as potential strong reducing agents for electrochemical-chemical (EC) redox cycling involving an electrode, a Tyr product, and a reducing agent. The combination of 4-methoxyphenol (MP) and ammonia-borane (AB) allows for (i) a high electrochemical signal level owing to rapid EC redox cycling and (ii) a low electrochemical background level owing to the slow oxidation of AB at a low applied potential and no reaction between MP and AB. When this combination was applied to an electrochemical immunosensor for parathyroid hormone (PTH) detection, a detection limit of 2 pg/mL was obtained. This detection limit is significantly lower than that obtained when a combination of phenol and AB was employed (300 pg/mL). It was also found that the developed immunosensor works well in PTH detection in clinical serum samples. This new phenolic substrate could therefore pave the way for Tyr to be more commonly used as a catalytic label in affinity-based biosensors.

Electrochemistry/Photoelectrochemistry-Based Immunosensing and Aptasensing of Carcinoembryonic Antigen

Recently, electrochemistry- and photoelectrochemistry-based biosensors have been regarded as powerful tools for trace monitoring of carcinoembryonic antigen (CEA) due to the fact of their intrinsic advantages (e.g., high sensitivity, excellent selectivity, small background, and low cost), which play an important role in early cancer screening and diagnosis and benefit people's increasing demands for medical and health services. Thus, this mini-review will introduce the current trends in electrochemical and photoelectrochemical biosensors for CEA assay and classify them into two main categories according to the interactions between target and biorecognition elements: immunosensors and aptasensors. Some recent illustrative examples are summarized for interested readers, accompanied by simple descriptions of the related signaling strategies, advanced materials, and detection modes. Finally, the development prospects and challenges of future electrochemical and photoelectrochemical biosensors are considered.

Aptamer and bifunctional enzyme co-functionalized MOF-derived porous carbon for low-background electrochemical aptasensing

To improve the efficiency of aptasensors, a signal amplification strategy by coupling tyrosinase (Tyr)-triggered redox cycling with nanoscale porous carbon (NCZIF) has been proposed. The NCZIF was obtained by calcining ZIF-8 crystals in an inert atmosphere. It had high surface areas, great biocompatibility, and ease of functionalization, which was beneficial for immobilizing sufficient Tyr and aptamer covalently. When the target prostate-specific antigen (PSA) was present, the NCZIF functionalized with Tyr and an aptamer bound to the aptamer-modified Au electrode specifically through the sandwich structure. Then, Tyr acted to oxidize the electroinactive phenol, which led to low-background signal, in the substrate to electroactive catechol, and triggered the redox cycling under the action of NADH. The low detection limit of the proposed electrochemical aptasensor for PSA was 0.01 ng mL^-1, and the wide detection range was from 0.01 to 50 ng mL^-1. The use of ZIF-8 derived porous carbon and Tyr-triggered redox cycling system provided a promising solution for the development of simple, rapid, reliable, and low-background aptasensing methods, which had great potential in the field of disease diagnosis and biomedicine.

Signal amplification of novel sandwich-type genosensor via catalytic redox-recycling on platform MWCNTs/Fe3O4@TMU-21 for BRCA1 gene detection

The MWCNTs/Fe₃O₄@TMU-21 as a novel electrochemical sandwich-type genosensor was fabricated to detect the BRCA1 gene using the redox-cycling ferrocene functionalized reporter label probe (r-Fc-DNA). In the designed genosensor, the capture probe (cDNA) and r-Fc-DNA were used to detect the BRCA1 gene in sandwich-type genosensor, in which DNA sequences are well -hybridized with the BRCA1 gene (t-DNA). The cDNA was immobilized on the multiwall carbon nanotube and metal-organic framework with Fe₃O₄ nanoparticle core, which is the sensor platform. Target DNA was assayed by redox-recycling reporter probe (r-Fc-DNA) using the electro-catalytic activity of ferri/ferrocyanide, which results in significantly enhanced the oxidation peak current of r-Fc-DNA. The electrochemical redox cycling led to a high signal-to-noise ratio for gene assay. MWCNTs and Fe₃O₄@TMU-21 were applied to increase the platform conductivity and suitable binding of the recognition elements. This constructed genosensor plays an influential role in increasing the sensitivity of BRCA1 gene sequence recognition. So that under optimal conditions, this genosensor illustrated a wide linear range from 1.0×10^-15 to 1.0×10^-10 M with a detection limit of 0.57 × 10^-15 M. Moreover, the genosensor exhibited high selectivity, stability, and reproducibility. The obtained recoveries (between 91 and 105%) of the BRCA1 gene assay in human blood samples satisfactory, which can be used for BRCA1 gene measurement in the laboratory.

Natural Molybdenite- and Tyrosinase-Based Amperometric Catechol Biosensor Using Acridine Orange as a Glue, Anchor, and Stabilizer for the Adsorbed Tyrosinase

To develop a natural mineral-based electrochemical enzyme biosensor, natural molybdenite (MLN), tyrosinase (TYR), and acridine orange (AO) were coadsorbed onto a glassy carbon electrode (GCE). The developed TYR/AO/MLN-GCE-based amperometric TYR biosensor exhibited excellent performance for highly sensitive determination of catechol (linear range, 0.1-80 μM; sensitivity, 0.0315 μA/μM; LOD, 0.029 μM; response time, <4 s) with good reproducibility and good operational and storage stabilities. The electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance with dissipation (QCM-D) revealed interesting roles of AO: (1) an efficient glue for enhancing the amount of the adsorbed TYR on the MLN-GCE, (2) an anchor for efficient orientation of the adsorbed TYR on the MLN-GCE, and (3) a stabilizer providing a suitable microenvironment for the adsorbed TYR on the MLN-GCE surface. This physical adsorption-based AO-coupled enzyme-modification strategy onto natural MLN would be a versatile strategy to develop cost-effective and environment-friendly natural mineral-based electrochemical biosensors and bioelectronic devices.

Integrated Experimental and Theoretical Studies on an Electrochemical Immunosensor

Electrochemical immunosensors (EIs) integrate biorecognition molecules (e.g., antibodies) with redox enzymes (e.g., horseradish peroxidase) to combine the advantages of immunoassays (high sensitivity and selectivity) with those of electrochemical biosensors (quantitative electrical signal). However, the complex network of mass-transfer, catalysis, and electrochemical reaction steps that produce the electrical signal makes the design and optimization of EI systems challenging. This paper presents an integrated experimental and modeling framework to address this challenge. The framework includes (1) a mechanistic mathematical model that describes the rate of key mass-transfer and reaction steps; (2) a statistical-design-of-experiments study to optimize operating conditions and validate the mechanistic model; and (3) a novel dimensional analysis to assess the degree to which individual mass-transfer and reaction steps limit the EI's signal amplitude and sensitivity. The validated mechanistic model was able to predict the effect of four independent variables (working electrode overpotential, pH, and concentrations of catechol and hydrogen peroxide) on the EI's signal magnitude. The model was then used to calculate dimensionless groups, including Damkohler numbers, novel current-control coefficients, and sensitivity-control coefficients that indicated the extent to which the individual mass-transfer or reaction steps limited the EI's signal amplitude and sensitivity.

Double-site DNA walker based ternary electrochemiluminescent biosensor

A novel biosensor was developed on the basis of Ru(dcbpy)(bpy)₂²⁺/tripropylamine (TPrA)/TiO₂ nanocrystallines (TiO₂ NCs) as efficient electrochemiluminescence (ECL) ternary system and enzyme-driven double-site DNA walker as signal amplification strategy for the sensitive detection of carcinoembryonic antigen (CEA). Specifically, coreaction accelerator anatase TiO₂ NCs with catalytic activity could accelerate the oxidization of TPrA for prominently stimulating the ECL performance of Ru(dcbpy)(bpy)₂²⁺/TPrA system to achieve the "signal on" state. Subsequently, numerous double-site walker DNA, converted from the target (CEA)-induced protein-aptamer cycle amplification, would trigger the detachment of Ru(dcbpy)(bpy)₂²⁺ to reach the state of "signal-off". Benefiting from the above advantages, the developed ECL biosensor achieved outstanding sensitivity with a linear range from 500 pg/mL to 50 fg/mL and a detection limit down to 10.5 fg/mL. More importantly, the proposed strategy opens a new path for employing the ECL ternary system for sensitive detection of biomolecules and disease diagnosis.

Catechol-Based Capacitor for Redox-Linked Bioelectronics

A common bioelectronics goal is to enable communication between biology and electronics, and success is critically dependent on the communication modality. When a biorelevant modality aligns with instrumentation capabilities, remarkable successes have been observed (e.g., electrodes provide a powerful tool to observe and actuate biology through its ion-based electrical modality). Emerging biological research demonstrates that redox is another biologically relevant modality, and recent research has shown that advanced electrochemical methods enable biodevice communication through this redox modality. Here, we briefly summarize the biological relevance of this redox modality and the use of redox mediators to enable access to this modality through electrochemical measurements. Next, we describe the fabrication of a catechol-chitosan redox capacitor that is redox-active but nonconducting and thus offers a unique set of molecular electronic properties that enhance access to redox-based information. Finally, we cite several recent studies that demonstrate the broad potential for this capacitor to access redox-based biological information. In summary, we envision the redox capacitor will become a vital component in the integrated circuitry of redox-linked bioelectronics.

Efficient Electrochemical Self-Catalytic Platform Based on l-Cys-hemin/G-quadruplex and Its Application for Bioassay

Commonly, in the artificial enzyme-involved signal amplification approach, the catalytic efficiency was limited by the relatively low binding affinity between artificial enzyme and substrate. In this work, substrate l-cysteine (l-Cys) and hemin were combined into one molecule to form l-Cys-hemin/G-quadruplex as an artificial self-catalytic complex for the improvement of the binding affinity between l-Cys-hemin/G-quadruplex and l-Cys. The apparent Michaelis-Menten constant ( K_m = 2.615 μM) on l-Cys-hemin/G-quadruplex for l-Cys was further investigated to assess the affinity, which was much lower than that of hemin/G-quadruplex ( K_m = 8.640 μM), confirming l-Cys-hemin/G-quadruplex possessed better affinity to l-Cys compared with that of hemin/G-quadruplex. Meanwhile, l-Cys bilayer could be further assembled onto the surface of l-Cys-hemin/G-quadruplex based on hydrogen-bond and electrostatic interaction to concentrate l-Cys around the active center, which was beneficial to the catalytic enhancement. Through this efficient electrochemical self-catalytic platform, a sensitive thrombin aptasensor was constructed. The results exhibited good sensitivity from 0.1 pM to 80 nM and the detection limit was calculated to be 0.032 pM. This self-catalytic strategy with improved binding affinity between l-Cys-hemin/G-quadruplex and l-Cys could provide an efficient approach to improve artificial enzymatic catalytic efficiency.

Photogenerated Hole-Induced Chemical Redox Cycling on Bi2S3/Bi2Sn2O7 Heterojunction: Toward General Amplified Split-Type Photoelectrochemical Immunoassay

This work reports the elegant bridging of enzymatic generation of electron donor with photogenerated hole-induced chemical redox cycling amplification (RCA) for innovative photoelectrochemical (PEC) immunoassay, by the aid of a heterojunction photoelectrode with split-type strategy. Specifically, the system was exemplified by the alkaline phosphatase (ALP) catalytic generation of ascorbic acid (AA), the redox cycling of AA by tris (2-carboxyethyl) phosphine (TCEP) as reductant, and the use of a novel Bi₂S₃/Bi₂Sn₂O₇ heterojunction and myoglobin (Myo) as the photoelectrode and the target, respectively. After the immunoreaction and ALP-induced production of AA, the subsequent oxidation of AA by the photogenerated holes of the Bi₂S₃/Bi₂Sn₂O₇ heterojunction could be cycled via the regeneration of AA by TCEP from the oxidized product of dehydroascorbic acid, leading to easy signal amplification for the sensitive immunoassay of Myo in real samples. It is believed that this work provided a basis for further design and development of general RCA-based PEC immunoassays.

Catechol-chitosan redox capacitor for added amplification in electrochemical immunoanalysis

Antibodies are common recognition elements for molecular detection but often the signals generated by their stoichiometric binding must be amplified to enhance sensitivity. Here, we report that an electrode coated with a catechol-chitosan redox capacitor can amplify the electrochemical signal generated from an alkaline phosphatase (AP) linked immunoassay. Specifically, the AP product p-aminophenol (PAP) undergoes redox-cycling in the redox capacitor to generate amplified oxidation currents. We estimate an 8-fold amplification associated with this redox-cycling in the capacitor (compared to detection by a bare electrode). Importantly, this capacitor-based amplification is generic and can be coupled to existing amplification approaches based on enzyme-linked catalysis or magnetic nanoparticle-based collection/concentration. Thus, the capacitor should enhance sensitivities in conventional immunoassays and also provide chemical to electrical signal transduction for emerging applications in molecular communication.

A novel photoswitchable enzyme cascade for powerful signal amplification in versatile bioassays

This report outlines the construction of an advanced, exquisite photoswitchable enzyme cascade on the basis that tyrosinase (TYR) catalyzes the generation of dihydroxyphenylalanine (DOPA) coordinated TiO₂ nanoparticles (NPs) to form a light responsive nano-trigger that subsequently photoactivates the enzymatic activity of horseradish peroxidase (HRP). This photoswitchable enzyme cascade has a powerful signal transduction/amplification ability in TYR-based bioassays, and holds great promise to be applied in versatile applications.

Embedded Disposable Functionalized Electrochemical Biosensor with a 3D-Printed Flow Cell for Detection of Hepatic Oval Cells (HOCs)

Hepatic oval cells (HOCs) are considered the progeny of the intrahepatic stem cells that are found in a small population in the liver after hepatocyte proliferation is inhibited. Due to their small number, isolation and capture of these cells constitute a challenging task for immunosensor technology. This work describes the development of a 3D-printed continuous flow system and exploits disposable screen-printed electrodes for the rapid detection of HOCs that over-express the OV6 marker on their membrane. Multiwall carbon nanotube (MWCNT) electrodes have a chitosan film that serves as a scaffold for the immobilization of oval cell marker antibodies (anti-OV6-Ab), which enhance the sensitivity of the biomarker and makes the designed sensor specific for oval cells. The developed sensor can be easily embedded into the 3D-printed flow cell to allow cells to be exposed continuously to the functionalized surface. The continuous flow is intended to increase capture of most of the target cells in the specimen. Contact angle measurements were performed to characterize the nature and quality of the modified sensor surface, and electrochemical measurements (cyclic voltammetry (CV) and square wave voltammetry (SWV)) were performed to confirm the efficiency and selectivity of the fabricated sensor to detect HOCs. The proposed method is valuable for capturing rare cells and could provide an effective tool for cancer diagnosis and detection.

Current Technologies of Electrochemical Immunosensors: Perspective on Signal Amplification

An electrochemical immunosensor employs antibodies as capture and detection means to produce electrical charges for the quantitative analysis of target molecules. This sensor type can be utilized as a miniaturized device for the detection of point-of-care testing (POCT). Achieving high-performance analysis regarding sensitivity has been one of the key issues with developing this type of biosensor system. Many modern nanotechnology efforts allowed for the development of innovative electrochemical biosensors with high sensitivity by employing various nanomaterials that facilitate the electron transfer and carrying capacity of signal tracers in combination with surface modification and bioconjugation techniques. In this review, we introduce novel nanomaterials (e.g., carbon nanotube, graphene, indium tin oxide, nanowire and metallic nanoparticles) in order to construct a high-performance electrode. Also, we describe how to increase the number of signal tracers by employing nanomaterials as carriers and making the polymeric enzyme complex associated with redox cycling for signal amplification. The pros and cons of each method are considered throughout this review. We expect that these reviewed strategies for signal enhancement will be applied to the next versions of lateral-flow paper chromatography and microfluidic immunosensor, which are considered the most practical POCT biosensor platforms.

An Integrated Redox Cycling for Electrochemical Enzymatic Signal Enhancement

Highly sensitive analytical methods for the detection of proteins are still an urgent demand in early medical diagnosis and the discovery of biomarkers with ultralow abundance. Here an integrated electrochemical-chemical-enzymatic redox cycling is designed for significant enhancement of electrochemical enzymatic signal in biorecognition. This strategy efficiently utilizes the high specificity of the outersphere to innersphere redox reaction to mediate the enzymatic redox cycling with the nonenzymatic redox cycling. The oxygenation activity of tyrosinase as a label of the biorecognition event ensures low background and generates outersphere-reaction-philic/innersphere-reaction-philic redox couples, which leads to 13 300× amplification of electrochemical signal. The mediation of nonenzymatic redox cycling in the integrated system produces a 14-fold improved ratio of signal to background. The practicality of the proposed approach with clinical samples demonstrates its potential in clinical diagnostic and therapeutic monitoring. This work opens a new avenue to design novel signal amplification strategies for ultrasensitive bioanalysis.

Cucurbituril and Azide Cofunctionalized Graphene Oxide for Ultrasensitive Electro-Click Biosensing

To achieve high selectivity and sensitivity simultaneously in an electrochemical biosensing platform, cucurbituril and azide cofunctionalized graphene oxide, a new functional nanomaterial that acts as a go-between to connect the recognition element with amplified signal architecture, is developed in this work. The cucurbituril and azide cofunctionalized graphene oxide features a high specific surface area with abundant levels of the two types of functional groups. Specifically, it emerges as a powerful tool to link recognition elements with simplicity, high yield, rapidity, and highly selective reactivity through azide-alkynyl click chemistry. Moreover, it possesses many host molecules to interact with guest molecules (also signal molecules)-grafted branched ethylene imine polymer, through which the detection sensitivity can be greatly improved. Together with electro-click technology, a highly controllable, selective, and sensitive biosensing platform can be easily created. For VEGF₁₆₅ protein detection, the electro-click assay has high selectivity and sensitivity; a dynamic detection range from 10 fg mL^-1 to 1 ng mL^-1 with a detection limit of 8 fg mL^-1 was achieved. The electro-click biosensing strategy based on cucurbituril and azide cofunctionalized graphene oxide would have great promise for other target analytes with a broad range of applications.

An ultrasensitive enzyme-free electrochemical immunosensor based on redox cycling amplification using methylene blue

We report a new enzyme-free electrochemical sensor for ultrasensitive measurements of protein biomarkers in plasma and whole blood samples based on a unique electrochemical-chemical-chemical (ECC) redox cycling signal amplification scheme. This scheme uses methylene blue (MB) as a redox indicator which undergoes an endergonic reaction with Ru(NH3)63+ and a highly exergonic reaction with tris(2-carboxyethyl)phosphine (TCEP). This approach offers improved detection sensitivity and sensor stability compared with enzyme-based ECC redox cycling techniques, while involving a simpler sensor modification process and detection protocol. This redox cycling scheme was combined with a robust immunosandwich assay for quantitative measurements of protein biomarkers. For proof of principle, Plasmodium falciparum histidine-rich protein 2 (PfHRP2) was measured in human plasma and whole blood samples, which could be detected down to 10 fg mL-1 and 18 fg mL-1, respectively. Furthermore, this immunosensor exhibits high selectivity, excellent reproducibility and good stability for up to 2 weeks, making it a promising platform for point-of-care testing, especially for detecting extremely low biomarker concentrations in raw biofluids.