Chronoamperometry-Based Redox Cycling for Application to Immunoassays

Constraint Coupling of Redox Cascade and Electron Transfer Synchronization on Electrode-Nanosensor Interface for Repeatable Detection of Tumor Biomarkers

Quantitative analysis of up-regulated biomarkers in pathological tissues is helpful to tumor surgery yet the loss of biomarker extraction and time-consuming operation limited the accurate and quick judgement in preoperative or intraoperative diagnosis. Herein, an immobilization-free electrochemical sensing platform is developed by constraint coupling of electron transfer cascade on electrode-nanosensor interface. Specifically, electrochemical indicator (Ri)-labeled single-stranded DNA on electroactive nanodonor (polydopamine, PDA) can be responsively detached by formation of DNA complex through the recognition and binding with targets. By applying the oxidation potential of Ri, nanosensor collisions on electrode surface trigger a cascade redox cycling of PDA and Ri through synchronous electron transfer, which boost the amplification of current signal output. The developed nanosensor exhibit excellent linear response toward up-regulated biomarkers (miRNA-21, ATP, and VEGF) with low detection limits (32 fM, 386 pM, and 2.8 pM). Moreover, background influence from physiological interferent is greatly reduced by restricted electron transfer coupling on electrode. The practical applicability is illustrated in sensitive and highly repeatable profiling of miRNA-21 in lysate of tumor cells and tumor tissue, beneficial for more reliable diagnosis. This electrochemical platform by employing electron transfer cascades at heterogeneous interfaces offers a route to anti-interference detection of biomarkers in tumor tissues.

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.

Electrochemical analysis of total phospholipids in human serum for severe sepsis diagnosis

Electrochemical analysis of total phospholipids was performed for the diagnosis of sepsis. The influence of electrode materials on the analysis of the chromogenic substrate was analyzed using Au, graphite, and pyrolyzed carbon electrodes. The total phospholipid analysis based on electrochemical analysis with pyrolyzed carbon was used for diagnosis of sepsis using sera from healthy volunteers, systemic inflammatory response syndrome (SIRS), and severe sepsis patients. The analysis results using the optical measurement and the electrochemical analysis were compared for the serum samples from sepsis patients and healthy controls. Additionally, the interference of human serum on the optical measurement and electrochemical analysis was estimated by signal-to-noise (S/N) calculation. The assay results of the levels of other biomarkers for sepsis (C-reactive protein and procalcitonin) and the total phospholipid levels obtained using the optical measurement and electrochemical analysis methods were statistically similar. Finally, the mortality of patients, indicated by the results of the total phospholipid assay performed using the electrochemical analysis of the patient samples collected daily (1, 3, and 7 day(s) after admission to hospital), was compared with the patient mortality assessed via conventional severity indexes, such as the SOFA and APACHE Ⅱ scores. The 28-day survival rate was estimated by Kaplan-Meier survival analysis based on the total phospholipid level of patient samples that were obtained after 1, 3, and 7 day(s) from hospital admission.

A wide-range UAC sensor for the classification of hyperuricemia in spot samples

Hyperuricemia (HUA) has received wide attention as an independent risk factor for various chronic diseases. HUA is usually asymptomatic, and the related damage can be reduced by effective classification and treatment according to uric acid clearance (UAC). UAC is a calculated ratio based on the uric acid level in blood and urine. This important method is not universally used due to the inconvenience of collecting 24-h urine samples in the clinic, and most sensors are limited by the need for wide ranges and for two testing samples. In this study, a pH-sensitive urate oxidase-modified electrochemical sensor with filter membrane was proposed to calculate UAC by detecting uric acid in blood and urine. The results demonstrated that the sensor had high selectivity for uric acid with a detection limit of 0.25 μM in 5 μL spot sample, the wide linear range was 2.5-7000 μM, and the impact of the sample pH was calibrated. The linear correlation of the measurement results between the UAC sensor and clinical instrument was higher than 0.980 for 87 patients. The change in UAC in spot urine may reflect alteration in body-transport mechanisms. Thus, the UAC sensor may open a new window for the management of HUA and broaden its application in point-of-care testing.

Bimetallic Cu-Zn Zeolitic Imidazolate Frameworks as Peroxidase Mimics for the Detection of Hydrogen Peroxide: Electrochemical and Spectrophotometric Evaluation

A copper incorporated zeolitic imidazolate framework-8 (ZIF-8) has been synthesized and demonstrated to be a potential material for a peroxidase mimic. The resultant bimetallic Cu-Zn incorporated MOF is used for the dual mode sensing of hydrogen peroxide by following electrochemical as well as spectrophotometric methods. Using 3,3',5,5'-tetramethylbenzidine (TMB) as a chromogenic substrate, spectrophotometric studies are carried out, and the steady state kinetic parameters are determined for two different concentrations of Cu incorporated ZIF-8 (viz Cu@ZIF-8-1 and Cu@ZIF-8-2). It is found that both Cu@ZIF-8-1 and Cu@ZIF-8-2 exhibit more affinity toward the TMB substrate than the horseradish peroxidase (HRP) enzyme as indicated by the low Km values obtained for the substrate. Also, as the concentration of incorporated Cu increases, Vmax values are also found to be enhanced. Electrochemically, the Cu@ZIF-8 modified glassy carbon electrode (GCE) showed a good response for peroxide detection in the concentration range from 0.5 mM to 5 mM at a working potential of -0.25 V in PBS (pH 7.0) with a limit of detection (LOD) value of 0.46 mM and a sensitivity of 20.25 μA/mM. Further, the chromogenic substrate TMB is successfully immobilized on the electrode surface and subsequently used for the peroxide detection along with Cu@ZIF-8. Here, TMB acts as a mediator and shifted the working potential to 0.1 V in acetate buffer (pH 5.0) in the concentration range from 0.5 mM to 5 mM with an LOD value of 0.499 mM and a sensitivity of 0.097 μA/mM. Interestingly, the same electrode in PBS of pH 7.0 showed a response to peroxide at a working potential of -0.1 V in the concentration range from 0.5 mM to 5 mM with an LOD value of 0.143 mM and a sensitivity of 0.33 μA/mM. Moreover, the applicability of this material for peroxide sensing is evaluated using milk samples, and the proposed material is able to recover the peroxide present in milk. Thus, the bimetallic Cu-Zn MOF can be utilized for the dual mode sensing of peroxide and can be extended for various real time applications.

Efficient Simultaneous Detection of Metabolites Based on Electroenzymatic Assembly Strategy

Objective and Impact Statement: We describe an electroenzymatic mediator (EM) sensor based on an electroenzymatic assembly peak separation strategy, which can efficiently realize the simultaneous detection of 3 typical cardiovascular disease (CVD) metabolites in 5 μl of plasma under one test. This work has substantial implications toward improving the efficiency of chronic CVD assessment. Introduction: Monitoring CVD of metabolites is strongly associated with disease risk. Independent and time-consuming detection in hospitals is unfavorable for chronic CVD management. Methods: The EM was flexibly designed by the cross-linking of electron mediators and enzymes, and 3 EM layers with different characteristics were assembled on one electrode. Electrons were transferred under tunable potential; 3 metabolites were quantitatively detected by 3 peak currents that correlated with metabolite concentrations. Results: In this study, the EM sensor showed high sensitivity for the simultaneous detection of 3 metabolites with a lower limit of 0.01 mM. The linear correlation between the sensor and clinical was greater than 0.980 for 242 patients, and the consistency of risk assessment was 94.6%. Conclusion: Metabolites could be expanded by the EM, and the sensor could be a promising candidate as a home healthcare tool for CVD risk assessment.

A vertically paired electrode for redox cycling and its application to immunoassays

An electrochemical immunoassay based on the redox cycling method was presented using vertically paired electrodes (VPEs), which were fabricated using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as an electrode material and parylene-C as a dielectric layer. For the application to immunoassays, different electrochemical properties of PEDOT:PSS were analyzed for the redox reaction of 3,3',5,5'-tetramethylbenzidine (TMB, the chromogenic substrate for enzyme-immunoassays) at different pH conditions, including the conductivity (σ), electron transfer rate constant (k_app), and double-layer capacitance (C_dl). The influencing factors on the sensitivity of redox cycling based on VPE based on PEDOT:PSS were analyzed for the redox reaction of TMB, such as the electrode gap and number of electrode pairs. Computer simulation was also performed for the redox cycling results based on VPEs, which had limitations in fabrication, such as VPEs with an electrode gap of less than 100 nm and more than five electrode pairs. Finally, the redox cycling based on VPE was applied to the medical diagnosis of human hepatitis-C virus (hHCV) using a commercial ELISA kit. The sensitivity of the redox cycling method for the medical diagnosis of hHCV was compared with conventional assay methods, such as TMB-based chromogenic detection, luminol-based chemiluminescence assay, and a rapid test kit (lateral flow immunoassay).

In Vivo Plant Bio-Electrochemical Sensor Using Redox Cycling

This work presents an in vivo stem-mounted sensor for Nicotiana tabacum plants and an in situ cell suspension sensor for Solanum lycopersicum cells. Stem-mounted sensors are mechanically stable and less sensitive to plant and air movements than the previously demonstrated leaf-mounted sensors. Interdigitated-electrode-arrays with a dual working electrode configuration were used with an auxiliary electrode and an Ag/AgCl quasi-reference electrode. Signal amplification by redox cycling is demonstrated for a plant-based sensor responding to enzyme expression induced by different cues in the plants. Functional biosensing is demonstrated, first for constitutive enzyme expression and later, for heat-shock-induced enzyme expression in plants. In the cell suspension with redox cycling, positive detection of the enzyme β-glucuronidase (GUS) was observed within a few minutes after applying the substrate (pNPG, 4-Nitrophenyl β-D-glucopyranoside), following redox reactions of the product (p-nitrophenol (pNP)). It is assumed that the initial reaction is the irreversible reduction of pNP to p-hydroxylaminophenol. Next, it can be either oxidized to p-nitrosophenol or dehydrated and oxidized to aminophenol. Both last reactions are reversible and can be used for redox cycling. The dual-electrode redox-cycling electrochemical signal was an order of magnitude larger than that of conventional single-working electrode transducers. A simple model for the gain is presented, predicting that an even larger gain is possible for sub-micron electrodes. In summary, this work demonstrates, for the first time, a redox cycling-based in vivo plant sensor, where diffusion-based amplification occurs inside a tobacco plant's tissue. The technique can be applied to other plants as well as to medical and environmental monitoring systems.

Interference of electrochemical ion diffusion in nanopore sensing

Stable and fast-responding ionic current is a prerequisite for reliable measurements of small objects with a nanopore. Here, we report on the interference of ion diffusion kinetics at liquid-electrode interfaces in nanopore sensing. Using platinum as electrodes, we observed a slow and large decrease in the ionic current through a nanopore in a salt solution suggestive of the considerable influence of the growing impedance at the liquid-metal interfaces via Cottrell diffusion. When detecting nanoparticles, the resistive pulses became weaker following the steady increase in the resistance at the partially polarizable electrodes. The interfacial impedance was also demonstrated to couple with the nanopore chip capacitance thereby degraded the temporal resolution of the ionic current measurements in a time-varying manner. These findings can be useful for choosing the suitable size and material of electrodes for the single-particle and -molecule analyses by ionic current.

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Ag/AgCl electrodes enable reliable resistive pulse detections of nanoparticles

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Pt electrodes induce ionic current decay by time via the Cottrell diffusion

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Cottrell diffusion deteriorates the nanopore sensor temporal resolution

Rapid and sensitive detection of viral particles by coupling redox cycling and electrophoretic enrichment

The COVID-19 pandemic has highlighted the need for rapid, low-cost, and sensitive virus detection platforms to monitor and mitigate widespread outbreaks. Electrochemical sensors are a viable choice to fill this role but still require improvements to the signal magnitude, especially for early detection and low viral loads. Herein, finite element analysis of a novel biosensor concept for single virion counting using a generator-collector microelectrode design is presented. The proposed design combines a redox-cycling amplified electrochemical current with electrophoresis-driven electrode-particle collision for rapid virus detection. The effects of experimental (e.g. scan rate, collector bias) and geometric factors are studied to optimize the sensor design. Two generator-collector configurations are explored: a ring-disk configuration to analyze sessile droplets and an interdigitated electrode (IDE) design housed in a microchannel. For the ring-disk configuration, we calculate an amplification factor of ∼5 and collector efficiency of ∼0.8 for a generator-collector spacing of 600 nm. For the IDE, the collector efficiency is even larger, approaching unity. The dual-electrode mode is critical for increasing the current and electric field strength. As a result, the current steps upon virus capture are more than an order of magnitude larger compared to single-mode. Additionally, single virus capture times are reduced from over 700 s down to ∼20 s. Overall, the frequency of virus capture and magnitude of the electrochemical current steps depend on the virus properties and electrode configuration, with the IDE capable of single virus detection within seconds owing to better particle confinement in the microchannel.

The heparinase-linked differential time method allows detection of heparin potency in whole blood with high sensitivity and dynamic range

Anticoagulation therapy with heparin is an effective treatment against thrombosis. Heparin tends to cause spontaneous bleeding and requires regular monitoring during therapy. Most high-sensitivity heparin sensors have focused on the concentration detection in clarified buffer solution. However, the pharmacodynamics of heparin vary depending on individual patient or disease, while potency detection with high sensitivity and dynamic range outperforms concentration detection in clinical diagnosis. In this study, a novel heparinase-linked differential time (HLDT) method was established with a two-zone of Graphene modified Carbon (GR-C) sensor, which was utilized to evaluate heparin potency in whole blood. It was based on electrochemical measurement of clotting time shifting associated with presence or absence of heparinase. Heparinase inhibits the anticoagulant ability of heparin by forming a heparin-antithrombin-thrombin complex during coagulation. And the intensity and peak time of electrochemical current were associated with thrombin activity and clotting on the electrode. The results demonstrated that the sensor had high selectivity for heparin potency in 10 μL of whole blood with a detection limit of 0.1 U/mL, and the linear detection range was 0.1-5 U/mL. The coefficient of variation (CV) of the peak time was less than 5%, and linear correlation between the GR-C sensor and the TEG-5000 instrument was 0.987. Thus, the HLDT method has better clinical application due to its good repeatability, high sensitivity and wide range in heparin potency evaluation.

Electrochemical One-Step Immunoassay Based on Switching Peptides and Pyrolyzed Carbon Electrodes

Switching peptides were designed to bind reversibly to the binding pocket of antibodies (IgG) by interacting with frame regions (FRs). These peptides can be quantitatively released when antigens bind to IgG. As FRs have conserved amino acid sequences, switching peptides can be used as antibodies for different antigens and different source animals. In this study, an electrochemical one-step immunoassay was conducted using switching peptides labeled with ferrocene for the quantitative measurement of analytes. For the effective amperometry of the switching peptides labeled with ferrocene, a pyrolyzed carbon electrode was prepared by pyrolysis of the parylene-C film. The feasibility of the pyrolyzed carbon electrode for the electrochemical one-step immunoassay was determined by analyzing its electrochemical properties, such as its low double-layer capacitance (C_dl), high electron transfer rate (k_app), and wide electrochemical window. In addition, the factors influencing the amperometry of switching peptides labeled with ferrocene were analyzed according to the hydrodynamic radius, the number of intrahydrogen bonds, dipole moments, and diffusion coefficients. Finally, the applicability of the electrochemical one-step immunoassay for the medical diagnosis of the human hepatitis B surface antigen (hHBsAg) was assessed.

Capacitive biosensor based on vertically paired electrodes for the detection of SARS-CoV-2

Vertically paired electrodes (VPEs) with multiple electrode pairs were developed for the enhancement of capacitive measurements by optimizing the electrode gap and number of electrode pairs. The electrode was fabricated using a conductive polymer layer of PEDOT:PSS instead of Ag and Pt metal electrodes to increase the VPE fabrication yield because the PEDOT:PSS layer could be effectively etched using a reactive dry etching process. In this study, sensitivity enhancement was realized by decreasing the electrode gap and increasing the number of VPE electrode pairs. Such an increase in sensitivity according to the electrode gap and the number of electrode pairs was estimated using a model analyte for an immunoassay. Additionally, a computer simulation was performed using VPEs with different electrode gaps and numbers of VPE electrode pairs. Finally, VPEs with multiple electrode pairs were applied for SARS-CoV-2 nucleoprotein (NP) detection. The capacitive biosensor based on the VPE with immobilized anti-SARS-CoV-2 NP was applied for the specific detection of SARS-CoV-2 in viral cultures. Using viral cultures of SARS-CoV-2, SARS-CoV, MERS-CoV, and CoV-strain 229E, the limit of detection (LOD) was estimated to satisfy the cutoff value (dilution factor of 1/800) for the medical diagnosis of COVID-19, and the assay results from the capacitive biosensor were compared with commercial rapid kit based on a lateral flow immunoassay.

Carbon electrode obtained via pyrolysis of plasma-deposited parylene-C for electrochemical immunoassays

In this study, parylene-C films from plasma deposition as well as thermal deposition were pyrolyzed to prepare a carbon electrode for application in electrochemical immunoassays.

A Label-Free Electrochemical Immunosensor for CEA Detection on a Novel Signal Amplification Platform of Cu2S/Pd/CuO Nanocomposites

Carcinoembryonic antigen (CEA) is regarded as one of the crucial tumor markers for colorectal cancer. In this study, we developed the snowflake Cu₂S/Pd/CuO nanocomposite to construct an original label-free electrochemical immunosensor for the ultrasensitive detection of CEA levels. The nanocomposite of cuprous sulfide (Cu₂S) with Pd nanoparticles (Pd NPs) was synthesized through an in situ formation of Pd NPs on the Cu₂S. Cuprous sulfide (Cu₂S) and CuO can not only be used as a carrier to increase the reaction area but also catalyze the substrate to generate current signal. Palladium nanoparticles (Pd NPs) have excellent catalytic properties and good biocompatibility, as well as the ability of excellent electron transfer. The immunosensor was designed using 5 mmol/L H₂O₂ as the active substrate by optimizing the conditions with a detection range from 100 fg/ml to 100 ng/ml and a minimum detection limit of 33.11 fg/ml. The human serum was detected by electrochemical immunoassay, and the results were consistent with those of the commercial electrochemical immunosensor. Therefore, the electrochemical immunosensor can be used for the detection of human serum samples and have potential value for clinical application.

Development of an Electrochemical CCL17/TARC Biosensor toward Rapid Triage and Monitoring of Classic Hodgkin Lymphoma

A point-of-care blood test for the detection of an emerging biomarker, CCL17/TARC, could prove transformative for the clinical management of classic Hodgkin lymphoma (cHL). Primary care diagnosis is challenging due to nonspecific clinical presentation and lack of a diagnostic test, leading to significant diagnostic delays. Treatment monitoring encounters false-positive and negative results, leading to avoidable chemotherapy toxicity, or undertreatment, impacting patient morbidity and mortality. Here, we present an amperometric CCL17/TARC immunosensor, based on the utilization of a thiolated heterobifunctional cross-linker and sandwich antibody assay, to facilitate novel primary care triage and chemotherapy monitoring strategies for cHL. The immunosensor shows excellent analytical performance for clinical testing; linearity (R² = 0.986), detection limit (194 pg/mL), and lower and upper limits of quantitation (387-50 000 pg/mL). The biosensor differentiated all 42 newly diagnosed cHL patients from healthy volunteers, based on serum CCL17/TARC concentration, using blood samples collected prior to treatment intervention. The immunosensor also discriminated between paired blood samples of all seven cHL patients, respectively, collected prior to treatment and during chemotherapy, attributed to the decrease in serum CCL17/TARC concentration following chemotherapy response. Overall, we have shown, for the first time, the potential of an electrochemical CCL17/TARC biosensor for primary care triage and chemotherapy monitoring for cHL, which would have positive clinical and psychosocial implications for patients, while streamlining current healthcare pathways.

Advanced Solid State Nano-Electrochemical Sensors and System for Agri 4.0 Applications

Global food production needs to increase in order to meet the demands of an ever growing global population. As resources are finite, the most feasible way to meet this demand is to minimize losses and improve efficiency. Regular monitoring of factors like animal health, soil and water quality for example, can ensure that the resources are being used to their maximum efficiency. Existing monitoring techniques however have limitations, such as portability, turnaround time and requirement for additional reagents. In this work, we explore the use of micro- and nano-scale electrode devices, for the development of an electrochemical sensing platform to digitalize a wide range of applications within the agri-food sector. With this platform, we demonstrate the direct electrochemical detection of pesticides, specifically clothianidin and imidacloprid, with detection limits of 0.22 ng/mL and 2.14 ng/mL respectively, and nitrates with a detection limit of 0.2 µM. In addition, interdigitated electrode structures also enable an in-situ pH control technique to mitigate pH as an interference and modify analyte response. This technique is applied to the analysis of monochloramine, a common water disinfectant. Concerning biosensing, the sensors are modified with bio-molecular probes for the detection of both bovine viral diarrhea virus species and antibodies, over a range of 1 ng/mL to 10 µg/mL. Finally, a portable analogue front end electronic reader is developed to allow portable sensing, with control and readout undertaken using a smart phone application. Finally, the sensor chip platform is integrated with these electronics to provide a fully functional end-to-end smart sensor system compatible with emerging Agri-Food digital decision support tools.

Microbial biosensor for Salmonella using anti-bacterial antibodies isolated from human serum

In this work, we present a novel microbial biosensor for Salmonella based on impedance spectrometry by using isolated antibodies against a specific bacterial strain from human serum. Anti-Salmonella (or BL21(DE3)) antibodies were isolated from human serum using S. enteritidis (or BL21(DE3)) and the mutant strain ClearColi. After the purification steps, the purification yield of the antibodies was calculated to be 0.2 %. From the FACS analysis, the isolated anti-Salmonella antibodies were estimated to have more than 6-fold higher binding affinity for S. enteritidis compared to antibodies against other kinds of Gram-negative bacterial strains, including HB101, ClearColi, JM110, DH5α, and BL21(DE3). Finally, the anti-Salmonella antibodies isolated herein were used for bacterial detection using electrochemical biosensors based on impedance spectrometry and the R_ct value of the antibodies was estimated for S. enteritidis from the Nyquist plot. The limit of detection of the isolated anti-Salmonella antibodies was estimated to be 1.0 × 10³ cells/mL for S. enteritidis and 1.0 × 10⁶ cells/mL for BL21(DE3), respectively.

Diagnosis of severe sepsis using phospholipids enzymatic assay based on cyclic voltammetry

In this work phospholipid quantification was carried out using an enzymatic assay based on cyclic voltammetry of the condensation product of N-ethyl-N-(2-hydroxy-3-sulfopropyl)-3,5-dimethoxyaniline sodium salt (DAOS) and 4-aminoantipyrine (4-AP) with a graphite electrode. For the optimization of electrochemical measurement for the product, electrochemical properties such as the electrochemical window, double layer capacitance (Cdl) and electron transfer rate (kapp) were analyzed for a graphite-electrode and Au-electrode. The phospholipid enzymatic assay based the on electrochemical measurement using the graphite electrode was applied to the diagnosis of sepsis for sera from healthy volunteers (n = 16), patients with systemic inflammatory response syndrome (SIRS, n = 16) and severe sepsis patients (n = 24). Finally, the phospholipid quantification results from the electrochemical measurement were statistically compared with the conventional method based on optical density measurement.