Determination of affinities and antigenic epitopes of bovine cardiac troponin I (cTnI) with monoclonal antibodies by surface plasmon resonance biosensor

Nano-biosensor for SARS-CoV-2/COVID-19 detection: methods, mechanism and interface design

The epidemic of coronavirus disease 2019 (COVID-19) was a huge disaster to human society. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which led to COVID-19, has resulted in a large number of deaths. Even though the reverse transcription-polymerase chain reaction (RT-PCR) is the most efficient method for the detection of SARS-CoV-2, the disadvantages (such as long detection time, professional operators, expensive instruments, and laboratory equipment) limit its application. In this review, the different kinds of nano-biosensors based on surface-enhanced Raman scattering (SERS), surface plasmon resonance (SPR), field-effect transistor (FET), fluorescence methods, and electrochemical methods are summarized, starting with a concise description of their sensing mechanism. The different bioprobes (such as ACE2, S protein-antibody, IgG antibody, IgM antibody, and SARS-CoV-2 DNA probes) with different bio-principles are introduced. The key structural components of the biosensors are briefly introduced to give readers an understanding of the principles behind the testing methods. In particular, SARS-CoV-2-related RNA mutation detection and its challenges are also briefly described. We hope that this review will encourage readers with different research backgrounds to design SARS-CoV-2 nano-biosensors with high selectivity and sensitivity.

Electrochemical Detection of Prostate Cancer Biomarker PCA3 Using Specific RNA-Based Aptamer Labelled with Ferrocene

This paper reports on a feasibility study of electrochemical in-vitro detection of prostate cancer biomarker PCA3 (prostate cancer antigen 3) in direct assay with specific RNA aptamer labelled with a redox group (ferrocene) and immobilized on a screen-printed gold electrode surface. The cyclic voltammograms and electrochemical impedance spectroscopy methods yield encouraging results on the detection of PCA3 in a range of concentrations from 1 μg/mL down to 0.1 ng/mL in buffer solutions. Both anodic and cathodic current values in cyclic voltammograms measurements and charge transfer resistance values in electrochemical impedance spectroscopy experiments correlate with the PCA3 concentration in the sample. Kinetics studies of the binding of the PCA3 to our aptamer demonstrated high specificity of the reaction with a characteristic affinity constant of approximately 4·10−10 molar. The results of this work provide a background for the future development of novel, highly sensitive and cost-effective diagnostic methodologies for prostate cancer detection.

An ultrasensitive electrochemical sensing platform for the detection of cTnI based on aptamer recognition and signal amplification assisted by TdT

We have developed an ultrasensitive and highly specific electrochemical sensing platform for the detection of cardiac troponin I (cTnI), a recognized biomarker for the diagnosis of acute myocardial infarction (AMI) and related cardiovascular diseases (CVDs). This strategy is based on the assists of terminal deoxynucleotidyl transferase (TdT)-mediated signal amplification and the specific recognition between cTnI and the aptamer of cTnI. In this experiment, we prepared a gold electrode that modified with probe 2 (P2), in the presence of cTnI, the aptamer of cTnI that in probe 1 (P1)/aptamer complexes bond with cTnI specifically and release the free P1. P1 would bind with P2, resulting in the formation of 3'-OH of DNA. In the presence of terminal deoxynucleotidyl transferase (TdT) and dTTP, TdT mediated P1 to extend and formed the structure of poly T. Methylene blue (MB)-poly A hybridized with the extended poly T and generated an electrochemical signal. The detection limit can be as low as 40 pg mL-1. This sensor was also successfully applied to the detection of cTnI in numerous spiked biological samples, and it can be a great reference for the clinical diagnosis, prognosis, and treatment of CVDs and AMI.

Ultrasensitive magnetic field-assisted surface plasmon resonance immunoassay for human cardiac troponin I

An ultrasensitive surface plasmon resonance (SPR) immunoassay was developed for the specific detection of human cardiac troponin I (cTnI), a principle diagnostic marker for myocardial damage. The thin gold film evaporated on a glass slate, which was employed as the SPR sensing film, was modified by hollow gold nanoparticles (HGNPs) and polydopamine (PDA) sequentially, and then was immobilized with antibodies for specific recognition of target analyte. Electronic coupling of the surface plasmon waves originating from the HGNPs and the gold film leads to the remarkable amplification of SPR response. The PDA film modified on the gold film via self-polymerization of dopamine (DA) facilitates the direct immobilization of capture antibodies (cAb). To separate and enrich the target analyte, PDA-wrapped magnetic multi-walled carbon nanotubes (MMWCNTs-PDA) were conjugated with detection antibodies (dAb) and used as the extracting agent for the magnetic extraction of cTnI in sample. Large surface area of MMWCNTs-PDA ensures its loading capacity for dAb, as well as its extraction efficiency for cTnI. By serving as the "vehicles" for fast delivering the concentrated analyte to the SPR sensing surface, MMWCNTs-PDA-dAb also overcomes the disadvantage of slow diffusion-limited mass transfer and matrix interference effect in regular patterns. The combination of the above improvements results in the significant sensitivity enhancement of the SPR immunoassay. The concentration of cTnI with minimum detectable SPR response obtained by the present assay is 1.25ngmL^-1, which is 1000-fold lower than that obtained by the traditional SPR immunoassay based on PDA-modified gold film.

Molecular interference in antibody-antigen interaction studied with magnetic force immunoassay

Molecular interferences are an important challenge in biotechnologies based on antibody-antigen interactions, such as sandwich immunoassays. We report how a sandwich immunoassay with magnetic particles as label can be used to probe interference by surfactants. Surfactants are often used to improve the performance of immunoassays, however the surfactants can affect the involved proteins and the mechanism of action of surfactant molecules on the antibody-antigen system is mostly unknown. As an example, we investigated molecular interference by a nonionic surfactant (Pluronic F-127) in a cardiac troponin (cTn) sandwich immunoassay with two monoclonal antibodies. The influence of the surfactant below the critical micelle concentration (0.00-0.04%) on dissociation properties was quantified in a magnetic tweezers setup, where a force is applied to the molecules via magnetic particle labels. The force-dependent dissociation curves revealed the existence of two distinct cTn-dependent bond types, namely a weak bond attributable to non-specific binding of cTn, and a strong bond attributable to the specific binding of cTn. The dissociation rate constant of the strong bonds increased with the surfactant concentration by about a factor of two. Circular dichroism spectroscopy data showed that the nonionic surfactant influences the conformation of cTn while not noticeably affecting the two monoclonal antibodies. This suggests that the surfactant-induced increase of the dissociation rate of the specific sandwich-type cTn binding may be related to a conformational change of the antigen molecule. The described methodology is an effective tool to study the influence of surfactants and other interferences on assays based on protein interactions.

Quantification of ligand binding to G-protein coupled receptors on cell membranes by ellipsometry

G-protein-coupled receptors (GPCRs) are prime drug targets and targeted by approximately 60% of current therapeutic drugs such as β-blockers, antipsychotics and analgesics. However, no biophysical methods are available to quantify their interactions with ligand binding in a native environment. Here, we use ellipsometry to quantify specific interactions of receptors within native cell membranes. As a model system, the GPCR-ligand CXCL12α and its receptor CXCR4 are used. Human-derived Ishikawa cells were deposited onto gold coated slides via Langmuir-Schaefer film deposition and interactions between the receptor CXCR4 on these cells and its ligand CXCL12α were detected via total internal reflection ellipsometry (TIRE). This interaction could be inhibited by application of the CXCR4-binding drug AMD3100. Advantages of this approach are that it allows measurement of interactions in a lipid environment without the need for labelling, protein purification or reconstitution of membrane proteins. This technique is potentially applicable to a wide variety of cell types and their membrane receptors, providing a novel method to determine ligand or drug interactions targeting GPCRs and other membrane proteins.

Analysis of protein interactions at native chloroplast membranes by ellipsometry

Membrane bound receptors play vital roles in cell signaling, and are the target for many drugs, yet their interactions with ligands are difficult to study by conventional techniques due to the technical difficulty of monitoring these interactions in lipid environments. In particular, the ability to analyse the behaviour of membrane proteins in their native membrane environment is limited. Here, we have developed a quantitative approach to detect specific interactions between low-abundance chaperone receptors within native chloroplast membranes and their soluble chaperone partners. Langmuir-Schaefer film deposition was used to deposit native chloroplasts onto gold-coated glass slides, and interactions between the molecular chaperones Hsp70 and Hsp90 and their receptors in the chloroplast membranes were detected and quantified by total internal reflection ellipsometry (TIRE). We show that native chloroplast membranes deposited on gold-coated glass slides using Langmuir-Schaefer films retain functional receptors capable of binding chaperones with high specificity and affinity. Taking into account the low chaperone receptor abundance in native membranes, these binding properties are consistent with data generated using soluble forms of the chloroplast chaperone receptors, OEP61 and Toc64. Therefore, we conclude that chloroplasts have the capacity to selectively bind chaperones, consistent with the notion that chaperones play an important role in protein targeting to chloroplasts. Importantly, this method of monitoring by TIRE does not require any protein labelling. This novel combination of techniques should be applicable to a wide variety of membranes and membrane protein receptors, thus presenting the opportunity to quantify protein interactions involved in fundamental cellular processes, and to screen for drugs that target membrane proteins.

Study of receptor-chaperone interactions using the optical technique of spectroscopic ellipsometry

This work describes a detailed quantitative interaction study between the novel plastidial chaperone receptor OEP61 and isoforms of the chaperone types Hsp70 and Hsp90 using the optical method of total internal reflection ellipsometry (TIRE). The receptor OEP61 was electrostatically immobilized on a gold surface via an intermediate layer of polycations. The TIRE measurements allowed the evaluation of thickness changes in the adsorbed molecular layers as a result of chaperone binding to receptor proteins. Hsp70 chaperone isoforms but not Hsp90 were shown to be capable of binding OEP61. Dynamic TIRE measurements were carried out to evaluate the affinity constants of the above reactions and resulted in clear discrimination between specific and nonspecific binding of chaperones as well as differences in binding properties between the highly similar Hsp70 isoforms.

Detection of β-amyloid peptide (1-16) and amyloid precursor protein (APP770) using spectroscopic ellipsometry and QCM techniques: a step forward towards Alzheimers disease diagnostics

A highly sensitive method of spectroscopic ellipsometry in total internal reflection mode (TIRE) was exploited for detecting β-amyloid peptide (Aβ(1-16)) in the direct immune reaction with monoclonal DE2 antibodies (raised against Aβ(1-16)) electrostatically immobilised on the surface of gold. A rapid detection of Aβ(1-16) in a wide range of concentrations from 5 μg/ml down to 0.05 ng/ml was achieved using a cost-effective and label-free direct immunoassay format. TIRE dynamic spectral measurements proved that the immune reaction between DE2 monoclonal antibodies and Aβ(1-16) is highly specific with the affinity constant K(D)=1.46×10(-8) mol/l. The same DE2 antibodies were utilised for detection of amyloid precursor protein APP(770), a larger protein containing Aβ(1-16) domain, using the quartz crystal microbalance (QCM) measurements in liquid. A combination of QCM and TIRE kinetics results allowed the evaluation of the originally unknown concentration of APP(770) in complete medium solution containing other proteins, salts, and amino acids.

Design and performances of immunoassay based on SPR biosensor with magnetic microbeads

A surface plasmon resonance (SPR) biosensor system was developed for immunoassay, based on the conjugates of magnetic microbeads coupling with antibody which could be trapped on the Au film firmly due to the magnetic force. The magnetic microbeads were used as the solid support for the heat shock protein 70 (Hsp 70) antibody and antibody immobilized magnetic microbeads were utilized instead of the single antibody for the determination of Hsp 70. Since the magnetic bead is coated with dextran, the antibodies and some specific biomolecular receptors can be immobilized using a variety of chemical reactions. Compared to traditional antibody immobilization on the sensing film, there is not a covalent link between the Au film and the antibody. There is a great advantage in that sensor can be stripped and reused, and the same chemistry used to derivative dextran-coated SPR sensors can be used for the magnetic bead-coated sensors. The sensing layer was formed well. Different dilution ratios (v/v) of the conjugates result in different detectable ranges. When the dilution ratios of the conjugate are 1:10 and 1:5, the lowest concentrations of Hsp 70 that can be detected are 1.50 and 0.30 microg ml(-1), respectively.

Surface plasmon resonance immunosensor for human cardiac troponin T based on self-assembled monolayer

The cardiac troponin T (cTnT) is specific biomarker important for trials of acute myocardial infarctions (AMI). In this paper, a SPR sensor in real time to detect the biomarker was developed on a commercially available surface plasmon resonance AUTOLAB SPIRIT. The cTnT receptor molecule was covalently immobilized on a gold substrate via a self-assembled monolayer (SAM) of thiols by using cysteamine-coupling chemistry. This biosensor presented a linear response range for cTnT between 0.05 and 4.5 ng/mL (r=0.997, p<<0.01) with a good reproducibility (CV=4.4%). The effect of the cysteamine (CYS) concentrations on the SAM coated gold sensor was studied as a function of the amount of the immobilized cTnT monoclonal antibodies. Analysis using serum samples undiluted was carried out at room temperature showing a well agreement with the ECLIA methods and the sensor surface could be regenerated by using a solution of 1% (w/v) sodium dodecyl sulphate (SDS) without losing the sensor immunoreactivity. These studies open new perspectives of using SAM to develop regenerable immunosensor with a good reproducibility allowing its use in the clinical applications.

An SPR immunosensor for human cardiac troponin T using specific binding avidin to biotin at carboxymethyldextran-modified gold chip

BACKGROUND

The cardiac troponin T (TnT) is a cardiospecific, highly sensitive marker for myocardial damage and is immediately released to bloodstream during the acute myocardial infarction (AMI). Surface plasmon resonance (SPR) immunosensor was developed for the quick detection of human cTnT in real time.

METHODS

An SPR sensor streptavidin terminated self-assembled monolayer, which was used to binding biotinylated anti troponin T monoclonal antibodies, was developed.

RESULTS

The cTnT was determined from successive injections with a linear range from 0.03 up to 6.5 ng/ml. The detection limit was 0.01 ng/ml corresponding to a resonant angle change of 1.28 millidegrees. The system presented a good repeatability with 3.4% of variation between run after regeneration of the coated surface with a solution of 1% SDS. The effects of blocking of non-specific adsorption using different solutions were studied. It was possible to measure the cTnT without dilution of the human serum with good specificity and reproducibility.

CONCLUSION

This sensor is practical and offer quick response in interval of 800 s.

Surface plasmon resonance biosensor based on Hg/Ag–Au film

Mercury or silver was electrodeposited on an Au surface to form an Hg-Au or Ag-Au film. Wavelength-modulation SPR biosensors based on this Hg/Ag-Au film were then used to determine human IgG and rabbit IgG. When direct immunoreactions were performed on the Au sensing surface, the range of concentrations of human IgG and rabbit IgG that could be determined were 2.00-40.00 microg/ml and 2.50-40.00 microg/ml, respectively. When Hg was electrodeposited onto the Au film for 1200 s, the range of concentrations of human IgG and rabbit IgG that could be determined were 0.50-40.00 microg/ml and 0.63-40.00 microg/ml, respectively. When Ag was electrodeposited onto the Au film for 1500 s, the range of concentrations of human IgG and rabbit IgG that could be determined were 0.25-20.00 and 0.42-20.00 microg/ml, respectively. The biosensor based on Ag-Au film was therefore found to be the most sensitive of the three types of biosensor tested, giving limits of determination that were up to eight times lower than those obtained with a biosensor based on Au film alone.

Registration of T-2 mycotoxin with total internal reflection ellipsometry and QCM impedance methods

A sensitive optical method of total internal reflection ellipsometry (TIRE) in conjunction with immune assay approach was exploited for the registration of T-2 mycotoxin in a wide range of concentrations from 100 microg/ml down to 0.15 ng/ml. Association constants of 1.4x10(6) and 1.9x10(7)mol(-1)s for poly- and monoclonal T-2 antibodies, respectively, were evaluated from TIRE kinetic measurements. According to TIRE data fitting, binding of T-2 molecules to antibodies (at saturation) has resulted in the increase in adsorbed layer thickness of 4-5 nm. The QCM impedance measurements data showed anomalously large mass increase and film softening, most likely, due to the binding of large T-2 aggregates to antibodies.

Bioenzymatic detection of troponin C using micro-opto-electro-mechanical systems

Diagnosis and monitoring of critical diseases such as acute myocardial infarction (AMI) require a quantitative analysis of biological molecules. A high-throughput identification of these biological molecules can be generated by using micro-electro-mechanical systems (MEMS) structures like simple cantilever beams, which respond to the intermolecular forces resulting from binding these molecules. Biochemical markers like troponin C are considered the primary markers for myocardial injury and have generated considerable interest. A 26-residue lytic membrane protein of bee venom melittin (ME) is chosen to interact with rabbit skeletal muscle troponin C (TnC) on the surface of the cantilever beams. An optical beam deflection method is employed to identify the enzymatic reaction on the surface of the cantilever due to these proteins. Identification of these proteins is also done using fluorescence spectroscopy (FS) to compliment the optical monitored deflection method. A second set of proteins like horse radish peroxide (HRP) and hydrogen peroxide (H2O2) are applied to atomic force microscopy (AFM) cantilever beams to study their behavior under the enzymatic reactions of proteins. Identification of these proteins is done using Fourier transform infrared spectroscopy (FTIR). An analytical model of the cantilever beam is developed, and its mode shapes are studied by employing orthogonal polynomials in the classic Rayleigh-Ritz method. The surface stress caused by the enzymatic reaction of the proteins that leads to pure bending on the top surface of the cantilever is evaluated. The information provided by the experimental and analytical modeling reported in this work will be useful in the development of a portable biosensor for the detection of AMI.

A Biosensor in the Time Domain Based on the Diffusion Coefficient Measurement: Intermolecular Interaction of an Intermediate of Photoactive Yellow Protein

A new type of biosensor is presented for the first time. This method is based on the diffusion measurement in the time domain by the transient grating method. As the first demonstration of this new method, various intermolecular interactions with a reaction intermediate of photoactive yellow protein, such as the protein-protein interaction, protein-DNA interaction, and protein-small molecule binding are detected. The characteristic advantages and limitations are summarized.

MCM-41 mesoporous material modified carbon paste electrode for the determination of cardiac troponin I by anodic stripping voltammetry

An anodic stripping voltammetric method for the determination of cardiac troponin I (cTnI) at a MCM-41 mesoporous material modified carbon paste electrode (MCM-MCPE) was investigated. The test was based on the dual monoclonal antibody "sandwich" principle using colloidal gold as a labeled substrate. Four main steps were carried out to obtain the analytical signal, i.e. electrode preparation, immunoreaction, silver enhancement, and anodic stripping voltammetric detection. The anodic stripping peak current increased linearly with the concentration of cTnI over the range of 0.8-5.0ng/ml. A detection limit of 0.5ng/ml was obtained. The established method was applied to detect cTnI in acute myocardial infarction (AMI) samples using routine enzyme-linked immunoadsorbent assay (ELISA) for comparison analysis, and good results were obtained.

Characterization of a self-assembled monolayer of thiol on a gold surface and the fabrication of a biosensor chip based on surface plasmon resonance for detecting anti-GAD antibody

A biosensor chip utilizing surface plasmon resonance (SPR) was fabricated for detecting anti-glutamic acid decarboxylase (GAD) antibody, which is an indicator of the presence of type I diabetes mellitus. The sensor surfaces were constructed from various thiol mixtures of different molar ratios of 3-mercaptopropionic acid (3-MPA) to 11-mercaptoundecanoic acid (11-MUA). To determine the surface characteristics of the different alkanethiol monolayers, several quantitative and kinetic measurements were carried out. The extent of immobilization of streptavidin (SA) and biotin-GAD (the anti-GAD receptor) and the immune response of anti-GAD antibody against GAD were measured using the SPR biosensor. The terminal functional group of a thiol has different effects on the adsorption and covalent binding of protein depending on the steric hindrance. The protein chip described herein permits simple, rapid detection of anti-GAD antibody.

Sensitivity-enhancement of wavelength-modulation surface plasmon resonance biosensor for human complement factor 4

Sandwich and colloidal Au techniques for enhancing the sensitivity of a wavelength-modulation surface plasmon resonance (SPR) immunosensor are demonstrated by the detection of human complement factor 4 (C4). The design of the wavelength-modulation SPR biosensor is based on fixing the incident angle of light and measuring the reflected intensity of light in the wavelength range spanning 500-900 nm simultaneously. The human C4 had good response in the concentration range 2-20 microg/mL in the direct assay. However, in the sandwich assay, the human C4 had good response in the concentration range 0.2-20 microg/mL and the lowest concentration is 10-fold lower than that obtained by the direct assay. With human C4-Au colloidal conjugate, the human C4 had good response in the concentration range 0.1-20 microg/mL and the lowest concentration is 20-fold lower than that obtained by the direct assay. In the colloidal-Au-enhanced sandwich assay, the human C4 had good response in the concentration range 0.05-5 microg/mL and the lowest concentration is 40-fold lower than that obtained by the direct assay. Under selected experimental conditions, the reproducibility, sensitivity, and reversibility of the enhanced SPR immunoassay are very satisfactory. The results represent potentially significant advantages in the sensitivity of SPR biosensors.

A novel sandwich immunosensing method for measuring cardiac troponin I in sera

Common methods for monitoring human cardiac troponin I (cTn I) are based on using antibodies against cTn I labeled with horseradish peroxidase, radioactive isotopes, or other labels. In this study, a novel label-free sandwich immunosensing method for measuring cTn I was developed. Three monoclonal antibodies (mAbs 9F5, 2F11, and 8C12) against human cTn I were generated by the commonly used hybridoma technique and characterized by a surface plasmon resonance (SPR) biosensor. An optimal pair of mAbs for measuring human cTn I was selected, as both mAbs have high affinities for cTn I and do not compete against each other for cTn I binding. An optical immunosensor for measuring cTn I in sera based on SPR was developed by using avidin as an intermediate layer and biotinylated-2F11 as the capturing antibody. Two detection methods for cTn I with the immunosensor were performed: (1) the direct detection of cTn I with a detection range of 2.5 to 40 microg/L and (2) the sandwich immunosensing method. In the sandwich assay mode, the second antibody 9F5 biologically amplified the sensor response. As a result, the sandwich assay showed a sensitivity of 0.25 microg/L and a detection range of 0.5 to 20 microg/L with within-run variation of 4.9 to 6.7% and between-run variation of 5.2 to 8.4%. This method has greatly enhanced the sensitivity for detection compared to that previously reported in the literatures.