A comparison of performances of four enzymes used in ELISA with special reference to beta-lactamase

Multi-channel prismatic localized surface plasmon resonance biosensor for real-time competitive assay multiple COVID-19 characteristic miRNAs

A multi-channel prismatic localized surface plasmon resonance (LSPR) biosensor was developed for quantitative and real-time detection of multiple COVID-19 characteristic miRNAs. The well-dispersed and dense gold nanoparticles (AuNPs) arrays for LSPR biosensing were fabricated through a nano-thickness diblock copolymer template (BCPT). Both theoretical and experimental analyses were conducted to investigate the effects of particle size, interparticle spacing, and surface coverage on LSPR sensing spectrum and intensity sensitivity of varied AuNPs sizes. A competitive assay strategy was proposed and used for non-amplification miRNA detection with a low limit detection of 3.41 nM, while a four-channel prismatic LSPR system enables parallel detection of multiple miRNAs. Furthermore, this sensing strategy can effectively and specifically identify target miRNA, distinguish mismatched miRNA and interfering miRNA, and exhibit low non-specific adsorption. This BCPT-based LSPR biosensor demonstrates the practicality and potential of a multi-channel, adaptable, and integrated prismatic sensor in medical testing and diagnostic applications.

Electrochemical Immuno- and Aptamer-Based Assays for Bacteria: Pros and Cons over Traditional Detection Schemes

Microbiological safety of the human environment and health needs advanced monitoring tools both for the specific detection of bacteria in complex biological matrices, often in the presence of excessive amounts of other bacterial species, and for bacteria quantification at a single cell level. Here, we discuss the existing electrochemical approaches for bacterial analysis that are based on the biospecific recognition of whole bacterial cells. Perspectives of such assays applications as emergency-use biosensors for quick analysis of trace levels of bacteria by minimally trained personnel are argued.

New optical method for the determination of β-galactosidase and α-fetoprotein based on oxidase-like activity of fluorescein

Fluorescein has been found as an efficient visible-light-induced oxidase mimic and its catalytic performance is group-dependent. Herein, a facile colorimetric strategy for β-galactosidase (β-gal) was developed using fluorescein di β-D-galactopyranoside (FDG) as a probe based on the analyte induced change in oxidase mimicking activity of fluorescein derivatives. FDG doesn't possess any visible-light-induced oxidase activity and can generate fluorescein and fluorescein mono β-D-galactopyranoside (FMG) in the presence of β-gal. The in situ generated fluorescein and FMG possess high oxidase-like activities under visible-light illumination and could catalyze the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB) upon short irradiation by light-emitting diode (LED) lamp. Thus, the β-gal activity can be selectively detected in linear range from 0.10 to 12.9 μg mL^-1 with a limit of detection (LOD) of 0.04 μg mL^-1. We further integrated with the visual detection of α-fetoprotein antigen (AFP) based on the corresponding colorimetric signal induced by β-gal-linked colorimetric immunoassay, a LOD of 0.08 ng mL^-1 could be achieved. Significantly, our proposed assay provides a facile sensing platform based on the change in enzyme mimicking activity induced by analytes. In addition, this optical method works without complex synthesis procedure and efficiently avoids participation of unstable H₂O₂ as an oxidant. Therefore, the present work not only shows the excellent assay performance in β-gal and tumor biomarker detection, but also opens up a new avenue for its application in practical optical sensing.

The Use of a β-lactamase-based Conductimetric Biosensor Assay to Detect Biomolecular Interactions

Biosensors are becoming increasingly important and implemented in various fields such as pathogen detection, molecular diagnosis, environmental monitoring, and food safety control. In this context, we used β-lactamases as efficient reporter enzymes in several protein-protein interaction studies. Furthermore, their ability to accept insertions of peptides or structured proteins/domains strongly encourages the use of these enzymes to generate chimeric proteins. In a recent study, we inserted a single-domain antibody fragment into the Bacillus licheniformis BlaP β-lactamase. These small domains, also called nanobodies, are defined as the antigen-binding domains of single chain antibodies from camelids. Like common double chain antibodies, they show high affinities and specificities for their targets. The resulting chimeric protein exhibited a high affinity against its target while retaining the β-lactamase activity. This suggests that the nanobody and β-lactamase moieties remain functional. In the present work, we report a detailed protocol that combines our hybrid β-lactamase system to the biosensor technology. The specific binding of the nanobody to its target can be detected thanks to a conductimetric measurement of the protons released by the catalytic activity of the enzyme.

Reagent pencils: a new technique for solvent-free deposition of reagents onto paper-based microfluidic devices

Custom-made pencils containing reagents dispersed in a solid matrix were developed to enable rapid and solvent-free deposition of reagents onto membrane-based fluidic devices. The technique is as simple as drawing with the reagent pencils on a device. When aqueous samples are added to the device, the reagents dissolve from the pencil matrix and become available to react with analytes in the sample. Colorimetric glucose assays conducted on devices prepared using reagent pencils had comparable accuracy and precision to assays conducted on conventional devices prepared with reagents deposited from solution. Most importantly, sensitive reagents, such as enzymes, are stable in the pencils under ambient conditions, and no significant decrease in the activity of the enzyme horseradish peroxidase stored in a pencil was observed after 63 days. Reagent pencils offer a new option for preparing and customizing diagnostic tests at the point of care without the need for specialized equipment.

Detection of femtomolar proteins by nonfluorescent ZnS nanocrystal clusters

Cation exchange (CX) in the nonfluorescent ZnS nanocrystal clusters (NCCs) was employed to detect trace biomolecules with immunoassays. The NCCs were porous and allowed fast cation exchange reaction to release an ultralarge number of Zn(2+) from each cluster that turned on the Zn-responsive dyes for fluorescence detection. The ZnS NCCs were highly stable in biological buffers and more biocompatible than quantum dots. Zn(2+) release efficiency and target binding by NCCs with average diameters of 44 nm, 86 nm, and 144 nm were investigated. The smallest NCCs exhibited the highest CX efficiency because of its larger surface area and bigger pores inside the cluster structure, and 71.0% of the enclosed Zn(2+) were freed by CX with 2-min microwave irradiation. They also experienced the least space hindrance and the fastest rate when binding to target molecules immobilized on surface. When the 44-nm NCCs were used to detect IgE in a sandwich assay, the limit of detection (LOD) was 5 pg/mL (33 fM), 1,000 times better than that of ELISA. Our results well demonstrate that CX in the ZnS NCCs is superior to the conventional signaling strategies in its high amplification efficiency, robustness, and biocompatibility.

Cation exchange in ZnSe nanocrystals for signal amplification in bioassays

ZnSe nanocrystals (NCs), possessing low native luminescence but high biocompatibility, were employed as labeling tags in bioassays. They were able to amplify each target recognition event thousands of times through a cation-exchange reaction (CXAmp) that released over 3000 encapsulated Zn(2+) from one single NC. The freed cations in turn triggered strong fluorescence from the Zn-responsive dyes. The present study demonstrated that CXAmp with ZnSe delivered superior detection performance in comparison to the conventional labeling methods. The overall fluorescence intensity of CXAmp using 5 nM ZnSe NCs was 30 times higher than that from 5 nM core-shell CdSe/ZnS quantum dots (QDs). The limit of detection (LOD) obtained with ZnSe-based CXAmp was 10-fold lower than with horseradish peroxidase (HRP) labeling, and the detection sensitivity, represented by the slope of the signal-versus-concentration curve, was 20-fold higher. When applied to detect immunoglobulin E (IgE) in a sandwich format, a LOD of 1 ng/mL was achieved. The highly sensitive CXAmp also allowed detection of the total IgE content in dilute human serum, in which the abundant matrix proteins exhibited less interference and more accurate quantification could be performed. Besides high signal amplification efficiency and good biocompatibility, CXAmp with ZnSe could be easily adapted to common laboratory settings and act as a universal labeling system for reliable detection of low-abundance targets.

A rapid and sensitive ELISA to quantify an HBsAg specific monoclonal antibody and a plant-derived antibody during their downstream purification process

An enzyme-linked immunosorbent assay to quantify the mAb CB.Hep-1 during downstream purification process was standardized and validated. This assay is characterized by a short time of incubation at high temperature, allowing the detection of this antibody with high specificity and sensitivity. Detection of antigen-antibody reaction was achieved using a horseradish peroxidase conjugated anti-mouse IgG whose enzyme activity was revealed with o-phenylenediamine substrate. The immunoassay is linear in a range between 3.12 and 50 ng/mL, with a recovery of 98.55-107.62%. According to results, it is possible to estimate the mAb CB.Hep-1 concentration with high precision and reproducibility. The intra- and interassay coefficient of variation ranged from 0.25 to 8.64% and 1.84 to 9.43%, respectively. Significant differences were not observed in the plant-derived antibody quantification by HRP-ELISA and PhoA-ELISA (n=18), demonstrating that plant endogenous peroxidases do not produce interferences in the quantification of this molecule. Therefore, both antibodies can be tested with the same immunoassay with high precision, specificity and accuracy during their respective purification processes without interference of the buffers and sample characteristics.

Electrochemical immunosensor array using a 96-well screen-printed microplate for aflatoxin B1 detection

A novel analytical immunosensor array, based on a microtiter plate coupled to a multichannel electrochemical detection (MED) system using the intermittent pulse amperometry (IPA) technique, is proposed for the detection of aflatoxin B1 (AFB1). In the present work, the electrochemical behaviour and electroanalytical performance of the thick-film carbon sensors (also designated as screen-printed electrodes) incorporated in the multichannel electrochemical plate were first evaluated. Then the 96-well screen-printed microplate was modified in accord with a competitive indirect enzyme-linked immunoassay (ELISA) format for aflatoxin B1 detection. The measurements were performed using both spectrophotometric and electrochemical procedures and the results of the calibration curves, detection limit (LOD), sensitivity and reproducibility of the respective assay systems were evaluated. The immunoassay was then applied for analysis of corn samples spiked with AFB1 before and after the extraction treatment, in order to study the extraction efficiency and the matrix effect, respectively. These studies have shown that using this system, AFB1 can be measured at a level of 30 pg/mL and with a working range between 0.05 and 2 ng/mL. Good recoveries (103+/-8%) were obtained, demonstrating the suitability of the proposed assay for accurate determination of the AFB1 concentration in corn samples. The specificity of the assay was assessed by studying the cross-reactivity of PAb relative to AFB1. The results indicated that the PAb could readily distinguish AFB1 from other aflatoxins, with the exception for AFG1.

Designing allosteric peptide ligands targeting a globular protein

Patented signal analytic algorithms applied to hydrophobically transformed, numerical amino acid sequences have previously been used to design short, protein-targeted, L or D retro-inverso peptides. These peptides have demonstrated allosteric and/or indirect agonist effects on a variety of G-protein and tyrosine kinase coupled membrane receptors with 30% to over 80% hit rates. Here we extend these approaches to a globular protein target. We designed eight peptide ligands targeting an ELISA antibody responsive protein, beta-galactosidase, betaGAL. Three of the eight 14mer peptides allosterically activated betaGAL with ELISA methodology. Using Bayesian statistics, this 38% hit rate would have occurred 2 x 10(-9) by chance. These peptides demonstrated binding site competitive or noncompetitive interactions, suggesting allosteric site multiplicity with respect to their betaGAL binding-mediated ELISA signal. Kinetic studies demonstrated the temperature dependence of the betaGAL peptide binding functions. Using the van't Hoff relation, we found evidence for enthalpy-entropy compensation. This relation is often found for hydrophobic interactions in aqueous media, and is consistent with the postulated hydrophobic series encoding underlying our protein-targeted, peptide design methods. It appears that our algorithmic, hydrophobic autocovariance eigenvector template approach to the design of allosteric peptides targeting membrane receptors may also be applicable to the design of peptide ligands targeting nonmembrane involved globular proteins.

Efficient labelling of antibodies with horseradish peroxidase using cyanuric chloride

An efficient and mild method for labelling of immunoglobulin G (IgG) with horseradish peroxidase (HRP) using cyanuric chloride (2,4,6-trichloro-1,3,5-triazine, CC) as a bridging molecule is described. The enzyme was treated first with cyanuric chloride to introduce dichloro triazine and after removal of excess reagent, the activated enzyme was mixed with the IgG preparation and incubated to effect linkages with amine groups in the antibody protein. Various amounts of coupling reagent were tested to optimise the conjugation method using commercially available enzyme and affinity-purified sheep IgG antibody preparations to three different test haptens. The conjugates were assessed by solid phase Enzyme Linked Immunosorbent Assays (ELISA) and commonly used peroxidase substrate preparations. The binding activity of the conjugates rose with increasing coupling reagent added during the enzyme activation step. Use of the conjugates prepared by the new method gave comparable sensitivity in direct competitive ELISAs for the three test haptens to assays carried out using indirect ELISA with commercial anti-sheep-HRP conjugates. No deterioration of enzyme activity or hapten-binding activity in the conjugates was observed after storage in 50% glycerol at -70 degrees C for up to 18 months. This study presents a relatively simple and efficient conjugating method for labelling antibodies with HRP and provides an additional and probably a better alternative to the periodate, glutaraldehyde and succinimide-maleimide procedures.

Comparative studies with penicillinase, horseradish peroxidase, and alkaline phosphatase as enzyme labels in developing enzyme immunoassay of cortisol

Relative merit of different enzyme labels for measuring cortisol directly in serum by competitive enzyme immunoassay (ELISA) was examined. Cortisol-21-hemisuccinate was labeled separately with penicillinase, horseradish peroxidase (HRP), and alkaline phosphatase (ALP) under identical reaction conditions. Antibody developed in rabbits against cortisol-3-0-(carboxymethyl)-oxime-bovine serum albumin was used to coat polystyrene tubes that were precoated with anti-rabbit gamma globulin (ARGG). Cortisol standards were prepared in steroid-free human serum in buffer (1:4) contaning 8-anilino-1-naphthalene sulfonic acid (8-ANS). Assay buffer also consisted 8-ANS. The assay involved adding standard cortisol or serum sample to antibody-coated tubes, followed by addition of enzyme label and buffer, and incubation for 2 h at 37 degrees C. The whole procedure took 3 h for completion. All three labels proved to be sensitive, with a slope around -2.0. Although penicillinase as an enzyme label was highly sensitive and stable compared with others, the assays were not always accurate and precise, especially at low concentrations of cortisol. This was mainly due to the color reagent used for measuring penicillinase activity. Serum samples that underwent 2-3 freeze-thaw cycles gave high values with HRP label compared with ALP. Therefore, utilizing ALP as an enzyme label, an ELISA was developed and its performance was comparable with some of the commercial kits already in the market.

Thermostable alpha-amylase conjugated antibodies as probes for immunodetection in ELISA

Thermostable alpha-amylase from B. licheniformis has been conjugated with high efficiency to goat antibodies against human, mouse, and rabbit immunoglobulins to prepare second-step reagents which can be used in Enzyme Linked Immunosorbent Assays (ELISA). Various conjugation methods, such as one- and two-step glutaraldehyde coupling and cross-linking, using heterobifunctional reagents such as sulfosuccinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carbonate (sulfo-SMCC) and N-succinimidyl-S-acetylthioacetate (SATA), yielded active alpha-amylase labeled second antibodies. Such conjugates had molecular sizes ranging between 200-300 kDa. Filter sterilized solutions of conjugates, when stored at 37 degrees C for two weeks, retained 32% of their biological activity and were thermostable even after keeping for 1 h at 90 degrees C.

Detection of hantavirus infection in hemolyzed mouse blood using alkaline phosphatase conjugate

Conventional methods such as ELISA and culture require a long time for the determination of viral infections. Fast and reliable instrumentation suitable for operation under field conditions that allows rapid detection of hantavirus in rodent populations in the wild, would be a substantial improvement over these methods. In response to this challenge, a prototype amperometric immunosensor based on highly dispersed immunoelectrode for rapid, sensitive and quantitative assay of hantavirus in mice blood has been developed. A sandwich scheme of immunoassay is used and the naphthol that is formed as a result of enzymatic hydrolysis of alpha-naphthyl phosphate in the presence of alkaline phosphatase (AP) label is quantified amperometrically. The main complication faced when testing the amperometric immunosensor for application to mouse blood samples under field conditions is the removal of the interference caused by the electroactive components due to the hemolysis of the samples. We studied the possibility of using other enzyme markers such as AP to replace the horseradish peroxidase (HRP) used earlier in testing for antibodies against hantavirus in human blood plasma. Best results were obtained when the reference electrode is covered with a thin Nafion layer. Further improvement of assay performance was achieved by the modification of the sensing element. The amperometric immunoelectrode showed significantly higher sensitivity (more than 10-fold) than the standard spectrophotometric detection ELISA method. It can also be used for rapid analysis in conventional and field conditions in biological, physiological and analytical practices.