Immunoassay development for environmental analysis

FRET Based Biosensor: Principle Applications Recent Advances and Challenges

Förster resonance energy transfer (FRET)-based biosensors are being fabricated for specific detection of biomolecules or changes in the microenvironment. FRET is a non-radiative transfer of energy from an excited donor fluorophore molecule to a nearby acceptor fluorophore molecule. In a FRET-based biosensor, the donor and acceptor molecules are typically fluorescent proteins or fluorescent nanomaterials such as quantum dots (QDs) or small molecules that are engineered to be in close proximity to each other. When the biomolecule of interest is present, it can cause a change in the distance between the donor and acceptor, leading to a change in the efficiency of FRET and a corresponding change in the fluorescence intensity of the acceptor. This change in fluorescence can be used to detect and quantify the biomolecule of interest. FRET-based biosensors have a wide range of applications, including in the fields of biochemistry, cell biology, and drug discovery. This review article provides a substantial approach on the FRET-based biosensor, principle, applications such as point-of-need diagnosis, wearable, single molecular FRET (smFRET), hard water, ions, pH, tissue-based sensors, immunosensors, and aptasensor. Recent advances such as artificial intelligence (AI) and Internet of Things (IoT) are used for this type of sensor and challenges.

Enhancing the detection sensitivity of nanobody against aflatoxin B1 through structure-guided modification

Nanobodies (Nbs) have shown great potential in immunodetection of small-molecule contaminants in food and environmental monitoring. However, the limited knowledge of the mechanism of Nbs binding to small molecules has hampered the development of high-affinity Nbs and assay improvement. We previously reported two homologous nanobodies Nb26 and Nb28 specific to aflatoxin B₁ (AFB₁), with the former exhibiting higher sensitivity in ELISA. Herein, Nb26 was selected as the model antibody to resolve its solution nuclear magnetic resonance (NMR) structure, and investigate its AFB₁ recognition mechanism. The results revealed that Nb26 exhibits a typical immunoglobulin fold and its AFB₁-binding interface is uniquely located in complementarity-determining region 3 (CDR3) and framework region 2 (FR2). This finding was applied to improve the binding activity of Nb28 against AFB₁ by constructing two Nb28-based mutants A⁵⁰V and S¹⁰²D, resulting in 2.3- and 3.3-fold sensitivity enhancement over the wild type, respectively. This study develops an NMR-based strategy to analyze the underlying mechanism of Nb against AFB₁, and successfully generated two site-modified Nbs with improved detection sensitivity. It is believed that this work could greatly expand the applications of Nbs by providing a way to enhance the binding activity.

Research on the Mechanism of Action of a Citrinin and Anti-Citrinin Antibody Based on Mimotope X27

Immunoassays are developed based on antigen–antibody interactions. A mimotope is an effective recognition receptor used to study the mechanism of action of antigens and antibodies, and is used for improving the sensitivity of the antibody. In this study, we built a 3D structure of the citrinin (CIT) mimotope X27 and anti-CIT single-chain antibody fragment (ScFv) through a “homologous modeling” strategy. Then, CIT and X27 were respectively docked to anti-CIT ScFv by using the “molecular docking” program. Finally, T28, F29, N30, R31, and Y32 were confirmed as the key binding sites in X27. Furthermore, the result of the phage-ELISA showed that the mutational phage lost the binding activity to the anti-CIT ScFv when the five amino acids were mutated to “alanine”, thereby proving the correctness of the molecular docking model. Lastly, a site-directed saturation strategy was adopted for the sites (T28, F29, N30, R31, and Y32). Eighteen different amino acids were introduced to each site on average. The activities of all mutants were identified by indirect competitive ELISA. The sensitivities of mutants T28F, T28I, F29I, F29V, N30T, and N30V were 1.83-, 1.37-, 1.70-, 2.96-, 1.31-, and 2.01-fold higher than that of the wild-type, respectively. In conclusion, the binding model between the CIT and antibody was elaborated for the first time based on the mimotope method, thereby presenting another strategy for improving the sensitivity of citrinin detection in immunoassays.

Hormones and Hormonal Anabolics: Residues in Animal Source Food, Potential Public Health Impacts, and Methods of Analysis

The demand for nutritious food, especially food of animal origin, is globally increasing due to escalating population growth and a dietary shift to animal source food. In order to fulfill the requirements, producers are using veterinary drugs such as hormones and hormone-like anabolic agents. Hormones such as steroidal (estrogens, gestagens, and androgens), nonsteroidal, semisynthetic, and synthetic or designer drugs are all growth-promoting and body-partitioning agents. Hence, in food animal production practice, farm owners use these chemicals to improve body weight gain, increase feed conversion efficiency, and productivity. However, the use of these hormones and hormonal growth-promoting agents eventually ends up with the occurrence of residues in the animal-originated food. The incidence of hormone residues in such types of food and food products beyond the tolerance acts as a risk factor for the occurrence of potential public health problems. Currently, different international and national regulatory bodies have placed requirements and legislative frameworks, which enable them to implement residue monitoring test endeavors that safeguard the public and facilitate the trading activity. To make the tests on the animal-origin food matrix, there are different sample extraction techniques such as accelerated solvent extraction, supercritical fluid extraction, solid phase extraction, solid-phase microextraction, and hollow-fiber liquid-phase microextraction. After sample preparation steps, the analytes of interest can be assayed by screening and confirmatory methods of analysis. For screening, immunological tests such as ELISA and radioimmunoassay are used. Detection and determination of the specific residues will be done by chromatographic or instrumental analysis. Mainly, among high-performance liquid chromatography, liquid chromatography with mass spectrometry (LC-MS, LC-MS/MS), and gas chromatography with mass spectrometry (GC-MS and GC-MS/MS) methods, LC-MS/MS is being preferred because of easier sample preparation without a derivatization step and high detection and quantification capacity.

Affimer-Enzyme-Inhibitor Switch Sensor for Rapid Wash-free Assays of Multimeric Proteins

Robust technology is required to underpin rapid point-of-care and in-field diagnostics to improve timely decision making across broad sectors. An attractive strategy combines target recognition and signal generating elements into an "active" enzyme-switch that directly transduces target-binding into a signal. However, approaches that are broadly applicable to diverse targets remain elusive. Here, an enzyme-inhibitor switch sensor was developed by insertion of non-immunoglobulin Affimer binding proteins, between TEM1-β-lactamase and its inhibitor protein, such that target binding disrupts the enzyme-inhibitor complex. Design principles for a successful switch architecture are illustrated by the rapid (min), simple (wash-free), and sensitive (pM) quantification of multimeric target analytes in biological samples (serum, plasma, leaf extracts), across three application areas. A therapeutic antibody (Herceptin), protein biomarker (human C-reactive protein), and plant virus (cow pea mosaic virus) were targeted, demonstrating assays for therapeutic drug monitoring, health diagnostics, and plant pathogen detection, respectively. Batch-to-batch reproducibility, shelf-life stability, and consistency with validated enzyme-linked immunosorbent assay analysis confirm that the principle of an Affimer-enzyme-inhibitor switch provides a platform for point-of-care and in-field diagnostics.

Development of a High-Quality ELISA Method for Dinotefuran Based on a Novel and Newly-Designed Antigen

The structure of hapten determines the performance of the antibody and the corresponding detection method. A new type of antigen was designed and synthesized to expose the spatial and characteristic structure of dinotefuran molecule, and a type of high-quality antibody was obtained. The IC50 value of the monoclonal antibody was 5.30 ng/mL and its cross-reactivity (CRs) was less than 2% when reacting with other structurally related analytes. The effects of spatial configurations of hapten on the antibody were visually analyzed while using the appropriate software according to the quality of the antibodies, which showed that the specificity of the antibody is closely related with the exposed structure of hapten. An ELISA assay with an IC50 of 5.66 ng/mL and a linear range of 1.95 to 16.29 ng/mL was developed. The results that were obtained from the ELISA and HPLC methods were equivalent. The results showed that spatial simulation is a crucial method that is used in the designing of hapten to obtain a sensitive and specific antibody. The application of this method will highlight the potential aim and improve the detection efficiency of ELISA.

Competitive and noncompetitive immunoassays for the detection of benzothiostrobin using magnetic nanoparticles and fluorescein isothiocyanate-labeled peptides

Phage-displayed peptides have been proven to be powerful reagents for competitive and noncompetitive immunoassays. However, they are unconventional reagents, which greatly limit their analytical commercial applications and require additional reagents for detection. In this work, the peptides that specifically bind with anti-benzothiostrobin monoclonal antibody (mAb) or benzothiostrobin-mAb immunocomplex were synthesized and conjugated with fluorescein isothiocyanate (FITC) as substitutes of the phage-displayed peptides to avoid their shortcomings and extend their applications. Competitive and noncompetitive fluorescence immunoassays (FIAs) for benzothiostrobin were developed by mAb coupling with magnetic nanoparticles as concentration elements and peptides conjugated with FITC as tracers. Compared with enzyme-linked immunosorbent assays, the FIAs reduced the number of steps from 6 to 2 and analysis time from more than 5 to 1.2 h. The competitive FIA showed the half-maximal inhibition concentration (IC₅₀) of 16.8 ng mL^-1 and detection range (IC₁₀-IC₉₀) of 1.0-759.9 ng mL^-1, while the concentration of analyte producing 50% saturation of the signal (SC₅₀) and detection range (SC₁₀-SC₉₀) of noncompetitive FIA were 93.4 and 5.9-788.2 ng mL^-1, respectively. The average spiked recoveries were 68.33-98.50% and 73.33-96.67% for competitive and noncompetitive FIAs, respectively. The FIAs showed good correlation with high-performance liquid chromatography for the detection of benzothiostrobin in authentic samples. Graphical abstract Development of competitive and noncompetitive fluorescence immunoassays for benzothiostrobin by using monoclonal antibody coupling with magnetic nanoparticles as concentration elements and peptides conjugated with fluorescein isothiocyanate as tracers.

Development of a phage chemiluminescent enzyme immunoassay with high sensitivity for the determination of imidaclothiz in agricultural and environmental samples

In this study, we isolated six phage-displayed peptides by biopanning phage-displayed peptide libraries on an immobilized anti-imidaclothiz monoclonal antibody. After analyzing the relative sensitivity of the individual phage-displayed peptides, we subsequently developed and optimized both a phage enzyme immunoassay (P-ELISA) and a phage chemiluminescent enzyme immunoassay (P-CLEIA) to improve the sensitivity and linear range of imidaclothiz assays. The P-CLEIA (50% inhibition concentration (IC₅₀) of 0.86ngmL^-1, linear range of 0.13-5.84ngmL^-1) was more sensitive and had a wider linear range compared to the P-ELISA (IC₅₀ of 1.45ngmL^-1, linear range of 0.55-3.82ngmL^-1). Besides, the sensitivities of the P-ELISA and P-CLEIA were increased by >4-fold and 8-fold, respectively as compared to homologous immunoassays developed using the same monoclonal antibody. Neither method had significant cross-reactivity with the analogues of imidaclothiz except for imidacloprid. Recoveries of the P-ELISA and P-CLEIA for imidaclothiz in paddy water, soil, cabbage, rice, apple, pakchoi, pear and tomato samples were 72.3-101.3% and 73.9-102.6%, respectively. The P-ELISA and P-CLEIA detected imidaclothiz in the authentic samples, and showed good correlation with results obtained from high-performance liquid chromatography (HPLC).

Direct competitive fluoroimmunoassays for detection of imidaclothiz in environmental and agricultural samples using quantum dots and europium as labels

A direct quantum dots-based fluoroimmunoassay (QDFIA) and a time-resolved fluoroimmunoassay (TRFIA) for imidaclothiz (IMI) were developed by using the quantum dots (QDs)-labeled antibody and the europium (Eu³⁺)-labeled antibody, respectively. After optimization, the half-maximal inhibition concentration (IC₅₀) and the limit of detection (LOD, IC₁₀) are 20.41 and 0.52μgL^-1 for the QDFIA, while 6.91 and 0.018μgL^-1 for the TRFIA, respectively. The cross-reactivities (CRs) with the analogues are negligible except for imidacloprid with CRs of 29.0% for the QDFIA and 26.6% for the TRFIA. The spiked recoveries of IMI in paddy water, soil, pear, tomato, rice, apple, cabbage and cucumber are 72.7-117.6% with a standard deviation (RSD) of 2.4-13.5% for the QDFIA, and 81.3-117.7% with a RSD of 1.6-7.5% for TRFIA. The detection results of the analyses for the real paddy water and pear samples are markedly correlated with these of high-performance liquid chromatography (HPLC).

Development and comparative study of chemosynthesized antigen and mimotope-based immunoassays for class-specific analysis of O,O-dimethyl organophosphorus pesticides

The multi-residue determination of organophosphorus pesticides (OPs) is an important task due to the wide application and high toxicity of OPs. However, there is no promising immunoassay to monitor the multi-residue of O,O-dimethyl OPs. In this study, a monoclonal antibody (mAb) against a generic hapten of O,O-dimethyl OPs (O,O-dimethyl O-(3-carboxyphenyl)phosphorothioate) was prepared. To develop an effective class-specific immunoassay, two strategies were performed to select the appropriate coating antigen or competing antigen. On the one hand, a total of 20 haptens were chemosynthesized, attached to ovalbumin for use as coating antigen candidates, and selected by direct competitive ELISA (dcELISA). As a second strategy, mimotopes of the mAb were selected from a random phage-display peptide library by panning, and the optimum mimotope was expressed as a fusion protein and biotinylated in vitro. Based on the selected chemosynthesized coating antigen and the biotinylated mimotope fusion protein, two sensitive broad-specificity dcELISAs were developed. The sensitivity, selectivity and practicability of the two immunoassays were compared. The results demonstrated that both methods showed similar selectivity and sensitivity and were reliable for O,O-dimethyl OP residues screening. However, the screening operation of mimotopes was much simpler and safer compared to the preparation of chemosynthesized coating antigens.

Signal amplification strategy using gold/N-trimethyl chitosan/iron oxide magnetic composite nanoparticles as a tracer tag for high-sensitive electrochemical detection

This study presents a novel signal amplification method for high-sensitive electrochemical immunosensing. Gold (Au)/N-trimethyl chitosan (TMC)/iron oxide (Fe3O4) (shell/shell/core) nanocomposite was used as a tracing tag to label antibody. The tag was shown to be capable of amplifying the recognition signal by high-density assembly of Au nanoparticles (NPs) on TMC/Fe3O4 particles. The remarkable conductivity of AuNPs provides a feasible pathway for electron transfer. The method was found to be simple, reliable and capable of high-sensitive detection of human serum albumin as a model, down to 0.2 pg/ml in the range of 0.25-1000 pg/ml. Findings of the present study would create new opportunities for sensitive and rapid detection of various analytes.

Modification of a deoxynivalenol-antigen-mimicking nanobody to improve immunoassay sensitivity by site-saturation mutagenesis

A nanobody (N-28) which can act as a deoxynivalenol (DON) antigen has been generated, and its residues Thr102-Ser106 were identified to bind with anti-DON monoclonal antibody by alanine-scanning mutagenesis. Site-saturation mutagenesis was used to analyze the plasticity of five residues and to improve the sensitivity of the N-28-based immunoassay. After mutagenesis, three mutants were selected by phage immunoassay and were sequenced. The half-maximal inhibitory concentrations of the immunoassay based on mutants N-28-T102Y, N-28-V103L, and N-28-Y105F were 24.49 ± 1.0, 51.83 ± 2.5, and 35.65 ± 1.6 ng/mL, respectively, showing the assay was, respectively, 3.2, 1.5, and 2.2 times more sensitive than the wild-type-based assay. The best mutant, N-28-T102Y, was used to develop a competitive phage ELISA to detect DON in cereals with high specificity and accuracy. In addition, the structural properties of N-28-T102Y and N-28 were investigated, revealing that the affinity of N-28-T102Y decreased because of increased steric hindrance with the large side chain. The lower-binding-affinity antigen mimetic may contribute to the improvement of the sensitivity of competitive immunoassays. These results demonstrate that nanobodies would be a favorable tool for engineering. Moreover, our results have laid a solid foundation for site-saturation mutagenesis of antigen-mimicking nanobodies to improve immunoassay sensitivity for small molecules.

DNA-Redox Cation Interaction Improves the Sensitivity of an Electrochemical Immunosensor for Protein Detection

A simple DNA-redox cation interaction enhancement strategy has been developed to improve the sensitivity of electrochemical immunosensors for protein detection. Instead of labeling with fluorophores or redox-active groups, the detection antibodies were tethered with DNA single strands. Based on the electrostatic interaction between redox cations ([Ru(NH₃)₆]³⁺) and negatively charged DNA backbone, enhanced electrochemical signals were obtained. Human chorionic gonadotropin (hCG) detection has been performed as a trial analysis. A linear response range up to the concentration of 25 mIU/mL and a detection limit of 1.25 mIU/mL have been achieved, both are comparable with the ultrasensitive enzyme-linked immunosorbent assay (ELISA) tests. The method also shows great selectivity towards hCG over other hormones such as thyroid stimulating hormone (TSH) and follicle stimulating hormone (FSH). By and large, our approach bears the merits of cost effectiveness and simplicity of instrumentation in comparison with conventional optical detection methods.

Enzyme-linked immunosorbent assay for triclocarban in aquatic environments

A sensitive, competitive indirect enzyme-linked immunosorbent assay (ELISA) was developed for the detection of triclocarban (TCC) in waters and sediments. Haptens were synthesized by derivatizing the paraposition of a phenyl moiety of TCC. The synthesized hapten was then coupled to bovine thyroglobulin to be used as an immunogen, based on which, a high affinity monoclonal antibody 4D5 was produced with the hybridoma technique. Under the optimized conditions, using the monoclonal antibody, excellent performances of the assay were obtained: satisfactory sensitivity (IC50 (50% inhibition concentration) value, 0.43 ng/mL; limit of detection, 0.05 ng/mL); good linear range (0.05-10 ng/mL); and satisfactory accuracy (recoveries 70.7-107% in waters; 74.8-98.3% in sediments). Furthermore, TCC was found with the concentration ranging from not detected to 422.12 ng/L in waters and from 6.68 ng/g to 78.67 ng/g in sediments in Yunliang River, Ancient Canal and Hongqiao Port in Zhenjiang City. In conclusion ELISA could be applied for monitoring TCC in aquatic environments.

An electrochemical immunosensor for ultrasensitive detection of carbohydrate antigen 199 based on Au@Cu(x)OS yolk-shell nanostructures with porous shells as labels

A novel and sensitive electrochemical immunosensor for ultrasensitive detection of pancreatic cancer biomarker carbohydrate antigen 199 (CA199) was proposed by using Au@Cu(x)OS yolk-shell nanostructures with porous shells as labels for signal amplification. Au@Cu(x)OS yolk-shell nanostructures exhibit high electrocatalytic activity toward the reduction of hydrogen peroxide (H2O2) as analytical signal. Moreover, secondary antibody (Ab2) can adsorb on the surface of Au@Cu(x)OS with porous shells which has large surface area and could greatly increase the probability of Ab2-antigen interactions thereby leading to higher sensitivity. Reduced graphene oxide-tetraethylene pentamine (rGO-TEPA), containing abundant amine groups, was supported Au nanoparticles as a support platform to immobilize the primary antibody (Ab1). The resulting sensing interface of rGO-TEPA/AuNPs could provide a large electroconductive surface area, allowing high loadings of the biological recognition elements as well as the occurrence of electrocatalytic and electron-transfer processes. Under optimal conditions, the immunosensor exhibited a wide linear response to CA199 ranging from 0.001 to 12 U/mL with a low detection limit of 0.0005 U/mL. The designed immunosensor displayed good precision, high sensitivity, acceptable stability and reproducibility, and has been applied to the analysis of serum with satisfactory results. The proposed method provides a new promising platform of clinical immunoassay for other biomolecules.

Magnetic nanoparticle-based immunosensor for electrochemical detection of hepatitis B surface antigen

An electrochemical immunosensor was developed for the detection of hepatitis B surface antigen (HBsAg). The biotinylated hepatitis B surface antibody was immobilized on streptavidin magnetic nanoparticles and used for targeting the HBsAg. By the addition of horseradish peroxidase conjugated with secondary antibody (HRP-HBsAb), a sandwich-type immunoassay format was formed. Aminophenol as substrate for conjugated HRP was enzymatically changed into 3-aminophenoxazone (3-APZ). This electroactive enzymatic production (3-APZ) was transferred into an electrochemical cell and monitored by cyclic voltammetry. Under optimal conditions, the cathodic current response of 3-APZ, which was proportional to the HBsAg concentration, was measured by a glassy carbon electrode. The immunosensor response was linear toward HBsAg in the concentration range from 0.001 to 0.015 ng/ml with a detection limit of 0.9 pg/ml at a signal/noise ratio of 3.

Determination of okadaic acid in shellfish by using a novel chemiluminescent enzyme-linked immunosorbent assay method

A direct competitive chemiluminescent enzyme-linked immunosorbent assay (CL-ELISA) was developed to determine okadaic acid (OA). Concentrations of the capture monoclonal anti-OA antibodies, conjugate of OA-HRP and a composition of blocking buffers were varied to optimize the assay condition. The values of IC10, IC50 and working range (IC20-IC80) for CL-ELISA were 0.01, 0.07, and 0.03-0.2 ng/mL, respectively. Additionally, the analytical recovery values of CL-ELISA from 3 shellfish spiked samples with OA concentrations of 0.03, 0.1 and 0.2 ng/mL ranged from 86.7% to 111.2%. Closely examining the OA concentrations in 19 various shellfish products performed by CL-ELISA revealed that OA concentrations in 6 of the 19 examined samples was undetected, whereas the 13 samples were contaminated with low levels of OA ranging from 1.2 to 8.0 ng/g.

Development of an immunochromatographic assay based on carbon nanoparticles for the determination of the phytoregulator forchlorfenuron

Rapid analytical methods enabling the determination of diverse targets are essential in a number of research areas, from clinical diagnostics to feed and food quality and safety. Herein, the development of a quantitative immunochromatographic assay for the detection of the synthetic phytoregulator forchlorfenuron (CPPU) is described. The competitive lateral flow immunoassay (LFIA) was based on the immobilization onto a nitrocellulose membrane of an ovalbumin-CPPU conjugate (test line) and on the use of an immunodetection ligand consisting of carbon nanoparticles labeled with an anti-CPPU monoclonal antibody through interaction with a secondary antibody. The presence of CPPU in horticultural samples was visually interpreted by the decrease in the black signal intensity of the test line, according to the competitive character of the format. The quantitative determination of the analyte was easily performed by a two-step procedure consisting of flatbed scanning of the strips followed by computer-based image analysis of the pixel gray volumes of the test lines. Under optimized conditions, the immunochromatographic test afforded a limit of quantification in buffer of 89 ng/L. The accuracy of the strip test was assessed by the analysis of fruit samples with incurred residues, and the obtained results were compared with those derived from two reference methods, ELISA and HPLC. The LOQ of the CPPU-specific LFIA in kiwifruits and grapes was established at 33.4 μg/kg. The excellent analytical performance of the developed strip test demonstrates the potential of immunochromatographic assays for the quantitative monitoring of small organic molecules in complex matrices.

Development of a solid-phase extraction coupling chemiluminescent enzyme immunoassay for determination of organophosphorus pesticides in environmental water samples

Solid-phase extraction (SPE) and direct competitive chemiluminescence enzyme immunoassay (dcCL-EIA) were combined for the detection of organophosphorus pesticides (OPs) in environmental water samples. dcCL-EIA based on horseradish peroxidase labeled with a broad-specificity monoclonal antibody against OPs was developed, and the effects of several physicochemical parameters on dcCL-EIA performance were studied. SPE was used for the pretreatment of water samples to remove interfering substances and to concentrate the OP analytes. The coupling of SPE and dcCL-EIA can detect seven OPs (parathion, coumaphos, phoxim, quinalphos, triazophos, dichlofenthion, and azinphos-ethyl) with the limit of quantitation below 0.1 ng/mL. The recoveries of OPs from spiked water samples ranged from 62.5% to 131.7% by SPE-dcCL-EIA and 69.5% to 112.3% by SPE-HPLC-MS/MS. The screening of OP residues in real-world environmental water samples by the developed SPE-dcCL-EIA and their confirmatory analysis using SPE-HPLC-MS/MS demonstrated that the assay is ideally suited as a monitoring method for OP residues prior to chromatographic analysis.

Hapten-antibody recognition studies in competitive immunoassay of α-zearalanol analogs by computational chemistry and Pearson Correlation analysis

The molecular recognition of hapten-antibody is a fundamental event in competitive immunoassay, which guarantees the sensitivity and specificity of immunoassay for the detection of haptens. The aim of this study is to investigate the correlation between binding ability of one monoclonal antibody, 1H9B4, recognizing and the molecular aspects of α-zearalanol analogs. The mouse-derived monoclonal antibody was produced by using α-zearalanol conjugated to bovine serum albumin as an immunogen. The antibody recognition abilities, expressed as IC(50) values, were determined by a competitive ELISA. All of the hapten molecules were optimized by Density Function Theory (DFT) at B3LYP/ 6-31G* level and the conformation and electrostatic molecular isosurface were employed to explain the molecular recognition between α-zearalanol analogs and antibody 1H9B4. Pearson Correlation analysis between molecular descriptors and IC(50) values was qualitatively undertaken and the results showed that one molecular descriptor, surface of the hapten molecule, clearly demonstrated linear relationship with antibody recognition ability, where the relationship coefficient was 0.88 and the correlation was significant at p < 0.05 level. The study shows that computational chemistry and Pearson Correlation analysis can be used as tool to help the immunochemistries better understand the processing of antibody recognition of hapten molecules in competitive immunoassay.

Paper-based chemiluminescence ELISA: lab-on-paper based on chitosan modified paper device and wax-screen-printing

A novel lab-on-paper device combining the simplicity and low-cost of microfluidic paper-based analytical devices (μPADs) and the sensitivity and selectivity of chemiluminescence ELISA (CL-ELISA) for the high-throughput, rapid, stable and reusable point-of-care testing is presented here. Chitosan was used to modify μPADs to covalently immobilize antibodies on μPADs. Thus, sandwich CL-ELISA on μPADs can be easily realized for further development of this technique in sensitive, specific and low-cost application. The paper device was fabricated by a low-cost, simple, and rapid wax-screen-printing method. Using tumor markers and paper microzone plate as model, the application test of this paper-based CL-ELISA was successfully performed with a linear range of 0.1-35.0 ng mL(-1) for α-fetoprotein, 0.5-80.0 U mL(-1) for cancer antigen 125 and 0.1-70.0 ng mL(-1) for carcinoembryonic antigen. Since the cutoff values of the three tumor markers in clinical diagnosis are 25 ng mL(-1), 35 U mL(-1) and 5 ng mL(-1), the sensitivity and linear ranges of the proposed method were enough for clinical application. In addition, this lab-on-paper immunodevice can provide reproducible results upon storage at 4 °C (sealed) for at least 5 weeks. Ultimately, this novel chitosan modification and wax-screen-printing methodology for μPADs can be readily translated to other signal reporting mechanism including electrochemiluminescence and photoelectrochemistry, and other receptors such as enzyme receptors and DNA receptors for determination of DNA, proteins and small molecules in point-of-care testing.

Comparison of enzyme-linked immunosorbent assays with chemiluminescent and colorimetric detection for the determination of ochratoxin A in food

A direct competitive chemiluminescent enzyme-linked immunosorbent assay (CL-ELISA) for the determination of ochratoxin A (OTA) was developed using soybean peroxidase (SbP) in combination with 3-(10'-phenothiazinyl)propane-1-sulfonate (SPTZ) and 4-morpholinopyridine (MORPH) as a detection system. By varying the concentrations of the capture monoclonal anti-OTA antibody, a conjugate of OTA with SbP, and the composition of blocking buffers, the conditions of the immunoassay were optimized. Advantages of CL-ELISA were demonstrated by comparison with ELISA with colorimetric detection (COL-ELISA). The values of IC₁₀, IC₅₀, and working range (IC₂₀-IC₈₀) for CL-ELISA and COL-ELISA were 0.01, 0.08, and 0.02-0.3 ng/mL and 0.08, 0.58, and 0.17-2.2 ng/mL, respectively. The recovery values of CL-ELISA from three soybean spiked samples with OTA concentrations of 0.07, 0.1, and 0.15 ng/mL ranged from 72 to 125%. Determination of OTA in 21 various agricultural commodities showed that OTA in 8 examined samples was not detected by COL-ELISA. Furthermore, it was found that in 4 of these 8 samples the developed CL-ELISA determined OTA at levels from 0.96 to 4.64 ng/g.

Miniaturised hybrid immunoassay for high sensitivity analysis of aflatoxin M1 in milk

An ultra-sensitive sandwich ELISA was developed for detection of AFM1 in milk. The assay involved the immobilization of rat monoclonal antibody of AFM1 in 384 microtiter plate to capture AFM1 antigen. This was detected by tracer secondary rabbit poly-clonal antibody labelled with horseradish peroxidase upon addition of a luminol-based substrate. Milk samples with different fat percentage were analyzed after pre-treatment. Linear range of AFM1 detection 250-6.25 pg/mL was achieved in 3% fat milk. The miniaturised assay (10 μL) enabled ultra trace analysis of AFM1 in milk with much improved lower limit of detection at 0.005 pg/mL. A sensitive magnetic nanoparticles (MNPs) based ELISA was also developed and coupled with micro plate ELISA for analysis in milk. The hybrid-assay, by coupling the 1°Ab immobilized MNPs column with microwell plate assay enabled simultaneous measurement of low (0.5 pg/mL) and high AFM1 contamination (200 pg/mL). The most promising feature of this MNPs-ELISA is the small column size, high capture efficiency and lower cost over other reported materials. The proposed assay can be deployed for simultaneous analysis and monitoring of AFM1 in milk.

Development of a monoclonal antibody-based enzyme immunoassay for the pyrethroid insecticide deltamethrin

With a screening hemisolid stem-cell culture, four positive hybridomas 2B(12), 2C(1), 2F(1), and 3D(4) were screened and used to prepare four correspondent monoclonal antibodies (McAbs) against the pyrethroid insecticide deltamethrin. These McAbs showed I(50) values in a range of 17-94 ng mL(-1), from among which the antibody 2B(12) with the lowest I(50) value was selected to develop an optimized enzyme-linked immunosorbent assay (ELISA). In the developed ELISA, the I(50) of deltamethrin was 17.0 +/- 3.3 ng mL(-1) and the limit of detection (LOD) was 1.2 +/- 1.3 ng mL(-1). There seemed to be little or no cross-reactivity with other tested pyrethroids and their metabolites. For validation of the assay method, environmental water samples fortified with deltamethrin were analyzed with the ELISA and gas chromatography (GC) methods. The recoveries of the developed ELISA ranged from 82 to 117%, which were close to those of the GC method (94-103%). These results suggested that the developed ELISA based on the McAb 2B(12) could be used for the rapid and sensitive determination of deltamethrin in environmental water.

On-chip immunoassay using electrostatic assembly of streptavidin-coated bead micropatterns

We propose an original concept for a sandwich immunoassay that is completely performed on-chip using streptavidin-coated beads as substrate. The latter are electrostatically self-assembled on aminosilane micropatterns at the bottom of a microfluidic channel. We use mouse IgG diluted in phosphate buffered saline (PBS) with 1% bovine serum albumin (BSA) solution as target antigen. The fluorescent sandwich immunocomplex is formed on the beads during the operation of the chip both in stop-flow and continuous-flow modes. Target mouse IgG antigen is detected down to a concentration of 15 ng/mL in stop-flow mode and 250 pg/mL in continuous-flow mode, using only 1300 nL of sample volume. We also demonstrate the possibility of simultaneous detection of two different antigens in a PBS-BSA solution using a dual microfluidic channel structure.

Electrochemical immunosensor of tumor necrosis factor α based on alkaline phosphatase functionalized nanospheres

A novel immunosensor for sensitive detection of tumor necrosis factor α was reported. First of all, gold nanoparticles were uniformly assembled on the surface of poly (styrene-acrylic acid) nanospheres, which was used as the matrix to conjugate alkaline phosphatase (ALP). And then, the obtained composite was used as multi-enzyme functionalized label for immunoassay. Biocompatible polyaniline doped with poly (acrylic acid) was electro-polymerized at the glass carbon electrode to construct the matrix for the immobilization of antibody TNF-α. After the sandwich immunoreaction, the labeled ALP was used to hydrolyze α-naphthyl phosphate to produce the electroactive α-naphthol, which could be amperometrically detected. The results showed that the electrochemical signals were proportional to the logarithm of the antigen concentration in the range of 0.02-200.00 ng/mL with the detection limit of 0.01 ng/mL. The developed immunoassay showed high sensitivity, acceptable stability and reproducibility, which might have potentially broad applications in protein diagnostics and bioassay.

Advantages of soybean peroxidase over horseradish peroxidase as the enzyme label in chemiluminescent enzyme-linked immunosorbent assay of sulfamethoxypyridazine

An indirect competitive chemiluminescent enzyme-linked immunosorbent assay (CL-ELISA) of sulfamethoxypyridazine (SMP) was developed. The conjugates of streptavidin with cationic horseradish peroxidase (HRP) and anionic soybean peroxidase (SbP) were used in CL-ELISA for the detection of biotinylated anti-SMP antibodies. For streptavidin-HRP conjugate-catalyzed chemiluminescence measured 20 s after the initiation of the enhanced chemiluminescence reaction (ECR), the limit of detection (IC(10)), the IC(50) value, and the working range in CL-ELISA of SMP are 0.3, 12.4, and 1.2-85.0 ng/mL, respectively. An increase in the time interval between the ECR initiation and the luminescence measurement results in the loss in the quality of analytical measurements because of the time-dependent quenching of chemiluminescence typical of the HRP-catalyzed ECR. In the case of SbP-based CL-ELISA of SMP, the limit of detection, the IC(50) value, and the working range (0.025, 0.17, and 0.045-0.63 ng/mL, respectively) are better than those for HRP-based CL-ELISA. Furthermore, the analytical parameters of SbP-based CL-ELISA remain unchanged during a long period of time (for at least 30 min). The recovery values from four spiked milk samples with different concentrations of SMP in SbP-based CL-ELISA vary from 70 to 130%.