High sensitivity immunoassays for small molecule compounds detection – Novel noncompetitive immunoassay designs

Methods and applications of noncompetitive hapten immunoassays

Hapten immunoassays have found extensive application across various domains such as disease diagnostics, environmental monitoring, as well as the evaluation of food and pharmaceutical safety. These techniques traditionally rely on competitive assay formats and often face challenges with sensitivity and specificity. This review focuses on the emergent noncompetitive immunoassay technologies that promise to transcend these limitations through innovative approaches. Noncompetitive immunoassays, leveraging novel elements such as anti-idiotype antibodies, anti-immunocomplex (IC) antibodies, and the strategic use of nanomaterial-enhanced signal detection, are setting new benchmarks for analytical performance. These advancements not only enhance the detection capabilities but also significantly improve specificity inherent in traditional methods. Moreover, the integration of novel materials and binding reagents in these assays offers substantial improvements in assay dynamics, providing faster, more accurate, and reliable results. This review consolidates the latest methodologies and their applications, underlining the transformative impact of noncompetitive technologies in the sensitive detection of haptens across various fields.

A novel sustainable immunoassay for sensitive detection of atrazine based on the anti-idiotypic nanobody and recombinant full-length antibody

Immunoassays have been widely used to determine small-molecule compounds in food and the environment, meeting the challenge of obtaining false positive or negative results because of the variance in the batches of antibodies and antigens. To resolve this problem, atrazine (ATR) was used as a target, and anti-idiotypic nanobodies for ATR (AI-Nbs) and a recombinant full-length antibody against ATR (ATR-rAb) were prepared for the development of a sustainable enzyme-linked immunosorbent assay (ELISA). AI-Nb-7, AI-Nb-58, and AI-Nb-66 were selected from an immune phage display library. ATR-rAb was produced in mammalian HEK293 (F) cells. Among the four detection methods explored, the assay using AI-Nb-66 as a coating antigen and ATR-rAb as a detection reagent yielded a half maximal inhibitory concentration (IC50) of 1.66 ng mL-1 for ATR and a linear range of 0.35-8.73 ng mL-1. The cross-reactivity of the assay to ametryn was 64.24%, whereas that to terbutylazine was 38.20%. Surface plasmon resonance (SPR) analysis illustrated that these cross-reactive triazine compounds can bind to ATR-rAb to varying degrees at high concentrations; however, the binding/dissociation kinetic curves and the response values at the same concentration are different, which results in differences in cross-reactivity. Homology modeling and molecular docking revealed that the triazine ring is vital in recognizing triazine compounds. The proposed immunoassay exhibited acceptable recoveries of 84.40-105.36% for detecting fruit, vegetables, and black tea. In conclusion, this study highlights a new strategy for developing sustainable immunoassays for detecting trace pesticide contaminants.

Development of a cloud-based flow rate tool for eNAMPT biomarker detection

Increased levels of extracellular nicotinamide phosphoribosyltransferase (eNAMPT) are increasingly recognized as a highly useful biomarker of inflammatory disease and disease severity. In preclinical animal studies, a monoclonal antibody that neutralizes eNAMPT has been generated to successfully reduce the extent of inflammatory cascade activation. Thus, the rapid detection of eNAMPT concentration in plasma samples at the point of care (POC) would be of great utility in assessing the benefit of administering an anti-eNAMPT therapeutic. To determine the feasibility of this POC test, we conducted a particle immunoagglutination assay on a paper microfluidic platform and quantified its extent with a flow rate measurement in less than 1 min. A smartphone and cloud-based Google Colab were used to analyze the flow rates automatically. A horizontal flow model and an immunoagglutination binding model were evaluated to optimize the detection time, sample dilution, and particle concentration. This assay successfully detected eNAMPT in both human whole blood and plasma samples (diluted to 10 and 1%), with the limit of detection of 1-20 pg/mL (equivalent to 0.1-0.2 ng/mL in undiluted blood and plasma) and a linear range of 5-40 pg/mL. Furthermore, the smartphone POC assay distinguished clinical samples with low, mid, and high eNAMPT concentrations. Together, these results indicate this POC assay, which utilizes low-cost materials, time-effective methods, and a straightforward immunoassay (without surface immobilization), may reliably allow rapid determination of eNAMPT blood/plasma levels to advantage patient stratification in clinical trials and guide ALT-100 mAb therapeutic decision-making.

Picomolar-Level Sensing of Cannabidiol by Metal Nanoparticles Functionalized with Chemically Induced Dimerization Binders

Simple and fast detection of small molecules is critical for health and environmental monitoring. Methods for chemical detection often use mass spectrometers or enzymes; the former relies on expensive equipment, and the latter is limited to those that can act as enzyme substrates. Affinity reagents like antibodies can target a variety of small-molecule analytes, but the detection requires the successful design of chemically conjugated targets or analogs for competitive binding assays. Here, we developed a generalizable method for the highly sensitive and specific in-solution detection of small molecules, using cannabidiol (CBD) as an example. Our sensing platform uses gold nanoparticles (AuNPs) functionalized with a pair of chemically induced dimerization (CID) nanobody binders (nanobinders), where CID triggers AuNP aggregation and sedimentation in the presence of CBD. Despite moderate binding affinities of the two nanobinders to CBD (equilibrium dissociation constants KD of ∼6 and ∼56 μM), a scheme consisting of CBD-AuNP preanalytical incubation, centrifugation, and electronic detection (ICED) was devised to demonstrate a high sensitivity (limit of detection of ∼100 picomolar) in urine and saliva, a relatively short sensing time (∼2 h), a large dynamic range (5 logs), and a sufficiently high specificity to differentiate CBD from its analog, tetrahydrocannabinol. The high sensing performance was achieved with the multivalency of AuNP sensing, the ICED scheme that increases analyte concentrations in a small assay volume, and a portable electronic detector. This sensing system is readily applicable for wide molecular diagnostic applications.

Affinity-Switchable Interaction of Biotin and Streptavidin for the Signal-ON Detection of Small Molecules

Lateral flow assay (LFA) based on gold nanoparticles (AuNPs) is a widely used analytical device for the rapid analysis of environmental hazards and biomarkers. Typically, a sandwich-type format is used for macromolecule detection, in which the appearance of a red test line indicates a positive result (Signal-ON). In contrast, small molecule detection usually relies on a competitive assay, where the absence of a test line indicates positive testing (Signal-OFF). However, such a "Signal-OFF" reading is usually detected within a narrower dynamic range and tends to generate false-negative signals at a low concentration. Moreover, inconsistent readings between macromolecule and small molecule testing might lead to misinterpretation when used by nonskilled individuals. Herein, we report a "Signal-ON" small molecule competitive assay based on the sterically modulated affinity-switchable interaction of biotin and streptavidin. In the absence of a small molecule target, a large steric hindrance can be imposed on the biotin to prevent interaction with streptavidin. However, in the presence of the small molecule target, this steric effect is removed, allowing the biotin to bind to streptavidin and generate the desired test line. In this article, we demonstrate the selective detection of two small molecule drugs, sulfonamides and trimethoprim, using this simple and modular affinity-switchable lateral flow assay (ASLFA). We believe that this affinity-switchable approach can also be adapted in drug discovery and clinical diagnosis, where the competitive assay format is always used for the rapid analysis of small molecules.

Quantitative detection of RAS and KKS peptides in COVID-19 patient serum by stable isotope dimethyl labeling LC-MS

Angiotensin and kinin metabolic pathways are reported to be altered by many diseases, including COVID-19. Monitoring levels of these peptide metabolites is important for understanding mechanisms of disease processes. In this paper, we report dimethyl labeling of amines in peptides by addition of formaldehyde to samples and deutero-formaldehyde to internal standards to generate nearly identical isotopic standards with 4 m/z units larger per amine group than the corresponding analyte. We apply this approach to rapid, multiplexed, absolute LC-MS/MS quantitation of renin angiotensin system (RAS) and kallikrein-kinin system (KKS) peptides in human blood serum. Limits of detection (LODs) were obtained in the low pg mL-1 range with 3 orders of magnitude dynamic ranges, appropriate for determinations of normal and elevated levels of the target peptides in blood serum and plasma. Accuracy is within ±15% at concentrations above the limit of quantitation, as validated by spike-recovery in serum samples. Applicability was demonstrated by measuring RAS and KKS peptides in serum from COVID-19 patients, but is extendable to any class of peptides or other small molecules bearing reactive -NH2 groups.

Picomolar-Level Sensing of Cannabidiol by Metal Nanoparticles Functionalized with Chemically Induced Dimerization Binders

Simple and fast detection of small molecules is critical to health and environmental monitoring. Methods for chemical detection often use mass spectrometers or enzymes; the former relies on expensive equipment and the latter is limited to those that can act as enzyme substrates. Affinity reagents like antibodies can target a variety of small-molecule analytes, but the detection requires successful design of chemically conjugated targets or analogs for competitive binding assays. Here, we developed a generalizable method for highly sensitive and specific in-solution detection of small molecules, using cannabidiol (CBD) as an example. Our sensing platform uses gold nanoparticles (AuNPs) functionalized with a pair of chemically induced dimerization (CID) nanobody binders (nano-binders), where CID triggers AuNPs aggregation and sedimentation in the presence of CBD. Despite moderate binding affinities of the two nano-binders to CBD (KDs of ~6 and ~56 μM), a scheme consisting of CBD-AuNP pre-analytical incubation, centrifugation, and electronic detection (ICED) was devised to demonstrate a high sensitivity (limit of detection of ~100 picomolar) in urine and saliva, a relatively short assay time (~2 hours), a large dynamic range (5 logs), and a sufficiently high specificity to differentiate CBD from its analog, tetrahydrocannabinol. The high sensing performance was achieved with the multivalency of AuNP sensing, the ICED scheme that increases analyte concentrations in a small assay volume, and a portable electronic detector. This sensing system is readily coupled to other binders for wide molecular diagnostic applications.

Low-Cost Microfluidic Systems for Detection of Neglected Tropical Diseases

Neglected tropical diseases (NTDs) affect tropical and subtropical countries and are caused by viruses, bacteria, protozoa, and helminths. These kinds of diseases spread quickly due to the tropical climate and limited access to clean water, sanitation, and health care, which make exposed people more vulnerable. NTDs are reported to be difficult and inefficient to diagnose. As mentioned, most NTDs occur in countries that are socially vulnerable, and the lack of resources and access to modern laboratories and equipment intensify the difficulty of diagnosis and treatment, leading to an increase in the mortality rate. Portable and low-cost microfluidic systems have been widely applied for clinical diagnosis, offering a promising alternative that can meet the needs for fast, affordable, and reliable diagnostic tests in developing countries. This review provides a critical overview of microfluidic devices that have been reported in the literature for the detection of the most common NTDs over the past 5 years.

A Chemiluminescence Enzyme Immunoassay Based on Biotinylated Nanobody and Streptavidin Amplification for Diazinon Sensitive Quantification

The advantages of genetic modification and preferable physicochemical qualities make nanobody (Nb) easy to develop a sensitive and stable immunosensor platform. Herein, an indirect competitive chemiluminescence enzyme immunoassay (ic-CLEIA) based on biotinylated Nb was established for the quantification of diazinon (DAZ). The anti-DAZ Nb, named Nb-EQ1, with good sensitivity and specificity, was obtained from an immunized library via a phage display technique, where the molecular docking results indicated that the hydrogen bond and hydrophobic interactions between DAZ and complementarity-determining region 3 and framework region 2 in Nb-EQ1 played a critical role in the Nb-DAZ affinity processes. Subsequently, the Nb-EQ1 was further biotinylated to generate a bi-functional Nb-biotin, and then an ic-CLEIA was developed for DAZ determination via signal amplification of the biotin-streptavidin platform. The results showed that the proposed method based on Nb-biotin had a high specificity and sensitivity to DAZ, with a relative broader linear range of 0.12-25.96 ng/mL. After being 2-folds dilution of the vegetable samples matrix, the average recoveries were 85.7-113.9% with a coefficient of variation of 4.2-19.2%. Moreover, the results for the analysis of real samples by the developed ic-CLEIA correlated well with that obtained by reference method GC-MS (R² ≥ 0.97). In summary, the ic-CLEIA based on biotinylated Nb-EQ1 and streptavidin recognition demonstrated itself to be a convenient tool for the quantification of DAZ in vegetables.

The Applications of Electrochemical Immunosensors in the Detection of Disease Biomarkers: A Review

Disease-related biomarkers may serve as indicators of human disease. The clinical diagnosis of diseases may largely benefit from timely and accurate detection of biomarkers, which has been the subject of extensive investigations. Due to the specificity of antibody and antigen recognition, electrochemical immunosensors can accurately detect multiple disease biomarkers, including proteins, antigens, and enzymes. This review deals with the fundamentals and types of electrochemical immunosensors. The electrochemical immunosensors are developed using three different catalysts: redox couples, typical biological enzymes, and nanomimetic enzymes. This review also focuses on the applications of those immunosensors in the detection of cancer, Alzheimer's disease, novel coronavirus pneumonia and other diseases. Finally, the future trends in electrochemical immunosensors are addressed in terms of achieving lower detection limits, improving electrode modification capabilities and developing composite functional materials.

Supersensitive detection of single-histamine molecule on nanoplates by turn-on small molecule fluorescence sandwich immunoassay

We develop a supersensitive "turn-on format" fluorescence sandwich immunoassay for detecting small single molecules. Gold nanoplate-based biotin antibodies and streptavidin-fluorophores were used instead of streptavidin-horseradish peroxidase reacting with a biotin tracer in a microplate-based competitive enzyme-linked immunosorbent assay (ELISA). Our platform showed a low detection limit of 5 zeptomolar (5 × 10^-21 M), 5.4 × 10¹⁰ times higher detection sensitivity than the conventional tune-off format ELISA.

Selective Detection and Characterization of Small Cysteine-Containing Peptides with Cluster-Modified Nanopore Sensing

It was recently demonstrated that one can monitor ligand-induced structure fluctuations of individual thiolate-capped gold nanoclusters using resistive-pulse nanopore sensing. The magnitude of the fluctuations scales with the size of the capping ligand, and it was later shown one can observe ligand exchange in this nanopore setup. We expand on these results by exploring the different types of current fluctuations associated with peptide ligands attaching to tiopronin-capped gold nanoclusters. We show here that the fluctuations can be used to identify the attaching peptide through either the magnitude of the peptide-induced current jumps or the onset of high-frequency current fluctuations. Importantly, the peptide attachment process requires that the peptide contains a cysteine residue. This suggests that nanopore-based monitoring of peptide attachments with thiolate-capped clusters could provide a means for selective detection of cysteine-containing peptides. Finally, we demonstrate the cluster-based protocol with various peptide mixtures to show that one can identify more than one cysteine-containing peptide in a mixture.

Identification and Application of Two Promising Peptide Ligands for the Immunodetection of Imidacloprid Residue

As the most widely used neonicotinoid insecticide, it is of great significance to explore the immunoreagents and immunoassays for imidacloprid (IMI) residue. In immunoassays, specific peptide ligands, such as peptidomimetic and anti-immunocomplex peptides, are regarded as promising substitutes for chemical haptens. In the present work, we identified thirty sequences of peptidomimetics and two sequences of anti-immunocomplex peptides for IMI from three phage pVIII display cyclic peptide libraries, in which the anti-immunocomplex peptides are the first reported noncompetitive reagents for IMI. The peptidomimetic 1-9-H and anti-immunocomplex peptide 2-1-H that showed the best sensitivity were utilized to develop competitive and noncompetitive phage enzyme-linked immunosorbent assays (P-ELISAs), with a half inhibition concentration of 0.55 ng/mL for competitive P-ELISA and a half-saturation concentration of 0.35 ng/mL for noncompetitive P-ELISA. The anti-immunocomplex peptide was demonstrated to greatly improve the specificity compared with competitive P-ELISA. In addition, the accuracy of proposed P-ELISAs was confirmed by recovery analysis and HPLC verification in agricultural and environmental samples. These results show that the peptide ligands identified from phage display library can replace chemical haptens in the immunoassays of IMI with the satisfactory performance.

Single-step noncompetitive immunocomplex immunoassay for rapid aflatoxin detection

Owing to the high carcinogenicity of aflatoxins, these toxic secondary metabolites pose a severe risk to human and animal health and can have major economic implications. Herein, we report the development of a noncompetitive immunoassay for aflatoxins based on a monoclonal capture antibody and a unique anti-immunocomplex (anti-IC) antibody fragment (scFv) isolated from a synthetic antibody repertoire. The anti-IC scFv recognizes the immunocomplex and enables the development of noncompetitive sandwich-type assays despite the small size of the analyte. The single-step assay developed in this work, with a detection limit of 70 pg mL^-1, could detect aflatoxins within 15 min. The assay was applied to the analysis of spiked food samples, and the results showed that the method could provide a rapid and simple tool for aflatoxin detection. Moreover, the work demonstrates the potential of anti-IC antibodies and non-competitive immunoassays for the analysis of small molecule contaminants.

Double Competitive Immunodetection of Small Analyte: Realization for Highly Sensitive Lateral Flow Immunoassay of Chloramphenicol

A new scheme of reagents interaction for lateral flow immunoassay (LFIA) is proposed, which combines the features of competitive and sandwich assay and provides highly sensitive detection of low-molecular-weight analytes. Namely, the antigen in the sample interferes with the formation of the antibody (on the membrane)–hapten-protein–antibody (on the nanoparticle-marker) complex, competing with hapten-protein conjugate in both reactions. The proposed scheme was modelled using COPASI software, with a prediction of limit of detection (LOD) decrease by one order of magnitude compared to the standard competitive LFIA. This feature was experimentally confirmed for the detection of chloramphenicol (CAP) in honey. When tested in spiked honey, the visual LOD was 50 ng/mL for the common scheme and 5 ng/mL for the proposed scheme. Instrumental LOD was 300 pg/mL (1.2 µg/kg in conversion per sample weight of honey) in the standard scheme and 20 pg/mL (80 ng/kg in conversion per sample weight of honey) in the proposed scheme.

Advances in Chicken IgY-Based Immunoassays for the Detection of Chemical and Biological Hazards in Food Samples

As antibodies are the main biological binder for hazards in food samples, their performance directly determines the sensitivity, specificity, and reproducibility of the developed immunoassay. The overwhelmingly used mammalian-derived antibodies usually suffer from complicated preparation, high cost, frequent bleeding of animals, and sometimes low titer and affinity. Chicken yolk antibody (IgY) has recently attracted considerable attention in the bioanalytical field owing to its advantages in productivity, animal welfare, comparable affinity, and high specificity. However, a broad understanding of the application of IgY-based immunoassay for the detection of chemical and biological hazards in food samples remains limited. Here, we briefly summarized the diversity, structure, and production of IgY including polyclonal and monoclonal formats. Then, a comprehensive overview of the principles, designs, and applications of IgY-based immunoassays for these hazards was reviewed and discussed, including food-borne pathogens, food allergens, veterinary drugs, pesticides, toxins, endocrine disrupting chemicals, etc. Thus, the trend of IgY-based immunoassays is expected, and more IgY types, higher sensitivity, and diversification of recognition-to-signal manners are necessary in the future.

Metabolomics for personalized medicine: the input of analytical chemistry from biomarker discovery to point-of-care tests

Metabolomics refers to the large-scale detection, quantification, and analysis of small molecules (metabolites) in biological media. Although metabolomics, alone or combined with other omics data, has already demonstrated its relevance for patient stratification in the frame of research projects and clinical studies, much remains to be done to move this approach to the clinical practice. This is especially true in the perspective of being applied to personalized/precision medicine, which aims at stratifying patients according to their risk of developing diseases, and tailoring medical treatments of patients according to individual characteristics in order to improve their efficacy and limit their toxicity. In this review article, we discuss the main challenges linked to analytical chemistry that need to be addressed to foster the implementation of metabolomics in the clinics and the use of the data produced by this approach in personalized medicine. First of all, there are already well-known issues related to untargeted metabolomics workflows at the levels of data production (lack of standardization), metabolite identification (small proportion of annotated features and identified metabolites), and data processing (from automatic detection of features to multi-omic data integration) that hamper the inter-operability and reusability of metabolomics data. Furthermore, the outputs of metabolomics workflows are complex molecular signatures of few tens of metabolites, often with small abundance variations, and obtained with expensive laboratory equipment. It is thus necessary to simplify these molecular signatures so that they can be produced and used in the field. This last point, which is still poorly addressed by the metabolomics community, may be crucial in a near future with the increased availability of molecular signatures of medical relevance and the increased societal demand for participatory medicine.

Monoplex and multiplex immunoassays: approval, advancements, and alternatives

Immunoassays are a powerful diagnostic tool and are widely used for the quantification of proteins and biomolecules in medical diagnosis and research. Enzyme-linked immunosorbent assay (ELISA) is the most commonly used immunoassay format and allows the detection of biomarkers at a very low concentration. The diagnostic platforms such as enzyme immunoassay (EIA), chemiluminescence (CL) assay, polymerase chain reaction (PCR), flow cytometry (FC), and mass spectrometry (MS) have been used to identify molecular biomarkers. However, these diagnostic tools requiring expensive equipment, long testing time, and qualified personnel that is not always available in small local hospitals with limited resources. The lateral flow immunoassay (LFIA) platform was developed for rapidly obtaining laboratory results and to make urgent decisions in emergency medicine, as well as the recently introduced concept of testing at the site of care (point-of-care, POC). The simultaneous measurement of different substances from a single sample called multiplex assays have become increasingly significant for in vitro quantification of multiple analytes in a single sample, thereby minimising cost, time, and volume. In multiplex immunoassays, the ligands are immobilized either in planar format (flat surface) or on microspheres in suspension that binds to target analytes in sample. The multiplex technology has established itself in proteomic networks and pathways, validation of genomic discoveries, and in the development of clinical biomarkers. In the present review article, various types of monoplex/simplex and complex/multiplex immunoassays have been analysed that are increasingly being applied in laboratory medicine. Also, some advantages and disadvantages of these multiplex assays have also been included such as experimental animals, in vitro tests using cell lines and tissue samples, 3-dimensional modelling and bioprinting, in silico tests, organ-on-chip, and computer modelling.

Immunoassays for rapid mycotoxin detection: state of the art

The widespread presence of mycotoxins in nature not only poses a huge health risk to people in terms of food but also causes incalculable losses to the agricultural economy. As a rapidly developing technology in recent years, the mycotoxin immunoassay technology has approached or even surpassed the traditional chromatography technology in some aspects. Using this approach, the lateral flow immunoassay (LFIA) has attracted the interest of researchers due to its user-friendly operation, short time consumption, little interference, low cost, and ability to process a large number of samples at the same time. This paper provides an overview of the immunogens commonly used for mycotoxins, the development of antibodies, and the use of gold nanoparticles, quantum dots, carbon nanoparticles, enzymes, and fluorescent microsphere labeling materials for the construction of LFIAs to improve detection sensitivity. The analytical performance, detection substrates, detection limits or detection ranges of LFIA for mycotoxins have been listed in recent years. Finally, we describe the future outlook for the field, predicting that portable mobile detection devices and simultaneous quantitative detection of multiple mycotoxins is one of the important directions for future development.

Competitive and noncompetitive fluorescence polarization immunoassays for the detection of benzothiostrobin using FITC-labeled dendrimer-like peptides

Peptides obtained from phage display libraries are valuable reagents for small-molecule immunoassays. However, their application in fluorescence polarization immunoassays (FPIAs) is limited by phage particles. Here, monomer, dendrimer-like dimer, tetramer peptidomimetic and anti-immunocomplex tracers were designed and synthesized using lysine as special scaffolds and spacers to develop competitive and noncompetitive FPIAs for benzothiostrobin. The affinity between tracers and monoclonal antibodies or immunocomplexes increased with the tracer valence. A higher signal-to-noise ratio and sensitivity could be generated in the FPIAs based on tetramer tracers. The sensitivities of competitive (50% inhibitory concentration) and noncompetitive (50% saturation concentration) FPIAs were 19.71 ± 4.65 and 40.43 ± 2.73 ng mL^-1, respectively. The spiked recoveries were 78.3%-105.2% with relative standard deviations (RSDs) of 0.7%-15.4% for the competitive FPIA, while 78.7%-115.3% with RSDs of 0.7%-12.5% for the noncompetitive FPIA. The amounts of benzothiostrobin in rice detected by the FPIAs were consistent with those detected by high performance liquid chromatography.

Fluorescence Signal Enhancement in Antibody Microarrays Using Lightguiding Nanowires

Fluorescence-based detection assays play an essential role in the life sciences and medicine. To offer better detection sensitivity and lower limits of detection (LOD), there is a growing need for novel platforms with an improved readout capacity. In this context, substrates containing semiconductor nanowires may offer significant advantages, due to their proven light-emission enhancing, waveguiding properties, and increased surface area. To demonstrate and evaluate the potential of such nanowires in the context of diagnostic assays, we have in this work adopted a well-established single-chain fragment antibody-based assay, based on a protocol previously designed for biomarker detection using planar microarrays, to freestanding, SiO₂-coated gallium phosphide nanowires. The assay was used for the detection of protein biomarkers in highly complex human serum at high dilution. The signal quality was quantified and compared with results obtained on conventional flat silicon and plastic substrates used in the established microarray applications. Our results show that using the nanowire-sensor platform in combination with conventional readout methods, improves the signal intensity, contrast, and signal-to-noise by more than one order of magnitude compared to flat surfaces. The results confirm the potential of lightguiding nanowires for signal enhancement and their capacity to improve the LOD of standard diagnostic assays.

Toxicant substitutes in immunological assays for mycotoxins detection: A mini review

Recurring mycotoxins contamination has posedaseriousthreatto food safety worldwide. Competitive immunoassays are widely used techniques for high-throughput mycotoxins detection in agricultural products and foods. However, the inevitable introduction of mycotoxin conjugates produced by chemical conjugation usually results in complicated by-products, large batch errors and threats to operators and environment. Biologically derived surrogates of mycotoxin conjugates or mycotoxin standards are renewable immunoreagents. They can serve the same function as the responding counterparts in the immunoassays. The substitute-based immunoassays exhibit satisfactory sensitivity, pose less health threats to operators and environment, and contribute to the standardization of immunoassays for mycotoxins. This review focuses on the current applications of substitute-based immunoassays, clarifies their underlying mechanisms and provides a careful comparison. Challenges and future prospects are discussed.

Recent advances in aptasensors for mycotoxin detection: On the surface and in the colloid

Mycotoxins are a great potential threat to human health, and the progress in the development of mycotoxin detection methods is of an escalating importance with the increasing emphasis on food safety. Aptamer, performing the same function as antibody in specific binding with targets, exhibits profound potential in biosensing since its debut in 1990. Recent years have witnessed the rapid development of aptasensors for mycotoxin detection with the achievement of ultralow limit of detection and high sensitivity in the lab. However, there is still no officially approved aptasensing methods in mycotoxin detection application. In order to provide researchers with inspirations in the design and development of aptasensors for mycotoxin detection, we divide these aptasensors into two types, namely "on the surface" and "in the colloid", according to the location where the key sensing reaction occurs. We also systematically review aptasensors reported in the past 5 years under the abovementioned criterion of classification, and compare the advantages and disadvantages of each kind of aptasensors. Finally, we discuss prospective directions in the development of aptasensors for mycotoxin detection. This paper will offer insight and motivation to practitioners working on the research and practical application of aptasensors in the detection of mycotoxins and other substances.

Application of phage-display developed antibody and antigen substitutes in immunoassays for small molecule contaminants analysis: A mini-review

Toxic small molecule contaminants (SMCs) residues in food threaten human health. Immunoassays are popular and simple techniques for SMCs analysis. However, immunoassays based on polyclonal antibodies, monoclonal antibodies and chemosynthetic antigens have some defects, such as complicated preparation of antibodies, risk of toxic haptens using for antigen chemosynthesis and so on. Phage-display technique has been proven to be an attractive alternative approach to producing antibody and antigen substitutes of SMCs, and opened up new realms for developing immunoassays of SMCs. These substitutes contain five types, including anti-idiotypic recombinant antibody (AIdA), anti-immune complex peptide (AIcP), anti-immune complex recombinant antibody (AIcA) and anti-SMC recombinant antibody (anti-SMC RAb). In this review, the principle of immunoassays based on the five types of substitutes, as well as their application and advantages are summarized and discussed.

Highly sensitive phage-magnetic-chemiluminescent enzyme immunoassay for determination of zearalenone

Here we demonstrate a novel phage-magnetic-chemiluminescent enzyme immunoassay (P-MCLEIA) for detection of zearalenone (ZEN). The P-MCLEIA was more efficient than conventional ELISA through several improvements. In the P-MCLEIA, magnetic nanoparticles were replaced of microplates as solid phases to reduce the whole incubation time within 40 min. Phage-mimotope was replaced of chemosynthetic antigen to improve the sensitivity of immunoassay. Chemiluminescence substrate was replaced of chromogenic substrate to further improve the sensitivity. The IC50 value of P-MCLEIA was 31.4 pg/mL, which was about 11 times lower than that of phage-magnetic-enzyme linked immunosorbent assay (P-MELISA) and 72 times lower than that of conventional ELISA. The LOD of P-MCLEIA was 4.3 pg/mL. Recovery study of P-MCLEIA was performed by analyzing ZEN levels in spiked corn samples, intra- and inter-assay recoveries were 80.0-119.8% and 82.7-112.7%, respectively. Furthermore, parallel analysis of natural corn samples showed a good correlation between the P-MCLEIA and high performance liquid chromatography.

Fluorescence immunoassay for multiplex detection of organophosphate pesticides in agro-products based on signal amplification of gold nanoparticles and oligonucleotides

Herein, we developed a multi-analyte fluorescence immunoassay for detection of three organophosphate pesticides (triazophos, parathion, and chlorpyrifos) in various agro-products (rice, wheat, cucumber, cabbage, and apple) using fluorescently labeled oligonucleotide and gold nanoparticle (AuNP) signal amplification technology. The AuNP probes for the three analytes were constructed by simultaneously modifying the corresponding antibodies and fluorescently labeled oligonucleotides on the probe surface. Three fluorophores (6-FAM, Cy3, and Texas red) with high fluorescence intensity and little overlap of excitation/emission wavelengths were selected. The method showed satisfactory linearity for triazophos, parathion, and chlorpyrifos in the ranges of 0.01-20, 0.05-50, and 0.5-1000 μg/L, respectively. For the 3 analytes, the limits of detection (LODs) were 0.007, 0.009, and 0.087 μg/L, respectively. The average recoveries were 77.7-113.6%, with relative standard deviations (RSDs) of 7.1-17.1% in various food matrices. The proposed method offers great potential in food safety surveillance, and could be used as well as a reference for multi-residue analysis of other small-molecule contaminants.

Bead-Based Immunocomplex Entrapment Assays for Rapid, Sensitive, and Multiplexed Detection of Disease Biomarkers with Minimal User Intervention

Current interest in at-home diagnostic devices derives from their potential to disrupt expensive and time-consuming hospital-based diagnostic practices. Conventional immunoassays are often touted for use in at-home diagnostic devices, although in practice they are slow, labor-intensive and require expensive equipment. Here, we introduce bead-based sensors as alternative biomarker detection platforms for at-home diagnostic devices. The immunocomplex entrapment assay (ICEA), and the related enzyme-linked ICEA (ELICEA) offer enhancements over conventional immunoassays in terms of their speed, and minimal requirements for user intervention and instrumentation. In particular, we designed bead-based sensors to entrap large molecular weight complexes between target molecules and signal-generating immunoconjugates while allowing any unbound conjugates to escape from the bead. Confocal fluorescence microscopy was used to demonstrate the sensitivity, robustness, and reproducibility of the ICEA and ELICEA platforms. For example, we showed the intensity of signals generated by entrapped immunoconjugate complexes correlate linearly with the concentration of target molecule in the sample. We employed ICEA, and ELICEA platforms to detect human forms of immunoglobulins, albumin, and κ light chain (KLC). For example, we used ICEA to detect KLC at 5 μg·mL^-1 in urine, which would allow for earlier diagnosis of Bence-Jones disease compared to conventional assays. In addition, we showed bead-entrapped phosphatases (AP) in immunocomplexes generate insoluble, blue-colored dyes from AP-substrates that accumulate in beads and allow for visual and cellphone camera-based detection of IgG to 10 ng·mL^-1 within 20 min. Finally, we described ICEA and ELICEA platforms to analyze multiple target proteins within individual beads.

Review: immunoassays in DNA damage and instability detection

The review includes information on the current state of knowledge of immunometric methods with emphasis on the possibility of deoxyribonucleic acid (DNA) damage detection. Beginning with basic immunoassay enzyme-linked immunosorbent assay (ELISA), this review describes methods such as tyramide signal amplification (TSA), enhanced polymer one-step staining (EPOS), and time resolved amplified cryptate emission (TRACE) as improvements of ELISA's developed over time to obtain more accurate results. In the second part of the review, surface plasmon resonance (SPR) and quantum dots (QDs) are presented as the newest outlooks in the context of immunoanalysis of biological material and molecular studies. The aim of this review is to briefly present immunoassays with emphasis on DNA damage detection; therefore, the types of methods are listed and described, types of signal indicators, basic definitions such as antigen and antibody are given. Every method is considered with an exemplary application focusing on DNA studies, DNA damage and instability detection.

Label-Free Direct Detection of Saxitoxin Based on a Localized Surface Plasmon Resonance Aptasensor

Seafood is an emerging health food, and interest in improving the quality of seafood is increasing. Saxitoxin (STX) is a neurotoxin produced by marine dinoflagellates that is accumulated in seafood. It can block the neuronal transmission between nerves and muscle cell membranes, resulting in the disturbance of neuromuscular transmission and subsequent voluntary muscle paralysis. Here, we developed a new aptamer for the detection of STX using graphene oxide–systematic evolution of ligands by exponential enrichment (GO-SELEX). Furthermore, we confirmed sensitivity and selectivity of the developed aptamer specific to STX using a localized surface plasmon resonance (LSPR) sensor. The sensing chip was fabricated by fixing the new STX aptamer immobilized on the gold nanorod (GNR) substrate. The STX LSPR aptasensor showed a broad, linear detection range from 5 to 10,000 μg/L, with a limit of detection (LOD) of 2.46 μg/L (3σ). Moreover, it was suitable for the detection of STX (10, 100, and 2000 μg/L) in spiked mussel samples and showed a good recovery rate (96.13–116.05%). The results demonstrated that the new STX aptamer-modified GNR chip was sufficiently sensitive and selective to detect STX and can be applied to real samples as well. This LSPR aptasensor is a simple, label-free, cost-effective sensing system with a wide detectable range.

Electrochemical Immuno- and Aptasensors for Mycotoxin Determination

Modern analysis of food and feed is mostly focused on development of fast and reliable portable devices intended for field applications. In this review, electrochemical biosensors based on immunological reactions and aptamers are considered in the determination of mycotoxins as one of most common contaminants able to negatively affect human health. The characteristics of biosensors are considered from the point of view of general principles of bioreceptor implementation and signal transduction providing sub-nanomolar detection limits of mycotoxins. Moreover, the modern trends of bioreceptor selection and modification are discussed as well as future trends of biosensor development for mycotoxin determination are considered.