One-Step Ultrasensitive Bioluminescent Enzyme Immunoassay Based on Nanobody/Nanoluciferase Fusion for Detection of Aflatoxin B1 in Cereal

Sequence-based design and construction of synthetic nanobody library

Nanobody (Nb), the smallest antibody fragments known to bind antigens, is now widely applied to various studies, including protein structure analysis, bioassay, diagnosis, and biomedicine. The traditional approach to generating specific nanobodies involves animal immunization which is time-consuming and expensive. As the understanding of the antibody repertoire accumulation, the synthetic library, which is devoid of animals, has attracted attention widely in recent years. Here, we describe a synthetic phage display library (S-Library), designed based on the systematic analysis of the next-generation sequencing (NGS) of nanobody repertoire. The library consists of a single highly conserved scaffold (IGHV3S65*01-IGHJ4*01) and complementary determining regions of constrained diversity. The S-Library containing 2.19 × 108 independent clones was constructed by the one-step assembly and rapid electro-transformation. The S-Library was screened against various targets (Nb G8, fusion protein of Nb G8 and green fluorescent protein, bovine serum albumin, ovalbumin, and acetylcholinesterase). In comparison, a naïve library (N-Library) from the source of 13 healthy animals was constructed and screened against the same targets as the S-Library. Binders were isolated from both S-Library and N-Library. The dynamic affinity was evaluated by the biolayer interferometry. The data confirms that the feature of the Nb repertoire is conducive to reducing the complexity of library design, thus allowing the S-Library to be built on conventional reagents and primers.

Nanobody mediated dual-mode immunoassay for detection of peanut allergen Ara h 3

To improve the performance of peanut allergen Ara h 3 detection, depending on boron and nitrogen carbon dots (B/N-CDs), a nanobody (Nb) mediated dual-mode immunoassay was established, which combines the dominance of colorimetry with ratiometric fluorescence techniques. With the catalysis of Horseradish peroxidase (HRP), the oxidization of o-phenylenediamine (o-PD) in the presence of H2O2, leading to the production of yellow 2,3-diaminophenolazine (DAP) with an absorption peak at 431 nm. Owing to inner filter effect (IFE), DAP quenched the fluorescence of B/N-CDs at 426 nm, and it emerged a new emission peak at 549 nm. The fluorescence intensity ratio and absorption intensity can be utilized for quantitative analysis of Ara h 3 concentration. Under optimal conditions, the detection limits were 6.61 and 9.79 ng·mL-1, respectively. The dual-mode immunoassay was assessed containing specificity, stability, reproducibility, and practicability. This method paved the way for sensitive detection of Ara h 3 without background Interference.

Multivalent nanobody-based sandwich enzyme-linked immunosorbent assay for sensitive detection of porcine reproductive and respiratory syndrome virus

The pandemic of the porcine reproductive and respiratory syndrome virus (PRRSV) has caused huge economic losses and continues to threaten the swine industry worldwide. Nucleocapsid protein (N protein) is the primary antigen of PRRSV for development of sensitive diagnostic assays. Two high affinity nanobodies against N protein, Nb12 and Nb35, were selected and employed to develop a sandwich ELISA. Further we improved the ELISA method to obtain greater sensitivity, a trivalent nanobody (3 × Nb35) and a bivalent nanobody-HRP fusion protein (2 × Nb12-HRP) were expressed and used. This modified ELISA was found to have high sensitivity for detecting PRRSV, with a detection limit of 10 TCID50/ml (median tissue culture infectious dose), which was approximately 200-fold greater than the single-copy nanobody-based sandwich ELISA. The developed assay shows high specificity and can detect almost all circulating lineages of PRRSV-2 in China. This study provides suggestions for reforming nanobodies and for the further development of multivalent nanobody-based ELISAs for other various viruses.

Immunoassays and Emerging Analytical Techniques of Fipronil and its Metabolites for Food Safety: A Review

Fipronil, classified as a phenylpyrazole insecticide, is utilized to control agricultural, public health, and veterinary pests. Notably, its unique ecological fate involves degradation to toxic metabolites, which poses the risk of contamination in water and foodstuffs and potential human exposure through the food chain. In response to these concerns, there is a pressing need to develop analytical methodologies for detecting fipronil and its metabolites. This review provides a concise overview of the mode of action, metabolism, and toxicology of fipronil. Additionally, various detection strategies, encompassing antibody-based immunoassays and emerging analytical techniques, such as fluorescence assays based on aptamer/molecularly imprinted polymer/fluorescent probes, electrochemical sensors, and Raman spectroscopy, are thoroughly reviewed and discussed. The focus extends to detecting fipronil and its metabolites in crops, fruits, vegetables, animal-derived foods, water, and bodily fluids. This comprehensive exploration contributes valuable insights into the field, aiming to foster the development and innovation of more sensitive, rapid, and applicable analytical methods.

Protein nanoscaffold enables programmable nanobody-luciferase immunoassembly for sensitive and simultaneous detection of aflatoxin B1 and ochratoxin A

Mycotoxins produced by fungi can contaminate various foods and pose significant health risks. Ensuring food safety demands rapid, highly sensitive analytical techniques. One-step Bioluminescent Enzyme Immunoassays (BLEIAs) employing nanobody-nanoluciferase fusion proteins have recently garnered attention for operational simplicity and heightened sensitivity. Nevertheless, fixed nanobody:nanoluciferase ratios in fusion proteins restrict the customization and sensitivity of traditional BLEIAs. In this study, we present a Scaffold Assembly-based BLEIA (SA-BLEIA) that overcomes these limitations through the programmable conjugation of nanobodies and luciferases onto 60-meric protein nanoscaffolds using SpyTag/SpyCatcher linkages. These nanoscaffolds facilitate the adjustable coupling of anti-aflatoxin B1 and anti-ochratoxin A nanobodies with luciferases, optimizing nanobody/luciferase ratios and diversifying specificities. Compared to conventional methods, SA-BLEIA demonstrates considerably elevated sensitivity for detecting both toxins. The elevated local concentration of luciferase significantly amplifies bioluminescence intensity, permitting reduced substrate consumption and cost-effective detection. The usage of dual-nanobody conjugates facilitates the quantification or simultaneous detection of both mycotoxins in a single test with shared reagents. The assay exhibits exceptional recovery rates in spiked cereal samples, strongly correlating with outcomes from commercial ELISA kits. Overall, this adaptable, highly sensitive, cost-effective, and multiplexed immunoassay underscores the potential of tunable scaffold assembly as a promising avenue for advancing bioanalytical diagnostic tools.

Molecular Evolution of Antiparathion Nanobody with Enhanced Sensitivity and Specificity Based on Structural Analysis

Nanobody (Nb) has gained significant attention in immunoassays owing to its numerous advantages, particularly its ease of molecular evolution. However, the limited understanding of how high sensitivity and specificity attained for antihapten Nbs hamper the development of high-performance Nbs. Herein, the antiparathion Nb (Nb9) we prepared previously was chosen as the model, and an approach based on X-ray crystallography, molecular docking, and rational site-directed saturation mutation for constructing a rapid and effective platform for nanobody evolution was described. Based on the structural analysis, two mutants, namely Nb-D5 (IC50 = 2.4 ± 0.2 ng/mL) and Nb-D12 (IC50 = 2.7 ± 0.1 ng/mL), were selected out from a six-sites directed saturation mutation library, 3.5-fold and 3.1-fold sensitivity enhancement over Nb9 to parathion, respectively. Besides, Nb-D12 exhibited improved sensitivity for quinalphos, triazophos, and coumaphos (5.4-35.4 ng/mL), indicating its broader detection potential. Overall, our study advances an effective strategy for the future rational evolution of Nbs with desirable performance.

Luciola mingrelica firefly luciferase as a marker in bioluminescent immunoassays

Chemical modification of the enzymes with biospecific macromolecules is used in various fields of biotechnology to impart new functions or improve their properties and is a fast and convenient way to get the final products. The preparation of highly active, stable, and functionally active conjugates of the thermostable luciferase through the NH2-groups or free SH-groups of the enzyme with target molecules of different molecular weight (albumin, avidin from chicken eggs, antibodies, and progesterone) is described. The obtained conjugates were successfully tested as a reporter in bioluminescent immunoassay for the detection of the molecules and pathogens. Thus, the luc-albumin (Luc-Alb) and luc-insulin (Luc-Ins) conjugates were used in competitive ELISA for the detection of an analyte (albumin or insulin) in the samples. Luc-progesterone (Luc-Pg) was used in the rapid homogeneous immunoassay of progesterone by the BRET technique with the detection limit of 0.5 ng/ml. Luciferase conjugates with avidin (Luc-Avi) and secondary and primary antibodies (Luc-RAM and Luc-Sal) were used for enzyme immunoassay detection of Salmonella paratyphi A cells with the cell detection limit of 5 × 104 CFU/ml. To reduce the detection limit of Salmonella cells, we developed a pseudo-homogeneous bioluminescent enzyme immunoassay of cells using a new matrix for the analyte capture-polystyrene microparticles coated with Pluronic F108, covalently labeled with Sal antibodies. This allowed to achieve efficient trapping of cells from solution, significantly reduced nonspecific sorption and decreased the cell detection limit to 2.7 × 103 CFU/ml without prior concentration of the sample. The methodology that was developed in this study can be applied for the development of novel bioanalytical systems based on firefly luciferases.

Structural Insights into the Stability and Recognition Mechanism of the Antiquinalphos Nanobody for the Detection of Quinalphos in Foods

Nanobodies (Nbs) have great potential in immunoassays due to their exceptional physicochemical properties. With the immortal nature of Nbs and the ability to manipulate their structures using protein engineering, it will become increasingly valuable to understand what structural features of Nbs drive high stability, affinity, and selectivity. Here, we employed an anti-quinalphos Nb as a model to illustrate the structural basis of Nbs' distinctive physicochemical properties and the recognition mechanism. The results indicated that the Nb-11A-ligand complexes exhibit a "tunnel" binding mode formed by CDR1, CDR2, and FR3. The orientation and hydrophobicity of small ligands are the primary determinants of their diverse affinities to Nb-11A. In addition, the primary factors contributing to Nb-11A's limited stability at high temperatures and in organic solvents are the rearrangement of the hydrogen bonding network and the enlargement of the binding cavity. Importantly, Ala 97 and Ala 34 at the active cavity's bottom and Arg 29 and Leu 73 at its entrance play vital roles in hapten recognition, which were further confirmed by mutant Nb-F3. Thus, our findings contribute to a deeper understanding of the recognition and stability mechanisms of anti-hapten Nbs and shed new light on the rational design of novel haptens and directed evolution to produce high-performance antibodies.

A highly sensitive nanobody-based immunoassay detecting SARS-CoV-2 nucleocapsid protein using all-recombinant reagents

Antigen tests have been crucial for managing the COVID-19 pandemic by identifying individuals infected with SARS-CoV-2. This remains true even after immunity has been widely attained through natural infection and vaccination, since it only provides moderate protection against transmission and is highly permeable to the emergence of new virus variants. For this reason, the widespread availability of diagnostic methods is essential for health systems to manage outbreaks effectively. In this work, we generated nanobodies to the virus nucleocapsid protein (NP) and after an affinity-guided selection identified a nanobody pair that allowed the detection of NP at sub-ng/mL levels in a colorimetric two-site ELISA, demonstrating high diagnostic value with clinical samples. We further modified the assay by using a nanobody-NanoLuc luciferase chimeric tracer, resulting in increased sensitivity (detection limit = 61 pg/mL) and remarkable improvement in diagnostic performance. The luminescent assay was finally evaluated using 115 nasopharyngeal swab samples. Receiver Operating Characteristic (ROC) curve analysis revealed a sensitivity of 78.7% (95% confidence interval: 64.3%-89.3%) and specificity of 100.0% (95% confidence interval: 94.7%-100.0%). The test allows the parallel analysis of a large number of untreated samples, and fulfills our goal of producing a recombinant reagent-based test that can be reproduced at low cost by other laboratories with recombinant expression capabilities, aiding to build diagnostic capacity.

Nanoassemblies Based on a Cationic Perylene Diimide Derivative and Sodium Dodecyl Sulfate: A Simple Fluorescent Platform for Efficient Analysis of Aflatoxin B1

Aflatoxin B₁ (AFB₁) is a kind of potently carcinogenic fungal metabolite in food threatening human health, and it is crucial and challenging to develop advanced nonimmune approaches for AFB₁ determination. Addressing this challenge, we successfully constructed a nanoassembly (PdE-PDI/SDS) by noncovalently coupling a cationic perylene diimide derivative (PdE-PDI) and sodium dodecyl sulfate (SDS), exhibiting high-density charges and a specific surface area for rapid sensing of AFB₁. The large electronic conjugate structure and rigid plane of PdE-PDI enable it to form more stable σ-π, π-π coordination, and hydrogen bonds with AFB₁. Additionally, the introduction of SDS significantly amplifies noncovalent interactions and enhances the quenching efficiency of PdE-PDI toward AFB₁. The proposed PdE-PDI/SDS exhibited excellent specificity to AFB₁ and showed dosage-sensitive detection with detection limit as low as 0.74 ng mL^-1. Finally, the PdE-PDI/SDS was successfully applied in cereal samples with good recoveries from 94.61 to 109.92%. To our knowledge, this is the first time a fluorescent strategy from the point of self-assembly for AFB₁ determination is reported, which holds great promise for wide applications of perylene diimide derivative in food safety.

Microelectromechanical Microsystems-Supported Photothermal Immunoassay for Point-of-Care Testing of Aflatoxin B1 in Foodstuff

Accurate identification of acutely toxic and low-fatality mycotoxins on a large scale in a quick and cheap manner is critical for reducing population mortality. Herein, a portable photothermal immunosensing platform supported by a microelectromechanical microsystem (MEMS) without enzyme involvement was reported for point-of-care testing of mycotoxins (in the case of aflatoxin B₁, AFB₁) in food based on the precise satellite structure of Au nanoparticles. The synthesized Au nanoparticles with a well-defined, graded satellite structure exhibited a significantly enhanced photothermal response and were coupled by AFB₁ antibodies to form signal conversion probes by physisorption for further target-promoted competitive responses in microplates. In addition, a coin-sized miniature NIR camera device was constructed for temperature acquisition during target testing based on advanced MEMS fabrication technology to address the limitation of expensive signal acquisition components of current photothermal sensors. The proposed MEMS readout-based microphotothermal test method provides excellent AFB₁ response in the range of 0.5-500 ng g^-1 with detection limits as low as 0.27 ng g^-1. In addition, the main reasons for the efficient photothermal transduction efficiency of Au with different graded structures were analyzed by finite element simulations, providing theoretical guidance for the development of new Au-based photothermal agents. In conclusion, the proposed portable micro-photothermal test system offers great potential for point-of-care diagnostics for residents, which will continue to facilitate immediate food safety identification in resource-limited regions.

Nanobody-based magnetic chemiluminescence immunoassay for one-pot detection of ochratoxin A

Ochratoxin A (OTA) contamination seriously threatens food safety and human health and requires sensitive and rapid tools for monitoring. In this study, a convenient enzyme-linked immunosorbent assay based on Avi-labeled nanobody Nb-2G/streptavidin-alkaline phosphatase and magnetic beads (MBS-ELISA) was established for the sensitive detection of OTA, which could be used for one-pot detection without immobilization. After optimization, the 50% inhibitory concentration (IC50) and the lowest limit of detection value of the MBS-ELISA was 1.17 ng/mL and 0.07 ng/mL and the linear range was 248.8 pg/mL-5.28 ng/mL, respectively, which accords with state criteria for food safety. The developed one-step MBS-ELISA was almost 20-times more sensitive than the classic BA-ELISA and could generate results within 15 min, which was significantly less than the classic BA-ELISA at approximately 3 h. The MBS-ELISA indicated good recovery (86.4-114.3%) in spiked sorghum, buckwheat, and mung bean. Thus, MBS-ELISA represents a very promising strategy for the simple, rapid, and accurate detection of OTA and other toxic and hazardous contaminants.

Production of Ribosomal Protein S12/Renilla Luciferase Fusion and Development of a Bioluminescent Method for Detection of Aminoglycosides in Pork and Studying Its Recognition Mechanism

In this study, the genes of Escherichia coli ribosomal protein S12 and renilla luciferase were linked and expressed to produce a fusion protein, and its intermolecular interactions and affinities with sevenaminoglycosides were studied. Then, the fusion protein was used as the core agent to develop a bioluminescent method on a conventional microplate for determination of the residues of thesevenaminoglycosides in pork. This method contained only one sample-loading step, and thus the assay was finished within 30 min. The limits of detection for the sevendrugs were in the range of 0.51-1.1 ng/mL, and the sensitivity for a specific drug was mainly determined by the receptordrug affinity but not related with the binding energy. After general comparison, the present method showed generally better performances than the previously reported enzyme-linked immunosorbent assays for aminoglycosides. This is the first study reporting the recognition mechanisms of Escherichia coli ribosomal protein S12 for aminoglycosides and developing a bioluminescent method for detection of aminoglycoside residues in pork samples.

A new nanobody-enzyme fusion protein-linked immunoassay for detecting antibodies against influenza A virus in different species

Circulation of influenza A virus (IAV), especially within poultry and pigs, continues to threaten public health. A simple and universal detecting method is important for monitoring IAV infection in different species. Recently, nanobodies, which show advantages of easy gene editing and low cost of production, are a promising novel diagnostic tool for the monitoring and control of global IAVs. In the present study, five nanobodies against the nucleoprotein of H9N2 IAV were screened from the immunized Bactrian camel by phage display and modified with horseradish peroxidase (HRP) tags. Out of which, we determined that H9N2-NP-Nb5-HRP can crossreact with different subtypes of IAVs, and this reaction is also blocked by positive sera for antibodies against different IAV subtypes. Epitope mapping showed that the nanobody-HRP fusion recognized a conserved conformational epitope in all subtypes of IAVs. Subsequently, we developed a nanobody-based competitive ELISA (cELISA) for detecting anti-IAV antibodies in different species. The optimized amount of coating antigen and dilutions of the fusion and testing sera were 100 ng/well, 1:4000, and 1:10, respectively. The time for operating the cELISA was approximately 35 min. The cELISA showed high sensitivity, specificity, reproducibility, and stability. In addition, we found that the cELISA and hemagglutination inhibition test showed a consistency of 100% and 87.91% for clinical and challenged chicken sera, respectively. Furthermore, the agreement rates were 90.4% and 85.7% between the cELISA and commercial IEDXX ELISA kit. Collectively, our developed nanobody-HRP fusion-based cELISA is an ideal method for monitoring IAV infection in different species.

A Nanobody-Based Immunoassay for Detection of Ustilaginoidins in Rice Samples

Ustilaginoidins are a class of bis-naphtho-γ-pyrone mycotoxins produced by the pathogen Villosiclava virens of rice false smut, which has recently become one of the most devastating diseases in rice-growing regions worldwide. In this research, the nanobody phage display library was established after an alpaca was immunized with the hemiustilaginoidin F-hapten coupled with bovine serum albumin (BSA). Heterologous antigen selection and combing trypsin with competition alternant elution methods were performed for nanobody screening. Two nanobodies, namely, Nb-B15 and Nb–C21, were selected for the establishment of indirect competitive enzyme-linked immunosorbent assays (ic-ELISAs). For Nb–B15 and Nb-C21, their IC₅₀ values were 11.86 μg/mL and 11.22 μg/mL, and the detection ranges were at 3.41–19.98 μg/mL and 1.17–32.13 μg/mL, respectively. Two nanobodies had a broad spectrum to quantify the contents of total ustilaginoidins in rice samples according to cross-reactivity. The recognition mechanisms of Nb-B15 and Nb-C21 against ustilaginoidin A were elucidated by molecular modeling and docking. The key amino acid sites for the binding of Nb–B15 or Nb–C21 to ustilaginoidin A were mainly located in the FR1 and CDR1 regions. As Nb-B15 was superior to Nb–C21 in the aspects of protein expression, ELISA titer, and tolerance to organic solvents, it was selected for application in the detection of actual contaminated rice samples. The total ustilaginoidin contents of rice samples were analyzed by Nb–B15-based ic–ELISA and HPLC-DAD, between which the results were found to be consistent. The developed immunoassay based on the nanobody from the alpaca can be employed as a rapid and effective method for detection of total utilaginoidins in contaminated rice samples.

Nanobody/NanoBiT system-mediated bioluminescence immunosensor for one-step homogeneous detection of trace ochratoxin A in food

Hazardous small molecules in food and environment seriously threatens human health, which requires sensitive and rapid tools for monitoring. Using a previously identified nanobody against ochratoxin A (OTA), we herein proposed a homogeneous sensing platform "nanobody/NanoLuc Binary Technology (NanoBiT) system" and developed a nanobody/NanoBiT system-mediated bioluminescence immunosensor (NBL-Immunosens) for OTA using LgBiT (Lg) and SmBiT (Sm), two subunits of the split nanoluciferase (NanoLuc). The core elements of NBL-Immunosens include Lg-nanobody fusion (NLg) and Sm-labeled OTA-bovine serum albumin conjugate (OSm). The antigen-antibody interaction between NLg and OSm triggers the reconstitution of NanoLuc for generating luminescence signals. Moreover, free OTA can compete with OSm for binding to NLg, resulting the decrease of dose-dependent signals. NBL-Immunosens can detect OTA in a one-step assay of 5 min without washing and exhibit a limit of detection of 0.01 ng/mL with a linear range of 0.04-2.23 ng/mL. It shows high selectivity for OTA and has good accuracy and precision in the spiking-and-recovery experiments. Furthermore, its effectiveness was evaluated with real cereal samples and confirmed by liquid chromatography tandem mass spectrometry and commercial ELISA kits. Hence, the NBL-Immunosens is a very promising tool for rapid, accurate, and selective detection of trace OTA in food.

Generation of bioluminescent enzyme immunoassay for ferritin by single-chain variable fragment and its NanoLuc luciferase fusion

Ferritin, widely present in liver and spleen tissue, is considered as a serological biomarker for liver diseases and cancers. The detection of ferritin may be an important tool in health diagnosis. In this study, 14 non-immunized chicken spleens were utilized to construct a single-chain fragment (scFv) phage library. After 4 rounds of panning, 7 unique clones were obtained. The optimal clone was further screened and combined with NanoLuc luciferase (Nluc) as a dual functional immunoprobe to bioluminescent enzyme immunoassay (BLEIA), which was twice as sensitive as its parental scFv-based double-sandwich enzyme-linked immunoassay (ds-ELISA). The cross-reactivity analysis revealed that the proposed methods were highly selective and suitable for clinical detection. To further verify the performance of the immunoassays, serum samples were tested by the proposed methods and a commercial ELISA kit, and there was a good correlation between the results. These results suggested that scFv fused with Nluc might be a powerful dual functional tool for rapid, practically reliable, and highly sensitive ferritin detection.

A novel magnetic metal-organic framework absorbent for rapid detection of aflatoxins B1B2G1G2 in rice by HPLC-MS/MS

In this study, a core-shell-structured magnetic metal-organic framework (MMOF) composite material (Fe₃O₄@UiO-66-NH₂) was synthesized by the solvothermal method. It was employed as a new absorbent in combination with high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) for the simultaneous detection of four aflatoxins (AFs) in rice. This method could shorten the pre-processing time by exploiting the advantageous characteristics of magnetic cores. The impurity was removed quickly. The effects of extraction solution, extraction time, adsorbent types, and amount of adsorbent on the extraction rate of target compounds were optimized. Under optimized conditions, AFs were validated and showed a good linear relationship within the 0.375-20 μg kg^-1 concentration range (r² > 0.9992). The limit of detection (LOD) was 0.0188-0.1250 μg kg^-1 and the limit of quantification (LOQ) was 0.0375-0.3750 μg kg^-1. At three spiking levels (0.375, 2, and 10 μg kg^-1), the average recovery values for the four AFs ranged from 85.1% to 111.0%. The relative standard deviation ranged from 3.4% to 7.7%. The new method proved to be simple, fast, efficient, and suitable for the determination of AFs in rice samples.

Nanobody and Nanozyme-Enabled Immunoassays with Enhanced Specificity and Sensitivity

Immunoassay as a rapid and convenient method for detecting a variety of targets has attracted tremendous interest with its high specificity and sensitivity. Among the commonly used immunoassays, enzyme-linked immunosorbent assay has been widely used as a gold standard method in various fields that consists of two main components including a recognition element and an enzyme label. With the rapid advances in nanotechnology, nanobodies and nanozymes enable immunoassays with enhanced specificity and sensitivity compared with conventional antibodies and natural enzymes. This review is focused on the applications of nanobodies and nanozymes in immunoassays. Nanobodies advantage lies in their small size, high specificity, mass expression, and high stability. Nanozymes with peroxidase, phosphatase, and oxidase activities and their applications in immunoassays are highlighted and discussed in detail. In addition, the challenges and outlooks in terms of the use of nanobodies and the development of novel nanozymes in practical applications are discussed.

Recent progress in fluorescent aptasensors for the detection of aflatoxin B1 in food

Aflatoxin B1 pollution is one of the most critical issues of food safety and has been categorized as a group I carcinogen by the International Agency for Research on Cancer. Aflatoxin B1 exists in various foods and feedstuff products and can be produced and contaminate food products in all processes, including growth, harvest, storage, or processing. Therefore, it is of great value for detecting and on-site monitoring aflatoxin B1. Aptamers are short single-stranded DNA or RNA obtained from the nucleic acid molecular library through SELEX. With advantages of high specificity, large affinity, and easy modification, aptasensors have become popular in a wide range of promising applications. This review focuses on recent advances on fluorescent aptamer sensors for the detection of aflatoxin B1, including their design strategies, working mechanisms, and applications to on-site detection. Finally, the current challenges and prospects are discussed.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Preparation of Specific Nanobodies and Their Application in the Rapid Detection of Nodularin-R in Water Samples

Nanobodies have several advantages, including great stability, sensibility, and ease of production; therefore, they have become important tools in immunoassays for chemical contaminants. In this manuscript, nanobodies for the detection of the toxin Nodularin-r (NOD-R), a secondary metabolite of cyanobacteria that could cause a safety risk for drinks and food for its strong hepatotoxicity, were for the first time selected from an immunized Bactrian camel VHH phage display library. Then, a sensitive indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) for NOD-R, based on the nanobody N56 with great thermostability and organic solvent tolerance, was established under optimized conditions. The results showed that the limit of detection for NOD-R was 0.67 µg/L, and the average spike recovery rate was between 84.0 and 118.3%. Moreover, the ic-ELISA method was validated with spiked water sample and confirmed by UPLC-MS/MS, which indicated that the ic-ELISA established in this work is a reproducible detection assay for nodularin residues in water samples.

A Novel Brighter Bioluminescent Fusion Protein Based on ZZ Domain and Amydetes vivianii Firefly Luciferase for Immunoassays

Immunoassays are widely used for detection of antibodies against specific antigens in diagnosis, as well as in electrophoretic techniques such as Western Blotting. They usually rely on colorimetric, fluorescent or chemiluminescent methods for detection. Whereas the chemiluminescence methods are more sensitive and widely used, they usually suffer of fast luminescence decay. Here we constructed a novel bioluminescent fusion protein based on the N-terminal ZZ portion of protein A and the brighter green-blue emitting Amydetes vivianii firefly luciferase. In the presence of D-luciferin/ATP assay solution, the new fusion protein, displays higher bioluminescence activity, is very thermostable and produces a sustained emission (t_1/2 > 30 min). In dot blots, we could successfully detect rabbit IgG against firefly luciferases, Limpet Haemocyanin, and SARS-CoV-2 Nucleoprotein (1–250 ng), as well as the antigen bound antibodies using either CCD imaging, and even photography using smartphones. Using CCD imaging, we could detect up to 100 pg of SARS-CoV-2 Nucleoprotein. Using this system, we could also successfully detect firefly luciferase and SARS-CoV-2 nucleoprotein in Western Blots (5–250 ng). Comparatively, the new fusion protein displays slightly higher and more sustained luminescent signal when compared to commercial HRP-labeled secondary antibodies, constituting a novel promising alternative for Western Blotting and immunoassays.

Enzyme cascade-amplified immunoassay based on the nanobody-alkaline phosphatase fusion and MnO2 nanosheets for the detection of ochratoxin A in coffee

Ochratoxin A (OTA) is a common food contaminant with multiple toxicities and thus rapid and accurate detection of OTA is indispensable to minimize the threat of OTA to public health. Herein a novel enzyme cascade-amplified immunoassay (ECAIA) based on the mutated nanobody–alkaline phosphatase fusion (mNb–AP) and MnO₂ nanosheets was established for detecting OTA in coffee. The detection principle is that the dual functional mNb–AP could specifically recognize OTA and dephosphorylate the ascorbic acid-2-phosphate (AAP) into ascorbic acid (AA), and the MnO₂ nanosheets mimicking the oxidase could be reduced by AA into Mn²⁺ and catalyze the 3,3′,5,5′-tetramethyl benzidine into blue oxidized product for quantification. Using the optimal conditions, the ECAIA could be finished within 132.5 min and shows a limit of detection of 3.38 ng mL⁻¹ (IC₁₀) with an IC₅₀ of 7.65 ng mL⁻¹ and a linear range (IC₂₀–IC₈₀) of 4.55–12.85 ng mL⁻¹. The ECAIA is highly selective for OTA. Good recovery rates (84.3–113%) with a relative standard deviation of 1.3–3% were obtained and confirmed by high performance liquid chromatography with a fluorescence detector. The developed ECAIA was demonstrated to be a useful tool for the detection of OTA in coffee which provides a reference for the analysis of other toxic small molecules.

Enzyme cascade-amplified immunoassay using nanobody–alkaline phosphatase fusion and MnO₂ nanosheets for sensitive and selective detection of ochratoxin A in coffee.

Preparation of Luciferase-fused Peptides for Immunoassay of Amyloid Beta

An immunoassay, such as the enzyme-linked immunosorbent assay (ELISA), is an analytical method that utilizes the interaction of antigens and antibodies. Enzyme-labeled antigens require both molecular recognition by the antibody and enzymatic activity as a reporter. We designed and constructed an immunodetection system for amyloid beta peptides (Aβ) using an enzyme-labeled antigen expressed from Escherichia coli. Aβ(1-16) fused with renilla luciferase was prepared as the enzyme-labeled antigen. In the presence of this luciferase-fused peptide, the luminescence of coelenterazine-h was observed. The influence of the fusion with Aβ on the luminescence reaction was insignificant. Surface plasmon resonance analysis indicated that the interaction between the luciferase-fused Aβ and anti-Aβ antibody was sufficiently strong. In the competitive ELISA assay for Aβ detection using the luciferase-fused Aβ, the luminescence intensity decreased as the Aβ concentration increased.

Enhanced Non-Toxic Immunodetection of Alternaria Mycotoxin Tenuazonic Acid Based on Ferritin-Displayed Anti-Idiotypic Nanobody-Nanoluciferase Multimers

The non-toxic immunoassay for mycotoxins is being paid more attention due to its advantages of higher safety and cost savings by using anti-idiotype antibodies to substitute toxins. In this study, with tenuazonic acid (TeA), a kind of highly toxic Alternaria mycotoxin as the target, an enhanced non-toxic immunoassay was developed based on the ferritin-displayed anti-idiotypic nanobody-nanoluciferase multimers. First, three specific β-type anti-idiotype nanobodies (AId-Nbs) bearing the internal image of TeA mycotoxin were selected from an immune phage display library. Then, the AId-Nb 2D with the best performance was exploited to generate a nanoluciferase (Nluc)-functionalized fusion monomer, by which a one-step non-toxic immunodetection format for TeA was established and proven to be effective. To further improve the affinity of the monomer, a ferritin display strategy was used to prepare 2D-Nluc fusion multimers. Finally, an enhanced bioluminescent enzyme immunoassay (BLEIA) was established in which the half maximal inhibitory concentration (IC₅₀) for TeA was 6.5 ng/mL with a 10.5-fold improvement of the 2D-based enzyme-linked immunosorbent assay (ELISA). The proposed assay exhibited high selectivities and good recoveries of 80.0-95.2%. The generated AId-Nb and ferritin-displayed AId-Nb-Nluc multimers were successfully extended to the application of TeA in food samples. This study brings a new strategy for production of multivalent AId-Nbs and non-toxic immunoassays for trace toxic contaminants.

Highly photoluminescent carbon dots-based immunosensors for ultrasensitive detection of aflatoxin M1 residues in milk

Here, a facile hydrothermal method was used to synthesize highly photoluminescent N-doped carbon dots, and the quantum yields reached 97.1%. Then, a label-free immunosensor based on the inner filter effect of carbon dots was developed for ultrasensitive detection of aflatoxin M₁ residues in milk. The detection limit was 0.0186 ng/mL (equivalents to 18.10 ng/kg), which satisfied the most stringent maximum tolerable limit value of 25 ng/kg. Besides, the immunosensor showed a good linear relationship from 0.003 ng/mL to 0.81 ng/mL, and the average recoveries ranged from 79.6% to 112.5% for spiked milk samples, with relative standard deviations ranging from 6.7% to 13.3%. Compared with other immunoassays, the inner filter effect-based immunosensor incorporating fluorescent detection into conventional enzymatic cascade amplification systems and could be a reliable on-site screening method for aflatoxin M₁ residue analysis.

Nanobody-Based Indirect Competitive ELISA for Sensitive Detection of 19-Nortestosterone in Animal Urine

Nanobody (Nb), a new type of biorecognition element generally from Camelidae, has the characteristics of small molecular weight, high stability, great solubility and high expression level in E. coli. In this study, with 19-nortestosterone (19-NT), an anabolic androgenic steroid as target drug, three specific Nbs against 19-NT were selected from camel immune library by phage display technology. The obtained Nbs showed excellent thermostability and organic solvent tolerance. The nanobody Nb2F7 with the best performance was used to develop a sensitive indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) for 19-NT detection. Under optimized conditions, the standard curve of ic-ELISA was fitted with a half-maximal inhibitory concentration (IC₅₀) of 1.03 ng/mL and a detection limit (LOD) of 0.10 ng/mL for 19-NT. Meanwhile, the developed assay had low cross- reactivity with analogs and the recoveries of 19-NT ranged from 82.61% to 99.24% in spiked samples. The correlation coefficient between ic-ELISA and the ultra-performance liquid chromatography/mass spectrometry (UPLC-MS/MS) method was 0.9975, which indicated that the nanobody-based ic-ELISA could be a useful tool for a rapid analysis of 19-NT in animal urine samples.

NanoLuc luciferase as a suitable fusion partner of recombinant antibody fragments for developing sensitive luminescent immunoassays

Enzyme-linked immunosorbent assays (ELISAs) are essential for monitoring various biomarkers. Competitive and noncompetitive (sandwich) assay formats are used to determine hapten and macromolecule levels, respectively. Both formats require more sensitive detection of reporter enzymes for greater assay sensitivities. We previously reported the utility of wild-type Gaussia luciferase (wtGLuc) as a fusion partner with antibody single-chain Fv fragments (scFvs) for developing sensitive luminescent ELISAs. Here, we evaluated utility of NanoLuc luciferase (NLuc), a recently developed luciferase, as fusion partner with scFvs from the view of comparison with wtGLuc and a mutant of alkaline phosphatase (ALP'). Thyroxine (T₄) and T₄-labeled albumin were chosen as model haptenic and macromolecular antigens, respectively. An in-house-prepared anti-T₄ scFv was fused with NLuc, wtGLuc, or ALP'. The scFv-NLuc fusion protein showed 47-fold and 29-fold lower limit of detection [LOD; 59 zmol (per assay)] than the wtGLuc- and ALP'-fusions, respectively. In a competitive T₄ ELISA, the NLuc-fusion showed 9.3- and 6.3-fold lower LOD, (0.67 pg) than the wtGLuc- and ALP'-fusions, respectively, with a higher specificity in clinical applications. A typical colorimetric ELISA using a peroxidase-labeled second antibody showed 70-fold higher LOD than NLuc-based ELISA. Another advantage of the NLuc-fusion was shown in the sandwich assays; the LOD of T₄-labeled albumin (5.0 fmol) was >6-fold lower than that of the other luminescent ELISAs. In an additional sandwich assay developed to count bacteriophage particles, NLuc enabled more sensitive determination than wtGLuc, whereas ALP' showed nearly equivalent performance. Its slowest alteration rate for light intensity after starting the enzyme reaction should enable robust batch-by-batch assay operations. Thus, we concluded that scFv-NLuc fusions serve as suitable probes in various types of immunoassays and may facilitate higher sensitivities with practical specificities.

Nanobodies targeting immune checkpoint molecules for tumor immunotherapy and immunoimaging (Review)

The immune checkpoint blockade is an effective strategy to enhance the anti-tumor T cell effector activity, thus becoming one of the most promising immunotherapeutic strategies in the history of cancer treatment. Several immune checkpoint inhibitor have been approved by the FDA, such as anti-CTLA-4, anti-PD-1, anti-PD-L1 monoclonal antibodies. Most tumor patients benefitted from these antibodies, but some of the patients did not respond to them. To increase the effectiveness of immunotherapy, including immune checkpoint blockade therapies, miniaturization of antibodies has been introduced. A single-domain antibody, also known as nanobody, is an attractive reagent for immunotherapy and immunoimaging thanks to its unique structural characteristic consisting of a variable region of a single heavy chain antibody. This structure confers to the nanobody a light molecular weight, making it smaller than conventional antibodies, although remaining able to bind to a specific antigen. Therefore, this review summarizes the production of nanobodies targeting immune checkpoint molecules and the application of nanobodies targeting immune checkpoint molecules in immunotherapy and immunoimaging.

Applications of Nanobodies

Unique, functional, homodimeric heavy chain-only antibodies, devoid of light chains, are circulating in the blood of Camelidae. These antibodies recognize their cognate antigen via one single domain, known as VHH or Nanobody. This serendipitous discovery made three decades ago has stimulated a growing number of researchers to generate highly specific Nanobodies against a myriad of targets. The small size, strict monomeric state, robustness, and easy tailoring of these Nanobodies have inspired many groups to design innovative Nanobody-based multi-domain constructs to explore novel applications. As such, Nanobodies have been employed as an exquisite research tool in structural, cell, and developmental biology. Furthermore, Nanobodies have demonstrated their benefit for more sensitive diagnostic tests. Finally, several Nanobody-based constructs have been designed to develop new therapeutic products.

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.

Coelenterazine-Dependent Luciferases as a Powerful Analytical Tool for Research and Biomedical Applications

: The functioning of bioluminescent systems in most of the known marine organisms is based on the oxidation reaction of the same substrate-coelenterazine (CTZ), catalyzed by luciferase. Despite the diversity in structures and the functioning mechanisms, these enzymes can be united into a common group called CTZ-dependent luciferases. Among these, there are two sharply different types of the system organization-Ca2+-regulated photoproteins and luciferases themselves that function in accordance with the classical enzyme-substrate kinetics. Along with deep and comprehensive fundamental research on these systems, approaches and methods of their practical use as highly sensitive reporters in analytics have been developed. The research aiming at the creation of artificial luciferases and synthetic CTZ analogues with new unique properties has led to the development of new experimental analytical methods based on them. The commercial availability of many ready-to-use assay systems based on CTZ-dependent luciferases is also important when choosing them by first-time-users. The development of analytical methods based on these bioluminescent systems is currently booming. The bioluminescent systems under consideration were successfully applied in various biological research areas, which confirms them to be a powerful analytical tool. In this review, we consider the main directions, results, and achievements in research involving these luciferases.

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.

Development of a horseradish peroxidase-nanobody fusion protein for visual detection of ochratoxin A by dot immunoassay

Ochratoxin A (OTA) is a common cereal mycotoxin that seriously threatens food safety and public health. Herein a horseradish peroxidase-nanobody fusion protein (HRP-Nb) retaining antibody and enzyme activity was obtained after inclusion body denaturation and renaturation and enzyme reconstitution, which served both as the primary antibody and reporter enzyme and was applied to develop a membrane-based dot immunoassay (HN-DIA) for OTA visual detection. Based on the optimal experimental conditions, the HN-DIA could be finished in 10 min with a cut-off limit of 50 μg kg-1 in rice and oat samples by eye. The HN-DIA showed high selectivity for OTA and had good accuracy and reproducibility in the recovery experiments. Spiked sample analysis results of the HN-DIA and high performance liquid chromatography (HPLC) correlated well with each other. Therefore, the proposed HN-DIA has the potential for rapid screening of OTA and other small molecule pollutants in food and the environment by naked eye.

Chemiluminescent Enzyme Immunoassay and Bioluminescent Enzyme Immunoassay for Tenuazonic Acid Mycotoxin by Exploitation of Nanobody and Nanobody-Nanoluciferase Fusion

The isolation of nanobodies (Nbs) from phage display libraries is an increasingly effective approach for the generation of new biorecognition elements, which can be used to develop immunoassays. In this study, highly specific Nbs against the Alternaria mycotoxin tenuazonic acid (TeA) were isolated from an immune nanobody phage display library using a stringent biopanning strategy. The obtained Nbs were characterized by classical enzyme-linked immunosorbent assay (ELISA), and the best one Nb-3F9 was fused with nanoluciferase to prepare an advanced bifunctional fusion named nanobody-nanoluciferase (Nb-Nluc). In order to improve the sensitivity and reduce the assay time, two different kinds of luminescent strategies including chemiluminescent enzyme immunoassay (CLEIA) and bioluminescent enzyme immunoassay (BLEIA) were established, respectively, on the basis of the single Nb and the fusion protein Nb-Nluc for TeA detection. The two-step CLEIA was developed on the basis of the same nanobody as ELISA, only with simple substrate replacement from 3,3',5,5'-tetramethylbenzidine (TMB) to luminol. In contrast with CLEIA, the novel BLEIA was conducted in one-step new strategy on the basis of Nb-Nluc and bioluminescent substrate coelenterazine-h (CTZ-h). Their half maximal inhibitory concentration (IC50) values were similar to 8.6 ng/mL for CLEIA and 9.3 ng/mL for BLEIA, which was a 6-fold improvement in sensitivity compared with that of ELISA (IC50 of 54.8 ng/mL). Both of the two assays provided satisfactory recoveries ranging from 80.1%-113.5% in real samples, which showed better selectivity for TeA analogues and other common mycotoxins. These results suggested that Nbs and Nb-Nluc could be used as useful reagents for immunodetection and that the developed CLEIA/BLEIA have great potential for TeA analysis.

An Ultrasensitive Bioluminescent Enzyme Immunoassay Based on Nanobody/Nanoluciferase Heptamer Fusion for the Detection of Tetrabromobisphenol A in Sediment

Tetrabromobisphenol A (TBBPA) is a flame retardant and has become a widely concerning environmental pollutant. An ultrasensitive nanobody-based immunoassay was developed to monitor the exposure of TBBPA in sediment. First, the anti-TBBPA nanobody was fused with nanoluciferase, and then a one-step bioluminescent enzyme immunoassay (BLEIA) was developed with high sensitivity for TBBPA, with a maximum half inhibition concentration (IC₅₀) at 187 pg/mL. Although approximately 10-fold higher sensitivity can be achieved by this developed BLEIA than by the classical two-step ELISA (IC₅₀ at 1778 pg/mL), it is still a challenge to detect trace TBBPA in sediment samples reliably due to the relatively high matrix effect. To further improve the performance of this one-step BLEIA, a C4b-binding protein (C4BP) was inserted as a self-assembling linker between the nanobody and nanoluciferase. Therefore, a heptamer fusion containing seven binders and seven tracers was generated. This reagent improved the binding capacity and signal amplification. The one-step heptamer plus BLEIA based on this immune-reagent shows an additional 7-fold improvement of sensitivity, with the IC₅₀ of 28.9 pg/mL and the limit of detection as low as 2.5 pg/mL. The proposed assay was further applied to determine the trace TBBPA in sediment, and the recovery was within 92-103%. Taking advantage of this heptamer fusion, one-step BLEIA can serve as a powerful tool for fast detection of trace TBBPA in the sediment samples.

Nanobody-based electrochemical competitive immunosensor for the detection of AFB1 through AFB1-HCR as signal amplifier

A novel nanobody (Nb)-based voltammetric immunosensor coupled with horseradish peroxidase concatemer-modified hybridization chain reaction (HRP-HCR) signal amplifying system is described to realize the rapid and ultrasensitive detection of AFB₁. To design such an immunoassay, anti-AFB₁ Nbs with smaller molecular size were coated densely onto the surface of Au nanoparticle-tungsten disulfide-multi-walled carbon nanotubes (AuNPs/WS₂/MWCNTs) functional nanocomposites as an effective molecular recognition element, whereas AFB₁-streptavidin (AFB₁-SA) conjugates were ingeniously bound with biotinylated HCR dsDNA nanostructures as the competitor, amplifier, and signal report element. In the presence of AFB₁ targets, a competitive immunoreaction was performed between the analyte and AFB₁-SA-labeled HCR (AFB₁-HCR) platform. Upon the addition of SA-modified polyHRP (SA-polyHRP), AFB₁-HCR nanostructures containing abundant biotins were allowed to cross-link to a quantity of HRP by streptavidin-biotin chemistry for signal amplification and signal conversion. Under optimal conditions, the immunosensor displayed a good linear correlation toward AFB₁ ranging from 0.5 to 10 ng mL^-1 with a sensitivity of 2.7 μA • (mL ng^-1) and an ultralow limit of detection (LOD) of 68 fg mL^-1. The specificity test showed that the AFB₁ immunosensor had no obvious cross-reaction with OTA, DON, ZEN, and FB₁. The signal of this sensor decreased by 10.18% in 4 weeks indicating satisfactory stability, and its intra- and inter-laboratory reproducibility was 3.42~10.35% and 4.03%~12.11%, respectively. This biosensing system will open up new opportunities for the detection of AFB₁ in food safety and environmental analysis and extend a wide range of applications in the analysis of other small molecules. Graphical abstract.

Nanobodies Based on a Sandwich Immunoassay for the Detection of Staphylococcal Enterotoxin B Free from Interference by Protein A

As one of the leading causes of food poisoning, staphylococcal enterotoxins (SEs) secreted by Staphylococcus aureus pose a serious threat to human health. The immunoassay has become the dominant tool used for the rapid detection of harmful bacteria and toxins as a result of its excellent specificity. However, with regard to SEs, staphylococcal protein A (SpA) is likely to bind with the fragment crystallizable (Fc) terminal of the traditional antibody and result in a false positive, limiting the practical application of this method. Therefore, to eliminate the bottleneck problem, the sandwich immunoassay was development by replacing the traditional antibody with a nanobody (Nb) that lacked a Fc terminal. Using 0.5 × 10⁷ colony-forming units, the Nb library was constructed using Bactrian camels immunized with staphylococcal enterotoxin B (SEB) to obtain a paired Nb against SEB with good affinity. A sandwich enzyme-linked immunosorbent assay (ELISA) was developed using one Nb as the capture antibody and a phage-displayed Nb with signal-amplifying properties as the detection antibody. In optimal conditions, the current immunoassay displayed a broad quantitative range from 1 to 512 ng/mL and a 0.3 ng/mL limit of detection. The recovery of spiked milk, milk powder, cheese, and beef ranged from 87.66 to 114.2%. The Nbs-ELISA was not influenced by SpA during the detection of SEB in S. aureus food poisoning. Therefore, the Nb developed here presented the perfect candidates for immunoassay application during SE determination as a result of the complete absence of SpA interference.

Recombinant expression of nanobodies and nanobody-derived immunoreagents

Antibody fragments for which the sequence is available are suitable for straightforward engineering and expression in both eukaryotic and prokaryotic systems. When produced as fusions with convenient tags, they become reagents which pair their selective binding capacity to an orthogonal function. Several kinds of immunoreagents composed by nanobodies and either large proteins or short sequences have been designed for providing inexpensive ready-to-use biological tools. The possibility to choose among alternative expression strategies is critical because the fusion moieties might require specific conditions for correct folding or post-translational modifications. In the case of nanobody production, the trend is towards simpler but reliable (bacterial) methods that can substitute for more cumbersome processes requiring the use of eukaryotic systems. The use of these will not disappear, but will be restricted to those cases in which the final immunoconstructs must have features that cannot be obtained in prokaryotic cells. At the same time, bacterial expression has evolved from the conventional procedure which considered exclusively the nanobody and nanobody-fusion accumulation in the periplasm. Several reports show the advantage of cytoplasmic expression, surface-display and secretion for at least some applications. Finally, there is an increasing interest to use as a model the short nanobody sequence for the development of in silico methodologies aimed at optimizing the yields, stability and affinity of recombinant antibodies.

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There is an increasing request for immunoreagents based on nanobodies.

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The multiplicity of their applications requires constructs with different structural complexity.

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Alternative expression methods are necessary to achieve such structural requirements.

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In silico optimization of nanobody biophysical characteristics becomes more and more reliable.

Ultrasensitive and rapid detection of ochratoxin A in agro-products by a nanobody-mediated FRET-based immunosensor

Ochratoxin A (OTA) is a major concern for public health and the rapid detection of trace OTA in food is always a challenge. To minimize OTA exposure to consumers, a nanobody (Nb)-mediated förster resonance energy transfer (FRET)-based immunosensor using quantum dots (Nb-FRET immunosensor) was proposed for ultrasensitive, single-step and competitive detection of OTA in agro-products at present work. QDs of two sizes were covalently labeled with OTA and Nb, acting as the energy donor and acceptor, respectively. The free OTA competed with the donor to bind to acceptor, thus the FRET efficiency increased with the decrease of OTA concentration. The single-step assay could be finished in 5 min with a limit of detection of 5 pg/mL, which was attributed to the small size of Nb for shortening the effective FRET distance and improving the FRET efficiency. The Nb-FRET immunosensor exhibited high selectivity for OTA. Moreover, acceptable accuracy and precision were obtained in the analysis of cereals and confirmed by the liquid chromatography-tandem mass spectrometry. Thus the developed Nb-FRET immunosensor was demonstrated to be an efficient tool for ultrasensitive and rapid detection of OTA in cereals and provides a detection model for other toxic small molecules in food and environment.

Bioluminescent Antibodies through Photoconjugation of Protein G-Luciferase Fusion Proteins

Bioluminescent antibodies represent attractive detection agents in both bioanalytical assays and imaging. Currently, their preparation relies on genetic fusion of luciferases to antibodies or nonspecific chemical conjugation strategies. Here, we report a generic method to generate well-defined covalent antibody-luciferase conjugates starting from commercially available monoclonal antibodies. Our approach uses fusion proteins consisting of the bright blue light-emitting luciferase NanoLuc (NL) and an Fc-binding protein domain (Gx) that can be photo-cross-linked to the antibody using UV light illumination. Green and red color variants were constructed by tight fusion of the NanoLuc with a green fluorescent acceptor domain and introduction of Cy3, respectively. To increase the already bright NanoLuc emission, tandem fusions were successfully developed in which the Gx domain is fused to two or three copies of the NanoLuc domain. The Gx-NL fusion proteins can be efficiently photo-cross-linked to all human immunoglobulin G (IgG) isotypes and most mammalian IgG's using 365 nm light, yielding antibodies with either one or two luciferase domains. The bioluminescent antibodies were successfully used in cell immunostaining and bioanalytical assays such as enzyme-linked immunosorbent assay (ELISA) and Western blotting.

Functional nanomaterials based immunological detection of aflatoxin B1: a review

Aflatoxin B1 (AFB1) is highly carcinogenic, mutagenic and teratogenic. Accordingly, sensitive, rapid and cost-effective techniques for detection of AFB1 is in urgent demand for food safety and the health of consumers. In this review, we report the current state of immunoassay formats and development, mainly based on nanomaterials for determination of AFB1. Following an introduction of the field, the microplate-, membrane- and microelectrode-based immunoassays are described. The relevant mechanisms, sensitivities, superiorities and deficiencies of each format are discussed. Finally, perspectives on the future development of nanomaterials-based immunoassays for AFB1 are provided.