Development of a nanobody-alkaline phosphatase fusion protein and its application in a highly sensitive direct competitive fluorescence enzyme immunoassay for detection of ochratoxin A in cereal

Hapten synthesis, monoclonal antibody production and immunoassay development for analyzing spiropidion residues

Spiropidion developed by Syngenta shows high insecticidal and acaricidal activity against a wide range of sucking pests. In this study, according to the structure of spiropidion, two haptens were synthesized by introducing carboxyl groups from the ester group. After cell fusion, a monoclonal antibody (mAb 8B5) of spiropidion was obtained. The IC50 of the established heterologous indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) was 7.36 ng/mL, and its working range was 1.75-34.92 ng/mL. The average recoveries were 76.05-124.78% in the Yangtze River and citrus samples. Moreover, the ic-ELISA results of 15 citrus samples agreed well with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Overall, the established ic-ELISA could be applied for the spiropidion residue monitor in food and agricultural samples.

Colorimetric and chemiluminescent enzyme immunoassays based on the alkaline phosphatase-tagged single-chain variable fragment fusion tracer for detecting zearalenone in agro-products

Zearalenone (ZEN) is a non-steroidal estrogenic mycotoxin and seriously threatens food safety, which requires rapid and sensitive detection methods for monitoring ZEN in agro-products. Herein, an alkaline phosphatase-tagged single-chain variable fragment fusion protein (ALP-scFv) was used as a bifunctional tracer to develop a colorimetric enzyme immunoassay (CEIA) and a chemiluminescent enzyme immunoassay (CLEIA) for ZEN. In addition, the interactions between scFv and ZEN were exploited by computer-assisted simulation, and four key amino acid sites were preliminarily identified. After optimization, the CEIA and CLEIA exhibited a limit of detection of 0.02 and 0.006 ng/mL, respectively. Furthermore, both methods showed favorable accuracy in recovery experiments and good selectivity in cross reactions. Moreover, the detection results of the actual samples from both methods correlated well with those from high-performance liquid chromatography. Overall, the ALP-scFv fusion tracer-based CEIA and CLEIA are demonstrated as reliable tools for ZEN detection in food.

Aptamers for nanobodies: A nontoxic alternative to toxic ochratoxin A in immunoassays

To measure toxins using immunoassays, hazardous toxin standards need to be added for quantification. To solve this problem, we propose to use aptamers as competitors to replace toxin standards. In this work, aptamers specific for ochratoxin A (OTA) nanobodies were selected using a DNA library containing a 36 nucleotide random region. The obtained sequences were highly aligned and the best competitor was identified to be a sequence named apt2-OT based on an indirect competitive enzyme-linked immunosorbent assay (ELISA). The Kd of apt2-OT was measured to be 2.86 μM using local surface plasmon resonance spectroscopy. The optimal apt2-OT was identified to substitute the OTA standard with a concentration needed for 50% inhibition of binding (IC50) of 3.26 μM based on a nontoxic direct competitive ELISA. The equivalence relationship between the aptamer and OTA was established in a flour sample, and a recovery experiment was performed. The detection limit for this method was 0.23 ng/mL, with a linear range from 0.25 to 10.50 ng/mL. The recovery rate was 97.5%-115.5%. This study provides a low-cost, rapid and environmentally friendly alternative to the development of immunoassays for toxins.

Combination of nanobody and peptidomimetic to develop novel immunoassay platforms for detecting ochratoxin A in cereals

Mimotope-based immunoassays for mycotoxins eliminate the requirement for large amounts of mycotoxin standards for the chemosynthesis of artificial antigens. Herein, the nanobody-based magnetic beads were used to screen the mimotope (peptidomimetic) of ochratoxin A (OTA) from the phage-displayed peptide library. The interactions between nanobody and the most sensitive Y4 peptidomimetic were investigated by computer-assisted simulation and compared with those between nanobody and OTA. By combining the nanobody, the phage-displayed Y4 and alkaline phosphatase-tagged Y4 fusion protein as the competing antigens, were used to develop two novel immunoassay platforms (PN-ELISA and APN-ELISA). The two methods are advantageous in the use of nontoxic substitutes of OTA and avoiding the use of monoclonal antibodies. Moreover, good analytical performances of both methods were obtained and confirmed by liquid chromatography tandem mass spectrometry. Therefore, the proposed novel methods based on nanobody and peptidomimetic were demonstrated to be highly reliable for detecting OTA in food.

Anti-Idiotypic Nanobody Alkaline Phosphatase Fusion Protein-Triggered On-Off-On Fluorescence Immunosensor for Aflatoxin in Cereals

Nanobodies (Nbs) are widely used in immunoassays with the advantages of small size and high stability. Here, the nanobody employed as the surrogate of aflatoxin antigen and the recognition mechanism of antiaflatoxin mAb with nanobody was studied by molecular modeling, which verified the feasibility of Nbs as antigen substitutes. On this basis, a nanobody-alkaline phosphatase fusion protein (Nb-AP) was constructed, and a highly sensitive "on-off-on" fluorescent immunosensor (OFO-FL immunosensor) based on the calcein/Ce3+ system was developed for aflatoxin quantification. Briefly, calcein serves as a signal transducer, and its fluorescence can be quenched after it is bound with Ce3+. In the presence of Nb-AP, AP catalyzed p-nitrophenyl phosphate to generate orthophosphate, which competes in binding with Ce3+, leading to fluorescence recovery. The method has a linearity range of 0.005-100 ng/mL, and the IC50 of the OFO-FL immunosensor was 0.063 ng/mL, which was 18-fold lower than that of conventional enzyme-linked immunosorbent assay. The assay was successfully applied in food samples with a recovery of 88-121%.

Efficient Hapten-Specific Biopanning Strategy Based on the Fe3O4@ENR-Functionalized Core-Shell Magnetic Nanoparticles

The biopanning of target-specific phages is one of the most critical steps in the preparation of single-domain antibodies. In the traditional biopanning of haptens, the nonspecific binding of library phages to macromolecular proteins is one of the most challenging problems in preparing single-domain antibodies. In this research, Fe3O4@ENR-functionalized core-shell magnetic nanoparticles (FMNPs) were silylated and aminated by tetraethyl orthosilicate and (3-aminopropyl)triethoxysilane, and target enrofloxacin was coupled onto the surface by the carbodiimide method. The magnetic nanoparticles were characterized by Fourier transform infrared spectroscopy, particle size distribution, zeta potential, transmission electron microscopy observation, and indirect enzyme-linked immunosorbent assay (ELISA). A biopanning strategy based on Fe3O4@ENR FMNPs was then established to solve the problem in the traditional solid-phase biopanning process. The results showed that a considerable number of enrofloxacin (ENR)-positive phages were screened by only one round of biopanning. Finally, two ENR-specific shark-derived single-domain genes were identified and validated by monoclonal phage ELISA, gene sequencing, and biolayer interferometry technology. Our study provides a new biopanning strategy based on Fe3O4@ENR FMNPs for efficiently providing phages specific to haptens.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

An "on-off-on" electrochemiluminescence aptasensor based on a self-enhanced luminophore for ochratoxin A detection

A highly selective and sensitive "on-off-on" electrochemiluminescence (ECL) aptasensor based on a self-enhanced luminophore was developed for the detection of ochratoxin A (OTA). Specifically, polyethyleneimine functionalized multi-walled carbon nanotubes decorated with gold nanoparticles (AuNPs-PEI-MWCNTs) were used as the electrode matrix to accelerate electron transfer and provide a favorable microenvironment for self-enhanced luminophore loading and ECL signal enhancement. In addition, black phosphorus quantum dots (BPQDs) were used as co-reactants of the ECL reagent tris (2,2'-bipyridyl) ruthenium(II) (Ru(bpy)32+) in ECL experiments, and the reaction mechanism was investigated. The self-enhanced luminophore Ru@SiO2-BPQDs was obtained by encapsulating Ru(bpy)32+ in silica (SiO2) nanoparticles and then combining it with BPQDs through electrostatic interaction. In conventional ECL systems, the emitter and its co-reactants reacted via the inter-nanoparticle pathway, leading to long distance electron transfer. However, the electron transfer distance in the self-enhanced luminophore was significantly shortened due to the intra-nanoparticle electron transfer pathway because BPQDs and oxidized Ru(bpy)32+ were bound within one nanoparticle, thereby improving ECL efficiency to achieve the first "switch-on" state. Then, the luminophore was quenched using ferrocenes (Fc) modified on an aptamer to achieve the "switch-off" state. Finally, OTA was specifically identified by the adapter, causing Fc to be released from the sensor interface, restoring the ECL intensity to achieve the second "switch-on" state. Under optimal conditions, the aptasensor exhibited good sensitivity, stability, and reproducibility, with a linear detection range from 0.1 to 320 ng/mL and a detection limit of 0.03 ng/mL. The novel ECL aptasensor provided a common analytical tool for the detection of mycotoxins and other small molecules.

Characterization, specific recognition, and the performance in fish matrix of a shark-derived single-domain antibody against enrofloxacin

The third generation of genetic engineering antibodies, single-domain antibodies, have been widely reported as potential biomaterials in recognizing small molecular hazards. In this study, a shark-derived single-domain antibody was used as the recognition element for the first time to detect enrofloxacin (ENR), one of the most representative hazards in aquaculture. An ENR-specific clone named 2E6 was isolated by phage display technology. Experimental results proved that 2E6 ssdAb showed high affinity to ENR-PEI complete antigen, with the highest OD₄₅₀ value of 1.348 in binding ELISA. Through icELISA, it was determined that the IC₅₀ of 2E6 ssdAb to ENR was 19.230 ng/mL, while the IC₁₀ was 0.975 ng/mL, with rare recognition to other fluoroquinolones, which showed high sensitivity and specificity to ENR. The 2E6 ssdAb also performed excellently in fish matrix immunoassay. Results showed that the ENR-negative fish matrix did not seriously interfere with the recognition of 2E6 ssdAb to ENR-OVA, with the matrix index between 4.85% and 11.75%, while the results of icELISA in ENR-spiked fish matrix showed that 2E6 ssdAb could recognize the target ENR in different ENR-spiked concentrations of the fish matrix (10-1000 ng/mL), with the recovery between 89.30% and 126.38% and the RSD between 1.95% and 9.83%. This study broadens the application scenario of shark-derived single-domain antibodies as small molecule recognition biomaterials, providing a new recognition element on ENR detection for immunoassay.

AuNPs and 2D functional nanomaterial-assisted SPR development for the cancer detection: a critical review

Cancer ranks as a leading cause of death and a huge obstacle to rising life expectancy. If cancers are spotted early there's a high chance of survival. The conventional methods relying on the phenotypic features of the tumor are not powerful to the early screening of cancer. Cancer biomarkers are capable of indicating specific cancer states. Current biochemical assay suffers from time and reagents consuming and discontinuous monitoring. Surface plasmon resonance (SPR) technology, a refractive index-based optical biosensor, has significant promise in biomarker detection because of its outstanding features of label-free, sensitivity, and reliability. The nanomaterial features exotic physical and chemical property work on the process of transferring biorecognition event into SPR signal and hence is functioned as signal enhancer. In this review, we mainly discussed the mechanism of gold nanoparticles (AuNPs) and two-dimensional (2D) functional nanomaterial for improving the SPR signal. We also introduced AuNPs and 2D nanomaterial assisted SPR technology in determining cancer biomarker. Last but not least, we discussed the challenges and outlooks of the aforementioned reformative SPR technology for cancer biomarker determination in the clinical trial.

Nanobody-based fluorescent immunoassay using carbon dots anchored cobalt oxyhydroxide composite for the sensitive detection of fenitrothion

Fenitrothion (FN) residue in food is a serious threat to public health. Consequently, a sensitive, cost-effective, and convenient immunoassay for FN urgently needs to be fabricated to safeguard human health. Herein, a nanobody-alkaline phosphatase fusion protein (Nb-ALP)-based fluorescent ELISA using red emissive carbon dots (r-CDs) anchored cobalt oxyhydroxide nanosheet (CoOOH NS) composite was developed for detecting FN. Briefly, a Nb-ALP was obtained by autoinduction expression and employed as a recognition, signal transduction, and amplification element. As the fluorescence signal source, r-CDs were assembled with CoOOH NS to yield the r-CDs@CoOOH NS composite, leading to the fluorescence quenching of r-CDs via Förster resonance energy transfer (FRET). After competitive immunoreaction, the Nb-ALP bounded to the immobilized antigen can mediate the production of ascorbic acid, which can reduce the CoOOH NS to Co²⁺, breaking the FRET between r-CDs and CoOOH NS, accompanied by the fluorescence recovery of r-CDs. This fluorescent ELISA is highly sensitive to FN with a detection limit of 0.14 ng mL^-1, which is 25-fold lower than that of conventional colorimetric ELISAs. The recovery test of food samples and the validation by GC-MS/MS further demonstrated the proposed assay was an ideal tool for detecting FN.

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.

Disposable Electrochemical Aptasensor Based on Graphene Oxide-DNA Complex as Signal Amplifier towards Ultrasensitive Detection of Ochratoxin A

Signal amplification is crucial in developing a reliable disposable screen-printed carbon electrodes (SPCEs)-based biosensor for analyte detection with a narrow detection window. This work demonstrated a novel label-free electrochemical aptasensor based on SPCEs for the ultrasensitive detection of ochratoxin A (OTA). The graphene oxide-DNA (GO-DNA) complex as a signal amplifier with easy preparation was investigated for the first time. The proposed aptasensor based on the SPCEs/GO/cDNA-aptamer/3D-rGO-AuNPs structure was formed through the hybridization of aptamer-linked 3D-rGO/AuNPs and its complementary DNA-linked GO (GO-cDNA). The presence of OTA was discerned by its specific aptamer forming a curled OTA-aptamer complex and releasing the GO-cDNA from the surface of SPCEs. The resulting OTA-aptamer complex hindered interfacial electron transfer on the sensing surface, leading to the decreased peak current. The GO-cDNA further amplified the peak current change. This electrochemical aptasensor showed a low limit of detection of 5 fg/mL as well as good reproducibility with the relative standard deviation (RSD) of 4.38%. Moreover, the detection result of OTA in the rice and oat samples was comparable with that of the enzyme-linked immunosorbent assay (ELISA) kit. In general, the OTA aptasensor used in this work with convenient preparation, low-cost, good selectivity, high sensitivity and acceptable reproducibility can be proposed as a reliable point-of-care (POC) technique for OTA determination.

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.

Developing of specific monoclonal recombinant antibody fused to alkaline phosphatase (AP) for one-step detection of fig mosaic virus

Present study was performed to develop a fusion recombinant monoclonal antibody for one-step and accurate detection of FMV with a specific single-chain variable fragment (scFv) fused to alkaline phosphatase (AP) named as scFv(FMV-NP)-AP. The gene encoding-specific scFv recombinant antibody binding to nucleocapsid protein of Fig Mosaic Virus (FMV-NP) was fused to upstream of AP gene and integrated in pET26b bacterial expression vector. As vector contain pelB signal peptide, the expressed protein is secreted into periplasmic compartment. Recombinant fusion protein was produced in transformed E. coli following induction by IPTG. Extraction and purification of fusion protein was performed under denatured condition. The results of SDS-PAGE and western blot analysis indicated high integrity and purity with a single band protein with expected size of 72 kDa. The total yield of purified scFv(FMV-NP)-AP fusion protein estimated around 0.5-1 mg/l cultured medium. Subsequent colorimetric analysis confirmed presence of alkaline phosphatase activity in prepared scFv-AP fusion protein. Specificity of generated recombinant fusion antibody against cognate antigen and the native virus presented in infected plant extracts was assessed by ELISA, western blot and dot blot assays. Results revealed that scFv(FMV-NP)-AP is able to detect the presence of FMV in infected fig plants. The novel approach, implementing specific recombinant fusion antibody developed in this research, leads to one-step detection of FMV in plants by avoiding the use of chemical enzyme-labeled secondary antibodies.

Generation of a nanobody-alkaline phosphatase heptamer fusion for ratiometric fluorescence immunodetection of trace alpha fetoprotein in serum

Alpha fetoprotein (AFP) is an important biomarker for diagnosis of hepatocellular carcinoma (HCC). Whereas, it is always a challenge to detect trace AFP in serum. In this work, a ratiometric fluorescence enzyme immunoassay (RFEIA) was developed using nanobody-alkaline phosphatase (Nb-AP) heptamer and MnFe layered double hydroxides nanoflakes (MnFe LDH) for ultrasensitive detection of AFP. The Nb-AP heptamer (Nb-C4bpα-AP) was constructed by fusion expression of Nb, AP, and α-chain of C4 binding protein (C4bpα), where the C4bpα contributed to multimerization through self-assembly. The dual functional Nb-C4bpα-AP can recognize AFP, dephosphorylate ascorbic acid-2-phosphate (AAP) into ascorbic acid (AA), and thus tune the MnFe LDH-mediated ratiometric fluorescence, which was generated from the oxidization of MnFe LDH on o-phenylenediamine (OPD) and the catalyzation of MnFe LDH on the cyclization reaction between AA and OPD. By integration of Nb-C4bpα-AP, MnFe LDH, AAP, and OPD, the RFEIA showed a limit of detection of 0.013 ng/mL with good selectivity, accuracy and precision. Furthermore, results of clinical serum samples tested by the RFEIA were well confirmed by the automated chemiluminescence immunoassay analyzer. Thus, this work demonstrated that the Nb-C4bpα-AP is a robust immunoreagent and the developed RFEIA could be a very promising tool for diagnosis of HCC.

Fluonanobody-based nanosensor via fluorescence resonance energy transfer for ultrasensitive detection of ochratoxin A

Ochratoxin A (OTA) contamination in food is a serious threat to public health. There is an urgent need for development of rapid and sensitive methods for OTA detection, to minimize consumer exposure to OTA. In this study, we constructed two OTA-specific fluonanobodies (FluoNbs), with a nanobody fused at the carboxyl-terminal (SGFP-Nb) or the amino-terminal (Nb-SGFP) of superfolder green fluorescence protein. SGFP-Nb, which displayed better fluorescence performance, was selected as the tracer for OTA, to develop a FluoNb-based nanosensor (FN-Nanosens) via the fluorescence resonance energy transfer, where the SGFP-Nb served as the donor and the chemical conjugates of OTA-quantum dots served as the acceptor. After optimization, FN-Nanosens showed a limit of detection of 5 pg/mL, with a linear detection range of 5-5000 pg/mL. FN-Nanosens was found to be highly selective for OTA and showed good accuracy and repeatability in recovery experiments using cereals with various complex matrix environments. Moreover, the contents of OTA in real samples measured using FN-Nanosens correlated well with those from the liquid chromatography with tandem mass spectrometry. Therefore, this work illustrated that the FluoNb is an ideal immunosensing tool and that FN-Nanosens is reliable for rapid detection of OTA in cereals with ultrahigh sensitivity.

Recombinant antibodies and their use for food immunoanalysis

Antibodies are widely employed as biorecognition elements for the detection of a plethora of compounds including food and environmental contaminants, biomarkers, or illicit drugs. They are also applied in therapeutics for the treatment of several disorders. Recent recommendations from the EU on animal protection and the replacement of animal-derived antibodies by non-animal-derived ones have raised a great controversy in the scientific community. The application of recombinant antibodies is expected to achieve a high growth rate in the years to come thanks to their versatility and beneficial characteristics in comparison to monoclonal and polyclonal antibodies, such as stability in harsh conditions, small size, relatively low production costs, and batch-to-batch reproducibility. This review describes the characteristics, advantages, and disadvantages of recombinant antibodies including antigen-binding fragments (Fab), single-chain fragment variable (scFv), and single-domain antibodies (VHH) and their application in food analysis with especial emphasis on the analysis of biotoxins, antibiotics, pesticides, and foodborne pathogens. Although the wide application of recombinant antibodies has been hampered by a number of challenges, this review demonstrates their potential for the sensitive, selective, and rapid detection of food contaminants.

Mimotopes for Mycotoxins Diagnosis Based on Random Peptides or Recombinant Antibodies from Phage Library

Mycotoxins, the small size secondary metabolites of fungi, have posed a threat to the safety of medicine, food and public health. Therefore, it is essential to create sensitive and effective determination of mycotoxins. Based on the special affinity between antibody and antigen, immunoassay has been proved to be a powerful technology for the detection of small analytes. However, the tedious preparation and instability of conventional antibodies restrict its application on easy and fast mycotoxins detection. By virtue of simplicity, ease of use, and lower cost, phage display library provides novel choices for antibodies or hapten conjugates, and lead random peptide or recombinant antibody to becoming the promising and environmental friendly immune-reagents in the next generation of immunoassays. This review briefly describes the latest developments on mycotoxins detection using M13 phage display, mainly focusing on the recent applications of phage display technology employed in mycotoxins detection, including the introduction of phage and phage display, the types of phage displayed peptide/recombinant antibody library, random peptides/recombinant antibodies-based immunoassays, as well as simultaneous determination of multiple mycotoxins.

Generation of a nanobody-alkaline phosphatase fusion and its application in an enzyme cascade-amplified immunoassay for colorimetric detection of alpha fetoprotein in human serum

Sensitive detection of liver disease biomarkers can facilitate the diagnosis of primary hepatoma and other benign liver diseases, and the alpha fetoprotein (AFP) was selected as the model macromolecule in this work. Herein an enzyme cascade-amplified immunoassay (ECAIA) based on the nanobody-alkaline phosphatase fusion (Nb-ALP) and MnO₂ nanoflakes was developed for detecting AFP. The bifunctional biological macromolecule Nb-ALP serves as the detection antibody and the reporter molecule. The MnO₂ nanoflakes mimic the oxidase for catalyzing the 3,3',5,5'-tetramethylbenzidine (TMB) into the blue oxidized TMB, which has a quantitative signal at the wavelength of 650 nm. Moreover, the Nb-ALP could dephosphorylate the ascorbic acid-2-phosphate (AAP) to form the ascorbic acid (AA) that can disintegrate the nanoflakes to reduce their oxidation capacity with the content decrease of the oxidized TMB. Using the constructed TMB-MnO₂ colorimetric sensing system for Nb-ALP and the optimized experimental parameters, the ECAIA has a limit of detection (LOD) of 0.148 ng/mL which is 18.7-fold lower than that of the p-nitrophenylphosphate (pNPP)-based method (LOD = 2.776 ng/mL). The ECAIA showed good selectivity for AFP with observed negligible cross-reactions with several common cancer biomarkers. The recovery rate for AFP spiked in human serum ranged from 94.8% to 113% with the relative standard deviation from 0.3% to 6.5%. For analysis of the actual human serum samples, a good linear correlation was found between the results tested by the ECAIA and the automatic chemiluminescence analyzer. Thus, the ECAIA was demonstrated to be a promising tool for highly sensitive and selective detection of AFP, providing a reference for analysis of other macromolecule biomarkers.

Development of a biotinylated nanobody for sensitive detection of aflatoxin B1 in cereal via ELISA

Aflatoxin B₁ (AFB₁) contamination is a severe threat to food safety and human health, and requires continuous monitoring. In this study, we developed a biotin-streptavidin-amplified enzyme-linked immunosorbent assay (BA-ELISA) by using biotinylated nanobody Nb26 and streptavidin-conjugated polymerized horseradish peroxide (SA-PolyHRP) for sensitive and rapid detection of AFB₁ in cereal. Under the optimal condition, the IC₅₀ value of the BA-ELISA was improved to 0.21 ng mL^-1 for AFB₁, satisfying the requirement of detection limit in practical applications. The total assay time of our strategy is reduced to 50 min from 2 h in conventional competitive ELISA. Additionally, the BA-ELISA saves as much as 98% of the antibody in comparison to the previous classic ELISA. Our work also demonstrated an interesting phenomenon that the biotinylated Nb26 achieved better selectivity to AFB₁, which could possibly result from the steric hindrance that interferes reaction between the Nb26 and the AFB₁ analogs. Furthermore, the assay was used to detect AFB₁ in two cereal samples, and the results were in good agreement with that obtained by high performance liquid chromatography. The developed BA-ELISA can be used for routine screening analysis of AFB₁, and offers a promising strategy for measuring low concentrations of food contaminants.

Development of Nanobodies against Mal de Río Cuarto virus major viroplasm protein P9-1 for diagnostic sandwich ELISA and immunodetection

Mal de Río Cuarto virus (MRCV) is a member of the genus Fijivirus of the family Reoviridae that causes a devastating disease in maize and is persistently and propagatively transmitted by planthopper vectors. Virus replication and assembly occur within viroplasms formed by viral and host proteins. This work describes the isolation and characterization of llama-derived Nanobodies (Nbs) recognizing the major viral viroplasm component, P9-1. Specific Nbs were selected against recombinant P9-1, with affinities in the nanomolar range as measured by surface plasmon resonance. Three selected Nbs were fused to alkaline phosphatase and eGFP to develop a sandwich ELISA test which showed a high diagnostic sensitivity (99.12%, 95% CI 95.21-99.98) and specificity (100%, 95% CI 96.31-100) and a detection limit of 0.236 ng/ml. Interestingly, these Nanobodies recognized different P9-1 conformations and were successfully employed to detect P9-1 in pull-down assays of infected maize extracts. Finally, we demonstrated that fusions of the Nbs to eGFP and RFP allowed the immunodetection of virus present in phloem cells of leaf thin sections. The Nbs developed in this work will aid the study of MRCV epidemiology, assist maize breeding programs, and be valuable tools to boost fundamental research on viroplasm structure and maturation.

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.

Carcinogenesis: Failure of resolution of inflammation?

Inflammation in the tumor microenvironment is a hallmark of cancer and is recognized as a key characteristic of carcinogens. However, the failure of resolution of inflammation in cancer is only recently being understood. Products of arachidonic acid and related fatty acid metabolism called eicosanoids, including prostaglandins, leukotrienes, lipoxins, and epoxyeicosanoids, critically regulate inflammation, as well as its resolution. The resolution of inflammation is now appreciated to be an active biochemical process regulated by endogenous specialized pro-resolving lipid autacoid mediators which combat infections and stimulate tissue repair/regeneration. Environmental and chemical human carcinogens, including aflatoxins, asbestos, nitrosamines, alcohol, and tobacco, induce tumor-promoting inflammation and can disrupt the resolution of inflammation contributing to a devastating global cancer burden. While mechanisms of carcinogenesis have focused on genotoxic activity to induce mutations, nongenotoxic mechanisms such as inflammation and oxidative stress promote genotoxicity, proliferation, and mutations. Moreover, carcinogens initiate oxidative stress to synergize with inflammation and DNA damage to fuel a vicious feedback loop of cell death, tissue damage, and carcinogenesis. In contrast, stimulation of resolution of inflammation may prevent carcinogenesis by clearance of cellular debris via macrophage phagocytosis and inhibition of an eicosanoid/cytokine storm of pro-inflammatory mediators. Controlling the host inflammatory response and its resolution in carcinogen-induced cancers will be critical to reducing carcinogen-induced morbidity and mortality. Here we review the recent evidence that stimulation of resolution of inflammation, including pro-resolution lipid mediators and soluble epoxide hydrolase inhibitors, may be a new chemopreventive approach to prevent carcinogen-induced cancer that should be evaluated in humans.

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.

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.

Gold-silver alloy film based surface plasmon resonance sensor for biomarker detection

In this study, we developed a gold‑silver alloy film based surface plasmon resonance (AuAg-SPR) sensor with wavelength interrogation to detect cancer antigen 125 (CA125) using a sandwich immunoassay. We first theoretically simulated the sensitivity of conventional gold film based SPR (Au-SPR) sensor and AuAg-SPR sensor, and conducted a series of experiments to investigate the sensitive characteristics of AuAg-SPR sensor, including the angle and refractive index (RI) sensitivity. We then conducted CA125 detection experiments on these two types of sensors. The results demonstrated that the limit of detection (LOD) of CA125 on the AuAg-SPR sensor was 0.1 U/mL (0.8 ng/mL) based on its direct reaction with an immobilised antibody, which was two orders of magnitude lower than that of the Au-SPR sensor (10 U/mL). The total changes in the resonance wavelength (∆λ_R) of the former were 1.7-fold those of the latter. The volume fractions of the adsorbates (f_ad) and effective RIs (n_adlayer) in each adlayer were then calculated and the effect of the antibody size on the detection results was analysed. The AuAg-SPR sensors had a higher sensitivity than the conventional Au-SPR sensors for detecting CA125 due to their electric field characteristics. Therefore, these will have better application prospects.

Development of Improved Double-Nanobody Sandwich ELISAs for Human Soluble Epoxide Hydrolase Detection in Peripheral Blood Mononuclear Cells of Diabetic Patients and the Prefrontal Cortex of Multiple Sclerosis Patients

Nanobodies have been progressively replacing traditional antibodies in various immunological methods. However, the use of nanobodies as capture antibodies is greatly hampered by their poor performance after passive adsorption to polystyrene microplates, and this restricts the full use of double nanobodies in sandwich enzyme-linked immunosorbent assays (ELISAs). Herein, using the human soluble epoxide hydrolase (sEH) as a model analyte, we found that both the immobilization format and the blocking agent have a significant influence on the performance of capture nanobodies immobilized on polystyrene and the subsequent development of double-nanobody sandwich ELISAs. We first conducted epitope mapping for pairing nanobodies and then prepared a horseradish-peroxidase-labeled nanobody using a mild conjugation procedure as a detection antibody throughout the work. The resulting sandwich ELISA using a capture nanobody (A9, 1.25 μg/mL) after passive adsorption and bovine serum albumin (BSA) as a blocking agent generated a moderate sensitivity of 0.0164 OD·mL/ng and a limit of detection (LOD) of 0.74 ng/mL. However, the introduction of streptavidin as a linker to the capture nanobody at the same working concentration demonstrated a dramatic 16-fold increase in sensitivity (0.262 OD·mL/ng) and a 25-fold decrease in the LOD for sEH (0.03 ng/mL). The streptavidin-bridged double-nanobody ELISA was then successfully applied to tests for recovery, cross-reactivity, and real samples. Meanwhile, we accidentally found that blocking with skim milk could severely damage the performance of the capture nanobody by an order of magnitude compared with BSA. This work provides guidelines to retain the high effectiveness of the capture nanobody and thus to further develop the double-nanobody ELISA for various analytes.

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.

A colorimetric biosensor for simultaneous ochratoxin A and aflatoxins B1 detection in agricultural products

Colorimetric biosensors have been widely applied to mycotoxins testing. However, the colorimetric assay previously reported used a single color to detect one mycotoxin, and there were few reports on the simultaneous detection of multiple mycotoxins. In this work, a colorimetric biosensor for dual mycotoxins detection was developed. A Fe₃O₄/GO based platform for aflatoxins B1 (AFB1) detection and a Fe₃O₄@Au based platform for ochratoxin A (OTA) detection were fabricated. The quantification of OTA and AFB1 was respectively achieved by the release of thymolphthalein under alkaline conditions and 3,3',5,5'-tetramethylbenzidine was catalyzed by Au NPs under acidic conditions. Because of different conditions, two sensing methods didn't interfere with each other but could provide a higher detection efficiency. The detection range of AFB1 is 5-250 ng·ml^-1 and that of OTA is 0.5-80 ng·ml^-1. This biosensor has been successfully applied in real sample detection, which has a broad application prospect in fields of food safety.

Development of a highly thermostable immunoassay based on a nanobody-alkaline phosphatase fusion protein for carcinoembryonic antigen detection

Carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM-5) assays are employed in routine clinical settings to diagnose tumor. We selected two nanobodies with high-affinity to CEACAM-5, termed Nb11C12 and Nb2D5, using phage-display technology. The Nb2D5 fused with calf intestinal alkaline phosphatase (CAP), human placental alkaline phosphatase (HAP), or Pyrococcus abyssi alkaline phosphatase (PAP) were expressed in human embryonic kidney (HEK293) cells. The enzymatic activity of Nb2D5-HAP fusion protein was the best and remained stable at 60 °C for 7 days. The affinity of Nb2D5-HAP fusion protein to CEACAM-5 reached 42 pM. A chemiluminescent enzyme immunoassay (CLEIA) based on Nb2D5-HAP fusion protein was established for quantitative CEACAM-5 assay in clinical settings. The CLEIA exhibited a wide linear range of 0.31-640 ng/mL toward CEACAM-5, with a limit of detection (LOD) of 0.85 ng/mL. No cross-reactivity occurred with CEACAM-1, CEACAM-3, CEACAM-6, or CEACAM-8, and no interference was observed with rheumatoid factors. The CLEIA based on Nb2D5-HAP fusion protein was stable for 8 weeks at 37 °C and 50% relative humidity. The CLEIA developed from Nb2D5-HAP fusion protein had much better stability and linearity with similar reproducibility compared with the enzyme-linked immunosorbent assay developed from conventional monoclonal antibodies, which have been widely used in clinics over the past several decades. Graphical abstract.

Nanobody-alkaline phosphatase fusion-mediated phosphate-triggered fluorescence immunoassay for ochratoxin a detection

Ochratoxin A (OTA) is a kind of mycotoxin that seriously harms the health of humans and animals. In this study, a nanobody-alkaline phosphatase fusion-mediated phosphate-triggered fluorescence immunoassay (Nb-AP-mediated PT-FIA) was developed for detecting OTA. Based on the constructed phosphate-triggered fluorescence sensing system for Nb-AP and the optimal working conditions, the Nb-AP-mediated PT-FIA has a half maximal inhibition concentration (IC₅₀) of 0.46 ng/mL, a limit of detection (IC₁₀) of 0.12 ng/mL, and a linear range (IC_20-80) of 0.2-1.26 ng/mL, respectively. The recovery experiment indicated acceptable accuracy and precision of the Nb-AP-mediated PT-FIA, and the results were validated by high performance liquid chromatography with fluorescence detector. Thus this proposed method is applicable to sensitive, rapid, and low-cost detection of OTA and other toxic analytes with low molecular weight in food and environment.

A rapid and simple fluorescence enzyme-linked immunosorbent assay for tetrabromobisphenol A in soil samples based on a bifunctional fusion protein

Tetrabromobisphenol A (TBBPA) is the largest brominated flame retardant which can be released to environment and cause long-term hazard. In this work, we developed a rapid and highly sensitive fluorescence enzyme-linked immunosorbent assay (FELISA) for monitoring of TBBPA in soil samples. TBBPA specific nanobody derived from camelid was fused with alkaline phosphatase to obtain the bi-functional fusion protein, which enable the specific binding of TBBPA and the generation of detection signal simultaneously. The assay showed an IC₅₀ of 0.23 ng g^-1, limit detection of 0.05 ng g^-1 and linear range from 0.1 to 0.55 ng g^-1 for TBBPA in soil samples. Due to the high resistance to organic solvents of the fusion protein, a simple pre-treatment by using 40% dimethyl sulfoxide (DMSO) as extract solvent can eliminate matrix effect and obtain good recoveries (ranging from 93.4% to 112.4%) for spiked soil samples. Good relationship between the results of the proposed FELISA and that of liquid chromatography tandem mass spectrometry (LC-MS/MS) was obtained, which indicated it could be a powerful analytical tool for determination of TBBPA to monitor human and environmental exposure.

Luminescent detection of Ochratoxin A using terbium chelated mesoporous silica nanoparticles

This work represents the time-resolved fluorescence detection of Ochratoxin A (OTA), a highly toxic and commonly found toxin in food stuffs, by a terbium (Tb³⁺) chelated nanoparticle sensor with high sensitivity and remarkable selectivity. The coordination of OTA to Tb³⁺ center on nanoparticle surface resulted in the significant enhancement of the fluorescence signal in nanomolar concentrations with a detection limit of 20 ppb. In contrast, no enhancements were observed in the presence of other common mycotoxins such as Aflatoxin B1, Zearalenone, Citrinin and Patulin. The results indicate that the Tb³⁺ chelated nanoparticle sensor has great potential for applications in food analysis and safety.

Investigation of the Small Size of Nanobodies for a Sensitive Fluorescence Polarization Immunoassay for Small Molecules: 3-Phenoxybenzoic Acid, an Exposure Biomarker of Pyrethroid Insecticides as a Model

Limited reports on the use of nanobodies (Nbs) in fluorescence polarization immunoassay (FPIA) aroused us to explore if the small size of Nbs is a drawback for the development of sensitive FPIA to small molecular compounds, particularly since FPIA is a technology strongly dependent on molecular weight. In the present work, three different molecular weight Nbs against 3-phenoxybenzoic acid (3-PBA), an exposure biomarker of pyrethroid insecticides, including bare Nbs (15 kDa), Nbs-Avidin (Nbs-AV, 60 kDa), and Nbs-Alkaline phosphatase (Nbs-AP, 130 kDa) were specifically generated to cover distinct regions on the polarization and molecular weight relationship curve for a fluorescein tracer. In competitive FPIA, similar half-maximal inhibitory concentrations (IC₅₀) of 3-PBA of 16.4, 12.2, and 14.8 ng mL^-1 were obtained for Nbs, Nbs-AV, and Nbs-AP, respectively, indicating that the size of Nbs in the range tested had no significant effect on the sensitivity of the resulting competitive FPIA. An IC₅₀ of 20.2 ng mL^-1 for an anti-3-PBA polyconal antibody based FPIA further demonstrated the performance of Nbs, which was comparable to that of traditional antibodies in FPIA. Spike-recovery studies showed good and reproducible recovery of 3-PBA in urine samples, demonstrating the applicability of Nb-based FPIA. Overall, our results show that Nb-based FPIA achieves sensitivity levels of FPIA based on conventional antibodies and further indicate that Nb absolutely meets the sensitivity requirement of FPIA.

Nanobody-based fluorescence resonance energy transfer immunoassay for noncompetitive and simultaneous detection of ochratoxin a and ochratoxin B

A noncompetitive and homogeneous fluorescence resonance energy transfer (FRET) immunoassay was developed using a nanobody (Nb) for highly sensitive and simultaneous detection of ochratoxin A (OTA) and ochratoxin B (OTB). The promoted intrinsic fluorescence (λ_ex: 280 nm) of tryptophan residues (donor) in Nb can excite the fluorescence of OTA and OTB (acceptor) for detection (λ_em: 430 nm). Using optimal conditions, the limits of detection of the Nb-based FRET immunoassay were 0.06 and 0.12 ng/mL for OTA and OTB, respectively. Minimal cross reactivity was detected for several analogues of OTA and OTB as well as nonspecific proteins and antibodies. Acceptable accuracy and precision were obtained in the spike and recovery study, and the results correlated well with those by HPLC. These results demonstrated that the developed method could be a useful tool for noncompetitive, homogeneous, and simultaneous detection of OTA and OTB as well as other environmental analytes with similar fluorescence properties.

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

Nanoluciferase (Nluc), the smallest luciferase known, was used as the fusion partner with a nanobody against aflatoxin B₁ to develop a bioluminescent enzyme immunoassay (BLEIA) for detection of the aflatoxin B₁ in cereal. Nanobody (clone G8) against aflatoxin B₁ was fused with nanoluciferase and cloned into a pET22b expression vector, and then transformed into Escherichia coli. The nanobody fusion gene contained a hexahistidine tag for purification by immobilized metal affinity chromatography, yielding a biologically active fusion protein. The fusion protein G8-Nluc retained binding properties of the original nanobody. Concentration of the coelenterazine substrate and buffer composition were also optimized to provide high intensity and long half-life of the luminescent signal. The G8-Nluc was used as a detection antibody to establish a competitive bioluminescent ELISA for the detection of aflatoxin B₁ in cereals successfully. Compared to classical ELISA, this novel assay showed more than 20-fold improvement in detection sensitivity, with an IC₅₀ value of 0.41 ng/mL and linear range from 0.10 to 1.64 ng/mL. In addition, the entire BLEIA detection procedure can be completed in one step within 2 h, from sample preparation to data analysis. These results suggest that nanobody fragments fused with nanoluciferase might serve as useful and highly sensitive dual functional reagents for the development of rapid and highly sensitive immunoanalytical methods.

Development of a biotin-streptavidin-amplified nanobody-based ELISA for ochratoxin A in cereal

A biotin-streptavidin-amplified enzyme-linked immunosorbent assay using a biotinylated nanobody (BA-Nb ELISA) was developed to detect ochratoxin A (OTA) in cereal. The limit of detection (LOD) of the BA-Nb ELISA, which equals to 10% maximal inhibitory concentration, was 0.011 ng/mL for OTA in buffer, and the sensitivity was approximately improved by one order of magnitude compared with the traditional Nb ELISA (LOD = 0.112 ng/mL). Under optimal conditions, the developed assay could be accomplished in 40 min with maximal inhibitory concentration of 0.138 ng/mL and the linear detection range of 0.034-0.460 ng/mL. The average recovery rate of the BA-Nb ELISA ranged from 92.8% to 114%, and the relative standard deviation was in the range of 2.04-9.85%. The developed BA-Nb ELISA was validated by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and the results indicated the reliability of BA-Nb ELISA for the detection of OTA in cereal.

Ligase amplification reaction-catalyzed assembly of a single quantum dot-based nanosensor for sensitive detection of alkaline phosphatase

We demonstrate the ligase amplification reaction-catalyzed assembly of a single quantum dot-based nanosensor for sensitive detection of alkaline phosphatase.

Nanobody-Alkaline Phosphatase Fusion Protein-Based Enzyme-Linked Immunosorbent Assay for One-Step Detection of Ochratoxin A in Rice

Ochratoxin A (OTA) has become one a focus of public concern because of its multiple toxic effects and widespread contamination. To monitor OTA in rice, a sensitive, selective, and one-step enzyme-linked immunosorbent assay (ELISA) using a nanobody-alkaline phosphatase fusion protein (Nb28-AP) was developed. The Nb28-AP was produced by auto-induction expression and retained an intact antigen-binding capacity and enzymatic activity. It exhibited high thermal stability and organic solvent tolerance. Under the optimal conditions, the developed assay for OTA could be finished in 20 min with a half maximal inhibitory concentration of 0.57 ng mL^-1 and a limit of detection of 0.059 ng mL^-1, which was 1.1 times and 2.7 times lower than that of the unfused Nb28-based ELISA. The Nb28-AP exhibited a low cross-reactivity (CR) with ochratoxin B (0.92%) and ochratoxin C (6.2%), and an ignorable CR (<0.10%) with other mycotoxins. The developed Nb-AP-based one-step ELISA was validated and compared with a liquid chromatography-tandem mass spectrometry method. The results show the reliability of Nb-AP-based one-step ELISA for the detection of OTA in rice.

Development of a Highly Sensitive Direct Competitive Fluorescence Enzyme Immunoassay Based on a Nanobody-Alkaline Phosphatase Fusion Protein for Detection of 3-Phenoxybenzoic Acid in Urine

3-Phenoxybenzoic acid (3-PBA) is a human urinary metabolite of many pyrethroid insecticides and can be used as a biomarker to monitor human exposure to these pesticides. A rapid and sensitive direct competitive fluorescence enzyme immunoassay (dc-FEIA) for detecting 3-PBA on the basis of a nanobody (Nb)-alkaline phosphatase (AP) fusion protein was developed. The anti-3-PBA Nb-AP fusion protein was expressed and purified. The 50% inhibitory concentration (IC₅₀) and linear range of dc-FEIA were 0.082 and 0.015-0.447 ng/mL, respectively, with a detection limit of 0.011 ng/mL. The IC₅₀ of dc-FEIA was improved by nearly ten times compared with those of one-step and three-step direct competitive enzyme-linked immunosorbent assay (dc-ELISA). Spiked urine samples were detected by both dc-FEIA and liquid chromatography-mass spectrometry (LC-MS), and the results showed good consistency between the two analysis methods, indicating the reliability of dc-FEIA based on the Nb-AP fusion protein for detecting 3-PBA in urine.

Genetic Fusion of an Anti-BclA Single-Domain Antibody with Beta Galactosidase

The Bacillus collagen-like protein of anthracis (BclA), found in Bacillus anthracis spores, is an attractive target for immunoassays. Previously, using phage display we had selected llama-derived single-domain antibodies that bound to B. anthracis spore proteins including BclA. Single-domain antibodies (sdAbs), the recombinantly expressed heavy domains from the unique heavy-chain-only antibodies found in camelids, provide stable and well-expressed binding elements with excellent affinity. In addition, sdAbs offer the important advantage that they can be tailored for specific applications through protein engineering. A fusion of a BclA targeting sdAb with the enzyme Beta galactosidase (β-gal) would enable highly sensitive immunoassays with no need for a secondary reagent. First, we evaluated five anti-BclA sdAbs, including four that had been previously identified but not characterized. Each was tested to determine its binding affinity, melting temperature, producibility, and ability to function as both capture and reporter in sandwich assays for BclA. The sdAb with the best combination of properties was constructed as a fusion with β-gal and shown to enable sensitive detection. This fusion has the potential to be incorporated into highly sensitive assays for the detection of anthrax spores.

Probing the Entropic Effect in Molecular Noncovalent Interactions between Resin-Bound Polybrominated Arenes and Small Substrates

The associative interaction between resin-bound polybrominated arenes and small molecules was analyzed by using various spectroscopic techniques as well as a synthetic molecular model to establish the thermodynamics. The binding in acetonitrile was three orders of magnitude stronger than that in methanol, partly owing to the tertiary conformational gating of the resin that controls the entropic terms. By using the entropic superiority, the associative binding of up to 3×10⁴  m^-1 is achieved with the non-biological system. A modified Hill plot for the quantitative analysis of bindings was also devised, which enabled the interactions at the molecular level to be elucidated.

Development of a Nanobody-AviTag Fusion Protein and Its Application in a Streptavidin-Biotin-Amplified Enzyme-Linked Immunosorbent Assay for Ochratoxin A in Cereal

Ochratoxin A (OTA) is a common food contaminant that threatens consumers' safety and health. A sensitive and selective biotin-streptavidin-amplified enzyme-linked immunosorbent assay (BA-ELISA) for OTA using a nanobody-AviTag fusion protein (Nb-AviTag) was developed in this study. The prokaryotic expression vector Nb28-AviTag-pAC6 for Nb-AviTag was constructed, followed by transformation to the AVB101 cells for antibody expression and in vivo biotinylation. The purified Nb28-AviTag was used to establish the BA-ELISA and the procedures for this Nb-AviTag-based BA-ELISA were optimized. The Nb-AviTag-based BA-ELISA exhibited the half maximal inhibitory concentration (IC₅₀) of 0.14 ng mL^-1 and the limit of detection (LOD = IC₁₀) of 0.028 ng mL^-1 for OTA basing on the optimized experiment parameters. The assay sensitivity was improved 4.6 times and 4.3 times compared to Nb-based ELISA, respectively. This method had LODs of 1.4 μg kg^-1 in barley, 0.56 μg kg^-1 in oats, and 0.84 μg kg^-1 in rice for OTA. The average recovery percent was in a range of 84-137%, and the relative standard derivation percent ranged from 0.64% to 7.8%. The content of OTA in contaminated cereal samples was determined by both the developed Nb-AviTag-based method and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results demonstrated that the Nb-AviTag was a robust and promising bioreceptor in highly sensitive detection of OTA and other low molecular weight compounds using BA system.