Target-initiated fluorescent aptasensor based on multisite strand displacement amplification for label-free detection of ochratoxin A

We developed a fluorescent aptasensor for label-free detection of ochratoxin A (OTA) based on TdT- and DNA polymerase-assisted multisite strand displacement amplification. This aptasensor exhibits good specificity and high sensitivity with a limit of detection (LOD) of 0.18 ng mL-1, and it can be further applied for the accurate quantification of OTA in complex real samples, holding promising applications in the field of food safety.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Fluorescence and absorbance dual-mode immunoassay for detecting Ochratoxin A

In this work, a simple dual-mode immunoassay for detecting Ochratoxin A (OTA) was developed by mixing G-quadruplex/N-methylmesoporphyrin IX (G4/NMM) and 3,3',5,5'-tetramethylbenzidine (TMB). The fluorescence of G4/NMM can be quenched by oxidized TMB (oxTMB) because the absorbance of oxTMB overlapped with the fluorescence emission of G4/NMM. In the absence of OTA, large amounts of oxTMB were formed with blue color and the fluorescence of G4/NMM was quenched. In the presence of OTA, the concentration of oxTMB was decreased, therefore the fluorescence of G4/NMM increased. The linear range of fluorescence immunoassay was 0.195-25 ng/mL, and the linear range of the absorbance immunoassay was 0.049-1.563 ng/mL. Thus, the linear range of this dual-mode immunoassay can be expanded to 0.049-25 ng/mL. Meanwhile, the new method showed good selectivity for OTA. Besides, the satisfactory recovery rates implied the new method had a potential value for practical sample detection.

Molybdenum blue mediated photothermal immunoassay for CEA detection based on Ag4P2O7@Ag nanocomposites

A photothermal immunoassay was built for tumor marker detection based on Ag₄P₂O₇@Ag nanocomposites. Ag₄P₂O₇@Ag nanomaterials were synthesized by precipitation-photoreduction reaction, and characterized by transmission electron microscope (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectra (XPS) and X-ray powder diffraction (XRD). Come about PO₄^3- derived from Ag₄P₂O₇@Ag under acidic conditions react with ammonium molybdate in the action of reductant generating molybdenum blue. The photothermal change is due to molybdenum blue solution depending on the concentration of carcinoembryonic antigen (CEA) in immunoassay. Under optimal conditions, there is a linear relation between ΔT and CEA concentration in the range of 1 pg mL^-1-40 ng mL^-1 with the detection limit of 0.33 pg mL^-1. Meanwhile, the developed photothermal immunoassay displays preferable selectivity, repeatability, and stability.

Establishment of a Chemiluminescence Immunoassay Combined with Immunomagnetic Beads for Rapid Analysis Of Ochratoxin A

BACKGROUND

Ochratoxins A (OTA), as the harmful mycotoxins, are toxic secondary metabolite produced by several species of Aspergillus and Penicillium. Hence, it is of vital important to establish a sensitive method to detect OTA in various grains.

OBJECTIVE

Herein a novel highly sensitive chemiluminescence immunoassay was developed to determination of ochratoxin A in wheat, corn, and poultry feed.

METHODS

The immunomagnetic beads were used as solid phase carrier and separator in the competitive chemiluminescence method, the monoclonal antibodies anti-OTA coated on immunomagnetic beads were applied as the capturing antibody, in which OTA would compete with Ochratoxin A-alkaline phosphatase (OTA-ALP) in binding with OTA antibodies.

RESULTS

The proposed method exhibited good linearity (R2 ≥ 0.999), adequate OTA recovery rate (83.60% to 102.50%), and good repeatability. The sensitivity of the proposed method was 2.05 pg/mL. In addition, the method was highly selective, it did not have cross-reaction with other mycotoxin (e.g., AFB1, DON, ZEN). Furthermore, the developed method was applied in the analysis of wheat, corn and two animal feeds successfully, the results obtained by the chemiluminescence method showed the high correlation with those obtained by HPLC method (correlation coefficient 0.9958) when testing each extract by both methods.

CONCLUSIONS

This strategy shows great potential application for other toxic and harmful mycotoxin detection.

HIGHLIGHTS

A novel highly rapid and sensitive chemiluminescence immunoassay was established to quantitation for ochratoxin A.

An immunoassay for ochratoxin A using tetramethylrhodamine-labeled ochratoxin A as a probe based on a binding-induced change in fluorescence intensity

In an immunoassay, ochratoxin A (OTA) competitively displaces the bound tetramethylrhodamine (TMR)-OTA fluorescent probe from the antibody, causing a decrease in fluorescence.

Development and Characterization of Monoclonal Antibodies for the Mycotoxin Citreoviridin

Citreoviridin (CTV) in an inhibitor of mitochondrial ATPase that has been isolated from molded yellow rice and linked to the human disease Shoshin-kakke (acute cardiac beriberi). The disease results from a deficiency of thiamine, however, purified CTV can reproduce the symptoms in experimental animals. The link between CTV and Shoshin-kakke has been difficult to resolve, in part because cases of the disease are rare. In addition to rice, CTV has been found in maize, pecan nuts, and wheat products. A method to screen for CTV and its geometric isomer, iso-CTV, in commodities was developed, based upon the isolation of two novel monoclonal antibodies (mAb). In an antigen-immobilized competitive enzyme-linked immunosorbent assay format (CI-ELISA), the observed IC₅₀s for CTV were 11 ng/mL and 18 ng/mL (mAbs 2-2 and 2-4, respectively). The assays were relatively tolerant to methanol and acetonitrile, which allowed their application to the detection of CTV in spiked polished white rice. For quantification, a standard mixture of CTV and iso-CTV was used, along with matrix matched calibration. The dynamic range of the ELISA using mAb 2-4 was equivalent to 0.23 to 2.22 mg/kg in rice. Recoveries over the range of 0.36 to 7.23 mg/kg averaged 97 ± 10%. The results suggest that the mAb 2-4-based immunoassay can be applied to the screening of white rice for CTV. Both mAbs were also observed to significantly enhance the fluorescence of the toxin.

DNA template-mediated click chemistry-based portable signal-on sensor for ochratoxin A detection

A novel signal-on portable sensing system has been developed for OTA detection using personal glucose meter (PGM) as signal transducer. In the study, we explore the potential of using a short dsDNA as template to trigger the "click" ligation of two DNA strands, further improve the stability of DNA strand on the magnetic beads (MBs) surface, and thereby reduce the background signal. Compared with no "click" ligation, the background signal decreases 7.5 times. Both the sensitivity and selectivity are greatly promoted. A high sensitivity with OTA detection down to 72 pg/mL is achieved, which is comparable with several existing detectors, such as fluorescence-based detectors and electrochemical detectors. The feasibility of the strategy in real samples is well verified and evaluated by detecting OTA in feed samples, indicating the potential application in the food safety field.

Development of an Anti-Idiotypic VHH Antibody and Toxin-Free Enzyme Immunoassay for Ochratoxin A in Cereals

Enzyme-linked immunosorbent assay (ELISA) test kits have been widely used for the determination of mycotoxins in agricultural products and foods, however, this test uses toxin standards with high toxicity and carcinogenicity that seriously threaten human health. In this work, the anti-idiotypic nanobody VHH 2-24 was first developed and then, using it as a surrogate standard, a toxin-free enzyme immunoassay for ochratoxin A (OTA) was established. The IC₅₀ value of the VHH 2-24 surrogate standard-based ELISA was 0.097 µg/mL, with a linear range of 0.027–0.653 µg/mL. The average recoveries were tested by spike-and-recovery experiments, and ranged from 81.8% to 105.0%. The accuracy of the developed ELISA for detecting OTA was further verified by using the high performance liquid chromatography (HPLC) method, and an excellent correlation was observed. In summary, the toxin-free ELISA established in this study proves the latent use of the anti-idiotypic VHH as a surrogate calibrator for other mycotoxins and highly toxic small molecule analysis to improve assay properties for highly sensitive analyte determination in agricultural products.

Label free aptasensor for ochratoxin A detection using polythiophene-3-carboxylic acid

This work demonstrates the development of electrochemical aptasensor using ochratoxin A (OTA) aptamers. Different aptamer coupling strategies were tested using polythiophene-carboxylic acid (PT3C) and polypyrrole-3-carboxylic acid (PP3C). The best sensitivity was recorded by polythiophene-3-carboxylic acid (PT3C) on screen-printed carbon electrode (SPCE) to attain the direct detection of OTA. The quantification of OTA was achieved by using electrochemical impedance spectroscopy. A good dynamic range 0.125-2.5 ng ml^-1 was obtained for OTA with limit of detection (LOD) 0.125 ng ml^-1 and Limit of quantification (LOQ) 0.3 ng ml^-1 respectively. The good reproducibility was recorded with RSD% of 3.68. The obtained straight line equation was y = 0.4061 × + 1.03, r = 0.99. For real sample applications, the developed aptasensors were demonstrated in coffee samples. The aptasensor displayed good recovery values in the range 88-89%, thus exhibited the effectiveness of proposed aptasensor for such complex matrices.

A fluorescence enhancement-based label-free homogeneous immunoassay of benzo[a]pyrene (BaP) in aqueous solutions

A fluorescence enhancement-based immunoassay has been developed for the detection of the polycyclic aromatic hydrocarbons (PAH), benzo[a]pyrene (BaP), in aqueous solutions. The results of this study show that BaP, which inefficiently fluoresces in aqueous solution, displays enhanced fluorescence when bound to the anti-BaP antibody (anti-BaP), as part of a label-free immunoassay system. Binding to anti-BaP results in a 3.12-fold increase in the fluorescence intensity of BaP, which emits at 435 nm when excited at 280 nm, due to the hydrophobic interaction and fluorescence resonance energy transfer (FRET) between antibody and antigen. As result of this phenomenon, the antibody-based fluorescence immunoassay system can be used to detect BaP specifically with a limit of detection (LOD) of 0.06 ng mL(-1). Finally, extraction recoveries of BaP from spiked wheat and barley samples were found to be in the range of 80.5-87.0% and 92.9-92.1%, respectively.

Recent Advances for the Detection of Ochratoxin A

Ochratoxin A (OTA) is one of the mycotoxins secreted by Aspersillus and Penicillium that can easily colonize various grains like coffee, peanut, rice, and maize. Since OTA is a chemically stable compound that can endure the physicochemical conditions of modern food processing, additional research efforts have been devoted to develop sensitive and cost-effective surveillance solutions. Although traditional chromatographic and immunoassays appear to be mature enough to attain sensitivity up to the regulation levels, alternative detection schemes are still being enthusiastically pursued in an attempt to meet the requirements of rapid and cost-effective detections. Herein, this review presents recent progresses in OTA detections with minimal instrumental usage, which have been facilitated by the development of OTA aptamers and by the innovations in functional nanomaterials. In addition to the introduction of aptamer-based OTA detection techniques, OTA-specific detection principles are also presented, which exclusively take advantage of the unique chemical structure and related physicochemical characteristics.

Ultra-sensitive immunoassay biosensors using hybrid plasmonic-biosilica nanostructured materials

We experimentally demonstrate an ultra-sensitive immunoassay biosensor using diatom biosilica with self-assembled plasmonic nanoparticles. As the nature-created photonic crystal structures, diatoms have been adopted to enhance surface plasmon resonances of metal nanoparticles on the surfaces of diatom frustules and to increase the sensitivity of surface-enhanced Raman scattering (SERS). In this study, a sandwich SERS immunoassay is developed based on the hybrid plasmonic-biosilica nanostructured materials that are functionalized with goat anti-mouse IgG. Our experimental results show that diatom frustules improve the detection limit of mouse IgG to 10 pg/mL, which is ˜100× better than conventional colloidal SERS sensors on flat glass. Ultra-sensitive immunoassay biosensor using diatom biosilica with self-assembled plasmonic nanoparticles.

A Microfluidic Love-Wave Biosensing Device for PSA Detection Based on an Aptamer Beacon Probe

A label-free and selective aptamer beacon-based Love-wave biosensing device was developed for prostate specific antigen (PSA) detection. The device consists of the following parts: LiTaO₃ substrate with SiO₂ film as wave guide layer, two set of inter-digital transducers (IDT), gold film for immobilization of the biorecongniton layer and a polydimethylsiloxane (PDMS) microfluidic channels. DNA aptamer, or “artificial antibody”, was used as the specific biorecognition probe for PSA capture. Some nucleotides were added to the 3'-end of the aptamer to form a duplex with the 3'-end, turning the aptamer into an aptamer-beacon. Taking advantage of the selective target-induced assembly changes arising from the “aptamer beacon”, highly selective and specific detection of PSA was achieved. Furthermore, PDMS microfluidic channels were designed and fabricated to realize automated quantitative sample injection. After optimization of the experimental conditions, the established device showed good performance for PSA detection between 10 ng/mL to 1 μg/mL, with a detection limit of 10 ng/mL. The proposed sensor might be a promising alternative for point of care diagnostics.

Stimulated mass enhancement strategy-based highly sensitive detection of a protein in serum using quartz crystal microbalance technique

A quartz crystal microbalance immunosensor is described for highly sensitive detection of interleukin-6 in serum using a magnetic bead-supported bienzyme-catalyzed stimulated mass enhancement strategy.

Detection of human immunoglobulin G by label-free electrochemical immunoassay modified with ultralong CuS nanowires

A novel label-free electrochemical immunoassay modified with ultralong CuS nanowires was developed for the detection of human immunoglobulin G.

Concave gold nanoparticle-based highly sensitive electrochemical IgG immunobiosensor for the detection of antibody–antigen interactions

A concave gold nanocuboid-based electrochemical sensor was developed for the highly sensitive detection of antibody–antigen interactions.

Homogeneous assay of target molecules based on chemiluminescence resonance energy transfer (CRET) using DNAzyme-linked aptamers

We have designed a single-stranded DNAzyme-aptamer sensor for homogeneous target molecular detection based on chemiluminescence resonance energy transfer (CRET). The structure of the engineered single-stranded DNA (ssDNA) includes the horseradish peroxidase (HRP)-like DNAzyme, optimum-length linker (10-mer-length DNA), and target-specific aptamer sequences. A quencher dye was modified at the 3' end of the aptamer sequence. The incorporation of hemin into the G-quadruplex structure of DNAzyme yields an active HRP-like activity that catalyzes luminol to generate a chemiluminescence (CL) signal. In the presence of target molecules, such as ochratoxin A (OTA), adenosine triphosphate (ATP), or thrombin, the aptamer sequence was folded due to the formation of the aptamer/analyte complex, which induced the quencher dye close to the DNAzyme structure. Consequently, the CRET occurred between a DNAzyme-catalyzed chemiluminescence reaction and the quencher dye. Our results showed that CRET-based DNAzyme-aptamer biosensing enabled specific OTA analysis with a limit of detection of 0.27ng/mL. The CRET platform needs no external light source and avoids autofluorescence and photobleaching, and target molecules can be detected specifically and sensitively in a homogeneous manner.

Binding-induced autonomous disassembly of aptamer-DNAzyme supersandwich nanostructures for sensitive electrochemiluminescence turn-on detection of ochratoxin A

The self-assembled DNA nanostructure has been one of the most interesting research areas in the field of nanoscience, and the application of the DNA self-assembled nanostructures in biosensing is still in the early stage. In this work, based on the target-induced autonomous disassembly of the aptamer-DNAzyme supersandwich nanostructures, we demonstrated a highly sensitive strategy for electrochemiluminescent (ECL) detection of ochratoxin A (OTA). The aptamer-DNAzyme supersandwich nanostructures, which exhibited significant ECL quenching effect toward the oxygen/persulfate (O2/S2O8(2-)) system, were self-assembled on the gold electrode surface. The presence of the target OTA and the exonuclease (RecJf) resulted in autonomous disassembly of the nanostructures and cyclic reuse of OTA, leading to efficient recovery of the ECL emission and highly sensitive detection of OTA. Our developed method also showed high selectivity against other interference molecules and can be applied for the detection of OTA in real red wine samples, which offers the proposed method opportunities for designing new DNA-based nanostructures for biosensing applications.

Impedimetric aptasensor for ochratoxin A determination based on Au nanoparticles stabilized with hyper-branched polymer

An impedimetric aptasensor for ochratoxin A (OTA) detection has been developed on the base of a gold electrode covered with a new modifier consisting of electropolymerized Neutral Red and a mixture of Au nanoparticles suspended in the dendrimeric polymer Botlorn H30®. Thiolated aptamer specific to OTA was covalently attached to Au nanoparticles via Au-S bonding. The interaction of the aptamer with OTA induced the conformational switch of the aptamer from linear to guanine quadruplex form followed by consolidation of the surface layer and an increase of the charge transfer resistance. The aptasensor makes it possible to detect from 0.1 to 100 nM of OTA (limit of detection: 0.02 nM) in the presence of at least 50 fold excess of ochratoxin B. The applicability of the aptasensor for real sample assay was confirmed by testing spiked beer samples. The recovery of 2 nM OTA was found to be 70% for light beer and 78% for dark beer.

Femtogram ultrasensitive aptasensor for the detection of Ochratoxin A

A simple and ultrasensitive method was developed for the detection of Ochratoxin A, utilizing an aptamer as a molecular recognition probe and real-time quantitative PCR (RT-qPCR) amplification of its complementary DNA as signal generators. Under the optimized conditions, the cycle threshold (Ct) increased linearly with 10-fold serial dilutions of Ochratoxin A (OTA) from 5×10⁻⁶ to 5 ng mL⁻¹, with a limit of detection (LOD) of 1 fg mL⁻¹. The specificity of this aptasensor was considered to be excellent, as when tested against four other toxins it produced no obvious Ct value change. Furthermore, a satisfactory analyte concentration recovery was obtained from a series of concentrations of OTA spiked into red wine. Therefore, this highly sensitive approach shows a significant potential in a wide range of target analytes.