Quality evaluation of five commercial enzyme linked immunosorbent assay kits for detecting aflatoxin b1 in feedstuffs

A novel highly active AgMOF-based silver single-atom catalyst and its application to the aptamer SERS/RRS for the determination of aflatoxin B1

A novel highly active silver single-atom catalyst (AgSAC) was prepared by a microwave-assisted solvothermal method using silver covalent organic frameworks (AgMOF) as precursors. It was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared (IR), and surface-enhanced Raman scattering (SERS). The experiment found that AgSAC has excellent catalytic performance and can heavily catalyze the nano-reaction of chloroauric acid-malic acid (HAuCl4-H2Mi) to generate gold nanoparticles (AuNPs). The produced AuNPs have strong SERS, resonance Rayleigh scattering (RRS) and surface plasmon resonance absorption (Abs) signals. Aflatoxin B1 aptamer (AptAFB1) can be adsorbed to the surface of AgSAC through electrostatic interaction, to reduce the catalytic activity of AgSAC and the SERS/RRS/Abs signal of the system. When the target molecule (AFB1) was added, it will specifically bind to AptAFB1 and release AgSAC, restoring the catalytic activity of AgSAC, thereby restoring the SERS/RRS/Abs signal of the system. Based on this, a simple and sensitive aptamer sensing analysis platform for trace AFB1 was established, and a reasonable catalytic amplification mechanism of AgSAC was proposed. The SERS method exhibited the highest sensitivity, with a linear range of 0.005-0.225 μg/L and a detection limit of 0.002 μg/L.

A Novel Preanalytical Strategy Enabling Application of a Colorimetric Nanoaptasensor for On-Site Detection of AFB1 in Cattle Feed

Aflatoxin contamination of cattle feed is responsible for serious adverse effects on animal and human health. A number of approaches have been reported to determine aflatoxin B1 (AFB1) in a variety of feed samples using aptasensors. However, rapid analysis of AFB1 in these matrices remains to be addressed in light of the complexity of the preanalytical process. Herein we describe an optimization on the preanalytical stage to minimize the sample processing steps required to perform semi-quantitative colorimetric detection of AFB1 in cattle feed using a gold nanoparticle-based aptasensor (nano-aptasensor). The optical behavior of the nano-aptasensor was characterized in different organics solvents, with acetonitrile showing the least interference on the activity of the nan-aptasensor. This solvent was selected as the extractant agent for AFB1-containing feed, allowing for the first time, direct colorimetric detection from the crude extract (detection limit of 5 µg/kg). Overall, these results lend support to the application of this technology for the on-site detection of AFB1 in the dairy sector.

Simple Design Concept for Dual-Channel Detection of Ochratoxin A Based on Bifunctional Metal-Organic Framework

A simple fluorescence and electrochemical dual-channel biosensor based on bifunctional Zr(IV)-based metal-organic framework (Zr-MOF) was proposed to detect Ochratoxin A (OTA). The bifunctional Zr-MOF, with photoluminescence properties and enormous electroactive ligands, was exploited to load OTA-specific aptamers for designing signal probes, greatly simplifying the probe-fabrication process and improving sensing reliability. Upon specific recognition of aptamer toward OTA, the anchored probe was released from the sensing interface into the reaction solution. In this circumstance, the increased amount of the signal probe in reaction solution led to an enhanced fluorescence response, while the decreased amount of the signal probe on the sensing interface resulted in a diminished electrochemical response. According to the dual-channel signal change with increasing OTA concentration, the visual fluorescence strategy was established for intuitive OTA detection, and meanwhile, sensitive electrochemical assay with a detection limit of 0.024 pg/mL was also achieved with the help of one-step electrodeposition as a sensing platform. Moreover, the proposed dual-channel assay has been successfully applied to determine OTA levels in corn samples with rapid response, superior accuracy, and high anti-interference capability, providing a promising method for food safety monitoring.

Real-time monitoring of active caspase 3 during AFB1 induced apoptosis based on SERS-fluorescent dual mode signals

Aflatoxin B1 (AFB1) is the most toxic mycotoxin. Usually, the toxin activated apoptosis is considered mostly through intrinsic mitochondrial pathway while the caspase family as promoter and executor plays a crucial role. In this paper, a real-time and in situ detection of caspase 3 in living cells based on SERS-fluorescence dual mode nanosensor was studied. Firstly, gold nanotriangles (AuNTs) modified with the caspase 3 specifically recognized polypeptide chain DEVD were synthesized as both SERS enhanced substrate and fluorescent quencher. Rhodamine B (Rb) as both Raman and fluorescent signal molecules was modified on the N end of DEVD chain. After active caspase 3 specifically cut off the recognition site in DEVD, partial peptide chain with Rb fell off from the surface of AuNTs. Thus, the Raman signal of Rb decreased while its fluorescent signal recovered. There was a good linear relationship between caspase 3 and both the SERS and fluorescence signals of Rb. The minimum detection limit was 0.001 nM. After cells were stimulated by AFB1, when Cyt C in the cytoplasm reached a certain level, caspase 3 was activated. This nanosensor was realized in certain living cells (HepG2, HeLa and A549). Based on monitoring the activation of specific apoptotic markers, the conduction of marker signals in real time can provide more detailed information for apoptosis.

No-wash point-of-care biosensing assay for rapid and sensitive detection of aflatoxin B1

In many cases of in-situ or point-of-care testing (POCT), the single pursuit of good detection performance cannot meet the testing requirements, and thus no-wash testing has become one of the most effective methods to develop sustainable biosensing assay, providing more convenient operation procedures and shorting the detection time. Herein, a disposable POC biosensing assay was prepared based on the RGB color detector software on the smartphone and the peroxide-like activity of gold nanoparticles (Au NPs) for aflatoxin B1 (AFB1) sensitive detection. Using syringe filters for a simple physical separation procedure can easily realize washing free detection, which is superior to most biosensing assays with cumbersome detection procedures. The syringe filters with 200 nm pore diameter could only pass through small Au NPs (30 nm) while the large-sized SiO₂ nanoparticles (300 nm) was blocked on the membrane. In this work, Au NPs utilized their inherent peroxidase-like activity to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ to ox-TMB under acidic conditions, which results in blue color in aqueous solution. The color change due to different antigen concentrations was quantitatively determined by measuring the color intensity with a smartphone and the RGB color detector. By measuring the color intensity, it was known that the linear concentration range of the biosensing assay was 100 fg mL^-1 to 50 ng mL^-1, and the detection limit of AFB1 was 33 fg mL^-1 (S/N = 3). Additionally, the designed biosensing assay exhibited excellent selectivity, storage stability and reproducibility. More importantly, the innovation of detecting and analyzing technology is the outstanding advantage of the biosensing assay, providing a more flexible and convenient strategy for some other small molecule analysis.

An Impedance Based Electrochemical Immunosensor for Aflatoxin B1 Monitoring in Pistachio Matrices

Aflatoxins are highly toxic fungal secondary metabolites that often contaminate food and feed commodities. An electrochemical immunosensor for the determination of aflatoxin B1 (AFB1) was fabricated by immobilizing monoclonal AFB1 antibodies onto a screen-printed gold electrode that was modified with carbo-methyldextran by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride/N-hydroxysuccinimide cross-linking. An electrochemical interfacial modelling of biomolecular recognition was suggested and reasonably interpreted. Impedance technology was employed for the quantitative determination of AFB1. The limit of detection concentration of AFB1 for standard solutions and spiked pistachio samples was 0.5 ng/mL and 1 ng/mL, respectively. The immunosensor was able to successfully determine AFB1 concentrations in the range of 4.56–50.86 ng/mL in unknown pistachio samples. Comparative chromatographic analysis revealed that AFB1 concentrations that were higher than 345 ng/mL were not within the immunosensor’s upper limits of detection. Selectivity studies against Ochratoxin A and Aflatoxin M1 demonstrated that the proposed AFB1 immunosensor was able to differentiate between these other fungal mycotoxins. The novel electrochemical immunosensor approach has the potential for rapid sample screening in a portable, disposable format, thus contributing to the requirement for effective prevention and the control of aflatoxin B1 in pistachios.

Simultaneous detection of zearalenone, citrinin, and ochratoxin A in pepper by capillary zone electrophoresis

In the present study, a simple and fast method for simultaneous detection of zearalenone, citrinin, and ochratoxin A utilising capillary zone electrophoresis with an ultraviolet detector was developed. The optimised approach was validated and applied using pepper samples. The proposed method yielded satisfactory linearity between the signal and the mycotoxin concentration in the range of 1.5-150 μg/kg for zearalenone, 4.5-150 μg/kg for citrinin, and 0.8-150 μg/kg for ochratoxin A. The limits of detection for these mycotoxins ranged from 0.3 to 1.5 μg/kg. The corresponding intra- and inter-day precisions were less than 3.5 % and 4.1 %, respectively. Moreover, the matrix effect was also assessed and the result was compared using the capillary zone electrophoresis and high-performance liquid chromatography methods. The developed approach could be used for simultaneous detection of zearalenone, citrinin, and ochratoxin A in pepper.

Gold nanoparticles mediated designing of versatile aptasensor for colorimetric/electrochemical dual-channel detection of aflatoxin B1

This work is aimed to develop of a new class of versatile aptasensor to specifically detect aflatoxin B1 (AFB1) using dual-channel detection method. To achieve this objective, gold nanoparticles (AuNPs) having peroxidase-like activity and capability of promoting silver deposition were used as the versatile label for both colorimetric and electrochemical techniques. First of all, aptamer (apt) modified Fe₃O₄@Au magnetic beads (MBs-apt) and cDNA modified AuNPs (cDNA-AuNPs) were prepared to use as capture probes and signal probes, respectively. Taking advantage of hybridization reaction between apt and cDNA, these two probes were coupled with each other to generate MBs-apt/cDNA-AuNPs bioconjugations. The high affinity between apt and AFB1 made cDNA-AuNPs detached from MBs-apt, and the released signal probes were separated and collected using an external magnetic field and used for both colorimetric and electrochemical detection channels. The dual-channel signals were directly proportional to logarithm of AFB1 concentration within the ranges of 5-200 ng mL^-1 and 0.05-100 ng mL^-1. The detection limit can reach as low as 35 pg mL^-1 and 0.43 pg mL^-1 for colorimetric and electrochemical channel, respectively. Moreover, the proposed aptasensor has been successfully applied to determine AFB1 in corn samples with satisfactory results. This dual-channel detection method can not only improve the detection precision and diversity significantly, but also can reduce the false-negative and-positive rates in food quality monitoring. We believe we have provided a general strategy with the convincing dual-readout mode which possess great promising in all of the aptamer related fields.

Current research and prevention of aflatoxins in China

Since their discovery in the 1960s, aflatoxins were found to have a considerable impact on the health of humans and animals as well as the country’s economy and international trade. Aflatoxins are often found in nuts, cereals and animal feeds, which has a significant danger to the food industry. Over the years, several steps have been undertaken worldwide to minimise their contamination in crops and their exposure to humans and animals. China is one of the largest exporters and importers of food and animal feed. As a result, many studies have been carried out in China related to aflatoxins, including their distribution, pollution, detection methods, monitoring, testing and managing. Chinese scientists studied aflatoxins in microbiological, toxicological, ecological effects as well as policies relating to their controlling. China has thus put into practice a number of strategies aiming at the prevention and control of aflatoxins in order to protect consumers and ensure a safe trade of food and feed, and the status and enlargement of these strategies are very important and useful for many consumers and stakeholders in China. Therefore, this article aims at the detriment assessments, regulations, distribution, detection methods, prevention and control of aflatoxins in China. It equally provides useful information about the recent safety management systems in place to fight the contamination of aflatoxins in food and feed in China.

Competitive horseradish peroxidase-linked aptamer assay for sensitive detection of Aflatoxin B1

Aflatoxin B1 (AFB1) is one of highly toxic mycotoxins and a known human carcinogen. The frequent contamination of AFB1 in food products and large health risk of AFB1 have raised global concerns. Sensitive detection of AFB1 is of vital importance and highly demanded. Herein, we reported a competitive horseradish peroxidase (HRP)-linked aptamer assay for AFB1, combining the advantages of aptamer for affinity binding and enzyme label for signal amplification. In this assay, free AFB1 in solution competed with a covalent conjugate of bovine serum albumin-AFB1 (BSA-AFB1) coated on the wells of microplate in binding to the HRP-labeled aptamer probe. HRP attached on BSA-AFB1 in the wells catalyzed the conversion of substrates into products, allowing the final detection of AFB1 through measurement of the generated products. When TMB (3,3',5,5'-tetramethylbenzidine dihydrochloride) was used as substrate, absorbance analysis of the product of enzyme reaction enabled the detection of AFB1 at 0.2nM. We further lowered the detection limit of AFB1 to 0.01nM through chemiluminescence analysis by using chemiluminescence substrate of HRP. This assay enabled the detection of AFB1 in complex sample matrix, such as diluted white wine and maize flour. This assay provides a simple, sensitive and rapid method for AFB1 determination.

A terminal antibody method based on multiple factors that influence ELISA results for measurement of antibody affinity in clinical specimens

OBJECTIVE

To establish a new method for the measurement of antibody affinity in clinical samples.

METHODS

Serial dilutions of antiserum samples were prepared to find the threshold concentration of antibody separating detectable from negative ELISA results. This threshold concentration was defined as the terminal antibody (TA) concentration, and a new method for measuring antibody affinity based on the effect of multiple factors that influence ELISA results at TA concentration was established, which we called the TA method. The TA method was used to measure low- and high-affinity antiserum samples to verify the validity of the method, and then was used to measure the affinity of the antibody to Hepatitis B surface antigen and affinity of antibody to Hepatitis C Virus in clinical serum specimens.

RESULTS

Low- and high-affinity antiserum samples could be clearly distinguished by TA method. The antibody affinity in anti-HBe positive group was significantly higher than that of anti-HBe negative group; the affinity of antibody to HCV in HCV-RNA negative group was significantly higher than that of HCV-RNA positive group.

CONCLUSIONS

The TA method for measuring antibody affinity in clinical specimens displayed its validation and that may have potential application value in clinical settings.

Aflatoxin B1 Detection Using a Highly-Sensitive Molecularly-Imprinted Electrochemical Sensor Based on an Electropolymerized Metal Organic Framework

A sensitive electrochemical molecularly-imprinted sensor was developed for the detection of aflatoxin B1 (AFB1), by electropolymerization of p-aminothiophenol-functionalized gold nanoparticles in the presence of AFB1 as a template molecule. The extraction of the template leads to the formation of cavities that are able to specifically recognize and bind AFB1 through π-π interactions between AFB1 molecules and aniline moities. The performance of the developed sensor for the detection of AFB1 was investigated by linear sweep voltammetry using a hexacyanoferrate/hexacyanoferrite solution as a redox probe, the electron transfer rate increasing when the concentration of AFB1 increases, due to a p-doping effect. The molecularly-imprinted sensor exhibits a broad linear range, between 3.2 fM and 3.2 µM, and a quantification limit of 3 fM. Compared to the non-imprinted sensor, the imprinting factor was found to be 10. Selectivity studies were also performed towards the binding of other aflatoxins and ochratoxin A, proving good selectivity.