Application of Aptamer Identification Technology in Rapid Analysis of Mycotoxins

Fundamentals of Biosensors and Detection Methods

Biosensors have a great impact on our society to enhance the life quality, playing an important role in the development of Point-of-Care (POC) technologies for rapid diagnostics, and monitoring of disease progression. COVID-19 rapid antigen tests, home pregnancy tests, and glucose monitoring sensors represent three examples of successful biosensor POC devices. Biosensors have extensively been used in applications related to the control of diseases, food quality and safety, and environment quality. They can provide great specificity and portability at significantly reduced costs. In this chapter are described the fundamentals of biosensors including the working principles, general configurations, performance factors, and their classifications according to the type of bioreceptors and transducers. It is also briefly illustrated the general strategies applied to immobilize biorecognition elements on the transducer surface for the construction of biosensors. Moreover, the principal detection methods used in biosensors are described, giving special emphasis on optical, electrochemical, and mass-based methods. Finally, the challenges for biosensing in real applications are addressed at the end of this chapter.

A Fiber-Based SPR Aptasensor for the In Vitro Detection of Inflammation Biomarkers

It is widely accepted that the abnormal concentrations of different inflammation biomarkers can be used for the early diagnosis of cardiovascular disease (CVD). Currently, many reported strategies, which require extra report tags or bulky detection equipment, are not portable enough for onsite inflammation biomarker detection. In this work, a fiber-based surface plasmon resonance (SPR) biosensor decorated with DNA aptamers, which were specific to two typical inflammation biomarkers, C-reactive protein (CRP) and cardiac troponin I (cTn-I), was developed. By optimizing the surface concentration of the DNA aptamer, the proposed sensor could achieve a limit of detection (LOD) of 1.7 nM (0.204 μg/mL) and 2.5 nM (57.5 ng/mL) to CRP and cTn-I, respectively. Additionally, this biosensor could also be used to detect other biomarkers by immobilizing corresponding specific DNA aptamers. Integrated with a miniaturized spectral analysis device, the proposed sensor could be applied for constructing a portable instrument to provide the point of care testing (POCT) for CVD patients.

Contamination, Detection and Control of Mycotoxins in Fruits and Vegetables

Mycotoxins are secondary metabolites produced by pathogenic fungi that colonize fruits and vegetables either during harvesting or during storage. Mycotoxin contamination in fruits and vegetables has been a major problem worldwide, which poses a serious threat to human and animal health through the food chain. This review systematically describes the major mycotoxigenic fungi and the produced mycotoxins in fruits and vegetables, analyzes recent mycotoxin detection technologies including chromatography coupled with detector (i.e., mass, ultraviolet, fluorescence, etc.) technology, electrochemical biosensors technology and immunological techniques, as well as summarizes the degradation and detoxification technologies of mycotoxins in fruits and vegetables, including physical, chemical and biological methods. The future prospect is also proposed to provide an overview and suggestions for future mycotoxin research directions.

Solid-phase extraction techniques based on nanomaterials for mycotoxin analysis: An overview for food and agricultural products

Mycotoxin contamination is a globally concerned problem for food and agricultural products since it may directly or indirectly induce severe threats to human health. Sensitive and selective screening is an efficient strategy to prevent or reduce human and animal exposure to mycotoxins. However, enormous challenges exist in the determination of mycotoxins, arising from complex sample matrices, trace-level analytes, and the co-occurrence of diverse mycotoxins. Appropriate sample preparation is essential to isolate, purify, and enrich mycotoxins from complicated matrices, thus decreasing sample matrix effects and lowering detection limits. With the cross-disciplinary development, new solid-phase extraction strategies have been exploited and integrated with nanotechnology to meet the challenges of mycotoxin analysis. This review summarizes the advance and progress of solid-phase extraction techniques as the methodological solutions for mycotoxin analysis. Emphases are paid on nanomaterials fabricated as trapping media of SPE techniques, including carbonaceous nanoparticles, metal/metal oxide-based nanoparticles, and nanoporous materials. Advantages and limitations are discussed, along with the potential prospects. This article is protected by copyright. All rights reserved.

Development of a DNA-based biosensor for the fast and sensitive detection of ochratoxin A in urine

In this paper, a novel DNA-based biosensor is proposed, which is based on paramagnetic microbeads carrying an ochratoxin A (OTA) capture aptamer. A sandwich-like detection complex is linked to the capture aptamer and is able to trigger, in presence of OTA, an isothermal rolling circle amplification (RCA) reaction. This latter generated autocatalytic units with a peroxidase activity (DNAzyme) that, in presence of a proper substrate, gave a blue-coloured product visible by the naked eye. The capture aptamer, blocked onto magnetic beads, allowed the specific capture of OTA in liquid samples. The modified detection aptamer, annealed to a circularized probe, was then used to detect the toxin capture event. Indeed, in the presence of OTA and an isothermal enzyme, the circular DNA was amplified, producing a single-stranded and tandem repeated long homologous copy of its sequence. In the DNA strand, a self-catalytic structure was formed with hemin as the catalytic core, inducing the development of blue colour in the presence of ABTS and hydrogen peroxide. The results showed that the biosensor has high sensitivity and selectivity for the detection of OTA, as low as 1.09 × 10^-12 ng/mL. Moreover, the proposed biosensor was successfully used for the detection of OTA in naturally contaminated rat urine. Accuracy and repeatability data obtained in recovery experiments were satisfying, being recoveries >95% with relative standard deviations in the range 3.6-15%. For the first time, an aptasensor was successfully applied to detect OTA in biological fluids. It can be used for mycotoxin biomonitoring and assessment of individual exposure.

Aptamers: an emerging class of bioaffinity ligands in bioactive peptide applications

The food and health applications of bioactive peptides have grown remarkably in the past few decades. Current elucidations have shown that bioactive peptides have unique structural arrangement of amino acids, conferring distinct functionalities, and molecular affinity characteristics. However, whereas interest in the biological potency of bioactive peptides has grown, cost-effective techniques for monitoring the structural changes in these peptides and how these changes affect the biological properties have not grown at the same rate. Due to the high binding affinity of aptamers for other biomolecules, they have a huge potential for use in tracking the structural, conformational, and compositional changes in bioactive peptides. This review provides an overview of bioactive peptides and their essential structure-activity relationship. The review further highlights on the types and methods of synthesis of aptamers before the discussion of the prospects, merits, and challenges in the use of aptamers for bioaffinity interactions with bioactive peptides.

Rapid Determination of Ochratoxin A in Grape and Its Commodities Based on a Label-Free Impedimetric Aptasensor Constructed by Layer-by-Layer Self-Assembly

A simple and sensitive label-free impedimetric aptasensor for rapid determination of ochratoxin A (OTA) has been developed, which was based on the combination between thiolated aptamer and gold nanoparticles by layer-by-layer self-assembly. Because of the interaction between aptamer and OTA, the relative normalized electron-transfer resistance (ΔR_ct) values obtained by electrochemical impedance spectroscopy (EIS) was proportional to the concentration of OTA and showed a good linear relationship from 0.1 to 10.0 ng/mL, with a lower detection limit (0.030 ng/mL) than one-step thiolated DNA aptasensor. The established method was successfully applied to detect and analyze OTA in table wine and grape juice, and the recovery was 90.56%–104.21% when PVP effective removed of phenolic substances. The label-free impedimetric aptasensor was used for rapid detection and quantitation of OTA in the inoculated grapes with the Aspergillus Nigri (H1), and the production of OTA (62.4 μg/kg, 20 μg/kg) far exceeded the maximum levels of 2 μg/kg after inoculation for three days. The developed method exhibited a good specificity, high sensitivity, time-efficient, and it could be applied to detect the OTA concentration in grape and its commodities.

Fumonisins and their analogues in contaminated corn and its processed foods - a review

One of the food security problems faced worldwide is the occurrence of mycotoxins in grains and their foods. Fumonisins (FBs) are mycotoxins which are prevalent in corn (Zea mays L.) and its based foods. Their intake and exposure have been epidemiologically and inconclusively associated with oesophageal cancer and neural tube defects in humans, and other harmful health effects in animals. The toxic effects of FBs can be acute or chronic and these metabolites bioaccumulate mainly in liver and kidney tissues. Among FBs, fumonisin B₁ (FB₁) is the most relevant moiety although the 'hidden' forms produced after food thermal processes are becoming relevant. Corn is the grain most susceptible to Fusarium and FBs contamination and the mould growth is affected both by abiotic and biotic factors during grain maturation and storage. Mould counts are mainly affected by the grain water activity, the environmental temperature during grain maturation and insect damage. The abiotic factors affected by climatic change patterns have increased their incidence in other regions of the world. Among FBs, the hidden forms are the most difficult to detect and quantify. Single or combined physical, chemical and biological methods are emerging to significantly reduce FBs in processed foods and therefore diminish their toxicological effects.

Aptamer-tagged DNA origami for spatially addressable detection of aflatoxin B1

A DNA origami-based platform for detecting aflatoxin B1 has been constructed with the assistance of aptamer probes and its complementary ssDNA-modified gold nanoparticles. This work may open new horizons for the application of DNA origami in the detection of a variety of small molecules.

Single-Step LRET Aptasensor for Rapid Mycotoxin Detection

Contamination of foods by mycotoxins is a common yet serious problem. Owing to the increase in consumption of fresh produce, consumers have become aware of food safety issues caused by mycotoxins. Therefore, rapid and sensitive mycotoxin detection is in great demand in fields such as food safety and public health. Here we report a single-step luminescence resonance energy transfer (LRET) aptasensor for mycotoxin detection. To accomplish the single-step sensor, our sensor was constructed by linking a quencher-labeled aptamer through a linker to the surface of upconversion nanoparticles (UCNPs). Our LRET aptasensor is composed of Mn²⁺-doped NaYF₄:Yb³⁺,Er³⁺ UCNPs as the LRET donor, and black hole quencher 3 (BHQ3) as the acceptor. The maximum quenching efficiency is obtained by modulating the linker length, which controls the distance between the quencher and the UCNPs. Our distinctive design of LRET aptasensor allows detection of mycotoxins selectively in colored food samples within 10 min without multiple bioassay steps. We believe our single-step aptasensor has a significant potential for on-site detection of food contaminants, environmental pollutants, and biological metabolites.

Optimization of Aflatoxin B1 Aptasensing

Combination of aptamers with DNAzymes attracted intense attention for development of DNA-based biosensors for detection of mycotoxins. In the present study a combination of aflatoxin B1 specific aptamer and HRP- (horseradish peroxidase-) mimicking DNAzyme was optimized for detecting aflatoxin B1. Detecting approach is based on the binding affinity of aflatoxin B1 to its specific aptamer and conversion of substrate to a detectable colorimetric signal by a linked DNAzyme. Compared to conventional methods for aflatoxin B1 detection, DNA-based assay has the advantages of low cost, long-term stability, and rapid, simple, and user-friendly steps.

Recent advances in mycotoxins detection

Mycotoxins contamination in both food and feed is inevitable. Mycotoxin toxicity in foodstuff can occur at very low concentrations necessitating early availability of sensitive and reliable methods for their detection. The present research thrust is towards the development of a user friendly biosensor for mycotoxin detection at both academic and industrial levels to replace conventional expensive chromatographic and ELISA techniques. This review critically analyzes the recent research trend towards the construction of immunosensor, aptasensor, enzymatic sensors and others for mycotoxin detection with a reference to label and label free methods, synthesis of new materials including nano dimension, and transuding techniques. Technological aspects in the development of biosensors for mycotoxin detection, current challenges and future prospects are also included to provide a overview and suggestions for future research directions.

Fluorescence polarisation immunoassays for rapid, accurate and sensitive determination of mycotoxins

Fluorescence polarisation immunoassay (FPIA) is a type of homogeneous assay. For low molecular weight antigens, such as mycotoxins, it is based on the competition between an unlabeled antigen and its fluorescent-labelled derivative (tracer) for an antigen-specific antibody. The antigen content is determined by measuring the reduction of fluorescence polarisation signal, which in turn is determined by the reduction of tracer molecules able to bind antibody in solution. To develop a competitive FPIA for mycotoxin measurement the tracer has to be synthesised and its binding response with a specific antibody should be tested. Selectivity and sensitivity of the FPIA methods are strictly related to the antibody/tracer combination used. Several FPIA methods for the detection of the major mycotoxins, including aflatoxins, fumonisins, ochratoxin A, deoxynivalenol, T-2 and HT-2 toxins and zearalenone in food and beverages have been developed in the last decade. Basic principles, key elements, advantages and limitations of these methods are reviewed. These FPIA methods are simple, readily automated, rapid, and suitable for high-throughput screening, as well as for the reliable quantitative determination of mycotoxins in foods and commodities.

Identification of allosteric nucleotide sites of tetramethylrhodamine-labeled aptamer for noncompetitive aptamer-based fluorescence anisotropy detection of a small molecule, ochratoxin A

Aptamer-based fluorescence anisotropy (FA) assay combines the advantages of affinity aptamers in good stability, easy generation, and facile labeling and the benefits of FA in homogeneous analysis, such as robustness, simplicity, and high reproducibility. By using a fluorophore-labeled aptamer, FA detection of a small molecule is not as easy as detection of protein because the binding of a small molecule cannot cause significant increase of molecular weight of the dye-labeled aptamer. The intramolecular interaction between labeled tetramethylrhodamine (TMR) and DNA aptamer bases dramatically affects the local rotation and FA of TMR. This intramolecular interaction can be altered by aptamer conformation change upon target binding, leading to a significant change of FA of TMR. Taking this unique feature of a TMR-labeled aptamer, we described a noncompetitive aptamer-based fluorescence anisotropy assay for detection of small molecules by using ochratoxin A (OTA) as a model. We successfully identified the specific TMR-labeling sites of aptamers with sensitive FA response to OTA from the 5'-end, 3'-end and the internal thymine (T) bases. The aptamer with a TMR labeled on the 10th T base exhibited a remarkable FA reduction response to OTA (Δr = 0.078), without requiring any proteins or nanomaterials as FA signal enhancers. This FA approach for OTA showed high sensitivity with a detection limit of 3 nM, a dynamic range from 3 nM to 3 μM, and good selectivity over the tested compounds with similar structures to OTA. The new strategy allowed the detection of OTA in diluted red wine and urine samples.

Fluorescent sensing ochratoxin A with single fluorophore-labeled aptamer

We explored a fluorescent strategy for sensing ochratoxin A (OTA) by using a single fluorophore-labeled aptamer for detection of OTA. This method relied on the change of the fluorescence intensity of the labeled dye induced by the specific binding of the fluorescent aptamer to OTA. Different fluorescein labeling sites of aptamers were screened, including the internal thymine bases, 3'-end, and 5'-end of the aptamer, and the effect of the labeling on the aptamer affinity was investigated. Some fluorophore-labeled aptamers showed a signal-on or signal-off response. With the fluorescent aptamer switch, simple, rapid, and selective sensing of OTA at nanomolar concentrations was achieved. OTA spiked in diluted red wine could be detected, showing the feasibility of the fluorescent aptamer for a complex matrix. This method shows potential for designing aptamer sensors for other targets.