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 Fluorescence Resonance Energy Transfer Aptasensor for Aflatoxin B1 Based on Ligand-Induced ssDNA Displacement

In this study, a fluorescence resonance energy transfer (FRET)-based aptasensor for the detection of aflatoxin B1 (AFB1) was designed using a carboxyfluorescein (FAM)-labeled aptamer and short complementary DNA (cDNA) labeled with low molecular quencher RTQ1. The sensing principle was based on the detection of restored FAM-aptamer fluorescence due to the ligand-induced displacement of cDNA in the presence of AFB1, leading to the destruction of the aptamer/cDNA duplex and preventing the convergence of FAM and RTQ1 at the effective FRET distance. Under optimal sensing conditions, a linear correlation was obtained between the fluorescence intensity of the FAM-aptamer and the AFB1 concentration in the range of 2.5-208.3 ng/mL with the detection limit of the assay equal to 0.2 ng/mL. The assay time was 30 min. The proposed FRET aptasensor has been successfully validated by analyzing white wine and corn flour samples, with recovery ranging from 76.7% to 91.9% and 84.0% to 86.5%, respectively. This work demonstrates the possibilities of labeled cDNA as an effective and easily accessible tool for sensitive AFB1 detection. The homogeneous FRET aptasensor is an appropriate choice for contaminant screening in complex matrices.

Effects of Aspergillus niger Infection on the Quality of Jujube and Ochratoxin A Cumulative Effect

The jujube is one of the most popular fruits in China because of its delicious taste and high nutritional value. It has a long history of usage as an important food or traditional medicine. However, the jujube is easily infected by fungi, which causes economic losses and threatens human health. When the jujube was infected by Aspergillus niger (H1), the changes in nutritional qualities were determined, such as the content of total acid, vitamin C, reducing sugar, etc. In addition, the ability of A. niger (H1) to produce ochratoxin A (OTA) in different inoculation times and culture media was evaluated, and the content of OTA in jujubes was also analyzed. After jujubes were infected by A. niger (H1), the total acid, and vitamin C contents increased, while the total phenol content decreased, and the reducing sugar content increased after an initial decrease. Although A. niger (H1) infection caused the jujubes to rot and affected its quality, OTA had not been detected. This research provides a theoretical foundation for maximizing edible safety and evaluating the losses caused by fungal disease in jujubes.

Recent Progress in Electrochemical Nano-Biosensors for Detection of Pesticides and Mycotoxins in Foods

Pesticide and mycotoxin residues in food are concerning as they are harmful to human health. Traditional methods, such as high-performance liquid chromatography (HPLC) for such detection lack sensitivity and operation convenience. Efficient, accurate detection approaches are needed. With the recent development of nanotechnology, electrochemical biosensors based on nanomaterials have shown solid ability to detect trace pesticides and mycotoxins quickly and accurately. In this review, English articles about electrochemical biosensors in the past 11 years (2011-2022) were collected from PubMed database, and various nanomaterials are discussed, including noble metal nanomaterials, magnetic metal nanoparticles, metal-organic frameworks, carbon nanotubes, as well as graphene and its derivatives. Three main roles of such nanomaterials in the detection process are summarized, including biomolecule immobilization, signal generation, and signal amplification. The detection targets involve two types of pesticides (organophosphorus and carbamate) and six types of mycotoxins (aflatoxin, deoxynivalenol, zearalenone, fumonisin, ochratoxin A, and patulin). Although significant achievements have been made in the evolution of electrochemical nano-biosensors, many challenges remain to be overcome.

Nucleic Acid Aptamers for Pesticides, Toxins, and Biomarkers in Agriculture

Nucleic acid aptamers are short single-stranded DNA/RNA (ssDNA/RNA) oligonucleotides that can selectively bind to the targets. They are widely used in medicine, biosensing, and diagnostic assay. They have also been identified and extensively used for various targets in agriculture. In this review we summarize the progress of nucleic acid aptamers on pesticides (herbicides, insecticides, and fungicides), toxins, specific biomarkers of crops, and plant growth regulators in agricultural field in recent years. The basic process of aptamer selection, the already identified DNA/RNA aptamers and the aptasensors are discussed. We also discuss the future perspectives and the challenges for aptamer development in agriculture.

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.

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.

Highly sensitive and specific assessment of ochratoxin A in herbal medicines via activator regeneration by electron transfer ATRP

A signal-off fluorescence biosensor for highly sensitive detection of OTA was constructed via the ARGET ATRP signal amplification strategy.

Determination and Comprehensive Risk Assessment of Dietary Exposure to Ochratoxin A on Fermented Teas

A specialized method for ochratoxin A (OTA) determination on fermented teas was developed and validated using ultraperformance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). Methodology results showed that recovery, relative standard deviation, accuracy, and precision were qualified. The limits of detection and quantification were 0.32 and 0.96 μg/kg, respectively. Two of 158 collected samples were screened for OTA contamination. Comprehensive risk assessment based on OTA contaminations of this study and other peer-reviewed publications was performed. The highest hazard quotient (HQ) value (8.86 × 10^-2) and the highest 1/MoE value (8.61 × 10^-5) in probabilistic assessment were equally below the recommended non-neoplastic and neoplastic thresholds, indicating no health risks. However, the HQ and 1/MoE values of the 95th percentiles in 20-39 and ≥50 years of age were close to thresholds of 1.0 and 1.0 × 10^-4, respectively. Under the extreme case, there were only a few scenarios (e.g., 40-49 years of age) of HQ values below the non-neoplastic threshold, but the 1/MoE value of each group exceeded the neoplastic threshold. This is the first extensive risk assessment on OTA from fermented teas worldwide, but the sample size is still limited, and a large number of samples is encouraged in a future study for a more accurate assessment.

Application of the Dimeric G-Quadruplex and toehold-mediated strand displacement reaction for fluorescence biosensing of ochratoxin A

Ochratoxin A (OTA) is one of the most toxic mycotoxins that exists in various agro-products and foods. Here, a non-label and enzyme-free fluorescence biosensor for highly specific detection of OTA has been developed by the combination of toehold-mediated strand displacement reaction (TMSD) and G-quadruplex dimer/ThT (G-dimer/ThT). The DNA duplex (aptamer-IP) is composed of the anti-OTA aptamer and a single stranded initiation probe (IP). In the presence of OTA, the attachment of target to aptamer leads to the liberation of the IP, which activates the cycle TMSD amplifications of two hairpin probes (H1 and H2) accompanied by the production of numerous H1-H2 assemblies. This double-stranded H1-H2 structure results in the proximity between the 5'-end overhang tail of H1 and the 3'-end stem of H2 to liberate the pre-blocked G-dimer sequence for lighting up ThT. In addition, the method displayed a stable fluorescence emission in the high-salt media. It was successfully applied to analyze OTA in real food samples. Hence, the constructed fluorescence biosensing platform might provide a new way for OTA and other toxin analysis detection.

Label-Free and Highly-Sensitive Detection of Ochratoxin A Using One-Pot Synthesized Reduced Graphene Oxide/Gold Nanoparticles-Based Impedimetric Aptasensor

Ochratoxin A (OTA) primarily obtained by the genera aspergillus and penicillium, is one of the toxic substances for different organs and systems of the human body such as the kidney, liver, neurons and the immune system. Moreover, it is considered to cause tumors and fetal malformation even at a very low concentration. Fast and sensitive assay for detection of OTA at ultralow levels in foods and agricultural products has been an increasing demand. In this study, a new label-free electrochemical biosensor based on three-dimensional reduced graphene oxide/gold nanoparticles/aptamer for OTA detection was constructed. The 3D-rGO/Au NPs nanocomposites were firstly synthesized using a one-pot hydrothermal process under optimized experimental conditions. The 3D-rGO/Au NPs with considerable particular surface area and outstanding electrical conductivity was then coated on a glass carbon electrode to provide tremendous binding sites for -SH modified aptamer via the distinctive Au-S linkage. The presence of OTA was specifically captured by aptamer and resulted in electrochemical impedance spectroscopy (EIS) signal response accordingly. The constructed impedimetric aptasensor obtained a broad linear response from 1 pg/mL to 10 ng/mL with an LOD of 0.34 pg/mL toward OTA detection, highlighting the excellent sensitivity. Satisfactory reproducibility was also achieved with the relative standard deviation (RSD) of 1.393%. Moreover, the proposed aptasensor obtained a good recovery of OTA detection in red wine samples within the range of 93.14 to 112.75% along with a low LOD of 0.023 ng/mL, indicating its applicability for OTA detection in real samples along with economical, specific, susceptible, fast, easy, and transportable merits.

Recent Advances in Mycotoxin Analysis and Detection of Mycotoxigenic Fungi in Grapes and Derived Products

Mycotoxins are secondary metabolites of filamentous fungi that can cause toxic effects in human and animal health. Most of the filamentous fungi that produce these mycotoxins belong to four genera, namely, Aspergillus, Penicillium, Fusarium, and Alternaria. Mycotoxigenic fungi, along with mycotoxins, create a constant and serious economic threat for agriculture in many terms, counting product losses due to crop contamination and food spoilage, as well malnutrition when considering nutritional quality degradation. Given the importance of robust and precise diagnostics of mycotoxins and the related producing fungi in the grape food chain, one of the most important agricultural sectors worldwide, the present review initially delivers a comprehensive presentation of mycotoxin reports on grape and derived products, including a wide range of commodities such as fresh grapes, raisins, wine, juices, and other processed products. Next, based on worldwide regulations’ requirements for mycotoxins, and referring to the relative literature, this work presents methodological approaches for mycotoxin determination, and stresses major methods for the detection of fungal species responsible for mycotoxin production. The principle of function and basic technical background on the available analytical and molecular biology techniques developed—including chromatography, mass spectrometry, immunochemical-based assays, biosensors, and molecular assays—is briefly given, and references for their application to grape and derived product testing are highlighted.

Toehold-mediated DNA strand displacement-driven super-fast tripedal DNA walker for ultrasensitive and label-free electrochemical detection of ochratoxin A

DNA walkers, as intelligent artificial DNA nanomachines, have been widely used as efficient nucleic acid amplification tools that the detection sensitivity can be improved by incorporating DNA walkers into DNA biosensors. Nevertheless, since the premature release or flameout in a region of locally exhausted substrate, the walking efficiency of DNA walkers remains unsatisfactory. In this work, we design a smart tripedal DNA walker that is formed by target-initiated catalyzed hairpin assembly (CHA), which can move along the DNA duplex tracks on electrode driven by toehold-mediated DNA strand displacement (TMSD) for transduction and amplification of electrochemical signals. Emphatically, this flexible tripedal DNA walker is capable of walking freely along the tracks with unconstrained walking range. Moreover, the design of multi-legged walker can weaken the derailment of leg DNA and shorten the moving time on electrode, ensuring the processive walking with high efficiency. Additionally, the persistent walking of tripedal walker is driven by cascading TMSD, which eliminates the defects of high cost and instability of enzyme-assisted amplification technology. Therefore, the tripedal DNA walker-based electrochemical biosensor has enormous potential for the applications of OTA detection, and reveals a new avenue for food safety analysis and clinical diagnosis.

A competitive fluorescence assay based on free-complementary DNA for ochratoxin A detection

An ultrasensitive, rapid, and specific method for Ochratoxin A (OTA) detection was designed using complementary sequence to aptamer as a target of molecular beacon (MB). The designed loop structure of the MB has the same sequence as the aptamer with a complementary DNA (cDNA) which translates the level of the target into a measurable response. The presence of the target holds aptamer at the corresponding amount and the additional cDNAs are consumed by unbound aptamers which avails free cDNAs that resulting in fluorescence rising due to unfolding of MBs. Under the optimized conditions, the fluorescence intensity increased linearly with OTA concentration over the range of 10 pg mL^-1-1 µg mL^-1 with the detection limit of 0.247 pg mL^-1. The application of this assay in wheat sample in comparison with HPLC-MS/MS method, demonstrated that the new assay could be a potential sensing platform for OTA detection.

A signal-on electrochemical aptasensor based on silanized cellulose nanofibers for rapid point-of-use detection of ochratoxin A

An innovative ultrasensitive electrochemical aptamer-based sensor was developed for ochratoxin A (OTA) detection in cold brew coffee through revolutionary combination of nanofibers, electrochemical method, and aptamer technologies. The assembly of the aptasensor was based on the activation of silanized cellulose nanofibrous membranes as a supporting matrix for methylene blue (MB) redox probe-labeled aptamer tethering. Cellulose nanofibrous membranes were regenerated by deacetylating electrospun cellulose acetate nanofibrous membranes with deacetylation efficacy of 97%, followed by silanization of the nanofiber surfaces by using (3-aminopropyl)triethoxysilane (APTES). A replacement of conventionally casted membranes by the nanofibrous membranes increased the active surface area on the working electrode of a screen-printed three-electrode sensor by more than two times, consequently enhancing the fabricated aptasensor performance. The developed aptasensor demonstrated high sensitivity and specificity toward OTA in a range 0.002-2 ng mL^-1, with a detection limit of 0.81 pg mL^-1. Moreover, the assembled aptamer-based sensor successfully detected OTA in cold brew coffee samples without any pretreatment. The aptasensor exhibited good reusability and stability over long storage time. Graphical abstract.

Modification performance and electrochemical characteristics of different groups of modified aptamers applied for label-free electrochemical impedimetric sensors

Amino and thiolated aptamers are the main aptamers used to construct label-free electrochemical impedimetric aptasensors. In this study, the modification performance and electrochemical properties of amino aptamers and thiolated aptamers were studied in the construction of label-free impedimetric sensors. The results showed that the initial modification density of amino aptamers was higher than that of thiol aptamers. Aptamers can recognize and bind OTA to generate electrical signals. The higher the density of aptamer modification was, the better the electric signals were. If only considering the initial modification density, amino aptamers were more suitable for the preparation of aptasensors than thiolated aptamers. However, the modification density of the amino aptamer decreased with the prolonged immersion time in 1 mM HCl solution, which suggests that the stability of this sensor was poor. However, the thiolated aptamer maintained relatively constant density and could be reused. Thus, the thiolated aptasensor had a wide range and good reproducibility and stability for the determination of ochratoxin A (OTA). In addition, this study proved that gold nanoparticles play an important role in signal amplification by increasing the effective gold surface to fix more aptamers in the process of sensor preparation.

Recent advances in nanocomposite-based electrochemical aptasensors for the detection of toxins

Toxins are one of the major threatening factors to human and animal health, as well as economic growth. There is therefore an urgent demand from various communities to develop novel analytical methods for the sensitive detection of toxins in complex matrixes. Among the as-developed toxin detection strategies, nanocomposite-based aptamer sensors (termed as aptasensors) show tremendous potential for combating toxin pollution; in particular electrochemical (EC) aptasensors have received significant attention because of their unique advantages, including simplicity, rapidness, high sensitivity, low cost and suitability for field-testing. This paper reviewed the recently published approaches for the development of nanocomposite-/nanomaterial-based EC aptasensors for the detection of toxins with high assaying performance, and their potential applications in environmental monitoring, clinical diagnostics, and food safety control by summarizing the detection of different types of toxins, including fungal mycotoxins, algal toxins and bacterial enterotoxins. The effects of nanocomposite properties on the detection performance of EC aptasensors have been fully addressed for supplying readers with a comprehensive understanding of their improvement. The current technical challenges and future prospects of this subject have also been discussed.

Recent Advances in Ochratoxin A Electrochemical Biosensors: Recognition Elements, Sensitization Technologies, and Their Applications

Ochratoxin A (OTA) is a class of mycotoxin that are mainly produced by Aspergillus and Penicillium and widely found in plant origin food. OTA-contaminated foods can cause serious harm to animals and humans, while high stability of OTA makes it difficult to remove in conventional food processing. Thus, sensitive and rapid detection of OTA undoubtedly plays an important role in OTA prevention and control. In this paper, the conventional and novel methods of OTA at home and abroad are summarized and compared. The latest research progress and related applications of novel OTA electrochemical biosensors are mainly described with a new perspective. We innovatively divided the recognition element into single and combined recognition elements. Specifically, signal amplification technologies applied to the OTA electrochemical aptasensor are proposed. Furthermore, summary of the current limitations and future challenges in OTA analysis is included, which provide reference for the further research and applications.

Advances in Analysis and Detection of Major Mycotoxins in Foods

Mycotoxins are the most widely studied biological toxins, which contaminate foods at very low concentrations. This review describes the emerging extraction techniques and the current and alternatives analytical techniques and methods that have been used to successfully detect and identify important mycotoxins. Some of them have proven to be particularly effective in not only the detection of mycotoxins, but also in detecting mycotoxin-producing fungi. Chromatographic techniques such as high-performance liquid chromatography coupled with various detectors like fluorescence, diode array, UV, liquid chromatography coupled with mass spectrometry, and liquid chromatography-tandem mass spectrometry, have been powerful tools for analyzing and detecting major mycotoxins. Recent progress of the development of rapid immunoaffinity-based detection techniques such as immunoassays and biosensors, as well as emerging technologies like proteomic and genomic methods, molecular techniques, electronic nose, aggregation-induced emission dye, quantitative NMR and hyperspectral imaging for the detection of mycotoxins in foods, have also been presented.