Sensitive quantitation of Ochratoxin A in cocoa beans using differential pulse voltammetry based aptasensor

Electrochemical Aptasensors for Food and Environmental Safeguarding: A Review

Food and environmental monitoring is one of the most important aspects of dealing with recent threats to human well-being and ecosystems. In this framework, electrochemical aptamer-based sensors are resilient due to their ability to resolve food and environmental contamination. An aptamer-based sensor is a compact analytical device combining an aptamer as the bio-sensing element integrated on the transducer surface. Aptamers display many advantages as biorecognition elements in sensor development when compared to affinity-based (antibodies) sensors. Aptasensors are small, chemically unchanging, and inexpensive. Moreover, they offer extraordinary elasticity and expediency in the design of their assemblies, which has led to innovative sensors that show tremendous sensitivity and selectivity. This review will emphasize recent food and environmental safeguarding using aptasensors; there are good prospects for their performance as a supplement to classical techniques.

Fabrication of magnetically assembled aptasensing device for label-free determination of aflatoxin B1 based on EIS

Aflatoxin B1 (AFB1), one of the most common mycotoxins in food matrixes, has been identified as the most toxic contaminant with mutagenic, teratogenic, immunosuppressive, and carcinogenic effects. In this work, a magnetically assembled aptasensing device has been designed for label-free determination of AFB1 by employing a disposable screen-printed carbon electrode (SPCE) covered with a designed polydimethylsiloxane (PDMS) film as the micro electrolytic cell. The magnetically controlled bio-probes were firstly prepared by immobilization of the thiolated aptamers on the Fe₃O₄@Au magnetic beads, which was rapidly assembled on the working electrode of SPCE within 10 s, by using a magnet placed at the opposite side. The PDMS film with a centered hole was covered on the SPCE surface to achieve a more practicable and flexible electrochemical measurement. In this effort, a label-free aptasensor for the sensitive and selective determination of AFB1 has been developed using electrochemical impedance spectroscopy upon the biorecognition between aptamers and the targets. The developed method had a wide linear range of 20 pg mL^-1-50 ng mL^-1 with a detection limit of 15 pg mL^-1 (S/N = 3) and succeeded in spiked samples of peanuts. The developed aptasensing device shows fantastic application prospect with simple design, easy operation, low cost, and high sensitivity and selectivity characteristics. This sensing strategy represents a promising path toward routine quality control of food safety and creates the opportunity to develop facile aptasensing device for other targets.

Homogeneous and label-free electrochemiluminescence aptasensor based on the difference of electrostatic interaction and exonuclease-assisted target recycling amplification

The difference of electrostatic interaction between free Ru(phen)₃²⁺ and Ru(phen)₃²⁺ embedded in double strand DNA (dsDNA) to the negatively charged indium tin oxide (ITO) electrode has been applied to develop a homogeneous and label-free electrochemiluminescence (ECL) aptasensor for the first time. Ochratoxin A (OTA) has been chosen as the model target. The OTA aptamer is first hybridized with its complementary single strand DNA (ssDNA) to form dsDNA and then interacted with Ru(phen)₃²⁺ via the grooves binding mode to form dsDNA-Ru(phen)₃²⁺ complex, which remains negatively charged feature as well as low diffusion capacity to the negatively charged ITO electrode surface owing to the electrostatic repulsion. Meanwhile, the intercalated Ru(phen)₃²⁺ in the grooves of dsDNA works as an ECL signal reporter instead of the labor-intensive labeling steps and can generate much more ECL signal than that from the labeling probe. In the presence of target, the aptamer prefers to form an aptamer-target complex in lieu of dsDNA, which induces the releasing of Ru(phen)₃²⁺ from the dsDNA-Ru(phen)₃²⁺ complex into the solution. With the assistance of RecJ_f exonuclease (a ssDNA specific exonuclease), the released ssDNA and the aptamer in the target-complex were digested into mononucleotides. In the meantime, the target can be also liberated from OTA-aptamer complex and induce target cycling and large amount of free Ru(phen)₃²⁺ present in the solution. Since Ru(phen)₃²⁺ contains positive charges, which can diffuses easily to the ITO electrode surface because of electrostatic attraction, causing an obviously enhanced ECL signal detected. Under the optimal conditions, the enhanced ECL of the system has a linear relationship with the OTA concentration in the range of 0.01-1.0 ng/mL with a detection limit of 2 pg/mL. This innovative system not only expands the immobilization-free sensors in the electrochemiluminescent fields, but also can be developed for the detection of different targets easily with the same strategy by changing the aptamer used.

A Lateral Flow Strip Based Aptasensor for Detection of Ochratoxin A in Corn Samples

Ochratoxin A (OTA) is a mycotoxin identified as a contaminant in grains and wine throughout the world, and convenient, rapid and sensitive detection methods for OTA have been a long-felt need for food safety monitoring. Herein, we presented a new competitive format based lateral flow strip fluorescent aptasensor for one-step determination of OTA in corn samples. Briefly, biotin-cDNA was immobilized on the surface of a nitrocellulose filter on the test line. Without OTA, Cy5-labeled aptamer combined with complementary strands formed a stable double helix. In the presence of OTA, however, the Cy5-aptamer/OTA complexes were generated, and therefore less free aptamer was captured in the test zone, leading to an obvious decrease in fluorescent signals on the test line. The test strip showed an excellent linear relationship in the range from 1 ng·mL⁻¹ to 1000 ng·mL⁻¹ with the LOD of 0.40 ng·mL⁻¹, IC₁₅ value of 3.46 ng·mL⁻¹ and recoveries from 96.4% to 104.67% in spiked corn samples. Thus, the strip sensor developed in this study is an acceptable alternative for rapid detection of the OTA level in grain samples.

Ensuring food safety using aptamer based assays: Electroanalytical approach

Aptamers, are being increasingly employed as favorable receptors for constructing highly sensitive biosensors, for their remarkable affinities towards certain targets including a wide scope of biological or chemical substances, and their superiority over other biologic receptors. The selectivity and affinity of the aptamers have been integrated with the wise design of the assay, applying suitable modifications, such as nanomaterials on the electrode surface, employing oligonucleotide-specific amplification strategies or, their combinations. After successful performance of the electrochemical aptasensors for biomedical applications, the food sector with its direct implication for human health, which demands rapid and sensitive and economic analytical solutions for determination of health threatening contaminants in all stages of production process, is the next field of research for developing efficient electrochemical aptasensors. The aim of this review is to categorize and introduce food hazards and summarize the recent electrochemical aptasensors that have been developed to address these contaminants.

Fluorescence resonance energy transfer aptasensor between nanoceria and graphene quantum dots for the determination of ochratoxin A

In the present work, colloidal cerium oxide nanoparticles (nanoceria) and graphene quantum dots (GQDs) were firstly synthesized by sol-gel method and pyrolysis respectively, which all have a uniform nano-size and significant fluorescence emission. Due to the fluorescence emission spectrum of nanoceria overlapped the absorption spectrum of GQDs, fluorescence resonance energy transfer (FRET) between nanoceria and GQDs could occur effectively by the electrostatic interaction. Based on it, a sensitive ratiometric fluorescence aptasensor for the determination of ochratoxin A (OTA), a small molecular mycotoxin produced by Aspergillus and Penicillium strains, has been successfully constructed. In which, probe DNA1@nanoceria and DNA2@GQD were designed to complementary with OTA aptamer, both could adsorb each other, leading to the occur of FRET. After adding of OTA aptamer and then introducing of OTA, the FRET would be interrupted/recovered due to the specific affinity of OTA and its aptamer, the fluorescence recovery value would increase with the addition of OTA. Under the optimal experimental conditions (pH 7, m_{GQD/nanoceria} 2, c_aptamer 100 nM, incubation time 30 min), the constructed ratiometric fluorescence aptasensor exhibited a satisfying linear range (0.01-20 ng mL^-1), low limit of detection (2.5 pg mL^-1) and good selectivity towards OTA, and has been successfully applied for the analysis of real sample peanuts with good accuracy of the recoveries ranged from 90 to 110%.

Electrochemical aptasensors for contaminants detection in food and environment: Recent advances

The growing number of contaminants requires the development of new analytical tools to meet the increasing demand for legislative actions on food safety and environmental pollution control. In this context, electrochemical aptamer-based sensors appear promising among all biosensors because they permit multiplexed analysis and provide fast response, sensitivity, specificity and low cost. The aim of this review is to give the readers an overview of recent important achievements in the development of electrochemical aptamer-based biosensors for contaminant detection over the last two years. Special emphasis is placed on aptasensors based on screen-printed electrodes which show a substantial improvement of analytical performances.

Ultrasensitive detection of ochratoxin A using aptasensors

Regarding teratogenic, carcinogenic, and immunotoxic nature of ochratoxin A (OTA), selective and sensitive monitoring of this molecule in food samples is of great importance. In recent years, various methods have been introduced for detection of OTA. However, they are usually time-consuming, labor-intensive and expensive. Therefore, these parameters limited their usage. The emerging method of detection, aptasensor, has attracted more attention for OTA detection, due to distinctive advantages including high sensitivity, selectivity and simplicity. In this review, the new developed aptasensors for detection of OTA have been investigated. We also highlighted advantages and disadvantages of different types of OTA aptasensors. This review also takes into consideration the goal to find out which designs are the most rational ones for highly sensitive detection of OTA.

Tuning the Aggregation/Disaggregation Behavior of Graphene Quantum Dots by Structure-Switching Aptamer for High-Sensitivity Fluorescent Ochratoxin A Sensor

The design of graphene quantum dots (GQDs)-aptamer bioconjugates as the new sensing platform is very important for developing high-sensitivity fluorescent biosensors; however, achieving new bioconjugates is still a great challenge. Herein, we report the development of a new high-sensitivity fluorescent aptasensor for the detection of ochratoxin A (OTA) based on tuning aggregation/disaggregation behavior of GQDs by structure-switching aptamers. The fluorescence sensing process for OTA detection involved two key steps: (1) cDNA-aptamer (cDNA, complementary to part of the OTA aptamer) hybridization induced the aggregation of GQD (fluorescence quenching) after cDNA was added into the GQDs-aptamer bioconjugate solution, and (2) the target of OTA triggered disaggregation of GQD aggregates (fluorescence recovery). Such new fluorescent sensing platform can be used to monitor OTA with a linear range of 0 to 1 ng/mL and very low detection limit of 13 pg/mL, which is among the best in all the developed fluorescent nanoparticles-based sensors. Such sensing strategy is also successful in analyzing OTA in practical red wine sample with 94.4-102.7% of recoveries and relative standard deviation in the range of 2.9-5.8%. The present works open a new way for signaling the target-aptamer binding event by tuning aggregation/disaggregation behavior of GQDs-bioconjugates.

A novel impedimetric aptasensor based on AuNPs–carboxylic porous carbon for the ultrasensitive detection of ochratoxin A

A novel impedimetric aptasensor based on AuNPs–carboxylic porous carbon for the ultrasensitive detection of OTA.

Magneto-controlled aptasensor for simultaneous electrochemical detection of dual mycotoxins in maize using metal sulfide quantum dots coated silica as labels

Currently there is an urgent need for multi-mycotoxin detection methods due to the co-occurrence of multiple mycotoxins in food raw materials and their augmented toxicity. Herein, a magneto-controlled aptasensor has been developed for simultaneous electrochemical detection of ochratoxin A (OTA) and fumonisin B1 (FB1), two typical mycotoxins found in food crops world-wide. This aptasensor was designed using the high specificity between the target and aptamer with heavy CdTe or PbS quantum dots (QDs) coated silica as labels and the complementary DNA functionalized magnetic beads as capture probes. In presence of targets, the aptamer preferred to form the target-aptamer binding which forced the partial release of the preloaded labels from the magnetic beads. After a one-step incubation and a simple magnetic separation, the electrochemical signals of Cd²⁺ and Pb²⁺ dissolved from the reserved labels which had negative correlation with targets contents, was measured based on the difference of peak potentials. This aptasensor provided a wide detection range of 10pgmL^-1 to 10ngmL^-1 for OTA and 50pgmL^-1 to 50ngmL^-1 for FB1, and succeeded in real maize samples. This method provides a new avenue for high throughput screen of mycotoxins due to the advantages of simple instrument, low sample consumption, short assay times, and lower detection costs per assay. Moreover, it could be readily expanded for the simultaneous detection of a large panel of mycotoxins by using different metal sulfide QDs when their specific aptamers are available.

A fluorescent aptasensor based on a DNA pyramid nanostructure for ultrasensitive detection of ochratoxin A

Analytical techniques for detection of ochratoxin A (OTA) in food products and blood serum are of great significance. In this study, a fluorescent aptasensor was developed for sensitive and specific detection of OTA, based on a DNA pyramid nanostructure (DPN) and PicoGreen (PG) dye. The designed aptasensor inherits characteristics of DPN, such as high stability and capacity for PG loading. PG, as a fluorescent dye, could bind to double-stranded DNA (dsDNA). In the absence of OTA, the pyramid structure of DPN remains intact, leading to a very strong fluorescence emission. Because of higher affinity of aptamer for its target relative to its complementary strand, upon addition of target, the pyramid structure of DPN is disassembled, leading to a weak fluorescence emission. The presented aptasensor showed high specificity toward OTA with a limit of detection (LOD) as low as 0.135 nM. Besides, the designed sensing strategy was successfully utilized to recognize OTA in serum and grape juice with LODs of 0.184 and 0.149 nM, respectively.

Sensitive analytical performance of folding based biosensor using methylene blue tagged aptamers

This work demonstrates the development of a folding based electrochemical aptasensor using methylene blue (MB) tagged anti-Ochratoxin A (OTA) aptamers. Different aptamer coupling strategies were tested using Hexamethylenediamine, polyethylene glycol, simple adsorption and diazonium coupling mechanism. The best sensitivity was recorded by oxidation of amines using hexamethylenediamine (HDMA) on screen printed carbon electrode (SPCE). To achieve the direct detection of OTA, aptamer conjugated redox probe was used and detection was demonstrated based on the conformational changes in aptamer structure upon OTA sensing. Signaling in this class of sensors arises from changes in electron transfer efficiency upon target-induced changes in the conformation/flexibility of the aptamer probe. These changes can be readily recorded electrochemically. The developed aptasensor is unique in its own mechanism as redox probe tagged aptamer coupling such as MB has never been tried to immobilize using long carbon chain spacers as, addition of spacers would provide more sensitive detection methods. A good dynamic range 0.01-5ng/ml was obtained for OTA with Limit of detection (LOD) 0.01ng/ml and Limit of quantification (LOQ) of 0.03ng/ml respectively. The good reproducibility was recorded with RSD% of 3.75. The obtained straight line equation was y=0.4035x+0.90311, r=0.9976. We believe that the sensor design guidelines outlined here represents a general strategy for developing new folding-based electrochemical aptasensors. The developed aptasensor was extended to screen cocoa samples for OTA contamination. The cocoa samples were extracted and purified using molecular imprinted polymer (MIP) columns. The aptasensor displayed good recovery values in the range 84-85% thus, exhibited the effectiveness of proposed aptasensor for such complex matrices.

A novel fluorescent aptasensor based on gold and silica nanoparticles for the ultrasensitive detection of ochratoxin A

Analytical approaches for the detection and quantitation of ochratoxin A (OTA) in blood serum and food products are high in demand. In this study, a fluorescent aptamer-based sensor (aptasensor) is developed for the selective and sensitive detection of OTA, based on a complementary strand of aptamer (CS) and two types of nanoparticles, gold nanoparticles (AuNPs) and silica nanoparticles (SNPs) coated with streptavidin. The fabricated aptasensor inherits the characteristics of SNPs, as enhancers of fluorescence intensity; AuNPs, such as large surface area and unique optical properties; and high affinity of the aptamer toward its target compared to its CS. In the absence of OTA, no FAM and biotin-labeled CS is in the environment of the SNPs coated with streptavidin, which leads to no fluorescence emission. In the presence of the target, an FAM and biotin-labeled CS-SNPs coated with streptavidin conjugate is formed, thus resulting in a very strong fluorescence emission. The designed fluorescent aptasensor exhibits high selectivity toward OTA with a limit of detection (LOD) as low as 0.098 nM. Furthermore, the fabricated aptasensor was successfully applied for the detection of OTA in grape juice and serum with LODs of 0.113 and 0.152 nM, respectively.

Homogeneous electrochemical detection of ochratoxin A in foodstuff using aptamer-graphene oxide nanosheets and DNase I-based target recycling reaction

A simple and feasible homogeneous electrochemical sensing protocol was developed for the detection of ochratoxin A (OTA) in foodstuff on the immobilization-free aptamer-graphene oxide nanosheets coupling with DNase I-based cycling signal amplification. Thionine-labeled OTA aptamers were attached to the surface of nanosheets because of the strong noncovalent binding of graphene oxide nanosheets with nucleobases and aromatic compounds. The electronic signal was acquired via negatively charged screen-printed carbon electrode (SPCE) toward free thionine molecules. Initially, the formed thionine-aptamer/graphene nanocomposites were suspended in the detection solution and far away from the electrode, thereby resulting in a weak electronic signal. Upon addition of target OTA, the analyte reacted with the aptamer and caused the dissociation of thionine-aptamer from the graphene oxide nanosheets. The newly formed thionine-aptamer/OTA could be readily cleaved by DNase I and released target OTA, which could retrigger thionine-aptamer/graphene nanocomposites with target recycling to generate numerous free thionine molecules. Free thionine molecules were captured by negatively charged SPCE, each of which could produce an electrochemical signal within the applied potentials. Under optimal conditions, graphene-based aptasensing platform could exhibit good electrochemical responses for the detection of OTA at a concentration as low as 5.6pg/mL. The reproducibility, precision and selectivity of the system were acceptable. Importantly, the method accuracy was comparable with commercialized OTA ELISA kit when using for quantitative monitoring of contaminated wheat samples.

Highly Sensitive Colorimetric Detection of Ochratoxin A by a Label-Free Aptamer and Gold Nanoparticles

A label-free aptamer-based assay for the highly sensitive and specific detection of Ochratoxin A (OTA) was developed using a cationic polymer and gold nanoparticles (AuNPs). The OTA aptamer was used as a recognition element for the colorimetric detection of OTA based on the aggregation of AuNPs by the cationic polymer. By spectroscopic quantitative analysis, the colorimetric assay could detect OTA down to 0.009 ng/mL with high selectivity in the presence of other interfering toxins. This study offers a new alternative in visual detection methods that is rapid and sensitive for OTA detection.

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.

Colorimetric aptasensing of ochratoxin A using Au@Fe3O4 nanoparticles as signal indicator and magnetic separator

Gold nanoparticles (Au NPs) doped Fe3O4 (Au@Fe3O4) NPs have been synthesized by a facile one-step solvothermal method. The peroxidase-like activity of Au@Fe3O4 NPs was effectively enhanced due to the synergistic effect between the Fe3O4 NPs and Au NPs. On this basis, an efficient colorimetric aptasensor has been developed using the intrinsic dual functionality of the Au@Fe3O4 NPs as signal indicator and magnetic separator. Initially, the amino-modified aptamer specific for a typical mycotoxin, ochratoxin A (OTA), was surface confined on the amino-terminated glass beads surafce using glutaraldehyde as a linker. Subsequently, the amino-modified capture DNA (cDNA) was labeled with the amino-functionalized Au@Fe3O4 NPs and the aptasensor was thus fabricated through the hybridization reaction between cDNA and the aptamers. While upon OTA addition, aptamers preferred to form the OTA-aptamer complex and the Au@Fe3O4 NPs linked on the cDNA were released into the bulk solution. Through a simple magnetic separation, the collected Au@Fe3O4 NPs can produce a blue colored solution in the presence of 3,3',5,5'-tetramethylbenzidine and H2O2. When the reaction was terminated by addition of H(+) ions, the blue product could be changed into a yellow one with higher absorption intensity. This colorimetric aptasensor can detect as low as 30 pgmL(-1) OTA with high specificity. To the best of our knowledge, the present colorimetric aptasensor is the first attempt to use the peroxidase-like activity of nanomaterial for OTA detection, which may provide an acttractive path toward routine quality control of food safety.