Simply amplified electrochemical aptasensor of ochratoxin A based on exonuclease-catalyzed target recycling

A Calibration Strategy for Silicon Nanowire Field-Effect Transistor Biosensors and Its Application in Ultra-Sensitive, Label-Free Biosensing

The silicon nanowire field-effect transistor (SiNW FET) has been developed for over two decades as an ultrasensitive, label-free biosensor for biodetection. However, inconsistencies in manufacturing and surface functionalization at the nanoscale have led to poor sensor-to-sensor consistency in performance. Despite extensive efforts to address this issue through process improvements and calibration methods, the outcomes have not been satisfactory. Herein, based on the strong correlation between the saturation response of SiNW FET biosensors and both their feature size and surface functionalization, we propose a calibration strategy that combines the sensing principles of SiNW FET with the Langmuir-Freundlich model. By normalizing the response of the SiNW FET biosensors (ΔI/I0) with their saturation response (ΔI/I0)max, this strategy fundamentally overcomes the issues mentioned above. It has enabled label-free detection of nucleic acids, proteins, and exosomes within 5 min, achieving detection limits as low as attomoles and demonstrating a significant reduction in the coefficient of variation. Notably, the nucleic acid test results exhibit a strong correlation with the ultraviolet-visible (UV-vis) spectrophotometer measurements, with a correlation coefficient reaching 0.933. The proposed saturation response calibration strategy exhibits good universality and practicability in biological detection applications, providing theoretical and experimental support for the transition of mass-manufactured nanosensors from theoretical research to practical application.

Recent Advances in Recognition Receptors for Electrochemical Biosensing of Mycotoxins-A Review

Mycotoxins are naturally occurring toxic secondary metabolites produced by fungi in cereals and foodstuffs during the stages of cultivation and storage. Electrochemical biosensing has emerged as a rapid, efficient, and economical approach for the detection and quantification of mycotoxins in different sample media. An electrochemical biosensor consists of two main units, a recognition receptor and a signal transducer. Natural or artificial antibodies, aptamers, molecularly imprinted polymers (MIP), peptides, and DNAzymes have been extensively employed as selective recognition receptors for the electrochemical biosensing of mycotoxins. This article affords a detailed discussion of the recent advances and future prospects of various types of recognition receptors exploited in the electrochemical biosensing of mycotoxins.

Development of a Soluble KIT Electrochemical Aptasensor for Cancer Theranostics

An electrochemical sensor based on a conformation-changing aptamer is reported to detect soluble KIT, a cancer biomarker, in human serum. The sensor was fabricated with a ferrocene-labeled aptamer (Kd < 5 nM) conjugated to a gold electrode. Quantitative KIT detection was achieved using electrochemical impedance spectroscopy (EIS) and square-wave voltammetry (SWV). EIS was used to optimize experimental parameters such as the aptamer-to-spacer ratio, aptamer immobilization time, pH, and KIT incubation time, and the sensor surface was characterized using voltammetry. The assay specificity was demonstrated using interfering species and exhibited high specificity toward the target protein. The aptasensor showed a wide dynamic range, 10 pg/mL-100 ng/mL in buffer, with a 1.15 pg/mL limit of detection. The sensor also has a linear response to KIT spiked in human serum and successfully detected KIT in cancer-cell-conditioned media. The proposed aptasensor has applications as a continuous or intermittent approach for cancer therapy monitoring and diagnostics (theranostics).

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.

A potential reusable fluorescent aptasensor based on magnetic nanoparticles for ochratoxin A analysis

An aptasensor for the detection of ochratoxin A (OTA) in environmental samples was developed. It displayed high sensitivity and good selectivity. Factors such as specific binding between a FAM (5-carboxyfluorescein)-labeled aptamer (f-RP) and OTA, and a magnetic property of a streptavidin magbeads-modified capture probe (bm-CP) resulted in aptasensor’s linear relationship between fluorescence intensity and the concentration of OTA. This characteristic is present at the OTA concentration ranges from 0.100 μM to 25.00 μM with a LOD (limit of detection) of 0.0690 μM. The bm-CP can be reused through melting, washing and magnetic separation, which contributes to cost reduction. In addition, the proposed method is simple and detection process is fast. The aptasensor can be used in real samples.

Duplexed aptamers: history, design, theory, and application to biosensing

Nucleic acid aptamers are single stranded DNA or RNA sequences that specifically bind a cognate ligand. In addition to their widespread use as stand-alone affinity binding reagents in analytical chemistry, aptamers have been engineered into a variety of ligand-specific biosensors, termed aptasensors. One of the most common aptasensor formats is the duplexed aptamer (DA). As defined herein, DAs are aptasensors containing two nucleic acid elements coupled via Watson-Crick base pairing: (i) an aptamer sequence, which serves as a ligand-specific receptor, and (ii) an aptamer-complementary element (ACE), such as a short DNA oligonucleotide, which is designed to hybridize to the aptamer. The ACE competes with ligand binding, such that DAs generate a signal upon ligand-dependent ACE-aptamer dehybridization. DAs possess intrinsic advantages over other aptasensor designs. For example, DA biosensing designs generalize across DNA and RNA aptamers, DAs are compatible with many readout methods, and DAs are inherently tunable on the basis of nucleic acid hybridization. However, despite their utility and popularity, DAs have not been well defined in the literature, leading to confusion over the differences between DAs and other aptasensor formats. In this review, we introduce a framework for DAs based on ACEs, and use this framework to distinguish DAs from other aptasensor formats and to categorize cis- and trans-DA designs. We then explore the ligand binding dynamics and chemical properties that underpin DA systems, which fall under conformational selection and induced fit models, and which mirror classical SN1 and SN2 models of nucleophilic substitution reactions. We further review a variety of in vitro and in vivo applications of DAs in the chemical and biological sciences, including riboswitches and riboregulators. Finally, we present future directions of DAs as ligand-responsive nucleic acids. Owing to their tractability, versatility and ease of engineering, DA biosensors bear a great potential for the development of new applications and technologies in fields ranging from analytical chemistry and mechanistic modeling to medicine and synthetic biology.

Label-free and sensitive detection of Ochratoxin A based on dsDNA-templated copper nanoparticles and exonuclease-catalyzed target recycling amplification

Ochratoxin A (OTA) is one of the most toxic mycotoxins and exists in various food commodities. Herein, a sensitive fluorescence method was developed for OTA detection which combines the advantages of label-free dsDNA-templated copper nanoparticles (CuNPs), high selectivity of OTA aptamer and high efficiency of exonuclease-catalyzed target recycling amplification. OTA aptamer was hybridized with its complementary DNA (cDNA), and the obtained dsDNA acted as the template for fluorescent CuNPs. In the presence of its target (OTA), the aptamer prefers to form an OTA-aptamer complex in lieu of an aptamer-DNA duplex, which results in the dissociation of the aptamer-DNA duplex. The released cDNA and aptamer could be digested into mononucleotides by the RecJf exonuclease (single-stranded DNA specific exonuclease), and the target was liberated and could participate in the next reaction cycle. The above process resulted in the degradation of a large amount of template dsDNA, which prevented the synthesis of CuNPs, thus resulting in low fluorescence of the system. Based on this strategy, a label-free and sensitive detection of OTA was developed with a low detection limit of 5 ng mL-1 (S/N = 3). Our strategy was further validated and evaluated successfully by assaying OTA in real samples. The proposed assay has great potential as an OTA quantification method for use in the food safety field.

Graphene quantum dots-based electrochemiluminescence detection of DNA using multiple cycling amplification strategy

In this study, a novel strategy for amplified electrochemiluminescence (ECL) detection of DNA was proposed based on excellent ECL activity of graphene quantum dots (GQDs) coupled with multiple cycling amplification technique. A new type of graphene QDs with well ECL property and uniform size were firstly synthesized, then the graphene QDs were assembled on the electrode by poly (diallyldimethylammonium chloride) (PDDA)-graphene oxide (GO) nanocomposites, which could greatly improve ECL signal and stability of QDs. A novel signal-on ECL biosensor for DNA analysis was designed by using ECL quenching of gold nanoparticles (NPs) to graphene QDs combined with endonuclease-aided cyclic amplification strategy. As a result, the proposed strategy can be conveniently expanded to other biosensing application, especially in clinical diagnoses.

Recent trends in electrochemiluminescence aptasensors and their applications

Aptamers are single stranded DNA or RNA ligands which can be selected for different targets from proteins to small organic dyes. In the past few years great progress has been accomplished in the development of aptamer based bioanalytical assays with different detection techniques. Among them, electrochemiluminescence (ECL) aptasensors are very promising because they have the advantages of both electrochemical and chemiluminescence biosensors, such as high sensitivity, low background, cost effectiveness, and ease of control. In this review, we summarize the recent efforts to construct novel and improved ECL aptasensors and their application.

Antibody immobilization strategy for the development of a capacitive immunosensor detecting zearalenone

A highly sensitive flow-injection capacitive immunosensor was developed for detection of the mycotoxin zearalenone (ZEN). Different strategies for immobilization of an anti-ZEN antibody on the surface of a gold electrode, i.e. polytyramine or self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (3-MPA) and lipoic acid (LA), were used and their performances were compared. The LA- and 3-MPA-based systems showed broad linear ranges for ZEN determination, i.e. from 0.010 nM to 10 nM and from 0.020 nM to 10 nM, respectively. Under optimal conditions, the LA-based immunosensor was capable of performing up till 13 regeneration-interaction cycles (with use of glycine HCl, pH 2.4) with a limit of detection (LOD) of 0.0060 nM, equivalent to 1.9 pg mL^-1. It also demonstrated a good inter-assay precision (RSD < 10%). However, the tyramine-based capacitive immunosensor showed a bad repeatability (only 4 regeneration-interaction cycles were possible) and inter-assay precision (RSD > 15%) which did not allow sensitive and precise measurements. The LA-based method was compared with a direct ELISA. These results demonstrated that the label-free developed capacitive immunosensor had a better sensitivity and shorter analysis time in comparison with the direct microwell-plate format.

Insight into the Molecular Mechanisms of AuNP-Based Aptasensor for Colorimetric Detection: A Molecular Dynamics Approach

Colorimetric aptasensor based on assembly of salt-induced gold nanoparticles (AuNPs) is a promising biosensor. However, the molecular mechanism of the aptasensor is far from being fully understood. Herein, molecular dynamics (MD) simulation was used to investigate molecular interactions in the detection of ochratoxin A (OTA) including the following: (i) the molecular recognition of the anti-OTA aptamer, (ii) OTA-aptamer interactions in monovalent (Na⁺) and divalent (Mg²⁺) electrolytes, (iii) the binding mode of citrate on the AuNP surface, (iv) interactions of the aptamer with citrate-capped AuNPs, and (v) a detailed mechanism of the aptasensor. Our MD simulations revealed a specific binding of the OTA-aptamer complex, compared with OTB and warfarin. Compared with Na⁺, Mg²⁺ ions exerted a more effective attractive force between OTA and anti-OTA aptamer. Three different binding modes of citrate on AuNP surfaces were found. The kinetics of the adsorption of unfolded aptamers onto the citrate-capped AuNP was also elucidated. Most importantly, our MD simulation revealed an insightful analysis of the molecular mechanisms in the AuNP-based aptasensor and paved the way for the design of a novel colorimetric aptasensor for other target molecules, which is not limited to OTA detection.

The determination of Ochratoxin A based on the electrochemical aptasensor by carbon aerogels and methylene blue assisted signal amplification

In this work, a novel aptamer-based electrochemical biosensor was developed for the determination of Ochratoxin A (OTA) by using carbon aerogels (CAs) and methylene blue (MB) as signal amplification strategy. CAs was used as carrier to load the abundant of complementary DNA (cDNA), which could enhance the hybridization between CAs-cDNA and aptamer immobilized on the electrode surface, thus provide more double-stranded DNA for MB intercalation. The current of MB on the CAs-cDNA/apt/AuE sensor was twice that on the cDNA/apt/AuE sensor, which indicated that the CAs with high surface area enabled a higher loading of the cDNA and absorbed more MB, thus realized the signal amplification strategy. The optimum experimental conditions including MB incubation time of 15 min, aptamer concentration of 4.0 μmol/L, hybridization time of 2.0 h, and OTA incubation time of 18 min were obtained. The change of peak current was linearly proportional to the OTA concentration in the range of 0.10–10 ng/mL with the actual detection limit of 1.0 × 10⁻⁴ ng/mL. The experimental results showed that the prepared CAs-cDNA/apt/AuE exhibited good specificity, acceptable reproducibility and repeatability. This sensor was applied to detect OTA in the spiked corn samples, and obtained an acceptable average recovery of 89%.

Nucleic acid aptamer-based methods for diagnosis of infections

Infectious diseases are a serious global problem, which not only take an enormous human toll but also incur tremendous economic losses. In combating infectious diseases, rapid and accurate diagnostic tests are required for pathogen identification at the point of care (POC). In this review, investigations of diagnostic strategies for infectious diseases that are based on aptamers, especially nucleic acid aptamers, oligonucleotides that have high affinities and specificities toward their targets, are described. Owing to their unique features including low cost of production, easy chemical modification, high chemical stability, reproducibility, and low levels of immunogenicity and toxicity, aptamers have been widely utilized as bio-recognition elements (bio-receptors) for the development of infection diagnostic systems. We discuss nucleic acid aptamer-based methods that have been developed for diagnosis of infections using a format that organizes discussion according to the target pathogenic analytes including toxins or proteins, whole cells and nucleic acids. Also included is, a summary of recent advances made in the sensitive detection of pathogenic bacteria utilizing the isothermal nucleic acid amplification method. Lastly, a nucleic acid aptamer-based POC system is described and future directions of studies in this area are discussed.

Fluorescence method for quickly detecting ochratoxin A in flour and beer using nitrogen doped carbon dots and silver nanoparticles

In this paper, a FRET (Forster resonance energy transfer) based fluorescence method was developed for the quickly detection of ochratoxin A (OTA) in agricultural products (e.g., flour and beer). A highly fluorescent nitrogen doped carbon dots (CD) were served as energy donor, the DNA and MCH (6-mercapto-1-hexanol) modified Ag nanoparticles were served as energy acceptor in the FRET system. OTA can be detected in a concentration range between 10 and 5000 nM, the limit of detection is 8.7 nM. This method has three advantages: (1) an enhanced fluorescent intensity can be acquired by utilizing the nitrogen doped CD synthesized by one-step approach without sophisticated modification of nanoparticles; (2) OTA detection was accomplished quickly (less than 30 min) by using MCH as assistant molecule; (3) an extended OTA detection linear range was acquired, which may facilitate the OTA detection in real agricultural samples, and is helpful for solving food safety problems.

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.

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.

A highly sensitive and reusable electrochemical mercury biosensor based on tunable vertical single-walled carbon nanotubes and a target recycling strategy

Conformational regulation of SAMs was proposed for controlled growth of v-SWCNTs, which were employed to construct a high-performance mercury biosensor using a target recycling strategy.

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.

Mycotoxin Determination in Foods Using Advanced Sensors Based on Antibodies or Aptamers

Mycotoxin contamination threatens health and life of humans and animals throughout the food supply chains. Many of the mycotoxins have been proven to be carcinogens, teratogens and mutagens. The reliable and sensitive sensing methods are requested to monitor mycotoxin contamination. Advanced sensors based on antibodies or aptamers boast the advantages of high sensitivity and rapidity, and have been used in the mycotoxin sensing. These sensors are miniaturized, thereby lowering costs, and are applicable to high-throughput modes. In this work, the latest developments in sensing strategies for mycotoxin determination were critically discussed. Optical and electrochemical sensing modes were compared. The sensing methods for single mycotoxin or multiple mycotoxins in food samples were reviewed, along with the challenges and the future of antibody or aptamer-based sensors. This work might promote academic studies and industrial applications for mycotoxin sensing.

Nuclease-aided target recycling signal amplification strategy for ochratoxin A monitoring

Ochratoxin A (OTA), a toxin produced by Aspergillus ochraceus and Penicillium verrucosum, is one of the most abundant food-contaminating mycotoxins worldwide. OTA mainly exerts nephrotoxicity, immunotoxicity, mutagenicity, carcinogenicity, teratogenicity, and neurotoxicity. This paper describes a simple and sensitive aptamer/single-walled carbon nanohorn (SWCNH)-based assay for OTA detection. SWCNHs can protect DNA from DNase I cleavage. However, aptamers can be detached from the surface of SWCNHs through specific target binding, exposing them to enzymatic cleavage and releases the target for a new cycle. Cycling of targets leads to significant signal amplification and low limit of detection (LOD), resulting in a nearly 20-fold reduction in LOD for OTA assay compared with non-target recycling under the same experimental parameters. This technique responded specifically to OTA without interference from other analogues (Ochratoxin B, Ochratoxin C, warfarin, and N-acetyl-l-phenylalanine). Moreover, the application of this technique in real sample has been verified using red wine samples spiked with a series of OTA concentrations. This aptasensor offers a great practical importance in food safety and can be widely extended for detection of other toxins by replacing the sequence of the recognition aptamer.

An enzyme-free catalytic DNA circuit for amplified detection of aflatoxin B1 using gold nanoparticles as colorimetric indicators

An enzyme-free biosensor for the amplified detection of aflatoxin B1 has been constructed based on a catalytic DNA circuit. Three biotinylated hairpin DNA probes (H1, H2, and H3) were designed as the assembly components to construct the sensing system (triplex H1-H2-H3 product). Cascaded signal amplification capability was obtained through toehold-mediated strand displacement reactions to open the hairpins and recycle the trigger DNA. By the use of streptavidin-functionalized gold nanoparticles as the signal indicators, the colorimetric readout can be observed by the naked eye. In the presence of a target, the individual nanoparticles (red) aggregate into a cross-linked network of nanoparticles (blue) via biotin-streptavidin coupling. The colorimetric assay is ultrasensitive, enabling the visual detection of trace levels of aflatoxin B1 (AFB1) as low as 10 pM without instrumentation. The calculated limit of detection (LOD) is 2 pM in terms of 3 times standard deviation over the blank response. The sensor is robust and works even when challenged with complex sample matrices such as rice samples. Our sensing platform is simple and convenient in operation, requiring only the mixing of several solutions at room temperature to achieve visible and intuitive results, and holds great promise for the point-of-use monitoring of AFB1 in environmental and food samples.

Novel electrochemical sensing platform for quantitative monitoring of Hg(II) on DNA-assembled graphene oxide with target recycling

This work designs a new electrochemical sensing platform for the quantitative monitoring of mercury ion (Hg(2+)) on poly-T(15) oligonucleotide-functionalized graphene oxide by coupling with DNase I-assisted target recycling amplification. The assay was carried out on the basis of T-Hg(2+)-T coordination chemistry by using target-induced dissociation of indicator-labeled poly-T(15) oligonucleotide from graphene oxide nanosheets. The electronic signal was amplified through DNase I-triggered target recycling. Experimental results indicated that the amperometric response of DNA-based sensing platform deceased with the increasing Hg(2+) concentration in the sample, and has a detection limit of 0.12nM with a dynamic working range of up to 50nM. Our strategy afforded exquisite selectivity for Hg(2+) against other environmentally related metal ions. More significantly, this methodology displayed high reproducibility and acceptable accuracy, thus representing an optional sensing scheme for the screening of Hg(2+) in environmental water samples.

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.

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.

Comparison of Electrochemical Immunosensors and Aptasensors for Detection of Small Organic Molecules in Environment, Food Safety, Clinical and Public Security

We review here the most frequently reported targets among the electrochemical immunosensors and aptasensors: antibiotics, bisphenol A, cocaine, ochratoxin A and estradiol. In each case, the immobilization procedures are described as well as the transduction schemes and the limits of detection. It is shown that limits of detections are generally two to three orders of magnitude lower for immunosensors than for aptasensors, due to the highest affinities of antibodies. No significant progresses have been made to improve these affinities, but transduction schemes were improved instead, which lead to a regular improvement of the limit of detections corresponding to ca. five orders of magnitude over these last 10 years. These progresses depend on the target, however.

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.

Nitrogen-Doped Graphene Quantum Dots@SiO2 Nanoparticles as Electrochemiluminescence and Fluorescence Signal Indicators for Magnetically Controlled Aptasensor with Dual Detection Channels

We proposed a facile method to prepare the nitrogen-doped graphene quantum dots (NGQDs) doped silica (NGQDs@SiO2) nanoparticles (NPs). The NGQDs@SiO2 NPs were further explored as a versatile signal indicator for ochratoxin A (OTA) aptasensing by combination with electrochemiluminescence (ECL) and fluorescence (FL) detection. In this strategy, the core-shell Fe3O4@Au magnetic beads (MBs) acted as a nanocarrier to immobilize the thiolated aptamer specific for OTA, and the amino modified capture DNA (cDNA) was efficiently tagged with NGQDs@SiO2 NPs. The multifunctional aptasensor was thus fabricated by assembly of the NGQDs@SiO2 NPs onto the surface of Fe3O4@Au MBs through the high specific DNA hybridization between aptamer and cDNA. Upon OTA incubation, the aptamer linked with Fe3O4@Au MBs preferred to form an aptamer-OTA complex, which resulted in the partial release of the preloaded NGQDs@SiO2 NPs. The more OTA molecules in the detection system, the more NGQDs@SiO2 NPs were released into the bulk solution and the less preloaded NGQDs@SiO2 NPs were accumulated on the magnetic electrode surface. This provided a dual channel for OTA detection by combination with the enriched solid-state ECL and homogeneous FL detection. The FL assay exhibits a wide dynamic range and is more reproducible due to the homogeneous detection while the ECL assay possesses a lower detection limit and is preferable by using a cheaper instrument. One can obtain a preliminary screen from FL assay and a more accurate result from ECL assay. Integrating the virtues of dual analytical modality, this aptasensing strategy well-balanced the rapidity, sensitivity, and dynamic range, making it promising to other targets with aptamer sequences.

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.

Exonuclease-Catalyzed Target Recycling Amplification and Immobilization-free Electrochemical Aptasensor

A simple, sensitive, and selective immobilization-free electrochemical aptasensor had been developed which combines the advantages of the discrimination of the aggregation of long and short DNA on a negatively charged indium tin oxide (ITO) electrode, high selectivity of the aptamer, and high efficiency of exonuclease-catalyzed target recycling amplification. Ochratoxin A (OTA), a type of mycotoxin, has been chosen as the model target. Methylene blue (MB) labeled probe DNA had been hybridized with the OTA aptamer first, which cannot diffuse freely to the negative charged ITO electrode surface due to the repulsion of the negative charges, since the hybridized DNA contains large negative charges. In the presence of target (OTA), the aptamer prefers to form an OTA-aptamer complex in lieu of an aptamer-DNA duplex, which results in the dissociation of probe DNA from the probe DNA-aptamer complex. The released probe DNA could be digested into mononucleotides, including a MB-labeled electroactive mononucleotide (eT), due to the employment of the RecJf exonuclease, a single-stranded DNA specific exonuclease. Since the eT contains little negative charge, it can diffuse easily to the negative charged ITO electrode surface, which results in the enhanced electrochemical response detected. At the same time, the aptamer in the OTA-aptamer complex can be digested by RecJf exonuclease also to liberate the target, which can participate in the next reaction cycling and realize the electrochemical signal amplification. Based on this strategy, an ultrasensitive homogeneous immobilization-free electrochemical aptasensor for OTA can be developed with a low detection limit (LOD) of 0.004 ng mL(-1) (S/N = 3). The proposed biosensor combines the advantages of the simplicity of immobilization-free homogeneous ITO based electrochemical determination, high efficiency of exonuclease-catalyzed target recycling, and high selectivity of the aptamer. The fabricated biosensor has been applied to detect OTA in real samples with satisfactory results. The same strategy can be applied to develop biosensors for diverse targets.

Isothermal Amplification of Nucleic Acids

Isothermal amplification of nucleic acids is a simple process that rapidly and efficiently accumulates nucleic acid sequences at constant temperature. Since the early 1990s, various isothermal amplification techniques have been developed as alternatives to polymerase chain reaction (PCR). These isothermal amplification methods have been used for biosensing targets such as DNA, RNA, cells, proteins, small molecules, and ions. The applications of these techniques for in situ or intracellular bioimaging and sequencing have been amply demonstrated. Amplicons produced by isothermal amplification methods have also been utilized to construct versatile nucleic acid nanomaterials for promising applications in biomedicine, bioimaging, and biosensing. The integration of isothermal amplification into microsystems or portable devices improves nucleic acid-based on-site assays and confers high sensitivity. Single-cell and single-molecule analyses have also been implemented based on integrated microfluidic systems. In this review, we provide a comprehensive overview of the isothermal amplification of nucleic acids encompassing work published in the past two decades. First, different isothermal amplification techniques are classified into three types based on reaction kinetics. Then, we summarize the applications of isothermal amplification in bioanalysis, diagnostics, nanotechnology, materials science, and device integration. Finally, several challenges and perspectives in the field are discussed.

Highly sensitive impedimetric aptasensor based on covalent binding of gold nanoparticles on reduced graphene oxide with good dispersity and high density

A series of gold nanoparticles (AuNPs) that were covalently bound to 2-aminothiophenol-functionalized reduced graphene oxide (Au-ATP-rGO) composites have been synthesized with well-dispersed and controllable surface coverage of AuNPs. Aptamer immobilization capacity studies demonstrated that the surface density of AuNPs played a key role in increasing the amount of anchoring aptamers to enhance the sensitivity of affinity based detection. With the composites possessing dense surface coverage of AuNPs as a versatile signal amplified platform, a label-free aptasensor for the sensitive and selective detection of small molecules (ochratoxin A in this case) has been developed using electrochemical impedance spectroscopy (EIS). A wide linear range of 0.1-200 ng mL(-1) was obtained with a low detection limit of 0.03 ng mL(-1) (S/N = 3). This work provides a universal strategy for the sensitive detection of a variety of targets in a truly label-free manner by means of changing the corresponding aptamer. The promising platform based on the combination of Au-ATP-rGO composites, EIS technique, and aptamers would have great potential applications in clinical diagnosis, environmental analysis, and food safety monitoring.

Colorimetric Detection of Small Molecules in Complex Matrixes via Target-Mediated Growth of Aptamer-Functionalized Gold Nanoparticles

A versatile and sensitive colorimetric assay that allows the rapid detection of small-molecule targets using the naked eye is demonstrated. The working principle of the assay integrates aptamer-target recognition and the aptamer-controlled growth of gold nanoparticles (Au NPs). Aptamer-target interactions modulate the amount of aptamer strands adsorbed on the surface of aptamer-functionalized Au NPs via desorption of the aptamer strands when target molecules bind with the aptamer. Depending on the resulting aptamer coverage, Au NPs grow into morphologically varied nanostructures, which give rise to different colored solutions. Au NPs with low aptamer coverage grow into spherical NPs, which produce red-colored solutions, whereas Au NPs with high aptamer coverage grow into branched NPs, which produce blue-colored solutions. We achieved visible colorimetric response and nanomolar detection limits for the detection of ochratoxin A (1 nM) in red wine samples, as well as cocaine (1 nM) and 17β-estradiol (0.2 nM) in spiked synthetic urine and saliva, respectively. The detection limits were well within clinically and physiologically relevant ranges, and below the maximum food safety limits. The assay is highly sensitive, specific, and able to detect an array of analytes rapidly without requiring sophisticated equipment, making it relevant for many applications, such as high-throughput drug and clinical screening, food sampling, and diagnostics. Furthermore, the assay is easily adapted as a chip-based platform for rapid and portable target detection.

A structure-switchable aptasensor for aflatoxin B1 detection based on assembly of an aptamer/split DNAzyme

An ultrasensitive, colorimetric and homogeneous strategy for aflatoxin B1 (AFB1) detection, which uses a DNA aptamer and two split DNAzyme halves, has been developed. Split halves of a hemin-binding DNAzymes is combined with an AFB1 aptamer to generate a homogeneous colorimetric sensor that undergoes an AFB1 induced DNA structural change. In the absence of AFB1, the split probes have peroxidase mimicking DNAzyme activity associated with catalysis of a color change reaction. Specific recognition of AFB1 by the aptamer component leads to structural deformation of the aptamer-DNAzyme complex, which causes splitting of the DNAzyme halves and a reduction in peroxidase mimicking activity. Therefore, a decrease of colorimetric signal arising from the catalytic process takes place upon in the presence of AFB1 in a concentration dependent manner in the 0.1-1.0 × 10(4) ng/mL range and with a colorimetric detection limit of 0.1 ng/mL. The new assay system exhibits high selectivity for AFB1 over other mycotoxins and can be employed detect the presence of AFB1 in ground corn samples. Overall, the strategy should serve as the basis for the development of rapid, simple and low-cost methods for detection of mycotoxins.

Single-Stranded DNA Aptamers against Pathogens and Toxins: Identification and Biosensing Applications

Molecular recognition elements (MREs) can be short sequences of single-stranded DNA, RNA, small peptides, or antibody fragments. They can bind to user-defined targets with high affinity and specificity. There has been an increasing interest in the identification and application of nucleic acid molecular recognition elements, commonly known as aptamers, since they were first described in 1990 by the Gold and Szostak laboratories. A large number of target specific nucleic acids MREs and their applications are currently in the literature. This review first describes the general methodologies used in identifying single-stranded DNA (ssDNA) aptamers. It then summarizes advancements in the identification and biosensing application of ssDNA aptamers specific for bacteria, viruses, their associated molecules, and selected chemical toxins. Lastly, an overview of the basic principles of ssDNA aptamer-based biosensors is discussed.

Application of Europium Multiwalled Carbon Nanotubes as Novel Luminophores in an Electrochemiluminescent Aptasensor for Thrombin Using Multiple Amplification Strategies

A novel electrochemiluminescent (ECL) aptasensor was proposed for the determination of thrombin (TB) using exonuclease-catalyzed target recycling and hybridization chain reaction (HCR) to amplify the signal. The capture probe was immobilized on an Au-GS-modified electrode through a Au-S bond. Subsequently, the hybrid between the capture probe and the complementary thrombin binding aptamer (TBA) was aimed at obtaining double-stranded DNA (dsDNA). The interaction between TB and its aptamer led to the dissociation of dsDNA because TB has a higher affinity to TBA than the complementary strands. In the presence of exonuclease, aptamer was selectively digested and TB could be released for target recycling. Extended dsDNA was formed through HCR of the capture probe and two hairpin DNA strands (NH2-DNA1 and NH2-DNA1). Then, numerous europium multiwalled carbon nanotubes (Eu-MWCNTs) could be introduced through amidation reaction between NH2-terminated DNA strands and carboxyl groups on the Eu-MWCNTs, resulting in an increased ECL signal. The multiple amplification strategies, including the amplification of analyte recycling and HCR, and high ECL efficiency of Eu-MWCNTs lead to a wide linear range (1.0×10(-12)-5.0×10(-9) mol/L) and a low detection limit (0.23 pmol/L). The method was applied to serum sample analysis with satisfactory results.

Label-Free Luminescent Switch-On Probe for Ochratoxin A Detection Using a G-Quadruplex-Selective Iridium(III) Complex

A library of six luminescent Ir(III) complexes were synthesized and studied for their capacity to function as probes for G-quadruplex DNA. The novel Ir(III) complex 1 was discovered to be selective for G-quadruplex structures and was subsequently used for the construction of a label-free G-quadruplex-based ochratoxin A (OTA) sensing platform in aqueous solution. The assay exhibited linearity for OTA in the range of 0 to 60 nM (R2=0.9933), and the limit of detection for OTA was 5 nM. Furthermore, this assay was highly selective for OTA over its structurally related analogues.

Electrochemiluminescence biosensor for ultrasensitive determination of ochratoxin A in corn samples based on aptamer and hyperbranched rolling circle amplification

Determination of ochratoxin A (OTA) is highly important for food safety control. In this study, a signal-on electrochemiluminescence (ECL) biosensor which combined the characteristics of high efficiency of hyperbranched rolling circle amplification (HRCA) and high selectivity of aptamer was developed for OTA determination. The capture probe DNA (CDNA) was firstly immobilized on the gold electrode surface through Au-S interaction, then the OTA aptamer was modified on the electrode surface through hybridization with CDNA. Since OTA can competitively bind with the aptamer due to their high affinity, which would induce the releasing of aptamer from the electrode surface. Subsequently, the free CDNA on the electrode surface can hybridize with the padlock probe and induce HRCA reaction subsequently. Thus, the HRCA products which contained large amount of double-stranded DNA (dsDNA) fragments can be accumulated on the electrode surface. Since Ru(phen)3(2+) can intercalate into the groove of dsDNA and acts as ECL indicator, high ECL intensity can be detected from the electrode surface. The enhanced ECL intensity has a linear relationship with OTA in the range of 0.05-500 pg/mL with a correlation coefficient of 0.9957, and the limit of detection (LOD) was 0.02 pg/mL. The developed biosensor has been applied to determine OTA concentration in the corn samples with satisfied results.

Magnetic-fluorescent-targeting multifunctional aptasensorfor highly sensitive and one-step rapid detection of ochratoxin A

A multifunctional aptasensor for highly sensitive and one-step rapid detection of ochratoxin A (OTA), has been developed using aptamer-conjugated magnetic beads (MBs) as the recognition and concentration element and a heavy CdTe quantum dots (QDs) as the label. Initially, the thiolated aptamer was conjugated on the Fe3O4@Au MBs through Au-S covalent binding. Subsequently, multiple CdTe QDs were loaded both in and on a versatile SiO2 nanocarrier to produce a large amplification factor of hybrid fluorescent nanoparticles (HFNPs) labeled complementary DNA (cDNA). The magnetic-fluorescent-targeting multifunctional aptasensor was thus fabricated by immobilizing the HFNPs onto MBs' surface through the hybrid reaction between the aptamer and cDNA. This aptasensor can be produced at large scale in a single run, and then can be conveniently used for rapid detection of OTA through a one-step incubation procedure. The presence of OTA would trigger aptamer-OTA binding, resulting in the partial release of the HFNPs into bulk solution. After a simple magnetic separation, the supernatant liquid of the above solution contained a great number of CdTe QDs produced an intense fluorescence emission. Under the optimal conditions, the fluorescence intensity of the released HFNPs was proportional to the concentration of OTA in a wide range of 15 pg mL(-1) -100 ng mL(-1) with a detection limit of 5.4 pg mL(-1) (S/N=3). This multifunctional aptasensor represents a promising path toward routine quality control of food safety, and also creates the opportunity to develop aptasensors for other targets using this strategy.

An exonuclease-assisted amplification electrochemical aptasensor of thrombin coupling "signal on/off" strategy

In this work, a dual-signaling electrochemical aptasensor based on exonuclease-catalyzed target recycling was developed for thrombin detection. The proposed aptasensor coupled "signal-on" and "signal-off" strategies. As to the construction of the aptasensor, ferrocene (Fc) labeled thrombin binding aptamer (TBA) could perfectly hybridize with the methylene blue (MB) modified thiolated capture DNA to form double-stranded structure, hence emerged two different electrochemical signals. In the presence of thrombin, TBA could form a G-quadruplex structure with thrombin, leading to the dissociation of TBA from the duplex DNA and capture DNA formed hairpin structure. Exonuclease could selectively digest single-stranded TBA in G-quadruplex structure and released thrombin to realize target recycling. As a consequence, the electrochemical signal of MB enhanced significantly, which realized "signal on" strategy, meanwhile, the deoxidization peak current of Fc decreased distinctly, which realized "signal off" strategy. The employment of exonuclease and superposition of two signals significantly improved the sensitivity of the aptasensor. In this way, an aptasensor with high sensitivity, good stability and selectivity for quantitative detection of thrombin was constructed, which exhibited a good linear range from 5 pM to 50 nM with a detection limit of 0.9 pM (defined as S/N=3). In addition, this design strategy could be applied to the detection of other proteins and small molecules.

Label-free colorimetric aptasensor for sensitive detection of ochratoxin A utilizing hybridization chain reaction

The combination of high selectivity of aptamer with the peroxidase-mimicking property of DNAzyme has presented considerable opportunities for designing colorimetric aptasensor for detection of ochratoxin A (OTA). The activities of both aptamer (as biorecognition element) and DNAzyme (as signal amplification element) are blocked via base pairing in the hairpin structure. Hybridization chain reaction (HCR) between two hairpin DNAs was employed to further improve the sensitivity of this method. The presence of OTA triggers the opening of the hairpin structure and the beginning of HCR, which results in the release of many DNAzyme, and generates enhanced colorimetric signals, which is correlated to the amounts of OTA with linear range between 0.01 to 0.32 nM, and the limit of detection is 0.01 nM under optimal conditions. OTA in yellow rice wine and wheat flour samples was also detected using this method. We demonstrate that a new colorimetric method for the detection of OTA has been established, which is simple, easy to conduct, label-free, sensitive, high throughput, and cost-saving.