Enzyme-free amplified and ultrafast detection of aflatoxin B1 using dual-terminal proximity aptamer probes

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.

An accurate and ultrasensitive ratiometric electrochemical aptasensor for determination of Ochratoxin A based on catalytic hairpin assembly

Ochratoxin A (OTA) pollution in agricultural products has raised the pressing to develop sensitive, accurate and convenient detection methods. Herein, an accurate and ultrasensitive ratiometric electrochemical aptasensor was proposed based on catalytic hairpin assembly (CHA) for OTA detection. In this strategy, the target recognition and CHA reaction were both accomplished in the same system, which avoided tedious multi-steps operation and extra reagents, providing the advantage of convenience with only a one-step reaction and without enzyme. The labeled Fc and MB were used as the signal-switching molecules, avoiding various interferences and greatly improving the reproducibility (RSD: 3.197%). This aptasensor achieved trace-level detection for OTA with LOD of 81 fg/mL in the linear range of lower concentration (100 fg/mL-50 ng/mL). Moreover, this strategy was successfully applied to OTA detection in cereals with comparable results of HPLC-MS. This aptasensor provided a viable platform for accurate, ultrasensitive, and one-step detection of OTA in food.

Switchable aptamer-fueled colorimetric sensing toward agricultural fipronil exposure sensitized with affiliative metal-organic framework

Persistent usage of pesticides in agriculture has posed serious damage to overall ecosystem and human health, and thereby it is imperative to develop sensitive and efficient tools to evaluate residual pesticides in food and environmental setting. Herein, we reported a switchable colorimetric probe toward fipronil residue sensitized with aptamer-fueled catalytic activity of affiliative ZIF-8. Innovatively, it was found that the attached aptamer preferred to adsorb 3,3',5,5'-tetramethylbenzidine (TMB) rather than 2,2-azinobis (3-ethylbenzothiazo-line-6-sulfonic acid) (ABTS), greatly promoting catalytic oxidation of ZIF-8 toward TMB for further improving sensitivity. Aiding with smartphone-based image acquisition, fipronil-responsive discoloration degree was converted into the ratio of green and blue (G/B) with limit of detection as low as 0.036 μM (0.016 μg·g^-1). Moreover, it allowed for fipronil analysis in water, soil and vegetable samples with good recovery between 87 % and 110 %, verifying extension application prospect of the aptamer-fueled colorimetry for on-field pesticide evaluation in food safety supervision.

Complementary DNA Significantly Enhancing Signal Response and Sensitivity of a Molecular Beacon Probe to Aflatoxin B1

This paper reported an improved molecular beacon method for the rapid detection of aflatoxin B1 (AFB1), a natural mycotoxin with severe carcinogenicity. With the assistance of a complementary DNA (cDNA) chain, the molecular beacon which consists of a DNA aptamer flanked by FAM and BHQ1 displayed a larger fluorescent response to AFB1, contributing to the sensitive detection of AFB1. Upon optimization of some key experimental factors, rapid detection of AFB1 ranging from 1 nM to 3 μM, within 20 min, was realized by using this method. A limit of detection (LoD) of 1 nM was obtained, which was lower than the LoD (8 nM) obtained without cDNA assistance. This aptamer-based molecular beacon detection method showed advantages in easy operation, rapid analysis and larger signal response. Good specificity and anti-interference ability were demonstrated. This method showed potential in real-sample analysis.

Structural insights into the AFB1 aptamer coupled with a rationally designed CRISPR/Cas12a-Exo III aptasensor for AFB1 detection

Aflatoxin B₁ (AFB₁) is a typical food contaminant. A truncated DNA aptamer of AFB₁ was reported by our team in previous work. However, the recognition mechanism between aptamer and AFB₁ was lacking, which was crucial for the design of related aptasensor. Herein, the binding of aptamer to AFB₁ was systematically studied and found that it was an exothermic process and the conformation of aptamer changed during the recognition process. Loop bases in the secondary structure of aptamer formed a special binding pocket to recognize AFB₁. Van der Waals and electrostatic interaction were the main driving forces. By blocking the stem bases guided by the structural investigation, a rationally designed CRISPR/Cas12a-Exo III aptasensor for AFB₁ detection was constructed, and the sensitivity was improved by target recycling. Under optimal conditions, the linear detection range for AFB₁ was 0.01-20 ng/mL, and AFB₁ was accurately determined in corn and wheat samples. This work laid a theoretical foundation for the design of AFB₁ aptasensor, and the developed detection model came up with new ideas for the development of CRISPR/Cas12a-based aptasensor.

Advances in functional photonic crystal materials for the analysis of chemical hazards in food

Chemical contaminants in food generally include natural toxins (mycotoxins, animal toxins, and phytotoxins), pesticides, veterinary drugs, environmental pollutants, heavy metals, and illegal additives. Developing a low-cost, simple, and rapid detection technology for harmful substances in food is urgently needed. Analytical methods based on different advanced materials have been developed into rapid detection methods for food samples. In particular, photonic crystal (PC) materials have a unique surface periodic structure, structural color, a large surface area, easy integration with photoelectronic and magnetic devices which have great advantages in the development of rapid, low-cost, and highly sensitive analytical methods. This review focuses on the PC materials in the view of their fabrication processes, functionalized recognition components for the specific recognition of hazardous substances, and applications in the separation, enrichment, and detection of chemical hazards in real samples. Suspension array based on three-dimensional PC microspheres by droplet-based microfluidic assembly is a great promising and powerful platform for food safety detection fields. For the PCs selective analysis, biological antibodies, aptamers, and molecularly imprinted polymers (MIPs) could be modified for specific recognition of target substances, particularly MIPs because of their low-cost and easy mass production. Based on these functional PCs, various toxic and hazardous substances can be selectively enriched or recognized in real samples and further quantified in combination of liquid chromatography method or optical detection methods including fluorescence, chemiluminescence, and Raman spectroscopy.

An Alkyne-Mediated SERS Aptasensor for Anti-Interference Ochratoxin A Detection in Real Samples

Avoiding interference and realizing the precise detection of mycotoxins in complex food samples is still an urgent problem for surface-enhanced Raman spectroscopy (SERS) analysis technology. Herein, a highly sensitive and specific aptasensor was developed for the anti-interference detection of Ochratoxin A (OTA). In this aptasensor, 4-[(Trimethylsilyl) ethynyl] aniline was employed as an anti-interference Raman reporter to prove a sharp Raman peak (1998 cm-1) in silent region, which could avoid the interference of food bio-molecules in 600-1800 cm-1. 4-TEAE and OTA-aptamer were assembled on Au NPs to serve as anti-interference SERS probes. Meanwhile, Fe3O4 NPs, linked with complementary aptamer (cApts), were applied as capture probes. The specific binding of OTA to aptamer hindered the complementary binding of aptamer and cApt, which inhibited the binding of SERS probes and capture probes. Hence, the Raman responses at 1998 cm-1 were negatively correlated with the OTA level. Under the optimum condition, the aptasensor presented a linear response for OTA detection in the range of 0.1-40 nM, with low detection limits of 30 pM. In addition, the aptasensor was successfully applied to quantify OTA in soybean, grape and milk samples. Accordingly, this anti-interference aptasensor could perform specific, sensitive and precise detection of OTA in real samples, and proved a reliable reference strategy for other small-molecules detection in food samples.

Recent Progress on Techniques in the Detection of Aflatoxin B1 in Edible Oil: A Mini Review

Contamination of agricultural products and foods by aflatoxin B1 (AFB1) is becoming a serious global problem, and the presence of AFB1 in edible oil is frequent and has become inevitable, especially in underdeveloped countries and regions. As AFB1 results from a possible degradation of aflatoxins and the interaction of the resulting toxic compound with food components, it could cause chronic disease or severe cancers, increasing morbidity and mortality. Therefore, rapid and reliable detection methods are essential for checking AFB1 occurrence in foodstuffs to ensure food safety. Recently, new biosensor technologies have become a research hotspot due to their characteristics of speed and accuracy. This review describes various technologies such as chromatographic and spectroscopic techniques, ELISA techniques, and biosensing techniques, along with their advantages and weaknesses, for AFB1 control in edible oil and provides new insight into AFB1 detection for future work. Although compared with other technologies, biosensor technology involves the cross integration of multiple technologies, such as spectral technology and new nano materials, and has great potential, some challenges regarding their stability, cost, etc., need further studies.

Development of a Lateral Flow Strip with a Positive Readout for the On-Site Detection of Aflatoxin B1

Aflatoxin B₁ is one of the contamination indicators for food safety monitoring. The rapid and effective assessment and determination of AFB₁ in food is of great importance to dietary safety. The lateral flow assay shows advantages in its simplicity, and rapidity, and provides a visual readout, while the available lateral flow assay for AFB₁ requires a competitive format that produces readings inversely proportional to the AFB₁ concentration, which is counterintuitive and may lead to a potential misinterpretation of the results. Herein, we developed a positive readout aptamer-based lateral flow strip (Apt-strip) for the detection of AFB₁. This Apt-strip relies on the competition between AFB₁ and fluorescein-labeled complementary DNA strands (FAM-cDNA) for affinity binding to limited aptamers against AFB₁ (AFB₁-Apt). In the absence of AFB₁, AFB₁-Apt hybridizes with FAM-cDNA. No signal at the T-line of the Apt-strip was observed. In contrast, AFB₁-Apt binds to AFB₁ in the sample, and then a part of the FAM-cDNA is hybridized with the free AFB₁-Apt, at which time the other unreacted FAM-cDNA is captured by A35-Apt on the T-line. The signal was observed. This method achieved fast detection of AFB₁ with a detection limit (DL) of 0.1 ng/mL, positive readout, and increased sensitivity.

A versatile Y shaped DNA nanostructure for simple, rapid and one-step detection of mycotoxins

A versatile Y shaped DNA nanostructure has been developed for simple, rapid and one-step simultaneous detection aflatoxin B1 (AFB1) and ochratoxin A (OTA). Y shaped duplex DNA arms was formed with two DNA tweezer at the ends. The aptamer sequence at the third end can bind to its target mycotoxins with strong affinity and then release the two DNA fragments. The released DNA fragments can open the DNA tweezers at the ends of Y shaped DNA arm. The amounts of AFB1 and OTA can be quantitative detection through the recovery of the fluorescent intensities. This strategy is simple and rapid with self-powered DNA hybridization reaction to control the "open" of Y shaped DNA tweezers. Furthermore, it can be finished in 60 min with only one-step of operation. The linear range of AFB1 was from 0.5 to 200 ng/mL (R² = 0.995) and linear relationship of OTA was obtained from 4 to 300 ng/mL (R² = 0.990). It also has been successfully applied for mycotoxins detection in real food samples. Importantly, the target mycotoxins can be extended to others by simply replacing the corresponding aptamer sequences.

A Simple Structure-Switch Aptasensor Using Label-Free Aptamer for Fluorescence Detection of Aflatoxin B1

Aflatoxin B1 (AFB1) is one of the mycotoxins produced by Aspergillus flavus and Aspergillus parasiticus, and it causes contamination in foods and great risk to human health. Simple sensitive detection of AFB1 is important and demanded for food safety and quality control. Aptamers can specifically bind to targets with high affinity, showing advantages in affinity assays and biosensors. We reported an aptamer structure-switch for fluorescent detection of aflatoxin B1 (AFB1), using a label-free aptamer, a fluorescein (FAM)-labeled complementary strand (FDNA), and a quencher (BHQ1)-labeled complementary strand (QDNA). When AFB1 is absent, these three strands assemble into a duplex DNA structure through DNA hybridization, making FAM close to BHQ1, and fluorescence quenching occurs. In the presence of AFB1, the aptamer binds with AFB1, instead of hybridizing with QDNA. Thus, FAM is apart from BHQ1, and fluorescence increases with the addition of AFB1. This assay allowed detection of AFB1 with a detection limit of 61 pM AFB1 and a dynamic concentration range of 61 pM to 4 μM. This aptamer-based method enabled detection of AFB1 in complex sample matrix (e.g., beer and corn flour samples).

A Simple Ratiometric Electrochemical Aptasensor Based on the Thionine–Graphene Nanocomposite for Ultrasensitive Detection of Aflatoxin B2 in Peanut and Peanut Oil

The accurate and reliable analysis of aflatoxin B2 (AFB2) is widely required in food and agricultural industries. In the present work, we report the first use of a ratiometric electrochemical aptasensor for AFB2 detection with high selectivity and reliability. The working principle relies on the conformation change of the aptamer induced by its specific recognition of AFB2 to vary the ratiometric signal. Based on this principle, the proposed aptasensor collects currents generated by thionine–graphene composites (ITHI) and ferrocene-labeled aptamers (IFc) to output the ratiometric signal of ITHI/IFc. In analysis, the value of ITHI remained stable while that of IFc increased with higher AFB2 concentration, thus offering a “signal-off” aptasensor by using ITHI/IFc as a yardstick. The fabricated aptasensor showed a linear range of 0.001–10 ng mL−1 with a detection limit of 0.19 pg mL−1 for AFB2 detection. Furthermore, its applicability was validated by using it to detect AFB2 in peanut and peanut oil samples with high rates of recovery. The developed ratiometric aptasensor shows the merits of simple fabrication and high accuracy, and it can be extended to detect other mycotoxins in agricultural products.

Sensitive colorimetric aptasensor based on stimuli-responsive metal-organic framework nano-container and trivalent DNAzyme for zearalenone determination in food samples

Zearalenone (ZEN) poses a serious threat to human and animal health. The development of sensitive determination methods for ZEN is of great significance for ensuring the quality and safety of food. Herein, based on a stimuli-responsive aptamer-functionalized metal-organic framework (MOF) nano-container and trivalent DNA peroxidase mimicking enzyme (DNAzyme), an efficient aptasensor was constructed initially for the colorimetric determination of ZEN. The proposed aptasensor only required simple operations but exhibited outstanding specificity, reproducibility, storage stability and reusability simultaneously. Under the optimal conditions, there was a good linear relationship between the changed absorbance and logarithm concentration of ZEN within 0.01-100 ng mL^-1, and the limit of detection (LOD) could reach 0.36 pg mL^-1. Moreover, the proposed aptasensor was reliable in quantifying ZEN in spiked food samples. The current bioassay provides a promising scheme for constructing stable, specific and rapid colorimetric platforms with potential applications in the fields of food safety.

Dual-Enzyme-Based Signal-Amplified Aptasensor for Zearalenone Detection by Using CRISPR-Cas12a and Nt.AlwI

Zearalenone (ZEN) is harmful to animals and human beings, so it is very important to develop a rapid and sensitive method for the detection of ZEN. In this paper, we proposed a novel ZEN-monitoring method using two aptamers as recognition elements and EnGen LbaCas12a and Nt.AlwI nicking endonuclease as signal amplifiers. When ZEN was present, it bound to the aptamer Z0 and, Z1 was released into solution. The solution was then separated and the Nt.AlwI enzyme was added in order to form a nicking-enzyme cycle, thereby producing large amounts of the ssDNA Z3 for 30 min. The Z3 formed a CRISPR-Cas12a-Z3 complex with CRISPR-Cas12a, activated the trans-cleavage ability of Cas12a, cleaved the Quenched Reporter for 20 min, and underwent fluorescence recovery. The aptasensor was able to sensitively detect ZEN in the linear range of 1–1000 pg/mL, with a detection limit as low as 0.213 pg/mL. The detection time lasted for 2 h. Additionally, this detection technology can also be used to monitor other hazards.

Temperature-robust and ratiometric G-quadruplex proximate DNAzyme assay for robustly monitoring of uranium pollution and its microbial biosorbents screening

Uranium pollution in environment and food chain is a serious threat to public security and human health. Herein, we proposed a temperature-robust, ratiometric, and label-free bioassay based on G-quadruplex proximate DNAzyme (G4DNAzyme), accommodating us to precisely monitor uranium pollution and biosorption. The proximity of split G-quadruplex probes was proposed to sense UO₂²⁺-activated DNAzyme activity, thus eliminating the use of chemically labeled nucleic acid probes. And the simultaneous monitoring of G-quadruplex and double-stranded structures of DNAzyme probes contributed to a ratiometric and robust detection of UO₂²⁺. Particularly, the separation of enzymatic digestion and fluorescence monitoring endued a robust and highly responsive detection of UO₂²⁺ upon the temperature of enzymatic digestion process ranged from 18° to 41 °C. Consequently, G4DNAzyme assay allowed a robust, label-free and ratiometric quantification of uranium. We demonstrated the feasibility of G4DNAzyme assay for estimating uranium pollution in water and aquatic product samples. Ultimately, G4DNAzyme assay was adopted to serve as the platform to screen bacterial species and conditions for uranium biosorption, promising its roles in uranium associated biosafety control.

Recent progress of personal glucose meters integrated methods in food safety hazards detection

Development of personal glucose meters (PGMs) for blood glucose monitoring and management by the diabetic patients has been a long history since its first invention in 1968 and commercial application in 1975. The main reasons for its wide acceptance and popularity can be attributed mainly to the easy operation, test-to-result model, low cost, and small volume of sample required for blood glucose concentration test. During past decades, advances in analytical techniques have repurposed the use of PGMs into a general point-of-care testing platform for a variety of non-glucose targets, especially the food hazards. In this review, we summarized the recent published research using PGMs to detect the food safety hazards of mycotoxins, illegal additives, pathogen bacteria, and pesticide and veterinary drug residues detection with PGMs. The progress on PGM-based detection achieved in food safety have been carefully compared and analyzed. Furthermore, the current bottlenecks and challenges for practical applications of PGM for hazards detection in food safety have also been proposed.

Graphene-nucleic acid biointerface-engineered biosensors with tunable dynamic range

Programmed biosensors with tunable quantification range and sensitivity would greatly broaden their application in medical diagnosis, food safety and environmental analysis. Herein, we proposed a graphene-nucleic acid biointerface-engineered biosensor, allowing target molecules to be detected with adjustable dynamic ranges and sensitivities. The biosensors were programmed by simply tuning the poly A tail of aptamer probes. The tuning of the poly A tail would allow the interaction between aptamer probes and graphene oxide (GO) to be modulated, in turn programing the competitive binding processes of aptamer probes to target molecules and GO. The biosensors, termed affinity-tunable aptasensors (atAptasensors) could be easily tuned with different dynamic ranges by using aptamer probes with different tail lengths, and the dynamic range could be extended to be over 3 orders by a combined use of multiple aptamer probes. Remarkably, the specificity of aptamer probes could be increased by increasing the interaction between aptamer probes and GO. Reliability of atAptasensor for ATP detection was tested in serum and milk samples, and we also applied atAptasensor for culture-independent analysis of microorganism pollution.

Interaction between the functionalized probes: The depressed efficiency of dual-amplification strategy on ratiometric electrochemical aptasensor for aflatoxin B1

Signal amplification is one of the most effective ways to develop the high-performance electrochemical sensors. However, it can be more complicated for ratiometric detections. Herein, a ratiometric electrochemical aptasensor for aflatoxin B1 (AFB1) was proposed by taking advantage of a dual-amplification strategy by coupling of DNA walker (DW) with hybridization chain reaction (HCR). The special binding of AFB1 with ferrocene (Fc)-labelled aptamer triggers DW on hairpin DNA (hDNA) tracks to produce abundant double-stranded DNA (dsDNA). HCR-based strand amplification occurs on these dsDNA to absorb more methylene blue (MB). Then current ratio of MB (I_MB) and Fc (I_Fc) is designed as a yardstick to detect AFB1. Our experiments reveal that the interaction between Fc and MB (i.e., steric hindrance, electron mediator) varies. In addition to steric hindrance, the presence of MB also acts as electron mediator, thereby facilitating the electron transfer between Fc and electrode. Such combined effect consequently depresses the efficiency of dual-amplification strategy to improve the detection. The developed ratiometric electrochemical aptasensor allows the accurate detection of AFB1 in the 0.003-3 pg mL^-1 range. Our work has shed light on the amplification strategy for ratiometric sensing, and provided a new route in integrating different amplification strategies.

Periodically programmed building and collapse of DNA networks enables an ultrahigh signal amplification effect for ultrasensitive nucleic acids analysis

ANALYSIS

of molecular species is needed for applications in diagnosis of infections and genetic diseases. Herein, we demonstrate a target DNA-responsive ultrahigh fluorescence signal-on DNA amplification system via periodically programmed building and collapse of DNA networks. In this system, a pair of oligonucleotides of padlock probe (PP) and palindromic hairpin probe (PHP) are utilized. The presence of target DNA firstly hybridizes with PP, allowing the occurrence of rolling circle amplification (RCA) to produce RCA products with tandem repeats in abundance to bind and unfold numbers of PHPs. The conformational change of PHPs enables the building of DNA networks via the intermolecular palindrome pairing, but then makes the DNA networks collapsed via the palindrome-induced strand displacement polymerization. The displaced RCA products are dynamically reused to undergo periodically programmed multiple rounds of DNA network building and collapse. Depend on the bidirectional DNA assembly and disassembly, a strikingly amplified fluorescence can be collected to ultrasensitive and specific detection of target DNA. The practicability has been demonstrated by evaluating target-spiked human serum, saliva, and urine samples with acceptable recoveries and reproducibility. Therefore, this newly explored method opens a promising avenue for the detection of nucleic acids with low abundance in biochemical analysis and diseases diagnosis.

Self-replicating catalyzed hairpin assembly for rapid aflatoxin B1 detection

Herein, a rapid signal amplified aflatoxin B1 (AFB1) detection system based on self-replicating catalyzed hairpin assembly (SRCHA) has been constructed. In this SRCHA system, trigger DNA was initially blocked and two split trigger DNA sequences were integrated into two hairpin auxiliary probes, H1 and H2, respectively. In the presence of AFB1, the aptamer sequence was recognized by AFB1 and trigger DNA was released, which can initiate a CHA reaction and lead to the formation of a helix DNA H1-H2 complex. Then this complex can dissociate double-stranded probe DNA (F-Q) and the fluorescence signal was recovered. Meanwhile, the two split trigger DNA sequences came into close-enough proximity and a trigger DNA replica was formed. Then the obtained replicas can trigger an additional CHA reaction, leading to the rapid and significant enhancement of the fluorescence signal, and AFB1 can be detected within 15 min with a detection limit of 0.13 ng mL-1. This AFB1 detection system exhibits potential application in the on-site rapid detection of AFB1.

Recent progress in mycotoxins detection based on surface-enhanced Raman spectroscopy

Mycotoxins are toxic compounds naturally produced by certain types of fungi. The contamination of mycotoxins can occur on numerous foodstuffs, including cereals, nuts, fruits, and spices, and pose a major threat to humans and animals by causing acute and chronic toxic effects. In this regard, reliable techniques for accurate and sensitive detection of mycotoxins in agricultural products and food samples are urgently needed. As an advanced analytical tool, surface-enhanced Raman spectroscopy (SERS), presents several major advantages, such as ultrahigh sensitivity, rapid detection, fingerprint-type information, and miniaturized equipment. Benefiting from these merits, rapid growth has been observed under the topic of SERS-based mycotoxin detection. This review provides a comprehensive overview of the recent achievements in this area. The progress of SERS-based label-free detection, aptasensor, and immunosensor, as well as SERS combined with other techniques, has been summarized, and in-depth discussion of the remaining challenges has been provided, in order to inspire future development of translating the techniques invented in scientific laboratories into easy-to-operate analytic platforms for rapid detection of mycotoxins.

Simple electrochemical sensing for mercury ions in dairy product using optimal Cu2+-based metal-organic frameworks as signal reporting

A convenient sensor is developed for electrochemical assay of Hg²⁺ in dairy product using the optimal Cu²⁺-based metal-organic frameworks (Cu-MOFs) as signal reporting. Benefiting from specific recognition between Hg²⁺ and thymine (T)-rich DNA strands, the interferences of milk matrices are effectively eliminated, thereby greatly improving the accuracy of test results. Moreover, the suitable Cu-MOFs offer an efficient carrier for probe design, and the contained Cu²⁺ ions could be directly detected to output electrochemical signal of Hg²⁺ presence without labor- or time-intensive operations. Compared with previous methods, this sensor substantially simplifies the process of electrochemical measurement and facilitates highly sensitive, selective and rapid analysis of Hg²⁺ with detection limit of 4.8 fM, offering a valuable means for monitoring dairy product contamination with Hg²⁺.

Amplified Fluorescent Aptasensor for Ochratoxin A Assay Based on Graphene Oxide and RecJf Exonuclease

In this study, we developed an aptamer-based fluorescent sensing platform for the detection of ochratoxin A (OTA) based on RecJ_f exonuclease-assisted signal amplification and interaction between graphene oxide (GO) and the OTA aptamer (OTA-apt). After optimizing the experimental conditions, the present aptamer-based sensing system can exhibit excellent fluorescent response in the OTA assay, with a limit of detection of 0.07 ng/mL. In addition to signal amplification, this strategy is also highly specific for other interfering toxins. Furthermore, this aptasensor can be reliably used for assessing red wine samples spiked with different OTA concentrations (2.4, 6 and 20 ng/mL). The proposed assay plays an important role in the field of food safety and can be transformed for detecting other toxins by replacing the sequence that recognizes the aptamer.

Metal-induced G-quadruplex polymorphism for ratiometric and label-free detection of lead pollution in tea

Lead pollution are critical concerns for food safety and human health. Herein, a ratiometric metal-induced G-quadruplex polymorphism was introduced to construct aptamer probes, enabling label-free and ratiometric detection of lead in tea, thus is promising for on-site detection of lead pollution. The key feature of the aptamer probe is the synergistic utilization of the dual-wavelength fluorescent signal outputs from a G-quadruplex specific dye and a DNA intercalation dye under a single-wavelength excitation, leading to a more stable and reliable recognition of Pb²⁺ than that of analyses based on single fluorescent reporter. The aptamer probe allowed to a mix-and-read, rapid, cost-effective detection of Pb²⁺ with high specificity and accuracy. Pb²⁺ analysis in tap water and tea exhibited good performance with recovery rates of 92.3%-109.0%. The adoption of ratiometric metal-induced G-quadruplex polymorphism would be a compelling design strategy for constructing robust aptasensor, facilitating the translation of aptamer for food safety control.

Simultaneous detection of aflatoxin B1 and ochratoxin A in food samples by dual DNA tweezers nanomachine

a dual DNA tweezers nanomachine was developed for one-step simultaneous detection of aflatoxin B1 (AFB1) and ochratoxin A (OTA) in food samples. The dual DNA tweezers are locked by the aptamers of mycotoxins, resulting the "turn off" of fluorescent signal. In the presence of AFB1 and OTA, the aptamers can bind with their corresponding targets, resulting the "open" of DNA tweezers and the "turn on" of the fluorescent signals. The limits of detections were 3.5 × 10^-2 ppb for AFB1 and 0.1 ppb for OTA. Moreover, the applicability of the method was further demonstrated by conducting a limited survey on 5 samples collected from various sources. The recoveries of this method change from 90.0% to 110.0% for simultaneous detection of AFB1 or OTA and the RSDs vary from 4.1% to 9.2%. Detection uncertainties were within 5% (with a 95% confidence level).

Surface-Confined Building of Au@Pt-Centered and Multi-G-Quadruplex/Hemin Wire-Surrounded Electroactive Super-nanostructures for Ultrasensitive Monitoring of Morphine

Overuse and abuse of morphine (MOP), one of the main components of pericarpium papaveris, have attracted increasing attention in the medical field owing to its pharmacological and toxicological activity. Herein, we proposed a new electrochemical nano-biosensor for MOP detection based on surface-confined building of Au@Pt-centered and multi-G-quadruplex/hemin wire-surrounded electroactive super-nanostructures. The center Au@Pt was flower-shaped and irregularly protruded, allowing substantial loading of multiple G-quadruplex wire/hemin complexes on its surface to accomplish the assembly of electroactive super-nanostructures. Interestingly, as the super-nanostructures were closely confined on the electrode surface, a significantly amplified electrochemical signal was thus obtained in the absence of MOP. In contrast, the introduction of target MOP can induce an intense competitive effect and strongly destroy the assembly process, resulting in the reduction of the electrochemical response that is correlated with the logarithmic concentration of MOP. Under optimal conditions, the electrochemical nano-biosensor is capable of highly sensitive detection of MOP in a dynamic concentration range from 1 ppt to 500 ppb. The limit of detection is achieved as low as 0.69 ppt, and the practical application was confirmed by examining MOP from chafing dish condiments. We expect the electrochemical platform utilizing this unique nanoarchitecture to provide rational guidelines to design high-performance analytical tools.

Directly profiling intact Staphylococcus aureus in water and foods via enzymatic cleavage aptasensor

Staphylococcus aureus (S. aureus) causes serious food-borne diseases, and tools able to directly profile intact S. aureus would greatly facilitate food safety and public health. Herein, we proposed a biosensing platform for culture-independent and separation-free profiling S. aureus, thus allow us to directly detect intact S. aureus in complex samples. The binding protection effect of aptamer-cell complex was introduced to construct the aptasensor, and it allowed to eliminate the optimization of aptamer probe sequences. The proposed aptasensor, terms enzymatic cleavage aptasensor could achieve a sensitive (a detection limit of 64 CFU/mL) and broad-concentration quantification (dynamic range 10²-10⁷ CFU/mL) of S. aureus. Furthermore, it could specifically identify intact S. aureus in complex samples, and the quantifying of S. aureus was achieved in tap water, milk and porker with high precision. Therefore, enzymatic cleavage aptasensor could be a good candidate for on-site biosensing platform of S. aureus, as well as other pathogens by replacing the aptamer sequences.

Selection of Specific DNA Aptamers for Hetero-Sandwich-Based Colorimetric Determination of Campylobacter jejuni in Food

Herein, a high-affinity single-stranded DNA aptamer (59 nt) against Campylobacter jejuni, defined as CJA1, was obtained using the whole-bacterium-based systemic evolution of ligands by exponential enrichment procedure. CJA1 was analyzed with a stable secondary structure and low dissociation constant (K_d) value of 1.37 ± 0.28 nM. The potential use of CJA1 was exemplified by the construction of a hetero-sandwich platform, in which C. jejuni was bound with a biotin-tagged CJA1 to perform a colorimetric reaction that is associated with visible color changes and detectable optical responses. Dependent upon this sensing platform, C. jejuni can be detected from 1.7 × 10¹ to 1.7 × 10⁶ colony-forming units (CFU)/mL. The limit of detection (LOD) is obtained as 10 CFU/mL in PBS. The specificity study showed that the sensing platform is easy to distinguish C. jejuni from other common pathogens. Moreover, the C. jejuni-contaminated milk samples can also be accurately probed (LOD = 13 CFU/mL) without sacrificing its assay abilities, indicating the promising prospect of CJA1 in the fields of biosensing and diagnostics.

A metal-organic framework/aptamer system as a fluorescent biosensor for determination of aflatoxin B1 in food samples

The demand of simple, sensitive, selective and reliable assay for aflatoxin B1 (AFB1) detection is ubiquitous in food safety, due to its high toxic. Herein, a novel fluorescent aptasensor using metal-organic frameworks (UiO-66-NH₂) and TAMRA label aptamer as sensing platform for AFB1 detection was developed. The TAMRA aptamer adsorbed on the surface of UiO-66-NH₂ via van der Waals force and its fluorescence was quenched for the charge transfer from fluorescence dye TAMRA to metal ions of UiO-66-NH₂. After introducing AFB1 to the system, the TAMRA aptamer binded to AFB1 and formed TAMRA aptamer/AFB1complex, making its conformation change and resulting in fluorescence recovery. Thus, the quantity of AFB1 could be analyzed according to the fluorescence signal change. Under optimize experimental conditions, the assay exhibited high sensitivity toward AFB1 in range of 0-180 ng mL^-1 with low limit of detection of 0.35 ng mL^-1 and good specificity against other toxins. Moreover, the aptamer/metal-organic frameworks sensing platform could be utilized to determine AFB1 content in food samples such as corn, rice and milk. It provided a reasonable method for other mycotoxin detection by changing the sequence of aptamer.

Aptamer-Based Biosensor for Detection of Mycotoxins

Mycotoxins are a large type of secondary metabolites produced by fungi that pose a great hazard to and cause toxic reactions in humans and animals. A majority of countries and regulators, such as the European Union, have established a series of requirements for their use, and they have also set maximum tolerance levels. The development of high sensitivity and a specific analytical platform for mycotoxins is much in demand to address new challenges for food safety worldwide. Due to the superiority of simple, rapid, and low-cost characteristics, aptamer-based biosensors have successfully been developed for the detection of various mycotoxins with high sensitivity and selectivity compared with traditional instrumental methods and immunological approaches. In this article, we discuss and analyze the development of aptasensors for mycotoxins determination in food and agricultural products over the last 11 years and cover the literatures from the first report in 2008 until the present time. In addition, challenges and future trends for the selection of aptamers toward various mycotoxins and aptasensors for multi-mycotoxins analyses are summarized. Given the promising development and potential application of aptasensors, future research studies made will witness the great practicality of using aptamer-based biosensors within the field of food safety.

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.

Comparative Study of Time-Resolved Fluorescent Nanobeads, Quantum Dot Nanobeads and Quantum Dots as Labels in Fluorescence Immunochromatography for Detection of Aflatoxin B1 in Grains

Label selection is an essential procedure for improving the sensitivity of fluorescence immunochromatography assays (FICAs). Under optimum conditions, time-resolved fluorescent nanobeads (TRFN), quantum dots nanobeads (QB) and quantum dots (QD)-based immunochromatography assays (TRFN-FICA, QB-FICA and QD-FICA) were systematically and comprehensively compared for the quantitative detection of aflatoxin B₁ (AFB₁) in six grains (corn, soybeans, sorghum, wheat, rice and oat). All three FICAs can be applied as rapid, cost-effective and convenient qualitative tools for onsite screening of AFB₁; TRFN-FICA exhibits the best performance with the least immune reagent consumption, shortest immunoassay duration and lowest limit of detection (LOD). The LODs for TRFN-FICA, QB-FICA and QD-FICA are 0.04, 0.30 and 0.80 μg kg⁻¹ in six grains, respectively. Recoveries range from 83.64% to 125.61% at fortified concentrations of LOD, 2LOD and 4LOD, with the coefficient of variation less than 10.0%. Analysis of 60 field grain samples by three FICAs is in accordance with that of LC-MS/MS, and TRFN-FICA obtained the best fit. In conclusion, TRFN-FICA is more suitable for quantitative detection of AFB₁ in grains when the above factors are taken into consideration.

Progress on Structured Biosensors for Monitoring Aflatoxin B1 From Biofilms: A Review

Aspergillus exists commonly in many crops and any process of crop growth, harvest, storage, and processing can be polluted by this fungus. Once it forms a biofilm, Aspergillus can produce many toxins, such as aflatoxin B1 (AFB1), ochratoxin, zearalenone, fumonisin, and patulin. Among these toxins, AFB1 possesses the highest toxicity and is labeled as a group I carcinogen in humans and animals. Consequently, the proper control of AFB1 produced from biofilms in food and feed has long been recognized. Moreover, many biosensors have been applied to monitor AFB1 in biofilms in food. Additionally, in recent years, novel molecular recognition elements and transducer elements have been introduced for the detection of AFB1. This review presents an outline of recent progress made in the development of biosensors capable of determining AFB1 in biofilms, such as aptasensors, immunosensors, and molecularly imprinted polymer (MIP) biosensors. In addition, the current feasibility, shortcomings, and future challenges of AFB1 determination and analysis are addressed.

Multicolor colorimetric detection of ochratoxin A via structure-switching aptamer and enzyme-induced metallization of gold nanorods

Colorimetric aptasensors have been intensively studied for the ochratoxin A (OTA) detection, but they mostly exhibit just one-color change, resulting in poor visual resolution and limited use for semi-quantitative analysis. Thus, we designed a high-resolution colorimetric assay on the basis of aptamer structural switching and enzyme-induced metallization of gold nanorods (AuNRs). DNA-alkaline phosphatase (ALP)-immobilized magnetic beads were prepared. The aptamer bounded to OTA to form G-quadruplexes, releasing ALP-labelled complementary DNA (cDNA-ALP). After magnetic separation, cDNA-ALP catalyzed the decomposition of ascorbic acid 2-phosphate to ascorbic acid that reduced Ag⁺, forming an Ag shell on the surface of AuNRs. This caused a blue-shift of the longitudinal local surface plasmon resonance peak of the AuNRs and a naked eye visible multicolor change. Under optimal conditions, the assay exhibited a 9.0 nM detection limit for OTA, with high specificity. This method is promising for the on-site visual semi-quantitative detection of mycotoxins in foods.

Label-free one-step fluorescent method for the detection of endonuclease activity based on thioflavin T/G-quadruplex

Endonucleases, one of the basic tool enzymes of modern molecular biology and medical genetics, have also been clarified as the potential targets for antimicrobial and antiviral drugs screening. However, traditional assays to monitor endonuclease activity can be expensive, time-consuming, or laborious. In order to provide new detective platform, we proposed a novel label-free one-step fluorescent method for the detection of endonuclease activity based on cleavage-induced G-quadruplex formation. In this detection system, a simple DNA probe can spontaneously form a duplex structure with recognition sites of EcoRI and prevent the generation of the G-quadruplex. Once EcoRI is present, the recognition sites in the duplex DNA are cleavage, producing a free guanine-rich sequence to form G-quadruplex. When thioflavin T (ThT) is added, a strong fluorescence signal is given by ThT/G-quadruplex, and therefore the EcoRI activity can be detected. After systematic investigation and optimization, this method has gained a sensitive limit of detection at 0.75 U/mL, and a wide detection range between 0.75 U/mL and 120 U/mL. Furthermore, the inhibitory effect of 5-fluorouracil on EcoRI activity was verified and IC50 was calculated. Taken together, these experimental results have proven that this turn-on fluorescent method has considerable analytical performances. As far as we are concerned, this method is the first reported EcoRI assay based on ThT/G-quadruplex, and only one kind of probe and one kind of dye are involved, providing one of the simplest detective strategies on EcoRI. More importantly, the convenience and cost make this one-step method quite attractive for application transformation. Therefore, we hope this method could be a hopeful option for EcoRI activity determination, and further to help monitoring the quality of tool enzymes and promote the development of high-through automatic drug screening system.

Engineering Multivalence Aptamer Probes for Amplified and Label-Free Detection of Antibiotics in Aquatic Products

Excessive use of antibiotics in aquatic products is a serious problem for food safety and human health, and on-site detection of antibiotics is highly demanded. Herein, we proposed multivalence aptamer probes, allowing sensitive, label-free, and homogeneous detection of antibiotics in different aquatic products. Compared to commonly used aptamers, multivalence aptamer probes can provide multiple binding sites and a higher affinity for target molecules, and the iterative binding on different binding sites contributes to an amplified recognition effect, sharply increasing the response and sensitivity of aptamer probes. The 2-valence aptamer probes conferred a limit of detection of 0.097 nM for kanamycin detection, where it is estimated that their sensitivity is enhanced 12 times compared to 1-valence aptamer probes. Meanwhile, multivalence aptamer probes allowed us to specifically identify kanamycin among other antibiotics. It could detect kanamycin residual in aquatic products including river eel and puffer fish, as well as tap water with high precision. A multivalence design strategy of aptamer probes would significantly improve the detection performance of aptamers, facilitating the translation of aptamer for food safety control.

Graphene/aptamer probes for small molecule detection: from in vitro test to in situ imaging

Small molecules are key targets in molecular biology, environmental issues, medicine and food industry. However, small molecules are challenging to be detected due to the difficulty of their recognition, especially in complex samples, such as in situ in cells or animals. The emergence of graphene/aptamer probes offers an excellent opportunity for small molecule quantification owing to their appealing attributes such as high selectivity, sensitivity, and low cost, as well as the potential for probing small molecules in living cells or animals. This paper (with 130 refs.) will review the application of graphene/aptamer probes for small molecule detection. We present the recent progress in the design and development of graphene/aptamer probes enabling highly specific, sensitive and rapid detection of small molecules. Emphasis is placed on the success in their development and application for monitoring small molecules in living cells and in vivo systems. By discussing the key advances in this field, we wish to inspire more research work of the development of graphene/aptamer probes for both on-site or in situ detection of small molecules and its applications for investigating the functions of small molecules in cells in a dynamic way. Graphical abstract Graphene/aptamer probes can be used to construct different platforms for detecting small molecules with high specificity and sensitivity, both in vitro and in situ in living cells and animals.

Predominant Mycotoxins, Pathogenesis, Control Measures, and Detection Methods in Fermented Pastes

Fermented pastes are some of the most popular traditional products in China. Many studies reported a strong possibility that fermented pastes promote exposure to mycotoxins, including aflatoxins, ochratoxins, and cereulide, which were proven to be carcinogenic and neurotoxic to humans. The primary mechanism of pathogenicity is by inhibiting protein synthesis and inducing oxidative stress using cytochrome P450 (CYP) enzymes. The level of mycotoxin production is dependent on the pre-harvest or post-harvest stage. It is possible to implement methods to control mycotoxins by using appropriate antagonistic microorganisms, such as Aspergillus niger, Lactobacillus plantarum, and Saccharomyces cerevisiae isolated from ordinary foods. Also, drying products as soon as possible to avoid condensation or moisture absorption in order to reduce the water activity to lower than 0.82 during storage is also effective. Furthermore, organic acid treatment during the soaking process reduces toxins by more than 90%. Some novel detection technologies based on magnetic adsorption, aptamer probes, and molecular-based methods were applied to rapidly and accurately detect mycotoxins in fermented pastes.

Aptamer-based Homogeneous Analysis for Food Control

Background:

Highly sensitive and rapid analysis of food contaminants is of great significance for food safety control. Aptamer is a new kind of recognition molecules which could be applied for constructing homogeneous analysis assays, potentially achieving highly sensitive, cheap and rapid profiling of food contaminants.

Methods:

An overview of the literature concerning the homogeneous analysis of food contaminations based on aptamers has been reviewed (focused on the most recent literature, 2000-2018).

Results:

Attributed to aptamer’s controllability, designability and feasibility for the adoption of nucleic acid amplification, rapid, highly sensitive homogeneous assay for various food contaminants could be constructed. The structure-switching aptamer probe would confer quick, efficient and specific response to target food contaminants. Besides, the capability of amplification of aptamer sequences or nucleic acid probes would lead to highly sensitive detection.

Conclusion:

Aptamer-based homogeneous analysis methods have already been applied to detect various food contaminations ranging from toxins, heavy metal and pesticide to allergen and pathogenic bacteria. However, it is still a challenge to achieve robust and accurate detection of food contaminants in complex food samples.

A Fluorescent DNA Hydrogel Aptasensor Based on the Self-Assembly of Rolling Circle Amplification Products for Sensitive Detection of Ochratoxin A

A sensitive fluorescent DNA hydrogel aptasensor based on the self-assembly of rolling circle amplification (RCA) products was developed for ochratoxin A (OTA) detection in beer. A competitive binding mode of aptamer, complementary sequence, and target was integrated into the DNA hydrogel for OTA detection. The OTA aptamer first combined with the primer to form the hybridized product. Then, in the presence of OTA, the aptamer combined with OTA, which released the primer. The released primer hybridized with the padlock probe to form a circular template, and the RCA reaction was initiated by adding ligase, polymerase, and dNTPs. The fluorescent DNA hydrogel was obtained by adding Cy3-dUTP together with dNTPs, and the fluorescence (FL) intensity of the DNA hydrogel was positively correlated with OTA concentration. Under the optimal experimental conditions, the linear range of the relationship varied from 0.05 ng/mL to 100 ng/mL with a detection limit for OTA of 0.01 ng/mL. The fluorescent DNA hydrogel aptasensor showed good specificity and stability in beer samples. Therefore, the fabricated DNA hydrogel aptasensor shows considerable potential applications in detecting OTA for food safety.