PVP-coated graphene oxide for selective determination of ochratoxin A via quenching fluorescence of free aptamer

Exo I-based cyclic digestion coupled with synergistic enhancement strategy for integrating dual-mode optical aptasensor platform

The improvement of dual-mode techniques was of particular interest to researchers, which might enhance the detection performance and applicability. Here, a dual-mode optical aptasensor (DO-aptasensor) platform based on exonuclease I (Exo I) cyclic digestion and synergistic enhancement strategy had proposed for zearalenone (ZEN). Following the preparation of dumbbell-shaped signal probe, the Exo I-based cyclic digestion amplification performed, and then the synergistic enhancement effect carried out to achieve the Poly-HRP-based colorimetry and FAM-SGI-based fluorescence. The efficient homogeneous system realized through the magnetic separation, while the signal interference further eliminated by the graphene oxide (GO). The LOD values were as low as 0.067 ng mL-1 for colorimetry mode and 0.009 ng mL-1 for fluorescence mode, which reduced 23-fold and 172-fold than ELASA by same ZEN-Apt. This promising platform gave rise to a dual-mode optical readout, improved sensitivity and positively correlated detection. Meanwhile, the DO-aptasensor also exhibited the acceptable specificity, desirable reliability and excellent practicability. This novel avenue of aptasensor platform hold great potential for dual-mode optical monitoring of other targets, which can further expand the application scope of Exo I-based signal amplification and synergistic enhancement effect.

Polycarboxyl ionic liquid functionalized Yb-MOFs nanoballs based dual-wavelength responsive photoelectrochemical aptasensor for the simultaneous determination of AFB1 and OTA

Developing an accurate and precise approach for the simultaneous detection of ochratoxin A (OTA) and aflatoxin B1 (AFB1) is significant for food safety surveillance. Herein, a photoelectrochemical sensing platform was constructed based on polycarboxylic ionic liquid functionalized metal-organic framework integrated with gold nanoparticles (Yb-MOFs@AuNPs). Sulfhydryl functionalized hairpin DNA (hDNA) was immobilized on a Yb-MOFs@AuNPs modified glassy carbon electrode (GCE) surface through Au-S bond. After blocking residual active binding sites with BSA, gold nanoparticles-labeled AFB1 aptamer (AuNPs-Apt 1) and gold nanorods-labeled OTA aptamer (AuNRs-Apt 2) were introduced to construct a photoelectrochemical aptasensor for the simultaneous determination of AFB1 and OTA. Due to the surface plasmon resonance effect and the nanometer size effect of gold nanomaterials, the photoelectrochemical aptasensor can output photocurrent responses as being excited with different wavelengths at 520 nm and 808 nm, respectively. When the AFB1 and OTA concentration in the range of 0.001-50.0 ng mL-1, a good linear relationship between the photocurrent difference (ΔI) before and after recognizing targets and the logarithm of AFB1 or OTA concentration was obtained. The detection limits for AFB1 and OTA were 0.40 pg mL-1 and 0.19 pg mL-1, respectively. AFB1 and OTA in corn samples were detected simultaneously by the photoelectrochemical aptasensor.

On-Site Fluorescent Detection of Sepsis-Inducing Bacteria using a Graphene-Oxide CRISPR-Cas12a (GO-CRISPR) System

Sepsis is an extremely dangerous medical condition that emanates from the body's response to a pre-existing infection. Early detection of sepsis-inducing bacterial infections can greatly enhance the treatment process and potentially prevent the onset of sepsis. However, current point-of-care (POC) sensors are often complex and costly or lack the ideal sensitivity for effective bacterial detection. Therefore, it is crucial to develop rapid and sensitive biosensors for the on-site detection of sepsis-inducing bacteria. Herein, we developed a graphene oxide CRISPR-Cas12a (GO-CRISPR) biosensor for the detection of sepsis-inducing bacteria in human serum. In this strategy, single-stranded (ssDNA) FAM probes were quenched with single-layer graphene oxide (GO). Target-activated Cas12a trans-cleavage was utilized for the degradation of the ssDNA probes, detaching the short ssDNA probes from GO and recovering the fluorescent signals. Under optimal conditions, we employed our GO-CRISPR system for the detection of Salmonella Typhimurium (S. Typhimurium) with a detection sensitivity of as low as 3 × 103 CFU/mL in human serum, as well as a good detection specificity toward other competing bacteria. In addition, the GO-CRISPR biosensor exhibited excellent sensitivity to the detection of S. Typhimurium in spiked human serum. The GO-CRISPR system offers superior rapidity for the detection of sepsis-inducing bacteria and has the potential to enhance the early detection of bacterial infections in resource-limited settings, expediting the response for patients at risk of sepsis.

Aptamer-based fluorescent sensor for highly sensitive detection of methamphetamine

The construction of a fluorescence aptamer sensor was achieved by employing the fundamental principle of fluorescence resonance energy transfer. By employing molecular modeling technologies to identify the binding site, the high-affinity aptamer APT-40nt was derived from the whole sequence and utilized on the graphene oxide (GO) fluorescent platform for the purpose of achieving a highly sensitive detection of methamphetamine (METH). The aptamer tagged with fluorescein (FAM) dye undergoes quenching in the presence of GO due to π-stacking interaction. With the addition of the target, the aptamer that has been tagged was detached from the GO surface, forming a stable complex with METH. This process resulted in fluorescence restoration of the system, and the degree of fluorescence restoration was proportional to METH concentration in the linear range of 1-50 and 50-200 nM. Notably, under optimized conditions, the detection limit of this aptasensor was as low as 0.78 nM, which meets the detection limit requirements of METH detection in saliva and urine in some countries and regions. Moreover, other common illicit drugs and metabolites had minimizing interference with the determination. The established aptasensor, therefore, has been successfully applied to detect METH in saliva and urine samples and exhibited satisfactory recoveries (87%-111%). This aptasensor has the advantages of low detection limit, excellent selectivity, ease of operation, and low cost, providing a promising strategy for on-site detection of METH in saliva and urine.

Alkaline phosphatase triggered gold nanoclusters turn-on fluorescence immunoassay for detection of Ochratoxin A

Ochratoxin A (OTA) is a highly toxic mycotoxin which can cause a variety of diseases. Sensitive detection of OTA is significant for food safety. Herein, a feasible and sensitive immunoassay was established for OTA detection by alkaline phosphatase (ALP) triggered gold nanoclusters (AuNCs) turn-on fluorescence. The fluorescence of the AuNCs can be quenched by Cr6+ induced aggregation of AuNCs and the fluorescence resonance energy transfer (FRET) between AuNCs and Cr6+. Under the catalytic action of ALP-labelled IgG (IgG-ALP), the ascorbic acid 2-phosphate (AA2P) was hydrolyzed to ascorbic acid (AA) for the reducing of Cr6+ to Cr3+. As a result, the degrees of AuNCs aggregation and FRET were weakened and the fluorescence of AuNCs was turned on. The amount of OTA in the sample was negatively correlated with the amount of IgG-ALP captured by anti-OTA monoclonal antibody (McAb) in the microplate. In optimal conditions, the turn-on fluorescence immunoassay had a good linear range of 6.25-100 ng/mL, and the detection limit was 0.693 ng/mL. The recoveries of OTA from corn were 95.89%-101.08% for the fluorescence immunoassay. This work provided a feasible, sensitive and good selectivity fluorescence method for OTA detection.

Target-initiated fluorescent aptasensor based on multisite strand displacement amplification for label-free detection of ochratoxin A

We developed a fluorescent aptasensor for label-free detection of ochratoxin A (OTA) based on TdT- and DNA polymerase-assisted multisite strand displacement amplification. This aptasensor exhibits good specificity and high sensitivity with a limit of detection (LOD) of 0.18 ng mL-1, and it can be further applied for the accurate quantification of OTA in complex real samples, holding promising applications in the field of food safety.

Exogenous interference and autofluorescence-free ratiometric aptasensor for detection of OTA based on dual-colored persistent luminescence nanoparticles

Accurate and sensitive detection of ochratoxin A (OTA) is highly necessary due to its high carcinogenicity, teratogenicity and mutagenicity. Herein, we reported an exogenous interference and autofluorescence-free ratiometric aptasensor based on dual-colored persistent luminescent nanoparticles for precise detection of OTA. Green-emitting ZnGeO:Mn bonded with OTA aptamer and BHQ1-modified complementary base was acted as detection and specific recognition probe (ZGM@BHQ1). Quaternary ammonium modified ZnGaGeO:Cr with red emission was employed as reference probe and further bonded to ZGM@BHQ1 through electrostatic interaction to construct the ratiometric aptasensor. The developed ratiometric aptasensor was free from real-time excitation, external interference and autofluorescence and gave low detection limit of 3.4 pg mL^-1, wide linearity in the range of 0.01-50 ng mL^-1 and high precision of 3.1 % (11 replicate determinations, at 1 ng mL^-1 level). The applicability of the aptasensor was successfully demonstrated by analyzing OTA in in grain samples with recoveries of 97.6 %-105.2 %.

Quantitative and qualitative analysis of ochratoxin-A using fluorescent CQDs@DNA-based nanoarchitecture assembly to monitor food safety and quality

Ochratoxin A (OTA), a mycotoxin formed by various fungi, such as Aspergillus and Penicillium species, is dangerous to human health. Thus, to circumvent the risk of OTA ingestion, the recognition and quantification of OTA levels are of great significance. A perusal of the literature has revealed that the integration of DNA/Carbon Quantum Dot (CQD)-based hybrid systems may exhibit the unique electronic and optical properties of nanomaterials/nanoarchitecture and consequent recognition properties. Herein, we developed the CQDs@DNA-based hybrid nanoarchitecture system for the selective detection of OTA, which exhibits modulation in the emission spectrum after interaction with OTA, with a significant binding constant (K_a = 3.5 × 10⁵ M^-1), a limit of detection of 14 nM, limit of quantification of 47 nM and working range of 1-10 μM. The mechanism for sensing the OTA has been corroborated using fluorescence, UV-visible absorption spectroscopy, and FTIR techniques, demonstrating the binding mode of CQD@DNA hybrid nano-architecture assembly with OTA. Further, we demonstrated the sensing ability of developed CQDs@DNA-based nanoarchitecture assembly towards the quantification of OTA in real food monitoring analysis for real-time applications, which makes this developed nanoarchitecture assembly the potential candidate to conveniently monitor food safety and quality for human health.

Synthesis and Functionalization of Graphene Materials for Biomedical Applications: Recent Advances, Challenges, and Perspectives

Since its discovery in 2004, graphene is increasingly applied in various fields owing to its unique properties. Graphene application in the biomedical domain is promising and intriguing as an emerging 2D material with a high surface area, good mechanical properties, and unrivalled electronic and physical properties. This review summarizes six typical synthesis methods to fabricate pristine graphene (p-G), graphene oxide (GO), and reduced graphene oxide (rGO), followed by characterization techniques to examine the obtained graphene materials. As bare graphene is generally undesirable in vivo and in vitro, functionalization methods to reduce toxicity, increase biocompatibility, and provide more functionalities are demonstrated. Subsequently, in vivo and in vitro behaviors of various bare and functionalized graphene materials are discussed to evaluate the functionalization effects. Reasonable control of dose (<20 mg kg^-1 ), sizes (50-1000 nm), and functionalization methods for in vivo application are advantageous. Then, the key biomedical applications based on graphene materials are discussed, coupled with the current challenges and outlooks of this growing field. In a broader sense, this review provides a comprehensive discussion on the synthesis, characterization, functionalization, evaluation, and application of p-G, GO, and rGO in the biomedical field, highlighting their recent advances and potential.

Detection of ochratoxin A using a "turn-on" fluorescence assay based on guanine quenching of the aptamer

Ochratoxin A (OTA) is a common mycotoxin with high carcinogenicity; therefore, it is crucial to establish a simple, rapid, and sensitive method for its detection. In this study, we developed a "turn-on" fluorescence assay for detecting OTA based on guanine quenching of the aptamer. The method uses fluorescein (FAM) fluorophore to label the complementary strand of the OTA aptamer, Fc-DNA. In the absence of OTA, the Fc-DNA hybridizes with the aptamer to form a double strand. Due to the occurrence of photo-induced electron transfer (PET), the FAM fluorescence signal is quenched as the FAM on the Fc-DNA approaches the guanine of the aptamer at the 5' end. When OTA is present, the aptamer binds to it and thus, is unable to hybridize with Fc-DNA to form a double strand; the FAM fluorescence signal is restored as FAM moves away from the guanine of the aptamer. The assay achieved OTA detection at a detection limit of 28.4 nM. The application of the original guanine of the aptamer as the quenching agent helps avoid the complex designing and labeling of the aptamer, which ensures the high affinity of the aptamer for OTA. Meanwhile, this "turn-on" detection mode helps avoid potential false-positive results as in the "turn-off" mode and improves the assay's sensitivity. Additionally, the method has good selectivity and can be used to detect OTA in traditional Chinese medicine. This method provides a simple, low-cost, and rapid method for OTA detection.

A fluorescence detection method for the determination of β-lactoglobulin in foods

A fluorescence detection method based on quantum dot-aptamer-graphene oxide probes (QD-Apt-GO) was developed to detect β-lactoglobulin (β-LG) in foods. When β-LG was present in the samples, it specifically bound to the aptamer, inhibiting the binding of probes to graphene oxide (GO), and the fluorescence of the probes could be detected. When β-LG was not present, the probes could bind to GO through π-π stacking, and the fluorescence was consequently quenched. The detection range of the optimized assay for β-LG detection was 0.36-500 mg L^-1. The limit of detection (LOD) for β-LG was 96.91 μg L^-1. The method was also validated for food sample detection. In the spike and recovery experiments of Neocate amino acid infant formula, infant millet cookies, and infant rice porridge, the recoveries were in the range of 83.33-114.53%, which met the required range of the addition recoveries. At the same time, the results were consistent with those of commercial ELISA kits. Three types of random food products purchased from a local market were analyzed for β-LG via the developed assay and using a commercial ELISA kit. The results showed good accuracy and consistency between the proposed method and the commercial ELISA kit.

Adsorption-desorption nano-aptasensors: fluorescent screening assays for ochratoxin A

In this study, a FRET-based fluorescent aptasensor for the detection of ochratoxin A (OTA) was optimized based on the quenching efficiency of single-walled carbon nanotubes (SWCNTs) and the binding affinity of aptamers. OTA aptamers were conjugated with quantum dots and adsorbed to the surface of both acid-modified and unmodified SWCNTs. The maximum fluorescence quenching efficiency of the SWCNTs were compared. Acid-modified SWCNTs (amSWCNTs) have moderate quenching efficiency, providing an optimal sensitivity for qualitative fluorescence-enhancement biosensor assays. The binding parameters of the QD-modified OTA aptamers (1.12.2 and A08min) on the surface of amSWCNTs were compared. Based on our results, the A08min aptamer is a better candidate for OTA detection. Using the A08min aptamer, the SWCNT method had a limit of detection (LOD) of 40 nM. The amSWCNT method had a significantly lower LOD of 14 nM. Turn-on fluorescent nano-aptasensors are emerging as an effective diagnostic tool for simple detection of mycotoxins. Nanocomplexes designed for the detection of mycotoxins in solution and paper-based tests have proven to be useful.

Molecular Diagnostic of Ochratoxin A With Specific Aptamers in Corn and Groundnut via Fabrication of a Microfluidic Device

Ochratoxin A (OTA) is one of the predominant mycotoxins that contaminate a wide range of food commodities. In the present study, a 36-mer aptamer was used as a molecular recognition element coupled with gold nanoparticles (AuNPs) for colorimetric detection of OTA in a microfluidic paper-based analytical device (μPADs). The μPADs consisted of three zones: control, detection, and sample, interconnected by channels. UV-vis spectroscopy (UV-vis), Dynamic Light Scattering (DLS), and Transmission Electron Microscopy (TEM) were used for characterization of AuNPs and AuNPs/Aptamer. According to the colorimetric assay, limit of detection (LOD) was found to be 242, 545.45, and 95.69 ng/mL in water, corn, and groundnut, respectively. The HPLC detection method achieved acceptable coefficient in standard curves (r 2 = 0.9995), improved detection range, and recovery rates in spiked corn and groundnut samples as 43.61 ± 2.18% to 87.10 ± 1.82% and 42.01 ± 1.31% to 86.03 ± 2.64% after multiple sample extractions and cleanup steps. However, the developed μPADs analytical device had the potent ability to rapidly detect OTA without any extraction pre-requirement, derivatization, and cleanup steps, thus illustrating its feasibility in the animal health sector, agricultural, and food industries.

A novel highly sensitive soy aptasensor for antigen β-conglycinin determination

β-Conglycinin, composed of three subunits (α', α and β), is the main allergen of soy protein which can cause severe allergic reactions, such as diarrhea, decreased growth performance and even death. Among them, the β subunit is more stable and difficult to remove, being one of the main nutritional inhibitors, which can be used to evaluate the concentration of β-conglycinin. However, there is no effective, accurate method for its β subunit rapid detection. Herein, we have successfully selected a high affinity β subunit aptamer (Kd = 6.9 nM) and developed a highly sensitive aptasensor. The aptasensor displayed high specificity and the β subunit at a concentration of 70-350 nM could be detected with a detection limit of 4.48 nM (3S/N). In addition, the recoveries of β subunit were more than 90%, demonstrating its practical properties for complicated conditions such as food quality control and disease diagnosis, without requiring expensive and sophisticated equipment.

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

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

A review on graphene-based electrochemical sensor for mycotoxins detection

This work focuses on the study of nanomaterial-based sensors for mycotoxins detection. Due to their adverse effects on humans and animals, mycotoxins are heavily regulated, and the foodstuff and feed stocks with a high probability of being contaminated are often analyzed. In this context, the recent developments in graphene-based electrochemical sensors for mycotoxins detection were examined. The mycotoxins' toxicity implications on their detection and the development of diverse recognition elements are described considering the current challenges and limitations.

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.

Detection Strategies of Zearalenone for Food Safety: A Review

Zearalenone (ZEN) is a toxic compound produced by the metabolism of fungi (genus Fusarium) that threaten the food and agricultural industry belonging to the in foods and feeds. ZEN has toxic effects on human and animal health due to its mutagenicity, teratogenicity, carcinogenicity, nephrotoxicity, immunotoxicity, and genotoxicity. To ensure food safety, rapid, precise, and reliable analytical methods can be developed for the detection of toxins such as ZEN. Different selective molecular diagnostic elements are used in conjunction with different detection strategies to achieve this goal. In this review, the use of electrochemical, colorimetric, fluorometric, refractometric as well as other strategies were discussed for ZEN detection. The success of the sensors in analytical performance depends on the development of receptors with increased affinity to the target. This requirement has been met with different immunoassays, aptamer-assays, and molecular imprinting techniques. The immobilization techniques and analysis strategies developed with the combination of nanomaterials provided high precision, reliability, and convenience in ZEN detection, in which electrochemical strategies perform the best.

Persistent luminescence nanorod based luminescence resonance energy transfer aptasensor for autofluorescence-free detection of mycotoxin

Serious ochratoxin A (OTA) contamination necessitates the development of rapid, sensitive and selective analytical methods for its determination in food safety. Herein, we report a persistent luminescence resonance energy transfer (LRET) based aptasensor for the autofluorescence-free detection of OTA. OTA aptamer functionalized persistent luminescence nanorod (PLNR) Zn₂GeO₄:Mn²⁺ and the aptamer complementary DNA modified gold nanoparticle (AuNP) were used as the donor and the acceptor, respectively. The developed LRET aptasensor integrated the advantages of the long-lasting persistent luminescence of PLNR, the high selectivity of aptamer and the low probe background of LRET sensors, allowing autofluorescence-free detection of OTA in biological samples with high sensitivity and selectivity. The developed LRET aptasensor gave an excellent linearity in the range of 0.01-10 ng mL^-1, the detection limit of 3 pg mL^-1 and the precision of 2.7% (RSD, n = 11) at 1 ng mL^-1 level. The applicability of the developed aptasensor was demonstrated by analyzing beer samples for OTA with the recoveries of 92.3%-104%.

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.

Unravelling the Aptamer-Analyte Interaction Dynamics through Fluorescence Quenching in Graphene Quantum Dots (GQDs) Based Homogeneous Assays

Graphene quantum dots (GQDs) are used here as a biosensing platform for the recognition of the major food contaminant ochratoxin A (OTA), with a fluorescently labelled DNA aptamer (FAM OTA aptamer) functioning as the biorecognition element. The detection principle lies in the formation of noncovalent interactions between the FAM OTA aptamer and the GQD surface, and the consequent fluorescence quenching. The further change in the fluorescence signal, induced by the formation of the FAM OTA Aptamer/OTA conjugate during the detection step, could then be correlated to the presence and concentration of the target analyte. Upon tuning the concentration of GQDs, a switch in the biorecognition mechanism occurred. Specifically, while a lower GQD concentration (0.060 mg/mL) resulted in a restoration of the fluorescence intensity upon incubation with OTA, a higher GQD concentration (0.150 mg/mL) provided a further quenching of the final fluorescence intensity. Upon further calibration study, it was discovered that the latter mechanism provided a better option in terms of linearity of response, detection limit and selectivity.

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.

A fluorometric aptamer-based assay for ochratoxin A by using exonuclease III-assisted recycling amplification

A fluorometric assay is described for ochratoxin A (OTA) using an aptamer. The method is based on exonuclease-assisted recycling amplification. The OTA-binding aptamer partially hybridizes with complementary DNA (cDNA) that is released when the aptamer recognizes OTA. Then, cDNA hybridizes with a specifically designed hairpin DNA. Next, short ssDNA and cDNA are, respectively, released by exonuclease III catalyzed hydrolysis of the dsDNA. The cDNA induces the next ring opening and digestion. The short ssDNA captures the sDNA that is labeled with fluorescent FAM and is absorbed on graphene oxide (GO). The green fluorescence of the sDNA/GO system is quenched but is recovered if the sDNA is released from GO. This assay is high sensitive, works in the 5 nM to 200 nM OTA concentration range and has a 0.96 nM lower detection limit. It was applied to the quantitation of OTA in spiked wine and coffee samples. Graphical abstractSchematic of a fluorometric assay based on exonuclease-assisted recycling amplification for quantitative monitoring of OTA without the need of sample separation and multiple washing steps.

Graphene quantum dots mediated electron transfer in DNA base pairs

Graphene quantum dots (GQDs) were connected to [Ru(bpy)3]2+ to sense DNA-mediated charge transfer. Interaction between abasic site double stranded DNA (Abasic-DNA) and [Ru(bpy)3-GQD]2+ was investigated by absorption spectroscopy, gel electrophoresis, circular dichroism, and melting temperature measurements. The results indicate that [Ru(bpy)3-GQD]2+ could be intercalated into double stranded DNA. Using [Ru(bpy)3-GQD]2+ as a signal molecule, the charge transfer performance of DNA-intercalated [Ru(bpy)3-GQD]2+ was determined using electrochemical and electrochemiluminescence measurements. Various DNA types were immobilized on Au electrodes via Au-S bonds. Electrochemiluminescence and electrochemical measurements indicate that [Ru(bpy)3-GQD]2+ could enhance DNA-mediated charge transfer when intercalated into an abasic site of double stranded DNA. And comparing with [Ru(bpy)3]2+, it can be concluded that GQDs intercalate into the DNA duplex by acting as a base analog, thus enhancing DNA charge transfer. These findings suggest that the DNA-GQD structure could aid the development of molecular devices and electric drivers, and broaden the application of DNA charge transfer.

Label-Free Fluorescent Aptasensor for Ochratoxin-A Detection Based on CdTe Quantum Dots and (N-Methyl-4-pyridyl) Porphyrin

With the widespread contamination of ochratoxin A (OTA), it is of significant importance for detecting OTA in foods and traditional Chinese medicine (TCM). In this study, a novel label-free fluorescent aptasensor utilizing the interaction between OTA-triggered antiparallel G-quadruplex and (N-methyl-4-pyridy) porphyrin (TMPyP) for the rapid and sensitive determination of OTA was established. The fluorescence of CdTe quantum dots (QDs) could be quenched by TMPyP. In the presence of analyte (OTA), the aptamer could recognize OTA and transform from a random coil to the antiparallel G-quadruplex. The interaction between G-quadruplex and TMPyP could release CdTe QDs from TMPyP, and thus recover the fluorescence of CdTe QDs. Under optimized conditions, the detection limit of the designed aptasensor was 0.16 ng mL⁻¹, with a linear range of 0.2 to 20 ng mL⁻¹. Furthermore, this aptasensor showed high selectivity toward OTA against other structural analogs and other mycotoxins, and was successfully applied in Astragalus membranaceus samples. The presented aptasensor for OTA detection could be a promising tool for the field monitoring of food and TCM.

A Water-Stable Luminescent Metal-Organic Framework for Rapid and Visible Sensing of Organophosphorus Pesticides

Metal-organic frameworks (MOFs) have shown considerable prospects for sensing pesticide residues. However, the low stability of MOFs in water hinders them from testing food and environmental samples. Herein, we report an easy and cost-efficient synthesis of a water-stable zirconium luminescent MOF (Zr-LMOF) and its application for rapid, sensitive, and in situ detection of organophosphorous pesticides (OPPs). The Zr-MOF is prepared using Zr(IV) and 1,2,4,5-tetrakis(4-carboxyphenyl)benzene. The synthesized Zr-LMOF rapidly absorbs trace amounts of OPP parathion-methyl and indicates its presence. A low limit of detection of 0.115 μg kg^-1 (0.438 nM) with a wide linear range from 70 μg kg^-1 to 5.0 mg kg^-1 was achieved. Satisfactory recoveries ranging from 78% to 107% were obtained for spiked food and environmental samples. Further, the Zr-LMOF was applied to imitate rapid and in situ imaging detection of pesticide residue on fresh produce nondestructively; visual signals appeared under ultraviolet light within 5 min. These results suggest that the Zr-LMOF has the potential for low-cost, rapid, and in situ imaging detection of OPPs contamination via easy-to-read visual signal.

Exonuclease I-assisted fluorescent method for ochratoxin A detection using iron-doped porous carbon, nitrogen-doped graphene quantum dots, and double magnetic separation

In this paper, a fluorescent method was developed for ochratoxin A (OTA) detection that uses iron-doped porous carbon (MPC) and aptamer-functionalized nitrogen-doped graphene quantum dots (NGQDs-Apt) as probes. In this method, the adsorbance of the NGQDs-Apt on the MPC due to a π-π interaction between the aptamer and the MPC results in the quenching of the fluorescence of the NGQDs-Apt. However, since OTA interacts strongly with the aptamer, the presence of OTA leads to the detachment of the NGQDs-Apt from the MPC, resulting in the resumption of fluorescence from the NGQDs-Apt. When exonuclease I (Exo I) is also added to the solution, this exonuclease specifically digests the aptamer, leading to the release of the OTA back into the solution. This free OTA then interacts with another MPC-NGQDs-Apt system, inducing the release of more NGQDs into the solution, which enhances the fluorescent intensity compared to that of the system with no Exo I. Utilizing this behavior of OTA in the presence of NGQDs-Apt, it was possible to detect concentrations of OTA ranging from 10 to 5000 nM, with a limit of detection of 2.28 nM. Our method was tested by applying it to the detection of OTA in wheat and corn samples. This method has four advantages: (1) the magnetic porous carbon is easy to prepare, its porosity enhances its loading capacity for NGQDs, it highly efficiently quenches the fluorescence of the NGQDs, and its magnetic properties facilitate the separation of the MPC from other species in solution; (2) applying double magnetic separation decreases the background signal; (3) Exo I digests the free aptamer effectively, which allows the resulting free OTA to induce the release of more NGQDs-Apt, ultimately enhancing the fluorescent signal; and (4) the proposed method presented high sensitivity and a wide linear detection range. This method may prove helpful in food safety analysis and new biosensor development (achieved by using different aptamer sequences to that used in the present work). Graphical abstract Exonuclease I (Exo I)-assisted fluorescent method for ochratoxin A (OTA) detection using magnetic porous carbon (MPC), nitrogen-doped graphene quantum dots (NGQDs), and double magnetic separation.

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.

A multiplexed FRET aptasensor for the simultaneous detection of mycotoxins with magnetically controlled graphene oxide/Fe3O4 as a single energy acceptor

A multiplexed FRET aptasensor was developed for the simultaneous detection of AFB1 and FB1 with magnetically controlled GO/Fe₃O₄ as a single energy acceptor.

A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

Enhancing the sensing specificity of a MoS2 nanosheet-based FRET aptasensor using a surface blocking strategy

Aptamer-based biosensing, which uses short, single-stranded nucleic acid segments to bind to a target, can be advantageous over antibody-based diagnostics due to the ease of synthesis and high stability of aptamers. However, the development of most aptamer-based sensors (aptasensors) is still in its initial stages and many factors affecting their performance have not been studied in great detail. Here, we enhance the sensing specificity of a fluorescence resonance energy transfer (FRET)-based MoS₂ nanosheet aptasensor in detecting the malarial biomarker Plasmodium lactate dehydrogenase (pLDH). In this sensing scheme, the presence of target is signaled by an increase in fluorescence when fluorescently-labeled aptamers bind to pLDH and release from a quenching material. Interestingly, unlike most of the reported literature on aptasensors, we observe that non-target proteins also cause a considerable increase in the detected fluorescence. This may be due to the nonspecific adsorption of proteins onto the fluorescence quencher, leading to the displacement of aptamers from the quencher surface. To reduce this nonspecific association and to enhance the sensor specificity, we propose the application of a surface blocking agent to the quenching material. Importantly, we demonstrate that the sensing specificity of the MoS₂ nanosheet-based aptasensor towards target pLDH biomolecules can be significantly enhanced through surface passivation, thus contributing to the development of highly selective and robust point-of-care malaria diagnostics.

A fluorometric aptasensor for patulin based on the use of magnetized graphene oxide and DNase I-assisted target recycling amplification

A fluorometric patulin (PAT) assay is presented that is based on the use of magnetic reduced graphene oxide (rGO) and DNase I. The fluorescence of the PAT aptamer labelled with 6-carboxyfluorescein (FAM) is quenched by magnetized reduced graphene oxide (rGO-Fe₃O₄) due to fluorescence resonance energy transfer (FRET). However, in the presence of PAT, the labelled aptamer is stripped off from rGO-Fe₃O₄. The rGO-Fe₃O₄ is then magnetically separated so that the fluorescence of free labelled PAT aptamer is restored. DNase I cannot hydrolyze the aptamer on rGO-Fe₃O₄, but it can cleave the free aptamer-PAT complex. This will release FAM and PAT which can undergo a number of additional cycles to trigger the cleavage of abundant aptamer. Recycling of DNase I-assisted target therefore leads to a strong amplification of fluorescence and consequently to an assay with low limit of detection. The detection limit for PAT is as low as 0.28 μg L^-1 which is about 13 times lower than that without using DNase I. The method offers a new approach towards rapid, sensitive and selective detection based on an aptamer. Conceivably, it has a wide scope in that it may be applied to numerous other analytes if appropriate aptamers are available. Abstract Schematic of a fluorometric assay based on the use of magnetic graphene oxide and DNase I. It was applied to the determination of patulin. DNase I was introduced for recycling amplification. The detection limit is about 13 times lower than that without using DNase I. Figure a contains poor quality of text in image. Otherwise, please provide replacement figure file.Thank you. I will provide the figure file.

Advances in aptasensors for the detection of food contaminants

Food safety is a global health objective, and foodborne diseases represent a major crisis in health. Techniques that are simple and suitable for fast screening to detect and identify pathogenic factors in the food chain are vital to ensure food safety. At present, a variety of analytical methods have been reported for the detection of pathogenic agents. Whereas the sensitivity of detection and quantification are still important challenges, we expect major advances from new assay formats and synthetic bio-recognition elements, such as aptamers. Owing to the specific folding capability of aptamers in the presence of an analyte, aptasensors have substantially and successfully been exploited for the detection of a wide range of small and large molecules (e.g., toxins, antibiotics, heavy metals, bacteria, viruses) at very low concentrations. Here, we review the use of aptasensors for the development of highly sensitive and affordable detection tools for food analysis.

Critical Review: DNA Aptasensors, Are They Ready for Monitoring Organic Pollutants in Natural and Treated Water Sources?

There is a growing need to monitor anthropogenic organic contaminants detected in water sources. DNA aptamers are synthetic single-stranded oligonucleotides, selected to bind to target contaminants with favorable selectivity and sensitivity. These aptamers can be functionalized and are used with a variety of sensing platforms to develop sensors, or aptasensors. In this critical review, we (1) identify the state-of-the-art in DNA aptamer selection, (2) evaluate target and aptamer properties that make for sensitive and selective binding and sensing, (3) determine strengths and weaknesses of alternative sensing platforms, and (4) assess the potential for aptasensors to quantify environmentally relevant concentrations of organic contaminants in water. Among a suite of target and aptamer properties, binding affinity is either directly (e.g., organic carbon partition coefficient) or inversely (e.g., polar surface area) correlated to properties that indicate greater target hydrophobicity results in the strongest binding aptamers, and binding affinity is correlated to aptasensor limits of detection. Electrochemical-based aptasensors show the greatest sensitivity, which is similar to ELISA-based methods. Only a handful of aptasensors can detect organic pollutants at environmentally relevant concentrations, and interference from structurally similar analogs commonly present in natural waters is a yet-to-be overcome challenge. These findings lead to recommendations to improve aptasensor performance.

A Label-Free Aptasensor for Ochratoxin a Detection Based on the Structure Switch of Aptamer

A label-free sensing platform is developed based on switching the structure of aptamer for highly sensitive and selective fluorescence detection of ochratoxin A (OTA). OTA induces the structure of aptamer, transforms into G-quadruplex and produces strong fluorescence in the presence of zinc(II)-protoporphyrin IX probe due to the specific bind to G-quadruplex. The simple method exhibits high sensitivity towards OTA with a detection limit of 0.03 nM and excellent selectivity over other mycotoxins. In addition, the successful detection of OTA in real samples represents a promising application in food safety.

Designed Strategies for Fluorescence-Based Biosensors for the Detection of Mycotoxins

Small molecule toxins such as mycotoxins with low molecular weight are the most widely studied biological toxins. These biological toxins are responsible for food poisoning and have the potential to be used as biological warfare agents at the toxic dose. Due to the poisonous nature of mycotoxins, effective analysis techniques for quantifying their toxicity are indispensable. In this context, biosensors have been emerged as a powerful tool to monitors toxins at extremely low level. Recently, biosensors based on fluorescence detection have attained special interest with the incorporation of nanomaterials. This review paper will focus on the development of fluorescence-based biosensors for mycotoxin detection, with particular emphasis on their design as well as properties such as sensitivity and specificity. A number of these fluorescent biosensors have shown promising results in food samples for the detection of mycotoxins, suggesting their future potential for food applications.

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.

A Rapid Label-Free Fluorescent Aptasensor PicoGreen-Based Strategy for Aflatoxin B₁ Detection in Traditional Chinese Medicines

Aflatoxin B₁ (AFB₁) is a very hazardous carcinogen, readily contaminating foodstuffs and traditional Chinese medicines (TCMs) that has inspired increasing health concerns due to dietary exposure. Colloidal nanocrystals have been proposed as optical labels for aptasensor assembly, but these typically require tedious multistep conjugation and suffer from unsatisfactory robustness when used for complex matrices. In the present study, we report a rapid and sensitive method for screening for trace AFB₁ levels in TCMs using a label-free fluorescent aptasensor PicoGreen dye-based strategy. Using PicoGreen to selectively measure complementary double-stranded DNA, fluorescence enhancement due to dsDNA is ‘turned off’ in the presence of AFB₁ due binding of aptamer target over complementary sequence. Self-assembly of a label-free fluorescent aptasensor based on AFB₁ aptamer and PicoGreen dye was performed. Due to competition between the complementary sequence and AFB₁ target, this rapid method was capable of highly sensitive and selective screening for AFB₁ in five types of TCMs. This proposed approach had a limit of detection as low as 0.1 μg·L⁻¹ and good linearity with a range of 0.1–10 μg·L⁻¹ (0.1–10 ppb). Among the 20 samples tested, 6 batches were found to be contaminated with AFB₁ using this method, which was confirmed using sophisticated liquid chromatography-electrospray ionization-tandem mass spectrometry/mass spectrometry analysis. The results of this study indicate the developed method has the potential to be a simple, quick, and sensitive tool for detecting AFB₁ in TCMs.

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.