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

Simultaneous detection of multiple microRNAs based on fluorescence resonance energy transfer under a single excitation wavelength

MicroRNAs (miRNAs) play a key role in physiological processes, and their dysregulation is closely related to various human diseases. Simultaneous detection of multiple miRNAs is pivotal to cancer diagnosis at an early stage. However, most multicomponent analyses generally involve multiple excitation wavelengths, which are complicated and often challenging to simultaneously acquire multiple detection signals. In this study, a convenient and sensitive sensor was developed to simultaneously detection of multiple miRNAs under a single excitation wavelength through the fluorescence resonance energy transfer between the carbon dots (CDs)/quantum dots (QDs) and graphene oxide (GO). A hybridization chain reaction (HCR) was triggered by miRNA-141 and miRNA-21, resulting in the high sensitivity with a limit of detection (LOD) of 50 pM (3σ/k) for miRNA-141 and 60 pM (3σ/k) for miRNA-21. This simultaneous assay also showed excellent specificity discrimination against the mismatch. Furthermore, our proposed method successfully detected miRNA-21 and miRNA-141 in human serum samples at a same time, indicating its diagnostic potential in a clinical setting.

A label-free aptasensor based on electrodeposition of gold nanoparticles on silver-based metal-organic frameworks for measuring ochratoxin A in black and red pepper

Since ochratoxin A (OTA) is immunotoxic, teratogenic and carcinogenic, it is very important to monitor this compound in food samples. In the present work, the development and fabrication of a label-free electrochemical aptasensor based on the gold nanoparticles/silver-based metal-organic framework (AuNPs/Ag-MOF) for the determination of ochratoxin A (OTA) is introduced. The aptasensor was fabricated by electrodeposition of AuNPs on a glassy carbon electrode modified with Ag-MOF. The characteristics of the synthesized Ag-MOF were determined by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and UV-Visible spectroscopy. The aptamer was immobilized on the modified electrode and then OTA was incubated on it. The process of different stages of the aptasensor construction has been confirmed by two methods of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) and using [Fe(CN)6]3-/4- as a redox probe. The EIS method has also been used for the OTA quantitative determination. The difference in charge transfer resistance (Rct) before and after the interaction of OTA with the immobilized aptamer was considered as the analytical response of the aptasensor. Using the developed aptasensor, it is possible to measure OTA in the concentration range of 1.0 × 10-3 to 200.0 ng mL-1 with a detection limit of 2.2 × 10-4 ng mL-1. Finally, the ability of the aptasensor to measure OTA in red and black pepper was investigated and completely satisfactory results were obtained.

N/S-Co-doped carbon dot-based FRET ratiometric fluorescence aptasensing platform modulated with entropy-driven DNA amplifier for ochratoxin A detection

This study proposes a nitrogen and sulfur co-doped carbon dot (N/S-CD)-based FRET ratiometric fluorescence aptasensing strategy modulated with entropy-driven DNA amplifier for sensitive and accurate detection of ochratoxin A (OTA). In the strategy, a duplex DNA probe containing OTA aptamer and complementary DNA (cDNA) is designed as a recognition and transformation element. Upon sensing of target OTA, the cDNA was liberated, and triggered a three-chain DNA composite-based entropy-driven DNA circuit amplification, making CuO probes anchor on a magnetic bead (MB). The CuO-encoded MB complex probe is finally turned into abundant Cu²⁺, which oxidizes o-phenylenediamine (oPD) to generate 2,3-diaminophenazine (DAP) with yellow fluorescence and further triggers FRET between the blue fluorescent N/S-CDs and DAP. The changes in ratiometric fluorescence are related to the OTA concentration. Originating from the synergistic amplifications from the entropy-driven DNA circuits and Cu²⁺ amplification, the strategy dramatically enhanced detection performance. A limit of detection as low as 0.006 pg/mL of OTA was achieved. Significantly, the aptasensor can visually evaluate the OTA via on-site visual screening. Moreover, the high-confidence quantification of the OTA in real samples with results consistent with that of the LC-MS method indicated that the proposed strategy has practical application prospects for sensitive and accurate quantification in food safety.

A label-free fluorescent aptasensor based on a novel exponential rolling circle amplification for highly sensitive ochratoxin A detection

Rapid and sensitive analysis of ochratoxin A (OTA) plays an important role in food safety. Here, an aptasensor based on novel exponential rolling circle amplification (ERCA) was proposed for ultrasensitive and label-free fluorescence detection of OTA. The attachment of OTA to its aptamer could release H and rapidly hybridize with CT to initiate rolling circle amplification (RCA). The amplicons could further displace H from APH to initiate recycled RCA, achieving exponential growth of amplification products that contained G4 dimers for lighting up ThT. Benefiting from the exponential amplification efficiency of the ERCA strategy and the high fluorescence quantum yield of G4 dimer/ThT, this strategy exhibited a wide linear range from 10 fg/mL to 10 ng/mL with a detection limit of 4.3 fg/mL. In addition, the aptasensor displayed satisfactory recoveries in real sample analysis. We believe that this novel aptasensor possesses promising application prospects in food safety and medicine detection.

A Hydrothermal Method to Generate Carbon Quantum Dots from Waste Bones and Their Detection of Laundry Powder

Surfactants are one of the major pollutants in laundry powder, which have an impact on the environment and human health. Carbon quantum dots (CQDs) are spherical zero-dimensional fluorescent nanoparticles with great potential for fluorescent probing, electrochemical biosensing and ion sensing. Herein, a bottom-up approach was developed for the synthesis of CQDs from biomass to detect laundry detergent and laundry powder. Waste chicken bones were used as carbon precursors after being dried, crushed and reacted with pure water at 180 °C for 4 h to generate CQDs, which exhibited a monodisperse quasi-spherical structure with an average particle size of 3.2 ± 0.2 nm. Functional groups, including -OH, C=O, C=C and C-O, were identified on the surface of the prepared CQDs. The optimal fluorescence excitation wavelength of the yellow-brown CQDs was 380 nm, with a corresponding emission peak at 465 nm. CQDs did not significantly increase cell death in multiple cell lines at concentrations of 200 µg·mL⁻¹. Fluorescence enhancement of CQDs was observed after addition of sodium dodecyl benzene sulphonate, a major anionic surfactant in laundry powder. A linear relationship between fluorescence enhancement CQDs and the concentration of laundry powder was established. Thus, a hydrothermal method was developed to generate CQDs from waste biomass that may be used as a fluorescent probe to detect laundry powder.

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.

Application of Nanomaterials for Coping with Mycotoxin Contamination in Food Safety: From Detection to Control

Mycotoxins, which are toxic secondary metabolites produced by fungi, are harmful to humans. Mycotoxin-induced contamination has drawn attention worldwide. Consequently, the development of reliable and sensitive detection methods and high-efficiency control strategies for mycotoxins is important to safeguard food industry safety and public health. With the rapid development of nanotechnology, many novel nanomaterials that provide tremendous opportunities for greatly improving the detection and control performance of mycotoxins because of their unique properties have emerged. This review comprehensively summarizes recent trends in the application of nanomaterials for detecting mycotoxins (fluorescence, colorimetric, surface-enhanced Raman scattering, electrochemical, and point-of-care testing) and controlling mycotoxins (inhibition of fungal growth, mycotoxin absorption, and degradation). These detection methods possess the advantages of high sensitivity and selectivity, operational simplicity, and rapidity. With research attention on the control of mycotoxins and the gradual excavation of the properties of nanomaterials, nanomaterials are also employed for the inhibition of fungal growth, mycotoxin absorption, and mycotoxin degradation, and impressive controlling effects are obtained. This review is expected to provide the readers insight into this state-of-the-art area and a reference to design nanomaterials-based schemes for the detection and control of mycotoxins.

A fluorescence aptasensor based on carbon quantum dots and magnetic Fe3O4 nanoparticles for highly sensitive detection of 17β-estradiol

Trace amounts of 17β-estradiol (E2) in food and the environment poses a threat to human health, which has created the demand for sensitive analytical methods to detect E2. In this study, a novel fluorescent aptasensor was developed for sensitive detection of E2 based on double-chain hybridization between carbon quantum dots-labelled with E2 aptamer (CQDs-aptamer) and Fe₃O₄ nanoparticles modified by complementary DNA (Fe₃O₄-cDNA). Under the optimal conditions, the aptasensor displayed a good linear range of 10^-11-10^-6 M for E2 with the coefficient of determination (R²) of 0.996, and a low detection limit of 3.48 × 10^-12 M was obtained. Besides, the aptasensor showed high selectivity and good reproducibility for E2 detection, which was successfully applied to the sensitive detection of E2 in milk as compared with tap water and lake water with satisfactory recoveries from 85.21% to 114.80%, suggesting the great significance of this aptasensor for detecting food contaminants in the food industry.

Nanomaterial-based aptamer biosensors for ochratoxin A detection: a review

Ochratoxin A (OTA) is a widely distributed mycotoxin that often contaminates food, grains and animal feed. It poses a serious threat to human health because of its high toxicity and persistence. Therefore, the development of an inexpensive, highly sensitive, accurate and rapid method for OTA detection is imperative. In recent years, various nanomaterials used in the establishment of aptasensors have attracted great attention due to their large surface-to-volume ratio, good stability and facile preparation. This review summarizes the development of nanomaterial-based aptasensors for OTA determination and sample treatment over the past 5 years. The nanomaterials used in OTA aptasensors include metal, carbon, luminescent, magnetic and other nanomaterials. Finally, the limitations and future challenges in the development of nanomaterial-based OTA aptasensors are reviewed and discussed.

Toehold-Mediated Cascade Catalytic Assembly for Mycotoxin Detection and Its Logic Applications

In this work, an enzyme-free biosensor is reported for mycotoxin detection based on a toehold-mediated catalytic hairpin assembly (CHA) and a DNAzyme-cascaded hydrolysis reaction. In the presence of a mycotoxin, the recognition between an aptamer and the mycotoxin releases the trigger DNA. The trigger DNA initiates the toehold-mediated CHA, generating large amounts of partial duplex B/C with four toeholds, which can be used to assemble the DNAzyme-cascaded hydrolysis reaction. Furthermore, through a collaborative autoassembly reaction among the B/C duplex, DNA1, and DNA2, supramolecular nanostructures corresponding to Mg²⁺-dependent DNAzymes can be formed. With the incubation of Mg²⁺, the dual-modified (TAMRA/BHQ2) substrate strand DNA2 will be cleaved into two fragments, yielding a high TAMRA fluorescence signal for mycotoxin testing. Under optimal conditions, the sensing system was ultrasensitive and showed low detection limits of 0.2 pM for ochratoxin A (OTA), 0.13 pM for aflatoxin B1 (AFB1), and 0.17 pM for zearalenone (ZEN). The mycotoxin aptasensor also exhibited high selectivity and was successfully applied for the quantitative analysis of OTA, AFB1, and ZEN in wine samples. Due to the advantages of flexibility and versatility, this mycotoxin platform was used to fabricate several concatenated logic gates including "AND-INHIBIT", "INHIBIT-OR", "OR-AND", and "OR-INHIBIT" logic biocomputings. Such multiple functions of the logic system provided a universal sensing strategy for the intelligent detection of multiplex mycotoxins, demonstrating considerable potential in food safety and environmental monitoring.

Simple Design Concept for Dual-Channel Detection of Ochratoxin A Based on Bifunctional Metal-Organic Framework

A simple fluorescence and electrochemical dual-channel biosensor based on bifunctional Zr(IV)-based metal-organic framework (Zr-MOF) was proposed to detect Ochratoxin A (OTA). The bifunctional Zr-MOF, with photoluminescence properties and enormous electroactive ligands, was exploited to load OTA-specific aptamers for designing signal probes, greatly simplifying the probe-fabrication process and improving sensing reliability. Upon specific recognition of aptamer toward OTA, the anchored probe was released from the sensing interface into the reaction solution. In this circumstance, the increased amount of the signal probe in reaction solution led to an enhanced fluorescence response, while the decreased amount of the signal probe on the sensing interface resulted in a diminished electrochemical response. According to the dual-channel signal change with increasing OTA concentration, the visual fluorescence strategy was established for intuitive OTA detection, and meanwhile, sensitive electrochemical assay with a detection limit of 0.024 pg/mL was also achieved with the help of one-step electrodeposition as a sensing platform. Moreover, the proposed dual-channel assay has been successfully applied to determine OTA levels in corn samples with rapid response, superior accuracy, and high anti-interference capability, providing a promising method for food safety monitoring.

Label-free bioassay with graphene oxide-based fluorescent aptasensors: A review

Bioassays using a fluorophore and DNA aptamer have been extensively developed due to the ultrasensitivity of fluorophores and recognition ability of DNA aptamers. Conventional fluorescent aptamer-based sensors (aptasensors) require chemical labeling between the fluorophore and aptamer and is technologically impracical for various sensing and assay applications. A simple "mix and go" strategy has been introduced that uses label-free technology as a platform for sensor development. The biosensors comprise a fluorophore, a ssDNA aptamer, and eco-friendly graphene oxide (GO). In the absence of the sensor target, GO quenches the fluorescence of the fluorophore and single-strand DNA aptamer complex. When the target is added, the DNA aptamer conformationally turns into a duplex, G-quadruplexe, or other secondary structure. This structure change leads to release of GO by the fluorophore-aptamer-target complex, generating dramatic fluorescence recovery and amplification. With this sensing method, the DNA aptamer does not need to be chemically labeled. Therefore, flexible fluorophore indicators and ssDNA aptamers can be used in this label-free aptasensing strategy. In this review, we discuss various unlabeled fluorophores, including synthetic small molecular fluorophores and genetically encoded fluorescent proteins, as indicators for generating GO-based fluorescent DNA aptasensors for label-free bioassay.

Sulphur-doped graphene quantum dot based fluorescent turn-on aptasensor for selective and ultrasensitive detection of omethoate

Development of selective, ultra-sensitive, rapid and facile methods for the detection of chemical residues of toxic pesticides and hazardous chemicals are quite important in food safety, environmental monitoring and safeguarding public health. Herein, we presented a fluorescent turn-on aptasensor based on sulphur-doped graphene quantum dot (S-GQD) utilizing specific recognition and binding property of aptamer for the ultra-sensitive and selective detection of omethoate (OM) which is a systemic organophosphorus pesticide. The detection method is based on tuning aggregation-disaggregation mechanism of S-GQD by way of conformational alteration of the recognition probe. Fluorescence 'turn-on' process includes aggregation-induced quenching of S-GQD with aptamer via S-GQD-aptamer complex formation and its subsequent fluorescence recovery with the addition of OM by structural switching of S-GQD-aptamer complex to aptamer-omethoate complex. The reported 'switch-on' aptasensor has exhibited a low limit of detection of 0.001 ppm with high selectivity for OM over other pesticides.

A Simple and Sensitive Nanogold RRS/Abs Dimode Sensor for Trace As3+ Based on Aptamer Controlled Nitrogen Doped Carbon Dot Catalytic Amplification

Using citric acid (CA) and ethylenediamine (EDA) as precursors, stable nitrogen-doped carbon dots (CD) nanosols were prepared by microwave procedure and characterized in detail. It was found that CD_Ns catalyze ethanol (Et)-HAuCl₄ to generate gold nanoparticles (AuNPs), which have strong surface plasmon resonance, Rayleigh scattering, (RRS) and a surface plasmon resonance (SPR) absorption (Abs) effect at 370 nm and 575 nm, respectively. Compled the new catalytic amplification indicator reaction with the specific As³⁺ aptamer reaction, a new RRS/Abs dual-mode aptamer sensor for the assay of trace As³⁺ was developed, based on the RRS/Abs signals increasing linearly with As³⁺ increasing in the ranges of 5-250 nmol/L and 50-250 nmol/L, whose detection limits were 0.8 nmol/L and 3.4 nmol/L As³⁺, respectively. This analytical method has the advantages of high selectivity, simplicity, and rapidity, and it has been successfully applied to the detection of practical samples.

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

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

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Simultaneous electrochemical aptasensing of patulin and ochratoxin A in apple juice based on gold nanoparticles decorated black phosphorus nanomaterial

Simultaneous detection of patulin (PAT) and ochratoxin A (OTA) in food products is in great demand, which can prevent toxins from being exposed to human and animal bodies. However, simultaneous detection of multiple targets still faces a challenge. Herein, we developed a novel electrochemical aptasensor for the simultaneous detection of PAT and OTA in apple juice based on gold nanoparticles decorated black phosphorus (AuNPs-BP) nanomaterial. AuNPs-BP function?/work? as a sensing platform for loading much different electrochemical signal molecules functionalized aptamers. In this context, methylene blue functionalized PAT aptamers (Mb-PAT-aptamers) and ferrocene functionalized OTA aptamers (Fc-OTA-aptamers) have been introduced here to fabricate the aptasensor. Fc close to electrode surface showed a strong signal, whereas Mb was far away from electrode surface so exhibited a weak signal in the absence of OTA and PAT. Two kinds of electrochemical signal changes have been recorded dependent on target of OTA and PAT concentrations. So, simultaneous detection of OTA and PAT is achieved. Under the optimum conditions, using this developed biosensor, PAT and OTA can be quantified at a linearity range of 0.01 × 10^-7 μg·mL^-1 ~ 0.10 μg·mL^-1. In addition, it also has good selectivity, stability and repeatability. For the practical application, it shows promising performance for the simultaneous detection of PAT and OTA in apple juice.

Electrochemical/visual dual-readout aptasensor for Ochratoxin A detection integrated into a miniaturized paper-based analytical device

Development of portable, sensitive and reliable devices for Ochratoxin A (OTA) detection is highly demanded, especially for resource-limited regions. Herein, a novel paper-based analytical device (PAD) is designed through wax printing and screen-printed technologies, which integrates sample flowing, electrode modification, cleaning and electrochemical (EC)/colorimetric signal output. To greatly enhance the detection sensitivity, we synthesized a chitosan functionalized MoS₂-Au@Pt (Ch-MoS₂-Au@Pt) via electrostatic self-assembly, and used it to immobilize the label aptamer (apta₂) for signal regulation and amplification. Concretely, with the addition of analytes, the Ch-MoS₂-Au@Pt-apta₂ could be combined on the sensing interface by specific biorecognition and catalyzed reduction of H₂O₂, resulting in a remarkable EC response. Meanwhile, the released hydroxyl radicals (·OH) flowed to the visualization zone and promoted the oxidation of 3,3',5,5'-tetramethylbenzidine for colorimetric detection. Consequently, the dual-mode PAD achieved acceptable prediction and accurate analysis in the range of 0.1-200 ng mL^-1 and 1 × 10^-4-200 ng mL^-1 by matching the visual and EC signal intensity, respectively. Compared with traditional single-mode sensor for OTA, the proposed dual-mode aptasensor featuring independent signal conversion and readout, not only avoided the false-positive signal associated with detection condition and operation, but also enlarged the detection ranges and improved the sensitivity. Furthermore, the consistency of EC/colorimetric assay was validated in real OTA samples. Overall, this work provided a portable, cost-effective, sensitive and visualized aptasensor platform, which could be extended to various other mycotoxins in the field of food safety.

Dual-color graphene quantum dots and carbon nanoparticles biosensing platform combined with Exonuclease III-assisted signal amplification for simultaneous detection of multiple DNA targets

Sensitive and simultaneous detection of multiple biomarkers such as target DNA or proteins using biocompatible materials with good analysis performance remains an important challenge. Herein, we successfully developed a signal "off-on" highly sensitive multiplex detection platform based on the combination of dual-color graphene quantum dots (blue GQDs and green GQDs) modified DNA probes with carbon nanoparticles (CNPs), which is a cheap, effective nonfluorescent quencher to simultaneously quench the fluorescence of both GQDs-DNA probes. The Exo III-assisted sequence-independent target recycling and signal amplification strategy was integrated into this sensing platform, which endows it with high sensitivity towards the multiplex detection of targets DNA. The detection limits of 6.6 pM for HIV and 9.5 pM for HBV were achieved respectively, which is about 60-fold lower than that of traditional unamplified homogeneous fluorescent assay methods. Our proposed multiplex detecting platform is advantageous in both respective and simultaneous detection of multiple targets and can also discriminate perfectly matched targets from mismatched targets in both PBS buffer and 1% human serum samples, demonstrating its potential to be a reliable strategy for highly sensitive simultaneous detection of multiple target genes in practical diagnosis applications.

Development of a novel label-free impedimetric electrochemical sensor based on hydrogel/chitosan for the detection of ochratoxin A

Ochratoxin A (OTA) is one of the most abundant mycotoxins that contaminate various food products. Herein, we propose a novel label-free impedimetric electrochemical sensor consisting of chitosan/dipeptide nanofibrous hydrogel and immobilized DNA probes with OTA aptamer for the detection of OTA. The thin film of chitosan/dipeptide nanofibrous hydrogel was used as sensing interface and carrier for hybridization chain reaction (HCR) of OTA aptamer and DNA2 strand to form DNA concatemer. The concatemer was dissociated to single-stranded DNA (ssDNA) in the presence of target OTA, and the signal amplification was further implemented by introducing RecJ_f exonuclease, which could digest the single-stranded DNA resulting in OTA recycle. Electrochemical impedance spectroscopy (EIS) has been employed to characterize the properties of the fabricated sensor. A linear detection range of 0.1-100 ng mL^-1 was obtained for OTA with a low detection limit of 0.03 ng mL^-1. Furthermore, the developed sensor was demonstrated in white wine to detect OTA, indicating that the proposed impedimetric sensor has a promising potential application in the food industry.

Cancer cell-targeted nanoprobe for multilayer imaging of diverse biomarkers and precise photodynamic therapy

A novel multifunctional nanoprobe was designed for cancer cell targeted multilayer imaging of two cancer biomarkers. Based on the proposed method, in situ imaging of membrane MUC1 mucin and cytoplasmic microRNA miR-21 coupled with precise photodynamic therapy was achieved.

Aggregation-induced responses (AIR) of 2D-derived layered nanostructures enable emerging colorimetric and fluorescence sensors

Layered nanostructures (LNs), including two-dimensional nanosheets, nanoflakes, and planar nanodots, show large surface-to-volume ratios, unique optical properties, and desired interfacial activities. LNs are highly promising as alternative probes and platforms due to numerous merits, e.g. signal amplification, improved recognition ability, and anti-interference capacity, for emerging sensing applications. Significantly, when stimuli-responsive aggregation occurs, the modified LNs show engineered morphologies, attractive optical absorption and fluorescence characteristics, which are remarkably programmable. On the basis of the altered aggregation behaviours of LNs, as well as their modulated physical and chemical characteristics, a series of novel sensing assays exhibiting enhanced sensitivity, simple operation, multiple functions, and improved anti-interference capacity are reported, contributing to both point-of-care testing and high-throughput measurements. Herein, the aggregation-induced response sensing strategies of LNs are comprehensively summarized with the classification of materials and variation of aggregated routes aiming at understanding dimension-dependent features, expanding nanoscale biosensor applications, and addressing key issues in disease diagnosis and environmental analysis.

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

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

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.

AuNanostar@4-MBA@Au Core-Shell Nanostructure Coupled with Exonuclease III-Assisted Cycling Amplification for Ultrasensitive SERS Detection of Ochratoxin A

The "turn-on" mode surface-enhanced Raman scattering (SERS) aptasensor for ultrasensitive ochratoxin A (OTA) detection was developed based on the SERS "hot spots" of AuNanostar@4-MBA@Au core-shell nanostructures (AuNS@4-MBA@Au) and exonuclease III (Exo III)-assisted target cycle amplification strategy. Compared with conventional gold nanoparticles, AuNS@4-MBA@Au provides a much higher SERS enhancement factor because AuNS exhibits a larger surface roughness and the lightning rod effect, as well as an excellent electromagnetic field between the AuNS core and the Au shell, which contribute to the superstrong SERS signal. Meanwhile, Exo III-assisted target cycle amplification can be used as an effective method for the further amplified detection of OTA. Additionally, the utilization of streptavidin magnesphere paramagnetic particles offers a green, economical, and facile technology for the accumulation and separation of the signal probe AuNS@4-MBA@Au from solution. All these factors lead to a significant enhancement of detectable signals and superhigh sensitivity. As a result, the limit of detection as low as 0.25 fg mL^-1 could be achieved, which was lower than that in the other reported literatures on SERS methods for OTA detection as we know. The developed SERS aptasensor also provides a promising tool for foodstuff detection.

Magneto-controlled aptasensor for simultaneous detection of ochratoxin A and fumonisin B1 using inductively coupled plasma mass spectrometry with multiple metal nanoparticles as element labels

The simultaneous detection of multiple mycotoxins is important for food safety. Here, a magneto-controlled aptasensor for quantitative analysis of ochratoxin A (OTA) and fumonisin B1 (FB1) using inductively coupled plasma mass spectrometry (ICP-MS) with multiple metal nanoparticles as element labels was proposed. Firstly, the OTA aptamer (Apt1) and the FB1 aptamer (Apt2) immobilized on the magnetic beads (MBs) were hybridized with probe DNA1-CdSe quantum dots (pDNA1-QDs) and probe DNA2-Ag nanoparticles (pDNA2-Ag NPs) labels, producing the MBs-Apt1-pDNA1-QDs and MBs-Apt2-pDNA2-Ag NPs conjugates, respectively. Then, the MBs-Apt1-OTA and MBs-Apt2-FB1 conjugates were generated with the addition of targets, resulting the pDNA1-QDs and pDNA2-Ag NPs labels released into the solutions. Finally, the signal intensities of ¹¹¹Cd and ¹⁰⁷Ag were detected by ICP-MS, achieving limits of detection of 0.10 and 0.30 ng mL^-1 for OTA and FB1, respectively. The assay showed high specificity and succeeded in wheat flour. The method provides an ideal model for sensitive analysis of multiple mycotoxins in food samples.

Dark field microscope-based single nanoparticle identification coupled with statistical analysis for ultrasensitive biotoxin detection in complex sample matrix

A novel approach for ultrasensitive ochratoxin A (OTA) detection is reported based on dark field microscope-based single nanoparticle identification coupled with a statistical analysis method. OTA aptamers were firstly hybridized with a single-stranded DNA (DNA1) to form an identification probe (DNA1-Apt). The aptamers separate from DNA1 in the presence of OTA and are released from the identification probe. Then, another single-stranded DNA (DNA2) hybridizes with DNA1 and result in the aggregation of gold nanoparticles (AuNPs). Therefore, the presence of AuNP aggregates is the evidence of the presence of OTA, while AuNP aggregates can be easily identified together with the monomers under dark field microscopic inspection. On the other hand, by counting the aggregation rate (the number of AuNP aggregates versus the number of AuNP monomers) with a statistical analysis method, OTA could be quantitatively detected. The detection range for OTA was 0.1 pg/mL ~ 30 ng/mL and the limit of detection was 0.1 pg/mL. The proposed sensor has comparative detection performance to sensors utilizing a number of signal amplification procedures, with the additional advantages of simplicity and high efficiency. The sensor can also be adopted for other target detection simply by replacing the identification probes. Graphical abstract The schematic of the AuNP aggregation for OTA detection. The OTA aptamers were competitively banded by OTA and induced form AuNP aggregation after adding DNA2 and AuNPs2. Subsequently, AuNPs were detected under dark field microscope and statistical analysis.

Target-responsive ratiometric fluorescent aptasensor for OTA based on energy transfer between [Ru(bpy)3]2+ and silica quantum dots

A ratiometric fluorescent aptasensor based on energy transfer between [Ru(bpy)₃]²⁺ and silica quantum dots (silica QDs) for assaying OTA was fabricated. The aptamer for OTA was used as the gate to shield the fluorescent reagent [Ru(bpy)₃]²⁺ into mesoporous silica nanoparticle (MSN). In the presence of OTA, the constrained [Ru(bpy)₃]²⁺ was released from MSN due to a target-induced aptamer conformational change. The released [Ru(bpy)₃]²⁺ adsorbed onto the negatively charged silica QDs through electrostatic interaction. This creates appearance of fluorescence from [Ru(bpy)₃]²⁺ at 625 nm and decrease of the fluorescence from silica QDs at 442 nm owing to the energy transfer. The value of FL_625nm/FL_442nm was in proportion to the concentration of OTA in the range 0.5~100 ng mL^-1 with a LOD of 0.08 ng mL^-1. Practical applicability of this method was validated by the determination of OTA in flour samples. Graphical abstract The sensing principle of this sensor.

Copper-Ion-Assisted Precipitation Etching Method for the Luminescent Enhanced Assembling of Sulfur Quantum Dots

In this study, we report a metal-ion-assisted precipitation etching strategy that can be used to manipulate the optical properties associated with the assembling of sulfur quantum dots (S dots) using copper ions. Transmission electron microscopy confirmed that the S dots were mainly distributed within 50-80 nm and that they exhibited an ambiguous boundary. After the post-synthetic Cu2+-assisted modification was completed, the assisted precipitation-etching S dots (APE-S dots) were observed to exhibit a relatively clear boundary with a high fluorescence (FL) quantum yield (QY) of 32.8%. Simultaneously, the Fourier transform infrared radiation, X-ray photoelectron spectra, and time-resolved FL decay spectra were used to illustrate the improvement in the FL QY of the APE-S dots.

A quencher-free 2-aminopurine modified hairpin aptasensor for ultrasensitive detection of Ochratoxin A

A sensitive, efficient and quencher-free fluorescence aptasensor to detect Ochratoxin A (OTA) based on aptamer, 2-aminopurine (2AP) labeled Oligonucleotide sequence, as well as exonuclease I (Exo I) activity was developed. In which the aptamer specific to OTA was modified into a hairpin structure, and 8 bases at the 3' ends are exposed (H); also, 2AP is embedded in the oligonucleotide complementary to the 8 bases (2AP-probe).The detection principle based on 2AP-probe could be bonded to its complementary sequence and quenches the fluorescence of 2AP; The aptamer has a stronger affinity for the target than its complementary sequence; Exo I can dissociate single-stranded DNA and has little effect on double-stranded DNA as well as folded DNA. In the absence of OTA, the fluorescence of 2AP is quenched due to the complementary pairing of H and 2AP-probe; in the presence of OTA, H selective binding target is detached from 2AP-probe, and the fluorescence of 2AP is slightly restored. Moreover, when the Exo I is added to the detection system, 2AP-probe is dissociated by the Exo I to release the free 2AP, and the fluorescence of the system is further enhanced thereby realizing the detection of OTA. The detection limit of the aptasensor was low as 0.03 nM with a linear range of 0.5-100 nM. Moreover, the aptasensor has good selectivity and practicability and also has good potential in realizing the detection of toxic and harmful substances in food complex matrices.

Perspective on the Future Role of Aptamers in Analytical Chemistry

It has been almost 30 years since the invention of Systematic Evolution of Ligands by Exponential Enrichment (SELEX) methodology and the description of the first aptamers. In retrospect over the past 30 years, advances in aptamer development and application have demonstrated that aptamers are potentially useful reagents that can be employed in diverse areas within analytical chemistry, biotechnology, biomedicine, and molecular biology. While often touted as artificial antibodies with an ability to be selected for any target, aptamer development, unfortunately, lags behind development of analytical methodologies that employ aptamers, hindering deeper integration into the application of analytical tool development. This perspective covers recent advances in SELEX methodology for improving efficiency of the SELEX procedure and enhancing affinity and specificity of the selected aptamers, what we view as a critical barrier in the future role of aptamers in analytical chemistry. We discuss postselection modifications that can be used for enhancing performance of the selected aptamers in an analytical device by including understanding intermolecular interaction forces in the binding domain. While highlighting promising properties of aptamers that enable several analytical advances, we provide discussion on the challenges of penetration of aptamers in the analytical field.

Highly selective antenna effect of graphene quantum dots (GQDs): A new fluorescent sensitizer for rare earth element terbium in aqueous media

This study focused on the fluorescence antenna-sensitizing effect of graphene quantum dots (GQDs) in a case on the detection of terbium ions. A simple one-step chemical oxidation method was applied for the preparation of GQDs starting from the regular multilayer graphene oxide (GO) via the refluxing in a concentrated mixture of strong acids. The as-prepared GQDs were further evaluated as a fluorescent sensitizer to the terbium ion. An expanded in-deep mechanism study on the fluorescence phenomena during the interaction of the as-prepared GQDs and REEs was made. The highly selective antenna effect of GQDs on one of REEs' aqueous media, which was terbium (III), was identified. The excited terbium ion emitted its long-living fluorescence based on its own characteristic line-typed f-f transition, contrasting to a undetectable fluorescence in a very poor quantum yield in its aqueous solution induced by water collisions. This study, in the first place, identified the significant sensitization effect of the as-prepared GQDs on the terbium ion in a high selectivity in aqueous media. The detectable linear range and the detection limit of the terbium ion was 0-30 × 10^-6 mol L^-1 (R² = 0.9960) of 0.3 × 10^-6 mol L^-1, respectively. The excitation wavelength and the optimal fluorescence wavelength were 230 nm 546 nm, respectively. Further material characterizations, involving XPS, FTIR, Raman and the Zeta potential, verified the important participation of carboxyl function groups on the as-prepared GQDs.

Plasmonic Au-Ag Janus Nanoparticle Engineered Ratiometric Surface-Enhanced Raman Scattering Aptasensor for Ochratoxin A Detection

Ochratoxin A (OTA), a toxic mycotoxin, poses severe risks to environment and human health. Herein, we develop a ratiometric surface-enhanced Raman scattering (SERS) aptasensor based on internal standard (IS) methods for the sensitive and reproducible quantitative detection of OTA. Au-Ag Janus nanoparticles (NPs) are successfully synthesized under the guidance of 2-mercaptobenzoimidazole-5-carboxylic acid (MBIA), which possesses intrinsic Raman signals, thus no additional modification with a Raman reporter on NPs is required. In addition, Au-Ag Janus NPs exhibit amplified and stable SERS activity. MXenes nanosheets generate a unique and stable Raman signal, making them an ideal IS for quantitative Raman analysis. In principle, Au-Ag Janus NPs are assembled with MXenes nanosheets depending on hydrogen bond and the chelation interaction between MXenes nanosheets and OTA aptamers. In the presence of OTA, Au-Ag Janus NPs are dissociated from MXenes nanosheets due to the formation of aptamer/OTA complex, leading to the attenuation of Raman signal of Au-Ag Janus NPs, and meanwhile, the signal of MXenes nanosheets remain constant. Quantitatively, upon correction by the IS Raman signals, sensitive and quantitative detection can be achieved with the limit of detection (LOD) of 1.28 pM for OTA. Our results suggest that this ratiometric SERS aptasensor is a powerful tool which shows great promise for applications in complex systems.

Review of Carbon and Graphene Quantum Dots for Sensing

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluorescent, photoluminescent, chemiluminescent, and electrochemiluminescent properties are what gives them potential in sensing. In this Review, we summarize the basic knowledge on CQDs and GQDs before focusing on their application to sensing thus far followed by a discussion of future directions for research into CQDs- and GQD-based nanomaterials in sensing. With regard to the latter, the authors suggest that with the potential of these nanomaterials in sensing more research is needed on understanding their optical properties and why the synthetic methods influence their properties so much, into methods of surface functionalization that provide greater selectivity in sensing and into new sensing concepts that utilize the virtues of these nanomaterials to give us new or better sensors that could not be achieved in other ways.

Advances in biosensors for the detection of ochratoxin A: Bio-receptors, nanomaterials, and their applications

Ochratoxin A (OTA) is a class of mycotoxin mainly produced by the genera Aspergillus and Penicillium. OTA can cause various forms of kidney, liver and brain diseases in both humans and animals although trace amount of OTA is normally present in food. Therefore, development of fast and sensitive detection technique is essential for accurate diagnosis of OTA. Currently, the most commonly used detection methods are enzyme-linked immune sorbent assays (ELISA) and chromatographic techniques. These techniques are sensitive but time consuming, and require expensive equipment, highly trained operators, as well as extensive preparation steps. These drawbacks limit their wide application in OTA detection. On the contrary, biosensors hold a great potential for OTA detection at for both research and industry because they are less expensive, rapid, sensitive, specific, simple and portable. This paper aims to provide an extensive overview on biosensors for OTA detection by highlighting the main biosensing recognition elements for OTA, the most commonly used nanomaterials for fabricating the sensing interface, and their applications in different read-out types of biosensors. Current challenges and future perspectives are discussed as well.

An "off-on" fluorescent sensor for copper ion using graphene quantum dots based on oxidation of l-cysteine

A simple and highly efficient "off-on" fluorescent sensor based on grapheme quantum dots (GQDs) for Cu²⁺ was developed. In this sensing platform, the fluorescence of GQDs was quenched in the presence of 2,4-dinitrophenylcysteine (DNPC), which is the reaction product of 1-chloro-2,4-dinitrobenzene (CDNB) and l-cysteine, owing to the spectral overlap between the absorption of DNPC and the excitation of GQDs. In the presence of Cu²⁺, l-cysteine was catalytically oxidized to l-cystine by O₂, resulting in the reduction of DNPC. Thus, the fluorescence of GQDs was recovery. Based on this, the fluorescent detection of Cu²⁺ could be achieved. The proposed sensing strategy offered a selective identification of Cu²⁺ with a detection limit of 4.5 nM. Additionally, the practical application of this assay for Cu²⁺ determination in real water samples was also demonstrated.