A homogeneous immunosensor for AFB1 detection based on FRET between different-sized quantum dots

The impact of aflatoxin B1 on animal health: Metabolic processes, detection methods, and preventive measures

Aflatoxin (AF) is a toxic metabolite produced by the fungus Aspergillus. The various subtypes of AFs include B1, B2, G1, G2, M1, and M2, with Aflatoxin B1 (AFB1) being the most toxic. These AFs are widespread in the environment, particularly in soil and food crops. The World Health Organization (WHO) has classified AFB1 as a highly potent natural Class 1A carcinogen. Excessive exposure to AFB1 can lead to poisoning in both humans and animals, posing substantial risks to food safety and livestock breeding industries. This review provides an overview of the metabolic processes, detection methods, and the detrimental impacts of AFB1 on animal reproduction, immunity, nerves, intestines, and metabolism. Furthermore, it explores the preventive and control capacities of natural active substances, trace elements, and microorganisms against AFB1. Ultimately, this paper serves as a reference for further research on the pathogenic mechanism of AFB1, the development of preventive drugs, and the selection of effective detoxification measures for AFB1 in animal feed.

Proximity-Unlocked Luminescence by Sequential Enzymatic Reactions from Antibody and Antibody/Aptamer (PULSERAA): A Platform for Detection and Visualization of Virus-Containing Spots

The SARS-CoV-2 pandemic has challenged more scientists to detect viruses and to visualize virus-containing spots for diagnosis and infection control; however, detection principles of commercially available technologies are not optimal for visualization. Here, a convenient and universal homogeneous detection platform named proximity-unlocked luminescence by sequential enzymatic reactions from antibody and antibody/aptamer (PULSERAA) is developed. This is designed so that the signal appears only when the donor and acceptor are in proximity on the viral surface. PULSERAA specifically detected in the range of 25-500 digital copies/mL of inactivated SARS-CoV-2 after simply mixing reagents; it is elucidated that the accumulation of chemical species in a limited space of the viral surface contributed to such high sensitivity. PULSERAA was quickly adapated to detect another virus variant, inactivated influenza A virus, and infectious SARS-CoV-2 in a clinical sample. Furthermore, on-site (direct, rapid, and portable) visualization of the inactivated SARS-CoV-2-containing spots by a conventional smartphone camera was achieved, demonstrating that PULSERAA can be a practical tool for preventing the next pandemic in the future.

Chimeric Protein Switch Biosensors

Rapid detection of protein and small-molecule analytes is a valuable technique across multiple disciplines, but most in vitro testing of biological or environmental samples requires long, laborious processes and trained personnel in laboratory settings, leading to long wait times for results and high expenses. Fusion of recognition with reporter elements has been introduced to detection methods such as enzyme-linked immunoassays (ELISA), with enzyme-conjugated secondary antibodies removing one of the many incubation and wash steps. Chimeric protein switch biosensors go further and provide a platform for homogenous mix-and-read assays where long wash and incubation steps are eradicated from the process. Chimeric protein switch biosensors consist of an enzyme switch (the reporter) coupled to a recognition element, where binding of the analyte results in switching the activity of the reporter enzyme on or off. Several chimeric protein switch biosensors have successfully been developed for analytes ranging from small molecule drugs to large protein biomarkers. There are two main formats of chimeric protein switch biosensor developed, one-component and multi-component, and these formats exhibit unique advantages and disadvantages. Genetically fusing a recognition protein to the enzyme switch has many advantages in the production and performance of the biosensor. A range of immune and synthetic binding proteins have been developed as alternatives to antibodies, including antibody mimetics or antibody fragments. These are mainly small, easily manipulated proteins and can be genetically fused to a reporter for recombinant expression or manipulated to allow chemical fusion. Here, aspects of chimeric protein switch biosensors will be reviewed with a comparison of different classes of recognition elements and switching mechanisms.

Engineering highly efficient NIR-II FRET platform for Background-Free homogeneous detection of SARS-CoV-2 neutralizing antibodies in whole blood

Förster or fluorescence resonance energy transfer (FRET) enables to probe biomolecular interactions, thus playing a vital role in bioassays. However, conventional FRET platforms suffer from limited sensitivity due to the low FRET efficiency and poor anti-interference of existing FRET pairs. Here we report a NIR-II (1000-1700 nm) FRET platform with extremely high FRET efficiency and exceptional anti-interference capability. This NIR-II FRET platform is established based on a pair of lanthanides downshifting nanoparticles (DSNPs) by employing Nd³⁺ doped DSNPs as an energy donor and Yb³⁺ doped DSNPs as an energy acceptor. The maximum FRET efficiency of this well-engineered NIR-II FRET platform reaches up to 92.2%, which is much higher than most commonly used ones. Owing to the all-NIR advantage (λ_ex = 808 nm, λ_em = 1064 nm), this highly efficient NIR-II FRET platform exhibits extraordinary anti-interference in whole blood, and thus enabling background-free homogeneous detection of SARS-CoV-2 neutralizing antibodies in clinical whole blood sample with high sensitivity (limit of detection = 0.5 μg/mL) and specificity. This work opens up new opportunities for realizing highly sensitive detection of various biomarkers in biological samples with severe background interference.

FRET Based Biosensor: Principle Applications Recent Advances and Challenges

Förster resonance energy transfer (FRET)-based biosensors are being fabricated for specific detection of biomolecules or changes in the microenvironment. FRET is a non-radiative transfer of energy from an excited donor fluorophore molecule to a nearby acceptor fluorophore molecule. In a FRET-based biosensor, the donor and acceptor molecules are typically fluorescent proteins or fluorescent nanomaterials such as quantum dots (QDs) or small molecules that are engineered to be in close proximity to each other. When the biomolecule of interest is present, it can cause a change in the distance between the donor and acceptor, leading to a change in the efficiency of FRET and a corresponding change in the fluorescence intensity of the acceptor. This change in fluorescence can be used to detect and quantify the biomolecule of interest. FRET-based biosensors have a wide range of applications, including in the fields of biochemistry, cell biology, and drug discovery. This review article provides a substantial approach on the FRET-based biosensor, principle, applications such as point-of-need diagnosis, wearable, single molecular FRET (smFRET), hard water, ions, pH, tissue-based sensors, immunosensors, and aptasensor. Recent advances such as artificial intelligence (AI) and Internet of Things (IoT) are used for this type of sensor and challenges.

A highly sensitive photothermal immunochromatographic sensor for detection of aflatoxin B1 based on Cu2-xSe-Au nanoparticles

In the study, Cu_2-xSe-Au nanoparticles (CSA) with a photothermal conversion efficiency of 60.78 % at 808 nm were applied to the construction of thermal analysis immunochromatographic test strips for the highly sensitive quantitative detection of aflatoxin B₁ (AFB₁) in grain. The CSA was coupled with the AFB₁ antibody to form a photothermal sensor probe by physical adsorption. The constructed immunosensor exhibited high sensitivity and a wide linear range from 0.01 to 10 μg/L in PBS. The detection limits of 0.00842 μg/L based on the thermal analysis was significantly improved by 11.88-fold compared with colorimetric results. No cross-reaction with the other mycotoxins was found except for aflatoxin B₂, aflatoxin M₁, aflatoxin G₁ and aflatoxin G₂. Applied to analysize grain sample, the method achieved the detection of AFB₁ ranging from 0.16 to 160 μg/kg.

An ultralow concentration of Al-MOFs for turn-on fluorescence detection of aflatoxin B1 in tea samples

A turn-on fluorescent sensing platform based on an ultralow concentration of Al-metal organic frameworks for the detection of aflatoxin B₁ has been developed for the first time. This fluorescence turn-on sensor exhibits the largest fluorescence enhancement (or quenching) constant value of 179404 M^-1 among all luminescence-based chemical sensors reported till date. Moreover, the sensor afforded a rapid detection of aflatoxin B₁, with a linear response in the concentration range of 0.05-9.61 μM and a low detection limit of 11.67 ppb. Additionally, the fabricated sensor showed good repeatability, reproducibility, stability, and selectivity. Most importantly, the practical application of this sensor has been demonstrated by detecting aflatoxin B₁ in complex tea samples with low relative standard deviation (≤7.72%; n = 3) and satisfactory recoveries. In summary, the proposed method has great potential as a simple, sensitive and selective strategy for monitoring aflatoxin B₁ in food samples.

An Immuno-Separated Assay for Ochratoxin Detection Coupled with a Nano-Affinity Cleaning-Up for LC-Confirmation

An immuno-separated assay for ochratoxin A detection coupled with a nano-affinity cleaning up for LC-confirmation was developed. Firstly, ochratoxin A was modified to quantum dot beads for immuno-fluorescent reporters. Secondly, Fe3O4 magnetic nanoparticles were conjugated with protein G for immuno-magnetic adsorbents. The immuno-separation of fluorescent reporters by magnetic adsorbents could be completed by ochratoxin A, so the fluorescent reporters released from the immune complex indicate a linear correlation with the concentration of ochratoxin A. Furthermore, the immuno-separated ochratoxin A can be eluted from magnetic adsorbent for LC-conformation. The optimized assay showed results as follows: the quantitative range of the immuno-separated assay was 0.03-100 ng mL-1 of ochratoxin A. The recoveries for spiked samples ranged from 78.2% to 91.4%, with the relative standard deviation (RSD) being 11.9%~15.3%. Statistical analysis indicated no significant difference between the HPLC-FLD results based on commercial affinity column and by nano-affinity cleaning up.

Fluonanobody-based nanosensor via fluorescence resonance energy transfer for ultrasensitive detection of ochratoxin A

Ochratoxin A (OTA) contamination in food is a serious threat to public health. There is an urgent need for development of rapid and sensitive methods for OTA detection, to minimize consumer exposure to OTA. In this study, we constructed two OTA-specific fluonanobodies (FluoNbs), with a nanobody fused at the carboxyl-terminal (SGFP-Nb) or the amino-terminal (Nb-SGFP) of superfolder green fluorescence protein. SGFP-Nb, which displayed better fluorescence performance, was selected as the tracer for OTA, to develop a FluoNb-based nanosensor (FN-Nanosens) via the fluorescence resonance energy transfer, where the SGFP-Nb served as the donor and the chemical conjugates of OTA-quantum dots served as the acceptor. After optimization, FN-Nanosens showed a limit of detection of 5 pg/mL, with a linear detection range of 5-5000 pg/mL. FN-Nanosens was found to be highly selective for OTA and showed good accuracy and repeatability in recovery experiments using cereals with various complex matrix environments. Moreover, the contents of OTA in real samples measured using FN-Nanosens correlated well with those from the liquid chromatography with tandem mass spectrometry. Therefore, this work illustrated that the FluoNb is an ideal immunosensing tool and that FN-Nanosens is reliable for rapid detection of OTA in cereals with ultrahigh sensitivity.

The application of the QDs/H2O2 chemiluminescence system in HRP assay and HRP-based immunoassay

Horseradish peroxidase (HRP) is usually used as a label enzyme in immunoassay so the method used for HRP detection in enzyme immunoassay (EIA) plays a key role in sensitivity and precision. The catalytic activity of HRP does not strictly follow classic Michaelis-Menten kinetics, probably due to the inactivation of the enzyme at high concentrations of H₂O₂. In this paper, a highly sensitive alternative procedure for the HRP assay using H₂O₂-sensitive CdTe quantum dots as a chemiluminescence (CL) system is reported. This method can measure a much more accurate and reliable value of K_m (187 mM H₂O₂) in comparison with the standard detection method. This system also was applied to thyroid hormone (T4) detection using HRP-based immunoassay. The QD/H₂O₂ system exhibits a higher linear range of 0.2-16 μg/dL with the improved LOD value of 0.06 μg/dL and selective response to T4, which was better than the commercial colorimetric immunoassay. Meanwhile, the proposed method has been successfully applied to the clinical determination of T4 in the serum samples, and the results confirmed an excellent correlation with the conventional ELISA method (R² = 0.9832), indicating the potential applications of the method for clinical diagnosis as well.

Quantitative detection of aflatoxin B1 using quantum dots-based immunoassay in a recyclable gravity-driven microfluidic chip

To achieve rapid and sensitive detection of aflatoxin B₁ (AFB₁), we developed a polydimethylsiloxane gravity-driven cyclic microfluidic chip using the two-signal mode strategy. The structural design of the chip, together with the two-wavelength quantum dot ratio fluorescence, effectively eliminates the influence of environmental factors, improves the signal stability, and ensures that the final detection result positively correlates with the target concentration. Moreover, the theoretical analysis performed for the established physical model of the three-dimensional reaction interface inside the chip confirmed the improved reaction rate of immune adsorption in the microfluidic strategy. Overall, the method exhibited a wide analytic range (0.2-500 ng mL^-1), low detection limit (0.06 ng mL^-1), high specificity, good precision (coefficient of variation < 5%), excellent reusability (20 times, 89.1%) and satisfactory practical sample analysis capacity. Furthermore, the reusability and designability of this chip provide a reliable scheme for field detection of AFB₁, analysis of other small molecules, and establishment of high-throughput detection systems under different conditions.

Recent Advances in Aflatoxins Detection Based on Nanomaterials

Aflatoxins are the secondary metabolites of Aspergillus flavus and Aspergillus parasiticus and are highly toxic and carcinogenic, teratogenic and mutagenic. Ingestion of crops and food contaminated by aflatoxins causes extremely serious harm to human and animal health. Therefore, there is an urgent need for a selective, sensitive and simple method for the determination of aflatoxins. Due to their high performance and multipurpose characteristics, nanomaterials have been developed and applied to the monitoring of various targets, overcoming the limitations of traditional methods, which include process complexity, time-consuming and laborious methodologies and the need for expensive instruments. At the same time, nanomaterials provide general promise for the detection of aflatoxins with high sensitivity, selectivity and simplicity. This review provides an overview of recent developments in nanomaterials employed for the detection of aflatoxins. The basic aspects of aflatoxin toxicity and the significance of aflatoxin detection are also reviewed. In addition, the development of different biosensors and nanomaterials for aflatoxin detection is introduced. The current capabilities and limitations and future challenges in aflatoxin detection and analysis are also addressed.

Enhanced sensitivity of VEGF detection using catalase-mediated chemiluminescence immunoassay based on CdTe QD/H2O2 system

Background

Since vascular endothelial growth factor (VEGF) is a significant regulator of cancer angiogenesis, it is essential to develop a technology for its sensitive detection. Herein, we sensitized a chemiluminescence (CL) immunoassay through the combination of H₂O₂-sensitive TGA-CdTe quantum dot (QD) as signal transduction, dextran as a cross-linker to prepare enzyme-labeled antigen and the ultrahigh bioactivity of catalase (CAT) as reporter enzyme.

Results

Under the optimized experimental conditions, the chemiluminescence enzyme-linked immunosorbent assay (CL-ELISA) method can detect VEGF in the excellent linear range of 2–35,000 pg mL⁻¹, with a detection limit (S/N = 3) of 0.5 pg mL⁻¹ which was approximately ten times lower than the commercial colorimetric immunoassay. This proposed method has been successfully applied to the clinical determination of VEGF in the human serum samples, and the results illustrated an excellent correlation with the conventional ELISA method (R² = 0.997). The suitable recovery rate of the method in the serum ranged from 97 to 107%, with a relative standard deviation of 1.2% to 13.4%.

Conclusions

The novel immunoassay proposes a highly sensitive, specific, and stable method for very low levels detection of VEGF that can be used in the primary diagnosis of tumors. With the well-designed sensing platform, this approach has a broad potential to be applied for quantitative analysis of numerous disease-related protein biomarkers for which antibodies are available.

Detection of Aflatoxin B1 Based on a Porous Anodized Aluminum Membrane Combined with Surface-Enhanced Raman Scattering Spectroscopy

An Aflatoxin B₁ (AFB₁) biosensor was fabricated via an Ag nanoparticles assembly on the surface of a porous anodized aluminum (PAA) membrane. First, the Raman reporter 4-Aminothiophenol (4-ATP) and DNA (partially complementary to AFB₁ aptamer) were attached to the surface of Ag nanoparticles (AgNPs) by chemical bonding to form a 4-ATP-AgNPs-DNA complex. Similarly, the surface of a PAA membrane was functionalized with an AFB₁ aptamer. Then, the PAA surface was functionalized with 4-ATP-AgNPs-DNA through base complementary pairing to form AgNPs-PAA sensor with a strong Raman signal. When AFB₁ was added, AgNPs would be detached from the PAA surface because of the specific binding between AFB₁ and the aptamer, resulting in a reduction in Raman signals. The detection limit of the proposed biosensor is 0.009 ng/mL in actual walnut and the linear range is 0.01-10 ng/mL. The sensor has good selectivity and repeatability; it can be applied to the rapid qualitative and quantitative detection of AFB₁.

Silver nanoprism-based plasmonic ELISA for sensitive detection of fluoroquinolones

Fluoroquinolones are synthetic antibiotics that are commonly used in animal husbandry, and the consumption of animal products with fluoroquinolone residues has imposed a serious threat to human health. Here, we report a plasmonic enzyme-linked immunosorbent assay (pELISA) method based on oxidative etching of silver nanoprisms (AgNPRs) for the quantitative and qualitative detection of danofloxacin (DAN), a fluoroquinolone antibiotic. AgNPRs that undergo colorimetric changes upon oxidative etching by H2O2 serve as the signal transducer in our design. An indirect competitive pELISA was constructed by introducing biotinylated monoclonal antibody (mAb), streptavidin and biotinylated glucose oxidase, which catalyzes the generation of H2O2 for etching AgNPRs. The quantitative detection limit of the proposed method was 0.24 ng mL-1 for DAN. The qualitative detection limit for DAN reached 0.32 ng mL-1, which was 32-fold lower than that of the assay using 3,3',5,5'-tetramethylbenzidine (TMB) as the signal transducer. The average recoveries of DAN in milk ranged from 103% to 121%, with a coefficient of variation of 0.6-3.41%. The recovery results were further confirmed using liquid chromatography-tandem mass spectrometry. In summary, the proposed AgNPR-etching pELISA exhibits high sensitivity, good accuracy and excellent reliability for the quantitative and qualitative detection of DAN in milk.

Highly sensitive phage-magnetic-chemiluminescent enzyme immunoassay for determination of zearalenone

Here we demonstrate a novel phage-magnetic-chemiluminescent enzyme immunoassay (P-MCLEIA) for detection of zearalenone (ZEN). The P-MCLEIA was more efficient than conventional ELISA through several improvements. In the P-MCLEIA, magnetic nanoparticles were replaced of microplates as solid phases to reduce the whole incubation time within 40 min. Phage-mimotope was replaced of chemosynthetic antigen to improve the sensitivity of immunoassay. Chemiluminescence substrate was replaced of chromogenic substrate to further improve the sensitivity. The IC50 value of P-MCLEIA was 31.4 pg/mL, which was about 11 times lower than that of phage-magnetic-enzyme linked immunosorbent assay (P-MELISA) and 72 times lower than that of conventional ELISA. The LOD of P-MCLEIA was 4.3 pg/mL. Recovery study of P-MCLEIA was performed by analyzing ZEN levels in spiked corn samples, intra- and inter-assay recoveries were 80.0-119.8% and 82.7-112.7%, respectively. Furthermore, parallel analysis of natural corn samples showed a good correlation between the P-MCLEIA and high performance liquid chromatography.

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.

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

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

Ultrasensitive and rapid detection of ochratoxin A in agro-products by a nanobody-mediated FRET-based immunosensor

Ochratoxin A (OTA) is a major concern for public health and the rapid detection of trace OTA in food is always a challenge. To minimize OTA exposure to consumers, a nanobody (Nb)-mediated förster resonance energy transfer (FRET)-based immunosensor using quantum dots (Nb-FRET immunosensor) was proposed for ultrasensitive, single-step and competitive detection of OTA in agro-products at present work. QDs of two sizes were covalently labeled with OTA and Nb, acting as the energy donor and acceptor, respectively. The free OTA competed with the donor to bind to acceptor, thus the FRET efficiency increased with the decrease of OTA concentration. The single-step assay could be finished in 5 min with a limit of detection of 5 pg/mL, which was attributed to the small size of Nb for shortening the effective FRET distance and improving the FRET efficiency. The Nb-FRET immunosensor exhibited high selectivity for OTA. Moreover, acceptable accuracy and precision were obtained in the analysis of cereals and confirmed by the liquid chromatography-tandem mass spectrometry. Thus the developed Nb-FRET immunosensor was demonstrated to be an efficient tool for ultrasensitive and rapid detection of OTA in cereals and provides a detection model for other toxic small molecules in food and environment.

Preparation of an Antidanofloxacin Monoclonal Antibody and Development of Immunoassays for Detecting Danofloxacin in Meat

Danofloxacin (DAF), a third-generation fluroquinolone (FQ), is widely used as a broad-spectrum antibacterial drug to prevent diseases in livestock and poultry. In this study, a highly specific and sensitive monoclonal antibody (mAb) against DAF was prepared. Also, the mAb was used for the indirect competitive enzyme-linked immunosorbent assay (icELISA) and immunochromatographic strip for the detection of DAF residues in meat. The IC₅₀ of the icELISA based on this mAb was 1.39 ng/mL, and the limit of detection was 0.2 ng/mL. According to the cross-reactivity (CR) experiment, the ELISA that we developed was highly specific and had low CR with other FQ analogues. Moreover, the cut-off of the immunochromatographic strip developed for detecting DAF in meat was 5 ng/mL. Overall, the developed ELISA and immunochromatographic strip based on the prepared mAb were proved reliable for the rapid detection of DAF in meat and can be considered as effective screening methods for food safety and quality management.

Nanobody-based fluorescence resonance energy transfer immunoassay for noncompetitive and simultaneous detection of ochratoxin a and ochratoxin B

A noncompetitive and homogeneous fluorescence resonance energy transfer (FRET) immunoassay was developed using a nanobody (Nb) for highly sensitive and simultaneous detection of ochratoxin A (OTA) and ochratoxin B (OTB). The promoted intrinsic fluorescence (λ_ex: 280 nm) of tryptophan residues (donor) in Nb can excite the fluorescence of OTA and OTB (acceptor) for detection (λ_em: 430 nm). Using optimal conditions, the limits of detection of the Nb-based FRET immunoassay were 0.06 and 0.12 ng/mL for OTA and OTB, respectively. Minimal cross reactivity was detected for several analogues of OTA and OTB as well as nonspecific proteins and antibodies. Acceptable accuracy and precision were obtained in the spike and recovery study, and the results correlated well with those by HPLC. These results demonstrated that the developed method could be a useful tool for noncompetitive, homogeneous, and simultaneous detection of OTA and OTB as well as other environmental analytes with similar fluorescence properties.

Highly sensitive aflatoxin B1 sensor based on DNA-guided assembly of fluorescent probe and TdT-assisted DNA polymerization

A novel aptasensor, based on a perylene probe (PAPDI; N,N'-bis(propylenetrimethylammonium)-3,4,9,10-perylenediimide), was developed for the detection of aflatoxin B1 (AFB1) in maize samples. AuNPs/DNA composites were synthesized and integrated with aptamers-modified magnetic nanoparticles (MNPs) via DNA hybridization. For AFB1 determination, AuNPs/DNA composites were released from MNPs through specific binding of AFB1 with the aptamer and used for assembly of the PAPDI probe. To enhance the method sensitivity, terminal deoxynucleotidyl transferase (TdT)-catalyzed DNA polymerization was performed to elongate DNA on AuNPs/DNA composites. As a result, more PAPDI probes were assembled on the AuNPs/DNA composites. Through a multiple signal amplification strategy, the proposed method exhibited high sensitivity towards AFB1, with a detection limit of 0.01 nM (3.1 pg/mL). In summary, the proposed method has great potential to be a universal strategy for monitoring AFB1 in food samples.

Simple "signal-on" photoelectrochemical aptasensor for ultrasensitive detecting AFB1 based on electrochemically reduced graphene oxide/poly(5-formylindole)/Au nanocomposites

A simple "signal-on" photoelectrochemical (PEC) aptasensor is constructed for Aflatoxin B1 (AFB1) detection based on electrochemically reduced graphene oxide/poly(5-formylindole)/Au (erGO/P5FIn/Au) nanocomposites. The nanocomposites are synthesized by simple electrochemical deposition method and show good photoelectrochemical performance. Poly(5-formylindole) (P5FIn) can generate electron-hole pairs under light irradiation, leading to the formation of robust cathode photocurrent. Au can be acted as signal amplifier due to the high conductivity. The erGO is used to immobilize AFB1 aptamer chain by π-π stacking interaction between the carbon six-membered ring in graphene and the C-N heterocyclic ring in nucleobases of ssDNA. After the insulating AFB1 aptamer chain is fixed to the electrode, the signal of PEC sensor is "OFF". In the process of AFB1 detection, the aptamer chain detaches from the surface of erGO, which results in "ON" of the sensor signal. Based on this design, this constructed PEC aptasensor shows a high sensitivity for AFB1 with a wide linear detection range (LDR) from 0.01 ng mL^-1 to 100 ng mL^-1. The limit of detection (LOD) is 0.002 ng mL^-1. This PEC sensor also exhibits good stability, selectivity, specificity, and satisfactory practical sample analysis ability. This work may provide a new promising PEC platform for AFB1 detection as well as some other small molecules analysis.

A label-free fluorescent aptasensor for the detection of Aflatoxin B1 in food samples using AIEgens and graphene oxide

The detection of Aflatoxin B1 (AFB1) has attracted extensive attention for food safety is a worldwide public health problem. Herein, a novel, simple and label-free fluorescent aptasensor, based on quaternized tetraphenylethene salt (TPE-Z), graphene oxide (GO) and AFB1 aptamer, has been constructed to detect AFB1. In the presence of AFB1, AFB1 aptamer undergoes a conformational switch from single stranded structure to the AFB1/AFB1 aptamer complex upon target binding, which induces the release of TPE-Z/AFB1 aptamer from the surface of GO. Thus, the fluorescence of TPE-Z/AFB1 aptamer is recovered. The assay can be performed by simply mixing TPE-Z, AFB1 aptamer, the GO and the AFB1 samples with a detection limit of 0.25 ng/mL. It is highly selective against other aflatoxins in foods and its performance has been verified in food samples (corn, milk and rice) with known concentration AFB1.

A water-stable luminescent metal–organic framework for effective detection of aflatoxin B1 in walnut and almond beverages

Sensitive and rapid detection of aflatoxin B1 (AFB1) without using antibody or biomolecular modifications in water is achieved using a novel water-stable luminescent metal–organic framework (LMOF) termed Zr-CAU-24. The 1,2,4,5-tetrakis(4-carboxyphenyl) benzene (H₄TCPB)-based LMOF with high water-stability has demonstrated drastic fluorescence fading in the presence of AFB1. The detection limit for AFB1 using this porous nanomaterial reaches as low as 19.97 ppb (64 nM), which is below the applicable action level for peanut and corn products set by the FDA and among the most sensitive sensors reported for AFB1. We further investigated its response to five other mycotoxins including AFB2, AFG1, AFG2, AFM and OTA and their Stern–Volmer quenching efficiencies are significantly below that of AFB1 (138 461 M⁻¹). The prepared water-stable LMOF was directly used for the detection of AFB1 in spiked walnut and almond beverages. High recovery rates (91–108%) were achieved in 5 min. We found that the quenching of H₄TCPB molecules towards mycotoxins was remarkably enhanced by anchoring them into the periodic framework and its mechanism was discussed. The presented method with acceptable detection limit is of potential for the development of low-cost, robust and sensitive sensors for the rapid detection of AFB1 in agricultural and food products.

Sensitive and rapid detection of aflatoxin B1 (AFB1) without using antibody or biomolecular modifications in water is achieved using a novel water-stable luminescent metal–organic framework (LMOF) termed Zr-CAU-24.

Cytometric Microbead Magnetic Suspension Array for High-Throughput Ultrasensitive Detection of Aflatoxin B1

High-throughput and low-cost detection of mycotoxins in complex matrices is becoming increasingly urgent but it is still challenging to perform ultrasensitive analyses. Here we report a green and practical cytometric microbead magnetic suspension array (CBMSA) strategy for rapid and economical detection of aflatoxin B₁ (AFB₁) in multiple batches of lotus seed samples. The protocol included (1) fabrication of suspension array chips by immobilizing biotin-modified bovine serum albumin-AFB₁ (antigen) onto the surface of streptavidin-coated magnetic microbeads in a multiwell array, (2) indirect immunocompetition of antigen and target of AFB₁ in lotus seed samples with the specific antibodies, (3) rapid magnetic separation regardless of complex pretreatment steps, and (4) ultrasensitive fluorescence detection of fluorescein isothiocyanate-labeled goat anti-mouse immunoglobulin G (FITC-IgG) probes. After systematic optimization of some crucial parameters, the developed CBMSA assay allowed for ultrasensitive detection of AFB₁ with limit of detection as low as 7.8125 pg·kg^-1. For high-throughput analysis, the CBMSA technique was capable of on-site co-instantaneous detection of 50-100 samples in one operation within 30 s, only needing a small amount (50 μL) of solution, which is much cheaper, greener, and more user-friendly than conventional techniques. Moreover, CBMSA with magnetic separation is free of multiple centrifugation and cleanup steps to avoid unpredictable loss of targets. Since various capture and fluorescent probes can be randomly constructed and bound onto the surface of magnetic microbeads to establish an ultrasensitive detection system, the CBMSA technique is very promising for more trace analytes in complex matrices and for broad point-of-need applications, such as drug screening and real-time high-throughput analysis.

Fluorescence immunoassay based on the enzyme cleaving ss-DNA to regulate the synthesis of histone-ds-poly(AT) templated copper nanoparticles

Herein, for the first time we report a novel competitive fluorescence immunoassay for the ultrasensitive detection of aflatoxin B1 (AFB1) using histone-ds-poly(AT) templated copper nanoparticles (His-pAT CuNPs) as the fluorescent indicator. In this immunoassay, glucose oxidase (Gox) was used as the carrier of the competing antigen to catalyze the formation of hydrogen peroxide (H2O2) from glucose. H2O2 was converted to a hydroxyl radical using Fenton's reagent, which further regulated the fluorescence signals of His-pAT CuNPs. Owing to the ultrahigh sensitivity of the ss-DNA to the hydroxyl radical, the proposed fluorescence immunoassay exhibited a favorable dynamic linear detection of AFB1 ranging from 0.46 pg mL-1 to 400 pg mL-1 with an good half maximal inhibitory concentration and limit of detection of 6.13 and 0.15 pg mL-1, respectively. The intra- and inter-assay showed that the average recoveries for AFB1 spiked corn samples ranged from 96.87% to 100.73% and 96.67% to 114.92%, respectively. The reliability of this method was further confirmed by adopting ultra-performance liquid chromatography coupled with the fluorescence detector method. In summary, this work offers a novel screening strategy with high sensitivity and robustness for the quantitative detection of mycotoxins or other pollutants for food safety and clinical diagnosis.

Portable Aptasensor of Aflatoxin B1 in Bread Based on a Personal Glucose Meter and DNA Walking Machine

Despite some recent developments on the portable on-site sensor of Aflatoxin B1 (AFB1), the complex and expensive preparation of recognition elements have still limited their wide applications. In this paper, using the fast, low-cost, and stable recognition of aptamer DNA-AFB1, a portable aptasensor was constructed for the on-site detection of AFB1 in food matrixes, with the readout of personal glucose meter (PGM) and DNA walking machine for signal probe separation. In such an assay protocol, the target could trigger the DNA walker to autonomously move on the electrode surface, propelled by unidirectional Pb²⁺-specific DNAzyme digestion, which could amplify the signal and separate the signal probe as well for further quantification by the PGM. Under optimized conditions, the increase of PGM signal was relative with the concentration of AFB1 ranging from 0.02 to 10 nM and the low limit of detection (LOD) was 10 pM (S/N = 3). With the features of portability, and cheapness, the presented user-friendly method could be extended to various other analytes for wide point-of-care applications.

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.

Affinity Biosensors for Detection of Mycotoxins in Food

This chapter reviews recent achievements in methods of detection of mycotoxins in food. Special focus is on the biosensor technology that utilizes antibodies and nucleic acid aptamers as receptors. Development of biosensors is based on the immobilization of antibodies or aptamers onto various conventional supports like gold layer, but also on nanomaterials such as graphene oxide, carbon nanotubes, and quantum dots that provide an effective platform for achieving high sensitivity of detection using various physical methods, including electrochemical, mass sensitive, and optical. The biosensors developed so far demonstrate high sensitivity typically in subnanomolar limit of detection. Several biosensors have been validated in real samples. The sensitivity of biosensors is similar and, in some cases, even better than traditional analytical methods such as ELISA or chromatography. We believe that future trends will be focused on improving biosensor properties toward practical application in food industry.

Ratiometric optical nanoprobes enable accurate molecular detection and imaging

Exploring and understanding biological and pathological changes are of great significance for early diagnosis and therapy of diseases. Optical sensing and imaging approaches have experienced major progress in this field. Particularly, an emergence of various functional optical nanoprobes has provided enhanced sensitivity, specificity, targeting ability, as well as multiplexing and multimodal capabilities due to improvements in their intrinsic physicochemical and optical properties. However, one of the biggest challenges of conventional optical nanoprobes is their absolute intensity-dependent signal readout, which causes inaccurate sensing and imaging results due to the presence of various analyte-independent factors that can cause fluctuations in their absolute signal intensity. Ratiometric measurements provide built-in self-calibration for signal correction, enabling more sensitive and reliable detection. Optimizing nanoprobe designs with ratiometric strategies can surmount many of the limitations encountered by traditional optical nanoprobes. This review first elaborates upon existing optical nanoprobes that exploit ratiometric measurements for improved sensing and imaging, including fluorescence, surface enhanced Raman scattering (SERS), and photoacoustic nanoprobes. Next, a thorough discussion is provided on design strategies for these nanoprobes, and their potential biomedical applications for targeting specific biomolecule populations (e.g. cancer biomarkers and small molecules with physiological relevance), for imaging the tumor microenvironment (e.g. pH, reactive oxygen species, hypoxia, enzyme and metal ions), as well as for intraoperative image guidance of tumor-resection procedures.

Quantum dots: from fluorescence to chemiluminescence, bioluminescence, electrochemiluminescence, and electrochemistry

During the past decade, nanotechnology has become one of the major forces driving basic and applied research. As a novel class of inorganic fluorochromes, research into quantum dots (QDs) has become one of the fastest growing fields of nanotechnology today. QDs are made of a semiconductor material with tunable physical dimensions as well as unique optoelectronic properties, and have attracted multidisciplinary research efforts to further their potential bioanalytical applications. Recently, numerous optical properties of QDs, such as narrow emission band peaks, broad absorption spectra, intense signals, and remarkable resistance to photobleaching, have made them biocompatible and sensitive for biological assays. In this review, we give an overview of these exciting materials and describe their potential, especially in biomolecules analysis, including fluorescence detection, chemiluminescence detection, bioluminescence detection, electrochemiluminescence detection, and electrochemical detection. Finally, conclusions are made, including highlighting some critical challenges remaining and a perspective of how this field can be expected to develop in the future.

A Novel Fluoroimmunoassay for Detecting Ruscogenin with Monoclonal Antibodies Conjugated with CdSe/ZnS Quantum Dots

Ruscogenin (RUS) is a steroidal sapogenin found in Ruscus aculeatus and Ophiopogon japonicus with several pharmacological activities. In the work reported herein, a novel method termed competitive fluorescence-linked immunosorbent assay (cFLISA) based on monoclonal antibodies (mAbs) coupled with quantum dots (QDs) was developed for the quick and sensitive determination of RUS in biological samples. The mAbs against RUS were conjugated with CdSe/ZnS QDs by the crossing-linking reagents and an indirect cFLISA method was developed. There was a good linear relationship between inhibition efficiency and logarithm concentration of RUS which was varied from 0.1 to 1000 ng/mL. The IC₅₀ and limit of detection (LOD) were 9.59 ng/mL and 0.016 ng/mL respectively, which much lower than the enzyme-linked immunosorbent assay (ELISA) method. The recoveries in plasma and tissues were ranged from 82.3% to 107.0% and the intra- and inter-day precision values were below 15%. The developed cFLISA has been successfully applied to the measurement of the concentrations of RUS in biological samples of rats, and showed great potential for the tissue distribution study of RUS. The cFLISA method may provide a valuable tool for the analysis of small molecules in biological samples and such an approach could be applied to other natural products.

A review of fluorescent signal-based lateral flow immunochromatographic strips

Fluorescent signal-based lateral flow immunochromatographic strips (FLFICS) have received great expectations since they combine the quantitative sensitivity of fluorescence analysis and the simplicity, rapidness, and portability of a common lateral flow immunochromatographic strip (LFICS).

A homogeneous assay for highly sensitive detection of CaMV35S promoter in transgenic soybean by förster resonance energy transfer between nitrogen-doped graphene quantum dots and Ag nanoparticles

In this work, a novel homogeneous assay for DNA quantitative analysis based on förster resonance energy transfer (FRET) was developed for cauliflwer mosaic virus 35s (CaMV35S) promoter of transgenic soybean detection. The homogenous FRET of fluorescence signal was fabricated by DNA hybridization with probe modified nitrogen-doped graphene quantum dots (NGQDs) and silver nanoparticles (AgNPs), which acted the donor-acceptor pairs for the first time. The highly efficient FRET and unique properties of the NGQDs made the proposed FRET system as a functionalized detection platform for labelling of DNA. Upon the recognition of specific target DNA (tDNA), the FRET between NGQDs and AgNPs was triggered to produce fluorescence quenching, which could be used for tDNA detection. The fabricated homogeneous FRET assay displayed a wide linear range of 0.1-500.0 nM and a low limit of detection 0.03 nM for the detection of CaMV35S (S/N = 3). This proposed biosensor revealed high specificity to detect tDNA, with acceptable intra-assay precision and excellent stability. This method was successfully applied to identify the real sample of 0.5% containing transgenic soybean, which achieved the most of national law regulations. This assay was further validated by polymerase chain reaction as the genetically modified organisms, suggesting that the proposed FRET system is a feasible tool for the further daily genetically modified organism detection.

Advantages of time-resolved fluorescent nanobeads compared with fluorescent submicrospheres, quantum dots, and colloidal gold as label in lateral flow assays for detection of ractopamine

Label selection is a critical factor for improving the sensitivity of lateral flow assay. Time-resolved fluorescent nanobeads, fluorescent submicrospheres, quantum dots, and colloidal gold-based lateral flow assay (TRFN-LFA, FM-LFA, QD-LFA, and CG-LFA) were first systematically compared for the quantitative detection of ractopamine in swine urine based on competitive format. The limits of detection (LOD) of TRFN-LFA, FM-LFA, QD-LFA, and CG-LFA were 7.2, 14.7, 23.6, and 40.1pg/mL in swine urine samples, respectively. The sensitivity of TRFN-LFA was highest. In the quantitative determination of ractopamine (RAC) in swine urine samples, TRFN-LFA exhibited a wide linear range of 5pg/mL to 2500pg/mL with a reliable coefficient of correlation (R²=0.9803). Relatively narrow linear ranges of 10-500pg/mL (FM-LFA) and 25-2500pg/mL (QD-LFA and CG-LFA) were acquired. Approximately 0.005µg of anti-RAC poly antibody (pAb) was used in each TRFN-LFA test strip, whereas 0.02, 0.054, and 0.15µg of pAb were used in each of the FM-LFA, QD-LFA, and CG-LFA test strips, respectively. In addition, TRFN-LFA required the least RAC-BSA antigens and exhibited the shortest detection time compared with the other lateral flow assays. Analysis of the RAC in swine urine samples showed that the result of TRFN-LFA was consistent with that of liquid chromatography-tandem mass spectrometry (LC-MS/MS) and a commercial enzyme-linked immunosorbent assay (ELISA) kit.