Bioinspired recognition elements for mycotoxin sensors

A review of biomolecules conjugated lanthanide up-conversion nanoparticles-based fluorescence probes in food safety and quality monitoring applications

Upconversion nanoparticles (UCNPs) are known to possess unique characteristics, which allow them to overcome a number of issues that plague traditional fluorescence probes. UCNPs have been employed in a variety of applications, but it is arguably in the realm of optical sensors where they have shown the most promise. Biomolecule conjugated UCNPs-based fluorescence probes have been developed to detect and quantify a wide range of analytes, from metal ions to biomolecules, with great specificity and sensitivity. In this review, we have given much emphasis on the recent trends and progress in the preparation strategies of bioconjugated UCNPs and their potential application as fluorescence sensors in the trace level detection of food industry-based toxicants and adulterants. The paper discusses the preparation and functionalisation strategies of commonly used biomolecules over the surface of UCNPs. The use of different sensing strategies namely heterogenous and homogenous assays, underlying fluorescence mechanisms in the detection process of food adulterants are summarized in detail. This review might set a precedent for future multidisciplinary research including the development of novel biomolecules conjugated UCNPs for potential applications in food science and technology.

A dual-signal aptasensor based on cascade amplification for ultrasensitive detection of aflatoxin B1

Aflatoxin B1 (AFB1) is considered as a serious carcinogenic mycotoxin that was widely detected in grains and foods, and its sensitive analysis is of key importance to avoid the health threats for consumers. In this study, a dual-signal aptasensor based on cascade of entropy-driven strand displacement reaction (ESDR) and linear rolling circle amplification (LRCA) was fabricated for ultrasensitive determination of AFB1. At the sensing system, the complementary strand would be released after the aptamer combined with AFB1, which will bring about the functional domains exposed, triggering the subsequent ESDR. Meanwhile, the two strands that were outputted by ESDR would incur the downstream LRCA reaction to produce a pair of long strands to assist in the generation of fluorescence and absorbance signals. Under the optimized conditions, the proposed aptasensor could achieve excellent sensitivity (limit of detection, 0.427 pg/mL) with satisfactory accuracy (recoveries, 92.8-107.9 %; RSD, 2.4-5.0 %), mainly ascribed to the cascade amplification. Importantly, owing to the flexibility design of nucleic acid primer, this analytical method can be applied in monitoring various hazardous substances according to the specific requirements. Our strategy provides some novel insights at signal amplification for rapid detection of AFB1 and other targets.

Rationally designed peptide conjugated to gold nanoparticles for detection of aflatoxin B1 in point-of-care dot-blot assay

Aflatoxin B1 (AFB1) is a hepatotoxic and carcinogenic food contaminant. Although on-site paper-based detection is sensitive it depends on expensive antibodies which are difficult to raise against mycotoxins. Here, we rationally designed a high binding octapeptide, N-KSGKSKPR-C peptide for AFB1 detection, by molecular docking, as confirmed by indirect ELISA (K_d 323 nM). Further, conjugation of octapeptide with gold nanoparticles (26 nm) permitted its use as a visual detection agent in rapid, sensitive dot-blot assay (LOD 0.39 μg/kg). The assay displayed negligible cross-reactivity with co-contaminating mycotoxins. AFB1 recovery from spiked wheat sample was comparable by dot-blot (78-91 %) and HPLC (65-87 %). Evaluation of dot-blot using certified reference material and 146 food and feed samples showed high correlation R² = 0.87 with HPLC. The assay displayed high accuracy (91 %), sensitivity (71 %) and specificity (96.5 %). Therefore, the developed dot-blot assay holds promise for monitoring AFB1 contamination in food and feed.

Homogeneous immunoassay for cyclopiazonic acid based upon mimotopes and upconversion-resonance energy transfer

Strains of Penicillium spp. are used for fungi-ripened cheeses and Aspergillus spp. routinely contaminate maize and other crops. Some of these strains can produce toxic secondary metabolites (mycotoxins), including the neurotoxin α-cyclopiazonic acid (CPA). In this work, we developed a homogeneous upconversion-resonance energy transfer (UC-RET) immunoassay for the detection of CPA using a novel epitope mimicking peptide, or mimotope, selected by phage display. CPA-specific antibody was used to isolate mimotopes from a cyclic 7-mer peptide library in consecutive selection rounds. Enrichment of antibody binding phages was achieved, and the analysis of individual phage clones revealed four different mimotope peptide sequences. The mimotope sequence, ACNWWDLTLC, performed best in phage-based immunoassays, surface plasmon resonance binding analyses, and UC-RET-based immunoassays. To develop a homogeneous assay, upconversion nanoparticles (UCNP, type NaYF₄:Yb³⁺, Er³⁺) were used as energy donors and coated with streptavidin to anchor the synthetic biotinylated mimotope. Alexa Fluor 555, used as an energy acceptor, was conjugated to the anti-CPA antibody fragment. The homogeneous single-step immunoassay could detect CPA in just 5 min and enabled a limit of detection (LOD) of 30 pg mL^-1 (1.5 μg kg^-1) and an IC₅₀ value of 0.36 ng mL^-1. No significant cross-reactivity was observed with other co-produced mycotoxins. Finally, we applied the novel method for the detection of CPA in spiked maize samples using high-performance liquid chromatography coupled to a diode array detector (HPLC-DAD) as a reference method.

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.

Design of a Quencher-Free Fluorescent Aptasensor for Ochratoxin A Detection in Red Wine Based on the Guanine-Quenching Ability

This study describes a quencher-free fluorescent aptasensor for ochratoxin A (OTA) detection using the specific quenching ability of guanine for fluorescein (FAM) molecules based on photo-induced electron transfer (PIET). In this strategy, OTA is detected by monitoring the fluorescence change induced by the conformational change of the aptamer after target binding. A new shorter OTA aptamer compromising three guanine bases at the 5' end was used in this study. This new aptamer, named G3-OTAapt1-FAM (F1), was labeled with FAM on the 3' end as a fluorophore. In order to increase the binding affinity of the aptamer and OTA, G3-OTAapt2-FAM (F2) was designed; this added a pair of complementary bases at the end compared with F1. To prevent the strong self-quenching of F2, a complementary chain, A13, was added. Although the F1 aptasensor was simpler to implement, the sensitivity of the F2 aptasensor with A13 was better than that of F1. The proposed F1 and F2 sensors can detect OTA with a concentration as low as 0.69 nmol/L and 0.36 nmol/L, respectively.

Three-Dimensional Ordered Macroporous Magnetic Inverse Photonic Crystal Microsphere-Based Molecularly Imprinted Polymer for Selective Capture of Aflatoxin B1

Development of an efficient detection method to monitor residual mycotoxins in food is very important to ensure food safety, but the complex food matrix seriously affects the detection sensitivity and accuracy. Here, using a three-dimensional ordered macroporous magnetic inverse photonic crystal microsphere (MPCM) as the supporting material, a molecularly imprinted polymer (MIP) that can selectively recognize aflatoxin B₁ (AFB₁) was synthesized through the dummy template imprinting strategy. The MPCM@MIP prepared by employing 5,7-dimethoxycoumarin as the template and methacrylic acid as the functional monomer displayed selectivity toward AFB₁ (imprinting factor of 1.5) and could be used as a solid-phase extraction material. By coupling with high-performance liquid chromatography, an analytical method targeting AFB₁ was established and displayed a wide linear range of 5-1000 ng/mL with a low detection limit of 0.4 ng/mL. The method showed a good recovery rate of 73-92% in AFB₁-spiked soy sauce and vinegar samples. Moreover, the MPCM@MIP could be separated from the sample solution easily because of its magnetic performance, displaying a promising future not only in the enrichment of AFB₁ to improve the detection sensitivity and accuracy but also in the removal of AFB₁ from food and environmental samples.

Phage-based technologies for highly sensitive luminescent detection of foodborne pathogens and microbial toxins: A review

Foodborne pathogens and microbial toxins are the main causes of foodborne illness. However, trace pathogens and toxins in foods are difficult to detect. Thus, techniques for their rapid and sensitive identification and quantification are urgently needed. Phages can specifically recognize and adhere to certain species of microbes or toxins due to molecular complementation between capsid proteins of phages and receptors on the host cell wall or toxins, and thus they have been successfully developed into a detection platform for pathogens and toxins. This review presents an update on phage-based luminescent detection technologies as well as their working principles and characteristics. Based on phage display techniques of temperate phages, reporter gene detection assays have been designed to sensitively detect trace pathogens by luminous intensity. By the host-specific lytic effects of virulent phages, enzyme-catalyzed chemiluminescent detection technologies for pathogens have been exploited. Notably, these phage-based luminescent detection technologies can discriminate viable versus dead microbes. Further, highly selective and sensitive immune-based assays have been developed to detect trace toxins qualitatively and quantitatively via antibody analogs displayed by phages, such as phage-ELISA (enzyme-linked immunosorbent assay) and phage-IPCR (immuno-polymerase chain reaction). This literature research may lead to novel and innocuous phage-based rapid detection technologies to ensure food safety.

Recent Progress in Rapid Determination of Mycotoxins Based on Emerging Biorecognition Molecules: A Review

Mycotoxins are secondary metabolites produced by fungal species, which pose significant risk to humans and livestock. The mycotoxins which are produced from Aspergillus, Penicillium, and Fusarium are considered most important and therefore regulated in food- and feedstuffs. Analyses are predominantly performed by official laboratory methods in centralized labs by expert technicians. There is an urgent demand for new low-cost, easy-to-use, and portable analytical devices for rapid on-site determination. Most significant advances were realized in the field bioanalytical techniques based on molecular recognition. This review aims to discuss recent progress in the generation of native biomolecules and new bioinspired materials towards mycotoxins for the development of reliable bioreceptor-based analytical methods. After brief presentation of basic knowledge regarding characteristics of most important mycotoxins, the generation, benefits, and limitations of present and emerging biorecognition molecules, such as polyclonal (pAb), monoclonal (mAb), recombinant antibodies (rAb), aptamers, short peptides, and molecularly imprinted polymers (MIPs), are discussed. Hereinafter, the use of binders in different areas of application, including sample preparation, microplate- and tube-based assays, lateral flow devices, and biosensors, is highlighted. Special focus, on a global scale, is placed on commercial availability of single receptor molecules, test-kits, and biosensor platforms using multiplexed bead-based suspension assays and planar biochip arrays. Future outlook is given with special emphasis on new challenges, such as increasing use of rAb based on synthetic and naïve antibody libraries to renounce animal immunization, multiple-analyte test-kits and high-throughput multiplexing, and determination of masked mycotoxins, including stereoisomeric degradation products.

Recombinant antibodies and their use for food immunoanalysis

Antibodies are widely employed as biorecognition elements for the detection of a plethora of compounds including food and environmental contaminants, biomarkers, or illicit drugs. They are also applied in therapeutics for the treatment of several disorders. Recent recommendations from the EU on animal protection and the replacement of animal-derived antibodies by non-animal-derived ones have raised a great controversy in the scientific community. The application of recombinant antibodies is expected to achieve a high growth rate in the years to come thanks to their versatility and beneficial characteristics in comparison to monoclonal and polyclonal antibodies, such as stability in harsh conditions, small size, relatively low production costs, and batch-to-batch reproducibility. This review describes the characteristics, advantages, and disadvantages of recombinant antibodies including antigen-binding fragments (Fab), single-chain fragment variable (scFv), and single-domain antibodies (VHH) and their application in food analysis with especial emphasis on the analysis of biotoxins, antibiotics, pesticides, and foodborne pathogens. Although the wide application of recombinant antibodies has been hampered by a number of challenges, this review demonstrates their potential for the sensitive, selective, and rapid detection of food contaminants.

Biomimetic Sensors to Detect Bioanalytes in Real-Life Samples Using Molecularly Imprinted Polymers: A Review

Molecularly imprinted polymers (MIPs) come with the promise to be highly versatile, useful artificial receptors for sensing a wide variety of analytes. Despite a very large body of literature on imprinting, the number of papers addressing real-life biological samples and analytes is somewhat limited. Furthermore, the topic of MIP-based sensor design is still, rather, in the research stage and lacks wide-spread commercialization. This review summarizes recent advances of MIP-based sensors targeting biological species. It covers systems that are potentially interesting in medical applications/diagnostics, in detecting illicit substances, environmental analysis, and in the quality control of food. The main emphasis is placed on work that demonstrates application in real-life matrices, including those that are diluted in a reasonable manner. Hence, it does not restrict itself to the transducer type, but focusses on both materials and analytical tasks.

Biosensing based on upconversion nanoparticles for food quality and safety applications

Food safety and quality regulations inevitably call for sensitive and accurate analytical methods to detect harmful contaminants in food and to ensure safe food for the consumer. Both novel and well-established biorecognition elements, together with different transduction schemes, enable the simple and rapid analysis of various food contaminants. Upconversion nanoparticles (UCNPs) are inorganic nanocrystals that convert near-infrared light into shorter wavelength emission. This unique photophysical feature, along with narrow emission bandwidths and large anti-Stokes shift, render UCNPs excellent optical labels for biosensing because they can be detected without optical background interferences from the sample matrix. In this review, we show how this exciting technique has evolved into biosensing platforms for food quality and safety monitoring and highlight recent applications in the field.

Mycotoxin extraction from edible insects with natural deep eutectic solvents: a green alternative to conventional methods

Edible insects are widely consumed in Africa, Asia, Oceania and Latin America, but less commonly so in Western countries. Since the turn of the millennium, however, entomophagy has aroused growing interest worldwide in response to the increasing scarcity of food resources. In fact, edible insects can be a source of high-quality protein, and also of fat, energy, minerals and vitamins. However, the lack of regulatory guidelines for microbiologically or chemically hazardous agents potentially present in these new foods (e.g., mycotoxins) may make their consumption unsafe. In this work, we developed an environmentally friendly analytical method using natural deep eutectic solvents (NADES or natural DES) in combination with ultra-high performance liquid chromatography/tandem mass spectrometry (UHPLC-MS/MS) for the simultaneous determination of six mycotoxins of great concern owing to their toxic effects on humans and animals (namely, fumonisin B₁, fumonisin B₂, T-2 toxin, HT-2 toxin, ochratoxin A and mycophenolic acid) in insect-based food products. The target mycotoxins were co-extracted from cricket flour by using the optimum DES composition (namely, a mixture of choline chloride and urea, in a 1:2 mole ratio, containing 15% water which resulted in the highest extraction recoveries for all toxins). An experimental design method (Fractional Factorial Design (FFD) was used to examine the influence of the operational variables DES volume and water content, amount of sample, extraction time and extraction temperature on the extraction efficiency for each mycotoxin. Under optimum conditions, extraction recoveries were close to 100% except for fumonisin B₂ (70%) and T-2 toxin (50%), with relative standard deviations (RSDs) below 13% in all cases. The proposed NADES-UHPLC-MS/MS method was validated in accordance with the European Commission 2002/657/EC and 2006/401/EC decisions, and used to determine the target compounds in cricket flour, silkworm pupae powder and black cricket powder.

An overview of nanomaterial based biosensors for detection of Aflatoxin B1 toxicity in foods

Aflatoxin B1 (AFB1) is one of the most potent mycotoxin contaminating several foods and feeds. It suppresses immunity and consequently increases mutagenicity, carcinogenicity, teratogenicity, hepatotoxicity, embryonic toxicity and increasing morbidity and mortality. Continuous exposure of AFB1 causes liver damage and thus increases the prevalence of cirrhosis and hepatic cancer. This article was planned to provide understanding of AFB1 toxicity and provides future directions for fabrication of cost effective and user-friendly nanomaterials based analytical devices. In the present article various conventional (chromatographic & spectroscopic), modern (PCR & immunoassays) and nanomaterials based biosensing techniques (electrochemical, optical, piezoelectrical and microfluidic) are discussed alongwith their merits and demerits. Nanomaterials based amperometric biosensors are found to be more stable, selective and cost-effective analytical devices in comparison to other biosensors. But many unresolved issues about their stability, toxicity and metabolic fate needs further studies. In-depth studies are needed for development of advanced nanomaterials integrated biosensors for specific, sensitive and fast monitoring of AFB1 toxicity in foods. Integration of biosensing system with micro array technology for simultaneous and automated detection of multiple AFs in real samples is also needed. Concerted efforts are also required to reduce their possible hazardous consequences of nanomaterials based biosensors.

Self-replicating catalyzed hairpin assembly for rapid aflatoxin B1 detection

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

Phage Display Selection of an Anti-Idiotype-Antibody with Broad-Specificity to Deoxynivalenol Mycotoxins

The use of synthetic antibody libraries and phage displays provides an efficient and robust method for the generation of antibodies against a wide range of targets with highly specific binding properties. As the in vitro selection conditions can be easily controlled, these methods enable the rapid generation of binders against difficult targets such as toxins and haptens. In this study, we used deoxynivalenol mycotoxin as a target to generate anti-idiotype-antibodies with unique binding properties from synthetic antibody libraries. The binding of the selected anti-idiotype antibodies can be efficiently inhibited with the addition of free isoforms of deoxynivalenol. The antibody was consecutively used to develop deoxynivalenol-specific ELISA and TRF-immunoassays, which can detect deoxynivalenol and two of the most common metabolic isoforms in the range of 78-115 ng/mL.

Immunoaffinity Extraction and Alternative Approaches for the Analysis of Toxins in Environmental, Food or Biological Matrices

The evolution of instrumentation in terms of separation and detection allowed a real improvement of the sensitivity and analysis time. However, the analysis of ultra-traces of toxins in complex samples requires often a step of purification and even preconcentration before their chromatographic analysis. Therefore, immunoaffinity sorbents based on specific antibodies thus providing a molecular recognition mechanism appear as powerful tools for the selective extraction of a target molecule and its structural analogs to obtain more reliable and sensitive quantitative analysis in environmental, food or biological matrices. This review focuses on immunosorbents that have proven their efficiency in selectively extracting various types of toxins of various sizes (from small mycotoxins to large proteins) and physicochemical properties. Immunosorbents are now commercially available, and their use has been validated for numerous applications. The wide variety of samples to be analyzed, as well as extraction conditions and their impact on extraction yields, is discussed. In addition, their potential for purification and thus suppression of matrix effects, responsible for quantification problems especially in mass spectrometry, is presented. Due to their similar properties, molecularly imprinted polymers and aptamer-based sorbents that appear to be an interesting alternative to antibodies are also briefly addressed by comparing their potential with that of immunosorbents.

Bioluminescent detection of zearalenone using recombinant peptidomimetic Gaussia luciferase fusion protein

The development of a bioluminescent immunosensor is reported for the determination of zearalenone (ZEA) based on a peptide mimetic identified by phage display. The peptide mimetic GW, with a peptide sequence GWWGPYGEIELL, was used to create recombinant fusion proteins with the bioluminescent Gaussia luciferase (GLuc) that were directly used as tracers for toxin detection in a competitive immunoassay without the need for secondary antibodies or further labeling. The bioluminescent sensor, based on protein G-coupled magnetic beads for antibody immobilization, enabled determination of ZEA with a detection limit of 4.2 ng mL^-1 (corresponding to 420 μg kg^-1 in food samples) and an IC₅₀ value of 11.0 ng mL^-1. The sensor performance was evaluated in spiked maize and wheat samples, with recoveries ranging from 87 to 106% (RSD < 20%, n = 3). Finally, the developed method was applied to the analysis of a naturally contaminated reference matrix material and good agreement with the reported concentrations was obtained.Graphical abstract.

Modification performance and electrochemical characteristics of different groups of modified aptamers applied for label-free electrochemical impedimetric sensors

Amino and thiolated aptamers are the main aptamers used to construct label-free electrochemical impedimetric aptasensors. In this study, the modification performance and electrochemical properties of amino aptamers and thiolated aptamers were studied in the construction of label-free impedimetric sensors. The results showed that the initial modification density of amino aptamers was higher than that of thiol aptamers. Aptamers can recognize and bind OTA to generate electrical signals. The higher the density of aptamer modification was, the better the electric signals were. If only considering the initial modification density, amino aptamers were more suitable for the preparation of aptasensors than thiolated aptamers. However, the modification density of the amino aptamer decreased with the prolonged immersion time in 1 mM HCl solution, which suggests that the stability of this sensor was poor. However, the thiolated aptamer maintained relatively constant density and could be reused. Thus, the thiolated aptasensor had a wide range and good reproducibility and stability for the determination of ochratoxin A (OTA). In addition, this study proved that gold nanoparticles play an important role in signal amplification by increasing the effective gold surface to fix more aptamers in the process of sensor preparation.

Advances in Optical Single-Molecule Detection: En Route to Supersensitive Bioaffinity Assays

The ability to detect low concentrations of analytes and in particular low-abundance biomarkers is of fundamental importance, e.g., for early-stage disease diagnosis. The prospect of reaching the ultimate limit of detection has driven the development of single-molecule bioaffinity assays. While many review articles have highlighted the potentials of single-molecule technologies for analytical and diagnostic applications, these technologies are not as widespread in real-world applications as one should expect. This Review provides a theoretical background on single-molecule-or better digital-assays to critically assess their potential compared to traditional analog assays. Selected examples from the literature include bioaffinity assays for the detection of biomolecules such as proteins, nucleic acids, and viruses. The structure of the Review highlights the versatility of optical single-molecule labeling techniques, including enzymatic amplification, molecular labels, and innovative nanomaterials.

Development of an Escherichia coli-based electrochemical biosensor for mycotoxin toxicity detection

Mycotoxin contamination in food and feed is a global concern because mycotoxin contamination can cause both acute and chronic health effects in humans and animals. In the present work, an Escherichia coli-based biosensor is described for the toxicity assessment of aflatoxin B₁ (AFB₁) and zearalenone (ZEN). In this electrochemical biosensor, E. coli is used as the signal recognition element, p-benzoquinone is used as the mediator, and a two-step reaction procedure has been developed to separate the mediator from the mycotoxins. The current value of the as-prepared microbial biosensor exhibits a linear decrease with concentrations of AFB₁ and ZEN in the range of 0.01-0.3 and 0.05-0.5 μg/mL, with detection limits reaching 1 and 6 ng/mL, respectively. The IC₂₅ values obtained by the present method are 0.25 and 0.40 μg/mL for AFB₁ and ZEN, which shows that the cytotoxicity of AFB₁ to E. coli is more severe than the cytotoxicity of ZEN to E. coli. The combined toxic effect of these two mycotoxins has also been explored, and synergistic biotoxicity has been observed. Moreover, the biosensor is successfully applied to the toxicity evaluation of mycotoxins in real samples, including peanut and corn oils. This work could provide new insight into mycotoxin and microorganism interactions and could establish a new approach for future mycotoxin detection.

Two-Dimensional Layered Nanomaterial-Based Electrochemical Biosensors for Detecting Microbial Toxins

Toxin detection is an important issue in numerous fields, such as agriculture/food safety, environmental monitoring, and homeland security. During the past two decades, nanotechnology has been extensively used to develop various biosensors for achieving fast, sensitive, selective and on-site analysis of toxins. In particular, the two dimensional layered (2D) nanomaterials (such as graphene and transition metal dichalcogenides (TMDs)) and their nanocomposites have been employed as label and/or biosensing transducers to construct electrochemical biosensors for cost-effective detection of toxins with high sensitivity and specificity. This is because the 2D nanomaterials have good electrical conductivity and a large surface area with plenty of active groups for conjugating 2D nanomaterials with the antibodies and/or aptamers of the targeted toxins. Herein, we summarize recent developments in the application of 2D nanomaterial-based electrochemical biosensors for detecting toxins with a particular focus on microbial toxins including bacterial toxins, fungal toxins and algal toxins. The integration of 2D nanomaterials with some existing antibody/aptamer technologies into electrochemical biosensors has led to an unprecedented impact on improving the assaying performance of microbial toxins, and has shown great promise in public health and environmental protection.

A signal-on electrochemical aptasensor for rapid detection of aflatoxin B1 based on competition with complementary DNA

Aflatoxin B1 (AFB1) is the most toxic mycotoxin, causing harmful effects on human and animal health, and the rapid and sensitive detection of AFB1 is highly demanded. We developed a simple electrochemical aptasensor achieving rapid detection of aflatoxin B1 (AFB1). A short anti-AFB1 aptamer having a methylene blue (MB) redox tag at the 3'-end was immobilized on the surface of a gold electrode. In the absence of AFB1, a complementary DNA (cDNA) strand hybridized with the MB-labeled aptamer, causing MB apart from the electrode surface and low current of MB. In the presence of AFB1, AFB1 competed with the cDNA in the binding to the MB-labeled aptamer, and the aptamer-AFB1 binding caused formation of a hairpin structure, making the MB close to the electrode surface and current of MB increase. Under optimized conditions, we achieved detection of AFB1 over dynamic concentration range of 2 nM-4 μM by using this signal-on electrochemical aptasensor. This method only required a simple 5-min incubation of sample solution prior to rapid electrochemical sensing, more rapid than other electrochemical aptasensors. The sensor could be well regenerated and reused. This sensor allowed to detect AFB1 spiked in 20-fold diluted beer and 50-fold diluted white wine, respectively. It shows potential for detection of AFB1 in wide applications.

Phage Display in the Quest for New Selective Recognition Elements for Biosensors

Phages are bacterial viruses that have gained a significant role in biotechnology owing to their widely studied biology and many advantageous characteristics. Perhaps the best-known application of phages is phage display that refers to the expression of foreign peptides or proteins outside the phage virion as a fusion with one of the phage coat proteins. In 2018, one half of the Nobel prize in chemistry was awarded jointly to George P. Smith and Sir Gregory P. Winter "for the phage display of peptides and antibodies." The outstanding technology has evolved and developed considerably since its first description in 1985, and today phage display is commonly used in a wide variety of disciplines, including drug discovery, enzyme optimization, biomolecular interaction studies, as well as biosensor development. A cornerstone of all biosensors, regardless of the sensor platform or transduction scheme used, is a sensitive and selective bioreceptor, or a recognition element, that can provide specific binding to the target analyte. Many environmentally or pharmacologically interesting target analytes might not have naturally appropriate binding partners for biosensor development, but phage display can facilitate the production of novel receptors beyond known biomolecular interactions, or against toxic or nonimmunogenic targets, making the technology a valuable tool in the quest of new recognition elements for biosensor development.

Immobilized Luminescent Bacteria for the Detection of Mycotoxins under Discrete and Flow-Through Conditions

A biosensitive element in the form of bacterial Photobacterium phosphoreum cells immobilized in poly(vinyl alcohol) cryogel was tested for the determination of different mycotoxins under discrete and flow-through analysis conditions. The immobilized bioluminescent cells made it possible to quantify the presence of Ochratoxin A, Sterigmatocystin, Zearalenone, and Deoxynivalenon in aqueous media in a wide range of their concentrations (0.017-56 mg/L, 0.010-33 mg/L, 0.009-14 mg/L, and 0.026-177 mg/L, respectively) via measuring the quenching of cell luminescence. The flow conditions allowed the analysis sensitivity to be improved by an order of magnitude in terms of detected concentrations. Using the immobilized luminescent bacterial cells, we have shown the possibility of evaluating the efficiency of the mycotoxins' hydrolysis under the action of enzymes. In this way, a 94 ± 4.5% efficiency of Zearalenone hydrolysis with hexahistidine-containing organophosphorus hydrolase for 1h-long treatment of the mycotoxin solution (100 mg/L) was shown.

Facile Fabrication of Surface-Imprinted Macroporous Films for Chemosensing of Human Chorionic Gonadotropin Hormone

We present an improved approach for the preparation of highly selective and homogeneous molecular cavities in molecularly imprinted polymers (MIPs) via the combination of surface imprinting and semi-covalent imprinting. Toward that, first, a colloidal crystal mold was prepared via the Langmuir-Blodgett (LB) technique. Then, human chorionic gonadotropin (hCG) template protein was immobilized on the colloidal crystal mold. Later, hCG derivatization with electroactive functional monomers via amide chemistry was performed. In a final step, optimized potentiostatic polymerization of 2,3'-bithiophene enabled depositing an MIP film as the macroporous structure. This synergistic strategy resulted in the formation of molecularly imprinted cavities exclusively on the internal surface of the macropores, which were accessible after dissolution of silica molds. The recognition of hCG by the macroporous MIP film was transduced with the help of electric transducers, namely, extended-gate field-effect transistors (EG-FET) and capacitive impedimetry (CI). These readout strategies offered the ability to create chemosensors for the label-free determination of the hCG hormone. Other than the simple confirmation of pregnancy, hCG assay is a common tool for the diagnosis and follow-up of ectopic pregnancy or trophoblast tumors. Concentration measurements with these EG-FET and CI-based devices allowed real-time measurements of hCG in the range of 0.8-50  and 0.17-2.0 fM, respectively, in 10 mM carbonate buffer (pH = 10). Moreover, the selectivity of chemosensors with respect to protein interferences was very high.

Electrochemical Immuno- and Aptasensors for Mycotoxin Determination

Modern analysis of food and feed is mostly focused on development of fast and reliable portable devices intended for field applications. In this review, electrochemical biosensors based on immunological reactions and aptamers are considered in the determination of mycotoxins as one of most common contaminants able to negatively affect human health. The characteristics of biosensors are considered from the point of view of general principles of bioreceptor implementation and signal transduction providing sub-nanomolar detection limits of mycotoxins. Moreover, the modern trends of bioreceptor selection and modification are discussed as well as future trends of biosensor development for mycotoxin determination are considered.

Developments in mycotoxin analysis: an update for 2017-2018

This review summarises developments that have been published in the period from mid-2017 to mid-2018 on the analysis of various matrices for mycotoxins. Analytical methods to determine aflatoxins, Alternaria toxins, ergot alkaloids, fumonisins, ochratoxins, patulin, trichothecenes, and zearalenone are covered in individual sections. Advances in sampling strategies are discussed in a dedicated section, as are methods used to analyse botanicals and spices, and newly developed comprehensive liquid chromatographic-mass spectrometric based multi-mycotoxin methods. This critical review aims to briefly discuss the most important recent developments and trends in mycotoxin determination as well as to address limitations of the presented methodologies.

Homogeneous Quenching Immunoassay for Fumonisin B1 Based on Gold Nanoparticles and an Epitope-Mimicking Yellow Fluorescent Protein

Homogeneous immunoassays represent an attractive alternative to traditional heterogeneous assays due to their simplicity, sensitivity, and speed. On the basis of a previously identified epitope-mimicking peptide, or mimotope, we developed a homogeneous fluorescence quenching immunoassay based on gold nanoparticles (AuNPs) and a recombinant epitope-mimicking fusion protein for the detection of mycotoxin fumonisin B₁ (FB₁). The fumonisin mimotope was cloned as a fusion protein with a yellow fluorescent protein that could be used directly as the tracer for FB₁ detection without the need of labeling or a secondary antibody. Furthermore, owing to the fluorescence quenching ability of AuNPs, a homogeneous immunoassay could be performed in a single step without washing steps to separate the unbound tracer. The homogeneous quenching assay showed negligible matrix effects in 5% wheat extract and high sensitivity for FB₁ detection, with a dynamic range from 7.3 to 22.6 ng mL^-1, a detection limit of 1.1 ng mL^-1, and IC₅₀ value of 12.9 ng mL^-1, which was significantly lower than the IC₅₀ value of the previously reported assay using the synthetic counterpart of the same mimotope in a microarray format. The homogeneous assay was demonstrated to be specific for fumonisins B₁ and B₂, as no significant cross-reactivity with other mycotoxins was observed, and acceptable recoveries (86% for FB₁ 2000 μg kg^-1 and 103% for FB₁ 4000 μg kg^-1), with relative standard deviation less than 6.5%, were reported from spiked wheat samples, proving that the method could provide a valuable tool for simple analysis of mycotoxin-contaminated food samples.

Optical Biosensors for Label-Free Detection of Small Molecules

Label-free optical biosensors are an intriguing option for the analyses of many analytes, as they offer several advantages such as high sensitivity, direct and real-time measurement in addition to multiplexing capabilities. However, development of label-free optical biosensors for small molecules can be challenging as most of them are not naturally chromogenic or fluorescent, and in some cases, the sensor response is related to the size of the analyte. To overcome some of the limitations associated with the analysis of biologically, pharmacologically, or environmentally relevant compounds of low molecular weight, recent advances in the field have improved the detection of these analytes using outstanding methodology, instrumentation, recognition elements, or immobilization strategies. In this review, we aim to introduce some of the latest developments in the field of label-free optical biosensors with the focus on applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed.

Development of aptamer fluorescent switch assay for aflatoxin B1 by using fluorescein-labeled aptamer and black hole quencher 1-labeled complementary DNA

Aflatoxin B1 (AFB1) is one of the most toxic mycotoxins and draws great concern in health and food safety. A DNA aptamer against AFB1 having a stem-loop structure shows high binding affinity to AFB1 and promise in assay development for AFB1 detection. Based on the structure-switching property of the aptamer, we report an aptamer fluorescence assay for AFB1 detection. Aptamer with fluorescein (FAM) label at 5' end was used as affinity ligand, while its short complementary DNA (cDNA) with BHQ1 (black hole quencher 1) label at 3' end was used as a quencher. In the absence of AFB1, FAM-labeled aptamer hybridized with BHQ1-labeled cDNA, forming a duplex of cDNA and aptamer, resulting in fluorescence quenching of FAM. When AFB1 bound with aptamer, the BHQ1-labeled cDNA was displaced from aptamer, causing fluorescence restoration of FAM. We tested a series of FAM-labeled aptamers and BHQ1-labeled cDNAs with different lengths. The lengths of the aptamer stem and the cDNA, Mg²⁺ in binding buffer, and temperature had significant influence on the performance of the assay. Under optimized conditions, we achieved sensitive detection of AFB1 by using a 29-mer FAM-labeled aptamer and a 14-mer BHQ1-labeled cDNA, and the detection limit of AFB1 reached 0.2 nM. The maximum fluorescence recovery rate of FAM-labeled aptamer caused by AFB1 was about 69-fold. This method enabled the detection of AFB1 in complex sample matrix, e.g., diluted wine samples and maize flour samples. This aptamer-based fluorescent assay for AFB1 determination shows potential for broad applications. Graphical abstract ᅟ.

Biosensing Based on Nanoparticles for Food Allergens Detection

Food allergy is one of the major health threats for sensitized individuals all over the world and, over the years, the food industry has made significant efforts and investments to offer safe foods for allergic consumers. The analysis of the concentration of food allergen residues in processing equipment, in raw materials or in the final product, provides analytical information that can be used for risk assessment as well as to ensure that food-allergic consumers get accurate and useful information to make their food choices and purchasing decisions. The development of biosensors based on nanomaterials for applications in food analysis is a challenging area of growing interest in the last years. Research in this field requires the combined efforts of experts in very different areas including food chemistry, biotechnology or materials science. However, the outcome of such collaboration can be of significant impact on the food industry as well as for consumer’s safety. These nanobiosensing devices allow the rapid, selective, sensitive, cost-effective and, in some cases, in-field, online and real-time detection of a wide range of compounds, even in complex matrices. Moreover, they can also enable the design of novel allergen detection strategies. Herein we review the main advances in the use of nanoparticles for the development of biosensors and bioassays for allergen detection, in food samples, over the past few years. Research in this area is still in its infancy in comparison, for instance, to the application of nanobiosensors for clinical analysis. However, it will be of interest for the development of new technologies that reduce the gap between laboratory research and industrial applications.

Molecular recognition by synthetic receptors: Application in field-effect transistor based chemosensing

Molecular recognition, i.e., ability of one molecule to recognize another through weak bonding interactions, is one of the bases of life. It is often implemented to sensing systems of high merits. Preferential recognition of the analyte (guest) by the receptor (host) induces changes in physicochemical properties of the sensing system. These changes are measured by using suitable signal transducers. Because of possibility of miniaturization, fast response, and high sensitivity, field-effect transistors (FETs) are more frequently being used for that purpose. A FET combined with a biological material offers the potential to overcome many challenges approached in sensing. However, low stability of biological materials under measurement conditions is a serious problem. To circumvent this problem, synthetic receptors were integrated with the gate surface of FETs to provide robust performance. In the present critical review, the approach utilized to devise chemosensors integrating synthetic receptors and FET transduction is discussed in detail. The progress in this field was summarized and important outcome was provided.