Aptamer binding to celiac disease-triggering hydrophobic proteins: a sensitive gluten detection approach

Parylene Double-Layer Coated Screen-Printed Carbon Electrode for Label-Free and Reagentless Capacitive Aptasensing of Gliadin

Celiac patients are required to strictly adhere to a gluten-free diet because even trace amounts of gluten can damage their small intestine and leading to serious complications. Despite increased awareness, gluten can still be present in products due to cross-contamination or hidden ingredients, making regular monitoring essential. With the goal of guaranteeing food safety for consuming labeled gluten-free products, a capacitive aptasensor was constructed to target gliadin, the main allergic gluten protein for celiac disease. The success of capacitive aptasensing was primarily realized by coating a Parylene double-layer (1000 nm Parylene C at the bottom with 400 nm Parylene AM on top) on the electrode surface to ensure both high insulation quality and abundant reactive amino functionalities. Under the optimal concentration of aptamer (5 μM) used for immobilization, a strong linear relationship exists between the amount of gliadin (0.01-1.0 mg/mL) and the corresponding ΔC response (total capacitance decrease during a 20 min monitoring period after sample introduction), with an R2 of 0.9843. The detection limit is 0.007 mg/mL (S/N > 5), equivalent to 0.014 mg/mL (14 ppm) of gluten content. Spike recovery tests identified this system is free from interferences in corn and cassava flour matrices. The analytical results of 24 commercial wheat flour samples correlated well with a gliadin ELISA assay (R2 = 0.9754). The proposed label-free and reagentless capacitive aptasensor offers advantages of simplicity, cost-effectiveness, ease of production, and speediness, making it a promising tool for verifying products labeled as gluten-free (gluten content <20 ppm).

Simple detection of gluten in commercial gluten-containing samples with a novel nanoflower electrosensor made of molybdenum disulfide with comparison of the ELISA method

In this study, a new electrochemical sensor based on molybdenum disulfide (MoS2) nanoflowers/glassy carbon electrode (GCE was created for the sensitive detection of gluten. The prepared nanocatalysts were characterized using scanning electron microscopy with energy dispersive spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. The effects of the prepared nanocatalysts, pH value, and dropping amounts on the results were examined in detail. The electrochemical performance of the developed sensor (MoS2 nanoflowers/GCE) was then evaluated using differential pulse voltammetry, and the sensor was found to have significant electrochemical activity against gluten. A substantial linear connection was observed in the range of 0.5-100 ppm of gluten concentration under optimum experimental circumstances, and the detection limit between peak current and gluten concentration was determined as 1.16 ppm. The findings showed that the MoS2 nanoflowers/GCE gluten sensor has exceptional selectivity and stability. Finally, the generated electrochemical sensor was effectively utilized for gluten detection in commercial gluten-containing materials with a detection limit of 0.1652 ppm. Thus, the developed MoS2 nanoflowers/GCE sensor offers a potential method for the detection of other molecules and is a promising candidate for gluten detection in commercial samples.

A review on magnetic beads-based SELEX technologies: Applications from small to large target molecules

This review summarizes the stepwise strategy and key points for magnetic beads (MBs)-based aptamer selection which is suitable for isolating aptamers against small and large molecules via systematic evolution of ligands by exponential enrichment (SELEX). Particularities, if any, are discussed according to the target size. Examples targeting small molecules (<1000 Da) such as xenobiotics, toxins, pesticides, herbicides, illegal additives, hormones, and large targets such as proteins (biomarkers, pathogens) are discussed and presented in tabular formats. Of special interest are the latest advances in more efficient alternatives, which are based on novel instrumentation, materials or microelectronics, such as fluorescence MBs-SELEX or microfluidic chip system-assisted MBs-SELEX. Limitations and perspectives of MBs-SELEX are also reviewed. Taken together, this review aims to provide practical insights into MBs-SELEX technologies and their ability to screen multiple potential aptamers against targets from small to large molecules.

Lable-free aptamer portable colorimetric smartphone for gliadin detection in food

For individuals with celiac disease (CD), the current clinical therapy option available is a lifelong gluten-free diet. Therefore, it is essential to swiftly and efficiently detect gluten in foods. A colorimetric sensor has been developed, which operates by regulating the aggregation and dispersion state of AuNPs induced by high concentration NaCl through the specific binding of gliadin and aptamer, thereby achieving rapid detection of gliadin in flour. It is found that the sensor exhibits good linearity in the concentration range of 0.67-10 μM and the LOD (3σ/S) is 12 nM. And it can accurately distinguish various types of free-gliadin samples, with a spiked recovery rate of 85%-122.3%. To make the detection process more convenient, the colorimetric results of the biosensor were translated into RGB color-gamut parameters by a smartphone color-picking program for further analysis. Gliadin can still be accurately quantified with the established smartphone platform, and a correlation coefficient of 0.988 was found. The proposed portable smartphone aptamer colorimetric sensing device has achieved satisfactory results in the rapid detection of gliadin in food.

A label-free aptamer-based colorimetric biosensor for rapid gliadin detection in foods: a focus on pasta, bread and cookies

Despite numerous advancements in gluten detection, a substantial need remains for innovative, cost-effective, in situ methods that can be employed without complex analytical instruments. Addressing this demand, this study introduces a pioneering label-free colorimetric biosensor for the in situ detection of gliadin, a major component of gluten, which is a prevalent trigger of food allergies. Our novel approach employs the strategic coating of gold nanoparticles (AuNP) with gliadin-specific aptamers. In the absence of gliadin, these aptamers stably disperse AuNP, preventing their aggregation. However, upon the introduction of gliadin and in the presence of sodium chloride, AuNP aggregate, yielding a measurable colorimetric signal that facilitates the precise quantification of gliadin. Under rigorously optimized conditions, this AuNP/aptamer-based colorimetric biosensor demonstrated exceptional sensitivity and selectivity, with a detection limit of 32.1 ng mL-1 and a linear response range of 0-300 ng mL-1. Critically, the sensor maintained reliable performance when applied to real-world food samples, including gluten-free bread, cookies, and pasta. Due to its simplicity, selectivity, speed, and cost-effectiveness, this assay represents a significant advancement over current gluten detection methods. Moreover, the developed AuNP/aptamer-based colorimetric biosensor design holds promising potential for adaptation to detect other food allergens or protein toxins through selective aptamer modifications.

Overview on the Design of Magnetically Assisted Electrochemical Biosensors

Electrochemical biosensors generally require the immobilization of recognition elements or capture probes on the electrode surface. This may limit their practical applications due to the complex operation procedure and low repeatability and stability. Magnetically assisted biosensors show remarkable advantages in separation and pre-concentration of targets from complex biological samples. More importantly, magnetically assisted sensing systems show high throughput since the magnetic materials can be produced and preserved on a large scale. In this work, we summarized the design of electrochemical biosensors involving magnetic materials as the platforms for recognition reaction and target conversion. The recognition reactions usually include antigen-antibody, DNA hybridization, and aptamer-target interactions. By conjugating an electroactive probe to biomolecules attached to magnetic materials, the complexes can be accumulated near to an electrode surface with the aid of external magnet field, producing an easily measurable redox current. The redox current can be further enhanced by enzymes, nanomaterials, DNA assemblies, and thermal-cycle or isothermal amplification. In magnetically assisted assays, the magnetic substrates are removed by a magnet after the target conversion, and the signal can be monitored through stimuli-response release of signal reporters, enzymatic production of electroactive species, or target-induced generation of messenger DNA.

Luminescent Aptamer-Based Bioassays for Sensitive Detection of Food Allergens

The presence of hidden allergens in food products, often due to unintended contamination along the food supply chain (production, transformation, processing, and transport), has raised the urgent need for rapid and reliable analytical methods for detecting trace levels of such species in food products. Indeed, food allergens represent a high-risk factor for allergic subjects due to potentially life-threatening adverse reactions. Portable biosensors based on immunoassays have already been developed as rapid, sensitive, selective, and low-cost analytical platforms that can replace analyses with traditional bench-top instrumentation. Recently, aptamers have attracted great interest as alternative biorecognition molecules for bioassays, since they can bind a variety of targets with high specificity and selectivity, and they enable the development of assays exploiting a variety of transduction and detection technologies. In particular, aptasensors based on luminescence detection have been proposed, taking advantage of the development of ultrasensitive tracers and enhancers. This review aims to summarize and discuss recent efforts in the field of food allergen analysis using aptamer-based bioassays with luminescence detection.

Recent Advances in Electrochemical Sensing Strategies for Food Allergen Detection

Food allergy has been indicated as the most frequent adverse reaction to food ingredients over the past few years. Since the only way to avoid the occurrence of allergic phenomena is to eliminate allergenic foods, it is essential to have complete and accurate information on the components of foodstuff. In this framework, it is mandatory and crucial to provide fast, cost-effective, affordable, and reliable analysis methods for the screening of specific allergen content in food products. This review reports the research advancements concerning food allergen detection, involving electrochemical biosensors. It focuses on the sensing strategies evidencing different types of recognition elements such as antibodies, nucleic acids, and cells, among others, the nanomaterial role, the several electrochemical techniques involved and last, but not least, the ad hoc electrodic surface modification approaches. Moreover, a selection of the most recent electrochemical sensors for allergen detection are reported and critically analyzed in terms of the sensors’ analytical performances. Finally, advantages, limitations, and potentialities for practical applications of electrochemical biosensors for allergens are discussed.

An Effective Label-Free Electrochemical Aptasensor Based on Gold Nanoparticles for Gluten Detection

Nanomaterials can be used to modify electrodes and improve the conductivity and the performance of electrochemical sensors. Among various nanomaterials, gold-based nanostructures have been used as an anchoring platform for the functionalization of biosensor surfaces. One of the main advantages of using gold for the modification of electrodes is its great affinity for thiol-containing molecules, such as proteins, forming a strong Au-S bond. In this work, we present an impedimetric biosensor based on gold nanoparticles and a truncated aptamer for the quantification of gluten in hydrolyzed matrices such as beer and soy sauce. A good relationship between the R_ct values and PWG-Gliadin concentration was found in the range between 0.1–1 mg L⁻¹ of gliadin (corresponding to 0.2–2 mg L⁻¹ of gluten) with a limit of detection of 0.05 mg L⁻¹ of gliadin (corresponding to 0.1 mg L⁻¹ of gluten). The label-free assay was also successfully applied for the determination of real food samples.

Aptamers targeting a tumor-associated extracellular matrix component: The human mature collagen XIα1

The extracellular matrix (ECM) plays an essential role in tumor progression and invasion through its continuous remodeling. The growth of most carcinomas is associated with an excessive collagen deposition that provides the proper environment for tumor development and chemoresistance. The α1 chain of a minor human collagen, type XI, is overexpressed in some tumor stroma, but not found in normal stroma. To test the clinical utility of this collagen as a cancer biomarker, specific receptors are needed. Available antibodies do not show enough selectivity or are directed toward the propeptide region that is cleaved when the protein is released to the ECM. Here we show the selection of an aptamer for the specific C-telopeptide region using a 16-mer peptide as the target for the SELEX. The aptamer selected with a K_d of ∼25 nM was able to capture the collagen XI from cell lysates. It was also used for target detection in a mixed antibody-aptamer sandwich assay showing it can be useful for diagnostic purposes in biological fluids.

Paper-based aptamer-antibody biosensor for gluten detection in a deep eutectic solvent (DES)

Paper has been widely employed as cheap material for the development of a great number of sensors such as pregnancy tests, strips to measure blood sugar, and COVID-19 rapid tests. The need for new low-cost analytical devices is growing, and consequently the use of these platforms will be extended to different assays, both for the final consumer and within laboratories. This work describes a paper-based electrochemical sensing platform that uses a paper disc conveniently modified with recognition molecules and a screen-printed carbon electrode (SPCE) to achieve the detection of gluten in a deep eutectic solvent (DES). This is the first method coupling a paper biosensor based on aptamers and antibodies with the DES ethaline. Ethaline proved to be an excellent extraction medium allowing the determination of very low gluten concentrations. The biosensor is appropriate for the determination of gluten with a limit of detection (LOD) of 0.2 mg L⁻¹ of sample; it can detect gluten extracted in DES with a dynamic range between 0.2 and 20 mg L⁻¹ and an intra-assay coefficient of 10.69%. This approach can be of great interest for highly gluten-sensitive people, who suffer from ingestion of gluten quantities well below the legal limit, which is 20 parts per million in foods labeled gluten-free and for which highly sensitive devices are essential.

Functional Nucleic Acids Under Unusual Conditions

Functional nucleic acids (FNAs), including naturally occurring ribozymes and riboswitches as well as artificially created DNAzymes and aptamers, have been popular molecular toolboxes for diverse applications. Given the high chemical stability of nucleic acids and their ability to fold into diverse sequence-dependent structures, FNAs are suggested to be highly functional under unusual reaction conditions. This review will examine the progress of research on FNAs under conditions of low pH, high temperature, freezing conditions, and the inclusion of organic solvents and denaturants that are known to disrupt nucleic acid structures. The FNA species to be discussed include ribozymes, riboswitches, G-quadruplex-based peroxidase mimicking DNAzymes, RNA-cleaving DNAzymes, and aptamers. Research within this space has not only revealed the hidden talents of FNAs but has also laid important groundwork for pursuing these intriguing functional macromolecules for unique applications.

A fluorogenic capped mesoporous aptasensor for gluten detection

Celiac disease is a complex and autoimmune disorder caused by the ingestion of gluten affecting almost 1% of global population. Nowadays an effective treatment does not exist, and the only way to manage the disease is the removal of gluten from the diet. Owing the key role played by gluten, clear and regulated labelling of foodstuff and smart methods for gluten detection are needed to fight frauds on food industry and to avoid the involuntary ingestion of this protein by celiac patients. On that scope, the development of a novel detection system of gluten is here presented. The sensor consists of nanoporous anodic alumina films loaded with a fluorescent dye and capped with an aptamer that recognizes gliadin (gluten's soluble proteins). In the presence of gliadin, aptamer sequences displace from the surface of anodic alumina resulting in pore opening and dye delivery. The dispositive shows a limit of detection (LOD) of 100 μg kg^-1 of gliadin, good selectivity and a detection time of approximately 60 min. Moreover, the sensor is validated in real food samples. This novel probe allows fast gluten detection through a simple signalling process with potential use for food control.

Truncated aptamers as selective receptors in a gluten sensor supporting direct measurement in a deep eutectic solvent

Enzyme-linked immunosorbent assays are currently the most popular methods to quantify gluten in foods. Unfortunately, the antibodies used as specific receptors in such methods are not compatible with the usual solvents for the extraction of gluten proteins. In consequence, commercial tests require a high dilution of the sample after the extraction, increasing the limit of quantification and decreasing convenience. In this work, we have rationally truncated an aptamer capable of recognizing gliadin in a deep eutectic solvent (DES). The truncated aptamer is a 19-nucleotides-long DNA that minimizes self-hybridization, allowing the development of an electrochemical sandwich-based sensor for the quantification of gluten in the DES ethaline. The sensor incorporates two identical biotin-labeled truncated aptamers, one of which is immobilized on a carbon screen-printed electrode and the other reports the binding of gliadin after incubation in streptavidin-peroxidase. This sensor can detect gliadin in DES, with a dynamic range between 1 and 100 μg/L and an intra-assay coefficient of variation of 11%. This analytical performance allows the quantification of 20 μg of gluten/kg of food when 1 g of food is extracted with 10 mL of ethaline. We demonstrate the ability of this method to achieve the measurement of gluten in food samples, after the extraction with pure ethaline. The assay is useful for the analysis of residual gluten levels in foods, thus facilitating the evaluation of any potential health risk associated with the consumption of such food by people with celiac disease or other gluten-related disorders.

Microfluidic opportunities in printed electrolyte-gated transistor biosensors

Printed electrolyte-gated transistors (EGTs) are an emerging biosensor platform that leverage the facile fabrication engendered by printed electronics with the low voltage operation enabled by ion gel dielectrics. The resulting label-free, nonoptical sensors have high gain and provide sensing operations that can be challenging for conventional chemical field effect transistor architectures. After providing an overview of EGT device fabrication and operation, we highlight opportunities for microfluidic enhancement of EGT sensor performance via multiplexing, sample preconcentration, and improved transport to the sensor surface.

Gluten Detection Methods and Their Critical Role in Assuring Safe Diets for Celiac Patients

Celiac disease, wheat sensitivity, and allergy represent three different reactions, which may occur in genetically predisposed individuals on the ingestion of wheat and derived products with various manifestations. Improvements in the disease diagnostics and understanding of disease etiology unveiled that these disorders are widespread around the globe affecting about 7% of the population. The only known treatment so far is a life-long gluten-free diet, which is almost impossible to follow because of the contamination of allegedly "gluten-free" products. Accidental contamination of inherently gluten-free products could take place at any level from field to shelf because of the ubiquity of these proteins/grains. Gluten contamination of allegedly "gluten-free" products is a constant threat to celiac patients and a major health concern. Several detection procedures have been proposed to determine the level of contamination in products for celiac patients. The present article aims to review the advantages and disadvantages of different gluten detection methods, with emphasis on the recent technology that allows identification of the immunogenic-gluten peptides without the use of antibodies. The possibility to detect gluten contamination by different approaches with similar or better detection efficiency in different raw and processed foods will guarantee the safety of the foods for celiac patients.

Recent advances in the development of electrochemical aptasensors for detection of heavy metals in food

Heavy metal contamination in environment and food has attracted intensive attention from the public since it poses serious threats to ecological system and human health. Traditional detection methods for heavy metals such as atomic absorption spectrometry have a fairly low detection limit, but the methods have many limitations and disadvantages. Therefore, it is of significance to develop a rapid technology for real-time and online detection of heavy metals. The electrochemical aptasensor-based technology is promising in the detection of heavy metals with advantages of high sensitivity, specificity, and accuracy. Although its development is rapid, more researches should be carried out before this technology can be used for on-site detection. In this review, the origin, basic principles and development of electrochemical aptasensors are introduced. The applications of nanomaterials and electrochemical aptasensors for the detection of heavy metals (mainly mercury, lead, cadmium, and arsenic) are summarized. The research and application tendency of electrochemical aptasensors for detection of heavy metals are prospected.

Genetic and environmental factors affecting the expression of α-gliadin canonical epitopes involved in celiac disease in a wide collection of spelt (Triticum aestivum ssp. spelta) cultivars and landraces

BACKGROUND

Celiac disease (CD) is an autoimmune disorder affecting genetically predisposed individuals whose dietary gluten proteins trigger an inflammatory reaction in the small intestine. Gluten is found in the seeds of cereals like bread wheat (Triticum aestivum ssp. aestivum) and spelt (Triticum aestivum ssp. spelta). The development of new varieties lacking immunogenic peptides is one of the strategies currently investigated to address the CD problem. Among gluten proteins, α-gliadins display the strongest immunogenicity with four main T-cell stimulatory epitopes. The objective of this work was to study the expression of α-gliadin epitopes related to CD in a wide collection of 121 spelt accessions (landraces and varieties, spring and winter accessions) from different provenances, and to analyze the correlation between the presence of epitope sequences in gDNA and their expression (cDNA). The effect of environmental factors (harvest year and N fertilization) on the epitope expression was also investigated.

RESULTS

TaqMan probes targeting the canonical form of the epitopes were used to evaluate the epitope expression levels. Significant variations in the amount of epitope transcripts were identified between accessions and according to the provenances. Spring accessions showed a significantly higher immunogenicity than winter ones and no influence of spelt breeding on the epitope expression levels could be assessed when comparing landraces and varieties from Northwestern Europe. No correlation was observed between quantitative PCR results obtained from cDNA and gDNA for 45 accessions tested, stressing the need to use markers focusing on epitope transcripts rather than on genomic sequences. A relative stability of the amount of epitopes expressed by a same accession across four harvest years was detected. The fertilization strategy, evaluated through seven N fertilization modalities applied to two commercial spelt varieties, did not influence the epitope expression of the first variety, whereas it had a slight effect for the second one.

CONCLUSIONS

The results obtained in this work showed that the CD-related epitope expression greatly fluctuated among the spelt accessions studied. This expression was not correlated to the epitope genomic occurrence and environmental factors had almost no influence on the amount of epitope transcripts.

Recent progress in nanomaterial-based assay for the detection of phytotoxins in foods

Phytotoxins refers to toxic chemicals derived from plants. They include both secondary metabolites that are dose-dependently toxic and allergens that can cause anaphylactic shock in sensitive individuals. Detecting phytotoxins in foods is increasingly important. Conventional methods for detecting phytotoxins lack sufficient sensitivity and operational convenience. Nanomaterial-based determination assays show great competence in fast and accurate sensing of trace substances. In the present review, representative phytotoxin categories of alkaloids, cyanides, and proteins are discussed. Application of notable nanomaterials, e.g. carbon nanotubes, graphene oxide, magnetic nanoparticles, metal-based nanotools, and quantum dots, in specific sensing strategies to fit the physiochemical properties of the target toxins are summarized. Nanomaterials mainly play four roles in phytotoxin detection: 1) analyte enricher; 2) sensor structure mediator; 3) target recognizer or reactant; 4) signaling agent. Great achievements have been made in the detection of trace plant-derived toxins in food matrices, yet there are still challenges awaiting further investigation.

Selection of Anti-gluten DNA Aptamers in a Deep Eutectic Solvent

Herein, we show the feasibility of using deep eutectic solvents as a faster way of selecting aptamers targeting poorly water-soluble species. This unexplored concept is illustrated for gluten proteins. In this way, aptamer-based gluten detection can be performed directly in the extraction media with improved detectability. We envision deep implications for applications not only in food safety control but also in biomedicine.

Rapid and selective luminescent sensing of allergenic gluten by highly phosphorescent switch-on probe

First time the luminescent switch-on probe using novel water-soluble cyclometallated iridium complex (Ir-dc) has been developed for sensitive and selective detection of gluten in the presence of several interfering elements. Linear concentration range of gluten is obtained from 5 to 200 µg/mL with a limit of detection 2.6 µg/mL. The Ir-dc complex responded to the broad pH range which is advantageous for the detection of gluten in various food samples. Additionally, It has been successfully employed for the detection of gluten in commercial food samples of wheat flour and oats with highest recovery values, indicating applicability of Ir-dc for practical usage.

Detection and Sourcing of Gluten in Grain with Multiple Floating-Gate Transistor Biosensors

We report a chemically tunable electronic sensor for quantitation of gluten based on a floating-gate transistor (FGT) architecture. The FGTs are fabricated in parallel and each one is functionalized with a different chemical moiety designed to preferentially bind a specific grain source of gluten. The resulting set of FGT sensors can detect both wheat and barley gluten below the gluten-free limit of 20 ppm (w/w) while providing a source-dependent signature for improved accuracy. This label-free transduction method does not require any secondary binding events, resulting in a ca. 45 min reduction in analysis time relative to state-of-the-art ELISA kits with a simple and easily implemented workflow.

Prospects in the use of aptamers for characterizing the structure and stability of bioactive proteins and peptides in food

Food-derived bioactive proteins and peptides have gained acceptance among researchers, food manufacturers and consumers as health-enhancing functional food components that also serve as natural alternatives for disease prevention and/or management. Bioactivity in food proteins and peptides is determined by their conformations and binding characteristics, which in turn depend on their primary and secondary structures. To maintain their bioactivities, the molecular integrity of bioactive peptides must remain intact, and this warrants the study of peptide form and structure, ideally with robust, highly specific and sensitive techniques. Short single-stranded nucleic acids (i.e. aptamers) are known to have high affinity for cognate targets such as proteins and peptides. Aptamers can be produced cost-effectively and chemically derivatized to increase their stability and shelf life. Their improved binding characteristics and minimal modification of the target molecular signature suggests their suitability for real-time detection of conformational changes in both proteins and peptides. This review discusses the developmental progress of systematic evolution of ligands by exponential enrichment (SELEX), an iterative technology for generating cost-effective aptamers with low dissociation constants (K _d) for monitoring the form and structure of bioactive proteins and peptides. The review also presents case studies of this technique in monitoring the structural stability of bioactive peptide formulations to encourage applications in functional foods. The challenges and potential of aptamers in this research field are also discussed. Graphical abstract Advancing bioactive proteins and peptide functionality via aptameric ligands.

Development of TaqMan probes targeting the four major celiac disease epitopes found in α-gliadin sequences of spelt (Triticum aestivum ssp. spelta) and bread wheat (Triticum aestivum ssp. aestivum)

BACKGROUND

Celiac disease (CD) is caused by specific sequences of gluten proteins found in cereals such as bread wheat (Triticum aestivum ssp. aestivum) and spelt (T. aestivum ssp. spelta). Among them, the α-gliadins display the highest immunogenicity, with four T-cell stimulatory epitopes. The toxicity of each epitope sequence can be reduced or even suppressed according to the allelic form of each sequence. One way to address the CD problem would be to make use of this allelic variability in breeding programs to develop safe varieties, but tools to track the presence of toxic epitopes are required. The objective of this study was to develop a tool to accurately detect and quantify the immunogenic content of expressed α-gliadins of spelt and bread wheat.

RESULTS

Four TaqMan probes that only hybridize to the canonical-i.e. toxic-form of each of the four epitopes were developed and their specificity was demonstrated. Six TaqMan probes targeting stable reference genes were also developed and constitute a tool to normalize qPCR data. The probes were used to measure the epitope expression levels of 11 contrasted spelt accessions and three ancestral diploid accessions of bread wheat and spelt. A high expression variability was highlighted among epitopes and among accessions, especially in Asian spelts, which showed lower epitope expression levels than the other spelts. Some discrepancies were identified between the canonical epitope expression level and the global amount of expressed α-gliadins, which makes the designed TaqMan probes a useful tool to quantify the immunogenic potential independently of the global amount of expressed α-gliadins.

CONCLUSIONS

The results obtained in this study provide useful tools to study the immunogenic potential of expressed α-gliadin sequences from Triticeae accessions such as spelt and bread wheat. The application of the designed probes to contrasted spelt accessions revealed a high variability and interesting low canonical epitope expression levels in the Asian spelt accessions studied.

Ensuring food safety using aptamer based assays: Electroanalytical approach

Aptamers, are being increasingly employed as favorable receptors for constructing highly sensitive biosensors, for their remarkable affinities towards certain targets including a wide scope of biological or chemical substances, and their superiority over other biologic receptors. The selectivity and affinity of the aptamers have been integrated with the wise design of the assay, applying suitable modifications, such as nanomaterials on the electrode surface, employing oligonucleotide-specific amplification strategies or, their combinations. After successful performance of the electrochemical aptasensors for biomedical applications, the food sector with its direct implication for human health, which demands rapid and sensitive and economic analytical solutions for determination of health threatening contaminants in all stages of production process, is the next field of research for developing efficient electrochemical aptasensors. The aim of this review is to categorize and introduce food hazards and summarize the recent electrochemical aptasensors that have been developed to address these contaminants.

Information Visualization and Feature Selection Methods Applied to Detect Gliadin in Gluten-Containing Foodstuff with a Microfluidic Electronic Tongue

The fast growth of celiac disease diagnosis has sparked the production of gluten-free food and the search for reliable methods to detect gluten in foodstuff. In this paper, we report on a microfluidic electronic tongue (e-tongue) capable of detecting trace amounts of gliadin, a protein of gluten, down to 0.005 mg kg^-1 in ethanol solutions, and distinguishing between gluten-free and gluten-containing foodstuff. In some cases, it is even possible to determine whether gluten-free foodstuff has been contaminated with gliadin. That was made possible with an e-tongue comprising four sensing units, three of which made of layer-by-layer (LbL) films of semiconducting polymers deposited onto gold interdigitated electrodes placed inside microchannels. Impedance spectroscopy was employed as the principle of detection, and the electrical capacitance data collected with the e-tongue were treated with information visualization techniques with feature selection for optimizing performance. The sensing units are disposable to avoid cross-contamination as gliadin adsorbs irreversibly onto the LbL films according to polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) analysis. Small amounts of material are required to produce the nanostructured films, however, and the e-tongue methodology is promising for low-cost, reliable detection of gliadin and other gluten constituents in foodstuff.

Electrochemical Affinity Biosensors Based on Disposable Screen-Printed Electrodes for Detection of Food Allergens

Food allergens are proteins from nuts and tree nuts, fish, shellfish, wheat, soy, eggs or milk which trigger severe adverse reactions in the human body, involving IgE-type antibodies. Sensitive detection of allergens in a large variety of food matrices has become increasingly important considering the emergence of functional foods and new food manufacturing technologies. For example, proteins such as casein from milk or lysozyme and ovalbumin from eggs are sometimes used as fining agents in the wine industry. Nonetheless, allergen detection in processed foods is a challenging endeavor, as allergen proteins are degraded during food processing steps involving heating or fermentation. Detection of food allergens was primarily achieved via Enzyme-Linked Immuno Assay (ELISA) or by chromatographic methods. With the advent of biosensors, electrochemical affinity-based biosensors such as those incorporating antibodies and aptamers as biorecognition elements were also reported in the literature. In this review paper, we highlight the success achieved in the design of electrochemical affinity biosensors based on disposable screen-printed electrodes towards detection of protein allergens. We will discuss the analytical figures of merit for various disposable screen-printed affinity sensors in relation to methodologies employed for immobilization of bioreceptors on transducer surface.

A microfluidic biosensor using graphene oxide and aptamer-functionalized quantum dots for peanut allergen detection

The increasing prevalence of food allergies and the intake of packing foods in the past two decades urge the need for more rapid, accurate, and sensitive assays to detect potential allergens in food in order to control the allergen content. Most of the commercial analytical tools for allergen detection rely on immunoassays such as ELISA. As far as disadvantages, ELISA can be time-consuming and expensive. Biosensors appear as a suitable alternative for the detection of allergens because they are rapid, highly sensitive, selective, less expensive, environmentally friendly, and easy to handle. In this study, we developed a microfluidic system integrated with a quantum dots (Qdots) aptamer functionalized graphene oxide (GO) nano-biosensor for simple, rapid, and sensitive food allergen detection. The biosensor utilized Qdots-aptamer-GO complexes as probes to undergo conformational change upon interaction with the food allergens, resulting in fluorescence changes due to the fluorescence quenching and recovering properties of GO by adsorption and desorption of aptamer-conjugated Qdots. This one-step 'turn on' homogenous assay in a ready-to-use microfluidic chip took ~10min to achieve a quantitative detection of Ara h 1, one of the major allergens appearing in peanuts. The results suggested this system had remarkable sensitivity and selectivity. The integration of a microfluidics platform in a homemade miniaturized optical analyzer provides a promising way for the rapid, cost-effective, and accurate on-site determination of food allergens. This biosensor can also be extended to the detection of other food allergens with a selection of corresponding aptamers.

Harnessing Aptamers to Overcome Challenges in Gluten Detection

Celiac disease is a lifelong autoimmune disorder triggered by foods containing gluten, the storage protein in wheat, rye, and barley. The rapidly escalating number of patients diagnosed with this disease poses a great challenge to both food industry and authorities to guarantee food safety for all. Therefore, intensive efforts are being made to establish minimal disease-eliciting doses of gluten and consequently to improve gluten-free labeling. These efforts depend to a high degree on the availability of methods capable of detecting the protein in food samples at levels as low as possible. Current analytical approaches rely on the use of antibodies as selective recognition elements. With limited sensitivity, these methods exhibit some deficiencies that compromise the accuracy of the obtained results. Aptamers provide an ideal alternative for designing biosensors for fast and selective measurement of gluten in foods. This article highlights the challenges in gluten detection, the current status of the use of aptamers for solving this problem, and what remains to be done to move these systems into commercial applications.

Competitive immunosensor based on gliadin immobilization on disposable carbon-nanogold screen-printed electrodes for rapid determination of celiotoxic prolamins

The first competitive disposable amperometric immunosensor based on gliadin-functionalized carbon/nanogold screen-printed electrodes was developed for rapid determination of celiotoxic prolamins. To date, no competitive spectrophotometric or electrochemical immunoassays have yet been successfully applied to gluten detection in processed food samples, which require the use of complex prolamin extraction solutions containing additives with denaturing, reducing and disaggregating functions. Thus, in this work, great effort was put into the optimization and performance evaluation of the immunosensor in terms of suitability as a screening tool for analysis of cereal-based food samples. For this purpose, aqueous ethanol or complex extraction mixtures, as the patented Cocktail Solution®, were proved effective in the extraction of gliadin. Good sensitivity was achieved after optimization of the immunocompetitive assay, giving limit of detection and limit of quantitation of 8 and 22 ng/ml of gliadin, respectively, for ethanol extracts. The immunosensor was proved to be suitable also for samples extracted with Cocktail Solution® after a proper dilution. Analysis of real samples of different flours proved the suitability of the immunosensing device as a powerful tool for safety assessment of raw materials used for the formulation of dietary products for celiac disease patients. This immunosensor combines good analytical performance using a very simplified set-up protocol with suitability for rapid screening analysis performed using inexpensive and portable instrumentation. Graphical abstract Depiction of the development and working principle of the competitive immunosensor.

SELEX Modifications and Bioanalytical Techniques for Aptamer-Target Binding Characterization

The quest to improve the detection of biomolecules and cells in health and life sciences has led to the discovery and characterization of various affinity bioprobes. Libraries of synthetic oligonucleotides (ssDNA/ssRNA) with randomized sequences are employed during Systematic Evolution of Ligands by Exponential Enrichment (SELEX) to select highly specific affinity probes called aptamers. With much focus on the generation of aptamers for a variety of target molecules, conventional SELEX protocols have been modified to develop new and improved SELEX protocols yielding highly specific and stable aptamers. Various techniques have been used to analyze the binding interactions between aptamers and their cognate molecules with associated merits and limitations. This article comprehensively reviews research advancements in the generation of aptamers, analyses physicochemical conditions affecting their binding characteristics to cellular and biomolecular targets, and discusses various field applications of aptameric binding. Biophysical techniques employed in the characterization of the molecular and binding features of aptamers to their cognate targets are also discussed.

Sensitive gluten determination in gluten-free foods by an electrochemical aptamer-based assay

Enzyme immunoassays are currently the methods of choice for gluten control in foods labelled as gluten free, providing a mechanism for assessing food safety for consumption by coeliac and other allergic patients. However, their limitations, many of them associated to the reactivity of the different antibodies used and their degree of specificity, have prevented the establishment of a standardised method of analysis. We explore new methods for quantitatively determining gluten content in foods based on the use of two recently described aptamers, raised against a 33-mer peptide recognised as the immunodominant fragment from α2-gliadin. The assays use the target peptide immobilised onto streptavidin-coated magnetic beads in combination with a limited amount of biotin-aptamer in a competitive format, followed by streptavidin-peroxidase labelling of the aptamer that remains bound to the magnetic beads. The enzyme activity onto the beads, measured by chronoamperometry in disposable screen-printed electrodes, is inversely related to the target concentration in the test solution. We find that while the assay using the aptamer with the highest affinity towards the target (Gli 4) achieves low detection limits (~0.5 ppm) and excellent analytical performance, when challenged in samples containing the intact protein, gliadin, it fails in detecting the peptide in solution. This problem is circumvented by employing another aptamer (Gli 1), the most abundant one in the SELEX pool, as a receptor. The proposed assays allow the convenient detection of the allergen in different kinds of food samples, including heat-treated and hydrolysed ones. The obtained results correlate with those of commercially available antibody-based assays, providing an alternative for ensuring the safety and quality of nominally gluten-free foods. Graphical Abstract Electrochemical magnetoassay for gluten determination using biotin-aptamers as receptors.

Affinity of aptamers binding 33-mer gliadin peptide and gluten proteins: Influence of immobilization and labeling tags

Aptamers are starting to increase the reagents tool box to develop more sensitive and reliable methods for food allergens. In most of these assays, aptamers have to be modified for detection and/or immobilization purposes. To take full advantage of their affinity, which decisively influence the detectability, these modifications must be faced rationally. In this work, a recently developed aptamer for an immunotoxic peptide of gliadin associated to celiac disease is used in different configurations and modified with various markers and anchored groups to evaluate the influence of such modifications on the real affinity. The interaction in solution with the peptide is strong for a relatively small molecule (Kd = 45 ± 10 nM, 17 °C) and slightly stronger than that for the immobilized intact protein due to a cooperative binding effect. Comparatively, while only minor differences were found when the peptide or the aptamer were immobilized, labeling with a biotin resulted preferable over fluorescein (Kd = 102 ± 11 vs 208 ± 54 nM, 25 °C). These findings are of prime importance for the design of an aptamer-based analytical method for gluten quantification.

Aptamer-based technology for food analysis

Aptamers are short and functional single-stranded oligonucleotide sequences selected from systematic evolution of ligands by exponential enrichment (SELEX) process, which have the capacity to recognize various classes of target molecules with high affinity and specificity. Various analytical aptamers acquired by SELEX are widely used in many research fields, such as medicine, biology, and chemistry. However, the application of this innovative and emerging technology to food safety is just in infant stage. Food safety plays a very important role in our daily lives because varieties of poisonous and harmful substances in food affect human health. Aptamer technique is promising, which can overcome many disadvantages of existing detection methods in food safety, such as long detection time, low sensitivity, difficult, and expensive antibody preparation. This review provides an overview of various aptamer screening technologies and summarizes the recent applications of aptamers in food safety, and future prospects are also discussed.