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Electrochemical genosensors have undergone an enormous development in the last decades, but only very few have achieved a quantification of target content in highly processed food samples. The detection of allergens, and particularly gluten, is challenging because legislation establishes a threshold of 20 ppm for labeling as gluten-free but most genosensors expresses the results in DNA concentration or DNA copies. This paper describes the first attempt to correlate the genosensor response and the wheat content in real samples, even in the case of highly processed food samples. A sandwich-based format, comprising a capture probe immobilized onto the screen-printed gold electrode, and a signaling probe functionalized with fluorescein isothiocyanate (FITC), both hybridizing with the target was used. The hybridization event was electrochemically monitored by adding an anti-FITC peroxidase (antiFITC-HRP) and its substrate, tetramethylbenzidine. Binary model mixtures, as a reference material, and real samples have been analyzed. DNA from food was extracted and a fragment encoding the immunodominant peptide of α2-gliadin amplified by a tailored PCR. The sensor was able to selectively detect toxic cereals for celiac patients, such as different varieties of wheat, barley, rye and oats, from non-toxic plants. As low as 0.001% (10 mg/kg) of wheat flour in an inert matrix was reliably detected, which directly compete with the current method of choice for DNA detection, the real-time PCR. A good correlation with the official immunoassay was found in highly processed food samples. [ABSTRACT FROM AUTHOR]

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. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Reliable detection of decreasing amounts of gluten in food is the only way of ensuring the safety of all coeliac patients. Results obtained with the method of choice, immunochemical assays, are not entirely comparable and many of them are sandwich assays that cannot recognize hydrolyzed proteins. In this work, we propose a competitive electrochemical sensor based on a recently described aptamer targeting the gliadin immunodominant peptide 33-mer that triggers the coeliac disease. The sensing layer is built on the surface of a screen-printed carbon electrode (SPCE) by adsorption of streptavidin and subsequent peptide immobilization. A competition between the peptide and gluten proteins from samples for a defined concentration of biotinylated Gli 4 aptamer is established. The aptamer bound to the peptide on the surface is finally measured after enzyme labelling and chronoamperometric detection of an enzymatically obtained electrochemically active product. This method is able to detect as low as 0.113 μg L −1 of gliadin, which corresponds to 380 μg kg −1 of gluten in food, taking all dilutions and conversion factors into consideration, with a reproducibility lower than 11%. The aptasensor was applied to food samples with gluten contents above and below the legislated threshold for gluten-free labelling in the EU, obtaining good agreement with the official R5 immunochemical method. [ABSTRACT FROM AUTHOR]

With this work, it is intended to propose a novel approach based on high resolution melting (HRM) analysis to detect wheat and discriminate it from other gluten-containing cereals. The method consisted of a real-time PCR assay targeting the gene encoding for the germ agglutinin isolectin A protein (Tri a 18 allergen), using the fluorescent Evagreen dye combined with HRM analysis. The results enabled wheat differentiation from other phylogenetically related cereals, namely barley, rye and oat with high level of confidence. Additionally, a quantitative real-time PCR approach was proposed, allowing detecting and quantifying wheat down to 20 mg/kg in rice flour and 20 pg of wheat DNA (∼1.1 DNA copies). Its application was successfully achieved in the analysis of processed foods to verify labelling compliance, being considered as a cost-effective tool for the specific detection of cereals in gluten-free foods. [ABSTRACT FROM AUTHOR]

Abstract: Electrochemical genosensors are becoming cost-effective miniaturizable alternatives to real-time PCR (RT-PCR) methods for the detection of sequence-specific DNA fragments. We report on the rapid detection of PCR amplicons without the need of purification or strand separation. A challenging target sequence for both PCR amplification and electrochemical detection allowed us to address some difficulties associated to hybridization on electrode surfaces. The target was a highly specific oligonucleotide sequence of wheat encoding the most immunogenic peptide of gliadin that triggers the immune response of celiac disease (CD), the 33-mer. With a sandwich assay format and a rational design of the capture and tagged-signaling probes the problems posed by the strong secondary structure of the target and complementary probes were alleviated. Using a binary self-assembled monolayer and enzymatic amplification, a limit of detection of 0.3nM was obtained. The genosensor did not respond to other gluten-containing cereals such as rye and barley. Coupling to PCR to analyze wheat flour samples required tailoring both the capture and signaling probes. This is the first time that deleterious steric hindrance from long single-stranded regions adjacent to the electrode surface is reported for relatively short amplicons (less than 200bp). The importance of the location of the recognition site within the DNA sequence is discussed. Since the selected gene fragment contains several repetitions of short sequences, a careful optimization of the PCR conditions had to be performed to circumvent the amplification of non-specific fragments from wheat flour. [Copyright &y& Elsevier]

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. Image 1 • First aptamer-based sensor directly deployed in a deep-eutectic solvent. • A truncated aptamer is designed to be applied in a sandwich format. • Electrochemical sensor exhibits a limit of quantification of 20 μg gluten/kg sample. • Detection of residual gluten levels in food for highly-sensitive celiac patients. [ABSTRACT FROM AUTHOR]