Optical immunosensors for the detection of harmful agents in food samples

Food quality and safety assessment is of major importance to both the food industry, the food producers and the consumers. For some foods, such as milk, it is necessary to test both the material and the processed products against harmful substances as quickly as possible. Such tests include the detection of aflatoxin M1 (AFM1), as well as of several pathogenic bacteria including Salmonella spp., Escherichia Coli O157: H7, Staphylococcus aureus, Bacillus cereus and Listeria monocytogenes. AFM1 is one of the most carcinogenic and toxic aflatoxins produced by fungi and therefore the European Union has set very strict maximum allowable limits for AFM1 in food. Simiral limits have been set regarding microorganisms in food for which in many cases the presence is completely prohibited due to the seriousness of the problems they might cause to human health. The detection of various harmful substances in food is carried out by analytical methods that are characterized by high sensitivity and accuracy, however they are quite time consuming, require expensive equipment and cannot be used for field analysis. For this reason, the use of immunosensors, and in particular optical immunosensors, has been explored as alternative that could replace existing methodologies. Optical immunosensors are divided into two categories, those that require the use of labels to detect the substance to be determined and the label-free ones. In the latter category are included those based on Mach-Zehnder interferometry. In this context, the project aims to develop optical immune sensors for the detection of harmful species (e.g., mycotoxins, bacteria) in food. More specifically, an immersible photonic chip is developed in which two interferometers are realized, one working and one reference interferometer. The sensor arm of the reference interferometer will be coated with an inert protein, while the sensor arm of the working interferometer will be modified with the toxin or microorganism whose detection in milk is targeted, following a competitive immunoassay. The chip integrates the complete Mach-Zehnder interferometers photonic circuit but the optical coupling and signal recording is done via a double optical fiber that connects the chip input with a white light source and its output with a digital spectral analyzer. For analysis, the chip will be immersed in a solution containing the sample and specific antibodies against each analyte without the need for microfluidic cells. The work is carried out in the context of the execution of the FOODSENS project (Τ2ΕΔΚ-01934) and is the subject of the doctoral thesis of Mrs. Dimitra Kourti. The experimental part of the work is carried out at the Immunoassays-Immunosensors Laboratory of IPRETEA in collaboration with the Department of Microelectronics of the Institute of Nanoscience & Nanotechnology of NCSR “Demokritos” and the Department of Chemistry of the University of Athens (Prof. A. Economou).

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