On-chip food safety monitoring: multi-analyte screening with imaging surface plasmon resonance-based biosensor

Food safety is an increasing health concern, recognised and promoted by many institutions across the globe. Food products can be contaminated with pathogenic microorganisms, environmental pollutants, veterinary drug residues, allergens and toxins. Public health concerns which have been raised in relation to hazardous agents found in food include, among others, increased cancer risk, endocrine, reproductive and neurobehavioral systems disruption, teratogenesis, antibiotic resistance and even death in cases of allergic reactions and acute poisoning. Some of the food hazardous agents (e.g. pathogenic microorganisms and toxins) can even be used as biological warfare, spread through food and agricultural chains. Thus, an adequate detection of these compounds is also important for biosecurity. In order to safeguard consumers’ health, legislations have been put in place both in the US and the EU. These laws specify for each health threatening compound the maximal acceptable amounts in different food products. Besides health issues, food safety and quality has an economical impact on the food industry, where quality control expenses amount to about 1.5 – 2 % of the total sales. Since more and more food products nowadays contain multiple and processed ingredients, which are often shipped from different parts of the world, and share common production lines and storage spaces, food safety and quality monitoring becomes a challenging task. Traditional analytical methods require dedicated laboratories, equipment and highly trained personnel for detection and identification of each type of hazardous agent (e.g. antibiotics, bacteria, allergens). These techniques are also time-consuming and often expensive. There is a growing need for multi-analyte screening methods, which will enable rapid and simultaneous detection of multiple compounds in complex food samples. In recent years, biosensors have been applied successfully to food analysis, incorporating the same bioassay principals as traditional methods with transducers (optical, electrochemical, etc) in novel, usually miniaturized, integrated analytical devices. However, most of these biosensors still lack the desired level of the multiplexicity. Recent developments in the field of Surface Plasmon Resonance (SPR) technology in the direction of high-throughput systems and multi-analyte measurements present a promising alternative for the existing systems. One of such systems is imaging SPR (iSPR); it enables real-time and label free read-out of spatially modified surfaces (e.g. microarrays). The aim of this study was to develop an iSPR–based biosensor, for simultaneous and quantitative detection of different health-threatening compounds in food. To obtain a comprehensive overview on the analytical applicability of such a system, several points were addressed. The intrinsic sensor properties, such as optical sensitivity and robustness, of the iSPR instrument were studied. Further on, both direct and competitive immunoassay formats for high and low molecular weight compounds detection using the iSPR platform were evaluated. Then, the iSPR-based biosensor was applied for detection of regulated substances in food such as antibiotic residues in milk and allergens in cookies and chocolates. Finally, the most common drawback of using SPR for screening in complex biological matrices, the nonspecific binding to the sensor chip surface, was tackled. The sensitivity of both high and low molecular weight compounds was proven to be sufficient for some of the hazardous agents detection at the maximum residue levels, established in the EU legislation, as was demonstrated by simultaneous detection of seven antibiotic residues in milk and twelve allergens in cookies and dark chocolates. The analysis time takes about 10 minutes and provides quantitative information on multiple targets, producing a fingerprint (allergenic fingerprint for instance) of the tested food. This detailed food profile contributes to the decision making process on the quality and safety of foods, basing it on the total picture of all target compounds present. In order for iSPR-based biosensing to reach its full potential and to become a widely applied routine analytical tool, the instrumental cost needs to be reduced and the analysis further simplified, becoming cost-effective and approachable to non-trained personnel. An additional drawback in analytical applications of a SPR sensor is the nonspecific binding of the matrix components of complex samples to the sensor surface. Many assays based on SPR fail due to inapplicability to measure in “real” samples. As a possible solution to this problem, sensor chip surface engineering was suggested in this thesis. A nanopatterned filter layer covering the sensor chip surface was found to be effective in reducing nonspecific binding when the measurements were performed in “raw” samples by keeping the non-soluble aggregates and big sample matrix components beyond the sensing region of the SPR. With respect to other existing biosensors, iSPR still lags behind in terms of sensitivity and portability. In summary, the results of this study demonstrate that iSPR-based biosensor is a versatile platform, which can be applied for a wide variety of fundamentally different analytes and offers several advantages over already existing methods. SPR detection principle eliminates the need in labelling and the instrumental set-up allows automated analysis. High multiplexing capabilities and short measurement times are obtained with no need for complex and time consuming sample preparation steps. By using iSPR-based biosensor, one can obtain robust and quantitative information on the target analyte concentration, in real time and with high specificity (or broad spectrum, depending on the assay). In conclusion, on-chip screening using iSPR, described here, presents a powerful analytical approach towards food safety and quality monitoring which satisfies the current need in rapid and multi-analytical devices.