Label-free detection and differentiation of foodborne pathogenic bacteria in fresh produce by surface-enhanced Raman spectroscopy
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Surface-enhanced Raman scattering (SERS) based platform has been previously used for the detection of bacteria. However, these studies had a generally poor sensitivity and specificity that are not practical for the use in food safety applications, and these studies have been conducted with pure lab strains without any consideration of the influence from food samples. This dissertation therefore focuses on the optimization of silver nanoraod (AgNR) array substrates to achieve better detection and differentiation of foodborne pathogenic bacteria, as well as its application in the fresh produce. The origins of the bacterial SERS signal have been studied by thoroughly investigating the impact of cell disruption on SERS signal of bacteria. Important bacterial cell components have been isolated and their SERS spectra are obtained. In addition, the effect of each cell component on the differentiation of the bacteria has also been studied. The AgNR substrates have been functionalized with vancomycin, and the sensitivity of the functionalized substrates has been optimized. The vancomycin coating on the substrates surface not only captures more bacteria from the solution, but also reduces the distance between bacteria and the substrate surface, resulting in a significant increase on the sensitivity. Such functionalized substrates also show improved specificity when differentiating bacteria, compared to the pristine AgNR substrates. The differentiation of bacteria between species, strains, and serotypes has been achieved with chemometric analysis, as well as the classification between Gram positive and Gram negative bacteria. The feasibility of utilizing the vancomycin functionalized AgNR substrates in the detection and differentiation of foodborne pathogenic bacteria from real food samples has also been investigated. Combining with simple filtration methods, low concentration of bacteria have been detected in various fresh produce samples, and the differentiation between bacteria has also been investigated. This SERS based method shows great agreement with conventional bacteria detection method, and the use of bench-top and handheld Raman spectrometers makes the system field-deployable. It has the potential to be used as an on-site pathogen detection method in the food industry.