Rapid detection of target bacteria is crucial to provide a safe food supply and to prevent foodborne diseases. Herein, we present an optical biosensor for identification and quantification of Escherichia coli (E. coli, used as a model indicator bacteria species) in complex food industry process water. The biosensor is based on a nanostructured, oxidized porous silicon (PSi) thin film which is functionalized with specific antibodies against E. coli. The biosensors were exposed to water samples collected directly from process lines of fresh-cut produce and their reflectivity spectra were collected in real time. Process water were characterized by complex natural micro-flora (microbial load of >107 cell/mL), in addition to soil particles and plant cell debris. We show that process water spiked with culture-grown E. coli, induces robust and predictable changes in the thin-film optical interference spectrum of the biosensor. The latter is ascribed to highly specific capture of the target cells onto the biosensor surface, as confirmed by real-time polymerase chain reaction (PCR). The biosensors were capable of selectively identifying and quantifying the target cells, while the target cell concentration is orders of magnitude lower than that of other bacterial species, without any pre-enrichment or prior processing steps.
A new hybrid guest-host material consisting of a Fabry-Pérot porous silicon (PSi) thin film, a nanostructured high surface-area matrix, and encapsulated fluorescent carbon quantum dots (C-dots) is described. The hybrid is synthesized by a facile in situ pyrolysis treatment of the carbonaceous precursor incorporated within the nanoscale pores of the inorganic host. The effects of nanoconfinement on the integrity of the C-dots and their optical properties are characterized. We show that the resulting hybrid allows for label-free optical detection of target molecules using two orthogonal modalities, that is, the white-light reflectivity of the PSi matrix and the fluorescence of the confined C-dots, and these two signals can be observed and collected simultaneously. The resulting hybrid system exhibits superior sensing performance in comparison with that of the individual components. Notably, we demonstrate that the confined C-dots exhibit greater sensitivity toward various analytes as well as an improved linear response, thus providing evidence of the impact of the host nanoscale porous scaffold on the optical properties of the C-dots. Moreover, we show that this orthogonal detection scheme increases the dynamic range of the sensor and minimizes falsenegative results.
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