Rapid detection of polycyclic aromatic hydrocarbons from cooking oil using on-chip ultra-thin layer chromatography with surface enhanced Raman spectroscopy
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Polycyclic aromatic hydrocarbons (PAHs) are a class of potent environmental pollutants which exhibit carcinogenic, mutagenic, and teratogenic properties. High levels of PAHs are found in food products cooked at high temperatures, such as fried foods and repeatedly used or illicitly recycled oils. Screening and detection of PAH compounds are an urgent demand to ensure cooking oil safety, but current techniques require labor-intensive and time-consuming sample preparation procedures. To shorten the detection time and avoid complicated sample pretreatment, a new technique combining ultra-thin layer chromatography (UTLC) and surface enhanced Raman spectroscopy (SERS) has been proposed and evaluated for rapid screening of PAHs in oil samples. The UTLC-SERS method utilizes the nanoporous and Raman enhancing properties of the silver nanorod (AgNR) substrates for chromatographic separation of mixture samples directly on the sensing surface within minutes. In this dissertation, the UTLC-SERS principles were demonstrated using model Raman reporter molecules, and subsequently applied in the detection of PAHs. The AgNR substrate was modified with mercaptoethanol (ME), which served as the stationary phase in UTLC. The mobile phase was optimized for the separation of three representative PAHs, benz(a)anthracene (BaA), benzo(a)pyrene (BaP), and pyrene (P). PAHs were extracted from artificially contaminated vegetable oil samples through a 1-min acetonitrile extraction procedure, and the organic phase was directly used for UTLC-SERS without further treatment. The UTLC process served to elute target PAHs from the oil matrix to near the solvent front, where the SERS signal of BaA, BaP, and P could be identified at concentrations as low as 50 µg PAH/mL oil. The UTLC-SERS approach was demonstrated to rapidly identify BaA, BaP, and P from vegetable oil at relatively high contamination levels without complicated sample preparation. To achieve lower detection limits and more efficient separation among the PAHs, more in-depth work is needed on improving the UTLC solvent migration as well as refining the sample preparation protocols.