Citrus quality and safety control - Fast and sensitive SERS detection of functional ingredients and pollutants

Food detection technologies play a vital role in ensuring food quality and safety. Especially for rapid and on-site detection methods, they have advantages of practical applications. Traditional methods, such as mass spectrometry and high-performance liquid chromatography have limitations for on-site detection as they are time-consuming, require professionals, etc. Surface enhanced Raman Spectroscopy (SERS) technology has the advantages to be non-destructive, fast, specific, and sensitive. SERS has been widely used in the detection of food functional ingredients and various pollutants, such as illegal additives and pathogenic bacteria. However, the practical application of SERS detection methods is less reported due to the detection specificity, sensitivity, and complex food matrix. In this thesis, we take citrus flavonoids and pathogens as the target, and tried to construct new SERS detection methods through the conjugation with other technique, fabricating new SERS substrate, and combining analysis methods, so as to promote the application of SERS detection technique in practice. Firstly, we tried to conjugate SERS with other technologies to establish a qualitative and quantitative analysis method for 14 citrus flavonoid analogues, which biological function are dramatically determined by structure and substitutes. SERS is a detection technique with no separation function. We first established two-dimensional thin layer chromatography (2D-TLC) elution conditions to separate most flavonoid analogs at the condition of dichloromethane: methanol = 20: 1 and petroleum ether: acetone = 6: 4. The separated flavonoids were detected by SERS based on silver dendrite substrate. Afterwards, SERS spectra were analyzed by principal component analysis and partial least-squares for qualification and quantification. The established TLC-SERS method realized the rapid and sensitive analysis of 14 citrus flavonoids for the first time. It exhibited higher sensitivity (LOD: 10.0-16.7 μM) than TLC (LOD: 0.1-5.0 mM) and comparable accuracy with high-performance liquid chromatography in real sample detection. Secondly, we fabricated novel organic and inorganic hybrid Au/Fe3+ nanoclusters (NCs) for sensitive and fast detection of E. coli O157:H7. There are antibodies on porous spongy Au/Fe3+ NCs for bio-specific capability for E. coli O157:H7. Fe3+ in NCs could produce Prussian blue (PB) via metathesis reaction which has signal characteristic Raman peak at 2150 cm-1. Gold nanoparticles (GNPs) in NCs could significantly enhance Raman signal of PB. Combining with antibody modified magnetic nanoparticles (MNPs), E. coli O157:H7 could be qualitative and quantitative analyzed via “sandwich” structure. The established SERS biosensor showed good linear response (101 to 106 cfu/mL), high detection sensitivity (2 cfu/mL), and good recovery rate (93.60–97.50%) in spiked food samples. Thirdly, we synthesized two novel kinds of morphologically controllable SERS tags for simultaneous detection of E. coli O157:H7 and S. typhimurium. The tags were fabricated via aptamers and the Raman reporters co-mediated gold nanorods (GNRs). The aptamers not only act as bio-recognition molecules, but along with the Raman reporters, induce GNRs to grow to specific shapes. As a result of co-mediated synthesis, the stable anchored aptamers and embedded Raman reporters could facilitate avoidance of signal interference during the simultaneous detection of pathogens. We combined the novel SERS tags with antibody-modified MNPs and achieved simultaneous detection of E. coli O157:H7 and S. typhimurium with good linear response (101 to 106 cfu/mL), high detection sensitivity (< 8 cfu/mL) and recovery rate (95.26%-107.88%) in spiked food samples. According to this dissertation, the three established SERS detection methods provide new insights and ideas for the detection of food functional components and pollutants, as well as the simultaneous detection of multiple targets. Despite the good performance of established SERS methods, these applications are still in the laboratory stage. More technique development is needed to realize application in practice, such as the popularization of high-quality handheld Raman instruments and the commercialization of substrates, which requires further research