Study On Lipids Molecular Structure Measurement By Liquid Interface Enhanced Raman Spectroscopy

As one of the basic food nutrients,lipids are rich in various nutrients beneficial to human health and are the main source of caloric energy in the human diet.The diversity and complexity of lipids structure play an essential role in maintaining body health,disease prevention,and food quality control.Therefore,characterizing the types and contents of lipids in food,tracking lipids adulteration,monitoring lipids changes,and ensuring lipids safety have important research value for analyzing food complexity and ensuring food quality and safety.However,current detection methods require complex preprocessing and special equipment,and the complexity of lipids molecular structure and chemical properties also increases the difficulty of analysis.Surface-enhanced Raman Spectroscopy（SERS）,as a unique fingerprint spectroscopy technique with ultra-high sensitivity,is expected to become an important detection tool for lipids molecular structure recognition.In this paper,based on liquid/liquid interface enhanced Raman spectroscopy（LLI-SERS）,the identification of edible oil in terms of the types,oxidation,and adulteration was firstly performed to establish a foundation for the subsequent realization of the identification of edible oil-related components.Then,a SERS library of edible oil quality-related components was constructed and combined with neural network algorithms to realize the rapid resolution of edible oil quality-related components;After that,a SERS library of edible oil-related components metabolites was established and combined with a clustering algorithm to realize the rapid analysis of edible oil-related components metabolites;Finally,highly sensitive detection of lipoid molecules was established,and the oxidation process of lipoid molecules was monitored.The main research works and results accomplished are summarized as follows:1.In terms of identification of oil types,oxidation,and adulteration,an LLI-SERS strategy was constructed for direct quantitative analysis of oil by portable Raman equipment.The results showed that the best SERS enhancement was obtained when the gold nanoparticles（GNPs）volume was 3 m L.The SERS spectra of two characteristic peaks at 1267 and 1658 cm^-1 were used to quickly identify the types of six edible oils（linseed oil,bean oil,sunflower oil,colza oil,corn oil and olive oil）combined with principal component analysis（PCA）.Then the oxidation degree of edible oils was investigated and the results shown that the oxidation degree of sunflower oil and corn oil was much higher than that of bean oil.Finally,the intensity difference between olive oil and bean oil at 1267 and 1658 cm^-1 was used to realize the identification of bean oil doping in olive oil,which promoted the practical application of SERS technology in oil quality characterization.2.Typical SERS analysis based on plasma nanoarrays at the solid/air interface is faced with the inherent bottleneck of extremely weak intensity and indistinguishable spectral fingerprints of lipids.Oil-related components fatty acids（FAs）exhibit ordered molecular orientation and unique adsorption patterns at an immiscible liquid/liquid interface,which promotes the co assembly of nanoparticles and significantly improves the sensitivity of SERS detection.Based on the dual-phase accessibility of the LLI-SERS arrays,a high-resolution Raman fingerprint library was built for the molecular chain length,C=C position,saturation,regional isomers,and stereoisomers of triglycerides（TAGs）and FAs,and the detection sensitivity was as low as 1 ppb.The LLI-SERS arrays have great potential in promoting molecular recognition ability down to the single-molecule level.The detection process does not require complicated sample purification,enrichment,or complex derivative processing,expanding the potential of LLI-SERS strategy in characterizing the detailed structure of other lipid-related components in food.3.It is still a challenge to accurately determine the structure and content of hydroxy fatty acids（HFA）.Here,a highly dense LLI-SERS array induced by tannic acid（TA）was constructed for the detection of lipids metabolite HFA.The influence of TA concentration,volume,and surface plasmon resonance（SPR）of particles on SERS detection was systematically studied.The results showed that the best enhancement was achieved at a TA concentration of 10^-4 M,a volume of 5μL,and a nanoparticle SPR of550 nm.The highly dense LLI-SERS array can sensitively identify the fingerprint spectra of a variety of Raman molecules,such as rhodamine 6G（R6G）,crystal violet（CV）,and thiabendazole（TBZ）,in two-phase and multi-component systems.The structure of the HFA isomer was analyzed in detail,which provided a new way the study complex lipids metabolites at the liquid interface.4.Based on the excellent detection performance of simple lipids at the liquid phase interface,we extended it to the SERS detection of lipoid cholesterol（Cho）without enzyme.A dimethyl carbonate（DMC）hydrolysis-driven LLI-SERS array was constructed by replacing chloroform with DMC.The detection limit of the Cho molecule was as low as 10^-8 M,and the oxidation of the Cho molecule was 0.75 h to achieve sensitive SERS monitoring.Meantime,the interference of biological molecules such as ascorbic acid（AA）,sucrose（SUC）,cysteine（Cys）,and common metal ions in the complex system was investigated.The strategy showed negligible interference,indicating the real-time availability of LLI-SERS arrays.The combination of the DD-SIMCA algorithm for the detection of Cho content in milk samples opens a new idea for the detection of Cho in food.