Constructing 3D Pickering Emulsion SERS System For Ultrasensitive Detection Of Endogenous And Exogenous Harmful Substances In Edible Oils

The safety of edible oils has attracted widespread attention.In particular,the harmful substances and illegal additives produced in the process of oil processing and consumption have seriously threaten people’s health.Typically,the physicochemical indexes of edible oils could be determined by many chromatographic analysis.As they are specific and accurate,but have great limitations in that it is a cumbersome,time-consuming,laborious process requiring skilled staff,and generates environmental pollution.Currently,few studies have reported rapid and sensitive techniques to identify potentially illegal cooking oils.Surface-enhanced Raman scattering（SERS）spectroscopy is based on the smart combination of nanomaterials and Raman spectrum technology and becomes a very important spectroscopic technique in modern analysis and detection fields with its single molecule level detection capability.However,in this detection system,the SERS"hot spots"need to be evenly distributed,and all the substances to be detected need to be adsorbed on the substrate.The preparation of oil-soluble SERS substrates for oil-soluble detection substances is cumbersome and costly.Therefore,in this study,a novel Pickering emulsion SERS detection system was constructed for trace detection of edible oil.The water-soluble metal nanoparticles are adsorbed on the oil-water interface and fully contacted with the oil-soluble detection substance,thereby enhancing the Raman signal.The main results are as follows:Gold nanoparticles（Au NPs）were prepared by a combination of chemical reduction and seed growth techniques.Through single factor experiments,the preparation conditions of Au NPs were optimized based on the characteristic peaks of 0.1ppm malachite green Raman spectrum.The optimal preparation conditions were:the boiling reaction time of seed crystals was 6min,the mass ratio of trisodium citrate to hydroxylamine hydrochloride was 1:1,the p H was kept at 2.0.On this basis,it was mixed with sodium caseinate（Na Cas）to form composite particles.The Na Cas-Au NPs-stabilized Pickering emulsion detection system was successfully prepared,and the system structure was characterized and optimized.This system was also optimized based on the characteristic peaks of 0.1ppm malachite green Raman spectrum.The optimal preparation conditions were:the mass percentage of Na Cas was 0.005%.The results of TEM and gold ICP-AES showed that in the Pickering emulsion system,Au NPs were indeed distributed in the emulsion layer,and the distribution was uniform.About 99.93%of the Au NPs in this system were distributed in the emulsion and played a role at the oil-water interface.Malachite green,crystal violet and methylene blue were used as Raman probes to detect the SERS performance of the constructed Pickering emulsion.SERS test of Malachite green results showed that:using this Pickering emulsion detection system,the detection limit of malachite green was 10ppb.Crystal violet exhibited linearity regression in the range of 0.1-10ppm,and the R² value was as high as 0.9954.It also had a good linear relationship with a detection limit of 10ppb.Methylene blue showed linearity regression in the range of 0.1-10ppm,and the R² value was as high as 0.9807,which also had a good linear relationship with a detection limit of 10ppb.The SERS repeatability test results showed that the eight repeatability experiments of the Raman probe were basically the same.The test results of the spiked recovery in tap water showed that:in the concentration range of 0.1ppm-10ppm,all spiked recoveries of malachite green and crystal violet in tap water were maintained between 90%-110%,and their RSD values were as low as 1%or less.We have used the constructed Pickering emulsion detection system to detect the oxidation of edible oils and the quality of illegal edible oils by SERS.The Raman detection results of edible oils after heating at 70°C for 11 days showed that the relative Raman intensity at 1658cm^-1 decreased by about 50%.The disappearance of the cis double bond peak at 1658 cm^-1 in sunflower oil and linseed oil was more obvious than that in soybean oil,which may be due to the fact that the total amount of four types of tocopherols in soybean oil is much larger than that in sunflower oil and linseed oil total tocopherols.Regression analysis of Malondialdehyde（MDA）in the range from 0.1 to1.5 ppm showed the limit of detection being 0.05 ppm in standard solution,R²value was as high as 0.99525.The detection limit of capsaicin,the main component of illegal cooking oil,was as low as 0.1 ppm,which is comparable to the detection limit of HPLC method.The recovery rate of capsaicin in edible oil was close to 100%（n=6）.