The Chemical Reaction Combined With The Surface-enhanced Raman Method Specifically Detects The Biomarker Malondialdehyde

Surface-enhanced Raman（SERS）spectroscopy provides fingerprint information of molecules.It is sensitive,rapid and low sample consumption.It can detect analytes with single molecule.Malondialdehyde（MDA）is a biomarker of lipid peroxidation and intracellular oxidative stress.In food,MDA can be used to monitor the freshness of food.MDA formed during intracellular oxidative stress can cause various diseases including depression,dental caries,chronic heart failure,hypertension,diabetes and atherosclerosis,and is also an indicator of aging.However,MDA has a small relative molecular mass and is difficult to be detected by SERS.Therefore,chemical reation-SERS method is introduced to provide corresponding groups through chemical reaction.In this work,the chemical reaction-SERS method was applied to detect MDA in the edible oil.The thiobarbituric acid（TBA）was used as the reaction reagent.Gold nanoparticles were enriched by Q agarose gel spheres（Au NPs@QSS）as the SERS substrate.The enriched products were detected in a liquid environment directly.Furthermore,4-aminothiophenol（PATP）was reacted with MDA to optimize the reaction conditions.Anhydrous magnesium sulfate（MgSO₄）was added to greatly increase the SERS signal of PATP-MDA products.To further optimize the reaction conditions,2,4-dinitrophenylhydrazine（DNPH）was reacted with MDA at room temperature and successfully applied to human saliva detection.The research contents in detial are as follows:Part 1.In this work,a new SERS substrate was constructed by enriching Au NPs on the surface of Q agarose gel spheres.Chemical reaction tandem SERS method was used to determine trace MDA.TBA is used as a derivatization reagent with MDA.The TBA-MDA adduct has a larger Raman scattering cross-sectional area than MDA and has a stronger binding capacity on the SERS substrate both than MDA and TBA.The Au NPs@QSS-SERS method can directly detect TBA-MDA adducts in the reaction medium with high sensitivity.QSS can not only adsorb Au NPs,enrich TBA-MDA adducts,but also improve the stability of adsorbed Au NPs and maintain its uniform distribution on the QSS surface in an acidic environment.The Au NPs@QSS-SERS method has a linear range of 0.33μM to 3.3 mM in a liquid environment.Limit of detection is 10^-1010 M in a dry state,and this method is successfully applied to the detection of MDA in oil samples.The method is reliable,simple,economical and convenient,and can be used for on-site analysis of food quality by means of a portable Raman system.Part 2.Based on the first part of the work,the reaction between PATP and MDA are milder（40°C）,faster（15 min）,lower of the detection limit(10^-10 M),and more novel.After reacting PATP with MDA at 40°C for 15 min,the Raman scattering cross-sectional area of MDA was greatly increased.A new peak appeared at 1658 cm^-1,and the peak of 1080 cm^-1 was used as the internal standard to improve the detection accuracy.The ratio of the intensity at 1658 cm^-1 and 1080 cm^-1 is used for quantitative MDA detection.Silver phytate（IP₆@Ag NPs）is used as the SERS substrate,which is more sensitive,more uniform and more stable than traditional Au NPs substrates.The addition of MgSO₄ not only promotes the reaction progress but also promotes the agglomeration of the substrates,which greatly improving the SERS signal of the PATP-MDA product.Finally,this method is successfully applied to human saliva detection and may be used to monitor diseases that can respond by MDA content such as dental caries.Part 3.In this work,on the basis of work 2,in order to make the reaction conditions milder and achieve on-site ultra-sensitive detection,DNPH and MDA were selected for derivatization reaction reacted at room temperature.A new SERS peak was appeared at 1386 cm^-1,which was used for MDA quantitative detection.IP₆@Ag NPs is used as the SERS substrate.Sodium chloride（NaCl）is added to promote a certain degree of agglomeration of the substrate and improve the SERS signal of the DNPH-MDA product.The reaction is carried out at room temperature without heating condition.What’s more,the reaction has stronger specificity,which improves the accuracy of the detection.This method has been successfully applied to the detection of human saliva.