Surface Modification Of Noble Metal Composite Nanostructures And Its Application In SERS-based Assay

Surface-enhanced Raman spectroscopy（SERS）has proved to be a powerful tool for detection of food,environment and biological systems at trace levels with the advantages of high sensitivity,fast response and characteristic fingerprint recognition.However,in practical applications,due to the interference of molecules in complex samples,sample preparation is inevitable,and it is unachievable for SERS to detect some small molecules that are difficult to"close"to the substrate or"oversized"biomolecules.Therefore,the substrate can obtain the functions of separation and enrichment,label recognition or specific response through surface modification,which undoubtedly brings a broader application prospect to SERS technology.This paper focuses on the functionalized surface modification of SERS substrates to achieve the enrichment and ultra-sensitive,rapid detection of a variety of synthetic pigments and negatively charged dyes,and achieve preliminary establishment of novel homogeneous immunoassay method based on functional SERS substrate of enzymatic cascade reaction.The specific research contents are as follows:1.Branched polyethyleneimine（PEI）-modified silver nanowires and gold nanostars were deposited onto paper to form a three-dimensional paper-based substrate,which was used as a solid-phase SERS active substrate to directly detect four synthetic pigments commonly used in food and beverage,including amaranth,alluring red,new coccine and tartrazine.Combined with portable Raman,this method can provide a potential analytical tool for on-site and rapid detection of illegal pigments in fast-selling products such as food and beverages.2.When using solid-phase substrate for SERS detection,it is necessary to consider the liquid phase mass transfer efficiency and the uniformity of the adsorption of analytes on surface of the substrate.And it is more advantageous to build the three-dimensional SERS substrate in solution and achieve enrichment and homogeneous detection.We continue to use PEI to modify the surface of nanostructures Fe₃O₄@Au@Ag to overcome the shortcomings of ordinary sol substrates that are unstable in complex environments.At the same time,it can quickly adsorb negatively charged dyes and construct a three-dimensional substrate in liquid phase under magnetic induction to realize highly sensitive and reproducible detection of the analyte in solution.Among them,the detection limit of amaranth with weak affinity to the metal surface reached 0.05μg/m L,and the detection sensitivity of alizarin red S was nearly5 times higher than that without the modified PEI.3.For biomolecules that are difficult to detect directly,we introduced the enzymatic cascade reaction of small molecules into the SERS system,and modified the substrate molecule p-aminophenyl phosphate sodium salt（PAPP）on the surface of the SERS substrate.The continuous change of the SERS signal monitors the enzymatic cascade reaction that alkaline phosphatase（ALP）and tyrosinase（TYR）catalyze PAPP,and carry out preliminary exploration on SERS immunoassay of two targets.This method can not only identify and separate the target through the immune structure,but also overcome the problems that large biomolecules are difficult to approach the substrate and cannot be directly detected because of poor spectral specificity.Moreover,a new functionalized substrate has been developed to enable simultaneous homogeneous detection of multiple targets,laying a foundation for a novel SERS immunoassay method.