SERS Mechanism Analysis Of Magnetic Composites And Its Application In PAHs Detection

Surface enhanced Raman scattering（SERS）technology expands the application of Raman spectrum detection through the amplification of Raman signal.Therefore,the research on SERS is of great significance to enrich the theoretical basis of Raman spectroscopy and improve the sensitivity of Raman spectroscopy.The research hotspots in the field of SERS mainly include the effective evaluation of enhancement effect,the preparation of substrate materials and the reasonable analysis of enhancement mechanism,as well as enriching the detection application of SERS.Among them,SERS enhancement factor is the most direct index to evaluate the enhancement effect.At present,there is still a lack of theoretical interpretation combining its electromagnetic enhancement mechanism with chemical enhancement mechanism,so it is difficult to predict and evaluate the enhancement effect of different molecules.Because of its excellent properties such as magnetic enrichment,magnetic composites have become the"dark horse"in SERS substrates.The rapid development of preparation research has also increased the desire for a clear analysis of its reinforcement mechanism.Moreover,the adsorption and capture of molecules by magnetic composites breaks through the problem that the traditional precious metal substrate can not detect hydrophobic organic molecules.Therefore,combined with the electromagnetic and chemical enhancement mechanism,this thesis establishes the theoretical prediction model of enhancement factor;constructing the magnetic composite SERS substrate,and theoretically analyzing its electromagnetic and chemical enhancement mechanism;realizing the detection of polycyclic aromatic hydrocarbons（PAHs）based on the substrate,which provides a feasible technical means for the detection of hydrophobic organic molecules.It has theoretical and practical significance for guiding the preparation experiment of substrate materials,analyzing the microstructure information and macro material properties.The specific research contents are as follows:1.Proposing a theoretical calculation model of SERS enhancement factor integrating electromagnetic enhancement mechanism and chemical enhancement mechanism.Realizing the enhancement prediction with molecular selectivity after introducing electronic structure energy.Analyzing electromagnetic and chemical enhancement by using the methods of electron dynamics and quantum chemistry and clarifying the changes of electric field intensity caused by the properties,size and shape of nanoparticles,as well as the changes of adsorption energy and Raman intensity caused by the adsorption site and cluster configuration on the substrate and intermolecular in SERS system.The results show that the increment of electronic structure energy can reflect the chemical enhancement and can be used to supplement the chemical enhancement factor.By comparing with the empirical formula and the theoretical model of electromagnetic enhancement to the fourth power,verifying the correctness of the model and confirming the molecular selectivity of the model.2.Constructing Fe₃O₄@GO@Ag magnetic composite structure,researching optical properties and the variation law of local electric field and analyzing its electromagnetic enhancement mechanism.Calculating the optical properties of Fe₃O₄ spherical particles and the local electric field distribution under different radii by finite element method to obtain the radius with the best enhancement effect;analyzing the influence of GO film thickness on electric field enhancement and far-field radiation to obtain the best film thickness;researching the effect of the change of nano Ag particle gap on the enhancement of local electric field to obtain the gap with the best enhancement effect.Establishing the F_e3O₄@GO@Ag model with the above optimized parameters and analyzing the influence of electric field enhancement effect and material superposition on optical properties to obtain the largest electric field enhancement which multi particle coupling produces,and Ag plays a major role in the whole structure.3.Researching Fe₃O₄@GO@Ag substrate and the chemical properties of PAHs--Fe₃O₄@GO@Ag system and analyzing the chemical enhancement mechanism.Through the quantum chemical method,analyzing the properties of Fe₃O₄@GO@Ag heterojunction,and obtaining the bond cooperation between layers to strengthen the adsorption and binding of structure;secondly,simulating and calculating the vibrational spectra of naphthalene,anthracene,phenanthrene,fluoranthene,pyrene and benzo[a]pyrene,confirming that the Raman activity of PAHs is better than that of infrared and determining the Raman characteristic peaks of six PAHs according to the quantitative vibrational mode;finally,the structure optimization and SERS calculation of PAHs--Fe₃O₄@GO@Ag system show that the adsorption configuration affects the chemical enhancement effect,and the direction of charge transfer is the charge transfer transition from Fe₃O₄@GO@Ag to PAHs.4.Preparing Fe₃O₄@GO@Ag magnetic composite and using it as substrates to realize the SERS detection of PAHs.Modifying Fe₃O₄ by PEI and using GO as interlayer to realize the close coating of Ag nanoparticles.Through systematic characterization and analysis,the prepared Fe₃O₄@GO@Ag particles have good dispersion and uniformity.The substrate material was used for SERS detectio n of PAHs.Compared with silver sol,Fe₃O₄@GO@Ag has good SERS performance.Detecting the concentration of six PAHs and obtaining that the Raman intensity increased with the increase of concentration,and the minimum detection concentration reached nm level,which realized the demand of trace detection.In the detection and analysis of PAHs mixed solution,realizing the qualitative identification of components in the mixture successfully through the identification of characteristic spectral peaks.