Synthesis And Surface Enhanced Raman Spectroscopic Studies Of Iron Oxide@Au Core-Shell Particles

Surface enhanced Raman scattering (SERS) spectroscopy acted as one of the most sensitive in-situ detection technologies in surface science and was widely applied in many relative scientific fields. The extension field is relevant directly to the preparation of SERS active substrates and the investigation of SERS mechanisms. With the rapid development of nanotechnology, it is possible to prepare the controllable SERS active substrates to improve SERS activity and detection sensitivity. However, up to now, it has not reached a common recognization about the complicate SERS mechanism, especially about the relationship about experiment and theoretical results due to the absence of simple experimental and theoretical model. Hence , this dissertation focused on the preparation of novel SERS active substrates with the special morphology to explore the mechanisms of SERS and attempt to investigate SERS spectra from a single particle or aggregate complex particles. The main results and conclusions of the dissertation are listed as follows:I. The preparation of superparamagnetic Fe3O4@Au core-shell nanoparticles and SERS investigationDue to the character of both superparamagnetism of core and function of metal shell, the dispersion and stability of magnetic particles had been improved. The enrichment of core-shell particles under the magnetic field, the modification and function of the shell and the independence of the function from both core and shell are beneficial to extend the application field. The magnetic core-Au shell particles with continuous shell were prepared successfully. The molecules adsorbed on the surface of the core-shell particles were enriched by the magnetic field, and thus to improve the detection sensitivity of the mono-antigen immunoassay. Main results are given as follows:1. The Fe3O4 nanoparticles with strong saturation magnetization and superparamagnetic Fe3O4@Au core-shell particles have been successfully prepared . The Fe3O4 particles were initially prepared as original cores. The 2nm Au nanoparticles attached onto the surface of Fe3O4 particles through organosilane molecules and acted as the seeds for the formation of core-shell particles. Continuous Au shell was formed onto the Fe3O4 core modified by Au nanoparticles through the adjusting reduction of HAuCl4. The thickness of Au shell was controlled through regulating the concentration of HAuCl4 solution. Structure and properties of the core-shell particles were characterized by XRD, UV-vis, VSM.2. The core-shell particles have the optimum properties of both magnetic core and Au shell , and can be served as the SERS active substrates. The SERS activity was dependent on the thickness of Au shell. The core-shell particles gathered in a certain non-uniform magnetic field, resulting in the enhancement of SERS signal. And proble molecules in solution adsorbed on the surface of particles were enriched to improve the detection sensitivity core-shell particles. The sandwich complex "solid phase antibody/antigen/ immunoassay core-shell particles " was built for the recognization of antigen. The results revealed that the magnetic core have no influence on the specific interaction between antibody and antigen.II. Polarization dependence of SERS on a single spindle Fe2O3@Au particleThere was a great difference in SERS effect for active substrates with a variety of sizes and shapes. As predicted early, ellipsoidal or spindle particles with a large aspect ratio would exhibit an ideal SERS effect. When the polarized direction of the incident laser matched the dimension size of the particles, the largest LSPR was excited which leaded to the strong SERS effect. According to the relationship of SERS and LSPR, it is worthing preparing certain nano-materials with special structures not only for extending application of SERS , but also for investigating the complicated mechanism of SERS.Previous SERS studies focused on the coupling effect among the particles. However, for a isolated single particle, SERS effect was mainly contributed by the particle itself . It is helpful to simplify the system and provid a model for understanding the complicated SERS mechanisms . Main results are as follows:1. Fe2O3@Au particles were composed of spindle hematite core and a continuous Au shell, and optical properties of core-shell particles can be tuned by changing the thickness of the Au shell. A larger aspect ratio of the spindle Fe2O3 particles were prepared by hydrolysis and Fe2O3@Au core-shell particles were synthesized via the growth of continuous Au nanoshells by reduction of HAuCl4 with weak reducing on the small Au nanoparticle attached onto the Fe2O3 Surface. Using thiophenol as a probe molecule, these particles showed very high SERS activities. With the increase of the thickness and roughness of Au layer, the SERS intensity was increased to 106 because of the electromagnetic enhancement.2. According to the theoretical simulation with FDTD, the surface local electric field about double particles couplling system and single-particle exhibited a characteristic distribution. For a single isolated particles, SERS effect is mainly attributed from the particle itself rather than from couplling effect between particles. The theoretical calculations of FDTD results showed that the maximum enhancement of SERS on the single particle strongly depended on the angle between the incident field and the long axis of the particle. For parallel polarization(θ=0°or 180°), the calculated maximum of local intensity enhancement was observed on the sites close to the end of the long axis, while for perpendicular polarization(θ=90°), the calculated local enhancement was very weak, it was about 10-7 times of the maximum. Theθdependent profile of local electric field was well fitted to the curve of cos4θ. The preliminary results provided the theoretical basis for predicting the polarized dependent SERS of anisotropic particles.3. The polarized SERS spectroscopy was observed form a single anisotropic core-shell particles for the first time. It provided the experimental basis and theoretical model of SERS mechanisms investigation. Dramatic variations in SERS intensity were observed when the single core-shell particle was oriented at different angles relative to the polarization of excitation laser. The maximum SERS intensity was detected when its long axis was paralleled to the polarization direction of incident field, and the minimum intensity was detected at the direction of the incident field which was perpendicular to the long axis of the particle. The results were generally consistent with FDTD theoretical calculations.4. There was difference of about 2-3 orders between of the strongest and the weakest of SERS signal intensity. In a certain direction of incident polarization field, hot spots for SERS are mainly locatted on the end of long axis of the anisotropy particle, while in the direction perpendicular to the long axis (i.e, parallel to the short-axis ), the weak signal was observed. Considering the SERS enhancement of electromagnetic field is mainly due to the lightning rod effect, the electromagnetic field enhanced five times at the end of the long axis comparing to that from vertical direction. It was consistent with the results of theoretical simulation.Combining with FDTD theoretical calculation and 2D-mapping investigation by mathematic resolution. The investigation on the polarized dependent SERS of a single spindle particle is beneficial to well understand the lighting rod effect of electromagnetic enhancement.Ⅲ. Synthesis and surface enhanced optical properties of multibranched spindle FeOOH@Au particles .In the view of the importance of anisotropic particles for SERS, multi-branched spindle particles based on single branch particle is expected to enhance the SERS activity effectively. However, the direct preparation of multi-branched of spindle-shaped coin metal is very difficult, the indirect methods were explored to prepare multi-branched of spindle Au particles through indirect methods. The synthesis mechanism of multi-branched particles and SERS effect were investigated. Main results are as follows:1. A facile approach has been developed for the synthesis of multi-branched spindle FeOOH particles as well as FeOOH@Au core-shell structures by regulating and controlling the component in the solution. The formation of unique multibranched spindle FeOOH particles depended on the slow dissociation of hydroxide ions by urea which facilitated the nucleation and growth rate of crystals. The addition of different metal ions and the concentration of oleic acid exhibited significant effect on the shape of the particles. It induced the formation of multibranched ends of the particles. Their core-shell structures were fabricated by similar method with the preparation of Fe2O3@Au. The FeOOH and FeOOH@Au core-shell particles were characterized by TEM, XRD and SERS.2. The specific core-shell multibranches particles exhibited optimum SERS activity. The results revealed that the SERS signal was mainly originated from the Au shell of the spindle Au core-shell structures. With the increase of the Au shell thickness , the SERS signal increased dramatically. By comparing the SERS effect the multibranches structures exhibited stronger effect which might contribute to the stronger lightening rod effect and the possible couplling effect between of the multi branches from the same particles.Ⅳ. Theoretical Raman spectra study of 1,4-BDT adsorbed on the Au shell.Combined with quantum chemical calculation, the Raman spectrum of 1,4-BDT molecule and the adsorption behavior have been studied on Au shell surface. Research contents are as follows:1.In order to investigate the mechanism and adsorption orientation of 1,4-BDT on the Fe2O3@Au core-shell particles, the optimized geometry of 1,4-BDT and Raman spectrum were calculated by using the density functional theory (DFT) at the B3LYP/6-311+G** level, which confined at the C2h point group symmetrical operation. By the definition of inner coordinate, the coordinates of the DFT force constants in Cartesian coordinates changed into a coordinate within the force constants. The theoretical frequencies were scaled according to the scaled quantum mechanical (SQM) procedure, compared with the experiment results. The result scaling factors are 0.9668 for C-C and C-H stretching modes, 0.974 for C-S stretching modes ,0.915 for S-H stretching modes and 0.974 for other modes, such as ring in-plane deformation, ring out-of-plane deformation, C–H in-plane bending and C–H out-of-plane bending modes. The final harmonic vibrational wavenumbers and the potential energy distributions (PEDs) were derived by the Wilson's GF matrix method from the SQMF-DFT force field. The simulated Raman spectrum is well accorded with the experimental data.2. The SERS spectra of 1,4-BDT molecular adsorbed on Fe2O3@ Au core-shell particles were investigated by DFT theory. For the complex of 1,4-BDT and a Au atom, there have two kinds of complex structure. The optimized geometries, vibrational force fields and Raman intensities were calculated by using B3LYP method with 6-311+G**(C, H, S) / LANL2DZ (Au) basis set. Based on the relative intensity , Raman shift of vibrational models , together with the NBO results and PEDs analysis of one of complex HSC6H4SAu(Ⅰ), the possible forms of 1,4-BDT adsorbed on gold shell particles in tilt orientation.In summary, the controlable synthesis of special-shaped particles core and their core-shell structure not only provide a new type of SERS substrate to extend the application of SERS, but also offer an experimental basis and simply model for theoretical simulation on the mechanisms of SERS.The allocation of single isolated particle and the polarized SERS investigation by combination of theoretical simulation and mathematic resolution could be developed to one of powerful technical tools for the exploration of SERS mechanism.