Multi-dimensional Construction Of "Hot Spots" And Its Surface Enhanced Raman Spectroscopic Studies

Surface enhanced Raman spectroscopy has been attracted considerable interest due to its high sensitivity upto single molecule detection level. It provided the rich fingerprint vibrational spectral features about the relevant molecules, which brought the information on the structure and the interaction with the surroundings and external field. Just like a magnifier, SERS exhibited the capability to magnify the weak signals to the researchers. Therefore, SERS, as a powerful tool for the surface and interface studies, has been already extended to the practical application in material science, chemistry, surface catalysis, enviorment and food science, and biomedicine. Generally, the giant SERS effect was mainly originated from the unique optical properties on the nanostructure surface, it was called as surface plasmon. At a certain condition, the surface plasmon resonance allowed the notable enhancement of electromagnetic field, which increased the surface Raman signal dramatically. However, the hetergenous distribution of surface nanostructure resulted in the nonuniformity of electromagnetic field enhancement. For example, abnormal enhancement was produced at some special regions, which was defined as “hot spots”. It contained less than 1% molecules in the hot spots, but contributed 70% of the overall SERS signal. Therefore, the tuning on the “hot spots” become the key issue for the fundamental investigation and practical application. In this dissertation, “hot spots” was tuned at multi-scale and multi-dimension. The static and dynamic “hot spots” were successfully constructed at nanometer scale. The optimal uniform SERS substrate was fabricated by the assembly of “hot spots” for extending its application. The main results were listed as follows:1. Developing a facile approach to fabricate Au@Si O2 dimers with single static “hot spots” through the steric hindrance effect, in which the Si O2 shell functioned as a block and a rigid dithiol molecule was employed as linker. The SEM and SERS mapping techniques were employed to position the single dimer for the SERS investigation. The thickness of the Si O2 shell played a critical role in improving the yield of dimers. When 1,4-benzenedithiol was used as linker molecule, the yield of dimers was ~30%. The low number of linker molecules on the exposed area of monodisperse single nanoparticles and the lack of LSPR coupling effect(“hot spots”) resulted in the disappearance of SERS signals of the linkers. The estimated SERS enhancement factor was about eight fold because of the strong coupling effect in the gap of the dimer with the distance of the dithiol molecular length. Moreover, the gap distance of the dimer was depended on the length of the linker. It opened a new window for quantitative investigation of “hot spots” of dimers.2. Developing the strategy for the construction of dynamic “hot spots” by applying external magnetic field on the Fe3O4@Au core-shell nanoparticles. The preparation of Fe3O4@Au spherical nanoparticles was optimized, and the balance was built between the magnetism of the core and SERS activities of shell by changing the thickness of Au shell. Exploring the approach for the self assembly of a two-dimensional monolayer film of Fe3O4@Au nanoparticles at a hexane/water interface. The Fe3O4@Au nanoparticles moved directionally in the monolayer by an external magnetic field. The balance between the electrostatic repulsive force, surface tension, and magnetic attractive force allowed observation of the magnetic-field-responsive SERS effect. Upon introduction of an external magnetic field, the SERS intensity reached maximum quickly and the observed variations in SERS intensities were fully reversible after removal of the external magnetic field. The reduction of interparticle spacing in response to a magnetic field resulted in about one order of magnitude of SERS enhancement. The combined use of the monolayer film and external magnetic field could be developed as a strategy to construct “hot spots” both for practical application of SERS and theoretical simulation of enhancement mechanisms.3. Developing the facial approach for assembling two-dimension Au nanoparticles monolayer film at air/liquid interface, and the “hot spots” was gathered at two or three dimension. By association with surfactant of PVP, the Au nanoparticles monolayer film was fabricated at centimeter scale. The film exhibited a two dimensional hexagonal close-packed structure having interparticle gaps smaller than 2 nm. These gaps generated numerous uniform “hot spots” for surface-enhanced Raman scattering(SERS) activity. The as-prepared monolayer film could be transferred to a solid substrate for use as a suitable SERS substrate with high activity, high uniformity, and high stability. The surface enhancement factor was six folders in magnitude and it depended on the compactness of the monolayer film. The cleaness of the substrate was improved after an electrochemical cleaning. The three dimensional “hot spots” was fabricated through the multi-transfer of monolayer onto the same solid substrates, in which the SERS effect was increased remarkably. The SERS effect reached the maximum as the size of probe molecules was comparable or less than that of the gap distance of “hot spots”.4. Exploring the approach for further treatment on Au nanoparticles monolayer film and for extension of its practical application. After heating and electrochemical deposition, the stability and cleaness were accordingly improved significantly. The dynamic SERS effect was observed during the drying processes of the Au monolayer film transferred to a solid substrate, which was mainly contributed by the dynamic change in the “hot spots” during the evaporation of water included in the monolayer film. The SERS effect achieved the maximum near the complete drying of this film. The Au monolayer could be served as an optimal SERS substrate for the untrasensitive detection of trace residues(pesticide and environmental pollution gas) and weak adsorption species. It also could be developed as a promising electrod material for the spectroelectrochemical investigation and as a potential substrate candidate for the surface plasmon catalyzed reaction.