Selective Molecular Raman Scattering Enhancement Via Built-in "Hot Spots" On Single Au@Ag Nanorods

The match between localized surface plasmon resonance (LSPR) peaks of gold nanorods (NRs) and laser excitation wavelength is considered to provide on-resonance field enhancement at the NR surface, and thus increase the Raman scattering of molecules in vicinity enormously. However, experimentally the intensity augment is difficult to be disentangled from the contribution of aggregation induced hot spots. Herein, we embedded Raman reporter molecules in the metal layers of the Au NR core and Ag NR shell to build "intraparticle" hot spots, which induce Raman signals much stronger than those caused by "interparticle" hot spots between Au@Ag NRs. With these specially designed plasmonic nanostructures, we observed maximum Raman signal enhancement at excitation wavelength off plasmon resonance frequency of Au@Ag nanorods. These findings are promising for potential applications in selective molecular sensing, multiplexed imaging and high-throughput detection.The thesis contains the following parts:In chapter1, we introduced the research background and reviewed previous literatures.In chapter2, the gold nanorods capped with CTAB were synthesized via the seed-growth method. The LSPR peaks of the gold nanorods were adjusted by changing the amount of AgNO3or using H2O2to oxidize the gold nanorods. In addition, the gold nanorods modified with thiols were selected as the seeds to synthesize the molecule-embedded gold core@silver shell nanorods with different LSPR peaks.In chapter3, we chose the514nm,633nm and785nm laser excitation wavelength to get the surface-enhanced Raman scattering spectra of p-aminothiophenol respectively by using the gold nanorods as the substrates. Experimentally the intensity augment is difficult to be disentangled from the contribution of aggregation induced hot spots. The relationship between the Raman scattering signals and the LSPR peaks of gold nanorods under different excitation wavelengths was investigated. It was found that the enhancement of Raman signals was not correlated directly with the LSPR peaks of the gold nanorods.In chapter4, we fabricated the Raman reporter-embedded gold nanorods core@silver shell nanostructures to construct built-in hot spots in order to further enhance molecular Raman scattering signals. The "interparticle" hot spots of Au@Ag nanorods increased the Raman scattering signal more drastically than "intraparticle" hot spots, and hence the correlation between the LSPR peaks of nanorods and the surface-enhanced Raman spectra was observed. The results herein imply that we can enhance the Raman scattering signal selectively by changing the excitation wavelength.In chapter5, we measured the nonlinear optical properties of water-soluble gold nanoparticles/nanorods and organic-soluble gold nanoparticles/nanoclusters by Z-scan technique. The fitting results showed that the nonlinear response of gold nanoparticles was not only caused by surface plasmon saturated absorption but also by the existing free-carrier. The nonlinear response can be affected by the morphology, size and surface plasmon properties of gold nanoparticles.