Spectroscopic Studies Of Collodial Silver Nanoparticles

The adsorption of molecules/anions onto metal surface leads to different photophysics and photochemistry than those found in pure molecules/anions or metal surfaces individually. One important consequence of this type of adsorption is enhancement of optical process such as surface-enhanced Raman scattering (SERS), enhanced absorption, enhanced and quenched fluorescence and so on. Different chemical reduction methods and laser ablation method were used to prepare Ag nanoparticles, and with transmission electron microscope (TEM), Raman detector, UV-Vis absorption and fluorescence spectrometers, we analyzed the influences of size, shape, concentration and added species to their absorptions, luminescence and SERS signals. The following topics related colloidal silver nanoparticles and their spectroscopies will be discussed.(1) Highly luminescent singles of colloidal silver nanoparticles in aqueous solution under blue excitation were detected. It was observed that Ag sol, composed of a large amount of silver atoms or particles have alterable surface energy bands different from plasmon resonance energy band of colloid due to the interactions between atoms and particles. Thus, the energy hybridization model of silver particles was deduced.(2) Laser ablated silver particles were prepared to characterize chemiadsorption of nucleophilic anion IO₃^-and its evocable spectroscopy. Besides the gradual decrease of particles' plasmon resonance absorption with the increase of anion, some new absorption bands, assigned to different mechanisms, appeared and shifted due to chemiadsorption effect and aggregation of particles. At the same time, gradual changes and new appearance of luminescence bands indicate the hybridization of surface energy states on modified metal surfaces did happen.(3) It was found that the 420nm fluorescence band of MO aqueous solution quenched sharply. While, the enhancement factor of the other fluorescence band at 530nm was up to 20, and at the same time, the band had a red shift of about 30nm. These notable phenomena are explained in the main aspects of the influence of fluorescence quenching and fluorescence enhancing by the radiationless energy transfer channel.(4) The correspondence found between SERS, absorption and fluorescence signals points out the importance of a first layer effect to account for the changes in intensity and spectral profile observed when adding Cl^-. A different response of Ag colloids to the excitation wavelength in presence or absence of Cl^-indicates the influence of the halide on the morphology of the metal particles and, in consequence, the surface-coverage of adsorbate. The differences observed in the spectral profile of first layer SERS spectra andthe multilayer ones may be attributed to a considerable contribution of the charge-transfer (CT) mechanism to the SERS effect, which is more efficient in the first layer.(5) The gradual changes of SERS signals indicate differing adsorption orientations of dyes on silver surface when pH changed and thus, results in different energy transfer efficiencies. Both experimental evidence and theoretic analysis indicate that, different molecular structures and differing adsorption orientations of dyes on silver surface were existence when pH changed.(6) UV-Visible absorption, fluorescence and surface-enhanced Raman scattering of methyl orange adsorbed on surfaces of both positively and negatively charged colloidal silver particles were observed respectively, and compared with each other. The results indicate that dye molecules may adsorb on these two kinds of silver surface in differing adsorption orientations and interactions with different groups, and thus caused the different phenomena.In all, different kinds of silver nanoparticles were prepared and with the change of their geography (size, shape) and surface conditions (surface electricity, added species), their spectroscopy and geography appeared disciplinarian change, which indicate different effects on optical property of colloidal Ag particles. These results may be good evidence and exploration for the application of silver nanoparticles to nanotechnology.