Optimization of SERS activities of gold nanoparticles and gold-core-palladium-shell nanoparticles by controlling size and shell thickness

The optimization of surface-enhanced Raman scattering (SERS) activity of gold nanoparticles is essential for further enhancing SERS capability in terms of high sensitivity, stability and reproducibility. Recently, we utilized a simple seed-mediated growth method to synthesize monodisperse Au nanoparticles with controllable size from about 16 to 160 nm, which were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-vis spectroscopy. These nanoparticles can be easily formed as a uniform thin film on glass carbon or gold substrates with an area larger than 1 mm.(2) The nanoparticle film with the size range of 120-135 nm showed the highest SERS activity with the excitation wavelength of 632.8 nm. Using pyridine as the probe molecule, the average enhancement factorcould reach Up to 10(7). Finite difference time domain (FDTD) calculation was employed to explain the size-dependent SERS activity. The optimum-sized Au nanoparticles were utilized to further prepare Au-Pd core-shell (Au@Pd)nanoparticles in order to greatly enhance the SERS activity of the Pd shell. The enhancement factor of the ultrathin Pd shell was found to be over 5 x 10(4). Thus the originally low enhancement factor of Pd could be improved substantially. Copyright (C) 2008 John Wiley & Sons, Ltd.