Ag-Clad Au Nanoparticles:  Novel Aggregation, Optical, and Surface-Enhanced Raman Scattering Properties

Abstract

Ag-coated Au colloidal particles have been prepared by reduction of Ag+ in the presence of preformed Au colloids. The composition of the Au100-x Ag x particles was varied from x = 0 to 80. SERS spectra of pyridine, p-nitroso-N,N‘-dimethylaniline (p-NDMA), and trans-1,2-bis(4-pyridyl)ethylene (BPE) have been obtained with these colloids. At monolayer Ag coverages (x < 10), the optical spectra of Ag-coated Au particles are indistinguishable from uncoated Au particles. However, the SERS behavior of aggregated colloids with 647.1 nm excitation is extremely dependent upon the Ag:Au ratio. Very small amounts of Ag (x ≤ 5) lead to an increase in SERS intensity, but further increases lead to complete loss of signal. For p-NDMA and pyridine, these data can be explained by Ag inhibition of adsorbate-induced aggregation. The initial increase in SERS intensity results from production of smaller aggregates that exhibit a surface plasmon band in better alignment with the excitation wavelength; higher ratios of Ag eliminate aggregation and all SERS enhancement. For BPE, the same Ag-induced loss of SERS is observed, even though each of the Au100-x Ag x colloidal solutions is clearly aggregated by BPE adsorption. This finding suggests that submonolayers of Ag modulate specific chemical interactions between the Au and BPE that are responsible for SERS.

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

“…It is well known that Au has lower EF values in the visible 37 and many attempts to obtain composite particles by depositing Ag on preformed Au particles have been extensively carried out. 38,39 So we hope our report could have some contributions to the research of the SERS enhancement mechanism on the Au surface.…”

Highly size-controlled synthesis of Au/Pd nanoparticles by inert-gas condensation

“…These properties are also strongly dependent on the size of the particles; in order to use efficiently these structures in the development of applications at industrial scales, the chosen synthesis method must allow a fair control of the mean size of the particle, with small deviations around this size. Of the different methods developed to produce metal nanoparticles, those based on chemical procedures are the most commonly used, [7][8][9][10] mainly because of their relative simplicity and economy. The bimetallic nanoparticles produced by chemical methods present in many cases a core-shell structure, as determined by transmission electron microscopy methods.…”

Surface-enhanced Raman scattering of silver-gold bimetallic nanostructures with hollow interiors

“…So many attempts to obtain silver/gold composite particles have been carried out in order to induce a greater SERS enhancement on gold surface. For example, Freeman et al 20 have studied the SERS properties of Ag-clad Au nanoparticles and proposed that deposition of submonolayers of Ag on preformed colloid Au leads to altered aggregation and SERS enhancement behavior. Rivas et al 21 have studied the SERS activity of mixed silver/gold colloids and investigated the activity of these substrates at different excitation wavelengths.…”