Mixed silver/gold colloids: A study of their formation, morphology, and surface-enhanced Raman activity

Ag-coated Au and Au-coated Ag colloidal particles were prepared by deposition of Ag or Au through chemical reduction on Au or Ag colloids, respectively. Different amounts of the depositing metal were employed in order to obtain different compositions of the Au100-xAgx and Ag100-yAuy resulting particles. The obtained colloids were characterized by UV-vis spectroscopy and transmission electron microscopy. A different formation mechanism was deduced for the different mixed colloids obtained by this procedure. Moreover, the activity of these colloids in surface-enhanced Raman spectroscopy (SERS), that is, the SERS enhancement factor, was checked by using pyridine as an adsorbate probe. The SERS excitation profiles for each colloid were investigated in order to obtain information about the activity of these substrates at different excitation wavelengths as well as to estimate the relative exposed area of each metal in every composite colloid.