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 depositi on of Ag or Au through chemical reduction on Au or Ag colloids, respectivel y. Different amounts of the depositing metal were employed in order to obta in different compositions of the Au100-xAgx and Ag100-yAuy resulting partic les. The obtained colloids were characterized by UV-vis spectroscopy and tr ansmission 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 a dsorbate probe. The SERS excitation profiles for each colloid were investig ated in order to obtain information about the activity of these substrates at different excitation wavelengths as well as to estimate the relative exp osed area of each metal in every composite colloid.