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Ag-coated Au colloidal particles have been prepared by reduction of Ag + in the presence of preformed Au colloids. The composition of the Au 100- 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 Au 100- 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.
Freeman et al. (Mon,) studied this question.