Surface -enhanced Raman spectroscopic studies of organonitriles on copper colloids

Dramatic differences are observed between the surface-enhanced Raman spectra (SERS) of organonitriles mixed with copper colloids, the Raman of the free parent molecules, and the SERS for adsorption on a gold colloid. Specifically, nu(C≡N) is missing in the SER spectra involving the copper colloid. The data are adequately explained by a transition metal assisted borohydride reduction of the cyano group to an amino group. SER spectra of the reduction products of the organonitriles in the presence of copper colloids are identical within experimental error. The nu(C≡N) signal was observed when a gold colloid was prepared by borohydride reduction, thereby indicating the metal selectivity of the reaction. Adsorption on silver colloids exhibit a complete disappearance of the nitrile stretching normal mode. The absence of the nitrile stretching signal is not due to a metal-assisted reduction of the nitrile group to an amino group. The observation of no nitrile stretching band for the silver colloids indicates that the silver metal is distinct from the copper and gold. SERS data collected for the isomeric forms of cyanopyridine (4-CP, 3-CP and 2-CP) mixed with the copper and gold colloids supports the theory that the degree of surface enhancement is lowered by the loss of molecular symmetry. Addition of selected sodium salts were used to determine the role of aggregation and co-adsorption in surface enhancement. Our SERS data demonstrate that the increase in surface enhancement is primarily influenced by the chemical interaction between the adsorbate and the metal surface. Although the molecular symmetry of the adsorbate, as well as the affects of aggregation, affect the degree of enhancement observed, the chemical adsorption onto the metal dominates the enhancement mechanism.*.;*Originally published in DAI Vol. 61, No. 3. Reprinted here with corrected author name