Surface-Enhanced Raman Scattering of Mercaptopyridines and Pyrazinamide Incorporated in Silver Colloid−Adsorbate Films

Abstract

Pyrazinamide (pza) and 2-mercaptopyridine (2-MPy) are shown to form Ag colloid−adsorbate films at the interface between an aqueous Ag colloid and a solution of an adsorbate in dichloromethane. By contrast, under the same conditions, 4-mercaptopyridine (4-MPy) forms Ag organosol aggregates by the interface transfer of adsorbate-covered Ag colloidal particles from the aqueous phase to the organic phase. This result confirms that formation of Ag colloid−adsorbate films is an adsorbate specific process. Transmission electron and optical microscopy and surface plasmon adsorption measurements are used to characterize the morphology of the deposited Ag colloid−adsorbate structures while SERS spectra of the incorporated adsorbates probe their internal structure. Furthermore, we demonstrate that, for a particular adsorbate, the process of Ag colloid−adsorbate film formation, the morphology of the deposited films, and the SERS spectra are substantially influenced by the Ag content of the parent colloid (Ag−pza and Ag−4-MPy films) and/or by the concentration of the adsorbate in the organic phase (Ag−2-MPy films). We propose that the observed differences in the rates of formation of the films, their morphology, and the orientation of the adsorbate with respect to the Ag surface are related to differences in the actual surface coverage of Ag colloidal particles by adsorbate molecules.