Understanding the Enhancement Mechanisms in the Surface-Enhanced Raman Spectra of the 1,10-Phenanthroline Molecule Adsorbed on a Au@Ag Bimetallic Nanocolloid

Adsorption of 1,10-phenanthroline (PHEN) on the Au-core-Ag-shell (Au@Ag) bimetallic nanocolloid surface has been investigated. Bimetallic nanocolloids have been synthesized using beta-cyclodextrin (beta-CD) in alkaline solution. Nanocolloids of different Au-core: Ag-shell ratio were synthesized keeping the same Au-core size. Again, for the preparation, the total metal concentrations were always kept constant. The structure and composition of the bimetallic particles were characterized by UV-vis and high-resolution transmission electron microscopy. Detailed normal Raman (NRS) and surface-enhanced Raman spectra (SERS) of PHEN in aqueous solution have been studied. The relative contributions of the electromagnetic (EM) and charge-transfer (CT) mechanism to the overall enhancement of the SEAS bands of the PHEN molecule have been estimated. PHEN molecules are adsorbed on the Ag-shell surface through both the nitrogen atoms with the molecular plane almost perpendicular to the surface, which has been confirmed from Ag-N stretching vibration. The three-dimensional finite difference time domain (3D-FDTD) method has been applied to simulate the local electric field on the spherical Au@Ag nanocolloid for various core/shell ratios. Comparative spectral information revealed the highest SEAS effect from the 1:4 Au-core-Ag-shell bimetallic nanocolloid of 29 nm size. This observation has also been supported by theory. Thus, experiment and theory keep abreast the attachment of PHEN molecules on a new substrate from SEAS studies.