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 β-cyclodextrin (β-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 SERS 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 SERS 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 SERS studies.