Determining the Molecular Orientation on the Metal Nanoparticle Surface through Surface-Enhanced Raman Spectroscopy and Density Functional Theory Simulations

We report here the efficacy of surface-enhanced Raman spectroscopy (SERS) measurements as a probe for molecular orientation. 4-Aminobenzoic acid (PABA) on a surface consisting of silver (Ag) nanoparticles (NPs) is investigated. We find that the orientation of the PABA molecule on the SERS substrate is estimated based on the relative change in the magnitude of the C-H stretching bands on the SERS substrate, and it is found that the molecule assumes a horizontal orientation on the Ag-NP surface. The strong molecule-metal interaction is determined by an abnormal enhanced SERS band appearing at 980 cm-1, and the peak is assigned to an out-of-plane amine vibrational mode, which is supported by our ab initio calculations. DFT-based Raman activity calculations corroborate the SERS results, revealing that (i) the PABA molecule attaches to the surface of Ag-NPs with its α dimers rather than single-molecule binding and (ii) the molecule preserves its α dimers in an aqueous environment. Our results demonstrate that SERS can be used to gain deeper insights into the molecular orientation on metal nanoparticle surfaces.