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

Abstract

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.

Focusing ion funnel-assisted ambient electrospray enables high-density and uniform deposition of non-spherical gold nanoparticles for highly sensitive surface-enhanced Raman scattering

“…To calculate the enhancement factor of the substrate, the most dominant Raman peak located at 1585 cm −1 was chosen. The enhancement factor is calculated by using the following formula (eqn (2)), 68,69 where I NR and I SERS are the Raman signal intensities for normal Raman and SERS measurements, respectively. N SERS and N NR are the average number of molecules within laser irradiation for normal Raman measurements.…”

“…To calculate the enhancement factor of the substrate, the most dominant Raman peak located at 1585 cm −1 was chosen. The enhancement factor is calculated by using the following formula (eqn (2)), 68,69…”

Electrochemical Surface-Enhanced Raman Spectroscopy for Aniline Adsorbed on Silver Electrodes: A DFT Study of the Anharmonic Effects of Amino Wagging Vibrational Modes

“…Aromatic amine molecules are an important class of organic molecules, and their adsorption on metal surfaces exhibits strong surface-enhanced Raman spectroscopy (SERS) signals. − This signal is mainly due to the surface plasmon resonance (SPR) effect of silver nanoclusters, which converts light from the far-field to the near field to enhance the light-molecule interaction, significantly enhances the inelastic light scattering signal, and generates strong SERS signals to obtain information of molecules adsorbed on the surface of complex nanostructures. − Based on the surface spectral selection rules of Moskovits and Creighton, it can be used to infer the adsorption orientation and bonding properties of molecules on metal surfaces.…”

Plasmonic Photoelectrochemical Coupling Reactions of para-Aminobenzoic Acid on Nanostructured Gold Electrodes

“…Many aromatic molecules on PMNS have been studied in depth using SERS. ,,, para -Aminobenzoic acid (PABA) is an aromatic molecule that contains amino and carboxylic groups, similar to amino acids, biomolecules, and many drugs. Thus, PABA is a highly desired vital nutrient for numerous microorganisms and a key active component in a wide range of personal care products as well as the production of esters, folic acid, and sunscreens. , It has also been applied as a probe molecule in nanoscience and surface science. − Moreover, a change in its orientation that results in the adsorption of the above-mentioned surface functional groups on the surface of metals was observed. These surface functional groups behave as adsorption sites, aiding the design of appropriate anode and cathode catalysts .…”

“…Thus, PABA is a highly desired vital nutrient for numerous microorganisms and a key active component in a wide range of personal care products as well as the production of esters, folic acid, and sunscreens. 39,40 It has also been applied as a probe molecule in nanoscience and surface science. 41−45 Moreover, a change in its orientation that results in the adsorption of the above-mentioned surface functional groups on the surface of metals was observed.…”