Importance of Tilt Angles of Adsorbed Aromatic Molecules on Nanoparticle Rattle SERS Substrates

Abstract

Solution-phase nanoparticles are extensively used as surface enhanced Raman scattering (SERS) substrates, but signal intensities depend on dynamic nanoparticle optical properties and stabilities as well as molecular identity and orientation. To evaluate how these contributions influence the detection of aromatic thiols, internally etched silica encapsulated gold-coated silver (IE Ag@Au@SiO2) nanoparticles are used. First, localized surface plasmon resonance (LSPR) spectroscopy is implemented to estimate molecular tilt angle. Different tilt angles are then related to functional group induced surface density differences. Next, evaluation of SERS intensities and vibrational modes suggest that molecular tilt angle and surface selection rules govern the behavior observed in SERS intensities. Finally, concentration-dependent SERS signals are modeled using the Langmuir adsorption model. Equilibrium constants and free energies associated with adsorption are consistent with differences from London dispersion force stabilization between the molecules and the metal surface. These studies suggest that the SERS intensities observed for these thiolated ligands are highly sensitive to adsorbate tilt angle relative to the nanoparticle surface, which are easily estimated because of the optical stability and controlled adsorbate interactions with IE Ag@Au@SiO2 nanoparticles and could be extended to other molecules in the future to better understand and evaluate reproducible applications using SERS.

Gold Nanoparticle Monolayers from Sequential Interfacial Ligand Exchange and Migration in a Three-Phase System

“…Although the EM-enhancement of incident light is considered as the largest contributor to the SERS intensity amplification, the chemical enhancement cannot be totally ruled out. Many studies have demonstrated that the SERS efficiency is closely related to non-EM factors, such as the adsorption of the analyte to the Au surface 76 77 , and the adsorbed analyte molecular orientation 78 . As mentioned earlier, the surface OA ligand in the Au NP monolayer has a different chain conformation than that of the alkylamine-Au NP film.…”

Irreversible accumulated SERS behavior of the molecule-linked silver and silver-doped titanium dioxide hybrid system

“…As shown in main text, 4MBA molecule is adsorbed on the surface of Ag/Ag-doped TiO 2 substrate by Ag-S bond, and the benzene ring and COOH group can rotate round the bond. Normally, 4MBA molecules should orient to the surface at an approximate angle of 57°for a close-packed selfassembled monolayers 60 . During light irradiation, the reorientation of 4MBA molecules are subject to the actions of electric-field force, damping fore, and restoring force.…”

Theoretical simulation of surface‐enhanced resonance Raman spectroscopy of cytosine and its tautomers

“…It can be found that interaction of cytosine through both N and O atoms with Ag 20 reproduce the experimental SERS spectra better than others. It should be mentioned that the dependence of SERS spectra on the orientations of adsorbent has been demonstrated previously . At last but not least, in order to find the dependence of shape of spectrum to overtone excitation, the Cyt─Ag 20 SERS spectrum was obtained (by considering the overtone excitations), and slight variation and modification in the intensity of high‐frequency region (>1,200 cm ‐1 ) was found in Figure S9.…”