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.