Concentration and Surface Chemistry Dependent Analyte Orientation on Nanoparticle Surfaces

The development of surface-enhanced Raman spectroscopy (SERS)-based sensors necessitates a deeper understanding of the analyte-nanoparticle interaction. For optimal reliability, factors that may affect the resulting spectra need to be understood. First and foremost, the signal enhancement (and hence the improved sensitivity) offered by these systems highly relies on the localization of molecules or moieties in molecules as close as possible to the nanoparticle surface and decreases the farther a molecule is from the surface. Furthermore, the relative peak intensity, and thus the possibility to rely on a specific peak (or set of peaks) to build a calibration curve, depends on the orientation of the molecule with respect to the metallic surface due to the tensorial nature of the Raman polarizability. As a consequence, a change in analyte orientation on a nanoparticle surface impacts the resulting spectral pattern. Herein, factors that affect analyte orientation on a nanoparticle surface and their effect on the resulting SERS spectra are investigated. To do so, two unique nanostar morphologies and three analytes were selected. SERS spectra were acquired at varying analyte concentrations, and deconvoluted. X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations were conducted to confirm the hypothesized adsorbate/nanostars environment. Our study reveals three factors theorized to impact the molecular orientations: (1) analyte concentration, (2) nanoparticle surface properties, and (3) analyte-nanoparticle bond nature. Results herein suggest that when the analyte concentration is sufficiently high, the molecules reorient from parallel to perpendicular or remain perpendicular relative to the nanoparticle surface compared to the situation at low concentration. The way in which the analyte and nanoparticle interact (e.g., physisorb or chemisorb) will determine the preferred analyte orientation at low concentration. If covalently bound, this preliminary orientation is believed to be dictated by the preferred bond angle between surface and bound moiety. If physisorbed, the analyte will be parallel relative to the nanostar surface at low concentrations and then reorient perpendicular at increased concentrations. The work presented here, explaining in detail the concentration-dependent nature of the analyte orientation, will aid in the development of more reliable SERS sensors.