Concentration and Surface Chemistry Dependent Analyte Orientation on Nanoparticle Surfaces

Abstract

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.

Self-Functional Off-Stoichiometry Polymeric Materials: Potential for Tunable Plasmonic Applications

“…Thus, this experimental observation supported the fact that we have successfully prepared a plasmonic surface using gold nanoparticles-to-thiol ligand exchanges. Because the −SH groups have a strong binding affinity toward AuNPs, making them easily accessible for active plasmonic surfaces, these thiol terminal groups experience higher affinity than other functional groups available on the OSTE+ polymer surface. − However, further, S 2p and S 2s energy states were analyzed from ranges between 190 and 168 eV to 222 and 234 eV to gain more profound information on the nature of bonds among nanoparticles and -SH groups (Figure b,c). This study revealed the types of chemical bonds that can exist between the immobilized nanoparticles and the functional polymer surfaces.…”

Shape-Induced Variations in Aromatic Thiols Adsorption on Gold Nanoparticle: A Novel Method for Accurate Evaluation of Adsorbed Molecules

“…5,6 Although adsorption of a self-assembly monolayer (SAM) of thiols on flat gold substrates and spherical nanoparticles has been comprehensively investigated, 7−13 the situation is much less clear for anisotropic particles such as nanostars. 14,15 It is known that thiol adsorption on gold surfaces is influenced by many factors, such as reaction media (including solvents, buffers, other analytes), thiol ligand chain size and functional groups (including charge), roughness of the gold surface, and the reversibility of adsorption of the thiol layer on gold films. 7−10 The concentration of thiols has been highlighted to be of particular importance.…”

“…Developing quantitative characterization methods for understanding the structure and chemistry of organic/inorganic-nanoparticle interfaces is critical across many applications such as chemical catalysis, , biomedicine, , and electronics. , Although adsorption of a self-assembly monolayer (SAM) of thiols on flat gold substrates and spherical nanoparticles has been comprehensively investigated, − the situation is much less clear for anisotropic particles such as nanostars. , …”

“…Only a few studies have been performed on the adsorption of thiols on gold nanoparticles (GNPs) with different sizes and asymmetric shapes. , Bulk characterization techniques such as light spectroscopy (UV–visible and fluorescence spectroscopy) lack the spatial resolution needed to study how molecules bind to nanoparticles with complex, highly curved geometries. Assumptions such as full surface coverage and monolayer adsorption have been used to estimate the number of molecules on surfaces. , While these assumptions may be valid for uniform spherical nanoparticles or flat surfaces, they are unlikely to be valid for anisotropic nanoparticles and nanoparticles that are not monodispersed.…”

Nanophotonics for Chemical Imaging and Spectroscopy

“…9 Indeed, this work dissects the contributions of different phenomena to SERS (and ambient TERS) signal fluctuations, which continue to puzzle practitioners, even at this (late) stage of evolution of both techniques. 25,26 Equally interesting contributions analyzed ligand binding on silver nanoparticles, 27,28 a detailed understanding of which is a prerequisite to rationalizing the enhanced optical spectra and chemical properties of molecules on plasmonic nanostructures. Finally, both conventional and machine-learning-assisted analyses of nanoparticles and nanoparticle−cyanobacteria assemblies were described, 29 with a goal of interfacing advanced theoretical treatments with SERS to significantly broaden the application space of this powerful technique.…”