Direct and Water-Mediated Adsorption of Stabilizers on SERS-Active Colloidal Bimetallic Plasmonic Nanomaterials: Insight into Citrate–AuAg Interactions from DFT Calculations

Abstract

In previous studies, AuAg colloidal nanostar formulations were developed with the two-fold aim of producing optimized surface-enhanced Raman spectroscopy (SERS) substrates and investigating the nature of the capping process itself. Findings demonstrated that the nanoparticle metals are alloyed and neutral, and capping by stabilizers occurs via chemisorption. This study utilizes citrate as the model stabilizer and investigates the mechanistic aspects of its interaction with mono- (Au20) and bimetallic (Au19Ag) surfaces by density functional theory (DFT) calculations. Citrate was modeled according to the colloid’s pH and surrounded by a water and sodium first solvation shell. A population of stable cluster–citrate structures was obtained, and energies were refined at the uB3LYP//LANL2TZ­(f)/cc-pVTZ level of theory. Solvation was accounted for both explicitly and implicitly by the application of the continuum model SMD. Results indicate that both direct binding and binding by water proxy through the charge-transfer complex formation are thermodynamically favorable. Water participation in citrate adsorption is supported by the adsorption behavior observed experimentally and the comparison between experimental and DFT-simulated IR spectra. Vibrational mode analysis suggests the possible presence of water within a crystal in dried nanostar residues. All ΔG ads(aq) indicate a weak chemisorptive process, leading to the hypothesis that citrate could be displaced by analytes during SERS measurements.

Challenges and opportunities for SERS in the infrared: materials and methods

“…Besides modulating colloidal stability, surface ligands can indeed minimize surface energy. Shape conversion of gold nanostars can thus be prevented by capping them with strongly interacting surfactants, such as CTAB, 83 weakly chemisorbed small molecules such as citrate, 53,143 or other ionic species such as HEPES 129 and carbonate. 53 It must be recalled that the choice of nanoparticle capping agents is not only critical for the retention of shape and colloidal stability, 53,140 but also for optimal signal enhancement 53 and biocompatibility.…”

Simulation Calculation Verification of Graphene Oxide-Decorated Silver Nanoparticles Growing on Titania Nanotube Array as SERS Sensor Substrate

“…The theoretical basis is still sufficient to support high sensing performance of GO/Ag/TiO 2 SERS sensor substrate. The simulation calculation on the basis of the Density Functional Theory (DFT) is an effective approach to disclose the electronic properties, which are related to Raman electromagnetic enhancement, and the interfacial adsorption energy, which is related to Raman chemical enhancement [26][27][28][29]. In this study, GO-decorated Ag nanoparticles/TiO 2 nanotube arrays (GO/Ag/TiO 2 NTA) were designed as active SERS sensor substrates for sensitive detection and determination of the organic compound BPA.…”

Virtual Issue on Experiment–Theory Synergies in the Study of Metal and Metal-Containing Clusters

“…Synergies also arise from theoretical studies inspired by experimental findings, such as the benchtop syntheses of atomically precise nanoparticles. There are several examples of theoretical studies on the mechanisms of these syntheses of pure and doped ligated particles, , which can inform experimental strategies for advanced syntheses. As another example, the theoretical framework presented to enable determination of magnetic coupling constants Fe­(III) 4 S 4 cubanes, the subject of a recent purely experimental paper, suggested the action of spin-frustration-driven Jahn–Teller distortion and hidden magnetic order within a manifold of singlet electronic states …”