Probing the structural properties of the water solvation shell around gold nanoparticles: A computational study

Abstract

While subjected to radiation, gold nanoparticles (GNP) have been shown to enhance the production of radicals when added to aqueous solutions. It has been proposed that the arrangement of water solvation layers near the water-gold interface plays a significant role. As such, the structural and electronic properties of the first water solvation layer surrounding GNP of varying sizes were compared to bulk water using classical molecular dynamics, quantum and semiempirical methods. Classical molecular dynamics was used to understand the change in macroscopic properties of bulk water in the presence of different sizes of GNP, as well as by including salt ions. The analysis of these macroscopic properties has led to the conclusion that larger GNPs induce the rearrangement of water molecules to form a 2D hydrogen-bond network at the interface. Quantum methods were employed to understand the electronic nature of the interaction between water molecules and GNP along with the change in the water orientation and the vibrational density of states. The stretching region of vibrational density of states was found to extend into the higher wavenumber region, as the size of the GNP increases. This extension represents the dangling water molecules at the interface, as a result of reorientation of the water molecules in the first solvation shell. This multi-level study suggests that in the presence of GNP of increasing sizes, the first water solvation shell undergoes a rearrangement to maximize the water-water interactions as well as the water-GNP interactions.

Highly Heterogeneous Polarization and Solvation of Gold Nanoparticles in Aqueous Electrolytes

“…39 Yet, a shortcoming of most previous studies is the absence of explicit metal polarizability and its concomitant interfacial effects on interface hydration, adsorption, polarization, and charge transfer. 40 The effects of induced charges in metal surfaces have long been known to be significant in vacuum from an electronic-structure perspective, 41 whereas the effects on interfaces with aqueous solutions, biomolecules, and ionic liquids have only started to be addressed recently in a quantitative manner, both from the quantum and classical perspectives. 27,32,42 Furthermore, recent studies of induction effects on the hydration of graphene by using the selfconsistent atomic polarization model of graphene 43 and the fluctuating-charge technique of constant applied potential molecular dynamics (CAPMD) 44 show the importance of explicit polarizability of quasi-metallic graphene.…”

Highly Heterogeneous Polarization and Solvation of Gold Nanoparticles in Aqueous Electrolytes

“…47 In order to inspect the microscopic structure of interfacial water at polarized AuNPs in more detail, we analyzed the water orientation with respect to the NP (Figure 2e), which can be determined by two angles, α and β, as proposed in a recent work. 37 Angle α is defined as the angle between the dipole of the water molecule and the vector formed between the center of the NP and the oxygen atom of the water molecule; whereas β denotes the angle between the normal of the water molecule and the vector formed between the center of the NP and the oxygen atom of the water molecule; see the schematic diagram in Figure S1 in the Supporting Information. According to this definition, the criteria for the 'up' and 'down' configurations are α < 0.4π and α > 0.6π, respectively, regardless of β; the flat configuration is characterized by 0.45π < α < 2 3 π and either β < 1 6 π or β > 5 6 π; and the dangling configuration corresponds to the remaining configurations.…”

“…Hence, water and ionic contributions are balancing each other, while polarizability influences the balance. In fact, classical non-polarizable MD studies have found that the rearrangement of water molecules at the interface is actually disrupted by the presence of ions, 37 showing that the proportion of dangling water molecules change according to the size of the NP and the concentration of ions. This has consequences for water hydrolysis in the presence of gold NPs as an example.…”

“…36 Yet, a shortcoming of most previous studies is the absence of explicit metal polarization and its effects present at the interfaces formed in such systems, e.g., on hydration, adsorption, and charge transfer. 37 The effects of induced charges in metal surfaces have long been known to be significant in vacuum from an electronic-structure perspective, 38 whereas the effects on interfaces with aqueous solutions, biomolecules, and ionic liquids have only started to be addressed recently in a quantitative manner both from the quantum and classical perspectives. 24,29,39 But so far, the influence of metal polarization in complex, heterogeneous NP interfaces in ion-specific electrolytes has not been fully scrutinized from an atomistic perspective.…”

“…24,29,39 But so far, the influence of metal polarization in complex, heterogeneous NP interfaces in ion-specific electrolytes has not been fully scrutinized from an atomistic perspective. 37 Noteworthy, recent simulation studies in that direction include a capacitancepolarizability interaction model to study water structure on AuNPs in neat water 30 or a rigid-rod model with the GolP-CHARMM force field to study AuNP-citrate interactions. 33 A reactive force field (ReaxFF) MD simulation was employed to study peptide adsorption on AuNPs.…”