Striped gold nanoparticles: New insights from molecular dynamics simulations

Influence of Temperature-Guided SAM Growth on Wetting and Its Mass Transfer Models

The formation and growth of self-assembled monolayers (SAMs) composed of amphiphiles have garnered significant attention due to their diverse technical applications. This article reports the findings of molecular dynamics simulations aimed at elucidating the intricate relationship between the wetting behavior of amphiphiles, specifically n-alkanols, and the growth of their SAMs on a mica surface under varying temperature conditions. The investigation quantifies the structural characteristics of the formed SAMs, including density profiles, in-plane radial distribution functions, order parameters, and end-to-end length distributions of n-alkanol molecules within the SAM. Thermodynamic properties, such as the second virial coefficient and excess entropy, are examined in relation to temperature and time. The growth of the SAM is assessed by analyzing characteristic time scales at different temperatures and in-plane diffusion of n-alkanol molecules and utilizing classical theories of mass transfer to quantify the growth rate as a function of temperature. These results are then correlated with changes in the contact angle and spreading coefficient of n-alkanol droplets on the mica surface over time, providing insights into the impact of SAM growth on the wetting behavior and the mass transfer model of such systems.

Computational approach to (ZnS)_{i} nanoclusters in ionic liquids

Unique and attractive properties have been predicted for II-VI-type semiconductor nanoclusters within the field of nanotechnology. However, the low reaction kinetics within the usual solvents gives only thermodynamic control during their production process, making the obtention of different metastable polymorphs extremely difficult. The use of ionic liquids as solvents has been proposed to overcome this problem. Identifying how these nanoclusters are solvated within ionic liquids is fundamental if this strategy is to be pursued. While computational chemistry tools are best suited for this task, the complexity and size of the system requires a careful design of the simulation protocol, which is put forward in this work. Taking as reference the (ZnS)_{12} nanocluster and the [EMIM][EtSO_{4}] ionic liquid, we characterize the interactions between the nanoparticle and first solvation shell by density functional theory calculations, considering most of the solvent implicitly. The DFT results are consistent through different theory levels showing a strong interaction between the Zn atoms of the nanocluster and the [EtSO_{4}^{-}] anion of the ionic liquid. A more realistic representation of the system is obtained by classical MD calculations, for which various classical force fields were considered and several atomic interactions parameterized. This new set of parameters correctly describes the interaction of different (ZnS) nanoclusters, supporting its transferability. The resulting MD simulation shows the formation of a structured ionic liquid solvation shell around the nanocluster with no exchange of ions for at least 5 ns, in agreement with the strong interactions observed in the density functional theory calculations.

Effect of Concentration, Chain Length, Hydrophobicity, and an External Electric Field on the Growth of Mixed Alkanethiol Self-Assembled Monolayers: A Molecular Dynamics Study

Growing functionalized self-assembled monolayers (SAMs) with fewer defects and lower cost is the focus of ongoing investigations. In the present study, molecular dynamics simulations were performed to investigate the process of SAM formation on a gold substrate from mixed alkanethiolates in ethanol solution. Using the mixed-SAM system of 11-mercaptoundecanoic acid (MUA) with either 1-decanethiol (C9CH3) or 6-mercaptohexanol (C6OH) in a 3:7 ratio as the standard SAM model, we systematically investigated the effects of the concentration, chain length, functional group, and an external electric field on SAM growth. The results showed that the initial growth rate and surface coverage of the SAM are dependent on the ligand concentration. At a certain high concentration (about 1.2-1.5 times the minimum concentration), the final surface coverage is optimal. Reducing the chain length and increasing the proportion of hydrophobic diluting molecules are effective ways to improve the surface coverage, but the compositional ligands have to be changed, which may not be desirable for the functional requirements of SAMs. Furthermore, by investigating the behavior of the alkanethiolates and ethanol solvent under an applied external field, we find that a strong electric field with a proper field direction can facilitate the generation of defect-free monolayers. These findings will contribute to the understanding of mixed-SAM formation and provide insight into experimental design for efficient and effective SAM formation.

What Does Nanoparticle Stability Mean?

The term "nanoparticle stability" is widely used to describe the preservation of a particular nanostructure property ranging from aggregation, composition, crystallinity, shape, size, and surface chemistry. As a result, this catch-all term has various meanings, which depend on the specific nanoparticle property of interest and/or application. In this feature article, we provide an answer to the question, "What does nanoparticle stability mean?". Broadly speaking, the definition of nanoparticle stability depends on the targeted size dependent property that is exploited and can only exist for a finite period of time given all nanostructures are inherently thermodynamically and energetically unfavorable relative to bulk states. To answer this question specifically, however, the relationship between nanoparticle stability and the physical/chemical properties of metal/metal oxide nanoparticles are discussed. Specific definitions are explored in terms of aggregation state, core composition, shape, size, and surface chemistry. Next, mechanisms of promoting nanoparticle stability are defined and related to these same nanoparticle properties. Metrics involving both kinetics and thermodynamics are considered. Methods that provide quantitative metrics for measuring and modeling nanoparticle stability in terms of core composition, shape, size, and surface chemistry are outlined. The stability of solution-phase nanoparticles are also impacted by aggregation state. Thus, collision and DLVO theories are discussed. Finally, challenges and opportunities in understanding what nanoparticle stability means are addressed to facilitate further studies with this important class of materials.

Molecular-Level "Observations" of the Behavior of Gold Nanoparticles in Aqueous Solution and Interacting with a Lipid Bilayer Membrane

We use coarse-grained molecular dynamics simulations to "observe" details of interactions between ligand-covered gold nanoparticles and a lipid bilayer model membrane. In molecular dynamics simulations, one puts the individual atoms and groups of atoms of the physical system to be "observed" into a simulation box, specifies the forms of the potential energies of interactions between them (ultimately quantum based), and lets them individually move classically according to Newton's equations of motion, based on the forces arising from the assumed potential energy forms. The atoms that are chemically bonded to each other stay chemically bonded, following known potentials (force fields) that permit internal degrees of freedom (internal rotation, torsion, vibrations), and the interactions between nonbonded atoms are simplified to Lennard-Jones forms (in our case) and coulombic (where electrical charges are present) in which the parameters are previously optimized to reproduce thermodynamic properties or are based on quantum electronic calculations. The system is started out at a reasonable set of coordinates for all atoms or groups of atoms, and then permitted to develop according to the equations of motion, one small step (usually 10 fs time step) at a time, for millions of steps until the system is at a quasi-equilibrium (usually reached after hundreds of nanoseconds). We then let the system play out its motions further for many nanoseconds to observe the behavior, periodically taking snapshots (saving all positions and energies), and post-processing the snapshots to obtain various average descriptions of the system. Alkanethiols of various lengths serve as examples of hydrophobic ligands and methyl-terminated PEG with various numbers of monomer units serve as examples of hydrophilic ligands. Spherical gold particles of various diameters as well as gold nanorods form the core to which ligands are attached. The nanoparticles are characterized at the molecular level, especially the distributions of ligand configurations and their dependence on ligand length, and surface coverage. Self-assembly of the bilayer from an isotropic solution and observation of membrane properties that correspond well to experimental values validate the simulations. The mechanism of permeation of a gold NP coated with either a hydrophobic or a hydrophilic ligand, and its dependence on surface coverage, ligand length, core diameter, and core shape, is investigated. Lipid response such as lipid flip-flops, lipid extraction, and changes in order parameter of the lipid tails are examined in detail. The mechanism of permeation of a PEGylated nanorod is shown to occur by tilting, lying down, rotating, and straightening up. The nature of the information provided by molecular dynamics simulations permits understanding of the detailed behavior of gold nanoparticles interacting with lipid membranes which in turn helps to understand why some known systems work better than others and aids the design of new particles and improvement of methods for preparing existing ones.

Gold nanoparticles with patterned surface monolayers for nanomedicine: current perspectives

Molecular self-assembly is a topic attracting intense scientific interest. Various strategies have been developed for construction of molecular aggregates with rationally designed properties, geometries, and dimensions that promise to provide solutions to both theoretical and practical problems in areas such as drug delivery, medical diagnostics, and biosensors, to name but a few. In this respect, gold nanoparticles covered with self-assembled monolayers presenting nanoscale surface patterns-typically patched, striped or Janus-like domains-represent an emerging field. These systems are particularly intriguing for use in bio-nanotechnology applications, as presence of such monolayers with three-dimensional (3D) morphology provides nanoparticles with surface-dependent properties that, in turn, affect their biological behavior. Comprehensive understanding of the physicochemical interactions occurring at the interface between these versatile nanomaterials and biological systems is therefore crucial to fully exploit their potential. This review aims to explore the current state of development of such patterned, self-assembled monolayer-protected gold nanoparticles, through step-by-step analysis of their conceptual design, synthetic procedures, predicted and determined surface characteristics, interactions with and performance in biological environments, and experimental and computational methods currently employed for their investigation.