Assembly of responsive-shape coated nanoparticles at water surfaces

Janus Ligand-Tethered Nanoparticles at Liquid-Liquid Interfaces

We investigate the structural properties of Janus ligand-tethered nanoparticles at liquid-liquid interfaces using coarse-grained molecular dynamics simulations. The effect of interactions between different chains and liquids is discussed. We consider the Janus particles with symmetrical interactions with the liquids which correspond to supplementary wettability and particles with uncorrelated interactions. Simulation results indicate that the Janus hairy particles trapped in the interface region have different configurations characterized by the vertical displacement distance, the orientation of the Janus line relative to the interface, and the particle shape. The Janus hairy particles present abundant morphologies, including dumbbell-like and typical core-shell, at the interface. The shape of adsorbed particles is analyzed in detail. The simulation data are compared with those predicted by a simple phenomenological approach. This work can promote the applications of Janus hairy particles in nanotechnology.

Hybrid Nanoparticles at Fluid-Fluid Interfaces: Insight from Theory and Simulation

Hybrid nanoparticles that combine special properties of their different parts have numerous applications in electronics, optics, catalysis, medicine, and many others. Of the currently produced particles, Janus particles and ligand-tethered (hairy) particles are of particular interest both from a practical and purely cognitive point of view. Understanding their behavior at fluid interfaces is important to many fields because particle-laden interfaces are ubiquitous in nature and industry. We provide a review of the literature, focusing on theoretical studies of hybrid particles at fluid-fluid interfaces. Our goal is to give a link between simple phenomenological models and advanced molecular simulations. We analyze the adsorption of individual Janus particles and hairy particles at the interfaces. Then, their interfacial assembly is also discussed. The simple equations for the attachment energy of various Janus particles are presented. We discuss how such parameters as the particle size, the particle shape, the relative sizes of different patches, and the amphiphilicity affect particle adsorption. This is essential for taking advantage of the particle capacity to stabilize interfaces. Representative examples of molecular simulations were presented. We show that the simple models surprisingly well reproduce experimental and simulation data. In the case of hairy particles, we concentrate on the effects of reconfiguration of the polymer brushes at the interface. This review is expected to provide a general perspective on the subject and may be helpful to many researchers and technologists working with particle-laden layers.

Functional-Group Effect of Ligand Molecules on the Aggregation of Gold Nanoparticles: A Molecular Dynamics Simulation Study

In this paper, atomistic molecular dynamics simulations are performed for the systems consisting of functionalized gold nanoparticles (NPs) in a toluene medium. Gold NPs are coated with ligand molecules that have different terminal groups, that is, polar carboxyl (COOH), hydroxyl (OH), amine (NH₂), and nonpolar methyl (CH₃). These functional groups are selected to understand the relation between the aggregation behavior of functionalized gold NPs in toluene and the polarity of terminal groups of ligand molecules. The center-of-mass distances between NP pairs, the radial distribution functions, the mean square displacements, the radius of gyration, and the number of hydrogen bonds (H-bond) between ligand molecules are computed. Our simulation results indicate that functionalized gold NPs exhibit different aggregation/dispersion behaviors depending upon the terminal group of ligands.

Patterning of Self-Assembled Monolayers of Amphiphilic Multisegment Ligands on Nanoparticles and Design Parameters for Protein Interactions

Functionalization of nanoparticles with specific ligands is helpful to control specific diagnostic and therapeutic responses such as protein adsorption, cell targeting, and circulation. Precision delivery critically depends on a fundamental understanding of the interplay between surface chemistry, ligand dynamics, and interaction with the biochemical environment. Due to limited atomic-scale insights into the structure and dynamics of nanoparticle-bound ligands from experiments, relationships of grafting density and ligand chemistry to observable properties such as hydrophilicity and protein interactions remain largely unknown. In this work, we uncover how self-assembled monolayers (SAMs) composed of multisegment ligands such as thioalkyl-PEG-(N-alkyl)amides on gold nanoparticles can mimic mixed hydrophobic and hydrophilic ligand coatings, including control of patterns, hydrophilicity, and specific recognition properties. Our results are derived from molecular dynamics simulations with the INTERFACE-CHARMM36 force field at picometer resolution and comparisons to experiments. Small changes in ligand hydrophobicity, via adjusting the length of the N-terminal alkyl groups, tune water penetration by multiples and control superficial ordering of alkyl chains from 0 to 70% regularity. Further parameters include the grafting density of the ligands, curvature of the nanoparticle surfaces, type of solvent, and overall ligand length, which were examined in detail. We explain the thermodynamic origin of the formation of heterogeneous patterns of multisegment ligand SAMs and illustrate how different degrees of ligand order on the nanoparticle surface affect interactions with bovine serum albumin. The resulting design principles can be applied to a variety of ligand chemistries to customize the behavior of functionalized nanoparticles in biological media and enhance therapeutic efficiency.

Predicting the Physicochemical Properties and Biological Activities of Monolayer-Protected Gold Nanoparticles Using Simulation-Derived Descriptors

Gold nanoparticles are versatile materials for biological applications because their properties can be modulated by assembling ligands on their surface to form monolayers. However, the physicochemical properties and behaviors of monolayer-protected nanoparticles in biological environments are difficult to anticipate because they emerge from the interplay of ligand-ligand and ligand-solvent interactions that cannot be readily inferred from ligand chemical structure alone. In this work, we demonstrate that quantitative nanostructure-activity relationship (QNAR) models can employ descriptors calculated from molecular dynamics simulations to predict nanoparticle properties and cellular uptake. We performed atomistic molecular dynamics simulations of 154 monolayer-protected gold nanoparticles and calculated a small library of simulation-derived descriptors that capture nanoparticle structural and chemical properties in aqueous solution. We then parametrized QNAR models using interpretable regression algorithms to predict experimental measurements of nanoparticle octanol-water partition coefficients, zeta potentials, and cellular uptake obtained from a curated database. These models reveal that simulation-derived descriptors can accurately predict experimental trends and provide physical insight into what descriptors are most important for obtaining desired nanoparticle properties or behaviors in biological environments. Finally, we demonstrate model generalizability by predicting cell uptake trends for 12 nanoparticles not included in the original data set. These results demonstrate that QNAR models parametrized with simulation-derived descriptors are accurate, generalizable computational tools that could be used to guide the design of monolayer-protected gold nanoparticles for biological applications without laborious trial-and-error experimentation.

Adsorption of Polymer-Tethered Particles on Solid Surfaces

We explore the behavior of polymer-tethered particles on solid surfaces using coarse-grained molecular dynamics simulations. Segment–segment, segment–core, and core–core interactions are assumed to be purely repulsive, while the segment–substrate interactions are attractive. We analyze changes in the internal structure of single hairy particles on the surfaces with the increasing strength of the segment–substrate interactions. For this purpose, we calculate the density profiles along the x, y, z axes and the mass dipole moments. The adsorbed hairy particles are found to be symmetrical in a plane parallel to the substrate but strongly asymmetric in the vertical direction. On stronger adsorbents, the particle canopies become flattened and the cores lie closer to the wall. We consider the adsorption of hairy nanoparticles dispersed in systems of different initial particle densities. We show how the strength of segment–substrate interactions affects the structure of the adsorbed phase, the particle–wall potential of the average force, the excess adsorption isotherms, and the real adsorption isotherms.

Adsorption on Ligand-Tethered Nanoparticles

We use coarse-grained molecular dynamics simulations to study adsorption on ligand-tethered particles. Nanoparticles with attached flexible and stiff ligands are considered. We discuss how the excess adsorption isotherm, the thickness of the polymer corona, and its morphology depend on the number of ligands, their length, the size of the core, and the interaction parameters. We investigate the adsorption-induced structural transitions of polymer coatings. The behavior of systems involving curved and flat "brushes" is compared.

The Interplay of Ligand Properties and Core Size Dictates the Hydrophobicity of Monolayer-Protected Gold Nanoparticles

The hydrophobicity of monolayer-protected gold nanoparticles is a crucial design parameter that influences self-assembly, preferential binding to proteins and membranes, and other nano-bio interactions. Predicting the effects of monolayer components on nanoparticle hydrophobicity is challenging due to the nonadditive, cooperative perturbations to interfacial water structure that dictate hydrophobicity at the nanoscale. In this work, we quantify nanoparticle hydrophobicity by using atomistic molecular dynamics simulations to calculate local hydration free energies at the nanoparticle-water interface. The simulations reveal that the hydrophobicity of large gold nanoparticles is determined primarily by ligand end group chemistry, as expected. However, for small gold nanoparticles, long alkanethiol ligands interact to form anisotropic bundles that lead to substantial spatial variations in hydrophobicity even for homogeneous monolayer compositions. We further show that nanoparticle hydrophobicity is modulated by changing the ligand structure, ligand chemistry, and gold core size, emphasizing that single-ligand properties alone are insufficient to characterize hydrophobicity. Finally, we illustrate that hydration free energy measurements correlate with the preferential binding of propane as a representative hydrophobic probe molecule. Together, these results show that both physical and chemical properties influence the hydrophobicity of small nanoparticles and must be considered together when predicting gold nanoparticle interactions with biomolecules.

Free Thiols Regulate the Interactions and Self-Assembly of Thiol-Passivated Metal Nanoparticles

Thiol ligands bound to the metallic core of nanoparticles determine their interactions with the environment and self-assembly. Recent studies suggest that equilibrium between bound and free thiols alters the ligand coverage of the core. Here, X-ray scattering and MD simulations investigate water-supported monolayers of gold-core nanoparticles as a function of the core-ligand coverage that is varied in experiments by adjusting the concentration of total thiols (sum of free and bound thiols). Simulations demonstrate that the presence of free thiols produces a nearly symmetrical coating of ligands on the core. X-ray measurements show that above a critical value of core-ligand coverage the nanoparticle core rises above the water surface, the edge-to-edge distance between neighboring nanoparticles increases, and the nanoparticle coverage of the surface decreases. These results demonstrate the important role of free thiols: they regulate the organization of bound thiols on the core and the interactions of nanoparticles with their surroundings.

Molecular dynamics simulations of mono-tethered particles at solid surfaces

We use molecular dynamics simulations to study the behavior of mono-tethered nanoparticles on solid surfaces. In our model particle-particle and particle-chain interactions are repulsive, while chain-chain interactions are attractive. Two surfaces are considered: the first one attracts particles and the other attracts chains. Excess adsorption isotherms are presented for both the surfaces and different lengths of tethers. The mechanism of adsorption is discussed. We find that depending on the assumed parameters the mono-tethered particles can be adsorbed as single particles or as different aggregates. Our main goal is to explore the structure of surface films. We show that the morphology of the adsorbed layer depends mainly on the type of the surface but the influence of the particle diameter, the chain length and the density is also important. We prove that the shape of aggregates changes near the substrate. For certain parameters the aggregates can break under the influence of the surface.

Self-assembly of hairy disks in two dimensions - insights from molecular simulations

We report the results of large scale molecular dynamics simulations conducted for sparsely grafted disks in two-dimensional systems. The main goal of this work is to show how the ligand mobility influences the self-assembly of particles decorated with short chains. We also analyze the impact of the chain length on the structure of dense phases. A crossover between the systems controlled by the core-core or by the segment-segment interactions is discussed. We prove that the ligand mobility determines the structure of the system. The particles with fixed tethers are found to order into different structures, an amorphous phase, hexagonal or honeycomb lattices, and a "spaghetti"-like phase containing single strings of cores, depending on the length of attached chains. The disks with mobile monomers assemble into a hexagonal structure, while the particles with longer mobile chains attached to them form a lamellar phase consisting of double strings of cores.

Size-dependent assembly of ligated gold nanocrystals in two dimensions

Brownian dynamics (BD) simulation based on a coarse-grained model is performed to study the assembly of dodecanethiol-ligated Au nanocrystals (NCs) at a toluene-air interface. With increasing diameter from 3 nm to 9 nm, the NCs are found to form three different phases-a dispersed phase without aggregation, a mixture of dispersed NCs and rounded clusters, and a compactly packed solid phase of the fractal structure. Such size dependence of the assembled phase is attributed to the variation in the well depth [Formula: see text] of the interaction potential between NCs, and the value of [Formula: see text] for 6 nm NCs turns out to be most suitable to form monolayers with hexagonal packing. The result is of universal importance for assembling complete monolayers, because the valley of the interaction potential can be well tailored by properly choosing the NC size, ligand length and solvent.

Understanding and Designing the Gold-Bio Interface: Insights from Simulations

Gold nanoparticles (AuNPs) are an integral part of many exciting and novel biomedical applications, sparking the urgent need for a thorough understanding of the physicochemical interactions occurring between these inorganic materials, their functional layers, and the biological species they interact with. Computational approaches are instrumental in providing the necessary molecular insight into the structural and dynamic behavior of the Au-bio interface with spatial and temporal resolutions not yet achievable in the laboratory, and are able to facilitate a rational approach to AuNP design for specific applications. A perspective of the current successes and challenges associated with the multiscale computational treatment of Au-bio interfacial systems, from electronic structure calculations to force field methods, is provided to illustrate the links between different approaches and their relationship to experiment and applications.

Reversible switching of structural and plasmonic properties of liquid-crystalline gold nanoparticle assemblies

Hybrid materials built of spherical gold nanoparticles with three different sizes covered with (pro)mesogenic molecules have been prepared. Small-angle X-ray diffraction studies showed that after thermal annealing most of the obtained materials formed long-range ordered assemblies. Variation of the (pro)mesogenic ligand architecture enabled us to achieve a switchable material, which could be reversibly reconfigured between 3D long-range ordered structures with tetragonal and face centred cubic symmetries. This structural reconfiguration induces changes to the plasmonic response of the material. This work demonstrates that it is possible to use LC-based self-assembling phenomena to prepare dynamic materials with structural properties important for the development of active plasmonic metamaterials.

Modeling the Assembly of Polymer-Grafted Nanoparticles at Oil-Water Interfaces

Using dissipative particle dynamics (DPD), I model the interfacial adsorption and self-assembly of polymer-grafted nanoparticles at a planar oil-water interface. The amphiphilic core-shell nanoparticles irreversibly adsorb to the interface and create a monolayer covering the interface. The polymer chains of the adsorbed nanoparticles are significantly deformed by surface tension to conform to the interface. I quantitatively characterize the properties of the particle-laden interface and the structure of the monolayer in detail at different surface coverages. I observe that the monolayer of particles grafted with long polymer chains undergoes an intriguing liquid-crystalline-amorphous phase transition in which the relationship between the monolayer structure and the surface tension/pressure of the interface is elucidated. Moreover, my results indicate that the amorphous state at high surface coverage is induced by the anisotropic distribution of the randomly grafted chains on each particle core, which leads to noncircular in-plane morphology formed under excluded volume effects. These studies provide a fundamental understanding of the interfacial behavior of polymer-grafted nanoparticles for achieving complete control of the adsorption and subsequent self-assembly.

Ligand structure and mechanical properties of single-nanoparticle-thick membranes

The high mechanical stiffness of single-nanoparticle-thick membranes is believed to result from the local structure of ligand coatings that mediate interactions between nanoparticles. These ligand structures are not directly observable experimentally. We use molecular dynamics simulations to observe variations in ligand structure and simultaneously measure variations in membrane mechanical properties. We have shown previously that ligand end group has a large impact on ligand structure and membrane mechanical properties. Here we introduce and apply quantitative molecular structure measures to these membranes and extend analysis to multiple nanoparticle core sizes and ligand lengths. Simulations of nanoparticle membranes with a nanoparticle core diameter of 4 or 6 nm, a ligand length of 11 or 17 methylenes, and either carboxyl (COOH) or methyl (CH(3)) ligand end groups are presented. In carboxyl-terminated ligand systems, structure and interactions are dominated by an end-to-end orientation of ligands. In methyl-terminated ligand systems large ordered ligand structures form, but nanoparticle interactions are dominated by disordered, partially interdigitated ligands. Core size and ligand length also affect both ligand arrangement within the membrane and the membrane's macroscopic mechanical response, but are secondary to the role of the ligand end group. Moreover, the particular end group (COOH or CH(3)) alters the nature of how ligand length, in turn, affects the membrane properties. The effect of core size does not depend on the ligand end group, with larger cores always leading to stiffer membranes. Asymmetry in the stress and ligand density is observed in membranes during preparation at a water-vapor interface, with the stress asymmetry persisting in all membranes after drying.

Thermodynamic insights into the self-assembly of capped nanoparticles using molecular dynamic simulations

Although the molecular modeling of self-assembling processes stands as a challenging research issue, there have been a number of breakthroughs in recent years. This report describes the use of large-scale molecular dynamics simulations with coarse grained models to study the spontaneous self-assembling of capped nanoparticles in chloroform suspension. A model system comprising 125 nanoparticles in chloroform evolved spontaneously from a regular array of independent nanoparticles to a single thread-like, ramified superstructure spanning the whole simulation box. The aggregation process proceeded by means of two complementary mechanisms, the first characterized by reactive collisions between monomers and oligomers, which were permanently trapped into the growing superstructure, and the second a slow structural reorganization of the nanoparticle packing. Altogether, these aggregation processes were over after ca. 0.6 μs and the system remained structurally and energetically stable until 1 μs. The thread-like structure closely resembles the TEM images of capped ZrO2, but a better comparison with experimental results was obtained by the deposition of the suspension over a graphene solid substrate, followed by the complete solvent evaporation. The agreement between the main structural features from this simulation and those from the TEM experiment was excellent and validated the model system. In order to shed further light on the origins of the stable aggregation of the nanoparticles, the Gibbs energy of aggregation was computed, along with its enthalpy and entropy contributions, both in chloroform and in a vacuum. The thermodynamic parameters arising from the modeling are consistent with larger nanoparticles in chloroform due to the solvent-swelled organic layer and the overall effect of the solvent was the partial destabilization of the aggregated state as compared to the vacuum system. The modeling strategy has been proved effective and reliable to describe the self-assembling of capped nanoparticles, but we must acknowledge the fact that larger model systems and longer timescales will be necessary in future investigations in order to assess structural and dynamical information approaching the behavior of macroscopic systems.

Mechanical properties of self-assembled nanoparticle membranes: stretching and bending

Monolayers composed of colloidal nanoparticles, with a thickness of less than ten nanometers, have remarkable mechanical strength and can suspend over micron-sized holes to form free-standing membranes. We discuss experiments probing the tensile strength and bending stiffness of these self-assembled nanoparticle sheets. The fracture behavior of monolayers and multilayers is investigated by attaching them to elastomer substrates which are then stretched. For different applied strain, the fracture patterns are imaged down to the scale of single particles. The resulting detailed information about the crack width distribution allows us to relate the measured overall tensile strength to the distribution of local bond strengths within a layer. We then introduce two methods by which freestanding nanoparticle monolayers can be rolled up into hollow, tubular “nano-scrolls”, either by electron beam irradiation during imaging with a scanning electron microscope or by spontaneous self-rolling. Indentation measurements on the nano-scrolls yield values for the bending stiffness that are significantly larger than expected from the response to stretching. The ability to stretch, bend, and roll up nanoparticle sheets offers new possibilities for a variety of applications, including sensors and mechanical transducers.

Temperature effects on nanostructure and mechanical properties of single-nanoparticle thick membranes

The properties of mechanically stable single-nanoparticle (NP)-thick membranes have largely been studied at room temperature. How these membranes soften as nanoparticle ligands disorder with increasing temperature is unknown. Molecular dynamics simulations are used to probe the temperature dependence of the mechanical and nanostructural properties of nanoparticle membranes made of 6 nm diameter Au nanoparticles coated with dodecanethiol ligands and terminated with either methyl (CH₃) or carboxyl (COOH) terminal groups. For methyl-terminated ligands, interactions along the alkane chain provide mechanical stiffness, with a Young's modulus of 1.7 GPa at 300 K. For carboxyl-terminated chains, end-group interactions are significant, producing stiffer membranes at all temperatures, with a Young's modulus of 3.8 GPa at 300 K. For both end-group types, membrane stiffness is reduced to zero at about 400 K. Ligand structure and mechanical properties of membranes at 300 K that have been annealed at 400 K are comparable to samples that do not undergo thermal annealing.

High strength, molecularly thin nanoparticle membranes

The unique strength observed in molecular thin films consisting of assemblies of nanoparticles encoded with short organic chains opens an intriguing new realm of controllable materials. Here the fundamental mechanisms underlying this unique mechanical strength are probed by molecular dynamics simulations. Using nanoparticles encoded with short hydrocarbon chains, we show that the mechanical response and failure of single nanoparticle thick membranes depend on subtle changes of the coating. Extremely high moduli were observed in agreement with experiment. We calculate Young's modulus for the membrane system based on properties of the individual components and find that ligand end-group interactions explain the observed changes in mechanical properties. Specifically, for dodecanethiol chains on 6 nm diameter gold cores, Young's modulus is 2.5 GPa for CH_{3} terminated chains and increases by 50% when end groups are replaced by COOH.

Effects of functional groups and ionization on the structure of alkanethiol-coated gold nanoparticles

We report classical atomistic molecular dynamics simulations of alkanethiol-coated gold nanoparticles solvated in water and decane, as well as at water/vapor interfaces. The structure of the coatings is analyzed as a function of various functional end groups, including amine and carboxyl groups in various ionization states. We study both neutral and charged end groups for two different chain lengths (9 and 17 carbons). For the charged end groups, we simulated both mono- and divalent counterions. For the longer alkanes, we find significant local bundling of chains on the nanoparticle surface, which results in highly asymmetric coatings. In general, the charged end groups attenuate this effect by enhancing the water solubility of the nanoparticles. On the basis of the coating structures and density profiles, we can qualitatively infer the overall solubility of the nanoparticles. This asymmetry in the alkanethiol coatings is likely to have a significant effect on aggregation behavior. Our simulations elucidate the mechanism by which modulating the end group charge state can be used to control coating structure and therefore nanoparticle solubility and aggregation behavior.