Three-body interaction of gold nanoparticles: the role of solvent density and ligand shell orientation.

Molecular dynamics simulations are used to study the effective interactions of alkanethiol passivated gold nanoparticles in supercritical ethane at two- and three-particle levels with different solvent densities. Effective interaction is calculated as the potential of mean force (PMF) between two nanoparticles, and the three-body effect is estimated as the difference in PMFs calculated at the two- and three-particle levels. The variation in the three-body effect is examined as a function of solvent density. It is found that effective interaction, which is completely repulsive at very high solvent concentrations, progressively turns attractive as solvent density declines. On the other hand, the three-body effect turns out to be repulsive and increases exponentially with decreasing solvent density. Further, the structure of the ligand shell is analyzed as a function of nanoparticle separation, and its relationship with the three-body effect is investigated. It is observed that the three-body effect arises when the ligand shell begins to deform due to van der Waals repulsion between ligand shells. The study provides a deep insight into good understanding of the solvent evaporation-assisted nanoparticle self-assembly and can aid in experiments.

Adsorption characteristics of peptides on ω-functionalized self-assembled monolayers: a molecular dynamics study.

Molecular dynamics simulations were employed to investigate the adsorption behavior of a variety of amino-acid side-chain analogs (SCAs) and a ß-hairpin (HP7) peptide on a series of liquid-like self-assembled monolayers (SAMs) with terminal functional groups of -OH, -OCH3, -CH3, and -CF3. The relationships between the adsorption free energy of the SCAs and the interfacial properties of water on the SAMs were examined to determine the acute predictors of protein adsorption on the SAM surfaces. The structural changes of HP7 on the SAM surfaces were also investigated to understand the relationship between the surface nature and protein denaturation. It was found that the adsorption free energy of the SCAs was linearly related to the surface hydrophobicity, which was computed as the free energy of cavity formation near the SAM-water interfaces. In addition, the hydrophobic -CH3 and -CF3 SAMs produced substantial conformational changes in HP7 because of the strong hydrophobic attractions to the nonpolar side chains. The hydrophilic surface terminated by -OH also promoted structural changes in HP7 resulting from the formation of hydrogen bonds between the hydrophilic tail and HP7. Consequently, the moderate amphiphilic surface terminated by -OCH3 avoided the denaturation of HP7 most efficiently, thus improving the biocompatibility of the surface. In conclusion, these results provide a deep understanding of protein adsorption for a wide range of polymeric surfaces, and they can potentially aid the design of appropriate biocompatible coatings for medical applications.

Hydrophobic association and solvation of neopentane in urea, TMAO and urea-TMAO solutions.

A detailed knowledge of hydrophobic association and solvation is crucial for understanding the con-formational stability of proteins and polymers in osmolyte solutions. Using molecular dynamics simulations, it is found that the hydrophobic association of neopentane molecules is greater in a mixed urea-TMAO-water solution in comparison to that in 8 M urea solution, 4 M TMAO solution and neat water. The neopentane association in urea solution is greater than that in TMAO solution or neat water. We find the association is even less in TMAO solution than pure water. From free energy calculations, it is revealed that the neopentane sized cavity creation in mixed urea-TMAO-water is most unfavorable and that causes the highest hydrophobic association. The cavity formation in urea solution is either more unfavorable or comparable to that in TMAO solution. Importantly, it is found that the population of neopentane-neopentane contact pair and the free energy contribution for the cavity formation step in TMAO solution are very sensitive towards the choice of TMAO force-fields. A careful construction of TMAO force-fields is important for studying the hydrophobic association. Interestingly it is observed that the total solute-solvent dispersion interaction energy contribution is always the most favorable in mixed urea-TMAO-water. The magnitude of this interaction energy is greater in urea solution relative to TMAO solution for two different force-fields of TMAO, whereas the lowest value is obtained in pure water. It is revealed that the extent of the overall hydrophobic association in osmolyte solutions is mainly governed by the cavity creation step and it nullifies the contribution coming from the solute-solvent interaction contribution.

Understanding the binary interactions of noble metal and semiconductor nanoparticles.

Molecular dynamics simulations are used to study the solvation and effective pair interactions of Au (1.2 nm) and CdSe (2.2 nm) nanoparticles passivated with alkanethiol and alkylamine ligands, respectively, for two different chain lengths in vacuum and n-hexane at 300 K. The solvation studies focus on quantifying the ligand and solvent shell structures, which are used to rationalize the interactions of nanoparticles in solution. To investigate the effective pair interactions, we compute the isotropic potential of mean forces (PMFs) between two nanoparticles and also analyze the anisotropy in the interactions that arises as a result of ligand shell fluctuations. Both isotropic and anisotropic contributions to the effective pair interactions between the two classes of nanoparticles are compared as a function of the ligand chain length and the solvent quality. It is demonstrated that the inclusion of the anisotropic aspect in the interparticle interactions is essential to properly describe the self-assembly thermodynamics of passivated nanoparticles. The implications of the coarse-grained modeling of the formation of binary nanocrystal superlattices (BNSLs) are considered.

Effects of Packing Density and Chain Length on the Surface Hydrophobicity of Thin Films Composed of Perfluoroalkyl Acrylate Chains: A Molecular Dynamics Study.

A good understanding of the surface hydrophobicity of fluorinated materials is useful for their application as coating materials. The present study investigates the surface hydrophobicity of perfluoroalkyl acrylate (PFA) thin films using molecular dynamics simulations. Surface hydrophobicity is characterized by examining the contact angle of a water droplet on PFA surfaces and the cavity formation free energy in the vicinity of the surface. It is found that the calculated microscopic contact angles are in good agreement with the experimental results and partially capture the difference in the hydrophobicity of the surface arising from the variation of packing density and side chain length of PFA. The variations of cavity formation free energy in the vicinity of the surface elucidate that the surface hydrophobicity is mainly governed by the packing density rather than the chain length of PFA. The hydrophobicity generally increases with decreasing the packing density to some extent and then turns to decrease as further reducing the packing density. At higher packing density, the surface hydrophobicity slightly decreases with increasing the chain length, while at the lower packing density, the surface hydrophobicity is increased when chain length of PFA is longer than six carbons. Furthermore, we found that the influence of packing density on the surface hydrophobicity is directly related to the variation of the surface roughness and chain flexibility, that is, the surface hydrophobicity increases with increase in the surface roughness, while the chain flexibility plays a secondary role in the enhancement by affecting the stability of water staying near the interface. The study provides a significant insight into the local hydrophobicity and microscopic structure of the PFA surfaces, which would be useful for the application of surface modification.

Thiolated gold nanoparticle solvation in near-critical fluids: The role of density, temperature, and topology.

We employ molecular dynamics simulations to study the structure and solvation thermodynamics of thiolated gold nanoparticles of size 1.2 and 1.6 nm with ligand of chain length 8-16 carbons in ethane and propane over a wide range of densities close to the critical isotherm. The Helmholtz free energy is estimated by explicitly calculating the change in entropy and internal energy of solvation, and the effect of density and temperature on fluctuation-driven inherent anisotropy in the ligand corona is characterized. Since the topological variation further accentuates this instantaneous asymmetry in the ligand cloud, the anisotropy with varying surface coverage and chain length is also studied including the solvent contributions to the entropic and energetic metrics. Our results are consistent with the experiment, suggesting a route of obtaining structural insights into solvation thermodynamics that could be useful for understanding the stability of nanoparticle dispersions.

Effective interactions between nanoparticles: Creating temperature-independent solvation environments for self-assembly.

The extent to which solvent-mediated effective interactions between nanoparticles can be predicted based on structure and associated thermodynamic estimators for bulk solvents and for solvation of single and pairs of nanoparticles is studied here. As a test of the approach, we analyse the strategy for creating temperature-independent solvent environments using a series of homologous chain fluids as solvents, as suggested by an experimental paper [M. I. Bodnarchuk et al., J. Am. Chem. Soc. 132, 11967 (2010)]. Our conclusions are based on molecular dynamics simulations of Au140(SC10H21)62 nanoparticles in n-alkane solvents, specifically hexane, octane, decane and dodecane, using the TraPPE-UA potential to model the alkanes and alkylthiols. The 140-atom gold core of the nanocrystal is held rigid in a truncated octahedral geometry and the gold-thiolate interaction is modeled using a Morse potential. The experimental observation was that the structural and rheological properties of n-alkane solvents are constant over a temperature range determined by equivalent solvent vapour pressures. We show that this is a consequence of the fact that long chain alkane liquids behave to a good approximation as simple liquids formed by packing of monomeric methyl/methylene units. Over the corresponding temperature range (233-361 K), the solvation environment is approximately constant at the single and pair nanoparticle levels under good solvent conditions. However, quantitative variations of the order of 10%-20% do exist in various quantities, such as molar volume of solute at infinite dilution, entropy of solvation, and onset distance for soft repulsions. In the opposite limit of a poor solvent, represented by vacuum in this study, the effective interactions between nanoparticles are no longer temperature-independent with attractive interactions increasing by up to 50% on decreasing the temperature from 361 K to 290 K, accompanied by an increase in emergent anisotropy due to correlation of mass dipoles on the two nanoparticles. One expects therefore that during self-assembly using solvent evaporation, temperature can be used as a structure-directing factor as long as good solvent conditions are maintained. It also suggests that disordered configurations may emerge as solvent quality decreases due to increasing role of short-range attractions and ligand fluctuation-driven anisotropy. The possibilities of using structural estimators of various thermodynamic quantities to analyse the interplay of ligand fluctuations and solvent quality in self-assembly as well as to design solvation environments are discussed.

Fluctuation-driven anisotropy in effective pair interactions between nanoparticles: thiolated gold nanoparticles in ethane.

Fluctuations within the ligand shell of a nanoparticle give rise to a significant degree of anisotropy in effective pair interactions for low grafting densities [B. Bozorgui, D. Meng, S. K. Kumar, C. Chakravarty, and A. Cacciuto, Nano Lett. 13, 2732 (2013)]. Here, we examine the corresponding fluctuation-driven anisotropy for gold nanocrystals densely passivated with short ligands. In particular, we consider gold nanocrystals capped by alkylthiols, both in vacuum and in ethane solvent at high density. As in the preceding study, we show that the anisotropy in the nanoparticle pair potential can be quantified by an angle-dependent correction term to the isotropic potential of mean force (PMF). We find that the anisotropy of the ligand shells is distance dependent, and strongly influenced by ligand interdigitation effects as well as expulsion of ligand chains from the interparticle region at short distances. Such fluctuation-driven anisotropy can be significant for alkylthiol-coated gold nanoparticles, specially for longer chain lengths, under good solvent conditions. The consequences of such anisotropy for self-assembly, specially as a function of grafting density, solvent quality and at interfaces, should provide some interesting insights in future work. Our results clearly show that an isotropic two-body PMF cannot adequately describe the thermodynamics and assembly behavior of nanoparticles in this dense grafting regime and inclusion of anisotropic effects, as well as possibly many-body interactions, is necessary. Extensions of this approach to other passivated nanoparticle systems and implications for self-assembly are considered.