Interacting nanoparticles with functional surface groups

Theoretical Modeling of Chemical Equilibrium in Weak Polyelectrolyte Layers on Curved Nanosystems

Surface functionalization with end-tethered weak polyelectrolytes (PE) is a versatile way to modify and control surface properties, given their ability to alter their degree of charge depending on external cues like pH and salt concentration. Weak PEs find usage in a wide range of applications, from colloidal stabilization, lubrication, adhesion, wetting to biomedical applications such as drug delivery and theranostics applications. They are also ubiquitous in many biological systems. Here, we present an overview of some of the main theoretical methods that we consider key in the field of weak PE at interfaces. Several applications involving engineered nanoparticles, synthetic and biological nanopores, as well as biological macromolecules are discussed to illustrate the salient features of systems involving weak PE near an interface or under (nano)confinement. The key feature is that by confining weak PEs near an interface the degree of charge is different from what would be expected in solution. This is the result of the strong coupling between structural organization of weak PE and its chemical state. The responsiveness of engineered and biological nanomaterials comprising weak PE combined with an adequate level of modeling can provide the keys to a rational design of smart nanosystems.

Electrostatics and Interactions of an Ionizable Silica Nanoparticle Approaching a Plasma Membrane

The surface charge of the plasma membrane (PM) and the large salt content of the extracellular space ensure a significant role of the electrostatic effect dictating the interaction between the PM and an approaching nanoparticle (NP). In this article, we theoretically study the case of an ionizable silica NP approaching the PM. We witness that the surface charge of the silica NP, dictated by the surface ionization of the silica in the electrostatic environment created by the PM surface charge and the extracellular ion concentration, decreases as it approaches the PM. In other words, a silica NP is more negative away from the PM than in close proximity to the PM. Accordingly, we witness a significantly lower repulsion between the PM and NP favoring the approach and the interactions of the silica NP with the PM. Additionally, the presence of the silica NP in the vicinity of the PM induces a large nonisopotentiality, even across a fully permeable PM. We anticipate that these findings will be critically important in the better design of the widely used silica NPs for targeted drug and gene deliveries.

Self-assembly and structural manipulation of diblock-copolymer grafted nanoparticles in a homopolymer matrix

Detailed coarse-grained molecular dynamics simulations are performed to investigate the structural and mechanical properties of nanoparticles (NPs) grafted with an amphiphilic AB diblock copolymer, with the A-block being compatible with NPs and the B-block being miscible with a homopolymer matrix.

Progress in the theory of electrostatic interactions between charged particles

In this perspective we examine recent theoretical developments in methods for calculating the electrostatic properties of charged particles of dielectric materials.

Interaction between functionalized gold nanoparticles in physiological saline

The interactions between functionalized noble-metal particles in an aqueous solution are central to applications relying on controlled equilibrium association. Herein, we obtain the potentials of mean force (PMF) for pair-interactions between functionalized gold nanoparticles (AuNPs) in physiological saline. These results are based upon >1000 ns experiments in silico of all-atom model systems under equilibrium and non-equilibrium conditions. Four types of functionalization are built by coating each globular Au144 cluster with 60 thiolate groups: GS-AuNP (glutathionate), PhS-AuNP (thiophenol), CyS-AuNP (cysteinyl), and p-APhS-AuNP (para-amino-thiophenol), which are, respectively, negatively charged, hydrophobic (neutral-nonpolar), hydrophilic (neutral-polar), and positively charged at neutral pH. The results confirm the behavior expected of neutral (hydrophilic or hydrophobic) particles in a dilute aqueous environment, however the PMF curves demonstrate that the charged AuNPs interact with one another in a unique way-mediated by H2O molecules and an electrolyte (Na(+), Cl(-))-in a physiological environment. In the case of two GS-AuNPs, the excess, neutralizing Na(+) ions form a mobile (or 'dynamic') cloud of enhanced concentration between the like-charged GS-AuNPs, inducing a moderate attraction (∼25 kT) between them. Furthermore, to a lesser degree, for a pair of p-APhS-AuNPs, the excess, neutralizing Cl(-) ions (less mobile than Na(+)) also form a cloud of higher concentration between the two like-charged p-APhS-AuNPs, inducing weaker yet significant attractions (∼12 kT). On combining one GS- with one p-APhS-AuNP, the direct, attractive Coulombic force is completely screened out while the solvation effects give rise to moderate repulsion between the two unlike-charged AuNPs.

Adsorption of acid and polymer coated nanoparticles: a statistical thermodynamics approach

A molecular theoretical description is developed to describe the adsorption of nanoparticles (NPs) that are coated with polymers and functionalized with (surface) acid groups. Results are presented for the adsorption onto both negatively and positively charged surfaces as a function of pH and salt concentration, polymer coating, and NP size. An important finding is that nanoparticles that are coated with weak charge regulating acid molecules such as citric acid develop an asymmetric charge distribution close to a charged surface, due to their finite size. Depending on the sign of the surface charge of the adsorbing surface, a nanoparticle close to the surface either gains more charge or loses charge compared to its "bulk" degree of charge. This in turn influences the amount of NPs that adsorb. The effect of adsorption of negatively charged NPs onto a positively charged surface shows a nonmonotonical variation with pH. The described charging mechanism reveals that details such as size of the NP and acid distribution on the NP need to be considered to provide an accurate understanding of the adsorption process.

Presentation of large DNA molecules for analysis as nanoconfined dumbbells

The analysis of very large DNA molecules intrinsically supports long-range, phased sequence information, but requires new approaches for their effective presentation as part of any genome analysis platform. Using a multi-pronged approach that marshaled molecular confinement, ionic environment, and DNA elastic properties-but tressed by molecular simulations-we have developed an efficient and scalable approach for presentation of large DNA molecules within nanoscale slits. Our approach relies on the formation of DNA dumbbells, where large segments of the molecules remain outside the nanoslits used to confine them. The low ionic environment, synergizing other features of our approach, enables DNA molecules to adopt a fully stretched conformation, comparable to the contour length, thereby facilitating analysis by optical microscopy. Accordingly, a molecular model is proposed to describe the conformation and dynamics of the DNA molecules within the nanoslits; a Langevin description of the polymer dynamics is adopted in which hydrodynamic effects are included through a Green's function formalism. Our simulations reveal that a delicate balance between electrostatic and hydrodynamic interactions is responsible for the observed molecular conformations. We demonstrate and further confirm that the "Odijk regime" does indeed start when the confinement dimensions size are of the same order of magnitude as the persistence length of the molecule. We also summarize current theories concerning dumbbell dynamics.

Ligand-mediated short-range attraction drives aggregation of charged monolayer-protected gold nanoparticles

Monolayer-protected gold nanoparticles (AuNPs) are a promising new class of nanomaterials with applications in drug delivery, self-assembly, and biosensing. The versatility of the AuNP platform is conferred by the properties of the protecting monolayer which can be engineered to tune the surface functionality of the nanoparticles. However, many applications are hampered by AuNP aggregation, which can inhibit functionality or induce particles to precipitate out of solution, even for water-soluble AuNPs. It is critical to understand the mechanisms of aggregation in order to optimally engineer protecting monolayers that both inhibit aggregation and maintain functionality. In this work, we use implicit solvent simulations to calculate the free energy change associated with the aggregation of two small, charged, alkanethiol monolayer-protected AuNPs under typical biological conditions. We show that aggregation is driven by the hydrophobic effect related to the amphiphilic nature of the alkanethiol ligands. The critical factor that enables aggregation is the deformation of ligands in the monolayer to shield hydrophobic surface area from water upon close association of the two particles. Our results further show that ligand deformation, and thus aggregation, is highly dependent on the size of the AuNPs, choice of ligands, and environmental conditions. This work provides insight into the key role that ligand-ligand interactions play in stabilizing AuNP aggregates and suggests guidelines for the design of protecting monolayers that inhibit aggregation under typical biological conditions.

Conformational transitions of weak polyacids grafted to nanoparticles

The charge distribution on polyelectrolytes is a key factor, which controls their conformation and interactions. In weak polyelectrolytes, this distribution is determined by a number of factors, including the solvent conditions and local environment. In this work, we investigate charge distributions of chains end-grafted on a spherical nanoparticle in a salt solution, using grand canonical titration Monte Carlo simulations of a coarse-grained polymer model. In this approach, the ionization state of each polymer bead fluctuates based on the dissociation constant, pH of the solution, and interactions with other particles in the system. We determine charge and polymer conformations as functions of the pH and solvent quality. We compare the results to a fixed charge model and also investigate the role of grafting density and the effect of curvature on the film morphologies.