Effect of nanoparticle polydispersity on the self-assembly of polymer tethered nanospheres

Antibiofilm potential of nanonized eugenol against Pseudomonas aeruginosa

AIMS

The purpose of this study was to synthesize a nanoform of eugenol (an important phytochemical with various pharmacological potentials) and to investigate its antibiofilm efficacy on Pseudomonas aeruginosa biofilm.

METHODS AND RESULTS

Colloidal suspension of eugenol-nanoparticles (ENPs) was synthesized by the simple ultrasonic cavitation method through the emulsification of hydrophobic eugenol into hydrophilic gelatin. Thus, the nanonization process made water-insoluble eugenol into water-soluble nano-eugenol, making the nanoform bioavailable. The size of the ENPs was 20-30 nm, entrapment efficiency of eugenol within gelatin was 80%, and release of eugenol from the gelatin cap was slow and sustained over 5 days. Concerning the clinically relevant pathogen P. aeruginosa, ENPs had higher antibiofilm (for both formation and eradication) activities than free eugenol. Minimal biofilm inhibitory concentration and minimal biofilm eradication concentration of ENP on P. aeruginosa biofilm were 2.0 and 4.0 mM, respectively. In addition, the measurement of P. aeruginosa biofilm biomass, biofilm thickness, amount of biofilm extra-polymeric substance, cell surface hydrophobicity, cell swarming and twitching efficiencies, cellular morphology, and biofilm formation in catheter demonstrated that the antibiofilm efficacy of nano-eugenol was 30%-40% higher than that of bulk eugenol.

CONCLUSION

These results signify that future pharmacological and clinical studies are very much required to investigate whether ENPs can act as an effective drug against P. aeruginosa biofilm-mediated diseases. Thus, the problem of intrinsic antibiotic tolerance of biofilm-forming cells may be minimized by ENPs. Moreover, ENP may be used as a potential catheter-coating agent to inhibit pseudomonal colonization on catheter surfaces and, therefore, to reduce catheter-associated infections and complications.

Even Strong Energy Polydispersity Does Not Affect the Average Structure and Dynamics of Simple Liquids

Size-polydisperse liquids have become standard models for avoiding crystallization, thereby enabling studies of supercooled liquids and glasses formed, e.g., by colloidal systems. Purely energy-polydisperse liquids have been studied much less, but provide an interesting alternative. We here study numerically the difference in structure and dynamics obtained by introducing these two kinds of polydispersity into systems of particles interacting via the Lennard-Jones and EXP pair potentials. To a very good approximation, the average pair structure and dynamics are unchanged even for strong energy polydispersity, which is not the case for size-polydisperse systems. When the system at extreme energy polydispersity undergoes a continuous phase separation into lower and higher particle-energy regions whose structure and dynamics are different from the average, the average structure and dynamics are still virtually the same as for the monodisperse system. Our findings are consistent with the fact that the distribution of forces on the individual particles do not change when energy polydispersity is introduced, while they do change in the case of size polydispersity. A theoretical explanation remains to be found, however.

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.

Size-encoded hierarchical self-assembly of nanoparticles into chains and tubules

Hierarchical structures with sophisticated patterns allow the emergence of challenging properties. However, the highly cooperative and specific interactions needed for assembly spanning different length scales are typically lacking in inorganic nanoparticles (NPs). Here we show that size can be a common structural driving force for controlling hierarchical assembly of inorganic NPs into anisotropic superstructures. It involves first the self-limiting assembly of small CdS NPs into large supraparticles and their subsequent spontaneous organization into chains and tubules hundreds of nanometers long. Our quantitative calculations based on DLVO theory reveals an intrinsic size effect relating to the dimension change of assembly units in accordance with a negative cooperativity. It is shown that the size increase in building blocks creates an effective kinetic barrier contrast at different attachment sites due to the increase of interparticle electrostatic repulsion, switching the assembly from thermodynamically preferred 3D to kinetically favored 1D pathway. The size-encoded hierarchical assembly is accompanied by the ligand-controlled Oswald ripening process, which is responsible for the variation of hierarchical patterns from chains to tubules. The general principle in governing multistage inorganic NP ordering represents an important guideline toward the complex mesoscale structures that may surpass traditional materials in both design and functionality.

DNA-mediated hierarchical organization of gold nanoprisms into 3D aggregates and their application in surface-enhanced Raman scattering

Colloidal crystallization using DNA provides a robust method for fabricating highly programmable nanoparticle superstructures with collective plasmonic properties. Here, we report on the DNA-guided fabrication of 3D plasmonic aggregates from polydisperse gold nanoprisms. We first construct 1D crystals via DNA-induced and shape-directed face-to-face assembly of anisotropic gold nanoprisms. Using the near-T_m thermal annealing approach that promotes long-range DNA-induced interaction and ordering, we then assemble 1D nanoprism crystals into a 3D nanoprism aggregate that exhibits a polycrystalline morphology with nanoscale ordering and microscale dimensions. The presence of closely packed nanoprism arrays over a large area gives rise to strong near-field plasmonic coupling and generates a high density of plasmonic hot spots within the 3D nanoprism aggregates that exhibit excellent surface-enhanced Raman scattering performance. The plasmonic 3D nanoprism aggregates demonstrate significant SERS enhancement (<10⁶), and low detection limits (10^-9M) with good sample-to-sample reproducibility (CV ∼ only 5.6%) for SERS analysis of the probe molecule, methylene blue. These findings highlight the potential of 3D anisotropic nanoparticle aggregates as functional plasmonic nanoarchitectures that could find applications in sensing, photonics, optoelectronics and lasing.

Self-Assembly of Single-Polymer-Tethered Nanoparticle Amphiphiles upon Varying Tail Length

We systematically investigated the roles of tail length on the self-assembly of shape amphiphiles composed of a hydrophobic polymer chain (tail) and a hydrophilic nanoparticle in selective solvent using Brownian dynamics simulations. The shape amphiphiles exhibited a variety of self-assembled aggregate morphologies which can be tuned by changing tail length (n) in combination with amphiphile concentration (φ) and system temperature (T*). Specifically, at high φ with T*=1.4, the morphology varied following the sequence "spheres → cylinders → vesicles" upon increasing n, agreeing well with experimental observations. At low φ with T*=1.4 or at high φ with T*=1.2, the morphology sequence becomes "spheres or spheres and cylinders mixture → cylinders → vesicles → spheres" upon increasing n, which has not been found experimentally. Two morphological phase diagrams depending on n and φ were constructed for T*=1.4 and 1.2, respectively. The rich phase behaviors on varying tail length could provide the feasible routes to fabricate target aggregate morphologies in various applications, especially for the vesicles with tunable thickness of membranes that are crucial in drug and gene delivery.

Tunable Assembly of Block Copolymer Tethered Particle Brushes by Surface-Initiated Atom Transfer Radical Polymerization

A strategy to synthesize SiO₂-g-PMMA/PMMA-b-PS mono- and bimodal block copolymer particle brushes by surface-initiated atom transfer radical polymerization (SI-ATRP) from silica particles is presented. First, PMMA blocks were prepared by normal ATRP with controlled degree of polymerizations and grafting density. In a second step, the PS block was synthesized through a chain extension using low parts per million of Cu catalyst. Variation of the SiO₂-g-PMMA-Br macroinitiator concentration had a pronounced effect on the modality of the chain extension product. In the limit of small concentration, partial termination resulted in bimodal brush architectures, while more uniform brush architectures were observed with increasing concentration of macroinitiator. Brush nanoparticles with bimodal architectures assembled into string-like aggregates that bore a resemblance to structures found in systems comprised of sparse (homopolymer) brush particles. The unexpected effect of modality on structure formation points to opportunities in controlling microstructures in brush particle materials.

Orientational phase behavior of polymer-grafted nanocubes

Surface functionalization of nanoparticles with polymer grafts was recently shown to be a viable strategy for controlling the relative orientation of shaped nanoparticles in their higher-order assemblies. In this study, we investigated in silico the orientational phase behavior of coplanar polymer-grafted nanocubes confined in a thin film. We first used Monte Carlo simulations to compute the two-particle interaction free-energy landscape of the nanocubes and identify their globally stable configurations. The nanocubes were found to exhibit four stable phases: those with edge-edge and face-face orientations, and those exhibiting partially overlapped slanted and parallel faces previously assumed to be metastable. Moreover, the edge-edge configuration originally thought to involve kissing edges instead displayed partly overlapping edges, where the extent of the overlap depends on the attachment positions of the grafts. We next formulated analytical scaling expressions for the free energies of the identified configurations, which were used for constructing a comprehensive phase diagram of nanocube orientation in a multidimensional parameter space comprising of the size and interaction strength of the nanocubes and the Kuhn length and surface density of the grafts. The morphology of the phase diagram was shown to arise from an interplay between polymer- and surface-mediated interactions, especially differences in their scalings with respect to nanocube size and grafting density across the four phases. The phase diagram provided insights into tuning these interactions through the various parameters of the system for achieving target configurations. Overall, this work provides a framework for predicting and engineering interparticle configurations, with possible applications in plasmonic nanocomposites where control over particle orientation is critical.

Assembling Ordered Crystals with Disperse Building Blocks

Conventional colloidal crystallization techniques typically require low dispersity building blocks in order to make ordered particle arrays, resulting in a practical challenge for studying or scaling these materials. Nanoparticles covered in a polymer brush therefore may be predicted to be challenging building blocks in the formation of high-quality particle superlattices, as both the nanoparticle core and polymer brush are independent sources of dispersity in the system. However, when supramolecular bonding between complementary functional groups at the ends of the polymer chains are used to drive particle assembly, these "nanocomposite tectons" can make high quality superlattices with polymer dispersities as large as 1.44 and particle diameter relative standard deviations up to 23% without any significant change to superlattice crystallinity. Here we demonstrate and explain how the flexible and dynamic nature of the polymer chains that comprise the particle brush allows them to deform to accommodate the irregularities in building block size and shape that arise from the inherent dispersity of their constituent components. Incorporating "soft" components into nanomaterials design therefore offers a facile and robust method for maintaining good control over organization when the materials themselves are imperfect.

Hierarchical self-assembly of hard cube derivatives

Hierarchical, self-assembled structures are ordered on multiple scales, and formed by objects comprised of even smaller elements. Such structures are widely reported for nanoparticles, macromolecules, and peptides, and even in entropy-driven hard particle assembly hierarchical colloidal crystals have been reported. Here we consider the hierarchical self-assembly of a cubic colloidal crystal from congruent hard cube derivatives, and investigate how various ways of slicing and dicing a cube can affect the ability of the pieces to entropically re-assemble the initial colloidal crystal formed from perfect cubes. We present design rules that support heuristics reported for different systems, and present evidence for a previously unreported cubatic phase from 2 : 1 rectangular prisms.

Tolerance in superstructures formed from high-quality colloidal ZnO nanoparticles with hexagonal cross-section

The order of periodic arrays of hexagonal ZnO nanoplates has been investigated in terms of polydispersity. A continuous transition from a crystalline to a glassy state has been found.

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.

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.

Soft self-assembly of Weyl materials for light and sound

Soft materials can self-assemble into highly structured phases that replicate at the mesoscopic scale the symmetry of atomic crystals. As such, they offer an unparalleled platform to design mesostructured materials for light and sound. Here, we present a bottom-up approach based on self-assembly to engineer 3D photonic and phononic crystals with topologically protected Weyl points. In addition to angular and frequency selectivity of their bulk optical response, Weyl materials are endowed with topological surface states, which allow for the existence of one-way channels, even in the presence of time-reversal invariance. Using a combination of group-theoretical methods and numerical simulations, we identify the general symmetry constraints that a self-assembled structure has to satisfy to host Weyl points and describe how to achieve such constraints using a symmetry-driven pipeline for self-assembled material design and discovery. We illustrate our general approach using block copolymer self-assembly as a model system.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Thermoswitchable Janus Gold Nanoparticles with Stimuli-Responsive Hydrophilic Polymer Brushes

Well-defined thermoswitchable Janus gold nanoparticles with stimuli-responsive hydrophilic polymer brushes were fabricated by combining ligand exchange reactions and the Langmuir technique. Stimuli-responsive polydi(ethylene glycol) methyl ether methacrylate was prepared by addition-fragmentation chain-transfer polymerization. The polymer brushes were then anchored onto the nanoparticle surface by interfacial ligand exchange reactions with hexanethiolate-protected gold nanoparticles, leading to the formation of a hydrophilic (polymer) hemisphere and a hydrophobic (hexanethiolate) one. The resulting Janus nanoparticles showed temperature-switchable wettability, hydrophobicity at high temperatures, and hydrophilicity at low temperatures, due to thermally induced conformational transition of the polymer ligands. The results further highlight the importance of interfacial engineering in the deliberate functionalization of nanoparticle materials.

Nonequilibrium structure in sequential assembly

The assembly of monomeric constituents into molecular superstructures through sequential-arrival processes has been simulated and theoretically characterized. When the energetic interactions allow for complete overlap of the particles, the model is equivalent to that of the sequential absorption of soft particles on a surface. In the present work, we consider more general cases by including arbitrary aggregating geometries and varying prescriptions of the connectivity network. The resulting theory accounts for the evolution and final-state configurations through a system of equations governing structural generation. We find that particle geometries differ significantly from those in equilibrium. In particular, variations of structural rigidity and morphology tune particle energetics and result in significant variation in the nonequilibrium distributions of the assembly in comparison to the corresponding equilibrium case.

Effect of Size Polydispersity on the Nature of Lennard-Jones Liquids

Polydisperse fluids are encountered everywhere in biological and industrial processes. These fluids naturally show a rich phenomenology exhibiting fractionation and shifts in critical point and freezing temperatures. We study here the effect of size polydispersity on the basic nature of Lennard-Jones (LJ) liquids, which represent most molecular liquids without hydrogen bonds, via two- and three-dimensional molecular dynamics computer simulations. A single-component liquid constituting spherical particles and interacting via the LJ potential is known to exhibit strong correlations between virial and potential energy equilibrium fluctuations at constant volume. This correlation significantly simplifies the physical description of the liquid, and these liquids are now known as Roskilde-simple (RS) liquids. We show that this simple nature of the single-component LJ liquid is preserved even for very high polydispersities (above 40% polydispersity for the studied uniform distribution). We also investigate isomorphs of moderately polydisperse LJ liquids. Isomorphs are curves in the phase diagram of RS liquids along which structure, dynamics, and some thermodynamic quantities are invariant in dimensionless units. We find that isomorphs are a good approximation even for polydisperse LJ liquids. The theory of isomorphs thus extends readily to size polydisperse fluids and can be used to improve even further the understanding of these intriguing systems.

Controlling the localization of nanoparticles in assemblies of amphiphilic diblock copolymers

We performed a dissipative particle dynamics (DPD) approach to study the self-assembly of AB diblock copolymer tethered nanoparticles (P) in dilute solutions. Different morphological aggregates, including spherical micelles, vesicles, disk-like micelles and rod-like micelles, were found by varying the interaction between block copolymers and nanoparticles. Most importantly, the nanoparticles can selectively localize in the different domains within the aggregates. When the repulsive interaction between block copolymers and nanoparticles aPA = aPB = 25, the nanoparticles are evenly distributed within the spherical micelles. While aPA or aPB increases, the nanoparticles gradually aggregate and separate from copolymers and then localize in the central portion of vesicular wall or disk-like and rod-like micelles. The degree of stretching of the tethered copolymer chains gradually grows with the increase of aPA or aPB, while the degree of stretching of solvophobic block B decreases when the morphologies change from spherical to disk-like micelles and further to rod-like micelles. This work illustrates that tuning the miscibility of copolymers and nanoparticles could be used to project the selective localization of nanoparticles within the aggregates self-assembled by diblock copolymer tethered nanoparticles in dilute solutions.

Multiscale active layer morphologies for organic photovoltaics through self-assembly of nanospheres

We address here the need for a general strategy to control molecular assembly over multiple length scales. Efficient organic photovoltaics require an active layer comprised of a mesoscale interconnected networks of nanoscale aggregates of semiconductors. We demonstrate a method, using principles of molecular self-assembly and geometric packing, for controlled assembly of semiconductors at the nanoscale and mesoscale. Nanoparticles of poly(3-hexylthiophene) (P3HT) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with targeted sizes. Nanoparticles containing a blend of both P3HT and PCBM were also fabricated. The active layer morphology was tuned by the changing particle composition, particle radii, and the ratios of P3HT:PCBM particles. Photovoltaic devices were fabricated from these aqueous nanoparticle dispersions with comparable device performance to typical bulk-heterojunction devices. Our strategy opens a revolutionary pathway to study and tune the active layer morphology systematically while exercising control of the component assembly at multiple length scales.

Assembly of responsive-shape coated nanoparticles at water surfaces

Nanoparticle (NP) assembly and aggregation can be controlled using a variety of organic coatings that bind to the nanoparticle surface and alter its affinity for solvent and other particles. We show that surprisingly simple short chain polymer coatings can be effectively used to selectively control the aggregation of very small nanoparticles by taking advantage of the environment-responsive shape produced by the coating's spontaneous asymmetry on high-curvature nanoparticles. Using extremely long molecular dynamics simulations of alkanethiol coated Au nanoparticles, we show that varying the terminal groups of a nanoparticle coating dramatically alters the coating shape at the water liquid-vapor interface, producing very different assembly morphologies. NPs with CH3-terminated coatings assemble into short linear groupings with a highly aligned structure at early time and then form more disordered clusters as these linear groupings further assemble. NPs with COOH-terminated coatings assemble into dimers and disordered clumps with no preferred alignment at short time and longer disordered chains of particles at longer times. We also find that the responsive shape of the coating continues to adapt to local environment during assembly. The orientations of chains within NP coatings are significantly different when the NPs are arranged in aggregates than when they are isolated.

Nanoparticles adsorbed at the water/oil interface: coverage and composition effects on structure and diffusion

Dissipative particle dynamics simulations are performed to study the structural and dynamical properties of various systems of nanoparticles accumulated at the water/oil interface. Homogeneous and Janus nanoparticles with different surface compositions are studied. For all nanoparticles, as the surface density increases, a transition from a liquidlike to a solidlike state is observed, as expected. At a high density of nanoparticles, hexagonal structures emerge and the nanoparticles' self-diffusion coefficient decreases because of caging effects. Similar results are observed for nanoparticles with different surface chemistry. Because different nanoparticles have different contact angles at the water/oil interface, the results obtained for systems containing mixed nanoparticles are more interesting. For example, our results show that the self-diffusion coefficient is not a monotonic function of the system composition, caused by the complex relation between hydrodynamic interactions and effective nanoparticle-nanoparticle interactions.