Janus Particles: Synthesis, Self-Assembly, Physical Properties, and Applications

Rational design and synthesis of Janus composites

Janus composites with two different components divided on the same object have gained growing interest in many fields, such as solid emulsion stabilizers, sensors, optical probes and self-propellers. Over the past twenty years, various synthesis methods have been developed including Pickering emulsion interfacial modification, block copolymer self-assembly, microfluidics, electro co-jetting, and swelling emulsion polymerization. Anisotropic shape and asymmetric spatial distribution of compositions and functionalities determine their unique performances. Rational design and large scale synthesis of functional Janus materials are crucial for the systematical characterization of performance and exploitation of practical applications.

The impact of Janus nanoparticles on the compatibilization of immiscible polymer blends under technologically relevant conditions

Several hundred grams of Janus nanoparticles (d ≈ 40 nm) were synthesized from triblock terpolymers as compatibilizers for blending of technologically relevant polymers, PPE and SAN, on industry-scale extruders. The Janus nanoparticles (JPs) demonstrate superior compatibilization capabilities compared to the corresponding triblock terpolymer, attributed to the combined intrinsic properties, amphiphilicity and the Pickering effect. Straightforward mixing and extrusion protocols yield multiscale blend morphologies with "raspberry-like" structures of JPs-covered PPE phases in a SAN matrix. The JPs densely pack at the blend interface providing the necessary steric repulsion to suppress droplet coagulation during processing. We determine the efficiency of JP-compatibilization by droplet size evaluation and find the smallest average droplet size of d ≈ 300 nm at 10 wt % of added compatibilizer, whereas at 2 wt %, use of JPs is most economic with reasonable small droplets and narrow dispersity. In case of excess JPs, rheological properties of the system is changed by a droplet network formation. The large-scale synthesis of JPs, the low required weight fractions and their exceptional stability against extensive shear and temperature profiles during industrial extrusion process make JP promising next generation compatibilizers.

Double-phase-functionalized magnetic Janus polymer microparticles containing TiO2 and Fe2O3 nanoparticles encapsulated in mussel-inspired amphiphilic polymers

Recently, anisotropic colloidal polymeric materials including Janus microparticles, which have two distinct aspects on their surfaces or interiors, have garnered much interest due to their anisotropic alignment and rotational orientation with respect to external electric or magnetic fields. Janus microparticles are also good candidates for pigments in "twisting ball type" electronic paper, which is considered promising for next-generation flexible display devices. We demonstrate here a universal strategy to encapsulate inorganic nanoparticles and to introduce different such inorganic nanoparticles into distinct polymer phases in Janus microparticles. TiO2 and Fe2O3 nanoparticles were separately encapsulated in two different mussel-inspired amphiphilic copolymers, and then organic-inorganic composite Janus microparticles were prepared by simple evaporation of solvent from the dispersion containing the polymer and nanoparticle. These Janus microparticles were observed to rotate quickly in response to applied magnetic fields.

Reversible Switching of Liquid Crystalline Order Permits Synthesis of Homogeneous Populations of Dipolar Patchy Microparticles

The spontaneous positioning of colloids on the surfaces of micrometer-sized liquid crystalline droplets and their subsequent polymerization offers the basis of a general and facile method for the synthesis of patchy microparticles. The existence of multiple local energetic minima, however, can generate kinetic traps for colloids on the surfaces of the liquid crystal (LC) droplets and result in heterogeneous populations of patchy microparticles. To address this issue, here we demonstrate that adsorbate-driven switching of the internal configurations of LC droplets can be used to sweep colloids to a single location on the LC droplet surfaces, thus resulting in the synthesis of homogeneous populations of patchy microparticles. The surface-driven switching of the LC can be triggered by addition of surfactant or salts, and permits the synthesis of dipolar microparticles as well as "Janus-like" microparticles. By using magnetic colloids, we illustrate the utility of the approach by synthesizing magnetically-responsive patchy microdroplets of LC with either dipolar or quadrupolar symmetry that exhibit distinct optical responses upon application of an external magnetic field.

Morphologies of Bottle-Brush Block Copolymers

We investigate the self-assembly of bottle-brush block copolymers into well-defined periodic morphologies by using molecular dynamics simulations of a bead-spring model. The microphase separation is driven by the chemical incompatibility between the different blocks of side chains leading to the formation of two- and three-domain lamellae and hexagonally packed cylinders. The molecular asymmetry required for the formation of cylindrical domains is not introduced by the difference in volume fractions, but by the asymmetry of the side chain lengths. Such behavior deviates significantly from what is typically known for linear block copolymers. The obtained morphology maps provide a genuine way of understanding the role of molecular architecture in achieving experimentally desired structures for nanoporous and photonic materials based on the self-assembly of bottle-brush block copolymers.

Orientational order of one-patch colloidal particles in two dimensions

We studied the orientational order of one-patch colloidal particles (Janus particles) in a close-packed monolayer. In an experiment on hemispherically patched particles, we realized a highly ordered zigzag stripe pattern by inducing directional growth of the pattern via a phase transition of the solvent. Upon spontaneous ordering by strengthening the inter-patch attraction, however, the particles are trapped in a poorly ordered zigzag pattern, illustrating the importance of controlling kinetics to attain a highly ordered state. The patch-size dependence of an equilibrium orientational order is experimentally observed under moderate inter-patch attraction. We also calculated the equilibrium order against the patch size and attraction in a Monte Carlo simulation. In the simulation, the rather discrete transition between a zigzag stripe, tiling of triangular trimers and tiling of dimers under strong attraction becomes continuous with weakening attraction. The experimental result not only coincides with the simulation qualitatively but also suggests that a particular cluster is selectively formed by nonuniform inter-patch attraction in the experiment. The effect of patch-substrate attraction and commonalities of the order with liquid crystals are also discussed.

Molecular dynamics simulations: insight into molecular phenomena at interfaces

Molecular dynamics simulations, when aptly devised, can enhance our fundamental understanding of a system, set up a platform for testing theoretical predictions, and provide insight and a framework for further experimental studies. This feature article highlights the importance of molecular dynamics simulations in understanding interfacial phenomena using three case studies involving liquid-liquid and solid-liquid interfaces. After briefly reviewing molecular dynamics methods, we discuss velocity slip at a liquid-liquid interface, the coalescence of liquid drops in suspension and in free space, and the behavior of colloidal nanoparticles at a liquid-liquid interface. We emphasize the utility of simple intermolecular potentials and generic liquids. The case studies exemplify the significant insight gained through the molecular modeling approach regarding the interfacial phenomena studied. We conclude the highlight with a brief discussion illustrating potential shortcomings and pitfalls of molecular dynamics simulations.

Self-assembly of amido-ended hyperbranched polyester films with a highly ordered dendritic structure

Self-assemblies fabricated from dendrimers and amphiphilic polymers have demonstrated remarkable performances and a wide range of applications. Direct self-assembly of hyperbranched polymers into highly ordered macrostructures with heat-resistance remains a big challenge due to the weak amphiphilicity of the polymers. Here, we report the self-assembly of amphiphilic amido-ended hyperbranched polyester (HTDA-2) into millimeter-size dendritic films using combined hydrogen bond interaction and solvent induction. The self-assembly process and mechanism have been studied. Hydrogen bond interaction between amido-ended groups assists the aggregation of inner and outer chains of the HTDA-2, resulting in phase separation and micelle formation. Some micelles attach to and grow on the glass substrate like seedlings. Other micelles move to the seedlings and connect with their branches via solvent induction and hydrogen bond interaction, leading to the fabrication of highly ordered crystalline dendritic films that show high heat-resistance. HTDA-2 can further self-assemble into sheet crystals on the dendritic films.

Form factor of asymmetric elongated micelles: playing with Russian dolls has never been so informative

Scattering techniques (i.e., static light scattering, small angle neutron scattering,11 or small angle X-ray scattering) are excellent tools to study nanoscopic objects in solution. However, to interpret the experimental data, one needs to use the appropriate form factor. While recent progress has been made in the writing of form factors for complex structures, there is still a need to develop a method to evaluate the form factor of inhomogeneous elongated scatterers. Here, we propose an approach based on the principle of "Russian dolls". Multiblock rods are represented as multi generations of rods (mother, daughter, granddaughter, etc.), where each rod is nested within the rod of the previous generation, like Russian dolls. A shift parameter is used to introduce asymmetry in the rod along its long axis. This approach not only allowed us to write the form factor of multiblock rods, but it also gave us the possibility to account for the polydispersity in length of each block and of the shift parameter. Finally, we applied these equations to the case of a series of solutions of triblock comicelles slightly polydisperse in length.

Insertion and confinement of hydrophobic metallic powder in water: the bubble-marble effect

Metallic powders such as thermite are known as efficient fuels also applicable in oxygen-free environments. However, due to their hydrophobicity, they hardly penetrate into water. This paper presents an effect that enables the insertion and confinement of hydrophobic metallic powders in water, based on encapsulating an air bubble surrounded by a hydrophobic metallic shell. This effect, regarded as an inverse of the known liquid-marble effect, is named here "bubble marble" (BM). The sole BM is demonstrated experimentally as a stable, maneuverable, and controllable soft-solid-like structure, in a slightly deformed hollow spherical shape of ∼1-cm diameter. In addition to experimental and theoretical BM aspects, this paper also demonstrates its potential for underwater applications, such as transportation of solid objects within BM and underwater combustion of thermite BM by localized microwaves. Hence, the BM phenomena may open new possibilities for heat and thrust generation, as well as material processing and mass transfer underwater.

Soft Janus particles: ideal building blocks for template-free fabrication of two-dimensional exotic nanostructures

The design and fabrication of two-dimensional (2D) well-ordered nanostructures by a facile and effective strategy remain a major scientific and technological challenge, hitherto achieved mainly through the aid of interfaces or substrates with an ordered arrangement. Here we introduce a new concept in achieving template-free fabrication of diverse 2D ordered nanostructures by utilizing anisotropic characteristics of soft triblock Janus particles. Our numerical investigation demonstrates how particle softness and controllable directional attraction interplay to generate a number of fascinating non-close-packed 2D nanostructures and even three-dimensional (3D) vesicles. These non-close-packed nanostructures are of great interest for scientific reasons and lead to promising applications in soft nanotechnology and biotechnology.

Sonication-induced formation of size-controlled self-assemblies of amphiphilic Janus-type polymers as optical tumor-imaging agents

In this study, amphiphilic Janus-type polymers were synthesized via ring-opening metathesis polymerization (ROMP), multiple vicinal diol formation, and grafting of poly(ethylene glycol) monomethyl ether (mPEG). These amphiphilic polymers formed self-assemblies, which were a mixture of micelles and multimicellar aggregates, in water. By choosing suitable Janus-type polymers and irradiating an aqueous solution of polymers using a sonicator, either small micelles or large multimicellar aggregates were obtained selectively. Hydrophobic substituents controlled the aggregation-disaggregation behavior, leading to the formation of metastable self-assemblies by sonication. The formation of self-assemblies with a uniform size was affected by ultrasonic frequency, rather than power. In vivo optical tumor imaging revealed that the large-size multimicellar aggregates persisting for a long time in blood circulation slowly accumulated in tumor tissues. In contrast, the tumor site was rapidly, clearly visualized using the small-size micelles.

Platelet Janus particles with hairy polymer shells for multifunctional materials

A novel approach is developed for the large-scale synthesis of Janus particles with platelet geometry and dense polymer shells by employing simultaneous "grafting from" of hydrophilic and hydrophobic polymers using surface-induced ATRP in emulsion. The method is based on the fabrication of an emulsion consisting of a water solution of a hydrophilic monomer and a solution of a hydrophobic monomer in an organic solvent, which is stabilized by initiator-modified kaolinite particles. Two polymers are grafted simultaneously on the opposite faces of the kaolinite particle during polymerization. The synthesized particles have a clear Janus character and are highly efficient for the stabilization of emulsions. Because of its simplicity, the method can readily be upscaled for the synthesis of large amounts of Janus particles, up to several grams.

Polymer-grafted lignin surfactants prepared via reversible addition-fragmentation chain-transfer polymerization

Kraft lignin grafted with hydrophilic polymers has been prepared using reversible addition-fragmentation chain-transfer (RAFT) polymerization and investigated for use as a surfactant. In this preliminary study, polyacrylamide and poly(acrylic acid) were grafted from a lignin RAFT macroinitiator at average initiator site densities estimated to be 2 per particle and 17 per particle. The target degrees of polymerization were 50 and 100, but analysis of cleaved polyacrylamide was consistent with a higher average molecular weight, suggesting not all sites were able to participate in the polymerization. All materials were readily soluble in water, and dynamic light scattering data indicate polymer-grafted lignin coexisted in isolated and aggregated forms in aqueous media. The characteristic size was 15-20 nm at low concentrations, and aggregation appeared to be a stronger function of degree of polymerization than graft density. These species were surface active, reducing the surface tension to as low as 60 dyn/cm at 1 mg/mL, and a greater decrease was observed than for polymer-grafted silica nanoparticles, suggesting that the lignin core was also surface active. While these lignin surfactants were soluble in water, they were not soluble in hexanes. Thus, it was unexpected that water-in-oil emulsions formed in all surfactant compositions and solvent ratios tested, with average droplet sizes of 10-20 μm. However, although polymer-grafted lignin has structural features similar to nanoparticles used in Pickering emulsions, its interfacial behavior was qualitatively different. While at air-water interfaces, the hydrophilic grafts promote effective reductions in surface tension, we hypothesize that the low grafting density in these lignin surfactants favors partitioning into the hexanes side of the oil-water interface because collapsed conformations of the polymer grafts improve interfacial coverage and reduce water-hexanes interactions. We propose that polymer-grafted lignin surfactants can be considered as random patchy nanoparticles with mixed hydrophilic and hydrophobic domains that result in unexpected interfacial behaviors. Further studies are necessary to clarify the molecular basis of these phenomena, but grafting of hydrophilic polymers from kraft lignin via radical polymerization could expand the use of this important biopolymer in a broad range of surfactant applications.

pH-responsive PDMS-b-PDMAEMA micelles for intracellular anticancer drug delivery

A series of poly(dimethysiloxane)-b-poly(2-(dimethylamino)ethyl methacrylate) (PDMS-b-PDMAEMA) block copolymers were synthesized with atom transfer radical polymerization (ATRP). In aqueous solution the polymers self-assembled into micelles with diameters between 80 and 300 nm, with the ability to encapsulate DOX. The polymer with the shortest PDMAEMA block (5 units) displayed excellent cell viability, while micelles containing longer PDMAEMA block lengths (13 and 22 units) led to increased cytotoxicity. The carriers released DOX in response to a decrease in pH from 7.4 to 5.5. Confocal laser scanning microscopy (CLSM) revealed that nanoparticles were taken up by endocytosis into acidic cell compartments. Furthermore, DOX-loaded nanocarriers exhibited intracellular pH-response as changes in cell morphology and drug release were observed within 24 h.

Equilibrium phases of one-patch colloids with short-range attractions

Inspired by experimental studies of short-ranged attractive patchy particles, we study with computer simulations the phase behavior and the crystalline structures of one-patch colloids with an interaction range equal to 5% of the particle diameter. In particular, we study the effects of the patch surface coverage fraction, defined as the ratio between the attractive and the total surface of a particle. Using free-energy calculations and thermodynamic integration schemes, we evaluate the equilibrium phase diagrams for particles with patch coverage fractions of 30%, 50% and 60%. For a 60% surface coverage fraction, we observe stable lamellar crystals consisting of stacked bilayers that directly coexist with a low density fluid. Inside the coexistence region, we observe the formation of lamellar structures also in direct NVT simulations, indicating that the barrier of formation is low and experimental realization is feasible. For sufficiently strong interactions, these structures spontaneously assemble from the fluid in simulations, suggesting that they might also easily form in experimental systems. In the Janus case, i.e. at 50% surface coverage fraction, no lamellar structures are formed, and the stable crystals are similar to those that have been found previously for a longer interaction range (i.e. 20% of the particle diameter). At 30% coverage fraction, we identify novel 'open' crystal structures with large unit cells of up to 14 particles that are stable in the strong interaction limit.

Self-organizing microfluidic crystals

We consider how to design a microfluidic system in which suspended particles spontaneously order into flowing crystals when driven by external pressure. Via theory and numerics, we find that particle-particle hydrodynamic interactions drive self-organization under suitable conditions of particle morphology and geometric confinement. Small clusters of asymmetric "tadpole" particles, strongly confined in one direction and weakly confined in another, spontaneously order in a direction perpendicular to the external flow, forming one dimensional lattices. Large suspensions of tadpoles exhibit strong density heterogeneities and form aggregates. By rationally tailoring particle shape, we tame this aggregation and achieve formation of large two-dimensional crystals.

Janus Polymer Single Crystal Nanosheet via Evaporative Crystallization

We show that liquid/liquid interface can guide polymer chain folding during crystallization. Evaporation-induced crystallization of telechelic dicarboxyl end-functionalized poly(ε-caprolactone) (COOH-PCL-COOH) at a water/pentyl acetate interface produced millimeter-scale, uniform polymer single crystal (PSC) films. Due to the asymmetric nature at the interface, the PSC nanosheets exhibited a Janus structure: the two surfaces of the crystal showed distinct water contact angle, which are quantitatively confirmed by in situ nanocondensation using environmental scanning electron microscopy (ESEM).

Janus nanodisc of diblock copolymers

Janus nanodiscs of diblock copolymers are prepared by stepwise disassembly of PS-b-P4VP disc-stacked particles. The Janus nanodiscs are uniform in thickness and regular in contour. By preferential growth of functional materials at the positively charged P4VP side, the composition, microstructure, and performance of the Janus nanodiscs are tunable.

Mimicking bubble use in nature: propulsion of Janus particles due to hydrophobic-hydrophilic interactions

Bubbles are widely used by animals in nature in order to fulfill important functions. They are used by animals in order to walk underwater or to stabilize themselves at the water/air interface. The main aim of this work is to imitate such phenomena, which is the essence of biomimetics. Here, bubbles are used to propel and to control the location of Janus particles in an aqueous medium. The synthesis of Janus SiO2-Ag and polystyrene-Ag (PS-Ag) particles through embedment in Parafilm is presented. The Janus particles, partially covered with catalytically active Ag nanoparticles, are redispersed in water and placed on a glass substrate. The active Ag sites are used for the splitting of H2O2 into water and oxygen. As a result, an oxygen bubble is formed on one side of the particle and promotes its propulsion. Once formed, the bubble-particle complex is stable and therefore, can be manipulated by tuning hydrophilic-hydrophobic interactions with the surface. In this way a transition between two- and three- dimensional motion is possible by changing the hydrophobicity of the substrate. Similar principles are used in nature.

Combining the best of two worlds: nanoparticles and nanofibers

The preparation and applications of nanoparticles and nanofibers are widely described in the literature. Both types of materials have specific advantages but also drawbacks. We discuss here the methods to fabricate nanofibers from nanoparticles and vice versa by template-free methods and colloid-electrospinning. Nanoparticles and nanofibers can be also synergistically combined to yield nanostructured constructs that display highly advantageous properties such as good mechanical integrity, double protection of encapsulated substances, or the possibility to co-encapsulate payloads with different polarities.

Preparation of stimuli-responsive "mushroom-like" janus polymer particles as particulate surfactant by site-selective surface-initiated AGET ATRP in aqueous dispersed systems

Micrometer-sized, monodisperse, "mushroom-like" Janus poly(methyl methacrylate)/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate)-graft-poly(2-(dimethyl amino)ethyl methacrylate) (PMMA/P(S-BIEM)-g-PDM) particles were successfully synthesized by site-selective surface-initiated activator generated by electron transfer for atom transfer radical polymerization in aqueous dispersed systems with spherical PMMA/P(S-BIEM) composite particles having controlled morphologies prepared using the solvent evaporation method. The anisotropic nonspherical shape of the obtained particles was controlled by changing the percentage of the surface area occupied by localized initiation sites (bromine group) at the surface of the PMMA/P(S-BIEM) composite particles with different P(S-BIEM) contents. Grafted PDM layer formed at the surface (contacting with water) of the P(S-BIEM) phase reversibly exhibited the volume phase transition in response to temperature and pH, which gave different nonspherical shapes ("open" or "closed" mushroom-cap). On the basis of such dual stimuli-responsive properties, the nonspherical particles effectively operated as particulate surfactant for Pickering emulsion, resulting in a stable 1-octanol-in-water emulsion at optimum temperature and pH value, and the Pickering emulsion could be easily unstabilized quickly by controlling them.

DNA as a powerful tool for morphology control, spatial positioning, and dynamic assembly of nanoparticles

Several properties of nanomaterials, such as morphologies (e.g., shapes and surface structures) and distance dependent properties (e.g., plasmonic and quantum confinement effects), make nanomaterials uniquely qualified as potential choices for future applications from catalysis to biomedicine. To realize the full potential of these nanomaterials, it is important to demonstrate fine control of the morphology of individual nanoparticles, as well as precise spatial control of the position, orientation, and distances between multiple nanoparticles. In addition, dynamic control of nanomaterial assembly in response to multiple stimuli, with minimal or no error, and the reversibility of the assemblies are also required. In this Account, we summarize recent progress of using DNA as a powerful programmable tool to realize the above goals. First, inspired by the discovery of genetic codes in biology, we have discovered DNA sequence combinations to control different morphologies of nanoparticles during their growth process and have shown that these effects are synergistic or competitive, depending on the sequence combination. The DNA, which guides the growth of the nanomaterial, is stable and retains its biorecognition ability. Second, by taking advantage of different reactivities of phosphorothioate and phosphodiester backbone, we have placed phosphorothioate at selective positions on different DNA nanostructures including DNA tetrahedrons. Bifunctional linkers have been used to conjugate phosphorothioate on one end and bind nanoparticles or proteins on the other end. In doing so, precise control of distances between two or more nanoparticles or proteins with nanometer resolution can be achieved. Furthermore, by developing facile methods to functionalize two hemispheres of Janus nanoparticles with two different DNA sequences regioselectively, we have demonstrated directional control of nanomaterial assembly, where DNA strands with specific hybridization serve as orthogonal linkers. Third, by using functional DNA that includes DNAzyme, aptamer, and aptazyme, dynamic control of assemblies of gold nanoparticles, quantum dots, carbon nanotubes, and iron oxide nanoparticles in response to one or more stimuli cooperatively have been achieved, resulting in colorimetric, fluorescent, electrochemical, and magnetic resonance signals for a wide range of targets, such as metal ions, small molecules, proteins, and intact cells. Fourth, by mimicking biology, we have employed DNAzymes as proofreading units to remove errors in nanoparticle assembly and further used DNAzyme cascade reactions to modify or repair DNA sequences involved in the assembly. Finally, by taking advantage of different affinities of biotin and desthiobiotin toward streptavidin, we have demonstrated reversible assembly of proteins on DNA origami.

Influence of photo-cross-linking on emulsifying performance of the self-assemblies of poly(7-(4-vinylbenzyloxyl)-4-methylcoumarin-co-acrylic acid)

Polymeric micelles could be used as model polymeric particulate emulsifiers to elucidate the correlation between the micellar structure and their emulsifying performance. Photo-cross-linkable and pH-responsive micelles were prepared with amphiphilic random copolymers, poly(7-(4-vinylbenzyloxyl)-4-methylcoumarin-co-acrylic acid) (PVMAA), via the self-assembly in selective-solvent DMF/H2O and then used as polymeric particulate emulsifiers to stabilize toluene-in-water emulsions. Primary micelles, based on PVMAA with 12 mol % of hydrophobic composition, were chosen as model to investigate the influence of photo-cross-linking on the emulsifying performance. The larger shrinkage degree by photo-cross-linking (SDC) the micelles have, the lower emulsifying efficiency the micelles exhibit. Furthermore, the structural transitions of micelles with SDC of 0% and 95% in response to pH change were comparatively confirmed by a combination of electrophoresis, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The micelles of various states, manipulated by photo-cross-linking and pH changes, were used as emulsifiers to stabilize toluene-in-water or styrene-in-water emulsions. For the un-cross-linked micelles, polymer chains gradually protrude from micelles with pH increasing, which benefits the increase in the emulsifying efficiency of micelles. However, as pH elevated over 8, the stability of emulsions significantly decreases due to the disintegration of micelles. On the contrary, micelles with SDC of 95% keep their structural integrity and become more rigid as pH increase, leading to lower emulsifying efficiency of micelles and worse stability of the emulsions. This paper provides a new insight into the principles governing the extremely high emulsifying efficiency of polymeric particulate emulsifiers and pH-dependent or pH-responsive properties of the formed emulsions.

Using Markov state models to study self-assembly

Markov state models (MSMs) have been demonstrated to be a powerful method for computationally studying intramolecular processes such as protein folding and macromolecular conformational changes. In this article, we present a new approach to construct MSMs that is applicable to modeling a broad class of multi-molecular assembly reactions. Distinct structures formed during assembly are distinguished by their undirected graphs, which are defined by strong subunit interactions. Spatial inhomogeneities of free subunits are accounted for using a recently developed Gaussian-based signature. Simplifications to this state identification are also investigated. The feasibility of this approach is demonstrated on two different coarse-grained models for virus self-assembly. We find good agreement between the dynamics predicted by the MSMs and long, unbiased simulations, and that the MSMs can reduce overall simulation time by orders of magnitude.

Giant negative mobility of Janus particles in a corrugated channel

We numerically simulate the transport of elliptic Janus particles along narrow two-dimensional channels with reflecting walls. The self-propulsion velocity of the particle is oriented along either its major (prolate) or minor axis (oblate). In smooth channels, we observe long diffusion transients: ballistic for prolate particles and zero diffusion for oblate particles. Placed in a rough channel, prolate particles tend to drift against an applied drive by tumbling over the wall protrusions; for appropriate aspect ratios, the modulus of their negative mobility grows exceedingly large (giant negative mobility). This suggests that a small external drive suffices to efficiently direct self-propulsion of rod-like Janus particles in rough channels.

Surface activity and collective behaviour of colloidally stable Janus-like particles at the air-water interface

In this work we report an experimental study on the surface activity and the collective behaviour of colloidally stable Janus-like silver particles at the air-water interface. The colloidal stability of silver nanoparticles has been enhanced using different capping ligands. Two polymers coated the silver particles: 11-mercaptoundecanoic acid and 1-undecanthiol. These capping ligands adsorbed onto the particle surface are spontaneously rearranged at the air-water interface. This feature leads to Janus behaviour in the silver particles with amphiphilic character. The surface activity of the silver particles at the air-water interface has been measured using pendant drop tensiometry. The Janus-like silver particles revealed a surface activity similar to that shown by conventional amphiphilic molecules but with much larger area per particle. The variation of the surface pressure with the area per particle was described properly using the Frumkin isotherm up to the collapse state. Furthermore, oscillating pendant drop tensiometry provided very useful data on the rheological properties of Janus particle monolayers; these properties depended on the lateral interactions between particles and were closely related to the monolayer microstructure. We revealed the close relationship between the collective behavior and the surface activity of Janus-like silver particles.

Ligand engineering of polymer nanocomposites: from the simple to the complex

One key to optimizing the performance of polymer nanocomposites for high-tech applications is surface ligand engineering of the nanofiller, which has been used to either tune the nanofiller morphology or introduce additional functionalities. Ligand engineering can be relatively simple such as a single population of short molecules on the nanoparticle surface designed for matrix compatibility. It can also have complexity that includes bimodal (or multimodal) populations of ligands that enable relatively independent control of enthalpic and entropic interactions between the nanofiller and matrix as well as introduce additional functionality and dynamic control. In this Spotlight on Applications, we provide a brief review into the use of brush ligands to tune the thermodynamic interactions between nanofiller and matrix and then focus on the potential for surface ligand engineering to create exciting nanocomposites properties for optoelectronic and dielectric applications.

Rapid control of phase growth by nanoparticles

Effective control of phase growth under harsh conditions (such as high temperature, highly conductive liquids or high growth rate), where surfactants are unstable or ineffective, is still a long-standing challenge. Here we show a general approach for rapid control of diffusional growth through nanoparticle self-assembly on the fast-growing phase during cooling. After phase nucleation, the nanoparticles spontaneously assemble, within a few milliseconds, as a thin coating on the growing phase to block/limit diffusion, resulting in a uniformly dispersed phase orders of magnitude smaller than samples without nanoparticles. The effectiveness of this approach is demonstrated in both inorganic (immiscible alloy and eutectic alloy) and organic materials. Our approach overcomes the microstructure refinement limit set by the fast phase growth during cooling and breaks the inherent limitations of surfactants for growth control. Considering the growing availability of numerous types and sizes of nanoparticles, the nanoparticle-enabled growth control will find broad applications.

Refining microstructure is an important goal in many material systems. Here, the authors report an approach for microstructure refinement based on nanoparticle self-assembling on a growing phase, which is shown to be effective for both metallic and organic systems.

Janus nanoparticles: materials, preparation and recent advances in drug delivery

INTRODUCTION

The term Janus particles was used to describe particles that are the combination of two distinct sides with differences in chemical nature and/or polarity on each face. Due to the exponential growth of interest on multifunctional nanotechnologies, such anisotropic nanoparticles are promising tools in the field of drug delivery.

AREAS COVERED

The main preparation processes and the materials used have been described first. Then a specific focus has been done on therapeutic and/or diagnostic applications of Janus particles.

EXPERT OPINION

Janus particles are demonstrated as interesting objects with advanced properties that combine features and functionalities of different materials in one single unit. Due to their dual structure, Janus particles are promising candidates for a variety of high-quality applications dealing with drug delivery purposes. Still, the main challenges for the future lie in the development of the preparation of shape-controlled and nano-sized particles with large-scale production processes and approved pharmaceutical excipients.

Facile colloidal coating of polystyrene nanospheres with tunable gold dendritic patches

Patchy particles comprise regions of differing material or chemical functionality on otherwise isotropic cores. To meet the great potential of these anisotropic structures in a wide range of application fields, completely new approaches are sought for the scalable and tunable production of patchy particles, particularly those with nanoscale dimensions. In this paper the synthesis of patchy particles via a simple colloidal route is investigated. Using surfactant-free cationic polystyrene nanospheres as core particles, gold patches are produced through the in situ reduction of chloroauric acid with ascorbic acid. The fact that such nanostructured metal patches can be heterogeneously nucleated on polymer nanospheres is related to the electrostatic interaction between core and metal precursor. Furthermore, the lateral expansion of the gold patches over the polystyrene surface is facilitated by an excess of ascorbic acid. The morphology of the patches is highly dendritic and process-induced variations in the structure are related to gold surface mobility using Monte Carlo simulations based on the diffusion limited aggregation principle. Considering the pH dependent behaviour of ascorbic acid it is possible to predict the moiety which most likely adsorbs to the polymer surface and promotes gold surface diffusion. This enables the judicious adjustment of the pH to also obtain non-dendritic patches. On account of the plasmonic behaviour of gold, the patchy particles have morphology-dependent optical properties. The systematic development of the synthetic approach described here is expected to lay a foundation for the development of functional materials based on the self- or directed-assembly of nanoscale building blocks with anisotropic interactions and properties.

Tri-Phasic Size- and Janus Balance-Tunable Colloidal Nanoparticles (JNPs)

These studies show synthesis of triphasic size- and Janus balance (JB)-tunable nanoparticles (JNPs) utilizing a two-step emulsion polymerization of pentafluorostyrene (PFS) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) and n-butyl acrylate (nBA) in the presence of poly(methyl methacrylate (MMA)/nBA) nanoparticle seeds. Each JNP consists of three phase-separated copolymers: p(MMA/nBA) core, temperature, and pH-responsive (p(DMAEMA/nBA)) phase capable of reversible size and shape changes, and shape-adoptable (p(PFS/nBA)) phase. Due to built-in second-order lower critical solution temperature (II-LCST) transition of p(DMAEMA/nBA) copolymer, macromolecular segments collapse when temperature increases from 30 to 45 °C, resulting in size and shape changes. The p(DMAEMA/nBA) and p(MMA/nBA) phases within each JNP assume concave, flat, or convex shapes, forcing p(PFS/nBA) phase to adopt convex, planar, or concave interfacial curvatures, respectively. As a result, the JB can be tuned from 3.78 to 0.72. The presence of pH-responsive DMAEMA component also facilitates the size and JB changes due to protonation of the tertiary amine groups of p(DMAEMA/nBA) backbone. Synthesized in this manner, JNPs are capable of stabilizing oil droplets in water at high pH to form Pickering emulsions, which at lower pH values release oil phase. This process is reversible and can be repeated many times.

Anisotropic particles from a one-pot double emulsion induced by partial wetting and their triggered release

A family of anisotropic particles is synthesized via a facile, scalable, and versatile method based on a conventional double emulsion, which is very well suited for practical production and applications. Partial wetting theory is well established as a powerful instrument to elucidate the controlled phase separation within the double emulsion. This theory is employed to assist the manipulation of the phase separation process for the formation of well-defined nonspherical particles as well as their triggered release.