Fabrication of Polyhedral Particles from Spherical Colloids and Their Self‐Assembly into Rotator Phases

Particle shape is a critical parameter that plays an important role in self‐assembly, for example, in designing targeted complex structures with desired properties. Over the last decades, an unprecedented range of monodisperse nanoparticle systems with control over the shape of the particles have become available. In contrast, the choice of micrometer‐sized colloidal building blocks of particles with flat facets, that is, particles with polygonal shapes, is significantly more limited. This can be attributed to the fact that in contrast to nanoparticles, the larger colloids are significantly harder to synthesize as single crystals. It is now shown that a very simple building block, such as a micrometer‐sized polymeric spherical colloidal particle, is already enough to fabricate particles with regularly placed flat facets, including completely polygonal shapes with sharp edges. As an illustration that the yields are high enough for further self‐assembly studies, the formation of three‐dimensional rotator phases of fluorescently labelled, micrometer‐sized, and charged rhombic dodecahedron particles was demonstrated. This method for fabricating polyhedral particles opens a new avenue for designing new materials.     

Anisotropic colloidal particles in critical fluids

We consider anisotropic colloidal particles with dumbbell or lens shapes that are immersed in a critical binary fluid mixture. The orientation-dependent long-ranged universal interactions mediated by the critical solvent between a particle and a wall or between two particles are investigated for mesoscopic particle sizes small compared to the correlation length and interparticle distances. Exact results are obtained using a "small particle operator expansion." The amplitudes of the isotropic and anisotropic operators in the expansion depend on the size and aspect ratio of the dumbbell or lens and are determined by density profiles in the Ising model at the critical point in a wedge geometry with symmetry-breaking fixed-spin boundary conditions. Dumbbells and ellipsoids with a symmetry preserving surface are also considered.     

Engineering strength, porosity, and emission intensity of nanostructured CdSe networks by altering the building-block shape

The effect of primary particle shape on the porosity, mechanical strength, and luminescence intensity of metal chalcogenide aerogels was probed by comparison of CdSe aerogels prepared from spherical and rod-shaped particles. Rod-shaped particles yield aerogels with polymeric morphologies in contrast to the colloidal morphology obtained from spherical particles. Relative to the colloidal analogues, the polymeric CdSe aerogels exhibit twice the surface area, a doubling of the complex viscosity for 5 wt % aerogel-PDMS composites, and a 25-fold increase in emission intensity. Altering the shape of the building block from which nanostructured networks are assembled is an effective way to tune the basic properties of metal chalcogenide semiconducting aerogels.     

Determining the phase behavior of nanoparticle‐filled binary blends

Nanoparticle additives provide a means of imparting the desired electrical, optical, or mechanical properties to a polymeric matrix. The difficulty faced in creating these composites is determining the optimal conditions for forming a thermodynamically stable mixture, where the particles will not phase separate from the matrix material. This challenge is even more daunting when the polymeric matrix is itself a multicomponent mixture, as is often the case in advanced materials. Ideally, the nanoparticles would not only contribute the needed physical properties, but also stabilize the mixture so that the entire system forms a single‐phase system. In this study, we use a free energy expression for a binary blend that contains nanoparticles and take the interaction parameters between the different species to be independent variables. Thus, the particles can have distinct enthalpic interactions with each of the polymeric components. Using this expression, we determine the conditions under which the mixture forms a stable, single‐phase material. In particular, we isolate how variations in the system's parameters (e.g., polymer composition, particle volume fraction, particle size, interaction energies) affect the phase diagrams. The findings provide guidelines for creating effective formulations and can allow researchers to understand how choices made in the nature of the components affect the overall macroscopic properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2389–2403, 2006     

Size-sorted anionic iron oxide nanomagnets as colloidal mediators for magnetic hyperthermia

Iron oxide colloidal nanomagnets generate heat when subjected to an alternating magnetic field. Their heating power, governed by the mechanisms of magnetic energy dissipation for single-domain particles (Brown and Néel relaxations), is highly sensitive to the crystal size, the material, and the solvent properties. This study was designed to distinguish between the contributions of Néel and Brownian mechanisms to heat generation. Anionic nanocrystals of maghemite and cobalt ferrite, differing by their magnetic anisotropy, were chemically synthesized and dispersed in an aqueous suspension by electrostatic stabilization. The particles were size-sorted by successive electrostatic phase separation steps. Parameters governing the efficiency of nanomagnets as heat mediators were varied independently; these comprised the particle size (from 5 to 16.5 nm), the solvent viscosity, magnetic anisotropy, and the magnetic field frequency and amplitude. The measured specific loss powers (SLPs) were in quantitative agreement with the results of a predictive model taking into account both Néel and Brown loss processes and the whole particle size distribution. By varying the carrier fluid viscosity, we found that Brownian friction within the carrier fluid was the main contributor to the heating power of cobalt ferrite particles. In contrast, Néel internal rotation of the magnetic moment accounted for most of the loss power of maghemite particles. Specific loss powers were varied by 3 orders of magnitude with increasing maghemite crystal size (from 4 to 1650 W/g at 700 kHz and 24.8 kA/m). This comprehensive parametric study provides the groundwork for the use of anionic colloidal nanocrystals to generate magnetically induced hyperthermia in various media, including complex systems and biological materials.     

A Two-Level, Discrete-Particle Approach for Simulating Ordered Colloidal Structures

We devise a new, two-level discrete-particle model to simulate ordered colloidal structures with vastly different scales. We use the molecular dynamics paradigm with a Lennard-Jones-type potential to define colloidal particle system and dissipative particle dynamics (DPD) to model the solvent. The initially mixed, disordered particle ensemble undergoes a phase transition. We observe the spontaneous creation of spherical or rod-like micelles and their crystallization in stable hexagonal or worm-like structures, respectively. The ordered arrays obtained by using the particle model are similar to the two-dimensional colloidal crystals observed in laboratory experiments. The micelle shape depends on the ratio between the scaling factors of the colloid-colloid to colloid-solvent particle interactions. The properties of the DPD solvent, such as the strongly variable viscosity and partial pressure, determine the speed of crystallization. The intriguing features of colloidal arrays and their exotic symmetries, which persist also over two-dimensional domains, can be simulated numerically by using the two-level discrete-particle approach and are illustrated here. Copyright 2000 Academic Press.     

Communication: Bulkiness versus anisotropy: the optimal shape of polarizable Brownian nanoparticles for alignment in electric fields

Self-assembly and alignment of anisotropic colloidal particles are important processes that can be influenced by external electric fields. However, dielectric nanoparticles are generally hard to align this way because of their small size and low polarizability. In this work, we employ the coupled dipole method to show that the minimum size parameter for which a particle may be aligned using an external electric field depends on the dimension ratio that defines the exact shape of the particle. We show, for rods, platelets, bowls, and dumbbells, that the optimal dimension ratio (the dimension ratio for which the size parameter that first allows alignment is minimal) depends on a nontrivial competition between particle bulkiness and anisotropy because more bulkiness implies more polarizable substance and thus higher polarizability, while more anisotropy implies a larger (relative) difference in polarizability.     

Hydrogel Microellipsoids that Form Robust String‐Like Assemblies at the Air/Water Interface

Soft colloidal particles such as hydrogel microspheres assemble at air/water or oil/water interfaces, where the soft colloids are highly deformed and their surface polymer chains are highly entangled with each other. Herein, we report the formation of robust one‐dimensional, string‐like colloidal assemblies through self‐organization of hydrogel microspheres with shape anisotropy at the air/water interface of sessile droplets. Shape‐anisotropic hydrogel microspheres were synthesized via two‐step polymerization, whereby a hydrogel shell was formed onto preformed rigid microellipsoids. The shape anisotropy of the hydrogel microspheres was confirmed by transmission electron microscopy and high‐speed atomic force microscopy as well as by light‐scattering measurements. The present findings are crucial for the understanding of natural self‐organization phenomena, where “softness” influences microscopic assembled structures such as those of Nostoc bacteria.     

Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy

Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles. Osmotic stress experiments were carried out at different ionic strengths. The osmotic pressure curves display a plateau whose beginning systematically coincides with the sol/gel transition determined by oscillatory stress measurements. The concentration corresponding to the sol/gel transition increases linearly with particle anisotropy, which shows that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, a reverse evolution should be observed for an I/N transition involving the individual clay particles. Still, when observed between crossed polarizer and analyzer, the gel samples exhibit permanent birefringent textures, whereas in the "sol" region, transient birefringence is observed when the samples are sheared. This suggests that interacting clay particles are amenable to generate, at rest and/or under shear, large anisotropic particle associations.     

Anomalous dependence of particle size on supersaturation in the preparation of iron nanoparticles from iron pentacarbonyl

Anisotropic colloidal particles consisting of different compositions and geometry are useful for various applications. These include optical biosensing, antireflective coatings and electronic displays. In this work we demonstrate a simple and cost-effective method for fabricating anisotropic colloidal particles bearing a snowman-like shape. This is achieved by first settling the positively-charged polystyrene latex (PSL) colloids and negatively-charged silica colloids in deionized water onto a glass substrate, forming heterodoublets. The temperature is then raised above the glass transition temperature of the polymer. As a result, the silica particle spontaneously rises to the top of the PSL particle forming a snowman like structure. We have extended this method to different sizes and shown that the structure of the hybrid particles can be tuned by adjusting the size ratio between the silica and the PSL colloids. The surface coverage of the PSL, and hence of the snowman particles, on the glass substrate can also be varied by changing the ionic strength of the solution during the adhesion of PSL to the glass.     

Pickering–Ramsden emulsions stabilized with chemically and morphologically anisotropic particles

Recent developments in nanotechnology have facilitated the use of surface-active colloidal particles with tailor-made anisotropic properties. These surface-active agents have introduced unprecedented emulsion systems that exhibit qualitatively different self-assembled/organized structures and material properties from those of emulsions with conventional surfactants or isotropic colloidal particles. The author highlights the recent experimental works that elucidate the fundamental roles of anisotropy in the self-assembly/organization in emulsions, while focusing predominantly on amphiphilicity and morphological anisotropy in a particle. The author also introduces recent works that harness these fundamental properties of anisotropy for realizing the characteristic emulsion state and its functionality, together with a work with large particles beyond colloidal scale.     

Preparation and characterization of acrylic‐based black particles of poly(methyl methacrylate‐co‐ethylene glycol dimethacrylate) by dispersion polymerization for electrophoretic displays

Ultrafine black particles, ranging in diameter from 1 to 3 μm, were prepared by dispersion polymerization in a methanol/water mixture with vinyl monomers, nonpolymerizable Sudan black B dyes, and fluorescein isothiocyanate labeled charge control additives. Both the ratio of the methanol to the water dispersion medium and the polymeric stabilizer concentration had significant effects on the particle size. The important role of the stabilizer concentration lay in the particle formation step, during which it determined the particle stability and final particle size. These could affect the extent of the aggregation of nuclei by changing the adsorption rate of the stabilizer and the viscosity of the dispersion medium, resulting in smaller particles. The fluorescent‐labeled charge control additives strongly affected the electrophoretic mobility. A small concentration of fluorescent‐labeled charge control additives increased the electrophoretic mobility. However, a further addition reduced the electrophoretic mobility of the polymer particles. The concentration dependence of the fluorescent‐labeled charge control additives on the deposition behavior in the polymer particles was successfully imaged and thereafter quantified by image analysis. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5608–5616, 2004     

Spontaneous transformation of CdTe nanoparticles into angled Te nanocrystals: from particles and rods to checkmarks, X-marks, and other unusual shapes

CdTe nanoparticles spontaneously transform into the branched Te nanocrystals with the unique, highly anisotropic shape of checkmarks after partial removal of the stabilizers of L-cysteine. The Te checkmarks are made in a relatively high yield and uniformity; the length of the arms is ca. 150 nm, whereas the angle between the arms is 74 degrees . Subsequent growth of the particle yields mothlike nanocrystals retaining geometrical anisotropy. Unlike the previous synthesis methods of branched nanocrystals, they are formed via a merger of individual rod-shaped crystallites. High-energy crystal faces on their apexes act as the sticky points causing the particles to join in the ends. This is the first demonstration of spontaneous transformation of binary semiconductor particles into highly anisotropic nanocolloids in an angled conformation. The end reactivity of starting Te rods can be used both for bottom-up fabrication of nanoscale electronics and relatively safe and nontoxic method of synthesis of Te-based optical and other materials.     

Rheology of binary colloidal structures assembled via specific biological cross-linking

The selectivity and range of energies offered by specific biological interactions serve as valuable tools for engineering the assembly of colloidal particles into novel materials. In this investigation, high affinity biological interactions between biotin-coated "A" particles (RA = 0.475 microm) and streptavidin-coated "B" particles (RB = 2.75 microm) drive the self-assembly of a series of binary colloidal structures, from colloidal micelles (a large B particle coated by smaller A particles) to elongated chain microstructures (alternating A and B particles), as the relative number of small (A) to large (B) particles (2 < or = NA/NB < or = 200) is decreased at a low total volume fraction (10(-4) < or = phiT < or = 10(-3)). At a significantly higher total volume fraction (phiT > or = 10(-1)) and a low number ratio (NA/NB = 2), the rheological behavior of volume-filling particle networks connected by streptavidin-biotin bonds is characterized. The apparent viscosity (eta) as a function of the shear rate gamma, measured for networks at phiT = 0.1 and 0.2, exhibits shear-rate-dependent flow behavior, and both the apparent viscosity and the extent of shear thinning increase upon an increase of a factor of 2 in the total volume fraction. Micrographs taken before and after shearing show a structural breakdown of the flocculated binary particle network into smaller flocs, and ultimately a fluidlike suspension, with increasing shear rate. Rheological measurements provide further proof that suspension microstructure is governed by specific biomolecular interactions, as control experiments in which the streptavidin molecules on particles were blocked displayed Newtonian flow behavior. This investigation represents the first attempt at measuring the rheology of colloidal suspensions where assembly is driven by biomolecular cross-linking.     

GALAMOST: GPU‐accelerated large‐scale molecular simulation toolkit

GALAMOST [graphics processing unit (GPU)‐accelerated large‐scale molecular simulation toolkit] is a molecular simulation package designed to utilize the computational power of GPUs. Besides the common features of molecular dynamics (MD) packages, it is developed specially for the studies of self‐assembly, phase transition, and other properties of polymeric systems at mesoscopic scale by using some lately developed simulation techniques. To accelerate the simulations, GALAMOST contains a hybrid particle‐field MD technique where particle–particle interactions are replaced by interactions of particles with density fields. Moreover, the numerical potential obtained by bottom‐up coarse‐graining methods can be implemented in simulations with GALAMOST. By combining these force fields and particle‐density coupling method in GALAMOST, the simulations for polymers can be performed with very large system sizes over long simulation time. In addition, GALAMOST encompasses two specific models, that is, a soft anisotropic particle model and a chain‐growth polymerization model, by which the hierarchical self‐assembly of soft anisotropic particles and the problems related to polymerization can be studied, respectively. The optimized algorithms implemented on the GPU, package characteristics, and benchmarks of GALAMOST are reported in detail. © 2013 Wiley Periodicals, Inc.     

Production and characterization of anisotropic particles from biodegradable materials

In recent years, production and characterization of anisotropic particles has become of interest in a wide range of scientific fields including polymer chemistry, drug delivery, electronics, energy, and nanotechnology. In this work, we demonstrate a novel formulation for production of anisotropic particles via an internal phase separation of biodegradable components. Specifically, binary mixtures of biodegradable polymers poly(lactic-co-glycolic acid), polycaprolactone, and biodegradable lipid Precirol (glyceryl palmitostearate) were dissolved in dichloromethane, emulsified, and prepared into anisotropic particles using a modified solvent evaporation technique. During the slow evaporation process the components self-assembled into anisotropic particles with distinct morphologies. Polymer/polymer formulations resulted in compartmentalized anisotropic heterodimer particles, while polymer/lipid combinations yielded "ice cream cone" shaped particles. It was found that addition of certain active pharmaceuticals resulted in an altered, pox-like segregation at the particle surface of polymer/polymer formulations. The anisotropic nature of the particles was subsequently characterized using optical microscopy, scanning electron microscopy, zeta potential, electrophoresis, and X-ray diffraction. Successful formulations presented here may potentially be employed as multicompartmental drug carriers with staggered drug release rates or alternatively as a colloidal excipient for an arsenal of pharmaceutical applications.     

Shape‐Shifting Patchy Particles

A facile method to synthesize shape‐shifting patchy particles on the colloidal scale is described. The design is based on the solvent‐induced shifting of the patch shape between concave and convex features. The initial concave patchy particles were synthesized in a water suspension by a swelling‐induced buckling process. Upon exposure to different solvents, the patches were tuned reversibly to be either concave or convex. These particles can be assembled into chained, branched, zigzag, and cyclic colloidal superstructures in a highly site‐specific manner by surface–liquid capillary bridging. The biphasic nature of the particles also enables site‐selective surface functionalization.     

Colloidal micro- and nano-particles as templates for polyelectrolyte multilayer capsules

Colloidal particles play an important role in various areas of material and pharmaceutical sciences, biotechnology, and biomedicine. In this overview we describe micro- and nano-particles used for the preparation of polyelectrolyte multilayer capsules and as drug delivery vehicles. An essential feature of polyelectrolyte multilayer capsule preparations is the ability to adsorb polymeric layers onto colloidal particles or templates followed by dissolution of these templates. The choice of the template is determined by various physico-chemical conditions: solvent needed for dissolution, porosity, aggregation tendency, as well as release of materials from capsules. Historically, the first templates were based on melamine formaldehyde, later evolving towards more elaborate materials such as silica and calcium carbonate. Their advantages and disadvantages are discussed here in comparison to non-particulate templates such as red blood cells. Further steps in this area include development of anisotropic particles, which themselves can serve as delivery carriers. We provide insights into application of particles as drug delivery carriers in comparison to microcapsules templated on them.     

Cooperative Self‐Assembly‐Assisted Formation of Monodisperse Optically Active Spherical and Anisotropic Nanoparticles

We report a new method in which spontaneous self‐assembly is employed to synthesize monodisperse polymer nanoparticles with controlled size (<50 nm), shape, tunable functionality, and enhanced solvent and thermal stability. Cooperative noncovalent interactions, such as hydrogen bonding and aromatic π–π stacking, assist self‐assembly of amphiphilic macromolecules (polystyrene‐block‐polyvinylpyridine, PS? PVP) and structure directing agents (SDAs) to form both spherical and anisotropic solid polymer nanoparticles with SDAs residing in the particle core surrounded by the polymers. Through detailed investigations by scanning electron microscopy and transmission electron microscopy (TEM), we have rationalized nanoparticle morphology evolution and dependence on factors such as SDA concentration and PVP size. By keeping the PS chain size constant, the particle morphology progresses from continuous films to spherical particles, and on to cylindrical nanowires or rods with increasing the PVP chain size. The final nanoparticles are very stable and can be redispersed in common solvents to form homogenous solutions and thin films of ordered nanoparticle arrays through solvent evaporation processes. These nanoparticles exhibit tunable fluorescent colors (or emissions) depending on the choices of the central SDAs. Our method is simple and general without requiring complicated synthetic chemistry, stabilizing surfactants, or annealing procedures (e.g., temperature or solvent annealing), making scalable synthesis feasible.     

Selective Janus Particle Assembly at Tipping Points of Thermally‐Switched Wetting

Thermal wetting can simply, selectively and reversibly join patchy particles into clusters (2D and 3D) and also colloidal crystals over the narrow temperature range of 1–2 °C. This is demonstrated with Janus particles (gold half‐coated silica spheres) immersed in a binary mixture of water/2,6‐lutidine, such that the relative strength of gold–gold bonding through hydrophobic interaction and silica–silica bonding through the wetting‐induced attraction is reversibly switched according to temperature.