Evaporative self-assembly from complex DNA-colloid suspensions

Evaporation-induced pattern formation has attracted considerable attention as a simple yet versatile method for generating self-assembled structures that have broad applications from photonic devices to biomacromolecular recognition. Previous study of evaporative self-assembly has mainly focused on single nonvolatile component systems, and the driving mechanisms have been extensively investigated. In contrast, pattern formation from evaporating multicomponent systems, despite its wide existence in nature and numerous engineering applications, has been rarely explored. In this work, we examine a DNA-colloid binary suspension as a model system to understand the evaporation-induced interfacial hydrodynamics and self-assembled morphology in multicomponent systems involving complex competing intermolecular and interfacial interactions. Direct microscopic observations show that the composition of the binary system plays a critical role in the multiple-ring formation upon evaporation: (1) suspensions with high DNA concentrations and low colloidal concentrations favor the formation of the multiple-ring pattern; (2) the size of colloidal particles added into DNA aqueous droplets can significantly disrupt smooth multiple rings to form rippled rings and curtain-like periodic patterns with a curious spoke-like structure as the size of colloidal particles increases; and (3) the enhancement of DNA-colloid interaction by oppositely charged colloidal particles results in considerably high irregularity of DNA stain ring spacing. We examine the disruption of the multiring morphology under varied conditions and attribute it to local hydrodynamics governed by colloid aggregation and sedimentation. Our results demonstrate the feasibility of fabricating periodic self-assembled hybrid structures via one-step evaporation of droplets consisting of multiple components.     

Effect of colloidal particle size on adsorbed monodisperse and bidisperse monolayers

Coating hydrogel films or microspheres by an adsorbed colloidal shell is one synthesis method for forming colloidosomes. The colloidal shell allows control of the release rate of encapsulated materials, as well as selective transport. Previous studies found that the packing density of self-assembled, adsorbed colloidal monolayers is independent of the colloidal particle size. In this paper we develop an equilibrium model that correlates the packing density of charged colloidal particles in an adsorbed shell to the particle dimensions in monodisperse and bidisperse systems. In systems where the molar concentration in solution is fixed, the increase in adsorption energy with increasing particle size leads to a monotonic increase in the monolayer packing density with particle radius. However, in systems where the mass fraction of the particles in the adsorbing solutions is fixed, increasing particle size also reduces the molar concentration of particles in solution, thereby reducing the probability of adsorption. The result is a nonmonotonic dependence of the packing density in the adsorbed layer on the particle radius. In bidisperse monolayers composed of two particle sizes, the packing density in the layer increases significantly with size asymmetry. These results may be utilized to design the properties of colloidal shells and coatings to achieve specific properties such as transport rate and selectivity.     

Self-assembly of poly(N-isopropylacrylamide)-carrying microspheres into two-dimensional colloidal arrays

Droplets containing polymer particles were deposited on a substrate. Poly(N-isopropylacrylamide) (PNIPAM) hydrogel and particles with PNIPAM graft chains on the surface self-assembled into a two-dimensional (2-D) superlattice when their dilute dispersions were dried on substrates. The capillary force between the particles induced ordered array formation during water evaporation. The presence of a PNIPAM layer on the particle surface gave the particles steric stability during ordered array formation. By grafting PNIPAM chains on particle surfaces by living radical polymerization, we successfully controlled the structural patterns of the colloidal arrays. These, controllable, 2-D colloidal arrays were generated on various substrates upon air-drying.     

Emulsified microemulsions and oil-containing liquid crystalline phases

Self-assembled nanostructures, such as inverted type mesophases of the cubic or hexagonal geometry or reverse microemulsion phases, can be dispersed using a polymeric stabilizer, such as the PEO-PPO-PEO triblock copolymer Pluronic F127. The particles, which are described in the present study, are based on monolinolein (MLO)-water mixtures. When adding tetradecane (TC) to the MLO-water-F127 system at constant temperature, the internal nanostructure of the kinetically stabilized particles transforms from a Pn3m (cubosomes) to a H2 (hexosomes) and to a water-in-oil (W/O, L2) microemulsion phase (emulsified microemulsion (EME)). To our knowledge, this is the first time that the formation of stable emulsified microemulsion (EME) systems has been described and proven to exist even at room temperature. The same structural transitions can also be induced by increasing temperature at constant tetradecane content. The internal nanostructure of the emulsified particles is probed using small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). At each investigated composition and temperature, the internal structure of the dispersions is observed to be identical to the corresponding structure of the nondispersed, fully hydrated bulk phase. This is clear evidence for the fact that the self-assembled inner particle nanostructure is preserved during the dispersion procedure. In addition, the internal structure of the particles is in thermodynamic equilibrium with the surrounding water phase. The internal structure of the dispersed, kinetically stabilized particles is a "real" and stable self-assembled nanostructure. To emphasize this fact, we denoted this new family of colloidal particles (cubosomes, hexosomes, and EMEs) as "ISASOMES" (internally self-assembled particles or "somes").     

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.     

Hierarchically structured porous cadmium selenide polycrystals using polystyrene bilayer templates

In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.     

Three-dimensional structure and defects in colloidal photonic crystals revealed by tomographic scanning transmission X-ray microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.     

熵驱大分子胶体粒子的柱状受限结晶

<正>熵是物理化学的基本状态参量,在统计力学和热力学中处于核心位置.按照玻尔兹曼的微观解释,熵可以由孤立系统微观状态的数目(W)给出,即S=k_BlnW,这里k_B为玻尔兹曼常数^[1,2].根据此公式,微观状态数越多,系统越混乱,熵越大,所以熵常被视作体系无序程度的度量.但熵增仅对应体系微观     

Complex collective dynamics of active torque-driven colloids at interfaces

Modern self-assembly techniques aiming to produce complex structural order or functional diversity often rely on non-equilibrium conditions in the system. Light, electric, or magnetic fields are predominantly used to modify interaction profiles of colloidal particles during self-assembly or induce complex out-of-equilibrium dynamic ordering. The energy injection rate, properties of the environment are important control parameters that influence the outcome of active (dynamic) self-assembly. The current review is focused on a case of collective dynamics and self-assembly of particles with externally driven torques coupled to a liquid or solid interface. The complexity of interactions in such systems is further enriched by strong hydrodynamic coupling between particles. Unconventionally ordered dynamic self-assembled patterns, spontaneous symmetry breaking phenomena, self-propulsion, and collective transport have been reported in torque-driven colloids. Some of the features of the complex collective behavior and dynamic pattern formation in those active systems have been successfully captured in simulations.     

Template-assisted self-assembly: a practical route to complex aggregates of monodispersed colloids with well-defined sizes, shapes, and structures

This paper describes a strategy that combines physical templating and capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures. When an aqueous dispersion of colloidal particles was allowed to dewet from a solid surface that had been patterned with appropriate relief structures, the particles were trapped by the recessed regions and assembled into aggregates whose structures were determined by the geometric confinement provided by the templates. We have demonstrated the capability and feasibility of this approach by assembling polystyrene beads and silica colloids (> or =150 nm in diameter) into complex aggregates that include polygonal or polyhedral clusters, linear or zigzag chains, and circular rings. We have also been able to generate hybrid aggregates in the shape of HF or H2O molecules that are composed of polymer beads having different diameters, polymer beads labeled with different organic dyes, and a combination of polymeric and inorganic beads. These colloidal aggregates can serve as a useful model system to investigate the hydrodynamic and optical scattering properties of colloidal particles having nonspherical morphologies. They should also find use as the building blocks to generate hierarchically self-assembled systems that may exhibit interesting properties highly valuable to areas ranging from photonics to condensed matter physics.     

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.     

Protein adsorption on polystyrene latex particles

Protein adsorption is the net result of various types of interaction between the different components present in the system, i.e. the sorbent surface, the protein(s), the solvent and any other solutes such as low molecular weight electrolyte. In this paper methods and techniques will be discussed to study the mechanism of protein adsorption on, in particular, colloidal particles. The discussion is based on experimental data obtained with relatively simple systems that contain one well-defined protein and well-characterized polystyrene latexes. Thus, the adsorption behavior of lysozyme and α-lactalbumin are compared, emphasizing the role of structural rearrangements in the protein molecule.     

Synthesis and characterization of novel core-shell colloidal particles ZnO/poly(ethylcyanoacrylate)

Here we report the synthesis and physicochemical characterization of novel hybrid core/shell type ZnO/poly(ethylcyanoacrylate) colloidal particles. It is expected that coating ZnO colloidal particles with biocompatible and biodegradable poly(alkylcyanoacrylates) will pave the way toward the potential application of ZnO colloidal particles in biomedical research. Recent findings of cell selective toxicity indicate a potential utilization of ZnO colloidal particles in the treatment of cancer. For this purpose, ZnO colloidal particles have to be selectively delivered to the site of action by a suitable biocompatible and biodegradable carrier system. Toward this goal, poly(alkylcyanoacrylates) meet ideally the requirements for carrier systems in drug delivery due to their biocompatibility, biodegradability, low toxicity, and ability to overcome the multidrug resistance in cancer cells.     

Fabrication of colloidal grid network by two-step convective self-assembly

We explored a "template-free" approach to arranging colloidal particles into a network pattern by a convective self-assembly technique. In this approach, which we call "two-step convective self-assembly," a stripe pattern of colloidal particles is first prepared on a substrate by immersing it in a suspension. The substrate with the stripes is then rotated by 90° and again immersed in the suspension to produce stripes perpendicular to the first ones, resulting in a grid-pattern network of colloidal arrays. The width of the colloidal grid lines can be controlled by changing the particle concentration while maintaining an almost constant spacing between the lines. On the basis of these results, we propose a mechanism for grid pattern formation. Our method is applicable to various types of particles. In addition, the wide applicability of this method was employed to create a hybrid grid pattern.     

TGA and magnetization measurements for determination of composition and polymer conversion of magnetic hybrid particles

Magnetic hybrid colloidal particles can be characterized using various techniques and numerous tools leading generally to particles size, size distribution, and electrokinetic properties. However, the chemical composition of these hybrid particles can be estimated using thermal gravimetric analysis (TGA). More interestingly, the combination of this quantitative technique with the magnetization measurement leads not only to chemical composition but also to the overall polymerization conversion and more precisely to the polymerization conversion on the seed particles. In fact, the TGA performed on dried magnetic particles leads to exact organic/inorganic chemical composition. Regarding the magnetization, the amount of magnetic material can be deduced, and consequently, the amount of non‐magnetic material can be also estimated. Thus, TGA and magnetization measurements are considered as complementary techniques for characterization of magnetic hybrid colloidal particles. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis of nanostructured magnetic photocatalyst by colloidal approach and spray-drying technique

Nanostructured particles with a magnetic core and a photocatalytic shell are very interesting systems for their properties to be magnetically separable (and so reusable) in photocatalytic water depuration implant. Here, a robust, low time-consuming, easily scale up method to produce Fe₃O₄/SiO₂/TiO₂ hierarchical nanostructures starting from commercial precursors (i.e. Fe₃O₄, SiO₂) by employing a colloidal approach (i.e. heterocoagulation) coupled with the spray-drying technique is presented. In particular, a self-assembled layer-by-layer methodology based on the coagulation of dissimilar colloidal particles was applied. First, a passive layer of silica (SiO₂, amorphous) was created on magnetite in order to avoid detrimental phenomena arising from the direct contact between magnetite and titania, then the deposition of titania onto silica-coated-magnetite was promoted. TiO₂, SiO₂ and Fe₃O₄ nanosols were characterized in terms of zeta potential, optimized and a self-assembled layer-by-layer approach was followed in order to promote the heterocoagulation of silica onto magnetite surface and of titania onto silica coated magnetite. Once optimized the colloidal route, the mixture was then spray-dried to obtain a granulated powder with nano-scale reactivity, easier to handle and re-disperse in comparison to starting nanopowders with the same surface properties. The nanostructured particles have been characterized by different techniques such as SEM, TEM, XDR and their magnetic properties have been investigated. Moreover, preliminary photocatalytic texts have been performed.     

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.     

The zeta potential of surface-functionalized metallic nanorod particles in aqueous solution

Metallic nanoparticles suspended in aqueous solutions and functionalized with chemical and biological surface coatings are important elements in basic and applied nanoscience research. Many applications require an understanding of the electrokinetic or colloidal properties of such particles. We describe the results of experiments to measure the zeta potential of metallic nanorod particles in aqueous saline solutions, including the effects of pH, ionic strength, metallic composition, and surface functionalization state. Particle substrates tested include gold, silver, and palladium monometallic particles as well as gold/silver bimetallic particles. Surface functionalization conditions included 11-mercaptoundecanoic acid (MUA), mercaptoethanol (ME), and mercaptoethanesulfonic acid (MESA) self-assembled monolayers (SAMs), as well as MUA layers subsequently derivatized with proteins. For comparison, we present zeta potential data for typical charge-stabilized polystyrene particles. We compare experimental zeta potential data with theoretically predicted values for SAM-coated and bimetallic particles. The results of these studies are useful in predicting and controlling the aggregation, adhesion, and transport of functionalized metallic nanoparticles within microfluidic devices and other systems.     

Colloidal self-assemblies used as templates to control size,shape and self-organization of nanoparticles

In this paper we show that the use of colloidal assemblies as templates favors the control of the size and shape of nanoparticles. As expected theoretically, the change in size and shape of copper metal nanosized particles induces changes in their optical properties. Cylindrical copper metal particles having the same size and shape can be obtained in various regions of the phase diagram when the template is made of interconnected cylinders. Self-assembly of silver metal nanoparticles is reported. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced, and, in these aggregates, the particles are highly organized and form pseudo-crystals with a face-centered cubic structure for various particles sizes. The optical properties of the silver nanoparticles isolated in micellar solution or self-assembled in 2D or 3D supperlattices are reported. Syntheses of magnetic fluids differing in their particle size are presented. The magnetic properties differ with the particle size.     

Applications of liquid crystals in biosensing and organic light-emitting devices: future aspects

ABSTRACT

This article summarises recent advances made in our laboratory towards the development of new technological applications, such as biosensors and organic light-emitting diodes (OLEDs) based on liquid crystals (LCs) other than LC displays. The study of biomolecular interaction using LC material relies on the specific interaction between the LC and the biomolecule of interest at interfaces that permit the biomolecular events to be amplified into easily measured signals for various sensing applications. In the first part, we emphases recent studies in the design and modulation of LC-based interfaces based on robust colloidal LC gels for biological amplification, qualitative and quantitative understanding of important biomolecular interactions at LC–aqueous interfaces for diagnostic and laboratory applications and design of LC droplets that hold promise to act as a marker for cells and cell-based interactions. In the second part, we described design of organic materials for application in OLEDs on various discotic monomers, dimers and oligomers. These molecules have the ability to transport charges, holes and electrons. In addition, because of the high conductivity and π–π stacking, they are considered as the advanced materials for practical applications. The technological advances in our laboratory using discotic LCs will be briefly presented in this article.