Spherical Colloidal Crystals Research Articles

Assembly of Nanometer-Sized Hollow Sphere Colloidal Crystals for Applications as Tunable Photonic Materials

Assembly of Nanometer-Sized Hollow Sphere Colloidal Crystals for Applications as Tunable Photonic Materials

Oil-Triggered and Template-Confined Dewetting for Facile and Low-Loss Sample Digitization.

This paper proposes a simple and robust method for spontaneously digitizing aqueous samples into a high-density microwell array. The method is based on an oil-triggered template-confined dewetting phenomenon. To realize the dewetting-induced sample digitization, an aqueous sample is first infused into a networked microwell array (NMA) through a pre-degassing-based self-pumping mechanism, and an immiscible oil phase is then applied over the surface of NMA chip to induce the templated dewetting. Due to periodic interfacial tension heterogeneity, such dewetting ruptures the sample at the thinnest parts (i.e., connection channels) and spontaneously splits the sample into droplets in individual microwells. Without requiring any complex pumping or valving systems, this method can discretize a sample into tens of thousands of addressable droplets in a matter of minutes with nearly 98% usage. To demonstrate the utility and universality of this self-digitization method, we exploited it to discretize samples into 40 233 wells for a digital PCR assay, the digital quantification of bacteria, the self-assembly of spherical colloidal photonic crystals, and the spherical crystallization of drugs. We believe this facile technique will provide a substantial benefit to many compartmentalized assays or syntheses where it is necessary to partition samples into a large number of small individual volumes.

Coloration in Supraparticles Assembled from Polyhedral Metal-Organic Framework Particles.

Supraparticles are spherical colloidal crystals prepared by confined self‐assembly processes. A particularly appealing property of these microscale structures is the structural color arising from interference of light with their building blocks. Here, we assemble supraparticles with high structural order that exhibit coloration from uniform, polyhedral metal–organic framework (MOF) particles. We analyse the structural coloration as a function of the size of these anisotropic building blocks and their internal structure. We attribute the angle‐dependent coloration of the MOF supraparticles to the presence of ordered, onion‐like layers at the outermost regions. Surprisingly, even though different shapes of the MOF particles have different propensities to form these onion layers, all supraparticle dispersions show well‐visible macroscopic coloration, indicating that local ordering is sufficient to generate interference effects.

Coloration in Supraparticles Assembled from Polyhedral Metal‐Organic Framework Particles

AbstractSupraparticles are spherical colloidal crystals prepared by confined self‐assembly processes. A particularly appealing property of these microscale structures is the structural color arising from interference of light with their building blocks. Here, we assemble supraparticles with high structural order that exhibit coloration from uniform, polyhedral metal–organic framework (MOF) particles. We analyse the structural coloration as a function of the size of these anisotropic building blocks and their internal structure. We attribute the angle‐dependent coloration of the MOF supraparticles to the presence of ordered, onion‐like layers at the outermost regions. Surprisingly, even though different shapes of the MOF particles have different propensities to form these onion layers, all supraparticle dispersions show well‐visible macroscopic coloration, indicating that local ordering is sufficient to generate interference effects.

Stimuli-Responsive Structural Colored Gel That Exhibits the Three Primary Colors of Light by Using Multiple Photonic Band Gaps Acquired from Photonic Balls.

A material that can capture changes in environmental stimuli as a color change can be used to develop sensors and displays. By producing an ordered structure in a polymer gel that reflects particular wavelengths of light, we can express the volume change that occurs based on the environment as the change in the wavelength of reflected light, i.e., structural color. To date, many systems have been developed to change the hue of the structural color as a function of temperature, pH, substance, applied force, and so on. However, as is expected from the principle of optical interference, the gel usually shows a red-shift with increasing volume. In this study, we propose a method for preparing structurally colored stimuli-responsive polymer gels that display appropriate color changes according to changes in environmental stimuli. For this purpose, we employ the photonic balls, which are spherical colloidal crystals consisting of monodisperse silica particles, as templates. By combining the wavelength-selective reflection generated from different photonic band gaps of the photonic balls, we succeeded in developing porous stimuli-responsive polymer gels that exhibited various types of color change, which are not observed in conventional systems.

Controlled Assembly of Icosahedral Colloidal Clusters for Structural Coloration

Icosahedral colloidal clusters are a new class of spherical colloidal crystals. This cluster allows for potentially superior optical properties in comparison to conventional onion-like colloidal su...

Reversible solid-state phase transitions in confined two-layer colloidal crystals

Using a combination of fluorescence and bright-field optical imaging, the solid-state packing structures of semi-confined two-layer spherical colloidal crystals were observed during modulation of an external AC electric field. Upon increasing field strength, the bottom layer of colloids (layer 1) transitioned from the entropically favored hexagonal packing structure with p6m symmetry to a square-packing structure with p4m symmetry. The packing structure of layer 2 was determined by the packing structure of layer 1, with layer 2 particles resting in, and moving in registry with, the low-energy interstitial sites of layer 1. Modulation of the field strength thus resulted in a reversible transition between a face-centered cubic crystal structure and a body-centered cubic crystal structure at low and high field strengths, respectively. These structures were found to be sensitive to the particle density in the wells, with vacancies and insertions leading to the formation of mixed crystal phases at high field strengths.

Optical Characterization of the Photonic Ball as a Structurally Colored Pigment.

A photonic ball is a spherical colloidal crystal. Because it can exhibit vivid structural colors, many attempts have been made to apply it as a structurally colored pigment. However, the optical properties of the photonic ball are complicated because different crystal planes can be involved in the coloration mechanism, depending on the size of the constituent colloidal particles. In this paper, we report a comparative study of photonic balls consisting of silica particles with sizes ranging from 220 to 500 nm. We first analyze the reflectance spectra acquired in a nearly backscattering geometry and confirm that Bragg diffraction from different crystal planes causes several spectral peaks. Second, the angular dependence of reflection is experimentally characterized and theoretically analyzed with appropriate models. These analyses and a comparison with a planar colloidal crystal reveal that the spherical shape plays an essential role in the minor iridescence of photonic balls. We finally discuss a method to enhance color saturation by incorporating small light-absorbing particles. We also discuss the iridescence of the photonic ball under directional and ambient illumination conditions.

Spherical Colloidal Photonic Crystals with Selected Lattice Plane Exposure and Enhanced Color Saturation for Dynamic Optical Displays.

While structural color materials have nonfading properties and contribute significantly to the sustainable development of pigments or dyes, they are plagued by low color saturation and limited color tunability. Here, we describe a new type of spherical colloidal photonic crystals (CPCs) prepared by a droplet-based microfluidic strategy, featuring enhanced color saturation and tunable structural colors. Methyl viologen (MV) functionalized SiO2 colloids were synthesized and used for the preparation of CPCs in microdroplets. Because of the absorption of incoherently scattered light by MV, the ratio of peak-to-background amplitude in the reflectance spectra of CPCs is increased, leading to brilliant structural color with enhanced saturation. The lattice plane exposure of spherical CPCs depends on the refractive index contrast between the filling medium and SiO2 building blocks, and this offers an alternative way to tune the structural color in a spherical CPC. Accordingly, a dynamic optical display was constructed, providing valuable insights to the future development of structural color-based sensors, surface coatings, or displays.

Competing factors in grain boundary loop shrinkage: Two-dimensional hard sphere colloidal crystals.

A grain boundary (GB) loop in a two-dimensional solid is the boundary of a domain or grain whose lattice orientation is mismatched with its uniform surroundings. Understanding the factors that influence the rate at which the interior of a GB loop relaxes to the orientation of its surroundings is an important step toward control and predictability of grain coarsening in general. Recent computational and experimental studies looking at the rate of GB loop shrinkage in two-dimensional colloidal hard sphere solids have uncovered contradictory trends: in experiments, GB loops with low misorientation angles shrank the fastest, while in simulations, they persisted the longest. In this study, the computational system's behavior is brought into qualitative agreement with the experimental results through increasing the lateral packing pressure, decreasing the domain size, and mimicking the experimental protocol used to form the GB loop. Small GB loops with the same misorientation, but displaying either a hexagonal or starlike grain shape depending on the orientation of their six dislocations, are shown to differ in their rates of shrinkage by two orders of magnitude. The evidence suggests that both the barrier to generating new dislocations as well as the pattern of dislocations formed by different GB loop preparation methods will determine which trend is observed.

Colorful Photonic Pigments Prepared by Using Safe Black and White Materials

The methods used to prepare vivid color materials that have supported our rich lifestyle are due for review, with the goal of using chemical substances that minimize the significant adverse effects on human health and the environment. In this research, we found that pigments of various colors can be obtained by utilizing submicron-sized amorphous silica fine particles and a complex of iron and tannic acid (Fe–TA complex), which are used for foods, cosmetics, etc. Silica fine particles with uniform submicron particle size appear as a white powder. When silica fine particles form spherical colloidal crystals with short-range order, long-distance order, and periodicity or form colloidal amorphous arrays with only short-range order, the fine particle arrays strongly scatter light with a specific wavelength in all directions. If the wavelength is in the visible light region, these arrays can be used as color materials displaying colors with low angle dependence. However, such arrays formed only from silica fin...

Facile synthesis of functionalized polymer microsphere with three-dimensionally ordered macroporous structure by a colloidal crystal templating method

Three-dimensionally ordered macroporous microspheres (3DOMMs) with copolymer skeletons have been successfully fabricated using a colloidal crystal templating method. The spherical SiO2 colloidal crystal was firstly self-assembled as template via the evaporation-induced self-assembly process in a water/oil suspension system. The 3DOMMs were obtained by sealing the silica colloidal crystal template with polyethylene glycol 1500 (PEG 1500) after the infiltration of the precursor into the interstice of the template. The compositions of the precursor had an important effect on the structural order and integrity of the 3DOMMs. The structural strength of the microspheres was obviously improved by adding 2-hydroxyethyl methacrylate (HEMA) and 3-(trimethoxysilyl)propyl methacrylate (KH570) into the precursor. The chromatographic column packed with the 3DOMMs showed higher permeability and lower backpressure compared with the column packed with the traditional bulk materials and was suitable for the fast chromatographic separation.

Interface and Morphology Control of the Thermal Conductivity in Core–Shell Particle Colloidal Crystals

AbstractThe thermal transport properties of nanostructured composite colloidal assemblies are investigated. These are of importance for future phase change material applications, which increasingly address the micrometer and sub‐micrometer ranges. Polystyrene silica core–shell colloidal particles sizes of 270–480 nm and shell thicknesses of 15–42 nm are used as a structurally well‑defined model system. This allows deducing precise structure property relationships with the lowest thermal conductivity being observed for particles with a large diameter, a thin shell thickness, and the highest polymer content. Importantly, clear evidence is found for polymer leakage through 15 nm silica shells when exceeding the glass transition temperature of the core polymer. This leads to a steady increase in thermal conductivity but also presents a lower limit for the silica shell thickness to contain the second phase. For a complete understanding, the findings are discussed in the light of colloidal crystals consisting of pure silica and polystyrene particles. Solid silica sphere colloidal crystals possess the highest thermal conductivity, and pure polymer beads the lowest. This demonstrates to which extent the thermal transport properties can be solely adjusted by the particle composition and morphology.

Assembly of the most topologically regular two-dimensional micro and nanocrystals with spherical, conical, and tubular shapes.

We consider how to control the extension of curvature-induced defects in the hexagonal order covering different curved surfaces. In these frames we propose a physical mechanism for improving structures of two-dimensional spherical colloidal crystals (SCCs). For any SCC comprising of about 300 or less particles the mechanism transforms all extended topological defects (ETDs) in the hexagonal order into the point disclinations. Perfecting the structure is carried out by successive cycles of the particle implantation and subsequent relaxation of the crystal. The mechanism is potentially suitable for obtaining colloidosomes with better selective permeability. Our approach enables modeling the most topologically regular tubular and conical two-dimensional nanocrystals including various possible polymorphic forms of the HIV viral capsid. Different HIV-like shells with an arbitrary number of structural units (SUs) and desired geometrical parameters are easily formed. Faceting of the obtained structures is performed by minimizing the suggested elastic energy.

Self-Assembly of Spherical Colloidal Photonic Crystals inside Inkjet-Printed Droplets

The manufacturing of three-dimensional colloidal structures on solid substrates is an important topic of applied research, aiming for photonic components especially in photovoltaic and sensor applications. Whereas conventional techniques such as wet self-assembly are based on engineering of the substrate surface energy, alternative strategies envisage the independence of the interfacial conditions. We report on inkjet printing of colloidal suspensions of monodisperse silica or polystyrene nanoparticles or both and their self-assembly to spherical colloidal photonic crystals. The formation process of the colloidal nanoparticles into stable spherical colloidal assemblies (SCAs) is achieved by a self-assembly process inside tiny droplets of a stochastic mist generated intentionally instead of a jet of individual single droplets using inkjet printing. The mist-jetted, shrinking droplets serve as confined geometries for the solidification of the nanoparticles during the evaporation; thus the particles are pack...

Direct fabrication of complex 3D hierarchical nanostructures by reactive ion etching of hollow sphere colloidal crystals.

Direct reactive ion etching (RIE) of hollow SiO2 sphere colloidal crystals (HSCCs) is employed as a facile, low-cost method to fabricate complex three-dimensional (3D) hierarchical nanostructures. These multilayered structures are gradually transformed into nanostructures of increasing complexity by controlling the etching time, without complicated procedures (no mask needed). The resulting 3D topologies are unique, and cannot be obtained through traditional approaches. The formation mechanism of these structures is explained in detail by geometrical modeling during the different etching stages, through shadow effects of the higher layers. SEM images confirm the modeled morphological changes. The nanostructures obtained by our approach show very fine features as small as ∼30 nm. Our approach opens new avenues to directly obtain complex 3D nanostructures from colloidal crystals and can find applications in sensing, templating, and catalysis where fine tuning the specific surface might be critical.

Relaxation of interstitials in spherical colloidal crystals

Spherical colloidal crystals (CCs) self-assemble on the interface between two liquids. These 2D structures unconventionally combine local hexagonal order and spherical geometry. Nowadays CCs are actively studied by altering their structures. However, the statistical analysis of such experiments results is limited by uniqueness of self-assembled structures and their short lifetime. Here we perform numerical experiments to investigate pathways of CC structure relaxation after the intrusion of interstitial. The process is simulated in the frames of overdamped molecular dynamics method. The relaxation occurs due to interaction with extended topological defects (ETDs) mandatory induced in spherical CCs by their intrinsic Gaussian curvature. Types of relaxation pathways are classified and their probabilities are estimated in the low-temperature region. To analyze the structural changes during the relaxation we use a parent phase approach allowing us to describe the global organization of spherical order. This organization is preserved by only the most typical relaxation pathway resulting in filling one of vacancies integrated inside the ETD areas. In contrast with this pathway the other ones shift the ETDs centers and can strongly reconstruct the internal structure of ETDs. Temperature dependence of the relaxation processes and the mechanism of dislocation unbinding are discussed. Common peculiarities in relaxation of spherical structures and particular fragments of planar hexagonal lattice are found.

Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies

Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, we show that a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal's curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies--potentially as more efficient mimics of structural color as it occurs in nature.

Inverse Opal Spheres Based on Polyionic Liquids as Functional Microspheres with Tunable Optical Properties and Molecular Recognition Capabilities

Based on the combination of the unique features of both polyionic liquids and spherical colloidal crystals, a new class of inverse opaline spheres with a series of distinct properties was fabricated. It was found that such photonic spheres could not only be used as stimuli-responsive photonic microgels, but also serve as multifunctional microspheres that mimic the main characteristics of conventional molecules, including intrinsic optical properties, specific molecular recognition, reactivity and derivatization, and anisotropy.

Photonic metal-organic framework composite spheres: a new kind of optical material with self-reporting molecular recognition.

Exploiting metal-organic framework (MOF) materials as novel building blocks to construct superstructures with extended and enhanced functions represents a big challenge. In biological systems, the ordering of many components is not achieved by interaction of the components with each other, but by interaction of each component with the host protein which provides a matrix to support the entire assembly. Inspired by biological systems, in this work, a general strategy for efficient spatial arrangement of MOF materials was developed by using spherical colloidal crystals as host matrices, affording a new class of highly tunable MOF composite spheres with a series of distinctive properties. It was found that the synergetic combination of the unique features of both MOF and photonic colloidal crystal imparted these hierarchically structured spheres intrinsic optical properties, specific molecular recognition with self-reporting signalling, derivatization capability, and anisotropy. More importantly, the unique photonic band-gap structure integrated in these composite spheres provides a more convenient means to manipulate the photophysical and photochemical behaviour of the trapped guest molecules in MOF nanocavities.