Deterministic Formation and Growth of Dendritic Crystals of Attractive Colloids.

Dendrites are ubiquitous crystals produced in supersaturated solutions and supercooled melts, but considerably less is known about their formation and growth kinetics. Here, the key factors are explored that dictate dendrite formation and growth, utilizing experimental colloidal models in which the particles act as molecules with Mie potential. Depletion attraction is employed to colloids and manipulate their strength to control supersaturation. Dendrites are predominantly produced under conditions of low supersaturation, where the separation between crystals is large due to slow nucleation. The dendrites do not emerge directly from nuclei. Instead, isotropic grains, initially produced from nuclei, morph into polygons. Arms then sprout from the vertices of these polygons, eventually giving rise to dendrites. Triggering this polygon-to-dendrite transformation requires a high diffusional flux. This necessitates a prolonged diffusion time to maintain a steep concentration gradient in the surrounding environment even after the transformation from circular grains to polygons.

Designing Tough Hydrogel Shells for Glucose Sensing.

Conventional hydrogel microcapsules often suffer from inadequate mechanical stability, hindering their use. Here, water-cored double-network (DN) hydrogel shells are designed, formed by polyacrylamide and calcium alginate networks using triple-emulsion templates. These DN hydrogel shells offer robust mechanical stability, optical transparency, and a precisely-defined cut-off threshold. The feasibility of this platform is demonstrated through the development of a fluorometric glucose sensor. Glucose oxidase is enclosed within the water core, while a pH-responsive fluorescent dye is incorporated into the DN shells. Glucose diffuses into the core through the DN shells, where the glucose oxidase converts glucose into gluconic acid, leading to pH reduction and a subsequent decrease in fluorescence intensity of DN shells. Additionally, the pH-sensitive colorant dissolved in the medium enables visual pH assessment. Thus, glucose levels can be determined using both fluorometric and colorimetric methods. Notably, the DN shells exhibit exceptional stability, enduring intense mechanical stress and cycles of drying and rehydration without leakage. Moreover, the DN shells act as effective barriers, safeguarding glucose oxidase against proteolysis by large disruptive proteins, like pancreatin. This versatile DN shell platform extends beyond glucose oxidase encapsulation, serving as a foundation for various capsule sensors utilizing enzymes and heterogeneous catalysts.

Centrifugation-Mediated Crystal Growth of Attractive Colloids for Band Edge Lasing.

Attractive depletion interactions are utilized to organize colloidal particles into crystalline arrays with high crystallinity through spontaneous phase separation. However, uncontrolled nucleation frequently leads to the formation of crystalline grains with varied crystal orientations, which hampers the optical performance of photonic crystals. Here, colloidal crystals have been engineered with uniform orientation and high surface coverage by applying centrifugal force during the depletion-induced assembly of polystyrene particles. The centrifugal force encourages the particles to move toward the bottom surface, which fosters heterogeneous nucleation and supports rapid crystal growth, yielding densely-packed and uniformly-arranged crystal grains with high reflectivity. This study has observed that the nucleation and crystal growth behavior is significantly influenced by the salt concentration. Based on the pair potentials, the transition boundary has been quantitatively analyzed between fluid and crystal phases and identified the threshold for homogeneous nucleation. Utilizing the high-reflectivity colloidal crystals, band-edge lasing is achieved by dissolving the water-soluble dye into the aqueous suspensions. Upon optical excitation, a lasing emission characterized is observed by a narrow spectral width at the short-wavelength band edge. Notably, the laser wavelength can be adjusted by altering the salt concentration or particle diameter, offering a versatile approach to tuning the optical properties.

Influence of Sphericity on Surface Termination of Icosahedral Colloidal Clusters.

Colloids self-organize into icosahedral clusters composed of a Mackay core and an anti-Mackay shell under spherical confinement to minimize the free energy. This study explores the variation of surface arrangements of colloids in icosahedral clusters, focusing on the determining factors behind the surface arrangement. To efficiently assemble particles in emulsion droplets, droplet-to-droplet osmotic extraction from particle-laden droplets to salt-containing droplets is used, where the droplets are microfluidically prepared to guarantee a high size uniformity. The icosahedral clusters are optimally produced during a 24-h consolidation period at a 0.04 m salt concentration. The findings reveal an increase in the number of particle layers from 10 to 15 in the icosahedral clusters as the average number of particles increases from 3300 to 11 000. Intriguingly, the number of layers in the anti-Mackay shells, or surface termination, appears to more strongly depend on the sphericity of the clusters than on the deviation in the particle count from an ideal icosahedral cluster. This result suggests that the sphericity of the outermost layer, formed by the late-stage rearrangement of particles to form an anti-Mackay shell near the droplet interface, may play a pivotal role in determining the surface morphology to accommodate a spherical interface.

Asymmetric Pairing of Cholesteric Liquid Crystal Droplets for Programmable Photonic Cross-Communication.

The photonic cross-communication between photonic droplets has provided complex color patterns through multiple reflections, potentially serving as novel optical codes. However, the cross-communication is mostly restricted to symmetric pairs of identical droplets. Here, a design rule is reported for the asymmetric pairing of two distinct droplets to provide bright color patterns through strong cross-communication and enrich a variety of optical codes. Cholesteric liquid crystal (CLC) droplets with different stopband positions and sizes are paired. The brightness of corresponding color patterns is maximized when the pairs are selected to effectively guide light along the double reflection path by stopbands of two droplets. The experimental results are in good agreement with a geometric model where the blueshift of stopbands is better described by the angles of refraction rather than reflection. The model predicts the effectiveness of pairing quantitatively, which serves as a design rule for programming the asymmetric photonic cross-communication. Moreover, three distinct droplets can be paired in triangular arrays, where all three cross-communication paths yield bright color patterns when three droplets are selected to simultaneously satisfy the rule. It is believed that asymmetric pairing of distinct CLC droplets opens new opportunities for programmable optical encoding in security and anti-counterfeiting applications.

Tomographic measurement of dielectric tensors at optical frequency.

The dielectric tensor is a physical descriptor of fundamental light-matter interactions, characterizing anisotropic materials with principal refractive indices and optic axes. Despite its importance in scientific and industrial applications ranging from material science to soft matter physics, the direct measurement of the three-dimensional dielectric tensor has been limited by the vectorial and inhomogeneous nature of light scattering from anisotropic materials. Here, we present a dielectric tensor tomographic approach to directly measure dielectric tensors of anisotropic structures including the spatial variations of principal refractive indices and directors. The anisotropic structure is illuminated with a polarized plane wave with various angles and polarization states. Then, the scattered fields are holographically measured and converted into vectorial diffracted field components. Finally, by inversely solving a vectorial wave equation, the three-dimensional dielectric tensor is reconstructed. Using this approach, we demonstrate quantitative tomographic measurements of various nematic liquid-crystal structures and their fast three-dimensional non-equilibrium dynamics.

Effect of surface tension on elastocapillary wrinkling of interfacially adsorbed hydrogel disks with photothermally programmed swelling profiles.

In this work, we study the influence of surface tension on light-induced wrinkling of hydrogel disks containing patterned regions of photothermally-active gold nanoparticles at the air-water interface. The disks, which are initially radially stretched by the air-water surface tension, undergo wrinkling under illumination through a radially nonuniform photothermal deswelling. By tuning the surface tension of the surrounding air-water interface through variations in concentration of a poly(vinyl alcohol) surfactant, we observe a critical threshold for wrinkling, followed by a monotonic decrease in wrinkle number with decreasing surface tension. Finite element simulations performed to better understand this behavior reveal qualitatively similar trends as the experiments. The insights provided into elastocapillarity-mediated wrinkling may guide future efforts to control interfacial behaviour of reconfigurable and shape-morphing films.

Direct Determination of the Phase Diagram of a Depletion-Mediated Colloidal System.

Depletion is one widely used potential to modulate colloidal interaction because it enables the production of a wide variety of crystalline and glassy phases of spherical and shape-tailored colloids. The attractive depletion potential gives rise to qualitatively new behavior. However, depletion-mediated phase behaviors have never been systematically investigated experimentally regarding pair potentials for aqueous suspensions. In this work, we implement three distinct phases of fluid, crystal, and glass by adjusting the concentrations of depletant and salt in the aqueous suspension of polystyrene particles. To define the phase boundary between the fluid and crystal, we calculate pair potential with a superposition of van der Waals, electrostatic, and depletion interactions. Two unknown parameters in the pair potential─the concentration of ionic impurities and the ratio of the molar concentration of depletant to osmolarity─are experimentally determined from sets of reflectance spectra. The interparticle spacing in the crystalline phase is extracted from the peak wavelength originating from Bragg diffraction, which corresponds to the interparticle separation at energy minimum in the pair potential. The boundary between the fluid and crystal is well defined with the depth of the energy well of 3kBT. By contrast, the onset of glass formation is better characterized by not the well depth but the assembly rate, which is estimated from the slope of the pair potential from force balance. Glasses are produced as the speed exceeds 300 µm/s. That is, crystals are produced by enthalpy gain overwhelming entropy loss, whereas glasses are kinetically produced due to fast jamming.

Dual-Colored Janus Microspheres with Photonic and Plasmonic Faces.

Photonic and plasmonic colors, stemming from nanostructures of dielectric materials and metals, are promising for pigment-free coloration. In particular, nanostructures with structural colors have been employed in stimuli-responsive Janus microparticles to provide active color pixels. Here, the authors report a simple strategy to produce electro-responsive Janus microspheres composed of photonic and plasmonic faces for active color change. The photonic microspheres are first prepared by self-assembly of silica particles in emulsion droplets of photocurable resin. The silica particles form 3D crystalline arrays in the interior and 2D hexagonal arrays on the interface. The emulsion droplets are photocured and the silica particles are selectively removed to make porous photonic microspheres with hexagonal arrays of dimples on the surface. Directional deposition of gold or aluminum on the photonic microsphere develops plasmonic color on the top hemisphere while maintaining photonic color on the bottom hemisphere. Moreover, the metal deposited on one side renders the Janus microspheres electro-responsive. Therefore, the photonic and plasmonic colors are switchable by the orientation control of the Janus microspheres with an external electric field. The photonic and plasmonic colors are independently adjustable by employing two different sizes of silica particles in core-shell emulsion drops.

Co-Assembly of Colloids and Eumelanin Nanoparticles in Droplets for Structural Pigments with High Saturation.

Colloidal crystals have been used to develop structural colors. However, incoherent scattering causes the colors to turn whitish, reducing the color saturation. To overcome the problem, light-absorbing additives have been incorporated. Although various additives have been used, most of them are not compatible with a direct co-assembly with common colloids in aqueous suspensions. Here, the authors suggest eumelanin nanoparticles as a new additive to enhance the color chroma. Eumelanin nanoparticles are synthesized to have diameters of several nanometers by oxidative polymerization of precursors in basic solutions. The nanoparticles carry negative charges and do not weaken the electrostatic repulsion among same-charged polystyrene particles when they are added to aqueous suspensions. To prove the effectiveness of eumelanin as a saturation enhancer, the authors produce photonic balls through direct co-assembly of polystyrene and eumelanin using water-in-oil emulsion droplets, while varying the weight ratio of eumelanin to polystyrene. The high crystallinity of colloidal crystals is preserved for the ratio up to at least 1/50 as the eumelanin does not perturb the crystallization. The eumelanin effectively suppresses incoherent scattering while maintaining the strength of structural resonance at an optimum ratio, improving color chroma without compromising brightness.

Photonic Microbeads Templated by Oil-in-Oil Emulsion Droplets for High Saturation of Structural Colors.

Photonic microbeads containing crystalline colloidal arrays are promising as a key component of structural-color inks for various applications including printings, paintings, and cosmetics. However, structural colors from microbeads usually have low color saturation and the production of the beads requires delicate and time-consuming protocols. Herein, elastic photonic microbeads are designed with enhanced color saturation through facile photocuring of oil-in-oil emulsion droplets. Dispersions of highly-concentrated silica particles in elastomer precursors are microfluidically emulsified into immiscible oil to produce monodisperse droplets. The silica particles spontaneously form crystalline arrays in the entire volume of the droplets due to interparticle repulsion which is unperturbed by the diffusion of the surrounding oil whereas weakened for oil-in-water droplets. The crystalline arrays are permanently stabilized by photopolymerization of the precursor, forming elastic photonic microbeads. The microbeads are transferred into the refractive-index-matched biocompatible oil. The high crystallinity of colloidal arrays increases the reflectivity at stopband and the index matching reduces incoherent scattering at the surface of the microbeads, enhancing color saturation. The colors can be adjusted by mixing two distinctly colored microbeads. Also, low stiffness and high elasticity reduce foreign-body sensation and enhance fluidity, potentially serving as pragmatic structural colorants for photonic inks.

Hydrogel-Assisted 3D Volumetric Hotspot for Sensitive Detection by Surface-Enhanced Raman Spectroscopy.

Effective hotspot engineering with facile and cost-effective fabrication procedures is critical for the practical application of surface-enhanced Raman spectroscopy (SERS). We propose a SERS substrate composed of a metal film over polyimide nanopillars (MFPNs) with three-dimensional (3D) volumetric hotspots for this purpose. The 3D MFPNs were fabricated through a two-step process of maskless plasma etching and hydrogel encapsulation. The probe molecules dispersed in solution were highly concentrated in the 3D hydrogel networks, which provided a further enhancement of the SERS signals. SERS performance parameters such as the SERS enhancement factor, limit-of-detection, and signal reproducibility were investigated with Cyanine5 (Cy5) acid Raman dye solutions and were compared with those of hydrogel-free MFPNs with two-dimensional hotspots. The hydrogel-coated MFPNs enabled the reliable detection of Cy5 acid, even when the Cy5 concentration was as low as 100 pM. We believe that the 3D volumetric hotspots created by introducing a hydrogel layer onto plasmonic nanostructures demonstrate excellent potential for the sensitive and reproducible detection of toxic and hazardous molecules.

Photoswitchable Surfactant-Driven Reversible Shape- and Color-Changing Block Copolymer Particles.

Polymer particles that switch their shape and color in response to light are of great interest for the development of programmable smart materials. Herein, we report block copolymer (BCP) particles with reversible shapes and colors activated by irradiation with ultraviolet (UV) and visible lights. This shape transformation of the BCP particles is achieved by a spiropyran-dodecyltrimethylammoium bromide (SP-DTAB) surfactant that changes its amphiphilicity upon photoisomerization. Under UV light (365 nm) irradiation, the hydrophilic ring-opened merocyanine form of the SP-DTAB surfactant affords the formation of spherical, onion-like BCP particles. In contrast, when exposed to visible light, surfactants with the ring-closed form yield prolate or oblate BCP ellipsoids with axially stacked nanostructures. Importantly, the change in BCP particle morphology between spheres and ellipsoids is reversible over multiple UV and visible light irradiation cycles. In addition, the shape- and color-switchable BCP particles are integrated to form a composite hydrogel, demonstrating their potential as high-resolution displays with reversible patterning capabilities.

Real-Time Monitoring of Colloidal Crystallization in Electrostatically-Levitated Drops.

Colloidal crystallization is analogous to the crystallization in bulk atomic systems in various aspects, which has been explored as a model system. However, a real-time probing of the phenomenon still remains challenging. Here, a levitation system for a study of colloidal crystallization is demonstrated. Colloidal particles in a levitated droplet are gradually concentrated by isotropic evaporation of water from the surface of the droplet, resulting in crystallization. The structural change of the colloidal array during crystallization is investigated by simultaneously measuring the volume and reflectance spectra of the droplet. The crystal nucleates from the surface of the droplet at which the volume fraction exceeds the threshold and then the growth proceeds. The crystal growth behavior depends on the initial concentrations of colloidal particles and salts which determine the overall direction of crystal growth and interparticle spacing, respectively. The results show that a levitating bulk droplet has a great potential as a tool for in situ investigation of colloidal crystallization.

Composite Microgels Created by Complexation between Polyvinyl Alcohol and Graphene Oxide in Compressed Double-Emulsion Drops.

Microgels, microparticles made of hydrogels, show fast diffusion kinetics and high reconfigurability while maintaining the advantages of hydrogels, being useful for various applications. Here, presented is a new microfluidic strategy for producing polymer-graphene oxide (GO) composite microgels without chemical cues or a temperature swing for gelation. As a main component of microgels, polymers that are able to form hydrogen bonds, such as polyvinyl alcohol (PVA), are used. In the mixture of PVA and GO, GO is tethered by PVA through hydrogen bonding. When the mixture is rapidly concentrated in the core of double-emulsion drops by osmotic-pressure-driven water pumping, PVA-tethered GO sheets form a nematic phase with a planar alignment. In addition, the GO sheets are linked by additional hydrogen bonds, leading to a sol-gel transition. Therefore, the PVA-GO composite remains undissolved when it is directly exposed to water by oil-shell rupture. These composite microgels can be also produced using poly(ethylene oxide) or poly(acrylic acid), instead of PVA. In addition, the microgels can be functionalized by incorporating other polymers in the presence of the hydrogel-forming polymers. It is shown that the multicomponent microgels made from a mixture of polyacrylamide, PVA, and GO show an excellent adsorption capacity for impurities.

Depletion-Mediated Interfacial Assembly of Semiconductor Nanorods.

Electronic devices comprised of nanocrystal (NC) thin film are projected to demonstrate enhanced figure of merit if NC building blocks self-assemble into highly uniform, 2-dimensional (2-D) superstructures with long-range order. Despite intensive research efforts and remarkable progress, long-range assembly of colloidal anisotropic NCs into thin films with orientational and positional order has remained to be addressed. One of the most promising approaches is to dissolve excess free molecules into NC solution, which has enabled the formation of NC monolayers with exceptional quality at air/solution interface. Nevertheless, the assembly mechanism and the role of free molecules have not been comprehensively elucidated, restricting the use of the approach. Here, we find that the interfacial assembly of CdSe/CdS core/shell nanorods (NRs) results in various ordered structures in the presence of free oleic acid molecules. The structures include a bundle of standing NRs, a belt of multilayered lying NRs, and a monolayer smectic phase, obtained by simple change in density of surface ligands on the NRs. Experimental observation and theoretical calculation reveal that the assembly is initiated at the air/solution interface due to the preferential depletion attraction of NRs to the interface. However, subsequent growth is significantly altered depending on the ligand density that determines the relative magnitude of interface-NR depletion attraction to inter-NR attraction. Highly ordered structures of NRs, especially for the monolayer smectic phase, are promising as a polarized light-emitting layer for thin-film optical devices. In addition, our findings on the depletion-mediated NR assembly provide important and universal design criteria for 2-D structuring of NCs with diverse geometries and compositions.

Active Patchy Colloids with Shape-Tunable Dynamics.

Controlling the complex dynamics of active colloids-the autonomous locomotion of colloidal particles and their spontaneous assembly-is challenging yet crucial for creating functional, out-of-equilibrium colloidal systems potentially useful for nano- and micromachines. Herein, by introducing the synthesis of active "patchy" colloids of various low-symmetry shapes, we demonstrate that the dynamics of such systems can be precisely tuned. The low-symmetry patchy colloids are made in bulk via a cluster-encapsulation-dewetting method. They carry essential information encoded in their shapes (particle geometry, number, size, and configurations of surface patches, etc.) that programs their locomotive and assembling behaviors. Under AC electric field, we show that the velocity of particle propulsion and the ability to brake and steer can be modulated by having two asymmetrical patches with various bending angles. The assembly of monopatch particles leads to the formation of dynamic and reconfigurable structures such as spinners and "cooperative swimmers" depending on the particle's aspect ratios. A particle with two patches of different sizes allows for "directional bonding", a concept popular in static assemblies but rare in dynamic ones. With the capability to make tunable and complex shapes, we anticipate the discovery of a diverse range of new dynamics and structures when other external stimuli (e.g., magnetic, optical, chemical, etc.) are employed and spark synergy with shapes.

Janus Microcarriers for Magnetic Field-controlled Combination Chemotherapy of Hepatocellular Carcinoma†.

Combination chemotherapy administering multiple chemo-agents is widely exploited for the treatment of various cancers in the clinic. Specially for hepatocellular carcinoma (HCC), one of the most common malignancies, a co-administration of combinational cytostatic multi-kinase inhibitors and cytotoxic chemo-agents has been suggested as a potential curative approach. Here, Janus microcarriers were developed for the controlled local combination chemotherapy of HCC. The Janus microcarriers are composed of polycaprolactone (PCL) compartment and magnetic nanoparticles-loaded poly(lactide-co-glycolic acid) (PLGA) compartment which contain hydrophobic regorafenib and hydrophilic doxorubicin, respectively. Exploiting the magnetic anisotropy, rotational motion of the Janus microcarriers is controlled with magnetic field, which enables the active co-release of dual chemo-agents. Furthermore, Janus microcarriers exhibit magnetic resonance (MR) contrast effect, supporting the successful transcatheter intra-arterial delivery of the combination chemo-agents loaded the microcarriers to the targeted tumor. This Janus microcarriers potentially serve as a general combinational chemo-therapeutic platform for the co-delivery of various combinations of multi-chemo-agents.

Smart Microcapsules with Molecular Polarity- and Temperature-Dependent Permeability.

Microcapsules with molecule-selective permeation are appealing as microreactors, capsule-type sensors, drug and cell carriers, and artificial cells. To accomplish molecular size- and charge-selective permeation, regular size of pores and surface charges have been formed in the membranes. However, it remains an important challenge to provide advanced regulation of transmembrane transport. Here, smart microcapsules are designed that provide molecular polarity- and temperature-dependent permeability. With capillary microfluidic devices, water-in-oil-in-water (W/O/W) double-emulsion drops are prepared, which serve as templates to produce microcapsules. The oil shell is composed of two monomers and dodecanol, which turns to a polymeric framework whose continuous voids are filled with dodecanol upon photopolymerization. One of the monomers provides mechanical stability of the framework, whereas the other serves as a compatibilizer between growing polymer and dodecanol, preventing macrophase separation. Above melting point of dodecanol, molecules that are soluble in the molten dodecanol are selectively allowed to diffuse across the shell, where the rate of transmembrane transport is strongly influenced by partition coefficient. The rate is drastically lowered for temperatures below the melting point. This molecular polarity- and temperature-dependent permeability renders the microcapsules potentially useful as drug carriers for triggered release and contamination-free microreactors and microsensors.

Structural Coloration with Nonclose-Packed Array of Bidisperse Colloidal Particles.

Colloidal crystals and glasses have their own photonic effects. Colloidal crystals show high reflectivity at narrowband, whereas colloidal glasses show low reflectivity at broadband. To compromise the opposite optical properties, a simple means is suggested to control the colloidal arrangement between crystal and glass by employing two different sizes of silica particles with repulsive interparticle potential. Monodisperse silica particles with repulsive potential spontaneously form crystalline structure at volume fraction far below 0.74. When two different sizes of silica particles coexist, the arrangement of silica particles is significantly influenced by two parameters: size contrast and mixing ratio. When the size contrast is small, a long-range order is partially conserved in the entire mixing ratio, resulting in a pronounced reflectance peak and brilliant structural color. When the size contrast is large, the long-range order is rapidly reduced along with mixing ratio. Nevertheless, a short-range order survives, which causes low reflectivity at a broad wavelength, developing faint structural colors. These findings offer an insight into controlling the colloidal arrangements and provide a simple way to tune the optical property of colloidal arrays for structural coloration.