Cover Gallery

Photonic Multishells: Photonic Multishells Composed of Cholesteric Liquid Crystals Designed by Controlled Phase Separation in Emulsion Drops (Adv. Mater. 30/2020)

Sihun Park, Sang Seok Lee*, and Shin‐Hyun Kim*

In article number 2002166, Sang Seok Lee, Shin‐Hyun Kim, and Sihun Park design onion‐like photonic multishells with distinct photonic bandgaps. The multilayered structure is spontaneously created in emulsion drops by consecutive steps of phase separation between cholesteric liquid crystals (CLCs) and hydrophilic liquid, triggered by material exchange with the continuous phase. During the phase separation, a chiral dopant is unevenly partitioned in the CLC shells, resulting in multiple stopbands. The photonic multishells provide multi‐wavelength lasing and advanced optical barcoding.

Plasmonic Microspheres: Plasmonic Janus Microspheres Created from Pickering Emulsion Drops (Adv. Mater. 26/2020)

Jong Bin Kim, Su Yeon Lee, Nam Gi Min, Seung Yeol Lee, and Shin‐Hyun Kim*

In article 2001384, Shin‐Hyun Kim and co‐workers design plasmonic Janus microspheres using emulsion drops whose interface is decorated by particles. The emulsion drops made of acrylate resin are photopolymerized and the particles are removed. The directional deposition of the metal renders the top hemispheres as plasmonically colored and the position of the particles at the interface is precisely controlled by aging, which enables the tuning of the plasmonic property. The plasmonic colors can be switched on and off by controlling the orientation of the Janus microspheres with an external electric field.

Hyerim Hwang, Yong Chan Cho, Sooheyong Lee, Tae Min Choi, Shin-Hyun Kim*, and Geun Woo Lee*, "Real-Time Monitoring of Colloidal Crystallization in Electrostatically-Levitated Drops", Small, 16, 1907478 (2020).

In article number 1907478, Shin‐Hyun Kim, Geun Woo Lee, and co‐workers present a unique platform of an electrostatically‐levitated spherical droplet for a real‐time in‐situ probing of colloidal crystallization. The gradual evaporation of the levitated droplet initiates the crystallization, of which detailed progression is monitored by simultaneous measurements of volume change and reflectance spectra of the droplet. This work provides an ideal testbed for the study of structural evolution of the colloidal assembly.

Dong Jae Kim, Sung-Gyu Park, Dong-Ho Kim, and Shin-Hyun Kim, "SERS-Active Charged Microgels for Size- and Charge-Selective Molecular Analysis of Complex Biological Samples", Small, 14, 1802520 (2018). (Co-corresponding author)

In article number 1802520, Dong‐Ho Kim, Shin‐Hyun Kim, and co‐workers design charged microgels to encapsulate agglomerates of gold nanoparticles for the direct analysis of biological samples through Raman analysis. The charged microgels concentrate oppositely charged small molecules while excluding large proteins, increasing Raman intensity and preventing gold contamination. Using the microgels, fipronil sulfone, a metabolite of a toxic insecticide dissolved in eggs, is directly detected without any sample pretreatment.

Sang Seok Lee, Hyeon Jin Seo,Yun Ho Kim, and Shin-Hyun Kim, "Structural Color Palettes of Core-Shell Photonic Ink Capsules containing Cholesteric Liquid Crystals", Advanced Materials, 29, 1606894 (2017).(Co-corresponding author)

In article number 1606894, structural colors are mixed by Yun Ho Kim, Shin-Hyun Kim, and co-workers to yield new colors by designing core–shell capsules that contain two distinct helical structures of cholesteric liquid crystals. The core reflects right-handed red light and the shell reflects left-handed green light, thereby mixing the two colors to develop yellow.

Soojeong Cho, Tae Soup Shim, Ju Hyeon Kim, Dong-Hyun Kim, and Shin-Hyun Kim, "Selective Coloration of Melanin Nanospheres through Resonant Mie Scattering", Advanced Materials, 29, 1700256 (2017).

Black melanin inks are prepared to selectively exhibit colors under strong light, inspired by human hair. High absorbance of melanin suppresses multiple scattering, causing resonant Mie scattering predominant. Various colors can be developed as the resonant wavelength dictated by nanosphere diameter. Therefore, the melanin inks can be used to encrypt and selectively disclose multicolor patterns for anti-counterfeiting applications.

Sang Seok Lee, Su Kyung Kim, Jong Chan Won, Yun Ho Kim, and Shin-Hyun Kim, "Reconfigurable Photonic Capsules Containing Cholesteric Liquid Crystals with Planar Alignment," Angewandte Chemie International Edition, In press (2015).

Cholesteric liquid crystals (CLCs) reflect selected wavelengths of light owing to their periodic helical structures. The encapsulation of CLCs leads to photonic devices that can be easily processed and might be used as stand-alone micro-sensors. However, when CLCs are enclosed by polymeric membranes, they usually lose their planar alignment, leading toa deterioration of the optical performance. A microfluidics approach was employed to integrate an ultra-thin alignment layer into microcapsules to separate the CLC core and the elastomeric solid membrane using triple-emulsion drops as the templates. The thinness of the alignment layer provides high lubrication resistance, preserving the layer integrity during elastic deformation of the membrane. The CLCs in the microcapsules can thus maintain their planar alignment, rendering the shape and optical properties highly reconfigurable.

Tae Soup Shim, Seung-Man Yang, and Shin-Hyun Kim, "Dynamic Designing of Microstructures by Chemical Gradient-Mediated Growth," Nature Communications, 6, 6584 (2015).

Hyelim Kang, Joon-Seok Lee, Won Seok Chang, and Shin-Hyun Kim, "Liquid-Impermeable Inverse Opals with Invariant Photonic Bandgap," Advanced Materials, 27, 1282−1287 (2015)

Omniphobic inverse opals are created by structurally and chemically modifying the surface of inverse opals through reactive ion etching. During the etching, void arrays of the inverse opal surface evolves to a triangular post array with re-entrant geometry. The elaborate structure can efficiently pin the air–liquid interface and retain air cavities against water and oil, thereby providing liquid-impermeable inverse opals with invariant photonic bandgap.

Sang Seok Lee, Bomi Kim, Su Kyung Kim, Jong Chan Won, Yun Ho Kim, and Shin-Hyun Kim, "Robust Microfluidic Encapsulation of Cholesteric Liquid Crystals toward Photonic Ink Capsules," Advanced Materials, 27, 627−633 (2015)

Robust photonic microcapsules are created by microfluidic encapsulation of cholesteric liquid crystals with a hydrogel membrane. The membrane encloses the cholesteric core without leakage in medium of water and the core exhibits pronounced structural colors. The photonic ink capsules which have precisely controlled bandgap position and size will provide new opportunity in colorimetric micro-thermometers and optoelectric applications.

Hyelim Kang, Shin-Hyun Kim, Seung-Man Yang, and Ji-Ho Park, "Bio-inspired nanotadpoles with component-specific functionality," Journal of Materials Chemistry B, 2, 6462-6466 (2014).

We report a new class of bio-inspired nanotadpoles (NTPs) with component-specific functionalities. The plasmonic NTPs with a gold-coated head and a reactive ion etching-treated tail showed the tail length dependence of their cellular uptake, enabling the photothermal treatment of cancer cells with high efficacy.

Hye Soo Lee, Ju Hyeon Kim, Joon-Seok Lee, Jae Young Sim, Jung Yoon Seo, You-Kwan Oh, Seung-Man Yang, and Shin-Hyun Kim, "Magnetoresponsive Discoidal Photonic Crystals towards Active Color Pigments," Advanced Materials, 26, 5801-5807 (2014).

Morpho butterflys show beautiful colors that arise from periodic nanostructures. Inspired by the butterfly, colloidal photonic crystals are designed to have a form of microdisks, which are then further rendered to be magnetoresponsive. S.-H. Kim and co-workers demonstrate that magnetic-field-controlled flipping of the photonic microdisks enables the switching of structural colors, thereby providing a photonic microdisk display.

Shin-Hyun Kim, Tae Yong Lee and Sang Seok Lee, "Osmocapsules for Direct Measurement of Osmotic Strength," Small, 10, 1155-1162 (2014)

Monodisperse microcapsules with ultra-thin membrane are microfluidically-designed to be highly sensitive to osmotic pressure, thereby providing a tool for direct measurement of osmotic strength. To make such osmocapsules, water-in-oil-in-water double-emulsion drops with ultra-thin shell are prepared as templates through emulsification of core-sheath biphasic flow in a capillary microfluidic device. By employing a set of distinct osmocapsules confining aqueous solutions with different osmotic strengths, the osmotic strength of unknown solutions can be estimatedthrough observation of the capsules which are selectively buckled. This approach provides the efficient measurement of osmotic strength with very small volume of liquid, thereby providing a useful alternative to other measurement methods which use complex setups. In addition, in-vivo measurement of the osmotic strength can be potentially accomplished by implanting these biocompatible osmocapsules into tissue, which is difficult to achieve in conventional methods.

Ju Hyeon Kim, Tae Yoon Jeon, Tae Min Choi, Tae Soup Shim, Shin-Hyun Kim and Seung-Man Yang, "Droplet Microfluidics for Producing Functional Microparticles,"Langmuir, 30, 1473-1488 (2014).

Recent advances in droplet microfluidics have enabled the production of monodisperse emulsions that yield highly uniform microparticles, albeit only on a drop-by-drop basis. In addition, microfluidic devices have provided a variety of means for particle functionalization through shaping, compartmentalizing, and microstructuring. These functionalized particles have significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications.significant potential for practical applications as a new class of colloidal materials. This feature article describes the current state of the art in the microfluidic-based synthesis of monodisperse functional microparticles. The three main sections of this feature article discuss the formation of isotropic microparticles, engineered microparticles, and hybrid microparticles. The complexities of the shape, compartment, and microstructure of these microparticles increase systematically from the isotropic to the hybrid types. Each section discusses the key idea underlying the design of the particles, their functionalities, and their applications.

Tae Soup Shim, Shin-Hyun Kim, and Seung-Man Yang, "Elaborate Design Strategies toward Novel Microcarriers for Controlled Encapsulation and Release," Particle & Particle System Characterization (2013). (Co-corresponding author)

Novel microcarriers for encapsulation of bioactive agents have been extensively investigated for therapeutic applications. Recent advances in microfluidics and other techniques have inspired the design of new microcarriers with precisely controlled size, shape, and function, which possibly allow for new medical and biological applications. Various types of novel microcarriers, including block-copolymer nanoparticles, cylindrical microparticles, dense-shell microcapsules, macroporous microcapsules, polymer vesicles, and foldable bilayer microparticles, are reviewed by Seung-Man Yang and co-workers on page 9.

Sujit S. Datta, Shin-Hyun Kim, Jayson Paulose, Alireza Abbaspourrad, David R. Nelson, and David A. Weitz, "Delayed buckling and guided folding of inhomogeneous capsules," Physical Review Letters 109, 134302(2012). (The first three authors contributed equally to this work.)

Optical microscope images of a colloidal capsule with an inhomogeneous shell, buckled under an external osmotic pressure. Numerically simulated shell similar to the experimental capsule; color reflects spatially varying shell thickness (bottom). Elapsed time increases from left to right. [Sujit S. Datta et al., Phys. Rev. Lett.109, 134302 (2012)]

Tae Soup Shim, Shin-Hyun Kim, Chul-Joon Heo, Hwan Chul Jeon and Seung-Man Yang, "Controlled Origami Folding of Hydrogel Bilayers with Sustained Reversibility for Robust Microcarriers," Angewandte Chemie International Edition, 51, 1420-1423 (2012).

Origami of hydrogel bilayers provides robust mircocapsules through anisotropic volume expansion. In their Communication on page 1420 ff., S.-M. Yang and co-workers show planar bilayer microparticles composed of active and passive layers that can transform into microcapsules with a closed compartment. The reversible transformation by folding and unfolding of microparticles enables in situ encapsulation and triggered release of micro- to nanoscopic encapsulants.

Shin-Hyun Kim, Woong Chan Jeong, Hyerim Hwang and Seung-Man Yang, “Robust Chirped Photonic Crystals Created by Controlled Colloidal Diffusion,” Angewandte Chemie International Edition, 50, 11649-11653 (2011).

An on-chip spectrometer has been developed using chirped 3D photonic crystals mounted on a complementary metal-oxide-semiconductor sensor array. In their Communication on page 11 649 ff., S.-H. Kim, S.-M. Yang, and co-workers show that colloidal diffusion in a photocurable medium created gradual variations in the lattice constant of the colloidal crystals. The variations in the lattice constant resulted in a color gradient that spanned the entire visible range.

Shin-Hyun Kim, Hyerim Hwang, Che Ho Lim, Jae Won Shim and Seung-Man Yang, “Packing of emulsion droplets: Structural and functional motifs for multi-cored microscapsules,” Advanced Functional Materials, 21, 1608-1615 (2011). (Co-corresponding author)

Multicompartment microcapsules with a unique configuration are presented by Shin-Hyun Kim, and Seung-Man Yang, and co-workers on page 1608. Photocurable densely confined core droplets within an oily shell droplet rearrange into a unique configuration that minimizes the interfacial energy. Photopolymerization of the shell phase results in microcapules that are capable of isolating active materials and releasing them in a controlled manner using well-defined nanohole arrays or photothermal nanoscopic silver architectures on thin membranes.

Shin-Hyun Kim, Jae Won Shim and Seung-Man Yang, “Microfluidic Multi-Color Encoding of Microspheres with Nanoscopic Surface Complexity for Multiplex Immunoassays,” Angewandte Chemie International Edition, 50, 1171-1174 (2011).

Transparent microspheres can be optically encoded with color core droplets, as shown by S.-H. Kim, J.W. Shim, and S.-M. Yang in their Communication on page 1171 ff. A microfluidic device produces and manipulates double-emulsion droplets containing the desired number of color core droplets with unprecedented controllability. The droplets are then photopolymerized to produce microspheres. Silica particle arrays on the surface of the encoded microspheres enable biomolecules to be immobilized.

Shin-Hyun Kim, Su Yeon Lee, and Seung-Man Yang, “Janus Microspheres for Highly Flexible and Impregnable Water-Repelling Interface,” Angewandte Chemie International Edition, 49, 2535-2538 (2010)

Janus microspheres composed of two hemispherical surfaces with distinctly different surface complexities show strongly contrasting water affinities between the two halves. As S.-H. Kim, S.-Y. Lee, and S.-M. Yang describe in their Communication on page 2535 ff., the microspheres were prepared by a simple process that commences with Pickering emulsion droplets. Placing the Janus particles at an air–water interface resulted in the formation of a highly flexible and robust superhydrophobic membrane.

Shin-Hyun Kim, Hyo Sung Park, Jae Hoon Choi, Jae Won Shim, and Seung-Man Yang, “Integration of Colloidal Photonic Crystals toward Miniaturized Spectrometers,” Advanced Materials, 22, 946-950 (2010)

The cover shows a schematic illustration of patterned colloidal photonic crystals with different bandgap positions. When an unknown light source impinges on the patterned photonic crystals, the light information can be identified from the reflection intensity profile of the constituent photonic crystals. The two optical microscopy images and background image display integrated photonic crystals with 20 different bandgaps spanning the entire visible range, and the SEM image shows the cross-section of the photonic crystal stripes, as reported by Shin-Hyun Kim, Seung-Man Yang, and co-workers on p. 946

Shin-Hyun Kim, Se-Heon Kim, and Seung-Man Yang, “Patterned Polymeric Domes with 3D and 2D Embedded Colloidal Crystals using Photocurable Emulsion Droplets,” Advanced Materials, 21, 3771-3775 (2009)

The inside cover shows a scheme for the preparation of photonic dome patterns, SEM images of a dome pattern, and a single dome decorated with 2D colloid array, as fabricated in work reported on p 3771 by Seung-Man Yang and co-workers. The background is an optical microscopy image of patterned photonic domes, which can be used as a near-field microlens array. The greenish color of the domes corresponds to the photonic bandgap.

Seung-Man Yang, Shin-Hyun Kim, Jong-Min Lim and Gi-Ra Yi, "Synthesis and assembly of structured colloidal particles", Journal of Materials Chemistry, 18, 2177-2190 (2008)

Synthesis and self-assembly of structured colloids is a nascent field. Recent advances in this area include the development of a variety of practical routes to produce robust photonic band-gap materials, colloidal lithography for nanopatterns, and hierarchically structured porous materials with high surface-to-volume ratios for catalyst supports. To improve their properties, non-conventional suprastructures have been proposed, which could be built up using binary or bimodal mixtures of spherical particles and particles with internal or surface nanostructures. This Feature Article will describe the state-of-the-art in colloidal particles and their assemblies. The paper consists of three main sections categorized by the type of colloid, namely shape-anisotropic particles, chemically patterned particles and internally structured particles. In each section, we will discuss not only synthetic routes to uniform colloids with a range of structures, features and shapes, but also self-organization of these colloids into macrocrystalline structures with varying nanoscopic features and functionalities. Finally, we will outline future perspectives for these colloidal suprastructures.

Shin-Hyun Kim, Young-Sang Cho, Seog-Jin Jeon, Tai Hee Eun, Gi-Ra Yi and Seung-Man Yang, “Microspheres with Tunable Refractive Index by Controlled Assembly of Nanoparticles,” Advanced Materials, 20, 3268-3273 (2008)

The cover presents a novel method for synthesizing microspheres with tunable refractive index by controlled assembly of multicomponent nanoparticles. The backdrop comprises highly monodisperse water-in-oil emulsion droplets in which nanoparticles such as silica, titania, and gold are encapsulated and dispersed finely. Subsequently, as water is removed from the emulsion droplets by evaporation, the encapsulated nanoparticles spontaneously assemble into microspheres of all equal size, as schematically illustrated on the cover. In their Communication on p. 3268, Seung-Man Yang and co-workers create composite microspheres by using colloidal mixture suspensions of different materials. This is technologically important because the functional properties such as refractive index can be tuned by changing the components and their mixing ratio.

Shin-Hyun Kim, Seog-Jin Jeon, Gi-Ra Yi, Chul-Joon Heo, Jae Hoon Choi and Seung-Man Yang, “Optofluidic Assembly of Colloidal Photonic Crystals with Controlled Sizes, Shapes and Structures,” Advanced Materials, 20, 1649-1655 (2008)

The cover shows a schematic of the optofluidic assembly of colloidal photonic crystals against a backdrop of arrays of blue, green, and red photonic spheres self-organized by in situ photo-induced solidification of crystalline colloidal arrays. This high-throughput optofluidic technique can create various structural motifs in photonic crystals, report Seung-Man Yang and co-workers on p. 1649; an example is photonic Janus spheres, as highlighted on the cover.