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Functional Microparticles

미세입자는 연성소재의 핵심이 되는 물질로써, 광범위한 분야에서 활용도가 매우 높은 물질이다. 균일한 크기의 미세입자는 유화중합이나 분산중합 혹은 솔-젤 반응 등을 통해 만들어져 왔다. 그러나 이러한 방법은 입자의 크기를 수 마이크로미터 이상으로 제조하기 힘들고, 입자는 등방성 구형입자로 제한이 된다. 이러한 한계를 넘어서고자, 본 연구실에서는 미세유체소자를 이용한 기능성 입자 제조 연구를 수행한다. 이는 입자의 화학적∙구조적 비등방성을 유도하여 기존의 등방 입자로 불가능하였던 다양한 응용을 가능케 하며, 새로운 물리 현상의 관찰을 가능케 한다. 예를 들어 양친성 표면을 갖는 초승달 형 입자를 짝지어진 액적으로부터 제조할 수 있고, 이를 이용해 오일과 물 및 에탄올의 혼합액 사이의 계면을 안정화 시킬 수 있다. 또한, 비슷한 방법으로 초소수성 표면과 친수성 표면을 동시에 갖는 입자를 제조하면, 이를 통해 공기와 물 사이의 계면을 안정화 및 고정화 시킬 수 있게 된다.

Anisotropic or functional microparticles have great potential as a new class of colloidal materials with advanced applications. For example, Janus particles can be used as active pigments in new types of display devices, while chemically-patterned microparticles can be used as building blocks to construct photonic structures through directional interactions. In addition, amphiphilic microparticles are useful for stabilization of the interface between two immiscible fluids. Therefore, there remains intense demand for new classes of microparticles. However, surface energy drives microparticles spherical and isotropic, which makes it difficult to design new types of microparticles. We address this problem using various drop-based approaches in microfluidics. A few examples are listed below.

Amphiphilic microparticles
Taking inspiration from superhydrophobic small objects such as the scales of butterflies or moths and the legs of water striders, we have fabricated and investigated superhydrophobic microspheres with a complex surface morphology in conjunction with hydrophobic surface moieties. The combination of superhydrophobic and hydrophilic surfaces enable the stabilization between air-water interface through strong adsorption of the particles at the interface. Of particular interest is that the high mobility of the superhydrophobic microspheres gives rise to unique interfacial properties which cannot be achieved using conventional superhydrophobic materials of solid film type.

Monodisperse amphiphilic microparticles which are highly anisotropic, with a crescent-moon shape, consisting of hydrophilic convex and hydrophobic concave surfaces can be used to stabilize oil-water interface. To make such particles, monodisperse paired oil droplets of fluorocarbon and photocurable monomer are prepared in water. The monomer droplets contain silica particles which spontaneously adsorb at the aqueous interface but do not adsorb at the fluorocarbon-oil interface. After polymerization of monomer droplets and subsequent removal of fluorocarbon droplets, two different surfaces remain on each microparticle, resulting in crescent-moon shaped amphiphilic microparticles which can be used to stabilize oil drops in water. Unlike molecular surfactants which lower interfacial tension, these amphiphilic particles are densely packed at the oil-water interface, thereby preventing direct contact between neighboring drops because they are immobilized.

1. 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).

2. Shin-Hyun Kim, Alireza Abbaspourrad, and David A. Weitz, “Amphiphilic Crescent-Moon-shaped Microparticles formed by Selective Adsorption of Colloids,” Journal of the American Chemical Society, 133, 5516-5524 (2011).

Magnetoresponsive microparticles
We have developed a simple and reliable method that enables both the introduction of surface complexity onto microparticles, as well as the remote control of the microparticle translational motion. In the proposed method, microparticles are synthesized using a combination of two distinct approaches: microfluidic synthesis to achieve size and shape uniformity, and colloidal self-organization to generate surface complexity.

1. Shin-Hyun Kim, Jae Young Sim, Jong-Min Lim and Seung-Man Yang, “ Magnetoresponsive Microparticles with Nanoscopic Surface Structures for Remote-Controlled Locomotion ,” Angewandte Chemie International Edition, 49, 3786-3790 (2010)

Microparticles with tunable refractive index
Large ensembles of microspheres are useful for creating optical resonators, microlenses, and a whole host of other photonic components. The refractive index of the microsphere material is a key property: a large index enables a broader range of applications. Titania (TiO2) has one of the highest refractive indices of all minerals, but in its most common form, rutile, it is difficult to shape into spheres. We have come up with a clever way around this problem: we use droplets of an emulsion to mould particles of titania into a spherical shape. The titania nanoparticles are suspended in a water-in-oil drop. When the water is evaporated away, the nanoparticles self assemble into identical spheres. The size of the microspheres can be controlled by the diameter of the pipette used to form the droplets and the number of nanoparticles inside. In this way the team manages to create spheres with a diameter as small as 7 μm. 
1. 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)

Photonic Microparticles
We have developed a novel and simple method for creating various structural motifs of colloidal crystals by photoinduced consolidation over fast time scales. Our strategy, in conjunction with a high-throughput optofluidic
technique, allows unprecedented control over the threedimensional organization of the colloidal particles, as well as
the combination of different materials over multiple length scales. Specifically, we demonstrate shaping colloidal crystals into various usable solid objects such as photonic (Janus) balls, cylinders, and films in a controlled manner, thus expanding the potential for specific applications such as mobile information displays, e-papers, photonic crystal sensors, and lightemission modulators.

1. 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)

2. Shin-Hyun Kim, Su Yeon Lee, Gi-Ra Yi, David J. Pine, and Seung-Man Yang, "Microwave-Assisted Self-Organization of Colloidal Particles in Confining Aqueous Droplets," Journal of the American Chemical Society, 128, 10897-10904 (2006)

Encoded microparticles and sensor particles

Suspension arrays have emerged as a promising tool for biological screening and multiplex immunoassays owing to several advantages over conventional 2D planar arrays such as faster binding kinetics, large surface areas, increased statistical power and low assay cost.Suspension arrays use identification tags on the microparticles rather than the spatial information employed in planar arrays.To produce such encoded microspheres, we used simple optofluidic approach, enabling the encoding with a photocurable double emulsion system. By using core droplets with three distinct colors—red, green and blue (RGB)—we generated optically identifiable codes by controlling the numbers of the RGB core droplets encapsulated in the shell droplet. Because the RGB cores captured in the transparent shell can be identified without spectral analysis, the information can be decoded simply by counting the respective numbers of encapsulated RGB core droplets, without the need for additional decoding equipment or devices. In addition, the abundance of achievable codes can be expressed using codes comprised of a few characters. On the other hand, the surface of each shell is decorated with an array of colloidal silica beads, which enables the creation of surface functional groups for immobilizing biomolecules.

Chemical and biomolecular detection and analysis are of increasing importance in a wide range of industrial and research fields, including medical diagnosis, drug discovery, and bioassays, as well as in the prevention and prediction of chemical-related emergencies. Surface-enhanced Raman scattering (SERS) have been emerged as a promising strategy for molecular detection that has high sensitivity and label-free identification of molecules. The excitation of localized surface plasmons and formation of charge-transfer complexes on the surface of metal nanostructures enhance the Raman signals to levels comparable to those of the fluorescence signals that have conventionally been employed in biosensor. We developed a facile approach to generating SERS-active microspheres in order to enhance the advantages and obviate the limitations of conventional SERS systems. Microfluidic emulsification was used to produce monodisperse microspheres, and a combination of colloidal self-assembly at the droplet interface and selective deposition of silver nanostructures was employed to create hierarchical metal nanopatterns on the microspheres. The resulting microspheres exhibited both significant densities of hot spots and fast binding kinetics, which can be attributed to the periodic metal nanopatterns and high mobility of the microspheres, respectively. 

1. 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 Edition50, 1171-1174 (2011). (Co-corresponding author)

2. Hyerim Hwang, Shin-Hyun Kim, and Seung-Man Yang, “Fabrication of SERS-Active Microspheres for Molecular Detection,” Lab on a Chip, 11, 87-92 (2011).