Chul-Joon Heo

Flexible, Angle-Independent, Structural Color Reflectors Inspired by Morpho Butterfly Wings

Thin-film color reflectors inspired by Morpho butterflies are fabricated. Using a combination of directional deposition, silica microspheres with a wide size distribution, and a PDMS (polydimethylsiloxane) encasing, a large, flexible reflector is created that actually provides better angle-independent color characteristics than Morpho butterflies and which can even be bent and folded freely without losing its Morpho-mimetic photonic properties.

Controlled Origami Folding of Hydrogel Bilayers with Sustained Reversibility for Robust Microcarriers

Microencapsulation and controlled release have long been studied because of the high demand for practical delivery systems in the pharmaceutics and cosmetics fields. Multiphase emulsion drops have provided efficient templates for microcapsules, and various feasible methods have been developed for controlled release. However, the emulsion-based approach has limitations for the in situ control of membrane permeability. Micro-origami has emerged as one of the most promising alternative approaches for producing tunable microcapsules with the potential to be applied, for example as drug carriers, actuators, microcontainers, and microrobots. Inspired by living organisms in nature such as the ice plant and Venus flytrap, two different micro-origami approaches have been employed to make various microstructures. One approach uses solid patches connected by active hinge materials. Typical examples use various metal– metal, metal–polymer, and polymer–polymer combinations. The patch and hinge system has enabled the capture, release, and gripping of target materials, showing the feasibility of micro-origami structures. However, the microcapsule is limited to polyhedral shapes in this approach, and complete sealing of the gaps between patches requires exquisite control of the folding angles. Moreover, the delicate and complex fabrication processes make practical applications difficult. The second approach uses a bilayer structure composed of two different materials. For example, a metal– polymer bilayer can show bending/unbending when the polymeric active layer suffers significant volume change, but the metal layer remains unchanged. 13] Polymer materials have been employed in both layers to make biocompatible microcapsules. 15] However, complete sealing of the gaps in the bilayer contact regions remains an important, yet unmet, need. In addition, a simple and effective method for the fabrication of practical microcapsules has not yet been developed, and remains highly desirable. This is the main thrust of the present study. Herein, we report the use of biocompatible bilayer structures for the fabrication of tunable microcapsules based on micro-origami. Monodisperse bilayer microstructures were prepared using a facile photolithographic procedure, without employing photomask alignment. In addition, highly flexible hydrogels were selected as both active and passive layers, facilitating tight contact between patches. The bilayer structure therefore enabled in situ encapsulation, through a reversible transformation to microcapsules with a closed compartment. The resultant microcapsules showed negligible leakage of encapsulants and triggered release of the encapsulants could be achieved simply by inducing the unfolding of the hydrogel bilayer. The essential strategy of our approach relies on the anisotropic volume change of a hydrogel bilayer. As shown in Scheme 1a, the active hydrogel layer shows significant volume expansion under external stimuli by swelling, whereas the passive hydrogel layer remains in a constant volume. Therefore, mechanical stress drives the bending of the bilayer, resulting in microcapsules with a closed compartment. The hydrogel swelling behavior is highly reversible, enabling repeated transformations. The hydrogel bilayer structure was prepared on a glass substrate, using photolithography with an amorphous silicon photomask, as shown in Scheme 1b. Here, we propose poly(2hydroxyethyl methacrylate-co-acrylic acid), p(HEMA-coAA), and poly(2-hydroxyethyl methacrylate), p(HEMA), as model components because they are widely used, FDAapproved (FDA = Food and Drug Administration) biocompatible materials. One monomer solution for p(HEMA-coAA) was infiltrated into the space between the photomask and a polydimethylsiloxane (PDMS) microchannel of 25 mm thickness; this monomer solution was then polymerized by UV irradiation through the photomask. The second monomer solution for p(HEMA) was infiltrated into the space between the same photomask and a PDMS microchannel 50 mm in thickness, after washing out the previously unpolymerized solution. Upon the second round of UV irradiation, bilayer structures consisting of a p(HEMA) layer on a p(HEMA-coAA) layer were formed; alignment of the photomask was unnecessary, because each layer was fabricated on the photomask. The resultant bilayer structures were released from the photomask through immersion in a pH 9 buffer solution. To exploit the structural transformation of the bilayer microparticles, we used two different shapes of microparticle: snowman-shaped and flower-shaped. The shape and dimensions of these microparticles were carefully determined to ensure a fully closed compartment in the swollen state; both the snowmanand flower-shaped microparticles were 50 mm [*] T. S. Shim, Dr. S.-H. Kim, Dr. C.-J. Heo, H. C. Jeon, Prof. S.-M. Yang National Creative Research Initiative Center for Integrated Optofluidic Systems and Department of Chemical and Biomolecular Engineering KAIST Daejeon, 305-701 (Korea) E-mail: smyang@kaist.ac.kr Homepage: http://msfl.kaist.ac.kr

Durable plasmonic cap arrays on flexible substrate with real-time optical tunability for high-fidelity SERS devices.

Active tunable plasmonic cap arrays were fabricated on a flexible stretchable substrate using a combination of colloidal lithography, lift-up soft lithography, and subsequent electrostatic assembly of gold nanoparticles. The arrangement of the plasmonic caps could be tuned under external strain to deform the substrate in reversible. Real-time variation in the arrangement could be used to tune the optical properties and the electromagnetic field enhancement, thereby a proving a promising mechanism for optimizing the SERS sensitivity.

Organic-on-silicon complementary metal-oxide-semiconductor colour image sensors

Complementary metal–oxide–semiconductor (CMOS) colour image sensors are representative examples of light-detection devices. To achieve extremely high resolutions, the pixel sizes of the CMOS image sensors must be reduced to less than a micron, which in turn significantly limits the number of photons that can be captured by each pixel using silicon (Si)-based technology (i.e., this reduction in pixel size results in a loss of sensitivity). Here, we demonstrate a novel and efficient method of increasing the sensitivity and resolution of the CMOS image sensors by superposing an organic photodiode (OPD) onto a CMOS circuit with Si photodiodes, which consequently doubles the light-input surface area of each pixel. To realise this concept, we developed organic semiconductor materials with absorption properties selective to green light and successfully fabricated highly efficient green-light-sensitive OPDs without colour filters. We found that such a top light-receiving OPD, which is selective to specific green wavelengths, demonstrates great potential when combined with a newly designed Si-based CMOS circuit containing only blue and red colour filters. To demonstrate the effectiveness of this state-of-the-art hybrid colour image sensor, we acquired a real full-colour image using a camera that contained the organic-on-Si hybrid CMOS colour image sensor.

High-fidelity optofluidic on-chip sensors using well-defined gold nanowell crystals.

Recent advances in nanofabrication techniques have enabled the creation of various metallic nanostructures in order to engineer the location and properties of electromagnetic hot spots in a controlled manner. However, most previous methods usually require complicated and time-consuming techniques, and the integration of metallic nanostructures into simple, low-cost devices for chemical or biological sensing is still challenging. Here, we report a promising new strategy for the fabrication of large-area gold nanowell arrays with novel geometric features that makes use of the trapping of self-assembled colloidal particles on a polymer surface. Through both systematic experimental and theoretical analysis, we confirm that the strong plasmon resonances of the proposed nanowell structures are associated with localized surface plasmon resonance (LSPR) on the brims of the nanoholes in the top gold films as well as in the bottom gold disks. In addition, we demonstrate a novel optofluidic platform with built-in subwavelength nanowell arrays that exhibits strong plasmon resonances within microfluidic chips. In our optofluidic systems, the plasmon coupling between the brims and the disks of nanowells makes the plasmon resonance more sensitive to surrounding materials. The dependence of the plasmon resonance on the refractive index of the surrounding medium is found to be as high as 570 nm RIU(-1) (refractive index units). These data lead to a figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 4.1.

Biofunctional colloids and their assemblies

Colloidal particles are used as elemental building blocks to construct biofunctional nanostructures. In particular, multidimensional periodic arrangements of colloidal particles such as planar arrays and spherical assemblies can be used in a wide range of biological fields. The spatial regularity of such structures at the submicron-scale gives rise to special features such as a photonic bandgap (PBG) and selective permeability, which cannot be achieved by single colloidal particles. Recent advances in microfluidics technologies enable the fabrication of designed microparticles of equal size and shape in a continuous manner. Such microparticles have great potential for use in high-throughput screening and immunoassays. In this article, we review the current state-of-the-art in regard to colloidal assemblies and microparticles prepared by microfluidics for biological applications. This review consists of five main sections: (1) surface modification methods, (2) two dimensional (2D) and (3) three dimensional (3D) colloidal assemblies, (4) confined regular structures, and (5) novel fabrication strategies for advanced colloidal assemblies. In each section, we discuss not only the fabrication routes for biofunctional materials but also the characteristics of the materials and their biological applications. Finally, we outline the future perspectives for biofunctional colloidal materials.

Photothermolysis of immobilized bacteria on gold nanograil arrays

Photothermolysis technique via array of gold nanograils had been developed by illuminating near infrared laser light onto captured bacteria in metal nanostructure. The strong electromagnetic field enhancement at the sharp edges of the gold nanograils produced local heating that was sufficient to break the thick cell walls of the gram-positive Staphylococcus aureus cells within a short time. Individual cells in the nanograil array can be selectively lysed by adjusting the laser scanning area to the micrometer scale.

Lithographically-featured photonic microparticles of colloidal assemblies

We have described a new and promising strategy for the fabrication of composite and porous photonic crystal microparticles that combines the self-assembly of colloidal particles with photolithography techniques. We fabricated silica/SU-8 composite microparticles with photonic bandgaps via four steps: (1) deposition of the silica colloidal crystals on the photoresist, (2) embedding of the colloidal crystals in the photoresist, (3) UV exposure through a photomask and subsequent development, and (4) release of the microparticles from the substrate. Embedding was performed above the glass transition temperature (T(g)) of uncrosslinked SU-8. At such temperatures, capillary forces on the silica particles facilitate the migration of colloidal crystals in the SU-8 matrix. Particle migration ceased when the top colloidal crystal layer was trapped at the interface between air and SU-8. In addition, we also prepared porous microparticles with an inverse opaline structure by dissolving the embedded silica particles from the composite structures. The porous microparticles showed enhanced reflectivity at the bandgap position due to the large refractive index contrast. The bandgap position of the microparticles was controlled by the size of the silica particles, which determined the lattice constant. Bilayered composite and porous microparticles with two distinct photonic bandgaps were also prepared by sequential deposition of colloidal crystals composed of two differently sized silica particles.

Stability enhancement of an electrically tunable colloidal photonic crystal using modified electrodes with a large electrochemical potential window

The color tuning behavior and switching stability of an electrically tunable colloidal photonic crystal system were studied with particular focus on the electrochemical aspects. Photonic color tuning of the colloidal arrays composed of monodisperse particles dispersed in water was achieved using external electric field through lattice constant manipulation. However, the number of effective color tuning cycle was limited due to generation of unwanted ions by electrolysis of the water medium during electrical switching. By introducing larger electrochemical potential window electrodes, such as conductive diamond-like carbon or boron-doped diamond, the switching stability was appreciably enhanced through reducing the number of ions generated.

Metal nanograil arrays with tunable multiple dipolar plasmon modes in integrated optofluidic devices for ultrasensitive sensing of biomolecules

Biomolecular detection using Localized Surface Plasmon Resonances (LSPR) has been extensively investigated because these techniques enable label-free detection. The high-density metal nanopatterns with tunable LSPR characteristics have been used as refractive index sensing because LSPR property is highly sensitive to refractive index change of surroundings. Meanwhile, Colloidal lithography is a robust method for fabricating regularly ordered nanostructures in a controlled and reproducible way using spontaneous assembly of colloidal particles. In this study, nanopatterns on UV-curable polymer were prepared via colloidal lithography. Then, metallic nanograil arrays with high density were fabricated by sputtering noble metals such as gold and subsequent removal of residual polymers and colloidal particles. From Finite-Difference Time-Domain Method (FDTD) simulations and reflectance spectra, we found that multiple dipolar plasmon modes were induced by gold nanograil arrays and each mode was closely related with structural parameters. LSPR characteristics of gold nanograil arrays could be tuned by varying the fabrication conditions to obtain optimal structures for LSPR sensing. Sensing behavior of gold nanograil arrays was tested by applying various solvents with different refractive indices and measuring the variations of LSPR dips. Finally, gold nanograil arrays as LSPR sensors were integrated in optofluidic devices and used to achieve real-time label-free monitoring of biomolecules.