Seung-Kon Lee

Bioinspired Holographically Featured Superhydrophobic and Supersticky Nanostructured Materials

In this Letter, we present an intriguing method for fabricating polymeric superhydrophobic surfaces by reactive-ion etching of holographically featured three-dimensional structures. Using the proposed strategy, we generated both lotus and gecko surfaces by simply controlling the incident angle of the laser beam during holographic lithography. The adhesion force of the gecko-state superhydrophobic surfaces was the highest yet reported for an artificial superhydrophobic surface. The well-controlled patterns enable an in-depth understanding of superhydrophobic and superadhesive surfaces. In particular, the present observations provide direct evidence of a high adhesive force resulting from surface-localized wetting, which is quite different from previously suggested mechanisms.

Holographic fabrication of three-dimensional nanostructures for microfluidic passive mixing†

In this study, we incorporated mixing units of three-dimensional (3D) interconnected pore network inside microfluidic channels by combining single prism holographic lithography and photolithography. 3D pore network structures were generated by the interference of four laser beams generated by a truncated triangular pyramidal prism. The levelling between the 3D porous structures and the channel walls was greatly improved by employing supercritical drying, which induced negligible internal capillary stresses and reduced substantially anisotropic volume shrinkage of 3D structures. Also, complete sealing of the microfluidic chips was achieved by attaching flexible PDMS cover substrates. Overall mixing performance of the systems with completely sealed mixing units was 84% greater than that obtained without such mixers. Splitting and recombination of flows in the 3D interconnected pore structures enhanced the mixing efficiency by decreasing the diffusion path and increasing the surface contact between two liquid streams. Because the flow splitting and recombination was developed through the 3D interconnected pore network, high mixing efficiency (>0.60) was achieved at low Reynolds numbers (Re < 0.05) and Péclet numbers in the regime of Pe < 1.4 x 10(3).

Optofluidic integration of a photonic crystal nanolaser

We demonstrate a new type of photonic crystal nanolaser incorporated into a microfluidic chip, which is fabricated by multilayer soft lithography. Experimentally, room-temperature continuous-wave lasing operation was achieved by integrating a photonic crystal nanocavity with a microfluidic unit, in which the flow medium both enhances the rate of heat removal and modulates the refractive index contrast. Furthermore, using the proposed system, dynamic modulation of the resonance wavelength and far-field radiation pattern can be achieved by introducing a bottom reflector across which various fluids with different refractive indices are forced to flow. In particular, by maintaining a gap between the reflector and the cavity equal to the emission wavelength, highly efficient unidirectional emission can be obtained. The proposed nanolasers are ideal platforms for highfidelity biological and chemical detection tools in micro-total-analytical or lab-on-a-chip systems.

One-step continuous synthesis of biocompatible gold nanorods for optical coherence tomography.

We present a novel one-step flow process to synthesize biocompatible gold nanorods with tunable absorption and biocompatible surface ligands. Photothermal optical coherence tomography (OCT) of human breast tissue is successfully demonstrated using tailored gold nanorods designed to have strong absorption in the near-infrared range.

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.

High throughput synthesis of uniform biocompatible polymer beads with high quantum dot loading using microfluidic jet-mode breakup.

Uniform polymer microbeads with highly loaded quantum dots (QDs) are produced using high-throughput coherent jet breakup of a biocompatible poly(ethylene glycol) diacrylate (PEGDA) prepolymer resin, followed by in-line photopolymerization. A spiraling and gradually widening channel enables maximum absorption of radiated UV light for the in-line photopolymerization without coalescence and clogging issues. Although the dripping mode in general provides superior uniformity to the jet mode, our nozzle design with tapered geometry brings controlled jet breakup leading to 3% of uniform particle size distribution, comparable to dripping-mode performance. We achieve a maximum production rate of 2.32 kHz, 38 times faster than the dripping mode, at a same polymer flow rate. In addition, the jet-mode scheme provides better versatility with 3 times wider range of size control as well as the compatibility with viscous fluids that could cause pressure buildup in the microsystem. As a demonstration, a QD-doped prepolymer resin is introduced to create uniform biocompatible polymer beads with 10 wt % CdSe/ZnSe QD loading. In spite of this high loading, the resulting polymer beads exhibits narrow bandwidth of 28 nm to be used for the ultrasensitive bioimaging, optical coding, and sensing sufficiently with single bead.

Photonic-crystal-based centrifugal microfluidic biosensors

In this paper, we report a fast and facile method for fabricating colloidal photonic crystals inside microchannels of radially symmetric microfluidic chips. As the suspension of monodisperse silica or polystyrene latex spheres was driven to flow through the channels under the centrifugal force, the colloidal spheres were quickly assembled into face centered cubic arrangement which had photonic stop bands. The optical reflectance spectrum was modulated by the refractive-index mismatch between the colloidal particles and the solvent filled in the interstices between the particles. Therefore, the present microfluidic chips with built-in colloidal photonic crystals can be used as in-situ optofluidic microsensors for high throughput screening, light filters and biosensors in integrated adaptive optical devices.

Microfluidic channel with built-in photonic crystal nanolaser

We propose and demonstrate a new type of a photonic crystal nanolaser integrated into a microfluidic chip, which is fabricated by multilayer soft lithography. Experimentally, continuous-wave operation of the lasing action has been observed owing to efficient water-cooling. Characteristics of wavelength tuning by the fluid are investigated using both theory and experiment. In addition, we propose that dynamic modulation of far-field radiation pattern can be achieved by introducing a bottom reflector and by flowing the fluid on it. Especially, by choosing effective one-wavelength distance between the reflector and the cavity, efficient unidirectional emission can be obtained.

Holographic fabrication of hierarchical nanostructures using microprism array toward optofluidic integration

Holographic lithography provides a highly compatible and facile way to fabricate multi-dimensional periodic nanostructures. Periodic nanostructures have useful applications not only as biological substrates or catalytic supports but also as nanophotonic devices with various photonic properties such as photonic band-gap (PBG), localized surface plasmon resonance (LSPR) or surface enhanced Raman scattering (SERS). In combination with single refracting prism holographic lithography and conventional photolithography, we could achieve the micrometer-scale patterns of periodic nanostructures which can be integrated in microfluidic chip. With the help of conventional MEMS technologies, Arrays of pyramid shape and top-cut pyramid shape microprism can be prepared. Single laser exposure step through the microprism arrays (MPAs) can be generate multiscale patterns of 2D and 3D nanostructures. As prepared nanostructures combined with microfluidic chip is a highly efficient optofluidic platform which is applicable to the chemical and biosensors.

Holographic fabrication of photonic nanostructures for optofluidic integration

Holographic lithography is one of the promising techniques that can create three-dimensional (3D) periodic nanostructures without extensive lithography and etching steps. This proceeding discusses novel hybrid lithographic methods based on the holographic lithography in conjunction with photolithography to generate hierarchically-patterned structures. Using various types of photoresists including positive, negative and hydrogel, we fabricated 3D nanopatterns by holographic lithography. Then, two-dimensional (2D) photolithography was combined to pattern the 3D structures. Eventually, we created a microfluidic channel with 3D periodic patterns. Since the 3D structure possess photonic bandgap properties as well as interconnected pore networks, this kind of microfluidic channel can be applied to optical sensors, mixers and filters.