Seung Hyeon Ko

Surface-Mediated DNA Self-Assembly

This communication reports a strategy for solid surface-mediated DNA self-assembly. DNA molecules weakly interact with solid surfaces; thus are confined to solid surfaces. The confinement reduces the flexibility of DNA nanomotifs and promotes the DNA 2D crystals to grow on solid surfaces. As a demonstration, periodic DNA nanoarrays have been directly assembled onto mica surfaces. Such in situ assembly eliminates the sample transfer process between assembly and characterization and possible applications.

Symmetry Controls the Face Geometry of DNA Polyhedra

Two complementary strategies have been developed to control the face geometry during the self-assembly of DNA polyhedra from branched DNA nanomotifs (tiles). In these approaches, any two interacting tiles are not equivalent in terms of either sequence or orientation; thus, each face must contain an even number of tiles. As a demonstration, DNA cubes, whose each face contains four tiles, have been assembled through these approaches.

Synergistic self-assembly of RNA and DNA molecules

DNA has recently been used as a programmable ‘smart’ building block for the assembly of a wide range of nanostructures. It remains difficult, however, to construct DNA assemblies that are also functional. Incorporating RNA is a promising strategy to circumvent this issue as RNA is structurally related to DNA but exhibits rich chemical, structural and functional diversities. However, only a few examples of rationally designed RNA structures have been reported. Herein, we describe a simple, general strategy for the de novo design of nanostructures in which the self-assembly of RNA strands is programmed by DNA strands. To demonstrate the versatility of this approach, we have designed and constructed three different RNA–DNA hybrid branched nanomotifs (tiles), which readily assemble into one-dimensional nanofibres, extended two-dimensional arrays and a discrete three-dimensional object. The current strategy could enable the integration of the precise programmability of DNA with the rich functionality of RNA.

Quantum-Dot Fluorescence Lifetime Engineering with DNA Origami Constructs†

The ability to organize nanostructures of disparate types and materials—such as metal nanoparticles and semiconductor quantum dots—is challenging but essential for the creation of novel materials and devices. Metal nanoparticles (NPs) have interesting individual plasmonic properties and can be organized to exhibit useful collective responses. Quantum dots (Qdots) provide a powerful means to optically access the nanoscale. Bringing the two together in a well-controlled manner can create structures with interesting properties such as fluorescence enhancement/quenching and high efficiency Fçrster resonance energy transfer. It also has been an area of intense study, both theoretical and experimental, for a wide range of applications including photodetectors, optical modulators and nanoscale lasers. In particular, changing the fluorescence intensity and lifetime of Qdots, when proximate to metal NPs can be used in sensing applications because of the strong distance dependence of the interaction between the Qdots and the ability to engineer the properties (e.g. size, absorbance/emission spectrum) of the individual components over wide ranges. Numerous strategies have been used to connect and control the distance between Qdots and NPs with nanometer precision. 3,7–9] The structures used previously, however, exhibit a limited persistence length (about 50 nm), making the construction of complex geometries required for eventual use in real devices challenging. DNA origami offers a platform with significant advantages: a more rigid scaffold to organize various moieties, increased geometrical complexity, no need to control the stoichiometry of the spacer per NP, and more control over distances. We have therefore chosen to exploit DNA origami for this purpose. We have developed a novel, flexible approach to fluorescence lifetime engineering of CdSe/ZnS (core/shell) Qdots by controlling their coupling to adjacent gold nanoparticles (AuNPs) at geometrically different locations on the DNA origami. To examine these templates in their native state in solution, we use a three-dimensional (3D), real-time, single-particle tracking system. We determine the influence of AuNPs on the Qdot fluorescence lifetime by systematically varying the location, number, and size of AuNPs as well as the interparticle distance and spectral overlap between AuNPs and Qdots. The DNA origami template serves as a programmable nano-pegboard for heterogeneous integration of Qdots and AuNPs wherein complex geometries are created by using modified staple strands on the DNA origami to capture specific nanoparticles (Figure 1). Herein, we manipulate and control the average photon count rate and lifetime of Qdots by varying the geometrical configuration of Qdot–AuNP conjugates on DNA origami and observe good agreement between theory and measurement. Solution-based measurements are important, because they provide insight into how the templates will behave in a biological environment. The single-particle tracking system enables us to follow the 3D motion of individual diffusing

DNA self-assembly: from 2D to 3D

This paper describes our recent efforts on the self-assembly of three-dimensional (3D) DNA nanostructures from DNA star motifs (tiles). DNA star motifs are a family of DNA nanostructures with 3, 4, 5, or 6 branches; they are named as 3-, 4-, 5-, 6-point-star motifs, respectively. Such motifs are programmed to further assemble into nanocages (regular polyhedra or irregular nanocapsules) with diameters ranging from 20 nm to 2 microm. Among them, DNA nanocages derived from 3-point-star motif consists of a group of regular polyhedra: tetrahedra, hexahedra (or cubes), dodecahedra and buckyballs (containing 4, 8, 20, and 60 units of the 3-point-star motif, respectively). An icosahedron consists of twelve 5-point-star motifs and is similar to the shapes of spherical viruses. 6-point-star motifs can not assemble into regular polyhedra; instead, some sphere-like or irregular cages with diameters about 1-2 microm will form. Similar large cages can also assemble from the 5-point-star motif when the DNA concentrations are higher than those for assembling regular icosahedra. In our study, we have identified several important factors for assembly of well-defined 3D nanostructures, including the concentration, the flexibility, and the arm length of the DNA tiles and the association strength between the DNA tiles.

Pt-NP–MWNT nanohybrid as a robust and low-cost counter electrode material for dye-sensitized solar cells

Pt-NPs hybridized inside and outside multi-walled carbon nanotubes (MWNTs) were successfully synthesized using a liquid plasma system with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide under atmospheric pressure. The Pt-NPs with a size of 3–4 nm were stably and uniformly hybridized on both inner and outer surfaces of the MWNTs. The nanohybrid materials were applied to the counter electrode of dye-sensitized solar cells (DSCs). Electrochemical impedance measurement of DSCs revealed that the charge-transfer resistance of a MWNT–Pt nanohybrid-coated electrode was less than that of Pt-sputtered and MWNT-coated electrodes. Due to the low charge-transfer resistance, the DSC exhibited fairly improved energy conversion efficiency compared to the DSCs equipped with Pt-sputtered and MWNT-coated counter electrodes.

Description and Theory of a Fiber-Optic Confocal and Super-Focal Raman Microspectrometer

A fiber-optic bundle, placed in the imaging plane of a microspectrometer, functions as a variable-size pinhole. This arrangement allows for conventional confocal measurements to be made by collecting the signal from the central fiber. On the other hand, measurements arising from a larger focal volume are made by integrating the signal from the entire bundle. This new “super-focal” imaging technique yields larger imaging depth without any loss in spectral resolution. The instrument design and performance are described, as well as geometric optics calculations which accurately predict the depth resolution and oscillations in the super-focal depth response. Raman scattering from a three-component layered sample is used to illustrate the extension of this technique to more complicated systems.

Large-Scale Fabrication of Commercially Available, Nonpolar Linear Polymer Film with a Highly Ordered Honeycomb Pattern

Highly ordered, hexagonally patterned poly(methyl methacrylate) (PMMA) thin film is successfully fabricated using an improved phase separation method. A mixture of chloroform and methanol, which is used as a volatile solvent/nonsolvent pair, effectively controls the surface morphology and sensitively determines the ordered pattern. In particular, the methanol accumulation, which induces the formation of a gel-like protective layer and enhances the lateral capillary force, is crucial in the formation of the highly ordered hexagonal pattern even when using a nonpolar polymer such as PMMA. The convergence of cost-effective and large-scale production of highly ordered micropatterned film has wide potential for application, and it can enable new prospects for the commercialization of future high-tech devices that require specific multifunctionality.

Super-amphiphilic surface of nano silica/polyurethane hybrid coated PET film via a plasma treatment

This study first reports the fabrication of a super-amphiphilic surface using PET films with a silica-polyurethane hybrid top-coat layer through a non-thermal, one-atmospheric-pressure plasma treatment. This surface displays contact angle close to zero with both aqueous and oily liquids, which has attracted enormous attention for a wide-range of practical applications. We systematically investigated the influence of the plasma treatment time on the wetting behavior of the silica-polyurethane coated PET surface. The changes in morphology and chemical composition of PET surfaces before and after a plasma treatment were analyzed. In order to gain an insight into the formation of a super-amphiphilic PET surface and optimize the conditions under which super-amphiphilicity can be realized, we used a hemi-wicking action as a theoretical model and experimentally verified it through determining the critical angle. We also proposed a guide for designing a nano-sphere patterned PDMS surface which can generate super-wetting properties after a plasma treatment.

Nanocage-Confined Synthesis of Fluorescent Polycyclic Aromatic Hydrocarbons in Zeolite

Polycyclic aromatic hydrocarbons (PAHs) attract much attention for applications to organic light-emitting diodes, field-effect transistors, and photovoltaic cells. The current synthetic approaches to PAHs involve high-temperature flash pyrolysis or complicated step-by-step organic reactions, which lead to low yields of PAHs. Herein, we report a facile and scalable synthesis of PAHs, which is carried out simply by flowing acetylene gas into zeolite under mild heating, typically at 400 °C and generates the products of 0.30 g g-1 zeolite. PAHs are synthesized via acetylene polymerization inside Ca2+-ion-exchanged Linde type A (LTA) zeolite, of which the α-cage puts a limit on the product molecular size as a confined-space nanoreactor. The resultant product after the removal of the zeolite framework exhibits brilliant white fluorescence emission in N-methylpyrrolidone solution. The product is separated into four different color emitters (violet, blue, green, and orange) by column chromatography. Detailed characterizations of the products by means of various spectroscopic methods and mainly mass spectrometric analyses indicate that coronene (C24H12) is the main component of the blue emitter, while the green emitter is a mixture of planar and curved PAHs. The orange can be attributed to curved PAHs larger than ovalene, and the violet to smaller molecules than coronene. The PAH growth mechanism inside Ca2+-exchanged LTA zeolite is proposed on the basis of mass spectral analyses and density functional theory calculations.