Do-Hyun Lee

Enhanced discrimination of normal oocytes using optically induced pulling-up dielectrophoretic force

We present a method to discriminate normal oocytes in an optoelectrofluidic platform based on the optically induced positive dielectrophoresis (DEP) for in vitro fertilization. By combining the gravity with a pulling-up DEP force that is induced by dynamic image projected from a liquid crystal display, the discrimination performance could be enhanced due to the reduction in friction force acting on the oocytes that are relatively large and heavy cells being affected by the gravity field. The voltage condition of 10 V bias at 1 MHz was applied for moving normal oocytes. The increased difference of moving velocity between normal and starved abnormal oocytes allows us to discriminate the normal ones spontaneously under the moving image pattern. This approach can be useful to develop an automatic and interactive selection tool of fertilizable oocytes.

Wave-front phase measurements of high-order harmonic beams by use of point-diffraction interferometry

We report the wave-front phase measurement of high-order harmonics employing point-diffraction interferometry. The high-order harmonics generated in a gas-filled hollow tube showed excellent spatial coherence over nearly the whole cross section of the harmonic beams. Using this coherent harmonic source in the extreme-ultraviolet-soft-x-ray region, we have demonstrated the operation of a point-diffraction interferometer and measured the wave-front phase of high-order harmonics.

Formation of hexagonal Gd disilicide nanowires on Si(100)

The growth of hexagonal Gd disilicide nanowires on Si(100) is studied by scanning tunneling microscopy. Gd disilicide nanowires are grown on Si(100) by submonolayer Gd deposition on the substrate at 600 °C. The formation of nanowires is shown to be due to anisotropic lattice mismatches between hexagonal Gd disilicide and Si. The nanowires have widths of several nanometers and lengths up to micrometer length scales. The top of the nanowires has a c(2×2) structure, indicating that the crystalline structure is Si-deficient Gd disilicide. The nanowires were shown to have metallic properties using scanning tunneling spectroscopy.

In situ analysis of heterogeneity in the lipid content of single green microalgae in alginate hydrogel microcapsules.

Microalgae, a group of microorganisms that grow using sunlight as the sole energy source and carbon dioxide as an only carbon source, have been considered as a feedstock of choice for the production of biofuels such as biodiesel. To explore the economic feasibility of such application, however, many technical hurdles must first be overcome; the selection and/or screening of competent species are some of the most important and yet challenging tasks. To greatly accelerate this rather slow and laborious step, we developed a droplet-based microfluidic system that uses alginate hydrogel microcapsules with a mean diameter of 26 μm, each of which is able to encapsulate a single microalgal cell. This novel device was successfully demonstrated using three microalgae species, namely, Chlorella vulgaris , Chlamydomonas sp., and Botryococcus braunii . In situ analysis of the lipid content of individual microalgal cells by nondestructive fluorescence staining using BODIPY (4,4-difluoro-1,3,5,7,-tetramethyl-4-bora-3a,4a-diaza-s-indacene) was possible. In all cases, we confirmed that the lipid content of microalgal species in alginate hydrogel microcapsules was comparable to that of free-living cells. Stochastic heterogeneity in the lipid content was verified under a highly viable physiological condition, implying that other analyses were possible after the determination of lipid content. Furthermore, the designed microwell arrays enabled us to distinguish the BODIPY fluorescence response of a single live alga within the microcapsules.

Enhancement of soft x-ray emission from a cryogenically cooled Ar gas jet irradiated by 25 fs laser pulse

Soft x-ray spectra (40–180 A) produced by the interaction of 25 fs laser pulses at an intensity of ∼7×1016 W/cm2 with a cryogenically cooled Ar gas jet have been measured. New spectral lines from Ar8+ and Ar9+ charge states appeared with decreasing gas temperature. Nonlinear increase of x-ray line emission from Ar7+, Ar8+, and Ar9+ was observed with cooling, which saturated below certain temperature. The drastic change in the spectrum is attributed to efficient collisional heating and collisional ionization of growing (102–103 atoms) Ar clusters from the cooled jet.

Generation of bright low-divergence high-order harmonics in a long gas jet

Guided propagation of intense 28 fs laser pulses through a long gas jet made it feasible to produce a bright 27th harmonic from argon with a divergence of 0.5 mrad. This harmonic saturated an x-ray charge-coupled device of an extreme ultraviolet spectrometer in a single laser shot of 5 mJ. The low-divergence harmonic generation resulted from a well-guided pumping laser pulse, achieved by balancing diffraction and plasma defocusing with nonlinear self-focusing, and a large harmonic-generation cross section.

Generation and manipulation of droplets in an optoelectrofluidic device integrated with microfluidic channels

This letter describes an optoelectrofluidic platform integrated with microfluidic channels for continuous generation and programmable manipulation of water-in-oil droplet emulsions. A microchannel was integrated into a film-based optoelectrofluidic device by selective perforation of poly(dimethylsiloxane) layer for simultaneously performing microfluidic generation and optoelectrofluidic manipulation of droplets in a single device. By using this device, we could continuously generate, interactively transport, and merge multiple droplets using optically induced virtual electrodes.

Low resistivity of Pt silicide nanowires measured using double-scanning- probe tunneling microscope

We measure the resistivity of platinum-silicide nanowires (Pt2Si NWs) epitaxially formed on a Si(100) surface using double-scanning-probe tunneling microscope. Despite the large Schottky barrier height reported on a macroscopic Pt2Si∕n-Si interface, leakage current through the substrate is observed in the resistance measurement, and is quantitatively estimated to be separated from the current through the nanowire. The measured resistivity of Pt2Si NWs is about half the reported resistivity of thick Pt2Si films, which could be due to additional conduction paths through surface or interface states on NWs.

Structure and electronic properties of self-assembled Pt silicide nanowires on Si(100)

We investigated the formation of Pt silicide nanowires on a Si(100) surface using scanning tunnelling microscopy and high-resolution photoemission spectroscopy. Pt silicide nanowires with a tetragonal Pt2Si structure are formed along the step edges of Si(100). Pt-induced c(4 × 2) reconstructions also appear adjacent to the tetragonal Pt2Si nanowires. Formation of the Pt2Si nanowires is attributed to the anisotropic lattice mismatches between the tetragonal Pt2Si structure and Si(100). Scanning tunnelling spectroscopy data show that the nanowires are metallic. The stoichiometry of Pt silicide is confirmed by high-resolution photoemission spectroscopy.

Microbridge structures for uniform interval control of flowing droplets in microfluidic networks

Precise temporal control of microfluidic droplets such as synchronization and combinatorial pairing of droplets is required to achieve a variety range of chemical and biochemical reactions inside microfluidic networks. Here, we present a facile and robust microfluidic platform enabling uniform interval control of flowing droplets for the precise temporal synchronization and pairing of picoliter droplets with a reagent. By incorporating microbridge structures interconnecting the droplet-carrying channel and the flow control channel, a fluidic pressure drop was derived between the two fluidic channels via the microbridge structures, reordering flowing droplets with a defined uniform interval. Through the adjustment of the control oil flow rate, the droplet intervals were flexibly and precisely adjustable. With this mechanism of droplet spacing, the gelation of the alginate droplets as well as control of the droplet interval was simultaneously achieved by additional control oil flow including calcified oleic acid. In addition, by parallel linking identical microfluidic modules with distinct sample inlet, controlled synchronization and pairing of two distinct droplets were demonstrated. This method is applicable to facilitate and develop many droplet-based microfluidic applications, including biological assay, combinatorial synthesis, and high-throughput screening.