Dokyoung Kim

Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods

We report metalorganic vapor-phase epitaxial growth and structural and photoluminescent characteristics of ZnO nanorods. The nanorods were grown on Al2O3(00⋅1) substrates at 400 °C without employing any metal catalysts usually needed in other methods. Electron microscopy revealed that nanorods with uniform distributions in their diameters, lengths, and densities were grown vertically from the substrates. The mean diameter of the nanorods is as narrow as 25 nm. In addition, x-ray diffraction measurements clearly show that ZnO nanorods were grown epitaxially with homogeneous in-plane alignment as well as a c-axis orientation. More importantly, from photoluminescence spectra of the nanorods strong and narrow excitonic emission and extremely weak deep level emission were observed, indicating that the nanorods are of high optical quality.

Intravitreal properties of porous silicon photonic crystals: a potential self-reporting intraocular drug-delivery vehicle.

Aim: To determine the suitability of porous silicon photonic crystals for intraocular drug-delivery. Methods: A rugate structure was electrochemically etched into a highly doped p-type silicon substrate to create a porous silicon film that was subsequently removed and ultrasonically fractured into particles. To stabilise the particles in aqueous media, the silicon particles were modified by surface alkylation (using thermal hydrosilylation) or by thermal oxidation. Unmodified particles, hydrosilylated particles and oxidised particles were injected into rabbit vitreous. The stability and toxicity of each type of particle were studied by indirect ophthalmoscopy, biomicroscopy, tonometry, electroretinography (ERG) and histology. Results: No toxicity was observed with any type of the particles during a period of >4 months. Surface alkylation led to dramatically increased intravitreal stability and slow degradation. The estimated vitreous half-life increased from 1 week (fresh particles) to 5 weeks (oxidised particles) and to 16 weeks (hydrosilylated particles). Conclusion: The porous silicon photonic crystals showed good biocompatibility and may be used as an intraocular drug-delivery system. The intravitreal injectable porous silicon photonic crystals may be engineered to host a variety of therapeutics and achieve controlled drug release over long periods of time to treat chronic vitreoretinal diseases.

In vivo two-photon fluorescent imaging of fluoride with a desilylation-based reactive probe

A two-photon excitable molecular probe for fluoride, developed based on a fluoride-specific desilylation reaction, is demonstrated to be useful for fluorescent imaging of fluoride ions in live zebrafish by one-photon as well as two-photon microscopy for the first time.

Two-Photon Absorbing Dyes with Minimal Autofluorescence in Tissue Imaging: Application to in Vivo Imaging of Amyloid-β Plaques with a Negligible Background Signal

Fluorescence imaging of tissues offer an essential means for studying biological systems. Autofluorescence becomes a serious issue in tissue imaging under excitation at UV-vis wavelengths where biological molecules compete with the fluorophore. To address this critical issue, a novel class of fluorophores that can be excited at ∼900 nm under two-photon excitation conditions and emits in the red wavelength region (≥600 nm) has been disclosed. The new π-extended dipolar dye system shows several advantageous features including minimal autofluorescence in tissue imaging and pronounced solvent-sensitive emission behavior, compared with a widely used two-photon absorbing dye, acedan. As an important application of the new dye system, one of the dyes was developed into a fluorescent probe for amyloid-β plaques, a key biomarker of Alzheimers disease. The probe enabled in vivo imaging of amyloid-β plaques in a disease-model mouse, with negligible background signal. The new dye system has great potential for the development of other types of two-photon fluorescent probes and tags for imaging of tissues with minimal autofluorescence.

Reaction-Based Two-Photon Probes for Mercury Ions: Fluorescence Imaging with Dual Optical Windows

For fluorescent imaging of mercury ions in living species, two-photon probes with dual optical windows are in high demand but remain unexplored. Several dithioacetals were evaluated, and a probe was found, which, upon reaction with mercury species, yielded a two-photon dye; this conversion accompanies ratiometric emission changes with a 97-nm shift, enabling fluorescent imaging of both the probe and mercury ions in cells by one- and two-photon microscopy for the first time.

Toward a selective, sensitive, fast-responsive, and biocompatible two-photon probe for hydrogen sulfide in live cells.

Hydrogen sulfide has emerged as an exciting endogenous gasotransmitter in addition to nitric oxide and carbon dioxide. Noninvasive detection methods for hydrogen sulfide thus become indispensable tools for studying its diverse roles in biological systems. Accordingly, fluorescent probes for hydrogen sulfide have received great attention in recent years. A practically useful fluorescent probe for bioimaging of hydrogen sulfide should be selective, sensitive, fast-responsive, biocompatible, observable in the biological optical window, and capable of deep-tissue imaging. These sensing properties, however, are extremely difficult to achieve at the same time. Disclosed here is the two-photon fluorescent probe that meets all of these criteria. The probe belongs to a Michael acceptor system, which raised a serious selectivity issue over the competing biothiols such as cysteine and glutathione. We have addressed the selectivity issue by optimizing the electronic and steric interactions between biothiols and the probe, in addition to achieving very high sensitivity, fast-response, and biocompatibility. Also, the sensing mechanism suggested in the literature was revised. The probe thus enables us to image the endogenously produced hydrogen sulfide with negligible interference from other biothiols in live cells. The excellent sensing properties of the probe combined with its capability of bioimaging thus make it a practically useful tool for further studying biological roles of hydrogen sulfide.

π-Expanded coumarins: synthesis, optical properties and applications

Coumarins fused with other aromatic units have recently emerged as a hot topic of research. Their synthesis is partly based on classical methodologies such as Pechmann reaction or Knoevenagel condensation, but it also sparked the discovery of completely new pathways. In very recent years so-called vertically expanded coumarins were synthesized, effectively expanding the portfolio of existing architectures. A subtle relationship exists between the structure of fused coumarins and their optical properties. Although absorption of UV-radiation and light is a unifying theme among these π-expanded coumarins, the fluorescence properties strongly depend on the structure. The mode of fusion, the type of additional ring and the presence of electron-donating and electron-withdrawing substituents all influence the photophysical parameters. Recent advances made it possible to modulate their absorption from 300 nm to 550 nm, resulting in new coumarins emitting orange light. This review serves as a guide through both synthesis strategies and structure–property relationship nuances. Strong intramolecular charge-transfer character made it possible to reach suitable values of two-photon absorption cross-section. Photophysical advantages of π-expanded coumarins have been already utilized in fluorescent probes and two-photon excited fluorescence microscopy.

An FITC-BODIPY FRET couple: application to selective, ratiometric detection and bioimaging of cysteine.

A novel FRET couple of fluorescein is disclosed, and it was readily constructed by conjugating an amino-BODIPY dye, a new FRET donor, with fluorescein isocyanate. Its potential was demonstrated by a fluorescence sensing system for cysteine, which was prepared by introducing acryloyl groups to the fluorescein moiety. The FRET probe exhibited promising ratiometric response to cysteine with high selectivity and sensitivity in a buffer solution containing acetonitrile at a physiological pH of 7.4, but showed slow reactivity. This slow response was solved by addition of a surfactant, thus allowing ratiometric imaging and determination of the endogenous level of cysteine in cells in HEPES buffer, by confocal fluorescence microscopy. Imaging experiments toward various cells suggested that such aryl acrylate type probes are vulnerable to the ubiquitous esterase activity. For the selected C6 cell line, in which the esterase activity was minimal, the ratiometric quantification of cysteine level was demonstrated. The FRET probe was also applied to determine the level of cysteine in human blood plasma.

Structure and decomposition behaviour of MgLiAl alloys

The addition of 4--16 wt% Li to Mg lowers the density from 1.7g/cm[sup 3] to 1.3--1.6g/cm[sup 3] and the h.c.p. [alpha]-Mg phase is replaced by the b.c.c. [beta] phase, leading to considerable low temperature formability. Addition of Al to Mg-Li alloys can improve strength by forming a distribution of intermetallic particles such as MgLi[sub 2]Al and AlLi. However, the microstructural characteristics of Mg-Li-Al alloys have not yet been investigated in detail. The aim of the present work was to study the microstructure of Mg-Li-Al alloys, mainly by transmission electron microscopy (TEM).

Effect of applied pressure during solidification on the microstructural refinement in an AlCu alloy

The solidification microstructure can be refined by increasing the cooling rate during solidification. Applying high pressure during solidification can promote the solidification rate by increasing the heat transfer coefficient at the metal/mold interface and changing thermodynamic properties such as solid-liquid transition temperature. Based upon these advantages of high pressure solidification, various investigators have reported substantial refinement of microstructures and improvement of mechanical properties. However, the cooling rate and the degree of the microstructural refinement have not been reported consistently. The purpose of the present study is to investigate the effects of pressure up to 1.7GPa during solidification of an hypoeutectic Al-Cu alloy. Experimental observation of the microstructural refinement in high pressure solidified Al-5.4wt%Cu was compared with the mathematical simulation of the heat transfer behavior during high pressure solidification.