Jong Hoon Hahn

Polypyrrole/Agarose-based electronically conductive and reversibly restorable hydrogel.

Conductive hydrogels are a class of composite materials that consist of hydrated and conducting polymers. Due to the mechanical similarity to biointerfaces such as human skin, conductive hydrogels have been primarily utilized as bioelectrodes, specifically neuroprosthetic electrodes, in an attempt to replace metallic electrodes by enhancing the mechanical properties and long-term stability of the electrodes within living organisms. Here, we report a conductive, smart hydrogel, which is thermoplastic and self-healing owing to its unique properties of reversible liquefaction and gelation in response to thermal stimuli. In addition, we demonstrated that our conductive hydrogel could be utilized to fabricate bendable, stretchable, and patternable electrodes directly on human skin. The excellent mechanical and thermal properties of our hydrogel make it potentially useful in a variety of biomedical applications such as electronic skin.

Poly(dimethylsiloxane) microchip for precolumn reaction and micellar electrokinetic chromatography of biogenic amines

We have demonstrated that precolumn derivatization and capillary electrophoresis separation on a poly(dimethylsiloxane) (PDMS) microchip can be realized as efficient as those on glass microchips. In an optimized condition of micellar electrokinetic chromatography (MEKC), using 25 mM sodium borate buffer (pH 10.0) with 25 mM sodium dodecyl sulfate (SDS) and 5% v/v methanol, the electroosmotic flow in an oxidized PDMS microchip is stabilized within 3% for days. By employing a fluorometric derivatization with o‐phthaldialdehyde (OPA) in an optimally designed reaction chamber, four most important biogenic amines occurring in foods, histamine, tyramine, putrescine, and tryptamine, are quantitatively determined in less than 1 min at the levels applicable to real samples. The migration behaviors of anionic OPA‐derivatized biogenic amines under the MEKC conditions are analyzed, and it has been found that under our separation conditions, the electrophoretic mobility of the SDS micelles is significantly greater than those of the anions in the aqueous phase. The channel manifold in a PDMS substrate is fabricated using replica molding against a thick photoresist, SU‐8, pattern generated by photolithography. The plate with the microchannel pattern is strongly, irreversibly bonded to another PDMS plate by using a new bonding technique, which employs surface oxidation by corona discharge generated from a cheap, handy source, Tesla coil.

Nanoband electrode for high-performance in-channel amperometric detection in dual-channel microchip capillary electrophoresis.

A nanoband electrode detector integrated with a dual‐channel polydimethylsiloxane microchip is proposed for in‐channel amperometric detection in microchip capillary electrophoresis. Gold nanoband electrodes, which were fabricated on SU‐8 substrates with a 100‐nm‐width gold layer, were introduced into the dual‐channel microchip to be an electrochemical detector. Due to the nano‐sized width of the detector, the noise of the amperometric detection was significantly reduced, and a high separation resolution was achieved for monitoring the analytes. The detection sensitivity of the system was improved by high signal‐to‐noise ratio, and a low detection limit on microchip was obtained for p‐aminophenol (2.09 nM). Because of the high resolution in measuring half‐peak width, the plate number that is used to evaluate the separation efficiency was 1.5‐fold higher than that using 50‐μm‐width electrochemical detector. The effect of sample injection time and data acquisition time on separation efficiency was investigated, and an attractive separation efficiency was achieved with a plate number up to 17 500.

Syntheses, X-ray structures and second harmonic generation efficiencies of fused ring heterocycles

Abstract Second harmonic generation efficiencies of a number of fused ring heterocyclic compounds bearing a nitro group were measured using the Kurtz powder method. Among them, 5-nitroindole has the highest SHG efficiency, 20 times as high as that of urea reference. Several 5-nitroindole derivatives were synthesized in the hope of improving the SHG efficiency. None of the derivatives, however, exhibited higher SHG efficiency than the mother compound. X-ray crystal structures of some of these compounds as well as that of 5-nitroindole itself were determined. In the crystal structure of 5-nitroindole the angle (θ) between the molecular charge transfer axis and the polar axis of the crystal is 52.72 °, which is close to the optimum value 54.74 ° predicted by theory. On the other hand, crystal structures of l,2-dimethyl-5-nitroindole and 1-ethyl-5-nitroindole revealed θ values far from the optimum one, which is consistent with their low SHG efficiencies.

Temperature-dependent second harmonic generation in 6LiRbSO4 and 7LiRbSO4

Abstract Temperature-dependent second harmonic generation efficiencies of 6 LiRbSO 4 and 7 LiRbSO 4 have been measured across the ferroelectric phase IV region (166 – 185 °C), using the crystalline powder of 38 – 63 μm particle size range. The transition temperatures have not been changed by the lithium isotope substitution. More polarizable 6 LiRbSO 4 has higher second harmonic generation efficiency than 7 LiRbSO 4 up to 10%. We obtain unbalanced twin-peak-shaped patterns of thermal hysteresis. The unique hysteresis pattern of LiRbSO 4 is ascribed to the very narrow temperature region of the phase IV, and to the overlapping of two hysteresis curves with very different widths.

Self-assembly of non-linear optical chromophores through ionic interactions

Ionic attractions were applied to the construction of non-linear optical (NLO) chromophoric monolayers. In order to fully utilize such an interaction, stilbene-type NLO chromophores having a long alkyl chain and an anionic group at one end of the chain were designed; of this family of chromophores, sodium 11-[4-(trans-4′-pyridylstyryl)oxy] undecan-1-yl sulfate (3) was synthesized. By allowing a few minutes, this anionic chromophore self-assembles successfully on a cationically charged surface, which is prepared by treating clean fused silica with 3-aminopropyltriethoxysilane and then with iodomethane. The pyridine moiety of the self-assembled chromophore can be methylated to augment the molecular hyperpolarizability, β. The characteristics of the monolayer were examined viacontact angle measurements, UV–VIS spectroscopy, grazing-angle FTIR spectroscopy, and NLO property measurements.

Adhesive and Dead Volume Free Interfacing between PDMS Microfluidic Channels

We have developed an adhesive and dead volume free poly(dimethylsiloxane) (PDMS) microfluidic channel interfacing technique through capillaries. Capillary zone electrophoresis (CZE) with a bare capillary and capillary electrochromatography (CEC) with a beads-packed separation column successfully separated fluorescein, FITC and coumarin440 and 450 respectively using the adhesive free feature.

Self-assembly of nonlinear optical chromophoric layers through the ionic interaction

Abstract The ionic attraction was applied to construct nonlinear optical (NLO) chromophoric layers. In order to fully utilize such an interaction, stilbene-type NLO chromophores having an anionic group at one end of the chromophores were designed. Among such type of the chromophores, trans-4′-(4-sulfonic acid sodium salt)butanoxystyrylpyridine ( 1 ) was synthesized. The pyridine moiety of the chromophore can be easily methylated to augment the molecular hyperpolarizability (β). The above anionic chromophore self-assembles on the cationic surface, which is prepared by treating clean fused silica surface with 3-aminopropyltriethoxysilane and subsequently iodomethane. Characteristics of the molecular layer have been examined with contact angle measurement, UV-Vis spectroscopy, grazing-angle FTIR spectroscopy, and NLO property measurement.

Fast capillary-scanning system for detecting fluorescently labelled DNA sequencing fragments separated by capillary gel electrophoresis

SummaryA capillary-scanning system for increasing the total throughput of DNA sequencing has been developed. The DNA sequencing method using the system features a spatial and temporal separation of laser-induced fluorescence detection from capillary gel electrophoresis. Fluorescently labelled adenine base fragments of pBluescript SK(−) were, produced in enzymatic sequencing reactions, and separated by capillary gel electrophoresis using UV-visible transparent capillaries filled with 8% T, 0% C polyacrylamide gel. The capillary containing all bands of the fragments was then scanned longitudinally with a 488 nm argon ion laser beam in 2.6 min. The adenine base sequence of the DNA was determined out to 400 bases by detecting fluorescence signals generated from the bands during the scan. The present scan speed is essentially limited by a slow strip-chart recorder and could be greatly increased by employing a fast data acquisition system.

Microchip-Based Simultaneous On-Line Monitoring of CR(III) and CR(VI) Using Highly Efficient Chemiluminescence Detection

We have developed a micromachined continuous flow analysis (CFA) device for the simultaneous detection of Cr(III) and Cr(VI) with chemiluminescence (CL). By selective injection of the reductant, we can determine the total Cr and Cr(III) quantitatively. Linearity for Cr(III) and Cr(VI) has been established over concentration ranges of 50 ∼ 500 ppb and 10 ∼ 1000 ppb, and detection limits (S/N = 3) are 10 ppb and 1.3 ppb respectively.