Doo Soo Chung

GUEST-INDUCED REORGANIZATION OF A SELF-ASSEMBLED PD(II) COMPLEX

Abstract A self-assembled two-dimensional Pd(II) complex derived from a bidentate ligand trans -1,2-bis(4-pyridyl)ethylene and (en)Pd(NO 3 ) 2 in D 2 O showed concentration dependent and guest-induced reorganization behavior. While the trimeric assembly is induced either at very low concentration or in the presence of guest complementary to the trimer cavity, the tetrameric assembly is induced either in less dilute solution or in the presence of guest with better van der Waals contacts with the tetramer cavity surface. Especially, in the case of using 1,3-adamantanedicar☐ylic acid disodium salts as a guest, all the oligomeric forms disappeared and only a single, tetrameric assembly was clearly induced.

Single drop microextraction using commercial capillary electrophoresis instruments.

Single drop microextraction (SDME), a simple and efficient sample preconcentration method for capillary electrophoresis (CE), was performed using two different commercial CE instruments. With a CE instrument providing adjustable forward and backward pressures, a single drop of an aqueous acceptor phase covered with a thin layer of an organic phase was formed at the capillary tip by programming the sample-handling functions of the instrument to perform 3-phase SDME where the organic film is essentially a membrane. Analytes from an acidic donor phase were concentrated into a basic acceptor phase yielding 190-fold enrichment in 10 min. When the donor phase was agitated using a microstirrer, a 2000-fold enrichment was obtained in 10 min. For a less flexible CE instrument, 2-phase SDME was carried out with a large volume pentanol drop held by a Teflon sleeve, yielding 110-fold enrichments in 30 min. We demonstrate a practical and automated SDME-CE technique with accuracy and reproducibility that is easy to practice to attain matrix isolation and high concentration sensitivity.

Large-volume stacking in capillary electrophoresis using a methanol run buffer

Highly sensitive nonaqueous capillary electrophoresis of weakly acidic organic compounds has been performed using methanol as the run buffer solvent. Methanol provided appropriate suppression of the electroosmotic flow and an increase in the electrophoretic mobilities of anionic solutes compared to water. These two effects allowed large‐volume stacking using the electroosmotic flow pump (LVSEP) to be achieved for larger anions using a bare fused‐silica capillary under an electric field of reverse polarity, whereas only fast‐moving small anions were previously known to be suitable for LVSEP in aqueous media. A field‐enhanced sample injection of an additional amount of analytes during the solvent plug removal further enhanced the limits of detection to below the nanomolar range with conventional UV absorption detection. Under optimum conditions, excellent linear responses and reproducibility in the migration times together with the corrected peak areas for ten analytes were obtained in the concentration range of 10–100 nM.

Determination of homocysteine and other thiols in human plasma by capillary electrophoresis

A new capillary electrophoresis (CE) assay for thiols in human plasma, including homocysteine, which is an indicator of several clinical states has been developed. The thiols were derivatized quantitatively at 50 degrees C, pH 8.0 with a fluorogenic reagent, ADB-F (4-aminosulfonyl-7-fluoro-2,1,3-benzoxadiazole), which is about 30 times faster compared to the other fluorogenic reagent, SBD-F (ammonium 7-fluoro-2,1,3-benzoxadiazole-4-sulfonate). The separation of ABD-thiols was performed in a 50 mM sodium phosphate buffer (pH 2.1) using a bare fused silica capillary (27 cm x 50 microns i.d.) at 25 degrees C. With the electric field of 560 V cm-1, the time needed for the separation of homocysteine, glutathione and cysteine was less than 8 min. A filter-type ultraviolet detector and a 512-channel diode-array detector (DAD) were employed for ABD-thiol analysis. DAD was used to confirm the ABD-thiol peaks. The limits of detection (S/N = 3) for homocysteine, glutathione, and cysteine were 0.5, 1 and 2 microM at 220 nm, respectively.

Selective preconcentration of amino acids and peptides using single drop microextraction in-line coupled with capillary electrophoresis

Single drop microextraction (SDME) can be in-line coupled with capillary electrophoresis by attaching a drop to the tip of a capillary. With a 2-layer drop comprised of an aqueous basic acceptor phase covered with a thin organic layer, acidic analytes in an aqueous acidic donor phase can be extracted into the organic layer and then back-extracted into the acceptor phase. However, preconcentration of amino acids and peptides by SDME is difficult since their zwitterionic properties prevent them from being partitioned in the middle organic phase. When amino acids were derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), amino acids without a charged side chain were converted to carboxylic acids. In the acidic donor phase, those NBD-amino acids were predominantly neutral and they were successfully concentrated into the basic acceptor phase. In the meantime, amino acids with a charged side chain after NBD-F derivatization were not concentrated via SDME. With this selective SDME, we were able to extract acidic and neutral amino acids obtaining several hundred-fold enrichments within 5 min at 25 degrees C, while leaving basic amino acids-Arg, Lys, and His-in the acidic donor phase. Furthermore, detection sensitivity was enhanced by employing laser-induced fluorescence detection. We then applied this technique to the selective concentration of peptides.

Acid Dissociation Constants of Melamine Derivatives from Density Functional Theory Calculations

Melamine and its hydrolysis products (ammeline, ammelide, and cyanuric acid) recently attracted great attention as major food contaminants. Developing analytical tools to quantify them requires exact knowledge of their acid dissociation constants (pK(a) values). Herein, we calculated the pK(a) values of these melamine derivatives in water, using a density functional theory quantum mechanical method [B3LYP/6-311++G(d,p)] in combination with the Poisson-Boltzmann continuum solvation model. The excellent agreement of the calculated values with the experimental ones shows that our method can be used to predict such properties of other food contaminants.

Chiral separation of gemifloxacin in sodium‐ containing media using chiral crown ether as a chiral selector by capillary and microchip electrophoresis

Chiral crown ether, (+)‐(18‐crown‐6)‐tetracarboxylic acid (18C6H4), is an effective chiral selector for resolving enantiomeric primary amines owing to the difference in affinities between 18C6H4 and each of the amine enantiomers. In addition to the destacking effect of sodium ion in the sample solution, the strong affinity of sodium ion to the polyether ring of crown ether is unfavorable to chiral capillary electrophoresis using 18C6H4 as a chiral selector. In this report, the chiral separation of gemifloxacin dissolved in a saline sample matrix using 18C6H4 was investigated. Adding a chelating agent, ethylenediaminetetraacetic acid (EDTA), to the run buffer greatly improved the separation efficiencies and peak shapes. The successful chiral separation of gemifloxacin in a urinary solution was demonstrated for both capillary and microchip electrophoresis.

Near infrared dye indocyanine green doped silica nanoparticles for biological imaging

Indocyanine green (ICG) is an FDA-approved near infrared (NIR) fluorescent dye used in clinical imaging. However, its applications remain limited due to its short half-life, nonspecific plasma binding, optical instability, and poor aqueous stability. Dye doped silica nanoparticles provide an effective barrier in keeping the dye away from the surrounding environment, but ICG cannot be encapsulated into silica easily by conventional methods. In this study, ICG molecules ion-paired with a cationic polymer polyethylenimine (PEI) were successfully encapsulated into a silica matrix to form ICG doped silica nanoparticles by using the Stöber method. Pairing with PEI reduced self-quenching of fluorescence by preventing the aggregation of ICG molecules in silica nanoparticles. Dye leakage was also reduced to the level of 3-6% loss in 5 days. NIR fluorescence images of ICG doped silica NPs below a 2.0 cm thick porcine muscle sample illuminated by NIR light were obtained.

Single-drop microextraction as a powerful pretreatment tool for capillary electrophoresis: A review

Single drop microextraction (SDME) is a convenient and powerful preconcentration and sample cleanup method for capillary electrophoresis (CE). In SDME, analytes are typically extracted from a sample donor solution into an acceptor drop hanging at the inlet tip of a capillary. The enriched drop is then introduced to the capillary for CE analysis. Since the volume of the acceptor drop can be as small as a few nanoliters, the consumption of solvents can be minimized and the preconcentration effect is enhanced. In addition, by covering the acceptor phase with an organic layer or by using an organic acceptor phase, inorganic ions such as salts in the sample solution can be blocked from entering the acceptor phase, providing desalting effects. Here, we describe the basic principles and instrumentation for SDME and its coupling with CE. We also review recent developments and applications of SDME-CE.

Capillary electrophoresis of nonprotein and protein amino acids without derivatization.

An efficient separation of eleven nonprotein amino acids (NPAAs) and three protein amino acids containing aromatic moieties was achieved by capillary electrophoresis without derivatization. The fourteen amino acids were well separated with a 100 mM sodium phosphate run buffer (pH 2.0) using a 57 cm fused‐silica capillary (50 μm ID, 50 cm effective length) at 20°C. With an electric field of 351 V/cm, the time needed for the separation was less than 20 min. Under optimum conditions, excellent linear responses were obtained in the concentration range of 5—100 μM, with the linear correlation coefficient ranging from 0.9785 or greater. The relative standard deviations of the migration times and the corrected peak areas were found to be 1.5—3.9% and 8.0—11.5%, respectively. In order to improve the limit of detection (LOD), simple stacking and large volume stacking using an EOF pump (LVSEP) methods were used. Improved LODs were about 300 nM in stacking and below 15 nM for five small NPAAs in LVSEP.