Kyung-Sun Lee

Electrophoretic analysis of food dyes using a miniaturized microfluidic system

A simple and sensitive on‐chip preconcentration, separation, and electrochemical detection (ED) method for the electrophoretic analysis of food dyes was developed. The microchip comprised of three parallel channels: the first two are for the field‐amplified sample stacking (FASS) and subsequent field‐amplified sample injection (FASI) steps, while the third one is for the micellar EKC with ED (MEKC‐ED) step. The food dyes were initially extracted from real samples by employing a method that was simpler, easier, and faster compared with a standard method. The extraction of the samples was characterized by UV–Vis and electrochemical experiments. The chronoamperometric detection was performed with a glassy carbon electrode coupled horizontally with the microchip at the separation channel exit. Experimental parameters affecting the analytical performance of the method were assessed and optimized. The sensitivity of the method was improved by ∼10 800‐fold when compared with a conventional MEKC‐ED analysis. Reproducible response was observed during multiple injections of samples with an RSD of <7.2% (n = 5). The calibration plots were linear (r2 = 0.998) within the range of 1.0 nM–1.0 μM for all food dyes. LODs were estimated between 1.0 and 5.0 nM, based on S/N = 3, for food dyes. The applicability of the method for the analysis of food dyes in real sample was demonstrated.

Total analysis of endocrine disruptors in a microchip with gold nanoparticles.

The development of a simple, sensitive, and direct method for the total analysis of certain endocrine disruptors was performed by integrating preconcentration steps to a separation step on a microchip through the modification of the field‐amplified sample stacking and field‐amplified sample injection steps. To improve the preconcentration and separation performances, the preconcentration and separation buffers were modified with citrate‐stabilized gold nanoparticles (AuNPs). For the detection of the separated samples, cellulose‐dsDNA/AuNPs‐modified carbon paste electrodes were used at the channel end. The experimental parameters affecting the analytical performances, such as the buffer concentration, water plug length, SDS concentration in the separation buffer, AuNPs concentration, preconcentration time, detection potential and electrode to channel distance, were examined. The detection limits of the test compounds were between 7.1 and 11.1 fM and that for 4‐pentylphenol was 7.1 (±1.1) fM. Dynamic ranges were in the range from 0.15 to 600.0 pM. The experiments with real samples were performed to evaluate the reliability of the proposed method.

An all-solid-state reference electrode based on the layer-by-layer polymer coating.

A solid-state reference electrode (SSRE) was fabricated by layering a silicone rubber (SR) film containing KCl on an AgCl surface, then a perfluorinated ionomer film, and finally a polyurethane-based membrane containing an ionophore, a lipophilic ionic additive, and a plasticizer, respectively. The addition of SiCl4 to the polyurethane-based membrane layer enhanced the strength of the membrane in an aqueous solution. The morphologies of the membranes were studied separately by SEM. The fabrication of the Ag/AgCl electrode through this layer-by-layer polymer coating improved the electrode stability enormously. In addition, the potential drift of the SSRE according to the pH of the medium was minimized by introducing a H+-ion-selective ionophore (tridodecylamine; TDDA) into the outmost polymer membrane. The cyclic voltammetric and potentiometric responses using the SSRE and a conventional reference electrode, respectively, were consistent. The SSRE exhibited little potential variation even in the case of the addition of very high concentrations of various salts, such as Na salicylate, LiCl, KCl, CaCl2, MgCl2, KNO3, NaCl, and NaHCO3. The practicability of the proposed SSRE was tested for the determination of blood pH and pCO2 in a flow cell system. The SSRE fabricated in the present study was stable over two years.

Characterization of protein-attached conducting polymer monolayer.

Cytochrome c (cyt c)-immobilized monolayers and multiple monolayers of a conducting polymer [poly(terthiophene-3-carboxylic acid) polymer (poly-TTCA)] were prepared, where the monolayer of monomer precursor was fabricated with the Langmuir-Blogett technique. Covalent immobilization of cyt c was achieved by the formation of an amide bond between the carboxylic groups of the conducting polymer and amines groups of lysine in cyt c. The monolayer of poly-TTCA and poly-TTCA/cyt c was characterized by cyclic voltammetry, XPS, EQCM, Auger electron spectra (AES), and atomic force microscopy (AFM). The immobilization of cyt c on the polymer layer reveals the direct electron-transfer processes of cyt c. Cyclic voltammetry of the poly-TTCA/cyt c-modified electrode showed a pair of reversible peaks at approximately +212/+201 mV (Epa/Epc) versus Ag/AgCl in a 0.2 M phosphate buffer solution (pH 7.0). The peak separation and the redox peak current of the poly-TTCA/cyt c-modified electrodes were gradually increased by increasing the number of poly-TTCA/cyt c layers on the electrode. The heterogeneous electron-transfer rate constant (ks) of cyt c at the poly-TTCA/cyt c-monolayer-modified electrode was estimated to be 0.874 s(-1). The method provides a novel route for the fabrication of protein (cyt c)-immobilized and/or lipid (palmitoyloleoylphosphatidic acid)-immobilized monolayers and multiple monolayers of a conducting polymer. Cyt c bonded on the conductive polymer layers was applied for bioelectronic devices with unique functionality.

Development of extraction and analytical methods of nitrite ion from food samples: microchip electrophoresis with a modified electrode.

Two simple and fast methods for the extraction of the nitrite ion (NO(2)(-)) from food samples have been developed. The methods were characterized by UV-visible spectroscopic and electrochemical measurements, and their performance for NO(2)(-) extraction was compared with a standard method. The extraction methods yielded relative recoveries between 100 and 120% with good reproducibility of 3.9% (RSD, n = 4) in UV-visible experiments. Microchip electrophoresis with electrochemical detection (MCE-ED) coupled with a copper (3-mercaptopropyl)trimethoxysilane [Cu(II)-MPS] complex-modified carbon paste electrode (CPE) has been employed to detect NO(2)(-) in extracted samples. The Cu(II)-MPS complex was synthesized and characterized by voltammetry, XPS, and FT-IR analyses. Experimental parameters affecting the separation and detection performances of the MCE-ED method were assessed and optimized. The potential for the electrocatalytic reduction of NO(2)(-) for MCE-ED was found to be -190 mV (vs Ag/AgCl). When extracted food samples were analyzed by the MCE-ED method, a reproducible response for the NO(2)(-) reduction (RSD of 4.3%) at the modified-CPE reflected the negligible electrode fouling. A wide dynamic range of 1.0-160 ppm was observed for analyzing standard NO(2)(-) with a sensitivity of 0.05106 ± 0.00141, and the detection limit, based on S/N = 3, was found to be 0.35 ± 0.05 ppm. No apparent interference from NO(3)(-), other inorganic ions, and biological compounds was observed under the optimal experimental conditions. A standard addition method for real samples showed wide concentration ranges of 1.10-155 and 1.2-150 ppm for analyzing NO(2)(-) in ham and sausage samples, respectively.

Electrophoretic total analysis of trace tetracycline antibiotics in a microchip with amperometry.

The preconcentration, separation, and electrochemical detection of a series of tetracycline (TC) antibiotics in a microfluidic channel were preformed. The electrophoretic experimental parameters to analyze TC, oxytetracycline, chlortetracycline, and doxycycline antibiotics were investigated with a cellulose‐dsDNA‐modified carbon paste electrode. Modification of the electrode improved detection performance by enhancing the signal‐to‐noise characteristics without surface fouling of the electrode. Field‐amplified sample stacking and field‐amplified sample injection techniques were employed for on‐chip preconcentration of the TC series. The sensitivity of the method was improved 10 900‐fold when compared with conventional MEKC‐electrochemical detection analysis. The overall recoveries for TC, oxytetracycline, chlortetracycline, and doxycycline were 87, 89, 87, and 81%, respectively, with 6.0% RSD. The limits of detection for the series were estimated between 1.5 and 4.3 nM. Applicability of the method to beef samples was successfully demonstrated.