Hyon Hee Yoon

Nickel/cobalt oxide-decorated 3D graphene nanocomposite electrode for enhanced electrochemical detection of urea

A NiCo2O4 bimetallic electro-catalyst was synthesized on three-dimensional graphene (3D graphene) for the non-enzymatic detection of urea. The structural and morphological properties of the NiCo2O4/3D graphene nanocomposite were characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The NiCo2O4/3D graphene was deposited on an indium tin oxide (ITO) glass to fabricate a highly sensitive urea sensor. The electrochemical properties of the prepared electrode were studied by cyclic voltammetry. A high sensitivity of 166 μAmM(-)(1)cm(-)(2) was obtained for the NiCo2O4/3D graphene/ITO sensor. The sensor exhibited a linear range of 0.06-0.30 mM (R(2)=0.998) and a fast response time of approximately 1.0 s with a detection limit of 5.0 µM. Additionally, the sensor exhibited high stability with a sensitivity decrease of only 5.5% after four months of storage in ambient conditions. The urea sensor demonstrates feasibility for urea analysis in urine samples.

Volatile fatty acids production from marine macroalgae by anaerobic fermentation.

Volatile fatty acids (VFAs) were produced from the marine macroalgae, Laminaria japonica, Pachymeniopsis elliptica, and Enteromorpha crinite by anaerobic fermentation using a microbial community derived from a municipal wastewater treatment plant. Methanogen inhibitor (iodoform), pH control, substrate concentration, and alkaline and thermal pretreatments affected VFA productivity. Acetic, propionic, and butyric acids were the main products. A maximum VFA concentration of 15.2g/L was obtained from 50 g/L of L. japonica in three days at 35°C and pH 6.5-7.0. Pretreatment with 0.5 N NaOH improved VFA productivity by 56% compared to control. The result shows the applicability of marine macroalgae as biomass feedstock for the production of VFAs which can be converted to mixed alcohol fuels.

Immobilization of iron storage protein on a gold electrode based on self-assembled monolayers.

Ferritin is a globular protein consisting of 24 subunits to form a hollow shell and is capable of storing iron in the cavity. Findings that the naturally existing iron core of ferritin can be readily extracted and replaced with a variety of electroactive materials make ferritin suitable for biosensor and biofuel cell applications. The immobilization of ferritin on the electrode surface is essential for various bioelectronic applications. In this work, based on self-assembled monolayers, ferritin was immobilized on a gold electrode through two different methods: chemisorption of thiolated ferritin onto bare gold electrodes and covalent binding of ferritin to succinimidyl alkanedisulfide-modified Au electrodes. Effects of experimental conditions on the ferritin immobilization were investigated. The ferritin immobilized on the gold electrode was characterized by atomic force microscopy and cyclic voltammetry.

Pretreatment of macroalgae for volatile fatty acid production

In this study, a novel method was proposed for the biological pretreatment of macroalgae (Laminaria japonica, Pachymeniopsis elliptica, and Enteromorpha crinita) for production of volatile fatty acid (VFA) by anaerobic fermentation. The amount of VFA produced from 40 g/L of L. japonica increased from 8.3 g/L (control) to 15.6 g/L when it was biologically pretreated with Vibrio harveyi. The biological treatment of L. japonica with Vibrio spp. was most effective likely due to the alginate lyase activity of Vibrio spp. However, a considerable effect was also observed after biological pretreatment of P. elliptica and E. crinita, which are red and green algae, respectively. Alkaline pretreatment of 40 g/L of L. japonica with 0.5 N NaOH resulted in an increase of VFA production to 12.2 g/L. These results indicate that VFA production from macroalgae can be significantly enhanced using the proposed biological pretreatments.

Fabrication of CNT/Ferrocene/Glucose Oxidase/Chitosan-Layered Bioanode for Glucose/Oxygen Biofuel Cells

An enzyme-modified electrode was fabricated by entrapping glucose oxidase (GOx) and ferrocene (Fc) onto a multiwall carbon nanotube (MWCNT)-coated electrode. The MWCNT, Fc, GOx, and chitosan (CHI) were sequentially coated on a glassy carbon electrode. The MWCNT/Fc/GOx/CHI electrode was characterized by scanning electron microscopy (SEM), and cyclic voltammetry (CV). The prepared electrode exhibited good electrochemical performance for the glucose analysis with a linear range of 0–60 mM glucose. It was found that the MWCNT film on the electrode remarkably enhanced the performance of the electrode. The MWCNT/Fc/GOx/CHI electrode was integrated with a bilirubin oxidase-immobilized cathode for a biofue cell application. The maximum power density at a glucose concentration of 10 mM was 13 μW/cm2 at a cell voltage of 0.19 V. The results of this study indicate that the MWCNT/Fc/GOx/CHI electrode could be applied in the development of biofuel cells and bisensors.

A bionanocomposite based on 1,4-diazabicyclo-[2.2.2]-octane cellulose nanofiber cross-linked-quaternary polysulfone as an anion conducting membrane

Anion conducting composite membranes were synthesized by cross-linking hydroxide conducting 1,4-diazabicyclo-[2.2.2]-octane (DABCO)–cellulose nanofibers (isolated from Citrus tangerine) with DABCO–polysulfone using 1,4-dibromo butane. The content of quaternized cellulose was adjusted to control the ion exchange capacity (IEC) and the ionic conductivity. The structural and morphological characteristics of the membranes were determined by Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance spectroscopy (1H-NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microphase structure of the membranes was studied by atomic force microscopy (AFM). The effects of the DABCO–cellulose on water uptake (WU), ion exchange capacity (IEC) and ionic conductivity were investigated. The cross-linking of the quaternized cellulose with the polymer main chains formed a bedform type structure, ensuing good chemical and excellent mechanical stability of the membranes in aqueous and alkaline media. The composite membranes showed conductivity in the range of ca. 39–74 mS cm−1 at 25 °C and reached 128 mS cm−1 at 80 °C, derived from the nanophase separation and densely distributed ionic channels. Such a strategy provides a valuable prospect to design high anion conducting membranes for fuel cell applications.

Simultaneous saccharification and fermentation of cellulose for lactic acid production

Lactic acid production from α-cellulose by simultaneous saccharification and fermentation (SSF) was studied. The cellulose was converted in a batch SSF using cellulase enzyme Cytolase CL to produce glucose sugar andLactobacillus delbrueckii to ferment the glucose to lactic acid. The effects of temperature, pH, yeast extract loading, and lactic acid inhibition were studied to determine the optimum conditions for the batch processing. Cellulose was converted efficiently to lactic acid, and enzymatic hydrolysis was the rate controlling step in the SSF. The highest conversion rate was obtained at 46°C and pH 5.0. The observed yield of lactic acid from α-cellulose was 0.90 at 72 hours. The optimum pH of the SSF was coincident with that of enzymatic hydrolysis. The optimum temperature of the SSF was chosen as the highest temperature the microorganism could withstand. The optimum yeast extract loading was found to be 2.5 g/L. Lactic acid was observed to be inhibitory to the microorganisms’ activity.

Direct Electron Transfer of Glucose Oxidase and Carbon Nanotubes Entrapped with Biocompatible Organic Materials

Efficient electron transfer between redox enzymes and electrodes is essential for enzyme-based biosensors, biofuel cells, and bioelectronic devices. Generally glucose oxidase (GOx) requires mediators for electrical communication with electrodes because the redox center of GOx is deeply buried in the insulating protein shell. In the present work, direct electron transfer (DET) between GOx and electrodes was attempted. GOx and carbon nanotubes (CNTs) were immobilized on a glassy carbon (GC) electrode by using biocompatible polymer, chitosan (CHI). Cyclic voltammograms revealed that the CHI/GOx/CNT-GC electrode showed a pair of well-defined redox peaks in 0.1 M phosphate buffer solution (pH 7.0) saturated with argon. Under the same conditions, no redox peak was observed in the absence of CNTs. The formal redox potential was −450 mV (vs. Ag/AgCl), which agreed well with that of FAD/FADH2, the redox center of GOx. This result clearly shows that the DET between the GOx and the electrode was achieved. The use of thin CNTs significantly improved the DET efficiency of the GOx. It was found that the GOx immobilized on the electrode retained catalytic activity for the oxidation of glucose.

A methane-fueled SOFC based on a thin BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte film and a LaNi0.6Co0.4O3 anode functional layer

A methane-fueled solid oxide fuel cell (SOFC) operating at 500–650 °C was fabricated using a thin BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) electrolyte film and a LaNi0.6Co0.4O3 anode functional layer. Thin BZCYYb electrolyte films (thickness = 3 μm) and NiO–BZCYYb anode supports were prepared by electron beam vapor deposition. A LaNi0.6Co0.4O3 catalyst layer was coated onto the anode support. The BZCYYb electrolyte film was characterized by X-ray diffraction and scanning electron microscopy. Current–voltage (I–V) curves and impedance spectra were measured to characterize the electrochemical performance of the cell. The NiO–BZCYYb anode exhibited a high coking resistance, and the cell was stable for 200 h when operated under methane at 550 °C. The presence of the LaNi0.6Co0.4O3 functional catalyst significantly enhanced the cell performance. The maximum power densities of the prepared cells were 0.98, 0.65, 0.51 and 0.40 W cm−2 at 650, 600, 550 and 500 °C, respectively, under methane fuel.

Immobilization and characterization of ferritin on gold electrode

Properties of ferritin, immobilized on dithiobis (N-succinimydyl propionate) (DTSP)-covered gold electrode, in 3-morpholino propanesulfonic acid buffer were investigated by AFM and FE-SEM. Electrochemical properties the ferritin was measured by a cyclic volatammetry. When the potentials of 0.2, 0.34, and 1.0V vs. Ag/AgCl were applied for 15h for the ferritin immobilization, the electrode potential of Fe(II)/Fe(III) in ferritin changed to negative values. Negative electrode potential shift of Fe(II)/Fe(III) in ferritin with respect to the applied potential could be attributed to the stabilized ferritin on DTSP-covered gold electrode. From AFM and SEM images, it was proven that ferritins fixed at below 0.34V were clusters in several micrometer and those fixed at higher applied potential than OCV were finely distributed particles in several tens of nanometer.