Woo-Jae Kim

Gachon University, Sŏngnam, Gyeonggi Province, South Korea

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Publications (46)159.28 Total impact

  • Journal of Industrial and Engineering Chemistry. 11/2014; 20(6):4183–4187.
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    ABSTRACT: Sidewall functionalization of single-walled carbon nanotubes (SWNTs) has previously been used to effectively attach a variety of functional groups onto SWNTs; however, there has been little investigation into the reversibility of these reactions, which is critical for the restoration of the original optical and electrical properties of SWNTs. In this study, we investigated the complete removal of functional groups attached to the sidewalls of SWNTs by a thermal annealing process at high temperature to restore the genuine optical and electrical properties of SWNTs. Hydroxyphenyl groups were covalently attached to SWNTs for this purpose. Functionalized SWNTs typically show a significantly larger sheet resistance and completely lose their typical characteristic optical properties due to the covalent attachment of the functional groups onto the SWNT surfaces. However, we found that all of the functional groups could be completely removed by an annealing temperature above 400oC in a nitrogen environment, as confirmed by FT-IR, Raman and UV-vis-nIR absorption analyses. Accordingly, the original optical and electrical properties were also recovered upon annealing at above 400oC, as confirmed by the recovery of the original sheet resistance value. To demonstrate the effectiveness of annealing, we evaluated transparent films, which were prepared predominantly with metallic SWNTs, using the separation technique, which is based on selective covalent functionalization. A functionalized metallic SWNT film recovers its original high conductivity upon annealing at 400oC by the removal of the functional groups, which demonstrates the effectiveness of this annealing process.
    RSC Advances 09/2014; · 3.71 Impact Factor
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    ABSTRACT: Composite materials of Sn-doped TiO2 nanoparticles (4–6 nm) uniformly anchored onto reduced graphene oxide (RGO) sheets were fabricated by a hydrothermal process. When compared with an undoped TiO2/RGO composite material, the partial substitution of Sn4+ for Ti4+ in TiO2/RGO in different doping concentrations not only increased the specific surface area, but also narrowed the band gap energy of the material. It also resulted in a shift in the photoresponse of the material into the visible light region, an increase in the relative intensity of its Raman (D/G) band, and changes in its X-ray photoelectron spectra; these results implied strong interactions between the Sn species and nanosized TiO2 as well as between the TiO2 and RGO layers. The presence of RGO led to enhanced adsorptivity of methyl orange onto the material compared with that of either pure TiO2 or Sn-doped TiO2. Increasing the Sn dopant content to optimal value facilitated photoactivity under both UV and visible light irradiation. The incorporation of both RGO and Sn dopants significantly modified the electronic structure of TiO2, altering the electron transfer direction and efficiently prohibiting the recombination of photo-induced charge carriers.
    Applied Catalysis A General 03/2014; 473:21–30. · 3.41 Impact Factor
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    ABSTRACT: La oxide is known to be the best promoter among reducible metal oxides for acetylene hydrogenation. However, it requires high-temperature reduction, which is not feasible in commercial processes. To maintain the enhanced catalytic performance by La oxide addition while lowering the reduction temperature for application in commercial process, we added Ti oxide as a second promoter, which has a higher reducibility than La oxide. The Ti oxide is added to the Pd surface, which has been partially covered by La oxide, and maintains the modified geometric and electronic structures of the Pd catalyst induced by the high-temperature-reduced La oxide even after low-temperature reduction, as confirmed by H2 chemisorption and X-ray photoelectron spectroscopy. Surprisingly, Ti oxide further modifies the electronic structure of Pd, even for low-temperature reduction, due to its high reducibility, leading to higher ethylene selectivity than when La oxide is used exclusively. We also confirmed that a similar additive effect also applies to other metal oxides, i.e., Nb2O5.
    Applied Catalysis A: General. 01/2014; 471:80–83.
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    ABSTRACT: Thermopower waves are a recently developed energy conversion concept utilizing dynamic temperature and chemical potential gradients to harvest electrical energy while the combustion wave propagates along the hybrid layers of nanomaterials and chemical fuels. The intrinsic properties of the core nanomaterials and chemical fuels in the hybrid composites can broadly affect the energy generation, as well as the combustion process, of thermopower waves. So far, most research has focused on the application of new core nanomaterials to enhance energy generation. In this study, we demonstrate that the alignment of core nanomaterials can significantly influence a number of aspects of the thermopower waves, while the nanomaterials involved are identical carbon nanotubes (CNTs). Diversely structured, large-area CNT/fuel composites of one-dimensional aligned CNT arrays (1D CNT arrays), randomly oriented CNT films (2D CNT films), and randomly aggregated bulk CNT clusters (3D CNT clusters) were fabricated to evaluate the energy generation, as well as the propagation of the thermal wave, from thermopower waves. The more the core nanostructures were aligned, the less inversion of temperature gradients and the less cross-propagation of multiple thermopower waves occurred. These characteristics of the aligned structures prevented the cancellation of charge carrier movements among the core nanomaterials and produced the relative enhancement of the energy generation and the specific power with a single-polarity voltage signal. Understanding this effect of structure on energy generation from thermopower waves can help in the design of optimized hybrid composites of nanomaterials and fuels, especially designs based on the internal alignment of the materials. More generally, we believe that this work provides clues to the process of chemical to thermal to electrical energy conversion inside/outside hybrid nanostructured materials.
    Nanoscale Research Letters 01/2014; 9(1):536. · 2.52 Impact Factor
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    ABSTRACT: Highly efficient exfoliation of individual single-walled carbon nanotubes (SWNTs) was successfully demonstrated by utilizing biocompatible phenoxylated dextran, a kind of polysaccharide, as a SWNT dispersion agent. Phenoxylated dextran shows greater ability in producing individual SWNTs from raw materials than any other dispersing agent, including anionic surfactants and another polysaccharide. Furthermore, with this novel polymer, SWNT bundles or impurities present in raw materials are removed under much milder processing conditions compared to those of ultra-centrifugation procedures. There exists an optimal composition of phenoxy groups (∼13.6 wt%) that leads to the production of high-quality SWNT suspensions, as confirmed by UV-vis-nIR absorption and nIR fluorescence spectroscopy. Furthermore, phenoxylated dextran strongly adsorbs onto SWNTs, enabling SWNT fluorescence even in solid-state films in which metallic SWNTs co-exist. By bypassing ultra-centrifugation, this low-energy dispersion scheme can potentially be scaled up to industrial production levels.
    Nanoscale 05/2013; · 6.73 Impact Factor
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    ABSTRACT: Diazonium salts preferentially react with metallic single-walled carbon nanotubes (SWNT) over semiconducting SWNT, enabling the separation of SWNT by electronic type. Therefore, the reaction selectivity of diazonium salts for metallic SWNT is crucial for high purity separation of both metallic and semiconducting SWNT. Herein, we developed an efficient method of increasing the reaction selectivity by manipulating the redox potential of diazonium salts. The electron affinity of diazonium salts is effectively lowered when the para-substituent of the diazonium salts is an electron-donating group, (i.e., 4-hydroxy and 4-propargyloxy) rather than an electron-withdrawing group (i.e., 4-nitro, 4-carboxy, and 4-cholro). The reduction potential of 4-hydroxyphenyl and 4-propargyloxyphenyl diazonium salt was greater than the oxidation potential of semiconducting SWNT; therefore, the electron transfer reaction between these two reagents was effectively suppressed, leading to a highly selective reaction for metallic SWNT. We confirmed that this highly selective reaction scheme can be used to separate SWNT, and high purity semiconducting SWNT can be obtained via density-induced separation.
    Chemistry of Materials. 10/2012; 24(21):4146–4151.
  • Woo-Jae Kim, Sang Heup Moon
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    ABSTRACT: This article reviews our previous studies to develop high performance catalysts for acetylene hydrogenation. Ethylene selectivity of Pd catalysts in acetylene hydrogenation was improved by adding Si, metal oxides showing SMSI behavior, and either Ag or Cu as promoters. The promoter effect was further enhanced by maximizing the interactions between Pd and added promoters. Chemical vapor deposition was used for the addition of Si, high-temperature reduction for the SMSI metal oxides, and a surface redox (SR) for Ag and Cu. Si modified the Pd surface geometrically, SMSI metal oxides modified Pd both electronically and geometrically, and Ag modified Pd largely electronically. Cu added by SR modified Pd electronically to a small extent, but preferentially decorated the low-coordination sites of Pd such that the ethylene selectivity of the optimum catalyst was significantly promoted from that of Pd, while its activity remained comparable to that of Pd. The sensitivity of ethylene selectivity to the surface structure of Pd was demonstrated using model catalysts containing uniform-sized Pd particles in either cubic or spherical shapes. The three stage deactivation of the Pd catalyst and the self-regenerative behavior in early stages of deactivation were also investigated.
    Catalysis Today - CATAL TODAY. 05/2012;
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    ABSTRACT: Electron beam deposition technique was used for formation of gadolinium Ca doped GDC electrolyte thin films. Ca_GDC electrolyte thin films were grown by evaporating Gd0.1Ce0.9O1.95 ceramic powder. The evaporating Ca_GDC electrolyte powder was synthesized by solid-state reaction method. The GDC thin films were deposited on porous Ni-GDC anode pellet. Operating technical parameters that influence thin film microstructure and crystallite size were studied. It was found that electron gun power (changed in the range of 0.60 to 1.05 kW) has the influence on the crystallite size of GDC thin films and decreased linearly increasing electron beam gun power. The influence of electron gun power and annealing temperature on thin film structure and surface morphology were investigated by X-ray diffraction (XRD) using CuKα-radiation in the range of 2 = 20∼80°C. The XRD peaks was showed formation of single-phase monoclinic structure was confirmed by X-ray diffraction (XRD) for the doped Ca GDC electrolyte. The patterns indicated the intensity of well-cubic fluorite structure phase.The morphology and the size of the prepared particles were investigated with a field-emission scanning electron microscope (FE-SEM). The performance of the cells was evaluated over 500∼800°C using humidified hydrogen as fuel and air as oxidant.
    Molecular Crystals and Liquid Crystals 01/2012; 568(1). · 0.53 Impact Factor
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    ABSTRACT: The aim of this work is to prevent the back transfer of electrons due to direct contact between the electrolyte and the conductive substrate by using TiO2 passivation. A thin TiO2 passivating layer was deposited on fluorine-doped tin oxide (SnO2:F, denoted FTO) glass by radio frequency (denoted RF) magnetron sputtering using different working pressures. The thickness and the crystalline structure were adjusted by applying various working conditions. The TiO2 films calcinated at low working pressure had an anatase phase, and they grew into a rutile phase with a decrease of the working pressure. The dye-sensitized solar cell using TiO2 passivating layer prepared at 0.5mTorr was measured the maximum conversion efficiency of 4.6% due to effective prevention of the electron recombination to electrolyte. It was found that the conversion efficiency of the dye sensitized solar cell (denoted DSSC) was highly affected by the crystalline structure of the passivating layer.
    Ceramics International - CERAM INT. 01/2012;
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    ABSTRACT: GAZO (Ga–Al doped ZnO)/Ag/GAZO multilayer films were prepared by Facing Target Sputtering (FTS) methods at room temperature. The GAZO multilayer films consisted of various thickness Ag and top GAZO thin film. The electrical, optical and structural properties of the films were investigated using a four-point probe, an UV/vis spectrometer, a X-ray diffractometer (XRD), a field emission scanning electron microscope (FE-SEM) and atomic force microscopy (AFM). For the multilayer film with top and bottom GAZO thickness of 50nm and intermediate Ag thickness of 12nm, it exhibits the maximum figure of merit of 73.05×10−3Ω−1 with sheet resistance of 9.1Ω/sq and transmittance of 96.4% at wavelength of 550nm.
    Microelectronic Engineering - MICROELECTRON ENG. 01/2012;
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    ABSTRACT: Molecularly hybridized materials composed of polymer semiconductors (PSCs) and single-walled carbon nanotubes (SWNTs) may provide a new way to exploit an advantageous combination of semiconductors, which yields electrical properties that are not available in a single-component system. We demonstrate for the first time high-performance inkjet-printed hybrid thin film transistors with an electrically engineered heterostructure by using specially designed PSCs and semiconducting SWNTs (sc-SWNTs) whose system achieved a high mobility of 0.23 cm(2) V(-1) s(-1), no V(on) shift, and a low off-current. PSCs were designed by calculation of the density of states of the backbone structure, which was related to charge transfer. The sc-SWNTs were prepared by a single cascade of the density-induced separation method. We also revealed that the binding energy between PSCs and sc-SWNTs was strongly affected by the side-chain length of PSCs, leading to the formation of a homogeneous nanohybrid film. The understanding of electrostatic interactions in the heterostructure and experimental results suggests criteria for the design of nanohybrid heterostructures.
    ACS Nano 12/2011; 6(1):662-70. · 12.03 Impact Factor
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    ABSTRACT: Exciton engineering with carbon nanotubes and graphene for solar energy conversion: from exciton antennae to nano-heterojunctions Michael S. Strano Charles and Hilda Roddey Associate Professor of Chemical Engineering 66-566 Department of Chemical Engineering 77 Massachusetts Avenue Cambridge, MA 02139-4307 Email: strano@MIT.EDU phone: (617) 324-4323 fax: (617) 258-8224 http://web.mit.edu/stranogroup/ Our laboratory has been interested in how low dimensional materials, such as single walled carbon nanotubes or graphene, can augment and enhance solar conversion efficiencies and demonstrate new photonic concepts. We will discuss two recent examples of our work in this space. In the first example, there is significant interest in combining carbon nanotubes with semiconducting polymers for photovoltaic applications because of potential advantages from smaller exciton transport lengths and enhanced charge separation. However, to date, bulk heterojunction (BHJ) devices have demonstrated relatively poor efficiencies, and little is understood about the polymer/nanotube junction. To investigate this interface, we fabricate a planar nano-heterojunction comprising well-isolated millimeter-long single-walled carbon nanotubes underneath a poly(3-hexylthiophene) (P3HT) layer (1). The resulting junctions display photovoltaic efficiencies per nanotube ranging from 3% to 3.82%, which exceed those of polymer/nanotube BHJs by a factor of 50-100. The increase is attributed to the absence of aggregate formation in this planar device geometry. It is shown that the polymer/nanotube interface itself is responsible for exciton dissociation. Typical open-circuit voltages are near 0.5 V with fill factors of 0.25-0.3, which are largely invariant with the number of nanotubes per device and P3HT thickness. A maximum efficiency is obtained for a 60 nm-thick P3HT layer, which is predicted by a Monte Carlo simulation that takes into account exciton generation, transport, recombination, and dissociation. We combine for the first time both optical T-matrix and kinetic Monte Carlo models to investigate the photocurrent generation in two state-of-the-art PHJ photovoltaics. The combined model takes into account the rates of exciton generation, transport, recombination and dissociation using literature values. By including the optical, electronic and structural properties of the different materials, we are able to predict the short-circuit current of recently reported P3HT/SWNT PHJ and also P3HT/PCBM PHJ solar cells from the literature. The experimental data for each of these devices show a maximum photocurrent output at a P3HT thickness of 60-65 nm, in contradiction to the expected value equal to the diffusion length of excitons in P3HT (8.5nm). The model demonstrates how a bulk exciton sink can explain this shifted maximum in the P3HT/SWNT case, whereas the maximum is mainly determined by PCBM interdiffusing in P3HT in the P3HT/PCBM case. This platform is promising for further understanding the potential role of polymer/nanotube interfaces for photovoltaic applications. In the second example, there has been renewed interest in solar concentrators and optical antennas for improvements in photovoltaic energy harvesting and new optoelectronic devices. We dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating it to the centre of core–shell structures with radial gradients in the optical bandgap (2). We find an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K, attributed to the strongly temperature-dependent second-orderAuger process. Core–shell structures consisting of annular shells of mostly (6, 5) SWNTs (Eg = 1.21 eV) and cores with bandgaps smaller than those of the shell (Eg = 1.17 eV (7,5)–0.98 eV (8,7)) demonstrate the concentration concept: broadband absorption in the ultraviolet–near-infrared wavelength regime provides quasi-singular photoemission at the (8, 7) SWNTs. This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways. * Ham MH, Paulus GLC, Lee CY, Song C, Kalantar-zadeh K, Choi W, Han JH and Strano MS: Evidence for High-Efficiency Exciton Dissociation at Polymer/Single-Walled Carbon Nanotube Interfaces in Planar Nano-heterojunction Photovoltaics. ACS NANO, 4 (2010) 6251-6259 * Han JH, Paulus GLC, Maruyama R, Jeng ES, Heller DA, Kim WJ, Barone PW, Lee CY, Choi JH, Ham MH, Song C, Fantini C, Strano MS: Exciton Antennas and Concentrators from Core-Shell and Corrugated Carbon Nanotube Filaments of Homogeneous Composition. NATURE MATERIALS, 9, 833 - 839 (2010).
    2011 AIChE Annual Meeting; 10/2011
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    ABSTRACT: This study investigates the crystal phase and microstructure of LSM (La1-xSrxMnO3) cathode material synthesized by solid state reaction method and combustion synthesis. It measures and compares their electrical properties. LSM is a lanthanum ferrite-based cathode material. It has an ABO3 perovskite structure; the A in this structure was substituted by Sr in this research to synthesize the LSM cathode powder. An electrolyte pellet was prepared using 8 mol% YSZ (yttria-stabilized zirconia) powder. The electrode was vapor deposited by screen printing. The crystal structure and morphology were measured by scanning electron microscopy (SEM) and X-ray diffraction (XRD) for the sintered samples collected. The complex impedance was measured in the temperature range 600–900°C in air (Computer Impedance Grain-Phase Analyzer). The electrical conductivity and polarization resistance of LSM were characterized systematically.
    Molecular Crystals and Liquid Crystals 05/2011; 539(1):50/[390]-57/[397]. · 0.53 Impact Factor
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    ABSTRACT: 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.
    Molecular Crystals and Liquid Crystals 05/2011; 539(1):238/[578]-246/[586]. · 0.53 Impact Factor
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    ABSTRACT: Novel hierarchical rose bridal bouquet- and humming-top-like nanostructured TiO(2) were successfully prepared by the simple process with the hydrothermal temperature as the morphology-controlling factor. The gradual transformation from layered titanate to brookite phase was well consistent with the formation mechanism of the hierarchical morphologies. The three-dimensional flower bouquets built from the bunches of roses with surrounding fern fronds displayed the best adsorptivity and completely degraded methylene blue within 60 min under UV irradiation, whereas the humming-top geometry composed of anisotropically elongated spindle-like crystallites was detrimental to the dye photodegradation.
    Journal of Colloid and Interface Science 04/2011; 356(1):138-44. · 3.55 Impact Factor
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    ABSTRACT: The performance of Ag-promoted Pd/Al2O3 catalysts, which were prepared by the selective deposition of Ag onto Pd using a surface redox (SR) method, during acetylene hydrogenation was compared with that of catalysts prepared by impregnation. The Pd surface was more effectively modified with Ag added by SR, even when small amounts of Ag were added. The catalyst prepared by SR showed a higher ethylene selectivity than the one prepared by impregnation, because SR allowed both the preferential deposition of Ag on the low-coordination sites of Pd and a greater electronic modification of Pd by Ag.
    Catalysis Communications - CATAL COMMUN. 01/2011; 12(13):1251-1254.
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    ABSTRACT: We prepared the indium tin oxide thin (ITO) film on the polymer substrate by using facing target sputtering method. To obtain a smooth surface of the ITO thin film for application of OLEDs, before deposition of the ITO thin film, the polymer substrate was given plasma surface treatment. The electrical and surface properties were measured by a Hall Effect measurement and a contact angle measurement. The structural and optical properties were evaluated by an X-ray diffractometer, an atomic force microscope and a UV/VIS spectrometer, respectively. All ITO thin films deposited on plasma-treated polymer substrate showed an average transmittance over 85% in visible range, and the lowest resistivity was 4.17×10−4Ωcm.
    Thin Solid Films 01/2011; 519(20):6844-6848. · 1.87 Impact Factor
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    ABSTRACT: Cu-promoted Pd/Al2O3 catalysts were prepared by selectively depositing Cu onto the Pd surface using a surface redox (SR) method, and their performance in the selective hydrogenation of acetylene was compared with that of Ag-promoted catalysts prepared by both the SR and the conventional impregnation method. The Cu-promoted catalysts prepared by SR showed higher ethylene selectivity and activity than Ag-promoted catalysts, particularly with small amounts of added promoter. The above results were obtained because Cu added by SR was deposited preferentially onto the low-coordination sites of Pd, which were detrimental to ethylene selectivity but took a small fraction of the Pd surface that was responsible for acetylene conversion, and also because Cu had an intrinsic activity for hydrogenation. The advantages of Cu-promoted catalysts prepared using Cu as a promoter and the SR process as the promoter-addition method were conclusively demonstrated in the present study.
    Applied Catalysis A-general - APPL CATAL A-GEN. 01/2011; 401(1):12-19.
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    ABSTRACT: Quantum dots have been attractive especially in the area of biosensors due to their peculiar optical properties. In this context, less toxic Zinc selenide (ZnSe) quantum dots were synthesized in the cavity of the apoferritin from horse spleen (HsAFr), and the two-dimensional ZnSe-ferritin nanodots were prepared on modified silicon surface. For utilizing the array as a biosensor, the photoluminescence (PL) spectrum change was investigated by accompanying its conjugation reaction with gamma-aminobutyric acid (GABA) and glutamic acid, where GABA is a major inhibitory neurotransmitter in the central nervous system and glutamic acid is its physiological precursor. The results revealed that the fluorescence intensity of ZnSe quantum dots in ferritin core is dependent on the concentration of GABA and the enhancement factor was a linear function of the GABA concentration in the range of 0.03 to 0.18 μM despite the presence of glutamic acid. Accordingly, the ZnSe-ferritin nanodot arrays can be employed as a useful sensing media for even very tiny concentration of neurotransmitter GABA.
    Molecular Crystals and Liquid Crystals. 12/2010; 519(1):27-35.

Publication Stats

321 Citations
159.28 Total Impact Points

Institutions

  • 2010–2014
    • Gachon University
      • Department of Environmental and Energy Engineering
      Sŏngnam, Gyeonggi Province, South Korea
  • 2008–2013
    • Massachusetts Institute of Technology
      • Department of Chemical Engineering
      Cambridge, MA, United States
  • 2006–2007
    • University of Illinois, Urbana-Champaign
      • Department of Chemical and Biomolecular Engineering
      Urbana, IL, United States
    • Sungkyunkwan University
      • Department of Chemical Engineering
      Seoul, Seoul, South Korea