Youngmi Lee

A beneficial role of exfoliated layered metal oxide nanosheets in optimizing the electrocatalytic activity and pore structure of Pt-reduced graphene oxide nanocomposites

Mesoporous nanocomposites of Pt-reduced graphene oxide (RGO)-layered titanate are synthesized by the reaction of a mixture of exfoliated layered titanate nanosheets, graphene oxide nanosheets, and H2PtCl6 with NaBH4 to investigate the effect of layered metal oxide nanosheets on the electrocatalyst performance of Pt–RGO nanocomposites. The obtained ternary nanocomposites are composed of a porous stacking assembly of layered titanate/RGO nanosheets with well-dispersed Pt nanocrystals whose particle size becomes smaller with the increase of titanate content. The incorporation of layered titanate nanosheets into the Pt–RGO nanocomposite induces a marked increase of surface area through the frustration of a strong π–π interaction between RGO nanosheets by the intervention of titanate nanosheets. In addition, the incorporation of layered titanate nanosheets gives rise to a remarkable improvement of electrocatalytic activity for oxygen reduction reaction (ORR). Of prime importance is that the present nanocomposites show a more positive onset potential for ORR than commercial Pt/carbon catalysts, underscoring a powerful role of titanate nanosheets in optimizing the electrocatalyst performance of Pt–RGO nanocomposites. This is the first report showing the usefulness of metal oxide nanosheets as an effective additive for enhancing the electrocatalyst performance of platinum nanoparticles. The observed enhancement of electrocatalytic activity upon the addition of titanate nanosheets is attributable to the decrease of Pt crystal size, the increase of surface area, and the increase of surface hydrophilicity. The present findings clearly demonstrate that the incorporation of layered titanate nanosheets is quite effective in improving the electrocatalytic functionality of Pt–RGO nanocomposites.

The Crystal Structure of Ferritin from Helicobacter pylori Reveals Unusual Conformational Changes for Iron Uptake

The crystal structure of recombinant ferritin from Helicobacter pylori has been determined in its apo, low-iron-bound, intermediate, and high-iron-bound states. Similar to other members of the ferritin family, the bacterial ferritin assembles as a spherical protein shell of 24 subunits, each of which folds into a four-alpha-helix bundle. Significant conformational changes were observed at the BC loop and the entrance of the 4-fold symmetry channel in the intermediate and high-iron-bound states, whereas no change was found in the apo and low-iron-bound states. The imidazole rings of His149 at the channel entrance undergo conformational changes that bear resemblance to heme configuration and are directly coupled to axial translocation of Fe ions through the 4-fold channel. Our results provide the first structural evidence of the translocation of Fe ions through the 4-fold channel in prokaryotes and the transition from a protein-dominated process to a mineral-surface-dominated process during biomineralization.

Improved electrochemical microsensor for the real-time simultaneous analysis of endogenous nitric oxide and carbon monoxide generation.

An amperometric dual NO/CO microsensor was developed on the basis of a working electrode incorporating dual Pt microdisks (each diameter, 76 μm) and a Ag/AgCl reference electrode covered with a gas permeable membrane. One of the Pt disks was sequentially electrodeposited with Pt and Sn; the other Pt disk was deposited with Pt-Fe(III) oxide nanocomposites. The first showed activity for the oxidation of both NO and CO; the second showed activity only for NO oxidation. In the copresence of NO and CO, the currents measured at each electrode, respectively, represented the concentrations of CO and NO. The sensor showed high stability during the monitoring of organ tissue for at least 2.5 h and high selectivity to NO over CO at the Pt-Fe(III) oxide working electrode. Real-time coupled dynamic changes of NO and CO generated by a living C57 mouse kidney were monitored simultaneously and quantitatively in response to a NO synthase inhibitor (N(G)-nitro-l-arginine methyl ester), for the first time. CO was found to increase and NO decreased upon addition of the inhibitor, suggesting a possible reciprocal interaction between these endogenous gases.

Hierarchically Driven IrO2 Nanowire Electrocatalysts for Direct Sensing of Biomolecules

Applying nanoscale device fabrications toward biomolecules, ultra sensitive, selective, robust, and reliable chemical or biological microsensors have been one of the most fascinating research directions in our life science. Here we introduce hierarchically driven iridium dioxide (IrO(2)) nanowires directly on a platinum (Pt) microwire, which allows a simple fabrication of the amperometric sensor and shows a favorable electronic property desired for sensing of hydrogen peroxide (H(2)O(2)) and dihydronicotinamide adenine dinucleotide (NADH) without the aid of enzymes. This rational engineering of a nanoscale architecture based on the direct formation of the hierarchical 1-dimensional (1-D) nanostructures on an electrode can offer a useful platform for high-performance electrochemical biosensors, enabling the efficient, ultrasensitive detection of biologically important molecules.

Single carbon fiber decorated with RuO2 nanorods as a highly electrocatalytic sensing element.

We demonstrate highly efficient electocatalytic activities of single crystalline RuO(2) nanorods grown on carbon fiber (CF), i.e., RuO(2) nanorod-CF hybrid microelectrode, prepared by a simple thermal annealing process from the Ru(OH)(3) precursor at 300 °C. The general electrochemical activity of a RuO(2) nanorod-CF microelectrode represents faster electron transfer for the [Fe(CN)(6)](3-/4-) couple than that of the bare CF microelectrode which are confirmed from the cyclic voltammetry (CV) measurement. Also, the amperometric response for the H(2)O(2) oxidation is remarkably facilitated at the RuO(2) nanorod-CF microelectrode by not only the enlarged surface area but the high electrocatalytic activity of the RuO(2) nanorod material itself. Furthermore, a single microelectrode of RuO(2) nanorod-CF exhibits the superior tolerance to Cl(-) ion poisoning unlike Pt-based electrocatalysts, indicating the promising sensor candidate in physiological conditions.

Spongelike nanoporous Pd and Pd/Au structures: facile synthesis and enhanced electrocatalytic activity.

This paper reports the facile synthesis and characterization of spongelike nanoporous Pd (snPd) and Pd/Au (snPd/Au) prepared by a tailored galvanic replacement reaction (GRR). Initially, a large amount of Co particles as sacrificial templates was electrodeposited onto the glassy carbon surface using a cyclic voltammetric method. This is the key step to the subsequent fabrication of the snPd/Au (or snPd) architectures by a surface replacement reaction. Using Co films as sacrificial templates, snPd/Au catalysts were prepared through a two-step GRR technique. In the first step, the Pd metal precursor (at different concentrations), K2PdCl4, reacted spontaneously to the formed Co frames through the GRR, resulting in a snPd series. snPd/Au was then prepared via the second GRR between snPd (prepared with 27.5 mM Pd precursor) and Au precursor (10 mM HAuCl4). The morphology and surface area of the prepared snPd series and snPd/Au were characterized using spectroscopic and electrochemical methods. Rotating disk electrode (RDE) experiments for oxygen reduction in 0.1 M NaOH showed that the snPd/Au has higher catalytic activity than snPd and the commercial Pd-20/C and Pt-20/C catalysts. Rotating ring-disk electrode (RRDE) experiments reconfirmed that four electrons were involved in the electrocatalytic reduction of oxygen at the snPd/Au. Furthermore, RDE voltammetry for the H2O2 oxidation/reduction was used to monitor the catalytic activity of snPd/Au. The amperometric i-t curves of the snPd/Au catalyst for a H2O2 electrochemical reaction revealed the possibility of applications as a H2O2 oxidation/reduction sensor with high sensitivity (0.98 mA mM(-1) cm(-2) (r = 0.9997) for H2O2 oxidation and -0.95 mA mM(-1) cm(-2) (r = 0.9997) for H2O2 reduction), low detection limit (1.0 μM), and a rapid response (<∼1.5 s).

In vitro assessment of clastogenicity of mobile-phone radiation (835 MHz) using the alkaline comet assay and chromosomal aberration test

Recently we demonstrated that 835‐MHz radiofrequency radiation electromagnetic fields (RF‐EMF) neither affected the reverse mutation frequency nor accelerated DNA degradation in vitro. Here, two kinds of cytogenetic endpoints were further investigated on mammalian cells exposed to 835‐MHz RF‐EMF (the most widely used communication frequency band in Korean CDMA mobile phone networks) alone and in combination with model clastogens: in vitro alkaline comet assay and in vitro chromosome aberration (CA) test. No direct cytogenetic effect of 835‐MHz RF‐EMF was found in the in vitro CA test. The combined exposure of the cells to RF‐EMF in the presence of ethylmethanesulfonate (EMS) revealed a weak and insignificant cytogenetic effect when compared to cells exposed to EMS alone in CA test. Also, the comet assay results to evaluate the ability of RF‐EMF alone to damage DNA were nearly negative, although showing a small increase in tail moment. However, the applied RF‐EMF had potentiation effect in comet assay when administered in combination with model clastogens (cyclophosphamide or 4‐nitroquinoline 1‐oxide). Thus, our results imply that we cannot confidently exclude any possibility of an increased risk of genetic damage, with important implications for the possible health effects of exposure to 835‐MHz electromagnetic fields.

Impact of Anions on Electrocatalytic Activity in Palladium Nanoparticles Supported on Ionic Liquid–Carbon Nanotube Hybrids for the Oxygen Reduction Reaction

A series of palladium nanoparticles supported on carbon nanotubes (CNTs), which were functionalized covalently with imidazolium polymer salts with different anions, Pd/polyIL(X)-CNTs (IL=ionic liquid; X=Cl, Br, I, ClO(4), BF(4), PF(6)), were prepared to investigate the influence of imidazolim salt anions on electrocatalytic activity in the oxygen reduction reaction (ORR). The anions of the imidazolium moiety significantly impacted on the ORR kinetics in a 0.1 M solution of HClO(4). The electronically active surface area results are in good agreement with the order of the ORR kinetic activity of the supported Pd/polyIL(X)-CNTs (X: Cl>ClO(4)>BF(4)>Br≈PF(6)≫I). In contrast, owing to the facile anion exchange of halide anions with hydroxide anions, anion-dependent catalytic activity has not been observed in 0.1 M NaOH. Iterative ORR experiments in acid-base solutions demonstrated anion exchange on the electrode. These results indicate that subtly varied structures of the IL moiety profoundly influence the performance of IL-CNT hybrid materials and molecular-level control of interfacial interactions between the support material, catalysts, and electrolytes is important in the design of supported metal nanoparticle catalysts for fuel cells.

Synthesis and electrocatalytic activity of highly porous hollow palladium nanoshells for oxygen reduction in alkaline solution.

A series of hollow Pd nanoshells are prepared by employing Co nanoparticles as sacrificial templates with different concentrations of a Pd precursor (1, 6, 12, 20, and 40 mM K2PdCl4), denoted hPd-X (X: concentration of K2PdCl4 in mM unit). The synthesized hPd series are tested as a cathodic electrocatalyst for oxygen reduction reaction (ORR) in alkaline solution. The morphology and surface area of the hPd catalysts are characterized using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), and cyclic voltammetry (CV). Rotating disk electrode (RDE) voltammetric studies show that the hPd-20 (prepared using 20 mM K2PdCl4) has the highest ORR activity among all the hPd series, while being comparable to commercial Pd and Pt catalysts (E-TEK). The more facilitated ORR at hPd-20 is presumably induced by the enhanced Pd surface area and efficiently high porosity of Pd nanoshells.

Real-Time in Vivo Simultaneous Measurements of Nitric Oxide and Oxygen Using an Amperometric Dual Microsensor

This paper reports a real-time study of the codynamical changes in the release of endogenous nitric oxide (NO) and oxygen (O(2)) consumption in a rat neocortex in vivo upon electrical stimulation using an amperometric NO/O(2) dual microsensor. Electrical stimulation induced transient cerebral hypoxia due to the increased metabolic demands that were not met by the blood volume inside the stimulated cortical region. A NO/O(2) dual microsensor was successfully used to monitor the pair of real-time dynamic changes in the tissue NO and O(2) contents. At the onset of electrical stimulation, there was an immediate decrease in the cortical tissue O(2) followed by a subsequent increase in the cortical tissue NO content. The averages of the maximum normalized concentration changes induced by the stimulation were a 0.41 (±0.04)-fold decrease in the O(2) and a 3.6 (±0.9)-fold increase in the NO concentrations when compared with the corresponding normalized basal levels. The peak increase in NO was always preceded by the peak decrease in O(2) in all animals (n = 11). The delay between the maximum decrease in O(2) and the maximum increase in NO varied from 3.1 to 54.8 s. This rather wide variation in the temporal associations was presumably attributed to the sparse distribution of NOS-containing neurons and the individual animals differences in brain vasculatures, which suggests that a sensor with fine spatial resolution is needed to measure the location-specific real-time NO and O(2) contents. In summary, the developed NO/O(2) dual microsensor is effective for measuring the NO and O(2) contents in vivo. This study provides direct support for the dynamic role of NO in regulating the cerebral hemodynamics, particularly related to the tissue oxygenation.