Jun Ho Shim

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.

Amperometric Nitric Oxide Microsensor Based on Nanopore-Platinized Platinum: The Application for Imaging NO Concentrations

This paper reports an amperometric nitric oxide (NO) microsensor based on a cone-shaped nanopore-platinized Pt working electrode. The senor was fabricated using the following procedure: (1) a parent nanodisk electrode was prepared by polishing an etched Pt wire (radius = 12.5 microm; dimension of etched tip end point <10 nm) embedded in a glass capillary, (2) the nanodisk Pt was further etched to produce a nanopore (pore opening radius <1 microm; pore depth approximately 30 microm), (3) the Pt base surface in the nanopore electrode was platinized electrochemically to improve the sensor sensitivity, and (4) silanization and further modification with the electropolymerized polymeric film [poly(5-amino-1-naphthol)] on the nanopore-platinized Pt electrode were carried out to obtain the sensor selectivity to NO. The analytical performance of the sensor was characterized. For example, a sensor with a pore opening radius of 797 nm exhibited a decent linear dynamic range (at least for 0.2-1.8 microM), detection limit of < approximately 32 nM, response time (t(90%)) of < approximately 5 s, and sensitivity of 6.5 +/- 0.02 pA/nM. This sensor was used successfully as a NO-selective probe tip in scanning electrochemical microscopy (SECM) to obtain a two-dimensional image of the local NO concentrations for an inlaid NO-emitting microdisk film (radius = 12.5 microm) on a glass substrate.

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).

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.

Oxidation-state dependent electrocatalytic activity of iridium nanoparticles supported on graphene nanosheets.

Nanocomposites of iridium nanoparticles (Ir NPs), supported on graphene nanosheets, are synthesized and their electrocatalytic acitivities in the oxygen reduction reaction (ORR) are studied depending on their Ir oxidation state. Graphene functionalized with poly(vinyl pyrrolidone) (pRGO) is a suitable support for Ir NPs, producing well-monodispersed Ir NPs anchored strongly on the pRGO surface (Ir NP/pRGO) with a very high density. This was confirmed by scanning electron microscopy and transmission electron microscopy. The ORR activity of the Ir NP/pRGO nanocomposites in 0.5 M H2SO4 solution was observed to be dependent on the oxidation state of the immobilized Ir NPs. In fact, the nanocomposite composed of Ir(0) metal NPs, rather than Ir oxide (IrOx) NPs, exhibits higher ORR activity, such as more positive onset potential, higher and flatter limiting current density, a greater n value, and a sharper curve shape in the rotating disk electrode voltammetry experiments. Higher ORR activity of Ir is ascribed to the stronger adsorption of oxygen on the surface of Ir compared to IrOx. The practical stability of the Ir NP/pRGO composite was also confirmed under O2 saturated/acidic conditions.

One dimensional Ag/Au/AgCl nanocomposites stemmed from Ag nanowires for electrocatalysis of oxygen reduction

This paper reports the synthesis and characterization of one dimensional nanocomposites composed of Ag, Au, and AgCl (denoted as Ag/Au/AgCl). The Ag/Au/AgCl composites with different compositional ratios are prepared by the galvanic replacement reaction between a Ag nanowire template and Au precursor at various concentrations (1, 10, 100 mM HAuCl4). As the Au precursor concentration increases, the structure of the resulting Ag/Au/AgCl changes from a core–shell solid wire to a porous hollow wire with a decrease in the Ag and AgCl contents and increase in the Au content. The catalytic activity of Ag/Au/AgCl for the oxygen reduction reaction (ORR) is investigated in 0.1 M NaOH solution. The nanocomposite with a Ag : Au : AgCl compositional ratio of 4 : 86 : 10 shows reasonable ORR activity (high ORR current, positive onset potential) along with sufficient stability and ethanol tolerance in alkaline media, suggesting that it could be used as a substitute for Pt-based cathode catalysts in fuel cells.