Juhyoun Kwak

Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles

Nanostructured carbon materials are potentially of great technological interest for the development of electronic, catalytic and hydrogen-storage systems. Here we describe a general strategy for the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters (typically 6 nanometres inside and 9 nanometres outside). These structures are formed by using ordered mesoporous silicas as templates, the removal of which leaves a partially ordered graphitic framework. The resulting material supports a high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials (such as carbon black, charcoal and activated carbon fibres). The platinum cluster diameter can be controlled to below 3 nanometres, and the high dispersion of these metal clusters gives rise to promising electrocatalytic activity for oxygen reduction, which could prove to be practically relevant for fuel-cell technologies. These nanomaterials can also be prepared in the form of free-standing films by using ordered silica films as the templates.

Time-Resolved In Situ Spectroelectrochemical Study on Reduction of Sulfur in N , N ′ -Dimethylformamide

This research was supported by a grant from KOSEF through the Center for Integrated Molecular Systems and a contract with Samsung SDI, Inc. Ltd. Graduate students were supported by the BK21 program of the Korea Research Foundation.

Faradaic impedance titration of pure 3-mercaptopropionic acid and ethanethiol mixed monolayers on gold

Abstract Interfacial proton transfer reactions of pure 3-mercaptopropionic acid (MPA) and ethanethiol (EtSH) mixed self-assembled monolayers (SAMs) have been studied using the faradaic impedance titration method. The Fe(CN) 6 3− is used to probe the charge development of the terminal COOH group of the monolayers. The charge-transfer resistance ( R ct ) is measured with the monolayer composition, adsorption coverage, and the ionic strength of pH solution. The surface p K determined for the pure MPA is 6.0 at a monolayer coverage and 0.1 M ionic strength. The in-plane electrostatic force effect, which causes a broadening of the titration curves, on the surface p K for the MPA SAM is not significant at an ionic strength over 0.1 M. The p K value for the pure MPA SAM shifts negatively as the surface coverage decreases, indicating that the in-plane interactions between acids and the hydrophobicity surrounding acids decrease at the same time. When the pure MPA SAM is compared with the EtSH mixed SAM, the surface p K of the mixed SAM is larger than that of the pure SAM. Such positive p K shifts are more pronounced at the pure MPA SAM with a lower coverage. This implies that the p K shifts of sparsely adsorbed acidthiol SAM are more sensitive to the introduction of hydrophobicity than to the decrease of the in-plane interactions.

Characterization and electrocatalytic properties of Prussian blue electrochemically deposited on nano-Au/PAMAM dendrimer-modified gold electrode

Gold electrode was modified with 3-mercaptopropionic acid (MPA) and further reacted with poly(amidoamine) (PAMAM) dendrimer (generation 4.0) then attached the nano-Au to obtain films on which Prussian blue (PB) was electrochemically deposited to afford much wider pH adaptive range, much better electrochemical stability and excellent electrochemical response. The microstructure and electrochemical behavior of Au/MPA/PAMAM/nano-Au/PB electrode were investigated by scanning electron microscopy (SEM) and cyclic voltammetry. The electrochemical response of the Au/MPA/PAMAM/nano-Au/PB-modified electrode for the electrocatalytic reduction of hydrogen peroxide was investigated, and it was found that the sensitivity as well as the corresponding detection limits were improved as compared to the voltammetric response of a Au/PB-modified electrode and Au/MPA/PAMAM/PB electrode. Based on this, a new electrochemical sensor for determination of hydrogen peroxide has been developed.

Label-free aptasensor for platelet-derived growth factor (PDGF) protein

A label-free aptasensor for platelet-derived growth factor (PDGF) protein is reported. The aptasensor uses mixed self-assembled monolayers (SAMs) composed of a thiol-modified PDGF binding aptamer and 6-mercaptohexanol (MCH) on a gold electrode. The SAMs were characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV) before and after binding of the protein using [Fe(CN)(6)](3-/4-), a redox marker ion as an indicator for the formation of a protein-aptamer complex. The CVs at the PDGF modified electrode showed significant differences, such as changes in the peak currents and peak-to-peak separation, before and after binding of the target protein. The EIS spectra, in the form of Nyquist plots, were analyzed with a Randles circuit while the electron transfer resistance R(ct) was used to monitor the binding of the target protein. The results showed that, without any modification to the aptamer, the target protein can be recognized effectively at the PDGF binding aptamer SAMs at the electrode surface. Control experiments using non-binding oligonucleotides assembled at the electrode surfaces also confirmed the results and showed that there was no formation of an aptamer-protein complex. The DPV signal at the aptamer functionalized electrode showed a linearly decreased marker ion peak current in a protein concentrations range of 1-40 nM. Thus, label-free detection of PDGF protein at an aptamer modified electrode has been demonstrated.

correction: Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles

This corrects the article DOI: 10.1038/35084046

An electrochemical immunosensor using ferrocenyl-tethered dendrimer

We report here an enzyme-amplified, sandwich-type immunosensor for detecting the biospecific interaction between an antibody and antigen using redox mediation. We employed biotin/anti-biotin IgG as a model immunosensing pair. Partially ferrocenyl-tethered dendrimer (Fc-D), whose ferrocene moiety acts as a redox mediator, was immobilized to the electrode surface by covalent binding between the dendrimer amines and the carboxylic acids of a self-assembled monolayer. The unreacted amines of the immobilized Fc-D were modified with biotin groups to allow the specific binding of goat anti-biotin IgG. Rabbit anti-goat IgG-conjugated alkaline phosphatase was bound to goat anti-biotin IgG to catalyze conversion of p-aminophenyl phosphate monohydrate to p-aminophenol. This product is oxidized to quinoimide by the reduction of ferrocenium back to ferrocene, producing an electrocatalytic anodic current. Cyclic voltammograms and surface plasmon resonance experiments showed that the binding of nonspecific proteins is not significant on the biotinylated Fc-D surface. We also examined the change in peak current according to the concentration of anti-biotin IgG and found that the detection range of this immunosensing scheme is between 0.1 and 30 microg mL(-1).

Polymer Films on Electrodes XXIV . Ellipsometric Study of the Electrochemical Redox Processes of a Polypyrrole Film on a Platinum Electrode

Ellipsometry was used to observe the in situ growth and electrochemical conversion redox processes of a polypyrrole film, which was electrochemically deposited to a thickness of ca. 100 nm on a platinum electrode in solution containing 50 mM pyrrole. To determine the spatial distribution of oxidized and reduced states during the reduction and oxidation of the film, the conversion process was simulated by a multilayer film model. Such simulations of the ellipsometric results show that the redox conversion of a polypyrrole film proceeds from the solution toward the electrode and is controlled by counterion movement. These results contrast with previous work on a thionine film where conversion proceeds from electrode to solution. The effective diffusion constant for charge transport based on ellipsometric simulation was estimated as .

Electrochemical insertion of lithium into polyacrylonitrile-based disordered carbons

Electrochemical lithium insertion into polyacrylonitrile (PAN)-based disordered carbons was studied using the techniques of discharge/charge tests, cyclic voltammetry, and {sup 7}Li nuclear magnetic resonance (NMR) spectroscopy. The PAN-based carbons were prepared by vacuum pyrolysis of PAN at 500, 800, and 1,000 C. They showed charge capacities between 254 and 380 mAh/g in the first cycle. {sup 7}Li NMR spectra showed two kinds of lithium insertion sites in the PAN-based carbons: a reversible site where lithium is removed in the subsequent charge process and an irreversible site where lithium remains intact. The NMR results suggest that lithium in fully Li-inserted PAN-based carbons has an ionic character, and reversible site lithium resides between negatively charged carbon layers.

Ion and water transports in Prussian blue films investigated with electrochemical quartz crystal microbalance

Water and ion transport in electrochemically prepared Prussian blue (PB, Iron(III) hexacyanoferrate(II)) films has been investigated with the electrochemical quartz crystal microbalance (EQCM) and the electrochemical/electrogravimetric impedance techniques. It is shown that the freshly prepared PB film is highly hydrated and that it undergoes an irreversible mass change during the first cathodic scan. The latter result supports the previously proposed structural reorganization scheme of the PB film from the insoluble form Fe4[Fe(CN)6]3·6H2O to the soluble form MFeFe(CN)6 (M is a monovalent cation). It is also shown that, during the first cathodic scan, a substantial amount of water is excluded from the PB film. After the structural reorganization, ion transport during the redox reaction of the PB film is cation-dominant with a small fraction of accompanying water transport.