Ichizo Yagi

Electrochemical Oxidation of NADH at Highly Boron-Doped Diamond Electrodes.

Conductive boron-doped chemical vapor-deposited diamond thin films, already known to have superior properties for general electroanalysis, including low background current and a wide potential window, are here shown to have additional advantages with respect to electrochemical oxidation of nicotinamide adenine dinucleotide (NADH), including high resistance to deactivation and insensitivity to dissolved oxygen. Cyclic voltammetry, amperometry, and the rotating disk electrode technique were used to study the reaction in neutral pH solution. Highly reproducible cyclic voltammograms for NADH oxidation were obtained at as-deposited diamond electrodes. The response was stable over several months of storage in ambient air, in contrast to glassy carbon electrodes, which deactivated within 1 h. The diamond electrode exhibited very high sensitivity for NADH, with an amperometric detection limit of 10 nM (S/N = 7). The response remained stable, even in the very low concentration range, for several months. In addition, interference effects due to ascorbic acid were minimal when the concentrations of NADH and ascorbic acid were comparable. An NADH-mediated dehydrogenese-based ethanol biosensor incorporating an unmodified diamond electrode is demonstrated. The present results indicate that diamond is a useful electrode material for the analytical detection of NADH, making it attractive for use in sensors based on enzyme-catalyzed reactions involving NADH as a cofactor.

Electrochemical selectivity for redox systems at oxygen-terminated diamond electrodes

Abstract Oxygen-terminated diamond electrodes were prepared by exposing as-grown hydrogen-terminated diamond thin films to oxygen plasma. The as-grown surfaces, which were highly hydrophobic, become hydrophilic after the oxygen plasma treatment. The apparent surface conductivity was not significantly changed after the oxygen plasma treatment. However, the electrochemical responses to several redox systems became remarkably different. For example, the cyclic voltammetric anodic–cathodic peak separations for the oxygen-terminated diamond electrodes became extremely large compared to those for the as-grown electrodes. This behavior was examined in comparison with as-grown diamond and glassy carbon electrodes.

Electroanalysis of dopamine and NADH at conductive diamond electrodes

Abstract Highly boron-doped diamond thin-film electrodes were examined for various possible applications in electroanalysis. Electrochemical oxidation of dopamine and NADH was investigated using cyclic voltammetry and chronoamperometry. Comparison experiments were performed using glassy carbon electrodes. Anodically treated diamond electrodes made it possible to determine dopamine selectively with high sensitivity in the presence of a large excess of ascorbic acid in acidic media. A detection limit of 50 nM was obtained using chronoamperometry. The treated electrodes were found to be stable for several months. Electrochemical oxidation of NADH was carried out at as-deposited diamond electrodes, with which very stable and reproducible cyclic voltammograms for NADH oxidation were obtained, unlike glassy carbon, at which a significant positive shift (∼200 mV) in the peak potential was observed within 1 h. The amperometric detection limit was found to be ∼10 nM. Interference of ascorbic acid was minimal using untreated electrodes when the concentration of ascorbic acid was comparable to the NADH concentration. Diamond microelectrodes small enough to consist of only one or two high quality microcrystals were fabricated in order to compare the electrochemical behavior with that of polycrystalline thin film electrodes, which contain large numbers of grain boundaries, at which non-diamond (sp 2 ) carbon can exist. This work demonstrates the potential of diamond electrodes for electroanalytical applications.

Synthesis of Well-Aligned Diamond Nanocylinders**

By Hideki Masuda,* Takashi Yanagishita, Kenji Yasui,Kazuyuki Nishio, Ichizo Yagi, Tata N. Rao, andAkira FujishimaDiamond and related materials have attracted intense inter-est because of several outstanding properties including nega-tive electron affinity (NEA) and high stability. These materi-als are expected to be promising for cold-cathode flat-panelelectronic displays and microelectronics.

Surface carbonyl groups on oxidized diamond electrodes

Abstract Oxygen-containing functional groups can be introduced onto the surface of polycrystalline boron-doped diamond electrodes by either anodic polarization or oxygen plasma treatment. Of these, the carbonyl groups are of particular interest and can be studied specifically by means of specific chemical modification with dinitrophenylhydrazine (DNPH). The modification of the surface carbonyl groups with DNPH retards the Fe2+/3+ redox reaction, which is known to be catalyzed by carbonyl groups. Anodic polarization generated a larger number of carbonyl groups per unit area that were reactive with DNPH than that generated by oxygen plasma treatment. The molar ratio of the DNPH-reactive carbonyl groups to the total of all types of oxygen atoms could be as high as 5% for electrochemically oxidized diamond surfaces. This value is reasonable in view of steric limitations. The total number of carbonyl groups per unit area introduced by oxygen plasma treatment was probably larger than that introduced by anodic polarization, but the number that can react with DNPH is lower due to disordering by energetic oxygen ions.

Control of the Dynamics of Photogenerated Carriers at the Boron‐Doped Diamond/Electrolyte Interface by Variation of the Surface Termination

The photoelectrochemical response of boron-doped diamond electrode/electrolyte interfaces was examined to establish the influence of the chemical termination at the diamond surface on the dynamics of photogenerated carriers. Photocurrent transients res ulting from pulsed excimer laser irradiation at hydrogen-terminated (as-deposited) diamond electrodes and at oxygen-terminated ele ctrodes, which were prepared either by exposing the surface to oxygen plasma or by anodic polarization in an electrolyte solution , were compared. A significant difference between the photocarrier dynamics for these two surfaces was observed in terms of the f ull width at half-maximum of the photocurrent transients. The much faster decay rate that was observed for the oxygen-terminated su rface is attributed to either the depassivation of deep trapping sites due to removal of subsurface hydrogen or to the introduct ion of defect recombination sites by the oxygen plasma.

Real time monitoring of electrochemical deposition of tellurium on Au(111) electrode by optical second harmonic generation technique

Abstract Electrochemical deposition of Te on a single crystal Au(111) electrode was monitored by the optical second harmonic generation (SHG) technique. The rotational anisotropic SHG measurement was used to estimate the overall symmetry of the electrode surface during the first underpotential deposition (upd), the second upd and bulk deposition of Te in real time. The formation of the first Te upd layer seemed to proceed in two steps as the SH rotational anisotropy became rather isotropic in the middle of the first upd and, then, the anisotropic character of the surface recovered again after the completion of the first upd. The experimental results suggest random deposition in the initial stage followed by the island growth, leading to the highly ordered ( 3 × 3 ) R 30° structure. It was also found that the anisotropic character of the overall surface symmetry was attenuated after the second upd and bulk deposition of Te.

Enhancement of Electrocatalytic Oxygen Reduction Activity and Durability of Pt–Ni Rhombic Dodecahedral Nanoframes by Anchoring to Nitrogen-Doped Carbon Support

Pt-based nanostructured electrocatalysts supported on carbon black have been widely studied for the oxygen reduction reaction (ORR), which occurs at the cathode in polymer electrolyte fuel cells. Because sluggish ORR kinetics are known to govern the cell performance, there is a need to develop highly active and durable electrocatalysts. The ORR activity of Pt-based electrocatalysts can be improved by controlling their morphology and alloying Pt with transition metals such as Ni. Improving the catalyst durability remains challenging and there is a lack of catalyst design concepts and synthetic strategies. We report the enhancement of the ORR activity and durability of a nanostructured Pt–Ni electrocatalyst by strong metal/support interactions with a nitrogen-doped carbon (NC) support. Pt–Ni rhombic dodecahedral nanoframes (NFs) were immobilized on the NC support and showed higher ORR electrocatalytic activity and durability in acidic media than that supported on a nondoped carbon black. Durability tests demonstrated that NF/NC showed almost no activity loss even after 50 000 potential cycles under catalytic conditions, and the Ni dissolution from the NFs was suppressed at the NC support, as confirmed by energy dispersive X-ray spectroscopy analysis. Physicochemical measurements including surface-enhanced infrared absorption spectroscopy of surface-adsorbed CO revealed that the strong metal/support interactions of the NF with the NC support caused the downshift of the d-band center position of the surface Pt. Our findings demonstrate that tuning the electronic structure of nanostructured Pt alloy electrocatalysts via the strong metal/support interactions with heteroatom-doped carbon supports will allow the development of highly active and robust electrocatalysts.