Kensuke Honda

Simultaneous detection of purine and pyrimidine at highly boron-doped diamond electrodes by using liquid chromatography

Highly boron-doped diamond (BDD) electrode, have been examined for simultaneous detection of purine and pyrimidine bases in mild acidic media by using HPLC with amperometric detection. Cyclic voltammetry at as-deposited (AD) and anodically oxidized (AO) BDD were used to study the electrochemistry and to optimize the condition for HPLC applications. At AO BDD electrode, due to its higher overpotential of oxygen evolution reaction, well-defined anodic peaks were observed for the oxidation of purine and pyrimidine bases in acid medium, whereas at AD BDD the oxidation peak of thymine was overlapped with the anodic current of oxygen evolution. The chromatograms of adenine, guanine, cytosine, thymine and 5-methylcytosine mixture were well resolved by using a silica-based column and a solution of 5% acetonitrile in 100mM ammonium acetate buffer (pH 4.25) as the mobile phase. The detection was carried out at AO BDD electrode at an applied potential of 1.6V versus Ag/AgCl. Linear calibration curves were obtained within the concentration range from 0.1 to 10microM with the limits of detection (S/N=3) ranging from 26.3 to 162.1nM, resulting in an order of magnitude higher sensitivities than those at conventional electrodes. HPLC analysis with diamond amperometric detector was successfully applied for determination of 5-methylcytosine in real DNA samples with high reproducibility. No deactivation of the electrode was found during cyclic voltammetric and HPLC measurements, indicating the high stability for analysis of biological samples.

Covalent Modification of Single-Crystal Diamond Electrode Surfaces

To develop the capability to create diamond electrodes with greater functionality, covalent modification was carried out on homoepitaxial single-crystal diamond electrode surfaces. (100) and (111) single-crystal boron-doped diamond electrodes were first prepared homoepitaxially, then subjected to oxidative treatments, and the functional groups on the oxidized surfaces were analyzed by employing X-ray photoelectron spectroscopy (XPS). Based on the results, we conclude that both singly and doubly bonded oxygen groups (C-O and C=O) were generated on the anodically treated (100) diamond electrode surfaces, whereas only singly bonded oxygen groups (C-O) were generated on the (111) surfaces. Second, selective surface modifications of anodically treated (100) and (111) diamond surfaces with 2,4-dinitrophenylhydrazine and 3-aminopropyltriethoxysilane moieties were carried out. Based on the tendencies of these surface modifications, together with the XPS results, we propose that carbonyl and hydroxyl groups are generated on anodically treated (100) diamond surfaces, and hydroxyl groups are mainly generated on anodically treated (111) diamond surfaces. The study of chemical modification of single-crystal diamond electrode surfaces should be useful, not only for creating new types of functional electrodes, but also for better understanding the properties of polycrystalline diamond electrodes and their possible applications.

Electrochemical Characterization of Carbon Nanotube/Nanohoneycomb Diamond Composite Electrodes for a Hybrid Anode of Li-Ion Battery and Super Capacitor

Carbon nanotube/nanohoneycomb diamond (CNT-NANO) composite electrodes were fabricated by introducing multiwalled carbon nanotubes into the pores of nanohoneycomb diamond of 400 nm diam using the chemical vapor deposition method. The electrochemical behavior of these electrodes was examined with cyclic voltammetry, electrochemical impedance, and galvanostatic measurements in LiClO 4 /propylene carbonate electrolyte. The behavior of Li + insertion into CNTs was observed in the cathodic sweep at -3.3 V (vs. Ag/Ag + ) in CV. AC impedance measurements have indicated that at the nanohoneycomb diamond densely deposited CNTs (HD CNT-NANO), only the Li + intercalation process was observed. In contrast, the nanohoneycomb diamond modified with CNTs in low-density (LD CNT-NANO) exhibited the combination behavior of Li + intercalation at CNTs and the electrochemical double-layer discharging on the diamond surface. In galvanostatic measurements, HD CNT-NANO behaved as a pure Li + ion battery anode, and the specific capacity (per I g of activated material) was found to be 894 mAh g -1 , which is higher than that obtained for mesophase carbon materials. For LD CNT-NANO, in the initial time following the start of discharging, the behavior of the double-layer discharging was observed in addition to Li + deintercalation. Suppression of the potential drops associated with Li + deintercalation by rapid discharging from the electrical double-layer could increase the specific power for LD CNT-NANO. The combination function of the super capacitor and the Li + -ion battery that work simultaneously supporting each other in one electrochemical cell suggests the possible realization of a hybrid electrode material with high energy density and high specific power.

Crystal-face-selective adsorption of Au nanoparticles onto polycrystalline diamond surfaces.

Crystal-face-selective adsorption of Au nanoparticles (AuNPs) was achieved on polycrystalline boron-doped diamond (BDD) surface via the self-assembly method combined with a UV/ozone treatment. To the best of our knowledge, this is the first report of crystal-face-selective adsorption on an inorganic solid surface. Hydrogen-plasma-treated BDD samples and those followed by UV/ozone treatment for 2 min or longer showed almost no adsorption of AuNP after immersion in the AuNP solution prepared by the citrate reduction method. However, the samples treated by UV/ozone for 10 s showed AuNP adsorption on their (111) facets selectively after the immersion. Moreover, the sample treated with UV/ozone for 40-60 s showed AuNP adsorption on the whole surface. These results indicate that the AuNP adsorption behavior can be controlled by UV/ozone treatment time. This phenomenon was highly reproducible and was applied to a two-step adsorption method, where AuNPs from different batches were adsorbed on the (111) and (100) surface in this order. Our findings may be of great value for the fabrication of advanced nanoparticle-based functional materials via bottom-up approaches with simple macroscale procedures.

Hybrid Electrochemical Treatment for Persistent Metal Complex at Conductive Diamond Electrodes and Clarification of Its Reaction Route

The hybrid electrochemical treatment for persistent organometallic complexes [Cu-ethylenediaminetetraacetic acid (EDTA)] that was a combination of the reduction recovery of the center metal as nanoparticles and the oxidative decomposition of the organic ligand was attempted using the potential cycling at the oxygen-terminated boron-doped diamond (BDD) electrode. In the step of the recovery of the center metal of the organo-metal complex in the potential cycling from the reduction potential for Cu 2+ , by stepping up the anodic potential limit to the potential region for oxygen evolution reaction, the reionization of electrodeposited Cu could be suppressed in the presence of the dissolved oxygen. Moreover, separation of the electrodeposited Cu as nanoparticles from the electrode surface could be achieved by introducing the oxygen-containing functional groups on the diamond surface. The oxidation of the particle surface by the dissolved oxygen might induce the formation of the insulating layer and accelerate the separation by the electrostatic repulsive force between the oxygen-terminated surface and the insulating layer on the Cu particles. The reaction pathway of the Cu-EDTA treatment was analyzed using the long-term potential cycling at the potential region between the reduction potential of Cu and the oxidation potential of the EDTA. In flow injection analysis with the UV detector, the formation of the Cu particles by the reduction reaction of the Cu 2+ ion from Cu-EDTA was clarified to mainly occur in the initial stage of the electrolysis. Moreover, in high-performance liquid chromatography analysis with the electrogenerated chemiluminescence detector for the treated solution, ethylenediaminetriacetic acid (ED3A) and ethylenediaminediacetic acid (EDDA) were observed. Therefore, it was confirmed that the liberated EDTA was oxidized to ED3A and EDDA through sequential removal of the acetate groups. An unidentified product (supposed to be the small size of a hydrocarbon) appeared after 100 h during the electrolysis. Although the concentration of this unidentified product was not decreased in the electrolysis with anodic potential limit of 2.3 V, the concentration decay for this product was observed for the potential cycling up to 3.0 V, suggesting that for the full oxidation of EDTA to CO 2 , it might be effective for the electrochemical oxidation by the OH radical available over 2.6 V at BDD.

Kinetic characteristics of enhanced photochromism in tungsten oxide nanocolloid adsorbed on cellulose substrates, studied by total internal reflection Raman spectroscopy

The nanostructured tungsten(VI) oxide (WO3)/cellulose derivatives (cellulose (CE) and triacetyl cellulose (TACE)) hybrid films were prepared by a solution-dipping adsorption process, and their structure and optical properties have been investigated. Various techniques, including adsorption isotherm, transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), in situUV-Vis absorption, and in situ total internal reflection Raman spectroscopy, were used for the characterization of the WO3/CE and WO3/TACE hybrid materials. Under UV irradiation, the photochromism (colorless → blue) was confirmed from the WO3/CE hybrid film, although no coloration of the WO3/TACE hybrid film was observed. This distinct difference in the coloration suggested that the interfacial interaction between hydroxyl groups present on the surface of the CE substrate and WO3 nanoparticlesviahydrogen bonding plays a major role in the enhancement of photochromism in the WO3/CE hybrid system. Moreover, the joint evidence in in situUV-Vis absorption and in situ total internal reflection Raman studies clearly revealed that the photogenerated coloration is related to a partial reduction of W6+ cations into W5+ cations in the WO3/CE hybrid film. The findings in this study have great implications for the development of the novel green-functional inorganic/organic hybrid materials in optical devices.

Controllable Electrochemical Activities by Oxidative Treatment toward Inner-Sphere Redox Systems at N-Doped Hydrogenated Amorphous Carbon Films

The electrochemical activity of the surface of Nitrogen-doped hydrogenated amorphous carbon thin films (a-CNH, N-doped DLC) toward the inner sphere redox species is controllable by modifying the surface termination. At the oxygen plasma treated N-doped DLC surface (O-DLC), the surface functional groups containing carbon doubly bonded to oxygen (C=O), which improves adsorption of polar molecules, were generated. By oxidative treatment, the electron-transfer rate for dopamine (DA) positively charged inner-sphere redox analyte could be improved at the N-doped DLC surface. For redox reaction of 2,4-dichlorophenol, which induces an inevitable fouling of the anode surface by forming passivating films, the DLC surfaces exhibited remarkably higher stability and reproducibility of the electrode performance. This is due to the electrochemical decomposition of the passive films without the interference of oxygen evolution by applying higher potential. The N-doped DLC film can offer benefits as the polarizable electrode surface with the higher reactivity and higher stability toward inner-sphere redox species. By making use of these controllable electrochemical reactivity at the O-DLC surface, the selective detection of DA in the mixed solution of DA and uric acid could be achieved.

Fabrication of silicon and carbon based wide-gap semiconductor thin films for high conversion efficiency

Nitrogen doped amorphous silicon carbide (N-doped a-SiC) thin films were fabricated by radio frequency plasma enhanced chemical vapor deposition (RF-PeCVD) method using mixed solution of tetramethylsilane (TES) and 1,1,1,3,3,3-hexamethyldisilazane (HMDS) as a liquid source. Chemical composition of N-doped a-SiC thin film was Si:C = 1:4 and sp2-bonded carbon ratio was 0.75. N-doped DLC were multi-phase structure including a-SiC phase, sp2 clusters and a-Si clusters. Optical gap and resistivity of the film were 1.68 eV and 4.32×104 Ω cm, respectively. From photocurrent measurement under UV exposure, it was clarified that the film functioned as n-type semiconductor materials with 4.87 % of quantum yield, which was on the same level as that obtained at anatase-type titanium oxide prepared by sol-gel method. To apply these films to solar cells, further improvements of optical gap and conductivity are necessary.

Enhancement of electrical conductivity and electrochemical activity of hydrogenated amorphous carbon by incorporating boron atoms

Conductive boron-doped hydrogenated amorphous carbon (B-DLC) thin films were successfully synthesized with RF plasma-enhanced CVD method. By incorporating boron atoms in amorphous carbon, conduction types were changed from n- to p-type, and volume resistivity was decreased from 30.4 (non-doped) to 6.36 × 10−2 Ω cm (B/C = 2.500 atom%). B-DLC film with sp2/(sp2 + sp3) carbons of 75 atom% exhibited high resistance to electrochemically-induced corrosion in strong acid solution. Furthermore, it was clarified that boron atoms in DLC could enhance kinetics of hydrogen evolution during water electrolysis at B-DLC surface. B-DLC is, therefore, a promising electrode material for hydrogen production by increasing the concentration of boron atoms in B-DLC and enhancing the reactivity of H2 evolution.

Torsional oscillator experiment on superfluid 4He confined in a porous alumina nanopore array

We studied superfluidity of liquid 4He confined in an array of well-characterized straight nanopores of porous alumina (PA). The PA plate sample of 45 nm pore size is set in an annular flow channel and the superflow is detected by torsional oscillator (TO) technique. Superfluid transition Tc in the nanopores is suppressed by 3.5 mK from the bulk λ point. Tc is consistent with the temperature at which the healing length is equal to the pore radius. We have observed many anti-crossing anomalies in the TO frequency associated with dissipation peaks, which are attributed to the coupling to second sound resonances.