Osamu Niwa

Quantitative analysis of reversible diffusion-controlled currents of redox soluble species at interdigitated array electrodes under steady-state conditions

Abstract Equations for diffusion-controlled currents at interdigitated array electrodes (IDA) were derived analytically for the reversible redox reactions of soluble species under steady-state conditions. The two-dimensional diffusion equation was solved by the Schwarz-Christoffel transformation. Current and concentration distributions were obtained. The total current was expressed by the ratios of the two complete elliptic functions and the geometric widths of the anode, the cathode and the gap. The current at each microband electrode of the IDA depended on the ratios of the three widths rather than the absolute values of the widths. Platinum IDAs were fabricated by photolithography on silicon wafers. The widths of the microband electrodes were 3–10 μm while those of the gap were 2–5 μm. Voltammograms of ferrocene were measured with a dual potentiostat under steady-state conditions. The collection efficiency was ca. 95%. The limiting current agreed with the theoretical prediction.

A Nanocarbon Film Electrode as a Platform for Exploring DNA Methylation

We describe the quantitative nonlabel electrochemical detection of both cytosine (C) and methylcytosine (mC) in oligonucleotides using newly developed nanocarbon film electrodes. The film consists of nanocrystalline sp2 and sp3 mixed bonds formed by employing the electron cyclotron resonance (ECR) sputtering method. We successfully used this film to develop a simple electrochemical DNA methylation analysis technique based on the measurement of the differences between the oxidation currents of C and mC since our ECR nanocarbon film electrode can directly measure all DNA bases more quantitatively than conventional glassy carbon or boron-doped diamond electrodes. The excellent properties of ECR nanocarbon film electrodes result from the fact that they have a wide potential window while maintaining the high electrode activity needed to oxidize oligonucleotides electrochemically. Proof-of-concept experiments were performed with synthetic oligonucleotides including different numbers of C and mC. This film allowed us to perform both C- and mC-positive assays solely by using the electrochemical oxidation of oligonucleotides without bisulfite or labeling processes.

Interdigitated array microelectrodes as electrochemical sensors

We investigated electrochemical measurements with interdigitated array (IDA) electrodes in both stationary solutions and flow systems. In a stationary solution, we achieved a very low detection limit of 10 pM of reversible redox species by using substitutional stripping voltammetry, which is a new type of stripping voltammetry using an IDA microelectrode. In flow systems, current enhancement by redox cycling is less effective than that in a stationary solution. The flow rate dependence of redox cycling is constant in the amperometric region, varies with coulometric yield in the quasi-amperometric region, and is inversely proportional to the 2/3 power of the volume flow rate in the coulometric region. A low detection limit of 5 fg (32 amol) is obtained for dopamine due to the high current density and low background noise level (0.1 pA) at the carbon-based IDA microelectrode used as a detector for liquid chromatography. A new separation approach is demonstrated which combines electrochemical detection and a molecular template. The electrode is first partly covered with print molecules and then modified with silane coupling reagent. The catechol-imprinted electrode shows the usual diffusion-limited cyclic voltammogram of catechol and has a diminished response against all catecholamines. The selectivity between catechol and epinephrine is about 100 when the electrode is used as an electrochemical detector in liquid chromatography.

Fabrication and characteristics of vertically separated interdigitated array electrodes

Description de la fabrication et des caracteristiques electrochimiques des electrodes ITA (Interdigitated array)

Determination of DNA Methylation Using Electrochemiluminescence with Surface Accumulable Coreactant

Cytosine methylation in DNA was determined by an enzyme linked immunosorbent assay (ELISA) with electrochemiluminescence (ECL) detection and employed for the DNA methylation assay of a long and real genomic sample for the first time. The developed method employed an antimethyl cytosine antibody labeled with acetylcholinesterase, which was added to recognize single methylated cytosine in a DNA oligomer. The acetylcholinesterase converted acetylthiocholine (substrate) to thiocholine (product), which was accumulated on a gold electrode surface via gold-thiol binding. This surface accumulated preconcentration made it possible to observe bright and distinctive ECL by applying a potential to the gold electrode in the presence of a tris(2,2-bipyridyl)ruthenium complex luminophore when the analyte DNA contained a methylation region. Methyl-cytosine was measured quantitatively in the 1-100 pmol range, which exhibits sufficiently high sensitivity to achieve real DNA measurements without amplification by a polymerase chain reaction (PCR). The proposed ECL method also exhibited high selectivity for methyl-cytosine against nonmethylated cytosine, guanine, thymine, and adenine nucleotides. Finally, original and methylated DNA samples were clearly distinguished with our method using a real DNA bacteriophage sample (48,502 base pairs).

A surface plasmon resonance immunosensor for detecting a dioxin precursor using a gold binding polypeptide

A surface plasmon resonance (SPR) based biosensor was developed for monitoring 2,4-dichlorophenol, a known dioxin precursor, using an indirect competitive immunoassay. The SPR sensor was fabricated by immobilizing a gold-thin layer on the surface of an SPR sensor chip with an anti-(2,4-dichlorophenol) antibody using a gold binding polypeptide (GBP) and protein G. The SPR response based on the antigen-antibody reaction in a flow system was measured by injecting a 2,4-dichlorophenol sample solution into the flow system in which the SPR sensor was located. In a direct immunoassay system using the modified sensor chip, no significant SPR angle shift less than 0.001 degrees was observed when a 25 ppm of 2,4-dichlorophenol solution was injected. In order to improve the sensitivity of the SPR sensor, an indirect competitive immunoassay method was used in conjunction with the SPR sensor system using 2,4-dichlorophenol conjugated with bovine serum albumin (BSA). In the competitive assay, a 350 ppm 2,4-dichlorophenol-BSA conjugate solution containing 2,4-dichlorophenol at various concentrations (10-250 ppb) were injected into the SPR sensor system. The sensitivity of this indirect immunoassay was found to be extremely sensitive, compared to the direct one, and a detection limit of 20 ppb was estimated. Verification that the use of GBP for immobilizing the antibody on the sensor chip enhanced the sensitivity to 2,4-dichlorophenol was obtained by comparing the procedure with another modification, in which BSA was used instead of GBP for immobilizing the antibody on the sensor chip. The affinity constant of 2,4-dichlorophenol and its conjugate to the antibody were estimated form the SPR response.

Characterization and electrochemical properties of highly dispersed copper oxide/hydroxide nanoparticles in graphite-like carbon films prepared by RF sputtering method

We prepared 4.5% and 2.6% copper oxide/hydroxide nanoparticle highly dispersed in graphite-like carbon film (Cu–NDC) by RF co-sputtering copper and carbon together. The two film structures were investigated by X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM) techniques. The nanoparticles size is about . The copper particles in the two different content films have different chemical state. The copper in 4.5% Cu–NDC film exists mainly as cupric hydroxide (Cu(OH)2), while in 2.6% Cu–NDC film cuprous oxide (Cu2O) is the predominant form. The electrochemical properties of the two kinds of Cu–NDC films were evaluated by detecting glucose in an alklaline medium at constant applied potentials in flow system. The 4.5% Cu–NDC film electrode has higher electrocatalytic ability than that of 2.6% Cu–NDC film electrode with regards to electrooxidation of glucose. Both of the two Cu–NDC film electrodes have good stability.

Anisotropic conductivity of polypyrrole-polyvinylchloride conducting polymer alloy film prepared on patterned electrode

Abstract A highly conducting polyvinylchloride (PVC) / polypyrrole (PPy) alloy films can be prepared by the electrochemical polymerization of pyrrole on a PVC coated electrode. When the electrode surface is fabricated to have desired conductive patterns, PPy is alloyed exclusively in the PVC portions corresponding to the electrode patterns. This process has revealed to be very useful to realize a highly anisotropic conductivity in a polymer film. The grating patterns provided one-dimensional conductivity, and circular dots patterns gave the PVC films a conductivity in a film thickness direction. A highly optical transparency was also obtained by the use of small dots patterns with large gaps. Anisotropic conductivity of neat PVC/PPy alloy films as well as the patterned films was studied.

Pd-Ni alloy nanoparticle/carbon nanofiber composites: preparation, structure, and superior electrocatalytic properties for sugar analysis.

Novel Pd-Ni alloy nanoparticle/carbon nanofiber (Pd-Ni/CNF) composites were successfully prepared by a simple method involving electrospinning of precursor polyacrylonitrile/Pd(acac)2/Ni(acac)2 nanofibers, followed by a thermal process to reduce metals and carbonize polyacrylonitrile. The nanostructures of the resulting Pd-Ni/CNF nanocomposites were carefully examined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM), energy dispersive X-ray (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray photoelectron spectra (XPS). For all the nanocomposites, the Pd-Ni alloy nanoparticles (NPs) were dispersed uniformly and embedded firmly within the framework or on the surface of CNF. The size, composition, and alloy homogeneity of the Pd-Ni alloy NPs could be readily tailored by controlling the feed ratio of metal precursors and the thermal treatment process. Cyclic voltammetric studies showed enhanced redox properties for Pd-Ni/CNF-based electrodes relative to the Ni-metal electrode and significantly improved electrocatalytic activity for sugar (e.g., glucose, fructose, sucrose, and maltose) oxidation. The application potential of Pd-Ni/CNF-based electrodes in flow systems for sugars detection was explored. A very low limit of detection for sugars (e.g., 7-20 nM), high resistance to surface fouling, excellent signal stability and reproducibility, and a very wide detection linear range (e.g., 0.03-800 μM) were revealed for this new type of Pd-Ni/CNF nanocomposite as the detecting electrode. Such detection performances of Pd-Ni/CNF-based electrodes are superior to those of state-of-the-art nonenzymatic sugar detectors that are commercially available or known in the literature.

Efficient Direct Electron Transfer with Enzyme on a Nanostructured Carbon Film Fabricated with a Maskless Top-Down UV/Ozone Process

We have developed a new carbon film electrode material with thornlike surface nanostructures to realize efficient direct electron transfer (DET) with enzymes, which is very important for various enzyme biosensors and for anodes or cathodes used in biofuel cells. The nanostructures were fabricated using UV/ozone treatment without a mask, and the obtained nanostructures were typically 2-3.5 nm high as confirmed by atomic force microscopy measurements. X-ray photoelectron spectroscopy and transmission electron microscopy revealed that these nanostructures could be formed by employing significantly different etching rates depending on nanometer-order differences in the local sp(3) content of the nanocarbon film, which we fabricated with the electron cyclotron resonance sputtering method. These structures could not be realized using other carbon films such as boron-doped diamond, glassy carbon, pyrolyzed polymers based on spin-coated polyimide or vacuum-deposited phthalocyanine films, or diamond-like carbon films because those carbon films have relatively homogeneous structures or micrometer-order crystalline structures. With physically adsorbed bilirubin oxidase on the nanostructured carbon surface, the DET catalytic current amplification was 30 times greater than that obtained with the original carbon film with a flat surface. This efficient DET of an enzyme could not be achieved by changing the hydrophilicity of the flat carbon surface, suggesting that DET was accelerated by the formation of nanostructures with a hydrophilic surface. Efficient DET was also observed using cytochrome c.