Shin Ichiro Imabayashi

Reductive desorption of carboxylic-acid-terminated alkanethiol monolayers from Au(111) surfaces

Abstract The peak potential E p of the reductive desorption for COOH-terminated alkanethiols at the gold(111)|0.1 M KOH solution interface has been studied at several different alkyl chain lengths and compared with those for n -alkanethiols. The desorption of COOH-terminated alkanethiols is also confirmed by the increase in the capacitance of gold|solution interface and the reflectivity of gold electrode surface. The E p for both CH 3 - and COOH-terminated alkanethiols shift by 15 mV to the negative direction per methylene group as the alkyl chain length n increases. The E p vs. n plots for the two series of alkanethiols are parallel with each other, suggesting that the chemical interaction of thiol head group with gold, the chain-chain attractive interaction, and the electrostatic repulsion between the carboxylate groups make an additive contribution to the determination of E p . The E p values for COOH-terminated alkanethiols are 110 mV more positive than those for n -alkanethiols having the same alkyl chains, owing in part to the repulsive interaction between the negatively-charged carboxylate groups in a COOH-terminated alkanethiol monolayer. The positive shift of E p for mercaptopropionic acid with decreasing concentration of KOH solution also suggests the primary role of the electrostatic repulsive interaction between carboxylate groups in the shift of E p .

Phase separation of binary self-assembled thiol monolayers composed of 1-hexadecanethiol and 3-mercaptopropionic acid on Au(111) studied by scanning tunneling microscopy and cyclic voltammetry

Abstract The mixing characteristics of binary self-assembled monolayers composed of 1-hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) on Au(111) have been studied by cyclic voltammetry and scanning tunneling microscopy. Two distinctive peaks, ∼0.45V apart, are observed over the entire range of surface composition on cyclic voltammograms for the reductive desorption of the adsorbed thiol molecules, which reflects the presence of two different types of phase-separated domains greater than several tens of nm 2 which can be imaged by scanning tunneling microscopy. The peak potential of 1-hexadecanethiol is nearly independent of the surface composition whereas a slight shift of the peak potential is observed in the case of MPA, suggesting that the HDT is slightly soluble in the MPA domains, while MPA is insoluble in the HDT domains. The minimum number of the adsorbed thiol molecules required for exhibiting two distinctive peaks (i.e. two-dimensional bulk properties) is estimated to be ca. 50 by comparing the cyclic voltammograms with the distribution of the domain size observed by scanning tunneling microscopy.

Long-range electron-transfer reaction rates to cytochromec across long- and short-chain alkanethiol self-assembledmonolayers: Electroreflectancestudies

The kinetics of electron transfer (ET) between cytochrome c and a gold (111) electrode through self-assembled monolayers of alkanethiols with terminal carboxylic acid groups, COOH(CH 2 ) n SH, have been studied for n=2–11 using an ac potential-modulated UV–VIS reflectance spectroscopic technique (electroreflectance spectroscopy, ER). For 9⩽n⩽11, the standard ET rate constant, k app , depends exponentially on the chain lengths and the exponential decay factor is 1.09 per methylene group; for n<9, however, k app deviates from the exponential plot. The ET reaction through short-chain alkanethiol monolayers is controlled by the preceding chemical reaction. The rate-controlling step is very likely to be the reorganization of cytochrome c to the favourable conformation for the ET reaction. The ET reaction rate constant from cytochrome c in the favourable conformation to the electrode surface obeys Marcus theory for long-range ET. The ET reaction through long-chain alkanethiol monolayers is controlled by the ET rate through alkanethiols.

Electroreflectance spectroscopic study of the electron transfer rate of cytochrome c electrostatically immobilized on the ω-carboxyl alkanethiol monolayer modified gold electrode

Abstract Horse heart cytochrome c was immobilized electrostatically on self-assembled monolayers (SAMs) of carboxylic-acid-terminated alkanethiols, HS(CH 2 ) n COOH ( n = 2, 10) on gold electrodes, The rate constants of the electrode reaction of cytochrome c at the SAM-modified gold electrodes were determined by the frequency dependence of a.c. modulated UV-visible electroreflectance signals. The apparent standard electrode reaction rate constants of cytochrome c were 880 s −1 and 72 s −1 at the HS(CH 2 ) 2 COOH and HS(CH 2 ) 10 COOH SAM modified gold electrodes respectively. The rate constant at HS(CH 2 ) 2 COOH was much smaller than expected from Marcus theory. However, the rate constant at HS(CH 2 ) 10 COOH could be explained in terms of the through-bond tunnelling mechanism with tunnelling parameter β = 8.2 nm −1 .

LCST-type liquid-liquid phase separation behaviour of poly(ethylene oxide) derivatives in an ionic liquid

We present a new series of polymer-ionic liquid solutions exhibiting LCST-type liquid-liquid phase separation behaviour, and reveal their phase behaviour and intermolecular interactions based on phase diagrams and NMR analysis.

Amperometric biosensor for polyphenol based on horseradish peroxidase immobilized on gold electrodes

Horseradish peroxidase (HRP) is covalently immobilized on a self-assembled monolayer of mercaptopropionic acid on vapor-deposited gold electrode. The electrode allows the polyphenol detection down to 2 µM with a linear relationship up to 25 µM. The reduction current of oxidized polyphenols, formed during the enzymatic oxidation of polyphenolic compounds in the presence of H2O2, is proportional to their concentration. The sensitivity of the detection of various polyphenols by the present method depends on both the electron-donating properties of polyphenols and the electron-accepting properties of oxidized polyphenols. The total amounts of polyphenols in several wine and tea samples detected by the present method are well correlated with those determined by the Folin-Ciocalteu method. In addition, this method has several advantages over the Folin-Ciocalteu method: shorter detection time, smaller sample volume, and more torelant to interference substances.

A directional electron transfer regulator based on heme-chain architecture in the small tetraheme cytochrome c from Shewanella oneidensis

The macroscopic and microscopic redox potentials of the four hemes of the small tetraheme cytochrome c from Shewanella oneidensis were determined. The microscopic redox potentials show that the order of reduction is from hemes in the C‐terminal domain (hemes 3 and 4) to the N‐terminal domain (heme 1), demonstrating the polarization of the tetraheme chain during reduction. This makes heme 4 the most efficient electron delivery site. Furthermore, multi‐step reduction of other redox centers through either heme 4 or heme 3 is shown to be possible. This has provided new insights into the two‐electron reduction of the flavin in the homologous flavocytochrome c–fumarate reductase.

The role of intramolecular association in the electrochemical reduction of viologen dimers and trimers

Abstract The electrochemical reduction of several viologen dimers (D) and trimers (T) was studied by absorption spectroscopy and differential pulse polarography in N , N -dimethylformamide. The viologen dimer and trimer linked by a propane chain (D3 and T3) exhibited efficient intramolecular association between two viologen radial cations, while the association in other viologens having longer alkyl spacer chains was less efficient. For D3 and T3, reduction of the second viologen group proceeded at a more positive potential compared with that of the first one, indicating that the association of the radical cations facilitated the reduction processes. For the viologens which showed inefficient intramolecular association, the four (for dimers) and six (for trimers) formal potentials were explained by taking statistical factors into account. The effects of the chemical structure of the viologens on both their spectroscopic and electrochemical behavior were also discussed.

Determination of the electrode kinetic parameters of a species immobilized on electrodes using the electroreflectance (ER) voltammogram

Abstract Ac-modulated UV-vis reflectance spectroscopy was applied to elucidate the electrode reaction rate of the species immobilized on electrode surfaces. The electroreflectance (ER) with respect to the electrode potential at constant wavelength, denoted as ER voltammetry, was analyzed theoretically using numerical calculation. In the present calculation, the amplitude of the ac modulation, the IR drop due to the solution resistance, and the double layer capacitance were taken into account. The results of the calculation were compared to the electrode reaction of cytochrome c immobilized on an SH(CH 2 ) 9 COOH self-assembled monolayer modified gold electrode, which has previously been studied. The standard rate constant of the electrode reaction of cytochrome c 3 adsorbed directly on gold electrode was determined to be 200 s −1 using non-linear least square fitting to the experimental data. This approach is applicable to unstable redox species at the electrode surface, e.g. cytochrome c 3 , which desorbs gradually from the gold electrode surface at negative electrode potential. That is, one can determine the kinetic parameters of adsorbed redox species by a single potential scan.

Polypyrrole / Polymer Electrolyte Composites Prepared by In-situ Electropolymerization of Pyrrole as Cathode / Electrolyte Material for Facile Electron Transfer at the Solid Interface

Polypyrrole (PPy)/ion-conducting polymer electrolyte (PE) composites are prepared by in situ electropolymerization of pyrrole in a solvent-free PE matrix which is formed by photocross-linking of triacrylated poly(ethylene oxide-co-propylene oxide) (TA). X-ray photoelectron spectroscopy measurements reveal the gradient structure at the PPy/PE interface where PPy is formed with the concentration gradient. For the composite film, a facile electron transfer across the PPy/PE interface is demonstrated by a cyclic voltammogram for the doping-undoping process, and a decrease in the charge-transfer resistance at the interface is proved by an electrochemical impedance analysis. This is in marked contrast to a combined bilayer film of PPy and TA-based PE for which an electron transfer across the interface between two polymer films is not clearly observed. The composite exhibits charge-discharge performance, which suggests a possibility of fabricating solid-state batteries composed of conductive polymer electrode/ion-conductive PE/Li by one-step electrolysis.