Satoshi Ueyama

Electrochemical behaviour of a flavin Langmuir-Blodgett monolayer on gold electrodes

Abstract A flavin monolayer of 7,8-dimethyl-3,10-dinonylisoalloxazine (DNI) was deposited on Au electrodes by the Langmuir-Blodgett method. Stable cyclic voltammograms of the DNI electrode were observed based on the flavin redox reaction and were examined upon changing the potential sweep rate. The half-height widths of the cathodic peaks are smaller than those of the anodic peaks, and the cathodic peak potential increases linearly with an increase in the logarithm of the sweep rate at rapid sweep rates (⩾ 4 V s−1). The half-height width of the cathodic peaks at rapid sweep rates (⩾ 8 V s−1) is about twice as great as that for an ideal reversible reaction. The cyclic voltammograms were analysed kinetically and it was found that the reactions at rapid sweep rates (⩾ 8 V s−1 are totally irreversible. The analysis also indicated that molecular interaction among the isoalloxazine rings in the DNI monolayer is rather strong and that the surface redox reaction is slow.

Photo-induced electron transfer in molecular heterojunctions using flavin-porphyrin Langmuir-Blodgett multilayers

Abstract Photo-induced electron transfer in hetero-type Langmuir-Blodgett (LB) multilayers using flavin and porphyrin, which are typical of the redox groups in the biological electron transfer system, has been investigated in terms of transient photocurrent properties. Transient photocurrent properties of a flavin-porphyrin molecular heterojunction cell, were elucidated by a kinetic model of photophysical processes (photo-excitation of flavin, charge separation between excited flavin and prophyrin, and inter-monolayer charge shift). Thus, we may conclude that the mechanism underlying the high photoconductivity of the molecular heterojunction cell is based on the redox potential difference between flavin and porphyrin.

Transient Photocurrent Based on Photoinduced Electron Transfer Processes in Flavin-Porphyrin Hetero Langmuir-Blodgett Monolayers

Dynamic processes of photoinduced electron transfer in flavin-porphyrin hetero Langmuir-Blodgett monolayers, which are characterized both by the electronic properties of functional groups and by the structure of molecular organization, have been studied in terms of transient photocurrent properties. From an analysis of transient photocurrent with a kinetic model based on the organic quantum well structure, we can conclude that the photocarrier generation process is mainly due to charge separation from photoexcited flavin to porphyrin at the flavin-porphyrin molecular heterojunction, and that the charge shift in monolayers and charge transfer between monolayers and an electrode are controlled by applied electric field.

Vector electron transfer from a flavin Langmuir-Blodgett monolayer to cytochrome c

Abstract The electron-transfer reaction between cytochrome c and flavin was studied electrochemically using Au electrodes modified by the Langmuir-Blodgett (LB) method with a flavin monolayer of [(7,8-di-methyl-3,10-dinonylisoalloxazine-8α-yl)thio]acetic acid. The redox potential of the flavin monolayer is −0.40 V vs. Ag/AgCl, which is ca. 0.45 V more negative than that of cytochrome c. A constant potential was applied to the flavin electrode in an aqueous solution of a mixture of ferricytochrome c (cytochrome cOx) and ferrocytochrome c (cytochrome CRed). Cytochrome CRed) was not oxidized on the flavin electrode at a potential ranging from −0.5 to +0.5 V vs. Ag/AgCl, but cytochrome COx) was reduced rapidly at a potential more negative than 0 V. This reaction mechanism was proposed and the result of the simulation indicates that the electron transfer from flavohydroquinone to cytochrome COx) is much faster than that from cytochrome CRed) to flavoquinone. Therefore, the electron transfer between Au and cytochrome c was found to be regulated in a one-way direction with the flavin LB monolayer. This is believed to make molecular rectifying devices or molecular diodes possible.

Interlayer electron transfer in porphyrin Langmuir-Blodgett multilayer-modified gold electrodes

Abstract Interlayer electron transfer in electroactive Langmuir-Blodgett multilayer-modified electrodes was studied. A model for the theoretical treatment of cyclic voltammetry (CV) was proposed by developing the models for ultra-thin film modified electrodes by Laviron and by Daifuku et al. Cyclic voltammetry was carried out with hematoporphyrin(IX) bis (tridecanoyl ether) Ru(II) (pyridine) 2 complex [RuHP(Py) 2 ] modified gold electrodes. The amount of charge flowing during the sweep is proportional to the number of monolayers. Then CV with a three-monolayer modified electrode was carried out at sweep rates from 0.05 to 102.4 V s −1 . Good agreement was obtained between the experimental and the simulated cyclic voltammograms, and the electron diffusion coefficient for the interlayer electron transfer was determined to be 0.8–1.6 × 10 −11 cm 2 s −1 .

Degreasing of Solid Surfaces by Microbubble Cleaning

It is increasingly required to reduce the environmental impact and cost in the field of industrial cleaning. As a substitute for conventional degreasing technology using organic solvents, acids, and alkalis, the authors have developed a new cleaning technology that uses microbubbles having an average diameter of about 70 µm. Grease being adsorbed onto a bubbles surface and grease being separated from a solid surface by its buoyancy were captured using a high-speed microscopic video camera to demonstrate the degreasing capability of bubbles. High-density microbubbles were generated by adding a trace amount of a specific chemical (0.1% weight or less). The cleaning performance using microbubbles was found to be highly improved compared with that using normal bubbles. It was also revealed that the grease removal efficiency was strongly dependent on the viscosity of the grease. Raising the temperature of the cleaning solution is an effective method of improving cleaning performance by reducing the viscosity. Finally, the degreasing of about 150 machining metal parts at the same time was demonstrated to exceed the common target cleaning level (5–20 µg/cm2) in only 2 min because of their large surface area. Furthermore, the high degreasing performance was maintained even after repeated use of the cleaning solution because of the separation of grease due to buoyancy.

One-way electron transfer from flavin to porphyrin in a Langmuir-Blodgett multilayer-modified electrode

Abstract Electron transfer between different redox molecules was studied using Langmuir-Blodgett (LB) multilayers. First, a model of the cyclic voltammetry (CV) for a modified electrode with a heterojunction of the LB multilayers was developed from the model for the multilayer given in the preceding paper. Next, porphyrin (hematoporphyrin(IX) bis (tridecanoyl ether) Ru(II) (pyridine) 2 complex RuHP(PY) 2 ) and flavin (7,8-dimethyl-3,10-dinonylisoalloxazine, DNI) LB multilayers were deposited on a gold electrode to form a porphyrin-flavin heterojunction, and the cyclic voltammograms were analysed based on the model. The electron diffusion coefficients for the electron transfer between porphyrin and flavin were determined according to the model to be ca. 1.9 × 10 −12 cm 2 s −1 from flavin to porphyrin and less than 1.5 × 10 −14 cm 2 s −1 for the reverse direction. This difference is considered to be based on the redox potential difference between DNI and RuHP(Py) 2 . This one-way electron transfer is the first step in the realization of molecular rectifying devices.

Photoelectric properties based on photo-induced electron transfer processes in flavin–porphyrin hetero-type Langmuir–Blodgett films

Abstract The photo-induced electron transfer processes were investigated in flavin–porphyrin hetero-type Langmuir–Blodgett (LB) films to clarify the photoelectric properties of metal-insulator-metal (MIM) devices composed of LB films sandwiched within aluminium electrodes. A hetero-type MIM device with a flavin and porphyrin molecular heterojunction (MHJ) showed highly efficient photovoltaic effects and high photoconductivity. In contrast, a homo-type MIM device with flavin and porphyrin LB films exhibited low photovoltaic effects and a short-circuit photocurrent density less than a tenth of that of the MHJ device. Furthermore, the transient photocurrent of the MHJ device showed that the time constant for the charge separation (CS) process in the MHJ device was of a sub-nanosecond order. This was more than two orders of magnitude shorter than the time constant for the CS process in the flavin homo-type MIM device. We concluded that the highly efficient photoelectric properties of the MHJ device were mainly attributable to the fast CS process from the photo-excited flavin to the porphyrin at the MHJ. Considering the long distance between the flavin and porphyrin at the MHJ and the moderate free energy difference, the mechanism underlying the fast CS process might be based on the quantum electron tunneling.

Molecular Orientation And Photoelectric Properties Of Molecular Heterojunction Using Flavin-cytochrome c Multilayers

In order to realize electronic devices based on molecular electronic function, it is of great importance to control the structure and the electron transport function of molecular organization. From the structural standpoint, cytochrome c (cyt c) adsorbed on flavin monolayers was found to show specific in-plane molecular orientation by the polarized fluorescence method. As for the electron transport function, the rectifying property was observed in the MIM (Metal-InsulatorMetal) cell using FMN / cyt c / EDTA solid film under illumination.

A Stark spectroscopic study on a flavin Langmuir-Blodgett film

The molecular orientation of a flavin Langmuir–Blodgett film was investigated by Stark spectroscopy. The first-harmonic Stark spectrum was found to be close to the first derivative of the absorbance, indicating that the flavin (isoalloxazine) ring took a specific orientation toward the direction of the applied electric field instead of a random orientation. From the analysis of the Stark spectrum, it was deduced that difference dipole moments for the S0 → S1 and S0 → S2 transitions were almost parallel to the substrate in the direction of approximately 35° from the long axis of the isoalloxazine ring.