Mineo Ikematsu

Effect of cations on the electrochemical behavior of p-toluenesulfonate-doped polypyrrole in various aqueous solutions

The electrochemical behavior of p-toluenesulfonate-doped polypyrrole (PPy/TsO−) in aqueous solution was analyzed by cyclic voltammetry. These films were prepared by the electropolymerization of pyrrole in aqueous solutions. It was found that in aqueous solutions containing alkali metal ions (M+), the dopant anions (TsO−) in the PPy film could be exchanged relatively easily with electrolyte anions in aqueous solutions by potential sweeping. However, PPy/TsO− was electrochemically inactivated in aqueous solutions containing alkaline earth metal ions (M2+) or tetraalkylammonium ions (R4N+). It was found that the inactivation in aqueous solutions containing M2+ or R4N+ could not be explained simply by the hypothesis that these cations were not incorporated into PPy/TsO− during the reduction process because of their large ion size. It was speculated that the anisotropic molecular organization of PPy/TsO− might play a key role in its inactivation of PPy/TsO−.

n-Type TiO2 Thin Films for Electrochemical Ozone Production

An electrode for the electrochemical production of ozone, which has the compositional sequence Si/TiO x /Pt/TiO 2 (TiO x is titanium oxide), was fabricated by sputtering TiO 2 thin film (the thickness is typically 300 nm) on a Si/TiO x /Pt substrate. The TiO 2 thin film was characterized by X-ray diffraction, transmission electron microscopy, UV photoelectron spectroscopy, UV-visible spectroscopy, and photoelectrochemical measurements: It is an n-type semiconductor of the rutile-type TiO 2 . The electrochemical ozone production (EOP) was realized, and a high current efficiency of 9% was achieved at a low current density of 8.9 mA cm -2 in 0.01 M HClO 4 at 15°C. The observed high efficiency of EOP was considered to originate from the electrocatalysis of the n-type TiO 2 in the dark when a large anodic bias was applied in which, based on the band structure of the n-type TiO 2 /HClO 4 solution interface, electron tunneling can take place through a deep depletion layer of the TiO 2 surface.

Si/TiOX/Pt/TaOX Electrodes Fabricated by Sputtering for Electrochemical Ozone Generation

We fabricated an electrode for electrochemical generation of ozone by sputtering. The vertical structure of the electrode was composed of silicon/titanium oxide/platinum/tantalum oxide (Si/TiOX/Pt/TaOX). The catalyst layer of the electrode was an insulator comprising only tantalum oxide. By using the electrode as the anode and perchloric acid as the electrolyte solution, ozone generation was observed at a current density of ca. 40 mA/cm2 and above. According to the electrochemical measurement, the potential of the electrode was ca. 8 V [vs a saturated calomel electrode (SCE)] at a current density of ca. 40 mA/cm2, and it was higher than the potential of the Pt electrode. The electrode was efficient for the electrochemical generation of ozone because of its high electrode potential.

Controlling the Orientation of Purple Membrane Fragments on an Air/Water Interface by a New Method of Direct Electric Field Application during Purple Membrane Spreading

The orientation of purple membrane (PM) fragments on an air/water interface was controlled by a direct electric field applied across the interface during spreading of a PM solution. We call this the direct electric field application Langmuir-Blodgett (LB) technique, and found it effective in increasing the population of preferentially oriented PM fragments at the interface. Using this technique with an electric field exceeding 4 kV/cm and an initial coverage C i of 0.2 (i.e., the ratio of the surface area of the air/water interface covered by PM fragments to the total surface area of the interface before the PM was spread over the interface), we produced a PM LB film in which all the constituent PM fragments were preferentially oriented. We successfully demonstrated that this technique is a powerful tool for producing LB films in which proteins are perfectly oriented (i.e., all the constituent PM fragments are preferentially oriented).

Determination of the amount of native structural bacteriorhodopsin in purple membrane Langmuir-Blodgett films by a spectroscopic surface denaturation quantifying technique

Purple membrane (PM) shows denaturation when spread over an air/water interface. We established a technique, which we call the spectroscopic surface denaturation quantifying (SSDQ) technique, that uses infrared linear dichroism to determine the amount of native structural bacteriorhodopsin (BR) in PM Langmuir-Blodgett (LB) films. Using the SSDQ technique we found that the conformational change after surface denaturation of BR was the same as that caused by ethanol treatment. By extrapolating the data of the amount of non-denatured BR molecules in PM LB films vs. the area of a single BR molecule on an air/water interface, we also found that the surface area of a single non-denatured BR molecule was 11.5 nm2, which is consistent with that determined by high-resolution electron cryo-microscopy and electron diffraction (EMD). These results demonstrate that the SSDQ technique is effective in quantifying the amount of native structural BR in PM LB films. The SSDQ technique is also applicable to other types of protein consisting of alpha-helical conformation.

Lipid bilayer formation in a microporous membrane filter monitored by ac impedance analysis and purple membrane photoresponses

Abstract We have measured and analyzed the time-dependent variation in the ac impedance of lipid-impregnated polycarbonate membrane filters (PCMs). The results showed that the electrical characteristics of PCMs gradually changed with time. From the results, we inferred the formation of bilayer-like micromembranes within the PCM pores and presented a model of the time-dependent thinning process of a lipid + decane micromembrane system. In addition, we measured the time-dependent variation of the photoresponse of purple membrane reconstituted onto the PCM system and showed that thinning of the micromembrane proceeded in all the PCM pores. The results provided information about what is occurring within lipid-impregnated PCMs. Hence such filters will help us explore model systems simulating certain biological membrane systems. The time-dependent variations in the characteristics of such lipid-impregnated PCM systems were previously unknown, although these systems are accepted as stable and feasible candidates for simulating biological functions.

Direct transmembraneous reconstitution of bacteriorhodopsin into planar phospholipid bilayers

Abstract A new method of incorporating bacteriorhodopsin molecules into planar lipid bilayers was developed and compared with a conventional system where purple membranes were adsorbed onto planar bilayers. By the new method, purple membrane sheets were first solubilized with a detergent and the solubilized fraction was directly added to an aqueous phase of a preformed planar bilayer membranes. Then, the solubilized bacteriorhodopsin molecules were spontaneously incorporated into the planar bilayers. Upon illumination, a steady state electric current was generated and the magnitude of the current was hardly affected by the presence of an uncoupler, FCCP. Furthermore, when bacteriorhodopsin molecules were incorporated into planar bilayers in high temperature range, a transient capacitive peak current was mostly eliminated by successive perfusions with a buffer at room temperature. These properties are in contrast with those of conventional method and strongly indicate that the bacteriorhodopsin molecules are transmembraneously reconstituted into planar bilayer membranes by the new method. The new method will prove useful in quantitative investigation of the properties of active ion transport.

Ti/Pt/(Pt-TaOX) Electrode Fabricated by Thermal Decomposition and Si/Pt/TaOX Electrode Fabricated by Sputtering for Ozone Generation

Two types of electrodes for electrochemical generation of ozone were fabricated by thermal decomposition and sputtering. Ozone generation was observed using both the electrodes. The catalyst layer of the electrode, fabricated by thermal decomposition, functions as an insulator comprising of tantalum oxide and platinum. It had many cracks, and some of them extended up to the platinum-buffered layer. However, we have not explained the ozone generation by the current concentration in the platinum-buffered layers due to the passage of an electrolyte through the cracks alone. On the other hand, the catalyst layer of the electrode fabricated by sputtering was also an insulator comprising only of tantalum oxide; this layer was also crack free. Because of the thin catalyst layer (ca. 20 nm), electrons drifted through it thereby causing electrolysis (i.e., ozone was generated). The results show that tantalum oxide functions as a catalyst of ozone generation. Therefore, with regard to electrolysis using the electrode fabricated by thermal decomposition, ozone generation by an electrochemical reaction is possible in both the thin layer of tantalum oxide, in which cracks on the inner wall extend up to the interior of the catalyst layer, and the platinum-buffered layer.

Time-dependent variation in a.c. impedance of lipid-impregnated membrane filters

Abstract We measured and analyzed time-dependent variation in the a.c. impedance of lipid-impregnated membrane filters. By changing the lipid concentration of the membrane and the salt concentration of the aqueous solution surrounding the filter, we could determine how the lipids in a decane solution rearange their order in lipid-impregnated membrane filters. The results showed that the electrical characteristics of such filters change gradually with time; from this we inferred the formation of bilayer-like micromembranes. In addition, we verified the effectiveness of cholesterol and octadecylamine in improving membrane stability, resulting in a longer life-span of the micromembranes. Finally, we present a scenario for the time-dependent behavior of a lipid-decane system that is within a skeleton consisting of a cellulose matrix of a membrane filter. The results yielded knowledge about what occurs within lipid-impregnated membrane filters and the mechanism for membrane stabilization. Hence, such filters will help us explore model systems that simulate certain biological membrane systems. The time-dependent variations in characteristics of such lipid-impregnated membrane systems were previously unknown, even though such systems have been recognized as stable and feasible candidates for simulating biological functions.

electrochemical behavior simulating the synaptic plasticity in the dopant exchange process of p-toluenesulfonate-doped polypyrrole

Abstract The unusual electrochemical behavior of p-toluenesulfonate-doped polypyrrole (PPy/TsO−) was studied in terms of its application to the device that can reversibly change the amplitude of the output signal according to the frequency of the input signal. The device simulates the synaptic plasticity (the efficiency of the signal transmission changes as a function of the frequency of the stimulus signal). Therefore we call this an electrically plastic device. PPy/TsO− films were prepared by electropolymerization of pyrrole in aqueous solutions. PPy/TsO− is electrochemically inactivated in aqueous solutions containing alkaline-earth metal ions or tetraalkylammonium ions. The inactivated PPy/TsO− is gradually activated however by potential sweeping or spontaneous anion exchange during immersion. Using both gradual activation by potential sweeping and preferential dopant exchange during immersion in a mixed electrolyte solution, reversible electrically plastic behavior was successfully developed. The possibility of developing a biomimetic device that uses PPy/TsO− as an artificial conducting polymer in an electrochemical system is discussed.