Yuki Kitazumi

Ultraslow response of interfacial tension to the change in the phase-boundary potential at the interface between water and a room-temperature ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide.

Ultraslow response, on the order of minutes, of the interfacial tension to the change in the phase-boundary potential at the interface between water and a room-temperature ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, has been demonstrated. This ultraslow relaxation, which is not observed at the interface between two immiscible electrolyte solutions made of molecular organic solvents, is likely to be due to the long-range and collective ordering of ions of the electrical double layer on the ionic liquid side of the interface.

Electrostatic interaction between an enzyme and electrodes in the electric double layer examined in a view of direct electron transfer-type bioelectrocatalysis

Effects of the electrode poential on the activity of an adsorbed enzyme has been examined by using copper efflux oxidase (CueO) as a model enzyme and by monitoring direct electron transfer (DET)-type bioelectrocatalysis of oxygen reduction. CueO adsorbed on bare Au electrodes at around the point of zero charge (E(pzc)) shows the highest DET activity, and the activity decreases as the adsorption potential (E(ad); at which the enzyme adsorbs) is far from E(pzc). We propose a model to explain the phenomena in which the electrostatic interaction between the enzyme and electrodes in the electric double layer affects the orientation and the stability of the adsorbed enzyme. The self-assembled monolayer of butanethiol on Au electrodes decreases the electric field in the outside of the inner Helmholtz plane and drastically diminishes the E(ad) dependence of the DET activity of CueO. When CueO is adsorbed on bare Au electrodes under open circuit potential and then is held at hold potentials (E(ho)) more positive than E(pzc), the DET activity of the CueO rapidly decreases with the hold time. The strong electric field with positive surface charge density on the metallic electrode (σ(M)) leads to fatal denaturation of the adsorbed CueO. Such denaturation effect is not so serious at E(ho)<<E(pzc), but the electric field with negative σ(M) induces an orientation inconvenient for the DET reaction during the adsorption process. A positively charged neomycin shows a promoter ability to CueO adsorbed at E(ad)<<E(pzc). The phenomenon is also explained on the proposed model.

Direct electron transfer-type dual gas diffusion H2/O2 biofuel cells

H2/O2 biofuel cells utilizing hydrogenases and multicopper oxidases as bioelectrocatalysts are clean, sustainable, and environmentally friendly power devices. In this study, we constructed a novel gas diffusion bioelectrode with a sheet of waterproof carbon cloth as the electrode base and optimized the hydrophilicity/hydrophobicity of the electrode for both high gas permeability and high direct electron transfer bioelectrocatalytic activity. The electrode exhibited a large current density of about 10 mA cm−2 in the steady-state for both H2 oxidation and O2 reduction. The biocathode and the bioanode were coupled to construct a gas diffusion H2/O2 biofuel cell. The dual gas diffusion system allowed the separate supply of gaseous substrates (H2 and O2) to the bioanode and biocathode, with consequent suppression of the oxidative inhibition of the hydrogenases. The cell exhibited a maximum power density of 8.4 mW cm−2 at a cell voltage of 0.7 V under quiescent conditions.

Sensitive d-amino acid biosensor based on oxidase/peroxidase system mediated by pentacyanoferrate-bound polymer.

A sensitive d-amino acid oxidase (DAAO)/peroxidase (POD) bienzyme biosensor is constructed, in which pentacyanoferrate-bound poly(1-vinylimidazole) polymer (PVI[Fe(CN)5]) is selected as a mediator. Reductive current of PVI[Fe(CN)5] related to the H2O2 concentration generated in the DAAO reaction was measured at -0.1V vs. Ag|AgCl with DAAO/POD/PVI[Fe(CN)5]-modified electrode. The result revealed that PVI[Fe(CN)5] is suitable as a mediator for this bienzyme system due to its appropriate formal potential and its extremely low reactivity against DAAO. The stability of DAAO was improved by adding free flavin adenine dinucleotide and the electrode composition was optimized for the detection of d-alanine. Nafion and ascorbate oxidase-immobilized films worked successfully to prevent severe interference from uric acid and ascorbic acid. The low detection limits of d-alanine (2μM) and d-serine (2μM) imply its possibility for the determination of extremely low concentration of d-amino acids in physiological fluids. The proposed bienzyme biosensor is proved to be capable of detecting d-amino acids in urine.

Potential-dependent adsorption of decylsulfate and decylammonium prior to the onset of electrochemical instability at the 1,2-dichloroethane|water interface.

The adsorption of decylsulfate (DeSO4(-)) and decylammonium (DeNH(3+)) at the 1,2-dichloroethane (DCE)|water(W) interface has been examined as a function of the phase-boundary potential by simultaneous recording of electrocapillary curves and voltammograms. The standard Gibbs energies for the adsorption of DeSO(4)(-) and DeNH(3)(+) at the DCE|W interface from the W phase depend linearly on the phase-boundary potential, having the slopes of 9.1 and-9.8 kJ mol-1 V-1, respectively. These values suggest that the charged head groups of adsorbed surface-active ions are located almost outside of the diffuse part of the electrical double layer in W. It has also been demonstrated that the interface enters into the mode of the electrochemical instability in the potential region where the current is slightly augmented in comparison with that of the diffusion-limited current, that is, prior to the appearance of irregularly increased currents on the voltammogram.

Voltammetric manifestation of the ultraslow dynamics at the interface between water and an ionic liquid.

The ultraslow relaxation (on the order of minutes) of the electrical double-layer structure, related to a change in the phase-boundary potential across the interface between water (W) and the ionic liquid (IL) trioctylmethylammonium bis(nonafluorobutanesufonyl)amide ([TOMA(+)][C(4)C(4)N(-)]) (Y. Yasui et al., J. Phys. Chem. B. 2009, 113, 3273), appears to be invisible in the transfer of tetrapropylammonium ions across the [TOMA(+)][C(4)C(4)N(-)]|W interface, provided that the charging current, which shows an unusual dependence on the voltage scan rate, is subtracted to obtain the faradaic current. This counterintuitive observation can be explained by the differences in the timescales of the fast and slow components of the relaxation dynamics of the electrical double layer on the IL side (ms and min). In contrast, the effect of the slow dynamics becomes surfaced in ion-transfer voltammetry when the ion is surface-active. The transfer of pentadecafluorooctanoate across the [TOMA(+)][C(4)C(4)N(-)]|W interface is irreversible, which is attributable to the self-inhibition of pentadecafluorooctanoate ions transferred to the IL phase. This process is likely to be affected by the ultraslow structural change of the IL side of the interface.

Electrocapillarity under Ultraslow Relaxation of the Ionic Liquid Double Layer at the Interface between Trioctylmethylammonium Bis(nonafluorobutanesulfonyl)amide and Water

Electrocapillarity has been studied in detail at the interface between a hydrophobic ionic liquid (IL), trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, and water, where the ultraslow relaxation of the structure of the electrical double layer on the IL side of the interface exists. The response of the interfacial tension and that of the charging current to the potential step can be fitted by a double exponential model having the relaxation time constants of a few seconds and 100 s. The hysteresis in the interfacial tension in the IL diluted with nitrobenzene persists even in an almost 1:1 mixture of the IL and nitrobenzene. The thermodynamically definable double layer capacitance (C(dl)) estimated from the equilibrated electrocapillary curve, that is, the interfacial tension versus potential curve, at the IL|water interface has a maximum value of about 60 microF cm(-2) in the vicinity of the potential of zero charge (pzc) when the aqueous phase is 0.1 mol dm(-3) LiCl. The C(dl) versus the potential plot shows a characteristic camelback shape, showing a shallow minimum at the pzc, where the C(dl) value is 42 microF cm(-2). This minimum seems to reflect the contribution of C(dl) on the aqueous side of the interface to the total capacitance.

Ionic liquid salt bridge based on tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide for stable liquid junction potentials in highly diluted aqueous electrolyte solutions.

A moderately hydrophobic ionic liquid, tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide ([TBMOEP(+)][C(2)C(2)N(-)]), shows a very stable liquid junction potential upon contact with an aqueous solution whose ionic strength is as low as 1 μ mol dm(-3). The stability with the maximum excursion of the potential within ± 0.5 mV for 30 min is very promising for accurate determination of pH and other single ion activities potentiometrically.

Imaging of the Liquid−Liquid Interface under Electrochemical Instability Using Confocal Fluorescence Microscopy

The onset of the electrochemical instability has been imaged at the 1,2-dichloroethane (DCE)|water (W) interface modified with a fluorescent phospholipid using confocal fluorescence microscopy (CFM). Heterogeneously fluorescent images are recorded successively under the transfer of dodecyl sulfate ions (DS-) across the interface, which induces the irregularly increased current in a voltammogram. The commencement of the hydrodynamic movement of the solutions due to the electrochemical instability has been detected as the appearance of dark domains at the edge of the interface, that is, the three-phase contact of the DCE-W-glass wall confining the interface. In the potential region around the midpoint potential of the transfer of DS-, the area of the dark domains fluctuates and gradually grows with time. These locally confined unstable domains give rise to slightly augmented current in the simultaneously recorded voltammogram. The interface in this potential range appears to be globally stable at the expense of small unstable domains. In the potential region where the irregularly increased current is visible in the voltammogram, the entire interface becomes unstable and the dark and bright regions move vertiginously due to the Marangoni convection of the adjacent solution phases.

Phase transition of a binary room-temperature ionic liquid composed of bis(pentafluoroethanesulfonyl)amide salts of tetraheptylammonium and N-tetradecylisoquinolinium and its surface properties at the ionic liquid|water interface.

A binary room-temperature ionic liquid (RTIL) composed of bis(pentafluoroethanesulfonyl)amide (C(2)C(2)N(-)) salts of tetraheptylammonium (THpA(+)) and N-tetradecylisoquinolinium (C(14)Iq(+)) undergoes a phase transition upon increasing the mole fraction of C(14)Iq(+) (x) in the bulk RTIL. The initial decrease with x of the interfacial tension (gamma) at the interface between water (W) and the binary RTIL reaches a break point at x approximately 0.2 irrespective of the values of the phase-boundary potential. The surface tension at RTIL|air interface and the conductivity of the binary RTIL support that the break point at x = 0.2 at the RTIL|W interface is attributable to the change of the bulk property. However, unlike the micelle formation of a surfactant solution, a further increase in x gives rise to a further change in gamma. Whereas the phase transition at x = 0.2 does not depend on the applied potential (E) across the RTIL|W interface, the mode of the change in gamma at x > 0.2 strongly depends on E and the apparent deficit of C(14)Iq(+) at the interface is more pronounced when E is closer to the point of zero charge.