Mao Fukuyama

Measurement of vibrational spectrum of liquid using monochromated scanning transmission electron microscopy–electron energy loss spectroscopy

Investigations on the dynamic behavior of molecules in liquids at high spatial resolution are greatly desired because localized regions, such as solid-liquid interfaces or sites of reacting molecules, have assumed increasing importance with respect to improving material performance. In application to liquids, electron energy loss spectroscopy (EELS) observed with transmission electron microscopy (TEM) is a promising analytical technique with the appropriate resolutions. In this study, we obtained EELS spectra from an ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (C2mim-TFSI), chosen as the sampled liquid, using monochromated scanning TEM (STEM). The molecular vibrational spectrum and the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the liquid were investigated. The HOMO-LUMO gap measurement coincided with that obtained from the ultraviolet-visible spectrum. A shoulder in the spectrum observed ∼0.4 eV is believed to originate from the molecular vibration. From a separately performed infrared observation and first-principles calculations, we found that this shoulder coincided with the vibrational peak attributed to the C-H stretching vibration of the [C2mim(+)] cation. This study demonstrates that a vibrational peak for a liquid can be observed using monochromated STEM-EELS, and leads one to expect observations of chemical reactions or aids in the analysis of the dynamic behavior of molecules in liquid.

Interfacial Phenomena and Fluid Control in Micro/Nanofluidics.

Fundamental aspects of rapidly advancing micro/nanofluidic devices are reviewed from the perspective of liquid interface chemistry and physics, including the influence of capillary pressure in microfluidic two-phase flows and phase transitions related to capillary condensation.

Microfluidic protein crystallisation controlled using spontaneous emulsification

Microdroplet-based protein crystallisation using spontaneous emulsification is proposed and demonstrated. The dependence of water transport on the concentration of Span 80 was investigated. Crystallisation of the lysozyme was then demonstrated. Based on the results obtained, the dependency of the crystal number in a single microdroplet on the surfactant concentration was discussed.

Enhanced chiral recognition by β-cyclodextrin at liquid/liquid interfaces as revealed by chromatographic and interfacial tension measurements

The chiral selectivity of β-cyclodextrin (β-CD) in the water/hexane (3.0% THF) two-phase system is studied with chromatography and interfacial tension measurements. Chromatography using silica gel impregnated with aqueous β-CDas the stationary phase reveals that the chiral selectivity of this system is higher than that predicted from the β-CD complexation that occurs in bulk water. The retention of the solutes and chiral selectivity can be explained by the adsorption of β-CD at the interface between the aqueous phase (AP) and hexane (3.0% THF) phase. The interfacial tension measurements suggest that β-CD molecules forms a monolayer at this interface. The interfacial complexation constants are larger than the corresponding bulk water constants by at least one order of magnitude. The β-CD molecules adsorbed at the interface are preferably oriented for the formation of inclusion complexes, and thereby solute molecules are directly accommodated in the CD cavity at the interface without partitioning into the AP. The present chromatography-based method probes the interfacial phenomena that are not accessible by other methods. Hence, the liquid/liquid interface is a new field for molecular recognition that does not occur in bulk solution phases.

Kinetic Switching of the Concentration/Separation Behavior of Microdroplets

This work demonstrates that the solute concentration inside 100 micrometer-sized aqueous microdroplets can be controlled by adjusting the time required for the aqueous nanometer-sized droplets (nanodroplet) or reverse micelles to pass over the surface of the microdroplet. The kinetics of molecular transport between the microdroplets and the nanodroplets was investigated by utilizing a microdroplet array, and on the basis of these results, a control over the concentration selectivity of the contents of the microdroplet was achieved. This method is operationally simple and can be potentially applied as a pretreatment method for microanalytical systems that require high-density microdroplet arrays. This method can also be utilized for parallel small sample analyses such as single cell analysis.

Distribution and Adsorption of Ionic Species into a Liposome Membrane and Their Dependence upon the Species and Concentration of a Coexisting Counterion

The distribution of ions into a bilayer lipid membrane (BLM) and their adsorption on the BLM are investigated by extracting a hydrophobic cation, rhodamine 6G (R6G+), into a liposome through the dialysis membrane method. R6G+ distribution mainly depends upon the concentration of the coexisting anion and its species (Cl-, Br-, BF4-, ClO4-, and picrate). On the other hand, R6G+ adsorption on the BLM surface follows the Langmuir adsorption model and is independent of the coexisting anion in the aqueous phase. We propose an extraction model of ionic species into the BLM, to explain the dependence of extraction of ionic species upon the coexisting anion. In this model, an ion is distributed with a coexisting counterion into the BLM and then forms an ion pair in the BLM. Here, the ion adsorption equilibrium on the BLM surface is independent of the species and concentration of the coexisting counterion under the same ionic strength. On the basis of this model, we estimate the distribution constant of R6G+ and anion (KD), the ion-pair formation constant in the BLM (Kip), and the R6G+ adsorption constant on the BLM surface (Kad). Even for an ultrathin membrane system, such as a BLM, R6G+ is distributed with a coexisting counterion and the distribution equilibrium of the ionic species at the water-BLM interface is analyzable similar to that at the water-organic solvent interface.

Zeta potential determination with a microchannel fabricated in solidified solvents

This paper proposes a simple and versatile method for the determination of the zeta potential of a channel wall and discusses the values measured for the surface of frozen solvents, which are not only of scientific interest but also of potential use for microfluidic platforms. The zeta potential of the solid surface is an important parameter for discussing its electrokinetic properties, the distribution and reaction of ions in an electric double layer, and the fluidic behavior in the space surrounded by the surface. While the zeta potential of colloidal matters can be determined from their electrophoretic mobility, it is often difficult to determine that of a bulk material. In this paper, the zeta potential of a microchannel fabricated in a frozen solvent is determined by measuring the apparent mobility of microparticles as the probe. The electrophoretic mobility of the microparticles has been measured in advance using microchip electrophoresis under various conditions. This approach allows us to determine the zeta potential of water-ice and frozen cyclohexane. We discuss the pH dependence of the zeta potential of ice and also effects of the NaCl concentration on that of ice and frozen cyclohexane.