Biomedical subjects

Takahiro Ohkubo

Publications and source records attributed to Takahiro Ohkubo.

13 recordsLinked to original sources

PubMed2006

RBM band shift-evidenced dispersion mechanism of single-wall carbon nanotube bundles with NaDDBS.

The dispersion process of single-wall carbon nanotube (SWNT) by using sodium dodecylbenzene sulfonate (NaDDBS) was studied by means of surface tension measurements, ultraviolet-visible (UV-Vis) spectroscopy, scanning electron microscopy (SEM), and transmission electron spectroscopy (TEM). The critical micelle concentration (CMC) and the concentration where the surface tension begins to drop increase by the presence of SWNT. The isotherm of NaDDBS amount adsorbed on SWNT shows the plateau region at 0.2-6 mM and the saturated region above 40 mM. The external surface of SWNT bundle is fully covered with adsorbed NaDDBS at the plateau region, showing that SWNTs can be dispersed with the bundle form. On the other hand, SWNTs are dispersed in individual tubes at the saturated region, where the adsorption amount corresponds to coating of individual tube surfaces with NaDDBS. This dispersion state was confirmed by SEM and TEM observations. The effect of the dispersion state of SWNTs on radial breathing mode in Raman spectrum gave inherent peak shifts, being the in situ evidences on the step-wise dispersion mechanism of the SWNT bundle to the individual tubes.

Journal Article↗

PubMed2006

Adsorption and aggregation properties of heterogemini surfactants containing a quaternary ammonium salt and a sugar moiety.

A novel heterogemini surfactant comprising two hydrocarbon chains and two different hydrophilic groups such as a quaternary ammonium cation and gluconamide nonion N,N-dimethyl-N-[2-(N'-alkyl-N'-gluconamide)ethyl]-1-alkylammonium bromides (2CnAmGlu, where n represents hydrocarbon chain lengths of 8, 10, 12, and 14) was synthesized by reacting N,N-dimethylethylenediamine with alkyl bromide, followed by a reaction with 1,5-D(+)-gluconolactone. The adsorption properties of 2CnAmGlu were characterized by surface tension measurements made using the Wilhelmy plate method, and their aggregation properties were investigated by dynamic light scattering and cryogenic transmission electron microscopy techniques. The relationship between the hydrocarbon chain length and the logarithm of the critical micelle concentration (cmc) for 2CnAmGlu exhibited a linear decrease when the chain length was increased up to 12 and then a departure from linearity at n=14. The surface tension reached 24-26 mN m-1 at each cmc, indicating high efficiency in lowering the surface tension of water. Furthermore, it was found that the structure of the aggregate formed for 2CnAmGlu in solution was influenced by the hydrocarbon chain length; that is, for n=10 and 12, micelles with a hydrodynamic radius of 2-5 nm were formed, whereas vesicles were also observed for n=14.

Adsorption↗

PubMed2006

Polymerizable cationic gemini surfactant.

A polymerizable cationic gemini surfactant, [CH(2)=C(CH(3))COO(CH(2))(11)N(+)CH(3))(2)CH(2)](2).2Br(-), 1 has been synthesized and its basic interfacial properties were investigated (in water and in the presence of 0.05 M NaBr). For comparison, the properties of monomeric surfactant corresponding to 1, CH(2)=C(CH(3))COO(CH(2))(11)N(+)(CH(3))(3).Br(-), 2, were also investigated. Parameters studied include cmc (critical micelle concentration), C(20) (required to reduce the surface tension of the solvent by 20 mN/m), gamma(cmc) (the surface tension at the cmc), Gamma(cmc) (the maximum surface excess concentration at the air/water interface), A(min) (the minimum area per surfactant molecule at the air/water interface), and cmc/C(20) ratio (a measure of the tendency to form micelles relative to adsorb at the air/water interface). For the polymerizable gemini surfactant, 1, the methacryloxy groups at the terminal of each hydrophobic group in a molecule have no contact with the air/water interface in the monolayer, whereas for the corresponding monomeric surfactant, 2, the methacryloxy group contacts at the interface forming a looped configuration like a bolaamphiphile. Polymerized micelles of the gemini surfactant are fairly small monodisperse and spherical particles with a mean diameter of 3 nm.

Journal Article↗

PubMed2006

Preparation of highly dispersed core/shell-type titania nanocapsules containing a single Ag nanoparticle.

Core/shell-type titania nanocapsules containing a single Ag nanoparticle were prepared. Ag nanoparticles were prepared using the reduction of silver nitrate with hydrazine in the presence of cetyltrimethylammonium bromide (CTAB) as protective agent. The sol-gel reaction of titanium tetraisopropoxide (TTIP) was used to prepare core/shell-type titania nanocapsules with CTAB-coated Ag nanoparticles as the core. TEM observations revealed that the size of the core (Ag particle) and the thickness of the shell (titania) of the core/shell particles obtained are about 10 nm and 5-10 nm, respectively. In addition, the nanocapsules were found to be dispersed in the medium as individual particles without aggregation. Moreover, titania coating caused the surface plasmon absorption of Ag nanoparticles to shift toward the longer wavelength side.

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PubMed2006

Montmorillonite alignment induced by magnetic field: evidence based on the diffusion anisotropy of water molecules.

Diffusion coefficients of water in Na-montmorillonite (Na-mon) suspensions have been determined by pulsed-field gradient spin-echo (PGSE) NMR spectroscopy for three directions (x, y, and z), where x and y mean the directions perpendicular to the static magnetic field, and z the direction parallel to it. Diffusion anisotropy was observed in the suspensions with Na-mon weight fractions of 0.63, 1.82, and 3.32%; i.e., the diffusivity of water in the z direction is faster than that in the x or y direction. The largest diffusion anisotropy of water was observed at the Na-mon fraction of 3.32%. However, diffusion anisotropy disappeared in the suspensions with Na-mon fraction more than 5.02%. The fast diffusivity in the z direction was slightly enhanced in a stronger static magnetic field (14.1 T). These results indicate that the platelike Na-mon particles are aligned with their platelike faces parallel to the static magnetic field of NMR. We also measured diffusion coefficients of water for the z direction in the temperature range from 24 to 85 degrees C. The plot of diffusion coefficients of water against reciprocal temperature showed a refraction point at 65 degrees C. This phenomenon explicitly means that the alignment is gradually relaxed at higher temperatures.

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PubMed2005

Direct synthesis of mesoporous titania particles having a crystalline wall.

In this study, we demonstrate a novel method for preparing crystallized mesoporous titania by using a low-temperature synthesis technique in the presence of cationic surfactant. XRD patterns showed that the titania particles obtained had both hexagonal structure and a wall with anatase crystalline structure. Transmission electron microscopy (TEM) observation and corresponding electron diffraction pattern confirmed that the calcined particles are crystallized mesoporous titania.

Journal Article↗

PubMed2005

Photoinduced reversible change of fluid viscosity.

We report a reversible photoinduced fluid viscosity change. A small amount of a "photoswitchable" azobenzene-modified cationic surfactant (4-butylazobenzene-4'-(oxyethyl)trimethylammonium bromide, AZTMA) was added to a wormlike micellar solution of cetyltrimethylammonium bromide (CTAB) containing sodium salicylate (NaSal). The trans-AZTMA solution had a remarkably high viscosity as a result of the entangled network of wormlike micelles. UV light irradiation on the trans-AZTMA solution remarkably decreased the viscosity of the solution because the bulky structure of cis-AZTMA is likely to disrupt the network structure of wormlike micelles. This photoinduced viscosity change is perfectly reversible between the trans- and cis-AZTMA solutions.

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PubMed2005

Synthesis of microtubes with a surface of "house of cards" structure via needlelike particles and control of their pore size.

The conditions for synthesizing microtubes with a surface of "house of cards" structure via needlelike particles were examined in detail. Magnesium carbonate trihydrate was formed as a metastable phase in the reaction process using magnesium hydroxide and carbon dioxide as starting materials. Subsequently, in the formation of basic magnesium carbonate from magnesium carbonate trihydrate, microtubes with a surface of house of cards structure were obtained via needlelike particles of magnesium carbonate trihydrate under certain conditions where the temperature and added amount of sodium hydroxide were properly controlled. The pore size of the microtubes could be controlled within a range of 0.5-6 microm by adjusting the condition of needlelike particle formation. In addition, the sustainability of naphthalene release from the microtube was found to be about 6 times higher than that from naphthalene crystal.

Journal Article↗

PubMed2005

Membrane properties of cationic liposomes composed of dipalmitoylphosphatidylcholine and dipalmityldimethylammonium bromide.

Cationic liposomes composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmityldimethylammmonium bromide (DPAB) were prepared by the Bangham method and the effect of DPAB on the membrane properties was examined in terms of liposomal shape, particle size, trapping efficiency, surface potential and dispersibility. The dispersibility of the mixed DPPC/DPAB liposomes (the mole fraction of DPAB (XDPAB)>==0.05) was excellent and the dispersibility was maintained for 6 months, since the zeta-potential of the mixed liposomes was approximately +40 mV. The trapping efficiency of the mixed DPPC/DPAB liposomes (XDPAB=0.05) was 10 times greater than that of the DPPC liposomes, and the value was largest among the mixed liposomes (XDPAB=0-1.0). Freeze-fracture electron micrographs indicated that the shape of the mixed DPPC/DPAB liposomes (XDPAB=0.05) was that of large unilamellar vesicles (LUVs) with a diameter of approximately 2 microm, while the shape of the DPPC liposomes was that of multilamellar vesicles (MLVs). The mixed liposomes had, therefore, a high trapping efficiency. Furthermore, the shape of the mixed DPPC/DPAB liposomes (XDPAB=0.75) was also that of LUVs with a diameter of approximately 2 microm and these had a high trapping efficiency. Whereas, the particle size (500 nm) of the mixed DPPC/DPAB liposomes (XDPAB=0.25) was smaller than that of the former and had the minimum trapping efficiency. The phase transition temperature of the liposomal bilayer membranes indicated a maximum value at 0.25-0.30 mole fractions of DPAB. These facts were considered to be due to the fact that DPPC and DPAB, whose molar ratio was 7.5:2.5, were tightly packed in the liposomal bilayer membranes and that the curvature of the liposomal particle was resultantly large. Nevertheless, LUVs having a high trapping efficiency were easily obtained by mixing a small amount of DPAB with the DPPC.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2004

Control of viscoelasticity using redox reaction.

The viscoelasticity of a fluid was tuned with the Faradaic reaction of (11-ferrocenylundecyl)trimethylammonium bromide (FTMA), a "redox-switchable" surfactant. An aqueous solution of the reduced form of FTMA exhibited a remarkable viscoelasticity in the presence of sodium salicylate (NaSal) because of the formation of three-dimensional entanglement of wormlike micelles. Electrolytic oxidation of FTMA caused the viscosity of the system to dramatically decrease and the elasticity to disappear. This drastic decrease in viscoelasticity arose from the disruption of wormlike micelles. This novel electrorheological phenomenon is expected to be applicable to ink for inkjet printers, the electrochemically controlled release of substances entrapped in wormlike micelles of FTMA, and fluid flow rate control using electric signals.

Journal Article↗

PubMed2004

Control of particle alignment in water by an alternating electric field.

We attempted to align a large number of silica particles dispersed in aqueous solution by controlling the alternating electric field between the two electrodes (400 microm apart). Relatively large particles (9.9 microm) were found to align forming strings in the direction parallel to the electric field while relatively small particles (2.0 and 4.9 microm) were observed to align making stripes in the direction perpendicular to the field. The number of stripes formed by particles between the electrodes increased with increasing frequency of the alternating field. This peculiar perpendicular particle alignment appeared when the contribution to particle alignment of electroosmotic flow exceeded that of dielectric polarization and the osmotic flow was found to be stronger around the particles than in the vicinity of the electrode surface.

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PubMed2004

Three-dimensional arrangements of polystyrene latex particles with a hyperbolic quadruple electrode system.

An attempt was made to arrange polystyrene latex particles (2, 5, and 10 microm in diameter) dispersed in aqueous media making use of their dielectrophoresis and electrophoresis with a hyperbolic quadruple electrode system. Application of a high-frequency ac field enabled the particles to arrange themselves between the electrodes forming a particle monolayer due to the negative dielectrophoretic force. Simultaneous application of high-frequency ac and dc fields caused the particles to gather in the region surrounded by the electrodes to form particle multilayers. Appropriate choice of the way of applying an electric field thus allowed the reversible control of particle arrangements (monolayer, multilayer, dispersion). Reapplication of an ac field to the particle layers produced highly dense particle multilayers.

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PubMed2002

Restricted hydration structures of Rb and Br ions confined in slit-shaped carbon nanospace.

Hydration structures of "nanosolutions" around both cation and anion for rubidium bromide confined in nanospaces (pore width = 1.1 nm) of activated carbon fiber were determined with EXAFS measurements and related analysis. The analytical results indicate that the ionic solutions confined in hydrophobic nanospaces tend to form an incomplete dehydration structure, because of the spatial restriction by the nanospace and capturing water molecules in the ordered structure.

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