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Bunsho Ohtani

Publications and source records attributed to Bunsho Ohtani.

11 recordsLinked to original sources
PubMed2006

Electric-field-induced changes in absorption and emission spectra of CdS nanoparticles doped in a polymer film.

Electric-field-induced changes in absorption and emission spectra of colloidal CdS nanoparticles ranging in size from 1.0 to 5.0 nm in diameter have been measured by using electric field modulation spectroscopy. The analysis of the electroabsorption spectra indicates that the dipole moment in the first exciton state becomes larger with increasing particle size. The presence of the large dipole moment following photoexcitation into the first exciton band suggests that the CdS nanoparticles have large CT character in the first exciton state. The quantum yields both of the exciton emission and of the trap emission are markedly reduced by application of an electric field. On the basis of the direct measurements of the field-induced change in emission decay profile, it is suggested that the field-induced de-enhancement of these emission yields results from the field-induced decreases both in lifetime and in initial population of each emitting state. It is also found that the emission intensity of CdS nanoparticles increases under the UV light irradiation in air and decreases in a vacuum condition and that fluorescence lifetime in the former case is longer than that in the latter. This enhancement and de-enhancement process in emission intensity is almost reversible at least in several cycles.

Journal Article↗
PubMed2006

Effect of ionic surfactants on the iridescent color in lamellar liquid crystalline phase of a nonionic surfactant.

A nonionic surfactant, n-dodecyl glyceryl itaconate (DGI), self-assembles into bilayer membranes in water having a spacing distance of sub-micrometer in the presence of small amounts of ionic surfactants, and shows beautiful iridescent color. Ionic surfactants have strong effects on this iridescent system. We have interestingly found that the iridescent color changes with time after mixing DGI and ionic surfactants and the color in equilibrium state changes greatly with concentration of the ionic surfactants. The time-dependent color change results from the transformation of DGI aggregate structure after being mixed with ionic surfactant. It is first found that the iridescent color of this nonionic system can be changed from red to deep blue by altering the concentration of ionic surfactants added even though the total concentration of surfactant is almost constant. Such large blue shift of the iridescent color in equilibrium state cannot be fully explained by the ordinary undulation theory applied so far for this phenomenon. The flat lamellar sheets tend to curve by increasing the concentration of ionic surfactants to form separated onion-like and/or myelin-like structures. These separated structures of lamellar system result in the decrease of spacing distance between bilayer membranes because some vacant spaces necessarily appear among these structures.

Journal Article↗
PubMed2006

Photochemical fine-tuning of luminescent color of cadmium selenide nanoparticles: fabricating a single-source multicolor luminophore.

Size-selective photoetching was applied to silica-coated cadmium selenide (SiO2/CdSe) nanoparticles to precisely control their photoluminescence properties. The absorption spectra of CdSe was blue-shifted by irradiation of monochromatic light, and finally, the absorption onset agreed with the wavelength of irradiation light, indicating that CdSe particles were photoetched to smaller ones until the irradiated photons were not absorbed by the photoetched particles and that the SiO2 shell layer surrounding the CdSe core prevented coalescence between the photoetched particles. Although as-prepared SiO2/CdSe did not exhibit photoluminescence, the application of size-selective photoetching to SiO2/CdSe resulted in the development of the band gap emission, with the degree being enhanced with progress of the photoetching. The peak wavelength of photoluminescence decreased with a decrease in the wavelength used for the photoetching, so that the luminescence color could be tuned between red and blue. Partial photoetching of SiO2/CdSe nanoparticle films produced intense band gap emission of CdSe at the photoetched area, while the remainder of the SiO2/CdSe films did not exhibit detectable photoluminescence, resulting in the formation of a clear photoluminescence image under UV irradiation. This technique makes it possible to produce a multicolored photoluminescence image by irradiation with monochromatic lights having various wavelengths using a single source material.

Cadmium Compounds↗
PubMed2005

Asymmetrically modified silica particles: a simple particulate surfactant for stabilization of oil droplets in water.

Spherical silica particles that are able to assemble at a phase boundary of a dual-phase mixture of water and an immiscible organic solvent were prepared by a partial modification of their surface hydroxyl groups with an alkylsilylation agent. Scanning electron microscopic observation of these particles in which their remaining surface hydroxyl groups had been selectively modified with colloidal gold particles revealed that each particle has an asymmetric surface structure: one side of the surface is hydrophilic and the other is hydrophobic. We found that these particles could form a micellar structure in water in the presence of an organic solution of a toluene/polystyrene mixture. The micellar structure was evidenced by formation of golf-ball-like polystyrene particles with dimples imprinting morphologies of the hydrophobic part of modified silica particles.

Journal Article↗
PubMed2004

Chiral discrimination between thalidomide enantiomers using a solid surface with two-dimensional chirality.

Chiral discrimination between thalidomide enantiomers was achieved using the self-assembled monolayer (SAM) of an atropisomeric compound, 1,1'-binaphthalene-2,2'-dithiol (BNSH), which takes a two-dimensional chiral arrangement on gold(111) surface. Interestingly, an "all-or-none" type enantioselectivity appears; one enantiomeric form of BNSH SAM allows the adsorption of only one enantiomer of thalidomide. In addition, the response of a racemic SAM of BNSH was revealed to be one-half of that caused by pure enantiomeric SAM.

Carboxylic Acids↗
PubMed2003

Photoinduced chemical reactions on natural single crystals and synthesized crystallites of mercury(II) sulfide in aqueous solution containing naturally occurring amino acids.

Photoirradiation at >300 nm of aqueous suspensions of several natural crystal specimens and synthesized crystallites of mercury(II) sulfide (HgS) induced deaminocyclization of optically active or racemic lysine into pipecolinic acid (PCA) under deaerated conditions. This is the first example, to the best of our knowledge, of photoinduced chemical reactions of natural biological compounds over natural minerals. It was found that the natural HgS crystals had activity higher than those of synthesized ones but lower than those of other sulfides of transition metals, e.g., CdS and ZnS, belonging to the same II-IV chalcogenides. In almost all of the photoreactions, decompostion of HgS occurred to liberate hydrogen sulfide (H(2)S) and Hg(2+), and the latter seemed to have undergone in-situ reductive deposition on HgS as Hg(0) after a certain induction period (24-70 h) during the photoirradiation, as indicated by the darkened color of the suspensions. The formation of PCA, presumably through combination of oxidation of lysine and reduction of an intermediate, cyclic Schiff base, could also be seen after a certain induction time of the Hg(0) formation. This was supported by the fact that the addition of small amount of Hg(2+) (0.5 wt % of HgS) increased the PCA yield by almost 2-fold. We also tried to elucidate certain aspects of the plausible stereochemical reactions in relation to the chiral crystal structure of HgS. Although, in some experiments, slight enantiomeric excess of the product PCA was observed, the excess was below or equal to the experimental error and no other supporting analytical data could not be obtained; we cannot conclude the enantiomeric photoproduction of PCA by the natural chiral HgS specimen.

Amino Acids↗
PubMed2003

Preparation of novel silica-cadmium sulfide composite nanoparticles having adjustable void space by size-selective photoetching.

The size-selective photoetching technique was used to control the size of a CdS nanoparticle inside a silica shell. With monochromatic light irradiation, the diffuse reflectance spectra of silica-coated CdS nanoparticles were blue-shifted, and the size of the resulting CdS nanoparticles incorporated in the silica shells was adjustable by varying the wavelength of irradiated light. TEM observation revealed that the original CdS nanoparticle seemed to be in close contact with the amorphous silica shell to leave almost no clearance, while the monochromatic light irradiation caused the decrease in the size of CdS particles, resulting in the formation of a void space between the photoetched CdS core particle and the silica shell. The average void spaces available in the shells were calculated to be ca. 1.4 and 2.4 nm with the irradiation at 514 and 458 nm, respectively. These results indicated that the size-selective photoetching technique enables the regulation of void space formed in the core-shell structure by choosing the wavelength of irradiation light.

Journal Article↗
PubMed2002

Enantioselective adsorption of phenylalanine onto self-assembled monolayers of 1,1'binaphthalene-2,2'-dithiol on gold.

Self-assembled monolayer of atropisomeric compound, 1,1'-binaphthalene-2,2'-dithiol (BNSH), provides a field for chiral discrimination. The two-dimensional chiral arrangement with screw-like units, each of which is composed of three BNSH molecules, plays an important role in the enantioselectivity of adsorption of phenylalanine, as revealed by a quartz crystal microbalance technique.

Adsorption↗