Toshio Nakashima

Quantum size effect in TiO2 nanoparticles prepared by finely controlled metal assembly on dendrimer templates.

The use of dendrimer templates to make metal-based nanoparticles of controlled size has attracted much interest. These highly branched macromolecules have well-defined structures that enable them to bind metal ions to generate precursors that can be converted into nanoparticles. We describe the sub-nanometre size control of both anatase and rutile forms of TiO2 particles with phenylazomethine dendrimers, leading to samples with very narrow size distributions. Such fine tuning is possible because both the number and location of metal ions can be precisely controlled in these templates. Quantum size effects are observed in the particles, and the energy gap between the conduction and valence bands exhibits a blueshift with decreasing particle size and is dependent on the crystal form of the material. The dependency of the bandgap energy on these factors is explained using a semi-empirical effective mass approximation.

Metastability of anatase: size dependent and irreversible anatase-rutile phase transition in atomic-level precise titania

Since crystal phase dominantly affects the properties of nanocrystals, phase control is important for the applications. To demonstrate the size dependence in anatase-rutile phase transition of titania, we used quantum-size titania prepared from the restricted number of titanium ions within dendrimer templates for size precision purposes and optical wave guide spectroscopy for the detection. Contrary to some theoretical calculations, the observed irreversibility in the transition indicates the metastablity of anatase; thermodynamics cannot explain the formation of metastable states. Therefore, we take into account the kinetic control polymerization of TiO6 octahedral units to explain how the crystal phase of the crystal-nucleus-size titania is dependent on which coordination sites, cis- or trans-, react in the TiO6 octahedra, suggesting possibilities for the synthetic phase control of nanocrystals. In short, the dendrimer templates give access to crystal nucleation chemistry. The paper will also contribute to the creation of artificial metastable nanostructures with atomic-level precision.

Potentiometric study on critical micellization concentrations (CMC) of sodium salts of bile acids and their amino acid derivatives

The critical micellization concentration (CMC)s of a group of sodium salts of bile acids, i.e., cholic acid (abbreviated as C), deoxycholic acid (DC), chenodeoxycholic acid (CDC), ursodeoxycholic acid (UDC), glycocholic acid (GC), glycodeoxycholic acid (GDC), glycochenodeoxycholic acid (GCDC), and some amino acid derivatives of C and DC were determined by potentiometric titration. An automatic titration system has been established to determine the apparent dissociation constants of bile acids below and above CMC. The CMCs of these bile salts were in accord with those values obtained by conventional methods such as surface tension and/or cholesterol solubilization methods. The CMCs of amino acid derivatives of cholic, and deoxycholic acids were observed to be much lower than those of the respective free bile acids. The reasons of the difference were described.

Determination of uranium(VI) in seawater by ion-exchanger phase absorptiometry with arsenazo III.

An ion-exchange phase absorptiometric method with Arsenazo III has been developed for the determination of uranium (VI). A flow cell with 0.1 ml of anion exchange resin was employed to achieve a detection limit for uranium of 0.16 microg/l. in 100 ml of a seawater sample. The sensitivity is about 300 times higher than for corresponding solution spectrophotometry.

Ligand-field control in the self-assembly of polymeric metal complexes: copper(II) complexes with quadridentate Schiff-base ligands involving an imidazole moiety

Ligand-field control in the self-assembly of polymeric metal complexes has been studied. A series of seven ‘ligand complexes’[Cu(HL)]+ containing an imidazole moiety have been prepared, where H2L are the quadridentate Schiff-base ligands of the 1 : 1 : 1 condensation products of 4-formylimidazole, a diamine (ethylenediamine, 1,3-diaminopropane or trans-cyclohexane-1,2-diamine), and a salicylaldehyde derivative (salicylaldehyde, 5-bromosalicylaldehyde, 5-methoxysalicylaldehyde, 3-methoxysalicylaldehyde, o-hydroxyacetophenone) or benzoylacetone. The single-crystal X-ray analyses of three complexes have confirmed that the co-ordination geometries are essentially square planar. From the deprotonation behaviour of the imidazole proton and the d–d band maxima, the complexes can be classified into two types; type A complexes composed of salicylaldehyde derivatives and type B complexes composed of benzoylacetone. Under alkaline conditions, the type A complexes easily produce a self-assembly process giving insoluble imidazolate-bridged polymeric species and their potentiometric pH titrations are irreversible in the forward and reverse titrations. The variable-temperature magnetic data of the deprotonated type A complexes are well reproduced using the Ising model based on H=–2ΣJijSiSj, and J values of –8.6, –1.8, –5.2, –13.0 and –4.5 cm–1 are obtained. On the other hand, under alkaline conditions, the type B complexes give mononuclear species soluble in common organic solvents and act as good ‘ligand complexes’. The potentiometric pH titration curves are almost the same in the forward and reverse titrations, indicating that protonation and deprotonation were reversible. The type A complexes exhibiting weaker ligand-field strength in the equatorial ligand are ‘self-assembly complexes’ and the type B complexes with stronger fields are ‘ligand complexes’.