Masahito Sano

Novel nucleotide-responsive Hydrogels designed from copolymers of boronic acid and cationic units and their applications as a QCM resonator system to nucleotide sensing

Hydrogels comprised of boronic acid monomer (3), cationic monomer (4), and crosslinker monomer (5) were prepared by radical copolymerization. These hydrogels could efficiently bind nucleotides such as AMP and ATP by a cooperative action of the boronic acid-cis-diol complexation and the electrostatic interaction between the cationic unit and the phosphate group. The binding processes were conveniently monitored by the swelling and deswelling behaviors of these hydrogels in aqueous solution. For the hydrogel with the specific monomer composition an interesting charge inversion was observable: with increasing AMP or ATP concentration, the cation-rich hydrogel was gradually charge neutralized, once shrunken at the neutral stage, and then swollen again because of the anion-rich charge state. These nucleotide-induced swelling and deswelling phenomena were reproduced on the gold surface of a QCM resonator. Therefore, the present system is not only interesting to consider nucleotide-induced mechanochemical properties, but also applicable as a sensor to the nucleotide detection.

ORGANIC GELS ARE USEFUL AS A TEMPLATE FOR THE PREPARATION OF HOLLOW FIBER SILICA

A novel mesoporous silica with a tubular structure has been prepared using organic gel fibers as a template.

Silica gel fabrication of [60]fullerene aggregates and carbon nanotubes utilizing the amphiphilic nature of poly(N-vinylpyrrolidone) as a ‘glue’

Sol–gel polycondensation of tetraethoxysilane in the presence of [60]fullerene/poly(N-vinylpyrrolidone) (PVP) composite or single-walled carbon nanotubes (SWNTs)/PVP composite yielded novel silica nanoparticles or silica nanorods, respectively. TEM and SEM observations and IR, UV-VIS and XPS spectroscopic analyses consistently revealed that these novel silica-based superstructures are composed of templates ([60]fullerene or SWNTs), PVP and a thin silica layer. From the silica nanoparticles it was possible to prepare the hollow silica particles by pyrolysis. In the silica nanorods, it was possible to remove PVP and then SWNTs at 500 and 1000 °C, respectively, by stepwise pyrolysis. TEM and SEM images of the silica nanrods obtained after removal of PVP showed a hollow structure with SWNTs sticking out of the inner tube, like the lead of an ‘automatic pencil’.

‘Molecular-imprinting’ in polyion complexes which creates the ‘memory’ for the AMP template

The formation of a polyion complex precipitate was applied to molecular imprinting for the first time. Polyanion containing boronic acid units can sustain AMP by a boronate–cis-diol interaction. When this polyanion forms a polyion complex with polycation according to 1∶1 cation–anion stoichiometry, the phosphate anionic charges introduced into the polyanion by the AMP complexation are also counted. Thus, after removal of AMP from the polyion complex a ‘cleft’ which has the memory for the AMP template is created. It was proved that this cleft shows high affinity with AMP. We also noticed that the removal and re-binding processes for AMP coincide with the swelling and deswelling phenomena of this polyion complex.

Facile construction of an ultra-thin [60]fullerene layer from [60]fullerene–homooxacalix[3]arene complexes on a gold surface

A hexacationic homooxacalix[3]arene–[60]fullerene 2∶1 complex can be deposited on an anion-coated gold surface as a monolayer (or at least as a monolayer-like ultra-thin film); as expected, this membrane efficiently shows a redox response in cyclic voltammetry and a photoelectrochemical response under visible light irradiation.

'Molecular-imprinting' of AMP utilising the polyion complex formation process as detected by a QCM system

The formation of a polyion complex was applied for molecular imprinting of AMP on a QCM resonator surface. The gold electrode on the resonator surface was modified with anionic thiols. Subsequently, layers of polycations and boronic acid-containing polyanions were adsorbed onto the QCM anionic surface utilising an alternating adsorption process. When the polymer adsorption was conducted in the presence of AMP, the anionic charges of the phosphate group introduced into polyanion by a boronate–cis-diol interaction altered the film growth behaviour: viz., excess polycationic units were adsorbed onto the polyanionic surface. After removal of the AMP from the surface polyion complex, a swollen gel layer with excess cationic charge is created. It was proven that this QCM system sensitively responds to AMP. The responsiveness is derived from the mass decrease in relation to the shrinking of the surface gel layer.

Catalytic Effects on Thermal Oxidation of Single-Walled Carbon Nanotubes by Alkali Metal Chlorides

Chemical reactions on single-walled carbon nanotubes (SWCNTs) are particularly important for purification and application. As-grown SWCNTs contain various forms of other carbons, to which we will refer herein as soot, and residual metal catalyst. One of the common purification procedures is to oxidize soot at high temperatures and to dissolve metals away chemically. For applications, because SWCNTs are difficult to disperse and mix with other materials, chemical modifications are often employed to improve surface properties for further processing. In practice, many chemical reactions involving SWCNTs depend on the tube diameter, defects, and the degree of bundling. There are numerous reports on the diameter dependence of reactions using thin SWCNTs, with diameters typically less than 2 nm, with the consistent result that tubes with smaller diameters are oxidized faster. Such results can be attributed to the increasing strain on carbon– carbon bonds as the tube diameter becomes smaller. Because thick SWCNTs, with diameters larger than approximately 2.5 nm, tend to collapse into flattened configurations as the opposing walls attract each other due to strong van der Waals forces, the carbon–carbon bonds at the folds are severely strained. Although reactions on thick SWCNTs have not been well characterized, we expect that the larger diameter tubes are more reactive for thick SWCNTs. Defects, which include any bonding sites where the carbon hexagonal continuum is broken and surface sites with chemically adsorbed species, are often terminated by various oxide groups and have the effect of localizing p electrons. Some of them can initiate or promote organic reactions, with the consequence that SWCNTs with more defects tend to be more reactive. Due to the close packing of tubes in bundles, reactant molecules cannot access tubes inside the bundles unless they are very small. As a result, bundling tends to lower the reactivity as a whole. In addition to these general tendencies, some reactions are dependent on the electronic states of SWCNTs, which have been applied to separate metallic and semiconducting tubes. Herein we report an exception to these ordinary characteristics. Thermal oxidation in the presence of alkali metal chlorides was found to be independent of these parameters. Using this result, unwanted selectivity during purification, for instance, for an evaluation of “true” chirality distribution of a given as-grown sample, could be minimized. Furthermore, the reaction was catalytic. Carbons were oxidized at much lower temperatures at faster rates. In particular, the reaction did not produce new defects on the SWCNTs. Such “clean” combustion avoids damage to the SWCNTs during purification processes, in addition to helping to save energy and time. The catalytic effects of metal oxide and chloride salts during combustion of soot have been known for some time, in relation to the environmental and health issues of diesel exhaust gases. Because the temperature of the exhaust gas is less than 670 K, it is necessary that combustion catalyst are effective below this temperature. These studies, however, are aimed at lowering the combustion temperatures of soot and the mechanism of catalysis remains largely unknown. Endo et al. investigated thermal oxidation of graphitized multiwalled carbon nanotubes (MWCNTs) in the presence of NaCl. Based on the kinetic study, they proposed two reaction steps; introduction of disorder as the first process and oxidation of the disordered tubes as the second process. They concluded that the main role played by NaCl was to increase the number of defects on the MWCNTs. We examined SWCNTs, which allowed us to obtain detailed information about the diameter, defects, and bundling from Raman spectroscopy. Scanning electron microscopy (SEM) revealed morphological changes taking place during oxidation. X-ray photoelectron spectroscopy (XPS) was used to look for specific interactions between the SWCNTs and the chloride salts. SWCNTs (HiPco) were rendered hydrophilic by sonication in a mixture of H2SO4 and HNO3 (3:1 v/v) for a few hours, followed by thorough washing with water to neutral pH. An aqueous dispersion of the acid-treated SWCNTs was mixed with an alkali metal chloride (NaCl, KCl, CsCl) solution (typically 0.1–10 mm) with brief sonication. The salt caused immediate aggregation of the SWCNTs. The mixture was cast on a glass plate for oxidation experiments. For thermogravimetric analysis (TGA), the solid was collected on a filter and dried. The exact same procedure without the addition of chloride salts was carried out for a control sample. As it turned out, the SWCNTs with chloride salts were oxidized so fast at low temperatures that significant oxidation would have taken place during the heating to the final temperature if we have had used an electric oven. Because the sample was taken out of the oven for each measurement, which was performed at room temperature, and brought back up to temperature repeatedly, the electric oven caused a problem. For this reason, we used a microwave oven for the thermal oxidation of SWCNTs. By microwave heating, temperatures above the combustion point could be reached in several seconds and then brought back down to room temperature on a similar time scale. Because the heating rate is much faster than the rate of oxygen intake, the fast heating and cooling minimized complications due to oxidation during the temperature rise. A drawback of using the microwave oven was that the exact temperature of the SWCNTs was unknown. [a] Y. Kobayashi, Prof. Dr. M. Sano Department of Polymer Science and Engineering, Yamagata University 3-4-16 Jyonan, Yonezawa, Yamagata 992-8510 (Japan) Fax: (+ 81) 238-26-3072 E-mail : mass@yz.yamagata-u.ac.jp

Length scales necessary for proper averaging to characterize polymerization in nanosystems: topochemical polymerization of diacetylene nanocrystals dispersed in a polystyrene matrix as probed by confocal Raman microscopy.

The scales necessary to make appropriate spatial averaging on solid-state polymerization were investigated by confocal Raman microscopy with a mapping resolution of 2 μm. Nanocrystals of an aliphatic diacetylene with an average size of 0.14 μm, each separated by 0.3 μm on average, were dispersed in a polystyrene matrix and were polymerized by UV irradiation. The distribution of nanocrystals was inhomogeneous over approximately 20 μm scale. A large crystal of the same monomer shows that photoinitiation is already averaged at the microscope resolution, while the color transition from the blue to the red form requires a scale greater than 5 μm. For the nanocrystals at low conversion, UV-vis absorption spectroscopy measured over a centimeter scale indicates linear polymerization kinetics and a higher polymer yield at a higher temperature. By contrast, the Raman microscopy reveals that, whereas the 20 μm region of high monomer concentrations yields more polymers at -24 °C, the region of low monomer concentrations gives more polymers at 20 °C. We propose thermal initiation, which is not efficient in the large crystals, as an additional initiation process for the apparent discrepancy, implying that the initiation process is not averaged below 20 μm scale for the dispersed nanocrystals.

Hierarchical structures in mesoscopic fibers that are self-organized by monolayer nano-clusters

Hierarchical structures in fibrous assemblies self-organized from nanometre-sized monolayer clusters are elucidated. Langmuir monolayers of a short chain carboxyazobenzene derivative feature mesoscopic fibrous structures, a monolayer thick, 80 nm apparent width, and over 100 μm long when transferred onto mica from the air–water interface. The effects of pH and salts on the large-scale morphology are followed by atomic force microscopy (AFM) and those of molecular-scale orientation are probed by UV-vis spectroscopy. The large morphological changes, despite no evidence of molecular reorientation, indicate that a small monolayer cluster, rather than a single molecule, is the relevant unit making up a fiber. Furthermore, detailed statistical analyses of the images reveal that nearly all fibers actually consist of a pair of strands, each strand 29 nm wide, that are imaged by the AFM tip, having an effective apex size of 21 nm. The results suggest the following hierarchical structure. Molecules aggregate into a monolayer cluster 29 nm wide. These clusters associate one-dimensionally to form a long monolayer strand. Finally, two strands join together to result in a fiber with a true width of 58 nm.

Facile construction of a novel metal-imprinted polymer surface without a polymerisation process

Porous thin films have been prepared by casting water–THF mixed solutions of poly(vinyl chloride-co-acrylic acid): when Cu2+ ion is imprinted in the casting process, the film obtains a memory for the original Cu2+ ion. The imprinting processes can be closely monitored by FTIR spectroscopy.