Ryohei Kokawa

Growth shape of isotactic polystyrene crystals in thin films

The crystal growth of isotactic polystyrene (it-PS) is investigated in very thin, 11 nm thick films. The it-PS crystals grown in the thin films show quite different morphology from that in the bulk. With decreasing crystallization temperature, the branching morphology in a diffusion field appears: dendrites and compact seaweed. The branching morphology is formed through a morphological instability caused by the gradient of film thickness around a crystal; the thicker the film thickness, the larger is the lateral growth rate of crystals. Regardless of the morphological change, the growth rate as well as the lamellar thickness depends on the crystallization temperature as predicted by the surface kinetics.

CRYSTAL GROWTH OF ISOTACTIC POLYSTYRENE IN ULTRATHIN FILMS: FILM THICKNESS DEPENDENCE

The film thickness dependence of crystal growth is investigated for isotactic polystyrene (it-PS) in thin films for thicknesses from 20 down to 4 nm. The single crystals of it-PS grown at 180°C in the ultrathin films show a morphology typical of diffusion-controlled growth: dense branching morphology and fractal seaweed. The characteristic length of the morphology, i.e., the width of the branch, increases with decreasing film thickness. The thickness dependence of the crystal growth rate shows a crossover around the lamellar thickness of 8 nm. The thickness dependences of the growth rate and morphology are discussed in terms of the diffusion of chain molecules in thin films.

Specific interaction between GroEL and denatured protein measured by compression-free force spectroscopy.

We investigated the interaction between GroEL and a denatured protein from a mechanical point of view using an atomic force microscope. Pepsin was bound to an atomic force microscope probe and used at a neutral pH as an example of denatured proteins. To measure a specific and delicate interaction force, we obtained force curves without pressing the probe onto GroEL molecules spread on a mica surface. Approximately 40 pN of tensile force was observed for approximately 10 nm while pepsin was pulled away from the chaperonin after a brief contact. This length of force duration corresponding to the circumference of GroELs interior cavity was shortened by the addition of ATP. The relation between the observed mechanical parameters and the chaperonins refolding function is discussed.

Solution–TiO2 Interface Probed by Frequency-Modulation Atomic Force Microscopy

The topography and solvation structure of a solution–TiO2 interface were observed in the dark using highly sensitive, frequency-modulated atomic force microscopy (FM-AFM). The nucleation and growth of an ionic solute, KCl, in this study, were observed in constant frequency-shift topography. The force applied to the tip was determined as a function of tip–surface distance. Modulations were identified on some force curves and were found to be related to the site-specific density of water molecules.

Development of method for evaluating cell hardness and correlation between bacterial spore hardness and durability

BackgroundDespite the availability of conventional devices for making single-cell manipulations, determining the hardness of a single cell remains difficult. Here, we consider the cell to be a linear elastic body and apply Young’s modulus (modulus of elasticity), which is defined as the ratio of the repulsive force (stress) in response to the applied strain. In this new method, a scanning probe microscope (SPM) is operated with a cantilever in the “contact-and-push” mode, and the cantilever is applied to the cell surface over a set distance (applied strain).ResultsWe determined the hardness of the following bacterial cells: Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and five Bacillus spp. In log phase, these strains had a similar Young’s modulus, but Bacillus spp. spores were significantly harder than the corresponding vegetative cells. There was a positive, linear correlation between the hardness of bacterial spores and heat or ultraviolet (UV) resistance.ConclusionsUsing this technique, the hardness of a single vegetative bacterial cell or spore could be determined based on Young’s modulus. As an application of this technique, we demonstrated that the hardness of individual bacterial spores was directly proportional to heat and UV resistance, which are the conventional measures of physical durability. This technique allows the rapid and direct determination of spore durability and provides a valuable and innovative method for the evaluation of physical properties in the field of microbiology.

Molecular resolution investigation of tetragonal lysozyme (110) face in liquid by frequency-modulation atomic force microscopy

In order to determine the molecular structure by x-ray diffraction analysis, it is very important to grow high quality protein crystals. The molecular resolution imaging of soluble protein crystals such as the tetragonal lysozyme (110) face in saturated solution is demonstrated using frequency-modulation atomic force microscopy (FM-AFM). The surface structure of the (110) face and the crystallographic position of individual molecules were determined from molecular resolution images. For observation of protein crystals, FM-AFM is a favorable technique as an alternative to contact mode or amplitude-modulation AFM.

Characterization of local electrical properties of polycrystalline silicon thin films and hydrogen termination effect by conductive atomic force microscopy

We observed local electrical properties of polycrystalline silicon films by conductive atomic force microscopy. Moreover, we investigated the effects of hydrogen termination on the polycrystalline silicon films. Before hydrogen termination, conductive regions in grain disappeared with the repeated scanning of the cantilever, while conductive regions in grain boundary almost unchanged. It is considered that hopping conduction is a major electrical conduction mechanism at grain boundaries. After 5 min hydrogen termination, locally nonterminated regions were observed near grain boundaries. This suggests that hydrogen termination of the polycrystalline silicon does not randomly progress, and there are regions that cannot be easily inactivated near grain boundaries.

Formation Mechanism of Incommensurate Epitaxial Crystals of Chloroiron(III) Derivative of Tetraphenylporphine on Alkali–Halide (001) Surfaces

We investigate the thickness dependence of the lattice constants of a chloroiron(III) derivative of tetraphenylporphine (ClTPPFe) epitaxial crystals grown on alkali–halide substrates, KCl, KBr and KI, in order to study the accommodation of the misfit strain, and do not observe any obvious dependence. Moreover, the orientational disturbance of the epitaxial crystals on the alkali halides and the formation of amorphous islands on an amorphous carbon film are observed. On the basis of these results, we propose the mechanism that the orientation of surface-diffusing molecules contributes significantly to the epitaxial growth of ClTPPFe on the alkali halides.

Atom-Resolved Analysis of an Ionic KBr(001) Crystal Surface Covered with a Thin Water Layer by Frequency Modulation Atomic Force Microscopy

An ionic KBr(001) crystal surface covered with a thin water layer was observed with a frequency modulation atomic force microscope (FM-AFM) with atomic resolution. By immersing only the tip apex of the AFM cantilever in the thin water layer, the Q-factor of the cantilever in probing the solid-liquid interface can be maintained as high as that of FM-AFM operation in air, leading to improvement of the minimum detection of a differential force determined by the noise. Two types of images with atom-resolved contrast were observed, possibly owing to the different types of ions (K(+) or Br(-)) adsorbed on the tip apex that incorporated into the hydration layers on the tip and on the sample surface. The force-distance characteristics at the solid-water interface were analyzed by taking spatial variation maps of the resonant frequency shift of the AFM cantilever with the high Q-factor. The oscillatory frequency shift-distance curves exhibited atomic site dependence. The roles of hydration and the ions on the tip and on the sample surface in the measurements were discussed.

With respect to coefficient of linear thermal expansion, bacterial vegetative cells and spores resemble plastics and metals, respectively.

BackgroundIf a fixed stress is applied to the three-dimensional z-axis of a solid material, followed by heating, the amount of thermal expansion increases according to a fixed coefficient of thermal expansion. When expansion is plotted against temperature, the transition temperature at which the physical properties of the material change is at the apex of the curve. The composition of a microbial cell depends on the species and condition of the cell; consequently, the rate of thermal expansion and the transition temperature also depend on the species and condition of the cell. We have developed a method for measuring the coefficient of thermal expansion and the transition temperature of cells using a nano thermal analysis system in order to study the physical nature of the cells.ResultsThe tendency was seen that among vegetative cells, the Gram-negative Escherichia coli and Pseudomonas aeruginosa have higher coefficients of linear expansion and lower transition temperatures than the Gram-positive Staphylococcus aureus and Bacillus subtilis. On the other hand, spores, which have low water content, overall showed lower coefficients of linear expansion and higher transition temperatures than vegetative cells. Comparing these trends to non-microbial materials, vegetative cells showed phenomenon similar to plastics and spores showed behaviour similar to metals with regards to the coefficient of liner thermal expansion.ConclusionsWe show that vegetative cells occur phenomenon of similar to plastics and spores to metals with regard to the coefficient of liner thermal expansion. Cells may be characterized by the coefficient of linear expansion as a physical index; the coefficient of linear expansion may also characterize cells structurally since it relates to volumetric changes, surface area changes, the degree of expansion of water contained within the cell, and the intensity of the internal stress on the cellular membrane. The coefficient of linear expansion holds promise as a new index for furthering the understanding of the characteristics of cells. It is likely to be a powerful tool for investigating changes in the rate of expansion and also in understanding the physical properties of cells.