Hirofumi Takikawa

Plasma irradiation of artificial cell membrane system at solid-liquid interface

We provide direct evidence of plasma-induced pore formation in a cell membrane model system. We irradiated plasma on the basis of the dielectric barrier discharge onto a supported lipid bilayer (SLB). Observation with a fluorescence microscope and atomic force microscope revealed the formation of pores on the order of 10 nm–1 µm in size. Capturing these micropores in a fluid lipid membrane is a significant advantage of the SLB system, and quantitative analysis of the pores was performed. Stimulation with equilibrium chemicals (HNO3 and H2O2) indicated that other transient active species play critical roles during the poration in the SLB.

Torsion fracture of carbon nanocoils

We fix a carbon nanocoil (CNC) on a substrate in a focused ion beam instrument and then fracture the CNC with a tensile load. Using the CNC spring index, we estimate the maximum to average stress ratio on the fractured surface to range from 1.3 to 1.7, indicating stress concentration on the coil wire inner edge. Scanning electron microscopy confirms a hollow region on the inner edge of all fractured surfaces.

Reduction in lateral lipid mobility of lipid bilayer membrane by atmospheric pressure plasma irradiation

Plasma medicine is an emerging research field in which various applications of electrical discharge, especially in the form of nonequilibrium plasma at atmospheric pressure, are examined, for example, the application of plasma to biological targets for various purposes such as selective killing of tumor cells and blood stanching. We have focused on the behavior of an artificial cell membrane system at the solid–liquid interface. To evaluate the lateral lipid mobility, we measured the diffusion coefficient of the supported lipid bilayer (SLB) composed of dioleoylphosphatidylcholine with fluorescence recovery after photobleaching by confocal laser scanning microscopy. It was found that the diffusion coefficient was decreased by plasma irradiation and that the diffusion coefficient decreasing rate proceeded with increasing plasma power. We investigated the effects of stimulation with an equilibrium chemical, H2O2, on the SLB and confirmed that the diffusion coefficient did not change at least up to a H2O2 concentration of 5 mM. These results indicate that transient active species generated by plasma play critical roles in the reduction in SLB fluidity. The effects of the two generated major oxidized lipid species, hydroxyl- or hydroperoxy-phosphatidylcholine (PC) and acyl-chain-truncated PCs terminated with aldehyde or carboxyl group, on lateral lipid mobility are discussed.

Supporting PtRu catalysts on various types of carbon nanomaterials for fuel cell applications

PtRu catalysts were supported on five types of carbon nanomaterials of various shapes, sizes, and graphitic properties and the catalyst supports evaluated. The carbon nanomaterial used included three types of nanoparticles: Arc Black (AcB), Vulcan XC-72 (Vulcan) and graphene oxide (GO), and two types of nanofibers: carbon nanocoil (CNC) and carbon nanotube (CNT). Pt and Ru were supported by the reduction method using sodium borohydride. The metal catalyst loading was confirmed by thermo-gravimetric analysis (TGA), electron microscopy, and X-ray diffraction (XRD). Transmission electron microscopy (TEM) and XRD revealed that the diameter of PtRu catalyst nanoparticles loaded on reduced GO (rGO) and AcB were ~2 nm and was the smallest among all the samples. Shifts in Pt (111) XRD peaks of CNC and CNT were larger than those of AcB, Vulcan, and rGO. These results suggest that the diameters of catalyst nanoparticles became smaller by loading on the carbon nanoparticles with a large surface area including rGO, AcB, and Vulcan. Loading onto the carbon nanofibers enhanced the degree of PtRu alloying.

Computational study of temporal behavior of incident species impinging on a water surface in dielectric barrier discharge for the understanding of plasma–liquid interface

A lipid bilayer is a basic structure of the cell membrane and is stable in liquid solution. In this study, we analyzed dielectric barrier discharge (DBD) containing water on a quartz substrate using a one-dimensional fluid model. To simulate atmospheric pressure plasma for practical use, a tiny amount of N2 gas (0.5 ppm) was added to He gas ambient as an impure gas. The calculated current–voltage (I–V) characteristics reproduced the measured ones qualitatively. We focused on the behavior of DBD at the plasma–liquid interface and analyzed the temporal behavior of the electric field strength and incident fluxes of charged, excited, and radical species on the water surface. By varying the gap length, it was shown that the maximum electric field strength in an AC cycle saturated at gap lengths ≥0.15 cm. The incident fluxes of N2+ and He2+ on the water surface are almost the same and show strong correlations with the electric field in the vicinity of the water surface.

Improved mechanical properties of bucky paper achieved via the addition of carbon nanocoils

The Young’s modulus of buckypaper (BP) was improved via the addition of carbon nanocoils (CNCs). The Young’s modulus was the highest when the added amount of CNCs of the mass of the bucky paper was 11.9%. It is likely that the improved Young’s modulus was due to the higher Young’s modulus of the CNCs, compared with the multi-walled carbon nanotube (MWCNT) bundles. The helical structure of the CNCs also contributed to the improvement in the Young’s modulus. In the BP containing CNCs, the CNCs were anchored in the MWCNT matrix with superior entanglement, and this effect improved the Young’s modulus of the BP.