Junya Suehiro

Fabrication of a carbon nanotube-based gas sensor using dielectrophoresis and its application for ammonia detection by impedance spectroscopy

This paper describes a new method for fabricating a gas sensor composed of multi-wall carbon nanotubes (MWCNTs) using dielectrophoresis (DEP). MWCNTs dispersed in ethanol were trapped and enriched in an interdigitated microelectrode gap under the action of a positive DEP force that drove the MWCNTs to a higher electric field region. During the trapping of MWCNTs, the electrode impedance varied as the number of MWCNTs bridging the electrode gap increased. After the DEP process, the ethanol was evaporated and the microelectrode retaining the MWCNTs was exposed to ammonia (NH3) gas while the electrode impedance was monitored. It was found that the electrode impedance was altered by ppm-levels of ammonia at room temperature. The ammonia exposure decreased the sensor conductance, while the capacitance increased. The sensor showed a reversible response with a time constant of a few minutes. The conductance change was proportional to ammonia concentration below 10 ppm and then gradually saturated at higher concentrations. Effects of the number of trapped MWCNTs on sensor response were also discussed.

Selective detection of viable bacteria using dielectrophoretic impedance measurement method

This paper describes a selective detection technique of viable bacteria based on dielectrophoresis and electrical impedance measurements. The authors have previously proposed a detection technique of biological particles called dielectrophoretic impedance measurement (DEPIM) method using positive dielectrophoretic force to capture biological cells in suspension onto an interdigitated microelectrode array. By combining antigen-antibody reaction with the DEPIM, selective detection of a particular species of bacteria was demonstrated. In this present work, the authors demonstrated another selective DEPIM method utilizing cell viability dependency of dielectrophoretic force without introducing the antigen-antibody reaction. It was found that dielectrophoresis of heat-treated Escherichia coli showed strong dependency on viability when applied field frequency was as high as 1 MHz. As a result, viable bacteria could be exclusively collected by positive dielectrophoresis and selectively detected by the DEPIM technique from a suspension also containing heat-treated nonviable cells. On the other hand, nonviable bacteria obtained by UV irradiation showed little dielectrophoresis dependency on viability. According to a theoretical analysis of the dielectrophoretic force, it is suggested that heat treatment alters the dielectric properties of treated cells. In particular, a decrease in cytoplasmic conductivity, which might be caused by heat-induced perforation of cell membrane, was expected to considerably affect dielectrophoresis characteristics. Proposed selective DEPIM method was also applied to evaluation of heat sterilization effect on a real time basis. It was experimentally proved that DEPIM could evaluate viable cell number variation with heat treatment time in a considerably shorter time than conventional microbiological method based on cell incubation.

Quantitative estimation of biological cell concentration suspended in aqueous medium by using dielectrophoretic impedance measurement method

We describe a new technique that realizes the quantitative estimation of the biological cell concentration in an aqueous medium. The proposed dielectrophoretic impedance measurement method utilizes the positive dielectrophoretic force to capture suspended biological particles onto an interdigitated micro-electrode array in pearl-chain formation. Higher cell concentration results in faster development of the pearl chains, which are electrically connected in parallel within the electrode gap to increase the conductance and capacitance between the electrodes. By monitoring temporal variation of the electrical impedance, it is possible to quantitatively evaluate the cell population according to a theoretical model of the cell collection process. It has been demonstrated that a suspension liquid of Escherichia coli could be accurately assayed in about 10 min at 105 cm-3 concentration.

Dielectrophoretic filter for separation and recovery of biological cells in water

Dielectrophoresis (DEP) is the electrokinetic motion of dielectrically polarized particles in nonuniform electric fields. DEP has found many useful technological applications including separation, levitation, and characterization of dielectric particles such as biological cells. In this study, the authors demonstrated continuous separation and recovery of biological cells suspended in water using a DEP filter. The DEP filter consists of an electrode system that is filled up with many glass beads. These glass beads modify the electric field distribution in the electrode system so that strong DEP force is generated on their surfaces. If the DEP force is stronger than drag force exerted by liquid flow in the filter, the suspended particles can be trapped and eliminated from the flowing liquid. The DEP filter can control trapping and releasing process just by changing electrode energizing AC signal and the resultant DEP force. It was experimentally confirmed that the DEP filter could continuously eliminate yeast cells suspended in water. The cell density decreased from 10/sup 6/ to 10/sup 1/ cells/mL in about 1 h. The electrical conductivity of the medium was a crucial parameter that influenced the liquid temperature by Joule heating and DEP force. Furthermore, the selective separation of viable and nonviable yeast cells was demonstrated by utilizing the viability dependency of the DEP force.

Selective detection of specific bacteria using dielectrophoretic impedance measurement method combined with an antigen-antibody reaction

Abstract This paper describes a new selective detection method for specific bacteria by using a dielectrophoretic impedance measurement method combined with an antigen–antibody reaction. The authors have previously proposed a bacteria detection technique called dielectrophoretic impedance measurement (DEPIM) using positive dielectrophoretic force to capture bacteria in suspension onto an interdigitated microelectrode array. In this paper, the authors propose a selective bacteria detection method using DEPIM combined with an antigen–antibody reaction. A suspension containing Escherichia coli (E. coli) and Serratia marcescens (Serratia) was tested as a sample specimen. Those two bacteria were captured onto a microelectrode by positive dielectrophoresis in almost equal amounts. Agglutination of target bacteria caused by an antigen–antibody reaction was combined with conventional DEPIM in two different ways. As a result of agglutination, target bacteria became larger than nontarget ones and could experience higher dielectrophoretic force. In one method, E. coli and Serratia were trapped together under positive dielectrophoresis and then agglutinated E. coli was selectively left in the electrode gap by washing process. In the other method, agglutination products, which had been produced in advance, were selectively trapped and detected by DEPIM. It was experimentally confirmed that proposed two methods could selectively detect E. coli from the mix suspension as long as E. coli population was more than that of Serratia.

Preparation of water-soluble carbon nanotubes using a pulsed streamer discharge in water

A novel technique for the preparation of water-soluble carbon nanotubes was demonstrated using a pulsed streamer discharge generated in water. The technique involved chemical reactions between radicals generated by the pulsed streamer discharge and carbon nanotubes. The pulsed streamer-treated carbon nanotubes were homogeneously dispersed and well solubilized in water for a month or longer. The mechanism of solubilization of carbon nanotubes by the pulsed streamer discharge is discussed based on FTIR spectroscopy and optical emission spectra measurements. FTIR spectroscopy revealed that -OH groups, which are known to impart a hydrophilic nature to carbon material, were introduced on the carbon nanotube surface. Optical emission spectra from the pulsed streamer plasma showed that highly oxidative O(*) and H(*) radicals were generated in water. These results suggest that the functionalization of the carbon nanotube surface by -OH group can be attributed to the O(*) and H(*) radicals. An advantage of the proposed method is that there is no need for any chemical agents or additives for solubilization. Chemical agents for solubilization are generated from the water itself by the electrochemical reactions induced by the pulsed streamer discharge.

Analysis of PD-generated SF/sub 6/ decomposition gases adsorbed on carbon nanotubes

Chemical byproducts analysis has been recognized as a powerful diagnosis method for SF6 gas-insulated switchgear (GIS). The authors have previously demonstrated that a carbon nanotube (CNT) gas sensor could detect partial discharge (PD) generated in SF6 gas. However, PD-generated decomposition gas species, which were responsible for the CNT gas sensor response, have not been identified yet. In this paper, two kinds of experiments were conducted in order to identify the responsible decomposition gas species. At first, the decomposition gas molecules adsorbed on CNTs were analyzed by Fourier transformation infrared (FTIR) spectroscopy. FTIR absorbance was observed around 735 cm-1 after CNTs were exposed to PD generated in SF6. In the second experiment, the CNT gas sensor responses to typical SF6 decomposition products (HF and SF4) were examined. The CNT gas sensor responded to these gases in the same way as to PD generated in SF6. SF4 response was larger than HF response. Based on these results, SF 4 and SOF2 emerged as candidates for the responsible decomposition gases. Electrochemical interactions between adsorbed gas molecules and CNT were discussed based on theoretical predictions of molecular orbital calculations. The calculation results suggested that both of SOF2 and SF4 could increase the CNT gas sensor conductance

Enhancement of microplasma-based water-solubilization of single-walled carbon nanotubes using gas bubbling in water

The authors have previously proposed a novel technique for the preparation of water-soluble carbon nanotubes (CNTs) using microplasma generated by a pulsed streamer discharge in water. This paper describes an improvement in the method of the microplasma-based CNT solubilization process by the use of gas bubbling in water. Oxygen, argon and nitrogen were used as bubbling gas in order to clarify the effects of the gas species on the single-walled CNT (SWCNT) solubilization efficiency. Ultraviolet‐visible absorption spectra of the SWCNT suspensions revealed that the SWCNT solubility was increased by more than two times by using gas bubbling together with microplasma treatment. No significant difference was observed among the three gas species tested. Fourier transform infrared (FTIR)spectroscopy and x-ray photoelectron spectroscopy (XPS) analysis showed that the number of ‐OH groups, introduced on the SWCNT surface by the microplasma treatment, was increased by gas bubbling. Optical emission measurements also showed that the number of highly oxidative oxygen and hydrogen radicals, which were generated by the microplasma, was also increased by gas bubbling. These results indicate that gas bubbling has positive effects on microplasma-based SWCNT solubilization as a result of enhanced radical formation and functionalization of the SWCNT surface. (Some figures in this article are in colour only in the electronic version)

Development of rapid oral bacteria detection apparatus based on dielectrophoretic impedance measurement method

In this study, a bacteria detection apparatus based on dielectrophoretic impedance measurement (DEPIM) method was demonstrated for rapid evaluation of oral hygiene. The authors integrated a micro electrode chip on which bacteria were captured by dielectrophoresis (DEP), an AC voltage source to induce DEP force, and an impedance measurement circuit to a portable instrument that enables rapid and automated oral bacterial inspection in hospitals and clinics. Special considerations have been made on effects of high electrical conductivity of oral samples on DEP force and DEPIM results. It was shown experimentally and theoretically that using a higher electric field frequency for the DEP bacteria trap and the impedance measurement could realise DEPIM application to bacteria inspection from oral samples with higher conductivity. Based on these investigations, the authors optimised the frequency condition of the DEPIM suitable for inspecting an oral sample along with the design and development of a portable DEPIM apparatus for on-site inspection of oral bacteria. Under the optimised frequency condition, DEPIM results were in good agreement with the conventional culture method showing significant applicability of the DEPIM apparatus for practical rapid oral bacteria inspection.

Breakdown characteristics of cryogenic gaseous nitrogen and estimation of its electrical insulation properties

The electrical properties of liquid nitrogen and its vaporized gas have become of great interest since the discovery of high-temperature superconducting materials. The authors present the breakdown characteristics of gaseous nitrogen from room temperature down to 74 K. With uniform field gap geometry, the validity of Paschens law is confirmed even under the saturated gas condition at 74 K. In the case of a nonuniform field gap, there is a remarkable a difference in the effects of positive and negative polarities on corona stabilization. A wider gap length is necessary for stable negative corona formation than for positive corona at temperatures >