Selective Detection of Nitrogen-Containing Compound Gases.

N-containing gaseous compounds, such as trimethylamine (TMA), triethylamine (TEA), ammonia (NH3), nitrogen monoxide (NO), and nitrogen dioxide (NO2) exude irritating odors and are harmful to the human respiratory system at high concentrations. In this study, we investigated the sensing responses of five sensor materials-Al-doped ZnO (AZO) nanoparticles (NPs), Pt-loaded AZO NPs, a Pt-loaded WO3 (Pt-WO3) thin film, an Au-loaded WO3 (Au-WO3) thin film, and N-doped graphene-to the five aforementioned gases at a concentration of 10 parts per million (ppm). The ZnO- and WO3-based materials exhibited n-type semiconducting behavior, and their responses to tertiary amines were significantly higher than those of nitric oxides. The N-doped graphene exhibited p-type semiconducting behavior and responded only to nitric oxides. The Au- and Pt-WO3 thin films exhibited extremely high responses of approximately 100,000 for 10 ppm of triethylamine (TEA) and approximately -2700 for 10 ppm of NO2, respectively. These sensing responses are superior to those of previously reported sensors based on semiconducting metal oxides. On the basis of the sensing response results, we drew radar plots, which indicated that selective pattern recognition could be achieved by using the five sensing materials together. Thus, we demonstrated the possibility to distinguish each type of gas by applying the patterns to recognition techniques.

SnO2 nanoslab as NO2 sensor: identification of the NO2 sensing mechanism on a SnO2 surface.

Among the various metal oxides, SnO2 has been most widely exploited as a semiconductor gas sensor for its excellent functionalities. Models illustrating the sensing mechanism of SnO2 have been proposed and tested to explain experimentally derived "power laws". The models, however, are often based on somewhat simplistic assumptions; for instance, the net charge transfer from an adsorbate to a sensor surface site is assumed to occur only by integer values independent of the crystallographic planes. In this work, we use layer-shaped SnO2 crystallites with one nanodimension (1ND-crystallites) as NO2 gas sensing elements under flat band conditions, and derive appropriate "power laws" by combining the dynamics of gas molecules on the sensor surface with a depletion theory of semiconductor. Our experimentally measured sensor response as a function of NO2 concentration when compared with the theoretically derived power law indicates that sensing occurs primarily through the chemisorption of single NO2 molecules at oxygen vacancy sites on the sensor surface.

A surface acoustic wave-based immunosensing device using a nanocrystalline ZnO film on Si.

The novelty of this study resides in the fabrication of a bio-sensing device, based on the surface acoustic wave (SAW) on a nanocrystalline ZnO film. The ZnO film was deposited using an rf magnetron sputtering at room temperature on silicon. The deposited films showed the c-axis-oriented crystallite with grain size of approximately 40 nm. The immunosensing device was fabricated using photolithographic protocols on the film. As a model biomolecular recognition and immunosensing, biospecific interaction between a 6-(2,4-dinitrophenyl)aminohexanoic acid (DNP) antigen and its antibody was employed, demonstrating the shifts of resonant frequencies on SAW immunosensing device. The device exhibited a linearity as a function of the antibody concentration in the range of 20-20,000 ng/ml.

Nanocrystalline ZnO film layer on silicon and its application to surface acoustic wave-based streaming.

A surface acoustic wave (SAW) device consisting of 1-6 microm-thick ZnO thin films deposited on Si wafer was designed, fabricated, and characterized in this study. Photolithographic protocols for interdigitated transducers (IDTs) and surface modification using fluoroalkylsilane are employed with the aim of droplet-based microfluidic actuations in bio-microsystems. A ZnO thin film was grown on a 4' silicon wafer with c-axis orientation, an average roughness of 11.6 nm, and a small grain size of 20 nm. It was found that the resonant frequencies (Rayleigh and Sezawa modes) of SAW devices move to a lower frequency range as the thickness of the ZnO thin films increases. Through the silane surface modification, a hydrophobic surface with a contact angle of 114 degrees was obtained. Finally, liquid streaming by acoustic wave was demonstrated by observing the actuation of SiO2 microparticles in a microfluidic drop.

On-chip deposition of carbon nanotubes using CMOS microhotplates.

The direct deposition of carbon nanotubes on CMOS microhotplates is demonstrated in this paper. Tungsten microhotplates, fabricated on thin SOI membranes aside CMOS control circuitry, are used to locally grow carbon nanotubes by chemical vapour deposition. Unlike bulk heating of the entire chip, which could cause degradation to CMOS devices and interconnects due to high growth temperatures in excess of 500 °C, this novel technique allows carbon nanotubes to be grown on-chip in localized regions. The microfabricated heaters are thermally isolated from the rest of the CMOS chip as they are on the membranes. This allows carbon nanotubes to be grown alongside CMOS circuitry on the same wafer without any external heating, thus enabling new applications (e.g. smart gas sensing) where the integration of CMOS and carbon nanotubes is required.

ZnO nanotips and nanorods on carbon nanotube/Si substrates: anomalous p-type like optical properties of undoped ZnO nanotips.

ZnO nanotips and nanorods were grown on screen-printed multi-walled carbon nanotube (MWCNT) films via thermal chemical vapor deposition at relative low growth temperatures of 400 and 500 °C. Uniform formation of ZnO nanotips and nanorods occurred on MWCNT-printed Si substrates, but were rarely observed on bare Si substrates at the same growth temperatures. In photoluminescence (PL) measurements, it was found that ZnO nanorods exhibit typical intrinsic optical properties, while ZnO nanotips revealed p-type like luminescence behavior. Acceptor-related emission bands originating from neutral acceptor-bound exciton, free-to-acceptor and donor-acceptor pair transitions are clearly observed in temperature-dependent PL spectra of ZnO nanotips.

Separation of apoptotic cells using a microfluidic device.

A highly sensitive microfluidic device has been developed to separate apoptotic cells. Apoptotic Jurkat cells were selectively labeled with magnetic beads (0.8 microm diam) using the C2A protein which recognizes phosphatidylserine. The cell mixture was flowed through a microfluidic channel and apoptotic cells were separated by a 0.3 T permanent magnet. Separations using our device showed 96% agreement with those of a commercial flow cytometer, indicating our device can be used to sort apoptotic cells in a miniaturized system.

Analysis of recombinant protein expression using localized surface plasmon resonance (LSPR).

The localized surface plasmon resonance (LSPR)-based optical biosensor using nano-structures of noble metals has been considered as a useful tool for label-free detection of DNA hybridization and protein-protein interactions. We fabricated LSPR-based optical biosensors using gold nano-islands (nominal thickness; 75 A) on glass substrates that were easily made using the conventional fabrication methods. The formation of gold nano-islands on glass substrates was realized by heat treatment of thin gold film deposited with a low deposition rate (approximately 0.05 A/s). The morphologies of sensor surfaces composed of gold nano-islands were observed using an atomic force microscope (AFM) with a non-contact mode. To investigate the sensing capacity of the gold nano-island sensor for the binding of proteins by affinity interactions, the streptavidin and biotin interaction was used as a model system. In addition, detection of recombinant glutathione-S-transferase (GST)-tagged human interleukin-6 (hIL6) expressed in Escherichia coli was carried out by LSPR. It is expected that the LSPR sensors composed of gold nano-islands can be an alternative to traditional methods such as SDS-polyacrylamide gel electrophoresis (SDS-PAGE) for fast analysis of protein expression.