Takaaki Otagawa

Planar microelectrochemical carbon monoxide sensors

Abstract A series of planar miniaturized electrochemical CO sensors have been fabricated, each comprising three platinum electrodes (sensing, counter and reference) and a solution-cast Nafion film as a solid polymer electrolyte. The Nafion film was chosen for it compatibility with microelectronic fabrication and for its outstanding inertness. The CO sensor response characteristics were evaluated using four different substrates (smooth glass, sandblasted glass, and rougher and smoother ceramic), three different kinds of solution-cast Nafion films, and five different gap sizes (between adjacent microelectrodes) from 5 to 50 μm. The best reproducible performance was obtained from the basic planar-type CO sensor with a 10-μm gap using the smooth ceramic substrate and a 1-μm-thick Nafion 1 film. An optimal ratio of signal to background current was obtained at an applied potential of approximately − 50 mV versus the platinum/air reference electrode, at which both the background current and the CO signal became almost humidity-independent. The response is linear with the CO concentration in air, which yields an estimated sensitivity in the order of 8 pA/ppm with a typical response time in the order of 70 s. Moreover, at the − 50 mV sensing potential, the CO sensor can exclude interfering signals from NO and NO 2 and, to a large extent, interfering signals from ethanol and H 2 . However, a long-term stability test showed a drift on the sensor performance as a function of time, which was most likely due to a drift of the actual potential of the platinum/air reference electrode.

Microelectrochemical sensor for nitrogen oxides

Abstract A sensor for nitrogen oxides is presented based on the Back Cell™ sensor design. This unique sensor design allows response times many times faster than those of conventional amperometric gas sensors to be achieved. Conventional amperometric NO sensors operating at an NO oxidation potential suffer from interference caused by oxidation of NO2, which occurs in the same potential range. In the present sensing scheme, the sensor is operated at two different potentials. At the first potential, both NO and NO2 are oxidized; the second potential produces a reduction current proportional to NO2 concentration alone. This signal can be subtracted from the first signal to obtain an interference-free measurement of the NO concentration. In practice, the oxidation current when both NO and NO2 are present is higher than the sum of their individual currents, indicating a synergistic effect between the two gases. This type of NOx sensor may find application in environmental monitoring.

Multilayer ionic devices fabricated by the plasma-spray method

Abstract Using the plasma spray method, ceramic films can be easily and quickly coated onto a substrate. Zirconia-based oxygen sensors, both potentiometric-mode and amperometric-mode, were fabricated by the plasma-spray method and their oxygen sensing ability was demonstrated. Stabilized zirconia films had sufficient ionic conductivity for use in oxygen sensors. The film permeability was controlled by changing spray conditions, providing flexibility in the design of amperometric-mode sensors. Combined with other thin- and thick-film techniques, and shadow mask technologies, the plasma spray method has great potential in the fabrication of multilayered ceramic devices, especially in batch processes.

Multilayer ionic devices fabricated by thin- and thick-film technologies

Abstract A variety of ionic devices based on thin- and thick-film technologies as well as micromachining techniques have been developed at TSDC, and are discussed in this review paper. Five ionic devices are exampled: a LaF 3 -based fuel cell, LaF 3 smart humidity/temperature sensor, room-temperature micro-electrochemical smart sensor (RT-MECSS), back cell electrochemical gas sensor, and air electrolyte gas sensor. The benefits and problems in merging thin- and thick-film technologies in developing these devices are discussed.

Electrolytic media for chemical sensors

Abstract Four types of all-solid-state chemical sensors are discussed. Three use introduced ionic media (hydrogels, solid-polymer electrolytes and composites, solid electrolytes) and one spaces the electrodes so close together (submicron spacing) that measurements can be made in air without any further ionic medium. The resulting chemical sensors have application in a variety of fields and the likehood of growth in the use of each type is assessed.

Catheter-Based Micromachined Electrochemical Sensors

In this paper, we discuss the recent progress in the development at SRI International of a catheter-based macromachined electrochemical sensor - a micron-sized, silicon-based electrochemical sensor for in-vivo monitoring of pH, CO2, and O2. The manufacture of the sensor is based on a generic technology for producing electrochemical sensors in silicon, which we refer to as Room Temperature Micro-Electronic Chemical Smart Sen-sor (RT-MECSS).