PLANAR MICROELECTROCHEMICAL CARBON-MONOXIDE SENSORS

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 NO2 and, to a large extent, interfering signals from ethanol and H2. 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. © 1990.