Solid-state microelectrode oxygen sensors

Two types of all-solid amperometric sensors, incorporating Nafion(R) as the solid ionic conductor and exhibiting microelectrode behaviour, have been u sed as gaseous oxygen probes in the 1-25% (v/v) concentration range. The fi rst one involved a Pt or Ag microdisc surrounded by a ring of the anode mat erial, the two electrodes being in the same plane and covered by a solution -recast Nafion(R) film. In a modified version of this type of sensor, a Au microband was used as the indicator electrode. Well-defined sigmoidal volta mmograms for oxygen reduction from the gas phase were obtained and, when us ed in a constant potential mode, the sensing device showed good linearity ( r(2)=0.9998-0.9988) with oxygen concentration in the gas stream and a satis factory sensitivity of 8x10(-5) A cm(-2) (%v/v)(-1). The high detection lim it of 2% (v/v), however, restricts the possible applications to crude monit oring in the higher oxygen concentration range. In the second type of senso r, Au electrodes were vacuum-deposited as thin layers on the same face of a Nafion(R) membrane and a strip of uncovered ionic polymer between the two metal layers ensured completion of the electrical circuit. The gas samples were in contact with the electrode layers and oxygen reduction led to an ex ponential current rise over a wide potential range indicating very high mas s transport rates. The sensitivity of the device was 20 nA (%v/v)(-1) and i t is suggested that with such a sensor configuration the electroactive gas reacts at the line formed by the gas/solid electrolyte/metal layer interfac e, i.e. at a virtual microband electrode. (C) 1999 Elsevier Science B.V. Al l rights reserved.