Amperometric limiting current oxygen sensor

For special investigations in biomedical research miniaturized amperometric oxygen sensors have been developed. At the sensor shown in Fig. 7 the diameter of the ft cathode is only 30 pm. The resulting limited current of such a sensor is in the range Id — nA at the oxygen partial pressure = 21 kPa. 

Previous efforts relevant to oxygen transport have used two-electrode amperometric sensors such as the Clark and Mackereth cells (Clark, 1959). These sensors are inexpensive, accurate, and small, but they require frequent calibration, consume oxygen, and suffer from long-term drift. Other options include cyclic voltammetric sensors with remote three-electrode cells, and variations on galvanic techniques to monitor limiting currents at steady state (Fan et al., 1991 Haug and White, 2000 Utaka et al., 2009). 

There are two major sensors classes that use liquid electrolytes amperometric and poten-tiometric sensors. The earliest example of an amperometric gas sensor, the Clark oxygen sensor used for the measurement of oxygen in the blood is more than 40 years old. The amperometric sensor produces current signal, which is related to the concentration of the analyte by Faraday s law and the laws of mass transport. It is operated in a region where mass transport is limiting and therefore has a linear response with concentration of the analyte. This type of sensor has now been developed in many different geometries with different types of material coated or uncoated to enhance sensitivity and selectivity (Figure 15.5). 

In amperometric sensors (Fig. 19.2 right) " raie electrode is covered by a diffusion-limiting layer (also a chamber with small holes can be used) so that the transport of oxygen to the electrode is the rate-limiting step. This is schematically shown in Fig. 19.3. When the electrode is polarised by an external voltage, the current increases as long as enough electrochemicaUy active species are present. 

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