Electrode Design and Examples

A very important amperometric sensor is the Clarksensor for determination of dissolved oxygen. The classical macroscopic design is used widely, but miniature forms are also common (Fig. 7.20). 

The Clark sensor is equipped with a gas-permeable membrane located in direct contact with the working electrode surface (Fig. 7.20). The membrane separates the sensor from the sample solution. In this way the electrode surface is protected. Selectivity is increased since only gaseous components can reach the active area. Membranes of macroporous polytetrafluorethylene (PTFE) are used preferably. The material is strongly hydrophobic. Water in liquid state is unable to permeate the membrane pores. Unlike liquid water, gases easily diffuse through the pores. Membranes of silicon rubber have also been used with the Clark sensor. They are permeable for oxygen since it is dissolved homogeneously in the matrix. 

The miniature form of the Clark sensor depicted in Fig. 7.20 contains an anode and a cathode as well as a small electrolyte stock (chloride solution containing gel-forming agents) behind the membrane. The technology for manufacturing such sensors differs from standard microelectronic techniques. It had to be developed especially for sensor applications (Suzuki 2000). 

Ligands belonging to the groups of calixarenes and cyclodextrines were mentioned in connection with potentiometric sensors (Fig. 7.8). They can be 

An intramolecular channel exists with SAMs formed by aggregation of a cyclodextrine derivate, the per-6-thio- -cyclodextrine at a gold electrode (D Aimibale et al. 1999). Ferrocenylcarboxylate or hydroquinone are used as electrochemicaUy active markers. Their signal decreases if certain neutral molecules like trimethylcetylammonium are present in the sample solution. The strongly selective complex formation with cones of cyclodextrines somehow clogs the internal channels. 

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