The Origin of Sensor Response

Discussion of general operation of conductometric sensors can begin by analyzing Fig. 8.1. Once again we realize that the electrochemical cell is a complex arrangement of resistances and capacitances. The primary interaction between the sample and the sensor involves the selective layer the sensory information is 

Because resistance is an intensive property (i.e., size-dependent), it is possible to localize the source of the signal at one electrode by making its contact area with the selective layer smaller than that of the other electrode. This is exactly the same argument as we have used in the discussion of the amperometric sensing circuit (Chapter 7). 

Turning to Fig. 8.1a, we can dissect the overall impedance between points A and B into partial impedances (Z) and admittances (Y) and represent them with an equivalent impedance circuit. The overall impedance of this circuit is then 

The same consideration applies to the impedance measurement according to Fig. 8.1b. It is a normal electrochemical interface to which the Warburg element (Zw) has been added. This element corresponds to resistance due to translational motion (i.e., diffusion) of mobile oxidized and reduced species in the depletion layer due to the periodically changing excitation signal. This refinement of the charge-transfer resistance (see (5.23), Chapter 5) is linked to the electrochemical reaction which adds a characteristic line at 45° to the Nyquist plot at low frequencies (Fig. 8.2) 

Equation (8.6) is a simplified approximation. For an exact definition, please see, for example, Bard and Faulkner (2001). It is mentioned here because it appears at low frequency (0-20 Hz) excitations. At higher ( 100Hz) frequencies, the second term can be neglected and the Nyquist plot reverts to the simple semicircle. 

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