Sensor signals, concentration dependence

Due to the proposed elementary steps of this sensing principle and the signal transduction from the surface reaction to a change of band structure and consequentially, to a change of resistance, as described in detail in (Weimar, 2002), the dependency of the sensor resistance from the concentration of analyte is logarithmic and the resistance change is dependent on the baseline value. In order to have a more transferable parameter for the characterization of sensor performance, the sensor signal S is defined as ... 

These examples are based on both electrodes operating in the activation polarization regime, in which the logarithm of the current is proportional to the overpotential. However, there are situations - particularly at low concentrations - in which the electrochemical reaction is limited by mass transport to the electrode surface. This is referred to as concentration polarization, and is illustrated in Figure 13.2d. In this case, above a critical overpotential the current becomes constant, which appears as a vertical line in the plot. A new mixed potential is established at the intersection of this vertical line and the cathode polarization for the oxygen reduction. This potential depends on the gas concentration, and thus can be used for the chemical sensor signal. 

During operation, Li2O dissolves in the carbonate electrode and the equilibrium between lithium and oxygen ions depends on the activity ofthe dissolved Li2O. Any consequent changes in the concentration of dissolved Li2O can lead to drifts in the sensor signal over time [209, 210],... 

Fig. 108. Concentration dependence of the signals for aspartame, its amino acid constituents, and glucose of a sensor containing B. subtilis cells grown on aspartame. (Redrawn from Renneberg et al., 1985).

The semilogarithmic plot of the sensor signal versus sulfate concentration showed that the inhibition was linearly dependent on PSO4" be-... 

This chapter primarily deals with the issue of correlation between sensor signal and analyte concentration in chemical measurements, i.e. the creation of a model based on standards, and the estimation of unknown samples based on that model. Sensor calibration is dependent upon the type of sensor signal such as linear versus non-linear response and sensor format involving one or many sensors simultaneously. The advantages of moving from one sensor to several sensors and from several sensors to several sensors coupled with analyte concentration modulation can yield remarkable information about a sample [1]. Each level of sensor system complexity, when coupled with the proper analysis tools, creates an unique situation which yields information about each component in a mixture and potential interferences. 

Fig. 15 Sensor signal (a) and its temperature dependence (b) for PNIPAAm (MBAAm 4) at different concentrations of NaCl and Nal salts. Reprinted from (Guenther et al. 2006,2007b) with kind permission from SPIE and Elsevier...

In practice, the dependence of the sensor signal on the concentration tp of certain components B in one of the electrode compartments, is of interest. It is useful to divide between the electrodes of the cell in order to ascertain how the signal function U (

defines electrode potentials and considers electrode potential equations which describe the dependence on temperature and composition quantities of the potential-forming substances. 

The ratio of the rate of the enzymatic reaction to that of diffusion indicates whether the process in an enzyme layer is determined by enzyme kinetics or by substrate diffusion. At low enzyme activity, the process is kinetically controlled. In this case the substrate concentration does not become zero in any part of the enzyme layer, that is, the enzyme sensor signal is mainly a function of the active enzyme concentration. Therefore, effectors (activators, inhibiting factors, including H+, and OH ) and, the amount of enzyme in front of the tranducer, as well as the time-dependent enzyme inactivation, may all directly effect the measuring signal. 

The same approach can be utilized in the construction of sensors cathodicaly monitoring the oxygen demand. Beside elimination of interfering effects, a signal is then produced which is directly glucose-concentration-dependent and, in a certain range, independent of oxygen concentratin in the electrolyte (a buffer solution with the compounds needed to obtain the optimal rate of enzymatic reaction). 

---