4.3.4. Potentiometric Biosensor
Characteristics of Potentiometric Sensor. The potentiometric sensor (the base sensor) has the characteristic that are different from amperometric sensors. These are:
(Note: for symbols refer to Fig. 2.15).
Examples of Potentiometric Biosensor. Many
biosensors have been developed using pH sensor as the base transducer. Some
of the examples and their performances are
illustrated in the following.
1. Penicillin biosensor
Penicillin -----------------------> penicillic acid + H+
Response: 52 mV/decade over 5x10-2 to 10-4 M.
2. Glucose biosensor
Glucose ------------------------> gluconic acid + H2O2
Response: log linear response from 0.1 mM to 1 mM
3. Urea biosensor
Urea ----------------------------- 2 NH4+ +NCO3-
Response: log linear response from 5x10-5 to 5x10-3
Configuration of Biosensor. For potentiometric
biosensors, the outer membrane is often not used.Often,
the inner membrane is not used either. In such a case, the enzyme is immobilized
directly on the surface of the potentiometric sensor.
Model geometry and assumptions used for the modeling
Same as those of the amperometric biosensor.
Steady State Solutions for the Case When [S] << KM
Equation and Bcs. The same equations (Eq. (8) through Eq. (12b)) are used to obtain the steady state solutions. The only change that has to be made is B.C.1 for the product. B.C. 1 has to be replaced by:
B.C. 1: @ x = 0, d[P]/dx = 0 (no transport; P is not consumed)
The solutions are:
Solution. Sensor signal output will be proportional to [P] at x = 0:
Sensor Output. For sensors under diffusion control (fE > 25):
It has to be noted that under this condition, the sensor output Eout is proportional to the natural log the analyte concentration S, while the temperature dependence is linear:
Effect of enzyme loading factor on sensor output
Fig. 4. 8 shows the effect of enzyme loading on sensor output.
Fig. 4.8. Enzyme loading effect on potentiometric biosensor.