4.3.3. Potentiometric Biosensor

Characteristics of Potentiometric Sensor. The potentiometric sensor (the base sensor) has the characteristic that are different from amperometric sensors. These are:

1. The measured species (such as H+, NH4+, etc.) is not consumed.
2. The sensor measures the activity (for dilute solutions, molar concentration can be used), of a specific ion (Co) in reference to its internal standard (Ci).
3. The output is in voltage (Em).
4. The ion specificity comes entirely from the membrane. For example, there is a membranes specific to H+, one for NH4+, one for Ca++, etc.
5. The output of the base sensor is in voltage; independent of the sensor size; and proportional to natural log of Co.

(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

Penicillinase

Penicillin -----------------------> penicillic acid + H+

Response: 52 mV/decade over 5x10-2 to 10-4 M.

2. Glucose biosensor

Glucose oxidase

Glucose ------------------------> gluconic acid + H2O2

Response: log linear response from 0.1 mM to 1 mM

3. Urea biosensor

Urease

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.