Conducting polymers such as polypyrrole (PPy), polythiophene (PT), polyaniline (PANI), and their derivatives, are truly multifunctional materials. The unique electrical, electrochemical and optical properties of conjugated polymers can be used to convert chemical information (concentration, activity, partial pressure) into electrical or optical signals in the solid state. Therefore, conjugated polymes are useful as transducers in chemical sensors. One of the key issues is to combine conjugated polymer transducers with selective molecular recognition in order to obtain durable chemical sensors.
Conducting polymer modified electrodes can be obtained by electrochemical polymerization of various monomers with different substituents and counterions. Alternatively, soluble conjugated polymers may be cast on the electrode surface from solution. Since conducting polymers are electroactive materials with mixed ionic and electronic conductivity, they can interact with various ions and redox species in the surrounding medium in a relatively non-selective manner. Therefore the goal is often to immobilize specific recognition sites (receptors) in the conducting polymer film in order to achieve selectivity to a certain target species. Such recognition sites can be immobilized as counterions or the recognition sites can be covalently bound to the conjugated polymer backbone.
Interactions between the target species and the conducting polymer film may influence the:
- optical properties of the polymer film
- electrical conductivity of the polymer film
- oxidation/reduction current at the polymer modified electrode
- equilibrium potential of the polymer modified electrode
The basic principles of chemical sensors based on conducting polymers will be illustrated in this tutorial lecture by considering the following three types of electrochemical sensors:
1. Conductimetric gas sensors
2. Amperometric biosensors
3. Potentiometric ion sensors