Nanotechnology-Enabled Sensors

394 Chapter 7: Organic Nanotechnology Enabled Sensors
Redox doping (ion doping): all conductive polymers can undergo either
p- and/or n- redox doping by chemical and/or electrochemical processes.
These doping processes change the number of electrons associated with
the polymer backbone; hence the conductivity of the polymer is also
changed.
Photo and charge-injection doping: when ICPs are exposed to radiation
whose energy is greater than its band-gap energy of the polymer, electrons
hop across the gap and the polymer undergoes photo-doping. In addition,
charge-injection doping can also be conducted using a metal/insulator/
semiconductor (MIS) configuration involving a metal and a conductive
polymer separated by a thin layer of an insulator with a high dielectric
dielectric strength.
Non-redox doping: in this process, the number of electrons present in a
polymer does not change but instead the energy levels of these electrons
are rearranged during doping. An example of this occurs when an ICP such
as emeraldine base form of polyaniline becomes highly conductive by immersing
the polymer in an acid. Emeraldine base interact with aqueous
protonic acids to produce the protonated emeraldine which is highly conductive.
48,49 This process can increase the conductivity of the polymer by
several orders of magnitude (as demonstrated in Fig. 7.20).
Example: polyaniline
Amongst intrinsically-conductive polymers, polyaniline is one of the
most attractive ones. 50 Polyaniline is easy to synthesis and process and is
environmentally stable. In order to create more efficient sensing properties
for polyaniline, it is possible to synthesis and use its different nanostructured
forms. Such a nanostructured polyaniline can be employed as a sensitive
film, where the smaller dimensions allow the target analyte to diffuse
faster and more easily into the sensitive layer and interacts with the
functional sites.
Polyaniline is composed of reduced benzenoid and oxidized quinoid
units. 50 It contains amine (-NH-) and imine (=N-) functional groups in
equal proportions. Polyaniline has the ability to exist in a wide range of
oxidation states. If we assume that the average oxidation state is given by
1-y as shown in Fig. 7.21 – a then: when y = 1, polyaniline is in the leucoemeraldine
(LM) base state (Fig. 7.21 – b). It is completely oxidized
when y = 0 as Fig. 7.21 – c and is called pernigraniline (PNA) base. The
form that can be doped into a highly conductive state is called emeraldine
base, EB (Fig. – d) with y = 0.5. In this state, the structure consists of an alternating
sequence of two benzenoid and one quinoid units. The emer