Nanotechnology-Enabled Sensors

7.3 Surface Materials and Surface Modification 397
Fig. 7.23 Polyaniline nanofibres grown by a chemical polymerization technique.
Angelopoulos et al have compared the IR spectra of emeraldine and leucoemeraldine
base polyaniline films (Fig. 7.24). 67,68 They concluded that
the major spectroscopic signatures for polyaniline are generally located between
600 to 1700 cm –1 and 3000 to 3500 cm –1 .
In order to investigate the directionality of polyaniline nanofibres IR
spectroscopy can also be employed. Liu et al 60 showed that for nanofibres
which are aligned to the surface, there is a strong p-polarized peak at
3350 cm –1 which cannot be observed in the bulk polyaniline spectrum.
This peak implies that, in the films that were deposited, the N-H bonds are
nearly perpendicular to the surface.
ICPs posses the ability to change color when their conductivity changes
during a redox process (after interacting with a target analyte) and this
property can be utilized in the fabrication of optical sensors. As can be
seen Fig. 7.25, the UV-visible absorption spectrum of the emeraldine base
polyaniline contains two peaks at approximately 330 nm (3.75 eV) and
650 nm (1.95 eV). The absorption spectra for the pernigraniline base state
show peaks at approximately 280 nm (4.3 eV), 330 nm (3.8 eV) and
530 nm (2.3 eV). For the leucoemeraldine base state only one peak at
340 nm (3.6 eV) is observed. 67 These values change dramatically after the
polyaniline is doped or when it is in nanofibre form.