Page 357 - Organic Electronics in Sensors and Biotechnology

334 Chapter Nine

570 nm, indicating that an aggregation of polyelectrolyte chains takes
place at alkaline pH but not at acidic pH.
The photoluminescence (fluorescence) efficiency of conjugated
polymers is also dependent on the geometry of the polymer back-
bone, especially the separation or aggregation of polymer chains. The
intensity of the fluorescence of the aggregated phase of polythio-
phene derivatives compared with the fluorescence of the single-chain
state has been shown to be weaker by approximately one order of
magnitude. 40–42 Similarly, studies of thin films of POWT (Fig. 9.1) have
shown an analogous trend in the photoluminescence upon separa-
tion and aggregation of the polymer chains. 43, 44 As the polymer chains
were separated, the photoluminescence maximum was also blue-
shifted by approximately 105 nm, compared to the dense packing of
the polymer chains. Similar changes also occur when placing POWT
36
in different buffer solutions (Fig. 9.2b). At pH 5, when the polymer
side chains largely have a neutral net charge, the polymer chains
adopt a nonplanar conformation, and the chains are separated, seen
as a blue-shifted emission maximum and an increase of the intensity
of the emitted light. In more alkaline pH (pH 8) the POWT peak emis-
sion is at a longer wavelength and with decreased intensity, related to
a more planar backbone and aggregation of polymer chains. At acidic
pH (pH 2), light with a slightly longer wavelength (relative to pH 5)
is emitted, but the intensity of the fluorescence is not decreasing in
the same way as observed for POWT in alkaline buffer solution.
Hence, an acidic pH seems to favor a more rod shape conformation of
the polymer chains, but aggregation of the polyelectrolyte chains is
presumably absent. A schematic drawing of the polymer chain con-
formations for POWT in different buffer solutions and the conforma-
tional induced optical transitions relating to these geometric changes
are shown in Fig. 9.2c. 36

9.2.3 Conjugated Polymers as Optical Sensors
The application of conjugated polymers for colorimetric detection of
biological targets (biochromism) was first described by Charych and
coworkers in 1993. The technique is utilizing a ligand-functionalized
45
conjugated polymer, which undergoes a colorimetric transition (coil-
to-rod transition of the conjugated backbone) upon interaction with a
receptor molecule of interest (Fig. 9.3). The specificity in this first gen-
eration of conjugated polymer-based biosensors is due to the covalent
integration of distinct ligands on the side chains of the conjugated
polymers. Ligand-functionalized versions of polydiacetylenes have
been used extensively for colorimetric detection of molecular interac-
tions, 45–49 and polythiophene derivatives that display biotin 21–23 and
24
different carbohydrates have been synthesized and shown to
undergo colorimetric transitions in response to binding of strepta-
vidin and different types of bacteria and viruses, respectively.