Optical Reporting by Conjugated Polymers via Conformational Changes - Advanced Fluorescence Reporters in Chemistry and Biology - page 394

changing the pH of the polymer solution [ 9 ]. The absorption spectra of POWT in
different buffer systems are shown in Fig. 5a . At pH 5, where most of the side
chains are zwitter-ionic (both negatively and positively charged), the polymer
backbone is in a nonplanar helical conformation, associated with a blue-shifted
absorption spectrum. When the pH is increased or decreased, the backbone of the
CP adopts a more planar conformation, seen as a red shift of the absorption
maximum. A closer look at the spectra for POWT in pH 2 and pH 8 buffer solutions
(Fig. 5a ) reveals an interesting observation. The absorption maximum is similar but
the spectrum recorded for POWT in the pH 8 buffer solution has a shoulder in the
region at 570 nm, indicating that an aggregation of polymer chains (stacking of the
thiophene rings) takes place at alkaline pH but not at acidic pH.
The emission of POWT is also dependent on the geometry of the polymer
backbone, especially the separation or aggregation of polymer chains. Studies of
thin films of POWT have shown that the intensity of the fluorescence for the
aggregated phase of POWT is weaker by approximately one order of magnitude
compared with the fluorescence for the single chain state [ 10 , 11 ]. 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 in different buffer solutions [ 9 ].
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 in the intensity of the
emitted light. In more alkaline pH (pH 8), the POWT emits light with a longer
wavelength and with decreased intensity, related to a more planar backbone and
aggregation of polymer chains. At acidic pH, light with a slightly longer wave-
length is emitted, but the intensity of the fluorescence is not decreasing in the same
way as observed for POWT in alkaline buffer solution. Consequently, an acidic pH
seems to favor a more rod shape conformation of the polymer chains, but aggrega-
tion of the polyelectrolyte chains is presumed to be absent. A schematic drawing of
the polymer chain conformations for POWT in different buffer solutions and the
conformation-induced optical transitions relating to these geometrical changes are
shown in Fig. 5c .
3 Covalent Ligand Sensors
The first generation of chromic sensors based on CPs was generally utilizing ligand-
functionalized CPs. Hence, the receptor (sensing element) was covalently attached
to the CP backbone (the transducer) and this receptor-functionalized CP undergoes
a colorimetric transition (coil-to-rod transition of the conjugated backbone) upon
interaction with a receptor molecule of interest. The specificity in this first genera-
tion of CP-based biosensors is due to the covalent integration of distinct ligands on
the side chains of the CPs, and this technique has been employed mainly for the
detection of specific ions and a wide range of biological targets.