Page 359 - Organic Electronics in Sensors and Biotechnology

336 Chapter Nine

in the geometry of the LCP. The most sensitive biosensors based on
conjugated polymers reported in the literature are utilizing changes
in the absorption or emission properties from the conjugated poly-
mers. Normally, fluorescence is the preferable method of choice for
detection, as it is a widely used and rapidly expanding method in
chemical sensing. Aside from inherent sensitivity, this method offers
diverse transduction schemes based upon changes in intensity, energy
transfer, wavelength (excitation and emission), polarization and life-
time. Optical sensors, based on LCPs, can be divided mainly into two
different types, depending on which detection scheme is used. Sche-
matic drawings of the two detection schemes for the detection of
DNA hybridization are shown in Fig. 9.3.
In the first approach, superquenching of the fluorescence from
the conjugated polymer chain is used, where a single site of quench-
ing causes loss of fluorescence from the complete chain. 8, 50, 54–63 The
quenching may be due to fluorescence resonance energy transfer
(FRET) or excitation quenching. If a biomolecule labeled with a
quencher is coordinated in close vicinity to the polymer chain by
multiple noncovalent interactions (electrostatic or hydrophobic inter-
actions), it is possible to detect the presence of a certain biomolecule
in a sample by the quenching of the emitted light from the LCP. A
wide range of biosensors, including sensors for DNA hybridization
as well as ligand-receptor interactions and enzymatic activity, utiliz-
ing the impact of biomolecules on these conditions for FRET or exci-
tation transfer have been reported. 64–79
The second type of biosensors is based on detection of biological
processes through their impact on the conformation and the geometry
of the conjugated polymer chains. 7, 13, 52, 53, 80–85 Similar to the first tech-
nique described above, a complex between the conjugated polymer
and a certain biomolecule is being formed due to noncovalent interac-
tions. The complex being formed can then be studied in situ as the
conformational flexibility of LCPs allows direct correlation between
the geometry of chains and the resulting electronic structure and opti-
cal processes such as absorption and emission. If conformational
changes of the biomolecule or other biomolecular events can lead to
different conformations of the associated polymer backbone, an altera-
tion of the absorption and emission properties of the polymer will be
observed. Hence, this phenomenon can be used as a sensing element
for a wide range of biological events, appropriate for making novel
biosensors. Similar to the quenching method described above, this sec-
ond technique has been used to detect DNA hybridization and ligand-
receptor interactions. 7, 53, 81, 83-86 However, utilizing the structurally
induced optical changes of the conjugated polymer backbone also
allows the tantalizing possibility to detect conformational changes of
biomolecules. This has been verified by using LCPs to detect confor-
mational changes in synthetic peptides, 87, 88 and calcium-induced con-
86
formational changes in calmodulin. The detection of these biological