Page 353 - Organic Electronics in Sensors and Biotechnology

330 Chapter Nine

or the conformation-sensitive optical properties of the conjugated
polymers. LCPs offer a diverse sensor platform and can be used in a
wide range of biomolecular recognition schemes to obtain sensory
responses. Biosensors based on conjugated polymers are sensitive to
very minor perturbations, due to amplification by a collective system
response, and offer a key advantage compared to small-molecules
based sensors.
Conjugated polymers are made of several repeating units, mers,
and a wide range of biological active polymers can also be found in
nature. For instance, the molecule carrying all the genetic informa-
tion, DNA, has a repetitive helical structure made up from four
nucleotides, whereas 20 amino acids are used to create a diversity of
polypeptide chains that are folded into functional proteins. This
molecular similarity between conjugated polymers and biological
polymers offers a great possibility to create simple versatile biosen-
sors, as these two classes of molecules are able to form strong com-
plexes with each other due to multiple noncovalent interactions. The
ability of conjugated polymers to noncovalently interact with individ-
ual biomolecules, such as proteins, and afford an optical fingerprint
corresponding to a distinct conformational state of this biomolecule
sets these molecules apart from conventional dyes and other sensor
technologies, potentially enabling novel technologies for studying
biological processes in a more refined manner. Most conventional
techniques are limited by their reliance on detecting a certain biomol-
ecule, whereas the LCPs are identifying a specific structural motif or
a distinct conformational state of a biomolecule. Hence, the LCPs
offer a possibility to monitor the biochemical activity of biological
events on the basis of a structure-function relationship rather than on
a molecular basis.
The unique conformational-sensitive optical properties of LCPs
have proved to be a great asset for studying protein misfolding and
aggregation. As the aggregation of proteins is associated with a wide
range of serious diseases, the LCP technique can also be used to gain
increasing knowledge regarding the pathological events of such
diseases. In this chapter, the molecular structure and the optical
properties of LCPs, as well as the use of LCPs as optical sensors for
biological events, especially protein aggregation, will be discussed.

9.2 Luminescent Conjugated Polymers

9.2.1 Definition and Examples
In the unsubstituted form, conjugated polymers are insoluble, but
with proper chemical modifications of the polymer backbone they
can be dissolved in organic solvents. However, the use of conjugated
polymers as detecting elements for biological molecules requires that