Optical Reporting by Conjugated Polymers via Conformational Changes - Advanced Fluorescence Reporters in Chemistry and Biology - page 390

1 Introduction

The art of detection is the heart of developing technologies within all scientific
fields. What can be “seen” serves as an indicator or parameter to measure variable
or invariable behavior. Within the fields of chemistry, biology, and medicine, the
detection of small molecules, specific proteins, and biological events are of central
importance. This has mainly been accomplished through means such as the use of
fluorophore-labeled antibodies specific for distinct biomolecules. Such molecules
integrate a sensing element (antibody) with a transducer (fluorophore), the compo-
nent responsible for reporting a change in status of the sensing element and thus
allowing for direct quantification of a distinct bimolecular target.
Conjugated polymers (CPs) represent a useful and interesting class of materials
well known for their abilities as transducers in the form of colorimetric and fluoro-
metric reporting. The CP can serve as a transducer covalently or noncovalently
associated with the biological component or in some instances the CP itself can act
to detect and transduce the signal. CPs provide various means of probing via a
change in their conductivity, potential, color, or fluorescence. Because fluorescence
measurements provide a number of parameters for observing a change in the system
- e.g., changes in intensity, wavelength, energy transfer, and emission lifetime - the
coupling of such measurements with the unique fluorescence reporting capabilities
of CPs has been successful in a number of systems and continues to develop as a
means for “seeing” the unseen. In this chapter, we will attempt to provide an
intuitive and intentionally simplistic description of CPs and their unique conforma-
tion-dependent optical properties. A brief review of their use in fluorescence
detection of small molecules and a diversity of biological processes will then
follow.

1.1 Conjugation and Conformation-Dependent Optical Properties

CPs are in many ways similar to conventional plastics but because of their unique
electronic configuration, they exhibit additional properties that make them useful in
a variety of applications ranging from organic electronics to biosensors. In noncon-
jugated “conventional” organic polymers, the carbon chain consists of sp 3 hybri-
dized carbon atoms with single

bonds (Fig. 1a ). Conjugated “conducting”
polymers consist of alternating single and double bonds and are thus characterized
by sp 2 hybridized carbon atoms with overlapping

s

p orbitals (Fig. 1b ). These over-

lapping orbitals allow for delocalization of electrons along the backbone of the

polymer chain (Fig. 1c ), similar to the free motion of electrons within a metal.
The movement of these electrons along the backbone of the polymer can be
facilitated or hindered depending on the ease with which the electron can migrate
from orbital to orbital. When the p z orbitals are aligned or “in-plane” and directly
overlapping (Fig. 2 , top), electron movement is facilitated. However, when the

p