Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 382

A 2004 study of the emission of PPE-biotin polymers mixed with fluorophore-
streptavadin showed that energy transfer did not occur for the fluorophore with best
spectral overlap but rather for the more planar hydrophobic fluorophore, which
could stack with the polymer backbone and allow direct orbital overlap [ 116 ]. Kim
and Swager developed a sensing approach for the detection of fluoride ion, which
relied on the Dexter mechanism rather than the F¨rster for energy transfer [ 75 ].
A PAE polymer was prepared with alternative rings linked to a coumarin precursor
via a thiophene group. In the presence of F , the precursor cyclized to form
coumarin, the coumarin lowered the band gap and formed an exciton trap, and
the emission peak shifted from the blue emission characteristic of the polymer to a
blue-green emission characteristic of coumarin. This approach showed that the
PAE “molecular wire” amplified the signal compared to the rise in emission from
the formation of plain coumarin (without the polymer attached) from its precursor.
Energy transfer from PDA to fluorophores [ 26 , 59 , 112 ], and from fluorophores
to PDA [ 26 , 117 , 118 ], has proven an effective way to improve the signal char-
acteristics of PDA sensors [ 5 ]. As noted above, PDA has a relatively low emission.
The overall quantum yield of PDA liposomes has been shown to be increased by
incorporation of a fluorophore that accepts energy from emissive PDA [ 26 ]. A study
of diacetylene/PDA liposomes with fluorophores, polymerized to different extents,
suggest that energy transfer between PDA chains competes with energy transfer to
fluorophores and that at greater polymerization extents the fluorophore emission is
greatly reduced or eliminated [ 112 ]. The density of PDA chains in a material can be
controlled by the UV dose used to form the polymer. In sensing schemes involving
PDA and energy transfer to fluorophores it is important to balance the desire for
greater conversion of monomers to polymer for material stability, and the need for
lower polymer densities to facilitate energy transfer to amplifying fluorophores.
6 Conclusion
Emissive conjugated polymers have shown considerable potential as the basis for
sensors for both biological and chemical targets. These macromolecular fluoro-
phores have features arising from the delocalized electronic states created by the
extended conjugation of the backbone that are not found in localized small-molecule
fluorophores and lead to high sensitivities. For example, in quenching-based detec-
tion the exciton travels past many binding sites allowing it to sample and respond to
targets captured by any site along its path, effectively allowing the conjugated
polymer segment to behave as a fluorophore with many binding groups rather than
the one or two found on small molecules. Changes in the polymer microstructure,
e.g., from coil to rod, affect backbone conjugation and can be used for signal
generation. Many complicating factors in using emissive conjugated polymer for
sensing, arising from unanticipated microstructure changes and chain aggrega-
tion that can affect emission, go hand in hand with the advantages of these
macromolecular materials. In addition, there is considerable variation between