Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 379

Fig. 1 ) first forms a low emission complex with a single DNA strand where the
polymer backbone is in the extended form [ 85 , 102 ]. Upon DNA hybridization, the
cationic PT wraps around the helix, kinking the backbone, reducing conjugation and
increasing its emission. To increase sensitivity further, Leclerc and co-workers
labeled the capture strand with a fluorophore that accepts energy from emissive
PT via FRET - this allowed the detection of as few as five strands in 3 mL [ 84 ]. The
photophysics leading to the exceptional sensitivity of this assay was further explored
by Leclerc et al. and attributed to aggregation of the duplexes prior to target transfer,
so that when the DNA hybridized the nonemissive PT in the triplex has multiple
fluorophores nearby to accept energy [ 103 ]. Variations of this approach have also
been used for the detection of K + [ 104 ] and thrombin [ 105 ] and have been adapted
for microarray screening of nucleic acids [ 43 , 106 ]. Nilsson and co-workers have
used a zwitterionic PT [ 107 ] in a similar approach for probing protein conforma-
tions; again, the PT chain conformation changes in response to changes in the target
3-D shape [ 108 ]; this and related work is described in more detail in [ 109 ].
PDA has also been used as the basis for colorimetric and fluorescent sensing
materials. Spectroscopic studies of PDA show that the blue, planar PDA undergoes
ultrafast relaxation of the excited state [ 110 ], making it effectively nonemissive.
The conversion of blue to red PDA is accompanied by a change in emission that has
been shown to be more responsive than the change in absorbance [ 5 , 26 ], and this
change has been used for the detection of enzymes, bacteria, and other biological
targets [ 5 ]. PDA materials can be formed with lipophilic binding groups and
substrates incorporated or they can be added later through insertion or surface
reactions. Interaction of the target with binding groups, substrate, etc. at the PDA
interface leads to changes in the polymer backbone translated via the methylene
groups between the surface and the polymer chain. While changes in the PDA
emission have been used for detection, a more common approach is to use energy
transfer to a small-molecule fluorophore, as discussed below. Emissive PDA has a
relatively low quantum yield (0.02 at room temperature for a red PDA film [ 111 ])
and the overall emissive output can be increased by the choice of an appropriate
acceptor fluorophore [ 112 ]. It should be noted that the conformational changes in
PT chains are often reversible, while for a PDA material, the switch from non-
emissive to emissive is usually not reversible after a certain amount of conversion
has taken place. This is a general consequence of the thermodynamic strain present
in the nonemissive form of PDA. This strain is a result of polymer formation
through a sterically demanding two-electron process, and is reduced by the back-
bone twisting that makes the polymer emissive [ 5 ].
5.3 Emission Wavelength Shifts and FRET
In the discussion of detection signals above, the focus has been primarily on
increases or decreases in the polymer emission at one wavelength. It is also possible
for the emission wavelength of the polymer itself to shift as result of interaction