Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 364

stamped in a pattern on a filter membrane electrospun fiber mesh beads suspension

Fig. 4 Cartoon of polymers on different surface types

polymer (e.g., coiled, extended, aggregated, etc.) can strongly influence the behav-
ior in a cast film; PPV films cast from a good solvent have morphology and
emission yields significantly different from PPV films cast from a poor solvent
[ 36 ]. Sensing polymers have been attached to various kinds of surfaces: planar
surfaces such as wells of microplates and glass slides; fibers; filter membranes; and
particles/beads (Fig. 4 ). Beads offer the option of suspending the sensing material
(on the bead) in solution, improving mixing; beads can also be arrayed to create
sensing chips [ 43 , 44 ]. Conjugated polymers have also been combined with nano-
particles to exploit the quenching abilities of the nanoparticles [ 45 , 46 ]. Attaching
the sensing polymer to a filter membrane offers the option of pulling the target
solution past the sensing surface and increasing exposure of the sensing polymer to
the target [ 47 , 48 ]. Conjugated polymers have been deposited on electrospun
nanofiber films to increase the surface area and to prevent

- stacking aggregation

p p

[ 49 , 50 ].

4.1 Materials for Aqueous Applications

Detection of biological targets usually requires sensing in water. One technique is
to synthesize the polymer as a polyelectrolyte with charged side chains so that it
becomes more water soluble. Nonspecific interactions are a perennial problem in
these systems for several reasons: (1) the hydrophobic nature of the polymer back-
bones makes the chains aggregation prone; (2) many biological species (proteins,
nucleic acids, etc.) are charged and will form hydrogen bonds or electrostatically
interact with the polymers affecting emission and/or causing aggregation [ 3 ]; and
(3) buffer salts will potentially affect the microstructure of the polymer and,
through charge screening, electrostatic interaction with the target. The ionic envi-
ronment along the polymer is significantly different than in the bulk solution; for
example, it has been shown that the pH in the vicinity of a weak polyelectrolyte is
3 units higher than the bulk solution pH [ 51 ].
Surfactants can be added to improve the behavior of polyelectrolytes. Early work
with anionic PPV showed that the addition of surfactants changed polymer aggrega-
tion and morphology, and increased the polymer quantum yield by reducing self-
quenching and increasing the conjugation length [ 52 ]. Bunz and co-workers
have shown how choice of surfactant can be made to tune emissive properties of