Chemistry Reference
In-Depth Information
Q
Q
Q
some fluorophores quenched
others not
Small molecule fluorophore solution
Q
Q
Q
versus
quenchers
(
(
Q
Q
Q
)
)
n
n
Conjugated polymer segments
segments completely quenched
by one quencher each
Fig. 8 Comparison of quenching of small-molecule fluorophores vs. conjugated polymers
environment for target binding than a small molecule, which can affect the binding
affinity. Frequently, researchers overlook that the emissive species for a conjugated
polymer is a collection of repeat units, and thus treat the polymer as though each
repeat unit is an individual fluorophore. Analysis of whether there is an enhance-
ment of binding to the polymer when compared with the small molecule needs to
take into account the exciton delocalization length to estimate how many binding
groups there are per emissive species. In the early work by Zhou and Swager, this
was achieved by synthesis of a series of polymers of increasing molecular weight
and examination of their Stern-Volmer quenching constants [
66
]. The
K
SV
values
rose with increased polymer length and then plateaued, leading the researchers to
conclude that the exciton was traveling on average over 134 repeat units. Since
every repeat unit had a binding group, the fact that the
K
SV
of the polymer was
60-fold greater than that of a comparison small-molecule fluorophore with one
binding group is not surprising.
Creating a polymer with many binding sites is more easily achieved for small
molecules and ion targets that can be captured by cyclophanes, bi- and tri-pyridines
and other binding units that are readily incorporated in the polymer, than for
biological targets that may require relatively large (and expensive) biomolecules
such as peptides, antibodies, or aptamers for target capture. If the target size is of a
scale comparable to the polymer (e.g., a protein) or larger, then the advantages of
super-quenching may be reduced, as the target can bind more small-molecule
fluorophores than macromolecule fluorophores (Fig.
9a
). Another consequence of
the “super-quenching” mechanism is while the materials are often extremely
sensitive it can also be easy to saturate the sensor (Fig.
9b
)[
3
].
Search WWH ::
Custom Search