Chemistry Reference
In-Depth Information
phospholipids with transition temperatures (
T
m
s) below room temperature [
59
,
61
]
allowing the possibility of forming fluid domains with more biomimetic character
in the material.
5 Signal Transduction
Fluorescence signal generation can be sorted into three rough categories: reduction
or quenching of emission by the target (“turn-off”), increase or restoration of
emission (“turn-on”), and changes in the emission wavelength arising from the
conformational changes in the polymer or energy transfer to another fluorophore.
More than one signal may be present in a sensing system; in some cases, the signal
with the largest change is measured and, in others, a ratio of signals may be used.
5.1 Turn-Off Sensing
Quenching of emission by a target is a well-known phenomenon in fluorescence
sensing. In conjugated polymer systems, there are three major ways that quenching
occurs in the presence of a target analyte: (1) the analyte quenches the polymer
emission directly; (2) the analyte causes aggregation of the polymer chains leading
to self-quenching; and (3) the analyte changes the polymer backbone conjugation
and decreases the emission (Fig.
6
). Direct quenching and aggregation often occur
together leading to enhanced quenching as described below. Aggregation also can
accompany the third path to quenching, in particular, in cases where the presence of
the analyte changes the polymer microstructure from coils to rigid-rods, which then
aggregate.
(
)
n
analyte
(
(
chain aggre
gation
& self-quench
(
)
n
)
n
(
)
n
)
n
)
n
Fig. 6 Illustration of three quenching pathways
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