Chemistry Reference
In-Depth Information
unbound the exciton trap disappears and emission is restored. Aggregation plays a
role in many quenching assays and may be deliberately incorporated in the sensing
scheme design, as it can amplify the signal through extending the exciton pathway
and through enabling polymer self-quenching.
5.2 Turn-On Sensing
The complement to turn-off sensing is turn-on sensing - situations where the
polymer emission goes up as a result of a target. Turn-on sensing may be achieved
by preparing a conjugated polymer with a quenching group that is removed by the
target, by changing the aggregation state of the polymer and thus reducing self-
quenching, or by using a conjugated polymer that has intrinsically emissive and
nonemissive states related by backbone twisting (Fig.
11
). Ideally, the polymer is in
a completely nonemissive state before being exposed to the target; the presence of
initial emission lowers the sensitivity of the sensor. As seen below, however,
obtaining a nonemissive state of many types of conjugated polymers is difficult,
and as a result, the turn-on sensors are often less sensitive than turn-off sensors.
Less-sensitive sensors, however, complement ultra-sensitive sensors and are use-
ful for applications where the target is present in concentrations where an ultra-
sensitive sensor
is
saturated. Another
important consideration is
that
the
a
(
Q
Q
(
Q
Q
quen
cher is rem
oved
)
n
)
n
b
(
(
(
)
n
)
n
(
chai
ns de-aggre
gate
)
n
)
n
n
c
ba
ckbone twis
ts
(
)
n
)
n
Fig. 11 Cartoons of “turn-on” mechanisms. The target (
in red
) may (a) remove a quencher,
(b) deaggregate chains, or (c) twist the backbone, leading to increases in emission
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