Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 372

between the boronic acids and the sugars. The work with sugars illustrates how
turn-off sensing can be achieved even though the target is not intrinsically a
quencher.

5.1.2 “Turn-Off” Detection of Ions

Developing emissive conjugated polymers for ion detection, primarily of cations, is
a significant research area [ 3 , 6 ]. Quenching of conjugated polymer emission by
cations [ 6 , 73 ] occurs by Dexter and PET electron transfer mechanisms, by through
space energy transfer (F ¨ rster), and by ion-induced aggregation [ 74 ]. There are far
fewer examples of detection of anions by conjugated polymers, and the examples
that exist tend to be either of fluoride, which has specific reactivity [ 20 , 75 , 76 ], or
of anionic transition metal complexes, or consist of nonselective sensors that
respond to a range of anionic species [ 3 ].
Polymers for cation detection have been designed both with the binding group
incorporated in the backbone [ 77 ] and with the binding group as a side group.
Designing the polymer with the binding group as a side chain allows more variation
in the choice of binding group, as the binding group does not have to have the

p
structure required for participation in the conjugation of the backbone. Incorpora-
tion of the binding group into the polymer backbone, however, can increase
sensitivity [ 78 ]. The nature of the linker between the backbone and the binding
group [ 73 ] has been shown to affect sensitivity with a vinyl linker leading to greater
quenching than an ether linker [ 79 ]. Similarly, when the binding group is
incorporated in the backbone, the nature of the linker between units also affects
performance, for example, fluorene/bipyridine copolymers showed greater sensi-
tivity when there was a lower barrier to chelation-induced rotation of the bipyridine
group [ 80 ].
The quenching effect of cations on conjugated polymer sensors is often ampli-
fied by cation-induced aggregation of the polymer chains [ 3 ]. Quenching may also
occur purely as a result of aggregation in cases where the target itself cannot
undergo electron or energy transfer with the polymer. K + , for example, was
detected by conjugated polymers with crown ethers attached as side chains; the
crown ethers sandwiched the K + , brought the chains together and induced self-
quenching [ 74 , 81 ].
It should be noted that most conjugated polymer ion sensors suffer from lack of
selectivity. Even when the polymer responds to a particular ion with greater
sensitivity than to other ions at the same concentration, it does not guarantee that
the polymer will be effective in a real-world setting where environmental samples
can contain many ions at a wide variety of concentrations. In cases where a few ion
species are present at much higher concentrations than the target ions, the sensor
response may be misleading. It is standard to invoke using sensor arrays (“tongues”
and “noses”) as a technique to avoid this issue; however, such claims should be
approached with some skepticism, as multivariate analysis of samples with ten or
more species is considerably more difficult than analysis of standard example