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benefits of higher signal-to-noise ratios, lower measurement uncertainties (e.g., due
to better counting statistics in a given time interval), and/or faster accumulation
times. Second, from the viewpoint of the analytical chemist, the generation of a
specific, enhanced output signal is preferable compared to unspecific quenching in
terms of species identification because it offers quantitative and qualitative infor-
mation, e.g., through a species-specific fluorescence lifetime of a certain host-guest
ensemble. Quenching analyses usually cannot distinguish between the desired,
analyte-related quenching effect and unspecific interactions by notorious quenchers
being present in a sample such as electron-rich, colored or heavy atom-containing
(naturally occurring or anthropogenic) compounds. Finally, this article concerns
probes for inorganic and organic small-molecule ionic or neutral analytes. Bio- or
other macromolecules are only very marginally covered here for illustrative
purposes in Sect.
4
.
2 Channels of Communication between Binding Site
and Fluorophore
Some of the best performing examples in terms of signal enhancement upon analyte
binding are truly “dinosaurs” among the fluorescent reporters known today, fluo-
rescent ligands such as 2,2
0
-bipyridyl (1)[
28
], 8-hydroxyquinoline (2)[
29
] or the
oldest fluorescent reporter described, morin (3), a flavone derivative [
30
] (Fig.
1
).
These ligands usually consist of an aromatic or a heterocyclic ring system, the
heteroatoms of which (mostly nitrogen and oxygen) are arranged in such a way that
they can form a chelate with a metal ion. Chromophore and binding unit are
identical. The major prerequisite for strong signal enhancement upon analyte
binding, the absence of or a weak fluorescence of the reporter in the unbound
state, is here usually accomplished through one (or more) structurally inherent,
efficient nonradiative relaxation pathways of the excited state, i.e., energetically
low-lying n
p
* states, excited-state intramolecular proton transfer
(ESIPT)
Fig. 1 (a) Chemical structures of fluorescent ligands; metal ion coordination sites are indicated in
blue. (b) Absorption (
solid
) and fluorescence spectra (
dotted
)of1 (
black
) and its Zn
2+
complex
(
red
) in water at pH 7; whereas uncomplexed 1 is virtually nonfluorescent (
10
5
), the
F
<
f
f
of 1-Zn
2+
amounts to 0.34 [
31
]
fluorescence quantum yield
F
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