Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 63

Fig. 14 Anion probes operating through the reinforcement of hydrogen bonds. For color code, see
Fig. 3
mechanism is outlined in Fig. 15 . In the absence of the analyte, the remaining
coordination site of the metal ion is occupied by a donor atom of the probe. Anion
addition then leads to the dissociation of the metal ion-fluorophore bond and
formation of the coordinative bond between metal ion and anion, which has a
pronounced electrostatic character [ 130 ]. Depending on the fluorescent probe
chosen, strong shifts and intensity modulations can be encountered. As the pep-
tide-appended derivative of 41 in Fig. 15b-d nicely shows, such a system can also
be implemented in more advanced sensing schemes [ 129 ]. Moreover, this
approach is not only suitable for stable metal ion complexes of ICT type but for
PET type probes as well [ 131 ]. Recently, a metal ion-fluorescent probe complex
has also been used in a redox-mediated fluorescence “light up” approach, in which a
paramagnetic Cu 2+ ion is bound close to the fluorophore and thus quenching the
emission is turned into a diamagnetic noninterfering Cu + ion through reaction with
iodide [ 132 ].
Related yet mechanistically different are two other types of metal complexes
with organic (fluorescent) receptors using lanthanide ions or lower-row transition
metal ions. The architecture of the lanthanide-based probes is very similar to the
ICT complex probes (cf. 42 and 41 in Fig. 15 ) though the organic chromophore
does not act as an emitter, but only as a light-harvesting antenna to efficiently
sensitize the Eu 3+ luminescence. Since the quantum yield of the latter is very
sensitive toward the type of interaction at the remaining coordination site(s) of
the metal ion, comparatively loosely coordinating species such as water molecules
usually quench the emission to a considerable degree. Exchange of weakly by a
strongly binding species such as an anion then reduces the probability of non-
radiative decay and entails an increase in the typical line-shape Eu 3+ luminescence
[ 133 ]. Examples of the latter type of metal-based probes are for instance Ru II -tris
(bipyridine) chromophores exhibiting metal-to-ligand CT luminescence. These
motifs are rather popular in anion probes though their luminescence effects
are usually only modest and include quenching as well as enhancement features.
However, if a deliberate quenching channel is introduced through remote
redox sites as in 43, anion binding in the receptor unit separating the Ru II (bipy) 3
emitter and the quinine quenchers can revive luminescence and yield strong signal
outputs [ 134 ].