Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 50

Fig. 5 Colorimetric ICT probes ( left ) and representative spectroscopic responses toward acetate
( right ). For color code and symbols, see Fig. 3
several colorimetric ICT probes for anions exist [ 57 ]. Without going too much into
speculations, this lack is most likely connected to the same reasons as outlined in
the previous section and is supported by findings that already the spectral shifts in
absorption observed for many ICT probes and monovalent cations such as Na + and
K + are rather weak and fluorescence modulations are often negligible. If one
considers that most colorimetric ICT probes for anions carry the strongly electron
accepting nitro group, which is known to quench many UV/vis fluorophores (see,
e.g., 11 and 12 in Fig. 5 [ 58 , 59 ]), the lack of fluorescent ICT anion probes is
comprehensible. The few strategies for anion sensing that have been developed on
the architectural basis of ICT probes but which operate through different mechan-
isms will be discussed in Sect. 3 .
2.2 Spacer-Separated Binding Site and Fluorophore
The separation of binding site and fluorophore by a nonconjugating spacer opens
the path to other mechanisms of communication, most prominently ET and exci-
mer/exciplex formation. In the first case, the electronic nature of both fluorophore
and receptor unit and the steric nature of the spacer are the important parameters for
signal generation. In the second case, for most systems the electronic nature of the
fluorophores and the steric nature of the receptor as well as its change upon analyte
binding determine the signal.
2.2.1 Single Binding Site-Single Fluorophore Architectures
Fluorescent reporter molecules consisting of a single receptor, a single fluorophore,
and a spacer electronically separating these two are popular probes since the