Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 75

Fig. 27 (a) Representative scheme for a displacement assay protocol in which first a fluorescent
indicator is coordinated to a host. As a consequence, its optical properties are altered. Second,
upon analyte addition, the higher affinity of the analyte for the host leads to dissociation of the
complex and displacement of the indicator. The original optical properties of the fluorophore are
restored, signaling indirectly the presence of the analyte; (b) Some examples of receptors and
fluorescent indicators reported in the literature for F-IDAs
the fluorescence is observed [ 166 ]. A similar approach was followed by the same
authors for the determination of inositol-triphosphate (IP 3 )[ 167 ].
In succession to this early example relying on a quenched signal, efforts have
been made to achieve “light-up” F-IDAs. One example was described by Fabbrizzi
et al. for the selective detection of carbonate in water, utilizing metal-ligand
(coordinative) interactions for binding and displacement [ 168 ]. A dicopper(II)
polyamino cage complex was chosen as the receptor and coumarin 343 as the
fluorescent indicator, because it possesses a carboxylate function capable of bridg-
ing the two Cu 2+ centers of the polyamino cage. Complete quenching of the
coumarin emission at 487 nm was observed upon titration of the fluorophore with
the polyamine-Cu 2+ receptor. Addition of carbonate to the resulting chemosensing
ensemble solution resulted in an almost complete recovery of the coumarin emis-
sion. Other anions tested like NCS ,NO 3 ,SO 4 2 , HPO 4 2 , HCOO , and
CH 3 COO , whose association constants are well below that of coumarin 343,
could not compete successfully with the indicator and could thus not be detected
fluorometrically. In a similar fashion, Smith et al. described an F-IDA for the
detection of pyrophosphate under physiological conditions [ 169 ]. Pyrophosphate
was capable of displacing a fluorescent coumarin indicator (69) from a bis-Zn 2+ -
dipicolylamine host (68) with association constants of 10 7 M -1 . The fluorescence of
coumarin was quenched when bound to receptor 68. When pyrophosphate dis-
placed 69 from the complex, a strong enhancement of the emission at 480 nm was
recorded. Remarkably, the chemosensing ensemble 68/69 was able to detect pyro-
phosphate almost 100-fold more sensitive than hydrogen phosphate. The same
coumarin indicator 69 and a similar Zn 2+ -dipicolylamine complex now integrating
a phenylboronic acid group for the coordination of sugars was used recently by Horie
and Kubo for the determination of phosphosugars in water at neutral pH [ 170 ].