Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 69

Fig. 21 Fluorescein-based chemodosimeter molecules for ROS. Whereas faintly colored and
weakly fluorescent 54 reacts rather nonspecifically to green and highly fluorescent 55 (a; hROS
¼ highly reactive oxygen species), colorless and nonfluorescent 56 is selectively converted into
green and highly fluorescent 57 in the presence of H 2 O 2
4.1.2 Catalytic Production of Fluorophores
Reaction-based indicators which are catalytically transformed by the analyte of
interest are an attractive alternative due to the simple reason mentioned in the
context of enzymes at the beginning of Sect. 4 : one analyte molecule is qualified to
produce a lot of fluorophores, increasing sensitivity dramatically. For an actual
quantitative determination, however, the same conditional constraints are relevant
like in enzyme-based methods, i.e., reproducible and defined incubation times,
temperature, pH, etc. Up to now, transition metal cations have been mostly
shown to be detectable by catalytic fluorophore production.
To circumvent some of the above-mentioned drawbacks of sulfur-based mercury
chemodosimeters, a system based on the alkyne oxymercuration of 58 has been
developed (Fig. 22 )[ 146 ]. 58 shows high selectivity, a limit of detection of ca.
8 ppm, resistance against strong oxidants, and a positive reaction even in the
presence of cysteine, which is known to form stable mercury complexes and is
used for the extraction of mercury from tissue samples. Another metal that is well-
known for its catalytic ability is palladium, catalyzing different reactions depending
on its oxidation state. Since this metal is toxic, assessment of the maximum
allowable concentration of Pd in consumer products such as pharmaceuticals
requires highly sensitive and selective detection schemes. For this purpose, indica-
tor 60 was conceived to undergo allylic oxidative insertion to the fluorescein