Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 66

4.1.1 Transformation of a Leuko 12 Dye into a Fluorophore
(Chemodosimeter)

A rather new approach in selective fluorescence detection is the production of
fluorophores in a chemical reaction. Compared to supramolecular reporters, for
which the target is interacting via noncovalent interactions such as hydrogen
bonding, interactions, or electrostatic attraction with a binding site that is

complementary to the analyte to modulate the fluorescence signal, the use of
specific reactions is more straightforward. In a simplified picture, only a leuko
dye is required which can be transformed into the respective fluorophore in a
chemical reaction that should be specific for the analyte of interest. A prominent
advantage of the reaction-based systems is that during the reaction, the site to which
the analyte is attracted and at which it binds is removed (in most cases) from the
fluorophore, enabling in a straightforward way the detection of notoriously quench-
ing analytes (e.g., the heavy and transition metal ions mentioned above) by strong
fluorescence signals. However, one disadvantage still resides with many reaction-
based systems, i.e., they can only act as chemodosimeters. Most reactions that
produce a fluorophore are essentially irreversible so that the indicator is consumed,
prohibiting the continuous monitoring or reuse of the system; the chemosensing
system is a disposable one and can only be applied in a single analysis.
For cations, various examples have been realized mostly concerned with the
detection of the already mentioned heavy and transition metal ions. One of the first
examples published involves the Cu 2+ -induced hydrolysis of a rhodamine B hydra-
zide 44 to rhodamine B 45, which can be employed in water to detect Cu 2+ down to
10 n M with a linear response up to 2

p - p

M (Fig. 16 )[ 135 ].
A very popular target for reaction-based sensing is Hg 2+ because of its outstand-
ing thiophilicity and ability to cleave C-S bonds [ 136 - 139 ]. A remarkable example
is the one shown in Fig. 17 . Use of a squaraine dye as the target structure allows
achieving intense absorption and emission bands in the far red region of the visible
spectrum, facilitating “naked-eye” monitoring and fluorescence detection without
the interference of autofluorescence and matrix absorption [ 140 ]. Moreover, due to

m

Fig. 16 Rhodamine-based Cu 2+ chemodosimeter; 44 is colorless and nonfluorescent, 45 is pink
and highly fluorescent

12 “Leukos” (Greek) means “white, colorless”. The term “leuko dye” is commonly used for a
colorless (or largely UV-absorbing) dye precursor.