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Fig. 17 Hg
2+
-induced transformation and thiol-mediated regeneration of 46 and 47; 46 is colorless
and nonfluorescent, 47 is blue and highly fluorescent
Fig. 18 (a) Rhodamine-based Ag
+
sensing system and (b) fluorescence response of 48 in the
presence of Ag
+
and the other cations mentioned in the text. (Reprinted in part with permission
from [
141
]. Copyright 2009 American Chemical Society)
the high electrophilicity of the squaraine's cyclobut-2-enone core, the leuko dye
can be regenerated with propanethiol, circumventing the problem of irreversibility
of the reaction. Sensing and regeneration of the system have been shown to work
with a derivative of 46 in a poly(ethylene-terephthalate) based dip-stick assay.
A remaining issue when using sulfur-based leuko dyes is their sensitivity to
oxidation, limiting long-term storage at ambient temperature. In addition, such
indicator systems are commonly not able to detect mercury in sulfur-rich environ-
ments, where it is often present.
Cu
2+
and Hg
2+
are perhaps the most obvious targets for reaction-based signaling
systems. However, this approach has also been realized for monovalent Ag
+
. Here,
again a rhodamine derivative (rhodamine B spirolactam 48, Fig.
18
) allows the
highly selective detection of Ag
+
with a detection limit of 14 ppb in water contain-
ing 20% ethanol. Silver nanoparticles, a topical analyte today, can also be detected,
after oxidation of the particles with H
2
O
2
/H
3
PO
4
[
141
]. The selectivity was tested
with Ca
2+
,Ba
2+
,Mg
2+
,Cr
2+
,Mn
2+
,Fe
3+
,Co
2+
,Ni
2+
,Cu
+
,Cu
2+
,Zn
2+
,Pd
2+
,Cd
2+
,
Hg
+
, and Hg
2+
and no interference was found.
For the reaction-based sensing of anions, the most considerable success has been
reached in cyanide sensing, basically due to the necessity for assessing this toxic
species and because it can be easily addressed in water due to its high nucleophi-
licity. For instance, the coumarin derivative 50 shows a dramatic fluorescence
increase after cyanide-induced cyanohydrin reaction (Fig.
19
), no response to
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