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SCHEME 11.8
Representative structure of debittering “click” CD polymer. (Adapted from
ref. 28.)
and limonoids (limonin) are responsible for the bitterness in citrus juice. To suppress
the bitterness in citrus juice, Binello et al. prepared a
-CD copolymer via “click”
HDC reaction, which can efficiently sequester naringin from aqueous solution [28].
They first converted two secondary hydroxyl groups of
-CD into alkynyl groups,
then copolymerize them with 1,3-bis(azidomethyl)benzene to give a reticulated water
insoluble polymer (Scheme 11.8). The debittering performance is better than that of
commercial porous polymer of Amberlite XAD-16, and can be used for at least three
entrapment/desorption cycles without appreciable loss of sequestering power.
11.3.3 Biological Sensors
Due to the inclusion ability and the multiple hydroxyl-decorated rims of CDs, another
exciting application area for these cyclic molecules is to develop biological sensors
to probe the existence and concentration of a particular substance with important
biological functions. These substances may include metal ions, organic pesticides,
and fatty acids.
Some metal ions play important roles in the normal functions of biological sys-
tems, with others being extremely toxic. Therefore, the development of effective
and efficient metal ion detecting methods becomes critically important for clinical
diagnosis of trace element deficiencies or heavy metal intoxication. Using
-CD,
Zhang et al. developed two effective fluorescent sensors for detection of cadmium
ion, which can cause acute and chronic toxicity in humans [29]. As the first step
of the sensor design, mono(6-deoxy-6-azido)-
-CD and 8-propargyloxyquinoline
were prepared (Scheme 11.9). They were then “clicked” together to yield the sensor
molecule, which has weak fluorescence. When exposed to Cd
2
+
, the sensor will
be able to emit significantly enhanced fluorescence signal (1.7-fold increase). The
researchers believed that the observed enhancement was attributed to the cooper-
ative coordination of Cd
2
+
, 8-hydroxyquinoline, and triazole, which blocked the
intramolecular photo-induced electron transfer. The sensor also showed good selec-
tivity with no other metal ion being able to generate such fluorescence enhancement.
Because of the high affinity of the
-CD annulus to adamantyl group, the sensor
can easily sequester 1-admantanecarboxylic acid sodium salt (ADCA). In the pres-
ence of ADCA, the authors observed a sevenfold fluorescence enhancement than the
sensor without ADCA. In these studies, it is very important to note that the HDC
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