Compared with other reported fluorescent sensors for Fe
3+
, this work showed a lower
detection limit, which was 6.06 Â 10
À8 M. Meantime, the quenched fluorescence could
be recovered by L-Cys, which acted as the competition of πÁÁÁπ stacking and cationÁÁÁπ
interactions in the gel state.
Wei and coworkers reported a pillar[5]arene-based supramolecular organic
framework for the detection of both metal ions and anionic molecules, in which
two kinds of pillar[5]arene were incorporated (Fig. 15) [31]. Firstly, the bisthioacetylhydrazine functionalized pillar[5]arene (DPSH) could self-assemble into
linear supramolecular polymer through the hydrogen bond. Then this
linear assembly could further bind the bis-bromohexane functionalized pillar[5]
arene (DPHB) through the host-guest interaction, which result in the successful
construction of supramolecular organic framework. This framework showed
a fluorescent response for Fe
3+ , Cr
3+ , Hg
2+ , and Cu
2+ ions; the fluorescence emission
of the framework was quenched. The lowest fluorescent response concentration of
Fe
3+ , Cr
3+ , Cu
2+ , and Hg
2+ for the framework was determined in the range of
1.0 Â 10
À6 M to 1.0 Â 10
À7 M by fluorescent titrations. Moreover, the framework
containing the metal cations could also be used as the fluorescence probe for
the anionic molecules for the competitive binding with the metal ions. For example,
to the framework solution containing Fe
3+ , only the F
À and L-Cys could induce
the emission of bright blue fluorescence at 456 nm. Other tested frameworks
exhibited similar phenomena, the framework containing Hg
2+ could selectively
sense Br
À
, and Cu-framework could selectively bind L-Cys. And the lowest fluorescent response concentrations were all determined to be in the range from
1.0 Â 10
À7 M to 1.0 Â 10
À8 M. This work showed a novel and efficient way for
the development of multifunctional framework materials for multi-guests detection
and recyclable separation.
13.3.2 Stimuli-Responsive Soft Materials
Gels are soft solids or solid-like materials that immobilize a large amount of solvent
in a three-dimensional network held together by covalent bonds and noncovalent or
topological interactions [32]. These materials are appealing to various applications
owing to their compositional and structural versatility that allows introducing
responsiveness to external stimuli.
Yang and coworkers prepared multiple stimuli-responsive supramolecular gels
using metal-ligand bonds and host-guest interactions, in which discrete hexakispillar[5]arene metallacycles with different diameters were prepared by metal-ligand
bonds between the di-pyridine modified pillar[5]arene and different linear di-Pt(II)
acceptors [19]. Then the addition of ditopic guest containing nitrile motif, acting as
the cross-linker, would result in the formation of cross-linked polymer, which further
convert into the gels. These obtained supramolecular gels not only exhibited the
thermal responsiveness but also exhibited responsibility to halide ions as well as
the competitive guest molecules. As well known, the metal-ligand bonds possessed
relatively moderate bond energies, which could be destroyed by the competitive
ligand. In this system, the gel to sol transition behavior was observed when bromide
360
X. Wu et al.
3+
, this work showed a lower
detection limit, which was 6.06 Â 10
À8 M. Meantime, the quenched fluorescence could
be recovered by L-Cys, which acted as the competition of πÁÁÁπ stacking and cationÁÁÁπ
interactions in the gel state.
Wei and coworkers reported a pillar[5]arene-based supramolecular organic
framework for the detection of both metal ions and anionic molecules, in which
two kinds of pillar[5]arene were incorporated (Fig. 15) [31]. Firstly, the bisthioacetylhydrazine functionalized pillar[5]arene (DPSH) could self-assemble into
linear supramolecular polymer through the hydrogen bond. Then this
linear assembly could further bind the bis-bromohexane functionalized pillar[5]
arene (DPHB) through the host-guest interaction, which result in the successful
construction of supramolecular organic framework. This framework showed
a fluorescent response for Fe
3+ , Cr
3+ , Hg
2+ , and Cu
2+ ions; the fluorescence emission
of the framework was quenched. The lowest fluorescent response concentration of
Fe
3+ , Cr
3+ , Cu
2+ , and Hg
2+ for the framework was determined in the range of
1.0 Â 10
À6 M to 1.0 Â 10
À7 M by fluorescent titrations. Moreover, the framework
containing the metal cations could also be used as the fluorescence probe for
the anionic molecules for the competitive binding with the metal ions. For example,
to the framework solution containing Fe
3+ , only the F
À and L-Cys could induce
the emission of bright blue fluorescence at 456 nm. Other tested frameworks
exhibited similar phenomena, the framework containing Hg
2+ could selectively
sense Br
À
, and Cu-framework could selectively bind L-Cys. And the lowest fluorescent response concentrations were all determined to be in the range from
1.0 Â 10
À7 M to 1.0 Â 10
À8 M. This work showed a novel and efficient way for
the development of multifunctional framework materials for multi-guests detection
and recyclable separation.
13.3.2 Stimuli-Responsive Soft Materials
Gels are soft solids or solid-like materials that immobilize a large amount of solvent
in a three-dimensional network held together by covalent bonds and noncovalent or
topological interactions [32]. These materials are appealing to various applications
owing to their compositional and structural versatility that allows introducing
responsiveness to external stimuli.
Yang and coworkers prepared multiple stimuli-responsive supramolecular gels
using metal-ligand bonds and host-guest interactions, in which discrete hexakispillar[5]arene metallacycles with different diameters were prepared by metal-ligand
bonds between the di-pyridine modified pillar[5]arene and different linear di-Pt(II)
acceptors [19]. Then the addition of ditopic guest containing nitrile motif, acting as
the cross-linker, would result in the formation of cross-linked polymer, which further
convert into the gels. These obtained supramolecular gels not only exhibited the
thermal responsiveness but also exhibited responsibility to halide ions as well as
the competitive guest molecules. As well known, the metal-ligand bonds possessed
relatively moderate bond energies, which could be destroyed by the competitive
ligand. In this system, the gel to sol transition behavior was observed when bromide
360
X. Wu et al.