Double metal ions competitively control the guest-sensing process: A facile approach to stimuli-responsive supramolecular gels

Article abstract of DOI:10.1002/chem.201403327

A facile approach to the design of stimuli-responsive supramolecular gels (SRSGs) termed double-metal-ion competitive coordination control is reported. By this means, the fluorescence signals and guest-selective responsiveness of the SRSGs are controlled

Full text of DOI:10.1002/chem.201403327

DOI: 10.1002/chem.201403327
Full Paper
&
Smart Materials
Double Metal Ions Competitively Control the Guest-Sensing
Process: A Facile Approach to Stimuli-Responsive Supramolecular
Gels
Qi Lin,* Bin Sun, Qing-Ping Yang, Yong-Peng Fu, Xin Zhu, Tai-Bao Wei, and You-
Ming Zhang*[a]
Abstract: A facile approach to the design of stimuli-respon-
sive supramolecular gels (SRSGs) termed double-metal-ion
competitive coordination control is reported. By this means,
the fluorescence signals and guest-selective responsiveness
of the SRSGs are controlled by the competitive coordination
of two different metal ions with the gelators and the target
guest. To demonstrate this approach, a gelator G2 based on
multiple self-assembly driving forces was synthesized. G2
could form Ca2+-coordinated metallogel CaG with strong
aggregation-induced emission (AIE). Doping of CaG with
Cu2+ results in AIE quenching of CaG and formation of Ca2+
- and Cu2+-based metallogel CaCuG. CaCuG could fluores-
cently detect CNÀ with specific selectivity through the com-
petitive coordination of CNÀ with the Cu2+ and the coordi-
nation of Ca2+ with G2 again. This approach may open up
routes to novel stimuli-responsive supramolecular materials.
only one kind of metal ions,[4,5] and only in a few of them are
two kinds of metal ions employed to extend the stimuli-re-
sponse properties. On the other hand, it is still a major chal-
lenge to design and synthesize novel SRSGs that can optically
sense a given chemical stimulus with specific selectivity. Could
the cooperation of two different kinds of metal ions in the
same gel system improve the signal reporting and selective re-
sponse abilities of the metallogel-based SRSGs? Reports on
such attempts are very rare.
Introduction
Stimuli-responsive supramolecular gels (SRSGs),[1] as a class of
smart and advanced materials, have attracted more and more
attention due to their promising applications, including che-
mosensors, optoelectronic devices, drug delivery, tissue engi-
neering, biomaterials, surface science, and displays.[2] SRSGs are
derived from the noncovalent self-assembly of small mole-
cules.[1,3] On account of the dynamic and reversible nature of
noncovalent interactions, the SRSGs can sense, process, and
actuate a response to an external change without assis-
tance.[1,4] Though SRSGs formed by organic molecules have
been widely reported, SRSGs based on metallogels have only
been a subject of study in the last few years.[4] Interestingly,
the tunable coordination binding strength, as well as the fasci-
nating redox, optical, electronic, and magnetic properties of
the metal ions, would benefit the application of metallogel-
based SRSGs in materials science.[4a,b,5]
In addition, due to the fundamental role of anions in chemi-
cal, biological, and environmental processes,[6] more and more
interest has focused on anion sensors based on SRSGs.[1] Al-
though a number of the reported organogels could sense
anions such as FÀ and AcOÀ[7] through their competitive bind-
ing with self-assembly sites of the organogels (e.g., hydrogen
bonds), there is still great demand for SRSGs that can specifi-
cally detect certain anions. For example, despite the fact that
CNÀ is well known as a hazardous chemical in biology and the
environment, and is extremely toxic to mammals,[8] to the best
of our knowledge there is no report on CNÀ-responsive supra-
molecular gels.
Although considerable efforts have been dedicated to met-
allogel-based SRSGs, most metallogel-based SRSGs contain
[a] Dr. Q. Lin, B. Sun,+ Q.-P. Yang,+ Y.-P. Fu, X. Zhu, Prof. T.-B. Wei,
Prof. Y.-M. Zhang
Herein, we provide a novel approach to SRSGs design
termed double-metal-ion competitive coordination control. To
demonstrate this method, organogelator G2 based on multiple
self-assembly driving forces, fluorescent signal groups, and co-
ordination binding sites was designed and synthesized
(Scheme 1). G2 could form stable supramolecular organogels
in various solvents at very low critical gelation concentrations
(CGCs). On addition of calcium perchlorate as Ca2+ source to
the G2 organogel (OG) in ethanol, a stable Ca2+-coordinated
metallogel (CaG)was formed, accompanied by strong brilliant
blue aggregation-induced emission (AIE) of fluorescence.[9] In
Key Laboratory of Eco-Environment-Related Polymer Materials
Ministry of Education of China
Key Laboratory of Polymer Materials of Gansu Province
College of Chemistry and Chemical Engineering
Northwest Normal University
Lanzhou, Gansu, 730070 (P. R. China)
E-mail: linqi2004@126.com
zhangnwnu@126.com
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403327.
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Results and Discussion
The synthesis of G2 is shown in
Scheme 2. We rationally intro-
duced multiple self-assembly
driving forces, namely, strong
van der Waals (vdW) interactions
of the long alkyl chains, hydro-
gen bonds of the acylhydrazone
groups, and p–p stacking of
naphthyl groups, to provide the
gelator with better gelation abili-
ty. The gelation ability of G2 was
examined in various solvents by
means of stability to inversion of
a test tube (Supporting Informa-
tion Table S1). G2 showed excel-
lent gelation ability in dimethyl
sulfoxide, ethanol, propyl alco-
hol, n-butyl alcohol, and isoamyl
alcohol. Among these solvents,
G2 showed the lowest CGC
(0.4% w/v, 10 mgmLÀ1 =1%)
and the highest gel–sol transi-
tion temperature Tgel in ethanol
(Figure 1 and Supporting Infor-
mation Table S1). Therefore, the
G2 organogel in ethanol is more
stable than the gels in other sol-
vents. Moreover, in ethanol, the
sol–gel transition is very rapid.
The whole transition process
took no more than 1 min after
heating of the ethanol solution
was stopped. In addition, with
increasing concentration of the
G2 in ethanol, Tgel reached
Scheme 1. Chemical structure of the G2 and the assumed self-assembly and stimuli-responsive mechanism.
addition, continuous addition of copper perchlorate as Cu2+
source to CaG could form a stable Ca2+/Cu2+-based metallogel
(CaCuG). The addition of Cu2+ resulted in fluorescence
quenching of the CaG, which was attributed to competitive
binding of Cu2+ with the orga-
a limit of 818C. This gel was found to be stable in closed tubes
for at least three months at room temperature.
Because organogel OG is more stable in ethanol than in
other solvents, we investigated the influence of metal ions on
nogelator G2 and the replace-
ment of Ca2+. More interestingly,
when CNÀ was added to the
CaCuG gel, its fluorescence com-
pletely recovered, which was at-
tributed to competitive binding
of CNÀ with the Cu2+, while
Ca2+ in the gel coordinated with
G2 again. During the whole
CNÀ-response process, the gel
did not undergo
a gel-to-sol
transition. Thus CaCuG could flu-
orescently detect CNÀ in the
gel–gel state with specific selec-
tivity.
Scheme 2. Synthetic route to organogelator G2.
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emission of OG by AIE, whereas
the formation of metallogel CuG
could not enhance the fluores-
cence emission of OG.
Since
Ca2+
belongs
to
Group 2 and Cu2+ to Group 11,
the coordination ability of Cu2+
with acylhydrazone is much
stronger than that of Ca2+
.
Therefore, we presumed that, on
adding Cu2+ to the metallogel
CaG, Cu2+ could competitively
coordinate with gelator G2 in
CaG and release Ca2+. This pro-
Figure 1. a) CGCs of G2 in different solvents. b) Plots of Tgel against the concentrations of organogel OG and met-
allogels CaG and CaCuG in ethanol (for CaG, G2:Ca2+ =1:1; for CaCuG, G2:Cu2+ :Ca2+ =1:2:1).
OG in ethanol. The addition and diffusion of one equivalent of
various metal ions (Mg2+, Ca2+, Cr3+, Fe3+, Co2+, Ni2+, Cu2+
cess could quench the AIE of CaG. The experimental results
confirmed this hypothesis. The addition and diffusion of two
equivalents of Cu2+ into CaG (1%) could form Cu2+/Ca2+
-based metallogel CaCuG (Scheme 1). On diffusion of Cu2+, the
fluorescence emission of CaG was quenched (Figure 2a). In ad-
dition, dissolution of G2, one equivalent of Ca2+, and two
equivalents of Cu2+ in hot ethanol also generated the same
CaCuG (1%). CaCuG showed very weak fluorescence emission,
similar to that of CuG (Figure 2a).
,
Zn2+, Ag+, Cd2+, Hg2+, and Pb2+) to OG (1%) generated the
corresponding metallogels (Supporting Information Figure S1).
Moreover, dissolution of gelator G2 and one equivalent of vari-
ous cations in heated ethanol could also form the same metal-
logels (1%). On excitation at 365 nm with a hand-held UV
lamp, organogel OG emitted very weak fluorescence, whereas
metallogel CaG emitted strong brilliant blue fluorescence
(Scheme 1 and Supporting Information Figure S1). In the corre-
sponding fluorescence spectra (Figure 2a and Supporting In-
The anion-response capability of metallogel CaCuG was pri-
marily investigated by adding five equivalents of various
anions (FÀ, ClÀ, BrÀ, IÀ, AcOÀ,
À
À
À
HSO4
,
H2PO4
,
N3
,
SCNÀ,
ClO4À, and CNÀ as 0.1 molLÀ1
aqueous solutions of their
sodium salts) to the CaCuG. As
expected, CaCuG showed excel-
lent fluorescence changes with
CNÀ as stimulus (Figure 3a). On
addition of CNÀ to CaCuG at
208C, CaCuG emitted strong
brilliant blue fluorescence at
470 nm. This fluorescence is sim-
ilar to that of CaG. These results
confirmed that CNÀ competitive-
ly bound to Cu2+, while the Ca2+
coordinated with gelator G2
again. Moreover, other anions
did not cause similar fluorescent
responses; therefore, CaCuG
could detect CNÀ with specific
Figure 2. a) Fluorescence spectra of organogel OG (1% in ethanol) and metallogels CaG, CuG, and CaCuG (1% in
ethanol; for CaG, G2:Ca2+ =1:1; for CuG, G2:Cu2+ =1:1; for CaCuG, G2:Cu2+:Ca2+ =1:2:1). b) Temperature-depen-
dent fluorescence spectra of CaG (1% in ethanol) during the gelation process (lex =350 nm).
formation Figure S2), the fluorescence intensity of CaG at
469 nm is about ten times higher than that of OG. More inter-
estingly, as shown in Figure 2b, CaG shows no fluorescence in
hot ethanolic solution (T>Tgel). However, when the tempera-
ture of a hot ethanol solution dropped below Tgel of CaG, the
emission intensity at 469 nm showed a sudden increase and
reached a steady state, which indicated that the fluorescence
of metallogel CaG was due to AIE. Meanwhile, the Cu2+-coordi-
nated metallogel CuG showed very weak fluorescence emis-
sion, which was similar to that of OG (Scheme 1 and Support-
ing Information Figure S1). These results illustrated that the
formation of metallogel CaG could enhance the fluorescence
selectivity. Interestingly, unlike most of the reported SRSGs,
which showed gel–sol phase transition on stimulation with
anions, the gel state of CaCuG did not show any gel-to-sol
changes during the whole CNÀ-response process. This special
stability could be attributed to the cooperation of the multiple
self-assembly forces that we rationally introduced into the ge-
lator. Simply stated, because there are three kinds of self-as-
sembly driving forces in gelator G2, even if the hydrogen
bonds were destroyed by metal ions, the two remaining forces
could maintain the gel state of the organogel.
CaCuG could act as a smart material for the detection of
CNÀ in aqueous solutions. For instance, as shown in Figure 4a,
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on the film with a brush dipped
in an aqueous solution of CNÀ,
a brilliant blue fluorescent image
appeared (Figure 4b). Moreover,
as shown in Figure 3b, on grad-
ual addition of CNÀ, the emis-
sion intensity at 470 nm in-
creased with increasing concen-
tration of CNÀ. The detection
limit of the fluorescence spectral
changes calculated on the basis
of 3sB/S, in which sB is the stan-
dard deviation of the blank
measures and S the sensitivity of
Figure 3. a) Fluorescence spectra of CaCuG (1% in ethanol, G2:Cu2+ :Ca2+ =1:1:1) in the presence of various
anions (5 equiv of FÀ, ClÀ, BrÀ, IÀ, AcOÀ, HSO4À, H2PO4À, N3À, SCNÀ, ClO4À, and CNÀ as 0.1 molLÀ1 aqueous solu-
tions of their sodium salts) at room temperature. b) Fluorescence spectra of CaCuG (1% in ethanol,
G2:Cu2+:Ca2+ =1:2:1) with increasing concentration of CNÀ (0.1 molLÀ1 aqueous solution of NaCN as CNÀ source).
the method, was 1.0ꢁ10À6
m
(1 mm) for the CNÀ anion.[10]
To investigate the self-assem-
bly of organogel OG, a series of experiments on metallogels
CaG and CaCuG as well as the CNÀ-response mechanism of
CaCuG was carried out. Firstly, in the concentration-dependent
1H NMR spectrum (Figure 5a–c) of G2, the NH (Ha) and N=CH
(Hb) signals showed obvious downfield shifts with increasing
concentration of G2. Moreover, in the FTIR spectrum (Support-
ing Information Figure S3) the C=O vibrational band of G2
powder shifted to lower wavenumber in the corresponding
OG xerogel. These results revealed that, in the gelation pro-
cess, the NH (Ha) and N=CH (Hb) groups formed hydrogen
bonds with the C=O groups on adjacent gelators. On the
other hand, as shown in Figure 5a–c, with gradual increase in
1
concentration, the H NMR signals of naphthyl protons (Hc, Hd,
He, and Hf) showed obvious upfield shifts, which indicate that
p–p stacking interactions between naphthyl groups were in-
volved in the gelation process.[11] Therefore, as illustrated in
Scheme 1, the gelator G2 self-assembled to organogel OG by
hydrogen bonding, p–p stacking, and vdW interactions of the
long alkyl chains.
The formation of metallogels was also investigated. As
shown in Figure 5d, after addition of one equivalent of Ca2+
to a solution of G2, the NH (Ha) and N=CH (Hb) signals showed
significant downfield shifts, and the signals of phenyl protons
Hg and Hh, which are adjacent to the acyl (ÀC=O) group shifted
upfield. In the IR spectra (Supporting Information Figure S4)
the C=O and C=N stretching bands of G2 shifted to lower
Figure 4. a) Photograph of metallogel CaCuG (1% in ethanol,
G2:Cu2+:Ca2+ =1:2:1) selectively detecting CNÀ (5 equiv, 0.1 mol LÀ1 aque-
ous NaCN solution as CNÀ source) in aqueous solution on a spot plate, illu-
minated at 365 nm. b) Photograph of a CaCuG-based film (obtained from
1% metallogel CaCuG in ethanol, G2:Cu2+ :Ca2+ =1:2:1) fluorescently de-
tecting CNÀ in aqueous solution (0.1 molLÀ1 NaCN), illuminated at 365 nm.
on adding aqueous solutions of
various anions to small amounts
of metallogel CaCuG on a spot
plate, only CNÀ could induce an
instant fluorescence emission of
CaCuG. To facilitate the use of
CaCuG,
a
CNÀ-detecting film
based on CaCuG was prepared
by pouring a hot ethanolic solu-
tion of CaCuG onto a clean glass
surface and drying in air. The
CaCuG film showed no fluores-
1
Figure 5. a)–c) Partial 1H NMR spectra of G2 in CDCl3 at different concentrations. d) Partial H NMR spectrum of G2
cence emission, but on writing mixed with 1 equiv of Ca2+ in CDCl3.
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wavenumbers when G2 interacted with one equivalent of
Ca2+. These phenomena indicated that in CaG Ca2+ coordinat-
ed with the nitrogen and oxygen atoms of the acylhydrazone
group (Scheme 1). In addition, after adding one equivalent of
Cu2+ to CaG, the C=N stretching bands of G2 shifted to lower
wavenumbers again, which indicated that in CaCuG the Cu2+
ions formed more stable coordination bonds with nitrogen
atoms of the acylhydrazone groups and Ca2+ was replaced by
Cu2+ (Scheme 1).
This hypothetical self-assembly and competitive coordina-
tion mechanism was also supported by the Tgel values of OG,
CaG, and CaCuG. For instance, as shown in Figure 1a, under
the same conditions, the Tgel value of OG is obviously higher
than those of CaG and CaCuG, which are similar to each other.
The large differences in Tgel between OG and CaG or CaCuG
are ascribed to the breakage of intermolecular hydrogen
bonds between N=CH groups and C=O groups of neighboring
gelator molecules in OG (Scheme 1) by coordination of Ca2+ or
Cu2+ with gelator G2.
Figure 7. SEM images of xerogels of a) OG (obtained from 1% ethanolic so-
lution of organogel), b) CaG (obtained from 1% ethanolic solution of metal-
logel CaG, G2:Ca2+ =1:1), c) CaCuG (obtained from 1% ethanolic solution of
metallogel CaCuG, G2:Cu2+:Ca2+ =1:2:1), and d) CaCuG xerogel treated
with CNÀ in situ.
Moreover, the XRD patterns (Figure 6) of OG, CaG, CaCuG,
and CaCuG treated with CNÀ showed peaks at 2q=23.76–
of OG showed an overlapping rugate layer structure. The thick-
ness of each layer was approximately 10–20 nm. The metallo-
gels CaG and CaCuG also showed overlapping rugate layer
structures. This indicated that although the gelator G2 coordi-
nated with Ca2+ or Cu2+, no significant changes took place in
the self-assembly states.
Conclusion
We have demonstrated a new strategy for the design of stimu-
li-responsive supramolecular gels termed double-metal-ion
competitive coordination control. The stimulus–response selec-
tivity and signal-reporting property of a supramolecular gel are
controlled by competitive coordination of the two different
metal ions. This is an effective way to achieve selective re-
sponse of a supramolecular gel to a certain chemical stimulus.
There are two key features of this approach. Firstly, the rational
introduction of two different metal ions with different coordi-
nation abilities into the same metallogel is crucial. In present
work, by competitive coordination of Ca2+ and Cu2+ with gela-
tor G2, the strong AIE of metallogel CaG was controlled in on–
off mode. In addition, through competitive coordination of
CNÀ with the Cu2+ in the metallogel CaCuG, Ca2+ coordinated
with gelator again and the AIE of the metallogel recovered.
Secondly, the introduction of multiple self-assembly driving
forces provided the gelator with stronger gelation abilities,
which ensured that the gelator self-assembled to organogel
and double-metal-based supramolecular gels. This concept of
double-metal-ion competitive coordination control may open
doors to novel metallogel-based smart materials.
Figure 6. Powder XRD patterns of xerogels of OG (obtained from 1% OG in
ethanol), CaG, CaCuG (obtained from 1% ethanolic solutions of metallogels;
for CaG, G2:Ca2+ =1:1; for CaCuG, G2:Cu2+:Ca2+ =1:2:1), and CaCuG xero-
gel treated with CNÀ (5 equiv, 0.1 molLÀ1 aqueous NaCN solution as CNÀ
source).
23.908 corresponding to d spacings of 3.71–3.74 ꢂ, which sug-
gested that p–p stacking existed between the naphthyl
groups of these gels. In addition, compared with OG, CaG
showed a new strong peak at 2q=20.988, which was attribut-
ed the coordination of Ca2+ with gelator G2. However, this
peak disappeared when CaG changed to CaCuG. Interestingly,
after adding an aqueous solution of CNÀ to CaCuG, the strong
peak at 2q=20.988 appeared again, which confirmed that the
CNÀ competitively bound to Cu2+, while Ca2+ coordinated
with the gelator G2 again.
Experimental Section
Materials and instruments
To get further insight into the morphological features of or-
ganogel OG and metallogels CaG and CaCuG, SEM studies
were carried out on their xerogels (Figure 7). The SEM image
All anions were used as the sodium salts, and all cations as the per-
chlorate salts, which were purchased from Alfa Aesar and used as
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received. Fresh double-distilled water was used throughout the ex-
periments. NMR spectra were recorded on Varian Mercury 400 and
Varian Inova 600 instruments. Mass spectra were recorded on
a Bruker Esquire 6000 MS instrument. XRD analysis was performed
in transmission mode on
a Rigaku RINT2000 diffractometer
equipped with graphite-monochromated CuKa radiation (l=
1.54073 ꢂ). The morphologies and sizes of the xerogels were char-
acterized by field-emission SEM (JSM-6701F) at an accelerating
voltage of 8 kV. The IR spectra were recorded on a Digilab FTS-
3000 FTIR spectrophotometer. Melting points were measured on
an X-4 digital melting-point apparatus (uncorrected). Fluorescence
spectra were recorded on a Shimadzu RF-5301PC spectrofluoro-
photometer. Elemental analyses were performed with a Thermo
Scientific Flash 2000 organic elemental analyzer.
Synthesis of gelator G2
3,4,5-Tris(hexadecyloxy)benzohydrazide was synthesized according
to literatures methods.[12] G2 was synthesized as follows: 1-naph-
thaldehyde
(1 mmol),
3,4,5-tris(hexadecyloxy)benzohydrazide
(1 mmol), and acetic acid (0.1 mL, as catalyst) were added to etha-
nol (20 mL). Then the reaction mixture was stirred under reflux
conditions for 24 h, after which removing the solvent yielded a pre-
cipitate, which was recrystallized from CHCl3/C2H5OH to give solid
1
G2. Yield: 75%, m.p. 89–918C H NMR (CDCl3, 400 MHz): d=9.77 (s,
1H, NH), 9.04 (s, 1H, N=CH), 8.85 (d, J=6.4 Hz, 1H, ArH), 7.99 (s,
1H, ArH), 7.87 (t, J=7.8 Hz, 2H, -ArH), 7.51–7.45 (m, 3H, ArH), 7.11
(s, 2H, ArH), 3.98 (t, J=6.4 Hz, 6H, OCH2), 1.77 (t, J=6.9 Hz, 6H,
OCH2CH2), 1.43–1.25 (m, 72H, CH2), 0.88 ppm (t, J=6.2 Hz, 9H,
CH3); 13C NMR (CDCl3, 100 MHz): 166.94, 152.77, 142.25, 141.33,
133.77, 131.10, 130.70, 129.05, 128.81, 127.73, 127.36, 127.00,
126.18, 125.26, 124.60, 107.81, 105.91, 105.12, 73.55, 73.45, 69.38,
69.10, 68.92, 31.93, 30.34, 29.72, 29.38, 26.08, 22.69, 14.13 ppm; IR
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À1
~
(KBr): n=3450 (NH), 1717 (C=O), 1649 cm (C=N); ESI-MS calcd for
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This work was supported by the National Natural Science
Foundation of China (NSFC) (Nos. 21064006; 21161018;
21262032), the Natural Science Foundation of Gansu Province
(1308RJZA221) and the Program for Changjiang Scholars and
Innovative Research Team in University of Ministry of Education
of China (IRT1177).
Keywords: aggregation · fluorescence · gels · self-assembly ·
smart materials
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Received: April 30, 2014
Published online on July 23, 2014
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