A novel strategy for the design of smart supramolecular gels: Controlling stimuli-response properties through competitive coordination of two different metal ions

Article abstract of DOI:10.1039/c4cc03753g

By rationally introducing Ca²⁺ and Fe³⁺ into a supramolecular gel, a bimetal-gel CaFeG was prepared. CaFeG could reversibly "turn-on" its fluorescence upon sensing H₂PO ₄^- with specific selectivity un

Full text of DOI:10.1039/c4cc03753g

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A novel strategy for the design of smart
supramolecular gels: controlling stimuli-response
properties through competitive coordination of
two different metal ions†
Cite this: Chem. Commun., 2014,
50, 10669
Received 16th May 2014,
Accepted 15th July 2014
Qi Lin,* Bin Sun,‡ Qing-Ping Yang,‡ Yong-Peng Fu, Xin Zhu, You-Ming Zhang and
DOI: 10.1039/c4cc03753g
Tai-Bao Wei*
www.rsc.org/chemcomm
By rationally introducing Ca2+ and Fe3+ into a supramolecular gel, a by rationally introducing two different metal ions into a supra-
bimetal–gel CaFeG was prepared. CaFeG could reversibly ‘‘turn-on’’ molecular gel, the stimuli-response properties of the gel could
À
its fluorescence upon sensing H2PO4 with specific selectivity under be accurately controlled by the competitive coordination of
gel–gel states through the competitive coordination of Ca2+ and Fe3+ these metal ions with gelators and guest.
with gelators and H2PO4À. Thus, CaFeG could act as a H2PO4À test kit
and could be utilised in rewritable security display materials.
To demonstrate this strategy, we designed and synthesized a
gelator G based on multi self-assembly driving forces, fluorescent
signal groups and coordination binding sites (Scheme 1). G could
Over the past few decades, stimuli-responsive gels, a kind of smart form stable supramolecular organogel in various solvents at very
materials, have attracted increasing attention because of their promis- low critical gelation concentrations (CGCs) (Table S1 in ESI†).
ing applications, including chemosensors, drug delivery, biomaterials, Among these solvents, the gelator G showed the lowest CGC
displays, etc.1–3 By taking advantage of the dynamic and reversible (0.4%, w/v%, 10 mg mLÀ1 = 1%) and the highest gel–sol transition
nature of noncovalent interactions, the stimuli-responsive supramole- temperature (Tgel) in ethanol (Fig. S1 and Table S1 in ESI†).
cular gels can sense, process, and actuate a response to an external Therefore, G-based supramolecular organogel OG in ethanol is
change without assistance.1,4,5 Recently, metal ions-coordinated supra- more stable than the gels in other solutions.
molecular gels have become a focus for research because the tunable
We investigated the influence of metal ions on G organogel
coordination binding strength, as well as the fascinating redox, optical, in ethanol. The addition and diffusion of 1 equiv. of various metal
electronic, or magnetic properties of the metal ions would benefit the ions (Mg2+, Ca2+, Cr3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+
application of these smart materials.6 However, although a significant and Pb2+, using their solid perchlorate salts as sources) to the OG
amount of effort has been devoted to the development of metal ions- (1%) generated the corresponding metallogels (Fig. S2, ESI†). The
coordinated supramolecular gels, to design and synthesize novel smart organogel OG emitted very weak fluorescence, while the Ca2+
supramolecular gels that can optically sense a given chemical stimulus coordinated metallogel CaG exhibited strong brilliant blue fluores-
with specific selectivity is still a big challenge.
cence emission (Scheme 1 and Fig. S2, ESI†). In the corresponding
Up to now, most of the reported metal ion-coordinated fluorescence spectra (Fig. S3 and S4, ESI†), the fluorescence intensity
supramolecular gels contain only one kind of metal ion,6 and a few of CaG at 469 nm is approx. 10 times higher than that of OG. More
of them employ two kinds of metal ions to extend their stimuli- interestingly, as shown in Fig. S5 (ESI†), CaG has no fluorescence in
response properties. An important question is whether the coopera- hot ethanol solution (T 4 Tgel). However, with the temperature of
tion of two different kinds of metal ions in the same supramolecular hot ethanol solution decreasing below the Tgel of CaG, the emission
gel system can improve stimuli-response abilities. The reports on such
attempts are very rare. Herein, we provide a novel strategy for the
design of stimuli-responsive supramolecular gels. In this manner,
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,
Scheme 1 Molecular structure of G and fluorescence photograph of
organogel OG (1%, in ethanol), metallogels CaG, CaFeG (1%, in ethanol,
for CaG, G : Ca2+ =À1 : 1; for CaFeG, G : Fe3+ : Ca2+ = 1 : 2 : 1) and CaFeG
treated with H2PO4 and illuminated at 365 nm.
weitaibao@126.com
† Electronic supplementary information (ESI) available: Experimental details,
synthesis of G, and other materials. See DOI: 10.1039/c4cc03753g
‡ These authors contributed equally.
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intensity at 469 nm showed a sudden increase and reached a steady
state, which indicated that the fluorescence of metallogel CaG
was aggregation-induced emission (AIE).7 Meanwhile, the Fe3+-
coordinated metallogel FeG did not show fluorescence emission
(Fig. S2 and S4, ESI†). These results illustrated that the formation
of metallogel CaG could enhance the fluorescence emission of
OG by AIE, while the formation of metallogel FeG could quench
the weak fluorescence emission of OG.
Because Ca2+ belongs to the IIA group and Fe3+ belongs to the
VIIIB group, the coordination ability of Fe3+ with acylhydrazone is
significantly stronger than Ca2+. Therefore, we presumed that by
adding Fe3+ to the metallogel CaG, Fe3+ could competitively coordi-
nate with the gelator G in CaG and release Ca2+. This process could
quench the AIE of the CaG. Interestingly, the experimental results
confirmed this assumption. The addition and diffusion of 2 equiv.
Fe3+ into the CaG (1%) could form Fe3+ and Ca2+-based metallogel
CaFeG (Scheme 1). With the diffusion of Fe3+, the fluorescence
emission of the CaG quenched (Scheme 1 & Fig. S4, ESI†) and the
white color of CaG changed to brown, which is similar to the color
of FeG. In the corresponding UV-vis spectra, a broad absorption at
ca. 374 nm appeared (Fig. S6, ESI†). In addition, the dissolution of
the gelator G with 1 equiv. of Ca2+ and 2 equiv. of Fe3+ cations in hot
ethanol also generated the same CaFeG (1%).
Fig. 1 (a) Fluorescence spectra of CaFeG (1%, in ethanol, CaFeG, G:Fe3+ :Ca2+
=
1 : 2 : 1) in the presence of various anions (5 equiv. of FÀ, ClÀ, BrÀ, IÀ, AcOÀ,
HSO4À, H2PO4À, N3À, SCNÀ, ClO4 and CNÀ, respectively, using their 1.0 Â
À
The anion response capability of the metallogel CaFeG was
primarily investigated by adding 5 equiv. of various aÀnions
(FÀ, ClÀ, BrÀ, IÀ, AcOÀ, HSO4À, H2PO4À, N3À, SCNÀ, ClO4 and
CNÀ using their sodium salts water solution as the sources) to
CaFeGÀ. As shown in Scheme 1 and Fig. 1a, upon the addition of
H2PO4 to CaFeG at 20 1C and with the diffusion of H2PO4À, the
CaFeG emitted strong brilliant blue fluorescence emission at
469 nm, while the UV-vis absorption at ca. 374 nm decreased. This
fluorescence emission is similar to that of the CaG. These results
10À3 M sodium salts water solution as the sources) at room temperature.
(b) Fluorescent ‘‘OFF–ON–OFF’’ cycles of CaFeG (1%, in ethanol, G:Fe3+ :Ca2+
=
1:2:1), controlled by the alternative addition of Fe3+ and H2PO4À, lex = 350 nm.
confirmed that the H2PO4 competitively bound to the Fe3+, while
À
the Ca2+ again coordinated with the gelator G. Interestingly, upon
the addition of Fe3+ into the H2PO4À-containing CaFeG, the fluores-
cence of CaFeG could be quenched, which was also attributed to
Fe3+ coordination with G. These properties enable CaFeG to act as a
H2PO4 and Fe3+ controlled ‘‘OFF–ON–OFF’’ fluorescent switch. By
À
alternating the addition of H2PO4 and Fe3+, the switch could be
À
Fig. 2 (a) Photograph of metallogel CaFeG (1%, in ethanol, CaFeG, G:Fe3+
:
reversibly performed at least for three cycles with slight loss in
fluorescent efficiency (Fig. 1b). Because other anions could not cause
similar fluorescent response, it was clear that the CaFeG could detect
Ca2+ = 1 : 2 : 1) selectively detects H2PO4À (5 equiv., using 0.1 mol LÀ1 NaH2PO4
À
water solution as the H2PO4 sources) in water solution on a spot plate,
illuminated at 365 nm. (b) Writing and erasing of a natural light invisible image
on a CaFeG supramolecular bimetal–gel film obtained from 1% ethanol
metallogel, CaFeG, G:Ca2+ :Fe3+ = 1 : 1 : 2. The photographs were taken at
room temperature under normal light and exposed to 365 nm ultraviolet light.
À
H2PO4À with specific selectivity (Fig. 1a and 2a). Moreover, the
H2PO4 response sensitivity of CaFeG was very high. As shown in
Fig. S7 (ESI†), with the gradual addition of H2PO4À, the emission
intensity at 469 nm increased along with increasing concentrations
of H2PO4À. The detection limit of the changes in fluorescence
spectra calculated on the basis of cL = ksB/S (where sB is the standard Simply, because there are three kinds of noncovalent motifs in
deviation of the blank measures, S is the seÀnsitivity of the method, the gelator G, even if the hydrogen bonds were destroyed by
k = 3)8 was 1.0 Â 10À6 M (1 mM) for H2PO4 anion.
metal ions, the other noncovalent interactions could maintain
It is very interesting that unlike most of the reported stimuli- the gel states of the organogels.
responsive gels that showed gel–sol phase transition according
To facilitate the use of the CaFeG, the H2PO4À response film
to the stimulation of anions, the gel state of CaFeG did not show based on CaFeG was prepared by pouring hot ethanol solution
any gel to sol changes in the entire H2PO4À response process. This of CaFeG onto a clean glass surface and then drying in the air.
special stability could be attributed to the cooperation of multi The CaFeG film has no fluorescence emission but when writing
self-assembly forces we rationally introduced to the gelators. on the film with a writing brush dipped in H2PO4À water solution,
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a brilliant blue fluorescent image of the writing appeared (Fig. 2b). Fig. S11 (ESI†), the SEM images of OG showed an overlapped rugate-
This fluorescent image could be erased by brushing Fe3+ on the layered structure. The thickness of each layer was approximately
film. Therefore, the CaFeG film could not only act as a convenient 10–20 nm. The metallogels CaG and CaFeG also showed overlapped
reversible H2PO4À detection test kit, but also act as an erasable rugate-layered structures. These results indicated that although
dual-channel security display material. It is worth noting that gelator G coordinated with Ca2+ or Fe3+, significant changes did
security display materials are composed of invisible substances not occur in the self-assembly states. Moreover, the XRD patterns
that provide printed images that cannot be photocopied and are (Fig. S12, ESI†) of the OG, FeG, CaFeG and the CaFeG treated
À
readable only under special environments. Although thermally with H2PO4 showed periodical diffraction peaks, indicating
rewritable printing media have already been developed, security that they assembled into the ordered structures. The peaks at 2y =
display materials that allow the rewriting of invisible printed 22.96–23.761 corresponded to d-spacing 3.73–3.87 Å, which sug-
images are very rare.9
gested that p–p stacking existed in the naphthyl groups of these gels.
The self-assembly and stimuli-response mechanism of the
In summary, we have demonstrated a new strategy for the
supramolecular gels were carefully investigated. In concentration designing of stimuli-responsive supramolecular gels. In this manner,
dependent 1H NMR (Fig. S8a–c, ESI†) of G, the –NH (Ha) and the stimuli-response selectivity and signal reporting property of the
–NQCH (Hb) resonance signals showed obvious downfield shifts as supramolecular gels are controlled by the competitive coordination of
the concentration of G increased. Moreover, in FT-IR (Fig. S9, ESI†), two different metal ions with gelators and guest. In the present work,
the CQO vibration absorption of OG shifted to a low wavenumber by the competitive coordination of Ca2+, Fe3+ with gelator and
in the corresponding OG xerogel. These results revealed that during H2PO4À, the strong aggregation-induced emission of the metallogel
gelation, the –NH (Ha) and –NQCH (Hb) groups formed hydrogen CaG was controlled as ‘‘OFF–ON–OFF’’. The CaFeG could not only act
bonds with the –CQO groups on adjacent gelators. On the other as a convenient highly selective and sensitive H2PO4À detection test
hand, as shown in Fig. S8a–c (ESI†), with a gradual increase in kit, but could also act as an erasable secret documentation medium.
1
concentration, the H NMR signal of naphthyl protons (Hc, Hd,
This work was supported by the National Natural Science
He and Hf) showed obvious upfield shift, indicating that the p–p Foundation of China (NSFC) (No. 21064006; 21161018;
stacking interactions between the naphthyl groups were involved 21262032), the Natural Science Foundation of Gansu Province
in gelation.10 Therefore, as illustrated in Scheme S2 (ESI†), the (1308RJZA221) and the Program for Changjiang Scholars and
gelator G self-assembled to organogel OG by hydrogen bonds, Innovative Research Team in University of Ministry of Educa-
p–p stacking, as well as the van der Waals force existing in the tion of China (IRT1177).
long alkyl chains.
The formation of metallogels was also investigated. As shown in
Notes and references
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