Designing isometrical gel precursors to identify the gelation pathway for nickel-selective metallohydrogels

Article abstract of DOI:10.1039/C6DT03828J

Two novel multi-responsive metallohydrogels, namely, 2-PF-Ni and 3-PF-Ni, were successfully constructed from phenylalanine derivatives. The 2-PF gelator shows specific responses to Ni²⁺; particularly, 2-PF-Ni is lavender colored, which has been rarely reported among hydrogels triggered by Ni²⁺. 3-PF-Ni is light green and exhibits perfect thixotropy. This paper provides insights into the gelation mechanisms of these two metallogels.

Full text of DOI:10.1039/C6DT03828J

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DOI: 10.1039/C6DT03828J
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COMMUNICATION
Designing Isometrical Gel Precursors to Identify the Gelation pathway for Nickel-
Selective Metallohydrogels
a,b
a
a
a
a
a‡
a
Xiaojuan Wang, Ting He, Lan Yang, Huiqiong Wu, Jiafu Yin, Rujuan Shen, Juan Xiang, Yi
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
Zhang, * and Chuanwan Wei, *
DOI: 10.1039/x0xx00000x
www.rsc.org/
3
0, 31
32–34
Two novel multi-responsive metallohydrogels, namely, 2–PF–Ni ions.
Studies on the pyridyl group as metal-binding moieties
3
5
and 3–PF–Ni, were successfully constructed from phenylalanine and terpyridine-based metallogelators have prompted our current
2
+
research. A series of regioisomers of phenylalanine derivatives
compounds 1, 2, and 3, named as 2–PF, 3–PF, and 4–PF,
respectively) were synthesized (Fig. 1A). As we expected, 3–PF self-
assembled to 3–PF–Ni gel in the presence of Ni and was confirmed
to be a perfect metallohydrogelator. 3–PF–Ni presented a green color,
derivatives. The 2–PF gelator shows specific responses to Ni ;
particularly, 2–PF–Ni is lavender, which has been rarely
reported among hydrogels triggered by Ni . 3–PF–Ni is light
green and exhibits perfect thixotropy. This paper provides
insights into the gelation mechanisms of these two metallogels.
(
2
+
2
+
2
+
36, 37
similar to a limited number of reported Ni metallohydrogels.
2
+
Stimuli-responsive supramolecular hydrogels have attracted Hitherto, the gels triggered by Ni are either blue or green in color.
widespread attention among biologists and material scientists owing Intriguingly, the 2–PF–Ni complex finally turned out to be a
to their various potential applications in biosciences and material lavender metallohydrogel after a series of tricky treatments,
sciences, such as drug delivery and release, wound healing, signal including sonication, heating, and long time incubation in a narrow
1
-9
2+
sensors and pollutant sequestering.
Conjugating metal–ligand pH range (Fig. 1B); the product is
a
rare Ni -triggered
coordination interactions with other non-covalent interactions (e.g., metallohydrogel.
hydrophobic interactions, hydrogen bonds, and π−π stacking, etc) is
2
+
one of the most straightforward and effective methods for
It is notable that 2–PF specifically binds Ni among commonly
10-12
2+
2+
2+
2+
2+
2+
3+
constructing functional and stimuli-responsive metallohydrogels.
used metal ions, such as Mn , Pb , Zn , Mg , Ni , Cd , Fe ,
2
+
Metallohydrogels can inherit catalysis/redox properties from their and Co to form a hydrogel (Fig. 1C). The as-prepared 2–PF–Ni gel
metal–ligand moieties as well as photoresponsiveness from other is sensitive to pH, but stable within pH 7.0 to 8.0 (Fig. S7†). The
1
3-16
moieties.
The cross-linked network structure of metallogels critical gelation concentration (CGC) was 0.08 M at room
formed through metal–ligand interaction is stable and can be easily
1
7, 18
tuned by changing the transition metal ions.
On the other hand,
although numerous metallogelators have been developed and
1
9, 20
studied,
establishing rational designs for multi-responsive
metallogelators remains challenging.
2
1, 22
To continue research on tryptophan-based metallogelators,
we
theoretically predicted that the 3–PF (Fig. 1A) conformation could
bind transition metal ions and lead to cross-linked networks, which
may consequently form
a
hydrogel. Short peptide-based
metallogelators are designed based on innovative works on amino
2
3-29
acid-based gelators.
This kind of gelators can gel different metal
a.
College of Chemistry and Chemical Engineering and ‡State Key Laboratory of
Powder Metallurgy, Central South University, Changsha 410083, China. E-
mail:yzhangcsu@mail.csu.edu.cn; FAX: 0086(731) 883-6964
College of Chemistry and Chemical Engineering, University of South China,
Hengyang 421000, China. E-mail: weichw@hotmail.com
Fig. 1 A) Chemical structures of the gelators; B) Digital photos of
metallohydrogels 2–PF–Ni(left) and 3–PF–Ni(middle), 4–PF–Ni
complex(right); C) Complex images of 2–PF and different metal
ions.
b.
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available.
See
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temperature when the molar ratio of 2–PF and Ni was 2:1. Ultra- network of nanofibers with 40–90 nm in diameteVriewanAdrticsleeOvnelrinael
sonication proved to be a key to accelerate the gelation of 2–PF–Ni.
micrometers in length (Figs. 3A and S12B†)D. OFiI:g1.0S.11013B9/†Cs6hDoTw03s8t2h8aJt
the 3–PF gelator self-assembled into non-intertwined and relatively
loose nanofibers below the gelation concentration (0.05 M, aqueous
solution).
The gelation of 3–PF–Ni is a facile process. The turquoise
metallohydrogel 3–PF–Ni was formed instantly just by mixing 3–PF
2
+
with Ni aqueous solutions within pH 4.0 to 8.0, without requiring
any additional treatments, such as ultrasound or heating (Figs. S8
The electron microscopy results provide a structural perspective to
and S9†), and the CGC was 0.05 M. It was observed that the understand the viscosity and stability of these metallogels. The
gelation occurred at all the gelation concentrations from 0.25 to 1 cross-linked network structure of 3–PF–Ni is the key to maintain the
2
+
equivalents of Ni ions with respect to 3–PF. By contrast, the 4– gel state under shearing force, whereas the stacked-rods structure of
PF–Ni complex presents as precipitate, confirming that the slight 2–PF–Ni is sensitive to shearing force and exhibits strong tendency
difference in molecular structure significantly changes the to self-aggregate. This finding further suggests that it is the
hydrogelation property.
conformation arrangement, not the density of gelator in solution (the
concentration of monomer) that plays the key role to determine the
gelation property of metallohydrogels.
Rheological tests of the metallohydrogels indicated that the
average values of the storage modulus (G′) of 2–PF–Ni and 3–PF–Ni
were always higher than the loss modulus (G″), respectively (Fig.
S10†). And the G′ and G″ values exhibited slight dependence on identify the coordination structures between Ni and the gelator
frequency over the entire measurement range (0.1–100 Hz), precursors. The peak at 3430 cm (ν-NH) shifted to 3390 cm , and
confirming the elastic nature of stable gel phases of the both the peak at 3062 cm (ν-COOH) disappeared (Fig. 2B). This finding
complexes. As expected, the G′ values of 3–PF–Ni are higher than indicates that the carboxyl oxygen and amino nitrogen atoms were
those of 2–PF–Ni. 3–PF–Ni metallohydrogel exhibited excellent involved in the coordination between Ni and 2–PF. Besides, the
Fourier-transform infrared spectra (FT-IR) were recorded to
2
+
−1
−1
−1
2
+
stability and did not change at room temperature for several months. peak of pyridine ring skeleton stretching vibration shifted from 1588
−1
−1
By contrast, 2–PF–Ni gel collapsed after two-three weeks' cm to 1602 cm , demonstrating that the presence of pyridine N
3
8
incubation.
atom is significant to the formation of 2–PF–Ni metallohydrogel.
Hence, it can be concluded that in addition to the carboxyl oxygen
and amino nitrogen atoms, the pyridine nitrogen atom participated in
The above results clearly suggest that the gelation pathways are
significantly different between 2–PF–Ni and 3–PF–Ni. Scanning
electron microscopy (SEM) and transmission electron microscopy
2
+
the coordination of Ni and 2–PF. For the 3–PF–Ni gel, the peak at
−1
−1
−1
3
431 cm (ν-NH) shifted to 3422 cm , and the peak at 3031 cm (ν
-
(
TEM) analyses were carried out to reveal the morphological details
COOH) disappeared, suggesting that the carboxyl oxygen and amino
nitrogen atoms participated in the coordination (Fig. 3B).
of 2–PF–Ni and 3–PF–Ni metallohydrogels (Figs. 2 and 3). The
comparison results indicated that 2–PF (0.08 M) self-assembled to
flake-like aggregates in the absence of Ni , (Fig. S11A†), whereas
2
+
2+
Fluorescence spectra demonstrated that the addition of Ni to the
2
–PF–Ni aggregated to rod-like clusters, which were densely stacked solutions of 2–PF and 3–PF caused immediate fluorescence
but not entangled with one another (Figs. 2A and S12A†). Under the quenching (Fig. 2C and S13†). This finding confirms the occurrence
2
+
same condition, 3–PF–Ni xerogel presents as an interconnected of energy transfer or electron transfer between Ni and the gelators,
3
8
which corresponding to a rapid non-radiative decay. The peak at
60 nm indicates the existence of strong aromatic interactions
3
3
9
among molecular backbones of the two gelators. These results are
consistent with those of the circular dichroism (CD) spectra (Fig.
S14†). It was observed that the strength of CD signals increase
between 240 nm and 300 nm when Ni was added to the gelator
solutions, suggesting that the enhanced π–π interactions facilitate the
formation of the ordered molecular arrangement.
2
+
X-ray diffraction (XRD) studies on 2–PF–Ni and 3–PF–Ni
xerogels were performed to obtain insights into the molecular
arrangements of the metallogels. The XDR results suggested that the
lyophilized skeleton of 2–PF–Ni presented an ordered structure (Fig.
2
D). The peak at a d-spacing of 3.2 Å (2θ = 28.4°) confirmed the
presence of metal–metal interactions during the gelling process. The
observed 5.67 Å peak is assigned to the stacking distance of the 2–
PF–Ni molecular backbones. Another peak at d=3.73 Å represents
Fig. 2 A)TEM image of 2–PF–Ni B) FT-IR spectra of 2–PF, and 2–
PF–Ni xerogel C) Fluorescence spectra of 2–PF (2 mM) and 2–PF–
Ni complex (the mole ratio of 2–PF to Ni is 2:1) and D) XRD
pattern of 2–PF–Ni xerogel.
4
0
2
+
the strong π−π interactions among the molecular benzene rings or
4
1, 42
pyridine rings.
For 3–PF–Ni, a common broad diffraction peak
2
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2
+
binds Ni to form a hexa-coordination compound. First, 2–PF
View Article Online
2
+
gelator self-assembled into flake-like fibersDinOI:th1e0.1p0r3e9s/eCn6cDeTo0f38N2i8J
and then the fibers aggregated to produce rod-like clusters by various
non-covalent interactions (e.g., metal-metal interactions, π−π
interactions, etc.). For 3–PF–Ni, 3–PF moiety may serve as a
2
+
bidentate ligand that binds Ni to form tetra-coordination compound
by carboxyl oxygen and amino nitrogen atoms. The pyridine
nitrogen atom that is not involved in the coordination can form
additional hydrogen bonds with water molecules, leading to the
formation of a network structure.
UV-vis diffuse reflectance spectroscopy (DRS) analysis was also
2
+
performed to infer the complex state of Ni in 2–PF–Ni and 3–PF–
Ni. The bands (Fig.S17A-a†) observed at 450 and 689 nm can be
assigned to the octahedral configuration of 2–PF–Ni (hexa-
coordination compound). For 3–PF–Ni, the typical DRS band of
Fig. 3 A)TEM image of 3–PF–Ni B)FT-IR spectra of 3–PF, and 3–
PF–Ni xerogel C)XRD pattern of 3–PF–Ni xerogel and
D)Oscillatory rheology of the 3–PF–Ni metallohydrogel prepared at
the CGC, step-strain measurements at 0.1 Hz.
2+
Ni appeared at 530 nm, which corresponds to a tetrahedral
symmetry (Fig. S17A-b†). These results are consistent with the
45
findings reported in the literature, hence, our hypothesis of
coordination approach to 2–PF–Ni and 3–PF–Ni is confirmed.
The as-proposed gelation pathway of 2–PF–Ni and 3–PF–Ni can
be used to explain their multi-responsiveness to various physical and
chemical stimuli (Figs. S18 and S19†). The cross-linked networks
structure plays an important role for the excellent thixotropy of 3–
PF–Ni gel that was investigated by a time-dependent step-strain
rheological experiment (Figs. 3D and S20†). The average values of
G′ and G′′ were not affected even if the cycle was repeated five
times; hence, the formed gel exhibits potential as an excellent soft
material with rapid thixotropic responsiveness. The rheological
viscosity of 2–PF–Ni gel is weaker because it has stacked skeleton
structure. This type of structure exhibits strong tendency to further
aggregate to bulk precipitates and leads to gel collapse.
at 20° was observed, which indicates the amorphous nature of the 3–
4
3
PF–Ni gel (Fig. 3C). In addition, a typical peak at a d-spacing of
4
4
2
.3 Å can be assigned to H-bonds in 3–PF–Ni gel networks.
The mass spectra [MS (ESI)] results indicated that the
2
+
coordination ratio between two kinds of ligands and Ni is 2:1,
respectively (Figs. S15 and S16†). Based on all of the above
observations, schematic representations of the probable local
structure of metallohydrogels of 2–PF–Ni and 3–PF–Ni are proposed
(
Fig. 4). The possible coordination approach of 2–PF–Ni is that 2–
PF as tridentate ligand
This proposal is also useful to understand significant chiral
sensitivity of 2–PF–Ni. As shown in Fig. S24†, the CD signals of
pure dextrorotatory (D-) and levorotatory (L-) ligands exhibited
perfect symmetry along the X axis, indicating that the chirality arises
from the PF moiety. Although L-2–PF–Ni and D-2–PF–Ni xerogels
possessed similar structures, the morphological structure of the
racemic mixtures were significantly changed (Fig. S25†). As the
result, the 2–PF–Ni maintained acceptable gel strength when the
enantiomeric excess (e.e) was 70% to 100%. The 2–PF–Ni gel
transformed to viscous fluid when the e.e ranged from 10% to 70%,
whereas, precipitate appeared when the e.e was below 10%. The
results elucidate that enantiomeric purity is the key to form stable
4
6
gels due to the chiral symmetry forbidden effect. By contrast, the
chirality of the gelator has no significant influence on the formation
of 3–PF–Ni metallogel. And 3–PF–Ni metallogel exhibited excellent
performance of adsorbing toxic dyes (methyl orange) (Figs. S26A
and S26B†), whereas the dyes-adsorbing ability of 2–PF–Ni
metallogel was weaker than that of 3–PF–Ni.
Fig. 4 Schematic representations of the probable local structure for
A)2–PF–Ni metallohydrogel B)3–PF–Ni metallohydrogel.
In summary, we have studied the gelling behaviours of three
2
+
simply positional isomers with Ni in water. The experimental
results revealed that these gelators showed utterly different gelling
2
+
abilities with Ni owing to the various positions of the pyridine
2
+
nitrogen atoms. Although two of the regioisomers can bind Ni to
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form metallohydrogels, the as-prepared metallohydrogels, 2–PF–Ni
and 3–PF–Ni exhibited dramatically distinguishing gelation
properties in terms of color, stability, rheological characteristics,
stimulus responsiveness, and chiral sensitivity. The coordination
19. M. J. Mayoral, C. Rest, V. Stepanenko, J. SchelVlhieewimArteicrl,eROn. lQine.
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2
1
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mode between Ni and the gelator precursors is the key to
determine the gelation pathway, which finally leads to the individual
functional properties of metallogels. Intriguingly, the formation of
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+
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metallohydrogel. We believe that 2–PF–Ni will serve as a valuable
archetypical template for designing excellent metallogelators.
Further studies regarding the potential applications of 2–PF–Ni and
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Acknowledgments
This work was financially supported by the NSFC of China
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21473257), National High Technology Research and Development
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Hunan Province of China (15C1168, 14C1004) and Doctoral
scientific research foundation of University of South China
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(2014XQD32). We are grateful for the funding from the State Key
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DOI: 10.1039/C6DT03828J
2+
Two positional isomers can bind Ni to form different metallohydrogels, which exhibited dramatically
distinguishing gelation properties.