Fabrication of well-defined crystalline azacalixarene nanosheets assisted by Se...N non-covalent interactions

Article abstract of DOI:10.1039/c2cc33760f

We have employed a selenium containing amphiphile to assist the formation of well-defined azacalixarene nanosheets, in which the Se...N non-covalent interaction plays an important role.

Full text of DOI:10.1039/c2cc33760f

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COMMUNICATION
Fabrication of well-defined crystalline azacalixarene nanosheets assisted
by SeÁ Á ÁN non-covalent interactionsw
Yu Yi,a Shixin Fa,b Wei Cao,a Lingwu Zeng,a Meixiang Wang,c Huaping Xu*a and
Xi Zhanga
Received 25th May 2012, Accepted 10th June 2012
DOI: 10.1039/c2cc33760f
We have employed a selenium containing amphiphile to assist
the formation of well-defined azacalixarene nanosheets, in which
the SeÁ Á ÁN non-covalent interaction plays an important role.
weak SeÁ Á ÁN non-covalent interaction for self-assembly, we
employ a Se containing amphiphile (SeG), which has three
hydrophilic branches with a Se atom in each branch, and an
azacalixarene APy6 with twelve nitrogen atoms which can
form both multiple SeÁ Á ÁN non-covalent interactions as well as
hydrophobic interactions with the SeG molecules.
In a typical procedure, 2 Â 10À6 mol SeG and 5 Â 10À7 mol
APy6 were mixed in dichloromethane, a good solvent for both
molecules. The solvent was then removed under reduced
pressure and 1 mL water was added. A turbid suspension
was obtained after sonication for 2 min and the resulting
solution was stood for 12 h before characterization.
Azacalixarene, a new kind of macrocycle with well-defined
conformations and multiple nitrogen atoms, has recently
drawn considerable attention in supramolecular recognition,
gas storage, catalysis and enzyme mimics.1–6 One of the
strategies to realize and enrich those functions is to prepare
well-defined nano-crystals and to use their ordered structures
and microenvironments.7 Until now, most of the well-defined
crystals of heteracalixaromatics have been obtained in
organic solvents. How to fabricate well-defined crystals of
heteroacalixaromatics with controlled manner in aqueous
solution remains a challenge.
To investigate the morphology of the nanosheets, transmis-
sion electron microscopy (TEM) and atomic force microscope
(AFM) were employed. As shown in Fig. 1a and b, well-
defined nanosheets which are 1–2 mm long and 300–500 nm
wide can be observed. Cryo-TEM images (Fig. S1, ESIw) also
support the result and indicate that the formation of the
nanosheets is in solution instead of on the substrates during
the drying process. With the same preparation procedure, the
amphiphilic SeG molecules form micelles in water with a
diameter of 140 nm (Fig. 1c and S2a, ESIw) when they are
above the critical micelle concentration (1.2 mM, Fig. S2b,
ESIw). The insoluble APy6 molecules in aqueous solution only
exist as irregular aggregates from TEM observations (Fig. 1d).
Therefore, it can be speculated that the well-defined nano-
sheets are formed by both SeG and APy6.
Selenium is an essential trace element and micronutrient for
the human body. Recent advances indicate that selenium has
the potential application in various fields, including quantum
dots,8 semiconducting conjugated materials for transistor and
solar cell applications,9–15 responsive polymeric biomaterials
for controlled drug delivery16–20 etc. Great effort has also been
seen with mimics of glutathione peroxidase (GPx),21–25
a
mammalian antioxidant seleno-enzyme that protects bio-
membranes and other cellular components from oxidative
damage. It is found that intra-molecular SeÁ Á ÁN non-covalent
interactions are important in GPx and its mimics.26–30 How-
ever, inter-molecular SeÁ Á ÁN non-covalent interactions have
rarely been used as driving force for controlled self-assembly.
In this communication, we reported the fabrication of well-
defined azacalix[6]pyridine (APy6) nanosheets in an aqueous
solution (Scheme 1), which was assisted by inter-molecular
SeÁ Á ÁN non-covalent interactions. In order to enhance the
In order to know whether the well-defined nanosheets are
crystalline or not, TEM electron diffraction and X-ray diffrac-
tion (XRD) measurements were carried out. The electron
diffraction pattern of the nanosheets suggest that they are a
a Key Lab of Organic Optoelectronics and Molecular Engineering,
Department of Chemistry, Tsinghua University, Beijing, 100084,
P. R. China. E-mail: xuhuaping@mail.tsinghua.edu.cn;
Tel: +86-10-62773672
b CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences, Beijing,
100190, P. R. China.
c Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology, Department of Chemistry, Tsinghua University, Beijing,
100084, P. R. China.
w Electronic supplementary information (ESI) available: Experimental
details for synthesis and characterization. See DOI: 10.1039/c2cc33760f
Scheme 1 Fabrication of well-defined azacalix[6]pyridine nanosheets
assisted by SeÁ Á ÁN non-covalent interactions.
c
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Chem. Commun., 2012, 48, 7495–7497 7495

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of the SeÁ Á ÁN non-covalent interaction is that the selenium
atom partially accepts the lone pair electrons of the nitrogen
atom,29 when SeÁ Á ÁN non-covalent interaction forms, the
binding energy of selenium atom in the XPS spectrum will
decrease. As expected, the Se 3d3 binding energy of SeG
molecule in XPS spectrum shifts from 51.80 eV down to
50.82 eV after the formation of the nanosheets (Fig. 3a).
Furthermore, FT-IR spectrum shows the vibration of a
C–Se bond, which is identified with the literature31 and
Gaussian calculations32 of a model molecule (see ESIw), shifts
from 561 cmÀ1 up to 568 cmÀ1 (Fig. 3b). In addition, a
SeG/APy6 1 : 1 complex (plus K+, m/z: 1471.1684, calcd:
1471.4107) is found in the ESI-Q-TOF spectrum (Fig. S5,
ESIw), indicating the complexation of SeG and APy6 mole-
cules. Moreover, the two dimensional structure of the nano-
sheets indicate that the SeG molecules prefer to bind with the
APy6 molecules’ face packing direction and inhibit the growth
of this dimension. Therefore, all these data suggest the
existence of SeÁ Á ÁN non-covalent interactions between the
SeG and APy6 molecules.
To further study the SeÁ Á ÁN non-covalent interactions
between the SeG and APy6 molecules, three control experi-
ments were carried out. Firstly, a non-selenium containing
amphiphile, 1,3,5-tri(2,5,8,11-tetraoxadodecyl)benzene (OG)
in which selenium atoms are replaced by oxygen atoms, is
synthesized. Irregular aggregates are observed (Fig. 4a) with
the same preparation procedure, indicating the important
role of Se atoms played in the formation of the nanosheets.
In addition, HCl (aq) and H2O2 (aq) were added to the
suspension of the nanosheets so the final pH was 2 and the
percentage of H2O2 was 1%. When treated by pH = 2 HCl
(aq) for 12 h, the nanosheets are decomposed (Fig. 4b) because
of the protonation of nitrogen atoms in the APy6 molecules.
When treated by 1% H2O2 (aq) for 12 h, the surfaces of the
nanosheets are partly destroyed (Fig. 4c) because of the
oxidation of the Se atoms.17,19,20 Both the protonation of
APy6 and the oxidation of SeG can disrupt the SeÁ Á ÁN non-
covalent interactions as well as hydrophobic interaction and
lead to the deformation of the nanosheets. These experiments
indicate that the SeG and APy6 molecules are mainly com-
bined in the outer layers of the nanosheets and the central part
of them are mainly composed of APy6 molecules.
Fig. 1 TEM (a) and AFM (b) images of the nanosheets. Well-defined
nanosheets which are 1–2 mm long and 300–500 nm wide are observed.
TEM images of (c) the micelles of SeG in water with an average
diameter of 140 nm, and (d) the irregular aggregates of the insoluble
APy6 molecules in aqueous solution.
triclinic single crystal structure (Fig. 2a). XRD result shows
the layer distance of the nanosheets are 0.93 nm (Fig. 2b),
which agrees well with the vertical distance of the cell length
corresponding to the face packing direction of APy6 molecules
in single crystals (calculated as 9.35 A, see ESIw).5 AFM was
employed to measure the thickness of the nanosheets. The
AFM height results show the thickness of the nanosheets
varies from 24 nm to 35 nm (Fig. S3, ESIw), indicating that
the nanosheets are composed of multi-layers. The results
above indicate that the nanosheets are mainly constructed by
the APy6 molecules as nano-crystalline structures.
We wondered whether SeG still existed in the nanosheets
and whether the SeÁ Á ÁN non-covalent interactions played an
important role in the formation of the nanosheets, TEM-based
energy dispersive spectroscopy (TEM-EDS), X-ray photo-
electron spectroscopy (XPS), Fourier transform infrared (FT-IR)
spectroscopy and ESI-Q-TOF spectrum were employed.
TEM-EDS shows Se atoms are contained in the nanosheets
even after being rinsed 5 times (Fig. S4, ESIw). Since the nature
A possible mechanism is proposed to explain the formation
of the nanosheets. It has been reported that the surfactants
such as sodium dodecyl sulfate could affect the crystallization
Fig. 2 TEM electron diffraction pattern (a) and XRD (b) of the
nanosheets. (a) The electron diffraction pattern of the nanosheets
suggest that they are a triclinic single crystal structure. (b) The 001
and 002 peaks are 9.481 and 19.01 respectively, indicating that the
distance between layers of the nanosheets is 0.93 nm.
Fig. 3 XPS analysis (a) and FT-IR spectrum (b) of the nanosheets on
Si substrates. After the formation of the nanosheets, (a) the Se 3d3
biding energy of SeG shifts from 51.80 eV to 50.82 eV, while (b) the
C–Se bond vibration shifts from 561 cmÀ1 to 568 cmÀ1, indicating the
existence of the SeÁ Á ÁN non-covalent interactions.
c
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the SeG molecules first serve as surfactants helping to dissolve
the APy6 molecules and facilitate the crystal growth. Then parts
of SeG and APy6 molecules are complexed together and
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of the face packing dimension to assist the formation of two
dimensional nanosheets. In addition, the adsorption of the SeG
molecules onto the nanosheets also prevents further growth of
the crystals and finally well-defined nanosheets are obtained.
In conclusion, we have successfully fabricated well-defined
azacalixarene nanosheets in aqueous solution. The experi-
mental results indicate that the formation of the nanosheets
is assisted by SeÁ Á ÁN non-covalent interactions. It is greatly
anticipated that SeÁ Á ÁN or SeÁ Á ÁX (X = Cl, Br, I) intra/inter
molecular interactions will act as new driving forces for
controlled self-assembly, leading to new GPx mimics or other
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We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (21074066),
the Foundation for Innovative Research Groups of the
National Natural Science Foundation of China (21121004),
Tsinghua University Initiative Scientific Research Program
(2009THZ02-2) and the NSFC-DFG joint grant (TRR 61).
We thank Prof. Zhibo Li and Mr Yu Liu (Chinese Academy of
Sciences) for Cryo-TEM and Ms Ling Hu (Tsinghua University)
for TEM-EDC and electron diffraction measurements. We
also appreciate the helpful discussions and suggestions from
Prof. Yapei Wang (Renmin University).
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