Fluorescent composite hydrogels of metal-organic frameworks and functionalized graphene oxide

Article abstract of DOI:10.1002/chem.201102603

GO MOFs! Azobenzoic acid functionalized graphene (A-GO) can act as a structure-directing template that influences hydrogel formation together with metal-organic frameworks (MOFs). Zn²⁺ MOFs of pyridine derivatives work as framework linkers between the A-GO sheets (MOF-A-GO, see figure). MOF-A-GO exhibits a strong fluorescence enhancement upon gel formation. In addition, MOF-A-GO selectively recognizes trinitrotoluene. Copyright

Full text of DOI:10.1002/chem.201102603

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DOI: 10.1002/chem.201102603
Fluorescent Composite Hydrogels of Metal–Organic Frameworks and
Functionalized Graphene Oxide
Ji Ha Lee,[a] Sunwoo Kang,[b] Justyn Jaworski,[a, c] Ki-Young Kwon,[a] Moo Lyong Seo,[a]
Jin Yong Lee,*[b] and Jong Hwa Jung*[a]
Graphene and its functionalized derivatives are versatile
building blocks for carbon-based materials, because of the
unique 2D or 3D structures and excellent physical and
chemical properties.[1–9] Recent work has demonstrated that
self-assembly is a powerful technique for constructing hier-
archical graphene-based nanomaterials with novel func-
tions.[10–14] In particular, self-assembly of nanosized graphene
into macroscopic materials can translate the properties of
individual graphene sheets into the resulting macrostruc-
tures, which show numerous breakthrough applications in
optoelectronics,[15] energy storage,[16–18] and biomedicine.[19–21]
For example, transparent conducting membranes[22–24] and
strong, layered, paperlike materials[25–27] have been prepared
by various 2D self-assembly methods, including flow-direct-
ed self-assembly, layer-by-layer deposition, and Langmuir–
Blodgett techniques.
Very recently, graphene-based metal–organic frameworks
(MOFs) have been used by several groups to demonstrate
3D macroassemblies, due to the high porosity, the adsorp-
tion capacity for specific gases, and the electric properties of
the MOFs.[28–31] For example, Loh and co-workers reported
the synthesis of a crystalline MOF–graphene oxide compo-
site.[28] Benzoic-acid-functionalized graphene acted as a
structure-directing template that influenced the crystal
growth of the MOF. The nanowire obtained from the benzo-
ic-acid-functionalized graphene with MOF structure also im-
parted new electrical properties, such as photoelectric trans-
port. However, the work on 3D self-assembly of graphene
and its functionalized derivatives is still limited.[27–30] More
facile and mild assembly strategies are needed for fabrica-
tion of multifunctional, 3D, graphene macrostructures. Al-
ternatively, graphene-oxide-based hydrogels with MOF
structures possess similar properties and are relatively rapid,
efficient, and easy to prepare under mild conditions com-
pared with crystalline MOFs. With this in mind, we report
herein the formation of MOF–azobenzoic-acid-functional-
ized graphene-oxide hydrogels (MOF–A-GO) in the pres-
ence of Zn2+ and its application as a chemosensor for the
detection of trinitrotoluene (TNT) molecules.
The preparation of azobenzoic acid (2)-functionalized gra-
phene oxide (A-GO) is shown in Scheme 1. Initially, epoxy,
and hydroxyl groups were removed by reduction with
NaBH4. Next, the chemically reduced GO (r-GO) was func-
tionalized with azobenzoic acid (Scheme 1 and S1 in the
Supporting Information) by using the diazonium grafting
method, and this allowed the basal planes to become ex-
tended by azobenzoic-acid groups. A-GO was characterized
by UV/Vis, FTIR, X-ray photoelectron spectroscopy (XPS),
SEM, and TEM. Figure S1 in the Supporting Information
shows the UV/Vis absorption spectra of GO, r-GO, and A-
GO in water. The redshifted p–p* absorption band of r-GO
at 260 nm compared with the band of GO at 241 nm is con-
sistent with the partial recovery of the conjugated network.
In addition, absorption appeared at 400 nm, which is a typi-
cal wavelength for the azobenzoic acid attached to r-GO;
this strongly indicates that the azobenzoic-acid moiety exists
on the surface of r-GO. Moreover, A-GO shows improved
dispersion in water compared with r-GO. The solution dis-
persability of A-GO was examined with UV/vis spectrosco-
py. A linear relationship between absorbance and concentra-
tion is observed in water; this is indicative of good disper-
sion of A-GO.
[a] J. H. Lee, Prof. Dr. J. Jaworski, Prof. Dr. K.-Y. Kwon,
Prof. Dr. M. L. Seo, Prof. Dr. J. H. Jung
Department of Chemistry and Research Institute
of Natural Sciences, Gyeongsang National University
Jinju 660-701 (Korea)
Figure S2 in the Supporting Information shows the FTIR
spectra of r-GO and A-GO. The vibrational peaks of r-GO
appeared at 1718, 1678, and 1060 cmÀ1 for C=O, C OH, and
À
À
C O, respectively. On the other hand, new vibrational peaks
Fax : (+82)55-772-1488
E-mail: jonghwa@gnu.ac.kr
of A-GO appeared at 3012, 1638, 1598, and 1430 cmÀ1 for
À
aromatic C H streches, C=O, and C=C ring stretches of the
[b] S. Kang, Prof. Dr. J. Y. Lee
Department of Chemistry and Institute of Basic Science
Sungkyunkwan University, Suwon 440-746 (Korea)
E-mail: jinylee@skks.edu
azobenzoic-acid groups, respectively. This is also indicative
of attachment of the azobenzoic-acid moiety to the surface
of r-GO by covalent bonding. r-GO was further confirmed
by XPS before and after immobilization of azobenzoic acid
(Figure S3 in the Supporting Information). The XPS spec-
trum of r-GO before immobilization of azobenzoic acid
shows C1s and O1s binding energy (Figure S3a in the Sup-
[c] Prof. Dr. J. Jaworski
Department of Chemical Engineering
Hanyang University, Seoul 133-791 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102603.
Chem. Eur. J. 2012, 18, 765 – 769
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765

Scheme 1. Synthetic route of A-GO.
portign Information), whereas N1s for nitrogen appeared in
A-GO (Figure S3b).
the A-GO monolayer is regular with a height of 1.0 nm, in-
dicating that A-GO consists of a monolayer sheet.
A-GO was further confirmed by TEM and electron
energy loss spectroscopy (EELS, Figure 1). The material
contained carbon, oxygen, and nitrogen components, sup-
porting the idea that azobenzoic acid is homogenously at-
tached to the surface of r-GO. Figure 1B shows the atomic
force microscopy (AFM) image of an A-GO sheet, and Fig-
ure 1C shows the corresponding height profile. It shows that
When the A-GO (10 mgmLÀ1) suspension with Zn2+ and
ligand 1 (Scheme S2 in the Supporting Information) was
mixed by ultrasonication for a few seconds, the MOF–A-
GO composite hydrogel was obtained as shown in Fig-
ure 2A. On the other hand, ligand 1 with Zn2+ did not form
the hydrogel at the same concentration. These findings sug-
Figure 2. A) Photograph of MOF-A-GO composites hydrogels; a) 1:1:1
(metal/ligand/GO), b) 1:1:2, c) 1:1:3 mole ratios, d) sample in (c) irradiat-
ed with UV lamp. B) SEM image of a MOF-A-GO composites hydrogel
formed by a 1:1:3 (metal/ligand/GO) mole ratio. C) UV/Vis spectra of
a) the MOF–A-GO composite hydrogel (1:1:3 mole ratio), b) r-GO, c) A-
GO, and d) ligand 1 obtained from (a) with curve fitting. D) Fluorescence
spectrum of the MOF–A-GO composite hydrogel (1:1:3 mole ratio).
Figure 1. A) EELS-TEM images of A-GO; a) zero-loss image, b) carbon,
c) oxygen, and d) nitrogen components. B) AFM image of A-GO and
C) the height profile.
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MOF–GO Hydrogels
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gest that A-GO acted as an interlayer to form the stable
MOF structure in the gel phase.
To gain insight into the morphology of the MOF–A-GO
composite hydrogel induced by both A-GO and ligand 1 in
the presence of Zn2+, we observed a xerogel of MOF–A-
GO by SEM (Figure 2B). The morphology of MOF–A-GO
is quite different from that of A-GO. MOF–A-GO has a
well-defined nanorod structure with a width of 100–150 nm
and a length of several micrometers. The sheet structure of
A-GO was not obtained in MOF–A-GO. The results sup-
port the view that A-GO is bound to ligand 1 by a coordina-
tion bond with Zn2+; this forms the MOF structure.
Figure 3. A) Representation of proposed bonding between 1 and A-GO
in the presence of Zn2+. B) The expanded A-GO with two Zn2+ and four
1 optimized by MM calculation. (The GO sheet is denoted as balls and
sticks. The rest of moieties are denoted as sticks.)
The absorption and emission properties of A-GO, sol 1,
and MOF–A-GO in the presence of Zn2+ were extensively
studied. The UV/Vis absorption band of MOF–A-GO with
Zn2+ appears at 302 nm and 367 nm, respectively (Fig-
ure 2C), typical of a p–p* transition for the self-assembled
ligand 1 and the azobenzoic-acid moiety.[32,33] The fluores-
cence spectrum of MOF–A-GO (lex =302 nm) was also ob-
tained (Figure 2D). Interestingly, MOF–A-GO exhibits a
strong blue emission with a maximum at l=425 nm. The
fluorescence intensity is enhanced drastically compared with
sol 1. The photoluminescence of MOF–A-GO can be seen
by the naked eye under UV light (Figure 2Ad). The fluores-
cence intensity of MOF–A-GO gradually increased with the
addition of metal ion until 1.0 equivalent had been added
and then remained constant upon further additions (Fig-
ure S4 in the Supporting Information). These results indicate
that one molecule of ligand 1 binds to one Zn2+ ion. The lu-
minescence properties of MOF–A-GO were studied by
time-resolved fluorescence confocal microscopy. The emis-
sion-decay profiles were monitored at l=405–490 nm for
MOF–A-GO. The fluorescence decay of MOF–A-GO was
fitted with a single exponential component, yielding a life-
time of 2.75 ns (Figure S5 in the Supporting Information),
which indicates that this emission is fluorescence. This result
reflects the fact that MOF–A-GO in the aggregate state is
more rigid and restricts the rotational and vibrational move-
ments of the molecules. The limited molecular motions de-
crease the nonradiative relaxation process; this leads to a
longer lifetime and fluorescence enhancement.[34] In con-
trast, the lifetime of sol 1 is much shorter than that of
MOF–A-GO.
By using the structural parameters and geometrical fea-
tures of the optimized structure, the expanded size of A-GO
with two Zn2+ and four 1 was generated (Figure 3B). This
expanded structure was simply optimized with MM calcula-
tions by employing a universal force field due to computa-
tional capabilities. The interatomic distance between the
two Zn2+ atoms was calculated to be 18.37 ꢂ and that be-
tween pyridine nitrogens of two molecules of ligand 1 was
calculated to be 8.82 ꢂ, which indicates that a sufficient
cavity size exsists. Consequently, it would be expected from
the structural feature that the cavity can act as a recognition
site for certain guest molecules through hydrogen binding or
p–p* stacking.
Fluorescence of MOF–A-GO was also measured as a
function of temperature (Figure S6 in the Supporting Infor-
mation). No significant spectral changes were observed
below 1008C. The fluorescence intensity of MOF–A-GO
slightly decreases at 1058C. Further increases in tempera-
ture resulted in a large decrease in emission. These results
suggest that the emission of MOF–A-GO decreases as it
starts to melt at 105–1108C. The optimal emission of MOF–
A-GO therefore occurs when the gel is completely formed
and decreases as the gel melts at higher temperature. This
striking observation may be attributed to the rigidification
of the media upon gelation, a process that slows down non-
radiative-decay mechanisms and leads to luminescence en-
hancement. Even though the gel dissociates from the aggre-
gated state and shows a drastic drop in fluorescence intensi-
ty, the complex still exhibits a considerable blue emission in
the solution state. This may be due to weak supramolecular
interactions while still in the solution state.
Zn2+ is known to have Td- or Oh-coordinated structures
with certain ligands.[35,36] To understand the nature of the co-
ordination conformation of Zn2+, the favored coordinated
structure needs to be considered. Hence, we have carried
out density functional theory (DFT) calculations for Td and
Oh structures by employing the hybrid functional of Beckeꢁs
three parameterized Lee–Yang–Parr with the 6-31G* basis
sets and a suite of Gaussian 09 programs.[37] From the opti-
mized structures, we concluded that the Td structure should
be the major product, as is shown in Figure 3. The intera-
tomic distances between the oxygen atoms of the ligands (1
and A-GO) and Zn2+ were calculated to be 1.937–1.976 ꢂ.
The substituted azobenzoic moieties break the planarity of
GO.
To gain insight into the thermally promoted stability of
MOF–A-GO, the sol–gel transition temperature (Tgel) of
MOF–A-GO was measured by differential scanning calorim-
etry (DSC, Figure S7 in the Supporting Information). The
MOF–A-GO composite gel showed a sharp phase transition
at 1078C; this indicated an endothermic reaction. This endo-
thermic thermogram was attributed to the change of MOF–
A-GO into a sol state (Figure S8 in the Supporting Informa-
tion). This Tgel of MOF–A-GO is consistent with the results
obtained by fluorescence spectroscopy.
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J. H. Jung, J. Y. Lee et al.
Rheological information aids in understanding the behav-
ior of gels when they are exposed to mechanical stress, espe-
cially the “storage” (or “elastic”) modulus G’, which repre-
sents the ability of the deformed material to “snap back” to
the original geometry, and the “loss” (or “viscous”) modulus
G’’, which represents the tendency of a material to flow
under stress. Two rheological criteria are required for a gel:
1) the dynamic G’ has to be independent of the oscillatory
frequency, and 2) G’ should exceed G’’ by about one order
of magnitude. We first used a dynamic strain sweep to deter-
mine the proper condition for the subsequent examination
of the gel under a dynamic frequency sweep. As shown in
Figure S8A in the Supporting Information, the values of G’
and G’’ exhibited a weak strain dependence from 0.1 to
1.0% (with G’ dominating G’’); this indicates that the
sample is a gel. After setting the strain amplitude to 0.8%
(within the linear response regime for strain amplitute), we
used a dynamic frequency sweep to further study the gel.
Figure S8B demonstrates that G’ and G’’ are almost con-
stant despite an increase in frequency from 0.1 to
100 radsÀ1. The value of G’ is about five times larger than
that of G’’ over the whole range (0.1–100 radsÀ1), suggesting
that the gel is fairly tolerant to external forces. Furthermore,
time-dependent oscillation measurements were used to
monitor the gelation process of MOF–A-GO, which forms
gradually upon mixing of A-GO and ligand 1 in the pres-
ence of Zn2+ (Figure S8C). The time-sweep results show the
rapid growth of G’ and G’’ in the initial stage of gelation fol-
lowed by a slower long-term approach to a final pseudo-
equilibrium plateau. At the end of the experiment, the value
of G’ was about an order-of-magnitude higher than G’’.
To investigate the applicability of MOF–A-GO as a porta-
ble chemosensor kit, the sensing ability was studied in the
film state by exposing films to trinitrotoluene (TNT) and di-
nitrotoluene (DNT) vapors (Figure 4). Films of MOF–A-
GO were drop casted from water, and the films showed
70% fluorescence quenching after one minute of exposure
to saturated TNT vapors. Nearly 98% quenching was ob-
served upon continuous exposure for 10 min (Figure 4b). As
expected, the MOF–A-GO film exhibited a gradual de-
crease in fluorescent intensity upon the addition of TNT;
this is attributed to charge-transfer interactions between the
electron-deficient aromatic ring of TNT and the electron-
rich aromatic group of 1 in MOF–A-GO. The use of MOF–
A-GO for the detection of DNT vapors was also studied. A
film of MOF–A-GO showed a quenching effect on fluores-
cence when exposed to saturated DNT vapors under the
same conditions; however, the quenching efficiency was
found to be much less than that of TNT vapors. After expo-
sure for 10 min, only 2% quenching was observed in the
presence of DNT compared with 98% quenching for TNT.
We also carried out DFT calculations for the charge-trans-
fer interaction between the TNT molecules and 1 in MOF–
A-GO by using Beckeꢁs three parameterized Lee–Yang–
Parr with the 6-31G* basis sets and a suite of Gaussian 09
programs.[37] According to preliminary computational simu-
lations, TNT molecules can be inserted into the cavity of the
Zn2+ complex, and binds tightly through the charge-transfer
interaction inside the cavity (Figure S9 in the Supporting In-
formation). In addition, the charge-transfer interaction be-
tween the TNT molecules and 1 in the MOF–A-GO compo-
site hydrogel was much stronger than that of DNT.
In conclusion, we have demonstrated that the formation
the MOF–A-GO composite hydrogel can be achieved. The
A-GO is shown to efficiently produce a hydrogel by simple
mixing with 1 in the presence of Zn2+. This is the first exam-
ple of a graphene-based MOF–GO composite hydrogel.
Photophysical studies show that the hydrogel exhibits a typi-
cal p–p* transition and gives rise to highly fluorescent be-
havior. Upon the formation of the MOF–A-GO composite
hydrogel, the complex shows a pronounced fluorescence en-
hancement with a long lifetime, compared with that of the
ligand. Together, measurements of dynamic oscillation and
steady shear indicate the formation of a weak and thermally
labile network for which the tenuous supramolecular struc-
ture is irreversibly disrupted by mechanical and thermal
treatments. In addition, MOF–A-GO composite hydrogels
can act as chemosensors, for example, for TNT detection.
Graphene-based MOF hydrogels provide a powerful strat-
egy for developing new molecularly defined materials in the
areas of biology, medicine, and material science. We believe
this could possibly provide a model system for sensors, catal-
ysis, and delivery.
Acknowledgements
This work was supported by a grant from the World Class University
(WCU) Program (R32-2008-000-20003-0) and the NRF (2011-0005689)
supported from the Ministry of Education, Science, and Technology, S.
Korea.
Figure 4. Fluorescence spectra of a MOF–A-GO composite hydrogel film
a) before and after immersion of b) TNT (1.0 mmol) and c) DNT
(1.0 mmol).
Keywords: graphene oxide
frameworks · trinitrotoluene
·
gels
·
metal–organic
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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