Instant visual detection of picogram levels of trinitrotoluene by using luminescent metal-organic framework gel-coated filter paper

Article abstract of DOI:10.1002/chem.201301507

There is an ongoing need for explosive detection strategies to uncover threats to human security including illegal transport and terrorist activities. The widespread military use of the explosive trinitrotoluene (TNT) for landmines poses another particula

Full text of DOI:10.1002/chem.201301507

FULL PAPER
DOI: 10.1002/chem.201301507
Instant Visual Detection of Picogram Levels of Trinitrotoluene by Using
Luminescent Metal–Organic Framework Gel-Coated Filter Paper
Ji Ha Lee,[a] Sunwoo Kang,[b] Jin Yong Lee,*[b] Justyn Jaworski,[c] and Jong Hwa Jung*[a]
Abstract: There is an ongoing need for
explosive detection strategies to uncov-
er threats to human security including
illegal transport and terrorist activities.
The widespread military use of the ex-
plosive trinitrotoluene (TNT) for land-
mines poses another particular threat
to human health in the form of con-
tamination of the surrounding environ-
ment and groundwater. The detection
of explosives, particularly at low pico-
lenge. Herein, we report on the use of
a fluorescent metal–organic framework
hydrogel that exhibits a higher detec-
tion capability for TNT in the gel state
compared with that in the solution
state. A portable sensor prepared from
filter paper coated by the hydrogel was
able to detect TNT at the picogram
level with a detection limit of 1.82 ppt
(parts per trillon). Our results present
a simple and new means to provide se-
lective detection of TNT on a surface
or in aqueous solution, as afforded by
the unique molecular packing through
the metal–organic framework structure
in the gel formation and the associated
photophysical properties. Furthermore,
the rheological properties of the MOF-
based gel were similar to those of a typ-
ical hydrogel.
Keywords: explosives · gels · met-
al–organic frameworks · sensors ·
supramolecular chemistry
gram levels, by using
a molecular
sensor is seen as an important chal-
Introduction
of conjugated polymers as a result of intermolecular exciton
migration and also through multi-photon fluorescence
quenching, thereby providing a means for ultrasensitive de-
tection of TNT. Consequently, several approaches using
quantum dots,[9,25] gold nanoparticles,[26,27] and molecular im-
printing with AuNPs[28,29] have been reported for the detec-
tion of TNT. Recently, Zang et al.[30] have also reported on
the selective detection of TNT by implementing nanoporous
nanofibers fabricated from carbazole-based tetracycles that
are strongly encapsulated in the nanopores.
Compared with individual aromatic molecules, the self-as-
sembly of nano-architectures from these building-block
components can have strikingly different properties.[31–35]
The property of fluorescence response, for example, can
often show significant changes depending on the surround-
ing medium. In some cases, molecules can even fluoresce as
a consequence of gelation and have shown remarkable mod-
ulations of the emission color and intensity, which are rever-
sible with external stimuli.[36,37] Self-assembled gels with
such fluorescence properties are, therefore, useful candi-
dates for the detection of various analytes, including explo-
sive compounds, cationic species, and anionic species.[38–43] In
particular, Berke et al.[44] has reported the selective detec-
tion of nitroaromatic compounds by using an Al-based
metal–organic framework gel. Whereas such organic conju-
gated polymers and nanomaterials have shown successful
detection of explosive compounds with high sensitivity, the
visual detection of explosives at the picogram level by using
metal–organic framework gels remains a valuable goal and
a challenge worth undertaking. With this in mind, we herein
report the formation of a metal–organic framework hydro-
Trinitrotoluene (TNT) is an explosive chemical, in wide use
by militaries, existing in a variety of formulations and degra-
dation products that are harmful to human health and the
environment.[1–5] As a result, the selective detection of TNT
has been a matter of great concern to scientists.[6] Although
a number of reports exist on the topic of explosive detec-
tion, a simple approach to the detection of TNT at extreme-
ly low concentrations still remains a challenge.[7–11] Due to
the relatively low vapor pressure of TNT, particularly in
comparison to other explosive chemicals, the detection of
low quantities of TNT is particularly challenging.[12] The
groups of Swager[13–19] and Trogler[20–24] have made significant
advances in the detection of TNT. Swager et al. reported im-
proved fluorescence quenching associated with the nitroaro-
matic binding events in aggregated systems and solid films
[a] J. H. Lee, Prof. Dr. J. H. Jung
Department of Chemistry and
Research Institute of Natural Sciences
Gyeongsang National University, Jinju 660-701 (Korea)
Fax : (+82)55-772-1488
E-mail: jonghwa@gnu.ac.kr
[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
[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.201301507.
Chem. Eur. J. 2013, 19, 16665 – 16671
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
16665

gel in the presence of Zn2+ and its application as a chemo-
sensor for the detection of low levels of TNT molecules.
Scanning electron microscopy (SEM) images of gel 1-
Zn2+ when it was coated onto filter paper are shown in Fig-
ure S1 (the Supporting Information). The SEM image of gel
1-Zn2+ revealed a typical fiber structure with diameter of
45–200 nm (Figure S1a, the Supporting Information). When
the gel was applied to the filter paper, the paper was com-
pletely covered by the entangled gel fibers (45–200 nm) of
the 1-Zn2+ molecules (Figure S1b, the Supporting Informa-
tion), which had a similar appearance to the gel 1-Zn2+
itself. This finding supports the view that gel 1-Zn2+ adheres
well to the filter paper.
Results and Discussion
In a solution of ethanol, compound 1 with Zn2+ was non-
fluorescent, but upon gelation it emitted blue fluorescence
(445 nm; Figure 1a and 1b). When gel 1-Zn2+ was coated
We prepared several samples of sol 1-Zn2+ in ethanol to
study the response of their fluorescence with different nitro-
aromatic compounds such as dinitrotoluene (DNT), o-nitro-
toluene (ONT), nitromethane (NM), nitrobenzoic acid
(NBA), methyl benzolate (MB), and isophthalate (IP; Fig-
ure S2, the Supporting Information). Our first attempt stud-
ied the fluorescence changes of 1 in the presence of Zn2+ in
ethanol. The fluorescence intensity of 1 with Zn2+ (1ꢁ
10À3 m) in a solution of ethanol was about 15-fold less than
that of gel 1-Zn2+, due to fast rotation of the phenyl rings.
The fluorescence response of sol 1 (2.0 wt%) with Zn2+
(2.0 equiv) in ethanol upon addition of TNT (2.0–10.0 mm;
Figure 2A) displayed a weak quenching of the fluorescence.
Figure 1. Photographs of (a) sol 1 (2.0 wt%) with Zn2+ (2.0 equiv) and
(b) hydrogel 1 (2.0 wt%) with Zn2+ (2.0 equiv); (c) Photographs of sol
1 with Zn2+ in ethanol and (d) hydrogel 1-Zn2+-coated filter papers;
(e) Fluorescence spectra of hydrogel 1-Zn2+ and sol 1 with Zn2+ in etha-
nol (lex =302 nm); (f) Fluorescence spectra of hydrogel 1-Zn2+at differ-
ent temperatures.
on filter paper, the blue emission persisted (Figure 1d). On
the other hand, sol
1
(2.0 wt%) with Zn2+-coated
(2.0 equiv) filter paper showed non-emission (Figure 1c). In
addition, the gel 1-Zn2+ exhibited a strong emission, where-
as the sol 1 with Zn2+ exhibited non-emission at the same
concentration (Figure 1e). Fluorescence of the coordination
polymer gel 1 also varied as a function of temperature (Fig-
ure 1 f). The fluorescence intensity of gel 1-Zn2+ slightly de-
creased at 658C. Further increases in temperature resulted
in a large decrease in emission. These results suggest that
the emission of gel 1-Zn2+ decreases as it starts to melt at
65–708C. The optimal emission of 1-Zn2+ therefore occurs
when the gel is completely formed and decreases as the gel
melts at higher temperature.
Figure 2. (A) Emission spectra of sol 1-Zn2+ upon addition of TNT (lex
=
302 nm). Inset: Plot of quenching (%) at 445 nm versus concentration of
TNT; (B) Fluorescence spectra of gel 1-Zn2+-coated filter paper upon ad-
dition of TNT (1.0ꢁ10À3 m); (C) Plot of fluorescence quenching (%) at
445 nm versus time in seconds; (D) Lifetime decay profiles of gel 1-Zn2+
(a) before and (b) after addition of TNT (1.0ꢁ10À3 m) at room tempera-
ture.
These observations indicated that TNT is not an efficient
quencher for 1 in the isotropic solution state and hence, this
setup of a non-gelled solution of 1 is not suitable for the de-
tection of TNT, but instead it requires the gel state. In addi-
tion, no significant quenching effects of the fluorescence of
sol 1 with Zn2+ were obtained by exposure to DNT, ONT,
NM, and NBA (Figure S2, the Supporting Information).
16666
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16665 – 16671

Instant Visual Detection of Trinitrotoluene
FULL PAPER
Figure 4. Confocal laser scanning microscopy (CLSM) images of hydrogel
1-Zn2+ upon addition of TNT (a) 0, (b) 10, and (c) 100 ppm.
Figure 3. Images of the filter paper sensor (A) and (B) after addition of
the hydrogel 1-Zn2+ ((a) gel 1-Zn2+
; (b) plastic mask, and (c) filter
paper); (C) Final sensor view.
nanofiber of gel 1-Zn2+ could sensitively detect TNT mole-
cules.
For immobilized gel-based experiments, we cut the gel 1-
Zn2+-coated filter paper into strips of 2ꢁ5 cm and partially
covered them with plastic masks of 3ꢁ6 cm on both sides
(Figure 3). Details of these portable sensor components and
assembly are schematically shown in Figure 3. When glued
together, the two plastic masks on either side of the filter
paper prevent any direct contact of gel 1-Zn2+ fibers with
impurities. The small orifice (0.5 cm diameter) in the front
mask allows room for casting the membrane. Then, a mea-
sured amount of hydrogel 1-Zn2+ (100 mL, 2.0 wt%) was
dropped at the orifices on filter paper (0.5 cm diameter) and
dried under vacuum (Figure 3). These filter papers were
used as conventional chemosensors for TNT. The hydrogel
1-Zn2+-coated filter paper was placed in a vial containing
TNT at room temperature. The emission spectra were mea-
sured by using a front-face technique after exposing the film
for specific intervals (Figure 2B). Surprisingly, significant
quenching of the fluorescence was observed (Figure 2B and
2C) upon exposure to TNT solution. Nearly (90Æ5)%
quenching was noticed within 30 s of exposure (Figure 2C),
indicating a fast response time and a detection limit of less
than the ppb concentration level. To understand the excited-
state behavior of the gel 1-Zn2+, the fluorescence lifetime
decay profiles before and after exposure to TNT were re-
corded. These data of hydrogel 1-Zn2+ (lex =303 nm) exhib-
ited a biexponential character with lifetimes of 5.30 and
120.68 ns when monitored at 440 nm (Figure 2D; a). Upon
exposure to TNT vapor, a fast biexponential decay with
time constants of 3.95 and 62.16 ns was observed (Fig-
ure 2D; b). The decrease in the average lifetime can be at-
tributed to the charge-transfer interaction of TNT with the
self-assembled gel fibrils (specifically between the aromatic
ring of TNT, which is electron-deficient, and the aromatic
group of the 1 in the metal–organic framework, which is
electron-rich).
During the preparation and packaging of explosive devi-
ces or during an explosion, particles of the explosive com-
pound can contaminate the human body, clothing, and other
materials in the surroundings. In such cases, it is desirable to
check for residual contamination by an explosive chemical
by using a contact-mode methodology. For this purpose, we
transferred 1-Zn2+ gel fibers onto a Whatman filter paper
by applying a hot solution of the gel to the paper (Figure S3,
the Supporting Information), followed by drying under
vacuum. The dried filter papers were then cut into strips.
We considered that a contact-mode test for TNT contamina-
tion could be performed by extracting the target material
with a suitable solvent, then dilution of the extract to the
appropriate volume, and spot testing by application of the
extract to the paper test strips.
We prepared acetonitrile solutions of different analytes
(acetonitrile) and 100 mL of each solution was placed on the
filter paper test strips coated by gel 1-Zn2+ to give a spot
area of around 0.18 cm2. The visual fluorescence response of
TNT at different concentrations by contact-mode detection
on Whatman filter paper test strips is shown in Figure 5A.
The minimum amount of TNT detectable by the naked eye
was as low as 5.0 mL of a 1.0ꢁ10À12 m solution, thereby re-
cording a detection limit of 1.82 parts per trillon (ppt). The
fluorescence spectral changes of the test strips on contact
with TNT for a wide range of concentration is shown in Fig-
ure 5B, and the corresponding fluorescence quenching (%)
at 460 nm is shown in Figure 5C. A comparison of the mini-
mum detectable amounts of the analytes estimated from the
fluorescence quenching indicates that nonaromatic explo-
sives and aromatic compounds without nitro groups are not
sensed by the test strips (Figure 5D). Fluorescence quench-
ing could be visually detected with these test strips to the
level of 1.82ꢁ10À12 g (nitrotoluene; TNT), 2.27ꢁ10À10 g (di-
nitrotoluene; DNT), 1.37ꢁ10À10 g (o-nitrotoluene; ONT),
1.67ꢁ10À8 g (nitrobenzoic acid; NBA), 1.66ꢁ10À8 g (nitro-
methane; NM), and 1.67ꢁ10À4 g (toluene; T) when 100 mL
of the analyte was spotted with a spread of about 0.18 cm2.
From these data, the detection limit of TNT is calculated as
around 1.82 pgcmÀ2. Thus, the sensitivity of gel 1 with Zn2+
for TNT was as much as ꢀ100 times higher than that of
DNT. Furthermore, in ambient light, the white color of the
gel 1-Zn2+ was changed into a red color when TNT solution
was added to the filter paper coated with the gel 1-Zn2+
(Figure S4, the Supporting Information), indicating that the
In confocal laser scanning microscopy (CLSM) images of
the hydrogel 1-Zn2+ before addition of a solution of TNT
(Figure 4), the nanofiber of gel 1-Zn2+ showed a strong blue
emission. At 10 s after the addition of TNT (10 ppm), the
fluorescence emission of the nanofibers of hydrogel 1-Zn2+
was slightly quenched (Figure 4b), and upon addition of
higher levels of TNT (100 ppm), almost complete quenching
was observed (Figure 4c). These findings indicate that the
fluorescence of gel 1-Zn2+ was more responsive in the gel
phase as compared to the solution state. In addition, a single
Chem. Eur. J. 2013, 19, 16665 – 16671
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16667

J. Y. Lee, J. H. Jung et al.
Table 1. Comparison of sensors for the detection of TNT in aqueous so-
lution.
Approach
LOD[a]
[ppb]
Ref.
AHCTUNGTRENNUNG
Imprinted electropolymerized
oligoaniline-cross-linked AuNPs
0.046
[28]
[47]
[48]
[49]
Anti-2,4,6-trinitrophenol antibody surface
plasmon resonance (SPR) immunoassay
Electrochemical sensor using mesoporous
SiO2 of MCM-41 as modified electrode material
Adsorptive stripping voltammetric
detection using a carbon nanotube-modified
glassy carbon electrode
0.09
ꢀ0.4
0.6
Commercially available colorimetric
kits for TNT detection
1
1
[50]
[51]
Adsorptive stripping voltammetry using
a nanocomposite-modified glassy carbon
electrode (GC) containing Cu nanoparticles
and SWCNT[b] solubilized in Nafion
Electrochemical detection by MWCNT[c]
functionalized electrodes
-
ꢀ5
[52]
[53]
[54]
Competitive immunoassay for TNT
detection as detected by QCM and SPR
Fiber-optic biosensor employing
fluorescent evanescent wave-sensing
with antibodies used as the
1–10
10
Figure 5. (A) Photographs of the fluorescence quenching of gel 1-Zn2+
-coated test strips by different concentration of TNT; (a) 0m, (b) 1.0ꢁ
10À12 m, (c) 1.0ꢁ10À10 m, (d) 1.0ꢁ10À8 m, and (e) 1.0ꢁ10À6 m; (B) Fluores-
cence spectral changes of the test strips on contact with added amount of
TNT (lex =302 nm); (C) Plot of the quenching (%) against concentration
of added TNT in acetonitrile; (D) Contact-mode detection of the lowest
amount of different analytes (1.0ꢁ10À12 m) by the emission quenching
test strip.
recognition molecule
Luminescence from colloidal
oligo(tetraphenyl)silole nanoparticle-
based sensors
Electrochemical sensor using multiple
carbon fibers as a working electrode
20
[22]
[55]
ꢀ30
[a] LOD=limit of detection. [b] SWCNT=single-wall carbon nanotubes.
[c] MWCNT=multi-walled carbon nanotubes.
gel 1-Zn2+ acted as both a chromogenic and fluorogenic
chemosensor for TNT. We have provided a comparison of
our gel 1-Zn2+-based detection system to several other ap-
proaches for TNT detection (Table 1), showing the merit of
our system in terms of improved sensitivity.[47–55]
To better understand, from a structural perspective, the
molecular recognition process between 1-Zn2+ complex and
TNT, we have carried out density functional theory (DFT)
calculations using B3LYP hybrid functional with 6-31G*
basis set implemented in Gaussian 09 programs (Figures 5
and S5 in the Supporting Information).[45] In our calcula-
tions, we reduced the size of the host molecule containing
two Zn2+ cations and six ligands due to the computational
capability. The optimized structures of 1-Zn2 as a host, 1-
Zn2+·TNT, appears in Figure 6. Similar to our previous
report,[46] each Zn2+ species was tetrahedrally coordinated
by the oxygen atoms of the ligands. The inter-atomic dis-
tance between the two Zn2+ cations (RZn···Zn) was calculated
to be 18.82 ꢂ and that between the two nitrogen atoms at
the pyridine moieties (RN···N) was 11.72 ꢂ in the host mole-
cule. Moreover, the distances of RO···O (oxygen atom at car-
boxyl group of ligand) were measured in the range of 3.050–
3.426 ꢂ. In binding with TNT, the value of RZn···Zn was calcu-
lated to be 18.18 ꢂ for 1-Zn2+·TNT, and the RN···N was calcu-
lated to be 12.46 ꢂ. Additionally, the RO···O values for 1-
Zn2+ with TNT were determined in the range of 3.152–
3.435 ꢂ. These results indicate that TNT induces a remark-
able structural change of the host molecule. Whereas hydro-
gen bonding (HB) may play an important role with respect
Figure 6. The optimized structure of
(a) before and (b) after addition of TNT.
a
model system of gel 1-Zn2+
to TNT in the cavity of the host, the calculated structure
does not appear to account for the charge-transfer interac-
tions observed. The detailed structural feature of HBs in
two recognition systems indicated that three HBs exist in 1-
Zn2+·TNT with NH···O distances of 2.255, 2.181, and
2.646 ꢂ. As expected, the calculated interaction energy be-
16668
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16665 – 16671

Instant Visual Detection of Trinitrotoluene
FULL PAPER
tween 1-Zn2+ and TNT was calculated to be 7.32 kcalmolÀ1.
The HBs existing in 1-Zn2+·TNT can be characterized as
typical HBs considering HB distances and interaction ener-
gies. It is important to note that other more preferred con-
figurations may be revealed in more accurate models for
London dispersion interaction. For example, one might have
expected other configurations allowing charge-transfer inter-
actions, such as p–p stacking, to be in the predicted lowest-
energy geometry. In addition, the small energy differences
between host and guest (DNT and TNT) molecules can rep-
resent the selectivity of an appropriate guest molecule. We
have only considered orbital energy levels of isolated DNT
and TNT molecules, respectively, because the HOMO
energy level of the isolated host is same. Thus, we have com-
pared the LUMO energy levels of DNT and TNT. The cal-
culated LUMO energy levels for DNT and TNT are À3.17
and À3.70 eV, indicating that the LUMO energy level of
TNT is closer in energy than that of DNT to the HOMO
energy level of the host.
peak appeared at 23.2q; this peak, corresponding to a d-
spacing of 3.86 ꢂ, could be the p-stacking distance between
the ligand molecule and the TNT molecule. On the other
hand, after exposure to DNT vapors, only negligible changes
were observed in the diffraction peaks, indicating that the
packing of gel 1 is not considerably disturbed by DNT
vapors. In addition, we measured the powder-XRD patterns
of gel 1-Zn2+ upon addition of methyl benzolate (MB), or
isophthalate (IP; Figure S6, the Supporting Information).
The powder XRD patterns of gel 1-Zn2+ upon addition of
methyl benzolate, or isophthalate were almost the same as
that of gel 1-Zn2+ itself. These results indicate that the TNT
molecules are trapped inside the interstitial space of the
ligand 1-Zn2+ complexes, inducing changes in the overall
molecular packing, and the observed fluorescence quenching
is due to excited-state processes.
Rheological information is an indicator of the behavior of
the gels when they are exposed to mechanical stress. The
“storage” (or “elastic”) modulus G’ represents the ability of
the deformed material to “snap back” to its original geome-
try, and the “loss” (or “viscous”) modulus G’’ represents the
tendency of a material to flow under stress. Two rheological
criteria required for a gel are: 1) the independence of the
dynamic elastic modulus, G’, with respect to the oscillatory
frequency, and 2) G’ must exceed the loss modulus G’’ by
about one order of magnitude. We first used dynamic strain
sweep to determine the proper conditions for the dynamic
frequency sweep of the gels 1-Zn2+. As shown in Fig-
ure S7A (the Supporting Information), the values of the
storage modulus (G’) and the loss modulus (G’’) exhibited
a weak dependence from 0.1 to 1.0% of strain (with G’
dominating G’’), indicating that the sample is a gel. The
value of G’ and of the gel 1-Zn2+ were about 10-fold higher
than that of G’’. We also used the dynamic frequency sweep
to study the gel after setting the strain amplitude at 0.1%
(within the linear response region of the strain amplitude).
The values of G’ and G’’ were almost constant with the in-
crease of frequency from 0.1 to 100 radsÀ1 (Figure S7B, the
Supporting Information). The value of G’ was about 5 times
larger than that of G’’ over the whole range (0.1–
100 radsÀ1), suggesting that the gel is fairly tolerant to exter-
nal force. Furthermore, time-dependent oscillation measure-
ments were used to monitor the gelation processes of gels 1-
Zn2+ (Figure S7C, the Supporting Information). The time
sweep shows the rapid increase of G’ and G’’ in the initial
stage of gelation, followed by a slower long-term approach
to a final pseudo-equilibrium plateau. At the end of the ex-
periment, the value of G’ was about an order of magnitude
higher than G’’.
The XRD patterns of gel 1 with Zn2+ before and after ex-
posure to TNT are shown in Figure 7. The main reflection
peaks appeared at 13.5, 22.0, 25.2, 29.2, and 39.5q for 6.55,
Figure 7. Powder XRD patterns of gel 1-Zn2+ (a) before and after expo-
sure to (b) TNT (3.0 equiv), (c) DNT (3.0 equiv), or (d) gel 1-Zn2+ in
acetonitrile.
4.03, 3.57, 3.03, and 2.03 ꢂ, respectively. The d-spacing of
8.13 ꢂ corresponds to the distance between the alternate
(top and bottom) ligand 1. Interestingly, gel 1-Zn2+-coated
filter paper after exposure to saturated TNT vapors exhibit-
ed a large change in the XRD pattern. The reflection peaks
were shifted to the small angle range. The largest d-spacing
of 9.67 ꢂ corresponds to the distance between the alternate
(top and bottom) ligand. In particular, the new reflection
Conclusion
Pictogram-level detection of TNT by using a fluorescent
metal–organic framework nanomaterial has been demon-
strated. The ligand 1 in the presence of Zn2+ in the solution
state is not efficient for detecting TNT, whereas in the
Chem. Eur. J. 2013, 19, 16665 – 16671
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16669

J. Y. Lee, J. H. Jung et al.
completion of the run, the imposed stress was withdrawn and the extent
of structure recovery was recorded for another 60 min (relaxation curve).
Dynamic and steady shear measurements were conducted in triplicate
and creep (transient) measurements in duplicate. The rheological proper-
ties of gel 1 (2.0 wt%) with Zn2+ (3.0 equiv) were measured at 25.08C.
metal–organic framework-based xerogel state we have ach-
ieved high sensitivity detection of TNT. These findings high-
light the unique capability of the self-assembled fibrous
structures as new materials with specific sensing applica-
tions. The detection level of metal–organic framework hy-
drogel 1 was in the ppt range in the contact mode. Pico-
gram-level detection with high sensitivity, achieved by using
portable filter paper-based test strips, allows a simple and
low-cost protocol for the on-site instant detection of TNT on
contaminated specimens. Such a strategy may be extended
to other fluorescence-based sensing materials as an ap-
proach for the selective detection of a specific analyte. Fur-
thermore, the rheological properties exhibited by gel 1-Zn2+
were similar to those of a typical hydrogel. The present re-
sults emphasize the validity of ligand design that includes
metal-binding in realizing metal–organic framework gel sys-
tems with versatile and stimuli-responsive properties. These
concepts should bring about the development of a wide
range of responsive soft materials.
X-ray powder diffraction: The X-ray powder diffraction (XRPD) experi-
ments were performed in a transmission mode with a Bruker GADDS
diffractometer equipped with graphite monochromated CuKa radiation
(l=1.54073 ꢂ). The samples were prepared by freeze-drying the hydro-
gel 1-Zn2+ (2.0 wt%).
SEM Observation: Scanning electron micrographs of the samples were
taken with a field emission scanning electron microscope (FE-SEM, Phi-
lips XL30 S FEG). The accelerating voltage of SEM was 5–15 kV and the
emission current was 10 mA.
Preparation of hydrogel 1-Zn2+: In a vial, ligand 1 (5 mg) was added to
0.1m NaOH solution (200 ml) of in the presence of Zn2+ (2.0 equiv). The
hydrogel was formed immediately after ultrasonication in ambient tem-
perature.
Preparation of filter paper test strips: Filter paper (10ꢁ2 cm) test strips
were prepared by coating with the melted hydrogel 1-Zn2+ (2.0 wt%)
followed by removal of solvent under vacuum at room temperature. The
gel-coated filter papers were then cut into 10 pieces (0.5ꢁ 0.5 cm) to
obtain the test strips, which were then used for the detection of explo-
sives.
Detection of explosive analytes by emission quenching: The required an-
alyte solutions of various concentrations (1ꢁ10À12–1ꢁ10À3 m) were added
to each strip, and the solvents were allowed to evaporate. The film was
placed in such a way that the excitation beam falls on the spot where ex-
plosive analyte is added. Emission was collected by a front-face tech-
nique by using a film sample holder. Emission of a blank sample was
monitored by the addition of solvent alone.
Experimental Section
General considerations: 2,4-dinitrotoluene (DNT) was purchased from
TCI and re-crystallized from ethanol. 2,4,6-Trinitrotoluene (TNT) was
prepared from DNT using literature procedures.[45] All other chemicals
were purchased from Aldrich, TCI, or Wako and used as received. 1H
and 13C NMR spectra were measured on a Bruker ARX 300 apparatus.
Compound 2: Methyl 4-(chlorocarbonyl)benzoate 4 (0.5 g, 2.5 mmol) in
THF was added to 2,6-diaminopyridine 3 (1.02 g, 1.25 mmol) and a solu-
tion of pyridine (0.148 mL, 1.25 mmol). The reaction mixture was stirred
at 258C for 2 h and then was removed solvent by evaporation. The crude
product was purified by column chromatography on silica gel with di-
chloromethane (Rf =0.5). Yield 63%. 1H NMR (300 MHz, [D6]DMSO):
d=10.7 (s, 2H, NH), 8.2 (m, 8H, Ar-H), 7.9 (m, 3H, Ar-H), 3.9 ppm (s,
6H, -CH3); 13C NMR ([D6]DMSO): d=166, 165, 160, 139, 138, 135, 133,
128, 99, 51 ppm; IR (KBr): n˜ =3337, 3009, 2950, 1708, 1639, 1590, 1511,
IR spectra were obtained for KBr pellets, in the range 400–4000 cmÀ1
,
with a Shimadzu FT-IR 8400S, and mass spectra were obtained by
a JEOL JMS-700 mass spectrometer. The optical absorption spectra of
the samples were obtained at 298 K using a UV/Vis spectrophotometer
(Hitachi U-2900). All fluorescence spectra were recorded in RF-5301PC
spectrophotometer. The accelerating voltage of SEM was 5–15 kV and
the emission current was 10 mA.
1442, 1314, 1274, 1096, 1008, 899, 870, 790, 731, 632 cmÀ1
; HRMS
Fluorescence lifetime microscopy (FLM) measurements: Fluorescence
lifetime images were acquired by an inverse time resolved fluorescence
microscope, MicroTime-200 (PicoQuantGmBH). The excitation wave-
length, the spatial resolution, and the time resolution were 405 nm,
0.3 mm, and 60–70 ps, respectively. The samples were prepared on one
side of microscope cover glasses. The manufacturerꢃs software was used
to analyze the data and calculate the lifetime maps. The color scales rep-
resent average lifetime and total number of counts is indicated by color
density at each point.
(FAB+): m/z calcd for C23H19N3O6: 434.3562 [M+H+]; found: 434.4135;
elemental analysis calcd (%) for C23H19N3O6: C 63.74, H 4.42, N 9.70,
O 22.15; found: C 63.92, H 4.23, N 9.68.
Compound 1: Compound 2 (0.3 g, 0.7 mmol) in THF (10 mL) was added
to a solution of NaOH (0.7 g, 3.5 mmol). The reaction mixture was
heated at reflux for 3–4 h at 708C and then cooled to room temperature.
Then, an aqueous solution of HCl (1.0m) was slowly added to the reac-
tion mixture (pH 3–4). The product was obtained as a white powder.
Yield 40%. 1H NMR (300 MHz, [D6]DMSO): d=13.25 (s, 2H, OH),
10.74 (s, 2H, NH), 8.2 (m, 8H, Ar-H), 7.9 (m, 3H, Ar-H); 13C NMR
([D6]DMSO): d=169, 165, 158, 139, 134, 130, 128, 127, 99 ppm;
IR (KBr): n˜ =3419, 1705 m 1542, 1421, 1232, 1110, 665 cmÀ1; HRMS
(FAB+): m/z calcd (%) for C21H15N3O6: 406.2968 [M+H+]; found:
406.3603; analysis calcd (%) for C21H15N3O6: C 62.22, H 3.73, N 10.37, O
23.68. found: C 62.48, H 3.72, N 10.42.
Fluorescence confocal microscopy: Images were recorded on Nikon Mi-
croscope ECLIPSE 80i using UV light (400 nm) as the excitation source,
and the emission was collected between 340–480 nm with 100ꢁ magnifi-
cation. Samples were prepared by drop-casting on a glass slide followed
by slow evaporation.
Rheological measurements: These were carried out on freshly prepared
gels by using a controlled stress rheometer (AR-1000N, TA Instruments
Ltd., New Castle, DE, USA). A parallel plate geometry of 40 mm diame-
ter and 1.5 mm gap was employed throughout. Following loading, the ex-
posed edges of samples were covered with a silicone fluid from BDH
(100 centistokes (cs)) to prevent water loss. Dynamic oscillatory work
kept a frequency of 1.0 radsÀ1. The following tests were performed: in-
creasing amplitude of oscillation up to 100% apparent strain on shear,
time, and frequency sweeps at 258C (60 min and from 0.1–100 radsÀ1, re-
spectively). Unidirectional shear routines were performed at 258C cover-
ing a shear-rate regime between 10À1 and 103 sÀ1. Mechanical spectrosco-
py routines were completed with transient measurements. In doing so,
the desired stress was applied instantaneously to the sample and the an-
gular displacement was monitored for 60 min (retardation curve). After
Acknowledgements
This work was supported by a grant from the Program (R32-2008-000-
20003-0) and the NRF (2012002547) supported from the Ministry of Edu-
cation, Science and Technology, Korea. In addition, this work was partial-
ly supported by a grant from the Next-Generation BioGreen 21 Program
(SSAC, grant No.: PJ009041022012), Rural development Administration,
Korea.
16670
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16665 – 16671

Instant Visual Detection of Trinitrotoluene
FULL PAPER
[35] H. Kim, T. Kim, M. Lee, Acc. Chem. Res. 2011, 44, 72.
[1] A. Fainberg, Science 1992, 255, 1531.
[36] A. Ajayaghosh, V. K. Praveen, C. Vijayakumar, S. J. George, Angew.
Chem. 2007, 119, 6376; Angew. Chem. Int. Ed. 2007, 46, 6260.
[37] C. Vijayakumar, V. K. Praveen, A. Ajayaghosh, Adv. Mater. 2009,
21, 2059.
[38] L. Zang, Y. Che, J. S. Moore, Acc. Chem. Res. 2008, 41, 1596.
[39] G. V. Zyryanov, M. A. Palacios, P. Anzenbacher, Org. Lett. 2008, 10,
3681.
[40] C. Vijayakumar, G. Tobin, W. Schmitt, M. J. Kima, M. Takeuchi,
Chem. Commun. 2010, 46, 874.
[41] B. An, S. Kwon, S. Jung, S. Y. Park, J. Am. Chem. Soc. 2002, 124,
14410.
[2] K. J. Albert, N. S. Lewis, C. L. Schauer, G. A. Sotzing, S. E. Stitzel,
T. P. Vaid, D. R. Walt, Chem. Rev. 2000, 100, 2595.
[3] T. H. Kim, B. Y. Lee, J. Jaworski, K. Yokoyama, W.-J. Chung, E.
Wang, S. Hong, A. Majumdar, S.-W. Lee, ACS Nano 2011, 5, 2824.
[4] A. L. Pinnaduwage, A. Gehl, D. L. Hedden, G. Muralidharan, T.
Thundat, R. T. Lareau, T. Sulchek, L. Manning, B. Rogers, M. Jones,
J. D. Adams, Nature 2003, 425, 474.
[5] W. E. Tenhaeff, L. D. McIntosh, K. K. Gleason, Adv. Funct. Mater.
2010, 20, 1144.
[6] A. W. Czarnik, Nature 1998, 394, 417.
[7] M. E. Germain, M. J. Knapp, Chem. Soc. Rev. 2009, 38, 2543.
[8] Y. Salinas, R. Martınez-Manez, M. D. Marcos, F. Sancenon, A. M.
Costero, M. Parra, S. Gil, Chem. Soc. Rev. 2012, 41, 1261.
[9] E. R. Goldman, I. L. Medintz, J. L. Whitley, A. Hayhurst, A. R.
Clapp, H. T. Uyeda, J. R. Deschamps, M. E. Lassman, H. Mattoussi,
J. Am. Chem. Soc. 2005, 127, 6744.
[42] H. Lee, S. H. Jung, W. S. Han, J. H. Moon, S. Kang, J. Y. Lee, J. H.
Jung, S. Shinkai, Chem. Eur. J. 2011, 17, 2823.
[43] K. K. Kartha, R. D. Mukhopadhyay, A. Ajayaghosh, Chimia 2013,
67, 51.
[44] S. Barman, J. A. Garg, O. Blacque, K. Venkatesan, H. Berke, Chem.
Commun. 2012, 48, 11127.
[10] A. Lan, K. Li, H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. Hong, J.
Li, Angew. Chem. 2009, 121, 2370; Angew. Chem. Int. Ed. 2009, 48,
2334.
[11] S. Pramanik, C. Zheng, X. Zhang, T. J. Emge, J. Li, J. Am. Chem.
Soc. 2011, 133, 4153.
[12] M. B. Pushkarsky, I. G. Dunayevskiy, M. Prasanna, A. G. Tsekoun,
R. Go, C. K. N. Patel, Proc. Natl. Acad. Sci. USA 2006, 103, 19630.
[13] J.-S. Yang, T. M. Swager, J. Am. Chem. Soc. 1998, 120, 11864.
[14] J.-S. Yang, T. M. Swager, J. Am. Chem. Soc. 1998, 120, 5321.
[15] D. T. McQuade, A. E. Pullen, T. M. Swager, Chem. Rev. 2000, 100,
2537.
[16] S. Zahn, T. M. Swager, Angew. Chem. 2002, 114, 4399; Angew.
Chem. Int. Ed. 2002, 41, 4225.
[17] A. Rose, Z. Zhu, C. F. Madigan, T. M. Swager, V. Bulovi, Nature
2005, 434, 876.
[18] S. W. Thomas, G. D. Joly, T. M. Swager, Chem. Rev. 2007, 107, 1339.
[19] T. M. Swager, Acc. Chem. Res. 2008, 41, 1181.
[20] H. Sohn, R. M. Calhoun, M. J. Sailor, W. C. Trogler, Angew. Chem.
2001, 113, 2162; Angew. Chem. Int. Ed. 2001, 40, 2104.
[21] H. Sohn, M. J. Sailor, D. Magde, W. C. Trogler, J. Am. Chem. Soc.
2003, 125, 3821.
[22] S. J. Toal, D. Magde, W. C. Trogler, Chem. Commun. 2005, 5465.
[23] J. C. Sanchez, A. G. DiPasquale, A. L. Rheingold, W. C. Trogler,
Chem. Mater. 2007, 19, 6459.
[45] F. D. Lewis, J. S. Yang, C. L. Stern, J. Am. Chem. Soc. 1996, 118,
12029.
[46] Gaussian 03, Revision B.05, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
ery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
[47] D. R. Shankaran, K. V. Gobi, T. Sakai, K. Matsumoto, K. Toko, N.
Miura, Biosens. Bioelectron. 2005, 20, 1750.
[48] H.-X. Zhang, A.-M. Cao, J.-S. Hu, L.-J. Wan, S.-T. Lee, Anal. Chem.
2006, 78, 1967.
[24] J. C. Sanchez, W. C. Trogler, J. Mater. Chem. 2008, 18, 3143.
[25] K. Zhang, H. Zhou, Q. Mei, S. Wang, G. Guan, R. Liu, J. Zhang, Z.
Zhang, J. Am. Chem. Soc. 2011, 133, 8424.
[26] M. Riskin, R. Tel-Vered, I. Willner, Adv. Mater. 2010, 22, 1387.
[27] W. G. Qu, B. Deng, S. L. Zhong, H. Y. Shi, S. S. Wang, A. W. Xu,
Chem. Commun. 2011, 47, 1237.
[28] M. Riskin, R. T. Vered, T. Bourenko, E. Granot, I. Willner, J. Am.
Chem. Soc. 2008, 130, 9726.
[29] M. Riskin, R. Tel-Vered, O. Lioubashevski, I. Willner, J. Am. Chem.
Soc. 2009, 131, 7368.
[49] J. Wang, S. B. Hocevar, B. Ogorevc, Electrochem. Commun. 2004, 6,
176.
[50] Strategic Diagnostics, Inc., Analysis of TNT and RDX in Ground-
water, http://www.sdix.com.
[51] S. Hrapovic, E. Majid, Y. Liu, K. Male, J. H. T. Luong, Anal. Chem.
2006, 78, 5504.
[52] H.-X. Zhang, J.-S. Hu, C.-J. Yan, L. Jiang, L.-J. Wan, Phys. Chem.
Chem. Phys. 2006, 8, 3567.
[53] Larsson, J. Angbrant, J. Ekeroth, P. Mansson, B. Liedberg, Sens. Ac-
tuators B 2006, 113, 730.
[54] L. C. Shriver-Lake, K. A. Breslin, P. T. Charles, D. W. Conrad, J. P.
Golden, F. S. Ligler, Anal. Chem. 1995, 67, 2431.
[30] K. K. Kartha, S. S. Babu, S. Srinivasan, A. Ajayaghosh, J. Am.
Chem. Soc. 2012, 134, 4834.
[31] A. Ajayaghosh, V. K. Praveen, Acc. Chem. Res. 2007, 40, 644.
[32] A. Ajayaghosh, V. K. Praveen, C. Vijayakumar, Chem. Soc. Rev.
2008, 37, 109.
[55] J. Wang, R. K. Bhada, J. Lu, D. MacDonald, Anal. Chim. Acta 1998,
361, 85.
[33] T. Cardolaccia, Y. Li, K. S. Schanze, J. Am. Chem. Soc. 2008, 130,
2535.
[34] S. S. Babu, K. K. Kartha, A. Ajayaghosh, J. Phys. Chem. Lett. 2010,
1, 3413.
Received: April 20, 2013
Revised: September 9, 2013
Published online: November 6, 2013
Chem. Eur. J. 2013, 19, 16665 – 16671
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16671