A fluorescent sensor for highly selective detection of nitroaromatic explosives based on a 2D, extremely stable, metal-organic framework

Article abstract of DOI:10.1002/chem.201304692

A 2D, extremely stable, metal-organic framework (MOF), NENU-503, was successfully constructed. It displays highly selective and recyclable properties in detection of nitroaromatic explosives as a fluorescent sensor. This is the first MOF that can distinguish between nitroaromatic molecules with different numbers of -NO₂ groups.

Full text of DOI:10.1002/chem.201304692

DOI: 10.1002/chem.201304692
Communication
&
Metal–Organic Frameworks
A Fluorescent Sensor for Highly Selective Detection of
Nitroaromatic Explosives Based on a 2D, Extremely Stable, Metal–
Organic Framework
Shu-Ran Zhang,[a] Dong-Ying Du,[a] Jun-Sheng Qin,[a] Shao-Juan Bao,[a] Shun-Li Li,[b] Wen-
Wen He,[a] Ya-Qian Lan,*[b] Ping Shen,[a] and Zhong-Min Su*[a]
Metal–organic frameworks (MOFs) have been extensively in-
vestigated in chemistry and materials science owing to, not
only the intriguing structural and chemical diversities, but also
the functional properties and potential applications of these
compounds.[5] Among these, multifunctional MOFs with func-
tional sites, such as open metal sites, catalytically active metal
sites, and photoactive metal sites,[6] have been of particular in-
terests because of the promising application as chemical sen-
sors.[7,8] Moreover, some ligands have also been designed, syn-
thesized, and explored as fluorescent sensors.[9] In general,
MOFs used to detect explosives are porous frameworks with
2D or 3D structures and nonporous frameworks with 2D
sheets or nanoscale materials.[10] The sensitivity and recyclabili-
ty of these MOFs, and especially the stability, need to be im-
proved. Therefore, it is important to synthesize stable MOFs to
be used for detection of explosives. In particular, the selection
of the ligand plays a crucial role in the design and construction
of the stable MOFs for fluorescence detection of explosives.
We selected 4,4’,4’’-((2,2’,2’’-(nitrilotris(methylene))tris(1H-ben-
zo[d]imidazole-2,1-diyl))tris(methylene))tribenzoic acid (H3L, see
the Supporting Information) and CdII to synthesize MOFs for
detection of explosives based on the following reasons: 1) Nu-
merous explosives are good electron acceptors with electron-
deficient ÀNO2 groups, including the hazardous explosives 2,4-
dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT), 2,4,6-trini-
trophenol (TNP), and 1,3-dinitrobenzene (1,3-DNB), which are
common chemical constituents of commercial explosives.[11]
H3L is a flexible tripodal ligand with aromatic p rings and nitro-
gen atoms with lone-pair electrons, which may help to adjust
the position of ligands to provide intraligand interactions and
promote luminescent character. 2) The coordination between
organic ligands and metal ions, not only adds flexibility and di-
versity in the geometric structures of the MOFs, but also alters
the electronic structures and surface functionalities of the
MOFs; this can facilitate efficient exciton migration between
MOFs and electron-deficient nitroaromatic analytes so as to
achieve the means of detection.[12] 3) In addition, d10 metal
ions, such as ZnII and CdII, usually show high complexation af-
finity to carboxylate and do not interfer with fluorescence, be-
cause they can display varied coordination numbers and geo-
metries, and exhibit outstanding luminescent properties.[13]
Herein, we report the synthesis of [Cd2Cl(H2O)(L)]·4.5DMA
(DMA=N,N-dimethylacetamide; NENU-503, NENU=Northeast
Normal University) by the solvothermal reaction of Cd-
(NO3)2·4H2O and H3L. NENU-503 displays high stability in air for
Abstract: A 2D, extremely stable, metal–organic frame-
work (MOF), NENU-503, was successfully constructed. It
displays highly selective and recyclable properties in de-
tection of nitroaromatic explosives as a fluorescent sensor.
This is the first MOF that can distinguish between nitroar-
omatic molecules with different numbers of ÀNO2 groups.
Chemical sensors for fast and highly selective detection of
high explosives and explosivelike substances have attracted in-
creasing attention concerning homeland security, environmen-
tal and humanitarian implications.[1] Although many explosive-
detection technologies, such as typical canines or sophisticated
instruments,[2] are widely utilized, these techniques are expen-
sive and may not be easily accessible in most cases. Alterna-
tively, fluorescence detection based on chemical sensing by
using fluorescent materials has proven to be an excellent can-
didate for the rapid detection of explosives. Fluorescence-
based detection has an advantage over traditional detection
methods in virtue of the high sensibility, simplicity, short re-
sponse time, and the ability to be applied in both solution and
solid phase.[3] Nowadays, new molecular, oligomeric, polymeric,
and nanoscale materials are usually used for fluorescence de-
tection. These materials are capable of quick, efficient, and
credible detection of various explosives.[4] However, there are
still some hindrances, such as stability, toxicity, sensitivity, and
biodegradability, thus it is a significant and challenging task to
synthesize novel materials for fluorescence detection of explo-
sives.
[a] Dr. S.-R. Zhang, Dr. D.-Y. Du, Dr. J.-S. Qin, Dr. S.-J. Bao, Dr. W.-W. He,
Dr. P. Shen, Prof. Z.-M. Su
Institute of Functional Material Chemistry
Faculty of Chemistry, Northeast Normal University
Changchun 130024, Jilin (P.R. China)
Fax: (+86)431-85684009
E-mail: zmsu@nenu.edu.cn
[b] Prof. S.-L. Li, Prof. Y.-Q. Lan
Jiangsu Key Laboratory of Biofunctional Materials
College of Chemistry and Materials Science
Nanjing Normal University
Nanjing 210023, Jiangsu (P.R. China)
E-mail: yqlan@njnu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304692.
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more than two weeks, and in both acidic and alkaline solu-
tions. Meanwhile, NENU-503 can detect nitroaromatic explo-
sives fast and sensitively through fluorescence quenching. This
is the first report on MOFs that can distinguish between nitro-
aromatic molecules with different numbers of ÀNO2 groups. Al-
though the toxicity of CdII cannot be ignored, especially when
inhaled or exposed to skin, our results demonstrate a promising
and proven approach for a highly selective detection of nitro-
aromatic explosives.
Single-crystal X-ray analysis reveals that NENU-503 crystal-
lizes in the monoclinic space group C2/c (see the Supporting
Information, Table S1). The asymmetric unit of NENU-503 in-
cludes two Cd2+ in different coordination environments, one
L3À ion, one ClÀ ion, and one coordinated H2O molecule (Fig-
ure 1a). The Cd1 atom is four-coordinate by three nitrogen
atoms from one L3À ion and a chlorine atom (CdÀN, 2.217–
2.606 ꢁ and CdÀCl, 2.413 ꢁ, see the Supporting Information,
Table S2). The Cd2 atom is seven-coordinate by six carboxylate
oxygen atoms from three L3À ligands and one oxygen atom
from a coordinated H2O molecule (Figure 1a). Three carboxyl-
ate groups from L3À can coordinate three Cd2 atoms (see the
Supporting Information, Figure S1a). Such connectivity modes
give rise to a 2D sheetlike structure with (3,3) topology (Fig-
ure 1b,c), which is packed along the c axis (see the Supporting
Information, Figure S2). Furthermore, adjacent layers are con-
nected through p–p stacking, yielding a 3D supramolecular
motif (see the Supporting Information, Figure S3). NENU-503
displays a microporous framework along the c axis (Figure 1d).
Moreover, under ambient conditions, that is, exposure to air
for more than two weeks, no major changes to either the ex-
ternal appearance or to the PXRD patterns of NENU-503 were
detected; confirming extreme framework stability (see the Sup-
porting Information, Figure S4).
Figure 1. a) Coordination environments of CdII atoms in NENU-503. Symme-
try code: #1 Àx+1, y, Àz+3/2. b) Ball-and-stick representation of the 2D
sheetlike structure in NENU-503. c) 2D sheet with (3,3) topology of NENU-
503. d) Space-filling representation of NENU-503 viewed from the c axis. All
the hydrogen atoms are omitted for clarity.
formation, Figure S7a). This emission band can be assigned to
H3L ligand-centered emission, because similar emission was
observed at 424 nm (lex =345 nm) for the free H3L ligand (see
the Supporting Information, Figure S7b). In addition, the fluo-
rescence properties of NENU-503 in different solvent emul-
sions were investigated (Figure 2a). The predominant feature
is that the PL intensities are largely dependent on the solvent
molecules, particularly in the case of nitrobenzene (NB), which
exhibits significant quenching behavior (see the Supporting In-
formation, Figure S8a). The physical interaction of the solute
and solvent plays a vital role in such fluorescence behavior. To
examine sensing sensitivity towards NB in more detail, a batch
of suspensions of NENU-503 with gradually increasing NB con-
Surprisingly, NENU-503 also ex-
hibits outstanding chemical sta-
bility and unaltered PXRD re-
sults, suggesting that the crystal-
linity is retained, after exposure
to aqueous solutions (HCl and
NaOH) with pH values in the
range of 2–12 (see the Support-
ing Information, Figure S5). Only
a few MOFs that are stable in
both acidic and basic solutions
have been reported previous-
ly.[14] The thermal stability of
NENU-503 was studied by ther-
mogravimetric analysis (TGA, see
the Supporting Information, Fig-
ure S6).
The photoluminescence (PL)
spectra of NENU-503 and H3L in
the solid state were recorded at
room temperature. NENU-503
exhibited an emission peak at
389 nm upon excitation at
Figure 2. a) Emission spectra of NENU-503 in different solvents (excited at 295 nm). b) Emission spectra of NENU-
503 in different concentrations of nitrobenzene in DMA (excited at 295 nm). Inset: a photograph taken under UV
light (365 nm), showing the fluorescence quenching upon addition of nitrobenzene to NENU-503. c,d) Emission
331 nm (see the Supporting In- spectra of NENU-503 in different concentrations of 1,3-DNB and 2,4-DNT in DMA (excited at 295 nm), respectively.
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Communication
tents in N,N-dimethylacetamide (DMA) was prepared to moni-
tor the emissive response (Figure 2b). The luminescence inten-
sity decreased to 50% at 20 ppm, and complete quenching
was received at 100 ppm (see the Supporting Information, Fig-
ure S8b). The finely ground powder of NENU-503 is dispersed
well in the solution. This enables NB to be closely adhered to
the surface of the MOF particles and facilitates possible host–
guest interactions. Therefore, electron transfer from the elec-
tron-donating framework to the highly electron-deficient NB
molecule can take place upon excitation, resulting in fluores-
cence quenching.
50 ppm for analytes in DMA (see the Supporting Information,
Figure S12) indicate that, among the different analytes tested,
NENU-503 displays the most effective detection for NB. Inter-
estingly, the color of the solution turned orange upon incre-
mental addition of TNT; this could be observed by the naked
eye (Figure 3a). Therefore, we can detect TNT with high selec-
tivity simply by registering the color change; this is extremely
convenient and fast. The color changes observed in the pres-
ence of TNT may be ascribed to the formation of charge-trans-
fer complexes between NENU-503 (electron donor) and TNT
(electron acceptor) through charge-transfer interactions upon
excitation.[15] Upon closer examination of the PL spectra, we
have found that the spectrum of 1,3-DNB is obviously blue-
shifted by 13 nm as compared with that of pure DMA (390 nm,
Figure 2c), whereas it is apparently redshifted by 84 nm for
TNP (Figure 3b) and only one emission band exists, namely, in
the spectrum for TNP in DMA without NENU-503 (see the Sup-
porting Information, Figure S13). As mentioned above, the
emission band of NB is at 390 nm. Therefore, NENU-503 can
be used to distinguish between nitroaromatic molecules with
different numbers of ÀNO2 groups, for example, NB, 1,3-DNB,
and TNP, through the shift of the PL spectra (Figure 3c). More-
over, the PL spectra do not shift when performing the same
Because the luminescence quenching is related to the elec-
tron-withdrawing ÀNO2 group, we investigated the potential
of NENU-503 towards sensing a series of nitroaromatic explo-
sives, such as 1,3-DNB, 2,4-DNT, TNT and TNP. High fluores-
cence quenching was monitored by PL spectroscopy (lex =
295 nm) when the analytes were added to DMA. The lowest
concentration of complete quenching is 300 ppm for 1,3-DNB
and 2,4-DNT (Figure 2c,d and Figure S9 in the Supporting In-
formation), and 400 ppm for TNT and TNP (Figure S10 in the
Supporting Information). The relationship between emission
intensity and different concentrations of analytes (see the Sup-
porting Information, Figure S11) and quenching efficiency at
Figure 3. Emission spectra of NENU-503 in different concentrations of a) TNT and b) TNP in DMA (excited at 295 nm). Inset in (a): color changes upon incre-
mental addition of TNT solution (left: TNT; middle: NENU-503 in DMA; right: NENU-503 in DMA containing TNT). c) Emission spectra of NENU-503 in NB, 1,3-
DNB, and TNP (10 ppm in DMA). d) Shapes of HOMO and LUMO of the molecular orbitals considered and the relative energy level investigated by the B3LYP/
6-31G** method.
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experiment with only H3L (see the Supporting Information, Fig-
ure S14). This indicates that this discrimination is due to NENU-
503. Most previous reports of MOFs for detection of nitro-con-
taining compounds investigate the effects of the concentration
on fluorescence quenching, whereas the distinction between
different nitro-containing compounds has rarely been report-
ed.[16] This is the first time that a MOF can distinguish nitroaro-
matic compounds from each other through shifts of PL spec-
tra. The shifts of the PL spectra may be due to the formation
of exciplexes (excited complexes) by the interaction of analytes
and MOFs in the excited states;[17] this phenomenon is less
studied in MOFs. To further explore the reason for the shift of
the PL spectra of TNP, we have monitored the PL spectra of
phenol and p-nitrophenol in the presence of NENU-503. The
emission band of phenol is at 390 nm, whereas the PL spectra
are obviously redshifted by 10 nm for p-nitrophenol (400 nm)
and 84 nm for TNP (474 nm, see the Supporting Information,
Figure S15). Therefore, the redshift of the PL spectra for TNP
results from ÀOH and ÀNO2 groups in combination with exci-
plexs. The framework is intact after detection of different ana-
lytes as confirmed by PXRD (see the Supporting Information,
Figure S16).
resonance energy transfer can be observed from the donor to
the acceptor. The efficiency and sensitivity of the fluorescence
quenching can therefore be significantly improved.[3,19] The ab-
sorption bands of the analytes have a considerable degree of
overlap with the PL spectrum of NENU-503 in DMA (see the
Supporting Information, Figure S19). As a result, the combina-
tion of photoinduced electron transfer and resonance energy
transfer gives rise to the fluorescence quenching.
We also monitored the sensitivity of H3L towards NB in DMA
(see the Supporting Information, Figure S20). The quenching
efficiency in the presence of NENU-503 is 45, 93, and 100% for
10, 50, and 100 ppm of NB, respectively, whereas for H3L it is
only 28, 56, and 78%. This result indicates that the coordina-
tion between H3L and CdII makes the electron transfer from
NENU-503 to analytes more effective. Furthermore, H3L dis-
solves well in DMA, thus the stability of NENU-503 is propi-
tious to separate and recycle. In particular, NENU-503 can be
regenerated by centrifugation of the solution after use and
washing several times with DMA.[20] The quenching efficiencies
of cycles 1–6 are basically unchanged at about 90%, displaying
high recyclability and stability for detection applications
(Figure 4). The PXRD pattern further confirms that the frame-
work is retained after cycle 6 (see the Supporting Information,
Figure S21).
The quenching efficiency can be quantitatively explained by
the Stern–Volmer equation: (I0/I)=Ksv[A]+1, where I0 and I are
the fluorescence intensities before and after addition of the an-
alyte, respectively, Ksv is the quenching constant (mÀ1), [A] is
the molar concentration of the analyte. The Stern–Volmer plots
for analytes are nearly linear at low concentrations. The plots
subsequently deviate from linearity at higher concentrations
(see the Supporting Information, Figure S17). The nonlinear
nature of the plot of the analytes may be ascribed to self-ab-
sorption or an energy-transfer process.[3,18] This suggests that
dynamic and static quenching coexist in the fluorescence
quenching process. Fluorescence decays of NENU-503 at dif-
ferent concentrations of analytes also verify this (see the Sup-
porting Information, Figure S18).[18b] The fluorescence attenua-
tion can be attributed to a photoinduced electron-transfer
(PET) mechanism. MOFs constructed from d10 metal ions with
highly localized electronic states are often characterized by
narrow energy bands. Therefore, they can be regarded as giant
“molecules” and the valence-band (VB) and conduction-band
(CB) energy levels can be described in a fashion similar to that
used for molecular orbitals (MOs).[10b] In general, the lowest un-
occupied MOs (LUMOs) of analytes are low-lying p*-type orbi-
tals stabilized by the ÀNO2 group through conjugation (Fig-
ure 3d); the energy levels of these orbitals are lower than the
CB energy levels of NENU-503. This can force electrons to
transfer from the CB of NENU-503 to the LUMO of the analyte,
thus leading to fluorescence quenching upon excitation.
Shapes and relative orbital energies of the highest occupied
MO (HOMO) and LUMO of the molecular considered were cal-
culated by density functional theory at the B3LYP/6-31G** level
(see the Supporting Information, Table S3). Another possible
reason for the quenching is resonance energy transfer. If the
emission spectrum of the fluorophore (donor) has a certain
degree of overlap with the absorption band of the analyte (ac-
ceptor), when the distance between them is appropriate, the
Figure 4. Reproducibility of the quenching ability of NENU-503 dispersed in
DMA and in the presence of 50 ppm NB. The dark-grey bars represent the
initial fluorescence intensity and the light-grey bars represent the intensity
upon addition a solution of 50 ppm NB in DMA. The percentages on the top
represent quenching efficiency of every cycle.
In conclusion, we have successfully synthesized extremely
stable NENU-503 for quick and sensitive detection of nitroaro-
matic explosives through fluorescence quenching. Notably, we
can easily detect TNT by the naked eye. This fluorescence
quenching is ascribed to photoinduced electron transfer and
resonance energy transfer. For the first time, a MOF can distin-
guish between NB, 1,3-DNB, and TNP with different numbers
of ÀNO2 groups by the shift of the PL spectra. High stability
and recyclability of NENU-503 make it an outstanding candi-
date in the field of detection of explosives. The present work
opens a promising approach to design MOF-based sensors for
explosives; this will probably be useful under more realistic
conditions in the future.
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Experimental Section
Synthesis of NENU-503
H3L (0.03 g, 0.04 mmol), Cd(NO3)2·4H2O (0.16 g, 0.52 mmol), DMA
(5 mL), H2O (3 mL), and four drops of HCl (6 mollÀ1) were sealed in
a Teflon-lined stainless steel container. The container was heated
to 1108C for four days, resulting in white crystals that were isolated
by washing with DMA and dried at room temperature. Yield: 64%
based on H3L. IR (KBr): n˜ =3434 (s), 2937 (m), 1625 (s), 1552 (s),
1401 (s), 1262 (m), 1187 (m), 1018 (s), 959 (w), 859 (w), 756 (m), 719
(w), 596 (m), 475 (w), 427 cmÀ1 (w); elemental analysis calcd (%) for
C66H77.5N11.5O11.5Cd2: C 53.69, H 5.30, N 10.91; found: C 53.62, H
5.38, N 11.01.
Acknowledgements
This work was financially supported by Pre-973 Program
(2010CB635114), the National Natural Science Foundation of
China (No. 21001020 and 21371099), the Jiangsu Specially Ap-
pointed Professor, the NSF of Jiangsu Province of China (No.
BK20130043), the Natural Science Research of Jiangsu Higher
Education Institutions of China (No. 13KJB150021), the Priority
Academic Program Development of Jiangsu Higher Education
Institutions and the Foundation of Jiangsu Collaborative Inno-
vation Center of Biomedical Functional Materials.
Keywords: explosives
·
fluorescence
·
metal–organic
frameworks · nitroaromatic molecules · sensors
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