A fluorescent lanthanide-organic framework for highly sensitive detection of nitroaromatic explosives

Article abstract of DOI:10.1016/j.poly.2016.12.030

A lanthanide-organic framework constructed from a new π-electron rich ligand 5,5′-(9H-carbazole-3,6-diyl)diisophthalic acid (H₄L) and Tb(NO₃)₃·6H₂O, namely, [(CH₃)₂NH₂][TbL] (1), has been synthesized under solvothermal condition. X-ray diffraction analysis reveals that complex 1 exhibits a 3D porous framework featuring a (4,8)-connected sqc net. The solid sample of 1 exhibits strong green fluorescence in the visible region upon excitation at 354?nm. The fine grinding particles of 1 dispersed in ethanol shows good sensitivity for picric acid (PA), which can be reused at least five times for detecting PA. The good sensitivity and recyclability of 1 make it a potential fluorescent sensor for explosives detection.

Full text of DOI:10.1016/j.poly.2016.12.030

Accepted Manuscript
A Fluorescent Lanthanide–organic Framework for Highly Sensitive Detection
of Nitroaromatic Explosives
Yajuan Mu, Yungen Ran, Jianlong Du, Xueyi Wu, Weiwei Nie, Jianhong Zhang,
Ying Zhao, Huan Liu
PII:
S0277-5387(16)30681-7
DOI:
http://dx.doi.org/10.1016/j.poly.2016.12.030
POLY 12388
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 October 2016
19 December 2016
21 December 2016
Please cite this article as: Y. Mu, Y. Ran, J. Du, X. Wu, W. Nie, J. Zhang, Y. Zhao, H. Liu, A Fluorescent
Lanthanide–organic Framework for Highly Sensitive Detection of Nitroaromatic Explosives, Polyhedron (2016),
doi: http://dx.doi.org/10.1016/j.poly.2016.12.030
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A Fluorescent Lanthanide–organic Framework for Highly
Sensitive Detection of Nitroaromatic Explosives
a
b
Yajuan Mu , Yungen Ran , Jianlong Du , Xueyi Wu , Weiwei Nie , Jianhong Zhang ,
c
a
a
a
∗
Ying Zhaoa, Huan Liua
a Department of Traditional Chinese Medicine, Hebei University, Baoding, 071000, P. R. China, E-mail:
muyajuan1027@163.com, Tel./fax: +86 312 5078519
b Hebei University, Baoding, 071000, P. R. China
c College of Chemistry & Environmental Science, Hebei University, Baoding, 071000, P. R. China
Abstract: A lanthanide-organic framework constructed from a new π-electron rich ligand
5,5’-(9H-carbazole-3,6-diyl)diisophthalic acid (H4L) and Tb(NO3)3·6H2O, namely,
[(CH3)2NH2][TbL] (1), has been synthesized under solvothermal condition. X-ray
diffraction analysis reveals that complex 1 exhibits a 3D porous framework featuring a
(4,8)-connected sqc net. The solid sample of 1 exhibits strong green fluorescence in the
visible region upon excitation at 354 nm. The fine grinding particles of 1 dispersed in ethanol
shows good sensitivity for picric acid (PA), which can be reused at least five times for
detecting PA. The good sensitivity and recyclability of 1 make it a potential fluorescent
sensor for explosives detection.
Keywords: Nitroaromatic explosives; Selective sensing; Lanthanide–organic framework;
Luminescence
1. Introduction
With the increasing use of nitroaromatic explosive materials in terrorism all over the
world, rapid detection methods for tracing nitroaromatic explosive materials are in strong
demand concerning security screening, homeland security, environmental monitoring and
human health [1-3]. Hence, various methods to detect nitroaromatic explosives have been
developed, such as gas chromatography coupled with mass spectrometry [4], surface
enhanced Raman spectroscopy [5], neutron activation analysis [6], ion mobility spectroscopy
∗
Corresponding author. Tel./fax: +86 312 5078519
E-mail: muyajuan1027@163.com
1

(IMS) [7] and electrochemical method [8]. These instrumental techniques are expensive and
may not always be easily accessible. Fluorescence quenching based sensing is a much simpler,
sensitive and convenient technique [9]. Electron donor conjugated polymers have proven to
be excellent candidates for the detection of nitroaromatic explosives. The delocalized π
electrons of such polymers can increase the electrostatic interaction between the polymers and
electron-deficient nitroaromatic analytes [10].
Recently, fluorescent MOFs have drawn considerable attention for nitroaromatic
explosive sensing [11,12]. Due to the controllable size and porosity, structural stability and
high surface area, fluorescent MOFs exhibit advantages over other fluorescent materials.
The fluorescent MOFs can allow more analytes to come into contact with the MOF surface,
which may improve the sensitivity. It is well-known that the fluorescent behaviour of a MOF
highly depends on the organic linker and metal ion. Therefore, enhanced performance can be
accomplished by elaborate selection of the organic linker and metal ions.
Here, we designed and synthesized a π-electron rich polycarboxylates ligand
5,5’-(9H-carbazole-3,6-diyl)diisophthalic acid (H4L). Considering that most of the Ln-MOFs
exhibit excellent luminescence properties, lanthanide Tb3+ was choosed as the metal connector
to synthesize fluorescent MOFs. We employed this ligand to react with lanthanide Tb3+ to
construct a newl luminescent 3D Tb-MOF [(CH3)2NH2][TbL] (1). Because the H4L ligand can
effectively sensitize the emission of Tb3+, the solid sample of 1 emits high bright green light
upon excitation at 354 nm. The fine grinding particles of 1 dispersed in ethanol exhibit
selective sensing of nitroaromatic explosives, indicating the Tb-MOF can be a good candidate
for fluorescent sensor.
2. Experimental section
2.1 General information and materials
Except for the H4L ligand, all other chemicals were commercially available and used
without further purification. IR data were recorded on a BRUKER TENSOR II
spectrophotometer with KBr pellets in the region of 400–4000 cm-1. Elemental analyses (C, H
and N) were carried out on a Flash EA 1112 elemental analyzer. Powder X-ray diffraction
2

(PXRD) measurements were finished with a Bruker AXS D8 Advance using Cu Kα radiation.
Thermal analyses were performed on a Netzsch STA 449C thermal analyzer at a heating rate
1
of 10 °C· min-1 in air. H NMR was determined on Bruker DPX 600 nuclear magnetic
resonance spectrometer. The luminescence spectra for the powdered solid samples were
measured at room temperature on a Hitachi F-7000 Fluorescence Spectrophotometer. The
excitation slit and the emission slit were 5 nm (PMT Voltage = 400V). The scan speed is 1200
nm/min. The fluorescent sensing properties of 1 were examined under the same conditions.
2.2. Synthesis
2.2.1 Synthesis of 5,5’-(9H-carbazole-3,6-diyl)diisophthalic acid (H4L) (Scheme 1)
3,6-Dibromo-9H-carbazole was prepared according to the literature [13].
Under a nitrogen atmosphere, to a solution of 3,6-dibromo-9H-carbazole (325 mg, 1
mmol)
in
DMF
solvent
(30
mL)
were
added
dimethyl
5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)isophthalate(640 mg, 2 mmol), Pd(PPh3)4
(93 mg, 0.05 mmol), K2CO3 (324 mg, 3 mmol). After the mixture was stirred at 100oC for
18 h, then filtered and concentrated. The crude product was purified by column
chromatography on silica gel with dichloromethane/methanol (100:5, V:V) to give pure
tetramethyl 5,5’-(9H-carbazole-3,6-diyl)diisophthalate as a gray solid. Yelid: 330 mg, 60%.
1H NMR (600 MHz, DMSO-d6): δ 11.557 (s, 1H), 8.858 (s, 2H), 8.559 (s, 4H), 8.439 (s, 2H),
7.832 (d, 2H), 7.645 (d, 2H), 4.519(s, 12H). MS m/z 550.2 (M + H+).
Tetramethyl 5,5’-(9H-carbazole-3,6-diyl)diisophthalate(551 mmg,1mmol) was dissolved
in 20 mL of mixed solvent of THF and MeOH (v/v =1:1). Then 10 mL of 20% NaOH
aqueous solution was added to the above solution. The mixture was constantly stirred at 80oC
for 24 h. After that, the organic phase was removed under reduced pressure. The remaining
aqueous phase was acidified by dilute hydrochloric acid to obtain a gray precipitate, which
1
was filtered and washed with water. Yield: 410 mg, 83%. H NMR (600 MHz, DMSO-d6): δ
13.394 (s, 4H), 11.557 (s, 1H), 8.858 (s, 2H), 8.559 (s, 4H), 8.439 (s, 2H), 7.832 (d, 2H),
7.645 (d, 2H). MS m/z 494.2 (M + H+).
2.2.2 Synthesis of [(CH3)2NH2][TbL] (1)
A mixture of Tb(NO3)3·6H2O (45.3 mg, 0.1 mmol), H4L (24.7 mg, 0.05 mmol) and
3

NaOH (8.0 mg, 0.2 mmol) in 5 mL of the component solvent (DMF:CH3CN:H2O = 3:1:1)
was sealed in a 25 mL Teflon-lined stainless steel vessel and heated to 160 °C for 72 h. After
the mixture was cooled to room temperature at a rate of 5 °C·h–1, colorless crystals of 1 were
obtained with a yield of 65% (based on Tb). Anal. Calcd for C18H17N2O4Tb (%): C, 51.74; H,
3.04; N, 4.02. Found: C, 51.80; H, 3.08; N, 4.03. IR (cm-1, KBr): 3423s, 1771w, 1734w,
1715w, 1698w, 1650w, 1620w, 1557w, 1539w, 1522w, 1508w, 1456w, 1287w, 849m, 820w,
785m, 718m, 595m.
2.3. Crystal structure determination
Single-crystal X-ray diffraction measurement was collected on a Bruker SMART
APEX-II CCD diffractometer for
1
at temperature of 23
1
°C using
graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å). Absorption corrections were
applied by using multi-scan program. The data were corrected for Lorentz and polarization
effects. The structure was solved by direct methods and refined with a full-matrix
least-squares technique based on F2 with the SHELXL-97 crystallographic software package
[14]. The hydrogen atoms were assigned with common isotropic displacement factors and
included in the final refinement by using geometrical restrains. The molecular formula of 1
was calculated and confirmed by the elemental analysis data and thermogravimetric analysis,
because the SQUEEZE function in PLATON was applied [15]. Crystallographic crystal data
and structure processing parameters for complex 1 are summarized in detail in Table 1.
Selected bond lengths and bond angles are listed in Table S1.
3. Results and discussion
3.1. Crystal structure description
Single-crystal X-ray analysis indicates that complex 1 crystallizes in the tetragonal
crystal system with space group of I-42d. The asymmetrical unit of 1 contains half a Tb3+ ion,
+
half a L4- ligand and the negative charge of the framework is balanced by half a (CH3)2NH2
cation, which was generated from the decomposed DMF. As shown in Fig.1a, the central Tb3+
ion possesses an eight-coordinated square antiprism coordination mode, being linked to eight
carboxylate oxygen atoms from six different L4- ligands. The Tb–O bond lengths vary from
2.260(5) to 2.501(5) Å, which are similar to those reported in other Tb3+ compounds [16].
4

Two symmetry related Tb3+ ions are bridged by carboxylate groups to form a noteworthy
paddle-wheel [Tb2(CO2)8] subunit with a Tb···Tb distance of 4.2938(17) Å. Each L4- anion
coordinates to 8 Tb3+ ions, with carboxylate groups adopting two kinds of coordination modes:
µ2-η1:η1 and µ1-η1:η1 mode. Finally, each binuclear [Tb2(CO2)8] subunit are linked by L4-
anions to generate a fascinating 3D porous framework (Fig.1b). The solvent-accessible
volume in 1 is 2206.6 Å3, equal to 36.6% of the volume of the unit cell calculated by the
PLATON program. Topological analysis was carried out to get insight into the structure [17].
As shown in Fig. 1c, the L4- anions and the [Tb2(CO2)8] subunits can be defined as 4- and
8-connected nodes, respectively. Therefore, complex 1 can be simplified as a (4,8)-connected
sqc network with Schläfli symbol of (43·63)(412·616).
3.2. PXRD patterns and thermogravimetric analysis
The PXRD patterns of complex 1 are shown in Fig. S1 in the Supporting Information.
The peak positions of experimental PXRD patterns are in line with the simulated ones,
showing the pure phase of the complex. As shown in Fig. S2, the TGA of 1 shows an initial
gradual weight loss of 6.12% between 85 °C to 242 °C, corresponding to the release of
+
(CH3)2NH2 (calcd: 6.62%). The compound 1 begins to decompose upon further heating and
underwent a rapid weight loss of 64.58% from 242 °C to 563 °C, corresponding to the
destruction of the H4L ligands.
3.3. Fluorescence behaviors and sensing properties
The lanthanide coordination polymers have displayed excellent fluorescence properties
in the repect of their high color purity [18]. So we investigated the ligand-assisted
fluorescence properties of powder samples of the lanthanide complex at room temperature
(Fig.S3). When excited at 354 nm, complex 1 exhibits the characteristic emissions of Tb3+
ions in the visible region. The emission peaks at 490, 545, 585 and 621 nm can be attributed
to the transitions of 5D4-7FJ (J = 6, 5, 4, 3) from Tb3+ ions, respectively. Upon excitation at 243
nm, the free H4L ligand exhibits a luminescent emission band at 388 nm, which is attributed
to the π–π* transition. The luminescent investigation on the complex 1 suggests that H4L
ligand may sensitize the luminescence of Tb3+ ions. The strongest emission in 1 appears at
545 nm which can be easily observed as bright green light by the naked eye. The remarkable
fluorescence offers the possibility of 1 being a fluorescence sensor.
5

To explore the potential photoluminescence sensing properties of 1, the fine grinding
sample of 1 (3 mg) was dispersed in 3 mL of various solvents (EtOH, MeCN, DMF, CH2Cl2,
THF, acetone, H2O, toluene (TO), benzene (BZ), nitrobenzene(NB)) sonicated for 30 min and
the emission was observed. The fluorescence intensities are mainly dependent on the solvent
molecules, which present different fluorescence quenching degree (Fig. 2). Among the chosen
solvents, the severe quenching of fluorescence intensity was observed in NB, indicating that 1
has a selective response to NB. The mechanism of fluorescence quenching might be attributed
to the photoinduced electron transfer from an excited MOF to the highly electron deficient
NB adsorbed on the surface of 1 [19]. The fluorescence quenching of 1 is similar to that
reported for many other fluorescent MOFs in order to account for their highly selective
sensing behaviour towards NB [20].
The sensing properties of 1 for NB prompted us to investigate its application in detecting
different electron-withdrawing nitroaromatics. The nitroaromatics such as 4-nitrotoulene
(4-NT),
2,4-dinitrotoluene
(2,4-DNT),
4-ni-tro-chlorobenzene
(4-NC),
2,4-dinitro-chlorobenzene (2,4-DNC), picric acid (PA) were added to a suspension of 1 (3 mg)
in ethanol (3 mL). As shown in Fig. 3, upon the addition of the same amount of nitroaromatics
([nitroaromatics] = 10 ppm), different degrees of fluorescence quenching of the suspension
were tested. The descending order of quenching efficiency was PA > 4-NC > 2,4-DNC >
2,4-DNT > 4-NT >NB. About quenching efficiency (QE = (1-F/F0) × 100% (F0 and F are the
fluorescent intensities before and after the addition of nitroaromatics, respectively.)) of 76.9%
for PA is observed, which is significantly more than that of 4-NC (29.2%), indicating that 1
shows a high selectivity for PA.
To further study the sensing capacity of 1 for PA, the emission spectra are monitored by
gradually increasing PA contents of a suspension of 3 mg of 1 dispersed in 3 mL ethanol. The
fluorescent intensity obviously decreased with the increasing PA concentration (Fig. 4). When
the concentration of PA reached 30 ppm, the quenching efficiency was 94.2% for 1, which is
higher than or comparable to other MOF sensors, polymeric nanocomposites [3a] and
conjugated polymer sensors for PA. For example, the micrometer-sized MOF
[Cd3(TPT)2(DMF)2]·(H2O)0.5 show an 82.5% quenching efficiency for PA [2c]. The
quenching efficiency of the reported carbon and silver nanocomposites for PA is 99% [3a].
6

The fluorescence intensity of conjugated polymer flim only decreased with 77-fold
fluorescence quenching [21]. After immersing into the PA solution (1 mM in ethanol) for 12h,
crystals of 1 are collected by filtration, washed with EtOH and air-dried. Then the PXRD
measurement was performed. The PXRD result indicates that the crystallinity of 1 was still
remained after the experiment (Fig. S4).
The reusability of a sensor is a critical problem for the long-term practical applications.
So, the recyclability experiments of the particles of 1 for sensing PA were carried out up to
five cycles. The particles of 1 could be recovered by centrifugation of the dispersed solution
and washing three times with ethanol. The luminescent intensity and quenching ability of 1
are essentially retained in 1–5 cycles (Fig. 5). The results demonstrated that the complex has
high reproducibility for explosive detection.
4. Conclusions
In conclusion, we have successfully synthesized a novel fluorescent Tb-MOF
[(CH3)2NH2][TbL] (1) with a new π-electron rich tetracarboxylic acid H4L. The complex
exhibits a 3D porous framework constructed via paddle-wheel [Tb2(CO2)8] subunit. The
solid-state fluorescent measurements indicated that the ligand H4L can sensitize the
fluorescence intensities of Tb3+ ions, resulting in the strong fluorescence emission of Tb-MOF.
The fine grinding particles of 1 dispersed in ethanol exhibits a high quenching efficiency for
PA. The application of 1 may enable further development of new chemosensors for
explosives.
Acknowledgment
We gratefully acknowledge the financial support by the National Natural Science Foundation
of China (No. 21301045), the Natural Science Foundation of Hebei province (No.
B2014201143) and the Natural Science Research Plan Project of Hebei University (No.
2012-243).
Appendix A. Supplementary data
CCDC 1469345 contains the supplementary crystallographic data for complex 1. These data
7

can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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Figure captions
Scheme 1 Design and synthesis of ligand H4L.
Fig.1 (a) Coordination environment of Tb3+ ion in 1. The hydrogen atoms are omitted for
clarity. Symmetry code: A = 1.5-x, y, 1.75-z; B = 1-x, -y, z; C = 1-y, 0.5-x, 0.25+z; D = 1-x,
-0.5+y, 2.25-z; E = x, 0.5-y, 2.25-z; F = y, -0.5+x, 0.25+z. (b) The 3D framework. (c) Crystal
structure of 1: Defined 4- and 8-connected nodes (above) and schematic representation of
topological net of 1 (below).
Fig. 2 Luminescence spectra of dispersed 1 in various pure solvents under λex = 354 nm.
5
Fig. 3 Reduction of the D4-7F5 transition intensities (plotted as quenching efficiency) of the
ethanol solution of 1 with 10 ppm of different analytes (excited and monitored at 354 nm and
545 nm, respectively). Error bars represent one standard deviation.
Fig. 4 Emission spectra of 1 dispersed in ethanol by the titration of PA in ethanol (1 mM).
Fig. 5 The quenching and recovery experiment of 1 dispersed in ethanol in presence of PA
solution. The red bars and green bars represent the fluorescence intensity before and after the
addition of 10 ppm PA (1 mM). Error bars represent one standard deviation.
11

Scheme 1 Design and synthesis of ligand H4L.
12

B
(a)
(b)
13

(c)
Fig.1 (a) Coordination environment of Tb3+ ion in 1. The hydrogen atoms are omitted for
clarity. Symmetry code: A = 1.5-x, y, 1.75-z; B = 1-x, -y, z; C = 1-y, 0.5-x, 0.25+z; D = 1-x,
-0.5+y, 2.25-z; E = x, 0.5-y, 2.25-z; F = y, -0.5+x, 0.25+z. (b) The 3D framework. (c) Crystal
structure of 1: Defined 4- and 8-connected nodes (above) and schematic representation of
topological net of 1 (below).
14

Fig. 2 Luminescence spectra of dispersed 1 in various pure solvents under λex = 354 nm.
15

80
60
40
20
0
PA
4-NC
2,4-DNC
2,4-DNT
4-NT
NB
5
Fig. 3 Reduction of the D4-7F5 transition intensities (plotted as quenching efficiency) of the
ethanol solution of 1 with 10 ppm of different analytes (excited and monitored at 354 nm and
545 nm, respectively). Error bars represent one standard deviation.
16

Fig. 4 Emission spectra of 1 dispersed in ethanol by the titration of PA in ethanol (1 mM).
17

1
0.8
0.6
0.4
0.2
0
1
2
3
4
5
Fig. 5 The quenching and recovery experiment of 1 dispersed in ethanol in presence of PA
solution. The red bars and green bars represent the fluorescence intensity before and after the
addition of 10 ppm PA (1 mM). Error bars represent one standard deviation.
18

Table 1 Crystal Data and Structure Refinement Details for Complex 1
compound
formula
fw
1
C30H21N2O8Tb
696.41
296(2)
0.71073
Tetragonal
I-42d
T/K
λ (Mo Kα), Å
cryst syst
space group
a/Å
13.799(2)
13.799(2)
31.702(12)
90
b/Å
c/Å
α/deg
90
β/deg
90
γ/deg
V/Å3
6036(3)
8
Z
25
2θmax (deg)
Dc/g·cm-3
Reflns collected /unique
R(int)
1.533
15165/ 2669
0.0682
2.393
Abs coeff/mm-1
2752
F(000)
1.075
GOF
R1 [I > 2σ(I)]a
wR2 (all data) b
0.0332
0.0942
2 2 1/2
aR1 = ∑׀׀Fo׀−׀Fc׀׀/∑׀Fo׀. bwR2 = [∑w(Fo2 −Fc2)2/∑w(Fo ) ] .
19

Graphical abstract: pictogram
Graphical abstract: Synopsis
A Fluorescent Lanthanide–organic Framework for Highly
Sensitive Detection of Nitroaromatic Explosives
Yajuan Mu, Yungen Ran, Jianlong Du, Xueyi Wu, Weiwei Nie, Jianhong Zhang, Ying
Zhao, Huan Liu
A novel fluorescent Tb-MOF with a new π-electron rich tetracarboxylic acid
5,5’-(9H-carbazole-3,6-diyl)diisophthalic acid has been synthesized under solvothermal
condition. The Tb-MOF exhibits a 3D porous framework constructed via paddle-wheel
[Tb2(CO2)8] subunit. The fine grinding particles of Tb-MOF dispersed in ethanol exhibits
high efficiency detection for picric acid (PA).
20