Selective Sensing of Fe3+ and Al3+ Ions and Detection of 2,4,6-Trinitrophenol by a Water-Stable Terbium-Based Metal-Organic Framework

Article abstract of DOI:10.1002/chem.201501162

A water-stable luminescent terbium-based metal-organic framework (MOF), {[Tb(L₁)_1.5(H₂O)]·3 H₂O}_n (Tb-MOF), with rod-shaped secondary building units (SBUs) and honeycomb-type tubular channels has been synthesized and structurally characterized by single-crystal X-ray diffraction. The high green emission intensity and the microporous nature of the Tb-MOF indicate that it can potentially be used as a luminescent sensor. In this work, we show that Tb-MOF can selectively sense Fe³⁺ and Al³⁺ ions from mixed metal ions in water through different detection mechanisms. In addition, it also exhibits high sensitivity for 2,4,6-trinitrophenol (TNP) in the presence of other nitro aromatic compounds in aqueous solution by luminescence quenching experiments.

Full text of DOI:10.1002/chem.201501162

DOI: 10.1002/chem.201501162
Full Paper
&
Metal–Organic Frameworks
3
+
3+
Selective Sensing of Fe and Al Ions and Detection of 2,4,6-
Trinitrophenol by a Water-Stable Terbium-Based Metal–Organic
Framework
[a]
[a]
[a]
[a]
[a, b]
Li-Hui Cao, Fang Shi, Wen-Min Zhang, Shuang-Quan Zang,* and Thomas C. W. Mak
Abstract: A water-stable luminescent terbium-based metal–
organic framework (MOF), {[Tb(L ) (H O)]·3H O} (Tb-MOF),
minescent sensor. In this work, we show that Tb-MOF can
3
+
3+
selectively sense Fe and Al ions from mixed metal ions
in water through different detection mechanisms. In addi-
tion, it also exhibits high sensitivity for 2,4,6-trinitrophenol
(TNP) in the presence of other nitro aromatic compounds in
aqueous solution by luminescence quenching experiments.
1
1.5
2
2
n
with rod-shaped secondary building units (SBUs) and honey-
comb-type tubular channels has been synthesized and struc-
turally characterized by single-crystal X-ray diffraction. The
high green emission intensity and the microporous nature
of the Tb-MOF indicate that it can potentially be used as a lu-
Introduction
RNA synthesis. Both excess and deficiency from the normal
[
6]
permissible limit can induce serious disorders. On the other
3
+
Porous metal–organic frameworks (MOFs) are an important
class of multifunctional materials used in many practical appli-
hand, the Al ion is not a necessary microelement for the
human body, and its concentration level has a direct impact
on human health. Because of the frequent use of aluminum
[
1,2]
cations,
including gas storage and separation, heterogene-
3
+
ous catalysis, and guest species recognition and adsorption,
etc. The tunable structures and properties of porous MOFs pro-
vide an important advantage to chemosensory and adsorptive
materials. Among the successful production of functional
porous MOFs, lanthanide metal–organic frameworks (Ln–
MOFs) are fascinating because of their versatile coordination
geometry, high stability, and unique luminescent and magnetic
cookware, the risk of the absorption of Al
ions by the
human body is increasing. Many diseases can be caused by
iron poisoning or aluminum toxicity, such as microcytic hypo-
chromic anemia, bone softening, encephalopathy, myopathy,
[
7]
and Alzheimer’s disease, etc. Therefore, the selective detec-
3
+
3+
tion of Fe and Al is very important for human health. Re-
cently, many excellent studies, based on Ln–MOFs or Ln@MOFs
[
3]
3+
2+
properties. Owing to their high color purity and the long life-
times of their excited states, the intriguing luminescence of
lanthanide ions has potential applications in optical communi-
that can selectively sense metal ions such as Fe , Cu ions
or organic molecules such as acetone, have been reported by
[
8]
[9]
our group and other groups.
[4]
cations as well as fluorescent probes. In the past few years,
fluorescent probes based on luminescent Ln–MOFs have been
widely investigated for the selective detection of metal ions or
small organic molecules as a result of their ability to provide
Nitro aromatic compounds such as nitrobenzene (NB), 1,3-di-
nitrobenzene (1,3-DNB), 1,4-dinitrobenzene (1,4-DNB), 2-nitro-
toluene (2-NT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene
(2,6-DNT), 2,4,6-trinitrotoluene (TNT), and 2,4,6-trinitrophenol
(TNP) are the primary components of industrial explosives, en-
vironmentally deleterious substances, and many unexploded
[5]
a simple, selective, and sensitive detection method.
Iron and aluminium are ubiquitous metals in our living envi-
ronment, and their cations also play crucial roles in the human
[
10]
land mines. Hence, for the sake of security, military applica-
tions, and environmental concerns, the reliable and convenient
3
+
body and other biological tissues. The Fe ion influences elec-
tron transfer and oxygen metabolism processes in DNA and
[
11,12]
detection of explosives has become a pressing issue.
Owing to the unique selectivity, sensitivity, short response
time, and convenient visual detection, fluorescence-based de-
tection of nitro aromatic explosives by using luminescent
metal–organic frameworks has attracted much attention since
[
a] L.-H. Cao, F. Shi, W.-M. Zhang, Prof. S.-Q. Zang, Prof. T. C. W. Mak
College of Chemistry and Molecular Engineering
Zhengzhou University
Zhengzhou (P. R. China)
Fax: (+86)371-6778-0136
[
13]
the first work reported in 2009. Of all the nitro aromatic ex-
plosives, TNP is one of the most powerful explosives, and is
E-mail: zangsqzg@zzu.edu.cn
[
14]
[
b] Prof. T. C. W. Mak
commonly used in dyes, fireworks, and pesticides. However,
owing to their strong electron affinities, it is still a challenge to
selectively detect TNP in the presence of other nitro aromatic
Department of Chemistry and Center of Novel Functional Molecules
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong SAR (P. R. China)
[
15]
explosives. Although some Ln–MOFs have been employed
for explosive detection, only a few reports of Ln-MOF or
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501162.
Chem. Eur. J. 2015, 21, 15705 – 15712
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Full Paper
Ln@MOFs based probes can selectively detect TNP in the pres-
[
16]
ence of other nitro aromatic compounds in aqueous media.
Because of the oxophilic nature of lanthanide ions, the use
of terephthalic acid and its derivatives to act as chromophoric
ligands is one of the most common and efficient methods to
construct novel lanthanide-containing porous MOFs with inter-
esting structures and high luminescence quantum yields in
[
11c,17]
previous reports.
um-based metal–organic framework, {[Tb(L ) (H O)]·3H O}
n
Herein, we reported a water-stable terbi-
1
1.5
2
2
(
Tb-MOF), which is based on a derivative of terephthalic acid,
with strong luminescence intensity and that possesses 1D rod-
shaped [Tb(CO ) ] building units and honeycomb-type tubular
2
3 n
3
+
3+
channels. Tb-MOF can rapidly sense Fe and Al ions from
mixed metal ions through different detection mechanisms. Tb-
MOF also exhibits high sensitivity for 2,4,6-trinitrophenol (TNP)
in the presence of other nitro aromatic compounds in aqueous
solution through luminescence quenching experiments.
Results and Discussion
[18]
Single-crystal X-ray diffraction (XRD) analysis revealed that
Tb-MOF crystallizes in the trigonal space group R 3¯ . As shown
in Figure S1 in the Supporting Information, the fundamental
3
+
building unit of Tb-MOF contains one Tb ion, one and a half
2
À
L1 ligands, and one coordinated water molecule. The three
2À
carboxylate groups of the one and a half L1 ligands have two
different coordination modes: one carboxylate bridges two
3
+
Tb ions in a bidentate fashion, whereas the other two car-
3
+
boxylate groups chelate and bridge two Tb centers (Fig-
3
+
ure S2 in the Supporting Information). The Tb center is coor-
2
À
dinated to eight carboxyl oxygen atoms from six L
ligands
1
3
+
and one water molecule. The nine-coordinate Tb ions are
further bridged by oxygen atoms from the carboxylate groups
to form infinite 1D rod-shaped [Tb(CO ) ] secondary building
2
3 n
units (SBUs) running along the c axis (Figure 1b). This repre-
2
À
Figure 1. (a) Structure of organic ligand L
[Tb(CO
with honeycomb-type channels. (d) Schematic representation of the 4,6-con-
1
. (b) The infinite 1D rod-shaped
sents an efficient strategy to construct robust porous MOFs by
[19]
2 3 n
) ] SBUs running along the c axis. (c) 3D coordination framework
utilizing rod-shaped SBUs. Each Tb–carboxylate infinite SBU
2À
is connected to three other chains through L1 linkers to gen-
erate an interesting 3D coordination network with honey-
comb-type channels propagated along the c axis (Figure 1c).
The diameter of the 1D channel is 9.735 Š (opposite C18 to
C18 distance), in which the vacancies are filled with solvent
4
2
9
6
nected stp network with a Schläfli symbol of {4 ·6 } {4 ·6 } .
3
2
ion upon excitation at 329 nm. Of these characteristic lines of
3
+
the Tb ion, the green luminescence peak at 541 nm is the
most prominent. In addition, it is worth noting that the ligand-
centered broad emission is not observed in the solid-state
photoluminescent emission spectrum of Tb-MOF, indicating
[
20]
molecules. PLATON
space in Tb-MOF is about 25.1% of the crystal volume
2539.2 Š of the 10114.1 Š unit-cell volume) when the guests
calculations indicate that the vacant
(
2À
and coordinated water molecules are removed. In addition, if
each metal center can be considered as a six-connected node
and the two kinds of ligands as four-connected spacers, the
whole architecture is a 4,6-connected stp net with a Schläfli
the occurrence of the “antenna effect”: L1 moieties adsorb
2À
3+
the energy and, by vibronic coupling between L
and Tb
,
1
3
+
transfer energy to Tb , leading to the strong luminescence of
3
+
Tb . The excitation spectrum collected at the wavelength of
541 nm displays a dominating broad band in the range of
4
2
9
6
[21]
symbol of {4 ·6 } {4 ·6 } calculated with TOPOS (Figure 1d).
3
2
2
À
2
50–400 nm, originating from the absorption of the L1 ligand
(
Figure S4 in the Supporting Information), providing further
Luminescence properties of Tb-MOF
evidence for the “antenna effect”.
The room-temperature solid-state photoluminescent excitation
and emission spectra of the Tb-MOF are shown in Figure 2. Tb-
MOF shows emission peaks at 488, 541, 585, and 619 nm aris-
Before studying the properties of Tb-MOF, we first examined
the water stability. Tb-MOF crystals were immersed in water for
45 days at room temperature. Powder XRD patterns of the as-
synthesized and water-treated samples completely overlap
5
7
3+
ing from the D ! F (J=6, 5, 4, and 3) transitions of the Tb
4
J
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Figure 2. Emission spectrum of Tb-MOF in the solid state at room tempera-
ture. The insets are the corresponding excitation spectrum and lumines-
cence picture under UV-light irradiation at 365 nm.
Figure 3. Fluorescence spectra of Tb-MOF (10 mg) in aqueous solutions of
various metal cations (3 mL, 10 mM). The inset is the corresponding photo-
graphs under UV-light irradiation at 365 nm.
3
+
that of the simulated pattern from single-crystal analysis (Fig-
ure S5 in the Supporting Information), indicating the retention
of the framework of Tb-MOF after water treatment. The good
stability of the Tb-MOF emission in the aqueous environment
tively sense Fe through fluorescence quenching of the char-
3
+
3+
acteristic lines of the Tb
ion and selectively detect Al
through fluorescence enhancement of the ligand, which is
quite rare in the previous reports on luminescent MOFs.
To validate the high selectivity of Tb-MOF for detection of
3
+
can probably be ascribed to the sufficient protection of Tb
3
+
3+
provided by the MOF scaffold. This compatibility with aqueous
environments makes Tb-MOF well suited for generating new
fluorescence probes for the aqueous environmental systems.
Fe
and Al , competitive experiments of coexisting ions
were performed under the same conditions. Upon the intro-
3
+
duction of Fe to the mixture of Tb-MOF and other metal cat-
ions, the fluorescence is significantly quenched (Figure S7a in
3
+
the Supporting Information). Similarly, when introducing Al
to the mixture of Tb-MOF and other metal cations except
Sensing of metal ions
3
+
To examine the potential of Tb-MOF for sensing of metal ions,
the as-synthesized samples were ground and immersed in
Fe , the fluorescence of the ligand is significantly enhanced
(Figure S7b in the Supporting Information). This reveals that
the interference from conventional metal cations can be ne-
+
+
aqueous solutions containing different metal ions (Li , Na ,
+
2+
2+
2+
2+
3+
3+
2+
2+
2+
2+
2+
3+
K , Mg , Ca , Sr , Ba , Al , Fe , Fe , Co , Ni , Cu
,
glected; only Cu might interfere with the detection of Fe
to a small degree, further confirming the selectivity of Tb-MOF
2
+
2+
+
2+
Zn , Cd , Ag , and Pb ) to form a metal-ion-incorporated
MOF suspension. The luminescent spectra and corresponding
excitation spectra were recorded and are compared in Figure 3
as well as Figure S6 in the Supporting Information. The emis-
sion spectra show that the various metal ions display markedly
3
+
3+
for Fe and Al detection and the potential of Tb-MOF for
practical use.
To better understand the fluorescence response of Tb-MOF
3
+
3+
to Fe
and Al , concentration-dependent luminescence
3
+
different effects on the luminescence of the Tb ion. For ex-
ample, the effect on the luminescence intensity of Tb-MOF at
measurements were also carried out. The as-synthesized Tb-
MOF solid samples were ground and immersed in different
+
+
+
2+
2+
2+
3+
3+
5
41 nm is negligible when Li , Na , K , Mg , Ca , Sr
,
concentrations of Fe or Al , and then their luminescence
spectra were recorded. As demonstrated in Figure 4, the emis-
sion intensity of the Tb-MOF suspension was quenched steadi-
2
+
2+
2+
2+
2+
2+
+
2+
Ba , Fe , Co , Ni , Zn , Cd , Ag , or Pb are involved,
3
+
2+
3+
whereas Al , Cu , and Fe
exhibit varying extents of
3
+
3+
quenching on the luminescence of Tb , and their emission
ly with increasing Fe concentration from 0–10 mm and en-
hanced accordingly with increasing Al concentration from 0–
2
+
3+
spectra are different and easily distinguished: Cu
has
3
+
a quenching effect on luminescence intensity to a certain
100 mm. As shown in Figure 4b for the Al loaded sample,
the luminescence intensity of Tb decreased slowly and the
3
+
3+
3+
degree; Al can quench the emission of Tb , whereas the
emission of the ligand has been enhanced to a large extent,
giving rise to a bright blue light (Figure 3). However, unlike
emission of the ligand increased gradually with an increase in
3
+
the concentration of Al . When the concentration reaches
2
+
3+
3+
3+
Cu or Al , Fe can completely quench the emission of Tb-
MOF. As a result, its emission color under UV light is dark. The
remarkable enhancing and quenching effects can be further
10 mm, the quenching effect on the luminescence of Tb is
clear. When the concentration reaches 100 mm, the lumines-
cence of Tb is nearly completely quenched and only the
emission of the ligand can be observed. As a result, under UV-
3
+
3
+
3+
confirmed by the photographs of the Al @Tb-MOF and Fe
Tb-MOF suspensions under UV-light irradiation (the inset pic-
tures in Figure 3). These results indicate that Tb-MOF can selec-
3
+
@
light irradiation at 365 nm, with an increase in Al , the colors
3
+
of the Al -loaded samples change from green to bright blue
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by the metal cations; (c) cation exchange between the central
cations of the frameworks and the targeted cations. Powder
XRD pattern studies of Tb-MOF and concentration-dependent
3
+
Fe @Tb-MOF (Figure S10 in the Supporting Information) dis-
closed that the crystalline structure of Tb-MOF was retained in
3
+
the Fe cation exchange process.
To further understand the process of cation exchange, we
À1
3+
choose the concentration of 0.1 molL Fe in aqueous solu-
tion for the next time-dependent experiments. The ground
3
+
powder samples of Tb-MOF were introduced into the Fe so-
lution for different amounts of time (10 min, 30 min, 1 h, 2 h,
3
+
5
h, 24 h) to obtain the time-dependent Fe @Tb-MOF sam-
ples, and then filtered and washed with fresh distilled water
several times until the filtrate was completely colorless; this
ensured that any metal cations physically absorbed on the sur-
face of the crystals had been washed off. Next, these time-de-
3
+
pendent Fe @Tb-MOF samples were dried in air. Then, we an-
alyzed the time-dependent inductively coupled plasma spec-
3
+
troscopy (ICP) of solid samples of Fe @Tb-MOF as well as the
ICP of their filtrate after “ion exchanging”. Time-dependent ICP
results (Table S3 in the Supporting Information) showed that
3
+
3+
the Tb content of the filtrate after “Fe exchange” contin-
3
+
ues to increase and the Fe concentration was decreased.
3
+
3+
Conversely, in the solid-state Fe @Tb-MOF, the Fe content
3
+
continually increased and the Tb content decreased with in-
creasing immersion time. In addition, the luminescence intensi-
ty decreased gradually with increasing concentrations of the
metal cations (Figure 4a). Combining these observations with
the above experimental results, we proposed that some of the
3
+
3+
Tb in the framework is substituted by Fe and is dissolved
3
+
in the Fe aqueous solution, which reduces the energy trans-
3
+
Figure 4. Emission spectra of Tb-MOF in aqueous solutions of various con-
fer efficiency from the ligands to Tb ions, thus leading to the
quenching effect.
3
+
3+
centrations of Fe (a) and Al (b) under excitation at 329 nm.
As shown in Figure S11 in the Supporting Information, when
3
+
2À
Al was added to an aqueous solution of L , the emission
1
2À
3+
(
Figure S8 in the Supporting Information). Simultaneously, we
intensity of L1 increased significantly. However, Tb
de-
but the characteristic
emission of Tb increased, owing to the antenna effect. When
2À
discovered that the framework of the Tb-MOF dissolved gradu-
ally in water and that the solution was completely clear when
creased the emission intensity of L
1
3
+
3
+
À1
3+
3+
the Al concentration reached 0.1 molL . Reversible experi-
ments were carried out, but unfortunately, the sensitivity of
both Al and Tb were added into the aqueous solution of
2
À
3+
L1 , the characteristic emission intensity of Tb
was en-
3
+
3+
3+
Tb-MOF for Fe and Al was not reversible (Figures S16 and
S17 in the Supporting Information), which limited its recycle
use.
hanced by the same amount as when only Tb was added,
and the emission intensity of the ligand was reduced, but less
3
+
3+
so than with Tb alone. So, when Al was added to the sus-
pension of Tb-MOF, the mixture turned clear gradually, while at
2
+
Also, it should be noted that the addition of Cu can cause
partial luminescence intensity quenching of Tb-MOF, which
2À
the same time, the emission intensity of L1 increased and the
3
+
3+
3+
may interfere with the detection of Fe . So the concentration-
characteristic emission intensity of Tb decreased when Al
reached a certain concentration. The emission spectrum was
2
+
dependent luminescence measurement of Cu was carried
out (Figure S9 in the Supporting Information). The results
3
+
3+
similar to that of both Al and Tb being added to the aque-
2
+
2À
3+
show that the quenching efficiency of Cu is only half that of
ous solution of L1 . The results showed that Al can collapse
3
+
3+
2À
Fe when the concentration is 10 mm. According to the corre-
sponding photographs under UV-light irradiation at 365 nm,
we also find that this small interference does not affect the de-
the crystal structure of Tb-MOF to Tb and L1 , which were
then dissolved in the aqueous solution.
3
+
tection of Fe (insert pictures in Figure S9).
Sensing of nitro explosives
According to previous reports, the quenching effect on lumi-
nescence MOFs by metal cations may be attributed to the fol-
To trace the ability of Tb-MOF to sense explosives, changes in
the fluorescence intensity of Tb-MOF dispersed in water on ad-
dition of different nitro aromatic compounds, including TNT,
[
3c,5f,9a,22–23]
lowing factors:
(a) interactions between metal cat-
ions and organic ligands; (b) collapse of the crystal structure
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NB, 1,3-DNB, 1,4-DNB, 2-NT, 2,4-DNT, 2,6-DNT, and TNP in aque-
ous solutions (1 mm), were investigated (Figure S12 in the Sup-
porting Information). The luminescence intensity of Tb-MOF at
5
41 nm is negligibly influenced by aqueous solutions of TNT,
NB, 1,3-DNB, 1,4-DNB, 2-NT, 2,4-DNT, and 2,6-DNT, whereas very
high levels of fluorescence quenching were observed in TNP
aqueous solution. The visible yellow–green emission of Tb-
MOF under UV light vanished upon the addition of the TNP so-
lution, which quenched nearly 93% of the initial fluorescence
intensity (Figure 5).
Figure 6. Emission spectra of Tb-MOF (10 mg) dispersed in water (3 mL)
upon incremental addition of TNP solution (10 mm) in water. Inset: Photo-
graphs showing the original and the quenched fluorescence upon addition
of TNP.
Figure 5. Quenching efficiency of Tb-MOF for different nitro analytes.
To explore the potential applications of Tb-MOF in the de-
tection of TNP, fluorescence-quenching titrations were per-
formed by gradual addition of TNP (10 mm in aqueous solu-
tion) to a dispersion of ground Tb-MOF in water. Very high
levels of fluorescence quenching were observed upon incre-
mental addition of TNP (Figure 6). In contrast, all of the other
nitro analytes had a minor effect on the fluorescence intensity
of Tb-MOF (Figure 5). This clearly indicates that Tb-MOF has
high selectivity towards TNP over other nitro analytes. In addi-
tion, reversible experiments were performed, but TNP adsorp-
Figure 7. Stern–Volmer plot of various nitro analytes in water.
3
+
tion was also found to be irreversible on adding excess Tb
Figure S18 in the Supporting Information), which limited its
(
recycle use.
fluorescence intensity in the presence of the respective ana-
Encouraged by these results, the selectivity of Tb-MOF to-
wards TNP in the presence of other nitro aromatic explosives
was investigated under the same conditions. Upon the intro-
duction of TNP to the mixture of Tb-MOF and other nitro aro-
matics, the fluorescence is quenched promptly (Figure S13 in
the Supporting Information). The results indicate that the inter-
ference from conventional nitro aromatic explosives can be ne-
glected, further confirming the high selectivity of Tb-MOF for
TNP detection and the potential of Tb-MOF for practical use.
These results demonstrate that Tb-MOF has high selectivity
for TNP compared with other nitro aromatics. Further, the fluo-
rescence quenching efficiency was analyzed by using the
Stern–Volmer (SV) equation (Figure 7), I /I=K [Q]+1, where, I
0
lyte, K is the quenching constant, and [Q] is the concentration
sv
of the analyte. As shown in Figure S14 in the Supporting Infor-
mation, the SV plot for TNP was nearly linear at low concentra-
tions and subsequently deviated from linearity, bending up-
wards at higher concentrations. The nonlinear nature of the SV
plot of TNP can be attributed to self-absorption, a combination
of static and dynamic quenching, or an energy-transfer process
[
19a,24]
between TNP and the MOF.
The quenching constant (K )
sv
4
À1
for TNP was found to be 7.47”10 m , as verified by linear fit-
ting of the SV plot at a low concentration range (0–0.04 mm,
[
15a,25]
Figure S14).
Results indicate that strong interactions occur
between TNP and the host framework.
Further studies of the UV/Vis absorption spectra of the nitro
aromatic explosives show that the substituents on the aromat-
0
sv
is the initial fluorescence intensity without any analyte, I is the
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ic moieties can cause a different amount of red-shift of the
p!p* transitions. Under excitation at 329 nm, TNP has
a higher absorbance and e (molar extinction efficient) in com-
parison to the other analytes (Figure S15 in the Supporting In-
formation), which can lead to lower absorption of the linker
ture. Luminescence spectra for the solid and suspension samples
were recorded on a Hitachi 850 fluorescence spectrophotometer.
Inductively coupled plasma spectroscopy (ICP) was performed on
a Thermo ICAP 6500 DUO spectrometer. UV/Vis absorption spectra
were measured with a TU-1901 double-beam UV/Vis spectropho-
tometer.
2
À
L1 moieties. This implies that TNP the stronger quenching
effect on the fluorescence intensity of Tb-MOF originates from
the energy-transfer process between the ligands and metal
centers. According to the above results, we can speculate the
possible sensing mechanism for TNP explosive: The lumines-
cence quenching of Tb-MOF by TNP is attributed to competi-
tion between the absorption of the light source energy and
Preparation of 2-(2-Acetoxy-propionylamino)-terephthalic
acid (H L) and 2-(2-Hydroxy-propionylamino)-terephthalic
2
acid (H L )
2
1
The H L ligand was prepared by reaction of 2-acetoxypropionyl
2
chloride with 2-amino-terephthalic acid (Scheme 1). 2-Acetoxypro-
pionyl chloride was synthesized according to the literature proce-
2
À
[11c]
the electronic interaction between TNP and L1 moieties.
TNP absorbs the excitation energy and decreases the light ad-
[26]
dure.
added dropwise to a solution of 2-amino-terephthalic acid (3 g,
6.5 mmol) in N,N’-dimethylacetamide (DMAc) (20 mL). The result-
2-Acetoxypropionyl chloride (6.30 mL, 49.5 mmol) was
2À
sorbed by the L1 moieties, so that the probability of energy
1
2
À
3+
transfer from L1 to Tb is reduced and subsequently the
ing mixture was stirred at about 508C for 5.5 h. Water (80 mL) was
added to the reaction mixture at room temperature, after which
the suspension formed was cooled to a temperature between 08C
and 58C and stirred for 30 min at this temperature. The suspension
was filtered and the solid was washed with distilled water. The
product was dried in a vacuum oven at 608C to yield 2-(2-acetoxy-
propionylamino)-terephthalic acid (3.93 g, 13.3 mmol, 80.6%). Ele-
mental analysis calcd (%) for C H O N (295.24): C 52.88, H 4.44, N
3
+
characteristic luminescence of Tb is quenched. The SV plot,
which deviates from linearity at high concentrations, also dem-
onstrates the presence of both static and dynamic quenching
mechanisms.
13
13
7
1
Conclusion
4
.74; found: C 53.21, H 4.09, N 4.91: H NMR (300 MHz, DMSO): d=
A honeycomb-type luminescent terbium-based metal–organic
framework (Tb-MOF) was synthesized and characterized. Tb-
9.12 (s, 1H), 8.09 (d, 1H), 7.70 (d, 1H), 5.16 (m, 1H), 2.20 (d, 3H),
1.46 ppm (d, 3H); IR (KBr): 2995 (w), 1753 (m), 1691 (vs), 1577 (s),
3
+
3+
1539 (s), 1417 (m), 1275 (s), 1219 (vs), 1080 (m), 914 (m), 775 (m),
MOF is highly selective and sensitive towards Fe and Al
ions through different detection mechanisms. Most important-
À1
5
36 cm (w).
3
+
3+
ly, the selectivity for Fe and Al ions is not subject to inter-
ference by other mixed metal ions. In addition, Tb-MOF also
shows highly selective and sensitive detection of TNP in aque-
ous media, even in the presence of other nitro aromatic com-
pounds. The occurrence of both electron and energy-transfer
processes, in addition to electrostatic interaction between the
MOF and TNP, contribute to the selective fluorescence quench-
ing. This work demonstrates the potential application of a fluo-
rescent Tb-MOF as a multiresponsive probe for detection of ex-
plosives in the aqueous phase. The present results may pro-
vide a facile route to design and synthesize functional porous
MOFs with applications in fluorescent sensors.
2 2 1
Scheme 1. The synthesis of ligands H L and H L .
Experimental Section
Materials and physical measurements
H L (3.0 g, 10.0 mmol) was dissolved in methanol (60 mL) with
2
a stoichiometric amount of CH ONa and the mixture was stirred
3
All the starting reagents and solvents employed in the present
work were of analytical grade as obtained from commercial sour-
ces and used without further purification. H NMR spectra were re-
for 20 h at room temperature. Subsequently, Amberlyst 15 (ion-ex-
change resin, 3.6 g) was added and stirred for 2 h, after filtration,
additional Amberlyst 15 (2 g) was added and stirred for another
1
corded with a Bruker-400 spectrometer and the chemical shifts
were reported in ppm using tetramethylsilane (TMS) as the internal
standard. Elemental analysis for C, H, and N were performed on
a PerkinElmer 240 elemental analyzer. The FTIR spectra were re-
2
h. After filtration, the solution was evaporated under reduced
pressure, and the resulting off-white solid, H L , was dried under
2
1
vacuum overnight. Yield: 2.33 g, 9.21 mmol, 92%. Elemental analy-
sis calcd (%) for C H O N (253.21): C 52.18, H 4.38, N 5.53; found:
À1
corded from KBr pellets in the range 4000–400 cm on a Nicolet
11 11
6
1
C 52.83, H 3.99, N 5.21; H NMR (300 MHz, DMSO): d=9.18 (s, 1H),
.08 (d, 1H), 7.59 (d, 1H), 4.14 (m, 1H), 1.33 ppm (d, 3H); IR (KBr):
NEXUS 470-FTIR spectrometer. Thermogravimetric analysis (TGA)
was performed on an SDT 2960 thermal analyzer, under air, from
8
À1
room temperature to 8008C with a heating rate of 208C min .
3436 (s), 1679 (m), 1635 (m), 1591 (s), 1513 (m), 1450 (w), 1417 (w),
À1
Powder X-ray diffraction data were collected on a Rigaku D/Max-
1363 (m), 1251 (m), 1110 (m), 759 (m), 601 cm (w).
2
2
500PC diffractometer with CuKa radiation (l=1.5406 Š) over the
q range of 5–508 with a scan speed of 58/min at room tempera-
Chem. Eur. J. 2015, 21, 15705 – 15712
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Synthesis of {[Tb(L ) (H O)]·3H O} (Tb-MOF)
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1.5
2
2
n
5
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3
3
2
2
H O/CH CN (4/2 mL) were mixed in a Teflon-lined stainless steel
2
3
vessel (25 mL). The mixture was heated under autogenous pressure
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n
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(
(
7
s), 1652 (m), 1538 (s), 1461 (m), 1422 (s), 1380 (s), 1303 (m), 1269
m), 1121 (w), 1084 (w), 1045 (w), 948 (w), 883 (w), 830 (w), 810 (w),
À1
73 (m), 528 cm (m). The deprotection process took place during
the synthesis of Tb-MOF: The acetyl group seems to be vital to
this process, as replacing H L with H L does not give crystals of
[
[
2
2 1
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the aqueous solutions of M(NO ) (0.02 molL , 1.5 mL) at room
3
+
x
x+
+
+
2+
2+
2+
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temperature (M =Li , Na , K , Mg , Ca , Sr , Ba , Cu
,
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À1
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Received: March 24, 2015
Published online on September 10, 2015
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