Highly Stable Zr(IV)-Based Metal-Organic Frameworks for the Detection and Removal of Antibiotics and Organic Explosives in Water

Article abstract of DOI:10.1021/jacs.6b01663

Antibiotics and organic explosives are among the main organic pollutants in wastewater; their detection and removal are quite important but challenging. As a new class of porous materials, metal-organic frameworks (MOFs) are considered as a promising platform for the sensing and adsorption applications. In this work, guided by a topological design approach, two stable isostructural Zr(IV)-based MOFs, Zr₆O₄(OH)₈(H₂O)₄(CTTA)_8/3 (BUT-12, H₃CTTA = 5′-(4-carboxyphenyl)-2′,4′,6′-trimethyl-[1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid) and Zr₆O₄(OH)₈(H₂O)₄(TTNA)_8/3 (BUT-13, H₃TTNA = 6,6′,6″-(2,4,6-trimethylbenzene-1,3,5-triyl)tris(2-naphthoic acid)) with the the-a topological structure constructed by D_4h 8-connected Zr₆ clusters and D_3h 3-connected linkers were designed and synthesized. The two MOFs are highly porous with the Brunauer-Emmett-Teller surface area of 3387 and 3948 m² g^-1, respectively. Particularly, BUT-13 features one of the most porous water-stable MOFs reported so far. Interestingly, these MOFs represent excellent fluorescent properties, which can be efficiently quenched by trace amounts of nitrofurazone (NZF) and nitrofurantoin (NFT) antibiotics as well as 2,4,6-trinitrophenol (TNP) and 4-nitrophenol (4-NP) organic explosives in water solution. They are responsive to NZF and TNP at parts per billion (ppb) levels, which are among the best performing luminescent MOF-based sensing materials. Simultaneously, both MOFs also display high adsorption abilities toward these organic molecules. It was demonstrated that the adsorption plays an important role in the preconcentration of analytes, which can further increase the fluorescent quenching efficiency. These results indicate that BUT-12 and -13 are favorable materials for the simultaneous selective detection and removal of specific antibiotics and organic explosives from water, being potentially useful in monitoring water quality and treating wastewater.

Full text of DOI:10.1021/jacs.6b01663

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Article
Highly Stable Zr(IV)-Based Metal-Organic Frameworks for the
Detection and Removal of Antibiotics and Organic Explosives in Water
Bin Wang, Xiu-Liang Lv, Dawei Feng, Lin-Hua Xie, Jian
Zhang, Ming Li, Yabo Xie, Jian-Rong Li, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01663 • Publication Date (Web): 19 Apr 2016
Downloaded from http://pubs.acs.org on April 23, 2016
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Journal of the American Chemical Society
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Highly Stable Zr(IV)-Based Metal-Organic Frameworks for the De-
tection and Removal of Antibiotics and Organic Explosives in Water
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Bin Wang, Xiu-Liang Lv, Dawei Feng, Lin-Hua Xie, Jian Zhang, Ming Li, Yabo Xie, Jian-
,†
‡
Rong Li,* and Hong-Cai Zhou
†
Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering,
College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China
‡
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
§
State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao Uni-
versity of Science and Technology, Qingdao 266042, P. R. China
KEYWORDS: Antibiotic, Organic Explosive, Metal-Organic Framework, Selective Detection, Adsorption
ABSTRACT: Antibiotics and organic explosives are among the main organic pollutants in wastewater; their detection and
removal are quite important but challenging. As a new class of porous materials, metal-organic frameworks (MOFs) are
considered as promising platform for the sensing and adsorption applications. In this work, guided by a topological de-
sign approach, two stable isostructural Zr(IV)-based MOFs, Zr O (OH) (H O) (CTTA) (BUT-12, H CTTA = 5′ -(4-
6
4
8
2
4
8/3
3
carboxyphenyl)-2′ ,4′ ,6′ -trimethyl-[1,1′ :3′ ,1′ ′ -terphenyl]-4,4′ ′ -dicarboxylic acid) and Zr O (OH) (H O) (TTNA) (BUT-
6
4
8
2
4
8/3
1
3, H TTNA = 6,6′ ,6′ ′ -(2,4,6-trimethylbenzene-1,3,5-triyl)tris(2-naphthoic acid)) with the the-a topological structure
3
constructed by D4h 8-connected Zr clusters and D 3-connected linkers were designed and synthesized. The two MOFs
6
3h
2
–1
are highly porous with the BET surface area of 3387 and 3948 m g , respectively. Particularly, BUT-13 features one of the
most porous water-stable MOFs reported so far. Interestingly, these MOFs represent excellent fluorescent properties,
which can be efficiently quenched by trace amounts of nitrofurazone (NZF) and nitrofurantoin (NFT) antibiotics, as well
as 2,4,6-trinitrophenol (TNP) and 4-nitrophenol (4-NP) organic explosives in water solution. They are responsive to NZF
and TNP at parts per billion (ppb) levels, which are among the best performing luminescent MOF-based sensing materials.
Simultaneously, both MOFs also display high adsorption abilities towards these organic molecules. It was demonstrated
that the adsorption plays an important role in the pre-concentration of analytes, which can further increase the fluores-
cent quenching efficiency. These results indicate that BUT-12 and -13 are favorable materials for the simultaneous selec-
tive detection and removal of specific antibiotics and organic explosives from water, being potentially useful in monitor-
ing water quality and treating wastewater.
Introduction
portant, but challenging. Till now, the detection of anti-
biotics and nitroaromatics is mainly based on instrumen-
tal methods such as liquid chromatography-tandem mass
With ever-increasing concern for public health and
water quality, there is now a much greater demand for the
detection and removal of pollutants from wastewater.
Antibiotics being used extensively for the treatment of
bacterial infections in humans and animals have been
noticed as a class of important organic pollutes in water.
The abuse of antibiotics has led to high level of antibiotic
residues. Various antibiotics have been detected in both
surface and ground water, as well as even in drinking wa-
ter. Recent research shows that the total antibiotic usage
in China in 2013 was approximately 162000 tons, and this
number was further increasing with the development of
3
4
spectrometry (LC-MS), LC with UV detection (LC-UV),
5
6
capillary electrophoresis (CE), mass spectrometry (MS),
7
raman spectroscopy (RS), and ion mobility spectrometry
8
(
IMS). However, all these methods are time-consuming,
expensive, and require complex equipment and trained
personnel. In addition, technologies for the removal of
antibiotics are not so mature yet, although a variety of
9
methods including photolysis, hydrolysis, and thermoly-
10
11
sis, technical oxidation processes, as well as biodegrada-
12
tion have been developed, which are all based on chemi-
cal treatment. Therefore, the development of portable,
reliable, and inexpensive methods/technologies for the
detection and removal of antibiotics and nitroaromatics
pollutants have been a matter of great concern to re-
searchers.
1,2
the industry and the increase of the populations. Simi-
larly, nitroaromatics widely used as explosives are also
undesirable organic pollutions in wastewater apart from
antibiotics. These chemicals are highly poisonous and
difficult to be degraded by nature. Monitoring and remov-
ing these specific pollutants from water are quite im-
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Alternately, optical sensing and adsorption based (1) incorporating hydrophobic groups near coordination
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methods have been considered as promising technolo-
gies in the detection and removal of antibiotics and or-
ganic explosives, respectively, because of some ad-
vantages, such as easy operation, energy-saving, high effi-
ciency, and so on. Although some progresses have been
made in this regards, great efforts are still required to put
the application into practice. The challenge of developing
these methods rests with the selection of materials, which
should incisively respond to the checked molecules. Up to
now, a lot of checked materials display only one function
as either the sensing materials or the adsorbent. Integrat-
ing these two functions into one material is clearly favor-
able, but rarely identified. Using such a multifunctional
material would be not only cost effective but also efficien-
cy improved. Besides, the adsorption process can allow
the pre-concentration of target analytes, thus increasing
sites or onto linkers through direct synthesis or postsyn-
thetic modification to enhance the hydrophobic property
of MOFs, thereby protecting coordination bonds from
3
8-44
hydrolysis
and (2) using high oxidation state metals
3
+
3+
3+
4+
(such as Cr , Fe , Al , and Zr ) to form strong coordina-
tion bonds with organic carboxylate ligands.
4
5-48
In this work, guided by a topological design approach,
two isostructural Zr-MOFs, Zr O (OH) (H O) (CTTA)
6
4
8
2
4
8/3
(
BUT-12, where BUT = Beijing University of Technology)
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and Zr O (OH) (H O) (TTNA) (BUT-13) were designed
6
4
8
2
4
8/3
and synthesized through the reaction of ZrCl with two
4
pre-designed
carboxyphenyl)-2′ ,4′ ,6′ -trimethyl-[1,1′ :3′ ,1′ ′ -terphenyl]-
,4′ ′ -dicarboxylic acid (H CTTA) and 6,6′ ,6′ ′ -(2,4,6-
fluorescent
ligand
acids,
5′ -(4-
4
3
trimethylbenzene-1,3,5-triyl)tris(2-naphthoic
acid)
1
5-17
(H TTNA), respectively. The framework structures of the
3
the detection ability of the material.
two MOFs exhibit the the-a topology with D4h 8-
connected Zr cluster nodes and D 3-connected ligand
Porous materials are promising candidates for inte-
grating these two functions. However, because of the dif-
ficulty in the modification of the pores, the application of
traditional porous materials, such as activated carbon,
zeolites, aluminosilicates, etc. is limited to some extent.
As a class of newly developed porous solids, metal-organic
frameworks (MOFs), constructed by metal ions or metal
clusters and organic ligands through coordination bonds,
are considered as favorable platform for the detec-
6
3h
linkers. Both MOFs show high surface area and moderate
pore sizes, as well as good stability in water, HCl solutions
(2 M, 6 M, and concentrated HCl), and NaOH solutions
(pH = 10). Particularly, they represent excellent fluores-
cent property. Based on their outstanding water stability,
porosity, and fluorescence, the detection and removal of
selected antibiotics and nitroaromatics were explored in
the two MOFs. Twelve antibiotics of five classes: nitrofu-
rans (NFs), nitroimidazole (NMs), sulfonamides (SAs),
chloramphenicols (CPs), and β-lactams, and eleven ni-
troaromatics were studied. It has been found that both
BUT-12 and -13 represent high fluorescence quenching
efficiency and high adsorption ability towards nitrofura-
zone (NZF) and nitrofurantoin (NFT) antibiotics and
2,4,6-trinitrophenol (TNP) and 4-nitrophenol (4-NP) ni-
troaromatics. The detection limits of BUT-12 towards NZF
and TNP are estimated to be 58 and 23 ppb, and those of
BUT-13 are 90 and 10 ppb, respectively. It has also been
demonstrated that the adsorption process indeed plays an
important role in the enrichment of NFs, TNP, and 4-NP,
which further increase the quenching efficiency of the
MOFs.
1
8-22
23,24
tion/sensing
and adsorption
applications because
of their specific electronic and optical properties, perma-
nent porosity, high surface area, and easily tailorable
structures and functions. Particularly, through ligand
modifications, various fluorophores can be rationally in-
troduced into the pores of MOFs, which makes them
show excellent fluorescent property without loss of poros-
25
ities. As a result, the simultaneous detection and remov-
al of specific chemical species using MOFs become possi-
ble and accessible.
Some MOFs have been explored for the detection of
explosives and shown excellent detection abilities and
1
8-20
selectivities.
However, most of them were checked in
organic solvents instead of water. And, to the best of our
knowledge, there is no report in using MOFs as sensing
materials to detect antibiotics. Moreover, documented
publications concerning aqueous-phase adsorption and
removal of organic contaminants using MOFs are limited
Results and discussion
Design and synthesis
Zr-MOFs have been extensively studied since the de-
velopment of using modulating reagents to assist single
2
6
27
28
to benzene, organic dyes, pharmaceuticals, phenol,
2
9
30
4
9,50
bisphenol A, 2,4-dichlorophenoxyacetic acid, and p-
cresol. Very few works have been reported in the remov-
al of antibiotics and explosives by using MOFs as adsor-
crystals production.
However, compared with a lot of
3
1
other metal species which can form many different clus-
ters with various symmetries and connectivities, the
3
2,33
bents.
overwhelming majority of Zr-MOFs are based on Zr car-
6
boxylate cluster, which severely limits their topology di-
versity, and thus requires judicious design of the overall
MOF structures rather than simply varying the symmetry
and connectivity of used ligands. Try-error could thus
easily lead to amorphous products due to the topological
incompatibility between the organic linkers and the Zr6
cluster. On the other hand, even though crystalline prod-
uct can be obtained, lack of large crystals suitable for sin-
In addition, for the applications of MOFs in water sys-
tem, such as the pollutants detection and removal men-
tioned herein, the water stability of their frameworks is of
34,35
the precondition.
Unfortunately, a large number of
reported MOFs are not stable in water, except for some
examples of ZIFs, MILs, CAUs, UiOs and other Zr(IV)-
36,37
based MOFs, and so on.
Generally, there are two ways
to improve the water stability of carboxylate-based MOFs:
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Journal of the American Chemical Society
gle crystal characterization still hinders the development
of Zr-MOFs with new structures. Therefore, the design of
Zr-MOFs with targeting structures is of great importance.
work, and then design specific organic linkers as the
compatible counter-parts.
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So far, Zr clusters with several connectivities have
6
4
7,53,54
54,55
52,54,56-
As aforementioned, Zr cluster is the dominant sec-
been reported, including twelve,
ten,
eight,
6
5
9
54,60,61
ondary building unit (SBU) core in Zr-MOFs. Despite of
and six.
Among them, D4h 8-connected node is
this, the high symmetry and connectivity enable Zr clus-
able to form several binodal networks with different link-
6
ter serving as different types of nodes in the resulting
networks via reducing the connectivity, making it com-
patible to form three dimensional (3D) periodic frame-
works with different linkers. As such, the possible sym-
ers, such as scu-a, flu-a, ocu-a, and the-a networks (Fig-
5
1
ure 1a). However, only flu-a network has been realized
5
2,54,59
in Zr-MOFs among those four.
For the ocu-a net-
work, it is challenging because D3d 6-connected linker is
required, which is not common for organic linkers. While
the scu-a and the-a networks are supposed to be more
feasible owing to the ease of their organic linker synthesis.
In the former, a D4h or D2h 4-connected linker
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metry and connectivity of Zr cluster can be easily pre-
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dicted. Since many binodal networks have been present-
5
1
ed, one of the most convenient approaches to construct
a new Zr-MOF is to fit Zr cluster with compatible sym-
6
metry and connectivity as one node into a binodal net-
Figure 1. (a) Topological analysis of binodal edge-transitive networks with D4h 8-connected Zr clusters and different linkers; (b)
6
structures of H TATB and PCN-777; (c) structures of H BTB and Zr-BTB; (d) structures H CTTA and BUT-12; and (e) structures
3
3
3
of H TTNA and BUT-13 (Color code: C, black; O, red; and Zr, green; H atoms on ligands are omitted for clarity; the large pink
3
and yellow spheres represent cage void regions inside the frameworks).
thermodynamically favored outcome. Consequently, the
scu-a network is missing in Zr-MOFs.
is required and in the latter it is a D3h 3-connected linker.
Nevertheless, 4-connected D4h and D2h linkers are also
compatible with many other Zr nodes, when the energy
6
In comparison, a D3h 3-connected linker required in
the the-a network is one type of the most frequently
adopted linkers in MOFs. However, although three Zr-
MOFs constructed with 3-connected linkers have been
of the linker and overall connectivity of the framework
are taken into account, scu-a network may not be the
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reported, namely MOF-808, PCN-777, and Zr-BTB,
a
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the-a type Zr-MOF is still missing. Actually, when the
entropy effect is considered, the the-a network is more
thermodynamically favored because of its higher connec-
tivity compared to those three MOFs. Therefore, it is puz-
zling that the the-a type Zr-MOF has not been synthe-
3
–
sized yet with benzene-1,3,5-tricarboxylate (BTC ),
3
–
4
1
,4′,4′′-s-triazine-2,4,6-triyltribenzoate (TATB ), and
3–
,3,5-benzenetribenzoate (BTB ) ligands, which suggests
there might be some key point missed in a traditional
topology guided design of the MOFs.
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In a typical topological simplification of a MOF, only
the connectivity and symmetry of each node in the net-
work are considered. After the simplification, the connec-
tion between different nodes is always represented by a
line without other details, such as the direction prefer-
ence of the coordination bond. However, in a reverse way
when topological analysis is used as a tool to design
MOFs, these details have to be considered to avoid the
high energy configuration of either the inorganic cluster
or the organic linker. Otherwise, the resulting product
5
3
Figure 2. (a, b) Polyhedral cages and their 3D packing in
BUT-12; (c, d) view of the [Zr O (OH) (H O) (COO) ] SBU
node and CTTA ligand in BUT-12, respectively, (e, f) the
natural tiling of polyhedral cages in BUT-12. (Color code: C,
black; O, red; and Zr, green; H atoms on ligands are omitted
for clarity).
will quite possibly be thermodynamically unfavored. In
6
4
8
2
4
8
the the-a network, when the D4h Zr cluster is placed in
6
3–
the position of the 8-connected node, it fits perfect in
terms of the connectivity and symmetry. However, if we
take a close look at the orientation of three carboxylates
on three neighboring Zr clusters, which are supposed to
6
be connected by a D3h node, they do not stay in the same
plane. Instead, those three carboxylates are perpendicular
to the same plane, which suggests that the D3h 3-
connected linker has to adopt the same configuration in
Hence, we designed and synthesized two carboxylate
3–
ligands preferring such a configuration, CTTA and
3–
3–
TTNA . The former is a derivative of BTB . Differently,
the central phenyl ring is functionalized with three me-
thyl groups, which can force three peripheral phenyl rings
perpendicular to the central one because of the steric
hindrance, while still gives rise to the D3h symmetry. Be-
sides, the presence of three methyl groups on the central
phenyl ring of the ligands can increase the hydrophobicity
of the framework, therefore probably leading to a further
enhanced water stability of resulting Zr-MOF. On the
other hand, fixing the terminal phenyl rings to a rigid
structure can efficiently eliminate non-radiative relaxa-
3
–
3–
order to fit the network. Whereas, in BTC and TATB ,
three carboxylate groups prefer to be coplanar due to
3
–
their large conjugation system (Figure 1b). In BTB , alt-
hough the perfect coplanar configuration of three carbox-
ylates is not the favorite, the energy of the ligand is much
higher when three carboxylates are perpendicular to the
central phenyl ring (Figure 1c). Therefore, none of these
three ligands prefer to stay in the the-a network with D4h
8-connected Zr6 cluster. To overcome the conflict be-
tween topological preference and ligand configuration
preference, a D3h 3-connected ligand, which allows three
carboxylates to stay perpendicularly to the same plane, is
necessary.
tion pathways, thus increase the fluorescent property of
62-64
itself as well as derived MOFs.
In addition, naphtha-
lene ring is recognized as a good fluorescent group, thus
3–
the replacement of phenyl rings in CTTA with naphtha-
lene rings will further improve the luminescent property
3–
of resulting MOF with TTNA (Figure 1d and e). As a re-
3–
sult, the lowest energetic configuration of CTTA and
3–
TTNA matches the symmetry and orientation require-
ment in the the-a network, making it thermodynamically
preferred product.
As we expected, solvothermal reactions of H CTTA or
3
H TTNA with ZrCl in the presence of formic acid or ace-
3
4
tic acid as competing reagents in DMF yielded cubic-
shaped single crystals of BUT-12 and -13, respectively. In
the synthesis of Zr-MOFs, it has been confirmed that ad-
ditional acid modulators play an important role in con-
4
9
trolling the nucleation rate of forming products. With-
out modulators, the Zr-MOFs precipitate as micro-sized
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OH/O atoms. Furthermore, eight of the twelve octahedral
aggregates of nanocrystals or disordered phases were usu-
3
–
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3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ally obtained. In our case, formic acid or acetic acid was
used as the modular reagent to obtain pure crystalline
samples of BUT-12 and -13, which were confirmed by
powder X-ray diffraction (PXRD) (Figure 3a and b). After
precisely tuning the amounts of the modulators, single
crystals big enough for the single-crystal X-ray diffraction
were finally obtained. In addition, in the FT-IR spectra of
BUT-12 and -13, slight blue shifts of carbonyl group char-
acteristic bands compared with corresponding ligands
were observed, illustrating the metal coordination of car-
boxylate groups in these ligands (Figure S4 in the Sup-
porting Information).
edges are connected to CTTA ligands through the car-
boxylate coordination, while the remaining Zr coordina-
tion sites are occupied by eight terminal –OH/H O
2
groups to form a [Zr O (OH) (H O) (CO ) ] SBU (Figure
6
4
8
2
4
2 8
5
6
2c). The same SBU was also observed in NU-1000 and
5
7
3–
PCN-222. These SBUs are linked by CTTA ligands to
form a 3D framework with two types of polyhedral cages.
One is octahedral, with six Zr(IV)-based SBUs occupying
3
–
the vertices and eight CTTA linkers covering the faces
(Figure 2a). The size across the edge of an octahedral cage
is about 17.5 Å (atom to atom distance). The other one is a
cuboctahedral cage constructed by twelve SBUs and eight
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
–
CTTA ligands. This cage consists eight triangular and six
square faces and can enclose a sphere of diameter 24.7 Å
(atom to atom distance) inside its pore (Figure 2a). In the
structure, one cuboctahedral cage is joined to six other
cuboctahedral cages through sharing square windows and
to eight octahedral cages through sharing triangular win-
dows to finish the whole framework construction. From
Structural description
Single-crystal X-ray diffraction reveals that BUT-12
crystallizes in the cubic space group Pm–3m with the lat-
tice parameter a = 28.199 (7) Å (Table S1 in the Supporting
Information). In the structure, there exist two crystallo-
graphically independent Zr atoms (Zr1 and Zr2), which are
all eight-coordinated in a tetragonal antidipyramid coor-
dination geometry (Figure 2c). Zr1 is coordinated by four
O atoms from different carboxylate groups and four μ3-
OH/O moieties, while Zr2 is coordinated by two O atoms
from different carboxylate groups, four μ -OH/O moie-
ties, and two terminal –OH/H O entities. Six Zr atoms
connected with each other by four μ -O and four μ -OH
groups to form a Zr O (OH) core. The core can be de-
the topological view point, the Zr cluster serves as a 8-
6
3
–
connected node (Figure 2c) and the CTTA ligand can be
seen as a 3-connected node (Figure 2d), the 3D structure
of BUT-12 can thus be simplified as a 3,8-c binodal net
3
8
4
12
4
with the point symbol of {4 }8{4 .6 .8 .10 }
, which corre-
3
3
sponds to the the-a topology, being first example among
2
65-67
Zr-MOFs (Figure 2f).
For BUT-13, due to its high po-
3
3
rosity, it is hard to collect a well enough diffraction data
set to solve its structure through single-crystal X-Ray dif-
fraction notwithstanding obtained single
6
4
4
scribed as a Zr octahedron, in which the vertices are oc-
cupied by Zr atoms and the faces are capped by eight μ3-
6
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Page 6 of 16
Figure 3. (a, b) PXRD patterns of BUT-12 and -13, respectively upon the treatment with water, boiling water, pH = 10 NaOH solu-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
tion, 2 M HCl, 6 M HCl, and concentrated HCl; (c, d) N adsorption isotherms of BUT-12 and -13 after treated in water, respec-
2
tively at 77 K (Inset shows DFT pore size distribution for the corresponding MOF evaluated by using N adsorption data meas-
2
ured at 77 K, respectively).
crystals are large (Figure S2 in the Supporting Infor-
mation). We thus constructed its structure by following a
reverse topological approach based on combining the
computational construction method and experimental
single-crystal X-Ray diffraction. The detailed construc-
tion is illustrated in Figure S1 of the Supporting Infor-
mation. The generated BUT-13 shares the same structure
as BUT-12 and has a crystal lattice parameter of 33.223(2)
Å. The size across the edge of an octahedral cage and the
internal pore diameter of the cuboctahedral cage in BUT-
with its high charge density (Z/r) can polarize the O at-
oms of the carboxylate groups to form strong Zr–O bonds
with significant covalent character. Besides the stable Zr6
cluster, the excellent water stabilities of BUT-12 and -13
might also be attributed to a combination of hydrophobic
effect and electronic effects of methyl groups in the lig-
ands. The methyl group is hydrophobic, which can en-
hance the hydrophobicity of the frameworks. On the oth-
er hand, as electron donor, methyl group can increase the
electron density of both the central and the terminal ben-
zene rings in the ligand, leading to an increase of the elec-
tron density of carboxyl O atoms, which can further in-
crease the Zr–O bond strength. To quantitatively demon-
strate such an electronic effect, DFT calculations were
performed to identify the electron density of different
atoms in the ligand acids by using the GAUSSIAN 03
package (for details, see Section 5 of the Supporting In-
formation). After addition of methyl groups, the charges
68
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
71
1
3 are 21.3 and 30.2 Å, respectively (atom to atom dis-
tance). After removing free solvent molecules, the total
solvent-accessible volumes of BUT-12 and -13 are estimat-
69
ed to be 79.1% and 84.6%, respectively by PLATON.
Pore characterization
The porosities of BUT-12 and -13 were examined by N2
adsorption at 77 K. Saturated N uptakes of 982 and 1422
2
3
–1
cm
g
(STP) are achieved, and evaluated Brunauer-
Emmett-Teller (BET) surface areas are 3387 and 3948 m
g , respectively (Figure 3c and d). The experimental total
pore volumes are 1.52 and 2.20 cm g for BUT-12 and -13,
being in close with the calculated values of 1.71 and 2.68
2
of O1 and O2 in the carboxyl group of H
–0.621 and –0.609 to –0.645 and –0.624 e in H
spectively. And, the charges of O1 and O2 in 6,6′ ,6′ ′ -
benzene-1,3,5-triyl)tris(2-naphthoic acid) (H BTNA)
3
BTB change from
–
1
3
CTTA, re-
3
–1
(
3
3
–1
change from –0.621 and –0.603 to –0.633 and –0.611 e in
cm g , respectively. Stepwise N adsorption/desorption
2
H TTNA, respectively (Table S3 and Figure S6 in the Sup-
3
isotherms were observed, which implies the meso-
5
7,70
porting Information). Enhanced electron densities of car-
boxyl O atoms in H CTTA and H TTNA were thus justi-
porosity or cage-type structure of the two MOFs.
Fur-
3
3
thermore, we compared the pore volumes and total sol-
vent-accessible volumes of the two MOFs with other Zr-
MOFs, as well as some other water-stable MOFs (Table S2
in the Supporting Information). It was found that the
pore volumes of BUT-12 and -13 are in the middle level.
However, the total solvent-accessible volumes of BUT-12
are comparable or higher than other MOFs. Particularly,
BUT-13 represents one of the most porous water-stable
fied.
In addition, as shown in Figure S3 of the Supporting
Information, the thermal gravimetric analysis (TGA)
curves indicate that BUT-12 and -13 are thermally stable
up to 320 and 430 °C, respectively. Generally, the thermal
stability of MOFs is related to the strength of the associ-
ated metal coordination bond, as well as the nature of
used ligands. Since the two MOFs have isoreticular
structures with close Zr–O bond strength, the different
thermal stability of them is probably caused by the differ-
ent decomposition temperatures of the two ligands,
which are related with their different structures and mo-
lecular weights.
72
MOFs reported so far. Based on the N sorption data, the
2
pore size distributions were calculated by density func-
tional theory (DFT) method, which gave two types of
pores of 13 and 21 Å for BUT-12, and 14 and 28 Å for BUT-
13 (Figure 3c and d, inset), being consistent with the crys-
tallographic structure determination in each case when
van der Waals contact is considered.
Detection of antibiotics and nitroaromatics
Stability test
Because of the high porosity and excellent water sta-
bility of BUT-12 and -13, we sought to explore their appli-
cation in monitoring antibiotics and nitroaromatics in
water through fluorescent sensing. The solid-state lumi-
nescent properties of H CTTA, H TTNA, BUT-12, and
In order to examine the chemical stability of BUT-12
and -13, the samples were checked in water, HCl solutions
2 M, 6 M, and concentrated HCl), and NaOH solution
pH = 10) at room temperature, as well as in boiling water.
(
(
3
3
After being immersed in these different solutions for 24 h,
the measured PXRD patterns show retained crystallinity
and unchanged structures, demonstrating their excellent
stability (Figure 3a and b). It should be pointed out that
there are limited MOFs showing good stability in boiling
water, as well as in aqueous solutions with such a wide
pH range (Table S2 in the Supporting Information). As is
well-known, the Zr6 cluster is one of the most stable
BUT-13 were firstly checked at room temperature. As
shown in Figure S7 (see the Supporting Information), the
ligand acids, H CTTA and H TTNA exhibit fluorescent
3
3
emissions at 381 and 399 nm upon the excitations at 312
and 324 nm, respectively. Compared with the free
H CTTA and H TTNA, BUT-12 and -13 show similar emis-
3
3
sions at 372 and 410 nm based on the same excitations as
their corresponding ligand acids, respectively, which indi-
cates that the fluorescence of BUT-12 and -13 is mainly
4
+
building units for MOF construction because the Zr
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Journal of the American Chemical Society
attributed to the emission of the organic ligands. Fur-
for three repeated fluorescent measurements of blank
solutions, the detection limit (3S /K ) of BUT-12 and -13
towards NZF were calculated to be 58 and 90 ppb, respec-
tively (Table S6 in the Supporting Information).
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
thermore, the fluorescent properties of BUT-12 and -13
dispersed in different solvents were investigated (Figure
S8 in the Supporting Information). It was found that the
fluorescent emissions of them have slight solvent-
dependence and both MOFs represent excellent fluores-
cent emissions in water, being promising candidates for
the detection applications in water system. The highly
water stability together with good fluorescent perfor-
mances of BUT-12 and -13 prompt us to explore their fluo-
rescent sensing properties in water.
b
sv
The selective detection of antibiotics in water system
is highly desirable for practical application. Above results
demonstrate that BUT-12 and -13 have high quenching
efficiencies towards NFs, but very poor towards THI and
PCL antibiotics. Motivated by these findings, we further
checked the detection selectivity for NFs in the presence
THI or PCL. In a control experiment, the fluorescence
spectra of BUT-12 and -13 dispersed in water were initially
recorded, respectively. To these systems, a saturated
aqueous solution of THI was initially added so that high
affinity binding sites would be accessible to THI and then
followed by NFs (1 mM); the corresponding emissions
were monitored (for detail, see the Experimental Section).
As can be seen from Figure S23-28 in the Supporting In-
formation, the emission intensity of the two MOFs only
shows slightly changes in the presence of excess THI. Up-
on introducing NFs into the mixture of the MOFs and
THI, the fluorescence was significantly quenched. This
result reveals that the interference from THI can be ne-
glected, convincing the high quenching selectivities of the
two MOFs towards NFs. Similarly, the addition of PCL
also showed negligible effect on the fluorescence intensi-
ty, whereas NFs can quench effectively the fluorescence of
the MOFs in the presence of PCL in water (Figure S23-28
in the Supporting Information). These results can be easi-
ly visualized by plotting the percentage fluorescence in-
tensity versus volume of antibiotic added, as shown in
Figure 4f and g. Where, the stepwise decrease in fluores-
cence intensity clearly demonstrates the selectivity of
BUT-12 and -13 towards NFs, even in the presence of a
higher concentration of THI or PCL. These highly selec-
tive detections in water system in the presence of THI or
PCL make BUT-12 and -13 reliable sensing materials for
NFs.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
To explore the ability of BUT-12 and -13 to sense a
trace quantity of antibiotics, fluorescence-quenching ti-
trations were performed with the piece by piece addition
of antibiotics to water where BUT-12 and -13 are dis-
persed. Five classes of frequently-used antibiotics, NFs
(
furazolidone, FZD; nitro-furazone, NZF; nitrofurantoin,
NFT), NMs (ronidazole, RDZ; metronidazole, MDZ;
dimetridazole, DTZ; ornidazole, ODZ), sulfonamides (sul-
fadiazine, SDZ; sulfamethazine, SMZ), chloramphenicols
(
chloramphenicol, CAP; thiamphenicol, THI), and β-
lactams (Penicillin, PCL) were checked (Figure S9 in the
Supporting Information). It was found that high fluores-
cence quenching of MOFs occurs upon the incremental
addition of NZF, while THI shows very low quenching
effect (Figure 4a-d). Figure 4e shows the percentage of
fluorescence quenching in terms of adding a certain
amount of different antibiotics at room temperature. Ob-
viously, NZF and NFT give rise to the highest quenching
efficiencies of 92 and 91% for BUT-12, and 95 and 94% for
BUT-13, respectively. In addition, FZD, ODZ, RDZ, and
DTZ also lead to relative high quenching efficiencies,
whereas quenching efficiencies are low for the remaining
antibiotics (Figure S10-21 in the Supporting Information).
The quenching efficiencies of BUT-12 for these antibiotics
follow the order of NZF > NFT > FZD > ODZ > DTZ >
RDZ > MDZ > CAP > SAM > SDZ> PCL > THI, and that of
BUT-13 is NZF > NFT > FZD > ODZ > DTZ > RDZ > CAP >
SAM > MDZ > SDZ > PCL > THI. It should also be pointed
out that the frameworks of the two MOFs are intact after
the sensing experiments, as confirmed by PXRD (Figure
S56 and 57 in the Supporting Information).
The fluorescent quenching efficiency can be quantita-
tively explained by the Stern-Volmer (SV) equation: (I /I)
0
–
1
=
1 + K [Q], where K is the quenching constant (M ),
sv sv
[
Q] is the molar concentration of the analyte, I and I are
0
the luminescence intensities before and after addition of
the analyte, respectively. As indicated in Figure 4a-d (in-
set), S10-21 (inset), and S22 of the Supporting Information,
the SV plots for NZF and NFT are nearly linear at low
concentration ranges, but subsequently deviate from line-
arity and bend upwards at higher concentrations. Such
phenomena of nonlinear SV plots might be due to self-
1
5,73-77
absorption or an energy-transfer process.
Whereas,
the other antibiotics gave linear SV plots. BUT-12 and -13
5
4
–1
have the highest K values of 1.1 × 10 and 7.5 × 10 M
sv
4
4
–1
towards NZF, and 3.8 × 10 and 6.0 × 10 M towards NFT,
respectively (Table S5 in the Supporting Information).
Based on the Ksv values and the standard deviations (Sb)
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Journal of the American Chemical Society
Page 8 of 16
quenching efficiencies of 98 and 96% and 97 and 95%
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
towards TNP and 4-NP, respectively. Other nitroaromat-
ics also give rise to high quenching efficiencies, but
quenching efficiencies for non-nitroaromatics and ali-
phatic nitro compounds are low. The quenching efficien-
cies of BUT-12 follow the order of TNP > 4-NP > NB > 2,4-
DNT > 2,6-DNT > BC > CB > PHL > NM > MB > DMNB,
and that of BUT-13 is TNP > 4-NP > NB > 2,4-DNT > 2,6-
DNT > DMNB > PHL > NM > BC > MB > CB (Figure 5e).
The frameworks of the two MOFs are also intact after
these detection experiments as confirmed by PXRD (Fig-
ure S56 and 57 in the Supporting Information).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Similar to those in detecting antibiotics, the SV plots
of TNP and 4-NP are also nearly linear at low concentra-
tion ranges, then subsequently deviate from linearity and
bend upwards at higher concentration ranges (Figure 5a-d
inset, S29-40 inset, and S41 in the Supporting Infor-
mation). Linear SV plots were observed for all the other
analytes over a wide concentration range (Figure S41 in
the Supporting Information). Both BUT-12 and -13 show
5
5
–1
the highest K values of 3.1 × 10 and 5.1 × 10 M for TNP,
sv
respectively. The detection limits of BUT-12 and -13 to-
wards TNP are estimated to be 23 and 10 ppb, respectively
(
Table S6 in the Supporting Information). It should be
pointed out that most of published works in the detection
of nitroaromatics with MOFs are based on the checks in
organic solvents such as CH OH, C H OH, DMF, and
3
2
5
CH CN. Only UiO-67@N showed a good detection ability
3
towards TNP in water. The K for TNP in UiO-67@N was
sv
4
–1
15
2
.9 × 10 M , far smaller than those of BUT-12 and -13. In
addition, the K values for TNP in the two MOFs are also
sv
larger than most traditionally used organic polymer sens-
ing materials, demonstrating a super-quenching ability of
them (Table S8 in the Supporting Information). The Ksv
values calculated for other nitroaromatics are smaller
than that of TNP (Table S5 in the Supporting Infor-
mation). Furthermore, the selective fluorescence quench-
ing of BUT-12 and -13 towards some nitroaromatics in the
presence MB or CB were also studied (Figure S42-47 in
the Supporting Information). As shown in Figure 5f and g,
the stepwise decrease in fluorescence intensity clearly
demonstrates the unprecedented selectivity of BUT-12
and -13 towards TNP, 4-NP, and NB, even in the presence
of MB or CB with a high concentration.
Figure 4. Effect on the emission spectra of (a, b) BUT-12 and
c, d) BUT-13 dispersed in water upon incremental addition
(
of a selected antibiotic (1 mM, 20 µL addition each time; in-
set: Stern-Volmer plots of selected antibiotics); (e) fluores-
cence quenching of BUT-12 and -13 by different antibiotics at
room temperature, and selective detection of NFs on (f)
BUT-12 and (g) BUT-13 in the presence of THI or PCL in wa-
ter.
On the other hand, until now a lot of MOFs have
shown excellent performances for the detection of organic
explosives and nitroaromatics based on their fluorescence
In order to better understand the fluorescence
quenching effect of BUT-12 and -13 towards NFs and ni-
troaromatics, the quenching mechanism was proposed.
Simply, MOFs can be regarded as large “molecules” and
the valence-band (VB) and conduction-band (CB) energy
18-20
quenching process.
However, most of them work in
organic solvent systems, but not in water. High water
stability of BUT-12 and -13 allows us to check their appli-
cation in the detection of nitroaromatics in water. For
comparison, some other aromatics and aliphatic nitro
compounds were also tested. Here, eleven analytes in-
cluding 2,4,6-trinitrophenol (TNP), 4-nitrophenol (4-NP),
nitrobenzene (NB), 2,4-dinitrophenol (2,4-DNT), 2,6-
dinitrophenol (2,6-DNT), benzoic acid (BC), chloroben-
zene (CB), phenol (PHL), nitromethane (NM),
methylbenzene (MB), and 2,3-dimethyl-2,3-dinitrobutane
levels can be described in a mode similar to that used for
15,73-77
molecular orbitals (MOs).
The CB of a MOF lies at
higher energy level than the lowest unoccupied molecular
orbitals (LUMOs) of an analyte, which leads to a driving
force for the electron transfer from the MOF to the ana-
lyte, thus resulting in the fluorescence quenching. Shapes
and relative orbital energies of the highest occupied mo-
lecular orbitals (HOMOs) and LUMOs of the analytes
were herein calculated by DFT (Figure S58, 59 and Table
S7 in the Supporting Information). These LUMO energy
(
DMNB) were checked (Figure S9 in the Supporting In-
formation). As shown in Figure 5a-d and S29-40 of the
Supporting Information, BUT-12 and -13 represent high
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Journal of the American Chemical Society
the fluorescence quenching processes. As we know, when
levels, which are arranged in a descending energy order,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
are expected to represent how easily an electron can be
transferred to the electron-deficient analyte in the fluo-
rescence quenching process. It was found that the LUMO
energies are in good agreement with the maximum
quenching efficiency observed for TNP, but the order of
observed quenching efficiency is not fully in accordance
with the LUMO energies of antibiotics and other nitroar-
omatics. These results indicate that the photo-induced
electron transfer is not the only mechanism for the fluo-
rescence quenching observed in these systems.
the absorption band of the analyte has an effective over-
lap with the emission band of the MOF, the resonance
energy can transfer from the MOF to the analyte, there-
fore the fluorescence quenching happening. The probabil-
ity of resonance energy transfer thus depends upon the
extent of spectral overlap between the absorption band of
the analyte and the emission band of the MOF. As shown
in the UV-vis absorption spectra of the analytes and the
two MOFs (Figure S60 and 61 in the Supporting Infor-
mation), the absorption band of NZF has the greatest de-
gree of overlapping with the emission spectra of BUT-12
and -13, followed by NFT, FZD, ODZ, DTZ, MDZ, RDZ,
SAM, CAP, SDZ, PCL, and THI. For nitroaromatics, TNP
has the greatest overlap, followed by 4-NP, NB, 2,4-DNT,
2,6-DNT, NM, THL, DMNB, CB, MB, and BC. Clearly, the
extents of the overlap are highly consistent with the
quenching efficiencies for both antibiotics and nitroaro-
matics as discussed above. As a result, the co-existence of
electron transfer and resonance energy transfer makes
NFs and nitroaromatics show higher photo-luminescence
quenching effect compared with other checked analytes.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The existence of energy transfer was also supported by
the preferential quenching of the 374 nm peak over 430
nm in BUT-12, and 390 nm peak over 440 nm in BUT-13
(
Figure 4a, c, 5a and c). The peaks at 374 nm of BUT-12
and 390 nm of BUT-13 have large spectral overlaps with
the absorption spectra of NFs and nitroaromatics, so that
the efficient quenching of the two peaks occurs, respec-
tively, thereby giving higher quenching response. Where-
as, the peaks at 430 nm of BUT-12 and 440 nm of BUT-13
have less overlap with the absorption spectra of them,
thus the quenching occurs only based on an electron
transfer mechanism, and a small quenching response to-
wards other antibiotics and non-nitroaromatics were ob-
served. These results also imply that the energy transfer is
predominant over the electron transfer in the fluores-
cence quenching of the two MOFs by these antibiotics
and nitroaromatics. For other analytes, the quenching
occurs only by an electron transfer process. In addition,
since the energy transfer is a long-range process, the
emission quenching by NFs and nitroaromatics are car-
ried over the surrounding fluorophores, thus amplifying
the quenching response of BUT-12 and -13. While the elec-
tron transfer is a short-range process, the emission
quenching by other analytes is limited to the fluorophore
that has direct interaction with the analytes. Thus, BUT-12
and -13 respond more selectively towards NFs and ni-
troaromatics than other analytes. It should also be point-
ed out that BUT-12 and -13 can be recovered and regener-
ated by the centrifugation of the solution after use and
washing with acetone several times. The quenching effi-
ciencies up to 6 cycles are basically unchanged, demon-
strating good recyclability and stability for these detec-
tion applications (Figure S62 and 63 in the Supporting
Information).
Figure 5. Effect on the emission spectra of (a, b) BUT-12 and
(
c, d) BUT-13 dispersed in water upon incremental addition
of a selected analyte (1 mM, 20 µL addition each time; inset:
SV plots of selected analyte); (e) fluorescence quenching of
BUT-12 and -13 by different analytes at room temperature;
and the selective detection of TNP, 4-NP or NB on (f) BUT-12
and (g) BUT-13 in the presence of CB or MB in water.
Another reason for the quenching might be the reso-
1
5,73,74
nance energy transfer.
The nonlinearity of the SV
Adsorption of antibiotics and nitroaromatics
plots for NZF, NFT, TNP, and 4-NP discussed above in-
deed suggests that such an energy transfer should exist in
Besides detection, the removal of antibiotics and ni-
troaromatics from wastewater are also important in water
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 16
o
treatment. Reported results have shown that MOFs can
be used as good adsorbents for the removal of organic
sorbents were activated at 100 C under vacuum for 10 h.
As shown in Figure S78-82 of the Supporting Information,
zeolites 5A, 3A, 13X, Na-LSX, and SBA-15 show trace ad-
sorption towards the two antibiotics. The hydrophilic
nature and/or unmatched pore size of these materials are
believed to be the main reason of the observed low up-
takes. The sizes of NZF and NFT are of 9.34 × 2.25 and
8.91 × 3.52 Å, respectively (Figure S9, and Table S4 in the
Supporting Information), implying that they should be
able to diffuse into the pores of 5A, 13X (ca. 8 Å), and Na-
LSX (ca. 10 Å) zeolites. However, the hydrophilic nature
of the zeolites makes water molecules preferentially oc-
cupy their pores during the adsorption processes, thus
limiting the access of the antibiotics. The interactions
between the antibiotic molecules and SBA-15 are probably
relatively weak because of its mesoporous nature (~60 Å),
thus resulting in low uptakes. Additionally, SBA-15 is also
moderately hydrophilic; the competitive adsorption of
water may further lead to its low adsorption towards an-
tibiotics. We thus propose that the suitable pore size and
hydrophobic pore surface of a material might play key
roles in the adsorption of these antibiotic molecules from
water. The presence of methyl groups in the ligands of
BUT-12 and -13 can increase the hydrophobicity of the two
MOFs, which weakens their interactions with water mol-
ecules thus increasing their adsorption ability for antibi-
otics and nitroaromatics. The high contact angles of water
1
2
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5
6
7
8
9
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1
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5
6
23
contaminants from water. However, to the best of our
knowledge, the relevant studies on antibiotics and ni-
troaromatics are still scarce up to now. Thus we checked
the adsorption performances of BUT-12 and -13 for some
selected antibiotics (NFs, NMs, SAM, and CAP) and ni-
troaromatics (TNP, 4-NP, 2,4-DNT, 2,6-DNT, and NB).
Freshly prepared BUT-12 and -13 were activated to remove
guest molecules accommodated in their pores and then
immersed in water solutions of these antibiotics and ni-
troaromatics at room temperature. The uptakes of these
analytes in BUT-12 and -13 were determined by using UV-
Vis spectroscopy. As shown in Figure 6a-d and S64-77 in
the Supporting Information, BUT-12 represents large ad-
sorption rates towards NZF and NFT antibiotics, and
TNP, 4-NP, 2,4-DNT, and 2,6-DNT nitroaromatics; while
BUT-13 can quickly adsorb NZF, NFT, ODZ, SAM, and
CAP antibiotics, and TNP, 4-NP, 2,4-DNT, 2,6-DNT, and
NB nitroaromatics. Different adsorption behaviors of
BUT-12 and -13 should be related with their different pore
sizes.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
3
4
5
6
7
8
9
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4
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6
7
8
9
0
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2
3
4
5
6
7
8
9
0
o
(138.7 and 118.3 ) suggest the hydrophobic property of
BUT-12 and -13 (Figure S83 in the Supporting Infor-
mation). Besides, water adsorption isotherms for BUT-12
and-13 were recorded at room temperature (Figure S84 in
the Supporting Information). It was found that the two
3
-1
MOFs have high water uptakes of 540 and 615 cm g at
P/P = 0.86, respectively, which is in accordance with
0
their high porosity. However, the water adsorption iso-
therms are stepwise with hysteresis loops and low up-
takes at lower pressure ranges (below P/P = 0.27 for BUT-
0
12, and P/P = 0.38 for BUT-13) followed by a steep rise of
0
uptake at higher pressures. Compared with the total wa-
3
ter uptakes, at low pressure ranges the uptakes (100.0 cm
-
1
3
-1
g at P/P = 0.27 for BUT-12 and 92.7 cm g at P/P = 0.38
0
0
for BUT-13) are relatively low (18.5 and 15.1% of the total
uptakes, respectively), which indicates that pore surfaces
of the two MOFs are dominatively hydrophobic notwith-
standing there are also small portion of hydrophilic ad-
5
4
sorption sites. This is consistent with the single crystal
structures of the two MOFs, which show that their pore
walls mostly consists of backbone of organic linkers with
Figure 6. Adsorptions of NZF (a) and TNP (b) in BUT-12, and
NZF (c) and TNP (d) in BUT-13, tracked by UV-vis spectra
change with respect to time; and adsorption isotherms of
TNP and 4-NP in BUT-12 (e) and BUT-13 (f) (adsorption con-
ditions: at 298 K, 50 mL of solution, 15 mg of MOFs, contact
time of 4 h).
hydrophobic methyl groups. The O/OH groups in Zr6
clusters and coordinated carboxylate O atoms in ligands
should be responsible to the small portion of hydrophilic
adsorption sites on their pore surface.
For nitroaromatics, particularly, TNP and 4-NP can be
completely adsorbed within 15 and 100 min in BUT-12, and
within 2 and 40 min in BUT-13, respectively (Figure 6b
and d). The faster adsorption kinetics in BUT-13 than that
in BUT-12 can be attributed to the larger pore size of the
former than that of the latter. With these results in mind,
we further explored the adsorption isotherms of TNP and
It should be pointed out that, up to now, few works
have been reported for antibiotic removal in water with
the adsorption method. For comparison, we also carried
out similar adsorption experiments of NZF and NFT for
zeolites (5A, 3A, 13X, and Na-LSX) and mesoporous SiO2
(SBA-15) in water. Before adsorption experiments, all ad-
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Journal of the American Chemical Society
4
-NP in BUT-12 and -13 at 298 K. As shown in Figure 6e
On the other hand, as discussed above, the fluorescence
of the two Zr-MOFs is mainly attributed to the emission
of their ligands; it thus allows us to estimate the pre-
concentration effect by comparing the quenching con-
1
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5
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9
1
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1
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2
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4
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5
5
5
6
and f, the maximum adsorption amounts of TNP and 4-
NP in BUT-12 are 708 and 414 mg/g, and those in BUT-13
are 865 and 560 mg/g, respectively. These values are
comparable and/or even higher than those in other po-
rous materials reported so far (Table S8 in the Supporting
Information). In addition, the results of FT-IR measure-
ments also confirm that TNP and 4-NP molecules are in-
deed adsorbed into the pores of the two MOFs (Figure
S85 and 86 in the Supporting Information). The superior
performances of the two MOFs in TNP and 4-NP adsorp-
tions could be ascribed to their large specific surface are-
as, suitable pore size, as well as the distributing OH
groups on pore surfaces, which endow the MOF strong
stants (K ) of the MOFs with those of their corresponding
sv
ligand acids. In this regard, if the Ksv value of a MOF is
larger than that of its corresponding ligand acid, then we
can say that the pores of the MOF indeed play an im-
portant role in the pre-concentration quenching. Fluores-
cence-quenching titration experiments of the ligand ac-
ids, H CTTA and H TTNA towards TNP and 4-NP were
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
3
thus carried out under the same conditions, and their Ksv
values were calculated (Figure S52-55 in the Supporting
Information). It concluded that the K values of H CTTA
sv
3
–1
78
4
4
interactions with the adsorbates. Moreover, Langmuir
towards TNP and 4-NP are 2.1 × 10 and 1.4 × 10 M , and
79
4
4
–1
and Freundlich models were used to fit and examine
above adsorption isotherms, respectively. The related
parameters are given in Table S9, as well as the details in
Figure S87-90 in the Supporting Information. Obviously,
the data are well fitted by the Langmuir model, indicating
a homogeneous and monolayer adsorption occurring in
BUT-12 and -13 with a finite number of identical sites.
Thus, it is expected that with further increasing their sur-
face areas, the number of adsorption sites will increase,
which will accordingly enhance their adsorption capaci-
ties. In addition, as shown in Figure S91, the two MOFs
almost regained their initial adsorption capacities over
three repeated cycles, demonstrating their high stability
and good reusability.
those of H TTNA are 1.0 × 10 and 1.4 × 10 M , respective-
3
ly. These values are clearly far smaller than those of their
corresponding MOFs, BUT-12 and -13 (Table S5 in the
Supporting Information), implying the effect of the pre-
concentration of analytes on the fluorescent quenching.
Conclusion
Two new chemically stable fluorescent Zr(IV)-based
MOFs have been designed, synthesized, and used in the
selective detection and removal of antibiotics and ni-
troaromatics in/from water, showing excellent perfor-
mances. Guided by a topological design approach, two
ligands were rationally designed and resulting MOFs rep-
resented an expected the-a topology, being first examples
among Zr-MOFs. Besides, the introduced methyl groups
into the ligands actually increased the steric hindrance to
fix the position of terminal phenyl rings thus eliminating
non-radiative relaxation pathways and increase the fluo-
rescence property of resulting MOFs. The two MOFs
showed excellent selective detection ability towards NZF
and NFT antibiotics and TNP and 4-NP organic explosives
over other partners based on their sensitive fluorescence
quenching. The detection limits of BUT-12 towards NZF
and TNP are estimated to be 58 and 23 ppb, and those of
BUT-13 are 90 and 10 ppb, respectively. The high quench-
ing efficiencies can be attributed to a combining effect
from electron and energy transfers in the host-guest sys-
tems. Both MOFs show also good adsorption ability to-
wards NZF, NFT, 4-NP, and TNP. Among them, the up-
takes for 4-NP and TNP are comparable to those reported
porous materials. Moreover, it was found that the adsorp-
tion process plays an important role in the pre-
concentration of the analytes in the pores of the two
MOFs, which makes the analytes contact with MOFs
more sufficiently, thus enhancing the detection efficiency.
Present study provides a new insight into the design of
MOFs for the simultaneously detection and removal of
contaminations in water. The resulting new MOF materi-
als are potentially useful for the water treatment applica-
tions.
Overall, above results demonstrate that BUT-12 and -13
have high selective fluorescence quenching efficiencies
and good adsorption abilities towards NZF, NFT, TNP,
and 4-NP. The detection sensitivities of the two MOFs are
believed to be related with the pre-concentration effect of
these analytes. That is, during the detection process,
when analytes are added into MOF-containing solutions,
part of the analytes are firstly adsorbed by the MOFs,
which makes the analytes contact with MOFs more suffi-
ciently, thereby leading to an enhanced florescent re-
sponse. To verify such a pre-concentration effect, solubili-
ty partition coefficients of TNP and 4-NP in water/MOFs
system were calculated. The solubility partition coeffi-
cient is the ratio of concentrations of the quencher mole-
17
cules within the MOFs and in the water. Based on ad-
sorption data, the concentrations of TNP and 4-NP in the
supernatant are known and the concentration of them in
the pores of BUT-12 and -13 were calculated based on the
equation (4) (see the Experimental Section). Thus, the
solubility partition coefficients can be calculated through
“
the concentrations of TNP or 4-NP in the pores of BUT-
1
2 and -13 divide their concentrations in the supernatants,
respectively”. It was found that the averaged solubility
partition coefficients of TNP and 4-NP in BUT-12/water
system are 6891 and 3635, and those of them in BUT-
1
3/water system are of 6217 and 3648, respectively. The
large solubility partition coefficients of TNP and 4-NP in
water/MOFs solution show that, in these systems, TNP
and 4-NP tend to preferentially access the pores of the
MOFs, thus demonstrating the pre-concentration effect.
Experimental section
Materials and instruments
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Journal of the American Chemical Society
Page 12 of 16
All general chemicals and solvents (AR grade) were
commercially available and used as received. H NMR
As-synthesized samples were soaked in fresh DMF for
24 h, and the extract was discarded. Fresh acetone was
1
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5
5
6
spectra were measured on Bruker Avance 400 MHz with
tetramethylsilane as the internal standard. FT-IR data
were recorded on an SHIMADZU IR Affinity-1 instrument.
Powder X-ray diffraction (PXRD) patterns were recorded
on a BRUKER D8-Focus Bragg-Brentano X-ray Powder
Diffractometer equipped with a Cu sealed tube (λ =
.54178) at room temperature. Simulation of the PXRD
spectra was carried out by the single-crystal data and dif-
fraction-crystal module of the Mercury program available
subsequently added, and the samples were guest ex-
changed for 12 h. This procedure was again repeated three
times. After decanting the acetone extract, the samples
–3
were dried under a dynamic vacuum (< 10 Torr) at room
temperature for 1 h. Before adsorption measurement, the
samples were further activated using the “outgas” func-
o
1
tion of the adsorption analyzer at 100 C for 10 h.
Single-crystal X-ray diffraction
0
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0
The diffraction data of BUT-12·S were collected in a
Rigaku Supernova CCD diffractometer equipped with a
mirror-monochromatic enhanced Cu-Kα radiation (λ =
free
of
charge
via
internet
at
http://www.ccdc.cam.ac.uk/mercury/. Thermogravimet-
ric analysis (TGA) data were obtained on a TGA-50
1
.54184 Å) at 100 K. The dataset was corrected by empiri-
(
SHIMADZU) thermogravimetric analyzer with a heating
–1
cal absorption correction using spherical harmonics, im-
rate of 10 °C min under air atmosphere. The contact an-
gles towards water were measured on Dataphysics tp50.
Gas adsorption isotherms were reported by a volumetric
method using a Micromeritics ASAP2020 surface area and
pore analyzer. Fluorescence spectra were recorded on an
F-4600 FL Spectrophotometer equipped with a xenon
lamp and quartz carrier at room temperature. UV/vis
spectra were obtained with a UV-2600 spectrophotometer
in the range of 250~800 nm at room temperature.
81
plemented in the SCALE3 ABSPACK scaling algorithm.
The structure was solved by direct methods and refined
2
by full-matrix least-squares on F with anisotropic dis-
82
placement by using the SHELXTL software package.
Non-hydrogen atoms were refined with anisotropic dis-
placement parameters during the final cycles. Hydrogen
atoms of ligands were calculated in ideal positions with
isotropic displacement parameters. Those in –OH/H O
2
groups of the Zr(IV)-based clusters were not added, but
were calculated into molecular formula of the crystal da-
ta. There is large solvent accessible pore volume in the
structure of BUT-12, which is occupied by highly disor-
dered solvent molecules. No satisfactory disorder model
for these solvent molecules could be achieved, and there-
fore the SQUEEZE program implemented in PLATON was
used to remove these electron densities of these disor-
Synthesis
The
trimethyl-[1,1':3',1''-terphenyl]-4,4''-dicarboxylic
(H CTTA) and 6,6',6''-(2,4,6-trimethylbenzene-1,3,5-
ligand
acids,
5'-(4-carboxyphenyl)-2',4',6'-
acid
3
triyl)tris(2-napthoic acid) (H TTNA) were synthesized by
3
following a previously reported procedures with some
8
0
modifications. The detailed description is provided in
the Supporting Information.
69
dered species. Thus, all of electron densities from free
solvent molecules have been “squeezed” out. The details
of crystal data and structural refinement can be found in
Tables S1 of the Supporting Information, and the provided
CIF files.
[
Zr O (OH) (H O) (CTTA)8/3]·S (BUT-12·S) (S repre-
6 4 8 2 4
sents non-assignable solvent molecules): ZrCl (48 mg, 0.2
4
mmol), H CTTA (40 mg, 0.08 mmol), and formic acid (8
3
mL)
were
ultrasonically
dissolved
in
N,N'-
Fluorescence measurements
dimethylformamide (DMF, 8 mL) in a 20 mL Pyrex vial.
o
Caution: TNP and 2,4-DNT are highly explosive and
should be handled carefully and in small amounts. In addi-
tion, TNP is also easy to form shock-sensitive compounds
when meet with heavy metals.
The vial was sealed and then heated at 120 C for 48 h in
an oven. After cooling to room temperature, the resulting
colorless crystals were harvested by filtration and washed
with DMF and acetone, and then dried in air (yielded 32
mg). For PXRD pattern of as-synthesized material, see
Figure 3a; for TGA, and FT-IR, see Figure S3 and 4 of the
Supporting Information, respectively.
In a typical experimental setup, 2 mg of BUT-12 or -13
sample was weighed and finely grounded, and then added
to a cuvette containing 2.5 mL of deionized water under
stirring. The fluorescence upon excitation at 312 nm of
BUT-12 and 324 nm of BUT-13 was measured in-situ after
incremental addition of freshly prepared analyte solutions
(1mM, 20 µL addition each time). The mixed solution was
stirred at constant rate during experiment to maintain its
homogeneity. All the experiments were performed in trip-
licate, and consistent results were reported.
[Zr O (OH) (H O) (TTNA)8/3]·S (BUT-13·S) (S repre-
6
4
8
2
4
sents non-assignable solvent molecules): ZrCl (48 mg, 0.2
4
mmol), H TTNA (40 mg, 0.06 mmol), and acetic acid (3.2
3
mL) were ultrasonically dissolved in 12 mL of DMF in a 20
mL Pyrex vial and sealed. The reaction system was then
o
heated at 120 C for 72 h in an oven. After cooling to room
temperature, the resulting colorless crystals were collect-
ed by filtration and washed with DMF and acetone, and
then dried in air (yielded 38 mg). For PXRD pattern of as-
synthesized material, see Figure 3b; for TGA, and FT-IR,
see Figure S3 and 4 of the Supporting Information, re-
spectively.
Similarly, in a selective detection experiment, 2 mg
finely grounded sample was added to a cuvette containing
2.5 mL of deionized water under stirring. Fluorescence of
the obtained suspension was recorded. Then, saturated
THI (or PCL, CB, and MB) and 1 mM NZF (or NFT, FZD,
TNP, and 4-TN) solutions were alternatively introduced
into the suspension in such a sequence: THI (20 ml), THI
Sample activation
ACS Paragon Plus Environment

Page 13 of 16
Journal of the American Chemical Society
(
20 ml), NZF (20 ml), NZF (20 ml), THI (20 ml), THI (20
The regenerated MOFs were used again for the adsorp-
tion of TNP or 4-NP up to three cycles.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ml)…, the process was repeated till the total volume of
added analytes reached 200 µL. After each addition, the
fluorescence of the suspension was monitored.
ASSOCIATED CONTENT
Aqueous-phase adsorption
Supporting Information
Freshly prepared BUT-12 or -13 sample (10 mg) was to-
tally activated and then transferred into water solutions
of different analytes with given concentrations in a vial,
respectively. UV-vis spectra of the solutions were record-
ed to characterize the adsorption performances of BUT-12
and -13 along with the soaking time at 298 K. The adsorp-
tion isotherms of TNP and 4-NP were obtained by mixing
Full details for the synthesis of H CTTA and H TTNA,
3
3
structure
refinement
and
construction,
general
characterizations, additional structural figures, DFT
calculation, detection of antibiotics and nitroaromatics,
adsorption towards selected antibiotics and nitroaromatics.
This material is available free of charge via the Internet at
http://pubs.acs.org.
0
1
2
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4
5
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8
9
0
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2
3
4
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9
0
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3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
1
5 mg MOFs with 50 mL TNP or 4-NP solution of different
–
1
concentrations from 100 to 1600 mg L at a constant tem-
perature of 298 K with stirring for 4 h. The amount of
TNP or 4-NP adsorbed on the MOFs was calculated using
the mass balance with Eq. 1:
AUTHOR INFORMATION
Corresponding Author
jrli@bjut.edu.cn
C ꢀ C
)V
(1)
(
o
e
Q =
e
Notes
M
–1
The authors declare no competing financial interest.
where Q (mg g ) is the equilibrium adsorbed amount; C
and C (mg L ) are the initial and equilibrium concentra-
e
0
–
1
e
ACKNOWLEDGMENTS
tions of solution; V (L) is the volume of solution; M (g) is
the mass of MOF. In order to assure the accuracy of
measurements, all the experiments were repeated at least
three times and the average values were reported. All ma-
terials were dried overnight under vacuum at 373 K before
each repeated use. The resulting isotherms are fitted by
Langmuir mode (Eq. 2):
We thank Dr. Hongliang Huang of Beijing University of
Chemical Technology for his discussion in computational
simulations and Prof. Daqiang Yuan of Fujian Institute of
Research on the Structure of Matter for his help in structural
analysis. This work was financially supported from the Natu-
ral Science Foundation of China (21322601, 21576006,
21271015) and the Program for New Century Excellent Talents
Ce
1
Ce
(2)
in University of China (NCET-13-0647).
=
+
Qe K Q Qm
L
m
REFERENCES
and Freundlich mode (Eq. 3):
(1)
Kümmerer, K. Chemosphere 2009, 75, 417.
1
(3)
lnQ = ln KF
+
lnCe
(2)
Zhang, Q.-Q.; Ying, G.-G.; Pan, C.-G.; Liu, Y.-S.; Zhao,
e
n
J.-L. Environ. Sci. Technol. 2015, 49, 6772.
-
1
where C (mg L ) is the equilibrium concentration of ad-
sorbate; Q (mg L ) is the adsorbed amount at equilibri-
um; Q (mg L ) is the maximum monolayer adsorption
capacity; K (L g mg L ) is the Langmuir constant related
e
(3)
1005.
(4)
Blasco, C.; Corcia, A. D.; Picó, Y. Food Chem. 2009, 116,
-
1
e
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1
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1
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(
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=
6
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(
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where CMOF (mg L ) is the concentration of adsorbent in
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