Selective sensing of CrVI and FeIII ions in aqueous solution by an exceptionally stable TbIII-organic framework with an AIE-active ligand

Article abstract of DOI:10.1016/j.cclet.2021.01.040

We herein report a new lanthanide metal-organic framework (MOF) that exhibits excellent chemical stability, especially in the aqueous solution over a wide pH range from 1 to 14. In contrast to many reported lanthanide MOFs, this Tb-based MOF emits cyan fluorescence inherited from the integrated AIE-active ligand, rather than Ln³⁺ ions. More remarkably, its fluorescence signal features a highly selective and sensitive “turn-off” response toward CrO₄^2?, Cr₂O₇^2? and Fe³⁺ ions, highlighted with the low detection limits down to 68.18, 69.85 and 138.8 ppm, respectively. Thus, the exceptional structural stability and sensing performance render this material able to be a superior luminescent sensor for heavy metal ions in wastewater.

Full text of DOI:10.1016/j.cclet.2021.01.040

G Model
CCLET-6117; No. of Pages 5
Chinese Chemical Letters xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Selective sensing of CrVI and FeIII ions in aqueous solution by an
exceptionally stable TbIII-organic framework with an AIE-active ligand
a,
a,b
Jing-Jing Panga, Rui-Huan Dua, Xin Liana, Zhao-Quan Yaoa, , Jian Xu , Xian-He Bu
*
*
a
School of Materials Science and Engineering, National Institute for Advanced Materials, Tianjin Key Laboratory of Metal and Molecule-Based Material
Chemistry, Nankai University, Tianjin 300350, China
b
State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
Article history:
We herein report a new lanthanide metal-organic framework (MOF) that exhibits excellent chemical
stability, especially in the aqueous solution over a wide pH range from 1 to 14. In contrast to many
reported lanthanide MOFs, this Tb-based MOF emits cyan fluorescence inherited from the integrated AIE-
active ligand, rather than Ln3+ ions. More remarkably, its fluorescence signal features a highly selective
Received 20 December 2020
Received in revised form 14 January 2021
Accepted 19 January 2021
Available online xxx
and sensitive “turn-off” response toward CrO42ꢀ, Cr2O7 and Fe3+ ions, highlighted with the low
2ꢀ
detection limits down to 68.18, 69.85 and 138.8 ppm, respectively. Thus, the exceptional structural
stability and sensing performance render this material able to be a superior luminescent sensor for heavy
metal ions in wastewater.
© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Lanthanide metal-organic framework
Aggregation-induced emission
Heavy metal ions
Selective luminescence sensing
Aqueous-phase sensing
To date, heavy metal ions are still one of the most common
sources of water contamination, posing a severe poisoning risk to
human beings and animals, and the booming industrialization has
continued to aggravate this situation. In this context, hexavalent
chromium (CrVI) is one of the well-known toxic heavy metal
pollutants [1–6]. The extensive use of CrVI in industry would
inevitably cause leakage accidents and bring hazardous environ-
mental pollutants in the form of Cr2O72ꢀ or CrO42ꢀ anion. Likewise,
iron as one of the most important metals used by modern industry
and technology is an essential trace element in the biochemical
processes, such as blood transmission and oxygen storage [7,8]. For
human health, either the deficiency or the overload of FeIII ion may
induce a variety of disorders, including hemochromatosis, anemia,
liver damage, diabetes, heart failure, and Parkinson’s disease [9,10].
At present, the prevailing detection techniques for heavy metal
ions mainly include inductively coupled plasma mass spectroscopy
(ICP-MS), atomic absorption spectroscopy, and electrochemical
analysis [2,11–13]. However, all of these approaches are high cost,
time consuming and often require expensive, non-portable appara-
tus. Therefore, for the sake of human health, it is highly demandable
to develop a more convenient and low-cost method for the sensitive
detection of heavy metal ions, especially in the aqueous phase.
Luminescence sensing is a potential solution due to its ready
operability, ease of visualization, and quick response [10]. In this
regard, metal-organic frameworks (MOFs) held together by the
coordination bonds between metal ions/clusters and organic
ligands are conceived of as a new promising class of sensing
materials [14,15]. The almost infinite combinations of organic and
inorganic moieties could endow them with designable pore
structures and good feasibility in a wide array of applications,
such as catalysis [16–18], gas storage and separation [19–22],
photovoltaic conversion [23], magnetism [24,25], conductivity
[26], polymerization [27–29], as well as sensing [30–39]. However,
from an application perspective, a majority of MOF-based sensors
are known to be intrinsically unstable in the aqueous solutions,
especially under strong acidic or basic condition, greatly limiting
their application scope [40].
Besides that, for MOFs, their luminescence can derive from metal
nodes, organic linkers or both [41]. Compared to inorganic metal
components, a huge diversity of organic ligands makes much more
room for the engineering of high-luminescence materials with
excellent sensing ability. Recently, employing a special kind of
organic luminogens with aggregation-induced emission (AIE)
activity to construct luminescent MOFs has aroused much attention.
Ingeneral, by virtueofcoordinativeimmobilizationoftheAIElinkers
in a rigid MOF matrix, the nonradiative dissipation of these linkers
can be efficiently reduced through suppressing their intramolecular
rotation, vibration and motion, thus beneficial to promoting the
luminescence efficiency by a MOF-promoted AIE effect.
* Corresponding authors.
E-mail addresses: lawyer185@163.com (Z.-Q. Yao), jxu@nankai.edu.cn (J. Xu).
https://doi.org/10.1016/j.cclet.2021.01.040
1001-8417/© 2021 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article as: J.-J. Pang, R.-H. Du, X. Lian et al., Selective sensing of CrVI and FeIII ions in aqueous solution by an exceptionally stable
TbIII-organic framework with an AIE-active ligand, Chin. Chem. Lett., https://doi.org/10.1016/j.cclet.2021.01.040

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Bearing this in mind, we herein modify a celebrated AIE-active
molecule, tetraphenylethene (TPE), as the target emissive ligand, as
TPE has been widely used as fluorescent sensors, bio-imaging
materials, etc. [42], and also been employed in fabricating MOFs
with excellent photophysical properties [43–52]. By solvothermal
reaction of this TPE-based organic linker, 4',4”',4””',4”””'-(ethene-
1,1,2,2-tetrayl)tetrakis(([1,10-biphenyl]-4-carboxylic acid)) (name-
ly, H4ETTC) with Tb3+ ion, we obtain a new luminescent MOF with
the formula of {H3Oꢁ[Tb(H2O)2(ETTC)]} (namely, 1). The powder X-
ray diffraction (PXRD) pattern of the as-synthesized sample of 1
matches well with the simulated result based on the single crystal
data, validating its crystallinity and phase purity (Fig. S5 in
Supportinginformation).Thiscomplexpossesseshighcoordination
number and strong coordination bond between carboxyl groups
and Ln3+ ion. As a result, itexhibits exceptional structuralstabilityin
the aqueous solution over a wide pH range of 1ꢀ14. Interestingly,
this lanthanide MOF emits the H4ETTC-based fluorescence, rather
than the characteristic green light of Tb3+ ion, due to the absence of
the antenna effect [53]. Further luminescence studies reveal that 1
displays a highly selective and sensitive quenching response to CrVI
anions and FeIII cation in the aqueous solutions. In addition, this
sensor material has a good recyclability and anti-interference
ability. Obviously, allof thesemerits renderthismaterialable tobea
promisingcandidatesensor forthe detectionofCr2O72ꢀ/CrO42ꢀ and
Fe3+ ions in wastewater.
node. The coordination configuration between the carboxyl groups
and the Tb3+ ion can be viewed as four propeller-shaped blades
(Fig. S6 in Supporting information). In 1, each mononuclear Tb
node connects four bidentate carboxyl groups from four ETTC4ꢀ
linkers, while each ETTC4ꢀ ligand links four Tb nodes to extend into
a two-dimensional (2D) network with the rhombic window of
22 Å ꢂ 14 Å in size (Figs. 1b and c). From the viewpoint of topology,
the Tb monomer can be simplified as a 4-connected node, and the
ETTC4ꢀ ligand can be viewed as a 4-connected linker. Thus, the 2D
layer can be identified as a 4,4-connected net with the sql topology
(Fig. 1d). Further, one set of parallel 2D nets are displaced with
each other by 7.1 Å in an AB stacking mode and intersect vertically
with another set of parallel 2D layers to generate a 2-fold
interpenetrated structure. And these interpenetrated 2D layers
encompass the 1D square channels with an aperture size of
2.5 Å ꢂ 2.5 Å (Figs. 1e and f). Overall, this framework exhibits a
2D-3D inclined interpenetrated structure [54–57].
The stability performance is one of the most important
prerequisites for practical applications of chemosensors. After
the treatment of exchanging DMAc with EtOH for 2 days, we
conduct the thermogravimetric analysis (TGA) on the activated
sample of 1. The result indicates that its framework does not
collapse unless the temperature reaches above 450 ꢃC, higher than
that of many reported MOFs (Fig. S7 in Supporting information).
Further, the variable temperature PXRD tests confirm that 1 can
retain its crystallinity until 340 ꢃC (Fig. S8 in Supporting informa-
tion). On the other hand, we also examine the chemical stability of
1 by immersing the as-synthesized sample (10 mg) in a series of
common organic solvents (10 mL; e.g., DMF, MeOH, EtOH, THF,
CH2Cl2, CH3CN, acetone and H2O) for 2 days, and in the aqueous
solutions (20 mL) with pH values ranging from 1 to 14 for 4 days.
Consequently, all the PXRD peaks are well remained (Fig. 2),
hinting that this MOF can maintain its crystallinity and structural
regularity in a broad range of chemical environments. To our
knowledge, such a degree of structural stability is rather rare in
MOF chemistry and it will undoubtedly pave the way to a wide
scope of applications.
Single-crystal X-ray diffraction analysis reveals that 1 crystal-
lizes in the tetragonal space group P4212 and the asymmetric unit
consists of one independent Tb3+ ion, a quarter of the H4ETTC
ligand, and two coordinated H2O molecules (Fig. 1a). Each Tb3+ ion
is ten-coordinated by four carboxyl groups from four individual
ETTC4ꢀ ligands and two water molecules, yielding an octahedral
Upon a 365 nm excitation, the as-obtained yellow-green block
crystal of 1 emits a bright fluorescence with the maximum around
477 nm (Fig. S9 in Supporting information), which is blue-shifted
by 50 nm relative to that of pure H4ETTC ligand (527 nm) [58]. This
is very common for TPE-based MOFs [34,36], as it indicates a more
twisted nonplanar structure of TPE moiety in rigid MOF
frameworks (Fig. S10 in Supporting information), due to the hinge
effect of metal-ligand coordination. It is worth noting that many
reported lanthanide MOFs exhibit the characteristic fluorescence
of Ln3+ ions, which is sensitized by the conjugated ligand via the
“antenna effect”. According to the Reinhold’s empirical rule, to
invoke the antenna effect, not only is the energy gap between the
singlet- and triplet-state of the ligand required to be larger than
5000 cmꢀ1, but also the energy gap between the triplet-state of the
ligand and the excited state of Ln3+ ion must be larger than
3000 cmꢀ1 [53,59,60]. However, for 1, the energy of the main
accepting level of Tb3+ ion is 20,500 cmꢀ1, while the HOMO-LUMO
bandgap of H4ETTC is only 2.49 eV (20,055 cmꢀ1) [58]. Evidently, 1
does not fulfill the theoretically lowest energy gap to activate the
antenna effect and thereby emits the fluorescence derived solely
from the AIE-active ligand.
Considering its strong luminescence and satisfactory water
resistance, we attempt to explore the sensing ability of 1 with
respect to a series of common ions in the aqueous solutions. A total
of thirteen anions (H2PO4ꢀ, Fꢀ, MoO42ꢀ, Clꢀ, NO3ꢀ, CO32ꢀ, Iꢀ, SO4
,
2ꢀ
WO42ꢀ, NO2ꢀ, Brꢀ, CrO42ꢀ, Cr2O72ꢀ) as well as twelve metal cations
(Pb2+, Li+, Mg2+, Zn2+, Cd2+, K+, Na+, Ca2+, Ni2+, Co2+, Cu2+, Fe3+) are
selected as the target analytes. For comparison, an equal amount of
finely ground powder sample of 1 is dispersed into the respective
Fig.1. (a) The asymmetric unit of 1 (A = ꢀy, ꢀx, ꢀz; B = y, x, ꢀz; C = ꢀx, ꢀy, z). (b) The
coordination environment of the ETTC4ꢀ ligand in 1. (c) The 2D layer in 1 with the sql
topology. (d) The simplified 2D layer. (e) The two-fold interpenetrated structure of 1
with the 1D rhombic channels running along the c direction. (f) The simplified two-
fold interpenetrated structure in 1. All the hydrogen atoms are omitted for clarity.
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Fig. 2. (a) The PXRD patterns of 1 soaked in different organic solvents for 2 days. (b)
The PXRD patterns of 1 immersed in HCl (pH 1ꢄ4) and NaOH aqueous solutions (pH
11ꢄ14) for 4 days.
aqueous solutions with different ions of the same concentration
(10 mmol/L). Under UV illumination at 365 nm, only the fluores-
cence signals of the suspensions containing Fe3+, CrO4
and
2ꢀ
Cr2O72ꢀ ions are acutely quenched, while the others display slight
or no discernible changes. Such a high-contrast “turn-off” signal
can be visually observed with the naked eyes (Fig. S11 in
Supporting information). As estimated based on the corresponding
emission spectra (Fig. S12 in Supporting information), the
2ꢀ
Fig. 3. Fluorescence titration experiments of 1 toward (a) CrO42ꢀ, (b) Cr2O7 and
quenching percentages (QPs) of the suspensions containing Fe3+
,
2ꢀ
2ꢀ
(c) Fe3+ in the aqueous solutions under a 365 nm UV lamp.
CrO4 and Cr2O7 are 99.7%, 99.8%, and 99.9%, respectively, far
beyond those of the other analytes, thus indicative of highly
selective quenching response of 1 toward different ions.
2ꢀ
CrO42ꢀ, Cr2O7 and Fe3+ show a linear relationship in the low
The sensing performances of 1 for CrVI and FeIII ions are further
assessed by the fluorescence titration experiments. As shown in
Fig. 3, the fluorescence intensity of the suspension decreases
concentration range, with the estimated Ksv values being
1.48 ꢂ 104 (R2 = 0.9925), 2.69 ꢂ 104 (R2 = 0.9920), and 3.50 ꢂ
103 L/mol (R2 = 0.9845), respectively, higher than many reported
2ꢀ
MOF-based sensors [62–64]. According to the formula of 3
s/Ksv
sharply and is fully quenched when the concentration of CrO4
reaches 31.3 ppm. The quenching trend of Cr2O7 is akin to the
case of CrO4 and the emission is entirely dimmed when the
(s
: standard error), the detection limits for CrO42ꢀ, Cr2O72ꢀ and Fe3
2ꢀ
+ are 68.18, 69.85 and 138.8 ppm, respectively, thus indicating that
1 can achieve the trace amount recognition of those heavy metal
ions. In addition to high selectivity and sensitivity, anti-interfer-
ence ability and recyclability are also important for practical
application of sensors. For 1, it is observed that the fluorescence
intensity of the suspension does not significantly alter in the
presence of other competing ions and reduces only after the
2ꢀ
2ꢀ
Cr2O7
concentration increases to 194 ppm. Slightly differing
from CrVI anions, the fluorescence signal of the suspension
containing Fe3+ moderately decreases in the early stage of the
titration and entirely turns off until the Fe3+ concentration is above
117 ppm. We also analyze the fluorescence quenching behavior of 1
in terms of the Stern-Volmer (S-V) equation: I0/I = Ksv[Q]+1. Herein,
I0 and I refer to the fluorescence intensity of the suspension before
and after the addition of the analyte, [Q] is the molar concentration
of the analyte, and Ksv denotes the quenching constant [61]. As
shown in Fig. S13 (Supporting information), all the S-V curves for
addition of CrO42ꢀ, Cr2O7 and Fe3+ ions (Fig. S14 in Supporting
2ꢀ
information). As for its recyclability, we testify that the QP of 1 can
keep almost unchanged during four consecutive sensing cycles
(Fig. S15 in Supporting information). These testaments strongly
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2ꢀ
suggest 1 to be a qualified fluorescent sensor toward CrO4
,
the selective luminescence sensing behavior of 1 to the synergistic
effect of competitive absorption and FRET mechanism [3].
Cr2O72ꢀ and Fe3+ ions. Since now, several luminescent MOFs have
been announced for detecting CrVI and FeIII ions, but many of them
are not applicable for the sensing in the aqueous environment, due
to their intrinsically weak stability [2,3,14,65].
In summary, we fabricate a new luminescent lanthanide MOF
by introducing an AIE-active ligand. Owing to the high coordina-
tion number of Tb3+ ion and its strong coordination bond with the
multidentate TPE-based ligand, this MOF (1) exhibits excellent
structural stability, which has been very rare in MOF chemistry.
Unlike many reported lanthanide MOFs, 1 emits the ligand-
centered fluorescence, as the excitation energy of the constituent
ligand is insufficient to sensitize the fluorescence of Tb3+ ion.
Notably, this complex displays a highly selective and sensitive
To unveil the underlying selective fluorescence “turn-off”
mechanism of 1, we first measure the PXRD patterns of the
undissolved sample after the immersion of 1 in the aqueous
solutions containing CrO42ꢀ, Cr2O7 and Fe3+ ions for 1 day. As
2ꢀ
illustrated by Fig. S16 (Supporting information), the sample
exhibits good crystallinity and phase purity, revealing that its
fluorescence “turn-off” sensing mechanism has no relation with
framework collapse. Next, the ICP analyses are also conducted on
the filtrate of the CrVI and FeIII-containing solutions before and
after adding 1, respectively. The results show that none or only a
negligible fraction of CrVI and FeIII ions enters into the channels of 1
after 24 h (Table S3 in Supporting information), ruling out the
influence of guest adsorption on the quenching behavior of 1.
Lastly, we compare the UV–vis absorption spectra of all the
analytes with reference to the excitation and emission spectra of 1
(Fig. 4). Evidently, in comparison to the negligible ones of the other
ions, the overlap between the absorption band of the Na2CrO4,
Na2Cr2O7 and Fe(NO3)3 solution and either the excitation or the
emission spectrum of 1 is considerably much larger. While the
former overlap (with the excitation spectrum of 1) indicates a
competitive absorption of the excitation energy between the
analytes and the MOF, the latter (with the emission band of 1)
implies the occurrence of fluorescence resonance energy transfer
(FRET) from the MOF to the analytes. Both the two types of spectral
overlap are fully coincident with the observed QPs trend of 1 in the
presence of different analytes. Therefore, we reasonably attribute
fluorescence quenching toward CrO42ꢀ, Cr2O7 and Fe3+, which
2ꢀ
can be visually observed without the interference by other
common ions. Thus, considering its exceptional stability
performance in the aqueous phase, we suggest that this material
has a good application prospect for sensing heavy metal ions in
wastewater.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21771113, 22001132), and China
Postdoctoral Science Foundation (No. 2019M651011).
Appendix A. Supplementary data
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
online version, at doi:https://doi.org/10.1016/j.cclet.2021.01.040.
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