Exploiting a new europium(III) coordination polymer based on a zwitterionic ligand as a fluorescent probe for uranyl cations

Article abstract of DOI:10.1177/1747519820932266

Two new isostructural lanthanide(III) coordination polymers based on an unreported zwitterionic ligand, namely, [Ln(ox)(L)]_n (ox = oxalate, HL = N,N'-dipropionic acid imidazolium, Ln = Eu or La), are synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, infrared spectroscopy, powder X-ray diffraction, and thermogravimetric analysis. The fluorescence properties of the europium coordination polymer are investigated. In addition, the europium-based coordination polymer is utilized for specific sensing of UO₂²⁺ ions, showing high selectivity, a fast response time (8 min) and high sensitivity with noticeable quenching (Ksv = 6.19 × 10⁴ M^?1) and limit of detection of 1.95 μM.

Full text of DOI:10.1177/1747519820932266

932266CHL0010.1177/1747519820932266Journal of Chemical ResearchCai et al.
research-article2020
Research Paper
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Exploiting a new europium(III) coordination
polymer based on a zwitterionic ligand as a
fluorescent probe for uranyl cations
https://doi.org/10.1177/1747519820932266
DOI: 10.1177/1747519820932266
journals.sagepub.com/home/chl
Bin Cai1 , Yu-Ning Meng2, Meng-En Zhu1 and Youming Yang2
Abstract
Two new isostructural lanthanide(III) coordination polymers based on an unreported zwitterionic ligand, namely, [Ln(ox)
(L)]n (ox = oxalate, HL = N,N'-dipropionic acid imidazolium, Ln = Eu or La), are synthesized under hydrothermal conditions
and characterized by single-crystal X-ray diffraction, infrared spectroscopy, powder X-ray diffraction, and thermogravimetric
analysis. The fluorescence properties of the europium coordination polymer are investigated. In addition, the europium-
based coordination polymer is utilized for specific sensing of UO22+ ions, showing high selectivity, a fast response time
(8min) and high sensitivity with noticeable quenching (Ksv = 6.19×104 M−1) and limit of detection of 1.95µM.
Keywords
crystal structure, fluorescence probe, lanthanides, uranyl cations, zwitterionic ligand
Date received: 24 February 2020; accepted: 15 May 2020
Two new LnCPs were synthesized under hydrothermal conditions and were characterized adequately. The LnCP 1 has
outstanding performance for the sensing of UO22+ ions with high selectivity, a fast response time, and a high sensitivity
with noticeable quenching, which could be an efficient fluorescence probe for monitoring of environmental water.
1School of Chemistry and Chemical Engineering, Zhoukou Normal
University, Zhoukou, P.R. China
Corresponding author:
Bin Cai, School of Chemistry and Chemical Engineering, Zhoukou
Normal University, Zhoukou 466001, P.R. China.
Email: caib@actinide.org
2School of Metallurgy and Chemical Engineering, Jiangxi University of
Science and Technology, Ganzhou, P.R. China

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Journal of Chemical Research 00(0)
Table 1. Crystallographic data for 1 and 2..
Introduction
Compound
1
2
Lanthanide coordination polymers (LnCPs, sometimes
including lanthanide organic frameworks (LnOFs)) have
attracted considerable attention from a scientific and tech-
nological point of view due to their distinctive optical, elec-
tronic, and magnetic properties.1–3 Many lanthanide ions,
particularly Eu(III) and Tb(III) ions, can act as the emissive
centers to afford types of phosphors,4 which have opened
many opportunities in growing fields of social and eco-
nomic impact, for example, as luminescent materials, in
medical diagnostics, in environmental sciences and in cell
biology.5–8 They have also been extensively used in many
practical fields such as lighting and displays. Luminescent
lanthanide ions or LnCPs possess several virtues, such as
strong emission bands for f–f transitions generated via the
“antenna effect”,9 wide spectral range, broad range life-
times, high quantum yields, and so on.10 Due to their unique
luminescence properties, LnCPs have potential applica-
tions as fluorescent probes, which have been extensively
studied for selectively probing nitro explosives, toxic
organic solvents, and heavy-metal ions.11–13
Temperature (K)
Formula
λ(Å)
296(2)
296(2)
C11H11LaN2O8
0.71073
438.12
monoclinic
P21/c
9.035(3)
16.026(5)
9.680(3)
90
105.395(5)
90
1351.3(7)
4
26.57
2.1534
3.207
C11H11EuN2O8
0.71073
451.18
Monoclinic
P21/c
9.0472(11)
15.5089(19)
9.4523(12)
90
105.062(3)
90
1280.7(3)
4
23.19
2.3398
4.945
Mr
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
Max 2θ (°)
ρ calcd/ (g cm−3)
μ(MoKα) (mm−1)
F(000)
Crystal size (mm3)
Collected reflections
Unique reflections (Rint)
Parameters
Goodness of fit (F2)
R1/wR2 [I > 2σ(I)]
R1/wR2 (all data)
872
5840
0.28×0.12×0.10 0.2×0.12×0.10
872
848
2399 (0.0367)
199
1.1103
0.0272/0.0647
0.0376/0.0750
Well known as the key raw material in the nuclear
energy industry and other important industrial applica-
tions, radioactive and chemically toxic uranium mainly
exists in its hexavalent state with very high solubility in
solution and is widely distributed in different environ-
ments such as various forms of water, air, dust, soil, and
2306 (0.0000)
187
1.0187
0.0331/0.0749
0.0424/0.0786
rock.14,15 The chemical toxicity and radioactivity of ura- a R1 = ∑||Fo|―|Fc||/∑|Fo|, b wR2 = {∑[w(Fo ―Fc2)2]/∑[w(Fo )2]}1/2
nium poses a significant threat to biological systems, lead-
.
2
2
ing to irreversible damage.16,17 Therefore, it is vital to
compounds are isostructural. The single-crystal X-ray dif-
fraction analysis revealed that 1 crystallizes in the mono-
clinic space group P21/c and is composed of binuclear
[Eu2(ox)4(COO)4] subunits that are interconnected by L
anions and oxalates. Stoichiometrically, the asymmetric
unit of 1 contains one Eu(III) center, one oxalate, and one L
anion with both ligand types being completely deproto-
nated. As shown in Figure 1(a), the central Eu1 atom is
located on a crystallographic center of symmetry and has a
nona-coordination environment formed by nine O atoms [
O1, O2, O3i, O4i, O5, O5ii, O6ii, O7iii, and O8iv; symmetry
codes: (i) x, 0.5-y, -0.5+z; (ii) 1-x, 1-y, -z; (iii) x, y, -1+z;
(iv) 1-x, 1-y, 1-z.] from four different oxalates and four dif-
ferent L anions, with Eu—O bond lengths in the range
2.410 (6)–2.706 (3) Å. As shown in Figure 1(b), the central
Eu1 ion and Eu1ii ion in the binuclear [Ln2(ox)4(COO)4]
SBUs (secondary building units) are bridged by two car-
boxylate groups in a µ2-1η:η1 coordination mode and by
two carboxylate groups in a µ2-2η:η1 coordination mode,
with Eu···Eu separation of 4.0882 (6) Å. The Eu1—O—
Eu1ii angle is 106.771(228)o for the µ2-2η:η1 carboxylate
groups. When oxalates adopt a bidentate mode and act as a
µ2-node coordinating with two Eu(III) ions, the L anions
adopt a µ4-node connecting four Eu(III) ions. Each
[Ln2(ox)4(COO)4] SBU in 1 and 2 is eight-coordinated to
six neighboring bi-lanthanide clusters by four oxalates and
four L anions. These interactions extend the structure into
develop effective, quick, and simple analytical methods
for detecting UO22+ in different samples.
There are a few reports describing the use of LnCPs to
2+
detect or uptake UO2 at low concentrations and with
high sorption capacity and high sensitivity by fluores-
cence quenching.18–23 One of the most important features
for sensing uranyl cations is the abundance of open Lewis
basic sites.18 Encouraged by these earlier reports, we
have synthesized two new LnCPs based on a novel zwit-
terionic N,N'-dipropionic acid imidazolium ligand as a
more flexible organic linker, together with oxalic acid
under hydrothermal conditions. The products, LnCPs,
[Ln(ox)(L)]n (Ln = Eu, 1 and La, 2) are highly thermal
stable and exhibit very strong fluorescence. To the best of
our knowledge, the two title LnCPs have not been
reported to date. Products 1 and 2 were characterized by
single-crystal X-ray diffraction, infrared (IR) spectros-
copy, powder X-ray diffraction, and thermogravimetric
analysis. Moreover, the fluorescence properties of 1 have
also been investigated. Compound 1 was also explored as
a fluorescence probe for detecting trace uranyl in solution
by fluorescence quenching.
Results and discussion
The crystal structures of complexes 1 and 2
The crystallographic data are shown in Table 1 for 1 and 2. an infinite 2D CP (coordination polymer) along the bc
Taking 1 as an example, owing to the fact that both title plane (Figure 1(c)). Besides, it can also be seen that

Cai et al.
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Figure 1. (a) The coordination environment of the Ln ion, with displacement ellipsoids at the 50% probability level. [Symmetry
codes: (i) x, 0.5-y, -0.5+z; (ii) 1-x, 1-y, -z; (iii) x, y, -1+z; (iv) 1-x, 1-y, 1-z.] (b) The binuclear [Ln2(ox)4(COO)4] SBU of [Ln(ox)(L)]n.
(c) The 2D structure of [Ln(ox)(L)]n along the bc plane. H atoms have been omitted for clarity.
[Ln2(ox)4(COO)4] SBUs are connected by oxalates to gen- originate from air because the zwitterionic ligand has a
erate a 2D pore structure and the L anions link the very strong ability to absorb moisture. The bands from
[Ln2(ox)4(COO)4] SBUs, and at the same time are used to 1630 to 1410cm−1 correspond to the v(C=C) and v(C=N)
fill the pores to strengthen the stability of the structure. It is vibrations of the zwitterionic ligands. The bands from 1360
noteworthy that there are no coordination or crystal water to 1160cm−1 correspond to the v(HC—H) vibrations of the
molecules in the structures of 1 and 2, which is a not very zwitterionic ligands. Furthermore, the absorption peak
common phenomenon in lanthanide complexes.24,25 This is located at 1160cm−1 can be attributed to the C—C vibration
one of the reasons why 1 and 2 have much high thermal of the zwitterionic ligand. Also, medium-intensity bands at
stability. However, we found that there is trace water in the 840–750cm−1 were assigned to the bending frequencies of
title LnCPs, which was confirmed by IR spectroscopy and the O—C=O group of the carboxyl groups.
thermogravimetric analysis. Meanwhile, the elemental
analysis results indicate that the measured value of hydro-
gen was a little higher than the theoretical value. A reason-
Powder X-ray diffraction (PXRD)
able explanation is that the zwitterionic ligand has a very The PXRD patterns for 1 and 2 are shown in Figure 2 and
strong ability to absorb moisture. All bond lengths and Supplemental Figure S2. The simulated patterns of 1 and 2
angles of the title compound are comparable to its were calculated from their single-crystal X-ray data with
analogs.26–28
Mercury 4.0 software. The diffraction peaks of the simu-
lated and the experimental patterns of 1 match well and the
result of 2 is the same as 1, which confirms the phase purity
of 1 and 2.
IR spectra
From Supplemental Figure S1, it is found that in the IR
spectra of zwitterionic ligand HL (HL = N,N'-dipropionic
acid imidazolium), 1 and 2, the peaks at 3450–3000cm−1
Thermogravimetric analysis (TGA)
appearing in the three samples are attributed to the O—H Because LnCPs 1 and 2 are isostructural, only the TGA of
stretching vibrations of water molecules that do not exist. A 1 was investigated. The TGA curve of 1 shows a two-step
reasonable explanation is that these water molecules weight loss in the range of 298–1073K (Supplemental

4
Journal of Chemical Research 00(0)
5D0→7F0, 5D0→7F1, 5D0→7F2, 5D0→7F3, and 5D0→7F4 tran-
sitions, respectively. For LnCP 1, the strongest band is the
5D0→7F1 transition at wavelength 614nm and the weakest
band is the 5D0→7F2 transition at wavelength 650nm.
Fluorescence probing studies
To investigate the influence of different metal ions on the
fluorescence of 1 and to explore the selectivity for uranyl
ions, a series of fluorescence spectra (emission at 320nm)
were recorded, with a total of 15 different competing metal
ions (0.5mM), including Co2+, K+, Ba2+, Ni2+, Mg2+,
Zn2+, Ca2+, Ag+, Na+, Pb2+, Cd2+, Cr3+, Cu2+, Al3+, and
UO22+. As demonstrated in Figure 4(a) and (b), there was a
2+
significant quenching effect with UO2 and all the com-
peting metal ions had little effect on the intensity of the
fluorescence of 1, which demonstrated the specific recogni-
tion of 1 toward UO22+ because of the presence of abundant
Lewis basic sites in LnCP 1. The quenching percentage of
polymer 1 toward UO22+ was evaluated to be 97.25%. It is
worth noting that the fluorescent intensities of 1 are highly
dominated by the concentration of the uranyl solution as
illustrated in Figure 4(c). The quenching progress could be
described by the Stern-Volmer equation. As shown in
Figure 4(d), the linear range of the curve was 5 to 95µM
and the R2 value was 0.999. The Ksv value of LnCP 1 for
uranyl cations calculated to be 6.19×104 M−1. To the best
of our knowledge, the value of Ksv in this work is much
larger than these Ksv values in earlier reports for detection
uranyl cations used LnCP or LnOF.21,23 The limit of detec-
tion (LOD, LOD = 3σ/slope, in which σ is the standard
deviation of multiple measurements of a blank sample and
the slope corresponds to the linear relationship between the
fluorescence intensity) was calculated to be 1.95µM, which
is lower than those of the previously reported LnCP- or
LnOF-based luminescent sensors.18,19,22,23 Also the detec-
tion of uranyl cations needs to be improved, LnCP 1 still
displays a good ability of the material to detect trace uranyl
in water solution. As shown in Figure 2, the PXRD patterns
of LnCP 1 treated by soaking in H2O or EtOH for 48h were
recorded. The PXRD results indicated that 1 can maintain
its crystallinity under such conditions, which attests to the
outstanding stability of LnCP 1.
Figure 2. The experimental, simulated, saturated in H2O for
48h and saturated in EtOH for 48h PXRD patterns for LnCP 1.
Figure 3. The fluorescence spectrum of 1 at room
temperature with excitation at 320nm.
Figure S3). The first step weight loss (3.63%) from room
temperature to 663K was assigned to the release of water
molecules from air because of its strong ability to absorb
moisture, which indicates the very high thermal stability
of 1. The second step weight loss is 61.06% (calcd
63.78%) attributed to the collapse of organic oxalate com-
ponents and zwitterionic ligands from 663 to 942K. When
Conclusion
the temperature is higher than 942K, there is no obvious Two new LnCPs were synthesized under hydrothermal
weight loss.
conditions using an unreported zwitterionic ligand and
oxalate as co-ligands. The two title compounds were char-
acterized adequately with highly thermal stability. Single-
crystal measurement revealed that the two LnCPs are 2D
Fluorescence properties
The fluorescence spectrum of 1 was measured at room tem- isostructural constructed by [Ln2(ox)4(COO)4] SBUs. The
perature. LnCP 1 displays strong red fluorescence upon LnCP 1 emits strong red fluorescence. The LnCP 1 was
excitation at wavelength 320nm. As shown in Figure 3, the exploited for sensing of UO22+ ions, resulted a high selec-
emission bands of 1 show five typical emission peaks tivity, a fast response time, and a high sensitivity with
(5D0→7FJ, J = 0-4) between 570 and 700nm, which are noticeable quenching. Thus, the LnCP 1 could be devel-
ascribed to the Eu3+ cation. The five bands due to the Eu3+ oped as an efficient fluorescence probe for monitoring of
ion at 592, 614, 650, 686, and 696nm are attributed to the environmental water.

Cai et al.
5
Figure 4. (a) Bar chart obtained for LnCP 1 upon addition of various metal ions. (b) Fluorescence spectra of 1 upon addition of
various metal ions when excited at 320nm. (c) Fluorescence spectra of 1 upon addition of various UO22+ concentrations when
excited at 320nm. (d) The linear fitting of the Stern–Volmer plot of UO22+
.
modification.29 At room temperature, 3-aminopropanoic
acid (8.9g, 100mmol) was mixed with 40% glyoxal (7.32g,
50mmol, w/w in water) and stirred under air. After 15min,
the reaction solution became cloudy and dark brown. After
stirring for 2h, the mixture was filtered, and washed with
ice-cold water (3×15mL). Following drying under vac-
uum, a yellow-brown powder was obtained, yield 56.8%
(based on γ-aminobutyric acid), which was characterized
by IR spectroscopy (Supplemental Figure S1).
Experimental
Materials and methods
All of the reagents are commercially available and were
used without further purification. All of the metal salts used
are nitrate salts. Other reagents and solvents were pur-
chased from commercial sources and were used without
further purification. Elemental analyses (C, H and N) were
carried out using a Vario EL III elemental analyzer. The IR
spectrum was recorded as a KBr pellet in the range 4000–
400cm−1 on a Smart Omni-Transmission spectrometer.
PXRD data were recorded on a Philips X-Pert-MPD dif-
Synthesis of [Ln(ox)(L)]n (1 and 2)
fractometer with CuKα (λ = 1.5406Å) radiation in the 2θ A mixture of Ln(NO3)3·6H2O (1 mmol, Ln = Eu, 446mg,
range (5–50o) at a rate of 5o min−1. Thermogravimetric 1; La, 433mg, 2), oxalic acid (1mmol, 90mg), HL (1mmol,
analysis was carried out on a Mettler TGA/SDTA 851 ther- 212mg), and H2O (10mL) was transferred to a 30mL
mal analyzer in the temperature range 298–1073K under an Teflon-lined stainless steel autoclave and was heated under
N2 flow with a heating rate of 10K min−1. The fluorescence autogenous pressure at 443K for 72h and then cooled to
spectrum of 1 was recorded on a HITACHI F-4600 spectro- room temperature naturally. Yellow block-shaped crystals
photometer at room temperature.
suitable for single-crystal X-ray diffraction were collected
by filtration, washed with distilled H2O and dried in air
(yield 81% and 76%, based on La and Eu, respectively).
Analysis for 1: C11H11EuN2O8: Calcd (%): C, 29.26; H,
Synthesis of ligand (HL)
The new zwitterionic ligand HL (Supplemental Scheme 2.44; N, 6.21. Found (%): C, 28.97; H, 2.69; N, 6.04. IR
S1) was synthesized according to the reference with slight (KBr, cm−1; s = strong, m = medium and w = weak): 3460

6
Journal of Chemical Research 00(0)
Table 2. Selected bond lengths (Å) for compounds 1 and 2.
1
2
Eu(1)—O(1)
Eu(1)—O(3)i
Eu(1)—O(5)
Eu(1)—O(6)ii
Eu(1)—O(8)iv
2.055(14)
2.083(12)
2.039(13)
2.068(14)
Eu(1)—O(2)
Eu(4)—O(1)i
Eu(1)—O(6)ii
Eu(1)—O(7)iii
1.971(13)
2.042(12)
2.071(14)
2.008(12)
La(1)—O(1)
La(1)—O(3)i
La(1)—O(5)ii
La(1)—O(6)
La(1)—O(8)iv
2.522(3)
2.512(4)
2.584(4)
2.756(3)
2.569(4)
La(1)—O(2)
La(1)—O(4)i
La(1)—O(6)ii
La(1)—O(7)iii
2.523(3)
2.531(3)
2.493(3)
2.573(4)
Symmetry codes: 1 and 2: #i x, 3/2-y, 1/2+z; #ii 1-x, 1-y, -z; #iii x, y, -1+z; #iv 1-x, 1-y, 1-z.
(s), 3420 (s), 3140 (s), 3090 (m), 3010 (w), 1670 (w), 1610 Declaration of conflicting interests
(s), 1440 (m), 1400 (w), 1310 (m), 1170 (s), 1130 (s), 1020
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
(m), 936 (m), 886(m), 867 (m), 837 (s), 778 (s), 630 (s), 482
(w), 424 (w). Analysis for 2: C11H11LaN2O8: Calcd (%): C,
30.13; H, 2.51; N, 6.39. Found (%): C, 29.76; H, 2.74; N,
6.12. IR: 3410 (s), 3140 (s), 3090 (m), 3030 (w), 1690 (w),
1620 (s), 1450 (m), 1380 (w), 1320 (m), 1180 (s), 1140 (s),
1010 (m), 946 (m), 906 (w), 886 (m), 847 (s), 788 (s), 640
(s), 493 (w), 424 (w).
Funding
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: This
study was supported by the Key Scientific and Technological
Research Projects in Henan Province (192102210028) and the
National Natural Science Foundation of China (51602358,
51564017, and 51774155).
Single-crystal structure determination
The X-ray diffraction data for 1 and 2 were collected on a
Bruker Smart Apex II CCD equipped with a Mo-Kα radi-
ation source (λ = 0.71073\%A). The data were integrated
by using the SAINT program.30 The program SADABS
was used for the absorption correction.31 The structures
were solved by direct methods and refined on F2 by full-
matrix least-squares methods using the Olex2 and
SHELXTL program package.32,33 All hydrogen atoms
were refined isotropically as a riding mode using the
default SHELXTL parameters. All non-hydrogen atoms
were refined anisotropically, with C—H = 0.93–0.97Å
and Uiso(H) = 1.2Ueq(C). The maximum residual electron-
density peak and hole were located at −0.783 and 1.049eÅ3
for the Eu1 atom and at −0.927 and 1.265eÅ3 for the La1
atom, respectively. The crystallographic data and struc-
tural refinements and the selected bond lengths for 1 and
2 are shown in Tables 1 and 2, respectively. CCDC
1833928 and 1833929 contain the supplementary crystal-
lographic data. The data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
ORCID iD
Bin Cai
https://orcid.org/0000-0002-3573-5651
Supplementary material
Supplemental material for this article is available online.
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form the tested suspensions, with the uranyl concentration
ranging from 0 to 95µM/L, after ultrasonic treatment for
5min. To examine the influence of competing metal ions,
the uranyl solution was replaced by 10mM of different
metal nitrates to form the tested suspensions with 50µM
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