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

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 (8 min) and high sensitivity with noticeable quenching (Ksv = 6.19 × 10⁴ M⁻¹) and limit of detection of 1.95 µM.

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.

Introduction

Lanthanide coordination polymers (LnCPs, sometimes including lanthanide organic frameworks (LnOFs)) have attracted considerable attention from a scientific and technological point of view due to their distinctive optical, electronic, 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 economic 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 lifetimes, high quantum yields, and so on. ¹⁰ Due to their unique luminescence properties, LnCPs have potential applications as fluorescent probes, which have been extensively studied for selectively probing nitro explosives, toxic organic solvents, and heavy-metal ions.^11–13

Well known as the key raw material in the nuclear energy industry and other important industrial applications, radioactive and chemically toxic uranium mainly exists in its hexavalent state with very high solubility in solution and is widely distributed in different environments such as various forms of water, air, dust, soil, and rock.^14,15 The chemical toxicity and radioactivity of uranium poses a significant threat to biological systems, leading to irreversible damage.^16,17 Therefore, it is vital to develop effective, quick, and simple analytical methods for detecting UO₂²⁺ in different samples.

There are a few reports describing the use of LnCPs to detect or uptake UO₂²⁺ at low concentrations and with high sorption capacity and high sensitivity by fluorescence 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 zwitterionic 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) spectroscopy, 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. Taking 1 as an example, owing to the fact that both title compounds are isostructural. The single-crystal X-ray diffraction analysis revealed that 1 crystallizes in the monoclinic 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 deprotonated. 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 O8^iv; 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 different 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 carboxylate 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 an infinite 2D CP (coordination polymer) along the bc plane (Figure 1(c)). Besides, it can also be seen that [Ln₂(ox)₄(COO)₄] SBUs are connected by oxalates to generate a 2D pore structure and the L anions link the [Ln₂(ox)₄(COO)₄] SBUs, and at the same time are used to fill the pores to strengthen the stability of the structure. It is noteworthy that there are no coordination or crystal water molecules in the structures of 1 and 2, which is a not very common phenomenon in lanthanide complexes.^24,25 This is one of the reasons why 1 and 2 have much high thermal stability. However, we found that there is trace water in the title LnCPs, which was confirmed by IR spectroscopy and thermogravimetric analysis. Meanwhile, the elemental analysis results indicate that the measured value of hydrogen was a little higher than the theoretical value. A reasonable explanation is that the zwitterionic ligand has a very strong ability to absorb moisture. All bond lengths and angles of the title compound are comparable to its analogs.^26–28

Table 1.

Crystallographic data for 1 and 2..

Compound	1	2
Temperature (K)	296(2)	296(2)
Formula	C11H11EuN2O8	C11H11LaN2O8
λ(Å)	0.71073	0.71073
Mr	451.18	438.12
Crystal system	Monoclinic	monoclinic
Space group	P21/c	P21/c
a (Å)	9.0472(11)	9.035(3)
b (Å)	15.5089(19)	16.026(5)
c (Å)	9.4523(12)	9.680(3)
α (°)	90	90
β (°)	105.062(3)	105.395(5)
γ (°)	90	90
V (Å3)	1280.7(3)	1351.3(7)
Z	4	4
Max 2θ (°)	23.19	26.57
ρ calcd/ (g cm−3)	2.3398	2.1534
μ(MoKα) (mm−1)	4.945	3.207
F(000)	872	5840
Crystal size (mm3)	0.28 × 0.12 × 0.10	0.2 × 0.12 × 0.10
Collected reflections	872	848
Unique reflections (Rint)	2306 (0.0000)	2399 (0.0367)
Parameters	187	199
Goodness of fit (F2)	1.0187	1.1103
R1/wR2 [I > 2σ(I)]	0.0331/0.0749	0.0272/0.0647
R1/wR2 (all data)	0.0424/0.0786	0.0376/0.0750

a R1 = ∑||Fo|―|Fc||/∑|Fo|, b wR₂ = {∑[w(F_o²―F_c²)²]/∑[w(F_o²)²]}^1/2.

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.

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 3450–3000 cm⁻¹ appearing in the three samples are attributed to the O—H stretching vibrations of water molecules that do not exist. A reasonable explanation is that these water molecules originate from air because the zwitterionic ligand has a very strong ability to absorb moisture. The bands from 1630 to 1410 cm⁻¹ correspond to the v(C=C) and v(C=N) vibrations of the zwitterionic ligands. The bands from 1360 to 1160 cm⁻¹ correspond to the v(HC—H) vibrations of the zwitterionic ligands. Furthermore, the absorption peak located at 1160 cm⁻¹ can be attributed to the C—C vibration of the zwitterionic ligand. Also, medium-intensity bands at 840–750 cm⁻¹ were assigned to the bending frequencies of the O—C=O group of the carboxyl groups.

Powder X-ray diffraction (PXRD)

The PXRD patterns for 1 and 2 are shown in Figure 2 and Supplemental Figure S2. The simulated patterns of 1 and 2 were calculated from their single-crystal X-ray data with Mercury 4.0 software. The diffraction peaks of the simulated 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.

Figure 2.

The experimental, simulated, saturated in H₂O for 48 h and saturated in EtOH for 48 h PXRD patterns for LnCP 1.

Thermogravimetric analysis (TGA)

Because LnCPs 1 and 2 are isostructural, only the TGA of 1 was investigated. The TGA curve of 1 shows a two-step weight loss in the range of 298–1073 K (Supplemental Figure S3). The first step weight loss (3.63%) from room temperature to 663 K 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 components and zwitterionic ligands from 663 to 942 K. When the temperature is higher than 942 K, there is no obvious weight loss.

Fluorescence properties

The fluorescence spectrum of 1 was measured at room temperature. LnCP 1 displays strong red fluorescence upon excitation at wavelength 320 nm. As shown in Figure 3, the emission bands of 1 show five typical emission peaks (⁵D₀→⁷F_J, J = 0-4) between 570 and 700 nm, which are ascribed to the Eu³⁺ cation. The five bands due to the Eu³⁺ ion at 592, 614, 650, 686, and 696 nm are attributed to the ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃, and ⁵D₀→⁷F₄ transitions, respectively. For LnCP 1, the strongest band is the ⁵D₀→⁷F₁ transition at wavelength 614 nm and the weakest band is the ⁵D₀→⁷F₂ transition at wavelength 650 nm.

Figure 3.

The fluorescence spectrum of 1 at room temperature with excitation at 320 nm.

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 320 nm) were recorded, with a total of 15 different competing metal ions (0.5 mM), 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 significant quenching effect with UO₂²⁺ and all the competing metal ions had little effect on the intensity of the fluorescence of 1, which demonstrated the specific recognition 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 95 µM and the R² value was 0.999. The K_sv value of LnCP 1 for uranyl cations calculated to be 6.19 × 10⁴ M⁻¹. To the best of our knowledge, the value of K_sv in this work is much larger than these K_sv values in earlier reports for detection uranyl cations used LnCP or LnOF.^21,23 The limit of detection (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 1.95 µM, which is lower than those of the previously reported LnCP- or LnOF-based luminescent sensors.^18,19,22,23 Also the detection 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 48 h 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 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 320 nm. (c) Fluorescence spectra of 1 upon addition of various UO₂²⁺ concentrations when excited at 320 nm. (d) The linear fitting of the Stern–Volmer plot of UO₂²⁺.

Conclusion

Two new LnCPs were synthesized under hydrothermal conditions using an unreported zwitterionic ligand and oxalate as co-ligands. The two title compounds were characterized adequately with highly thermal stability. Single-crystal measurement revealed that the two LnCPs are 2D isostructural constructed by [Ln₂(ox)₄(COO)₄] SBUs. The LnCP 1 emits strong red fluorescence. The LnCP 1 was exploited for sensing of UO₂²⁺ ions, resulted a high selectivity, a fast response time, and a high sensitivity with noticeable quenching. Thus, the LnCP 1 could be developed as an efficient fluorescence probe for monitoring of environmental water.

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 purchased 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 4000–400 cm⁻¹ on a Smart Omni-Transmission spectrometer. PXRD data were recorded on a Philips X-Pert-MPD diffractometer with CuKα (λ = 1.5406 Å) radiation in the 2θ range (5–50^o) at a rate of 5^o min⁻¹. Thermogravimetric analysis was carried out on a Mettler TGA/SDTA 851 thermal analyzer in the temperature range 298–1073 K under an N₂ flow with a heating rate of 10 K min⁻¹. The fluorescence spectrum of 1 was recorded on a HITACHI F-4600 spectrophotometer at room temperature.

Synthesis of ligand (HL)

The new zwitterionic ligand HL (Supplemental Scheme S1) was synthesized according to the reference with slight modification. ²⁹ At room temperature, 3-aminopropanoic acid (8.9 g, 100 mmol) was mixed with 40% glyoxal (7.32 g, 50 mmol, w/w in water) and stirred under air. After 15 min, the reaction solution became cloudy and dark brown. After stirring for 2 h, the mixture was filtered, and washed with ice-cold water (3 × 15 mL). Following drying under vacuum, a yellow-brown powder was obtained, yield 56.8% (based on γ-aminobutyric acid), which was characterized by IR spectroscopy (Supplemental Figure S1).

Synthesis of [Ln(ox)(L)]_n (1 and 2)

A mixture of Ln(NO₃)₃·6H₂O (1 mmol, Ln = Eu, 446 mg, 1; La, 433 mg, 2), oxalic acid (1 mmol, 90 mg), HL (1 mmol, 212 mg), and H₂O (10 mL) was transferred to a 30 mL Teflon-lined stainless steel autoclave and was heated under autogenous pressure at 443 K for 72 h and then cooled to room temperature naturally. Yellow block-shaped crystals 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, 2.44; N, 6.21. Found (%): C, 28.97; H, 2.69; N, 6.04. IR (KBr, cm⁻¹; s = strong, m = medium and w = weak): 3460 (s), 3420 (s), 3140 (s), 3090 (m), 3010 (w), 1670 (w), 1610 (s), 1440 (m), 1400 (w), 1310 (m), 1170 (s), 1130 (s), 1020 (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).

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 radiation 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 = 0.93–0.97 Å and U_iso(H) = 1.2U_eq(C). The maximum residual electron-density peak and hole were located at −0.783 and 1.049 eų for the Eu1 atom and at −0.927 and 1.265 eų for the La1 atom, respectively. The crystallographic data and structural 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 crystallographic data. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Table 2.

Selected bond lengths (Å) for compounds 1 and 2.

1				2
Eu(1)—O(1)	2.055(14)	Eu(1)—O(2)	1.971(13)	La(1)—O(1)	2.522(3)	La(1)—O(2)	2.523(3)
Eu(1)—O(3)i	2.083(12)	Eu(4)—O(1)i	2.042(12)	La(1)—O(3)i	2.512(4)	La(1)—O(4)i	2.531(3)
Eu(1)—O(5)	2.039(13)	Eu(1)—O(6)ii	2.071(14)	La(1)—O(5)ii	2.584(4)	La(1)—O(6)ii	2.493(3)
Eu(1)—O(6)ii	2.068(14)	Eu(1)—O(7)iii	2.008(12)	La(1)—O(6)	2.756(3)	La(1)—O(7)iii	2.573(4)
Eu(1)—O(8)iv				La(1)—O(8)iv	2.569(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.

Fluorescence probing measurement

Finely ground crystalline material of 1 (5 mg) was dispersed in EtOH (5 mL) to make a homogeneous suspension after ultrasonic treatment for about 15 min. Different volumes of 10 mM UO₂²⁺ aqueous solution were added to form the tested suspensions, with the uranyl concentration ranging from 0 to 95 µM/L, after ultrasonic treatment for 5 min. To examine the influence of competing metal ions, the uranyl solution was replaced by 10 mM of different metal nitrates to form the tested suspensions with 50 µM competing metal ions, respectively. To eliminate errors, all of the fluorescence spectra were measured in parallel three times to obtain average data under the same conditions.

Supplemental Material

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

Supplemental material, CaiB for Exploiting a new europium(III) coordination polymer based on a zwitterionic ligand as a fluorescent probe for uranyl cations by Bin Cai, Yu-Ning Meng, Meng-En Zhu and Youming Yang in Journal of Chemical Research

Supplemental Material

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

Supplemental material, checkcif for Exploiting a new europium(III) coordination polymer based on a zwitterionic ligand as a fluorescent probe for uranyl cations by Bin Cai, Yu-Ning Meng, Meng-En Zhu and Youming Yang in Journal of Chemical Research

Supplemental Material

Supporting_Information_3 – Supplemental material for Exploiting a new europium(III) coordination polymer based on a zwitterionic ligand as a fluorescent probe for uranyl cations

Supplemental material, Supporting_Information_3 for Exploiting a new europium(III) coordination polymer based on a zwitterionic ligand as a fluorescent probe for uranyl cations by Bin Cai, Yu-Ning Meng, Meng-En Zhu and Youming Yang in Journal of Chemical Research