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Efficient and selective sensing of Cu2+ and UO22+ by a europium metal-organic framework
Talanta 196 (2019) 515–522
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Efficient and selective sensing of Cu2+ and UO22+ by a europium metal-organic framework
T
Wei Liu, Yanlong Wang, Liping Song, Mark A. Silver, Jian Xie, Linmeng Zhang, Lanhua Chen, ⁎ Juan Diwu, Zhifang Chai, Shuao Wang State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China
ARTICLE INFO
ABSTRACT
Keywords: Luminescent metal organic frameworks Adsorption Copper and uranium contamination Detection
We report here the investigation of using a luminescent europium organic framework, [Eu2(MTBC) (OH)2(DMF)3(H2O)4]·2DMF·7H2O (denoted as compound 1), for detecting of both Cu2+ and UO22+ with high sensitivity. Based on the spectroscopy analysis, compound 1 could selectively respond to Cu2+ and UO22+ ions among other selected monovalent, divalent, trivalent metal cations based on a turn-off mechanism. The detection limit of compound 1 towards Cu2+ ion was as low as 17.2 μg/L, which is much lower than the maximum tolerable concentration of Cu2+ in drinking water (2 mg/L) defined by United States Environmental Protection Agency. On the other hand, the detection limit towards UO22+ ions is 309.2 μg/L, which could be used for detecting uranium in relative severely contaminated areas. The concentration-dependent luminescence intensity evolution process could be fully understood by the absorption kinetics and isotherm investigations. Furthermore, the quenching mechanism was elucidated by the UV-vis, excitation, luminescence, and lifetime studies. Compound 1, as the first MOF based luminescence probe for both Cu2+ and UO22+ ions, provides insight into developing MOF-based multifunctional sensors for both nonradioactive and radioactive elements.
1. Introduction Metal-organic frameworks (MOFs) are freshly raised extended crystalline structures which assemble by replaceable cation nodes and organic linkers [1]. Owing to their unparalleled tunability and structural diversity, MOFs are extensively realized for a variety of applications, such as gas storage and separation [2,3], controlled drug delivery and release [4,5], heterogeneous catalysis [6], proton conductivity [7,8], as well as luminescent materials [9–11]. Particularly, luminescent MOFs, which generally involve an emissive organic ligand or metal center, display promising potential as luminescent probes toward heavy metal cations [12,13], toxic organic pollutants [14,15], UV light [16], X-rays and γ-radiation [17], and so on. These MOF-based probes are primarily aimed at developing eco-friendly, low cost, versatile, sensitive, and multifunctional utility [18,19]. Amongst them, multifunctional MOFs have been widely investigated for various
applications, especially for luminescent MOF-based metal cation probes. However, these probes are rarely utilized for the recognition of both radioactive and nonradioactive elements. Contamination from uranium sources has become a global concern due to the rapid expansion of nuclear science and technology during the last century. This element could be readily found in various publically accessible industries, such as uranium mining and processing, glass manufacturing, and nuclear power stations, which all share a significant hazard to human health. Radiotoxicity accompanied by chemotoxicity is what makes the ingestion or inhalation of uranium a problem that could lead to serious kidney damage [20], disruption of bioactive molecules (enzymes) [21], as well as a series of other violent health problems [22–24]. Contrary to uranium, copper is nonradioactive, however, it can still could induce serious health issues because of its heavy metal nature. Excess intake of copper may cause severe neurodegenerative diseases, such as hematological
Abbreviations: MOFs, Metal-organic frameworks; ICP-MS, inductively coupled plasma mass spectrometry; ICP-AES, inductively coupled plasma atomic emission spectroscopy; H4MTBC, 4′,4′,4′,4′-methanetetrayltetrakis-[1,1′-biphenyl]-4-carboxylic acid; DMF, N,N-dimethylformamide; SCXRD, Single crystal X-ray diffraction; TGA, thermogravimetric analysis; KSV, quenching constants; DT, detection limit; UV-vis, Ultraviolet–visible spectroscopy; E, efficiency of energy transfer ⁎ Corresponding author. E-mail addresses: [email protected] (W. Liu), [email protected] (Y. Wang), [email protected] (L. Song), [email protected] (M.A. Silver), [email protected] (J. Xie), [email protected] (L. Zhang), [email protected] (L. Chen), [email protected] (J. Diwu), [email protected] (Z. Chai), [email protected] (S. Wang). https://doi.org/10.1016/j.talanta.2018.12.088 Received 10 August 2018; Received in revised form 21 December 2018; Accepted 25 December 2018 Available online 27 December 2018 0039-9140/ © 2018 Elsevier B.V. All rights reserved.
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manifestations, Parkinson's and Wilson's diseases [25–27]. On the other hand, copper is a crucial micronutrient which is essential for a number of enzymes and other cellular activities [28]. It is therefore important to develop a proper strategy to assess the contamination conditions of these metal ions in various public resources and utilities. Nowadays, metal cation detection techniques are primarily classified into two groups: instrumental and chemical methods. Instrumental methods include, but are not limited to, inductively coupled plasma mass spectrometry (ICP-MS) [29,30], X-ray fluorescence spectroscopy [31,32], and inductively coupled plasma atomic emission spectroscopy (ICP-AES) [33,34], which typically suffer from being expensive and requiring sophisticated operation by experienced personnel. Chemical methods are a much more facile and cheaper route; however, their effectiveness may still be limited by the production of secondary pollutants, the inability to operate as a multifunctional material under a variety of conditions, and so forth. As a consequence, developing an efficient probe for detecting and quantifying both Cu2+ and UO22+ within one chemical platform, whose performance abides by being low cost, highly responsive, and without hazardous waste or the need of special instrumentation is highly desirable in mineral prospecting and environmental contamination monitoring. Herein, we present the investigation of a luminescent europium metal-organic framework, compound 1, for multifunctional applications as the first PL sensor for Cu2+ and UO22+ cations. Compound 1 bears two hydroxyl groups in each asymmetric unit that provides the opportunity for fast adsorption of metal cations, which directly affects the luminescence signal with rapid response. Notably, the luminescence is highly sensitive and selective towards Cu2+ and UO22+ and the detection limit is determined to be 17.2 μg/L and 309.2 μg/L for each ion, respectively. The detection limit for Cu2+ falls below the tolerable maximum contamination concentration of 2 mg/L Cu2+ in drinking water as defined by U.S. Environmental Protection Agency [35]. Additionally, the quenching of the luminescence in compound 1 by Cu2+ and UO22+ adsorption adopt completely different mechanisms. The quenching effect of Cu2+ manifests in the re-absorption of emitted light from Eu3+, while at the same time UO22+ impairs the luminescence intensity by competing for absorption of the excitation light. The energy transfer efficiency is determined to be 0.57 and 0.16 between the framework and Cu2+ and UO22+ ions, respectively, which further explains the inherent reason for the much lower detection limit toward Cu2+ ions. These mechanisms provide considerable properties for designing multifunctional luminescent MOF probe for both radioactive and nonradioactive elements.
technique and a CMOS detector at 273 K. The data collection was carried out using the program APEX3 and processed using the SAINT routine. The structure of compound 1 was solved by direct methods and refined using the full-matrix least squares on F2 in the SHELXTL2014 program. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms attached to carbon atoms were placed in geometrically idealized positions. Crystallographic data of compound 1 are summarized in Tables S1 and S2 [36]. 2.3. Cation exchange kinetics studies Cation exchange experiments were conducted at room temperature in DMF:H2O = 1:1 (v/v), with a solid/liquid ratio of 1 g L-1. The adsorption kinetics experiment was conducted by immersing 10 mg of compound 1 as a finely ground powder into 10 mL of 10 mg/L UO22+ and Cu2+ nitrate solution. This mixture was stirred for 24 h before sampling. All solution samples were filtered through a 0.22 µm nylon membrane filter. The exchange process was monitored with ICP-AES. 2.4. Sorption isotherm experiments The adsorption isotherm was conducted by immersing 10 mg of compound 1 as a finely ground powder into 10 mL of DMF:H2O = 1:1 (v/v) containing various concentration of UO22+ and Cu2+ ions (from 0.2 mg/L to 500 or 800 mg/L, respectively). The mixture was then shaken for 24 h to ensure the adsorption process reached equilibrium. The resulting solutions were filtered with a 0.22 µm nylon membrane filter. The concentration of the solution was monitored by ICP-AES. 2.5. Uranium and copper concentration-dependent luminescence spectra The material of compound 1 was finely ground before used. 10 mg of compound 1 was dispersed into 2 mL of DMF:H2O = 1:1 (v/v), then sonicated for 3 min to prepare a homogeneous suspension. 50 μL of the suspension was added into UO22+ and Cu2+ solutions with concentrations varying from 1 to 300 mg/L and 1–800 mg/L, respectively. The mixture was sonicated for another 3 min to form a homogeneous suspension and aged for an hour. Luminescence spectra of these suspensions were then collected. All spectra were collected three times and the averaged spectra were used to establish the curve. 2.6. Influence of competing cations Certain amounts of compound 1 (2 mg) was dispersed into 2 mL of 300 mg/L M(NO3)x•n(H2O) (M = Na+, Mg2+, Ni2+, Co2+, Pd2+, Sr2+, Zn2+, Cu2+, Cr3+, and UO22+) solutions (DMF:H2O = 1:1 v/v). The mixtures were then sonicated for 3 min to prepare a homogeneous suspension. Luminescence spectra for all samples were collected. 2 mg of compound 1 was dispersed into 2 mL DMF:H2O = 1:1 (v/v) solution without the addition of a metal cation to use as a comparable blank sample. On the other hand, in order to demonstrate the selectivity in real sample matrixes. We also checked the selectivity in a mixture of Dushu lake water and DMF solution containing 100 mg/L metal cations through similar method. All spectra were collected three times and the averaged spectra were used.
2. Experimental section 2.1. Synthesis procedure Compound 1 was synthesized through our previously reported method [36]. A mixture of 0.12 mmol (40 mg) of Eu(CH3COO)3, 0.025 mmol (20 mg) of 4′,4′,4′,4′-methanetetrayltetrakis-[1,1′-biphenyl]-4-carboxylic acid (H4MTBC), 4 mL of DMF (N,N-dimethylformamide) and 45 μL of concentrated HCl solution (37.5 wt%) were added to a 10 mL glass vial and heated at 100 °C for 12 h. In order to obtain a pure phase, the reaction vial was shaken for 10 s every 4 h. The reaction system was cooled to room temperature at a rate of 0.6 °C min-1. The needle-like colourless crystals that were obtained were washed with ethanol three times and then dried in air. Elemental analysis results: calcd. C, 48.75%; N, 4.30%; H, 5.59%; found C 46.92%; N, 4.20%; H, 4.83%.
3. Results and discussion 3.1. Structure and general characterizations SCXRD analysis reveals that compound 1 crystallizes in the triclinic space group, P-1. There are two independent Eu3+ centers in each asymmetric unit, both of which are eight-coordinate and linked by two MTBC ligands. As depicted in Fig. 1a, the coordination environment of Eu1 is composed of four oxygen atoms from two carboxylate groups, two terminating water molecules, one coordinating hydroxyl group
2.2. X-ray crystallography studies Single crystal X-ray diffraction (SCXRD) data were collected using a Bruker D8-Venture diffractometer with a Turbo X-ray Source (Mo Kα radiation, λ = 0.71073 Å) adopting the direct-drive rotating anode 516
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Fig. 1. Crystal structure depictions of compound 1, all hydrogen atoms have been omitted for clarity: (a) Coordination environment of Eu1 and Eu2. (b) The asymmetric unit. (c) 2D layer structure. (d) The undulate-layer packed structure with ABAB••••• packing mode. Atom colors:Eu = orange, O = light blue, C = gray, N = blue.
(O3, BVS results are shown in Table S3) and one DMF molecule [36]. The coordination geometry of Eu2 differs slightly from Eu1 in that one additional DMF molecule coordinates in place of one water molecule. For both Eu3+ environments, the carboxylate groups from MTBC ligands are chelating. There is one tetradentate MTBC ligand in the asymmetric unit of compound 1 adopting a tetrahedral configuration (Fig. 1b). The angles of L1-Eu1-L1 and L2-Eu2-L2 are approximately 110.2° and 109.6°, respectively, based on the axes between two independent MTBC ligands and the Eu vertex. The connections of the neighboring Eu and ligands result in a two-dimensional layer structure. Furthermore, a large window with an effective size of ca. 21 Å × 21 Å appears within a single layer of compound 1 (Fig. 1c); however, the sheets pack into a pseudo-3D structure, thereby shrinking the channels to ca. 6.5 Å × 6.2 Å in order to avoid the large void space in the layered structure (Fig. 1d). Consequently, the overall structure of compound 1 can be described as a 2D layer-packing framework with ABAB••••• packing mode. To further understand the complicated packing motif, the MTBC ligand can be simplified to a 4-connection node, and each Eu metal is reduced to a “V”-like connection mode with Eu at the vertex of “V”, the 2D layer of compound 1 was finally come down to an “undulate-layer”.
corresponds to the loss of coordinated DMF molecules. The framework completely collapsed when the temperature reached 600 °C (Fig. S1) [36]. The phase purity of the bulk sample was confirmed by the PXRD study. As shown in Fig. S2 the PXRD pattern of the as-synthesized material exhibits good agreement with the simulated pattern obtained from the single crystal data. These results also suggested that compound 1 possesses good hydrolytic stability in various metal cation solutions. The PXRD patterns of the samples after treatment with 300 mg/L metal ions solutions in Fig. S2 reveal that compound 1 maintains its integrity under these conditions, which is an important feature for a detection probe. 3.3. Sensing and adsorption properties The metal cation detection capacity was first investigated by treating compound 1 with a series of metal cations in mixed solution of DMF/H2O = 1:1. Finely ground powder was immersed into 300 mg/L M (NO3)x·n(H2O) solutions (M = Na+, Sr2+, Co2+, Cr3+, Ni2+, Mg2+, Pd2+, Zn2+, Cu2+, or UO22+), in addition to a blank for comparison. As shown in Fig. S3, the impact that all metal cation solutions had on the shape of the emission spectra were negligible. This phenomenon could also be observed in real natural water sample indicating the real application of the material (Fig. S4). The luminescence spectra of all samples displayed the characteristic transitions for trivalent europium at 592, 617, and 651 nm (5D0 → 7FJ, J = 1, 2, 3, respectively) [37]. The intensity of the luminescence from Eu3+ in compound 1, however, is observably affected in solutions containing Cu2+ and UO22+. As shown in Fig. 2a, this phenomenon occurs only in the presence of Cu2+ and UO22+, and the influence from other metal cations on the luminescence of compound 1 is negligible. The ability to perceive this
3.2. Stability test The reaction of H4MTBC and Eu(CH3COO)3 under solvothermal condition produces colourless needle-like crystals. The molecular formula of compound 1 was determined to be [Eu2(MTBC) (OH)2(DMF)3(H2O)4]·2DMF·7H2O, based on the combination of SCXRD, thermogravimetric and elemental analysis. TGA showed ~ 16.37% weight loss until 112 °C which can be correlated to the loss of solvent molecules within the lattice. The subsequent weight loss (~ 12.48%) 517
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[Cu2+] = 60 mg/L, and is almost completely quenched at [Cu2+] = 500 mg/L. This, again, can be clearly witnessed with the naked eye under 365 nm light, as depicted in the inset of Fig. 3a, which bolsters the described sensitive luminescence response by compound 1 towards Cu2+. Interestingly, as shown in Fig. 3b, when the quenching ratio (denoted as (I0-I)/I0, %) was plotted against the respective [Cu2+], the generated curve could be simulated by a Langmuir model (R2 = 0.996). This phenomenon clearly demonstrates the relationship between luminescence evolution and the uptake process. More importantly, when the equation was mathematically transformed into the correlation between C/[(I0-I)/I0] (%) and [Cu2+], a linear plot (R2 = 0.999) was established, whose equation can be used to quantify the concentration of Cu2+ accurately within a relatively wide concentration range (0–500 mg/L). Similar trends from the concentration-dependent luminescence data of compound 1 immersed in UO22+ was also observed as discussed below, supporting a quenching law that is identical for these metal cations and compound 1. Based on our previous work involving the use of MOFs to detect metal cations, we proposed that the mechanism of distorting the luminescence in compound 1 could be used to detect Cu2+ and UO22+ and that the mechanism is directly correlated to the adsorption process of these two cations [39]. Hence, the adsorption property of compound 1 for Cu2+ and UO22+ were initially evaluated by EDX mapping experiments. As shown in Fig. 4a and b, the adsorbed Cu2+ and UO22+ ions on the surface of the crystals of compound 1 are as homogeneously distributed as the in-situ incorporated Eu3+ within the framework, revealing an ideal adsorption property of compound 1 toward these two cations. This supports the assumption that compound 1 should possess proficient adsorption ability for both Cu2+ and UO22+ cations. To bolster this further, the Cu2+ and UO22+ adsorption kinetics and isotherms in compound 1 were investigated by immersing the powder of compound 1 in 10 mg/L solutions of either cation. Because the sorption of these two cations relies on the same monolayer adsorption mechanism common in MOF-based adsorbents [40], an explanation of the kinetics of Cu2+ will suffice in developing our understanding of the concentration-dependent luminescence previously observed. As shown in Fig. 5a, the adsorption of Cu2+ undergoes a similar process as that of the luminescence experiments (Fig. 3b). The uptake of Cu2+ ions reaches 81.3% of the maximum adsorption and equilibrates within 30 min, indicating that the adsorption of Cu2+ could reach equilibrium in a relatively short time. This property is imperative to improving the response efficiency of MOF-based luminescence probes. The adsorption isotherm, as shown in Fig. 5b, could be fitted well using a Langmuir model (R2 = 0.99), revealing the monolayer adsorption process of compound 1. Just as the equilibrated [Cu2+] increased from 1 to ~550 mg/L, so too did the adsorption capacity, until reaching a maximum value of ca. 91.8 mg(Cu)/g. These results indicate a commendable pre-concentration capacity in compound 1 towards Cu2+, and with knowledge of the luminescence quenching process undergoing a similar tendency which could be fitted by a Langmuir model, these properties further support the use of this material as a highly-responsive luminescent probe for determining Cu2+ concentration. The uranium detection capacity in terms of concentration dependent luminescence spectra, detection curve establishing, adsorption kinetics and isotherm investigation were also studied. As shown in Fig. 6a and b, compound 1 exhibits characteristic transition peak of Eu3+ at 592, 616 and 653 nm as in the uranyl solutions which are ascribed to the 5D0 → 7FJ (J = 1–3) transitions. The peak shape transformation was not observed which initially indicating that compound 1 can maintain its integrity under these conditions. As shown in Fig. 6a, similar to the Cu2+ detection process, with the increase of the uranium concentration, the luminescence intensity of compound 1 attenuated sharply as well. The quenching process could also be more quantitatively described by plotting the quenching ratio (denoted as (I0-I)/I0%) against uranium concentration as exhibits in Fig. 6b. Besides, the
Fig. 2. (a) Luminescence intensity (λEm = 616 nm, λExc = 345 nm) of compound 1 immersed in 300 mg/L M solutions (M = Na+, Co2+, Mg2+, Ni2+, Pd2+, Sr2+, Zn2+, Cr3+, Mn2+, UO22+, and Cu2+). (b) Corresponding photograph of the luminescence of compound 1 immersed in various metal cation solutions.
interaction with the naked eye under 365 nm light, as shown in Fig. 2b, alludes the use of compound 1 as a luminescent probe for Cu2+ and UO22+ through exploitation of this cogent phenomenon. In order to accurately evaluate the sensitivity and selectivity of compound 1 as a Cu2+ and UO22+ probe, the quenching constants (KSV) of compound 1 treated with various metal cation solutions were determined by Stern−Volmer equation [38]:
I0 /I = KSV [C] + 1
(1)
Here, I0 is the luminescence intensity of compound 1 in the blank solution, I represents the luminescence intensity of compound 1 in various metal ions solutions, and [C] is the molarity of the metal cations. As depicted in Table S4, KSV of compound 1 toward Cu2+ and UO22+ are 2251.4 and 3631.5 M-1, respectively, indicating a selective quenching capacity for these cations. Besides, the luminescence intensity of the material also exhibits less dependent on the pH values and several selected organic compounds which implies the material may potentially be used under environmental related conditions (Figs. S5 and S6). Since compound 1 demonstrates excellent selectivity and sensitivity in detecting of Cu2+ and UO22+, concentration-dependent luminescence spectra were collected to further investigate the evolution of this optical phenomenon and confirm the detection limits for these cations. Samples of solid powder of compound 1 were immersed into Cu2+ solutions (DMF/H2O = 1:1) of various concentrations ranging from 0 to 500 mg/L before collecting photoluminescence spectra. As shown in Fig. 3a, the luminescence spectra display the three characteristic transitions of Eu3+ metal centered at 592, 617 and 651 nm. The distinct difference is the gradual interference that Cu2+ has on the intensity of these transitions inherent of Eu3+ after immersion of compound 1 in increasingly concentrated Cu2+ solutions. The same behaviour is observed for samples of compound 1 immersed in UO22+ solutions, therefore a similar process is considered to occur between compound 1 and these metal cations. Moving forward, the emission intensity was impaired by ca. 80.85% of the initial luminescence when 518
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Fig. 3. (a) Luminescence spectra of compound 1 in solutions of varying concentrations of Cu2+. Inset: photograph of the luminescence of compound 1 immersed in 0 and 200 mg/L Cu2+ solutions (λExc = 345 nm). (b) The correlation between quenching ratio versus concentration simulated by a Langmuir model. Inset: correlation between C/[(I0− I)/I0] (%) versus [Cu2+].
quenching process could also be well fitted by Langmuir model with R2 = 0.95. The uranium adsorption kinetics and isotherms were also investigated to confirm the correlation between the luminescence evolution and uranium adsorption process. As shown in Fig. 6c, with the contact time increase the adsorption of uranium increase as well, it only takes about 60 min to reach the equilibrium. As for the sorption isotherm demonstrated in Fig. 6d, it is obvious that similar evolution process was observed for both the concentration dependent luminescence evolution and adsorption isotherm which is a direct evidence for the inherent correlation between these two processes. The high KSV value also indicative of the high sensitivity of compound 1 toward Cu2+ and UO22+ ions. Hence, the detection limit of compound 1 toward Cu2+ and UO22+ were calculated based on the above concentration dependent luminescence data. Here we define the decrease of the luminescence intensity three times higher than the standard deviation as the spectrograph detectable signal. And the detection limit (DT) was derived from the following equations [41]:
DT = 3 /k = 100 × (ISE / I0)
Where, the standard deviation (σ) is defined by 100 × (ISE/I0), ISE is the emission intensity of the used solution without compound 1 and any cations (monitored at 616 nm). I0 is the initial luminescence intensity of compound 1 before treated with metal solutions. k is the slope obtained by the linear fitting of the concentration dependent luminescence intensity evolution in the low concentration range. The detection limit toward Cu2+ and UO22+ ions are determined as ca. 17.2 μg/L and 309.2 μg/L. The detailed determination procedure is displayed in Figs. S7-S9. Clearly, the detection limit of Cu2+ even lower than the tolerable maximum contamination concentration of 2 mg/L and 1.3 mg/L Cu2+ in drinking water defined by U.S. EPA and WHO which confirms that compound 1 could be viewed as a powerful luminescence Cu2+ probe. However, the detection limit of UO22+ is relative higher than our recently reported value of 0.9 μg/L in aqueous solution [39]. This may induced by the inefficient energy transfer from the ligand to the guest UO22+ ions. Nevertheless, it is still promising for the application in high level contaminated areas. Moreover, the detection limit of compound 1 toward Cu2+ and UO22+ are all at a very low level compared with the reported MOF based Cu2+ and UO22+ probes. Most of these reported MOFs are lack of selectivity or stability which can not be utilized for real application (Tables S5-S6). On the other hand, the method
(2) (3)
Fig. 4. (a) Left to right: Microphotograph of Cu2+-loaded compound 1, Eu and Cu elemental mapping of Cu2+-loaded compound 1. (b) Left to right: Microphotograph of the UO22+-loaded compound 1, Eu and U elemental mapping of UO22+-loaded compound 1. 519
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Fig. 5. (a) The adsorption kinetics of compound 1 in 10 mg/L Cu2+ solution. (b) Adsorption isotherm for Cu2+ sorption on compound 1.
proposed in this contribution could be directly used for probing both Cu2+ and UO22+ contamination in a wide concentration range (17.2 μg/L-500 mg/L for Cu2+ and 309.2 μg/L to 800 mg/L for UO22+). Note that this can not be achieved by traditional analytical methods. In order to confirm the performance of the material in real environmental sample for the determination of Cu2+ and UO22+ contamination. The contamination level of the artificial Cu2+ and UO22+ contaminated
sample (prepared by Dushu lake water, Jiangsu Province, China) was evaluated by ICP-AES and our method. As shown in Fig. S10 and S11, the concentration of Cu2+ determined by ICP-AES and the presented method are 4.02 mg/L and 4.43 mg/L, respectively. On the other hand, the UO22+ concentration is determined to be 18.70 mg/L and 19.19 mg/L respectively. These values are actually very close which indicative of the potential real application of the material.
Fig. 6. (a) Luminescence spectra of compound 1 in varied concentrations of UO22+. (b) The correlation between the quenching ratio versus concentration simulated by a Langmuir model. The inset is the correlation between C/[(I0− I)/I0]% versus UO22+ concentration. (c) The UO22+ adsorption kinetics curve (C0 = 10 mg/L). (d) Adsorption isotherm curve of UO22+ simulated by a Langmuir model. 520
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Fig. 7. (a) Combined UV-vis absorption spectra of compound 1, and Cu2+- and UO22+-loaded species, as well as photoluminescence spectrum of compound 1. Inset: excitation spectrum of compound 1 monitored at 616 nm. (b) The photoluminescence decay time of Eu3+ centers in free, Cu2+- and UO22+-loaded compound 1 monitored at 616 nm.
Generally, several popular quenching mechanisms in luminescent MOFs have been proposed by the former researchers: (a) the degradation of the framework, (b) the substitution of the metal nodes by the guest metal, (c) and the interaction between the framework and the guest molecules that results in efficient energy transfer that diminishes the intensity of luminescence [42,43]. Based on stability investigations from the PXRD pattern, it is clear that the framework retains its integrity even in highly concentrated metal solutions and that the quenching process does not develop from the collapse of the framework. Additionally, these data also confirm the inability for metal nodes to exchange throughout these particular experiments. Hence, the most logical mechanism which could be construed from the aforementioned data is an energy transfer process that occurs from the interaction of Cu2+ and UO22+ with the framework. In order to confirm this speculation, the UV-vis, excitation, and luminescence spectra were further investigated systematically. As shown in the inset of Fig. 7a, the excitation spectrum indicates that the luminescence of compound 1 responds to 345 nm light. At the same time, UO22+ strongly absorbs 345 nm UV light, which creates competition between UO22+ and compound 1 for optical excitation. Going further, no absorption peak appears throughout the entire emission range of compound 1 which demonstrates that the re-absorption of the emitted light does not occur under these conditions. Hence, the competition for absorption of the excitation light will more or less contribute to the quenching effect observed by uranyl ions. In contrast to UO22+, Cu2+-loaded material does not absorb light at 345 nm and therefore cannot have the same effect as that of uranyl. Notably, the mantle area between the UV-vis spectrum of Cu2+-loaded sample and the emission spectrum of compound 1 clearly demonstrates the re-absorption of the emitted light from compound 1 which may also contribute to the luminescence quenching phenomenon observed. However the above speculations should not represent the overall mechanism of the quenching effect because re-absorption of the emitted light and the competition for absorption of the excitation light cannot lead to the change of luminescence lifetime. As illustrated in Fig. 7b, the lifetimes of the excited state 5D0 (Eu3+) within the initial, and Cu2+and UO22+-loaded MOFs were monitored at 616 nm. The 5D0 lifetime of the unloaded material was determined to be ca. 341 μs. However, when Cu2+ or UO22+ were loaded into compound 1, the lifetime significantly decreased by approx. 15.8% and 57.2%, respectively. This establishes that energy transfer occurs throughout the adsorption process. In order to quantify the energy transfer process, the efficiency of energy transfer (E) between the host framework and guest ions were calculated using the following equation [44]:
E = 1 –
da
/
d
(4)
Where τda and τd are the lifetime of the excited state of compound 1 in the presence and absence of either acceptor (Cu2+ or UO22+), respectively. As calculated, the energy transfer efficiency between compound 1 and Cu2+ and UO22+ are 0.57 and 0.16, respectively. Clearly, the energy transfer is much more efficient between compound 1 and Cu2+ than UO22+. This also could explain why the detection limit toward Cu2+ is much lower than UO22+. 4. Conclusions In summary, a highly luminescent europium(III) organic framework (compound 1) was successfully synthesized through conventional solvothermal methods and exhibits an interesting sensing dynamic towards nonradioactive Cu2+ and radioactive UO22+ ions. In order to explore the potential application as a Cu2+ and UO22+ probe, the selectivity, concentration-dependent luminescence intensity, and detection limit were systematically investigated. The calculated detection limits toward Cu2+ and UO22+ were 17.2 μg/L and 309.2 μg/L, respectively. The detection capacity of compound 1 for Cu2+ is much lower than the EPA standard for Cu2+ content in drinking water. However, compound 1 exhibits less detection capacity for UO22+, but this does not discount its use at heavily contaminated sites, such as uranium mines and processing facilities. Furthermore, the batch experiments and spectroscopy studies were comprehensively conducted to confirm the internal detection mechanism with compound 1. In conclusion, compound 1 represents a rare case in which the luminescence of a MOF could serve as a probe for both nonradioactive and radioactive heavy metal cations, providing an additional dimension by which these versatile materials can be exploited to. Acknowledgements We appreciate the National Natural Science Foundation of China (21790370, 21790374, 21601131, 21761132019), the Science Challenge Project (TZ2016004), the "Young Thousand Talented Program", and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for providing financial support of this work. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2018.12.088 521
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Efficient and selective sensing of Cu2+ and UO22+ by a europium metal-organic framework
T
Wei Liu, Yanlong Wang, Liping Song, Mark A. Silver, Jian Xie, Linmeng Zhang, Lanhua Chen, ⁎ Juan Diwu, Zhifang Chai, Shuao Wang State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China
ARTICLE INFO
ABSTRACT
Keywords: Luminescent metal organic frameworks Adsorption Copper and uranium contamination Detection
We report here the investigation of using a luminescent europium organic framework, [Eu2(MTBC) (OH)2(DMF)3(H2O)4]·2DMF·7H2O (denoted as compound 1), for detecting of both Cu2+ and UO22+ with high sensitivity. Based on the spectroscopy analysis, compound 1 could selectively respond to Cu2+ and UO22+ ions among other selected monovalent, divalent, trivalent metal cations based on a turn-off mechanism. The detection limit of compound 1 towards Cu2+ ion was as low as 17.2 μg/L, which is much lower than the maximum tolerable concentration of Cu2+ in drinking water (2 mg/L) defined by United States Environmental Protection Agency. On the other hand, the detection limit towards UO22+ ions is 309.2 μg/L, which could be used for detecting uranium in relative severely contaminated areas. The concentration-dependent luminescence intensity evolution process could be fully understood by the absorption kinetics and isotherm investigations. Furthermore, the quenching mechanism was elucidated by the UV-vis, excitation, luminescence, and lifetime studies. Compound 1, as the first MOF based luminescence probe for both Cu2+ and UO22+ ions, provides insight into developing MOF-based multifunctional sensors for both nonradioactive and radioactive elements.
1. Introduction Metal-organic frameworks (MOFs) are freshly raised extended crystalline structures which assemble by replaceable cation nodes and organic linkers [1]. Owing to their unparalleled tunability and structural diversity, MOFs are extensively realized for a variety of applications, such as gas storage and separation [2,3], controlled drug delivery and release [4,5], heterogeneous catalysis [6], proton conductivity [7,8], as well as luminescent materials [9–11]. Particularly, luminescent MOFs, which generally involve an emissive organic ligand or metal center, display promising potential as luminescent probes toward heavy metal cations [12,13], toxic organic pollutants [14,15], UV light [16], X-rays and γ-radiation [17], and so on. These MOF-based probes are primarily aimed at developing eco-friendly, low cost, versatile, sensitive, and multifunctional utility [18,19]. Amongst them, multifunctional MOFs have been widely investigated for various
applications, especially for luminescent MOF-based metal cation probes. However, these probes are rarely utilized for the recognition of both radioactive and nonradioactive elements. Contamination from uranium sources has become a global concern due to the rapid expansion of nuclear science and technology during the last century. This element could be readily found in various publically accessible industries, such as uranium mining and processing, glass manufacturing, and nuclear power stations, which all share a significant hazard to human health. Radiotoxicity accompanied by chemotoxicity is what makes the ingestion or inhalation of uranium a problem that could lead to serious kidney damage [20], disruption of bioactive molecules (enzymes) [21], as well as a series of other violent health problems [22–24]. Contrary to uranium, copper is nonradioactive, however, it can still could induce serious health issues because of its heavy metal nature. Excess intake of copper may cause severe neurodegenerative diseases, such as hematological
Abbreviations: MOFs, Metal-organic frameworks; ICP-MS, inductively coupled plasma mass spectrometry; ICP-AES, inductively coupled plasma atomic emission spectroscopy; H4MTBC, 4′,4′,4′,4′-methanetetrayltetrakis-[1,1′-biphenyl]-4-carboxylic acid; DMF, N,N-dimethylformamide; SCXRD, Single crystal X-ray diffraction; TGA, thermogravimetric analysis; KSV, quenching constants; DT, detection limit; UV-vis, Ultraviolet–visible spectroscopy; E, efficiency of energy transfer ⁎ Corresponding author. E-mail addresses: [email protected] (W. Liu), [email protected] (Y. Wang), [email protected] (L. Song), [email protected] (M.A. Silver), [email protected] (J. Xie), [email protected] (L. Zhang), [email protected] (L. Chen), [email protected] (J. Diwu), [email protected] (Z. Chai), [email protected] (S. Wang). https://doi.org/10.1016/j.talanta.2018.12.088 Received 10 August 2018; Received in revised form 21 December 2018; Accepted 25 December 2018 Available online 27 December 2018 0039-9140/ © 2018 Elsevier B.V. All rights reserved.
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manifestations, Parkinson's and Wilson's diseases [25–27]. On the other hand, copper is a crucial micronutrient which is essential for a number of enzymes and other cellular activities [28]. It is therefore important to develop a proper strategy to assess the contamination conditions of these metal ions in various public resources and utilities. Nowadays, metal cation detection techniques are primarily classified into two groups: instrumental and chemical methods. Instrumental methods include, but are not limited to, inductively coupled plasma mass spectrometry (ICP-MS) [29,30], X-ray fluorescence spectroscopy [31,32], and inductively coupled plasma atomic emission spectroscopy (ICP-AES) [33,34], which typically suffer from being expensive and requiring sophisticated operation by experienced personnel. Chemical methods are a much more facile and cheaper route; however, their effectiveness may still be limited by the production of secondary pollutants, the inability to operate as a multifunctional material under a variety of conditions, and so forth. As a consequence, developing an efficient probe for detecting and quantifying both Cu2+ and UO22+ within one chemical platform, whose performance abides by being low cost, highly responsive, and without hazardous waste or the need of special instrumentation is highly desirable in mineral prospecting and environmental contamination monitoring. Herein, we present the investigation of a luminescent europium metal-organic framework, compound 1, for multifunctional applications as the first PL sensor for Cu2+ and UO22+ cations. Compound 1 bears two hydroxyl groups in each asymmetric unit that provides the opportunity for fast adsorption of metal cations, which directly affects the luminescence signal with rapid response. Notably, the luminescence is highly sensitive and selective towards Cu2+ and UO22+ and the detection limit is determined to be 17.2 μg/L and 309.2 μg/L for each ion, respectively. The detection limit for Cu2+ falls below the tolerable maximum contamination concentration of 2 mg/L Cu2+ in drinking water as defined by U.S. Environmental Protection Agency [35]. Additionally, the quenching of the luminescence in compound 1 by Cu2+ and UO22+ adsorption adopt completely different mechanisms. The quenching effect of Cu2+ manifests in the re-absorption of emitted light from Eu3+, while at the same time UO22+ impairs the luminescence intensity by competing for absorption of the excitation light. The energy transfer efficiency is determined to be 0.57 and 0.16 between the framework and Cu2+ and UO22+ ions, respectively, which further explains the inherent reason for the much lower detection limit toward Cu2+ ions. These mechanisms provide considerable properties for designing multifunctional luminescent MOF probe for both radioactive and nonradioactive elements.
technique and a CMOS detector at 273 K. The data collection was carried out using the program APEX3 and processed using the SAINT routine. The structure of compound 1 was solved by direct methods and refined using the full-matrix least squares on F2 in the SHELXTL2014 program. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms attached to carbon atoms were placed in geometrically idealized positions. Crystallographic data of compound 1 are summarized in Tables S1 and S2 [36]. 2.3. Cation exchange kinetics studies Cation exchange experiments were conducted at room temperature in DMF:H2O = 1:1 (v/v), with a solid/liquid ratio of 1 g L-1. The adsorption kinetics experiment was conducted by immersing 10 mg of compound 1 as a finely ground powder into 10 mL of 10 mg/L UO22+ and Cu2+ nitrate solution. This mixture was stirred for 24 h before sampling. All solution samples were filtered through a 0.22 µm nylon membrane filter. The exchange process was monitored with ICP-AES. 2.4. Sorption isotherm experiments The adsorption isotherm was conducted by immersing 10 mg of compound 1 as a finely ground powder into 10 mL of DMF:H2O = 1:1 (v/v) containing various concentration of UO22+ and Cu2+ ions (from 0.2 mg/L to 500 or 800 mg/L, respectively). The mixture was then shaken for 24 h to ensure the adsorption process reached equilibrium. The resulting solutions were filtered with a 0.22 µm nylon membrane filter. The concentration of the solution was monitored by ICP-AES. 2.5. Uranium and copper concentration-dependent luminescence spectra The material of compound 1 was finely ground before used. 10 mg of compound 1 was dispersed into 2 mL of DMF:H2O = 1:1 (v/v), then sonicated for 3 min to prepare a homogeneous suspension. 50 μL of the suspension was added into UO22+ and Cu2+ solutions with concentrations varying from 1 to 300 mg/L and 1–800 mg/L, respectively. The mixture was sonicated for another 3 min to form a homogeneous suspension and aged for an hour. Luminescence spectra of these suspensions were then collected. All spectra were collected three times and the averaged spectra were used to establish the curve. 2.6. Influence of competing cations Certain amounts of compound 1 (2 mg) was dispersed into 2 mL of 300 mg/L M(NO3)x•n(H2O) (M = Na+, Mg2+, Ni2+, Co2+, Pd2+, Sr2+, Zn2+, Cu2+, Cr3+, and UO22+) solutions (DMF:H2O = 1:1 v/v). The mixtures were then sonicated for 3 min to prepare a homogeneous suspension. Luminescence spectra for all samples were collected. 2 mg of compound 1 was dispersed into 2 mL DMF:H2O = 1:1 (v/v) solution without the addition of a metal cation to use as a comparable blank sample. On the other hand, in order to demonstrate the selectivity in real sample matrixes. We also checked the selectivity in a mixture of Dushu lake water and DMF solution containing 100 mg/L metal cations through similar method. All spectra were collected three times and the averaged spectra were used.
2. Experimental section 2.1. Synthesis procedure Compound 1 was synthesized through our previously reported method [36]. A mixture of 0.12 mmol (40 mg) of Eu(CH3COO)3, 0.025 mmol (20 mg) of 4′,4′,4′,4′-methanetetrayltetrakis-[1,1′-biphenyl]-4-carboxylic acid (H4MTBC), 4 mL of DMF (N,N-dimethylformamide) and 45 μL of concentrated HCl solution (37.5 wt%) were added to a 10 mL glass vial and heated at 100 °C for 12 h. In order to obtain a pure phase, the reaction vial was shaken for 10 s every 4 h. The reaction system was cooled to room temperature at a rate of 0.6 °C min-1. The needle-like colourless crystals that were obtained were washed with ethanol three times and then dried in air. Elemental analysis results: calcd. C, 48.75%; N, 4.30%; H, 5.59%; found C 46.92%; N, 4.20%; H, 4.83%.
3. Results and discussion 3.1. Structure and general characterizations SCXRD analysis reveals that compound 1 crystallizes in the triclinic space group, P-1. There are two independent Eu3+ centers in each asymmetric unit, both of which are eight-coordinate and linked by two MTBC ligands. As depicted in Fig. 1a, the coordination environment of Eu1 is composed of four oxygen atoms from two carboxylate groups, two terminating water molecules, one coordinating hydroxyl group
2.2. X-ray crystallography studies Single crystal X-ray diffraction (SCXRD) data were collected using a Bruker D8-Venture diffractometer with a Turbo X-ray Source (Mo Kα radiation, λ = 0.71073 Å) adopting the direct-drive rotating anode 516
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Fig. 1. Crystal structure depictions of compound 1, all hydrogen atoms have been omitted for clarity: (a) Coordination environment of Eu1 and Eu2. (b) The asymmetric unit. (c) 2D layer structure. (d) The undulate-layer packed structure with ABAB••••• packing mode. Atom colors:Eu = orange, O = light blue, C = gray, N = blue.
(O3, BVS results are shown in Table S3) and one DMF molecule [36]. The coordination geometry of Eu2 differs slightly from Eu1 in that one additional DMF molecule coordinates in place of one water molecule. For both Eu3+ environments, the carboxylate groups from MTBC ligands are chelating. There is one tetradentate MTBC ligand in the asymmetric unit of compound 1 adopting a tetrahedral configuration (Fig. 1b). The angles of L1-Eu1-L1 and L2-Eu2-L2 are approximately 110.2° and 109.6°, respectively, based on the axes between two independent MTBC ligands and the Eu vertex. The connections of the neighboring Eu and ligands result in a two-dimensional layer structure. Furthermore, a large window with an effective size of ca. 21 Å × 21 Å appears within a single layer of compound 1 (Fig. 1c); however, the sheets pack into a pseudo-3D structure, thereby shrinking the channels to ca. 6.5 Å × 6.2 Å in order to avoid the large void space in the layered structure (Fig. 1d). Consequently, the overall structure of compound 1 can be described as a 2D layer-packing framework with ABAB••••• packing mode. To further understand the complicated packing motif, the MTBC ligand can be simplified to a 4-connection node, and each Eu metal is reduced to a “V”-like connection mode with Eu at the vertex of “V”, the 2D layer of compound 1 was finally come down to an “undulate-layer”.
corresponds to the loss of coordinated DMF molecules. The framework completely collapsed when the temperature reached 600 °C (Fig. S1) [36]. The phase purity of the bulk sample was confirmed by the PXRD study. As shown in Fig. S2 the PXRD pattern of the as-synthesized material exhibits good agreement with the simulated pattern obtained from the single crystal data. These results also suggested that compound 1 possesses good hydrolytic stability in various metal cation solutions. The PXRD patterns of the samples after treatment with 300 mg/L metal ions solutions in Fig. S2 reveal that compound 1 maintains its integrity under these conditions, which is an important feature for a detection probe. 3.3. Sensing and adsorption properties The metal cation detection capacity was first investigated by treating compound 1 with a series of metal cations in mixed solution of DMF/H2O = 1:1. Finely ground powder was immersed into 300 mg/L M (NO3)x·n(H2O) solutions (M = Na+, Sr2+, Co2+, Cr3+, Ni2+, Mg2+, Pd2+, Zn2+, Cu2+, or UO22+), in addition to a blank for comparison. As shown in Fig. S3, the impact that all metal cation solutions had on the shape of the emission spectra were negligible. This phenomenon could also be observed in real natural water sample indicating the real application of the material (Fig. S4). The luminescence spectra of all samples displayed the characteristic transitions for trivalent europium at 592, 617, and 651 nm (5D0 → 7FJ, J = 1, 2, 3, respectively) [37]. The intensity of the luminescence from Eu3+ in compound 1, however, is observably affected in solutions containing Cu2+ and UO22+. As shown in Fig. 2a, this phenomenon occurs only in the presence of Cu2+ and UO22+, and the influence from other metal cations on the luminescence of compound 1 is negligible. The ability to perceive this
3.2. Stability test The reaction of H4MTBC and Eu(CH3COO)3 under solvothermal condition produces colourless needle-like crystals. The molecular formula of compound 1 was determined to be [Eu2(MTBC) (OH)2(DMF)3(H2O)4]·2DMF·7H2O, based on the combination of SCXRD, thermogravimetric and elemental analysis. TGA showed ~ 16.37% weight loss until 112 °C which can be correlated to the loss of solvent molecules within the lattice. The subsequent weight loss (~ 12.48%) 517
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[Cu2+] = 60 mg/L, and is almost completely quenched at [Cu2+] = 500 mg/L. This, again, can be clearly witnessed with the naked eye under 365 nm light, as depicted in the inset of Fig. 3a, which bolsters the described sensitive luminescence response by compound 1 towards Cu2+. Interestingly, as shown in Fig. 3b, when the quenching ratio (denoted as (I0-I)/I0, %) was plotted against the respective [Cu2+], the generated curve could be simulated by a Langmuir model (R2 = 0.996). This phenomenon clearly demonstrates the relationship between luminescence evolution and the uptake process. More importantly, when the equation was mathematically transformed into the correlation between C/[(I0-I)/I0] (%) and [Cu2+], a linear plot (R2 = 0.999) was established, whose equation can be used to quantify the concentration of Cu2+ accurately within a relatively wide concentration range (0–500 mg/L). Similar trends from the concentration-dependent luminescence data of compound 1 immersed in UO22+ was also observed as discussed below, supporting a quenching law that is identical for these metal cations and compound 1. Based on our previous work involving the use of MOFs to detect metal cations, we proposed that the mechanism of distorting the luminescence in compound 1 could be used to detect Cu2+ and UO22+ and that the mechanism is directly correlated to the adsorption process of these two cations [39]. Hence, the adsorption property of compound 1 for Cu2+ and UO22+ were initially evaluated by EDX mapping experiments. As shown in Fig. 4a and b, the adsorbed Cu2+ and UO22+ ions on the surface of the crystals of compound 1 are as homogeneously distributed as the in-situ incorporated Eu3+ within the framework, revealing an ideal adsorption property of compound 1 toward these two cations. This supports the assumption that compound 1 should possess proficient adsorption ability for both Cu2+ and UO22+ cations. To bolster this further, the Cu2+ and UO22+ adsorption kinetics and isotherms in compound 1 were investigated by immersing the powder of compound 1 in 10 mg/L solutions of either cation. Because the sorption of these two cations relies on the same monolayer adsorption mechanism common in MOF-based adsorbents [40], an explanation of the kinetics of Cu2+ will suffice in developing our understanding of the concentration-dependent luminescence previously observed. As shown in Fig. 5a, the adsorption of Cu2+ undergoes a similar process as that of the luminescence experiments (Fig. 3b). The uptake of Cu2+ ions reaches 81.3% of the maximum adsorption and equilibrates within 30 min, indicating that the adsorption of Cu2+ could reach equilibrium in a relatively short time. This property is imperative to improving the response efficiency of MOF-based luminescence probes. The adsorption isotherm, as shown in Fig. 5b, could be fitted well using a Langmuir model (R2 = 0.99), revealing the monolayer adsorption process of compound 1. Just as the equilibrated [Cu2+] increased from 1 to ~550 mg/L, so too did the adsorption capacity, until reaching a maximum value of ca. 91.8 mg(Cu)/g. These results indicate a commendable pre-concentration capacity in compound 1 towards Cu2+, and with knowledge of the luminescence quenching process undergoing a similar tendency which could be fitted by a Langmuir model, these properties further support the use of this material as a highly-responsive luminescent probe for determining Cu2+ concentration. The uranium detection capacity in terms of concentration dependent luminescence spectra, detection curve establishing, adsorption kinetics and isotherm investigation were also studied. As shown in Fig. 6a and b, compound 1 exhibits characteristic transition peak of Eu3+ at 592, 616 and 653 nm as in the uranyl solutions which are ascribed to the 5D0 → 7FJ (J = 1–3) transitions. The peak shape transformation was not observed which initially indicating that compound 1 can maintain its integrity under these conditions. As shown in Fig. 6a, similar to the Cu2+ detection process, with the increase of the uranium concentration, the luminescence intensity of compound 1 attenuated sharply as well. The quenching process could also be more quantitatively described by plotting the quenching ratio (denoted as (I0-I)/I0%) against uranium concentration as exhibits in Fig. 6b. Besides, the
Fig. 2. (a) Luminescence intensity (λEm = 616 nm, λExc = 345 nm) of compound 1 immersed in 300 mg/L M solutions (M = Na+, Co2+, Mg2+, Ni2+, Pd2+, Sr2+, Zn2+, Cr3+, Mn2+, UO22+, and Cu2+). (b) Corresponding photograph of the luminescence of compound 1 immersed in various metal cation solutions.
interaction with the naked eye under 365 nm light, as shown in Fig. 2b, alludes the use of compound 1 as a luminescent probe for Cu2+ and UO22+ through exploitation of this cogent phenomenon. In order to accurately evaluate the sensitivity and selectivity of compound 1 as a Cu2+ and UO22+ probe, the quenching constants (KSV) of compound 1 treated with various metal cation solutions were determined by Stern−Volmer equation [38]:
I0 /I = KSV [C] + 1
(1)
Here, I0 is the luminescence intensity of compound 1 in the blank solution, I represents the luminescence intensity of compound 1 in various metal ions solutions, and [C] is the molarity of the metal cations. As depicted in Table S4, KSV of compound 1 toward Cu2+ and UO22+ are 2251.4 and 3631.5 M-1, respectively, indicating a selective quenching capacity for these cations. Besides, the luminescence intensity of the material also exhibits less dependent on the pH values and several selected organic compounds which implies the material may potentially be used under environmental related conditions (Figs. S5 and S6). Since compound 1 demonstrates excellent selectivity and sensitivity in detecting of Cu2+ and UO22+, concentration-dependent luminescence spectra were collected to further investigate the evolution of this optical phenomenon and confirm the detection limits for these cations. Samples of solid powder of compound 1 were immersed into Cu2+ solutions (DMF/H2O = 1:1) of various concentrations ranging from 0 to 500 mg/L before collecting photoluminescence spectra. As shown in Fig. 3a, the luminescence spectra display the three characteristic transitions of Eu3+ metal centered at 592, 617 and 651 nm. The distinct difference is the gradual interference that Cu2+ has on the intensity of these transitions inherent of Eu3+ after immersion of compound 1 in increasingly concentrated Cu2+ solutions. The same behaviour is observed for samples of compound 1 immersed in UO22+ solutions, therefore a similar process is considered to occur between compound 1 and these metal cations. Moving forward, the emission intensity was impaired by ca. 80.85% of the initial luminescence when 518
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Fig. 3. (a) Luminescence spectra of compound 1 in solutions of varying concentrations of Cu2+. Inset: photograph of the luminescence of compound 1 immersed in 0 and 200 mg/L Cu2+ solutions (λExc = 345 nm). (b) The correlation between quenching ratio versus concentration simulated by a Langmuir model. Inset: correlation between C/[(I0− I)/I0] (%) versus [Cu2+].
quenching process could also be well fitted by Langmuir model with R2 = 0.95. The uranium adsorption kinetics and isotherms were also investigated to confirm the correlation between the luminescence evolution and uranium adsorption process. As shown in Fig. 6c, with the contact time increase the adsorption of uranium increase as well, it only takes about 60 min to reach the equilibrium. As for the sorption isotherm demonstrated in Fig. 6d, it is obvious that similar evolution process was observed for both the concentration dependent luminescence evolution and adsorption isotherm which is a direct evidence for the inherent correlation between these two processes. The high KSV value also indicative of the high sensitivity of compound 1 toward Cu2+ and UO22+ ions. Hence, the detection limit of compound 1 toward Cu2+ and UO22+ were calculated based on the above concentration dependent luminescence data. Here we define the decrease of the luminescence intensity three times higher than the standard deviation as the spectrograph detectable signal. And the detection limit (DT) was derived from the following equations [41]:
DT = 3 /k = 100 × (ISE / I0)
Where, the standard deviation (σ) is defined by 100 × (ISE/I0), ISE is the emission intensity of the used solution without compound 1 and any cations (monitored at 616 nm). I0 is the initial luminescence intensity of compound 1 before treated with metal solutions. k is the slope obtained by the linear fitting of the concentration dependent luminescence intensity evolution in the low concentration range. The detection limit toward Cu2+ and UO22+ ions are determined as ca. 17.2 μg/L and 309.2 μg/L. The detailed determination procedure is displayed in Figs. S7-S9. Clearly, the detection limit of Cu2+ even lower than the tolerable maximum contamination concentration of 2 mg/L and 1.3 mg/L Cu2+ in drinking water defined by U.S. EPA and WHO which confirms that compound 1 could be viewed as a powerful luminescence Cu2+ probe. However, the detection limit of UO22+ is relative higher than our recently reported value of 0.9 μg/L in aqueous solution [39]. This may induced by the inefficient energy transfer from the ligand to the guest UO22+ ions. Nevertheless, it is still promising for the application in high level contaminated areas. Moreover, the detection limit of compound 1 toward Cu2+ and UO22+ are all at a very low level compared with the reported MOF based Cu2+ and UO22+ probes. Most of these reported MOFs are lack of selectivity or stability which can not be utilized for real application (Tables S5-S6). On the other hand, the method
(2) (3)
Fig. 4. (a) Left to right: Microphotograph of Cu2+-loaded compound 1, Eu and Cu elemental mapping of Cu2+-loaded compound 1. (b) Left to right: Microphotograph of the UO22+-loaded compound 1, Eu and U elemental mapping of UO22+-loaded compound 1. 519
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Fig. 5. (a) The adsorption kinetics of compound 1 in 10 mg/L Cu2+ solution. (b) Adsorption isotherm for Cu2+ sorption on compound 1.
proposed in this contribution could be directly used for probing both Cu2+ and UO22+ contamination in a wide concentration range (17.2 μg/L-500 mg/L for Cu2+ and 309.2 μg/L to 800 mg/L for UO22+). Note that this can not be achieved by traditional analytical methods. In order to confirm the performance of the material in real environmental sample for the determination of Cu2+ and UO22+ contamination. The contamination level of the artificial Cu2+ and UO22+ contaminated
sample (prepared by Dushu lake water, Jiangsu Province, China) was evaluated by ICP-AES and our method. As shown in Fig. S10 and S11, the concentration of Cu2+ determined by ICP-AES and the presented method are 4.02 mg/L and 4.43 mg/L, respectively. On the other hand, the UO22+ concentration is determined to be 18.70 mg/L and 19.19 mg/L respectively. These values are actually very close which indicative of the potential real application of the material.
Fig. 6. (a) Luminescence spectra of compound 1 in varied concentrations of UO22+. (b) The correlation between the quenching ratio versus concentration simulated by a Langmuir model. The inset is the correlation between C/[(I0− I)/I0]% versus UO22+ concentration. (c) The UO22+ adsorption kinetics curve (C0 = 10 mg/L). (d) Adsorption isotherm curve of UO22+ simulated by a Langmuir model. 520
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Fig. 7. (a) Combined UV-vis absorption spectra of compound 1, and Cu2+- and UO22+-loaded species, as well as photoluminescence spectrum of compound 1. Inset: excitation spectrum of compound 1 monitored at 616 nm. (b) The photoluminescence decay time of Eu3+ centers in free, Cu2+- and UO22+-loaded compound 1 monitored at 616 nm.
Generally, several popular quenching mechanisms in luminescent MOFs have been proposed by the former researchers: (a) the degradation of the framework, (b) the substitution of the metal nodes by the guest metal, (c) and the interaction between the framework and the guest molecules that results in efficient energy transfer that diminishes the intensity of luminescence [42,43]. Based on stability investigations from the PXRD pattern, it is clear that the framework retains its integrity even in highly concentrated metal solutions and that the quenching process does not develop from the collapse of the framework. Additionally, these data also confirm the inability for metal nodes to exchange throughout these particular experiments. Hence, the most logical mechanism which could be construed from the aforementioned data is an energy transfer process that occurs from the interaction of Cu2+ and UO22+ with the framework. In order to confirm this speculation, the UV-vis, excitation, and luminescence spectra were further investigated systematically. As shown in the inset of Fig. 7a, the excitation spectrum indicates that the luminescence of compound 1 responds to 345 nm light. At the same time, UO22+ strongly absorbs 345 nm UV light, which creates competition between UO22+ and compound 1 for optical excitation. Going further, no absorption peak appears throughout the entire emission range of compound 1 which demonstrates that the re-absorption of the emitted light does not occur under these conditions. Hence, the competition for absorption of the excitation light will more or less contribute to the quenching effect observed by uranyl ions. In contrast to UO22+, Cu2+-loaded material does not absorb light at 345 nm and therefore cannot have the same effect as that of uranyl. Notably, the mantle area between the UV-vis spectrum of Cu2+-loaded sample and the emission spectrum of compound 1 clearly demonstrates the re-absorption of the emitted light from compound 1 which may also contribute to the luminescence quenching phenomenon observed. However the above speculations should not represent the overall mechanism of the quenching effect because re-absorption of the emitted light and the competition for absorption of the excitation light cannot lead to the change of luminescence lifetime. As illustrated in Fig. 7b, the lifetimes of the excited state 5D0 (Eu3+) within the initial, and Cu2+and UO22+-loaded MOFs were monitored at 616 nm. The 5D0 lifetime of the unloaded material was determined to be ca. 341 μs. However, when Cu2+ or UO22+ were loaded into compound 1, the lifetime significantly decreased by approx. 15.8% and 57.2%, respectively. This establishes that energy transfer occurs throughout the adsorption process. In order to quantify the energy transfer process, the efficiency of energy transfer (E) between the host framework and guest ions were calculated using the following equation [44]:
E = 1 –
da
/
d
(4)
Where τda and τd are the lifetime of the excited state of compound 1 in the presence and absence of either acceptor (Cu2+ or UO22+), respectively. As calculated, the energy transfer efficiency between compound 1 and Cu2+ and UO22+ are 0.57 and 0.16, respectively. Clearly, the energy transfer is much more efficient between compound 1 and Cu2+ than UO22+. This also could explain why the detection limit toward Cu2+ is much lower than UO22+. 4. Conclusions In summary, a highly luminescent europium(III) organic framework (compound 1) was successfully synthesized through conventional solvothermal methods and exhibits an interesting sensing dynamic towards nonradioactive Cu2+ and radioactive UO22+ ions. In order to explore the potential application as a Cu2+ and UO22+ probe, the selectivity, concentration-dependent luminescence intensity, and detection limit were systematically investigated. The calculated detection limits toward Cu2+ and UO22+ were 17.2 μg/L and 309.2 μg/L, respectively. The detection capacity of compound 1 for Cu2+ is much lower than the EPA standard for Cu2+ content in drinking water. However, compound 1 exhibits less detection capacity for UO22+, but this does not discount its use at heavily contaminated sites, such as uranium mines and processing facilities. Furthermore, the batch experiments and spectroscopy studies were comprehensively conducted to confirm the internal detection mechanism with compound 1. In conclusion, compound 1 represents a rare case in which the luminescence of a MOF could serve as a probe for both nonradioactive and radioactive heavy metal cations, providing an additional dimension by which these versatile materials can be exploited to. Acknowledgements We appreciate the National Natural Science Foundation of China (21790370, 21790374, 21601131, 21761132019), the Science Challenge Project (TZ2016004), the "Young Thousand Talented Program", and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for providing financial support of this work. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2018.12.088 521
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